Patent Description:
This application relates to the field of secondary battery technologies, and in particular, to a preparation method for a positive electrode active material.

Lithium-ion secondary batteries are rechargeable batteries that operate mainly depending on migration of lithium ions between a positive electrode and a negative electrode. They are a form of clean energy that is widely used currently. As an important part of a lithium-ion secondary battery, a positive electrode active material provides the lithium ions that move back and forth between the positive and negative electrodes in a battery charge/discharge process. Therefore, the positive electrode active material is of great importance to battery performance.

Lithium nickel cobalt manganese oxides have high theoretical capacity, and a lithium-ion secondary battery using a lithium nickel cobalt manganese oxide as a positive electrode active material is expected to obtain high energy density. However, how to make such lithium-ion secondary battery have both high energy density and good high-temperature cycling performance has become a technical issue that urgently needs to be resolved. Among the prior art, <NPL> discloses the effect of Nb doping in LiNi<NUM>Co<NUM>Mn<NUM>O<NUM> on the cycling stability and voltage decay at high rates. Moreover, <CIT> discloses a positive electrode active material for a lithium secondary battery including secondary particles capable of being doped and undoped with lithium ions, wherein the electrode active material has an α-NaFeO<NUM> crystal structure.

A first aspect of this application provides a method for preparing positive electrode active material according to claim <NUM>. The positive electrode active material which is mixed in said method includes a lithium nickel cobalt manganese oxide. In the lithium nickel cobalt manganese oxide, the number of moles of nickel accounts for <NUM>% to <NUM>% of the total number of moles of nickel, cobalt, and manganese. The lithium nickel cobalt manganese oxide has a layered crystal structure with a space group R<NUM>m. The lithium nickel cobalt manganese oxide includes a doping element. When the positive electrode active material is in a <NUM>% delithiated state, the doping element has two or more different valence states, and the amount of doping element in a highest valence state accounts for <NUM>% to <NUM>% of the total amount of doping element.

In the method for preparing a positive electrode active material provided in this application, the lithium nickel cobalt manganese oxide with high nickel content has a relatively high specific capacity, so that a lithium-ion secondary battery can have relatively high capacity performance and energy density. The lithium nickel cobalt manganese oxide is further doped with a polyvalent element (that is, an element that may have two or more different valence states). In addition, in the positive electrode active material in the <NUM>% delithiated state, the percentage of the amount of doping element in the highest valence state in the total amount of doping element is properly controlled, so that the positive electrode active material can be driven to release more lithium ions, and structural stability and high-temperature cycle stability of the positive electrode active material can be significantly improved. Therefore, with the positive electrode active material obtained by using the preparation method provided in this application this application, capacity performance and energy density of the lithium-ion secondary battery can be further improved, and high-temperature cycling performance of the lithium-ion secondary battery is improved.

In some embodiments, when the positive electrode active material is in the <NUM>% delithiated state, the amount of doping element in the highest valence state accounts for <NUM>% to <NUM>%, optionally, <NUM>% to <NUM>%, of the total amount of doping element. When the positive electrode active material satisfies the foregoing condition, the energy density and the high-temperature cycling performance of the battery can be further improved.

In any one of the foregoing embodiments, when the positive electrode active material is in the <NUM>% delithiated state, one part of the doping element has one or more valences of +<NUM>, +<NUM>, and +<NUM>, and the other part of the doping element has one or more valences of +<NUM>, +<NUM>, +<NUM>, +<NUM>, and +<NUM>. The doping element can contribute more electrons to the positive electrode active material, and can strengthen binding with oxygen atoms. With the positive electrode active material, the energy density and the high-temperature cycling performance of the battery can be improved.

In the aforementioned method, the doping element is as defined in claim <NUM>. The doping element can better provide a charge compensation effect and stabilize a structure of the positive electrode active material, thereby further improving the energy density and the high-temperature cycling performance of the battery.

In the positive electrode material obtained by the method of the present invention, a relative deviation of a local mass concentration of the doping element in a particle of the positive electrode active material may be <NUM>% or below. Distribution uniformity of the doping element in the particle is relatively high, so that structural stability of all locations in the particle can be improved, and the positive electrode active material can also have relatively high lithium ion transport performance, thereby further improving the energy density and the high-temperature cycling performance of the battery.

In the positive electrode material obtained by the method of the present invention, a true doping concentration ω of the positive electrode active material may satisfy <NUM>µg/cm<NUM> ≤ ω ≤ <NUM>µg/cm<NUM>, optionally, <NUM>µg/cm<NUM> ≤ ω ≤ <NUM>µg/cm<NUM>, or optionally, <NUM>µg/cm<NUM> ≤ ω ≤ <NUM>µg/cm<NUM>. The true doping concentration of the positive electrode active material is within an appropriate range, thereby helping ensure that the amount of doping element in the highest valence state accounts for <NUM>% to <NUM>% of the total amount of doping element in the positive electrode active material in the "<NUM>% delithiated state". In addition, the doping element can contribute more electrons to the positive electrode active material, and better stabilize the structure of the positive electrode active material, thereby further improving the energy density and the high-temperature cycling performance of the battery.

In the positive electrode material obtained by the method of the present invention, a deviation ε of a mass concentration of the doping element in the positive electrode active material relative to an average mass concentration of the doping element in a particle of the positive electrode active material may be <NUM>% or below, optionally, <NUM>% or below. When ε of the positive electrode active material is within the foregoing range, macroscopic and microscopic consistency of the positive electrode active material is relatively good, and particle stability is high, thereby helping the positive electrode active material achieve higher capacity performance and normal-temperature and high-temperature cycling performance.

In the positive electrode material obtained by the method of the present invention, in differential scanning calorimetry analysis with the positive electrode active material in the <NUM>% delithiated state, an onset exothermic temperature of a main exothermic peak may be <NUM> or above, and an integral area of the main exothermic peak is <NUM> J/g or below. Optionally, a peak width at half height of the main exothermic peak is <NUM> or below. Optionally, a maximum exothermic temperature of the main exothermic peak is <NUM> or above. When the positive electrode active material satisfies the foregoing condition, the positive electrode active material can have higher structural stability, thereby further improving the high-temperature cycling performance of the battery. Further, the battery can achieve higher energy density.

The true density ρtrue of the positive electrode active material obtained according to the method of the invention may satisfy <NUM>/cm<NUM> ≤ ρtrue ≤ <NUM>/cm<NUM>. The positive electrode active material has a relatively high specific capacity, so that the energy density of the battery can be improved.

In the positive electrode material obtained by the method of the present invention, a volume average particle size Dv<NUM> of the positive electrode active material may be <NUM> to <NUM>, optionally, <NUM> to <NUM>, and further optionally, <NUM> to <NUM>. When Dv<NUM> of the positive electrode active material is within the foregoing range, cycling performance and rate performance of the lithium-ion secondary battery can be improved, and the energy density of the battery can also be improved.

In the positive electrode material obtained by the method of the present invention, a specific surface area of the positive electrode active material may be <NUM><NUM>/g to <NUM><NUM>/g, optionally, <NUM><NUM>/g to <NUM><NUM>/g. When the specific surface area of the positive electrode active material is within the foregoing range, the battery can achieve relatively high energy density and cycling performance.

In the positive electrode material obtained by the method of the present invention, tap density of the positive electrode active material is <NUM>/cm<NUM> to <NUM>/cm<NUM>. When the tap density of the positive electrode active material is within the foregoing range, the lithium-ion secondary battery can have relatively high energy density.

In the positive electrode material obtained by the method of the present invention, compacted density of the positive electrode active material under a pressure of <NUM> tons (equivalent to <NUM> kN) may be <NUM>/cm<NUM> to <NUM>/cm<NUM>. When the compacted density of the positive electrode active material is within the given range, the lithium-ion secondary battery can achieve relatively high energy density, and can also achieve relatively high cycling performance.

In any one of the foregoing embodiments, the lithium nickel cobalt manganese oxide may satisfy a chemical formula Li<NUM>+a[NixCoyMnzM<NUM>b]O<NUM>-pXp, where M<NUM> is the doping element, M<NUM> is selected from one or more polyvalent elements of Ti, V, Cr, Se, Nb, Mo, Ru, Rh, Pd, Sb, Te, Ce and W, X is selected from one or more of F, N, P and S, <NUM> ≤ x ≤ <NUM>, <NUM> < y < <NUM>, <NUM> < z < <NUM>, -<NUM> ≤ a < <NUM>, <NUM>< b < <NUM>, x + y + z + b = <NUM>, and <NUM> ≤ p < <NUM>. Alternatively, the lithium nickel cobalt manganese oxide may satisfy a chemical formula Li<NUM>+c[Nir-dCosMntM<NUM>daO<NUM>-qX'q, where M<NUM> is the doping element, M<NUM> is selected from one or more polyvalent elements of Ti, V, Cr, Se, Nb, Mo, Ru, Rh, Pd, Sb, Te, Ce and W, X' is selected from one or more of F, N, P and S, <NUM> ≤ r -d ≤ <NUM>, <NUM> < s < <NUM>, <NUM> < t < <NUM>, -<NUM> ≤ c < <NUM>, <NUM> < d < <NUM>, r + s + t = <NUM>, and <NUM> ≤ q < <NUM>.

The present invention provides a method for preparing positive electrode active material according to claim <NUM>. The method includes the following steps: mixing a positive electrode active material precursor, a lithium source, and a doping element precursor to obtain a mixture, where the positive electrode active material precursor is selected from one or more of an oxide, hydroxide, or carbonate that contains Ni, Co, and Mn, the number of moles of nickel accounts for <NUM>% to <NUM>% of the total number of moles of nickel, cobalt, and manganese, and at least part of the doping element in the doping element precursor has not reached a highest valence state; and sintering the mixture in an oxygen-containing atmosphere at a temperature of <NUM> to <NUM> to obtain a positive electrode active material, where the positive electrode active material includes a lithium nickel cobalt manganese oxide, in the lithium nickel cobalt manganese oxide, the number of moles of nickel accounts for <NUM>% to <NUM>% of the total number of moles of nickel, cobalt, and manganese, and the lithium nickel cobalt manganese oxide has a layered crystal structure with a space group R<NUM>m; and the lithium nickel cobalt manganese oxide includes a doping element, and when the positive electrode active material is in a <NUM>% delithiated state, the doping element has two or more different valence states, and the amount of doping element in a highest valence state accounts for <NUM>% to <NUM>% of the total amount of doping element.

The positive electrode active material obtained by using the preparation method provided in this application contains the lithium nickel cobalt manganese oxide with high nickel content. The lithium nickel cobalt manganese oxide is further doped with a polyvalent element. In addition, in the positive electrode active material in the <NUM>% delithiated state, the percentage of the amount of doping element in the highest valence state in the total amount of doping element is properly controlled, so that the positive electrode active material can be driven to release more lithium ions, structural stability and high-temperature cycle stability of the positive electrode active material can be significantly improved, and a lithium-ion secondary battery can achieve relatively high energy density and high-temperature cycling performance.

In the method of the present invention, the doping element precursor is selected from one or more of a titanium oxide, a vanadium oxide, a chromium oxide, a selenium oxide, a niobium oxide, a molybdenum oxide, a ruthenium oxide, a rhodium oxide, a palladium oxide, an antimony oxide, a tellurium oxide, a cerium oxide, and a tungsten oxide. Optionally, the doping element precursor is selected from one or more of TiO<NUM>, TiO, V<NUM>O<NUM>, V<NUM>O<NUM>, V<NUM>O<NUM>, CrO<NUM>, Cr<NUM>O<NUM>, SeO<NUM>, Nb<NUM>O<NUM>, NbO<NUM>, MoO<NUM>, MoO<NUM>, RuO<NUM>, Rh<NUM>O<NUM>, PdO, Sb<NUM>O<NUM>, Sb<NUM>O<NUM>, TeO<NUM>, CeO<NUM>, WO<NUM>, and WO<NUM>.

In the method of the present invention, the doping element precursor includes a first doping element oxide and a second doping element oxide, a valence α of a doping element in the first doping element oxide is less than or equal to <NUM>, and a valence β of a doping element in the second doping element oxide is greater than or equal to <NUM>; and β > α.

In any one of the foregoing embodiments, an oxygen concentration of a sintering atmosphere may be <NUM>% to <NUM>%, optionally, <NUM>% to <NUM>%.

In any one of the foregoing embodiments, a sintering temperature may be <NUM> to <NUM>.

In any one of the foregoing embodiments, sintering duration is <NUM> to <NUM>, optionally, <NUM> to <NUM>.

The positive electrode material produced by the method of the present invention may be used for a positive electrode plate (not according to the invention). The positive electrode plate includes a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector. The positive electrode active material layer includes the positive electrode active material in the first aspect of this application, or the positive electrode active material obtained by using the preparation method in the second aspect of this application.

Because the positive electrode plate of this application includes the positive electrode active material, a lithium-ion secondary battery in which the positive electrode plate is used can have relatively high energy density and high-temperature cycling performance.

The positive electrode material produced by the method of the present invention may be used for a lithium-ion secondary battery (not claimed). The lithium-ion secondary battery includes the positive electrode plate in the third aspect of this application.

The lithium-ion secondary battery of this application includes the positive electrode plate, and therefore can have relatively high energy density and high-temperature cycling performance.

The positive electrode material produced by the method of the present invention may be used for a battery module (not claimed). The battery module includes the lithium-ion secondary battery in the fourth aspect of this application.

The positive electrode material produced by the method of the present invention may be used for a battery pack (not claimed). The battery pack includes the lithium-ion secondary battery in the fourth aspect of this application, or the battery module in the fifth aspect of this application.

The positive electrode material produced by the method of the present invention may be used for an apparatus (not claimed). The apparatus includes at least one of the lithium-ion secondary battery in the fourth aspect of this application, the battery module in the fifth aspect of this application, or the battery pack in the sixth aspect of this application.

The above-described battery module, the battery pack, and the apparatus of this application can include the above lithium-ion secondary battery of this application, and therefore have at least an effect the same as or similar to that of the lithium-ion secondary battery.

To describe the technical solutions in the embodiments of this application more clearly, the following briefly describes the accompanying drawings required for describing the embodiments of this application. Apparently, the accompanying drawings in the following description show merely some embodiments of this application, and a person of ordinary skill in the art may still derive other drawings from the accompanying drawings without creative efforts.

To make the invention objectives, technical solutions, and beneficial technical effects of this application clearer, the following further describes this application in detail with reference to embodiments. It should be understood that the embodiments described in this specification are merely intended to explain this application.

For simplicity, only some numerical ranges are explicitly disclosed in this specification. However, any lower limit may be combined with any upper limit to form an unspecified range, any lower limit may be combined with another lower limit to form an unspecified range, and similarly, any upper limit may be combined with any other upper limit to form an unspecified range. In addition, although not explicitly stated, each point or a single numerical value between end points of a range is included in the range. Therefore, each point or a single numerical value may serve as its own lower limit or upper limit and be combined with any other point or single numerical value or another lower limit or upper limit to form an unspecified range.

In the descriptions of this specification, it should be noted that "above" and "below" include the number itself, "more types" in "one or more types" means at least two types, and "more" in "one or more" means at least two, unless otherwise specified.

In the descriptions of this application, the term "or (or)" is inclusive, unless otherwise specified. For example, the phrase "A or (or) B" means "A, B, or both A and B". More specifically, all the following conditions fall within the condition "A or B": A is true (or present), and B is false (or not present); A is false (or not present), and B is true (or not present); or both A and B are true (or present).

The foregoing summary of this application is not intended to describe each disclosed embodiment or each implementation of this application. The following descriptions illustrate example embodiments more specifically by using example. In many places throughout this application, guidance is provided through a series of embodiments. These embodiments may be used in various combinations. In each embodiment, enumeration is only representative but should not be interpreted as exhaustive.

In the method of the invention, a positive electrode active material precursor is obtained. The positive electrode active material includes a lithium nickel cobalt manganese oxide. In the lithium nickel cobalt manganese oxide, the number of moles of nickel accounts for <NUM>% to <NUM>% of the total number of moles of nickel, cobalt, and manganese. The lithium nickel cobalt manganese oxide has a layered crystal structure with a space group R<NUM>m. The lithium nickel cobalt manganese oxide includes a doping element. When the positive electrode active material is in a <NUM>% delithiated state, the doping element has two or more different valence states, and the amount of doping element in a highest valence state accounts for <NUM>% to <NUM>% of the total amount of doping element.

According to the method of the invention, the positive electrode active material includes the lithium nickel cobalt manganese oxide, and the number of moles of nickel accounts for <NUM>% to <NUM>% of the total number of moles of nickel, cobalt, and manganese. The lithium nickel cobalt manganese oxide with a high nickel content is characterized by a relatively high specific capacity, allowing the battery to present relatively high capacity performance and energy density.

The positive electrode active material is doped with a polyvalent element. When the positive electrode active material is in a <NUM>% delithiated state, the doping element has two or more different valence states, and the amount of doping element in a highest valence state accounts for <NUM>% to <NUM>% of the total amount of doping element. The percentage herein is a mass percentage.

The inventor finds that, compared with a positive electrode active material doped with a non-polyvalent element in a cycling process, the doping element can contribute more electrons to the positive electrode active material, thereby enabling the positive electrode active material to release more lithium ions, and further improving capacity performance and energy density of the battery. In addition, a doping element in a higher valence state can be bonded to an oxygen atom more strongly. To be specific, bond energy for combining with the oxygen atom is higher, and the oxygen atom can be effectively bound, thereby preventing the positive electrode active material from releasing oxygen in a heating and high-temperature cycling process after delithiation, and suppressing an irreversible structural phase change. Therefore, structural stability and high-temperature cycle stability of the positive electrode active material are greatly improved, so that high-temperature cycling performance of a lithium-ion secondary battery can be improved.

The inventor further finds that, when the amount of doping element in the highest valence state accounts for <NUM>% to <NUM>% of the total amount of doping element in the positive electrode active material in the "<NUM>% delithiated state", the doping element can prevent a component segregation phenomenon in the positive electrode active material while effectively improving a specific capacity and structural stability of the positive electrode active material. Therefore the positive electrode active material can maintain relatively high lithium ion transport performance, and the battery can have relatively high energy density and cycling performance. In addition, internal stress of the positive electrode active material is distributed uniformly, thereby further improving structural stability and high-temperature cycle stability of the positive electrode active material, and effectively improving high-temperature cycling performance of the battery.

In some embodiments, in the positive electrode active material in the "<NUM>% delithiated state", the amount of doping element in the highest valence state accounts for <NUM>% to <NUM>%, optionally, <NUM>% to <NUM>%, <NUM>% to <NUM>%, <NUM>% to <NUM>%, or <NUM>% to <NUM>%, of the total amount of doping element.

In this specification, the "<NUM>% delithiated state" is a state in which the number of moles of lithium released from the positive electrode active material accounts for <NUM>% of a theoretical amount of lithium in a battery charge process. In an actual use process of the positive electrode active material, usually, a "fully charged state" is set, and a "charge cut-off voltage" is set, to ensure safe use of the battery. The "fully charged state" means that a state of charge (State of Charge, SOC) of a secondary battery is <NUM>%. In other words, a secondary battery composed of a positive electrode including the foregoing positive electrode active material is charged to the cut-off charge voltage within a reversible charge and discharge range. The "fully charged state" or the "charge cut-off voltage" varies due to a difference in the positive electrode active material or a difference in safety requirement. When the secondary battery prepared from the positive electrode active material including the lithium nickel cobalt manganese oxide is in the "fully charged state", the delithiation of the positive electrode active material is usually around the "<NUM>% delithiated state" to ensure normal use.

In this specification, with reference to mappings between "delithiated states" and "voltages", the positive electrode active material in the "<NUM>% delithiated state" is obtained for research. Specifically, the positive electrode active material is used to prepare a series of standard button half-cells that are separately charged to <NUM> V, <NUM> V, <NUM> V, <NUM> V, <NUM> V, <NUM> V, <NUM> V, and <NUM> V at a rate of <NUM>. Then a positive electrode plate of the battery is taken, and an electrolyte is removed through washing, to digest the positive electrode active material. An inductively coupled plasma-optical emission spectrometer (inductively coupled plasma-Optical Emission spectrometer, ICP-OES) is used to test mass concentrations of Li, transition metals (Ni, Co, and Mn), and an O element of the positive electrode active material, to calculate the "delithiated state". Mappings between charging voltages and "delithiated states" are obtained, and then a charging voltage corresponding to the "<NUM>% delithiated state" is obtained. Then the button half-cells are charged to the voltage corresponding to the "<NUM>% delithiated state", and the positive electrode active material in the "<NUM>% delithiated state" may be obtained through disassembly for further research.

A valence of the doping element in the positive electrode active material in the "<NUM>% delithiated state" may be obtained through an X-ray photoelectron spectroscopy (XPS) test, and more accurately, may be obtained through a synchrotron radiation photoelectron spectroscopy (Synchrotron radiation photoelectron spectroscopy, SRPES) test.

The inventor further finds that the doping element has two or more different valence states in the positive electrode active material in the "<NUM>% delithiated state", and the amount of doping element in the highest valence state accounts for <NUM>% to <NUM>% of the total amount of doping element, so that an onset exothermic temperature and a maximum exothermic temperature of a main exothermic peak in a DSC spectrogram of the positive electrode active material in the "<NUM>% delithiated state" are both increased, and an integral area and a peak width at half height of the main exothermic peak can be both reduced.

The DSC spectrogram of the positive electrode active material in the "<NUM>% delithiated state" is obtained through measurement by using a differential scanning calorimeter. <FIG> is a schematic DSC spectrogram of a positive electrode active material obtained according to the method of this application. As shown in <FIG>, the DSC spectrogram is a curve graph drawn with a heat flow rate as an ordinate and a temperature as an abscissa. In this specification, the main exothermic peak is a peak with a largest integral area in the DSC spectrogram, and is an exothermic peak resulting from a phase change of a layered phase structure of the positive electrode active material due to oxygen release. The onset exothermic temperature of the main exothermic peak is an intersection (that is, an extrapolation starting point) A between a tangent to a maximum-slope point on a low-temperature side of the main exothermic peak and an extension of a baseline ef before the peak. The integral area of the main exothermic peak is an area of a region enclosed by the exothermic peak and an inscribed baseline fg, and is used to indicate heat emitted by the positive electrode active material per unit weight in this interval. The peak width at half height of the main exothermic peak is a peak width at half of a peak height mn, where n is an intersection between the inscribed baseline fg and a straight line. The straight line is perpendicular to the abscissa and starts from a peak top m of the main exothermic peak.

The inventor finds through research that, in the DSC spectrogram of the positive electrode active material in the "<NUM>% delithiated state", the onset exothermic temperature of the main exothermic peak is relatively high, so that during a high-temperature cycling process or under a heating condition, the positive electrode active material can have relatively high structural stability and always remain in a layered phase state with strong electrochemical activity, in which oxygen atoms are not likely to deviate from original lattice locations. The positive electrode active material is not prone to irreversible structural phase changes, and presents relatively high thermal stability and high-temperature cycle stability, thereby improving high-temperature cycling performance and safety performance of the battery. In some embodiments, in the DSC spectrogram of the positive electrode active material in the "<NUM>% delithiated state", the onset exothermic temperature of the main exothermic peak is <NUM> or above, optionally, <NUM> or above, <NUM> or above, or <NUM> or above. When in the lithium nickel cobalt manganese oxide, the number of moles of nickel accounts for <NUM>% to <NUM>% of the total number of moles of nickel, cobalt, and manganese, in the DSC spectrogram of the positive electrode active material in the "<NUM>% delithiated state", optionally, the onset exothermic temperature of the main exothermic peak may be alternatively <NUM> or above, <NUM> or above, or <NUM> or above.

A peak temperature (namely, the maximum exothermic temperature) of the main exothermic peak is relatively high, so that the positive electrode active material in the "<NUM>% delithiated state" is not prone to oxygen release in a heating or high-temperature cycling process, thereby effectively suppressing an irreversible phase change of the positive electrode active material after delithiation. The positive electrode active material has relatively high thermal stability, thereby improving high-temperature cycling performance of the battery. In some embodiments, in the DSC spectrogram of the positive electrode active material in the "<NUM>% delithiated state", the peak temperature of the main exothermic peak may be <NUM> or above. When in the lithium nickel cobalt manganese oxide, the number of moles of nickel accounts for <NUM>% to <NUM>% of the total number of moles of nickel, cobalt, and manganese, in the DSC spectrogram of the positive electrode active material in the "<NUM>% delithiated state", optionally, the peak temperature of the main exothermic peak may be alternatively <NUM> or above, <NUM> or above, or <NUM> or above.

The integral area of the main exothermic peak is relatively small, so that an amount of heat released by the positive electrode active material in the "<NUM>% delithiated state" during a high-temperature cycling process or under a heating condition is reduced. This indicates that the positive electrode active material is subject to a relatively slight irreversible reaction or relatively small structural damage during the high-temperature cycling process or under the heating condition. Thermal stability and high-temperature cycle stability of the positive electrode active material are effectively improved. In some embodiments, in the DSC spectrogram of the positive electrode active material in the "<NUM>% delithiated state", the integral area of the main exothermic peak is <NUM> J/g or below, optionally, <NUM> J/g or below, <NUM> J/g or below, <NUM> J/g or below, <NUM> J/g or below, <NUM> J/g or below, or <NUM> J/g or below.

The peak width at half height of the main exothermic peak is relatively small, so that an irreversible reaction or structural damage of the positive electrode active material in the "<NUM>% delithiated state" during a high-temperature cycling process or under a heating condition is significantly relieved. Thermal stability and high-temperature cycle stability of the positive electrode active material are greatly improved, thereby greatly improving battery performance.

The peak width at half height of the main exothermic peak is relatively small, so that doping modification of the positive electrode active material does not result in obvious new phases. The doping element is essentially for doping and substitution in one or more of nickel sites, manganese sites, and cobalt sites. The positive electrode active material maintains a good layered crystal structure. The positive electrode active material can provide a good carrier for deintercalation of lithium ions, thereby facilitating intercalation and deintercalation of lithium ions, preventing reversible lithium ions from being consumed on an electrode surface or in an electrolyte, and effectively reducing an irreversible capacity. Therefore, the positive electrode active material has a relatively high initial capacity and cycling capacity retention rate, thereby improving energy density and normal-temperature and high-temperature cycling performance of the battery.

In some embodiments, in the DSC spectrogram of the positive electrode active material in the "<NUM>% delithiated state", the peak width at half height of the main exothermic peak is <NUM> or below, optionally, <NUM> or below, <NUM> or below, or <NUM> or below.

In the positive electrode active material obtained by using the preparation method provided in this embodiment of this application, the doping element may be selected from one or more of transition metal elements other than nickel, cobalt, and manganese, and elements of groups IIIA to VIA other than carbon, nitrogen, oxygen, and sulfur. In some embodiments, in the positive electrode active material in the "<NUM>% delithiated state", one part the doping element has one or more valences of +<NUM>, +<NUM>, and +<NUM>, and the other part of the doping element has one or more valences of +<NUM>, +<NUM>, +<NUM>, +<NUM>, and +<NUM>. Optionally, in the positive electrode active material in the "<NUM>% delithiated state", one part of the doping element has one or more valences of +<NUM> and +<NUM>, and the other part of the doping element has one or more valences of +<NUM>, +<NUM>, +<NUM>, and +<NUM>. Further optionally, in the positive electrode active material in the "<NUM>% delithiated state", one part of the doping element has one or more valences of +<NUM> and +<NUM>, and the other part of the doping element has one or more valences of +<NUM> and +<NUM>.

In the positive electrode active material in the "<NUM>% delithiated state", a part of the doping element has a relatively high valence state, that is, the doping element contributes more electrons to the positive electrode active material and can strengthen binding on oxygen atoms. With the positive electrode active material, energy density and high-temperature cycling performance of the battery can be improved.

In particular, in the positive electrode active material in the "<NUM>% delithiated state", a low-valence doping element can also more effectively contribute electrons to the positive electrode active material, especially when a highest-valence doping element and a lowest-valence doping element have a large difference in valence (for example, the difference is <NUM> or above), thereby further improving energy density and high-temperature cycling performance of the battery.

In some optional embodiments, the doping element may include one or more polyvalent elements of Ti, V, Cr, Se, Nb, Mo, Ru, Rh, Pd, Sb, Te, Ce, and W. Optionally, the doping element may include one or more of Se, Nb, Mo, Ru, Rh, Sb, Te, Ce, and W. Optionally, the doping element may include one or more of Se, Nb, Mo, Sb, and W. An appropriate doping element can better provide a charge compensation effect and stabilize a structure of the positive electrode active material, thereby further improving the energy density and the high-temperature cycling performance of the battery.

In some optional embodiments, a relative deviation of a local mass concentration of the doping element in a particle of the positive electrode active material is <NUM>% or below, further optionally, <NUM>% or below, particularly optionally, <NUM>% or below, <NUM>% or below, <NUM>% or below, or <NUM>% or below.

In this specification, the local mass concentration of the doping element in the particle of the positive electrode active material is a mass concentration of the doping element in all elements in a finite volume element of any selected location in the particle, and may be obtained by testing element concentration distribution through EDX (Energy Dispersive X-Ray Spectroscopy, energy dispersive X-ray spectroscopy) or EDS element analysis in combination with TEM (Transmission Electron Microscope, transmission electron microscope) or SEM (Scanning Electron Microscope, scanning electron microscope) single-point scanning, or in other similar manners. When testing is performed through EDX or EDS element analysis in combination with TEM or SEM single-point scanning, mass concentrations of the doping element in µg/g at different locations in the particle of the positive electrode active material are denoted as η<NUM>, η<NUM>, η<NUM>,. , ηn, where n is a positive integer greater than <NUM>.

An average mass concentration of the doping element in the particle of the positive electrode active material is a mass concentration of the doping element in all elements in one or more particles of the positive electrode active material, and may be obtained by testing element concentration distribution through EDX or EDS element analysis in combination with TEM or SEM surface scanning, or in other similar manners. When testing is performed in the manner of testing the element concentration distribution through EDX or EDS element analysis in combination with TEM or SEM surface scanning, a test surface includes all test locations in the foregoing single-point test. The average mass concentration of the doping element in the particle of the positive electrode active material is denoted as η , with a unit of µg/g.

The relative deviation σ of the local mass concentration of the doping element in the particle of the positive electrode active material is calculated based on a formula (<NUM>).

The relative deviation of the local mass concentration of the doping element in the particle of the positive electrode active material is within the foregoing range, indicating that distribution uniformity of the doping element in the particle is relatively high. Properties of all locations in the particle of the positive electrode active material remain the same, and the doping element can improve structural stability of each location in the particle, and suppress oxygen release and a structural phase change of each location in the particle. The onset exothermic temperature and the maximum exothermic temperature of the main exothermic peak in the DSC spectrogram of the positive electrode active material in the "<NUM>% delithiated state" are both further increased, and the integral area and the peak width at half height of the main exothermic peak are both further reduced. In other words, stability of the positive electrode active material during a high-temperature cycling process or under a heating condition is further improved, so that overall performance of the battery can be better improved.

In the uniformly doped positive electrode active material, a migration and diffusion capability of lithium ions in different regions inside a particle can be at the same level, so that the positive electrode active material can have relatively high lithium ion transport performance, thereby improving capacity performance, energy density, and cycling performance of the battery. All locations in the particle of the uniformly doped positive electrode active material have similar structural stability and anti-deformation capabilities, so that stress in the particle is uniformly distributed. Therefore, the particle of the positive electrode active material is not prone to cracking, thereby preventing side reactions and capacity and cycling performance deterioration caused by exposure of fresh surfaces due to cracking, and improving high-temperature cycling performance of the battery.

In some embodiments, a true doping concentration ω of the positive electrode active material may satisfy <NUM>µg/cm<NUM> ≤ ω ≤ <NUM>µg/cm<NUM>, optionally, <NUM>µg/cm<NUM> ≤ ω ≤ <NUM>µg/cm<NUM>, and further optionally, <NUM>µg/cm<NUM> ≤ ω ≤ <NUM>µg/cm<NUM>, <NUM>µg/cm<NUM> ≤ ω ≤ <NUM>µg/cm<NUM>, <NUM>µg/cm<NUM> ≤ ω ≤ <NUM>µg/cm<NUM>, or <NUM>µg/cm<NUM> ≤ ω ≤ <NUM>µg/cm<NUM>.

In this specification, the true doping concentration ω of the positive electrode active material is calculated by using a formula (<NUM>).

In the formula (<NUM>), ω is the true doping concentration of the positive electrode active material, with a unit of µg/cm<NUM>.

ρtrue is true density of the positive electrode active material, with a unit of g/cm<NUM>, and is equal to a ratio of mass of the positive electrode active material to a true volume of the positive electrode active material, where the true volume is an actual volume of solid matter, and does not include pores inside or between particles of the positive electrode active material. ρtrue may be measured by using a well-known instrument and method in the art, for example, a gas volume method that may be performed by using a powder true density tester.

ϕ is a mass concentration of the doping element in the positive electrode active material, with a unit of µg/g, that is, mass of a doping element contained in each gram of the positive electrode active material. ϕ indicates doping element contained in the global macroscopic positive electrode active material, including doping element distributed into particles of the positive electrode active material, doping element concentrated in other phases on surfaces of particles of the positive electrode active material, and doping element located between particles of the positive electrode active material. ϕ may be measured by using a solution absorption spectrum of the positive electrode active material, for example, by using an ICP (Inductive Coupled Plasma Emission Spectrometer, inductive coupled plasma emission spectrometer) or XAFS (X-ray absorption fine structure spectroscopy, X-ray absorption fine structure spectroscopy).

The true doping concentration of the positive electrode active material is within an appropriate range, thereby helping ensure that the doping element of the positive electrode active material in the "<NUM>% delithiated state" has two or more different valence states, and the amount of doping element in the highest valence state accounts for <NUM>% to <NUM>% of the total amount of doping element. The doping element can contribute more electrons to the positive electrode active material, and fully stabilize a structure of the positive electrode active material, thereby better improving energy density and high-temperature cycling performance of the battery. In addition, the doping element can further increase the onset exothermic temperature and the maximum exothermic temperature of the main exothermic peak in the DSC spectrogram of the positive electrode active material in the "<NUM>% delithiated state", and further reduce the integral area and the peak width at half height of the main exothermic peak.

In addition, the true doping concentration of the positive electrode active material is within the foregoing range, thereby further ensuring that the doping element is for doping in a transition metal layer, preventing the doping element from entering a lithium layer, and ensuring that the particle of the positive electrode active material has a relatively high lithium ion transport and diffusion capability, so that the battery has relatively high capacity performance and cycling performance.

In some preferred embodiments, a deviation ε of the mass concentration ϕ of the doping element in the positive electrode active material relative to the average mass concentration η of the doping element in the particle of the positive electrode active material satisfies ε < <NUM>%. Optionally, ε ≤ <NUM>%. Further optionally, ε ≤ <NUM>%, ≤ <NUM>%, ≤ <NUM>%, ≤ <NUM>%, or ≤ <NUM>%.

ε is calculated by using the following formula (<NUM>).

ε of the positive electrode active material is within the foregoing range, indicating that successful doping of the doping element into particles of the positive electrode active material, and there is a relatively small amount of doping element distributed in other phases on surfaces of particles of the positive electrode active material, and doping element embedded in gaps between particles of the positive electrode active material. The positive electrode active material has relatively good macroscopic and microscopic uniformity, and a uniform structure. In a charge and discharge cycling process, swelling and contraction degrees of particles of the positive electrode active material can remain the same, and the particles have high stability, so that the positive electrode active material has relatively high capacity performance and normal-temperature and high-temperature cycling performance.

The positive electrode active material obtained by using the preparation method provided in this embodiment of this application can be uniformly doped within the foregoing proper true doping concentration range, and consistency between microscopic distribution and macroscopic content of the doping element is ensured, so that thermal stability and high-temperature cycle stability of the positive electrode active material can be more effectively improved, thereby better improving overall performance of the battery.

In some embodiments, true density ρtrue of the positive electrode active material may satisfy <NUM>/cm<NUM> ≤ ρtrue ≤ <NUM>/cm<NUM>. The positive electrode active material has a relatively high specific capacity, thereby improving capacity performance and energy density of the battery.

In some optional embodiments, in the lithium nickel cobalt manganese oxide ,the number of moles of nickel accounts for <NUM>% to <NUM>%, for example, <NUM>% to <NUM>%, of the total number of moles of nickel, cobalt, and manganese. The positive electrode active material with high nickel content is characterized by a higher specific capacity and has higher cycling stability, so that energy density and cycling performance of the lithium-ion secondary battery can be improved.

In some examples, the lithium nickel cobalt manganese oxide may be a compound represented by Li<NUM>+a[NixCoyMnzM<NUM>b]O<NUM>-pXp. In the expression, M<NUM> is a doping element, and is for doping and substitution in one or more of nickel sites, cobalt sites, and manganese sites; X is an oxygen site substituent element, and may be selected from one or more of F, N, P and S; and <NUM> ≤ x ≤ <NUM>, <NUM> ≤ y < <NUM>, <NUM> ≤ z < <NUM>, -<NUM> ≤ a < <NUM>, <NUM> < b < <NUM>, x + y + z + b = <NUM>, and <NUM> ≤ p < <NUM>. With the ternary positive electrode active material with high nickel content, the battery can have relatively high capacity performance, energy density, and normal-temperature and high-temperature cycling performance.

Optionally, <NUM> ≤ x ≤ <NUM>, <NUM> < y < <NUM>, and <NUM> < z < <NUM>. The ternary positive electrode active material with high nickel content has high energy density and good structural stability, thereby enabling the battery to have high energy density and a long cycle life.

Optionally, M<NUM> is selected from one or more polyvalent elements of Ti, V, Cr, Se, Nb, Mo, Ru, Rh, Pd, Sb, Te, Ce, and W. Optionally, M<NUM> may include one or more of Se, Nb, Mo, Ru, Rh, Sb, Te, Ce, and W. Optionally, M<NUM> may include one or more of Se, Nb, Mo, Sb, and W.

Alternatively, the lithium nickel cobalt manganese oxide may be a compound represented by Li<NUM>+c[Nir-dCosMntM<NUM>d]O<NUM>-qX'q. In the expression, M<NUM> is a doping element, and is partially substituted in a nickel site; X' is an oxygen site substituent element, and is preferably selected from one or more of F, N, P and S; and <NUM> ≤r - d ≤ <NUM>, <NUM> ≤ s < <NUM>, <NUM> ≤ t < <NUM>, -<NUM> ≤ c < <NUM>, <NUM> < d < <NUM>, r + s + t = <NUM>, and <NUM> ≤ q < <NUM>. With the ternary positive electrode active material with high nickel content, the battery can have relatively high capacity performance, energy density, and normal-temperature and high-temperature cycling performance.

Optionally, <NUM> ≤ r - d ≤ <NUM>, <NUM> < s < <NUM>, and <NUM> < t < <NUM>. The ternary positive electrode active material with high nickel content has high energy density and good structural stability, thereby enabling the battery to have high energy density and a long cycle life.

The lithium nickel cobalt manganese oxide in the foregoing examples may be independently used for the positive electrode active material, or a combination of any two or more of the foregoing lithium nickel cobalt manganese oxides may be used for the positive electrode active material.

Optionally, a volume average particle size Dv<NUM> of the positive electrode active material is <NUM> to <NUM>, further optionally, <NUM> to <NUM>, and further optionally, <NUM> to <NUM>. Dv<NUM> of the positive electrode active material is within an appropriate range, and a migration path of lithium ions and electrons in the material is relatively short, thereby further improving transport and diffusion performance of lithium ions and electrons in the positive electrode active material, reduce battery polarization, and improving cycling performance and rate performance of the lithium-ion secondary battery. In addition, the positive electrode active material can further have relatively high compacted density, thereby improving energy density of the battery.

Dv<NUM> of the positive electrode active material is within the foregoing range, thereby helping reduce side reactions of an electrolyte on a surface of the positive electrode active material and reduce agglomeration between particles of the positive electrode active material, and improving normal-temperature and high-temperature cycling performance and safety performance of the positive electrode active material.

Optionally, a specific surface area of the positive electrode active material is <NUM><NUM>/g to <NUM><NUM>/g, further optionally, <NUM><NUM>/g to <NUM><NUM>/g. The specific surface area of the positive electrode active material is within an appropriate range, thereby ensuring that the positive electrode active material has a relatively large active specific surface area, helping reduce side reactions of an electrolyte on the surface of the positive electrode active material, and improving capacity performance and a cycle life of the positive electrode active material. In addition, agglomeration between particles of the positive electrode active material in a slurry preparation or charge and discharge process can be suppressed, thereby improving energy density and cycling performance of the battery.

Optionally, tap density of the positive electrode active material is <NUM>/cm<NUM> to <NUM>/cm<NUM>. The tap density of the positive electrode active material enables the battery to have relatively high capacity performance and energy density.

Optionally, compacted density of the positive electrode active material under a pressure of <NUM> tons (equivalent to <NUM> kN) is <NUM>/cm<NUM> to <NUM>/cm<NUM>. The compacted density of the positive electrode active material is relatively high, thereby enabling the battery to have relatively high capacity performance and energy density, and enabling the battery to have good normal-temperature cycling performance and high-temperature cycling performance.

Optionally, a shape of the particles in the positive electrode active material obtained by using the preparation method of this application is one or more of a spherical shape and a spheroidal shape.

In some embodiments, the positive electrode active material includes a secondary particle aggregated from primary particles. In these embodiments, the aforementioned "particle" includes the secondary particle.

In this specification, Dv<NUM> of the positive electrode active material has a meaning well-known in the art, and is also referred to as a median particle size. Dv<NUM> indicates a particle size corresponding to <NUM>% of volume distribution of particles in the positive electrode active material. Dv<NUM> of the positive electrode active material may be measured by using a well-known instrument and method in the art, for example, may be conveniently measured by using a laser particle size analyzer (such as Mastersizer <NUM> of Malvern Instruments, UK).

The specific surface area of the positive electrode active material has a meaning well-known in the art, and may be measured by using a well-known instrument and method in the art, for example, may be measured by using a nitrogen-adsorption specific surface area analysis test method and calculated by using a BET (Brunauer Emmett Teller) method. The nitrogen-adsorption specific surface area analysis test may be performed by using a NOVA 2000e specific surface area and aperture analyzer of Quantachrome Instruments (US). In a specific example, the test method is as follows: <NUM> to <NUM> of positive electrode active material is taken by using a weighed empty sample tube. The positive electrode active material is well stirred and weighed. The sample tube is put into a NOVA 2000e degassing station for degassing. Total mass of a degassed positive electrode active material and the sample tube is obtained through weighing. Mass G of the degassed positive electrode active material is obtained by subtracting mass of the empty sample tube from the total mass. The sample tube is put into NOVA 2000e. An adsorption amount of nitrogen on a surface of the positive electrode active material under different relative pressures is measured. A monolayer adsorption amount is calculated based on a Brunauer-Emmett-Teller multilayer adsorption theory and a formula thereof. Then a total surface area A of the positive electrode active material is calculated, and a specific surface area of the positive electrode active material is calculated by using A/G.

The tap density of the positive electrode active material has a meaning well-known in the art, and may be measured by using a well-known instrument and method in the art, for example, conveniently measured by using a tap density measuring instrument (such as FZS4-4B).

The compacted density of the positive electrode active material has a meaning well-known in the art, and may be measured by using a well-known instrument and method in the art, for example, conveniently measured by using an electronic pressure testing machine (such as UTM7305).

The following schematically describes a method for preparing a positive electrode active material. According to the preparation method, any one of the foregoing positive electrode active materials can be prepared. The preparation method includes:
mixing a positive electrode active material precursor, a lithium source, and a doping element precursor, and performing sintering to a positive electrode active material.

The positive electrode active material precursor may be one or more of an oxide, hydroxide, or carbonate that contains Ni, Co, and Mn according to a stoichiometric ratio, for example, hydroxide that contains Ni, Co, and Mn according to a stoichiometric ratio.

The positive electrode active material precursor may be obtained by using a known method in the art, for example, prepared by using a co-precipitation method, a gel method, or a solid phase method.

In an example, a Ni source, a Co source, and a Mn source are dispersed in a solvent to obtain a mixed solution; the mixed solution, a strong base solution, and a complexing agent solution are all pumped into a stirring reactor through continuous cocurrent reactions, where a pH value of the reaction solution is controlled to be <NUM> to <NUM>, a temperature in the reactor is <NUM> to <NUM>, and an inert gas is introduced for protection during a reaction process; after the reaction is completed, aging, filtration, washing, and vacuum drying are performed to obtain hydroxide that contains Ni, Co, and Mn.

The Ni source may be soluble nickel salt, for example, one or more of nickel sulfate, nickel nitrate, nickel chloride, nickel oxalate, and nickel acetate; for another example, one or more of nickel sulfate and nickel nitrate; for another example, nickel sulfate. The Co source may be soluble cobalt salt, for example, one or more of cobalt sulfate, cobalt nitrate, cobalt chloride, cobalt oxalate, and cobalt acetate; for another example, one or more of cobalt sulfate and cobalt nitrate; for another example, cobalt sulfate. The Mn source may be soluble manganese salt, for example, one or more of manganese sulfate, manganese nitrate, manganese chloride, manganese oxalate, and manganese acetate; for another example, one or more of manganese sulfate and manganese nitrate; for another example, manganese sulfate.

The strong base may be one or more of LiOH, NaOH, and KOH, for example, NaOH. The complexing agent may be one or more of ammonia, ammonium sulfate, ammonium nitrate, ammonium chloride, ammonium citrate, and ethylenediaminetetraacetic acid (EDTA), for example, ammonia.

There are no special limitations on solvents of the mixed solution, the strong base solution, and the complexing agent solution. For example, the solvents of the mixed solution, the strong base solution, and the complexing agent solution each are one or more of deionized water, methanol, ethanol, acetone, isopropyl, and hexanol, for example, deionized water.

The inert gas introduced during the reaction process is, for example, one or more of nitrogen, argon, and helium.

The lithium source may be one or more of lithium oxide (Li<NUM>O), lithium phosphate (Li<NUM>PO<NUM>), lithium dihydrogen phosphate (LiH<NUM>PO<NUM>), lithium acetate (CH<NUM>COOLi), lithium hydroxide (LiOH), lithium carbonate (Li<NUM>CO<NUM>), and lithium nitrate (LiNO<NUM>). Further, the lithium source is one or more of lithium carbonate, lithium hydroxide, and lithium nitrate. Furthermore, the lithium source is lithium carbonate.

The doping element precursor may be one or more of an oxide, nitric acid compound, carbonic acid compound, hydroxide compound, or acetic acid compound that contains a doping element. In the doping element precursor, at least part of doping element is still capable of contributing electrons, that is, having not reached a highest valence state of the element.

Further, the doping element precursor may be alternatively a combination of a precursor containing a low-valence doping element and a precursor containing a high-valence doping element. The precursor containing a low-valence doping element may be one or more of an oxide, nitric acid compound, carbonic acid compound, hydroxide compound, or acetic acid compound of a doping element whose valence is less than or equal to <NUM>. The precursor containing a high-valence doping element may be one or more of an oxide, nitric acid compound, carbonic acid compound, hydroxide compound, or acetic acid compound of a doping element whose valence is greater than or equal to <NUM>.

The doping element oxide may be, for example, one or more of a titanium oxide (such as TiO<NUM> or TiO), a vanadium oxide (such as V<NUM>O<NUM>, V<NUM>O<NUM>, or V<NUM>O<NUM>), a chromium oxide (such as CrO<NUM> or Cr<NUM>O<NUM>), a selenium oxide (such as SeO<NUM>), a niobium oxide (such as Nb<NUM>O<NUM> or NbO<NUM>), a molybdenum oxide (such as MoO<NUM> or MoO<NUM>), a ruthenium oxide (such as RuO<NUM>), a rhodium oxide (such as Rh<NUM>O<NUM>), a palladium oxide (such as PdO), an antimony oxide (such as Sb<NUM>O<NUM> or Sb<NUM>O<NUM>), a tellurium oxide (such as TeO<NUM>), a cerium oxide (such as CeO<NUM>) and a tungsten oxide (such as WO<NUM> or WO<NUM>).

The positive electrode active material precursor, the lithium source, and the doping element precursor may be mixed by using a ball mill mixer or a high-speed mixer. For example, the high-speed mixer is used to mix the materials for <NUM> to <NUM>.

The mixed materials are added to an atmosphere sintering furnace for sintering. A sintering atmosphere is an oxygen-containing atmosphere, for example, an air atmosphere or an oxygen atmosphere. Optionally, an oxygen concentration of the sintering atmosphere is <NUM>% to <NUM>%, for example, <NUM>% to <NUM>%. A sintering temperature is <NUM> to <NUM>. Optionally, the sintering temperature is <NUM> to <NUM>. This enables the doping element to have relatively high distribution uniformity. Sintering duration may be adjusted based on an actual situation, for example, <NUM> to <NUM>; for another example, <NUM> to <NUM>.

For elements that are relatively difficult in doping, such as doping elements with a large atomic radius, doping uniformity may be improved by increasing the sintering temperature and/or extending the sintering duration.

It should be noted that, during preparation of the positive electrode active material, there are a plurality of theoretically feasible manners of controlling a valence and a proportion of a doping element in the positive electrode active material that includes a lithium nickel cobalt manganese oxide and that is in a "<NUM>% delithiated state", for example, a valence state of the doping element precursor, a doping element amount, oxidability of a sintering atmosphere during doping, a quantity of times of sintering, uniformity of mixing, a sintering temperature, and sintering duration. In this application document, a manner of controlling a doping precursor type and overall sintering duration and temperature in solid-phase sintering and doping is listed, and the positive electrode active material that includes the lithium nickel cobalt manganese oxide and in which the doping element has preset valence distribution in the "<NUM>% delithiated state" is obtained. The positive electrode active material is characterized by high energy density, thermal stability, and high-temperature cycle stability. Preferably, through further control on doping uniformity, the positive electrode active material with DSC characteristics in the <NUM>% delithiated state has a better effect. It should be understood that the method described in this specification is merely intended to explain this application.

In some embodiments, a sintered product may be further crushed and sieved to obtain a positive electrode active material with an optimized particle size distribution and specific surface area. There are no special limitations on a crushing manner. The crushing manner may be selected according to an actual requirement, for example, a particle crusher is used.

This application provides a positive electrode plate, and the positive electrode plate uses any one or more positive electrode active materials of this application.

Because the positive electrode plate of this application uses the positive electrode active material of this application, a battery using the positive electrode plate can have both good normal-temperature and high-temperature cycling performance and relatively high energy density.

Specifically, the positive electrode plate includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector. For example, the positive electrode current collector includes two opposite surfaces in its thickness direction, and the positive electrode active material layer is stacked on either or both of the two surfaces of the positive electrode current collector.

The positive electrode active material layer includes the positive electrode active material of this application.

In addition, the positive electrode active material layer may further include a conductive agent and a binder. Types of the conductive agent and the binder in the positive electrode active material layer are not specifically limited in this application, and the conductive agent and the binder may be selected according to an actual requirement.

In an example, the conductive agent may be one or more of graphite, superconducting carbon, acetylene black, carbon black, Ketjen black, a carbon dot, a carbon nanotube, graphene, and carbon nanofiber; and the binder may be one or more of styrene-butadiene rubber (SBR), water-based acrylic resin (water-based acrylic resin), carboxymethyl cellulose (CMC), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl butyral (PVB), ethylene vinyl acetate (EVA), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, a fluorine-containing acrylic resin, and polyvinyl alcohol (PVA).

The positive electrode current collector may be a metal foil or a porous metal plate with good conductivity and mechanical properties. The positive electrode current collector may be made of one or more of aluminum, copper, nickel, titanium, silver, and their respective alloys. The positive electrode current collector is, for example, an aluminum foil.

The positive electrode plate may be prepared according to a conventional method in the art. For example, the positive electrode active material, the conductive agent, and the binder are dispersed in a solvent, where the solvent may be N-methylpyrrolidone (NMP) or deionized water, to form a uniform positive electrode slurry. The positive electrode slurry is applied to the positive electrode current collector, and processes such as drying and roll-in are performed to obtain the positive electrode plate.

This application provides a lithium-ion secondary battery. The lithium-ion battery includes a positive electrode plate, a negative electrode plate, a separator, and an electrolyte, where the positive electrode plate is any positive electrode plate of this application.

The lithium-ion secondary battery uses the positive electrode active material of this application, and therefore can have both good normal-temperature and high-temperature cycling performance and relatively high energy density.

The negative electrode plate may be a lithium metal plate.

Alternatively, the negative electrode plate may include a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector. For example, the negative electrode current collector includes two opposite surfaces in its thickness direction, and the negative electrode active material layer is stacked on either or both of the two surfaces of the negative electrode current collector.

The negative electrode active material layer includes the negative electrode active material. A type of the negative electrode active material is not specifically limited in this embodiment of this application, and the negative electrode active material may be selected according to an actual requirement. In an example, the negative electrode active material may be one or more of natural graphite, artificial graphite, mesocarbon microbeads (MCMB), hard carbon, soft carbon, silicon, a silicon-carbon composite, SiOm (<NUM> < m < <NUM>, for example, m = <NUM>), a Li-Sn alloy, a Li-Sn-O alloy, Sn, SnO, SnO<NUM>, spinel-structure lithium titanate Li<NUM>Ti<NUM>O<NUM>, a Li-Al alloy, and lithium metal.

The negative electrode active material layer may further include a conductive agent and a binder. Types of the conductive agent and the binder in the negative electrode active material layer are not specifically limited in this embodiment of this application, and the conductive agent and the binder may be selected according to an actual requirement. In an example, the conductive agent is one or more of graphite, superconducting carbon, acetylene black, carbon black, Ketjen black, a carbon dot, a carbon nanotube, graphene, and carbon nanofiber; and the binder is one or more of styrene-butadiene rubber (SBR), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl butyral (PVB), and water-based acrylic resin (water-based acrylic resin).

Optionally, the negative electrode active material layer may further include a thickener, for example, sodium carboxymethyl cellulose (CMC-Na).

The negative electrode current collector may be a metal foil or a porous metal plate with good conductivity and mechanical properties. The negative electrode current collector may be made of one or more of copper, nickel, titanium, iron, and their respective alloys. The negative electrode current collector is, for example, a copper foil.

The negative electrode plate may be prepared according to a conventional method in the art. For example, the negative electrode active material, the conductive agent, the binder, and the thickener are dispersed in a solvent, where the solvent may be N-methylpyrrolidone (NMP) or deionized water, to form a uniform negative electrode slurry. The negative electrode slurry is applied to the negative electrode current collector, and processes such as drying and roll-in are performed to obtain the negative electrode plate.

In the lithium-ion secondary battery in this embodiment of this application, the electrolyte may be a solid electrolyte, for example, a polymer electrolyte or an inorganic solid electrolyte, but is not limited thereto. Alternatively, the electrolyte may be a liquid electrolyte. The liquid electrolyte includes a solvent and a lithium salt dissolved in the solvent.

The solvent may be a non-aqueous organic solvent, for example, one or more, for another example, at least two, of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethylene propyl carbonate (EPC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), and ethyl butyrate (EB).

The lithium salt may be one or more of LiPF<NUM> (lithium hexafluorophosphate), LiBF<NUM> (lithium tetrafluoroborate), LiClO<NUM> (lithium perchlorate), LiAsF<NUM> (lithium hexafluoroarsenate), LiFSI (lithium bisfluorosulfonimide), LiTFSI (lithium bis-trifluoromethanesulfonimide), LiTFS (lithium trifluoromethanesulfonate), LiDFOB (lithium difluorooxalate borate), LiBOB (lithium bisoxalate borate), LiPO<NUM>F<NUM> (lithium difluorophosphate), LiDFOP (lithium difluorooxalate phosphate), and LiTFOP (lithium tetrafluorooxalate phosphate), for example, one or more of LiPF<NUM> (lithium hexafluorophosphate), LiBF<NUM> (lithium tetrafluoroborate), LiBOB (lithium bisoxalate borate), LiDFOB (lithium difluorooxalate borate), LiTFSI (lithium bis-trifluoromethanesulfonimide), and LiFSI (lithium bisfluorosulfonimide).

Optionally, the liquid electrolyte may further contain another additive, for example, one or more of vinylene carbonate (VC), vinyl ethylene carbonate (VEC), fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), trifluoropropylene carbonate (TFPC), succinonitrile (SN), adiponitrile (ADN), glutaronitrile (GLN), hexanetrinitrile (HTN), <NUM>,<NUM>-propane sultone (<NUM>,<NUM>-PS), ethylene sulfate (DTD), methylene methanedisulfonate (MMDS), <NUM>-propene-<NUM>,<NUM>-sultone (PST), propane <NUM>,<NUM>-cyclic sulfate (PCS), prop-<NUM>-ene-<NUM>,<NUM>-sultone (PES), <NUM>-propyl-[<NUM>,<NUM>,<NUM>]dioxathiolane-<NUM>,<NUM>-dioxide (PEGLST), propylene sulfate (TS), <NUM>,<NUM>-butane sultone (<NUM>,<NUM>-BS), ethylene sulfite (DTO), dimethyl sulfite (DMS), diethyl sulfite (DES), sulfonate cyclic quaternary ammonium salt, tris(trimethylsilyl) phosphate (TMSP), and tris(trimethylsilyl) borate (TMSB), but is not limited thereto.

The lithium-ion secondary battery in this embodiment of this application has no special limitations on the separator, and any well-known separator with a porous structure and with electrochemical stability and mechanical stability may be selected, for example, a single-layer or multi-layer film made of one or more of glass fiber, nonwoven fabric, polyethylene (PE), and polyvinylidene fluoride (PVDF).

The positive electrode plate and the negative electrode plate are alternately stacked, and the separator is disposed between the positive electrode plate and the negative electrode plate for separation, to obtain a battery cell, or a battery cell may be obtained through winding. The battery cell is placed in a housing, an electrolyte is injected, and the housing is sealed to obtain the lithium-ion secondary battery.

There are no special limitations on a shape of the lithium-ion secondary battery in this application, and the lithium-ion secondary batter may be cylindrical, square, or in any other shape. <FIG> shows a lithium-ion secondary battery <NUM> of a square structure as an example.

In some embodiments, the secondary battery may include an outer package. The outer package is used to package a positive electrode plate, a negative electrode plate, and an electrolyte.

In some embodiments, referring to <FIG>, the outer package may include a housing <NUM> and a cover plate <NUM>. The housing <NUM> may include a base plate and a side plate connected to the base plate. The base plate and the side plate encircle an accommodating cavity. The housing <NUM> has an opening communicating with the accommodating cavity, and the cover plate <NUM> can cover the opening to close the accommodating cavity.

The positive electrode plate, the negative electrode plate, and a separator may be wound or laminated to form a battery cell <NUM>. The battery cell <NUM> is packaged in the accommodating cavity. The electrolyte may be a liquid electrolyte, and the battery cell <NUM> is infiltrated in the liquid electrolyte. There may be one or more battery cells <NUM> in the lithium-ion secondary battery <NUM>, and their quantity may be adjusted as required.

In some embodiments, the outer package of the lithium-ion secondary battery may be a hard shell, for example, a hard plastic shell, an aluminum shell, or a steel shell. Alternatively, the outer package of the secondary battery may be a soft package, for example, a soft bag. The soft bag may be made of plastic, for example, may include one or more of polypropylene (PP), polybutylene terephthalate (PBT), polybutylene succinate (PBS), and the like.

In some embodiments, the lithium-ion secondary battery may be assembled into a battery module, and the battery module may include a plurality of lithium-ion secondary batteries. A specific quantity may be adjusted based on a use case and a capacity of the battery module.

<FIG> shows a battery module <NUM> used as an example. Referring to <FIG>, in the battery module <NUM>, a plurality of lithium-ion secondary batteries <NUM> may be sequentially arranged in a length direction of the battery module <NUM>. Certainly, the lithium-ion secondary batteries may alternatively be arranged in any other manner. Further, the plurality of lithium-ion secondary batteries <NUM> may be fixed by using fasteners.

Optionally, the battery module <NUM> may further include a housing with an accommodating space, and the plurality of lithium-ion secondary batteries <NUM> are accommodated in the accommodating space.

In some embodiments, the battery module may be further assembled into a battery pack, and a quantity of battery modules included in the battery pack may be adjusted based on application and a capacity of the battery pack.

<FIG> show a battery pack <NUM> used as an example. Referring to <FIG>, the battery pack <NUM> may include a battery box and a plurality of battery modules <NUM> disposed in the battery box. The battery box includes an upper box body <NUM> and a lower box body <NUM>. The upper box body <NUM> can cover the lower box body <NUM> to form enclosed space for accommodating the battery modules <NUM>. The plurality of battery modules <NUM> may be arranged in the battery box in any manner.

This application further provides an apparatus (not claimed). The apparatus includes at least one of the lithium-ion secondary battery, the battery module, or the battery pack of this application. The lithium-ion secondary battery, the battery module, or the battery pack may be used as a power source of the apparatus, or may be used as an energy storage unit of the apparatus. The apparatus may include, but is not limited to, a mobile device (for example, a mobile phone or a notebook computer), an electric vehicle (for example, a battery electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf vehicle, or an electric truck), an electric train, a ship, a satellite, an energy storage system, and the like.

A lithium-ion secondary battery, a battery module, or a battery pack may be selected for the apparatus according to a use requirement of the apparatus.

<FIG> shows an apparatus used as an example. The apparatus is, for example, a battery electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet a requirement of the apparatus for high power and high energy density of the secondary battery, a battery pack or a battery module may be used.

In another example, the apparatus may be a mobile phone, a tablet computer, or a notebook computer. The apparatus usually requires lightness and thinness, and the lithium-ion secondary battery may be used as a power source.

The following examples more specifically describe the content disclosed in this application, and these examples are merely intended for illustrative description, because various modifications and changes made within the scope of the content disclosed in this application are obvious to a person skilled in the art. Unless otherwise specified, all portions, percentages, and ratios reported in the following examples are based on a weight, all reagents used in the examples are commercially available or are synthesized according to a conventional method, and can be directly used without further processing, and all instruments used in the examples are commercially available.

A positive electrode active material precursor [Ni<NUM>Co<NUM>Mn<NUM>](OH)<NUM>, lithium hydroxide LiOH, molybdenum trioxide MoO<NUM>, and molybdenum dioxide MoO<NUM> were added to a high-speed mixer for mixing for <NUM> to obtain a mixture. A molar ratio Li/Me of the positive electrode active material precursor to the lithium hydroxide was <NUM>, where Me indicated total molar mass of Ni, Co, and Mn in the positive electrode active material precursor, and MoO<NUM> accounted for <NUM>% of total mass of MoO<NUM> and MoO<NUM>. The mixture was put into an atmosphere sintering furnace for sintering, where a sintering temperature was <NUM>, and a sintering atmosphere was an oxygen-containing atmosphere with an O<NUM> concentration of <NUM>%, and sintering duration was <NUM>. A sintered product was crushed and sieved to obtain a ternary positive electrode active material with high nickel content.

Based on testing, a true doping concentration of Mo in the positive electrode active material was <NUM>µg/cm<NUM>, and Mo with a valence of +<NUM> (that is, a highest valence state) in the positive electrode active material in a <NUM>% delithiated state accounted for <NUM>% of total Mo content. As shown in <FIG>, the doping element Mo was in uniform doping in a bulk structure of particles in the positive electrode active material.

EC, DEC, and DMC were mixed at a volume ratio of <NUM>:<NUM>:<NUM> to obtain a solvent. Then a lithium salt LiPF<NUM> was dissolved in the solvent to obtain an electrolyte, where a concentration of LiPF<NUM> was <NUM> mol/L.

The prepared positive electrode active material, conductive carbon black, and a binder PVDF were dispersed in a solvent N-methylpyrrolidone (NMP) at a weight ratio of <NUM>:<NUM>:<NUM>, and were well mixed to obtain a positive electrode slurry. The positive electrode slurry was evenly applied to a positive electrode current collector aluminum foil, followed by drying and cold pressing, to obtain a positive electrode plate. In a button battery box, the positive electrode plate, a separator, and a lithium metal plate were stacked in sequence, and the electrolyte was injected, to obtain a button battery through assembly.

The prepared positive electrode active material, a conductive agent acetylene black, and a binder PVDF were dispersed in a solvent NMP at a weight ratio of <NUM>:<NUM>:<NUM>, and were well mixed to obtain a positive electrode slurry. The positive electrode slurry was evenly applied to a positive electrode current collector aluminum foil, followed by drying and cold pressing, to obtain a positive electrode plate.

A negative electrode active material artificial graphite, hard carbon, a conductive agent acetylene black, a binder styrene-butadiene rubber (SBR), and a thickener sodium carboxymethyl cellulose (CMC-Na) were dispersed in deionized water at a weight ratio of <NUM>:<NUM>:<NUM>:<NUM>:<NUM>, and were well mixed to obtain a negative electrode slurry. The negative electrode slurry was evenly applied to a negative electrode current collector aluminum foil, followed by drying and cold pressing, to obtain a negative electrode plate.

A polyethylene (PE) porous polymer film was used as a separator. The positive electrode plate, the separator, and the negative electrode plate were stacked in sequence to obtain a bare cell. The bare cell was placed in an outer package, the electrolyte was injected, and the bare cell was packaged. Then processes such as formation were performed to obtain a full battery.

A difference from Example <NUM> lies in that related parameters in the preparation steps of the positive electrode active material were modified to obtain a positive electrode active material with a predetermined doping element type, doping amount, and doping uniformity. For details, refer to Table <NUM>.

Comparative Examples <NUM> to <NUM> and Examples <NUM> and <NUM> differ from Example <NUM> in that MoO<NUM> added in a sintering step accounted for <NUM>%, <NUM>%, <NUM>%, <NUM>%, and <NUM>% of total mass of MoO<NUM> and MoO<NUM>, respectively.

In Comparative Example <NUM>, no doping element was added.

In Comparative Example <NUM>, a doping element source was Y<NUM>O<NUM>.

In Examples <NUM> to <NUM> and Examples <NUM> to <NUM>, in a sintering step, a common oxide containing elements with a valence greater than +<NUM> and a corresponding low-valence oxide were selected as doping element sources, where doping element sources with a valence greater than +<NUM> accounted for <NUM>% of total mass of doping element sources.

In Examples <NUM> to <NUM>, Sb<NUM>O<NUM> added during sintering accounted for <NUM>% of total mass of Sb<NUM>O<NUM> and Sb<NUM>O<NUM>, and a total doping amount was controlled to obtain positive electrode active materials with different true doping concentrations.

In Examples <NUM> to <NUM>, WO<NUM> added during sintering accounted for <NUM>% of total mass of WO<NUM> and WO<NUM>.

In Comparative Example <NUM>, a positive electrode active material precursor [Ni<NUM>Co<NUM>Mn<NUM>](OH)<NUM> was used, and no doping element was added.

In Comparative Example <NUM>, a positive electrode active material precursor [Ni<NUM>Co<NUM>Mn<NUM>](OH)<NUM> was used, and a doping element source was Y<NUM>O<NUM>.

In Comparative Examples <NUM> and <NUM> and Example <NUM>, a positive electrode active material precursor [Ni<NUM>Co<NUM>Mn<NUM>](OH)<NUM> was used, and Sb<NUM>O<NUM> added during sintering accounted for <NUM>%, <NUM>%, and <NUM>% of total mass of Sb<NUM>O<NUM> and Sb<NUM>O<NUM>, respectively.

In Example <NUM>, a positive electrode active material precursor [Ni<NUM>Co<NUM>Mn<NUM>](OH)<NUM> was used, and WO<NUM> added during sintering accounted for <NUM>% of total mass of WO<NUM> and WO<NUM>.

A relative deviation σ of a local mass concentration of a doping element in a particle of the positive electrode active material, and a deviation ε, of a mass concentration of a doping element in the positive electrode active material from an average mass concentration of the doping element in the particle of the positive electrode active material may be adjusted by adjusting the doping element type, the doping amount, the sintering temperature, and/or the sintering duration.

At <NUM>, eight button batteries were separately charged to an upper limit of a cut-off charge/discharge voltage at a constant current of 1C, then charged to a current less than or equal to <NUM> mA at a constant voltage, then left aside for <NUM> minutes, and then discharged to a lower limit of the cut-off charge/discharge voltage at a constant current of 1C.

Then the eight charged and discharged button batteries were charged to <NUM> V, <NUM> V, <NUM> V, <NUM> V, <NUM> V, <NUM> V, <NUM> V, and <NUM> V at a rate of <NUM>. 1C, respectively. Each charged button battery was taken, and a positive electrode plate was taken as a sample in a drying room. The sample was weighed, mass was recorded, and the sample was put into a digestion tank. <NUM> of aqua regia was slowly added as a digestion reagent. The sample was assembled and put into a CEM-Mars5 microwave digestion instrument, and digestion performed at a microwave emission frequency of <NUM>. A digested sample solution was transferred to a volumetric flask for a good shaking. A sample was taken and put into a sample feeding system of a 7000DV inductively coupled plasma-optical emission spectrometer (ICP-OES) of PE (US). Mass concentrations of Li, O, Ni, Co, Mn, and the doping element in the positive electrode active material were tested under an argon pressure of <NUM> MPa and a radio frequency power of <NUM> W. A chemical formula under each voltage was obtained through conversion based on a mass concentration of each element, and then a delithiated state under each voltage was obtained. If a chemical formula of the positive electrode active material obtained through conversion at a voltage of <NUM> V was Li<NUM>Ni<NUM>Co<NUM>Mn<NUM>O<NUM>, a corresponding delithiated state was (<NUM> - <NUM>) × <NUM>% = <NUM>% delithiated state, that is, the <NUM>% delithiated state corresponded to a battery voltage of <NUM> V.

The button battery was charged to the voltage corresponding to the <NUM>% delithiated state at <NUM> and at a rate of <NUM>. 1C, to obtain a sample in the <NUM>% delithiated state. Then the following operations were performed.

<NUM> to <NUM> of positive electrode active material powder in the "<NUM>% delithiated state" was taken and put into a high-pressure crucible of a STA449F3-QMS403C thermal-mass spectrometry system for sealing. The sample was heated at a heating rate of <NUM> /min, and heat flow changes of the sample along with temperature changes were recorded, to obtain a DSC spectrogram, and also obtain an onset exothermic temperature, a peak width at half height, an integral area, and a maximum exothermic temperature of a main exothermic peak.

<NUM> of a positive electrode active material powder sample was weighed, and the sample was evenly sprinkled on a sample stage with conductive adhesive, and then the powder was lightly pressed to fix it. Alternatively, an electrode plate of <NUM> × <NUM> was cut from the positive electrode plate of the battery and pasted to the sample stage as a to-be-tested sample. The sample stage was put into a vacuum sample chamber and properly fixed. An IB-09010CP cross-section polisher of Japan Electron Optics Laboratory (JEOL) was used to prepare a cross section of a particle in the positive electrode active material (as shown in <FIG>). Points were taken with reference to <NUM> locations in the cross section of the particle shown in <FIG>, and an area of each point is <NUM> × <NUM>. An X-Max energy dispersive spectroscopy (EDS) of Oxford Instruments (UK) and a Sigma-<NUM>-<NUM> scanning electron microscope (SEM) of ZEISS (Germany) were used together to test mass concentrations of doping element at the <NUM> locations. A test method was as follows: Li, O, Ni, Co, Mn, and the doping element were selected as to-be-tested elements. SEM parameters were set as follows: An accelerating voltage was <NUM> kV, an aperture was <NUM>, an operating distance was <NUM>, and a current was <NUM> A. When an EDS test was performed, the test did not stop until a spectrogram area reached <NUM>,<NUM> cts or above (controlled by collection duration and a collection rate), and data was collected to obtain mass concentrations of the doping element at all locations, which were denoted as η<NUM>, η<NUM>, η<NUM>,. , and η<NUM>.

Method for testing an average mass concentration η of the doping element in a particle of the positive electrode active material: The foregoing EDS-SEM test method was used. As indicated by a dashed-line box in <FIG>, a test area covered all points of spot scanning for the particle of the positive electrode active material, and did not go beyond a cross section of the secondary particle.

Then the relative deviation σ of the local mass concentration of the doping element in the particle of the positive electrode active material was calculated based on the formula (<NUM>).

To test a positive electrode active material in a battery, the battery might be disassembled in a drying room. A middle part of a positive electrode plate was taken and put into a beaker. An appropriate amount of high-purity anhydrous dimethyl carbonate DMC was added, and the DMC was replaced every <NUM> hours. The middle part was cleaned for consecutive three times, then put into a vacuum standing box in the drying room, and dried for <NUM> hours in a vacuum state (-<NUM> MPa). After drying, an electrode plate sample with a size of <NUM> × <NUM> or above was cut, and the electrode plate sample was pasted to a sample stage with conductive adhesive; or <NUM> of positive electrode active material powder was taken through scraping in the drying room as a to-be-tested sample, and a test was performed according to the foregoing method.

A TD2400 powder true density tester of Beijing Builder Electronic Technology Co. was used to test true density ρtrue of the positive electrode active material. A test method was as follows: At <NUM>, a positive electrode active material with specific mass was taken and put into a sample cup, and the mass m of the positive electrode active material was recorded. The sample cup containing the positive electrode active material was put into a test chamber of the true density tester, the test system was sealed, and an inert gas with a small molecular diameter such as helium or nitrogen was introduced. Gas pressures in the sample chamber and a swelling chamber were measured. Then a true volume V of the tested material was measured according to the Bohr's theory PV = nRT, and true density ρtrue of a secondary particle was calculated by using m/V. n was molar mass of a gas in the sample cup. R was an ideal gas constant, which was <NUM>. T was an ambient temperature, which was <NUM>.

A 7000DV inductively coupled plasma-optical emission spectrometer (ICP-OES) of PE (US) was used to test a mass concentration ϕ of a doping element in the positive electrode active material. A test method was as follows: An electrode plate containing the positive electrode active material was taken and punched into a wafer whose total mass was greater than <NUM>, or at least <NUM> of a positive electrode active material powder sample was taken. The sample was weighed, and mass of the sample was recorded. Then the sample was put into a digestion tank, and <NUM> of aqua regia was slowly added as a digestion reagent. Then the sample was put into a Mars5 microwave digestion instrument of CEM (US), and digested at a microwave emission frequency of <NUM>. A digested sample solution was transferred to a volumetric flask for a good shaking. A sample was taken and put into a sample feeding system of an ICP-OES. The mass concentration of the doping element in the positive electrode active material was tested under an argon pressure of <NUM> MPa and a radio frequency power of <NUM> W.

Then the true doping concentration ω of the positive electrode active material was calculated by using the formula (<NUM>).

At <NUM>, the battery was charged to an upper limit of a cut-off charge/discharge voltage at a constant current of <NUM>. 1C, then charged to a current less than or equal to <NUM> mA at a constant voltage, then left aside for <NUM> minutes, and then discharged to a lower limit of the cut-off charge/discharge voltage at a constant current of <NUM>. A discharge capacity in this case was the initial gram capacity of the button battery.

At <NUM>, the battery was charged to an upper limit of a cut-off charge/discharge voltage at a constant current of <NUM>/3C, then charged to a current less than or equal to <NUM> mA at a constant voltage, then left aside for <NUM> minutes, and then discharged to a lower limit of the cut-off charge/discharge voltage at a constant current of <NUM>/3C. A discharge capacity in this case was the initial gram capacity of the full battery.

At <NUM>, the battery was charged to an upper limit of a cut-off charge/discharge voltage at a constant current of 1C, then charged to a current less than or equal to <NUM> mA at a constant voltage, then left aside for <NUM> minutes, and then discharged to a lower limit of the cut-off charge/discharge voltage at a constant current of 1C. This was a charge and discharge cycle, and a discharge capacity in this case was recorded as a discharge specific capacity Di in the first cycle. <NUM> charge and discharge cycles were performed on the battery according to the foregoing method, and a discharge specific capacity D<NUM> in the <NUM>th cycle was recorded.

In tests <NUM>, <NUM>, <NUM>, and <NUM>:.

Test results of examples and comparative examples are shown in Table <NUM> to Table <NUM>.

In Table <NUM>, the "lowest valence state" is a smallest value in valence distribution of a doping element in a positive electrode active material in a "<NUM>% delithiated state"; the "highest valence state" is a largest value in the valence distribution of the doping element in the positive electrode active material in the "<NUM>% delithiated state"; and the "highest-valence percentage" is a percentage of the amount of doping element in a highest valence state in the positive electrode active material in the "<NUM>% delithiated state" in the total amount of doping element.

From the results of Examples <NUM> to <NUM> and Comparative Examples <NUM> to <NUM> and the results of Example <NUM> and Comparative Examples <NUM> to <NUM>, it can be learned that the positive electrode active material included a lithium nickel cobalt manganese oxide, in the lithium nickel cobalt manganese oxide, the number of moles of nickel accounted for <NUM>% to <NUM>% of the total number of moles of nickel, cobalt, and manganese, a polyvalent element was included in doping in a transition metal layer, the doping element in the positive electrode active material in the <NUM>% delithiated state had two or more different valence states, and the amount of doping element in a highest valence state accounted for <NUM>% to <NUM>% of the total amount of doping element, so that the lithium-ion secondary battery not only had a relatively high initial capacity, but also had relatively good high-temperature cycling performance.

From the results of Examples <NUM>, <NUM> to <NUM>, and <NUM> to <NUM> and the results of Examples <NUM> and <NUM>, it can be learned that selecting a better doping element type could better improve an initial capacity and high-temperature cycling performance of the battery.

From the results of Examples <NUM> to <NUM>, it can be learned that controlling the true doping concentration ω of the positive electrode active material in an appropriate range helped improve an initial capacity and high-temperature cycling performance of the battery.

From the results of Examples <NUM> to <NUM>, it can be learned that controlling the relative deviation σ of the local mass concentration of the doping element in the particle of the positive electrode active material to be <NUM>% or below, especially below <NUM>%, could better improve an initial capacity and high-temperature cycling performance of the battery.

Claim 1:
A method for preparing positive electrode active material, comprising the following steps:
mixing a positive electrode active material precursor, a lithium source, and a doping element precursor to obtain a mixture, wherein the positive electrode active material precursor is selected from one or more of an oxide, hydroxide, or carbonate that contains Ni, Co, and Mn, the number of moles of nickel accounts for <NUM>% to <NUM>% of the total number of moles of nickel, cobalt, and manganese, and at least part of doping element in the doping element precursor has not reached its highest valence state; and
sintering the mixture in an oxygen-containing atmosphere at a temperature of <NUM> to <NUM> to obtain a positive electrode active material, wherein
the positive electrode active material comprises a lithium nickel cobalt manganese oxide, wherein in the lithium nickel cobalt manganese oxide, the number of moles of nickel accounts for <NUM>% to <NUM>% of the total number of moles of nickel, cobalt, and manganese, and the lithium nickel cobalt manganese oxide has a layered crystal structure with a space group R<NUM>m; and
the lithium nickel cobalt manganese oxide comprises a doping element, wherein when the positive electrode active material is in a <NUM>% delithiated state, the doping element has two or more different valence states, and the amount of doping element in a highest valence state accounts for <NUM>% to <NUM>% of the total amount of doping element;
wherein the doping element precursor is selected from the group, consisting of one or more of a titanium oxide, a vanadium oxide, a chromium oxide, a selenium oxide, a niobium oxide, a molybdenum oxide, a ruthenium oxide, a rhodium oxide, a palladium oxide, an antimony oxide, a tellurium oxide, a cerium oxide, and a tungsten oxide;
wherein the doping element precursor comprises a first doping element oxide and a second doping element oxide, a valence α of a doping element in the first doping element oxide is less than or equal to <NUM>, and a valence β of a doping element in the second doping element oxide is greater than or equal to <NUM>; and β > α.