Patent Description:
A secondary battery is a battery that can be repeatedly charged and discharged, and is widely applied to portable electronic communication devices such as camcorders, mobile phones, and laptop computers with the development of information communication and display industries. Examples of the secondary battery may include a lithium secondary battery, a nickel-cadmium battery, a nickel-hydrogen battery and the like. Among them, the lithium secondary battery has a high operating voltage and a high energy density per unit weight, and is advantageous in terms of a charging speed and light weight. In this regard, the lithium secondary battery has been actively developed and applied as a power source.

The lithium secondary battery may include an electrode assembly including a cathode, an anode, and a separation membrane (separator), and an electrolyte immersing the electrode assembly. In addition, the lithium secondary battery may further include an outer case having, e.g., a pouch-shaped for accommodating the electrode assembly and the electrolyte are housed.

For example, the electrode may include electrode active material particles capable of intercalating and deintercalating lithium ions. When repeatedly charging/discharging the secondary battery, mechanical and chemical damage such as crack of the particles may occur, and contact between the active material particles may be deteriorated and short circuit may occur.

When changing the composition and structure of the electrode active material in order to improve stability of the active material particles, conductivity may be reduced to cause a decrease in an output of the secondary battery. Therefore, development of a secondary battery electrode capable of securing life-span stability and output/capacity characteristics is required.

For example, <CIT> discloses an electrode assembly for a lithium secondary battery and a lithium secondary battery including the same, but there is a limitation in securing sufficient high rate and high capacity characteristics. <CIT> further discloses electrodes for lithium secondary batteries.

According to an aspect of the present disclosure, there is provided an electrode for a lithium secondary battery having improved electrical properties and stability.

According to an aspect of the present disclosure, there is provided a lithium secondary battery including the electrode having improved electrical properties and stability.

To achieve the above objects, according to an aspect of the present invention, there is provided an electrode for a lithium secondary battery including: an electrode current collector; an electrode active material layer which is disposed on at least one surface of the electrode current collector and includes electrode active material particles; and a lithium salt-containing coating which is formed on at least portions of surfaces of the electrode active material particles or at least a portion of a surface of the electrode active material layer, wherein the electrode has a surface arithmetic average roughness (Ra or Ra) represented by Equation <NUM> below:.

Wherein, in Equation <NUM>, Ra is a value obtained by calculating an arithmetic average of surface roughness values measured in <NUM> or more measurement regions of the surface of the electrode for a lithium secondary battery using an atomic force microscopy in a scan range of <NUM> × <NUM>.

For example, the surface arithmetic average roughness (Ra) may be <NUM> to <NUM>.

In some embodiments, a surface roughness value measured in each of the measurement regions may be an arithmetic average value of roughness values excluding a maximum value and a minimum value among the surface roughness values when the surface roughness value is measured <NUM> or more times in a scan range of <NUM> × <NUM> for each of the measurement regions.

In some embodiments, a standard deviation of the surface roughness values measured in the measurement regions of the surface of the electrode for a lithium secondary battery may be <NUM> or less.

In some embodiments, the lithium salt-containing coating may have a continuous film shape which covers the surfaces of the electrode active material particles or the surface of the electrode active material layer.

In some embodiments, the lithium salt-containing coating may have a thickness of <NUM> to <NUM>,<NUM>.

In some embodiments, the lithium salt-containing coating may include at least one of LiCl, LiF, Li<NUM>PO<NUM>, LiBO<NUM>, LiIO<NUM>, Li<NUM>CO<NUM>, Li<NUM>B<NUM>O<NUM>, Li<NUM>SO<NUM>, LiBr, LiI, LiNO<NUM>, Li(CF<NUM>CO<NUM>), Li((CH<NUM>)<NUM>SiO), Li(CH<NUM>O), LiCH<NUM>COO, Li(CO<NUM>CH<NUM>), (CH<NUM>)<NUM>CHOLi (lithium isopropoxide), CH<NUM>CH(OH)COOLi (lithium lactate), Li<NUM>S, LiOH, Li<NUM>O, Li<NUM>O<NUM>, CH<NUM>CH<NUM>OLi (lithium ethoxide), C<NUM>H<NUM>OLi (lithium phenoxide) and C<NUM>H<NUM>LiO<NUM> (lithium benzoate).

In some embodiments, a content of the lithium salt-containing coating measured by high performance liquid chromatography (HPLC) may be <NUM> ppm to <NUM> ppm based on a total weight of the electrode active material layer.

In some embodiments, the lithium salt-containing coating may be formed on both the surfaces of the electrode active material particles and the surface of the electrode active material layer.

In some embodiments, the lithium salt-containing coating may have a lithium ion conductivity of <NUM> x <NUM>-<NUM> S/cm or more and an electronic conductivity of <NUM> x <NUM>-<NUM> S/cm or less.

In some embodiments, the electrode for a lithium secondary battery may be an anode or a cathode.

According to another aspect of the present invention, there is provided a lithium secondary battery including: a cathode; and an anode disposed to face the cathode, wherein at least one of the cathode and the anode is the above-described electrode for a lithium secondary battery.

In some embodiments, the lithium secondary battery may further include an electrolyte in which the cathode and the anode are impregnated, wherein the lithium salt-containing coating has a solubility of <NUM>/L or less in the electrolyte.

The electrode for a lithium secondary battery according to example embodiments may include a lithium salt-containing coating formed on the surface of an electrode active material or the surface of an electrode active material layer. Direct contact between the electrode active material and the electrolyte may be suppressed by the lithium salt-containing coating, and volume expansion and structural collapse of the electrode active material may be prevented.

The electrode for a lithium secondary battery may have a predetermined surface arithmetic average roughness. Accordingly, a side reaction between the electrode active material and the electrolyte may be further suppressed. Thus, consumption of the electrolyte and irreversible capacity loss may be reduced, and rapid charging and cycle properties of the lithium secondary battery may be improved.

The lithium salt-containing coating has high lithium ion conductivity and low electronic conductivity, thereby life-span characteristics and structural stability of the electrode active material may be improved even during repeated charging/discharging behaviors under severe high temperature/high humid conditions. Accordingly, durability and charge/discharge capacities of the lithium secondary battery may be improved without decreasing initial efficiency and high charging speed.

According to embodiments of the present disclosures, an electrode for a lithium secondary battery including a lithium salt-containing coating and having a predetermined surface arithmetic average roughness value is provided.

In addition, a lithium secondary battery including the above-described electrode for a lithium secondary battery is provided.

Hereinafter, various features of the disclosed technology will be described in detail with reference to embodiments and the accompanying drawings.

The electrode for a lithium secondary battery includes an electrode current collector and an electrode active material layer disposed on at least one surface of the electrode current collector and including electrode active material particles. A lithium salt-containing coating is formed on at least portions of surfaces of the electrode active material particles and/or at least a portion of a surface of the electrode active material layer.

For example, the lithium salt-containing coating may be formed on at least a portion of the surface of the electrode active material. For example, the lithium salt-containing coating may be formed on at least a portion of the surface of the electrode active material layer. For example, the term "surface of the electrode active material layer" means the outside part or uppermost layer of the electrode active material layer which preferably faces the electrolyte in use.

The lithium salt-containing coating is formed on the surface of the electrode active material and/or on the surface of the electrode active material layer, so that direct exposure of the electrode active material particles to an electrolyte may be prevented. Therefore, a side reaction between the electrode active material particles and the electrolyte may be suppressed thus to prevent irreversible decomposition of the electrolyte.

The electrode for a lithium secondary battery has a surface arithmetic average roughness (Ra or Ra) represented by Equation <NUM> below.

In Equation <NUM>, the surface arithmetic average roughness (Ra) is obtained from surface roughness values measured on the surface of the electrode active material layer using an atomic force microscopy (AFM) in a scan range of <NUM> × <NUM>. For example, Ra measures the deviation of a surface from a mean height, i.e., Ra is the average, or arithmetic average of profile height deviations from the mean line.

The scan range of <NUM> × <NUM> may be a range of a portion of the surface of the electrode for a lithium secondary battery or a portion of the surface of the electrode active material particle designated in a surface direction of the electrode active material layer when observed the surface with the atomic force microscopy (AFM).

The Ra of the electrode for a lithium secondary battery is an arithmetic average value of surface roughness values measured in predetermined regions of the surface of the electrode active material layer using the atomic force microscopy (AFM).

In one embodiment, the Ra of the electrode for a lithium secondary battery may be obtained by measuring the surface roughness values of at least three regions of the electrode. For example, the surface arithmetic average roughness (Ra) may be obtained by selecting three or more regions of the surface of the electrode active material layer to measure surface roughness values in each region, and calculating an arithmetic average of the measured surface roughness values.

In one embodiment, measuring the surface arithmetic average roughness (Ra) may be performed by arbitrary (randomly) designating <NUM> or more regions, <NUM> or more regions, <NUM> or more regions, <NUM> or more regions, or <NUM> to <NUM> regions of the surface of the electrode active material layer. As the number of measurement regions is increased, accuracy and reliability of measurement may be more improved. An area of each measurement region may be in a size of about <NUM> × about <NUM>.

In one embodiment, the surface roughness value of each of the measurement regions may be obtained by measuring roughness values in the measurement region multiple times in a scan range of <NUM> × <NUM> for each of the measurement regions, and calculating an arithmetic average of the measured roughness values.

For example, the roughness value may be obtained from electrode active material particles present in each measurement region, and may mean a center line average roughness in a direction perpendicular to a surface of the measurement region.

The roughness values in each measurement region may be measured in several directions for each measurement region. For example, the surface roughness value of the measurement region may be obtained by measuring roughness values in the measurement region in each of <NUM> or more directions and calculating an arithmetic average of the measured roughness values. For example, the roughness values of each measurement region may be measured <NUM> times or more, <NUM> times or more, or <NUM> to <NUM> times. The measurement direction in the measurement region may be randomly selected.

In an embodiment, the surface roughness value of each measurement region may be obtained by calculating an arithmetic average of roughness values excluding a maximum value and a minimum value among the measured roughness values in consideration of measurement error and dispersion.

As the electrode for a lithium secondary battery has a surface arithmetic average roughness value within the above range, a surface area of the electrode exposed to the electrolyte may be reduced, and a side reaction within the electrode may be suppressed thus to improve life-span characteristics.

For example, when the surface roughness of the electrode active material and/or the electrode active material are/is high, the surface area exposed to the electrolyte may be increased due to an uneven structure. Accordingly, a side reaction between the electrolyte and the electrode active material may be increased, and structural collapse of the electrode active material and depletion of the electrolyte may occur due to repeated charging and discharging.

The lithium salt-containing coating is formed on the surfaces of the electrode active material particles and/or the surface of the electrode active material layer, and the surface arithmetic average roughness of the electrode for a lithium secondary battery is adjusted within the above-described range, thereby a side reaction between the electrode active material and the electrolyte may be further suppressed.

For example, as the surface area of the electrode active material is reduced, contact between the electrode active material and the electrolyte may be suppressed, and as the electronic conductivity is decreased by the lithium salt-containing coating, a side reaction with the electrolyte may be prevented. In addition, the surface arithmetic average roughness value may be easily adjusted by the lithium salt-containing coating.

In some embodiments, the surface arithmetic average roughness of the electrode for a lithium secondary battery may be <NUM> to <NUM>, and for example, <NUM> to <NUM> or <NUM> to <NUM>. Within the above range, a stable solid electrolyte interface (SEI) layer may be easily formed on the surface of the electrode active material layer during initial driving, and reactivity between the electrode active material and the electrolyte may be further reduced. Accordingly, the life-span characteristics of the battery may be more improved by further preventing the collapse/reproduction of the SEI layer and the consumption of the electrolyte due to the same during repeated charging and discharging.

In some embodiments, a standard deviation of the surface roughness values measured in the measurement regions of the surface of the electrode for a lithium secondary battery may be <NUM> or less. For example, the standard deviation may be less than <NUM>, <NUM> or less, or <NUM> or less.

The standard deviation may be an index representing uniformity of the roughness of the surface of the electrode for a lithium secondary battery. For example, as the surface roughness values measured in an arbitrary region of the surface of an electrode for a lithium secondary battery have a low standard deviation or a narrow distribution, a roughness deviation between local regions may be reduced.

For example, as the above-described standard deviation is increased, a portion of the region of the electrode surface may have relatively high roughness. Therefore, since a reaction between the electrode active material and the electrolyte can be concentrated in the corresponding region, such that structural stability of the electrode and life-span characteristics of the electrode active material may be relatively decreased.

According to example embodiments, as both the arithmetic average value (Ra) and the standard deviation of the surface roughness values measured from the surface of the electrode for a lithium secondary battery have a low value, the surface of the electrode may have a low and uniform roughness as a whole. Accordingly, the stability and life-span characteristics of the lithium secondary battery may be further improved.

In some embodiments, the lithium salt-containing coating may cover at least a portion of the surface of the electrode active material and/or at least a portion of the surface of the electrode active material layer. For example, the lithium salt-containing coating may have a continuous film shape which covers a part or all of the surface of the electrode active material and/or the surface of the electrode active material layer.

<FIG> is a schematic cross-sectional view illustrating an electrode active material included in the electrode for a lithium secondary battery according to example embodiments.

Referring to <FIG>, the electrode for a lithium secondary battery may include an electrode active material particle <NUM> and a lithium salt-containing coating <NUM> which surrounds the surface of the electrode active material particle <NUM>.

As the lithium salt-containing coating <NUM> is formed on the surface of the electrode active material particle <NUM>, direct contact between the electrode active material particle <NUM> and the electrolyte may be prevented, and mechanical/thermal shocks from an outside may be alleviated. Therefore, structural defect and damage to the electrode active material particles <NUM> may be prevented.

For example, the lithium salt-containing coating <NUM> may be formed by a wet coating method. For example, the lithium salt-containing coating <NUM> may be formed on the surfaces of the electrode active material particles <NUM> by mixing and stirring a mixed solution containing a lithium salt and an organic solvent with the electrode active material particles <NUM>, followed by drying the mixture to evaporate the organic solvent. A heat treatment step may be additionally performed after the drying.

<FIG> is a schematic cross-sectional view illustrating an electrode for a lithium secondary battery according to example embodiments.

Referring to <FIG>, an electrode <NUM> for a lithium secondary battery may include an electrode current collector <NUM>, an electrode active material layer <NUM> formed on at least one surface of the electrode current collector <NUM>, and a lithium salt-containing coating <NUM> which covers at least a portion of the surface of the electrode active material layer <NUM>.

In some embodiments, the electrode active material layer <NUM> may be formed on both surfaces of the electrode current collector <NUM>. For example, the electrode active material layer <NUM> may be coated on upper and lower surfaces of the electrode current collector <NUM>, respectively. The electrode active material layer <NUM> may be directly coated on the surface of the electrode current collector <NUM>.

The lithium salt-containing coating <NUM> may be formed to cover the electrode active material layer <NUM>. For example, the lithium salt-containing coating <NUM> may be formed by a wet coating method.

In one embodiment, the lithium salt-containing coating <NUM> may be formed by applying the mixed solution containing a lithium salt and an organic solvent to the electrode active material layer <NUM>, followed by drying the mixture. A heat treatment step may be additionally performed after the drying.

In one embodiment, the lithium salt-containing coating <NUM> may be formed on the electrode active material layer <NUM> by immersing the electrode active material layer <NUM> in a solution in which lithium salts are dissolved, then taking out the electrode active material layer <NUM> and drying. A heat treatment step may be additionally performed after the drying.

In some embodiments, the lithium salt-containing coating may have a thickness of <NUM> to <NUM>,<NUM>, and for example, <NUM> to <NUM>. In some embodiments, the lithium salt-containing coating on the electrode active material particles may have a thickness of <NUM> to <NUM>,<NUM>, and for example, <NUM> to <NUM> or <NUM> to <NUM> and/or the lithium salt-containing coating on the electrode active material layer may have a thickness of <NUM> to <NUM>,<NUM>, and for example, <NUM> to <NUM> or <NUM> to <NUM>.

Within the above ranges, depletion of the electrolyte and oxidation/reduction of the electrode active material due to the contact between the electrode active material and the electrolyte may be more suppressed. In addition, as a lithium movement path in the lithium salt-containing coating is shortened, ionic conductivity may be improved, and the lithium secondary battery may have higher initial efficiency and charge/discharge capacities.

For example, if the thickness of the lithium salt-containing coating is less than <NUM>, it may be relatively difficult to block permeation of the electrolyte due to the thin coating, and a side reaction between the electrode active material and the electrolyte cannot be effectively suppressed. If the thickness of the lithium salt-containing coating is greater than <NUM>,<NUM>, resistance may be relatively increased due to the thickly formed coating, and initial efficiency and performance of the lithium secondary battery may be relatively decreased.

In some embodiments, the lithium salt-containing coatings may be formed on both the surfaces of the electrode active material particles and the surface of the electrode active material layer. For example, the lithium salt-containing coating may include a first lithium salt-containing coating which covers the surfaces of the electrode active material particles and a second lithium salt-containing coating which covers the surface of the electrode active material layer.

As the lithium salt-containing coatings are formed on both the surfaces of the electrode active material particles and the surface of the electrode active material layer, penetration of the electrolyte into the electrode active material particles is doubly blocked, thereby further improving life-span characteristics. In addition, since high ion conductivity may be secured, output characteristics and charge/discharge capacities may be further improved.

In some embodiments, a part of the lithium salt-containing coating may be individually distributed and disposed in an island shape on the surface of the electrode active material and/or the surface of the electrode active material layer.

In one embodiment, the lithium salt-containing coating may be disposed in an area of <NUM>% or more of the surface of the electrode active material and/or an area of <NUM>% or more of the surface of the electrode active material layer, and for example, may be disposed in an area of <NUM>% or more, or <NUM>% or more. Accordingly, deterioration of the electrode active material due to the external environment and charging/discharging may be prevented while maintaining high lithium ion conductivity, and thus initial efficiency and capacity retention rate may be further improved.

In some embodiments, the lithium salt-containing coating may include a lithium salt having low solubility and swelling characteristics in an organic solvent. Accordingly, the lithium salt-containing coating may have low reactivity to the electrolyte, high insulation properties, and improved mechanical/chemical stabilities.

For example, since the lithium salt has low electronic conductivity and high stability even under severe high temperature/high humid conditions, a side reaction between the electrode active material and the electrolyte may be suppressed. Therefore, oxidation/reduction of the electrode active material by the electrolyte may be prevented, and structural collapse and crack of the electrode active material due to driving at high temperature/high voltage and external physical impact may be prevented.

The lithium salt may improve a rate of intercalation and deintercalation of lithium ions, and thus, irreversible reaction and capacity loss due to overvoltage occurring during high speed charging may be prevented. In addition, while initial efficiency and high speed charging characteristics are improved due to high lithium ion conductivity and low reactivity to the electrolyte, capacity retention rate and cycle characteristics may be improved.

For example, the lithium salt-containing coating may include LiCl, LiF, Li<NUM>PO<NUM>, LiBO<NUM>, LiIO<NUM>, Li<NUM>CO<NUM>, Li<NUM>B<NUM>O<NUM>, Li<NUM>SO<NUM>, LiBr, LiI, LiNO<NUM>, Li(CF<NUM>CO<NUM>), Li((CH<NUM>)<NUM>SiO), Li(CH<NUM>O), LiCH<NUM>COO, Li(CO<NUM>CH<NUM>), (CH<NUM>)<NUM>CHOLi (lithium isopropoxide), CH<NUM>CH(OH)COOLi (lithium lactate), Li<NUM>S, LiOH, Li<NUM>O, Li<NUM>O<NUM>, CH<NUM>CH<NUM>OLi (lithium ethoxide), C<NUM>H<NUM>OLi (lithium phenoxide), C<NUM>H<NUM>LiO<NUM> (lithium benzoate) and the like. These may be used alone or in combination of two or more thereof.

In one embodiment, the lithium salt-containing coating may include C<NUM>H<NUM>LiO<NUM> (lithium benzoate). C<NUM>H<NUM>LiO<NUM> has low reactivity to the electrolyte or organic solvent and has a high transport capacity for lithium ions, such that the charging characteristics and life-span characteristics of the lithium secondary battery may be further improved.

In some embodiments, the lithium salt-containing coating may consist of a lithium salt. For example, the lithium salt-containing coating may not include a polymeric component or a polymer such as a binder resin. In this case, a decrease in the lithium ion conductivity and an increase in the resistance due to other components (e.g., the polymeric component or polymer) may be prevented, and electrochemical characteristics of the lithium secondary battery may be further improved.

In some embodiments, a content of the lithium salt-containing coating may be <NUM> ppm or more based on a total weight of the electrode active material layer. The content of the lithium salt-containing coating may be measured through high performance liquid chromatography (HPLC) analysis.

For example, a ratio of the lithium salt-containing coating to the electrode active material may be calculated by analyzing a peak area (e.g., peak area %) of the lithium salt compound measured through an HPLC analysis graph.

If the content of the lithium salt-containing coating measured by HPLC peak area is less than <NUM> ppm, the lithium salt-containing coating may not be substantially present on the surface of the electrode active material or the surface of the electrode active material layer. For example, a peak area of less than <NUM> ppm may be a value measured from a by-product produced from the electrolyte or a cathode.

In one embodiment, the content of the lithium salt-containing coating measured by HPLC may be about <NUM> to <NUM> ppm, <NUM> to <NUM> ppm, or <NUM> to <NUM> ppm based on the total weight of the electrode active material layer. The above-described range may be a content measured after a formation process or initial charging and discharging of the lithium secondary battery. For example, the content of the lithium salt-containing coating measured after fully charging and discharging the lithium secondary battery may be <NUM> to <NUM> ppm.

Within the above range, a coating coverage rate of the electrode active material or the electrode active material layer may be increased while further suppressing an increase in the resistance due to the lithium salt-containing coating.

For example, when the content of the lithium salt-containing coating included in the electrode measured after initial charging and discharging is greater than <NUM> ppm based on the total weight of the electrode active material layer, the capacity and energy density of the lithium secondary battery may be relatively reduced.

In some embodiments, the content of the lithium salt-containing coating measured before the formation process of the lithium secondary battery or before initial charging and discharging may be <NUM> to <NUM> parts by weight ("wt. parts") based on <NUM> wt. parts of the electrode active material layer, and for example, may be <NUM> to <NUM> wt.

For example, if the content of the lithium salt-containing coating measured before the formation process is less than <NUM> wt. parts, the coating coverage rate may be decreased due to the decomposition of the lithium salt-containing coating that occurs during an initial charging and discharging process, and life-span characteristics and stability of the electrode for a lithium secondary battery may be relatively decreased.

For example, if the content of the lithium salt-containing coating measured before the formation process is greater than <NUM> wt. parts, a thick film may be formed on the electrode active material during the initial charging and discharging process, thereby causing an increase in the internal resistance, and a relative decrease in the electrochemical performance.

In one embodiment, the content of the lithium salt-containing coating in the electrode active material layer may be measured through high performance liquid chromatography (HPLC) analysis. For example, <NUM> of an electrode active material sample, which is obtained from the electrode active material layer and has a lithium salt-containing coating formed thereon, may be added to <NUM> of DI water (<NUM>% trifluoroacetic acid (TFA)), then subjected to ultrasonic extraction for <NUM> minutes. Thereafter, the sample may be pretreated by mixing the extracted solution for <NUM> hours and filtering the mixture with a syringe filter (Whatman, <NUM>). Then, a content of the lithium salt-containing coating may be measured by performing HPLC analysis on the pretreated sample under the following analysis conditions. Specific compounds corresponding to each peak in the HPLC graph may be analyzed through an MS detector.

In some embodiments, the lithium salt-containing coating may have a form of a solid electrolyte having high lithium ion conductivity and low electronic conductivity. For example, the lithium salt-containing coating may have a lithium ion conductivity of <NUM> x <NUM>-<NUM> S/cm or more and an electronic conductivity of <NUM> x <NUM>-<NUM> S/cm or less. For example, the lithium ion conductivity and electronic conductivity may be calculated by measuring a resistance with electrochemical impedance spectroscopy (EIS) and then converting it into a thickness and an area of the lithium salt-containing coating.

In one embodiment, the lithium ion conductivity of the lithium salt-containing coating may be <NUM> x <NUM>-<NUM> S/cm to <NUM> x <NUM>-<NUM> S/cm, and for example, <NUM> x <NUM>-<NUM> S/cm to <NUM> x <NUM>-<NUM> S/cm. As the lithium salt-containing coating has high lithium ion conductivity, an increase in the resistance in the coating layer may be suppressed, and thus life-span characteristics, initial efficiency, and stability of the secondary battery may be further improved.

In one embodiment, the electronic conductivity of the lithium salt-containing coating may be <NUM> x <NUM>-<NUM> S/cm or less, and for example, <NUM> x <NUM>-<NUM> S/cm or less. As the lithium salt-containing coating has the above-described low electronic conductivity, oxidation/reduction or side reaction of the electrode active material may be prevented, and cycle characteristics and operation stability of the lithium secondary battery may be further improved.

In some embodiments, the electrode for a lithium secondary battery may be a cathode including a cathode active material. For example, the electrode current collector may be a cathode current collector, and the electrode active material layer may be a cathode active material layer.

In some embodiments, the electrode for a lithium secondary battery may be an anode including an anode active material. For example, the electrode current collector may be an anode current collector, and the electrode active material layer may be an anode active material layer.

In some embodiments, the cathode active material may include a lithium metal oxide, for example, a lithium (Li)-nickel (Ni) oxide or a lithium iron phosphate compound (LFePO<NUM>). For example, the lithium (Li)-nickel (Ni) oxide included in the cathode active material layer may be represented by Formula <NUM> below.

[Formula <NUM>]     Li<NUM>+aNi<NUM>-(x+y)CoxMyO<NUM>.

In Formula <NUM>, a, x and y may be in a range of -<NUM>≤a≤<NUM>, <NUM>≤x≤<NUM>, <NUM>≤y≤<NUM>, <NUM>≤x+y≤<NUM>, respectively, and M may be one or more elements selected from the group consisting of Mg, Sr, Ba, B, Al, Si, Mn, Ti, Zr and W. In one embodiment, x and y may be in a range of <NUM>≤x≤<NUM>, <NUM>≤y≤<NUM>, respectively.

In one embodiment, in Formula <NUM>, M may be manganese (Mn). In this case, nickel-cobalt-manganese (NCM) lithium oxide may be used as the cathode active material.

In some embodiments, the anode active material may include carbon-based materials such as crystalline carbon, amorphous carbon, carbon composite, carbon fiber, etc.; a lithium alloy; silicon or tin.

Examples of the amorphous carbon may include hard carbon, cokes, mesocarbon microbead (MCMB), mesophase pitch-based carbon fiber (MPCF) or the like.

Examples of the crystalline carbon may include graphite-based carbon such as natural graphite, graphite cokes, graphite MCMB, graphite MPCF or the like. As an element included in the lithium alloy, Al, Zn, Bi, Cd, At, Si, Pb, Sn, Ga or In may be used.

A lithium secondary battery includes a cathode and an anode disposed to face the cathode, and the electrode for a lithium secondary battery including a lithium salt-containing coating is at least one of the cathode and the anode. For example, the lithium salt-containing coating may be included in the cathode, or included in the anode. For example, the lithium salt-containing coating may be included in both the cathode and the anode.

<FIG> are a schematic plan view and a cross-sectional view illustrating the secondary battery according to example embodiments, respectively. For example, <FIG> is a cross-sectional view taken on line I-I' shown in <FIG>.

Referring to <FIG>, the secondary battery may include an electrode assembly <NUM> and a case <NUM> in which the electrode assembly <NUM> is housed. The electrode assembly <NUM> may include a cathode <NUM>, an anode <NUM> and a separation membrane <NUM>.

The cathode <NUM> may include a cathode current collector <NUM> and a cathode active material layer <NUM> formed on at least one surface of the cathode current collector <NUM>. In one embodiment, the above-described lithium salt-containing coating may be formed on at least a portion of the surface of the cathode active material layer <NUM>.

In some embodiments, the cathode active material layer <NUM> may be formed on both surfaces (e.g., upper and lower surfaces) of the cathode current collector <NUM>. For example, the cathode active material layer <NUM> may be coated on the upper and lower surfaces of the cathode current collector <NUM>, respectively, or may be directly coated on the surface of the cathode current collector <NUM>.

The cathode current collector <NUM> may include, for example, stainless steel, nickel, aluminum, titanium, copper, or an alloy thereof, and preferably includes aluminum or an aluminum alloy.

In some embodiments, in the case of the cathode active material layer <NUM>, a cathode slurry may be coated on the cathode current collector <NUM>, followed by compressing and drying to form the cathode active material layer <NUM>. For example, the cathode slurry may be prepared by mixing the cathode active material with a binder, a conductive material and/or a dispersant in a solvent, followed by stirring the same. In some embodiments, the above-described lithium salt-containing coating may be formed on at least a portion of the surface of the cathode active material.

The binder may include, for example, an organic binder such as vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, etc., or an aqueous binder such as styrene-butadiene rubber (SBR), and may be used together with a thickener such as carboxymethyl cellulose (CMC).

For example, a PVDF-based binder may be used as a binder for forming the cathode. In this case, an amount of the binder for forming the cathode active material layer <NUM> may be reduced and an amount of the cathode active material or lithium metal oxide particles may be relatively increased. Thereby, the output and capacity of the secondary battery may be further improved.

The conductive material may be included to facilitate electron transfer between the active material particles. For example, the conductive material may include a carbon-based conductive material such as graphite, carbon black, graphene, or carbon nanotubes and/or a metal-based conductive material such as tin, tin oxide, titanium oxide, or a perovskite material such as LaSrCoO<NUM>, and LaSrMnO<NUM>, etc..

In some embodiments, the cathode <NUM> may have an electrode density of <NUM> to <NUM>/cc, and for example, <NUM> to <NUM>/cc.

The anode <NUM> may include an anode current collector <NUM> and an anode active material layer <NUM> formed on at least one surface of the anode current collector <NUM>. In some embodiments, the lithium salt-containing coating may be formed on at least a portion of the surface of the anode active material layer <NUM>.

In some embodiments, the anode active material layer <NUM> may be formed on both surfaces (e.g., upper and lower surfaces) of the anode current collector <NUM>. The anode active material layer <NUM> may be coated on the upper and lower surfaces of the anode current collector <NUM>, respectively, and may be in direct contact with the surface of the anode current collector <NUM>.

The anode current collector <NUM> may include gold, stainless steel, nickel, aluminum, titanium, copper or an alloy thereof, and preferably includes copper or a copper alloy.

In some embodiments, in the case of the anode active material layer <NUM>, an anode slurry may be applied (coated) to the anode current collector <NUM>, followed by compressing and drying to form the anode active material layer <NUM>. For example, the anode slurry may be prepared by mixing the anode active material with a binder, a conductive material and/or a dispersant in a solvent, followed by stirring the same. In some embodiments, the above-described lithium salt-containing coating may be formed on at least a portion of the surface of the anode active material.

Materials substantially the same as or similar to those used for forming the cathode <NUM> may be used as the binder and the conductive material. In some embodiments, the binder for forming the anode <NUM> may include, for example, styrene-butadiene rubber (SBR) or an acrylic binder for consistency with the graphite-based active material, and may be used together with a thickener such as carboxymethyl cellulose (CMC).

In some embodiments, the anode active material layer <NUM> may have a density of <NUM> to <NUM>/cc.

In some embodiments, the anode <NUM> may have an area (e.g., a contact area with the separation membrane <NUM>) and/or volume larger than those/that of the cathode <NUM>. Thereby, lithium ions generated from the cathode <NUM> may smoothly move to the anode <NUM> without being precipitated in the middle, such that output and capacity characteristics may be further improved.

The separation membrane <NUM> may be interposed between the cathode <NUM> and the anode <NUM>. The separation membrane <NUM> may include a porous polymer film made of a polyolefin polymer such as ethylene homopolymer, propylene homopolymer, ethylene/butene copolymer, ethylene/hexene copolymer, ethylene/methacrylate copolymer. The separation membrane may include a nonwoven fabric made of glass fiber having a high melting point, polyethylene terephthalate fiber or the like.

The separation membrane <NUM> may extend between the cathode <NUM> and the anode <NUM>, and may be folded and wound in a thickness direction of the lithium secondary battery. Accordingly, a plurality of cathodes <NUM> and anodes <NUM> may be laminated in the thickness direction with the separation membrane <NUM> interposed therebetween.

In some embodiments, an electrode cell is defined by the cathode <NUM>, the anode <NUM>, and the separation membrane <NUM>, and a plurality of electrode cells are laminated to form, for example, a jelly roll type electrode assembly <NUM>. For example, the electrode assembly <NUM> may be formed by winding, lamination, folding, or the like of the separation membrane <NUM>.

The electrode assembly <NUM> is housed in the case <NUM>, and an electrolyte may be injected into the case <NUM> together. The case <NUM> may include, for example, a pouch, a can, or the like in shape.

In some embodiments, a non-aqueous electrolyte may be used as the electrolyte.

The non-aqueous electrolyte includes a lithium salt of an electrolyte and an organic solvent, the lithium salt is represented by, for example, Li+X-, and as an anion (X-) of the lithium salt, F-, Cl-, Br-, I-, NO<NUM>-, N(CN)<NUM>-, BF<NUM>-, ClO<NUM>-, PF<NUM>-, (CF<NUM>)<NUM>PF<NUM>-, (CF<NUM>)<NUM>PF<NUM>-, (CF<NUM>)<NUM>PF<NUM>-, (CF<NUM>)<NUM>PF-, (CF<NUM>)<NUM>P-, CF<NUM>SO<NUM>-, CF<NUM>CF<NUM>SO<NUM>-, (CF<NUM>SO<NUM>)<NUM>N-, (FSO<NUM>)<NUM>N-, CF<NUM>CF<NUM>(CF<NUM>)<NUM>CO-, (CF<NUM>SO<NUM>)<NUM>CH-, (SF<NUM>)<NUM>C-, (CF<NUM>SO<NUM>)<NUM>C-, CF<NUM>(CF<NUM>)<NUM>SO<NUM>-, CF<NUM>CO<NUM>-, CH<NUM>CO<NUM>-, SCN- and (CF<NUM>CF<NUM>SO<NUM>)<NUM>N-, etc. may be exemplified.

As the organic solvent, for example, propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), methylpropyl carbonate, dipropyl carbonate, dimethyl sulfoxide (DMSO), acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate, sulforane, γ-butyrolactone, propylene sulfite, tetrahydrofurane, and the like may be used. These compounds may be used alone or in combination of two or more thereof.

In some embodiments, the lithium salt-containing coating in the electrolyte may have a solubility of <NUM>/L or less, and preferably <NUM>/L or less. For example, the lithium salt-containing coating may have the above-described low solubility in the organic solvent included in the electrolyte.

Accordingly, the lithium salt-containing coating may have improved chemical stability, and low reactivity to the electrolyte, such that deterioration of the electrode active material layer by the electrolyte may be suppressed. Accordingly, a lithium secondary battery having an improved capacity retention rate and high efficiency even during repeated charging/discharging behaviors may be provided.

As shown in <FIG>, electrode tabs (a cathode tab and an anode tab) may protrude from the cathode current collector <NUM> and the anode current collector <NUM>, respectively, which belong to each electrode cell, and may extend to one side of the case <NUM>. The electrode tabs may be fused together with the one side of the case <NUM> to form electrode leads (a cathode lead <NUM> and an anode lead <NUM>) extending or exposed to an outside of the case <NUM>.

<FIG> illustrates that the cathode lead <NUM> and the anode lead <NUM> are formed on the same side of the lithium secondary battery of the case <NUM>, but these electrode leads may be formed on sides opposite to each other.

For example, the cathode lead <NUM> may be formed on one side of the case <NUM>, and the anode lead <NUM> may be formed on the other side of the case <NUM>.

The lithium secondary battery may be manufactured, for example, in a cylindrical shape using a can, a square shape, a pouch type or a coin shape.

Hereinafter, experimental examples including specific examples and comparative examples are proposed to facilitate understanding of the present invention.

<NUM> of artificial graphite (D50: <NUM>) was prepared as an anode active material.

Thereafter, a process of forming a lithium salt-containing coating on the electrode active material particles was performed (see also <FIG>). Specifically, a coating solution, in which <NUM> wt. % of lithium benzoate (C<NUM>H<NUM>LiO<NUM>) was dissolved in a mixed solvent of water and ethanol, was put into a vacuum planetary mixer (Kurabo mixer manufactured by KURABO), and the prepared anode active material was added, followed by mixing at a stirring speed of <NUM> for <NUM> hours. Then, the mixture was dried for <NUM> hours in a vacuum state and at a temperature of <NUM> to form a lithium salt-containing coating having a thickness shown in Table <NUM>, <NUM>nd column, below.

In Table <NUM>, <NUM>nd column, below, "O" denotes an anode active material in which the lithium salt-containing coating is formed. "X" denotes an anode active material in which the lithium salt-containing coating is not formed. The anode active material, in which the lithium salt-containing coating is not formed, was prepared in the same manner except for omitting the process of forming a lithium salt-containing coating on the electrode active material particles.

A composition for an anode was prepared by mixing the prepared anode active material, conductive material (carbon black), binder (SBR), and thickener (CMC) in a ratio of <NUM>:<NUM>:<NUM>:<NUM>. The prepared composition for an anode was applied to a Cu foil, dried and rolled to prepare an anode active material layer having a slurry density of <NUM>/cm<NUM> and <NUM>/cc.

Thereafter, a process of forming a lithium salt-containing coating was performed (see also <FIG>). Specifically, a coating solution, in which <NUM> wt. % of lithium benzoate (C<NUM>H<NUM>LiO<NUM>) was dissolved in a mixed solvent of water and ethanol, was applied to the anode active material layer, and then dried in a convection oven under a temperature condition of <NUM> for <NUM> hours to form a lithium salt-containing coating having a thickness listed in Table <NUM>, <NUM>th column, below.

A coin cell type secondary battery was manufactured using an Li foil as a counter electrode and an electrolyte containing <NUM> LiPF<NUM> in a mixed solvent of ethyl carbonate (EC) and ethylmethyl carbonate (EMC) (EC:EMC = <NUM>:<NUM>).

In Table <NUM>, <NUM>th column, below, "O" denotes an anode active material layer in which the lithium salt-containing coating is formed. "X" denotes an anode active material layer in which a lithium salt-containing coating is not formed. The anode active material layer, in which a lithium salt-containing coating is not formed, was prepared in the same manner except for omitting the process of forming a lithium salt-containing coating on the anode active material layer.

<NUM> or more arbitrary regions of an upper surface of the prepared anode were designated and the roughness (i.e., center line average roughness) in each region was measured <NUM> times using an atomic force microscopy (Icon, Bruker Co. ) under a condition of an XY scan range of <NUM> × <NUM>. The surface roughness value in each region was calculated as an arithmetic average value of values excluding the maximum and minimum values among the measured values.

Thereafter, the Ra value of the anode was calculated as the arithmetic average value of the surface roughness values of each region.

<FIG> is a graph illustrating the distribution of surface roughness values measured on anodes according to Example <NUM> and Comparative Example <NUM>.

Referring to <FIG>, the anode of Example <NUM> has a low arithmetic average value of the surface roughness values measured therefrom as a whole, and a relatively narrow distribution. Accordingly, the anode may have a low standard deviation value of the surface roughness values, and a relatively constant roughness over the entire area of the surface thereof.

However, the anode according to Comparative Example <NUM> has a high arithmetic average value of the surface roughness values measured therefrom as a whole, and a relatively wide distribution. Therefore, it could be confirmed that the deviation of the roughness values for each region of the anode surface was high, and the roughness uniformity of the anode was low.

The standard deviation of the surface roughness values measured on the anode according to Example <NUM> was <NUM>, and the standard deviation of the surface roughness values measured on the anode according to Comparative Example <NUM> was <NUM>.

After performing charging (CC / CV <NUM> C <NUM> V <NUM> C CUT-OFF) and discharging (CC <NUM> C <NUM> V CUT-OFF) on the prepared secondary battery, <NUM> of an anode active material sample was obtained from the anode active material layer.

The obtained sample was added to <NUM> of DI water (<NUM>% trifluoroacetic acid (TFA)) and subjected to ultrasonic extraction for <NUM> minutes, then the extracted solution was mixed for <NUM> hours and filtered through a syringe filter. HPLC analysis was performed on the sample filtered under the following HPLC analysis conditions to measure a content of the lithium salt-containing coating.

<FIG> is an atomic force microscopy (AFM) image illustrating an upper surface of the anode according to Example <NUM>, and <FIG> is an atomic force microscopy image illustrating an upper surface of the anode according to Comparative Example <NUM>.

Referring to <FIG>, the anode according to Example <NUM> exhibited that the arithmetic average roughness was <NUM> or less, and the upper surface of the anode was uniform.

Referring to <FIG>, the anode according to Comparative Example <NUM> exhibited that the lithium salt-containing coating was not formed on both the anode active material and the anode active material layer, and the upper surface of the anode was non-uniform.

Charging (CC/CV <NUM> C <NUM> V <NUM> C CUT-OFF) and discharging (CC <NUM> C <NUM> V CUT-OFF) were performed on the secondary batteries according to the examples and comparative examples to measure initial charge/discharge capacities (CC: Constant Current, CV: Constant Voltage). The initial efficiency was evaluated as a percentage of the value obtained by dividing the initial discharge capacity by the initial charge capacity.

<NUM> cycles of charging/discharging were repeatedly performed on the secondary batteries according to the examples and comparative examples in a way of executing charging (CC/CV method, current rate <NUM> C, upper limit voltage <NUM> V, cut-off current <NUM> C) and discharging (CC, <NUM> C, lower limit voltage <NUM> V cut-off) at <NUM> was set to be one cycle. Thereafter, the capacity retention rate was evaluated as a percentage of the value obtained by dividing the discharge capacity at <NUM> cycles by the discharge capacity at one cycle.

Evaluation results are shown in Table <NUM> below.

Referring to Table <NUM>, the secondary batteries of the examples, which include the lithium salt-containing coatings, exhibited that the surface arithmetic average roughness of the anode was <NUM> or less, and the initial efficiency and capacity retention rate were improved.

On the other hand, the secondary battery of Comparative Example <NUM>, which does not include the lithium salt-containing coating, exhibited that the surface arithmetic average roughness value of the anode was greater than <NUM>, and the initial efficiency and capacity retention rate were decreased.

In addition, the secondary batteries of Comparative Examples <NUM> and <NUM> exhibited that the surface roughness values were high, and the initial efficiency and cycle characteristics were decreased.

Claim 1:
An electrode for a lithium secondary battery comprising:
an electrode current collector;
an electrode active material layer disposed on at least one surface of the electrode current collector and including electrode active material particles; and
a lithium salt-containing coating formed on at least portions of surfaces of the electrode active material particles and/or on at least a portion of a surface of the electrode active material layer,
wherein the electrode has a surface arithmetic average roughness (Ra) represented by Equation <NUM>: <MAT>
wherein, in Equation <NUM>, Ra is a value obtained by calculating an arithmetic average of surface roughness values measured in <NUM> or more measurement regions of the surface of the electrode using an atomic force microscopy in a scan range of <NUM> × <NUM>,
wherein a surface roughness value measured in each of the measurement regions is an arithmetic average value of roughness values excluding a maximum value and a minimum value among the surface roughness values when the surface roughness value is measured <NUM> or more times in a scan range of <NUM> × <NUM> for each of the measurement regions.