Patent Description:
Currently, lithium ion batteries use a layered compound LiMeO<NUM> in which Me is a cation selected to have a +III valency on average and necessarily contains a redox cation; a spinel compound LiMeQO<NUM> in which Q is a cation selected to have a +IV valency on average); an olivine compound LiX<NUM>X<NUM>O<NUM> in which X<NUM> is a cation selected to have a +II valency and necessarily contains a redox cation, and X<NUM> is a cation selected to have a +V valency); or a fluorite compound Li<NUM>MeO<NUM>, as a positive electrode active material. On the other hand, electrolytic solutions or other constituent requirements have been improved year by year in order to utilize its characteristics.

However, for the lithium ion batteries, most of the electrolytes are organic compounds, and even if a flame-retardant compound is used, any risk of causing a fire could not be completely eliminated. As an alternative candidate for such liquid-type lithium ion batteries, all-solid lithium ion batteries having a solid electrolyte have been attracting attention in recent years. Among others, an all-solid lithium ion battery containing a sulfide such as Li<NUM>S-P<NUM>S<NUM> or the sulfide to which a lithium halide is added, as a solid electrolyte, is becoming mainstream.

In the all-solid lithium ion batteries, it is common to cover a surface of a positive electrode active material in order to prevent a high-resistivity layer from being produced at an interface between the positive electrode active material and the sulfide-based solid electrolyte.

For example, Patent Literature <NUM> discloses that a positive electrode active material is covered with a reaction-inhibiting layer of Li<NUM>BO<NUM>-Li<NUM>SiO<NUM> containing carbon.

<CIT> discloses a composite cathode active material for a solid state secondary battery, including a core particle, a first coating layer and a second coating layer, wherein the core particle includes a cathode active material, the first and second coating layers cover a surface of the core particle, the first coating layer includes a first lithium-containing compound which includes Zr, Nb, Ti, Al or a combination thereof, the second coating layer includes a second lithium-containing compound which includes Ge, Ni, Ga, or a combination thereof, and the first lithium-containing compound is different from the second lithium-containing compound.

<CIT> discloses a positive electrode active material for a nonaqueous electrolyte secondary battery, comprising lithium nickel composite oxide particles, each having a lithium compound layer on at least part of its surface and a composition given by the formula Lit1Ni<NUM>-x-yCoxM1yM2zO<NUM> (where M1 and M2 represent at least one element selected from a group consisting of Mg, Al, Ca, Ti, V, Cr, Mn, Zr, Nb, Mo and W, <NUM> < t1 ≤ <NUM>, <NUM> ≤ x ≤ <NUM>, <NUM> ≤ y ≤ <NUM>, <NUM> < z ≤ <NUM>. The lithium compound layer comprises a lithium compound including Li and M2, has a thickness of <NUM>-<NUM> and a concentration gradient such that the concentration of M2 lowers in a direction from the outermost surface of the composite oxide particle toward the centre thereof.

<CIT> discloses a manganese-lithium phosphate composite material for a cathode of a lithium ion battery, comprising a LiMnPO<NUM> core, a LiNbO<NUM> first coating layer and a C second coating layer, wherein the LiNbO<NUM> layer coats the surface of the LiMnPO<NUM> core, and the C layer coats the surface of the LiNbO<NUM> layer.

However, the carbon in the coated layer contains not only a simple substance but also oxides, hydroxides, or the like, so that the resistance is higher and sufficient output characteristics cannot be obtained. Further, the effect of suppressing the reaction with the sulfide-based solid electrolyte is reduced due to the carbon in the coated layer, resulting in insufficient suppression of generation of the high resistivity layer.

An object of the present invention is to provide a positive electrode active material for all-solid lithium ion batteries, which suppresses generation of ahigh-resistivity layer at an interface with a sulfide-based solid electrolyte, has improved electron conductivity, and exhibits good output characteristics when applied to all-solid lithium ion batteries; a positive electrode for all-solid lithium ion batteries using the same; an all-solid lithium ion battery; and a method for producing a positive electrode active material for all-solid lithium ion batteries.

As a result of various studies, the present inventors have found that the above problems can be solved by a positive electrode active material for all-solid lithium ion batteries including a core positive electrode active material having a predetermined composition and a coated portion formed on a surface of the core positive electrode active material, in which the coated portion is formed of lithium niobate and carbon.

In one aspect, the present invention completed on the basis of the above findings relates to a positive electrode active material for all-solid lithium ion batteries, the positive electrode active material comprising: a core positive electrode active material having a composition represented by the following formula: LiaNibCocMdO<NUM> in which M is at least one element selected from Mn, V, Mg, Ti and Al, <NUM> ≤ a ≤ <NUM>, <NUM> ≤ b ≤ <NUM>, and b + c + d = <NUM>; and a coated portion formed on a surface of the core positive electrode active material, wherein the coated portion comprises a layer of lithium niobate and a carbon layer in this order, from the surface of the core positive electrode active material, and wherein a ratio of a content of Nb in the coated portion is from <NUM> to <NUM> mol% relative to the total content of Ni, Co and M in the core positive electrode active material, and a ratio of a content of carbon in the coated portion is from <NUM> to <NUM> mol% relative to the content of Nb in the coated portion.

In an embodiment of the positive electrode active material for all-solid lithium ion batteries according to the present invention, the carbon of the coated portion is elemental carbon.

In another aspect, the present invention relates to a positive electrode for all-solid lithium ion batteries, comprising the positive-electrode active material for all-solid lithium ion batteries according to the first aspect of the present invention.

In yet another aspect, the present invention provides an all-solid lithium ion battery, comprising: the positive electrode for all-solid lithium ion batteries according to the preceding aspect of the present invention; a negative electrode; and a solid electrolyte.

In still another aspect of the present invention, there is provided a method for producing the positive electrode active material for all-solid lithium ion batteries according to the first aspect of the present invention, the method comprising: coating the entire surface of the core positive electrode active material with lithium niobate; and after coating the entire surface with the lithium niobate, further coating it with carbon using a carbon target material by means of barrel sputtering.

According to the present invention, it is possible to provide a positive electrode active material for all-solid lithium ion batteries, which suppresses generation of a high-resistivity layer at an interface with a sulfide-based solid electrolyte, has improved electron conductivity, and exhibits good output characteristics when applied to all-solid lithium ion batteries; a positive electrode for all-solid lithium ion batteries using the same; an all-solid lithium ion battery; and a method for producing a positive-electrode active material for all-solid lithium ion batteries.

A positive electrode active material for all-solid lithium ion batteries according to the present invention includes: a core positive electrode active material having a composition represented by the following formula:.

in which M is at least one element selected from Mn, V, Mg, Ti and Al, <NUM> ≤ a ≤ <NUM>, <NUM> ≤ b ≤ <NUM>, and b + c + d = <NUM>; and a coated portion formed on a surface of the core positive electrode active material, wherein the coated portion comprises a layer of lithium niobate and a carbon layer in this order, from the surface of the core positive electrode active material, wherein a ratio of a content of Nb in the coated portion is from <NUM> to <NUM> mol% relative to the total content of Ni, Co and M in the core positive electrode active material, and wherein a ratio of a content of carbon in the coated portion is from <NUM> to <NUM> mol % relative to the content of Nb in the coated portion.

In the all-solid lithium ion batteries, it is conventionally said that the coating of LiNbO<NUM> on surfaces of positive electrode active material particles improves battery characteristics as compared with a case of no coating. This is because although an energy gap at a positive electrode-electrolyte interface tends to increase when a general oxide positive electrode active material or a sulfide-based solid electrolyte is used as an electrolyte, the total energy gap at the positive electrode-electrolyte interface is decreased by placing LiNbO<NUM> having a crystal lattice relaxation effect therebetween. However, there are problems that although the LiNbO<NUM> coating suppresses the reaction between the sulfide-based electrolyte and the positive electrode material and formation of the high-resistivity layer can be suppressed, the resistance of the entire active material including the coated layer is higher and sufficient output characteristics and cycle characteristics cannot be obtained.

Further, conventionally, there is a technique of coating a positive electrode active material with a reaction-inhibiting layer of carbon-containing Li<NUM>BO<NUM>-Li<NUM>SiO<NUM> to provide a positive electrode having improved electron conductivity. However, since the carbon in the coated layer includes not only a single substance but also oxides and hydroxides, the resistance is higher so that sufficient output characteristics cannot be obtained. Further, the effect of suppressing the reaction with the sulfide-based solid electrolyte is reduced due to the carbon of the coated layer, resulting in insufficient suppression of generation of the high-resistivity layer.

On the other hand, the coated portion of the positive electrode active material for all solid-state lithium ion batteries according to the present invention has a layer of lithium niobate and a layer of carbon in this order from the surface of the core positive electrode active material. In other words, the positive electrode active material for all-solid lithium ion batteries according to the present invention has a structure in which the surface of the core positive electrode active material is covered with lithium niobate as a reaction-inhibiting layer and the surface of the lithium niobate is covered with carbon. According to this structure, the core positive electrode active material is coated with carbon-free lithium niobate, which forms an inner layer of the coated portion, so that the formation of the high-resistivity layer is suppressed. Furthermore, electronic conductivity can be improved by coating an outer layer of the coated portion with carbon which forms an outer layer of the coated portion. Then, the ionic and electronic conductivity at the interface between the positive electrode active material and the solid electrolyte is improved, whereby an all-solid battery having improved output and cycling characteristics can be obtained.

For the positive electrode active material for all solid-state lithium ion batteries according to the invention, a ratio of a content of Nb in the coated portion is preferably from <NUM> to <NUM> mol% relative to the total content of Ni, Co and M in the core positive electrode active material. According to this configuration, when used in an all-solid lithium ion battery, the positive electrode active material can lead to decreased capacity degradation of that battery, resulting in improved output and cycling characteristics. The ratio of the content of Nb in the coated portion may be <NUM> mol% or more, <NUM> mol % or more, or <NUM> mol % or more, and <NUM> mol % or less, <NUM> mol % or less, or <NUM> mol % or less, relative to the total content of Ni, Co and M in the core positive electrode active material.

For the positive electrode active material for all solid-state lithium ion batteries according to the invention, a ratio of a content of carbon in the coated portion is preferably from <NUM> to <NUM> mol% relative to the content of Nb in the coated portion. The ratio of the content of carbon in the coated portion of <NUM> mol% or more relative to the content of Nb in the coated portion can provide further improvement of electronic conductivity at the interface between the positive electrode active material and the solid electrolyte. Further, the ratio of the content of carbon in the coated portion of <NUM> mol% or less relative to the content of Nb in the coated portion can provide further improvement of ionic conductivity at the interface between the positive electrode active material and the solid electrolyte. The ratio of the content of carbon in the coated portion may be <NUM> mol% or more, or <NUM> mol% or more, and <NUM> mol% or less, or <NUM> mol% or less, relative to the content of Nb in the coated portion.

If the coated portion also contain oxides, hydroxides or the like, the resistance will be increased, so that sufficient output characteristics may not be obtained. Therefore, for the positive electrode active material for all solid-state lithium-ion batteries according to an embodiment of the present invention, the carbon of the coated portion is preferably elemental carbon.

For the positive electrode active material for all-solid lithium ion batteries according to the present invention, the coated portion has a layer of lithium niobate and a layer of carbon in this order from the surface of the core positive electrode active material. According to the structure, the carbon in the upper layer of the coated portion can further improve electronic conductivity, thereby further reducing the resistance of the positive electrode active material. Further, since the core positive electrode active material is coated with lithium niobate in the lower layer of the coated portion, the formation of the high-resistivity layer by the reaction with the sulfide-based solid electrolyte is more satisfactorily suppressed. As a result, the ionic and electronic conductivity at the interface between the positive electrode active materials and at the interface between the positive electrode and solid electrolyte can be further improved, thereby providing an all-solid battery having improved output characteristics.

A positive electrode for all-solid lithium ion batteries positive electrode can be produced using the positive electrode active material for all-solid lithium ion batteries according to the present invention, and an all-solid lithium ion battery can be further produced using the positive electrode for all-solid lithium ion batteries, a negative electrode, and a solid electrolyte.

Next, a method for producing the positive electrode active material for all-solid lithium ion batteries according to an embodiment of the method aspect of the present invention will be described in detail.

The method for producing the core positive electrode active material according to the embodiment of the present invention starts from preparation of a Ni/Co/M composite hydroxide having a Ni composition of <NUM> or more in molar ratio, or a precursor of the Ni/Co/M composite hydroxide. Herein, M is at least one element selected from Mn, V, Mg, Ti and Al. Subsequently, a Li source (Li carbonate, Li hydroxide, etc.) is mixed with the composite hydroxide by dry mixing with a Henschel mixer or the like in an adjusted mixing ratio of the respective raw materials, and then fired at a temperature of <NUM> to <NUM> for <NUM> to <NUM> hours to obtain a fired body. If necessary, the fired body is then pulverized using, for example, a pulverizer, to obtain powder of the core positive electrode active material.

In a coating method according to an embodiment of the present invention, the powder of the above core positive electrode active material is fully coated with lithium niobate. The coating method is not particularly limited, but, for example, lithium niobate can be coated on the surface of the core positive electrode active material using a solution containing LiOC<NUM>H<NUM> and Nb(OC<NUM>H<NUM>)<NUM> by means of a certain coating device.

Sputtering is then carried out using a carbon target material by means of barrel sputtering apparatus, thereby further coating with carbon the core positive electrode active material which has been fully coated with lithium niobate. The positive electrode active material for all-solid lithium ion batteries according to the embodiment of the present invention can be thus produced.

While Examples are provided below for better understanding of the invention and its advantages, the present invention is not limited to these Examples.

Commercially available nickel sulfate, cobalt sulfate, and manganese sulfate were mixed as an aqueous solution such that a molar ratio of Ni, Co, and M would reach a predetermined value, and co-precipitated with an alkaline (sodium hydroxide) solution with sufficiently stirring, and then filtred and washed. The reaction method was carried out according to the standard method. Subsequently, the above coprecipitate was mixed with lithium hydroxide monohydrate such that a molar ratio of Li to the total of Ni, Co and M (Li / (Ni + Co + M)) would reach a predetermined value, and fired in a roller hearth kiln (at a calcination temperature of <NUM> for a calcination time of <NUM> hours), and then pulverized using a roll mill and pulverizer to obtain a core positive electrode active material having a particle diameter (D50) of <NUM>.

The surface of the core positive electrode active material Li<NUM>Ni<NUM>CO<NUM>Mn<NUM>O<NUM> was coated with <NUM> mol% of LiNbO<NUM> using a solution containing LiOC<NUM>H<NUM> and Nb(OCC<NUM>H<NUM>)<NUM> by means of a rolling fluidized bed coating apparatus. The powder coated with LiNbO<NUM> was then placed in the barrel sputtering apparatus, and barrel sputtering was carried out using a carbon (C) target material under a condition of an output of <NUM> to 700W to coat elemental carbon so as to have 30mol% relative to Nb. A heat treatment was carried out at <NUM> for one hour to obtain a positive electrode active material in which the core positive electrode active material was coated with lithium niobate and elemental carbon.

A positive electrode active material in which lithium niobate and elemental carbon were coated on the core positive electrode active material was obtained in the same method as that of Example <NUM>, with the exception that elemental carbon was coated so as to have <NUM> mol% relative to Nb.

A positive electrode active material in which lithium niobate and elemental carbon were coated on the core positive electrode active material was obtained in the same method as that of Example <NUM>, with the exception that <NUM> mol% of LiNbO<NUM> was coated.

A positive electrode active material in which lithium niobate and elemental carbon were coated on the core positive electrode active material was obtained in the same method as that of Example <NUM>, with the exception that the composition of the core positive electrode active material was Li<NUM>Ni<NUM>Co<NUM>Mn<NUM>O<NUM>.

A positive electrode active material in which lithium niobate and elemental carbon were coated on the core positive electrode active material was obtained in the same method as that of Example <NUM>, with the exception that the composition of the core positive electrode active material was Li<NUM>Ni<NUM>Co<NUM>V<NUM>O<NUM>.

A positive electrode active material in which lithium niobate and elemental carbon were coated on the core positive electrode active material was obtained in the same method as that of Example <NUM>, with the exception that the composition of the core positive electrode active material was Li<NUM>Ni<NUM>CO<NUM>Mg<NUM>O<NUM>.

A positive electrode active material in which lithium niobate and elemental carbon were coated on the core positive electrode active material was obtained in the same method as that of Example <NUM>, with the exception that the composition of the core positive electrode active material was Li<NUM>Ni<NUM>Co<NUM>Ti<NUM>O<NUM>.

A positive electrode active material in which lithium niobate and elemental carbon were coated on the core positive electrode active material was obtained in the same method as that of Example <NUM>, with the exception that the composition of the core positive electrode active material was Li<NUM>Ni<NUM>Co<NUM>Al<NUM>O<NUM>.

A core positive electrode active material Li<NUM>Ni<NUM>Co<NUM>Mn<NUM>O<NUM> was prepared in the same method as that of Example <NUM>, and a surface of the core positive electrode active material was coated with <NUM> mol% of LiNbO<NUM> using a solution containing LiOC<NUM>H<NUM> and Nb(OC<NUM>H<NUM>)<NUM> by means of a rolling fluidized bed coating apparatus. A heat treatment was then carried out at <NUM> for one hour to obtain a positive electrode active material in which lithium niobate and elemental carbon were coated on the core positive electrode active material.

Each Evaluation was carried out under the following conditions using samples of the respective Examples and Comparative Examples thus produced.

The compositions of the core positive electrode active material and a coated layer <NUM> (a layer of lithium niobate) of the coated portion were evaluated using an inductively coupled plasma (ICP) emission spectroscopy analyzer. A coated layer <NUM> (a layer of elemental carbon) of the coated portion was evaluated by a non-dispersive infrared analysis method using a CHN analyzer.

Each of the positive electrode active materials of Examples and Comparative Examples and LPS (<NUM>. 75Li<NUM>S-<NUM>. 25P<NUM>S<NUM>) were weighed in a ratio of <NUM>:<NUM> and mixed to obtain a positive electrode mixture. A mold having an inner diameter of <NUM> was filled with a Li-In alloy, LPS (<NUM>. 75Li<NUM>S-<NUM>. 25P<NUM>S<NUM>), the positive electrode mixture, and an Al foil in this order, and pressed at <NUM> MPa. The resulting pressed body was restrained at <NUM> MPa using a metallic jig to produce an all-solid lithium ion battery. For the battery, an initial capacity (at <NUM>, a charge upper limit voltage of <NUM> V, a discharge lower limit voltage of <NUM> V) obtained at a charge/discharge rate of <NUM> C was measured, which was defined as a discharge capacity <NUM>. The charging/discharging was then repeated ten times at a charge/discharge rate of <NUM> C (at <NUM>, a charge upper limit voltage of <NUM> V, a discharge lower limit voltage of <NUM> V). The capacity obtained by the first discharge at the charge/discharge rate of <NUM> C was defined as a discharge capacity <NUM>, and a ratio of (discharge capacity <NUM>) / (discharge capacity <NUM>) was determined to be an output characteristic (%) in percentage.

Table <NUM> shows the evaluation conditions and results.

Claim 1:
A positive electrode active material for all-solid lithium ion batteries, the positive electrode active material comprising:
a core positive electrode active material having a composition represented by the following formula:

        LiaNibCocMdO<NUM>

in which M is at least one element selected from Mn, V, Mg, Ti and Al, <NUM> ≤ a ≤ <NUM>, <NUM> ≤ b ≤ <NUM>, and b + c + d = <NUM>; and
a coated portion formed on a surface of the core positive electrode active material,
wherein the coated portion comprises a layer of lithium niobate and a carbon layer in this order, from the surface of the core positive electrode active material,
wherein a ratio of a content of Nb in the coated portion is from <NUM> to <NUM> mol% relative to the total content of Ni, Co and M in the core positive electrode active material, and
wherein a ratio of a content of carbon in the coated portion is from <NUM> to <NUM> mol% relative to the content of Nb in the coated portion.