Positive electrode active material for lithium-ion secondary batteries, method for manufacturing the same, and lithium-ion secondary battery

Abstract

To provide a positive electrode active material which is capable of achieving high output and high durability when being used for a positive electrode of a lithium ion secondary battery.SOLUTION: A positive electrode active material for a lithium ion secondary battery is a positive electrode active material containing a lithium-sodium-nickel-cobalt composite oxide. The lithium-sodium-nickel-cobalt composite oxide has a layered crystal structure, represented by general formula (1): LixNayNi1-a-bCoaMbO2 (in the general formula (1), 0.05≤a≤0.95, 0≤b≤0.60, a+b<1, 0.96≤x+y≤1.20 and 0<y≤0.1 are satisfied, and an element M represents at least one element selected from among Mn, W, Mo, V, Mg, Ca, Al, Ti, Cr, Zr, La and Ta), and contains sodium in the 3a site that is a lithium site.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a positive electrode active material for a lithium ion secondary battery, a method for producing the same, and a lithium ion secondary battery.

Background Art

[0002] In recent years, with the spread of portable electronic devices such as smartphones, tablet terminals, digital cameras, and notebook personal computers, the development of small and lightweight non-aqueous electrolyte secondary batteries having a high energy density has been strongly desired. Also, the development of high-output secondary batteries as batteries for electric vehicles including hybrid vehicles has been strongly desired.

[0003] As a secondary battery that satisfies such requirements, there is a lithium ion secondary battery. A lithium ion secondary battery is composed of a negative electrode, a positive electrode, a non-aqueous electrolyte, etc., and materials into which lithium can be desorbed and inserted are used as the active materials of the negative electrode and the positive electrode.

[0004] Regarding lithium ion secondary batteries, research and development are currently being actively conducted. Among them, lithium ion secondary batteries using a lithium metal composite oxide having a layered or spinel-type crystal structure as a positive electrode active material are being put into practical use as batteries having a high energy density because a high voltage of 4V class can be obtained.

[0005] Examples of the positive electrode active materials proposed so far include lithium cobalt composite oxide (LiCoO2) that is relatively easy to synthesize, lithium nickel composite oxide (LiNiO2) using nickel that is cheaper than cobalt, and lithium nickel cobalt manganese composite oxide (LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2), etc.

[0006] For example, in the development for automotive applications, a secondary battery having higher durability and higher output than the current state is required. Also, in the positive electrode active material, a lithium metal composite oxide having a higher discharge capacity retention rate in a cycle test and a low positive electrode interface resistance is demanded.

[0007] For example, Patent Document 1 proposes a lithium ion secondary battery including a lithium transition metal oxide coated with a lithium ion conductive member as a positive electrode active material. Further, in the examples of Patent Document 1, using a nanoparticle composite device, lithium nickel 0.5 Mn 1.5 O4) coated with Li 1.3 Al 0.3 Ti 1.7 (PO4)3 or Li3PO4 is disclosed as the positive electrode active material. According to Patent Document 1, in this lithium ion secondary battery, the life characteristics (cycle characteristics, durability) are said to be improved.

[0008] Also, Patent Document 2 proposes a positive electrode active material in which a compound containing lithium and tungsten is formed on the surface of primary particles of a lithium metal composite oxide. Further, Patent Document 2 proposes a positive electrode active material powder in which a tungsten compound is added and mixed when washing a lithium metal composite oxide powder represented by Li 1.03 Ni 0.82 Co 1.5 Al 0.03 O2 with water. According to Patent Document 2, in the secondary battery, the positive electrode resistance is reduced and higher output is obtained.

[0009] Furthermore, Patent Document 3 proposes a positive electrode for a non-aqueous electrolyte secondary battery, having a positive electrode on the surface of a positive electrode active material made of a lithium metal composite oxide, on which a coating layer of a compound that allows lithium ions to diffuse in multiple directions is formed, and the coating layer contains an amorphous form of the compound that allows lithium ions to diffuse in multiple directions, and it is stated that the coating layer is formed from lithium tungstate. According to Patent Document 3, for example, by depositing a Li2WO4 film on LiCoO2 using pulsed laser deposition (PLD), lithium diffusion at the positive electrode / electrolyte interface is improved, interfacial resistance is reduced, and the amorphous state allows the lithium diffusion path to work effectively, promoting the resistance reduction effect and improving the output characteristics.

[0010] Furthermore, Patent Document 4 contains the general formula: Li x (Ni 1-y Co y ) 1-z M z A positive electrode active material for non-aqueous electrolyte secondary batteries has been proposed, represented as O2 (0.98≦x≦1.10, 0.05≦y≦0.4, 0.01≦z≦0.2, M=one or more of Al, Zn, Ti, and Mg), having a Li site occupancy rate of 98.5% or more at Li sites in the crystal as determined by Rietveld analysis, and a metal site occupancy rate of 95% or more and 98% or less at metal sites. According to Patent Document 4, this positive electrode active material is said to be able to achieve both high capacity and high power output simultaneously. [Prior art documents] [Patent Documents]

[0011] [Patent Document 1] Japanese Patent Publication No. 2016-51566 [Patent Document 2] Japanese Patent Publication No. 2016-167439 [Patent Document 3] Japanese Patent Publication No. 2017-63015 [Patent Document 4] Japanese Patent Publication No. 2008-218122 [Overview of the Initiative] [Problems that the invention aims to solve]

[0012] However, the improvement of output characteristics in the positive electrode active material described in Patent Document 1 has not been investigated. Furthermore, Patent Document 2 has not investigated a method for mixing the positive electrode active material powder and the tungsten compound with a smaller amount of water.

[0013] Furthermore, lithium-ion secondary batteries require further improvements in durability and output characteristics, and positive electrode active materials also require higher discharge capacity retention rates and further reductions in positive electrode interface resistance.

[0014] In view of the aforementioned problems, the present invention aims to provide a positive electrode active material that has a high discharge capacity retention rate when used in a secondary battery, or a positive electrode active material that has a high discharge capacity retention rate and low positive electrode interface resistance. Furthermore, the present invention aims to provide a method for easily manufacturing such a positive electrode active material in industrial-scale production.

[0015] Furthermore, none of the above patent documents describe or suggest anything about improving the discharge capacity retention rate or reducing the positive electrode interface resistance when other metal elements are present in the lithium site of a lithium metal composite oxide. In addition, Patent Document 4 describes that if metal atoms remain as defects in the Li site, these remaining metal atoms hinder the diffusion of Li within the Li layer, creating resistance, which leads to a decrease in output when used as a battery.

[0016] Furthermore, the aforementioned patent document makes no mention whatsoever of the effects of coating lithium metal composite oxides containing other metal elements at the lithium sites with an ion-conducting oxide. [Means for solving the problem]

[0017] In a first aspect of the present invention, the positive electrode active material contains a lithium sodium nickel cobalt composite oxide, wherein the lithium sodium nickel cobalt composite oxide has a layered crystalline structure and has the general formula (1):Lix Na y Ni 1-a-b Co a M b O2 (In the general formula (1), 0.05 ≦ a ≦ 0.95, 0 ≦ b ≦ 0.60, a + b < 1, 0.96 ≦ x + y ≦ 1.20, 0 < y ≦ 0.1, and the element M is at least one element selected from Mn, W, Mo, V, Mg, Ca, Al, Ti, Cr, Zr, La, and Ta), and a positive electrode active material for a lithium-ion secondary battery containing sodium at the 3a site which is a lithium site is provided.

[0018] Also, it is preferable that the site occupancy of sodium at the 3a site obtained from Rietveld analysis by X-ray diffraction exceeds 0% and is 10% or less. Further, it contains a lithium sodium nickel cobalt composite oxide and a lithium ion conductive oxide, and the lithium ion conductive oxide covers at least a part of the surface of the lithium sodium nickel cobalt composite oxide and may be composed of a compound containing tungsten. Also, the lithium ion conductive oxide is preferably composed of a compound containing tungsten and lithium. Also, the lithium ion conductive oxide is preferably lithium tungstate containing at least one selected from the group consisting of Li2WO4, Li6W2O9, Li4WO5, and 7Li2WO4·4H2O. Also, it is preferable that the amount of tungsten contained in the lithium ion conductive oxide is 0.1 atomic % or more and 1.0 atomic % or less with respect to the total number of atoms of nickel, cobalt, and the element M contained in the positive electrode active material.

[0019] A second aspect of the present invention relates to a method for producing a positive electrode active material containing a lithium sodium nickel cobalt composite oxide, comprising the steps of: mixing a nickel cobalt composite oxide, a lithium compound, and a sodium compound to obtain a lithium mixture; and calcining the lithium mixture to obtain a lithium sodium nickel cobalt composite oxide, wherein the nickel cobalt composite oxide contains nickel, cobalt, and optionally element M, and the atomic ratio of each metal element is Ni:Co:M=(1-ab):a:b (0.05≦a≦0.95, 0≦b≦0.60, a+b<1), and element M is Mn, W, A method for producing a positive electrode active material for a lithium-ion secondary battery is provided, wherein the lithium sodium nickel cobalt composite oxide is represented by at least one element selected from Mo, V, Mg, Ca, Al, Ti, Cr, Zr, La, and Ta, the total amount of lithium and sodium in the lithium mixture is 96 atomic% to 120 atomic% relative to the total amount of nickel, cobalt, and element M, and the amount of sodium is greater than 0 atomic% and less than or equal to 10 atomic% relative to the total amount of nickel, cobalt, and element M, and the lithium sodium nickel cobalt composite oxide has a layered crystalline structure and contains sodium at the 3a site which is a lithium site.

[0020] Furthermore, the firing is preferably carried out at a temperature of 700°C to 850°C for 5 to 15 hours. The process also preferably includes a step of mixing a lithium sodium nickel cobalt composite oxide, a tungsten-containing compound, and an aqueous solution, and then heat-treating the mixture, thereby coating the surface of the lithium sodium nickel cobalt composite oxide with a conductive oxide made from the tungsten-containing compound. The aqueous solution is preferably mixed with the lithium sodium nickel cobalt composite oxide in an amount of 2% to 10% by mass. The heat treatment is preferably carried out at a temperature of 50°C to 200°C for 1 hour or more. The amount of tungsten contained in the positive electrode active material is preferably 0.1% to 1.0% of the total number of atoms of nickel, cobalt, and element M contained in the positive electrode active material. The tungsten-containing compound is preferably lithium tungstate containing at least one selected from the group consisting of Li2WO4, Li6W2O9, Li4WO5, and 7Li2WO4·4H2O.

[0021] A third aspect of the present invention provides a lithium-ion secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the positive electrode contains the positive electrode active material for lithium-ion secondary batteries described above. [Effects of the Invention]

[0022] The positive electrode active material of the present invention contains a lithium sodium nickel cobalt composite oxide. This allows for a high discharge capacity retention rate when used in a secondary battery. In another embodiment of the positive electrode active material of the present invention, at least a portion of the surface of the lithium sodium nickel cobalt composite oxide is coated with a lithium ion conductive oxide. This allows for a high discharge capacity retention rate and low positive electrode interface resistance when used in a secondary battery. Furthermore, the manufacturing method of the positive electrode active material of the present invention allows for the easy production of these positive electrode active materials on an industrial scale. [Brief explanation of the drawing]

[0023] [Figure 1]Figure 1(A) shows an example of a positive electrode active material according to this embodiment, and Figure 1(B) shows another example of a positive electrode active material according to this embodiment. [Figure 2] Figure 2 shows an example of a method for producing the positive electrode active material according to this embodiment. [Figure 3] Figure 3 shows an example of the tungsten coating process according to this embodiment. [Figure 4] Figure 4 shows an example of the tungsten coating process according to this embodiment. [Figure 5] Figure 5 is a schematic cross-sectional view of the coin cell used to evaluate its characteristics. [Figure 6] Figure 6 shows an example of a Nyquist plot (top) and equivalent circuit (bottom) used for impedance evaluation. [Modes for carrying out the invention]

[0024] In the following, this embodiment will first describe the positive electrode active material for lithium-ion secondary batteries, followed by a description of its manufacturing method and a lithium-ion secondary battery using the positive electrode active material.

[0025] 1.Cathode active material Figures 1(A) and 1(B) show an example of a positive electrode active material for a lithium-ion secondary battery according to this embodiment (hereinafter also referred to as "positive electrode active material"). As shown in Figures 1(A) and 1(B), the positive electrode active materials 1A and 1B contain a lithium sodium nickel cobalt composite oxide 10 having a layered crystalline structure.

[0026] The positive electrode active materials 1A and 1B, by containing lithium sodium nickel cobalt composite oxide 10, exhibit a high discharge capacity retention rate and improved durability when used in secondary batteries. Furthermore, by adjusting the sodium content of the lithium sites, the positive electrode active materials 1A and 1B can reduce the positive electrode interface resistance in secondary batteries, thereby achieving high output characteristics.

[0027] As shown in Fig. 1(A), the positive electrode active material 1A may be composed of a lithium sodium nickel cobalt composite oxide 10. Further, as shown in Fig. 1(B), the positive electrode active material 1B may contain a lithium sodium nickel cobalt composite oxide 10 and a lithium ion conductive oxide 20. The lithium ion conductive oxide 20 is composed of a compound containing tungsten and covers at least a part of the surface of the lithium sodium nickel cobalt composite oxide 10.

[0028] By covering at least a part of the surface of the lithium sodium nickel cobalt composite oxide 10 with the lithium ion conductive oxide 20, the positive electrode active material 1B can not only reduce the positive electrode interface resistance and improve the output characteristics, but also suppress the decrease in the capacity retention rate accompanying the charge-discharge cycle. Hereinafter, each component constituting the positive electrode active materials 1A and 1B will be described.

[0029] (1) Lithium sodium nickel cobalt composite oxide The positive electrode active materials 1A and 1B contain a lithium sodium nickel cobalt composite oxide 10 having a layered (layered rock salt type) crystal structure. The lithium sodium nickel cobalt composite oxide 10 has the general formula (1): Li x Na y Ni 1-a-b Co a M b O2 (in the general formula (1), 0.05 ≦ a ≦ 0.95, 0 ≦ b ≦ 0.60, a + b < 1, 0.96 ≦ x + y ≦ 1.20, 0 < y ≦ 0.1, and the element M is at least one element selected from Mn, W, Mo, V, Mg, Ca, Al, Ti, Cr, Zr, La, and Ta). Further, the lithium sodium nickel cobalt composite oxide 10 contains sodium in the lithium site.

[0030] Generally, dissolving a heterogeneous element with an ionic radius close to lithium or a transition metal element in the cathode active material tends to cause cation mixing and easily lead to deterioration of battery characteristics. On the other hand, in the cathode active materials 1A and 1B according to this embodiment, by including the lithium-sodium-nickel-cobalt composite oxide 10, when used in a secondary battery, the discharge capacity retention rate can be improved and high durability can be achieved. Although the details of this mechanism are unclear, it is considered that sodium, which has a larger ionic radius than lithium or nickel, is contained in the lithium (3a) site of the layered oxide, which not only expands the lithium diffusion path but also suppresses cation mixing associated with the mixing of transition metals.

[0031] Also, in the lithium-sodium-nickel-cobalt composite oxide 10, as will be described later, by adjusting the site occupancy of sodium in the lithium site, the cathode interface resistance in the secondary battery can be further reduced and high output characteristics can be achieved.

[0032] Hereinafter, the composition of the lithium-sodium-nickel-cobalt composite oxide 10 will be described. The lithium-sodium-nickel-cobalt composite oxide 10 is represented by the general formula (1): Li x Na y Ni 1-a-b Co a M b O2 (in the general formula (1), 0.05 ≤ a ≤ 0.95, 0 ≤ b ≤ 0.60, a + b < 1, 0.96 ≤ x + y ≤ 1.20, 0 < y ≤ 0.1, and the element M is at least one element selected from Mn, W, Mo, V, Mg, Ca, Al, Ti, Cr, Zr, La, and Ta).

[0033] [(Li + Na) / T M )] In the above general formula (1), the sum (x + y) of x indicating the amount of lithium and y indicating the amount of sodium is the atomic ratio [(Li + Na) / T M of lithium and sodium (Li + Na) to the sum (T M) is shown. Also, [(Li + Na) / T M ) is 0.96 or more and 1.20 or less.

[0034] (Li + Na) / T M When (Li + Na) / T is less than 0.96, the reaction resistance (interface resistance) of the positive electrode in the lithium-ion secondary battery using the positive electrode active materials 1A and 1B increases, resulting in a decrease in the output of the secondary battery. Also, when (Li + Na) / T M exceeds 1.20, the discharge capacity of the positive electrode active material decreases, and the reaction resistance of the positive electrode also increases. Also, from the viewpoint of obtaining a larger discharge capacity, (Li + Na) / T M is preferably 1.10 or less. Also, in the general formula (1) above, x indicating the amount of lithium may exceed 1.

[0035] [Sodium] In the general formula (1) above, y indicating the amount of sodium (Na) is 0 < y ≤ 0.1, preferably 0.001 ≤ y ≤ 0.1, and more preferably 0.002 ≤ y ≤ 0.05.

[0036] Also, sodium is contained in the lithium site (3a site). The site occupancy of sodium in the lithium site is preferably more than 0% and 10% or less, more preferably 0.1% or more and 10% or less, and even more preferably 0.1% or more and 5% or less. When the amount of sodium is within the above range, the discharge capacity maintenance rate can be improved, and the positive electrode interface resistance can be reduced.

[0037] [Nickel] In the general formula (1) above, (1 - a - b) indicating the amount of nickel (Ni) is 0 < (1 - a - b) ≤ 0.95. The amount of nickel can be appropriately adjusted according to the required battery characteristics. (1 - a - b) may be, for example, 0.5 or more, or 0.6 or more, from the viewpoint of high battery capacity.

[0038] [Cobalt] In the general formula (1) above, a, which represents the amount of cobalt (Co), is 0.05 ≤ a ≤ 0.95. When a is 0.05 or greater, the cycle characteristics and thermal stability are excellent. Furthermore, from the viewpoint of improving battery capacity, x may be 0.5 or less.

[0039] [Element M] Element M is at least one element selected from Mn, W, Mo, V, Mg, Ca, Al, Ti, Cr, Zr, La, and Ta. In the above general formula (1), b, which represents the amount of element M, is 0 ≤ b ≤ 0.60. When element M is included (when b is greater than 0), battery characteristics such as output characteristics and thermal stability can be improved. On the other hand, when b is greater than 0.60, the amount of Ni that contributes to the Redox reaction decreases, so the battery capacity decreases. For example, element M may also include Al. In the above general formula (1), when the amount of Al contained in element M is b1, it is preferably 0.01 ≤ b1 ≤ 0.1.

[0040] Furthermore, the lithium sodium nickel cobalt composite oxide may contain small amounts of elements other than the aforementioned metal elements (Li, Na, Ni, Co, element M) and oxygen, as long as they do not hinder the effects of the present invention. It is also preferable that the lithium sodium nickel cobalt composite oxide contains secondary particles formed by the aggregation of multiple primary particles. Including such secondary particles increases the contact area with the electrolyte in the secondary battery, which is advantageous for improving output characteristics. In addition to secondary particles, the lithium sodium nickel cobalt composite oxide may also contain small amounts of individual primary particles. Furthermore, the positive electrode active materials 1A and 1B may contain other compounds besides the lithium sodium nickel cobalt composite oxide and lithium ion conductive oxide described above.

[0041] (2) Lithium-ion conductive oxides The lithium-ion conductive oxide 20 coats at least a portion of the surface of the lithium sodium nickel cobalt composite oxide 10.

[0042] Normally, if the surface of the positive electrode active material is completely covered with a different compound, the movement of lithium ions (intercalation) is greatly restricted, resulting in the loss of the positive electrode active material's advantage of high capacity. On the other hand, compounds with high lithium ion conductivity have the effect of promoting lithium ion movement. Therefore, by covering the surface of the positive electrode active material with a material that has high lithium ion conductivity, it is possible to promote intercalation on the surface of the positive electrode active material.

[0043] The lithium-ion conductive oxide 20 consists of a compound containing tungsten and lithium, and is more preferably lithium tungstate. Because lithium tungstate has the properties of a lithium-ion conductor, lithium insertion and deinsertion at the interface of the positive electrode active material 1B is promoted, and the output characteristics of the secondary battery are greatly improved.

[0044] Furthermore, it is preferable that the lithium tungstate includes at least one selected from the group consisting of Li2WO4, Li6W2O9, Li4WO5, and 7Li2WO4·4H2O.

[0045] Furthermore, the amount of tungsten contained in the lithium-ion conductive oxide 20 is preferably 0.1 atomic% to 1.0 atomic%, and more preferably 0.1 atomic% to 0.5 atomic%, relative to the total number of atoms of nickel, cobalt, and element M contained in the positive electrode active material 1B. When the amount of tungsten is within the above range, both high output characteristics and durability can be achieved.

[0046] On the other hand, if the tungsten content is less than 0.1 atomic%, the improvement in output characteristics may not be sufficient, and if the tungsten content exceeds 1.0 atomic%, the amount of lithium tungstate becomes too high, which can inhibit Li conduction between the positive electrode active material 1B and the electrolyte, potentially degrading battery performance.

[0047] Furthermore, it is preferable that the lithium-ion conductive oxide 20 uniformly coats the surface of the lithium sodium nickel cobalt composite oxide 10. If the surface of the lithium sodium nickel cobalt composite oxide 10 is unevenly coated with the lithium-ion conductive oxide 20, the movement of lithium ions between the particles of the lithium metal composite oxide will be uneven, which may cause stress on specific positive electrode active material particles, leading to deterioration of cycle characteristics and an increase in reaction resistance. By using the positive electrode active material manufacturing method described later, the surface of the lithium sodium nickel cobalt composite oxide 10 can be easily and uniformly coated with the lithium-ion conductive oxide 20.

[0048] [Specific surface area] The positive electrode active materials 1A and 1B have a specific surface area of ​​0.3 m². 2 / g or more 2m 2 It is preferable that the specific surface area is less than or equal to / g. By setting the specific surface area within the above range, contact with the electrolyte can be increased, resulting in better output characteristics and battery capacity, as well as ensuring thermal stability. On the other hand, if the specific surface area is 0.3m² 2 If the specific surface area is less than 2 m², sufficient contact with the electrolyte may not be achieved, which may reduce output characteristics and battery capacity. 2 If the concentration exceeds [amount] / g, the decomposition of the electrolyte may be accelerated, leading to a decrease in thermal stability.

[0049] 2. Method for manufacturing positive electrode active material As shown in Figure 2, the method for producing the positive electrode active material according to this embodiment comprises a mixing step (S10) of mixing nickel-cobalt composite oxide, a lithium compound, and a sodium compound to obtain a lithium mixture, and a calcination step (S20) of calcining the lithium mixture to obtain lithium sodium nickel-cobalt composite oxide 10.

[0050] These steps (S10, S20) allow for the easy acquisition of a positive electrode active material 1A containing lithium sodium nickel cobalt composite oxide 10.

[0051] Furthermore, as shown in Figure 3, the method for producing the positive electrode active material according to this embodiment may further include a tungsten coating step (S30), in which the lithium sodium nickel cobalt composite oxide 10, a compound containing tungsten, and an aqueous solution are mixed and then heat-treated.

[0052] This process (S30) makes it easy to obtain the above-mentioned positive electrode active material 1B, in which the surface of the lithium sodium nickel cobalt composite oxide 10 is coated with a lithium ion conductive oxide 20. Each step will be described below.

[0053] [Mixing process (S10)] The mixing step (S10) is a step in which nickel-cobalt composite oxide, lithium compound, and sodium compound are mixed to obtain a lithium mixture.

[0054] (Nickel-cobalt composite oxide) Nickel-cobalt composite oxide contains nickel, cobalt, and optionally element M, with the atomic ratio of each metallic element being represented as Ni:Co:M=(1-ab):a:b (0.05≦a≦0.95, 0≦b≦0.60, a+b<1, element M is at least one element selected from Mn, W, Mo, V, Mg, Ca, Al, Ti, Cr, Zr, La, and Ta).

[0055] Nickel-cobalt composite oxides can be obtained by known manufacturing methods, for example, by heat-treating nickel-cobalt composite hydroxides obtained by crystallization. The heat treatment temperature in this case is, for example, between 105°C and 700°C.

[0056] (Lithium compounds) The lithium compound is not particularly limited, and any known lithium-containing compound can be used, such as lithium carbonate, lithium hydroxide, and lithium nitrate. Among these, lithium carbonate and lithium hydroxide are preferred from the viewpoint of having less influence from residual impurities and dissolving at the calcination temperature.

[0057] (Sodium compounds) As sodium compounds, compounds containing sodium can be used, such as sodium carbonate, sodium bicarbonate, sodium percarbonate, sodium fluoride, sodium oxide, sodium citrate, sodium acetate, sodium chloride, sodium hydroxide, and sodium sulfate.

[0058] The total amount of lithium and sodium in the lithium mixture is 96 atomic% to 120 atomic%, preferably 100 atomic% to 115 atomic%, relative to the total amount of nickel, cobalt, and element M.

[0059] Furthermore, the total amount of sodium in the lithium mixture is greater than 0 atomic% and less than or equal to 10 atomic% relative to the total amount of nickel, cobalt, and element M, preferably greater than 0.1 atomic% and less than or equal to 10 atomic%, and more preferably between 0.1 atomic% and 5 atomic%.

[0060] The method of mixing the nickel-cobalt composite oxide, lithium compound, and sodium compound is not particularly limited; it is sufficient that these compounds are thoroughly mixed without destroying the nickel-cobalt composite oxide and other components. For example, the mixing can be done using a general-purpose mixer, such as a shaker mixer, Redigge mixer, Julia mixer, or V-blender.

[0061] [Firing process (S20)] The firing process (S20) is a process of firing the above lithium mixture. When the lithium mixture is fired, lithium from the lithium compound and sodium from the sodium compound diffuse into the nickel-cobalt composite oxide, and a lithium-sodium-nickel-cobalt composite oxide having a layered structure can be obtained. The firing conditions can be adjusted as appropriate depending on the composition of the lithium-sodium-nickel-cobalt composite oxide. An example of firing conditions is described below.

[0062] The firing temperature is preferably between 700°C and 900°C. When firing at the above temperature, melting of the lithium compound and sodium compound occurs, promoting the penetration and diffusion of lithium and sodium into the nickel-cobalt composite oxide.

[0063] The firing time is preferably 5 hours or more and 16 hours or less. If the firing time is insufficient, the formation of lithium sodium nickel cobalt composite oxide may not occur sufficiently.

[0064] [Tungsten coating process (S30)] The tungsten coating process (S30) comprises a step of mixing lithium sodium nickel cobalt composite oxide 10, a compound containing tungsten, and an aqueous solution (S31), and a step of heat treatment (S32).

[0065] The tungsten-containing compound reacts with the lithium compound present on the surface of the lithium sodium nickel cobalt composite oxide 10 through a mixing step (S31) and a heat treatment step (S32) to form a lithium ion conductive oxide 20 consisting of a compound containing tungsten and lithium.

[0066] [Mixing process (S31)] In the mixing step (S31), as shown in Figure 4, it is preferable to mix the lithium sodium nickel cobalt composite oxide 10 and the tungsten-containing compound while stirring (mixing) them, and then further mix them by spraying an aqueous solution.

[0067] When only the lithium sodium nickel cobalt composite oxide 10 and the tungsten-containing compound are stirred, the reaction between the lithium compound present on the surface of the lithium sodium nickel cobalt composite oxide 10 and the tungsten-containing compound does not proceed easily. Therefore, stirring is performed while spraying an aqueous solution. This allows the lithium ion conductive oxide 20 to be more uniformly coated on the surface of the lithium sodium nickel cobalt composite oxide 10.

[0068] (Mixture containing tungsten) The tungsten-containing compound can be any compound that can react with the lithium compound present on the surface of the lithium sodium nickel cobalt composite oxide 10 to form tungsten and lithium. Examples of tungsten-containing compounds include tungsten oxide, tungstic acid, ammonium paratungstate, and sodium tungstate, among which tungsten oxide (WO3) or tungstic acid (WO3·H2O) is preferred, and tungsten oxide is more preferred.

[0069] The tungsten-containing compound can be mixed in such an amount that the amount of tungsten in the resulting positive electrode active material is 0.1 atomic% to 1.0 atomic%, preferably 0.1 atomic% to 0.5 atomic%, relative to the total number of atoms of nickel, cobalt, and element M contained in the positive electrode active material.

[0070] (water, aqueous solution) The water or aqueous solution can be any aqueous solution capable of dissolving the tungsten-containing compound and reacting it with the lithium compound present on the surface of the lithium sodium nickel cobalt composite oxide 10. Examples of aqueous solutions include water, aqueous ammonia, and aqueous sodium hydroxide solution.

[0071] The amount of aqueous solution mixed (amount sprayed) is preferably 2% by mass or more and 10% by mass or less relative to 10% by mass of lithium sodium nickel cobalt composite oxide.

[0072] (Lithium compounds) Furthermore, in the mixing step (S32), a lithium compound may be added as a lithium source for forming the lithium-ion conductive oxide 20. By adding a lithium compound, a lithium-ion conductive oxide 20 consisting of Li4WO5 with high lithium-ion conductivity can be easily formed. Examples of lithium compounds that can be used include lithium carbonate and lithium hydroxide.

[0073] For stirring (mixing), a general-purpose stirrer can be used, such as a Henschel mixer or a drum mixer. Mixing and stirring should be done to the extent that the tungsten-containing compound is thoroughly stirred (mixed) without destroying the structure of the lithium sodium nickel cobalt composite oxide 10.

[0074] [Heat treatment process (S32)] The heat treatment step (S32) is a step in which the mixture obtained in the mixing step (S31) is heat-treated. The heat treatment is preferably carried out at a temperature of 50°C to 200°C for 1 hour or more, and more preferably at a temperature of 100°C to 200°C for 1 hour or more.

[0075] By performing the heat treatment step (S32), the tungsten-containing compound and the lithium compound present on the surface of the lithium sodium nickel cobalt composite oxide 10 react sufficiently to form a compound containing tungsten and lithium.

[0076] The tungsten and lithium-containing compound formed is preferably lithium tungstate, and more preferably contains at least one selected from the group consisting of Li2WO4, Li6W2O9, Li4WO5, and 7Li2WO4·4H2O. When the surface of the lithium sodium nickel cobalt composite oxide 10 is coated with these lithium tungstates, the positive electrode interface resistance in the secondary battery can be reduced and the output characteristics can be improved.

[0077] Furthermore, the effects obtained by coating with a compound containing tungsten and lithium can be applied not only to the lithium cobalt aluminum composite oxide according to this embodiment, but also to commonly used positive electrode active materials for lithium secondary batteries.

[0078] 3. Lithium-ion rechargeable batteries The lithium-ion secondary battery according to this embodiment (hereinafter also referred to as "secondary battery") comprises a positive electrode containing the positive electrode active material described above, a negative electrode, and a non-aqueous electrolyte. The secondary battery comprises, for example, a positive electrode, a negative electrode, and a non-aqueous electrolyte. Alternatively, the secondary battery may comprise, for example, a positive electrode, a negative electrode, and a solid electrolyte. Furthermore, the secondary battery may be any secondary battery that performs charging and discharging by desorption and insertion of lithium ions, and may be, for example, a non-aqueous electrolyte secondary battery or an all-solid-state lithium secondary battery. Note that the embodiments described below are merely illustrative, and the secondary battery according to this embodiment may be applied to various modified and improved forms based on the embodiments described herein.

[0079] [Components] (positive electrode) First, the positive electrode active material, conductive material, and binder are mixed, and activated carbon and solvents for viscosity adjustment or other purposes are added as needed. This mixture is then kneaded to produce a positive electrode composite paste. At this time, the mixing ratio of each component in the positive electrode composite paste can be adjusted as appropriate according to the desired performance of the secondary battery. For example, if the solid content of the positive electrode composite excluding the solvent is 100 parts by mass, the content of the positive electrode active material may be 60 parts by mass or more and 95 parts by mass or less, the content of the conductive material may be 1 part by mass or more and 20 parts by mass or less, and the content of the binder may be 1 part by mass or more and 20 parts by mass or less.

[0080] As conductive materials, for example, graphite (natural graphite, artificial graphite, and expanded graphite, etc.) or carbon black-based materials such as acetylene black and Ketjenblack can be used.

[0081] The binder serves to hold the active material particles together, and examples of binders that can be used include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluororubber, ethylene propylene diene rubber, styrene butadiene, cellulose resins, and polyacrylic acid.

[0082] If necessary, a solvent that disperses the positive electrode active material, conductive material, and activated carbon, and dissolves the binder, may be added to the positive electrode mixture. Specifically, organic solvents such as N-methyl-2-pyrrolidone can be used as the solvent. In addition, activated carbon may be added to the positive electrode mixture to increase the electrical double layer capacity.

[0083] The resulting positive electrode composite paste is applied to the surface of a current collector, for example, made of aluminum foil, and dried to allow the solvent to evaporate. If necessary, pressure may be applied using a roll press or the like to increase the electrode density. In this way, a sheet-like positive electrode can be produced. The sheet-like positive electrode can be cut to an appropriate size depending on the intended battery and used to manufacture the battery. Note that the method for producing the positive electrode is not limited to the method described above and other methods may be used.

[0084] (Negative electrode) As the negative electrode, metallic lithium or lithium alloy may be used. Alternatively, as the negative electrode, a negative electrode composite material may be used, which is prepared by mixing a binder with a negative electrode active material capable of intercalating and deintercalating lithium ions, adding a suitable solvent to form a paste, applying this paste to the surface of a metal foil current collector such as copper, drying it, and compressing it as needed to increase the electrode density.

[0085] As the negative electrode active material, for example, natural graphite, artificial graphite, and calcined organic compounds such as phenolic resin, and powdered carbon materials such as coke can be used. In this case, as with the positive electrode, a fluororesin such as PVDF can be used as the negative electrode binder, and an organic solvent such as N-methyl-2-pyrrolidone can be used as the solvent for dispersing these active materials and binders.

[0086] (Separator) A separator is placed between the positive and negative electrodes as needed. The separator separates the positive and negative electrodes and holds the electrolyte. A thin membrane made of polyethylene or polypropylene with numerous tiny pores can be used.

[0087] (Non-aqueous electrolyte) As a non-aqueous electrolyte, a non-aqueous electrolyte solution can be used. For example, a non-aqueous electrolyte solution may be one in which a lithium salt is dissolved in an organic solvent as a supporting salt. Alternatively, a non-aqueous electrolyte solution may be one in which a lithium salt is dissolved in an ionic liquid. An ionic liquid is a salt composed of cations and anions other than lithium ions, and which remains liquid at room temperature.

[0088] As the organic solvent, one of the following may be used alone or in combination: cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and trifluoropropylene carbonate; linear carbonates such as diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, and dipropyl carbonate; ether compounds such as tetrahydrofuran, 2-methyltetrahydrofuran, and dimethoxyethane; sulfur compounds such as ethyl methyl sulfone and butanesultone; and phosphorus compounds such as triethyl phosphate and trioctyl phosphate.

[0089] As supporting salts, LiPF6, LiBF4, LiClO4, LiAsF6, LiN(CF3SO2)2, and their composite salts can be used. Furthermore, the non-aqueous electrolyte may contain radical scavengers, surfactants, and flame retardants.

[0090] Furthermore, solid electrolytes may be used as non-aqueous electrolytes. Solid electrolytes have the property of being able to withstand high voltages. Examples of solid electrolytes include inorganic solid electrolytes and organic solid electrolytes.

[0091] As inorganic solid electrolytes, oxide-based solid electrolytes, sulfide-based solid electrolytes, and the like can be used.

[0092] The oxide-based solid electrolyte is not particularly limited and can be used as long as it contains oxygen (O) and has lithium ion conductivity and electronic insulating properties. Examples of oxide-based solid electrolytes include lithium phosphate (Li3PO4) and Li3PO4N. X LiBO2N X , LiNbO3, LiTaO3, Li2SiO3, Li4SiO4-Li3PO4, Li4SiO4-Li3VO4, Li2O-B2O3-P2O5, Li2O-SiO2, Li2O-B2O3-ZnO, Li 1+X Al X Ti 2-X (PO4)3(0≦X≦1), Li 1+X Al X Ge 2-X (PO4)3(0≦X≦1), LiTi2(PO4)3, Li 3X La 2 / 3-X TiO3(0≦X≦2 / 3), Li5La3Ta2O 12 Li7La3Zr2O 12 Li6BaLa2Ta2O 12 Li 3.6 Si 0.6 P 0.4 Examples include O4.

[0093] The sulfide-based solid electrolyte is not particularly limited and can be used as long as it contains sulfur (S) and has lithium-ion conductivity and electronic insulation properties. Examples of sulfide-based solid electrolytes include Li2S-P2S5, Li2S-SiS2, LiI-Li2S-SiS2, LiI-Li2S-P2S5, LiI-Li2S-B2S3, Li3PO4-Li2S-Si2S, Li3PO4-Li2S-SiS2, LiPO4-Li2S-SiS, LiI-Li2S-P2O5, and LiI-Li3PO4-P2S5.

[0094] Furthermore, other inorganic solid electrolytes may be used besides those mentioned above; for example, Li3N, LiI, Li3N-LiI-LiOH, etc., may be used.

[0095] The organic solid electrolyte is not particularly limited as long as it is a polymer compound exhibiting ionic conductivity; for example, polyethylene oxide, polypropylene oxide, or copolymers thereof can be used. The organic solid electrolyte may also contain a supporting salt (lithium salt). When using a solid electrolyte, the solid electrolyte may also be mixed into the cathode material to ensure contact between the electrolyte and the cathode active material.

[0096] (Battery shape, composition) The configuration of a secondary battery is not particularly limited and may consist of a positive electrode, a negative electrode, a separator, a non-aqueous electrolyte, etc., as described above, or it may consist of a positive electrode, a negative electrode, a solid electrolyte, etc. Furthermore, the shape of the secondary battery is not particularly limited and can be in various shapes such as cylindrical or stacked.

[0097] For example, in the case of a secondary battery using a non-aqueous electrolyte, the positive and negative electrodes are stacked with a separator in between to form an electrode body, the resulting electrode body is impregnated with a non-aqueous electrolyte, the positive electrode current collector is connected to the positive electrode terminal that is open to the outside, and the negative electrode current collector is connected to the negative electrode terminal that is open to the outside using current collector leads, and the battery is sealed in a battery case to complete the secondary battery.

[0098] (characteristic) The lithium-ion secondary battery according to this embodiment can have high output and high durability by including positive electrode active material 1A and / or positive electrode active material 1B in the positive electrode. When the positive electrode active materials 1A and 1B obtained in the preferred form are used, for example, in the positive electrode of a 2032 type coin cell battery used in the example, a low positive electrode interface resistance (positive electrode resistance) and a high discharge capacity retention rate after cycle testing can be obtained. [Examples]

[0099] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited in any way by these examples. The evaluation methods used in the examples and comparative examples are as follows.

[0100] (Analysis of composition) The results were measured by quantitative analysis using inductively coupled plasma (ICP) emission spectrometry. A Shimadzu ICPE-9000 ICP emission spectrometer was used for the analysis. (Site occupancy rate) The sodium site occupancy rate of lithium sites was determined by Rietveld analysis using X-ray diffraction. A Spectris X'PertPRO XRD system was used for the XRD, and JADE-XRD analysis software from MDI (USA) was used for the Rietveld analysis.

[0101] (Specific surface area of ​​positive electrode active material) The measurement was performed using the BET method with nitrogen adsorption. A MacSorb 1200 series (fluidized nitrogen gas adsorption method) manufactured by Mountec Co., Ltd. was used as the specific surface area analyzer.

[0102] (Manufacturing of secondary batteries) The battery characteristics of the obtained positive electrode active material for lithium-ion secondary batteries were evaluated by fabricating an evaluation coin-type battery (CBA) as shown in Figure 5.

[0103] 52.5 mg of the obtained positive electrode active material, 15 mg of acetylene black, and 7.5 mg of polytetrafluoroethylene resin (PTFE) were mixed and press-molded at a pressure of 100 MPa to a diameter of 11 mm and a thickness of 100 μm to produce a positive electrode (evaluation electrode) PE.

[0104] Next, the prepared positive electrode PE was dried in a vacuum dryer at 120°C for 12 hours. Using the dried positive electrode PE, negative electrode NE, separator SE, and electrolyte, a coin-type battery CBA, as shown in Figure 5, was fabricated in a glove box with an Ar atmosphere where the dew point was controlled to -60°C.

[0105] The negative electrode NE used a negative electrode sheet made of copper foil coated with graphite powder with an average particle size of approximately 20 μm, punched into a disc shape with a diameter of 14 mm, and polyvinylidene fluoride. The electrolyte was an equal mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (manufactured by Ube Industries, Ltd.) with 1 M LiPF6 as the supporting electrolyte. The separator SE used a polyethylene porous membrane with a thickness of 25 μm. The coin-type battery CBA had a gasket GA and a wave washer WW, and was assembled into a coin-shaped battery with the positive electrode can PC and the negative electrode can NC.

[0106] (Positive electrode interface resistance) The positive electrode interface resistance (positive electrode resistance) was evaluated using a coin-type battery CBA by the following method. First, the coin-type battery CBA was charged at a charging potential of 4.0V, and the impedance spectrum (Nyquist plot) shown in Figure 6 was obtained by measuring the AC impedance using a frequency response analyzer and a potentiometer galvanostat (Solartron 1255B) via the AC impedance method. The obtained Nyquist plot shows two semicircles in the high-frequency and intermediate-frequency regions, and a straight line in the low-frequency region. Based on this Nyquist plot, the positive electrode interface resistance was calculated by fitting calculations using the equivalent circuit model shown in Figure 6. In Figure 6, Rs is the bulk resistance, R1 is the positive electrode film resistance, Rct is the electrolyte / positive electrode interface resistance (Li+ migration resistance at the interface), W is the Warburg component, and CPE1 and CPE2 are the phase-constant elements. In Tables 1 and 2, the positive electrode interface resistance is shown as a relative value with Comparative Example 1 set to 1 (reference).

[0107] (Discharge capacity maintenance rate) The discharge capacity retention rate was evaluated using coin-type batteries (CBA) based on the capacity retention rate after a charge-discharge cycle test. The charge-discharge cycle test involved 100 cycles of charging and discharging in a 60°C environment within a voltage range of 3.0-4.2V. The discharge capacity after the 100th cycle was measured, and the capacity retention rate (%) was calculated as the percentage of the discharge capacity after the 100th cycle relative to the discharge capacity after the first cycle (initial discharge capacity).

[0108] (Example 1) A nickel-cobalt oxide was obtained by heating a nickel-based hydroxide, obtained using known techniques, at 600°C. This nickel-cobalt oxide was mixed with lithium hydroxide as a lithium compound and sodium carbonate as a sodium compound to obtain a lithium mixture. The sodium carbonate was added to the lithium mixture so that the amount of sodium was 1 atomic percent relative to the total number of nickel, cobalt, and aluminum atoms contained in the nickel-cobalt oxide. The above lithium mixture was calcined at 750°C for 12 hours to obtain a lithium-sodium-nickel-cobalt composite oxide, which was used as the positive electrode active material.

[0109] The composition of the obtained lithium sodium nickel cobalt composite oxide is Li 1.04 Na 0.01 Ni 0.82 Co 0.15 Al 0.03 It is O2, and its specific surface area is 0.5 m². 2 The concentration was / g. The sodium content (sodium site occupancy) in the lithium site (3a site) of the positive electrode active material was 0.95%. Furthermore, the battery characteristics were evaluated using the obtained positive electrode active material. The evaluation results are shown in Tables 1 and 2.

[0110] (Example 2) The resulting lithium sodium nickel cobalt composite oxide is Li 1.03 Na 0.02 Ni 0.82 Co 0.15 Al 0.03 A positive electrode active material was prepared in the same manner as in Example 1, except that the mixing ratios of lithium and sodium compounds in the lithium mixture were adjusted to achieve O2. The sodium content in the lithium sites of the positive electrode active material was 1.9%. The battery characteristics were then evaluated using the obtained positive electrode active material. The evaluation results are shown in Table 1.

[0111] (Example 3) The resulting lithium sodium nickel cobalt composite oxide is Li 1.00 Na 0.05 Ni 0.82 Co 0.15 Al 0.03A positive electrode active material was prepared in the same manner as in Example 1, except that the mixing ratios of lithium and sodium compounds in the lithium mixture were adjusted to achieve O2. The sodium content in the lithium sites of the prepared positive electrode active material was 1.9%. The battery characteristics were then evaluated using the obtained positive electrode active material. The evaluation results are shown in Table 1.

[0112] (Example 4) The resulting lithium sodium nickel cobalt composite oxide is Li 0.95 Na 0.10 Ni 0.82 Co 0.15 Al 0.03 A positive electrode active material was prepared in the same manner as in Example 1, except that the mixing ratios of lithium and sodium compounds in the lithium mixture were adjusted to obtain O2. The sodium content in the lithium sites of the prepared positive electrode active material was 9.0%. The battery characteristics were evaluated using the obtained positive electrode active material. The evaluation results are shown in Table 1.

[0113] (Example 5) The lithium sodium nickel cobalt composite oxide (base material) obtained in Example 1 and tungsten oxide were mixed and thoroughly stirred using a mixing stirrer, and 8% by mass of water was sprayed onto the surface of these powders relative to the lithium sodium nickel cobalt composite oxide. Subsequently, the resulting tungsten mixture was heat-treated at 150°C for 1 hour to coat the surface of the base material with a tungsten-containing compound (W compound) to obtain a positive electrode active material.

[0114] Analysis of the W content in the obtained positive electrode active material using ICP revealed that it was 0.1 atomic percent relative to the total number of nickel, cobalt, and aluminum atoms. Furthermore, XRD analysis of the coated W compound confirmed the formation of Li2WO4. The battery characteristics were evaluated using the obtained positive electrode active material. The evaluation results are shown in Table 2.

[0115] (Example 6) A positive electrode active material was prepared in the same manner as in Example 5, except that the W content in the positive electrode active material was set to 0.25 atomic percent. The coated W compound was in the form of Li2WO4. The battery characteristics were evaluated using the obtained positive electrode active material. The evaluation results are shown in Table 2.

[0116] (Example 7) A positive electrode active material was prepared in the same manner as in Example 5, except that the W content in the positive electrode active material was set to 0.50 atomic percent. The coated W compound was in the form of Li2WO4. The battery characteristics were evaluated using the obtained positive electrode active material. The evaluation results are shown in Table 2.

[0117] (Example 8) A positive electrode active material was prepared in the same manner as in Example 5, except that the W content in the positive electrode active material was set to 0.95 atomic percent. The coated W compound was in the form of Li2WO4. The battery characteristics were evaluated using the obtained positive electrode active material. The evaluation results are shown in Table 2.

[0118] (Example 9) The positive electrode active material was prepared in the same manner as in Example 7, except that the heat treatment temperature was set to 50°C. The morphology of the coated W compound was (Li2WO4)·4H2O. The battery characteristics were evaluated using the obtained positive electrode active material. The evaluation results are shown in Table 2.

[0119] (Example 10) A positive electrode active material was prepared in the same manner as in Example 7, except that the heat treatment temperature was set to 25°C. The morphology of the coated W compound was WO3. The battery characteristics were evaluated using the obtained positive electrode active material. The evaluation results are shown in Table 2.

[0120] (Example 11) A positive electrode active material was prepared in the same manner as in Example 7, except that the amount of water sprayed was 0.1% by mass and the heat treatment temperature was 100°C. The morphology of the coated W compound was WO3. The battery characteristics were evaluated using the obtained positive electrode active material. The evaluation results are shown in Table 2.

[0121] (Example 12) The positive electrode active material was prepared in the same manner as in Example 7, except that lithium hydroxide was added when mixing lithium sodium nickel cobalt composite oxide (base material) and tungsten oxide. The morphology of the coated W compound was Li4WO5. The battery characteristics were evaluated using the obtained positive electrode active material. The evaluation results are shown in Table 2.

[0122] (Comparative Example 1) Nickel-cobalt oxide, obtained by heating nickel-based hydroxide obtained by known techniques at 600°C, was mixed with lithium hydroxide as a lithium compound, and calcined at 750°C for 12 hours to obtain a lithium-nickel composite oxide (positive electrode active material). The composition of the obtained positive electrode active material was Li 1.05 Ni 0.82 Co 0.15 Al 0.03 It was O2. The specific surface area of ​​this lithium nickel composite oxide was 0.5 m². 2 It was / g.

[0123] (Comparative Example 2) A positive electrode active material was prepared in the same manner as in Example 5, except that the W content in the positive electrode active material was set to 0.50% by mass, using the lithium nickel composite oxide (positive electrode active material) of Comparative Example 1. The form of the coated W compound was Li2WO4. The battery characteristics were evaluated using the obtained positive electrode active material. The evaluation results are shown in Table 2.

[0124] (Comparative Example 3) The positive electrode active material was prepared in the same manner as in Comparative Example 2, except that the heat treatment temperature was set to 25°C. The morphology of the coated W compound was WO3. The battery characteristics were evaluated using the obtained positive electrode active material. The evaluation results are shown in Table 2.

[0125] [Table 1]

[0126] [Table 2]

[0127] (evaluation) As shown in Table 1, the positive electrode active materials of Examples 1 to 4, which consist of lithium sodium nickel cobalt composite oxide containing sodium at the lithium sites, showed improved discharge capacity retention compared to Comparative Example 1, which did not contain sodium. In particular, the positive electrode active materials of Examples 1 to 3, in which the site occupancy of sodium was 5% or less, showed reduced positive electrode interface resistance and high discharge capacity retention.

[0128] Furthermore, as shown in Table 2, the positive electrode active materials of Examples 5-9 and 12 were found to have lower positive electrode interface resistance and higher discharge capacity retention compared to the positive electrode active materials of Comparative Examples 1-3, demonstrating superior battery characteristics. In particular, the positive electrode active material of Example 12, in which a lithium compound was added when mixing the tungsten compound, exhibited good positive electrode interface resistance.

[0129] In Example 11, it was confirmed that the positive electrode interface resistance was lower and the discharge capacity retention rate was higher compared to Comparative Example 3, which was manufactured in the same manner as Example 10 except that it did not contain sodium. However, in the positive electrode active materials of Examples 10 and 11, the surface of the base material was coated with tungsten oxide, which has low lithium ion conductivity, so compared to Comparative Examples 1 and 2, the positive electrode interface resistance was higher and the discharge capacity retention rate was lower.

[0130] Based on the above results, it was confirmed that the lithium-ion secondary battery using the positive electrode active material of the present invention has low positive electrode interface resistance and good durability, resulting in a battery with excellent characteristics. [Industrial applicability]

[0131] The lithium-ion secondary battery of the present invention is suitable for powering small portable electronic devices (such as notebook computers, smartphones, tablet devices, and digital cameras) that always require high capacity, and is also suitable for electric vehicle batteries that require high output and high durability.

[0132] Furthermore, the lithium-ion secondary battery of the present invention has excellent safety, can be miniaturized, and can achieve high output, making it suitable as a power source for electric vehicles where mounting space is limited.

[0133] Furthermore, the present invention can be used not only as a power source for electric vehicles driven by electrical energy, but also as a power source for so-called hybrid vehicles and plug-in hybrid vehicles that are used in combination with combustion engines such as gasoline engines and diesel engines. [Explanation of symbols]

[0134] 1A, 1B...Cathode active material 10…Lithium sodium nickel cobalt composite oxide 20…Lithium-ion conductive oxide CBA… Coin-type battery (for evaluation) PE... Positive electrode (evaluation electrode) NE...negative electrode SE... Separator GA... Gasket WW... Wave Washer PC... Positive electrode can NC... Negative electrode can

Claims

1. A positive electrode active material containing a lithium sodium nickel cobalt composite oxide, The aforementioned lithium sodium nickel cobalt composite oxide is It has a layered crystalline structure, General formula (1): Li ｘ Na ｙ Ni １－ａ－ｂ Co ａ M ｂ O ２ (In the general formula (1) above, 0.05 ≤ a ≤ 0.5, 0 ≤ b ≤ 0.60, 0.5 ≤ (1 - a - b), 0.96 ≤ x + y ≤ 1.20, 0 < y ≤ 0.1, and element M is represented by at least one element selected from Mn, W, Mo, V, Mg, Ca, Al, Ti, Cr, Zr, La and Ta), and, The lithium site, site 3a, contains sodium. The site occupancy of sodium at the 3a site, as determined by Rietveld analysis using X-ray diffraction, is between 0.95% and 1.9%. The positive electrode interface resistance of a 2023-type coin cell using the aforementioned lithium-sodium-nickel-cobalt composite oxide as the positive electrode is lower than that of a 2023-type coin cell using lithium-nickel-cobalt composite oxide as the positive electrode, except that a lithium raw material containing the same number of moles of lithium as sodium in the sodium raw material was used instead of a sodium raw material. Positive electrode active material for lithium-ion secondary batteries.

2. A method for producing a positive electrode active material containing the lithium sodium nickel cobalt composite oxide described in Claim 1, A process of obtaining a lithium mixture by mixing nickel-cobalt composite oxide, a lithium compound, and a sodium compound, The process includes the step of calcining the lithium mixture to obtain a lithium sodium nickel cobalt composite oxide, The nickel-cobalt composite oxide contains nickel, cobalt, and optionally element M, and the atomic ratio of each metal element is expressed as Ni:Co:M = (1-a-b):a:b (0.05 ≤ a ≤ 0.5, 0 ≤ b ≤ 0.60, 0.5 ≤ (1-a-b), where element M is at least one element selected from Mn, W, Mo, V, Mg, Ca, Al, Ti, Cr, Zr, La, and Ta). The total amount of lithium and sodium in the lithium mixture is 96 atomic percent or more and 120 atomic percent relative to the total amount of nickel, cobalt, and element M, and the amount of sodium is greater than 0 atomic percent and less than or equal to 10 atomic percent relative to the total amount of nickel, cobalt, and element M. The lithium sodium nickel cobalt composite oxide has a layered crystalline structure and contains sodium at the 3a site, which is a lithium site, The sodium compound is one or more selected from sodium carbonate, sodium bicarbonate, sodium percarbonate, sodium fluoride, sodium oxide, sodium citrate, sodium acetate, sodium chloride, sodium hydroxide, and sodium sulfate. A method for producing positive electrode active material for lithium-ion secondary batteries.

3. The method for producing a positive electrode active material for a lithium-ion secondary battery according to claim 2, wherein the firing is performed at a temperature of 700°C or higher and 900°C or lower for 5 hours or higher and 15 hours or lower.

4. It comprises a positive electrode, a negative electrode, and a non-aqueous electrolyte. The positive electrode comprises the positive electrode active material for a lithium-ion secondary battery described in claim 1, wherein the positive electrode is a lithium-ion secondary battery.

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