WO2015004705A1 - Positive-electrode active material for lithium-ion secondary batteries - Google Patents

Abstract

This positive-electrode active material for lithium-ion secondary batteries is characterized by consisting of particles that contain lithium, nickel, and manganese and is also characterized in that: each of said particles has an interior and a surface layer; the composition of the particle interiors can be represented by the composition formula Li_xNi_aMn_bM_cO₂ (with 1 < x ≤ 1.2, 0.2 ≤ a ≤ 0.4, 0.4 ≤ b ≤ 0.6, c ≤ 0.02, a+b+c = 0.8, and M representing a transition-metal element); the atomic ratio of lithium to nickel and manganese in the surface layers is lower than the atomic ratio of lithium to nickel and manganese in the particle interiors; and the surface layers contain an added metal element. This invention makes it possible to provide a positive-electrode active material for lithium-ion secondary batteries having a high discharge capacity and a low irreversible capacity.

Description

Positive electrode active material for lithium ion secondary battery

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 using the same.

The problem with electric vehicles is that the energy density of the driving battery is low and the travel distance for one charge is short. Therefore, there is a demand for an inexpensive secondary battery with high energy density.

Lithium ion secondary batteries have a higher energy density per weight than secondary batteries such as nickel hydrogen batteries and lead batteries, but in order to meet the demands of electric vehicles, it is necessary to further increase the energy density. In order to realize high energy density of the battery, it is necessary to increase the energy density of the positive electrode and the negative electrode, and high capacity of the active material is required. Layered solid solution is a positive electrode active material that can be expected to have a high capacity.

Patent Document 1 describes a positive electrode material in which a lithium composite oxide having a layered layered rock salt type crystal structure of a hexagonal system and a lithium manganese oxide having a spinel structure belonging to a space group R-3m is mixed. There is. Patent Document 1, the composition formula _{Li a M1 b M2 c O d} (M1 is Mn, Ni, 2 or more elements including at least Mn of Co, M2 is Al, Ti, Mg, of B, at least one Element, 1.1 <a <1.3, 0.7 <b + c <1.1, 0 <c <0.a> b + c) and at least a portion of the surface of the positive electrode material By providing a covering layer including the third element as a constituent element while covering, the capacity retention rate at the time of cycling is improved.

JP 2012-142154 A

The positive electrode material disclosed in Patent Document 1 can obtain a high capacity, but the irreversible capacity is large because the content of Mn is large. Since the battery needs a negative electrode matched to the charge capacity of the positive electrode, when using a positive electrode active material having a large irreversible capacity, a negative electrode having a capacity corresponding to the first large charge capacity is required. However, since the discharge capacity that can actually be used is smaller than the charge capacity, the unit weight and the capacity per unit volume decrease as the amount of the negative electrode increases. Therefore, an object of the present invention is to provide a layered solid solution positive electrode active material having a small irreversible capacity and a high discharge capacity.

In order to solve the above problems, the positive electrode active material for a lithium ion secondary battery of the present invention is a particle containing Li, Ni, and Mn, the particle has an inner portion and a surface portion, and the inner composition is a composition Formula Li _x Ni _a Mn _b M _c O ₂ (1 <x ≦ 1.2, 0.2 ≦ a ≦ 0.4, 0.4 ≦ b ≦ 0.6, c ≦ 0.02, a + b + c = 0. 8, M is a transition metal element), the content ratio of Li to Ni and Mn in the surface layer (Li / (Ni + Mn)) is lower than the internal Li / (Ni + Mn), and the surface layer is an additive metal element M 'is included.

According to the present invention, it is possible to provide a layered solid solution positive electrode active material having a small irreversible capacity and a high discharge capacity.

It is sectional drawing which shows the structure of a lithium ion secondary battery typically. It is sectional drawing which shows typically the structure of the particle | grains of the positive electrode active material which concerns on this invention. It is a figure which shows the first time charge-discharge curve of the conventional layered solid solution.

<Positive electrode active material>
When using a lithium ion secondary battery for an electric vehicle, it is required that the energy density is high, the travel distance per charge is long, and the life is long. In the lithium ion secondary battery, this characteristic is closely related to the positive electrode active material. In order to improve the energy density of the battery, it is required to increase the capacity of the positive electrode active material.

The positive electrode active material for a lithium ion secondary battery according to the present invention is a particle containing Li, Ni, and Mn. The particle has an inner portion and a surface portion, and the surface portion is a range of 1/10 of the particle diameter from the surface of the particle, and the inner portion is a portion other than the surface portion of the particle. FIG. 2 shows the structure of the particle according to the present invention. In FIG. 2, 11 is the inside, 12 is the surface layer. The inside of the particle has the composition formula Li _x Ni _a Mn _b M _c O ₂ (1 <x ≦ 1.2, 0.2 ≦ a ≦ 0.4, 0.4 ≦ b ≦ 0.6, c ≦ 0. 02, a + b + c = 0.8, M is a transition metal element such as Al or Mg). Here, M is an additive or an impurity contained at a composition ratio of 0.02 or less, and does not greatly affect the capacity and irreversible capacity of the positive electrode active material. By satisfying the above composition, high capacity can be achieved in the potential range of 3.0 to 4.6 V.

X in the composition formula represents the proportion of Li in Li _x Ni _a Mn _b M _c O ₂ . If x is less than 1, the amount of Li contributing to the reaction is reduced and a high capacity can not be obtained. On the other hand, when x is larger than 1.2, the crystal lattice becomes unstable and the discharge capacity is reduced.

In the composition formula, a represents the content ratio (atomic weight ratio) of Ni of the positive electrode active material. If a is smaller than 0.2, the content of Ni, which is mainly responsible for charge and discharge, decreases, so the capacity decreases. On the other hand, when a is larger than 0.4, the valence number of Ni increases, the charge / discharge capacity involving Ni decreases, and a high capacity can not be obtained.

In addition, although the positive electrode active material according to the present invention contains Li, Ni and Mn as transition metals, it does not contain expensive Co. The positive electrode active material in the present embodiment has an advantage of low cost in addition to high energy density.

The positive electrode active material according to the present invention is characterized in that the atomic ratio of Li to Ni and Mn (Li / (Ni + Mn)) in the surface layer portion is lower than the internal Li / (Ni + Mn). The irreversible capacity can be reduced because the content ratio of Li to Ni and Mn in the surface layer portion is lower than the content ratio of Li to Ni and Mn inside.

The initial charge / discharge curve of the conventional layered solid solution positive electrode active material is shown in FIG. In FIG. 3, the vertical axis represents voltage, the horizontal axis represents capacity, 13 is a charge curve, and 14 is a discharge curve. It can be seen from FIG. 3 that there is a large difference 15 between the initial charge capacity and the discharge capacity. This difference 15 is the irreversible capacity. When a layered solid solution is used as the positive electrode active material, it is thought that a reaction in which Li ₂ O is eliminated from the positive electrode occurs at the time of initial charge. It is inferred that Li ₂ O which was desorbed at the time of initial charge does not return to the positive electrode by discharge, which causes irreversible capacity. Therefore, when the positive electrode active material in which Li in the surface layer portion is deficient is used in advance, desorption of Li ₂ O can be suppressed at the time of initial charge, and irreversible capacity can be reduced.

Li in the surface layer can be lost by surface treatment with an acid. The method of deficient Li in the surface layer is not limited to this, and the Li content in the surface layer may be lower than the internal Li content.

The positive electrode active material according to the present invention is characterized in that the surface layer portion contains the additive metal element M ′. The surface layer portion having a smaller content of Li than the inside of the particles is devoid of Li ₂ O and becomes active. In the activated part, the decomposition reaction of Li salt which is an electrolyte is likely to occur. Therefore, by covering the active part of the surface layer with the phase containing M ', it is possible to improve the capacity retention rate upon cycling.

M 'is a metal element which does not exist in the inside of the particle, and examples thereof include Mg, Fe, Zn, Al, Mo and the like. These may coat the surface layer as an oxide. The content of M ′ is characterized in that it is 1 to 5% by mass ratio to the positive electrode active material. If the content of M 'is less than 1%, there is no effect on the improvement of the capacity retention rate during cycling. On the other hand, when the content is more than 5%, the resistance becomes high, and the content of Li, Ni, and Mn in the positive electrode active material decreases, so the capacity decreases.

The positive electrode active material according to the present invention can be produced by a method generally used in the technical field to which the present invention belongs. For example, it can be produced by mixing and firing compounds containing Li, Ni, and Mn in appropriate proportions. The composition of the positive electrode active material can be appropriately adjusted by changing the ratio of the compound to be mixed.

Examples of the compound containing Li include lithium acetate, lithium nitrate, lithium carbonate, lithium hydroxide, lithium oxide and the like. Examples of the compound containing Ni include nickel acetate, nickel nitrate, nickel carbonate, nickel sulfate, nickel hydroxide and the like. As a compound containing Mn, manganese acetate, manganese nitrate, manganese carbonate, manganese sulfate, manganese oxide etc. can be mentioned, for example.

The composition of the positive electrode material can be determined by, for example, elemental analysis by inductively coupled plasma (ICP) or the like, and elemental analysis from the surface to the inside by Auger analysis.

<Lithium ion secondary battery>
The lithium ion secondary battery according to the present invention is characterized by containing the above-mentioned positive electrode active material. By using the above-mentioned positive electrode active material for the positive electrode, a lithium ion secondary battery with high energy density can be obtained. Since the battery requires an amount of the negative electrode matched to the charge capacity of the positive electrode, when using a positive electrode active material having a large irreversible capacity, a negative electrode having a capacity corresponding to the first large charge capacity is required. However, since the discharge capacity that can actually be used is smaller than the charge capacity, the larger amount of the negative electrode results in wasting the negative electrode. By using a positive electrode active material with a small irreversible capacity, the amount of wasted anode can be reduced, and the volume and weight per unit volume can be reduced. As a result, a lithium ion secondary battery with high energy density can be obtained. The lithium ion secondary battery according to the present invention can be preferably used, for example, for an electric vehicle.

The lithium ion secondary battery is composed of a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, a separator, an electrolytic solution, an electrolyte and the like.

The negative electrode active material is not particularly limited as long as it can absorb and release lithium ions. Materials generally used in lithium ion secondary batteries can be used as the negative electrode material. For example, graphite, lithium alloy and the like can be exemplified.

As a separator, those generally used in lithium ion secondary batteries can be used. For example, a microporous film or non-woven fabric made of polyolefin such as polypropylene, polyethylene, and a copolymer of propylene and ethylene can be exemplified.

As the electrolytic solution and the electrolyte, those generally used in lithium ion secondary batteries can be used. For example, as the electrolytic solution, diethyl carbonate, dimethyl carbonate, ethylene carbonate, propylene carbonate, vinylene carbonate, methyl acetate, ethyl methyl carbonate, methyl propyl carbonate, dimethoxyethane and the like can be exemplified. Further, as the _{electrolyte, LiClO 4, LiPF 6, LiBF} 4, LiAsF 6, LiSbF 6, LiCF 3 SO 3, LiC 4 F 9 SO 3, LiCF 3 CO 2, Li 2 C 2 F 4 (SO 3) 2, LiN _{(CF 3 SO 2) 2,} LiC (CF 3 SO 2) can be exemplified ₃ or the like.

One embodiment of a structure of a lithium ion secondary battery according to the present invention will be described with reference to FIG. The lithium ion secondary battery 10 includes an electrode group including a positive electrode 1 having a positive electrode active material coated on both sides of a current collector, a negative electrode 2 having a negative electrode active material coated on both sides of the current collector, and a separator 3. The positive electrode 1 and the negative electrode 2 are wound via the separator 3 to form a wound electrode group. The wound body is inserted into the battery can 4.

The negative electrode 2 is electrically connected to the battery can 4 via the negative electrode lead piece 6. A sealing lid 7 is attached to the battery can 4 via a packing 8. The positive electrode 1 is electrically connected to the sealing lid 7 through the positive electrode lead piece 5. The wound body is insulated by the insulating plate 9.

The electrode group may not be a wound body shown in FIG. 1, and may be a laminate in which the positive electrode 1 and the negative electrode 2 are stacked via the separator 3.

Hereinafter, the present invention will be described in more detail using Examples and Comparative Examples, but the technical scope of the present invention is not limited thereto.

<Preparation of positive electrode active material>
Lithium carbonate, nickel carbonate and manganese carbonate were mixed in a ball mill to obtain a precursor. The obtained precursor was calcined at 500 ° C. for 12 hours in the air to obtain a lithium transition metal oxide. The obtained lithium transition metal oxide was pelletized and then fired at 850 to 1050 ° C. for 12 hours in the air. The fired pellets were crushed in an agate mortar and classified with a 45 μm sieve to obtain a positive electrode active material.

The following two-step method was used as the surface treatment of the obtained positive electrode active material. In the first process, the positive electrode active material was acid-treated to deplete Li in the surface layer. The positive electrode active material was added to the acidic solution and stirred while heating at 100 to 130.degree. Thereafter, it was heated at 200 to 400 ° C., and after the temperature dropped, it was washed with purified water. Here, as an acidic solution, nitric acid, hydrochloric acid, sodium sulfate, ammonium sulfate etc. can be mentioned.

In the second-stage process, the surface part lacking Li was covered with a phase containing an additive metal element. The positive electrode active material after the first-step treatment was placed in a solution containing the elemental species to be coated, and stirred while heating at 100 to 130.degree. Thereafter, the resultant was heated at 300 to 600 ° C. to obtain a surface-coated positive electrode active material. As a solution containing an additive metal element, a solution in which magnesium acetate, iron acetate, zinc acetate, aluminum lactate, molybdic acid and the like are dissolved in purified water or citric acid can be mentioned.

The composition of the positive electrode active material used in each example and comparative example is shown in Table 1.

<Production of a prototype battery>
In each example and comparative example, a positive electrode was produced using 18 types of positive electrode active materials produced as mentioned above, and 18 types of trial manufacture batteries were produced.

The positive electrode active material, the conductive agent, and the binder were uniformly mixed to prepare a positive electrode slurry. The positive electrode slurry was applied onto a 20 μm thick aluminum current collector foil, dried at 120 ° C., and compression molded by a press so that the electrode density was 2.2 g / cm ³ to obtain an electrode plate. Thereafter, the electrode plate was punched into a disk shape having a diameter of 15 mm to produce a positive electrode.

The negative electrode was produced using metallic lithium. As a non-aqueous electrolytic solution, one in which LiPF ₆ was dissolved at a concentration of 1.0 mol / L in a mixed solvent of ethylene carbonate and dimethyl carbonate at a volume ratio of 1: 2 was used.

<Charge / discharge test>
In each of the examples and the comparative examples, charge and discharge tests were performed on the 18 types of prototype batteries manufactured as described above.

The charge and discharge test was performed on the prototype battery. The charging was constant current constant voltage charging (CC-CV mode), and the upper limit voltage was 4.6V. The discharge was constant current discharge (CC mode), and the lower limit voltage was 2.5V. The charge / discharge current was equivalent to 0.05 C, and the charge cutoff current was equivalent to 0.005 C. In each of the examples and the comparative examples, a value obtained by dividing the discharge capacity in the area of 4.6 to 3.0 V by the discharge capacity in the area of 4.6 to 3.0 V in the comparative example 1 is defined as a discharge capacity ratio. The results are shown in Table 2.

Moreover, in each Example and Comparative Example, a value obtained by dividing the irreversible capacity obtained by subtracting the discharge capacity from the initial charge capacity by the irreversible capacity of Comparative Example 1 was defined as an irreversible capacity ratio. The results are shown in Table 2.

<Capacity maintenance rate after cycle>
Initialization was performed for two cycles under the above-described charge and discharge test conditions. After that, cycle test was performed under the following conditions. The charging was in CC-CV mode, and the upper limit voltage was 4.6V. The discharge was in CC mode and the lower limit voltage was 2.5V. The charge / discharge current was equivalent to 1 C, and the charge cutoff current was equivalent to 0.2 C. A value obtained by dividing the discharge capacity at 100th cycle at 1 C by the initial discharge capacity is defined as a capacity retention rate. A value obtained by dividing the capacity retention rate in each example and comparative example by the capacity retention rate in comparative example 1 is defined as a capacity retention rate ratio. The results are shown in Table 2.

As shown in Table 2, in Examples 1 to 11, the irreversible capacity ratio is lower and the capacity retention ratio is higher than in Comparative Example 1. Since the positive electrode active materials of Examples 1 to 11 were subjected to the surface treatment, the content ratio of Li to Ni and Mn in the surface layer portion is low, and the surface layer portion contains the additive metal element. Therefore, the irreversible capacity can be reduced and the discharge capacity can be improved by using the positive electrode active material in which the surface layer portion contains an additive metal element, which has a lower Li content in the surface layer than the internal Li content.

When Example 2 and Comparative Example 6 are compared, Example 2 has a smaller irreversible capacity ratio than Comparative Example 6. The positive electrode active material of Example 2 has a structure in which the surface treatment with acid is performed and Li in the surface layer is lost, while the positive electrode active material of Comparative Example 6 is not treated with acid, so the surface layer is Li is not deficient. Therefore, it was found that the irreversible capacity can be reduced by using the positive electrode active material in which the Li content to the internal Ni and Mn is lower than the Li content to the Ni and Mn in the surface layer.

When Example 1 is compared with Comparative Example 7, Example 1 has a larger capacity retention ratio than Comparative Example 7. The positive electrode active material of Example 1 contains the additive metal element in the surface layer portion, but the surface layer portion of the positive electrode active material of Comparative Example 7 is not coated. Therefore, in Comparative Example 7, the capacity retention ratio was reduced. Therefore, it was found that the particle surface can be protected and the cycle characteristics can be improved by including the metal element not present inside in the surface layer portion.

When Examples 1 to 4 and Comparative Examples 2 to 4 are compared, the discharge capacity ratio of Examples 1 to 4 is higher than the discharge capacity ratio of Comparative Examples 2 to 4. This is because the composition of the positive electrode active material of Comparative Examples 2 to 4 is not included in the range of 1 <x ≦ 1.2, 0.2 ≦ a ≦ 0.4, 0.4 ≦ b ≦ 0.6. It is for. In Comparative Example 2, since the amount of Li of the positive electrode active material is small, the capacity is reduced and the irreversible capacity is high. In Comparative Example 3, since the amount of Li of the positive electrode active material was too large, the crystal lattice became unstable and the capacity decreased. In Comparative Example 4, the capacity decreased because the amount of Ni of the positive electrode active material was too large.

When Examples 1, 5 and 6 and Comparative Example 5 are compared, the discharge capacity ratio of Examples 1, 5 and 6 is higher than the discharge capacity ratio of Comparative Example 5. In Comparative Example 5, since the content of the additive metal element was too large, the resistance increased, and the contents of Li, Ni, and Mn decreased, so the capacity decreased. Therefore, it was found that when the content of the additive metal element is 1 to 5% by mass ratio to the positive electrode active material, both the cycle characteristics and the high capacity can be compatible.

As described above, the surface treatment of the layered solid solution positive electrode active material in which the composition inside the particle is in the range of 1 <x ≦ 1.2, 0.2 ≦ a ≦ 0.4, 0.4 ≦ b ≦ 0.6 The irreversible capacity can be reduced and the discharge capacity can be improved by covering the surface layer portion with a phase containing a metal element that is not present inside the particles after depleting Li in the surface layer portion. Also, cycle characteristics can be improved.

DESCRIPTION OF SYMBOLS 1 positive electrode 2 negative electrode 3 separator 4 battery can 5 positive electrode lead piece 6 negative electrode lead piece 7 sealing lid 8 packing 9 insulating plate 10 lithium ion secondary battery 11 internal 12 surface portion 13 charge curve 14 discharge curve 15 irreversible capacity

Claims (15)

A positive electrode active material for a lithium ion secondary battery, which occludes and releases lithium ions,
The positive electrode active material is a particle containing Li, Ni, or Mn,
The particles have a surface portion and an interior,
The surface layer portion is in the range of a depth of 1/10 of the particle diameter from the surface of the particle, and
The inside is a portion other than the surface layer portion of the particle,
The composition of the inside is a composition formula Li _x Ni _a Mn _b M _c O ₂ (1 <x ≦ 1.2, 0.2 ≦ a ≦ 0.4, 0.4 ≦ b ≦ 0.6, c ≦ 0 .02, a + b + c = 0.8, M is a transition metal element),
The atomic ratio (Li / (Ni + Mn)) of Li to Ni and Mn in the surface layer portion is lower than the internal Li / (Ni + Mn),
The said surface layer part contains additive metal element M ', The positive electrode active material for lithium ion secondary batteries characterized by the above-mentioned. A positive electrode active material for a lithium ion secondary battery according to claim 1,
The M 'is at least one element selected from Mg, Fe, Zn, Al and Mo. A positive electrode active material for a lithium ion secondary battery according to claim 1,
The said surface layer part contains the oxide containing said M ', The positive electrode active material for lithium ion secondary batteries characterized by the above-mentioned. A positive electrode active material for a lithium ion secondary battery according to claim 1,
The positive electrode active material for a lithium ion secondary battery, wherein a content of the M ′ is 1 to 5% by mass with respect to the positive electrode active material. A positive electrode active material for a lithium ion secondary battery according to claim 1,
The composition of the inside is a composition formula Li _x Ni _a Mn _b O ₂ (1 <x ≦ 1.2, 0.2 <a ≦ 0.4, 0.4 ≦ b <0.6, a + b = 0.8 The positive electrode active material for lithium ion secondary batteries characterized by being represented by these. A positive electrode active material for a lithium ion secondary battery according to claim 1,
The positive electrode active material for a lithium ion secondary battery, wherein Li / (Ni + Mn) in the surface layer portion is 5 to 10 at% lower than the internal Li / (Ni + Mn). A positive electrode for a lithium ion secondary battery comprising the positive electrode active material for a lithium ion secondary battery according to any one of claims 1 to 6. Compositional formula Li _x Ni _a Mn _b M _c O ₂ (1 <x ≦ 1.2, 0.2 ≦ a ≦ 0.4, 0.4 ≦ b ≦ 0.6, c ≦ 0.02, a + b + c = 0 .8, M is a transition metal element), and the lithium complex oxide is acid-treated to desorb a part of Li;
Adding the lithium composite oxide from which a part of Li has been eliminated to a solution containing the additive metal element, heating, and coating with an oxide containing the additive metal element;
A method for producing a positive electrode active material for a lithium ion secondary battery, comprising: A lithium ion secondary battery comprising a positive electrode containing a positive electrode active material and a negative electrode containing a negative electrode active material,
The positive electrode active material is a particle containing Li, Ni, or Mn,
The particles have a surface portion and an interior,
The surface layer portion is in a range of 1/10 of the particle diameter from the surface of the particle,
The inside is a portion other than the surface layer portion of the particle,
The composition of the inside is a composition formula Li _x Ni _a Mn _b M _c O ₂ (1 <x ≦ 1.2, 0.2 ≦ a ≦ 0.4, 0.4 ≦ b ≦ 0.6, c ≦ 0 .02, a + b + c = 0.8, M is a transition metal element),
The atomic ratio (Li / (Ni + Mn)) of Li to Ni and Mn in the surface layer portion is lower than the internal Li / (Ni + Mn),
The surface layer portion contains an additive metal element M ′. It is a lithium ion secondary battery according to claim 9,
The lithium ion secondary battery according to claim 1, wherein the M 'is at least one element selected from Mg, Fe, Zn, Al and Mo. It is a lithium ion secondary battery according to claim 9,
The surface layer part contains the oxide containing said M ', The lithium ion secondary battery characterized by the above-mentioned. It is a lithium ion secondary battery according to claim 9,
The lithium ion secondary battery, wherein a content of the M ′ is 1 to 5% by mass with respect to the positive electrode active material. It is a lithium ion secondary battery according to claim 9,
The composition of the inside is a composition formula Li _x Ni _a Mn _b O ₂ (1 <x ≦ 1.2, 0.2 <a ≦ 0.4, 0.4 ≦ b <0.6, a + b = 0.8 A lithium ion secondary battery characterized by being represented by It is a lithium ion secondary battery according to claim 9,
A lithium ion secondary battery characterized in that Li / (Ni + Mn) in the surface layer portion is lower by 5 to 10 at% than Li / (Ni + Mn) in the inside. A lithium ion secondary battery according to any one of claims 9 to 14, wherein
A lithium ion secondary battery characterized in that it is used in a potential range of 3.0 to 4.6 V.

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