JP6460575B2 - Positive electrode active material for lithium secondary battery, electrode for lithium secondary battery, and lithium secondary battery - Google Patents

Description

  The present invention relates to a positive electrode active material for a lithium secondary battery containing a novel lithium transition metal composite oxide, an electrode for a lithium secondary battery containing the positive electrode active material, a lithium secondary battery including the electrode, and a battery thereof. The present invention relates to an assembled power storage device.

Conventionally, as a positive electrode active material for a lithium secondary battery, a “LiMeO ₂ type” active material (Me is a transition metal) having an α-NaFeO ₂ type crystal structure has been studied, and lithium secondary batteries using LiCoO ₂ have been widely put into practical use. It was converted. However, the discharge capacity of LiCoO ₂ was about 120 to 130 mAh / g. As Me, it has been desired to use abundant Mn as a global resource. However, the “LiMeO ₂ type” active material containing Mn as Me, when the molar ratio of Mn to Me, Mn / Me exceeds 0.5, the structure changes to spinel type when charged, Since the structure could not be maintained, there was a problem that the charge / discharge cycle performance was extremely inferior.

Accordingly, various “LiMeO ₂ type” active materials having a Mn to Me molar ratio Mn / Me of 0.5 or less and excellent in charge / discharge cycle performance have been proposed and partially put into practical use. For example, LiNi _1/2 Mn _1/2 O ₂ and LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ have a discharge capacity of 150 to 180 mAh / g.

In recent years, there has been proposed an active material that contains Ni, Co, and Mn as Me, has a molar ratio of Mn to Me, Mn / Me of 0.5 or more, and can maintain an α-NaFeO ₂ structure even when charged.

Patent Documents 1 and 2 include 4.3 V (vs) as a battery manufacturing method using an active material containing Ni, Co, and Mn as Me and having a molar ratio Mn to Me of Mn / Me of 0.5 or more. .Li / Li ⁺⁾ of more than 4.8V below (appearing in the positive electrode potential range of vs.Li/Li ^+), by providing a manufacturing step of performing at least throughout charging potential change to a relatively flat region, using Even when a charging method is adopted in which the maximum potential of the positive electrode during charging is 4.3 V (vs. Li / Li ⁺ ) or less or less than 4.4 V (vs. Li ^/ Li ⁺ ) It is described that a battery capable of obtaining a discharge capacity of 200 mAh / g or more can be manufactured.

Thus, unlike the case of the conventional “LiMeO ₂ type” positive electrode active material, the positive electrode active material containing Ni, Co and Mn as Me, and the molar ratio of Mn to Me, Mn / Me is 0.5 or more. In at least the first charge, a relatively high potential exceeding 4.3 V, in particular, a potential of 4.4 V or higher is achieved, whereby a high discharge capacity can be obtained.

In this material, the raw materials are mixed so that the composition ratio Li / Me of lithium (Li) to the ratio of transition metal (Me) is greater than 1, for example, Li / Me is 1.25 to 1.6. Since it is synthesized, it is also called a “lithium-excess type” active material, and the composition after synthesis can be expressed as Li _{1 + α} Me _1-α O ₂ (α> 0). Here, when the composition ratio Li / Me of lithium (Li) with respect to the ratio of the transition metal (Me) is β, β = (1 + α) / (1-α), and thus, for example, Li / Me is 1.5. In this case, α = 0.2.

In Patent Document 3, “a method for producing a positive electrode active material for obtaining a positive electrode active material from a lithium-containing oxide, including a step of treating the lithium-containing oxide with an acidic aqueous solution, ₁ + includes _{x (Mn y M 1-y} ) 1-x O 2 (0 <x <0.4,0 <y ≦ 1), wherein M includes at least one transition metal except manganese, the acidic aqueous solution The amount of hydrogen ions therein is xmol or more and less than 5xmol with respect to 1 mol of the lithium-containing oxide. "(Claim 5) invention is described, and excellent load characteristics are described. In addition, it is described that a lithium secondary battery having high initial charge / discharge efficiency can be manufactured.

Patent Document 4 discloses Li (Li _x Mn _y Me _z ) O _p F _q (Me is at least one element selected from Co, Ni, Cr, Fe, Al, Ti, Zr, and Mg, 0.09 <x <0.3, lithium in which 0.4 ≦ y / (y + z) ≦ 0.8, x + y + z = 1, 1.9 <p <2.1, 0 ≦ q ≦ 0.1) An oxide of at least one metal element selected from Zr, Ti, Sn, Mg, Ba, Pb, Bi, Nb, Ta, Zn, Y, La, Sr, Ce, In and Al on the surface of the composite oxide containing A positive electrode active material for a lithium ion secondary battery comprising particles to which fine particles adhere is described.

In Patent Document 5, Li _{(1 + x)} Co _(1-y) M _y O _(2-z) (M is Mg, Al, B, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn) , Mo, Sn, Ca, Sr, W, Y, Zr, at least one element selected from the group consisting of: −0.10 ≦ x ≦ 0.10, 0 ≦ y <0.50, z = −0 .10 ≦ z ≦ 0.20), an oxide coating layer containing Li, Ni and / or Mn, and an oxide containing a lanthanoid element provided on the coating layer. A positive electrode active material for a non-aqueous electrolyte secondary battery having an object surface layer, which suppresses binding between particles and prevents exposure of active composite oxide particle surfaces due to destruction of the coating layer or destruction of the coating layer It is described.

Patent Document 6 has an α-NaFeO ₂ type crystal structure and a composition formula Li _x Mn _a Ni _b Co _c O _d (0 ≦ x ≦ 1.3, a + b + c = 1, | a−b | ≦ 0.03, 0 ≦ c <1, 1.7 ≦ d ≦ 2.3) on the surface in contact with the electrolyte of the base metal which is a lithium transition metal composite oxide. It is described that it is a positive electrode active material in which an element is present, and can provide a lithium secondary battery excellent in storage stability and charge / discharge cycle performance in a charged state.

WO2012 / 091015 WO2013 / 084923 JP 2009-4285 A JP 2012-138197 A JP 2009-4316 A WO2005 / 008812

Although the discharge capacity of the “Li-excess type” active material is generally larger than that of the “LiMeO ₂ type” active material, it is known that the initial charge / discharge efficiency (hereinafter referred to as “initial efficiency”) is low. Among these, as described in Patent Document 3, it is known that the load characteristics and the initial efficiency are improved by acid treatment of the active material. However, in recent years, a lithium secondary battery used in the automobile field such as an electric vehicle, a hybrid vehicle, and a plug-in hybrid vehicle has been required to have a positive electrode active material having a high discharge capacity maintenance rate as well as an initial efficiency.

  Patent Document 4 describes that, for an “Li-excess type” active material, by attaching fine particles of an oxide of a metal element, the effect on cycle characteristics is improved. Is confirmed only in the case of Zr.

  Patent Document 5 describes an example of an active material in which a surface layer containing cerium is formed on the surface of a lithium cobalt composite oxide particle that can contain Mn and Ni. This surface layer is a binding between particles. It is intended for prevention, and does not indicate the problem of increasing the initial efficiency and the discharge capacity maintenance rate in the “lithium-excess type” active material.

  The active material described in Patent Document 6 is not supposed to be “lithium-excess type”, and does not indicate the problem of increasing the initial efficiency and the discharge capacity retention rate.

  In view of the above problems, the present invention includes a positive electrode active material for a lithium secondary battery having excellent initial efficiency and a high discharge capacity retention rate, an electrode for a lithium secondary battery containing the positive electrode active material, and the electrode. An object is to provide a lithium secondary battery.

In order to achieve the above object, the present inventors adopt the following means.
(1) A positive electrode active material including a lithium transition metal composite oxide having an α-NaFeO ₂ structure, wherein the transition metal (Me) includes Co, Ni, and Mn, Li and The molar ratio of transition metal (Me) (Li / Me) is 1 <Li / Me, the molar ratio of Mn to transition metal (Me) (Mn / Me) is 0.5 <Mn / Me, and Ce is A positive electrode active material for a lithium secondary battery.
(2) The positive electrode active material according to (1), wherein the lithium-transition metal composite oxide has a metal-converted Ce content ratio of 0.15 to 3.37% by mass.
(3) In the X-ray diffraction pattern analysis using a CuKα tube, the half width (FWHM) of the diffraction peak attributed to the (104) plane is 0.269 ≦ FWHM ≦ 0.273 The positive electrode active material according to (1) or (2).
(4) The electrode for lithium secondary batteries containing the positive electrode active material for lithium secondary batteries in any one of said (1)-(3).
(5) A lithium secondary battery comprising the lithium secondary battery electrode of (4).

  According to the present invention, a positive electrode active material for a lithium secondary battery having excellent initial efficiency and a high discharge capacity maintenance rate, an electrode for a lithium secondary battery containing the positive electrode active material, and a lithium secondary battery including the electrode Can be provided.

1 is an external perspective view showing an embodiment of a lithium secondary battery according to the present invention. Schematic showing a power storage device in which a plurality of lithium secondary batteries according to the present invention are assembled

  The present inventors can improve the initial efficiency by acid-treating the “Li-excess type” active material, but the reason why the discharge capacity retention rate after the charge / discharge cycle cannot be increased is as follows. It was inferred that the treatment promoted the elution of Mn from the active material accompanying the charge / discharge cycle.

  Therefore, by protecting the particle surface of the “Li-excess type” active material, the elution of Mn from the active material during charge / discharge can be relaxed, and the decrease in discharge capacity associated with the charge / discharge cycle can be suppressed. As a result, when treatments with various metals were attempted, it was found that Ce was added to the acid treatment aqueous solution, acid treatment was performed, and then heating was performed to impart Ce to the active material.

  Hereinafter, although the positive electrode active material suitable for this invention and its manufacturing method are explained in full detail, this invention is not limited to this.

(Positive electrode active material)
The positive electrode active material of the present invention is typically Li _{1 + α} (Co _a Ni _b Mn _c ) _1-α O ₂ , where α> 1, a + b + c = 1, a> 0, b> 0, c> 0. In order to obtain a lithium secondary battery excellent in initial efficiency and high-rate discharge performance, it is a composite oxide composed of Li, Co, Ni, and Mn. The molar ratio Li / Me is larger than 1.2 and smaller than 1.6, that is, 1.2 <(1 + α) / (1−α) <1 in the composition formula Li _{1 + α} Me _1-α O ₂ . 6 is preferable. Among these, from the viewpoint that a lithium secondary battery having a particularly large discharge capacity and excellent initial efficiency and high rate discharge performance can be obtained, the one having the Li / Me of 1.25 to 1.5 should be selected. Is preferred. In the present invention, the molar ratio Li / Me is after acid treatment, and is slightly higher in the starting material before acid treatment.

In addition, in the positive electrode active material of the present invention, the molar ratio Mn / Me of the transition metal element Me is preferably 0.5 or more in order to improve the initial efficiency and high rate discharge performance of the lithium secondary battery. . In the “LiMeO ₂ type” active material, when the molar ratio Mn / Me exceeds 0.5, the structure changes to the spinel type when charged and does not have a structure attributed to the α-NaFeO ₂ structure. However, the “lithium-excess type” active material has a problem as an active material for a lithium secondary battery, and the α-NaFeO ₂ structure when charged even when the molar ratio Mn / Me exceeds 0.5. Can be maintained. Therefore, the configuration where the molar ratio Mn / Me exceeds 0.5 characterizes the “lithium-rich” active material. The molar ratio Mn / Me is more preferably 0.51 to 0.75.

  In order to improve the initial efficiency and high rate discharge performance of the lithium secondary battery, the molar ratio Co / Me of the Co to the transition metal element Me is preferably 0.05 to 0.40, and 0.10 More preferably, it is set to ˜0.30.

  The lithium transition metal composite oxide according to the present invention is essentially a composite oxide composed of Li, Co, Ni, and Mn, but it is preferable to contain 1000 ppm or more of Na in order to improve the discharge capacity. As for content of Na, 2000-10000 ppm is more preferable.

  In order to contain Na, in the step of preparing a carbonate precursor, which will be described later, a sodium compound such as sodium carbonate is used as a neutralizing agent, and Na is left in the washing step. A method of adding a sodium compound such as sodium can be employed.

  In addition, a small amount of other metals such as alkali metals other than Na, alkaline earth metals such as Mg and Ca, transition metals typified by 3d transition metals such as Fe and Zn, and the like are included as long as the effects of the present invention are not impaired. It does not exclude doing.

The lithium transition metal composite oxide according to the present invention has an α-NaFeO ₂ structure. The lithium transition metal composite oxide after synthesis (before charging and discharging) is attributed to the space group P3 ₁ 12 or R3-m. Among these, those belonging to the space group P3 ₁ 12 include a superlattice peak (Li [Li _1/3 Mn _2/3 ] O around 2θ = 21 ° on an X-ray diffraction diagram using a CuKα tube. _A peak observed in type ₂ monoclinic crystal) is confirmed. However, when charging is performed once and Li in the crystal is desorbed, the symmetry of the crystal changes, whereby the superlattice peak disappears and the lithium transition metal composite oxide belongs to the space group R3-m. Will come to be. Here, P3 ₁ 12 is a crystal structure model in which the atomic positions of the 3a, 3b, and 6c sites in R3-m are subdivided, and when ordering is recognized in the atomic arrangement in R3-m, the P3 ₁ 12 model Is adopted. Note that “R3-m” should be represented by adding a bar “-” on “3” of “R3m”.

The lithium transition metal composite oxide according to the present invention has a half width of the diffraction peak attributed to the (003) plane of 0.202 when the space group R3-m is used as a crystal structure model based on the X-ray diffraction pattern. It is preferable to be within the range of ° to 0.265 °. Moreover, it is preferable that the half width of the diffraction peak attributed to the (104) plane is in the range of 0.265 ° to 0.285 °. By doing so, it is possible to increase the initial efficiency of the positive electrode active material. Note that diffraction peaks existing at 2θ = 44 ° ± 1 ° and 18 ° ± 1 ° on the X-ray diffraction diagram are respectively present on the (114) plane and the (003) plane in the Miller index hkl in the space group P3 ₁ 12 respectively. The space group R3-m is indexed on the (104) plane and the (003) plane in the Miller index hkl.

In addition, the lithium transition metal composite oxide according to the present invention preferably has a crystal structure that does not change when heat treatment is performed at 1000 ° C. This can be confirmed by observation as a single phase (α-NaFeO ₂ structure) belonging to the space group R3-m on the X-ray diffraction diagram when heat treatment is performed at 1000 ° C. Thereby, a lithium secondary battery excellent in high rate discharge performance can be obtained.

Further, in the lithium transition metal composite oxide, the oxygen position parameter obtained from the crystal structure analysis by the Rietveld method based on the X-ray diffraction pattern is 0.262 or less at the end of discharge and 0.267 or more at the end of charge. Is preferred. Thereby, a lithium secondary battery excellent in high rate discharge performance can be obtained. Note that the oxygen positional parameter is Me (transition metal) spatial coordinates (0, 0, 0) for the α-NaFeO ₂ type crystal structure of the lithium transition metal composite oxide belonging to the space group R3-m, This is the value of z when the spatial coordinates of Li (lithium) are defined as (0, 0, 1/2) and the spatial coordinates of O (oxygen) are defined as (0, 0, z). In other words, the oxygen position parameter is a relative index indicating how far the O (oxygen) position is from the Me (transition metal) position (see Patent Documents 1 and 2).

  The lithium transition metal composite oxide and its carbonate precursor according to the present invention preferably have a 50% particle diameter (D50) of 5 to 18 μm in the particle size distribution measurement. When producing a lithium transition metal composite oxide from a hydroxide precursor, excellent performance cannot be obtained unless the particle size is controlled to be smaller, but by producing from a carbonate precursor, 50% in the particle size distribution measurement. Even when the particle diameter (D50) is about 5 to 18 μm, a positive electrode active material having a large discharge capacity can be obtained.

BET specific surface area of the positive electrode active material according to the present invention, the initial efficiency, in order to obtain a lithium secondary battery having excellent high rate discharge performance, preferably at least ^{1m 2 / g, 2~7m 2 /} g is more preferable.
Further, the tap density is preferably 1.25 g / cc or more, and more preferably 1.7 g / cc or more in order to obtain a lithium secondary battery excellent in high rate discharge performance.

  The lithium transition metal composite oxide according to the present invention has a pore region having a pore diameter within a range of up to 60 nm, which indicates the maximum value of the differential pore volume determined by the BJH method from the adsorption isotherm using the nitrogen gas adsorption method ( It is preferable that the pore volume be 0.055 cc / g or more in the pore region (up to 60 nm). Thereby, a lithium secondary battery excellent in initial efficiency and high rate discharge performance can be obtained. In addition, by setting the pore volume to 0.08 cc / g or less, a lithium secondary battery having particularly excellent high rate discharge performance can be obtained. Therefore, the pore volume is 0.055 to 0.08 cc / g. It is preferable that it is g.

(Method for producing positive electrode active material)
Next, a method for producing the active material for a lithium secondary battery of the present invention will be described.
The active material for a lithium secondary battery of the present invention basically includes a raw material containing a metal element (Li, Mn, Co, Ni) constituting the active material according to the composition of the active material (oxide). It can be obtained by preparing and baking this. However, with respect to the amount of the Li raw material, it is preferable to add an excess of about 1 to 5% in view of the disappearance of a part of the Li raw material during firing.
In producing an oxide having a desired composition, a so-called “solid phase method” in which salts of Li, Co, Ni, and Mn are mixed and fired, or Co, Ni, and Mn were previously present in one particle. A “coprecipitation method” is known in which a coprecipitation precursor is prepared, and a Li salt is mixed and fired therein. In the synthesis process by the “solid phase method”, especially Mn is difficult to uniformly dissolve in Co and Ni, so it is difficult to obtain a sample in which each element is uniformly distributed in one particle. In literatures and the like, many attempts have been made to dissolve Mn in a part of Ni or Co (LiNi _1-x Mn _x O ₂ etc.) by solid phase method, but the “coprecipitation method” is selected. It is easier to obtain a homogeneous phase at the atomic level. Therefore, the “coprecipitation method” is employed in the examples described later.

When preparing a coprecipitation precursor, Mn is easily oxidized among Co, Ni and Mn, and it is not easy to prepare a coprecipitation precursor in which Co, Ni and Mn are uniformly distributed in a divalent state. Uniform mixing at the atomic level of Co, Ni and Mn tends to be insufficient. In particular, in the composition range of the present invention, since the Mn ratio is higher than the Co and Ni ratios, it is particularly important to remove dissolved oxygen in the aqueous solution. Examples of the method for removing dissolved oxygen include a method of bubbling a gas not containing oxygen. The gas not containing oxygen is not limited, but nitrogen gas, argon gas, carbon dioxide (CO ₂ ), or the like can be used. Among these, when preparing a coprecipitated carbonate precursor as in the examples described later, it is preferable to employ carbon dioxide as a gas not containing oxygen because an environment in which carbonate is more easily generated is provided. .

  The pH in the step of producing a precursor by co-precipitation of a compound containing Co, Ni and Mn in a solution is not limited, but let's make the co-precipitation precursor as a co-precipitation carbonate precursor. In this case, it can be set to 7.5 to 11. In order to increase the tap density, it is preferable to control the pH. By setting the pH to 9.4 or less, the tap density can be set to 1.25 g / cc or more, and the high rate discharge performance can be improved. Furthermore, since the particle growth rate can be accelerated by adjusting the pH to 8.0 or less, the stirring continuation time after the raw material aqueous solution dropping is completed can be shortened.

  The coprecipitation precursor is preferably a compound in which Mn, Ni, and Co are uniformly mixed. In the present invention, in order to obtain an active material for a lithium secondary battery having a large discharge capacity, the coprecipitation precursor is preferably a carbonate. In addition, a precursor having a larger bulk density can be produced by using a crystallization reaction using a complexing agent. At that time, a higher density active material can be obtained by mixing and firing with a Li source, so that the energy density per electrode area can be improved.

  The raw material of the coprecipitation precursor is manganese oxide, manganese carbonate, manganese sulfate, manganese nitrate, manganese acetate, etc. as the Mn compound, and nickel hydroxide, nickel carbonate, nickel sulfate, nickel nitrate, nickel acetate as the Ni compound. As examples of the Co compound, cobalt sulfate, cobalt nitrate, cobalt acetate, and the like can be given as examples.

  In the present invention, a reaction crystallization method is employed in which a raw material aqueous solution of the coprecipitation precursor is supplied dropwise to a reaction tank maintaining alkalinity to obtain a coprecipitation carbonate precursor. Here, lithium compounds, sodium compounds, potassium compounds, and the like can be used as the neutralizing agent, but it is preferable to use sodium carbonate, sodium carbonate and lithium carbonate, or a mixture of sodium carbonate and potassium carbonate. In order to leave Na at least 1000 ppm, Na / Li, which is the molar ratio of sodium carbonate to lithium carbonate, or Na / K, which is the molar ratio of sodium carbonate to potassium carbonate, is 1/1 [M] or more. Is preferred. By setting Na / Li or Na / K to 1/1 [M] or more, it is possible to reduce the possibility that Na will be excessively removed in the subsequent washing step and become less than 1000 ppm.

  The dropping speed of the raw material aqueous solution greatly affects the uniformity of element distribution in one particle of the coprecipitation precursor to be generated. In particular, Mn is difficult to form a uniform element distribution with Co and Ni, so care must be taken. The preferred dropping rate is influenced by the reaction vessel size, stirring conditions, pH, reaction temperature, etc., but is preferably 30 ml / min or less. In order to improve the discharge capacity, the dropping rate is more preferably 10 ml / min or less, and most preferably 5 ml / min or less.

  In addition, when a complexing agent is present in the reaction tank and a certain convection condition is applied, the particle rotation and revolution in the stirring tank are promoted by continuing the stirring after the dropwise addition of the raw material aqueous solution. In this process, the particles grow concentrically in stages while colliding with each other. That is, the coprecipitation precursor undergoes a reaction in two stages: a metal complex formation reaction when the raw material aqueous solution is dropped into the reaction tank, and a precipitation formation reaction that occurs while the metal complex is retained in the reaction tank. It is formed. Therefore, a coprecipitation precursor having a target particle size can be obtained by appropriately selecting a time for continuing stirring after the dropping of the raw material aqueous solution.

  The preferable stirring duration after completion of dropping of the raw material aqueous solution is influenced by the size of the reaction vessel, stirring conditions, pH, reaction temperature, etc., but 0.5 h or more is required to grow the particles as uniform spherical particles. Preferably, 1 h or more is more preferable. Moreover, in order to reduce the possibility that the output performance in the low SOC region of the battery is not sufficient due to the particle size becoming too large, it is preferably 30 h or less, more preferably 25 h or less, and most preferably 20 h or less.

  In addition, the preferable stirring duration for setting D50, which is a particle diameter at which the cumulative volume in the particle size distribution of the secondary particles of the carbonate precursor and the lithium transition metal composite oxide is 50%, to 5 to 18 μm is controlled by pH It depends on. For example, when the pH is controlled to 7.5 to 8.2, the stirring duration is preferably 1 to 15 h, and when the pH is controlled to 8.3 to 9.4, the stirring duration is 3 to 3. 20h is preferred.

  When the carbonate precursor particles are prepared using a sodium compound such as sodium carbonate as a neutralizing agent, sodium ions adhering to the particles are washed away in the subsequent washing step. It is preferable to wash and remove under conditions such that Na remains at 1000 ppm or more. For example, when the produced carbonate precursor is removed by suction filtration, a condition that the number of washings with 200 ml of ion-exchanged water is 5 times can be employed.

The carbonate precursor is preferably dried at 80 ° C. to less than 100 ° C. in an air atmosphere under normal pressure. By drying at 100 ° C. or higher, more water can be removed in a short time, but by drying at 80 ° C. for a long time, an active material having more excellent electrode characteristics can be obtained. Although the reason is not necessarily clear, since the carbonate precursor is a porous body having a specific surface area of 50 to 100 m ² / g, it has a structure that easily adsorbs moisture. Therefore, by drying at a low temperature, a state in which a certain amount of adsorbed water remains in the pores in the state of the precursor is removed from the pores in the firing step of mixing with the Li salt and firing. The invention may be because molten Li can enter the pores so as to replace the adsorbed water, and thereby, an active material having a more uniform composition can be obtained compared with the case of drying at 100 ° C. Have guessed. In addition, although the carbonate precursor obtained by drying at 100 ° C. exhibits a black brown color, the carbonate precursor obtained by drying at 80 ° C. exhibits a skin color, so depending on the color of the precursor Can be distinguished.

  The active material for a lithium secondary battery of the present invention can be suitably prepared by mixing the carbonate precursor and the Li compound and then heat-treating them. As a Li compound, it can manufacture suitably by using lithium hydroxide, lithium carbonate, lithium nitrate, lithium acetate, etc. However, with respect to the amount of the Li compound, it is preferable to add an excess of about 1 to 5% in view of the disappearance of a part of the Li compound during firing.

  In the present invention, in order to make the content of Na in the lithium transition metal composite oxide 1000 ppm or more, even if Na contained in the carbonate precursor is 1000 ppm or less, the Na compound is added together with the Li compound in the firing step. By mixing with the carbonate precursor, the amount of Na contained in the active material can be 1000 ppm or more. As the Na compound, sodium carbonate is preferable.

The firing temperature affects the reversible capacity of the active material.
When the firing temperature is too high, the obtained active material collapses with an oxygen releasing reaction, and in addition to the hexagonal crystal of the main phase, the monoclinic Li [Li _1/3 Mn _2/3 ] O ₂ type is obtained. The defined phase tends to be observed as a phase separation rather than as a solid solution phase. If too many such phase separations are contained, it is not preferable because it leads to a reduction in the reversible capacity of the active material. In such materials, impurity peaks are observed around 35 ° and 45 ° on the X-ray diffraction pattern. Therefore, the firing temperature is preferably less than the temperature at which the oxygen release reaction of the active material affects. The oxygen release temperature of the active material is approximately 1000 ° C. or higher in the composition range according to the present invention. However, there is a slight difference in the oxygen release temperature depending on the composition of the active material. It is preferable to keep it. In particular, it is confirmed that the oxygen release temperature of the precursor shifts to the lower temperature side as the amount of Co contained in the sample increases. As a method for confirming the oxygen release temperature of the active material, a mixture of a coprecipitation precursor and a lithium compound may be subjected to thermogravimetric analysis (DTA-TG measurement) in order to simulate the firing reaction process. In this method, the platinum used in the sample chamber of the measuring instrument may be corroded by the Li component volatilized, and the instrument may be damaged. Therefore, a composition in which crystallization is advanced to some extent by adopting a firing temperature of about 500 ° C. in advance. Goods should be subjected to thermogravimetric analysis.

On the other hand, if the firing temperature is too low, crystallization does not proceed sufficiently and the electrode characteristics tend to deteriorate. In the present invention, the firing temperature is preferably at least 800 ° C. or higher. By sufficiently crystallizing, the resistance of the crystal grain boundary can be reduced and smooth lithium ion transport can be promoted.
In addition, the inventors have analyzed the half width of the diffraction peak of the active material of the present invention in detail, and in the sample synthesized at a temperature up to 750 ° C., strain remains in the lattice, and at a temperature higher than that, It was confirmed that almost all strains could be removed by synthesis. The crystallite size was increased in proportion to the increase in the synthesis temperature. Therefore, even in the composition of the active material of the present invention, a favorable discharge capacity can be obtained by aiming at a particle having almost no lattice distortion in the system and having a sufficiently grown crystallite size. Specifically, it is preferable to employ a synthesis temperature (firing temperature) and a Li / Me ratio composition in which the strain amount affecting the lattice constant is 2% or less and the crystallite size is grown to 50 nm or more. all right. Although changes due to expansion and contraction are observed by charging and discharging by molding these as electrodes, it is preferable as an effect that the crystallite size is maintained at 30 nm or more in the charging and discharging process.

As described above, the firing temperature is related to the oxygen release temperature of the active material. However, even if the firing temperature does not reach the firing temperature at which oxygen is released from the active material, the crystal due to large growth of primary particles above 900 ° C. A phenomenological phenomenon is observed. This can be confirmed by observing the fired active material with a scanning electron microscope (SEM). The active material synthesized through a synthesis temperature exceeding 900 ° C. has primary particles grown to 0.5 μm or more, which is disadvantageous for Li ⁺ movement in the active material during the charge / discharge reaction, and has a high rate discharge performance. descend. The size of the primary particles is preferably less than 0.5 μm, and more preferably 0.3 μm or less. Further, at a synthesis temperature exceeding 900 ° C., the pore volume of the active material is less than 0.055 cc / g in the pore region up to 60 nm, and the initial efficiency and high rate discharge performance are lowered.

  Therefore, in order to improve initial efficiency and high rate discharge performance, when the lithium transition metal composite oxide according to the present invention of 1.2 <molar ratio Li / Me <1.6 is used as the positive electrode active material, the firing temperature is It is preferable to set it as 800-900 degreeC.

  The active material after firing is desirably an average particle size of 100 μm or less, and particularly desirably 10 μm or less for the purpose of improving high output characteristics. In order to obtain the powder in a predetermined shape, a pulverizer or a classifier is used. For example, a mortar, a ball mill, a sand mill, a vibrating ball mill, a planetary ball mill, a jet mill, a counter jet mill, a swirling air flow type jet mill or a sieve is used. At the time of pulverization, wet pulverization in the presence of water or an organic solvent such as hexane may be used. There is no particular limitation on the classification method, and a sieve, an air classifier, or the like is used as needed for both dry and wet methods.

(Cerium-added acid treatment)
In the present invention, the method for applying cerium is not limited. In the examples described later, a method was adopted in which Ce was added by performing an acid treatment on the lithium transition metal composite oxide not containing cerium prepared as described above.
This acid treatment can be performed, for example, by introducing a lithium transition metal composite oxide into an acid solution obtained by adding sulfuric acid to an aqueous solution of a cerium compound such as cerium sulfate. As the acid, sulfuric acid or phosphoric acid is preferable. It is preferable to avoid the use of hydrochloric acid as the acid, and this can reduce the risk of the crystal structure before the acid treatment being broken. Further, although boric acid may be used as the acid, it is preferable to avoid the use of a weak acid such as boric acid, so that the amount of lithium desorbed from the active material is too small to sufficiently improve the initial efficiency. It is possible to reduce the possibility of being not played.
It is preferable not to make the acid treatment time too long, and it is preferable not to make the pH value too small, thereby reducing the possibility that the amount of lithium desorbed from the active material becomes too large and the reversible capacity is not sufficient. it can. It is preferable not to make the acid treatment time too short, and it is preferable not to make the pH value too large, and thereby the amount of lithium released from the active material is so small that the effect of the initial efficiency may not be sufficiently achieved. Can be reduced. From the above viewpoint, the acid treatment time is preferably from 30 seconds to 10800 seconds, and the pH of the acid solution is preferably from 0.5 to 3.
After the acid treatment, the lithium transition metal composite oxide containing Ce on the filter paper is recovered by suction filtration or the like, and then dried and heat-treated to obtain an active material to which Ce is added.

  It is preferable to perform heat treatment after the acid treatment. The heat treatment temperature is preferably 300 to 600 ° C. The holding time is preferably 2 to 10 hours. The heat treatment stabilizes the support of Ce on the active material surface, prevents local Mn elution associated with charge and discharge, and improves the discharge capacity retention rate. By not setting the treatment temperature too high, the possibility that the crystal structure changes and the characteristics of the positive electrode active material are deteriorated can be reduced. Moreover, the possibility that the loading of Ce will not be sufficient can be reduced by not setting the treatment temperature too low. Even if the holding time is too long, the effect of the present invention is not affected, but 10 h or less is preferable in order to avoid wasteful consumption of energy and decrease in productivity.

The amount of Ce contained in the lithium transition metal composite oxide according to the present invention can be adjusted by the concentration of cerium ions in the acid solution. Here, the concentration of cerium ions is preferably 0.0005 to 0.025M. A larger amount of Ce contained in the lithium transition metal composite oxide according to the present invention is preferable because the initial efficiency is improved. In addition, it is preferable not to increase the amount of Ce excessively, because Ce adhering to the active material surface can inhibit the insertion and release of lithium ions and reduce the active material high rate discharge performance.
In addition, the addition amount of Ce can be calculated | required by the ICP analysis about the active material which concerns on this invention. In addition, the fact that the active material according to the present invention contains Ce is estimated by observing the strongest peak attributed to CeO ₂ in the vicinity of 2θ = 29 ± 1 ° in the XRD pattern using the CuKα radiation source. can do. Further, when the active material according to the present invention is fired at 1000 ° C. in air, the crystallinity of CeO ₂ is increased. Therefore, in addition to the diffraction peak around 2θ = 29 ± 1 °, 2θ = around 33 ± 1 °, 47 Since diffraction peaks near ± 1 ° and 56 ± 1 ° are manifested, the presence of CeO ₂ can be estimated more reliably.

(Negative electrode active material)
The negative electrode material is not limited, and any negative electrode material may be selected as long as it can release or occlude lithium ions. For example, titanium-based materials such as lithium titanate having a spinel crystal structure represented by Li [Li _1/3 Ti _5/3 ] O ₄ , alloy-based materials such as Si, Sb, and Sn-based lithium metal, lithium alloys (Lithium metal-containing alloys such as lithium-silicon, lithium-aluminum, lithium-lead, lithium-tin, lithium-aluminum-tin, lithium-gallium, and wood alloys), lithium composite oxide (lithium-titanium), silicon oxide In addition, an alloy capable of inserting and extracting lithium, a carbon material (for example, graphite, hard carbon, low-temperature fired carbon, amorphous carbon, etc.) can be used.

(Positive electrode / Negative electrode)
The positive electrode active material and the negative electrode material, which are the main components of the positive electrode and the negative electrode, have been described in detail above. In addition to the main components, the positive electrode and the negative electrode include a conductive agent, a binder, a thickener, and a filler. Etc. may be contained as other constituents.

  The conductive agent is not limited as long as it is an electron conductive material that does not adversely affect the battery performance. Usually, natural graphite (such as scaly graphite, scaly graphite, earthy graphite), artificial graphite, carbon black, acetylene black, Conductive materials such as ketjen black, carbon whisker, carbon fiber, metal (copper, nickel, aluminum, silver, gold, etc.) powder, metal fiber, and conductive ceramic material can be included as one kind or a mixture thereof. .

  Among these, as the conductive agent, acetylene black is desirable from the viewpoints of electron conductivity and coatability. The addition amount of the conductive agent is preferably 0.1% by weight to 50% by weight, and particularly preferably 0.5% by weight to 30% by weight with respect to the total weight of the positive electrode or the negative electrode. In particular, it is desirable to use acetylene black by pulverizing into ultrafine particles of 0.1 to 0.5 μm because the required carbon amount can be reduced. These mixing methods are physical mixing, and the ideal is uniform mixing. Therefore, powder mixers such as V-type mixers, S-type mixers, crackers, ball mills, and planetary ball mills can be mixed dry or wet.

  The binder is usually a thermoplastic resin such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butadiene. Polymers having rubber elasticity such as rubber (SBR) and fluororubber can be used as one kind or a mixture of two or more kinds. The addition amount of the binder is preferably 1 to 50% by weight, particularly preferably 2 to 30% by weight, based on the total weight of the positive electrode or the negative electrode.

  As the filler, any material that does not adversely affect the battery performance may be used. Usually, olefin polymers such as polypropylene and polyethylene, amorphous silica, alumina, zeolite, glass, carbon and the like are used. The addition amount of the filler is preferably 30% by weight or less with respect to the total weight of the positive electrode or the negative electrode.

  The positive electrode and the negative electrode are prepared by mixing the main constituents (positive electrode active material in the positive electrode, negative electrode material in the negative electrode) and other materials into a mixture and mixing with an organic solvent such as N-methylpyrrolidone or toluene or water. After that, the obtained mixed solution is applied on a current collector such as an aluminum foil, or is pressure-bonded and heat-treated at a temperature of about 50 ° C. to 250 ° C. for about 2 hours. About the application method, for example, it is desirable to apply to any thickness and any shape using means such as roller coating such as applicator roll, screen coating, doctor blade method, spin coating, bar coater, etc. It is not limited.

(Nonaqueous electrolyte)
The nonaqueous electrolyte used for the lithium secondary battery according to the present invention is not limited, and those generally proposed for use in lithium batteries and the like can be used. Examples of the nonaqueous solvent used for the nonaqueous electrolyte include cyclic carbonates such as propylene carbonate, ethylene carbonate, butylene carbonate, chloroethylene carbonate, and vinylene carbonate; cyclic esters such as γ-butyrolactone and γ-valerolactone; dimethyl carbonate, Chain carbonates such as diethyl carbonate and ethyl methyl carbonate; chain esters such as methyl formate, methyl acetate and methyl butyrate; tetrahydrofuran or derivatives thereof; 1,3-dioxane, 1,4-dioxane, 1,2-dimethoxy Ethers such as ethane, 1,4-dibutoxyethane and methyldiglyme; Nitriles such as acetonitrile and benzonitrile; Dioxolane or derivatives thereof; Ethylene sulfide, sulfolane, sultone or derivatives thereof Examples thereof include a conductor alone or a mixture of two or more thereof, but are not limited thereto.

Examples of the electrolyte salt used for the nonaqueous electrolyte include LiClO ₄ , LiBF ₄ , LiAsF ₆ , LiPF ₆ , LiSCN, LiBr, LiI, Li ₂ SO ₄ , Li ₂ B ₁₀ Cl ₁₀ , NaClO ₄ , NaI, NaSCN, NaBr. , KClO ₄ , KSCN, and other inorganic ion salts containing one of lithium (Li), sodium (Na), or potassium (K), LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiC (CF ₃ SO ₂ ) ₃ , LiC (C ₂ F ₅ SO ₂ ) ₃ , (CH ₃ ) ₄ NBF ₄ , ( CH ₃ ) ₄ NBr, (C ₂ H ₅ ) ₄ NClO ₄ , (C ₂ H ₅ ) ₄ NI, (C ₃ H ₇ ) ₄ NBr, (n-C ₄ H ₉ ) ₄ NClO ₄ , (n-C _{4 H 9) 4 NI, (} C 2 H 5) 4 N-mal _{ate, (C 2 H 5)} 4 N-benzoate, (C 2 H 5) 4 N-phthalate, lithium stearyl sulfonate, lithium octyl sulfonate, organic ion salts of lithium dodecyl benzene sulfonate, and the like. These These ionic compounds can be used alone or in admixture of two or more.

Furthermore, by mixing and using LiPF ₆ or LiBF ₄ and a lithium salt having a perfluoroalkyl group such as LiN (C ₂ F ₅ SO ₂ ) ₂ , the viscosity of the electrolyte can be further reduced. The low temperature characteristics can be further improved, and self-discharge can be suppressed, which is more desirable.

  Moreover, you may use normal temperature molten salt and an ionic liquid as a nonaqueous electrolyte.

  The concentration of the electrolyte salt in the non-aqueous electrolyte is preferably 0.1 mol / l to 5 mol / l, more preferably 0.5 mol / l to 2 in order to reliably obtain a non-aqueous electrolyte battery having high battery characteristics. .5 mol / l.

(Separator)
As the separator, it is preferable to use a porous film or a non-woven fabric exhibiting excellent high rate discharge performance alone or in combination. Examples of the material constituting the separator for a nonaqueous electrolyte battery include polyolefin resins typified by polyethylene and polypropylene, polyester resins typified by polyethylene terephthalate and polybutylene terephthalate, polyvinylidene fluoride, and vinylidene fluoride-hexa. Fluoropropylene copolymer, vinylidene fluoride-perfluorovinyl ether copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-trifluoroethylene copolymer, vinylidene fluoride-fluoroethylene copolymer, fluorine Vinylidene fluoride-hexafluoroacetone copolymer, vinylidene fluoride-ethylene copolymer, vinylidene fluoride-propylene copolymer, vinylidene fluoride-trifluoropropylene copolymer, vinylidene fluoride - tetrafluoroethylene - hexafluoropropylene copolymer, vinylidene fluoride - ethylene - can be mentioned tetrafluoroethylene copolymer.

  The porosity of the separator is preferably 98% by volume or less from the viewpoint of strength. Further, the porosity is preferably 20% by volume or more from the viewpoint of charge / discharge characteristics.

  The separator may be a polymer gel composed of a polymer such as acrylonitrile, ethylene oxide, propylene oxide, methyl methacrylate, vinyl acetate, vinyl pyrrolidone, polyvinylidene fluoride, and an electrolyte. Use of the non-aqueous electrolyte in the gel state as described above is preferable in that it has an effect of preventing leakage.

  Furthermore, it is desirable that the separator be used in combination with the above-described porous film, non-woven fabric, or the like and a polymer gel because the liquid retention of the electrolyte is improved. That is, by forming a film in which the surface of the polyethylene microporous membrane and the microporous wall are coated with a solvophilic polymer having a thickness of several μm or less, and holding the electrolyte in the micropores of the film, Gels.

  Examples of the solvophilic polymer include polyvinylidene fluoride, an acrylate monomer having an ethylene oxide group or an ester group, an epoxy monomer, a polymer having a monomer having an isocyanate group, and the like crosslinked. The monomer can be subjected to a crosslinking reaction using a radical initiator in combination with heating or ultraviolet rays (UV), or using an actinic ray such as an electron beam (EB).

(Configuration of lithium secondary battery)
The configuration of the lithium secondary battery of the present invention is not particularly limited, and examples thereof include a cylindrical battery having a positive electrode, a negative electrode, and a roll separator, a square battery, and a flat battery. FIG. 1 shows an example of a square battery. An electrode group 2 composed of a positive electrode and a negative electrode wound with a separator interposed therebetween is housed in a rectangular battery container 3. A positive electrode terminal 4 is connected via a positive electrode lead 4 ′, and a negative electrode terminal 5 is connected via a negative electrode lead 5 ′. It is led out of the battery container.

(Configuration of power storage device)
The lithium secondary battery of the present invention is configured by assembling a plurality of lithium secondary batteries, particularly when used as a power source for an automobile such as an electric vehicle (EV), a hybrid vehicle (HEV), and a plug-in hybrid vehicle (PHEV). It can be mounted as a power storage device (battery module).
FIG. 2 shows an example of a power storage device 30 in which the power storage units 20 in which the lithium secondary batteries 1 are assembled are further assembled.

(Charge potential)
Both the conventional positive electrode active material and the active material of the present invention can be charged / discharged when the positive electrode potential reaches around 4.5 V (vs. Li ^/ Li ⁺ ). However, depending on the type of nonaqueous electrolyte used, if the positive electrode potential during charging is too high, the nonaqueous electrolyte may be oxidized and decomposed, resulting in a decrease in battery performance. Therefore, in use, a lithium secondary battery capable of obtaining a sufficient discharge capacity even when a charging method is adopted in which the maximum potential of the positive electrode during charging is 4.3 V (vs. Li ^/ Li ⁺ ) or less. May be required. When the active material of the present invention is used, for example, 4.4 V (vs. Li ^/ Li) such that the maximum potential of the positive electrode during charging is lower than 4.5 V (vs. Li ^/ Li ⁺ ) during use. ⁺ ) Or less and 4.3 V (vs. Li ^/ Li ⁺ ) or less, even if a charging method is adopted, it is possible to take out a discharge electric quantity exceeding the capacity of the conventional positive electrode active material of about 200 mAh / g or more. Is possible.

Example 1
<Synthesis of lithium transition metal composite oxide>
Cobalt sulfate heptahydrate (14.08 g), nickel sulfate hexahydrate (21.00 g) and manganese sulfate pentahydrate (65.27 g) were weighed, and all of these were dissolved in 200 ml of ion-exchanged water, and Co: Ni: Mn A 2.0 M aqueous sulfate solution having a molar ratio of 12.50: 19.94: 67.56 was prepared. On the other hand, 750 ml of ion exchange water was poured into a ₂ L reaction tank, and CO ₂ gas was bubbled for 30 minutes to dissolve CO _{2 in the} ion exchange water. The temperature of the reaction vessel was set to 50 ° C. (± 2 ° C.), and the aqueous sulfate solution was stirred at a rate of 3 ml / min while stirring the inside of the reaction vessel at a rotational speed of 700 rpm using a paddle blade equipped with a stirring motor. It was dripped. Here, during the period from the start to the end of the dropwise addition, an aqueous solution containing 2.0 M sodium carbonate and 0.4 M ammonia is appropriately dropped, so that the pH in the reaction tank is always 7.9 (± 0. 05). After completion of the dropping, stirring in the reaction vessel was further continued for 5 hours. After the stirring was stopped, the mixture was allowed to stand for 12 hours or more.
Next, using a suction filtration device, the coprecipitated carbonate particles produced in the reaction vessel are separated, and sodium ions adhering to the particles are washed away using ion-exchanged water, and an electric furnace is used. Then, it was dried at 100 ° C. for 20 hours in an air atmosphere under normal pressure. Then, in order to arrange | equalize a particle size, it grind | pulverized for several minutes with the smoked automatic mortar. In this way, a coprecipitated carbonate precursor was produced.
Lithium carbonate is added to 2.278 g of the coprecipitated carbonate precursor so that the molar ratio of Li: (Co, Ni, Mn) is 1.44 and mixed well using a smoked automatic mortar. The body was prepared. Using a pellet molding machine, molding was performed at a pressure of 6 MPa to obtain pellets having a diameter of 25 mm. The amount of the mixed powder subjected to pellet molding was determined by conversion so that the mass of the assumed final product was 2 g. One pellet was placed on an alumina boat having a total length of about 100 mm, placed in a box-type electric furnace (model number: AMF20), and heated in an air atmosphere at normal pressure to a temperature of 900 ° C. over 10 hours. And calcined for 10 hours at the temperature after the temperature elevation. The box-type electric furnace has internal dimensions of 10 cm in length, 20 cm in width, and 30 cm in depth, and heating wires are inserted at intervals of 20 cm in the width direction. After firing, the heater was turned off and allowed to cool naturally with the alumina boat placed in the furnace. As a result, the temperature of the furnace decreases to about 200 ° C. after 5 hours, but the subsequent temperature decrease rate is somewhat moderate. After the passage of day and night, it was confirmed that the furnace temperature was 100 ° C. or lower, and then the pellets were taken out and pulverized for several minutes in a smoked automatic mortar in order to make the particle diameter uniform. In this manner, a starting lithium transition metal composite oxide Li _1.18 Co _0.10 Ni _0.17 Mn _0.55 O ₂ containing 2100 ppm Na and having a D50 of 13 μm was prepared.

<Cerium-added acid treatment>
To a 300 ml Erlenmeyer flask, 0.0406 g of cerium sulfate tetrahydrate and ion exchange water were added and dissolved to prepare a 0.0005 M aqueous solution of cerium sulfate. To the above aqueous solution, 98 wt% sulfuric acid was added and mixed until the pH reached 1.6 to prepare an acid solution. Since the amount of sulfuric acid added for adjusting the pH is very small, 0.3 ml or less, the cerium ion concentration in the acid solution is equal to the cerium ion concentration in the aqueous solution within the range of significant figures. While the acid solution was being stirred at 25 ° C. and 400 rpm using a stirrer, 5.0 g of the lithium transition metal composite oxide was added. The lithium transition metal composite oxide was collected on the filter paper by suction filtration after 30 seconds from the introduction of the lithium transition metal composite oxide.
This lithium transition metal composite oxide is dried in air at atmospheric pressure and 80 ° C. for 16 to 18 hours using a dryer (manufactured by Yamato Kagaku Co., Ltd.) to obtain a lithium transition metal composite oxide treated with a cerium sulfate aqueous solution. It was.

  Add 5.0 g of the lithium transition metal composite oxide treated with the above acid solution to the lid of the alumina crucible and place it in a box-type electric furnace (model number: AMF20). In the air, the heating rate is 5 ° C./min. A lithium transition metal composite oxide containing Ce was prepared by heat treatment under conditions of a temperature of 400 ° C. and a holding time of 4 hours.

(Examples 2 to 5)
Examples 2 to 5 are the same as Example 1 except that 0.001M, 0.005M, 0.01M, and 0.025M aqueous solutions were used as the cerium sulfate aqueous solution used in preparing the acid solution. A lithium transition metal composite oxide according to the present invention was prepared.

(Comparative Example 1)
A lithium transition metal composite oxide according to Comparative Example 1 was produced in the same manner as in Example 1 except that the acid treatment was not performed.

(Comparative Example 2)
A lithium transition metal composite oxide according to Comparative Example 2 was prepared in the same manner as in Example 1 except that a sulfuric acid aqueous solution having a hydrogen ion concentration of 0.05 M was used as the acid solution.

(Comparative Examples 3 to 8)
Comparative Example as in Example 1 except that 0.0005M, 0.001M, 0.005M, 0.01M, 0.025M and 0.05M tin sulfate aqueous solutions were used instead of the cerium sulfate aqueous solution, respectively. Lithium transition metal composite oxides according to 3 to 8 were produced.

(Comparative Example 9)
A lithium transition metal composite oxide according to Comparative Example 9 was produced in the same manner as in Example 1 except that a 0.1 M aqueous iron sulfate solution was used instead of the cerium sulfate aqueous solution.

(Comparative Example 10)
A lithium transition metal composite oxide according to Comparative Example 10 was produced in the same manner as in Example 1 except that a 0.01 M zirconium sulfate aqueous solution was used instead of the cerium sulfate aqueous solution.

<Measurement of half width>
The lithium transition metal composite oxides according to all Examples and Comparative Examples were subjected to X-ray diffraction measurement according to the following conditions and procedures, and the half width was determined. Powder X-ray diffraction measurement was performed using an X-ray diffractometer (manufactured by Rigaku, model name: MiniFlex II). The radiation source was CuKα, and the acceleration voltage and current were 30 kV and 15 mA, respectively. The sampling width was 0.01 deg, the scanning time was 14 minutes (scanning speed was 5.0), the divergence slit width was 0.625 deg, the light receiving slit width was open, and the scattering slit was 8.0 mm. For the obtained X-ray diffraction data, the half width of the diffraction peak at 2θ = 44 ° ± 1 ° on the X-ray diffraction diagram was determined using “PDXL” which is software attached to the X-ray diffractometer.

<Metal content confirmation test>
The amount of metal contained in the positive electrode active material after acid treatment was quantified by ICP emission spectroscopic analysis. In addition, the positive electrode active material powder which concerns on Examples 1-5 was disperse | distributed in water, and ultrasonic vibration was given for 2 minutes using the ultrasonic cleaner. When the particle size distribution measurement was performed using this dispersion as a sample, particles corresponding to the particle diameter of CeO ₂ considered to be precipitated were not observed. Therefore, it was found that the Ce compound was stably supported on the surface of the positive electrode active material.

<Production of lithium secondary battery>
Using the lithium transition metal composite oxides according to all of the examples and comparative examples as positive electrode active materials for lithium secondary batteries, lithium secondary batteries were fabricated according to the following procedure.

  Using N-methylpyrrolidone as a dispersion medium, an active material, acetylene black (AB), and polyvinylidene fluoride (PVdF) were kneaded and dispersed at a mass ratio of 90: 4: 6. The coating paste was applied to one side of an aluminum foil current collector having a thickness of 20 μm to produce a positive electrode plate. In addition, the mass and coating thickness of the active material applied per fixed area were standardized so that the test conditions were the same among the lithium secondary batteries according to all the examples and comparative examples.

  For the purpose of accurately observing the single behavior of the positive electrode, metallic lithium was used in close contact with the nickel foil current collector for the counter electrode, that is, the negative electrode. Here, a sufficient amount of metallic lithium was disposed on the negative electrode so that the capacity of the lithium secondary battery was not limited by the negative electrode.

As an electrolytic solution, LiPF ₆ was dissolved in a mixed solvent in which ethylene carbonate (EC) / ethyl methyl carbonate (EMC) / dimethyl carbonate (DMC) had a volume ratio of 6: 7: 7 so that the concentration was 1 mol / l. The solution was used. As the separator, a polypropylene microporous film whose surface was modified with polyacrylate was used. A metal resin composite film made of polyethylene terephthalate (15 μm) / aluminum foil (50 μm) / metal-adhesive polypropylene film (50 μm) is used for the outer package, and the electrodes are exposed so that the open ends of the positive and negative terminals are exposed to the outside And a hermetic seal in which the inner surfaces of the metal resin composite film face each other are hermetically sealed except for a portion serving as a liquid injection hole. After injecting the electrolytic solution, the injection hole was sealed to prepare a lithium secondary battery.

<Evaluation of lithium secondary battery>
(Initial efficiency)
The lithium secondary battery produced by the above procedure was subjected to an initial charge / discharge process at 25 ° C. Charging was performed at a constant current and a constant voltage with a current of 0.1 CA and a voltage of 4.6 V, and the charge termination condition was the time when the current value was attenuated to 1/5. The discharge was a constant current discharge with a current of 0.1 CA and a final voltage of 2.0 V. This charge / discharge was performed for one cycle. Here, a pause process of 10 minutes was provided after charging and discharging, respectively. A value (%) obtained by dividing the discharge capacity in the charge / discharge test by the amount of charge was recorded as the initial charge / discharge efficiency (“initial efficiency” in Table 1).

(Discharge capacity maintenance rate)
Next, a 30-cycle charge / discharge test was performed. The charging / discharging test was performed at a constant current / constant voltage with a current of 0.2 CA and a voltage of 4.45 V. The charge termination condition was when the current value was attenuated to 1/10. The discharge was a constant current discharge with a current of 0.5 CA and a final voltage of 2.0 V. The ratio (%) of the discharge capacity at the 30th cycle to the discharge capacity at the 1st cycle was recorded as the discharge capacity retention rate.

Lithium transition metal composite oxides according to Examples and Comparative Examples, and
Table 1 shows the test results of the lithium secondary battery used as the positive electrode active material for the lithium secondary battery. The Ce, Sn, Fe, or Zr ion concentration in the acid solution used for the acid treatment is shown in Table 1 as “metal ion concentration (M)”.

According to Table 1, Comparative Example 2, in which the “lithium-excess type” active material was subjected to sulfuric acid treatment, had improved initial charge / discharge efficiency than Comparative Example 1, in which acid treatment was not performed, but the discharge capacity retention rate was decreased. doing.
Even when Sn addition (Comparative Examples 3 to 8) or Fe addition (Comparative Example 9) is performed on the active material by acid treatment with addition of metal ions, the discharge capacity maintenance rate of Comparative Example 2 subjected to sulfuric acid treatment is exceeded. There is nothing. In Comparative Example 10 in which Zr was added, the discharge capacity retention rate was improved as compared with Comparative Example 2 in which sulfuric acid treatment was performed, but the initial efficiency was lowered.

On the other hand, Examples 1 to 5 in which Ce ions were added and subjected to acid treatment were comparative examples 1 in which initial efficiency and discharge capacity maintenance rate were not subjected to acid treatment, and comparative examples in which sulfuric acid treatment was performed. 2 and Comparative Examples 3 to 9 where acid treatment was performed by adding Sn ions or Fe ions. Further, compared with Comparative Example 10 in which Zr was added and acid treatment was performed, the initial efficiency was excellent, although no particular difference was observed in the discharge capacity retention rate.
In Examples 1 to 5, the full width at half maximum of the peak attributed to the (104) plane existing at 2θ = 44 ° ± 1 ° in the X-ray diffraction data is 0.269 to 0.273 °, and is moderate by acid treatment. It can be confirmed that an active material having excellent crystallinity was obtained and the initial efficiency was improved. It was also confirmed that an active material having excellent initial efficiency and a high discharge capacity retention rate can be obtained by containing 0.15 to 3.37% by mass of Ce. It is considered that the elution of Mn from the positive electrode active material was suppressed when the positive electrode active material contained Ce.

The molar ratio Li / Me in the positive electrode active material after the charge / discharge cycle was confirmed by the following procedure. After the charge / discharge test, the battery in the end-discharge state was disassembled, the positive electrode was taken out alone, a cell was assembled using metallic lithium as a counter electrode, and the constant voltage was charged at a current of 0.1 CmA at a charge voltage of 4.3V. Thereafter, constant current discharge was performed at 0.1 CmA up to 2.0 V with a pause of 30 minutes, and a final discharge state was obtained. The positive electrode plate taken out again was washed with dimethyl carbonate (DMC) and vacuum-dried at room temperature for 30 minutes. Positive electrode mixture 50 in which positive electrode active material, AB, and PVdF are mixed from the positive electrode after drying
The mg was peeled off, added to 35 wt% hydrochloric acid, and boiled at 150 ° C. for 10 minutes to dissolve only the positive electrode active material. AB and PVdF were removed by filtering this solution. The obtained filtrate was subjected to ICP emission spectroscopic analysis. As a result, the molar ratio Li / Me was 97% with respect to the molar ratio Li / Me of the lithium transition metal composite oxide after synthesis (before charging and discharging).
Therefore, when Li / Me of the lithium transition metal composite oxide determined by the amount of raw material charged is used for the battery electrode as an active material, the amount of Li changes depending on the charge / discharge state. The amount of Li / Me after synthesis of the positive electrode active material (before charge / discharge) can be estimated by adding 3% to the amount of Li measured through the above.

  Moreover, when the amount of Ce was calculated | required by the ICP emission-spectral-analysis about the positive electrode and negative electrode after a charging / discharging cycle, almost 100% was detected from the positive electrode side. Therefore, it can be seen that Ce does not disappear from the positive electrode active material and exists stably even in the battery after the charge / discharge cycle.

  As described above, the present invention provides a positive electrode active material for a lithium secondary battery that has excellent initial efficiency and a high discharge capacity retention rate, an electrode for a lithium secondary battery containing the positive electrode active material, and lithium including the electrode. Since a secondary battery can be provided, it can be expected to be used as a battery for hybrid vehicles and electric vehicles.

DESCRIPTION OF SYMBOLS 1 Lithium secondary battery 2 Electrode group 3 Battery container 4 Positive electrode terminal 4 'Positive electrode lead 5 Negative electrode terminal 5' Negative electrode lead 20 Power storage unit 30 Power storage device

Claims (4)

A positive electrode active material comprising a lithium transition metal composite oxide having an α-NaFeO ₂ structure,
In the lithium transition metal composite oxide, the transition metal (Me) includes Co, Ni and Mn,
The molar ratio (Li / Me) between Li and the transition metal (Me) is 1 <Li / Me,
Mn and transition metal (Me) molar ratio (Mn / Me) is 0.5 <Mn / Me,
Containing Ce ,
In the X-ray diffraction pattern analysis using a CuKα tube, the half width (FWHM) of the diffraction peak attributed to the (104) plane is 0.269 ≦ FWHM ≦ 0.273, Positive electrode active material.   2. The positive electrode active material according to claim 1, wherein the lithium-transition metal composite oxide has a metal-converted Ce content ratio of 0.15 to 3.37 mass%. An electrode for a lithium secondary battery, comprising the positive electrode active material for a lithium secondary battery according to claim 1 . A lithium secondary battery comprising the electrode for a lithium secondary battery according to claim 3 .

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