JP4228287B2 - Evaluation method of positive electrode active material for lithium secondary battery - Google Patents

Description

【００３８】
図２及び表１から明らかなように、Ｗｉｌｓｏｎ法により算出されたマンガン酸リチウム結晶子サイズと、正極容量とは高い相関性をもつことが分かった。つまり、実際に電池を製造しなくても、マンガン酸リチウムについて結晶子サイズを測定すれば、そのマンガン酸リチウムを正極に用いた電池の電池容量を評価することができることが分かった。
【００３９】
〈試験２〉
試験１における試料１及び２のマンガン酸リチウムを用いた電池について、それぞれ３．０〜４．２Ｖ間で充放電を行うことで劣化させて電池容量を０ｍＡｈとした。その電池を分解して正極を取り出してエタノールで洗浄した後に試験１と同様の方法（Ｘ線回折法及びＷｉｌｓｏｎ法）で結晶子サイズを測定した。
【００４０】
その結果、それぞれの結晶子サイズは１２．０ｎｍ及び１０．０ｎｍであり、図１に当てはめると、実際の電池容量（０ｍＡｈ）が推測できた。つまり、一度電池（正極合材）の状態にしたマンガン酸リチウムであっても本評価方法を適用することで電池容量（言い換えると、劣化状態）を評価することができることが分かった。
【００４１】
【発明の効果】
本発明のリチウム二次電池用正極活物質の評価方法は、結晶子サイズを測定することによって、そのリチウム二次電池用正極活物質を電池に適用した場合の電池容量を評価することが可能となるという効果を有する。
【図面の簡単な説明】
【図１】実施例における回折線をＷｉｌｓｏｎ法に基づいてプロットしたグラフである。
【図２】実施例における電池容量（正極容量）と結晶子サイズとの関係を示したグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for evaluating a positive electrode active material for a lithium secondary battery containing lithium manganate as a main component, and in particular, accurately estimates the positive electrode capacity when applied to a lithium secondary battery without assembling an actual battery. The present invention relates to a method for evaluating a positive electrode active material for a lithium secondary battery.
[0002]
[Prior art]
Along with the downsizing of personal computers, video cameras, mobile phones, etc., lithium secondary batteries with high energy density have been put into practical use and widely used as power sources used in the fields of information related devices and communication devices. On the other hand, in the field of automobiles, the development of electric vehicles is urgently caused by environmental problems and resource problems, and lithium secondary batteries are being studied as a power source for electric vehicles.
[0003]
Lithium secondary batteries have a 4V-class operating voltage. LiMn ₂ O _{4 with} a spinel structure, LiCoO ₂ and LiNiO _{2 with} a layered rock salt structure, and lithium transitions obtained by substituting some of these elements with other elements A positive electrode active material made of a metal composite oxide is well known. Among these lithium transition metal composite oxides, LiMn ₂ O ₄ has advantages such as abundant manganese resources and high stability in the charged state.
[0004]
By the way, these positive electrode active materials made of lithium transition metal composite oxides such as LiMn ₂ O _{4 have} subtle variations in synthesis conditions, such as composition composition of raw materials, heat treatment temperature, atmosphere (oxygen concentration, dew point, and CO ₂ content). The crystal structure changes with the amount) and time. Even if the positive electrode active material having a changed crystal structure has the same composition ratio, the state of insertion and extraction of lithium is greatly different, and the performance when applied to a battery is greatly different. In particular, the battery capacity is fundamental in the battery performance, and it is natural that the manufactured battery has the battery capacity as designed.
[0005]
Therefore, before a battery using the produced positive electrode active material for each manufacturing lot, it is necessary to evaluate whether or not having the proper properties when used in a battery.
[0006]
Conventional positive electrode active methods who evaluate the substance had been produced electrode performance testing of the actual battery such test cells.
[0007]
[Patent Document 1]
Japanese Patent Laid-Open No. 2001-332261
[Problems to be solved by the invention]
However, although the electrode performance test performed after the battery is manufactured is a reliable inspection method, since the battery is actually manufactured, it is considerably complicated. In addition, it is necessary to perform a long-time test in order to perform various characteristic evaluations. In other words, it is difficult to apply to uses such as lot inspection.
[0009]
In view of the above problems, the present invention particularly for positive electrode active material mainly composed of lithium manganate, that that provides a means of evaluation rapidly and conveniently the battery capacity when applied (positive electrode capacity) in the lithium secondary battery Is a problem to be solved.
[0010]
[Means for Solving the Problems]
In order to solve the above problems, the present inventor has intensively studied and completed the following invention. That is, the evaluation method of the positive electrode active material for a lithium secondary battery of the present invention is an evaluation method of a positive electrode active material for a lithium secondary battery composed of a lithium transition metal composite oxide mainly composed of manganese as a transition metal,
A crystallite size measurement step of measuring a crystallite size of the positive electrode active material for a lithium secondary battery based on an X-ray diffraction method;
Possess an estimation step of estimating the positive electrode capacity from said binding crystallite size, a,
The crystallite size measurement step includes diffraction lines measured by X-ray diffraction on the (311), (222), (400), (331), (511), (440), (531), and (444) planes. The crystallite size is calculated by the Wilson method based on four or more diffraction lines selected from the above, and the obtained crystallite size is characterized in that lattice strain components are separated. (Claim 1).
[0011]
In other words, the present inventor discovered the correlation between the crystallite size and the positive electrode capacity for lithium manganate, and obtained the knowledge that the crystallite size of lithium manganate can be measured with high accuracy by the X-ray diffraction method. Completed the invention.
An approximate straight line can be obtained if the number of diffraction lines used in the Wilson method is two or more, but four or more diffraction lines are used in order to improve measurement accuracy. In particular, as a result of considering the overlap between diffraction lines, the diffraction lines in (311), (222), (400), (331), (511), (440), (531) and (444) are selected.
In addition, when the crystallite size is measured by X-ray diffraction method, the influence of the lattice distortion component appears on the crystallite size, but it is estimated that the lattice distortion component does not have a large influence on the positive electrode performance. . Here, since lithium manganate has an isotropic crystal structure, it is relatively easy to separate lattice distortion components.
Patent Document 1 discloses a relationship between crystallite size and battery durability of a lithium secondary battery using a lithium transition metal composite oxide (mainly lithium nickelate) as a positive electrode active material.
[0012]
The crystallite size measurement step was performed by X-ray diffraction on the (311), (222), (400), (331), (511), (440), (531), and (444) planes. It is preferable that the crystallite size is calculated by the Wilson method based on all diffraction lines .
[0014]
DETAILED DESCRIPTION OF THE INVENTION
A positive electrode active material to which the method for evaluating a positive electrode active material for a lithium secondary battery of the present invention can be applied is composed of a lithium transition metal composite oxide containing manganese as a main component as a transition metal. For example, in addition to pure lithium manganate (LiMn ₂ O ₄ ), lithium manganese composite oxide in which a part of manganese is substituted with other elements (LiMn _2−x Me _x O ₄ , 0 ≦ x ≦ 2, Me is Ti, Cr, Fe, Co, Ni, Cu, Zn, etc.).
[0015]
Positive active material is synthesized from a compound of an element to be contained other compound of the compound and lithium Mn. The synthesis of the positive electrode active material is generally a solid phase method in which a powder in which raw materials containing constituent elements are mixed is fired. In addition to the solid phase method, there are a melt impregnation method, a hydrothermal synthesis method, an ion exchange method, a liquid phase method and the like.
[0016]
The melt impregnation method is a method in which a lithium salt such as LiNO ₃ is melted and impregnated in a manganese oxide such as MnOOH and then heat treatment is performed. The hydrothermal synthesis method is a method in which an aqueous solution of a raw material composed of γ-MnOOH, LiOH or the like is reacted at a relatively low temperature under pressure.
[0017]
The ion exchange method is a method in which lithium is introduced into a manganese composite oxide synthesized in advance by ion exchange and calcined. Liquid phase methods include ultrasonic pyrolysis and sol-gel method.
[0018]
The starting material for synthesis is appropriately selected from the viewpoints of price, quality stability, handleability, impurity content, supply stability, reactivity, and the like. Examples of the Mn compound include electrolytic manganese dioxide and chemical manganese dioxide, and examples of the lithium compound include Li ₂ CO ₃ and LiNO ₃ . In the solid phase method, these raw materials are fired in the air.
[0019]
The positive electrode active material has a large influence on the positive electrode performance because the crystallite size, particle size, particle shape, specific surface area, etc. vary greatly depending on the synthesis method, synthesis conditions and starting material. In particular, the influence of the crystallite size on the positive electrode performance is very large. In this specification, “crystallite” refers to a single crystal region constituting a lithium transition metal composite oxide. The lattice is disturbed between adjacent crystallites.
[0020]
[Valuation method of the positive electrode active material for lithium secondary battery]
The evaluation method of this embodiment includes a crystallite size measurement step and an estimation step. This evaluation method can be performed with the positive electrode active material manufactured, and can also be applied after the positive electrode is formed. Further, the positive electrode can be taken out and evaluated after being assembled and used in the state of a battery. The performance of the positive electrode active material manufactured by applying this evaluation method as it is in the state of manufacturing the positive electrode active material or in the state of the positive electrode can be evaluated. Moreover, the deterioration state of a battery can be evaluated by evaluating about the positive electrode taken out from the inside of the battery after use .
[0021]
A general positive electrode is prepared by mixing the above-mentioned lithium transition metal composite oxide with additives as necessary, such as a binder and a conductive material, and adding a suitable solvent as necessary to form a paste-like positive electrode mixture. This is formed by applying and drying on the surface of a current collector made of a metal foil such as aluminum, and then increasing the active material density by pressing. The binder plays a role of connecting the active material particles and the conductive material particles, and a fluorine-containing resin such as polytetrafluoroethylene, polyvinylidene fluoride, and fluororubber, and a thermoplastic resin such as polypropylene and polyethylene are used. it can. An organic solvent such as N-methyl-2-pyrrolidone can be used as a solvent for dispersing these active material, conductive material, and binder.
[0022]
The crystallite size measurement step is a step of measuring the crystallite size of the positive electrode active material for a lithium secondary battery based on the X-ray diffraction method. In the X-ray diffraction method, the average value of the crystallite size can be measured. In order to achieve the object of the evaluation method of this embodiment, it is sufficient that the average value of the crystallite size can be measured.
[0023]
Moreover, the measurement conditions in the X-ray diffraction method are not particularly limited. For example, an easy-to-handle X-ray having a wavelength satisfying a measurable Bragg condition and a measurable intensity in the target lithium transition metal composite oxide is selected. For example, CuKα rays or the like can be used at an arbitrary intensity.
[0024]
As a method for calculating the crystallite size from the relationship between the diffraction angle measured by the X-ray diffraction method and the width of the diffraction line, a method in which lattice strain components are separated is employed . The Wilson method can be used as a method for calculating the crystallite size by separating the lattice distortion components.
[0025]
The Wilson method is a method of approximating the spread of diffraction lines due to the crystallite size with a Cauchy function and approximating the spread of diffraction lines due to a lattice distortion component with a Gauss function. That is, it is a method of approximating the spread of diffraction lines by a linear combination of a Cauchy function and a Gaussian function. Specifically, β ² / (tan θ · sin θ) derived from the peak position θ (Bragg angle: rad) of the diffraction line and the integral width β (rad) of the diffraction line is set to β ² / Plot tan ² θ on the Y axis.
[0026]
Here, the integral width means the width of the diffraction line at a height that halves the area when the diffraction line is divided into two in the vertical direction, and is the width obtained by subtracting the optical spread of the X-ray diffraction measurement device from the measured value. It is. The points plotted for multiple diffraction lines are approximated with a straight line, the crystallite size is calculated from the slope of the straight line (related to the Cauchy function parameter), and the lattice distortion component from the Y-axis intercept (related to the Gaussian function parameter) (Crystallite size and lattice strain program instruction manual, Rigaku Corporation). Specifically, it is calculated from the equation (1): (crystallite size) = 1.05 × 1.54 / (gradient of approximate straight line) / 10 (nm).
[0027]
The number of diffraction lines used in Wilson method can determine the approximate line as long as it is two or more, but in order to improve the measurement accuracy Ru using four or more diffraction lines. In particular result of considering the overlap between the diffraction lines, (311), (222), (400), (331), (511), (440), select from the diffraction line at (531) and (444). It is desirable to use all these eight diffraction lines.
[0028]
【Example】
<Test 1>
(Sample preparation)
20 to 25 g of a mixture of lithium compound (LiNO ₃ ) and manganese compound (electrolytic manganese dioxide: MnO ₂ ) (molar ratio 1: 2) was fired in an alumina crucible at 900 ° C. for 24 hours in an air atmosphere. The firing operation was performed 9 times with separate mixtures to obtain 9 types of lithium manganate powders (samples 1 to 9). The firing temperature was considered to vary about 50 ° C. at the maximum. The particle size and specific surface area of each sample 1-9 were almost the same.
[0029]
(Measurement of crystallite size)
For Samples 1 to 9, diffraction lines were measured based on the X-ray diffraction method. As measurement conditions, a CuKα ray was used as the X-ray source, an X-ray tube voltage of 50 kV and an X-ray tube current of 300 mA were selected. As diffraction lines used for the analysis, eight diffraction lines (311), (222), (400), (331), (511), (440), (531) and (444) were used.
[0030]
The crystallite size was calculated by the Wilson method based on these measurement data. Specifically, for each diffraction line, β / (tan θ · sin θ) on the X axis and β ² / tan ² θ on the Y axis from the peak position θ (rad) of the diffraction line and the integration width β (rad). Each diffraction line was plotted (♦: Fig. 1). The integral width was obtained by subtracting the optical spread of the X-ray diffractometer from the measured value. The optical spread of the X-ray diffraction measurement apparatus was obtained by measuring the standard sample Si using the X-ray source used in the X-ray diffraction measurement apparatus used. The points plotted for the eight diffraction lines were approximated by a straight line (□: FIG. 1), and the crystallite size of lithium manganate of each sample was calculated by substituting the slope of the straight line into the above formula (1).
[0031]
[Create test battery]
The lithium secondary battery of this test example is a lithium secondary battery using lithium manganate represented by the composition formula LiMn ₂ O ₄ as a positive electrode active material and graphite as a negative electrode active material.
[0032]
(Positive electrode)
The lithium manganate powder of each sample 1-9, polyvinylidene fluoride (PVDF) as a binder, and graphite powder as a conductive material were in a mass ratio of 90: 5: 5 (mass ratio). A paste was prepared by dispersing in N-methylpyrrolidone. The paste was applied to the surface of an aluminum foil-like current collector with a doctor blade and dried to obtain a sheet-like positive electrode having an active material layer formed on the surface of the current collector.
[0033]
(Negative electrode)
A paste was prepared by dispersing graphite powder having an average particle diameter of 50 nm and PVDF in N-methylpyrrolidone so as to have a mass ratio of 95: 5. The paste was applied and dried on the surface of the Cu foil-like current collector with a doctor blade to obtain a sheet-like negative electrode having an active material layer formed on the current collector surface.
[0034]
(Battery assembly)
The positive electrode and the negative electrode were each cut into a predetermined size, and the cut positive electrode and negative electrode were wound with a 25 μm thick polyethylene separator interposed therebetween to form a roll-shaped electrode body. At this time, there were two negative electrodes for one positive electrode. Since the battery capacity can be regulated as the positive electrode capacity by making the negative electrode excessive from the positive electrode, the positive electrode capacity can be measured by measuring the battery capacity normally.
[0035]
These electrode bodies were provided with current collecting leads, inserted into a 18650 type battery case, and then a non-aqueous electrolyte was injected into the battery case. As the non-aqueous electrolyte, a solution obtained by dissolving LiPF ₆ at a concentration of 1 mol / L in a mixed solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at a volume ratio of 3: 7 was used. Finally, the battery case was sealed to complete the lithium secondary battery (1000 mAh) of each test example.
[0036]
(Evaluation)
Each test battery was conditioned by charging / discharging between 3.0 and 4.2 V at a current density of 0.1 mA / cm ² . Then, about each test battery, the discharge capacity (percentage with respect to 1000 mAh) when charging / discharging was measured. The results are shown in Table 1 and FIG. The battery capacity measured here is the positive electrode capacity.
[0037]
[Table 1]

[0038]
As is clear from FIG. 2 and Table 1, it was found that the lithium manganate crystallite size calculated by the Wilson method has a high correlation with the positive electrode capacity. That is, it was found that the battery capacity of a battery using the lithium manganate as the positive electrode can be evaluated by measuring the crystallite size of the lithium manganate without actually manufacturing the battery.
[0039]
<Test 2>
About the battery using the sample 1 and the lithium manganate of 2 in Test 1, it deteriorated by charging / discharging between 3.0-4.2V, respectively, and battery capacity was set to 0 mAh. The battery was disassembled, the positive electrode was taken out and washed with ethanol, and then the crystallite size was measured by the same method as in Test 1 (X-ray diffraction method and Wilson method).
[0040]
As a result, the crystallite sizes were 12.0 nm and 10.0 nm, respectively. When applied to FIG. 1, the actual battery capacity (0 mAh) could be estimated. That is, it has been found that even when lithium manganate is once in a battery (positive electrode mixture) state, the battery capacity (in other words, the deterioration state) can be evaluated by applying this evaluation method.
[0041]
【The invention's effect】
The method for evaluating a positive electrode active material for a lithium secondary battery according to the present invention can evaluate the battery capacity when the positive electrode active material for a lithium secondary battery is applied to a battery by measuring the crystallite size. It has the effect of becoming.
[Brief description of the drawings]
FIG. 1 is a graph in which diffraction lines in an example are plotted based on the Wilson method.
FIG. 2 is a graph showing the relationship between battery capacity (positive electrode capacity) and crystallite size in Examples.

Claims (2)

A method for evaluating a positive electrode active material for a lithium secondary battery comprising a lithium transition metal composite oxide containing manganese as a main component as a transition metal,
A crystallite size measurement step of measuring a crystallite size of the positive electrode active material for a lithium secondary battery based on an X-ray diffraction method;
Possess an estimation step of estimating the positive electrode capacity from said binding crystallite size, a,
The crystallite size measurement step includes diffraction lines measured by X-ray diffraction on the (311), (222), (400), (331), (511), (440), (531), and (444) planes. A step of calculating a crystallite size by a Wilson method based on four or more diffraction lines selected from the above, wherein the obtained crystallite size has a lattice distortion component separated therefrom; Evaluation method of positive electrode active material.   The crystallite size measurement step includes all measurements measured by X-ray diffraction on the (311), (222), (400), (331), (511), (440), (531), and (444) planes. The method for evaluating a positive electrode active material for a lithium secondary battery according to claim 1, wherein the crystallite size is calculated by a Wilson method based on diffraction lines.

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