JP2017062997A - Method for evaluating positive electrode material for nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for visualizing the lithium ion diffusibility in an electrode and evaluating a self-diffusion coefficient of lithium having influence on an output characteristic.SOLUTION: A method for evaluating a positive electrode material for a nonaqueous electrolyte secondary battery comprises the steps of: charging and discharging a nonaqueous electrolyte secondary battery including a positive electrode having a lithium metal composite oxide with its surface coated with a lithium ion-conducting oxide, in which the lithium metal composite oxide and the lithium ion-conducting oxide are labeled withLi, an electrolytic solution labeled withLi and a negative electrode; and then, performing a secondary ion mass spectrometry on the positive electrode to obtain diffusion profiles of 6 Li and 7 Li in the positive electrode. Thus, it becomes possible to check the change in lithium ion behavior in a simple and convenient manner. The method can be used for search for a coating layer expected to have an effect for a high output characteristic, or performance evaluation.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for evaluating a positive electrode material for a non-aqueous electrolyte secondary battery.

In recent years, with the widespread use of portable electronic devices such as mobile phones and laptop computers, development of small and lightweight non-aqueous electrolyte secondary batteries having high energy density is strongly desired. In addition, development of a high output secondary battery is strongly desired as a battery for electric vehicles including hybrid vehicles. As a secondary battery satisfying such requirements, there is a lithium ion secondary battery.
A lithium ion secondary battery includes a positive electrode having a positive electrode active material as a main component, a negative electrode having a negative electrode active material as a main component, and a non-aqueous electrolyte. The negative electrode and the positive electrode active material remove lithium. A material that can be separated and inserted is used.

Such lithium ion secondary batteries are currently being actively researched and developed, and lithium ion secondary batteries using a layered lithium metal composite oxide as a positive electrode material have a high voltage of 4V. Therefore, practical use is progressing as a battery having a high energy density.
The materials proposed so far include lithium cobalt composite oxide (LiCoO ₂ ) that is relatively easy to synthesize, lithium nickel composite oxide (LiNiO ₂ ) using nickel that is cheaper than cobalt, and lithium nickel cobalt manganese composite. An oxide (LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ) can be used.

  In order to develop the lithium composite oxide for use in automobiles, it is important to improve the material to obtain a higher output than the current state, that is, to lower the resistance of the positive electrode material.

  In Patent Document 1, an alkaline solution in which a tungsten compound is dissolved is added to and mixed with lithium nickel composite oxide powder, and the surface of the lithium nickel composite oxide particles is coated with lithium tungstate to reduce the positive electrode resistance and output. It is reported that the characteristics are improved. In this proposal, it is stated that covering with lithium tungstate having good lithium conductivity leads to a reduction in resistance, but there is no description showing what kind of action the resistance reduction is realized.

  In the development for improving the output characteristics as described above, it is necessary to analyze the behavior of lithium ions. Examples of methods for directly evaluating lithium ions include inductively coupled plasma emission spectrometry and secondary ion mass spectrometry. It is done. However, in the case of inductively coupled plasma optical emission spectrometry, as described in Patent Document 2, the entire amount of the positive electrode is dissolved to analyze elements such as lithium, so that the diffusion of lithium in the electrode cannot be evaluated. .

  Non-Patent Document 1 evaluates the lithium diffusion coefficient by directly detecting lithium of lithium lithium manganate using a secondary ion mass spectrometer. In this proposal, only the lithium diffusion coefficient of the positive electrode is analyzed using a secondary ion mass spectrometer, and no examination is made on the evaluation of the behavior of lithium ions for improving the output characteristics.

JP 2013-125732 A JP 2010-78381 A

Journal of Power Sources 189 (2009) 643.

  In view of the above circumstances, the present invention visualizes the diffusibility of lithium ions in the positive electrode and evaluates the change in the behavior of lithium ions by the coating layer on the surface of the positive electrode active material that affects the output characteristics. It is an object to provide a method for evaluating a positive electrode material for a non-aqueous electrolyte secondary battery that can be used for improvement

In order to solve the above-mentioned problems, the present inventor has made extensive studies on the evaluation of lithium diffusion in lithium metal composite oxides using labeling of stable enriched isotope ⁶ Li. As a result, the present inventors formed a lithium metal composite oxide on the surface. By labeling the coated layer with ⁶ Li and evaluating the change in the behavior of lithium ions, the inventors have obtained the knowledge that effective data can be obtained for studying the improvement of the output characteristics, thereby completing the present invention.

The method for evaluating a positive electrode material for a non-aqueous electrolyte secondary battery according to the first invention is such that the surface of a lithium metal composite oxide is coated with a lithium ion conductive oxide, the lithium metal composite oxide and the lithium ion conductive oxide Is charged and discharged with a non-aqueous electrolyte secondary battery including a positive electrode labeled with ⁶ Li, an electrolyte solution labeled with ⁷ Li, and a negative electrode. Ion mass spectrometry is performed to obtain a diffusion profile of ⁶ Li and ⁷ Li in the positive electrode.
The method for evaluating a positive electrode material for a non-aqueous electrolyte secondary battery according to a second aspect of the present invention is the first aspect, wherein the lithium isotope ratio in the lithium metal composite oxide and the lithium ion conductive oxide is ⁶ Li / ⁷ Li ≧ It is 90.0 / 10.0.
The method for evaluating a positive electrode material for a non-aqueous electrolyte secondary battery according to the third aspect of the present invention is the first or second aspect, wherein the electrolyte solution labeled with the stable concentrated isotope ⁷ Li has a lithium isotope ratio of ⁷ Li / ⁶ Li ≧ 90.0 / 10.0.
The positive electrode material for a non-aqueous electrolyte secondary battery according to a fourth aspect of the invention is any one of the first to third aspects, wherein the negative electrode is labeled with a stable concentrated isotope ⁷ Li, and the isotope ratio of lithium in the negative electrode ⁷ Li / ⁶ Li ≧ 90.0 / 10.0.
The positive electrode material for a non-aqueous electrolyte secondary battery according to a fifth aspect of the invention is characterized in that, in any of the first to fourth aspects, the positive electrode is a thin film, and the thin film is produced by a physical film formation method. And
The method for evaluating a positive electrode material for a non-aqueous electrolyte secondary battery according to a sixth aspect of the invention is characterized in that, in the fifth aspect, the lithium ion conductive oxide constituting the thin film has a thickness of 100 to 500 nm.
The method for evaluating a positive electrode material for a non-aqueous electrolyte secondary battery according to a seventh aspect of the invention is characterized in that, in the sixth aspect, the variation width of the thickness of the lithium ion conductive oxide is within 10%.

According to the first invention, a positive electrode in which a lithium metal composite oxide and a lithium ion conductive oxide are labeled with ⁶ Li, an electrolyte solution labeled with a stable concentrated isotope ⁷ Li, and a negative electrode are included. After charging / discharging the non-aqueous electrolyte secondary battery composed of the above, ⁶ Li and ⁷ Li are subjected to secondary ion mass spectrometry, and the diffusion profile is obtained to understand the behavior of Li during charge / discharge. It becomes possible to do. That is, by analyzing the diffusion profile, the lithium ion self-diffusion coefficient of the lithium ion conductive oxide can be evaluated, and useful data for studying the improvement of output characteristics can be obtained. This makes it possible to investigate changes in the behavior of lithium ions when a lithium ion conductive oxide coating layer is formed on the surface of a lithium metal composite oxide used as a positive electrode active material, which is effective for high output characteristics. It can be used to search for potential coating layers and evaluate performance, and its value is great.
According to the second invention, when the isotope ratio of lithium in the positive electrode is ⁶ Li / ⁷ Li ≧ 90.0 / 10.0, ⁷ Li inserted at the time of discharge becomes clearer and finer It becomes possible to grasp the change, which is useful for examining the improvement of output characteristics.
According to the third invention, the electrolyte solution labeled with the stable concentrated isotope ⁷ Li has a lithium isotope ratio of ⁷ Li / ⁶ Li ≧ 90.0 / 10.0. Mixing of ⁶ Li into Li inserted into the metal is reduced, and only approximately ⁷ Li can be inserted, so that even finer changes can be grasped.
According to the fourth invention, the negative electrode is also labeled with the stable concentrated isotope ⁷ Li, and the lithium isotope ratio is ⁷ Li / ⁶ Li ≧ 90.0 / 10.0, so that it is inserted at the time of discharge. ⁶ Li can be reduced.
The positive electrode thin film prepared by a physical film forming method such as a PLD method according to the fifth invention is preferable because it can be easily labeled with ⁶ Li.
According to the sixth aspect of the invention, the difference between the constituent materials of the positive electrode material can be clearly grasped by setting the thickness of the lithium ion conductive oxide to 100 to 500 nm.
According to the seventh aspect of the invention, it is possible to eliminate influences such as a contact state with the electrolytic solution due to a variation in thickness and a variation in Li diffusion rate, and an accurate evaluation can be obtained.

It is explanatory drawing of the principle of the evaluation method of this invention. It is the schematic of the cross section which shows the structure of the positive electrode of a non-aqueous electrolyte secondary battery. It is the schematic of the cross section which shows the coin-type cell structure of a non-aqueous electrolyte secondary battery. It is an impedance spectrum of Examples 1-3. This is an equivalent circuit used for analysis. It is Li diffusion profile in the electrode before and behind an electrochemical test. It is Li diffusion profile in the electrode of Examples 1-3 after a tracer diffusion experiment.

The method for evaluating a positive electrode material for a non-aqueous electrolyte secondary battery according to the present invention (hereinafter simply referred to as “evaluation method”) is a method in which the surface of a lithium metal composite oxide is coated with a lithium ion conductive oxide. A non-aqueous electrolyte secondary battery comprising a cathode, a positive electrode in which the lithium ion conductive oxide is labeled with ⁶ Li, an electrolyte solution labeled with a stable concentrated isotope ⁷ Li, and a negative electrode After charging and discharging, secondary ion mass spectrometry is performed on the positive electrode, and a diffusion profile of ⁶ Li and ⁷ Li in the positive electrode is obtained.

Lithium (Li) has ⁶ Li and ⁷ Li as natural isotopes, which are atoms with the same atomic number but different mass numbers. Secondary ion mass spectrometry can be used to analyze even the smallest amount in a substance. Is possible. In the evaluation method of the present invention, ⁶ Li is used as a label (tracer), and the behavior of Li is evaluated by secondary ion mass spectrometry. Here, “labeling” means that the ratio of one isotope is within a predetermined numerical value for one atom constituting an object, and that the isotope can be evaluated as a label. say.

  To improve the output characteristics of non-aqueous electrolyte secondary batteries, it is effective to coat the surface of the lithium metal composite oxide used as the positive electrode active material with a lithium ion conductive oxide. For this purpose, the movement of Li between the electrolytic solution and the positive electrode active material, that is, the movement of Li between the electrolytic solution and the lithium ion conductive oxide, and between the lithium ion conductive oxide and the lithium metal composite oxide, It is important to know in detail.

In the evaluation method of the present invention, by using ⁶ Li and ⁷ Li for labeling, it is possible to easily grasp the movement of Li between the electrolytic solution and the positive electrode active material. Li is extracted from the positive electrode active material during charging and diffused into the electrolytic solution, and inserted into the positive electrode active material from the electrolytic solution during discharging. Therefore, as shown in FIG. 1, Li contained in the lithium metal composite oxide constituting the positive electrode active material and the lithium ion conductive oxide on its surface is ⁶ Li, Li in the electrolytic solution is ⁷ Li, and charging / discharging By performing secondary ion mass spectrometry of ⁶ Li and ⁷ Li before and after, it is possible to grasp the Li behavior during discharge that governs the output characteristics.

That is, most of Li in the lithium metal composite oxide and the lithium ion conductive oxide exists as ⁶ Li. On the other hand, since a large amount of ⁷ Li is present in the electrolytic solution, even if ⁶ Li is drawn out during charging and diffuses into the electrolytic solution, the majority is ⁷ Li, and ⁷ Li is inserted during discharging. It becomes. Therefore, by secondary ion mass spectrometry of ⁶ Li and ⁷ Li after charge and discharge, it becomes possible to grasp the influence of the lithium ion conductive oxide on the movement of Li when charged and discharged, and the diffusion profile is By analyzing, it is possible to evaluate the self-diffusion coefficient of lithium ion of the lithium ion conductive oxide, and data useful for studying improvement of output characteristics can be obtained.

Here, even if only the lithium metal composite oxide is labeled with ⁶ Li, the movement of Li in the lithium ion conductive oxide that greatly affects the movement of Li in the positive electrode active material becomes unclear. Therefore, when Li constituting the lithium metal composite oxide and the lithium ion conductive oxide is labeled as ⁶ Li, clear data including movement of Li in the lithium ion conductive oxide can be obtained. It becomes possible.

In order to perform labeling, it is sufficient that ⁶ Li is contained to such an extent that Li movement in the lithium metal composite oxide and the lithium ion conductive oxide can be confirmed, but the lithium isotope ratio is ⁶ Li / ⁷ Li ≧ It is preferably 90.0 / 10.0. Thereby, ⁷ Li inserted at the time of discharge becomes clearer, and it becomes possible to grasp a finer change, which is useful for studying improvement of output characteristics. In each of the lithium metal composite oxide and the lithium ion conductive oxide, it is preferable that the isotope ratio of lithium is ⁶ Li / ⁷ Li ≧ 90.0 / 10.0. There is no problem even if the lithium isotope ratio in the combined state is ⁶ Li / ⁷ Li ≧ 90.0 / 10.0.

In addition, the electrolyte solution labeled with the stable concentrated isotope ⁷ Li preferably has a lithium isotope ratio of ⁷ Li / ⁶ Li ≧ 90.0 / 10.0. Thereby, mixing of ⁶ Li into Li inserted into the positive electrode active material at the time of discharge can be reduced, and only ⁷ Li can be inserted, and a more minute change can be grasped. From the viewpoint of reducing the mixing of ⁶ Li into Li inserted during discharge, the negative electrode is also labeled with the stable concentrated isotope ⁷ Li, and the isotope ratio of lithium is ⁷ Li / ⁶ Li ≧ 90.0 / It is preferable that it is 10.0.
In the above embodiment has been labeled a positive electrode with ⁶ Li, and labeled with ⁷ Li, a negative electrode and an electrolyte may be labeled with ⁶ Li. That is, the Li behavior can be grasped by labeling the positive electrode, the negative electrode, and the electrolyte with different isotopes of Li.

The evaluation method of the present invention is applicable to any nonaqueous electrolyte secondary battery (hereinafter simply referred to as “battery”) having a positive electrode capable of being labeled with ⁶ Li and ⁷ Li and capable of secondary ion mass spectrometry after charge and discharge. In order to more easily analyze the movement of Li, a positive electrode material for a non-aqueous electrolyte secondary battery (hereinafter simply referred to as “positive electrode material”) is evaluated using a battery having the following configuration. It is preferable.

  A preferred embodiment of the battery used in the evaluation method of the present invention is a positive electrode, separator, and negative electrode using a lithium metal composite oxide whose surface is modified with a lithium ion conductive oxide on the surface of the lithium metal composite oxide as a positive electrode material. , A coin-type non-aqueous electrolyte secondary battery (hereinafter simply referred to as “coin-type cell”) composed of an electrolyte.

The lithium metal composite oxide used for the positive electrode material may be an object for studying improvement of output characteristics, but as an object, for example, a high voltage of 4V class is obtained, and layered lithium having excellent charge / discharge characteristics is obtained. A composite oxide is mentioned. And it targets materials such as lithium cobalt composite oxide (LiCoO ₂ ), lithium nickel composite oxide (LiNiO ₂ ), and lithium nickel cobalt manganese composite oxide (LiNi _1/3 Co _1/3 Mn _1/3 ). However, when examining the mechanism for improving the output characteristics, LiCoO ₂ is preferable because it is relatively easy to synthesize and has a simple composition.

Furthermore, the positive electrode material is preferably composed of a thin film of a lithium metal composite oxide and a lithium ion conductive oxide, and this thin film is preferably produced by a physical film formation method. For example, after sintering the above lithium metal complex oxide powder labeled with ⁶ Li to produce a target, a pulse laser deposition (PLD) method is used to form a Pt / Cr / SiO ₂ or other smooth conductive substrate. After depositing a lithium metal composite oxide on the substrate, a lithium ion conductive oxide is deposited in the same manner as a coating layer to obtain a positive electrode thin film. Such a positive electrode thin film by the PLD method is preferable because it is easy to label with ⁶ Li.

As the lithium ion conductive oxide serving as the coating layer for the positive electrode thin film, for example, a material made of a metal element having a valence of 4 or more that is stable against electrochemical oxidation is selected. Examples of such a material include tungsten such as Li ₂ WO ₄ , Li ₄ WO ₅ , Li ₆ W ₂ O ₉ , 7 (Li ₂ WO ₄ ) · 4H ₂ O, and tungsten having a valence of six. A lithium acid is mentioned. As the other coating layer material, an oxide material that does not contain lithium may be selected. Examples of such a material include MgO, ZnO, and ZrO ₂ . These coating layers may be in a crystalline state or an amorphous state.

  The thickness of the lithium ion conductive oxide thin film constituting the positive electrode thin film is preferably 100 to 500 nm. By setting the thickness to 100 to 500 nm, it is possible to clearly grasp the difference between the constituent materials of the positive electrode material. If the thickness is less than 100 nm, the effect of the lithium ion conductive oxide may not be sufficiently obtained depending on the film forming conditions, and if the thickness is 500 nm, the movement of Li may be inhibited. May not be obtained. Furthermore, the thickness of the lithium ion conductive oxide preferably has a fluctuation range of 10% or less. As a result, it is possible to eliminate influences such as a contact state with the electrolytic solution due to a variation in thickness and a variation in Li diffusion rate, and an accurate evaluation can be obtained.

Below, each structure of the battery using a positive electrode thin film is demonstrated in detail.
(1) Positive electrode The positive electrode thin film electrode which forms a positive electrode is demonstrated. The material constituting the positive electrode is composed of a positive electrode thin film and a substrate.
After sintering the lithium metal composite oxide powder labeled with ⁶ Li as a raw material to produce a target, a coin cell is previously formed using a physical film-forming method such as PLD method, sputter deposition method or molecular beam epitaxy method. A thin film of lithium metal composite oxide is deposited on a conductive substrate such as Pt / Cr / SiO ₂ or Pt that has been cut to a size suitable for the above.
Next, a lithium ion conductive oxide labeled with ⁶ Li is deposited on the lithium metal composite oxide thin film using a physical film forming method.
Examples of the lithium composite oxide include lithium cobalt composite oxide (LiCoO ₂ ), lithium nickel composite oxide (LiNiO ₂ ), and lithium nickel cobalt manganese composite oxide (LiNi _1/3 Co _1/3 Mn _1/3 ). Can be mentioned.
Specific examples of the lithium ion conductive oxide include a lithium composite oxide containing at least one selected from Ti, V, Nb, Ta, Mo, and W. More specific examples of the ion conductive oxide include lithium composite oxides such as lithium tungstate, lithium molybdate, lithium titanate, lithium niobate, and lithium tantalate.

(2) Negative electrode The negative electrode is preferably labeled with ⁷ Li as described above, and it is preferable to use metallic lithium or a lithium alloy. The thickness of the metal lithium or lithium alloy constituting the negative electrode is preferably in the range of 0.5 to 2.0 mm so that the coin cell does not swell. It is preferable to shape the circular shape with a diameter of about 5 to 15 mm so as to fit in the coin cell, and the negative electrode preferably has a larger area than the positive electrode.

(3) Separator A separator is interposed between the positive electrode and the negative electrode. The separator has functions such as insulation between the positive electrode and the negative electrode and further holding the electrolyte, and those used in general non-aqueous electrolyte secondary batteries can be used. For example, a porous film such as polyethylene (PE), polypropylene (PP), or a laminate thereof may be used as long as it has the necessary function, and measured with a separator used in a general non-aqueous electrolyte secondary battery. As long as no interfering element is contained, there is no particular limitation.

(4) Non-aqueous electrolyte The non-aqueous electrolyte is obtained by dissolving a lithium salt as an electrolyte in an organic solvent. Examples of the organic solvent include cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and trifluoropropylene carbonate; chain carbonates such as diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, and dipropyl carbonate; and tetrahydrofuran, 2- One kind selected from ether compounds such as methyltetrahydrofuran and dimethoxyethane, sulfur compounds such as ethylmethylsulfone and butanesultone, phosphorus compounds such as triethyl phosphate and trioctyl phosphate, etc. are used alone or in admixture of two or more. be able to.
As an electrolyte, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiN (CF ₃ SO ₃ ) labeled with a stable concentrated isotope ⁷ Li or a natural isotope ( ⁷ Li / ⁶ Li = 92.5 / 7.5) ₂ ) ₂ etc. and their complex salts can be used. Furthermore, the non-aqueous electrolyte solution may contain a radical scavenger, a surfactant, a flame retardant, and the like.

(5) Battery configuration A positive electrode and a negative electrode are laminated via a separator to form an electrode body, and this electrode body is impregnated with a non-aqueous electrolyte. Each of the positive electrode and the negative electrode is connected to an external terminal to conduct. The coin cell is manufactured by putting the above structure into a metal container such as stainless steel.

Example 1
In FIG. 2, the schematic of the cross section of the positive electrode 11 used for the evaluation method of this invention is shown.
In Example 1, a LiCoO ₂ thin film labeled with a stable concentrated isotope ⁶ Li was used as the positive electrode active material 13. The LiCoO ₂ thin film was produced by the PLD method. Li ₂ CO ₃ (manufactured by Cambridge Isotop Laboratories, ⁶ Li / ⁷ Li = 95/5) and Co ₃ O ₄ mixed with ⁶ Li so as to have a composition of LiCoO ₂ were mixed at 980 ° C. in an oxygen atmosphere. Firing was performed to prepare LiCoO ₂ powder. Thereafter, LiCoO ₂ powder was sintered at 1000 ° C. to produce pellets. Using this pellet as a target, a LiCoO ₂ (LCO) thin film having an area of 8 mm × 8 mm was formed to a thickness of 300 nm on a Pt / Cr / SiO ₂ substrate 12 in an oxygen atmosphere at 500 ° C.

Thereafter, a lithium tungstate (LWO) thin film was formed as the lithium ion conductive oxide 14 on the surface of the prepared LiCoO ₂ thin film. The PLD method was used for the production of the thin film. After mixing LiOH.H ₂ O labeled with ⁶ Li (manufactured by Sigma-Aldrich, ⁶ Li / ⁷ Li = 95/5) and WO ₃ at a molar ratio of Li: W = 2: 1, 120 ° C. Was dried for 24 hours to obtain a Li—W—O powder serving as a target for producing an LWO thin film. This powder was sintered at 650 ° C. to make a pellet as a target. Using this target, an LWO thin film was further formed at a thickness of 300 nm at 500 ° C. on the LiCoO ₂ thin film obtained above to produce a positive electrode thin film electrode 11 shown in FIG. When analyzed by X-ray diffraction (XRD), it was crystallized Rhohedral type Li ₂ WO ₄ .

For the evaluation of the positive electrode thin film, a coin-type cell shown in FIG. 3 was used. As shown in FIG. 3, the coin cell includes a case 2 and an electrode 3 accommodated in the case 2.
The case 2 has a positive electrode can 2a that is hollow and open at one end, and a negative electrode can 2b that is disposed in the opening of the positive electrode can 2a. When the negative electrode can 2b is disposed in the opening of the positive electrode can 2a, A space for accommodating the electrode 3 is formed between the negative electrode can 2b and the positive electrode can 2a.
The electrode 3 includes a positive electrode (positive electrode thin film) 3a, a separator 3c, and a negative electrode 3b, which are stacked in this order. The positive electrode 3a contacts the inner surface of the positive electrode can 2a, and the negative electrode 3b contacts the inner surface of the negative electrode 2b. It is accommodated in the case 2 so as to come into contact.
The case 2 includes a gasket 2c, and relative movement is fixed by the gasket 2c so as to maintain a non-contact state between the positive electrode can 2a and the negative electrode can 2b.

The coin cell used was prepared as follows.
The produced positive electrode thin film was used as the positive electrode 3a, and using the negative electrode 3b, the separator 3c, and the electrolyte, the coin-type cell of FIG. 3 was produced in a glove box in an Ar atmosphere in which the dew point was controlled to −60 ° C. or less.
Note that the negative electrode 3b using metallic lithium natural isotopes 0.5mm pressure was punched into a disc having a diameter of ^{13mm (7 Li / 6 Li =} 92.5 / 7.5).
Further, a polyethylene porous film having a film thickness of 25 μm was used for the separator 3c.

The electrolytes are ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) using 1M LiPF ₆ of natural isotope ( ⁷ Li / ⁶ Li = 92.5 / 7.5) as a supporting electrolyte. The equivalent liquid mixture (made by Ube Industries, Ltd.) was used.

The positive electrode interface resistance is obtained by charging a coin-type cell to a charging potential of 4.0 V, measuring an AC impedance using a frequency response analyzer and a potentio galvanostat, and obtaining the impedance spectrum shown in FIG. A positive electrode interface resistance was analyzed using a circuit model. Here, Rs is a bulk resistance, R1 is a positive electrode film resistance, Rct is an electrolyte / positive electrode interface resistance (Li ⁺ movement resistance at the interface), W is a Warburg component, and CPE1 and CPE2 are constant phase elements. Table 1 shows the positive electrode interface resistance of Example 1.

Next, the diffusibility of lithium in the positive electrode of Example 1 was evaluated using secondary ion mass spectrometry (SIMS).
The coin cell in which the AC impedance measurement was performed was discharged to 3.0 V, the coin cell was disassembled in a glove box in an Ar atmosphere, only the positive electrode thin film was taken out, washed with DMC to remove the attached LiPF ₆ , dried, and SIMS A sample was prepared. For comparison, an LWO-coated LCO positive electrode thin film immediately after film formation without electrochemical measurement was also prepared. The sample is transferred from a glove box in an Ar atmosphere to a SIMS device (made by CAMECA, IMS-7f) using a transfer vessel so as not to be exposed to the atmosphere, and irradiated with Cs ⁺ ions accelerated to 15 kV on the sample surface. The secondary ions of ⁶ Li and ⁷ Li released were detected and analyzed in the depth direction by SIMS, and the Li diffusion profile shown in FIG. 6B was obtained. For comparison, FIG. 6A shows a Li diffusion profile immediately after film formation. 6A to 6D, the vertical axis represents the lithium isotope density, the horizontal axis represents the thickness direction of the positive electrode, the left side is the coating layer, and the right side is the positive electrode active material, with the horizontal axis being 300 nm. In addition, ⁶ Li is represented by a solid line and ⁷ Li is represented by a dotted line.

As shown in FIG. 6 (A), in the sample immediately after film formation, the ratio of ⁶ Li / ⁷ Li in the whole thickness direction of the positive electrode (the left-right direction in FIG. 6) is constant at 0.95 / 0.05, It can be seen that the LWO coated LCO thin film electrode is labeled with the stable enriched isotope ⁶ Li. On the other hand, in the Li diffusion profile of Example 1, as shown in FIG. 6B, about 20% of ⁶ Li is exchanged with the natural isotope ⁷ Li in the electrolyte solution in the LWO coating layer. It can be seen that is exchanged for 50% or more and ⁷ Li. This is considered to be due to the fact that a large amount of ⁷ Li present in the electrolyte during discharge is stagnating in the LWO layer.

Next, a tracer diffusion experiment was performed to evaluate the Li self-diffusion coefficient of Limboedral type Li ₂ WO ₄ coated. A certain amount of the electrolytic solution was dropped onto the LWO-coated LCO thin film electrode of Example 1 immediately after film formation and left at 25 ° C. for 1 hour. Thereafter, the sample was washed with DMC to obtain a sample for SIMS, and a depth direction analysis was performed by SIMS in the same manner as described above, and a ⁷ Li diffusion profile shown in FIG. 7 was obtained.

The self-diffusion coefficient of ⁷ Li diffusing from the electrolytic solution into the LWO layer was calculated by fitting the diffusion profile shown in FIG. 7 using equation (1) derived from the diffusion equation. The self-diffusion coefficient of Li in the obtained Rhombohedral type Li ₂ WO ₄ was 4.0 × 10 ⁻¹⁵ cm ² / sec.
Where Ds is the self-diffusion coefficient of lithium ions, x is the distance from the LWO surface, t is the time in contact with the electrolyte during the tracer diffusion experiment, and C (x, t) is an arbitrary distance x and time t. , C ₀ is the lithium ion concentration when x = 0, and C ₁ is the lithium ion concentration when t = 0.

(Example 2)
LiOH.H ₂ O ( ⁶ Li / ⁷ Li = 95/5) and WO ₃ were mixed at a molar ratio of Li: W = 4: 1 to obtain a Li—W—O powder serving as a target for LWO thin film production. Except for the above, the positive electrode thin film electrode 11 was obtained and evaluated in the same manner as in Example 1. When the state of LWO of the positive electrode thin film was confirmed by XRD, it was crystallized Tetragonal type Li ₂ WO ₄ .

When the battery performance was evaluated, as shown in FIG. 3 and Table 1, it was confirmed that the LWO-coated LCO thin film of Example 2 had a lower positive electrode interface resistance than that of Example 1 and improved output characteristics. . Further, when the Li diffusibility in the electrode was evaluated, as shown in FIG. 6C, the Li diffusion profile of Example 2 shows that ⁶ Li enriched in the LWO coating layer is a natural isotope in the electrolyte. ⁷ Li and has been replaced about 80%, LCO region is seen that they are almost completely replaced with ⁷ Li of natural isotopes in the electrolyte. This indicates that ⁶ Li in the electrode is replaced with ⁷ Li in the electrolytic solution as compared with Example 1, and it was confirmed that Li diffusibility in the electrode was improved.

Furthermore, as a result of the tracer diffusion experiment, as shown in FIG. 7, the tetragonal Li ₂ WO ₄ of Example ₂ was transferred to the LWO coating layer by ⁷ Li in the electrolyte solution rather than the Rhombedral type Li ₂ WO ₄ of Example 1. You can see that it is spreading. Moreover, when the self-diffusion coefficient of lithium was calculated, as shown in Table 1, the self-diffusion coefficient of Tetragonal Li was 6.5 × 10 ⁻¹⁴ cm ² / sec, and the self-diffusion coefficient of Li was high. confirmed.

(Example 3)
LiOH.H ₂ O ( ⁶ Li / ⁷ Li = 95/5) and WO ₃ were mixed at a molar ratio of Li: W = 4: 1 to obtain a Li—W—O powder serving as a target for LWO thin film production. In addition, the positive electrode thin film electrode 11 was obtained and evaluated in the same manner as in Example 1 except that the LWO thin film was formed at 25 ° C. to a thickness of 300 nm. When the LWO state of the positive electrode thin film electrode 11 was confirmed by XRD, it was in an amorphous state.

When the battery performance was evaluated, as shown in FIG. 3 and Table 1, the LWO-coated LCO thin film of Example 3 had a lower positive electrode interface resistance than Examples 1 and 2, and the output characteristics were greatly improved. Was confirmed. Further, when the Li diffusibility in the electrode was evaluated, as shown in FIG. 6D, the Li diffusion profile of Example 3 shows that the Li in the electrode is a natural isotope ratio ( ⁷ Li / ⁶ Li = 92. 5 / 7.5), Li in the electrode labeled with ⁶ Li is completely exchanged with natural Li isotope ⁷ Li in the electrolyte, and lithium ions are smoothly diffused without stagnation. It was confirmed that the internal Li diffusibility was further improved.

  Next, using the produced positive electrode thin film, coin type cells were produced in the same manner as in Examples 1 and 2, and the battery performance was compared. As a result, as shown in FIG. 3 and Table 1, it can be seen that the LWO-coated LCO thin film of Example 3 has a lower positive electrode interface resistance than that of Examples 1 and 2, and significantly improved output characteristics.

Next, using the produced positive electrode thin film, Li diffusibility in the electrode was investigated in the same manner as in Examples 1 and 2. As a result, as shown in FIG. 6D, when the Li diffusion profile of Example 3 is seen, Li in the electrode is constant at a natural isotope ratio ( ⁷ Li / ⁶ Li = 92.5 / 7.5). Thus, it can be seen that Li in the electrode labeled with ⁶ Li was completely exchanged with natural Li isotope ⁷ Li in the electrolyte, and lithium ions were smoothly diffused without stagnation.

Furthermore, as a result of the tracer diffusion experiment, as shown in FIG. 7, in the amorphous LWO of Example 3, ⁷ Li in the electrolyte solution diffuses into the LWO coating layer more than the LWO in the crystalline state shown in Examples 1 and 2. You can see that Further, when the self-diffusion coefficient of lithium was calculated, it was 3.0 × 10 ⁻¹² cm ² / sec as shown in Table 1, and the Li self-diffusion coefficient was higher than the LWO in the crystalline state of Examples 1 and 2. It was confirmed to be high.
As described above, the evaluation method of the present invention makes it possible to visualize the diffusivity of lithium ions in the lithium metal composite oxide coated with the lithium ion conductive oxide, and to change the behavior of lithium ions affecting the output characteristics. It was confirmed that it was possible to evaluate.

(Comparative example)
Evaluation was performed in the same manner as in Example 1 except that a LiCoO ₂ thin film labeled with a stable concentrated isotope ⁶ Li was used as a positive electrode active material having no coating layer of a lithium ion conductive oxide. It was confirmed that the positive electrode resistance was increased and the output characteristics were lower than those in Examples 2 and 3.

  The method for evaluating a positive electrode material for a non-aqueous electrolyte secondary battery according to the present invention includes various methods used to coat the surfaces of a positive electrode and a negative electrode in a lithium ion secondary battery for electric vehicles and hybrid vehicles that require high output. It is suitable for the evaluation of coating layers and the elucidation of phenomena occurring in electrodes. In the future, the present invention will be useful not only for lithium ion secondary batteries but also for development of new all-solid-state battery materials.

DESCRIPTION OF SYMBOLS 1 Coin type cell 2 Case 2a Positive electrode can 2b Negative electrode can 2c Gasket 3 Electrode 3a Positive electrode 3b Negative electrode 3c Separator 11 Positive electrode thin film electrode 12 Substrate 13 Positive electrode active material
14 Lithium ion conductive oxide

Claims (7)

A surface of the lithium metal composite oxide is coated with a lithium ion conductive oxide, the lithium metal composite oxide and a positive electrode in which the lithium ion conductive oxide is labeled with ⁶ Li;
An electrolyte labeled with ⁷ Li;
After charging and discharging a non-aqueous electrolyte secondary battery comprising a negative electrode,
A method for evaluating a positive electrode material for a non-aqueous electrolyte secondary battery, wherein secondary ion mass spectrometry is performed on the positive electrode to obtain a diffusion profile of ⁶ Li and ⁷ Li in the positive electrode. 2. The non-lithographic composition according to claim 1, wherein an isotope ratio of lithium in the lithium metal composite oxide and the lithium ion conductive oxide is ⁶ Li / ⁷ Li ≧ 90.0 / 10.0. Evaluation method of positive electrode material for aqueous electrolyte secondary battery. 3. The electrolyte solution labeled with the stable concentrated isotope ⁷ Li has a lithium isotope ratio of ⁷ Li / ⁶ Li ≧ 90.0 / 10.0. 4. Evaluation method of positive electrode material for non-aqueous electrolyte secondary battery. The negative electrode is labeled with the stably concentrated isotope ⁷ Li, and the isotope ratio of lithium in the negative electrode is ⁷ Li / ⁶ Li ≧ 90.0 / 10.0. The evaluation method of the positive electrode material for nonaqueous electrolyte secondary batteries in any one of 3. The method for evaluating a positive electrode material for a nonaqueous electrolyte secondary battery according to claim 1, wherein the positive electrode is a thin film, and the thin film is produced by a physical film formation method. 6. The method for evaluating a positive electrode material for a non-aqueous electrolyte secondary battery according to claim 5, wherein the lithium ion conductive oxide constituting the thin film has a thickness of 100 to 500 nm. The method for evaluating a positive electrode material for a non-aqueous electrolyte secondary battery according to claim 6, wherein the thickness fluctuation range of the lithium ion conductive oxide is within 10%.