JP2021157957A - Composite lithium salt, positive electrode, and lithium ion secondary battery - Google Patents

Abstract

To provide a composite lithium salt capable of suppressing an increase in electrode resistance after pre-doping in a lithium ion secondary battery, and also to provide a positive electrode including the composite lithium salt and a lithium ion secondary battery using the positive electrode.SOLUTION: A composite lithium salt includes a lithium salt and a coating layer covering at least part of the surface of the lithium salt. Here, the composite lithium salt is characterized in that the lithium salt is lithium oxocarbonate and the coating layer covering at least part of the lithium salt includes a carbon material.SELECTED DRAWING: Figure 1

Description

The present invention relates to a composite lithium salt, a positive electrode, and a lithium ion secondary battery.

As a battery for many electronic devices such as mobile phones and personal computers, a lithium ion secondary battery that is small in size, lightweight, and has a high energy density is used. Furthermore, in recent years, applications have expanded to applications that require higher energy densities, such as automobiles and drones, and lithium-ion batteries are required to have higher performance.

Although there is a particularly strong demand from the market for higher energy density, the energy density has almost reached the theoretical limit when using existing material systems, and the material system should be changed for further high energy density. Is required.

Of these, graphite has been widely used as a negative electrode material in conventional batteries, but silicon-based materials are expected as alternative materials. While this material has a capacity far exceeding that of graphite, it produces a large capacity called irreversible capacity that does not contribute to charging and discharging, and as a result, there is a problem that the capacity of the battery is impaired.

In order to solve this problem, a technique called lithium predoping has been proposed in which lithium that is lost due to its irreversible capacity is supplemented in advance. For example, Patent Document 1 discloses a technique for adding lithium oxalate, Non-Patent Document 1 discloses lithium carbonate, and Non-Patent Document 2 discloses a technique for adding lithium sulfide to a positive electrode. These lithium salts decompose during charging to release lithium, and play a role in offsetting the irreversible capacitance of the negative electrode. Since these lithium salts have a higher capacity than the commonly used positive electrode material, the irreversible capacity of the negative electrode can be efficiently offset, and as a result, the energy density of the battery can be increased.

JP-A-7-254436

Nano Research, 2016, 9 (12), 3903-3913 Electrochimica Acta, 255 (2017), 212-219

However, the conventional method does not satisfy the characteristics, and there is a problem that the resistance increases especially in the electrode after pre-doping and the output characteristics deteriorate.

The present invention has been made in view of the above-mentioned problems of the prior art, and an object of the present invention is to provide a composite lithium salt, a positive electrode, and a lithium ion secondary battery capable of suppressing an increase in resistance of an electrode after predoping.

In order to solve the above problems, the composite lithium salt according to the present invention is a composite lithium salt having a lithium salt and a coating layer covering at least a part of the surface of the lithium salt, and the lithium salt is oxocarbon. It is lithium acid, and the coating layer is characterized by containing a carbon material.

According to this, by further coating lithium oxocarbonate, in which carbon remains after decomposition, with a carbon material, the conductivity of the positive electrode can be sufficiently maintained even after predoping, and an increase in resistance can be suppressed.

The composite lithium salt according to the present invention preferably further contains the carbon material in an amount of 0.1% by mass or more and 50.0% by mass or less based on the total amount of the composite lithium salt.

According to this, it is suitable as the amount of carbon material, and the increase in resistance can be further suppressed. If the amount of carbon is too large, the capacity of the composite lithium salt as a whole decreases, which is not preferable.

The positive electrode according to the present invention is a positive electrode having a positive electrode active material layer containing the composite lithium salt and a positive electrode active material mainly composed of a lithium transition metal oxide, and the average particle size of the composite lithium salt is A. When the average particle size of the positive electrode active material is B, the B / A is 2.5 or more and 150 or less.

According to this, it is suitable as a ratio of the average particle size of the positive electrode active material and the composite lithium salt. In this case, since the positive electrode active material is covered with the carbon component remaining after the composite lithium salt is decomposed and the auxiliary agent, the conductivity of the positive electrode can be sufficiently maintained and the increase in resistance can be suppressed.

The lithium ion secondary battery according to the present invention is characterized by including the above-mentioned positive electrode.

According to the present invention, there are provided a composite lithium salt, a positive electrode, and a lithium ion secondary battery capable of suppressing an increase in resistance of an electrode after predoping.

It is a schematic cross-sectional view of the lithium ion secondary battery of this embodiment.

Hereinafter, preferred embodiments according to the present invention will be described with reference to the drawings. The present invention is not limited to the following embodiments. In addition, the components described below include those that can be easily conceived by those skilled in the art and those that are substantially the same. Further, the components described below can be combined as appropriate.

<Lithium-ion secondary battery>
As shown in FIG. 1, the lithium ion secondary battery 100 according to the present embodiment is arranged adjacent to each other between the plate-shaped negative electrode 20 and the plate-shaped positive electrode 10 and the negative electrode 20 and the positive electrode 10. One end is electrically connected to a laminate 30 including a plate-shaped separator 18, an electrolyte solution containing lithium ions, a case 50 containing these in a sealed state, and a negative electrode 20. It includes a lead 62 whose other end protrudes to the outside of the case, and a lead 60 whose one end is electrically connected to the positive electrode 10 and whose other end protrudes to the outside of the case.

The positive electrode 10 has a positive electrode current collector 12 and a positive electrode active material layer 14 formed on the positive electrode current collector 12. Further, the negative electrode 20 has a negative electrode current collector 22 and a negative electrode active material layer 24 formed on the negative electrode current collector 22. The separator 18 is located between the negative electrode active material layer 24 and the positive electrode active material layer 14.

<Composite lithium salt>
The composite lithium salt according to the present embodiment is a composite lithium salt having a lithium salt and a coating layer covering at least a part of the surface of the lithium salt, wherein the lithium salt is lithium oxocarbonate, and the above. The coating layer contains a carbon material.

According to this, by further coating lithium oxocarbonate, in which carbon remains after decomposition, with a carbon material, the conductivity of the positive electrode can be sufficiently maintained even after predoping, and an increase in resistance can be suppressed.

The method for measuring the resistance of the positive electrode after predoping is not particularly limited, but for example, a battery using a commercially available digital multimeter (for example, manufactured by Hioki Electric Co., Ltd.) or an impedance analyzer (for example, manufactured by Bio-Logic). By measuring the resistance of the above, the effect according to the present embodiment can be sufficiently confirmed. For more detailed analysis, for example, the battery is disassembled in a glove box in an inert atmosphere, and only the positive electrode is taken out to produce a symmetric cell, or a commercially available electrode resistance measuring instrument (for example, Hioki Electric Co., Ltd.) It is also possible to confirm the increase in resistance of only the positive electrode by measuring the resistance of the positive electrode taken out using (manufactured by the company).

Further, it is more preferable that the lithium oxocarbonate contains at least one of the group consisting of lithium deltaate, lithium squaric acid, and lithium croconate.

The composite lithium salt according to the present embodiment further preferably contains the above carbon material in an amount of 0.1% by mass or more and 50.0% by mass or less based on the total amount of the composite lithium salt.

According to this, it is suitable as the amount of carbon material, and the increase in resistance can be further suppressed. If the amount of carbon is too large, the capacity of the composite lithium salt as a whole decreases, which is not preferable.

The method for producing the composite lithium salt according to the present embodiment is not particularly limited, but for example, a lithium oxocarbonate surface is subjected to mechanical milling treatment of lithium oxocarbonate and a conductive auxiliary agent using a ball mill, and a surface of lithium oxocarbonate is used by CVD. Is coated with carbon, lithium oxocarbonate and a conductive additive are dispersed in an appropriate solvent, and then the solvent is removed by drying.

<Positive electrode>
The positive electrode according to the present invention is a positive electrode having a positive electrode active material layer containing the composite lithium salt and a positive electrode active material mainly composed of a lithium transition metal oxide, and the average particle size of the composite lithium salt is A. When the average particle size of the positive electrode active material is B, the B / A is 2.5 or more and 150 or less.

According to this, it is suitable as a ratio of the average particle size of the positive electrode active material and the composite lithium salt. In this case, since the positive electrode active material is covered with the carbon component remaining after the composite lithium salt is decomposed and the auxiliary agent, the conductivity of the positive electrode can be sufficiently maintained and the increase in resistance can be suppressed.

(Positive current collector)
The positive electrode current collector 12 may be any conductive plate material, and for example, a thin metal plate (metal foil) such as aluminum or an alloy thereof or stainless steel can be used.

(Positive electrode active material layer)
The positive electrode active material layer 14 is mainly composed of a positive electrode active material, a positive electrode binder, and a positive electrode conductive auxiliary agent.

(Positive electrode active material)
As the positive electrode active material, occlusion of lithium ions and release, desorption and insertion of lithium ions (intercalation), or counter anions of the lithium ions (e.g., PF 6 ^_-) to doping and dedoping of reversibly As long as it can be advanced, it is not particularly limited, and a known electrode active material can be used. For example, lithium cobalt oxide (LiCoO _2), lithium nickel oxide (LiNiO _2), lithium manganese spinel (LiMn ₂ O _4), and the chemical _{formula: LiNi x Co y Mn z M} a O 2 (x + y + z + a = 1,0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ z ≦ 1, 0 ≦ a ≦ 1, M is a compound represented by one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, and Cr). Metal oxide, lithium vanadium compound Li _a (M) _b (PO ₄ ) _c (However, M = VO or V, and 0.9 ≦ a ≦ 3.3, 0.9 ≦ b ≦ 2.2, 0 .9 ≤ c ≤ 3.3), olivine type LiMPO ₄ (where M indicates one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, Zr), titanium lithium acid _{(Li 4 Ti 5 O 12)} , LiNi x Co y Al z O 2 composite metal oxide such as (0.9 <x + y + z <1.1) , and the like.

(Binder for positive electrode)
The positive electrode binder binds the positive electrode active materials to each other, and also bonds the positive electrode active material layer 14 and the positive electrode current collector 12. The binder may be any one capable of the above-mentioned bonding, for example, fluororesins such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE), cellulose, styrene / butadiene rubber, ethylene / propylene rubber, and polyimide. A resin, a polyamide-imide resin, or the like may be used. Further, an electron conductive conductive polymer or an ionic conductive polymer may be used as the binder. Examples of the electron-conducting conductive polymer include polyacetylene, polythiophene, polyaniline and the like. Examples of the ionic conductive polymer include a composite of a polyether polymer compound such as polyethylene oxide and polypropylene oxide and a lithium salt such as _{LiClO 4} , LiBF ₄ , and LiPF _6. Be done.

The content of the binder in the positive electrode active material layer 14 is not particularly limited, but when added, it is preferably 0.5 to 5 parts by mass with respect to 100 parts by mass of the positive electrode active material.

(Conductive aid for positive electrode)
The conductive auxiliary agent for the positive electrode is not particularly limited as long as it improves the conductivity of the positive electrode active material layer 14, and a known conductive auxiliary agent can be used. Examples thereof include carbon-based materials such as graphite and carbon black, metal fine powders such as copper, nickel, stainless steel and iron, and conductive oxides such as ITO.

<Negative electrode>
(Negative electrode current collector)
The negative electrode current collector 22 may be any conductive plate material, and for example, a thin metal plate (metal foil) such as copper can be used.

(Negative electrode active material layer)
The negative electrode active material layer 24 is mainly composed of a negative electrode active material, a negative electrode binder, and a negative electrode conductive auxiliary agent.

(Negative electrode active material)
The negative electrode active material is not particularly limited as long as it can reversibly proceed with the occlusion and release of lithium ions and the desorption and insertion (intercalation) of lithium ions, and a known electrode active material can be used. .. For example, carbon-based materials such as graphite and hard carbon _{, silicon-based materials such as silicon oxide (SiO x} ) and metallic silicon (Si), metal oxides such as lithium titanate (LTO), and metals such as lithium, tin and zinc. Materials can be mentioned.

When no metal material is used as the negative electrode active material, the negative electrode active material layer 24 may further contain a negative electrode binder and a negative electrode conductive auxiliary agent.

(Binder for negative electrode)
The binder for the negative electrode is not particularly limited, and the same binder as the binder for the positive electrode described above can be used.

(Conductive auxiliary agent for negative electrode)
The conductive auxiliary agent for the negative electrode is not particularly limited, and the same conductive auxiliary agent for the positive electrode described above can be used.

<Electrolytic solution>
The electrolytic solution according to the present invention is mainly composed of a solvent and an electrolyte.

(solvent)
As the solvent, a solvent generally used for a lithium ion secondary battery can be mixed and used at an arbitrary ratio. For example, cyclic carbonate compounds such as ethylene carbonate (EC), propylene carbonate (PC) and butylene carbonate, chain carbonate compounds such as diethyl carbonate (DEC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC), γ-butyrolactone. Cyclic ester compounds such as (GBL), chain ester compounds such as propyl propionate (PrP), ethyl propionate (PrE), and ethyl acetate can be mentioned.

(Electrolytes)
The electrolyte is not particularly limited as long as it is a lithium salt used as an electrolyte for a lithium ion secondary battery. For example, an inorganic acid anion salt such as _{LiPF 6} , LiBF _{4 , or lithium bisoxalate boron, LiCF 3} SO ₃ , Organic acid anionic salts such as (CF ₃ SO ₂ ) ₂ NLi and (FSO ₂ ) _{2 NLi can be used.}

Although the preferred embodiment of the present invention has been described above, the present invention is not limited to the above embodiment.

Hereinafter, the present invention will be described in more detail based on Examples and Comparative Examples, but the present invention is not limited to the following Examples.

[Example 1]
(Preparation of composite lithium salt)
Lithium squarate was used as the lithium salt, and carbon black was used as the carbon material. Lithium squarate: carbon black = 99: 1 (parts by _{volume) was mixed, and then mixed powder: 2} mmφZrO 2 beads = 1: 1 (parts by volume) to a ball mill pot so that the filling ratio was 60%. A composite lithium salt was prepared by filling and ball milling at a rotation speed of 240 rpm for 8 hours.

(Measurement of average particle size of composite lithium salt)
The obtained composite lithium salt was observed at a magnification of 5,000 using a scanning electron microscope (SEM) manufactured by JEOL Ltd., and 500 or more composite lithiums were randomly extracted on the SEM image. The salt was subjected to binarization analysis using image analysis software, and the average particle size was measured. As a result, the average particle size A of the obtained composite lithium salt was 1.0 um.

(Preparation of positive electrode)
_{LiCoO 2} having an average particle size of 5.0 um was used as the positive electrode active material, carbon black was used as the conductive auxiliary agent, and PVDF was used as the binder. LiCoO ₂ : Composite lithium salt: Carbon black: PVDF = 70: 10: 5: 5 (parts by mass) was mixed and dispersed in N-methyl-2-pyrrolidone (NMP) using a hybrid mixer to activate the positive electrode. The slurry for forming the material layer was prepared. This slurry was applied to an aluminum foil having a thickness of 20 μm so as to have a coating amount of 12.0 mg / cm ^2, and dried at 100 ° C. to form a positive electrode active material layer. Then, this was pressure-molded by a roller press to prepare a positive electrode.

(Preparation of negative electrode)
Graphite and metallic silicon (Si) were used as the negative electrode active material, carbon black was used as the conductive auxiliary agent, and polyimide (PI) was used as the binder. Graphite: Si: Carbon black: PI = 64: 16:10: 10 (parts by mass) was mixed and dispersed in N-methyl-2-pyrrolidone (NMP) using a hybrid mixer to form a negative electrode active material layer. The slurry for was prepared. This slurry was applied to a copper foil having a thickness of 20 μm so as to have a coating amount of 8.0 mg / cm ^2, and dried at 100 ° C. to form a negative electrode active material layer. Then, this was pressure-molded by a roller press to prepare a negative electrode.

(Preparation of electrolytic solution)
Ethylene carbonate (EC) and diethyl carbonate (DEC _{) were used as the solvent, lithium hexafluorophosphate (LiPF 6} ) was used as the supporting salt, and fluoroethylene carbonate (FEC) was used as the additive. The mixture was mixed so that EC: DEC = 50: 50 (volume part), and LiPF ₆ was dissolved therein so as to have a concentration of 1.0 mol / L. Subsequently, FEC was added to the solution so as to have a concentration of 3.0% by mass to prepare an electrolytic solution.

(Manufacturing of lithium-ion secondary battery for evaluation)
The positive electrode and the negative electrode prepared above were sequentially laminated via a polyethylene separator. The tab leads were ultrasonically welded to this laminate and then packaged in an aluminum laminate pack. Then, the electrolytic solution prepared above was injected and vacuum-sealed to prepare a lithium ion secondary battery for evaluation.

(Manufacture of lithium-ion secondary battery for resistance comparison)
In order to measure the rate of increase in resistance, a lithium ion secondary battery containing no composite lithium salt in the positive electrode was manufactured. Specifically, in the production of the positive electrode, the active material composition is LiCoO ₂ : carbon black: PVDF = 80: 5: 5 (parts by mass), and the other production methods are the same as above. The next battery was manufactured.

(Measurement of battery capacity)
The evaluation lithium ion secondary battery and the resistance comparison lithium ion secondary battery produced above were placed in a constant temperature bath set at 25 ° C. and evaluated with a charge / discharge test device manufactured by Hokuto Denko Co., Ltd. First, the battery was charged with a constant current charge having a current density of 0.80 mAh / cm ^{2 until the battery voltage became 4.6 V.} Regarding the lithium ion secondary battery for evaluation, the cross section of the electrode was observed by SEM after charging, and it was confirmed that the composite lithium salt in the positive electrode was decomposed and the predoping reaction proceeded. Subsequently, the battery was discharged with a constant current discharge of a current density of 0.80 mAh / cm ^{2 until the battery voltage became 2.8 V.} The discharge capacity obtained at this time was taken as the battery capacity, and the values are shown in Table 1.

(Measurement of resistance increase rate)
The evaluation lithium-ion secondary battery and the resistance comparison lithium-ion secondary battery charged and discharged above are placed in a constant temperature bath set at 25 ° C., and the DC resistance is measured using a digital multimeter manufactured by Hioki Electric. It was measured. The resistance of the lithium ion secondary battery for evaluation _{R 1,} the resistance of the resistor comparative lithium ion secondary battery and _{R 2,} was defined as the resistance increase rate _{= (R 1 / R 2 -1} ) × 100 [%]. The obtained values are shown in Table 1.

[Example 2]
In the preparation of the composite lithium salt, the amount of carbon black used was changed to the value shown by the amount of carbon in Table 1. In addition, the amount of the composite lithium salt used in the preparation of the positive electrode was changed to the value shown in Table 1. A lithium ion secondary battery for evaluation of Example 2 was produced in the same manner as in Example 1 except for the above.

[Example 3]
In the preparation of the composite lithium salt, the amount of carbon black used was changed to the value shown by the amount of carbon in Table 1. In addition, the amount of the composite lithium salt used in the preparation of the positive electrode was changed to the value shown in Table 1. A lithium ion secondary battery for evaluation of Example 3 was produced in the same manner as in Example 1 except for the above.

[Example 4]
In the preparation of the composite lithium salt, the amount of carbon black used was changed to the value shown by the amount of carbon in Table 1. In addition, the amount of the composite lithium salt used in the preparation of the positive electrode was changed to the value shown in Table 1. A lithium ion secondary battery for evaluation of Example 4 was produced in the same manner as in Example 1 except for the above.

[Example 5]
In the preparation of the composite lithium salt, the amount of carbon black used was changed to the value shown by the amount of carbon in Table 1. In addition, the amount of the composite lithium salt used in the preparation of the positive electrode was changed to the value shown in Table 1. A lithium ion secondary battery for evaluation of Example 5 was produced in the same manner as in Example 1 except for the above.

[Example 6]
In the preparation of the composite lithium salt, the amount of carbon black used was changed to the value shown by the amount of carbon in Table 1. In addition, the amount of the composite lithium salt used in the preparation of the positive electrode was changed to the value shown in Table 1. A lithium ion secondary battery for evaluation of Example 6 was produced in the same manner as in Example 1 except for the above.

[Example 7]
In the preparation of the composite lithium salt, the amount of carbon black used was changed to the value shown by the amount of carbon in Table 1. In addition, the amount of the composite lithium salt used in the preparation of the positive electrode was changed to the value shown in Table 1. A lithium ion secondary battery for evaluation of Example 7 was produced in the same manner as in Example 1 except for the above.

[Example 8]
In the preparation of the composite lithium salt, the amount of carbon black used was changed to the value shown by the amount of carbon in Table 1. In addition, the amount of the composite lithium salt used in the preparation of the positive electrode was changed to the value shown in Table 1. A lithium ion secondary battery for evaluation of Example 8 was produced in the same manner as in Example 1 except for the above.

[Example 9]
In the preparation of the composite lithium salt, the lithium salt used was changed to the compound shown in Table 1. A lithium ion secondary battery for evaluation of Example 9 was produced in the same manner as in Example 1 except for the above.

[Example 10]
In the preparation of the composite lithium salt, the lithium salt used was changed to the compound shown in Table 1. A lithium ion secondary battery for evaluation of Example 10 was produced in the same manner as in Example 4 except for the above.

[Example 11]
In the preparation of the composite lithium salt, the lithium salt used was changed to the compound shown in Table 1. A lithium ion secondary battery for evaluation of Example 11 was produced in the same manner as in Example 1 except for the above.

[Example 12]
In the preparation of the composite lithium salt, the lithium salt used was changed to the compound shown in Table 1. A lithium ion secondary battery for evaluation of Example 12 was produced in the same manner as in Example 4 except for the above.

[Example 13]
In the preparation of the composite lithium salt, the lithium salt used was changed to the compound shown in Table 1. A lithium ion secondary battery for evaluation of Example 13 was produced in the same manner as in Example 1 except for the above.

[Example 14]
In the preparation of the composite lithium salt, the lithium salt used was changed to the compound shown in Table 1. A lithium ion secondary battery for evaluation of Example 14 was produced in the same manner as in Example 4 except for the above.

[Example 15]
In the preparation of the composite lithium salt, the average particle size A of the lithium salt used was changed to the compound shown in Table 2. A lithium ion secondary battery for evaluation of Example 15 was produced in the same manner as in Example 1 except for the above.

[Example 16]
In the preparation of the composite lithium salt, the average particle size A of the lithium salt used was changed to the compound shown in Table 2. A lithium ion secondary battery for evaluation of Example 16 was produced in the same manner as in Example 1 except for the above.

[Example 17]
In the preparation of the positive electrode, the average particle size B of the positive electrode active material used was changed to the compound shown in Table 2. A lithium ion secondary battery for evaluation of Example 17 was produced in the same manner as in Example 1 except for the above.

[Example 18]
In the preparation of the positive electrode, the average particle size B of the positive electrode active material used was changed to the compound shown in Table 2. A lithium ion secondary battery for evaluation of Example 18 was produced in the same manner as in Example 16 except for the above.

[Example 19]
In the preparation of the positive electrode, the average particle size B of the positive electrode active material used was changed to the compound shown in Table 2. A lithium ion secondary battery for evaluation of Example 19 was produced in the same manner as in Example 16 except for the above.

[Comparative Example 1]
In the preparation of the composite lithium salt, the lithium salt used was changed to the compound shown in Table 1. A lithium ion secondary battery for evaluation of Comparative Example 1 was produced in the same manner as in Example 1 except for the above.

[Comparative Example 2]
No carbon material was added in the preparation of the composite lithium salt, and the uncomposited lithium salt was used in the preparation of the positive electrode. A lithium ion secondary battery for evaluation of Comparative Example 1 was produced in the same manner as in Example 1 except for the above.

For the evaluation lithium ion secondary batteries prepared in Examples 2 to 19 and Comparative Examples 1 and 2, the battery capacity and the resistance increase rate were measured in the same manner as in Example 1. The results are shown in Table 1.

In all of Examples 1 to 8, the rate of increase in resistance was suppressed as compared with Comparative Example 1 in which lithium oxocarbonate was not used as the lithium salt. It is considered that the conductivity of the positive electrode was maintained even after predoping due to the carbon produced by the decomposition of lithium oxocarbonate in addition to the carbon material.

Further, it was confirmed that in Examples 9 to 14 in which lithium oxocarbonate was changed, the resistance increase rate was suppressed as compared with Comparative Example 1, and the improvement effect common to lithium oxocarbonate was obtained. In addition, from the results of Examples 1 to 14, it was clarified that lithium squalate had the most excellent improvement effect, and lithium deltate and lithium crocate had the next improvement effect.

Further, from the results of Examples 15 to 19, it is shown that the optimum ratio B / A of the average particle size A of the composite lithium salt and the average particle size B of the positive electrode active material is 2.5 or more and 150 or less. Was done.

In Comparative Example 2, although the rate of increase in resistance was suppressed, the battery capacity was extremely low, and predoping did not proceed satisfactorily. From this, it was suggested that when lithium oxocarbonate is used as a pre-doped material, it is necessary to combine it with a carbon material.

INDUSTRIAL APPLICABILITY The present invention provides a composite lithium salt, a positive electrode, and a lithium ion secondary battery capable of suppressing an increase in resistance of an electrode after predoping.

10 ... Positive electrode, 12 ... Positive electrode current collector, 14 ... Positive electrode active material layer, 18 ... Separator, 20 ... Negative electrode, 22 ... Negative electrode current collector, 24 ... Negative electrode active material layer, 30 ... Laminated body, 50 ... Case, 60 , 62 ... Reed, 100 ... Lithium ion secondary battery.

Claims (4)

A composite lithium salt having a lithium salt and a coating layer covering at least a part of the surface of the lithium salt.
The lithium salt is lithium oxocarbonate,
A composite lithium salt, wherein the coating layer contains a carbon material. The composite lithium salt according to claim 1, wherein the carbon material is contained in an amount of 0.1% by mass or more and 50.0% by mass or less based on the entire composite lithium salt. A positive electrode having a positive electrode active material layer containing the composite lithium salt according to claim 1 or 2 and a positive electrode active material mainly composed of a lithium transition metal oxide.
A positive electrode having a B / A of 2.5 or more and 150 or less, where A is the average particle size of the composite lithium salt and B is the average particle size of the positive electrode active material. A lithium ion secondary battery including the positive electrode according to claim 3.

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