JP2019036455A - Non-aqueous electrolyte secondary battery - Google Patents

Abstract

To improve the storage characteristics in a non-aqueous electrolyte secondary battery containing 1,2-dimethoxyethane in an electrolyte.SOLUTION: A non-aqueous electrolyte secondary battery includes at least a positive electrode, a negative electrode, and an electrolyte. The electrolyte includes at least 1,2-dimethoxyethane. The positive electrode includes at least a positive electrode active material, a conductive carbon material, and a coating. The coating is formed at least on the surface of the conductive carbon material. The coating includes a component derived from at least one selected from the group consisting of propane sultone, vinylene carbonate, and ethylene sulfite.SELECTED DRAWING: None

Description

  The present disclosure relates to a non-aqueous electrolyte secondary battery.

  Japanese Unexamined Patent Publication No. 9-092265 (Patent Document 1) discloses a combination of a positive electrode active material coated with a carbon material and an electrolytic solution containing 1,2-dimethoxyethane (DME).

Japanese Patent Laid-Open No. 9-092265

  In the electrolytic solution, DME is a solvent that dissolves the supporting salt. The use of DME is expected to reduce resistance. It is considered that DME has a large number of donors and a low viscosity. That is, it is considered that the use of DME facilitates dissociation of the supporting salt and facilitates movement of the ions after dissociation.

  However, DME is inferior in oxidation resistance. Therefore, when the charged battery is stored in a high temperature environment, it is considered that DME is oxidatively decomposed at the positive electrode. As a result, the battery capacity is considered to decrease.

  An object of the present disclosure is to improve storage characteristics in a non-aqueous electrolyte secondary battery including DME in the electrolyte.

  Hereinafter, the technical configuration and effects of the present disclosure will be described. However, the mechanism of action of the present disclosure includes estimation. The scope of the claims should not be limited by the correctness of the mechanism of action.

The non-aqueous electrolyte secondary battery of the present disclosure includes at least a positive electrode, a negative electrode, and an electrolytic solution.
The electrolytic solution contains at least 1,2-dimethoxyethane.
The positive electrode includes at least a positive electrode active material, a conductive carbon material, and a coating.
The coating is formed on at least the surface of the conductive carbon material.
The coating includes a component derived from at least one selected from the group consisting of propane sultone (PS), vinylene carbonate (VC), and ethylene sulfite (ES).

  Conventional positive electrodes include a conductive carbon material (for example, carbon black) in addition to the positive electrode active material. This is to assist electronic conduction in the positive electrode. The conductive carbon material is also referred to as “conductive aid”. The conductive carbon material has a large specific surface area as compared with the positive electrode active material. Therefore, it is considered that oxidative decomposition of DME is likely to occur on the surface of the conductive carbon material.

  According to the new knowledge of the present disclosure, the coating containing the specific component is formed on the surface of the conductive carbon material, so that oxidative decomposition of DME on the surface of the conductive carbon material can be suppressed. That is, improvement in storage characteristics is expected. The specific component is derived from at least one selected from the group consisting of PS, VC and ES. Hereinafter, at least one selected from the group consisting of PS, VC and ES may be abbreviated as “PS etc.”.

FIG. 1 is a schematic diagram illustrating an example of the configuration of the battery according to the present embodiment. FIG. 2 is a schematic diagram illustrating an example of the configuration of the electrode group. FIG. 3 is a conceptual diagram showing composite particles.

  Hereinafter, an embodiment of the present disclosure (referred to as “the present embodiment” in the present specification) will be described. However, the following description does not limit the scope of the claims.

<Nonaqueous electrolyte secondary battery>
Hereinafter, the non-aqueous electrolyte secondary battery may be abbreviated as “battery”.
The battery of this embodiment is typically a lithium (Li) ion secondary battery. However, the battery of this embodiment may be a sodium ion secondary battery, for example.

"Case"
FIG. 1 is a schematic diagram illustrating an example of the configuration of the battery according to the present embodiment. Battery 100 includes a case 90. The case 90 has a rectangular shape (flat rectangular parallelepiped). However, the case should not be limited to a square. The case may be cylindrical, for example. Case 90 includes a container 91 and a lid 92. The lid 92 is joined to the container 91 by laser welding, for example. The lid 92 includes a positive electrode terminal 81 and a negative electrode terminal 82. The lid 92 may include, for example, a liquid injection hole, a gas discharge valve, a current interruption mechanism (CID), and the like.

  Case 90 is sealed. Case 90 may be made of, for example, an aluminum (Al) alloy. However, as long as a predetermined sealing property can be ensured, the case (that is, the exterior body) may be a pouch made of an aluminum laminate film, for example. The case 90 houses the electrode group 50 and the electrolytic solution. A one-dot chain line in FIG. 1 indicates a liquid level of the electrolytic solution. The electrolytic solution is also present in the electrode group 50.

<Electrode group>
FIG. 2 is a schematic diagram illustrating an example of the configuration of the electrode group. The electrode group 50 is a wound type. That is, the electrode group 50 is configured by stacking the positive electrode 10, the separator 30, the negative electrode 20, and the separator 30 in this order and winding them in a spiral shape. The positive electrode 10 is electrically connected to the positive electrode terminal 81 (FIG. 1). The negative electrode 20 is electrically connected to the negative electrode terminal 82 (FIG. 1). In the battery 100, the positive electrode 10, the separator 30, and the negative electrode 20 are each impregnated with an electrolytic solution. That is, the battery 100 includes at least the positive electrode 10, the negative electrode 20, and the electrolytic solution. The electrode group may be a stacked type (stacked type). The stacked electrode group can be configured by alternately stacking the positive electrode and the negative electrode while the separator is sandwiched between the positive electrode and the negative electrode.

《Positive electrode》
In the present embodiment, the positive electrode 10 is a strip-shaped sheet. The positive electrode 10 includes a positive electrode current collector 11 and a positive electrode mixture layer 12. The positive electrode current collector 11 may be, for example, an Al foil. The positive electrode current collector 11 may have a thickness of 10 to 100 μm, for example.

  The positive electrode mixture layer 12 is porous. The positive electrode mixture layer 12 is formed on the surface of the positive electrode current collector 11. The positive electrode mixture layer 12 may be formed on both the front and back surfaces of the positive electrode current collector 11. The positive electrode mixture layer 12 may have a thickness of 10 to 200 μm, for example. The positive electrode mixture layer 12 includes at least a positive electrode active material, a conductive carbon material, and a coating. That is, the positive electrode 10 includes at least a positive electrode active material, a conductive carbon material, and a film.

(Positive electrode active material)
The positive electrode mixture layer 12 may include, for example, 80 to 99% by mass of a positive electrode active material. The positive electrode active material can be a particle. The positive electrode active material may have an average particle diameter of 1 to 30 μm, for example. The “average particle size” in the present specification indicates the particle size at which the cumulative particle volume from the fine particle side is 50% of the total particle volume in the volume-based particle size distribution measured by the laser diffraction scattering method. The particle size is also referred to as “D50”.

The positive electrode active material may have a specific surface area of, for example, 0.1 to ⁵ m ² / g. The “specific surface area” in the present specification is a value calculated by a BET multipoint method based on an isothermal adsorption curve measured by a nitrogen gas adsorption method. The specific surface area can be measured at least three times. An arithmetic average of at least three times can be adopted as the measurement result. The positive electrode active material may have a specific surface area of, for example, 0.1 to ¹ m ² / g.

The positive electrode active material electrochemically occludes and releases charge carriers (typically Li ions). The positive electrode active material should not be particularly limited. The positive electrode active material is, for example, LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , LiNi _x Co _y M _z O ₂ (wherein M is Mn or Al, and x, y, and z are 0 <x <1, 0 <y <1, 0 <z <1, x + y + z = 1), LiFePO _4, or the like. One kind of positive electrode active material may be used alone. Two or more positive electrode active materials may be used in combination.

(Conductive carbon material)
The positive electrode mixture layer 12 may include, for example, 0.5 to 10% by mass of a conductive carbon material. The conductive carbon material may have a ratio of, for example, 1% by mass to 8% by mass with respect to the total of the positive electrode active material and the conductive carbon material. The conductive carbon material may have a ratio of, for example, 3% by mass or more with respect to the total of the positive electrode active material and the conductive carbon material. Thereby, for example, improvement of output characteristics (reduction of resistance) is expected. The conductive carbon material may have a ratio of, for example, 5% by mass or less with respect to the total of the positive electrode active material and the conductive carbon material. Thereby, for example, improvement of storage characteristics is expected.

  The conductive carbon material may be, for example, carbon black, graphite, vapor grown carbon fiber (VGCF), carbon nanotube, or the like. The carbon black may be, for example, acetylene black (AB), ketjen black (registered trademark) (KB), or the like. That is, the conductive carbon material may be at least one selected from the group consisting of AB and KB.

The conductive carbon material can have a large specific surface area as compared with the positive electrode active material. The conductive carbon material may have a specific surface area of, for example, 10 to 2000 m ² / g. The conductive carbon material may have a specific surface area of, for example, 10 to 100 m ² / g. The specific surface area is expected to improve storage characteristics, for example.

(Coating)
The coating is formed on at least the surface of the conductive carbon material. Thereby, oxidative decomposition of DME on the surface of the conductive carbon material can be suppressed. That is, improvement in storage characteristics is expected. The coating may be formed also on the surface of the positive electrode active material, for example.

  The coating includes a component derived from at least one selected from the group consisting of PS, VC, and ES. The coating can be a degradation product such as PS. The coating is considered to have ionic conductivity. For example, a film can be formed by oxidatively decomposing PS or the like added to the electrolytic solution on the surface of the conductive carbon material or the like. However, it is generally considered that the oxidative decomposition of PS or the like and the formation of a coating hardly progress on the surface of the conductive carbon material. For example, it is considered that a film is not substantially formed on the surface of the conductive carbon material simply by dispersing the conductive carbon material in the positive electrode mixture layer 12. It is considered that the coating that can suppress the oxidative decomposition of DME is preferentially formed on the surface of the positive electrode active material.

(Composite particles)
As a method for forming a film on the surface of the conductive carbon material, for example, it is conceivable to coat the surface of the positive electrode active material with a conductive carbon material. That is, the positive electrode active material and the conductive carbon material may constitute composite particles. FIG. 3 is a conceptual diagram showing composite particles. Composite particle 200 includes base material particle 201 and coating layer 202. Base material particle 201 contains a positive electrode active material. The covering layer 202 includes a conductive carbon material.

  According to the composite particle 200, it is expected that the oxidative decomposition of PS or the like and the formation of the coating 300 are promoted in the conductive carbon material (coating layer 202). That is, the coating 300 is expected to be formed on the surface of the conductive carbon material. When the conductive carbon material includes pores, the coating 300 is expected to be formed on the inner walls of the pores.

  When the coating 300 is also formed on the surface of the positive electrode active material (base material particle 201), the resistance may increase. This is probably because the electronic conductivity of the coating 300 is low. According to the composite particle 200, it is expected that the conductive carbon material (the coating layer 202) supplements the electron conduction on the surface of the positive electrode active material. That is, an increase in resistance accompanying the formation of the coating 300 can be suppressed.

  The composite particle 200 can be formed by, for example, a mechanochemical method. In the composite particle 200, the coating layer 202 may cover the entire surface of the base material particle 201 (positive electrode active material) or may partially cover the surface of the base material particle 201. Composite particle 200 may have a coverage of 40% or more and 100% or less, for example. The composite particle 200 may have a coverage of 50% or more, for example, or may have a coverage of 80% or more. For example, an improvement in output characteristics is expected due to the coverage. The composite particle 200 may have a coverage of 90% or less, for example. The coverage is expected to improve storage characteristics, for example.

  The “coverage” in this specification can be measured by an electron microscope-energy dispersive X-ray analyzer (SEM-EDS). That is, an SEM image of the composite particle 200 is acquired. In the SEM image of the composite particle 200, elemental mapping of carbon is performed. The area of the part where carbon is detected is calculated. The ratio (percentage) of the area of the part where carbon is detected to the area of the entire composite particle 200 is the coverage. The coverage can be measured on 10 composite particles 200. An arithmetic average of 10 coverages can be adopted as a measurement result.

  The coating layer 202 may have a thickness of 0.12 μm or more and 0.6 μm or less, for example. Covering layer 202 may have a thickness of, for example, 0.25 μm or more. The thickness is expected to improve output characteristics, for example. The covering layer 202 may have a thickness of 0.37 μm or less, for example. The thickness is expected to improve the storage characteristics, for example.

  “Coating layer thickness” herein may be measured in a cross-sectional SEM image of the composite particle 200. That is, in the cross-sectional SEM image of the composite particle 200, the thickness of the coating layer 202 is measured at 10 points that are randomly extracted. An arithmetic average of 10 points can be adopted as a measurement result.

The composite particles 200 may have a specific surface area of, for example, 0.6 m ² / g or more and 3 m ² / g or less. Composite particle 200 may have a specific surface area of, for example, 1.3 m ² / g or more. The specific surface area is expected to improve output characteristics, for example. Composite particle 200 may have a specific surface area of, for example, 2 m ² / g or less. The specific surface area is expected to improve storage characteristics, for example.

(Binder)
The positive electrode mixture layer 12 may further include a binder. The positive electrode mixture layer 12 may include, for example, a binder of 0.5 to 10% by mass. The binder should not be particularly limited. Examples of the binder include polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), vinylidene fluoride-hexafluoropropylene copolymer [poly (VDF-co-HFP)], polyacrylic acid (PAA), carboxymethylcellulose. (CMC) or the like may be used. One kind of binder may be used alone. Two or more binders may be used in combination.

<Negative electrode>
In the present embodiment, the negative electrode 20 is a strip-shaped sheet. The negative electrode 20 includes a negative electrode current collector 21 and a negative electrode mixture layer 22. The negative electrode current collector 21 may be, for example, a copper (Cu) foil. The negative electrode current collector 21 may have a thickness of 10 to 100 μm, for example.

  The negative electrode mixture layer 22 is porous. The negative electrode mixture layer 22 is formed on the surface of the negative electrode current collector 21. The negative electrode mixture layer 22 may be formed on both the front and back surfaces of the negative electrode current collector 21. Negative electrode composite material layer 22 may have a thickness of 10 to 200 μm, for example.

  The negative electrode mixture layer 22 may include, for example, 80 to 99.5% by mass of the negative electrode active material. The negative electrode active material can be a particle. The negative electrode active material may have an average particle diameter of 1 to 30 μm, for example. The negative electrode active material electrochemically occludes and releases charge carriers. The negative electrode active material should not be particularly limited. The negative electrode active material may be, for example, a carbon material including a graphite structure, silicon, silicon oxide, a silicon-based alloy, tin, a tin-based alloy, or the like. The graphite structure indicates a crystal structure in which carbon hexagonal network surfaces are stacked. Examples of the carbon material containing a graphite structure include graphite, graphitizable carbon, and non-graphitizable carbon. One kind of negative electrode active material may be used alone. Two or more negative electrode active materials may be used in combination.

  The negative electrode mixture layer 22 may include, for example, 0.5 to 20% by mass of a binder. The binder should not be particularly limited. The binder may be, for example, CMC, styrene butadiene rubber (SBR), PAA, or the like. One kind of binder may be used alone. Two or more binders may be used in combination.

<< Separator >>
In this embodiment, the separator 30 is a strip-shaped sheet. The separator 30 is disposed between the positive electrode 10 and the negative electrode 20. The separator 30 is electrically insulating. The separator 30 electrically isolates the positive electrode 10 and the negative electrode 20. The separator 30 is porous. The separator 30 allows the electrolytic solution to pass therethrough. Separator 30 may have a thickness of 10 to 50 μm, for example.

  The separator 30 may be a porous film made of, for example, polyethylene (PE) or polypropylene (PP). Separator 30 may have a single layer structure, for example. Separator 30 may be constituted only by a porous film made of PE, for example. Separator 30 may have, for example, a multilayer structure (for example, a three-layer structure). The separator 30 may be configured, for example, by stacking a PP porous film, a PE porous film, and a PP porous film in this order.

<Electrolyte>
The electrolytic solution includes a solvent and a supporting salt. The solvent is aprotic. That is, the electrolytic solution is a non-aqueous electrolytic solution. The solvent contains at least DME. That is, the electrolytic solution contains at least DME. When the electrolytic solution contains DME, a reduction in resistance is expected. That is, an improvement in output characteristics is expected.

  In the solvent, DME may have a ratio of, for example, 30% by volume or less. In the solvent, DME, for example, may have a ratio of 1% by volume or more, may have a ratio of 10% by volume or more, or may have a ratio of 20% by volume or more.

  The solvent may further contain a cyclic carbonate. In the solvent, the cyclic carbonate may have a ratio of, for example, 10 to 40% by volume. The cyclic carbonate may be, for example, ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), fluoroethylene carbonate (FEC), and the like. One cyclic carbonate may be used alone. Two or more cyclic carbonates may be used in combination.

  The solvent may further contain a chain carbonate. The sum of the chain carbonate and DME in the solvent may have a ratio of 60 to 90% by volume. The chain carbonate may be dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC) or the like. One type of chain carbonate may be used alone. Two or more chain carbonates may be used in combination.

The electrolytic solution may contain a supporting salt of 0.5 to 2 mol / l, for example. The supporting salt may be, for example, LiPF ₆ , LiBF ₄ , Li [N (SO ₂ F) ₂ ], Li [N (CF ₃ SO ₂ ) ₂ ] and the like. One supporting salt may be used alone. Two or more supporting salts may be used in combination.

  The electrolytic solution may further include at least one selected from the group consisting of PS, VC, and ES. PS etc. in electrolyte solution may be the remainder of PS etc. consumed for formation of a coat. The electrolytic solution may contain, for example, 0.1 to 5% by mass of PS. The electrolytic solution may contain, for example, PS exceeding 0% by mass and less than 1% by mass.

<Method for producing non-aqueous electrolyte secondary battery>
The battery 100 of this embodiment can be manufactured, for example, by the following manufacturing method.
(Α) A positive electrode including at least a positive electrode active material and a conductive carbon material is prepared.
(Β) Prepare an electrolytic solution containing at least DME and an additive.
(Γ) A battery including at least a positive electrode, a negative electrode, and an electrolytic solution is manufactured.
(Δ) A film is formed at least on the surface of the conductive carbon material.
The coating includes a component derived from the additive.
The additive is at least one selected from the group consisting of PS, VC and ES.

  In the above (α), a positive electrode including composite particles may be prepared. The composite particles include base material particles and a coating layer. The base material particles include a positive electrode active material. The covering layer includes a conductive carbon material. The composite particles can be formed by, for example, a mechanochemical method. The coating can be formed, for example, by storing the battery in a charged state.

<Applications>
The battery 100 of this embodiment can have excellent output characteristics. Furthermore, the battery 100 of the present embodiment can also have excellent storage characteristics. As an application in which these characteristics are utilized, for example, a driving power source for a hybrid vehicle (HV), a plug-in hybrid vehicle (PHV), an electric vehicle (EV), or the like can be considered. However, the use of the battery 100 of this embodiment should not be limited to the power source for driving a car. The battery 100 of this embodiment is applicable to all uses.

  Hereinafter, examples of the present disclosure will be described. However, the following description does not limit the scope of the claims.

<Experiment 1>
Example 1
The following materials were prepared:
Positive electrode active material: LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (D50 10 μm, specific surface area 0.5 m ² / g)
Conductive carbon material: AB (specific surface area 68 m ² / g)
Binder: PVdF
Solvent: N-methyl-2-pyrrolidone Positive electrode current collector: Al foil (thickness 15 μm)

(Coating treatment)
A circulation type mechanofusion system (manufactured by Hosokawa Micron Corporation) was prepared. A positive electrode active material and a conductive carbon material were charged into the rotating container. A shearing force was applied to the positive electrode active material and the conductive carbon material between the inner wall of the rotating container and the press head (inner piece). Thereby, composite particles were formed. That is, composite particles were formed by a mechanochemical method. The rotation speed of the rotating container was 2000 rpm. The processing time was 30 minutes. The composite particles include base material particles and a coating layer. The base material particles include a positive electrode active material. The covering layer includes a conductive carbon material.

(Preparation of positive electrode)
A positive electrode paste was prepared by mixing the composite particles, the binder, and the solvent. The positive electrode paste was applied to the surface (both front and back surfaces) of the positive electrode current collector and dried to form a positive electrode mixture layer. The positive electrode mixture layer was compressed. Thereby, the positive electrode was prepared. The positive electrode was cut into a strip shape.

The positive electrode has a thickness of 0.16 mm. The positive electrode mixture layer has a density of 3.5 g / cm ³ . The positive electrode mixture layer includes 96% by mass of a positive electrode active material, 1% by mass of a conductive carbon material, and 3% by mass of a binder. The conductive carbon material has a ratio of 1% by mass with respect to the total of the positive electrode active material and the conductive carbon material.

(Preparation of negative electrode)
The following materials were prepared:
Negative electrode active material: Graphite (D50 15 μm)
Binder: CMC, SBR
Solvent: Water Negative electrode current collector: Cu foil (thickness 10 μm)

  A negative electrode paste was prepared by mixing a negative electrode active material, a binder, and a solvent. The negative electrode paste was applied to the surface (both front and back surfaces) of the negative electrode current collector and dried to form a negative electrode mixture layer. The negative electrode mixture layer was compressed. This prepared the negative electrode. The negative electrode was cut into a strip shape.

The negative electrode has a thickness of 0.17 mm. The negative electrode mixture layer has a density of 1.5 g / cm ³ . The negative electrode mixture layer includes 98% by mass of a negative electrode active material and 2% by mass of a binder.

(Formation of electrode group)
A strip separator was prepared. The separator has a three-layer structure. That is, the separator is configured by laminating a porous film made of PP, a porous film made of PE, and a porous film made of PP in this order. The positive electrode, the separator, the negative electrode, and the separator were laminated in this order and wound in a spiral shape. Thereby, a cylindrical electrode group was formed. The electrode group was formed into a flat shape.

(Manufacture of batteries)
A square case was prepared. The positive electrode was electrically connected to the positive terminal. The negative electrode was electrically connected to the negative terminal. The electrode group was stored in the case. An electrolyte was prepared. The electrolytic solution includes the following components.

Solvent: [EC: EMC: DME = 3: 4: 3 (volume ratio)]
Supporting salt: LiPF ₆ (1 mol / l)
Additive: PS (1% by mass)

  80 g of electrolyte was poured into the case. The case was sealed. Thus, a battery (rectangular lithium ion secondary battery) was produced. The battery is designed to have a rated capacity of 25 Ah in the voltage range of 3 to 4.1V. Hereinafter, “C” is used as the unit of current. A current of 1 C discharges the rated capacity in 1 hour.

(aging)
In a 25 ° C. environment, the battery was charged to 4.1 V by constant current charging at 0.2 C. The battery was stored for 10 hours in a 60 ° C. environment. Thus, it is considered that a film was formed on the surface of the positive electrode active material and the surface of the conductive carbon material. The coating is considered to contain a component derived from PS.

<< Examples 2 to 4 >>
As shown in Table 1 below, batteries were respectively produced by the same production method as in Example 1 except that the content of the conductive carbon material, the thickness of the coating layer, and the coverage were changed.

<< Comparative Example 1 >>
As shown in Table 1 below, the battery was manufactured by the same manufacturing method as in Example 1 except that the coating treatment was not performed and an electrolytic solution containing no additive (PS) was used. . In this experiment, for the sample that is not subjected to the coating treatment, a mixture is prepared by simply mixing the positive electrode active material and the conductive carbon material. A positive electrode paste is prepared by mixing the mixture, binder and solvent.

<< Comparative Example 2 >>
As shown in Table 1 below, a battery was manufactured by the same manufacturing method as Comparative Example 1 except that the content of the conductive carbon material was changed.

<< Comparative Examples 3 and 4 >>
As shown in Table 1 below, batteries were produced by the same production method as in Examples 1 and 2 except that an electrolyte solution containing no additive (PS) was used.

<< Comparative Examples 5 and 6 >>
As shown in Table 1 below, batteries were manufactured by the same manufacturing method as in Examples 1 and 2 except that the coating treatment was not performed.

《Reference example》
In the reference example, an electrolyte solution containing no DME was used. The solvent composition of the electrolytic solution is [EC: EMC = 3: 7 (volume ratio)]. Except for this, a battery was manufactured by the same manufacturing method as Comparative Example 1.

<Evaluation>
<Total thickness of coating layer, coverage, specific surface area>
According to the above-mentioned method, the thickness of the coating layer and the coverage of the composite particles were measured. The thickness of the coating layer and the coverage of each sample are shown in Table 1 below.

  According to the above-described method, the specific surface area of the mixture of the positive electrode active material and the conductive carbon material or the specific surface area of the composite particles was measured. The specific surface area of each sample is shown in Table 1 below. In the following Table 1 and Table 2 (described later), the value with “*” in the column of “total surface area” indicates the specific surface area of the composite particles. The value without “*” indicates the specific surface area of the mixture of the positive electrode active material and the conductive carbon material. The mixing ratio of the mixture is shown in each table.

<Output characteristics>
In a 25 ° C. environment, the battery was discharged to 3 V by a constant current discharge of 0.2 C. In the same environment, the SOC (State of charge) of the battery was adjusted to 60%. The battery was discharged by a current of 20C. The amount of voltage drop 10 seconds after the start of discharge was measured. The resistance was calculated by dividing the voltage drop by the current. In this specification, the resistance is referred to as “IV resistance”. The IV resistance of each sample is shown in Table 1 below. In Table 1 below, the value shown in the column of “IV resistance” is a relative value when the IV resistance of the reference example is 100%. The lower the value, the better the output characteristics.

<Storage characteristics>
After the evaluation of the output characteristics, the battery was charged to 4.1 V by constant voltage charging of 4.1 V in a 25 ° C. environment. Thereby, the charge capacity was measured. The battery was stored for 14 days in a 60 ° C. environment. After 14 days, the battery was discharged to 3 V by 0.2 C constant current discharge in a 25 ° C. environment. Thereby, the discharge capacity was measured. The capacity retention rate (percentage) was calculated by dividing the discharge capacity by the charge capacity. The capacity retention rate of each sample is shown in Table 1 below. The higher the capacity retention rate, the better the storage characteristics.

<Result of Experiment 1>
The reference example has high resistance. This is probably because DME is not used.

  Comparative Examples 1 to 4 have a low capacity retention rate. Since no film is formed on the surface of the conductive carbon material, it is considered that oxidative decomposition of DME is promoted during high temperature storage.

  In Comparative Examples 5 and 6, the capacity retention rate is improved compared to Comparative Examples 1 to 4. Since the film containing the component derived from PS is formed on the surface of the positive electrode active material, it is considered that the oxidative decomposition of DME on the surface of the positive electrode active material was suppressed. However, in Comparative Examples 5 and 6, the conductive carbon material is simply dispersed in the positive electrode mixture layer. Therefore, it is considered that a film is not substantially formed on the surface of the conductive carbon material. Therefore, in Comparative Examples 5 and 6, it is considered that the capacity retention rate is lower than in Examples 1 to 4.

  In Examples 1 to 4, the capacity retention rate is improved compared to Comparative Examples 1 to 6. That is, the storage characteristics are improved. This is presumably because a film containing a component derived from PS is formed on the surface of the conductive carbon material. In Examples 1 to 4, since the surface of the positive electrode active material is covered with the conductive carbon material, it is considered that a film is also formed on the surface of the conductive carbon material.

  In addition, it is estimated that the film containing the component derived from VC and ES also has the same effect | action as the film containing the component derived from PS. This is because VC and ES, like PS, can suppress oxidative decomposition of the electrolytic solution by forming a film on the surface of the positive electrode active material. Therefore, it is considered that the coating may contain a component derived from at least one selected from the group consisting of PS, VC and ES.

The following tendency is recognized in Examples 1-4. When the composite particles have a coverage of 80% or more, output characteristics are improved. When the composite particles have a specific surface area of 1.3 m ² / g or more, the output characteristics are improved. When the coating layer has a thickness of 0.25 μm or more, the output characteristics are improved.

<Experiment 2>
Example 5
A battery is manufactured by the same manufacturing method as in Example 1 except that KB (specific surface area 1500 m ² / g) is used as the conductive carbon material instead of AB (specific surface area 68 m ² / g). It was.

Example 6
As shown in Table 2 below, a battery was manufactured by the same manufacturing method as in Example 5 except that the content of the conductive carbon material was changed.

<< Comparative Example 7 >>
As shown in Table 2 below, a battery was manufactured by the same manufacturing method as in Example 5 except that the coating treatment was not performed.

<Evaluation>
Similar to Experiment 1, the thickness, coverage, specific surface area, output characteristics and storage characteristics of the coating layer were evaluated. The results are shown in Table 2 below.

<Result of Experiment 2>
As shown in Table 2 above, even when the conductive carbon material is KB, the storage characteristics are improved by forming a film on the surface of the conductive carbon material. Therefore, it is considered that the conductive carbon material may be at least one selected from the group consisting of AB and KB.

  The embodiments and examples disclosed herein are illustrative and non-restrictive in every respect. The technical scope defined by the description of the claims includes all modifications within the meaning and scope equivalent to the description of the claims.

  DESCRIPTION OF SYMBOLS 10 Positive electrode, 11 Positive electrode collector, 12 Positive electrode compound material layer, 20 Negative electrode, 21 Negative electrode current collector, 22 Negative electrode compound material layer, 30 Separator, 50 Electrode group, 81 Positive electrode terminal, 82 Negative electrode terminal, 90 Case, 91 Container , 92 Lid, 100 battery (non-aqueous electrolyte secondary battery), 200 composite particles, 201 base material particles, 202 coating layer, 300 coating film.

Claims (1)

Including at least a positive electrode, a negative electrode and an electrolyte,
The electrolytic solution includes at least 1,2-dimethoxyethane,
The positive electrode includes at least a positive electrode active material, a conductive carbon material, and a coating,
The coating is formed on at least the surface of the conductive carbon material,
The coating includes a component derived from at least one selected from the group consisting of propane sultone, vinylene carbonate, and ethylene sulfite.
Non-aqueous electrolyte secondary battery.

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