JP2018037289A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery in which a negative electrode has high thermal stability, and the worsening of cycle characteristics is suppressed.SOLUTION: A nonaqueous electrolyte secondary battery comprises: a positive electrode; a negative electrode; and a nonaqueous electrolyte solution. The negative electrode includes graphite. The nonaqueous electrolyte solution includes: 0.01-0.05 mol/l of a first component; 0.01-0.75 mol/l of a second component; and lithium ions. The first component is at least one of cesium ions and rubidium ions. The second component is at least one kind selected from a group consisting of vinylene carbonate, fluoroethylene carbonate, vinylethylene carbonate and ethylene sulfite.SELECTED DRAWING: None

Description

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

Japanese Patent Application Laid-Open No. 2007-018945 (Patent Document 1) discloses a nonaqueous electrolytic solution containing LiPF ₆ as a supporting electrolyte salt and further added with cesium ions and rubidium ions.

JP 2007-018945 A

Graphite is known as a negative electrode active material for non-aqueous electrolyte secondary batteries. The charge carrier of the non-aqueous electrolyte secondary battery is typically lithium ion (Li ⁺ ). Li ⁺ in the non-aqueous electrolyte is reversibly inserted and desorbed between the graphite layers in the ionic state. Therefore, the nonaqueous electrolyte secondary battery can operate without producing metallic lithium (Li) having low thermal stability.

However, for example, during overcharge, Li ⁺ may become metal Li and precipitate on the surface of the negative electrode. It is known that metal Li grows in a dendrite shape. Dendritic metal Li has a large surface area and is highly reactive. Therefore, when the metal Li grows in a dendrite shape, the thermal stability of the negative electrode is lowered.

Cesium ions (Cs ⁺ ) and rubidium ions (Rb ⁺ ) have been found to have an effect of suppressing the growth of metal Li in a dendritic form. That is, when Li ⁺ and Cs ^{+ and the} like coexist in the non-aqueous electrolyte, the metal Li is likely to grow in a planar shape rather than a dendrite shape. This is expected to improve the thermal stability of the negative electrode.

However, Cs ⁺ and the like can be inserted between the graphite layers as well as Li ⁺ . When Cs ⁺ or the like is inserted between the graphite layers, the cycle characteristics of the battery are significantly deteriorated.

  Therefore, the present disclosure aims to provide a non-aqueous electrolyte secondary battery in which the negative electrode has high thermal stability and the deterioration of cycle characteristics is suppressed.

  The non-aqueous electrolyte secondary battery of the present disclosure includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. The negative electrode contains graphite. The non-aqueous electrolyte contains a first component of 0.01 mol / l or more and 0.05 mol / l or less, a second component of 0.01 mol / l or more and 0.75 mol / l or less, and lithium ions. The first component is at least one of cesium ions and rubidium ions. The second component is at least one selected from the group consisting of vinylene carbonate, fluoroethylene carbonate, vinyl ethylene carbonate, and ethylene sulfite.

In the non-aqueous electrolyte secondary battery of the present disclosure, the non-aqueous electrolyte contains a first component (Cs ⁺ , Rb ⁺ ). The first component has an action of growing the metal Li in a planar shape. Thereby, the thermal stability of the negative electrode is improved.

The present disclosure provides new knowledge regarding the first component (Cs ⁺ , Rb ⁺ ). That is, a film formed by a specific compound on the surface of graphite allows the passage of Li ⁺ but inhibits the transmission of Cs ⁺ and Rb ⁺ . As such compounds, vinylene carbonate (VC), fluoroethylene carbonate (FEC), vinyl ethylene carbonate (VEC), and ethylene sulfite (ES) have been found. Therefore, the nonaqueous electrolytic solution of the present disclosure contains these compounds as the second component. When the non-aqueous electrolyte contains the second component, the first component (Cs ⁺ , Rb ⁺ ) is suppressed from being inserted between the graphite layers.

  However, the content of the first component is 0.01 mol / l or more and 0.05 mol / l or less, and the content of the second component is 0.01 mol / l or more and 0.75 mol / l or less.

When the second component is less than 0.01 mol / l, the film is not sufficiently formed, and the first component (Cs ⁺ , Rb ⁺ ) may be inserted between the graphite layers. When the second component exceeds 0.75 mol / l, the coating becomes excessively dense, so that the penetration of Li ⁺ is also inhibited. Thereby, battery resistance may increase.

  On the other hand, if the first component is less than 0.01 mol / l, the effect of growing the metal Li in a planar shape may not be obtained. When the first component exceeds 0.05 mol / l, even if the second component is 0.75 mol / l (upper limit), a part of the first component may be inserted between the graphite layers.

FIG. 1 is a conceptual diagram illustrating a configuration of a nonaqueous electrolyte secondary battery according to an embodiment of the present disclosure.

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

<Nonaqueous electrolyte secondary battery>
The nonaqueous electrolyte secondary battery of the present embodiment has high thermal stability and suppresses deterioration of cycle characteristics. Such characteristics are suitable, for example, for in-vehicle applications that require extremely high safety and long life. In-vehicle use refers to a power source for an electric vehicle such as a hybrid vehicle (HV). However, the use of the nonaqueous electrolyte secondary battery of the present embodiment is not limited to such in-vehicle use. The non-aqueous electrolyte secondary battery of this embodiment can be applied to all uses.

  FIG. 1 is a conceptual diagram showing the configuration of the non-aqueous electrolyte secondary battery according to this embodiment. In the following description, the nonaqueous electrolyte secondary battery may be abbreviated as “battery”. The battery 10 includes a battery case 11. The battery case 11 in FIG. 1 has a rectangular shape (flat rectangular parallelepiped). However, the battery case 11 is not limited to a square shape, and may be, for example, a cylindrical shape. The battery case 11 may be made of, for example, aluminum (Al) or Al alloy. However, the battery case 11 may be, for example, an aluminum laminate film bag or the like as long as it has a predetermined sealing property. Although not shown, the battery case 11 may be provided with a liquid injection port, a safety valve, a pressure-sensitive current interruption mechanism (CID), and the like.

  In the battery case 11, an electrode group 12 and a non-aqueous electrolyte solution 13 are disposed. That is, the battery 10 includes a nonaqueous electrolytic solution 13. A part of the non-aqueous electrolyte 13 is stored at the bottom of the battery case 11 and the other is held in the electrode group 12. The electrode group 12 is led out of the battery case 11 by a current collecting terminal 14. The electrode group 12 includes a positive electrode, a negative electrode, and a separator. That is, the battery 10 includes a positive electrode and a negative electrode.

  The electrode group 12 may be a wound electrode group or a stacked electrode group. The wound electrode group is an electrode group formed by winding a belt-like positive electrode and a belt-like negative electrode. The stacked electrode group is an electrode group configured by alternately stacking a rectangular positive electrode and a rectangular negative electrode. In any electrode group, a separator is disposed between the positive electrode and the negative electrode.

《Positive electrode》
The positive electrode can be a strip-like, rectangular or other sheet member. The positive electrode includes a current collector and a positive electrode mixture. The current collector may be, for example, an Al foil. The positive electrode mixture is, for example, applied in layers on the surface of the current collector. The positive electrode mixture contains a positive electrode active material. The positive electrode active material is, for example, a lithium-containing metal oxide. Examples of the lithium-containing metal oxide include LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , LiNi _1/3 Co _1/3 Mn _1/3 O _{2, and the} like. Alternatively, the positive electrode active material may be a lithium-containing metal phosphate such as LiFePO ₄ .

  In addition to the positive electrode active material, the positive electrode mixture may contain, for example, a conductive material, a binder, and the like. For example, the positive electrode mixture contains 80 to 98% by mass of a positive electrode active material, 1 to 10% by mass of a conductive material, and 1 to 10% by mass of a binder. Examples of the conductive material include acetylene black (AB), thermal black, vapor grown carbon fiber (VGCF), and the like. Examples of the binder include polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), polyacrylic acid (PAA), and the like.

<Negative electrode>
The negative electrode can be a strip-like, rectangular or other sheet member. The negative electrode includes a current collector and a negative electrode mixture. The current collector may be, for example, a copper (Cu) foil. The negative electrode mixture is, for example, applied in layers on the surface of the current collector.

  The negative electrode mixture contains graphite. That is, the negative electrode contains graphite. Graphite functions as a negative electrode active material. As long as the negative electrode contains graphite, the negative electrode may contain other negative electrode active materials. Examples of other negative electrode active materials include graphitizable carbon, non-graphitizable carbon, silicon, silicon oxide, silicon carbide, tin, and tin oxide.

  The negative electrode mixture may contain, for example, a binder in addition to the negative electrode active material. The negative electrode mixture contains, for example, 90 to 99% by mass of a negative electrode active material and 1 to 10% by mass of a binder. Examples of the binder include carboxymethyl cellulose (CMC) and styrene butadiene rubber (SBR).

<Non-aqueous electrolyte>
The non-aqueous electrolyte includes an aprotic solvent and a supporting electrolyte salt. The supporting electrolyte salt is dissolved in an aprotic solvent.

(Aprotic solvent)
The aprotic solvent may be, for example, a mixture of cyclic carbonate, chain carbonate and the like. Examples of the cyclic carbonate include ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, and the like. Examples of the chain carbonate include ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and diethyl carbonate (DEC). The mixing ratio of the cyclic carbonate and the chain carbonate may be, for example, about “cyclic carbonate: chain carbonate = 1: 9 to 5: 5” in volume ratio. The aprotic solvent may contain ethers such as 1,2-dimethoxyethane (DME), esters such as methyl propionate (MP), and the like.

(Supporting electrolyte salt)
The supporting electrolyte salt is a Li salt. That is, the nonaqueous electrolytic solution contains Li ⁺ and a counter anion. The non-aqueous electrolyte contains, for example, Li ⁺ of 0.5 mol / l or more and 2.0 mol / l or less. The counter anion compensates for the charge of the metal cation (Li ⁺ , Cs ⁺ , Rb ⁺ ) in the non-aqueous electrolyte. The counter anion may be, for example, PF ₆ ⁻ , BF ₄ ⁻ , [N (SO ₂ F) ₂ ] ⁻ , [N (SO ₂ CF ₃ ) ₂ ] ⁻ or the like. The nonaqueous electrolytic solution may contain two or more kinds of counter anions.

(First component)
The nonaqueous electrolytic solution contains a first component of 0.01 mol / l or more and 0.05 mol / l or less. The first component is at least one of Cs ⁺ and Rb ⁺ . The nonaqueous electrolytic solution may contain one of Cs ⁺ or Rb ⁺ , or may contain both Cs ⁺ and Rb ⁺ . However, if the non-aqueous electrolyte solution contains both Cs ⁺ and Rb ^+, requires that the sum of Cs ⁺ and Rb ⁺ is less than 0.01mоl / l or more 0.05mоl / l.

  The first component has an action of growing the metal Li in a planar shape. Thereby, improvement of the thermal stability of the negative electrode is expected. If the first component is less than 0.01 mol / l, the desired effect may not be obtained. When the first component exceeds 0.05 mol / l, a part of the first component may be inserted between the graphite layers even when the non-aqueous electrolyte contains a second component (VC or the like) described later. . When the first component is inserted between the graphite layers, the cycle characteristics are deteriorated. From the viewpoint of thermal stability of the negative electrode, the non-aqueous electrolyte preferably contains a first component of 0.03 mol / l or more and 0.05 mol / l or less.

  The first component is added to the non-aqueous electrolyte in the form of a salt, for example. The counter anion of the first component may be the same as or different from the counter anion of the Li salt described above.

(Second component)
The nonaqueous electrolytic solution contains a second component of 0.01 mol / l or more and 0.75 mol / l or less. The second component is a specific compound. That is, the second component is at least one selected from the group consisting of VC, FEC, VEC and ES. The nonaqueous electrolytic solution may contain one kind of compound alone among these compounds, or may contain two or more kinds of compounds. However, when the non-aqueous electrolyte contains two or more compounds, the total of the two or more compounds needs to be 0.01 mol / l or more and 0.75 mol / l or less. The film formed by VC has particularly good Li ⁺ permeability. Therefore, the second component is particularly preferably VC.

The second component is reduced and decomposed on the surface of graphite to form a film. The film formed by the second component allows the permeation of Li ⁺ but inhibits the permeation of the first component (Cs ⁺ , Rb ⁺ ). That is, the film formed by the second component suppresses the first component from being inserted between the graphite layers. If the second component is less than 0.01 mol / l, the film is not sufficiently formed, and the first component may be inserted between the graphite layers. When the second component exceeds 0.75 mol / l, the coating becomes excessively dense, so that the penetration of Li ⁺ is also inhibited. Thereby, battery resistance may increase. From the viewpoint of battery resistance, the nonaqueous electrolytic solution preferably contains a second component of 0.15 mol / l or more and 0.40 mol / l or less.

(Other ingredients)
The nonaqueous electrolytic solution may contain other components as long as it contains the first component, the second component, and Li ⁺ . Examples of other components include gas generating agents such as cyclohexylbenzene and biphenyl. The gas generant promotes the operation of the pressure sensitive CID during overcharge.

<< Separator >>
The separator is disposed between the positive electrode and the negative electrode. The separator holds the non-aqueous electrolyte in the internal gap while isolating the positive electrode and the negative electrode.

  The separator is typically a resinous porous membrane. The separator can be, for example, a polyethylene (PE) porous film, a polypropylene (PP) porous film, or the like. The separator may have a multilayer structure. For example, the separator may be configured by laminating a porous film of PP, a porous film of PE, and a porous film of PP in this order. Furthermore, the separator may include a heat resistant layer on the surface thereof. The heat-resistant layer includes inorganic particles such as alumina, for example.

  Examples will be described below. However, the following examples do not limit the scope of the invention of the present disclosure.

<Manufacture of non-aqueous electrolyte secondary batteries>
Nonaqueous electrolyte secondary batteries according to Examples and Comparative Examples were manufactured as follows.

Example 1
1. Preparation of positive electrode The following materials were prepared.

(Positive electrode material)
Cathode active material: LiNi _1/3 Co _1/3 Mn _1/3 O ₂
Conductive material: AB
Binder: PVdF
Solvent: N-methyl-2-pyrrolidone (NMP)
Current collector: Al foil

A positive electrode mixture slurry was prepared by mixing a positive electrode active material, a conductive material, a binder, and a solvent. The composition of the positive electrode mixture was “positive electrode active material: conductive material: binder = 93: 4: 3” by mass ratio. The positive electrode mixture slurry was applied to the surface (both front and back surfaces) of the current collector and dried. As a result, the positive electrode mixture was applied in layers on the surface of the current collector. As a result, a positive electrode including a current collector and a positive electrode mixture was obtained. The coating amount of the positive electrode mixture was 19 mg / cm ² per side. The positive electrode was processed to have a predetermined dimension. Thereby, the positive electrode which is a strip-shaped sheet member was obtained.

2. Production of Negative Electrode The following materials were prepared.

(Negative electrode material)
Negative electrode active material: Graphite Binder: CMC and SBR
Solvent: Water Current collector: Cu foil

A negative electrode mixture slurry was prepared by mixing a negative electrode active material, a binder, and a solvent. The composition of the negative electrode mixture was “negative electrode active material: CMC: SBR = 99: 0.5: 0.5” by mass ratio. The negative electrode mixture slurry was applied to the surface (both front and back surfaces) of the current collector and dried. As a result, the negative electrode mixture was applied in layers on the surface of the current collector. As a result, a negative electrode including a current collector and a negative electrode mixture was obtained. The amount of the negative electrode mixture applied was 9 mg / cm ² per side. The negative electrode was processed to have a predetermined dimension. Thereby, the negative electrode which is a strip | belt-shaped sheet | seat member was obtained.

3. Preparation of Nonaqueous Electrolytic Solution A nonaqueous electrolytic solution having the following composition was prepared. Li ⁺ and Cs ⁺ were added to the nonaqueous electrolyte in the form of a salt (LiPF ₆ , CsPF ₆ ) and dissolved in the nonaqueous electrolyte.

(Composition of non-aqueous electrolyte)
Solvent: EC / DMC / EMC = 3/4/3 (v / v / v)
First component (Cs ⁺ ): 0.05 mol / l
Second component (VC): 0.15 mol / l
Li ⁺ : 1.1 mol / l
PF ₆ ⁻ : 1.15 mol / l

4). Assembly A separator having a thickness of 20 μm was prepared. This separator has a three-layer structure in which a PP porous film, a PE porous film, and a PP porous film are laminated in this order.

  The electrode group was configured by winding the belt-like positive electrode and the belt-like negative electrode. The electrode group was configured to have a flat outer shape. A separator was disposed between the positive electrode and the negative electrode.

  A current collecting terminal was attached to the electrode group. The electrode group was housed in a battery case made of Al. This battery case is a rectangular case having a width of 148 mm, a height of 90 mm, and a depth of 27 mm. A predetermined amount of non-aqueous electrolyte was injected from a liquid injection port provided in the battery case. The injection port was sealed with a stopper. From the above, the nonaqueous electrolyte secondary battery according to Example 1 was manufactured. This battery is designed to operate in the range of 3.0-4.1V. The design capacity of the battery is 35 Ah.

<< Comparative Example 1 >>
A non-aqueous electrolyte secondary battery according to Comparative Example 1 was manufactured by the same procedure as Example 1 except that a non-aqueous electrolyte not containing the first component was used.

<< Example 2, Example 3, Comparative Example 2 >>
As shown in Table 1 below, Example 2, Example 3 and Comparative Example 2 were performed in the same procedure as Example 1 except that the content of the first component in the non-aqueous electrolyte was changed. A non-aqueous electrolyte secondary battery according to was manufactured.

<< Comparative Example 3 >>
A non-aqueous electrolyte secondary battery according to Comparative Example 3 was manufactured by the same procedure as Example 1 except that a non-aqueous electrolyte not containing the second component was used.

<< Examples 4 to 6, Comparative Example 4 >>
As shown in Table 1 below, Examples 4 to 6 and Comparative Example 4 are performed according to the same procedure as Example 1 except that the content of the second component in the nonaqueous electrolytic solution is changed. A non-aqueous electrolyte secondary battery was manufactured.

Example 7
A nonaqueous electrolyte secondary battery according to Example 7 was manufactured by the same procedure as Example 1 except that the first component was changed from Cs ⁺ to Rb ⁺ .

<< Example 8, Example 9, Example 10 >>
Except that the second component is changed from VC to VEC, FEC, and ES, respectively, the non-aqueous electrolyte secondary solution according to Examples 8, 9, and 10 is performed in the same procedure as in Example 1. A battery was manufactured.

<Evaluation>
Each battery was evaluated as follows.

<Measurement of battery resistance>
The battery was charged to 3.7 V in a 25 ° C. environment. The charged battery was discharged at a current of 35 A for 10 seconds. The amount of voltage drop during discharge (ΔV) was measured. The battery resistance was calculated by dividing the amount of voltage drop (ΔV) by the current (35 A) during discharge. The results are shown in Table 1 above.

<Evaluation of thermal stability of negative electrode>
The battery was overcharged to 4.4V with a current of 35A. The overcharged battery was disassembled and the negative electrode was recovered. Thermal analysis of the negative electrode was performed by DSC (Differential Scanning Calorimetry). Thereby, the temperature of the exothermic peak of the negative electrode was measured. The results are shown in Table 1 above. The higher the temperature of the exothermic peak, the higher the thermal stability of the negative electrode.

<Evaluation of cycle characteristics>
The battery cycle characteristics were evaluated in a 25 ° C. environment. The following “CC charge → pause → CC discharge → pause” was defined as one cycle, and the cycle was repeated 1000 times. Here, “CC (Constant Current)” indicates a constant current method.

(Cycle conditions)
CC charging: Current = 35A, end voltage = 4.1V
Rest: 10 minutes CC discharge: Current = 35A, end voltage = 3.0V
Rest: 10 minutes

  The capacity retention rate after the cycle (percentage) was calculated by dividing the discharge capacity at the 1000th cycle by the discharge capacity at the 1st cycle. The results are shown in Table 1 above. It shows that the higher the post-cycle capacity retention rate, the more the degradation of cycle characteristics is suppressed.

<Result>
As shown in Table 1 above, in Examples 1 to 3, the thermal stability of the negative electrode was improved as compared with Comparative Example 1. This is probably because Cs ⁺ (first component) suppressed the growth of metal Li in a dendrite shape. From the results of Example 3, it can be seen that if the first component is 0.01 mol / l or more, improvement in thermal stability of the negative electrode can be expected.

Examples 1 to 3 showed cycle characteristics comparable to Comparative Example 1. It is considered that the coating with VC (second component) suppresses the insertion of Cs ⁺ between the graphite layers.

From the results of Comparative Example 2, it can be seen that when the content of Cs ⁺ exceeds 0.05 mol / l, the cycle characteristics may be deteriorated. If Cs ⁺ is present excessively, it is considered that a part of Cs ⁺ is inserted between the graphite layers even if a film of VC is formed.

  From the results of Example 4, it can be seen that a decrease in cycle characteristics is sufficiently suppressed with a VC (second component) of 0.01 mol / l. Therefore, the second component may be 0.01 mol / l or more. From the results of Example 5, Example 6, and Comparative Example 4, it can be seen that when the VC (second component) exceeds 0.75 mol / l, the battery resistance increases. Therefore, the second component should be 0.75 mol / l or less.

From the results of Example 7, it can be seen that Rb ⁺ has the same effect as Cs ⁺ . Therefore, the first component may be at least one of Cs ⁺ and Rb ⁺ .

  From the results of Example 1 and Examples 8 to 10, it can be seen that VEC, FEC and ES also have the same effect as VC. Therefore, the second component may be at least one selected from the group consisting of VC, FEC, VEC and ES.

  The embodiments and examples disclosed herein are illustrative in all respects and should not be construed as being restrictive. The scope of the invention of the present disclosure is shown not by the above description but by the scope of claims, and is intended to include all modifications within the meaning and scope equivalent to the scope of claims.

  DESCRIPTION OF SYMBOLS 10 Battery (nonaqueous electrolyte secondary battery), 11 Battery case, 12 Electrode group, 13 Nonaqueous electrolyte, 14 Current collection terminal.

Claims (1)

Positive electrode,
A negative electrode and a non-aqueous electrolyte,
The negative electrode contains graphite,
The non-aqueous electrolyte contains a first component of 0.01 mol / l or more and 0.05 mol / l or less, a second component of 0.01 mol / l or more and 0.75 mol / l or less, and lithium ions,
The first component is at least one of a cesium ion and a rubidium ion,
The second component is at least one selected from the group consisting of vinylene carbonate, fluoroethylene carbonate, vinyl ethylene carbonate, and ethylene sulfite.
Non-aqueous electrolyte secondary battery.

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