WO2013008439A1

WO2013008439A1 - Non-aqueous electrolyte and non-aqueous electrolyte secondary cell employing same

Info

Publication number: WO2013008439A1
Application number: PCT/JP2012/004411
Authority: WO
Inventors: 出口　正樹
Original assignee: パナソニック株式会社
Priority date: 2011-07-08
Filing date: 2012-07-06
Publication date: 2013-01-17

Abstract

Provided is a non-aqueous electrolyte containing a non-aqueous solvent, and a solute dissolved in the non-aqueous solvent. The non-aqueous solvent contains ethylene carbonate, propylene carbonate, and a fluorinated ester having three fluorine atoms, all of the fluorine atoms in the fluorinated ester being bonded to terminal carbon atoms not situated at the α position of the carbonyl group, and the carbon number, excluding the ester group of the fluorinated ester, being 5 or fewer. The weight proportion W_EC of the ethylene carbonate in the total of the ethylene carbonate, the propylene carbonate, and the fluorinated ester is 5-30 wt%, the weight proportion W_PC of the propylene carbonate in the total is 20-55 wt%, and the weight proportion W_FE of the fluorinated ester in the total is 20-50 wt%.

Description

Nonaqueous electrolyte and nonaqueous electrolyte secondary battery using the same

The present invention relates to a non-aqueous electrolyte and a non-aqueous electrolyte secondary battery, and more particularly to a composition of the non-aqueous electrolyte.

A nonaqueous electrolyte contained in a nonaqueous electrolyte secondary battery represented by a lithium ion secondary battery includes a nonaqueous solvent and a solute dissolved in the nonaqueous solvent. As the solute, lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), or the like is used.

Nonaqueous solvents include chain carbonates, cyclic carbonates, cyclic carboxylic acid esters, chain ethers, cyclic ethers and the like. Examples of the chain carbonate include diethyl carbonate (DEC) and ethyl methyl carbonate (EMC). Examples of the cyclic carbonate include ethylene carbonate (EC), propylene carbonate (PC), vinylene carbonate (VC), and the like. Cyclic carbonates such as EC and PC have a high dielectric constant and are advantageous for obtaining high lithium ion conductivity, but have high viscosity. Therefore, it is common to use a mixture with a chain carbonate such as DEC or EMC having a low viscosity.

In a nonaqueous electrolyte secondary battery, a carbon material is generally used as a negative electrode material. The carbon material may cause a side reaction with the non-aqueous electrolyte as described above, and may deteriorate battery characteristics. In order to suppress a side reaction between the carbon material and the nonaqueous electrolyte, it is important to form a coating (SEI: solid-electrolyte-interface) on the electrode surface. Since the coating affects battery characteristics, it is important to control its structure and properties.

As a technique for forming a film having high chemical stability on the electrode surface, Patent Document 1 discloses that 1,3-propane sultone (PS) and VC are predetermined for non-aqueous solvents including PC, EC and DEC. It is proposed to be included as an additive in the composition of

On the other hand, addition of a fluorine compound such as a fluorinated ester to the non-aqueous electrolyte is also being studied (Patent Documents 2 and 3). In Patent Document 2, 0.01 to 50% by weight of a specific fluorine compound is added to the nonaqueous electrolyte for the purpose of suppressing the decomposition of the nonaqueous electrolyte at the interface between the positive electrode and the separator. For example, 0.01 to 15% by weight of a fluorinated ester compound is added to a mixed solution containing EC and EMC in a volume ratio of 4: 6.

Patent Document 3 proposes that a non-aqueous solvent contains a chain fluorinated ester having a specific composition. According to Patent Document 3, the chain fluorinated ester and light metal ions are appropriately solvated and stabilized, so that the precipitation of dendritic metal in the negative electrode is suppressed.

JP 2004-355974 A JP 2002-343424 A Japanese Patent Laid-Open No. 08-298134

Among cyclic carbonates, it is desirable to use PC having a high oxidation resistance and a low melting point as a main solvent. However, when the non-aqueous electrolyte contains a large amount of PC, the negative electrode is likely to deteriorate along with the reductive decomposition of PC. Therefore, in order to improve the cycle characteristics of the nonaqueous electrolyte secondary battery, it is important to suppress the reduction reaction of PC at the negative electrode, and to suppress the deterioration and gas generation of the negative electrode.

In Patent Document 1, since the reduction potentials of PS and PC are close to each other, reduction decomposition of PC is likely to occur preferentially over film formation by PS. Therefore, it is difficult to sufficiently suppress the side reaction between the negative electrode and PC.

Patent Document 1 proposes a non-aqueous electrolyte having a large weight ratio of DEC. A chain carbonate such as DEC has an oxidation potential lower than that of a cyclic carbonate, and is easily oxidatively decomposed on the positive electrode side. Further, the chain carbonate is easily reductively decomposed because the molecular structure has a low electron density of carbonyl carbon. Therefore, when DEC is used as the main solvent as in Patent Document 1, a large amount of gas is generated during the charge / discharge cycle, and the charge / discharge capacity of the battery tends to decrease.

In Patent Documents 2 and 3, the combination of the fluorinated ester and other components, the number of fluorine atoms of the fluorinated ester, the position where the fluorine atoms are bonded, etc. are not fully considered, and the oxidation resistance and non-aqueous electrolyte are not considered. Compatibility with other components contained tends to decrease.

Furthermore, the non-aqueous electrolyte proposed in Patent Document 2 contains EMC which is a chain carbonate as a main solvent. EMC is susceptible to oxidative degradation and reductive degradation, like DEC. Therefore, in the proposal of patent document 2, suppression of gas generation is insufficient.

Therefore, an object of the present invention is to provide a nonaqueous electrolyte capable of suppressing gas generation during a charge / discharge cycle of a nonaqueous electrolyte secondary battery.

One aspect of the present invention includes a nonaqueous solvent and a solute dissolved in the nonaqueous solvent. The nonaqueous solvent includes ethylene carbonate, propylene carbonate, and a fluorinated ester having three fluorine atoms. In the fluorinated ester, all the fluorine atoms are bonded to the terminal carbon atom not located at the α-position of the carbonyl group, and the number of carbon atoms excluding the carbon atom contained in the ester group of the fluorinated ester is 5 or less, ethylene carbonate, propylene carbonate, the weight ratio W _EC ethylene carbonate to the total of the fluorinated ester is from 5 to 30 wt%, the weight ratio W _PC propylene carbonate relative to the total of 20 to 55 wt% Further, the present invention relates to a non-aqueous electrolyte in which the weight ratio W _FE of the fluorinated ester in the total is 20 to 50% by weight.

Another aspect of the present invention includes a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and the nonaqueous electrolyte described above, and the negative electrode is attached to the negative electrode core material and the negative electrode core material. A non-aqueous electrolyte, wherein the negative electrode mixture layer includes graphite particles, a water-soluble polymer that coats the surface of the graphite particles, and a binder that bonds the graphite particles coated with the water-soluble polymer. The present invention relates to a secondary battery.

According to the present invention, gas generation during the charge / discharge cycle of the nonaqueous electrolyte secondary battery can be effectively suppressed.

While the novel features of the invention are set forth in the appended claims, the invention will be further described by reference to the following detailed description, taken in conjunction with the other objects and features of the invention, both in terms of construction and content. It will be well understood.

1 is a longitudinal sectional view schematically showing a configuration of a nonaqueous electrolyte secondary battery according to an embodiment of the present invention.

The nonaqueous electrolyte according to the present invention includes a nonaqueous solvent and a solute dissolved in the nonaqueous solvent. The non-aqueous solvent contains ethylene carbonate (EC), propylene carbonate (PC), and a fluorinated ester having three fluorine atoms as a main solvent. The main solvent preferably occupies 80% by weight or more and 100% by weight or less of the whole nonaqueous electrolyte, and more preferably 90-100% by weight. With such a composition, since the non-aqueous solvent contains only a small amount of chain carbonate that is relatively easily decomposed, the effect of suppressing gas generation is enhanced.

EC and PC have a high dielectric constant and are advantageous for improving lithium ion conductivity. Since PC has high oxidation resistance, it is particularly advantageous for suppressing gas generation at the positive electrode. Further, since PC (melting point: −49 ° C.) has a lower melting point than EC (melting point: 37 ° C.), it is advantageous for improving rate characteristics at low temperatures. On the other hand, EC forms a lithium ion conductive film on the negative electrode. This coating is considered to play a role of enabling insertion and desorption reactions of lithium ions with respect to the negative electrode, and improves the charge acceptance of the negative electrode. Further, since EC forms a relatively dense film, an effect of suppressing the reductive decomposition of the nonaqueous electrolyte can be obtained. In addition, since the EC-derived film alone has low heat resistance, it is easily peeled off at high temperatures. On the other hand, a hybrid film derived from EC and a fluorinated ester is relatively dense and has high heat resistance.

Here, cyclic carbonates such as EC and PC have a relatively high viscosity. Therefore, when the non-aqueous electrolyte contains a large amount of cyclic carbonate, lithium ion conductivity at a low temperature may be lowered. For this reason, it is considered desirable to mix cyclic carbonates such as EC and PC and chain carbonates (DEC, EMC, etc.) having a low viscosity.

However, chain carbonates are prone to oxidative decomposition and reductive decomposition, causing gas generation. Therefore, in the present invention, a predetermined fluorinated ester having 5 or less carbon atoms excluding carbon atoms contained in the ester group (—COO—) is added. Such a fluorinated ester is chemically stable and has a low viscosity as compared with a chain carbonate. Therefore, in the present invention, the non-aqueous electrolyte may not contain a chain carbonate. Therefore, the viscosity of the nonaqueous electrolyte can be sufficiently lowered while suppressing gas generation. In addition, the rate characteristics at low temperatures can be improved while suppressing gas generation during charge / discharge cycles and during storage in a high temperature environment. In the fluorinated ester, the number of carbon atoms excluding the ester group is more preferably 3 to 4 in terms of viscosity and compatibility with other components.

The fluorinated ester according to the present invention forms a film containing LiF on the negative electrode during the initial charge / discharge of the battery. A film containing LiF has high heat resistance and is difficult to peel off even at high temperatures. A film containing LiF alone is a relatively sparse film, but is considered to be a dense and highly heat-resistant film by being mixed with a film derived from EC or an additive. For example, EC forms a relatively dense film, but a film derived from EC has low heat resistance, and thus is easily peeled off at high temperatures. On the other hand, a hybrid film derived from EC and a fluorinated ester is relatively dense and has high heat resistance. Therefore, the hybrid coating is difficult to peel off even at high temperatures and can suppress the reductive decomposition of the nonaqueous electrolyte, particularly PC, regardless of the temperature.

In addition, by using the fluorinated ester, gas generation can be greatly suppressed even at the end of the charge / discharge cycle of the nonaqueous electrolyte secondary battery. This is considered to be because even if lithium is deposited on the negative electrode surface at the end of the cycle, a film containing LiF is formed on the surface of lithium by the action of the fluorinated ester, and the reaction between lithium and the nonaqueous electrolyte is suppressed. .

The mechanism by which a film containing LiF is formed on the negative electrode is considered as follows. Since the fluorine atom has a high electronegativity, the electron density of the carbon atom to which the fluorine atom of the fluorinated ester is bonded is low. Starting from this carbon atom, the fluorinated ester is susceptible to reductive decomposition. Fluoride ions desorbed by reductive decomposition are combined with lithium ions and deposited on the negative electrode surface. As a result, it is considered that a film containing LiF is formed.

The coating containing LiF can be confirmed using X-ray photoelectron spectroscopy (XPS). Specifically, the battery after charge / discharge is disassembled, the negative electrode is sampled, and the sample is washed with a solvent such as EMC. Thereby, the components of the remaining nonaqueous electrolyte are removed. The washed sample is dried. Thereafter, XPS measurement is performed to obtain an energy spectrum of emitted photoelectrons. By analyzing the obtained spectrum, the Li and F elements present on the surface of the sample can be identified. Moreover, an element can be quantified from the ratio of the peak areas of each element. Furthermore, since the peak position of each element shifts due to the difference in chemical state, the peak derived from the LiF bond can be identified from the shift amount.

The fluorinated ester according to the present invention has three fluorine atoms. Such a fluorinated ester is excellent in compatibility with other components contained in the non-aqueous solvent and oxidation resistance. If the number of fluorine atoms contained in the molecule of the fluorinated ester is 2 or less, sufficient oxidation resistance cannot be obtained. Moreover, it is thought that the component of the film containing LiF decreases and the heat resistance of the whole film falls. On the other hand, if the number of fluorine atoms contained in the fluorinated ester molecule is 4 or more, sufficient compatibility with other components contained in the non-aqueous solvent cannot be obtained, and it is difficult to prepare a stable non-aqueous electrolyte. become. Moreover, there are too many components of the film containing LiF, and the denseness of the film is lowered. Therefore, it is important that the number of fluorine atoms contained in the fluorinated ester is 3.

The fluorine atom contained in the fluorinated ester has a strong electron withdrawing property. Therefore, when a fluorine atom and a hydrogen atom are bonded to the same carbon atom, the acidity of the hydrogen atom is increased and a gas such as hydrogen fluoride is easily generated. Therefore, in the fluorinated ester according to the present invention, all the fluorine atoms are bonded to the terminal carbon atom not located at the α-position of the carbonyl group. That is, the fluorinated ester according to the present invention has only one terminal CF ₃ that is not located at the α-position of the carbonyl group. Since fluorine atoms and hydrogen atoms are not bonded to the same carbon atom, generation of hydrogen fluoride as described above is suppressed.

When a group such as CF ₃ or CH ₂ FCF ₂ is present at the α-position of the carbonyl group, the electron density of the carbonyl group becomes too low and reductive decomposition becomes large. Further, a group such as CH ₂ FCF ₂ CH ₂ is less likely to form a good film as compared with groups such as CF ₃ CH ₂ and CF ₃ CH ₂ CH ₂ having all the fluorine atoms at the terminals. When all the fluorine atoms are present at the terminal, it is considered that there is no steric hindrance and the reduction reaction of the carbon atom having a low electron density to which the fluorine atom is bonded proceeds more easily.

The weight ratio W _FE of the fluorinated ester in the total of EC, PC and fluorinated ester is 20 to 50% by weight. By making _WFE 20% by weight or more, in addition to forming a film on the negative electrode, the ratio of fluorinated ester having high oxidation resistance in the non-aqueous solvent increases, and gas generation can be suppressed more favorably. In addition, the rate characteristics at a low temperature can be improved. By making _WFE 50% by weight or less, good ion conductivity can be secured, and an appropriate amount of a stable coating can be formed on the negative electrode. _WFE is preferably 25% by weight or more, and more preferably 30% by weight or more. Moreover, it is preferable that it is 45 weight% or less, and it is more preferable that it is 40 weight% or less. These upper and lower limits can be arbitrarily combined.

The fluorinated ester is represented by the general formula: R ₁ —COO—R ₂ , R ₁ is a carbon chain having 2 to 4 carbon atoms, and three fluorine atoms are bonded to the terminal carbon atom. , R ₂ preferably has a structure having a carbon chain having 1 to 3 carbon atoms. Since the fluorinated ester having such a structure is easy to form a stable film and has high oxidation resistance, gas generation can be satisfactorily suppressed. Moreover, since such a fluorinated ester has a low viscosity, the rate characteristics at low temperature are further improved. Specifically, CF ₃ CH ₂ CH ₂ —COO—CH ₂ CH ₃ , CF ₃ CH ₂ —COO—CH ₃ , CF ₃ CH ₂ —COO—CH ₂ CH ₃ , CF ₃ CH ₂ —COO—CH ₂ Examples thereof include compounds such as CH ₂ CH ₃ , CF ₃ CH ₂ CH ₂ —COO—CH ₃ , and CF ₃ CH ₂ CH ₂ CH ₂ —COO—CH ₃ . R ₁ is more preferably a carbon chain having 3 to 4 carbon atoms, and particularly preferably a carbon chain having 3 carbon atoms. R ₂ is more preferably a carbon chain having 1 to 2 carbon atoms.

The fluorinated ester is represented by the general formula: R ₃ —COO—R ₄ , R ₃ is a carbon chain having 1 to 3 carbon atoms, R ₄ is a carbon chain having 2 to 4 carbon atoms, and The terminal carbon atom may have a structure in which three fluorine atoms are bonded. Since the fluorinated ester having such a structure also easily forms a stable film and has high oxidation resistance, gas generation can be satisfactorily suppressed. Moreover, since the viscosity is low, the rate characteristics at low temperature are further improved. Specifically, CH ₃ CH ₂ CH ₂ —COO—CH ₂ CF ₃ , CH ₃ —COO—CH ₂ CF ₃ , CH ₃ —COO—CH ₂ CH ₂ CF ₃ , CH ₃ —COO—CH ₂ CH ₂ Examples thereof include compounds such as CH ₂ CF ₃ , CH ₃ CH ₂ —COO—CH ₂ CF ₃ , and CH ₃ CH ₂ —COO—CH ₂ CH ₂ CF ₃ . R ₃ is more preferably a carbon chain having 2 to 3 carbon atoms. R ₄ is more preferably a carbon chain having 2 to 3 carbon atoms.
The above fluorinated esters may be used alone or in any combination of a plurality of types. Among these, at least one of CF ₃ CH ₂ CH ₂ —COO—CH ₂ CH ₃ and CF ₃ CH ₂ —COO—CH ₂ CH ₃ is preferably used. The carbon chain having n carbon atoms is a group having n carbon atoms, specifically, an alkyl group or a fluoroalkyl group.

EC and, PC and, PC weight ratio W _PC to the total of the fluorinated ester is 20 to 55% by weight. From the viewpoint of suppressing gas generation due to oxidative decomposition of EC, it is preferable that the weight ratio of PC is relatively larger than the weight ratio of EC. Furthermore, since the melting point of PC is lower than that of EC, solidification of the nonaqueous electrolyte at a low temperature can be suppressed by increasing the weight ratio of PC. Therefore, it is advantageous in terms of the low temperature characteristics of the nonaqueous electrolyte secondary battery. W _PC is preferably 30 wt% or more, more preferably 40 wt% or more. Moreover, it is preferable that it is 55 weight% or less, and it is more preferable that it is 50 weight% or less. These upper and lower limits can be arbitrarily combined.

When a relatively large amount of PC is contained at 20 to 55% by weight, the oxidation resistance of the nonaqueous electrolyte and the rate characteristics of the battery are improved, but the negative electrode tends to deteriorate with the reduction decomposition of PC. However, in the present invention, since a film containing LiF is formed on the negative electrode, even when a relatively large amount of PC is contained as described above, the reductive decomposition of PC and the deterioration of the negative electrode can be satisfactorily suppressed. Further, when the nonaqueous electrolyte contains an appropriate amount of EC, the effect of suppressing the reductive decomposition of PC is increased.

When W _PC is less than 20 wt%, the amount of EC in the nonaqueous solvent is relatively large, it may not be sufficiently suppress the generation of gas. When W _PC exceeds 55 wt%, PC in the negative electrode is reduced and decomposed, there is a case where _{_{_{CH 4, C 3 H 6,}}} C 3 H 8 , etc. gases. By setting the weight ratio of PC within the above range, the generation of gas derived from EC can be suppressed and the reductive decomposition of PC can be suppressed.

The ratio W _PC / W _FE of the weight ratio W _PC of the PC to the total weight ratio W _FE of the fluorinated ester in the total of EC, PC and fluorinated ester is 0.6 to 2.2. It is preferable that By setting W _PC / W _FE to be 0.6 or more, it becomes easy to form an appropriate amount of a stable film on the negative electrode. By setting W _PC / W _FE to 2.2 or less, gas generation derived from reductive decomposition of PC at the negative electrode can be more effectively suppressed. W _PC / W _FE is more preferably 1.0 to 1.7.

The EC weight ratio W _{EC in} the total of EC, PC and fluorinated ester is 5 to 30% by weight. If _WEC is less than 5% by weight, a coating derived from EC may not be sufficiently formed on the negative electrode. As a result, lithium ions are less likely to be occluded or released from the negative electrode, and charge acceptance may be reduced. When W _EC exceeds 30% by weight, in particular occur oxidative decomposition of EC at the positive electrode, there are cases where the greater the amount of gas generation. Further, when W _EC exceeds 30% by weight, is formed excessive amount of the coating on the negative electrode reduces the charge acceptance, there is a case where Li is easily precipitated. When _WEC is 5 to 30% by weight, preferably 10 to 15% by weight, the amount of gas generated due to oxidative decomposition of EC is reduced, and an appropriate amount of a stable coating is formed on the negative electrode. The charge / discharge capacity and rate characteristics of the nonaqueous electrolyte secondary battery are greatly improved.

The non-aqueous electrolyte of the present invention may contain at least one selected from a cyclic carbonate having a C═C unsaturated bond and a sultone compound as an additive. When the additive includes a cyclic carbonate having a C═C unsaturated bond, a stable film is formed mainly on the negative electrode, and decomposition of the nonaqueous electrolyte is more effectively suppressed. Furthermore, when the additive contains a sultone compound, a better film is formed on the negative electrode, and a film is also formed on the positive electrode. By forming a film on the positive electrode, it is possible to effectively suppress oxidative decomposition of the nonaqueous solvent at the positive electrode in a high-temperature environment. In addition, the effect of suppressing the reductive decomposition of the nonaqueous solvent (particularly PC) by the negative electrode coating is enhanced.

The amount of additive, ie, the total amount of sultone compound and cyclic carbonate having a C═C unsaturated bond, preferably occupies 0.2 to 5% by weight of the whole non-aqueous electrolyte, 0.5 to 5% by weight or It is more preferably 1 to 5% by weight, and more preferably 2 to 4% by weight. By including an additive in the above range, an appropriate amount of a stable film can be formed on the positive electrode and the negative electrode. As a result, reductive decomposition of PC at the negative electrode and oxidative decomposition of EC at the positive electrode are further effectively suppressed.

The above additive forms a dense film with a small amount of addition, so it is effective in suppressing the reductive decomposition of PC, but it has low heat resistance and is therefore easily peeled off at high temperatures. On the other hand, the film derived from the additive is considered to be a dense and highly heat-resistant film by being mixed with the film derived from the fluorinated ester. That is, the hybrid (hybrid) film derived from the additive and the fluorinated ester is hardly peeled off even at a high temperature and can suppress the reductive decomposition of the nonaqueous electrolyte, particularly PC, regardless of the temperature.

By adjusting the total amount of the sultone compound and the cyclic carbonate having a C═C unsaturated bond to 0.2% by weight or more of the whole non-aqueous electrolyte, the non-aqueous electrolyte containing EC, PC, and fluorinated ester has an appropriate amount. An amount of a stable coating is likely to be formed. Moreover, in the nonaqueous electrolyte containing EC, PC and fluorinated ester, the total amount of the sultone compound and the cyclic carbonate having a C═C unsaturated bond is 5% by weight or less of the whole nonaqueous electrolyte. Thus, it is difficult to form a coating film excessively, and lithium ion insertion and desorption reactions are not inhibited, and sufficient charge acceptability is easily obtained.

Ratio of weight ratio W _{C of} cyclic carbonate having C═C unsaturated bond in additive and weight ratio W _SL of sultone compound: W _C / W _{SL satisfies} 0.5 ≦ W _C / W _SL ≦ 3 It is preferable to satisfy. When W _C / W _SL is 0.5 or more, it becomes difficult for the sultone compound to form an excessive film on the negative electrode, and a film made of a cyclic carbonate having a C═C unsaturated bond is also easily formed on the negative electrode. As a result, good charge acceptability is ensured, and cycle characteristics are unlikely to deteriorate. In addition, the film resistance of the negative electrode is not increased, and the discharge characteristics at low temperatures are less likely to occur.

On the other hand, when W _C / W _SL is 3 or less, oxidative decomposition and gas generation of the cyclic carbonate having a C═C unsaturated bond are suppressed. Moreover, it becomes easy to obtain the effect of suppressing the reductive decomposition at the negative electrode of PC by the sultone compound and the effect of suppressing the oxidative decomposition of the cyclic carbonate having a C═C unsaturated bond at the positive electrode. W _C / W _SL is more preferable to satisfy the _{_{0.75 ≦ W C / W SL ≦}} 1.5.

Specific examples of the cyclic carbonate having a C═C unsaturated bond include vinylene carbonate (VC), vinyl ethylene carbonate (VEC), and divinyl ethylene carbonate (DVEC). These cyclic carbonates having a C═C unsaturated bond may be used alone or in combination of two or more. Among these, vinylene carbonate is more preferable because a thin and dense film can be formed on the negative electrode and the film resistance is low.

Specific examples of the sultone compound include 1,3-propane sultone (PS), 1,4-butane sultone, 1,3-propene sultone (PRS), and the like. A sultone compound may be used individually by 1 type, and may be used in combination of 2 or more type. Of these, 1,3-propane sultone is more preferable because it has a high effect of suppressing reductive decomposition of PC.

Among these, it is particularly preferable that the additive contains both vinylene carbonate and 1,3-propane sultone. As a result, a coating film derived from 1,3-propane sultone is formed on the positive electrode, and a coating film derived from vinylene carbonate and a coating derived from 1,3-propane sultone are formed on the negative electrode. Since the coating derived from vinylene carbonate can suppress an increase in coating resistance, the charge acceptability is improved. Therefore, deterioration of cycle characteristics can be suppressed. The coating derived from 1,3-propane sultone can further suppress the reductive decomposition of PC at the negative electrode and greatly reduce the generation of gases such as CH ₄ , C ₃ H ₆ , and C ₃ H ₈ .

When only vinylene carbonate is added, since vinylene carbonate has low oxidation resistance, it may be oxidized and decomposed at the positive electrode to generate gas such as CO ₂ . By adding 1,3-propane sultone together with vinylene carbonate, 1,3-propane sultone forms a film on the surface of the positive electrode, and oxidative decomposition of vinylene carbonate can be suppressed. This makes it possible to greatly suppressed the generation of gas such as CO _2.

As described above, by adding at least one of a sultone compound and a cyclic carbonate having a C═C unsaturated bond to a nonaqueous solvent containing EC, PC, and a fluorinated ester, a more excellent film A non-aqueous electrolyte can be obtained in which is preferentially formed on the electrode. Such a coating does not hinder charge acceptance and is stable.

The additive is not limited to the above sultone compound and a cyclic carbonate having a C═C unsaturated bond. Further, the nonaqueous electrolyte may further contain other compounds. Other compounds are not particularly limited, and examples thereof include fluorinated aromatic compounds such as fluorobenzene (FB), cyclic sulfones such as sulfolane, fluorine-containing compounds such as fluorinated ethers, and cyclic carboxylic acid esters such as γ-butyrolactone. Can be mentioned. In the whole nonaqueous electrolyte, the weight ratio of these other additives is preferably 10% by weight or less. These other additives may be used alone or in combination of two or more.

The viscosity of the nonaqueous electrolyte at 25 ° C. is, for example, 3 to 7 mPa · s. Thereby, the fall of the rate characteristic especially at low temperature can be suppressed. The viscosity is measured using a rotary viscometer and a cone plate type spindle.

The solute of the nonaqueous electrolyte is not particularly limited. Examples thereof include inorganic lithium fluorides such as LiPF ₆ and LiBF ₄ and lithium imide compounds such as LiN (CF ₃ SO ₂ ) ₂ and LiN (C ₂ F ₅ SO ₂ ) ₂ .

The nonaqueous electrolyte secondary battery of the present invention will be described.
The nonaqueous electrolyte secondary battery includes a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and the nonaqueous electrolyte. The non-aqueous electrolyte secondary battery is preferably charged and discharged at least once before use. Charging / discharging is preferably performed in a range where the potential of the negative electrode is 0.05 to 1.5 V with respect to lithium. By performing such charge and discharge, at least a part of the fluorinated ester contained in the nonaqueous electrolyte is decomposed to form a film containing LiF on the negative electrode. The amount W _FE of the fluorinated ester in the non-aqueous electrolyte contained in the battery after charging and discharging is, for example, 18 to 48% by weight.

In a preferred embodiment, the negative electrode includes a negative electrode core material and a negative electrode mixture layer attached to the negative electrode core material. The negative electrode mixture layer includes graphite particles, a water-soluble polymer that covers the surface of the graphite particles, and a water-soluble material. And a binder for adhering the graphite particles coated with the polymer.

By coating the surface of graphite particles with a water-soluble polymer, a non-aqueous electrolyte containing a fluorinated ester can easily penetrate into the negative electrode. As a result, the non-aqueous electrolyte can be present almost uniformly on the surface of the graphite particles, and a film containing LiF can be easily and uniformly formed during initial charging. Therefore, an appropriate amount of a stable film is formed on the negative electrode, and the reductive decomposition of PC can be satisfactorily suppressed. That is, by using the water-soluble polymer and the non-aqueous electrolyte in combination, gas generation can be significantly suppressed as compared with the case where each is used alone.

The type of the water-soluble polymer is not particularly limited, and examples thereof include cellulose derivatives, polyacrylic acid, polyvinyl alcohol, polyvinyl pyrrolidone, and derivatives thereof. Of these, the water-soluble polymer preferably contains a cellulose derivative or polyacrylic acid. As the cellulose derivative, methyl cellulose, carboxymethyl cellulose, Na salt of carboxymethyl cellulose and the like are preferable. The molecular weight of the cellulose derivative is preferably 10,000 to 1,000,000. The molecular weight of polyacrylic acid is preferably from 5,000 to 1,000,000.

The amount of the water-soluble polymer contained in the negative electrode mixture layer is preferably 0.4 to 2.8 parts by weight, more preferably 0.5 to 1.5 parts by weight per 100 parts by weight of the graphite particles. ˜1 part by weight is particularly preferred. When the amount of the water-soluble polymer is within the above range, the water-soluble polymer can cover the surface of the graphite particles with a high coverage. Moreover, the graphite particle surface is not excessively covered with the water-soluble polymer, and the increase in the internal resistance of the negative electrode is also suppressed.

The binder to be included in the negative electrode mixture layer is not particularly limited, but is preferably a particulate binder having rubber elasticity. The average particle diameter of the particulate binder is preferably 0.1 μm to 0.3 μm, more preferably 0.1 to 0.26 μm, and particularly preferably 0.1 to 0.15 μm. Preferably, it is 0.1 to 0.12 μm. The average particle size of the binder is, for example, an SEM photograph of 10 binder particles taken with a transmission electron microscope (manufactured by JEOL Ltd., acceleration voltage 200 kV), and the average of these maximum diameters. Calculate as a value.

As the binder, which is particulate, has rubber elasticity, and an average particle size of 0.1 μm to 0.3 μm, a polymer containing a styrene unit and a butadiene unit is particularly preferable. Such a polymer is excellent in elasticity and stable at the negative electrode potential.

The amount of the binder contained in the negative electrode mixture layer is preferably 0.4 to 1.5 parts by weight, more preferably 0.4 to 1 part by weight, and more preferably 0.4 to 0.1 parts by weight per 100 parts by weight of the graphite particles. 7 parts by weight is particularly preferred. When the water-soluble polymer coats the surface of the graphite particles, the slippage between the graphite particles is good, so that the binder attached to the surface of the graphite particles coated with the water-soluble polymer has sufficient shear. It receives force and acts effectively on the graphite particle surface. In addition, a particulate binder having a small average particle size increases the probability of contact with the surface of graphite particles coated with a water-soluble polymer. Therefore, sufficient binding properties are exhibited even with a small amount of the binder.

A metal foil or the like is used as the negative electrode core material. When producing the negative electrode of a lithium ion secondary battery, generally copper foil, copper alloy foil, etc. are used as a negative electrode core material. Of these, copper foil (which may contain components other than copper of 0.2 mol% or less) is preferable, and electrolytic copper foil is particularly preferable.

The water permeation rate of the negative electrode mixture layer is preferably 3 to 40 seconds. The water penetration rate of the negative electrode mixture layer can be controlled by, for example, the coating amount of the water-soluble polymer. When the water permeation speed of the negative electrode mixture layer is 3 to 40 seconds, the non-aqueous electrolyte particularly easily penetrates into the negative electrode. Thereby, reductive decomposition of PC can be suppressed more favorably. The water penetration rate of the negative electrode mixture layer is more preferably 10 to 25 seconds.

The water permeation rate of the negative electrode mixture layer is measured in an environment of 25 ° C., for example, by the following method.
2 μl of water is dropped to bring the droplet into contact with the surface of the negative electrode mixture layer. By measuring the time until the contact angle θ of water with respect to the surface of the negative electrode mixture layer becomes smaller than 10 °, the water permeation rate of the negative electrode mixture layer is obtained. The contact angle of water with the surface of the negative electrode mixture layer may be measured using a commercially available contact angle measuring device (for example, DM-301 manufactured by Kyowa Interface Science Co., Ltd.).

The porosity of the negative electrode mixture layer is preferably 24 to 28%. By controlling the porosity of the negative electrode mixture layer containing graphite particles whose surfaces are coated with water-soluble polymer to 24 to 28%, the nonaqueous electrolyte can more easily penetrate into the negative electrode. Thereby, since a uniform film is easily formed on the negative electrode, the reductive decomposition of PC can be suppressed more favorably.

The negative electrode contains graphite particles as a negative electrode active material. Here, the graphite particles are a general term for particles including a region having a graphite structure. Thus, the graphite particles include natural graphite, artificial graphite, graphitized mesophase carbon particles, and the like.

The diffraction image of graphite particles measured by the wide-angle X-ray diffraction method has a peak attributed to the (101) plane and a peak attributed to the (100) plane. Here, the ratio of the peak intensity I (101) attributed to the (101) plane and the peak intensity I (100) attributed to the (100) plane is 0.01 <I (101) / I. (100) <0.25 is preferably satisfied, and 0.08 <I (101) / I (100) <0.2 is more preferably satisfied. The peak intensity means the peak height.

The average particle diameter of the graphite particles is preferably 14 to 25 μm, more preferably 16 to 23 μm. When the average particle diameter is within the above range, the slipping property of the graphite particles in the negative electrode mixture layer is improved, the filling state of the graphite particles is improved, and it is advantageous for improving the adhesive strength between the graphite particles. The average particle diameter means the median diameter (D50) in the volume particle size distribution of the graphite particles. The volume particle size distribution of the graphite particles can be measured by, for example, a commercially available laser diffraction type particle size distribution measuring apparatus.

The average circularity of the graphite particles is preferably 0.9 to 0.95, and more preferably 0.91 to 0.94. When the average circularity is included in the above range, the slipping property of the graphite particles in the negative electrode mixture layer is improved, which is advantageous in improving the filling properties of the graphite particles and the adhesion strength between the graphite particles. The average circularity is represented by 4πS / L ² (where S is the area of the orthographic image of graphite particles, and L is the perimeter of the orthographic image). For example, the average circularity of 100 arbitrary graphite particles is preferably in the above range.

The specific surface area S of the graphite particles is preferably 3 to 5 m ² / g, more preferably 3.5 to 4.5 m ² / g. When the specific surface area is included in the above range, the slipperiness of the graphite particles in the negative electrode mixture layer is improved, which is advantageous for improving the adhesive strength between the graphite particles. Further, the preferred amount of the water-soluble polymer that covers the surface of the graphite particles can be reduced.

In order to coat the surface of the graphite particles with a water-soluble polymer, it is desirable to produce a negative electrode by the following production method.
A preferred method includes a step (step (i)) of mixing graphite particles, water, and a water-soluble polymer dissolved in water, and drying the resulting mixture to obtain a dry mixture. For example, a water-soluble polymer is dissolved in water to prepare a water-soluble polymer aqueous solution. The obtained water-soluble polymer aqueous solution and graphite particles are mixed, and then the water is removed and the mixture is dried. Thus, once the mixture is dried, the water-soluble polymer efficiently adheres to the surface of the graphite particles, and the coverage of the graphite particle surface with the water-soluble polymer is increased.

The viscosity of the water-soluble polymer aqueous solution is preferably controlled to 1000 to 10,000 mPa · s at 25 ° C. The viscosity is measured using a B-type viscometer at a peripheral speed of 20 mm / s and using a 5 mmφ spindle. The amount of graphite particles mixed with 100 parts by weight of the water-soluble polymer aqueous solution is preferably 50 to 150 parts by weight.

The drying temperature of the mixture is preferably 80 to 150 ° C., and the drying time is preferably 1 to 8 hours.
Next, the obtained dry mixture, the binder, and the liquid component are mixed to prepare a negative electrode mixture slurry (step (ii)). By this step, the binder adheres to the surface of the graphite particles coated with the water-soluble polymer. Because of the good slippage between the graphite particles, the binder attached to the surface of the graphite particles coated with the water-soluble polymer receives sufficient shearing force and is effective on the surface of the graphite particles coated with the water-soluble polymer. Act on.

Then, the negative electrode mixture slurry obtained is applied to a negative electrode core material and dried to form a negative electrode mixture layer, whereby a negative electrode is obtained (step (iii)). The method for applying the negative electrode mixture slurry to the negative electrode core material is not particularly limited. For example, the negative electrode mixture slurry is applied in a predetermined pattern on the raw material of the negative electrode core material using a die coat. The drying temperature of the coating film is not particularly limited. The dried coating film is rolled with a rolling roll and controlled to a predetermined thickness. By the rolling process, the adhesive strength between the negative electrode mixture layer and the negative electrode core material and the adhesive strength between the graphite particles coated with the water-soluble polymer are increased. The negative electrode mixture layer thus obtained is cut into a predetermined shape together with the negative electrode core material, whereby the negative electrode is completed.

The liquid component used for preparing the negative electrode mixture slurry by the above method is not particularly limited, but water, an aqueous alcohol solution, and the like are preferable, and water is most preferable. However, N-methyl-2-pyrrolidone (hereinafter referred to as NMP) may be used.

A positive electrode will not be specifically limited if it can be used as a positive electrode of a nonaqueous electrolyte secondary battery. For the positive electrode, for example, a positive electrode mixture slurry containing a positive electrode active material, a conductive agent such as carbon black, and a binder such as polyvinylidene fluoride is applied to a positive electrode core material such as an aluminum foil, dried, and rolled. Can be obtained. As the positive electrode active material, a lithium-containing transition metal composite oxide is preferable. Representative examples of the lithium-containing transition metal composite oxide include LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , LiMnO _{2 and the} like.

Especially, it is preferable that a positive electrode contains the complex oxide containing lithium and nickel from the point from which the effect which suppresses gas generation | occurrence | production while ensuring high capacity | capacitance is acquired more notably. In this case, the molar ratio of nickel to lithium contained in the composite oxide is preferably 30 to 100 mol%.

The composite oxide preferably further contains at least one selected from the group consisting of manganese and cobalt, and the total molar ratio of manganese and cobalt to lithium is preferably 70 mol% or less.

The composite oxide further preferably contains an element M other than Li, Ni, Mn, Co and O, and the molar ratio of the element M to lithium is preferably 1 to 10 mol%.

Specific lithium nickel-containing composite oxides include, for example, the general formula (1):
_{_{_{_{Li x Ni y M z Me 1-}}}} (y + z) O 2 + d (1)
(M is at least one element selected from the group consisting of Co and Mn, Me is at least one element selected from the group consisting of Al, Cr, Fe, Mg, and Zn; 98 ≦ x ≦ 1.1, 0.3 ≦ y ≦ 1, 0 ≦ z ≦ 0.7, 0.9 ≦ (y + z) ≦ 1, −0.01 ≦ d ≦ 0 .01).

As the separator, a microporous film made of polyethylene, polypropylene or the like is generally used. The thickness of the separator is, for example, 10 to 30 μm.

The present invention can be applied to non-aqueous electrolyte secondary batteries having various shapes such as a cylindrical shape, a flat shape, a coin shape, and a square shape, and the shape of the battery is not particularly limited.

Next, the present invention will be specifically described based on examples and comparative examples. However, the present invention is not limited to the following examples.

Example 1
(A) Production of negative electrode Step (i)
First, carboxymethylcellulose (hereinafter referred to as CMC, molecular weight 400,000), which is a water-soluble polymer, was dissolved in water to obtain an aqueous solution having a CMC concentration of 1% by weight. While mixing 100 parts by weight of natural graphite particles (average particle size 20 μm, average circularity 0.92, specific surface area 4.2 m ² / g) and 100 parts by weight of CMC aqueous solution, the temperature of the mixture is controlled at 25 ° C. Stir. Thereafter, the mixture was dried at 120 ° C. for 5 hours to obtain a dry mixture. In the dry mixture, the amount of CMC per 100 parts by weight of graphite particles was 1 part by weight.

Step (ii)
101 parts by weight of the obtained dry mixture, 0.6 parts by weight of a binder (hereinafter referred to as SBR) having a rubber elasticity, which is in the form of particles having an average particle size of 0.12 μm, and containing styrene units and butadiene units; .9 parts by weight of carboxymethyl cellulose and an appropriate amount of water were mixed to prepare a negative electrode mixture slurry. SBR was mixed with other components in an emulsion using water as a dispersion medium (BM-400B (trade name) manufactured by Nippon Zeon Co., Ltd., SBR weight ratio: 40% by weight).

Step (iii)
The obtained negative electrode mixture slurry was applied to both surfaces of an electrolytic copper foil (thickness 12 μm) as a negative electrode core material using a die coat, and the coating film was dried at 120 ° C. Thereafter, the dried coating film was rolled with a rolling roller at a linear pressure of 0.25 ton / cm to form a negative electrode mixture layer having a thickness of 160 μm and a graphite density of 1.65 g / cm ³ . The negative electrode mixture layer was cut into a predetermined shape together with the negative electrode core material to obtain a negative electrode.

The water penetration rate of the negative electrode mixture layer was measured by the following method.
2 μl of water was dropped to bring the droplet into contact with the surface of the negative electrode mixture layer. Thereafter, using a contact angle measuring device (DM-301 manufactured by Kyowa Interface Science Co., Ltd.), the time until the contact angle θ of water with respect to the negative electrode mixture layer surface at 25 ° C. was smaller than 10 ° was measured. The water penetration rate of the negative electrode mixture layer was 15 seconds.

Further, the porosity of the negative electrode mixture layer was calculated from the true density of each material constituting the negative electrode mixture and found to be 25%.

(B) Preparation of positive electrode 4 parts by weight of polyvinylidene fluoride (PVDF) as a binder was added to 100 parts by weight of LiNi _0.80 Co _0.15 Al _0.05 O _{2 as} a positive electrode active material, and an appropriate amount of N-methyl- Mixing with 2-pyrrolidone (NMP) to prepare a positive electrode mixture slurry. The obtained positive electrode mixture slurry was applied to both surfaces of a 20 μm thick aluminum foil as a positive electrode core material using a die coat, the coating film was dried, and further rolled to form a positive electrode mixture layer. . The positive electrode mixture layer was cut into a predetermined shape together with the positive electrode core material to obtain a positive electrode.

(C) Preparation of non-aqueous electrolyte Ethylene carbonate (EC), propylene carbonate (PC), and methyl-2,2,2-trifluoropropionate (FE) were mixed with W _EC : W _PC : W _FE = 10. : LiPF ₆ is dissolved at a concentration of 1 mol / liter in a mixed solvent containing a weight ratio of 50:40, and 1% by weight of 1,3-propane sultone, which is a sultone compound, has a C═C unsaturated bond. A non-aqueous electrolyte was prepared by containing 2% by weight of vinylene carbonate, which is a cyclic carbonate. When measured with a rotational viscometer (cone plate type, cone plate radius: 24 mm), the viscosity of the nonaqueous electrolyte at 25 ° C. was 4.4 mPa · s.

(D) Battery assembly A square lithium ion secondary battery as shown in FIG. 1 was produced.
A negative electrode and a positive electrode are wound through a separator (A089 (trade name) manufactured by Celgard Co., Ltd.) made of a polyethylene microporous film having a thickness of 20 μm between the negative electrode and the positive electrode. Group 21 was configured. The electrode group 21 was housed in an aluminum square battery can 20. The battery can 20 has a bottom part and a side wall, the top part is opened, and the shape is substantially rectangular. The thickness of the main flat part of the side wall was 80 μm. Thereafter, an insulator 24 for preventing a short circuit between the battery can 20 and the positive electrode lead 22 or the negative electrode lead 23 was disposed on the electrode group 21. Next, a rectangular sealing plate 25 having a negative electrode terminal 27 surrounded by an insulating gasket 26 in the center was disposed in the opening of the battery can 20. The negative electrode lead 23 was connected to the negative electrode terminal 27. The positive electrode lead 22 was connected to the lower surface of the sealing plate 25. The end of the opening and the sealing plate 25 were welded with a laser to seal the opening of the battery can 20. Thereafter, 2.5 g of nonaqueous electrolyte was injected into the battery can 20 from the injection hole of the sealing plate 25. Finally, the liquid injection hole was closed by welding with a plug 29 to complete the prismatic lithium ion secondary battery 1 having a height of 50 mm, a width of 34 mm, an inner space thickness of about 5.2 mm, and a design capacity of 850 mAh.

<Battery evaluation>
(1) Evaluation of cycle capacity maintenance rate The battery charge / discharge cycle of battery 1 was repeated at 45 ° C. In the charge / discharge cycle, in charging, constant current charging with a charging current of 600 mA and a final voltage of 4.2 V was performed, and then constant voltage charging was performed at 4.2 V up to a charging cut current of 43 mA. The rest time after charging was 10 minutes. On the other hand, in the discharge, constant current discharge was performed with a discharge current of 850 mA and a discharge end voltage of 2.5V. The rest time after discharge was 10 minutes.
The discharge capacity at the third cycle was regarded as 100%, and the discharge capacity when 500 cycles passed was defined as the cycle capacity maintenance rate [%]. The results are shown in Table 1.

(2) Evaluation of battery swelling after cycle In the state after charging in the third cycle and in the state after charging in the 501st cycle, the central portion perpendicular to the maximum plane (vertical 50 mm, horizontal 34 mm) of the battery 1 The thickness was measured. From the difference in battery thickness, the amount of battery swelling [mm] after the charge / discharge cycle at 45 ° C. was determined. The results are shown in Table 1.

(3) Evaluation of low-temperature discharge characteristics For battery 1, the battery charge / discharge cycle was repeated three times at 25 ° C. Next, after charging at the fourth cycle at 25 ° C., the battery was left at 0 ° C. for 3 hours and then discharged at 0 ° C. as it was. The discharge capacity at the third cycle (25 ° C.) was regarded as 100%, the discharge capacity at the fourth cycle (0 ° C.) was expressed as a percentage, and this was defined as the low temperature discharge capacity maintenance rate [%]. The results are shown in Table 1. The charging / discharging conditions were the same as those in the evaluation (1) except for the rest time after charging.

(4) Evaluation of battery swelling after storage at high temperature First, the battery 1 was charged and discharged at 25 ° C. for 2 cycles, and then charged in the third cycle. The thickness of the central part was measured. Next, after the battery 1 was allowed to stand at 85 ° C. for 3 days and then cooled at 25 ° C. for 3 hours, the battery thickness was measured in the same manner. From the difference in battery thickness, the amount of battery swelling [mm] after high temperature storage at 85 ° C. for 3 days was determined. The results are shown in Table 1.

Example 2
Table 1 shows W _EC : W _PC : W _FE , which is a weight ratio of ethylene carbonate (EC), propylene carbonate (PC), and methyl-2,2,2-trifluoropropionate (FE). A nonaqueous electrolyte was prepared in the same manner as in Example 1 except that the amount was changed. Batteries 2 to 14 were produced in the same manner as in Example 1 except that the obtained nonaqueous electrolyte was used.

In addition, a nonaqueous electrolyte was prepared in the same manner as in Example 1 except that fluorobenzene (FB) was further added at the weight ratio (W _FB ) shown in Table 1, and the same as in Example 1 was used. Thus, batteries 15 and 16 were produced.

Furthermore, a non-aqueous electrolyte containing EC and EMC was prepared at a weight ratio (W _EC : W _EMC ) shown in Table 1, and a battery 17 was produced in the same manner as in Example 1 using this.
Batteries 2, 8-9, 14, 16 and 17 are comparative examples. The obtained battery was evaluated in the same manner as in Example 1. The results are shown in Table 1.

Example 3
A nonaqueous electrolyte was prepared in the same manner as in Example 1 except that the fluorinated ester shown in Table 2 was used instead of methyl-2,2,2-trifluoropropionate. Batteries 18 to 33 were produced in the same manner as in Example 1 except that the obtained nonaqueous electrolyte was used. The batteries 27 to 33 are comparative examples. The obtained battery was evaluated in the same manner as in Example 1. The results are shown in Table 2.

Example 4
In the non-aqueous electrolyte, an additive containing 1,3-propane sultone, which is a sultone compound, and vinylene carbonate, which is a cyclic carbonate having a C═C unsaturated bond, in a weight ratio of 1: 2, was included in the amounts shown in Table 3. A nonaqueous electrolyte was prepared in the same manner as in Example 1 except that. Batteries 34 to 40 were produced in the same manner as in Example 1 except that the obtained nonaqueous electrolyte was used. The obtained battery was evaluated in the same manner as in Example 1. The results are shown in Table 3.

Example 5
Batteries 41 to 44 were produced in the same manner as in Example 1 except that the water-soluble polymer shown in Table 4 was used.

The present invention is, for example, a non-aqueous electrolyte secondary used for power supplies of electronic devices such as mobile phones, personal computers, digital still cameras, game devices, and portable audio devices, and vehicles such as electric vehicles and hybrid vehicles (HEV). Although useful in batteries, the field of application of the present invention is not limited to these.

Although the present invention has been described in terms of the presently preferred embodiments, such disclosure should not be construed as limiting. Various changes and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains after reading the above disclosure. Accordingly, the appended claims should be construed to include all variations and modifications without departing from the true spirit and scope of this invention.

20 Battery Can 21 Electrode Group 22 Positive Electrode Lead 23 Negative Electrode Lead 24 Insulator 25 Sealing Plate 26 Insulating Gasket 29 Sealing

Claims

A non-aqueous solvent, and a solute dissolved in the non-aqueous solvent,
The non-aqueous solvent contains ethylene carbonate, propylene carbonate, and a fluorinated ester having three fluorine atoms as a main solvent,
In the fluorinated ester, all the fluorine atoms are bonded to the terminal carbon atom not located at the α-position of the carbonyl group, and the number of carbon atoms excluding the carbon atom contained in the ester group of the fluorinated ester is 5 or less. And
The ethylene carbonate, the propylene carbonate, and the fluorinated ester have a weight ratio W EC of the ethylene carbonate in the total of 5 to 30% by weight,
The propylene carbonate weight ratio W PC in the total is 20 to 55% by weight,
A non-aqueous electrolyte in which a weight ratio W FE of the fluorinated ester in the total is 20 to 50% by weight.
The fluorinated ester is represented by the general formula: R 1 —COO—R 2 , R 1 is a carbon chain having 2 to 4 carbon atoms, and the three fluorine atoms are bonded to a terminal carbon atom. The nonaqueous electrolyte according to claim 1, wherein R 2 is a carbon chain having 1 to 3 carbon atoms.
The fluorinated ester is represented by the general formula: R 3 —COO—R 4 , R 3 is a carbon chain having 1 to 3 carbon atoms, and R 4 is a carbon chain having 2 to 4 carbon atoms, The non-aqueous electrolyte according to claim 1, wherein the three fluorine atoms are bonded to a terminal carbon atom.
The fluorinated ester is CF 3 CH 2 CH 2 —COO—CH 2 CH 3 , CF 3 CH 2 —COO—CH 3 , CF 3 CH 2 —COO—CH 2 CH 3 , CF 3 CH 2 —COO—CH. 2 CH 2 CH 3 , CF 3 CH 2 CH 2 —COO—CH 3 , CF 3 CH 2 CH 2 CH 2 —COO—CH 3 , CH 3 CH 2 CH 2 —COO—CH 2 CF 3 , CH 3 —COO —CH 2 CF 3 , CH 3 —COO—CH 2 CH 2 CF 3 , CH 3 —COO—CH 2 CH 2 CH 2 CF 3 , CH 3 CH 2 —COO—CH 2 CF 3 , and CH 3 CH 2 — The nonaqueous electrolyte according to claim 1, wherein the nonaqueous electrolyte is at least one selected from the group consisting of COO-CH 2 CH 2 CF 3 .
The nonaqueous electrolyte according to claim 4, wherein the fluorinated ester comprises at least one of CF 3 CH 2 CH 2 —COO—CH 2 CH 3 and CF 3 CH 2 —COO—CH 2 CH 3 .
The non-aqueous solvent includes an additive composed of at least one of a sultone compound and a cyclic carbonate having a C═C unsaturated bond,
The nonaqueous electrolyte according to any one of claims 1 to 5, wherein the additive occupies 0.2 to 5% by weight of the entire nonaqueous electrolyte.
Weight ratio W FE of the fluorinated ester is 30 to 40 wt%, the non-aqueous electrolyte according to any one of claims 1 to 6.
A positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and the nonaqueous electrolyte according to any one of claims 1 to 7,
The negative electrode includes a negative electrode core material and a negative electrode mixture layer attached to the negative electrode core material,
The negative electrode mixture layer includes graphite particles, a water-soluble polymer that coats the surface of the graphite particles, and a binder that bonds the graphite particles coated with the water-soluble polymer. Electrolyte secondary battery.
A non-aqueous electrolyte secondary battery obtained by charging and discharging the battery according to claim 8 at least once.
The nonaqueous electrolyte secondary battery according to claim 8 or 9, wherein a film containing lithium fluoride is formed on a surface of the negative electrode mixture layer.
The non-aqueous electrolyte secondary battery according to any one of claims 8 to 10, wherein the water-soluble polymer contains a cellulose derivative or polyacrylic acid.