JP2009163971A - Non-aqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem that while an electrolyte containing a mixed solvent made of a cyclic carbonate and a chain carbonate is used frequently for a nonaqueous electrolyte secondary battery, carrying out battery charge and discharge using this electrolyte results in extremely intensive polarization at the negative electrode in a discharge process, thus making charge/discharge impossible in the end. <P>SOLUTION: A nonaqueous electrolyte secondary battery to be provided is a battery having a constituent element of a negative electrode containing a material that takes in lithium ions in a charging process while releasing lithium ions in a discharging process. The nonaqueous electrolyte secondary battery uses an electrolyte made by dissolving a lithium salt in a solvent composed only of the chain carbonate. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a non-aqueous electrolyte secondary battery, and particularly to a battery using a suitable electrolyte.

  Non-aqueous electrolyte using a transition metal oxide such as lithium cobaltate, lithium nickelate, lithium manganate, and lithium iron phosphate as the positive electrode active material, and a layered compound such as artificial graphite and natural graphite as the negative electrode active material Secondary batteries, so-called lithium ion batteries, have been developed.

  In order to further increase the capacity of the non-aqueous electrolyte secondary battery, it is necessary to fill more positive electrode active material and negative electrode active material in a limited volume of the battery container. Alternatively, it is necessary to increase the capacity of the positive electrode active material and the negative electrode active material.

  Filling a limited volume with a large amount of active material results in a decrease in porosity in the positive electrode and the negative electrode, and a decrease in the amount of electrolyte that can be injected. Therefore, the diffusion of ions in the electrode and between the electrodes becomes dull. As a technique for reducing the diffusion resistance, it is common to prepare a low-viscosity electrolytic solution.

  Patent Document 1 discloses a mixed solvent of fluoroethylene carbonate (hereinafter abbreviated as FEC), which is a cyclic carbonate, and dimethyl carbonate (hereinafter abbreviated as DMC), which is a chain carbonate, or ethyl methyl carbonate (hereinafter abbreviated as EMC). It is disclosed that the safety and load characteristics of the battery are improved.

  Patent Document 2 discloses an electrolytic solution that uses a mixed solvent composed of FEC and diethyl carbonate (hereinafter abbreviated as DEC), and discloses that the initial charge / discharge efficiency of the battery and the capacity retention rate during charge / discharge are improved. Has been.

  Patent Document 3 describes an electrolytic solution that uses a mixed solvent composed of FEC and DMC, and discloses that the initial charge / discharge efficiency of the battery and the capacity retention rate during charge / discharge are improved.

  In addition, looking at the mixing ratio of FEC and DMC etc., in Patent Document 1, FEC / DMC = 25/75 (volume ratio) and FEC / EMC = 10/90 (volume ratio) are described, In patent document 3, FEC / DEC = 50/50 (mass ratio) is described. On the other hand, other than Patent Documents 1 to 3, a disclosure example of a non-aqueous electrolyte secondary battery using a solvent consisting only of a chain carbonate as an electrolyte was sought, but could not be found.

Further, looking at the materials used for the negative electrode of the nonaqueous electrolyte secondary battery, in Patent Document 1, the carbon material is the example, in Patent Document 2, the silicon is the example, and in Patent Document 3, the copper- Tin alloys, cobalt-tin-carbon containing materials, etc. are described in the examples.
Special table 2007-504628 JP 2006-216450 A JP 2006-134719 A

Electrolyte that uses a mixed solvent of cyclic carbonate and chain carbonate is often used in non-aqueous electrolyte secondary batteries. However, when the battery is charged and discharged, the negative electrode polarization eventually becomes extremely large, and the battery is charged and discharged. Can no longer do. This is due to the following reason.

  The chain carbonate is decomposed on the negative electrode to become an alkoxide or the like. Alkoxides are strong bases that open the cyclic carbonate and facilitate the decomposition of the cyclic carbonate on the positive and negative electrodes. The cycle efficiency (ratio of discharge capacity to charge capacity) of the negative electrode decreases due to such decomposition of the chain carbonate itself and decomposition of the cyclic carbonate induced by the decomposition. And a decomposition product accumulates on a negative electrode, and the electrode reaction in a negative electrode comes to be inhibited now.

  Therefore, in order to improve the life of the charge / discharge cycle, the amount of the chain carbonate is reduced, a low viscosity solvent is used instead of the chain carbonate, or an additive that prevents the decomposition of the chain carbonate is added to the electrolyte. It is necessary to adopt a technique such as adding.

  In patent documents 1-3, in order to suppress decomposition | disassembly on the negative electrode of a chain carbonate, FEC is used. However, since the viscosity of the electrolyte containing a large amount of FEC is increased, it is difficult to maintain the cycle life and load characteristics of the battery against the decrease in the amount of the electrolyte due to the demand for higher capacity of the battery.

  The non-aqueous electrolyte secondary battery of the present invention is a battery having a negative electrode that uses a material that takes in lithium ions by charging and releases lithium ions by discharging, and includes a lithium salt in a solvent composed of only a chain carbonate. Use an electrolyte solution.

  The cycle efficiency of the negative electrode is improved by the electrolytic solution of the present invention, and the charge / discharge cycle life of the non-aqueous electrolyte secondary battery can be extended. Moreover, since the electrolytic solution of the present invention has a low viscosity, it facilitates the diffusion of ions in and between the electrodes, and makes the reaction inside the electrodes uniform. Therefore, excellent cycle efficiency of the negative electrode can be obtained even at a high load.

  The electrolytic solution of the present invention is an electrolytic solution in which a lithium salt is dissolved in a solvent composed only of a chain carbonate. The chain carbonate is easily decomposed on the negative electrode, and the decomposition product accumulates on the negative electrode, thereby inhibiting the reaction of the negative electrode. On the other hand, chain carbonate is a low-viscosity solvent and easily penetrates into the electrode to promote the electrode reaction. In particular, by using a porous electrode, a combination with a cyclic carbonate having a high viscosity can be avoided, and sufficient performance as a solvent for an electrolytic solution can be obtained. Examples of the chain carbonate include DMC, EMC, propyl methyl carbonate (hereinafter abbreviated as PMC), butyl methyl carbonate (hereinafter abbreviated as BMC), and a chain carbonate having at least one methyl group is preferable. DMC can be used as a single solvent. As a mixed solvent of a plurality of chain carbonates, a combination of DMC and EMC is preferable. A suitable mixing range is a chain carbonate other than DMC> DMC in a molar ratio, and preferably a chain carbonate other than 10/0 ≧ DMC / DMC ≧ 8/2 in a molar ratio.

  Use of chain carbonates such as DEC, dipropyl carbonate (hereinafter abbreviated as DPC), dibutyl carbonate (hereinafter abbreviated as DBC), etc., in addition to its low viscosity, wettability of electrolyte to polyolefin separators Is effective. However, since the chain carbonate having a plurality of alkyl groups having 2 or more carbon atoms is more easily decomposed on the negative electrode than the chain carbonate having a methyl group, the mixing ratio is preferably small.

Examples of the lithium salt dissolved in the electrolyte include LiPF6, LiBF4, LiClO4, lithium bis [oxalate (2-)] borate (hereinafter abbreviated as LiBOB), lithium bis [trifluoromethanesulfonyl] imide (hereinafter abbreviated as LiTFSI). ), Lithium bis [pentafluoroethanesulfonyl] imide (hereinafter abbreviated as LiBETI), lithium [trifluoromethanesulfonyl] [nonafluorobutanesulfonyl] imide (hereinafter abbreviated as LiMBSI), lithium cyclohexafluoropropane-1 , 1,3-bis [sulfonyl] imide (hereinafter, LiCHPI hereinafter), lithium trifluoromethyl trifluoroborate (LiBF ₃ CF _3), lithium pentafluoroethyl trifluoroborate (LiBF ₃ C ₂ F ₅ , Lithium heptafluoropropyl trifluoroborate _{(LiBF 3 C 3 F 7)} , lithium tris [pentafluoroethyl] trifluoroacetic phosphate _{(LiPF 3 (C2F 5) 3} ) or the like can be used. Among these, LiBOB is preferable because it forms a film on the negative electrode that suppresses the decomposition of the chain carbonate even when added in a small amount.

  The negative electrode material combined with the electrolytic solution of the present invention is preferably a material containing an element that is electrochemically alloyed with lithium. Conventionally, layered carbon materials such as artificial graphite and natural graphite have been used as the negative electrode material for lithium ion batteries. However, when combined with an electrolytic solution mainly composed of chain carbonate, the cycle efficiency of the negative electrode is greatly reduced. To do. In addition, gases such as methane and ethane are likely to be generated.

Active materials used for the negative electrode include metals that can be alloyed with lithium (for example, Ag, Au, Zn, Al, Ga, In, Si, Ge, Sn, Pb, Bi), oxides such as Si and Sn, and lithium-containing materials. Transition metal oxides (for example, Li ₄ Ti ₅ O ₁₂ ) can be used. Further, it may be a metal oxide such as CoO, NiO, MnO, Fe ₂ O ₃ which reacts with lithium and decomposes into a metal and lithium oxide. In particular, it is preferable to use a material containing oxygen and an element that forms an alloy with lithium, for example, an oxide such as Si or Sn for the negative electrode active material. Since the lithium oxide film is formed on the negative electrode surface by the first charge, the decomposition of the chain carbonate such as DMC is further suppressed.

  The active material used for the negative electrode is preferably formed directly on the current collector rather than in powder form. Direct formation means formation by a physical method such as vapor deposition or sputtering. If the active material is in a powder form, it must be mixed with a conductive material such as carbon black in order to obtain good electrode characteristics. However, a chain carbonate such as DMC tends to decompose on the carbon black. In addition, even a material formed by a physical method is preferably an aggregate of columnar active materials. This is because a space having a porosity of about 30% or more is generated in the electrode, the penetration of the electrolytic solution is improved, and a uniform reaction occurs even inside the electrode.

  Examples of the active material used for the positive electrode include lithium-containing transition metal oxides such as lithium cobaltate, lithium nickelate, lithium manganate, and lithium iron phosphate. Although the reaction form is different from these transition metal oxides, carbon materials such as activated carbon, carbon black, non-graphitizable carbon, artificial and natural graphite, carbon nanotube, and fullerene may be used. In addition, conductive polymers such as polyacetylene, polypyrrole, and polythiophene can also be used. Here, the reaction forms in lithium-containing transition metal oxides and carbon materials and conductive polymers, but in lithium-containing transition metal oxides, lithium ions in the transition metal oxides are released into the electrolyte by charging, and discharge In the case of carbon materials and conductive polymers, the anions in the electrolyte are taken into the carbon material by charging, and the carbon material is discharged into the carbon material. It is a reaction in which the anions in the etc. are released into the electrolyte. However, in any material, the negative electrode is a reaction in which lithium ions in the electrolytic solution are taken in by charging and lithium ions are released by discharging.

  Examples relating to the present invention will be described below.

Example 1
(I) Preparation of Nonaqueous Electrolytic Solution As shown in Table 1, LiPF ₆ and chain carbonate were mixed to prepare 1 to 14 electrolytic solutions.

(Ii) Production of negative electrode Silicon was deposited on a roughened copper foil having a thickness of 35 μm while introducing a small amount of oxygen into the vapor deposition chamber by using an electron beam vapor deposition method. When the material deposited on the copper foil was analyzed, the composition was SiO _0.56 , and the form was a state in which columnar ones were planted starting from the protrusion of the copper foil. More specifically, columnar ones with a thickness of 10 to 20 μm and a height of about 25 μm are formed so as to stand on a copper foil, one by one, or several in a bundle. It was. The interval between the columnar members is several to 10 μm, and the porosity of the electrode plate is about 40%.
Met. This value is about twice the porosity of the negative electrode in a lithium ion battery using a coated electrode.

  The copper foil on which this silicon oxide was deposited was cut into a size of 35 mm × 35 mm and ultrasonically welded to a copper plate with leads to obtain a negative electrode.

(Iii) Production of counter electrode Lithium foil having a thickness of 300 μm was pressure-bonded to a copper plate with leads to obtain a counter electrode.

(Iv) Preparation of Reference Electrode A lithium foil having a thickness of 300 μm and a width of 5 mm was pressure-bonded to the tip of a nickel ribbon with a lead to obtain a reference electrode.

(V) Assembly of cell for measuring cycle efficiency of negative electrode A separator made of polyethylene was sandwiched between the negative electrode and the counter electrode so that silicon oxide and lithium were opposed to each other, and taped and integrated. Next, the integrated product and the reference electrode were placed in a cylindrical aluminum laminate bag having both ends open, and one opening of the bag was welded at the lead portion. Then, 2.4 g of the electrolyte prepared from the other opening was dropped. Finally, degassing was performed at 10 mmHg for 5 seconds, and the injected opening was sealed by welding.

(Vi) Measurement of negative electrode cycle efficiency (in the case of low load)
Using the measurement cell assembled in this manner, a constant current of 4.1 mA was passed to the negative electrode at 20 ° C. In charging (a state in which a cathode current flows to the negative electrode and lithium ions are taken into the negative electrode), the energizing time of the constant current was set to 10 hours. In discharging (a state in which an anode current flows to the negative electrode and lithium ions are released from the negative electrode), a constant current was supplied until the potential of the negative electrode became 1.5 V with respect to the reference electrode. Then, this charge and discharge cycle was repeated, and the charge electricity amount and the discharge electricity amount in each cycle were obtained.

The cycle efficiency of the negative electrode in each cycle was calculated by one formula.
Cycle efficiency = (discharged electricity / charged electricity) × 100 (%) (1)
(Vii) Negative electrode cycle efficiency (in the case of low load)
Table 1 summarizes the cycle efficiency of the negative electrode at the 20th cycle. Cycle efficiency of about 99.6% or more in the case of an electrolytic solution composed of a DMC single solvent and a mixed solvent of a solvent selected from DMC and {EMC, PMC, BMC} and mainly composed of DMC was gotten. In an electrolytic solution using a mixed solvent having a large EMC ratio and an electrolytic solution using PMC or BMC as a single solvent, a good value of about 99.5% or more was obtained although the cycle efficiency was slightly lowered.

  On the other hand, in the electrolytic solution (12 to 14) using a solvent containing a chain carbonate in which two alkyl groups each have 2 or more carbon atoms, the cycle efficiency was about 99.3 to 99.5%. This is because a chain carbonate having no methyl group is easily decomposed on the negative electrode.

(Viii) Measurement of negative electrode cycle efficiency (in the case of high load)
At 20 ° C., the current during charging and discharging was 23.5 mA. The energization time during charging was 1 hour 45 minutes, and the upper limit potential during discharging was 1.5 V with respect to the reference electrode. Then, this charge and discharge cycle was repeated, and the charge electricity amount and the discharge electricity amount in each cycle were obtained. The cycle efficiency of the negative electrode in each cycle was calculated by one formula.

(Ix) Negative electrode cycle efficiency (in the case of high load)
Table 1 summarizes the cycle efficiency of the negative electrode at the 20th cycle. Compared with the cycle efficiency in the case of a low load with 4.1 mA flowing, although the electrolyte efficiency (12-14) using a chain carbonate having no methyl group was slightly lowered, the obtained cycle efficiency was almost the same. there were.

  That is, with the electrolytic solution of the present invention, the cycle efficiency is unlikely to decrease even at high loads. This is because the electrolyte solution of the present invention is an electrolyte solution composed of a chain carbonate only solvent, so that the viscosity is lowered, and the use of a chain carbonate having a methyl group as the main solvent suppresses decomposition at the negative electrode. It is thought to be done.

(Comparative Example 1)
As shown in Table 1, LiPF ₆ was mixed with chain carbonate and methyl ester to prepare 15 to 17 electrolytes. Here, MP is methyl propionate, MB is methyl butyrate, and MV is an abbreviation for methyl valerate. These methyl esters are low viscosity solvents like chain carbonates such as DMC. Further, LiPF ₆ , chain carbonate and FEC were mixed to prepare 18 electrolytes.

  The cycle efficiency of the negative electrode was measured in the same manner as in Example 1 except that the electrolytic solution was changed to 15-18.

  When the cycle efficiency of the negative electrode is compared with the electrolytic solutions (1 to 14) of the present invention, it can be seen that when a solvent containing a methyl ester is used for the electrolytic solution, the efficiency decreases at both low and high loads. This is because methyl ester is easily decomposed on the negative electrode. In addition, the cycle efficiency at high load is significantly lower than the cycle efficiency at low load. The electrode reaction is uneven due to the deposit of methyl ester decomposed at the negative electrode, and the active material is unevenly expanded. It is thought that the contraction occurred and the active material collapsed from the current collector.

  It can also be seen that when a solvent containing FEC is used for the electrolytic solution, the efficiency is improved at a low load as compared with the electrolytic solution of the present invention, but the efficiency is lowered at a high load. This is because FEC forms a film that suppresses the decomposition of chain carbonate on the negative electrode, so that the efficiency is improved at low loads, but the film inhibits the release of lithium ions taken into the negative electrode at high loads. It is.

  In brief, the significance of the value of 0.1% means that 0.1% of the battery capacity is lost in one cycle. Therefore, 10% of the battery capacity is lost after 100 cycles and 50% of the battery capacity is lost after 500 cycles. Improving the cycle efficiency of the negative electrode by 0.1% has great significance. Furthermore, if it adds about the current accuracy of the used charging / discharging apparatus, the difference | error with respect to the set electric current value is 0.02% or less, and cycle efficiency is significant to one decimal place.

(Example 2)
(I) Preparation of non-aqueous electrolyte As shown in Table 2, LiPF ₆ and other lithium salts and chain carbonates were mixed to prepare 19 to 30 electrolytes.

(Ii) Assembly of cell for measuring cycle efficiency of negative electrode A measurement cell was assembled in the same manner as in Example 1 except that the electrolytic solution having the above composition was used.

(Iii) Measurement of cycle efficiency of negative electrode Using the measurement cell assembled in this manner, a constant current of 4.1 mA was applied to the negative electrode at 20 ° C. In charging, the energizing time of the constant current was 10 hours. In discharging, a constant current was applied until the potential of the negative electrode became 1.5 V with respect to the reference electrode. Then, this charge and discharge cycle was repeated, and the charge electricity amount and the discharge electricity amount in each cycle were obtained. The cycle efficiency of the negative electrode in each cycle was calculated by one formula.

(Iv) Cycle Efficiency of Negative Electrode Table 2 summarizes the cycle efficiency of the negative electrode at the 20th cycle. Generally good value. It can be seen that the efficiency tends to increase when the lithium salt contains a high number of fluorine atoms or a high number of oxygen atoms. This is because these lithium salts decompose on the negative electrode to form a film that suppresses the decomposition of DMC. In particular, LiBOB is found to give good cycle efficiency.

(Example 3)
(I) Preparation of Nonaqueous Electrolytic Solution LiPF ₆ and DMC were mixed at a molar ratio of 1/10 to prepare an electrolytic solution.

(Ii) Production of negative electrode Granular Pb was melted to produce a flat ingot. This ingot was rolled to produce a Pb foil having a thickness of 50 μm. The produced Pb foil was cut out to 35 mm × 35 mm, sandwiched with nickel expanded metal, and then a lead was attached to form a negative electrode.

  Similarly to Pb, a Sn foil negative electrode was prepared using granular Sn.

  A commercially available aluminum foil with a thickness of 50 μm was used. After sandwiching with nickel expanded metal, a lead was attached to form a negative electrode.

  Si was formed into a thin film by sputtering on a copper foil having a rough surface. In the same manner as in Example 1, a copper foil having a thin film was cut out and ultrasonically welded to a copper plate with leads to obtain a negative electrode.

Similarly to Si, a negative electrode was prepared for SnO ₂ by sputtering.

When the forms of Si and SnO ₂ produced by the sputtering method were observed, columnar ones were founded starting from the protrusion of the copper foil as the growth starting point.

(Iii) Assembly of cell for measuring cycle efficiency of negative electrode A measurement cell was assembled in the same manner as in Example 1 except that the negative electrode using the material as described above was used.

(Iv) Measurement of cycle efficiency of negative electrode Using the measurement cell assembled in this manner, a constant current of 4.1 mA was applied to the negative electrode at 20 ° C. The amount of electricity at the time of charging was set to 1 mol of lithium with respect to 1 mol of the metal element, and the upper limit potential at the time of discharging was set to 1.5 V with respect to the reference electrode. Then, this charge and discharge cycle was repeated, and the charge electricity amount and the discharge electricity amount in each cycle were obtained.

  The cycle efficiency of the negative electrode in each cycle was calculated by one formula, and the value at which the cycle was maximized by repeating the cycle was defined as the cycle efficiency possible with the material.

Table 3 summarizes the cycle efficiency of negative electrodes using various materials. From Table 3, it can be seen that the negative electrode using a material containing a metal element that takes in lithium ions by charging and releases lithium ions by discharging has good cycle efficiency. In particular, it can be seen that oxides of Si and Sn give high cycle efficiency. The reason for this is that a large amount of Li ₂ O generated when a cathode current is passed becomes a film, and the decomposition of DMC on the negative electrode is suppressed. Moreover, the cycle efficiency in the negative electrode produced by rolling Pb or the like is slightly lower than the cycle efficiency in the negative electrode such as Si produced by the sputtering method. This is considered to be because in the negative electrode using Si, since Si is columnar, the space inside the electrode is large, and a uniform electrode reaction occurs.

(Comparative Example 2)
As the electrolytic solution, a mixture of LiPF ₆ and DMC at a molar ratio of 1/10 was used.

  A copper plate with a lead was used as the negative electrode plate.

  Otherwise, a cell for measuring the cycle efficiency of the negative electrode was assembled in the same manner as in Example 1.

  The cycle efficiency of the negative electrode was measured as follows. A cathode current of 4.1 mA was passed through the negative electrode plate to deposit lithium. The amount of electricity of the cathode current was 1.67 mAh / cm2. Subsequently, the deposited lithium was dissolved by passing an anode current of 4.1 mA. The upper limit potential of the negative electrode plate was 1.5V. Such precipitation and dissolution of lithium were repeated 20 times. The cycle efficiency of the negative electrode was the value obtained by dividing the amount of anode electricity until the upper limit potential was reached by the amount of cathode electricity at the 20th cycle.

  As shown in Table 3, the cycle efficiency of the negative electrode was 8.1%, which was extremely lower than the cycle efficiency of the negative electrode using a metal element that takes in and releases lithium ions by charge and discharge. The reason for this is that the deposited lithium reacts with the electrolytic solution and becomes passivated.

  As described above, the use of the electrolytic solution of the present invention improves the cycle efficiency of the negative electrode, and also has excellent load characteristics due to its low viscosity. Therefore, it is a highly reliable non-aqueous solution with a long charge / discharge cycle life. An electrolyte secondary battery can be obtained.

Claims (6)

  A battery comprising a negative electrode using a material that takes in lithium ions by charging and releases lithium ions by discharging, and includes an electrolyte solution in which a lithium salt is dissolved in a solvent composed of only one or a plurality of chain carbonates. A non-aqueous electrolyte secondary battery.   The non-aqueous electrolyte secondary battery according to claim 1, wherein at least one of the one or more chain carbonates is a chain carbonate having at least one methyl group.   The non-aqueous electrolyte secondary battery according to claim 2, wherein the chain carbonate having at least one methyl group is dimethyl carbonate.   The plurality of chain carbonates are composed of dimethyl carbonate and one or more chain carbonates other than dimethyl carbonate, and the molar concentration of the dimethyl carbonate is larger than the molar concentration of the chain carbonate other than the one or more dimethyl carbonates. The non-aqueous electrolyte secondary battery according to claim 1.   5. The non-aqueous electrolysis according to claim 1, wherein the material that takes in lithium ions by charging and releases lithium ions by discharging is a material that includes at least an element that is electrochemically alloyed with lithium. Liquid secondary battery.   6. The non-aqueous electrolyte according to claim 5, wherein the material that takes in lithium ions by charging and releases lithium ions by discharging is a material that includes at least an element that is electrochemically alloyed with lithium and oxygen. Secondary battery.