JP2005071617A - Nonaqueous electrolyte secondary battery and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery which can improve a cycle property by protecting a positive electrode active material from the action of a hydrofluoric acid. <P>SOLUTION: In this nonaqueous electrolyte secondary battery provided with a nonaqueous electrolyte comprising LiPF6 as electrolyte salt, the nonaqueous electrolyte contains lithium borate. By this constitution, the hydrofluoric acid generated by the decomposition of the LiPF6 reacts with the lithium borate to precipitate as a boric acid compound not giving an adverse affect to the positive electrode active material and to be removed. Thereby, the cycle property can be improved. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a non-aqueous electrolyte secondary battery and a method for manufacturing the same.
[0002]
[Prior art]
In a lithium ion secondary battery, since a lithium compound is used as an active material, an aqueous solvent highly reactive with lithium cannot be used. For this reason, a nonaqueous electrolytic solution in which a lithium salt is dissolved as an electrolyte salt in a nonaqueous solvent that does not chemically react with lithium is used. For this reason, this type of battery is referred to as a “non-aqueous electrolyte secondary battery”. Here, as a lithium salt widely used, for example, LiPF ₆ is cited.
[0003]
[Patent Document 1]
Japanese Patent Laid-Open No. 7-122297
[Problems to be solved by the invention]
However, this type of battery has a problem that the discharge capacity decreases due to repeated charge and discharge. In particular, the discharge capacity was significantly reduced under high temperature conditions.
[0005]
The reason for this is considered as follows. LiPF ₆ used as an electrolyte salt is decomposed by reaction with moisture mixed from the outside during repetition of production or use, or during production or use, and generates hydrofluoric acid. This hydrofluoric acid is considered to have a negative effect on the positive electrode active material, thereby reducing the discharge capacity. In particular, when lithium manganese composite oxide is used as the positive electrode active material, manganese ions are eluted from the lithium manganese composite oxide by the action of hydrofluoric acid, so that the capacity is likely to decrease.
[0006]
The present invention has been made in view of the above circumstances, and an object thereof is to provide a non-aqueous electrolyte secondary battery that can improve cycle characteristics by protecting the positive electrode active material from the action of hydrofluoric acid. It is in.
[0007]
[Means for Solving the Problems]
As a result of diligent research, the present inventor has added lithium borate to the electrolytic solution, and the resulting hydrofluoric acid reacts with this lithium borate (reaction formula (1)) to form a positive electrode active material. It was found that it precipitates and is removed as a boric acid compound that does not have an adverse effect.
[0008]
_{Li 2 B 4 O 7 · 10H} 2 O + 12HF → Li 2 O · 4BF 3 + 16H 2 O ... (1)
[0009]
That is, the present invention is a non-aqueous electrolyte secondary battery comprising a non-aqueous electrolyte containing LiPF ₆ as an electrolyte salt in a non-aqueous solvent, wherein the non-aqueous electrolyte contains lithium borate.
The non-aqueous electrolyte secondary battery manufacturing method of the present invention is a method for manufacturing a non-aqueous electrolyte secondary battery comprising a non-aqueous electrolyte solution containing LiPF ₆ as an electrolyte salt in a non-aqueous solvent, It is characterized in that lithium borate is added to the water electrolyte so that the concentration becomes 0.1% by weight or more.
[0010]
Here, for the purpose of removing hydrofluoric acid as a boric acid compound, it is possible to use all borates. However, if a cation other than lithium ions such as sodium ions is present in the electrolyte, the battery reaction may be adversely affected. For this reason, it is necessary to use lithium borate as a borate.
The amount of lithium borate added varies depending on the type and mixing ratio of the nonaqueous solvent, the concentration of the electrolyte salt, and the like, and cannot be limited unconditionally. Note that the lithium phosphate content may be equal to or higher than the saturation concentration.
The amount of lithium borate added to the non-aqueous electrolyte is, for example, the total amount of boron in the non-aqueous electrolyte measured by IPC (High Frequency Inductively Coupled Plasma) emission spectrometry or borate ion determined by ion chromatography. It can obtain | require by converting from the quantity of.
[0011]
[Action and effect of the invention]
According to the present invention, in the non-aqueous electrolyte secondary battery, lithium borate is added to the non-aqueous electrolyte. According to such a structure, the generated hydrofluoric acid reacts with this lithium borate, precipitates and is removed as LiBF ₄ that does not adversely affect the positive electrode active material. Thereby, cycle characteristics can be improved.
[0012]
【Example】
1. Test method <Example 1>
(1) Production of lithium ion secondary battery (1) Production of positive electrode Lithium manganate is used as a positive electrode active material, polyvinylidene fluoride as a binder for the positive electrode active material, and acetylene black as a conductive agent. The mixture was mixed at a weight ratio of 87: 8: 5, and N-methylpyrrolidone was added to prepare a positive electrode mixture paste. This paste was uniformly applied to both sides of a current collector made of an aluminum foil having a thickness of 20 μm, dried and then pressed to produce a strip-like positive electrode sheet provided with a positive electrode active material layer. A positive electrode lead was welded to one end of the positive electrode sheet.
[0013]
(2) Preparation of negative electrode Graphite is used as a negative electrode active material, carboxymethyl cellulose as a binder and styrene butadiene rubber are mixed at a weight ratio of 95: 2: 3 to this graphite, and appropriate moisture is added. A negative electrode mixture paste was prepared. This paste was uniformly applied to both surfaces of a current collector made of a copper foil having a thickness of 15 μm, and a strip-shaped negative electrode sheet was produced by the same method as that for the positive electrode sheet. A negative electrode lead was welded to one end of the negative electrode sheet.
[0014]
(3) Preparation of electrolytic solution Ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at a volume ratio of 3: 7 to prepare a nonaqueous solvent. To this non-aqueous solvent, LiPF ₆ which is a lithium salt as an electrolyte was added to a concentration of 1.0 mol / l to prepare a non-aqueous electrolyte. Next, lithium borate was added to the non-aqueous electrolyte so as to have a saturated concentration (0.5 wt%).
[0015]
{Circle over (4)} Production of Square Battery A battery 1 having the structure shown in FIG. 1 was produced.
The power generation element 2 was produced by laminating the positive electrode sheet 3 produced as described in (1) above and the negative electrode sheet 4 produced as described above in (2) through a separator 5 and wound into an oval spiral. The separator 5 was a polyethylene microporous film having a thickness of 20 μm.
The power generation element 2 was housed in a rectangular battery case 6, and the negative electrode lead 11 was connected to the negative electrode terminal 9 provided in the battery lid 7. Further, the positive electrode lead 10 was connected to the battery lid 7. The battery lid 7 was attached to the opening of the battery case 6 by laser welding. Into the battery case 6, the electrolyte solution 12 prepared in the above (3) was vacuum injected from the injection port provided in the battery lid 7 to an extent that it does not become excessive. In this way, a square battery having a width of 30 mm, a height of 48 mm, and a thickness of 5 mm was assembled. The battery lid 7 is provided with a safety valve 8.
[0016]
(2) Cycle test The battery produced by the above method was charged to 4.1 V at a constant current of 800 mA (1 CA) in an atmosphere of 60 ° C., and charged at a constant voltage of 4.1 V for 3 hours after the start of charging. Went. Thereafter, this battery was discharged at a constant current of 800 mA to 2.75 V, and the initial discharge capacity was measured. This was defined as one cycle, and the discharge capacity was measured by repeating a predetermined cycle.
[0017]
<Comparative Example 1>
A battery was prepared and tested in the same manner as in Example 1 except that lithium borate was not added.
[0018]
2. Results and Discussion FIG. 2 shows a graph showing the relationship between the number of cycles and the capacity retention rate in the batteries of Example 1 and Comparative Example 1. In the figure, symbol Δ represents the battery of Example 1, and symbol O represents the battery of Comparative Example 1.
The capacity retention rate (%) was obtained by dividing the discharge capacity after each cycle by the initial discharge capacity.
[0019]
In the battery of Example 1 to which lithium borate was added, the capacity retention after 50 cycles was 81%, indicating good cycle characteristics. On the other hand, in the battery of Comparative Example 1 in which no lithium borate was added, the capacity retention after 50 cycles was reduced to 78%. From FIG. 2, it is estimated that when the number of cycles is further increased, the difference in capacity maintenance ratio between the battery of Example 1 and the battery of Comparative Example 1 at the same number of cycles is further increased.
[0020]
Thus, the battery in which lithium borate was added to the nonaqueous electrolytic solution showed better cycle characteristics than the battery without addition. This is presumably because lithium borate has removed hydrofluoric acid produced by the decomposition of LiPF ₆ that is an electrolyte salt.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view of the battery of this example. FIG. 2 is a graph showing the relationship between the number of cycles and the capacity retention rate in the batteries of Example 1 and Comparative Example 1.
1 ... Battery (non-aqueous electrolyte secondary battery)
3 ... Positive electrode sheet (positive electrode)
4 ... Negative electrode sheet (negative electrode)
12 ... Non-aqueous electrolyte

Claims (3)

A non-aqueous electrolyte secondary battery comprising a non-aqueous electrolyte containing LiPF ₆ as an electrolyte salt in a non-aqueous solvent,
The non-aqueous electrolyte secondary battery, wherein the non-aqueous electrolyte contains lithium borate. The nonaqueous electrolyte secondary battery according to claim 1, wherein the lithium borate concentration in the electrolytic solution is 0.1 wt% or more. A method for producing a non-aqueous electrolyte secondary battery comprising a non-aqueous electrolyte containing LiPF ₆ as an electrolyte salt in a non-aqueous solvent,
A method for producing a non-aqueous electrolyte secondary battery, wherein lithium borate is added to the non-aqueous electrolyte so as to have a concentration of 0.1% by weight or more.

Images