JPS62219476A - Secondary cell of lithium - Google Patents

Abstract

PURPOSE:To improve the charge/discharge characteristic of lithium, by using at least one of derivatives of a 1,3-dioxolan for the electrolytic solution of a secondary cell of lithium, and controlling the quantities of water and non- water impurities. CONSTITUTION:At least one of derivatives of a 1,3-dioxolan having a general structure shown by a formula is used as an organic solvent for an electrolytic solution in which a lithium salt is dissolved in the solvent. For example, the derivatives of the 1,3-dioxolan are 2-methyl-1,3-dioxolan, 4-methyl-1,3-dioxolan, 2,4-dimethyl-1,3-dioxolan and 2-ethyl-1,3-dioxolan. The water content and non- water content of the electrolytic solution are not more than 100ppm and 1,000ppm, respectively.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a lithium secondary battery, and more particularly to a lithium secondary battery having a good electrolyte.

[Background of the invention]

Lithium batteries have a high standard unipolar potential, -3.03V based on standard hydrogen electrodes, and have an extremely strong reducing power.Also, because the atomic weight is small at 6.941, the capacity density per weight is 3.
．． It is large at 86Ah/g. For this reason, batteries that use lithium as the negative electrode active material (hereinafter referred to as lithium batteries) are being researched as small, high-energy-density batteries, and batteries that use manganese dioxide, graphite fluoride, etc. as the positive electrode active material are already commercially available. ing. However, these commercially available lithium batteries are primary batteries, and at present, a rechargeable and dischargeable lithium secondary battery for practical use has not been realized. If lithium batteries can be recharged while taking advantage of the discharge characteristics of high energy density, a battery with extremely superior characteristics compared to conventional battery systems will be realized, and it will be useful for portable electronic devices and other devices. The effects on industry are high.

In order to secondaryize lithium batteries, there are many issues that need to be resolved, such as the selection of positive electrode active materials and battery construction methods. In particular, the selection of electrolyte is an important issue, and the electrolyte for secondary batteries is required to have high lithium charging and discharging efficiency.

LiCl is an electrolytic solution with high charge/discharge efficiency for lithium.
04 A 2-methyltetrahydrofuran-based electrolyte has been proposed (see U.S. Pat. No. 4,086,403), but its properties are not necessarily sufficient, and there is a need for an electrolyte for lithium secondary batteries with even better properties. It is being

[Summary of the invention]

The present invention has been made in view of the current situation, and its purpose is to provide a lithium secondary battery with excellent lithium charge/discharge characteristics.

Therefore, in the lithium secondary battery according to the present invention, the negative electrode active material is lithium or a lithium alloy that allows lithium ions to be discharged, the positive electrode active material is a substance that electrochemically performs a reversible reaction with lithium ions, and the electrolyte is In a lithium secondary battery in which a lithium salt is dissolved in an organic solvent, the organic solvent of the electrolyte is selected from a 1,3-dioxolane derivative having the general structural formula shown in the following formula (1). one or more types, the water content of the electrolyte is 100 ppm or less, and the content of impurities other than water is 11000 ppm.
It is characterized by the following:

R1-C11-0 (However, at least one of R1-R3 has 1 to 1 carbon atoms.
According to the present invention, one or more types of 1,3-dioxolane derivatives are used as an electrolyte for a lithium secondary battery, and the amount of water and impurities other than water is controlled. The main point is to
This realizes a lithium secondary battery with high conductivity and charge/discharge efficiency.

[Detailed description of the invention]

The present invention will be explained in more detail.

In lithium secondary batteries, the negative electrode active material is lithium or a lithium alloy that can discharge lithium ions, and the positive electrode active material is a material that electrochemically undergoes a reversible reaction with lithium ions. According to the present invention, a type of 1,3-dioxolane derivative having the general structure shown in formula (1) is used as the organic solvent of the electrolyte solution in which a lithium salt is dissolved in an organic solvent. The above is used.

R, -C■□0 (However, at least one of R1 to R3 has a carbon number of 1 to
4 is an alkyl group and the others are hydrogen) as described above [
1.3-dioxolane has the general formula (1) above.

In this formula (1), R and ~R3 each have 1 carbon number.
-4 alkyl groups and hydrogen, and at least one of R and -123 is the above-mentioned alkyl group having 1 to 4 carbon atoms.

This is because if the number of carbon atoms in the alkyl group is 5 or more, the conductivity will be low and the viscosity will be increased, making it undesirable as an electrolytic solution.

Specific derivatives of 1,3-dioxolane include, for example, 2-methyl-1,3-dioxolane, 4-methyl-1,3-dioxolane, and 2,4-dimethyl-1,3-dioxolane.
Dioxolane, 2-ethyl-1,3-dioxolane, 4
-ethyl-1,3-dioxolane, 2゜4-diethyl-
1,3-dioxolane, 2-methyl-4-ethyl-1,
It can be one or more of 3-dioxolane and the like.

In order to increase the Li charging and discharging efficiency of the electrolyte used in lithium secondary batteries, it is necessary to select a solvent with low electron transfer reactivity from Li to the solvent, and to select a solvent in which the Li salt in the solvent system is easily dissociated and It is believed that high L bioion mobility is required. Conventionally, 1,3-dioxolane has been well known as an electrolyte solvent for lithium batteries, but it has a practical problem that it is very easily oxidized and therefore easily polymerized. Therefore, 1,3-dioxolane It is expected that by introducing an alkyl group, which is a large substituent, into the 1,3-dioxolane, polymerization of 1,3-dioxolane is prevented, and the reactivity is suppressed due to the electron-inducing effect of the alkyl group through a single bond. Therefore, Li charging/discharging efficiency is high and a stable electrolytic solution can be provided.

The lithium salt dissolved in one or more of the 1,3-dioxolane derivatives is not fundamentally limited in the present invention. For example, LiAsF e , LiClO4, L
i5bF 6 , LiPF6 , Li^]C14, Li
One or more types of CF3SO3, LiCF3CO2, etc. can be effectively used.

Such a lithium salt is added to the mixed solvent in an amount of 0.5 to 2.
It is preferable to add 0 mol/6 (M). This is because, if it deviates from this range, not only the conductivity will decrease, but also the lithium charging/discharging efficiency may decrease significantly.

As is clear from Example 1 and Table 1, which will be described later, it has become clear that the lower the water content of such an electrolytic solution, the better the charging and discharging efficiency. That is, 1 according to the present invention,
When one or more 3-dioxolane derivatives are used, the water content is preferably 100 ppm or less, preferably 30 ppm or less. This is because if the water content exceeds 100 ppm, the charge/discharge efficiency will drop significantly.

Further, as is also clear from Example 1 and Table 1, which will be described later, better charge/discharge efficiency can be obtained when the content of impurities other than water is also small. That is, the content of impurities other than water is 11,000 ppn or less, preferably 700 ppn.
+ or less. This is because if the content of the impurities exceeds 11,000 pp, the charge/discharge efficiency will be significantly impaired.

For such electrolytes, in order to improve the solubility of solutes, the volume mixing ratio to the total amount of electrolyte is 5.
less than 0% (i.e. the volumetric mixing ratio of the above mixed solvent is 50%)
(above) additives can be used. Examples of such additives include hexamethylphosphoric triamide, N,N,N',N'-tetramethylethylenediamine, diglyme, trigetime, tetraglyme, 1.
2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, tetrahydrobilane, 1.2-
One or more compounds selected from jetoxyethane and the like can be used. If the content of such additives is 50% or more, the additives will become the main component, and there is a possibility that the 1,3-dioxolane derivative will not be able to exhibit sufficient effects as an electrolytic solution.

The negative electrode active material used in the lithium secondary battery according to the present invention is basically not limited, and the negative electrode active material used in conventional lithium batteries, that is, lithium or a lithium alloy that can discharge lithium ions, is used. be able to.

Similarly, the positive electrode active material used in the present invention is not fundamentally limited, and may be a positive electrode active material used in conventional lithium secondary batteries, that is, a material that electrochemically undergoes a reversible reaction with lithium ions. be able to.

Among such positive electrode active materials, in the lithium secondary battery of the present invention, an amorphous material whose main component is vanadium oxide such as (1) 205, for example, v205 alone, (2)
205 to P 205, TeO$1, 5bsT03
, Big 03, Ge02, B S! 03.MoO
Particularly preferred are materials to which one or more of a, WO3, TiOsl, etc. are added, but as mentioned above, the material is not limited thereto, and inorganic or organic positive electrode active materials can be effectively used.

An amorphous material containing the above-mentioned glue 205 as a main component and adding P2O5 or the like can be obtained by mixing a component to be mixed with V, O, such as Psis, and then melting and rapidly cooling the mixture.

Examples will be described below.

Example 1 1.5 M LiAsF s 2-methyl-1,3-
An electrolytic solution with controlled impurities in dioxolane was prepared, and the lithium charge/discharge efficiency was measured using the method described below.

The charge/discharge efficiency (Ea) was measured as follows using a battery using a platinum electrode as a working electrode, lithium as a counter electrode, and lithium as a reference electrode. For the measurement, first, lithium was deposited on a platinum electrode for 80 minutes at a constant current of 0.5 mA/ad (2,4 C/cd), and then a part of the deposited lithium (0,6 C/J) was deposited on the platinum electrode. A cycle test was repeated in which the battery was discharged as Li0 ions and then further discharged at a capacity of 0.6 C/-.

The charge/discharge efficiency (Ha) is determined from the change in potential of the platinum electrode,
Assuming that the number of cycles showing an apparent efficiency of 100% is n, the above-mentioned Ea can be determined from the following formula (I).

In Table 1, when electrolyte solution 1 and electrolyte solution 2 are compared, it can be seen that the lower the water content of the electrolyte solution, the higher the value of charge/discharge efficiency. Also, electrolyte 3 and electrolyte 4 in Table 1
It was found that the lower the content of impurities other than water, the higher the charge/discharge efficiency.

It is clear from Table 1 that the charge/discharge efficiency is significantly improved by removing water and other impurities. According to this, the water content of the electrolyte is 100 p
pm or less, preferably 30 ppm or less, and when impurities other than water are 1000 ppm or less, preferably 700 ppm or less, it has been found that particularly high charge/discharge efficiency can be obtained.

In the examples shown below, unless otherwise specified,
An electrolyte having a composition very similar to the electrolyte 4 is used as the electrolyte.

Table 1 (Li in 1,5M LiAsF s MeDOL
Negative electrode charge/discharge efficiency) Electrolyte Impurities ppm Charge/discharge efficiency 1/1-E (%) E: Charge/discharge water Others Value obtained by dividing efficiency by 100 Electrolyte 1 90 2000 88.9 9.
0 Electrolyte 2 120 1200 90.5
10.5 Example 2 1.5 M LiAsF a 2-methyl-1°3-dioxolane and 1.511 LiAs as electrolytes
Li charge/discharge efficiency was measured in the same manner as in Example 1 using F e 2-methyltetrahydrofuran. 1.5
M LiAsFe-2-methyltetrahydrofuran is an electrolytic solution known to have extremely high Li charge/discharge efficiency as described above, and was studied here as a comparative example to demonstrate the effects of the present invention.

The results are shown in Table 2. It was found that when 2-methyl-1,3-dioxolane was used, higher Li charge/discharge efficiency was exhibited than that of 2-methyltetrahydrofuran.

Table 2: MeTHF: 2-methyltetrahydrofuran.

Example 3 1.5 M LiAsF s 2-methyl-1,3-dioxolane was used as the electrolyte, and 95 mo I% V t Os 5 mol was used as the active material in the positive electrode.
70% by weight of amorphous v905 with a composition of %P, Qs
A positive electrode mixture pellet (16 mmφ) prepared with a mixing ratio of 25% by weight of acetylene black as a conductive agent and 5% by weight of Teflon as a binder was used, metal lithium (16 mmφ, 90mAh) was used as a negative electrode, and a fine powder was used as a separator. Using porous polypropylene sheet,
Coin type lithium battery (23a+mφ, thickness 2II11
) was manufactured.

At room temperature, charging current 1 s + A/(j, discharging current 3 mA/c
d. A charge/discharge test was conducted in a voltage range of 2 to 3.5 V to evaluate the charge/discharge characteristics of the electrolytic solution.

The results are shown in Figure 1. As is clear from this figure 1,
It was found that the 2-methyl-1,3-dioxolane type had a larger discharge capacity for each cycle and exhibited better cycle characteristics than the 2-methyltetrahydrofuran type.

〔Effect of the invention〕

As explained above, the lithium secondary battery according to the present invention has the advantage that it is a small, high energy density battery with a large charge/discharge capacity and excellent cycle life, and can be widely used in various fields. .

[Brief explanation of drawings]

Figure 1 shows the range of 2 to 3.5 V.
FIG. 3 is a diagram showing the relationship between the battery capacity and the number of charge/discharge cycles when a Pgos battery was subjected to a charge/discharge test.

Claims (3)

[Claims] (1) The negative electrode active material is lithium or a lithium alloy that enables lithium ions to be discharged, and the positive electrode active material is a material that electrochemically undergoes a reversible reaction with lithium ions.
In a lithium secondary battery in which the electrolytic solution is a lithium salt dissolved in an organic solvent, the organic solvent of the electrolytic solution is a 1,3-dioxolane derivative having the general structural formula shown in the following formula (1). , the water content of the electrolyte is 100p
pm or less, and impurities other than water are 1000 ppm or less. ▲There are mathematical formulas, chemical formulas, tables, etc.▼ (However, at least one of R_1 to R_3 is an alkyl group with 1 to 4 carbon atoms, and the others are hydrogen.) (2) The electrolyte is LiAsF_6, LiClO_4
, LiBF_4, LiPF_6, LiAlCl_4, L
iCF_3SO_3, LiCF_3CO_2 and Li
A lithium secondary battery according to claim 1, characterized in that the lithium secondary battery contains 0.5 to 2.0 mol/l of one or more lithium salts selected from the group consisting of SbF_6. (3) The positive electrode active material is V_2O_5 alone or V_2O
P_2O_5, TeO_2, Sb_2O_3, B to_5
i_2O_3, GeO_2, B_2O_3, MoO_3
, WO_3, and TiO_2. A lithium secondary battery according to claim 1 or 2, characterized in that the lithium secondary battery is an amorphous material to which one or more of the following is added.