JPS62222575A - Lithium secondary battery - Google Patents

Abstract

PURPOSE:To increase conductivity and charge-discharge efficiency and to make it possible to use in a wide temperature range by using a specific mixed solvent in an electrolyte, and specifying the contents of moisture and the other impurities. CONSTITUTION:A negative active material is lithium or a lithium alloy capable of discharge of a lithium ion. A positive active material is a substance which performs reversible reaction electrochemically with a lithium ion. An electrolyte is a solution obtained by dissolving a lithium salt in an organic solvent. As the organic solvent of the electrolyte, a mixed solvent of ethylene carbonate and one or more solvents selected from a group of compounds having polar double bond of >C=O and/or >S=O, in a volume ratio of 40-90% is used. The moisture contents in the electrolyte are 150ppm or less and the impurity contents other than moisture are 1,000ppm or less. Thereby, charge-discharge capacity, cycle life, and energy density are increased.

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 a negative electrode active material (hereinafter referred to as lithium batteries) are being researched as small-sized batteries with high energy density, and batteries that use manganese dioxide, graphite fluoride, etc. as positive electrode active materials 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. Although it is desirable from a practical standpoint to use a non-aqueous electrolyte in a lithium secondary battery that operates at room temperature, the conductivity of the electrolyte is one or two orders of magnitude lower than that of the aqueous solution system used in conventional battery systems. Because of these drawbacks, it is essential to improve the conductivity of the electrolyte in order to improve the discharge utilization rate of the battery. At the same time, in order to apply it to secondary batteries, it is natural that lithium is required to have high charging and discharging efficiency in the non-aqueous electrolyte. That is, the electrolytic solution used in a lithium secondary battery needs to satisfy two requirements at the same time: (1) having high conductivity and (2) having high lithium charging and discharging efficiency.

In order to increase the conductivity of the electrolyte used in lithium secondary batteries and the charging/discharging efficiency of Li, it is necessary to select a solvent that has low reactivity for electron transfer from I to the solvent, and to increase the Lf salt in the solvent system. It is thought that it is necessary to be able to dissociate easily and to have high mobility of Li+ ions. From this point of view, it is expected that the use of a high dielectric constant solvent is advantageous in improving the properties of the electrolytic solution.

Ethylene carbonate has a high dielectric constant (relative permittivity of 89°■
, 40°C), but its melting point is about 36°C, which makes it difficult to use alone at room temperature, and its viscosity is high (1.9 cP, 40°C).

[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 high conductivity and 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 the group of compounds having ethylene carbonate and a polar double bond of >C=0 and/or >S=0. The main component is a mixed solvent with a volume mixing ratio of 40 to 90% with one or more selected types, and the water content of the electrolyte is less than 100 ppm, and the content of impurities other than water is 1)000 ppm or less. This is a characteristic feature.

According to the present invention, ethylene carbonate and >C=0 and/or >S=0 are used as an electrolyte for a lithium secondary battery.
The most important points are to use a mixed solvent with a volume mixing ratio of 40 to 90% with one or more compounds selected from the group of compounds having polar double bonds, and to control the amount of water and impurities other than water. This makes it possible to realize a lithium secondary battery that has high conductivity and charge/discharge efficiency, and can be used over a wide temperature range.

[Detailed description of the invention]

The present invention will be explained in more detail.

In a lithium secondary battery, 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 material that electrochemically performs a reversible reaction with lithium ions, and the electrolyte is an organic According to the present invention, ethylene carbonate and a polar double of >C=0 and/or >S=0 are used as the organic solvent of the electrolyte in which a lithium salt is dissolved in an organic solvent. A mixed solvent having a volume mixing ratio of 40 to 90% with one or more selected from the group of compounds having a bond is used.

As mentioned above, ethylene carbonate has the advantage of a high dielectric constant, but has the disadvantage of a high melting point, making it difficult to use at low temperatures. The molar freezing point depression of ethylene carbonate is 5.5°C 1 mol, and in order to compensate for the disadvantages without impairing the advantages of ethylene carbonate, in the present invention, ° is >C=0 and/or >S =0
It contains one or more compounds having a polar double bond and a relatively high dielectric constant.

Examples of compounds having polar double bonds of >C=0 and/or >S=O include propylene carbonate, T-butyl lactone, T-valerolactone, γ-octanoic lactone, and 3-methyl- It is possible to use at least one selected from 2-oxazolidine, sulfolane, 3-methylsulfolane, dimethylsulfoxide, methyl acetate, methyl formate, and the like.

Ethylene carbonate and >C=0 and/or >S=
The volume mixing ratio with one or more compounds selected from compounds having 0 polar double bonds is 40 to 90%, preferably 60 to 70%, but the volume mixing ratio of ethylene carbonate exceeds 90%. , there is not much change from ethylene carbonate alone, while if it is less than 40%, >C
-0 and/or >S-0 polar double bonds. .

The lithium salt dissolved in the above-mentioned mixed solvent is not fundamentally limited in the present invention.

For example, LiAsF 6 , LtClOa, LiBF
s, LiPFe, Li^1cI4, LiCF3SO
3. One or more of LiCF3 CO2, LiSbF6, etc. can be effectively used.

Such a lithium salt is added to the mixed solvent in an amount of 0.5 to 2.
0 moles/7! (M) It is good to add. 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 mentioned above, the organic solvent of the electrolyte used in the present invention is ethylene carbonate and >C-0 and/or >
The main component is a mixed solvent with one or more compounds selected from compounds having a polar double bond with S=0.

For such mixed solvents, for reasons such as improving the solubility of the solute, the volume mixing ratio of the total amount of electrolyte is less than 50% (i.e., the volume mixing ratio of the above mixed solvent is 50%).
% or more) can be used. Examples of such additives include hexamethylphosphoric triamide, N,N,N',N'-tetramethylethylenediamine, diglyme, trigetime, tetraglyme,
1,2-dimethoxyethane, tetrahydrofuran, 2-
Methyltelorahydrofuran, tetrahydropyran, 1,
One or more compounds selected from 2-jethoxyethane and the like can be used. If the amount of this additive is 50% or more, it may become the main component and may not be effective as a mixed solvent.

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, when using a mixed solvent of ethylene carbonate according to the present invention and one or more compounds having polar double bonds of >C=O and/or >S=0, the water content is 150 ppn+ or less, preferably 50 ppm or less. It's good to have one. Water content is tso p
This is because if it exceeds pm, the charging/discharging efficiency will be significantly reduced.

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 1) 000 ppm or less, preferably 700 ppm
It is as follows. This is because if the content of the impurities exceeds 1)000 pp, the charge/discharge efficiency will be significantly impaired.

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 monolithium rechargeable battery of the present invention, an amorphous material containing vanadium oxide as a main component such as V2O5, for example, V2O5 alone, V2O5 with Pieces, ↑eo t, Sb2O a ,
Bi2O3, GeO1)-, B2O3% MoO
Particularly preferred are materials to which one or more of s, WO3, Tooe, 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 ν2os and the like as a main component and adding P2O5 and the like can be obtained by mixing a component to be mixed with 2O5, such as P2O5, and then melting and rapidly cooling the mixture.

Examples will be described below.

Example 1 1.5 M LiAsF s-ethylene carbonate/
7” l:1 pyrene carbonate (volume mixing ratio 1/1
) An electrolytic solution with controlled impurities was prepared, and a lithium secondary battery as described below was fabricated.

The positive electrode contains 95mol%V2O55m as an active material.
70i of amorphous V2O5 with a composition of o1% P2O5
! 1%, acetylene black as a conductive agent 25i1f
ffH% and Teflon as a binder at a mixing ratio of 5% by weight, a positive electrode mixture pellet (16III#Iφ) was used, and as a negative electrode, metallic lithium (17m-φ, 15mAh
) and a microporous polypropylene sheet as a separator to produce a coin-type lithium battery.

This lithium battery was heated at room temperature with a current value of 1 mA, 2 to 3
．． A charge/discharge test was conducted in a voltage range of 5 V to evaluate the charge/discharge characteristics of the electrolyte.

The results are shown in Figure 1. As is clear from this figure 1,
Excess lithium that can participate in discharge is consumed on the negative electrode side, and the discharge capacity gradually decreases depending on the charge/discharge efficiency. That is, ・E is the charging/discharging efficiency of the negative electrode, C
When n is the first discharge capacity, Cn = E XCn −1 holds true, and from this, the relationship 1nCn = (n −1) 1nE + Inet can be found, and the charging and discharging efficiency can be calculated.

The above ethylene carbonate: propylene carbonate =
Table 1 below shows the results of calculating the charge/discharge efficiency using the above formula while varying the amounts of water and impurities in the electrolyte using a 1:1 mixed solvent.

In Table 1, electrolyte 1, electrolyte 3, electrolyte 4,
Comparing electrolyte 5, the lower the water content of the electrolyte, the higher the E
It can be seen that the value of becomes high. Also, electrolyte 2 in Table 1
Comparing Electrolyte 4 with Electrolyte 4, it was found that the value of E increases as the content of impurities other than water decreases.

From Table 1, it is clear that by removing water and other impurities, the charging and discharging efficiency is greatly improved. According to this, the water content of the electrolyte is 150 p
It has been found that particularly high charge/discharge efficiency can be obtained when the concentration of impurities other than water is 1) 000 ppm or less, preferably 700 ppm or less.

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

PC: Akutobilene carbonate Example 2 1.5% of the electrolyte was added to a mixed solvent of ethylene carbonate and propylene carbonate (volume mixing ratio 1/1).
A solution of M LiAsFe was used. The conductivity of this electrolyte at 25°C is approximately 20% higher than propylene carbonate alone, or 6.2xio-3
S crm-' was shown. Also, 1.5 M LiAs
Although Fs-ethylene carbonate begins to solidify at temperatures below 2O<0>C and cannot be used as an electrolyte, the electrolyte according to the present invention has the advantage of exhibiting high conductivity even at low temperatures, as shown in FIG.

Example 3 As an electrolyte, 1.5 M LiAsF a-ethylene carbonate/propylene carbonate (volume mixing ratio l
/1) to measure the charging and discharging efficiency of lithium. 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. In the measurement, first, lithium was deposited on a platinum electrode for 80 minutes at a constant current of 0.5 mA/cnl (2,4C/, ff1), and then a portion of this deposited lithium (0,6C/cIA> A cycle test was repeated in which the battery was discharged as Li'' ions and then further discharged at a capacity of 0.6 C/cd.

The charge/discharge efficiency (Ea) 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 Ha can be determined from the following formula <1).

The results are shown in Table 2. It was found that the ethylene carbonate/propylene carbonate mixed system [Table 2 (A)] exhibited higher lithium charge/discharge efficiency than propylene carbonate [Table 2 (B'')] alone.

Example 4 As an electrolyte, 1,5 M LiAsF a-ethylene carbonate/dimethyl sulfoxide (volume mixing ratio: 1
The charging and discharging efficiency of lithium <Ea) was measured in the same manner as in Example 3, except that Ea) was used.

The results are shown in Table 2. From this table, it can be seen that the ethylene carbonate/dimethyl sulfoxide mixed system [Table 2 (C)] exhibits higher lithium charge/discharge efficiency than dimethyl sulfoxide [Table 2 (B)] alone.

Example 5 The charging and discharging efficiency of lithium (1! a ) was measured.

The results are shown in Table 2. From this table, ethylene carbonate/methyl formate mixed system [Table 2 (E)] is different from methyl formate [Table 2 (E)].
Table 2 (F)] It can be seen that the charging and discharging efficiency of lithium is higher than that of lithium alone.

Example 6 Lithium charge/discharge efficiency (Ba) was determined in the same manner as in Example 3, except that 1.5 M LiAsF (3-ethylene carbonate/methyl acetate (volume mixing ratio: 2/3) was used as the electrolyte. was measured.

The results are shown in Table 2. From this table, ethylene carbonate/methyl acetate mixed system [Table 2 (G)] is different from methyl acetate [Table 2 (G)].
Table 2 (■)] It can be seen that the charging and discharging efficiency of lithium is higher than that of lithium alone.

(Margin below) Table 2 Example 7 1.5 M LiAsF s-ethylene carbonate/propylene carbonate (volume mixing ratio 1/
1) was used, and the positive electrode contained 95mo 1%V as an active material.
Amorphous V2 with a composition of 2O5-5 TsuruO1%P2O5
A positive electrode mixture pellet (161φ) made with a mixing ratio of 70% by weight of O5, 25% by weight of acetylene black as a conductive agent, and 5% by weight of Teflon as a binder was used, and metallic lithium (16miφ, 90mAh) was used as a negative electrode. Furthermore, a coin-type lithium battery (23a+n+φ, thickness 21II1) was manufactured using a microporous polypropylene sheet as a separator.

At 25℃, charging current 1 m^/cd, discharging current 3-
A charge/discharge test was conducted in a voltage range of 2 to 3.5 V to evaluate battery characteristics.

The results are shown in Figure 3. In Figure 3, 1.5MLiA
sF e-ethylene carbonate/propylene carbonate (volume mixing ratio 1/1) mixed system and 1.5
The results are shown for the case of M LiAsF a /propylene carbonate alone system.

It was found that the ethylene carbonate/propylene carbonate mixed system exhibited better cycle characteristics than the propylene carbonate single system.

Example 8 Lithium/V2Os P
A g05 battery was produced. A discharge test was conducted on this battery at 25° C. with a discharge current density in the range of 0.5 to 6 n+A/cd. FIG. 4 shows the relationship between V*Os P IIOs, discharge capacity per weight, and discharge current density until the discharge voltage reaches 2V in this test.

FIG. 4 shows, as a comparative example to show the effect of the present invention,
Also shown are the results when 1.5 M LiAsFe-propylene carbonate was used as the electrolyte. As can be seen from FIG. 4, the obtained current value of the battery is improved by using the mixed solvent electrolyte according to the present invention.

〔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]

Fig. 1 beam, 1 in 5 M LiAsFe-ethylene carbonate/propylene carbonate (1/1) system
1 a+A of battery using 5s+Ah Li, 2-3
．． Figure 2 shows the change in discharge capacity in a cycle test in the 5ν range. Figure 2 shows the relationship between conductivity and temperature for 1.5 M LiAsFe - ethylene carbonate/propylene carbonate. Figure 3 shows the relationship between the conductivity and temperature of Li/ in the range of .5 V
Figure 4 shows the relationship between the battery capacity and the number of charge/discharge cycles when performing a charge/discharge test on a V2O5-P2O5 battery. It is a figure showing the relationship of discharge current density. Figure 1 Iiguru number of times (n) Figure 2 Kuko - 10O102O30

Claims (3)

[Claims] (1) The negative electrode active material is lithium or a lithium alloy that enables lithium ions to be discharged, the positive electrode active material is a material that electrochemically performs a reversible reaction with lithium ions, and the electrolyte is a lithium salt dissolved in an organic solvent. In the lithium secondary battery, the organic solvent of the electrolyte is a combination of ethylene carbonate and one or more compounds selected from the group of compounds having polar double bonds of >C=0 and/or >S=0. A lithium secondary battery, characterized in that the main component is a mixed solvent with a volumetric mixing ratio of 40 to 90%, the electrolytic solution has a water content of 150 ppm or less, and impurities other than water are 1000 ppm or less. (2) The electrolyte is LiAsF_6, LiClO_4,
LiBF_4, LiSbF_6, LiPF_6, LiA
lCl_4, LiCF_3SO_3, LiCF_3CO
A lithium secondary battery according to claim 1, characterized in that the lithium secondary battery is one or more lithium salts selected from the group consisting of _2 dissolved at 0.5 to 2.0 mol/l. (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
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 , WO_3 and TiO_2 are added.