JPH05299118A - Nonaque0us solvent for electrolyte of battery - Google Patents

Abstract

(57) [Summary] [Purpose] In addition to the function as a viscosity-reducing diluent of a non-aqueous solvent for electrolytes consisting of PC or a compound equivalent thereto, the structure itself is a structure that is difficult to oxidize, and Provided is a non-aqueous solvent for a battery electrolyte, which improves cycle characteristics in the battery and is also suitable as an electrolyte solvent for a primary battery. [Structure] Represented by the following structural formula: (In the formula, R ₁ and R ₂ may be the same or different and each represents a saturated hydrocarbon having 1 or more carbon atoms), and the carbonic acid ester compound is mixed with another non-aqueous solvent and used. When the function for reducing the viscosity of PC or a compound having physicochemical properties corresponding to PC is emphasized, it is desirable that the number of carbon atoms of R ₁ and R ₂ is 3 or less,
It is not limited to this as long as the viscosity can be reduced by adding the third diluent.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous electrolyte secondary battery such as a lithium secondary battery, or a non-aqueous solvent for an electrolyte used in a lithium primary battery, and more particularly to propylene carbonate (abbreviation PC: PC hereinafter). (Abbreviated) or a compound equivalent thereto, and a non-aqueous solvent suitable as a diluent for reducing the viscosity.

[0002]

2. Description of the Related Art As a non-aqueous electrolyte secondary battery, MnO ₂ , V ₂ O ₅ , MoO ₃ , V ₆ O ₁₃ and Ti are used as positive electrode active materials.
Secondary batteries using metal oxides or transition metal oxides or sulfides such as S ₂ and MoS _{2 and} metal lithium or an alloy thereof such as a wood alloy or a lithium-aluminum alloy as a negative electrode are known.

[0003] In the cell of this type, in order to achieve the recent high energy density, and material generating a higher voltage is studied, for example, the general formula: Li _x M _y O ₂
(M is Co or Ni, x is 0.8 or less, y is almost 1)
A secondary battery composed of an ionic conductor shown by is disclosed in JP-B-63-59507.

Using this active material, the open circuit voltage of a battery constructed with Li as the negative electrode and LiBF ₄ (1M) / PC as the electrolyte reaches 4 V or more. Further, the actual operating voltage range can be about 3.4 to 4.6V.

A battery having such a high battery voltage characteristic is suitable as a high energy density battery as a power source for various electronic devices which have been remarkably developed in recent years, and it is lightweight, compact, and highly functional. Further promote

In addition to the above-mentioned PC, various solvents have been studied as a solvent for non-aqueous electrolyte batteries.
Among them, γ-butyrolactone (abbreviation: γ-BL, the same applies hereinafter), ethylene carbonate (EC), sulfolane (SL), 1,3-dioxolane (DO), 1,2-dimethoxyethane (DME), 2-methyltetrahydrofuran. (2-MeTHF), diethyl carbonate (DE
C) and the like have been used alone or as a mixture as particularly useful solvents.

However, in the secondary battery system using these electrolytes, the low charge / discharge efficiency of the negative electrode is a serious problem. Regarding the charge and discharge efficiency of the positive electrode, there is a phenomenon that the utilization rate decreases at the beginning of the cycle, but eventually the charge and discharge efficiency reaches a value close to 100%.

On the other hand, regarding the negative electrode, the charge / discharge efficiency is up to 97% for metallic lithium, and up to 99% even if a lithium alloy is used. This means that if the positive and negative electrode capacities are the same, the negative electrode capacity decreases by 2 to 3% per cycle, and if the charge and discharge at a deep depth is repeated, a cycle life of several tens of times is exhibited. Stay in.

It is considered that the main reason for this is that the lithium that moves during charging and discharging is very active and decomposes the solvent, resulting in the conversion of lithium into an electrochemically inactive compound. ing.

Although the detailed mechanism is not clear, the properties of a certain film on the surface of lithium (physical properties, ionic conductivity, electronic conductivity, etc.) formed by the reaction of the components constituting the solvent with lithium. Is closely related to the mechanism of the reaction between the solvent and lithium for reduction, or the effect of the solvent on the charge / discharge efficiency of lithium.

In the lithium primary battery, it is not necessary to consider the charging problem, but there is a problem in that the battery performance is deteriorated due to the deterioration of the solvent due to the presence of active lithium during discharging.

By the way, PC, which is industrially most often used as a solvent for non-aqueous electrolyte secondary batteries, has a problem in that when it is used in a high operating voltage range, it causes the above-mentioned decrease in charge and discharge efficiency. There is also a problem that conductivity is low because the viscosity of itself is large. That is, when the viscosity is high, the movement of ions becomes slower, and the battery performance is affected accordingly.

That is, in order to increase the conductivity of the electrolytic solution, the movement of the ions moving in the electrolytic solution must be fast,
The high viscosity of the solvent is disadvantageous in this respect. In the case of PC alone, the viscosity at 25 ° C. is 2.53 cP, which is a considerably high value, but normally a viscosity of about 1 cP is desirable.

Therefore, for this PC, ethers such as DO, DME, THF, etc. have been particularly often used as the solvent for diluting for decreasing the viscosity from the above-mentioned solvent for the electrolytic solution, and the capacity has been increased. It has been practiced to lower the desired viscosity by mixing in a ratio of about 1: 1.

[0015]

However, it is generally said that the oxidation potential of the electrolytic solution must be high in order that the electrolytic solution does not react with the positive electrode active material. Although the above-mentioned solvent has a low viscosity, It is a substance that is easily oxidized. Note that there is a correlation between the reduction potential of the organic compound and the LUMO (lowest unoccupied molecular orbital) energy, and the higher the LUMO energy, the lower the reduction potential, that is, the more difficult it is to reduce. The oxidation potential of the solvent corresponds to the HOMO (highest occupied molecular orbital) energy, and the lower the HOMO energy, the more difficult it is to oxidize. Therefore, the ease of redox is determined from these calculation results.

The LUMO and HOMO energies of PC and the solvent for dilution are calculated as shown in Table 1 below.
become that way.

[0017]

[Table 1] That is, these solvents are required to be substances that are difficult to oxidize for the above-mentioned reason, in addition to the function as a diluent that lowers the viscosity of PC.

However, in reality, all of the above-mentioned solvents are substances that are easily reduced, but are easily oxidized, and in a secondary battery using an electrolytic solution composed of a mixed solvent of these solvents, more than in the case of PC alone. The problem of a decrease in charge / discharge efficiency has become prominent. Further, also in the primary battery, in the region where the operating voltage is high, the performance is deteriorated for the same reason.

The present invention solves the above problems,
Its purpose is not only to function as a diluent for reducing the viscosity of a non-aqueous solvent for an electrolytic solution composed of PC or a compound equivalent thereto, but also to have a structure in which the structure itself is difficult to oxidize, and a cycle characteristic in a secondary battery. And to provide a non-aqueous solvent for a battery electrolyte, which is also suitable as an electrolyte solvent for a primary battery.

[0020]

To achieve the above object, the present invention is represented by the following structural formula: (In the formula, R ₁ and R ₂ may be the same or different and each represents a saturated hydrocarbon having 1 or more carbon atoms), and the carbonic acid ester compound is mixed with another non-aqueous solvent and used.

When the function for reducing the viscosity of PC or a compound having physicochemical properties corresponding to PC is emphasized, it is desirable that the carbon number of R ₁ and R ₂ be 3 or less. However, it is not limited to this as long as the viscosity can be reduced by adding the third diluent.

The carbonic acid ester compound of the present invention is particularly suitable for PC.
It is used as a diluent for not only PC but also other P
It is possible to mix and use a compound having a physicochemical property corresponding to C, but it is necessary to select a compound which is adjusted to have generally preferable properties as a non-aqueous solvent for a battery by mixing. Is.

The non-aqueous solvent for an electrolytic solution of the present invention uses a metal oxide or a sulfide as a positive electrode active material, and a negative electrode made of metallic lithium or lithium alloy or a carbonaceous material which absorbs and releases lithium ions. In addition to being used as a non-aqueous solvent for an electrolytic solution of a non-aqueous electrolytic solution secondary battery, it can be used as a non-aqueous solvent for an electrolytic solution of a lithium primary battery.

[0024]

The carbonic acid ester compound having the above structure has a low viscosity and an oxidation potential higher than that of the conventional one. Therefore, the viscosity of an electrolytic solution using a mixed solvent of these carbonate compounds and PC or a compound equivalent thereto is lower than that of PC alone, and a charge / discharge cycle of a lithium secondary battery using the above mixed solvent. The characteristics are remarkably improved as compared with those using a mixed solution of PC under the same conditions and a conventional diluting solvent. Further, the characteristics of the primary battery can be improved.

[0025]

EXAMPLES Next, examples of the present invention will be described. However, the present invention is not limited to the examples described below.

Example 1. In the above structural formula, R ₁ ,
Ester compounds in which R ₂ was variously changed as shown in Table 2 below were synthesized, and LUMO (lowest unoccupied molecular orbital) energy and HOMO (highest maximum coverage) of PC and the conventionally used diluent solvent were synthesized. The orbital energy) and the viscosity at room temperature of 25 ° C. were measured.

[0027]

[Table 2] In the table, * D represents CH (CH ₃ ) ₂ .

As is clear from Table 2, the HOMO energy of the compound according to the present invention is a substance that is less likely to be oxidized than the conventionally used diluent solvent. The viscosity also shows a value suitable as a solvent for dilution.

FIG. 1 shows the relationship between the mixing ratio (volume ratio) of, and PC and the viscosity. Usually, the preferable solvent viscosity is in the range of 1.0 to 1.2 cP, so that the volume ratio is 5
It was confirmed that 0:50 was a preferable mixing ratio and that the mixing ratio could be easily controlled.

Example 2. (Example of Application to Lithium Secondary Battery) Next, Example 1. The non-aqueous electrolytic solution obtained by mixing the solvent obtained in 1) with PC and DM, which is a conventional non-aqueous solvent, with PC.
A coin type 2016 type secondary battery was assembled using a non-aqueous electrolyte for comparison in which E and DO were mixed, and the cycle characteristics of each were examined.

The composition of the solvent of the non-aqueous electrolyte is, as shown in Table 3 below, eight kinds of combinations including the present invention,
All the solutes were LiPF ₆ 1 mol / l.

[0032]

[Table 3] The specifications of the battery excluding the non-aqueous electrolyte are as follows.
As a positive electrode, LiMnO ₂ , carbon powder as a conductive material, and Teflon powder as a binder were used in a weight ratio of 10.
The mixture was mixed at a ratio of 0: 10: 4, and the mixture was pressure-molded into pellets and then heat-treated to remove water and used. A lithium-aluminum alloy was used as the negative electrode. The atomic ratio of lithium to aluminum is 1: 1. A polypropylene microporous film (thickness 0.025 mm) was used as the separator. The positive electrode is a circular pellet having a diameter of 15 mm and a height of 0.47 mm, and the negative electrode has a diameter of 15.
It is a circular pellet with a height of 4 mm and a height of 0.9 mm. The nominal capacity of this battery is 20 mAh.

Next, when the charge / discharge cycle characteristics of the secondary battery using the above eight kinds of electrolytic solutions were examined, the results shown in FIG. 2 were obtained. The test conditions are 1 mA for charging current and 2 mA for discharging current.
Constant current charging and discharging. The discharge end voltage was 2.0 V and the charge voltage was 3.4 V. The figure shows the change in capacity for each cycle due to the difference in the composition of the electrolyte solution when the initial discharge capacity is 100. As is clear from the results shown in FIG. 2, the cycle characteristics are remarkably improved in the battery using the electrolytic solution containing the non-aqueous solvent of the present invention. Further, this suggests that the solvent of the present invention is less likely to be decomposed than the conventionally used diluent solvent and that the conductivity of the electrolytic solution is increased.

Example 3. (Evaluation test in test cell) Example 2. No. The test cell shown in FIG. 3 was assembled using the nonaqueous electrolytic solutions 1 to 8. As the positive electrode 1,
The weight ratio of LiCoO ₂ , carbon powder as a conductive material, and Teflon powder as a binder was 100: 10: 6.
The mixture was rolled into a sheet shape and pressed onto the titanium net of the current collector 2. Further, the negative electrode 3 is made by mixing carbonaceous powder obtained by firing pitch-based carbon fiber and EPDM (ethylene propylene diene monomer) as a binder to 100: 7 and rolling to form a sheet. It was crimped to the Ni net of No. 4.
The size of each of the positive electrode and the negative electrode is a square of 10 × 10 mm, the thickness of the positive electrode is 0.25 mm, and the thickness of the negative electrode is 0.40 mm.

The theoretical filling capacity of the positive electrode is 8.2 mAh,
The negative electrode is 7 mAh. The reason for increasing the theoretical capacity of the positive electrode is that after the first charge, the amount of lithium that can be involved in the next discharge is less than the charge capacity, and this is limited to the first charge / discharge cycle. This is because a certain amount of lithium is taken into the carbonaceous negative electrode and it cannot be discharged from the next cycle.

Next, the charge / discharge cycle characteristics of the test cell using the above eight kinds of electrolytes were examined, and the results shown in FIG. 4 were obtained. The test conditions are 1 mA for charging current and 2 mA for discharging current.
Constant current charging and discharging. The discharge end voltage was 2.8 V and the charge voltage was 4.2 V as the upper limit. The figure shows the change in capacity for each cycle due to the difference in the composition of the electrolyte solution when the initial discharge capacity is 100. As is clear from the results shown in FIG. 4, the battery using the electrolytic solution of the present invention has markedly improved cycle characteristics. It was confirmed that the same effect as was obtained. Especially in the case of this battery system, the battery voltage is 4
The average operating voltage, which exceeds V, is 3.6 V, which is higher than that of the conventional battery. Therefore, the conventional No. In the batteries using the electrolyte solutions of Nos. 7 and 8, the deterioration of the characteristics is significantly recognized. This is DM
It is considered that this is a result of the fact that E and DO are oxidized and the solvent is partially decomposed.

Example 4. (Example of Application to Lithium Primary Battery) Example 2. No. AA so-called inside-out type lithium primary batteries were assembled using the non-aqueous electrolyte solutions 1 to 8.

As a positive electrode, LiMnO ₂ , carbon powder as a conductive material, and Teflon powder as a binder were mixed in a weight ratio of 100: 10: 4, and the mixture was pressure-molded and then heat-treated. The water was removed before use. A lithium sheet was used as the negative electrode. As the separator, a polypropylene non-woven fabric having a thickness of 0.2 mm was used. The positive electrode is formed in the shape of a hollow cylinder having an outer diameter of 13.7 mm, a length of 40 mm, and an inner diameter of 9 mm, and is housed so as to keep the connection with the positive electrode also serving as a container. The negative electrode is formed by forming a lithium sheet having a thickness of 1.1 mm, a width of 40 mm and a length of 24 mm into a cylindrical shape, and is fitted to face the inside of the positive electrode via a separator. The nominal capacity of this battery is 1.9 Ah.

Next, the discharge characteristics of the primary battery using the above eight kinds of electrolytic solutions were examined, and the results shown in FIG. 5 were obtained.
The test conditions are a temperature of 20 ° C. and a discharge current of 1 mA.

In this example, as is clear from the results shown in FIG. 5, the characteristics are remarkably improved when the electrolytic solution containing the non-aqueous solvent of the present invention is used. Further, this suggests that the solvent of the present invention does not easily deteriorate even if it is brought into contact with lithium for a long period of time, and maintains stability.

[0041]

INDUSTRIAL APPLICABILITY As described in detail in each of the above examples, by using the carbonate compound according to the present invention as a solvent constituent for diluting PC or a compound equivalent to PC, discharge characteristics and The charge / discharge cycle characteristics can be remarkably improved and it is useful as a solvent constituent substance of an electrolytic solution.

Also, there is an advantage that the discharge characteristics can be improved when applied to a primary battery.

[Brief description of drawings]

FIG. 1 Example 1. 6 is a graph showing the relationship between the mixing ratio of PC and the solvent of the present invention and viscosity in FIG.

[Fig. 2] Example 2. 5 is a graph comparing charge / discharge cycle characteristics in FIG.

FIG. 3 Example 3. 3 is a schematic diagram of a test cell in FIG.

FIG. 4 is a third example. 5 is a graph comparing charge / discharge cycle characteristics in FIG.

FIG. 5: Example 4. 5 is a graph comparing the discharge characteristics in FIG.

[Explanation of symbols] 1 positive electrode 2,4 current collector 3 negative electrode

Claims (5)

[Claims] 1. Represented by the following structural formula: (Wherein R ₁ and R ₂ may be the same or different and each represents a saturated hydrocarbon having 1 or more carbon atoms), and the carbonic acid ester compound is used in a mixture with another non-aqueous solvent. Non-aqueous solvent for electrolyte of battery. _2. The non-aqueous solvent for an electrolytic solution of a battery according to claim ₁ , wherein the carbon numbers of R ₁ and R ₂ are 3 or less. 3. The electrolytic solution for a battery according to claim 1, wherein the other non-aqueous solvent mixed with the carbonic acid ester compound is propylene carbonate or a compound having physicochemical properties corresponding to propylene carbonate. Non-aqueous solvent. 4. A non-aqueous electrolyte secondary using a metal oxide, a sulfide or the like as a positive electrode active material and a metallic lithium or lithium alloy or a carbonaceous material capable of absorbing and releasing lithium ions as a negative electrode. It is used as a non-aqueous solvent for an electrolytic solution of a battery, and the non-aqueous solvent for an electrolytic solution of a battery according to any one of claims 1 to 3. 5. The method according to any one of claims 1 to 3, which is used as a non-aqueous solvent for an electrolytic solution of a lithium primary battery.
A nonaqueous solvent for an electrolytic solution of a battery according to any one of 1 to 4 above.