JPH05290882A - Nonaqueous solvent for battery electrolyte - Google Patents

Abstract

PURPOSE:To prevent the deterioration of characteristics of a primary battery at the time of the long-term discharge with a light load by using a mixture of ring carbonic ester compound of a specific structure expression and the other nonaqueous solvent to increase the charge and discharge cycle characteristics of a secondary battery. CONSTITUTION:When a mixture of a ring carbonic ester compound represented by a structure expression and the other nonaqueous solvent is used, the charge and discharge characteristics of a secondary battery is remarkably increased. When it is applied to a primary battery, the deterioration of characteristics thereof at the time of the long-term discharge may be prevented. In the expression, R1 to R2: alkyl group represented by a general formula CnH2n+1, n=1 to 4; R4: hydrogen or alkyl group represented by a general formula CnH2n+1, n=1 to 4.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous electrolytic solution secondary battery such as a lithium secondary battery or a non-aqueous solvent for an electrolytic solution used in a lithium primary battery.

[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.
A secondary battery is known in which a metal oxide such as S ₂ or MoS ₂ or a transition metal oxide or a sulfide is used, and metal lithium or an alloy thereof such as a wood alloy or a lithium aluminum alloy is used for a negative electrode.

[0003] To achieve the recent high energy density in batteries of this type, and material generating a higher voltage is studied, for example, the general _{formula: Li x M y O 2 (} M is C
O or Ni, x is 0.8 or less, and y is almost 1).
It is disclosed in Japanese Patent Publication No. 9507.

The open circuit voltage of a battery constructed by using this active material for the negative electrode of lithium and the electrolyte of LiBF ₄ (1M) / propylene carbonate 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 power source for various electronic devices, which have been remarkably developed in recent years, as a high energy density battery, and the device is lightweight, compact, and highly functional. Further promote

However, in these secondary battery systems,
The low charge and discharge efficiency of the negative electrode is a big 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, with respect to the negative electrode, the charge and discharge efficiency is 97% at maximum for metallic lithium and 99% at maximum even when a lithium alloy is used. This means that if the positive and negative electrodes have the same capacity, the negative electrode capacity will decrease by 2 to 3% per cycle, and if the charge and discharge at a deep depth is repeated, the cycle life will be only tens of cycles. ..

The main cause of this is Japanese Patent Laid-Open No. 1-286263.
As described in Japanese Patent Publication, it is thought that this is because the lithium deposited during charging is very active and reduces the solvent, and as a result, lithium changes into an electrochemically inactive compound. There is.

Although the detailed mechanism is not clear, the properties (physical properties, ionic conductivity,
It is believed that the electron conductivity and the like) are closely related to the mechanism of the reaction between the solvent and lithium to be reduced, and the influence of the solvent on the charge and discharge efficiency of lithium.

By the way, various solvents have been studied as solvents for non-aqueous electrolyte batteries, and among them, γ-butyrolactone (abbreviation: γ-BL, the same applies hereinafter), ethylene carbonate (EC), propylene carbonate (P
C), sulfolane (SL), 1,3-dioxolane (D
O), 1,2-dimethoxyethane (DME), 2-methyltetrahydrofuran (2-MeTHF), diethyl carbonate (DEC) and the like have been used as a particularly useful solvent, either alone or as a mixture thereof.

Of these, PC is most often used as a solvent for a non-aqueous electrolyte secondary battery both as a single solvent and as a mixed solvent. However, by repeatedly charging and discharging, PC reacts with negative electrode lithium to give a reduction product. Occurs, which accumulates on the surface of the negative electrode and causes a decrease in charge / discharge efficiency.

The above-mentioned various solvents, especially PC, are also used in the lithium primary battery. In a primary battery, it is not necessary to consider the problem of charging, but there is a problem that the solvent is reduced due to the presence of active lithium at the time of discharging and the performance of the battery is deteriorated. Particularly, when a weak discharge is performed at a high temperature for a long time, the reaction between active lithium and a solvent is performed for a long time, and this tendency becomes remarkable, which may lead to a significant deterioration in battery performance. ..

Therefore, the above-mentioned Japanese Patent Laid-Open No. 1-286263
In the publication, as a practical solution for making it difficult to react with lithium, carbonic acid as an additive is added to a known cyclic carbonic acid ester such as PC and BC, or a solvent containing a mixed solvent thereof and other ester or ether. The one to which lithium is added is used.

This is because the solvent containing cyclic carbonate such as PC reacts with lithium to produce lithium carbonate, but Li ₂ CO ₃ cannot discharge Li, so that lithium as a negative electrode active material is consumed and charged and discharged. Since the cycle becomes short, in order to prevent this, the Li ₂ CO
_The addition of ₃ is intended to prevent an increase in lithium carbonate due to the reaction.

[0015]

However, even if such an additive is added, it is still not a sufficient countermeasure against deterioration of characteristics as a solvent for a lithium secondary battery, and it is considered that the reduction of the solvent is the cause. The deterioration of the cycle characteristics is recognized. This means that even if additives are added,
It is suggested that the easiness of reduction of these cyclic carbonic acid ester itself cannot be sufficiently modified.

For a PC, for example, the following structural formula:

[Chemical 2] In the above, R ₁ is a methyl group and the other R _{2 to} R ₄ are substituted with hydrogen. Therefore, the PC has only 1 in C ^4.
Since it has only one methyl group and is easily cleaved between the oxygen and carbon of O ¹ -C ⁵ , it is easily reduced. Also B
In the case of C, the present inventors hypothesized that in the above structural formula, each of C ⁴ and C ⁵ has one methyl group, and it is slightly less likely to be reduced than PC.

The present invention has been made by paying attention to the above points, and its purpose is to reduce the structure itself at the negative electrode more than the conventionally used cyclic carbonic acid ester including PC and the like. It is an object of the present invention to provide a non-aqueous solvent for an electrolytic solution of a battery, which has a difficult structure and completely satisfies other physicochemical characteristics required as an electrolytic solution solvent of the battery.

[0018]

To achieve the above object, the present invention is represented by the following structural formula:

[Chemical 3] (In the formula, R ₁ , R ₂ and R ₃ are represented by the general formula C _n H _{2n + 1} (n = 1 to 4
), An alkyl group represented by the formula (4), R ₄ is hydrogen or a cyclic carbonic acid ester compound of the general formula C _n H _{2n + 1} (n = 1 to 4) is mixed with another non-aqueous solvent. It is characterized by being used.

The carbon number n of the substituent of R _{1 to} R ₃ (R ₄ ) is preferably about 1 to 4. This does not suppress the property of being easily reduced even if the carbon number is further increased, and is meaningless, as well as other physical properties,
This is because problems such as viscosity and melting point occur as a solvent for the electrolytic solution.

Particularly in the present invention, 4,4,5-trimethyl-1,3-dioxolan-2- _one wherein R ₁ , R ₂ and R ₃ are CH ₃ and R ₄ is hydrogen, or the above It is preferable to use 4,4,5,5-tetramethyl-1,3-dioxolan-2-one in which R _{1 to} R ₄ are all CH ₃ .

The cyclic carbonic acid ester compound is generally obtained by the following chemical reaction formula.

[0022]

[Chemical 4] The other non-aqueous solvent mixed with the cyclic carbonic acid ester compound can be selected from the above-mentioned known esters and ethers, but not only has the property of being less likely to be reduced to the negative electrode, but also the cyclic carbonic acid ester compound. It is necessary to select, as a non-aqueous solvent for a battery, one which is adjusted to have generally preferable physicochemical properties by mixing.

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 carbonaceous material capable of inserting and extracting lithium metal or lithium alloy or lithium ion into the negative electrode. It can be used as a non-aqueous solvent for an electrolytic solution of a non-aqueous electrolytic solution secondary battery using the material and also as a non-aqueous solvent for an electrolytic solution of a lithium primary battery.

[0024]

Non-aqueous solvent comprising a cyclic carbonate compound of the effects of the present invention, two to C ⁴ in the structural formula, have one or more alkyl groups in C ^5, O ¹ -C This alkyl group
Cleavage between oxygen and carbon of ⁵ , O ³ -C ⁴ is unlikely to occur.
Therefore, the solvent is less likely to be reduced than PC and BC. The improvement of the charge / discharge cycle characteristics of the lithium secondary battery using such a non-aqueous solvent can be achieved under the same conditions of PC and BC.
It is demonstrated by being remarkable compared with the one using. Further, in the primary battery, it is possible to prevent deterioration of battery characteristics particularly during long-term discharge under a low load, and increase in discharge capacity demonstrates the usefulness of using the non-aqueous solvent of the present invention.

[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 ,
R ₂ and R ₃ are CH ₃ and R ₄ is hydrogen, 4,
4,5-Trimethyl-1,3-dioxolan-2-one was synthesized according to the above chemical reaction formula. The physical properties of this product were a solid at room temperature with a melting point of 56-60 ° C.

Example 2. In the above structural formula, R ₁ ~
4,4,5,5-Tetramethyl-1,3-dioxolan-2-one in which all R _{4 s} are CH ₃ was synthesized according to the above chemical reaction formula. This product has a melting point of 170 to 17
Solid at 4 ° C.

Next, in order to examine the difference between the reduction potentials of the compounds obtained in Examples 1 and 2 and conventional PC and BC, the LUMO (lowest unoccupied molecular orbital) energy (eV) of the molecular orbital of each compound. Then, the following result was obtained. The reduction potential of organic compounds and LUM
There is a correlation with the O energy, and the higher the LUMO energy, the lower the reduction potential, that is, the more difficult it is to reduce.

LUMO energy (eV) PC ... 1.234 BC ... 1.288 Example 1. ... 1.350 Example 2. ... 1.410 As is apparent from these results, Example 1. , 2. The compound by is PC
Alternatively, it has a larger LUMO energy than BC, indicating that it is a substance that is difficult to reduce. Further, other properties also show suitable values as the nonaqueous solvent for the electrolytic solution.

Example 3. (Example of Application to Lithium Secondary Battery) Next, Example 1. , 2. A non-aqueous electrolyte solution obtained by mixing the solvent obtained in (1) and another non-aqueous solvent and a non-aqueous electrolyte solution obtained by mixing PC, BC and another non-aqueous solvent are used to form coin-type lithium secondary batteries ( 2016 type) was assembled and the cycle characteristics of each were investigated. The solvent was mixed in the non-aqueous electrolyte solution in eight kinds of combinations including the following conventional example and the present invention, and the solute was all LiPF ₆ 1 mol / l.

Formulation No. No. 1 ... PC + DME Capacity ratio 1: 1 Comparative example No. 2 ... PC + * EMC capacity ratio 1: 1 Comparative example No. 3 ... BC + DME Capacity ratio 1: 1 Comparative example No. 4 ... BC + EMC capacity ratio 1: 1 Comparative example No. 5 ... actual 1 + DME capacity ratio 1: 1 The present invention No. 6 ... Actual 1 + EMC capacity ratio 1: 1 The present invention No. 7 ... actual 2 + DME capacity ratio 1: 1 The present invention No. 8: Actual 2 + EMC capacity ratio 1: 1 of the present invention * EMC is an abbreviation for ethyl methyl carbonate Further, the specifications of the battery excluding the non-aqueous electrolyte are as follows. As a positive electrode, LiMnO ₂ , carbon powder as a conductive agent, and Teflon powder as a binder were mixed in a weight ratio of 100: 10: 40, and the mixture was pressure-molded into pellets and heat-treated to remove water. 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 has a diameter of 15 mm and a height of 0.
47mm circular pellets, the negative electrode has a diameter of 15.4mm,
It is a circular pellet with a height of 0.9 mm. The nominal capacity of this battery is 20 mAh.

Next, the charge and discharge cycle characteristics of the battery of the above specifications using the above eight kinds of electrolytes were examined.
Got the result. The test conditions are 1 mA for charging current and 2 for discharging current.
A constant current charge / discharge of mA was used. 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. 1, the cycle characteristics are remarkably improved in the battery using the electrolytic solution containing the non-aqueous solvent of the present invention. In addition, this means that the solvent of the present invention is more durable than PC and BC,
This suggests that reduction by the negative electrode is difficult.

Example 4. (Evaluation test in test cell) Example 3. No. The test cell shown in FIG. 2 was assembled using the non-aqueous electrolyte solutions 1 to 8. As the positive electrode 1,
LiCoO ₂ , carbon powder as a conductive agent, and Teflon powder as a binder in a weight ratio of 100: 10:
The mixture was mixed at a ratio of 6 and rolled into a sheet shape, which was pressed onto a titanium net as 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 in a ratio of 100: 7 and rolling to form a sheet. It was crimped to a Ni net as No. 4. The size of the positive and negative electrodes is a square of 10 x 10 mm. The thickness of the positive electrode is 0.25 mm and the thickness of the negative electrode is 0.40 mm.
Is. The distance between the electrodes 1 and 3 was 2 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 because the lithium was charged only in the first charge / discharge cycle. This is because a certain amount of lithium is taken into the carbonaceous negative electrode, and it will not be possible to discharge from the next time.

Next, when the charge and discharge cycle characteristics of the test cell of the above specifications using the above eight kinds of electrolytes were examined, the results of FIG. 3 were obtained. The test conditions were constant current charging / discharging with a charging current of 1 mA and a discharging current of 2 mA. The discharge end voltage is 2.
The upper limit of the charging voltage was 8V and 4.15V. 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. 3, in the battery using the electrolytic solution containing the non-aqueous solvent of the present invention, the cycle characteristics were remarkably improved. It was confirmed that the same effect as was obtained.

Example 5. (Example of Application to Lithium Primary Battery) Example 3. No. AA so-called inside-out type lithium primary batteries were assembled using the non-aqueous electrolyte solutions 1 to 8.

As the positive electrode, LiMnO ₂ , carbon powder as a conductive agent, and Teflon powder as a binder were used in a weight ratio of 1
The mixture was mixed at a ratio of 00:10:40, pressure-molded, and then heat-treated to remove water and used. 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 has an outer diameter of 13.7 mm,
It is formed into a hollow cylinder having a length of 40 mm and an inner diameter of 9 mm, and is housed in a positive electrode which also serves as a container so as to keep the connection. For the negative electrode, a lithium sheet having a thickness of 1.1 mm, a width of 40 mm, and a length of 24 mm is formed into a cylindrical shape, and the lithium sheet is fitted to face the inside of the positive electrode via a separator. The nominal capacity of this battery is 1.9 Ah.

Next, when the discharge characteristics of the primary battery of the above specifications using the above eight kinds of electrolytic solutions were examined, the results shown in FIG. 4 were obtained. The test conditions were a temperature of 60 ° C. and a discharge current of 10
μA, and as described above, when a long-term discharge is performed under a low-rate load under such a temperature condition, the active lithium at the time of discharge and the solvent are in contact with each other for a long time, so that the deterioration of the electrolytic solution progresses. However, it is said that the life is shortened.

In this Example, as is clear from the results shown in FIG. 4, when the electrolytic solution containing the non-aqueous solvent of the present invention was used, the characteristics were low even with such a low rate discharge. Noticeably improved. This also means that the solvent of the present invention is P
This suggests that it is more durable than C and BC and that it is less likely to deteriorate even if it is in contact with lithium for a long period of time.

[0040]

As described in detail in the above examples, by using the cyclic carbonic acid ester according to the present invention as the solvent constituent substance of the electrolytic solution, the charge / discharge cycle characteristics can be remarkably improved in the secondary battery. Therefore, it is useful as a solvent constituent substance of the electrolytic solution. In addition, when applied to a primary battery, there is a feature that deterioration of characteristics during discharge over a long period under a low load can be prevented.

[Brief description of drawings]

FIG. 1 Example 3. 5 is a graph comparing charge / discharge cycle characteristics in FIG.

FIG. 2 Example 4. 3 is a schematic diagram of a test cell in FIG.

FIG. 3 Example 4. 5 is a graph comparing charge / discharge cycle characteristics in FIG.

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

Claims (5)

[Claims] 1. Represented by the structural formula: (In the formula, R ₁ , R ₂ and R ₃ are represented by the general formula C _n H _{2n + 1} (n = 1 to 4
), An alkyl group represented by the formula (4), R ₄ is hydrogen or a cyclic carbonic acid ester compound of the general formula C _n H _{2n + 1} (n = 1 to 4) is mixed with another non-aqueous solvent. A non-aqueous solvent for a battery electrolyte, which is used. 2. R ₁ , R ₂ and R ₃ are CH ₃ .
4,4,5-trimethyl-1,3-wherein R ₄ is hydrogen
The non-aqueous solvent for an electrolyte of a battery according to claim 1, which is dioxolan-2-one. 3. R _{1 to} R ₄ are all CH ₃ .
The non-aqueous solvent for an electrolytic solution of a battery according to claim 1, which is 4,4,5,5-tetramethyl-1,3-dioxolan-2-one. 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.