JP4915025B2 - Nonaqueous electrolyte and lithium secondary battery - Google Patents

Description

【００３３】
比較例１
表１に示す組成をもつ電解液を用いた以外は、実施例１と同様にして平板電池Ｒ１〜Ｒ6を作製した。これらの電池（Ｒ１〜Ｒ6）について、実施例１と同様の試験を行った。結果を表１にまとめて示す。
【００３４】
実施例２
エチレンカーボネート、ビニレンカーボネート、ジメチルカーボネート、エチルメチルカーボネートの体積比１７：３：３０：５０とした組成をもつ電解液を用いて、実施例１と同様にしてガス捕集治具を取り付けたステンレス製の容器を用い、平板電池Ａ７を作製した。
作製した電池を用い、室温において電池を４ｍＡ、上限電圧４．２Ｖの定電流・定電圧充電を４時間行った後、６０℃の雰囲気中で１０日間保存した後のガス発生量を測定したところ、０．８ｃｍ³であった。
【００３５】
比較例２
エチレンカーボネート、ビニレンカーボネート、ジメチルカーボネート、エチルメチルカーボネートの体積比１７：３：６０：２０とした以外は、実施例２と同様にして平板電池Ｒ7を作製した。この電池について、実施例２と同様の試験を行ったところ、ガス発生量は１．２ｃｍ³であった。
【００３６】
【発明の効果】
本発明の非水電解液を用いたリチウム二次電池は、充放電サイクル中の放電特性、低温時および大電流放電時の放電特性並びに高温で長期保存したときの容量保存特性、高温で保存したときのガス発生量が小さいこと、がバランス良く優れている。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a non-aqueous electrolyte and a lithium secondary battery.
[0002]
[Prior art]
Since lithium secondary batteries have a high energy density, the field of use is expanding, especially in portable electronic devices. And since the material of a lithium secondary battery dislikes a water | moisture content, the aprotic organic solvent is used as a solvent of electrolyte solution.
[0003]
Since the aprotic organic solvent has a conductivity that is almost an order of magnitude lower than that of the aqueous electrolyte, the composition of the electrolyte that improves the ionic conductivity has been studied.
In addition, the electrolyte solution of the battery is required to have excellent oxidation resistance and reduction resistance, not to be a solid or gas within the operating temperature range of the battery, and to have relatively low toxicity.
[0004]
In addition, lithium secondary batteries have good discharge characteristics during charge / discharge cycles, good discharge characteristics at low temperatures and large current discharges, good capacity storage characteristics when stored for long periods, and gas generation when stored at high temperatures. A small amount is desired, and a non-aqueous electrolyte that satisfies these requirements is required.
[0005]
In order to satisfy the above various requirements, it is difficult to use a non-aqueous solvent composed of only one type of compound, and studies have been made to use a mixture of two or more types of compounds as a non-aqueous solvent.
[0006]
For example, propylene carbonate + acyclic carbonate (JP-A-2-10666), ethylene carbonate or propylene carbonate + diethyl carbonate (JP-A-2-12777), cyclic carbonate + acyclic carbonate (JP-A-2-110). 172162, JP-A-22-172163, JP-A-4-171474, JP-A-5-013088, JP-A-5-121097, JP-A-6-150968), cyclic carbonate + asymmetric Chain carbonate (JP-A-6-290809), propylene carbonate, ethylene carbonate or γ-butyrolactone + dimethyl carbonate (JP-A-5-217602), ethylene carbonate + low viscosity ester or ether (JP-A-8 190933), dimethyl carbonate + methyl ethyl carbonate (JP-A-7-14607), ethylene carbonate + dimethyl carbonate (US Pat. No. 5,192,629), ethylene carbonate + dimethyl carbonate + diethyl carbonate (JP-A 5-283104). No.), ethylene carbonate + dimethyl carbonate + methyl ethyl carbonate (JP-A-7-45304), vinylene carbonate + dimethyl carbonate (JP-A-6-52887), ethylene carbonate + propylene carbonate, butylene carbonate or vinylene carbonate + Dimethyl carbonate or diethyl carbonate (Japanese Patent Application Laid-Open No. 6-84542), vinylene carbonate + methyl ethyl carbonate (Japanese Patent Laid-Open No. 11-185806) and the like are disclosed.
[0007]
However, the discharge characteristics during charge / discharge cycles, the discharge characteristics at low temperatures and large current discharges are good, the capacity storage characteristics are good when stored for a long time, and the amount of gas generated when stored at high temperatures is small. The requirements have not necessarily been realized, and there has been a demand for an electrolyte solution that satisfies these requirements in a well-balanced manner.
[0008]
[Problems to be solved by the invention]
The object of the present invention is that the discharge characteristics during the charge / discharge cycle, the discharge characteristics at low temperature and large current discharge, the capacity storage characteristics when stored for a long time at high temperatures, and the amount of gas generated when stored at high temperatures is small. An excellent non-aqueous electrolyte with good balance and a lithium secondary battery using the same are provided.
[0009]
[Means for Solving the Problems]
As a result of intensive studies, the present inventors have used a non-aqueous solvent containing ethylene carbonate, vinylene carbonate, dimethyl carbonate, and ethyl methyl carbonate within a specific composition range as a non-aqueous solvent for the non-aqueous electrolyte. Thus, the inventors have found that the above-mentioned problems can be solved, and have completed the present invention.
[0010]
That is, the present invention comprises [1] a nonaqueous solvent containing ethylene carbonate, vinylene carbonate, dimethyl carbonate and ethyl methyl carbonate and an electrolyte, and the sum of the volumes of ethylene carbonate, vinylene carbonate, dimethyl carbonate and ethyl methyl carbonate is 100 volumes. %, Ethylene carbonate is 5 to 30% by volume, vinylene carbonate is 1 to 10% by volume, dimethyl carbonate is 5 to 30% by volume, ethyl methyl carbonate is 40 to 90% by volume, and the volume of ethylene carbonate and vinylene carbonate is The present invention relates to a nonaqueous electrolytic solution having a sum of 6 to 30% by volume and a sum of volumes of dimethyl carbonate and ethyl methyl carbonate of 70 to 94% by volume.
The present invention also provides [2] a non-aqueous solution comprising a non-aqueous solvent and an electrolyte as an electrolyte, using a lithium-doped / de-doped carbon material for the negative electrode, a composite oxide of lithium and a transition metal for the positive electrode. The present invention relates to a lithium secondary battery comprising an electrolytic solution, wherein the nonaqueous electrolytic solution is the nonaqueous electrolytic solution of [1].
Furthermore, the present invention provides [3] a non-aqueous electrolysis comprising a non-aqueous solvent and an electrolyte as an electrolytic solution, using a lithium-doped / de-doped carbon material for the negative electrode, a composite oxide of lithium and a transition metal for the positive electrode. The present invention relates to a lithium secondary battery in which these power generation elements are stored in a sealed battery can using a liquid, and the nonaqueous electrolyte before being stored in the battery can is the nonaqueous electrolyte of [1].
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail.
The non-aqueous solvent used in the non-aqueous electrolyte of the present invention is ethylene carbonate, vinylene carbonate, dimethyl carbonate, and ethyl methyl carbonate, where the total volume is 100% by volume, ethylene carbonate is 5-30% by volume, and vinylene carbonate is 1 to 10% by volume, 5 to 30% by volume of dimethyl carbonate, 40 to 90% by volume of ethyl methyl carbonate, 6 to 30% by volume of the volume of ethylene carbonate and vinylene carbonate, and the volume of dimethyl carbonate and ethyl methyl carbonate The sum is 70 to 94% by volume.
Preferably, ethylene carbonate is 5 to 20% by volume, vinylene carbonate is 1 to 7% by volume, dimethyl carbonate is 5 to 25% by volume, ethyl methyl carbonate is 55 to 90% by volume, and the sum of the volumes of ethylene carbonate and vinylene carbonate is 6 to 25% by volume, and the sum of the volumes of dimethyl carbonate and ethyl methyl carbonate is 75 to 94% by volume.
More preferably, ethylene carbonate is 10 to 20% by volume, vinylene carbonate is 2 to 7% by volume, dimethyl carbonate is 5 to 25% by volume, ethyl methyl carbonate is 60 to 80% by volume, and the sum of the volumes of ethylene carbonate and vinylene carbonate. Is 12 to 25% by volume, and the sum of the volumes of dimethyl carbonate and ethyl methyl carbonate is 75 to 88% by volume.
[0012]
When ethylene carbonate is less than 5% by volume, the discharge characteristics during the charge / discharge cycle are not good, and when it exceeds 30% by volume, the discharge characteristics at low temperatures are not good. Further, when vinylene carbonate is less than 1% by volume, capacity storage characteristics during charge / discharge cycles and during long-term storage are not good, and when it exceeds 10% by volume, discharge characteristics at low temperatures and during large current discharges. Not good. Furthermore, when dimethyl carbonate is less than 5% by volume, the discharge characteristics at the time of large current discharge are not good, and when it exceeds 30% by volume, the amount of gas generated when stored at high temperature becomes large. Next, when ethylmethyl carbonate is less than 40% by volume, the discharge characteristics at low temperature are not good, and when it exceeds 90% by volume, the discharge characteristics at high current discharge are not good.
[0013]
Further, when the sum of the volumes of ethylene carbonate and vinylene carbonate is less than 6% by volume, the capacity storage characteristics during long-term storage at a charge / discharge cycle and at a high temperature are not good, and when it exceeds 30% by volume, at a low temperature Also, the discharge characteristics during large current discharge are not good.
[0014]
Furthermore, when the sum of the volumes of dimethyl carbonate and ethyl methyl carbonate is less than 70% by volume, the discharge characteristics at low temperature and during large current discharge are not good, and when it exceeds 94% by volume, the charge / discharge cycle and at high temperatures are prolonged. The capacity storage characteristics when stored are not good.
[0015]
The nonaqueous solvent used for the nonaqueous electrolytic solution of the present invention may contain a nonaqueous solvent other than ethyl methyl carbonate, vinylene carbonate, ethylene carbonate, and dimethyl carbonate. Examples of the nonaqueous solvent include cyclic carbonates such as propylene carbonate, acyclic carbonates such as methylpropyl carbonate and methylisopropyl carbonate, cyclic ethers such as tetrahydrofuran, acyclic ethers such as methylbutyl ether, and γ-butyrolactone. Examples include cyclic esters, acyclic esters such as methyl acetate and methyl propionate.
[0016]
As the electrolyte used in the nonaqueous electrolytic solution of the present invention, conventionally known electrolytes can be used. For example, LiAsF ₆ , LiPF ₆ , LiBF ₄ , LiClO ₄ , LiCF ₃ SO ₃ , LiN (SO ₂ C _n F _{2n + 1} ) ₂ , LiC (SO ₂ C _n F _{2n + 1} ) ₃ (where n represents an integer of 1 to 10) and the like, LiPF ₆ , LiBF ₄ , LiN (SO ₂ C _n F _{2n + 1} ) ₂ and LiC (SO ₂ C _n F _{2n + 1} ) ₃ are preferable because the capacity of the battery is increased, and LiPF ₆ is more preferable in view of excellent large current characteristics. These electrolytes may be used individually by 1 type, and may be used together 2 or more types as needed.
[0017]
In the nonaqueous electrolytic solution of the present invention, the electrolyte concentration is preferably 0.5 mol / liter to 2 mol / liter, and further, the ionic conductivity is high, so that 0.7 mol / liter to 1.5 mol. It is more preferable that the ratio is 1.0 mol / liter to 1.4 mol / liter because the cycle characteristics and the low-temperature discharge characteristics are good.
[0018]
Next, the lithium secondary battery of the present invention will be described.
The lithium secondary battery of the present invention [2] uses a lithium-doped / undopeable carbon material for the negative electrode, a composite oxide of lithium and a transition metal for the positive electrode, and a non-aqueous solvent and an electrolyte as an electrolyte. The nonaqueous electrolytic solution is a nonaqueous electrolytic solution according to the present invention [1].
The lithium secondary battery of the present invention [3] uses a lithium-doped / undopeable carbon material for the negative electrode, a composite oxide of lithium and a transition metal for the positive electrode, and a non-aqueous solvent as an electrolyte. The non-aqueous electrolyte made of an electrolyte is used, and these power generation elements are housed in a sealed battery can. The non-aqueous electrolyte before being housed in the battery can is the non-aqueous electrolyte of the present invention [1]. It is characterized by that.
[0019]
The composite oxide used for the positive electrode of the lithium secondary battery of the present invention may be a composite oxide of lithium and a transition metal, such as LiMO ₂ having a layered structure or LiM ₂ O ₄ having a spinel structure (where M is At least one transition metal). Here, examples of M include cobalt, manganese, nickel, iron, vanadium, and titanium.
Among these, a composite oxide represented by LiNiO ₂ , LiCoO ₂ , LiCo _X Ni _1-X O ₂ (where 0 <X <1) is more preferable in terms of discharge capacity and cycle characteristics.
[0020]
Specifically, as the positive electrode, composite oxide powder, auxiliary conductive agent powder, binder for binding these powders and the like are mixed uniformly and then pressure-molded, or paste using a solvent or the like And a structure in which the current collector is fixed to the current collector sheet by, for example, pressing on the current collector after being coated and dried.
[0021]
The auxiliary conductive agent powder used for the positive electrode has a conductive effect and may have any resistance to the non-aqueous electrolyte to be used or the electrochemical reaction at the positive electrode. For example, artificial graphite, natural graphite, etc. Examples thereof include graphite-based conductive agents, carbon black-based conductive agents such as acetylene black and ketjen black, coke-based powders, conductive polymers, or mixtures thereof.
The amount of the auxiliary conductive agent is preferably about 1 to 20 parts by weight with respect to 100 parts by weight of the active material powder to be used.
[0022]
The carbon material used for the negative electrode of the lithium secondary battery according to the present invention may be any carbon material that can be doped and dedoped with lithium. In addition to natural graphite, various organic polymer compounds or composites thereof may be used. Examples thereof include pyrolytic carbon, carbon fiber, coke, glassy carbon, and artificial graphite that are carbonized or graphitized by the above method. Among these, graphite is preferable because crystallization is developed to a certain degree or more, the amount of occlusion / release of lithium is large, and the operating potential is base and flat over the entire region.
[0023]
The negative electrode is configured, for example, by mixing the carbon material with a binder and, if necessary, a solvent, kneading and granulating. Examples of the binder include fluorinated resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), cellulose resins, polyolefins, latex, polyimides, and the like.
[0024]
As the current collector used for the positive electrode and the negative electrode, any material may be used as long as it has resistance to the non-aqueous electrolyte to be used and electrochemical reaction in the positive electrode and the negative electrode. For example, nickel, titanium, stainless steel, Aluminum, copper, etc. are mentioned.
The thickness of the current collector is preferably as thin as possible so that the volume energy density of the battery increases, and is preferably about 5 to 100 μm.
As the current collector of the positive electrode, an aluminum foil is preferable in that it can be easily processed into a thin film and is inexpensive. As the current collector of the negative electrode, a copper foil is preferable in that it is difficult to make an alloy with lithium and it is easy to process into a thin film.
[0025]
In the lithium secondary battery of the present invention, as the separator, the electrodes are prevented from being in contact with each other, have an insulating property, hold a non-aqueous electrolyte, and have a function of allowing lithium ions to pass therethrough. As long as it has resistance and resistance to the electrochemical reaction at the positive electrode and negative electrode, examples thereof include olefin resins such as fluorine resin, polyethylene and polypropylene, nonwoven fabrics such as nylon, aromatic polyamide and polyimide, and woven fabrics. it can. The thickness of the separator is preferably as thin as possible as long as the mechanical strength is maintained in that the volume energy density of the battery is increased and the internal resistance is reduced, and is preferably about 10 to 200 μm.
[0026]
【Example】
EXAMPLES The present invention will be described in more detail with reference to examples below, but the present invention is not limited to the examples.
[0027]
Example 1
LiCoO ₂ was used as the positive electrode active material. This was mixed with artificial graphite as a conductive agent and PVdF as a binder dissolved in N-methylpyrrolidone to obtain a slurry for a positive electrode. The weight ratio of LiCoO ₂ : artificial graphite: PVdF was 88: 5: 7. The slurry was applied to a current collector made of an aluminum foil having a thickness of 20 μm, dried and rolled, and a vacuum-dried sheet-like electrode cut into a circle having a diameter of 2 cm was used as a positive electrode sheet electrode.
[0028]
Also, natural graphite, styrene-butadiene rubber latex, and sodium salt of carboxymethyl cellulose (CMC) are made into a slurry form like water using a water and applied to a copper foil (negative electrode current collector) having a thickness of 10 μm. The same treatment as in the case of the positive electrode was performed to obtain a negative electrode sheet electrode. Here, the natural graphite: styrene-butadiene rubber latex: carboxymethylcellulose sodium salt was 97: 1: 2 by weight.
[0029]
The positive electrode sheet-like electrode and the negative electrode sheet-like electrode obtained as described above were opposed to each other through a porous polypropylene separator, housed in a stainless steel container, and an electrolytic solution was injected to prepare a flat battery.
[0030]
Here, as the nonaqueous electrolytic solution, a solution obtained by dissolving LiPF ₆ as an electrolyte in a solvent in which ethylene carbonate (EC), vinylene carbonate (VC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) are combined. Using. These compositions are shown in Table 1. The concentration of LiPF ₆ is 1 mol / liter.
[0031]
This battery (A) was subjected to the following test.
Standard discharge capacity: Discharge capacity when the battery was subjected to a constant current / constant voltage charge of 4 mA and an upper limit voltage of 4.2 V for 4 hours at room temperature and then a discharge test was performed at a constant current of 4 mA and a final voltage of 3 V at room temperature.
Cycle characteristics: The first time when the battery was subjected to a constant current / constant voltage charge of 4 mA and an upper limit voltage of 4.2 V for 4 hours at room temperature and then a discharge test was performed 30 times at a constant current of 8 mA and a final voltage of 3 V at room temperature. Of discharge capacity at the 30th time with respect to the discharge capacity (%). The larger this ratio, the better the discharge characteristics during the charge / discharge cycle.
High load characteristics: 4 mA at room temperature, 4 mA constant current / constant voltage charging with an upper limit voltage of 4.2 V for 4 hours, then 2 mA when conducting a discharge test at room temperature with a constant current of 8 mA and 2 mA and a final voltage of 3 V Ratio (%) of 8 mA discharge capacity to discharge capacity. It shows that the discharge characteristic at the time of large current discharge is so favorable that this ratio is large.
Low temperature characteristics: When a battery was charged at a constant current / constant voltage of 4 mA and an upper limit voltage of 4.2 V for 4 hours at room temperature, and then a discharge test was performed at a constant current of 4 mA and a final voltage of 3 V at room temperature and −10 ° C. Ratio of discharge capacity at −10 ° C. to discharge capacity at room temperature (%). Larger ratios indicate better discharge characteristics at low temperatures.
Storage characteristics: After 4 hours of constant current / constant voltage charging at room temperature for 4 mA and upper limit voltage of 4.2 V, storage for 10 days in an atmosphere at 60 ° C., then returning to room temperature, 4 mA constant current, termination The ratio (%) of the standard discharge capacity to the discharge capacity when the discharge test was conducted at a voltage of 3V. Larger ratios indicate better capacity storage characteristics when stored for long periods.
These results are summarized in Table 1.
[0032]
[Table 1]

[0033]
Comparative Example 1
Except having used the electrolyte solution which has a composition shown in Table 1, it carried out similarly to Example 1, and produced flat battery R1-R6. About these batteries (R1-R6), the test similar to Example 1 was done. The results are summarized in Table 1.
[0034]
Example 2
Made of stainless steel with a gas collecting jig attached in the same manner as in Example 1 using an electrolytic solution having a volume ratio of 17: 3: 30: 50 of ethylene carbonate, vinylene carbonate, dimethyl carbonate, and ethyl methyl carbonate A flat battery A7 was produced using the above container.
The amount of gas generated after storage for 10 days in an atmosphere at 60 ° C. after 4 hours of constant current / constant voltage charging with a battery of 4 mA and an upper limit voltage of 4.2 V at room temperature was measured. 0.8 cm ³ .
[0035]
Comparative Example 2
A flat battery R7 was produced in the same manner as in Example 2 except that the volume ratio of ethylene carbonate, vinylene carbonate, dimethyl carbonate, and ethyl methyl carbonate was 17: 3: 60: 20. When this battery was tested in the same manner as in Example 2, the gas generation amount was 1.2 cm ³ .
[0036]
【Effect of the invention】
The lithium secondary battery using the non-aqueous electrolyte of the present invention was stored at a high temperature, a discharge characteristic during a charge / discharge cycle, a discharge characteristic at a low temperature and a large current discharge, a capacity storage characteristic when stored for a long time at a high temperature. The small amount of gas generated is excellent in a well-balanced manner.

Claims (7)

It consists of a nonaqueous solvent containing ethylene carbonate, vinylene carbonate, dimethyl carbonate and ethyl methyl carbonate and an electrolyte. -30% by volume, vinylene carbonate 1-10% by volume, dimethyl carbonate 5-30% by volume, ethyl methyl carbonate 40-90% by volume, the sum of the volume of ethylene carbonate and vinylene carbonate 6-30% by volume, dimethyl A non-aqueous electrolyte for a lithium secondary battery, wherein the sum of the volume of carbonate and ethyl methyl carbonate is 70 to 94% by volume. The electrolyte is LiPF ₆ , LiBF _Four , LiN (SO ₂ C _n F _{2n + 1} ) ₂ , And LiC (SO ₂ C _n F _{2n + 1} ) _Three The nonaqueous electrolytic solution for a lithium secondary battery according to claim 1, wherein at least one selected from the group consisting of (where n represents an integer of 1 to 10). 3. The non-aqueous electrolyte for a lithium secondary battery according to claim 1, wherein the electrolyte concentration is 0.5 mol / liter to 2.0 mol / liter. A lithium-doped / de-doped carbon material is used for the negative electrode, a composite oxide of lithium and a transition metal is used for the positive electrode, and a non-aqueous electrolyte composed of a non-aqueous solvent and an electrolyte is used as the electrolyte. lithium secondary battery, characterized by the electrolyte is non-aqueous electrolyte according to any one of claims 1 to 3. Using a carbon material that can be doped / undoped with lithium for the negative electrode, a composite oxide of lithium and transition metal for the positive electrode, and a non-aqueous electrolyte consisting of a non-aqueous solvent and electrolyte as the electrolyte, these power generation elements A lithium secondary battery, wherein the lithium secondary battery is stored in a sealed battery can and the nonaqueous electrolyte before being stored in the battery can is the nonaqueous electrolyte according to any one of claims 1 to 3. . 6. The lithium secondary battery according to claim 4 , wherein the carbon material is graphite. LiM ₂ O ₄ composite oxide has a LiMO ₂ or spinel structure having a layered structure (where M is at least one transition metal) according to any one of claims 4-6, characterized in that the Lithium secondary battery.