CN102569885A - Non-aqueous electrolyte for lithium ion battery and lithium ion secondary battery - Google Patents

Abstract

The invention provides non-aqueous electrolyte for a lithium ion battery and a lithium ion secondary battery prepared from the non-aqueous electrolyte. The non-aqueous electrolyte for the lithium ion battery comprises cyclic carboxylic ester and/or cyclic carbonate ester, cyclic sulfate, an electrolyte salt, fluoroether with the following structural formula: Rf1-O-Rf2, and a fluorocarbon surfactant, wherein the Rf1 is a fluorine-containing alkyl group of which the carbon atom number is 3 to 4; the Rf2 is a fluorine-containing alkyl group of which the carbon atom number is 2 to 5; the mass percentage of the fluoroether in the electrolyte is 10 to 50 percent; and the fluorocarbon surfactant is used for further improving the performances of the battery. The high temperature performance and the safety of the lithium ion secondary battery prepared from the non-aqueous electrolyte can be kept at a higher level as a whole, and the cycling performance is improved.

Description

Non-aqueous electrolyte solution for lithium ion battery and lithium ion secondary battery

technical field

The invention relates to a lithium-ion battery non-aqueous electrolyte and a lithium-ion secondary battery made thereof.

Background technique

Lithium-ion battery is the most competitive battery of the new generation, known as "green energy", and is the preferred technology to solve contemporary environmental pollution and energy problems. In recent years, lithium-ion batteries have achieved great success in the field of high-energy batteries, but consumers still expect batteries with higher comprehensive performance, which depends on the research and development of new electrode materials and electrolyte systems. Lithium-ion battery electrolyte system, as an important part of lithium-ion batteries, is a key material necessary for lithium-ion batteries, and its performance is a great constraint on the development of lithium-ion batteries. At present, the electrolyte of lithium-ion batteries is composed of flammable organic solvents and lithium salts. When lithium-ion secondary batteries are overcharged and discharged, short-circuited, and operated for a long time with a high current, a large amount of heat is released, which causes the battery The internal temperature of the system is too high, which becomes a safety hazard of the flammable electrolyte, which may cause catastrophic thermal breakdown, or even battery explosion [Zheng Honghe. Lithium-ion battery electrolyte [M]. Beijing: Chemical Industry Press, 2006, 134.]. Therefore, safety issues have become an important prerequisite for innovation in the lithium-ion battery market, especially in the field of power lithium-ion batteries for electric vehicles (EV) and hybrid electric vehicles (HEV). requirements.

At present, there are two main technical routes for the development of flame-retardant electrolytes in the industry. On the one hand, by adding phosphorus-based flame retardants, halogen-based flame retardants, composite flame retardants and other new flame retardants to conventional electrolytes, flammable organic electrolytes can be turned into flammable or non-flammable electrolytes , reduce the heat release value of the battery and the self-heating rate of the battery, increase the thermal stability of the electrolyte itself, so as to avoid the combustion or explosion of the battery under overheating conditions. In order not to reduce the performance of the electrolyte and to improve its flame retardancy, the patents CN101490893A and CN101584075A applied by Daikin Corporation in China proposed to add fluorine-containing ethers. On the other hand, try to use high boiling point, high flash point organic solvents to replace low boiling point, low flash point linear carbonate solvents, so as to improve the safety performance and high temperature storage performance of the electrolyte, and some progress has been made. South Korea's Samsung SDI Co., Ltd. patent CN1577944 discloses an electrolyte for lithium-ion batteries, which contains gamma-butyrolactone with high boiling point and high flash point, cyclic carbonates and ester compounds with electron-withdrawing groups. The electrolyte system has good safety and good storage properties. Kejha J B disclosed a high-performance and safer electrolyte system suitable for lithium-ion batteries in the patent US2006204857 (A1). This system uses LiBF ₄ as lithium salt, with a high flash point and high boiling point The γBL (or PC or BC) plus 70-90% EC is an organic solvent, and the obtained electrolyte is difficult to ignite, and its electrochemical performance can be comparable to that of a conventional electrolyte using a linear carbonate solvent.

In recent years, with the increase in battery energy, the battery case is required to be light in weight and thin in thickness, so that the battery can expand more easily. In practical applications, various performance requirements for the battery usually change corresponding to the use conditions, one of which is the high-temperature storage performance of the battery. To improve the high-temperature storage performance of the battery, an electrolyte solvent with a high boiling point and a low vapor pressure can be used, or a method of inhibiting the decomposition of the non-aqueous electrolyte on the surface of the positive and negative electrodes can be used (CN1282272C). When using a solvent with a high boiling point and a low vapor pressure, there are generally problems such as an increase in the viscosity of the solvent, a decrease in the conductivity of the non-aqueous electrolyte, and a decrease in the discharge characteristics. (2000-235868) proposed to use γ-butyrolactone (GBL) with high dielectric constant and high boiling point, etc. However, the electrochemical oxidation resistance and reduction resistance of GBL-based electrolytes are inferior to those obtained by mixing ethylene carbonate and low-viscosity solvents. The GBL-based electrolyte film on the carbon negative electrode is loose, porous, rough, and has poor stability. It is easy to cause reduction and decomposition reactions on the negative electrode during charging. Due to the action of the decomposition products, the surface resistance of the negative electrode increases, and at the same time, it causes the diaphragm. Since the micropores are clogged, there is a problem that the battery capacity significantly decreases when charging and discharging are repeated. At the same time, the GBL-based electrolyte in the charging state is more unstable when stored at high temperature, and it is prone to side reactions with the positive and negative electrodes, resulting in a significant increase in internal resistance. At the same time, the SEI film is severely damaged, resulting in a decrease in the retention capacity and recovery capacity, and severe battery swelling. Therefore, the GBL-based electrolyte has the problem of rapid capacity decay when used in a battery with graphite as the negative electrode; the problem of battery capacity retention at high temperature needs to be further improved. In addition, in the case of excess additives in the electrolyte, the excess additives will oxidize and decompose on the positive electrode and generate gas when placed at high temperature, which will cause significant expansion of the battery due to the increase in internal pressure, all of which are is the problem.

Contents of the invention

The inventors have found through a lot of experiments that the problems of capacity fading and high-temperature storage performance degradation of γBL-based electrolytes can be successfully solved by adding cyclic sulfuric acid esters and linear fluoroethers.

Based on the above findings, the present invention aims to solve the related technical problems existing in the above electrolyte. Provided is a non-aqueous electrolyte solution for a lithium ion battery, which enables the lithium ion battery to obtain excellent electrochemical performance while having high safety, including excellent high-temperature storage performance and good cycle performance.

This kind of non-aqueous electrolyte solution for lithium-ion batteries uses an organic solvent with a high flash point and a high boiling point, and then adds a halogenated flame retardant (linear fluoroether) or further adds a fluorocarbon surfactant. An electrolyte solution obtained based on this idea is beneficial to improving the cycle performance and high-temperature storage performance of lithium-ion batteries, and the electrolyte solution is especially suitable for capacity batteries at low and medium rates.

The invention provides a non-aqueous electrolytic solution for a lithium ion battery, the electrolytic solution contains:

Lithium salt;

Cyclic carboxylates, cyclic carbonates or mixtures of cyclic carboxylates and cyclic carbonates;

Fluoroethers as shown in structural formula (I):

R _f1 -OR _f2 structural formula (I)

In the formula, R _f1 is a fluorine-containing alkyl group with 3 to 4 carbon atoms, and R _f2 is a fluorine-containing alkyl group with 2 to 5 carbon atoms;

Cyclic sulfuric acid ester as shown in structural formula (II):

Structural formula (II)

In the formula, the value of n is 0 or 1, and R ₁ , R ₂ , R ₃ and R ₄ each independently represent a hydrogen atom or an alkyl group with 1 to 5 carbon atoms.

Among them, the cyclic carboxylic acid ester is preferably selected from: γ-butyrolactone, γ-valerolactone, δ-valerolactone, halogenated γ-butyrolactone, nitro γ-butyrolactone, cyano γ- One or more of butyrolactone and α-acetyl-γ-butyrolactone.

Wherein, the cyclic carbonate is preferably selected from one or more of ethylene carbonate, propylene carbonate, butylene carbonate or halogenated ethylene carbonate.

Wherein, the mass percentage of the fluoroether in the electrolyte is preferably 10-50%.

Wherein, the mass percentage of the cyclic sulfuric acid ester in the electrolyte is preferably 0.01%-2%.

Preferably, the electrolyte also includes a fluorocarbon surfactant of the following formula (III):

R _f3 X(CH ₂ CH ₂ O) _n R ₁ or R _f3 X(CHCH ₃ CH ₂ O) _n R ₁ structural formula (III)

Among them, R _f3 is a fluorine-containing alkyl group with 2 to 18 carbon atoms, X is oxygen (-O-), sulfur (-S-), amine oxide (- ⁺ NO ^- -), amide (-CONH-) Or a sulfonamide (-SO ₂ N-) functional group, R ₁ is a hydrogen atom or an alkyl group with 1-4 carbon atoms, n=1-25.

Preferably, the electrolyte also contains one or more of chain carbonates, chain carboxylates or chain sulfites.

Wherein said chain carbonate is preferably selected from: one or more in dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, dipropyl carbonate ;

The chain carboxylate is preferably selected from: methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, a kind of in ethyl butyrate or more;

The chain sulfite is preferably selected from one or more of dimethyl sulfite, diethyl sulfite, and ethyl methyl sulfite.

Among them, the lithium salt is preferably selected from: LiPF ₆ , LiBF ₄ , LiClO ₄ , LiBOB, LiODFB, LiN(SO ₂ CF ₃ ) ₂ , LiN(SO ₂ C ₂ F ₅ ) ₂ , LiN(SO ₂ F) ₂ One or more of them, the concentration of the lithium salt in the electrolyte is 0.6-2 mol/L in terms of lithium ions.

Preferably, the electrolyte solution also contains additives, and the additives are preferably selected from one of: vinylene carbonate, vinyl sulfite, fluoroethylene carbonate, vinylethylene carbonate, 1,3-propane sultone One or more, the mass percentage of the additive in the electrolyte is 0.1-10%.

The present invention also provides a lithium ion secondary battery based on the above electrolytic solution, which includes a positive electrode, a negative electrode and an electrolytic solution.

Preferably, the positive electrode has a lithium salt active material containing a transition metal oxide, and the lithium salt includes LiCoO ₂ , LiMn ₂ O ₄ , LiNi _1-xy Co _x Mn _y O ₂ (0<x<1, 0<y <1), one or more of LiNi _1-x Co _x O ₂ (0<x<1), LiFePO ₄ ; the active material of the negative electrode is graphite, an alloy material containing Si or Sn, or lithium titanate.

Compared with the prior art, this technical solution has the following beneficial effects:

1. Cyclic carboxylate, cyclic carbonate or a mixture of cyclic carboxylate and cyclic carbonate and fluoroether with high boiling point, low vapor pressure solvent plus film-forming additives to suppress high-temperature gas production, high-temperature storage performance excellence. Therefore, the electrolyte solution composed of high flash point and low vapor pressure solvent of the present invention produces less gas during high-temperature storage, which is beneficial to improve high-temperature storage performance. In order to cooperate with the solvent, the present invention forms a stable SEI film on the electrode by adding a small amount of additive cyclic sulfate, so that the decomposition of the non-aqueous electrolyte is kinetically inhibited and is more conducive to improving the high-temperature storage performance.

2. By adding cyclic sulfuric acid ester and linear fluoroether in combination, it can successfully solve the unstable film formation of γBL-based electrolyte and co-intercalation of PC-based electrolyte in batteries with graphite as the negative electrode, resulting in battery capacity attenuation question. The use of fluoroether as a solvent can reduce the reactivity between the electrolyte and the electrode, and inhibit the heat generation at the electrode interface; the cyclic sulfuric acid ester has good film-forming properties. Experiments have found that the combination of the two can inhibit high flash points and high boiling points such as γBL and PC. The side reaction of the solvent in the graphite negative electrode greatly improves the high-temperature storage performance, and becomes an effective guarantee for further improving the safety and performance of the battery.

3. The electrolyte composed of high flash point and high boiling point solvent has high flash point and boiling point; it is not easy to ignite, and fluoroether flame retardant is added. Fluoroether itself contains the flame retardant element fluorine, which has a high flash point and is difficult to burn. , added to the electrolyte to make the flammable organic electrolyte into a flammable or non-flammable electrolyte, which is effective for flame retardancy; even in a very harsh environment, the electrolyte reaches its flash point and is ignited, There is also the flame-retardant and self-extinguishing effect of fluoroether, which has two more safety guarantees than conventional electrolytes, and more effectively improves the safety performance of the battery.

4. In order to further optimize battery performance, the dispersibility of fluorocarbon surfactants to fluoroethers and other component solvents is beneficial to improve the compatibility of fluoroethers with high boiling point and high flash point solvents. Fluorocarbon surfactants are highly effective in improving the wetting properties of solvents, especially for high-viscosity solvent systems composed of high boiling point and high flash point solvents. The addition of surfactants can improve the wettability between non-aqueous electrolytes and electrodes And ion diffusivity at the interface; under the premise of ensuring that the electrolyte is flammable or non-flammable, it also has excellent battery characteristics.

Therefore, by replacing the low-flash point linear carbonate or carboxylate with a high-flash point, high-boiling point cyclic solvent; adding a technical solution of cyclic sulfuric acid ester and halogenated flame retardant fluoroether, the flame retardancy can be obtained An electrolyte solution for lithium secondary batteries that is excellent in battery characteristics (high temperature storage performance, cycle performance).

Detailed ways

In order to describe in detail the technical content, structural features, achieved objectives and effects of the present invention, the following will be described in detail in conjunction with the embodiments.

One. The preparation method of embodiment electrolyte

In an argon-filled glove box (H ₂ O<10ppm), the cyclic carbonate, cyclic carboxylate, linear fluoroether, lithium salt, cyclic sulfate, film-forming additive and fluorocarbon surfactant Prepare according to the electrolyte mass ratio listed in each embodiment and comparative example in Table 1. The above-mentioned raw materials are added in sequence and fully stirred evenly to obtain the lithium-ion battery electrolyte of the present invention, which is used for flammability test and battery performance test.

Two. The manufacture method of embodiment lithium-ion battery

The non-aqueous electrolyte secondary battery of the present invention is composed of the above-mentioned non-aqueous electrolyte, a negative electrode and a positive electrode.

The active material constituting the positive electrode can be LiCoO ₂ , LiMn ₂ O ₄ , LiNi _1-xy Co _x Mn _y O ₂ (0<x<1, 0<y<1), LiNi _1-x Co _x O ₂ (0<x<1), LiFePO ₄ and so on.

The active material constituting the negative electrode may be graphite, an alloy material containing Si or Sn, or lithium titanate.

LiCoO ₂ , conductive agent acetylene black and polyvinylidene fluoride were mixed in a weight ratio of 8:1:1, then 1-methyl-2-pyrrolidone was added to form a slurry, which was then coated on an aluminum foil, then dried and Molded to form the cathode.

Mix natural graphite and polyvinylidene fluoride at a weight ratio of 9:1, then add 1-methyl-2-pyrrolidone to form a slurry, which is then coated on a copper foil, dried, pressed and heat-treated to form an anode. Use polypropylene porous membrane as the separator, then wind the anode sheet, cathode sheet and separator to form a winding body, or stack the sheets into an electrode group, and package the above components together with the electrolyte prepared above in a metal casing to make a square shape Lithium Ion Battery.

Three. Formation and test method of embodiment lithium-ion battery:

The chemical conversion steps adopted in the present invention: primary chemical conversion conditions: 0.05C, 3min; 0.2C, 5min; 0.5C, 25min. Then replenish the liquid and seal it, charge it to 4.2V with a constant current of 0.2C, and age at room temperature for 24 hours, then supplement it with a constant current and voltage (4.2V) at 0.2C, and discharge it to 3.0V with a constant current of 0.2C .

The present invention evaluates the charging and discharging performance of the lithium-ion battery electrolyte: inject the prepared lithium-ion battery electrolyte into a 1000mAh aluminum shell square _LiCoO2 battery, charge and discharge at 1C with a voltage range of 3.0-4.2V Cycle test.

The present invention tests the high-temperature storage performance of lithium-ion battery electrolyte:

First charge and discharge the battery once at 1C at room temperature (25°C±2°C), record the discharge capacity at room temperature as C ₁ , then fully charge the battery at 1C constant current and constant voltage, and test the thickness of the battery at full charge D _1. Carry out a high-temperature storage test on a fully charged battery. After the battery is completely cooled, test the thickness D ₂ of the battery again; charge and discharge the removed battery as follows:

a. 1C constant current discharge to the end voltage of 2.75V, and the discharge capacity is recorded as C ₂ .

b. Leave it on hold for 5 minutes.

c, 1C constant current and constant voltage charge to 4.2V, cut-off current 0.02C.

d. Leave it on hold for 5 minutes.

e. Discharge at a constant current of 1C to a termination voltage of 2.75V, and record the discharge capacity as C ₃ .

Capacity retention rate after high temperature storage = C ₂ /C ₁ ×100%, capacity recovery rate = C ₃ /C ₁ ×100%,

Thickness expansion ratio = (D ₂ -D ₁ )/D ₁ ×100%.

The present invention is to the test method of the flame retardant property of lithium battery electrolyte: with long 50mm, wide 5mm, thick 1.65mm bubble film nickel is immersed in the embodiment that the present invention mentions or the electrolyte of comparative example, then take out with tweezers , close to the flame of the igniter, stay for 2s, then remove the flame to observe the phenomenon and record the self-extinguishing time.

Four. In the embodiment, the code name of the organic matter is explained

1. Fluoroethers in each embodiment:

S ₁ is HCF ₂ CF ₂ CH ₂ OCF ₂ CF ₂ H

S ₂ is HCF ₂ CF ₂ CH ₂ OCF ₂ CFHCF ₃

S ₃ is CF ₃ CF ₂ CH ₂ OCF ₂ CFHCF ₃

S ₄ is CF ₃ CF ₂ CH ₂ OCF ₂ CF ₂ CF ₂ CF ₂ H

S ₅ is HCF ₂ CF ₂ CF ₂ CH ₂ OCH ₂ CF ₂ CF ₂ CF ₂ CF ₂ H

S ₆ is HCF ₂ CF ₂ CF ₂ CH ₂ OCF ₂ CF ₂ H

S ₇ is (CH ₂ F) ₂ CHOC(CF ₃ ) ₃

2. the cyclic sulfuric acid ester in each embodiment:

A ₁ is ethylene glycol sulfate

A ₂ is 1,2-propanediol sulfate

A ₃ is 1,2-butanediol sulfate

A ₄ is 1,3-butanediol sulfate

A ₅ is 2,3-butanediol sulfate

A ₆ is 1,2-heptanediol sulfate

A ₇ is

3. the fluorocarbon surfactant in each embodiment:

a ₁ is CF ₃ (CF ₂ ) ₄ CH ₂ O(CH ₂ CH ₂ O) ₃ H,

a ₂ is C ₆ F ₁₃ CH ₂ CH ₂ S(CH ₂ CH ₂ O) ₃ H,

a ₃ is C ₈ F ₁₇ CH ₂ CH ₂ SO ₂ N(CH ₃ )CH ₂ CH ₂ OH,

a ₄ is CF ₃ CHFCF ₂ CH ₂ O[CH(CH ₃ )CH ₂ O] ₂ H

a ₅ is CF ₃ CH ₂ NO(CH ₂ CHO) ₂₅ CH ₃

a ₆ is CH ₂ F(CH ₂ ) ₁₆ CH ₂ CONH(CH(CH ₃ )CH ₂ O) ₁₀ (CH ₂ ) ₃ CH ₃

4. Description of other organic component codes

PC: Propylene carbonate GBL: γ-butyrolactone VC: vinylene carbonate EC: Vinyl carbonate EMC: Ethyl Methyl Carbonate EB: Ethyl butyrate DMS: Dimethyl sulfite ES: Vinyl sulfite 1,3-PS: 1,3-propane sultone BC Butylene carbonate DMC: Dimethyl carbonate FEC: Fluoroethylene carbonate DEC: Diethyl carbonate MPC: Methyl Propyl Carbonate DVL: δ-valerolactone DES:   Diethyl sulfite BA: Methyl butyrate VEC: Ethylene carbonate

MES: Ethyl methyl sulfite

Example 1:

In an argon-filled glove box (H ₂ O<10ppm), the organic solvent is mixed with lithium hexafluorophosphate (1.2M) at a mass ratio of EC:GBL:S ₁ =1:1:1, and the additive is cyclic sulfate A ₁ , accounting for 1% of the total weight of the electrolyte. The above-mentioned raw materials are added in sequence, fully stirred evenly, and the lithium-ion battery electrolyte solution (free acid<30ppm, moisture<10ppm) of the present invention is obtained. Electrolyte is used for flammability test and battery performance test. The test results of flammability and the capacity retention rate at the 100th cycle of normal temperature cycle; the capacity retention rate, capacity recovery rate and thickness expansion rate after storage at 60°C for 30 days are shown in Table 1.

Example 2:

The process is the same as in Example 1, except that the fluoroether is S ₂ , and the cyclic sulfate is A ₂ .

Example 3:

The process is the same as in Example 1, except that the electrolyte solution ratio is: EC:GBL:S ₄ =1:1:1, 1.0M LiPF ₆ and 0.2M LiBF ₄ , A ₃ : 1%.

Example 4:

The same process as in Example 3, except that the fluoroether is S ₅ , and the cyclic sulfate is A ₄ .

Example 5:

The same process as in Example 3, except that the fluoroether is S ₆ , and the cyclic sulfate is A ₅ .

Embodiment 6:

The same process as in Example 1, except that the electrolyte ratio is: EC:PC:GBL:S ₃ =30:30:10:30, 1.0M LiPF ₆ and 0.2M LiBF ₄ , A ₁ : 1% .

Embodiment 7:

The process is the same as in Example 1, except that 2% VC is added on the basis of the electrolyte ratio in Example 1.

Embodiment 8:

The process is the same as in Example 1, except that the electrolyte ratio is: EC:GBL:MB:S ₆ =30:40:10:20, 0.8M LiBF ₄ and 0.2M LiN(SO ₂ CF ₃ ) ₂ , VC: 1%, A ₅ : 2%.

Embodiment 9:

The process is the same as in Example 1, except that the electrolyte ratio is: PC:S ₂ =60:40, 1.2M LiPF ₆ and 0.2M LiBOB, VC: 1%, A ₂ : 0.8%.

Example 10:

The process is the same as that of Example 1, except that the electrolyte ratio is: GBL:S ₁ =50:50, 1.4M LiBF ₄ , VC: 1%, A ₄ : 0.8%.

Example 11:

The same process as in Example 1, except that the electrolyte ratio is: PC:S ₂ =60:40, 1.2M LiPF ₆ and 0.2M LiBOB, VC: 1%, ES: 2%, A ₂ : 0.8 %.

Example 12:

The process is the same as in Example 1, except that the electrolyte ratio is: GBL:S ₁ =50:50, 1.4M LiBF ₄ , VC: 1%, 1,3-PS: 2%, A ₄ : 0.8 %.

Example 13:

The process is the same as that of Example 1, except that the electrolyte ratio is: EC:PC:EMC:S ₁ =30:20:10:40, 1.2M LiPF ₆ , A ₅ : 0.6%.

Example 14:

The same process as in Example 1, except that the electrolyte ratio is: EC:PC:EMC:S ₁ =30:20:10:40, 1.2M LiPF ₆ , VC: 1%, A ₅ : 0.6% .

Example 15:

The same process as in Example 1, except that the electrolyte ratio is: EC:PC:EMC:S ₁ =30:20:10:40, 1.2M LiPF ₆ , VC: 1%, A ₅ : 0.6% , a ₁ : 0.1%.

Example 16:

The same process as in Example 1, except that the electrolyte ratio is: EC:PC:DMS:S ₃ =20:20:20:40, 1.2M LiPF ₆ , VC: 1%, A ₁ : 1% .

Example 17:

The same process as in Example 1, except that the electrolyte ratio is: EC:PC:DMS:S ₃ =20:20:20:40, 1.2M LiPF ₆ , VC: 1%, A ₁ : 1% , a ₂ : 0.1%.

Example 18:

The process is the same as in Example 1, except that the electrolyte ratio is: EC:PC:EB:S ₂ =30:20:20:30, 1.2M LiPF ₆ , VC: 1%, A ₂ : 1.5% .

Example 19:

The process is the same as in Example 1, except that the electrolyte ratio is: EC:PC:EB:S ₂ =30:20:20:30, 1.2M LiPF ₆ , VC: 1%, A ₂ : 1.5% ,, a ₃ : 0.1%.

Example 20:

The same process as in Example 1, except that the electrolyte ratio is: EC:PC:EMC:S ₅ =40:5:15:40, 1.4M LiPF ₆ , VC: 1%, A ₂ : 1.5% ,, a ₄ : 0.1%.

Example 21:

The same process as in Example 1, except that the electrolyte ratio is: BC:DEC:MPC:DES:S ₇ =20:10:10:10:50; LiClO ₄ : 0.6M; A ₆ : 0.01% ; FEC: 0.1%; a ₅ : 0.2%.

Example 22:

The process is the same as in Example 1, except that the electrolyte ratio is: FEC:DVL:BA:MES:S ₁ =25:10:20:15:30; LiN(SO ₂ F) ₂ : 2.0M; A ₇ : 2%; VEC: 10%; a ₆ : 0.02%.

Comparative example 1 (reference sample):

The process is the same as in Example 1, except that the electrolyte solution ratio is: EC:PC:DMC=40:5:55, 1.2M LiPF ₆ , VC: 2%.

Comparative example 2:

The same process as in Example 1, except that the electrolyte ratio is: EC:PC:EMC:S ₅ =40:5:15:40, 1.4M LiPF ₆ , VC: 1%, a ₄ : 0.1% .

Comparative example 3:

The process is the same as in Example 1, except that the electrolyte solution ratio is: EC:PC:EMC:S ₁ =30:20:10:40, 1.2M LiPF ₆ , VC: 1%.

Comparative example 4:

The process is the same as in Example 1, except that the electrolyte solution ratio is: EC:GBL:DMC=1:1:1, 1.2M LiPF ₆ , VC: 1%, A ₁ : 1%.

Comparative example 5:

The process is the same as that of Example 1, except that the electrolyte ratio is: PC:DMC=60:40, 1.2M LiPF ₆ and 0.2M LiBOB, A ₂ : 0.8%.

Comparative example 6:

The process is the same as that of Example 1, except that the electrolyte ratio is: GBL:S ₁ =50:50, 1.4M LiBF ₄ , VC: 1%, 1,3-PS: 2%.

Five. Beneficial effect analysis:

From the analysis of electrolyte flammability performance test result table 1 in each embodiment and comparative example, it is known that: comparative example 1, comparative example 4 and comparative example 5 are due to containing a large amount of low-flash point DMC, and electrolyte is flammable, and other each embodiment and Because the comparative examples contain fluoroether, they all show a certain degree of flame retardant and self-extinguishing effect. Among them, Examples 1-7 and Examples 9-12 do not contain low-flash point linear carbonate or carboxylate or sulfite, and the electrolyte is difficult to ignite in a short time, which shows that the flame retardant effect is better.

From the high temperature storage performance parameters of each embodiment and comparative example in Table 1, it can be seen that the capacity retention rate and capacity recovery rate of each embodiment are higher than that of the reference sample (comparative example 1), and the thickness expansion rate is obviously lower than that of the reference sample (comparative example 1). . Comparing Example 1 which replaces DMC in Comparative Example 4 with fluoroether, the capacity retention and recovery rate of Comparative Example 4 are lower than that of Example 1, while the thickness expansion rate of Example 1 is significantly lower than that of Comparative Example 4, indicating that the fluoroether Existence can suppress the heat generation between the electrode and the electrolyte interface, which is beneficial to high-temperature storage. Further compare comparative example 2 and embodiment 20, comparative example 3 and embodiment 14 with the same solvent ratio, wherein embodiment 20 (or embodiment 14) is added with cyclic sulfuric acid ester A ₂ (or A ₅ ) in the electrolyte , its capacity retention rate and capacity recovery rate are higher than that of Comparative Example 2 (or Comparative Example 3), and the thickness expansion rate is lower than that of Comparative Example 2 (or Comparative Example 3), indicating that the addition of cyclic sulfate is beneficial to improve the high temperature of the battery Storage performance, can inhibit the gas production of the battery when stored at high temperature. The high-temperature storage performance can be improved by using a cyclic sulfate ester compound. Although the reason for this is not clear, it is presumed that it is due to the formation of a good SEI film on the surface of the electrode material with the sulfate ester compound, thereby inhibiting the oxidation of the solvent on the surface of the positive electrode. and reductive decomposition on the surface of the negative electrode, resulting in the generation of gas.

It can be seen from the table that the capacity retention rate of Example 7, Example 14, Example 15, Example 17 and Example 19 at room temperature cycle 100 is higher than or equivalent to that of the conventional electrolyte (Comparative Example 1). Comparing Example 1 and Comparative Example 4; Example 9 and Comparative Example 5, it can be seen that the capacity retention rate of Example 1 and Example 9 containing fluoroether is much higher than that of Comparative Example 4 and Comparative Example 5 not containing fluoroether. It can be seen that the presence of fluoroether has a certain inhibitory effect on the side reactions caused by the co-intercalation of the PC group and the poor film stability of the GBL-based electrolyte, which is conducive to improving the battery cycle performance, so its capacity retention rate is above 80% after 100 weeks . Further compare comparative example 2 and embodiment 20, comparative example 3 and embodiment 14, comparative example 6 and embodiment 12 with the same solvent proportioning, wherein in the electrolyte of embodiment 20, embodiment 14 and embodiment 12, add Cyclic sulfates A ₂ , A ₅ , and A ₄ have higher capacity retention rates in the 100th cycle at room temperature than those of Comparative Example 2, Comparative Example 3, and Comparative Example 6, indicating that the addition of cyclic sulfates can improve high-temperature storage performance. It is also beneficial to improve the normal temperature cycle performance of the battery.

As can be seen from Table 1, the embodiment 15, embodiment 17, embodiment 19, embodiment 20 containing surfactant, its capacity retention rate is higher than embodiment 14, embodiment 16, embodiment 18 with the same solvent composition, illustrates The addition of a small amount of fluorocarbon surfactant can improve the cycle performance of the battery; on the basis of embodiment 1, add 2% VC embodiment 7, on the basis of embodiment 13, add VC embodiment 14 cycle performance is improved, in the implementation On the basis of Example 9 and Example 10, Example 11 with ES added and Example 12 with 1,3-PS added had increased capacity retention at the 100th cycle of normal temperature cycle, and the high-temperature storage performance was further improved. It can be seen that the performance of the battery can be further optimized by adding film-forming additives to the electrolyte of the present invention.

Table 1 The ratio of the electrolytes of each embodiment and comparative example, the flammability and the capacity retention rate after 100 weeks and the capacity retention rate after high temperature storage, the capacity recovery rate, and the test results of the thickness expansion rate

The above is only an embodiment of the present invention, and does not limit the patent scope of the present invention. Any equivalent structure or equivalent process conversion made by using the content of the description of the present invention, or directly or indirectly used in other related technical fields, shall be The same reasoning is included in the patent protection scope of the present invention.

Claims (12)

1. a non-aqueous electrolytic solution for a lithium ion battery, characterized in that the electrolytic solution contains: Lithium salt; Cyclic carboxylates, cyclic carbonates or mixtures of cyclic carboxylates and cyclic carbonates; Fluoroethers as shown in structural formula (I): R _f1 -OR _f2 structural formula (I) In the formula, R _f1 is a fluorine-containing alkyl group with 3 to 4 carbon atoms, and R _f2 is a fluorine-containing alkyl group with 2 to 5 carbon atoms; Cyclic sulfuric acid ester shown in structural formula (II):

Structural formula (II) In the formula, the value of n is 0 or 1, and R ₁ , R ₂ , R ₃ and R ₄ each independently represent a hydrogen atom or an alkyl group with 1 to 5 carbon atoms. 2. The non-aqueous electrolytic solution for lithium-ion batteries according to claim 1, wherein the cyclic carboxylic acid ester is selected from the group consisting of: gamma-butyrolactone, gamma-valerolactone, delta-valerolactone, One or more of halogenated γ-butyrolactone, nitro γ-butyrolactone, cyano γ-butyrolactone, and α-acetyl-γ-butyrolactone. 3. The non-aqueous electrolytic solution for lithium-ion batteries according to claim 1, wherein the cyclic carbonate is selected from: ethylene carbonate, propylene carbonate, butylene carbonate or halogenated ethylene carbonate one or more of . 4. The non-aqueous electrolyte solution for lithium ion batteries according to claim 1, characterized in that the mass percentage of said fluoroether in the electrolyte solution is 10-50%. 5. The non-aqueous electrolyte solution for lithium-ion batteries according to claim 1, wherein the mass percentage of the cyclic sulfuric acid ester in the electrolyte solution is 0.01% to 2%. 6. nonaqueous electrolytic solution for lithium ion battery as claimed in claim 1, is characterized in that, described electrolytic solution also comprises the fluorocarbon surfactant of structural formula following formula (III): R _f3 X(CH ₂ CH ₂ O) _n R ₁ or R _f3 X(CHCH ₃ CH ₂ O) _n R ₁ structural formula (III) Among them, R _f3 is a fluorine-containing alkyl group with 2 to 18 carbon atoms, X is oxygen (-O-), sulfur (-S-), amine oxide (- ⁺ NO ^- -), amide (-CONH-) Or a sulfonamide (-SO ₂ N-) functional group, R ₁ is a hydrogen atom or an alkyl group with 1-4 carbon atoms, n=1-25. 7. The non-aqueous electrolytic solution for lithium-ion batteries according to claim 1, wherein the electrolytic solution also contains one or more of chain carbonates, chain carboxylates or chain sulfites. kind. 8. lithium-ion battery non-aqueous electrolytic solution according to claim 7, is characterized in that, The chain carbonate is selected from one or more of: dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, dipropyl carbonate; The chain carboxylate is selected from: methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, a kind of in ethyl butyrate or more; The chain sulfite is selected from one or more of dimethyl sulfite, diethyl sulfite and ethyl methyl sulfite. 9. The non-aqueous electrolyte solution for lithium-ion batteries according to claim 1, wherein the lithium salt is selected from the group consisting of: LiPF ₆ , LiBF ₄ , LiClO ₄ , LiBOB, LiODFB, LiN(SO ₂ CF ₃ ) ₂ , One or more of LiN(SO ₂ C ₂ F ₅ ) ₂ and LiN(SO ₂ F) ₂ , the concentration of the lithium salt in the electrolyte is 0.6-2 mol/L in terms of lithium ions. 10. The non-aqueous electrolytic solution for lithium ion batteries as claimed in any one of claims 1 to 9, wherein the electrolytic solution also contains additives, and the additives are selected from the group consisting of: vinylene carbonate, vinyl sulfite , one or more of fluoroethylene carbonate, ethylene ethylene carbonate, and 1,3-propane sultone, and the mass percentage of the additive in the electrolyte is 0.1-10%. 11. A lithium ion secondary battery comprising a positive electrode, a negative electrode and an electrolyte, characterized in that the electrolyte is the nonaqueous electrolyte for lithium ion batteries according to any one of claims 1 to 10. 12. The lithium ion secondary battery according to claim 11, characterized in that: the positive electrode has a lithium salt active material containing transition metal oxides, and the lithium salt includes LiCoO ₂ , LiMn ₂ O ₄ , LiNi _1-xy One or more of Co _x Mn _y O ₂ (0<x<1, 0<y<1), LiNi _1-x Co _x O ₂ (0<x<1), LiFePO ₄ ; the negative electrode has an active material It is graphite, an alloy material containing Si or Sn, or lithium titanate.