WO2018176981A1 - Electrolyte solution for lithium ion battery, and battery - Google Patents

Abstract

Provided is an electrolyte solution, comprising a lithium salt, an additive and an organic solvent, wherein the additive is poly(ethylene glycol)-poly(propylene glycol)-poly(ethylene glycol). Also provided is a battery, wherein same uses the electrolyte solution provided in the present invention. The electrolyte solution and the battery provided in the present invention can effectively control a thermal runaway phenomenon that may occur in the process of using the battery, and at the same time, there is little negative influence on the battery itself.

Description

Lithium ion battery electrolyte and battery

This application claims priority to Chinese Patent Application No. 200910204908.8, entitled "Li-ion Battery Electrolyte and Battery" by the Chinese Patent Office on March 31, 2017, the entire contents of which are incorporated herein by reference. In the application.

Technical field

The present application relates to the field of battery safety, and in particular to a lithium ion battery electrolyte and a battery.

Background technique

As people pay more and more attention to the environment and become more environmentally aware, the demand for new energy vehicles that replace traditional fuel vehicles is increasing. With the increasing number of new energy vehicles on the road, safety, especially the safety of power batteries as the power source of new energy vehicles, has become more and more important.

In power batteries, usually lithium-ion batteries, due to the use of flammable carbonate-based electrolytes; may be overcharged or over-discharged during use, or due to accidental impact (especially acupuncture) Under the influence), the internal reaction of the single cell is intensified, causing the internal temperature of the battery to rise, which leads to the occurrence of safety problems such as thermal runaway, natural and explosion of the battery. Therefore, how to improve the safety performance of the battery under overcharge, over discharge or extreme conditions has become a major problem to limit the development of electric vehicles.

Under normal circumstances, the power battery is provided with a safety valve. When the internal pressure is too large, the safety valve will open due to internal pressure, releasing the pressure inside the battery and improving the safety of the battery. However, in actual use, it is often the case that a violent reaction has occurred inside the battery, and the safety valve is not opened; or the violent reaction inside the battery does not cause the safety valve to be opened for the first time, resulting in safety measures being activated. Delays can adversely affect the battery.

In the prior art, part of the practice is to add a flame retardant additive to the electrolyte to prevent combustion inside the battery; in part, a substance that is thermally decomposed and can generate water is introduced into the battery system, and in the case of abnormal battery, let A large amount of gas is formed inside the battery to assist in opening the safety valve.

However, the addition of a flame retardant additive to the electrolyte has a large viscosity due to the flame retardant additive, which seriously affects the lithium ion transport performance of the electrolyte, thereby reducing the electrochemical performance such as the rate performance of the lithium battery.

In the battery system, a substance which is thermally decomposed to generate water is introduced. In this method, it is usual to mix the substance into a slurry of a positive electrode or a negative electrode and then apply it to a current collector. Since these materials are non-electrochemically active and have extremely low electrical conductivity, they can significantly affect the energy density of the battery. In addition, these materials usually use crystalline water compounds, which will contain a small amount of adsorbed water, which will affect The electrolyte system leads to poor performance of the lithium battery. The disadvantage of this method is that the decomposition of the substance is slow to the thermal response inside the battery, and the thermal runaway cannot be suppressed in time.

Summary of the invention

In order to solve the solution in the prior art, the present invention provides an electrolyte and a battery which can effectively control the thermal runaway phenomenon and have a negative influence on the performance of the battery itself at the same time.

The invention provides a lithium ion battery electrolyte comprising a lithium salt, an additive and an organic solvent, wherein the additive is polyethylene glycol-polypropylene glycol-polyethylene glycol.

In the present invention, when the internal temperature of the battery is increased due to overcharge, overdischarge or acupuncture, the additive in the electrolyte polyethylene glycol-polypropylene glycol-polyethylene glycol (PEO-PPO-PEO) will It reacts with lithium salt and its decomposition products due to heat, and forms a gel after the reaction; and the newly formed gel will cover the surface of the positive and negative electrodes to form a protective film, which sharply increases the internal resistance of the lithium battery and avoids positive and negative Extremely direct contact, which suppresses thermal runaway and improves battery safety. At the same time, the thermal response temperature of the above additive PEO-PPO-PEO is generally between 90 ° C and 120 ° C, and has little effect on the performance of the electrolyte itself at normal room temperature. For example, the influence of the conductivity of the electrolyte at room temperature is very small. Therefore, in the electrolyte provided by the invention, the addition of the additive can well control the internal thermal runaway phenomenon of the battery and improve the safety of the battery; on the other hand, the negative impact on the performance of the battery itself is very small, very good The technical problems existing in the background art of the present invention are solved.

Further, the present invention provides a battery comprising a housing, a pole core housed in the housing, and an electrolyte, which is the lithium ion battery electrolyte provided by the present invention.

DRAWINGS

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the embodiments or the description of the prior art will be briefly described below. Obviously, the drawings in the following description are only It is an embodiment of the present invention, and those skilled in the art can obtain other drawings according to the provided drawings without any creative work.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a graph showing temperature changes of acupuncture experiments of Examples 1-6 and Comparative Example 1.

2 is an SEM image of a pole piece after heat treatment of Comparative Example 1.

Fig. 3 is a SEM image of a pole piece after heat treatment in Example 3.

detailed description

The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It is understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

The present invention provides an electrolyte comprising a lithium salt, an additive and an organic solvent, wherein the additive is polyethylene glycol-polypropylene glycol-polyethylene glycol.

In the present invention, when the internal temperature of the battery is increased due to overcharge, overdischarge or acupuncture, the additive in the electrolyte polyethylene glycol-polypropylene glycol-polyethylene glycol (PEO-PPO-PEO) will It interacts with the electrolyte system due to heat (for example, phosphorus pentafluoride produced by thermal decomposition of lithium hexafluorophosphate is a strong Lewis acid; the electrolyte system of other lithium salts is also thermally decomposed to produce a strong Lewis acid group like this. The group, while PEO-PPO-PEO reacts with these strong Lewis acids), reacts to form a jelly product; and the newly formed gel coats the surface of the positive and negative electrodes to form a protective film, which sharply increases the internal resistance of the lithium battery. To avoid direct contact between the positive and negative electrodes, thereby suppressing thermal runaway and improving the safety of the battery. At the same time, the heat corresponding temperature of the above additive PEO-PPO-PEO is generally between 90 ° C and 120 ° C, and has little effect on the performance of the electrolyte itself at normal room temperature. For example, the influence of the conductivity of the electrolyte at room temperature is very small. Therefore, in the electrolyte provided by the invention, the addition of the additive can well control the internal thermal runaway phenomenon of the battery and improve the safety of the battery; on the other hand, the negative impact on the performance of the battery itself is very small, very good The technical problems existing in the background art of the present invention are solved.

At the same time, PEO-PPO-PEO is widely used in other chemical fields due to its low price and non-toxic superior performance.

In a preferred embodiment of the invention, the electrolyte contains from 0.5% to 20% by volume of the additive based on the total volume of the electrolyte. The additives in this range can achieve good results without affecting the performance of the battery.

In order to better reflect the effect of the additive and achieve a better effect, it is preferred that the additive of the present invention has an average molecular weight of 1800 to 10,000, which can further improve the performance of the battery of the present invention.

In the present invention, preferably, the lithium salt is selected from one or more of LiPF ₆ , LiBOB, LiTFSI or LiFSI.

In the present invention, preferably, the concentration of the lithium salt is from 0.5 mol/L to 3 mol/L.

In the present invention, in order to further improve the mutual solubility of PEO-PPO-PEO and an organic solvent, it is preferred that the organic solvent is one or more selected from the group consisting of a cyclic carbonate or a chain carbonate. PEO-PPO-PEO has the properties of a surfactant, which is especially compatible with carbonate electrolytes; at the same time, it can also improve the contact between active material particles and electrolyte during long-period cycling.

More specifically, the cyclic carbonate is selected from the group consisting of ethylene carbonate (EC) and/or propylene carbonate (PC). EC and PC have a higher ignition point and can form a stable SEI film at the negative electrode.

Further, the chain carbonate is selected from one or more of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC) or methyl ethyl carbonate (EMC). . The above chain carbonates have a lower viscosity, can improve the problem of excessive viscosity of EC or PC, and improve the lithium ion diffusion ability of the mixed electrolyte.

In addition, in order to improve the stability of the negative electrode SEI film, a negative electrode film-forming additive is further added to the electrolyte, and the mass percentage of the negative electrode film-forming additive contained in the electrolyte is 0.5% to 2% based on the total mass of the electrolyte. .

More specifically, the negative film forming additive is vinylene carbonate (VC) or fluoroethylene carbonate (FEC).

In some embodiments of the invention, the organic solvent is a mixed solution of ethylene carbonate, diethyl carbonate and methyl ethyl carbonate, the ethylene carbonate, diethyl carbonate and methyl ethyl carbonate The volume ratio of the ester was 1:1:1.

Further, the present invention provides a battery comprising a casing, a pole core housed in the casing, and an electrolyte, wherein the electrolyte is the electrolyte provided by the present invention.

In the present invention, the polar core is formed by winding a positive electrode sheet, a negative electrode sheet, and a separator.

The preparation method of the battery provided by the invention:

Step 1, preparing an electrolyte: the electrolyte is the electrolyte provided by the present invention described above;

Step 2: preparing a polar core: the positive electrode sheet, the separator and the negative electrode sheet are sequentially laminated and then wound to obtain a polar core;

Step 3, packaging the battery: the pole core prepared in step 2 is loaded into the housing;

Step 4. Injecting liquid and sealing: Injecting an electrolyte into the casing and sealing it to obtain a battery.

Example 1

(1) Preparation of positive electrode

Lithium cobaltate, acetylene black, polytetrafluoroethylene and N-methylpyrrolidone are mixed and stirred into a slurry at a weight ratio of 100:3:2:50, and the slurry is uniformly coated on a conductive base aluminum foil. On both sides, the film was dried, rolled, and cut at 110 ° C to obtain a positive electrode sheet having a size of 485 mm × 44 mm × 0.140 mm.

(2) Preparation of negative electrode

The natural graphite, carboxymethyl cellulose, styrene-butadiene rubber and water are thoroughly mixed and stirred in a weight ratio of 100:2:2:180 to obtain a uniform slurry, and the slurry is uniformly coated on a conductive base copper foil of 0.008 mm. Both sides were dried at 100 ° C, and finally, a negative electrode sheet having a size of 480 mm × 45 mm × 0.156 mm was obtained by cutting.

(3) Assembly of the battery

The positive electrode sheet, the negative electrode sheet and the polypropylene film prepared above were wound into a polar core of a prismatic lithium ion battery, and then the electrolytic solution was injected into the aluminum plastic film at a rate of 3.6 g/Ah, and sealed to prepare a soft-package lithium ion. Battery S1. The composition of the electrolyte is: 45 ml of organic solvent, and a mixed organic solvent containing EC, EMC and DEC is selected, wherein the volume ratio of EC, EMC and DEC is 1:1:1; the additive PEO-PPO-PEO is 5 ml, of which PEO-PPO The average molecular weight of PEO is 2900; a certain amount of LiPF _{6 is} added at a concentration of 1 mol/L.

Example 2

The battery S2 was prepared in the same manner as in Example 1, except that the composition of the electrolyte was 42.5 ml of an organic solvent, and a mixed organic solvent containing EC, EMC and DEC was selected, wherein the volume ratio of EC, EMC and DEC was 1:1:1; additive PEO-PPO-PEO 7.5ml, wherein the average molecular weight of PEO-PPO-PEO is 2900; a certain amount of LiPF _{6 is} added at a concentration of 1 mol/L.

Example 3

The battery S3 was prepared in the same manner as in Example 1, except that the composition of the electrolyte was 40 ml of an organic solvent, and a mixed organic solvent containing EC, EMC and DEC was selected, wherein the volume ratio of EC, EMC and DEC was 1. : 1:1; additive PEO-PPO-PEO 10ml, wherein the average molecular weight of PEO-PPO-PEO is 2900; a certain amount of LiPF _{6 is} added at a concentration of 1 mol/L.

Example 4

Battery S4 was prepared in the same manner as in Example 1, except that the additive (PEO-PPO-PEO had an average molecular weight of 3,500.

Example 5

Battery S5 was prepared in the same manner as in Example 1, except that the organic solvent was acetonitrile.

Example 6

Battery S6 was prepared in the same manner as in Example 1, except that the composition of the electrolyte further contained a negative electrode film-forming additive of VC, and the mass fraction of VC was 1.5%.

Comparative example 1

The battery DS1 was prepared in the same manner as in Example 1, except that the electrolyte: 50 ml of an organic solvent selected from a mixed organic solvent containing EC, EMC and DEC, wherein the volume ratio of EC, EMC and DEC was 1:1:1; a certain amount of LiPF _{6 was} added at a concentration of 1 mol/L.

Performance Testing:

1. The batteries DS1 and S6 prepared in the above Examples 1-6 and Comparative Example 1 were prepared to obtain a battery DS1. The batteries DS1 were prepared by first preparing the batteries S1-S6 prepared in the above Examples 1-6 and Comparative Example 1. Charge and discharge, charge and discharge current is 0.1C, charge and discharge voltage range is 2.65V-4.35V; after 0.1C cycle, charge to 4.35V, 2.5Ah capacity, puncture test. At the same time, the temperature change of the battery in the puncture experiment was detected. A detailed description of the puncture experiment is as follows:

1).0.2C current is charged to the upper limit voltage 4.20V (4.35V/4.40V), the cutoff condition is 0.02C;

2). Use a steel needle with a diameter of 3 mm to completely penetrate the center of the battery core at a speed of 150 mm/s to maintain the piercing state;

3) The test is terminated when the surface temperature of the cell drops below 35 °C;

Eligibility criteria: Cell temperature <200 ° C, no explosion, no fire, no smoke.

2. SEM (Scanning Electron Microscope) Test: The surface of the pole piece was observed by instrument model JSM-7600F, FESEM/EDS-field emission scanning electron microscope and accessory energy spectrometer. The battery is charged and discharged in a range of 2.75 to 4.35 V, and charged and discharged at a rate of 1 C for 50 cycles, and then recharged to 100% SOC; then the battery is disassembled in an argon-filled glove box to separate the positive electrode sheets; Finally, after sealing treatment in an aluminum plastic film sealed bag in a glove box, SEM related characterization was performed.

The test results are shown in Table 1.

Battery sample	Puncture experiment
S1	qualified
S2	qualified
S3	qualified
S4	qualified
S5	qualified
S6	qualified
DS1	Failed

The batteries prepared in Example 1, Example 2, Example 3, Example 4, Example 5 and Example 6 did not explode and did not burn vigorously;

The battery prepared in the comparative example, the battery burned violently, exploded, and the puncture failed.

As shown in Fig. 1, when the puncture test is performed on the above seven batteries, the obtained battery temperature change diagram is detected; as can be seen from the figure, the first embodiment, the second embodiment, the third embodiment, the fourth embodiment, and the fifth embodiment And in Example 6 during the puncture experiment, the battery temperature changes slowly with time and does not rise to too high temperature; while the comparative example during the puncture experiment, the battery temperature rises sharply with time and reaches 600 °C. Above the high temperature.

In the comparative example, when the steel needle passes through the battery in a fully charged state, since the steel needle is an electronic conductor and directly contacts the positive and negative electrodes, the battery in the comparative example is internally short-circuited, releasing heat for a short time, releasing the combustible gas, and the diaphragm. Shrinkage, large-area contact between positive and negative electrodes, causing thermal runaway; and in Example 1, Example 2, Example 3, Example 4, Example 5, and Example 6, PEO-PPO-PEO was added to the electrolyte. When the steel needle passes through the battery to cause internal short-circuit heat generation, PEO-PPO-PEO will preferentially react with strong Lewis acid such as PF5 and solvent to form a gel covering the surface of the positive and negative electrodes, as shown in Figures 2 and 3. 2 is an SEM image of the comparative example, and FIG. 3 is an SEM image of the third embodiment; from the comparison of FIG. 2 and FIG. 3, the SEM image of the comparative example of FIG. 2 has a relatively bulky molecular structure and is relatively large. The gap of the embodiment 3 of Fig. 3 is relatively dense because the surface forms a gel and covers the surface. Fig. 2 and Fig. 3 show that the surface of the pole piece of the comparative example of Fig. 2 is loose and porous, and the contact area with the electrolyte is very large. When the internal temperature of the battery rises, the rate of oxidation of the electrolyte is increased, thereby accelerating the electricity. The thermal runaway of the core; and the surface of the pole piece of Embodiment 3 of Figure 3, there is a flat covering, which isolates the contact with the electrolyte, thereby suppressing the rate at which the electrolyte is oxidized, alleviating the thermal runaway of the battery, and increasing The internal resistance of the battery prevents the direct contact between the positive and negative electrodes, and the PEO-PPO-PEO has a higher ignition point, is not easy to be ignited, and is safer.

The above is only the preferred embodiment of the present invention, and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection of the present invention. Within the scope.

Claims (11)

1. A lithium ion battery electrolyte comprising a lithium salt, an additive and an organic solvent, characterized in that the additive is polyethylene glycol-polypropylene glycol-polyethylene glycol.
2. The lithium ion battery electrolyte according to claim 1, wherein the electrolyte contains 0.5% to 20% by volume of an additive based on the total volume of the electrolyte.
3. The lithium ion battery electrolyte according to claim 1, wherein the additive has an average molecular weight of from 1800 to 10,000.
4. The lithium ion battery electrolyte according to claim 1, wherein the lithium salt is one or more selected from the group consisting of LiPF ₆ , LiBOB, LiTFSI, or LiFSI.
5. The lithium ion battery electrolyte according to claim 1 or 4, wherein the lithium salt has a concentration of 0.5 mol/L to 3 mol/L.
6. The lithium ion battery electrolyte according to claim 1, wherein the organic solvent is one or more selected from the group consisting of a cyclic carbonate or a chain carbonate.
7. The lithium ion battery electrolyte according to claim 6, wherein the cyclic carbonate is ethylene carbonate and/or propylene carbonate; the chain carbonate is selected from the group consisting of dimethyl carbonate and diethyl carbonate. One or more of an ester, dipropyl carbonate or methyl ethyl carbonate.
8. The lithium ion battery electrolyte according to claim 6, wherein the organic solvent is a mixed solution composed of ethylene carbonate, diethyl carbonate and methyl ethyl carbonate, the vinyl carbonate and the carbonic acid. The volume ratio of ethyl ester to methyl ethyl carbonate is 1:1:1.
9. The lithium ion battery electrolyte according to claim 1, wherein the electrolyte further contains a negative electrode film-forming additive, wherein the electrolyte contains 0.5% by mass based on the total mass of the electrolyte. 2% negative film forming additive.
10. The lithium ion battery electrolyte according to claim 9, wherein the negative electrode film forming additive is vinylene carbonate or fluoroethylene carbonate.
11. A battery comprising a housing, a pole core housed in the housing, and an electrolyte, wherein the electrolyte is the lithium ion battery electrolyte according to any one of claims 1-10.

Images