WO2023198004A1

WO2023198004A1 - Battery electrolyte, and lithium ion battery comprising same

Info

Publication number: WO2023198004A1
Application number: PCT/CN2023/087373
Authority: WO
Inventors: 张水蓉
Original assignee: 北京车和家汽车科技有限公司
Priority date: 2022-04-11
Filing date: 2023-04-10
Publication date: 2023-10-19

Abstract

The present disclosure relates to a battery electrolyte and a lithium ion battery comprising same. Components of the electrolyte comprise propylene carbonate, fluoroethylene carbonate and lithium difluorophosphate, wherein in terms of the total mass of the electrolyte being 100%, the content of the propylene carbonate is a%, the content of the fluoroethylene carbonate is b%, and the content of the lithium difluorophosphate is c%, a, b and c meeting the following relationship: 0.1≤(b/a)/c≤10, and 3≤b≤13.

Description

Battery electrolyte and lithium-ion battery containing the same

Cross-references to related applications

This application is filed based on a Chinese patent application with application number 202210375429.3 and a filing date of April 11, 2022, and claims the priority of the Chinese patent application. The entire content of the Chinese patent application is hereby incorporated into this application as a reference.

Technical field

The present disclosure relates to the technical field of electrochemical devices, and in particular, to a battery electrolyte and a lithium-ion battery containing the same.

Background technique

With the continuous popularity of electric vehicles, lithium-ion batteries have become one of the most widely used secondary batteries due to their advantages of high voltage, high energy density and long cycle life. However, with the advancement of technology, more stringent requirements have been put forward for the energy density and storage performance of lithium-ion batteries under cycling and high-temperature conditions.

Among currently known anode materials, silicon has the highest theoretical capacity (4200mAh/g); much higher than the 372mAh/g of graphite anode, the capacity of silicon-carbon anode materials mixed with silicon can reach 400-650mAh/g. level, therefore, in order to improve the energy density of lithium-ion batteries, more and more attention has been paid to the application of silicon.

At the same time, in order to increase the energy density as much as possible, the most ideal method at present is to use high-voltage ternary cathode materials with silicon anodes. However, due to the large volume expansion of silicon (up to 300% expansion), the silicon The solid electrolyte interface (SEI) film of the negative electrode continues to rupture due to the expansion of silicon, and a new interface leaks out to re-form the SEI film. The electrolyte needs to be continuously consumed, eventually causing the electrolyte to dry up and the cycle to decay rapidly. At the same time, due to the increase in voltage of high-voltage ternary cathode materials, the interface between the cathode and the electrolyte is unstable. Side reactions between the cathode and the electrolyte are prone to occur, which further accelerates the consumption of the electrolyte, and the occurrence of side reactions is easy to generate gas. This causes the lithium-ion battery to bulge and poses a safety hazard.

Therefore, for lithium-ion batteries using high-voltage ternary cathode materials and silicon systems, how to improve cycle and gas production is an urgent problem that needs to be solved.

Contents of the invention

In order to solve the above technical problems, the present disclosure provides a battery electrolyte and a lithium-ion battery containing the same. Using the electrolyte provided by the present disclosure in a lithium-ion battery using a high-voltage ternary cathode material and a silicon system can significantly improve the cycle performance and reduce the reaction between the cathode and the electrolyte, thus reducing the occurrence of gas production and improving the performance of the battery. safety.

In a first aspect, the present disclosure provides an electrolyte for batteries, the composition of which includes propylene carbonate, Fluoroethylene carbonate and lithium difluorophosphate, based on the total mass of the electrolyte being 100%, the content of the propylene carbonate is a%, the content of fluoroethylene carbonate is b%, and the lithium difluorophosphate The content is c%, and the a, b and c satisfy the following relationship: 0.1≤(b/a)/c≤10, and 3≤b≤13.

The electrolyte provided by the present disclosure includes three components: propylene carbonate (PC), fluoroethylene carbonate (FEC) and lithium difluorophosphate (LiPO ₂ F ₂ ). When the high-voltage ternary cathode material and silicon When the electrolyte provided by the present disclosure is used in a lithium-ion battery with a negative electrode of the system, propylene carbonate provides a smaller viscosity at low temperature, but has poor compatibility with the negative electrode. Propylene carbonate and fluorinated ethylene carbonate have excellent film-forming properties. Used together, fluoroethylene carbonate can not only form an excellent SEI film on the surface of the silicon system negative electrode, but also quickly repair the newly exposed interface caused by silicon expansion and contraction, and at the same time suppress silicon expansion; at the same time, The addition of lithium difluorophosphate can form a protective film on the positive electrode and improve high-temperature performance. Therefore, when the three are used together, the cycle capacity retention rate of the obtained lithium-ion battery cell can be better, and the problem of gas production during high-temperature storage can be eliminated. significantly improved.

If the content of propylene carbonate is high, in order to avoid the disadvantage of poor compatibility with the negative electrode material, the content of fluorinated ethylene carbonate needs to be increased accordingly. However, if the content of fluorinated ethylene carbonate is increased, it will easily Decomposition produces hydrogen fluoride, which will damage the surface of the cathode, especially the high-voltage lithium-nickel composite oxide cathode, thus deteriorating high-temperature storage performance. At the same time, although the addition of lithium difluorophosphate can improve high-temperature performance, when its content is increased, it will affect the negative electrode SEI film. Therefore, the addition amounts of PC, FEC and LiPO ₄ F ₂ need to be within the limited range of this disclosure. Able to achieve better application results.

(b/a)/c described in this disclosure can be 0.2, 0.5, 0.8, 1, 1.2, 1.5, 1.8, 2, 2.2, 2.5, 2.8, 3, 3.2, 3.5, 3.7, 4, 4.2, 4.5, 4.8 ,5,6,7,8,9, etc.

In the electrolyte of the present disclosure, 3≤b≤13, and b can be 3.2, 3.5, 3.8, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, 10.5, 11, 11.5, 12, etc.

As a preferred technical solution of the present disclosure, in the electrolyte solution, 0.8≤(b/a)/c≤6, such as 0.9, 1.0, 1.2, 1.5, 1.8, 2, 2.2, 2.5, 2.8, 3, 3.2, 3.5, 3.8, 4.0, 4.2, 4.5, 4.8, 5.0, 5.2, 5.5, 5.8, etc.

In order to further improve the application effect of the electrolyte, as a preferred technical solution of the present disclosure, in the electrolyte, 7≤a≤25, and a in the present disclosure can be 8, 9, 10, 11, 12, 13 , 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, etc.

As a preferred technical solution of the present disclosure, in the electrolyte solution, 0.1≤c≤1, and c in the present disclosure can be 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, etc.

As a specific embodiment of the present disclosure, based on the total mass of the electrolyte being 100%, the content of propylene carbonate is 7-25%, the content of fluorinated ethylene carbonate is 3-13%, and the content of diethylene carbonate is 3-13%. The content of lithium fluorophosphate is 0.1-1%, and 0.1≤(content of fluoroethylene carbonate/content of propylene carbonate)/content of lithium difluorophosphate≤10.

In the battery electrolyte, a conductive lithium salt should also be added. As a preferred technical solution of the present disclosure, the electrolyte also includes lithium hexafluorophosphate (LiPF ₆ ).

As a preferred technical solution of the present disclosure, in the electrolyte, the concentration of lithium hexafluorophosphate is 0.7-1.3mol/L, such as 0.8mol/L, 0.9mol/L, 1.0mol/L, 1.1mol/L , 1.2mol/L, etc.

As a preferred technical solution of the present disclosure, the electrolyte solution also includes chain carbonate, and the chain carbonate The preferred carbonate is ethyl methyl carbonate (EMC).

As a preferred technical solution of the present disclosure, in the electrolyte, the content of the ethyl methyl carbonate is 20-65wt%, such as 25wt%, 30wt%, 35wt%, 40wt%, 45wt%, 50wt%, 55wt%, 60wt%, etc.

As a preferred technical solution of the present disclosure, the electrolyte solution also includes other non-aqueous solvents.

As a preferred technical solution of the present disclosure, the non-aqueous solvent is selected from ethylene carbonate (EC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC) or ethyl carbonate. Any one or a combination of at least two of propyl esters (EPC).

In a second aspect, the present disclosure provides a lithium-ion battery including the electrolyte described in the first aspect.

As a preferred technical solution of the present disclosure, the lithium ion battery further includes a positive electrode and a negative electrode.

In a preferred technical solution of the present disclosure, the positive electrode includes a positive electrode current collector and a positive electrode active material layer located on one side of the positive electrode. The components of the positive electrode active material layer include a positive electrode active material and a conductive agent.

As a preferred technical solution of the present disclosure, the positive electrode includes a positive active material, and the composition of the positive active material includes a lithium-nickel composite oxide, and the lithium-nickel composite oxide contains divalent nickel and trivalent nickel, The content ratio of the divalent nickel to trivalent nickel on the surface of the positive electrode active material is greater than the content ratio of the divalent nickel to trivalent nickel in the center of the positive electrode active material.

The lithium-nickel composite oxide described in the present disclosure is a high-voltage, high-nickel ternary cathode material. For example, currently commercially available cathode materials 6 series or 8 series, such as NCM811, etc., can meet the limitations of the present disclosure and realize the present disclosure. purpose of invention.

The positive electrode active material surface mentioned in this disclosure refers to the outermost 50 nm area of the positive electrode active material particle toward the center direction as the surface.

The center of the positive active material in this disclosure refers to the center of the positive active material particle within a region of 50 nm from the center toward the surface.

In the cathode active material provided by the present disclosure, its metal elements not only include lithium and nickel, but also contain metal elements such as manganese and cobalt in the high-voltage ternary cathode material. As a preferred technical solution of the present disclosure, in In the positive active material, based on the total mole number of metal elements other than lithium being 100%, the sum of the nickel content is not less than 60%, such as 62%, 65%, 68%, 70%, 72%, 75%, 78%, etc.

In a preferred technical solution of the present disclosure, the negative electrode includes a negative electrode current collector and a negative electrode active material layer located on one side of the negative electrode.

As a preferred technical solution of the present disclosure, the negative electrode is composed of silicon material and graphite material.

As a preferred technical solution of the present disclosure, based on the total mass of the negative electrode being 100%, the addition amount of the silicon material is 0-20%, excluding 0, such as 1%, 2%, 5%, 8%, 10%, 12%, 15%, 18%, etc.

As a preferred technical solution of the present disclosure, the silicon material is SiO _z , where 0.5<z<1.5.

As a preferred technical solution of the present disclosure, the lithium ion battery includes a positive electrode, a negative electrode, a separator and the electrolyte described in the first aspect.

When the cathode of a lithium-ion battery is a high-voltage ternary cathode material and the cathode is a silicon-carbon anode material (silicon system anode), the electrolyte provided by the present disclosure can be used as the electrolyte, which can significantly improve the cycle performance of the lithium-ion battery, and its storage Produce The gas properties can also be improved and are less likely to bulge, which can improve the safety of lithium-ion batteries.

As a preferred technical solution of the present disclosure, the full charge voltage of the lithium-ion battery is ≥4.25V.

Compared with the existing technology, the technical solution provided by the embodiments of the present disclosure has the following advantages:

(1) Using the electrolyte provided by the present disclosure in a lithium-ion battery using a high-voltage ternary cathode material and a silicon system can significantly improve cycle performance and reduce the reaction between the cathode and the electrolyte, thus reducing the occurrence of gas production. Improve battery safety;

(2) The battery electrolyte and lithium-ion battery preparation method provided by the present disclosure are simple and easy to implement, and can be suitable for industrial production.

Detailed ways

In order to understand the above objects, features and advantages of the present disclosure more clearly, the solutions of the present disclosure will be further described below. It should be noted that, as long as there is no conflict, the embodiments of the present disclosure and the features in the embodiments can be combined with each other.

Many specific details are set forth in the following description to fully understand the present disclosure, but the present disclosure can also be implemented in other ways different from those described here; obviously, the embodiments in the description are only part of the embodiments of the present disclosure, and Not all examples.

Examples 1-10

This embodiment provides a lithium-ion battery, and the preparation method is as follows:

(1) Preparation of positive electrode

LiNi _0.8 Co _0.1 Mn _0.1 (NCM811), conductive carbon (SP), carbon nanotubes (CNTs) and polyvinylidene fluoride (PVDF) were mixed in the solvent N-format at a weight ratio of approximately 96:1:0.5:2.5 pyrrolidone, stir evenly to obtain the positive electrode slurry, and then use aluminum foil as the positive electrode current collector. Bake the positive electrode current collector coated with the slurry at 120°C for 1 hour, and then cold press, cut into pieces, and slit to prepare Get the positive pole;

(2) Preparation of negative electrode

Mix artificial graphite, SiO (mass ratio 92:8), sodium carboxymethylcellulose (CMC) and styrene-butadiene rubber (SBR) in deionized water at a weight ratio of approximately 96:2:2, stir evenly, and obtain a negative electrode. For the slurry, copper foil is used as the negative electrode current collector. The negative electrode current collector coated with the negative electrode slurry is baked at 120°C for 1 hour, and then cold pressed, cut into pieces, and slit to prepare the negative electrode;

(3) Preparation of electrolyte

In an argon atmosphere, add the additives and lithium salt to the mixed solvent and mix evenly. Control the added amount of lithium salt to 1 mol/L to obtain an electrolyte;

(4)Assembly

Use polyethylene film as the isolation film, stack the positive electrode, isolation film and negative electrode in order, so that the isolation film is between the positive and negative electrodes to play the role of isolation, then roll it, put it into the aluminum plastic film, and heat it at 80℃ After drying, the electrolyte is injected, and through processes such as vacuum packaging, standing, formation, and shaping, a lithium-ion battery is obtained.

Performance test 1

Refer to the proportion of each component of the electrolyte provided in Table 1, and refer to the preparation method provided in the embodiment to prepare a lithium-ion battery, and perform a performance test on the prepared lithium-ion battery. The method is as follows:

(1) Cycle test: Place the finished lithium-ion battery in a constant temperature box at 45°C for 30 minutes, charge it to 4.25V at a constant charging rate of 1.0C, then charge it at a constant voltage until the charging rate is 0.05C, let it stand for 5 minutes, and then Discharge to 3.0V at a constant discharge rate of 1.0C, recorded as the initial discharge capacity D ₀ , and then perform a cycle test as follows:

1) Let it sit for 5 minutes;

2) Charge to 4.25V at a constant charging rate of 1.0C, and charge to a charging rate of 0.05C at a constant voltage;

3) Let it sit for 5 minutes;

4) Discharge to 3.0V at a constant discharge rate of 1.0C;

5) Repeat step 1) to step 4) 1000 times to obtain capacity D ₁ ;

6) Calculate the capacity retention rate of lithium-ion battery cycles, where the calculation formula is:
Capacity retention rate (%)=D ₁ /D ₀ ×100%

(2) High temperature storage performance:

1) The finished lithium-ion battery is placed in a constant temperature box at 25°C for 30 minutes, charged to 4.25V at a constant charging rate of 1.0C, then charged at a constant voltage to a charging rate of 0.05C, left for 5 minutes, and the cell thickness is tested H ₀ ;

2) Then discharge to 3.0V at a constant discharge rate of 0.2C, let it stand for 5 minutes, and charge to 4.25V at a constant charge rate of 1.0C;

3) Place the battery core at 80°C, store it for 24 hours, and test the battery core thickness H ₁ ;

4) Calculate the high-temperature expansion rate of lithium-ion batteries, where the calculation formula is:
High temperature expansion rate (%) = (H ₁ -H ₀ )/H ₀ ×100%

The test results are shown in Table 1:

Table 1

It can be seen from the examples and performance tests that when the electrolyte provided by the present disclosure is used in a lithium-ion battery using a high-voltage ternary cathode material and a silicon system negative electrode, the cycle capacity retention rate of the battery core can be better, and the high-temperature storage yield can be improved. Qi was significantly improved.

At the same time, it can be seen from the comparison of Examples 7-9 that when the value of (b/a)/c is larger, as the ratio of (b/a)/c approaches 10, because the balance of the three is affected, As a result, the circulation improvement effect is reduced, and the high-temperature storage expansion rate increases.

Comparative Example 3-4

This comparative example provides a lithium ion battery.

The difference from Example 4 is that in the electrolyte used in this comparative example, the added amount of fluoroethylene carbonate is 1% (Comparative Example 3) and 15% (Comparative Example 4).

Comparative Example 5-6

This comparative example provides a lithium ion battery.

The difference from Example 4 is that the composition of the electrolyte provided in this comparative example is shown in Table 2.

Performance test 2

Perform performance tests on the lithium-ion batteries provided in Comparative Examples 3-6 with reference to the method provided in Performance Test 1. The results are shown in Table 2:

Table 2

It can be seen from the comparison between the examples and Comparative Examples 3-6 that in the present disclosure, the amount of fluoroethylene carbonate needs to be in the range of 3-13wt%, and (b/a)/c needs to be in the range of 0.1-10. As a result, the final lithium-ion battery has a better capacity retention rate and a lower high-temperature expansion rate.

Comparative example 7

This comparative example provides a lithium ion battery.

The difference from Example 4 is that in the electrolyte used in this comparative example, propylene carbonate (PC) is replaced by carbon Vinyl acid ester (EC).

Comparative example 8

This comparative example provides a lithium ion battery.

The difference from Example 4 is that in the electrolyte used in this comparative example, fluoroethylene carbonate (FEC) is replaced by ethylene carbonate (EC)

Performance test 3

Perform a performance test on the lithium-ion battery provided in the comparative example with reference to the method provided in Performance Test 1. The results are shown in Table 3:

table 3

It can be seen from the comparison between Example 4 and Comparative Examples 7-8 that the present disclosure uses propylene carbonate, fluoroethylene carbonate and lithium difluorophosphate at the same time, and all three are indispensable.

Comparative example 9

This comparative example provides a lithium ion battery.

The difference from Example 4 is that in this comparative example, the preparation method of the negative electrode is as follows:

Mix artificial graphite, sodium carboxymethylcellulose (CMC) and styrene-butadiene rubber (SBR) in deionized water at a weight ratio of approximately 96:2:2, stir evenly, and obtain a negative electrode slurry. Use copper foil as the negative electrode collector. Fluid, bake the negative electrode current collector coated with the negative electrode slurry at 120°C for 1 hour, and then perform cold pressing, cutting, and slitting to prepare the negative electrode;

Performance test 4

Perform a performance test on the lithium-ion battery provided in the comparative example with reference to the method provided in Performance Test 1. The results are shown in Table 4:

Table 4

It can be seen from the comparison between Example 4 and Comparative Example 9 that the electrolyte provided by the present disclosure can significantly improve the capacity retention rate and high-temperature cycle performance of lithium-ion batteries when used in conjunction with high-voltage ternary cathode materials and negative electrodes containing silicon elements.

It should be noted that, as used herein, the terms "include", "comprises" or any other variation thereof are intended to cover a non-exclusive inclusion, such that a process, method, article or device that includes a series of elements not only includes those elements, and also includes other elements not expressly listed or inherent to such process, method, article or equipment elements. Without further limitation, an element defined by the statement "comprises a..." does not exclude the presence of additional identical elements in a process, method, article, or apparatus that includes the stated element.

The above descriptions are only specific embodiments of the present disclosure, enabling those skilled in the art to understand or implement the present disclosure. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be practiced in other embodiments without departing from the spirit or scope of the disclosure. Therefore, the present disclosure is not to be limited to the embodiments described herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

An electrolyte for batteries, wherein the components of the electrolyte include propylene carbonate, fluoroethylene carbonate and lithium difluorophosphate, and based on the total mass of the electrolyte being 100%, the propylene carbonate The content is a%, the content of fluoroethylene carbonate is b%, and the content of lithium difluorophosphate is c%. The a, b and c satisfy the following relationship: 0.1≤(b/a)/c≤10, and 3≤b≤13.
The electrolyte solution according to claim 1, wherein in the electrolyte solution, 0.8≤(b/a)/c≤6.
The electrolyte solution according to claim 1 or 2, wherein in the electrolyte solution, 0.1≤c≤1;

and/or, in the electrolyte, 7≤a≤25

And/or, in the electrolyte solution, 3≤b≤13.
The electrolyte according to any one of claims 1-3, wherein the composition of the electrolyte further includes lithium hexafluorophosphate;

Preferably, the concentration of lithium hexafluorophosphate in the electrolyte is 0.7-1.3 mol/L.
The electrolyte solution according to any one of claims 1 to 4, wherein the components of the electrolyte solution further include chain carbonate, and the chain carbonate is preferably methyl ethyl carbonate.
The electrolyte solution according to claim 5, wherein the content of the ethyl methyl carbonate in the electrolyte solution is 20-65 wt%.
The electrolyte solution according to any one of claims 1-6, wherein the composition of the electrolyte solution further includes other non-aqueous solvents;

Preferably, the non-aqueous solvent is selected from any one or a combination of at least two of ethylene carbonate, diethyl carbonate, dipropyl carbonate, methylpropyl carbonate or ethylpropyl carbonate.
A lithium-ion battery, wherein the lithium-ion battery includes the electrolyte solution according to any one of claims 1-7.
The lithium ion battery according to claim 8, wherein the lithium ion battery further includes a positive electrode, the positive electrode includes a positive electrode active material, the composition of the positive electrode active material includes a lithium nickel composite oxide, the lithium nickel composite oxide The material contains divalent nickel and trivalent nickel, and the content ratio of the divalent nickel and trivalent nickel on the surface of the positive electrode active material is greater than the content ratio of the divalent nickel and trivalent nickel in the center of the positive electrode active material. content ratio;

Preferably, in the cathode active material, based on a total mole number of metal elements other than lithium being 100%, the sum of the contents of nickel elements is not less than 60%.
The lithium-ion battery according to claim 8, wherein the lithium-ion battery further includes a negative electrode, and the composition of the negative electrode includes silicon material and graphite material;

Preferably, based on the total mass of the negative electrode being 100%, the added amount of the silicon material is 0-20%, and does not include 0;

Preferably, the silicon material is SiO z , where 0.5<z<1.5.