CN114695959A - A kind of high voltage electrolyte and lithium ion battery - Google Patents

Abstract

The invention discloses a high-voltage electrolyte, which comprises a main lithium salt and an organic solvent, and further comprises: a first additive, which is at least one of thiophene phosphate compounds represented by the following formula (I):

in the formula, R₁、R₂Independently selected from C1-C12 hydrocarbyl, C1-C12 fluoro hydrocarbyl; r₃Selected from a direct bond, C1-C12 alkylene or C1-C12 fluoroalkylene; and the second additive is selected from at least one of lithium salt additives or organic ester additives. The high-voltage electrolyte disclosed by the invention has the advantages of good compatibility of a negative electrode interface, high voltage stability of a battery cell, long cycle performance and the like, and is effectively improved.

Description

A kind of high voltage electrolyte and lithium ion battery

technical field

The invention relates to the field of lithium ion batteries, in particular to an electrolyte and a lithium ion battery suitable for a high voltage and high temperature environment.

Background technique

Increasing the voltage is an effective way to increase the energy density of lithium-ion batteries, but as the energy density of the battery increases, it will not only cause the oxidative decomposition of the electrolyte at the cathode/electrolyte interface, but also accelerate the dissolution of the metal cations in the cathode in the electrolyte. , resulting in decreased battery cycle performance and safety.

The prior art reports that additives containing sulfur, borate esters, acid anhydrides, and nitrile can form films on the surface of the positive electrode material, thereby stabilizing the positive electrode/electrolyte interface. However, with the continuous increase of the charge cut-off voltage, such as when the voltage is >4.35V, the existing positive electrode film-forming additives have very limited stabilizing effects on high-temperature storage and cycling performance under high voltage, and need to be further improved.

On the other hand, most of the interfacial films formed by the existing positive electrode film-forming additives have high interfacial impedance and impedance growth rate. "diving" phenomenon, and cathode film-forming additives usually have poor compatibility with the anode interface, and it is difficult to guarantee long-term cycle performance.

Therefore, the development of an electrolyte system with long cycle stability, both high and low temperature and low internal resistance is the key factor for the commercial application of high-voltage power batteries.

SUMMARY OF THE INVENTION

In order to solve the above-mentioned technical problems, the present invention proposes a high-voltage electrolyte that has good compatibility at the negative electrode interface, and effectively improves the high-voltage stability and long-cycle performance of the cell.

The purpose of this invention is to realize through the following technical solutions:

A high-voltage electrolyte, comprising a main lithium salt and an organic solvent, the electrolyte further comprising:

The first additive, the first additive is at least one of the thiophene phosphate compounds represented by the following formula (I):

In the formula, R ₁ and R ₂ are independently selected from C1-C12 hydrocarbon group, C1-C12 fluorohydrocarbon group; R ₃ is selected from direct bond, C1-C12 alkylene group or C1-C12 fluoroalkylene group;

The second additive, the second additive is selected from at least one of lithium salt additives or organic ester additives.

Preferably, R ₁ and R ₂ are independently selected from C1-C6 hydrocarbon group and C1-C6 fluorohydrocarbon group; R ₃ is selected from direct bond, C1-C4 alkylene group or C1-C4 fluoroalkylene group.

More preferably, the first additive is selected from at least one of the following structures:

The lithium salt additive of the present invention is selected from lithium bisfluorosulfonimide (LiFSI), lithium difluorophosphate (LiDFP), lithium difluorobisoxalate phosphate (LiDFOP), lithium difluorooxalate borate (LiDFOB), bis-oxalic acid At least one of lithium borate (LiBOB), lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), lithium tetrafluoroborate (LiBF4), lithium tetrafluorooxalate phosphate (LiTFOP), and lithium trioxalate phosphate (LiTOP) The organic ester additive is selected from the group consisting of vinyl sulfate (DTD), 1,3-propane sultone (PS), methylene methane disulfonate (MMDS), vinyl sulfite (ES), Of 1,4-butane sultone (BS), propylene sulfate (PSA), tris(trimethylsilyl) phosphate (TMSP), tris(trimethylsilyl) borate (TMSB) at least one.

In the high-voltage electrolyte of the present invention, the addition amount of the first additive is 0.1-10.0% of the total amount of the electrolyte, and the addition amount of the second additive is 0.1-10.0% of the total amount of the electrolyte. Preferably, the added amount of the first additive is 0.2-3.0% of the total amount of the electrolyte, and the added amount of the second additive is 0.2-3.0% of the total amount of the electrolyte.

The main lithium salt in the present invention may be the lithium salt commonly used in the electrolyte. Preferably, the main lithium salt is selected from lithium hexafluorophosphate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium perchlorate, lithium bis-oxalate borate, lithium difluorooxalate borate, lithium difluorophosphate, bisfluorosulfonimide At least one of lithium and lithium bis-trifluoromethanesulfonimide, the molar concentration of which is 0.1-4.0 mol/L, and the main lithium salt is different from the second additive. More preferably, the main lithium salt is selected from lithium hexafluorophosphate and/or lithium bisfluorosulfonimide, and its molar concentration is 0.8-1.5 mol/L.

The organic solvent in the present invention can be any organic solvent commonly used in the electrolyte. Preferably, the organic solvent is selected from at least one of C3-C6 carbonate compounds, C3-C8 carboxylate compounds, sulfone compounds, and ether compounds. further,

The C3～C6 carbonate or fluorocarbonate compound is selected from ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, dipropyl carbonate, At least one in methyl propyl carbonate, ethyl propyl carbonate;

The C3～C8 carboxylate or fluorocarboxylate compound is selected from γ-butyrolactone, methyl acetate, methyl propionate, methyl butyrate, ethyl acetate, ethyl propionate, butyric acid At least one of ethyl ester, propyl acetate, and propyl propionate;

The sulfone compound is selected from at least one of sulfolane, dimethyl sulfoxide, dimethyl sulfone, and diethyl sulfone;

The ether compounds are selected from triglyme and/or tetraglyme.

The first additive of the present invention can not only participate in the formation of the negative electrode SEI film, but also act on the positive electrode interface, especially to form a dense and stable interface film on the positive electrode interface, which can inhibit the electrolyte solution on the surface of the high voltage positive electrode during the charging process. Oxidative decomposition. At the same time, the sulfur atom with strong electron-withdrawing effect in the structure of the thiophene phosphate compound can be oxidized and decomposed on the surface of the positive electrode to generate LiSO ₄ R (lithium sulfate) compounds, which have a three-dimensional pore structure and accelerate the lithium ions in the protective film. Compared with the existing technology, the shuttle has lower internal resistance of the battery, thereby effectively improving the cycle performance of the lithium-ion battery.

In the present invention, the second additive is used in combination with the first additive, which can further improve the compatibility between the positive electrode film-forming additive and the negative electrode interface. When the second additive is a lithium salt additive, an organic-inorganic inter-cross-linked interface film component can be formed on the surface of the positive electrode, and the organic-inorganic inter-cross-linked interface film component and the thiophene phosphate compound are decomposed to form LiSO ₄ The R (lithium sulfate) compounds together form a protective film on the positive electrode, which further reduces the impedance and improves the high-voltage cycling stability. When the second type of additive is an organic ester additive, the formed protective film contains more organic components, and the organic components and the thiophene phosphate compound are decomposed to form LiSO ₄ R (lithium sulfate) compounds together in the positive electrode A protective film is formed, the toughness of the protective film is improved, the rupture and re-formation of the protective film are avoided, and the resistance growth rate during the cycle is reduced.

The present invention also provides a lithium ion battery, the lithium ion battery includes a positive electrode, a negative electrode, a separator, and any one of the above-mentioned high-voltage electrolytes.

The positive electrode material is selected from nickel-cobalt-manganese ternary material, nickel-cobalt-aluminum ternary material or lithium cobalt oxide material.

Further, the nickel-cobalt-manganese ternary material is Li(Ni _x Co _y Mn _z )O ₂ , x≥0.5, y>0, z>0, x+y+z=1; the nickel-cobalt-aluminum three The primary material is Li(Ni _x Co _y Al _z )O ₂ , x≥0.8, y>0, z>0, x+y+z=1; the lithium cobalt oxide material is LiCoO ₂ .

The negative electrode active material is selected from graphite, silicon carbon, silicon oxide, silicon, tin, metallic lithium or composite materials thereof.

Compared with the prior art, the present invention has the following beneficial effects:

1. The present invention solves the problems of high impedance and poor high-voltage cycle stability when the traditional additive is used, and simultaneously improves the rate and high and low temperature performance of the battery through the combined use of the first additive and the second additive.

2. The electrolyte of the present invention is especially suitable for a battery system in a high voltage (>4.35V) high temperature (>45°C) environment. In this environment, the long-cycle performance of the battery is significantly improved compared to the existing electrolyte.

Description of drawings

Fig. 1 is the LSV oxidation curve of comparative example 1 electrolyte of the present invention;

Fig. 2 is the LSV reduction curve of the electrolyte of Comparative Example 1 of the present invention.

Detailed ways

The present invention will be further described below with reference to specific embodiments, but the present invention is not limited to these specific embodiments. Those skilled in the art should realize that the present invention covers all alternatives, modifications and equivalents that may be included within the scope of the claims.

1. Preparation of electrolyte

Preparation of basic electrolyte: In an argon-filled glove box (moisture <5ppm, oxygen <10ppm), ethylene carbonate (EC), diethyl carbonate (DEC) and ethyl methyl carbonate (EMC) were mixed by mass The ratio is EC:DEC:EMC=3:2:5, and the mixture is uniformly mixed, and lithium hexafluorophosphate (LiPF ₆ ) is slowly added to the mixed solution until the molar concentration of LiPF ₆ is 1.0 mol/L to obtain a basic electrolyte.

Example 1: In the base electrolyte, 1.0% of compound 1 and 1.0% of lithium difluorophosphate (LiDFP) were added to obtain the electrolyte of this example.

Example 2: In the base electrolyte, 1.0% of compound 1 and 0.5% of lithium difluorophosphate (LiDFP) were added to obtain the electrolyte of this example.

Example 3: In the base electrolyte, 1.0% of compound 1 and 2.0% of lithium difluorophosphate (LiDFP) were added to obtain the electrolyte of this example.

Example 4: In the base electrolyte, 0.5% of compound 1 and 1.0% of lithium difluorophosphate (LiDFP) were added to obtain the electrolyte of this example.

Example 5: In the base electrolyte, 2.0% of compound 1 and 1.0% of lithium difluorophosphate (LiDFP) were added to obtain the electrolyte of this example.

Example 6: In the base electrolyte, 1.0% of compound 2 and 1.0% of lithium difluorophosphate (LiDFP) were added to obtain the electrolyte of this example.

Example 7: In the base electrolyte, 1.0% of compound 4 and 1.0% of lithium difluorophosphate (LiDFP) were added to obtain the electrolyte of this example.

Example 8: In the base electrolyte, 1.0% of compound 5 and 1.0% of lithium difluorophosphate (LiDFP) were added to obtain the electrolyte of this example.

Example 9: In the base electrolyte, 1.0% of compound 8 and 1.0% of lithium difluorophosphate (LiDFP) were added to obtain the electrolyte of this example.

Example 10: In the base electrolyte, 1.0% of compound 1 and 1.0% of vinyl sulfate (DTD) were added to obtain the electrolyte of this example.

Example 11: In the base electrolyte, 1.0% of compound 2 and 1.0% of vinyl sulfate (DTD) were added to obtain the electrolyte of this example.

Example 12: In the base electrolyte, 1.0% of compound 1 and 1.0% of lithium bisfluorosulfonimide (LiFSI) were added to obtain the electrolyte of this example.

Example 13: In the base electrolyte, 1.0% of compound 1 and 1.0% of lithium difluorobisoxalate phosphate (LiDFOP) were added to obtain the electrolyte of this example.

Example 14: In the base electrolyte, 1.0% of compound 1 and 1.0% of lithium difluorooxalate borate (LiDFOB) were added to obtain the electrolyte of this example.

Example 15: In the base electrolyte, 1.0% of compound 1 and 1.0% of 1,3-propane sultone (PS) were added to obtain the electrolyte of this example.

Example 16: In the base electrolyte, 1.0% of compound 1 and 1.0% of 1,4-butane sultone (BS) were added to obtain the electrolyte of this example.

Example 17: In the base electrolyte, 1.0% of compound 1 and 1.0% of tris(trimethylsilyl) phosphate (TMSP) were added to obtain the electrolyte of this example.

Comparative Example 1: In the base electrolyte, 1.0% of compound 1 was added to obtain the electrolyte of this comparative example.

Comparative Example 2: In the base electrolyte, 1.0% lithium difluorophosphate (LiDFP) was added to obtain the electrolyte of this comparative example.

Comparative Example 3: In the base electrolyte, 1.0% of compound 2 was added to obtain the electrolyte of this comparative example.

Comparative Example 4: In the base electrolyte, 1.0% of compound 7 was added to obtain the electrolyte of this comparative example.

Comparative Example 5: In the basic electrolyte, 1.0% of compound 10 was added to obtain the electrolyte of this comparative example.

Comparative Example 6: In the basic electrolyte, 1.0% lithium bisfluorosulfonimide (LiFSI) was added to obtain the electrolyte of this comparative example.

Comparative Example 7: In the base electrolyte, 1.0% lithium difluorobisoxalate phosphate (LiDFOP) was added to obtain the electrolyte of this comparative example.

Comparative Example 8: In the basic electrolyte, 1.0% of vinyl sulfate (DTD) was added to obtain the electrolyte of this comparative example.

Comparative Example 9: In the basic electrolyte, 1.0% of 1,3-propane sultone (PS) was added to obtain the electrolyte of this comparative example.

Comparative Example 10: In the base electrolyte, 1.0% lithium difluorophosphate (LiDFP) and 1.0% 1,3-propane sultone (PS) were added to obtain the electrolyte of this comparative example.

Comparative Example 11: In the base electrolyte, 1.0% of lithium difluorophosphate (LiDFP) and 1.0% of vinyl sulfate (DTD) were added to obtain the electrolyte of this comparative example.

Comparative Example 12: In the basic electrolyte, 1.0% of compound 1 and 1.0% of vinyl ethylene carbonate (VEC) were added to obtain the electrolyte of this comparative example.

Comparative Example 13: In the base electrolyte, 1.0% of compound 1 and 1.0% of 1,3-propene sultone (PST) were added to obtain the electrolyte of this comparative example.

The mass percentages of the first additive, the second additive and other additives in the above examples and comparative examples are shown in Table 1 below:

The usage amount of additives in the embodiment and comparative example of table 1

2. Performance test

1) Add 1.0% compound 1 to the base electrolyte, that is, the electrolyte of Comparative Example 1, and perform LSV redox potential test on it.

Figure 1 shows the LSV oxidation curve. As shown in Figure 1, there is a preferential oxidation peak at around 5.0V. It is speculated that compound 1 may have a positive electrode film-forming effect, forming a sulfur- and phosphorus-containing cathode protective film.

Figure 2 shows the reduction curve of LSV. As shown in Figure 2, there is a preferential reduction peak at around 1.5V. It is speculated that compound 1 may have a negative electrode film-forming effect. Therefore, it is considered that Compound 1 participates in both the positive electrode film formation and the negative electrode film formation.

2) The lithium ion battery electrolyte of the above-mentioned embodiment and the comparative example is made into a soft pack capacity 1260mAh lithium ion power battery respectively, and the lithium ion power battery comprises a positive pole piece, a negative pole piece, a diaphragm, an electrolyte and a battery auxiliary material, The positive electrode material is a nickel-cobalt-manganese ternary material (LiNi _0.6 Co _0.2 Mn _0.2 O ₂ ), and the negative electrode material is graphite.

The preparation process is as follows: the positive pole piece, the separator and the negative pole piece are wound together into a roll core, sealed with an aluminum plastic film, and then baked to make the electrode moisture meet the requirements. The finished soft-packed cells are obtained through the processes of placement, formation, volume separation, and aging.

The performance test of the prepared lithium-ion power battery (soft-packed battery) mainly includes the following contents:

(1) Battery capacity test: Charge the divided battery with a constant current of 0.33C to 4.35V, and then continue to charge at a constant voltage until the current of 0.05C is cut off; leave it for 30 minutes; discharge with a constant current of 1C to 2.8 V, the discharge capacity of the single cell is obtained.

(2) -20℃ battery discharge DCIR test: adjust the battery to 50% SOC state with 0.33C current, leave the battery at -20℃ for 5 hours to depolarize the battery, record the open circuit voltage OCV1 after the end of the leave, and use 3C Discharge with current for 10s, put it on hold for 10min, and test the voltage OCV2 at the moment of termination of high-current discharge. According to the formula DCIR=(OCV1-OCV2)/3C, the low-temperature discharge DCIR of the single battery is obtained.

(3) -20℃ battery discharge capacity test: put the fully charged battery at -20℃ for 5 hours, and discharge it to 2.8V with a current of 0.5C to obtain the low temperature discharge capacity of the single battery.

(4) High temperature storage test at 60°C: Charge the battery to 100% SOC, put it in an oven at 60±2°C for 1 month, test the volume change before and after storage, and obtain the volume change rate of the single battery before and after storage at 60°C.

(5) 45°C high temperature cycle test: The battery is cycled in a 45±2°C oven with a charge and discharge current of 1C/1C, the weekly charge capacity and discharge capacity are calculated, and the DCIR change during the battery cycle is monitored every 100 weeks. and volume change to obtain the capacity retention rate, DCIR growth rate and volume change rate of the single cell after 500 cycles of cycling at 45°C.

Table 2 shows the test results of the basic performance (ACR internal resistance and initial capacity) and low temperature performance of soft-packed cells prepared with different electrolyte formulations in the embodiments of the present invention and comparative examples; Table 3 shows the embodiments of the present invention The storage performance at 60°C (volume change rate before and after storage, internal resistance growth rate before and after storage), 45°C cycle performance (volume change rate, DCIR internal resistance growth rate, capacity retention) test results. Each electrolyte formula is prepared into two identical soft-packed cells for parallel testing, as shown in Tables 2 and 3 below:

Table 2 Basic performance and low temperature performance test results

Table 3 Test results of high temperature storage at 60℃ and high temperature cycle performance at 45℃

According to the test results in Table 2 and Table 3, it can be known that:

1. Comparing Example 1 with Comparative Example 1, Comparative Example 2, Comparative Example 10 with Comparative Example 1, Comparative Example 8, and Comparative Example 15 with Comparative Example 1 and Comparative Example 9, it can be seen that under the high voltage and high temperature test conditions, When the composition of the first additive and the second additive is added to the electrolyte, compared with using the first additive or the second additive alone, the battery cell has better high-voltage and high-temperature performance, and can better inhibit high-temperature storage gas generation and High temperature cycle gas production, while taking into account the low temperature performance.

2. Comparing Examples 1, 6 to 17 and Comparative Examples 12 to 13, it can be seen that under the high voltage and high temperature test conditions, the combination of the first additive and the second additive of the present invention is better than other high voltage/high temperature protective additives. The combination of (such as VEC and PST) has lower cell impedance, better high-temperature storage performance and high-temperature cycle performance, which can solve the problems of high impedance and poor high-voltage cycle stability when using traditional additives, and can also take into account the battery high and low temperature performance.

3. Comparing Examples 1, 6-17 and Comparative Examples 10-11, it can be seen that under the high voltage and high temperature test conditions, the combination of the first additive and the second additive of the present invention is compared with the lithium salts in the second additive. When the additive is used in combination with the organic ester additive, it has better high temperature storage stability and the long cycle performance of the battery under high voltage is significantly improved.

To sum up, in a high-voltage battery system, the combined additive of the first additive and the second additive of the present invention has good compatibility at the negative electrode interface, significantly improves the high-voltage stability performance, suppresses high-temperature cycling and storage gas production, and at the same time Take into account the effect of low temperature discharge performance.

Claims (11)

1. A high voltage electrolyte comprising a primary lithium salt, an organic solvent, characterized in that: the electrolyte further includes:

a first additive, which is at least one of thiophene phosphate compounds represented by the following formula (I):

in the formula, R₁、R₂Independently selected from C1-C12 hydrocarbyl, C1-C12 fluoro hydrocarbyl; r₃Selected from a direct bond, C1-C12 alkylene or C1-C12 fluoroalkylene;

and the second additive is selected from at least one of lithium salt additives or organic ester additives.

2. The high voltage electrolyte of claim 1, wherein: r₁、R₂Independently selected from C1-C6 hydrocarbyl, C1-C6 fluoro hydrocarbyl; r₃Selected from a direct bond, C1-C4 alkylene or C1-C4 fluoroalkylene.

3. The high voltage electrolyte of claim 2, wherein: the first additive is selected from at least one of the following structures:

4. the high voltage electrolyte of claim 1, wherein: the lithium salt additive is selected from at least one of lithium bis (fluorosulfonyl) imide, lithium difluorophosphate, lithium difluorobis (oxalato) phosphate, lithium difluorooxalato borate, lithium bis (oxalato) borate, lithium bis (trifluoromethylsulfonyl) imide, lithium tetrafluoroborate, lithium tetrafluorooxalato phosphate and lithium tris (oxalato) phosphate; the organic ester additive is selected from at least one of vinyl sulfate, 1, 3-propane sultone, methylene methane disulfonate, ethylene sulfite, 1, 4-butane sultone, allyl sulfate, tri (trimethylsilyl) phosphate and tri (trimethylsilyl) borate.

5. The high voltage electrolyte of any one of claims 1 to 4, wherein: the addition amount of the first additive accounts for 0.1-10.0% of the total amount of the electrolyte, and the addition amount of the second additive accounts for 0.1-10.0% of the total amount of the electrolyte.

6. The high voltage electrolyte of claim 5, wherein: the addition amount of the first additive accounts for 0.2-3.0% of the total amount of the electrolyte, and the addition amount of the second additive accounts for 0.2-3.0% of the total amount of the electrolyte.

7. The high voltage electrolyte of claim 1, wherein: the main lithium salt is at least one selected from lithium hexafluorophosphate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium perchlorate, lithium bis (oxalate) borate, lithium difluoro (oxalate) borate, lithium difluorophosphate, lithium bis (fluorosulfonyl) imide and lithium bis (trifluoromethylsulfonyl) imide, and the molar concentration of the main lithium salt is 0.1-4.0 mol/L.

8. The high voltage electrolyte of claim 1, wherein: the organic solvent is at least one selected from C3-C6 carbonate compounds, C3-C8 carboxylic ester compounds, sulfone compounds and ether compounds.

9. The high voltage electrolyte of claim 8, wherein:

the carbonate or fluoro carbonate compound of C3-C6 is at least one selected from ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl propyl carbonate and ethyl propyl carbonate;

the carboxylic ester or fluorinated carboxylic ester compound of C3-C8 is at least one selected from gamma-butyrolactone, methyl acetate, methyl propionate, methyl butyrate, ethyl acetate, ethyl propionate, ethyl butyrate, propyl acetate and propyl propionate;

the sulfone compound is at least one selected from sulfolane, dimethyl sulfoxide, dimethyl sulfone and diethyl sulfone;

the ether compound is selected from triglyme and/or tetraglyme.

10. A lithium ion battery comprises a positive electrode, a negative electrode and a diaphragm, and is characterized in that: the lithium ion battery further comprises the high voltage electrolyte of any one of claims 1-9.

11. The lithium ion battery of claim 10, wherein: the anode material is selected from a nickel-cobalt-manganese ternary material, a nickel-cobalt-aluminum ternary material or a lithium cobaltate material.

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