CN118099533A - Electrolyte and rechargeable battery - Google Patents

Abstract

The invention provides an electrolyte, which comprises lithium salt and a nonaqueous solvent, wherein the nonaqueous solvent comprises a compound A shown as follows: Wherein R ₁ is selected from one of hydrogen atom and hydrocarbon group with carbon number of 1; r ₂ is independently selected from one of hydrocarbon groups with carbon atoms of 1-4. The invention also comprises a rechargeable battery containing the electrolyte, and the electrolyte has the characteristics of low cost, wide liquid range and high oxidation stability, has higher commercialization value and popularization value, and is expected to be applied to the next generation of high specific energy lithium battery in a large scale.

Description

Electrolyte and rechargeable battery

Technical Field

The present invention relates to the technical field of lithium ion batteries, and in particular to an electrolyte and a rechargeable battery.

Background technique

Lithium-ion batteries have been widely used in new energy vehicles and 3C digital products due to their advantages such as high energy density, high operating voltage, and long cycle life. The "blood" of lithium-ion batteries - electrolytes, plays a vital role in the performance of lithium-ion batteries. Although traditional carbonate-based electrolytes have achieved great success and occupied a monopoly position, their poor compatibility with lithium metal negative electrodes, poor high-voltage stability (>4.3V vs.Li ⁺ /Li), and narrow liquid range are becoming increasingly prominent. Specifically, in high-voltage lithium metal batteries, carbonate-based electrolytes with conventional lithium salt concentrations will be continuously oxidized and decomposed on the cathode surface, generating a large amount of by-products, which seriously reduces the cycle stability of the battery; at the same time, it will also lead to the growth of lithium dendrites at the anode, and the coulombic efficiency (CE) of the battery is low.

Unlike carbonate-based solvents, ether solvents exhibit good reduction stability on lithium metal. At the same time, they can also form a strong SEI layer on the surface of the lithium negative electrode, thereby inhibiting the formation of lithium dendrites and promoting the uniform deposition and stripping of lithium ions. The final result is that lithium metal batteries using ether-based electrolytes have excellent cycle stability and high coulombic efficiency. However, ether solvents generally have low oxidation stability (<4.0V vs.Li ⁺ /Li), which limits their application in high-voltage lithium metal batteries. The recent hot design concept of high-concentration lithium salts and locally high-concentration lithium salts, although it makes ether solvents have the potential to be used on lithium metal cobalt oxide positive electrodes up to 4.6V, the resulting high cost problem has become an irreconcilable contradiction.

Moreover, in the prior art, most of the solvents of lithium-ion battery electrolytes are selected from compounds containing a single functional group, such as carbonates (carbonate groups), carboxylates (ester groups) and ethers (ether bonds). However, the above single-functional compounds all have their obvious advantages and disadvantages, and it is difficult to take them into account. The common multi-solvent mixing method has improved their respective shortcomings to a certain extent, but it has greatly increased the complexity of the electrolyte composition, which is not conducive to research and application.

In summary, the existing lithium battery electrolytes with conventional lithium salt concentrations are difficult to simultaneously solve the problems of poor high-voltage stability, low coulombic efficiency, and narrow liquid range. Therefore, it is urgent to develop an "all-round" electrolyte.

Summary of the invention

The technical problem to be solved by the present invention is to provide an electrolyte to solve the problems of poor high-voltage stability, low coulombic efficiency and narrow liquid range of traditional electrolytes in the prior art.

To solve the above problems, the present invention provides an electrolyte, comprising a lithium salt and a non-aqueous solvent, wherein the non-aqueous solvent comprises a compound A as shown below, and the general formula is as follows:

In the formula, _R1 is selected from a hydrogen atom and a hydrocarbon group having 1 carbon atom; _R2 is independently selected from a hydrocarbon group having 1 to 4 carbon atoms;

As a preferred solution, the compound A is selected from at least one of the following compounds 1 to 8:

As a preferred embodiment, the non-aqueous solvent of the electrolyte contains, in addition to compound A, one or more of carbonate solvents, carboxylate solvents, cyclic lactone solvents, ether solvents, ionic liquids, phosphate solvents, fluorinated carbonate solvents, fluorinated carboxylate solvents, and fluorinated ether solvents;

As a preferred solution, the compound A accounts for 5 to 100% of the total mass of the non-aqueous solvent, specifically 5%, 10%, 20%, 50%, 70%, 80%, 90%, 100%, and any proportion between 5 and 100%;

As a preferred embodiment, the lithium salt is selected from at least one of lithium hexafluorophosphate (LiPF ₆ ), lithium perchlorate (LiClO ₄ ), lithium difluorophosphate (LiPO ₂ F ₂ ), lithium bis(oxalatoborate) (LiBOB), lithium difluorooxalatoborate (LiDFOB), lithium difluorobis(oxalatophosphate) (LiF2PO4(C2O4)2), lithium tetrafluoroborate (LiBF ₄ ), lithium tetrafluorooxalatophosphate (LiTFOP), lithium bis(trifluoromethylsulfonyl imide) (LiTFSI), and lithium bis(fluorosulfonyl imide) (LiFSI);

As a preferred solution, the lithium salt accounts for 5%-100% of the total mass of the electrolyte;

As a preferred embodiment, the electrolyte further includes an additive, and the additive is selected from one or more of lithium bis(oxalatoborate), lithium difluoro(oxalatoborate), vinylene carbonate, fluoroethylene carbonate, vinylene carbonate, vinyl sulfite, 1,3-propane sultone, 1,4-butane sultone, tetravinylsilane, vinyl sulfate, p-phenylene diisocyanate, 2,4-toluene diisocyanate, succinonitrile, adiponitrile, 1,3,6-hexanetrinitrile, vinyl disulfate, phenyl methanesulfonate, triallyl phosphate, tripropargyl phosphate, methylene methanedisulfonate, and tris(trimethylsilyl)borate;

As a preferred solution, the additive accounts for 0.01%-30% of the total mass of the electrolyte;

The present invention also provides a rechargeable battery, comprising a positive electrode, a diaphragm, a negative electrode, an electrolyte and a battery casing, wherein the battery casing wraps the positive electrode, the diaphragm, the negative electrode and the electrolyte, the diaphragm and the electrolyte are arranged between the positive electrode and the negative electrode, and the electrolyte is selected from the above-mentioned non-aqueous electrolyte.

As a preferred solution, the positive electrode includes a positive electrode material, and the positive electrode material contains one or more of lithium cobalt oxide, lithium iron phosphate, lithium nickel cobalt manganese oxide, lithium vanadium phosphate, lithium manganese oxide, and lithium iron manganese phosphate;

As a preferred scheme, the negative electrode includes a negative electrode material, and the negative electrode material includes at least one of a carbon-based negative electrode, a silicon-based negative electrode, a tin-based negative electrode and a lithium negative electrode. The carbon-based negative electrode includes one or more of natural graphite, artificial graphite, hard carbon, soft carbon, graphene, and mesophase carbon microspheres. The silicon-based negative electrode includes one or more of silicon materials, silicon oxides, silicon-carbon composite materials, and silicon alloy materials. The silicon-based negative electrode includes one or more of tin, tin carbon, tin oxygen, and tin metal compounds. The lithium negative electrode includes one or more of metallic lithium and lithium-silicon alloys.

The present invention provides an electrolyte and a rechargeable battery. The electrolyte has the advantages of low cost, high conductivity, wide liquid range, etc. at conventional lithium salt concentrations; and can stably circulate in a Li|| _LiCoO2 battery with a charging cut-off voltage of up to 4.6V; compared with the prior art, the present invention is the first to use a bifunctional compound A containing both an ester group and an ether bond as the main solvent of a lithium ion battery electrolyte, or even as a single solvent. According to the frontier orbital theory, the highest occupied orbital (HOMO) energy level can qualitatively measure the oxidation stability of a molecule. Generally, the smaller the HOMO energy level value, the better the oxidation stability of the molecule.

The present invention uses Gaussian 09 program to perform density functional theory calculations, takes 6-31++G(d,p) as the functional basis group, and optimizes the molecular structure of compound A at the B3LYP theoretical level. The calculation results show that although compound 4 has a higher HOMO energy level (-7.36eV) than compounds 1 to 8, it is still less than ethylene glycol dimethyl ether (DME, -7.12eV). Even among compounds 1 to 8, compound 5 (propylene glycol methyl ether acetate, PEGDMA) with the smallest HOMO energy level has a HOMO energy level as low as -7.56eV, showing excellent oxidation stability. The experimental results also show that under conventional lithium salt concentrations, the Li||LiCoO2 _secondary battery assembled with an electrolyte containing the compound 5 has the ability to cycle stably for a long time at a charge cut-off voltage of up to 4.6V. In view of the characteristics of low cost, wide liquid range, and high oxidation stability of the electrolyte, it is expected to be widely used in the next generation of high specific energy lithium batteries.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a graph showing the relationship between the number of cycles and the discharge specific capacity/charge and discharge efficiency of Example 4 at 4.45V and Comparative Example 2;

FIG2 is a graph showing the relationship between the number of cycles and the discharge specific capacity/charge and discharge efficiency of Example 4 at 4.6V;

Detailed ways

The technical solution of the present invention will be described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

The present invention provides an electrolyte comprising a lithium salt and a non-aqueous solvent, wherein the non-aqueous solvent comprises a compound A as shown below:

In the formula, _R1 is selected from a hydrogen atom and a hydrocarbon group having 1 carbon atom; _R2 is independently selected from a hydrocarbon group having 1 to 4 carbon atoms;

Preferably, the compound A is selected from at least one of the following compounds 1 to 8:

Preferably, the non-aqueous solvent of the electrolyte contains, in addition to compound A, one or more of carbonate solvents, carboxylate solvents, lactone solvents, ether solvents, ionic liquids, phosphate solvents, fluorocarbonate solvents, fluorocarboxylate solvents, and fluoroether solvents;

Preferably, the compound A accounts for 5-100% of the total mass of the non-aqueous solvent, specifically 5%, 10%, 20%, 50%, 70%, 80%, 90%, 100%, and any proportion between 5% and 100%;

Preferably, the lithium salt is selected from at least one of lithium hexafluorophosphate (LiPF ₆ ), lithium perchlorate (LiClO ₄ ), lithium difluorophosphate (LiPO ₂ F ₂ ), lithium bis(oxalatoborate) (LiBOB), lithium difluorooxalatoborate (LiDFOB), lithium difluorobis(oxalatophosphate) (LiF2PO4(C2O4)2), lithium tetrafluoroborate (LiBF ₄ ), lithium tetrafluorooxalatophosphate (LiTFOP), lithium bis(trifluoromethylsulfonyl imide) (LiTFSI), and lithium bis(fluorosulfonyl imide) (LiFSI);

Preferably, the lithium salt accounts for 5%-100% of the total mass of the electrolyte;

Preferably, the electrolyte further comprises an additive, and the additive is selected from one or more of lithium bis(oxalatoborate), lithium difluoro(oxalatoborate), vinylene carbonate, fluoroethylene carbonate, vinylene carbonate, vinyl sulfite, 1,3-propane sultone, 1,4-butane sultone, tetravinylsilane, vinyl sulfate, p-phenylene diisocyanate, 2,4-toluene diisocyanate, succinonitrile, adiponitrile, 1,3,6-hexanetrinitrile, vinyl disulfate, phenyl methanesulfonate, triallyl phosphate, tris(trimethylsilyl)phosphate, methylene methanedisulfonate, and tris(trimethylsilyl)borate;

Preferably, the additive accounts for 0.01%-30% of the total mass of the electrolyte;

The present invention also provides a rechargeable battery, comprising a positive electrode, a separator, a negative electrode, an electrolyte and a battery casing, wherein the electrolyte is selected from the above-mentioned non-aqueous electrolyte.

Preferably, the positive electrode comprises a positive electrode material, and the positive electrode material comprises one or more of lithium cobalt oxide, lithium iron phosphate, lithium nickel cobalt manganese oxide, lithium vanadium phosphate, lithium manganese oxide, and lithium iron manganese phosphate;

Preferably, the negative electrode includes a negative electrode material, and the negative electrode material includes at least one of a carbon-based negative electrode, a silicon-based negative electrode, a tin-based negative electrode and a lithium negative electrode, and specifically may be but is not limited to at least one of natural graphite, artificial graphite, hard carbon, soft carbon, graphene, mesophase carbon microspheres, silicon materials, silicon oxides, silicon-carbon composite materials, silicon alloy materials, tin, tin carbon, tin oxide, tin metal compounds, metallic lithium, and lithium silicon alloys.

In order to facilitate the understanding of the present invention, the following examples are listed. It should be clear to those skilled in the art that these examples are only to help understand the present invention and should not be regarded as specific limitations of the present invention.

In the case where specific conditions are not specified in the examples, the operations were carried out under conventional conditions. All reagents used were commercially available products.

Example 1

1.1 Preparation of electrolyte

In a glove box filled with argon (H ₂ O, O ₂ <0.1ppm), use a pipette to quantitatively measure the required compound A and other non-aqueous solvents, and add them to a high-density polyethylene bottle with a rated capacity of 50mL. Shake gently to mix them evenly. Afterwards, quantitatively weigh the required lithium salt, use a stainless steel spoon to slowly add it to the above non-aqueous solvent, and shake while adding to fully dissolve the lithium salt. The same is true for the addition of additives. Unless otherwise specified, the electrolytes of the following different embodiments are prepared in this way.

1.2 Selection or preparation of positive electrode, negative electrode and separator

The positive electrode uses a commercially available lithium cobalt oxide positive electrode. It uses a 14-micron thick aluminum foil as the current collector, coated on one side, with a coating surface density of 10.7 mg cm ^-2 , an active material ratio of 96.4%, and a compaction density of 3.5 g cm ^-3 . It is punched into a small disc with a diameter of 12 mm using a pole piece press for standby use. The negative electrode uses a 450-micron thick lithium sheet, and the separator model is celgard2320.

Unless otherwise specified, the above products are used in the following different embodiments. The electrolyte components and contents of Examples 2-5 and Comparative Examples are shown in Table 1:

Table 1 Electrolyte components and contents of various embodiments and comparative examples

The electrolytes of Examples 2 to 5 and Comparative Examples 1 to 2 were assembled into Li|| _LiCoO2 secondary batteries in the manner of Example 1, and the battery shell specification was CR2032.

The assembled button cell was first placed in a constant temperature box at 25°C for more than 9 hours. Then, a constant current charge and discharge test was performed for 2 cycles at a charge and discharge rate of 0.1C (1C = 160mAh g ^–1 , the same below). The charge cut-off voltage of each battery is shown in the table below, and the discharge cut-off voltage is uniformly 3V. After that, different batteries were subjected to long-term cycles at different voltages or constant current charge and discharge rates according to the needs of the test. The maximum discharge capacity in the 3rd to 10th cycles was taken as the reference value for calculating the capacity retention rate.

The 25°C room temperature cycling performance of the Li|| _LiCoO2 battery assembled with the electrolytes of the embodiments and comparative examples is shown in Table 2.

Table 2 Cyclic performance of Li|| _LiCoO2 assembled with electrolytes of various embodiments and comparative examples at 25°C

In particular, Figure 1 shows the 100-cycle cycle performance of Li||LiCoO2 button cells assembled using the electrolytes of Example 4 and Comparative Example ₂ at 25°C, 0.25C rate, and 4.45V charge cut-off voltage. As can be seen from the figure, the discharge capacity of Comparative Example 2 using the classic electrolyte formula has begun to fluctuate greatly after 70 cycles; less than 80 cycles, the discharge capacity has begun to decay rapidly. Finally, after 100 cycles, the capacity retention rate was less than 45%, and the average coulomb efficiency was only 97.5%, reflecting its poor cycle stability. The battery using the electrolyte of Example 4 has excellent cycle stability, with a capacity retention rate of 98.73% after 100 cycles and an average coulomb efficiency of 99.66%.

Moreover, as shown in FIG2 , the button cell using the electrolyte of Example 4 still has a capacity retention rate of 94.42% and an average coulombic efficiency of 99.49% after 100 cycles at a high cut-off voltage of 4.6V and a large constant current charging rate of 0.45C. The above data fully demonstrate that the electrolyte provided by the present invention has good oxidation stability and excellent cycle stability.

Further testing of the above-mentioned embodiments and comparative examples also proves that the electrolyte prepared by the present invention is applied to the battery and has the ability of long-term stable circulation at a charging cut-off voltage of up to 4.6V. Compared with comparative examples 1/2, the embodiments of the present invention have more excellent performance in terms of stability, retention rate and coulombic efficiency comprehensive performance. This also further illustrates that the electrolyte of the present invention has the characteristics of low cost, wide liquid range and high oxidation stability, and is expected to be widely used in the next generation of high specific energy lithium batteries.

Although the disclosure is disclosed as above, the protection scope of the disclosure is not limited thereto. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the disclosure, and these changes and modifications will fall within the protection scope of the present invention.

Claims (10)

1. An electrolyte, characterized in that the electrolyte comprises a lithium salt and a non-aqueous solvent, the non-aqueous solvent comprises a compound A, and the general formula of the compound A is shown in general formula A: Wherein, _R1 is selected from a hydrogen atom and a hydrocarbon group having 1 carbon atom; and _R2 is selected from a hydrocarbon group having 1 to 4 carbon atoms. 2. The electrolyte according to claim 1, characterized in that the compound A comprises at least one of compound 1 to compound 8, and the chemical formula of the compound 1 to compound 8 is: 3. The electrolyte according to claim 1 is characterized in that: the non-aqueous solvent also includes one or more of carbonate solvents, carboxylate solvents, cyclic lactone solvents, ether solvents, ionic liquids, phosphate solvents, fluorinated carbonate solvents, fluorinated carboxylate solvents, and fluorinated ether solvents. 4. The electrolyte according to claim 1, characterized in that the mass of the compound A is 5%-100% of the total mass of the non-aqueous solvent. 5. The electrolyte according to claim 1 is characterized in that: the lithium salt comprises one or more of lithium hexafluorophosphate, lithium perchlorate, lithium difluorophosphate, lithium bis(oxalatoborate), lithium difluorooxalatoborate, lithium difluorooxalatophosphate, lithium tetrafluoroborate, lithium tetrafluorooxalatophosphate, lithium bis(trifluoromethylsulfonylimide), and lithium bis(fluorosulfonylimide), and the mass of the lithium salt is 5%-100% of the total mass of the electrolyte. 6. The electrolyte according to claim 1 is characterized in that: the electrolyte also includes additives, and the additives include one or more of lithium bis(oxalatoborate), lithium difluoro(oxalatoborate), vinylene carbonate, fluoroethylene carbonate, vinylene carbonate, vinyl sulfite, 1,3-propane sultone, 1,4-butane sultone, tetravinylsilane, vinyl sulfate, p-phenylene diisocyanate, 2,4-toluene diisocyanate, succinonitrile, adiponitrile, 1,3,6-hexanetrinitrile, vinyl bissulfate, phenyl methanesulfonate, triallyl phosphate, tripropargyl phosphate, methylene methanedisulfonate and tris(trimethylsilyl)borate; the mass of the additive is 0.01%-30% of the total mass of the electrolyte. 7. A rechargeable battery, characterized in that it comprises a positive electrode, a separator, a negative electrode, an electrolyte and a battery casing, wherein the electrolyte is the electrolyte according to any one of claims 1 to 6. 8. The rechargeable battery according to claim 7 is characterized in that the positive electrode comprises a positive electrode material, and the positive electrode material comprises one or more of lithium cobalt oxide, lithium iron phosphate, lithium nickel cobalt manganese oxide, lithium vanadium phosphate, lithium manganese oxide, and lithium iron manganese phosphate. 9. The rechargeable battery according to claim 7, characterized in that the negative electrode comprises a negative electrode material, and the negative electrode material comprises one or more of a carbon-based negative electrode, a silicon-based negative electrode, a tin-based negative electrode and a lithium negative electrode. 10. The rechargeable battery according to claim 9 is characterized in that the carbon-based negative electrode includes one or more of natural graphite, artificial graphite, hard carbon, soft carbon, graphene, and mesophase carbon microspheres; the silicon-based negative electrode includes one or more of silicon materials, silicon oxides, silicon-carbon composite materials, and silicon alloy materials; the silicon-based negative electrode includes one or more of tin, tin-carbon, tin oxide, and tin metal compounds; and the lithium negative electrode includes one or more of metallic lithium and lithium-silicon alloys.