CN113745659A

CN113745659A - Composite high-safety electrolyte and lithium ion battery

Info

Publication number: CN113745659A
Application number: CN202110973838.9A
Authority: CN
Inventors: 项宏发; 刘永超; 孙毅; 梁鑫
Original assignee: Hefei University of Technology
Current assignee: Hefei University of Technology
Priority date: 2021-08-23
Filing date: 2021-08-23
Publication date: 2021-12-03

Abstract

The invention discloses a composite high-safety electrolyte and a lithium ion battery. The electrolyte comprises lithium salts, organic solvents, phosphate ester additives, silylacetamide additives and metal ion compounds. The present invention uses phosphate compounds as additives to make the electrolyte have flame retardant or non-flammable characteristics; silylacetamide additives can spontaneously react with trace water in the electrolyte to form cyclic molecules, and metal cation compounds can promote the cyclic molecules Ring-opening polymerization forms an ultra-thin, uniform, and highly ion-conductive interface film on the surface of the electrodes (positive and negative electrodes), which effectively improves the compatibility of the flame-retardant electrolyte with the positive and negative electrodes, and ensures that the high-safety electrolyte has good properties. electrochemical performance. At the same time, the silane-based acetamide additive consumes trace water in the electrolyte during the reaction, which can avoid the reduction of battery capacity, increase in internal resistance, and battery swelling due to the presence of trace water, which further improves the battery synthesis. performance.

Description

Composite high-safety electrolyte and lithium ion battery

Technical Field

The invention belongs to the technical field of lithium ion batteries, and particularly relates to a composite high-safety electrolyte and a lithium ion battery.

Background

The lithium ion battery has the advantages of high energy density, long cycle life, environmental protection and the like, and is widely applied to the fields of portable electronic products, smart homes, power devices and energy storage. In recent years, with the development of various industries, higher requirements are put on lithium ion batteries: higher energy density and higher safety. Meanwhile, the working temperature of the lithium ion battery also needs to be widened to meet the application in special fields. At present, the solvent of the lithium ion battery electrolyte is mainly an inflammable carbonate organic solvent, and under a long-time working state, the heat energy cannot be released in time to cause the short circuit of the battery, so that serious safety problems such as fire, combustion and even explosion can be caused. Furthermore, the battery can cause lithium precipitation at the negative side under low temperature or overcharge conditions, and lithium dendrites puncture the separator causing short circuits or failure, which in turn raises safety concerns.

In order to improve the safety of lithium ion batteries, a great deal of work has been done on the aspects of positive and negative electrodes, electrolytes, separator materials, battery structures, and the like. The electrolyte of the lithium ion battery is of great importance to the safety of the battery, and an effective modification means is realized by reducing or replacing a combustion-supporting solvent in the electrolyte or adding a flame-retardant additive in the electrolyte. In view of economy and practicality, a flame retardant additive is generally added to a lithium ion electrolyte to improve the safety of a battery. Common flame retardant additives comprise phosphate esters, but phosphate esters have high viscosity, low solubility of lithium salt and poor compatibility with a negative electrode, and although a large amount of the flame retardant additives (the addition amount is more than or equal to 20 wt.%) have a flame retardant effect, the electrochemical performance of a lithium ion battery can be seriously damaged. In order to improve the compatibility of the flame-retardant electrolyte with the electrode, film-forming additives such as fluoroethylene carbonate (FEC), Vinylene Carbonate (VC), 1, 3-Propane Sultone (PS), Vinyl Ethylene Carbonate (VEC), vinyl sulfate (DTD), etc. are generally added to the electrolyte. The additives can only form an interfacial film on the negative electrode, and the interfacial film is thick and has high impedance, so that the safety and the electrochemical performance of the lithium ion battery cannot be considered when the additives are matched with the flame retardant additives.

Disclosure of Invention

Aiming at the defects of the prior art, the invention aims to provide a composite high-safety electrolyte and a lithium ion battery so as to solve the problem of poor compatibility of a flame retardant additive and the electrochemical performance of the battery.

In order to achieve the purpose, the invention adopts the technical scheme that:

the invention provides a composite high-safety electrolyte, which comprises lithium salt, an organic solvent, a phosphate additive, a silyl acetamide additive and a metal ion compound; the chemical structural formula of the silylacetamide additive is shown as the following formula:

wherein: r1, R2 and R3 are independently selected from H, F, CH₃、C₂H₅、C₃H₇、C₄H₉、CF₃OR; the OR is OH OR OCH₂Or OC₂H₅。

Further, the mass fraction of the silylacetamide additive is 0.01-1%. Within the above-described preferable ratio range, the effect of the composite type high-safety electrolyte can be made more excellent.

Further, the lithium salt includes lithium hexafluorophosphate (LiPF)₆) Lithium hexafluoroarsenate (LiAsF)₆) Lithium tetrafluoroborate (LiBF)₄) Lithium perchlorate (LiClO)₄) Lithium trifluoromethanesulfonate (LiCF)₃SO₃) Lithium difluorophosphate (LiPO)₂F₂) Lithium 2-trifluoromethyl-4, 5-dicyanoimidazolium (LiN)₄C₆F₃) Lithium difluoroborate (LiBC)₂O₄F₂) Lithium chlorotrifluoroborate (LiBF)₃Cl), lithium phosphate (LiP (CO)₂CO₂)₃) Lithium tetrafluoro oxalate phosphate (LiPF)₄(CO₂CO₂) Lithium bis (oxalato) borate (LiB (C))₂O₄)₂) One or two of them; the concentration of the lithium salt is 0.5-2.0 mol/L。

Further, the organic solvent includes at least one of Ethylene Carbonate (EC), Propylene Carbonate (PC), and γ -butyrolactone (GBL). Still further, the organic solvent further includes at least one of butylene carbonate, fluoroethylene carbonate (FEC), ethylmethyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, 1, 3-propane sultone, methyl propionate, methyl butyrate, ethyl acetate, ethyl propionate, propyl propionate, and ethyl butyrate.

Further, the phosphate additive is at least one of methyl dimethyl phosphate, trimethyl phosphate, triethyl phosphate, tributyl tris (2,2, 2-trifluoroethyl) phosphate and tris (2,2, 2-trifluoroethyl) phosphite; the mass fraction of the phosphate additive is 5-20%. Such a high phosphate content can ensure high flame-retardant efficiency and low cost advantage, and in the above preferable ratio range, the effect of the high-safety electrolyte can be made more excellent.

Further, the metal ion compound comprises at least one of ionic compounds of Na, K, Rb, Cs, Mg and Ca; the mass fraction of the metal ion compound is 0.01-1%. Within the above-described preferable ratio range, the effect of the highly safe electrolyte can be made more excellent. Preferably, the metal ion compound is a cesium salt. Still further preferably, the cesium salt is at least one of cesium tetrafluoroborate, cesium perchlorate, cesium nitrate, cesium carbonate, cesium acetate, cesium bistrifluoromethylsulfonyl imide, cesium triflate, cesium difluorooxalate borate, cesium dioxalate borate and cesium methanesulfonate, cesium hexafluorophosphate, cesium tetrafluoroborate, cesium perchlorate, cesium nitrate, cesium carbonate, cesium acetate, cesium bistrifluoromethylsulfonyl imide, cesium trifluoromethanesulfonate, cesium difluorooxalate borate, cesium dioxalate borate, cesium methanesulfonate.

The invention also provides a lithium ion battery which comprises the electrolyte, wherein the electrolyte is the composite high-safety electrolyte.

The invention has the beneficial effects that:

the invention takes the phosphate compound with high flame retardance as an additive, so that the electrolyte has the characteristics of flame retardance or non-combustion; meanwhile, the silyl acetamide additive and the metal ion compound are added to promote film formation on the surfaces of the positive electrode and the negative electrode in a matching manner, so that the defect of poor compatibility between the phosphate compound and the electrode is overcome. The action principle diagram of the silylamide additive and the metal ion compound in the electrolyte is shown in fig. 1, wherein the common structural motif of the silylamide additive is a silicon-based group (Si-N) connected with a nitrogen atom, and the silylamide additive is broken after meeting water due to the fact that the bond energy of the Si-N is weak to generate a cyclic product; therefore, the silyl acetamide additive can spontaneously react with trace water in the electrolyte to form cyclic molecules, and the metal cation compound promotes the ring-opening polymerization of the cyclic molecules to form an ultrathin, uniform and high-ion-conduction interface film on the surfaces of electrodes (a positive electrode and a negative electrode), so that the compatibility of the flame-retardant electrolyte to the positive electrode and the negative electrode is effectively improved, and the electrolyte has good electrochemical performance; on the other hand, the trace amount of water in the electrolyte reacts with lithium salts such as hexafluorophosphoric acid and the like to generate corrosive hydrofluoric acid (HF), the consumption of the lithium salts is accelerated, and structures such as electrodes and current collectors in the battery are damaged.

Drawings

FIG. 1 is a schematic diagram of the operation of the composite high-safety electrolyte in the invention in ring-opening film formation on the surfaces of positive and negative electrodes;

FIG. 2 is a graph showing a comparison of combustion performance of the composite type high-safety electrolyte and the base electrolyte in example 1;

FIG. 3 is a graph showing the first charge-discharge curve of the composite high-safety electrolyte in a graphite negative half-cell in example 2;

fig. 4 is a first charge-discharge curve diagram of the composite high-safety electrolyte in the lithium iron phosphate/graphite full cell in example 3;

FIG. 5 is a first charge-discharge curve diagram of the composite high-safety electrolyte in the ternary/lithium metal soft package battery in example 4;

fig. 6 is a graph of the cycle performance of the composite safety electrolyte in the lithium iron phosphate/graphite full cell in example 5.

Detailed Description

The present invention will be further described with reference to the following examples. It is to be understood that the embodiments described are only a few embodiments of the present invention, and not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

The raw materials used in the following examples and comparative examples are all commercially available products.

Example 1

In the composite high-safety electrolyte of the present embodiment, the lithium salt is LiPF₆The organic solvent is a mixed solvent composed of EC and EMC according to the mass ratio of 3: 7. The preparation method comprises the following specific steps: in an argon-protected glove box, LiPF is added₆Adding the solution into a mixed solvent consisting of EC and EMC to prepare a basic electrolyte with the lithium salt concentration of 1mol/L, and marking the basic electrolyte as a sample 1; then, 10% trimethyl phosphate additive, 0.5% N-methyl-N- (trimethylsilyl) trifluoroacetamide, and 0.5% cesium hexafluorophosphate were added to the base electrolyte, and shaken up to prepare a composite high-safety electrolyte, which was labeled as sample 2.

Example 2

To the base electrolyte in example 1 was added 5% trimethyl phosphate additive, 0.1% N-methyl-N- (trimethylsilyl) trifluoroacetamide and 0.1% cesium hexafluorophosphate, all other things being the same as in example 1, which was labeled as sample 3.

Example 3

To the base electrolyte of example 1 were added 5% triethyl phosphate additive, 0.5% trimethylsilylacetamide and 0.5% cesium hexafluorophosphate, all other things being the same as in example 1, which was labeled as sample 4.

Example 4

To the base electrolyte in example 1 were added 10% triethyl phosphate additive, 0.1% trimethylsilylacetamide and 0.1% cesium nitrate, all other things being the same as in example 1, which was labeled as sample 5.

Example 5

To the base electrolyte in example 1 was added 20% trimethyl phosphate additive, 1% trimethylsilylacetamide and 1% cesium nitrate, all other things being the same as in example 1, which was labeled as sample 6.

Example 6

In the composite high-safety electrolyte of the present embodiment, the lithium salt is LiBF₆The organic solvent is a mixed solvent composed of GBL, PC and DMC according to the mass ratio of 1:1: 1. The preparation method comprises the following specific steps: in an argon-protected glove box, LiPF is added₆The mixture was added to the above mixed solvent to prepare a base electrolyte solution having a lithium salt concentration of 1mol/L, and then 10% trimethyl phosphate additive, 0.5% N-methyl-N- (trimethylsilyl) trifluoroacetamide and 0.5% cesium hexafluorophosphate were added to the base electrolyte solution, and shaken up to prepare a high safety electrolyte solution, which was labeled as sample 7.

Example 7

In the composite high-safety electrolyte of the present embodiment, the lithium salt is LiPF₆The organic solvent is a mixed solvent composed of GBL, FEC and DMC according to the mass ratio of 5:3: 2. The preparation method comprises the following specific steps: in an argon-protected glove box, LiPF is added₆The mixed solvent was added to prepare a base electrolyte solution having a lithium salt concentration of 1mol/L, and then 10% triethyl phosphate additive, 0.5% N-methyl-N- (trimethylsilyl) trifluoroacetamide and 0.5% cesium hexafluorophosphate were added to the base electrolyte solution, and shaking was carried out to prepare a high safety electrolyte solution, which was labeled as sample 8.

Comparative example 1

To the base electrolyte in example 1 was added 10% trimethyl phosphate and 0.5% N-methyl-N- (trimethylsilyl) trifluoroacetamide additive, all other things being the same as in example 1, which was labeled as sample 9.

Comparative example 2

To the base electrolyte in example 1 were added 10% trimethyl phosphate and 0.5% cesium hexafluorophosphate, all other things being the same as in example 1, which was labeled as sample 10.

Comparative example 3

To the base electrolyte in example 1 was added 0.5% N-methyl-N- (trimethylsilyl) trifluoroacetamide and 0.5% cesium hexafluorophosphate, all other things being the same as in example 1, which was labeled as sample 11.

The following performance tests were performed on the products prepared in the above examples and comparative examples, respectively:

(1) electrolyte combustion experiment

And taking out the composite high-safety electrolyte from the glove box, soaking the rubbed glass cotton balls with the mass of 10mg and the diameter of about 3mm in the composite electrolyte, rolling the glass cotton balls on filter paper after taking out, removing the composite electrolyte on the surface, controlling the mass of the cotton balls to be 0.1000g, igniting the cotton balls by open fire, and testing the extinguishing time of the glass cotton balls.

Fig. 2 is a graph comparing the combustion performance of the composite type high-safety electrolyte and the base electrolyte in example 1, and it can be seen from fig. 2 that the average self-extinguishing time of the base electrolyte is as high as 125s/g, which shows a significant flammability characteristic, while the self-extinguishing time of the composite type high-safety electrolyte is about 3s/g, which shows a significant flame-retardant or even non-flammable characteristic.

(2) And (3) electrochemical performance testing:

the compound high-safety electrolyte prepared in the embodiment is utilized to assemble a button full cell or a button half cell, wherein the preparation of the positive electrode comprises the following steps: with LiFePO₄(LFPO) or LiNi_0.8Co_0.1Mn_0.1O₂(NCM811) as an active material, SP as a conductive agent, PVDF as a binder, N-methyl-2-pyrrolidone (NMP) as a dispersant, according to the formula LiFePO₄(or NCM811) and mixing the slurry with SP: PVDF (84: 8: 8) in a mass ratio, and coating the slurry on an aluminum foil to prepare a positive plate; preparation of a negative electrode: graphite is used as an active material, acetylene black is used as a conductive agent, PVDF is used as a binder, NMP is used as a dispersing agent, and the weight percentages of the active material: acetylene black: after the PVDF is mixed with the slurry according to the mass ratio of 8:1:1, coating the slurry on a copper foil to prepare a negative plate; a battery is assembled by taking a polypropylene microporous membrane as a diaphragm, a half-battery counter electrode as a lithium sheet and a composite high-safety electrolyte as an electrolyte.

And (3) carrying out electrochemical performance test at normal temperature, wherein the charge-discharge voltage range of the negative half battery is 0.005-3.0V, the charge-discharge voltage range of the graphite | | | LFPO full battery is 2.2-3.8V, the charge-discharge voltage range of the Li | | | NMC811 metal battery is 2.8-4.3V, and the cycle is carried out for 500 times under the multiplying power of 1C. The negative half-cell impedance spectrum was tested using an electrochemical workstation.

Fig. 3 is a first charge-discharge curve diagram of a graphite negative electrode half-cell assembled by the composite high-safety electrolyte in example 2, and it can be seen from the diagram that the first discharge capacity is close to 348.5mAh/g, the coulombic efficiency is about 95.5%, and the interfacial impedance is 15ohm, while the first coulombic efficiency in the similar graphite negative electrode half-cell in comparative examples 1 and 2 is 85% and 75% respectively, and the interfacial impedance is 40ohm and 82ohm respectively, so that it can be seen that the composite high-safety electrolyte is formed into a low-impedance interfacial film due to the combined action of the silylacetamide and the metal cesium ions, stabilizes the interface between the composite high-safety electrolyte and the graphite negative electrode, and shows good compatibility with the graphite negative electrode.

Fig. 4 is a second-cycle charge-discharge curve of the composite high-safety electrolyte in the lithium iron phosphate/graphite all-cell in example 3, wherein the coulombic efficiency is 97.5%, and it can be seen that the composite high-safety electrolyte can simultaneously maintain good electrochemical stability with the positive and negative electrode materials.

Fig. 5 is a first charge-discharge curve of the composite high-safety electrolyte in the NCM811 positive electrode/lithium metal soft-package battery (> 3.5Ah) in example 4, and it can be seen that the first coulombic efficiency is 91%, indicating that the composite high-safety electrolyte has good compatibility with the ternary positive electrode and the lithium metal negative electrode.

Fig. 6 is a graph of the cycle performance of the composite high-safety electrolyte in the lithium iron phosphate/graphite full cell in example 5, wherein the capacity retention rate after 350 cycles is 84%, and it can be seen that the composite high-safety electrolyte has excellent electrochemical performance while having significant flame retardancy, and maintains good compatibility with the positive and negative electrodes in the lithium ion cell, and the cell performance is good.

The technical features of the embodiments described above may be arbitrarily combined, and for the sake of brevity, all possible combinations of the technical features in the embodiments described above are not described, but should be considered as being within the scope of the present specification as long as there is no contradiction between the combinations of the technical features.

Claims

1. a composite type high-safety electrolyte, is characterized in that: comprise lithium salt, organic solvent, phosphate ester additive, silylacetamide additive and metal ion compound; The chemical structural formula of described silylacetamide additive As shown in the following formula:

Wherein: R1, R2, R3 are each independently selected from any one of H, F, CH ₃ , C ₂ H ₅ , C ₃ H ₇ , C ₄ H ₉ , CF ₃ , OR; the OR is OH, OCH ₂ or OC ₂ H ₅ .

2 . The composite high-safety electrolyte according to claim 1 , wherein the mass fraction of the silylacetamide additive is 0.01-1%. 3 .

3. The composite high-safety electrolyte according to claim 1, wherein the lithium salt comprises lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium tetrafluoroborate, lithium perchlorate, lithium trifluoromethanesulfonate , Lithium difluorophosphate, Lithium 2-trifluoromethyl-4,5-dicyanoimidazolate, Lithium difluorooxalate borate, Lithium chlorotrifluoroborate, Lithium trioxalate phosphate, Lithium tetrafluorooxalate phosphate, Lithium bis-oxalate borate One or both of the above; the concentration of the lithium salt is 0.5 to 2.0 mol/L.

4 . The composite high-safety electrolyte according to claim 1 , wherein the organic solvent comprises at least one of ethylene carbonate, propylene carbonate and γ-butyrolactone. 5 .

5. The composite high-safety electrolyte according to claim 4, wherein the organic solvent further comprises butylene carbonate, fluoroethylene carbonate, ethyl methyl carbonate, dimethyl carbonate, dicarbonate Ethyl ester, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, 1,3-propane sultone, methyl propionate, methyl butyrate, ethyl acetate, ethyl propionate, propyl propionate At least one of ester and ethyl butyrate.

6. The composite high-safety electrolyte according to claim 1, wherein the phosphate ester additive is dimethyl methyl phosphate, trimethyl phosphate, triethyl phosphate, and tributyl phosphate At least one of (2,2,2-trifluoroethyl) phosphate and tris(2,2,2-trifluoroethyl) phosphite; the mass fraction of the phosphate additive is 5-20 %.

7 . The composite high-safety electrolyte according to claim 1 , wherein the metal ion compound comprises at least one of ionic compounds of Na, K, Rb, Cs, Mg, and Ca; the metal ion compound comprises: 8 . The mass fraction of the ionic compound is 0.01 to 1%.

8 . The composite high-safety electrolyte according to claim 1 , wherein the metal ion compound is a cesium salt. 9 .

9 . The composite high-safety electrolyte according to claim 8 , wherein the cesium salt is cesium tetrafluoroborate, cesium perchlorate, cesium nitrate, cesium carbonate, cesium acetate, and bistrifluoromethyl. 10 . Cesium sulfonimide, cesium trifluoromethanesulfonate, cesium difluorooxalate borate, cesium dioxalate borate and cesium methanesulfonate, cesium hexafluorophosphate, cesium tetrafluoroborate, cesium perchlorate, cesium nitrate, cesium carbonate, At least one of cesium acetate, cesium bis-trifluoromethanesulfonimide, cesium trifluoromethanesulfonate, cesium difluorooxalate borate, cesium dioxalate borate, and cesium methanesulfonate.

10. A lithium ion battery, comprising an electrolyte, wherein the electrolyte is the composite high-safety electrolyte according to any one of claims 1-9.