CN115966769A - Local high-concentration lithium metal battery electrolyte and preparation method and application thereof - Google Patents
Local high-concentration lithium metal battery electrolyte and preparation method and application thereof Download PDFInfo
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Abstract
The invention discloses a local high-concentration lithium metal battery electrolyte and a preparation method and application thereof. The electrolyte comprises lithium salt, organic solvent, additive and diluent; the organic solvent comprises at least one of methyl formate, methyl acetate, methyl propionate, methyl butyrate, ethyl propionate, ethyl acetate, ethyl butyrate, methyl trifluoroacetate and ethyl trifluoroacetate; the additive comprises at least one of fluoroethylene carbonate, lithium difluorobis (oxalato) phosphate and lithium difluorophosphate; the diluent is a fluorine-containing ether compound. The local high-concentration electrolyte has the advantages of high voltage resistance of the high-concentration electrolyte and inhibition of corrosion of the sulfimide electrolyte to aluminum foil, simultaneously overcomes the problems of high viscosity, low conductivity, poor wettability with a diaphragm and the like of the high-concentration electrolyte, obviously improves the cycle performance and low-temperature performance of the lithium metal battery, and is particularly suitable for being applied to low-temperature and high-pressure lithium batteries. The preparation method is simple, low in cost and suitable for industrial production.
Description
Technical Field
The invention relates to a metal battery electrolyte, in particular to a local high-concentration lithium metal battery electrolyte, and also relates to a preparation method and application thereof, belonging to the technical field of lithium metal batteries.
Background
The lithium ion battery has the advantages of high energy density, long cycle life, no memory effect and the like, and is widely applied to small electronic equipment such as mobile phones, notebook computers and the like. The method is also popularized in the fields of large-scale energy storage such as smart power grids and electric automobiles. Lithium metal batteries are considered to be next generation high energy density memory devices due to their extremely high theoretical specific capacity (3860 mAh/g) and lowest electrochemical potential (-3.040V compared to standard hydrogen electrode), and in particular high voltage lithium metal batteries assembled from a high voltage positive electrode and lithium metal are receiving increasing attention.
However, at a relatively low temperature, particularly below 0 ℃, the discharge capacity and discharge voltage of the lithium battery are rapidly reduced, and the increasingly growing requirements of portable electronic products and electric automobiles under a low-temperature condition cannot be met. Meanwhile, in the process of charging and discharging the lithium metal battery, the application of high-voltage LMBs is limited due to the uncontrollable growth of lithium dendrites and low Coulombic Efficiency (CE). In addition, with the continuous development of high-voltage positive electrode materials, such as high-nickel LiNi x Co y Mn z O 2 (x + y + z = 1). The electrolyte also plays a very important role in the battery system as a tie connecting the cathode and the anode. Whereas conventional commercial carbonate electrolytes have limited oxidative stability (about 4.3V), limiting their use in high voltage batteries. In addition, under the condition of low temperature, the lower limit of the temperature operating window of the traditional commercial electrolyte is lower (more than or equal to-20 ℃), the lithium ion migration rate is reduced, the impedance is high, and the provided capacity is smaller. Therefore, it is important to develop an electrolyte that is compatible with high voltage materials at low temperature.
Along with the increase of the concentration of the lithium salt, the interaction between the cations and anions of the lithium salt and the solvent is enhanced, the content of free-state solvent molecules is greatly reduced or even disappears, a special salt-solvent coordination structure is formed, a novel electrolyte with a special structure is obtained, the electrochemical window can be widened, the migration rate of the lithium ions is improved, and the reaction kinetic rate is improved, so that the electrochemical window is improved, the migration rate of the lithium ions is reduced, and the reaction kinetic rate is improvedIt is good for high-voltage and low-temperature performance of lithium ion battery. However, the high concentration electrolyte has certain disadvantages, such as high viscosity, low ionic conductivity, high cost of lithium salt, etc., which also limits its further application. The local high-concentration electrolyte is an improvement on the basis of high-concentration electrolyte, i.e. adding insoluble Li into the solution + To reduce the apparent concentration. The method not only ensures that the microstructure of the high-concentration electrolyte is not damaged, but also reduces the viscosity and economic cost of the electrolyte, improves the ionic conductivity and increases more possibilities of the application of the electrolyte in the low-temperature high-voltage field.
Disclosure of Invention
The invention aims to provide a local high-concentration lithium metal battery electrolyte, aiming at the defects of high viscosity, poor wettability, low ionic conductivity, poor low-temperature adaptability and the like of the existing lithium metal battery electrolyte. The electrolyte has high electrochemical window, good conductivity, good low-temperature and high-pressure adaptability, and can be widely applied to low-temperature and high-pressure lithium metal batteries.
The second purpose of the invention is to provide a method for preparing the electrolyte of the local high-concentration lithium metal battery. The method is simple and easy to implement, has low cost and is suitable for industrial production.
The third purpose of the invention is to provide the application of the electrolyte of the local high-concentration lithium metal battery. The electrolyte is used as the lithium metal battery electrolyte, so that the lithium metal battery has high discharge capacity, stable long cycle performance, high coulombic efficiency and good safety performance under the extremely low temperature condition.
In order to achieve the above technical objects, the present invention provides a local high concentration lithium metal battery electrolyte comprising a lithium salt, an organic solvent, an additive and a diluent; the organic solvent comprises at least one of methyl formate, methyl acetate, methyl propionate, methyl butyrate, ethyl propionate, ethyl acetate, ethyl butyrate, methyl trifluoroacetate and ethyl trifluoroacetate; the additive comprises at least one of fluoroethylene carbonate, lithium difluorobis (oxalato) phosphate and lithium difluorophosphate; the diluent is a fluorine-containing ether compound.
According to the invention, a reagent with high dielectric constant, low melting point and low viscosity is used as a solvent of the electrolyte, so that on one hand, more lithium salt can be dissolved in the electrolyte with high local concentration, and the concentration of the lithium salt is improved; on the other hand, the local high-concentration electrolyte is still in a liquid state at a very low temperature state (-70 ℃), the electrolyte is not solidified, and the conductivity is high and the viscosity is low. Meanwhile, by introducing an additive containing special components as a film forming agent and a fluoroether compound as a diluent, the film forming additive can form a stable interfacial film on the surface of the electrode, so that the electrode/electrolyte interface can be effectively stabilized, and the cycle stability of the lithium metal battery is improved; the diluent solves the problems of high viscosity, low conductivity and poor wettability with a diaphragm of a high-concentration electrolyte, improves the cycle performance and low-temperature performance of the lithium metal battery, and can form a layer of passivated solid electrolyte membrane with high fluorine content on the surface of an electrode by the degradation and reduction of the fluoroether compound, thereby effectively improving the cycle stability and coulombic efficiency of the battery. Through the synergistic effect of the special organic solvent, the additive and the diluent, the local high-concentration lithium metal battery electrolyte has better electrochemical window and conductivity, and can adapt to low-temperature and high-pressure conditions.
As a preferred embodiment, the lithium salt includes lithium bistrifluoromethanesulfonylimide and/or lithium bisfluorosulfonylimide.
As a preferred embodiment, the organic solvent comprises methyl propionate. The solvent has higher dielectric constant and low melting point, and ensures that more lithium salt is dissolved as far as possible while not solidifying at low temperature.
Preferably, the additive comprises fluoroethylene carbonate to protect to improve the intercalation kinetics of Li +, change the chemical composition and properties of the interfacial film on the surfaces of the positive electrode and the negative electrode, and facilitate the operation of the battery under the conditions of low temperature and high voltage.
As a preferred embodiment, the diluent comprises 2, 2-trifluoroethyl-1, 2-tetrafluoroethyl ether.
In a preferred embodiment, the local concentration of the lithium salt in the electrolyte is 2 to 7mol/L, preferably 3 to 5mol/L, more preferably 3 to 4mol/L, and still more preferably 3mol/L.
As a preferable scheme, the additive accounts for 1 to 20 percent of the volume of the electrolyte, and is preferably 10 percent.
As a preferable scheme, the diluent accounts for 30-70% of the electrolyte by volume, and preferably 45-55% of the electrolyte by volume.
The invention also provides a preparation method of the local high-concentration lithium metal battery electrolyte, which comprises the steps of mixing lithium salt and an organic solvent to form a lithium salt solution, and then sequentially adding an additive and a diluent to obtain the local high-concentration lithium metal battery electrolyte.
The local high-concentration lithium metal battery electrolyte developed and prepared by the invention has the conductivity of 0.655-0.22 ms/cm at the temperature of minus 40 ℃.
In the preparation process, the lithium salt is coordinated with the organic solvent to form a stable lithium salt solution, the additive and the diluent can be mutually dissolved with the organic solvent, the viscosity of the electrolyte can be reduced, the conductivity of the electrolyte and the wettability of a diaphragm can be improved, and the cycle performance and the low-temperature performance of the lithium metal battery can be improved, wherein the additive can form a stable interface film on the surface of an electrode, so that the electrode/electrolyte interface can be effectively stabilized, and the cycle stability of the lithium metal battery can be improved; the degradation and reduction of the fluoroether compound can form a layer of passivated solid electrolyte membrane with high fluorine content on the surface of the electrode, and can effectively improve the cycling stability and coulomb efficiency of the battery.
Preferably, the concentration of the lithium salt solution is 3 to 5mol/L.
The invention also provides an application of the local high-concentration lithium metal battery electrolyte, which is used as the lithium metal battery electrolyte. The electrolyte is used as the lithium metal battery electrolyte, so that the lithium battery has high discharge capacity, stable long cycle performance, high coulombic efficiency and good safety performance under the extremely low temperature condition, and the defects of low-temperature energy loss, small voltage range, serious lithium dendrite and the like of the lithium metal battery are effectively overcome.
The electrolyte developed by the invention is used for constructing an NCM811 Li battery, and the battery discharges at the temperature of minus 40 ℃ and the discharge rate of 0.2C, and the specific discharge capacity of the battery is 155-143 mAh/g.
The electrolyte developed by the invention is used for constructing an NCM811 Li battery, and when the volume ratio of the solvent, the additive and the diluent is 4:1: and 5, when the concentration of the lithium salt in the organic solvent is 3mol/L, the specific discharge capacity of the obtained product at the temperature of 50 ℃ below zero is 115mAh/g.
The electrolyte developed by the invention is used for constructing an NCM811| | | Li battery, when the volume ratio of the solvent, the additive and the diluent is 4:1: and 5, when the concentration of the lithium salt in the organic solvent is 3mol/L, the obtained product can still be stably and rapidly charged and discharged under the conditions of-40 ℃ and the voltage range of 2.8-4.6V, the specific capacity reaches 152mAh/g, the long-term circulation can be carried out for 50 circles, and the coulombic efficiency is more than 99.9%. The effect is far superior to the prior art and other schemes in the development process of the technology.
The lithium metal battery provided by the invention is based on the fact that a ternary material is used as a positive electrode material, a metal lithium sheet is used as a negative electrode, and stable, rapid and reversible long-acting charge-discharge circulation is carried out at the temperature of minus 40 ℃ and the voltage range of 2.8-4.6V.
Compared with the prior art, the invention has the following beneficial effects:
(1) The local high-concentration electrolyte has the advantages of high voltage resistance of the high-concentration electrolyte and inhibition of corrosion of the sulfimide electrolyte to aluminum foil, and meanwhile, the problems of high viscosity, low conductivity, poor wettability with a diaphragm and the like of the high-concentration electrolyte are solved, the cycle performance and low-temperature performance of the lithium metal battery are greatly improved, the defects of low-temperature energy loss, small voltage range, serious lithium dendrites and the like of the lithium metal battery are effectively improved, and the local high-concentration electrolyte is particularly suitable for being applied to low-temperature high-voltage lithium batteries;
(2) The method selects a reagent with high dielectric constant, low melting point and small viscosity as a lithium salt solvent, can dissolve more lithium salt, improves the concentration of local lithium salt, ensures that the electrolyte is still liquid at a very low temperature state (-70 ℃), does not generate a solidification phenomenon, and has high conductivity and low viscosity;
(3) The electrolyte is introduced with a film forming additive with specific components and a fluoroether compound diluent, wherein the film forming additive can form a stable interfacial film on the surface of an electrode, so that the electrode/electrolyte interface can be effectively stabilized, and the cycle stability of the lithium metal battery is improved; the degradation and reduction of the fluoroether compound can form a layer of passivated solid electrolyte membrane with high fluorine content on the surface of the electrode, thereby effectively improving the cycling stability and coulomb efficiency of the battery;
(4) The preparation process is simple, the cost is low, and the method is suitable for industrial production.
Drawings
FIG. 1 is a graph showing electrochemical windows of the electrolytes obtained in examples 1 to 3 and comparative example 4.
FIG. 2 is a graph showing the conductivity measurements of the electrolytes prepared in examples 1 to 3 at different temperatures.
FIG. 3 is a graph showing discharge characteristics at-40 ℃ at 0.2C discharge rate of NCM 811. Mu.l Li batteries using the electrolytes prepared in examples 1 to 3 and comparative examples 1 and 4.
Fig. 4 is a comparison graph of 0.2C rate discharge test curves at different temperatures for NCM811| | | Li batteries using the electrolyte prepared in example 1.
FIG. 5 is a graph showing cycle performance and coulombic efficiency of the NCM 811I Li battery using the electrolyte prepared in example 1 at-40 ℃ and a voltage range of 0.2C to 2.8-4.6V.
FIG. 6 is a top scanning electron micrograph of copper plated with lithium using the electrolyte prepared in example 1; (b) Is a top scanning electron micrograph of copper plated with lithium using the electrolyte prepared in example 2; (c) Is a top scanning electron micrograph of copper plated with lithium using the electrolyte prepared in comparative example 3.
FIG. 7 is a graph showing discharge characteristics at-40 ℃ at 0.2C discharge rate of NCM 811. Mu.l Li batteries using the electrolytes prepared in example 1 and comparative examples 5 and 6.
FIG. 8 is a graph showing discharge characteristics at-40 ℃ at 0.2C discharge rate of NCM 811. Mu.l Li batteries using the electrolytes prepared in example 1 and comparative example 7.
Detailed Description
The present invention is further illustrated by the following examples, which are not intended to limit the invention to these embodiments. It will be appreciated by those skilled in the art that the present invention encompasses all alternatives, modifications and equivalents as may be included within the scope of the claims.
Example 1
A local high-concentration electrolyte at low temperature and high pressure takes lithium bistrifluoromethanesulfonylimide as lithium salt, methyl propionate as a solvent, fluoroethylene carbonate as an additive and 2, 2-trifluoroethyl-1, 2-tetrafluoroethyl ether as a diluent. The preparation method comprises the following steps: dissolving lithium bis (trifluoromethyl) sulfonyl imide in dimethyl carbonate to reach the concentration of 3mol/L, and stirring and dissolving the lithium salt to obtain the high-concentration electrolyte. Subsequently, fluoroethylene carbonate was added to the solution and stirred until dissolved. And finally, adding 2, 2-trifluoroethyl-1, 2-tetrafluoroethyl ether into the solution, and uniformly stirring to obtain a local high-concentration electrolyte, wherein the obtained local high-concentration electrolyte is referred to as 3MMFH electrolyte. Wherein the volume ratio of the solvent to the additive to the diluent is 4:1:5.
example 2
A high-pressure low-temperature local high-concentration electrolyte takes bis (trifluoromethane) sulfonyl imide lithium as lithium salt, methyl propionate as a solvent, fluoroethylene carbonate as an additive and 2, 2-trifluoroethyl-1, 2-tetrafluoroethyl ether as a diluent. The preparation method comprises the following steps: dissolving lithium bis (trifluoromethyl) sulfonyl imide in dimethyl carbonate to reach the concentration of 4mol/L, and uniformly stirring and dissolving lithium salt to obtain the high-concentration electrolyte. Subsequently, fluoroethylene carbonate was added to the solution and stirred until dissolved. And finally, adding 2, 2-trifluoroethyl-1, 2-tetrafluoroethyl ether into the solution, and uniformly stirring to obtain a local high-concentration electrolyte, wherein the obtained local high-concentration electrolyte is referred to as 4MMFH electrolyte. Wherein the volume ratio of the solvent to the additive to the diluent is 4:1:5.
example 3
A high-pressure low-temperature local high-concentration electrolyte takes lithium bistrifluoromethanesulfonylimide as a lithium salt, methyl propionate as a solvent, fluoroethylene carbonate as an additive and 2, 2-trifluoroethyl-1, 2-tetrafluoroethyl ether as a diluent. The preparation method comprises the following steps: dissolving lithium bis (trifluoromethyl) sulfonyl imide in dimethyl carbonate to reach the concentration of 5mol/L, and stirring and dissolving the lithium salt to obtain the high-concentration electrolyte. Subsequently, fluoroethylene carbonate was added to the solution and stirred until dissolved. And finally, adding 2, 2-trifluoroethyl-1, 2-tetrafluoroethyl ether into the solution, and uniformly stirring to obtain a local high-concentration electrolyte, wherein the obtained local high-concentration electrolyte is referred to as 5MMFH electrolyte. Wherein the volume ratio of the solvent to the additive to the diluent is 4:1:5.
comparative example 1
A common low-temperature electrolyte is prepared from lithium hexafluorophosphate as lithium salt and ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate as solvent. The preparation method comprises the following steps: ethylene carbonate, dimethyl carbonate and methyl ethyl carbonate are mixed according to the volume ratio of 1:1:1, and then dissolving lithium bistrifluoromethylsulfonyl imide in the solution to make the concentration of the lithium bistrifluoromethylsulfonyl imide reach 1mol/L, wherein the obtained electrolyte is abbreviated as EDD111.
Comparative example 2
A low-concentration electrolyte takes bis (trifluoromethane sulfonyl) imide lithium as a lithium salt, methyl propionate as a solvent and fluoroethylene carbonate as an additive. The preparation method comprises the following steps: lithium bistrifluoromethylsulfonyl imide was dissolved in dimethyl carbonate to a concentration of 1mol/L. Subsequently, fluoroethylene carbonate is added to the solution, and the solution is uniformly stirred to obtain a low-concentration electrolyte. The resulting low concentration electrolyte is referred to as 1MMF electrolyte. Wherein the volume ratio of the solvent to the additive is 4:1.
comparative example 3
A low-concentration electrolyte is prepared from lithium bis (trifluoromethane sulfonyl) imide as lithium salt, methyl propionate as solvent, fluoroethylene carbonate as additive and 2, 2-trifluoroethyl-1, 2-tetrafluoroethyl ether as diluent. The preparation method comprises the following steps: lithium bistrifluoromethylsulfonyl imide was dissolved in dimethyl carbonate to a concentration of 1mol/L, and the lithium salt was uniformly stirred and dissolved. Subsequently, fluoroethylene carbonate and 2, 2-trifluoroethyl-1, 2-tetrafluoroethyl ether were added to the solution, and the mixture was stirred uniformly to obtain a low-concentration electrolyte. The obtained low-concentration electrolyte is called as 1MMFH electrolyte for short. Wherein the volume ratio of the solvent to the additive to the diluent is 4:1:5.
comparative example 4
A low-concentration electrolyte is prepared from lithium bis (trifluoromethane sulfonyl) imide as lithium salt, methyl propionate as solvent, fluoroethylene carbonate as additive and 2, 2-trifluoroethyl-1, 2-tetrafluoroethyl ether as diluent. The preparation method comprises the following steps: lithium bistrifluoromethylsulfonyl imide was dissolved in dimethyl carbonate to a concentration of 2mol/L, and the lithium salt was uniformly stirred and dissolved. Subsequently, fluoroethylene carbonate and 2, 2-trifluoroethyl-1, 2-tetrafluoroethyl ether were added to the solution, and the mixture was stirred uniformly to obtain a low-concentration electrolyte. The obtained low-concentration electrolyte is called 2MMFH electrolyte for short. Wherein the volume ratio of the solvent to the additive to the diluent is 4:1:5.
comparative example 5
A local high-electrolyte is prepared from lithium bis (trifluoromethane sulfonyl) imide as lithium salt, methyl propionate as solvent, fluoroethylene carbonate as additive and 2, 2-trifluoroethyl-1, 2-tetrafluoroethyl ether as diluent. The preparation method comprises the following steps: dissolving lithium bis (trifluoromethyl) sulfonyl imide in dimethyl carbonate to reach the concentration of 3mol/L, and stirring and dissolving the lithium salt to obtain the high-concentration electrolyte. Subsequently, fluoroethylene carbonate was added to the solution and stirred until dissolved. Finally, 2-trifluoroethyl-1, 2-tetrafluoroethyl ether is added into the solution and uniformly stirred to obtain a local high-concentration electrolyte, and the obtained local high-concentration electrolyte is referred to as a comparative example 5 electrolyte. Wherein the volume ratio of the solvent, the additive and the diluent is 2:1:7.
comparative example 6
A local high-electrolyte is prepared from lithium bis (trifluoromethane sulfonyl) imide as lithium salt, methyl propionate as solvent, fluoroethylene carbonate as additive and 2, 2-trifluoroethyl-1, 2-tetrafluoroethyl ether as diluent. The preparation method comprises the following steps: dissolving lithium bis (trifluoromethyl) sulfonyl imide in dimethyl carbonate to reach the concentration of 3mol/L, and stirring and dissolving the lithium salt to obtain the high-concentration electrolyte. Subsequently, fluoroethylene carbonate was added to the solution and stirred until dissolved. Finally, 2-trifluoroethyl-1, 2-tetrafluoroethyl ether is added into the solution and uniformly stirred to obtain a local high-concentration electrolyte, and the obtained local high-concentration electrolyte is referred to as the electrolyte of comparative example 6. Wherein the volume ratio of the solvent to the additive to the diluent is 6:1:3.
comparative example 7
A local high-electrolyte is prepared from lithium bis (trifluoromethane sulfonyl) imide as lithium salt, methyl propionate as solvent and 2, 2-trifluoroethyl-1, 2-tetrafluoroethyl ether as diluent. The preparation method comprises the following steps: dissolving lithium bis (trifluoromethyl) sulfonyl imide in dimethyl carbonate to reach the concentration of 3mol/L, and stirring and dissolving the lithium salt to obtain the high-concentration electrolyte. Subsequently, 2-trifluoroethyl-1, 2-tetrafluoroethyl ether was added to the solution, and the mixture was stirred uniformly to obtain a locally high-concentration electrolyte, and the obtained locally high-concentration electrolyte was referred to as a comparative example 7 electrolyte. Wherein the volume ratio of the solvent to the diluent is 4:5.
as shown in fig. 1, there is a graph of electrochemical windows obtained by a linear voltammetry scan test using a local high concentration electrolyte of examples 1,2, 3 of the present invention and comparative example 4. As can be seen from the figure, the prepared local high concentration electrolyte is very stable under 5V and does not decompose, the upper limit of the electrochemical window is increased and the oxidation resistance of the electrolyte is increased compared with the electrochemical window of the conventional commercial carbonate of 4.3V, which has been reported. Meanwhile, with the increase of the concentration of lithium salt, the upper limit of an electrochemical window is increased, the oxidation resistance of the electrolyte is increased, and the passivation performance of the electrolyte on the aluminum foil is also enhanced.
FIG. 2 is a graph showing the conductivity of the locally high-concentration electrolyte used in examples 1,2 and 3 of the present invention. At-40 ℃, the conductivities of the examples 1,2 and 3 are respectively 0.652, 0.351 and 0.223ms/cm, the ionic conductivity is very considerable, and the batteries made of the electrolyte can provide good electrochemical performance at extremely low temperature. Meanwhile, when the temperature is reduced to-70 ℃, the observed electrolyte is still not in a liquid state, and the phenomena of solidification and the like do not occur, so that the smooth operation of the battery in an extremely low temperature state is guaranteed.
The electrolytes of examples 1 to 3 and comparative examples 1 and 4 were used to construct NCM811 Li batteries, and the batteries were tested for discharge performance at-40 ℃ at a discharge rate of 0.2C. As shown in figure 3, the low-temperature discharge performance of the battery is greatly improved by adopting the local high-concentration electrolyte, the specific discharge capacities of 3MMFH, 4MMFH and 5MMFH are respectively 155mAh/g, 143mAh/g and 74mAh/g, while the low-concentration electrolyte 2MMFH used as a comparison is only 113mAh/g, and the low-temperature electrolyte EDD111 of a common comparative example 1 is only 40mAh/g. When the temperature is reduced to-50 ℃, the specific discharge capacity of the local high-concentration electrolyte 3MMFH is still 115mAh/g, while the comparative example 1 can not normally discharge at-50 ℃.
The electrolyte of example 1 was used to construct an NCM811 Li battery and the battery was tested for discharge performance at different temperatures at a current density of 0.2C. As shown in FIG. 4, the specific discharge capacity of the NCM 811I Li metal battery adopting the 3MMFH local high-concentration electrolyte at-20 ℃, 30 ℃ and 40 ℃ is respectively 194, 174 and 155mAh/g, and respectively 92%, 82% and 73% of the room-temperature capacity, and the specific discharge capacity at-50 ℃ can still reach 115mAh/g. Meanwhile, as shown in fig. 5, the NCM811| | | Li metal battery manufactured by using the local high-concentration electrolyte in example 1 can still perform stable and rapid charge and discharge at-40 ℃ and a voltage range of 2.8 to 4.6V, has a specific capacity of 152mAh/g, can perform long-term circulation for 50 cycles, and has a coulombic efficiency of more than 99.9%. Under the condition of room temperature, in the long circulation process with the current density of 0.5C and the voltage range of 2.8-4.6V, the comparative example 2 hardly reaches the charging voltage of 4.6V, the coulomb efficiency of the comparative example 3 in the circulation process is gradually reduced, even the overcharge phenomenon caused by the short circuit of the battery occurs, and finally the long-term circulation cannot be effectively carried out. In contrast, in examples 1 and 2, the coulombic efficiency is stable at room temperature and in the voltage range of 2.8-4.6V, and stable long-cycle operation can be performed.
The protective effect of examples 1 and 2 and comparative example 3 on lithium metal negative electrodes was investigated by making Li | | Cu cells. The current density was set to 1mA cm -2 Capacity of 5mAh cm -2 The Li | | Cu battery is disassembled, scanning Electron Microscope (SEM) analysis is carried out on Cu, and the deposition morphology of Li on the surface of Cu is analyzed. As shown in fig. 6, fig. 6 (c) shows that a large amount of needle-shaped dendrites and moss-shaped lithium are present in comparative example 3 to form a loose and thick deposit form, and the lithium easily penetrates the separator to consume an electrolyte, causing a short circuit and large polarization of the battery. Compared withIn fig. 6 (a) and 6 (b) examples 1 and 2, a flat and thin profile with a uniform and flat surface is obtained. The uniform and dense lithium deposition makes it less likely to penetrate the separator, reducing the reaction area of the lithium with the electrolyte. The electrolytes in example 1 and comparative examples 5 and 6 were used to construct NCM811 Li batteries, and the batteries were tested for discharge performance at-40 ℃ at a 0.2C discharge rate. As shown in FIG. 7, the local high concentration electrolyte 3MMFH prepared by adopting the optimized proportion can release a discharge capacity of 155mAh/g at-40 ℃, while the comparative example 5 with less solvent and more diluent can only release a discharge capacity of 139mAh/g at-40 ℃; when the solvent is more and the diluent is less, the electrolyte still remains in a high-concentration electrolyte state, the viscosity is higher, the conductivity is lower, the special local high-concentration electrolyte is not completely formed, and the electrochemical performance is reduced. While comparative example 6, which has a larger amount of solvent and a smaller amount of diluent, emits a discharge capacity of only 142mAh/g at-40 ℃. Because the diluent can not dissolve the lithium salt, but can affect the network structure of the solvent, when the solvent is less and the diluent is more, the more diluent destroys the structure of the solvent, thereby affecting the special coordination between the salt and the solvent, and reducing the electrochemical performance.
The electrolytes in example 1 and comparative example 7 were used to construct NCM811 Li batteries and the batteries were tested for discharge performance at-40 ℃ at 0.2C discharge rate. As shown in FIG. 8, the partial high concentration electrolyte 3MMFH prepared using the preferred ratio can discharge a discharge capacity of 155mAh/g at-40 deg.C, and comparative example 7, in which no additive fluoroethylene carbonate was added, can discharge a discharge capacity of only 133mAh/g at-40 deg.C. After a better cathode film forming additive fluoroethylene carbonate is added, a passivation film for inhibiting the corrosion of the aluminum current collector can be generated preferentially, and better high-pressure stability is realized to a certain extent; on the other hand, it can also change the uniformity of the CEI film, thereby optimizing the electrochemical stability and low temperature performance of the electrolyte.
Therefore, the lithium metal battery provided by the invention is based on the ternary material as the anode material and the metal lithium sheet as the cathode, and performs stable, rapid and reversible long-acting charge-discharge cycle at the temperature of minus 40 ℃ and the voltage range of 2.8-4.6V.
The above embodiments are merely preferred embodiments of the present invention, and the present invention is not limited to the above embodiments, and modifications and changes thereof may be made by those skilled in the art within the scope of the claims of the present invention.
Claims (10)
1. A local high-concentration lithium metal battery electrolyte is characterized in that: the electrolyte comprises lithium salt, organic solvent, additive and diluent; the organic solvent comprises at least one of methyl formate, methyl acetate, methyl propionate, methyl butyrate, ethyl propionate, ethyl acetate, ethyl butyrate, methyl trifluoroacetate and ethyl trifluoroacetate; the additive comprises at least one of fluoroethylene carbonate, lithium difluorobis (oxalato) phosphate and lithium difluorophosphate; the diluent is a fluorine-containing ether compound;
the concentration of the lithium salt in the organic solvent is 2-7 mol/L, preferably 3-5 mol/L;
the additive accounts for 1-20% of the electrolyte by volume;
the diluent accounts for 30 to 70 percent of the volume of the electrolyte;
the lithium salt comprises at least one of lithium bistrifluoromethanesulfonylimide, lithium tetrafluoroborate, lithium difluorooxalate borate and lithium dioxalate borate, and the lithium salt is insoluble in a diluent.
2. The local high concentration lithium metal battery electrolyte of claim 1, wherein: the fluorine-containing ether compound comprises 2, 2-trifluoroethyl-1, 2-tetrafluoroethyl ether 1, 2-tetrafluoroethyl-2, 3, -tetrafluoropropyl ether, 1,1,1,3,3,3-hexafluoroisopropyl methyl ether 2, 2-trifluoroethyl ether and 1, 2-tetrafluoroethyl methyl ether.
3. The local high concentration lithium metal battery electrolyte of claim 1, wherein: the concentration of the lithium salt in the organic solvent is 3-4 mol/L.
4. The local high concentration lithium metal battery electrolyte of claim 1, wherein: the additive accounts for 10% of the volume of the electrolyte.
5. The local high concentration lithium metal battery electrolyte of claim 1, wherein: the diluent accounts for 45-55% of the electrolyte by volume.
6. The method of preparing a localized high concentration lithium metal battery electrolyte of any of claims 1 to 5, wherein: mixing lithium salt and an organic solvent to form a lithium salt solution, and then sequentially adding an additive and a diluent to obtain a local high-concentration lithium metal battery electrolyte; the concentration of the lithium salt in the organic solvent is 2 to 7mol/L, preferably 3 to 5mol/L, and more preferably 3 to 4mol/L.
7. Use of a localized high concentration lithium metal battery electrolyte as claimed in any one of claims 1 to 6 wherein: as an electrolyte for lithium metal batteries.
8. Use of a localized high concentration lithium metal battery electrolyte as claimed in any one of claims 1 to 6 wherein: the developed electrolyte is used for constructing an NCM811 Li battery, and the battery discharges at the temperature of minus 40 ℃ and the discharge rate of 0.2C, and the specific discharge capacity of the battery is 155-143 mAh/g.
9. Use of a localized high concentration lithium metal battery electrolyte according to claim 8, wherein: the developed electrolyte is used for constructing an NCM811 Li battery, and when the volume ratio of the solvent, the additive and the diluent is 4:1: and 5, when the concentration of the lithium salt in the organic solvent is 3mol/L, the specific discharge capacity of the obtained product at the temperature of 50 ℃ below zero is 115mAh/g.
10. Use of a localized high concentration lithium metal battery electrolyte according to claim 8, wherein: the developed electrolyte is used for constructing an NCM811 Li battery, and when the volume ratio of the solvent, the additive and the diluent is 4:1: and 5, when the concentration of the lithium salt in the organic solvent is 3mol/L, the obtained product can still perform stable and rapid charge and discharge under the conditions of-40 ℃ and the voltage range of 2.8-4.6V, the specific capacity reaches 152mAh/g, the lithium salt can be circulated for 50 circles in a long term, and the coulombic efficiency is more than 99.9 percent.
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