A method for manufacturing a lithium metal battery and a lithium metal battery

Abstract

The application discloses a lithium metal battery preparation method and a lithium metal battery. The lithium metal battery preparation method comprises the following steps: S1, taking a to-be-processed positive electrode and a graphite negative electrode to assemble a double-electrode system, wherein the to-be-processed positive electrode is a working electrode, the graphite is a counter electrode, and the double-electrode system comprises an electrolyte; the electrolyte is composed of specific components; S2, after a preset number of charge-discharge cycles, a stable CEI film is formed on the surface of the positive electrode, then the positive electrode is taken out, washed and dried to remove the residual electrolyte on the surface, the CEI film is preserved, and the treatment of the positive electrode of the lithium metal battery is completed; S3, the treated positive electrode and a lithium metal negative electrode are assembled into a lithium metal battery; and S4, without formation, direct capacity distribution is performed. Through the steps of pre-assembling the positive electrode into a graphite system double-electrode, performing charge-discharge cycles, pre-forming a stable CEI film on the surface of the positive electrode, and assembling the positive electrode with the surface film into a lithium metal battery, the cycle performance of the lithium metal battery is effectively improved.

Description

Technical Field

[0001] This invention relates to the field of lithium battery technology, and in particular to a method for preparing a lithium metal battery and the lithium metal battery itself. Background Technology

[0002] Lithium-ion batteries, which use graphite as the negative electrode, no longer meet the growing energy density demands. Lithium metal batteries, using lithium metal as the negative electrode, achieve a specific capacity of 3860 mAh / g, more than ten times that of graphite. Furthermore, the use of metallic lithium compensates for lithium loss at the positive electrode, improving lithium utilization.

[0003] However, lithium metal batteries have poor cycle performance, with only about one-third the number of cycles compared to lithium-ion batteries. The industry generally attributes this decline in cycle count to the efficiency of the lithium metal anode. However, through long-term experimental research, the inventors have reached a conclusion contrary to current industry consensus: the fundamental cause of lithium metal battery cycle degradation lies not in the lithium metal anode, but in the deterioration of the composition of the positive electrode CEI film.

[0004] The specific mechanism is as follows: Lithium ions, solvent molecules, and anions form a lithium solvation structure, which first reacts with lithium metal to form a large amount of sulfur-containing selenium ion (SEI) film on the lithium metal surface. Because a large amount of sulfur in the lithium solvation structure is consumed by the lithium metal, the sulfur content in the CEI film formed at the positive electrode is very low, leading to a rapid decline in the cycle performance of the lithium metal battery. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a method for preparing a lithium metal battery and a lithium metal battery, so as to solve the problem that the low fluorine content in the CEI film of the positive electrode of existing lithium metal batteries leads to the deterioration of the cycle performance of lithium metal batteries.

[0006] The technical solution adopted by the present invention to solve the above-mentioned technical problems is as follows:

[0007] A method for preparing a lithium metal battery includes the following steps:

[0008] S1. Take the positive electrode to be treated and the graphite negative electrode and assemble them into a dual electrode system, wherein the positive electrode to be treated is the working electrode and the graphite is the counter electrode. The dual electrode system contains an electrolyte, which is composed of specific components.

[0009] S2. After a preset number of charge-discharge cycles, a stable CEI film is pre-formed on the surface of the positive electrode. Then, the positive electrode is removed, cleaned, and dried to remove any electrolyte residue on the surface, while the CEI film is preserved, thus completing the treatment of the positive electrode of the lithium metal battery.

[0010] S3. Assemble the treated positive electrode and lithium metal negative electrode into a lithium metal battery.

[0011] S4. No formation is required; direct separation is performed.

[0012] Preferably, the electrolyte comprises a lithium salt and a solvent, wherein the lithium salt comprises one or more of LiPF6, LiFSI, LiTFSI, LiDFOB, LiBOB and LiBF4.

[0013] Preferably, the solvent includes one or more of the cyclic carbonate solvent FEC and the chain carbonate solvents FEMC, TFEC, DMC, EMC, and DEC; the volume ratio of the cyclic carbonate to the total volume of the cyclic carbonate and chain carbonate is in the range of 20% to 100%.

[0014] Preferably, the molar concentration of lithium salt in the electrolyte is 1M to 5M.

[0015] Preferably, the electrolyte further includes a diluent, which includes one or more of hydrofluoroethers such as TTE, TFEO, and BTFE, and fluoroaromatic compounds such as 1,2-2-fluorobenzene, and the diluent accounts for 0-90% of the electrolyte volume.

[0016] Preferably, the electrolyte further includes additives for inhibiting the dissolution of metal ions, the additives including one or more of TMSB, TMSP, TMSPi, succinic acid, adiponitrile, 1,3,6-hexanetricarbonyl (HTCN), and EGBE.

[0017] Preferably, the additive accounts for 0-5% of the total mass of the electrolyte.

[0018] Preferably, the charge-discharge cycle process is as follows: first, charge the voltage to the rated voltage with a small constant current of 0.01C-0.1C, then maintain the voltage at a constant voltage for 1 hour, and then discharge the voltage to 3V with a larger current of 0.1C-1C, with the cycle number being 1 to 3 times.

[0019] Preferably, the cleaning process in step S2 is carried out in a dew point room, the cleaning agent is pure DMC solvent, the temperature is room temperature, the drying environment is a pure oxygen atmosphere, the temperature is 40℃-60℃, and the drying time is 24h.

[0020] A lithium battery comprising a lithium metal battery prepared by the method described above.

[0021] Compared with the prior art, the advantages of the lithium metal battery preparation method and the lithium metal battery of the present invention are as follows:

[0022] (1) By pre-assembling the positive electrode and graphite into a dual-electrode system for charge-discharge cycling, a fluorine-rich CEI film is pre-formed on the surface of the positive electrode, which improves the problem of deterioration of the cycle performance of the positive electrode CEI film due to lack of fluorine caused by directly using the positive electrode to assemble lithium metal batteries, thereby improving the cycle performance of lithium metal batteries.

[0023] (2) The applicant discovered that by using an electrolyte containing fluorinated lithium salt and a graphite counter electrode, the positive electrode CEI film can be enriched with fluorine during the pretreatment process, and the higher the concentration of the fluorinated lithium salt, the higher the fluorine content in the positive electrode CEI film. By adding a high concentration of lithium salt, the fluorine content in the positive electrode CEI film can be maintained at a high level.

[0024] (3) This application uses cyclic carbonate solvent FEC instead of EC, which differs from the traditional commercial EC plus linear carbonate. Traditional commercial EC results in a large amount of coordination between EC and lithium ions in the lithium solvation structure, forming a large amount of harmful phases such as lithium carbonate in the cathode CEI film. At the same time, the fluorine element can only be obtained through additives and has a low content. In addition, EC and the cathode undergo side reactions, leading to a deterioration of the CEI film. However, by using FEC, the product in the cathode CEI becomes LiF, a dense and flexible beneficial phase, instead of lithium carbonate. At the same time, it reduces the side reactions with the cathode, thereby greatly improving the composition of the cathode CEI film.

[0025] (4) By using a diluent, the shortcomings of high-concentration solvent viscosity and poor wettability are improved, ensuring cycle stability during the positive electrode treatment. In addition, the diluent contains a large amount of F element, which can form an F-rich CEI film during the charging process through oxidation and decomposition of the positive electrode.

[0026] (5) The addition of additives improves the problem of excessive metal ion dissolution that may occur in the positive electrode during the treatment process. Furthermore, the introduction of elements such as B, N, or P into the positive electrode CEI film increases the ionic conductivity of the CEI film and further improves the cycle performance of lithium metal batteries.

[0027] (6) During the positive electrode treatment, a small current charging method is used to allow the positive electrode CEI film to form slowly and achieve a uniform effect. When the voltage reaches the rated voltage, it is maintained at a constant voltage for 1 hour to ensure that the CEI film is sufficiently rich and stable. A large current discharge is used to shorten the time required for the CEI film dissolution process, resulting in less dissolution. Cycling for 1-3 weeks further stabilizes the positive electrode CEI film. This charge-discharge method ensures a uniform, rich, and stable positive electrode CEI film.

[0028] (7) The dry environment is selected as pure oxygen atmosphere, and the temperature is selected as 40℃-60℃. On the one hand, it can remove the residual DMC cleaning agent on the positive electrode surface, and on the other hand, it can protect the CEI film and prevent the CEI film from being reduced by reducing substances such as CO in the air, so that the CEI film is more stable.

[0029] (8) This preparation method does not require a chemical formation process and can be directly divided into volumes, saving chemical formation costs. Detailed Implementation

[0030] The present invention will be further described in detail below with reference to the embodiments.

[0031] Example

[0032] A method for preparing a lithium metal battery includes the following steps:

[0033] S1. Take the positive electrode to be treated and the graphite negative electrode and assemble them into a dual electrode system, wherein the positive electrode to be treated is the working electrode and the graphite is the counter electrode. The dual electrode system contains an electrolyte, which is composed of specific components.

[0034] S2. After a preset number of charge-discharge cycles, a stable CEI film is pre-formed on the surface of the positive electrode. Then, the positive electrode is removed, cleaned, and dried to remove any electrolyte residue on the surface, while the CEI film is preserved, thus completing the treatment of the positive electrode of the lithium metal battery.

[0035] S3. Assemble the treated positive electrode and lithium metal negative electrode into a lithium metal battery.

[0036] S4. No formation is required; direct separation is performed.

[0037] The charge-discharge cycle process in step S2 is as follows: first, charge the voltage to the rated voltage with a small constant current of 0.01C-0.1C, then maintain the voltage for 1 hour, and then discharge to 3V with a larger current of 0.1C-1C. The number of cycles is 1 to 3.

[0038] The electrolyte includes lithium salt, solvent, and additives for inhibiting the dissolution of metal ions. The lithium salt includes, but is not limited to, one or more of LiPF6, LiFSI, LiTFSI, LiDFOB, LiBOB, and LiBF4. The molar concentration of the lithium salt in the electrolyte is 1M to 5M.

[0039] The solvents include, but are not limited to, one or more of the following: cyclic carbonate solvents FEC and chain carbonate solvents FEMC, TFEC, DMC, EMC, and DEC; wherein the volume ratio of cyclic carbonates to the total volume of cyclic carbonates and chain carbonates ranges from 20% to 100%.

[0040] The additives include, but are not limited to, one or more of TMSB, TMSP, TMSPi, succinic acid, adiponitrile, 1,3,6-hexanetricarbonyl (HTCN), and EGBE, and the additives account for 0 to 5% of the total mass of the electrolyte.

[0041] The electrolyte may also include a diluent, which includes, but is not limited to, one or more of hydrofluoroethers such as TTE, TFEO, BTFE, and fluoroaromatic compounds such as 1,2-2-fluorobenzene, and the diluent accounts for 0 to 90% of the electrolyte volume.

[0042] The cleaning process in step S2 is carried out in a dew point room using pure DMC solvent as the cleaning agent at room temperature. The drying environment is a pure oxygen atmosphere at 40℃-60℃ for 24 hours.

[0043] Example 1

[0044] A lithium metal battery includes a lithium metal battery assembled using a treated positive electrode. A dual-electrode system is assembled from the positive electrode to be treated, a graphite negative electrode, a beaker, and an electrochemical workstation, wherein the positive electrode to be treated is the working electrode, and the graphite is the counter electrode. An electrolyte of a specific composition is then injected into the beaker. The electrolyte includes lithium salts of LiPF6, LiFSI, and LiTFSI, with a molar concentration of 2M for the lithium salts. The solvent is a mixture of FEC and DMC, wherein the volume ratio of cyclic carbonate FEC to the total volume of chain carbonate DMC is in the range of 60%. TMSB is used as an additive, accounting for 2% of the total mass of the electrolyte.

[0045] The aforementioned dual-electrode system underwent charge-discharge cycling. Specifically, it was first charged to the rated voltage using a small constant current of 0.01C and held at that voltage for 1 hour. Then, it was discharged to 3V using a larger current of 0.1C. After two cycles, the positive electrode was removed, cleaned, and dried. The cleaning process was carried out in a dew point chamber using pure DMC solvent at a room temperature of 25°C. The drying environment was a pure oxygen atmosphere at 50°C for 24 hours. Finally, the treated positive electrode was assembled with a lithium metal negative electrode to form a lithium metal battery.

[0046] Example 2

[0047] A lithium metal battery, comprising a lithium metal battery assembled using a treated positive electrode, differs from Example 1 in that the molar concentration of lithium salt in the electrolyte is 1M.

[0048] Example 3

[0049] A lithium metal battery, comprising a lithium metal battery assembled using a treated positive electrode, differs from Example 1 in that the molar concentration of lithium salt in the electrolyte is 5M.

[0050] Example 4

[0051] A lithium metal battery, comprising a lithium metal battery assembled using a treated positive electrode, differs from Example 1 in that the solvent is a mixed solvent of FEC and DMC, wherein the volume ratio of FEC to the total volume of the mixed solvent of FEC and DMC is in the range of 20%.

[0052] Example 5

[0053] A lithium metal battery, comprising a lithium metal battery assembled using a treated positive electrode, differs from Example 1 in that the solvent is FEC.

[0054] Example 6

[0055] A lithium metal battery, comprising a lithium metal battery assembled using a treated positive electrode, differs from Example 1 in that the additive used is TMSB, and the additive accounts for 0.1% of the total mass of the electrolyte.

[0056] Example 7

[0057] A lithium metal battery, comprising a lithium metal battery assembled using a treated positive electrode, differs from Example 1 in that the additive used is TMSB, and the additive accounts for 1% of the total mass of the electrolyte.

[0058] Example 8

[0059] A lithium metal battery, comprising a lithium metal battery assembled using a treated positive electrode, differs from Example 1 in that the additive used is TMSB, and the additive accounts for 5% of the total mass of the electrolyte.

[0060] Example 9

[0061] A lithium metal battery includes a lithium metal battery assembled using a treated positive electrode. The difference from Example 1 is that the dual electrode system is subjected to charge-discharge cycles. Specifically, the battery is first charged to the rated voltage using a small constant current of 0.05C and then kept at a constant voltage for 1 hour. Then, it is discharged to 3V using a larger current of 0.5C. After one cycle, the positive electrode is removed, cleaned, and dried to complete the treatment of the positive electrode of the lithium metal battery.

[0062] Example 10

[0063] A lithium metal battery includes a lithium metal battery assembled using a treated positive electrode. The difference from Example 1 is that the dual electrode system is subjected to charge-discharge cycles. Specifically, the battery is first charged to the rated voltage with a small constant current of 0.1C and then kept at a constant voltage for a period of time. Then, it is discharged to 3V with a larger current of 1C. After three cycles, the positive electrode is removed, cleaned and dried to complete the treatment of the positive electrode of the lithium metal battery.

[0064] Example 11

[0065] A lithium metal battery, comprising a lithium metal battery assembled using a treated positive electrode, differs from Example 3 in that it further comprises a diluent, wherein the diluent is TTE and the diluent accounts for 30% of the electrolyte volume.

[0066] Example 12

[0067] A lithium metal battery, comprising a lithium metal battery assembled using a treated positive electrode, differs from Example 3 in that it further comprises a diluent, wherein the diluent is TTE and the diluent accounts for 60% of the electrolyte volume.

[0068] Example 13

[0069] A lithium metal battery, comprising a lithium metal battery assembled using a treated positive electrode, differs from Example 3 in that it further comprises a diluent, wherein the diluent is TTE and the diluent accounts for 90% of the electrolyte volume.

[0070] Example 14

[0071] A lithium metal battery, comprising a lithium metal battery assembled using a treated positive electrode, differs from Example 1 in that the cleaning process is carried out in a dew point chamber, the cleaning agent is pure DMC solvent, and the temperature is room temperature (25°C). The drying environment is a pure oxygen atmosphere, the temperature is 40°C, and the drying time is 24 hours.

[0072] Example 15

[0073] A lithium metal battery, comprising a lithium metal battery assembled using a treated positive electrode, differs from Example 1 in that the cleaning process is carried out in a dew point chamber, the cleaning agent is pure DMC solvent, and the temperature is room temperature (25°C). The drying environment is a pure oxygen atmosphere, the temperature is 60°C, and the drying time is 24 hours.

[0074] Comparative Example 1

[0075] A lithium metal battery, comprising a lithium metal battery assembled using a treated positive electrode, differs from Example 1 in that the solvent is a mixed solvent of EC and DMC, wherein EC accounts for a volume ratio of 60% of the total volume of the mixed solvent.

[0076] Comparative Example 2

[0077] A lithium metal battery, comprising a lithium metal battery assembled using a treated positive electrode, differs from Example 1 in that the dry environment is an air atmosphere.

[0078] Comparative Example 3

[0079] A lithium metal battery is disclosed, which does not employ a positive electrode pretreatment method, but directly assembles the positive electrode and lithium metal negative electrode into a lithium metal battery. The battery is then formed using a 0.01C / 0.1C process.

[0080] The lithium metal batteries in Examples 1-15 and the comparative examples were subjected to charge-discharge cycles at a charging rate of 0.33C / 0.33C. The positive electrode of the lithium metal battery was NCM622, and the electrolyte was 1M LiPF6-FEC / EMC (1:1). The number of cycles was recorded when the capacity retention reached 80%. Three sets of batteries were selected from each example and comparative example, and the average test data is shown in the table below:

[0081]

[0082]

[0083]

[0084] Comparing Examples 1, 2, and 3, it can be seen that the higher the lithium salt concentration, the more F element is contained in the positive electrode CEI, and the better the cycle number of the lithium metal battery assembled from the treated positive electrode.

[0085] Comparing Examples 1, 4, and 5, it can be seen that the effect is better when the content of FEC is within a certain suitable range. This may be due to the combined factors of FEC having a large dielectric constant and high solvation ability, but also high viscosity and poor lithium-ion conductivity.

[0086] Comparing Examples 1 and 6-8, it can be seen that the higher the content of the additive (below 5%), the better the effect. This is because the additive inhibits the dissolution of positive electrode metal ions, resulting in a more stable CEI film.

[0087] Comparing Examples 1, 9 and 10, it can be seen that the smaller the charging current, the better the effect. This is because the smaller the charging current, the more uniform the CEI film will be.

[0088] Comparing Examples 3 and 11-13, it can be seen that the use of diluent in electrolyte has certain beneficial effects. This is because the diluent reduces the viscosity of electrolyte at high concentrations without changing the solvation structure, and improves ionic conductivity and electrolyte wettability. Moreover, the abundant F element in the diluent increases the proportion of F element in the electrolyte, thereby further increasing the F element content in the positive electrode CEI film and further improving the positive electrode treatment effect.

[0089] Comparing Example 1 and Comparative Example 1, it can be seen that the electrolyte solvent has a certain impact on the positive electrode treatment. The higher the content of F element in the solvent, the higher the content of F in the CEI formed on the positive electrode, and the higher the cycle number of the assembled lithium metal battery.

[0090] Comparing Examples 1, 14, and 15, it can be seen that the drying temperature has a certain impact on the positive electrode treatment. When the temperature is within a suitable range, it is beneficial to remove the residual DMC cleaning agent on the positive electrode surface, thereby making the CEI film more stable.

[0091] Comparing Example 1 and Comparison 2, it can be seen that the gas atmosphere of the dry environment also has a certain impact on the positive electrode treatment. This is because the atmosphere of pure oxygen is conducive to protecting the CEI membrane and preventing the CEI membrane from being reduced by reducing substances such as CO in the air, thereby making the CEI membrane more stable.

[0092] Although preferred embodiments of the present invention have been described in detail above, it should be clearly understood that various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a lithium metal battery, characterized in that: Includes the following steps: S1. Take the positive electrode to be treated and the graphite negative electrode and assemble them into a dual electrode system, wherein the positive electrode to be treated is the working electrode and the graphite is the counter electrode. The dual electrode system contains an electrolyte, which is composed of specific components. S2. After a preset number of charge-discharge cycles, a stable CEI film is pre-formed on the surface of the positive electrode. Then, the positive electrode is removed, cleaned, and dried to remove any electrolyte residue on the surface, while the CEI film is preserved, thus completing the treatment of the positive electrode of the lithium metal battery. S3. Assemble the treated positive electrode and lithium metal negative electrode into a lithium metal battery. S4. No formation is required; direct capacity separation can be performed. The charge-discharge cycle process is as follows: first, charge to the rated voltage with a small constant current of 0.01C-0.1C, then maintain the constant voltage for 1 hour, and then discharge to 3V with a larger current of 0.1C-1C. The number of cycles is 1 to 3 times. The electrolyte comprises a lithium salt and a solvent, wherein the lithium salt comprises one or more of LiPF6, LiFSI, LiTFSI, LiDFOB, LiBOB and LiBF4; The electrolyte also includes a diluent, which includes one or more of hydrofluoroethers such as TTE, TFEO, and BTFE, and the diluent accounts for 0-90% of the electrolyte volume.

2. The method for preparing a lithium metal battery according to claim 1, characterized in that: The solvent includes one or more of the following: cyclic carbonate solvent FEC and chain carbonate solvents FEMC, TFEC, DMC, EMC, and DEC; the volume ratio of the cyclic carbonate to the total volume of the cyclic carbonate and chain carbonate is in the range of 20% to 100%.

3. The method for preparing a lithium metal battery according to claim 1, characterized in that: The molar concentration of lithium salt in the electrolyte is 1M to 5M.

4. The method for preparing a lithium metal battery according to claim 1, characterized in that: The electrolyte also includes additives for inhibiting the dissolution of metal ions, the additives including one or more of TMSB, TMSP, TMSPi, succinate, adiponitrile, 1,3,6-hexanetricarbonyl nitrile, and EGBE.

5. The method for preparing a lithium metal battery according to claim 4, characterized in that: The additive accounts for 0-5% of the total mass of the electrolyte.

6. The method for preparing a lithium metal battery according to claim 1, characterized in that: The cleaning process in step S2 is carried out in a dew point room. The cleaning agent is pure DMC solvent, the temperature is room temperature, the drying environment is a pure oxygen atmosphere, the temperature is 40℃-60℃, and the drying time is 24 hours.

7. A lithium metal battery, characterized in that: The lithium metal battery includes those prepared by the method according to any one of claims 1-6.

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