Interface layer constructed by ultralow-concentration electrolyte, preparation method of interface layer and lithium battery

Abstract

The invention relates to a battery technology, in particular to an interface layer constructed by an ultralow-concentration electrolyte, a preparation method of the interface layer and a lithium battery. According to the preparation method of the interface layer constructed by the ultralow-concentration electrolyte, in a low-concentration electrolyte system, the ultralow-concentration electrolyte can greatly reduce the viscosity of the electrolyte, the ultralow-concentration electrolyte can generate overpotential in the formation process, and meanwhile very small formation current can be kept in the constant-voltage process; by constructing a uniform solid-liquid interface electrolyte membrane with an inorganic inner layer and an organic outer layer, the DCR of the battery can be remarkably reduced, the discharge capacity of the battery can be improved, and meanwhile, the cost can be further reduced by the low-concentration electrolyte.

Description

An interface layer constructed with an ultra-low concentration electrolyte, its preparation method, and a lithium battery Technical Field

[0001] This invention relates to battery technology, and more particularly to an interface layer constructed with an ultra-low concentration electrolyte, a method for preparing the same, and a lithium battery. Background Technology

[0002] As the performance indicators of lithium batteries become increasingly stringent, adjusting the electrolyte additives to construct the positive and negative electrode interface layer can achieve the indicators with the highest efficiency. However, the composition of the interface layer constructed by the additives is often disordered. The mixing of organic and inorganic components cannot maximize the synergistic effect of organic / inorganic components, or the interface layer has poor stability. These factors make it difficult to achieve the battery's electrical performance indicators or achieve high costs, thus hindering its widespread use.

[0003] CN114464903A introduces a metal battery electrolyte and its preparation and application methods. By reducing the concentration of electrolyte metal salts in the electrolyte and increasing the proportion of inert solvents, a more stable negative electrode solid electrolyte interface film is constructed, thereby improving the coulombic efficiency, cycle stability, and safety of the metal battery. Simultaneously, the use of ultra-low concentration electrolytes can significantly reduce the cost of the electrolyte, which is beneficial for promoting the practical application of metal batteries. CN117525611A discloses a method for constructing a lithium battery interface layer and a lithium battery. The construction method includes steps such as primary electrolyte injection, room temperature settling, high temperature settling, secondary electrolyte injection, small current perturbation, and conventional formation. During the high temperature settling process, an organic layer is formed in situ, and during the small current perturbation process, an inorganic layer is formed, thus constructing an interface layer structure with an inner organic layer and an outer inorganic layer. This inner organic interface structure can effectively withstand the effects of positive and negative electrode expansion and contraction during charging and discharging, reducing the generation of new interfaces and improving long-term cycle stability; while the outer inorganic interface structure reduces interface impedance, and the combination of organic / inorganic interface layers further enhances high-temperature stability and high-voltage stability.

[0004] The existing technical solution CN114464903A uses a homogeneous electrolyte system formed by ether solvents compatible with fluorinated ethers to construct a stable interfacial film for lithium metal batteries. Conventional ester and sulfone electrolytes cannot be used, and ether solvents are not commonly used in lithium batteries, which may increase the cost of the battery and limit its applicability. The existing technical solution CN117525611A uses polymerizable monomers and initiators to construct an organic interfacial layer through in-situ polymerization. It requires additional polymerizable monomers and initiators, which may result in poor uniformity of the formed interfacial layer, leading to unsatisfactory electrical performance of the battery. Summary of the Invention

[0005] This invention utilizes conventional ester solvents in lithium batteries, making it widely applicable. Through a two-stage electrolyte injection process, an ultra-low lithium salt concentration electrolyte is used during the first injection. A controllable inner inorganic and outer organic positive and negative electrode interface layer (SEI) is constructed using a small current, significantly reducing the battery's DC internal resistance (DCR) and increasing its discharge capacity. Simultaneously, the low-concentration electrolyte further reduces costs. This application provides an interface layer constructed with an ultra-low concentration electrolyte, its preparation method, and a lithium battery.

[0006] In a first aspect, this application provides a method for preparing an interface layer constructed from an ultra-low concentration electrolyte:

[0007] A method for preparing an interface layer constructed with an ultra-low concentration electrolyte includes the following steps:

[0008] S1: Perform the first electrolyte injection on the dry cell, inject electrolyte A, precharge to the upper limit of the electrochemical window after injection, and maintain constant voltage for 1 to 24 hours;

[0009] The electrolyte A contains solvents and additives;

[0010] S2: After aging and vacuuming the cell after constant voltage in step S1, perform a second electrolyte injection using electrolyte B. After electrolyte injection, age and vacuum again at high temperature, and then charge to the upper limit of the electrochemical window under a small current to complete the construction of the lithium battery interface layer.

[0011] The electrolyte B contains solvent, additives and lithium salt.

[0012] In the technical solution of this application, after the initial injection of low-concentration electrolyte, pre-charging is performed. The battery will exhibit a very large concentration polarization phenomenon, causing the voltage to quickly reach the upper voltage limit. Then, during the process of maintaining constant voltage, the current will drop sharply. Maintaining constant voltage for 1-24 hours can generate a more stable inorganic interface layer with better adhesion at extremely low current during the pre-charging process. In other words, pre-charging to the upper limit of the electrochemical window and maintaining constant voltage for 1-24 hours is to form a more stable inorganic film under low current conditions.

[0013] Compared to the traditional lithium battery interface layer construction, which uses monomers to generate polymer interface layers, the present invention first generates an inorganic interface layer on the surface, which makes the interface layer more stable. Compared to the technical solution of using initiator polymerization, the solvent and lithium salt are matched during the pre-charge formation process, which makes the solvent more stably composited on the inorganic film, which is convenient, fast and cost-effective.

[0014] Furthermore, the solvent includes any one or a combination of at least two of ethylene carbonate, propylene carbonate, diethyl carbonate, dimethyl carbonate, ethylene glycol dimethyl ether, 1,3-dioxolane, dimethyl sulfoxide, vinylene carbonate, methyl ethyl carbonate, and fluoroacetate; the additive includes any one or a combination of at least two of vinylene carbonate, 1,3-propanesulfonate lactone, 1,4-butanesulfonate lactone, fluoroethylene carbonate, propylene sulfite, ethylene ethylene sulfate, and ethylene sulfate; the lithium salt includes any one or a combination of at least two of lithium hexafluorophosphate, lithium difluorooxalate borate, lithium bis(oxalate borate), lithium difluorophosphate, lithium tetrafluoroborate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluorosulfonyl)imide.

[0015] Furthermore, electrolyte A also contains lithium salt, with the lithium salt concentration of electrolyte A being 0–1 mol / L and the lithium salt concentration of electrolyte B being 0.8–1.5 mol / L.

[0016] Furthermore, the lithium salt concentration of the electrolyte A is 0–0.5 mol / L.

[0017] Furthermore, in step S1, the mass of injected electrolyte A accounts for 10% to 60 wt% of the total mass of the two injections; in step S2, the mass of injected electrolyte B accounts for 40% to 90 wt% of the total mass of the two injections.

[0018] Furthermore, step S1 also includes, after injecting electrolyte A, before pre-charging to the upper limit of the electrochemical window, allowing it to stand for 5 to 24 hours at 25±3°C;

[0019] In step S1, the cell needs to be clamped by a fixture during pre-charging, with a fixture pressure of 8 to 12 psi; the upper limit of the electrochemical window is 2 to 3.6 V; and the pre-charging current is 0.1 to 0.5 C.

[0020] Furthermore, in step S2, the aging temperature is 38±3℃, the aging time is 5~48h; the small current is 0.01~0.1C, and the upper limit of the electrochemical window is 2~3.6V.

[0021] Furthermore, the method for preparing the interface layer constructed from the ultra-low concentration electrolyte further includes the following steps:

[0022] S3: Perform low-current charging and discharging on the battery cell for which the battery interface layer has been constructed in S2 to improve the stability of the battery interface layer; the number of cycles for the low-current charging and discharging is 1 to 5.

[0023] Secondly, this application provides an interface layer constructed with an ultra-low concentration electrolyte:

[0024] The interface layer was prepared using the method described in this application for preparing an interface layer using an ultra-low concentration electrolyte.

[0025] Thirdly, this application provides a lithium battery:

[0026] A lithium battery, wherein the interface layer is prepared by the method for preparing the interface layer constructed with the ultra-low concentration electrolyte described in this application, or includes the interface layer constructed with the ultra-low concentration electrolyte described in this application.

[0027] Beneficial effects:

[0028] This invention discloses a method for constructing an interface layer with an ultra-low concentration electrolyte. In the low-concentration electrolyte system of this invention, the ultra-low concentration electrolyte can greatly reduce the electrolyte viscosity. The ultra-low concentration electrolyte can generate overpotential during the formation process, while the constant voltage process can maintain a very small formation current. By constructing a uniform solid-liquid interface electrolyte film with an inner inorganic layer and an outer organic layer, the battery's discharge capacity can be significantly reduced and the battery's discharge capacity can be increased. At the same time, the low-concentration electrolyte can also further reduce costs. Detailed Implementation

[0029] To make the technical solution of the present invention clearer, specific embodiments are provided for further detailed description. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they shall be performed in accordance with the techniques or conditions described in the literature in the field or in accordance with the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0030] The numerical range described in this invention includes not only the point values ​​listed above, but also any point values ​​within the numerical ranges not listed above. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific point values ​​included in the range.

[0031] Example 1: A method for preparing an interface layer constructed with an ultra-low concentration electrolyte, comprising the following steps:

[0032] Preparation of dry cell batteries:

[0033] (1) Preparation of positive electrode sheet: The positive electrode active material lithium iron phosphate (LiPF6), binder polyvinylidene fluoride (PVDF), conductive agent carbon black and carbon nanotubes are mixed in a mass ratio of 97:1:0.5:0.5. N-methylpyrrolidone (NMP) is added and stirred in a vacuum mixer until stable and uniform to obtain the positive electrode material. After the viscosity is tested and found to be qualified, the obtained positive electrode slurry is coated on an aluminum foil with a thickness of 12μm. The coated aluminum foil is dried in an oven at 100℃ and then rolled and punched to obtain the positive electrode sheet.

[0034] (2) Preparation of negative electrode sheet: The negative electrode active material graphite, the binder styrene-butadiene rubber (SBR), the thickener sodium carboxymethyl cellulose (CMC), the conductive agent carbon black and carbon nanotubes are mixed in a ratio of 98:0.5:0.5:0.5:0.5, deionized water is added, and the mixture is stirred in a vacuum mixer until it is stable and uniform to obtain a negative electrode slurry. After the viscosity is tested and found to be qualified, the obtained negative electrode slurry is coated on a copper foil with a thickness of 8μm. The copper foil after coating with slurry is dried in an oven at 100℃, and then the negative electrode sheet is obtained by rolling and punching.

[0035] (3) Dry cell assembly: The prepared positive and negative electrode sheets and separators are stacked according to the design capacity, and then packaged with aluminum-plastic film to obtain dry cells, which are then vacuum dried at 85°C.

[0036] (4) Preparation of non-aqueous electrolyte: Anhydrous ethylene carbonate and dimethyl carbonate were mixed in a mass ratio of 3:7 without adding lithium salt to obtain electrolyte A0; another group was mixed with 1M lithium hexafluorophosphate to obtain electrolyte B1.

[0037] To represent the lithium salt concentration in electrolyte A and electrolyte B, this embodiment uses electrolyte A. n and electrolyte B n Indicates. Electrolyte A n and electrolyte B n In this context, n represents the lithium salt concentration in the electrolyte, such as electrolyte A. 0.04 Electrolyte A and electrolyte B are obtained by adding 0.04 mol / L lithium hexafluorophosphate. 0.04 Electrolyte B is obtained by adding 0.04 mol / L lithium hexafluorophosphate.

[0038] Construction of the lithium battery interface layer:

[0039] S1: The dry cell prepared above was injected with electrolyte An for the first time. After injection, it was left to stand at 25°C for 20 hours. The clamp (clamp pressure was 10 psi) was pre-charged to the upper limit of the electrochemical window (the upper limit of the electrochemical window was set to 3.6V; the pre-charge current was 0.3C) and kept at constant voltage for 12 hours.

[0040] S2: After pre-charging, the cells of S1 are aged (aged at 38°C for 27 hours) and evacuated. Then, a second electrolyte injection is performed, with electrolyte Bn injected. After the electrolyte injection, the cells are aged at high temperature again and evacuated. Then, they are charged to the upper limit of the electrochemical window (set to 3.6V) under a small current (0.05C) to complete the formation of the lithium battery interface layer.

[0041] S3: The battery cell for which the battery interface layer has been constructed in S2 is charged and discharged with a small current (0.05C) for 3 cycles to complete the construction of the lithium battery interface layer and obtain a lithium battery.

[0042] Following steps S1, S2, and S3, we obtain the activated lithium battery and the DCR test results.

[0043] The DCR test method is as follows: rest for 20 seconds, record the voltage V1 after resting, then discharge with a current of 5C for 20 seconds, record the voltage V2 after discharge, and the DCR value = (V1-V2) / discharge current.

[0044] Examples 2-5 and Comparative Examples 1-5 differ from Example 1 only in electrolyte A. n and B n The lithium salt concentrations are detailed in the table.

[0045]

[0046] As can be seen from the table above, the 1 / 3C discharge capacity and DCR of the lithium batteries in each embodiment of the present invention are significantly improved compared with the comparative examples. Among them, in Examples 1-5, the lower the lithium salt content in the first injection, the better the construction of the SEI film with the inner inorganic and outer organic layers, the discharge capacity gradually increases, the DCR gradually decreases, and the performance is optimal. Compared with Comparative Examples 1-5, the second injection is more conducive to the construction of a controllable SEI film with an inner inorganic and outer organic layer than the first injection. Moreover, when the total lithium salt content in the first injection is low (refer to Comparative Examples 1-3), the capacity is not fully utilized, indicating that the construction of the SEI layer cannot be separated from the participation of lithium salt, and that the second injection is more advantageous than the first injection.

[0047] Specifically, based on the data from Examples 1-5, it can be seen that during the first electrolyte injection, the extremely low lithium salt content can greatly reduce the electrolyte viscosity. The ultra-low concentration electrolyte can generate overpotential during the formation process, while the constant voltage process can maintain a very small formation current. By constructing a uniform solid-liquid interface electrolyte film with an inner inorganic layer and an outer organic layer, the battery's DCR can be significantly reduced and the battery's discharge capacity can be increased. At the same time, the low concentration electrolyte can also further reduce costs.

[0048] Comparative Examples 1-3: No test data were available for 1 / 3C discharge capacity and DCR. If a normal concentration electrolyte is not injected during the second electrolyte injection, the battery cannot complete the normal interface layer formation. This indicates that a low concentration electrolyte needs to be injected initially, and the inorganic interface layer needs to be generated using the designed pre-charge formation process. Then, a normal concentration electrolyte needs to be injected a second time to complete the interface layer formation in order to form a relatively stable lithium battery interface layer.

[0049] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A method for preparing an interface layer constructed with an ultra-low concentration electrolyte, characterized in that, Includes the following steps: S1: Perform the first electrolyte injection on the dry cell, inject electrolyte A, precharge to the upper limit of the electrochemical window after injection, and maintain constant voltage for 1 to 24 hours; the electrolyte A contains solvent and additives. S2: After aging and vacuuming the cell after constant voltage in step S1, a second electrolyte injection is performed, using electrolyte B. After injection, the cell is aged at high temperature and vacuumed again, and then charged to the upper limit of the electrochemical window under a small current to complete the construction of the lithium battery interface layer. The electrolyte B contains solvent, additives and lithium salt.

2. The method for preparing an interface layer constructed from an ultra-low concentration electrolyte according to claim 1, characterized in that, The solvent includes any one or a combination of at least two of ethylene carbonate, propylene carbonate, diethyl carbonate, dimethyl carbonate, ethylene glycol dimethyl ether, 1,3-dioxolane, dimethyl sulfoxide, vinylene carbonate, methyl ethyl carbonate, and fluoroacetate; the additive includes any one or a combination of at least two of vinylene carbonate, 1,3-propanesulfonate lactone, 1,4-butanesulfonate lactone, fluoroethylene carbonate, propylene sulfite, ethylene ethylene sulfate, and ethylene sulfate; the lithium salt includes any one or a combination of at least two of lithium hexafluorophosphate, lithium difluorooxalate borate, lithium bis(oxalate borate), lithium difluorophosphate, lithium tetrafluoroborate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluorosulfonyl)imide.

3. The method for preparing an interface layer constructed from an ultra-low concentration electrolyte according to claim 1, characterized in that, Electrolyte A also contains lithium salt, and the lithium salt concentration of electrolyte A is 0-1 mol / L, while the lithium salt concentration of electrolyte B is 0.8-1.5 mol / L.

4. The method for preparing an interface layer constructed from an ultra-low concentration electrolyte according to claim 3, characterized in that, The lithium salt concentration of electrolyte A is 0–0.5 mol / L.

5. The method for preparing an interface layer constructed from an ultra-low concentration electrolyte according to claim 1, characterized in that, In step S1, the mass of injected electrolyte A accounts for 10% to 60 wt% of the total mass of the two injections; in step S2, the mass of injected electrolyte B accounts for 40% to 90 wt% of the total mass of the two injections.

6. The method for preparing an interface layer constructed from an ultra-low concentration electrolyte according to claim 1, characterized in that, Step S1 further includes, after injecting electrolyte A, before pre-charging to the upper limit of the electrochemical window, standing for 5 to 24 hours at 25±3℃; in step S1, the cell also needs to be clamped by a clamp during the pre-charging, with a clamp pressure of 8 to 12 psi; the upper limit of the electrochemical window is 2 to 3.6V; the pre-charging current is 0.1 to 0.5C.

7. The method for preparing an interface layer constructed from an ultra-low concentration electrolyte according to claim 1, characterized in that, In step S2, the aging temperature is 38±3℃, the aging time is 5~48h, the small current is 0.01~0.1C, and the upper limit of the electrochemical window is 2~3.6V.

8. The method for preparing an interface layer constructed from an ultra-low concentration electrolyte according to claim 1, characterized in that, It also includes the following step: S3: Perform low-current charging and discharging on the battery cell for which the battery interface layer has been constructed in S2 to improve the stability of the battery interface layer; the number of cycles of the low-current charging and discharging is 1 to 5.

9. An interface layer constructed with an ultra-low concentration electrolyte, characterized in that, The interface layer is prepared using the method described in any one of claims 1-8 for preparing an interface layer constructed with an ultra-low concentration electrolyte.

10. A lithium battery, characterized in that, The lithium battery uses the method described in any one of claims 1-8 to prepare the interface layer, or includes the interface layer prepared by the ultra-low concentration electrolyte as described in claim 9.

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