Method for evaluating stability of CEI membrane and application thereof

Abstract

The invention provides a method for evaluating the stability of a CEI membrane and application of the method, and relates to the technical field of batteries. According to the method for evaluating the stability of the CEI membrane, the membrane forming quality is visually reflected from the aspect of electrochemical physical quantity, and the method can be used for rapid evaluation of the stability of the CEI membrane and screening of an electrolyte, a positive electrode membrane forming additive or a positive electrode material.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and in particular to a method for evaluating the stability of CEI films and its applications. Background Technology

[0002] In lithium manganese oxide batteries, intrinsic structural characteristics of the material, such as the Jan Taylor effect, cause Mn metal dissolution, which, during charging and discharging, deposits at the negative electrode, damaging the SEI film and leading to gas generation and rapid performance degradation. The key to improving this problem lies in optimizing the positive electrode CEI film to fundamentally inhibit metal dissolution. Adding highly efficient positive electrode film-forming additives such as lithium bis(oxalato)borate (LiBOB) to the electrolyte is a common method. Studies have shown that LiBOB preferentially decomposes on the positive electrode surface to form a stable passivation film (CEI film), effectively inhibiting the oxidative decomposition of the electrolyte and the dissolution of transition metals from the positive electrode material. Simultaneously, LiBOB can also act as an HF scavenger, reducing the hydrofluoric acid (HF) content in the electrolyte, preventing HF corrosion of the positive and negative electrode materials, and maintaining the battery structural integrity. However, excessively high LiBOB content can lead to excessive impedance, deteriorated kinetic performance, and poor interface stability. Evaluating the stability of the CEI film is crucial for judging battery performance.

[0003] In view of this, the present invention is hereby proposed. Summary of the Invention

[0004] The first objective of this invention is to provide a method for evaluating the stability of CEI membranes in order to solve the above-mentioned technical problems.

[0005] A second objective of this invention is to provide the application of the above-described method in the screening of electrolytes, cathode film-forming additives, or cathode materials.

[0006] To achieve the above objectives, the following technical solution is adopted: In a first aspect, the present invention provides a method for evaluating the stability of a CEI membrane, comprising the following steps: A coin cell was assembled from a positive electrode, a lithium electrode, a separator, and an electrolyte. After formation, a CEI film was constructed. The coin cell was then discharged at a constant current to 0% SOC and then charged at a constant current to V1. After resting to eliminate the effects of concentration polarization, it was charged at a constant voltage at potential V1, and the current change over time was recorded. The stability of the CEI film was evaluated based on at least one of the following evaluation parameters: a. Calculate the amount of charge of the side reactions during the constant voltage charging process. The smaller the amount of charge of the side reactions, the better the stability of the CEI film. b. In the initial stage of the constant voltage charging, the faster the current decay rate, the better the stability of the CEI film; c. After the constant voltage charging reaches a stable current value, the smaller the stable current value, the better the stability of the CEI film; The voltage V1 is 0.1-0.2V higher than the full-charge voltage; The electrolyte contains a positive electrode film-forming additive.

[0007] As a further technical solution, the positive electrode includes a current collector and an active material layer coated on the current collector; The active material layer includes a positive electrode active material, a conductive agent, and a binder.

[0008] The current collector includes aluminum foil; The positive electrode active material includes LMO, NMC811, NMC622 or NCA; The conductive agent includes conductive carbon black; The adhesive includes PVDF.

[0009] As a further technical solution, the solvent of the electrolyte includes EC and EMC, and the solute includes positive electrode film-forming additives and 1.0M LiPF6.

[0010] As a further technical solution, the positive electrode film-forming additive includes TMSP, FEC, LiBOB or LiODFB.

[0011] As a further technical solution, the formation step includes: charging the battery to full charge at a constant current of 0.02-0.1C.

[0012] As a further technical solution, the button cell is a CR2032 type button cell.

[0013] As a further technical solution, the formation process also includes aging the battery. The aging process takes 18-24 hours.

[0014] As a further technical solution, the resting time is 20-40 minutes.

[0015] As a further technical solution, the constant voltage charging time is 12-24 hours.

[0016] Secondly, the present invention provides the application of the above method in the screening of electrolytes, positive electrode film-forming additives, or positive electrode materials.

[0017] Compared with the prior art, the present invention has the following beneficial effects: The method for evaluating the stability of CEI films provided by this invention can intuitively reflect the film quality from the perspective of electrochemical physical quantities. It can be used for rapid evaluation of CEI film stability and screening of electrolytes, cathode film-forming additives or cathode materials. Attached Figure Description

[0018] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0019] Figure 1 The change of current over time during the constant voltage phase for batteries group 9 and group 11; Figure 2 The results are for the charge-discharge cycle tests of the 9th and 11th battery groups. Detailed Implementation

[0020] The embodiments and examples of the present invention will be described in detail below. However, those skilled in the art will understand that the following embodiments and examples are for illustrative purposes only and should not be considered as limiting the scope of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention. Unless otherwise specified, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0021] In a first aspect, the present invention provides a method for evaluating the stability of a CEI membrane, comprising the following steps: A coin cell was assembled from a positive electrode, a lithium electrode, a separator, and an electrolyte. After formation, a CEI film was constructed. The coin cell was then discharged at a constant current to 0% SOC and then charged at a constant current to V1. After resting to eliminate the effects of concentration polarization, it was charged at a constant voltage at potential V1, and the current change over time was recorded. The stability of the CEI film was evaluated according to at least one of the following AC evaluation parameters: a. Calculate the amount of charge of the side reaction during the constant voltage charging process (the amount of charge of the side reaction can be obtained from the curve of the current changing with time during the constant voltage charging process, and the area under the curve is the amount of charge of the side reaction). The smaller the amount of charge of the side reaction, the better the stability of the CEI film. b. In the initial stage of the constant voltage charging, the faster the current decay rate, the better the stability of the CEI film; c. After the constant voltage charging reaches a stable current value, the smaller the stable current value, the better the stability of the CEI film; The V1 is 0.1-0.2V higher than the full-charge voltage, for example, but not limited to 0.1V, 0.18V or 0.2V; The electrolyte contains a positive electrode film-forming additive.

[0022] The CEI film effectively prevents electrons from flowing from the cathode material to the electrolyte. However, a very small number of electrons can still pass through the CEI film via quantum tunneling and reach the electrolyte / CEI interface, where they can participate in side reactions. After prolonged constant-voltage charging at a relatively high voltage to eliminate the influence of the non-Faraday current, the leakage current (Faraday current) will slowly decrease over time, eventually reaching a very small stable value. Measuring the leakage current indirectly assesses the integrity and long-term stability of the CEI film under continuous high-voltage stress.

[0023] The method for evaluating the stability of CEI films provided by this invention can intuitively reflect the film quality from the perspective of electrochemical physical quantities. It can be used for rapid evaluation of CEI film stability and screening of electrolytes, cathode film-forming additives or cathode materials.

[0024] In some alternative embodiments, the positive electrode includes a current collector and an active material layer coated on the current collector; The active material layer includes a positive electrode active material, a conductive agent, and a binder.

[0025] The current collector includes aluminum foil; The positive electrode active material includes, but is not limited to, LMO, NMC811, NMC622 or NCA; The conductive agent includes, but is not limited to, conductive carbon black; The adhesive includes, but is not limited to, PVDF.

[0026] In some alternative embodiments, the electrolyte solvent includes EC and EMC, and the solute includes positive electrode film-forming additives and 1.0M LiPF6.

[0027] In some alternative embodiments, the cathode film-forming additive includes, but is not limited to, TMSP, FEC, LiBOB or LiODFB, or other cathode film-forming additives well known to those skilled in the art.

[0028] In some alternative implementations, the formation step includes charging the battery to full charge at a constant current of 0.02C-0.1C (e.g., but not limited to 0.02C, 0.05C, or 0.1C).

[0029] In some alternative embodiments, the button cell is a CR2032 type button cell.

[0030] In some optional embodiments, the formation process further includes aging the battery. The aging process can take, for example, but is not limited to, 18h, 20h, or 24h.

[0031] In some alternative implementations, the resting time may be, for example, but not limited to, 20 minutes, 30 minutes, or 40 minutes.

[0032] In some alternative implementations, the constant voltage charging time may be, for example, but not limited to, 12h, 16h or 24h.

[0033] Secondly, the present invention provides the application of the above method in the screening of electrolytes, positive electrode film-forming additives, or positive electrode materials.

[0034] The present invention will be further illustrated below with specific embodiments. However, it should be understood that these embodiments are merely for the purpose of more detailed illustration and should not be construed as limiting the present invention in any way.

[0035] Example 1 Preparation of positive electrode sheet: NMC811, LMO or surface-modified LMO (LJM-05 from Xiangtan Lijin New Material Co., Ltd.) are used as positive electrode active materials, mixed with conductive carbon black and PVDF binder at a mass ratio of 94:3:3, coated on aluminum foil, dried and punched into round sheets with a diameter of 14mm.

[0036] Electrolyte: includes base electrolyte and target film-forming additives; The base electrolyte is EC / EMC (3:7, v / v) containing 1.0 M LiPF6. The target film-forming additives are TMSP, FEC, LiBOB or LiODFB.

[0037] Battery assembly: In an argon glove box, CR2032 coin cells are assembled using the positive electrode as the working electrode and the lithium metal sheet as the counter electrode and reference electrode.

[0038] Formation: All cells were charged at a constant current of 0.05C to 4.2V (full charge voltage), and then at a constant voltage until the current dropped below 0.01C, completing the initial CEI film construction. Testing was conducted after 24 hours of resting.

[0039] Constant current charging: Discharge the battery to 0% SOC with constant current, then charge it to a high potential of 4.4V with constant current. Let it rest for 30 minutes at the end to eliminate the effects of concentration polarization.

[0040] Constant voltage charging: Charge at a high potential (4.4V) for 24 hours and record the change in current over time, then plot the data.

[0041] Constant current discharge: Discharge the current and end the leakage current test.

[0042] Evaluation criteria: (1) Plot the current versus time in the constant voltage section and integrate to obtain the charge Q of the side reaction. The smaller the charge of the side reaction, the better. (2) The faster the initial leakage current decays, the more quickly and completely the CEI film can cover the exposed surface; (3) Compare the magnitude of the stable leakage current I (during the constant voltage charging stage, the current will tend to stabilize as time goes on, and the current at this time is the stable leakage current). The smaller the stable current value, the better the final protection capability of the CEI membrane.

[0043] Experimental Example 1 Batteries were fabricated using different cathode materials and electrolytes according to the embodiments (Table 1), and the current variation over time was recorded. The amount of side reaction charge and the stable current value were calculated, and the results are shown in Table 1. Then, charge-discharge cycle tests were performed on each group of batteries (45℃, cycle rate 1C), and the capacity retention results are shown in Table 1 (the area under the curve represents the amount of side reaction charge). The current variation over time during the constant voltage stage for batteries in groups 8 and 10 is shown below. Figure 1 As shown, the charge-discharge cycle test results are as follows: Figure 2 As shown.

[0044] Table 1

[0045] Note: The side reaction charge and stable leakage current in the table are the average values ​​of 5 parallel sample test results.

[0046] The addition of film-forming additives significantly improved the stability of the cathode film formation, reducing both the side reaction charge and the stable leakage current. The improved cycle performance of the corresponding battery further corroborates the enhanced film formation stability.

[0047] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method of assessing stability of a CEI membrane, characterized by, The method comprises the following steps: Assembling a positive electrode sheet, a lithium sheet, a separator and an electrolyte into a button cell, constructing a CEI film after formation, then discharging the button cell at a constant current to 0% SOC, charging it to V1 at a constant current, then eliminating the effect of concentration polarization after standing treatment, and recording the data of current change with time at V1 potential, and evaluating the stability of the CEI film according to at least one of the following evaluation parameters: a. Calculate the side reaction charge during the constant voltage charging process, the smaller the side reaction charge, the better the stability of the CEI film; b. The faster the current attenuation rate at the initial stage of the constant voltage charging, the better the stability of the CEI film; c. The smaller the stable current value after the constant voltage charging, the better the stability of the CEI film; The V1 is higher than the full voltage by 0.1-0.2V; The electrolyte contains a positive electrode film-forming additive.

2. The method of claim 1, wherein, The positive electrode sheet comprises a current collector and an active material layer coated on the current collector; The active material layer comprises a positive electrode active material, a conductive agent and a binder; The current collector comprises an aluminum foil; The positive electrode active material comprises LMO, NMC811, NMC622 or NCA; The conductive agent comprises conductive carbon black; The binder comprises PVDF.

3. The method of claim 1, wherein, The solvent of the electrolyte comprises EC and EMC, and the solute comprises a positive electrode film-forming additive and 1.0M LiPF6.

4. The method of claim 1, wherein, The positive electrode film-forming additive comprises TMSP, FEC, LiBOB or LiODFB.

5. The method of claim 1, wherein, The step of formation comprises charging the battery at a constant current of 0.02-0.1C to full power.

6. The method of claim 1, wherein, The button cell is a CR2032 type button cell.

7. The method of claim 1, wherein, After formation, the battery is also subjected to aging treatment; The time of the aging treatment is 18-24h.

8. The method of claim 1, wherein, The time of the standing is 20-40min.

9. The method of claim 1, wherein, The time of the constant voltage charging is 12-24h.

10. The method of any one of claims 1-9 is used in the screening of electrolyte, positive electrode film-forming additive or positive electrode material.

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