A high-stability oxide solid electrolyte membrane, a preparation method therefor, and an application thereof

Abstract

The application provides a high-stability oxide solid electrolyte film and a preparation method and application thereof. The oxide solid electrolyte film comprises an oxide solid electrolyte with a chemical formula of Li 7‑x La3Zr 2‑x M x O 12 , wherein M is at least one of Al, Ta, Nb, W, Ga, Y and Te, 0<=x<=1; EC; a lithium salt; and a PVDF polymer. The content of the oxide solid electrolyte is 10%-30%; the content of the EC is 5%-35%, preferably 10%-28%; the content of the PVDF polymer is 20%-40%; and the content of the lithium salt is 20%-40%. The ion conductivity of the high-stability oxide solid electrolyte film reaches 10 ‑3 S / cm, and the stability to lithium is high. The high-stability oxide solid electrolyte film can reduce the defluorination side reaction of the PVDF polymer caused by the interface instability of LLZTO, thereby ensuring the stability of the film. The oxide solid electrolyte film with the specific components can be used instead of a separator and can also serve as an ion conductor, thereby reducing the use of electrolyte.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries, in particular to a high-stability oxide solid electrolyte film and a preparation method and application thereof. BACKGROUND

[0002] The garnet-type oxide solid electrolyte Li7La3Zr2O 12 (LLZO), Li 6.4 La3Zr 1.4 Ta 0.6 O 12 (LLZTO) and the like are extremely stable to metallic lithium, the stable electrochemical window can reach 5V, have strong ionic conductivity, extremely low electronic conductivity, and are easy to synthesize and extremely stable. Compared with other oxide solid electrolytes, the grain and grain boundary impedance of the garnet-type electrolyte are in the same order of magnitude, so the overall ionic conductivity is high, which can meet the requirements of polycrystalline solid electrolyte on ionic conductivity, and is a very promising type of solid electrolyte.

[0003] However, the garnet compound is not stable to carbon dioxide and water vapor in the air, and after the reaction, a layer of passivation layer with low ionic conductivity is formed on the surface of the electrolyte. In the past decade, great progress has been made in material preparation, cation substitutes and dopants, ion transport mechanism, ionic conductivity and interface engineering, but less attention has been paid to the air stability of LLZTO-type oxide solid electrolyte.

[0004] The application of garnet-type oxide solid electrolyte represented by LLZTO is mainly limited by poor interface stability, which is due to the fact that the interface of the electrolyte is easy to form lithium carbonate and hydroxide (such as Li2CO3 and LiOH) with water and carbon dioxide in the air, and the catalytic activity of La element inside the electrolyte is strong, making such side reactions very easy to occur. In addition, because the mechanical hardness of LLZTO film is strong, the electrolyte film is easy to break, so it is preferred to be used in combination with a polymer electrolyte, and the polymer electrolyte needs to meet the requirements of ionic conductivity and adhesive properties. Among many polymer electrolytes, PVDF (polyvinylidene fluoride) polymer has high dielectric constant and low glass transition temperature, which is helpful for the dissociation of lithium salt, and is a relatively ideal polymer matrix for preparing polymer electrolyte. However, when the LLZTO-type oxide solid electrolyte (including LLZTO and LLZTO) is compounded, it is easy to promote the self-coupling defluorination side reaction of PVDF-type polymer, which affects the comprehensive performance of lithium ion battery.

[0005] In summary, the high interface impedance of LLZTO-based oxide solid electrolyte limits its performance in battery applications. Furthermore, the PVDF-based polymer composite film of LLZTO-based oxide solid electrolyte has serious side reactions when applied to batteries, which can cause excessive self-discharge and poor cycle performance of the battery. SUMMARY

[0006] The present application addresses the problems in the prior art by providing a high-stability oxide solid electrolyte film, its preparation method and application. The high-stability oxide solid electrolyte film has high ionic conductivity and strong lithium stability, which improves the interface stability of the electrolyte and reduces the occurrence of PVDF-based polymer defluorination side reactions during film formation, ensuring the stability of the film.

[0007] Specifically, in one aspect, the present application provides a high-stability oxide solid electrolyte film, which comprises an oxide solid electrolyte with a chemical formula of Li 7-x La3Zr 2-x M x O 12 wherein M is at least one of Al, Ta, Nb, W, Ga, Y, and Te, 0≤x≤1; EC (ethylene carbonate); a lithium salt; and a PVDF-based polymer.

[0008] Further, the oxide solid electrolyte is Li7La3Zr2O 12 (LLZO) or Li 6.4 La3Zr 1.4 Ta 0.6 O 12 (LLZTO).

[0009] Further, the oxide solid electrolyte is in the form of powder or slurry, and has a particle size of 200 nm to 2 μm. Within the above particle size range, nano-sized particles provide higher ionic conductivity. Therefore, the particle size of the oxide solid electrolyte is preferably 200-400 nm.

[0010] Further, the PVDF-based polymer is preferably PVDF or PVDF-HFP (polyvinylidene fluoride-hexafluoropropylene copolymer).

[0011] Further, the lithium salt is one or more of lithium hexafluorophosphate, lithium difluorophosphate, lithium nitrate, lithium perchlorate, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, lithium difluoro-bis(oxalato)phosphate, lithium tetrafluoro(oxalato)phosphate, lithium tetrafluoroborate, lithium bis(fluorosulfonyl)imide, and lithium bis(fluorosulfonyl)imide.

[0012] Further, the mass content of the oxide solid electrolyte in the overall oxide solid electrolyte film is 10%-30%; the mass content of the EC in the overall oxide solid electrolyte film is 5%-35%; the mass content of the PVDF polymer in the overall oxide solid electrolyte film is 20%-40%; and the mass content of the lithium salt in the overall oxide solid electrolyte film is 20%-40%.

[0013] Further, the content of the EC additive is preferably 10%-28%, in which range the ionic conductivity of the electrolyte is high and the performance is stable. By adding EC, the residual alkali in the electrolyte and EC preferentially react, and the high dielectric constant of EC and the strong Li ion dissociation ability can maintain the stability of the LLZTO oxide solid electrolyte and greatly improve the ionic conductivity. However, the amount of EC needs to be within a certain range, and if the amount is too large, the high viscosity of EC will increase the internal resistance of the film and reduce the ionic conductivity. It should be noted that the EC additive cannot be replaced by other F-containing additives, and the EC additive cannot be replaced by a linear structure electrolyte additive, because FEC (fluorinated ethylene carbonate) will cause a superimposed defluorination reaction, and the low dielectric constant of the linear structure electrolyte additive is not conducive to the dissociation of lithium ions.

[0014] Specifically, in another aspect, the application provides a method for preparing the high-stability oxide solid electrolyte film, which mainly comprises the following steps:

[0015] S1, mixing the oxide solid electrolyte and EC in an organic solvent; adding a certain amount of EC additive to the high-stability oxide solid electrolyte film, so that the EC cyclic ester structure first contacts the LLZTO oxide solid electrolyte, eliminating the negative effects of the defluorination side reaction of PVDF caused by the LLZTO oxide solid electrolyte, preventing the slurry from gelling and discoloring, and ensuring the stability of film formation at room temperature or during drying and heating;

[0016] S2, adding the PVDF polymer and lithium salt to the mixture of S1 to obtain a uniformly mixed slurry;

[0017] S3, uniformly applying the slurry obtained in S2 to a substrate and placing it in a drying device to form a high-stability oxide solid electrolyte film with uniform thickness after drying.

[0018] Further, the slurry obtained in S2 can be directly coated on the positive electrode sheet and the negative electrode sheet to serve as a bottom coating layer of the electrode sheet, and then applied to various lithium battery systems. The coating thickness of the slurry obtained in S2 is preferably 3-100 μm, such as 3 μm, 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, but is not limited to the listed values, and other values within the above range are also within the protection scope of the technical scheme of the present application.

[0019] Further, the coating substrate used in S3 can be selected from a material that does not react with the raw material used for preparing the oxide solid electrolyte film and is easy to coat.

[0020] Specifically, in another aspect, the present application provides an application of the above high-stability oxide solid electrolyte film, i.e., using the high-stability oxide solid electrolyte film as a separator of a lithium ion battery. The high-stability oxide solid electrolyte film is used to replace the separator used in a general lithium battery, especially in a metal lithium negative electrode battery. The LLZTO type oxide solid electrolyte has strong stability to lithium and its performance is better than that of other electrolytes.

[0021] The present application provides a high-stability oxide solid electrolyte film, a preparation method and an application thereof. The ion conductivity of the high-stability oxide solid electrolyte film reaches 10 -3 S / cm, has strong stability to lithium, improves the interface stability of the electrolyte, and reduces the promotion of the side reaction of defluorination of PCDF type polymer caused by the interface instability of LLZTO, such as residual alkali, thereby ensuring the stability of the film formation. Under a specific ratio, the ion conductivity of the high-stability oxide solid electrolyte film is high, which can be used to replace the separator and also can act as an ion conductor to reduce the use of electrolyte. In addition, the process for preparing the high-stability oxide solid electrolyte film is simple and has low cost. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 A schematic diagram of the effect of EC content on ion conductivity in the experiment of the present application;

[0023] Figure 2 A photo of the oxide solid electrolyte film of experiments 1-7 of the present application;

[0024] Figure 3 A CV graph of experiment 5 of the present application;

[0025] Figure 4 A CV graph of experiment 1 of the present application;

[0026] Figure 5The figure for the electronic conductivity test of Experiment 5 of the present application;

[0027] Figure 6 The figure for the lithium stability test of Experiment 5 of the present application. DETAILED DESCRIPTION

[0028] In order to facilitate the understanding of the present application, the present application will be described more fully below with reference to the preferred embodiments of the present application. However, it should be understood that these embodiments are merely used for a more detailed description and should not be construed as limiting the present application in any form, i.e., not intended to limit the scope of protection of the present application.

[0029] In order to further verify the technical effects of the technical solutions of the present application, the following will be described in detail by a series of specific experiments as implementation examples; among them, Experiment 1 and Experiment 7 are comparative examples of the present application with different parameter settings, and Experiments 2-6 are embodiments of the present application. Specifically, the preparation process and steps of the oxide solid electrolyte film of Experiments 1-7 are as follows:

[0030] Experiment 1: At room temperature, LLZTO, EC, PVDF-HFP, LiFSI are dispersed in DMF solvent in a mass ratio of 2:0:3:3 to form a uniformly mixed slurry. The slurry is uniformly coated on a smooth copper foil, and after drying in a blast drying oven at 70°C for 24h, a uniform thickness film is formed. The above electrolyte film is cut into a circular piece, and two steel sheets are assembled into a button cell to test the alternating current impedance EIS, and the internal resistance value R(Ω) is obtained. The thickness L(cm) and area S(cm 2 ) of the circular piece are recorded, and the ionic conductivity A(S / cm) of the pure electrolyte film is obtained according to

[0031] Experiment 2: At room temperature, LLZTO, EC, PVDF-HFP, LiFSI are dispersed in DMF solvent in a mass ratio of 2:0.5:3:3 to form a uniformly mixed slurry. The slurry is uniformly coated on a smooth copper foil, and after drying in a blast drying oven at 70°C for 24h, a uniform thickness film is formed. The above electrolyte film is cut into a circular piece, and two steel sheets are assembled into a button cell to test the alternating current impedance EIS, and the internal resistance value Ω is obtained. The thickness L and area S of the circular piece are recorded, and the ionic conductivity of the pure electrolyte film is obtained according to L / Ω / S.

[0032] ​Experiment 3: At room temperature, LLZTO, EC, PVDF-HFP, LiFSI were dispersed in DMF solvent to form a mixed uniform slurry according to the mass ratio of 2:1:3:3. The slurry was uniformly coated on the smooth copper foil, and after drying in the air drying oven at 70°C for 24h, a uniform thickness film was formed. The above electrolyte film was cut into a round piece, and two pieces of steel were assembled into a button cell to test the alternating current impedance EIS, and the internal resistance value Ω was obtained. The thickness L and area S of the round piece were recorded, and according to L / Ω / S, the ionic conductivity of the pure electrolyte film was obtained.

[0033] Experiment 4: At room temperature, LLZTO, EC, PVDF-HFP, LiFSI were dispersed in DMF solvent to form a mixed uniform slurry according to the mass ratio of 2:2:3:3. The slurry was uniformly coated on the smooth copper foil, and after drying in the air drying oven at 70°C for 24h, a uniform thickness film was formed. The above electrolyte film was cut into a round piece, and two pieces of steel were assembled into a button cell to test the alternating current impedance EIS, and the internal resistance value Ω was obtained. The thickness L and area S of the round piece were recorded, and according to L / Ω / S, the ionic conductivity of the pure electrolyte film was obtained.

[0034] Experiment 5: At room temperature, LLZTO, EC, PVDF-HFP, LiFSI were dispersed in DMF solvent to form a mixed uniform slurry according to the mass ratio of 2:3:3:3. The slurry was uniformly coated on the smooth copper foil, and after drying in the air drying oven at 70°C for 24h, a uniform thickness film was formed. The above electrolyte film was cut into a round piece, and two pieces of steel were assembled into a button cell to test the alternating current impedance EIS, and the internal resistance value Ω was obtained. The thickness L and area S of the round piece were recorded, and according to L / Ω / S, the ionic conductivity of the pure electrolyte film was obtained.

[0035] Experiment 6: At room temperature, LLZTO, EC, PVDF-HFP, LiFSI were dispersed in DMF solvent to form a mixed uniform slurry according to the mass ratio of 2:4:3:3. The slurry was uniformly coated on the smooth copper foil, and after drying in the air drying oven at 70°C for 24h, a uniform thickness film was formed. The above electrolyte film was cut into a round piece, and two pieces of steel were assembled into a button cell to test the alternating current impedance EIS, and the internal resistance value Ω was obtained. The thickness L and area S of the round piece were recorded, and according to L / Ω / S, the ionic conductivity of the pure electrolyte film was obtained.

[0036] Experiment 7: At room temperature, LLZTO, FEC, PVDF-HFP, LiFSI were dispersed in DMF solvent to form a mixed uniform slurry according to the mass ratio of 2:3:3:3. The slurry was uniformly coated on the smooth copper foil, and after drying in the air drying oven at 70°C for 24h, a uniform thickness film was formed. The above electrolyte film was cut into a round piece, and two pieces of steel were assembled into a button cell to test the alternating current impedance EIS, and the internal resistance value Ω was obtained. The thickness L and area S of the round piece were recorded, and according to L / Ω / S, the ionic conductivity of the pure electrolyte film was obtained.

[0037] The ion conductivity of the oxide solid electrolyte films in the above experiments 1-7 with three different thicknesses was tested, and the results are shown in Table 1 and Figure 1 The film formation morphology of different experiments is shown in Figure 2 The lithium ion battery cycle test was performed on experiments 1-7, and the specific test conditions are as follows: (1) stand for 5 min; (2) 0.5C constant current constant voltage charging to 4.3V; (3) stand for 5 min; (4) 0.5C constant current discharging to 3.0V; (5) cycle for 500cls; (6) end. The test results are shown in Table 2.

[0038] Table 1

[0039]

[0040]

[0041] Table 2

[0042] Serial number Cycle number Capacity retention rate Experiment 1 200 53.2％ Experiment 2 200 87.4％ Experiment 3 200 88.7％ Experiment 4 200 90.4％ Experiment 5 200 95.0％ Experiment 6 200 66.0％ Experiment 7 200 49.5％

[0043] In the film formation of oxide solid electrolyte, the LLZTO type oxide solid electrolyte causes the defluorination side reaction of PVDF type polymer, which can deepen the color of the electrolyte film. It can be verified from Figure 2 that when the LLZTO type oxide solid electrolyte formula does not contain EC (such as experiment 1), the color of the film can be obviously seen as brown, and this discoloration is caused by the defluorination side reaction of LLZTO to PVDF-HFP; and when EC is added to the LLZTO type oxide solid electrolyte (such as experiments 2-6), the electrolyte film is obviously white, which is because EC reacts with the residual alkali on the surface of LLZTO during the preparation of the electrolyte, preventing the reaction of residual alkali and PVDF; when FEC is used instead of EC, the electrolyte film will also discolor (such as experiment 7), but the color is not as deep as the oxide solid electrolyte without EC, which shows that FEC can alleviate the influence of residual alkali to some extent, but due to the F contained in FEC, the defluorination side reaction will still occur during the film formation process.

[0044] It can be verified from Figure 3 and Figure 4 that when EC is not added to the oxide solid electrolyte film (such as Figure 4 ), oxidation reaction occurs obviously at 3.5V potential, and the electrochemical window is obviously smaller than that of the oxide solid electrolyte film with EC (such as Figure 3 )(~4.9V), which verifies that the addition of EC in the LLZTO type oxide solid electrolyte film can indeed hinder the occurrence of defluorination side reaction.

[0045] In addition, it can be verified from Table 2 that, compared with the LLZTO-based oxide solid electrolyte without EC (such as Experiment 2) or with FEC (such as Experiment 7), the capacity retention rate of the lithium ion battery can be obviously improved by adding EC in the LLZTO-based oxide solid electrolyte, and the capacity retention rate of the lithium ion battery in Experiment 5 is still 95% after 200 cycles at room temperature at 0.3C, which shows excellent cycle performance.

[0046] The mass content of EC in the oxide solid electrolyte film according to the application can be 5%-35%, and the above-mentioned Experiments 2-6 are further analyzed in this range. It can be verified from Table 1 and the graph that, in the above-mentioned seven experiments, the ionic conductivity of Experiment 5 (LLZTO-based / EC / PVDF-HFP / LiFSI, mass ratio 2:3:3:3, and the content of EC is about 27%) is the highest, reaching 10 -3 S / cm; when the content of EC exceeds about 27% (such as Experiment 6, the content of EC is about 33%), the ionic conductivity decreases with the increase of the content of EC, which shows that there is an inflection point in the oxide solid electrolyte when the content of EC is about 27%-33%, which is presumably because when the content of EC in the oxide solid electrolyte film is too high, the film resistance increases due to the large viscosity of EC, thereby reducing the ionic conductivity. Therefore, the upper limit of the mass content of EC in the oxide solid electrolyte film according to the application is preferably 28%. Meanwhile, referring to the variance results of the ionic conductivity in Experiments 3, 4 and 5, it can be verified that when the content of EC in the oxide solid electrolyte film is too low, the ionic conductivity performance of the film is unstable, which is presumably because the low content of EC cannot fully react with the residual alkali in the electrolyte, thereby causing the LLZTO-based oxide solid electrolyte to promote the defluorination side effect of PVDF in film forming, which shows that the lithium ion conductivity of the electrolyte film is low when the content of EC is low, and the degree of reaction between EC and the residual alkali in the electrolyte is unpredictable, thereby leading to poor stability of the ionic conductivity between different test groups when the content of EC in the LLZTO-based oxide solid electrolyte film is low. Therefore, the lower limit of the mass content of EC in the oxide solid electrolyte film according to the application is preferably 10%. Preferably, the mass content of EC in the oxide solid electrolyte film is 10%-28%.

[0047] Further, the electronic conductivity test results of the oxide solid electrolyte film in the more optimal Experiment 5 in Experiments 1-7 are as follows Figure 5 Further, the lithium stability of the oxide solid electrolyte film in Experiment 5 was tested, and the test method was as follows: ① the oxide solid electrolyte film in Experiment 5 and a lithium metal sheet were assembled into a symmetrical battery Li||SSE||Li; ② under the condition of 25±2℃, the cycle charging and discharging was carried out by an electrochemical workstation at a current of I=0.5mA / cm2, and the single-step time was 30min, and 1 hour was 1 cycle of charging and discharging; ③ whether the cycle overpotential curve is stable was observed, and the test results are as followsFigure 6 .

[0048] Depend on Figure 5 It can be verified that the oxide solid electrolyte membrane of the superior experiment 5 has strong electrical insulation. Those skilled in the art will understand that the oxide solid electrolyte membrane of the present invention with this specific ratio has high ionic conductivity. It can not only be used as a separator in the lithium-ion battery manufacturing process without the need for other separators to prevent short circuits between the positive and negative electrodes, but also act as an ion conductor, reducing the use of electrolyte.

[0049] Depend on Figure 6 It can be confirmed that the preferred scheme can maintain stable lithium cycling stability for more than 1600 cycles, indicating that the technical scheme in Experiment 5 exhibits strong stability against metallic lithium. In the prior art, other types of electrolytes, such as LATP electrolyte, although having high ionic conductivity, are prone to reacting with metallic lithium, while the experiments of this invention have shown high ionic conductivity and strong stability against metallic lithium.

[0050] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A high-stability oxide solid-state electrolyte film, characterized by, An oxide solid electrolyte including an oxide having a chemical formula of Li 7-x La3Zr 2-x M x O 12 wherein M is at least one of Al, Ta, Nb, W, Ga, Y, Te, 0≤x≤1; ethylene carbonate EC; a lithium salt; a PVDF-based polymer; the oxide solid electrolyte has a mass content of 10-30% of the overall oxide solid electrolyte film; the EC has a mass content of 10-28% of the overall oxide solid electrolyte film; the PVDF-based polymer has a mass content of 20-40% of the overall oxide solid electrolyte film; and the lithium salt has a mass content of 20-40% of the overall oxide solid electrolyte film. The high-stability oxide solid electrolyte film is prepared by the following steps: S1, mixing the oxide solid electrolyte and ethylene carbonate EC in an organic solvent; S2, adding the PVDF-based polymer and lithium salt additive to the mixture prepared in S1 to obtain a uniformly mixed slurry; S3, uniformly coating the slurry obtained in S2 on a substrate, and placing it in a drying device to form a high-stability oxide solid electrolyte film with uniform thickness after drying.

2. The high-stability oxide solid-state electrolyte film according to claim 1, characterized by, The oxide solid state electrolyte is Li7La3Zr2O 12 or Li 6.4 La3Zr 1.4 Ta 0.6 O 12 .

3. The high-stability oxide solid-state electrolyte film according to claim 2, characterized by, The oxide solid electrolyte is in powder form, and the particle size is 200 nm-2 μm.

4. The high-stability oxide solid-state electrolyte film according to claim 3, characterized by, The particle size of the oxide solid electrolyte is 200-400 nm.

5. The highly stable oxide solid state electrolyte film according to claim 1, wherein The PVDF-based polymer is PVDF or PVDF-HFP.

6. The highly stable oxide solid state electrolyte film of claim 1, wherein, The lithium salt is one or more of lithium hexafluorophosphate, lithium difluorophosphate, lithium nitrate, lithium perchlorate, lithium bisoxalate borate, lithium difluoro oxalate borate, lithium difluoro bisoxalate phosphate, lithium tetrafluoro oxalate phosphate, lithium tetrafluoroborate, and lithium bisfluorosulfonylimide.

7. A method for producing a high-stability oxide solid electrolyte film according to claim 1, characterized by, The method comprises the following steps: S1, mixing the oxide solid electrolyte and ethylene carbonate EC in an organic solvent; S2, adding the PVDF-based polymer and lithium salt additive to the mixture prepared in S1 to obtain a uniformly mixed slurry; S3, uniformly coating the slurry obtained in S2 on a substrate, and placing it in a drying device to form a high-stability oxide solid electrolyte film with uniform thickness after drying.

8. The method of claim 7, wherein the oxide solid state electrolyte film is formed by a method comprising: The slurry obtained in S2 can be coated on the pole piece as a bottom coating layer of the pole piece.

9. Use of the high-stability oxide solid electrolyte film according to any one of claims 1-6 as a separator for a lithium ion battery.

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