Polymer-based composite solid electrolyte, preparation method and lithium solid-state battery

Abstract

This invention relates to a polymer-based composite solid-state electrolyte, its preparation method, and a lithium solid-state battery. By weight, the polymer-based composite solid-state electrolyte comprises 5-15 parts of polymer, 5-15 parts of lithium salt, and 0.5-2.5 parts of composite filler; the composite filler includes barium titanate, with lithium tungstate coated on its surface. The polymer-based composite solid-state electrolyte of this invention exhibits high ionic conductivity and electrochemical stability, with an ionic conductivity as high as 2.86 mS / cm and an electrochemical stability window of 5.4 V. Simultaneously, the interface between the polymer-based composite solid-state electrolyte and the electrode is more stable, resulting in excellent cycle performance.

Description

Technical Field

[0001] This invention relates to a polymer-based composite solid electrolyte, its preparation method, and a lithium solid-state battery, belonging to the field of solid-state battery technology. Background Technology

[0002] While traditional liquid lithium-ion batteries are widely used in electric vehicles, energy storage facilities, and consumer electronics, their energy density is nearing its limit, and their safety shortcomings are becoming increasingly apparent. In recent years, solid-state batteries have become a research hotspot, offering advantages such as high energy density and high safety. The core of solid-state batteries is the solid electrolyte. Common solid electrolytes include four types: sulfide electrolytes, oxide electrolytes, halide electrolytes, and polymer electrolytes. Among them, polymer solid electrolytes have attracted increasing attention due to their excellent flexibility and processability. However, polymer solid electrolytes also suffer from relatively low room-temperature ionic conductivity and poor mechanical properties. Currently, one common strategy is to add inorganic fillers to polymer electrolytes to improve their performance. Common inorganic fillers include Al2O3, SiO2, TiO2, and BaTiO3, which can typically improve certain properties of polymer electrolytes to some extent. However, the effect of adding a single inorganic filler is limited and difficult to improve the overall performance of solid-state batteries.

[0003] Chinese invention patent application CN201410683144.1 discloses an all-solid-state polymer electrolyte comprising polyethylene oxide, lithium salt, inorganic nanoparticles, and an ionic liquid. The mass ratio of lithium salt to polyethylene oxide is 0.1–0.5, and the sum of the mass of inorganic nanoparticles and the ionic liquid is 10%–30% of the mass of the all-solid-state polymer electrolyte. The lithium salt includes one or more of lithium bis(trifluoromethanesulfonyl)imide, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium trifluoromethanesulfonate, and lithium dioxalateborate. The inorganic nanoparticles include one or more of nano-alumina, nano-silica, nano-zirconia, and nano-barium titanate. In this patent, the role of the nano-inorganic particles is to improve mechanical properties, and the role of the room-temperature ionic liquid is to provide conductivity. Through the combined effect of the two, the all-solid-state polymer electrolyte possesses both good mechanical strength and high ionic conductivity. However, ionic liquids are liquid and will not evaporate even after prolonged baking, which poses a risk of leakage to all-solid polymer electrolytes and may affect the normal performance of the polymer electrolytes. In addition, ionic liquids are expensive, which increases the cost of all-solid polymer electrolytes.

[0004] Chinese invention patent application CN202010141024.4 discloses a composite solid electrolyte, the components of which include: polyethylene oxide, a polymer, an electrolyte, inorganic filler particles, and a solvent. The polymer is one or more of polyethylene glycol (PEG), polyvinylidene fluoride (PVDF), sodium carboxymethyl cellulose (CMC), and polyvinylpyrrolidone (PVP). The electrolyte is one or more of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium hexafluorophosphate (LiPF6), and lithium perchlorate (LiClO4). The inorganic filler particles are one or more of titanium dioxide (TiO2), silicon dioxide (SiO2), alumina (Al2O3), and barium titanate (BaTiO3). However, the room temperature ionic conductivity of this composite solid electrolyte is only 3.83 × 10⁻⁶. -5 There is still considerable room for improvement in S / cm. Summary of the Invention

[0005] To address the shortcomings of existing technologies, one objective of this invention is to provide a polymer-based composite solid electrolyte with high ionic conductivity; another objective is to provide a method for preparing a polymer-based composite solid electrolyte; and a third objective is to provide a lithium solid-state battery.

[0006] To solve the above-mentioned technical problems, the technical solution of the present invention is as follows:

[0007] A polymer-based composite solid electrolyte, by mass, comprises 5-15 parts polymer, 5-15 parts lithium salt, and 0.5-2.5 parts composite filler; the composite filler comprises barium titanate, the surface of which is coated with lithium tungstate.

[0008] Optionally, the polymer-based composite solid electrolyte comprises 8-12 parts of polymer, 8-12 parts of lithium salt, and 0.8-2.2 parts of composite filler.

[0009] Further, the polymer-based composite solid electrolyte comprises 9.5-10.5 parts of polymer, 9.5-10.5 parts of lithium salt, and 1-2 parts of composite filler; and / or, the polymer-based composite solid electrolyte is in the form of a film with a thickness of 20-150 μm, more specifically 40-130 μm, and even more specifically 50-100 μm.

[0010] Furthermore, the polymer-based composite solid electrolyte comprises 9.8-10.2 parts of polymer, 9.8-10.2 parts of lithium salt, and 1.2-1.7 parts of composite filler.

[0011] Furthermore, in the composite filler, the mass ratio of lithium tungstate to barium titanate is 5-20:100, more preferably 8-18:100, even more preferably 10-15:100, even more preferably 11-14:100, and even more preferably 12-13:100.

[0012] Furthermore, the preparation method of the composite filler includes the following steps: adding barium titanate powder to a mixed solution, dispersing it ultrasonically, allowing it to stand under negative pressure, drying the solvent, calcining it, grinding it, and sieving it to obtain the composite filler;

[0013] The mixed solution contains tungstate and lithium ions; the molar ratio of tungstate to lithium ions in the mixed solution is 1:2; and the solvent of the mixed solution is water.

[0014] Barium titanate is insoluble in the above solvents and can be used as a core. In the above process, lithium tungstate generated in situ can be loaded onto barium titanate, so that lithium tungstate is coated on barium titanate to obtain a composite filler.

[0015] Optionally, the pH of the mixed solution is 7.0 ± 0.3, more specifically 7.0 ± 0.1.

[0016] During in-depth research, the applicant unexpectedly discovered that the composite filler described in this invention can effectively improve the ionic conductivity of polymer-based composite solid electrolytes and broaden their electrochemical window, resulting in superior cycle performance of the assembled lithium solid-state batteries. However, the above-mentioned improvement effect cannot be obtained when lithium tungstate or lithium titanate is used alone or when lithium tungstate and lithium titanate are simply combined as fillers.

[0017] Optionally, during ultrasonic dispersion, the ultrasonic power is 350-450W.

[0018] Optionally, the particle size of the barium titanate powder is <300 nm.

[0019] Optionally, the barium titanate powder is nano-barium carbonate powder.

[0020] Optionally, ultrasonic dispersion is performed for 30-90 minutes, or further, ultrasonic dispersion for 45-75 minutes.

[0021] Optionally, it can be left to stand for 8-12 hours under negative pressure (optionally -0.15 to -0.01 MPa).

[0022] Optionally, grinding may be performed for 0.3-0.8 hours.

[0023] Furthermore, it is first allowed to stand at -0.01 to -0.06 MPa for 1 to 3 hours, then at -0.09 to -0.07 MPa for 2 to 4 hours, and then at -0.11 to -0.09 MPa for 4 to 6 hours.

[0024] Optionally, calcination is carried out at 600-700℃ for 2-6 hours, and further, calcination is carried out at 620-670℃ for 3-5 hours.

[0025] Further, a water-soluble tungsten source and a water-soluble lithium source are dissolved in water to obtain a mixed solution; wherein the water-soluble tungsten source is ammonium dodecyltungstate ((NH4)2). 10 W 12 O 41 Ammonium metatungstate ((NH4)6H2W) 12 O 40 One or more of the following: lithium acetate and lithium hydroxide; the water-soluble lithium source is one or more of lithium acetate and lithium hydroxide. Therefore, the composite filler can be prepared without introducing impurity elements, which helps to obtain polymer-based composite solid electrolytes of superior quality.

[0026] Furthermore, the polymer is one or more of polyvinylidene fluoride, poly(vinylidene fluoride-hexafluoropropylene) copolymer, and poly(vinylidene fluoride-trifluoroethylene) copolymer.

[0027] Optionally, the weight-average molecular weight of the polymer is 300,000 to 1,000,000, further 400,000 to 900,000, and even further 500,000 to 800,000.

[0028] Further, the lithium salt is one or more of lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium hexafluorophosphate, lithium difluorophosphate, lithium tetrafluoroborate, lithium difluorooxalate borate, and lithium bis(oxalate borate).

[0029] Optionally, the polymer-based solid electrolyte may further include a polar solvent.

[0030] Optionally, the room temperature ionic conductivity of the polymer-based composite solid electrolyte is ≥1 mS / cm, preferably ≥1.5 mS / cm, more preferably ≥2 mS / cm, and even more preferably ≥2.5 mS / cm.

[0031] Optionally, the electrochemical stability window of the polymer-based composite solid electrolyte is ≥5V, preferably ≥5.2V, and more preferably ≥5.4V.

[0032] Based on the same inventive concept, the present invention also provides a method for preparing the polymer-based composite solid electrolyte as described above, comprising the following steps:

[0033] S1. Mix the polymer, lithium salt, composite filler and polar solvent in a ratio of 5-15g: 5-15g: 0.5-2.5g: 20-100mL to obtain a polymer solution.

[0034] S2. The polymer solution is coated onto the template, dried, and demolded to obtain a polymer-based composite solid electrolyte.

[0035] Further, in S1, the polymer, lithium salt, composite filler and polar solvent are mixed evenly in a ratio of 8-12g: 8-12g: 0.8-2.2g: 30-90mL.

[0036] Further, in S1, the polymer, lithium salt, composite filler and polar solvent are mixed evenly in a ratio of 9-11g: 9-11g: 1-2g: 40-80mL. Even further, the polymer, lithium salt, composite filler and polar solvent are mixed evenly in a ratio of 9.5-10.5g: 9.5-10.5g: 1.2-1.8g: 50-70mL.

[0037] Further, in S1, the polar solvent is one or more of N-methylpyrrolidone, acetone, N,N-dimethylformamide, and N,N-dimethylacetamide.

[0038] Further, in step S1, the polymer, composite lithium salt, filler, and polar solvent are mixed and stirred at 45-85℃ for 4-12 hours to obtain a polymer solution. Optionally, the polymer, lithium salt, and polar solvent are first mixed evenly, and then the composite filler is added and mixed evenly to obtain a polymer solution.

[0039] Furthermore, in S2, the template is a glass plate.

[0040] Optionally, in S2, a film is formed by coating with a doctor blade.

[0041] Furthermore, the gap between the scrapers is 200-1000μm.

[0042] Optionally, in S2, the product is dried at 25-80℃ for 0.5-8 hours, and further, at 35-65℃ for 1.5-6 hours.

[0043] Based on the same inventive concept, the present invention also provides: a lithium solid-state battery, comprising a negative electrode sheet, a polymer-based composite solid electrolyte as described above or a polymer-based composite solid electrolyte prepared by the preparation method described above, and a positive electrode sheet arranged in sequence.

[0044] Optionally, the active material of the positive electrode sheet is a conventional commercial lithium-ion battery positive electrode material, and the other components are binders, conductive agents and / or functional additives.

[0045] Optionally, the negative electrode sheet is a common commercially available negative electrode such as graphite, silicon carbide, or lithium metal.

[0046] Optionally, the lithium solid-state battery can be stably cycled for 500 cycles at 0.5C rate at room temperature with a capacity retention of ≥89%, preferably ≥90%, and more preferably ≥95%.

[0047] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0048] (1) The polymer-based composite solid electrolyte of the present invention has high ionic conductivity and electrochemical stability. The ionic conductivity of the polymer-based composite solid electrolyte is as high as 2.86 mS / cm, and the electrochemical stability window is 5.4 V. At the same time, the interface between the polymer-based composite solid electrolyte and the electrode is more stable, and the cycle performance is excellent.

[0049] (2) The lithium solid-state battery assembled using the polymer-based composite solid electrolyte of the present invention can be stably cycled for 500 cycles at room temperature and 0.5C rate with a capacity retention rate of up to 89.4%, which has good prospects for industrial application.

[0050] (3) The preparation process of the present invention is simple and efficient, which is conducive to its promotion and application. Attached Figure Description

[0051] Figure 1 This is an SEM image of the composite filler in Example 1 of the present invention.

[0052] Figure 2 This is the elemental analysis energy spectrum of the composite filler in Example 1 of the present invention.

[0053] Figure 3 These are photographs of the polymer-based composite solid electrolyte membranes prepared in Examples 1 and 3 of the present invention.

[0054] Figure 4 The graph shows the test results of the ionic conductivity of the polymer-based composite solid electrolytes prepared in Examples 1-2 and Comparative Examples 1 and 3 of this invention.

[0055] Figure 5 These are test results of the electrochemical windows of the polymer-based composite solid electrolytes prepared in Examples 1-3 and Comparative Examples 1-3 of the present invention.

[0056] Figure 6 The graph shows the cycle performance test results of the button cells prepared in Embodiment 1 and Comparative Examples 1 and 3 of the present invention. Detailed Implementation

[0057] The present invention will be described in detail below with reference to embodiments. It should be noted that, unless otherwise specified, the embodiments and features in the embodiments of the present invention can be combined with each other.

[0058] Example 1

[0059] The preparation method of the polymer-based composite solid electrolyte in this embodiment includes the following steps:

[0060] (1) Add 25.35g (0.1 / 12mol) ammonium dodecyltungstate and 13.2g (0.2mol) lithium acetate to 100mL of water, stir for 2h to dissolve, then add ammonia to adjust the pH of the solution to 7.0±0.1, and stir for 0.5h. Then, add 218.45g barium titanate powder (insoluble in water) to the solution, sonicate at 400W for 1h (ice bath temperature control <30℃), then put it into a vacuum oven, evacuate to -0.05Mpa and stand for 2h, then evacuate to -0.08Mpa and stand for 3h, then evacuate to -0.1Mpa and stand for 5h. Dry the solvent in a forced-air oven at 100℃, then calcine in a tube furnace at 650℃ for 4h, then grind with an agate mortar for 0.5h and then sieve through a 100-mesh sieve, and take the sieve material to obtain the composite filler.

[0061] (2) In an argon atmosphere glove box, weigh 1g of poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP), 1g of lithium bis(trifluoromethylamide)imine (LiTFSI), 2mL of N,N-dimethylformamide (DMF), and 5mL of acetone with a weight average molecular weight of 400,000. Then place the above substances in a serum bottle and magnetically stir at 600 rpm at 60°C until completely dissolved. Then take 0.15g of the composite filler obtained in step (1) and add it to the serum bottle. After ultrasonic dispersion for 30 min, stir at 800 rpm at 60°C for 8 h to obtain a polymer solution.

[0062] (3) The polymer solution was poured onto a glass plate, coated into a film using a scraper with a gap of 1000 μm, and then placed in a vacuum oven. After standing in vacuum at room temperature for 0.5 h, the temperature was raised to 45 °C and baked for 2 h. The film was then peeled off to obtain a polymer-based composite solid electrolyte membrane (see Figure 3 ).

[0063] The obtained polymer-based composite solid electrolyte membrane was cut into discs with a diameter of 19 mm, and its room temperature ionic conductivity and electrochemical window were tested using an electrochemical workstation. The test results are shown in Table 1.

[0064] Coin cells were assembled according to the stacking sequence of negative electrode (lithium sheet, 0.45 mm thick, 15.4 mm in diameter), polymer-based composite solid electrolyte membrane and positive electrode, and the cycle performance of the cells was tested. The test results are shown in Table 1.

[0065] The preparation method of the positive electrode sheet is as follows: The positive electrode material (a 6-series ternary cathode, purchased from Henan Tianli, chemical formula LiNi) is prepared... 0.6 Mn 0.2 Co 0.2O2), conductive agent (SP), binder (PVDF5130), and NMP (N-methylpyrrolidone) are mixed evenly in a mass ratio of 9:0.5:0.5:10. The mixture is then coated onto an aluminum current collector (aluminum foil) using a scraper with a gap of 100μm. After being placed in a vacuum oven and vacuum dried at 80℃ for 8 hours, the mixture is removed and cut into small circles with a diameter of 12mm to obtain the positive electrode sheet.

[0066] Depend on Figure 1 and Figure 2 It can be seen that the composite filler in Example 1 is in the form of nanoparticles, and the elements such as titanium, barium, tungsten, and oxygen are uniformly distributed. The distribution density of titanium, barium, and oxygen is relatively large, while the distribution density of tungsten is relatively small. This is related to the relatively small amount of ammonium dodecyltungstate and lithium acetate added in step (1), which also indicates that lithium tungstate was successfully coated onto the surface of barium titanate.

[0067] Example 2

[0068] Repeat Example 1, except that in step (2), 0.05g of composite filler is added.

[0069] Example 3

[0070] Repeat Example 1, except that in step (2), 0.25g of composite filler is added.

[0071] Comparative Example 1

[0072] Repeat Example 1, except that step (1) is omitted and barium titanate of equal mass is used to replace the composite filler in step (2).

[0073] Comparative Example 2

[0074] Repeat Example 1, except that step (1) is omitted and in step (2) the composite filler is replaced with an equal mass of lithium tungstate.

[0075] Comparative Example 3

[0076] Repeat Example 1, except that step (1) is omitted and in step (2) the composite filler is replaced with lithium tungstate powder and barium titanate powder. The ratio of the added lithium tungstate powder and barium titanate powder is the same as the ratio of lithium tungstate powder and lithium titanate powder in the composite filler in Example 1.

[0077] The performance test results of each embodiment and comparative example are shown in Table 1.

[0078] Table 1. Comparison of performance test results for each embodiment and comparative example.

[0079]

[0080] As shown in Table 1, using barium titanate and lithium tungstate alone as fillers results in low ionic conductivity, a narrow electrochemical window, and poor cycle performance of the assembled battery (see Comparative Examples 1 and 2). Even using a simple combination of barium titanate and lithium tungstate as fillers results in low ionic conductivity of the prepared polymer-based solid electrolyte, even lower than that of barium titanate alone (see Comparative Examples 1-3). This indicates that the simple combination of barium titanate and lithium tungstate exhibits an antagonistic effect, which is detrimental to improving the ionic conductivity of the polymer-based solid electrolyte. When using the composite filler of this invention, the prepared polymer-based composite solid electrolyte exhibits significantly higher ionic conductivity, demonstrating a synergistic effect between barium titanate and lithium tungstate. Furthermore, the prepared polymer-based composite solid electrolyte has a wider electrochemical window, and the cycle performance of the assembled battery is significantly better. The possible reason is that in a simple compound, barium titanate and lithium tungstate are dispersed in the polymer-based composite solid electrolyte. Since the addition amount of both is low, they are difficult to exert a synergistic effect. Therefore, when barium titanate and lithium tungstate are simply compounded as fillers, the resulting polymer-based composite solid electrolyte has poor ionic conductivity, electrochemical window, and corresponding battery cycle performance, which are close to or even worse than when barium titanate and lithium tungstate are used as fillers alone. However, when using the composite filler of this invention, the surface of barium titanate is coated with lithium tungstate. In the polymer-based composite solid electrolyte, barium titanate and lithium tungstate are in contact with each other, and they can exert a synergistic effect. This results in the polymer-based composite solid electrolyte exhibiting excellent ionic conductivity and electrochemical window, and the assembled battery exhibiting excellent cycle performance.

[0081] The above embodiments should be understood as being used only to illustrate the present invention more clearly, and not to limit the scope of the present invention. After reading the present invention, any modifications of the present invention in various equivalent forms by those skilled in the art fall within the scope defined by the appended claims.

Claims

1. A polymer-based composite solid electrolyte, characterized in that, The product comprises, by weight, 5-15 parts of polymer, 5-15 parts of lithium salt, and 0.5-2.5 parts of composite filler; the composite filler includes barium titanate, the surface of which is coated with lithium tungstate.

2. The polymer-based composite solid electrolyte according to claim 1, characterized in that, The polymer-based composite solid electrolyte comprises 8-12 parts of polymer, 8-12 parts of lithium salt, and 0.8-2.2 parts of composite filler; and / or, the polymer-based composite solid electrolyte is in the form of a film with a thickness of 20-150 μm.

3. The polymer-based composite solid electrolyte according to claim 1, characterized in that, In the composite filler, the mass ratio of lithium tungstate to barium titanate is 5-20:

100.

4. The polymer-based composite solid electrolyte according to any one of claims 1-3, characterized in that, The preparation method of the composite filler includes the following steps: adding barium titanate powder to a mixed solution, dispersing it ultrasonically, allowing it to stand under negative pressure, drying the solvent, calcining it, grinding it, and sieving it to obtain the composite filler; The mixed solution contains tungstate and lithium ions; the molar ratio of tungstate to lithium ions in the mixed solution is 1:2; and the solvent of the mixed solution is water.

5. The polymer-based composite solid electrolyte according to claim 4, characterized in that, A water-soluble tungsten source and a water-soluble lithium source are dissolved in water to obtain a mixed solution; wherein the water-soluble tungsten source is one or more of ammonium dodecyltungstate and ammonium metatungstate; and the water-soluble lithium source is one or more of lithium acetate and lithium hydroxide.

6. The polymer-based composite solid electrolyte according to any one of claims 1-3, characterized in that, The polymer is one or more of polyvinylidene fluoride, poly(vinylidene fluoride-hexafluoropropylene) copolymer, and poly(vinylidene fluoride-trifluoroethylene) copolymer; the lithium salt is one or more of lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium hexafluorophosphate, lithium difluorophosphate, lithium tetrafluoroborate, lithium difluorooxalate borate, and lithium dioxalate borate.

7. The method for preparing the polymer-based composite solid electrolyte according to any one of claims 1-6, characterized in that, Includes the following steps: S1. Mix the polymer, lithium salt, composite filler and polar solvent in a ratio of 5-15g: 5-15g: 0.5-2.5g: 20-100mL to obtain a polymer solution. S2. The polymer solution is coated onto the template, dried, and demolded to obtain a polymer-based composite solid electrolyte.

8. The preparation method according to claim 7, characterized in that, In S1, the polar solvent is one or more of N-methylpyrrolidone, acetone, N,N-dimethylformamide, and N,N-dimethylacetamide.

9. The preparation method according to claim 7, characterized in that, In S2, the template is a glass plate.

10. A lithium solid-state battery, characterized in that, It includes a negative electrode sheet stacked in sequence, a polymer-based composite solid electrolyte as described in any one of claims 1-6 or a polymer-based composite solid electrolyte prepared by the preparation method described in any one of claims 7-9, and a positive electrode sheet.

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