PVDF-based composite solid electrolyte, preparation method and application thereof, and lithium battery

Abstract

The invention belongs to the technical field of lithium batteries, and particularly relates to a PVDF-based composite solid electrolyte, a preparation method and application thereof, and a lithium battery. The composite solid electrolyte comprises a PVDF-based polymer, a lithium salt, an amide substance, a nano metal oxide and a residual organic solvent. The nano metal oxide and the amide substance jointly regulate and control the solvation structure of lithium ions, weaken the acting force of a solvent and the lithium ions and the acting force of [Li (residual solvent) x] < + > and a polymer, construct a weak interaction environment of the lithium ions, promote the transmission of Li < + >, and convert an electrode electrolyte interface phase derived from the solvent into an anion-dominated derivative phase, so that the lithium ion battery is obtained. On the other hand, the nano metal oxide reduces the crystallinity of the PVDF polymer, inhibits the formation of spherulites and densification of an electrolyte membrane, facilitates the construction of a uniform and continuous ion transmission channel, improves the mechanical property, effectively inhibits the growth of lithium dendrites, and improves the cycle stability of the battery.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium batteries, and particularly relates to a PVDF-based composite solid electrolyte, a preparation method and application thereof, and a lithium battery. Background Art

[0002] With the rapid development of technical fields such as electric vehicles, grid energy storage, and intelligent devices, new expectations and requirements for the energy density and safety of rechargeable batteries are continuously put forward. Metallic lithium has a high theoretical capacity of 3860 mAh / g and a lowest reduction potential of -3.04 V (relative to the standard hydrogen electrode), and is an ideal negative electrode for high-energy density batteries. The combination of high-voltage ternary materials and metallic lithium is the development trend of high specific energy lithium batteries. However, problems such as the extremely high electrochemical reaction activity of metallic lithium and the growth of lithium dendrites, as well as the safety problems of commercial lithium battery electrolytes, have hindered the further development of lithium metal batteries (LMBs). Replacing liquid electrolytes (LE) and separators with solid electrolytes (SEs) with high ionic conductivity, high stability, and high mechanical properties is expected to obtain solid-state lithium batteries that take into account high safety and high energy density.

[0003] PVDF-based polymer electrolytes mainly composed of polyvinylidene fluoride (PVDF) and its copolymers (polyvinylidene fluoride-hexafluoropropylene, polyvinylidene fluoride-trifluoroethylene) have some ideal electrolyte properties, such as both mechanical strength and toughness, a wide voltage window, good thermal stability, high electrochemical stability, no adverse reactions with other materials, and good processability. However, solid electrolytes have problems of low ionic conductivity and large interfacial impedance at room temperature. In the system of PVDF-based polymer + lithium salt + a small amount of residual solvent, the residual solvent, etc. combine with lithium ions (Li + ) to form a solvation structure of [Li(residual solvent) x + to ensure sufficient room temperature ion transport in the system. However, due to the strong cation-dipole interaction and the strong dipole-dipole interaction with polymer segments of the solvation structure of [Li(residual solvent) x + , it shows poor positive and negative electrode interfacial stability, and the plasticizing effect of the residual solvent leads to a significant decrease in the mechanical properties of the electrolyte, making it difficult to inhibit the growth of lithium dendrites and seriously affecting the long-term cycle stability of the system. Summary of the Invention

[0004] In view of this, the purpose of the present invention is to provide a PVDF-based composite solid electrolyte, a preparation method and application thereof, and a lithium battery. Using the PVDF-based composite solid electrolyte of the present invention, the assembled lithium metal battery has a high specific capacity, excellent rate performance, and good cycle stability.

[0005] ​​The present invention provides a PVDF-based composite solid electrolyte, which comprises a PVDF-based polymer, and a lithium salt, an amide substance, a nano metal oxide and a residual organic solvent dispersed in the PVDF-based polymer;

[0006] The structural formula of the amide substance is R-CO-NH2 or R-CO-NH-R', where R and R' are independently alkyl groups.

[0007] Preferably, the amide substance comprises one or more of N-methylformamide, N-methylacetamide, N-methylpropanamide, formamide, acetamide and propanamide.

[0008] Preferably, the molar ratio of the amide substance to the lithium salt is (1-8):1.

[0009] Preferably, the nano metal oxide is a nano single metal oxide, and the nano single metal oxide comprises one or more of titanium dioxide, zirconium dioxide, vanadium pentoxide, chromium trioxide, molybdenum trioxide and tungsten trioxide.

[0010] Preferably, the mass ratio of the nano metal oxide to the PVDF-based polymer is (0.03-0.2):1.

[0011] Preferably, the PVDF-based polymer comprises a PVDF homopolymer or a PVDF copolymer, the PVDF copolymer comprises a binary copolymer or a ternary copolymer, the binary copolymer comprises poly(vinylidene fluoride-trifluoroethylene), poly(vinylidene fluoride-hexafluoropropylene) or poly(vinylidene fluoride-trifluorochloroethylene), and the ternary copolymer comprises poly(vinylidene fluoride-trifluoroethylene-trifluorochloroethylene).

[0012] Preferably, the lithium salt comprises one or more of lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium hexafluorophosphate, lithium perchlorate, lithium difluoro(oxalato)borate and lithium nitrate; the mass ratio of the lithium salt to the PVDF-based polymer is (0.6-1.2):1.

[0013] The present invention also provides a preparation method of the PVDF-based composite solid electrolyte described in the above technical solution, comprising the following steps:

[0014] Mix the PVDF-based polymer, the lithium salt, the amide substance, the nano metal oxide and the organic solvent to obtain a precursor liquid;

[0015] Cast the precursor liquid and remove the organic solvent to obtain the PVDF-based composite solid electrolyte.

[0016] The present invention also provides an application of the PVDF-based composite solid electrolyte described in the above technical solution or the PVDF-based composite solid electrolyte obtained by the above preparation method in a lithium battery.

[0017] The present invention also provides a lithium battery, comprising a positive electrode, a negative electrode and an electrolyte, wherein the electrolyte is the PVDF-based composite solid electrolyte described in the above technical solution or the PVDF-based composite solid electrolyte obtained by the above preparation method.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] The present invention provides a PVDF-based composite solid electrolyte, comprising a PVDF-based polymer, and a lithium salt, an amide substance, a nano metal oxide and a residual organic solvent dispersed in the PVDF-based polymer; the structural formula of the amide substance is R-CO-NH2 or R-CO-NH-R', wherein R and R' are independently alkyl groups.

[0020] The PVDF-based composite solid electrolyte of the present invention comprises an amide substance and a nano metal oxide. The C=O in the amide substance can participate in the solvation structure of [Li(residual solvent) x + The amino group or imino group provides a hydrogen bond donor; the nano metal oxide has a phase structure and rich surface active sites. The amide substance and the nano metal oxide with rich surface defects jointly regulate the solvation structure of lithium ions, weaken the interaction between the solvent and lithium ions, and [Li(residual solvent) x + The interaction with the polymer, construct a weak interaction environment for lithium ions, promote the transport of Li + At the same time, the electrode electrolyte interface phase derived from the solvent is converted into an anion-dominated derived phase, improving the interface stability. On the other hand, the nano metal oxide filler reduces the crystallinity of the PVDF polymer, inhibits the formation of spherulites, densifies the electrolyte membrane, helps to construct a uniform and continuous ion transport channel, improves the mechanical properties, and effectively inhibits the growth of lithium dendrites, thereby greatly improving the cycle stability of the battery.

[0021] The solid polymer electrolyte of the present invention has high ionic conductivity and good electrochemical stability at room temperature, and has good compatibility with the positive and negative electrodes. The battery assembled with the solid polymer electrolyte of the present invention has a high specific capacity and excellent rate performance, and shows excellent cycle stability. The assembled ternary Li[Ni6Co2Mn2]O2 / lithium battery has a specific capacity of up to 140 mAh g at a rate of 1C and can be cycled more than 300 times. -1 . Description of the Drawings

[0022] ​​To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required in the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0023] Figure 1 Photograph of the composite solid electrolyte prepared in Example 1;

[0024] Figure 2 Scanning electron microscope image of the composite solid electrolyte prepared in Example 1;

[0025] Figure 3 Stress-strain curve of the composite solid electrolyte prepared in Example 1;

[0026] Figure 4 Ionic conductivity - Arrhenius curve of the composite solid electrolyte prepared in Example 1;

[0027] Figure 5 Linear sweep voltammogram of the steel sheet || Li battery with the composite solid electrolyte prepared in Example 1;

[0028] Figure 6 I - t curve and impedance diagrams before and after polarization of the Li || Li symmetric battery with the composite solid electrolyte prepared in Example 1;

[0029] Figure 7 Curve of voltage change with time at a constant current of the Li || Li symmetric battery with the composite solid electrolyte prepared in Example 1;

[0030] Figure 8 Rate cycling diagram of the button battery assembled in Application Example 1;

[0031] Figure 9 Cycling diagram of the button battery assembled in Application Example 1 at 1C;

[0032] Figure 10 Ionic conductivity - Arrhenius curve of the composite solid electrolyte prepared in Comparative Example 1;

[0033] Figure 11 SEM image of the composite solid electrolyte prepared in Comparative Example 1;

[0034] Figure 12 Cycling diagram at 1C of the button battery assembled with the composite solid electrolyte prepared in Comparative Example 1;

[0035] Figure 13 Stress-strain curve of the composite solid polymer electrolyte prepared in Example 2;

[0036] Figure 14 Cycling diagram of the button cell assembled for Application Example 2 at 1C;

[0037] Figure 15 Stress-strain curve of the composite solid polymer electrolyte prepared in Example 3;

[0038] Figure 16 Cycling diagram of the button cell assembled for Application Example 3 at 1C;

[0039] Figure 17 Cycling diagram of the button cell assembled with the composite solid electrolyte prepared in Comparative Example 2 at 1C. Detailed implementation manner

[0040] The present invention provides a PVDF-based composite solid electrolyte, comprising a PVDF-based polymer, and a lithium salt, an amide substance, a nano metal oxide and a residual organic solvent dispersed in the PVDF-based polymer;

[0041] The chemical formula of the amide substance is R-CO-NH2 or R-CO-NH-R', where R and R' are independently alkyl groups.

[0042] In the present invention, unless otherwise specified, the materials and equipment used are commercially available products in the art.

[0043] In the present invention, the PVDF-based composite solid electrolyte takes the PVDF-based polymer as the main body, the lithium salt, amide molecules and nano metal oxides are uniformly dispersed in the polymer matrix, and contains a small amount of residual solvent. The PVDF-based composite solid electrolyte of the present invention has a high room temperature ionic conductivity, excellent mechanical properties and good electrochemical stability.

[0044] In the present invention, the PVDF-based polymer preferably includes a PVDF homopolymer or a PVDF copolymer, and the PVDF copolymer preferably includes a binary copolymer or a terpolymer. The PVDF homopolymer is polyvinylidene fluoride, and its structural formula is as shown in I. The weight-average molecular weight is preferably 50,000 to 2,000,000; the binary copolymer preferably includes poly(vinylidene fluoride-trifluoroethylene), poly(vinylidene fluoride-hexafluoropropylene) or poly(vinylidene fluoride-chlorotrifluoroethylene), and their structural formulas are respectively as shown in formulas II, III and IV. The weight-average molecular weight is independently preferably 50,000 to 2,000,000. When synthesizing the binary copolymer, the molar ratio of other comonomers to the vinylidene fluoride monomer is preferably (0.01 to 0.2):1, and the other comonomers include trifluoroethylene, hexafluoropropylene or chlorotrifluoroethylene monomers; the terpolymer preferably includes poly(vinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene), and its structural formula is as shown in V. The weight-average molecular weight is preferably 10,000 to 200,0000. When synthesizing poly(vinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene), the molar ratio of trifluoroethylene monomer, chlorotrifluoroethylene monomer to vinylidene fluoride monomer is preferably (0.01 to 0.1):(0.01 to 0.1):1. The PVDF-based polymer is the main material of the composite solid electrolyte, and all are mainly based on -CH2-CF2-, [Li(residual solvent) x + The solvation structure of interacts with the PVDF chain to transport Li + . The addition of comonomers can improve the flexibility and crystallization characteristics of PVDF and enhance the ability of the polymer to promote the dissociation of lithium salts.

[0045]

[0046] In the present invention, the lithium salt preferably includes one or more of lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium hexafluorophosphate (LiPF6), lithium perchlorate (LiClO4), lithium difluoro(oxalato)borate (LiODFB) and lithium nitrate (LiNO3).

[0047] In the present invention, the mass ratio of the lithium salt to the PVDF-based polymer is preferably (0.6 to 1.2):1, more preferably (0.65 to 1):1, still more preferably (0.7 to 0.9):1, and specifically can be 0.8:1. The mass ratio of the lithium salt to the PVDF polymer in the present invention can ensure that the ionic conductivity of the electrolyte meets the performance requirements of lithium batteries, and can avoid excessive lithium salts resulting in too much residual solvent and a decrease in the mechanical properties of the polymer electrolyte.

[0048] ​In the present invention, the nano metal oxide is preferably a nano single metal oxide, and the nano single metal oxide preferably includes one or more of titanium dioxide (TiO2), zirconium dioxide (ZrO2), vanadium pentoxide (V2O5), chromium trioxide (Cr2O3), molybdenum trioxide (MoO3), and tungsten trioxide (WO3). The tungsten trioxide is preferably in the form of nanosheets or nanorods, and the titanium dioxide is preferably in the form of nanoparticles.

[0049] In the present invention, the size of the nano metal oxide is preferably 1 - 100 nm. When the nano metal oxide is in particulate form, the size refers to the particle diameter; when it is a nanosheet, the size refers to the thickness; when it is a nanorod, the size refers to the diameter of the circular cross-section. The nano metal oxide has a phase structure, abundant surface active sites, low cost, and simple synthesis.

[0050] In the present invention, the mass ratio of the nano metal oxide to the PVDF-based polymer is preferably (0.03 - 0.2):1, more preferably (0.05 - 0.15):1, and still more preferably (0.05 - 0.1):1. The nano metal oxide is a filler. The mass ratio of the nano metal oxide to the PVDF-based polymer in the present invention can effectively reduce the crystallinity of the polymer matrix, promote the dissociation of lithium salts, weaken the Li + solvation environment, and improve the transport of Li + .

[0051] In the present invention, the amide substances preferably include one or more of N-methylformamide, N-methylacetamide, N-methylpropanamide, formamide, acetamide, and propanamide.

[0052] In the present invention, the molar ratio of the amide substances to the lithium salt is preferably (1 - 8):1, more preferably (2 - 6):1, and still more preferably (3 - 5):1. Specifically, it can be 5:1, 4:1, or 2:1. For the molar ratio of the amide substances to the lithium salt in the present invention, the amide substances can participate in the [Li(residual solvent) x + solvation structure, compete with the residual solvent, weaken the binding of the residual solvent molecules to lithium ions, and the weak hydrogen bond interaction with the polymer chain promotes the transport of the solvation structure in the chain segments.

[0053] In the present invention, the organic solvents preferably include one or more of N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), and N-methylpyrrolidone (NMP).

[0054] The present invention also provides a method for preparing the PVDF-based composite solid electrolyte described in the above technical solution, which includes the following steps:

[0055] ​Mix a PVDF-based polymer, a lithium salt, an amide substance, a nano metal oxide, and an organic solvent to obtain a precursor liquid.

[0056] Cast the precursor liquid and remove the organic solvent to obtain a PVDF-based composite solid electrolyte.

[0057] In the present invention, a PVDF-based polymer, a lithium salt, an amide substance, a nano metal oxide, and an organic solvent are mixed to obtain a precursor liquid.

[0058] In the present invention, the mass ratio of the organic solvent to the PVDF-based polymer is preferably (7 - 14):1, more preferably (8 - 12):1, and still more preferably (8.5 - 10):1.

[0059] In the present invention, the mixing of the PVDF-based polymer, the lithium salt, the amide substance, the nano metal oxide, and the organic solvent preferably includes: first mixing the nano metal oxide and the organic solvent, and second mixing the obtained first mixture with the PVDF-based polymer, the lithium salt, and the amide substance. The first mixing is preferably carried out under ultrasonic conditions, and the ultrasonic time is preferably 3 - 6 h. The ultrasonic treatment can initially disperse the nano metal oxide filler; the second mixing is preferably carried out under stirring conditions, and the stirring is preferably mechanical stirring. The stirring speed is preferably 300 - 600 rpm, specifically 400 rpm or 500 rpm; the stirring time is preferably 6 - 18 h, more preferably 8 - 15 h, specifically 12 h; the stirring temperature is preferably 20 - 30 °C, specifically 25 °C. The stirring speed, temperature, and time in the present invention can make each component mix evenly.

[0060] After obtaining the precursor liquid, in the present invention, the precursor liquid is cast and the organic solvent is removed to obtain a PVDF-based composite solid electrolyte.

[0061] The present invention has no special requirements for the casting amount of the precursor liquid, and it can be selected according to the required thickness of the electrolyte.

[0062] In the present invention, the casting is preferably carried out on a glass plate, and the method for removing the organic solvent is preferably drying. The drying is preferably carried out in an oven, and the temperature of the oven is preferably 50 - 80 °C, specifically 60 °C; the drying time is preferably 16 - 28 h, more preferably 18 - 26 h, and still more preferably 20 - 24 h. During the drying process, the organic solvent can be basically removed. Due to intermolecular interactions, a small amount of the organic solvent is retained in the electrolyte, and the electrolyte presents a solid state characteristic. The drying temperature and time in the present invention can obtain a film-like composite solid electrolyte that is easy to peel off.

[0063] The present invention also provides an application of the PVDF-based composite solid electrolyte described in the above technical solution or the PVDF-based composite solid electrolyte obtained by the above preparation method in a lithium battery.

[0064] The present invention also provides a lithium battery, comprising a positive electrode, a negative electrode and an electrolyte, wherein the electrolyte is the PVDF-based composite solid electrolyte described in the above technical solution or the PVDF-based composite solid electrolyte obtained by the above preparation method.

[0065] The present invention has no special requirements for the compositions of the positive electrode and the negative electrode of the lithium battery, and the lithium battery positive electrode materials and negative electrode materials well-known to those skilled in the art can be used. Specifically, the positive electrode active material is ternary Li[Ni6Co2Mn2]O2, the current collector is aluminum foil, the conductive agent is acetylene black, the binder is polytetrafluoroethylene, and the negative electrode is metallic lithium; the mass ratio of Li[Ni6Co2Mn2]O2, acetylene black and polytetrafluoroethylene is preferably 8:1:1.

[0066] The present invention has no special requirements for the preparation method of the lithium battery, and the preparation methods of lithium batteries well-known to those skilled in the art can be used.

[0067] The lithium battery of the present invention has excellent rate performance, high charge-discharge specific capacity and excellent cycle stability.

[0068] In order to further illustrate the present invention, the PVDF-based composite solid electrolyte provided by the present invention, its preparation method and application, and the lithium battery will be described in detail below with reference to the accompanying drawings and embodiments, but they should not be construed as limiting the protection scope of the present invention.

[0069] Example 1

[0070] (1) 0.01 g of WO3 nanosheets were dispersed in 1.6 g of N,N-dimethylacetamide. After ultrasonic treatment for 3 h, 0.2 g of poly(vinylidene fluoride-hexafluoropropylene), 0.165 g of lithium bis(trifluoromethanesulfonyl)imide and 0.21 g of N-methylacetamide were added, and the mixture was stirred at 400 r / min at 25 °C for 12 h to obtain a precursor solution.

[0071] (2) The precursor solution was poured onto a glass plate and placed in an oven at a temperature of 60 °C for 20 h; then the composite solid electrolyte was peeled off and cut into appropriate sizes for assembling the battery.

[0072] Figure 1 Photograph of the composite solid electrolyte prepared for Example 1.

[0073] Figure 2 SEM image of the composite solid electrolyte prepared for Example 1. From Figure 2It can be seen that the surface of the composite solid electrolyte prepared in Example 1 presents a dense and uniform structure without obvious typical PVDF spherulites, indicating that the crystallinity of poly(vinylidene fluoride - hexafluoropropylene) (PVDF - HFP) is inhibited.

[0074] Figure 3 Figure 4 shows the stress - strain curve of the composite solid polymer electrolyte prepared in Example 1. From Figure 3 it can be seen that the tensile strength and fracture strain of the composite solid polymer electrolyte prepared in Example 1 are 2.0 MPa and 180% respectively, showing good mechanical strength, which is beneficial to inhibiting the growth of lithium dendrites.

[0075] Figure 4 Figure 5 shows the ionic conductivity - Arrhenius curve of the composite solid electrolyte prepared in Example 1. The room - temperature ionic conductivity of the composite solid electrolyte prepared in Example 1 is 9.6×10 -4 S·cm -1 , and the activation energy is 0.11 eV, which reflects excellent ion - transport ability and low migration barrier.

[0076] The electrochemical window of the composite solid electrolyte prepared in Example 1 was tested, and the results are as Figure 5 shown. The oxidation potential is 4.57 V, which can match the ternary cathode.

[0077] The I - t curve of the Li||Li symmetric battery assembled with the composite solid electrolyte prepared in Example 1 and the impedance before and after battery polarization were tested, and the results are as Figure 6 shown. From Figure 6 it can be seen that the lithium - ion transference number of the composite solid electrolyte prepared in Example 1 at room temperature is 0.67.

[0078] The voltage - time variation curve of the Li||Li symmetric battery assembled with the composite solid electrolyte prepared in Example 1 under constant - current conditions was tested, and the results are as Figure 7 shown. Under the conditions of 0.1 mA cm -2 and 0.1 mAh cm -2 , the polarization voltage of the battery is only 40 mV, and no short - circuit occurs after stable cycling for 1200 h, indicating the interfacial stability towards the lithium anode.

[0079] Application Example 1

[0080] A button battery was assembled with the composite solid electrolyte prepared in Example 1 as the electrolyte. The positive - electrode active material is ternary Li[Ni6Co2Mn2]O2, the current collector is aluminum foil, the conductive agent is acetylene black, and the binder is polytetrafluoroethylene. The mass ratio of each component in the positive electrode is Li[Ni6Co2Mn2]O2︰acetylene black︰polytetrafluoroethylene = 8:1:1; the negative electrode is metallic lithium.

[0081] The obtained button cells were subjected to rate cycling tests, and the results are as follows Figure 8 shown. The obtained button cells have excellent rate performance. They were cycled 5 times at different rates. At high rates of 1C and 2C, the discharge specific capacities can reach 140 and 120 mAh g -1 , respectively. And after restoring to the low rate of 0.1C, the specific capacity can be maintained at 170 mAh g -1 .

[0082] The obtained button cells were subjected to a 1C long cycle test, and the results are as follows Figure 9 shown. The obtained button cells were cycled 300 times at 1C, and the capacity retention rate was 80%.

[0083] Comparative Example 1

[0084] The composite solid electrolyte was prepared by the method described in Example 1, but without adding WO3 nanosheets. The remaining processes were the same as those in Example 1, and the electrolyte Comparative Example 1 was obtained.

[0085] Figure 10 is the ionic conductivity - Arrhenius curve of the composite solid electrolyte prepared in Comparative Example 1. The room temperature ionic conductivity of the electrolyte prepared in Comparative Example 1 is 7.7×10 -4 S·cm -1 , and the activation energy is 0.138 eV. The ionic conductivity is lower than that of Example 1, and the activation energy for ion migration is larger.

[0086] Figure 11 is the SEM image of the composite solid electrolyte prepared in Comparative Example 1. It can be seen that the electrolyte shows an obvious spherulite morphology, and the spherulite diameter is large, resulting in more pores, which is not conducive to the continuous and uniform transmission of lithium ions in the electrolyte. In Example 1, due to the addition of WO3 nanosheets, the crystallinity is inhibited, and the surface of the electrolyte membrane becomes denser.

[0087] Figure 12 is the cycle diagram of the button cell assembled with the composite solid electrolyte prepared in Comparative Example 1 at 1C. Due to the holes in the electrolyte and the non-uniform lithium ion transmission, after 80 cycles, dendrite growth leads to internal micro-short circuit and overcharge phenomenon. And the decomposition of the unstable solvation structure and the large activation energy for ion migration result in continuous capacity decay after the battery starts to cycle.

[0088] Example 2

[0089] (1) 0.01 g of WO3 nanorods were dispersed in 1.4 g of N,N-dimethylacetamide. After ultrasonic treatment for 3 h, 0.2 g of poly(vinylidene fluoride - hexafluoropropylene), 0.165 g of lithium bis(trifluoromethanesulfonyl)imide, and 0.136 g of acetamide were added, and the mixture was stirred at a speed of 400 r / min at 25°C for 12 h to obtain a precursor solution;

[0090] (2) Pour the precursor solution onto a glass plate and place it in an oven at a temperature of 60 °C for 24 h; then peel it off to obtain a composite solid electrolyte, which is cut into a suitable size for assembling the battery.

[0091] The room-temperature ionic conductivity of the composite solid electrolyte prepared in Example 2 is 6.8×10 -4 S·cm -1 .

[0092] Figure 13 is the stress-strain curve of the composite solid polymer electrolyte prepared in Example 2. It can be seen from Figure 13 that the tensile strength and fracture strain of the composite solid polymer electrolyte prepared in Example 2 are 2.5 MPa and 190% respectively, showing good mechanical strength, which is beneficial to inhibiting the growth of lithium dendrites.

[0093] Application Example 2

[0094] Assemble a button battery with the composite solid electrolyte prepared in Example 2 as the electrolyte. The positive electrode active material is ternary Li[Ni6Co2Mn2]O2, the current collector is aluminum foil, the conductive agent is acetylene black, and the binder is polytetrafluoroethylene. The mass ratio of each component of the positive electrode is Li[Ni6Co2Mn2]O2︰acetylene black︰polytetrafluoroethylene = 8:1:1; the negative electrode is metallic lithium.

[0095] Perform a 1C long-cycle test on the obtained button battery. The results are as Figure 14 shown. The obtained button battery is cycled 500 times at 1C, and the capacity retention rate is 85%.

[0096] Example 3

[0097] (1) Disperse 0.006 g of nano-particle TiO2 in 2 g of N,N-dimethylformamide. After ultrasonic treatment for 3 h, add 0.2 g of PVDF, 0.12 g of lithium bis(trifluoromethanesulfonyl)imide, and 0.049 g of acetamide, and stir at a speed of 500 r / min at 25 °C for 12 h to obtain a precursor solution;

[0098] (2) Pour the precursor solution onto a glass plate and place it in an oven at a temperature of 60 °C for 24 h; then peel it off to obtain a composite solid electrolyte, which is cut into a suitable size for assembling the battery.

[0099] The room-temperature ionic conductivity of the composite solid electrolyte prepared in Example 3 is 4.3×10 -4 S·cm -1 . Compared with Examples 1 and 2, the ionic conductivity of the electrolyte is relatively low, mainly due to the relatively small addition amounts of the lithium salt and the nano-metal oxide.

[0100] Figure 15 The stress-strain curve of the composite solid polymer electrolyte prepared in Example 3. The tensile strength and fracture strain of the composite solid polymer electrolyte prepared in Example 3 are 1.69 MPa and 150% respectively.

[0101] Application Example 3

[0102] A button cell was assembled with the composite solid electrolyte prepared in Example 3 as the electrolyte. The positive electrode active material is ternary Li[Ni6Co2Mn2]O2, the current collector is aluminum foil, the conductive agent is acetylene black, and the binder is polytetrafluoroethylene. The mass ratio of each component of the positive electrode is Li[Ni6Co2Mn2]O2︰acetylene black︰polytetrafluoroethylene = 8:1:1; the negative electrode is metallic lithium.

[0103] The obtained button cell was subjected to a 1C long cycle test, and the results are as Figure 16 shown. The obtained button cell was cycled 150 times at 1C, and the capacity retention rate was 80%.

[0104] Comparative Example 2

[0105] A composite solid electrolyte was prepared by the method described in Example 3, but without adding TiO2 nanoparticles, and the rest of the process was the same as in Example 3, and the electrolyte Comparative Example 2 was obtained.

[0106] The room temperature ionic conductivity of the composite solid electrolyte prepared in Comparative Example 2 is 1.8×10 -4 S·cm -1 . Compared with Example 3, the ionic conductivity is low, mainly because the content of lithium salt and free amide molecules is low, and at the same time, nano metal oxides are not added, and the PVDF electrolyte has high crystallinity, all of which are not conducive to lithium ion transport.

[0107] Figure 17 This is the cycle diagram of the button cell assembled with the composite solid electrolyte prepared in Comparative Example 2 at 1C. Due to the low ionic conductivity and the decomposition of the unstable solvation structure, the battery capacity decays after 50 cycles.

[0108] Although the above embodiments have described the present invention in detail, they are only a part of the embodiments of the present invention, rather than all embodiments. People can also obtain other embodiments according to the embodiments of the present invention without creative labor, and these embodiments all belong to the protection scope of the present invention.

Claims

1. A PVDF-based composite solid electrolyte, characterized in that, It includes a PVDF-based polymer, as well as a lithium salt, an amide substance, a nano metal oxide, and a residual organic solvent dispersed in the PVDF-based polymer; The structural formula of the amide substance is R-CO-NH2 or R-CO-NH-R', where R and R' are independently alkyl groups.

2. The PVDF-based composite solid electrolyte according to claim 1, wherein The amide substance includes one or more of N-methylformamide, N-methylacetamide, N-methylpropanamide, formamide, acetamide, and propanamide.

3. The PVDF-based composite solid electrolyte according to claim 1 or 2, characterized in that, The molar ratio of the amide substance to the lithium salt is (1-8):

1.

4. The PVDF-based composite solid electrolyte according to claim 1, wherein The nano metal oxide is a nano single metal oxide, and the nano single metal oxide includes one or more of titanium dioxide, zirconium dioxide, vanadium pentoxide, chromium trioxide, molybdenum trioxide, and tungsten trioxide.

5. The PVDF-based composite solid electrolyte according to claim 1 or 4, characterized in that, The mass ratio of the nano metal oxide to the PVDF-based polymer is (0.03-0.2):

1.

6. The PVDF-based composite solid electrolyte according to claim 1, wherein The PVDF-based polymer includes a PVDF homopolymer or a PVDF copolymer. The PVDF copolymer includes a binary copolymer or a ternary copolymer. The binary copolymer includes poly(vinylidene fluoride-trifluoroethylene), poly(vinylidene fluoride-hexafluoropropylene), or poly(vinylidene fluoride-trifluorochloroethylene). The ternary copolymer includes poly(vinylidene fluoride-trifluoroethylene-trifluorochloroethylene).

7. The PVDF-based composite solid electrolyte according to claim 1, characterized in that The lithium salt includes one or more of lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium hexafluorophosphate, lithium perchlorate, lithium difluoro(oxalato)borate, and lithium nitrate; the mass ratio of the lithium salt to the PVDF-based polymer is (0.6-1.2):

1.

8. The preparation method of the PVDF-based composite solid electrolyte according to any one of claims 1 to 7, characterized in that, It includes the following steps: Mix the PVDF-based polymer, the lithium salt, the amide substance, the nano metal oxide, and the organic solvent to obtain a precursor liquid; Cast the precursor liquid and remove the organic solvent to obtain the PVDF-based composite solid electrolyte.

9. Application of the PVDF-based composite solid electrolyte according to any one of claims 1-7 or the PVDF-based composite solid electrolyte obtained by the preparation method according to claim 8 in a lithium battery.

10. A lithium battery, comprising a positive electrode, a negative electrode and an electrolyte, characterized in that, The electrolyte is the PVDF-based composite solid electrolyte according to any one of claims 1-7 or the PVDF-based composite solid electrolyte obtained by the preparation method according to claim 8.

Images