A method for modifying the interface of a graphite negative electrode of a solid-state lithium ion battery by using carbon-coated bismuth titanate

Abstract

The application discloses a modification method for a graphite negative electrode interface of a full solid-state lithium ion battery by using carbon-coated bismuth titanate, and aims at solving the problems of uneven lithium deposition, large interface impedance and poor contact caused by large polarization of a polymer electrolyte and a negative electrode interface. The specific steps are as follows: a certain mass ratio of carbon-coated bismuth titanate and a solid electrolyte precursor slurry is uniformly mixed to obtain an interface modification slurry, a modification layer is coated on the electrolyte by using a doctor blade method, and the electrolyte film with the interface modification layer is obtained by vacuum drying. In the application, the carbon-coated bismuth titanate is used as a modification layer of the polymer electrolyte, the interface of the electrolyte is improved by coating, lithium ions are uniformly induced to deposit and embed into the graphite negative electrode, the structure of the graphite is ensured to be complete, and the stable circulation of the battery is ensured, so that the battery has excellent long-term cycle capacity. The application is used in the field of lithium ion batteries.

Description

Technical Field

[0001] This invention belongs to the field of polymer electrolyte interface modification for all-solid-state lithium-ion batteries, specifically relating to a method for modifying the graphite anode interface of all-solid-state lithium-ion batteries using carbon-coated bismuth titanate. Background Technology

[0002] All-solid-state batteries are considered the most promising next-generation batteries due to their high energy density and safety. Using lithium metal as the anode in all-solid-state batteries is an ideal choice. However, lithium metal anodes have poor stability and can cause internal short circuits during cycling due to dendrite formation. Therefore, carbon has attracted widespread attention as an anode material for lithium-ion batteries in recent years. Carbon as an anode material avoids the thermal runaway induced by the highly reactive lithium and reduces the formation of lithium dendrites on the anode surface, thus achieving higher safety and superior cycle performance.

[0003] Graphite, a carbon material, is considered an ideal anode material for lithium-ion batteries due to its high stability, good conductivity, wide availability, high bulk capacity (LiC6), and low electrode potential (0.01–0.2V). However, the most significant drawback of using graphite as an anode is its low efficiency during the first charge-discharge cycle. Furthermore, repeated insertion and extraction during charge-discharge cycles can directly lead to the peeling or even pulverization of the graphite layer. The repeated rupture and formation of the solid electrolyte interfacial film also causes irreversible loss of active materials. As the solid electrolyte interfacial film gradually thickens, its ion-electron conduction effect gradually weakens, thereby increasing the irreversible capacity of the graphite electrode and impairing its rate performance and cycle performance (Sci.Rep.-UK, 2018, 8(1):3182.).

[0004] Solid-state lithium-ion batteries have three main components: cathode, anode, and solid electrolyte. During discharge, lithium ions and electrons migrate in opposite directions, accompanied by cathode reduction and anode oxidation. The electrolyte interface in a solid-state lithium-ion battery involves the following reaction steps: (i) lithium-ion diffusion in the electrolyte, (ii) charge transfer processes, (iii) lithium-ion diffusion in the electrode, and (iv) interfacial reactions. A stable and closely contacting interface is required to ensure the smooth progress of the above reaction steps. Gu et al. addressed this issue by using a solid electrolyte to address the problem of lithium-ion ion diffusion in solid-state lithium-ion batteries. 1.3 Al 0.3 Ti 1.7Poor contact between (PO4)3 (LATP) and the lithium electrode leads to side reactions, resulting in uneven lithium plating and high interfacial resistance, which significantly hinders the practical application of LATP in high-energy-density solid-state lithium-ion batteries (Adv. Energy Mater., 2021, e12531). A barium titanate / polyvinylidene fluoride-trifluoroethylene-trichlorofluoroethylene) composite electrolyte was constructed between LATP and the negative electrode. This not only improved interfacial stability and maintained a tight interfacial contact, but also significantly reduced the interfacial resistance by two orders of magnitude. The assembled symmetrical battery can achieve a voltage drop of 0.2 mA cm⁻¹. -2 and 0.5mA cm -2 Stable cycling performance was achieved for 1800 hours and 1000 hours under certain conditions. The assembled solid-state full cell also exhibited excellent cycling performance of up to 250 cycles at 0.5C and room temperature. Li et al. constructed a highly efficient artificial solid electrolyte interface film protective layer on graphite electrodes using LiPF6 based on ethyl methyl carbonate (EMC) electrolyte, which significantly improved the structure and cycling stability of graphite anode coupled with lithium (Adv. Mater., 2018, 1804766). At a current density of 200 mA g... -1 At an upper cutoff voltage of 5.0V, the battery's specific capacity reaches 84.5mAh g. -1 It is significantly higher than the unmodified 75.2mAh g. -1 Furthermore, it exhibits better cycle stability than unmodified graphite batteries at any cutoff voltage. For example, it retains up to 98% of its capacity after 2000 cycles at an upper cutoff voltage of 4.9V, while unmodified batteries fail after only 180 cycles. A stable interface is crucial for the electrochemical performance of solid-state lithium-ion batteries. Summary of the Invention

[0005] This invention addresses the problems of high interfacial impedance, uneven lithium-ion deposition, and significant interfacial polarization in existing polymer electrolytes by proposing a method for modifying the graphite anode interface of all-solid-state lithium-ion batteries using carbon-coated bismuth titanate. Compared to the original polymer electrolyte membrane, the modified polymer electrolyte membrane exhibits higher ionic conductivity, lower impedance, and superior cycle performance, characterized by sufficient contact between the anode and the polymer electrolyte membrane surface during full-cell cycling, and the formation of stable and rapid lithium-ion channels. The modified polymer electrolyte membrane prepared using this method can improve the charge / discharge specific capacity and cycle stability of the corresponding all-solid-state lithium-ion battery.

[0006] The objective of this invention can be achieved through the following method: a method for modifying the graphite anode interface of an all-solid-state lithium-ion battery using carbon-coated bismuth titanate, characterized in that the specific steps of the method are: adding a polymer electrolyte precursor solution to carbon-coated bismuth titanate, adding an organic solvent and stirring evenly, preparing a modified coating on the surface of the polymer electrolyte, and drying it in an oven to obtain a polymer electrolyte with a carbon-coated bismuth titanate modified interface layer; thus completing the method for modifying the graphite anode interface of an all-solid-state lithium-ion battery using carbon-coated bismuth titanate.

[0007] The bismuth titanate is prepared by a hydrothermal method, followed by washing with distilled water, centrifugal drying, and annealing in air. Since bismuth titanate is a strong ferroelectric material, the modified layer prepared using bismuth titanate not only induces uniform lithium-ion deposition but also increases the cycle life of the graphite anode. The ferroelectric modified layer exhibits a spontaneous dipole moment, which reverses in the battery's electric field, generating a reverse polarization electric field. This reverse polarization electric field neutralizes locally concentrated electric fields, mitigating the "sharp effect" and ensuring uniform lithium-ion distribution, enabling horizontal migration of lithium ions along its surface. During electroplating deposition, the electric field distribution is uneven near high curvature locations. Essentially, lithium ions move towards protrusions under the influence of a localized strong electric field near high curvature locations, resulting in irregular lithium-ion deposition. Due to the uniform lithium-ion transport provided by the carbon-coated bismuth titanate interlayer, lithium ions are uniformly embedded in the graphite anode, ensuring the long-term excellent performance of the solid-state battery.

[0008] The mass ratio of carbon-coated bismuth titanate to polymer electrolyte precursor solution is 1-5:5 to prepare a polymer electrolyte with a mixed conductive layer. Under certain conditions, if the electrolyte has a high ionic conductivity, its ion-conducting ability is strong, and the voltage difference between the positive and negative electrodes is small. It is easier to adjust the current density by negative electrode polarization to make the current density distributed uniformly on the positive electrode. If the electrolyte polarization is large, increasing the conductivity of the electroplating solution will not significantly improve the uniform deposition ability and depth deposition ability. Therefore, a polymer electrolyte with not only good conductivity but also a modified layer with a large polarization effect to form a mixed conductive layer is required.

[0009] It is further specified that the carbon-coated bismuth titanate is coated onto bismuth titanate using a sol-gel method, which involves coating bismuth titanate with one or more of amines, proteins, or sugars. These substances contain nitrogen, and the introduction of heteroatomic nitrogen can effectively disrupt the electroneutrality of nearby carbon atoms, thereby causing an asymmetry in charge distribution and resulting in defects at adjacent sites. This has advantages such as excellent conductivity and charge transport.

[0010] Furthermore, the molar ratio of bismuth titanate to carbon used in the preparation of carbon-coated bismuth titanate is 1:1-40. If the bismuth titanate content is too low, it will not be able to neutralize the locally concentrated electric field, alleviate the "sharp effect", and make the lithium ion distribution uniform. If the carbon content is too low, the interface polarization will increase. Therefore, it is necessary to effectively control the ratio of the two to achieve the optimal effect.

[0011] Furthermore, the carbon-coated bismuth titanate is prepared via a sol-gel method followed by carbonization at 600-900℃. The main reason for the poor cycle stability of the battery is the large internal polarization, which causes irreversible capacity loss and rapid performance degradation. Ensuring both interfacial ion conduction and electron conduction is crucial for reducing polarization. Coating the surface of bismuth titanate with a layer of carbon enhances interfacial electron conduction, which helps to improve the electrochemical reaction kinetics at the battery interface and delay battery aging.

[0012] It is specified that the organic solvent is one or more of styrene, trichloroethylene, tetrahydrofuran, triethanolamine, N-methylpyrrolidone, N,N-dimethylformamide, or N,N-dimethylacetamide, as an effective solvent for preparing polymer electrolyte precursor solutions.

[0013] Further specifying, the organic solvent is added at a rate of 5-30 drops / min to prevent the drug from adhering to the wall and causing unevenness of the polymer electrolyte precursor solution. The stirring time is 8-14 hours. The dropping rate and stirring are key factors in obtaining a uniform modified coating. The combination of the two with the polymer electrolyte and the modified coating will prepare a uniform modified coating, making the lithium ion deposition current density uniform, ensuring that lithium ions are uniformly embedded in the graphite anode, and ensuring the long-term cycle stability of the lithium-ion solid-state battery.

[0014] It is specified that the thickness of the modified coating is 10-50μm. This thickness can ensure the uniformity and flatness of the modified layer, form good polarization resistance, balance the current density, and enable uniform deposition of lithium ions.

[0015] Furthermore, the vacuum drying temperature is specified as 100-140℃. This vacuum drying temperature is higher than the boiling point of the solvent but much lower than the electrolyte decomposition temperature, allowing the solvent in the polymer electrolyte to completely evaporate into an all-solid electrolyte. During evaporation, micropores are formed, creating more lithium-ion transport channels, improving the conductivity of the all-solid polymer electrolyte, and ensuring the stability of the polymer electrolyte.

[0016] It is defined as an all-solid-state lithium-ion battery composed of a polymer electrolyte with a carbon-coated bismuth titanate modified interface layer.

[0017] Furthermore, it is specified that during battery assembly, the polymer electrolyte with the carbon-coated bismuth titanate modified interface layer comes into contact with the graphite anode. The ferroelectric bismuth titanate coating interface can neutralize locally concentrated deposition electric fields, generating a very uniform electric field distribution. This uniform electric field distribution leads to a more uniform lithium-ion concentration distribution at the interface. The uniform electric field and lithium-ion concentration distribution under strong ferroelectric effect are beneficial to interface deposition behavior, and constructing a strong ferroelectric intermediate layer can achieve stable and efficient ion transport at the interface. The carbon layer, which conducts electrons in the modified layer, is used in conjunction with the electrolyte layer, which is equivalent to connecting an equivalent resistance in series between the electrode and electrolyte interface, promoting uniform electron distribution and thus obtaining a uniform lithium deposition layer.

[0018] Compared to existing technologies, this invention effectively reduces the bulk and interfacial impedance of the polymer electrolyte by introducing carbon-coated bismuth titanate, enabling uniform lithium ion deposition and embedding in the graphite. This prevents structural collapse of the graphite anode, improving the charge / discharge specific capacity and capacity retention of the all-solid-state lithium-ion battery, achieving an initial discharge specific capacity of 164.8 mAh g at 0.5 rate. -1 After 100 cycles, it remained at 126.5 mAh g. -1 The attenuation is relatively small. Attached Figure Description

[0019] To more clearly illustrate the modified results of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below.

[0020] Figure 1 The image shows the XRD pattern of the graphite anode before pre-lithiation in the modification method of the present invention.

[0021] Figure 2 The image shows the XRD pattern of the graphite anode after pre-lithiation using the modification method of this invention.

[0022] Figure 3 This is a SEM image of the graphite anode before pre-lithiation using the modification method of the present invention.

[0023] Figure 4 This is a SEM image of the graphite anode after pre-lithiation using the modification method of the present invention.

[0024] Figure 5 The image shows the XRD pattern of bismuth titanate in the modification method of this invention.

[0025] Figure 6 This is a SEM image of bismuth titanate in the modification method of the present invention;

[0026] Figure 7 This is a SEM image of carbon-coated bismuth titanate in the modification method of the present invention;

[0027] Figure 8 This is a cross-sectional SEM image of the polymer electrolyte in the comparative example of this invention;

[0028] Figure 9 This is a cross-sectional SEM image of the polymer electrolyte in the modification method of the present invention;

[0029] Figure 10 This is the bulk impedance diagram of the polymer electrolyte in the comparative example of the present invention;

[0030] Figure 11 This is a bulk impedance diagram of the polymer electrolyte in the modification method of the present invention;

[0031] Figure 12 This is the bulk impedance diagram of the polymer electrolyte in Example 2 of the modification method of the present invention;

[0032] Figure 13 This is the bulk impedance diagram of the polymer electrolyte in Example 3 of the modification method of the present invention;

[0033] Figure 14 This is the bulk impedance diagram of the polymer electrolyte in Example 4 of the modification method of the present invention;

[0034] Figure 15 This is the bulk impedance diagram of the polymer electrolyte in Example 5 of the modification method of the present invention;

[0035] Figure 16 This is an impedance diagram of a polymer electrolyte assembled full cell in the comparative example of this invention;

[0036] Figure 17 The impedance diagram is shown for assembling a full cell using the polymer electrolyte in the modified method of this invention.

[0037] Figure 18 The figure shows the specific capacity-efficiency diagram of the comparative assembly of the full cell of the present invention under 0.5 rate cycling. From top to bottom, the figures represent the coulombic efficiency and the charge / discharge specific capacity.

[0038] Figure 19 The specific capacity-efficiency diagram of the full cell assembled by the modified method of the present invention is shown in the figure at 0.5 rate cycling. The figures from top to bottom represent the coulombic efficiency and charge / discharge specific capacity, respectively.

[0039] Figure 20 The diagram shows the specific capacity-efficiency of the assembled full cell in Example 6 of the modified method of the present invention during cycling at 0.5 rate. From top to bottom, the diagram represents the coulombic efficiency and the charge / discharge specific capacity.

[0040] Figure 21 The diagram shows the specific capacity-efficiency of the full cell assembled in Example 7 of the modified method of the present invention during cycling at 0.5 rate. From top to bottom, the diagram represents the coulombic efficiency and the charge / discharge specific capacity.

[0041] Figure 22The specific capacity-efficiency diagram of the full cell assembled by the modified method of the present invention is shown in the figure at 2% rate cycling. The figures from top to bottom represent coulombic efficiency and charge / discharge specific capacity, respectively.

[0042] Figure 23 This is a SEM image of the graphite surface after 50 cycles of the assembled full battery according to the present invention.

[0043] Figure 24 SEM image of the graphite surface after 50 cycles of a full battery assembled using the modified method of this invention. Detailed Implementation

[0044] The following embodiments further illustrate the above-mentioned content of the present invention in detail. However, the subject matter of the present invention is not limited to the following embodiments, and all technologies implemented based on the above-mentioned content of the present invention fall within the scope of the present invention.

[0045] Experimental drugs

[0046]

[0047] Experimental equipment

[0048]

[0049] Comparative Example 1

[0050] The polymer electrolyte precursor solution was adjusted to an appropriate concentration using N,N-dimethylformamide and dried in an oven at 120°C. The resulting polymer electrolyte was then assembled into a lithium iron phosphate-graphite full cell.

[0051] Example 1

[0052] Bismuth titanate prepared and annealed via hydrothermal method was mixed with melamine at a molar ratio of 1:20 and carbonized at 800℃ to obtain carbon-coated bismuth titanate powder. 0.1 g of the carbon-coated bismuth titanate powder was weighed into a weighing bottle and mixed thoroughly with a polymer electrolyte precursor slurry at a mass ratio of 2:5. N,N-dimethylformamide was added dropwise to adjust the concentration to an appropriate level. The mixture was then coated onto the surface of a polymer electrolyte film with a thickness of 50 μm and dried in an oven at 120℃. The resulting polymer electrolyte was assembled into a lithium iron phosphate-graphite full battery.

[0053] Example 2

[0054] Bismuth titanate prepared and annealed via hydrothermal method was mixed with melamine at a molar ratio of 1:1 and carbonized at 800℃ to obtain carbon-coated bismuth titanate powder. 0.1 g of the carbon-coated bismuth titanate powder was weighed into a weighing bottle and mixed thoroughly with a polymer electrolyte precursor slurry at a mass ratio of 2:5. N,N-dimethylformamide was added dropwise to adjust the concentration to an appropriate level. The mixture was then coated onto the surface of a polymer electrolyte film with a thickness of 50 μm and dried in an oven at 120℃. The resulting polymer electrolyte was assembled into a lithium iron phosphate-graphite full battery.

[0055] Example 3

[0056] Bismuth titanate prepared and annealed via hydrothermal method was mixed with melamine at a molar ratio of 1:10 and carbonized at 800℃ to obtain carbon-coated bismuth titanate powder. 0.1g of the carbon-coated bismuth titanate powder was weighed into a weighing bottle and mixed evenly with polymer electrolyte precursor slurry at a mass ratio of 2:5. N,N-dimethylformamide was added dropwise to adjust the concentration to an appropriate level. The mixture was then coated onto the surface of a polymer electrolyte film with a thickness of 50μm and dried in an oven at 120℃. The resulting polymer electrolyte was assembled into a lithium iron phosphate-graphite full battery.

[0057] Example 4

[0058] Bismuth titanate prepared and annealed via hydrothermal method was mixed with melamine at a molar ratio of 1:30 and carbonized at 800℃ to obtain carbon-coated bismuth titanate powder. 0.1 g of the carbon-coated bismuth titanate powder was weighed into a weighing bottle and mixed thoroughly with a polymer electrolyte precursor slurry at a mass ratio of 2:5. N,N-dimethylformamide was added dropwise to adjust the concentration to an appropriate level. The mixture was then coated onto the surface of a polymer electrolyte film with a thickness of 50 μm and dried in an oven at 120℃. The resulting polymer electrolyte was assembled into a lithium iron phosphate-graphite full cell.

[0059] Example 5

[0060] Bismuth titanate prepared and annealed via hydrothermal method was mixed with melamine at a molar ratio of 1:40 and carbonized at 800℃ to obtain carbon-coated bismuth titanate powder. 0.1 g of the carbon-coated bismuth titanate powder was weighed into a weighing bottle and mixed thoroughly with a polymer electrolyte precursor slurry at a mass ratio of 2:5. N,N-dimethylformamide was added dropwise to adjust the concentration to an appropriate level. The mixture was then coated onto the surface of a polymer electrolyte film with a thickness of 50 μm and dried in an oven at 120℃. The resulting polymer electrolyte was assembled into a lithium iron phosphate-graphite full battery.

[0061] Example 6

[0062] Bismuth titanate prepared and annealed via hydrothermal method was mixed with melamine at a molar ratio of 1:20 and carbonized at 800℃ to obtain carbon-coated bismuth titanate powder. 0.1 g of the carbon-coated bismuth titanate powder was weighed into a weighing bottle and mixed evenly with polymer electrolyte precursor slurry at a mass ratio of 2:5. N,N-dimethylformamide was added dropwise to adjust the concentration to an appropriate level. The mixture was then coated onto the surface of a polymer electrolyte film with a thickness of 10 μm and dried in an oven at 120℃. The resulting polymer electrolyte was assembled into a lithium iron phosphate-graphite full battery.

[0063] Example 7

[0064] Bismuth titanate prepared and annealed via hydrothermal method was mixed with melamine at a molar ratio of 1:20 and carbonized at 800℃ to obtain carbon-coated bismuth titanate powder. 0.1 g of the carbon-coated bismuth titanate powder was weighed into a weighing bottle and mixed thoroughly with a polymer electrolyte precursor slurry at a mass ratio of 2:5. N,N-dimethylformamide was added dropwise to adjust the concentration to an appropriate level. The mixture was then coated onto the surface of a polymer electrolyte film with a thickness of 30 μm and dried in an oven at 120℃. The resulting polymer electrolyte was assembled into a lithium iron phosphate-graphite full battery.

[0065] Performance characterization was performed on the above embodiments and comparative examples.

[0066] 1) Impedance Measurement. The resistance of polymer electrolytes is an important standard for evaluating their performance and has a significant impact on the charge-discharge performance of batteries. Electrochemical impedance spectroscopy (EIS) was used to measure the resistance of polymer electrolytes. The instrument was a Shanghai Chenhua CHI760E electrochemical workstation with a frequency range of 0.01-100000Hz. Before testing, the polymer electrolyte was dried, and stainless steel (SS) sheets were used as inert electrodes to assemble stainless steel symmetrical blocking batteries for testing.

[0067] 2) Charge and discharge test. Charge and discharge tests are used to obtain many important parameters of the battery during cycling, such as charge / discharge specific capacity, charge / discharge efficiency, voltage plateau, etc. The instrument used is the LAND Battery Testing System CT2001A, with the voltage set at 2.0V-3.8V and the current rate at 0.5C. Polymer electrolytes are assembled into lithium iron phosphate-graphite full cells for testing.

[0068] 3) Interfacial impedance testing. Electrochemical impedance spectroscopy was used to test the interfacial impedance between the polymer electrolyte film and the electrode to determine the effect of the modified layer on the stability of the electrolyte-electrode interface. The electrochemical workstation was a CHI760E with a frequency of 0.01-100000Hz. Lithium iron phosphate-graphite full cells were assembled for testing.

[0069] 4) Scanning Electron Microscopy (SEM) Testing. The surface morphology of the lithium sheet was observed using a scanning electron microscope (SEM). The instrument model was FEI sirion200, the accelerating voltage was 0.2-30kV, and the resolution was 20kV. The prepared sample was dried, quenched in liquid nitrogen, and the resulting sample was attached to a sample holder coated with conductive adhesive for testing.

[0070] 5) X-ray diffraction (XRD) test. The phase and crystallinity of the prepared sample can be determined by comparing it with a standard card. X-ray diffraction of bismuth titanate material is performed using an X-ray crystal diffractometer, and the obtained data is used to determine the phase and whether it is a pure phase.

[0071] Figure 1 The image shows the XRD pattern of the graphite anode before pre-lithiation in the modified method of this invention. The XRD pattern shows that the diffraction peaks correspond to hexagonal graphite carbon (PDF#01-089-8487) and cubic copper foil current collector (PDF#01-085-1326), respectively.

[0072] Figure 2 The image shows the XRD pattern of the graphite anode after pre-lithiation using the modification method of this invention. The diffraction peaks correspond to cubic LiC6 (PDF#00-034-1320) and cubic copper foil current collector (PDF#00-004-0836), respectively.

[0073] Figure 3 This is a SEM image of the graphite anode before pre-lithiation using the modification method of this invention. The graphite surface is relatively flat and uniform.

[0074] Figure 4 This is a SEM image of the graphite anode after pre-lithiation using the modification method of this invention. The pre-lithiated graphite forms LiC6, which can replenish the active material lost during subsequent battery cycling.

[0075] Figure 5 The image shows the XRD pattern of bismuth titanate in the modification method of this invention. Its diffraction peaks correspond to the cubic phase of bismuth titanate (PDF#01-085-1326), and no other impurity phases are present, indicating that pure-phase bismuth titanate was successfully prepared.

[0076] Figure 6 This is a SEM image of bismuth titanate in the modification method of this invention. As can be seen from the image, bismuth titanate first forms a spherical structure, and then nanosheet-like structures grow on its surface. The presence of the nanosheet-like structures provides the shortest migration path for lithium ions, thereby increasing the migration rate of lithium ions.

[0077] Figure 7This is a SEM image of carbon-coated bismuth titanate in the modification method of this invention. As can be seen from the image, the bismuth titanate is completely encapsulated by carbon, providing an effective conductive pathway at the interface, thus effectively reducing polarization and ensuring long-term stable cycling of the battery.

[0078] Figure 8 This is a cross-sectional SEM image of the polymer electrolyte in the comparative example of this invention. The unmodified polymer electrolyte does not exhibit a layered structure.

[0079] Figure 9 This is a cross-sectional SEM image of the polymer electrolyte in the modification method of this invention. A 10 μm interface layer is clearly visible in the cross-section of the polymer electrolyte modified with carbon-coated bismuth titanate. The presence of this interface layer can reduce polarization and enable stable battery cycling.

[0080] Figure 10 This is the bulk impedance diagram of the polymer electrolyte in the comparative example of this invention. The impedance of the polymer electrolyte is approximately 24 Ω, and the calculated ionic conductivity is 3.82 × 10⁻⁶. -4 S cm -1 .

[0081] Figure 11 This is the bulk impedance diagram of the polymer electrolyte in the modification method of this invention. The bulk impedance of the polymer electrolyte after modification with bismuth titanate is approximately 13 Ω, and the calculated ionic conductivity is 4.03 × 10⁻⁶. -4 S cm -1 The results showed that the carbon-coated bismuth titanate interface layer was more conducive to lithium ion migration.

[0082] Figures 12 to 15 The figures show the bulk impedance diagrams of the polymer electrolytes in Examples 2 to 5 of the modification method of this invention. The calculated ionic conductivity of Examples 2, 3, 4, and 5 is 2.98 × 10⁻⁶. -4 3.48×10 -4 3.25×10 -4 and 3.11×10 -4 S cm -1 All were lower than Figure 11 The ionic conductivity of Example 1 shows that the optimal ratio of bismuth titanate to carbon is 1:20.

[0083] Figure 16 This is an impedance diagram of the polymer electrolyte-assembled full cell in the comparative example of this invention. The interfacial impedance (R0) of the unmodified polymer electrolyte-assembled lithium iron phosphate-graphite full cell is shown. f The Ω is 1326Ω.

[0084] Figure 17This is an impedance diagram of a full cell assembled with the polymer electrolyte using the modification method of this invention. The interfacial impedance (R0) of the lithium iron phosphate-graphite full cell assembled with the interface-modified polymer electrolyte is shown. f The impedance is 483 Ω, which is much smaller than that before modification with carbon-coated bismuth titanate. This indicates that the occurrence of interfacial side reactions is suppressed by the modified layer. The induced uniform deposition of lithium ions effectively reduces interfacial impedance and improves interfacial stability.

[0085] Figure 18 This is a capacity-efficiency graph showing the specific capacity of the comparative assembled full cell of this invention during cycling at 0.5 rate. From top to bottom, the graph represents coulombic efficiency and charge / discharge specific capacity. The initial discharge specific capacity of the polymer electrolyte at 0.5 rate is 115 mAh g⁻¹. -1 After 100 cycles, the capacity decayed to 42 mAh g. -1 Its capacity decays relatively quickly.

[0086] Figure 19 The graph shows the specific capacity-efficiency of the full cell assembled using the modified method of this invention during cycling at 0.5 rate. From top to bottom, the graph represents coulombic efficiency and charge / discharge specific capacity. The initial discharge specific capacity of the interface-modified polymer electrolyte at 0.5 rate is 164.8 mAh g⁻¹. -1 After 100 cycles, the mAh capacity was 126.5 g. -1 The attenuation is relatively small.

[0087] Figure 20 and Figure 21 The diagram shows the specific capacity-efficiency of the full cell assembled by the modified method of the present invention during cycling at 0.5 rate. From top to bottom, the diagram represents the coulombic efficiency and the charge / discharge specific capacity. Figure 20 and Figure 21 The interface-modified polymer electrolyte exhibits an initial discharge specific capacity of 127.9 mAh g at 0.5 rate. -1 and 133.6mAh g -1 After 100 cycles, the concentration was 100.6 mAh g. -1 and 81.5mAh g -1 Its cycle performance is poor.

[0088] Figure 22 The graph shows the specific capacity-efficiency of the full cell assembled using the modified method of this invention during cycling at 2% rate. From top to bottom, the graph represents coulombic efficiency and charge / discharge specific capacity. The initial discharge specific capacity of the interface-modified polymer electrolyte at 2% rate is 92.3 mAh g⁻¹. -1 After 100 cycles, the mAh g content was 97.8 mAh. -1Overall, the performance remained stable. This further confirms that the carbon-coated bismuth titanate coating can form an effective ion-electron dual-conductivity channel at the interface, reducing polarization and thus improving the battery's cycle performance.

[0089] Figure 23 This is a SEM image of the graphite surface after 50 cycles of the comparative assembly of the full battery of this invention. The degree of lithium deposition is a key factor in evaluating the effect of the modified coating on the polymer electrolyte and electrode interface. Characterization results of 50 cycles of the unmodified polymer electrolyte-assembled lithium iron phosphate-graphite full battery show that, with the progress of cycling, the dendrite growth on the graphite surface is uneven, indicating that the lithium ion deposition in the unmodified polymer electrolyte is uneven, which can easily cause the structural collapse or even pulverization of the graphite anode, leading to battery failure.

[0090] Figure 24 This is a SEM image of the graphite surface after 50 cycles of a full battery assembled using the modified method of this invention. After 50 cycles, the graphite surface of the lithium iron phosphate-graphite full battery assembled with the bismuth titanate-modified polymer electrolyte showed no large, protruding dendrites, and the dendrites were uniformly distributed. This indicates that lithium dendrites still grow on the graphite surface after bismuth titanate modification, but the dendrites are uniformly distributed, flatter, and denser, and will not puncture the film to cause a short circuit, thus solving the problem of lithium dendrite aggregation and growth in lithium-ion batteries. During cycling, due to uneven lithium-ion deposition, lithium ions tend to aggregate on the formed dendritic surface. With increasing deposition, lithium plating becomes severe, not only causing loss of active material but also easily puncturing the film, causing safety issues. The bismuth titanate-modified polymer electrolyte promotes uniform lithium-ion deposition and embedding into the graphite anode, delaying the structural collapse of the graphite anode. Combined with charge-discharge performance, this shows that the interface layer modified with bismuth titanate can perfectly solve the lithium plating problem and has good electrode / electrolyte interface compatibility.

Claims

1. A method for modifying the graphite anode interface of an all-solid-state lithium-ion battery using carbon-coated bismuth titanate, characterized in that... The specific steps of the method are as follows: add polymer electrolyte precursor solution to carbon-coated bismuth titanate, add organic solvent and stir evenly, prepare a modified coating on the surface of polymer electrolyte, and dry in an oven to obtain polymer electrolyte with carbon-coated bismuth titanate modified interface layer; thus completing the method of modifying the graphite anode interface of all-solid-state lithium-ion battery using carbon-coated bismuth titanate. The bismuth titanate is prepared by hydrothermal method, followed by washing with distilled water, centrifugal drying, and annealing in air; The mass ratio of the carbon-coated bismuth titanate to the polymer electrolyte precursor solution is 1-5:

5.

2. The method for modifying the graphite anode interface of an all-solid-state lithium-ion battery using carbon-coated bismuth titanate according to claim 1, characterized in that, The carbon-coated bismuth titanate is produced by coating one or more of amines, proteins, or sugars onto bismuth titanate using a sol-gel method.

3. A method for modifying the graphite anode interface of an all-solid-state lithium-ion battery using carbon-coated bismuth titanate according to claim 1 or 2, characterized in that, The molar ratio of bismuth titanate to carbon used in the preparation of carbon-coated bismuth titanate is 1:1-40.

4. A method for modifying the graphite anode interface of an all-solid-state lithium-ion battery using carbon-coated bismuth titanate according to claim 1 or 2, characterized in that, The carbon-coated bismuth titanate is prepared by sol-gel method and then carbonized at 600-900℃.

5. The method for modifying the graphite anode interface of an all-solid-state lithium-ion battery using carbon-coated bismuth titanate according to claim 1, characterized in that, The organic solvent is one or more of styrene, trichloroethylene, tetrahydrofuran, triethanolamine, N-methylpyrrolidone, N,N-dimethylformamide, or N,N-dimethylacetamide.

6. The method for modifying the graphite anode interface of an all-solid-state lithium-ion battery using carbon-coated bismuth titanate according to claim 1, characterized in that, The organic solvent is added dropwise at a rate of 5-30 drops / min, and the stirring time is 8-14 hours.

7. The method for modifying the graphite anode interface of an all-solid-state lithium-ion battery using carbon-coated bismuth titanate according to claim 1, characterized in that, The thickness of the modified coating is 10-50 μm.

8. The method for modifying the graphite anode interface of an all-solid-state lithium-ion battery using carbon-coated bismuth titanate according to claim 1, characterized in that, The drying in the oven is achieved through vacuum drying, and the vacuum drying temperature is 100-140℃.

9. An all-solid-state lithium-ion battery composed of a polymer electrolyte with a carbon-coated bismuth titanate modified interface layer prepared as described in claim 1.

10. The all-solid-state lithium-ion battery according to claim 9, characterized in that, In the assembly of the battery, the polymer electrolyte with the carbon-coated bismuth titanate modified interface layer comes into contact with the graphite negative electrode through the modified interface.

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