Preparation method of composite polymer solid-state electrolyte, and solid-state battery

Abstract

The application provides a preparation method of a composite polymer solid electrolyte and a solid-state battery, and belongs to the technical field of solid electrolytes. In the preparation process of the solid electrolyte, trace impurities in the system are removed in situ through the synergistic effect of tris(trimethylsilyl) phosphite and modified quantum dots, and the dissolution of transition metals on the positive electrode side is inhibited. Meanwhile, through the induction effect of the quantum dots, a stable deposition interface is constructed on the negative electrode side, the mechanical performance deterioration and the side reaction on the negative electrode side caused by the introduction of TMSPi are overcome, and long cycle stability of a high-voltage solid-state battery is achieved.

Description

Technical Field

[0001] This invention belongs to the field of high-nickel ternary solid-state battery technology, specifically relating to a method for preparing a composite polymer solid electrolyte and a solid-state battery. Background Technology

[0002] With the rapid development of new energy vehicles and large-scale energy storage technologies, the energy density and safety of lithium-ion batteries have become research hotspots. Among these, the use of high-nickel ternary cathode materials (such as LiNi) is particularly important. 0.8 Co 0.1 Mn 0.1 All-solid-state lithium batteries (NCM811) with matching solid-state electrolytes (O2) are considered an important direction for next-generation high-energy batteries due to their combination of high specific capacity and high safety. A key component of all-solid-state lithium batteries is the solid-state electrolyte. Among various solid-state electrolytes, polymer-based electrolytes, represented by polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE), have attracted widespread attention due to their excellent machinability, high dielectric constant, and good interfacial contact performance. However, high-nickel cathode materials have high surface activity and are prone to severe interfacial side reactions with the polymer matrix during cycling, leading to a sharp increase in interfacial impedance and a decline in battery cycle life. This has become a major bottleneck limiting the commercial application of this system.

[0003] To improve interface stability, functional additives have been widely studied and applied in the traditional field of liquid lithium-ion batteries. For example, trimethylsilane phosphite (TMSPi) is a well-known electrolyte additive. Existing technologies (such as Angew. Chem. Int. Ed. 2022, e202216354 and Acta Chimica Sinica 2018, 76(4): 259-277) show that in liquid carbonate or ether electrolyte systems, TMSPi mainly plays two roles: first, as a scavenger, it removes trace amounts of water and hydrofluoric acid (HF) in the system through chemical reactions, inhibiting the decomposition of electrolyte salts; second, it participates in the film-forming reaction on the electrode surface to form a solid electrolyte interphase (SEI / CEI) film rich in silicon (Si) and phosphorus (P), thereby inhibiting the continuous decomposition of solvent molecules.

[0004] Despite the significant performance of TMSPi in liquid battery systems, there are few research reports on its application in solid-state electrolytes, especially PVDF-based polymer electrolytes. This is mainly because the physicochemical environment of solid-state electrolytes differs significantly from that of liquid systems, and direct conversion faces many uncertainties: First, the direct introduction of liquid small-molecule additives can disrupt the intermolecular forces between polymer chains, leading to a significant decrease in the mechanical strength of the electrolyte membrane, softening, or even excessive viscosity, making it difficult to meet the structural stability requirements of solid-state batteries. Second, there are limitations to single modification methods. Existing modification strategies for solid-state electrolytes mainly focus on adding inert inorganic fillers (such as TiO2 and SiO2) to reduce crystallinity. Although this physical doping can improve ionic conductivity, it cannot actively suppress the HF removal reaction of the PVDF matrix, nor can it effectively capture dissolved transition metal ions. While TMSPi, if used alone, may suppress HF, the accumulation of its decomposition products at the solid-state interface may lead to poor interfacial contact and is prone to uncontrollable side reactions with lithium on the negative electrode side.

[0005] Therefore, existing PVDF-based solid electrolyte technologies still struggle to simultaneously meet the comprehensive requirements of excellent mechanical properties, high ionic conductivity, and a wide electrochemical window needed to match high-nickel cathodes. Summary of the Invention

[0006] To address the problems of existing PVDF-based solid electrolytes in high-voltage ternary cathode (e.g., NCM811) systems, such as poor oxidation resistance, susceptibility to hydrofluoric acid (HF) corrosion leading to transition metal dissolution, and the tendency of introducing a single acid-removing additive (e.g., TMSPi) to induce side reactions at the lithium metal anode interface, resulting in lithium dendrite growth and increased interfacial impedance, this invention provides a composite solid electrolyte containing tris(trimethylsilane) phosphite and modified quantum dots as synergistic additives, along with its preparation method. This solid electrolyte can remove trace impurities in situ from the system, suppress transition metal dissolution on the cathode side, and simultaneously construct a stable deposition interface on the anode side through the induction effect of quantum dots. This overcomes the mechanical performance degradation and anode-side side reactions caused by the introduction of TMSPi, achieving long-cycle stability of high-voltage solid-state batteries.

[0007] To achieve the above objectives, the technical solution of the present invention is as follows:

[0008] A method for preparing a composite polymer solid electrolyte includes the following steps:

[0009] Step 1. Histidine carbon quantum dots are prepared using a one-pot method;

[0010] Step 2. Add the polymer matrix, lithium salt, plasticizer, and film-forming additive to an organic solvent and stir until homogeneous to obtain solution A; the polymer matrix is ​​PVDF-TrFE and the film-forming additive is fluoroethylene carbonate (FEC).

[0011] Step 3. Add histidine carbon quantum dots to solution A, stir to disperse evenly, and then add tris(trimethylsilane) phosphite ester TMSPI to obtain a mixed slurry;

[0012] Step 4. The mixed slurry is evenly coated on the substrate to form an electrolyte membrane, and then dried. After drying, the membrane is peeled off from the surface of the substrate to obtain the desired PVDF-TrFE polymer solid electrolyte.

[0013] Furthermore, the specific process of step 1 is as follows:

[0014] The raw materials are dissolved in deionized water and reacted at 120-200℃ for 2-6 hours. After the reaction is completed, the reaction product is naturally cooled, purified by dialysis, and freeze-dried to obtain histidine-functionalized carbon quantum dot powder with uniform particle size.

[0015] The raw materials include carbon and nitrogen sources;

[0016] The carbon source is citric acid (CA).

[0017] The nitrogen source is histidine (His).

[0018] Furthermore, the molar ratio of citric acid to histidine is 1:0.06.

[0019] Further, in step 2, the lithium salt is any one of lithium bis(fluorosulfonyl)imide (LiFSI), lithium hexafluorophosphate (LiPF6), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI); the organic solvent is any one of dimethylformamide (DMF), acetonitrile (AN), N-methylpyrrolidone (NMP), acetone, tetrahydrofuran (THF), dimethyl sulfoxide (DMSO), and trimethyl phosphate (TEP); and the plasticizer is any one of succinic anionyl (SN), adiponitrile (ADN), glutaronitrile (GN), ethylene carbonate (EC), sulfolane (SL), trimethylolpropane (TMP), and polyethylene glycol (PEG).

[0020] Furthermore, the mass percentage of fluoroethylene carbonate to the polymer matrix is ​​5%-10%; the mass percentage of histidine-functionalized carbon quantum dots to the polymer matrix is ​​2%-3%.

[0021] Furthermore, the mass percentage of TMSPI to the polymer matrix is ​​1%-5%.

[0022] The present invention also provides a solid-state battery prepared based on the above-mentioned composite polymer solid-state electrolyte, comprising a positive electrode, a negative electrode and a composite polymer solid-state electrolyte;

[0023] The positive electrode is NCM811; the negative electrode is lithium metal.

[0024] The mechanism of this invention is as follows:

[0025] The synergistic enhancement mechanism of the electrolyte system in this invention is primarily manifested in its chemical protection and stabilization effect on the high-nickel cathode interface. The introduced TMSPI molecules, anchored in the PVDF-TrFE polymer matrix, maintain excellent chemical activity. Addressing the trace HF acidic impurities in the battery system caused by residual solvents (such as DMF) and residual moisture or component decomposition, the highly active Si-OP bonds in the TMSPI molecules can actively capture and consume HF using the extremely high bond energy of the Si-F bonds, thus blocking the lattice corrosion of the NCM811 cathode material by acidic species at the source and inhibiting the dissolution of transition metal ions. Simultaneously, in the initial stage of high-voltage (≥4.2V) cycling, the TMSPI dispersed at the cathode interface preferentially undergoes oxidative decomposition before the solvent, constructing a dense cathode-electrolyte interface (CEI) film rich in Si and P elements in situ. This effectively blocks continuous side reactions between the electrolyte and the cathode material, ensuring the battery's cycling stability under high voltage.

[0026] On the negative electrode side and within the polymer matrix, this system significantly enhances lithium-ion deposition behavior and transport kinetics through multi-component synergy. The abundant imidazole groups on the His-CQDs surface exhibit significant lithiophilicity, effectively homogenizing the lithium-ion flow distribution on the negative electrode surface. Through synergy with LiF, a decomposition product of TMSPI—LiF providing high interfacial mechanical strength and imidazole groups providing excellent adsorption sites—the system jointly induces dense, dendrite-free lithium-ion deposition. Furthermore, in this system of His-CQDs and TMSPI synergy, TMSPI, as a small organic molecule, acts as a plasticizing agent similar to a "molecular wedge," penetrating between long polymer chains and effectively increasing the distance between them, thereby disrupting the spatial conditions required for long-range ordered chain arrangement. Simultaneously, the nanoscale His-CQDs, with their extremely large specific surface area, act as a heterogeneous physical barrier, interrupting the growth of continuous crystalline regions. More importantly, the imidazole groups modified on the His-CQDs surface possess electron-rich properties, enabling them to generate strong Lewis acid-base interactions or dipole-dipole interactions with the highly electronegative fluorine atoms (-F) on the PVDF-TrFE backbone. This microscopic "chemical anchoring" effect restricts the free creep and rearrangement of polymer segments during film formation, making it difficult for them to form regular spherulitic structures through folding. The dual introduction of His-CQDs and TMSPI significantly modifies the semi-crystalline polymer PVDF-TrFE. As heterogeneous nucleation sites or intermolecular barriers, both effectively disrupt the regular arrangement of PVDF-TrFE molecular chains, reducing the crystallinity of the polymer and significantly increasing the proportion of amorphous regions. This provides richer channels for lithium ion migration between polymer segments, significantly improving the room-temperature ionic conductivity of the solid electrolyte.

[0027] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are:

[0028] This invention utilizes PVDF-TrFE as the polymer matrix in its solid electrolyte. By introducing TMSPI and imidazole-functionalized carbon quantum dots (His-CQDs) as synergistic modifiers, the synergistic effect of these two modifiers disrupts polymer crystallization, increases the proportion of amorphous regions, and improves room-temperature ionic conductivity. Simultaneously, TMSPI's acid removal and in-situ film-forming capabilities effectively protect the positive electrode interface, while the lithiophilicity of His-CQDs and the LiF decomposition product of TMSPI induce uniform lithium-ion deposition on the negative electrode side. This results in a comprehensive performance improvement, including high ion transport efficiency (ionic conductivity 0.5408 mS / cm), excellent interface stability, and long cycle life (80% capacity retention after 100 cycles when matched with an NCM811 positive electrode). Attached Figure Description

[0029] Figure 1 The images show physical images of the composite polymer solid electrolytes prepared in Example 1 and Comparative Example 1.

[0030] Figure 2 shows the LSV curves of the solid electrolyte obtained with different TMSPI addition amounts.

[0031] Figure 3 The ionic conductivity of solid electrolytes obtained with different amounts of TMSPI is shown in the graph.

[0032] Figure 4 SEM images of the lithium anode surface after 100 h of cycling of the Li-Li symmetric batteries assembled with polymer solid electrolytes prepared in Example 1 and Comparative Example 1.

[0033] Figure 5 Images obtained by inductively coupled plasma atomic emission spectrometry (ICP-AES) of NCM811|electrolyte|Li lithium metal batteries assembled with solid electrolytes obtained in Example 1 and Comparative Example 1 after 100 cycles.

[0034] Figure 6 The 0.5C cycling curves of NCM811-Li half-cells assembled with solid electrolytes prepared in Example 1 and Comparative Example 1 are shown.

[0035] Figure 7 Images of the NCM811 positive electrode side interface film (CEI film) after 60 cycles of the NCM811-Li half-cell assembled with the solid electrolytes prepared in Example 1 and Comparative Example 1. Detailed Implementation

[0036] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the embodiments and accompanying drawings.

[0037] Example 1

[0038] A method for preparing a composite polymer solid electrolyte includes the following steps:

[0039] Step 1. Prepare histidine carbon quantum dots using a one-pot method:

[0040] 19.2123 g of citric acid and 0.9310 g of histidine were dissolved in 40 mL of deionized water and reacted at 180 °C for 3 hours. After the reaction was completed, the reaction product was naturally cooled, purified by dialysis, and freeze-dried to obtain histidine-functionalized carbon quantum dot powder with uniform particle size.

[0041] Step 2. Add 1.00g of polymer matrix, 0.50g of LiTFSI, 0.20g of SN, and 0.01g of film-forming additive to 4.5556g of DMF, stir until homogeneous, and obtain solution A; the polymer matrix is ​​PVDF-TrFE, and the film-forming additive is fluoroethylene carbonate (FEC).

[0042] Step 3. Add 0.02g of histidine carbon quantum dots to solution A, stir and disperse evenly, then add TMSPI to obtain a mixed slurry;

[0043] Step 4. The mixed slurry is evenly coated on the substrate to form an electrolyte membrane. After rapid drying in a 65°C forced-air oven for 20 minutes, it is dried at 55°C and a vacuum of -85 kPa for 2 hours. After drying, the membrane is peeled off from the substrate surface to obtain the desired PVDF-TrFE polymer solid electrolyte.

[0044] Example 2

[0045] The solid electrolyte was prepared according to the steps of Example 1, except that the amount of TMSPI in step 3 was adjusted to 1%.

[0046] Comparative Example 1

[0047] Solid electrolytes were prepared according to the steps of Example 1, except that the amount of TMSPI in step 3 was adjusted to 0%, 5%, and 10%.

[0048] Figure 1 Physical images of the solid electrolytes prepared in Examples 1, 2, and Comparative Example 1. Figure 1 As shown in the figure, the solid electrolytes prepared in Examples 1 and 2 have smooth surfaces and are complete, dry films; while the solid electrolytes prepared in Comparative Example 1 with higher contents of 5% and 10% TMSPI have poor processability and are not easy to peel off from the substrate, and are prone to stringing and breakage.

[0049] Figure 2 shows the LSV curves of the solid electrolyte obtained with different TMSPI addition amounts. LSV can reveal the redox characteristics, reactivity, and electrochemical stability of the electrode. In this study, a CHI660D electrochemical workstation was used for LSV testing, and a steel sheet was used as the working electrode, while lithium sheets were used as the reference and auxiliary electrodes, respectively, to construct a steel-to-lithium stainless steel / solid electrolyte / lithium sheet coin cell for linear sweep voltammetry (LSV) testing. During the test, the voltage scan range was set from 0 to 6 V to cover the possible operating range of the battery. To ensure the accuracy and reliability of the test, the voltage scan rate was set to 0.1 mV / s, thereby systematically evaluating the electrochemical response of the electrolyte over a wide voltage range. The upper limit of the electrochemical window corresponds to the oxidation decomposition potential of the electrolyte. The wider the window, the higher the oxidation potential, and the higher the charge cut-off voltage the electrolyte can withstand. This allows the battery to be matched with a high-voltage cathode material (NCM811), thereby directly improving the overall energy density of the battery. Figure 2 As shown, with the increase of the amount added, the electrochemical window gradually widens and the electrolyte oxidation decomposition potential gradually increases, which means that the high voltage resistance gradually becomes stronger.

[0050] Figure 3 shows the ionic conductivity diagram. The bulk impedance of the polymer solid electrolyte membrane can be measured by applying a sinusoidal voltage signal with a frequency range of 0.1–1 MHz and a perturbation voltage of 10 mV to the assembled stainless steel / solid electrolyte / stainless steel membrane at a test temperature of 25 °C. The test results are based on the formula: σ = d / (S × R) b ) calculate, where d is the actual thickness of the polymer semi-solid electrolyte; S is the effective relative area of ​​the two electrodes; R b This represents the bulk impedance of the polymer semi-solid electrolyte. Ionic conductivity can effectively reflect the ion transport characteristics of polymer electrolytes, providing important information for optimizing electrolyte performance. For example... Figure 3 As shown, with the increase of TMSPI addition, the ionic conductivity does not increase in a straight line, but rather increases first and then decreases. This is because the introduction of a small amount of TMSPI can increase the amorphous region of the electrolyte and improve ionic conductivity. When the addition is excessive, the viscosity characteristics of TMSPI itself begin to dominate. Due to the intermolecular forces of TMSPI and its interaction with PVDF segments (potentially dipole-dipole interactions), excessive TMSPI no longer acts as a lubricant; instead, it may act like "glue," increasing the internal friction of polymer chain movement. The electrolyte film becomes sticky, the film formation effect is poor, the internal ion transport channels are blocked, and the ionic conductivity decreases.

[0051] Figure 4 shows the SEM images of the lithium anode surface of the Li-Li symmetric battery assembled based on the polymer solid electrolytes prepared in Example 1 and Comparative Example 1 after 100 hours of cycling. It can be clearly seen from the figure that, with the addition of quantum dots, lithium is deposited more uniformly on the lithium metal surface during lithium stripping and deposition cycles. This avoids localized over-deposition of lithium, which can subsequently grow into lithium dendrites and cause short circuits in the battery.

[0052] Figure 5 shows the ICP-OES (Inductively Coupled Plasma Emission Spectrometry) test image of the lithium metal surface after 100 cycles of the NCM811|electrolyte|Li lithium metal battery. The ICP test aims to detect the content of trace transition metals "escaping" from the electrolyte due to cathode corrosion. The data clearly shows that without TMSPI, the Mn element dissolution rate was as high as 73.13 mg / kg, while after adding 3% TMSPI, it significantly decreased to around 5.26 mg / kg. This directly proves that TMSPI can effectively inhibit the dissolution and loss of cathode materials (especially manganese). This protective effect stems from TMSPI's excellent "acid removal" capability. TMSPI acts as a dual "fire extinguisher": on the one hand, its Si groups can neutralize alkaline impurities on the cathode surface through Lewis acid-base interactions, inhibiting the initiation of HF removal reactions; on the other hand, it can directly remove HF already generated in the system (forming Si-F bonds), cutting off the path of acid corrosion of the cathode, thus preventing transition metal dissolution through a two-pronged approach.

[0053] Figure 6The cycling curves of the NCM811-Li half-cell assembled based on the polymer solid electrolytes prepared in Example 1 and Comparative Example 1 are shown. The specific assembly steps of the NCM811-Li half-cell are as follows: In an argon glove box with a water and oxygen content below 0.1 ppm, using a CR2032 positive electrode shell as a base, NCM811 electrodes, electrolyte membranes, lithium sheets, stainless steel gaskets, and spring sheets are stacked sequentially from bottom to top. After covering with the negative electrode cap, the battery is sealed using an encapsulation machine under appropriate pressure to obtain a lithium symmetric battery. The long-term cycling performance of the NCM811||Li battery was tested using a Blue Battery Testing System (CT3002AU) at a test temperature of 27±2 ℃. The NCM811||Li battery underwent 0.5C charge-discharge cycling, requiring 5 cycles of low-rate 0.2C charge-discharge activation before testing. The cycling test data of NCM811 show that the sample of Example 1 with the addition of 3% TMSPI exhibits significantly superior electrochemical stability. Compared to Comparative Example 1, the TMSPI-3% group maintained a higher specific capacity throughout 100 cycles, and its coulombic efficiency was more stable, remaining almost at 100% with no decay. This performance improvement is directly attributed to the protective effect of TMSPI on the interface: by removing HF generated by the defluorination reaction of PVDF-TrFE induced by alkaline impurities on the cathode surface, it cuts off the source of acid corrosion, thereby effectively inhibiting the dissolution of transition metal ions and ensuring the structural integrity of the cathode material during long-term cycling.

[0054] Figure 7 The image shows the NCM811 positive electrode side interface film (CEI film) after 60 charge-discharge cycles. The left side is the Comparative Example 1 sample without TMSPI, and the right side is the Example 1 sample with 3% TMSPI. It can be clearly seen that the left side, after 60 charge-discharge cycles, forms a very uneven and very thick interface film (CEI film) on the positive electrode side surface with high interfacial impedance. In contrast, the right side sample with 3% TMSPI can form a uniform interface film with lower thickness and lower interfacial impedance.

[0055] The above description is merely a specific embodiment of the present invention. Any feature disclosed in this specification may be replaced by other equivalent or similar features unless otherwise specified. All disclosed features, or steps in all methods or processes, may be combined in any way except for mutually exclusive features and / or steps.

Claims

1. A method for preparing a composite polymer solid electrolyte, characterized in that, Includes the following steps: Step 1. Histidine carbon quantum dots are prepared using a one-pot method; Step 2. Add the polymer matrix, lithium salt, plasticizer, and film-forming additive to an organic solvent and stir until homogeneous to obtain solution A; the polymer matrix is ​​PVDF-TrFE and the film-forming additive is fluoroethylene carbonate. Step 3. Add histidine carbon quantum dots to solution A, stir to disperse evenly, then add tris(trimethylsilane) phosphite ester TMSPI to obtain a mixed slurry; Step 4. The mixed slurry is evenly coated on the substrate to form an electrolyte membrane, and then dried. After drying, the membrane is peeled off from the surface of the substrate to obtain the desired PVDF-TrFE polymer solid electrolyte.

2. The preparation method according to claim 1, characterized in that, The specific process of step 1 is as follows: The raw materials are dissolved in deionized water and reacted at 120-200℃ for 2-6 hours. After the reaction is completed, the reaction product is naturally cooled, purified by dialysis, and freeze-dried to obtain histidine-functionalized carbon quantum dot powder with uniform particle size. The raw materials include carbon and nitrogen sources; The carbon source is citric acid; The nitrogen source is histidine.

3. The preparation method according to claim 2, characterized in that, The molar ratio of citric acid to histidine is 1:0.

06.

4. The preparation method according to claim 1, characterized in that, In step 2, the lithium salt is any one of lithium bis(fluorosulfonyl)imide, lithium hexafluorophosphate, and lithium bis(trifluoromethanesulfonyl)imide; the organic solvent is any one of dimethylformamide, acetonitrile, N-methylpyrrolidone, acetone, tetrahydrofuran, dimethyl sulfoxide, and trimethyl phosphate; and the plasticizer is any one of succinic anion, adiponitrile, glutaronitrile, ethylene carbonate, sulfolane, trimethylolpropane, and polyethylene glycol.

5. The preparation method according to claim 1, characterized in that, The mass percentage of fluoroethylene carbonate to the polymer matrix is ​​5%-10%; the mass percentage of histidine-functionalized carbon quantum dots to the polymer matrix is ​​2%-3%.

6. The preparation method according to claim 1, characterized in that, The mass percentage of TMSPI to the polymer matrix is ​​1%-5%.

7. A solid-state battery prepared based on a composite polymer solid-state electrolyte, characterized in that, The solid-state battery includes a positive electrode, a negative electrode, and a composite polymer solid electrolyte obtained by the preparation method according to any one of claims 1-6; The positive electrode is NCM811; the negative electrode is lithium metal.

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