| Lithium metal is of great interest as anode material for secondary rechargeable batteries due to its low density(0.53 g cm-3),low redox voltage(-3.04 V vs.Li/Li+)and high theoretical capacity(3860 mAh g-1).The electrolyte acts as an ion transport carrier and has a profound impact on the battery electrochemical performance.Conventional organic electrolytes have excellent ion conductivity and good contact with electrodes,but their inherent problems of flammability,leakage and volatility seriously threaten the safe operation of batteries.In addition,due to the uneven distribution of lithium ions on the lithium metal surface,the lithium cathode is prone to form dendritic lithium dendrites during the deposition/exfoliation process,resulting in cycle life degradation or even short circuit.Therefore,the use of a solid electrolyte with non-flammable,excellent encapsulation and good mechanical properties is expected to eliminate the safety problems associated with conventional electrolytes.Among the most commonly studied polymers,polyacrylonitrile(PAN)has good antioxidant properties,thermal stability,mechanical properties,and a wide electrochemical stability window,making PAN an ideal candidate for solid-state electrolytes.However,the use of pure PAN-based polymer electrolytes for lithium metal cathodes has many problems,such as functional groups such as cyano or carboxyl groups in polyacrylonitrile are prone to reduction reactions with lithium metal to form imine or imide compounds,and these reactions can cause changes in the molecular structure of the polymer,which in turn affects the long-cycle stability of the battery.Therefore,when PAN is used as a solid polymer electrolyte for lithium metal batteries,treatments such as surface modification or structural modification are required to improve the stability with lithium metal.In this study,firstly,different types of ionic liquids(IL)were introduced based on PAN to compare the effects of various types of ionic liquids on the electrochemical properties of electrolyte membranes,and the optimal ratio of PAN/LiTFSI/IL was also investigated to prepare a monolayer PAN solid-state electrolyte membrane with better overall performance;on this basis,PEO-based solid-state electrolytes were introduced to improve the electrolyte-negative lithium metal interface for solving the In order to solve the problems of low ionic conductivity and low ion mobility of PEO electrolyte film,ionic liquid and metal organic framework(MOF)were introduced and a bilayer electrolyte structure was designed to construct a solid electrolyte film with high ionic conductivity,wide electrochemical window and stability to lithium metal.The addition of ionic liquid significantly enhanced the ionic conductivity and electrochemical window of the PAN polymer matrix.By comparing the effects of three ionic liquids,imidazole,pyrrole and piperidine,on the electrochemical performance of the electrolyte films,the composite electrolyte films prepared with the pyrrole-based ionic liquid Pyr13TFSI showed the best overall performance in terms of electrochemical window and ionic conductivity.On this basis,we investigated the effect of the ratio of PAN,LiTFSI and Pyr13TFSI on the performance of the solid electrolyte film and determined an optimal ratio to make the monolayer PAN solid electrolyte.When the ratio of the three is 1:0.6:1,the electrolyte film has good electrochemical stability with an electrochemical window of 4.55 V,ionic conductivity of 2.53×10-4 S cm-1 at room temperature and ion mobility number of 0.437.In the solid-state lithium battery assembled with lithium metal as the negative electrode and NCM811 as the positive electrode,the battery has poor cycling stability at room temperature due to the instability of PAN to lithium,the first turn discharge specific capacity was 143 mAh g-1,and the 50-turn capacity retention was 76%.PEO has good stability in lithium metal anode.However,its poor oxidation stability can lead to their incompatibility with high-voltage cathodes.To address the above problems,a two-layer solid electrolyte film is proposed with PEO-based polymer for lithium and PAN-based polymer for cathode as a way to improve the electrochemical stability window and cycling stability.The PEO-side solid electrolyte introduces Li-IL@HKUST-1 as an active filler to improve the ionic conductivity of the PEO-side electrolyte and anchor the lithium with the help of the cationic active site exposed by HKUST-1 salt anion to enhance the ion mobility number of the electrolyte.The solid electrolyte film prepared using the bilayer cast method has a high electrochemical stability window of 4.85 V and an ionic conductivity of 1.25×10-4 S cm-1 at room temperature,and assembles NCM811/PAN-PEO/Li full cells with a specific capacity of 183 mAh g-1 at 60℃ and a capacity retention rate of 87%after 50 cycles,which is significantly higher than that of the single-layer PAN film in terms of cycling specific capacity and capacity retention.In addition,after preparing PAN and PEO membranes separately,the full cell with a simple stacked assembly of bilayer solid electrolyte exhibited 173 mAh g-1 specific capacity and 78%capacity retention after 50 cycles,demonstrating the superior electrochemical performance of the bilayer solid electrolyte prepared by the bilayer cast method. |
PDF Full Text Request