Replacing traditional liquid electrolytes with high-performance solid electrolytes to assemble a cell can not only eliminate the risk of electrolyte leakage, effectively suppress the growth of lithium dendrites in lithium batteries during the charging and discharging process, and improve the safety of battery usage, but also can directly use metal lithium as cathode of the battery, then greatly improving the energy density of the batter1, 2. To research and develop the high-performance all-solid-state rechargeable batteries with good safety, high energy density and wide operating temperature range becomes a mainstream direction for the next generation saving energy devices. A large number of studies and experiments in the early stage have formed a consensus that the ion transport performance of solid-state electrolytes at room temperature or below and the interface issue between the solid-state electrolytes and electrodes are the bottleneck restricting the further development of all-solid-state batteries3. Therefore, it is of practical application and academic valuable to study high-performance all-solid-state electrolytes and the strategies to regulate ion transport in solid-state electrolytes4, 5.
Commonly, solid electrolytes are able to be divided into three types based on their composition, i.e. Solid inorganic electrolyte (SIE), Solid polymer electrolyte (SPE), and Solid organic-inorganic composite electrolyte (CE). SIEs have good thermal stability, excellent mechanical strength, and high ionic conductivity at room temperature, but their brittleness and poor process-ability are obvious, which is hard to large-scale production. SPEs have good interface contact with the electrodes and are prone to plasticity, making them suitable for large-scale production. However, their ionic conductivity at room temperature is low; organic-inorganic composite solid electrolytes can collect the advanced performance of polymer electrolytes and inorganic solid electrolytes together, and now it has been widely used to improve the interface and mechanical properties of PEO-based electrolyte systems, the related research has received widespread attention from researchers6.
As a common organic component in CE, polyethylene oxide (PEO) based polymers have good solvation for most lithium salts and are the most widely studied polymer electrolyte substrates in published literature7. Lithium ions are transported by coordination/dissociation with PEO ether oxygen atoms and accompanied by motion of PEO segment8. However, the high crystallinity of PEO at room temperature limits the motion of PEO polymer segments, resulting in low ionic conductivity at room temperature(10−8~10−6 S cm−1)9, 10; On the other hand, the Electrochemical Stability Window (ESW) of PEO is usually lower than 3.7 V, which restricts the application of some advanced cathode materials with high potential in batteries, thus limiting the improvement of the batteries performance, such as specific capacity, energy density, charge discharge and cycle performance11, 12. Polyacrylonitrile (PAN) has high dielectric constant, excellent mechanical and thermal performances, flame retardancy, wide electrochemical window, and is easy to form films13. There is not only interaction between Li+ and nitrile group (-CN), but also PAN can promote the dissociation of lithium salts due to its high dielectric constant and increase the concentration of charge carrier Li+, which is beneficial for reducing concentration polarization14. SiO2, TiO2, LZTO, etc. are commonly inorganic filler used in organic-inorganic composite electrolyte systems15, the inert filler can promote the dissociation of lithium salts and hinder the migration of anion in the electrolyte through Lewis acid-base interaction16, 17, even forming a long-range "space charge zone" to promote the transportation of lithium ions18; inorganic active fillers can independently participate in the transportation of lithium ions, but the cost is too high19. Lin et al. obtained PEO:LiClO4/SiO2 composite electrolyte( EO/Li = 8:1,mass ratio) by adding 10 wt.% nano-SiO2 to PEO, which has a ionic conductivity of 1.2 mS cm-1 at 60℃ and a wide electrochemical window of 5.5 V 20. Based on this fact, it can be seen that the combination of solid polymer electrolytes and inorganic nanoparticles can significantly improve the Li+conductivity and Li+ transference number of solid polymer electrolytes 21, 22.
In this manuscript, the cross-linked copolymer poly (PEGDA-co-AN) is prepared by a free radical random copolymerization using polyethylene glycol diacrylate (PEGDA) and acrylonitrile (AN) as the monomers, and 2,2-azodiisobutyronitrile (AIBN) as a initiator, and a composite polymer electrolyte poly (PEGDA-co-AN) /LiTFSI/nano SiO2 with high entropy structure and high conductivity has been designed and fabricated. The influence of Lewis acid-base interaction between nano SiO2 additive and -CN or C-O-C on Li+ transport has been investigated, and a new idea is proposed to improve the lithium ion transport in the composite polymer electrolytes by adjusting the local charge environment of polymer electrolytes, and the composite polymer electrolyte poly (PEGDA-co-AN) /LiTFSI/nano SiO2 obtained exhibits a room temperature ionic conductivity of 3.5×10−3 S cm−1, Li+ transference number of 0.58, and the electrochemical stability window greater than 5 V.