Investigation On The Fabrication And Performance Of Quasi-solid Polymer Electrolytes

Solid polymer electrolyte(SPE)can effectively eliminate the safety hazards in lithium(Li)metal battery due to its inflammability and dimensional stability with respect to liquid electrolyte.Unfortunately,the intrinsic low ion conductivity of the SPE and high interface impedance between the SPE and electrodes at room temperature hinders its practical applications.In contrast,quasi solid polymer electrolyte(QSPE)has higher ionic conductivity and better interface contact performance,making it more promising for the practical applications in solid-state batteries.Polyethylene oxide(PEO)is the most widely studied polymer electrolyte materials,but the extremely low ion conductivity at room temperature prevents its wide application.Adding a large amount of succinonitrile(SCN)to PEO matrix can form a continuous network for fast Li+ion transport,where the ion transport mechanism is similar to that in QSPE.The ionic conductivity can reach 10-4 S/cm at ambient temperature.However,the subsequent deterioration of mechanical properties and poor interface stability lead to limited cycling performance.Therefore,modifying PEO-based electrolyte and passivating the surface of Li anode are synergistically done to enhance the cycling performance of the battery.By simultaneously introducing SCN plasticizer and PAN nanofiber membranes into the PEO matrix,a sandwiched composite electrolyte with high ionic conductivity of 1.8×10-4 S/cm and robust mechanical strength at 25℃ is obtained.Meanwhile,the Li anode is coated by a dense Li-Sn alloy layer to slow down the side reactions,inducing the uniform deposition of Li ions and inhibiting the growth of Li dendrites.Consequently,the symmetric cell has a long cycle life of 2400 h under 0.1 mA/cm2 and 0.1 mA h/cm2 at 25℃.And the assembled LiFePO4(LFP)cell also shows prolongated cycling performance with 85%capacity retention after 500 cycles at 0.5℃.PEO electrolyte based Li metal batteries can demonstrate an expanded cycle lifetime at room temperature,which can only be achieved under low current density since the bigger thickness and poor contact between the electrode and PEO electrolyte must promote the bulk and interface impedance rapidly.Therefore,in-situ polymerization strategy is developed to synthesize 1,3-dioxolane(DOL)/1,2-dimethoxyethane(DME)based QSPE under initiation of aluminum trifluoromethanesulfonate(Al(OTF)3).At 25℃,the ionic conductivity of the QSPE can reach 2.0 × 10-4 S/cm with the membrane thickness of only 26 μm.So,the intrinsic impedance of the thin electrolyte in the battery is reduced significantly.In addition,QSPE has excellent contact with the electrodes,resulting in a lower interface impedance and enabling the cell operation under harsh conditions.The assembled Li symmetric cell can work under 3 mA/cm2 and 1 mA h/cm2 showing the maximum cycle lifetime of 540 h.The assembled LFP cell shows great cycling performance which can keep a capacity retention of 81.6%even after 500 cycles at 1 C.