Research On Composite Polymer Electrolytes For Lithium-ion Secondary Batteries

As an importantly electrochemical energy storage devices,lithium ion batteries(LIBs)have the advantages of high energy density,fast charging speed,low self-discharge rate,long cycle life,and no memory effect.It has been widely used in portable electronic devices such as smart phones and bracelets.LIBs are mainly composed of three parts:positive electrode,negative electrode and electrolyte.The electrolyte plays a very important role as a channel for Li⁺to be conducted between positive and negative electrodes.The performance of the electrolyte directly affects the performance indicators such as the capacity,service temperature,safety and cycle performance of LIBs.Commercial lithium batteries use organic liquid electrolytes.It is prone to side reactions during charging and discharging,resulting in irreversible battery capacity decline.Meanwihle,the organic liquid electrolytes will evaporate and dry up,leakage and other phenomena affect battery life during long-term service.Replacing liquid electrolyte with solid electrolyte is one of the main ways to obtain solid lithium battery with high energy density,high safety and long cycle life.In this paper,a series of composite polymer solid electrolytes composed of PVDF-HFP,PEGDA,LLZO and Li TFSI are prepared to replace the electrolyte and membrane in traditional liquid batteries,and lithium iron phosphate|composite electrolyte|Li solid-state batteries are assembled.To discuss the electrochemical performance of PVDF-HFP/PEGDA/LLZO composite polymer electrolyte,the ion mobility number T⁺_Li is 0.33,the ion conductivity is 1.58×10^–4 S·cm^–1,and the activation energy of ion mobility is 0.413e V.The chemical window is 5.3V;the prepared lithium iron phosphate|composite polymer solid electrolyte|lithium metal all-solid-state lithium battery shows good cycle performance and lithium metal compatibility:The discharge specific capacity at 0.1C,0.2C,0.3C,0.5C,1C,1.5C,2C is 150.91 m Ah·g^-1,154.13 m Ah·g^-1,154.51 m Ah·g^-1,153.93 m Ah·g^-1,148.13m Ah·g^-1,147.24 m Ah·g^-1,140.73 m Ah·g^-1,the coulombic efficiency is close to 100%in the whole cycle.At a large discharge rate of 2C,it still has a capacity of 140m Ah·g^-1.It can be cycled stably for 180 cycles during the cycle without significant degradation of battery capacity,which is of great significance for improving the energy density of lithium batteries.The research results show that the weakening of the crystallinity of the polymer is beneficial to increase the ionic conductivity;the composite polymer solid electrolyte can effectively improve the mechanical properties and thermal stability of the electrolyte;in addition,the addition of LLZO can further improve the ionic conductivity and resistance of the solid electrolyte and the stability of lithium metal.