Researches On Polymer Electrolytes For Lithium Metal Batteries

Nowadays,the applications of lithium-ion batteries(LIBs)are expanded from traditional electronic devices such as mobile phones to electric vehicles and smart wearable devices.The coming demand on enhancing energy density of LIBs boosts the development of higher mass density LIBs,while traditional LIBs can no longer meet the need of the expanding market.Hence,long history lithium metal batteries(LMBs)have re-entered people's vision due to the super high theoretical specific capacity(completely above 3860 m Ah g^-1)and super low electrochemical potential(-3 V vs SCE).However,there are two problems hindering the commercialization of lithium metal batteries:(1)the previously reported LMBs generally use a large amount of polymer electrolytes mainly owing to the inferior compatibility with lithium metal anodes,which decreases the battery energy density;(2)thermal runaway of the battery easily occurs under abuse conditions,causing safety accidents.In view of these,we proposed the corresponding solutions.1.The previously reported LMBs generally used a large amount of polymer electrolytes,which decreased the energy density of batteries.The underlying reason behind this fact is the inferior compatibility between previously reported polymer electrolytes and lithium metal anodes.Hence,it was very important to develop a novel polymer electrolyte that could construct a mechanically stable solid electrolyte interphase(SEI)to prevent further reactions between the electrolyte and the lithium anode.It had been previously reported that the polymer-reinforced SEI layer could effectively prevent the side reactions between the electrolyte and the lithium metal anode and thus improved the cycling stability of the LMBs.Compare with the polymer-based artificial SEI layer,the SEI layer formed in-situ was facile to prepare and had high affinity with lithium metal anodes.Futhermore,it could effectively inhibit the growth of lithium dendrites and buffered the volume expansion effect during charging and discharging,thereby effectively improved cycle performance of LMBs.Here,a P(CUMA-CUEM)-based polymer electrolyte that was obtained by in-situ copolymerization of carbamate and urea-containing acrylate was designed and prepared.It was investigated that during cycling,P(CUMA-CUEM)-PE had higher reducibility due to the presence of its carbamate and urea segments,so it was easier to form a mechanically stable layer on the surface of the lithium metal anode than the liquid electrolyte.The stable SEI layer could prevent further side reactions between the electrolyte and the lithium metal anode.In addition,P(CUMA-CUEM)-PE could endow LMBs with good cycle stability under an electrolyte load of 7.5?L m Ah^-1.Raman spectroscopy and ¹H-¹⁹F HOSEY two-dimensional NMR spectra showed that the amide hydrogen on the polymer skeleton could interact with the anion on the lithium salt to increase the lithium ion transference number of P(CUMA-CUEM)-PE and to regulate the deposition/dissolution behavior of lithium ions on the surface of lithium anodes.The results of XPS,SEM,¹H NMR,AFM,in-situ optical microscopy and so on showed that the SEI components on the surface of lithium metal anode were mainly comprised of carbonate and its decomposition products,thus delivering high interface stability.2.Due to the vast volume change of lithium metal anodes upon cycling,SEI layer destruction/regeneration accompanied by severe electrolyte decomposition occurs continuously,which decreases the thermal runaway temperature of LMBs.In addition,this process generally induces lithium dendrite growth,which can penetrate though separators rendering battery short circuit.To circumvent the safety hazards mentioned above,we envisioned that the development of thermal shutdown typed polymer electrolytes by the repolymerization of polymer electrolytes were viable solutions.Give that the in-situ polymerization of vinylene carbonate(VC)and 2-ethylhexanoate always delivered low polymerization conversion(?40%),and the remaining large amount of monomers can experience repolymerization at high temperatures(?150°C),thus we chose this polymer electrolyte as the research model.By in-situ copolymerization of vinylene carbonate and2-ethylhexanoate,the polymer electrolyte P(VC-EAVE)was obtained.The polymer electrolyte-assembled Li Fe PO₄/Li LMBs delivered excellent cycling capabilities(capacity retention of 98.12%after 400 cycles).In addition,the extremely high temperature charge-discharge experiments and thermal abused experiments proved that the electrolyte could promote the safety of LMBs.Through the further analyses of XPS,¹H NMR,SEM and so on,it could be proved that the enhanced safety of P(VC-EAVE)-based LMBs was ascribed to the fact that the remaining VC monomers could continue to polymerize at the interface of lithium metal anode when the LMB was overheated,thereby evidently increased the overall interface impedance of the battery and preventing the further high-temperature decomposition reaction between lithium metal and electrolyte.