Preparation And Wide Temperature Range Electrochemical Performance Of Zinc Ion Polymer Electrolytes

As the exploration of polar,ocean,and tropics in severely colder or hot regions has risen,traditional electrochemical energy storage devices are difficult to be safely applied in extreme environments,the Zn-based electrochemical energy storage devices could be promising under harsh environments because of their high safety and low-cost.However,there are significant disadvantages to aqueous Zn-based energy storage devices such as solvent evaporation,drastic hydrogen evolution reaction,corrosion,and dendritic formation at high temperatures.Furthermore,because of their high resistance,electrolyte freeze,and poor interface contact between electrolyte and electrode,they show poor performance at low temperatures.Consequently,it is necessary to develop novel,efficient Zn-ion electrolytes that can operate under extreme hot or cold environments.In this consideration,polymer electrolytes with high mechanical properties outperform liquid electrolytes under extreme environments.Therefore,in this thesis,the anti-freezing and heat-resistant Zn-ion polymer electrolytes were prepared,respectively,to aid in the development of wide temperature range Zn-based energy storage devices for applications in extreme environments.The electrochemical behavior and related mechanisms of the as-developed Zn-ion polymer electrolytes were investigated at room temperature as well as under extreme environments.In order to improve the performance of hydrogel electrolytes at low temperatures,an anti-freezing gel electrolyte based on polyvinyl alcohol（PVA）was prepared by the freezing-thawing method,where,ethylene glycol was used as the anti-freezing additive,and zinc trifluoromethanesulfonate（Zn（OTf）₂）as a salt.Owing to the interaction between the ethylene glycol,PVA molecular chains,and water molecules,the activity of water has been inhibited,thereby,improving the anti-freezing property of the electrolyte,and demonstrating better low-temperature electrochemical performance.The ionic conductivity of the optimal PVA-based antifreeze gel electrolyte reached 9.05×10^-3 S cm^-1 even at-20^οC.In addition,the gel electrolyte effectively inhibited the hydrogen/oxygen evolution reactions,and corrosion,which is conducive to the Zn²⁺uniform deposition.The gel electrolyte displays good stability with Zn anode at both room temperature and-20^οC.The Zn-ion hybrid supercapacitor,assembled with the gel electrolyte and graphene electrode,exhibited a high specific capacitance value～202.8 F g^-1at-20^οC,which retains 82%of the capacitance at room temperature.In addition,there is almost no capacity decay over 30,000 charge-discharge cycles at-20^οC.However,owing to the weak salt tolerance of PVA,the low-temperature performance of PVA-based anti-freezing gel electrolytes could not be enhanced further.To further improve the anti-freezing ability and electrochemical performance of hydrogel electrolytes at low-temperature,the PAM/CMC-based anti-freezing gel electrolytes were prepared by free radical polymerization using high salt-tolerance polyacrylamide（PAM）as host polymer,carboxymethyl cellulose（CMC）as reinforcing agent,chaotropic zinc perchlorate（Zn（Cl O₄）₂）salt,and ethylene glycol as anti-freezing additive.A stable hydrogen bond network is formed between ethylene glycol,water molecules,and polymer molecular chains.Meanwhile,the Cl O₄^-disrupts the hydrogen bond between water molecules due to the Hofmeister effect,considerably improving the anti-freezing ability and electrochemical performance of the electrolyte at low temperature.The optimal PAM/CMC-based anti-freezing gel electrolyte showed a high ionic conductivity of 4.12×10^-3S cm^-1 even at-50^οC.Furthermore,the hydrogen/oxygen evolution reactions,corrosion,and dendrites growth were effectively inhibited due to the stable hydrogen bond network and the formation of the Cl^--rich electrode-electrolyte interface protective layer,which promotes the stability between electrolytes and Zn anode at both room temperature and-40℃.The assembled Zn||polyaniline device displayed a high specific capacitance value of 254.3 F g^-1even at-40^οC,retaining 70%of the capacitance at room temperature.Moreover,the capacity of the device shows no evident decay after 30,000 charge-discharge cycles at-40^οC.The hydrogel electrolytes are difficult to use for long-term application at high-temperature,hence it is necessary to develop polymer electrolytes,which can be applied in the high temperature environments.The PVDF-HFP-based heat-resistant polymer electrolyte was prepared using the solution casting method with poly（vinylidene fluoride-co-hexafluoropropylene）（PVDF-HFP）as the host polymer,and Zn（OTf）₂ as a salt,and 1-Methyl-1-propylpyrrolidinium Bis（trifluoromethanesulfonyl）imide(Pyr₁₃TFSI)ionic liquid with high thermal and chemical stability.Owing to the interaction between Pyr₁₃TFSI ionic liquid,Zn salt,and PVDF-HFP chains,the crystallinity of the electrolyte reduces,and its free surface area increases,resulting in an optimal electrolyte with a high room temperature ionic conductivity(1.52×10^-3S cm^-1)and a low ion conduction activation energy（0.19 e V）.Besides,the electrochemical stability window of the electrolyte reaches 2.1 V even at 100℃.In addition,the electrolyte effectively prevents the hydrogen/oxygen evolution reactions,corrosion,and formation of zinc dendrites due to the water-free solvent and the formation of the electrode-electrolyte interface protective layer.Consequently,the electrolyte shows good stability with the Zn anode both at room temperature and 80^οC.The Zn-ion hybrid supercapacitor is assembled with the gel electrolyte and graphene cathode which exhibits a high discharge specific capacitance of～195.0 F g^-1 with the capacity retention of 88%over 10,000 charging-discharging cycles at room temperature.Furthermore,the device can charge-discharge over 30,000 cycles even at 80^οC,demonstrating good electrochemical properties and cycle performance.