Progress in Gel Polymer Electrolytes for Sodium-Ion Batteries

Sodium-ion battery is a potential application system for large-scale energy storage due to the advantage of higher nature abundance and lower production cost of sodium-based materials.However,there exist inevitably the safety problems such as flammability due to the use of the same type of organic liquid electrolyte with lithium-ion battery.Gel polymer electrolytes are being considered as an effective solution to replace conventional organic liquid electrolytes for building safer sodium-ion batteries.In this review paper,the authors present a comprehensive overview of the research progress in electrochemical and physical properties of the gel polymer electrolyte-based sodium batteries.The gel polymer electrolytes based on different polymer hosts namely poly(ethylene oxide),poly(acrylonitrile),poly(methyl methacrylate),poly(vinylidene fluoride),poly(vinylidene fluoride-hexafluoro propylene),and other new polymer networks are summarized.The ionic conductivity,ion transference number,electrochemical window,thermal stability,mechanical property,and interfacial issue with electrodes of gel polymer electrolytes,and the corresponding influence factors are described in detail.Furthermore,the ion transport pathway and ion conduction mechanism are analyzed and discussed.In addition,the advanced gel polymer electrolyte systems including flame-retardant polymer electrolytes,composite gel polymer electrolytes,copolymerization,single-ion conducting polymer electrolytes,etc.with more superior and functional performance are classified and summarized.Finally,the application prospects,development opportunities,remaining challenges,and possible solutions are discussed.

Optimization Strategies Toward Functional Sodium-Ion Batteries

Exploration of alternative energy storage systems has been more than necessary in view of the supply risks haunting lithium-ion batteries.Among various alternative electrochemical energy storage devices,sodium-ion battery outstands with advantages of cost-effectiveness and comparable energy density with lithium-ion batteries.Thanks to the similar electrochemical mechanism,the research and development of lithium-ion batteries have forged a solid foundation for sodium-ion battery explorations.Advancements in sodium-ion batteries have been witnessed in terms of superior electrochemical performance and broader application scenarios.Here,the strategies adopted to optimize the battery components(cathode,anode,electrolyte,separator,binder,current collector,etc.)and the cost,safety,and commercialization issues in sodium-ion batteries are summarized and discussed.Based on these optimization strategies,assembly of functional(flexible,stretchable,self-healable,and self-chargeable)and integrated sodium-ion batteries(−actuators,−sensors,electrochromic,etc.)have been realized.Despite these achievements,challenges including energy density,scalability,trade-off between energy density and functionality,cost,etc.are to be addressed for sodium-ion battery commercialization.This review aims at providing an overview of the up-to-date achievements in sodium-ion batteries and serves to inspire more efforts in designing upgraded sodium-ion batteries.

Unraveling the reaction mechanisms of electrode materials for sodiumion and potassium‐ion batteries by in situ transmission electron microscopy

Sodium ion batteries(SIBs)and potassium ion batteries(PIBs)have caught numerous attention due to the low cost and abundant availability of sodium and potassium.However,their power density,cycling stability and safety need further improvement for practical applications.Investigations on the reaction mechanisms and structural degradation when cycling are of great importance.In situ transmission electron microscopy(TEM)is one of the most significant techniques to understand and monitor electrochemical processes at an atomic scale with real-time imaging.In this review,the current progress in unraveling reaction mechanisms of electrode materials for SIBs and PIBs via in situ TEM is summarized.First,the importance of in situ TEM is highlighted.Then,based on the three types of electrochemical reaction,i.e.,intercalation reac-tion,conversion reaction and alloying reaction,the structural evolution and reaction kinetics at atomic resolution,and their relation to the electrochemical performance of electrode materials are reviewed and described in detail.Fi-nally,future directions of in situ TEM for SIBs and PIBs are proposed.Therefore,the in‐depth understanding revealed by in situ TEM will give an instructive guide in rational design of electrode materials for high performance electrode materials of SIBs and PIBs.

In Situ Electrochemical Transmission Electron Microscopy for Sodium-Ion Batteries

Sodium-ion batteries(SIBs)possess promising application prospects for large-scale energy storage systems due to the abundance of sodium ions as a resource and their low cost.Development of advanced SIBs requires a clear understanding of the structures and kinetic/dynamic processes occurring in the cells during the charging/discharging process.In situ transmission electron microscopy(TEM)is a powerful tool for direct visualization of the phase transitions as well as morphological and structural evolutions of the electrodes during the electrochemical reaction process.Herein,we summarize the state-of-the-art in situ TEM studies on SIBs with a specific focus on real-time observations of the electrochemical behavior of battery materials.This review emphasizes the necessity of in situ TEM to elucidate fundamental issues regarding the reaction mechanism,phase transformation,structural evolution,and performance degradation of SIBs.Finally,critical challenges and emerging opportunities for in situ TEM research about SIBs are discussed.