Synthesis And Characterizations Of Cathode Materials For Al-Se Batteries

Rechargeable aluminum ion batteries（RAIBs）have attracted substantial interest due to their high theoretical energy density(2980 m Ah g^-1,8056 m Ah cm^-3),low cost,and high security.However,the low-energy density and high-cost of the reported cathode materials hinder their further development.Therefore,it is an urgent need to find a new cathode with high capacity and good stability,then explore the energy storage mechanism and the capacity fading mechanism of cathode material.Selenium is a promising cathode material for RAIBs due to the abundant reserves,low cost,non-toxic.More importantly,Se can achieve multi-electrons transfer reactions brings hope for solving the above challenges that most cathode materials could not match the advantages of RAIBs.In this paper,Se-based cathodes were selected to reveal the reversible 6-electron transfer reaction（Se（-II）（Al₂Se₃）（?）Se（0）（?）Se（IV）（Se Cl₃⁺））,using Al Cl₃/triethylamine hydrochloride（Et₃NHCl）ionic liquid electrolyte.This Se-based 6-electron transfer reaction process holds an ultra-high theoretical capacity of 2036 m Ah g^-1 with a real long discharge voltage plateau at au.1.9 V.Furthermore,the capacity fading mechanism of Se-based cathodes was also revealed,with the Se-based cationic clusters,such as Se_n²⁺and Se Cl₃⁺,could shuttle into the ionic liquid electrolyte,resulting in the serious capacity decay.To enhance the electrochemical performance of battery,we combined Se with carbon material to improve the utilization of Se cathode and used a carbon-based modified separator with oxygen-containing functional groups to promote the stable for Al-Se battery.The main contents and innovation points are as follows:In this section,we choose graphene aerogel（GA）as the host for Se to load,then the Se/graphene aerogel（Se/GA）composite as the cathode to study the electrochemical reactions of Al-Se battery.To thoroughly understanding the electrochemical reaction process of Se,Ex-situ XPS was carried out.Upon an anodic sweep to 0.01 V,Se was reduced to Se（-II）.Upon a cathodic sweep to 2.3 V,Se was oxidized to Se（IV）.Based on the above results,this Al-Se battery achieves a Se-based reversible 6-electron transfer reaction（Se（-II）（?）Se（0）（?）Se（IV））over a voltage range of 0.01-2.3 V,delivers a theoretical specific capacity of up to 2036 m Ah g^-1,which is the highest in RAIBs currently.Furthermore,the capacity fading mechanism of Al-Se battery was also revealed,with the Se-based cationic clusters,such as Se_n²⁺and Se Cl₃⁺,could shuttle into the ionic liquid electrolyte,resulting in the serious capacity decay.In this section,we explored a new way to enhance the cycling performance.Density functional theory（DFT）calculations were firstly utilized and predicted that oxygen-containing functional groups,especially for the withdrawing electron group carboxyl（-COOH）,can chemically adsorb the soluble Se-based cationic clusters powerfully.Then,the separator modified by oxidization CNT（O-CNT）with gradient content of-COOH verified such a principle that-COOH can suppress the shuttle effect of Se-based cationic clusters effectively,with extended applications in other types of aluminum ion batteries with related energy storage mechanism.As a consequence,this novel Al-Se battery delivered an initial specific capacity of1599 m Ah g^-1 at 100 m A g^-1(1957 Wh kg^-1)with high discharge platform at 1.9 V and high Se utilization of 78.5%.After 200 cycles,the battery still exhibited 395 m Ah g^-1 at 1000 m A g^-1(421 Wh kg^-1).These results indicated that the cycling stability of Al-Se battery was enhanced a lot by introducing a separator with high amount of carboxyl.In this section,SeS₂/GA composite as cathode to conduct the electrochemical test,using Al Cl₃/triethylamine hydrochloride（Et₃NHCl）ionic liquid electrolyte.As a consequence,the battery delivered an initial charge capacity of 666 m Ah g^-1 and discharge capacity of 427 m Ah g^-1 at 100 m A g^-1.After 200 cycles,the battery still exhibited 173 m Ah g^-1 at 100 m A g^-1 with superior rate performance(104 m Ah g^-1 at 1000 mA g^-1).Therefore,SeS₂ has a great potential in RAIBs.