Conventional batteries incorporate two electrodes - an anode and a cathode. Anode materials, such as silicon (Si), aluminum (Al), Stannum (Sn), are promising candidates to replace traditional graphite.  

However, the cycling stability of these alloy-type anode materials is unsatisfactory due to their huge volume expansion when lithiated, which causes solid electrolyte interface (SEI) damage, and eventually the pulverization of electrode materials. 

A research group led by Prof. TANG Yongbing and his team members (Dr. OU Xuewu, Zhang Ge, etc.) from the Shenzhen Institutes of Advanced Technology (SIAT) of the Chinese Academy of Sciences proposed a novel energy storage devices, combining the pre-alloying and artificial SEI together to enhance the cycling stability of Al anode. Their study was published in Energy Storage Materials.

The purpose of the pre-alloying treatment was to compensate for the irreversible loss of Li⁺ and the formation of dead Li, while the artificial SEI originated from the reduction of electrolyte additive contributed to enhancing the structural stability of the anode.  

After investigating the matching behaviors of several additives with different anodes, the researchers chose lithium difluoro (oxalate) borate (LiDFOB) as the optimal additive since its oxalato-borate structure can be reduced and polymerized via the ring-opening reaction at the anode side. The fluorine atoms in LiDFOB were useful to form artificial SEI and produce lithium fluoride (LiF)-rich component, improving stability of SEI. 

Researchers have constructed Li hybrid capacitor (LHC) by combining the achieved pre-alloyed Al-LiDFOB anode (so as the current collector) and environmental friendly activated carbon cathode. 

The LHC exhibited a specific capacity of 123.6 mAh g^-1 with a capacity retention of 85.6% after 2000 cycles. The study showed potential applications for high-performance energy storage devices.

(a) Proposed mechanism of simultaneous pre-alloying and artificial SEI strategy to improve the cycling stability of Al anode. (b) The reduction schematics of DFOB^- at Al anode and the differential charge density of DFOB^- after accepting an electron. (c) The reduction and polymerization reactions of DFOB^- via the ring-opening reaction. (Image by OU Xuewu)