Improved Cycling Performance of Intermetallic Anode By Minimized SEI Layer Formation

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Alloy anodes including intermetallic anode, such as Si, Sn, Sb or Al, have been widely studied to replace the standard graphite-based anode, because these anode active materials theoretically have much higher specific capacity than the graphite anodes. The biggest challenge of the alloy anode is its cycleability. Huge volume expansion/shrinkage during the lithiation/delithiation process leads the pulverization of the active material particles resulting in loss of the electric contact among active in the electrode. Since the volume expansion/shrinkage also initiates formation/dissipation of electrode/electrolyte interphase, the SEI layer is supposed to have physical flexibility. However, the required physical properties of the SEI layer is not understood yet. We think the analogy of Mg-based system is useful, because the organohaloaluminate-based electrolyte solution of the Mg-based system does not form the typical SEI layer. In the present study, electrochemical properties of bismuth composite electrode are investigated in magnesium and lithium system. The bismuth composite electrode shows good cycleability in a magnesium half-cell with an organohaloaluminate electrolyte solution, while sever capacity decay is observed in a lithium system with a conventional carbonate-based electrolyte solution. In the carbonate-based electrolyte solution, the bismuth composite electrode shows low coulombic efficiency of < 90% due to the formation of solid electrolyte interphase (SEI) layer, while the high coulombic efficiency is maintained in the magnesium system. The comparison of the two system suggests that the formation of the SEI layer accelerates the capacity fading. Another lithium-based electrolyte solution: 2M LiBH4 in THF is proposed as a minimized SEI formation electrolyte solution. The LiBH4-based electrolyte solution shows excellent cycleability with high coulombic efficiency. A spectroscopic study using in situ FTIR proved that the LiBH4-based electrolyte solution does not form the typical thick SEI layer.