Complex hydride for composite negative electrode - Applicable to bulk-type all-solid-state Li-ion battery with wide temperature operation

A composite negative electrode for use in bulk-type all-solid-state batteries was developed by combining the solid electrolyte Li₄(BH₄)₃I (LiBH₄ doped with LiI), acetylene black (AB) (as a conductive additive), and lithium titanate (Li₄Ti₅O₁₂, LTO). Because of the highly deformable nature of the complex hydride, hand milling and subsequent uniaxial pressing were sufficient to ensure tight interfacial contact. As a result, the composite negative electrode allowed successful operation of a bulk-type all-solid-state battery from room temperature (23 °C) to 150 °C. The composite negative electrode was characterized via microstructural observation and using electrochemical techniques. In the prepared composite negative electrode, LTO, Li₄(BH₄)₃I, and AB were homogeneously dispersed and formed tightly contacted interfaces. At 150 °C, the battery exhibited high discharging capacities of 170 and 158 mAh g^{- 1} for the 1st and 2nd cycles, respectively, which corresponded to LTO utilization ratios of 97% and 90%. Moreover, it retained 140 mAh g^{- 1} at the 100th cycle, which equates to a 90% capacity retention ratio from the 2nd cycle. Furthermore, the battery was successfully operated at room temperature (23 °C), exhibiting discharging capacities of 122 mAh g^{- 1} for the 1st cycle and 111 mAh g^{- 1} for the 5th cycle (capacity retention ratio of 91%). At both temperatures, the coulombic efficiency was nearly 100%, indicating that the charge/discharge reactions proceeded without any significant side-reactions. This good performance is due to the small charge-transfer resistance at the LTO interface resulting from the tight contact between the constituents of the composite electrode, and suggests that LiI-doped LiBH₄ is a promising solid electrolyte for practical composite electrodes.