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.