Surface Treatments of Metal Hydride Anode Material for Next-Generation Nickel-Metal Hydride Battery

Abstract

The development of the emergency call system (eCall system) was promoted by increasingly harsh and strict policies in many countries to enhance safety of drivers and passengers. A reliable battery technology with advantages of wide operating temperature range, long shelf life, ease of maintenance, and safety is essential to power and implement the eCall system. Nickel metal hydride (NiMH) battery and lithium-ion battery (LIB) are two of the most mature rechargeable battery technologies and candidates for eCall system. Compared to LIB, NiMH battery is a safer choice due to the intrinsic non-flammability of aqueous electrolyte. Moreover, NiMH battery has better low temperature performance than LIB because ions are more mobile in aqueous electrolytes than in organic electrolytes. Hence, NiMH battery is a promising alternative for safety-sensitive application. A limitation of today’s NiMH technology is that anode materials capable of discharging at low temperature (-40°C) have short cycle and calendar life. This is due to the fracture and dissolution of active materials during cycling. In this work being presented, we have coated the metal hydride anode material with lanthanum trifluoride (LaF3), which is insoluble in alkaline electrolyte and inert at anode operating potential window, to encapsulate the particle and improve capacity retention. The coated particle achieved stable cycling over 200 cycles at room temperature. Moreover, the LaF3 coating is electrically conductive and does not affect the kinetics. The coated particle still achieved more than 150 mAh/g at extremely low operating temperature (-40°C). Thorough material characterization has been carried out to investigate the structure of the coating and correlate to the performance enhancement.