Recent advances in NiMH battery technology

Nickel-metal hydride (NiMH) is a commercially important rechargeable battery technology for both consumer and industrial applications due to design flexibility, excellent energy and power, environmental acceptability and cost. [1] From the initial product introduction in 1991 of cylindrical cells having an energy of 54 Wh kg−1, todayʹs small consumer cells have a specific energy over 100 Wh kg−1. Numerous licensed manufacturers produce a myriad of NiMH products ranging from 30 mAh button cells to a wide variety of consumer cylindrical products, prismatic cells up to 250 Ah for electric buses and 6 Ah multicell modules for hybrid electric vehicles. Power has increased from under 200 to 1200 W kg−1 commercially and up to 2000 W kg−1 at a development level [2]. Early NiMH batteries had limited operating temperatures while todayʹs batteries can provide excellent power at cold temperatures of −30 °C and provide over 90% capacity at 70 °C. Many of these product performance advances are a result of innovations to the metal hydride and nickel hydroxide materials. We will report on some of these key material advances which provide todayʹs NiMH performance and new materials to allow higher energy, power and significant cost reduction.