NiMH (NiMH)

Nickel-Metal Hydride (NiMH) is a rechargeable battery chemistry that uses nickel hydroxide (Ni(OH)2) as the positive electrode, a hydrogen-absorbing metal alloy (typically AB5-type LaNi5 or AB2-type Laves-phase intermetallic) as the negative electrode, and an aqueous alkaline electrolyte (potassium hydroxide, 30% KOH). It was commercialised in 1989 by Sanyo and Matsushita, dominated portable consumer electronics through the 1990s-early 2000s, and remains the standard battery in hybrid electric vehicle (HEV) traction packs (Toyota Prius family, Honda Insight, Ford Escape Hybrid) where it offers proven longevity and safety advantages over early-generation lithium-ion at the cost of lower energy density.

The performance envelope sits between alkaline primary and lithium-ion. Specific energy of 60-120 Wh/kg (Li-ion 150-260 Wh/kg, NiCd 40-60 Wh/kg). Voltage 1.2 V per cell. Cycle life 500-2,000 cycles to 80% capacity retention (Li-ion 500-3,000, NiCd 1,000-2,000). Self-discharge 1-5% per day for standard NiMH, reducing to 0.1-0.5% per day for low-self-discharge variants (Eneloop, Sanyo's 2005 innovation). Operating temperature range -20 to +50°C — wider than Li-ion at the cold end, narrower at the hot end. No memory effect (the legacy NiCd cycling problem), and no thermal runaway risk above 100°C (the dominant Li-ion safety concern).

The hazard profile is much friendlier than both NiCd and Li-ion. No cadmium — the carcinogenic heavy metal that made NiCd a Stockholm Convention restricted chemistry. No flammable electrolyte — the aqueous KOH cannot sustain combustion, eliminating the principal Li-ion fire risk; NiMH cells vent under abuse but do not ignite. No lithium plating — the dendrite-driven internal short circuit that affects Li-ion under freezing-charge conditions does not occur. The safety advantages explain why Toyota persisted with NiMH for the Prius HEV battery through 2024 model years even as plug-in hybrids and BEVs moved to lithium-ion.

For Indian battery recyclers, NiMH recovery is technically the cleanest of the major rechargeable chemistries. Shredding is safe in air (no inert atmosphere required); the cell breach releases water vapour and dilute KOH, both managed by spray-water containment. Hydrometallurgy: dilute sulphuric acid leaching dissolves nickel and the rare-earth alloy components (lanthanum, cerium, neodymium, praseodymium — typically 8-15% of cell mass) into solution; selective precipitation and solvent extraction separate Ni from rare earths and from minor cobalt content. Pyrometallurgy: smelting at 1,500-1,700°C yields a ferro-nickel alloy with rare earths reporting to the slag (recoverable by hydrometallurgical re-treatment). Recovered nickel grades 99% or better; rare earth oxides at 95%+ purity. The economic case improves materially if rare earth recovery is included: NiMH cell rare earth content is 5-8x richer per kg than primary rare earth ore concentrates, making spent NiMH a strategic source of light rare earths in the context of China's progressive REE export restrictions. The trade-off is that current Indian rare earth refining infrastructure is limited (IREL's Manavalakurichi and OSCOM plants handle monazite primary ore but not NiMH secondary feed), so recovered rare earth mixtures typically go to international refiners (Solvay La Rochelle, Rhodia Salt Lake City) rather than domestic users.