The Nickel Metal Hydride battery has a nickel-hydroxide cathode, a metal hydride (a variety of metal alloys are used) anode, and aqueous potassium hydroxide electrolyte. This is a rechargeable battery chemistry that has been superseded by lithium ion, but has seen a lot of use in Toyota hybrids.
The overall reaction is:
2NiOOH + MH <=> Ni(OH)2 + M
Advantages:
- High efficiency
- Environmentally friendly
- Wide operating temperature range: -30°C to +65°C
- High abuse tolerance to overdischarging and overcharging
- Relatively maintenance free
- Long cycle life: 600 to 1200 cycles to 80% DoD
- Below 40% SoC the internal resistance increases due to electrode and transport kinetics which reduces the available power
- Above 80% SoC pressure within the cells causes the onset of hydrogen exchange which decreases the overall specific power
- The Toyota Prius kept the cells within a 5% window around 56% SoC so as to prolong life, increase safety and ensure a flat voltage profile.
Disadvantages:
- Low energy density compared to Lithium Ion