Nickel-metal hydride (NiMH) batteries are high-performance rechargeable batteries. They are classified into high-voltage and low-voltage types. The positive electrode active material is Ni(OH)₂ (called the NiO electrode), and the negative electrode active material is a metal hydride, also known as a hydrogen storage alloy (the electrode is called the hydrogen storage electrode). The electrolyte is a 6 mol/L potassium hydroxide solution. NiMH batteries are attracting increasing attention as an important direction for hydrogen energy applications.
Nickel-metal hydride (NiMH) batteries use Ni(OH)₂ (called the NiO electrode) as the positive electrode active material and a metal hydride, also known as a hydrogen storage alloy (the electrode is called the hydrogen storage electrode), as the electrolyte is a 6 mol/L potassium hydroxide solution. The main manufacturing processes for the active materials forming the electrode sheets include sintering, paste drawing, nickel foam, nickel fiber, and intercalation. Electrodes prepared using different processes vary significantly in capacity and high-current discharge performance. Batteries are generally manufactured according to the specific application conditions. Consumer batteries, such as those used in communications, mostly employ a paste-drawn negative electrode and a nickel foam positive electrode.
The chemical reactions for charging and discharging a nickel-metal hydride battery are as follows:
Positive electrode: Ni(OH)₂ + OH⁻ = NiOOH + H₂O + e⁻
Negative electrode: M + H₂O + e⁻ = MHab + OH⁻
Overall reaction: Ni(OH)₂ + M = NiOOH + MH
Note: M: hydrogen alloy; Hab: adsorbed hydrogen; the process from left to right in the reaction equation is the charging process; the process from right to left in the reaction equation is the discharging process.
During charging, Ni(OH)₂ and OH⁻ at the positive electrode react to form NiOOH and H₂O, while simultaneously releasing e⁻ to form MH and OH⁻. The overall reaction is Ni(OH)₂ and M to form NiOOH, thus the hydrogen storage alloy stores hydrogen. During discharging, the process is reversed: MHab releases H⁺, which reacts with OH⁻ to form H₂O and e⁻. NiOOH, H₂O, and e⁻ then recombine to form Ni(OH)₂ and OH⁻. The standard electromotive force of the battery is 1.319V.
Nickel-metal hydride (NiMH) batteries are already a mature product, with approximately 700 million units produced annually in the international market. my country possesses abundant rare earth metal resources for manufacturing NiMH batteries, with proven reserves accounting for over 80% of the world's total. Domestically developed NiMH battery raw material processing technologies are also becoming increasingly mature. NiMH batteries are interchangeable with zinc-manganese and nickel-cadmium batteries. Future development of cylindrical batteries will focus on product specification diversity and commercialization, while the development focus of prismatic batteries will be on their use as a power source for vehicles.
