Nickel-metal hydride battery

Guoan Energy Technology (Dongguan) Co., Ltd. R&D Center Editor

Nickel-metal hydride (NiMH) batteries are high-performance rechargeable batteries. They are classified into high-pressure and low-pressure types. The positive active material is Ni(OH)2 (NiO electrode), the negative active material is a metal hydride, also known as a hydrogen storage alloy (hydrogen storage electrode), and the electrolyte is a 6mol/L potassium hydroxide solution. NiMH batteries are receiving increasing attention as an important area of hydrogen energy application.

Emergence of Nickel-Metal Hydride Batteries:

With the dwindling supply of fossil fuels due to large-scale human exploitation, the development and utilization of hydrogen energy have received increasing attention in recent years. NiMH batteries are receiving increasing attention as an important area of hydrogen energy application. Although NiMH batteries are indeed high-performance rechargeable batteries, aerospace NiMH batteries are high-pressure types. The high-pressure hydrogen storage in thin-walled containers poses an explosion risk. Furthermore, NiMH batteries require precious metal catalysts, increasing their cost and hindering widespread civilian adoption. Therefore, since the 1970s, research into low-pressure NiMH batteries for civilian use has been explored abroad. NiMH batteries are classified into high-pressure and low-pressure types. High-pressure NiMH batteries were first developed in the early 1970s by M. Klein and J. F. Stockel in the United States. A trend of replacing cadmium-nickel batteries with NiMH batteries in various satellites has emerged.

Structure and Principle:

The positive active material of a NiMH battery is Ni(OH)2 (called NiO electrode), the negative active material is a metal hydride, also known as a hydrogen storage alloy (the electrode is called a hydrogen storage electrode), and the electrolyte is a 6mol/L potassium hydroxide solution. The methods for forming the active material into electrode plates mainly include sintering, slurry coating, nickel foam, nickel fiber, and impregnation. Electrodes prepared by different processes have significant differences in capacity and high-current discharge performance. Batteries are generally produced using different processes according to different usage conditions. Most civilian batteries for communication, etc., use slurry-coated negative electrodes and nickel foam positive electrodes. The charge-discharge chemical reaction is as follows [1]:

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 reaction from left to right is the charging process; The reaction from right to left is the discharging process.

During charging, Ni(OH)2 and OH- react at the positive electrode to produce NiOOH and H2O, while releasing e- to generate MH and OH-. The overall reaction is that Ni(OH)2 and M generate NiOOH, and the hydrogen storage alloy stores hydrogen. Discharging is the reverse process, with MH releasing H+, H+ and OH- generating H2O and e-, and NiOOH, H2O, and e- regenerating Ni(OH)2 and OH-. The standard electromotive force of the battery is 1.319V.

Classification:

Nickel-metal hydride batteries are classified into high-pressure and low-pressure types.

Low-pressure NiMH batteries have the following characteristics:

(1) Battery voltage is 1.2~1.3V, comparable to cadmium-nickel batteries;

(2) High energy density, more than 1.5 times that of cadmium-nickel batteries;

(3) Capable of fast charge and discharge, good low-temperature performance;

(4) Sealable, strong overcharge and over-discharge resistance;

(5) No dendritic crystal formation, preventing internal short circuits;

(6) Safe and reliable, environmentally friendly, no memory effect, etc. [1]

High-pressure NiMH batteries have the following characteristics:

(1) High reliability. It has good over-discharge and overcharge protection, can withstand high charge and discharge rates, and does not form dendrites. It has good specific characteristics. Its mass specific capacity is 60A·h/kg, 5 times that of cadmium-nickel batteries.

(2) Long cycle life, up to several thousand cycles.

(3) Compared with nickel-cadmium batteries, it is fully sealed and requires less maintenance.

(4) Excellent low-temperature performance, with no significant change in capacity at -10℃.

Battery Maintenance:

NiMH batteries require attention during use.

(1) Avoid overcharging. Overcharging should be avoided within the cycle life, as it can cause expansion of the positive and negative electrodes, leading to active material shedding, diaphragm damage, destruction of the conductive network, and increased battery ohmic polarization.

(2) Prevent electrolyte degradation. Hydrogen evolution should be suppressed during the cycle life of the NiMH battery.

(3) Storage of NiMH batteries. NiMH batteries should be stored fully charged. Long-term storage without sufficient charge will weaken the function of the negative electrode hydrogen storage alloy and shorten the battery life.

Development Trends:

Nickel-metal hydride (NiMH) batteries are a mature product. The international market produces approximately 700 million NiMH batteries annually. Japan consistently ranks among the top countries in NiMH battery production and industry scale, followed by the United States and Germany, both of which have also been developing and researching NiMH batteries for many years. China is rich in rare earth metal resources used in the manufacturing of NiMH batteries, with proven reserves accounting for over 80% of the world's total proven reserves. Domestically developed processing technologies for NiMH battery raw materials are also becoming increasingly mature. NiMH batteries can be used interchangeably with zinc-manganese and cadmium-nickel batteries. In the future, cylindrical batteries will primarily focus on product diversification and commercialization, while rectangular batteries will focus on development as power sources for vehicles.

Guoan Energy Technology (Dongguan) Co., Ltd. R&D Center Editor