The NiMH battery is considered to be a successor to the long-time market dominator the Nickel Cadmium (Ni-Cd) battery system. These cells have been in existence since the turn of the century. The Ni-Cd battery system started with a modest beginning, but with significant advances in the last four decades since the 1950s, the specific capacity of the batteries has improved fourfold. A strong growth of the rechargeable battery consumer appliance market for laptop computers, mobile phones, and camcorders pushed the battery performance requirements particularly service output duration even further. This factor, along with environmental concerns, has accelerated the development of the alternate NiMH system. Since its inception in the early to mid 1980s, the market share of the rechargeable NiMH battery has grown to 35% and the capacity, particularly the high-load capability, has been improving dramatically.
The scientific publications and patent literature provide an extensive number of reports regarding the different aspects of NiMH batteries, including chemistry and hydrogen storage properties of cathode materials.
However, it is important to understand design criteria that optimize performance and extend the cycle life of NiMH batteries.
AB5 (LaNi5) and AB2 (TiN2) alloy compounds have been studied as part of NiMH battery design. Both these alloys have almost similar hydrogen storage capacities, approximately 1.5% by weight. The theoretical maximum hydrogen storage capacities of AB2 alloys is slightly higher, 2% by weight than the maximum of 1.6% by weight for AB5 alloys. The higher AB2 hydrogen storage capacity by weight can be exploited only if the battery size is made larger. This becomes an undesirable factor for compact EV battery designs. The basic concept of the NiMH battery cathode results from research of metallic alloys that can capture (and release) hydrogen in volumes up to a thousand times of their own. The cathode mainly consists of a compressed mass of fine metal particles.
The much smaller hydrogen atom, easily absorbed into the interstices of a bimetallic cathode is known to expand up to 24 volume percent.
The hydride electrode has capacity density of up to 1,800mAh/cm3. Thus for the smaller size NiMH battery, the higher energy density for AB5 alloys, about 8–8.5g/cm3 compared to relatively lower energy density for AB2 alloys, about 5–7g/cm3 results in a battery with comparable energy density.
The conventional, although not cost-effective processing method for manufacturing the AB5 battery materials includes:
Step 1: Melting and rapidly cooling of large metals ingots
Step 2: Extensive heat treatment to eliminate microscopic compositional inhomogeneities
Step 3: Breaking down the large metal ingots into smaller pieces by the hydriding and dehydriding process
Step 4: Grinding of the annealed ingots pieces into fine powders
This four-step manufacturing process is the key-limiting factor to widespread commercialization of NiMH batteries. This process can be eliminated and replaced by a single step using rapid solidification processing of AB5 powders using high-pressure gas atomization. The H2 gas absorption and desorption behavior of the high-pressure gas atomization processed alloy is also significantly improved with the annealing of the powder.
The scientific publications and patent literature provide an extensive number of reports regarding the different aspects of NiMH batteries, including chemistry and hydrogen storage properties of cathode materials.
However, it is important to understand design criteria that optimize performance and extend the cycle life of NiMH batteries.
AB5 (LaNi5) and AB2 (TiN2) alloy compounds have been studied as part of NiMH battery design. Both these alloys have almost similar hydrogen storage capacities, approximately 1.5% by weight. The theoretical maximum hydrogen storage capacities of AB2 alloys is slightly higher, 2% by weight than the maximum of 1.6% by weight for AB5 alloys. The higher AB2 hydrogen storage capacity by weight can be exploited only if the battery size is made larger. This becomes an undesirable factor for compact EV battery designs. The basic concept of the NiMH battery cathode results from research of metallic alloys that can capture (and release) hydrogen in volumes up to a thousand times of their own. The cathode mainly consists of a compressed mass of fine metal particles.
The much smaller hydrogen atom, easily absorbed into the interstices of a bimetallic cathode is known to expand up to 24 volume percent.
The hydride electrode has capacity density of up to 1,800mAh/cm3. Thus for the smaller size NiMH battery, the higher energy density for AB5 alloys, about 8–8.5g/cm3 compared to relatively lower energy density for AB2 alloys, about 5–7g/cm3 results in a battery with comparable energy density.
The conventional, although not cost-effective processing method for manufacturing the AB5 battery materials includes:
Step 1: Melting and rapidly cooling of large metals ingots
Step 2: Extensive heat treatment to eliminate microscopic compositional inhomogeneities
Step 3: Breaking down the large metal ingots into smaller pieces by the hydriding and dehydriding process
Step 4: Grinding of the annealed ingots pieces into fine powders
This four-step manufacturing process is the key-limiting factor to widespread commercialization of NiMH batteries. This process can be eliminated and replaced by a single step using rapid solidification processing of AB5 powders using high-pressure gas atomization. The H2 gas absorption and desorption behavior of the high-pressure gas atomization processed alloy is also significantly improved with the annealing of the powder.