Chemistry
Nickel-zinc is one member of a family of aqueous alkaline chemistries, with the nickel positive electrode and an electrolyte composed primarily of potassium hydroxide being the commonalities within the family. This family includes nickel-iron, nickel-metal hydride and the most well-known variant, nickel-cadmium. The nickel electrode is designed to determine the amp-hour limit of the battery, while the counter electrode (in this case, metallic zinc) determines the operating voltage, as well as the size and weight of the cell.
The chemistry of nickel-zinc is similar to nickel-cadmium, except that the cadmium electrode is replaced by a high surface area metallic zinc electrode. The electrolyte is a potassium hydroxide solution which has typically been modified with additives to help control the charge-discharge reactions of the zinc electrode.
Zn + 2NiOOH + H2O » ZnO + 2Ni(OH)2 E°= 1.73V
During discharge, zinc oxidises to zinc oxide on the negative electrode providing electrons to the external circuit; the electrons then travel to the positive electrode where they reduce nickel oxyhydroxide to nickel hydroxide. This exothermic reaction generates a usable current until all the nickel oxyhydroxide has been converted. The above chemical reaction is reversible, this endothermic reaction uses an external power source to recharge the cell. A charging current is used to oxidise the nickel hydroxide; electrons are removed from the positive electrode and travel through the external circuit to the negative electrode where they reduce the zinc oxide back to zinc. During the discharge/charge reactions hydroxide ions are produced/reacted at the electrode-electrolyte interface and flow between the positive and negative electrodes, completing the electrical circuit.
Environment
NiZn is one of the most recyclable battery chemistries on the market. Unlike many other materials, both nickel and zinc can be recycled again and again while maintaining their physical and chemical properties, as there remains the same amount of each element at the end of a particular product cycle as at the beginning.
NiZn has a similar level of water usage and energy input (and therefore CO2 generation) per kWh during manufacture to lead acid batteries; these values are significantly smaller than Li-ion technologies. NiZn also uses much less cobalt than Li-ion and no volatile organic compounds.
NiZn Technology Benefits
By significantly extending the cycling capability, the NiZn battery has a wide range of customer benefits.
High Energy Density – NiZn technology offers higher energy density than lead-acid batteries and comparable energy density to high power lithium-ion batteries when measured by either weight (Watt hours per kilogram) or by volume (Watt hours per litre).
Superior Power Density – NiZn technology delivers a higher voltage in a smaller and lighter package than other rechargeable batteries. This reduces the size of the NiZn battery to less than half the size of a typical lead-acid battery.
Safety - NiZn battery chemistry has environmental and physical safety advantages over lead-acid and lithium-ion batteries. Both nickel and zinc are easily recycled. The zinc electrode contains no lead, cadmium or mercury, and presents no threat to the environment. NiZn batteries are non-flammable and fail-safe.
Charge/Discharge Rate - The NiZn battery chemistry excels at high discharge rates while retaining thermal stability and is also capable of fast recharging, a key benefit where high charge acceptance is necessary.
