The nickelâ€"zinc battery (sometimes abbreviated to the chemical symbols for the elements "NiZn") is a type of rechargeable battery that may be used in cordless power tools, cordless telephones, digital cameras, battery operated lawn and garden tools, professional photography, flashlights, electric bikes, and light electric vehicle sectors, among other uses.
Larger nickelâ€"zinc battery systems have been known for over 100 years. Since 2000, development of a stabilized zinc electrode system has made this technology viable and competitive with other commercially available rechargeable battery systems.
History
In 1901, Thomas Alva Edison was awarded U.S. Patent 684,204 for a rechargeable nickelâ€"zinc battery system.
The battery was later developed by the Irish chemist, Dr. James J. Drumm (1897â€"1974) and installed in four 2-car Drumm railcar sets between 1932 and 1948 for use on the Dublinâ€"Bray railway line. Although successful, they were withdrawn when the batteries wore out. Early nickelâ€"zinc batteries were plagued by a limited number of discharge/recharge cycles. In the 1960s, nickelâ€"zinc batteries were investigated as an alternative to silverâ€"zinc batteries for military applications, and in the 1970s, were again of interest for electric vehicles. A company called Evercel Inc. developed and patented several improvements in nickelâ€"zinc batteries, but withdrew from that area in 2004.
Applications
Nickelâ€"zinc batteries have a charge/discharge curve similar to 1.2 V NiCd or NiMH cells, but with a higher 1.6 V nominal voltage.
Nickel-zinc AA and AAA cells are available in some stores, often offered for digital cameras. Other sizes are uncommon in the consumer market. Both D-cells and sub-C cells are currently used in commercial applications.
Nickelâ€"zinc batteries perform well in high-drain applications, and may have the potential to replace leadâ€"acid batteries because of their higher energy-to-mass ratio and higher power-to-mass ratio (up to 75% lighter for the same power). NiZn are cheaper than nickel-cadmium batteries, and are expected to be priced somewhere in between NiCd and leadâ€"acid types. NiZn may be used as a substitute for nickel-cadmium. The European Parliament has supported bans on cadmium-based batteries, and nickelâ€"zinc offers the European power tool industry a good alternative.
After about 30 charges, NiZn batteries tend to self-discharge more quickly. Therefore, when a high-power, high-voltage battery is needed with less concern for longevity, NiZn batteries become a good choice to use.
Electrochemistry
Water is consumed and generated on the discharge and charge cycles respectively.
Discharge Reaction is left to right:
2H2O + Zn + 2NiOOH â†" Zn(OH)2 +2Ni(OH)2.
Electrochemical open circuit voltage potential: ~1.73 V.
Battery life
Compared with cadmium hydroxide, the tendency of zinc hydroxide to dissolve into solution and not fully migrate back to the cathode during recharging has, in the past, presented challenges for the commercial viability of the NiZn battery. The zinc's reluctance to fully return to the solid electrode adversely manifests itself as shape change and dendrites (or "whiskers"), which may reduce the cell discharging performance or, eventually, short out the cell, resulting in a low cycle life.
Recent advances have enabled manufacturers to greatly reduce this problem. These advances include improvements in electrode separator materials, inclusion of zinc material stabilizers, and electrolyte improvements (i.e. by using phosphates). One manufacturer, (PowerGenix), which has developed 1.6V batteries, has claimed battery cycle life comparable to NiCd batteries.
Battery cycle life is most commonly specified at a discharge depth of 80 percent of rated capacity and assuming a one-hour discharge current rate. If the discharge current rate is reduced, or if the depth of discharge is reduced, then the number of charge-discharge cycles for a battery increases. When comparing NiZn to other battery technologies, cycle life specifications may vary with other battery technologies, depending on the discharge rate and depth of discharge that were used.
Advantages
Nickelâ€"zinc cells have an open circuit voltage of 1.85 volts when fully charged, and a nominal voltage of 1.65 V. This makes NiZn an excellent replacement for electronic products that were designed to use alkaline primary cells (1.5 V). NiCd and NiMH both have nominal cell voltages of 1.2 V, which may cause some electronic equipment to shut off prior to a complete discharge of the battery because the minimal operating voltage is not provided.
Newer, more powerful cells with up to 800 cycles/life can be an alternative to Li-Ion batteries for electric vehicles. Due to their higher voltage, fewer cells are required (compared to NiCd and NiMH) to achieve a given battery-pack voltage, reducing pack weight and size and improving pack reliability. They also have low internal impedance (typically 5 milliohms), which allows for high battery discharge rates, up to 50C. (C is battery capacity in Ah, divided by one hour.)
NiZn batteries do not use mercury, lead, or cadmium, or metal hydrides that are difficult to recycle. Both nickel and zinc are commonly occurring elements in nature, and can be fully recycled.
NiZn cells use no flammable active materials or organic electrolytes, and the newest models use polymeric separators to resolve the dendrites problem.
Properly designed NiZn cells can have very high power density and low temperature discharging performance, and also can be discharged to 100% and recharged without problems. They are now available in sizes up to F and 50Ah/ prismatic cell.
Zinc is cheap and abundant metal (24th most abundant element in the Earth's crust) and it is not dangerous to health. Common oxidation is +2 so charge discharge move 2 electrons instead of 1 as in NiMH batteries.
Disadvantages
NiZn batteries cannot be discharged to 0V, end-of-discharge voltage must be 1.3V or the cell will be damaged.
Reverse Polarity
When used in series connection, a reverse-protection is needed to avoid gas formation. This is not really a problem, only need appropriate electronic or load that cut-out before voltage becomes too low.
Cost
Current Nickel Zinc batteries use nickel foam and copper foam current collectors for active materials; they are expensive, but new cheaper carbon-based current collectors are under development. Moreover, special rechargers must be used for NiZn batteries, rechargers for NiCd or NiMH are not suitable and will destroy the NiZn cell.
Charging
NiZn technology is well suited for fast recharge cycling, as optimum charge rates of C or C/2 are preferred.
Known charging regimes include a constant current of C or C/2 to cell voltage = 1.9 V. Maximum charge time is given variously as 2½ hours and 3 hours. Trickle charging is not recommended, as recombination is not provided for, and excess hydrogen will eventually vent, adversely affecting battery cycle life.
See also
- Zincâ€"air battery
- Nickelâ€"cadmium battery
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