74 Chapter 7 Other Nickel-based chemistries, such as nickel zinc (NiZn) and nickel hydrogen (NiH 2 ) have found use in many applications. NiZn has been integrated into applications such as power tools, lawn and garden equipment, light electric vehicles (LEVs), consumer cells (AA, AAA, etc.), and even in some HEV demonstration applications. NiZn chemistries are relatively abuse tolerant with high discharge capabilities and are relatively low cost. More importantly, NiZn are more environmentally friendly than the NiCd or even NiMh cells that are available today.
NiH 2 has found a very good home in aerospace satellite applications for both low earth orbit (LEO) and geosynchronous earth orbit (GEO) satellites. While NiH 2 has a higher initial cost and relatively low energy density, it is also capable of surviving long cycle life and calendar life while in orbital applications. LEO satellite applications typically require over 35,000 cycles and calendar life of over 6 years. GEO satellites require somewhat shorter cycle life, but still need about 2000 cycles but must survive much longer calendar life, up to 20 years in some applications (Brill, 2011).
Various other nickel-based chemistries have been tested and evaluated over the years, how- ever, these four represent the most commonly used nickel-based chemistries that are available in the market today. Sodium-Based Chemistries Another set of chemistries that has gained recent interest are the sodium-based batteries. Sodium batteries are often referred to as thermal batteries due to the fact that they must operate at very high temperatures in order to maintain the sodium (salt) electrolytes in a molten (liquid) state. One of the earliest developments was by a research team in South Africa called the Zeolite Battery Research Africa Project (ZEBRA).
The work of this group eventu- ally became the basis for most of the current sodium batteries on the market. Companies such as ZEBRA technologies, General Electric (GE), SONICK-FIAMM (MES-DEA) are produc- ing ZEBRA-type sodium metal halide batteries ( Table 4 ). One of the interesting characteristics of the sodium chemistries is that they need to operate at very high temperatures, typically between 350 °C–700 °C. This means that they operate in a molten form, which in itself creates some challenges. In the typical molten sodium battery, the anode and cathode are separated by a solid ceramic separator.
In these batteries, the electrolyte is typically a molten sodium of either aluminum chloride, nickel chloride, or similar chemistry. The positive electrode is made of nickel and the negative is the molten sodium. The ceramic separator separates the anode from the cathode, but is only activated once the sodium reaches its melting point, which may be as low as 150 °C but is typically closer to around 350 °C. These chemistries are very attractive because of the inherently low cost of the materials involved and for the ZEBRA-type sodium nickel–metal halide battery the inherently safe failure mode. The NaS batteries are more energetic if the separator fails.
One of the chal- lenges with this chemistry is that if it is allowed to cool below about 160 °C, the sodium will