High-temperature batteries
This category includes sodium sulfur, sodium-nickel chloride and lithium-metal disulfide
batteries. All high-temperature batteries suffer from the fact that temperature must be maintained
at about 300°C, which requires a sophisticated thermal management system and battery insulation
and imposes a lack of packaging flexibility. Moreover, thermal losses must be compensated by
electrical heating when the vehicle is not in use, so that these electrical losses are similar to self
discharge. Hence, these losses may significantly increase total electrical consumption for lightly
used vehicles. Meanwhile, these batteries offer much higher levels of energy storage performance
than lead acid or alkaline systems and are insensitive to ambient temperature effects.
Sodium sulfur batteries have been in operation for more than a decade in Europe and offer high
specific energy (100 Wh/kg) with relatively low-cost battery materials. They have the favorable
characteristic of their specific power’s not declining significantly with the state-of-charge,
although the specific power value is a relatively low 130 W/kg.^77 More recently, Silent Power has
unveiled a new design, the MK6, with a specific energy of 120 Wh/kg and specific power of about
230 W/kg.^78 However, the corrosivity of the battery materials at high temperature has led to
limited calendar life (to date), and reliability is affected if the battery “freezes.” Even now, a
leading manufacturer, ABB, claims a battery life of less than three years for its sodium sulfur-
battery. Silent Power has estimated a selling price of $250/kWh in volume production of 1050
units/month for its MK6 battery.
Sodium-nickel chloride batteries have many of the sodium sulfur batteries’ favorable
characteristics along with reduced material corrosivity, so that they may have longer calendar life.
These batteries are being extensively tested in Europe, and the latest versions (dubbed ZEBRA in
Europe) have shown energy densities over 80 Wh/kg and specific power of over 110 W/kg at full
charge.^79 Other advancements are expected to increase both specific energy and specific power.
However, specific power drops to nearly half the fully charged value at 80 percent DoD, and
possibly is also reduced with age or cycles used. Despite this problem, this battery type has
emerged as a leading contender in Europe owing to its potential to meet a life goal of five years.
Lithium-metal sulfide bipolar batteries hold the promise of improvements in specific energy and
power relative to the other “hot” batteries, but they are in a very early stage of their development.
Work by Argonne National Laboratories has shown very good prospects for this type of battery.
It is lithium’s low equivalent weight that gives lithium batteries their high-energy content of three
to five times that of a lead acid battery. Research efforts on lithium-metal sulfide batteries of the
bipolar type are being funded by the USABC, and battery developers hope to achieve specific
energy levels of over 125 Wh/kg and power levels of 190 W/kg.^80 Initial tests on cells have
indicated approximately constant power output with battery DoD, and the system also holds the
potential for long life and maintenance free operation, but substantial research is still required to