Page 203 - Battery Reference Book

Sodium-sulphur secondary batteries 15/3
The sodium-sulphur battery is a relatively high This battery has attracted worldwide commercial
ampere hour rechargeable battery, envisaged as a interest because of the low cost and ready availability
power source on electric vehicles and trains. It was of its chemical components and its very high energy
patented by the Ford Motor Company of Dearborn, density, some five times greater than that of conven-
USA in 1967. Chloride UK and British Rail have tional lead-acid batteries. In the USA, beta batter-
also taken out patents on the sodium-sulphur battery. ies are being developed mainly for load levelling in
Although developments have been in progress for over the electricity supply industry; 100 MW h units with
10 years, batteries are not expected to be commercially 10MW capacity are being designed and tested. In
available before the end of the 1980s. Its operation Europe the main thrust of research is directed towards
relies on the recently discovered property of B- automotive traction applications, especially for urban
alumina, namely that it combines very low electronic transport and delivery vehicles and also for rail cars.
conductivity with an unusually high ionic conductivity, The engineering and materials problems associated
which is specific to sodium ions as charge carriers. with this battery are severe, and research involving
The cell voltage, 2.08 V, is derived from the chem- industrial, government-funded and university labora-
ical reaction between sodium and sulphur to produce tories has been in progress for many years in order to
sodium polysulphide; and the theoretical energy den- improve battery lifetimes and to increase the commer-
sity, about 750 W h/kg, is high compared with that of cial attractiveness of the system.
the lead-acid battery (about 170 W h/kg). The novelty The characteristics of sodium-sulphur batteries are
of the properties discovered in sodium 0-alumina may listed in Table 15.1. The open circuit voltage remains
be estimated from the fact that, although its melting 16 2.075 V for about 70% of the discharge then it starts
point is around 2000"C, several amps per square cen- to decline as higher sodium polysulphides are formed
timetre of current may be passed across the electrolyte and discharge is terminated when the voltage reaches
at 300°C. The cell operates at about 350°C. The key 1.75 to 1.9 V. Up to 1000 charge/discharge cycles have
to the rapid sodium ion mobility lies in the crystal now been achieved. The battery has a virtually zero
structure and the low potential energy path for sodium self discharge rate.
migration that this produces.

Table 15.1 Characteristics of sodium-sulphur batteries
Open Volts Discharge Gravimetric Volumetric Gravimetric
circuit under voltage energy energy power
volts load projile density density density
(VI (V) (W h/kg-') (W Wdn-3) (Wkg-')
2.076 at 350°C 1.95 C13 Rate Linear reduction 188 377 210 at 66% of open
circuit voltage

Volumetric Storage Ope ration Self Cycle Calendar Types
power temperature temperature discharge life life available
densio ("C) ("C) rate
(W cm3)
440 at 66% of Insensitive Insensitive Zero 8000, Ey, cell 20AhPB
open circuit cell
voltage
500, lY,
battery battery