25 Sodiumbased Batteries

2.5.1 Introduction

In the 1980s a range of batteries which use a liquid sodium negative electrode were developed. These batteries differ from other batteries in so much as they run at high temperatures. They also have the interesting features of using one or more liquid electrodes in the form of molten sodium and using a solid ceramic electrolyte. Because of the need to operate at high temperatures they are only practical for large systems, such as electric cars; they are not suitable for scooters and cycles. They are rather more exotic than other types, as they will never be used in mobile phones or laptop computers, unlike the other types of battery that we will consider in this chapter. This limitation on their market has rather impeded their commercial development.

2.5.2 Sodium sulphur batteries

The development of these batteries started in the 1970s, and they run at temperatures between 300°C and 350°C. In order to keep the heat in the battery, the cells are enclosed in an evacuated case. The basic sodium sulphur cells, have a high specific energy, six times that of lead acid cells but in experimental batteries the mass of the enclosure typically halves this potential improvement.

The negative electrode in the cells consists of molten sodium, and the positive electrode consists of molten sulphur polysulphides. The electrolyte is a solid beta alumina ceramic, which conducts the sodium ions and also separates the two electrodes. The actual cells are kept fairly small and they are joined together and placed in an evacuated chamber to cut down heat losses. The design of the container needs careful thought as it can double the mass of the battery. Before the batteries can be used they have to be heated slowly to their working temperature. When in use the cells are essentially self-heating due to the electrical current passing through the battery internal resistance. When not in use for more than a day the battery interior has to be kept hot by the use of electrical heaters. The electrical energy is obtained by combining sodium with sulphur to form sodium sulphide.

The basic chemical formula for the reaction is

Table 2.4 Nominal battery parameters for sodium sulphur batteries. As always, the performance figures depend on usage

Specific energy

100 Wh.kg-1 (Potentially 200Wh.kg-1)

Energy density

150 Wh.L-1

Specific power

200 W.kg-1

Nominal cell voltage

2 V

Amphour efficiency

Very good

Internal resistance

Broadly similar to NiCad

Commercially available

Not on the market at all

Operating temperature

300-350°C

Self-discharge

Quite low, but when not in use energy must be

supplied to keep the battery warm

Number of life cycles

-1000 to 80% capacity

Recharge time

8 h

The overall characteristics of the battery are given in Table 2.4. Because of the need for good thermal insulation small batteries are impractical. The battery heating and cooling needs careful design and management. Although the sodium sulphur battery has considerable promise, worries about the safety of two reactive materials separated by a brittle ceramic tube have largely resulted in the batteries not appearing on the commercial market. These fears were boosted by spontaneous fires involving test vehicles during trials.

2.5.3 Sodium metal chloride (Zebra) batteries

The sodium metal chloride or Zebra3 battery is in many ways similar to the sodium sulphur battery and has many of this battery's advantages. However, with this system most (and some would say all) of the safety worries associated with the sodium sulphur battery have been overcome. The principle reason for the greater safety of the Zebra cells is the use of the solid positive electrolyte which is separated from the molten sodium metal by both solid and liquid electrolytes. It is certainly the case that prototype Zebra batteries have passed qualification tests for Europe, including rigorous tests such as crashing the cell at 50kph into a steel pole (Vincent and Scrosati 1997, p. 272). This battery has considerable promise and it can be obtained commercially.

The Zebra cell uses solid nickel chloride for the positive electrode and molten sodium for the negative electrode. Two electrodes are used, a beta ceramic electrode surrounding the sodium and a secondary electrolyte, sodium-aluminium chloride, is used in the positive electrode chamber. Chlorine ions are the mobile ion in the electrolyte. The electrical energy on discharge is obtained by combining sodium with nickel chloride to give nickel and sodium chloride. The overall chemical reaction which takes place in the zebra battery is:

3 Zebra is an acronym for Zero Emissions Battery Research Association. However it has now rather lost this connection, and is used as a name for this type of battery.

Figure 2.10 The reactions at each electrode of the sodium metal chloride battery during discharge

Figure 2.10 shows the reactions at each electrode during the middle and early part of the discharge of the cell. This reaction produces an open circuit voltage of about 2.5 V per cell. In the later stages of the discharge the reactions become more complex, involving aluminium ions from the electrolyte, and resulting in a lower voltage. Indeed an unfortunate feature of this type of cell is the way that the cell voltage falls during discharge, from about 2.5 V down to around 1.6 V. The internal resistance of the cell also increases, further affecting the output voltage. Nevertheless, as can be seen from the data in Table 2.5, the specific energy is very high, even with these effects.

A major problem with the Zebra battery is that it needs to operate at a temperature of about 320°C, similar to the sodium sulphur. Heat insulation is maintained by the use of a double skinned stainless steel box, with 2-3 cm of insulation between the two skins. All the air is removed from the insulation, and the vacuum is maintained for several years. Nevertheless, unless it is for a very short period, a few hours, these batteries need to be kept connected to a mains supply when not in use. This is to keep the battery hot, and is a major limitation to their application. As an example, the battery shown in Figure 2.11, which fits neatly under the seat of a battery electric car, holds an impressive 17.8 kWh of energy. However, when not in use it consumes about 100 W

Table 2.5 Nominal battery parameters for sodium metal chloride (Zebra) batteries

Specific energy

100 Wh.kg-1

Energy density

150 Wh.L-1

Specific power

150 W.kg-1

Nominal cell voltage

~2V average (2.5 V when fully charged)

Amphour efficiency

Very high

Internal resistance

Very low, but higher at low levels of charge

Commercially available

Available commercially, but very few suppliers

Operating temperature

300-350°C

Self-discharge

When not in use energy must be continually used to keep

the battery up to temperature, corresponding to a

self-discharge of about 10% per day

Number of life cycles

>1000

Recharge time

8 h

Figure 2.11 A commercial Zebra battery fitted neatly under the seat of an experimental battery electric vehicle by MES-DEA. The battery stores about 18kWh of electrical energy

of power keeping up to temperature. So, in a 24 hour period the heating will require 0.1 x 24 = 2.4 kWh of energy, corresponding to about 13% of the stored energy. In energy terms, this corresponds to the self-discharge of other types of battery, and is quite a high figure.

Zebra batteries can be allowed to cool, but if this happens they must be reheated slowly and steadily, a process typically taking about 24 hours.

They are available as tried and tested units with well-established performance criteria, though only in a very limited range of size. An example is shown in Figure 2.11.

The overall characteristics of the battery are given in Table 2.5. These are taken from the 17.8kWh (-280 V, 64 Ah, 180kg, 32kW peak power) unit manufactured by MES-DEA of Switzerland.

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