Na-S cell

The Na—S system is expected to provide significant iacreases ia energy density for sateUite battery systems (69). In-house testing of Na—S cells designed to simulate midaltitude (MAO) and geosynchronous orbits (GEO) demonstrated over 6450 and over 1400 cycles, respectively. 

Figure 10. Emf of an Na/S cell at 350 °C versus DOD. Reproduced with permission of Chapman and Hall, London.

A prerequisite of long-life sodium/sulfur batteries is that the cells contain suitable corrosion-resistant materials which withstand the aggressively corrosive environment of this high—temperature system. Stackpool and Maclachlan have reported on investigations in this field [17], The components in an Na/S cell are required to be corrosion-resistant towards sodium, sulfur and especially sodium polysulphides. Four cell components suffer particularly in the Na/S environment the glass seal, the anode seal, the cathode seal, and the current collector (in central sodium arrangements, the cell case). 

Figure 30. Open circuit voltage of the Na/S cell versus Na content (x) of the cathode.63...

A secondary electrochemical cell is simply one that can be recharged as in the case of the Na/S cell discussed below (in contrast a primary cell, such as the common torch battery, is exhausted after use and cannot be recharged). During charging the chemical reaction is driven in reverse by applying an e.m.f. in the sense to oppose the forward direction e.m.f. 

Elementary Na/S battery science The basic science of the Na/S cell is identical to that of the fuel cell. In both cases the essential is an electrolyte. On one side of the electrolyte membrane there is a source of sodium ions at a particular chemical potential, and on the other side a sink for the ions at a relatively lower chemical potential. The elements of the process are illustrated in Fig. 4.28. 

Na/S cell There are various cell designs and details can be found elsewhere [7]. The principle of operation is illustrated in Fig. 4.28 and a schematic of one design of cell shown in Fig. 4.34. The cell contains just the amount of sodium for it all to be combined with the sulphur for complete discharge. The small space between the stainless steel liner and the electrolyte contains a fine stainless steel mesh which, by capillarity, can raise the height of the liquid sodium which enters the gap through the small hole at the bottom of the liner. The sodium can be pulled to a height of at least 200 mm at 300 °C. 

If the lifetime of Li-based batteries (the term lithium ion batteries for batteries with polar Li-compounds as negative electrodes is very unfortunate) can be further enhanced, they will be also of importance for electrotraction. The classical battery type used in automobiles, viz, the lead-acid accumulator, is distinctly superior in terms of long-time stability but possesses too low an energy content per unit weight as to drive automobiles. Driving car of sensible size and performance with this alone requires a battery weight on the order of 11, (This problem is not removed by using Ni-Cd accumulators,) Much effort has been undertaken to develop a sodium-sulphur cell. In the Na-S cells ... 

The Na ions can move quite freely within the twin plane between the spinel layers as a result the cation conductivity is high within these planes but negligible in the perpendicular directioa The high ion conductivity makes these ceramics of interest for battery applications, and this has been exploited in the Na-S cell. This cell was developed around 1965 by Ford Motor Co., but has not been used in production. The main difficulty is that the cell must be kept at an operating temperature of 350°C to keep the electrode molten. 

The question of a suitable container for the cathode melt is one of the key problem areas in developing the Na/S cell research at present is concentrating on the development... 

The satisfactory sealing of Na/S cells is another difficult technical problem It is necessary to provide three separate seals - to seal both the sodium and the sulphur electrodes from the atmosphere and also from each other In addition to being leak-tight, these seals must be chemically and electrochemically compatible with the reactants they contain (sodium and S/Na S, respectively) at temperatures up to 400 C and for periods of several years Ideally, the seals will have some flexibility to compensate for the rigidity and brittle nature of the ceramic tube ... 

The assembly of these cells into a battery must take account of electrical engineering considerations, thermal control problems, questions of accessibility and maintenance and safety factors. With as many as 500-1000 cells involved it is evident that a series/parallel network will be required the design of this network poses particular problems for Na/S cells because of two characteristics they possess ... 

The internal resistance of a Na/S cell is higher than that of a comparable lead/acid cell. This higher resistance leads to reduced voltage at high current drains and limits the voltage efficiency of the Na/S battery to, typically, 65-85% depending upon the power output. This is not entirely wasteful as some heat output is required, in any event, to maintain the battery at operating temperature-... 

Using thin-film technology, sohd state microbatteries are compatible with microelectronics. One can in principle prepare an on-chip microbattery capable of maintaining its memory during a power outage. The combination of the thin-film configuration and the low current drain requirements (1 to 10 riA cm for a C-MOS memory) allows for the use of electrolytes with much lower conductivities than are necessary in the Na/S cell apphcations, for example. 

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