VoxSolaris: The Voice of the Sun
The Sodium Sulphur Battery
 
Anatomy of sodium sulphur cell

The battery has a sodium (Na) anode and a sulphur (S) cathode. The electrolyte is the ceramic, soduim beta alumina (approx 11 parts aluminium oxide Al2O3 and 1 part sodium oxide NaO). For the cell to work, both the soduim and the sulphur must be a liquid and the electrolyte must be at a temperature at which it can work as an ionic conductor. Sodium melts at 98C and sulphur at 113C but the electrolyte does not operate effectively until the temperature reaches around 350C. This then is the operating temperature of the battery. The electrolyte remains solid at this temperature as its melting point is 2100C. The cells are comonly constructed as a pair of concentric tubes in which the inner tube made of the electrolyte contains one electrode while the outer tube made of a metal such as aluminium, contains the other electrode. The cell is completely enclosed and does not give off any gases.

During discharge, electrons are striped from the sodium atoms and flow from the sodium anode through the external load to the sulphur cathode. The positively charged sodium ions move through the electrolyte where they react with the sulphur and the electrons to produce sodium polysulphide. During recharge, the applied voltage strips electrons from the sodium polysulphide turning it back into sulphur and sodium ions. The sodium ions now cross the electrolyte into the sodium where they are reunited with their missing electrons to form sodium atoms.
 
  Performance of sodium sulphur call

The cell has actually has a very high electrical efficiency of the order of 85% but the electrical efficiency of sodium sulphur batteries as a whole are normally quoted as being around 75%. This lower figure is actually an inappropriate use of averages. In the electrical sense the efficiency is 85% comprising a columbic efficiency of virtually 100% and a charge/discharge voltage ratio of 0.85 or better. The cells also have a very low self discharge rate. Both the high columbic efficiency and low self discharge are due to beta alumina being an extreamly poor conductor of electrons. However the high operating temperature introduces a different form of energy inefficiency and self discharge. The cell must be at its operating temperature in order to deliver or accept current and unless the user is prepared for very long startup times, the temperature must be permanently maintained. No matter how good the insulation, this requires energy and is thus a form of self discharge.

The sodium sulphur battery can have very high energy and power densities because of the chemistries of alkali metals of which sodium is a member. Reported figures differ widely, mostly because of differences in the construction of working systems. Mostly this is down to diiferent approaches to insulation but also to such factors as thickness of electolyte and cell walls. The lowest energy densities are around 50Wh/Kg and the highest are around 200Wh/Kg. Power densities range from about 100W/Kg to 200W/Kg.

Potential low cost

Probably the biggest advantage is that all the materials are cheap although Beta alumina is difficult to work with and can be expensive to engineer to a high quality. The materials are also very abundant and would not become scarce even if every house and every car in the world had a large sodium sulphur battery. The fact that sodium sulphur batteries are expensive today is purely because they are not mass produced. The materials needed for a one ton 100KWhr battery cost around $1,000 and the techniques for producing high quality Beta Alumina do lend themselves to mass production. Should mass production ever materialize prices could ultimately tumble to $75 per KWhr for car batteries and $25 per KWhr for large stationary applications.

Limited Shelflife

The battery has a limited shelflife, typically of the order of 2 to 5 years. While this may be reasonable compared to say lithium ion, it is a real disadvantage in both the stationary and transport markets. As the battery required bulky insulation these are the only markets it can operate in. The limited shelflife is virtually independant of the number of charge/discharge cycles and is largely a function of how long the electrolyte has sat in the rather hostile environment of molton sodium and sulphur. The electolyte degenerates over time, becoming a conductor of electrons and giving rise to lower columbic efficiency and self discharge. Ultimately this renders the cells useless.

Across the industry, current research into sodium sulphur is rather limited but is not surprisingly, focused on the electrolyte degeneration issue. The degeneration occurs because the aluminium oxide is gradually reduced by the passing sodium ions to aluminium and because sodium metal accumulates in the electrolyte fissures. It is hard to say how this research might progress in the future and obviously, any potential improvements in the electrolyte structure would take a minimum of 2 years of testing to show a positive result.

In spite of this there are companies who claim that they have sodium sulphur cells with lifespans of 15 years. For comercial reasons they tend to be somewhat secrative and use vague language such as 'we have found that deterioration of beta alumina is virtually eliminated by high quality of raw materials and high standards of production'. Umm maybe. Certainly there are things that can be done to improve the construction of beta alumina extrusions beyond what the standard method of sintering can achieve. Direct casting of the alumina produces a far superior result but the melting point of 2100C makes this a less than straightforward option. Whatever the reality behind these claims, there is a small but growing install base of large load levelling and UPS systems based on sodium sulphur so hopefully the evidence in favor of the technology is mounting. Our advice for now would be as follows:- Don't buy one without a 10 year waranty and make sure the waranty is from a manufacturer that is simply too large to go bust!

Exotherm Prevention and Detection

The electrolyte is very brittle and thus the sodium sulphur cell does suffer from poor shock resistance. It also suffers from thermal cycling problems although this is not an issue if the temperature is maintained. The poor shock resistance is less of a problem in stationary applications but it is a serious problem in the transport market where a crash could occur. If the sodium were to come in direct contact with the sulphur, an exotherm will result. In a crash such an exotherm could be massive and would easily breach the integrity of the outer casing of the battery. In such an event you can forget the occupants of the car. Even as a bystander the box at your funeral would owe its presence purely to tradition.

Even in a stationary application safety is an issue and again the quality of the beta alumina electrolyte is in the spotlight. If micro fissures in the electrolyte allow even tiny quantities of the electrodes to mix, an exotherm will result. If the exotherm serves to widen the fissure it could run away and lead rapidly to cell explosion. This could threaten the integrity of neighboring cells in the battery and ultimately the integrity of the outer casing of the battery. This would result in a high intensity fire if not an actual explosion.

Prevention of fissures and thus exotherms, involves careful manufacture and extensive ultrasound scanning of the electroyte before it is used in cells. It also means cells must be replaced regularly (in better designs cell can be 'hot swapped'). Exotherm detection during operation can be facilitated by monitoring one or more of three parameters, the voltage, the cell pressure and to a lesser extent, the temperature. None are reliable but all add to the cost! The voltage will drop accros a cell once the electrolye is breached but in the early stages of a disaster, this could be minimal. The oportunity for testing voltage also has to be created. There is little point testing the voltage accross all the cells because that won't pinpoint the faulty cell. In any case voltage depression will apply whenever a load is present. The cells have to be tested individually and while offline and that requires complex switching. Switching has to be facilitated mechanically because of the temperature. Monitoring the temperature of cells by such as a thermocouple is also frought with probems as placing cells in thermal isolation of each other would complicate the design. It is simpler and cheaper to have all the cells in a central high temperature location. In any event the temperature will rise while current is flowing. The only practical parameter to measure is the cell presure.

Then there is the issue of a corrective measure. Obviously the battery can be shut down if any of the cells are misbehaving but it is very easy to get a false positive and find the battery constantly shuts down. As a 'failsafe' measure, cells can be constructed such that in the event of a rise in pressure, the bottom of the cell would detach itself and vent the material outside the electrolyte to a sump in the bottom of the battery. This is not as simple as it sounds. The cell casing has to be manufactured to a high degree of accuracy in order to ensure that venting does happen when needed and does not happen otherwise. Additionally the sump would have to cope with the possibilty of a major electrolyte breach leading to both sodium and sulphur being vented from the cell. Smaller cells mitigate containment of such events but also drive up costs.

In practice large sodium sulphur installations operate successfully without constantly shuting down and the risk from run away exotherms is very small. However it is easy to see why sodium sulphur has not yet reached mass production and why it is the companies that make them tend to be staffed by people with lots of scientific and engineering accreditations to thier name.

The Zebra Battery

The zebra battery is a very close relative of the sodium sulpur battery. It has a sodium (Na) anode and a Nickel chloride (NiCl) cathode. The electrolyte is the same beta alumina used in the sodium sulphur cell. Like the sodium sulphur battery, the zebra ia a high temperature battery although at 270 - 300C it is slightly cooler. The nickel chloride remains a solid at these temperatures so between it and the beta alumina, an intermeadiate molten salt electrolyte of sodium tetrachloroaluminate is used.

The zebra having one electrode as a solid does not offer quite as many charge cycles as sodium sulphur due to the need of the electrode to regrow to something akin to its original shape. The usual solid electrode problem of dendrite formation appears to be mitigated by the temperature. Nickel chloride is significantly more expensive than sulphur but is still not expensive and the design is inherently safer. The onset of fissures in the electrolyte will result in much less violent exotherms because the concentration of nickel chloride in the sodium tetrachloroaluminate is low compared to the equivelent situation in the sodium sulphur cell. The slightly lower temperature also leads to lower thermal losses. The voltage is also higher, 2.58 volts as opposed to the 2.1 of the sodium sulphur.

Like sodium sulphur, the zebra has an electrical efficiency of around 85% with near 100% columbic efficiency and very low self discharge (because of beta alumina). It also shares sodium sulphur's high energy and power densities of around 100Wh/Kg and 150W/Kg respectively. These figures are less varied than the sodium sulphur because the zebra has been standardized for production. It is much closer to being rolled out to consumers than sodium sulphur. Indeed, the zebra is actually on sale, a rare feat for advanced batteries. It costs a staggering $600 per kWhr and is available in 20 kWhr units. These costs are expected to fall dramatically. The systems on offer come with expected lifespans of between 2 and 5 years reflecting the aforementioned problems with the deterioration of the beta alumina electrolyte. Obviously the manufacturer concerned, like ourselves, has not found that 'the deterioration of beta alumina is virtually eliminated by high quality of raw materials and high standards of production'. The zebra looks likely to capture a slice of the electric vehicle market and has already extensively tested in electric buses.

General construction

The general construction is to place the cells in a cage supported by spacers and seal the cage in an evacuated box. Typically the case will have several vacuum layers as this can dramatically improve the insulator performance as disscussed in our page on superinsulators. The diagram below shows a typical outline of a sodium sulphur or zebra battery.


With this agressive approach to insulation we do have a problem getting rid of heat when a heavy load is applied. This can be solved by variety of means, such as an electromechanical bridge to turn a conduction path on or off (not shown in the diagram). In a large installation it would even be possible to use cogeneration.

The Vox-Solaris Advanced Sodium Sulphur Electrochemical Reactor

Although we like the simplicity of the standard design, one branch of our research centered on redisigning the battery as a flow battery. What we were seeking to achieve was a safer battery for electric vehicles. We figured that in the event of a crash, large exotherms resulting from mass breaches of the beta alumina could be mitigated if the bulk of the active materials were stored in separate resevoirs away from the electrolyte. As we put together initial drafts of the design it became clear better saftey in a crash was not the only benefit on offer. Please read the page on Sodium Sulphur Electrochemical Reactor. Decide for yourself if this device is what the electric car has been waiting for.