Electrical energy is now accepted as a clean and universally available source of energy in almost all areas of life One exception is solar panels brisbane road traffic. Despite the fact that EVs dominated the early development of motor cars, the ICE has prevailed because of the high energy density of petrol.
Meanwhile, the increasing weight of traffic in built-up areas is today producing an intolerable level of environmental pollution from exhaust and noise, endangering the health of those affected and causing considerable damage to buildings and cultural heritage. This trend can, and must, be countered by EVs using batteries as their source of energy.
Initially, electrically driven vehicles used lead-acid batteries as their source of power. The limited range of these vehicles, however, remains unacceptable. Today, significant efforts are being directed towards the development of improved batteries with a higher energy storage-density as well as preparation for their economic production (Table 1).
The sum of the properties of the material system, namely common salt (NaCl) and nickel (Ni), used in ZEBRA batteries has proved to be eminently suitable, so now renewed interest is being shown in electrically driven vehicles as a means of protecting the environment. In addition, ZEBRA batteries can also considerably extend the operating range of electrically driven industrial vehicles.
Each ZEBRA battery is made up of individual cells (Figure 1). The chemical reaction in the battery converts common salt and nickel to nickel chloride and sodium during the charging phase. As it is discharged, the reaction is reversed.
Each cell is enclosed in a robust steel case. The NiCl2 and Na electrodes are separated by a ceramic partition that will conduct sodium ions but acts as a barrier to electrons. The cell reaction can therefore only rake place when an electric current flowing outside the cell is equal to the internal current of sodium ions, the current being controlled during discharge by the resistance of the load.
A molten salt, sodium aluminium chloride, which will conduct sodium ions, forms a conductor between the inner ceramic wall and the porous solid NiCl2 electrode in an electronically conducting nickel matrix. As a result, the total positive mass is involved in the cell reaction.
Apart from the reversible cell reaction, there are no side reactions, so that the coulometric efficiency of the ZEBRA cell is 100 per cent. The completely maintenance-free cells are hermetically sealed by a metal/ceramic combination.
Measurement of the cumulative charge from which the number of rated cycles is calculated;
Measurement of the insulation resistance between battery poles and battery case. For safety reasons a fault is reported if the insulation resistance falls below the lower limit;
Measurement of the battery terminal voltage;
Hearing of the battery during operation.
These data are available to the vehicle management system via a serial interface. The battery is monitored via this interface for prohibited operating conditions. Battery control unit and heating are supplied from the mains or from the battery system, whereby the mains power supply is given priority.
The battery control unit is directly plugged into the interface box which contains the two-pole main circuit breaker and the pre-charge resistor with switch. Therefore, the main circuit breaker is an integral part of the battery system and allows disconnection of the battery voltage from the vehicle. It is operated by the battery control unit and external signals, like a crash sensor.
Reliability and useful life.
These are the most important characteristics for the battery’s use in traffic. Its reaction during normal operation, to possible misuse and to accident situations, must also be taken into consideration.
The useful life is expressed both in the number of cycles and in calendar periods. The cycle life, based on statistically reliable battery test-rig results, is 1,000 full cycles; i.e. the battery can be fully charged and discharged at least 1,000 times before the capacity falls to below 80 per cent of the nominal value. This is independent of the length and nature of the individual cycles. At the same time, durability tests are being conducted in vehicles that agree with the test-rig results and these are being continued. The development target is to achieve more than 1,000 cycles, which equates to a driven distance of approximately 150,000km.
The calendar period, which is influenced by time-dependent changes in the battery, can be estimated from existing extrapolation to be at least five years, as the materials used in the batteries are not subject to any detectable corrosion. Durability tests are also being continued in this respect.
The battery is robust and fault-tolerant. Should a cell break down, it goes over to a low-ohm condition, i.e. it is bridged by a short circuit formed internally. This enables the battery to be operated easily with up to five per cent lost cells. An unexpected battery failure is therefore unlikely. The ZEBRA battery can supply high currents for short periods, as the liquid electrolyte ensures a uniform distribution of current in the cell.
Misuse can never entirely be excluded with batteries. It is thus important to allow for possible faults in the monitoring and charging units. Here too the ZEBRA batteries present no problems. The chemical reactions that occur in the battery as a result of excessive charging and discharging are illustrated in Figure 6.
The standard cell reaction takes place at a cell voltage of 2.58V. When, in the case of overcharge, the available salt is exhausted, NiCl2 continues to be produced at high voltage by the reaction of the nickel with the liquid electrolyte. A current of sodium ions is thereby maintained that protects the ceramic electrolyte against fracture by excess voltage. Over-charging should be avoided wherever possible. If this does occur, however, for example as a result of a fault in the charging unit, the battery is in no way endangered. Once again, this has been demonstrated in excess charge tests. Similarly, the liquid electrolyte develops a protective reaction during excess discharge that, for a few Ah, is reversible. Should a cell develop a fault, e.g. a crack in the ceramic, the same reaction will ensure that the total sodium content is converted. The reaction products (aluminium and common salt) lead to a cell short circuit, are not corrosive and, even at high temperatures, have an insignificant vapour pressure, all of which justifies the categorisation of the ZEBRA battery as extremely safe. The battery is tolerant of excess current, so that a brief short-circuit – which can occur during installation for example -does not immediately result in cell failure. The ZEBRA battery tolerates repeated cooling and, within certain limits, overheating as the steel cell cases will normally remain intact even at temperatures of 500 to 600C with no reactant leakage.
The cells are so constructed that any violent distortion of the cell case will first break the ceramic. Any liquid sodium present mixes with the liquid electrolyte and rapidly reacts to form common salt and aluminium, thus virtually excluding the possibility of any spillage of liquid sodium. This reaction releases only about two-thirds as much energy as the normal discharge reaction that is to a large extent prevented.
In serious accident situations, the whole battery could be mechanically destroyed. Even in such cases however, the battery should not constitute any additional source of danger. To this end, safety test programmes were run in which a fully charged battery, operational at 300C, was dropped on to a telegraph pole crash barrier with a final velocity of 50km/h. The barrier damaged the battery by about 30cm, destroying the cells in the damage zone (Figure 7).
Figure 7: Crashtest of a ZEBRA Z12 battery.
Practically the whole or the stored energy was converted to heat that raised the temperature inside the battery to approximately 700C. The effective thermal insulation kept the outer surface of the battery at a considerably lower temperature. There was no leakage of reactant from the battery, so the battery presented no risk. This test demonstrated the high safety standard of the ZEBRA battery. Further tests are being carried out in connection with applications and new battery types.
Materials, production costs and recycling
The materials required for battery manufacture are freely available in large quantities, so that the raw material supply for a major production series of ZEBRA batteries is secured in the long term.
The production costs are therefore very largely dependent upon economies of scale. An automatic pilot production line has been constructed that is produc-ing a limited number of batteries for trials in vehicle fleets and small series applications. At the same time, series production procedures are being further developed so that significant cost reductions, down to the level of lead-traction batteries, will be possible in subsequent series production.
The best form of recycling is a completely closed material cycle. With the ZEBRA battery, this is also made economically attractive by the use of nickel as the active material. It has already been proven that the nickel in used cells can be easily recovered in a pure form. During recycling, the cells are first heated to operating temperature and short-circuited. This ensures that the remaining sodium is converted to common salt and the nickel chloride to nickel, thus avoiding the need to handle liquid sodium in the recycling process. The industrial recycling process is being so developed that the relevant equipment for large quantities of used batteries will be available in time.
ZEBRA batteries are suitable for all mobile and stationary applications that require chargeable batteries with an energy content of more than 15kWh.
Currently, interest is primarily focused on electrically driven cars and minibuses, as ZEBRA batteries are particularly suited for ZEVs that meet the Californian clean air legislation requirements. However, industrial vehicles and mains energy storage units are also interesting future application areas.