There are several different types of electrochemical battery technologies available for energy storage. Batteries for energy storage applications that involve multiple charging/discharging cycles can also be classed as secondary (re-chargeable) batteries rather than primary batteries, which are designed to be used once and then disposed of. Battery storage technology currently provides the most widespread and satisfactory methods of storing relatively small amounts of energy for powering portable electrical devices. Several variants have also been used for grid applications, especially for power quality, UPS and short spinning reserve. There are also several trials in systems for energy management. An electrochemical battery contains a negative and a positive activation species, with different battery variants using different activation species. During charging the positive active species is oxidised, while the negative is reduced. There are a wide range of battery technologies available.
Lead-acid batteries are commercially mature re-chargeable batteries. They generally consist of lead metal and lead dioxide electrodes immersed in a sulphuric acid electrolyte (in the charged state). Both electrodes are converted to lead sulphate during discharge and the concentration of the sulphuric acid electrolyte is reduced as it becomes largely water. The reactions are as follows:
Positive Electrode: Pb + HSO4– ? PbSO4 + H+ + 2e– (discharge)
Negative Electrode: PbO2 + HSO4– + 3H+ ? PbSO4 + 2H2O (discharge)
Lead-acid batteries are used in stationary energy storage applications, especially as a DC auxiliary. Large lead acid batteries of 1-5MW have also have an established history in submarine use. They are suitable for power quality, UPS and spinning reserve applications. There is over 100MW of battery power capacity currently installed for standby duty throughout the National Power and PowerGen power companies in the United Kingdom . The largest current lead-acid battery installation is a 40 MWh system in Chino, California, with a rated power of 10 MW for 4 hours. Some authors predict a short-medium term market for lead-acid batteries in solar applications where high specific energy is not required due to their low cost.
The drawbacks of lead-acid batteries are primarily to do with low cycling capacity, high charge time and careful maintenance requirements coming largely from the evolution of H2 and water loss. This renders them largely unsuitable for energy management applications. They also have a low energy density to weight ratio (currently around 40 Wh/kg), due to the high density of lead.
Current research effort is targeted at the inclusion of carbon into the negative plate of the battery to improve the peak power handling capacity of the cells, reduce the need for maintenance and improve the deep discharge capability and cycle life of the technology.
Nickel based batteries (Ni-Cd, Ni-MH, NaNiCl2 etc.)
There are several nickel battery variants. Nickel-Cadmium batteries are the most mature of the Nickel variants. NiCd batteries contain a nickel hydroxide positive electrode, a cadmium hydroxide negative electrode, a separator, and an alkaline electrolyte, often KOH. They usually have a metal case with a sealing plate equipped with a self-sealing safety valve. The reactions are as follows:
Positive Electrode: 2NiOOH + H2O ? 2Ni(OH)2 + OH– (discharge)
Negative Electrode: Cd + 2OH– ? Cd(OH)2 + 2e– (discharge)
Ni-Cd batteries have been used in some utility scale applications, notably the 14,000 cell Ni-Cd storage system installation run by GVEA in Fairbanks, Alaska which currently holds the record for the world’s most powerful battery. The 6.5 MWh battery took the world record when it discharged at 46MW for 5 min during a maximum limit test. Ni-Cd batteries usually have slightly higher energy density than lead-acid types (around 50Wh/kg), can tolerate a deep state of discharge for relatively long periods, and require less maintenance than lead-acid batteries. Their cycling ability is regarded as a little higher than lead-acid but still relatively low (up to 2000 cycles). Although they require less maintenance than lead-acid, they require careful management to avoid the “memory effect” and to avoid over-charging. There are safety and environmental waste issues with Ni-Cd batteries because Cadmium is a very toxic substance.
The toxicity of the Cadmium in Ni-Cd batteries makes it very hard to envision a large amount of further research in this technology. They have largely been superseded by NiMH battery variants.
Nickel Metal Hydride batteries
Nickel Metal Hydride (NiMH) batteries have largely replaced Ni-Cd batteries for use in portable re-chargeable batteries. Developed in the 1990’s these batteries have a significantly higher energy density than Ni-Cd batteries (around 80 Wh/kg), have less environmental issues and are cheaper. However their main disadvantages are high self-discharge rates and a relatively low cycling capacity. Although they have yet to be used in grid energy storage applications, their use in hybrid vehicles merits their mention in this report.
Sodium Nickel Chloride batteries
The sodium-nickel chloride battery is better known as ZEBRA battery. It is a high temperature system, operating around 300C, much like the sodium sulphur battery, however it has a lower energy density (around 100 Wh/kg) and specific power (around 150W/kg) . The negative electrode is made of molten sodium while the positive electrode is nickel in the discharged state and nickel chloride in the charged state. The electrolyte is a molten sodium salt. The reactions are as follows:
Positive Electrode: NiCl2 + 2Na+ + 2e– ? Ni + 2NaCl (discharge)
Negative Electrode: 2Na ? 2Na+ + 2e– (discharge)
Recently there has been a growing interest in the NaNiCl2 batteries for grid and EV energy storage applications, and these batteries are being developed and manufactured by General Electric (after acquiring Beta R&D in 2007) and Fiamm SoNick. NaNiCl2 batteries have a much lower self-discharge rate than and better cycling capabilities than the other Nickel battery variants. Accordingly they show potential promise for stationary grid energy storage applications as well as for electric vehicles.
Lithium ion batteries are now the dominant type of batteries found in small portable electronic applications due to their high energy density, light-weight and high efficiencies. The negative electrode in these batteries is a lithiated metal oxide ((LiCoO2, LiMO2, LiNiO2) and the positive electrode is made of graphitic carbon with a layered structure. Electrolytes generally consist of lithium salts dissolved in organic carbonates, i.e. LiPF6 in ethylene carbonate. When the battery is being charged, the lithium atoms in the positive electrode become ions and migrate through the electrolyte toward the negative carbon electrode where they combine with external electrons and are deposited between carbon layers as lithium atoms. Discharging reverses the process. Examples of the reactions that occur in a LiCoO2 and LiC6 battery are:
Positive Electrode: CoO2 + Li+ + e– ? LiCoO2 (discharge)
Negative Electrode: LiC6 ? C6 + Li+ + e– (discharge)
These batteries have high energy densities (in excess of 150Wh/kg), high efficiency, a low rate of self-discharge (around 5% per month) and a good cycle life provided they aren’t fully discharged. Their disadvantages are a higher cost than other battery technologies, an inability to be fully discharged and a limited lifespan.
Figure: Illustrating the charge mechanism for a Li-ion battery (borrowed from howstuffworks).
Lithium ion batteries are set to be the dominant battery for the electric vehicle market, and their development for this market is driving their costs down. However their costs are still too high for grid energy storage applications, although there are several grid energy storage trials currently underway using small 5 to 10-kW/20-kWh distributed systems and larger systems providing around 1MW for 15 min – 1 hour for fast-responding systems for frequency regulation. There is speculation that due to their low rates of self-discharge and high efficiency that they could be used to provide energy management storage for up to 10 days with renewable energy systems. In June 2011 the successful commissioning of the first Li-ion energy storage system on the UK grid was announced, a 200 kWh capacity system installed by ABB. The system is an adaptation of the company’s existing reactive power compensation systems and works in conjunction with a super-capacitor bank to provide both reactive and active power compensation, aiding grid penetration of a local wind-farm.
There is currently a very large research effort concentrated on Lithium-ion batteries. The current trend is in nano-scale research and vastly increased electrode surface areas. This has the potential to provide significant improvements in power, capacity, cost, materials and sustainability, and promising more still. Polymer electrolyte batteries are also currently being developed to alleviate problems with liquid electrolytes such as internal shorting, leakage and flammability.
One other issue with Li-ion batteries is the lack of a viable recycling process. This is another topic of current research.
Sodium Sulphur (NaS) batteries are high temperature molten metal batteries. The negative electrode is made of liquid sodium while the electrolyte is solid beta-aluminium (a type of aluminium oxide). The positive electrode is molten sulphur. In order to keep the electrodes in their liquid states the system must be regulated at around 300oC. The solid electrolyte is known as a BASE (Beta-Aluminium Solid Electrolyte) and it selectively conducts sodium ions.
During discharge, the sodium atom in the negative electrode gives up an electron and migrates to the positive electrode. Here an electron reacts with the Sulphur and then combines with the sodium to form sodium polysulphide. The process is reversible and charging causes the polysulphide to release the electrons and the sodium ions migrate back through the electrolyte to recombine with the electrons at the negative electrode forming elemental sodium. The discharge reaction is as follows:
2Na + 4S ? Na2S4 (discharge)
Once running the high temperature is self-sustaining from the heat given off by the reactions, and when the battery is cooled self-discharge cannot occur as no chemical reactions can take place. However, maintaining the temperature while the battery is neither charging or discharging is a form of energy loss.
Due to the temperature requirements these type of cells become more economical with bigger size as the relative heat conservation increases. They have a very small self-discharge because the electrolyte is a very poor conductor of electrons.
Na-S batteries have high efficiencies (85-92%) and high energy and power densities (150-240 Wh/kg and 150-230 W/kg) . There are more than 316 MW installed globally at 221 sites, representing 1896 MWh of installed storage capacity . They are most suited to stationary grid applications due to high operating temperatures and corrosive nature of sodium polysulphide. Safety is also an issue as elemental sodium will ignite in contact with air or moisture.
The prospect of Na-S batteries has also been dealt a blow by the 2011 fire in the 2 MW Mitsubishi battery. The cause of this fire has not yet been published and dominant market supplier NGK has halted production and advised customers not to use their batteries until this safety issue has been resolved.
Metal-air batteries are unique because one of the reactants (the air) doesn’t have to be stored in the battery and hence this type of battery has a very high specific energy density. Over the last decade, the Zn-air and Li-air variants have attracted the most attention, though many other metals have also been found appropriate for use. Metal-air batteries consist of an exposed porous carbon electrode (called the air cathode) which is separated from the metal anode by an electrolyte. The exposed carbon electrode traps oxygen atoms from the air which react with the positive metal ions from the anode. Research into solid, liquid, aqueous and organic electrolytes has been undertaken but at present the non-aqueous electrolyte is the most developed. The reaction for the non-aqueous Lithium-air battery is as follows:
2Ni + O2 ? Li2O2 (discharge)
The main advantage of this type of battery is the huge increase in energy density over more conventional batteries; it is suggested that an energy density of up to 3kWh/kg may be achievable although the laboratory maximum in 2011 was 362 Wh/kg . However, this is a technology firmly in Research and Development and at present metal-air batteries have poor efficiency and cycling capability. A recent EPRI report into electricity storage options suggests that early stage deployments of this technology may appear in the 2015-2020 period. Electric car manufacturer Tesla has recently expressed an interest in metal-air batteries through the filing of several patents in 2013. A 2014 review into the state of Li-air batteries by Imanishi and Yamamoto suggested that there is mixed opinion about the future of the technology, safety issues with flammable electrodes for EV applications and that getting the required charging rates for EV vehicles may be the biggest hurdle yet.
Summary of characteristics
|Type||Typical Capacity||Typical Power||Efficiency (%)||Storage Duration||$/kWh||$/kW||Lifespan||Cycling capacity||Comments|
|Pb-A||Up to 40MWh||Up to 20MW||75-90 70-90 
|Seconds – days||200-400 
|Up to 20 years||500-2000 cycles||40 Wh/kg, s.d. 2-5%/month|
|Ni based||Up to 20 MWh||Up to 50MW||72-78 ||Seconds-days||200-600  (NiCd)
800-1500  (NiCd)
100-200  (NaNiCl2)
150-300  (NaNiCl2)
|Up to 20 years||1500-3000||50 Wh/kg, s.d. 5-20%/month NiCd|
|Li-ion||Up to 50MW||85 75-90 ||Seconds – hours||950-1400 
|1200-4000 ||5-15 years||3000 cycles at 80% D.O.D.||90–190 Wh/kg,s.d. 1-5%/month, currently large systems only for frequency reg.|
|Na-S||Up to 50MWh||Up to 10MW||89 
|Seconds – hours||300-500 
|5-15 years||3000 cycles 100% D.O.D.||100Wh/kg|
|Metal-air||–||–||50 ||Seconds – days||10-60 ||100-250 
|–||100-300 cycles||Lab test at 350Wh/kg, Unproven technology, recharging currently inefficient|
Table: Common electrochemical battery chemistries and characteristics
As well as the costs presented in the table (the costs per unit power ($/kW) and energy ($/kWh)), the present value installed costs – basically a measure of the required CAPEX – are very important, especially to investors. An unparalleled resource from this perspective is the DOE/EPRI 2013 Electricity Storage Handbook in Collaboration with NRECA. The table below is taken from this and shows the present value installed costs for Lithium ion batteries for utility energy storage applications. A recent paper by Dimitri Mignard uses the cost estimates from the DOE/EPRI Electricity Storage handbook to establish a relationship between the energy storage capacity of Li-ion batteries and the cost per installed kWh.
 Kaldellis, J. K. & Zafirakis, D., 2007. Optimum energy storage techniques for the improvement of renewable energy sources-based electricity generation economic efficiency.. Energy, Volume 32, p. 2295–2305.