Batteries today

Two hundred years after Volta's invention of the first electrochemical power source, Ron Dell reviews progress in battery technology

	Battery specifications
	High cell voltage and stable voltage plateau over most of the discharge High stored energy content per unit mass (Wh kg–1) and per unit volume (Wh dm–3) Low cell resistance (milliohms) High peak power output per unit mass (W kg–1) and per unit volume (W dm–3) High sustained power output Wide temperature range of operation Long inactive shelf-life (years) Long operational life Low initial cost Reliable in use Sealed and leak-proof Rugged and resistant to abuse Safe in use and under accident conditions Made of readily available materials that are environmentally benign Suitable for recycling Secondary batteries High electrical efficiency (Wh output/Wh input) Capable of many charge–discharge cycles Ability to accept fast recharge Will withstand overcharge and overdischarge Sealed and maintenance-free

The year 2000 is the bicentenary of Volta's pile, the first source of continuous or current electricity. From small beginnings, the applications for portable electrical power have mushroomed in recent years. Sixty years ago domestic uses for batteries were largely confined to flashlamps, radio sets and starter batteries for cars and motorcycles. Modern households typically have 40-50, hidden away in all sorts of consumer products - from clocks and watches to personal CD players and mobile phones. Away from the home there are many other applications, particularly for large batteries. Examples include the standby batteries for emergency use in hospitals, hotels, department stores, telephone exchanges etc; traction batteries for electric vehicles (tugs, tractors, forklift trucks, wheelchairs, golf carts); batteries for solar panels or wind generators; defence batteries in armaments, missiles, submarines, torpedos. Many of these applications demand a performance that is barely matched by traditional batteries - which explains the ever-present demand for new and better varieties.

Batteries are of two general types: primary cells that are discharged once and then discarded and secondary batteries that are recharged and used again. One of the interesting features of batteries is the very wide range of sizes in which they are manufactured, from a stored energy content of ca 0.1 watt-hour (Wh) for a watch or calculator battery, to 100MWh for a load-levelling battery in the electrical supply industry (Table 1). Can there be any other industry in which products are produced in such a size range? Estimating the market for batteries is notoriously difficult. In 1991 the world battery market was estimated at US$21,000m, with 40 per cent by value attributable to primary batteries and 60 per cent secondaries. Undoubtedly, the market has grown since then. A more recent (1999) estimate of the Japanese market is ca US$8000m, of which 25 per cent is due to primary cells and 75 per cent to secondary batteries.¹ Even if these figures are only approximate, this is clearly a major industry worldwide.

Table 1. Battery sizes and applications
Battery type	Stored energy/Wh	Applications
Miniature/button cells	0.1-5	Watches, calculators, heart pacemakers
Portable communications	2-100	Mobile phones, laptops
Domestic uses	2-100	Portable radio and TV, flashlamps, toys, video cameras, powertools
Automotive	102-103	Starting batteries for cars, trucks, buses, boats etc. Traction batteries for lawnmowers, golfcarts, invalid chairs etc
Remote area power supply	103-105	Lighting, water pimping, telecommunications etc
Traction	104-106	Electric vehicles, forklift trucks, tractors, torpedoes
Stationary	104-106	Standby batteries, un-interruptable power supply (UPS)
Submarine	106-107	Underwater propulsion
Load levelling	107-108	Electicity supply industry, load levelling, peak shaving, spinning reserve

Battery addicts

Someone once described a battery as 'a livelihood to the manufacturer, an irritation to the user and an addiction to the researcher'. If the latter attribute is true of primary batteries, how much more so is it for rechargable batteries where the technical challenge of developing a battery capable of being charge-discharge cycled hundreds or thousands of times is formidable. And we must meet this goal within a framework of producing a product that is economically viable, safe to use and environmentally friendly.

The Box above sets out in more detail the specifications sought in a battery. The problem (and the challenge) of developing new and better batteries lies in the complexity of this specification. Not only are there some 20 criteria for a secondary battery, but these are often highly interactive. For example, the available stored energy and the peak power output both depend on the temperature; the peak power also depends on the state of charge of the battery; the charge-discharge cycle life of a secondary battery depends critically on the depth to which it is discharged in each cycle, and so on. All of these factors need to be quantified before we can decide whether a battery is likely to be commercially viable for a particular application. However, key factors are the stored energy per unit mass and volume and it is here that modern batteries, such as lithium-based batteries, come into their own.

Smooth operation

How does a battery operate? A battery is basically a simple electrochemical device to store electrical energy as chemicals. It has certain essential components. The negative electrode consists of a current collector and an active component, often a metal such as finely divided zinc, lead or cadmium, which is capable of being oxidised with the release of electrons. The positive electrode also consists of a metallic current collector and an active component, generally a higher valent metallic oxide (eg MnO₂, PbO₂, NiOOH, AgO) that is capable of being reduced. The electrodes are separated by an electrolyte that conducts ions, but which must be an electronic insulator to avoid internal short-circuits.

Fig 1. Discharging an electrochemical cell

In most conventional batteries the electrolyte is an aqueous solution such as ZnCl₂, KOH or H₂SO₄, although some advanced batteries use ion-conducting ceramics, polymers or molten salts. During discharge, electrons flow from the negative electrode to the positive via this 'external load', thereby doing electrical work.

The charging of a secondary battery is the reverse of discharging (Fig 1). Given the apparent simplicity of this scheme, and the wide range of elements to choose from in the Periodic Table, at first sight it is a puzzle why developing new batteries has proved to be so difficult. Indeed, it seems ludicrous when considering the sophistication of the microchip in a modern laptop computer that the simple battery should be the largest and heaviest component. It is the complex and demanding user specification (see above) that poses the challenge. The more the specification can be relaxed for a particular application, the better the chance of meeting it.

Primary batteries

By far the most common primary cells are based on the zinc-manganese dioxide couple, either so-called zinc-carbon cells (Leclanché cells) or alkaline manganese cells. These both give 1.5V open circuit, but differ in a number of important respects. Zinc-carbon cells (Fig 2a) have a central carbon current collector immersed in the positive cathode (a mixture of impure MnO₂ and carbon), a container of metallic zinc as the anode, and an electrolyte of aqueous NH₄Cl and/or ZnCl₂. These cells are traditional and inexpensive.

Alkaline manganese cells (Fig 2b), a superior and more expensive product, use finely divided zinc powder as the anode and this fills the centre of the cell, with a brass pin to make contact with the base. The electrolyte is concentrated KOH solution and the cathode material - a mix of chemically or electrochemically prepared MnO₂ and carbon - forms a concentric annulus around the zinc powder and the separator. Alkaline manganese cells have a long shelf life and are particularly useful for high drain (power) applications, where their useful life is several times that of zinc-carbon.

Fig 2. (a) Zinc-carbon battery; (b) Alkaline manganese dioxide battery

The cheaper zinc-carbon cells are adequate for low drain applications and for intermittent use (such as in flashlights) where there is recovery time between uses, to allow diffusion processes to remove polarisation at the electrodes and restore equilibrium. Both types of cell are made by most manufacturers in a variety of standard sizes and shapes. The prismatic 9V cells, as used in smoke detectors, contain six small cells wired in series.

Several manufacturers are now offering 3V lithium-MnO₂ cells. These employ a lithium foil negative and an ion-conducting organic electrolyte. They are available as cylindrical cells, using spiral-wound electrodes ('jelly roll' configuration), or as button cells. Their advantages include high gravimetric and volumetric energy densities, high pulse rate capability, long shelf life and the ability to operate over a wide temperature range (-40°C to +60°C).

Button and coin cells are used widely in watches and pocket calculators. They may be either alkaline manganese cells (1.5V), zinc-silver oxide cells (1.5V), or 3V lithium cells with several possible cathodes (usually MnO₂ or CF_X). There are also zinc-air button cells, employing a fuel-cell type air cathode, which find their main application in hearing aids. Altogether there are over 40 different sizes and chemistries of button and coin cells.

Secondary batteries

Lead-acid batteries

The lead-acid battery, invented by Planté in 1859 and further improved by Fauré in 1881, is the most widely used secondary battery. The electrode reactions of the cell are unusual because the electrolyte, sulphuric acid, is one of the reactants, as seen in the following equations for discharge:

anode:

Pb + H2SO4 « PbSO4 + 2e- + 2H+

E0 = 0.356V

cathode:

PbO2 + H2SO4 + 2H+ + 2e- « PbSO4 + 2H2O

E0 = 1.685V

overall:

Pb + PbO2 + 2H2SO4 « 2PbSO4 + 2H2O

E0 = 2.041V

On discharge, sulphuric acid is consumed and water is formed, with the converse on charging. We can therefore determine the state of charge of the battery by measuring the relative density of the electrolyte (1.28-1.30 for a fully charged cell).

Energy storage options		Degradation modes
Fossil fuels Ever since the start of the industrial revolution, fossil fuels have provided the principal source of the world's energy. Initially, solid fuels (coal) were employed exclusively, then in the late 19th and early 20th centuries liquid fuels (petroleum) progressively took over an increasing share of the market, particularly for transport, and finally in the latter half of the 20th century gaseous fuels (natural gas) assumed increasing importance. Fossil fuels are versatile in that they may be combusted to provide heat, burnt in an internal combustion engine to provide mechanical energy/power (eg for transport) or used to generate electricity in a power station. An important feature of fossil fuels is that they are not only concentrated sources of energy, but are also readily transportable energy stores. Nuclear fuels Nuclear fuels (uranium, plutonium, thorium) are of use only for the central generation of electricity in a nuclear reactor. Although nuclear fuels themselves may be stored, the electricity produced from them cannot be stored directly. Renewable energy sources Renewable energy sources (wind energy, solar PV energy, wave energy, biomass, tidal energy, geothermal energy) are also best exploited via electricity generation, although in a dispersed mode and on a smaller scale than nuclear electricity. Again, electricity from these sources is not readily stored. Electricity storage As concern grows over fossil fuel usage, in terms of global warming and resource depletion, there will be a progressive swing to renewable energy. This will necessitate the development of improved methods of storing electricity, from periods when it is available (eg sunny or windy days) to when it is needed (night-time or periods of calm weather). Because electricity cannot be stored directly (except on a very small scale in capacitors), it must first be converted to some other energy form for storage. There are four options: Potential energy: Pumped-hydro schemes, as currently operated by electricity utilities in mountainous regions; compressed air storage. Kinetic energy: Storage in high-speed flywheels of advanced design, and made from fibre-reinforced composites. Thermal energy: Night storage heaters of high thermal capacity, as commonly used in the UK for space heating. Chemical energy: Conversion to fuels such as hydrogen or methanol, or storage as chemicals in batteries. In the 21st century, the requirement for electricity storage will grow and it is likely that batteries will play a key role.		If a battery is to be charged-discharged for hundreds, or even thousands, of cycles, it is essential that the chemical reactions that take place at the electrodes are quantitatively reversible. Even if as little as 0.1 per cent irreversibility (or side reaction) occurs, this will soon add cumulatively to a major loss in capacity. Many, if not most, electrode reactions involve a reconstructive phase change in the crystal chemistry of the active materials. A typical positive electrode reaction would be: Solid (A) + Anion « Solid (B) + e- (charged) (eg Ni(OH)2 + OH- « NiOOH + H2O + e-) This involves ionic diffusion processes in the crystal structure of the solids, leading to phase change and recrystallisation. From the viewpoint of the solid state chemist, the requirement to reverse this reaction quantitatively during each cycle is exceedingly demanding. The severity of the specification is apparent when one considers the many possible processes or side reactions leading to battery deterioration and failure. These include: densification and swelling of the electroactive material with loss of porosity; progressive formation of inactive phases, isolating active material; growth of metallic needles (dendrites) at the negative electrode, causing internal short circuits; mechanical shedding of active material from electrode plates; separator dry-out through over-heating; corrosion of current collectors, resulting in increased internal resistance; and gassing of electrode plates on overcharge. These and other degradation processes may result in precipitous battery failure, through an internal short circuit, or may lead to progressive loss in capacity and performance. Generally the degradation steps are interactive and accumulative, so that when the performance starts to deteriorate it soon accelerates and the battery becomes unusable. Despite this gloomy prognosis, some remarkable success has been achieved in designing batteries of long cycle life (ca 1000 cycles) for several different chemistries. The nickel-hydrogen battery has been demonstrated to last for >20000 cycles and is the preferred type in low Earth orbit satellites, as used for meteorology and Earth surveillance. For this application the batteries are required to undergo 16 charge-discharge cycles per day (5840 cycles per year, with no loss in capacity and no opportunity to exchange failed batteries!). Modern weather forecasts and military defences are dependent on the performance of the batteries in these satellites.

Lead-acid batteries find wide application in vehicles. Originally known as 'starting, lighting and ignition batteries', they are now more commonly referred to as 'automotive batteries' because of the range of other duties they must perform in the modern car. Other engine starting applications are in aircraft, boats and stationary engines for local electricity generation. These batteries are recharged by the engine's alternator and in normal use are not subjected to 'deep' discharge (see Glossary). They are of the 'pasted plate' (Fauré) design, in which the positive active material is pasted on to a lead grid current collector. This design is cheap to construct and gives a high power output, but the life of the battery is considerably shortened by repeated deep discharge. A modified version of the pasted plate battery is the so-called 'leisure battery' used in caravans, boats and so on for supplying the 'house electrics'. This is essentially an improved pasted plate battery which, at higher cost, will give a reasonable life when subjected to deep discharge duties.

Finally, there is the traction battery as used to propel electric vehicles (milk floats, tractors, fork-lift trucks etc). This is a more expensive battery type in which the positive active material (PbO₂) is contained in a row of polyester or braided glass fibre tubes. Co-axially, in the centres of the tubes, are vertical lead alloy spines that act as the current collectors. Figure 3 shows the two principal lead-acid types.

Fig 3. Lead-acid batteries with (a) flat plates and (b) tubular plates

Over the years, many improvements have been made to the lead-acid battery. Although the essential electrochemistry remains unchanged, the modern battery bears little resemblance to that of 50 years ago. Major advances have been made in the lead alloys used, in the materials and design of the separators, in the packaging (polypropylene containers rather than glass or hard rubber/pitch), and in the methods of construction. All these changes have led to batteries of improved performance, lower mass and lower cost. In recent years sealed lead-acid batteries have been developed that require no maintenance and may be used in any orientation.

Alkaline electrolytes

Rechargeable alkaline electrolyte batteries were invented at the end of the 19th century by Jungner in Sweden and Edison in the US. These were based on nickel oxide cathodes and either iron or cadmium anodes, and are popularly known as the nickel-iron and nickel-cadmium batteries, respectively. The electrolyte is concentrated KOH solution. The overall chemistry of each cell is analogous:

Fe + 2NiOOH + 4H2O « Fe(OH)2 + 2Ni(OH)2.H2O

E0 = 1.37V

Cd + 2NiOOH + 4H2O « Cd(OH)2 + 2Ni(OH)2.H2O

E0 = 1.30V

Both batteries were commercialised early in the 20th century, though the nickel-cadmium battery has proved more successful. This is because the iron electrode is more susceptible to corrosion and to self-discharge on standing. Also, the electrical efficiency is poor and a low over-potential for hydrogen evolution leads to excessive gassing during recharge.

Nickel-cadmium batteries are best known as small (AA size) rechargeable cells for use in childrens' toys. Much larger 6V batteries (5 cells in series) are available for engine-starting, for stationary battery applications, and for electric traction. These have several advantages over lead-acid: stable discharge voltage, long operational life (ca 1000 cycles), low maintenance, faster discharge rate, better low-temperature performance and excellent reliability. However, they are considerably more expensive and there are environmental concerns over the disposal of batteries containing toxic cadmium. This disposal problem may be easier to solve for the larger batteries used by industry, which can easily be recycled, than for domestic AA size cells that tend to be discarded with the domestic refuse.

For this reason, at least, two recent developments in rechargeable alkaline batteries are welcome. Traditionally, alkaline Zn-MnO₂ cells have always been seen as primary cells, but rechargeable cells of this type are now being marketed. This has come about as a result of advances in separator materials, which prevent the formation of elongated zinc needles or dendrites, which lead to internal short-circuiting. Changes in cell design prevent discharge from occurring beyond the first electron removal step (Mn⁴⁺ ® Mn³⁺). In addition it is necessary to use special chargers that taper-charge the cell to a maximum of 1.7V per cell, to prevent over-charge and gassing. Such cells are capable of relatively few charge-discharge cycles, but have several times the capacity of comparably sized Ni-Cd cells and are cheaper. They may be seen as intermediate between primary alkaline manganese cells and rechargeable Ni-Cd cells.

The second major advance in rechargeable alkaline batteries was the development of the nickel-metal hydride battery. This retains the nickel oxide positive electrode and the KOH electrolyte, but uses a metallic hydride rather than cadmium. Effectively, the negative electrode is hydrogen (as in a fuel cell), immobilised in the form of a metallic hydride. The hydride is a complex alloy of rare-earth elements and other metals that may be decomposed and reformed reversibly. The operating voltage of a Ni-MH cell is almost the same as that of Ni-Cd (1.2-1.3V), making for ready interchangeability. The specific energy of Ni- MH batteries (60-70 Whkg^-1) is up to double that of Ni-Cd and their specific power may be as high as 250Wkg^-1. These batteries are resilient to overcharge and overdischarge and operate from -30 to +45°C. Cells of both cylindrical and prismatic design are now manufactured in a range of sizes; small cells are used in portable electronic devices (eg mobile phones), while prismatic cells of 100Ah capacity are available for assembly into 12-14V modules (eg for use as traction batteries). The new material technology involved in developing Ni- MH batteries is almost entirely associated with the hydride negative electrode.

Lithium batteries

Lithium, with an atomic mass of 6.94, is the lightest of all the metals and is therefore an obvious candidate for battery use. It has a high specific capacity (3.86Ahg^-1) and a much higher electrochemical reduction potential (-3.045V) than zinc (-0.76V). The problems in developing lithium batteries stem from the high reactivity of lithium metal. It is necessary to use a non-aqueous electrolyte, which may be either an organic liquid or a solid polymer - each with a dissolved lithium salt to make it ionically conducting - or a fused lithium salt.

	Some key manufacturers
	Zinc-carbon and alkaline manganese primary batteries Duracell, Ever Ready (Energizer), Panasonic, UCAR, Vidor, Varta, Kodak, Rayovac Lithium primary cells Duracell, Ultralife, Energizer, Sanyo Button and coin cells Duracell, Varta, Hitachi Rechargeable alkaline manganese Rayovac Lead-acid Hawker (Tungstone), CMP, Johnson Controls, Varta, Tudor, CEAC Rechargeable nickel-cadmium batteries SAFT, Varta, Eagle-Picher Nickel-metal hydride batteries SAFT, GM-Ovonics, Varta, Panasonic, Matsushita Lithium ion AEA Technology, Sony, Sanyo, Varta

Primary lithium batteries using a lithium foil negative electrode, an organic liquid electrolyte and any one of several positive electrode materials are commercially available from several suppliers. The difficulties arise when one tries to develop a rechargeable lithium battery of this type. Much work has been done in this field with oniy limited success. In general, lithium is not electrodeposited as a smooth layer on the metal current collector, but as a mossy deposit.

Lithium foil that has been exposed to air is covered with a thin layer of hydroxide-nitride, which limits its reactivity. Freshly electrodeposited lithium is finely divided and highly reactive, and decomposes the electrolyte. Some of the deposit becomes electrically isolated from the electrode and so capacity is lost rapidly. Lithium metal may also be plated out as crystalline dendrites that ultimately penetrate the separator and cause an internal short-circuit of the cell. Finally, these processes constitute a fire risk and cells have been known to ignite spontaneously during recharge. For all these reasons, the commercial prospects for rechargeable cells based on liquid electrolytes and lithium metal negatives do not seem too bright, although much research is still in progress. The lithium ion battery, the rising star of the 1990s, circumvents most of these problems.

The essential feature of the lithium ion battery is that at no stage in the charge-discharge cycle should there be any lithium metal present. Rather, lithium ions are intercalated into the positive electrode in the discharged state and into the negative electrode in the charged state and move from one to the other across the electrolyte. The latter is a solution of a lithium salt in an organic solvent. The origin of the cell voltage is then the difference in free energy between Li⁺ ions in the crystal structures of the two electrode materials.

Commercial cells use carbon as the negative electrode: lithium ions will intercalate readily into graphite up to a composition approaching C₆Li, at a voltage of zero to 1V with respect to a lithium reference electrode. Using a positive electrode of LiCoO₂ or LiNiO₂, the cells are assembled in the discharged state and a 3V lithum ion cell results. After a few initial cycles, approximately half of the intercalated lithium may be removed reversibly, as shown:

charged

Li0.55CoO2 + 0.45Li+ + 0.45e- « LiCoO2 (123mAhg-1)

discharged

charged

Li0.35NiO2 + 0.5Li+ + 0.5e- « Li0.85NiO2 (135mAhg-1)

discharged

The solid state chemistry of the LiNiO₂ structure is more complex than that of LiCoO₂ and the fully lithiated compound is not stable during electrochemical cycling. Nevertheless, the practical Ah capacities are much the same, as are the cell voltages. When cycling Li⁺ ion cells it is important to control the top-of-charge voltage carefully (4.1V for LiNiO₂ and 4.2V for LiCoO₂). Failure to do so results in decomposition of the 'positives' to give oxygen gas and Co₃O₄ or LiNi2O4, a hazardous situation in a sealed cell. For this reason, lithium ion cells must be recharged using a specially designed charger incorporating both voltage and temperature control. Over-discharge must also be avoided and it is usual to have a limiting cut-off voltage on discharge of ca 2.7V. (In this regard the NiMH battery has the advantage of being much better able to withstand overcharge and overdischarge.)

The Sony Corporation in Japan first commercialised lithium ion cells in the early 1990s, and they have since been marketed by many other battery manufacturers. They are extensively employed in laptop computers, mobile phones and other portable electronic equipment. In 1997 alone it is estimated² that 190m cells were manufactured in Japan with a value exceeding US$2000m. With worldwide R&D in progress, we may hope for future improvements in performance as well as price reductions for lithium ion cells.

From this brief survey it is clear that battery research is a dynamic and challenging field for chemists, working closely with material scientists and design engineers. The rapid advances of the past 20 years augur well for new power sources in the 21st century.

Ronald Dell worked in applied electrochemistry for 20 years, and may be contacted at 2 Tullis Close, Sutton Courtenay, Abingdon, Oxfordshire OX14 4BD.

References

1. K. Tamura and T. Horiba, J. Power Sources. 1999, 81-82, 156.
2. T. Kodama and H. Sakaebe, J. Power Sources, 1999, 81-82, 144.

Battery bibliography

Journals Batteries International. London: Euromoney Publications. Electrochimica Acta. London: Elsevier Science. Journal of Applied Electrochemistry. The Netherlands: Kluwer Academic. Journal of the Electrochemical Society USA. Pennington, NJ: The Electrochemical Society. Journal of Power Sources. London: Elsevier Science. Proceedings of the ninth international meeting on lithium batteries, J. Power Sources, vol 81-82, 1999. Solid State lonics. London: Elsevier Science. Books D. Berndt, Maintenance-free batteries, 2nd edn. Taunton: Research Studies Press, 1997. S. U. Falk and A. J. Salkind, Alkaline storage batteries. Chichester: John Wiley, 1969. Lithium batteries, J.-P. Gabano (ed). Maidenhead: Academic Press, 1983. Handbook of batteries and fuel cells, 2nd edn, D. Linden (ed). New York: McGraw-Hill, 1995. D. A. J. Rand, R. Woods and R. M. Dell, Batteries for electric vehicles. Taunton: Research Studies Press, 1998. P. Reasbeck and J. G. Smith, Batteries for automotive use. Taunton: Research Studies Press, 1997. Solid state batteries, C. A. C. Sequeira and A. Hooper (eds). NATO ASI series. The Netherlands: Martinus Nijhoff International, 1985. J. L. Sudworth and A. R. Tilley, The sodium sulfur battery. London: Chapman & Hall, 1985. Modern battery technology, C. D. S. Tuck (ed). Chichester: Ellis Horwood, 1991. C. A. Vincent, Modern batteries, 2nd edn. Maidenhead: Edward Arnold, 1998.

Useful websites AEA Technology Duracell Electric Fuel Energizer Hawker Rayovac SAFT Ultralife Varta International Power Sources Symposium Royal Society of Chemistry links The Society's Electrochemistry Group, part of Faraday Division, represents and coordinates the interests of a wide range of scientists involved in all aspects of electrochemistry. The group organises events, promotes the involvement of postgraduate students in regional, national and international conferences, informs members and provides a mechanism for discussion. To advise research councils and industries, the group coordinates reports on current and future trends in research and applications of electrochemistry. Further information about the group's activities can be found at: www.rsc.org/lap/rsccom/dab/fara005.htm A brief review by the group about the future prospects for electrochemistry, Scientific forward look, can be found at: www.rsc.org/pdf/forwardlook/electrochemgrp.pdf Also of interest is the Society's Energy Sector, part of the Industrial Affairs Division. The mission of this sector is to provide a forum for members of the sector, division, Society and others to access knowledge and express views on chemical, legislative, educational and other matters relating to energy, and to promote the interests, both within the Society and externally, of the members of the sector and the division. More information on the Energy Sector can be found at: www.rsc.org/lap/rsccom/dab/ind010.htm

Glossary
Active material: The electrode material that takes part in the electrochemical reactions that store/deliver electrical energy. Active material utilisation: The fraction of active material that reacts during discharge before the battery can no longer deliver the required current at a useful voltage. Anode: The negative electrode from which electrons flow during discharge. Battery management: The regulation of charging and discharging conditions (eg control of temperature, cut-off voltages, current). Capacity: The amount of charge (measured in ampere-hours, Ah) that can be withdrawn from a fully charged battery under specified conditions. Cathode: The positive electrode to which electrons flow during discharge. Current collector: The metallic part of an electrode which conducts electrons to and from the active material. Depth of discharge: The ratio of the ampere-hours discharged from a battery to the available capacity measured at the same temperature and discharge rate. Energy density: The energy output from a battery per unit volume, expressed in Wh dm–3. Energy efficiency: The fraction of the energy used in charging the battery, expressed in watt-hours, which is available on discharge.	Open-circuit voltage: The voltage of a battery when there is no net current flowing. Over-discharge: The discharge of a battery beyond the level specified for correct operation. Passivation: The formation of a surface layer that impedes the electrochemical reactions at an electrode. Power density: The power output of a battery per unit volume, usually expressed in W dm–3 and quoted at 80 per cent depth of discharge. Self-discharge: The loss of capacity of a battery under open-circuit conditions as a result of internal chemical reactions and/or short-circuits. Separator: An electronically non-conducting, but ion-permeable, material that prevents electrodes of opposite polarity making contact. Shelf-life: The period over which a battery may be stored and still meet specified performance criteria. Specific energy: The energy output of a battery per unit weight, usually expressed as Wh kg–1. Specific power: The power output of a battery per unit weight, usually expressed as W kg–1.

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