It has long been realised that many types of cell chemistries should be able to achieve over 100 Wh/kg at battery level based on 100% of capacity. One type uses lithium as the negative electrode, a non-aqueous electrolyte, and any of a number of positive electrode materials. Although work on lithium cells has been continuous since the early seventies, successful commercial application of rechargeable lithium cells has been limited by poor cycle life at most a few hundred cycles and safety concerns. In contrast lithium primary cells have found extensive application, although safety concerns limit their domestic use in large capacity versions.

The poor cycle life was due to the difficulty of re-plating lithium onto the negative electrode during recharge. A significant proportion of the lithium re-plated during each cycle is not adequately integrated into the electrode and can no longer be discharged. Hence the loss in capacity. It was long known that this might be overcome by switching from metallic lithium to another material that could 'dissolve' lithium in a solid structure within which the lithium atoms are nevertheless mobile one of the authors worked on this approach in the late seventies. The problem was that the mass of lithium which could be reversibly dissolved into the materials that were tried was too small compared to the mass of the electrode, negating most of the energydensity advantage one was trying to obtain.

A major breakthrough was achieved by Sony when, in 1991, it introduced cells in which the lithium metal was replaced by a layered-structure carbon electrode into which lithium ions can pass reversibly and in large quantity roughly one lithium atom per six carbon atoms. This idea had been tried in the past without success, but the breakthrough was in finding the right form of carbon and the necessary pre-treatments for it to function reversibly. The use of carbon in place of lithium metal still does not come without penalties. It reduces the cell potential by about 0.3 volts and adds to the mass of the negative electrode. Fortunately, developments in positive electrode materials had shown how to compensate for this voltage disadvantage and, perhaps still more importantly, opened the way to simpler cell manufacture.

Unlike the negative electrode, positive electrodes have nearly always been 'solid solution electrodes' in which lithium ions are free to move within a layer structure, usually a transition metal oxide or sulphide. The content of lithium ions can be varied over a wide range of composition because the lithium ion charge can be balanced by the variable charge of the transition metal ions. Earlier lithium cells were made in the charged state, i.e. by combining lithium-metal negative electrodes with positive electrodes containing no lithium. As the cell was discharged, the voltage associated with the positive electrode fell as lithium ions were introduced.

The breakthrough in positive electrodes came from a UK Atomic Energy Authority now AEA Technology programme, as part of which J. Goodenough at Oxford University devised a new class of positive solid solution electrode materials that were already lithiumcontaining as synthesised. Combined with the new carbon positive electrodes, this brought two important advantages. Firstly, it enabled cells to be assembled in the discharged state so that it was no longer necessary to handle metallic lithium or the still very air and watersensitive lithiumcarbon compound during manufacture. This greatly simplified the manufacturing process. Secondly, when lithium was removed from the positive electrodes during charge, the voltage associated with the electrode increased, largely compensating the voltage penalty associated with the negative.

Any cells based on lithium-carbon negative electrodes and solid solution positive electrodes are referred to as lithium-carbon or lithium-ion cells. The latter name refers to the fact that the cell can be regarded as a concentration cell in which lithium remains in the form of ions and the voltage is due to the difference in chemical activity between the positive and negative electrodes. . They are now under vigorous development by most major battery manufacturers around the world and especially in Japan for use in portable electrical equipment such as computers, power tools and for electric vehicles. In fact, lithium-ion cells are not restricted to graphite as the anode host to the lithium ions, and alternatives are also under development.

Recent developments include batteries with embedded functionality such as a built-in charger and USB connector within the AA format, enabling the battery to be charged by plugging into a USB port without a charger, and low self-discharge LSD mix chemistries such as Hybrio, ReCyko, and Eneloop, where cells are precharged prior to shipping.