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New materials for batteries As a result of the interest in advanced battery materials stimulated, on the one hand, by the discovery of beta alumina and the development of the sodium-sulphur battery, and, on the other hand, by the energy crises of the Os, many new electrolytes for lithium and sodium conduction, and also new electrode materials have been discovered.
Several of the complex oxides are not thermodynamicahy stable against metallic lithium, although LisN, LiI Al,O, and several glasses are. From the point of view of electrode behav- iour, the smaller lithium ion is also more readily able to insert into frame- work materials such as V60 is. The lack of msertion electrodes for sodium is a continuing disadvantage in the development of ambient or moderate tem- perature sodium-based batteries.
Much work has been done on systems having lithium anodes, TiSz or analo- gous cathode materials, and a liquid electrolyte, consisting of a polar organic liquid containing a dissolved lithium salt. Once again, however, in spite of great expectations, fundamental problems persist with this type of cell for rechargeable applications, notably concerned with the rechargeability of the lithium anode and subsequent capacity loss on cycling.
For the latter two systems, sealing, self-passivation and, particularly, safety may pose difficulties for widespread commercial application. Interesting developments continue to be made in lithium primary battery technology, for example, the thin cells now in use with instant cameras and portable miniature televisions. However, in spite of the work and enthusiasm of the last decade or so there has seemed, until recently, very little prospect either of alkali-metal secondary systems entering the markets presently dominated by primaries, or of their entering new markets or existing ones presently catered for by nickel-cadmium or lead-acid.
The work was shared between Harwell, universities in the U. The scientific basis of the work at the time was described in a review article in Nature [l] and the rationale for the work, which was ad- dressed at the use of batteries for energy conservation and storage in electric vehicle traction and load levelling applications, was also reviewed [2].
In addition to the technical programme, a detailed market assessment and feasibility study was carried out which provided a very useful basis for future technical planning [ The technical objective of the programme was to examine the proper- ties and behaviour of several promising solid electrolytes and insertion elec- trodes described in the literature to obtain a realistic assessment of their properties, the problems in their use in batteries and their compatability with other materials in cells.
Hence a more realistic determination of which materials might be technologically useful for electric vehicle batteries of the future could be made. It was hoped, thereby, to obtain a fairly hard-headed assessment of whether alkali metal batteries could be developed to realise their potential energy density advantages and to identify which materials could best be chosen for future cell development studies.
A temperature range of - Phase I of this programme ended in [4,5]. The work was success- ful and clear choices of materials for further work and for incorporation in test cells were available. A Phase II programme now concerned primarily with cell testing and evaluation rather than with materials research started in and will continue until early The prospective at the beginning of Phase II has been described [6].
All-solid-state cells were seen as the only practical way forward for am- bient and moderate temperature cells, given the persisting difficulties with organic liquid electrolyte batteries.
The cells in the Phase II programme utilize a lithium or lithium-alloy anode and an insertion cathode. Although the Phase I programme studied in depth the very interesting ceramic lithium-ion-conducting electrolytes Li,N [ 7,8 J and LiI Al,Os [9, lo], the choice of this type of cell was made more realistic by the discovery of polymer-based solid electrolytes by Armand and co-workers in France [ Some polar organic materials such as poly ethylene oxide will take alkali salts into solution and manifest rapid alkali-ion conductivity.
Equally importantly, the plasticity of polymers overcomes the other major problem with solid- state battery systems, that of maintaining good interfacial contact. The progress to date has recently been reviewed [lo]. Techniques for continuous fabrication of electrolyte and electrolyte- containing composite cathode films have been developed Figs. The fabrication of an all-solid-state composite cathode using a continuous-casting, solvent-evaporation technique. The components of an all-solid-state rechargeable cell utilizing a poly ethylene oxide -based electrolyte.
Typical electrolyte and cathode thicknesses are 35 and 70 pm. Develop- ments in the fabrication technology and the results from cell testing have allowed realistic predictions to be made of ultimate battery performance and of materials costs.
For electric vehicle batteries, it is estimated that the selling price may be within the present range for lead-acid batteries.
The outstanding feature of the solid-state battery, however, as presently projected, is in the extremely high energy densities which may be achieved. These fiis include bipolar connectors, busbars and cell casing.
For electric vehicle application, the temperature is almost ideal, being low enough not to pose serious materials or heat conservation prob- lems and high enough to be independent of ambient temperature and not to require cooling in summer conditions. Many properties of materials are affected by their crystal structure.
This structure can be investigated using a range of crystallographic techniques, including X-ray crystallography, neutron diffraction and electron diffraction. The sizes of the individual crystals in a crystalline solid material vary depending on the material involved and the conditions when it was formed.
Most crystalline materials encountered in everyday life are polycrystalline, with the individual crystals being microscopic in scale, but macroscopic single crystals can be produced either naturally e.
Real crystals feature defects or irregularities in the ideal arrangements, and it is these defects that critically determine many of the electrical and mechanical properties of real materials. Properties of materials such as electrical conduction and heat capacity are investigated by solid state physics. An early model of electrical conduction was the Drude model, which applied kinetic theory to the electrons in a solid.
By assuming that the material contains immobile positive ions and an 'electron gas' of classical, non-interacting electrons, the Drude model was able to explain electrical and thermal conductivity and the Hall effect in metals, although it greatly overestimated the electronic heat capacity.
Arnold Sommerfeld combined the classical Drude model with quantum mechanics in the free electron model or Drude-Sommerfeld model. Here, the electrons are modelled as a Fermi gas, a gas of particles which obey the quantum mechanical Fermi—Dirac statistics.
The free electron model gave improved predictions for the heat capacity of metals, however, it was unable to explain the existence of insulators.
The nearly free electron model is a modification of the free electron model which includes a weak periodic perturbation meant to model the interaction between the conduction electrons and the ions in a crystalline solid. By introducing the idea of electronic bands, the theory explains the existence of conductors, semiconductors and insulators. The solutions in this case are known as Bloch states.
Since Bloch's theorem applies only to periodic potentials, and since unceasing random movements of atoms in a crystal disrupt periodicity, this use of Bloch's theorem is only an approximation, but it has proven to be a tremendously valuable approximation, without which most solid-state physics analysis would be intractable. Deviations from periodicity are treated by quantum mechanical perturbation theory.
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