Doping strategies

The use of framework structures to minimize AH for alkali-ion electrolytes has been demonstrated to provide a means of opening up the bottlenecks to cation motion in a number of oxides (Goodenough, Hong and Kafalas, 1976). Framework structures may provide one-dimensional tunnels as in hollandite, two-dimensional transport in planes as in the )S-aluminas, or three-dimensional transport as in NASICON and LISICON. Since one-dimensional tunnels are readily blocked, the two-and three-dimensional conductors are the more interesting. 

The )5-aluminas are described in some detail in Chapter 2, only a few specific features are noted here. In the -aluminas, the spinel blocks are stacked in such a way that the energetically equivalent sites occupied by Na ions are ideally just half-filled in the -aluminas the spinel blocks are stacked so as to distinguish two types of Na -ion sites of different potential energy, the Beevers-Ross (BR) and anti-Beevers-Ross (aBR) sites. In the Na-O planes, the shortest bottleneck distance 2.7 A is just a little greater than the sum of the ionic radii, 2.4 A, at room temperature, so a small value of AH can be anticipated. The discovery of fast Na -ion conductivity in the Na j5-aluminas (Yao and Kummer, 1967 Kummer and Weber, 1967) led to the invention of the Na/S battery that triggered extensive interest in the solid-electrolyte problem. 

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UC nanoparticles. In the end, this chapter points out the challenges and perspectives of the developing impurity-doping strategy. 

Alternative doping strategies have yielded a range of compounds containing Zr, Ta and Nb " " on the octahedrally coordinated position occupied by Ti" in the parent material. " Several of these compounds show a total lithium ion conductivity that is equal to that of the Li3, cLa cTi03 parent phases but are likely to show considerably greater electrochemical stability. Of particular interest is the report of... 

Doping with other metal elements is a demonstrated successful strategy to improve the struetural stability of LiNi02-based cathode materials. Many studies have deployed this doping strategy by replacing part of the nickel with other metal elements sueh as cobalt, magnesium, aluminum, or titanium... 

These authors have reviewed positional disorder in stoichiometric compounds in relation to crystal stmcture, materials exhibiting first-order phase transformations, and diffuse, so-called Faraday, transitions, extrinsic disorder, doping strategies, and solid solutions. 

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