Page 207 - Battery Reference Book

Other fast-ion conducting solid systems 16/24
Many research centers in the USA and Europe have One of the most attractive solid-solution electrodes
started programmes to discover new fast-ion conduct- so far reported is based on the semiconducting lay-
ing solids. To date, this search has not been very ered transition metal dichalcogenide titanium disul-
successful. No sodium or other alkali metal ion con- phide, TiS2. This is a layer material with adjacent
ducting materials have yet been found that show signif- layers of sulphur atoms held together only by van der
icantly improveld electrical properties over /3-alumina, Waals bonding. Titanium disulphide, and other sim-
and none has as low an activation energy for ionic ilar chalcogenides of the transition metals, can absorb
mobility (0.13 eV). alkaline cations between the sulphur layers. There is
One or two fast-iron conductors have been known a slight expansion of the crystal axis perpendicular
for a considerable time. Thus, p-silver iodide trans- to the layers but otherwise no change in the crystal
forms to a-silver iodide at 147°C and the first-order structure. For lithium passing into titanium disulphide
phase charge is accompanied by a 1000-fold increase there is a continuous region of non-stoichiometry from
in ionic conductivity. The structure of the phase was Tis1 to LiTiS2. Moreover, the partial free energy of
intercalation is relatively constant across the whole
studied in 1934 by Strock, who postulated that the sil-
ver ions were distributed over a large number of nearly range. Titanium disulphide is quite light, cheap and
readily available and the lithium-titanium sulphide
equivalent sites, whereas the anions formed a regular battery developed by Exxon. which has an organic
well ordered lattice. The entropy change accompany- electrolyte and works at room temperature, has a the-
ing this transiti.on can be thought of as due to the oretical energy density of 480 W Wkg. This is already
‘melting’ of the ordered Agf sublattice of the ,&phase. quite close to that of the sodium-sulphur couple, and
A similar phenomenon was found with lithium sul- there is the advantage of room temperature operation,
phate and extensively studied in the 1960s. The aim the absence of liquid alkaline metals and, possibly,
of most modern research into fast-ion conductors is considerably reduced problems in fabrication and cor-
to produce materials that show highly conductive rosion.
behaviour at reiatively low temperatures for conve- A wide range of solid-solution electrode mixed
nient incorporation into electrochemical devices. Thus, conductors has been investigated, including:
in 1967, RbAg41j and KAg41j, which have conduc-
tances at ambient temperatures of over lOs/m, were 1. Tungsten and vanadium bronzes which have chan-
described -- the same value as that of molar aque- nels that can incorporate metal atoms.
ous solutions of potassium chloride. These are opti- 2. Non-stoichiometric silver sulphide, which has a
mized Ag’ conductors having features in common remarkably high diffusion coefficient for silver, but
with a-silver iodide. a very limited range of composition.
One new and important concept which has emerged, 3. Alloys such as Li,Al and &Si.
involving fast-ion conductivity, is the insertion com- 4. Alkali metal p-ferrites. which are isomorphic with
pound or solid-.solution electrode. In a solid-solution j3-alumina.
electrode there is both rapid ion transport and elec- 5. Graphite and modified graphites such as (CF,),.
tronic conductivity, but a third vital ingredient is the Recently a great deal of interest has been shown in
presence of a range of stoichiometry involving the polyacetyiene-films that can incorporate alkali metal
mobile ion. Fast-ion conducting materials of this type, ions reversibly at a cathode to form electronically
therefore, can be used not as electrolytes, as is the case conducting compounds of the form (CHNa,),. An ‘all-
for /3-alumina, but a battery electrodes with either a polymer’ solid-state battery has been developed in
liquid electrolyte or a solid electrolyte. In the latter which the electrolyte is a sodium iodide-polyethylene
case fast-ion conducting materialls are used to make an oxide and the electrodes are doped polyacetylenes:
all-solid-state battery. In the former case, although the
arrangement of solid and liquid phases is formally sim- (CHNar),(s) Note. PEO.NaI(s)l (CHI, ),(s)
ilar to that found in conventional lead-acid or alkaline Such a power source has a relatively high energy
batteries, tlie energy densities obtainable using modem density but the overall internal resistance of the cell
materials are much higher than was previously thought is high.
possible. Solid-solution electrodes therefore provide a field of
In contrast to solid electrolytes, where sodium is complementary scientific interest to solid electrolytes
generally found to be the most mobile of the alkali and have at least equal potential application in bat-
ions, most good solid-solution electrode materials so tery systems. Apart from the presence of rapid ion
far discovered act best as conductors of lithium ions: transport, the extra keys to the utility of solid-solution
electrodes lie in the interfacial aspects of ions being
xLi+ + Tis2 + x e F==+ Li,TiSz (16.1) able to pass directly between the electrolyte phase and
the electrode without any change in crystal structure
although this distinction may not survive further of the electrode or the necessity to form a new com-
research. pound by electrocrystallization (compare the electrode