Materials Research Activities

Titanium disulfide

Titanium disulfide: the prototype cathode material

Titanium disulfide was for a long time the most important intercalation material used for cathodes in rechargeable batteries. Much of the research on intercalation has been done on this material. In the 1990s it has probably been superseded by other dichalcogenide compounds and entirely different materials.

Titanium

Titanium has only been available commercially since the end of World War II, and it remained expensive until the 1950s. Pure titanium and titanium alloys are useful wherever high strength and low density are desired. Its atomic number is 22 and its atomic weight is 47.90, making it one of the lightest metals. Its density is roughly 56% that of steel. (For more physical and chemical properties of titanium, you can consult commercial sites such as Rembar's.) Hence, the industrial development of the metal was prompted by demands in aircraft and jet engine design, armor plating, and other military hardware. Titanium is now utilized in gas turbine engines, heat transfer, desalination, electrowinning of metals, chemical processing, steam turbines, automotive, airframes, space structures, flue gas desulphurization, and nuclear waste storage, to name but the most prominent. As the titanium industry grew, the price dropped , and titanium has found applications in other fields too. For example, due to its corrosion resistance, it is useful in many devices of the chemical industry, such as pump valves, centrifuges, pipes and electrodes; in foils for hydrofoil boats; and in heat exchangers for nuclear systems.

Year Price in US$ per kilogram of Ti sponge
1959

12.00

1968
2.53
1971
1.36

Titanium was discovered in 1791 by William Gregor. It was named after the mythological Titans, the powerful first sons of the Earth. and in 1910, and titanium compounds were studied all along. U. A. Hunter developed a means of separating pure metallic titanium in 1910, but in the absence of a useful industrial technique for doing so, it remained comparatively little studied and used. But in 1936, W. I. Kroll published a method by reacting titanium chloride with molten magnesium:

TiCl4 + 2Mg = Ti + 2MgCl2

Kroll's method became the basis of industrial development of titanium (a description may be found on the commercial site mentioned above: Rembar). Once a method for winning titanium industrially had developed it became significant that titanium is comparatively abundant in the earth's crust. It is the ninth most abundant of the elements and seventh among the metals. It is about 33 times as abundant as zinc, for example. Titanium may be found in small concentrations in almost any rock and soil. There are also ores with high content of two titanium minerals: ilmenite (FeTiO2) and rutile (TiO2). Such ores have been found in many countries including the USA, Canada, Russia (Soviet Union), and India. The development of the titanium industry in the wake of WWII was pioneered by the U.S. Bureau of Mines. Once its military applications had become clear, the Department of Defense also supported research and development. In 1953, Defense contracted the Battelle Memorial Institute in Columbus, Ohio, to collect, store, and disseminate information on titanium technology. International symposia took place in 1968, 1972, and 1976. Titanium research in the USSR lagged behind that of the USA by only a little. In the mid-1950s, the Baikov Metallurgical Institute of the USSR Academy of Sciences, in Moscow, investigated titanium's structure, physico-chemical and mechanical properties. Production of titanium tripled in the USSR between 1961 and 1965. Intensive studies of the production and refinement of titanium in fused salt media took place in Leningrad's Aluminium-Magnesium Institute. In the early 1970s, a special Titanium Institute was inaugurated there.

Titanium disulfide

The electrochemistry of titanium was widely investigated in the context of industrial methods of titanium production from its ores. The accumulation of this knowledge revealed the utility of titanium for the use in batteries by the mid-1970s. It seemed promising in several contexts: as a material for anode, electrolyte, and cathode. Titanium disulfide revealed itself to be a good intercalation compound, and this material was crucial in the development of intercalation compounds generally from the mid-1970s onwards. In fact, this was the moment when the terms intercalation compound and intercalation chemistry were introduced, the intercalation referring to the insertion of ions between the layers of the compound.

Intercalation

Intercalation of lithium ions into titanium disulfide. The vertical measures of length indicate the slight expansion of the crystal upon intercalation. The forces between layers are van der Waals forces, the weakness of which allow for such stretching.

By permission of McGraw Hill. Source: McGraw-Hill Encyclopedia of Science and Technology, entry on Intercalation.

A simultaneous development had taken place in the field of ionic conduction in the wake of a 1967 announcement from a Ford research group that beta-alumina allowed for good ionic conduction and that this could be put to good use in battery design. Stanley Whittingham at Exxon's Corporate Research Laboratories in Linden, New Jersey, was the most influential individual in the history of intercalation chemistry. He took out a Belgian (1975) and a US (1977) patent for a high-energy density reversible battery with a lithium anode and a titanium disulfide cathode in 1973. A string of papers from his and his collaborators' pen examined the properties of layered titanium disulfide with intercalated lithium:

  • M. S. Whittingham, Materials Research Bulletin, 9 (1974), 1981
  • M. S. Whittingham and F. R. Gamble, Materials Research Bulletin, 10 (1975), 363
  • B. G. Silbernagel and M. S. Whittingham, Materials Research Bulletin, 11 (1976), 29
  • M. S. Whittingham, Journal of the Electrochemical Society, 123 (1976), 315
  • M. S. Whittingham, Journal of the Electrochemical Society, 124 (1977), 1387
  • M. S. Whittingham, Science, 192 (1976), 1126

Whittingham et al initially examined all the dichalcogenides of the transition metals of groups IVB and VB of the periodic table because of their high conductivity. They chose TiS2 particularly because it was the lightest and cheapest of all the layered dichalcogenides, in addition to its high conductivity. Nuclear magnetic resonance studies also revealed that lithium self-diffuses most rapidly in TiS2. Exxon developed a lithium-titanium disulfide battery with an organic electrolyte working at room temperature.

Many followed in Whittingham et al's footsteps. Researchers on fast ion transport in solids had gelled into something like a proto-discipline at a 1972 conference in Belgirate, Italy, sponsored by NATO. A proceedings has been edited by W. van Gool and published by North-Holland. By 1980, North-Holland maneouvered Stanley Whittingham into editing a new journal, named Solid State Ionics, providing another step on the road towards an independent discipline. Fast ion conduction was now known to be a common phenomenon and not restricted to a few exotic materials, as was rather believed in the late 1960s, when only beta-alumina were known. The European Community, for example, set up a battery research program in 1976. Many insertion electrodes were examined within the project, primarily layered metallic oxides. But the open-circuit voltage of a TiS2 battery was also measured as a function of the amount of lithium already intercalated into the TiS2. The TiS2 was explicitly referred to as the 'Exxon compound'! (J. Jensen, "Solid electrolyte battery research within the EEC research programme on energy conservation", Solid State Ionics, 5 (1981), 9-14.) The following statement on the historical relationship between solid state ionics and the price of oil was inspired by Whittingham's insights (cf. the interview).

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Oil prices 1982-2000

Much of the research on batteries and fuel cells in the 1970s and 1980s was due to the promise that they might replace gasoline as the power for automobiles. The EEC research program mentioned in the previous paragraph was initiated soon after the political dependence of Europe upon volatile Middle Eastern oil producers had become acute. For this reason, the funding of battery research was sensitive to the price of oil. When the price dipped slightly in the early 1980s, much large-scale R&D was curtailed. By 1984, the chief shortcoming of the electrical vehicle was that battery systems' energy density remained at 30-35 Wh/kg against that of the conventional gasoline powered vehicle stood at 15kWh/kg, approximately 500 times as much. ("Round table - Materials in Energy", Solid State Ionics, 12 (1984), 527-528.) This was true of the most advanced battery systems, including those using TiS2 intercalation compound as the cathode material.

(Source of illustration: Datastream, Morgan Stanley Dean Witter Research Estimates)

During the 1980s, much research was also done on the physics and chemistry of titanium disulfide. The expansion (seen in the illustration at the top of this page) of TiS2 layers was analysed not only in terms of the spatial accommodation of the intercalated lithium ions but also in terms of the effect of charge transfer of a lithium 2s electron into the vacant d-band of TiS2. (Thompson & Symon, Solid State Ionics, 3-4 (1981), 175.) It was generally found that the so-called rigid band model (RBM) was a useful one for describing the changes in electronic properties of the host lattice as a result of intercalation. This model assumes that the electronic band structure is not altered by intercalation - and this is tantamount to assuming that no hybrid electron wave functions are created between intercalant and host. But, as shown in the following illustration of the population of energy states according to RBM, higher energy states are filled due to the extra charges contributed by intercalants.

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Rigid Band Model

The two graphs illustrate the energy states occupied in titanium disulfide with and without an intercalant. In TiS2 without intercalant (left) only energy states below the Fermi level are occupied. There is a gap between the energy of the p orbital and that of the conduction band. The charges contributed by the intercalant occupy the lowest levels of just this band (right). The rigid band model assumes that intercalation does not change the electronic band structure itself (the shape of the curves in the illustration).

(Source of illustration: Julien & Nazri, Solid State Batteries: Materials Design and Optimization, Kluwer, 1994, p. 25)

In the 1980s, many of TiS2's physical and chemical qualities were examined as a function of temperature and extent of intercalation. The partial free energy turned out to be fairly constant over the entire range of intercalation, which entails that a battery using these materials will discharge evenly. The most common tools for investigating TiS2 included infrared reflection, conductivity, Hall effect, magnetoresistance, thermoelectricity, and measurement of the lattice parameters (usually x-ray analysis). Throughout the 1980s and 1990s, intercalation research was expanded to other materials. TiS2 is no longer the most promising cathode material for all types of rechargeable batteries, especially because every type of battery has to suit the particularly needs of small niches in the market. Nonetheless, TiS2 remains the most important conceptually. Partly this is because it was the earliest prominent candidate and a lot of research was conducted upon it. TiS2 is the prototype cathode material. In addition, the intercalation of lithium into TiS2 results in only very simple changes (otherwise RMB would not be a good model). TiS2 is unusual among cathodic materials in that it does not undergo any phase changes during intercalation. Many of the other candidate materials are made up from chemical elements in the same groups of the periodic table as titanium and sulphur. Titanium is a transition metal, and the other transition metals include Zr, Hf, V, Nb, Ta, Cr, Mo, and W. Sulphur is found in group VI (the chalcogens) that also includes Se and Te. All these candidate materials resembling TiS2 consist of one tranistion metal and two chalcogens. In other words, they are transition metal dichalcogenides. All have a layered structure made up of a sheet of metal atoms sandwiched between two sheets of chalcogen atoms. The layers of chalcogen-metal-chalcogen are held together by weak van der Waals forces, enabling an intercalant (alkaline cations, such as Li+) to enter.

Return to our solid-state batteries introduction, to the tutorial on solid-state battery technology, or to Whittingham.

This page was written and last updated by Arne Hessenbruch on 14 June 2001.