Science and Applications of Mixed Conductors for Lithium Batteries
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Applications of Mixed Conductors for Lithium Batteries
Michael M. Thackeray, John O. Thomas, and M. Stanley Whittingham Introduction Mixed conductors show significant mobility of both electronic and ionic species and were the subject of an earlier review in MRS Bulletin.1 The current review is restricted to those mixed conductors of interest for use in lithium batteries, with an emphasis on commercialization. The first lithium batteries were primary cells using pure lithium anodes and carbon monofluoride or manganese oxide as the cathode. Both were developed in Japan, the former for use in fishing floats and the latter for calculators and similar small devices. Such primary cells based mainly on MnO2 or FeS2 cathodes are still extensively used in watches, cameras, and so on. Lithium primary cells are also the main power source for many medical devices, such as pacemakers. In some of these applications, silver vanadate is the cathode. The recognition of the role of intercalation/insertion reactions in battery electrodes just over 25 years ago2 allowed significant progress to be made in secondary (rechargeable) lithium batteries. From the first prototypical titanium disulfide cells, the technology has more recently been commercialized by Sony Corp. in the Li-ion cell, using a cobalt oxide insertion cathode and a carbon insertion anode. In the former, commercialized by Exxon, the reaction is the simple intercalation of lithium ions between sheets of TiS2 , as shown schematically in Figure 1. As will be discussed in the section on “The Cathode,” TiS2 is the prototypical cathode material, showing all the desired characteristics except for overall cell voltage. Exxon combined TiS2 with a LiAl
MRS BULLETIN/MARCH 2000
anode3 and an organic-based electrolyte to give safe, rechargeable cells.4 In these cells, lithium ions were shuttled between the aluminum and titanium disulfide electrodes. These electrode materials are often referred to as the host material and the lithium ion as the guest, as the structure of the host remains essentially unchanged
Figure 1. Schematic diagram of lithium intercalation into TiS2.
during reaction except for a slight expansion or contraction. The electrodes are either reduced or oxidized as the lithium guest ions enter or leave the host structure. Both of these electrodes show high electronic conductivity and high ionic conductivity and are therefore known as mixed conductors—the subject of this review. In the case of the LiAl/TiS2 cells, the aluminum alloy becomes brittle upon cycling, and cell performance deteriorates; however, if LiAl alloys could be optimized, they would offer the highest energy density currently available and at the lowest cost. The Sony cells,5 which use LiCoO2 as the cathode and intercalated “graphite” as the anode, have proven to be very reliable in operation. They have become successful in many areas and now dominate as the power source for small devices such as cellular phones and portable computers. However, their energy density is no higher than the titanium su
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