Summary

The principal challenges facing the development of batteries for electric vehicles are cost, safety, cell energy density (voltage capacity), rate of charge/discharge, and service life. Automation of manufacturing, material selection, and service life are the keys to lower costs; the availability of Li need not be a problem. Long service life requires elimination of unwanted chemical reactions between the electrodes and the electrolyte as well as retention over many charge/discharge cycles of the elec- tronic contact between the active particles of an electrode and the current collector. The latter requirement restricts the volume change versus state of charge that can be tolerated in an electrode unless the active electrode par- ticles are tethered to a current collector. This attachment may be made directly by the growth of nanowires of active material on a current-collector substrate; it may also be done by bonding the active particles to a conductive polymer having a conduction band that overlaps the μ_A or μ_C of the electrode. The former rate is being researched

with Si nanowires,⁹⁵ the latter has been accomplished with

^{polypyrrole bonded to carbon-coated LiFePO}₄^.102,103

Safety is related to the flammability of the electrolyte,

the rate of charge and/or discharge, and the engineering of the battery pack. A hybrid ionic liquid and organic liquid as the electrolyte solvent can be made nonflam- mable without too great a compromise of the electrolyte σ_Li. Both the LiFePO₄ cathode and the Li₄Ti₅O₁₂ anode have demonstrated safe and rapid charge and discharge over many cycles where the μ_C and μ_A, respectively, are located within the window of the electrolyte, which removes the requirement of a passivating layer. However, location of the μ_C and μ_A within the electrolyte window is not sufficient if the electrode is operating at its voltage limit with a redox couple pinned to the top of an anion p band or overlapping a cation 4s band. An electrolyte with a larger window, especially one with a higher LUMO, would be helpful; but EC and/or VC in the electrolyte solvent do allow use of a carbon anode with a limited charging rate. Finally, engineering the battery-pack design to allow failure of one cell without compromising the entire stack can also build in safety.

A greater energy density requires both a larger voltage and larger capacity. To achieve a greater energy density

with inexpensive electrodes having a long service life is a challenge for the materials scientist. On the anode side, it will be difficult to have a better capacity than that of carbon, and the passivation layer makes it possible to take advantage of a μ_A above the LUMO of the electrolyte, albeit at the expense of a reduced charging rate. On the cathode side, a material having a redox couple pinned at the top of the O 2p band, but with two valence states accessible without exceeding the intrinsic voltage limit of the oxide, would offer the maximum voltage and capa- city. The upper intrinsic voltage limit of such an oxide can be raised by increasing the covalent component of the M-O bond of a countercation to lower the energy E_V of the top of the O 2p bands; but this tuning of E_V must not also push the pinned redox couple further than the change in E_V. Comparison of the upper voltage limits of the nickel oxides having Li in octahedral versus tetrahedral sites illustrates a tuning of E_V by 1.0 V. However, to take advantage of the availability of two redox couples on a single transition-metal atom, it is necessary to have a host structure that can accept two Li atoms per transition-metal cation. Can nature and/or the imagination of the solid state chemist identify such an electrode material with a μ_C matched to the electro- lyte window? An example of the imaginative materials design that may be needed is given by a recent report from Sun et al.¹⁰⁴

Acknowledgment. This work was supported by the office of FreedomCAR and Vehicle Technologies of the U.S. Department of Energy under contract no. DE-AC03-