The most convenient form of energy storage is portable chemical energy. The battery provides the portability of stored chemical energy with the ability to deliver this energy as electrical energy with a high conversion efficiency and no gaseous exhaust. A battery consists of a group of interconnected electrochemical cells. Of particular interest is a low-cost, safe, rechargeable battery of high voltage, charge capacity, and rate. The higher stored volume and energy density of a lithium battery has been intrinsic to the development of the mobile phone and lap-top computer.

Key requirements in the future for this industry focus on:

• Good capacity to weight ratios
• No memory effect
• Slow loss of charge

Lithium metal having a high specific capacity, and standard oxidation potential (smaller work function) became an obvious choice for the anode in the development of high-energy-density batteries. Lithium batteries consisting of a lithium anode and a transition metal, lithium insertion compound with a larger work function the cathode, offers higher cell voltages. Lithium as a result, remains the preferred material in battery applications as it is smaller and so move easily from anode to cathode, which is the key to charge capacity.

The electrolyte is specifically designed for a particular battery application. Room-temperature ionic liquids, such as phosphonium salts, have recently been considered as alternative electrolyte solvents for lithium-ion batteries as they offer several advantages over ther more standard, carbonate-based electrolytes. Such organic liquids have a high oxidation potential, non-flammability, low vapor pressures, better thermal stability, low toxicity, high boiling points, and a high lithium-salt solubility.

Transition metal insertion compounds generally incorporate molybdenum, cobalt, iron, vanadium or tungsten as sulfides. The transition metal ion in the insertion compound cathode generally should have a larger work function to maximize the cell voltage, and allow an insertion/extraction of a large amount of lithium to maximize the cell capacity. A high cell capacity together with a high cell voltage maximizes the energy density. The lithium insertion/extraction process should be reversible with minimal changes to the host structure. The insertion compound should also support good electron and lithium ion conductivity to minimize cell polarization. Such high conductivity is essential to support a large current density and so to provide a high power density. Finally, from a commercial point of view, the insertion compound should be inexpensive, chemically stable over the entire voltage range, environmentally friendly, and lightweight to minimize the battery weight.

For example, exfoliated MoS2 and polyethylene oxide in its various forms have combined as negative ion electrodes, where very high specific capacities (three times that of MoS2 alone) can be achieved. Such nanocomposites are promising alternative electrode materials as they form layers that are loosely coupled by weak Van der Waals forces, and so any lithium ions can migrate in the layers without inducing large volume changes.

The design of an electrode involves tailoring of the electrochemical potential of the anode and that of the cathode to the LUMO or HOMO of the lithium-ion electrolyte to be used. The electrode must also be chemically stable in that electrolyte. The energy of a given electrochemical potential corresponds to the energy of a redox couple of a transition-metal cation. Tailoring of the energy of a redox couple depends not only on the formal valence state of the cation, but also on the covalent component of its nearest-neighbor bonding, which is influenced by the structure, through the placement and character of any counter cations, as well as by the Madelung energy of the ionic component of the bonding.

Any element as host intended for use as an anode gives a voltage less than the voltage of the LUMO of a room temperature ionic liquid electrolyte. Therefore, a passivating layer must protect such an anode against chemical reaction with the electrolyte. The most successful elemental host is carbon. However, more lithium can be inserted into silicon than into carbon, which makes it potentially an anode of exceptionally high capacity in future materials selection.

Gelest, Inc. manufactures a wide range of materials suitable for use in batteries. Typical applications include: electrolytes, solvents and electrode precursors.

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