Archive for November, 2009

Lithium versus Lithium Ion – The difference is in the Anode

Monday, November 2nd, 2009

Metallic lithium rechargeable battery technology was first developed prior to 1970 and is still being pursued today by companies such as the Bolloré Group of France who recently acquired Avestor from Hydro Quebec in March of 2007 and Sion Power of Arizona with their Lithium Sulfur system. The defining aspect of lithium battery technology is that the anode itself is made of pure lithium metal in the form of a foil. During discharge, lithium ions dissolve from the surface of the foil and transfer to the cathode via the electrolyte. During charge, the lithium ions transfer back to the anode to be electroplated back onto the surface of the lithium foil, reforming as pure lithium metal once again, losing their status as “ions”.

Benefits of metallic lithium anodes are they are light weight and have high reversible capacity of 3,860 mAh/g. Problems are they are highly alkali in nature causing them to react with the organic electrolyte forming a passivation layer on their surface which leads to non-uniform plating of lithium during the charging process and formation of dendrites causing short circuits and serious safety problems due to localized hot spots. To overcome these problems, researchers in the 1970’s began studying the use of anode intercalation materials to replace metallic lithium. The new anode materials operate in the same fashion as existing cathode materials in that they hold the lithium atoms by insertion site diffusion, except that the anode materials do it at a much lower voltage closer to that of metallic lithium. Hence the lithium “ion” battery was born, where lithium atoms remain separate from one another at all times while residing in either the anode or cathode electrodes, eliminating the trouble of uneven metallic plating and its associated problems.

Lithium ion cells are termed rocking-chair cells because the lithium ions rock back and forth during charging and discharging between the anode and the cathode intercalation materials. Anode intercalation materials have much lower reversible capacities compared to metallic lithium, but the benefits of improved safety and much higher cycle life quickly outweigh the drawbacks in most applications.

The electrochemical potential of anode and cathode materials are measured relative to pure metallic lithium reference electrodes representing zero Volts, such that when a cell is constructed from an anode material with 0.3 Volts potential and a cathode material with 4.0 Volts potential, relative to metallic lithium, the resulting cell voltage is calculated by the difference, 4.0–0.3=3.7 Volts.

The most common anode material in use today is carbon in its layered form as graphite or in its glassy amorphous form as hard carbon. Carbon is cheap, light, environmentally friendly, has high reversible capacity of 372 mAh/g,  excellent cycling characteristics, and low electrochemical potential relative to metallic lithium in the range of 0.2-1.0 Volts, helping to maintain an overall high cell voltage when mated with other various cathode materials. Problems with carbon anodes are they are voluminous and have high irreversible first charge capacity loss in the range of 20%.

A less popular anode intercalation material is lithium titanate used by Toshiba and Altairnano, these metal oxide materials have low capacities of only 150 mAh/g and high electrochemical potentials of around 1.5 Volts resulting in a much lower energy density cell. Benefits are very good cycle life, stable electrolyte, and high power characteristics.

New silicon anode materials have very high theoretical capacities up to 4,200 mAh/g, exceeding that of even pure lithium, and voltage potentials below 1.0 Volts, but suffer from high mechanical stressing during the lithiation-delithiation processes, resulting in rapidly fading capacity loss during cycling. Other promising areas for new anode material developments include other silicides, nitrides, and lithium metal alloys.