Characteristics of Lithium rechargeable cells
Lithium is the smallest and lightest metallic element. It has the most negative electrochemical potential (vs standard hydrogen electrode) which makes it one of the strongest reducing agents and therefore the most electro-active of metals. Secondary (rechargeable) cells, which use these properties of lithium as an intercalation guest atom, give rise to batteries with very high power and energy densities highly suited for use in automotive and standby power applications.
Lithium-ion cells typically use a carbon based anode, although lithium titanate anodes have recently become commercially available. Various compounds can be used for the cathode, each of which offers different characteristics and electrochemical performance (see below). The electrolyte is usually a lithium salt dissolved in a non-aqueous inorganic solvent. Lithium battery technology is still developing, and there is considerable potential for further enhancements. Research has focused heavily on the development of the cathode material. The cathode materials typically have an ability to store charge (in the form of lithium), per unit mass and volume, which is significantly inferior to that of materials available to form the negative electrode (the anode). Therefore the most gains in performance, in terms of greater energy and power densities, are to be made by the development of new advanced, high capacity, positive electrode materials.
Different lithium chemistries
Lithium Iron Phosphate - LiFePO4
Phosphate based technology possesses superior thermal and chemical stability which provides better safety characteristics than those of other Lithium-ion technologies. Lithium phosphate cells are incombustible in the event of mishandling during charge or discharge, they are more stable under overcharge or short circuit conditions and they can withstand high temperatures without decomposing. When abuse does occur, the phosphate-based cathode material will not release oxygen, will not combust and is therefore much less susceptible to thermal runaway. Lithium Iron Phosphate cells also offer a longer cycle life (1000 - 2500 cycles).
Lithium Iron Phosphate batteries have lower energy density than cobalt, but they can support higher currents and thus greater power. They are a significant improvement over lithium cobalt oxide cells in terms of the cost, safety and toxicity.
Lithium Cobalt Oxide - LiCoO2
Lithium Cobalt Oxide has been the most widely used cathode material in lithium batteries for many years. It provides moderate cycle life (>500 cycles) and energy density. However, the chemistry is less thermally stable than other transition metal oxide or phosphate chemistries making it highly combustible under extreme abuse conditions. These characteristics make them unattractive for use in Electric and Hybrid Electric Vehicles.
Lithium Manganese Oxide Spinel - LiMn2O4
Lithium Manganese Oxide Spinel provides a higher cell voltage than Cobalt-based chemistries and thermally is more stable. However the energy density is about 20% less. Manganese, unlike Cobalt, is a safe and more environmentally benign cathode material due to its low toxicity. Other benefits include lower cost and higher temperature performance.
Lithium (NCM) - Nickel Cobalt Manganese - Li(NiCoMn)O2
Batteries which employ lithium nickel cobalt manganese oxide are a compromise of electrochemical performance, combined with lower cost. Electrochemically the performance is superior to LiFePO4 and LiCoO2 in terms of capacity and therefore energy density. In terms of rate capability and therefore power density the electrochemical performance is better than LiCoO2 but not as high as LiFePO4.
Lithium Titanate Oxide (LTO) - Li4Ti5O12
These cells replace the graphite anode with lithium titanate. This anode is compatible with any of the above cathodes, but is generally used in conjunction with high voltage Manganese-based materials due to its high potential vs Li/Li+ redox couple. They offer superior rate capability and power combined with wide operating temperature range. They are considered a safer alternative to the graphite anode due to higher potential and therefore inbuilt overcharge protection. Also they are a ‘zero-strain’ insertion material that does not form a large passivating layer with the electrolyte, thus giving rise to long cycle life. However, lithium titanate batteries tend to have a slightly lower energy density than graphite-based systems.
Applications of Lithium-ion batteries
Lithium batteries are ideally suited for automotive use, for both Electric Vehicles and Hybrid Electric Vehicles.
Electric Vehicle batteries are designed for maximum energy content and to deliver full power even with deep discharge (80% DoD) to ensure long range.
Batteries for Hybrid Electric Vehicles must deliver high power in repetitive shallow (50%) discharges (micro cycles) and accept high recharging rate from both engine and brakes. Their operating point is between 15% and 50% DoD to allow for regenerative braking.
High cell voltage of up to a nominal 3.7 Volts means that fewer cells and associated connections and electronics are needed for high voltage batteries (one Lithium cell can replace three NiCd or NiMH cells which have a cell voltage of only 1.2 Volts).
Very high energy density (about 4 times better than Lead acid).
Very high power density.
Can be optimised specifically for capacity or power requirement application.
Can be discharged at a rate of up to 40C0. The high discharge rate means that for automotive use the required cold cranking power or boost power for hybrid vehicles can be provided by a lower capacity battery.
Fast charge possible.
Batteries can be cycled to deep depths of discharge without adversely affecting the cycle life or ability to provide high power output.
Very low self discharge rate (3 to 5% per month). Can retain charge for up to ten years.
Very high coulombic efficiency (discharge capacity /charge capacity) of almost 100%. Thus very little capacity is lost during charge/discharge cycling.
No memory effect. No reconditioning required to maintain cycle life.
Long cycle life. Cycle life can be extended significantly by using protective circuits to limit the permissible DoD of the battery. This mitigates against the high initial costs of the battery.
Variants of the basic cell chemistry allow the performance to be tuned for specific applications.
Available in a wide range of cell constructions with capacities of single cells from less than 50mAh up to 1000Ah, from a large number of world-wide suppliers.
Very small batteries are also available. Electrode materials, and ceramic electrolyte can be sputtered onto solid (alumina/silica) or flexible(acrylic) substrates to form high energy density “all solid state” thin film batteries. Can be formed in-situ on micro-electron devices.