Other Lithium Cathode Chemistry Variants
Numerous variants of the basic Lithium-ion cell chemistry have been developed. Lithium Cobalt and Lithium Manganese were the first to be produced in commercial quantities but Lithium Iron Phoshate is taking over for high power applications because of its improved safety performance. The rest are either at various stages of development or they are awaiting investment decisions to launch volume production.
Doping with transition metals changes the nature of the active materials and enables the internal impedance of the cell to be reduced.
The operating performance of the cell can also be be "tuned" by changing the identity of the transition metal. This allows the voltage as well as the specific capacity of these active materials to be regulated. Cell voltages in the range 2.1 to 5 Volts are possible.
While the basic technology is well known, there is a lack of operating experience and hence system design data with some of the newer developments which also hampers their adoption. At the same time patents for these different chemistries tend to be held by rival companies undertaking competitive developments with no signs of industry standardisation or adoption of a common product. (The original patent on Lithium Cobalt technology has now expired which is perhaps one explanation for its popularity).
Lithium Cobalt LiCoO2
Lithium Cobalt is a mature, proven, industry-standard battery technology that provides long cycle life and very high energy density. The polymer design makes the cells inherently safer than "canned" construction cells that can leak acidic electrolyte fluid under abusive conditions. The cell voltage is typically 3.7 Volts. Cells using this chemistry are available from a wide range of manufacturers.
The use of Cobalt is unfortunately associated with environmental and toxic hazards.
Lithium Manganese LiMn2O4
Lithium Manganese provides a higher cell voltage than Cobalt based chemistries at 3.8 to 4 Volts but the energy density is about 20% less. It also provides additional benefits to Lithium-ion chemistry, including lower cost and higher temperature performance. This chemistry is more stable than Lithium Cobalt technology and thus inherently safer but the trade off is lower potential energy densities. Lithium Manganese cells are also widely available but they are not yet as common as Lithium Cobalt cells.
Manganese, unlike Cobalt, is a safe and more environmentally benign cathode material.
Manganese is also much cheaper than Cobalt, and is more abundant.
Lithium Nickel LiNiO2
Lithium Nickel based cells provide up to 30% higher energy density than Cobalt but the cell voltage is lower at 3.6 Volts. They also have the highest exothermic reaction which could give rise to cooling problems in high power applications. Cells using this chemistry are therefore not generally available.
Lithium (NCM) Nickel Cobalt Manganese - Li(NiCoMn)O2
Tri-element cells which combine slighlty improved safety (better than Cobalt oxide) with lower cost without compromising the energy density but with slightly lower voltage. Different manufacturers may use different proportions of the three constituent elements, in this case Ni, Co and Mn.
Lithium (NCA) Nickel Cobalt Aluminium - Li(NiCoAl)O2
As above, another tri-element chemistry which combines slighlty improved safety (better than Cobalt oxide) with lower cost without compromising the energy density but with slightly lower voltage.
Lithium Iron Phosphate LiFePO4
Phosphate based technology possesses superior thermal and chemical stability which provides better safety characteristics than those of Lithium-ion technology made with other cathode materials. 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 burn and is not prone to thermal runaway. Phosphate chemistry also offers a longer cycle life.
Recent developments have produced a range of new environmentally friendly cathode active materials based on Lithiated transition metal phosphates for Lithium-ion applications.
Phosphates significantly reduce the drawbacks of the Cobalt chemistry, particularly the cost, safety and environmental characteristics. Once more the trade off is a reduction of 14% in energy density, but higher energy variants are being explored.
Due to the superior safety characteristics of phosphates over current Lithium-ion Cobalt cells, batteries may be designed using larger cells and potentially with a reduced reliance upon additional safety devices.
The use of Lithium Iron Phosphate chemistry is the subject of patent disputes and some manufacturers are investigating other chemistry variants mainly to circumvent the patent on the LiFePO4 chemistry.
Lithium Metal Polymer
Developed specifically for automotive applications employing 3M polymer technology and independently in Europe with technology from the Fraunhofer Institute, they have been trialled successfully in PNGV project demonstrators in the USA. They use metallic Lithium anodes rather than the more common Lithium Carbon based anodes and metal oxide (Cobalt) cathodes.
Some versions need to work at temperatures between 80 and 120ºC for optimum results although it is possible to operate at reduced power at ambient temperature.
The Fraunhofer technology uses an organic electrolyte and the cell voltage is 4 Volts. It is claimed that their the cell chemistry is more tolerant to abuse.
These products are not yet in volume production.
Lithium Sulphur Li2S8
Lithium Sulphur is a high energy density chemistry, significantly higher than Lithium-ion metal oxide chemistries. This chemistry is under joint development by several companies but it is not yet commercially available. A major issue is finding suitable electrolytes which are not subject to the numerous unwanted side reactions which plague the current designs.
Lithium Sulphur cells are tolerant of over-voltages but current versions have limited cycle life. The cell voltage is 2.1 Volts
See also Dissolution of the Electrodes on the New Cell Designs and Chemistries page.
Alternative Anode Chemistry (LTO)
The anodes of most Lithium based secondary cells are based on some form of carbon (graphite or coke). Recently Lithium Titanate Spinel (Li4Ti5O12) has been introduced for use as an anode material providing high power thermally stable cells with improved cycle life.
This has the following advantages
Does not depend on SEI Layer for stability
No restriction on ion flow hence significantly higher charge and discharge rates possible as well as better low temperature performance.
Lower internal impedance of the cell
Higher temperatures can be tolerated.
No SEI build up over time means very long cycle life possible (10,000 deep cycles)
Public domain technology (No patent disputes)
Lower anode reactivity means cell voltage reduced to 2.25 Volts when used with Spinel cathode. (Other cathode chemistries possible)
25% to 30% Lower energy density hence bulkier cells
Lithium Air Cells
Originally conceived as primary cells (see Lithium Pimary Cells), Lithium air cells offer a very high energy density. Rechargeable versions are now under development which promise energy densities of 10 times more than the current generation of Lithium cells, approaching that of Gasoline/Petrol.
The anode is Lithium and the cathode is not air but in fact gaseous Oxygen from the air. Because the cell does not have a solid cathode in the conventional sense it eliminates the weight and volume of the cathode as well as its mechanical supporting structure.
This would enable very small batteries to be made with the same range as current technology, or alternatively, electric drive ranges of several hundred miles could be obtained from batteries the same physical size as those available today.
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