Lithium is the lightest metallic element and generates a high voltage vs. the standard hydrogen electrode (i.e. -3.045 V). Early attempts used lithium metal in combination with a transition metal oxide or sulphide (e.g. Li/MoS2 the Molicell) intercalation compound.
These cells, although exhibiting the high energy densities expected, suffered from two main problems, caused mainly by the lithium anode: limited cycle life and poor safety.
The lithium anode caused both the poor safety and limited cycle life. During repeated discharge and charge cycles, the lithium stripping and replatingprocess was not 100% efficient and this created high surface area, particulate lithium which gradually consumes the lithium metal foil anode. The presence of particulate lithium caused increased internal resistance that limits the cycle life.
In addition, and more importantly, the particulate lithium creates major safety problems and renders the cell unsafe.
The solution to the so-called ‘lithium metal’ problem was to replace the metal anode with a second intercalation compound which can reversibly intercalate Li+. This material is carbon and this is now used as the active anode material in all commercially available Li-ion cells.
Since the anode is carbon, the active material of the cathode must be a compound which already contains Li+ and moreover the Li+ must be easily removed without a change in its molecular (crystal) structure.
There are presently two types of compounds which meet this requirement: these are the layered transition metal oxides LiMO2 (where M = Co, Ni or Mn) – examples include LiCoO2 (lithium cobaltite) and LiNiO2 (lithium nickelite) – and the spinel material LiMn2O4.
The use of LiCoO2, LiNiO2 and mixed compounds of such as LiNi1-xCoxO2 is protected by several patents owned by AEA Technology.
All the major companies which manufacture Li-ion cells have found that LiCoO2 offers superior reversibility, discharge capacity, charge/discharge efficiency and have thus adopted it as the cathode material of choice for small cells used in portable electronics.
As Li-ion cells are charged and discharged Li+ ions are transported between their carbon-based anode and their LiCoO2-based cathode, with electrons exchanged as a result of lithium ion insertion (doping) and of lithium ion extraction (undoping).
During charging, the cathode is undoped (i.e. the lithium is removed), and the anode, which consist of carbon with a layered structure are doped (i.e. lithium ions are inserted).
During discharge (when electrical energy is spontaneously released) lithium is removed from the carbon layers of the anode and inserted into the layers of the cathode compound. When Li-ion cells are first charged, lithium ions are transferred from the layers of the lithium cobaltite to the carbon material which forms the anode.
This is illustrated below.
LiCoO2 + 6C –> Li1-xCoO2 + LixC6
Subsequent discharge and charge reactions are then based on the motion of lithium ions between anode and cathode.
Li1-xCoO2 + LixC <—-> Li1-x+dxCoO2 + Lix-dxC
In order to achieve a high energy density, the capacity of the carbon anode must be as high as possible. To this end, carbon materials with large lithium ion doping capacities are required and the stoichiometric LiC6 lithium-graphite interlated compound composition (corresponding to 372 mA h g-1) were selected.
At present, there are three kinds of carbon which are used in the anode of Li-ion cells:
Graphite types – highly structured
Coke types – less structured but easily transformed into graphite by heating
Non-graphitizable (hard) carbon types – highly disordered.
Of these carbon types Sony Energytec originally adopted a non-graphitizable (hard) carbon for use in the anode.
In contrast, the other leading Li-ion cell manufacturers such as Sanyo, Matushita (Panasonic) and Japan Storage Battery Co., Ltd. (JSB) have adopted a graphitic type carbon for the anode. Although the stored capacity of both graphite and hard carbon cells is similar, the average discharge voltage of the graphite cell (3.7 V) is slightly higher than for hard carbon (3.6 V). The delivered energy of the graphite technology is therefore higher for the same cell capacity due to its flatter discharge characteristic.
Recently, non-carbon active anode materials consisting of amorphous tin composite oxide (ATCO) materials have been used by Fujifilm Celltec. These materials have advantages in bulk density and reversible specific capacity compared to graphite. However, the major disadvantage with these materials is a large irreversible capacity that has to date limited their successful introduction in to commercial Li-ion products.
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