Cathode materials

LIB cathodes consist of a current conductor (usually aluminum foils) on which an active material is deposited, in which the current and the lithium ions can be stored. Various of these active materials are described below.

Lithium cobalt oxide

Abbreviations: LCO or as chemical formula: LiCoO2

LCO ensures a relatively short lifetime (500 to 1,000 cycles) and high energy density. However, LCO cathodes are not as resistant to temperatures as other types of lithium-ion cathodes and are therefore more dangerous in case of misuse. If, for example, too high an electrical voltage is applied or the cell is damaged, this can quite quickly lead to the battery melting or even burning. This is why this type of cathode is not suitable for electric vehicles in particular, which need a lot of cells. Another disadvantage is the limited load capacity (specific power). The LCO material was used in early LIB, modern systems contain nickel, manganese and/or aluminium to improve, among other things, service life, costs and charging properties.

Batteries with LCO cathodes (often in combination with a graphite anode) should not be charged or discharged with currents higher than 1C (cf. C coefficient), because this can lead to overheating and damage to the battery. For fast charging, manufacturers recommend charging currents of 0.8C or about 2,000 mA. Mandatory battery protection circuits limit the charging and discharging rates to 1C for safety reasons.

Lithium manganese oxide spinel Abbreviations: LMS, LMO or as chemical formula: LiMn2O4

LMS cathodes offer higher cell voltage than cobalt-based materials and are more resistant to heat. However, their energy density is about 20 times lower. Manganese compounds, unlike cobalt compounds, are a safer and more environmentally friendly cathode material. Other advantages include a low price and better performance at high outdoor temperatures.

Lithium nickel cobalt manganese oxides

Abbreviations: NMC, NCM or as chemical formula: Li(NiCoMn)O2

NMC cathodes are currently the most successful lithium-ion system. Like LMS systems, NMC systems can be designed for electrical power or high capacities. For example, there are 18650 cells that are designed for moderate charging conditions and have capacities of 2,800 mAh. They deliver currents between 4 and 5 A. However, the same cell type can also be optimized for particularly high, continuous discharge currents of 20 A, but then only offers a capacity of around 2,000 mAh. In combination with silicon-based anodes, the capacities could be increased to more than 4,000 mAh, but these are still associated with shorter lifetimes and poorer charging performance. However, many scientific projects are working on improving these disadvantages.

NMC cathodes offer a good compromise between good general electrochemical performance, high energy densities and costs. The specific energy density is better than LFP, LMO and LCO. The discharge rate is better than LCO cathodes, but not better than LFP cathodes.

The success of NMC cathodes is due to the combination of nickel and manganese. Nickel brings the high specific energy density to the material, but is also responsible for a rather unstable electrode structure. Manganese, on the other hand, has the property of forming stable so-called spinel structures and ensures low internal resistances. However, it has poor specific energy densities. When chemically combined, the metals reduce each other's weaknesses.

To create new cathode materials with even higher energy densities, attempts are being made to introduce more nickel into the NMC material. Such novel materials are called nickel-rich NMC materials. Similarly, attempts are being made to make materials richer in lithium and manganese. Standard NMC cathode material contains nickel, cobalt, and manganese in equal proportions and is therefore also referred to as 1:1:1 NMC (111). Nickel-rich variants are 5:3:1, (531) 6:2:2 and 8:1:1 (811) 6:2:2-NMC (622) has already reached market maturity and is used in some electric vehicles.

A lot of research is also currently going into new electrolytes and additives to be able to charge NMC cathodes up to voltages of 4.4 V and higher. This also leads to higher capacities in the batteries. Conventional electrolytes are no longer stable in this voltage range.

Lithium iron phosphate

Abbreviations: LFP or as chemical formula: LiFePO4

Phosphate-based materials have better thermal and chemical electrochemical stabilities than other lithium-ion cathode materials. LFP cells are very safe batteries. They are fire-resistant in case of overcharging and more resistant in case of short circuits. In case of misuse, this cathode material does not release oxygen, does not burn and is accordingly insensitive to heat. Lithium iron phosphate cells also have a longer life (1000 to 3000 cycles). However, due to their low nominal voltage of 3.2 V, they have a lower energy density than many other cathode materials such as NMC or NCA. Another disadvantage is that the self-discharge is higher than that of other LIB types. But LFP cathodes support higher voltages and correspondingly higher currents, which makes them suitable for use in fast-charging LIBs.

Lithium nickel cobalt aluminum oxide

Abbreviations: NCA or as chemical formula: LiNiCoAlO2

NCA has been available as a cathode material since 1999 - at least for special applications. Like NMC, it has a high specific energy density, good performance and a long lifetime.

Other research cathode materials

with manganese (LMnP) other lithium metal phosphates with nickel (LNiP) Vanadium oxide Metal sulphides Metal silicates with manganese and iron (LMFP) Lithium- and manganese-rich compounds such as LMNO Blends (mixtures of different cathode materials): such as NMC with LFP or NMC with LMFP Metal fluorides with cobalt (LcoP) Iron fluoride Copper fluoride Iron copper fluoride