Components of a Lithium-Ion Cell – Part 1 | Cathode

A Lithium-ion cell is built of various components and sub-components. This article discusses the functionality and importance of selecting the right type of (sub) components.

A Lithium-ion cell has four major components:

• Cathode – Positive electrode
• Anode – Negative electrode
• Electrolyte – Medium for the movement of lithium ions
• Separator – Prevents contact between cathode and anode

Part 1 in this series talks about the Cathode and its sub-components.

Cathode

Lithium-ion cells are generally named after their cathode active material. Popular cathode active materials include LCO (Lithium Cobalt Oxide), LFP (Lithium Iron Phosphate), NMC (Lithium Nickel Manganese Cobalt Oxide) and NCA (Lithium Nickel Cobalt Aluminium Oxide). Also, new generation cathodes are being commercialized at a pilot scale and are expected to be seen more commonly in the future; these include cathode active materials such as NCMA, LNMO and LMFP. Let’s talk about the cathode active materials in the popular cells used in the Indian market, such as LCO, LFP and NMC.

LCO – Lithium-ion cell with an LCO cathode active material has a higher range of working voltage and has been there for quite some time. It was commercialized by Sony in 1991 in 18650 cylindrical form factor to be able to fit in digital cameras. It is popularly used in cell phones and laptops. However, its safety and cycle life are on the lower side. Additionally, LCO uses high amounts of Cobalt, which makes LCO cathode active material the most expensive of all the variants of cathode active materials in Lithium-ion cells.

LFP – Lithium-ion cell with LFP cathode active material has a long cycling ability, and it’s a relatively stable chemistry. It has the ability to charge up to 60°C and discharge at temperatures beyond that. LFP has a good combination of being affordable, long-lasting and safe. But it has the disadvantage of having a relatively lower gravimetric and volumetric energy density. It also has difficulty charging at sub-zero temperatures. Hence, an LFP battery pack needs a heating mechanism to increase the temperature of the battery pack. It starts charging once its temperature comes to a positive value of degree Celsius. It is popularly used in low-range electric vehicles and energy storage systems. LFP has low tap density* and low conductivity; hence, it is manufactured with a uniform coating of conductive carbon. Generally speaking, 2.3 kg – 2.5 kg of LFP cathode active material is used per kWh.

*Tap density is the density (mass divided by volume) of a material after it has been beaten down for a particular period of time.

NMC – Lithium-ion cell with NMC cathode is preferred in electric vehicles, which are expected to deliver a higher range, as NMC batteries tend to occupy less space for the same energy output as compared to LFP. Additionally, NMC cells are lighter than LFP cells. Moreover, NMC cells have a steep discharge voltage curve, making its SoC (state-of-charge) estimation easier than LFP cells when using the direct measurement method to directly relate SoC with the OCV (open circuit voltage) of the cell/battery.

NMC cells are unsafe to charge above 45°C and discharge above 60°C. Thermal runaway in NMC leads to fire, and it needs effective thermal management in place to ensure battery safety. The amount of NMC cathode used per kWh varies because of the varieties in NMC, such as NMC 111, NMC 442, NMC 532, NMC 622 and NMC 811.

The trend in NMC shows a reduction in cobalt content and an increase in nickel content. This allows for an increase in the discharge capacity of the cathode active material (in terms of mAh/g). However, it also leads to the following compromise:

NMC 811 is considered not very stable as compared to the previous NMC versions. Cells using NMC 811 have seen fires across the globe.

Let us compare the data of the most commonly used cells in India –

LFP cylindrical 32700 and NMC cylindrical 18650

The cathode has the following sub-components:

• Cathode active material such as LCO, LFP, NMC, etc.

• Conductive agent – Conductive agents such as Super P carbon, Super C-65, etc. enhance the conductivity of the cathode active material. It has to be evenly coated over the cathode active material, and a good coating thickness is supposed to be maintained. The coating plays a vital role in the cycle life of the cell.

• Organic solvent – which is usually N-methyl-2-pyrrolidone or NMP. It is recovered in the drying process, which is after the coating process through an NMP recovery system. This recovery system allows reusing almost 99% of the NMP solvent.

• Binder – which is usually polyvinylidene fluoride or PVDF; it can only work in an organic solvent such as NMP). Its peel strength plays a vital role in cell functionality. There is a lot of research being done on using a liquid binder, which can completely do away with the usage of NMP solvent.

Part 2 of this series discusses Anode and its sub-components.

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Also Read :

1. Components of a Lithium-ion Cell – Part 2 | Anode
2. Components of a Lithium-ion Cell – Part 3 | Electrolyte
3. Components of a Lithium-ion cell – Part 4 | Separator