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 talked about the Cathode and its sub-components. In this article (part 2), author Rahul Bollini discusses the anode.
Anode stores lithium ions that come from the cathode during charging and releases lithium ions during discharge. It also passes the currents through an external circuit. The anode plays a vital role in defining the cycle life of a Lithium-ion cell.
Anode has the following sub-components:
- Anode active material – such as natural graphite, artificial graphite, silicon graphite, LTO.
- Conductive agents – such as Super P Carbon, Super C-65, etc. that enhance the conductivity of the anode active material.
- Solvent – Graphite anode coating is usually done with an aqueous-based process using deionized water. Because of this, the coating process of Graphite anode can be carried out in a not so strict dry room conditions. However, strict dry room conditions are required for the NMP solvent-based slurry coating process followed for the cathode. There have been successful demonstrations of aqueous-based cathode coating process, and it is expected to be adopted in mass manufacturing very soon by many companies.
- Binder – which is usually CMC (Carboxymethyl Cellulose) and SBR (Styrene-butadiene Rubber). It works with aqueous anode slurry and is pretty much the industry standard.
The way the future expansion of cell manufacturing capacity is being planned, my observation is that there is a lot of planning being done for the expansion of cathode manufacturing plants; however, the capacity expansion for battery-grade graphite is not being planned at a similar scale which might lead to a supply crunch for battery-grade graphite going forward.
Anode active material
1. Most Lithium-ion cells use carbon-based anodes, preferably graphite – natural or artificial. Battery-grade natural graphite is priced lower than battery-grade artificial graphite (usually 10-20% lower).
- There is a trade-off between the capacity a kind of anode can deliver as opposed to the cycles it will last.
Natural graphite tends to deliver higher capacity, but it is known to have a shorter cycle life (which causes higher irreversible capacity loss of the cell). On the other hand, artificial graphite tends to last for a higher life cycle but has a slightly lower specific capacity.
- Surface area (in sq.m/g) is an important factor for the anode, which decides how fast the intercalation of the Lithium ions happens, which in turn determines the speed of the cell charging and discharging.
The tap density of the ESS (energy storage system) grade anode active material tends to be higher, but it has a lower surface area than the EV grade anode active material. For example, Graphite provides a surface area of around 3 sq.m/g, which is much lower than the 100 sq.m/g provided by the LTO anode. Hence, electrons enter and leave slower in graphite anode when compared to LTO anode.
2. LTO (Lithium Titanium Oxide) – Usually, Lithium-ion cells are named after the cathode active material they use. LTO is the only Lithium-ion cell type named after its anode active material. LTO cells are Lithium-ion cells that use an LTO anode; these cells can use any cathode of choice.
Advantages of LTO anode
- Fast charging: LTO cells can charge as fast as 10C rate – practically from 0% to 80% in 6 minutes. Most chemistry Lithium-ion batteries max out at around a 1C rate of charging.
- High Power Discharge: LTO can discharge as high as 10C rate. It can power applications that need high power for a short period of time. For example, UPS for data centers, where the backup requirement is for a very little time.
- Wide Operating Temperatures: LTO batteries can operate at -40°C to 60°C temperatures. It has the widest range of operating temperature when compared with any other type of Lithium-ion cell chemistry.
- High-Temperature Operation: LTO batteries can operate at high temperatures. Unlike other Lithium-ion cell chemistries, which face serious irreversible capacity fade after 45°C, LTO batteries have a slower irreversible capacity fade when operating above 45°C.
- Safety: LTO batteries do not catch fire (like in LCO, NMC, and NCA batteries) and do not release smoke (like in LFP batteries) when damaged.
- Cycle Life: LTO batteries are advertised with the capability to have over 40,000 cycles at a 1C rate of charge and 1C rate of discharge. The cycle life comes down when charging and discharging at higher than 1C rates. Its calendar aging life is expected to be 30 years, which paves the way for the second life usage of LTO batteries.
Disadvantages of LTO anode
- Gravimetric Energy Density (Specific Energy): The discharge capacity of LTO is 175mAh/g at 0.1C rate compared to graphite which has a discharge capacity of more than 350mAh/g at 0.1C rate. The amount of anode active material loading tends to be more, and this increases the overall weight of the cell. Its gravimetric energy density is more than Lead Acid batteries but lower than LFP batteries that use graphite anode.
- Cost: LTO batteries are so expensive that their price cannot be compared with traditional NMC and LFP batteries sold in the market. The higher battery price and weight are some of the reasons why LTO battery manufacturers have not expanded their production capacity as much as manufacturers of other Li-ion chemistries.
3. Silicon Graphite – Silicon can deliver more than 3000mAh/g specific capacity. It is mixed with graphite in small quantities to achieve a Silicon-graphite composite anode that has a specific capacity of more than what is delivered by traditional graphite.
Advantages of Silicon Graphite anode
- Fast Charging
- High specific capacity
- Helps achieve higher gravimetric energy density (Wh/Kg) of the cell
- Helps achieve higher volumetric energy density (Wh/L) of the cell
- Brings down the overall cost of the cell
Disadvantages of Silicon Graphite anode
- Expansion of Silicon happens during the cycling of the overall cell
- Silicon has low electrical conductivity compared to graphite
- SEI (solid electrolyte interphase) layer tends to be unstable because of the expansion and contraction of Silicon in the anode during the cycling.
About the author
Rahul Bollini is a Lithium-ion cell and battery pack R&D expert with an industrial experience of over 7 years. He can be reached at +91-7204957389 and firstname.lastname@example.org.
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