As India gears up for the massive adoption of electric mobility, there is a lot of debate on the risks involved with using an EV. When it comes to building a battery pack for EVs, the cell selection process becomes important to ensure high performance while at the same time ensuring safety. One must look at multiple factors while selecting a cell like the underlying chemistry, form factor and other cell characteristics.
Cells are manufactured with different chemistries of anode and cathode materials to achieve performance at varying levels. Although there might be many cell chemistries under research, following are the cell chemistries commercially available in the market currently:
- Lithium Ferrous Phosphate (LFP)
- Nickel Manganese Cobalt (NMC)
- Lithium Cobalt Oxide (LCO)
- Nickel Cobalt Aluminum (NCA)
- Lithium Titanium Oxide (LTO)
- Lithium Manganese Oxide (LMO)
There is no straightforward answer to which cell chemistry might be the best. Simply put, there is no one-size-fits-all in the battery industry. A battery engineer has to undergo a tedious research process and come to a conclusion, meaning the selection of a cell is application-specific and is not about one chemistry being better than the other. However, there are metrics one can consider to compare cell chemistries and arrive at a conclusion.
Fig. 1: Comparisons of different types of Li-ion cells
- Specific Energy: It is a measure of the energy stored per unit mass. Its unit is Wh/kg. Higher specific energy lets you pack more energy into your battery pack
- Specific Power: It is a measure of power per unit mass, meaning the amount of power that can be delivered per kilogram
- Safety: Can’t have enough of this. Depends on your operational requirements and varying external conditions under which the cell will be put in use
- Performance: It reflects the condition of a cell when driving the load in varying external conditions
- Lifespan: It reflects cycle count and longevity of the cell
- Cost: Cell selection should consider economic viability for mass usage
There is no such Li-ion chemistry available in the market which has aced all the six metrics mentioned. Figure 1 compares the advantages and disadvantages of selecting any particular cell chemistry for a given application. NMC and LFP tend to be the widely preferred choice for EV application owing to their overall characteristics providing a better life cycle, better charge and discharge rates without compromising on operating safety, all this at viable costs.
Classification of cells based on shapes
There are mainly three types of cells available in the market based on their shape
- Prismatic cells
- Cylindrical cells
- Pouch cells
Fig. 2: Types of Li-ion cells based on Shape
- Cylindrical cells are one of the most used packaging styles for Li-ion cells because of ease of manufacturing, mechanical stability, and ability to withstand high internal pressures without deforming making them widely usable in automotive, medical equipment, and power tools.
- Pouch cells are commonly found in handheld devices like mobile phones and tablets because it offers a simple, flexible, and lightweight solution to battery design.
- Prismatic cells can be used in heavy electric vehicles like buses and cars. These cells have higher volume utilization compared to cylindrical cells and are less efficient in thermal management and can be prone to swelling.
There are also different cell characteristics that one must consider in the cell-selection process. These are defined in a straightforward manner.
- Nominal Voltage: It is the rated voltage of a cell
- Capacity: It is defined as the total amount of stored energy and is expressed in the unit of Ampere-hours (Ah)
- Internal Resistance: The resistance offered by the Li-ion cell
- Charge voltage: Maximum voltage up to which a cell can be charged.
- Discharge cut-off voltage: Minimum voltage up to which a cell can be discharged.
The framework for cell-selection given here is good primer for cell selection. Suitable chemistry, geometry, performance requirements, costs, safety parameters, ease of assembly, etc. all play a crucial role in the battery engineering process. A well-informed choice of a cell is the only way towards building a safe and high-performance battery pack!
Author: Mohammed Suffiyan, Battery Engineer, RACEnergy and Gautham Maheswaran, CTO & Co-founder, RACEnergy
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