21700 Cylindrical cells – Comparison of economical vs premium models • EVreporter

21700 cell, as the name suggests, stands for a cylindrical cell with 21mm width and 70mm height. It was first introduced in 2017 by a Tesla and Panasonic collaboration. 21700 was introduced as an alternative to the long-running 18650 model, which was introduced by Sony in 1991.

The 21700 cell has been mass-produced lately by many companies who were producing 18650, and some of these companies have completely moved to 21700 production. 21700 cells have better energy density (gravimetric and volumetric), higher cycle life, and lower cost per kWh than 18650 cells. 21700 cells make the battery pack more reliable due to the lower number of cells in parallel and reduce costs at the battery pack level.

21700 comes in two models, one with graphite and silicon oxide composite anode and the other with graphite anode only. Silicon’s discharge capacity (ability to hold Lithium-ions) is more than ten times that of graphite; hence, compositing little silicon oxide with graphite reduces the overall anode loading in the cell and allows for more space to load cathode and allows to achieve more capacity of the cell.

Mass production models of 21700 cells with graphite-only anode have reached 4500mAh capacity. On the other hand, 21700 cells with silicon oxide anode have various capacity ranges, starting from 5000mAh and 5800mAh. Higher than 5800mAh cell capacities are expected to launch in the coming times, by simply using higher Nickel content cathode and increasing the content of silicon oxide anode in the cell. These cells will be more expensive, have less cycle life and safety.

4500mAh cells could be considered an economy model because they take a value-for-money approach, skipping silicon oxide (which costs more than four times as much as graphite), while 5000mAh cells can be considered a premium model. Below is a table comparing both models.

Premium models such as 5000mAh cells use higher cost electrolyte and higher cost silicon oxide anode and sometimes use more Nickel content cathode than NCA or NMC (which is 80%). This combination allows for the achievement of a true capacity of 5000mAh; however, it also leads to higher costs, reduced safety, and shorter cycle life. Some companies using NMC cathodes with 80% nickel content struggle to consistently produce the 5000mAh true capacity. As a workaround, they may resort to thicker cell casings, which enable the incorporation of more active materials.

On the other hand, 4500mAh cells use NMC cathode with 80% Nickel content and have a higher cycle life than 5000mAh capacity cells and have a comparable cycle life to 2600mAh 18650 cells (which uses NMC 532).

The 18650 cell using NMC cathode with 80% Nickel content achieves 2900mAh capacity, but it is more expensive at a per kWh cost and has lower cycle life than 21700 4500mAh cells.

The key takeaway is that compared to 18650 cells, larger cells such as 21700 have a higher energy density, higher cycle life, lower costs and increased system reliability because of fewer cells in parallel connection. Hence, some companies are planning for even bigger cells, such as 4680.

To accommodate an additional 500mAh capacity (using 5000mAh over 4500mAh) per cell, the user ends up spending more than 25% at a per kWh level, settles for lower cycle life, pays attention to higher safety precautions and adds additional cost to the battery pack. Similar logic applied to 18650 cells while choosing 2900mAh capacity over 2600mAh capacity, which came at a higher cost and lower cycle life. Perhaps choosing higher capacity is not necessary for low and medium range vehicles. NMC cells play a crucial role in cold places (during winters) because LFP can’t charge below 0°C.

Users shifting to 21700 from 18650 must note that not all 21700 cell companies provide true capacity and mostly hang near minimum capacity. Hence, it is very important to study the cell capacity since some 21700 cell manufacturing companies claim that the true capacity will only come at a 0.2C discharge rate. This is unacceptable for companies that are accustomed to higher than rated capacity even at a 0.5C rate in 18650 cells. This holds very true for 2600mAh 18650 cell users, and they should exercise caution and verify the cell capacity while moving to 21700 cells.

21700 cells come with two types of tab designs.

• One model with a single positive tab and single negative tab provides ACIR below 30mΩ

• Another model with a single positive tab and double negative tab provides ACIR below 20mΩ.

The latter model is expensive to produce and can sometimes have lower discharge capacity because of having an additional anode tab. All the above-discussed 21700 models are called energy-type cells, and then there are power-type 21700 models, which have a higher number of tabs, and even tablets models have been launched to provide massive power and improve cycle life.

About the author

Rahul Bollini is an R&D expert in Lithium-ion cells with 10 years of experience. He founded Bollini Energy to assist in deep understanding of the characteristics of Lithium-ion cells to EV, BESS, BMS and battery data analytics companies across the globe. Rahul can be reached at +91-7204957389 and bollinienergy@gmail.com.

Also read: Introduction to NFPP (Sodium-ion) batteries and comparison with LFP (Lithium-ion)