Li-Ion cell formation is a critical step in the manufacturing process that turns a freshly assembled cell into something capable of storing an electrical charge. Before the forming process the Li-ion cell is merely an assembly of its components; mainly an anode, a cathode, a separator, terminals, electrolyte, and packaging that holds it all together. Then, during forming, a series of charge-rest-discharge-rest cycles are applied in a specific way to make it capable of storing a charge. Subtle differences in the forming process and equipment can either enhance or be a detriment to the performance and quality of the cell. Keysight recognized this in the development of their BT2200 Charge-Discharge Platform, making certain to provide the flexibility that allows cell manufacturers to fully optimize their cell forming process.
There is a lot more going on in the forming process than just putting an electrical charge into the cell. Forming is the process of growing a passivation layer on the anode of the cell. This passivation layer is known as the solid electrolyte interface, or SEI, layer, and is a vital part of what makes a Li-Ion cell work.
Proper SEI layer formation is essential to the performance and life of the cell. When a cell is properly formed, only some of the electrolyte and lithium is consumed during formation in creating the SEI layer. The formed SEI layer then serves as a barrier between the anode and electrolyte, inhibiting further SEI layer formation along with more consumption of the electrolyte and lithium. Ideally it needs to be as thin and uniform as practical without any porosity or other defects. Defects lead to ongoing SEI formation, diminishing the cell’s performance and capacity over time.
It generally turns out that to get a good SEI layer to form, the cell must be formed very slowly, typically at a C-rate on the order of 0.1C. This translates to charging the cell over a 10-hour period. The cell is then allowed to rest, and then it gets discharged at low rate and allowed to rest again. The cell will go typically through two or more charge-rest-discharge-rest cycles, as the SEI layer is completely formed. Both the amp-hours of charge put into the cell and then discharged back out are carefully measured for each cycle. The difference in the two values indicates how much charge was used in forming the SEI layer for each cycle. Depending on several factors specific to the cell design, 10 to 20% of the charge is consumed during forming. The manufacturer knows what to expect and if the amount of charge loss is different, it is a clear indicator something is not right!
It’s not hard to see, given the cells are being charged, rested, discharged, and rested again for several cycles at such low C-rates, that the forming process may take several days to complete. This represents a significant amount of the process time, work-in-progress, investment in equipment, and ongoing cost of electricity in manufacturing. As such, it is a top priority for the manufacture to optimize the forming process to minimize the time it takes while still producing high quality cells, to be competitive.
As Li-Ion cell manufacturers gain more experience optimizing their overall forming process, they are finding more advanced capabilities over the charging and discharging to be an essential part of this. To assist the manufacturers in this regard, Keysight has recently introduced the BT2200 Charge-Discharge Platform. This modular platform consists of the BT2202A and BT2203A 8-slot Charge-Discharge Chassis and BT2203A 32-channel Charge-Discharge Modules, providing up to 256 channels per set up. The BT2200 is shown in Figure 1.
Figure 1: The Keysight BT2200 Charge-Discharge Platform for Li-Ion cell forming
Keysight has worked with several manufacturers to identify and incorporate key features into the B2200 to enhance and optimize the cell formation process. Examples include:
- Energy-efficient regenerative platform that achieves up to 90% efficiency during charging. Energy regenerated during discharge can be used by other cells or put back onto the AC line. This reduces the amount of net energy consumed, a large expense in cell forming.
- Provides up to 256 channels in one compact unit, reducing floor space needed for the forming area, reducing manufacturing floor space and overhead.
- Each channel provides up to 6 volts and 6.25 amps. Up to 32 channels can be paralleled into a single channel providing up to 200 amps in a single channel. This provides flexibility and “future-proofing” to accommodate a wide range of cell capacities within the manufacturing process.
- Voltage and current are sampled with high accuracy every second, providing a way to better measure and discern subtle differences that might occur during the forming process.
- Similarly, high accuracy voltage and current programming controls provide tight process control during forming.
- 256 sequence steps are provided for configuring complex, dynamic charge and discharge forming profiles. This allows the user to optimize the forming process to yield a higher quality SEI layer in less time.
- An extensive number of parameters are monitored in real time, providing greater options for making conditional logical decisions during the forming process. This provides greater flexibility to dynamically make changes that can further enhance the forming process.
If Li-Ion cell manufacturing is going to be an important part of your responsibilities, you should find it beneficial to learn more about Keysight’s solutions here. To learn more, click on the following link“The BT2200 Charge-Discharge Platform”to take you to its product page where you will find a lot of relevant and useful information on this solution.