Electrolyte Filling - Battery Design

Up until now in the cell assembly process the cell is dry, now we look at electrolyte filling. A partial vacuum is created in the cell and a pre-determined quantity of electrolyte is delivered to the cell. The partial vacuum helps the distribution and hence wetting of all layers within the cell.

The electrolyte is dispensed based on a defined volume of liquid. A second quality check is the weight of the cell before and immediately after filling.

What are the Electrolyte Fill Requirements for a cell versus chemistry, capacity, format, lifetime and other parameters?

The calculation is based on the porosity of the cathode, anode and separator. Added to this is the free volume and then a multiplier to account for losses in the filling process.

Image: Thomas Knoche, Florian Surek, Gunter Reinhart, A process model for the electrolyte filling of lithium-ion batteries, 48th CIRP Conference on MANUFACTURING SYSTEMS – CIRP CMS 2015, Procedia CIRP 41 ( 2016 ) 405 – 410

Vacuum Filling Physics

The vacuum applied during filling is doing two separate jobs. Before dosing, it pulls dissolved and entrained gas out of the cell and the porous electrode/separator stack, removing the counter-pressure that trapped air would otherwise exert against the incoming liquid. Once the electrolyte is dosed, what happens next is governed less by the vacuum pump than by capillary action: the electrolyte does not need to be pushed into the pores of the electrode coating and separator, because the pore-scale capillary pressure itself pulls it forward. Filling-process models typically treat the advancing wetting front as a pressure-saturation problem, with flow through the porous stack following a Darcy-type relationship and capillary pressure supplying the driving force [2].

This is why filling parameters beyond the initial fill vacuum level matter in production. Experimental and simulation studies on cylindrical cell filling report that raising electrolyte temperature and holding a moderate, rather than the lowest achievable, wetting-stage pressure both speed up how fast the capillary front advances through the stack — warm electrolyte has lower viscosity, and an overly aggressive vacuum held too long can pull gas pockets deeper into the winding instead of clearing them [3]. Getting this balance wrong at production scale is what later shows up as the local under-wetting covered below.

Challenges

Environment ppm control
“vacuum” injection pressure integrity
The electrolyte needs to be in the very low ppb range for H₂O.
- Higher levels of H₂O creates HF not only is a safety hazard, but it also eats the battery from the inside out.
Mass flow injection (as opposed to vol flow injection)
Traceability finesse of the injection tanks, purge control, downtime in pipework etc
Injection and feeder tank residues build up (preventative maintenance control and frequency)
Temporary seal integrity checks
Wetting of all layers within the jelly roll or stack with electrolyte
- May require rolling / rotation protocol to enhance wetting

Wettability

Wettability refers to the ability of the electrolyte to spread and penetrate the porous electrode materials, crucial for battery performance and safety. Poor wettability can lead to issues like reduced capacity, increased resistance and safety problems.

Sponge-Inspired Electrolyte Wetting

When we wash dishes, we never wait for water to slowly creep into a sponge. We just give it a squeeze, and it wets almost instantly.

Ultrasonic Wetting Diagnostics

Because wetting happens inside a sealed cell, confirming that it has actually finished (rather than just assuming it has once the standard rest time has elapsed) is a separate measurement problem. Ultrasonic transmission scanning has become the main non-destructive way to do this: a transducer pair is scanned across the cell and the amplitude of the ultrasonic signal that passes through it is recorded at each point. Electrolyte conducts sound through the stack far better than the gas-filled pores of a dry or partially-wetted region do, so a dry pouch cell returns a visibly weaker, patchier transmitted signal than a fully wetted one, and the scan effectively produces a map of which areas of the jelly roll or stack are still dry [4].

The same technique is used further downstream of filling as well: because it is sensitive to how well the electrolyte is filling the pore structure, not just whether liquid is present, ultrasonic scanning can also pick up localised electrolyte dry-out or “de-wetting” that develops later in cell life as gas generation or gradual electrolyte consumption redistributes liquid away from part of the stack, which is a useful cross-check against the assumption that a cell which wetted correctly at end of line stays that way.