Formation & Aging

Formation

Battery cell Formation is the process of initially charging and discharging the cell after it has been assembled. So named because this process “forms” the electrochemical system. This step is really important as it sets up the electrochemical system for it’s future thousands of charge/discharge cycles, it’s rate capability and safety [1].

The formation process is not very well understood and yet there are huge pressures to reduce the time and cost.

The influencing factors include [1]:

• material and cell design
  • electrolyte
  • active material
  • potential prelithiation
  • surface properties
  • cell format
  • separator
  • electrode design
• process conditions
  • temperature
  • pressing pressure
  • forming cyclisation
  • degassing

SEI formation step: how that electrolyte reduction plays out isn’t just a function of chemistry and voltage window, the temporal shape of the current during that first charge measurably affects how uniform and stable the resulting SEI film is. A flat CC/CV ramp treats the reaction as though it responds linearly to current, but the interfacial kinetics are nonlinear, so the same average current delivered differently over time produces different SEI thickness and coverage.

That’s part of why the aging step afterward exists, it’s partly compensating for whatever unevenness got locked in during formation. Getting the first-charge current profile matched to the cell’s own kinetics can shrink how much aging has to correct for. Ibrahim Karakoc

Fast Formation Protocols

One of the slowest and most capital-intensive steps on the line: a conventional constant-current formation protocol can run for days, and every cell has to sit in a dedicated cycling channel for the whole of that time. One recent review of gigafactory process equipment put formation and its associated aging step at up to 30% of a plant’s production capital expense and around a quarter of its floor space, which is why shortening formation time without degrading the interphase it is meant to build has become a significant area of manufacturing process research.

Aging

After the formation process, the battery goes through a period of aging, which involves repeated cycles at different rates and rest times. The purpose of aging is to stabilize the battery’s electrochemical performance and make its voltage more accurate. Aging can be done at room temperature or at a higher temperature.