Think Like An Ion
Formation isn’t just a process step. It decides whether lithium ions move cleanly through the system or get slowed, trapped, or consumed along the way.
So instead of explaining formation like a textbook, we’re going to follow a single Li⁺ as it moves through the cell and show two realities at every step:
what success looks like
what failure actually feels like inside the system
Because most battery problems aren’t theoretical. They are transport problems in disguise.
Lithium ion from cathode to electrolyte
Success: Ion is dry, and all of a sudden, it gets soaked in a sweet, carbonate electrolyte. It remains stationary as the soaking lasts for 24 hours. It feels an abrupt pull - some may say electric - out of its olivine bed. Ion is dragged out to shore and released into a vast pool of electrolyte. Somewhere, an electron feels a similar pull, and mimics the ion’s travel plans through a drier pathway.
Duress: Ion is dry, but sees nearby ions in their olivine beds abruptly drenched in electrolyte. Ion experiences partial electrolyte wetting, but is unable to decide whether it should give in and leave its olivine structure. It deliberates for some time before deciding on following the pathway, though at a sluggish pace and may take some detours
Explanation:
Conventional li-ion cells are injected with electrolyte and allowed to soak for a 12-24 hour rest before commencing formation to ensure sufficient wetting and robust SEI formation
External power source creates an electric potential, forcing ion out of LFP structure = endothermic driving force pushing from low energy cathode state to high energy anode state
Cathode is getting oxidized during the first charge, losing electrons
Voltage of battery = difference in chemical potential of lithium between materials, this barrier or difference must be overcome in order to store energy
Lithium ion from electrolyte to separator, back to electrolyte
Success: Ion wades through the pool, making some friends along the way. The group of friends gets nicknamed “solvation sheath” (#squadgoals) for some odd reason. They come upon a massive polymer barrier, almost splitting the pool in half.
Duress: Sluggish ion meanders over to the same polymer barrier, but arrives at a much later time than the others.
Explanation:
Polymer separators are used in li-ion batteries to prevent short circuiting = electronically insulating, ionic conducting porous membrane
An electrochemical potential gradient is created between the anode and cathode upon charging/discharging, which is what allows the li-ions to move through the separator
Li-ions must solvate in a solvation sheath (Li+ coordinated with solvents), facilitating ion transport through the electrolyte. The separator must also be wetted (electrolyte in pores) to ensure ion transport energy barriers can be overcome. The thermodynamics of solvation, including the binding energy between Li+ and solvent molecules, influences ion transport
Temperature and salt concentration of the electrolyte will determine the thermodynamic driving force of li-ions in solution
Li-ion transport is determined by separator thickness, resistance, and tortuosity. Separator’s porosity and path length will facilitate transport kinetics (high porosity = faster transport). Pore size also manages uniform flow of ions at equal rates, mitigating growth of lithium dendrites
Li-ion transport is influenced by ionic conductivity as well. Electrolyte conductivity and certain coated separators (ceramic) can increase the concentration of free Li+, enhancing li-ion transport rate. High Li+ transport rate = high transference number
Transport kinetics are influenced by temperature: higher temperatures and less viscous electrolytes = faster kinetics
Lithium ion from electrolyte to anode (pair with electrons)
Success: Ion keeps swimming and sees a black sand shoreline in sight. It might finally be time to relax and take some PTO. It climbs onto the black sand, forgoing its solvation sheath, and finds the perfect hexagonal spot. Each of its friends do the same. Electrons have also reached the black sand, creating electrical current through an external circuit.
Duress: Ion reaches graphite shoreline, but has difficulty releasing its solvation sheath. Energetically, it feels as though it must overcome a very high threshold before fully resting on graphite. The ion has faced quite a few hurdles on its journey interacting with electrode and electrolyte interfaces.
Explanation:
Graphite anode gets reduced during charging (accepts electrons)
Graphite anode forms LiC6 structure with lithium intercalation upon charge
Lithium ions must de-solvate before intercalating into the graphite (lose their solvation sheath). Overcoming this energy barrier can impede tests such as fast charging, especially if there is electrochemical polarization (transferring electrons and desolvating li-ions).
Lithium ion during SEI formation
Success: Carbonate sisters (Ethylene, Propylene, Vinylene) get sacrificed and become splatter on the shore. Other Li-ions around our protagonist go missing as well, with 5% of the overall population decimated.
Duress: Other Carbonate family members get sacrificed unnecessarily along with higher numbers of Li-ions. Higher temperatures lead to thicker, less stable SEI layers. Same with higher current densities.
Explanation:
Electrolyte undergoes solvent reduction at graphite anode to form a multi-layer passivation film that is mostly composed of electrolyte additives as well as lithium ions from the cathode, consuming capacity (around 5%). Additives are sacrificial to create a thin, robust, and flexible SEI
SEI blocks electron transfer to prevent continuous electrolyte degradation, but is ionically conductive for li-ions
Electrolyte spontaneously reduces due to the low operating potential of anodes, driving force for electrolyte instability
For SEI formation
solvent diffusion to the anode surface
electron transfer from the anode
decomposition of electrolyte components
Lithium ions deintercalate from anode back to cathode
Success: Ion completes the journey backwards, moving from the anode through the Carbonate splatter on shore to rejoining its solvation sheath squad, journeying through the polymer membrane back to its olivine crystal bed. Electrons externally travel back to the cathode as well.
Duress: Ion attempts the journey backwards, moving through a thick carbonate splatter on shore, hindering its path. It gets consumed by the thick splatter, ending the return trip.
Explanation:
Lithium ions de-intercalate from graphite upon discharging, returning to the LFP cathode. Electrons do the same through the external circuit, cathode is now getting reduced on the discharge step.
Lithium ions have to solvate again to travel through the electrolyte
Non-uniform wetting / unstable thick SEI formation will create conditions where lithium will get repeatedly consumed in subsequent cycles with a li-ion cell losing capacity to the resistive SEI.
Zooming Out From the Cell
And every lithium ion that didn’t make it back?
That’s your lost capacity.
That’s lithium inventory you don’t get back.
Formation didn’t just start the system.
It decided how much lithium you’d lose before you ever shipped the cell.