Battery Electrochemistry: What Happens Inside a Li-ion Cell

An electrochemical cell, not a fuel tank

A lithium-ion cell stores energy by moving lithium ions between two host materials — typically a graphite negative electrode and a metal-oxide positive electrode — through an electrolyte, while electrons take the separate path through your external circuit. Charging forces ions to intercalate (slot into layered graphite sheets) at the negative electrode; discharging lets them flow back to the positive electrode, driving current through whatever the battery is powering. Nothing burns and nothing is consumed in an ideal cell — the same lithium ions simply shuttle back and forth, which is why the process is reversible over hundreds or thousands of cycles rather than being a one-shot chemical reaction like a disposable battery.

Reading a discharge curve

Cell voltage is not constant during discharge: it starts near the cell's nominal voltage, holds relatively flat across most of the usable capacity (the plateau reflects the thermodynamics of the intercalation reaction), and then falls sharply once lithium in the active material is nearly depleted — the knee that marks the end of useful capacity. The discharge C-rate — how fast you draw current, expressed as a multiple of the cell's rated capacity per hour — matters because internal resistance eats voltage as an IR drop proportional to current: a 2C discharge draws down the voltage curve faster and reaches the cutoff at less delivered capacity than a leisurely 0.2C discharge, which is exactly the family of curves a Ragone plot (energy density against power density) summarizes for comparing battery chemistries.

Butler-Volmer: the kinetics at the electrode surface

Getting an ion to cross the electrode-electrolyte interface requires overcoming an activation energy barrier, and the rate that happens at is described by the Butler-Volmer equation, which relates the net current density to how far the electrode potential is pushed away from its equilibrium value (the overpotential η):

i = i₀ · [ exp(αₐFη / RT) − exp(−α_c Fη / RT) ]

i₀ = exchange current density (intrinsic reaction speed at equilibrium)
η  = overpotential = applied potential − equilibrium potential
α_a, α_c = anodic/cathodic transfer coefficients (typically ≈ 0.5 each)

At small overpotential the relationship is nearly linear (an effective "charge-transfer resistance"); at large overpotential in either direction one exponential term dominates and current grows roughly exponentially with overpotential — which is why pushing a battery to charge or discharge faster costs disproportionately more overpotential, heat and eventual degradation for each extra increment of current.

Why capacity fades: SEI growth and lithium plating

A fresh graphite electrode reacts with the electrolyte on its first few cycles to form a passivating solid electrolyte interphase (SEI) layer, which is essential — it prevents continued electrolyte decomposition — but keeps slowly growing over the cell's life, each layer of growth consuming a small, permanently unrecoverable pool of active lithium. That slow SEI growth is the dominant capacity-fade mechanism under normal use. Under harsher conditions — charging too fast, especially at low temperature — lithium ions can arrive at the graphite surface faster than they can intercalate and instead deposit as metallic lithium on top of it (lithium plating), which is both a fast, severe capacity loss and, because plated lithium can grow dendritic structures that pierce the separator, a genuine safety hazard, which is why fast-charging protocols throttle current at low temperatures and near full charge.

Impedance and the Nyquist plot

Feeding a cell a small sinusoidal current at many frequencies and measuring the resulting voltage phase and magnitude — electrochemical impedance spectroscopy — separates different physical processes by their characteristic timescales. Plotted as a Nyquist plot (imaginary versus real impedance), the signature shape is a high-frequency intercept (pure ohmic resistance of electrolyte and contacts), a semicircle at mid-frequencies (charge-transfer resistance in parallel with the double-layer capacitance predicted by Butler-Volmer kinetics), and a rising line at low frequency (diffusion-limited transport of lithium through the solid electrode material, a Warburg-type behaviour) — a single sweep that fingerprints internal resistance, reaction kinetics and diffusion limits separately instead of lumping them into one number.

Frequently asked questions

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