SEI Growth - Battery Design

The Solid Electrolyte Interphase (SEI) is the film that builds up on a graphite anode when the electrolyte is reduced during the first charge. A cell cannot work without it: the SEI lets lithium ions through, keeps electrons away from the electrolyte and so stops the solvent being consumed on every cycle. The catch is that it never stops growing completely. Each extra nanometre locks up lithium that can no longer be cycled and adds resistance to the ion path, so slow SEI growth is one of the main reasons graphite-anode cells lose capacity and power over their life ^[5].

Key to the SEI growth diagrams: graphite, inner inorganic SEI, outer porous organic SEI, fresh SEI and electrons, lithium ions, solvent molecules and lithium metal

The six panels below follow one patch of graphite surface from a fresh cell to the point where lithium plating begins. They are schematics. A real SEI is of the order of tens of nanometres thick on a particle measuring 10–20 µm, so the film is drawn many times thicker than it really is. The bar under each panel shows the trend in cyclable lithium; the lengths are illustrative, not measured.

SEI growth in six stages

Stage 1: Pristine graphite

SEI growth stage 1: bare graphite surface with electrolyte solvent molecules and lithium ions above it and no SEI layer

Before the first charge the graphite surface is bare. It sits in the electrolyte, typically LiPF₆ dissolved in a blend of a cyclic carbonate (EC) and one or more linear carbonates such as DMC or EMC (Electrolyte Solvent Ratios).

An uncharged graphite electrode sits close to 3 V vs Li/Li⁺, well above the potential at which these solvents break down. Nothing reacts until the cell is charged, and every lithium ion loaded into the cathode is still available for cycling.

Stage 2: SEI nucleation during formation

SEI growth stage 2: electrons leave the graphite and reduce solvent molecules at the surface, forming a thin first SEI film

On the first charge the anode potential falls. From roughly 0.8 V vs Li/Li⁺ downwards, EC and other electrolyte components start to accept electrons from the graphite and decompose ^[3]. Their products (lithium alkyl carbonates, Li₂CO₃, LiF from the salt) settle on the surface as the first thin film, and gases such as ethylene are released.

Each electron used here is paired with a lithium ion drawn from the cathode, so the cell gives up part of its capacity before it has done any useful work. That first-cycle loss, and the quality of the film left behind, is why current, temperature and pressure during formation are controlled so closely (Fast Formation Protocols) ^[8].

Stage 3: Stable, passivating SEI

SEI growth stage 3: two-layer SEI with a dense inorganic inner layer and a porous organic outer layer; lithium ions pass through

A well-formed SEI has two layers. Against the graphite is a thin, dense inorganic layer (LiF, Li₂CO₃, Li₂O). On top of that is a thicker, more open layer of organic compounds and polymers. Peled’s mosaic model treats the film as a patchwork of these phases rather than a uniform coating ^[1]^[2].

The useful property is that the film conducts lithium ions while being a poor conductor of electrons. Once it covers the surface, electrons can no longer reach the electrolyte easily and the side reaction slows sharply. From here on, the growth rate is set by how quickly solvent or electrons can work their way through the film that already exists.

Stage 4: Cracking and repair

SEI growth stage 4: graphite expands during cycling, cracking the SEI; fresh SEI forms at the bottom of each crack

Graphite swells as lithium moves in between its layers. A fully lithiated particle can occupy about 10% more volume than an empty one ^[4]. The SEI is far less compliant than the graphite beneath it, so repeated expansion and contraction opens cracks, typically at particle edges and wherever the film is already thin (Cell Expansion).

Every crack exposes fresh graphite to the electrolyte, and a new patch of SEI grows to seal it. This is the same reaction as in formation, so each repair consumes more cyclable lithium and more electrolyte. It helps explain why a cell that is cycled deeply ages faster than one stored at the same average state of charge.

Stage 5: Thick, porous SEI

SEI growth stage 5: thick, uneven and porous SEI; only some lithium ions reach the graphite

Over hundreds of cycles or years of storage the film keeps thickening. Each new layer has to be built from species that diffuse through the layers beneath, so growth slows as the film gets thicker. The outcome is the familiar square-root-of-time shape of capacity fade early in a cell’s life, described in more detail below ^[5].

There is a power penalty as well. Ions have to cross a thicker, more tortuous and partly blocked layer, so internal resistance rises. On an impedance spectrum this shows up as a growing SEI contribution in the high-to-mid-frequency region (Electrochemical Impedance Spectroscopy).

Stage 6: Lithium plating

SEI growth stage 6: fragmented SEI with metallic lithium plated on the graphite surface, mossy lithium growth and isolated dead lithium particles

Plating is not SEI growth as such, but an aged SEI makes it more likely. Normally lithium ions arriving at the anode slot in between the graphite layers. If they arrive faster than they can be absorbed (during fast charging, at low temperature or close to full charge), the local anode potential can fall below 0 V vs Li/Li⁺ and metallic lithium deposits on the surface instead ^[6]. The extra resistance of a thick SEI adds to the overpotential and lowers the charge current at which this happens.

Plated lithium is very reactive and quickly grows its own SEI. Some can be stripped back during discharge, but any fragment that loses electrical contact becomes “dead lithium” and is lost for good. Plating can also block pores in the electrode, which pushes more current through the remaining paths and encourages further plating. This feedback is one of the mechanisms behind the sudden “knee” in many ageing curves ^[7]. Dendritic deposits also raise the risk of an internal short circuit.

Why growth slows: the square-root-of-time law

When growth is limited by transport through the existing film, the rate of thickening is inversely proportional to the thickness. Integrating that relationship gives an SEI thickness, and therefore a lithium loss, that rises with the square root of time ^[5]:

Q_{\mathrm{loss}}(t) \approx k_{\mathrm{SEI}}\sqrt{t}

Q_loss is the capacity lost to SEI growth (Ah or % of initial capacity)
t is time in storage, or accumulated time or charge throughput in cycling
k_SEI is a rate constant that depends on temperature (usually fitted with an Arrhenius term) and on the anode potential, and therefore state of charge

Two practical consequences follow. First, doubling the time only increases the loss by about 40% (since √2 ≈ 1.41), which is why fade is quickest in the early months and then eases. Second, plotting capacity against √t should give a roughly straight line while SEI growth dominates. A curve that bends away from that line points to another mechanism taking over, such as plating, loss of active material or cathode degradation.

What speeds up SEI growth

Temperature: the side reactions are thermally activated, so time spent hot has a much bigger effect than the same time at room temperature (Calendar Ageing vs Chemistry).
High state of charge: a fully lithiated anode sits at its lowest potential, where the electrolyte is least stable. Storage near 100% SOC ages a cell faster than storage at mid SOC.
Cycling depth and rate: larger swings in lithiation mean more expansion and contraction, more cracking and more repair (stage 4). Anodes containing silicon expand far more than graphite and suffer from this much more (Silicon Anode).
Electrolyte formulation: film-forming additives such as VC and FEC are used to build a thinner, tougher and more stable SEI during formation (Electrolyte Additives).

Design and BMS implications

Formation: the formation protocol sets the starting quality of the SEI. Faster formation saves factory time and capital but has to be proven against life testing.
Lithium inventory: Prelithiation can offset the lithium consumed in formation, and the N/P ratio provides margin against plating as the cell ages.
Operating limits: avoiding long periods at high SOC and high temperature slows calendar fade. Charge-current limits need to fall at low temperature and should be reviewed as resistance rises with age (Establishing Fast Charge Current Limits).
Monitoring: trending capacity, DC resistance and differential voltage features over life separates SEI-driven lithium loss from other mechanisms, and gives early warning of a move towards plating.