How batteries work Chapter 10 The SEI

Chapter 10

The solid electrolyte interphase (SEI): formation, passivation and growth

What is the solid electrolyte interphase, and why does every cell need one?

Figure 10.1 The first charge of a lithium-ion cell. On the right the potential of the graphite falls from high on the scale toward 0.2 volts versus lithium, below the 1.1 volt lower edge of the carbonate electrolyte's stability window. On the left, once it is below that edge, electrolyte molecules are reduced at the graphite surface and a layer, the SEI, grows; a bar shows part of the lithium from the positive electrode locked in that layer for good.
electrolyte moleculeSEIour ionelectron
The first charge, and the layer that costs lithium mostly once and then protects. Try this: watch the black bar on the right; nothing happens at the surface until it crosses the green edge.
SourceAfter Goodenough and Park 2013 (the graphite at about 0.2 V vs Li/Li+ lies outside the carbonate window, whose lower edge is about 1.1 V; the SEI forms on the first charge, consumes lithium from the positive electrode irreversibly and adds interfacial impedance), Winter and Brodd 2004, section 2.4 (the SEI is electronically insulating and passes Li+) and Tarascon and Armand 2001 (ethylene carbonate forms it on graphite). The starting potential of the empty graphite is not given in the sources and the axis is unnumbered there; the thickness of the layer and the share of lithium it locks up are schematic.

The first charge

So far we have taken the hosts as we find them in a working cell. But a cell has a beginning, and the beginning leaves a mark. A lithium-ion cell is assembled discharged, with all its lithium in the positive electrode. On the first charge the graphite is driven down to about 0.2 V vs Li/Li+, below the lower edge of the window in which the carbonate electrolyte is stable (chapter 3). The electrolyte is reduced at the graphite surface, and its products form a passivating layer, the solid-electrolyte interphase or SEI. Forming it consumes lithium from the positive electrode irreversibly, and the layer adds to the interfacial impedance; it also changes as the cell cycles.1

What is reduced, and what does the layer contain? Among the solvents, ethylene carbonate is reduced first, from about 0.8 V vs Li/Li+ (with LiPF6 the anion starts earlier; see Go deeper). Two EC molecules, two lithium ions and two electrons give lithium ethylene dicarbonate (Li2EDC) and ethylene gas; the layer that forms has a dense inorganic inner part next to the graphite (Li2CO3, LiF, Li2O) and a porous organic outer part.2,3

How thick does it get? A crystalline layer only 2 to 3 nm thick would already stop electrons tunnelling through, but on graphite the SEI soon grows past 10 nm and reaches about 50 nm after cycling.2,4

Why the layer helps

We have paid lithium for this layer; let us see what we get back. What makes the SEI useful is its selectivity: it is electronically insulating, so it stops further reduction, yet it conducts Li+, so the graphite keeps filling.5 Of the main solvents, only ethylene carbonate forms a protective layer on graphite, which is why it is in almost every commercial electrolyte; propylene carbonate does not, and as late as 2001 the reason was still described as unclear.6

Why the difference? The explanation usually given, which goes back to Dahn and co-workers in 1990, is that EC decomposes first and builds an SEI that stops PC from slipping into the graphite together with the lithium;7 PC holds on to its Li+ more strongly, goes into the graphite with it, and peels the graphite layers apart.2

By 1994 the picture could be put simply: the layer forms during the first insertion, and after it intercalation is completely reversible.8 In chapter 16 we shall see that this is only nearly true. Conversely, an electrode that works inside the window needs no SEI. Lithium titanate at 1.5 V forms none, and in one report electrodes of lithium titanate and LiFePO4 survived 30 000 cycles at 5C.1

Steering the SEI on purpose

If the layer matters this much, we want to control it. Today the SEI is also steered on purpose, with small amounts of additives that decompose first and leave a thin, controlled layer. The best known is vinylene carbonate (VC): reduced at the graphite above 1 V vs Li/Li+, it forms a passivation layer, and it suppresses the decomposition of PC.9

Why new cells are “formed”. Because the SEI decides how the cell will age, a factory does not leave its growth to chance: near the end of making a cell, it is cycled slowly under precisely defined conditions, for up to a day, and let the gas out before the cell is sealed for good.10 It is expensive, about a third (32 %) of the cost of making a cell, mostly for keeping cells warm while they condition.3 And the first cycle is where the big loss happens: in one recent laboratory cell the first cycle returned about 82.5 % of the charge put in, the second about 97.5 %.3

Not only on graphite. A positive electrode pushed above the upper edge of the window, about 4.3 V, oxidizes the electrolyte instead; LiNi0.5Mn1.5O4 at about 4.75 V is protected by a passivation layer of its own, also called an SEI.1

The SEI costs most of its lithium on the first charge, and then lets the cell work outside its electrolyte’s window.

Figure 10.2 A cross-section of graphite layers with the SEI on their surface: an inner layer of inorganic lithium compounds and an outer layer of organic compounds. Steps add solvent pushed between the layers and metal ions arriving from the positive electrode. A temperature slider thickens the SEI above 45 degrees and makes lithium plate on the surface in the cold.
inorganic SEIorganic SEIgraphiteplated lithiummetal ions from the positive side
Graphite’s skin: the SEI, and the ways a graphite electrode ages. Try this: step through the three callouts, then slide the temperature from 25 °C to 55 °C and down to −10 °C.
SourceThe SEI and its mosaic model (inorganic Li2CO3, Li2O and LiF near the graphite, semi-carbonates and polyolefins further out), exfoliation by co-intercalated solvent in ethers, cross-talk from transition-metal ions, an even and porous SEI at 25 °C against a thick, non-porous one above 45 °C, and lithium plating in the cold, after Rahman et al. 2025, section 3, Table 3 and Figs. 4 and 5b. Layer thicknesses are schematic; the 5 °C threshold for plating is the figure’s, not the source’s.

Graphite, still the workhorse

Of the three mechanisms, intercalation is the one we met in chapter 7, so let us look at graphite first. Sony’s first lithium-ion cell of 1991 paired graphite with LiCoO2, and three decades later graphite, alone or with some silicon, is still the negative electrode of high-energy cells, now facing nickel-rich layered oxides.11 It is cheap, conducts electrons well, and lithium goes in at about 0.15 V with only 10 to 50 mV of hysteresis.11

Its one real weakness is that it works below the potential at which the electrolyte is stable. On the first charge the electrolyte is reduced on its surface and leaves the SEI (chapter 10), consuming lithium; once the layer is complete it protects the graphite and lets it cycle.11 Changes at this interface are a major cause of ageing in commercial cells, through five routes: exfoliation and cracking of the graphite, an SEI that keeps forming and dissolving, SEI conversion and growth, SEI dissolution, and lithium plating.11 Graphite particles also crack along their grain boundaries as they expand and shrink, exposing new surface that builds more SEI.11

Temperature decides which route dominates: hot, the SEI grows thicker and consumes lithium and electrolyte; cold, lithium plates on the graphite instead of entering it.11 We follow the growth of the SEI over time in chapter 16.

Where graphite comes from. Natural graphite costs about $8000 to $11 000 per tonne, synthetic graphite about $20 000; China produces nearly 75 % of the world’s graphite, which is why the United States and Europe are building their own supply. Demand is expected to reach about 1800 kt by 2030.11

Go deeper: why the SEI forms where it does

We can draw the electrolyte as having its own window: an energy gap between its lowest unoccupied and highest occupied molecular orbitals. A negative electrode whose electrochemical potential μA lies above the lowest unoccupied orbital reduces the electrolyte; a positive electrode whose μC lies below the highest occupied orbital oxidizes it, unless a passivating layer blocks the electron transfer. For the carbonate electrolytes the lowest unoccupied orbital sits about 1.1 eV below the lithium level and the practical limit at the top is about 4.3 eV below it, which on the voltage scale of chapter 3 is the window from about 1.1 to about 4.3 V vs Li/Li+. Graphite, at about 0.2 V, is outside it, and so works only behind its SEI.1

The orbital picture is a simplification. Real reduction and oxidation potentials depend on the salt and on the molecules around each one: estimates from isolated molecules can be far off: oxidizing EC next to PF6−, with a hydrogen passed to a neighbour, takes almost 2 V less than oxidizing an isolated EC molecule,12 and in an EC/EMC electrolyte reduction is measured to start at about 1.7 V with LiPF6 but about 0.8 V with LiBF4.2 Most solvents and salt anions are thermodynamically unstable where lithium inserts into graphite; the electrolyte survives there only kinetically, behind the insulating interphase it has built.12

What a good SEI has to do comes down to four things: block electrons, so that no more electrolyte is reduced; let Li+ through without much hindrance; stay chemically stable, neither reacting with nor dissolving into the electrolyte; and stay mechanically intact while the electrode under it swells and shrinks.2 Peled introduced the idea of an electronically insulating, ionically conducting interphase in 1979.2

References for this chapter

Numbers in the text refer to this list. All references of the course, with the chapters that use each one.

  1. 1 J. B. Goodenough, K.-S. Park, "The Li-Ion Rechargeable Battery: A Perspective", Journal of the American Chemical Society 135, 2013, 1167-1176. doi:10.1021/ja3091438
  2. 2 A. Wang, S. Kadam, H. Li, S. Shi, Y. Qi, "Review on modeling of the anode solid electrolyte interphase (SEI) for lithium-ion batteries", npj Computational Materials 4, 2018, 15. doi:10.1038/s41524-018-0064-0
  3. 3 K. Manmi, M. Tuchel, E. Kendrick, F. Brosa Planella, "A Comparison of Standard SEI Growth Models in the Context of Battery Formation", Journal of The Electrochemical Society 171, 2024, 100530. doi:10.1149/1945-7111/ad8548
  4. 4 B. Horstmann, F. Single, A. Latz, "Review on multi-scale models of solid-electrolyte interphase formation", Current Opinion in Electrochemistry 13, 2019, 61-69. doi:10.1016/j.coelec.2018.10.013
  5. 5 M. Winter, R. J. Brodd, "What Are Batteries, Fuel Cells, and Supercapacitors?", Chemical Reviews 104, 2004, 4245-4269. doi:10.1021/cr020730k
  6. 6 J.-M. Tarascon, M. Armand, "Issues and challenges facing rechargeable lithium batteries", Nature 414, 2001, 359-367. doi:10.1038/35104644
  7. 7 H. Zhang, C. Li, G. G. Eshetu, S. Laruelle, S. Grugeon, K. Zaghib, C. Julien, A. Mauger, D. Guyomard, T. Rojo, N. Gisbert-Trejo, S. Passerini, X. Huang, Z. Zhou, P. Johansson, M. Forsyth, "From Solid-Solution Electrodes and the Rocking-Chair Concept to Today's Batteries", Angewandte Chemie International Edition 59, 2020, 534-538. doi:10.1002/anie.201913923
  8. 8 K. Brandt, "Historical development of secondary lithium batteries", Solid State Ionics 69, 1994, 173-183. doi:10.1016/0167-2738(94)90408-1
  9. 9 K. Abe, "Nonaqueous Electrolytes and Advances in Additives", in T. R. Jow, K. Xu, O. Borodin, M. Ue (eds.), Electrolytes for Lithium and Lithium-Ion Batteries, Modern Aspects of Electrochemistry 58, Springer, New York, 2014, 167-207. doi:10.1007/978-1-4939-0302-3_3
  10. 10 M. Fichtner, K. Edström, E. Ayerbe, M. Berecibar, A. Bhowmik, I. E. Castelli, S. Clark, R. Dominko, M. Erakca, A. A. Franco, A. Grimaud, B. Horstmann, A. Latz, H. Lorrmann, M. Meeus, R. Narayan, F. Pammer, J. Ruhland, H. Stein, T. Vegge, M. Weil, "Rechargeable Batteries of the Future—The State of the Art from a BATTERY 2030+ Perspective", Advanced Energy Materials 12, 2022, 2102904. doi:10.1002/aenm.202102904
  11. 11 M. M. Rahman, U. Nisar, A. Abouimrane, I. Belharouak, R. Amin, "Valuation of Anode Materials for High-Performance Lithium Batteries: From Graphite to Lithium Metal and Beyond", Electrochemical Energy Reviews 8, 2025, 14. doi:10.1007/s41918-025-00249-w
  12. 12 O. Borodin, X. Ren, J. Vatamanu, A. von Wald Cresce, J. Knap, K. Xu, "Modeling Insight into Battery Electrolyte Electrochemical Stability and Interfacial Structure", Accounts of Chemical Research 50, 2017, 2886-2894. doi:10.1021/acs.accounts.7b00486

Cite this page

Using this page in teaching, a thesis or an article? Cite it as follows; the date is the last update of the course.

Shyma Sajeevan, A. (2026). The solid electrolyte interphase (SEI): formation, passivation and growth. How batteries work. https://asajeevan.github.io/batteries/sei/

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