How batteries work Chapter 2 Cell anatomy

Chapter 2

Anatomy of a lithium-ion cell: components, cell geometry and the rocking-chair principle

If we cut open the battery in a phone, what would we find?

Where we are. Chapter 1 showed the four jobs in a cell: give electrons, take electrons, carry ions, keep the two electrodes apart. Now the parts that do those jobs in a real cell, and why they have the shapes they have.

Figure 2.1 A coin cell drawn in cross-section and pulled apart into its layers: aluminium-coated can, positive electrode, separator soaked with electrolyte, graphite negative electrode on copper, stainless-steel spacer, spring, and cap with gasket; a small top view shows the round shape; a strip below shows the potential jumping at the two electrodes.
positive active materialseparatorelectrolyte in the poresgraphitecopper foilaluminium foilsteel can and cappotential
A coin cell, the kind used to test new materials in research laboratories, cut through the middle and pulled apart layer by layer. Try this: hover or tap each layer to read its job; drag the slider to close the stack; then find the two layers that decide the voltage.
SourceStack order, materials and the assembly lessons from Murray, Hall and Dahn 2019, Figure 1 and text (electrolyte 1 mol/L LiPF6 in EC:DEC 1:1, about 38 mg per cell); jobs from the definitions in Winter and Brodd 2004, section 1.2 and 2.2; copper and aluminium collectors from Goodenough and Park 2013, Figure 1. Thicknesses not to scale; a CR2032 coin cell is 20 mm across (the number in its name). Potential strip after Bard, Faulkner and White 2022, Figure 1.1.2; the split of the voltage between the two jumps is schematic.

Every cell has the same four jobs. The negative electrode gives up electrons: it is a good reducing agent, such as lithium, zinc or lead. The positive electrode takes them: an electron acceptor, such as lithium cobalt oxide, manganese dioxide or lead oxide.1 The electrolyte “provides pure ionic conductivity” between the two: it lets ions through and blocks electrons.1

The separator is a porous barrier, permeable to ions and inert in the cell, that keeps the electrodes from touching.1 If they touched, the whole stored energy would be released as heat inside the cell.1

Anode or cathode? You will meet both words, and they name a reaction, not a piece of metal: the anode is the electrode where oxidation releases electrons into the external circuit, the cathode the electrode where reduction takes them back.1 On discharge the negative electrode is oxidized and the positive electrode is reduced, so the negative electrode is the anode and the positive electrode the cathode; that is the battery convention.1 On charge the charger drives the current the other way:1 electrons are pushed into the negative electrode, which is now reduced, and pulled out of the positive electrode, which is now oxidized, so by the reaction definition the names swap, and the graphite that was the anode while you used the phone is the cathode while it charges. The metals stay where they are and keep their signs; only the labels move. To avoid the confusion this page says negative electrode and positive electrode from here on.

In the coin cell above, graphite sits on copper foil and the lithium host on aluminium, exactly as in the first commercial lithium-ion cell.2 Each foil is a current collector: a metal path for the electrons between the coating and the terminal.2 The electrolyte is a liquid, added drop by drop during assembly: in Murray, Hall and Dahn’s research cells, about 38 mg of a 1 mol/L solution of LiPF6 in a carbonate solvent, enough to soak the separator and wet both electrodes.3

New positive-electrode materials are usually tested in a “half cell” against a piece of lithium foil; such cells are easy to make but can fail to predict how the material behaves against graphite in a full cell, which is why full coin cells like this one are built.3

Every part has one job, and the electrolyte’s job is to let ions through and block electrons.

Figure 2.2 A flat stack of copper foil, graphite, separator with electrolyte, lithium cobalt oxide and aluminium foil, which winds itself into a spiral as the slider moves, showing that a cylindrical cell is the same stack rolled up.
copper foilgraphiteseparator and electrolyteLiCoO2aluminium foil
Why cells are built from thin, wide layers, and how the layers fit in a can. Try this: drag the slider slowly and watch the five layers keep their order as they curl.
SourceLayer order and collectors after Goodenough and Park 2013, Figure 1, and their argument that a cell needs large-area electrodes and a thin electrolyte; wound construction after Winter and Brodd 2004, Figure 20. Schematic, not to scale.

Ions move through an electrolyte far more slowly than electrons move through a metal (chapter 0 showed why: an ion drags its way through a liquid at a terminal speed set by its mobility). So a cell is built with large-area electrodes separated by a thin electrolyte, and metallic current collectors carry the electrons between the electrodes and the terminals.2 A cylindrical cell is exactly that stack, wound into a roll; coin, prismatic and flat cells hold the same layers in other shapes.1,4

Worked example. Why area matters: the resistance of a layer is R = ρl/A (chapter 0).5 Keep the electrolyte layer as thin as the separator allows (small l) and make the electrodes as wide as will fit (large A), and the ionic resistance of the whole cell falls with both. A wound strip is the way to put a large A inside a small can.
Figure 2.3 Four cell formats drawn side by side in their real materials: a steel coin cell, a cylindrical cell in a steel can, a prismatic cell in an aluminium case with a vent, and a flat pouch cell in a silvery laminate with two tabs. Choosing a format shows how its name is decoded, where its name encodes size.
stainless or nickel-plated steelaluminiumcopper tabprinted wrap
Four formats, and how their names are read. Try this: decode CR2025 and 504050, then see why the cylindrical code is left alone.
SourceFormats, their construction and the coin and prismatic codes after Arote 2022, section 1.7; the response to vibration after Faruque et al. 2026, section 8.5. Shapes drawn in proportion only roughly; materials in their real colours.

Four formats

The same layers come in four common shapes, and some of them are named by their size.6 A coin cell is a flat stainless-steel case, wider than it is high, with a gasket, a cap, a spacer plate and a spring holding the layers together; in the laboratory its negative electrode is often lithium foil. Its name, standardized by the IEC, is a code: in CR2025, C stands for a lithium chemistry, R for round, 20 for the diameter in millimetres and 25 for the height in tenths of a millimetre, so 2.5 mm. CR2025 is the most common design at laboratory scale.6

In a cylindrical cell the electrodes and separator are wound into a spiral inside a cylindrical case, the roll of figure 2.2: good cycling stability, but round cells pack less densely. Its five-digit name, such as 18650 or 21700, gives its size. A prismatic cell holds large stacked sheets in a rectangular case of aluminium, steel or semi-hard plastic, usually with a vent, and suits uses where swelling matters; its six digits give the thickness in tenths of a millimetre, then the width and the length in millimetres, so 504050 is 5.0 by 40 by 50 mm. A pouch cell stacks its electrodes in a sealed, soft, flat case: light and cheap to make.6

Shape matters in use as well. In electric vehicles, vibration and shock speed up the loss of capacity and the rise of resistance, through the loss of active material and of lithium inventory of chapter 16; a 2026 review reports cylindrical 18650 cells as particularly susceptible, and pouch and prismatic cells as more tolerant when tightly packed.7

More names, for reference. A coin cell’s shape letter can be R for round, F for flat, S for square or P for not round; besides CR2025 there are CR2477, CR1616, CR1620, CR1632, CR2016 and CR2032, and such cells power watches, toys, LED lights, thermometers, heart-rate and glucose monitors, calculators, remote controls and car keys.6 Cylindrical cells may carry the letters LIR, for lithium-ion rechargeable; besides the 18650 there are 14500, 14650, 18500, 21700, 26650 and 32650 cells, used in security and communication systems, medical equipment, laptops, e-bikes and power tools.6

Figure 2.4 A zoom into an electrode coating on its foil: irregular active particles, a connected network of carbon dots linking every particle to the foil, short binder strands at the contacts, and pores filled with electrolyte; electrons arrive along the carbon from the foil and a lithium ion arrives through the pores from the separator side, meeting at one particle.
active materialconductive carbonbinderpores with electrolyteelectronour ion
An electrode is a coating, not a block. Try this: follow our ion from the top down through the pores, and the electrons up the carbon from the foil, and find the particle where they meet.
SourceComposition and the porosity of about 30 % from Winter and Brodd 2004, section 1.6; the two roads after their Figure 9. Schematic, not to scale; particle shapes are illustrative.

An electrode is not a solid block of the active material. It is a coating: particles of active material, a conductive diluent such as carbon powder, and a polymer binder that holds the mixture together and bonds it to the current collector. Typically about 30 % of its volume is pores, and the pores are filled with electrolyte.1

The porous structure extends the surface area where the reaction can happen and lowers the current density, so the cell works more efficiently.1 A particle can only react where electrons reach it through the carbon and ions reach it through the electrolyte; a later module returns to what happens where one of them does not.

Figure 2.5 The layers of a lithium-ion cell pulled apart: aluminium foil, positive electrode, separator, negative electrode and copper foil, with the electrolyte that fills the pores drawn behind the middle three. Choosing a layer lists the materials it is commonly made of, and one option for the next generation.
What each layer can be made of. Try this: find the layer whose menu holds four salts and five solvents, then the one that a carbide might replace.
SourceComponents and materials after Arote 2022, sections 1.4.2, 1.8 and 1.9; that list gives LiMn2O4 twice, as spinel manganese oxide and as LMO, and it appears here once. Layer order as in figure 2.2. Schematic, not to scale; the layers are drawn in the colours of their materials.

What each part is made of

We have met the parts of a lithium-ion cell one at a time. Let us put names to the materials. The positive electrode is typically a layered lithium metal oxide such as LiCoO2, coated on aluminium foil; the negative electrode is carbon or graphite on copper foil; the electrolyte is a lithium salt in organic solvents; and the separator is a polyethylene or polypropylene membrane.6 The positive electrode needs lattice sites that take lithium in and give it back reversibly, and the larger the difference in electrochemical potential between the two electrodes, the higher the energy of the cell.6

Each part has a menu. Positive electrodes: LiCoO2, the spinel LiMn2O4, LiFePO4, the nickel-manganese-cobalt oxides (NMC), LiNi0.8Co0.15Al0.05O2 (NCA) and layered dichalcogenides. Negative electrodes: graphitic and non-graphitic carbons, silicon-based alloys, tin alloys such as Cu6Sn5, Ni3Sn2 and Co3Sn2, and the titanium oxides Li4Ti5O12 and TiO2. Electrolytes: LiPF6, LiBF4, LiClO4 or LiAsF6 in a mixture of PC, EC, DMC, DEC and EMC, or, in a lithium polymer cell, a gel based on PEO, PAN, PVDF or PMMA. Separators: microporous polypropylene, non-woven mats, ceramic-based materials and copolymers of vinylidene fluoride. Current collectors: aluminium and copper, as foil, mesh or foam, etched or coated.6

Which way next? The same textbook names layered lithium-rich positive electrodes, silicon-based negative electrodes, electrolytes that are more stable and conduct ions better, and current collectors based on titanium carbide (MXene), lighter, cheaper and flexible. It adds recycling, both to cut electronic waste and as a second source of materials (chapter 18).6

Figure 2.6 The rocking-chair cell: graphite sheets on copper foil on the left, LiCoO2 layers on aluminium foil on the right, electrolyte between. On discharge lithium ions leave the gaps between the carbon sheets, cross the electrolyte and slip between the oxide layers while electrons go round through a lamp; on charge a charger drives both back. A bar shows that the same lithium only moves from one side to the other.
Li+ (our ion is labelled)electronCoO2 layergraphite sheetcopper foilaluminium foil
The rocking chair: graphite against LiCoO2, today’s form of the carbon | LiCoO2 cell that Sony put on the market in 1991 (Sony’s first cell used a coke-type carbon). Try this: let it run until the graphite is empty, then press Charge and follow our ion back home.
SourceAfter Tarascon and Armand 2001, Figure 2b, and Goodenough and Park 2013, Figure 1 (graphite on copper, LiCoO2 on aluminium). Insertion and intercalation as defined by Winter and Brodd 2004, section 2.4. The charger is drawn above the open-circuit voltage as in Winter and Brodd, Figure 11. Twelve ions stand for the movable lithium; numbers of ions, sites and layers are not to scale. Schematic.

Let us open your cell and look at the two electrodes. The first thing to notice is that neither of them is used up. Both are insertion hosts: solids that conduct ions and electrons and let guest ions in and out by a solid-state redox reaction. Because their structure and shape survive, insertion electrodes are highly reversible; for layered hosts the process is called intercalation.1

This is what sets the cell apart from its predecessors. Replacing their lithium-metal negative electrode with a second insertion host gave the lithium-ion or rocking-chair cell: demonstrated by Murphy and by Scrosati, put on the market by Sony in June 1991 as carbon against LiCoO2.4 In the graphite, lithium sits between the carbon sheets, up to one lithium per six carbons, LiC6.2

In LiCoO2 it sits between the oxide layers, and here the host sets a limit: LiCoO2 still gives up only about half its lithium in practice,2,4 about 135 to 140 mAh/g,8,9 because its structure degrades at high states of charge:10 once more than about 0.55 of the lithium is removed, the oxide starts to give off oxygen.2,4 Consumer cells have been pushing the upper cut-off voltage higher to win more capacity.8

Worked example. Let us count the travellers. How many ions rock in a 3 Ah phone cell on each full discharge? charge = 3 Ah × 3600 s/h = 10 800 C ions = 10 800 C ÷ 1.602 × 10−19 C = 6.7 × 1022, that is 10 800 ÷ 96 485 = 0.112 mol of lithium mass of that lithium = 0.112 mol × 6.94 g/mol = 0.78 g Less than a gram of lithium travels back and forth; the rest of the cell is the hosts, the electrolyte and the packaging that let it do so.4,5

On charge the cobalt is oxidized, Co3+ to Co4+, as lithium leaves LiCoO2 for the graphite; on discharge it is reduced again, and the process is fully reversible.6 The electrolyte only passes the lithium along: its lithium content stays the same whatever the state of charge, while that of the two electrodes changes.6 That is the rocking chair. While current flows the salt is unevenly spread across the cell, as figure 9.2 shows, but none is gained or lost.

Nothing is consumed in an ideal lithium-ion cell: the ions only change address.

Figure 2.7 A timeline from 1972 to 1996 with nine events: the Belgirate conference of 1972; lithium against TiS2 in 1976; the rocking-chair proposal in the late 1970s and the first rocking-chair cell soon after; LiCoO2 at 4 volts around 1980; carbon negative electrodes in 1983; the protective film from ethylene carbonate on graphite in 1990; Sony's cell in June 1991 at about 80 Wh/kg; the LiPF6 in EC/DMC electrolyte in 1993; LiFePO4 in 1996.
From a chemist’s curiosity to a product, and on to today’s cell. Try this: step through the nine events and find the one that removed lithium metal from the cell, and the one that made graphite usable.
SourceEvents, names, years and values after Zhang et al. 2020 (the history from solid-solution electrodes to today’s cells), Goodenough 2018, Shanmukaraj et al. 2020 and Brandt 1994, consistent with Goodenough and Park 2013 and Tarascon and Armand 2001. Zhang and co-workers give the 1991 cell as 80 Wh/kg and 200 Wh/L; Brandt’s 1994 table of carbon | LiCoO2 cells of 1990 to 1993 agrees (64 to 100 Wh/kg). TiS2 at 2.2 V after Goodenough and Park 2013.

We now know what is inside the cell: two hosts, a rocking guest and a protective layer. How did it come together? The pieces arrived one at a time. In 1972, at a conference at Belgirate, Steele singled out layered dichalcogenides such as TiS2 as positive electrodes and Armand set out what an intercalation electrode needs; in 1976 lithium went reversibly into TiS2 against a lithium-metal electrode.11,12 Exxon’s cell then failed on dendrites that shorted it and ignited the electrolyte.13 Armand’s answer in the late 1970s was the rocking chair, and Lazzari and Scrosati built the first one, LiₓWO2 against TiS2, at about 2 V.11,12

Around 1980 Goodenough’s group showed that over half the lithium could be taken out of LiCoO2 reversibly, at about 4 V, so that a cell could be built discharged.11,13 In 1983 Yoshino found that petroleum coke cycled in a carbonate electrolyte and proposed carbon against LiCoO2; Sony’s cell of June 1991 delivered about 80 Wh/kg and 200 Wh/L, more than any other rechargeable of the time.11

With the LiPF6 in EC/DMC electrolyte proposed in 1993 and decades of refinement, the same chemistry reached about 250 Wh/kg and 600 Wh/L by 2020.11 Today, cells with NMC-type (lithium nickel manganese cobalt oxide) positive electrodes typically reach 300 Wh/kg and above 700 Wh/L, while state-of-the-art LiFePO4 cells rose from 90 Wh/kg in 2010 to 160 Wh/kg in 2022.10 Since its launch the lithium-ion cell has gained a factor of three to four in energy density while its price fell by a factor of 18.14

You can now explain the parts of a cell and their jobs, why the separator must never let the electrodes touch, why cells are built from thin, wide layers, and why an electrode is a porous coating rather than a block.

Two of those parts decide how hard the push is.

Go deeper: what a research coin cell taught its makers

Murray, Hall and Dahn built full coin cells with graphite and a nickel-rich oxide and found that three “identical” cells kept 95 %, 90 % and 80 % of their capacity after 100 cycles. Taking them apart showed the cause: the positive and negative discs were slightly misaligned, and lithium had plated on the parts of the positive electrode not covered by the negative one. The spring and spacer had been dropped in at an angle from tweezers; placing them from directly above with a vacuum pen, and using a thicker, more compliant separator, brought the cells to the reproducibility of machine-made pouch cells.3 Why plated lithium is lost for good is a story for a later module.

The mathematics of this chapter

Rion = l / (κA)the ionic resistance of the electrolyte layer between the electrodes: thin (small l) and wide (large A) is the whole design brief of figure 2.2. Winter and Brodd give κ of order 1 S/cm for aqueous and 10−2 to 10−3 S/cm for the organic electrolytes of lithium cells.

κeff = κ εα, α ≈ 1.5inside a porous electrode only the pore fraction ε (about 0.3, figure 2.4) carries ions, along winding paths; the Bruggeman relation gives the effective conductivity of the electrolyte in the pores. Boz and co-workers quote α = 1.5, and 1.53 measured for LiFePO4 electrodes.

Deff = D ε/τ, τ = ε1−αthe same correction for diffusion, with τ the tortuosity of the pore network

With ε = 0.3 and α = 1.5, κeff = 0.31.5 κ = 0.16 κ: the electrolyte inside a coating conducts about a sixth as well as the free liquid. The module on composite electrodes, still to come, builds on this.

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 M. Winter, R. J. Brodd, "What Are Batteries, Fuel Cells, and Supercapacitors?", Chemical Reviews 104, 2004, 4245-4269. doi:10.1021/cr020730k
  2. 2 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
  3. 3 V. Murray, D. S. Hall, J. R. Dahn, "A Guide to Full Coin Cell Making for Academic Researchers", Journal of The Electrochemical Society 166, 2019, A329-A333. doi:10.1149/2.1171902jes
  4. 4 J.-M. Tarascon, M. Armand, "Issues and challenges facing rechargeable lithium batteries", Nature 414, 2001, 359-367. doi:10.1038/35104644
  5. 5 A. J. Bard, L. R. Faulkner, H. S. White, "Electrochemical Methods: Fundamentals and Applications", Wiley, 3rd edition, 2022.
  6. 6 S. A. Arote, "Fundamentals and perspectives of lithium-ion batteries", chapter 1 of Lithium-ion and Lithium–Sulfur Batteries: Fundamentals to Performance, IOP Publishing, 2022, pp. 1-1 to 1-26. doi:10.1088/978-0-7503-4881-2ch1
  7. 7 O. Faruque, M. J. Isalm, O. Faruq, M. M. Alam, "A comprehensive review of past and present developments of Li-ion batteries", Discover Sustainability 7, 2026, 881. doi:10.1007/s43621-026-02920-8
  8. 8 J. T. Frith, M. J. Lacey, U. Ulissi, "A non-academic perspective on the future of lithium-based batteries", Nature Communications 14, 2023, 420. doi:10.1038/s41467-023-35933-2
  9. 9 A. Manthiram, "A reflection on lithium-ion battery cathode chemistry", Nature Communications 11, 2020, 1550. doi:10.1038/s41467-020-15355-0
  10. 10 J.-J. Marie, S. Gifford, "Developments in lithium-ion battery cathodes", Faraday Insights, Issue 18 (The Faraday Institution), 2023, September 2023. link
  11. 11 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
  12. 12 D. Shanmukaraj, P. Ranque, H. Ben Youcef, T. Rojo, P. Poizot, S. Grugeon, S. Laruelle, D. Guyomard, "Review—Towards Efficient Energy Storage Materials: Lithium Intercalation/Organic Electrodes to Polymer Electrolytes—A Road Map (Tribute to Michel Armand)", Journal of The Electrochemical Society 167, 2020, 070530. doi:10.1149/1945-7111/ab787a
  13. 13 J. B. Goodenough, "How we made the Li-ion rechargeable battery", Nature Electronics 1, 2018, 204. doi:10.1038/s41928-018-0048-6
  14. 14 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
  15. 15 B. Boz, T. Dev, A. Salvadori, J. L. Schaefer, "Review: Electrolyte and Electrode Designs for Enhanced Ion Transport Properties to Enable High Performance Lithium Batteries", Journal of The Electrochemical Society 168, 2021, 090501. doi:10.1149/1945-7111/ac1cc3

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). Anatomy of a lithium-ion cell: components, cell geometry and the rocking-chair principle. How batteries work. https://asajeevan.github.io/batteries/cell-anatomy/

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