Battery materials: how a lithium-ion cell works and what it is made of

Every battery cell stores electrical energy as chemical energy in two electrodes: a reductant, the anode, and an oxidant, the cathode. Between them an electrolyte carries the ionic part of the reaction inside the cell and forces the electronic part to flow through the outside circuit, where it does work (Goodenough & Park 2013).

In a lithium-ion cell both electrodes are hosts. The cathode is a framework into which Li⁺ is inserted reversibly over a range of compositions (Goodenough & Park 2013), and the anode is usually graphite, which takes lithium between its carbon layers up to LiC₆ (Eshetu & Figgemeier 2019). On charging, Li⁺ is pulled out of the cathode, crosses the electrolyte and enters the anode; on discharge it returns (Lin et al. 2026). This “rocking chair” shuttle (Qiao et al. 2023) is called intercalation.

Between the electrodes sits a porous separator, usually polyethylene or polypropylene: it takes no part in the chemistry but keeps the electrodes apart to prevent a short circuit while letting Li⁺ pass through the liquid electrolyte in its pores (Heidari & Mahdavi 2020).

The cell voltage is set by the difference between the redox energies of the anode and the cathode (Manthiram 2020), but its useful upper limit is set by the electrolyte window, the gap between the electrolyte’s LUMO and HOMO. Water limits a cell to about 1.5 V; lithium-ion cells use an organic electrolyte with a larger window (Goodenough & Park 2013), typically a lithium salt such as LiPF₆ in carbonate solvents (as in the cells of Hashem et al. 2019).

The graphite anode works at a potential outside that window, so it is protected by a passivating surface layer, the solid–electrolyte interphase (SEI), which must let Li⁺ through. Forming it consumes lithium from the cathode on the first charge, and it adds impedance (Goodenough & Park 2013). At high voltage a similar cathode–electrolyte interphase (CEI) grows on the cathode (Lin et al. 2026).

All three practical classes of oxide cathodes came from John Goodenough’s group, at Oxford and later in Texas, and remain the only practical cathode classes for lithium-ion batteries (Manthiram 2020):

• Layered oxides. In LiCoO₂, Li⁺ and Co³⁺ order on alternate planes of a close-packed oxide, giving fast two-dimensional lithium diffusion and about 4 V; oxygen loss beyond half delithiation limits it to about 140 mAh g⁻¹ (Manthiram 2020). NMC replaces much of the cobalt with nickel and manganese (Manthiram 2020; Hashem et al. 2019).
• Spinels. LiMn₂O₄ has a three-dimensional framework of edge-sharing MnO₆ octahedra with lithium in tetrahedral sites: 4 V, below 130 mAh g⁻¹, fast rates and cheap manganese, but Mn dissolves into the electrolyte and poisons the graphite anode (Manthiram 2020).
• Polyanion oxides. Phosphate, sulfate and silicate groups raise the voltage through the inductive effect; olivine LiFePO₄ was identified in 1997. They conduct electrons poorly, so the particles are made small and coated with carbon, and they are less dense, but oxygen bound to phosphorus makes them thermally stable and safe, and they use abundant iron (Manthiram 2020).

• Graphite is the commercial anode, with a theoretical capacity of 372 mAh g⁻¹ as LiC₆ (Lee et al. 2024; Eshetu & Figgemeier 2019). Pairing a lithium-containing oxide cathode with lithium-free graphite is what made the modern lithium-ion cell possible (Manthiram 2020).
• Silicon alloys with lithium to about 4200 mAh g⁻¹, roughly ten times graphite (Eshetu & Figgemeier 2019), but expands by about 300% (Shen et al. 2023), which pulverizes particles and keeps breaking the SEI (Zhang et al. 2022). Silicon–carbon composites, in which carbon buffers the expansion, are the main remedy (Zhang et al. 2022).
• Lithium metal has the highest capacity, 3860 mAh g⁻¹, and the lowest potential, −3.04 V vs SHE, but grows dendrites and forms an unstable SEI (Bae et al. 2025). Dendrites causing internal shorts already ended the first lithium-metal batteries of the 1970s (Manthiram 2020).

Commercial lithium-ion cells use organic liquid electrolytes, which need stringent safety precautions in large systems (Kamaya et al. 2011). The sulfide Li₁₀GeP₂S₁₂ reached 12 mS cm⁻¹ at room temperature, higher than liquid organic electrolytes, while being non-volatile and non-explosive (Kamaya et al. 2011). The three main families each have a weakness: sulfides conduct well but have a narrow electrochemical window and allow dendrites, oxides have a wide window but high interfacial resistance with lithium metal, and polymers are flexible but conduct poorly (Lee et al. 2024). All-solid-state batteries attract interest for lithium metal anodes because they promise higher energy density and better safety (Lee et al. 2024).

Concerns over the supply and price of lithium have revived interest in sodium-ion batteries: sodium is widely available and its chemistry is similar to lithium’s (Hwang et al. 2017). Sodium layered oxides, phosphates and organic compounds serve as cathodes (Hwang et al. 2017), and the layered and polyanion cathode classes developed for lithium are the basis of sodium cathodes too (Manthiram 2020). The anode is harder: the Na⁺ ion is large (1.07 Å) and neither graphite nor silicon stores it reversibly, so carbons, oxides, alloys and organic materials are studied instead (Qiao et al. 2023; Hwang et al. 2017).

• LiCoO₂ (lithium cobalt oxide). The layered cathode of the first commercial lithium-ion cells, Sony 1991 (Manthiram 2020); about 140 of its theoretical 274 mAh g⁻¹ are used at 4.2 V (Lin et al. 2026).
• NMC (LiNiₓMnᵧCo₁₋ₓ₋ᵧO₂). Layered oxides in which nickel and manganese replace much of the cobalt; more nickel means more capacity and less cobalt but less stability (Manthiram 2020). Nickel-rich NMC powers millions of electric vehicles (Lu et al. 2024).
• LiFePO₄ (lithium iron phosphate). The olivine polyanion cathode (1997): thermally stable, safe and iron-based, but less dense and needing carbon coating (Manthiram 2020).
• LiMn₂O₄ (spinel). A cheap manganese spinel with three-dimensional lithium paths and 4 V operation, limited by manganese dissolution (Manthiram 2020).
• Graphite anode. The standard anode, storing lithium between its layers up to LiC₆, 372 mAh g⁻¹ (Eshetu & Figgemeier 2019).
• Silicon anode. About 4200 mAh g⁻¹ but about 300% volume change, usually used in silicon–carbon composites (Eshetu & Figgemeier 2019; Shen et al. 2023; Zhang et al. 2022).
• Solid electrolytes. Sulfide, oxide and polymer ion conductors for all-solid-state batteries (Lee et al. 2024); Li₁₀GeP₂S₁₂ conducts 12 mS cm⁻¹ (Kamaya et al. 2011).
• Sodium-ion batteries. The same rocking-chair principle with abundant, low-cost sodium (Hwang et al. 2017; Qiao et al. 2023).

A clear lithium-ion schematic shows the cathode crystal on one side, the anode on the other, the electrolyte and separator between them, and arrows for Li⁺ moving through the electrolyte and electrons through the external circuit, reversed for charge and discharge. Draw the cathode as its crystal structure: alternating layers for LiCoO₂ and NMC, a 3D framework for the spinel, PO₄ tetrahedra and FeO₆ octahedra for LiFePO₄. The LabCanvas figure editor’s Library has battery crystal objects, including a layered oxide and NMC811, that drop into a figure as high-resolution vector drawings you can recolour and label, and the LabCanvas 3D Studio has battery models you can rotate, show as polyhedra and export as high-resolution images.