Intercalation

Everyday term

In plain words

Slipping atoms or molecules into the gaps between the layers of a layered material, like sliding cards between the pages of a book. It can change a material’s properties completely, or push the layers apart so they separate more easily. A battery charges this way, with lithium slipping in between the carbon layers of its graphite electrode.

Going deeper

Guests inserted between layers do not spread evenly: they fill some gaps completely and leave others empty, in ordered sequences called stages. What they leave behind is a material with different carrier density, sometimes a different phase, and always a wider spacing.

Guests between the layers

Because the gap between layers is held only by , atoms, ions and molecules can be pushed into it without breaking the layers themselves. Alkali metals donate electrons to the host; and acids take them away; solvent molecules and long organic ions simply prise the layers apart. The chemistry of graphite intercalation compounds, reviewed in detail by Dresselhaus and Dresselhaus in 1981, set the vocabulary for the whole field.

Insertion is done electrochemically, by driving ions in with a voltage, from solution, or from the vapour. It is usually reversible, which is exactly what a battery needs.

Staging, and what it means

Guests do not distribute themselves one per gap at low concentration. Instead they fill certain gaps completely and leave others untouched, in an ordered sequence: stage 1 has a guest layer in every gap, stage 2 in every second, stage 3 in every third. The reason is elastic – opening a gap costs energy, so it is cheaper to fill a few gaps fully than to open many slightly.

Stages are visible in as a new series of sharp peaks with a longer repeat, and they change colour and conductivity; stage-1 lithium-graphite, LiC₆, is gold. Tracking the stages during charging is a standard way to follow what a battery electrode is doing.

What it is used for

Three uses dominate. First, energy storage: lithium moving in and out of graphite is the negative electrode of a lithium-ion battery, and ion insertion between layers carries much of the charge in their . Second, : intercalation can raise carrier density far beyond what a gate can reach, which is how intercalated graphite and heavily doped are made. Third, : swelling the gaps with ions or solvent molecules weakens the coupling so that layers separate easily, which is the basis of electrochemical and chemical exfoliation routes to .

In MoS₂, lithium insertion also drives the structure into the 1T′ form, a that survives after washing and is used to make metallic contacts and . The trade-offs are damage and instability: aggressive intercalation introduces defects, and many intercalated phases revert or degrade in air.

For specialists

Insertion of guest species – ions such as Li⁺, molecules or solvents – into the of a layered host. It changes carrier density, stacking and phase, for example driving 2H-to-1T′ transitions in MoS₂; it enables electrochemical exfoliation and underlies ion storage in batteries and MXene supercapacitors.

Where this comes from

Intercalation compounds of graphite Dresselhaus and Dresselhaus · Advances in Physics 30, 139 (1981) cited by 2,126
Photoluminescence from chemically exfoliated MoS₂ Eda et al. · Nano Letters 11, 5111 (2011) cited by 3,889

Battery electrode

A battery stores energy by moving ions – in phones and cars, lithium ions – between two electrodes, the anode and the cathode. Charging pushes the ions into the anode, where they slip between the layers of a material such as graphite; using the battery lets them flow back. Layered and 2D materials matter because ions can enter and leave the gaps between layers, and because thin conductive flakes can help electrons reach every grain of an electrode.

A lithium- or sodium-ion cell shuttles ions through an electrolyte between a negative electrode (the anode on discharge) and a positive one (the cathode), while electrons pass through the external circuit. Graphite, the standard anode, intercalates lithium between its layers up to LiC₆, about 372 mAh/g; layered oxides such as LiCoO₂ and NMC dominate cathodes. 2D materials enter as conductive additives (graphene flakes replacing part of the carbon black), as higher-capacity anodes that alloy or convert (phosphorus, antimony, SnS₂, SnO), as hosts for larger ions such as sodium, which graphite takes up poorly, and as MXene electrodes. Their weaknesses are large volume change, irreversible first-cycle loss from the high surface area, low packing density and cost, so gains at cell level are usually far smaller than those claimed for the material.

Full explanation

Chalcogenide, halide and pnictide

Family names for compounds, taken from the element in them that carries the negative charge. Chalcogenides contain sulfur, selenium or tellurium; halides contain fluorine, chlorine, bromine or iodine; pnictides contain phosphorus, arsenic, antimony or bismuth. Most layered crystals belong to one of these families, and the catalogue groups many of them that way.

Compounds named for their most electronegative constituent: chalcogenides of group 16 (S, Se, Te; oxides are conventionally counted apart), halides of group 17 (F, Cl, Br, I) and pnictides of group 15 (P, As, Sb, Bi; nitrides usually keep their own name). Down each group the anion grows larger and less electronegative, so bonding turns more covalent, gaps shrink and spin–orbit coupling grows. Chalcogenides give the MX₂ semiconductors and metals, halides mostly ionic, often magnetic insulators built from edge-sharing octahedra, and pnictides many metals, semimetals and the parents of the iron-based superconductors; mixed-anion compounds such as oxychalcogenides and chalcohalides combine two.

Full explanation

Metal, semimetal and insulator

The three basic ways a solid treats electricity, set by how its electrons fill the energies open to them. In a metal they fill a band of energies only partly, so the smallest push sets them moving, and it conducts at any temperature. In an insulator they fill a band completely and a wide gap separates it from the next, so nothing can move. A semimetal sits in between: two bands just touch or overlap a little, leaving a small number of mobile electrons and holes. NbSe₂ is a metal, graphene a semimetal, hBN an insulator.

A classification by where the Fermi level falls in the band structure. In a metal it crosses a partly filled band, leaving a Fermi surface, a carrier density of order one per atom (about 10²² cm⁻³) and a resistivity that falls on cooling. In a band insulator it lies in a gap of several electronvolts; a semiconductor is an insulator whose gap is small enough to be doped or gated. A semimetal has a slight overlap between conduction and valence bands (graphite, bismuth, bulk WTe₂) or bands that touch at isolated points (graphene), giving low and nearly equal densities of electrons and holes. Interactions can open gaps that band theory does not predict: Mott insulators from half-filled bands, excitonic insulators from semimetals.

Full explanation

Photocatalysis and electrocatalysis

Using light, or an applied voltage, to drive a chemical reaction at a surface – most often splitting water to make hydrogen. Thin materials are natural candidates because nearly every atom is a surface atom, and because their edges are usually far more active than their flat faces.

Surface reactions driven by photons (photocatalysis) or by electrode potential (electrocatalysis), judged by overpotential, Tafel slope and turnover per site rather than by raw current. In MoS₂ the basal plane is close to inert while the edge sites carry the hydrogen-evolution activity – the result that made layered chalcogenides catalyst candidates – so activity scales with edge length, defect density and phase, and any comparison needs an active-site count rather than a geometric area.

Full explanation