# 8.5: Layered Structures and Intercalation Reactions

Layered structures are characterized by strong (and typically covalent) bonding between atoms in two dimensions and weaker bonding in the third. A broad range of compounds including metal halides, oxides, sulfides, selenides, borides, nitrides, carbides, and allotropes of some pure elements (B, C, P, As) exist in layered forms. Structurally, the simplest of these structures (for example binary metal halides and sulfides) can be described as having some fraction of the octahedral and/or tetrahedral sites are filled in the fcc and hcp lattices. For example, the CdCl2 structure is formed by filling all the octahedral sites in alternate layers of the fcc lattice, and the CdI2 structure is the relative of this structure in the hcp lattice.

In the CdCl2 structure, the stacking sequence of anion layers is ABCABC...
In the CdI2 structure, the anion stacking sequence is ABAB..., and all the cations are eclipsed along the stacking axis.

 Comparison of the CdCl2 (left) and CdI2 (right) crystal structures

These are examples of 6-3 structures, because the cations are coordinated by an octahedron of six anions, and the anions are coordinated by three cations to make a trigonal pyramid (like NH3). Another way to describe these structures is to say that the MX6 octahedra each share six edges in the MX2 sheets.

 Polyhedral drawing of one layer of the CdCl2 or CdI2 structure showing edge-sharing MX6 octahedra

Because these structures place the packing atoms (the anions) in direct van der Waals contact, they are most stable for relatively covalent compounds. Otherwise, the electrostatic repulsion between contacting anions would destabilize the structure energetically. More ionic MX2 compounds tend to adopt the fluorite (CaF2) or rutile (TiO2) structures, which are not layered.

Despite the fact that these two structure types are the same at the level of nearest and next-nearest neighbor ions, the CdI2 structure is much more common than the CdCl2 structure.

CdCl2 structure:

MCl2 (M = Mg, Mn, Fe, Co, Ni, Zn, Cd)

NiBr2, NiI2, ZnBr2, ZnI2

CdI2 structure:

MCl2 (M = Ti, V)

MBr2 (M = Mg, Fe, Co, Cd)

MI2 (M = Mg, Ca, Ti, V, Mn, Fe, Co, Cd, Ge, Pb, Th)

M(OH)2 (M = Mg, Ca, Mn, Fe, Co, Ni, Cd)

MS2 (M = Ti, Zr, Sn, Ta, Pt)

MSe2 (M = Ti, Zr, Sn, V, Pt)

MTe2 (M = Ti, Co, Ni, Rh, Pd, Pt)

Physically, layered compounds are soft and slippery, because the layer planes slide past each other easily. For example, graphite, MoS2, and talc (a silicate) are layered compounds that are used widely as lubricants and lubricant additives.

An important reaction of layered compounds is intercalation. In intercalation reactions, guest molecules and ions enter the galleries that separate the sheets, usually with expansion of the lattice along the stacking axis. This reaction is typically reversible if it does not perturb the bonding within the sheets. Often the driving force for intercalation is a redox reaction, i.e., electron transfer between the host and guest. For example, lithium metal reacts with TiS2, MoS2, and graphite to produce LiTiS2, LixMoS2 (x < 1), and LiC6. In these compounds, lithium is ionized to Li+ and the sheets are negatively charged. Oxidizing agents such as Br2, FeCl3, and AsF5 also react with graphite. In the resulting intercalation compounds, the sheets are positively charged and the intercalated species are anionic.

Intercalation reactions are especially important for electrochemical energy storage in secondary batteries, such as lithium ion batteries, nickel-metal hydride batteries, and nickel-cadmium batteries. The reversible nature of the intercalation reaction allows the electrodes to be charged and discharged up to several thousand times without losing their mechanical integrity. In lithium ion batteries, the negative electrode material is typically graphite, which is intercalated by lithium to make LiC6. Several different oxides and phosphates containing redox active transition metal ions (Mn, Fe, Co, Ni) are used as the positive electrode materials.

 Oxidative or reductive intercalation involves the placement of anions or cations between sheets.

Lithium ion batteries based on CoO2 were first described in 1980[1] by John B. Goodenough's research group at Oxford. In batteries based on CoO2, which has the CdI2 structure, the positive electrode half-reaction is:

$\ce{LiCoO2 \leftrightharpoons Li_{1-x}CoO_{2} + xLi^{+} + xe^{-}}$

The negative electrode half reaction is:

$\ce{xLi^{+} + xe^{-} + xC_{6} \leftrightharpoons xLiC6}$

The battery is fully charged when the positive electrode is in the CoO2 form and the negative electrode is in the LiC6 form. Discharge involves the motion of Li+ ions through the electrolyte, forming LixCoO2 and graphite at the two electrodes.

 Blue plaque erected by the Royal Society of Chemistry commemorating the development of cathode materials for the lithium-ion battery
 Crystal structure of LiCoO2[2]

The lithium ion battery is a "rocking chair" battery, so named because charging and discharging involve moving Li+ ions from one side to the other. CoO2 is one example of a positive electrode material that has been used in lithium ion batteries. It has a high energy density, but batteries based on CoO2 have poor thermal stability. Safer materials include lithium iron phosphate (LiFePO4), and LiMO2 (M = a mixture of Co, Mn, and Ni). These batteries are used widely in laptop computers, portable electronics, cellular telephones, cordless tools, and electric and hybrid vehicles.

A similar intercalation reaction occurs in nickel-cadmium batteries and nickel-metal hydride batteries, except in this case the reaction involves the movement of protons in and out of the Ni(OH)2 lattice, which has the CdI2 structure:

$\ce{NiO(OH) + H2O + e^{-} -> Ni(OH)2 + OH^{-}}$

There are many layered compounds that cannot be intercalated by redox reactions, typically because some other stable product is formed. For example, the reaction of layered CdI2 with Li produces LiI (NaCl structure) and Cd metal.