Battery electrode

Everyday term

In plain words

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 matter because ions can enter and leave the gaps between layers, and because thin conductive can help electrons reach every grain of an electrode.

Going deeper

On charging, lithium ions move from a layered oxide into the gaps between the graphene sheets of a graphite anode, and back on discharge. 2D flakes help today as conductive additives that wire up every grain of the electrode; anodes that store far more charge per gram also swell far more, so a battery has to be judged as a whole cell.

Ions between layers

Every rechargeable lithium-ion battery already relies on layered materials. Its anode is usually graphite: on charging, lithium ions leave the cathode, cross the electrolyte and slip between the graphene sheets of the graphite – up to one lithium for every six carbon atoms – while electrons arrive through the external circuit. On discharge they flow back. The cathode is itself typically a layered oxide such as lithium cobalt oxide, from which lithium can be removed and put back. This – moving ions into and out of the gaps between layers without breaking the layers – is what lets a cell be charged hundreds or thousands of times.

Sodium is cheaper and more plentiful than lithium, but sodium ions sit poorly in graphite, which is one reason other layered and 2D hosts are studied for sodium-ion cells.

What 2D materials offer

The most successful use so far is modest: a small amount of graphene flakes added to an electrode as a , alongside or instead of carbon black, giving electrons a path to every grain. More ambitious are anodes that hold more lithium or sodium than graphite. Black phosphorus, antimony and bismuth nanosheets, SnS₂ and SnO react or with the ions and store several times more charge per gram. and other conductive layered compounds can also store charge quickly at their surfaces, blurring the line between batteries and .

The catch is that materials that take up much more charge also swell much more – phosphorus to about four times its volume – and crack, while their large surfaces use up lithium in a surface film during the first charge.

Reading the claims

Headlines about a graphene battery with several times the capacity usually mean a normal lithium-ion cell with a little graphene in it, or a capacity measured per gram of active material in a small laboratory cell. What decides a real battery is the energy per kilogram and per litre of the whole cell, the number of cycles, the loss on the first cycle, how fast it can be charged and what it costs. Thin flakes pack loosely, so a high capacity per gram can turn into a disappointing capacity per litre, and gains measured in a coin cell often shrink in a full cell.

For specialists

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.

Where this comes from

The Li-ion rechargeable battery: a perspective Goodenough and Park · Journal of the American Chemical Society 135, 1167 (2013)
Graphene, related two-dimensional crystals, and hybrid systems for energy conversion and storage Bonaccorso et al. · Science 347, 1246501 (2015)
A phosphorene–graphene hybrid material as a high-capacity anode for sodium-ion batteries Sun et al. · Nature Nanotechnology 10, 980 (2015)

Composites and coatings

The biggest market for 2D materials so far: flakes of graphene or other layered materials mixed in small amounts into plastics, rubber, concrete, paints or battery electrodes, to make them stiffer, tougher, more conductive or less permeable. Sold by the tonne, these products use cheap flakes made in bulk rather than perfect single layers. Whether a few percent of flakes really helps depends on how well they are spread out, how they line up and how firmly they hold on to the material around them.

Bulk uses in which exfoliated or chemically derived flakes – graphene nanoplatelets, reduced graphene oxide, hBN, MXenes, clays – are dispersed in a matrix or applied as a film. In polymer composites a few weight percent can raise stiffness and toughness, cross the percolation threshold for electrical conduction – as low as about 0.1 percent by volume for thin, wide flakes – and raise thermal conductivity; aligned flakes in barrier coatings lengthen the diffusion path of water, oxygen and ions and slow corrosion; flakes used as conductive additives replace part of the carbon black in battery electrodes; MXene films give strong electromagnetic shielding. Performance depends on flake size and thickness, dispersion without restacking, alignment and bonding to the matrix, and on powders whose properties vary widely between suppliers.

Full explanation

Alloy and solid solution

A crystal in which two similar kinds of atom share the same sites at random – some molybdenum sites taken by tungsten, say, or some sulfur sites by selenium – so that one crystal structure holds a mixture. By choosing the mix, its properties can be set almost anywhere between those of the two pure compounds, much as mixing two paints gives any shade between them. Chemists call it a solid solution; alloy is the everyday word.

A single-phase crystal in which two or more species share a sublattice at random, such as Mo_1−xW_xS₂ or MoS_2(1−x)Se_2x, with lattice constants close to Vegard’s law and band gaps that vary continuously with x, often with a small bowing. Complete miscibility needs similar radii, the same structure type and favourable mixing energetics; otherwise the mixture separates into phases or orders. Random disorder adds alloy scattering and inhomogeneous broadening.

Full explanation