Anthracite For The Production Of Batteries: Casings And Anodes
Anthracite The Material
Anthracite is one of the purest and highly valued forms of coal. As a carbonaceous material that has been millions of years in the making, its high energy density leads it to be the preferred type of coal for power generation applications. Aside from other wide-ranging applications in refractories and the direct manufacture of commodity and laboratory chemicals(1), anthracite finds essential uses as vital components in both traditional and contemporary battery technologies. The internal structure of anthracite is composed of graphene-like sheets, forming packets with clearly defined orientations(2). Across all applications, anthracite’s purity straight out of the ground, wide availability and low price make it an attractive material to use.
Density is the ongoing scourge of the modern battery maker. Heavy batteries mean a lower overall energy density given a constant charge state. As such, low-density materials as fillers for the manufacture of casings/electrodes are targeted. Anthracite has a density of 1.3 to 1.8 g/cm3, which is less than carbon black and less than half that of aluminium oxide - making it a good choice even before considering other advantages(3). Utilising low-density anthracite can substantially bring down battery weight, compared to an average density across all common fillers of 2.6 to 2.9 g/cm3.
For any application involving the storage of electrical energy, it is crucial to ascertain conductivity and resistivity for anthracite. Naturally, conductivity can vary as a function of temperature, but battery cells are closed systems so for all intents and purposes, once sealed within a cell conductivity of a particular material will remain constant. An important consideration, too, for coatings/casings is whether short circuiting is a possibility. Anthracite in untreated form is a poor electrical conductor, rendering it suitable for casings with little to no processing aside from grinding(4). For electrode applications, calcination (and thus subsequent graphitisation) is required to improve electrical conductivity. Phenomena such as the Rapoport effect are considered as non-issues for these applications(5).
Strongly related to hardness is the idea of workability, as in whether a product can be machined/milled into a usable material(6). Overall, anthracite is not regarded as a ‘hard’ material, with a Mohs hardness in the region of 2-3, compared to diamond at 10. Anthracite - before and after calcination/graphitisation - is easily worked with. Anthracite has reduced levels of hydrophobicity compared to some carbon forms(7).
Applications In Casings
Perhaps counterintuitively, anthracite has been utilised in battery casings as a vital filler component in the formation of certain types of synthetic rubber. Rubber - and similar plastics - have been long used for the containment of lead acid-type batteries used in the automotive and aviation sectors. Such high capacity, rechargeable cells require casings that are long lasting and relatively hard(8). An element of brittleness in such applications can be tolerated as the batteries themselves tend to be well supported, not be load bearing, and have little to no mechanical forces exerted on them. Anthracite is a suitable material as it is electrically resistive, in addition to providing many other benefits.
Rubber And Plastics
Rubber is composed of polymerised isoprene sourced from the latex of Hevea brasiliensis, whereas synthetic rubber uses a man-made isoprene obtained from petroleum products. Bulking agents, known as fillers, are commonly used to make rubber go further, add unique properties and to aid in manufacturing.
Ground anthracite is a popular choice of filler owing to its easy workability and low cost, whilst being largely contaminant free at source - anthracite being the purest form of coal. Milled anthracite is added to rubber manufacture in amounts of up to 80% by weight. Such additions not only render the new material in a black colouration, but ensure low weight on account of anthracite’s low density. Anthracite has been used as an effective filler in hard rubber/plastics since at least the early 1940s(9).
The Impact Of Anthracite On Rubber
Aside from the bulk effects of acting as a filler, i.e. providing most of the substance’s physical size, the use of anthracite as a filler for rubber causes a notable increase in tensile strengths in a variety of rubbers. Research has shown that in styrene-butadiene type synthetic rubbers, milled anthracite at a particle size of ca. 3 µm used at a 30% by weight quantity afforded a rubber with tensile strengths some 15% greater than when carbon black had been used as a filler(10). Authors note, however, that strength increases with greater anthracite loading to an extent - brittleness becomes noticeable at the highest anthracite levels.
A notable benefit of using anthracite as a filler for rubber and similar thermoplastics is the increased thermal stability of the finished material, resulting from an in situ generation of a polymer-carbonaceous material composite (11). Thermal stability in the finished rubber/plastic is of particular utility to materials used for battery casings as the batteries are often found in warm environments, such as in the engine bay of an automobile, and so long-term resistance to temperature is valued for ongoing battery life without leakages. Compared to other - lower - coal types, anthracite is much more easily milled to the ultra-fine particle sizes required(12) and it enjoys a physical structure similar to graphite(13), therefore requires little processing ahead of use.
Formation of rubber using high proportions of carbonaceous material such as anthracite fillers does lead to a degree of pore formation, but these are not penetrative through the bulk material, and often have pore sizes smaller than 2 nm(14) and so is not recognised as a problem. In fact such nano/micropores are advantageous; rubber is formed by a process known as vulcanisation, and the elastomers formed by vulcanisation fill the pores(15).
When forming composites like rubber, tensile stress is reduced by the rubber/plastic’s molecular chains ‘sliding’ on the milled anthracite when put under external stress - such ‘sliding’ reduces the overall stressful impacts and thus can be said to reinforce the structure(16). It is reported that milled anthracite has a stronger binding interaction with rubber molecules than other carbonaceous materials.
Considerations In Applications
Typical grind size for anthracite for rubber is 325 mesh (44 μm)(17). Crucial for longevity of service life - especially in the automotive sectors - is using the highest quality carbon for rubber/plastic battery container production. Anthracite is recognised as such a material, and affords strength properties to the overall casing. Notwithstanding this, batteries with carbon-rubber/plastic casings should be secured by more than one mounting point to prevent undue stress(18), if stresses are likely to occur, especially with high carbon contents exceeding 75%. It is postulated that electrical conductivity in rubber casings is increased via the inclusion of secondary fillers such as clay, and thus these should be avoided(19).
Applications In Anodes
A rapidly-growing use for ground and milled anthracite is in the production of high-performance, low-weight anodes for modern battery systems. Relying on anthracite that is highly conducting (achieved by calcination and graphitisation, see below), emerging technologies use the material frequently as anodes in premium lithium-ion cells(20), through to electric vehicle batteries(21) and high-end sodium-vanadium-phosphate cells(22) which claim to afford enhanced levels of energy density and rapid charging capabilities. Such usage manifolds depend upon the clearly defined molecular structure of anthracite, its low density thus lighter weight, and its low electrical resistance when calcined. Calcined anthracite can be used as part of the electrode - often as more than 50% of the electrode mass - or as a coating.
Calcination And Graphitised/Graphitizable Forms Of Carbon
Calcined anthracite has a wide array of uses across foundry, refractory and metal production applications(23,24) - and calcination is essential for anthracite to be used as an electrode. Treatment of crushed/powdered/milled anthracite is often the first process in producing anthracite best suited for electrical energy storage applications such as batteries.
Calcination decreases the electrical resistance of the material and removes residual volatile organic compounds. Non-calcined, as-mined, anthracite is an electrical insulator. Anthracite that has been calcined at temperatures as low as 900 °C shows a substantial boost to its electrical conductivity, with resistivity of just 1,000 µΩ at 1,300 °C(25).
Graphitised carbon refers to a form of carbon that has been heated to a certain degree upon which it takes on the properties of graphite, via the formation of a graphite-like molecular structure. Graphite is a superior electrical conductor, and thus if one can afford graphite-like properties to a cheap and widely available material such as anthracite, a highly performing electrical conductor will be realised. Complete graphitisation typically occurs when a solid carbonaceous material is heated beyond 2,000 °C, with partial graphitisation in a material occurring from around 1,400 °C onwards. Calcination can provide such temperatures, particularly at the lower end(26).
- Anthracite is one of the purest and most widely available forms of coal
- Its optimal density, hardness and carbon structure make it useful for a variety of applications
- Milled anthracite is used as a filler in plastic/hard rubber coatings, containers and casings for high-performance batteries
- Calcined anthracite can be used as anodes for high-tech batteries
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