Graphite materials – Production from biomass?

Graphite materials show high electrical and thermal conductivity making them useful in electronics both as electrical conductor, but as of today primarily used as a thermal conductor for thermal management and as the dominating anode material in lithium ion batteries. The conductivities depend on for example the degree of graphitisation, that is how close the material is to perfect graphite. Graphite materials can occur naturally in the earth’s bedrock and can thus be extracted by mining and is then called natural graphite. Graphitic carbon materials can also be synthesised and are then usually referred to a synthetic or artificial graphite, even though they should be referred to as graphite materials if being strict, as they never reach the structure of perfect graphite and always contain some defects and irregularities. This report starts with a short description of all carbon allotropes, i.e. structurally different forms of the same element due to how the atoms are chemically bonded to each other. It then continues with an overview of how graphitic carbon materials can and should be characterised, as well as analytical methods for making this characterisation. After this a section on production methods for graphite materials follows, that dependent on the principles they operate by are divided into: • Mining for graphite that occurs naturally in the earth’s bedrock. • High temperature heat-treatment, so called carbonisation, hydrothermal carbonisation if done in water, and graphitisation. • Chemical vapour deposition, i.e. depositing molecules or atoms in gas phase on a solid surface, that is used to synthesise pyrolytic carbon and graphite. • Extraction from a steelmaking by-product called Kish to obtain so called Kish graphite. • Thermal decomposition of carbides. This is followed by a section on the today most common and important graphite materials, which are: natural graphite (mined), anisotropic synthetic graphite, isotropic synthetic graphite, pyrolytic carbon and graphite. This section also includes specific production process details for the above listed graphite materials, their main properties, advantages, and common uses. Two of the most common and important uses of graphite materials, i.e. as anode in lithium ion batteries and for thermal management in electronics, are described somewhat more in depth. The focus of this report is biomass derived graphitic materials and this focus start fully first in section number four, which compares published values on electrical and thermal conductivity of different fossil and bio-based graphitic carbon materials. This comparison clearly shows that it is very challenging to derive graphitic carbon materials with high conductivities from biomass. This is because essentially all biomass is so-called non-graphitising or hard carbon precursor meaning that it is not transformed into highly graphitic carbon no matter how high temperature it is heated to. Catalytic graphitisation using metals salts or oxides can increase the degree of graphitisation that can be achieved, but all substances used for catalysing graphitisation forms solid nanoparticles which leaves voids when removed by for example acid dissolution, making the resulting graphitic material porous which in turn limits its electrical and thermal conductivity. Of all production processes reviewed here to create highly electrically and thermally conductive graphitic carbon materials from biomass, requiring a high degree of graphitisation and dense material, two methods stand out as especially interesting: • Chemical vapour deposition on suitable substrate (carbon materials, metals or ceramics) using biomass as carbon source. • Resistive heating of biomass derived films/objects. Bio-based free-standing graphene film with very high electrical and thermal conductivity have been produced using chemical vapour deposition technique. From a practical handling perspective, it would be beneficial to create thicker highly graphitic carbon films to make them stronger, although it may reduce the conductivities of the material. Methods based on chemical vapour deposition may be improved to be able to produce thicker graphitic films. Resistive heating of a film made of e.g. biobased lignin, mixed with mined graphene to 2192 °C have been shown to create a highly graphitic carbon film with the excellent electrical conductivity of 4480 S/cm. By substituting the mined graphene to bio-based ditto may open up for the production of a fully biobased, highly graphitic film with excellent conductive properties. It is suggested that the way to achieve fully biobased highly graphitic and dense films is to further refine the chemical vapour deposition and the resistive heating method