Although attempts have been made to find suitable replacements,
currently only carbonaceous materials are used in commercial
anodes [192]. Carbonaceous materials include graphites
(natural graphite and HOPG), modified graphites (MCMB, carbon
fiber, metal deposited carbon fiber), and non-graphitic carbons
[193]. There are a number of reviews on anode materials [193–196]
and many of them focus on both carbon and inorganic materials.
Conduction properties of carbonaceous materials are primarily
reviewed here; other anode materials display conduction mechanisms
similar to those described for cathode materials.
Conduction in graphite anodes is complex due to continuous
phase transformations and the formation of the SEI layer.
Phase transformations are reflected in the open-circuit voltage
(OCV) curve as distinct plateaus [197]. Also conduction is strongly
dependent upon the degree of crystallinity. As the fraction (f) of
amorphous phases (fraction of crystalline phases, 1−f) increases,
electrical conductivity decreases and diffusivity increases. This
indicates the possibility of optimizing conduction properties of carbonaceous
materials by varying the fraction of each phase [198].
Non-graphitic carbonaceous materials do not undergo phase transformations
and therefore do not show distinctive stages in the OCV
curve [197]. The SEI layer displays much lower ionic and electronic
conductivity than the bulk electrode. To elucidate the mechanisms
related to the properties and performance of Li-ion batteries, precise
investigation of the electronic state and the diffusion process
in the carbon and the SEI layer is still required.