Biomedical Engineering Reference
In-Depth Information
materials focuses mainly on the compounds of the following three
categories: nongraphitized carbon materials, metal oxides, and
alloys [19].
6.2.1 Graphite
Graphite is the first kind of anode material for commercial
lithium-ion batteries, and it is still widely used. However, the
co-intercalation of solvents such as propylene carbonate during
lithium insertion may lead to the decomposition and intensive
exfoliation of graphene sheets [20]. Besides, graphite is very
sensitive to the electrolyte, and if not properly dealt with, it
may even hinder lithium ions from reversibly intercalating into
graphite thus causing great capacity loss in the first several cycles
[21]. Therefore, to improve the safety and cycling performance of
lithium-ion battery, solvent co-intercalation and thermal stability of
graphite anode must be taken into consideration, and modifications
should be made on the material; yet much work has been done on
this front.
6.2.2 Nongraphiized Carbon Materials
Generally, nongraphitized carbon materials consist of two
categories: soft carbon and hard carbon. Although the soft carbon
material possesses a very high reversible Li-storage capacity, it
suffers from a serious voltage hysteresis during delithiation; the
hard carbon material, on the other hand, with its relatively high
capacity (200-600 mAh g
) over a wide voltage range, shows
deficiencies in its initial coulombic efficiency and tap density
[22, 23]. Among all the carbon materials, spherical hard carbon
materials seem to be promising. However, they were difficult to
prepare until 2001, when a hydrothermal method to prepare hard
carbon spheres (HCSs) from sugar solution was introduced [24].
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1
With sugar as the starting material,
a series of HCS materials with
a perfect spherical morphology and a controllable size ranging
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