An Unexpected Source of Hard Carbon, Rice Hull Ash, Provides Unexpected Li$^+$ Storage Capacities

Hard carbon (HC) anodes used in secondary batteries have attracted
increasing recent attention in particular to transition to new energy storage
formats. To date, HC is produced exclusively by charring organic precursors in
inert atmospheres. One would not expect to find HC in rice hull ash (RHA),
the byproduct of rice hull combustion processes. However, in developing
approaches to depolymerize RHA SiO$_2$ (90:10 wt% SiO$_2$:C) to produce
silica-depleted RHA or SDRHA$_{40-60}$ (40–60 wt% SiO$_2$) to tailor C:SiO$_2$ ratios for carbothermal reduction reactions, the SDRHA carbon component is recently
revisited. In more detailed efforts to characterize the form of carbon present in
SDRHA, a series of analyses reveal graphitized carbon domains in amorphous
matrices, i.e., HC, despite RHA being produced via combustion in an oxidizing
atmosphere. Comprehensive electrochemical analyses on SDRHA$_{40-60}$ find
unexpected capacities far in excess (>700 mAh g$^{−1}$) of reported values for HC
and graphite. Electrochemical and STEM characterization suggest that the
unexpected capacity may come from the nanoscale morphology of the
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amorphous carbon component. Given that RHA is a biowaste generated in
kilotons/year worldwide, there seems to be an opportunity to develop
sustainable high-capacity anode materials for alkali-ion storage systems.