Graphite intercalation compounds
(GICs) have attracted tremendous attention due to their exceptional
properties that can be finely tuned by controlling the intercalation
species and concentrations. Here, we report for the first time that
potassium (K) ions can electrochemically intercalate into graphitic
materials, such as graphite and reduced graphene oxide (RGO) at ambient
temperature and pressure. Our experiments reveal that graphite can
deliver a reversible capacity of 207 mAh/g. Combining experiments
with <i>ab initio</i> calculations, we propose a three-step
staging process during the intercalation of K ions into graphite:
C → KC<sub>24</sub> (Stage III) → KC<sub>16</sub> (Stage
II) → KC<sub>8</sub> (Stage I). Moreover, we find that K ions
can also intercalate into RGO film with even higher reversible capacity
(222 mAh/g). We also show that K ions intercalation can effectively
increase the optical transparence of the RGO film from 29.0% to 84.3%.
First-principles calculations suggest that this trend is attributed
to a decreased absorbance produced by K ions intercalation. Our results
open opportunities for novel nonaqueous K-ion based electrochemical
battery technologies and optical applications