  Potassium-ion ($\rm K$-ion) rechargeable batteries; considered to be
lucrative low-cost battery options for large-scale and capacious energy storage
systems, have been garnering tremendous attention in recent years. However, due
to the scarcity of cathode materials that can condone the reversible
re-insertion of the large $\rm K$-ions at feasible capacities, the viability of
$\rm K$-ion batteries has been greatly undercut. In this paper, we explore a
potential cathode material in the $\rm K_2O$-$\rm Fe_2O_3$-$\rm MnO_2$ ternary
phase system, that not only demonstrates reversible $\rm K$-ion reinsertion but
also manifests relatively fast rate capabilities. The titled cathode compound,
$\rm K_{0.4}Fe_{0.5}Mn_{0.5}O_2$, demonstrates a reversible capacity of
approximately $\rm120 $ mAh g$^{-1}$ at $\rm 10$ hours of (dis)charge ($viz.$,
$\rm C/10$ rate) with $ca.$ $\rm 85$% of the capacity being retained at a $\rm
1$ hour of (dis)charge ($\rm 1 C$ rate) which is considered to be good capacity
retention. Additionally, both hard and soft X-rays have been employed to
unravel the mechanism by which $\rm K$-ion is reversibly inserted into $\rm
K_{0.4}Fe_{0.5}Mn_{0.5}O_2$. The results revealed a cumulative participation of
both manganese cations and oxygen anions in $\rm K_{0.4}Fe_{0.5}Mn_{0.5}O_2$
illustrating its potential as a high-capacity $\rm K$-ion battery cathode
material that relies on both anion and cation redox. Further development of
related high-capacity cathode compositions can be anticipated.
