  Given increasing energy storage demands and limited natural resources of Li,
K-ion batteries (KIBs) could be promising next-generation systems having
natural abundance, similar chemistry and energy density. Here, we have
investigated the P3-type K$_{0.5}$TMO$_2$ (where TM = Ti, V, Cr, Mn, Co, or Ni)
systems using density functional theory calculations, as potential positive
intercalation electrodes (or cathodes) for KIBs. Specifically, we have
identified the ground state configurations, and calculated the average
topotactic voltages, electronic structures, on-site magnetic moments, and
thermodynamic stabilities of all P3-K$_{0.5}$TMO$_2$ compositions and their
corresponding depotassiated P3-TMO$_2$ frameworks. We find that K adopts the
honeycomb or zig-zag configuration within each K-layer of all P3 structures
considered, irrespective of the TM. In terms of voltages, we find the Co- and
Ti-based compositions to exhibit the highest (4.59 V vs. K) and lowest (2.24 V)
voltages, respectively, with the TM contributing to the redox behavior upon K
(de-)intercalation. We observe all P3-K$_{0.5}$TMO$_2$ to be (meta)stable, and
hence experimentally synthesizeable according to our 0 K convex hull
calculations, while all depotassiated P3-TMO$_2$ configurations are unstable
and may appear during electrochemical cycling. Also, we verified the stability
of the prismatic coordination environment of K compared to octahedral
coordination at the K$_{0.5}$TMO$_2$ compositions using Rouxel and cationic
potential models. Finally, combining our voltage and stability calculations, we
find P3-K$_x$CoO$_2$ to be the most promising cathode composition, while
P3-K$_x$NiO$_2$ is worth exploring. Our work should contribute to the
exploration of strategies and materials required to make practical KIBs.
