  Selenium (Se) cathodes are an exciting emerging high energy density storage
system for Potassium ion batteries(KIB), where potassiation reactions are less
understood. Here, we present an atomic-level investigation of KxSe cathode
enclosed in hexagonal lattices of carbon(C) characteristic of multilayered
graphene matrix and multiwalled carbon nanotubes (MW-CNTs). Microstructural
changes directed by graphene substrate in KxSe cathode are contrasted with
graphene-free cathode. Graphene's binding affinity for long-chain polyselenides
(Se-Se-Se = -2.82 eV and Se-Se = -2.646 eV) and ability to induce reactivity
between Se and K are investigated. Furthermore, intercalation voltage for
graphene enclosed KxSe cathode reaction intermediates are calculated with K2Se
as the final discharged product. Our results indicate a single-step reaction
near a voltage of 1.55 V between K and Se cathode. Our findings suggest that
operating at higher voltages (~2V) could result in the formation of reaction
intermediates where intercalation/deintercalation of K could be a challenge,
and therefore cause irreversible capacity losses in the battery. Primary issues
are the high binding energy of long-chain polyselenides with graphene that
discourage K storage and Se-Se bond dissociation at low K concentrations. A
comparison with graphene-free cathode highlights the substantial changes a van
der Waals (vdW) graphene interface can bring in atomic-structure and
electrochemistry of the KxSe cathode.
