Dynamic Stability: Phonon spectra were calculated for the lowest energy phases of KnC6O6 (where n=3, 4, 5 and 6) using the finite displacement (2x2x2) supercell method. Finite displacements were calculated with the Parlinski-Li-Kawazoe methodology 106 implemented in PHONOPY version 2.10. 107 These calculations relied on k-mesh densities ≤ 2π•0.03 Å -1 , and a kinetic energy cutoff of 800 eV. While no imaginary frequencies were found for K3C6O6, small ones, attributable to numerical error, are present for K6C6O6. In contrast, the lowest energy phases of K4C6O6 and K5C6O6 proved dynamically unstable. The discussed K3.5C6O6 phase is unfortunately of too low symmetry and has a too large unit cell to be feasibly calculated by us (the super cell contains 294 atoms). Phonon spectra are reproduced in Figures S4-S7.