Several previously unknown phases have been found to be on or close to the convex hull. The Li 2 Fe 4 S 3 O 2 (space group C2) and Li 2 Fe 3 S 2 O 2 (space group Pm) are shown in Fig. respectively. The former is on the convex hull while the latter is only 3 meV (per atom) above it. They can be regarded as cation deficient variants of anti-Ruddlesden-Popper and anti-perovskite structures. In both cases, the oxygen atoms are pyrimidally coordinated by two Li atoms and three Fe atoms, rather than the octahedral coordination in original archetype structures. The cation deficiency can be attributed to the reduced Li-to-Fe ratio. Since a Fe 2+ cation carries more positive charges, fewer cations are needed to balance the negative charge of the anions. Unfortunately, the reduced Lito-Fe ratios also means reduced specific capacities, making them less attractive as cathode materials.