The cathode is a critical component for lithium-ion batteries (LIBs) to achieve high energy density, safe operation, and economical viability. Despite many technological advances in LIBs, the cathode materials have barely evolved in the last 15 years. Those found in commercialised LIBs are still based on one of the three systems: layered LiCoO 2 , spinel LiMn 2 O 4 2 and olivine LiFePO 4 , all of which were first proposed more than two decades ago. Iron-based cathode materials are very attractive as Fe is one of the most abundant transition metal in the upper crust with very low production costs. LIBs with olivine LiFePO 4 cathodes are nowadays used in applications including electric vehicles, but they suffer from relatively low specific energies . Other iron based polyanion cathodes, such as Li 2 FeSiO 4 , LiFeSO 4 F have also being proposed and shown to have comparable performance to LiFePO 4 . The use of larger-sized polyanion groups, however, places an upper-limit on the energy density, which in turn offsets some of the benefits of the low cost. On the other hand, iron-oxide based cathodes suffer from the low voltage due to the Fe 2+ /Fe 3+ redox , while attempts to utilize the Fe 3+ /Fe 4+ redox often leads to decomposition into other phases upon charging . Substituting one element with another has been a fruitful strategy for increasing the performance of cathode materials. Over the years, most of the research interests have been focusing on cationic substitution, and materials such as layered LiNi x Mn y Co z O 2 have been reported to have improved reversible capacity over LiCoO 2 -only half of the Li can be extracted in the latter. In the meantime, anionic substitution, or mixed anion materials have been largely unexplored until recently . With different sizes, valence charge, electronegativity and polarizability of the anions, hetero-anionic materials are known to form layered structures due to preferential cation-anion bonding .