The Li-Fe-O-S chemical spaces have been explored using ab initio random structure searching (AIRSS), and a series of potentially synthesisable phases that are close to or on the convex hull have been predicted. Among these candidate phases, two anti-RP structures materials Li 2 Fe 2 S 2 O and Li 4 Fe 3 S 3 O 2 are particularly promising for cathode applications. Density functional theory calculations show that they have voltages that are similar to that of the previously reported Li 2 FeSO phase, and their theoretical charge capacities exceeds the latter. Fast Li conduction channels are revealed using climing-image nudged elastic band calculations. If successfully synthesised, they can be good contenders to the commercialised LiFePO 4 cathodes. Even though the low voltages (∼2.5 V) are nowhere near that of of LiFePO 4 , they are compensated by much increased theoretical gravimetric capacities (∼250 mAh/g vs ∼170 mAh/g). Our work also demonstrates that first-principles structure prediction is an effective way to explore complex quaternary chemical spaces where many new materials are waiting to be discovered for battery cathode applications and beyond.