Alternative Polymorph of the Hydroxysulfate Li x FeSO 4 OH Yields Improved Lithium-Ion Cathodes

Use of sustainable electrode components in Li-ion battery technology is essential for large-scale applications while addressing environmental concerns. Considering elemental abundance, Fe-based compounds can, in principle, work as the most economic cathodes. Fe-based hydroxysulfates LixFeSO$_4$OH (x = 0 –1) can be harnessed as low-cost, sustainable, high-voltage, and moisture-resistant battery cathode materials. In this system, monoclinic (m) FeSO$_4$OH and layered m-FeSO4OH were previously reported as Li-ion battery cathode materials. Here, we introduce orthorhombic (o) FeSO4OH as a potential low-cost cathode for Li-ion batteries synthesized by using a facile low-temperature hydrothermal route. The o-FeSO$_4$OH cathode delivers a reversible capacity of 100 mA h/g at a current rate of C/20 (1e– = 159 mAh/g) at a working potential of ca. 3.2 V vs Li$^+$/Li. A higher overpotential and faster rate kinetics compared with that of m-FeSO4OH stem from the subtle deviations in the structural framework affecting the Li coordination environment. Operando analytical tools, electrochemical titration techniques, and computational modeling are combined to characterize the complex phase transformation during the (de)lithiation process.