Alternative\nPolymorph of the Hydroxysulfate Li<sub><i>x</i></sub>FeSO<sub>4</sub>OH Yields Improved Lithium-Ion\nCathodes

Use of sustainable electrode components in Li-ion battery\ntechnology\nis essential for large-scale applications while addressing environmental\nconcerns. Considering elemental abundance, Fe-based compounds can,\nin principle, work as the most economic cathodes. Fe-based hydroxysulfates\nLi<sub><i>x</i></sub>FeSO<sub>4</sub>OH (<i>x</i> = 0 –1) can be harnessed as low-cost, sustainable, high-voltage,\nand moisture-resistant battery cathode materials. In this system,\nmonoclinic (<i>m</i>) FeSO<sub>4</sub>OH and layered m-FeSO<sub>4</sub>OH were previously reported as Li-ion battery cathode materials.\nHere, we introduce orthorhombic (<i>o</i>) FeSO<sub>4</sub>OH as a potential low-cost cathode for Li-ion batteries synthesized\nby using a facile low-temperature hydrothermal route. The o-FeSO<sub>4</sub>OH cathode delivers a reversible capacity of 100 mA h/g at\na current rate of C/20 (1e<sup>–</sup> = 159 mAh/g) at a working\npotential of ca. 3.2 V vs Li<sup>+</sup>/Li. A higher overpotential\nand faster rate kinetics compared with that of m-FeSO<sub>4</sub>OH\nstem from the subtle deviations in the structural framework affecting\nthe Li coordination environment. Operando analytical tools, electrochemical\ntitration techniques, and computational modeling are combined to characterize\nthe complex phase transformation during the (de)lithiation process.