Synthesis and Electrochemical Performance of the Orthorhombic Li2Fe(SO4)2 Polymorph for Li-Ion Batteries

To enhance the safety, cost, and energy density of Li-ion batteries, significant research efforts have been devoted to the search for new positive electrode materials that exhibit high redox potentials and are composed of low-cost, earth-abundant elements. Sulfate chemistry has yielded promising results for iron-based polyanionic electrode materials using the FeIII+/FeII+ redox couple, including the recent discovery of a monoclinic marinite Li2Fe­(SO4)2 phase (3.83 V vs Li+/Li0). Here, we report the ball-milling synthesis and electrochemical properties of a new orthorhombic polymorph of Li2Fe­(SO4)2, which reversibly reacts with lithium through a two-step redox process (3.73 and 3.85 V vs Li+/Li0) with an overall sustained capacity of about 90 mAh/g. Using similar synthesis conditions, the cobalt-, zinc-, magnesium-, and nickel-based orthorhombic analogues were also obtained, though no electrochemical activity was observed for these phases. Overall, our results demonstrate that polymorphism can play a crucial role in the search for new battery electrode materials and emphasize the need to understand and master synthetic control.