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

Abstract

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.

Sulfate‐Based Cathode Materials for Li‐ and Na‐Ion Batteries

“…SO 4 ‐based compounds are known to present a rich crystal chemistry and often occur as polymorphs (e. g. tavorite/triplite LiFeSO 4 F, tavorite/layered LiFeSO 4 OH). This is also the case for Li 2 Fe(SO 4 ) 2 , where besides the monoclinic phase an orthorhombic counterpart was reported, which is isostructural to Li 2 Ni(SO 4 ) 2 . Orthorhombic Li 2 Fe(SO 4 ) 2 is prepared via a mechanochemical approach, where FeSO 4 and Li 2 SO 4 are intensively ball‐milled for 10 h. A similar protocol was also applied for the synthesis of other orthorhombic Li 2 M (SO 4 ) 2 ( M =Co, Zn, Ni) homologues.…”

Magnetic Structures of Orthorhombic Li2M(SO4)2 (M = Co, Fe) and LixFe(SO4)2 (x = 1, 1.5) Phases

“…The NPD patterns were refined against the orthorhombic structure (space group Pbca) initially proposed by Isasi et al for the nickel analogue Li 2 Ni(SO 4 ) 2 and as previously reported by our group. 17,20,24 The results of these refinements as well as their structures are summarized in Figure SI1-2 and Table SI1-3 (Supporting Information). The structural models derived from XRD were here fully confirmed by NPD measurements indicating that the structures were preserved between room temperature and the above-mentioned temperatures.…”

“…29 However, in the present case, we could not identify any monoclinic distortion based on the Rietveld refinements of the XRD patterns of Li 2 Fe(SO 4 ) 2 and Li 1.5 Fe(SO 4 ) 2 . 17 Indeed, the Bragg peaks present an intrinsic broadening caused by the mechano-chemical synthesis approach, and refining the structural model in a monoclinic symmetry is almost impossible as the number of parameters is increased a lot and they may be highly correlated.…”

Advanced Nanostructured Anode Materials for Sodium‐Ion Batteries

“…Anyhow, many great successes of the fundamental understanding have been achieved based on the advanced characterization methods and technological optimizations . Various sodium‐insertion compounds have been studied for the cathode materials to NIBs, including layered oxides, tunnel oxides, olivine/sodium (Na) super ionic conductor (NASICON)‐structure phosphates, and polyanionic‐structure compounds . NaCoO 2 as an analog of LiCoO 2 (the current start‐of‐the‐art cathode material for LIBs) was the first reported layered oxides (Na x MeO 2 , Me = transition metal elements) for NIB cathode materials .…”