A 3.90 V iron-based fluorosulphate material for lithium-ion batteries crystallizing in the triplite structure

Abstract

Li-ion batteries have empowered consumer electronics and are now seen as the best choice to propel forward the development of eco-friendly (hybrid) electric vehicles. To enhance the energy density, an intensive search has been made for new polyanionic compounds that have a higher potential for the Fe²⁺/Fe³⁺ redox couple. Herein we push this potential to 3.90 V in a new polyanionic material that crystallizes in the triplite structure by substituting as little as 5 atomic per cent of Mn for Fe in Li(Fe(1-δ)Mn(δ))SO₄F. Not only is this the highest voltage reported so far for the Fe²⁺/Fe³⁺ redox couple, exceeding that of LiFePO₄ by 450 mV, but this new triplite phase is capable of reversibly releasing and reinserting 0.7-0.8 Li ions with a volume change of 0.6% (compared with 7 and 10% for LiFePO₄ and LiFeSO₄F respectively), to give a capacity of ~125 mA h g⁻¹.

Systematic Studies on “Abundant” Battery Materials: Identification and Reaction Mechanisms

“…Once these problems are solved, borates, rather than heavier and larger phosphates, could be a new avenue for the exploration of novel electrode materials. 27,28 This trend roughly follows the formal charges of the central atoms in the polyanions, which is consistent with the idea of the inductive effect. More essentially, the voltage is defined as the difference between the lithium chemical potential in the cathode and in the anode, leading to the simple thermodynamic definition ignoring PV and TS terms (P = pressure, V = volume, T = temperature, and S = entropy),…”

An intuitive and efficient method for cell voltage prediction of lithium and sodium-ion batteries

“…In his work on the Li x Mn 2 O 4 phases, Goodenough 2 also invoked the role of the Li-sites coordination to explain the 1 V shift in potential experimentally observed for this electrode material, which clearly corroborates the first-order contribution of the alkali ions. The potentials of the contentious LiFeSO 4 F tavorite (3.6 V) and disordered triplite, (3.9 V) polymorphs 12,[16][17][18] are well-reproduced by our model (see red plain circles in Fig. 4).…”

“…For the tavorite-LiFeSO 4 OH and Li 2 FePO 4 F phases, for which no experimental crystal structure is available in the literature, the most probable Li-sites were obtained from the Connolly surface of the Li-poor phase and the lowest structure energy was then used to compute Bader charges and Madelung fields. For the LiFeSO 4 F triplite and the Li 2 FeP 2 O 7 phases in which Li/M intermixing occurs 16,29 , averaged formal charges were considered (for example, 50% Li þ and 50% Fe 2 þ resulting in (Li/Fe) 1.5 þ ). For the specific case of the LiFeSO 4 F triplite, the 11-ordered structures proposed in the recent work by Ben Yahia et al 12 were considered to access average values of the Madelung fields in Figs 3 and 4.…”

“…14) and the more recent triplite LiFeSO 4 F (refs 16,17) exhibit potentials versus Li þ /Li 0 (the counter electrode currently used in laboratory half-cells) as high as 3.45 V and 3.9 V, respectively, compared with o3 V for standard Fe-based oxides. Although very powerful, this approach is also perfectible in particular to address the polymorphism issue recently raised by the LiFeSO 4 F tavorite versus triplite polymorphs whose potentials versus lithium are 3.6 V and 3.9 V, respectively, despite equivalent inductive ligands 16,18 .…”

High electrochemical performance of 3.9 V LiFeSO4F directly synthesized by a scalable solid- state reaction within 1 h

“…16,24,25 In solid-state reactions, the formation of tavorite depends on the precursors, indicating that tavorite is not thermodynamically stable at T A ¼ 250 C.…”