An electrochemical and structural investigation of two transition metal oxohalides has been undertaken. As negative electrodes for Li-ion batteries, these materials exhibit a combination of both conversion and alloying reactions within the same cell, giving rise to interesting electrochemical performances. Alloying-type reactions, typically involving Si, Sn or - as in this case - Sb, generally occur at low potentials vs. Li⁺/Li and show relatively good reaction kinetics. The main concern regarding alloying reactions, however, is the expansion/contraction exerted by repeated insertion of Li⁺, which leads to poor cycleability over time. Storage of Li in the family of conversion materials is based on the conversion of metal oxides into metallic nano-clusters and Li₂O.¹ This generally occurs at higher potentials and with larger voltage polarization between the charge and discharge processes. Here, these two types of reactions are exemplified using Ni₃Sb₄O₆F₆² and Mn₂Sb₃O₆Cl³ with the intent of finding a compromise between the alloying and conversion mechanisms. The inclusion of a halide allows for a more careful tuning of the crystal structure. In this respect, the relatively open structures are believed to at least partly affect the insertion of Li, and thereby the efficiency of the conversion process in the early stages of lithiation. The choice of the transition metal (i.e. Mn or Ni) and its specific redox potential vs. Li⁺/Li effectively determines the order in which the (transition metal oxide-conversion) ↔ (antimony conversion-alloying) reaction sequence occurs, with possibly crucial consequences.

Both compounds are shown to react reversibly with Li, with a good cycleability, in complex systems where multiple reactions occur sequentially and/or simultaneously via the in-situ formation of a nano-composite material. Operando XRD complemented with ex-situ XRD, XANES, TEM and an extensive electrochemical analysis have been used to illuminate the mechanisms involved.

1. Poizot, P.; Laruelle, S.; Grugeon, S.; Dupont, L.; Tarascon, J. M., Nano-sized transition-metal oxides as negative-electrode materials for lithium-ion batteries. Nature 2000, 407, 496-499.

2. Hu, S.; Johnsson, M.; Lemmens, P.; Schmid, D.; Menzel, D.; Tapp, J.; Möller, A., Acentric Pseudo-Kagome Structures: The Solid Solution (Co_1–xNi_x)₃Sb₄O₆F₆. Chemistry of Materials 2014, 26 (12), 3631-3636.

3. Zimmermann, I.; Johnsson, M., A Synthetic Route toward Layered Materials: Introducing Stereochemically Active Lone-Pairs into Transition Metal Oxohalides. Crystal Growth & Design 2014, 14 (10), 5252-5259.