Facile synthesis of β-Li3VF6: A new electrochemically active lithium insertion material

The β-modification of Li3VF6 is easily prepared following a precipitation reaction at moderate temperature avoiding the high temperature route normally used for preparing related fluorides. Precipitation conditions for Li3VF6 have been optimized for two different alcohols to obtain better electrochemical performances. Temperature has been found to play an important role in the progress of precipitation reaction and in the phase purity of the resulting product. Best results were achieved for 2-propanol based solutions at 60 °C and for ethanol based solutions at 45 °C containing VCl3, HF(aq) and Li2CO3 as starting compounds. In both cases large agglomerates, though of different size and shape, are obtained. These are composed of smaller particles of 20–30 nm. Electrochemical performances of as prepared Li3VF6 materials as electrode in lithium rechargeable batteries are modest with a maximum capacity of only 30 mAh g−1. However, processing these materials by ball milling with carbon allows the capacity to increase to its theoretical value, 144 mAh g−1, accordingly to the complete reduction of V3+ to V2+. The observed average reduction potential for V3+ to V2+, 2 V, may be certainly higher than that found in oxides but not high enough to be considered for application as the positive electrode. However, Li3VF6 is then another example of the β-Li3MF6 type-structure hosting lithium through an electrochemical insertion reaction. On the other hand, reversible oxidation of V3+ through lithium de-insertion, which would lead to the formation of Li3−xVF6 phases has not been successful yet. Taking into account the redox intercalation chemistry of vanadium in oxides, the reversible reduction of V3+ to V2+ and the irreversible oxidation of V3+ to V4+ in the herein presented fluoride make a significant difference derived from the nature of the non metal X. Then, research of fluorides is a path to find new chemistries for lithium batteries.

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► Nanometric lithium vanadium fluoride has been obtained.
► 20–30 nm particles form 500–900 nm diameter spherical-like agglomerates.
► Breaking of agglomerates by ball milling reduces the size below 200 nm.
► Full specific capacity is obtained in ball milled samples.
► The non usual V3+/V2+ redox couple is operating reversibly.