Lithium-Rich Rock-Salt-Type Vanadate as Energy Storage Cathode: Li 2-x VO 3 Valerie Pralong,* ,† Venkatesh Gopal, † Vincent Caignaert, † Victor Duffort, † and Bernard Raveau † Laboratoire de Crystallographie et Sciences des Mate ́ riaux, ENSICAEN, Universite ́ de Caen, CNRS, 6 Bd Mare ́ chal Juin, F-14050 Caen 4, France KEYWORDS: vanadium oxides, Li 2 VO 3 , rock salt structure, Li ion batteries, lithium vanadate L ithium intercalation in oxides is a topic of the highest importance in view of the commercial realization of batteries for electric vehicles (EV) and hybrid electric vehicles (HEV) as well as microtechnology (Mems, 2D and 3D microbatteries for pacemakers, hearing aids, smart cards, remote sensors, etc.). In this context, the nature of the cathode material has been the object of numerous investigations showing that LiCoO 2 -based oxides 1 and LiFePO 4 phosphate 2 are today the best candidates which can be used for this technology. However, the cost and environmental concerns of the cobalt based oxides and the difficulty of optimization and of utilization of LiFePO 4 necessitate continuous search for new electrode materials that can mitigate these weak points. The renewed interest in the search for new compounds that can be used as potential electrode materials for Li-ion batteries is due to the safety concerns associated with the redox chemistry of the electrode materials. In addition, large scale mobile applications require robust and low cost systems. Several families of materials are screened for this purpose. 1 Intercalation reactions are generally possible in a 3D framework containing interconnected tunnels or on a 2D layered structure. Thus, besides cobalt and iron based oxides, other families of transition metal oxides, involving nickel, manganese, vanadium, titanium, niobium, tungsten, and molybdenum, are of great interest due to their ability to exhibit a mixed valence, with redox potential values in adequation with the batteries applications. Among the numerous transition metal oxides that have been explored, 3 vanadium oxides appear as attractive electrode materials as they offer the advantages of low cost and abundant sources. This is the case of V 2 O 5 , 4 which has been studied for more than 30 years, and of the vanadate LiV 3 O 8 5 which received considerable attention as an insertion material. However, the performances of these materials are still unsatisfactory. For instance, attempts to improve the performances of V 2 O 5 by combining it with more conductive materials such as carbon or nickel show that the resulting composites suffer from low cycling stability. 6 This can be understood by the study carried out by Delmas et al., 7 which shows that the fully lithiated end member Li 3 V 2 O 5 of the Li x V 2 O 5 family exhibits the rock salt structure, very different from the layered character of V 2 O 5 . Moreover, two redox couples, V 5+ /V 4+ and V 4+ /V 3+ , are involved successively during the lithium intercalation-deintercalation process. As a consequence, the combination of the potential difference between the two redox couples and the structural transition makes the intercalation process difficult to control, leading to its amorphization on cycling and loss of capacity, so that V 2 O 5 cannot be used directly as an electrode material. In contrast, the layered structure of LiV 3 O 8 8 built up of VO 6 octahedra and VO 5 pyramids, held together by Li + cations, can intercalate two additional Li + cations in its tetrahedral sites, leading to the formula Li 3 V 3 O 8 . 9 This process involves only one redox couple V 5+ /V 4+ and has the advantage of being totally reversible. Nevertheless, this composition, despite much effort to be optimized, 10,11 exhibits a much lower specific capacity of 200 (mA h)/g, compared to Li 3 V 2 O 5 (290 (mA h)/g). Interestingly, two additional Li + cations can be inserted into this structure, leading to the end member Li 5 V 3 O 8 with a rock-salt-type structure. 10,11 Unfortunately, its preparation requires a nanoscale synthesis and a partial substitution of vanadium by chromium. On the basis of the above observations, we have revisited the Li-V-O system. In the latter, the vanadate LiVO 3 appears as a potential material for lithium insertion due to the unidi- mensionnal character of its structure (Figure 1) 12 built up of chains of corner sharing VO 4 tetrahedra, interconnected through LiO 6 octahedra. This idea is also supported by the fact that this oxide is a good ionic conductor, 13 suggesting a Received: November 2, 2011 Revised: November 30, 2011 Published: December 6, 2011 Figure 1. Structure view of LiVO 3 along the a axis. Communication pubs.acs.org/cm © 2011 American Chemical Society 12 dx.doi.org/10.1021/cm203281q | Chem. Mater. 2012, 24, 12-14