A comparative structural and electrochemical study of monoclinic Li 3 Fe 2 (PO 4 ) 3 and Li 3 V 2 (PO 4 ) 3 Se ´bastien Patoux a , Ca ˘lin Wurm a , Mathieu Morcrette a , Gwenae ¨lle Rousse b , Christian Masquelier a,* a Laboratoire de Re ´activite ´ et de Chimie des Solides, CNRS UMR 6007, Universite ´ de Picardie Jules Verne, 33 rue Saint-Leu, 80039 Amiens Cedex, France b Institut Laue Langevin, BP 156, F-38042 Grenoble Cedex 9, France Abstract Pure monoclinic Li 3 M 2 (PO 4 ) 3 (M: Fe, V) powders (<1 mm in diameter) were obtained by an original route that involved initial homogenization of precursors in aqueous solution followed by slow evaporation and annealing under controlled atmosphere at moderate temperatures. The crystal structure of Li 3 V 2 (PO 4 ) 3 was determined for the first time through Rietveld refinements of neutron diffraction data. As for Li 3 Fe 2 (PO 4 ) 3 , Li is distributed within three crystallographic sites, fully occupied at room temperature. The values of the temperature factors on Li(2) and Li(3) sites (five-fold coordination) were found significantly higher than that of Li(1) (four-fold coordination). Li 3 V 2 (PO 4 ) 3 shows four reversible redox phenomena upon insertion of two Li þ (V 3þ /V 2þ couple), at 1.98, 1.88, 1.73 and 1.70 V vs. Li. By comparison, Li 3 Fe 2 (PO 4 ) 3 shows two reversible redox phenomena upon insertion of two Li þ (Fe 3þ /Fe 2þ couple), at 2.88 and 2.73 V vs. Li. Experimental capacities close to the theoretical ones were obtained after optimal composite electrode preparation through ball-milling. In situ X-ray diffraction showed very minor changes from Li 3 M 2 (PO 4 ) 3 to Li 5 M 2 (PO 4 ) 3 . Additionally, Li is extracted from Li 3 V 2 (PO 4 ) 3 towards V 2 (PO 4 ) 3 (V 4þ /V 3þ and V 5þ /V 4þ couples) through four redox phenomena at 3.59, 3.67, 4.06 and 4.35 V vs. Li. Despite all these phase transitions, the [M 2 (PO 4 ) 3 ] framework is remarkably stable on cycling, particularly for M: Fe, while partial vanadium dissolution into the electrolyte occurs either on deep reduction to 1.5 V or deep oxidation to 4.6 V vs. Li. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Structural study; Monoclinic; Crystallographic 1. Introduction Recently, polyanion 3-D structures built of PO 4 tetrahedra and FeO 6 octahedra have engendered much interest for their potential use as cheap positive electrodes for lithium rechargeable batteries [1]. Efforts towards this relatively novel class of intercalation hosts for lithium have focused on systems such as the olivine Li 1x FePO 4 [2,3] and NASICON compositions Li x Fe 2 (SO 4 ) 3 [4] or Li 3þx Fe 2 (PO 4 ) 3 [5,6] into which reduction/oxidation of Fe 3þ /Fe 2þ occurs at potentials close to 3.43, 3.55 and 2.8 V vs. Li/Li þ , respectively. For applications, the material of choice of the so-called poly- anionic structures is with no doubt the olivine LiFePO 4 that, since the pioneering discovery of Padhi et al. [2] has gained much interest [7–9]. Recent optimization of LiFePO 4 has focused on: (i) increasing the electronic conductivity of composite electrodes through carbon coating on the active material particles [7,8]; and (ii) lowering the synthesis temperature to 400 8C with the use of strongly reactive iron oxalate under N 2 [9]. LiFePO 4 stands as a serious candidate for the next generation of Li-based polymer or Li-ion batteries. Materials of general composition A x MM’(PO 4 ) 3 (A: Li, Na, ...; M and M 0 : transition metal element) adopt either the ‘‘A’’-form or ‘‘B’’-form (NASICON) that differ in the 3-D connectivity between MM 0 (PO 4 ) 3 units. Among this very rich family of compositions and crystal structures, the NASICON Li 3 Fe 2 (PO 4 ) 3 [5,6], LiTi 2 (PO 4 ) 3 [10] and Li 3 V 2 (PO 4 ) 3 [11,12] were shown to react electrochemically with lithium at 2.8, 2.5 and 3.7 V vs. Li þ /Li, for the Fe 3þ/2þ , Ti 4þ/3þ and V 3þ/4þ couples, respectively. Delmas and co- workers were the first to demonstrate nice reversibility through a two-phase process between LiTi 2 (PO 4 ) 3 and Li 3 Ti 2 (PO 4 ) 3 (138 mAh/g) [10] but the practical use of these materials was still under question, due to their low intrinsic electronic conductivity. We recently paid particular atten- tion to circumvent this handicap and reported [13,24] that Journal of Power Sources 119–121 (2003) 278–284 * Corresponding author. E-mail address: christian.masquelier@sc.u-picardie.fr (C. Masquelier). 0378-7753/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0378-7753(03)00150-2