No abstract

No abstract

No abstract

We describe the preliminary performance characteristics of a novel LiVPO 4 F ʈ LiVPO 4 F lithium-ion cell. The electrochemical evaluation of the LiVPO 4 F phase in metallic lithium half cells indicates that this triclinic fluorophosphate undergoes reversible lithium extraction/insertion reactions at electrode potentials of 1.8 and 4.2 V vs. Li. These insertion properties allow the LiVPO 4 F phase to function successfully as both the positive and negative electrode material in a lithium-ion configuration. We suggest that this type of battery may be particularly useful in commercial applications where considerations such as cost, ease of construction, and safety are at a premium.Phosphate-based insertion phases such as LiFePO 4 and Li 3 V 2 ͑PO 4 ͒ 3 have been identified as electroactive materials for lithium-ion battery applications. 1,2 More recently, the structural and lithium insertion properties of the lithium vanadium fluorophosphate, LiVPO 4 F have been described in detail by this group. 3-7 X-ray powder diffraction studies indicate that the LiVPO 4 F is isostructural with the naturally occurring minerals Tavorite, LiFePO 4 · OH, 8 and Amblygonite, LiAlPO 4 ͑OH,F͒, 9 crystallizing with a triclinic structure ͑space group P1͒. The LiVPO 4 F structure comprises a three-dimensional framework built up from ͓PO 4 ͔ tetrahedra and ͓VO 4 F 2 ͔ octahedra 6,7 with the oxygen atoms shared between the two environments.The reversible lithium extraction/insertion reaction for Li 1−x VPO 4 F, based on the V 3+/4+ redox couple operates at around 4.2 V vs. Li, and as such this fluorophosphate phase may be employed as the positive electrode in conventional graphite-based lithium-ion cells. 6,7 More recent investigations 10 have established an additional lithium insertion reaction ͑composition corresponding to Li 1+x VPO 4 F where x = 0-1͒ at around 1.8 V vs. Li, this time based on the V 2+/3+ redox couple. Based on this information, we report here on the fabrication of symmetrical LiVPO 4 F ʈ LiVPO 4 F lithium-ion cells, in which identical electrode stock is utilized as both the positive and negative electrode material. Separate studies are being carried out by this group to establish the safety characteristics of the LiVPO 4 F material. These will be published in due course. 10 ExperimentalThe LiVPO 4 F used in this study was prepared by a carbothermal reduction ͑CTR͒ method, 11,12 utilizing orthorhombic VPO 4 as reaction intermediate. The precise preparative conditions have been described in detail elsewhere. 5-7 For electrochemical testing, composite electrodes were fabricated using 84 wt % active material, 5 wt % Super P ͑conductive carbon͒, and 11 wt % PVdF-HFP co-polymer ͑Elf Atochem͒ binder. For all electrodes, an aluminum current collector was employed. The electrolyte comprised a 1 M LiPF 6 solution in ethylene carbonate/dimethyl carbonate ͑2:1 by weight͒ while a dried glass fiber filter ͑Whatman, Grade GF/A͒ was used as electrode separator. High-resolution electrochemical measurements were performed using the electr...

In this study it is demonstrated that the lithium vanadium fluorophosphate, LiVPO 4 F phase, offers considerable promise as a viable cathode material for commercial lithium-ion batteries. The active material was synthesized using a reaction scheme based on carbothermal reduction. The LiVPO 4 F is isostructural with the known mineral ambygonite, LiAlPO 4 F, crystallizing with a triclinic structure ͑space group, Py͒. Performance evaluation of graphite//LiVPO 4 F lithium-ion cells indicates a reversible material utilization for the LiVPO 4 F of around 130 mAh/g and an average discharge voltage of 4.06 V. Long-term cycling ͑C/5 rate at 23°C͒ of this lithium-ion configuration predicts a cycle life Ͼ500 cycles to around 90% of the original discharge capacity.