Synthesis, redox potential evaluation and electrochemical characteristics of NASICON-related-3D framework compounds

• Published in: Solid state ionics Vol. 92; no. 1; pp. 1 - 10
• Main Authors: Nanjundaswamy, K.S., Padhi, A.K., Goodenough, J.B., Okada, S., Ohtsuka, H., Arai, H., Yamaki, J.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 01.11.1996; Elsevier Science
• Subjects: Applied sciences; Direct energy conversion and energy accumulation; Electrical engineering. Electrical power engineering; Electrical power engineering; Electrochemical conversion: primary and secondary batteries, fuel cells; Exact sciences and technology; Ferric sulfate; Li-battery cathode; Metal phosphates; Metal sulfates; NASICON-related compounds; Polyanion framework; Metal sulfates; Ferric sulfate; Polyanion framework; NASICON-related compounds; Li-battery cathode; Metal phosphates; Lithium Ions; Polyanion; Insertion; Differential thermal analysis; Electrode material; Discharge charge cycle; Titanium Sulfates; Quaternary compound; Energetic capacity; Three dimensional structure; Preparation; Battery; Iron Sulfates; Thermal analysis; Redox potential; Vanadium Sulfates; Electrical characteristic; Crystalline structure
• ISSN: 0167-2738; 1872-7689
• DOI: 10.1016/S0167-2738(96)00472-9

The framework compounds M 2(SO 4) 3 with M = ( Ti Fe), ( V Fe), Fe and Li x M 2( PO 4) 3 with M = Ti, ( V Fe), Fe, were synthesized and electrochemically characterized by the coin-cell method. Use of larger (XO 4) n− polyanions not only allows fast Li +-ion conduction in an open three-dimensional framework that is selective for the working alkali ion on discharge; it also stabilizes operative redox potentials Fe 3+ Fe 2+ , Ti 4+ Ti 3+ and V 3+ V 2+ that give open-circuit voltages V oc > 2.5 V as well as access to V 4+ V 3+ , Ti 3+ Ti 2+ and Fe 2+ Fe + couples. Separation of the V 4+ V 3+ and V 3+ V 2+ couples were found to be 2.0 V. Fe 2(SO 4) 3 has both monoclinic and rhombohedral modifications that give a flat open-circuit voltage V oc = 3.6 V versus Li and a reversible capacity for ~ 1.8 lithium atoms per formula unit. Li x Fe 2(SO 4) 3 shows an abrupt voltage drop occurring for x > 2 that can be held in check by the addition of buffers such as Li 3Fe 2(PO 4) 3, FeV(SO 4) 3 and LiTi 2(PO 4) 3. Changing the polyanion group from (SO 4) 2− to (PO 4) 3− in these framework compounds decreases the redox potentials from 3.2 to 2.5 V for the Ti 4+ Ti 3+ couple, 2.5 to 1.7 V for the V 3+ V 2+ couple and 3.6 to 2.8 V for the Fe 3+ Fe 2+ couple. Comparative advantages and disadvantages of framework cathodes for Li rechargeable battery applications are discussed.