Designing Nanocrystal Electrodes by Supercritical Fluid Process and Their Electrochemical Properties

Abstract

Modern society’s dependence on the electrical appliances and portable electronics has made rigorours development in battery technology. After three decades of development in battery technology, the Li-ion battery technology has emerged as one of the most popular battery technology [1-3]. They are widely used in varieties of electronic devices due to their good cycle life, high energy density and high capacity over any battery technology. The Li-ion battery technology that now dominates much of the portable battery business has matured enough over the last five years to be considered for the short-term implementation in Hybrid Electric Vehicles (HEV) and Electric Vehicles (EV) applications [1]. However, high cost, safety hazards, and chemical instability of cathode materials have been major concern in the battery industries, which prohibits wider application in the automotive industry [4]. In addition, the volatility of the Li-ion battery technology, has made a massive product recall from the battery industries in recent years. Therefore, increased public awareness about these battery issues solemnly demanding for Li ion batteries with reliability and safety materials. Among the battery components, the cathode materials are the one which are crucial in determining the high power, safety, longer life and cost of the battery that satisfy the requirement of the larger battery system that can be applicable to electric vehicles, power tools, energy storage equipment and so on [5, 6]. In this context, the olivine-type materials based on lithium transition metal phosphates (LiMPO4 with M=Fe, Mn, Co, Ni) has been emerging as a potential cathode candidate for high power batteries [7]. When compared to the well known layered structure (e.g. LiCoO2) and spinel structure (e.g. LiMn2O4) based cathode materials, the olivine structured LiMPO4 cathode materials exhibit a flat voltage profile at 3.45-5.1 V vs Li+/Li. Among these, the LiFePO4 and LiMnPO4 cathodes shows the theoretical discharge capacity of about 170 mAhg-1, which make this material a safe cathode material with good cycle life and high power density [5, 6]. However, LiMPO4 materials are basically insulating in nature, hence, has a very poor lithium ionic and electronic conductivity (10-9 to 10-12 S cm-1). This is because of separation of MO6 octahedron by the PO4 tetrahedra, and the one-dimensional chains formed by the edge-sharing LiO6 octahedra along the b-axis of the orthorhombic structure. Since the first report by Padhi et al., a lot of