LiFePO4 BASED COMPOSITE ELECTRODES FOR Li-ION BATTERY APPLICATION

Abstract

The climate change due to excessive emission of greenhouse gases (GHG) in the last two to three decades has adversely affected ecological life due to ocean pollution and global warming. It is an urgent requirement to find an alternative energy sources to replace conventional fossil fuel energy sources. However, most of the renewable energy sources like solar energy, wind energy and hydro energy are dependent on the natural conditions, e.g. solar energy depends on sun light and wind energy has dependence on wind flow. Therefore, to ensure the continuous energy supply, this has become mandatory to have energy storage system with high safety and affordable cost. Among all available energy storage technologies, electrochemical energy storage is the most beneficial technology which stores energy in batteries, fuel cells, and electrochemical capacitors or supercapacitors. However, batteries have advantages over other electrochemical systems due to large area of applications from portable electronics to hybrid electric vehicles (HEVs) and grid level storage. In past several decades, many battery systems such as lead-acid, Ni-Metal Hydride, Ni-Cd, Li-metal, and Li-ion battery were investigated depending on the applications. However, among various rechargeable batteries, lithium-ion batteries have high gravimetric and volumetric energy densities, long cycle life and very low self-discharge. The basic principle of Li-ion battery is to convert chemical energy into electrical energy through exothermic reaction. A Li-ion battery cell consists of several components such as anode, cathode, electrolyte, separator and current collectors. However, the performance of Li-ion battery mainly depends on the thermodynamics and kinetics of anode, cathode, electrolyte and their compatibility among each other. The energy of Li-ion battery is also affected by internal resistances develop inside the cell which is due to polarization in the cell. The output voltage of a battery cell depends on the voltage window of electrolyte (energy gap between the lowest unoccupied molecular orbital and highest occupied molecular orbital) and intrinsic potential of electrodes. The designing of electrodes of high capacity and their electrochemical potential lying within the voltage window of the electrolyte is a main challenge. For the safety and life of a Liion cell, electrodes must be chemically stable with electrolyte. Most of the electrodes (anode/cathode) used for practical application have host structure into/from which Li+ ions are inserted/extracted repeatedly during charging and discharging of the battery. Although, elemental lithium metal (Li) can be considered as an ideal anode for Li-ion battery but chemical potential of lithium lies outside of the presently available electrolytes. The mismatch in the energy of lithium metal anode and electrolyte can cause of short-circuits in Liion battery. The graphitic carbon anode is found much safer as compared to lithium metal. However, the redox potential plays an important role in development of the new cathode ii materials. The position of the M(n+1)+/Mn+ redox couple (for example, M(n+1)+/Mn+ = Co3+/Co2+, Fe3+/Fe2+ etc.) relative to the Li/Li+ couple of pure lithium metal defines the cell voltage. The redox potential values of Fe, Mn, Co and Ni are 3.45 V, 4.1 V, 4.8 V and 5.1 V, respectively. The liquid organic carbonate-based electrolytes in Li-ion battery are decomposed above 4.5 V, which limits the investigation of high voltage cathodes. There are various electrode materials introduced for the Li-ion battery till date but only a few of them are successfully commercialized. Since voltage of the battery cell is decided by cathode materials, therefore rigorous research has been done on cathode materials and various materials were proposed, so far for the cathode. The most successful cathode materials are LiCoO2, LiMn2O4 and LiFePO4, and other cathode materials LiNi1/3Co1/3Mn1/3O2, V2O5 and LiV3O8 are also proposed.