Cobalt free nanomaterials as positive electrodes for Lithium ion battery

Abstract

Cathode materials with improved specific energy, energy density, safety and reduced cost are essential for developing next generation Li-ion battery technology. This can be made possible by exploring new chemistries of cathode materials, which exhibit either higher operating voltages or higher specific capacity compared to the materials currently available in the market. As a result, new materials’ utilizing multielectron step redox reaction has gained attention. Such materials can exchange more than one Li-ion per formula unit and therefore, specific energy is improved. In my thesis, I focus on fundamental understanding of such cathode materials and establish a strong relationship between the physical properties of the active cathode materials and their electrochemical behaviour. This study leads to a proposition that nano-sizing is the key to further improve the specific energy of cathode materials for Li-ion batteries. My thesis provides a scope to further analyse and optimize materials as well as electrode preparation methods for nano-crystalline materials, and thus realizing a high-energy Li-ion cell with materials using different redox steps. Among the polyanion-based cathode materials, layered Li9V3(P2O7)3(PO4)2 (LVPP) is investigated as a high voltage cathode. In comparison to the commercially available LiFePO4, where 1 Li-ion per formula unit is exchanged, LVPP is theoretically predicted to exchange 6 Li-ions from the structure utilizing V3+ to V5+ redox reactions at an average potential of 4.2 V, thus delivering specific capacity of 173 mA h g -1. The feasibility of multi-electron step reactions in phosphate-based LVPP cathode and the factors influencing the overall electrochemical behaviour has been explored. In addition, the mechanism of Li extraction/insertion during charge and discharge is investigated and the structural transformations are studied by means of in situand ex situ X-ray diffraction. The results obtained suggest that by optimizing the material synthesis for crystallite size and using specific cycling conditions facilitates multielectron step redox reactions in LVPP. Among the oxide based cathode materials, LiNi0.5Mn1.5O4 (LMNO) is one of the most promising cathode materials because of its high operating potential at 4.7 V, and a theoretical specific capacity of 146 mA h g -1 . Furthermore, LMNO is completely cobalt-free, which makes it cost-effective compared to the commercially available cathode materials. Even though LMNO consists of two active redox centres such as Ni and Mn, only the Ni (II)  Ni (IV) redox reaction has received considerable attention. v However, when additionally utilizing the Mn redox reactions much improvement in the capacity can be obtained. The feasibility of multi-electron step reactions involving both Mn and Ni redox centres and the factors influencing the reversibility, overall capacity and kinetics have been studied. The electrochemical investigations on the Mn (IV) to Mn (III) redox reactions in LMNO clearly suggest that optimizing two important factors such as crystallite size and potential window are very important to improve both the specific capacity and kinetics. In the potential window of 2.4 – 4.9 V specific capacities from both Mn and Ni redox plateaus are exploited and a very high reversible capacity of 250 mA h g-1 has been demonstrated. This is put forth as the state of art for the cobalt free LMNO based cathodes.