The increasing intent to proliferate electric mobility vehicles and produce storage means for renewable energy (solar & wind) harvesting systems has increased attention on developing high energy density Li-ion batteries (LIBs). Present Li-ion battery technology lacks one or more of the required battery properties for large scale storage applications. They are extended cycle life, the capability to store large energy density, operating with acceptable safety norms, and production capability at a lower cost. The deterioration of LIB electrochemical performance over long-term cycling is caused both by the active (cathode, anode & electrolyte) and inactive (carbon black, PVDF & current collector) components escalated by unwanted chemical reactions. Among many of these contributions, the most critical deterioration is due to the transition metal ion dissolution through the secondary reaction of the surface structure of cathode materials with acidic HF in the electrolyte. This leads to an unstable electrode-electrolyte interface accompanied by the surface structure phase transition, thus, significan tly affecting the capacity and voltage fading of Li-ion cells. This thesis work focuses on mitigating the degradation of cathode material by employing a surface coating process, specifically testing layered oxide-based cathodes, upon extended electrochemical charge-discharge cycling.
In this thesis, a novel in-situ carbon encapsulation process on layered oxides during the solid-state reaction of carbon source pillared transition metal hydroxide and lithium hydroxide is demonstrated. The effect of carbon encapsulation on the electrochemical properties, of two different compositions viz. (i) LiNi1/3Co1/3Mn1/3O2 (LNMCO) and (ii) Li-rich layered oxide; Li1.15(Ni0.23Co0.08Mn0.54)O2 (LLO) are studied. The hydroxides pillared by ethylene glycol (EG) in between the interlayer structures, facilitated by the smaller size of the monomer act as an internal source of carbon encapsulation during the crystallization and growth of lithium transition metal oxides. The method uses a single-step approach whereby EG pillared transition metal hydroxide and lithium precursor at high-temperature heat treatment yield carbon-coated LiNi1/3Mn1/3Co1/3O2 and Li1.15Ni0.23Co0.08Mn0.54O2. This process is simple and uses a novel concept in contrast to methods that involve pyrolysis of c arbon source mixed with oxide cathode material under inert conditions as an additional step. The carbon encapsulation on layered oxide materials does not affect the phase formation and oxidation states of the transition metals. The electrochemical properties obtained on carbon-coated LiNi1/3Mn1/3Co1/3O2 were shown to be superior with 80 % capacity retention after 1000 cycles of charge/discharging in full cell configuration.
Adopting this surface coating technique on high voltage lithium-rich layered oxide materials has drastically reduced the first cycle irreversible capacity loss. Carbon coating has increased the reversibility of anionic redox reaction and reduced the nominal voltage fading with repeated cycling by suppressing surface phase transitions. At full cell level electrochemical charge/discharging of carbon-coated Li1.15(Ni0.23Co0.08Mn0.54)O2 could deliver energy density of above 500 Wh/kg even after 100 cycles.

Keywords: Li-ion Battery, Cathode Materials, Layered and Lithium rich Layered Oxides, Ethylene Glycol, Carbon Coating.