Thesis (Ph. D.)--University of Rochester. Materials Science Program, 2017.
Sodium-ion batteries (SIB) have been considered as a possible supplemental to lithium-ion batteries due to the low cost and high abundance of Na. Among all the cathode materials that have been studied in SIB, layered transition metal oxides, NaxMO2 (M= Mn, Ni, Co, Fe) with their binary and ternary derivatives are of great interest due to their high specific capacities. In order to get a better understanding of NaxMO2, it is important to decouple the mixing transition metals and carry out a thorough investigation on the end member of the series, such as NaxMnO2. On top of that, it is particularly attractive idea of the possibility in enabling NaxMnO2 or Mn based oxides as cathode material for SIBs due to its benefit of low cost and environmental friendly nature.
The relationship between the synthetic conditions through solid-state synthesis route and the structure of as-prepared cathode material as well as the structure and the electrochemical performance was investigated using NaxMnO2 model material. The structural change during solid-state synthesis of P- and Oꞌ3-type NaxMnO2 were unveiled for the first time by taking advantage of in situ high energy X-ray diffraction (HEXRD) and in situ X-ray absorption near edge spectroscopy (XANES) technique. Electrochemical performance for NaxMnO2 with P2- and Oꞌ3-type were tested. Na2/3MnO2 with P2-type structure shows good reversible capacity and capacity retention but low Na stoichiometry, whereas NaMnO2 with Oꞌ3-type structure exhibits high Na stoichiometry and reversible capacity at beginning but fast capacity decay as cycle progresses. Moreover, the fundamental reasons behind the capacity fading was investigated. A few factors have been found that could responsible for the fast capacity decay, including irreversible phase transitions occurred at about 3.6 V, and slow kinetics of sodium insertion at the end of discharge.
At last, a Ni and Co co-doping method was utilized in improving structural stability at high voltage for Mn rich sodium transition metal oxides. It was found out that the NaxNi0.19Co0.28Mn0.53O2 consists mixed P2/P3 structure, and its electrochemical performance cycled at a relatively high voltage, 2-4.5 V, can be further improved by a simple washing procedure. It is speculated that the preferred formation of transition metal vacancy around Ni for the sample after rinsing treatment is the key for better electrochemical performance.
