Development of Sodium/Lithium-based Metal Oxides as Cathodes for Na-ion Batteries

Abstract

In the present thesis, several sodium/lithium based metal oxides were investigated as cathodes in Na-ion batteries, with the purposes of understanding the relationship between crystal structure and electrochemical behavior, improving long-term cycle performance through stabilizing structure and exploring oxygen redox reaction as another mean of charge transfer. The research started with spinel Li4Mn5O12 to study the electrochemical behavior of spinel structure. Li4Mn5O12 is a well-known Li-ion battery cathode material with good cycling stability and excellent rate capability. In principle, this spinel structure also has open channels to accommodate and transport Na ions. The electrochemical behaviors of Li4Mn5O12 in Li-ion battery and Na-ion battery were evaluated and compared. Na ions can be reversibly inserted into and extracted from the three-dimensional spinel structure. However, unlike in Li-ion battery, the available capacity in Na-ion battery is strongly dependent on the particle size and current rate due to the sluggish Na-ion transport in solid phase. Cycle performance of Li4Mn5O12 in Na-ion battery is also inferior to that in Li-ion battery. Gradual loss of crystallinity, irreversible expansion of the crystal lattice and removal of Li ions from the material upon charging/discharging are found to be responsible for the capacity decay of Li4Mn5O12 in Na-ion battery.
Spinel Li4Mn5O12 is able to accommodate Na ions upon charging/discharging, but its available capacity and cycle performance cannot meet practical demands. Hence, the investigation was then moved on to layered Na0.7MnO2, which is a potential high-capacity cathode material (~200 mAh g-1) for Na-ion battery. Typical Na0.7MnO2 material exhibits a P2 structure with orthorhombic distortion, which is regarded as an adverse factor for Na storage performance. In order to study how the stability of this material is affected by its crystal structure, Na0.7MnO2 was made into an ideal P2 structure by directly adding Li into the precursor during synthesis. Electrochemical tests and ex-situ X-ray diffraction studies reveal that the phase-pure material exhibits fewer phase transitions and less structural degradation upon Na ion de-intercalation/intercalation, resulting in enhanced cycle performance. In addition, Na0.7MnO2 is not stable during natural cooling process after annealing and in ambient air, while the phase-pure Li-additional Na0.7MnO2 can be synthesized with natural cooling, and shows much better stability in air.
Li addition significantly improves the cycle performance of Na0.7MnO2 at the expense of first discharge capacity. In order to minimize the sacrifice in first discharge capacity and improve cycle performance simultaneously, surface coating of Na0.7Ni0.33Mn0.67O2 was performed for Na0.7MnO2. On one hand, electrochemically active Na0.7Ni0.33Mn0.67O2 also contributes capacity, so the loss of first capacity can be minimized. One the other hand, Na0.7Ni0.33Mn0.67O2 is stable in both of electrolyte and air, so the dissolution of Mn ions from Na0.7MnO2 into electrolyte and reaction towards air can be suppressed, thereby enhancing cycle performance and stability against air exposure. Na0.7MnO2/Na0.7Ni0.33Mn0.67O2 materials exhibit pure and well-crystallized P2 structure as well as suppressed Na+/vacancy ordering. Some Ni ions are speculated to be doped into Na0.7MnO2. Cycle performance of Na0.7MnO2 in Na-ion battery and stability of Na0.7MnO2 against air exposure are significantly improved by surface coating/doping. Ex-situ X-ray diffraction and electrochemical impedance spectra studies reveal that surface coating can maintain the crystallinity of the structure and reduce impedance change upon cycling.
After understanding more about the oxide materials that involve the redox of transition metals, we shifted our study to other oxides which are possible to trigger oxygen redox reactions and achieve relatively large amount of charge transfer from oxygen atoms, so as to further increase the capacity of cathode materials. Stoichiometric NaVO3 cathode without excess sodium is demonstrated to be possible to activate oxygen redox reactions in the last part of this thesis. Charge transfer is demonstrated with charge-discharge tests. De-intercalation/intercalation of Na ions from/into NaVO3 after charging and discharging is confirmed by inductively coupled plasma-atomic emission spectrometry. X-ray photoelectron spectroscopy and X-ray absorption spectroscopy results show that the oxidation state of vanadium remains unchanged with charging, which indicates that the charge transfer is due to oxygen ions. The theoretical calculation results of spin density further support the oxygen participation in electrochemical reaction. Ex-situ XRD results show that the structure of the material remains the same in the first cycle regardless of Na content, but obvious loss of crystallinity occurs after 10 cycles. Electrochemical behavior of NaNbO3 and Na2RuO3 is similar to that of NaVO3, suggesting that their reversible capacity may also come from oxygen redox reaction.

Date of Award	1 Sept 2017
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Yau Wai Denis YU (Supervisor)