Electrochemical Properties Of NaMO₂Compounds As Cathode Materials For Sodium-ion Batteries

Currently, lithium-ion batteries, fuel cells and hydrogen storage materials are gradually replacing the traditional energy. Rechargeable lithium batteries, commonly known as lithium-ion batteries, are key components of the portable, entertainment, computing and telecommunication equipment required by today’s information-rich, mobile society. With the developments of electric vehicles and other power systems, the requirements of the secondary batteries are demanded from small electronic devices to high-energy density. Prices of lithium will rise if a large number of electric cars and hybrid electric vehicles are needed in the future. These factors will limit the application and development of Li ion batteries. In response to the great interest in searching new advanced batteries for electric vehicles (EV) and smart grids, sodium ion secondary batteries (SIBs) have drawn wide attention due to the abundant resources and low costs.In the early1980s, Miyazaki et al. examined the electrochemical performances of NaCoO2, NaCrO2, NaFeO2, NaNiO2. However, the work on sodium ion secondary batteries (SIBs) progressed at a slow pace. This thesis will focus on the investigations of NaM02materials synthesized by a solid-state reaction and sol-gel method. Charge/discharge measurements and cyclic voltammograms were used to study cycle performances and electrochemical properties of the materials. X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectra (XAS) were employed to detect the composition and structure information of the materials in certain electrochemical states. Thereby the electrochemical reaction mechanism were discussed. The following systems were included:1. Electrochemical Properties of P2-Na0.74CoO2Compounds as Cathode Material for Rechargeable Sodium-ion Batteries. The P2-Na0.74CoO2as cathode material for sodium-ion batteries has been successfully synthesized by a solid-state reaction in air. It exhibits better electrochemical performance in NaPF6than NaClO4. SEM images of the pristine Na0.74CoO2powders indicate micro-sized agglomeration with irregular shapes and reduce the size of powders by add citric acid. The XPS, XRD and XAS results provide strong evidences on that the deintercalation/intercalation of Na ions from/into the layered structure proceeds with the Co3+/Co4+redox reaction.2. Cycle Performance Improvement of NaCrO2Cathode by Carbon Coating for Sodium Ion Batteries. NaCrO2particles with a uniform carbon coating have been successfully synthesized using citric acid as the carbon source. The carbon-coated NaCrO2electrode at rate of5mAg-1exhibits the larger discharge capacities of117mAhg-1if comparing with those of the naked sample which is107mAhg-1. The results suggested that the carbon-coated NaCrO2exhibits better cycle performance with the less capacity fading compared to the naked one. Previous studies on the carbon coated cathode material for Li ion batteries showed that carbon coating can prevent the active material from direct contact with the electrolyte and suppress the undesired side reactions between active material and electrolyte, as well as slowing down the SEI formation on the electrode surface. In addition, the coated carbon layer can enhance the electronic conductivity and decrease the polarization of the electrode. The similar improvement can also be applied on the electrode materials for storage sodium.3. In Situ XRD and XAS Studies of NaNio.5Mno.5O2for Sodium-Ion Batteries. XRD shows all the diffraction peaks can be indexed as hexagonal a-NaFeO2structure. The galvanostatic cycling profiles of the Na1-xNi0.5Mno.5O2/Na cell cycled between2.0and3.8V under a current density of8mA g-1indicates the first discharge capacity of the sample is125mAhg-1. In situ XAS shows the divalent nickel [Ni2+] is oxidized to tetravalent nickel [Ni4+] through an intermediate stage of trivalent nickel [Ni3+]. In situ X-ray diffraction pattern shows the structure of NaNio.5Mno.5O2changes from hex.03to mon. P3.In addition, appendix research works I made are introduced. It contains two topics:1. Layered LiNi1/4Mn1/2Co1/302as Cathode Material for All-solid-state Thin Film Rechargeable Lithium-ion Batteries. Li(Ni1/4Mn1/2Co1/3)O2thin film electrodes have been successfully fabricated by magnetron sputtering for the first time. The electrochemical behaviors of thin film electrodes have been investigated in liquid and solid electrolytes. The interface resistant between LiPON and cathode is quite large initially and then decrease gradually in the following cycles. So the cycle behavior of all solid-state thin film lithium battery should be related the interface feature between LiPON and Li(Ni1/4Mn1/2Co1/3)O2. 2. Ga2Se3Thin Film as a Negative Electrode Material for Lithium-Ion Batteries. The electrochemical properties of Ga2Se3thin films prepared by thermal co-evaporation technique have been investigated for the first time. The reversible discharge capacity of712mAh g-1was achieved for Li/Ga2Se3cells cycled between0and3.0V at0.1C. The discharge capacity after the100th cycles was about569.92mAh g-1. By using ex situ HRTEM and SAED measurements, the electrochemical reaction mechanism of Ga2Se3thin film with lithium involve both alloying/dealloying between Li2Ga and Ga, and selenylation/reduction reaction between Ga2Se3and Ga.These results provide some significance and reference value for the Na ion batteries and Li ion batteries.