Study on the nanocomposites as cathodes for next-generation battery

Abstract

Sodium and potassium ion batteries are the most promising candidate to the high price of current lithium-ion batteries. In general, the cathode material determines the performance of the entire battery. However, the electrode materials for sodium and potassium ion batteries reported so far have limited performance and require a search for new cathode materials. In this thesis, I present a new design strategy to develop cathode materials for sodium and potassium ion batteries using nanocomposite between alkali metal compounds and transition metal compounds. Also, the study on the origin of the overpotential in nanocomposite electrodes is presented. In chapter 2, the new type of sodium ion battery cathode materials is realized with NaF-FeF2 nanocomposites. The new host structure for sodium ions is formed during battery operation. The origin of electrochemical activity is investigated with ex-situ X-ray diffraction (XRD) analysis and X-ray absorption near edge structure (XANES) analysis which reveals that the NaF decomposition during charge activates the nanocomposite electrode and Fe2+/Fe3+ redox couple is responsible for electrochemical activity. The host formation behavior during first charge is analyzed with the transmission electron microscopy (TEM) analysis which reveal that the transformation of FeF2 into FeF3 during charge occurs firstly at the surface of FeF2 and propagates into the bulk. In chapter 3, iron oxyfluoride with cubic symmetry, which is a new host structure for sodium ions, is realized with NaF-FeO nanocomposite. The Fe K-edge XANES reveal that the Fe2+/Fe3+ redox reaction occurs reversibly during charge/discharge. The ex-situ XRD and atomic resolution TEM analysis reveal that the host structure formed here is iron oxyfluoride with cubic symmetry. Interestingly, the electrochemical profile is gradually changed as cycle proceeds. The F K-edge XANES analysis at various cycles reveal that the host structure is gradually formed as the cycle proceeds which coincident with the gradual changes in electrochemical profile. In chapter 4, the high energy density cathode for potassium ion battery is realized with KF-MnO nanocomposite. The KF-MnO nanocomposite follows surface conversion reaction which fluorine incorporation into MnO and oxidation of Mn mainly occur at the surface of MnO. The surface concentrated reaction is observed with TEM electron energy loss spectroscopy (EELS) and Mn L-edge XANES analysis. The high utilization of potassium ion per manganese results in high capacity and one of the highest energy density cathode in potassium ion battery ever reported. In chapter 5, the origin of the overpotential in nanocomposite electrodes is studied with the MFx-MnO (M=Li, Na, K, Rb, Cs, Mg, Ca, and Al) model system. As the MFx compounds varies, the activity of the nanocomposite electrode also varies. The origin of the overpotential is analyzed with various tools such as galvanostatic intermittent titration technique (GITT), X-ray photoelectron spectroscopy (XPS), and Rietveld refinement with Williamson-Hall plot which indicate that the lattice energy of MFx is highly correlated with the activity of the nanocomposite electrodes. The lattice energy can be indicated as F 1s binding energy of MFx and the binding energy and electrochemical activity are successfully tuned by making solid-solution between LiF and CsF. The work presented in chapter 4 not only reveal the origin of overpotential but also shows expandability to other battery system such as Ca, Mg, Al battery.