Infiltrated positive electrodes for all-solid-state sodium batteries

Abstract

All-solid-state batteries (ASSBs) are regarded as promising candidates for next-generation energy storage systems due to several superior properties in comparison to state-of-the-art batteries using liquid electrolytes. ASSBs have advantages with respect to no-leaking electrolytes, wider temperature window, and potentially higher volumetric energy density than those batteries using liquid electrolyte. In addition, ceramic ion-conducting oxides, as electrolyte materials, offer further benefit from their high conductivity, as well as good mechanical, chemical, and thermodynamic stability. Though great achievements have been made in the development of all-solid-state batteries, there are still a number of problems to be solved, especially with respect to the positive electrodes. The rigid nature of solid-state electrolyte materials restricts the electrolyte-electrode contact and causes problems during the fabrication of components. In the positive electrodes, contacts between randomly-arranged grains of Na-ion conductors and electrode active materials are not efficient for both ion conduction and electrochemical reaction. These rigid contacts are further damaged by volume changes of electrode active materials during electrochemical cycling. These electrode problems above are reported to be more severe in Na-based ASSBs (Na-ASSBs) due to the larger ionic radius of Na+ and thus larger volume changes of the electrode materials. In order to solve the problems associated with the positive electrodes of Na-ASSBs, a new electrode design is needed together with a practical preparation method. In this thesis, Na-ASSBs have been built using NaSICON-type Na3.4Zr2Si2.4P0.6O12 as the electrolyte material. A material with similar structure as the electrolyte, Na3V2P3O12, was chosen as the positive electrode material. Na3V2P3O12 has been widely studied as the electrode material for batteries with liquid electrolyte, but only a few unsuccessful attempts have been made to apply the material in ASSBs. As a starting point, the basic properties of Na3V2P3O12 were studied, from the synthesis to the characterization of the pure material. Then model ASSBs were built based on a back-bone design of the positive electrode. A new method, named chemical infiltration, has been developed combining the ideas of infiltration with in situ synthesis of the electrode material. In situ synthesis of the electrode material was performed to improve the interface between electrode and electrolyte materials, and was realized by infiltrating a precursor solution and forming the active electrode phase by a chemical reaction on the pore walls of the electrolyte. In ASSBs made by chemical infiltration, the electrode material showed excellent adhesion to the electrolyte, resulting in a low internal resistance of the cell. Additionally, this electrode structure ensures a high tolerance to the volume change of the electrode material. As a result, the Na-ASSBs have highly stable performance with low internal resistance and low fading rate. Na-ASSBs in this thesis became the first successful examples showing that contact problems in positive electrode can be solved without using any soft phase (liquid, polymers, ionic liquids etc.) as a compensator of volume changes or wetting medium. This method also enables the fabrication of very thick electrode layers for high capacities. Based on the experimental studies in this work, the future developments of ASSBs, including the possibilities and limitations, can be better assessed.