Development of layered sodium transition metal oxides as cathode materials for sodium-ion batteries

With the objective to reach net-zero carbon emissions, mobile and stationary electrical energy storage is gaining significant importance. Currently these applications are dominated by lithium-ion batteries, however, concerns about vulnerable supply chains and potential supply shocks call for a complementary battery technology. Sodium-ion batteries (SIB) are currently developed and commercialized by academia as well as industry and hold the potential as a cost-effective and complementary battery technology to today’s most affordable lithium-ion batteries, if the energy density of sodium-ion batteries can be improved further. This thesis addresses various current challenges of layered oxide cathode materials for the next generation of high-energy sodium-ion batteries in three scientific publications. The first publication, presented in Section 3.1, addresses Na+/vacancy orderings in P2-type layered oxides as cathode active material (CAM) for SIBs. By comparing the closely related materials P2-NaxNi1/3Mn2/3O2 and P2-NaxMn3/4Ni1/4O2, the chemical and structural context of Na+/vacancy orderings are uncovered using various experimental and computational methods. A guideline to estimate the tendency for Na+/vacancy orderings is proposed and crosschecked against various cathode compositions and literature reports. The guideline can serve to design new P2-type layered oxide cathode materials with improved cycling stability based on the absence of Na+/vacancy orderings. The second publication, presented in Section 3.2, takes a closer look at P2-NaxMn3/4Ni1/4O2 addressing the synthesis, structure, electrochemistry and robustness against storage in moist air. This publication explores the relationship between composition, temperature and crystal structure and investigates the synthesis route for a phase pure P2-type material in detail. The as-prepared material exhibits stacking faults, which are described and quantified for the first time and discussed with respect to the synthesis route. The P2-NaxMn3/4Ni1/4O2 CAM exhibits attractive performance in electrochemical half-cells, however, a significant capacity decay occurs during the first cycles. Lastly, a model storage experiment was performed to evaluate the reactivity of the developed P2-NaxMn3/4Ni1/4O2 cathode material with moist air. The third publication, presented in Section 3.3, addresses the origin of capacity fade in P2-NaxMn3/4Ni1/4O2. Three main aging mechanisms are isolated and their origin uncovered, namely surface densification, intra-crystalline cracking and Jahn-Teller distortion. To the best of my knowledge, this is the first scientific report providing proof for surface densification in P2-type layered oxides. The findings are then discussed in Section 4 in the context of recent scientific literature. Guidelines for the design of high-performance layered oxide cathode materials are proposed and as-designed cathode materials are presented. Finally, future directions for the development of powerful layered oxide cathode materials for the next generation of high-energy SIBs are suggested in Section 5.