Developments of Cathodes and Solid Conductors towards All-Solid-State Na Batteries

Lithium-ion batteries (LIBs) have been the most successful energy storage technology in modern society. However, the skyrocketing demand of LIBs boosted by the global decarbonization trend and emerging electrical vehicle (EV) market raises the concern about the cost and availability of Lithium. Na-ion batteries (NIBs) utilizing the second lightest alkali metal as working ions, appear naturally as low-cost alternatives to LIBs because of the natural abundance of Na resources. Similar working principle and cell construction between Li- and Na-ion batteries make the latter developed quickly in the last decade, yet most NIBs are not comparable to LIBs in terms of energy density, due to the inherent heavier weight and larger size of Na compounds./nRecently, the remarkable success in the discovery of solid-state superionic conductors brings up the idea of all-solid-state batteries, which also provides an opportunity for NIBs to compete with the current Li-ion technologies. All-solid-state Na batteries could potentially achieve high energy density based on two aspects: 1) densified solid-state conductors as electrolyte can intuitively better prevent dendrite propagation, which makes the utilization of high-energy Na metal anode possible; 2) because of the higher chemical and electrochemical stability, many solid-state electrolytes are compatible with high-voltage Na-ion cathodes, thus could increase the energy density by enlarging the electrochemical window. Therefore, high-voltage Na-ion cathodes and solid-state Na conductors are two key components of all-solid-state Na batteries. Herein, aiming at constructing high-energy all-solid-state Na batteries, this dissertation focuses on the development of both cathode and solid-state electrolyte materials, including materials design, synthesis, characterization and in-depth electrochemical investigation.Developing novel materials with tailored electrochemical properties for battery applications has been mostly performed through experimental trial-and-error approach, requiring a significant time and cost. Such difficulty stems from the lack of a fundamental understanding of the synthesis mechanism. The development of more precise synthesis guideline is thus critical in accelerating the materials discovery. Here, we first use in situ synchrotron X-ray diffraction to investigate the multistage crystallization pathways of the important two-layer (P2) sodium layered cathode materials Na0.67MO2 (M = Co, Mn), and present a theoretical framework to rationalize the observed phase progression. Such understandings reveal the insights of intricate competition between thermodynamics and kinetics during materials synthesis, which can help to realize the rational synthesis design of target materials./nTwo types of Na-ion cathode materials are discussed in this dissertation: Na layered oxides and polyanionic compounds. Though Na layered oxides usually exhibit large specific capacity, we found that this class of materials in general have low operating voltages, which is determined by the inherent strong Na+-Na+ electrostatic interaction within the two-dimensional Na layer. As a piece of evidence, P2-NaxNiyCo1-yO2 cathode containing high-voltage Ni3+/4+ and Co3+/4+ redox were synthesized, however, its operating voltage is not significantly higher than other Na layered oxides containing low-voltage transition metal redox. Consequently, we turned our attention to Na polyanionic compounds, which are able to achieve high voltage because of the less significant Na+-Na+ interaction and the inductive effect of electronegative polyanions. Novel polyanionic cathodes Na4MnCr(PO4)3 with a Sodium (Na) Super Ionic Conductors (NASICON)-type structure was then developed. It delivers a large capacity at a high operating voltage around 3.6 V, leading to the highest energy density among all reported NASICON-type Na cathodes./nIn the meantime, NASICON materials with redox-inactive metals are promising candidates for solid-state Na conductors as well. In order to search for novel NASICONs, we present in the last chapter a high throughput phase diagram dataset consisting of 3881 computed NASICON materials, interpret the thermodynamic stability of NASICON phase from a chemical origin, as well as propose a phenomenological “tolerance factor” to efficiently predict new NASICON materials. With our theoretical predictions, we have successfully synthesized several novel NASICONs with high Na ionic conductivity and high stability against Na metal. Lastly, three main factors (i.e., Na content, cation radius and polyanion chemistry) that influence the ionic conductivity were systematically investigated and rationalized as design principles in order to guide the development of high-performance NASICON conductors in the future.

Advisor: Ceder, Gerbrand GC