UC San Diego

The alkali-ion batteries are the key to unlock the bottleneck of the renewable energy storage and pave the way for a renewable-powered future. Battery technologies for grid-scale energy storage systems requires low costs, safety, high efficiency and high sustainability. In this dissertation, we present not only in-depth understandings of the electrode working mechanism but also develop novel cathode materials for alkali-ion batteries using first principles calculations. We divide the dissertation into four project-based parts.

In the first project, we performed a comprehensive study of Prussian blue and its analogues (PBAs) cathodes in aqueous sodium-ion batteries. Using density functional theory calculations, we proposed a general rule of the phase transition that dry PBAs generally undergo a phase transition from a rhombohedral Na2PR(CN)6 (where P and R are transition metals) to a tetragonal/cubic PR(CN)6 during Na extraction, which is in line with experimental observations. Using a grand potential phase diagram construction, we show that existence of lattice water and Na co-intercalation contribute to both higher energy density and better cycling stability. We also identified four new PBA compositions {Na2CoMn(CN)6, Na2NiMn(CN)6, Na2CuMn(CN)6 and Na2ZnMn(CN)6 – that show great promise as cathodes for aqueous rechargeable Na-ion batteries.

In the second project, we developed design rules for aqueous sodium-ion battery cathodes through a comprehensive density functional theory study of the working potential and aqueous stability of known cathode materials. These design rules were applied in a high-throughput screening of Na-ion battery cathode materials for application in aqueous electrolytes. Five promising cathode materials - NASICON-Na3Fe2(PO4)3, Na2FePO4F, Na3FeCO3PO4, alluadite-Na2Fe3(PO4)3 and Na3MnCO3PO4, were identified as hitherto unexplored aqueous sodium-ion battery cathodes, with high voltage, good capacity, high stability in aqueous environments and facile Na-ion migration. These findings pave the way the practical cathode development for large-scale energy storage systems based on aqueous Na-ion battery chemistry.

Then in the third project, we constructed a large database of aqueous Na-ion battery cathodes (Na-ion Aqueous Electrode Database, or NAED) based on the developed design rules in the second project. By screening and analyze the data in the database, we identified two promising candidates, NaMn2O4 and Na2(FeVO4)3 for synthesis and experimentation in aqueous sodium-ion batteries.

The final project presents a comprehensive study of Li insertion mechanism in DRX - Li3V2O5 anode in Li-ion batteries. Using a combination of first-principles calculations, cluster expansion and machine learning methods, we show that during discharge, Li ions mainly intercalate into tetrahedral sites, while the majority of Li and V ions in octahedral sites remain stable. Furthermore, its fast-charging nature is attributed to the facile diffusivity of Li ions via a correlated "octahedral - tetrahedral - octahedral" Li diffusion.