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Multi-electron Polyanion Cathode, VOPO4, for High Energy Density Alkali-ion Batteries


Today’s commercial lithium-ion battery cathodes function on the basis of a single-electron transfer per transition metal. In order to attain significantly higher capacities, particularly in polyanionic compounds with higher voltages, achieving reversible multi-electron transfer per transition metal is necessary. In this thesis, we focused on the study of one of the promising multielectron cathodes, AVOPO4. (A = Li, Na). The aim of this thesis is to use first-principles calculations to predict the thermodynamic stability and kinetics and provide insights to improve the electrochemical performance of AVOPO4, as well as explain its experimental findings. This thesis comprises three complementary projects.

In the first project, we demonstrated the stable cycling of more than one Li in solid-state-synthesized ε-LiVOPO4 over more than 20 cycles for the first time. Using joint first-principles calculations and experimental measurements, we presented a comprehensive analysis of the thermodynamics, kinetics, and structural evolution of ε-LixVOPO4 over the entire lithiation range (x=0 ~ 2). We unveiled two intermediate phases at x = 1.5 and 1.75 in the low-voltage regime (x=1 ~ 2). We showed that the capacity limitation in the high-voltage region is likely driven by Li mobility limitations whereas the increasing polarization in the low-voltage region is the result of structural changes. Finally, we predicted that ε-LixVOPO4 is likely a pseudo-1D ionic diffuser with low electronic conductivity using DFT calculations, which suggests that nanosizing and carbon coating are crucial to achieve good electrochemical performance in this material.

In the second project, we conducted a combined first-principles and experimental study to evaluate the thermodynamic stability, voltage, band gap and diffusion kinetics for Li and Na intercalation in the β-, ε- and αI- polymorphs of VOPO4. We found that all VOPO4 polymorphs remain reasonably stable with one alkali-ion insertion, but are significantly destabilized with two alkali-ion insertion. We predicted that the αΙ polymorph has higher Li+ migration barriers and larger band gaps than the other polymorphs, which accounts for the relatively worse electrochemical cycling performance observed. On the other hand, only the layered αΙ polymorph exhibits reasonably low barriers for Na+ migration, which are consistent with observed electrochemical performances reported thus far in the literature. We also showed that differences in voltage, kinetics and rate capability of these different polymorphs for alkali-ion insertion can be ascribed to their fundamentally different VO6/VO5-PO4 frameworks.

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