Synthesis, Structure and Electrochemistry of Positive Electrode Materials for Rechargeable Magnesium and Lithium Ion Batteries: Mechanistic Investigations

Abstract

To meet the requirements for high energy density storage systems, rechargeable batteries based on the “beyond lithium ion” technologies have been widely investigated. The magnesium battery is a promising candidate benefiting from the utilization of a Mg metal negative electrode, which offers high volumetric capacity (3833 mAh mL-1), low redox potential (-2.37 V vs. S.H.E.), non-dendritic growth, low price and safe handling in atmosphere. However, the discovery of potential positive electrode materials beyond the seminal Mo6S8 has been limited, mainly due to the sluggish mobility of a divalent Mg2+ ion in solid frameworks. This thesis presents the research on both finding new positive electrode materials and investigating mechanisms to understand the limitation. Two structures of titanium sulfide are identified as the second family of Mg2+ insertion positive electrodes, offering almost twice the capacity of the benchmark Mo6S8. The facile Mg2+ solid diffusion is mainly supported by the polarizable lattices, while the crystal structure plays a critical rule on the specific diffusion mechanism, which further influences the electrochemistry. While sulfides provide moderate energy density, it can be largely increased by shifting to oxide materials. However, poor electrochemistry has been widely observed for oxide based Mg positive electrode materials. In the present thesis work, a case study with birnessite MnO2 identifies desolvation as a key factor limiting Mg2+ insertion into oxides from nonaqueous electrolytes, while another study with Mg2Mo3O8 demonstrates the strong influence of transition states on setting the magnitude of migration barriers. Those limitations have to be overcome to allow facile Mg2+ insertion into oxides. Alternative setups which would accomplish the advantages of a Mg negative electrode and avoid the sluggish Mg2+ solid diffusion include the Mg-Li hybrid system. Two “high voltage” Prussian blue analogues (average 2.3 V vs. Mg/Mg2+) are investigated as positive electrode materials in the thesis, both showing promising energy density and cycle life. Finally, novel positive electrode materials for Li-ion batteries are examined. The possibility of stabilizing lithium transition-metal silicate in the olivine structure is studied by combined atomistic scale simulation and solid state synthesis, suggesting a potential solution by cation substitution.