Sulfide Anode Materials and Thiostannate Solid Electrolytes for Sodium-Ion Batteries : Investigation of Electrochemical Properties, Structure-Property Relationships, and Elucidation of Sodium Storage Mechanisms

The electrochemical properties of the potential anode active materials CuCrS2, MCr2S4 (M = Ti, Cr, Fe, Ni), and Fe3S4 were tested in sodium-ion battery cells in the first part of this thesis. New charge storage mechanisms were elucidated for CuCrS2 and NiCr2S4: electrochemical insertion of Na induces the reduction of Cu+/Ni2+ ions, which are expelled from the pseudo-layered parent compounds via formation of nanocrystalline Cu0/Ni0 and intermediate NaCrS2. This is particularly remarkable since the sulfides MCr2S4 (M = Ti, Cr, Fe) are directly converted into the metal elements during sodium uptake without intermediate formation. All chromium sulfides lose their structural integrity after full discharge vs Na+/Na, resulting in the generation of amorphous products and nanocrystalline Na2S. Moreover, a multi-step conversion mechanism was revealed for Na/Fe3S4 cells, which is fully reversible.

The second part of this thesis addresses structure-property relationships of ion-conducting sodium thiostannates. Directed thermal dehydration of appropriate hydrated precursors yields the new sodium thiostannates Na4SnS4 and Na4Sn2S6, respectively, both of which exhibit a decent ionic conductivity. Profound studies of the crystal structures and local ionic environments are supported by quantum mechanical calculations and explain the enhanced Na+ ion conduction properties of the new kinetically stabilized Na4SnS4 polymorph compared to the known thermodynamically stabilized form as well as superionic conductivity achieved by aliovalent substitution of Sn4+ by Sb5+ ions.


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