Rechargeable lithium ion battery (LIB) and sodium ion battery (NIB) anodes are dominated by carbon-based materials with relatively low capacities. To further increase the energy density of LIB and NIB, alloying anodes such as tin and tin dioxide have been proposed as potential candidates to replace graphite and hard carbon electrodes due to their outstanding theoretical capacities. However, the high capacity is accompanied by a large volume expansion during lithiation and sodiation, therby leading to rapid capacity loss. Here, tin and tin dioxide nanoparticles embedded in various asymmetric membrane structures were synthesized using a straightforward phase inversion method combined with sol-gel chemistry to accommodate the significant volume expansion for long cycle life. The asymmetric membranes consist of a thin, nanoporous top layer and a thick, macroporous bottom layer. The nanoporous top layer can prohibit the loss of fractured electrode materials, while the macroporous bottom layer can accommodate the large volume changes of the active materials. It was demonstrated that thin film SnO₂ asymmetric membrane LIB anode has a specific capacity of 500 mAh g^-1 a current density of 280 mA g^-1 (~0.5C) with >96% capacity retention after 400 cycles. When the current density is increased from 28 to 560 mA g^-1, more than 65% capacity can be retained. Thin film Sn asymmetric membrane NIB anode demonstrated 762 mA h g⁻¹ reversible capacity at 50 mA g⁻¹ and 92.1% capacity retention at 100 mA g⁻¹ over 100 cycles. Different polymers (polysulfone and polyacrylonitrile) and tin precursors (tin (IV) 2-ethylhexanoate diisopropoxide and tin (IV) tert-butoxide) were selected to investigate their impact on membrane porosity, membrane mechanical strength, capacity, rate performance and cyclability. We also studied the effect of carbonization temperature, concentration of active materials, thickness and geometry of asymmetric membranes (thin film vs. fiber) on chemical composition, structural and electrochemical properties by using extensive instrumental characterization that ranges from scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS), Raman spectroscopy, powder X-ray diffractometer (PXRD), thermogravimetric analyzer (TGA), surface area analyzer (BET method) to X-ray photoelectron spectroscopy (XPS), as well as various electrochemical tests such as cycling and rate performance, cyclic voltammetry, and electrochemical impedance spectroscopy.

Jianlin Li

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