Lithium ion batteries (LIBs) are approaching their theoretical capacity limit but an exponentially increasing demand for rechargeable electrochemical storage solutions for technologies ranging from portable devices, electric vehicles to large-scale energy storage systems is motivating intensive research for viable post-lithium ion battery chemistries. It is in this regard that sodium, due to its electrochemically similar storage behavior to Li but vastly lower precursor prices, is being explored as a feasible alternative. Graphite, anode material of choice for LIBs, is incompatible with sodium ion batteries (SIBs) due to larger ionic size of Na which greatly lowers the battery capacity. The use of Na metal as anode can help to realize highest theoretical capacity for the anode but mossy...[ Read more ]

Lithium ion batteries (LIBs) are approaching their theoretical capacity limit but an exponentially increasing demand for rechargeable electrochemical storage solutions for technologies ranging from portable devices, electric vehicles to large-scale energy storage systems is motivating intensive research for viable post-lithium ion battery chemistries. It is in this regard that sodium, due to its electrochemically similar storage behavior to Li but vastly lower precursor prices, is being explored as a feasible alternative. Graphite, anode material of choice for LIBs, is incompatible with sodium ion batteries (SIBs) due to larger ionic size of Na which greatly lowers the battery capacity. The use of Na metal as anode can help to realize highest theoretical capacity for the anode but mossy, dendritic growth of Na during plating/stripping process poses severe challenges to electrochemical stability and battery safety. Various approaches are being pursued to alleviate these challenges such as (ⅰ) the use of three-dimensional (3D) current collectors, (ⅱ) nanostructured hosts, (ⅲ) artificial secondary electrolyte interphase (ASEI) layers, and (ⅳ) engineered electrolytes. Rationally designed nanostructured carbon hosts are one of the most promising approach to enable Na metal anodes. Therefore, this thesis is dedicated to the investigation of electrospun 3D host structures and especially the correlation between surface morphology and chemistry and their interplay with the generation of SEI layer and electrochemical performance.

Metal organic framework-induced mesoporous, defect-engineered carbon nanofibers (PCNFs) are electrospun as a sodiophilic host for Na metal anode. Abundant mesopores are introduced by decomposition of bimetallic metal organic framework (MOF) nanoparticles during carbonization, which create uniformly distributed active sites for Na nucleation and allow facile electrolyte access to the entire electrode. The nitrogen and oxygen functional groups on fiber surface reduce the Na adsorption energy barrier, ameliorating the nucleation and deposition of Na, as corroborated by the density functional theory (DFT) calculations. The anode is cycled for over 4,000 hr at 1mA cm^-2 and over 2,000 hr at a higher current density of 3mA cm^-2 with stable voltage profiles and extremely low hysteresis owing to full utilization of carbon matrix. Furthermore, the Na-C/Na₃V₂(PO₄)₃ (NVP) full cell endures 1,000 cycles with Coulombic efficiencies greater than 98%.

To unveil the correlation between host morphology, surface chemistry, and electrochemical performance a hollow, mesoporous carbon nanofiber (HpCNF) structure was designed using co-axial electrospinning. The combined in situ TEM and cryogenic microscopy along with theoretical simulations reveal that the highly sodiophilic HpCNFs with abundant defects and nitrogen functional groups enable compact, uniform plating of Na with excellent reversibility aided by a thin, resilient, fluorine-rich SEI layer. Thanks to the optimized Na deposition in the entire structure, the Na@HpCNF anodes present an average Coulombic efficiency of 99.7% after 1,400 cycles at a current density of 3 mA cm^-2 and a plating/striping capacity of 6 mAh cm^-2. Their symmetric cell maintains stable cycles for over 1,000 hr at 5 mA cm^-2 and 5 mAh cm^-2, which is among the best when compared with state-of-the-art electrodes. The full cells paired with a Na₃V₂(PO₄)₂F₃ cathode deliver remarkable specific capacities of 115 and 93 mAh cm^-2 after 500 cycles at 1C and 200 cycles at 4C, respectively.

To augment the economic benefits of sodium batteries a sustainable carbon structure is fabricated by using aqueous electrospinning of low-cost, renewable lignin biopolymer. A unique super-sodiophilic, defect-rich and hierarchically porous skeletal carbon nanofiber (SCNF) host structure is fabricated that enables SCNF@Na composite anodes by rapid Na melt infiltration. The uniform nucleation and plating of Na inside the hierarchically porous structure coupled with defect-induced formation of a resilient, F-rich SEI layer offers excellent protection to the metallic anode. The beneficial role of defect-rich surface and porous structure in mediating dense Na nucleation, planar growth and electrochemical stability is demonstrated via theoretical and experimental measurements and analysis. The SCNF@Na composite anode maintains high Coulombic efficiencies ≈ 99.6% after 500 cycles in asymmetric cell at a current density of 2 mA cm^-2 for a plating capacity of 2 mAh cm^-2. The symmetric cells achieve excellent electrochemical reversibility with low overpotentials, and negligible impedance increase for 1,000 hr at a current density of 3 mA cm^-2 for a plating capacity of 3 mAh cm^-2. Full cells prepared by interfacing the SCNF@Na anode with Na₃V₂(PO₄)₂F₃ (NVPF) cathode exhibit excellent rate capability, enhanced capacity retention, and long cycle life. Such rationally architectured sustainable carbon hosts derived from biopolymers open new vistas for the exploration of wettable host structures for safe and low-cost metal batteries.