UC San Diego

Sodium-ion batteries (NIBs) are a promising solution for grid storage because they are inexpensive, sustainable, and have suitable energy densities. Exploration of energy-dense NIB cathode materials that include abundant and inexpensive elements has led to the investigation of sodium iron manganese oxides. However, these materials suffer from poor capacity retention, and their reaction mechanisms remain that presents an obstacle to their optimization. Herein we optimized a novel co-precipitation method that can be used to synthesize sphere-like meso-structured sodium transition metal (TM) oxide cathode materials. The key parameter for controlling the meso-structure was found to be the synthesis cooling rate and the sphere-like meso-structure improved the capacity and lifetime of the cathodes. Using this same synthesis route, P2-Na2/3FexMn1-xO2 (NFMO) cathodes with three Fe:Mn ratios were prepared. The redox mechanisms dependence on the Fe content were explored with X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations. A Fe:Mn ratio less than 1:2 was found to promote capacity retention. One pathway to improve the energy density of NIB cathodes is to enable oxygen redox activity. Oxygen redox activity was triggered in Na0.8Li0.12Ni0.22Mn0.66O2 (NLNMO) by TM layer ordering, however it was irreversible after the first charge. The mechanism of irreversible oxygen activity in NLNMO was observed with XAS, synchrotron X-ray diffraction (sXRD), pair distribution function (PDF), and DFT. Ni migration was found to be detrimental to the reversibility of oxygen redox activity. In addition to NIB cathodes, the anode performances need to be improved. Specifically, hard carbon (HC), one of the most attractive anode materials for NIB, has a low first cycle coulombic efficiency and poor rate capability. These drawbacks appear to be electrolyte dependent, since ether-based electrolytes can largely improve the HC performance compared to carbonate electrolytes. Using titration gas chromatography (TGC), Raman spectroscopy, cyro-transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS), we found that the electrolyte controls the quality of the solid electrolyte interphase (SEI), which in turn, controls the first cycle coulombic efficiency and the rate capability. Overall, this work provides insight on how to improve the design of inexpensive, sustainable, energy-dense NIBs for grid storage.