Highlights: Further performance improvements of sodium-ion batteries require better-performing electrode materials, particularly anodes. The layered lepidocrocite-type sodium titanate (Na0.74Ti1.815□0.185O4·1.27H2O), showing a high Na+ storage capacity of 229 mAh g−1 at relatively low average voltage of ca. 0.6 V vs. Na+/Na, is a promising candidate anode material. Abstract: A lepidocrocite-structured sodium titanate prepared by ion-exchange of a Cs-containing precursor shows promise as an anode material for sodium ion batteries, with a discharge capacity of up to 229 mAh g−1 at an average potential of about 0.6 V vs. Na+/Na. Titanium vacancies in the metal oxide layers provide additional sites for sodium intercalation in addition to interlayer sites, which accounts for the higher capacity compared to other previously reported lepidocrocite-structured titanates. By screening a series of electrolyte formulations and binders, we were able to improve the first-cycle coulombic efficiency to 81.8% and 94.7% respectively using CMC/SBR-based and binder-free electrodes in ether electrolytes. The electrochemical consequences of short-term air-exposure on the electrodes are also discussed. Graphic abstract: [Figure not available: see fulltext.]

Sodium titanate anodes synthesized via “chimie-douce” methods generally require a post-synthesis dehydration step. Their crystal structures are sensitive to the heating conditions such as temperature, resulting in distinct sodium storage performance. Herein, using in situ and ex situ high-temperature X-ray diffraction, we studied in detail the structural changes of Na0.74Ti1.815□0.185O4·1.27H2O (NTO) under various heating conditions. Electrochemical characterization revealed that NTOs heated at lower temperatures with layered structures deliver larger capacities than those heated at high temperatures with tunnel structures. Heat treatment at moderate temperatures greatly improves the capacity retention of NTO, especially stabilizing the high voltage processes that are major contributors to the capacity fading, because of the modified crystal structure and surface chemistry. Our findings suggest that post-synthesis heating is a simple and effective strategy to tailor the crystal structure, surface chemistry, and electrochemical properties of sodium titanate anodes.

NaTi3O6(OH)·2H2O, also known as “sodium nonatitanate” (NNT) can undergo reversible sodium (de)insertion at low potentials centered around 0.3 V. The low average insertion potential and high theoretical capacity (∼200 mA h g−1based on site considerations) suggest that it can be a promising high energy density anode material for sodium-ion batteries. However, its low practical capacity, poor capacity retention, and low initial coulombic efficiency require further material and electrode optimization. Herein, the optimization of the material properties of NNT as well as electrode engineering were used to improve these aspects of the electrochemical performance. Characterization tools including pair distribution function analysis, synchrotron X-ray diffraction, and soft and hard X-ray absorption spectroscopy were utilized to probe details of the crystal and electronic structure. Upon drying, rearrangement of the sodium ions in the interlayer space and formation of O-Na-O bridges occur. Hard and soft X-ray absorption spectroscopy show that charge transfer occurs upon discharge of the material in sodium half-cells, consistent with a reversible reductive intercalation mechanism. The best-performing electrodes were dehydrated at 500 °C, and the highest initial capacities of about 200 mA h g−1were obtained when a CMC binder was used and NNT was carbon-coated. Wrapping NNT with only 1 wt% graphene also resulted in improved performance.