Electrochemically induced amorphous-to-rock-salt phase transformation in niobium oxide electrode for Li-ion batteries.

Intercalation-type metal oxides are promising negative electrode materials for safe rechargeable lithium-ion batteries due to the reduced risk of Li plating at low voltages. Nevertheless, their lower energy and power density along with cycling instability remain bottlenecks for their implementation, especially for fast-charging applications. Here, we report a nanostructured rock-salt Nb2O5 electrode formed through an amorphous-to-crystalline transformation during repeated electrochemical cycling with Li+. This electrode can reversibly cycle three lithiums per Nb2O5, corresponding to a capacity of 269 mAh g-1 at 20 mA g-1, and retains a capacity of 191 mAh g-1 at a high rate of 1 A g-1. It exhibits superb cycling stability with a capacity of 225 mAh g-1 at 200 mA g-1 for 400 cycles, and a Coulombic efficiency of 99.93%. We attribute the enhanced performance to the cubic rock-salt framework, which promotes low-energy migration paths. Our work suggests that inducing crystallization of amorphous nanomaterials through electrochemical cycling is a promising avenue for creating unconventional high-performance metal oxide electrode materials.

A Comparison of Solid Electrolyte Interphase Formation and Evolution on Highly Oriented Pyrolytic and Disordered Graphite Negative Electrodes in Lithium-Ion Batteries.

The presence and stability of solid electrolyte interphase (SEI) on graphitic electrodes is vital to the performance of lithium-ion batteries (LIBs). However, the formation and evolution of SEI remain the least understood area in LIBs due to its dynamic nature, complexity in chemical composition, heterogeneity in morphology, as well as lack of reliable in situ/operando techniques for accurate characterization. In addition, chemical composition and morphology of SEI are not only affected by the choice of electrolyte, but also by the nature of the electrode surface. While introduction of defects into graphitic electrodes has promoted their electrochemical properties, how such structural defects influence SEI formation and evolution remains an open question. Here, utilizing nondestructive operando electrochemical atomic force microscopy (EChem-AFM) the dynamic SEI formation and evolution on a pair of representative graphitic materials with and without defects, namely, highly oriented pyrolytic and disordered graphite electrodes, are systematically monitored and compared. Complementary to the characterization of SEI topographical and mechanical changes during electrochemical cycling by EChem-AFM, chemical analysis and theoretical calculations are conducted to provide mechanistic insights underlying SEI formation and evolution. The results provide guidance to engineer functional SEIs through design of carbon materials with defects for LIBs and beyond.

Morphology and Crystallinity Effects of Nanochanneled Niobium Oxide Electrodes for Na-Ion Batteries.

Niobium pentoxide (Nb2O5) is a promising negative electrode for sodium ion batteries (SIBs). By engineering the morphology and crystallinity of nanochanneled niobium oxides (NCNOs), the kinetic behavior and charge storage mechanism of Nb2O5 electrodes were investigated. Amorphous and crystalline NCNO samples were made by modulating anodization conditions (20-40 V and 140-180 °C) to synthesize nanostructures of varying pore sizes and wall thicknesses with identical chemical composition. The electrochemical energy storage properties of the NCNOs were studied, with the amorphous samples showing better overall rate performance than the crystalline samples. The enhanced rate performance of the amorphous samples is attributed to the higher capacitive contributions and Na-ion diffusivity analyzed from cyclic voltammetry (CV) and the galvanostatic intermittent titration technique (GITT). It was found that the amorphous samples with smaller wall thicknesses facilitated improved kinetics. Among samples with similar pore size and wall thickness, the difference in their power performance stems from the crystallinity effect, which plays a more significant role in the resulting kinetics of the materials for Na-ion batteries.

Origins of Irreversibility in Layered NaNixFeyMnzO2 Cathode Materials for Sodium Ion Batteries.

Layered NaNixFeyMnzO2 cathode (NFM) is of great interest in sodium ion batteries because of its high theoretical capacity and utilization of abundant, low-cost, environmentally friendly raw materials. Nevertheless, there remains insufficient understanding on the concurrent local environment evolution in each transition metal (TM) that largely influences the reversibility of the cathode materials upon cycling. In this work, we investigate the reversibility of TM ions in layered NFMs with varying Fe contents and potential windows. Utilizing ex situ synchrotron X-ray absorption near-edge spectroscopy and extended X-ray absorption fine structure of precycled samples, the valence and bonding evolution of the TMs are elucidated. It is found that Mn is electrochemically inactive, as indicated by the insignificant change of Mn valence and the Mn-O bonding distance. Fe is electrochemically inactive after the first five cycles. The Ni redox couple contributes most of the charge compensation for NFMs. Ni redox is quite reversible in the cathodes with less Fe content. However, the Ni redox couple shows significant irreversibility with a high Fe content of 0.8. The electrochemical reversibility of the NFM cathode becomes increasingly enhanced with the decrease of either Fe content or with lower upper charge cutoff potential.

Fully inkjet-printed multilayered graphene-based flexible electrodes for repeatable electrochemical response.

Graphene has proven to be useful in biosensing applications. However, one of the main hurdles with printed graphene-based electrodes is achieving repeatable electrochemical performance from one printed electrode to another. We have developed a consistent fabrication process to control the sheet resistance of inkjet-printed graphene electrodes, thereby accomplishing repeatable electrochemical performance. Herein, we investigated the electrochemical properties of multilayered graphene (MLG) electrodes fully inkjet-printed (IJP) on flexible Kapton substrates. The electrodes were fabricated by inkjet printing three materials - (1) a conductive silver ink for electrical contact, (2) an insulating dielectric ink, and (3) MLG ink as the sensing material. The selected materials and fabrication methods provided great control over the ink rheology and material deposition, which enabled stable and repeatable electrochemical response: bending tests revealed the electrochemical behavior of these sensors remained consistent over 1000 bend cycles. Due to the abundance of structural defects (e.g., edge defects) present in the exfoliated graphene platelets, cyclic voltammetry (CV) of the graphene electrodes showed good electron transfer (k = 1.125 × 10-2 cm s-1) with a detection limit (0.01 mM) for the ferric/ferrocyanide redox couple, [Fe(CN)6]-3/-4, which is comparable or superior to modified graphene or graphene oxide-based sensors. Additionally, the potentiometric response of the electrodes displayed good sensitivity over the pH range of 4-10. Moreover, a fully IJP three-electrode device (MLG, platinum, and Ag/AgCl) also showed quasi-reversibility compared to a single IJP MLG electrode device. These findings demonstrate significant promise for scalable fabrication of a flexible, low cost, and fully-IJP wearable sensor system needed for space, military, and commercial biosensing applications.