Bryan W. Byles

Effect of aging and hydrothermal treatment on electrochemical performance of chemically pre-intercalated Na–V–O nanowires for Na-ion batteries

A chemical pre-intercalation approach was used to synthesize Na-containing vanadium oxide nanowires for use as Na-ion battery cathodes. The synthesis method was based on a sol–gel process followed by aging and/or hydrothermal treatment. We have for the first time shown that addition of sodium salt into the hydrothermally treated precursor mixture leads to a higher content of chemically pre-intercalated Na+ ions in the crystal structure of produced layered vanadium oxides. Further, the inclusion of an aging step was found to be crucial for the formation of bilayered NaxV2O5 phase with high capacity in Na-ion cells. Single-phase bilayered NaxV2O5 nanowires, obtained after the hydrothermal treatment of four-day aged precursor, demonstrated a record high initial discharge capacity of 365 mA h g−1. The hydrothermal treatment was shown to improve crystallinity of nanowires, leading to the better electrochemical stability of electrodes. Our results demonstrate the potential of chemical pre-intercalation synthesis method to develop high-capacity Na-ion battery cathodes. The ability to control various parameters in the multi-step chemical pre-intercalation approach opens a door to employing this method for the synthesis of electrode materials for other beyond lithium-ion electrochemical systems, such as Mg-ion and K-ion batteries.

Todorokite-type manganese oxide nanowires as an intercalation cathode for Li-ion and Na-ion batteries

Extended hydrothermal treatment at an elevated temperature of 220 °C allowed high yield synthesis of manganese oxide nanowires with a todorokite crystal structure suitable for ions intercalation. The flexible, high aspect ratio nanowires are 50–100 nm in diameter and up to several microns long, with 3 × 3 structural tunnels running parallel to the nanowire longitudinal axis. The tunnels are occupied by magnesium ions and water molecules, with the chemical composition found to be Mg0.2MnO2·0.5H2O. The todorokite nanowires were, for the first time, electrochemically tested in both Li-ion and Na-ion cells. A first discharge capacity of 158 mA h g−1 was achieved in a Na-ion system, which was found to be greater than the first discharge capacity in a Li-ion system (133 mA h g−1). Despite large structural tunnel dimensions, todorokite showed a significant first cycle capacity loss in a Na-ion battery. After 20 cycles, the capacity was found to stabilize around 50 mA h g−1 and remained at this level for 100 cycles. In a Li-ion system, todorokite nanowires showed significantly better capacity retention with 78% of its initial capacity remaining after 100 cycles. Rate capability tests also showed superior performance of todorokite nanowires in Li-ion cells compared to Na-ion cells at higher current rates. These results highlight the difference in electrochemical cycling behavior of Li-ion and Na-ion batteries for a host material with spacious 3 × 3 tunnels tailored for large Na+ ion intercalation.

The role of electronic and ionic conductivities in the rate performance of tunnel structured manganese oxides in Li-ion batteries

Single nanowires of two manganese oxide polymorphs (α-MnO2 and todorokite manganese oxide), which display a controlled size variation in terms of their square structural tunnels, were isolated onto nanofabricated platforms using dielectrophoresis. This platform allowed for the measurement of the electronic conductivity of these manganese oxides, which was found to be higher in α-MnO2 as compared to that of the todorokite phase by a factor of ∼46. Despite this observation of substantially higher electronic conductivity in α-MnO2, the todorokite manganese oxide exhibited better electrochemical rate performance as a Li-ion battery cathode. The relationship between this electrochemical performance, the electronic conductivities of the manganese oxides, and their reported ionic conductivities is discussed for the first time, clearly revealing that the rate performance of these materials is limited by their Li+ diffusivity, and not by their electronic conductivity. This result reveals important new insights relevant for improving the power density of manganese oxides, which have shown promise as a low-cost, abundant, and safe alternative for next-generation cathode materials. Furthermore, the presented experimental approach is suitable for assessing a broader family of one-dimensional electrode active materials (in terms of their electronic and ionic conductivities) for both Li-ion batteries and for electrochemical systems utilizing charge-carrying ions beyond Li+.

Prediction of optimal structural water concentration for maximized performance in tunnel manganese oxide electrodes

Crystal water has been shown to stabilize next-generation energy storage electrodes with structural tunnels to accommodate cation intercalation, but the stabilization mechanism is poorly understood. In this study, we present a simple physical model to explain the energetics of interactions between the electrode crystal lattice, structural water, and electrochemically cycled ions. Our model is applied to understand the effects of crystal water on sodium ion intercalation in a tunnel manganese oxide structure, and we predict that precisely controlling the crystal water concentration can optimize the ion intercalation voltage and capacity and promote stable cycling. The analysis yields a critical structural water concentration by accounting for the interplay between elastic and electrostatic contributions to the free energy. Our predictions are validated with first-principles calculations and electrochemical measurements. The theoretical framework used here can be extended to predict critical concentrations of stabilizing molecules to optimize performance in next-generation battery materials.

Improved electrochemical cycling stability of intercalation battery electrodes via control of material morphology

AbstractUsing a model tunnel manganese oxide with the todorokite crystal structure (T-MnO2), we demonstrate that controlling the morphology of the active material can improve the cycling stability of intercalation battery electrodes. The T-MnO2 structure is built from tunnels that provide spacious 1D diffusion channels for charge-carrying ions. Taking advantage of the unique ability to synthesize T-MnO2 in the form of both highly crystalline two-dimensional (2D) nanoplatelets and one-dimensional (1D) nanowires through a facile hydrothermal growth method, we investigated the effect of nanoscale particle dimensions on reversible battery cycling. Insertion of ions into the tunnels results in anisotropic expansion of the structure, making T-MnO2 with different morphologies an excellent model platform to understand how intercalation-induced volume change, typically leading to the deterioration of the electrode performance over extended cycling, can be controlled through synthesis of targeted morphologies. T-MnO2 nanowires showed not only significantly improved capacity retention but also substantially higher specific capacity than the T-MnO2 nanoplatelets. The enhanced electrochemical properties of the nanowire electrodes could be attributed to the larger surface-to-volume ratio than that of nanoplatelets, resulting in higher contact area with electrolyte for the nanowires. Moreover, due to the smaller cross-sectional area of the nanowires, volume expansion and contraction perpendicular to the structural tunnels induced by reversible ion intercalation occurs in a more facile fashion. This work shows that chemically controlling morphology and producing particles with nanostructure dimensionality replicating that of atomic structure (i.e., 1D morphology and 1D structure) makes it possible to enhance material performance. Graphical abstractᅟ

A 3-D nanoelectrokinetic model for predictive assembly of nanowire arrays using floating electrode dielectrophoresis

Floating electrode dielectrophoresis (FE-DEP) presents a promising avenue for scalable assembly of nanowire (NW) arrays on silicon chips and offers better control in limiting the number of deposited NWs when compared with the conventional, two-electrode DEP process. This article presents a 3D nanoelectrokinetic model, which calculates the imposed electric field and its resultant NW force/velocity maps within the region of influence of an electrode array operating in the FE-DEP configuration. This enables the calculation of NW trajectories and their eventual localization sites on the target electrodes as a function of parameters such as NW starting position, NW size, the applied electric field, suspension concentration, and deposition time. The accuracy of this model has been established through a direct quantitative comparison with the assembly of manganese dioxide NW arrays. Further analysis of the computed data reveals interesting insights into the following aspects: (a) asymmetry in NW localization at electrode sites, and (b) the workspace regions from which NWs are drawn to assemble such that their center-of-mass is located either in the inter-electrode gap region (desired) or on top of one of the assembly electrodes (undesired). This analysis is leveraged to outline a strategy, which involves a physical confinement of the NW suspension within lithographically patterned reservoirs during assembly, for single NW deposition across large arrays with high estimated assembly yields on the order of 87%.

Layered manganese oxides as electrodes for water desalination via hybrid capacitive deionization

This work explores the ion removal performance of Na-birnessite and Mg-buserite during extended cycling in NaCl and MgCl2 solutions in a hybrid capacitive deionization (HCDI) cell. These two layered manganese oxides (LMOs) contain two-dimensional diffusion pathways and thus present the potential for enhanced ion diffusion and higher performance in HCDI. Correlation between stabilizing ions and ions removed from solution are investigated. In NaCl solution, Mgbuserite shows the largest ion removal capacity of 37.2 mg g-1 while the reverse is true in MgCl2 solution, where Nabirnessite delivers a capacity of 50.2 mg g-1. Furthermore, ex-situ XRD after 200 cycles revealed the changes in the structures of the two materials after repeated ion removal-ion release. These results demonstrate that materials with twodimensional crystal structures can demonstrate high capacities in HCDI and show that interlayer content and spacing can dramatically impact material stability in electrochemical water desalination.

Diffusion of charge-carrying ions in tunnel manganese oxides: effect of 1D tunnel size and ionic content

The low cost, environmental friendliness, and high electrochemical activity of manganese oxides make them attractive candidates for electrodes in intercalation-based battery systems. Tunnel manganese oxides are a subset of this materials family built from corner and edge sharing MnO6 octahedra arranged around stabilizing cations and water molecules to form tunnels of various size and shape. Here, we synthesize three tunnel manganese oxides with different 1D diffusion channel size and ionic content. The apparent Li+ ion and Na+ ion diffusion coefficients are calculated from the galvanostatic intermittent titration technique to understand the effect of tunnel size and ionic content on diffusion of charge-carrying ions through the one-dimensional structural tunnels. In LIBs, the material with the largest tunnels demonstrated the highest Li+ ion diffusion coefficient, while in SIBs the material stabilized by Na+ ions (the same as the charge-carrying ions) demonstrated the highest rate performance, revealing the significance of ionic content in the structural tunnels. These results highlight the importance of the relationship between tunnel size and charge-carrying ion size and provide insight into the design and selection of tunnel manganese oxides for improved diffusion of charge carrying species.

Ion Removal Performance, Structural/Compositional Dynamics, and Electrochemical Stability of Layered Manganese Oxide Electrodes in Hybrid Capacitive Deionization

Hybrid capacitive deionization (HCDI) is a derivative of capacitive deionization (CDI) method for water desalination, in which one carbon electrode is replaced with a redox-active intercalation electrode, resulting in substantial improvements in ion removal capacity over traditional CDI. The search for high-performing intercalation host compounds is ongoing. In this study, two-layered manganese oxides (LMOs), with sodium (Na-birnessite) and magnesium (Mg-buserite) ions stabilizing the interlayer region, were for the first time evaluated as HCDI electrodes for the removal of ions from NaCl and MgCl2 solutions to understand structural/compositional dynamics and electrochemical stability of LMO electrodes over extended cycling. Both materials demonstrated excellent initial ion removal performance with the highest capacities of 37.2 mg g-1 (637 μmol g-1) exhibited by Mg-buserite in NaCl solution and 50.2 mg g-1 (527 μmol g-1) exhibited by Na-birnessite in MgCl2 solution. The performance decay observed over the course of 200 ion adsorption/ion release cycles was attributed to two major phenomena: oxidation of carbon electrode and evolution of the structure/composition of LMO electrodes. The latter involves disorder in stacking of Mn-O layers and changes in the interlayer spacing/interlayer ions reflecting the composition of the solution being desalinated. This work highlights the importance of understanding the interactions between the HCDI electrodes and solutions containing different ions and the structural analysis of redox-active material in intercalation electrodes over the course of operation for gaining insight into the fundamental processes governing desalination performance and developing next-generation HCDI systems with long-term electrochemical stability.

Electrochemically-correlated measurement of electronic transport in battery nanoelectrodes using single nanowire devices

In order to achieve electrochemical cycling at high rates for batteries with improved power densities, battery electrodes need to support the efficient insertion / extraction of both, electrons and intercalation ions within their constituent material systems. This paper presents an on-chip, single nanowire electrochemical device for measuring the electrical properties of a battery nanoelectrode as a function of its electrochemical state of charge (SOC). These measurements are performed on alpha-phase manganese oxide nanowires, which present crystalline tunnels along their longitudinal axis for ionic intercalation. Our results show a degradation in the electrical conductance of nanowire electrodes as the amount of lithium loading is increased and points to the need for optimizing crystal structures so that the electrical transport property of the host material is not degraded during electrochemical intercalation.