Ekaterina Pomerantseva

The role of vacancies and defects in Na0.44MnO2 nanowire catalysts for lithium–oxygen batteries

Na0.44MnO2 nanowires were acid leached in nitric acid, and dehydrated by heat treatment to induce controllable defect formation as monitored by high resolution TEM studies. The charge–discharge tests using these materials as catalysts (or “promoters”) in rechargeable lithium–oxygen batteries (in non-carbonate electrolytes) showed that a high defect concentration results in a doubling of the reversible energy storage capacity up to 11u2006000 mA h g−1, and lowered overpotentials for oxygen evolution. The role of the defects/vacancies in determining oxygen reduction behavior is highlighted.

Hierarchical Three-Dimensional Microbattery Electrodes Combining Bottom-Up Self-Assembly and Top-Down Micromachining

The realization of next-generation portable electronics and integrated microsystems is directly linked with the development of robust batteries with high energy and power density. Three-dimensional micro- and nanostructured electrodes enhance energy and power through higher surface area and thinner active materials, respectively. Here, we present a novel approach for the fabrication of hierarchical electrodes that combine benefits of both length scales. The electrodes consist of self-assembled, virus-templated nanostructures conformally coating three-dimensional micropillars. Active battery material (V(2)O(5)) is deposited using atomic layer deposition on the hierarchical micro/nanonetwork. Electrochemical characterization of these electrodes indicates a 3-fold increase in energy density compared to nanostructures alone, in agreement with the surface area increase, while maintaining the high power characteristics of nanomaterials. Investigation of capacity scaling for varying active material thickness reveals underlying limitations in nanostructured electrodes and highlights the importance of our method in controlling both energy and power density with structural hierarchy.

Composite Manganese Oxide Percolating Networks As a Suspension Electrode for an Asymmetric Flow Capacitor

In this study, we examine the use of a percolating network of metal oxide (MnO2) as the active material in a suspension electrode as a way to increase the capacitance and energy density of an electrochemical flow capacitor. Amorphous manganese oxide was synthesized via a low-temperature hydrothermal approach and combined with carbon black to form composite flowable electrodes of different compositions. All suspension electrodes were tested in static configurations and consisted of an active solid material (MnO2 or activated carbon) immersed in aqueous neutral electrolyte (1 M Na2SO4). Increasing concentrations of carbon black led to better rate performance but at the cost of capacitance and viscosity. Furthermore, it was shown that an expanded voltage window of 1.6 V could be achieved when combining a composite MnO2-carbon black (cathode) and an activated carbon suspension (anode) in a charge balanced asymmetric device. The expansion of the voltage window led to a significant increase in the energy density to ∼11 Wh kg(-1) at a power density of ∼50 W kg(-1). These values are ∼3.5 times and ∼2 times better than a symmetric suspension electrode based on activated carbon.

Tobacco mosaic virus: A biological building block for micro/nano/bio systems

Tobacco mosaic virus (TMV) has the potential to be an ideal candidate for a building block of the next-generation micro/nano/bio systems. The TMV virion is a high-aspect ratio rigid nanotube that is robust and compatible with some conventional microfabrication processes. TMV can be chemically and genetically modified to enhance its physical properties and tailor them to specific applications. This review covers the use of TMV nanostructures in a wide range of micro/nano/bio systems. TMV has been utilized in the production of nanowires, nanostructured thin films, biomimetic surfaces, novel sensors, high performance microbatteries, solid-state electronics, and engineered biosystems. The work highlighted here is meant to give a perspective of the entire breadth of the properties of these virions, from their synthesis and functionalization to assembly and patterning, as well as feature works that represent key milestones in the field of biofabrication and biomaterial integration. The advantages already demons...

Cathodic ALD V2O5 thin films for high-rate electrochemical energy storage

Atomic layer deposition (ALD) is attractive for next-generation electrical energy storage in forming passivation layers and more recently active storage material. Here we report a detailed study of ALD V2O5 as a high capacity cathode material, using vanadium tri-isopropoxide (VTOP) precursor with both O3 and H2O as oxidant. The O3-based process produces polycrystalline films with generally higher storage capacity than the amorphous films resulting from the H2O-based process over extended cycling (100 cycles). High capacities are achieved in V2O5 because of the ability to incorporate up to three Li per V2O5 formula unit. To address the central need for both high power and high energy, we identified the crucial tradeoff between higher gravimetric capacity with thinner films and higher material mass with thicker films. For the thickness regime 10–120 nm, we chose areal energy and power density as a useful metric for this tradeoff and found that it is optimized at 60 nm for the O3-VTOP ALD V2O5 films. We believe the control of material quality, thickness, and conformality achievable with ALD processes is valuable as new nanoarchitectures for electrochemical energy storage come into sight.

Solid flexible electrochemical supercapacitor using Tobacco mosaic virus nanostructures and ALD ruthenium oxide

An all-solid electrochemical supercapacitor has been developed using a nanostructured nickel and titanium nitride template that is coated with ruthenium oxide by atomic layer deposition (ALD). The electrode morphology was based on a high surface area biotemplate of genetically modified Tobacco mosaic virus. The biotemplate automatically self-assembles at room temperature in aqueous solution. Nafion R perfluorosulfonate ionomer dispersion was cast on the electrodes and used as a solid proton-conducting electrolyte. A 5.8 F g −1 gravimetric capacity (578 μ Fc m −2 based on footprint) was achieved in Nafion electrolyte, and the device retained 80% of its capacity after 25000 cycles. The technology presented here will enable thin, solid, flexible supercapacitors that are compatible with standard microfabrication techniques. (Some figures may appear in colour only in the online journal)

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+.

A MEMS platform for in situ, real-time monitoring of electrochemically induced mechanical changes in lithium-ion battery electrodes

We report the first successful demonstration of an optical microelectromechanical systems (MEMS) sensing platform for the in situ characterization of electrochemically induced reversible mechanical changes in lithium-ion battery (LIB) electrodes. The platform consists of an array of flexible membranes with a reflective surface on one side and a thin-film LIB electrode on the other side. The membranes deflect due to the active battery material volume change caused by lithium intercalation (expansion) and extraction (contraction). This deflection is monitored using the Fabry?Perot optical interferometry principle. The active material volume change causes high internal stresses and mechanical degradation of the electrodes. The stress evolution observed in a silicon thin-film electrode incorporated into this MEMS platform follows a ?first elastic, then plastic? deformation scheme. Understanding of the internal stresses in battery electrodes during discharge/charge is important for improving the reliability and cycle lifetime of LIBs. The developed MEMS platform presents a new method for in situ diagnostics of thin-film LIB electrodes to aid the development of new materials, optimization of electrode performance, and prevention of battery failure.