Session H54: Materials for Energy Storage Devices I

Dual Electrospray Pyrolysis for Mixed Metal Oxide (and Carbon) Composite Nanoparticle Synthesis with Applications in Energy Storage

We present a novel approach to synthesizing mixed metal oxide nanoparticles with a continuous, scalable aerosol flow process using the electrospray. The electrospray is a liquid atomization technique that generates a monodisperse population of highly charged liquid droplets over a broad size range (nanometric to tens of microns). Each liquid droplet serves as a micro-reactor, containing a payload of suitable precursors (such as metal nitrides), allowing for precise control over particle composition and size. By using two electrosprays of opposite polarities, the two highly charged droplets plumes are electrostatically mixed to produce a charge-neutral aerosol. Electrostatically driven droplet-droplet collisions can also be used to control morphology to some degree. This aerosol is passed through a tubular furnace via carrier gas, pyrolizing the precursors to synthesize nanomaterials. We apply this approach to manganese oxide, cobalt oxide, and carbon composite nanoparticles for use in energy storage applications.   

The Basic Understanding of Lithium Superoxide in Li-O$_{\mathrm{2}}$ Battery

The electrochemical and chemical processes that involved in Li-O$_{\mathrm{2}}$ battery are complex, and depend heavily on electrode materials, electrolytes, interfaces, and cell operating conditions. In non-aqueous Li-O$_{\mathrm{2}}$ battery, the main discharge products are commonly known to be lithium peroxide (Li$_{\mathrm{2}}$O$_{\mathrm{2}})$, and possibly some other parasitic components (i.e. Li$_{\mathrm{2}}$CO$_{\mathrm{3}}$, LiOH, Li$_{\mathrm{2}}$O). However, the superoxide intermediates and lithium superoxide (O$_{\mathrm{2}}^{\mathrm{-}}$, LiO$_{\mathrm{2}})$ which are commonly known to be metastable can also be found as reported [1, 2]. Relative to these compounds (i.e. Li$_{\mathrm{2}}$CO$_{\mathrm{3}}$, Li$_{\mathrm{2}}$O, LiOH, Li$_{\mathrm{2}}$O$_{\mathrm{2}})$ in discharge products, little is known about LiO$_{\mathrm{2}}$. To have a basic understanding of lithium superoxide, both theoretical studies and experimental characterizations are important. In this presentation, the recent developments, studies and findings of this exotic species will be discussed. References: 1. D. Zhai$^{\mathrm{+}}$, K.C. Lau$^{\mathrm{+}}$, H. Wang, J. Wen, D. Miller, J. Lu, F. Kang, B. Li, W. Yang, J. Gao, E. Indacochea, L.A. Curtiss, K.A. Amine, Nano Lett. 15 (2), 1041-1046 (2015). 2. J. Lu$^{\mathrm{+}}$, Y.J. Lee$^{\mathrm{+}}$, X. Luo$^{\mathrm{+}}$, K.C. Lau$^{\mathrm{+}}$, M. Asadi$^{\mathrm{+}}$, et. al. Nature (accepted).   

3D strain engineered self-rolled thin-film architecture for high-energy density lithium-ion batteries

Recently, multiple 3D geometries have been implemented into energy storage devices ($e.g.$ nanowire anodes and arrays of interdigitated rods) in order to better accommodate the large volume expansion experienced by the anode during lithiation and to increase the structure energy density. However, most approached structures are difficult to scale up. Here we show how self-rolled thin-films can maintain a high energy density and can potentially accommodate the volume expansion suffered by the anode. The self-rolled tubes are fabricated by physical deposition of the active layers, creating a stress gradient between thin-film stack due to differences in coefficient of thermal expansion. Upon a sacrificial layer removal, the thin-film rolls to relieve this built-in stress. We predict the final dimension of self-rolled battery tubes using known elastic properties of materials commonly used as the active layers of the device. We will discuss an appropriate figure-of-merit that defines how the winding process can ultimately affect the volumetric capacity of 3D self-rolled batteries.   

Electrochemical properties of Li2FeSiO4/C nanocomposites prepared by sol-gel and hydrothermal methods

Li$_{2}$FeSiO$_{4}$ is considered as potential cathode material for next generation lithium ion batteries because of its high specific theoretical capacity, low cost, and safety. However, it suffers from poor electronic conductivity and slow lithium ion diffusion in the solid phase. To address these issues, we have studied mesoporous Li$_{2}$FeSiO$_{4}$/C composites synthesized by sol-gel (SG) and hydrothermal (HT) methods using tri-block copolymer (P123) as carbon source and structure directing agent. The structure and morphology of the composites were characterized by XRD, SEM and TEM and the surface area and pore size distribution were measured by using N$_{2}$ adsorption/desorption. Galvanostatic cycling, electrochemical impedance spectroscopy, and cyclic voltammetry were used to evaluate the electrochemical performance of the Li$_{2}$FeSiO$_{4}$/C composites. The Li$_{2}$FeSiO$_{4}$/C (HT) composites show a superior electrochemical performance compared to Li$_{2}$FeSiO$_{4}$/C (SG). At C/30 rate, the discharge capacity of Li$_{2}$FeSiO$_{4}$/C (HT) reached \textasciitilde 276 mAh/g in the 1.5-4.6 V window and shows better rate capability and stability at high rates. We attribute the improved electrochemical performance of Li$_{2}$FeSiO$_{4}$/C (HT) to its large surface area and reduced particle size. The details of the study will be presented.   

Transmission Electron Microscopy and First Principle Studies Investigating Intercalation Phenomenon Of Vanadium Pentoxide(V$_{2}$O$_{5})$ nanowire cathode

Vanadium Pentoxide(V$_{2}$O$_{5})$ is an attractive intercalation compound due to its characteristic layered structure from weak vanadium-oxygen bonding which enables the intercalation of ions between the layers. Here, we will discuss an in-situ transmission electron microscopy and electron energy-loss spectroscopy approach investigating lithiation of orthorhombic $\alpha $-V$_{2}$O$_{5}$ nanowires where the center of the nanowire undergoes a transformation to $\gamma $-Li$_{2}$V$_{2}$O$_{5\, }$phase. Since V$_{2}$O$_{5}$ has also been predicted as a potential cathode host for magnesium ion intercalation, we also investigate Mg intercalation in $\alpha $-V$_{2}$O$_{5}$ nanowire and determine if our reaction pathway leads to the formation of $\varepsilon $-Mg$_{0.5}$V$_{2}$O$_{5\, }$ phase, as predicted by density functional theory calculations. In-situ Li and Mg intercalation experiments into the new tunnel structured $\zeta $- V$_{2}$O$_{5\, }$nanowires will also be presented and the resulting phases will be compared with theoretical predictions.   

Dry Pressed Holey Graphene Composites for Li-air Battery Cathodes

Graphene is considered an ``omnipotent'' material due to its unique structural characteristics and chemical properties. By heating graphene powder in an open-ended tube furnace, a novel compressible carbon material, holey graphene (hG), can be created with controlled porosity and be further decorated with nanosized catalysts to increase electrocatalytic activity. All hG-based materials were characterized using various microscopic and spectroscopic techniques to obtain morphological, topographical, and chemical information as well as to identify any disordered/crystalline phases. In this work, an additive-free dry press method was employed to press the hG composite materials into high mass loading mixed, sandwich, and double-decker Li-air cathode architectures using a hydraulic press. The sandwich and double-decker (i.e. Big Mac) cathode architectures are the first of its kind and can be discharged for more than 200 hours at a current density of 0.2 mA/cm$^{2}$. The scalable, binderless, and solventless dry press method and unique Li-air cathode architectures presented here greatly advance electrode fabrication possibilities and could promote future energy storage advancements.   

Importance of liquid fragility for energy applications of ionic liquids

Ionic liquids (ILs) are salts that are liquid at ambient temperatures. The strong electrostatic forces between their molecular ions result, e.g., in low volatility and high stability for many members of this huge material class [1]. For this reason they bear a high potential for new advancements in applications, e.g., as electrolytes in energy-storage devices such as supercapacitors or batteries, where the ionic conductivity is an essential figure of merit.\newline Most ILs show dynamic properties typical for glassy matter, which dominate many of their physical properties. An important method to study these dynamical glass-properties is dielectric spectroscopy that can access relaxation times of dynamic processes and the conductivity in a broad frequency and temperature range. In the present contribution, we present results on a large variety of ionic liquids showing that the conductivity of ILs depends in a systematic way not only on their glass temperature but also on the so-called fragility, characterizing the non-canonical super-Arrhenius temperature dependence of their ionic mobility [2].\newline [1] D. R. MacFarlane, \textit{et al.}, Energy Environ. Sci. \textbf{7,} 232–250 (2014).\newline [2] P. Sippel \textit{et al.}, Sci. Rep. \textbf{5,} 13922 (2015).   

A joint \emph{first principles} and ATR-IR study of the vibrational properties of interfacial water at semiconductor-water solid-liquid interfaces

Despite the importance of understanding the structural and bonding properties of solid-liquid interfaces for a wide range of (photo-)electrochemical applications, there are presently no experimental techniques available to directly probe the microscopic structure of solid-liquid interfaces. We carried out joint ATR-IR spectroscopy measurements and \emph{ab initio} molecular dynamics simulations of the vibrational properties of interfaces between liquid water and prototypical semiconductor substrates. In particular, the Ge(100)/H$_2$O interface is shown to feature a reversible bias potential dependent surface phase transition. Our study highlights the key role of coupled theory-experimental investigations on well controlled and characterized interfaces, in order to develop robust strategies to interpret experiments and validate theory. The authors wish to thank T. A. Pham for helpful discussions. G. G. and F. G. acknowledge DOE-BES Grant No. DE-SS0008939.   

Effect of Metal Ion Intercalation on the Structure of MXenes and its Impact on the Dynamics of Water in MXenes

MXenes are two-dimensional materials of sheet-like morphology invented as an alternative to graphene with a potential for energy applications. Because of the heterogeneous bonding between different species and the presence of surface functionalities, MXenes can be intercalated with different chemical species including metal ions and water. The presence of water in MXenes even at ambient conditions impacts their properties relevant to technical applications. Therefore, it is important to understand how intercalants change the structure of MXene and the behavior of water in these materials. Here, using different scattering techniques (x-ray and neutron), we found that intercalation of MXenes with potassium ion increases the c- lattice parameter, yielding a more homogeneous structure with higher water uptake compared to pristine MXenes. In the latter, inhomogeneous structure was observed, with most water residing between the MXenes stacks rather than in between the layers. We found a two orders of magnitude reduction in the diffusion coefficient of water resulting from potassium intercalation, which is in good agreement with the values predicted from ReaxFF simulation. Consequences of improved homogeneity on the water dynamics following metal ion intercalation will be discussed..   

Low Temperature Synthesis of Cubic-phase Fast-ionic Conducting Bi-doped Garnet Solid State Electrolytes.

We report on the synthesis of cubic-phase fast ionic conducting garnet solid state electrolytes based on LiLaZrO (LLZO) at unprecedented low synthesis temperatures. Ionic conductivities around 1.2 x 10-4 S/cm are readily achieved. Bismuth aliovalent substitution into LLZO utilizing the Pechini processing method is successfully employed to synthesize LiLaZrBiO compounds. Cubic phase LiLaZrBiO powders are generated in the temperature range 650C to 900C in air. In contrast, in the absence of Bi and under identical synthesis conditions, the cubic phase of LiLaZrO is not formed below 750C and a transformation to the poor ionically conducting tetragonal phase is observed at 800C for the undoped compound. The critical role of Bi in lowering the formation temperature of the garnet cubic phase and the improvements in ionic conductivity are elucidated in this work through microstructural and electrochemical studies.   

Electrophoretic deposition of RuO2/HRGO composites for flexible supercapacitor electrodes

Flexible energy storage devices are essential for the development of wearable electronics, such as bendable displays and wearable multi-media systems. A subset of these energy storage devices, flexible supercapacitors have received increased attention because of their long cycle life, low cost, and easy fabrication. Herein, we report an easy and low cost method to fabricate bendable ruthenium oxide (RuO$_{\mathrm{2}})$/ holey reduced graphene oxide (HRGO) electrodes using electrophoretic deposition. Analysis of the surface morphology using scanning electron microscopy (SEM) shows a highly nanoporous structure with pores ranging from 2 to 3 nm. The obtained RuO$_{\mathrm{2}}$/HRGO supercapacitor exhibited excellent electrochemical capacitive performance in a PVA-H$_{\mathrm{2}}$SO$_{\mathrm{4}}$ gel electrolyte, with a specific capacitance of 418.5F/g. Additionally, a high rate performance with capacitance retention of 85{\%} was observed when the current was increased by a factor of 20 from 1.0 to 20.0 A/g. The supercapacitor exhibited an exceptional cycling stability of 88.5{\%} after 10,000 cycles, indicating excellent long term electrochemical stability.   

Li$+$ ion diffusion in nanoscale alumina coatings

Nanoscale coatings of alumina are used to stabilize surfaces for a variety of technologies. Diffusion of ions through these coatings is of primary importance: in some cases, diffusion is unwanted (e.g. corrosion) and in others (e.g. electrode materials), it is necessary. In this work DFT and AIMD calculations are used to investigate Li+ ion diffusion through a nano-layer of alumina, examining the phase (alpha, gamma, and amorphous), ion concentration, and electron count dependence. We look at the role of the surface itself in promoting diffusion. One of our main findings is that as the number of ions or charge increases, the diffusivity rises. We show how our data can explain electrochemical data from coated LiCoO$_2$ cathodes and may point toward better and more efficient coatings for stabilizing electrodes.   

Finding out the optimal boron concentration in BC$_x$ sheets for high capacity anode material in Li-ion batteries

Boron doped graphene shows better adsorption of Li compared to pristine graphene and has been investigated as a potential anode material for Li-ion batteries. Using first principles density functional theory calculations, we investigate the effect of increasing boron concentration on the gravimetric capacity of mono-layered boron doped graphene sheets, BC$_x$ (x = 7, 5, 3, 2 and 1). Li storage capacity increases with the increase in boron concentration giving highest capacity for monolayer BC$_2$ ($\sim$ 1400 mAh/g), and is about 1.6 times higher than previously reported capacity of BC$_3$. This is due to the more number of available empty states above the Fermi level in BC$_2$ compared to other sheets. Moreover, owing to a very low Li diffusion barrier, the Li kinetics in BC$_2$ is also found to be better among all the layered boron doped carbon sheets. Further enhancement of B concentration, as in BC, leads to strong binding of Li, thereby hindering the delithiation processes. Hence, BC$_2$ with optimal concentration of B among the BC$_x$ phases, emerges as a promising choice for anode material in rechargeable Li ion battery.