Session B26: Focus Session: Physics of Energy Storage Materials - Advanced Materials

How can one tell if a lithium-ion battery will last for 10 years in experiments that only take a few weeks?

Lithium-ion batteries are now being used in electric vehicles. There are four main factors which will determine the success of Li-ion batteries in this application: a) safety; b) cost; c) performance and d) lifetime. Each of the factors is presently the subject of much debate and much R+D. I will only speak about lifetime here. Testing the lifetime of Li-ion batteries for automotive applications under \textbf{realistic} conditions of temperature and number of cycles per day is very time consuming. In fact, such a test should take a decade or more, if the batteries are expected to last a decade in the field. Tests of such duration slow the product improvement cycle immensely. In this lecture, I will discuss how high precision measurements of the coulombic efficiency of Li-ion cells and batteries can be used to predict the relative lifetimes, on the decade-long scale, of these devices in measurements that only take a few weeks. The measurements enable the rapid comparison of technologies using new electrode materials, electrolyte additives and cell designs so that the product improvement cycle can be significantly shortened. I will describe the requirements of the instruments needed to make these measurements and point out that nothing suitable is, as yet, commercially available. There is a major need for such equipment and an associated business opportunity.   

Organic electrodes for high rate capability Lithium-ion batteries

Lithium-ion batteries (LIBs) for power-intensive applications such as in electric vehicles require high discharge rate, i.e., high Li diffusion rate (or low Li diffusion barrier) in electrode and electrolyte materials. Based on first-principles calculations, we found that organic salt, di-lithium terephthalate (Li2TPA), a promising anode material recently tested in experiment, could have high rate capability. We further predict that di-potassium terephthalate (K2TPA) could exhibit even lower Li diffusion barrier. The calculated Li diffusion barrier in fully lithiated K2TPA is only 150 meV, which yields Li diffusion rate orders of magnitude higher than that in Li-intercalated graphite at room temperature. The calculated anode voltage vs metal lithium and specific energy density are 0.62 V and 209 mAh/g, respectively. In addition, the volume change of K2TPA in charging/discharging is only 5{\%}, much smaller than that in Li-intercalated graphite. These unique advantages call for further investigation of the organic salts, both the TPA-based and beyond, for power-intensive LIB applications either as anode or cathode materials.   

First Principles Study for Lithium Intercalation and Diffusion Behavior in Orthorhombic Nb2O5 Electrochemical Supercapacitor

Unlike batteries, electrochemical supercapacitors require not only high energy density, but also very fast rates of electronic and ionic transport. Experimental results show that niobium oxide exhibits an outstanding power density and fast ionic charging rates. We investigate lithium intercalation and diffusion behavior in orthorhombic niobium oxide (T-Nb2O5) by using first-principles density-functional theory (DFT) calculations. We find that the Li ions can only intercalate in the {001} family of lattice planes with the lowest niobium occupancy due to electrostatic-repulsion between Li+ and Nb5+. Besides, since Li diffusion along the z direction is hindered by a high diffusion barrier (2.64 eV), the overall Li intercalation and diffusion can only occur within {001} planes. Furthermore, the diffusion barriers within the {001} planes are found to have a broad distribution of values form around 50 meV to 1000 meV; the diffusion barrier is determined by the neighboring oxygen-oxygen distance. The barrier remains low (around 60 meV) when the neighboring oxygen distance along the diffusion path is larger than 3.9 {\AA}, thus leading to fast Li ion diffusion. These results explain the excellent performance of Nb2O5 as a cathode material for electrochemical supercapacitors.   

Multiscale Simulations of Energy Storage in Polymers

Polypropelene is the most used capacitor dielectric for high energy density storage. However, exotic materials such as copolymerized PVDF and, more recently, polythiourea, could potentially lead to an order of magnitude increase in the stored energy density [1,2]. In our previous investigations we demonstrated that PVDF-CTFE possesses non-linear dielectric properties under applied electric field. These are characterized by transitions from non-polar to polar phases that lead enhanced energy density. Recent experiments [3] have also suggested that polythiourea may be another potential system with high energy-density storage and low loss. However, the characteristics of this emerging material are not yet understood and even its preferred crystalline phases are not known. We have developed a multiscale approach to predicting polymer self-organization using the REAX force field and molecular dynamics simulations. We find that polythiourea chains tend to coalesce in nanoribbon-type structures and prefer an anti-polar interchain ordering similar to PVDF. These results suggest a possible role of topological phase transitions in shaping energy storage in this system.\\[4pt] [1] B. Chu et al, Science 313, 334 (2006).\\[0pt] [2] V. Ranjan et al., PRL 99, 047801 (2007).\\[0pt] [3] Q. Zhang, private communication   

Addressing the challenges of solar thermal fuels via atomic-scale computational design and experiment

By reversibly storing solar energy in the conformations of photo-isomers, solar thermal fuels (STFs) provide a mechanism for emissions-free, renewable energy storage and conversion in a single system. Development of STFs as a large-scale energy technology has been hampered by technical challenges that beset the photo-isomers of interest: low energy density, storage lifetime, and quantum yield; UV absorption; and irreversible degradation upon repeated cycling. In this talk, we discuss our efforts to design new STFs that overcome these hurdles. We present computational results on various STFs based on our recently proposed photo-isomer/template STF concept [Kolpak and Grossman, Nano Letters 11, 3156 (2011)], as well as new experimental results on azobenzene-functionalized carbon nanotube STFs. Our approach yields significant improvements with respect to STFs studied in the past, with energy densities similar to Li-ion batteries, storage lifetimes $>$ 1 year, and increased quantum yield and absorption efficiency. Our strategy also suggests mechanisms for inhibiting photo-isomer degradation. With a large phase space yet to be explored, there remain numerous possibilites for property enhancement, suggesting that STFs could become a competitive renewable energy technology.   

Photoswichable Molecular Rings for Solar-Thermal Energy Storage

Solar-thermal fuels reversibly store solar energy in the chemical bonds of molecules by photoconversion, and can release this stored energy in the form of heat upon activation. Many conventional photoswichable molecules could be considered as solar thermal fuels, although they suffer from low energy density and short lifetime in the photo-excited state, rendering their practical use unfeasible. We present a new approach to design systems for solar thermal fuel applications, wherein well-known photoswitchable molecules are connected by different linker agents to form molecular rings. This approach allows for a significant increase in both the amount of stored energy per molecule and the stability of the fuels. Our results suggest a range of possibilities for tuning the energy density and thermal stability as a function of the type of the photoswitchable molecule, the ring size, and/or the type of linkers.   

Physics of Materials for Sodium-Ion Batteries

Sodium is used as a functional ionic species in a variety of electrochemical energy storage devices. This talk will examine several manifestations of sodium ion use, and will focus deeply on a novel class of aqueous electrolyte sodium ion batteries that have been developed over the past 5 years. While these new systems are typically lower in energy density, they are very robust and low in cost, making them appealing for a number of stationary energy storage applications. Specific topics to be covered include: sodium ion intercalation compounds, sodium/carbon interaction at potentials below 0V vs NHE, the behavior of porous thick electrodes in different electrolyte solutions, and the future outlook for sodium ion battery research.   

Computational studies of carbon-onions for electrochemical capacitor applications

Supercapacitors bridge the gap between conventional batteries and electrolytic capacitors. Recently, onion-like carbon structures have [1] shown to have capacitances four orders of magnitude higher and energies an order of magnitude higher than conventional capacitors, making them the fastest growing competitors for energy storage. We study the formation of carbon-onions from nanodiamonds using reactive force-fields [2]. Our study suggests that the temperature and mechanical stability as well as the final-equilibrium structure are strongly dependent on the inclusion of long-range forces. We are currently developing reactive-force fields to allow mesoscopic modeling of reactions of carbon nanostructures with aqueous electrolytes. Progress along these lines will also be presented. This material is based upon work supported as part of the Fluid Interface Reactions, Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.\\[4pt] [1] D. Pech et. al, Nature Nanotechnology 5, 651 (2010)\\[0pt] [2] Adri C. T. van Duin et.al, J. Phys. Chem. A 105, 9396 (2001)   

Supercapacitor Electrodes with High-energy and power densities prepared from monolithic NiO/Ni Nanocomposite

Despite significant progresses in the development for high-performance supercapacitors, it lacks techniques to realize the full potential of electrode material by achieving simultaneously tailored pore structure, electrode conductivity, and crystallinity. Moreover, the problem of being difficult for industrial scale manufacture still exists. For an attempt to address all these issues, we recently developed a simple and cost-effective process, which is also scalable, for achieving supercapacitor electrodes with both high energy and power densities. We first produce nickel nanoparticles with a modified polyol method. A simple mechanical compaction of nanoparticles and a followed thermal treatment result in compact, stable, highly porous Ni/NiO electrodes that do not require a support. During the charging process, OH$^{-}$ electrolytic ions are bound to the NiO, giving off electrons. The process is reversed when the stored electrical energy is drawn off as current. The high granularity NiO provides a large inner surface area, and the conductive network of the metal particles is maintained. High energy density of about 60 Wh kg$^{-1}$ and power density of 10 kW kg$^{-1}$ were simultaneous achieved with a slow charge/fast discharge process.   

An ab-initio approach to modeling high temperature thermodynamics of non stoichiometric ceria

Ceria is a very promising material for fuel cell electrolytes because of its high oxygen ion diffusivity. Also, its ability to thermodynamically create oxygen vacancies in its structure at high temperatures and get oxidized at low temperatures has found use in thermochemical splitting of water to generate hydrogen. Although the experimental phase diagram for ceria is well established in the composition of interest, there have not been significant attempts at studying the oxygen vacancy thermodynamics at elevated temperatures from first principles. We performed GGA+$U$ calculations ceria to study the electronic structure and ground state energies of various concentrations and configurations of oxygen vacancies in ceria. The energies are then fitted to a Cluster expansion Hamiltonian to efficiently model the interactions between the different species : Ce$^{3+}$, Ce$^{4+}$ , O$^{2-}$ and oxygen vacancies . Lattice Monte Carlo simulations are then performed to obtain the free energy as a function of temperature and oxygen chemical potential through thermodynamic integration. We are also investigating the effect of lattice vibrational contribution to the phase diagram by including a temperature dependent cluster expansion.   

Defect-Induced Segregation and Lattice Stability of BSCF Perovskites

Among novel advanced materials for clean energy, \textbf{Ba}$_{x}$\textbf{Sr}$_{1-x}$\textbf{Co}$_{1-y}$\textbf{Fe}$_{y}$\textbf{O}$_{3-\delta }$(\textbf{BSCF}) are considered as promising materials for cathodes in solid oxide fuel cells (SOFC) and oxygen permeation membranes. BSCF exhibits a good oxygen exchange performance, mixed ionic and electronic conductivity, high oxygen vacancy concentration, and low diffusion activation barrier, which largely define the oxygen reduction chemistry. However, understanding the interplay between the structural disorder and crystalline stability in BSCF is extraordinarily complex and essentially unexplored. We present first principles calculations of an ideal BSCF crystal and the crystal containing point defects, Frenkel and Schottky disorders, cation and antisite exchanges, and a set of relevant solid-solid solutions. We discuss possible mechanisms of defect-induced (in)stability, solid-state decomposition reactions, and phase transitions of the BSCF lattice as a function of oxygen vacancy concentration for cubic and hexagonal BSCF in the context of available experiments. This research explains the observed SOFC performance reduction and provides insights on enhancing energy conversion.