Session F52: Physics of Batteries II

First principles studies of structure stability and lithium intercalation of ZnCo2O4

Among the metal oxides, which are the most widely investigated alternative anodes for use in lithium ion batteries (LIBs), binary and ternary transition metal oxides have received special attention due to their high capacity values. ZnCo2O4 is a promising candidate as anode for LIB, and one can expect a total capacity corresponding to 7.0 - 8.33 mol of recyclable Li per mole of ZnCo2O4. Here we studied the structural stability, electronic properties, lithium intercalation and diffusion barrier of ZnCo2O4 through density functional calculations. The calculated structural and energetic parameters are comparable with experiments. Our theoretical studies provide insights in understanding the mechanism of lithium ion displacement reactions in this ternary metal oxide.   

\textbf{First-principles investigations of ionic conduction in Li and Na borohydrides}

Recent experimental studies have identified a family of alkali borohydride materials that exhibit superionic transition temperatures approaching room temperature and ionic conductivities exceeding 0.1 S/cm$^{\mathrm{-1}}$, making them highly promising solid electrolytes for next-generation batteries. Despite the rapid advances in improving the superionic conductivity in these materials, an understanding of the exact mechanisms driving the transport remains unknown. Here we use \textit{ab initio} molecular dynamics calculations to address this issue by characterizing the diffusivity of the Li and Na species in a representative set of closoborane ionic conductors. We investigate both the Na and Li-containing borohydrides with icosahedral (B$_{\mathrm{12}}$H$_{\mathrm{12}})$ and double-capped square antiprism (B$_{\mathrm{10}}$H$_{\mathrm{10}})$ anion species and discuss the trends in ionic conductivity as a function of stoichiometry and the incorporation of various dopants. Our results support the borohydrides as a subset of a larger family of very promising solid electrolytes and identify strategies to improving the conductivity in these materials. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.   

First Principles Study of Electrochemical and Chemical Stability of the Solid Electrolyte-Electrode Interfaces in All-Solid-State Li-Ion Batteries

All-solid-state Li-ion battery is a promising next-generation energy-storage technology. Using novel ceramic solid electrolyte materials, all-solid-state battery has advantages of intrinsic safety and high energy density compared to current Li-ion batteries based on organic liquid electrolyte. However, the power density achieved in all-solid-state battery is still unsatisfactory. The high interfacial resistance at electrode-electrolyte interface is one of the major limiting factors. Here we demonstrated a computational approach based on first principles calculation to systematically investigate the chemical and electrochemical stability of solid electrolyte materials, and provide insightful understanding of the degradation and passivation mechanisms at the interface. Our calculation revealed that the intrinsic stability of solid electrolyte materials and solid electrolyte-electrode interfaces is limited and the formation of interphase layers are thermodynamically favorable. Our study demonstrated a computational scheme to evaluate the electrochemical and chemical stability of the solid interfaces. Our newly gained understanding provided principles for developing solid electrolyte materials with enhanced stability and for engineering interfaces in all-solid-state Li-ion batteries.   

Using Defects in Materials to Store Energy: a Theoretical Study

We study the energy stored by defects in materials using density functional theory (DFT) calculations. Leveraging experimental data to estimate the energy density of defects, expressed as the defect formation energy per unit volume (units of MJ/L) or weight (units of MJ/kg), we identify candidates for high energy density storage, including tungsten, diamond, graphite, silicon, and graphene. DFT calculations are applied to these materials to study the formation energy of vacancies, interstitials, and Frenkel pairs. Our results indicate that the energy density stored by defects in these materials, with experimentally accessible non-equilibrium defect concentrations, can be higher than that of common energy storage technologies such as lithium batteries and supercapacitors. We discuss storage of solar energy and electrical energy (through ion bombardment) using defects.   

Strain-induced tuning of surface energy, electron conductivity, and reduction drive in spinel LiMn$_2$O$_4$ cathodes

LiMn$_2$O$_4$ (LMO) implementation in rechargeable Li-ion batteries (LIBs) for stationary storage is hampered by the limited lifetime of the material and its interfaces, starting from the Solid Electrolyte Interphase [1,2]. Recent experiments [2] and Density Functional Theory (DFT) simulations [3] indicate that the formation and effectiveness of the SEI on LMO are related to the surface orientation and reduction drive. In this context, we analyse the role of geometrical strain for the relative energy, magnetic ordering and the reduction drive of several LMO surfaces. DFT simulations reveal LMO surfaces to be markedly sensitive to geometrical strain. Strain lower than 10\% can induce insulator-metal and ferromagnetic-antiferromagnetic transitions, alter the relative energy of LMO surfaces, and induce changes as large as 1.0 eV in the surface chemical potential, thence reduction drive. Prompted by advances in the synthesis of metal-oxide core-shell nanostructures [4], use of strained LMO coating as SEI-formation agent is put forward towards engineering of longer lived SEI on LMO substrates.\\ \\ 1. JCPC 2012, 116, 9852-9861\\ 2. J. Am. Chem. Soc. 2010, 132, 15268-15276\\ 3. J. Phys. Chem. C 2015, 119, 21358-21368 \\ 4. ACS Nano 2012, 6, 5531   

Atomic dynamics in PrBaCo2O6

We have used a combination of lattice dynamics and \textit{ab initio} molecular dynamics (MD) to study atomic dynamics in PrBaCo$_{2}$O$_{6}$, a prototype material for a large class of layered compounds of both fundamental and technological interest. With the layered structure as the framework for understanding the dynamics, our analysis reveals clear signatures of this structural motif in the overall atomic dynamics, especially for O atoms. In particular, we find that O atom dynamics in the PrO layer is predominantly in-plane (\textit{ab}-plane) in contrast to the predominantly out-of-plane dynamics in the CoO$_{2}$ layer. This finding suggests that the oxide ionic conductivity is dominated by the O atoms in the PrO layer. Additionally, we find sharp low-energy modes below $20$ meV for both Ba and Pr atoms, reminiscent of rattler modes known for reducing thermal conductivity in cage compounds.   

MnO$_{\mathrm{2}}$ Encapsulated Electrospun TiO$_{\mathrm{2\thinspace }}$Nanofibers: A Strategic Approach towards the Development of Aqueous Electrolyte Based Asymmetric Supercapacitors

An aqueous electrolyte based asymmetric supercapacitor was designed from MnO$_{\mathrm{2}}$ coated TiO$_{\mathrm{2}}$ nanofibers which were prepared by electrospinning and post hydrothermal process. The core shell fiber architecture exhibit highest specific capacitance of 868 F/g in aqueous Na$_{\mathrm{2}}$SO$_{\mathrm{4}}$ electrolyte as compared to the similar structures. The Asymmetric supercapacitor (ASC) fabricated based on these core shell fibers demonstrates large voltage window of 2.6 V which is one of the widest voltage window among aqueous electrolyte based asymmetric supercapacitors. In addition, the ASC delivers large specific capacitance and energy density as revealed by the electrochemical studies. The thin MnO$_{\mathrm{2}}$ shell, of thickness 6 nm, contributes to the extraordinary electrochemical performance for charge storage by redox reaction and intercalation mechanisms, while the anatase phase TiO$_{\mathrm{2}}$ core provides an easy pathway for electronic transport with additional electrochemical stability over thousands of charge discharge cycles.   

Helically coiled carbon nanotube forests for use as electrodes in supercapacitors

Supercapacitors are a class of devices which combine the high energy density of batteries with the power delivery of capacitors, and have benefitted greatly from the incorporation of carbon nanomaterials. In an effort to improve the specific capacitance of these devices, we have produced binder-free electrodes composed of helically coiled carbon nanotube forests grown on stainless steel current collectors with a performance superior to traditional carbon nanomaterials. By virtue of their helicity, the coiled nanotubes provide a greater surface area for energy storage than their straight counterparts, thus improving the specific capacitance. Furthermore, we used an Ar plasma treatment to increase the electronic density of states, and thereby the quantum capacitance, through the introduction of defects.   

High-Energy-Density Cost-Effective Graphene Supercapacitors

We introduce a cost-effective graphene platelet composite material as a replacement of an expensive reduced graphene oxide for electrodes in high energy density supercapacitors. We have tested a low size supercapacitor prototypes with the graphene platelets electrodes and newly developed polymer-gel Li$+$ ion electrolyte. We discuss the ways how to increase the capacitance and the energy densities of the supercapacitor significantly. A working prototype for testing the concept of the high voltage supercapacitor has been developed as well. The first test done up to 10 V showed excellent performance of the multy-cell multi-layer high voltage test assembly.   

Dipolar self-consistent field theory for ionic liquids between charged plates: Effects of dielectric contrast between cation and anion under external electrostatic fields

We develop a new dipolar self-consistent field theory (DSCFT) for both incompressible and compressible ionic liquids under external electrostatic fields. Our theory accounts for the difference between the dipole moments and the molecular volumes of the cation and anion, and the double layer caused by the strong association of the ions with the electrodes. To date, few theoretical studies have considered the dielectric contrast between the cation and anion. Thus, our study focuses on the effect of the dielectric inhomogeneity on the ion distribution and the capacitance. Our theory shows that the capacitance changes with the applied voltage in agreement with experimental observations. Importantly, the dielectric contrast and the difference in molecular volumes between the cation and anion have equal effects on the magnitude of the capacitance. We also consider compressible ionic liquids by developing a hybrid of DSCFT combined with Monte Carlo simulations. We then demonstrate that the hard-core nature of the ions causes oscillations in the density profile and dielectric value near the charged plates. Accordingly, the dielectric constants derived from the classical theories of Onsager and Kirkwood are shown to be gross approximations of the true situation in nanochannels.   

Theoretical Investigation of oxides for batteries and fuel cell applications

I will present theoretical studies of Li-ion and proton-conducting oxides using a combination of theory and computations that involve Density Functional Theory based atomistic modeling, cluster-expansion based studies, global optimization, high-throughput computations and machine learning based investigation of ionic transport in oxide materials. In Li-ion intercalated oxides, we explain the experimentally observed (Nature Materials 12, 518–522 (2013)) 'intercalation pseudocapacitance' phenomenon, and explain why $Nb_{2} O_{5}$ is special to show this behavior when Li-ions are intercalated (J. Mater. Chem. A, 2013,1, 14951-14956), but not when Na-ions are used. In addition, we explore Li-ion intercalation theoretically in $VO_{2}(B)$ phase, which is somewhat structurally similar to $Nb_{2}O_{5}$ and predict an interesting role of site-trapping on the voltage and capacity of the material, validated by ongoing experiments. Computations of proton conducting oxides explain why $Y$-doped $BaZrO_{3}$, one of the fastest proton conducting oxide, shows a decrease in conductivity above 20\% $Y$-doping. Further, using high throughput computations and machine learning tools we discover general principles to improve proton conductivity. Acknowledgements: LDRD at ORNL and CNMS at ORNL   

Atomic-Scale Mechanisms for Electrolyte Decomposition in Li-ion Battery Cathodes

Li-ion batteries using high energy density LiCoO$_{\mathrm{2}}$ (LCO) intercalation cathodes are known to generate gaseous species inside the cell, which can lead to venting flammable solvent vapor. It has been hypothesized that reactions at the cathode/electrolyte interface catalyze the production of these gaseous species. To elucidate the underlying reaction mechanism, first principles calculations were used to model interactions between LCO surfaces and Ethylene Carbonate (EC), a commonly used solvent in Li-ion batteries. A Metropolis Monte Carlo algorithm was used to identify likely low energy adsorption configurations for EC on the (10-14) surface of LCO. Several of these geometries were further analyzed with DFT. The thermodynamics and kinetics of EC decomposition were evaluated for plausible reaction pathways and associated various solvent decomposition mechanisms, such as hydrogen abstraction. Preliminary results indicate that hydrogen abstraction may lead to the spontaneous decomposition of EC into CO and other adsorbed species at the surface.