Johannes Voss

Interfacial challenges in solid-state Li ion batteries.

F over 2 decades, Li ion batteries have enabled the rise of portable electronics and dominated the battery market. The principal reason for this is that of all electrically rechargeable batteries with an adequate cycle life, the Li ion battery can store the most electrical energy, both in terms of weight (specific energy Wh/kg) and in terms of volume (energy density Wh/L). There has been steady but slow evolution of the important parameters describing Li ion battery performance since its commercial introduction in 1991 by Sony, for example, calendar and cycle lifetimes, the two energy densities (Wh/kg and Wh/L), power density, cost, and safety. Unfortunately, growth rates of the two energy densities have only been occurring at ∼7−8%/year. Today, cell-level-specific energies of ∼200 Wh/kg and energy density of ∼500 Wh/L are typical and are acceptable for most portable electronics applications. The electrification of light vehicle road transportation is generally considered the next important frontier for electrochemical energy storage, and it is currently debated whether Li ion batteries will ever be good enough for mass-market full electrification. At present, hybrids (HEVs) represent only ∼3% of new car sales in the U.S. market and plug-in hybrids (PHEVs) plus full electric vehicles (EVs) represent only ∼0.7% of U.S. new car sales. The principal issue inhibiting the massmarket electrification is simply a battery problem, that is, developing a cost-effective, safe, and long-lived battery with sufficient energy storage (both in terms of weight and volume) to give enough range for daily driving so that charging can be accomplished overnight at home. At present, the Li ion is the only practical battery for EVs and PHEVs, although this currently presents a difficult weight/volume−range−cost tradeoff in the design of the EV. It is projected that retail costs for Li ion batteries may decrease significantly in the future, from ∼

High-Pressure Single-Crystal Structures of 3D Lead-Halide Hybrid Perovskites and Pressure Effects on their Electronic and Optical Properties

400/kWh at present to ∼

Density functional theory based screening of ternary alkali-transition metal borohydrides: A computational material design project

100/kWh in the 2030 time frame (especially with buildup of Tesla style battery “Gigafactories”). This would make EVs cost-competitive with traditional gasoline-powered cars. However, this still does not solve the weight/volume-range issue because of the projected limited increases in specific energy and energy density of conventional Li ion. In addition, because Li ion batteries use flammable nonaqueous liquid electrolytes, there is a serious safety issue in their use, especially for the large-format battery packs in EVs (e.g., Tesla fires). While some believe that these Li ion issues can all be tamed, for example, Elon Musk, others believe that mass-market electrification will require a totally different battery chemistry with significantly higher energy densities, so-called beyond Li ion (Li−S, Li−air, Mg ion, etc.). Of course these latter currently have many technical challenges; therefore, it is not at all clear if they will ever become practical batteries for use in EVs. It seems to us that a wave of optimism is also building in the battery community that most of the limitations of the conventional Li ion batteries for EVs can be addressed by using a solid-state electrolyte in place of the traditional liquid one. Ideally, the solid-state Li ion battery replaces the intercalated lithium graphite anode (LiC6) in the conventional Li ion battery with Li metal and the liquid electrolyte with a solid-state electrolyte (SSE) but keeps the conventional intercalation cathode (C), for example, LiCoO2. We therefore view the battery as a Li|SSE|C thin-film stack (or series of repeated stacks). Unfortunately, much of the current development of solid-state Li ion batteries is being done by startups, so that, although the claims are quite impressive for the solid-state Li ion batteries, their current status is unclear. For example, several companies quote an energy density (Wh/L) several times higher than conventional Li ion, but no mention is made of their power density or capacity. Nevertheless, optimism seems high enough that at least three major car-related companies have invested significantly in U.S. solid-state battery startups, VW in Quantumscape, GM in Sakti3, and most recently Bosch in Seeo. In addition, Toyota is also investing heavily in their own solid-state battery program. A solid-state Li ion battery could, in principle, yield many advantages relative to the current conventional Li ion battery. Perhaps most important is in terms of safety by removing the flammable liquid electrolyte. Second, using Li metal instead of LiC6 (because the SSE hopefully suppresses Li dendrite formation) and a higher-voltage cathode (because many of the SSEs have electrochemical windows of >5 V) could allow higher energy densities (however, see Table 1). Finally, because

An orbital-overlap model for minimal work functions of cesiated metal surfaces.

We report the first high-pressure single-crystal structures of hybrid perovskites. The crystalline semiconductors (MA)PbX3 (MA = CH3NH3+, X = Br– or I–) afford us the rare opportunity of understanding how compression modulates their structures and thereby their optoelectronic properties. Using atomic coordinates obtained from high-pressure single-crystal X-ray diffraction we track the perovskites’ precise structural evolution upon compression. These structural changes correlate well with pressure-dependent single-crystal photoluminescence (PL) spectra and high-pressure bandgaps derived from density functional theory. We further observe dramatic piezochromism where the solids become lighter in color and then transition to opaque black with compression. Indeed, electronic conductivity measurements of (MA)PbI3 obtained within a diamond-anvil cell show that the material’s resistivity decreases by 3 orders of magnitude between 0 and 51 GPa. The activation energy for conduction at 51 GPa is only 13.2(3) meV, suggesting that the perovskite is approaching a metallic state. Furthermore, the pressure response of mixed-halide perovskites shows new luminescent states that emerge at elevated pressures. We recently reported that the perovskites (MA)Pb(BrxI1–x)3 (0.2 < x < 1) reversibly form light-induced trap states, which pin their PL to a low energy. This may explain the low voltages obtained from solar cells employing these absorbers. Our high-pressure PL data indicate that compression can mitigate this PL redshift and may afford higher steady-state voltages from these absorbers. These studies show that pressure can significantly alter the transport and thermodynamic properties of these technologically important semiconductors.

Thermionic current densities from first principles

We present a computational screening study of ternary metal borohydrides for reversible hydrogen storage based on density functional theory. We investigate the stability and decomposition of alloys containing 1 alkali metal atom, Li, Na, or K (M(1)); and 1 alkali, alkaline earth or 3d/4d transition metal atom (M(2)) plus two to five (BH(4))(-) groups, i.e., M(1)M(2)(BH(4))(2-5), using a number of model structures with trigonal, tetrahedral, octahedral, and free coordination of the metal borohydride complexes. Of the over 700 investigated structures, about 20 were predicted to form potentially stable alloys with promising decomposition energies. The M(1)(Al/Mn/Fe)(BH(4))(4), (Li/Na)Zn(BH(4))(3), and (Na/K)(Ni/Co)(BH(4))(3) alloys are found to be the most promising, followed by selected M(1)(Nb/Rh)(BH(4))(4) alloys.

Theoretical Investigations of the Electrochemical Reduction of CO on Single Metal Atoms Embedded in Graphene

We introduce a model for the effect of cesium adsorbates on the work function of transition metal surfaces. The model builds on the classical point-dipole equation by adding exponential terms that characterize the degree of orbital overlap between the 6s states of neighboring cesium adsorbates and its effect on the strength and orientation of electric dipoles along the adsorbate-substrate interface. The new model improves upon earlier models in terms of agreement with the work function-coverage curves obtained via first-principles calculations based on density functional theory. All the cesiated metal surfaces have optimal coverages between 0.6 and 0.8 monolayers, in accordance with experimental data. Of all the cesiated metal surfaces that we have considered, tungsten has the lowest minimum work function, also in accordance with experiments.

Γ-point lattice free energy estimates from O(1) force calculations

We present a density functional theory-based method for calculating thermionic emission currents from a cathode into vacuum using a non-equilibrium Greens function approach. It does not require semi-classical approximations or crude simplifications of the electronic structure used in previous methods and thus provides quantitative predictions of thermionic emission for adsorbate-coated surfaces. The obtained results match well with experimental measurements of temperature-dependent current densities. Our approach can thus enable computational design of composite electrode materials.

From the Sabatier principle to a predictive theory of transition-metal heterogeneous catalysis

Single transition metal atoms embedded at single vacancies of graphene provide a unique paradigm for catalytic reactions. We present a density functional theory study of such systems for the electrochemical reduction of CO. Theoretical investigations of CO electrochemical reduction are particularly challenging in that electrochemical activation energies are a necessary descriptor of activity. We determined the electrochemical barriers for key proton–electron transfer steps using a state-of-the-art, fully explicit solvent model of the electrochemical interface. The accuracy of GGA-level functionals in describing these systems was also benchmarked against hybrid methods. We find the first proton transfer to form CHO from CO to be a critical step in C1 product formation. On these single atom sites, the corresponding barrier scales more favorably with the CO binding energy than for 211 and 111 transition metal surfaces, in the direction of improved activity. Intermediates and transition states for the hydrogen evolution reaction were found to be less stable than those on transition metals, suggesting a higher selectivity for CO reduction. We present a rate volcano for the production of methane from CO. We identify promising candidates with high activity, stability, and selectivity for the reduction of CO. This work highlights the potential of these systems as improved electrocatalysts over pure transition metals for CO reduction.

Tunneling and Polaron Charge Transport through Li2O2 in Li–O2 Batteries

We present a new method for estimating the vibrational free energy of crystal (and molecular) structures employing only a single force calculation, for a particularly displaced configuration, in addition to the calculation of the ground state configuration. This displacement vector is the sum of the phonon eigenvectors obtained from a fast-relative to, e.g., density-functional theory (DFT)-Hessian calculation using interatomic potentials. These potentials are based here on effective charges obtained from a DFT calculation of the ground state electronic charge density but could also be based on other, e.g., empiric approaches.