Emulating synaptic behavior in surface-functionalized MoS2 through modulation of interfacial charge transfer via external stimuli.

Neuromorphic computing requires materials able to yield electronic switching behavior in response to external stimuli. Transition-metal dichalcogenides surfaces covered by partial or full monolayers of molecular species have shown promise due to their potential for tunable interfacial charge transfer. Here, we demonstrate a class of molecules able to position MoS2 surfaces on the cusp of electronic instabilities. Density functional theory (DFT) calculations and ab initio molecular dynamics simulations are used to study the interaction of four reduced pyridinium-derived pi-conjugated molecules with the pristine basal planes of MoS2, by exploring the dynamical evolution of the system at room temperature with regards to the effective band gap, radius of gyration (rog), and charge transfer. Computed rog profiles show that low concentrations of small reduced methyl viologen molecules have high mobilities on top of the surface of the basal plane at room temperature leading to unstable surface deposition, whereas a full monolayer of larger fused-ring molecules deposited on the basal surface shows greater thermal stability. DFT analyses show these larger reduced pyridinium derivatives promote n-type doping on the basal plane due to a built-in electric field, which can be systematically tuned to induce a switching effect, opening and closing a bandgap and providing a fundamental means of driving electronic instabilities needed for emulating neuronal functionality.

Highly Reversible Aqueous Zinc Batteries enabled by Zincophilic-Zincophobic Interfacial Layers and Interrupted Hydrogen-Bond Electrolytes.

Aqueous Zn batteries promise high energy density but suffer from Zn dendritic growth and poor low-temperature performance. Here, we overcome both challenges by using an eutectic 7.6 m ZnCl2 aqueous electrolyte with 0.05 m SnCl2 additive, which in situ forms a zincophilic/zincophobic Sn/Zn5 (OH)8 Cl2 ⋅H2 O bilayer interphase and enables low temperature operation. Zincophilic Sn decreases Zn plating/stripping overpotential and promotes uniform Zn plating, while zincophobic Zn5 (OH)8 Cl2 ⋅H2 O top-layer suppresses Zn dendrite growth. The eutectic electrolyte has a high ionic conductivity of ≈0.8 mS cm-1 even at -70 °C due to the distortion of hydrogen bond network by solvated Zn2+ and Cl- . The eutectic electrolyte enables Zn∥Ti half-cell a high Coulombic efficiency (CE) of >99.7 % for 200 cycles and Zn∥Zn cell steady charge/discharge for 500 h with a low overpotential of 8 mV at 3 mA cm-2 . Practically, Zn∥VOPO4 batteries maintain >95 % capacity with a CE of >99.9 % for 200 cycles at -50 °C, and retain ≈30 % capacity at -70 °C of that at 20 °C.

Fluoroethylene Carbonate as a Directing Agent in Amorphous Silicon Anodes: Electrolyte Interface Structure Probed by Sum Frequency Vibrational Spectroscopy and Ab Initio Molecular Dynamics.

Fluorinated compounds are added to carbonate-based electrolyte solutions in an effort to create a stable solid electrolyte interphase (SEI). The SEI mitigates detrimental electrolyte redox reactions taking place on the anode's surface upon applying a potential in order to charge (discharge) the lithium (Li) ion battery. The need for a stable SEI is dire when the anode material is silicon as silicon cracks due to its expansion and contraction upon lithiation and delithiation (charge-discharge) cycles, consequently limiting the cyclability of a silicon-based battery. Here we show the molecular structures for ethylene carbonate (EC): fluoroethylene carbonate (FEC) solutions on silicon surfaces by sum frequency generation (SFG) vibrational spectroscopy, which yields vibrational spectra of molecules at interfaces and by ab initio molecular dynamics (AIMD) simulations at open circuit potential. Our AIMD simulations and SFG spectra indicate that both EC and FEC adsorb to the amorphous silicon (a-Si) through their carbonyl group (C═O) oxygen atom with no further desorption. We show that FEC additives induce the reorientation of EC molecules to create an ordered, up-right orientation of the electrolytes on the Si surface. We suggest that this might be helpful for Li diffusion under applied potential. Furthermore, FEC becomes the dominant species at the a-Si surface as the FEC concentration increases above 20 wt %. Our finding at open circuit potential can now initiate additive design to not only act as a sacrificial compound but also to produce a better suited SEI for the use of silicon anodes in the Li-ion vehicular industry.

Tuning the Solid Electrolyte Interphase for Selective Li- and Na-Ion Storage in Hard Carbon.

Solid-electrolyte interphase (SEI) films with controllable properties are highly desirable for improving battery performance. In this paper, a combined experimental and theoretical approach is used to study SEI films formed on hard carbon in Li- and Na-ion batteries. It is shown that a stable SEI layer can be designed by precycling an electrode in a desired Li- or Na-based electrolyte, and that ionic transport can be kinetically controlled. Selective Li- and Na-based SEI membranes are produced using Li- or Na-based electrolytes, respectively. The Na-based SEI allows easy transport of Li ions, while the Li-based SEI shuts off Na-ion transport. Na-ion storage can be manipulated by tuning the SEI layer with film-forming electrolyte additives, or by preforming an SEI layer on the electrode surface. The Na specific capacity can be controlled to < 25 mAh g-1 ; ≈ 1/10 of the normal capacity (250 mAh g-1 ). Unusual selective/preferential transport of Li ions is demonstrated by preforming an SEI layer on the electrode surface and corroborated with a mixed electrolyte. This work may provide new guidance for preparing good ion-selective conductors using electrochemical approaches.

Facet-Dependent Thermal Instability in LiCoO2.

Thermal runaways triggered by the oxygen release from oxide cathode materials pose a major safety concern for widespread application of lithium ion batteries. Utilizing in situ aberration-corrected scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) at high temperatures, we show that oxygen release from LixCoO2 cathode crystals is occurring at the surface of particles. We correlated this local oxygen evolution from the LixCoO2 structure with local phase transitions spanning from layered to spinel and then to rock salt structure upon exposure to elevated temperatures. Ab initio molecular dynamics simulations (AIMD) results show that oxygen release is highly dependent on LixCoO2 facet orientation. While the [001] facets are stable at 300 °C, oxygen release is observed from the [012] and [104] facets, where under-coordinated oxygen atoms from the delithiated structures can combine and eventually evolve as O2. The novel understanding that emerges from the present study provides in-depth insights into the thermal runaway mechanism of Li-ion batteries and can assist the design and fabrication of cathode crystals with the most thermally stable facets.

Scaling Atomic Partial Charges of Carbonate Solvents for Lithium Ion Solvation and Diffusion.

Lithium-ion solvation and diffusion properties in ethylene carbonate (EC) and propylene carbonate (PC) were studied by molecular simulation, experiments, and electronic structure calculations. Studies carried out in water provide a reference for interpretation. Classical molecular dynamics simulation results are compared to ab initio molecular dynamics to assess nonpolarizable force field parameters for solvation structure of the carbonate solvents. Quasi-chemical theory (QCT) was adapted to take advantage of fourfold occupancy of the near-neighbor solvation structure observed in simulations and used to calculate solvation free energies. The computed free energy for transfer of Li+ to PC from water, based on electronic structure calculations with cluster-QCT, agrees with the experimental value. The simulation-based direct-QCT results with scaled partial charges agree with the electronic structure-based QCT values. The computed Li+/PF6- transference numbers of 0.35/0.65 (EC) and 0.31/0.69 (PC) agree well with NMR experimental values of 0.31/0.69 (EC) and 0.34/0.66 (PC) and similar values obtained here with impedance spectroscopy. These combined results demonstrate that solvent partial charges can be scaled in systems dominated by strong electrostatic interactions to achieve trends in ion solvation and transport properties that are comparable to ab initio and experimental results. Thus, the results support the use of scaled partial charges in simple, nonpolarizable force fields in future studies of these electrolyte solutions.