Sean P. Culver

Influence of Lattice Polarizability on the Ionic Conductivity in the Lithium Superionic Argyrodites Li6PS5X (X = Cl, Br, I)

In the search for novel solid electrolytes for solid-state batteries, thiophosphate ionic conductors have been in recent focus owing to their high ionic conductivities, which are believed to stem from a softer, more polarizable anion framework. Inspired by the oft-cited connection between a soft anion lattice and ionic transport, this work aims to provide evidence on how changing the polarizability of the anion sublattice in one structure affects ionic transport. Here, we systematically alter the anion framework polarizability of the superionic argyrodites Li6PS5X by controlling the fractional occupancy of the halide anions (X = Cl, Br, I). Ultrasonic speed of sound measurements are used to quantify the variation in the lattice stiffness and Debye frequencies. In combination with electrochemical impedance spectroscopy and neutron diffraction, these results show that the lattice softness has a striking influence on the ionic transport: the softer bonds lower the activation barrier and simultaneously decrease the prefactor of the moving ion. Due to the contradicting influence of these parameters on ionic conductivity, we find that it is necessary to tailor the lattice stiffness of materials in order to obtain an optimum ionic conductivity.

Earth abundant CuSbS2 thin films solution processed from thiol–amine mixtures

Solution processing is a practical low-cost strategy for depositing semiconductor thin films. A binary thiol–amine solvent mixture dissolves bulk Cu2S and Sb2S3 under ambient conditions, allowing for solution deposition and low temperature recovery of CuSbS2. The resulting films of earth abundant CuSbS2 possess optoelectronic properties suitable for photovoltaic applications.

The Detrimental Effects of Carbon Additives in Li10GeP2S12 based Solid-State Batteries

All-solid-state batteries (SSBs) have recently attracted much attention due to their potential application in electric vehicles. One key issue that is central to improve the function of SSBs is to gain a better understanding of the interfaces between the material components toward enhancing the electrochemical performance. In this work, the interfacial properties of a carbon-containing cathode composite, employing Li10GeP2S12 as the solid electrolyte, are investigated. A large interfacial charge-transfer resistance builds up upon the inclusion of carbon in the composite, which is detrimental to the resulting cycle life. Analysis by X-ray photoelectron spectroscopy reveals that carbon facilitates faster electrochemical decomposition of the thiophosphate solid electrolyte at the cathode/solid electrolyte interface-by transferring the low chemical potential of lithium in the charged state deeper into the solid electrolyte and extending the decomposition region. The occurring accumulation of highly oxidized sulfur species at the interface is likely responsible for the large interfacial resistances and aggravated capacity fading observed.

Structural disorder in AMoO4 (A = Ca, Sr, Ba) scheelite nanocrystals.

The crystal structure of sub-15 nm AMoO4 (A = Ca, Sr, Ba) scheelite nanocrystals has been investigated using a dual-space approach that combines Rietveld and pair distribution function (PDF) analysis of synchrotron X-ray diffraction data. Rietveld analysis yields an average crystal structure in which the Mo-O bond distance exhibits an anomalously large contraction (2.8%) upon chemical substitution of Ba(2+) for Ca(2+). Such a dependence on chemical composition contradicts the well-known rigid character of Mo(VI)-O bonds and the resulting rigidity of MoO4 tetrahedra in scheelites. Unlike Rietveld, PDF analysis yields a local crystal structure in which the Mo-O bond distance shows a negligible contraction (0.4%) upon going from Ba(2+) to Ca(2+) and, therefore, appears independent of the chemical composition. Analysis of the anisotropic displacement parameters of the oxygen atom reveals that the disagreement between the average and local structural models arises from the presence of static orientational disorder of the MoO4 tetrahedra. Rietveld analysis averages the random rotations of the MoO4 tetrahedra across the scheelite lattice yielding an apparent Mo-O bond distance that is shorter than the true bond distance. In contrast, PDF analysis demonstrates that the structural integrity of the MoO4 tetrahedra remains unchanged upon chemical substitution of the alkaline-earth cation, and that their orientational disorder is accommodated through geometric distortions of the AO8 dodecahedra.

Lithium ion conductivity in Li2S–P2S5 glasses – building units and local structure evolution during the crystallization of superionic conductors Li3PS4, Li7P3S11 and Li4P2S7

Motivated by the high lithium ion conductivities of lithium thiophosphate glasses, a detailed study is performed on the local chemical nature of the thiophosphate building units within these materials. Using Raman and 31P MAS NMR (Magic Angle Spinning – Nuclear Magnetic Resonance) spectroscopy, the continuous change from dominant P2S74− (di-tetrahedral) anions to PS43− (mono-tetrahedral) anions with increasing Li2S fraction in the (Li2S)x(P2S5)(100−x) glasses is observed. In addition, synchrotron pair distribution function analysis (PDF) of synchrotron X-ray total scattering data is employed to monitor in situ crystallization and phase evolution in this class of materials. Depending on the composition, different crystalline phases evolve, which possess different decomposition temperatures into less conducting phases. The results highlight the critical influence of the local anionic building units on the cation mobility and thermal stability, with PS43− tetrahedra forming the most thermally robust glass ceramics with the highest ionic conductivity.

Synthesis, Structural Characterization, and Lithium Ion Conductivity of the Lithium Thiophosphate Li2P2S6

Inspired by the ongoing search for new superionic lithium thiophosphates for use in solid-state batteries, we present the synthesis and structural characterization of Li2P2S6, a novel crystalline lithium thiophosphate. Whereas M2P2S6 with the different alkaline elements (M = Na, K, Rb, Cs) is known, the lithium counterpart has not been reported yet. Herein, we present a combination of synchrotron pair distribution function analysis and neutron powder diffraction to elucidate the crystal structure and possible Li+ diffusion pathways of Li2P2S6. Additionally, impedance spectroscopy is used to evaluate its ionic conductivity. We show that Li2P2S6 possesses P2S62- polyhedral units with edge-sharing PS4 tetrahedra and only one-dimensional diffusion pathways with localized Li-Li pairs, leading to a low ionic conductivity for lithium.

Lanthanide-activated scheelite nanocrystal phosphors prepared by the low-temperature vapor diffusion sol–gel method

A series of Eu3+-, Tb3+-, and Tm3+-doped CaWO4 phosphor nanocrystals have been synthesized under benign conditions using the vapor diffusion sol-gel method. The high degree of synthetic flexibility inherent to this approach has enabled the synthesis of a CaWO4:(Eu,Tb) dual-sensitized white light emitting nanocrystal phosphor upon commercial UV excitation at 366 nm with a long lifetime exceeding 1 ms.

Thermally activated rotational disorder in CaMoO4 nanocrystals

A dual-space approach, combining Rietveld and pair distribution function (PDF) analyses, has been applied to temperature-dependent synchrotron X-ray total scattering data collected on vapor diffusion sol–gel derived CaMoO4 nanocrystals. A sharp transition in Ca–O bond distances in the range of 151–163 K was identified by PDF analysis, which is attributed to the thermal activation of rotational disorder associated with the rigid MoO4 tetrahedra.

Degradation Mechanisms at the Li10GeP2S12/LiCoO2 Cathode Interface in an All-Solid-State Lithium-Ion Battery

All-solid-state batteries (ASSBs) show great potential for providing high power and energy densities with enhanced battery safety. While new solid electrolytes (SEs) have been developed with high enough ionic conductivities, SSBs with long operational life are still rarely reported. Therefore, on the way to high-performance and long-life ASSBs, a better understanding of the complex degradation mechanisms, occurring at the electrode/electrolyte interfaces is pivotal. While the lithium metal/solid electrolyte interface is receiving considerable attention due to the quest for high energy density, the interface between the active material and solid electrolyte particles within the composite cathode is arguably the most difficult to solve and study. In this work, multiple characterization methods are combined to better understand the processes that occur at the LiCoO2 cathode and the Li10GeP2S12 solid electrolyte interface. Indium and Li4Ti5O12 are used as anode materials to avoid the instability problems associated with Li-metal anodes. Capacity fading and increased impedances are observed during long-term cycling. Postmortem analysis with scanning transmission electron microscopy, electron energy loss spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy show that electrochemically driven mechanical failure and degradation at the cathode/solid electrolyte interface contribute to the increase in internal resistance and the resulting capacity fading. These results suggest that the development of electrochemically more stable SEs and the engineering of cathode/SE interfaces are crucial for achieving reliable SSB performance.

Effect of Si substitution on the structural and transport properties of superionic Li-argyrodites

Inspired by the recent interest in lithium ion conducting argyrodites as solid electrolytes for solid-state batteries, we have investigated the influence of aliovalent substitution in Li6PS5Br. Using Rietveld refinements against X-ray and neutron diffraction, coupled with impedance spectroscopy, we monitor the influence of Si4+ substitution for P5+ in Li6+xP1−xSixS5Br on the structure and ionic transport properties. A step-wise incorporation of Si4+ leads to an expansion of the unit cell, as well as the inclusion of additional Li+ within the structure. The increasing Li content occupies the structural transition state and, in combination with the structural changes, leads to a three-fold improvement of the ionic conductivity. This work demonstrates that the argyrodite material class can be optimized through aliovalent substitution, thereby making argyrodites an ideal system for studying solid electrolytes within the field of solid-state batteries.