Dominik A. Weber

Dynamic formation of a solid-liquid electrolyte interphase and its consequences for hybrid-battery concepts

The discharging and charging of batteries require ion transfer across phase boundaries. In conventional lithium-ion batteries, Li(+) ions have to cross the liquid electrolyte and only need to pass the electrode interfaces. Future high-energy batteries may need to work as hybrids, and so serially combine a liquid electrolyte and a solid electrolyte to suppress unwanted redox shuttles. This adds new interfaces that might significantly decrease the cycling-rate capability. Here we show that the interface between a typical fast-ion-conducting solid electrolyte and a conventional liquid electrolyte is chemically unstable and forms a resistive solid-liquid electrolyte interphase (SLEI). Insights into the kinetics of this new type of interphase are obtained by impedance studies of a two-chamber cell. The chemistry of the SLEI, its growth with time and the influence of water impurities are examined by state-of-the-art surface analysis and depth profiling.

Interfacial Reactivity Benchmarking of the Sodium Ion Conductors Na3PS4 and Sodium β-Alumina for Protected Sodium Metal Anodes and Sodium All-Solid-State Batteries

The interfacial stability of solid electrolytes at the electrodes is crucial for an application of all-solid-state batteries and protected electrodes. For instance, undesired reactions between sodium metal electrodes and the solid electrolyte form charge transfer hindering interphases. Due to the resulting large interfacial resistance, the charge transfer kinetics are altered and the overvoltage increases, making the interfacial stability of electrolytes the limiting factor in these systems. Driven by the promising ionic conductivities of Na3PS4, here we explore the stability and viability of Na3PS4 as a solid electrolyte against metallic Na and compare it to that of Na-β″-Al2O3 (sodium β-alumina). As expected, Na-β″-Al2O3 is stable against sodium, whereas Na3PS4 decomposes with an increasing overall resistance, making Na-β″-Al2O3 the electrolyte of choice for protected sodium anodes and all-solid-state batteries.

Interfacial Processes and Influence of Composite Cathode Microstructure Controlling the Performance of All-Solid-State Lithium Batteries

All-solid-state lithium-ion batteries have the potential to become an important class of next-generation electrochemical energy storage devices. However, for achieving competitive performance, a better understanding of the interfacial processes at the electrodes is necessary for optimized electrode compositions to be developed. In this work, the interfacial processes between the solid electrolyte (Li10GeP2S12) and the electrode materials (In/InLi and LixCoO2) are monitored using impedance spectroscopy and galvanostatic cycling, showing a large resistance contribution and kinetic hindrance at the metal anode. The effect of different fractions of the solid electrolyte in the composite cathodes on the rate performance is tested. The results demonstrate the necessity of a carefully designed composite microstructure depending on the desired applications of an all-solid-state battery. While a relatively low mass fraction of solid electrolyte is sufficient for high energy density, a higher fraction of solid electrolyte is required for high power density.

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.

(Electro)chemical expansion during cycling: monitoring the pressure changes in operating solid-state lithium batteries

Solid-state lithium-ion batteries (SSBs) are a promising concept for future energy storage applications. Interestingly, the mechanical effects during operation of SSBs, and their correlation to the electrochemical performance, have rarely been investigated. In such systems, the rigid mechanical coupling between the active phases and the solid electrolyte will lead to more complex non-local strain effects than in the common liquid electrolyte-based lithium-ion batteries, where the chemical expansion or compression of the active phases is accommodated by the liquid electrolyte, and only local mechanical strain within the electrode particles exists. In this work we report on the pressure and height changes within typical solid-state batteries, which were measured in situ during galvanostatic cycling conditions. The continuous volume changes of both the anode and the cathode during lithiation/delithiation are responsible for a highly reproducible cycle of pressure changes during the operation of the solid-state battery cell. Bending and cracking of the solid-state battery cells are observed with X-ray tomography and provide evidence for the critical role of the macroscopic strain generated during cycling. Furthermore, these pressure and dilatometry measurements as well as X-ray tomography underline the importance of external confinement and pressure control for SSBs.

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.

Correlating Transport and Structural Properties in Li1+xAlxGe2–x(PO4)3 (LAGP) Prepared from Aqueous Solution

Li1+ xAl xGe2- x(PO4)3 (LAGP) is a solid lithium-ion conductor belonging to the NASICON family, representing the solid solution of LiGe2(PO4)3 and AlPO4. The typical syntheses of LAGP either involve high-temperature melt-quenching, which is complicated and expensive, or a sol-gel process requiring costly organic germanium precursors. In this work, we report a simple method based on aqueous solutions without the need of ethoxide precursors. Using synchrotron and neutron diffraction, the crystal structure, the occupancies for Al and Ge, and the distribution of lithium were determined. Substitution of germanium by aluminum allows for an increased Li+ incorporation in the material and the actual Li+ content in the sample increases with the nominal Li+ content and a solubility limit is observed for higher aluminum content. By means of impedance spectroscopy, an increase in the ionic conductivity with increasing lithium content is observed. Whereas the lithium ionic conductivity improves, due to the increasing carrier density, the bulk activation energy increases. This correlation suggests that changes in the transport mechanism and correlated motion may be at play in the Li1+ xAl xGe2- x(PO4)3 solid solution.

Molybdenum Oxide Nitrides of the Mo2(O,N,□)5 Type: On the Way to Mo2O5

Blue-colored molybdenum oxide nitrides of the Mo2(O,N,□)5 type were synthesized by direct nitridation of commercially available molybdenum trioxide with a mixture of gaseous ammonia and oxygen. Chemical composition, crystal structure, and stability of the obtained and hitherto unknown compounds are studied extensively. The average oxidation state of +5 for molybdenum is proven by Mo K near-edge X-ray absorption spectroscopy; the magnetic behavior is in agreement with compounds exhibiting MoVO6 units. The new materials are stable up to ∼773 K in an inert gas atmosphere. At higher temperatures, decomposition is observed. X-ray and neutron powder diffraction, electron diffraction, and high-resolution transmission electron microscopy reveal the structure to be related to VNb9O24.9-type phases, however, with severe disorder hampering full structure determination. Still, the results demonstrate the possibility of a future synthesis of the potential binary oxide Mo2O5. On the basis of these findings, a tentative suggestion on the crystal structure of the potential compound Mo2O5, backed by electronic-structure and phonon calculations from first principles, is given.