Christian Dietrich

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.

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.

Redox-active cathode interphases in solid-state batteries

All-solid-state batteries are expected to provide a next-generation solution for energy storage. Employing fast conducting lithium thiophosphates as a replacement for liquid electrolytes in conventional lithium ion batteries has shown great promise, however, capacity fading and the underlying interfacial side reactions of thiophosphates and cathode active materials are not yet understood well. In this study, we charge solid-state batteries to different cut-off potentials and find the formation of a redox-active resistive layer in the solid electrolyte, which impedes the conductivity depending on the state-of-charge of the battery. Using electrochemical impedance spectroscopy as well as depth profiling with X-ray photoelectron spectroscopy we find a thick passivation layer at the current collector and decomposition products within the cathode composite. In addition, an in situ electrochemical experiment during X-ray photoelectron spectroscopy shows that the solid electrolyte is redox active at the cathode/solid electrolyte interface in solid-state batteries. This work highlights the importance of protecting interface layers at the current collector, and the influence of the resulting electric potential drop, as well as provides insight into the redox chemistry of lithium conducting thiophosphates.

Li18P6N16—A Lithium Nitridophosphate with Unprecedented Tricyclic [P6N16]18− Ions

Li18 P6 N16 was synthesized by reaction of LiPN2 and Li7 PN4 at 5.5 GPa and 1273 K employing the multi-anvil technique. It is the first lithium nitridophosphate obtained by high-pressure synthesis. Moreover, it is the first example received by reaction of two ternary lithium nitrides. The combination of high-pressure conditions with a Li3 N flux enabled a complete structure determination using single-crystal X-ray diffraction. The hitherto unknown tricyclic [P6 N16 ]18- anion is composed of six vertex-sharing PN4 tetrahedra forming one vierer- and two additional dreier-rings. To confirm the structure, Rietveld refinement, 7 Li and 31 P solid-state NMR spectroscopy, FTIR spectroscopy and EDX measurements were carried out. To validate the ionic properties, the migration pathways of the Li+ ions were evaluated, and the conductivity and its temperature dependence were determined by impedance spectroscopy measurements. In order to obtain a clearer picture of the formation mechanism of this compound class, different synthetic approaches were compared, enabling targeted syntheses of unprecedented P/N-anion topologies with intriguing properties.

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.

Li+ Ion Conductors with Adamantane-type Nitridophosphate Anions - β-Li10P4N10 and Li13P4N10X3 with X = Cl, Br

β-Li10 P4 N10 and Li13 P4 N10 X3 with X=Cl, Br have been synthesized from mixtures of P3 N5 , Li3 N, LiX, LiPN2 , and Li7 PN4 at temperatures below 850 °C. β-Li10 P4 N10 is the low-temperature polymorph of α-Li10 P4 N10 and crystallizes in the trigonal space group R3. It is made up of non-condensed [P4 N10 ]10- T2 supertetrahedra, which are arranged in sphalerite-analogous packing. Li13 P4 N10 X3 (X=Cl, Br) crystallizes in the cubic space group Fm3‾m . Both isomorphic compounds comprise adamantane-type [P4 N10 ]10- , Li+ ions, and halides, which form octahedra. These octahedra build up a face-centered cubic packing, whose tetrahedral voids are occupied by the [P4 N10 ]10- ions. The crystal structures have been elucidated from X-ray powder diffraction data and corroborated by EDX measurements, solid-state NMR, and FTIR spectroscopy. Furthermore, we have examined the phase transition between α- and β-Li10 P4 N10 . To confirm the ionic character, the migration pathways of the Li+ ions have been evaluated and the ion conductivity and its temperature dependence have been determined by impedance spectroscopy. XPS measurements have been carried out to analyze the stability with respect to Li metal.

Li47B3P14N42—A Lithium Nitridoborophosphate with [P3N9]12−, [P4N10]10−, and the Unprecedented [B3P3N13]15− Ion

Li47 B3 P14 N42 , the first lithium nitridoborophosphate, is synthesized by two different routes using a Li3 N flux enabling a complete structure determination by single-crystal X-ray diffraction data. Li47 B3 P14 N42 comprises three different complex anions: a cyclic [P3 N9 ]12- , an adamantane-like [P4 N10 ]10- , and the novel anion [P3 B3 N13 ]15- . [P3 B3 N13 ]15- is the first species with condensed B/N and P/N substructures. Rietveld refinement, 6 Li, 7 Li, 11 B, and 31 P solid-state NMR spectroscopy, FTIR spectroscopy, EDX measurements, and elemental analyses correspond well with the structure model from single-crystal XRD. To confirm the mobility of Li+ ions, their possible migration pathways were evaluated and the temperature-dependent conductivity was determined by impedance spectroscopy. With the Li3 N flux route we gained access to a new class of lithium nitridoborophosphates, which could have a great potential for unprecedented anion topologies with interesting properties.

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.