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 LiPSX 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.

For the development of next-generation lithium batteries, major research effort is made to enable a reversible lithium metal anode by the use of solid electrolytes. However, the fundamentals of the solid− solid interface and especially the processes that take place under current load are still not well characterized. By measuring pressure-dependent electrode kinetics, we explore the electrochemo-mechanical behavior of the lithium metal anode on the garnet electrolyte Li 6.25 Al 0.25 La 3 Zr 2 O 12 . Because of the stability against reduction in contact with the lithium metal, this serves as an optimal model system for kinetic studies without electrolyte degradation. We show that the interfacial resistance becomes negligibly small and converges to practically 0 Ω•cm 2 at high external pressures of several 100 MPa. To the best of our knowledge, this is the smallest reported interfacial resistance in the literature without the need for any interlayer. We interpret this observation by the concept of constriction resistance and show that the contact geometry in combination with the ionic transport in the solid electrolyte dominates the interfacial contributions for a clean interface in equilibrium. Furthermore, we show thatunder anodic operating conditionsthe vacancy diffusion limitation in the lithium metal restricts the rate capability of the lithium metal anode because of contact loss caused by vacancy accumulation and the resulting pore formation near the interface. Results of a kinetic model show that the interface remains morphologically stable only when the anodic load does not exceed a critical value of approximately 100 μA•cm −2 , which is not high enough for practical cell setups employing a planar geometry. We highlight that future research on lithium metal anodes on solid electrolytes needs to focus on the transport within and the morphological instability of the metal electrode. Overall, the results help to develop a deeper understanding of the lithium metal anode on solid electrolytes, and the major conclusions are not limited to the Li|Li 6.25 Al 0.25 La 3 Zr 2 O 12 interface.

Developing reversible lithium metal anodes with high rate capability is one of the central aims of current battery research. Lithium metal anodes are not only required for the development of innovative cell concepts such as lithium–air or lithium–sulfur batteries, they can also increase the energy density of batteries with intercalation-type cathodes. The use of solid electrolyte separators is especially promising to develop well-performing lithium metal anodes, because they can act as a mechanical barrier to avoid unwanted dendritic growth of lithium through the cell. However, inhomogeneous electrodeposition and contact loss often hinder the application of a lithium metal anode in solid-state batteries. In this review, we assess the physicochemical concepts that describe the fundamental mechanisms governing lithium metal anode performance in combination with inorganic solid electrolytes. In particular, our discussion of kinetic rate limitations and morphological stability intends to stimulate further progress in the field of lithium metal anodes.

enable the lithium metal anode with high rate capability. [1][2][3][4] While in LIBs with liquid electrolytes, lithium dendrite growth and low Coulombic efficiency prevent the use of lithium metal as an anode material, [3,[5][6][7][8][9][10][11] solid electrolytes (SEs) had been predicted to be able to block dendrite growth due to their high shear modulus. [12,13] In this context, Li 7 La 3 Zr 2 O 12 (LLZO) type garnet SEs [14] have attracted great attention as they combine high ionic conductivity with sufficient electrochemical stability against lithium metal, which prevents fast degradation and growth of a resistive interphase. [15,16] Nevertheless, certain issues at the lithium|solid electrolyte interface remain unsolved. [17,18] Lithium penetration through garnet-type SEs currently limits the possible charge rates. [19][20][21][22][23] In this context, it was found that good contact to a small reservoir of lithium metal is highly beneficial to prevent inhomogeneous lithium nucleation, which then reduces the lithium penetration susceptibility. [24] All previous results underline the need for sufficient and homogeneous contact between metal and SE during battery operation. Thus, it is of upmost importance for lithium metal solid-state battery development to prevent pore formation and growth at the anode interface during battery discharge. [24][25][26] Indeed, while the intrinsic charge transfer kinetics of the lithium|LLZO interface was found to be sufficiently fast for practical applications (R int < 2 Ωcm²), [26,27] recent work shows that the morphological instability of the (pure) lithium metal anode on solid electrolytes under anodic load is an inherent, fundamental problem that needs to be solved for battery designs that do not allow high operation pressures in the MPa range. [26,28] The morphological instability stems from the vacancy injection into lithium metal during anodic dissolution, which is a general phenomenon of parent metal electrodes. [29,30] It leads to contact loss and unwanted local current constriction during cell discharge. Therefore, transport of lithium in the lithium metal anode itself needs to be better understood and tuned to further increase the rate capability of cells with a lithium metal anode (i.e., to per-cycle areal capacities of 5 mAh cm −2 at current densities ranging to 10 mA cm −2 ). [31] However, the currently run, predominantly short-term lithium shuttling experiments onThe morphological instability of the lithium metal anode is the key factor restricting the rate capability of lithium metal solid state batteries. During lithium stripping, pore formation takes place at the interface due to the slow diffusion kinetics of vacancies in the lithium metal. The resulting current focusing increases the internal cell resistance and promotes fast lithium penetration. In this work, galvanostatic electrochemical impedance spectroscopy is used to investigate operando the morphological changes at the interface by analysis of the interface capacitances. Therewith, the effect of temper...

Li 7 La 3 Zr 2 O 12 (LLZO)-based garnet materials are recently being investigated as suitable electrolytes for solid-state batteries with lithium-metal electrodes. Unfortunately, lithium-metal penetration through polycrystalline garnet-type electrolytes limits the electric current density during cell charging. In this study, we introduce an electrochemical operando approach that is well suited to get insights into the early stage of lithium-metal penetration that was yet only accessible with very elaborate neutron measurements. Combined with in situ as well as ex situ electron microscopic techniques, we investigate the morphological instability of the lithium-metal anode on garnet-type solid electrolytes under cathodic load and demonstrate the inter-relationship between microkinetic aspects and lithium-penetration susceptibility.

Li+- and Na+-conducting thiophosphates have attracted much interest because of their intrinsically high ionic conductivities and the possibility to be employed in solid-state batteries. Inspired by the recent finding of the influence of changing lattice vibrations and induced lattice softening on the ionic transport of Li+-conducting electrolytes, here we explore this effect in the Na+ conductor Na3PS4–x Se x . Ultrasonic speed of sound measurements are used to monitor a changing lattice stiffness and Debye frequencies. The changes in the lattice dynamics are complemented by X-ray diffraction and electrochemical impedance spectroscopy. With systematic alteration of the polarizability of the anion framework, a softening of the lattice can be observed that leads to a reduction of the activation barrier for migration as well as a decreased Arrhenius prefactor. This work shows that, similar to Li+ transport, the softening of the average vibrational frequencies of the lattice has a tremendous effect on Na+-ionic transport and that ion–phonon interactions need to be considered in solid electrolytes.

A rapidly approaching theoretical limit of Li-ion batteries pushes the desire for next-generation energy storage devices [1]. One of the promising candidates is the all-solid-state battery with inorganic solid ion conductors. By replacing the currently employed liquid electrolyte, this battery architecture is thought to pave the way for a significant enhancement in the energy density with a Li-metal anode, as well as increase the battery safety [1][2][3][4]. The superior thermal stability of solid electrolytes enables operation without cooling, leading to a further gain in energy density when it comes to the device integration. Utilization of Na-ions may even enhance environmental friendliness. The solid ionic conductor performs the function of the separator, as well as the electrolyte in the electrode composites. Therefore, when replacing the liquid-solid interfacial contact that already proves cumbersome in today's lithium-ion batteries, solid-solid interfaces will show different degradation kinetics [5,6]. Although the diffusion kinetics in electrode materials matters in practice [7], as only one type of charge carrier is available effectively leading to cation transference numbers of unity, fast-charging seems possible due to minute cell polarization at high currents [8]. However, instability of the ionic conductors towards the electrodes makes protection concepts necessary [9][10][11][12]. While the interfacial stability and battery architecture are still open questions in the field, high ionic conductivity is paramount for all-solid-state battery operation [2,13]. Conductivities at room temperature above 1 mS cm −1 are typically considered to be sufficient for building research-based devices, whereas likely higher conductivities >10 mS cm −1 will be needed for high energy densities with thick electrode configurations and fast charging/discharging [14].The process of ionic conduction in solids has received attention over decades due to the possible application of oxide ion conductors in sensors and fuel cells and cation conductors in batteries, for instance as electrode materials [15][16][17][18][19][20]. The understanding of lithium and sodium solid-state ionic conductors grew when Na β″alumina, the NASICON (Na SuperIonic CONductor), and LISICON (Li SuperIonic CONductor) structures were found [21][22][23]. However, grain boundaries and mechanical brittleness of these materials have limited the

Recent work on superionic conductors has demonstrated the influence of lattice dynamics and the softness of the lattice on ionic transport. When examining either the changes in the acoustic phonon spectrum or the whole phonon density of states, both a decreasing activation barrier of migration and a decreasing entropy of migration have been observed, highlighting that the paradigm of "the softer the lattice, the better" does not always hold true. However, both approaches to monitor the changing lattice dynamics probe different frequency ranges of the phonon spectrum, and thus, it is unclear if they are complementary. In this work, we investigate the lattice dynamics of the superionic conductor Na 3 PS 4−x Se x by probing the optical phonon modes and the acoustic phonon modes, as well as the phonon density of states via inelastic neutron scattering. Notably, Raman spectroscopy shows the evolution of multiple local symmetry reduced polyhedral species, which likely affect the local diffusion pathways. Meanwhile, density functional theory and the ionic transport data are used to compare the different approaches for assessing the lattice dynamics. This work shows that, while acoustic and inelastic methods may be used to experimentally assess the overall changing lattice stiffness, calculations of the average vibrational energies between the mobile ions and the anion framework are important to assess and computationally screen for ionic conductors.