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Interdiffusion of transition metals across the cathode-electrolyte interface is identified as a key challenge for the practical realization of solid-state batteries. This is related to the formation of highly resistive interphases impeding the charge transport across the materials thus limiting the battery performance. Herein, we investigate the hypothesis that formation of interphases is associated with the incorporation of Co into the LLZO lattice representing the starting point of a cascade of degradation processes. It is shown that Co incorporates into the garnet structure preferably four-fold coordinated as Co2+ or Co3+ depending on oxygen fugacity. The solubility limit of Co is determined to be around 0.16 pfu, whereby concentrations beyond this limit causes a cubic-to-tetragonal phase transition. Moreover, the temperature-dependent Co diffusion coefficient is determined, e.g., D700 °C = 9.46 × 10-14 cm2/s and an activation energy Ea = 1.65 eV, suggesting that detrimental cross diffusion will take place at any relevant process condition. Additionally, the optimal protective Al2O3 coating thickness for relevant temperatures is studied, which allows to create a process diagram to mitigate any degradation with a minimum compromise on electrochemical performance. This study provides a tool to optimize processing conditions toward developing high energy density solid-state batteries.

Lithium-ion insertion/deinsertion in anode at slow rates limits the power performance of energy storage devices. Here, a new pseudocapacitive electrode with high reversible capacity during cycling has been proposed for a lithium-ion capacitor. The lithium-fluoride garnet, namely Na3Fe2Li3F12, is obtained via precipitation from an aqueous solution at room temperature using abundant materials and exhibits a high discharge capacity of 746 mAh/g. After the first charging cycle, energy is stored via fast pseudocapacitive faradaic reactions which are facilitated by the nanocrystalline transport pathways with no structural modification to the electrode. The high stability window of F-garnet allows extracting cell voltages of 2.2—3.2 V in a lithium-ion capacitor where it is coupled with a porous carbon-based positive electrode, with a high energy efficiency of 93% maintained for 10000 charge/discharge cycles. This study opens new research direction concerning pseudocapacitive anode materials for the enhanced power performance and even replacing the traditional battery-like anode materials.

Tantalum-doped garnet (Li6.5La3Zr1.5Ta0.5O12, LLZTO) is a promising candidate to act as a solid electrolyte in all-solid-state batteries owing to both its high Li+ conductivity and its relatively high robustness against the Li metal. Synthesizing LLZTO using conventional solid-state reaction (SSR) requires, however, high calcination temperature (>1000 °C) and long milling steps, thereby increasing the processing time. Here, we report on a facile synthesis route to prepare LLZTO using a molten salt method (MSS) at lower reaction temperatures and shorter durations (900 °C, 5 h). Additionally, a thorough analysis on the properties, i.e., morphology, phase purity, and particle size distribution of the LLZTO powders, is presented. LLZTO pellets, either prepared by the MSS or the SSR method, that were sintered in a Pt crucible showed Li+ ion conductivities of up to 0.6 and 0.5 mS cm-1, respectively. The corresponding activation energy values are 0.37 and 0.38 eV, respectively. The relative densities of the samples reached values of approximately 96%. For comparison, LLZTO pellets sintered in alumina crucibles or with γ-Al2O3 as sintering aid revealed lower ionic conductivities and relative densities with abnormal grain growth. We attribute these observations to the formation of Al-rich phases near the grain boundary regions and to a lower Li content in the final garnet phase. The MSS method seems to be a highly attractive and an alternative synthetic approach to SSR route for the preparation of highly conducting LLZTO-type ceramics.

Tantalum-doped garnet (Li

High cost and low electrochemical stability of the interconnection in Proton Exchange Membrane Fuel Cell (PEMFC) in the presence of H2SO4 are one of the main issues hindering the commercialization of these devices. This manuscript presents the utilization of cost-effective steel in an attempt to minimize the PEMFC interconnection costs with a thin-film solid oxide coating (TFSOC) providing sufficient corrosion resistance for efficient long-term operation. Novel Ti0.50-y/2Si0.50-y/2Nby1,2O2 as TFSOC was deposited on the C45E steel as a metal interconnect utilizing a sol&ndash, gel process at various annealing temperatures. The analysis of the phase and surface morphology demonstrates that lower annealing temperatures developed nanometric crystallite size of 68 nm, more uniform structure and higher corrosion resistance. Under standard test conditions, the TFSOC demonstrated high polarization resistance (1.3 kΩ cm2) even after 720 hours (h). Electrical conductivity of the TFSOC as low as 1.4 &times, 10&minus, 2 (Ω m)&minus, 1 and activation energy of 0.20 eV were achieved, which helps to maintain the PEMFC output power.

Cubic Li7La3Zr2O12(LLZO), stabilized by supervalent cations, is one of the most promising oxide electrolyte to realize inherently safe all-solid-state batteries. It is of great interest to evaluate the strategy of supervalent stabilization in similar compounds and to describe its effect on ionic bulk conductivity σ′bulk. Here, we synthesized solid solutions of Li7–xLa3M2–xTaxO12 with M = Hf, Sn over the full compositional range (x = 0, 0.25...2). It turned out that Ta contents at x of 0.25 (M = Hf, LLHTO) and 0.5 (M = Sn, LLSTO) are necessary to yield phase pure cubic Li7–xLa3M2–xTaxO12. The maximum in total conductivity for LLHTO (2 × 10–4 S cm–1) is achieved for x = 1.0; the associated activation energy is 0.46 eV. At x = 0.5 and x = 1.0, we observe two conductivity anomalies that are qualitatively in agreement with the rule of Meyer and Neldel. For LLSTO, at x = 0.75 the conductivity σ′bulk turned out to be 7.94 × 10–5 S cm–1 (0.46 eV); the almost monotonic decrease of ion bulk conductivity from x = 0.75 to x = 2 in this series is in line with Meyer–Neldel’s compensation behavior showing that a decrease in Ea is accompanied by a decrease of the Arrhenius prefactor. Altogether, the system might serve as an attractive alternative to Al-stabilized (or Ga-stabilized) Li7La3Zr2O12 as LLHTO is also anticipated to be highly stable against Li metal.

Cubic Li7La3Zr2O12(LLZO), stabilized by supervalent cations, is one of the most promising oxide electrolyte to realize inherently safe all-solid-state batteries. It is of great interest to evaluate the strategy of supervalent stabilization in similar compounds and to describe its effect on ionic bulk conductivity σ'bulk. Here, we synthesized solid solutions of Li7-x La3M2-x Ta x O12 with M = Hf, Sn over the full compositional range (x = 0, 0.25...2). It turned out that Ta contents at x of 0.25 (M = Hf, LLHTO) and 0.5 (M = Sn, LLSTO) are necessary to yield phase pure cubic Li7-x La3M2-x Ta x O12. The maximum in total conductivity for LLHTO (2 × 10-4 S cm-1) is achieved for x = 1.0; the associated activation energy is 0.46 eV. At x = 0.5 and x = 1.0, we observe two conductivity anomalies that are qualitatively in agreement with the rule of Meyer and Neldel. For LLSTO, at x = 0.75 the conductivity σ'bulk turned out to be 7.94 × 10-5 S cm-1 (0.46 eV); the almost monotonic decrease of ion bulk conductivity from x = 0.75 to x = 2 in this series is in line with Meyer-Neldel's compensation behavior showing that a decrease in Ea is accompanied by a decrease of the Arrhenius prefactor. Altogether, the system might serve as an attractive alternative to Al-stabilized (or Ga-stabilized) Li7La3Zr2O12 as LLHTO is also anticipated to be highly stable against Li metal.

Cubic Li7La3Zr2O12 (LLZO) garnets have attracted lots of attention in previous years as they show promising properties as a solid electrolyte for Solid-state Li batteries (SSLB), such as high ionic conductivity in the range of 1 mS cm-1 and high electrochemical stability up to 6 V.1 Despite of these promising prerequisites, its implementation in SSLB leads to high interfacial resistance that limits its current use in such devices. This high interface resistance relates to the required high temperature treatment to form a good contact between LLZO and, e.g., LiCoO2.2 It is known that this is related to the formation of interdiffusion layers that imped the ionic transport across the interface. Herein, we present that Co does not only lead to the formation of resistive interlayers, such as La2CoO4, but also leads to the incorporation of Co into the garnet lattice. Since, Co is a transition metal its incorporation could change, e.g., (i) the electronic conductivity, being suggested to play a critical role for dendrite formation within the garnet, and (ii) the band gap, hence, the electrochemical stability. Moreover, we also observed, that La diffuses into LiCoO2, which potentially changes the electrochemical properties of the cathode material as well.3 , 4 We mirrored the properties of the interface by incorporating Co from LiCoO2 powder into Czochralski-grown Li6.4Ga0.2La3Zr2O12 single crystals over the gas phase at high temperature. Interestingly, the color changed from yellow, orange to dark blue depending on the heating history. Thereafter we applied a wide spectrum of techniques, such as SC XRD, UV-VIS, TOF-SIMS, 57Emission Mößbauer spectroscopy, DFT/AIMD simulations, impedance spectroscopy and cyclic voltammetry to understand the role of Co in LLZO. For example, we found that the band gap decreases up to 2 eV by the incorporation of 0.1 Co per formula unit (pfu) and a La deficit of 0.12 pfu. This change in the band gap manifests itself in a decreased electrochemical stability of LLZO indicated by an oxidative peak below 4 V as observed in the CV measurement. Moreover, we found that the incorporation of Co leads to a significant change in the Li ion transport. The impedance analysis revealed a low activated (E a = 0.43 eV) process at temperatures between -100 °C and 110 °C and another highly activated (E a = 0.7 eV) diffusion process predominantly present at temperatures > 160 °C. By extrapolating the highly activated process to room temperature, an ionic conductivity σ = 2.91 x 10-6 S cm-1 could be calculated which is about two orders of magnitude lower than values found for pristine LLZO. References: Hofstetter, K. et al. Electrochemical Stability of Garnet-Type Li7La2.75Ca0.25Zr1.75Nb0.25O12 with and without Atomic Layer Deposited-Al2O3 under CO2 and Humidity . J. Electrochem. Soc. 166, A1844–A1852 (2019). Park, K. et al. Electrochemical Nature of the Cathode Interface for a Solid-State Lithium-Ion Battery: Interface between LiCoO2 and Garnet-Li7La3Zr2O12. Chem. Mater. 28, 8051–8059 (2016). Kim, K. H. et al. Characterization of the interface between LiCoO2 and Li7La3Zr2O12 in an all-solid-state rechargeable lithium battery. J. Power Sources 196, 764–767 (2011). Han, F. et al. High electronic conductivity as the origin of lithium dendrite formation within solid electrolytes. Nat. Energy 4, 187–196 (2019).

The dependence of ionic transport on crystal orientations in NaSICON-type solid electroytes is studied on flux-grown M3Sc2(PO4)3 (M = Na, Ag) single crystals with well-defined facets. Herein, we provide the first impedance spectroscopy study to characterize ion conduction along different crystallographic orientations in this important class of materials for electrochemical energy storage systems. Moreover, we used single crystal X-ray diffraction, differential scanning calorimetry, 23Na NMR spin-lattice relaxation measurements, and ab initio molecular dynamics simulations to study the interplay of structure and ion transport taking place at different length scales. We conclude that the phase behavior in NaSICON-type materials is strongly linked to ion diffusion. At room temperature, ionic conductivity is slightly anisotropic along the crystallographic orientations [001] and [100]. The slightly different activation energies are related to diffusion bottlenecks solely changing along [001]. This change is caused by anisotropic thermal lattice expansion. With increasing temperature, ion transport increasingly becomes isotropic finally resulting in an order-disorder phase transition from C2/c to R−3c. This phase transition is associated with a clear change in activation energy solely along [001]; it can be traced back to the increasing jump distance along this crystal orientation with temperature. Astonishingly, changing the ionic charge carrier, i.e., when going from Na+ to Ag+, shifts the phase transition temperature by 140 K toward lower temperature. The Arrhenius behaviour remains, however, similar. This finding is related to the higher mobility of Ag+ in the NaSICON framework leading to isotropic ion diffusion at much lower temperatures. Overall, flux-grown M3Sc2(PO4)3 allowed us to show that ionic transport parameters and phase stability sensitively depend on crystal chemistry.

Summary Ceramic-polymer electrolytes are expected to improve safety, energy density, and power of next-generation battery technologies. The realization of this type of battery is, however, hindered by the high interfacial resistance across the ceramic-polymer interface. Here, we report a surface-modification strategy to lower the interfacial resistance by more than four orders of magnitude. For this purpose, we activate the surface-terminated oxygen of Li6.4La3Zr1.4Ta0.6O12 (LLZTO) particles by plasma etching and functionalize them by immersing the LLZTO particles in a (3-glycidyloxypropyl)trimethoxysilane (Si-R) solution to form covalently bonded Si-R layers. The Si-Rs are terminated by an epoxy group that reacts with the hydroxyl group of the poly(ethylene oxide) (PEO) via a ring-opening reaction. The modifications improve the screening of the oxygen polarity of LLZTO particles and lower the free volume between both components, resulting in a LLZTO|PEO interface resistance of 500 Ω cm2 at 20°C, the lowest value reported so far to the best of our knowledge.