materials are a particularly promising class of solid electrolytes for all-solidstate lithium metal batteries, as they are predicted to have a wide electrochemical stability window, [5,6] can be synthesized with very high density (>97%) [7,8] and, through aliovalent doping, can achieve room temperature Li-ion conductivities as high as ≈1.0 mS cm −1 with negligible electronic conductivity. [9] However, significant fundamental issues remain unresolved for garnet-based all-solid-state batteries, including low accessible current densities, [10] the persistence of Li dendrite formation, [11,12] and perhaps most importantly, ambiguities as to whether the interfaces between LLZO and both Li metal [13,14] and high voltage oxide cathodes [15,16] are stable over extended cycling. Indeed, developing deep understanding of the intrinsic reactivity between solid electrolytes and relevant electrode materials is crucial to developing high voltage solidstate batteries with long lifetimes, as the presence of any significant (electro)chemical reactivity will ultimately lead to premature cell failure during extended cycling.Understanding interfacial stability is an especially challenging issue common to all solid-state battery systems due to the inability of many experimental techniques to adequately interrogate the chemical properties of buried interfaces. Such studies are further complicated when one or both materials at the interface are unstable to exposure to air, water, etc., as Li 7 La 3 Zr 2 O 12 (LLZO) garnet-based materials doped with Al, Nb, or Ta to stabilize the Li + -conductive cubic phase are a particularly promising class of solid electrolytes for all-solid-state lithium metal batteries. Understanding of the intrinsic reactivity between solid electrolytes and relevant electrode materials is crucial to developing high voltage solid-state batteries with long lifetimes. Using a novel, surface science-based approach to characterize the intrinsic reactivity of the Li-solid electrolyte interface, it is determined that, surprisingly, some degree of Zr reduction takes place for all three dopant types, with the extent of reduction increasing as Ta < Nb < Al. Significant reduction of Nb also takes place for Nb-doped LLZO, with electrochemical impedance spectroscopy (EIS) of Li||Nb-LLZO||Li symmetric cells further revealing significant increases in impedance with time and suggesting that the Nb reduction propagates into the bulk. Density functional theory (DFT) calculations reveal that Nb-doped material shows a strong preference for Nb dopants toward the interface between LLZO and Li, while Ta does not exhibit a similar preference. EIS and DFT results, coupled with the observed reduction of Zr at the interface, are consistent with the formation of an "oxygen-deficient interphase" (ODI) layer whose structure determines the stability of the LLZO-Li interface.

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All solid-state Li-ion batteries offer unprecedented improvements in energy density and safety compared to contemporary Li-ion batteries. As one of the most common oxide cathode materials for traditional Li-ion batteries, LiCoO 2 (LCO) is also under consideration for use in all solid-state batteries. However, differences in the coefficients of thermal expansion (CTE) between LCO and the solid electrolyte during composite electrode fabrication, and the differential expansion and contraction during electrochemical cycling resulting from Li insertion and de-insertion, will cause stresses resulting in battery capacity fade. To characterize the thermo-mechanical properties, this study utilized hot pressing to prepare high relative density (95%) LCO polycrystalline pellets. The elastic modulus (E), shear modulus (G), hardness (H), and Poisson's ratio () of LCO were determined to be ∼191 GPa, ∼80 GPa, ∼8.2 GPa (at peak indentation loads ≤5 mN), and 0.24, respectively. The CTE was determined to be ∼1.3 × 10 −5 / • C over the temperature range 50-400 • C. We believe these data are important to predict or increase the cycle life of commercially available LCO as a cathode material for state-of-the-art Li-ion and advanced solid-state batteries.

The bottom up approach suggests that atomic scale mixing should permit optimal control of processing for many types of materials in terms of densification rates, final and average grain sizes; and, thereafter, global properties thereby minimizing processing conditions, capital equipment requirements, and energy consumption. The literature indicates that, to date, this axiom has not been tested although numerous researchers have whole‐heartedly adopted the concept. Liquid‐feed flame spray pyrolysis (LF‐FSP) provides atomically mixed NiO·3Al2O3 nanopowders(30–40 nm average particle size, APS) that are a single phase, spinel solid solution. Sintering 56 ± 1 wt% CIPped pellets to 95 ± 1% theoretical density using two different heating schedules produces α‐Al2O3/NiAl2O4 composites with grain sizes of 0.9 ± 0.2 µm. For comparative purposes, ball milled Al2O3 are synthesized from both highly atomically‐mixed single phase metastable spinel nanopowders and NiAl2O4 30–40 nm APS pellets of the same composition with a submicro­meter length scale mixing and similar green densities are also sintered to 95 ± 2% TD. In both instances, the same microstructures are realized despite the great difference in length scale of mixing. This contrasts greatly with the expectation that the atomically mixed materials would give finer grain sizes at the same densities and with faster sintering times, suggesting that the bottom up approach is not always valid.