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.

The development of alternative energy systems for the clean production, storage, and conversion of energy is strongly dependent on our ability to understand, at atomic molecular levels, the functional links between the activity and stability of electrochemical interfaces. Whereas structure− activity relationships are rapidly evolving, the corresponding structure− stability relationships are still missing. This is primarily because there is no adequate experimental approach capable of monitoring the stability of welldefined single crystals in situ. Here, by utilizing the power of inductively coupled plasma mass spectrometry (ICP-MS) connected to a stationary probe and coupling this technique to the rotating disk electrode method, it was possible to simultaneously measure the dissolution rates of surface atoms (as low as 0.4 pg cm −2 s −1 ) and correlate them with the kinetic rates of electrochemical reactions in real time. Making use of this unique probe, it was possible to establish almost "atom by atom" structure−stability−activity relationships for platinum single crystals in both acidic and alkaline environments. We found that the degree of stability is strongly dependent on the coordination of surface atoms (less coordinated yields less stable), the nature of covalent and noncovalent interactions (i.e., adsorption of hydroxyl groups, oxygen atoms, and halide species vs interactions between hydrated Li cations and surface oxide), the thermodynamic driving force for Pt complexation (Pt ion speciation in solution), and the nature of the electrochemical reaction (the oxygen reduction/evolution and CO oxidation reactions). These findings open new opportunities for elucidating key fundamental descriptors that govern both activity and stability trends and will ultimately assist in the development of real energy conversion and storage systems.

Over the past few years, in situ transmission electron microscopy (TEM) studies of lithium ion batteries using an open-cell configuration have helped us to gain fundamental insights into the structural and chemical evolution of the electrode materials in real time. In the standard open-cell configuration, the electrolyte is either solid lithium oxide or an ionic liquid, which is point-contacted with the electrode. This cell design is inherently different from a real battery, where liquid electrolyte forms conformal contact with electrode materials. The knowledge learnt from open cells can deviate significantly from the real battery, calling for operando TEM technique with conformal liquid electrolyte contact. In this paper, we developed an operando TEM electrochemical liquid cell to meet this need, providing the configuration of a real battery and in a relevant liquid electrolyte. To demonstrate this novel technique, we studied the lithiation/delithiation behavior of single Si nanowires. Some of lithiation/delithation behaviors of Si obtained using the liquid cell are consistent with the results from the open-cell studies. However, we also discovered new insights different from the open cell configuration-the dynamics of the electrolyte and, potentially, a future quantitative characterization of the solid electrolyte interphase layer formation and structural and chemical evolution.

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Developing a new generation of battery chemistries is a critical challenge to moving beyond current Li-ion technologies. In this work, we introduce a surfacescience-based approach for understanding the complex phenomena controlling the reversibility of Mg anodes for Mg-ion batteries. We identify the profound impact of trace levels of H 2 O (≤3 ppm) on the kinetics of Mg deposition and determine that passive films of MgO and Mg(OH) 2 are formed only af ter Mg deposition ceases, rather than continuously during Mg reduction. We also find that Cl − inhibits passivation through the formation of adsorbed Cl − (Mg−Cl(ad)) and/or MgCl 2 on the surface, as well as through a dynamic competition with H 2 O in the double layer. This surface-science-based approach goes well beyond Mg anodes, highlighting the need for more in-depth understanding of electrolyte chemistries before a new generation of efficient and reversible battery technologies can be realized.