The garnet with nominal composition Li7La3Zr2O12 (LLZO) has attracted attention due to its potential use as a solid electrolyte in Li-based solid-state batteries. This interest reflects the high ionic conductivity of the cubic phase of LLZO, which can approach 1 mS/cm (at room temperature) when doped with supervalent cations such as Al or Ta. In addition to possessing high ionic conductivity, a viable solid electrolyte must also exhibit favorable interfacial properties with electrodes. The present study examines the cross-over of Co transition metal ions from a LiCoO2 (LCO) cathode to a LLZO solid electrolyte at the LLZO/LCO interface. Hot-pressing of LLZO and LCO pellets results in a color change in LLZO from white to blue in regions near the interface. Electron probe micro-analysis indicates that the color change coincides with the diffusion of Co from LCO to LLZO, while Raman spectra suggest that Co occupies sites on the Li-sublattice of LLZO. To understand how TM impurities in LLZO impact Li-ion mobility, molecular dynamics simulations were used to investigate the transport rates of Co and three other transition metals, Mn, Fe, and Ni, in Al-doped LLZO. These calculations reveal that the TM ions investigated exhibit lower diffusivities in LLZO compared to Li, with Fe (Mn) impurities being the most (least) mobile. Most importantly, the presence of TM ions in LLZO slows Li-ion migration, with the impact on Li mobility being largest for the slowest-diffusing TMs. Because the TMs migrate along the Li-sublattice, slower-moving TMs impede Li-ion motion via a traffic-jam-like process. This behavior suggests that care should be exercised in the synthesis of solid-solid LLZO/cathode interfaces: although high-temperature processing improves interfacial contact, these temperatures also increase the likelihood for TM crossover from the cathode to the solid electrolyte, thereby decreasing the solid electrolyte's Li-ion conductivity.

We present a linear scaling atomic orbital based algorithm for the computation of the most expensive exchange-type RI-MP2-F12 term by employing numerical quadrature in combination with CABS-RI to avoid six-center-three-electron integrals. Furthermore, a robust distance-dependent integral screening scheme, based on integral partition bounds [Thompson, T. H.; Ochsenfeld, C. J. Chem. Phys. 2019, 150, 044101], is used to drastically reduce the number of the required three-center-one-electron integrals substantially. The accuracy of our numerical quadrature/CABS-RI approach and the corresponding integral screening is thoroughly assessed for interaction and isomerization energies across a variety of numerical integration grids. Our method outperforms the standard density fitting/CABS-RI approach with errors below 1 μEh even for small grid sizes and moderate screening thresholds. The choice of the grid size and screening threshold allows us to tailor our ansatz to a desired accuracy and computational efficiency. We showcase the approach's effectiveness for the chemically relevant system valinomycin, employing a triple-ζ F12 basis set combination (C54H90N6O18, 5757 AO basis functions, 10,266 CABS basis functions, 735,783 grid points). In this context, our ansatz achieves higher accuracy combined with a 135× speedup compared to the classical density fitting based variant, requiring notably less computation time than the corresponding RI-MP2 calculation. Additionally, we demonstrate near-linear scaling through calculations on linear alkanes. We achieved an 817-fold acceleration for C80H162 and an extrapolated 28,765-fold acceleration for C200H402, resulting in a substantially reduced computational time for the latter─from 229 days to just 11.5 min. Our ansatz may also be adapted to the remaining MP2-F12 terms, which will be the subject of future work.

Ceramic oxides are promising solid electrolytes for Lithium metal solid-state batteries (SSBs) because of their high ionic conductivity and stability under ambient conditions. Nonetheless, the need for high-temperature densification approaches challenges their processability towards upscaling and their implementation into practical SSB devices. Here, we investigate a High-Pressure Low-Temperature (HPLT) processing technique for the densification of NASICON-based Li1+xAlxTi2-x(PO4)(3) (LATP) solid electrolyte, providing an understanding of several key parameters such as temperature, pressure and time. Our results show that nanometric LATP can be densified using the HPLT technique at 200 degrees C within 2 min delivering an ionic conductivity of 6.15x10(-5) Scm(-1), which is the highest reported value at this temperature without the addition of solvents. On the other hand, by combining HPLT with a short post-heat treatment at 700 or 800 degrees C for 1 hour, highly dense pellets with an ionic conductivity of 10(-4) Scm(-1) can be obtained. This represents a reduction of the densification temperature of 400 degrees C compared to conventional high-temperature sintering and shows the potential of HPLT to overcome current densification constraints derived from the use of very high temperatures.

This work introduces a polymeric backbone eutectogel (P-ETG) hybrid solid-state electrolyte with an N-isopropylacrylamide (NIPAM) backbone for high-energy lithium-ion batteries (LIBs). The NIPAM-based P-ETG is (electro)chemically compatible with commercially relevant positive electrode materials such as the nickel-rich layered oxide LiNi0.6Mn0.2Co0.2O2 (NMC622). The chemical compatibility was demonstrated through (physico)chemical characterization methods. The nonexistence (within detection limits) of interfacial reactions between the electrolyte and the positive electrode, the unchanged bulk crystallographic composition, and the absence of transition metal ions leaching from the positive electrode in contact with the electrolyte were demonstrated by Fourier transform infrared spectroscopy, powder X-ray diffraction, and elemental analysis, respectively. Moreover, the NIPAM-based P-ETG demonstrates a wide electrochemical stability window (1.5-5.0 V vs Li+/Li) and a reasonably high ionic conductivity at room temperature (0.82 mS cm-1). The electrochemical compatibility of a high-potential NMC622-containing positive electrode and the P-ETG is further demonstrated in Li|P-ETG|NMC622 cells, which deliver a discharge capacity of 134, 110, and 97 mAh g-1 at C/5, C/2, and 1C, respectively, after 90 cycles. The Coulombic efficiency is >95% at C/5, C/2, and 1C. Hence, gaining scientific insights into the compatibility of the electrolytes with positive electrode materials that are relevant to the commercial market, like NMC622, is important because this requires going beyond the electrolyte design itself, which is essential to their practical applications.

We present a method to simulate ultrafast pump-probe time-resolved circular dichroism (TRCD) spectra based on time-dependent density functional theory trajectory surface hopping. The method is applied to simulate the TRCD spectrum along the photoinduced ring-opening of provitamin D. Simulations reveal that the initial decay of the signal is due to excited state relaxation, forming the rotationally flexible previtamin D. We further show that oscillations in the experimental TRCD spectrum arise from isomerizations between previtamin D rotamers with different chirality, which are associated with the helical conformation of the triene unit. We give a detailed description of the formation dynamics of different rotamers, playing a key role in the natural regulation vitamin D photosynthesis. Going beyond the sole extraction of decay rates, simulations greatly increase the amount of information that can be retrieved from ultrafast TRCD, making it a sensitive tool to unravel details in the sub-picosecond dynamics of photoinduced chirality changes.

Temperature-assisted densification methods are typically used in oxide-based solid-state batteries to suppress resistive interfaces. However, chemical reactivity among the different cathode components (which include a catholyte, the conducting additive, and the electroactive material) still represents a major challenge and processing parameters need thus to be carefully selected. In this study, we evaluate the impact of temperature and heating atmosphere in the LiNi0.6Mn0.2Co0.2O2 (NMC), Li1+xAlxTi2-xP3O12 (LATP), and Ketjenblack (KB) system. A rationale of the chemical reactions between components is proposed from the combination of bulk and surface techniques and overall involves a cation redistribution in the NMC cathode material that is accompanied by the loss of lithium and oxygen from the lattice enhanced by LATP and KB, which act as lithium and oxygen sinks. The final result is the formation of several degradation products, starting at the surface, that lead to a rapid capacity decay above 400 °C. Both the reaction mechanism and threshold temperature depend on the heating atmosphere, with the air atmosphere being more favorable compared to oxygen or any other inert gases.

One of the main technological challenges oxide-based solid-state batteries face today is the densification of their components to reach good interfacial contact. The most common approach requires co-sintering of the different components (electroactive material, catholyte and conducting additive) at high temperatures which often results in the inter-diffusion of elements that deteriorate the overall cathode performance. In this work, the impact of different carbon grades in the thermal response of LATP-NMC622-Carbon electrodes is evaluated and shown to significantly influence the chemical compatibility between components. By means of a combination of bulk and surface characterization techniques including gas adsorption, X-ray diffraction, X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy and thermogravimetric analysis, it is shown that carbons with low surface area are more adequate as result in higher oxidation temperatures and hence are less reactive.

Ceramic oxides are promising solid electrolytes for Lithium metal solid-state batteries (SSBs) because of their high ionic conductivity and stability under ambient conditions and against lithium metal anode. Nonetheless, the need for high-temperature densification approaches challenges their processability towards upscaling and their implementation into practical SSB devices. Here, we investigate a High-Pressure Low-Temperature (HPLT) processing technique for the densification of NASICON-based Li1+xAlxTi2-x(PO4)3 (LATP) solid electrolyte, providing an understanding of several key parameters such as temperature, pressure and time. Our results show that nanometric LATP can be densified using the HPLT technique at 200˚C within 2 minutes delivering an ionic conductivity of 6.15 x 10-5 Scm-1, which is the highest reported value at this temperature without the addition of solvents. On the other hand, by combining HPLT with a short post-heat treatment at 700 or 800 ˚C for 1 hour, highly dense pellets with an ionic conductivity of 10-4 Scm-1 can be obtained. This represents a reduction of the densification temperature of more than 400˚C compared to conventional high-temperature sintering and shows the potential of HPLT to overcome current densification constraints derived from the use of very high temperatures.

The demand for energy storage systems has exponentially grown over the last decade. Li-ion battery, containing liquid electrolyte, is the most advanced and implemented technology for transport and stationary markets.[1] However, safety limitations of these systems mainly due to flammable and volatile electrolyte represent the major threat to reach complete market maturity.[2] Replacing liquid by safe solid electrolyte is foreseen as a realistic strategy while maintaining high energy and power densities. Among solid-state electrolyte materials, oxide-based ceramic electrolytes show high ionic conductivity, intrinsic safety and wide electrochemical window. The development of all-inorganic solid batteries is still at its infancy and presents various challenges due to the mechanical properties and processing of these materials.[3] High processing temperatures is certainly needed leading to chemical reactions and elemental diffusion, detrimental to maintain the battery performances. Developing a fully inorganic solid-state battery, consisting of a composite cathode, solid electrolyte and lithium metal anode by co-sintering the first two components, remains a great challenge. The composite cathode comprises the active material, and an ionic and an electronic conductive filler, that should form stable interfaces under processing as well as during cycling. But densification of oxide-based ceramic electrolytes requires temperatures as high as 1000 °C and co-sintering at these temperatures can induce numerous chemical side- reactions at composite cathode[4] which is detrimental to maintain the battery performance. To control the sintering process, the threshold temperature at which the material is stable and maintains the electrochemical performance needs to be defined and the reaction byproducts need to be identified to prevent and control their impact. In this work we will present the impact of heat treatment on the stability of composite cathode mixture with LiNi0.6Co0.2Mn0.2O2 (NMC), Li1+xAlxTi2-xP3O12 (LATP) and Ketjen black (KB). The study optimizes the threshold conditions such as heating atmosphere and temperature in determining the chemical and electrochemical compatibility of the composite and explains the reaction mechanism at threshold limits. Surface based analysis of the active material demonstrates that the NMC surface reconstruction acts as barrier to electrochemistry after heat treatment at temperatures above threshold limits. Also, it underlines the fact that each element of the composite has inevitable contribution to the reaction mechanism and is determined by both heating atmosphere and temperature. Alternatives to enhance the threshold limits will be proposed. Reference: [1] M. Armand and J. M. Tarascon, “Building better batteries,” Nature, vol. 451, no. 7179. Nature Publishing Group, pp. 652–657, 07-Feb-2008, doi: 10.1038/451652a. [2] D. Lisbona and T. Snee, “A review of hazards associated with primary lithium and lithium-ion batteries,” Process Saf. Environ. Prot., vol. 89, no. 6, pp. 434–442, Nov. 2011, doi: 10.1016/j.psep.2011.06.022. [3] K. Kerman, A. Luntz, V. Viswanathan, Y.-M. Chiang, and Z. Chen, “Review—Practical Challenges Hindering the Development of Solid State Li Ion Batteries,” J. Electrochem. Soc., vol. 164, no. 7, pp. A1731–A1744, Jun. 2017, doi: 10.1149/2.1571707jes. [4] L. Miara et al., “About the Compatibility between High Voltage Spinel Cathode Materials and Solid Oxide Electrolytes as a Function of Temperature,” vol. 8, no. 40, pp. 26842–26850, Oct. 2016, doi: 10.1021/acsami.6b09059.

We present a highly efficient and asymptotically linear-scaling graphic processing unit accelerated seminumerical exact-exchange method (sn-LinK). We go beyond our previous central processing unit-based method ( Laqua , H. ; Kussmann , J. ; Ochsenfeld , C. J. Chem. Theory Comput . 2018 , 14, 3451 - 3458 ) by employing our recently developed integral bounds ( Thompson , T. H. ; Ochsenfeld , C. J. Chem. Phys . 2019 , 150, 044101 ) and high-accuracy numerical integration grid ( Laqua , H. ; Kussmann , J. ; Ochsenfeld , C. J. Chem. Phys . 2018 , 149, 204111 ). The accuracy is assessed for several established test sets, providing errors significantly below 1mEh for the smallest grid. Moreover, a comprehensive performance analysis for large molecules between 62 and 1347 atoms is provided, revealing the outstanding performance of our method, in particular, for large basis sets such as the polarized quadruple-zeta level with diffuse functions.

The recent interest in solid state batteries has been motivated by the potential for higher energy density, longer cycle life, and improved safety compared to liquid electrolyte based Li-ion batteries. Deposition of thin solid electrolytes on 3D structures could enable a wide range of thin film and bulk battery architectures that offer energy and power density improvements compared with planar geometries. Traditional deposition methods are unable to form dense, uniform, pinhole-free films on complex non-planar geometries, however Atomic Layer Deposition (ALD) excels at conformally coating even ultrahigh aspect ratio substrates with uniform thickness and composition, even multi-component films.1 To date, the development of ionically conductive solid state electrolytes by ALD has failed to produce films with ionic conductivities comparable to that of sputtered LiPON (~2*10-6 S/cm at 298K), the current state-of-the-art in thin-film batteries.2–4 Despite the limited ionic conductivities, several reports have demonstrated the potential of ALD films for both interfacial engineering of bulk batteries, and for thin film batteries.5,6 Films with improved conductivity would enable faster charging rates, more robust electrolyte films, and serve as a better platform for 3D battery development. Here, we demonstrate a novel ALD process for ternary lithium borate thin films with ionic conductivities above 10-6 S/cm at 298K. This represents almost a 2x improvement over the previous best reported value in an ALD film.2 The stability and structure of the deposited films are characterized and compared with those calculated with Density Functional Theory and Molecular Dynamics. The film remains an ionic conductor when in contact with metallic Li, and displays stable cycling when paired with a thin-film cathode and Li metal anode. The composition, conductivity, and stability are studied as a function of deposition temperature showing tradeoffs between process conditions and performance, and demonstrating the precise control afforded by the ALD process.7 References (1) Kazyak, E.; Chen, K.-H.; Wood, K. N.; Davis, A. L.; Thompson, T.; Bielinski, A. R.; Sanchez, A. J.; Wang, X.; Wang, C.; Sakamoto, J.; Dasgupta, N. P. Atomic Layer Deposition of the Solid Electrolyte Garnet Li 7 La 3 Zr 2 O 12. Chem. Mater. 2017, 29 (8), 3785–3792. (2) Kozen, A. C.; Pearse, A. J.; Lin, C.-F.; Noked, M.; Rubloff, G. W. Atomic Layer Deposition of the Solid Electrolyte LiPON. Chem. Mater. 2015, 27 (15), 5324–5331. (3) Cao, Y.; Meng, X.; Elam, J. W. Atomic Layer Deposition of LixAlyS Solid-State Electrolytes for Stabilizing Lithium-Metal Anodes. ChemElectroChem 2016, 3 (6), 858–863. (4) Xie, J.; Sendek, A. D.; Cubuk, E. D.; Zhang, X.; Lu, Z.; Gong, Y.; Wu, T.; Shi, F.; Liu, W.; Reed, E. J.; Cui, Y. Atomic Layer Deposition of Stable LiAlF4Lithium Ion Conductive Interfacial Layer for Stable Cathode Cycling. ACS Nano 2017, 11 (7), 7019–7027. (5) Pearse, A. J.; Schmitt, T. E.; Fuller, E. J.; El-Gabaly, F.; Lin, C. F.; Gerasopoulos, K.; Kozen, A. C.; Talin, A. A.; Rubloff, G.; Gregorczyk, K. E. Nanoscale Solid State Batteries Enabled by Thermal Atomic Layer Deposition of a Lithium Polyphosphazene Solid State Electrolyte. Chem. Mater. 2017, 29 (8), 3740–3753. (6) Kazyak, E.; Wood, K. N.; Dasgupta, N. P. Improved Cycle Life and Stability of Lithium Metal Anodes through Ultrathin Atomic Layer Deposition Surface Treatments. Chem. Mater. 2015, 27 (18), 6457–6462. (7) Kazyak, E.; Yu, S.; Chen, K.H.; Davis, A.L.; Sanchez; Lasso, J.; Thompson, T.; Sakamoto, J.; Siegel, D. J.; Dasgupta, N. P., Atomic Layer Deposition of Ultrathin Lithium Borate Solid Electrolytes, Submitted

Differences in thermal contraction/expansion and volume changes during discharge/charge cycles lead to internal stresses that ultimately cause degradation of solid-state composite cathodes and hinder the realization of their practical applications. We employ the smoothed boundary method to solve the mechanical equilibrium equation for the residual stresses induced by cooling from the sintering temperature and by (de)lithiation in polycrystalline composite microstructures that are similar to realistic solid-state composite cathodes. The overall deformations of the composite slabs under these fabrication and operation conditions are also evaluated. The effects of cathode thickness and selections of different cathode materials on the resulting residual stresses and deformations are examined. We find that the (de)lithiation stresses during cycling are more than twice the thermal residual stresses after sintering. Furthermore, the maximum (de)lithiation and thermal residual stresses are sensitive to the cathode thickness only when the cathode-layer thickness is comparable to that of the electrolyte separator layer. We also investigate the impact of the lithium site fraction of the cathode particles prior to sintering on the cycling stresses and deformations, which may pave the path toward an approach to mitigating mechanically induced degradation.

High concentration Electrolytes (HCEs) exhibit a small molar ratio of solvent to salt, usually in the range of 2…4:1. Localized Superconcentrated Electrolytes (LSEs) are derived from HCEs but contain a co-solvent that is used to dissipate the solvent-salt complexes, leading to significantly reduced viscosity and therefore improved conductivity. One of these co-solvents is 1,1,2,2-Tetrafluoroethyl 2,2,3,3-Tetrafluoropropyl Ether (TTE). They can be prepared using solvents that have a high anodic stability such as Sulfolane (SL) making them ideal candidates for cycling lithium metal in conjunction with advanced cathode materials above 4,5 V. Since the introduction of the LSE concept, proposed by Jiangfeng et al. [1] most published variations used Lithium bis(fluorosulfone)imid (LiFSI) as the conducting lithium salt [2]. The structurally similar salt Lithium bis-(trifluoromethanesulforyl)imid (LiTFSI) is commonly used in classical electrolytes, especially for Lithium-Sulfur batteries because of its good ion separation and resulting high conductivity. It has however rarely been used in LSEs to date [3]. Although LiFSI and LiTFSI are very similar in structure, in contact with lithium metal their decomposition products display significant differences and can therefore lead to drastic divergencies in lithium passivation and deposition behavior [4, 5]. In this study we present a comparison of LSEs containing LiFSI and/or LiTFSI. The kinetics of Solid Electrolyte Interphase (SEI) formation is investigated by Electrochemical Impedance Spectroscopy (EIS) and differences in SEI composition are discerned using X-Ray Photoelectron Spectroscopy (XPS). Resulting divergencies in SEI stability and morphology of cycled Lithium metal are being investigated by Galvanostatic Cycling and Scanning Electron Microscopy (SEM) while principal conclusions are being drawn from Nernst-Potentials and solvent complex geometries obtained by Density Functional Theory (DFT). Our results show that the main LiTFSI decomposition fragment form a SEI that is unsuitable for efficient cycling of lithium metal in a HCE. This however can be remedied by applying the LSE principle to it where the stoichiometric addition of TTE improves the electrolytes electrochemical performance, so that a stable SEI can be formed on lithium metal. Because of its comparatively high conductivity it would be advantageous to use the LiTFSI based LSE over a FSI-based one. To further enhance the LSE multiple commonly employed electrolyte additives are being considered. In this regard results from EIS, XPS and galvanostatic cycling are correlated with theoretical predictions obtained from DFT calculations. In conclusion, this study highlights the influence of TTE on the formation of the SEI in the presence of LiTFSI. The co-solvent TTE serves a double purpose: dissipating solvent-salt complexes while simultaneously acting as additive during the initial passivation phase. Further, our results show that additives, that are applied in common electrolytes do not necessarily work the same way when used in an LSE, sometimes deteriorating the SEI rather than improving it. The reason for this is that the chemical nature of a SEI significantly differs when formed from SL based electrolytes compared to ones containing 1,3-Dioxolan or ethylene carbonate. Qian J, Henderson WA, Xu W et al. (2015) High rate and stable cycling of lithium metal anode. Nat Commun 6: 6362. doi: 10.1038/ncomms7362 Ren X, Chen S, Lee H et al. (2018) Localized High-Concentration Sulfone Electrolytes for High-Efficiency Lithium-Metal Batteries. Chem 4(8): 1877–1892. doi: 10.1016/j.chempr.2018.05.002 Piwko M, Thieme S, Weller C et al. (2017) Enabling electrolyte compositions for columnar silicon anodes in high energy secondary batteries. Journal of Power Sources 362: 349–357. doi: 10.1016/j.jpowsour.2017.07.046 Howlett PC, Izgorodina EI, Forsyth M et al. (2006) Electrochemistry at Negative Potentials in Bis(trifluoromethanesulfonyl)amide Ionic Liquids. Zeitschrift für Physikalische Chemie 220(10): 1483–1498. doi: 10.1524/zpch.2006.220.10.1483 Shkrob IA, Marin TW, Zhu Y et al. (2014) Why Bis(fluorosulfonyl)imide Is a “Magic Anion” for Electrochemistry. J. Phys. Chem. C 118(34): 19661–19671. doi: 10.1021/jp506567p Figure 1

A generalization of the Schwarz bound employed to reduce the scaling of quantum-chemical calculations is introduced in the context of non-Hermitian methods employing complex-scaled basis functions. Non-Hermitian methods offer a treatment of molecular metastable states in terms of L2-integrable wave functions with complex energies, but until now, an efficient upper bound for the resulting electron-repulsion integrals has been unavailable due to the complications from non-Hermiticity. Our newly formulated bound allows us to inexpensively and rigorously estimate the sparsity in the complex-scaled two-electron integral tensor, providing the basis for efficient integral screening procedures. We have incorporated a screening algorithm based on the new Schwarz bound into the state-of-the-art complex basis function integral code by White, Head-Gordon, and McCurdy [J. Chem. Phys. 142, 054103 (2015)]. The effectiveness of the screening is demonstrated through non-Hermitian Hartree-Fock calculations of the static field ionization of the 2-pyridoxine 2-aminopyridine molecular complex.

We introduce tight upper bounds for a variety of integrals appearing in electronic structure theories. These include electronic interaction integrals involving any number of electrons and various integral kernels such as the ubiquitous electron repulsion integrals and the three- and four-electron integrals found in explicitly correlated methods. Our bounds are also applicable to the one-electron potential integrals that appear in great number in quantum mechanical (QM), mixed quantum and molecular mechanical (QM/MM), and semi-numerical methods. The bounds are based on a partitioning of the integration space into balls centered around electronic distributions and their complements. Such a partitioning leads directly to equations for rigorous extents, which we solve for shell pair distributions containing shells of Gaussian basis functions of arbitrary angular momentum. The extents are the first general rigorous formulation we are aware of, as previous definitions are based on the inverse distance operator 1/r12 and typically only rigorous for simple spherical Gaussians. We test our bounds for six different integral kernels found throughout quantum chemistry, including exponential, Gaussian, and complementary error function based forms. We compare to previously developed estimates on the basis of significant integral counts and their usage in both explicitly correlated second-order Møller-Plesset theory (MP2-F12) and density functional theory calculations employing screened Hartree-Fock exchange.

: Mg/O 2 cells employing a MgCl 2 /AlCl 3 /DME (MACC/DME) electrolyte are cycled and compared to cells with modified Grignard electrolytes, showing that performance of magnesium/oxygen batteries depends strongly on electrolyte composition. Discharge capacity is far greater for MACC/DME-based cells, while rechargeability in these systems is severely limited. The Mg/O 2 -MACC/DME discharge product comprises a mixture of Mg(ClO 4 ) 2 and MgCl 2 , with the latter likely formed from slow decomposition of the former. The presence of Cl in these compounds suggests that the electrolyte participates in the cell reaction, or reacts readily with the initial electrochemical products. A rate study suggests that O 2 diffusion in the electrolyte limits discharge capacities at higher currents. Formation of an insulating product film on the positive electrodes of Mg/O 2 -MACC/DME cells following deep discharge increases cell impedance substantially, and likely explains the poor rechargeability. An additional impedance rise consistent with film formation on the Mg negative electrode suggests the presence of detrimental O 2 crossover. Minimizing O 2 crossover and bypassing charge transfer through the discharge product would improve battery performance.