The interfaces between many solid-state electrolytes (SSEs) and lithium metal are (electro)­chemically unstable, and improved understanding of how interfacial transformations influence electrochemical degradation is necessary to stabilize these interfaces and therefore enable a wider range of viable SSEs for batteries. Here, the (electro)­chemical reaction processes that occur at the interface between Li1.4Al0.4Ge1.6(PO4)3 (LAGP) electrolyte and lithium are studied using in situ transmission electron microscopy and ex situ techniques. The reaction of lithium with LAGP causes amorphization and volume expansion, which induce mechanical stress and fracture of the SSE along with a massive increase in impedance. The evolved interphase has a nonuniform morphology at high currents, which causes accelerated chemo-mechanical failure. This work demonstrates that the current-dependent nature of the reaction at the SSE/Li interface plays a crucial role in determining chemo-mechanical degradation mechanisms, with implications for understanding and controlling degradation in a wide variety of SSE materials with unstable interfaces.

Operation of Li-ion batteries below −20 °C is hindered by low electrolyte conductivity and sluggish solid-state diffusion in electrodes. Li metal anodes show promise for low-temperature operation, but few electrolyte compositions exhibit high conductivity at reduced temperature while also allowing Li electrodeposition/stripping with high Coulombic efficiency. Here, we show that the Coulombic efficiency of Li metal anodes can be substantially improved at low temperatures (−60 °C) by tailoring the solid-electrolyte interphase (SEI) structure through the use of two classes of electrolyte solvents: cyclic carbonates and ethers. Cryogenic transmission electron microscopy and other methods show that fluoroethylene carbonate (FEC) induces temperature-dependent changes in the chemistry and structure of the SEI to be abundant with LiF and Li2CO3, while 17O nuclear magnetic resonance and molecular dynamics calculations show that FEC affects the solvation behavior and SEI formation process in this new electrolyte system. Our results demonstrate the promise of rechargeable Li-metal batteries to enable energy storage over a broad temperature range.

Nanoscale transition-metal dichalcogenide (TMDC) materials, such as MoS, exhibit promising behavior in next-generation electronics and energy-storage devices. TMDCs have a highly anisotropic crystal structure, with edge sites and basal planes exhibiting different structural, chemical, and electronic properties. In virtually all applications, two-dimensional or bulk TMDCs must be interfaced with other materials (such as electrical contacts in a transistor). The presence of edge sites vs basal planes (i.e., the crystallographic orientation of the TMDC) could influence the chemical and electronic properties of these solid-state interfaces, but such effects are not well understood. Here, we use in situ X-ray photoelectron spectroscopy (XPS) to investigate how the crystallography and structure of MoS influence chemical transformations at solid-state interfaces with various other materials. MoS materials with controllably aligned crystal structures (horizontal vs vertical orientation of basal planes) were fabricated, and in situ XPS was carried out by sputter-depositing three different materials (Li, Ge, and Ag) onto MoS within an XPS instrument while periodically collecting photoelectron spectra; these deposited materials are of interest due to their application in electronic devices or energy storage. The results showed that Li reacts readily with both crystallographic orientations of MoS to form metallic Mo and LiS, while Ag showed very little chemical or electronic interaction with either type of MoS. In contrast, Ge showed significant chemical interactions with MoS basal planes, but only minor chemical changes were observed when Ge contacted MoS edge sites. These findings have implications for electronic transport and band alignment at these interfaces, which is of significant interest for a variety of applications.

MoS 2 has important applications in (electro)catalysis and as a semiconductor for electronic devices. Other chemical species are commonly added to MoS 2 to increase catalytic activity or to alter electronic properties through substitutional or adsorption-based doping. While groundbreaking work has been devoted to determining the atomic-scale structure of MoS 2 and other layered transition-metal dichalcogenides (TMDCs), there is a lack of understanding of the dynamic processes that govern the evolution of these materials during synthesis. Here, in situ transmission electron microscopy (TEM) heating, in combination with larger length scale ex situ experiments, is used to investigate the effects of added Ni on the growth of MoS 2 during the thermolysis of the solid-state (NH 4 ) 2 MoS 4 precursor. Low concentrations of Ni are observed to cause significant differences in the MoS 2 crystallization and growth process, leading to an increase in MoS 2 crystal size. This is likely a result of the altered mobility of interfaces between crystals during growth. These findings demonstrate the important role of additional elements in controlling the evolution of TMDCs during synthesis, which should be considered when designing these materials for a variety of applications.

Next-generation batteries with high energy density rely on high-capacity electrode materials, but large volume changes and mechanical fracture in these materials during charge and discharge limit cycle life. Here, we discover that FeS 2 electrode materials are more mechanically resilient during reaction with larger alkali ions (sodium and potassium) compared with lithium, despite larger volume changes. These findings are important since they suggest that various largevolume-change electrode materials could enable stable cycling performance in next-generation sodium-and potassium-ion batteries.