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.

The making of BaZrS 3 thin films by molecular beam epitaxy (MBE) is demonstrated. BaZrS 3 forms in the orthorhombic distorted-perovskite structure with corner-sharing ZrS 6 octahedra. The single-step MBE process results in films smooth on the atomic scale, with near-perfect BaZrS 3 stoichiometry and an atomically sharp interface with the LaAlO 3 substrate. The films grow epitaxially via two competing growth modes: buffered epitaxy, with a self-assembled interface layer that relieves the epitaxial strain, and direct epitaxy, with rotated-cube-on-cube growth that accommodates the large lattice constant mismatch between the oxide and the sulfide perovskites. This work sets the stage for developing chalcogenide perovskites as a family of semiconductor alloys with properties that can be tuned with strain and composition in highquality epitaxial thin films, as has been long-established for other systems including Si-Ge, III-Vs, and II-VIs. The methods demonstrated here also represent a revival of gas-source chalcogenide MBE.

Copper sulfide is found to exhibit excellent cycle life in Na-ion batteries, and multiscale in situ techniques (TEM and X-ray diffraction) are used to reveal unique aspects of the electrochemical reaction of sodium with this material.

Alloying anode materials offer high capacity for nextgeneration batteries, but the performance of these materials often decays rapidly with cycling because of volume changes and associated mechanical degradation or fracture. The direct measurement of crystallographic strain evolution in individual particles has not been reported, however, and this level of insight is critical for designing mechanically resilient materials. Here, we use operando X-ray diffraction to investigate strain evolution in individual germanium microparticles during electrochemical reaction with lithium. The diffraction peak was observed to shift in position and diminish in intensity during reaction because of the disappearance of the crystalline Ge phase. The compressive strain along the [111] direction was found to increase monotonically to a value of −0.21%. This finding is in agreement with a mechanical model that considers expansion and plastic deformation during reaction. This new insight into the mechanics of large-volume-change transformations in alloying anodes is important for improving the durability of high-capacity batteries.

Solid oxide fuel cells have the potential to render the conversion from fuel to electrical energy more efficienct while lowering emissions. The technology, however, suffers from performance degradation due to...