Many transition metal sulfides and phosphides are susceptible to surface oxidation under ambient conditions. The formed surface oxidation layer, which is likely to further restructure under reaction conditions, alters the chemical properties of the pristine material but has not been well studied. In this work, we for the first time use X-ray photoelectron spectroscopy to quantify the natural surface oxidation of transition metal phosphide and sulfide nanoparticles and employ a simplified Deal-Grove model to analyze the kinetics. We show that CoS₂ oxidizes faster than CoS whereas CoP₂ is more difficult to oxidize compared to CoP, and there exists an inverse correlation between the surface oxidation rate and the Co-S/P distance in the pristine structure. More inclusive investigation unveils different types of surface oxidation behavior: CoS, NiS and FeS are limited by their reactivity with oxygen; CoS₂ is the most reactive and its oxidation is governed by oxygen diffusion; CoP₂ is influenced by both reactivity and diffusion; CoP, Ni₂P, Cu₃P and MoP exhibit high initial oxidation degrees and the kinetics are not well-defined; MoS₂ is largely stable against oxidation.

Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP) is a solid-state electrolyte with high ionic conductivity and air stability but poor chemical stability and high interfacial impedance when directly contacted with Li metal. In this work, we develop an inorganic/polymer hybrid interlayer composed of Li bis(trifluoromethylsulfonyl)imide/poly(vinylene carbonate) polymer electrolyte and SiO₂ submicrospheres to stabilize the Li/LAGP interface. The polymeric component renders high ionic conductance and low interfacial resistance, whereas the inorganic component imparts flame retardancy and a physical barrier to the known Li-LAGP side reaction, together enabling stable Li stripping/plating for more than 1,500 h at room temperature. With this interlayer at both electrodes, all-solid-state Li||LiFePO₄ full cells with stable cycling performance are also demonstrated.

Despite promising characteristics such as high specific energy and low cost, current Li-S batteries fall short in cycle life. Improving the cycling stability of S cathodes requires immobilizing the lithium polysulfide (LPS) intermediates as well as accelerating their redox kinetics. Although many materials have been explored for trapping LPS, the ability to promote LPS redox has attracted much less attention. Here, we report for the first time on transition metal phosphides as effective host materials to enhance both LPS adsorption and redox. Integrating MoP-nanoparticle-decorated carbon nanotubes with S deposited on graphene oxide, we enable Li-S battery cathodes with substantially improved cycling stability and rate capability. Capacity decay rates as low as 0.017% per cycle over 1, 000 cycles can be realized. Stable and high areal capacity (> 3 mAh·cm^-2) can be achieved under high mass loading conditions. Comparable electrochemical performance can also be achieved with analogous material structures based on CoP, demonstrating the potential of metal phosphides for long-cycle Li-S batteries.