Based on symmetry analysis and lattice model calculations, we demonstrate that Dirac nodal line (DNL) can stably exist in two-dimensional (2D) nonmagnetic as well as antiferromagnetic systems. We focus on the situations where the DNLs are enforced by certain symmetries and the degeneracies on the DNLs are inevitable even if spin–orbit coupling is strong. After thorough analysis, we find that five space groups, namely 51, 54, 55, 57 and 127, can enforce the DNLs in 2D nonmagnetic semimetals, and four type-III magnetic space groups (51.293, 54.341, 55.355, 57.380) plus eight type-IV magnetic space groups (51.299, 51.300, 51.302, 54.348, 55.360, 55.361, 57.387 and 127.396) can enforce the DNLs in 2D antiferromagnetic semimetals. By breaking these symmetries, the different 2D topological phases can be obtained. Furthermore, by the first-principles electronic structure calculations, we predict that monolayer YB₄C₄ is a good material platform for studying the exotic properties of 2D symmetry-enforced Dirac node-line semimetals.

Traditional lithium-ion batteries with graphite anodes have gradually been limited by the glass ceiling of energy density. As a result, lithium metal batteries (LMBs), regarded as the ideal alternative, have attracted considerable attention. However, lithium is highly reactive and susceptible to most electrolytes, resulting in poor cycle performance. In addition, lithium grows Li dendrites during charging, adversely affecting the safety of LMBs. Therefore, LMBs are more sensitive to the chemical composition of electrolytes and their relative ratios (concentrations). Recently, concentrated electrolytes have been widely demonstrated to be friendly to lithium metal anodes (LMAs). This review focuses on the progress of concentrated electrolytes in LMBs, including the solvation structure varying with concentration, unique functions in stabilizing the LMA, and their interfacial chemistry with LMA.

Despite the potential advantages promised by solid-state batteries, the success of solid-state electrolytes has not yet been fully realised. This is due in part to the lower ionic conductivity of solid electrolytes. In many solid superionic conductors, grain boundaries are found to be ionically resistive and hence contribute to this lower ionic conductivity. Additionally, in spite of the hope that solid electrolytes would inhibit lithium filaments, in most scenarios their growth is still observed, and in some polycrystalline systems this is suggested to occur along grain boundaries. It is apparent that grain boundaries affect the performance of solid-state electrolytes, however a deeper understanding is lacking. In this perspective, the current theories relating to grain boundaries in solid-state electrolytes are explored, as well as addressing some of the challenges which arise when trying to investigate their role. Glasses are presented as a possible solution to reduce the effect of grain boundaries in electrolytes. Future research directions are suggested which will aid in both understanding the role of grain boundaries, and diminishing their contribution in cases where they are detrimental.