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Recognition of unusual optoelectronic properties for twodimensional (2D) layered organic−inorganic lead(II) halide materials (C n H 2n+1 NH 3 ) 2 PbX 4 (X = I, Br, and Cl) has attracted intense renewed interest in this class of materials. Single crystals of the 2D layered materials (C 10 H 21 NH 3 ) 2 PbBr 4 and pseudo-alloy (C 10 H 21 NH 3 ) 2 PbI 2 Br 2 were grown for photophysical evaluation. A 10-carbon alkylammonium cation was selected for investigation to provide strong dielectric screening in order to highlight quantum confinement effects of the anionic (PbX 4 2− ) semiconductor layer. Single crystals of the 2D layered (C 10 H 21 NH 3 ) 2 PbBr 4 compound display a characteristic free exciton with a binding energy of ca. 280 meV. Observation of a short photoluminescence lifetime of 2.8 ± 0.2 ns suggests that this electronic transition for the PbBr 4 -based layered material has only singlet character. Sheets of (C 10 H 21 NH 3 ) 2 PbBr 4 with thicknesses of a few layers were fabricated, and the dimensions were verified by AFM experiments. Excitonic emissions from (C 10 H 21 NH 3 ) 2 PbBr 4 and (C 10 H 21 NH 3 ) 2 PbI 4 exhibit relatively small spectral shifts from the bulk down to a thickness of five layers indicative of the strong confinement effect of the 10-carbon alkylammonium spacers. Single crystals of the pseudo-alloy (C 10 H 21 NH 3 ) 2 PbBr 2 I 2 give an excitonic absorption peak close to that of the tetrabromide (C 10 H 21 NH 3 ) 2 PbBr 4 and an emission peak with a large Stokes shift to a position similar to that of the tetraiodide (C 10 H 21 NH 3 ) 2 PbI 4 .

The practical application of lithium metal batteries is impeded by the growth of dendrites and decomposition of electrolytes especially at high temperature in normal carbonate-based electrolytes. Herein, a novel urea-based molecule, 1,3-dimethyl-2-imidazolidinone (DMI), with a high donor number is proposed, which exhibits an extraordinary solubility of LiNO3 of over 5 M. As a result, a sufficient amount of LiNO3 is readily introduced into the carbonate electrolytes with DMI as an additive, and an average coulombic efficiency of 99.1% for lithium plating/stripping is achieved due to a stable solid electrolyte interphase (SEI) rich in inorganic-rich lithium salts. The Li||Li symmetric cell achieves a stable operation for over 2500 h at 0.5 mA cm–2 and 1 mAh cm–2, and a granular shape of deposited Li metal is still preserved even at a high current density of 10 mA cm–2. Besides, the decomposition of LiPF6 is inhibited benefiting from its enhanced dissociation after the addition of DMI/LiNO3 and DMI’s function as a PF5 scavenger. Consequently, the Li||LiFePO4 cell succeeds to achieve an excellent capacity retention of 95.6% after 2200 cycles at a high rate of 5C, and a stable operation is realized at a high temperature of 60 °C even under harsh conditions (45 μm ultrathin Li and ∼1.5 mAh cm–2 LiFePO4). This work enriches the solvents and additives pool for stable and high-performance lithium metal batteries and will shed light on future developments of advanced battery electrolytes.