Chloride oxidation has tremendous utility in the burgeoning field of chlorine-mediated C–H activation, yet it remains a challenging process to initiate with light because of the exceedingly positive one-electron reduction potential, E° (Cl•/–), beyond most common transition-metal photooxidants. Herein, two photocatalytic chloride oxidation pathways that involve either one- or consecutive two-photon excitation of N-phenylphenothiazine (PTH) are presented. The one-photon pathway generates PTH• + by oxidative quenching that subsequently disproportionates to yield PTH2+ that oxidizes chloride; this pathway is also accessed by the electrochemical oxidation of PTH. The two-photon pathway, which proceeded through the radical cation excited state, 2PTH• +*, was of particular interest as this super-photooxidant was capable of directly oxidizing chloride to chlorine atoms. Laser flash photolysis revealed that the photooxidation by the doublet excited state proceeded on a subnanosecond timescale through a static quenching mechanism with an ion-pairing equilibrium constant of 0.36 M–1. The PTH photoredox chemistry was quantified spectroscopically on nanosecond and longer time scales, and chloride oxidation chemistry was revealed by reactivity studies with model organic substrates. One- and two-photon excitation of PTH enabled chlorination of unactivated C­(sp3)–H bonds of organic compounds such as cyclohexane with substantial yield enhancement observed from inclusion of the second excitation wavelength. This study provides new mechanistic insights into chloride oxidation catalyzed by an inexpensive and commercially available organic photooxidant.

Lithium dendrites (filaments) propagation in the solid electrolyte (SEs) leading to short circuits is one of the biggest obstacles to the application of all-solid-state lithium metal batteries (ASSLMBs). Due to the lacking of operando techniques that can provide nano-scale spatial resolution, the insufficient knowledge of the lithium dendrite growth inside SEs makes it difficult to suppress dendrite formation. To reveal the mechanism of Li filament growth in SEs, we achieved real-time monitoring of the nanoscale Li filament growth by operando small-angle neutron scattering (SANS) in a Cu|LLZNO|Li cell. On continuous plating, Li filament growth is not simply an accumulation of Li, but there is a dynamic evolution due to the competition between Li filament formation and self-healing. This dynamic evolution of Li filaments has been proved to be prevalent in typical SEs. With the aid of simulations and experiments, this dynamic competition was demonstrated to be highly dependent on the temperature variation. The enhanced self-healing ability of Li at elevated temperatures plays a positive role in suppressing Li filament formation. The cycle life of the cell was improved by heat therapy, which provided a brand-new insight for suppressing Li filament formation. Operando SANS with high Li sensitivity and high spatial resolution provides a new platform for investigating Li filaments in SEs.