In organic solar cells, the singlet and triplet excitons dissociate into free charge carriers with different mechanisms due to their opposite spin state. Therefore, the ratio of the singlet and triplet excitons directly affects the photocurrent. Many methods were used to optimize the performance of the low-efficiency solar cell by improving the ratio of triplet excitons, which shows a long diffusion length. Here we observed that in high-efficiency systems, the proportion of singlet excitons under linearly polarized light excitation is higher than that of circularly polarized light. Since the singlet charge transfer state has lower binding energy than the triplet state, it makes a significant contribution to the charge carrier generation and enhancement of the photocurrent. Further, the positive magnetic field effect reflects that singlet excitons dissociation plays a major role in the photocurrent, which is opposite to the case of low-efficiency devices where triplet excitons dominate the photocurrent.

Zinc-air batteries (ZABs) are widely studied because of their high theoretical energy density, high battery voltage, environmental protection, and low price. However, the slow kinetics of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) on the air electrode limits the further application of ZABs, so that how to develop a cheap, efficient, and stable catalyst with bifunctional catalytic activity is the key to solving the development of ZABs. Transition metal compounds are widely used as cathode materials for ZABs due to their low cost, high electrocatalytic activity, and stable structure. This review summarizes the research progress of transition metal compounds as bifunctional catalysts for ZABs. The development history, operation principle, and mechanism of ORR and OER reactions are introduced first. The application and development of transition metal compounds as bifunctional catalysts for ZABs in recent years are systematically introduced, including transition metal oxides (TMOs), transition metal nitrides (TMNs), transition metal sulfides (TMSs), transition metal carbides (TMCs), transition metal phosphates (TMPs), and others. In addition, the shortcomings of transition metal compounds as bifunctional catalysts for ZABs were summarized and reasonable design strategies and improvement measures were put forward, aiming at providing a reference for the design and construction of high-performance ZABs cathode materials. Finally, the challenges and future in this field are discussed and prospected.

Unexpected intercalation-dominated process is observed during K⁺ insertion in WS₂ in a voltage range of 0.01–3.0 V. This is different from the previously reported two-dimensional (2D) transition metal dichalcogenides that undergo a conversion reaction in a low voltage range when used as anodes in potassium-ion batteries. Charge/discharge processes in the K and Na cells are studied in parallel to demonstrate the different ion storage mechanisms. The Na⁺ storage proceeds through intercalation and conversion reactions while the K⁺ storage is governed by an intercalation reaction. Owing to the reversible K⁺ intercalation in the van der Waals gaps, the WS₂ anode exhibits a low decay rate of 0.07% per cycle, delivering a capacity of 103 mAh·g^-1 after 100 cycles at 100 mA·g-¹. It maintains 57% capacity at 800 mA·g^-1 and shows stable cyclability up to 400 cycles at 500 mA·g-¹. Kinetics study proves the facilitation of K⁺ transport is derived from the intercalation-dominated mechanism. Furthermore, the mechanism is verified by the density functional theory (DFT) calculations, showing that the progressive expansion of the interlayer space can account for the observed results.

A novel hierarchical electrode material for Na-ion batteries composed of Sb nanoplates on Ni nanorod arrays is developed to tackle the issues of the rapid capacity fading and poor rate capability of Sb-based materials. The three- dimensional (3D) Sb-Ni nanoarrays as anodes exhibit the synergistic effects of the two-dimensional nanoplates and the open and conductive array structure as well as strong structural integrity. Further, their capacitive behavior is confirmed through a kinetics analysis, which shows that their excellent Na-storage performance is attributable to their unique nanostructure. When used as binder-free sodium-ion battery (SIB) anodes, the nanoarrays exhibit a high capacity retention rate (more than 80% over 200 cycles) at a current density of 0.5 A·g^–1 and excellent rate capacity (up to 20 A·g^–1), with their capacity being 580 mAh·g^–1. Moreover, a P2-Na_2/3Ni_1/3Mn_2/3O₂//3D Sb-Ni nanoarrays full cell delivers a highly reversible capacity of 579.8 mAh·g^–1 over 200 cycles and an energy density as high as 100 Wh·kg^–1. This design strategy for ensuring fast and stable Na storage may work with other electrode materials as well.