Electrochemical energy storage devices are pivotal in achieving “carbon neutrality” by enabling the storage of energy generated from renewable sources. To facilitate the development of these devices, it is important to gain insight into the underlying the single-/multi-electron transfer process. This can be achieved through in-time detection under operational conditions, but there are limited tools available for monitoring electron transfer under operando conditions. Electron paramagnetic resonance (EPR) is a powerful technique that can meet these expectations, as it is highly sensitive to unpaired electrons and can detect changes of paramagnetic centres. Despite the long history of in situ electrochemical EPR research, its potential has been surprisingly underutilized due to the need for strict operando cell design under special testing conditions. This review comprehensively summarizes recent efforts to understand energy storage mechanisms using in situ/operando EPR, with the aim of drawing researchers’ attention to this powerful technique. After introducing the fundamental principles of EPR, we describe the critical advances made in detecting batteries using operando EPR, along with the remaining challenges and opportunities for future development of this technology in batteries. We emphasize the need for strict operando cell design and the importance of designing experiments that closely mimic real-world conditions. We believe that this review will provide innovative solutions to solve tough problems that researchers may encounter during their battery research, and ultimately contribute to the development of more efficient and sustainable energy storage devices.

Aqueous rechargeable Zn//MnO₂ batteries show promising prospects for grid-scale energy storage due to their intrinsic safety, abundant resource, and potential high performance. Unfortunately, the real capability of these devices is far from satisfactory thanks to the low capacity and sluggish kinetics of the MnO₂ cathode. Herein, we report a dual cation doping strategy by synthesis of MnO₂ in the presence of Ti₃C₂X MXenes and Ni²⁺ ions to essentially address these drawbacks. Such a process contributes to a Ti,Ni co-doped α-MnO₂ anchored on MXenes. The Ti³⁺ ions incorporated in the framework allow a partial multivalent variation for a large capacity while the Ni²⁺ ions promote the H⁺ transfer within the MnO₂ matrix via the Grotthuss proton transport manner. As a result, the optimal dual cation doped MnO₂ exhibits a large reversible capacity of 378 mAh·g⁻¹ at 0.1 C and a high rate capability. Moreover, capacity retention as high as 92% is observed after cycling at 4 C for 1000 times, far superior to many of the previously reported results. This facile strategy demonstrated here may shed new insight into the rational design of electrodes based on high-performance Zn//MnO₂ batteries.

The edge S sites of thermodynamically stable 2H MoS₂ are active for hydrogen evolution reaction (HER) but the active sites are scarce. Despite the dominance of the basal S sites, they are generally inert to HER because of the low p-band center. Herein, we reported a synergistic combination of phase engineering and NH₄⁺ intercalation to promote the HER performance of MoS₂. The rational combination of 1T and 2H phases raises the p-band center of the basal S sites while the intercalated NH₄⁺ ions further optimize and stabilize the electronic band of these sites. The S sites with regulated band structures afford moderate hydrogen adsorption, thus contributing to excellent HER performance over a wide pH range. In an acid medium, this catalyst exhibits a low overpotential of 169 mV at 10 mA·cm⁻² and Tafel slope of 39 mV·dec⁻¹ with robust stability, superior to most of recently reported MoS₂-based non-noble catalysts. The combined use of in/ex-situ characterizations ravels that the appearance of more unpaired electrons at the Mo 4d-orbital reduces the d-band center which upshifts the p-band center of the adjacent S for essentially improved HER performance. This work provides guidelines for the future development of layered transition-metal-dichalcogenide catalysts.

Carbonaceous materials represent the dominant choice of materials for anodic lithium storage in many energy storage devices. Nevertheless, the nonpolar carbonaceous materials offer weak adsorption toward Li⁺ that largely denies the high-rate Li⁺ storage. Herein, the atomic Fe sites decorated carbon nanofibers (AICNFs) facilely produced by electrospinning are reported for kinetically accelerated Li⁺ storage. Theoretical calculation reveals that the atomic Fe sites possess coordination unsaturated electronic configuration, enabling suitable bonding energy and facilitated diffusion path of Li⁺. As a result, the optimal structure displays a high capacitive contribution up to 95.9% at a scan rate of 2.0 mV·s⁻¹. In addition, ultrahigh capacity retention of 97% is afforded after 5,000 cycles at a current density of 3 A·g⁻¹. Moreover, the interlaced fiber structure enabled by electrospinning benefits structural stability and improved conductivity even at thick electrodes, thus allowing a high areal capacity of 1.76 mAh·cm⁻² at a loading of 8 mg·cm⁻². Because of these structure and performance merits, the lithium-ion capacitor containing the AICNF-based anode delivers a high energy density and large power density.

Nanoscale hierarchically porous metal–organic frameworks (NHP-MOFs) have received unprecedented attention in many fields owing to their integration of the strengths of nanoscale size (< 1 ​μm) and hierarchical porous structure (micro-, meso- and/or macro-pores) of MOFs. This review focuses on recent advances in the main synthetic strategies for NHP-MOFs based on different metal ions (e.g., Cu, Fe, Co, Zn, Al, Zr, and Cr), including the template method, composite technology, post-synthetic modification, in situ growth and the grind method. In addition, the mechanisms of synthesis, regulation techniques and the advantages and disadvantages of various methods are discussed. Finally, the challenges and prospects of the commercialisation of promising NHP-MOFs are also presented. The purpose of this review is to provide a road map for future design and development of NHP-MOFs for practical application.