Spinels have been widely concerned as a promising class of electrocatalysts due to their appealing catalytic properties and the tunability of their compositions and structures. Ligand field theory (LFT), which describes the origins and the consequences of metal-ligand interactions, offers crucial insights for the design of spinel-type electrocatalysts. In this review, we timely summarize the research progress of spinel electrocatalysts that leverage LFT for structure-property insights, providing a pioneering perspective in this field. This review explores how LFT plays a pivotal role in optimizing the electrocatalytic properties of spinels. It covers important aspects such as identifying the origin of the catalytic properties, tuning the number of active sites, manipulating the e_g-filling and the spin state of metal cations, and modulating the 2p band of ligands. We anticipate that this review will provide valuable theoretical guidance and inspire creative spinel designs that excel in electrocatalytic applications.

The global annual production of poly(ethylene terephthalate) (PET) has reached 82 million tons, yet only a small fraction (less than 20%) is recycled. The ultra-slow degradation rate of PET results in the accumulation of PET waste in the environment, causing serious plastic pollution and posing severe challenges to ecosystems. In response, great efforts have been directed toward developing a cascade degradation and electrocatalytic upcycling strategy, which serves as a “waste-to-wealth” pathway. This strategy involves electro-reforming PET-hydrolyzed intermediates or using PET pyrolyzed products as electrocatalysts to generate high-value products. This review provides an overview of the state-of-the-art strategies for the “degradation-electrocatalytic upcycling (De-eUp)” of PET waste. Initially, an introduction to the strategy is provided, categorizing it into two main frameworks: “pyrolysis-electrocatalytic upcycling” and “hydrolysis-electrocatalytic upcycling”. The section on “pyrolysis-electrocatalytic upcycling” delves into the degradation methods for designing derived carbon nanomaterials and their utilization as high-performance electrocatalysts. The “hydrolysis-electrocatalytic upcycling” section discusses recent advancements in electro-reforming of PET hydrolyzed intermediates for the production of C₁ and C₂ products. The review concludes by examining the challenges and future prospects in developing an efficient and economical PET upcycling strategy. It is anticipated that this review will stimulate further progress in plastic waste valorization.

With many merits such as facile synthesis, economy, and relatively high theoretical capacity, Prussian blue analogs (PBAs) are considered promising cathode materials for sodium-ion batteries (SIBs). However, their practical applications still suffer from a low actual specific capacity and inferior stability owing to the imperfect crystallinity, irreversible phase transition, and low intrinsic conductivity. Herein, a surface-modification technique for vapor-phase molecular self-assembly was developed to prepare Fe-based PBAs, specifically sodium iron hexacyanoferrate (NaFeHCF), with a uniform conductive polymer protective layer of polypyrrole (PPy) on the surface, resulting in NaFeHCF@PPy. The incorporation of a PPy protective layer not only improves the electronic conductivity of NaFeHCF@PPy, but also effectively mitigates the dissolution of Fe-ions during cycling. Specifically, this advanced vapor-phase technique avoids Fe²⁺ oxidation and Na⁺ loss during liquid-phase surface modification. The NaFeHCF@PPy exhibited a remarkably enhanced cycling performance, with capacity retentions of 85.6% and 69.1% over 500 and 1000 cycles, respectively, at 200 mA/g, along with a superior rate performance up to 5 A/g (fast kinetics). Additionally, by adopting this strategy for Mn-based PBAs (NaMnHCF@PPy), we further demonstrated the universality of this method for PBA cathodes in SIBs.

As one of the most promising cathodes for sodium-ion batteries (SIBs), the layered transition metal oxides have attracted great attentions due to their high specific capacities and facile synthesis. However, their applications are still hindered by the problems of poor moisture stability and sluggish Na⁺ diffusion caused by intrinsic structural Jahn–Teller distortion. Herein, we demonstrate a new approach to settle the above issues through introducing K⁺ into the structures of Ni/Mn-based materials. The physicochemical characterizations reveal that K⁺ induces atomic surface reorganization to form the birnessite-type K₂Mn₄O₈. Combining with the phosphate, the mixed coating layer protects the cathodes from moisture and hinders metal dissolution into the electrolyte effectively. Simultaneously, K⁺ substitution at Na site in the bulk structure can not only widen the lattice-spacing for favoring Na⁺ diffusion, but also work as the rivet to restrain the grain crack upon cycling. The as achieved K⁺-decorated P2-Na_0.67Mn_0.75Ni_0.25O₂ (NKMNO@KM/KP) cathodes are tested in both coin cell and pouch cell configurations using Na metal or hard carbon (HC) as anodes. Impressively, the NKMNO@KM/KP||Na half-cell demonstrates a high rate performance of 50 C and outstanding cycling performance of 90.1% capacity retention after 100 cycles at 5 C. Furthermore, the NKMNO@KM/KP||HC full-cell performed a promising energy density of 213.9 Wh·kg⁻¹. This performance significantly outperforms most reported state-of-the-art values. Additionally, by adopting this strategy on O3-NaMn_0.5Ni_0.5O₂, we further proved the universality of this method on layered cathodes for SIBs.

Searching for novel solid electrolytes is of great importance and challenge for all-solid-state Mg batteries. In this work, we develop an amorphous Mg borohydride ammoniate, Mg(BH₄)₂·2NH₃, as a solid Mg electrolyte that prepared by a NH₃ redistribution between 3D framework-γ-Mg(BH₄)₂ and Mg(BH₄)₂·6NH₃. Amorphous Mg(BH₄)₂·2NH₃ exhibits a high Mg-ion conductivity of 5 × 10⁻⁴ S cm⁻¹ at 75 ℃, which is attributed to the fast migration of abundant Mg vacancies according to the theoretical calculations. Moreover, amorphous Mg(BH₄)₂·2NH₃ shows an apparent electrochemical stability window of 0–1.4 V with the help of in-situ formed interphases, which can prevent further side reactions without hindering the Mg-ion transfer. Based on the above superiorities, amorphous Mg(BH₄)₂·2NH₃ enables the stable cycling of all-solid-state Mg cells, as the critical current density reaches 3.2 mA cm⁻² for Mg symmetrical cells and the reversible specific capacity reaches 141 mAh g⁻¹ with a coulombic efficiency of 91.7% (first cycle) for Mg||TiS₂ cells.