Doping-Induced Electronic/Ionic Engineering to Optimize the Redox Kinetics for Potassium Storage: A Case Study of Ni-Doped CoSe2.

Heteroatom doping effectively tunes the electronic conductivity of transition metal selenides (TMSs) with rapid K+ accessibility in potassium ion batteries (PIBs). Although considerable efforts are dedicated to investigating the relationship between the doping strategy and the resulting electrochemistry, the doping mechanisms, especially in view of the ion and electronic diffusion kinetics upon cycling, are seldom elucidated systematically. Herein, the crystal structure stability, charge/ion state, and bandgap of the active materials are found to be precisely modulated by favorable heteroatom doping, resulting in intrinsically fast kinetics of the electrode materials. Based on the combined mechanisms of intercalation and conversion reactions, electron and K+ ion transfer in Ni-doped CoSe2 embedded in carbon nanocomposites (Ni-CoSe2 @NC) can be significantly enhanced via electronic engineering. Benefiting from the synthetic controlled Ni grains, the heterointerface formed by the intermediate products of electrochemical reactions in Ni-CoSe2 @NC strengthens the conversion kinetics and interdiffusion process, developing a low-barrier mesophase with optimized potassium storage. Overall, an electronic tuning strategy can offer deeper atomic insights into the conversion reaction of TMSs in PIBs.

Controllable Heterojunctions with a Semicoherent Phase Boundary Boosting the Potassium Storage of CoSe2 /FeSe2.

Heterostructure construction is an efficient method for reinforcing K+ storage of transition metal selenides. The spontaneously developed internal electric fields give a strong boost to charge transport and significantly reduce the activation energy. Nevertheless, perfection of the interfacial region based on the energy level gradient and lattice matching degree is still a great challenge. Herein, rich vacancies and ultrafine CoSe2 -FeSe2 heterojunctions with semicoherent phase boundary are simultaneously obtained, which possess unique electronic structures and abundant active sites. When employed as anodes for potassium-ion batteries (PIBs), CoSe2 -FeSe2 @C composites display a reversible potassium storage of 401.1 mAh g-1 at 100 mA g-1 and even 275 mAh g-1 at 2 A g-1 . Theoretical calculation also reveals that the potassium-ion diffusion can be dramatically promoted by the controllable CoSe2 -FeSe2 heterojunction.

Large Interlayer Spacing of Few-Layered Cobalt-Tin-Based Sulfide Providing Superior Sodium Storage.

Mixed transition metal sulfides (MTMSs) have been regarded as a potential anode material for sodium-ion batteries (SIBs) due to their high reversible specific capacity. Herein, nanoflower-like few-layered cobalt-tin-based sulfide (F-CoSnS) with a large interlayer spacing is synthesized via a facile route for superior sodium storage. The growth mechanism of this unique F-CoSnS is systematically studied. Such distinctive nanostructured engineering synergistically combines a broad interlayer spacing (∼ 0.85 nm), the functionalities of few (2-3) layers, and the introduction of heterogeneous metal atoms, reducing the ion diffusion energy barrier for high-efficiency intercalation/deintercalation of Na+ ions, as revealed by density functional theory (DFT) calculations. With further incorporation of a three-dimensional (3D) conductive network, the F-CoSnS@C electrode shows a large sodium storage capacity (493.4 mAh g-1 at 50 mA g-1), remarkable rate capability (316.1 mAh g-1 at 1600 mA g-1), and superior cycling stability (450 mAh g-1 at 50 mA g-1 with 91.2% capacity retention, 0.044% fading rate per cycle, and approximately 100% Coulombic efficiency after 200 cycles). This work demonstrates that the few-layered ternary MTMSs are highly applicable for the development of advanced SIB anode materials with high performance.

Superior Selectivity and Tolerance towards Metal-Ion Impurities of a Fe/N/C Catalyst for CO2 Reduction.

The electrochemical CO2 reduction reaction (CO2 RR) in aqueous solution inevitably competes with the hydrogen evolution reaction (HER), which results in a difficult separation of the complex products. In this study, a Fe/N/C catalyst derived from Fe(SCN)3 (labelled SMFeSCN) revealed a high CO Faradaic efficiency (FE) of 99 % at a moderate overpotential of 0.44 V. CO2 RR and HER competed with each other for active sites on Fe/N/C. The high FE for CO production originated from the high content of micropores on the catalyst, which could suppress the side reactions by increasing CO2 uptake. More importantly, excellent tolerance towards metal-ion impurities was demonstrated in Fe/N/C, which was primarily owing to the high specific surface area with scattered active sites. Thus, the Fe/N/C catalyst showed good activity for CO2 RR without influencing the electrolyte purity, thus raising the possibility of its practical application.