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The abundant reserve and low price of potassium resources promote K-ion batteries (KIBs) becoming a promising alternative to Li-ion batteries, while the large ionic radius of K-ions creates a formidable challenge for developing suitable electrodes. Here Ni-substituted Prussian blue analogues (PBAs) are investigated comprehensively as cathodes for KIBs. The synthesized K1.90Ni0.5Fe0.5­[Fe­(CN)6]0.89·0.42H2O (KNFHCF-1/2) takes advantage of the merits of high capacity from electrochemically active Fe-ions, outstanding electrochemical kinetics induced by decreased band gap and K-ion diffusion activation energy, and admirable structure stability from inert Ni-ions. Therefore, a high first capacity of 81.6 mAh·g–1 at 10 mA·g–1, an excellent rate property (53.4 mAh·g–1 at 500 mA·g–1), and a long-term lifespan over 1000 cycles with the lowest fading rate of 0.0177% per cycle at 100 mA·g–1 can be achieved for KNFHCF-1/2. The K-ion intercalation/deintercalation proceeds through a facile solid solution mechanism, allowing 1.5-electron transfer based on low- and high-spins FeII/FeIII couples, which is verified by ex situ XRD, XPS, and DFT calculations. The K-ion full battery is also demonstrated using a graphite anode with a high energy density of 282.7 Wh·kg–1. This work may promote more studies on PBA electrodes and accelerate the development of KIBs.

Synthetic procedure of three-dimensional porous graphene-like sheets via low-temperature graphitization.

Phase engineering of nanomaterials provides a promising way to explore the phase-dependent physicochemical properties and various applications of nanomaterials. A general bottom-up synthesis method under mild conditions has always been challenging globally for the preparation of the semimetallic phasetransition-metal dichalcogenide (1T′-TMD) monolayers, which are pursued owing to their unique electrochemical property, unavailable in their semiconducting 2H phases. Here, we report the general scalable colloidal synthesis of nanosized 1T′-TMD monolayers, including 1T′-MoS 2 , 1T′-MoSe 2 , 1T′-WS 2 , and 1T′-WSe 2 , which are revealed to be of high phase purity. Moreover, the surfactantreliant stacking-hinderable growth mechanism of 1T′-TMD nano-monolayers was unveiled through systematic experiments and theoretical calculations. As a proof-of-concept application, the 1T′-TMD nano-monolayers are used for electrocatalytic hydrogen production in an acidic medium. The 1T′-MoS 2 nano-monolayers possess abundant in-plane electrocatalytic active sites and high conductivity, coupled with the contribution of the lattice strain, thus exhibiting excellent performance. Importantly, the catalyst shows impressive endurability in electroactivity. Our developed general scalable strategy could pave the way to extend the synthesis of other broad metastable semimetallic-phase TMDs, which offer great potential to explore novel crystal phase-dependent properties with wide application development for catalysis and beyond.

KFeII[FeIII(CN)6] with a symmetric cubic structure exhibits exceptional electrochemical performance based on a solid solution mechanism, and its high structural stability and electrochemical reversibility.

The cost-efficient and plentiful Na and K resources motivate the research on ideal electrodes for sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs). Here, MoSe2 nanosheets perpendicularly anchored on reduced graphene oxide (rGO) are studied as an electrode for SIBs and PIBs. Not only does the graphene network serves as a nucleation substrate for suppressing the agglomeration of MoSe2 nanosheets to eliminate the electrode fracture but also facilitates the electrochemical kinetics process and provides a buffer zone to tolerate the large strain. An expanded interplanar spacing of 7.9 Å is conducive to fast alkaline ion diffusion, and the formed chemical bondings (C–Mo and C–O–Mo) promote the structure integrity and the charge transfer kinetics. Consequently, MoSe2@5%rGO exhibits a reversible specific capacity of 458.3 mAh·g–1 at 100 mA·g–1, great cyclability with a retention of 383.6 mAh·g–1 over 50 cycles, and excellent rate capability (251.3 mAh·g–1 at 5 A·g–1) for SIBs. For PIBs, a high first specific capacity of 365.5 mAh·g–1 at 100 mA·g–1 with a low capacity fading of 51.5 mAh·g–1 upon 50 cycles and satisfactory rate property are acquired for MoSe2@10%rGO composite. Ex situ measurements validate that the discharge products are Na2Se for SIBs and K5Se3 for PIBs, and robust chemical bonds boost the structure stability for Na- and K-ion storage. The full batteries are successfully fabricated to verify the practical feasibility of MoSe2@5%rGO composite.

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