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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.

A 3D hierarchical structure of MoSe2/NiSe2 exhibits superior HER activity because of more active sites and faster transfer of electrons.

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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