Bond modulation of MoSe_2+<i>x</i>driving combined intercalation and conversion reactions for high-performance K cathodes

The urgent demand for large-scale global energy storage systems and portable electronic devices is driving the need for considerable energy density and stable batteries. Here, Se atoms are introduced between MoSe₂ layers (denoted as MoSe_2+<i>x</i> ) by bond modulation to produce a high-performance cathode for potassium-ion batteries. The introduced Se atoms form covalent Se-Se bonds with the Se in MoSe₂, and the advantages of bond modulation are as follows: (i) the interlayer spacing is enlarged which increases the storage space of K⁺; (ii) the system possesses a dual reaction mechanism, and the introduced Se can provide an additional conversion reaction when discharged to 0.5 V, which improves the capacity further; (iii) the Se atoms confined between MoSe₂ layers do not give rise to the shuttle effect. MoSe_2+<i>x</i> is compounded with rGO (MoSe_2+<i>x</i> -rGO) as a cathode for potassium-ion batteries and displays an ultrahigh capacity (235 mA h g^-1 at 100 mA g^-1), a long cycle life (300 cycles at 100 mA g^-1) and an extraordinary rate performance (135 mA h g^-1 at 1000 mA g^-1 and 89 mA h g^-1 at 2000 mA g^-1). Pairing the MoSe_2+<i>x</i> -rGO cathode with graphite, the full cell delivers considerable energy density compared to other K cathode materials. The MoSe_2+<i>x</i> -rGO cathode also exhibits excellent electrochemical performance for lithium-ion batteries. This study on bond modulation driving combined intercalation and conversion reactions offers new insights into the design of high-performance K cathodes.