Synthesis, structure, and electrochemical performances of a novel three-dimensional framework K2[Cu(SO4)2]

Abstract Among Cu2+-based electrode materials, copper sulfates have demonstrated significantly diverse electrochemical performances upon reactions with Li+ and Na+. Here we report a new, pure 3D framework K2[Cu(SO4)2] (1) bearing channels of both 12- and 8-membered rings and a new, facile synthesis of a Natrochalcite-type, 2D framework K[Cu2(OH)(H2O)(SO4)2] (2). To our best knowledge, the solvothermal synthesis of 1 represents the first example of making a pure-phase of such bimetallic copper sulfates containing no H2O and/or OH− groups via solution routes. This synthetic method can be generalized to develop more bimetallic sulfates of transition metal ions. Their electrochemical performances upon reactions with Na+ were preliminarily investigated and compared to that of Krohnkite, 1D framework Na2[Cu(SO4)2(H2O)2] (3). 1–3 decompose in the first discharge below 0.70 V (vs. Na+/Na) to deliver 62, 97, 67 mAh/g and continue to decompose in subsequent cycling, leading to low stable capacities of 12, 16, and 13 mAh/g respectively. Here we initiate the use of the HSAB, i.e., hard and soft (Lewis) acid and base, principle to explain the cycling stability difference among Cu-based frameworks and the difference between Cu-based and non-Cu-based frameworks in general in the hope to find directions to the development of Cu-based electrode materials.