The shape-controlled synthesis and novel lithium storage mechanism of as-prepared CuC2O4·xH2O nanostructures

•CuC2O4·xH2O nanostructures are facilely obtained in the absence of additives.•Structural properties of CuC2O4·xH2O depend upon the polarity of reaction medium.•A novel lithium storage mechanism of CuC2O4·xH2O has been investigated.•Crystal water induces an uptrend of CuC2O4·xH2O reversible capacity on cycling.•The 100th discharge capacity of CuC2O4·xH2O reaches 970 mAh g−1 at 200 mA g−1.

In a hydrothermal and solvothermal system at 120 °C, cylinder-like and rod-like nanostructures of hydrate copper oxalate (CuC2O4·xH2O) can be thoroughly synthesized in the absence of any shape-controlling additives, respectively. The self-assembly of primary nanocrystals has been investigated considering the polarity of reaction medium, and in the chemical formula of CuC2O4·xH2O the average x value of crystal water is estimated to discuss the superior lithium storage capability of hydrate products. The results show that cylinder-like aggregate of CuC2O4·xH2O (x ∼ 0.14) possesses an initial discharge capacity of 920.3 mAh g−1 with a residual capacity of 970.0 mAh g−1 at 200 mA g−1 over 100 discharge–charge cycles, while rod-like aggregate with a x value of ∼0.53 per chemical formula exhibits a higher initial capacity of 1211.3 mAh g−1 and a lower retention of 849.3 mAh g−1 under the same conditions. Furthermore, time-dependent measurements present a novel crystal-to-amorphous transformation of active substances, suggesting a positive effect of unavoidable crystal water on the superior lithium storage capability of nanostructured CuC2O4·xH2O.