Self-standing P2/P3 heterostructured Na0.7CoO2 nanosheet arrays as 3D cathodes for flexible sodium-ion batteries

The increasing power demand for flexible electronics stimulates the development of flexible batteries with high performance, light weight, and low cost. However, the fabrication of flexible cathodes for sodium-ion batteries is still a great challenge. In this work, freestanding three-dimensional P2/P3 heterostructured Na0.7CoO2 nanosheet arrays are grown on carbon cloth via a “solvothermal sodiation” reaction followed by calcination in argon atmosphere. The P2 and P3 phases have coherent interface perpendicular to the [00ℓ] direction in the P2/P3–Na0.7CoO2 nanosheets, which effectively suppresses the interlayer variation during charge/discharge, resulting in smaller volume change and improved structure stability with repeated Na+ intercalation/deintercalation. The self-standing P2/P3–Na0.7CoO2 nanosheet array electrode exhibits high specific capacity (126.2 mAh g−1 at 0.1 C and 85.5 mAh g−1 at 10 C) and superior cycle performance (92.4% after 300 cycles at 1 C). By coupling with hard carbon (HC) coated carbon cloth as anode, a P2/P3–Na0.7CoO2//HC flexible sodium-ion battery device is successfully constructed, delivering large reversible capacity of 120.5 mAh g−1 at 0.1 C (based on the weight of cathode material). The present fabrication of the three-dimensional flexible P2/P3–Na0.7CoO2 cathode provides new opportunity for the construction of advanced flexible sodium-ion batteries.