Ultralong Life Symmetric Potassium Ion Batteries Using a Bipolar Cr/Ti Based Layered Material

Based on low cost and earth abundance potassium resources,potassium-ion batteries have been regarded as the potential candidate for large-scale energy storage applications for renewable energy and smart grid.Although some earlier works have proposed reversible insertion/extraction of K^(+)ions in K cells,their cycle instability is insufficient for the further application of potassium-ion batteries.Herein,we report a symmetric potassium-ion battery with ultralong life employing a bipolar Cr/Ti based layered oxide,which contains two electrochemically active transition metals with the redox couples of Cr^(4+)/Cr^(3+)and Ti^(4+)/Ti^(3+)working on the cathode and anode side,respectively.Moreover,the(de)intercalation process in the structure and the in-situ electrochemical exchange process are investigated by ex-situ XRD.As a result,the symmetric cell based on the obtained K-based bipolar layered material exhibits a reversible capacity of 102 mA·h/g between 0.5 V and 3.5 V at 20 mA/g,an ultralong cycle life of 2000 cycles with a capacity retention of 80%and high rate capabilities.The outstanding cycling stability and rate performance endow this work with promising advantages for the future development of the novel energy storage system.