Remarkable Improvements in Volumetric Energy and Power of 3D MnO2 Microsupercapacitors by Tuning Crystallographic Structures

• Published in: Advanced functional materials Vol. 26; no. 11; pp. 1830 - 1839
• Main Authors: Li, Ying-Qi, Shi, Xiang-Mei, Lang, Xing-You, Wen, Zi, Li, Jian-Chen, Jiang, Qing
• Format: Journal Article
• Language: English
• Published: Blackwell Publishing Ltd 15.03.2016
• Subjects: crystallographic structure; internal resistance; microsupercapacitor; MnO2; nanoporous metals
• ISSN: 1616-301X; 1616-3028
• DOI: 10.1002/adfm.201504886

Transition‐metal oxides as faradaic charge‐storage intermediates sandwiched between conductor and electrolyte are key components to store/deliver high‐density energy in microsupercapacitors for many applications in miniaturized portable electronics and microelectromechanical systems. While the conductor facilitating their electron transports, they generally suffer from a switch of rate‐determining step to their sluggish redox reactions in pseudocapacitive energy storage, during which poor cation accessibility and diffusion leads to high internal resistances and lowers volumetric capacitance and rate performance. Here it is shown that the faradaic processes in a model system of MnO2 can be radically boosted by tuning crystallographic structures from cryptomelane (α‐MnO2) to birnessite (δ‐MnO2). As a result of greatly enhanced Na+ accessibility and diffusion, 3D layered crystalline δ‐MnO2 microelectrodes exhibit volumetric capacitance as high as ≈922 F cm−3 (≈1.5‐fold higher than α‐MnO2, ≈617 F cm−3) and excellent rate performance. This enlists δ‐MnO2 microsupercapacitor to deliver ultrahigh stack electrical powers (up to ≈295 W cm−3) while maintaining volumetric energy density much higher than that of thin‐film lithium battery. 3D MnO2 microsupercapacitors (micro‐SCs) are constructed by seamlessly integrating pseudocapactive MnO2 with 3D nanoporous current collectors and tuning their crystallographic structure. As a result of the minimization of internal resistance, the micro‐SCs exhibit exceptionally high stack electrical power while maintaining volumetric energy density much higher than that of thin‐film lithium battery.