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

Abstract

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.

A Highly Durable, Transferable, and Substrate‐Versatile High‐Performance All‐Polymer Micro‐Supercapacitor with Plug‐and‐Play Function

“…[1] On the other hand, the microfabrication technology opens up the possibilities for fabrication of microenergy-storage-devices with planar interdigitated structure. [20][21][22][23][24][25] Conductive polymers after being doped, advantageous to other pseudocapacitive materials (metal oxides), [26,27] offer low cost, high conductivity, lightweight, and excellent flexibility. Although these μSCs show excellent energy-storage performance (e.g., excellent power density and long cycle life) and great feasibility to be integrated to miniaturized devices, the usage scenarios are still limited due to the following disadvantages.…”

Three‐Dimensional Binder‐Free Nanoarchitectures for Advanced Pseudocapacitors

“…Firstly, new technologies with low cost for large-scale production of conductive integrated frameworks with a high effective surface area, as well as coating techniques of redox-active materials on the framework with seamless and Ohmic contact, are required. [23] Ni foam/CNTs/Ni(OH) 2 Chemical bath deposition 3300 F g −1 16 F cm −2 (1 mV s −1 ) 3000 (17% loss) [24] NPG/MnO 2 electroless plating 1145 F g −1 1160 F cm −3 (50 mV s −1 ) 1000 (15% loss) [27] NPG/(NiOH) 2 electrochemical deposition 3168 F g −1 2223 F cm −3 (5 A g −1 ) 3000 (10% loss) [32] MnO 2 /NPG/MnO 2 Electrochemical depostion 916 F g −1 (5 mV s −1 ) 3000 (2.9% loss) [12] NPG/δ-MnO 2 Electrochemical deposition 1049 F g −1 922 F cm −3 (5 mV s −1 ) 20000 (12% loss) [33] NPG/PPy Electrochemical plating 1300 F g −1 (3 A g −1 ) 3000 (15% loss) CVD-G is still relative expensive for large-scale industrial energy-storage application.…”

“…The chemical bonding, induced by electron transfer between the deposited MnO 2 and the ligament surface of the NPG, not only satisfies the interfacial charge transport but also enhances the mechanical stability of the thick electrode for an enhanced cycling life (97% retention after 3000 cycles). [33] The well-matched atom distance between δ-MnO 2 (012) (0.2948 nm) and Au (111) (0.2884 nm) not only contributes to promoting the stability of interface structure but also to raising the electron transfer between the δ-MnO 2 and the Au substrate, which means that δ-MnO 2 -based devices can deliver a high power density of 295 W cm −3 with volumetric energy density (24.3 mWh cm −3 ) much higher than that of other pseudocapacitive electrodes prepared using a nanoporous metal, as well as improved cyclic stability (only 12% loss after 20000 cycles). Following the above strategy, many other pseudocapacitive materials, such as SnO 2 , [20e] RuO 2 , [29] Co(OH) 2 , [30] Ppy, and PANI, [31] have been successfully deposited into the NPG and shown excellent energy-storage performance, indicating the universality and indispensable role of NPG in high-performance pseudocapacitive electrodes.…”

Tuning the interlayer of transition metal oxides for electrochemical energy storage

“…On the other hand, nanosheets can be highly advantageous for small-scale energy storage devices. 106 Also, restacked exfoliated nanosheets have recently been reported as precursors for the synthesis of Li-ion battery cathode materials, presenting an interesting strategy for the development of metastable layered oxides. 107 …”