Birnessite-Type Layered Manganese Oxides Nanosheets for 18650-Type Cylindrical Aqueous Supercapacitors

Abstract

The manganese oxide (MnO2) is one of the promising pseudocapacitive materials due to its high specific capacitance, low-cost, and low-toxicity.1 Especially, the birnessite-type MnO2 with 2D-layer structure demonstrates a high specific capacitance because the charge storage can occur at the active site within the interlayer region. Herein, the influence of hydration energy of structural cations (Li-, Ca-, Sr-, and Ba) within the birnessite-type MnOx is demonstrated on charge storage mechanisms via de-/intercalation processes upon redox reaction with ion-exchange process. Interestingly, the Ca-MnOx demonstrates the superior specific capacitance even though the Li-MnOx has higher redox activity and Mn utilization as observed by the ex situ X-ray absorption spectroscopy. The Ca-MnOx has higher hydration energy of Ca2+ (500 kcal mol-1) as compared to -465, -436 and -204 kcal mol-1 of Sr- MnOx, Ba-MnOx and Li-MnOx, respectively, which for the higher hydration energy leads to the easier cation exchange.2 Hence, the ion-exchange affinity within the birnessite structure is dominant which can further confirm by inductively coupled plasma - optical emission spectrometry technique. Moreover, the cycling stability of the as-prepared sample is retained at 96% after 11,000 cycles at current density of 5 A g-1. Up to now, there is no Ca-MnOx-based electrode material in real practical supercapacitor devices; therefore, we fabricated the first prototype of cylindrical-cell as well as further studied its performance.