P-n junction built-in electric field and electrochemical in-situ intercalation enabled ultra-stable and high-energy ammonium-ion storage

Abstract

Ammonium-ion (NH₄⁺) is a promising non-metallic charge carrier in aqueous energy storage with sustainability and environmental benignity. In spite of the unique H-bond mechanism between NH₄⁺ and host material, the anisotropy caused from tetrahedral structure of NH₄⁺ essentially limits its diffusion ability in host materials, still resulting in unsatisfied storage behavior. Herein, the built-in electric field (BIEF) mechanism has been first introduced towards NH₄⁺ hybrid supercapacitor (HSC) by constructing MnO_x/MnS₂ p-n junction. The p-n junction BIEF has a reversibly changed field direction and provides extra inside coulombic force to boost the NH₄⁺ diffusion kinetics, resulting in outstanding capacity of 838.56 F g^-1 (186.35 mAh g^-1) at 1 A g^-1 in 0.5 M NH₄Ac, which simultaneously outperforms than those in metallic cation electrolyte due to the existed H-bond. Besides, an interlayer pillars effect induced by electrochemical in-situ intercalation of NH₄⁺ stabilizes the layered matrix structure of MnO_x/MnS₂. As a result, the synergistical optimization of ion kinetics and crystal architectonics enables an ultra-stable NH₄⁺ storage of 96.42% capacitance retention upon 40000 cycles. The fabricated HSC delivers a high energy density of 79.57 Wh kg^-1 at the power density of 850 W kg^-1, of which the pouch-type device further manifests the practical applicability via powering real-life electric product, such as smartphone and ipad. This work provides new insight into improving NH₄⁺ intercalation chemistry and developing advanced host materials for aqueous energy storage.