High-capacity K+-pillared layered manganese dioxide as cathode material for high-rate aqueous zinc-ion battery.

Sluggish kinetics and severe structural instability of manganese-based cathode materials for rechargeable aqueous zinc-ion batteries (ZIBs) lead to low-rate capacity and poor cyclability, which hinder their practical applications. Pillaring manganese dioxide (MnO2) by pre-intercalation is an effective strategy to solve the above problems. However, increasing the pre-intercalation content to realize stable cycling of high capacity at large current densities is still challenging. Here, high-rate aqueous Zn2+ storage is realized by a high-capacity K+-pillared multi-nanochannel MnO2 cathode with 1 K per 4 Mn (δ-K0.25MnO2). The high content of the K+ pillar, in conjunction with the three-dimensional confinement effect and size effect, promotes the stability and electron transport of multi-nanochannel layered MnO2 in the ion insertion/removal process during cycling, accelerating and accommodating more Zn2+ diffusion. Multi-perspective in/ex-situ characterizations conclude that the energy storage mechanism is the Zn2+/H+ ions co-intercalating and phase transformation process. More specifically, the δ-K0.25MnO2 nanospheres cathode delivers an ultrahigh reversible capacity of 297 mAh g-1 at 1 A g-1 for 500 cycles, showing over 96 % utilization of the theoretical capacity of δ-MnO2. Even at 3 A g-1, it also delivered a 63 % utilization and 64 % capacity retention after 1000 cycles. This study introduces a highly efficient cathode material based on manganese oxide and a comprehensive analysis of its structural dynamics. These findings have the potential to improve energy storage capabilities in ZIBs significantly.

Spectroscopic Monitoring of the Electrode Process of MnO2@rGO Nanospheres and Its Application in High-Performance Flexible Micro-Supercapacitors.

Structural instability is a major obstacle to realizing the high performance of a MnO2-based pseudocapacitor material. Understanding its structure transformation in the process of electrochemical reaction, therefore, plays an important role in the efficient enhancement of rate capacity and stability. Herein, a stable MnO2@rGO core-shell nanosphere is first synthesized by a liquid-liquid interface deposition further combined with the electrostatic self-assembly method. The structural transformation process of the MnO2@rGO electrode is monitored by ex situ Raman and X-ray diffraction spectroscopy during the charging-discharging process. It is found in the first discharging process that layered-MnO2 transforms into the spinel-Mn3O4 phase with K+ ion intercalation. From the second charging, the spinel-Mn3O4 phase is gradually adjusted to a more stable λ-MnO2 with a three-dimensional tunnel structure, finally realizing the reversible intercalation/deintercalation of K+ ions in the λ-MnO2 tunnel structure during subsequent cycling, which can be attributed to the presence of oxygen vacancies formed by the lengthening of the Mn-O bond and losing oxygen in the MnO6 octahedral unit with K+ ion intercalation/deintercalation. Meanwhile, the MnO2@rGO electrode demonstrates a high specific capacitance of 378 F g-1 at 1 A g-1 and excellent cycling stability with a capacitance retention of up to 89.5% after 10 000 cycles at 10 A g-1. Furthermore, the assembled symmetric micro-supercapacitor delivers a high areal energy density of 1.01 µWh cm-2, superior cycling stability with no significant capacity decay after 8700 cycles, and a capacity retention rate of almost 100% after 2000 bending cycles, showing great mechanical flexibility and practicability.