Zhao Jinghao, Ma Zhipeng, Qiao Chunting, Fan Yuqian, Qin Xiujuan, Shao Guangjie
Hebei Key Laboratory of Applied Chemistry, Hebei Key Laboratory of Heavy Metal Deep-Remediation in Water and Resource Reuse, College of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, China.
State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China.
ACS Appl Mater Interfaces. 2022 Aug 3;14(30):34686-34696. doi: 10.1021/acsami.2c06850. Epub 2022 Jul 25.
Structural instability is a major obstacle to realizing the high performance of a MnO-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 MnO@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 MnO@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-MnO transforms into the spinel-MnO phase with K ion intercalation. From the second charging, the spinel-MnO phase is gradually adjusted to a more stable λ-MnO with a three-dimensional tunnel structure, finally realizing the reversible intercalation/deintercalation of K ions in the λ-MnO 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 MnO octahedral unit with K ion intercalation/deintercalation. Meanwhile, the MnO@rGO electrode demonstrates a high specific capacitance of 378 F g at 1 A g and excellent cycling stability with a capacitance retention of up to 89.5% after 10 000 cycles at 10 A g. Furthermore, the assembled symmetric micro-supercapacitor delivers a high areal energy density of 1.01 μWh cm, 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.
结构不稳定性是实现MnO基赝电容器材料高性能的主要障碍。因此，了解其在电化学反应过程中的结构转变，对于有效提高倍率性能和稳定性具有重要作用。在此，首先通过液-液界面沉积并结合静电自组装法合成了稳定的MnO@rGO核壳纳米球。采用非原位拉曼光谱和X射线衍射光谱对MnO@rGO电极在充放电过程中的结构转变过程进行了监测。发现在首次放电过程中，层状MnO随着K离子的嵌入转变为尖晶石MnO相。从第二次充电开始，尖晶石MnO相逐渐调整为具有三维隧道结构的更稳定的λ-MnO，最终在随后的循环中实现了K离子在λ-MnO隧道结构中的可逆嵌入/脱嵌，这可归因于随着K离子的嵌入/脱嵌，MnO八面体单元中Mn-O键的延长和氧的损失形成了氧空位。同时，MnO@rGO电极在1 A g时表现出378 F g的高比电容，以及优异的循环稳定性，在10 A g下循环10000次后电容保持率高达89.5%。此外，组装的对称微型超级电容器具有1.01 μWh cm的高面积能量密度、优异的循环稳定性，在8700次循环后无明显容量衰减，在2000次弯曲循环后容量保持率几乎为100%，显示出极大的机械柔韧性和实用性。
