Key Takeaways & Executive Findings
- •• Simple hydrothermal synthesis achieves controllable phase transition of MnO2 from β to δ via Al3+ doping. • Al3+ doping reduces crystallinity and increases specific surface area, providing more active sites for electrochemical reactions. • The optimized MnO2-3 electrode delivers high specific capacitance and rate performance; an asymmetric supercapacitor reaches 18.4 W·h/kg energy density and retains 90% capacitance after 20,000 cycles. • The study provides practical guidance for designing high-performance, low-cost MnO2-based supercapacitor electrodes.
Abstract
Al-doped manganese dioxide (MnO2) was synthesized by simple hydrothermal method, and a controllable phase transition of the MnO2 crystal phase from β to δ was achieved. The effects of Al doping concentration on the structure and electrochemical properties of electrode materials were studied in detail. The results show that the controlled synthesis requires a synergy between KMnO4, MnCl2 and AlCl3, and that Al3+ plays an important role. Compared with the pure phase MnO2, the crystallinity of Al-doped MnO2 decreases and the specific surface area increases, which provides more active sites for the electrode material. When 3 mmol Al3+ is added, the prepared MnO2-3 has the largest specific capacitance and the highest rate performance. The energy density of the asymmetric supercapacitor (ASC) with MnO2-3 as the positive electrode and activated carbon (AC) as the negative electrode can reach 18.4 W·h/kg at the power density of 400 W/kg, and the capacity can maintain 90% of the initial value after 20000 cycles, indicating that Al-doped MnO2 has certain practical application value. This study provides favorable guidance for MnO2 as a high performance electrode material.
1. Introduction
Supercapacitor is a kind of energy storage device between conventional physical capacitor and secondary battery. Due to its advantages of high power, large energy and long cycle life, it has broad application prospects in mobile communications, electric vehicles and national defense technology and so on. Electrode material is an important component of supercapacitors and a key factor affecting the production cost and performance of supercapacitors [1 −3]. The traditional electrode material RuO2 has the advantages of high specific capacitance, good conductivity, stability in electrolyte, etc., and can be well applied in supercapacitors [4, 5]. However, Ru has limited resources and high price, which makes it not widely used. Therefore, the research and development of high performance and low cost electrode materials is an important content in the research field of supercapacitors.
MnO2 has the advantages of low cost, environmental protection, easy preparation, etc., and is a promising electrode material for supercapacitors [6]. However, the specific capacitance of MnO2 is low, and improving its electrochemical performance is the focus of the next research. MnO2 has a variety of crystal type, such as α, β, γ and δ, etc [7]. It is generally believed that the crystal structure of MnO2 has a great influence on its performance. Among them, δ-MnO2 has a layered structure that is conducive to ion diffusion, and it has become one of the most potential electrode materials for supercapacitors [8, 9]. At present, hydrothermal method is the most effective way to synthesize MnO2 nanomaterials. However, the hydrothermal synthesis of MnO2 is affected by various factors, such as reaction temperature, time, pH value of solution and reactant concentration. Therefore, the controlled synthesis of MnO2 nanomaterials with different crystal structures and morphologies remains a challenge.
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CHENG Xiao-yang, ZHANG Li-hua, LI Ling-yan, WU Hao, ZHENG Jin-feng, YAO Jia-rong, LI Gui-fang (2025). Controllable phase transition of Al3+ induced manganese dioxide from β- to δ-type and their Al-doped δ-type manganese dioxide for high-performance asymmetric supercapacitors. Journal of Central South University. https://doi.org/10.1007/s11771-025-5996-1
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Frequently Asked Questions
What is the role of Al3+ in the phase transition of MnO2?
Al3+ doping induces a controllable phase transition from β-MnO2 to δ-MnO2 during hydrothermal synthesis. The presence of Al3+ reduces crystallinity and increases the specific surface area, which provides more active sites for electrochemical reactions and enhances supercapacitor performance.
How does Al-doped δ-MnO2 improve supercapacitor performance?
Al-doped δ-MnO2 exhibits lower crystallinity and higher specific surface area compared to pure MnO2. This structural modification facilitates ion diffusion and increases active sites, leading to higher specific capacitance and better rate capability.
What are the key electrochemical results of the MnO2-3 electrode?
The MnO2-3 electrode (with 3 mmol Al3+) shows the largest specific capacitance and highest rate performance among the tested samples. It was used as the positive electrode in an asymmetric supercapacitor, achieving an energy density of 18.4 W·h/kg at a power density of 400 W/kg.
What is the cycling stability of the asymmetric supercapacitor?
The asymmetric supercapacitor retains 90% of its initial capacitance after 20,000 charge-discharge cycles, indicating excellent long-term cycling stability and practical application potential.
What synthesis method was used for Al-doped MnO2?
A simple hydrothermal method was used, combining KMnO4, MnCl2, and AlCl3. The controlled synthesis relies on the synergy of these precursors, with Al3+ playing a key role in directing the phase transition and structural properties.
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