Author: Li, Ying
Title: Mn-based prussian blue cathode materials for sodium-ion batteries
Advisors: Lam, Kwok-ho (EE)
Cheng, Ka-wei Eric (EE)
Degree: Ph.D.
Year: 2022
Subject: Sodium ion batteries
Sodium ion batteries -- Materials
Hong Kong Polytechnic University -- Dissertations
Department: Department of Electrical Engineering
Pages: xvii, 166 pages : color illustrations
Language: English
Abstract: Prussian blue and its analogues (PBAs) have been widely investigated as the prominent cathode candidates for sodium-ion batteries (SIBs) due to its stable 3D metal-organic-frameworks and large interstitial sites for Na⁺ diffusion. Among the different transition metal-based PBAs, the Mn-based hexacyanides (MnHCF) are more competitive due to the high redox potential of Mn2⁺/Mn3⁺ (3.5 V) and large theoretical capacity (170 mA h g-1). Unfortunately, the conventional synthetic method always lead to large amounts of Fe(CN)6 vacancies and interstitial and coordinated water in the crystal structure of MnHCF materials. As a result, the relatively low actual capacity and poor cycling stability contributed by the structural instability are still problematic for this kind of materials.
In this thesis, the modified coprecipitation method was adopted to prepare the MnHCF materials. It is well known that the synthesis conditions have a great influence on the microstructure of the target material. Consequently, the influence of reactant concentration and aging time on the microstructures and electrochemical performance of MnHCF was firstly investigated systematically. The sample prepared with the optimal reactant concentration (10 mM for Mn²⁺) and aging time (24 h) exhibited the best electrochemical performance with a large initial specific discharge capacity of 140.3 mA h g-1 and a high capacity retention of 80.61% after 100 cycles at a current density of 20 mA g-1. This should be ascribed to the regular cubic morphology of particles with the optimal size (1 µm) and less defects. Furthermore, the better structure stability was found for K-MnHCF than Na-MnHCF at the same synthesis conditions. The synergistic effect of K⁺ and Na⁺ in the structure contributed to the improvement of the electrochemical performance. However, the cycling performance of the hybrid KNa-MnHCF was still unsatisfied for practical application even though better than the pure Na-MnHCF. Then the carbon nanotube (CNT) was implied to further slow down the nucleation of KNa-MnHCF and promote homogeneous and near-stoichiometric crystal growth. The well-designed hybrid KNa-MnFe(CN)6@CNT electrode showed the excellent cycling stability in the hybrid SIBs, yielding a very high initial discharge capacity of 164.5 mA h g-1 with high initial coulombic efficiency (ICE) of 96.3% and high capacity retention of 82.0% after 100 cycles at 20 mA g-1. The good structural reversibility of the KNa-MnHCF@CNT electrode was confirmed during the extraction/insertion process of alkali metal ions. In addition, the Cu²⁺ was adopted to modify the Na-MnHCF by regulating the microstructures. The as-prepared MnCuHCF composite exhibited uniform element distribution in either nanoscale or microscale which largely contributed to the improved electrochemical performance. Especially, a very high capacity retention of 86.38% after 100 cycles at 20 mA g-1 was obtained for the MnCuHCF composite benefited from the improved electrochemical kinetics due to the synergistic effect of the MnHCF and CuHCF phases. It is the first study about the Cu²⁺ modified MnHCF materials, exhibiting a great influence on the structural modification of the Na-MnHCF materials.
In summary, this thesis provides a very valuable and practical guidance for the synthesis of high-performance MnHCF cathode material for SIBs.
Rights: All rights reserved
Access: open access

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