钾离子电池以资源丰富、成本低、能量密度高等优势在大规模储能装置中具有广阔的应用市场。目前，钾离子电池的能量密度主要受到正极制约，较大半径的K+易诱发传统的正极材料在循环中结构坍塌、循环容量衰减等隐患。因此，本论文以具有稳定结构、高工作电压的普鲁士蓝（PB）正极为研究对象，提出一种自发实现气氛保护的简易合成方案，制备出电化学性能优异的PB正极材料，并探究碳酸酯类电解液对PB正极的性能影响。论文首先设计了一种不借助外部保护气氛的引入、装置内自发形成正压气氛的合成方案。利用柠檬酸和铁粉的反应产物——氢气和柠檬酸亚铁，足量的氢气可防止空气进入反应装置氧化PB粉体；柠檬酸亚铁降低了形核速率，减少晶体结构缺陷。同时发现合成中原料浓度、烘干温度、洗涤剂种类会直接影响样品的形貌和电化学性能。通过优化工艺，制备出结构缺陷少、纳米尺寸、循环性能稳定的K1.62Fe[Fe(CN)6]0.92?0.33H2O材料。该正极的初始容量为120.9 mAh/g，循环100圈容量保持率高达98.2%。X光衍射分析发现K+在样品晶体内的穿梭不会引起结构相变，证实稳定的晶体结构是优异电化学性能的基础。为了提高钾离子电池的库伦效率和循环寿命，本论文系统地分析了以KPF6盐配合不同碳酸酯溶剂的电解液在PB//K电池中的稳定性。其中碳酸丙烯酯（PC）和碳酸二乙酯（DEC）的稳定性最差：PC的分解产物会在K表面形成稳定但离子电导低的电极电解液界面（SEI），增大内部阻抗并降低可逆容量；DEC分解形成的SEI易溶于电解液，导致电池库伦效率低并出现周期波动。添加剂氟代碳酸乙烯酯（FEC）作用在K表面形成稳定的SEI，以减缓界面反应、提高库伦效率，但FEC形成的低离子电导的SEI会增大内部极化并导致容量显著衰减。为了解决上述问题，论文提出添加耐高压金属盐KClO4提高溶剂的抗氧化电位，或使用界面稳定的碳负极替换金属钾，可有效提高钾离子正极的库伦效率和容量保持率。最后为了改善纳米尺寸普鲁士蓝的界面活性，论文在氧化石墨烯（GO）片层间原位合成了高分散、纳米尺寸的PB@GO样品。GO的片层结构有利于原位生长的PB颗粒的分散，均匀分散的PB颗粒与电解液的界面反应活性降低。PB@GO在长循环上具备更好的稳定性，300 mA/g电流下500圈的容量保持率为98.9%。
Potassium-ion batteries (PIBs) are promising for the large-scale energy storage application. PIBs benefit from high energy density and low cost, as result of rich potassium resources in the Earth’s crust. At present, the cathode materials restrict the energy density of the PIBs. The large radius of K+ ion induces structure collapse of traditional cathode materials during cycling processes. In this paper, a facile precipitation method with gas spontaneously protection is proposed to synthesize a Prussian blue (PB) material. The cathode has stable open framework and exhibits an excellent electrochemical performance. Meanwhile, the effect of carbonate-based electrolyte on performance of PB cathode is investigated. First, a facile and low-cost precipitation method is proposed to synthesize the K-rich PB. This method utilizes the reaction between iron powder and citric acid, which generates hydrogen and ferrous citrate. The hydrogen atmosphere achieved protective atmosphere without introduction of external inert atmosphere, thus preventing the as-prepared PB from being oxidized. Meanwhile, the ferrous citrate reduces the nucleation rate, leading to decreased crystal defects. The factors including concentration of solutions, drying temperature and the cleaning solvents are found to affect the morphologies and electrochemical performances of as-prepared samples. Based on the optimized parameters, a nano-sized K1.62Fe[Fe(CN)6]0.92?0.33H2O material with less defects is prepared, which delivers a high capacity of 120.9 mAh/g and a capacity retention of 98.2% after 100 cycles. X-ray diffraction suggests a stable framework with cubic phase maintained during K+ extraction/insertion, which contributes to the superior electrochemical performances of this material.To improve Coulombic efficiency (CE) and lifespan of PIBs, the electrochemical performances of PB//K cells are investigated in different KPF6-based carbonate electrolytes. Especially, the propylene carbonate (PC) and diethyl carbonate (DEC) electrolytes undergo severe decomposition. The decomposition of PC causes the formation of a solid electrolyte interface (SEI) with poor K+ conductivity on K anode, resulting in the increased internal impedance and the capacity decay. SEI derived from DEC decomposition can easily dissolve in electrolyte, leading to a fluctuating and low-value CE. Fluoroethylene carbonate (FEC) is helpful to suppressing side reactions and enhancing CEs of cells, as a result of forming a robust and insoluble SEI on K anode. However, this ionic-conductivity SEI is poor, which deteriorates the capacity. In order to address above issues, two strategies are proposed to improve CE and capacity retention of PB//K half-cells. One is adding the anti-oxidation salt additive KClO4, the other is using carbon anode to replace the highly reactive potassium metal.Finally, in order to improve the interfacial activity of nano-sized PB, a highly dispersed, nano-sized PB@GO composite is synthesized in situ between graphene oxide (GO) sheets. The layered GO suppresses the agglomeration of PB, reducing the interfacial reactivity between PB and electrolyte. Therefore, the PB@GO cathode delivers a long-term cycling stability with a capacity retention of 98.9% over 500 cycles at 300 mA/g.
