【作者】 范佳鑫；

【导师】 袁文辉；

【作者基本信息】 华南理工大学 ， 化学工程， 2019， 博士

【摘要】 为解决当前社会所面临环境污染和能源短缺问题,推动社会的可持续发展,必须加快可再生能源的利用和大规模储能技术的开发。电化学储能特别是锂离子电池在便携式移动电子设备和电动汽车得到了快速发展,但锂资源短缺、分布不均、价格较高和安全隐患等问题限制了锂离子电池的大规模应用。因此,研发具有低成本和良好性能新型电池体系是当前能源科学领域的热点课题。基于上述研究背景,本文致力于探索一种廉价、安全、环保和性能良好的双离子二次电池储能体系。具体研究内容如下:（1）构建了基于纯离子液体电解质的双石墨电池。采用人造复合石墨同时作为电池的正负极电极材料,纯离子液体作为电池的电解质。通过比较不同的离子液体电解质体系,得到如下实验结果:含六氟磷酸根的离子液体（BMImPF₆）具有较高的粘度而导致电池的极化严重,放电比容量较低;四氟硼酸类离子液体（BMImBF₄）有较低的粘度,但电池放电平台较低,循环性能较差;三氟甲磺酸类离子液体（EMImTfO）电池展现出较高的放电平台和较好的循环性能,但该离子液体的电化学窗口相对较窄;双（三氟甲烷磺酰）亚胺（EMImTFSI）电池的放电容量和循环性能都较为优异。同时,对比了三种双（三氟甲烷磺酰）亚胺离子液体,研究表明PP₁₄TFSI双石墨电池展现出更良好的电化学性能:在30 mA/g的电流密度下,电池的放电平台介于3.3-4.3 V之间,放电比容量为82.0 mAh/g,能量密度为256 Wh/kg;在300 mA/g的电流密度下,电池循环600次后几乎没有容量的衰减,展现出较高的循环稳定性。然而,由于离子液体中PP⁺₁₄和TFSI^-均具有较大的离子尺寸,所以嵌入石墨后会导致石墨的膨胀。特别由于PP⁺₁₄离子具有更大的尺寸和不规则的三维环状立体构型,以至于负极石墨的膨胀更加严重,甚至出现石墨颗粒的脱落。（2）为改善纯离子液体电解质中石墨负极的体积膨胀问题,构建了以二硫化钼（MoS₂）为负极材料和天然石墨为正极材料的双离子电池体系。MoS₂具有类似于石墨的多层堆叠结构和高结晶度,层间距为0.62 nm,大于石墨的0.355 nm。采用水热法和高温热解法制备了两种类型的MoS₂负极材料:水热法制备的MoS₂呈花瓣状结构,片层薄且柔软,出现团聚现象;高温热解法制备的MoS₂呈片状结构,分散均匀,具有较强的（002）XRD峰,表明MoS₂具有高度堆叠S-Mo-S层面构型。恒电流充放电、Raman和SEM测试表明,花瓣状MoS₂的放电平台、放电容量和循环稳定性均不如高温热解片状MoS₂。片状MoS₂/EMImTFSI/石墨双离子电池在0.5-3.6 V电压窗口和50 mA/g的充放电电流条件下,表现出两个较高放电平台介于3.2-3.3 V和2.2-3.0 V,最大放电比容量为77.0 mAh/g,对应的能量密度为176 Wh/kg。在400 mA/g的电流下循环300次后,电池表现出良好的稳定性,具有较高的容量保持率84.3%和较高的库伦效率96.1%。MoS₂层间距大于石墨,可以有效容纳充放电时大尺寸EMIm⁺离子可逆嵌脱,且MoS₂独特的“三明治”状层结构可以提供足够的活性位点和离子嵌入空间,不仅能够实现快速的离子扩散,还能承受充放电过程的机械应力和体积变化,从而使其展现出良好的电化学性能和结构稳定性。（3）为降低双离子电池的自放电率和提高电池的放电比容量,构建了基于铁氰化铁FeFe（CN）₆负极材料和NaTFSI/EMImTFSI（1.0 mol/L）电解质的双离子电池。采用简单的液相法制备了FeFe（CN）₆纳米颗粒,该材料具有立方框架结构,低自旋的Fe1原子与六个碳原子相连,高自旋的Fe2原子与六个氮原子结合。循环伏安（CV）和恒电流充放电测试表明电解质中Na⁺能够可逆嵌脱于FeFe（CN）₆晶体中,而离子液体EMImTFSI的阳离子EMIm⁺不会嵌入到FeFe（CN）₆中。FeFe（CN）₆-石墨双离子电池在0.1-2.0 V的充放电电压区间的最大放电比容量为93.0 mAh/g,高于前面所述的的双离子电池系统,该容量所对应的Na⁺嵌脱FeFe（CN）₆的化学计量为Na_1.08FeFe（CN）₆。在0.4 mA/cm²电流密度下,电池展现出极高的99.9%库伦效率;即使在较低的0.05 mA/cm²电流下,电池仍然具有较高的98.5%库伦效率,表明该电池体系具有较低的自放电率。自放电测试中,静置110 h电池的容量保持率为63.6%,平均自放电率为0.32%/h,小于前面所述的两种双离子电池体系,这主要和Na⁺与FeFe（CN）₆晶体具有较高的结合力有关,从而降低了电池的自放电速率。（4）为进一步提高电池的放电比容量和减轻负极的膨胀,构建了能可逆沉积电解质中金属阳离子的金属负极-石墨双离子电池。首先研究了锌-石墨双离子系统,三电极循环伏安（CV）测试表明Zn²⁺能够从0.2 mol/L的Zn（TfO）₂/EMImTfO中可逆沉积到铜电极上;同时,TfO^-能可逆嵌入到石墨电极中。锌-石墨电池展现出平均电压为2.0 V的放电平台,高于工作电压为1.5 V的水系锌电池;放电比容量为33.7 mAh/g,所对应的能量密度为65.1 Wh/kg。为进一步提高电池的放电比容量,探讨了基于锡负极的钠双离子电池,该电池展现出较高4.5-3.5 V的放电平台,电池的放电比容为99.0 mAh/g,100次循环后的容量保持性能比锌-石墨电池高。锌-石墨电池33.7 mAh/g的放电比容量所对应的TfO^-嵌石墨的化学计量为C₆₆TfO,锡-石墨电池99.0 mAh/g比容量所对应的TFSI^-嵌石墨的计量为C_22.5TFSI。虽然TfO^-含有原子数比TFSI^-少,但石墨却具有更高的嵌TFSI^-的化学计量,这一方面与TfO^-离子的氧化电位较低有关,高电压会导致TfO^-的分解;另一方面与离子的空间立体构型和嵌入石墨后离子间的斥力有关。SEM测试表明,多次循环后,两种电池金属负极上沉积物分布较为均匀,均没有发生枝晶现象。更多还原

【Abstract】 In order to solve the current problems of environmental pollution and energy shortage,and promote the sustainable development of society,it is necessary to accelerate the utilization of renewable energy and promote the development of large-scale energy storage technology.Electrochemical energy storage systems,especially lithium-ion batteries,have been rapidly developed in portable electronic devices and electric vehicles.However,the shortage of lithium resources,uneven distribution,high prices and potential safety hazards have limited the large-scale applications of lithium-ion batteries.Therefore,the development of new battery systems with low-cost and good performance is a hot topic in the field of energy science research.Based on the above research background,this paper aims to explore cheap,safe,environmentally friendly and well-performing secondary dual-ion battery systems.The specific research contents are as follows:（1）A dual-graphite battery based on pure ionic liquid electrolyte was constructed.The composite graphite was used as the positive and negative electrodes,and pure ionic liquids were used as the electrolyte.By comparing dual-ion batteries with different ionic liquid electrolytes,the following experimental results are obtained:hexafluorophosphate-containing ionic liquid（BMImPF6）has a higher viscosity,which results in serious polarization of the battery and lower specific discharge capacity;tetrafluoroborate-based ionic liquid（BMImBF4）has a lower viscosity,but the battery discharge platform is lower and the cycling performance is limited;the trifluoromethanesulfonate ionic liquid（EMImTfO）battery exhibits a higher discharge platform and better cycle performance,but the electrochemical window of the liquid is relatively narrow;the bis（trifluoromethanesulfonyl）imide（EMImTFSI）battery has good discharge capacity and cycling performance.Furthermore,three kinds of bis（trifluoromethanesulfonyl）imide ionic liquids were compared.The results show that the PP14TFSI dual-graphite battery exhibits even better electrochemical performance:at a current density of 30 mA/g,the discharge platform is between 3.3 and 4.3 V,the discharge specific capacity is 82.0 mAh/g and the energy density is 256 Wh/kg.At a current density of 300mA/g,the battery exhibits high cycling stability with almost no capacity attenuation after 600cycles.However,since both PP14⁺and TFSI^-in the ionic liquid have a large ion size,the graphite is expanded due to the intercalation reactions.In particular,as the PP14⁺ion has a three-dimensional（3D）annular stereo configuration and even larger size,the expansion of the negative electrode graphite is even more serious,resulting in the exfoliation of the graphite particles.（2）In order to improve the volume expansion of graphite anode in pure ionic liquid electrolyte,a dual-ion battery system with molybdenum disulfide（MoS2）anode material was constructed.MoS2 has a graphite-like multilayer stack structure and high crystallinity with a layer spacing of 0.62 nm,which is larger than 0.355 nm of graphite.Two types of MoS2anode materials were prepared by hydrothermal method and high temperature pyrolysis method,respectively.The hydrothermal-MoS2 is flowerlike,with thin and soft petals,and agglomeration occurs.The pyrolysis-MoS2 has a flaky structure,with uniform particle dispersion.The strong（002）peak tested by XRD indicates highly stacked S-Mo-S layers.Galvanostatic charge-discharge,Raman and SEM tests show that the flake-MoS2 has even better discharge platform,specific capacity and cycling stability than those of the flowerlike-MoS2.At a voltage window of 0.5-3.6 V and a charge-discharge current of 50mA/g,the flake-MoS2/EMImTFSI/graphite dual-ion battery exhibits two higher discharge platforms between 3.2-3.3 V and 2.2-3.0 V,with a specific discharge capacity of 77.0 mAh/g and a corresponding energy density of 176 Wh/kg.At 400 mA/g,the battery exhibits good stability of 300 cycles,with a high capacity retention of 84.3%and a high coulombic efficiency of 96.1%.MoS2 has a larger interlayer spacing than that of graphite,which can effectively accommodate the reversible insertion/extraction of large EMIm⁺cations during charge-discharge.Meanwhile,the unique"sandwich"layer structure of MoS2 can provide sufficient active sites and intercalation spaces,which can not only achieve rapid ion diffusion,but also withstand the mechanical stress and volume change during the charge-discharge process,ensuring good electrochemical performance and high structural stability.（3）In order to reduce the battery self-discharge rate and increase the discharge specific capacity,a dual-ion battery with an iron ferricyanide FeFe（CN）6 anode and a NaTFSI/EMImTFSI（1.0 mol/L）electrolyte was constructed.Prepared by a simple liquid reaction method,the FeFe（CN）6 nanoparticles have a cubic frame structure,in which the low-spin Fe1 atoms are attached to six carbon atoms,while the high-spin Fe2 atoms are attached to six nitrogen atoms.Cyclic voltammetry（CV）and constant current charge-discharge tests show that Na⁺in electrolyte can reversibly insert into FeFe（CN）6crystal,while EMIm⁺in EMImTFSI ionic liquid cannot intercalate in FeFe（CN）6.The FeFe（CN）6-graphite dual-ion battery has a maximum discharge specific capacity of 93.0mAh/g between 0.1-2.0 V,which is higher than the dual ion batteries described above.The stoichiometry of Na⁺-embedded FeFe（CN）6 corresponding to 93.0 mAh/g capacity is Na1.08FeFe（CN）6.At a current of 0.4 mA/cm²,the battery exhibits an extremely high 99.9%Coulombic efficiency.Even at a lower current of 0.05 mA/cm²,the battery still has a higher98.5%Coulombic efficiency,indicating its lower self-discharge rate.In the self-discharge tests,after resting for 110 h,the capacity retention is 63.6%,and the average self-discharge rate is as low as 0.32%/h,which is mainly related to the high binding force between Na⁺and FeFe（CN）6 crystals during the intercalation process,thus reducing the self-discharge rate of the battery.（4）In order to further increase the specific discharge capacity and reduce the negative electrode expansion in dual-ion batteries,metal negative electrodes were introduced,which involve reversible deposition of metal cations on anode from the electrolyte.Firstly,the zinc-graphite dual-ion battery was studied.Three-electrode cyclic voltammetry（CV）tests show that Zn²⁺can be reversibly deposited from the 0.2 mol/L Zn（TfO）2/EMImTfO electrolyte,and the TfO^-can be reversibly embedded into the graphite electrode.The zinc-graphite dual-ion battery exhibits a medium discharge platform of 2.0 V and a specific discharge capacity of 33.7 mAh/g.To further improve the discharge capacity,the sodium based dual-ion battery with tin anode was explored,which exhibits a higher discharge platform of 4.5-3.5 V,with a high discharge capacity of 99.0 mAh/g.The capacity retention after 100 cycles is better than that of the zinc-graphite battery.The stoichiometry of TfO-graphite compound corresponding to 34 mAh/g capacity is C66TfO,while that of TFSI-graphite composition to 99.0 mAh/g is C22.5TFSI.Although TfO^-contains fewer atoms than TFSI^-,graphite has a higher amount of TFSI^-intercalation,which can be related to the lower oxidation stability of TfO^-,leading to decomposition reaction under high voltage.On the other hand,it can be assigned to the stereo 3D structure of the two ions,as well as the repulsive forces between the ions and graphite cathodes.The SEM tests show that after repeated cycles,the deposits on the metal negative electrodes are evenly distributed,without the hint of dendrites,indicating a feasible process.更多还原