【作者】 王晓；

【导师】 贾玉玺； 熊胜林；

【作者基本信息】 山东大学 ， 材料学， 2021， 博士

【摘要】 能源危机和环境污染已发展成为制约全球化进程的主要因素,为了保护人类的共同家园,发展并合理利用可持续的清洁能源如风能、太阳能、潮汐能等以及建立大规模的储能体系已在全球达成了共识。作为电化学储能体系的代表,锂离子电池在过去的几十年里取得了巨大的成功,完全地改变了我们的生活方式,目前已被广泛地应用于我们的日常生活中,如手机、便携式电脑等。然而,锂和钴资源的日益耗竭使得锂离子电池的成本逐年上升,加上锂离子电池使用易燃的有机电解液存在着大的安全隐患,使其远远不能满足随之而来的人们对于大规模电化学储能设备爆发性增长的需求特别是考虑到近年来电动汽车市场的急剧膨胀。因此,发展新型的安全环保低成本高性能的先进电池技术已成为广大科研工作者的研究目标。近年来,水系锌离子电池凭借着高安全性和令人印象深刻的电化学性能迅速受到了极大的关注,得益于锌金属的低的氧化还原电势(-0.76Vvs.标准氢电极)和在水中高的电化学稳定性,其可以直接采用锌金属作为负极,水系溶液作为电解液,嵌锌材料作为正极。尽管多种材料已被报道可以用于水系锌离子电池正极,但是仍存在着不少问题如复杂的储锌机制、正极材料的溶解、低的能量密度和短的循环寿命,这些仍然是制约水系锌离子电池大规模应用的关键因素。水系锌离子电池的发展还处于初期阶段,正极嵌锌材料的选择决定着该电池技术的比容量和能量密度。因此,对正极材料的结构优化和新材料的继续探索仍然是当前阶段需要重点关注的目标。在诸多正极材料中,钒基氧化物表现出令人印象深刻的比容量和循环寿命,但是低的能量密度和钒溶解限制了它的继续发展。为此,我们聚焦于新型钒基正极材料的探索,以提高能量密度和循环稳定性为目标,揭示储锌机制及钒溶解的本质,为钒基正极材料的发展提供实验支持和理论依据。本论文的主要内容包括:(1)采用温和的一步水热法制备得到水合铵离子稳定的V3O8型层状(NH4)2V6O16·1.5H2O单晶纳米带并将其用于水系锌离子电池正极材料,其中共边的VO5四面锥和VO6八面体共角连接并沿着z轴排列形成了典型的V3O8层状结构,层间NH4+的存在起到了层间支柱的作用,稳定了层状结构保证了电池的长循环稳定性;层间水分子通过起到电荷屏蔽的作用加速了Zn2+的扩散动力学。用其组装成的Zn电池展现出了优异的电化学性能,0.1 A g-1的电流密度下展现出了高达479 mA h g-1的放电比容量和371.5 W h kg-1的能量密度、以及长的循环寿命(5 A g-1的电流密度下经过3000圈循环后放电比容量为152 mA h g-1)。电化学动力学分析发现该正极存在着较大的赝电容电荷存储和高的固相扩散系数,这得益于大的层间距和带状形貌提供的快速离子迁移和电子传输能力。通过对储锌机制的探究得知,该正极材料展现出了典型的H+/Zn2+共嵌入机制,同时伴随着一个新的Zn3(OH)2V2O7·2H2O相的可逆出现与消失,展现出了高的可逆性。(2)在第一个工作基础上,想知道改变活性材料的形貌和晶体结构是否会对储锌性能及机制产生较大的影响。于是,进一步地通过简单的水热合成方法合成了一种层层堆垛状的水合铵离子稳定的V4O9型(NH4)2V4O9-0.5H2O纳米片组装体并用作水系锌离子电池正极材料,高达8.98 A的层间距为Zn2+的脱嵌提供了足够的传输通道,层间NH4+的存在稳定了层状结构。层层堆垛状的纳米片提供了足够的活性位点,使得该正极可以输出超高的倍率性能(15 A g-1电流密度下放电比容量可达101 mAh g-1),高的放电比容量374.3 mAh g-1(0.2 A g-1电流密度下)以及优异的长循环性能(1 A g-电流密度下循环400圈后容量保留率89%,5 Ag-1电流密度下循环1000圈后容量保留率84%),这得益于大的赝电容电荷存储和快速的反应动力学。通过对储锌机制的研究发现了放电过程中Zn4SO4(OH)6-xH2O的出现,证明了 H+/Zn2+的共嵌入机制。此外,该电池可以输出309 Wh kg-1的能量密度和9324 W kg-1的功率密度,展现出了良好的应用潜力。(3)水系电解液中钒的溶解问题既降低了活性材料的利用率,又严重损害了电池的循环稳定性。针对这一问题,通过调整钡前驱体的量采用一步水热合成的方法可控地得到了三种钒酸钡,Ba1.2V6O16-3H2O、BaV6O16-3H2O和BaxV2O5·nH2O,这三种活性材料均展现出了可观的储锌能力,但是却表现出了相差较大的电化学性能。得益于稳定的层状结构,层状的Ba1.2V6O16·3H2O纳米带展现出了最好的倍率性能(108.8mAhg-1@10Ag-1)和循环稳定性(2000圈循环后容量保留率为95.6%)。通过三种材料储锌机制及静态溶解实验的对比发现,Ba1.2V6O16·3H2O电极由于其本征稳定的层状结构有效地抑制了正极材料的溶解,同时减少了 Zn4SO4(OH)6·xH2O副产物的生成,从而实现了快速而又稳定的锌离子反应动力学。该工作揭示了不同结构构筑成的钒基化合物在作为水系锌离子电池正极材料时在钒溶解上面之间的区别。(4)在前面的工作基础上,合成了一种V2O6型本征结构稳定的层状MgV2O6·1.7H2O纳米带,并将其用于水系锌离子电池正极材料。该电极表现出了高的可逆容量425.7 mAh g-1(0.2 A g-1),优异的倍率性能(10 A g-1电流密度下放电比容量为182.1 mAh g-1)和超稳定的循环(4 A g-1电流密度下循环1500圈后没有衰减)。通过对储锌机制的研究发现该材料虽然基于典型的H+/Zn2+共嵌入机制,但展现出了一种非典型的Zn2+嵌入化学,在首圈放电过程中存在着部分且不可逆的Mg2+-Zn2+交换反应。得益于这种特殊的电荷存储机制和稳定的层状结构,该正极展现出高达88.4%的赝电容电荷存储(0.6 mV s-1扫描速度下),有助于快速和稳定的离子迁移和电子传输。理论计算证明了 Zn2+在MgV2O61.7H2O中具有更小的扩散能垒,从而实现了 Zn2+的快速反应动力学。该工作可以为更好地理解钒基氧化物的储锌机制提供了新的视角。更多还原

【Abstract】 The energy crisis and environmental pollution have become a leading factor that restricts the process of globalization.A consensus has been reached by developing and taking full advantage of those sustainable and clean energy resources such as wind,solar,and tidal power,as well as establishing grid scale energy storage systems to protect the commn home of mankind.We have witnessed an enormous success in the applications of lithium-ion batteries that serve as a typical representative of electrochemical storage technique in the last few decades,which have completely changed our lifestyle by powering mobile and portable computers.However,scarce lithium and cobalt sources and increasing cost,as well as the severe security risks stemming from the flammable nature of the organic electrolytes,have become a sort of disadvantages that should not be neglected,making them uncompetitive to cope with the upcoming challenges for the demand of scalable energy devices especially the booming electric vehicle propulsion.Accordingly,advanced battery techniques with high safety,environmental benigh,low cost,and high performance have developed to be the subject of intensive investigations.Aqueous zinc-ion batteries(AZIBs)have drawn extensive attention in the last few years owing to their high safety and impressive electrochemical performance,benefiting from the low redox potential(-0.76 V vs.standard hydrogen electrode)and excellent electrochemical stability in water of Zn metal that make it become few of the candidates that could be able to directly serve as a safe anode in a metal-ion battery.In addition to a Zn metal anode,this technology is composed of aqueous electrolyte and a cathode material capable of reversible Zn2+insertion/extraction.Though massive efforts have been dedicated to the explorations of cathode materials,the scalable applications of AZIBs are still plagued by some intractable issues such as elusive charge storage mechanism,cathode dissolution,inadequate energy density and undesirable life span.The structural optimization and continuous exploition of cathode materials should be focused on since it can largely determine the specific capacity and energy density of the cell,particularly considering that the development of this technology is still in the incipient stage.Among various candidates,vanadium-based oxide cathodes deliver impressive capacities and relatively high cycle stability,but their development are still troubled in the limited energy density and cycle life.Herein,we focus on the exploration of novel vanadium-based cathode materials aimed at achieving high energy density and extended cycle life and provide important experimental assistance and theoretical basis by unraveling their respective charge storage mechanisms as well as the origins that cause the vanadium dissolution.The main contents are as follows:(1)Layered(NH4)2V6O16·1.5H2O single crystal nanobelts with V3O8-type construction stabled by hydrated ammonium ions were synthesized via a facile one-step hydrothermal method and served as a cathode material for AZIBs.In the constrction,the edge-sharing VO5 square pyramids and VO6 octahedral chains construct the V3O8 layers along the z axis by sharing their corners.NH4+ in the interstitial sites stabilize the layered structure by acting as the "pillars",thereby ensuring long-term cycling stability,while the structural water molecules facilitate the Zn2+diffusion kinetics by serving as the charge screening.The assembled Zn cell delivers a superior reversible specific capacity of 479.4 mA h g-1 at 0.1 A g-1 with a desirable energy density of 371.5 W h kg 1 and exhibits an impressive cycling stability of more than 3000 cycles(152.1 mA h g-1 retained after 3000 cycles at 5 A g-1).The electrochemical kinetics analyses reveal that this active material exhibits large pseudocapacitance charge storage behavior and Zn2+solid-state diffusion coefficient because of the fast ion migration and electron transfer provided by the expanded interlayer distance and nanobelt morphology.The investigation of charge storage mechanism reveals that the electrode exhibits highly reversible H+/Zn2+co-insertion behavior,companied by formation and disapprearance of a new phase Zn3(OH)2V2O7·2H2O during the discharge and charge process.(2)Based on the above results,I wonder whether the morphology and crystal structures of vanadium-based oxide cathode materials have a large effect on the electrochemical performance and Zn-ion strorage mechanism.Herein,a layer-by-layer stacked(NH4)2V4O9·0.5H2O nanosheet assemblies with V4O9-type construction stabled by hydrated ammonium ions were engineered and prepared by a mild hydrothermal protocol.A considerable interlayer spacing of 8.98 A favors the ingress/egress of Zn2+and the pillared hydrate NH4+stabilizes the layered structure.Stacked nanosheets provide abundant active sites,which enable them to deliver high rate capability(101 mA h g-1 at 15 A g-1)and long-term stability(89%and 84%capacity retention after 400 cycles at 1 A g-1 and 1000 cycles at 5 A g-1,respectively),befeniting from dominated pseudocapacitance charge storage and facialiated redox reaction kinetics.The investigation of charge storage mechanism reveals that the electrode exhibits highly reversible H+/Zn2+co-insertion behavior companied by formation and disapprearance of a new phase Zn4SO4(OH)6·xH2O during the discharge and charge process.In addition,the Zn cell could power a high energy density of 309 Wh kg-1 and a power density of 9324 W kg-1,pushing the potential in practical application to a high level.(3)The vanadium dissolution in vanadium-based compounds when subjected to an aqueous electrolyte environment not only reduces the utilization of active materials but also lowers the cycle stability.To address this issue,we reported several barium vanadate nanobelt cathodes constructed of two sorts of architectures,i.e.,Ba1.2V6O16·3H2O and BaV6O16·3H2O(V3O8-type)and BaxV2O5·nH2O(V2O5-type),which are controllably synthesized by tuning the amount of barium precursor.The three active materials efficiently achieve reversible zinc storage but yet exhibit remarkable differences in performance.The Ba1.2V6O16·3H2O electrode exhibits deliver superior rate capability(108.8 mAh g-1 at 10 A g-1)and long-term cyclability(95.6%capacity retention over 2000 cycles)as a typical consequence of fast and stable zinc-ion kinetics provided by the intrinsically roust layered architecture,which could efficiently suppress the cathode dissolution as well as greatly eliminate the generation of byproduct Zn4SO4(OH)6·xH2O during cycling.This work reveals the difference between the vanadium oxides constructed with different crystal structures and the vanadium dissolution behaviors in AZIBs.(4)According to previous results,another vanadium-based oxide cathode based on MgV2O6·1.7H2O nanobelts with V2O6-type and intrinsically robust constructions,which delivers a high capacity(425.7 mAh g-1 at 0.2 A g-1),a robust rate capability(182.1 mAh g-1 at 10 A g-1),and an ultra-stable cycle up to 1500 cycles without any visiable deterioration,as well as an adequate energy density(331.6 Wh kg-1),was developed.Such excellent electrochemical Zn-ion storage performance is believed to result from the fast ion migration and electron transfer boosted by a stable layered structure and an ultra-high intercalation pseudocapacitance reaction(88.4%capacity contribution ratio at 0.6 mV s-1),which are also benefited by a typical H+/Zn2+co-insertion mechanism,accompanied by an atypical Zn2+ intercalation chemistry with a partial but irreversible Mg2+-Zn2+ion-exchange reaction during the initial discharge.DFT calculations unravel that a much lower Zn2+diffusion energy barrier could be realized in the MgV2O6·1.7H2O crystal structure,thereby leading to the fast reaction kinetics.This work could provide new insights to better understand the charge storage mechanism in vanadium-based oxide cathodes.更多还原