【作者】 张旭；

【导师】 张可敏；

【作者基本信息】 上海工程技术大学 ， 材料学， 2021， 硕士

【摘要】 锂离子电池因其优越的高比能量和长循环稳定等特性成功的实现了应用,然而锂资源的有限和锂离子电池整体造价成本昂贵依旧制约着其进一步的拓展。相比而言,镁离子电池安全、成本低,且镁负极枝晶形成倾向低、容量大、资源丰富,被认为是锂离子电池的理想替代品。现如今,镁离子电池的首要研究方向是寻求合适的正极材料和与之匹配的电解液。解决该问题的一种替代策略是构建镁锂双离子电池。镁锂双离子电池利用了金属镁负极和含锂电解液,锂离子理论上可以实现正极材料具有更快的反应动力学,且不影响镁离子的脱嵌与沉积,有助于提供高可逆容量和长循环寿命,因此被广泛研究。在本研究中,我们提出了一种镁锂双离子电池,以镁箔为负极,三维（3-D）花状硫化钴（CoS）为正极,全苯基配合物（APC）、氯化锂盐溶解于四氢呋喃（THF）的生成的APC-x LiCl/THF(x=0-0.8 mol L^-1)溶液为电解液。（1）在电解液氯化锂浓度研究中,镁锂双离子电池的电化学性能随氯化锂浓度增加而提高。镁锂双离子电池在APC-0.8 LiCl/THF电解液中实现最优的电化学性能。电化学动力学研究表明,在初始循环过程中电荷转移电阻的急剧下降,对容量的提高起着重要作用。与镁离子相比,锂离子的扩散速率更快,体积更小,会有更多的锂离子参与正极反应,因此随着氯化锂浓度的增加,电池比容量有所提高。（2）通过使用APC-0.8 LiCl/THF电解液,镁锂双离子电池在0.1 A g^-1循环30次后,容量逐渐增加到高容量507 m Ah g^-1;在0.5 A g^-1循环500次后,可逆容量保持为219 m Ah g^-1。此外,这些电池比容量呈现出先下降,后上升,接着稳定的趋势,这体现了电极材料显著的活化过程。这可以解释为,溶解的Li₂S、Mg S在电解液中逐渐减少,镁离子、锂离子和CoS正极的反应可逆性的增加,以及由于体积膨胀产生的间隙使得更多活性位点暴露,并充分参与氧化还原反应进而促使整个体系活化并趋于稳定。（3）活化后的CoS正极,在1 A g^-1的电流密度下,经过1000次充放电循环后仍可实现318 m Ah g^-1的稳定放电比容量,表现出优异倍率性能和高循环寿命特性。通过赝电容分析,快速的电容式储能和较慢的电池型储能共同贡献了镁锂双离子电池正极的比容量,其中较大比例的电容式储能为正极提供了较为优异的倍率性能。（4）预嵌入锂离子的CoS正极材料,配以APC/THF电解液展示出良好的电化学性能,其首圈充电过程中大量的锂离子脱离CoS正极材料,在随后的循环过程中继而发生转化反应,即整个循环过程中的电化学反应是通过锂离子和镁离子的协同作用实现的。（5）X射线衍射和透射电镜表征表明锂离子和镁离子均可以促进CoS转化为Co₉S₈,进而反应形成Co、Li₂S和Mg S。Co₉S₈作为反应中间体在循环过程中生成,并将其作为镁锂双离子电池正极材料研究发现在其整个循环过程不存在活化现象,同时可实现高的可逆充放电比容量(500 m Ah g^-1),表明在CoS正极反应中自发产生的有利中间相Co₉S₈具有良好的电化学性能。本文所获得的研究结果对于开发镁锂双离子电池金属硫化物正极材料及其电解液具有一定的启发意义。更多还原

【Abstract】 Lithium-ion batteries（LIBs）with high energy density and long-term cycle life have been applied in advanced energy storage equipment.However,limited resources and high cost still restrict the further development of lithium-ion batteries.Magnesium ion batteries（MIBs）are safe,low cost.Moreover,magnesium anode has low dendrite formation tendency,large capacity and abundant resources,which is considered as an ideal substitute for LIBs.Now,the main focus of research on MIBs is to find suitable cathode materials and electrolytes.Fortunately,an alternative strategy to solve the aforementioned issues by constructing magnesium lithium double ion batteries（MLIBs）has been proposed.MLIBs use Mg anode and electrolyte containing Li⁺ions.Theoretically,Li⁺ions can realize faster reaction kinetics of cathode materials and not affect the intercalation and deposition of Mg²⁺ions.But it can help to provide high reversible capacity and long-term cycling,so,MLIBs have been thoroughly studied.In this study,we designed MLIBs using Mg anodes,three-dimensional（3-D）flower-like cobalt sulfide（CoS）cathodes,all-phenyl complex（APC）and lithium chloride dissolved in tetrahydrofuran（THF）derived APC-x LiCl/THF(x=0-0.8 mol L^-1)electrolytes.（1）The electrochemical performance of MLIBs is improved with the increasing LiCl concentration owing to the increasing redox kinetics.MLIBs achieve optimal electrochemical performance using APC-0.8 LiCl/THF electrolyte.Electrochemical kinetic studies of MLBs indicate that the charge transfer resistance undergo a drastic decrease during initial cycling,taking a significant role in capacity increase.More Li⁺ions will participate in the cathode reaction owing to Li⁺ions for its higher diffusion rate and smaller size than Mg²⁺ions.Consequently,there is an improvement in specific capacity with the increasing LiCl concentration.（2）By virtue of APC-0.8 LiCl/THF electrolyte,the capacity of MLIBs increases gradually to a high capacity of 507 m Ah g^-1 at 0.1 A g^-1 after 30 cycles,indicating an activation process during initial cycling.It also realizes a reversible capacity of 219 m Ah g^-1 at 0.5 A g^-1 after 500 cycles.In addition,capacities of these MLIBs drop firstly,then rise gradually and finally stabilize,showing a significant activation process during battery cycling,which is attributed to the reducing dissolution of Li₂S and Mg S,the increasing reaction reversibility of Li⁺and Mg²⁺ions on CoS cathode and more exposion of CoS materials to electrolyte with the gap caused by volume expansion during cycling.（3）The stable discharge capacity of 318 m Ah g^-1 can still be achieved after 1000cycles at the current density of 1 A g^-1 for activated CoS electrode,which demonstrate better rate performance and long cycle performance.The pseudocapacitance analysis showed that the fast capacitive energy storage and the slow battery energy storage contribute to the specific capacity of the cathode of MLIBs,and the larger proportion of capacitive energy storage provides a better rate performance for the cathode.（4）The prelithiation CoS cathode shows good electrochemical performance using APC/THF electrolyte.Li⁺ions deintercalate from the CoS cathode in the first charging circle.The convert reaction occurs in the subsequent cycle,and the co-conversion mechanism of Li⁺and Mg²⁺ions is achieved for the whole cycle process.（5）X-ray diffraction and Transmission electron microscope indicate that Li⁺ions with rapid dynamics and Mg²⁺ions can promote the conversion of CoS to Co₉S₈,which leads to the formation of Co,Li₂S and Mg S.The reaction intermediate of Co₉S₈ is also generated during cycling,which can achieve a high reversible charge-discharge specific capacity(500 m Ah g^-1).Co₉S₈ cathode has better reaction kinetics than CoS cathode,which indicates that the favorable interphase Co₉S₈ spontaneously generated in the CoS cathode has good electrochemical performance.The systems studied here may offer insights and inspiration in the design of MLIBs based on metal sulfide materials and electrolytes.更多还原