【作者】 崔鹏辉；

【导师】 曹殿学；

【作者基本信息】 哈尔滨工程大学 ， 化学工程与技术， 2024， 硕士

【摘要】 锂离子电池在日常生活中的应用与日俱增,不过目前商业常见的负极材料石墨已经难以满足人们对电池高功率密度高能量密度的需求。而二维过渡金属碳化物、氮化物（MXenes）具有大比表面积,高导电性的特点,能够构建较快的离子传输通道,且能够提供较多的锂离子活性位点,有望成为新一代高容量快充电极材料,提高循环稳定性和倍率性能。然而,由于MXenes制备阶段往往需要用到高浓度的强酸、强碱,或者特定的刻蚀环境,极大的制约了其制备效率以及商业化发展;在性能提升方面,MXenes表面复杂的官能团为电极反应过程提供了大量的不确定性。因此,本文利用新型的二维材料制备改良方法-瞬时焦耳热技术,利用电容放电轰击引发焦耳热冲击以提高MXenes材料的刻蚀效率,并实现安全、规模化的批量制备路线。研究揭示FJH技术的作用机理,并利用其对MXenes进行表面官能团的定向修饰,以及结构优化,实现材料电化学性能的改善。对Ti₃AlC₂材料进行高能电击,发现高能电子和瞬时高温的作用下引起的焦耳热冲击能够破坏MAX相中较弱的Ti-Al键,降低了层与层之间范德华力的影响,并能够使层间的Al向外表面析出。通过高通量实验验证合适的电击能量为500 J,该条件下制备的样品具有层间距大、表面多孔的形貌,有利于刻蚀剂的深入刻蚀。缩短刻蚀时间至15 h,和正常刻蚀40 h的Ti₃C₂T_x相比,在2000 m A g^-1电流密度的循环测试中,放电比容量分别为102.8 m Ah g^-1和75.8 m Ah g^-1。研究表明,FJH预处理能够极大提高Ti₃AlC₂的刻蚀效率,并能够提高获得的MXenes的储锂性能和改善动力学。使用氟化物混合Ti₃AlC₂进行电击,在200 J电击能量下能够形成沸点较低的氟化铝,在瞬时加热过程中加速Al从MAX层间析出。通过电化学分析发现了电击后由于Al析出,出现的金属空位可以为Li⁺提供反应活性位点,且随着锂化和去锂化过程进行电极材料发生刻蚀行为。并验证了LiF和NH₄HF₂对加速Ti₃C₂T_x制备的促进效果,发现使用LiF仅能加速Ti₃AlC₂结构边缘的Al的析出,从而制备多层形貌的FF-Ti₃C₂T_x-LiF,后续可通过在低浓度HCl中浸泡10 h即可获得少层形貌的FF-Ti₃C₂T_x-LiF-10。而混合NH₄HF₂进行电击能够利用气体膨胀和NH₄⁺插层,在简单清洗副产物后即可获得少层的FF-Ti₃C₂T_x-NH₄HF₂。通过对储锂性能的研究发现:FF-Ti₃C₂T_x-LiF由于多为多层形貌,储锂性能不理想;而FF-Ti₃C₂T_x-LiF-10在小电流密度下有较好的性能,但倍率性能不稳定;FF-Ti₃C₂T_x-NH₄HF₂在500 m A g^-1电流密度下的放电比容量能达到456.07 m Ah g^-1,而在10000 m A g^-1电流密度下的放电比容量仍能保持在88.89 m Ah g^-1,展示了较好的倍率性能和电化学稳定性。使用FJH技术改善Ti₃C₂T_x表面官能团,通过第一性原理计算可知Ti₃C₂T_x表面的-OH、-F稳定性较差,能够利用热冲击过程产生的瞬时高热去除。获得的低-F、-OH含量的F-Ti₃C₂具有独特的三维形貌,并在1000 m A g^-1电流密度的循环测试中展现为226.41 m Ah g^-1的放电比容量。后续通过在电击时混合ⅤA、ⅥA、ⅦA的九种不同元素,实现单元素和多元素修饰。对制备的F-Ti₃C₂-F、Cl、Br、I、O、S、Se、N、P样品进行电化学分析,发现第ⅤA元素修饰样品要优于第ⅦA和ⅥA。其中各族元素中,-N、-S、-O在1000 m A g^-1电流密度下展现了454.11、349.68、271.67 m Ah g^-1的储锂容量。而F-Ti₃C₂-Se、S、Br展示了多储锂过程,其中F-Ti₃C₂-S、Br具有较快的Li⁺扩散速率,极大的提高了储锂能量密度。更多还原

【Abstract】 While commercial application of lithium-ion batteries is increasing,the current common anode material,graphite,has struggled to meet the demand for high power density and high energy density in the battery.The two-dimensional transition metal carbides and nitrides（MXenes）with large specific surface area and high conductivity can form faster ion transport channels and provide more lithium-ion active sites,which is expected to become a new generation of high-capacity.However,the preparation of MXenes often requires the use of high concentrations of strong acids and bases,or special etching environments,which severely limits their preparation efficiency and commercial development;in terms of performance enhancement,the complex functional groups on the surface of MXenes introduce a high degree of uncertainty into the electrode reaction process.Therefore,in this dissertation,we use the Flash Joule Heating（FJH）technique,which involves Joule thermal shock by capacitive discharge,to work on improving the etching efficiency of MXenes materials and achieving safe,scalable batch transient preparation.To elucidate the mechanism of action of FJH,and to use FJH technology to modify the surface functional groups of MXenes and optimise the structure to improve the electrochemical properties of the materials.High energy electroshock of the Ti₃AlC₂ material shows that the Joule thermal shock induced by the action of high energy electrons and transient high temperatures is capable of breaking the weaker Ti-Albonds in the MAX phase,reduced the effect of van der Waals forces in a layer,and was able to precipitate the Albetween the layers to the outer surface.It was demonstrated that the product has a large layer spacing at 500 J electroshock energy and a surface porous morphology through high throughput experiments,which contribute to the diffusion kinetics of the etchant.The total etching time of the samples can be reduced to 15 h after the electric shock,and the discharge specific capacity is 102.8 m Ah g^-1 in the cycling test at 2000 m A g^-1 current density,compared to75.8 m Ah g^-1 of Ti₃C₂T_x with normal etching for 40 h.The FJH pretreatment is also a good choice for the lithium storage capacity of Ti₃AlC₂,as it can be used for the lithium storage of MXenes.The results show that pre-treatment with FJH can significantly increase the etching efficiency of Ti₃AlC₂,enhance the lithium storage capacity and improve the kinetics of the obtained MXenes.Fluoride doping of Ti₃AlC₂ followed by electroshock was able to form aluminium fluoride with a lower boiling point at 200 J shock energy,which accelerated the precipitation of Alfrom the MAX interlayer during transient heating.The metal vacancies created by precipitation of Alafter electroshock can provide reactive sites for Li⁺through electrochemical analysis,and the electrode material was etched as the lithiation and delithiation process.The promoting effect of LiF and NH₄HF₂ in accelerating the preparation of Ti₃C₂T_x was also verified,and FF-Ti₃C₂T_x-LiF with multilayer morphology was prepared,because the use of LiF only accelerated the precipitation of Alat the edge of the Ti₃AlC₂ structure,whereas the FF-Ti₃C₂T_x-LiF with fewer layers still needed to be immersed in a low concentration of HCl for 10 h to be obtained.When doped with NH₄HF₂ for electroshock,the FF-Ti₃C₂T_x-NH₄HF₂ obtained after a simple purification of the by-products shows a two-dimensional lamellar morphology due to gas expansion and NH₄⁺intercalation during the reaction process.The electrochemical performances were the following:the lithium storage capacity of FF-Ti₃C₂T_x-LiF is not high due to its mainly multi-layer morphology,while FF-Ti₃C₂T_x-LiF-10 has a better cycling performance at small current densities,but the multiplicity performance is unstable.At a current density of 500 m A g^-1,the discharge specific capacity of FF-Ti₃C₂T_x-NH₄HF₂ could reach 456.07 m Ah g^-1,and the discharge specific capacity can still be maintained at 88.89 m Ah g^-1 at 10000 m A g^-1 current density,which shows better multiplicity performance and electrochemical stability.In addition,FJH can also be used to modify the surface functional groups of Ti₃C₂T_x.The poor stability of-OH and-F on the surface of Ti₃C₂T_x surface can be observed from first-principles calculations,and the transient hyperthermia generated by FJH can be used to remove these functional groups.The resulting product F-Ti₃C₂,with low-F and-OH content,has a unique three-dimensional morphology and exhibits a specific discharge capacity of 226.41 m Ah g^-1 at a current density of 1000 m A g^-1.Subsequently,single-element and multi-element modifications were achieved by doping nine different elements of VA,VIA and VIIA during electroshock.Electrochemical analyses of the prepared F-Ti₃C₂-F,Cl,Br,I,O,S,Se,N and P samples showed that the elemental modification samples of VA were superior to those ofⅦA andⅥA.Among the family elements,-N,-S and-O exhibited lithium storage capacities of 454.11,349.68 and271.67 m Ah g^-1 at 1000 m A g^-1 current density.And F-Ti₃C₂-Se,-S and-Br showed multiple lithium storage processes,where F-Ti₃C₂-S and-Br had faster Li⁺diffusion rates,which greatly improved the lithium storage energy density.更多还原