【作者】 张国梁；

【导师】 党锋；

【作者基本信息】 山东大学 ， 材料与化工（专业学位）， 2024， 博士

【摘要】 锂-氧气电池因其超高的理论能量密度受到广泛研究关注,然而目前锂-氧气电池因其能量效率、放电比容量和循环寿命等电化学性能较差,仍不能满足实际应用需要,开发设计高活性正极催化剂是提升锂-氧气电池性能的重要手段之一。二维材料因其独特的层状结构、均一表面状态和可调节性、高比表面积等特性,在高性能锂-氧气电池正极催化剂研发方面极具潜力。本研究论文以二维硒/硫化物为目标材料,对其在锂-氧气电池正极催化性能进行了系列研究,结合密度泛函理论计算揭示了二维材料催化机制,总结了二维材料不同堆叠结构间的催化特性,提出并验证了离子插层、构建异质结、表面结构设计等改性策略对提升二维材料电化学性能的作用,建立了二维材料催化能力与结构设计和电化学性能之间的构效关系。主要的研究内容如下:(1)二维类黑磷结构SnSe锂-氧气电池正极催化剂表面催化特点研究。通过水热法制备了均匀二维表面和堆叠侧面暴露的SnSe纳米片,分析暴露晶面在反应过程中的催化特性与差异,结果表明放电产物生成与分解过程主要集中于SnSe的二维表面。其表面均一的电子结构有利于稳定Li2-xO2等放电产物,促进氧还原/氧析出反应过程中的电子转移,从而表现出较好的催化活性。对于含范德华力的堆叠侧面,研究证明其独特的电子结构分布使其对Li2O2等放电产物或含氧中间体具有限域吸附特性,限制了产物的成核与生长过程,采用该催化剂作为正极获得了高放电容量20783 mAh g-1和长循环寿命380圈,对二维材料不同晶面在锂-氧气电池正极中的催化行为特点进行了深入分析。(2)Ag插层策略提升SnSe2二维表面催化活性,减小催化各向异性影响。基于上述研究中二维材料表面与侧面催化行为差异以及堆叠侧面的限域吸附特性对电化学性能的抑制,进一步分析了1T相SnSe2中表面与堆叠侧面的催化各向异性,根据前期研究与已报道文献,总结了二维材料的共性催化特性。考虑范德华层间的电子缺失,提出了离子插层策略改善二维材料堆叠侧面催化活性。通过水热法制备了具有Ag+层间插层的1T相SnSe2,结果表明Ag+插入可以实现堆叠层间的电子桥联作用,实现层间均一的电子结构分布,有效缓解堆叠侧面对放电产物的限域吸附效应,从而改善二维材料二维表面与堆叠侧面的催化各向异性。结果表明Ag+插层显著提升了 SnSe2正极电化学性能,在100 mA g-1的电流下放电容量为16871 mAh g-1,同时在600 mAh g-1容量下循环寿命超2300小时,是未掺杂样品性能的2.4倍。(3)MnS/MoS2异质结催化性能研究。通过构建异质结可以在材料内部形成内置电场促进电荷转移,提高材料催化活性。因此以MnMoO4作为前驱体在水热过程中合成MnS/MoS2异质结,并与纯相材料进行对比。异质结中MnS可以锚定在2H相MoS2表面,从而破坏二维材料的层间周期性堆叠生长,避免侧面的过度暴露对催化活性的抑制。异质结改善了材料表面与界面处的电荷分布,增强了表面对反应物的吸附强度,调节放电反应路径从MoS2表面的溶液介导路径转变到MnS/MoS2表面的表面生长路径,相应放电产物形貌从表面盘状颗粒变为薄膜状形貌。膜状放电产物能充分利用MnS/MoS2丰富的催化活性位点,促进其在充电过程被催化分解,提高电化学稳定性。电化学测试表明MnS/MoS2电极相比单相MnS和MoS2表现出更优异的性能,在100 mAg-1电流下放电容量约为11696mAhg-1,在600mAhg-1容量下可工作约1800小时。(4)金属性NbSe2二维表面结构调控与催化性能构效关系研究。二维材料中元素种类与配位结构不同导致了 d轨道分裂与电子排布的差异,从而决定材料的金属性与半导体性。基于VB族元素Nb外部4d5s1电子排布在层状硒化物中呈金属性特点,通过热注入法合成类石墨烯NbSe2纳米片,调节反应时间从0.5至1.5小时可以改变NbSe2纳米片厚度,薄层NbSe2约3.7 nm能充分发挥二维材料表面催化活性,避免侧面的影响,同时由于层内原子生长速率的差异会在表面形成台阶式结构。结果表明NbSe2电极在100mA g-1电流下放电容量为13630mAh g-1,在600mAhg-1容量下可以工作1600小时。理论研究证实金属性NbSe2对O2的弱吸附特性可以加快Li2O2分解过程决速步反应速率,同时台阶结构处应变降低了材料d带中心,削弱放电产物的吸附强度,有利于产物在充电过程被分解。该工作为二维催化剂结构设计和电子结构分析提供了研究思路。更多还原

【Abstract】 Li-O2 batteries have received much attention due to their ultra-high theoretical energy density.However.there are still some severe problems with the electrochemical performance of Li-O2 batteries,such as low energy efficiency and discharge/charge capacities,and short cycle life,which lead to that they cannot meet the requirements of practical applications.Developing a highly efficient cathode catalyst is crucial to improve the electrochemical performance.Two-dimensional(2D)materials have great potential as cathode catalysts for the development of high-performance Li-O2 batteries due to their unique layered structure,homogeneous and tunable surface state as well as high specific surface area.In this thesis,a series of research was conducted to study the electrochemical performance of 2D selenides and sulfides.Meanwhile.we further revealed the catalytic mechanisms combining the density functional theory(DFT)calculation and summarized the catalytic characteristics of 2D materials with different stacked structures.A series modulation strategy in 2D materials was applied to improve the electrochemical performance.including ion intercalation,heterostructure construction.and surface structural designments.Therefore,in this thesis,we build the structure-property relationship in 2D materials between catalytic capability,structural designments,and electrochemical performance.The main research contents are as follows:(1)The facet-dependent catalytic characteristic and electrochemical performance of black phosphorus structure SnSe cathode catalysts in Li-O2 batteries.SnSe nanosheets with uniform exposed surface and edge plane were synthesized by a typical hydrothermal reaction.The catalytic performance and behavior were evaluated between the two different planes.The generation and decomposition processes of discharge products are mainly concentrated on the 2D surface plane of SnSe.where the uniform electronic structure can stabilize Li2-xO2 intermediates,accelerating the electron transference during the oxygen reduction/evolution reaction processes.For the stacked edge plane,the unique electron distribution can confine the adsorption of Li2O2 and oxygenated intermediates,restricting the growth of discharge products.For the electrochemical performance.the SnSe cathode exhibits a high specific capacity of over 20783 mAh g-1 and ultralong cycle life of about 380 cycles This work provides in-depth insight into the elusive electrocatalytic characteristics of 2D materials in Li-O2 batteries.(2)The Ag+intercalation enhances catalytic performance for the surface plane of SnSe2 and diminishes the effect of catalytic anisotropy.Based on the above results about the catalytic characteristics between the surface and edge plane as well as the confinement effect in 2D materials,the catalytic anisotropy was studied in the surface and edge plane of 1T SnSe2.According to the results and previous reports,we summarize the general catalytic properties of 2D materials.Considering the lack of electrons in the 2D layer space,the ion intercalation strategy was provided to improve the catalytic performance of the stacked edge plane.Ag+intercalated SnSe2 was prepared by hydrothermal methods.Ag+can act as an electronic "bridge"to enhance the interaction between the adjacent layers and uniform the electron distribution,reducing the confinement effect of the reaction species on the edge plane and diminishing the catalytic anisotropy between the surface and edge plane.As a consequence,the Ag-intercalated SnSe2 cathode exhibits a superior specific capacity of 16871 mAh g-1 and a stable cycle life over 2300 h at a current density of 100 mA g-1 with a limited capacity of 600 mAh g-1,which is 2.4 times higher than the performance of SnSe2 cathode.(3)The research on catalytic performance of MnS/MoS2 heterostructure.Constructing the heterostructure will build inside electric field to promote the electron transfer.thus enhancing the catalytic performance.In this work,the MnS/MoS2 heterostructure were synthesized in the hydrothermal process with MnMoO4 nanorods precursor.The introduced MnS can be anchored on the surface of 2H MoS2,which can disturb the periodic layer growth of MoS2 and avoid the inhibition of catalytic performance by the stack edge plane with a large exposure area.The heterostructure can improve the charge distribution at the surface and interface,enhancing the adsorption strength of the surface to the reaction species.Benefit of these.the reaction path will transform from the solution-mediated path on the MoS2 to the surface growth path on the MnS/MoS2 surface.This also corresponds to the transition of morphologies from toroidal-like to film-like products.The film-like discharge products can tightly contact the MnS/MoS2 surface and fully utilize the active sites,which is beneficial for the decomposition of the products and electrochemical stability.In consequence,the MnS/MoS2 heterostructure displayed a higher performance than that of MnS or MoS2 cathode.It exhibits a high specific capacity of 11696.0 mAh g-1 and good cycle stability over 1800 h with a fixed specific capacity of 600 mAh g-1 at a current density of 100 mA g-1.(4)The structure-property relationship of structural design and catalytic performance for metallic graphene-like NbSe2 cathode catalysts.For 2D materials,the different element types and coordination structures lead to the difference in d orbital splitting and electron arrangement,which determine the metallicity or semi-conductivity of the materials.The group VB Nb with the configuration 4d45s1 displays metallic properties in the NbSe2 coordination.Here,NbSe2 nanosheets were prepared via a one-pot solvothermal reaction,and the thickness of the nanosheets can be varied by adjusting the reaction time from 0.5 to 1.5 hours.The obtained NbSe2 nanosheets display a thin thickness of about 3.7 nm and a high specific surface area,which can fully play intrinsic catalytic properties.Meanwhile,the step structure was formed on the surface due to the different growth rates in the layer.The NbSe2 cathode can deliver a high specific capacity of 13630 mAh g-1 and a good cycle stability for 1600 h with a limited specific capacity of 600 mAh g-1 at a current density of 100 mA g-1 The calculated results confirm that the weak O2 adsorption on the NbSe2 surface is conductive to the decomposition of Li2O2,where the desorption of O2 is the rate determination.Meanwhile,for the step structure,the surface strain will affect the electron structure and downshift the d-band center,further weakening the adsorption strength of discharge products and accelerating the decomposition of products.This work provides insights into surface structure design and electronic analysis of 2D materials catalysts.更多还原