【作者】 葛翔；

【导师】 谷长栋；

【作者基本信息】 浙江大学 ， 材料加工工程， 2017， 博士

【摘要】 过渡金属化合物在电化学能源的储存与转换中具有关键的作用。本论文针对过渡金属化合物的可控合成及电化学性能优化等关键科学问题,创制出深共熔溶剂的非水体系中对过渡金属化合物结构、成分和电化学性能的调控途径,并总结出对于一类层状过渡金属化合物,其结构和电化学性能关系的一般性规律。并进一步针对纳米快速充放电材料振实密度过低的问题,提出基于晶体工程的设计方案改善材料中的电荷及物质输运,克服了常规的纳米化或复合化材料改性手段造成的有效物质在电极中所占体积比过低的问题,达到提升过渡金属化合物在电化学能源存储和转换中性能优化的目的。重点研究了层状金属氢氧化物及其衍生物超结构的生成规律、分子尺度构建层状双金属氢氧化物和石墨烯超晶格复合材料的方法与电化学性能、基于T-Nb2O5的高体积能量密度的嵌入式赝电容电极设计等关键问题。通过原位X射线衍射及透射电子显微观察等先进测试技术,对过渡金属化合物在电化学反应中的结构演变和电化学综合性能进行了揭示。本文主体部分分为三个章节:第三章节中,我们首先介绍了通常被作为惰性溶剂使用的深共熔溶剂DES(deepeutecticsolvent)研究背景,并探索其在高温下作为自反应活性溶剂使用。我们发现在ChCl/urea型DES中溶解过渡金属盐,并在不外加其他任何反应剂的情况下,在密闭条件下直接加热,可获得一些具有特殊结构的过渡金属化合物,包括:具有特殊镂空八面体结构的Ni[NH3]6Cl2,calcite型MnCO3介晶等。此外,这类晶体热转变时其产物形貌与其前驱体具有一定的拓扑关系。本章中进一步探讨了溶剂的高离子强度,软模板效应,及强配位环境对材料生长的影响。第四章节中,我们设计了热注入法,在开放条件下预加热溶有金属离子的ChCl/urea一段时间后,快速注入水,可以合成一系列层状双金属氢氧化物LDHs(layered double hydroxides)类材料,该方法能将材料形核过程和生长过程分离,并且反应时间极短,因此能够得到尺寸均匀并且极小的LDHs,从DES中生长的LED具有一系列特殊性质:(a)通过热注入法可得到α相的Ni(OH)2,在一定情况下,所得到的α-Ni(OH)2具有自还原性,在惰性气氛下退火能直接在其中形成Ni单质,并且Ni单质为常规条件下难以形成的h.c.p.相,原位嵌入的h.c.p.Ni对材料在不同应用中的电化学性能有着普遍的提高,我们认为ChCl/urea中形成的α-Ni(OH)2的自还原性来源于溶剂分解产物氰酸根NCO-与Ni2+直接配位形成较弱的Ni-N键所致,同时ChCl/urea中可能有具有还原性的分解产物插入产物中,;(b)为了验证ChCl/urea溶剂环境的关键作用,我们使用NCO-直接与Ni2+或Co2+发生双水解反应合成其氢氧化物,验证了水溶液中生成的金属氢氧化物不具备自还原性,同时也发现这种基于双水解反应的合成方法简单高效,可在室温下合成具有一系列特殊形貌的金属氢氧化物,我们对产物的自组装机制,以及结构与性能的关系进行了研究。(c):在DES中通过热注入法合成仅含Co以及含CoFe的LDHs时,在合适的反应条件下,所生成的LDHs具有更大的层间距,大层间距的LDHs在电化学应用中具有更好的活性。本章节的研究中,我们发现具有特殊性质的LDHs的形成机制和ChCl/urea中形成的NCO-以及其他分解产物与金属离子形成四面体配位体有关。LDHs的特殊结构对其电化学性能具有深远的影响。此外,以大层间距的LDHs作为前驱体退火合成其氧化物时,能够生成尖晶石物相的介孔纳米片,我们对前躯体影响产物结构和电化学性能的规律进行了探讨。第五章节中,我们围绕高体积功率密度的材料设计展开研究,材料工作时其反应速率受限于多种原因,而纳米化,复合化改性手段往往会极大的牺牲活性材料在单位体积中的占据比。针对该问题我们进行了以下研究:(a)针对LDHs导电性差的问题,我们将分别剥离为单层的带正电的LDHs与带负电的GO可控的自组装,形成周期性堆叠的超晶格材料,并还原为LDHs/rGO,通过这种从晶体工程角度出发的设计方法改善导电性,回避了纳米化设计多孔化,保持了活性材料单位体积占据比,该研究中我们设计了共滴定的方法来创造温和的反应环境促使材料可控的自组装,并系统性的研究了合成参数对产物结构的影响。(b)我们围绕着不受限于近表面反应的具有嵌入式赝电容机制的T-Nb2O5设计了“取出法”,引入氧缺陷从而不牺牲有效物质所占体积,并进一步使用原位表征技术验证了其嵌锂过程为类似于固溶体的连续变化过程。(c)围绕直接与电解液接触时T-Nb2O5循环性能欠佳的问题,我们通过预压烧结法合成了具有大尺寸的T-Nb2O5,再通过在其表面包覆极薄的导电层,从而获得具有高有效物质占据体积比的复合赝电容材料,我们进一步用原位表征手段证实了 T-Nb2O5的嵌入式赝电容储锂机制不受限于其颗粒尺寸。更多还原

【Abstract】 Transition metal compounds play the key role in electrochemical energy conversion and storage.This dissertation focuses on the fundamental science in the field of controllable synthesis of transition metal compounds and their electrochemical applications.Firstly,we proposed and systematically optimized a methodolgy to synthesize a series of transition metal compounds from a non-aqoueous deep eutectic solvent(DES)system.We studied how to controllably tune the composition and structure of the materials,thus further optimizing their electrochemical performances.In this process,we attempt to shed light on the general rule governing the electrochemical performance of a family of layered transition metal compounds.Furthermore,we will focus on the challenges of using traditional nano technology to modify materials.That is,introducing pathway for electron/ion transfer would inevitably sacrifice the volumetric loading of active materials,resulting in low tapping density(especially for materials working with their near-surface).We try to facilitate the transfer of ion/electron based on the philosophy of crystal engineering,thus avoiding the aforementioned problem when using nano technology.This strategy aims at optimizing the electrochemical performance of transition metal compounds for practical applications.Experimentally,we studied the structural and electrochemical characteristics of layered metal hydroxides(LDHs)and their derived materials with unconventional structures.We investigated the method to alternately stack LDH and graphene oxide(GO)on the molecular scale to form super lattice materials,as well as their electrochemical performance.We further investigated a T-Nb2O5 phase with intercalation pseudocapacitive mechanism,which should potentially enable fast charge/discharge materials with high volumetric loading.We combined advanced characterization tools including in-situ XRD in-situ TEM to gain insight on the structural evolution and the dynamic electrochemical properties.The main body of this dissertation proceeds with three chapters:In chapter three,we first introduce the research background of DES which had been mainly used as inert solvent.We then explored its potential as reactive media at high temperature.From a ChCl/urea typed DES dissolved with metal ion,some transition metal compounds with unique structure can directly grow from this solution when the system is heated under sealed condition.This process needs no extra reagent.Such DES-thermal protocol could produce materials including Ni[NH3]6Cl2 with open octahedral morphology,as well as calcite type MnCO3 mesocrystals.These crystals,when used as precursors,can transform to their derived compounds with a topotactic transformation process.We propose that the physiochemical properties of DESs including high ionic strength,soft template effect and their coordination environment would determine the material growth process.In chapter four,we proposed a hot-injection method.Briefly,injecting water into a pre-heated DESs dissolved with metal ions would produce a series of layered double hydroxides(LDHs).This strategy leads to the separation of the nucleation and the growth process.Besides,instantaneous nucleation happens in seconds.These contribute to the small and uniform size of the LDHs.We discovered that LDHs synthesized from DES have the following special properties:(a)under certain condition,the a-Ni(OH)2 produced by hot injection method can have anomalous self-reduction capability.We noticed the formation of elementary nickel when annealing the a-Ni(OH)2 under Ar atmosphere.Beisdes,the formed Ni is in unusual h.c.p.phase.The embedded nickel would have a general improvement on the electrochemical performance of the annealed NiO matrix.We attribute such self-reductivity is related with the weak Ni-N bond,which forms when the decomposed product NCO-directly coordinate with Ni2+.Meamwhile,some other decomposed species that are reductive could also insert into the layered materials.(b)To verify the unique solvent effect of ChCl/urea,we used NCO-to react directly with Ni2+ or Co2+ in aqueous solution based on a double hydrolysis mechanism.Indeed,the produced hydroxides don’t have self-reductivity.During this experiment,we found that such double hydrolysis reaction is simple and effective to produce a series of hydroxides with special morphology.Therefore,we further investigated the assembly mechanism and how the structure would influence the electrochemical behavior.(c)The Co or CoFe LDHs produced with hot-injection could have larger interlayer space.The expanded interlayer space is beneficial to electrochemical applications in a general way.This chapter emphasize the potential of using DES to produce LDHs with special properties.The solvent effect comes from the decomposed product including NCO-and other molecules/ions.These ions can bond with metal ions and form tetrahedral coordination.The special structure of LDHs can have profound influence on their electrochemical properties.Furthermore,the LDHs with expanded interlayer space can act as precursor to produce spinel oxides with mesoporous nanosheets morphology.The precursor effect is also obvious in determining the structure and electrochemical properties of the annealed oxides.Chapter five focuses on how to design materials with high volumetric power density.To facilitate electron/ion transfer in the materials,traditional nano technology would inevitably sacrifice the volumetric ratio of the active materials.We made the following attempts to tackle this problem:(a)to improve the conductivity of LDHs,we exfoliated LDHs into positively charged single layers and exfoliated graphene oxide(GO)into negatively charged single layers,respectively.Then we alternately stack these single layers to form a periodic superlattice material.Such ideal of crystal engineering is different from typical nano technology in that the materials hybrid in a dense way,resulting in high volumetric occupancy of active materials.In this project,we designed a co-feeding protocol to create a mild reaction environment and systematically researched how various synthesis parameter would influence the self-assembly process.(b)We proposed a "take-out" strategy to modify the T-Nb2O5 phase that work with its bulk instead of near-surface.By introducing oxygen vancancy into such a intercalation pseudocapacitive type material,the volumetric loading of active material can maintain.Further in-situ characterization tools reveal that the structure of this material experiences a solid-solution-like process when storing lithium ion.(c)T-Nb2O5 has unsatisfactory cycling performance when it’s in direct contact with electrolye.We designed a pre-press assisted annealing protocol to fabricate T-Nb2O5 with larger size.By further coating it with an ultra-thin conductive layer,we can obtain a pseudocapacitive material with high volumetric ratio of active material.Further in-situ characterization results verify that the intercalation pseudocapacitive mechanism of T-Nb2O5 still work regardless of its larger grain size.更多还原