| 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. |
