Single-nanoparticle Level Studies On The Electrochemical Activity Of Ion Battery Cathode Materials

Facing with the rapid development of electronic information,battery capacity and performance need to be significantly improved to meet the growing demand for electronic devices.At present,the ion battery is the most important secondary battery,which plays a leading role in the batteries.The present bottlenecks that limit the development of battery include easy aging and slow charging rate.In depth,in situ study of the dynamic structural change of ion battery electrode materials during charging and discharging is a key prerequisite for further improving its diffusion rate and cycle performance.The traditional ion battery research technology is often focusing on the electrode interface containing numerous particles.The corresponding results reflect the ensemble average behavior of all the particles in the system.However,due to the limitation of material manufacturing and processing technology,electrochemically active particles are significantly heterogeneous in terms of both chemical structure and electrochemical activity.The measurement and characterization of the electrochemical activity of ion battery electrode materials at single particle level is one of the most effective approaches to clarify the behavior and mechanism of the micro and nanoscale electron transfer and to explore the relationship between structure and activity,which provides a new perspective for the rational design and optimization of the electrode materials.In this dissertation,we established a new method toward single particle electrochemical measurements with surface plasmon resonance(SPR)microscope.The study of the electrochemical reaction kinetics of single particle is achieved,since the single particle SPR optical signal is extremely sensitive to its dielectric constant.The latter is a function of its chemical composition and thus the electrode potential.Furthermore,we measured the diffusion coefficient of electrochemically active ions in a single particle.This study strongly demonstrated the remarkable advantages of the optical microscope in the study of ion battery electrode materials,such as in situ,nondestructive,high time resolution and generality,opening up new opportunities for the study of nano electrochemical applications includingion batteries.The main researches and highlights are summarized as follows:1)Optical imaging of phase transition of single LiCoO2 nanoparticles during electrochemical cyclingLiCoO2 is the first commercialized Li-ion electrode material,and it is also one of the most successful Li-ion cathode materials so far.The understanding on its properties plays a decisive role in improving its performance.Both experimental and theoretical studies show that the phase transition process of LiCoO2 nanoparticles occurs during charging and discharging.It is therefore believed that its optical properties will also change greatly.It was found that the dielectric constant of LiCoO2 nanoparticles was closely related to the state of lithiation,and the intensity of optical signal reflected the Li amount within a single nanoparticle.The change diagram of the SPR of a single LiCoO2 nanoparticle with the voltage was measured during the electrochemical cycle.Optical cyclic voltammogram of the single nanoparticle was resolved by analyzing the SPRM images.Compared with the traditional analytical methods based on electrochemical characterizations,the single particle electrochemistry technology built on optical imaging has the advantages of high sensitivity and high throughput,and benefits to the establishment of the structure-activity relationship.2)Measure of the Li ion diffusion coefficient of a single LiCoO2 nanoparticleThe diffusion coefficient of the electrode material seriously affects the performance of the electrode materials.Therefore,understanding the mechanism of the migration and diffusion of the electroactive ions in the layered nanomaterials is crucial to the design and optimization of electrode materials.The traditional technology often shows the average diffusion coefficient of ions in the macro interface(such as film)and is thus unable to reveal the migration behavior and laws in the micro and nanoscale scenarios.A feasible method for measuring the diffusion coefficient of Li ions in a single particle was proposed,and we realized the quantitative measurements on the diffusion coefficient of Li ion in single particles during lithiation and delithiation processes.The experimental results show that the diffusion coefficient of Li ions is of the intrinsic heterogeneity among individuals.3)High-resolution imaging of Li ion diffusion paths in single LiCoO2 particlesThe micro migration of Li ions inside the energy storage materials dominantly influences the materials performance in energy storage,and thus detecting the migration process plays an extremely important role in further improving the storage performance.Unlike the remarkable volume expansion of anode materials during lithiation and delithiation processes,the electrochemical process of cathode materials is usually not associated with obvious changes in volume.Therefore,the traditional in situ transmission electron microscopy method is difficult to study the migration path of Li ion in the cathode material.We developed a two-dimensional image correlation analysis method based on the results discussed above,bringing about the first realization of the super-resolution imaging of the Li ion diffusion in a single LiCoO2 particle(the localization precision was better than 10 nm).This work provided a novel characterization and research technology toward in-situ study of the ion diffusion kinetics in cathode materials.4)Film Electrochemistry of Single Prussian Blue NanoparticlesPrussian blue(PB)nanomaterials show different colors with different electrochemical conditions,and accordingly the refractive index of Prussian blue should also change greatly.We further expanded the application system of SPR microscope,for the measure of K ion insertion and removal in single PB nanoparticles,which provided a new technology for exploring PB-based K ion battery.On the basis,we successfully observed the thin-film electrochemistry properties of single PB nanoparticles at different scanning speeds(10-100 mV/s)and found the heterogeneity of the electrochemical current.This work not only demonstrated that SPRM method was powerful and reliable in studying electrode materials again,but also further expanded the applicable electrode materials,with deep implications in both fundamental nanoscale electrochemistry and the applications in ion battery.