Researches For Behavior Of Solid State Diffusion In Lithium Iron Phosphate And Pseudocapacitances In Cobalt (?,?) Oxide,Ferric Oxide

Electrochemical performances of lithium ion batteries?LIBs?are determined with the electrode materials.The superior electrochemical performances of electrode materials turn to be the precondition of the commercialization for LIBs.For the gradual maturation of LIBs,it is necessary to perceive and master the electrochemical behavior of electrode materials.There are abundant varieties of electrode materials for LIBs.Such as the phosphate,nickel-cobal-manganese ternary materials for the cathode and carbon,transition metal oxide,tin-based materials and silicon-based materials for the anode.Reports on excellent electrochemical performances and characteristics of electrode materials keep emerging one after another.The developments of brand new materials and systems are continously progressing.LIBs possess a promising future in many fields,especially in the application of energy storage and power supply.For the time being,researches associated with the electrode materials are mainly focusing on the optimizations of electrochemical performances.Compared to the enhancement of macroscopical performances,the understanding of basic theories is developing less efficiently,regulations of electrochemical behavior including the diffusion mode and rate of Li⁺that associated with macroscopical performances in particular.Discussion on the electrochemical behavior is limited with a certain kind of specific material for electrodes.Comprehensive understanding of kinetic behavior and thermodynamic characteristics with wide applicability are barely formed as the result of the shortage of mutual references among all kinds of research achievements and measurement technologies developed in the exploration of each kind of electrode materials.Therefore,decent explanation and understanding of common issues about LIBs are pending further development.In this dissertation,the explorations are carried out around LiFePO₄ cathode,transition metal oxide anodes including Co₃O₄ and Fe₂O₃.Phase changes of the electrochemical systems were analyzed from the angle of the diffusion of elementary particles.Reaction mechanisms,basic procedures of and influence factors on charge transfer of electrode materials were discussed with many kinds of electrochemical measurements.Solutions were introduced after the discovery of some basic issues emerged in the production applications of electrode materials.Primary researches conducted were summarized in 4 aspects as follows:1.Electrochemical characteristics of three-dimensional interconnected Li FePO₄/C core shell structure were analyzed based on“Radius Model”.Approach to improve the cycling performances as well as practical capacity density of LiFePO₄ at the same time was proposed and verified.?1?Three-dimensional interconnected LiFePO₄/C core shell structure was fabricated with a sol-gel process.The oxidative polymerization of carbon source aniline was promoted with the reduction of Fe³⁺on the surfaces of the sol molecules formed.Carbon layer coated on the surfaces of LiFePO₄,which was uniform and complete,prevented agglomeration,enhanced the electric conductivity,shortened the length of Li⁺diffusion paths by limiting the growth of LiFe PO₄ particles,protected LiFePO₄ from oxidation and HF corrosion.Hence,recommendable performances of LiFePO₄/C particles with the diameter of⁵0 nm were ensured.With a potential difference as small as 120 mV between the charge and discharge plateaus at the galvanostatic charging and discharging?GCD?process under current density of 34 mA/g,and a decrease of 200mV of the peak potential difference in cyclic voltammetry?CV?,the reversibility of the electrode reactions were proved to be improved with the special three-dimensional interconnected core shell structure.Electrochemical impedance spectroscopy?EIS?and related calculations of kinetic parameters indicated that the charge transfer impedance reduced from 488.2?to 41.3?.And there were promotions of Li⁺diffusion coefficient(8.3×10^-15 to 1.4×10^-11 cm²/s),conductivity(2.5×10^-5 to 3.0×10^-4 S/cm)as well as exchange current density(5.3×10^-5 to 6.2×10^-4 mA/cm²)for several order of magnitudes.Thus,after the GCD testing at 850 mA/g for 2000 cycles,90.5%of the capacities were remained,indicating decent cycling performances.?2?Reasons for the relatively low practical capacities were discovered with classical“Radius Model”.“Ultimate interface area”was proposed to determine the ratio of active materials unused.For higher utilization of active LiFePO₄,triple-layer coaxial structure was designed for the replacement of active LiFePO₄ within the“ultimate interface area”with conductive carbon,aiming at modifying the cycling performances and specific capacities at the same time.CV results of coaxial structural LiFePO₄without conducting agent and binder showed a peak potential difference of only 270mV,confirming the effectiveness of special structure designed.2.The solid phase diffusion procedure of Li⁺was analyzed based on the classic“Domino-Cascade Model”.Hence,“Anisotropic”nanocrystallization was designed to improve the rate performances of LiFePO₄.?1?Bare granular LiFePO₄ with a diameter of³00 nm fabricated with hydrothermal process was proved to possess high purity.Combination of GCD and EIS tests were conducted on the LiFePO₄ obtained hydrothermally to reveal the variation of Li⁺diffusion coefficients during the first 15 GCD cycles.It was found that,Li⁺diffusion coefficients within the sections of two phase transformations were much higher than those of the single phase solid solution processes.The former ones were more than 100 times higher than the later ones.?2?With the analysis of migration properties of Li⁺in the crystal of LiFe PO₄ based on the“Domino-Cascade Model”,two opposite effects of nanocrystallization for the enhancements of performances were pointed out:on one hand,nanocrystallization reduces the amount of“inactive Li⁺”,which improves the utilization of LiFePO₄ and increases the specific capacities;on the other hand,rate performances will be influenced as nanocrystallization decreases the regions of two phase transformation process with much higher Li⁺diffusion coefficients.?3?In order to make full use of nanocrystallization and balance its two opposite effects,“anisotropic nanocrystallization”was proposed.Namely,shrinking the crystal dimension along b-axis and maintaining relative larger sizes in a-axis and c-axis directions to build nanosheet structure with b-axis orientation.In this way,the specific capacities,rate and cycling performances of LiFePO₄ can be optimized synchronously,benefiting the exploitation of commercialized LiFePO₄ with promising comprehensive electrochemical performances3.Intermediate phase and behavior characteristics of the electrochemical reactions were clarified employing Co₃O₄ tubular structure with balsam pear-shaped surfaces in micrometer scale fabricated employing“Kirkendall effect”.?1?Scanning electron microscope?SEM?and transmission electron microscope?TEM?indicated that the diameter of Co₃O₄ tubes ranges from 0.5-3.0?m.And the length is in the district of 5-10?m.Calcining temperature was proved to be essential for the formation of tubular structure.Preferable tubes obtained with the calcining temperature of 350 ^oC possess large amount of voids generated from the hollow,porous and scobinate structure,which enhanced the cycling and rate performances by relieving the negative effects of huge volume changes during the electrode reactions.With the Coulombic efficiency remaining 100%,the tubular structural Co₃O₄ delivered the specific capacity as high as 447 mAh/g after being cycled for 40 times in the GCD tests under 0.05 mA/cm².GCD tests for the 1st cycle under different current densities?0.05,0.50,1.00,2.50 mA/cm²?indicated that the capacities barely decayed with the increasement of current densities.Potential of charging/discharging plateaus seldom changed,neither.Discharging and charging capacities for the 1st cycle under higher current density?5.00 mA/cm²?reached 907 and 651 mAh/g,respectively.?2?With the combination of XRD and GCD,intermediate phase of Co₃O₄ tubes during the first discharge process was found to be Li_1.47Co₃O₄.Benefiting from the determination of intermediate phase,reaction details were clarified on one hand;On the other hand,crystal volume expansion of⁸.6%in the initial reaction stage was calculated with the lattice parameters of different phases.?3?By combining EIS and GCD,the changes of metallic Co and inactive Li₂O,together with their impacts on the conductivity of electrode were analyzed for the 1st cycle.4.After clarifying the causes and properties of reversible capacities exceeding the theoretical value of transition metal oxides such as Fe₂O₃ and Co₃O₄,several electrochemical technologies were employed to explore the pseudocapacitive characteristic of transition metal oxides,which is the main source of the reversible capacities exceeding theoretical value.?1?GCD tests at high current density of 10 mA/cm² and differential capacity curve analysis were carried out on Fe₂O₃ nanoparticles of high quality and with steady electrochemical performances to find out the pseudocapacitive contribution of capacities.During the initial 5 cycles,solid electrolyte interface?SEI?made contributions to the amount of capacities.While in the following cycles,effects of SEI tended to be stable.However,constant capacity of 120 mAh/g?¹0%of the total capacities?contributed by pseudocapacitive processes for the following 100 cycles was found.?2?According to the power law relation between response current of CV tests at various scan rates and the scan rates?i=a?^b?,pseudocapacitive processes were proved to take place mainly inside the sections of 2.2-3.0 V and 0.4-0.8 V with CV results obtained with the scan rates under 1.0 mV/s.?3?Properties of Bode plots were summarized with several kinds of equivalent circuit diagrams.Connections between the components of equivalent circuit diagrams and electrode processes,including the faradaic procedure controlled by solid phase diffusion,pseudocapacitance procedure and so forth,were built.Apparent pseudocapacitive characteristics of Fe₂O₃ and Co₃O₄ were discovered with phase angle Bode plots??-logf?.Additionally,with the determination of frequency range in which pseudocapacitance took effect employing the phase angle Bode plots,pseudocapacitances and electric double layer capacitances were given for Fe₂O₃ and Co₃O₄.Namely,at the charging state of the testing batteries,pseudocapacitance and electric double layer capacitance of Fe₂O₃ was 246.87?F/cm² and 3.31?F/cm²,respectively.For Co₃O₄,it was 180.28?F/cm² for pseudocapacitance and 1.32?F/cm²for electric double layer capacitance.In this dissertation,electrochemical behavior of LiFePO₄ cathode and Co₃O₄,Fe₂O₃ anode were discussed from the angle of combining the changes in thermodynamic properties?energies?and transformation of dynamic procedures?migration of Li⁺?,in the order of:?1?crystal structures,?2?properties and their connection with crystal structures,?3?key factors limiting the performances,?4?traditional methods to optimize electrochemical performances and their functional mechanisms as well as limitations,?5?possible further develop directions in the future.Specific experimental results and systematic theoretical analysis were both emphasized.Large amounts of traditional electrochemical measurements were applied.What's more,electrochemical characterization systems for the explorations of pseudocapacitances in electrode materials for LIBs were established.Some contents in this dissertation are of reference significance for the developments of electrode materials of LIBs.