Electrode/electrolyte Interfaces Reaction Mechanisms Studies Of Cathodes In Lithium Ion Batteries

The lithium-ion battery, with the high capacity, high energy density and rapidcharge/discharge, is one of the important directions for green secondary energy in thefuture. The charge/discharge capacity, cycle ability and rate capability are associatedwith the electrochemical reactions at the electrode/electrolyte interfaces, so it isnecessary to understand the electrode/electrolyte interfaces reaction mechanisms forclarifying the capacity fading mechanisms, improving the charge/discharge capacityand the rate capability of the electrode material.In order to provide the theoretical evidence for improvement of commercialapplications, the LiCoO₂material with commercialization level was synthesized byhigh temperature solid-state reaction. The electrode/electrolyte interfaceelectrochemical reactions in the LiCoO₂cathode were investigated by electrochemicalimpedance spectroscopy （EIS）. The Nyquist plots involved a semicircle attributed tothe electronic properties of the material. At intermediate degrees of delithiation, thespectra exhibited three semicircles and a slightly inclined line that appearedsuccessively as the frequency decreases. Based on detail analysis of the change ofkinetic parameters obtained from simulating the experimental EIS data as functions ofpotentials, the high-frequency, the middle-frequency, and the low-frequencysemicircles can be attributed to the migration of the lithium ions through the SEI film,the electronic properties of the material and the charge transfer step, respectively. Theslightly inclined line arises from the solid state diffusion process. At early delithiationthere was a dramatic change in the electrical conductivity of the layered LiCoO₂thatwas caused by an insulator-to-metal transition, which would be weak after the firstdelithiation/lithiation process.The LiNi_1/3Co_1/3Mn_1/3O₂material with commercialization level was alsosynthesized by high temperature solid-state reaction. The EIS of thedelithiation/lithiation process in LiNi_1/3Co_1/3Mn_1/3O₂cathode were obtained atdifferent potentials during the first cycle. The EIS spectra exhibited three semicirclesand a slightly inclined line that appeared successively along with decrease infrequency. The middle-frequency semicircle should be attributed to the electronicproperties of the material. Along with raising potential from3.5to4.4V, the electricalconductivities of LiNi_1/3Co_1/3Mn_1/3O₂increased by two orders of magnitude, from1.3×10^-6to7.3×10^-4S·cm^-1. It was firstly proved that at early delithiation a dramaticchange in the electrical conductivity of the LiNi_1/3Co_1/3Mn_1/3O₂electrode was caused by an insulator-to-metal transition.The spinel Li₄Ti₅O₁₂, suggested as one of the most promising alternatives forgraphite anode, was synthesized by high temperature solid-state reaction. Atintermediate degrees of lithiation process, the EIS spectra exhibited three semicirclesand a slightly inclined reflecting solid state diffusion process. It had been revealedthese three semicircles appearing successively along with decrease in frequencyshould be attributed respectively to the Schottky contact, the electronic properties ofthe material and the charge transfer step. At intermediate degrees of delithiationprocess, above1.55V, an interesting and significant phenomenon was that themiddle-frequency semicircle turn into two joint semicircles, and this phenomenon wasattributed to the gas generation in LTO cells, due to the chemical reduction reaction ofsolvent by Li7Ti5O12with the help of LiPF₆and release H2. The fresh and after gasgeneration electrodes were investigated by galvanostatic cycling and FTIRS, and theresults illustrated that the electrochemical properties were strongly influenced by theresidue after gas generation.The new generation of Li-rich solid solution materials are received extensiveattention due to their high capacity and high energy density. But their newelectrochemical charge/discharge mechanism is not clear. The cathode materialsLi[Ni1/3Li1/9Mn5/9]O₂and Li[Ni1/2Mn1/2]O₂were synthesized by co-precipitationreaction. The differential capacitance curves showed three major reactions in the firstcharge process of the Li-rich material: the oxidation of Ni^2+/4+below4.5V, theextraction of oxygen and the exchange of proton above4.5V, and the capacitycontribution of them were65.2%,25.6%and9.2%, respectively. The oxidation ofNi^2+/4+is the only reaction in the following charge processes. The processes of thefirst delithiation/lithiation of Li[Ni_xLi_{（1/3-2x/3）}Mn_{（2/3-x/3）}]O₂（x=1/3） cathode wereinvestigated by electrochemical impedance spectroscopy （EIS）. The EIS results showthat Li-rich material can form a stable SEI film in the charge/discharge process eventhough the breakdown or dissolution of the resistive SEI film occurs in the highvoltage; the electronic properties change with oxidation reactions and phase transition;the continuing increase of the charge transfer resistance may be the major reason thatthe Li-rich materials have a poor performance in the high current rate.To further elucidate the delithiation/lithiation mechanisms of the intercalationcompound, EIS was used to measure variations of spectra of LiMn₂O₄electrode withthe temperature in the range-20²0℃in1mol/L LiPF₆-EC: DEC: DMC,1mol/L LiPF₆-EC: DEC: EMC and1mol/L LiPF₆-EC: DMC electrolyte solutions. Theresistance of the SEI film, the electronic resistance as well as the charge transferreaction resistance with variations of temperature were studied and the related kineticparameters the energy barriers for the ion jump relating to migration of Li-ion throughthe SEI film, the thermal active energy of the electronic conductivities and thedelithiation/lithiation reaction active energies were determined. In above threeelectrolyte solutions, LiMn₂O₄electrode had the largest kinetic parameters in1mol/LLiPF₆-EC: DMC electrolyte solution, which was consistent with the worstelectrochemical properties in this electrolyte solution.Influences of VC in the delithiation/lithiation process of LiNi_1/3Co_1/3Mn_1/3O₂electrode with different experiment condition were investigated by EIS from the pointof kinetic view. It was found that VC was favor of producing an stable SEI film on theelectrode, increasing the ion conductivity and the diffusion process inside of theelectrode. In addition, the activation energy of the ion jump, the thermal activationenergy of the electrical conductivity as well as the activation energy of thedelithiation/lithiation reaction of LiCoO₂electrode were determined to be37.7,39.1and69.0kJ/mol respectively with the temperature in the range0³0℃in1mol/LLiPF₆-EC: DMC electrolyte solution.