Transient electrochemical methods for investigation of porous battery electrodes
Abstract: This thesis treats electrochemical methods used to studybattery materials. Two different types of materials are used asmodel systems in this study, metal hydride alloys used in thenickel-metal hydride battery and LiMn2O4suggested for use in the lithium-ion battery. Thetwo model systems are used to critically analyse differentexperimental methods. The three methods under study have beenelectrochemical impedance spectroscopy (eis), potential step and linear sweep voltammetry. Threedifferent experimental systems are studied, the singleparticle, the porous electrode and the cell.Using a commonly accepted kinetic model for the metalhydride, it was shown that a surface phase transfer reactionhas to be included in the analysis of potential step data. Theconsequences of this are discussed. The local kinetics of LiMn2O4were studied in order to investigate thepossibility to use a simplified mathematical model. It washowever found that this model had limited applicability.Methods suitable to study porous electrodes are also developed.It is shown that an average particle size cannot replace thatof a size distributed model in eithereisor potential step investigations. The potentialdistribution in the porous structure was investigated and amethod that makes it easier to separate between effects of theporous electrode and the local kinetic reaction is presented.This method utilises the impedance measured at the back of theporous electrode in addition to that measured at the front. Themethod was applied both to a porous metal hydride- and a LiMn2O4-electrode.Five different metal hydride materials were investigated andit was found that the material properties of metal hydridematerials are strongly influenced by production method.Important rate constants such as diffusion coefficient andsurface phase transfer parameter were determined both frommeasurements on porous electrodes and single particles. It wasfound that the values were distributed within a sample ofsingle particles. It was concluded that one reason is particlecracking and that this will have consequences for the studiesof porous electrodes. Important differences were also found bystudying single particles and porous electrodes and it wasconcluded that there are differences in material behaviourwhere additional processes seem to take place in the porouselectrode.Eiswas also used for determination of state of chargein commercial nickel-metal hydride cells. A statisticalevaluation that makes it possible to determine state of chargeboth at open circuit and during discharge is presented.Keywords: Electrochemical impedance spectroscopy, LiMn2O4,mathematical modelling, metal hydride,microelectrode, porous electrode, potential step, State ofcharge.
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