Electrochemical Kinetic Model And Electrochemical Window Of Organic Electrolytes

Electrochemical window（EW）of electrolytes is regard as the bottleneck of the working voltage range of lithium batteries,which affects the charge and discharge energy of battery.However,EWs are often overestimated in the previous theoretical studies,which deviate significantly from the experimental results,hindering the prediction and designing of electrolyte performance from theory.In addition,the actual working potential ranges of electrolytes depend not only on the electrolyte itself,but also on factors such as the catalytic activity of the positive electrode surface and the solid electrolyte interphase（SEI）at the anode interface.Therefore,further investigation and clarification are needed on the intrinsic factors that determine the electrochemical window of electrolytes and the apparent effects on the interface.In response to the inaccurate prediction of the electrochemical window of electrolytes by traditional calculations,this dissertation proposes an innovative combined method without empirical parameters based on the thermodynamic and the Marcus-Gerischer theory of electrochemical kinetic reactions,and combining condensation effects,molecular thermal motion,solvent effects,interfacial electric fields,surface catalysis,and passivation effects.Quantitative calculations are performed to recur the linear scanning voltammetry（LSV）current curves for commonly used organic electrolytes,polyethylene oxide（PEO）and ethylene carbonate（EC）-dimethyl carbonate（DMC）,in lithium batteries,to analyze the source composition of the electrolyte oxidation-reduction current curve and distinguish the decomposition mechanism at different potentials.At the same time,the mathematical expressions of the intrinsic and apparent electrochemical windows of organic electrolytes are solved.Since the algorithm extrapolates the results to the condensed phase instead of only describing isolated molecular systems,the predicted electrochemical window compared with the LSV experimental results can reduce the error to within 0.1 V or less.Analysis from the model shows that the intrinsic EW on the oxidation side of the electrolyte is restricted by the solvent effect of the Marcus theory,for example,the intrinsic EW on the oxidation side is 4.06 V（relative to Li⁺/Li）for the PEO,and 5.12V for EC-DMC.However,since the electron occupancy state distribution caused by the solvent effect on the oxidation side has the same average position as the highest occupied molecular orbital（HOMO）energy level,the intrinsic electrochemical window on the oxidation side is also constrained by the position of HOMO.But the intrinsic electrochemical window on the reduction side is usually only constrained by the solvent effect,regardless of the lowest unoccupied molecular orbital（LUMO）.This is because the increase of the system free energy with decreasing external potential significantly initiates decomposition before the LUMO level constraint due to free energy instability.In addition,for the cathode with catalytic activity,a surface model was established to study the valence change of local transition metals on the surface and the difference of the Li_xCo O₂ surficial potential during the quasi-lithiation and delithiation process.And the inherent catalysis of the Li_xCo O₂ surface is corrected into the above calculation model.The calculations show that the catalysis of the Li_xCo O₂ cathode and the distribution of HOMO for EC-DMC electrolyte jointly limit the apparent EW to 4.48 V on the oxidation side,which is consistent with the LSV experimental curve results.The anode surface has complex passivation and SEI,so the surface potential of anode is different from the electrode potential.Therefore,there is no exact position of the apparent EW in principle.The passivation effects generated by various systems have great influences on the apparent EW and need to be studied separately.However,the lowest LUMO energy level in the electrolyte and its decomposition products can be used as a reference for EW.For example,acetaldehyde,a decomposition product of EC generated by the solvent effect,has a lower LUMO level than EC’s LUMO,constraining the apparent EW on the EC-DMC electrolyte reduction side to about 0.30 V without lithium salt.Additionally,based on this new model,a new single electron reduction decomposition mechanism for EC is revealed on the anode side dominated by solvent effects.It occurs in the range of about 1.50 to 2.35 V,i.e.,outside the intrinsic EW and within the two-electron reduction window.The predicted products are acetaldehyde and CO₂,also confirmed by gas chromatography-mass spectrometry（GC-MS）.The difference from other reduction mechanisms is not caused by electron gain first and then decomposition.Instead,the state first moves on the neutral potential energy surface,when it moves to the high energy position intersecting with the reduced potential energy surface,electrons are gained and complete the reduction with the bond cleavage process,which is consistent with the characteristics of the solvent effect theory.Therefore,the reaction products are different,and this reaction does not require the participation of lithium salt.When lithium salt participates in the process of forming passivation layers,it is easy to form a stable SEI that can shield the electrode potential and greatly widen the apparent EW.Since the intrinsic EWs on the reduction side of most solvents are often relatively narrow,the passivation effect plays a crucial role in lithium batteries.However,the influence of lithium salt on the electrochemical properties is complex and diverse,and further in-depth research is still needed.