A Study On Performance Failure Of Lithium Ion Batteries Via Anodic Side Reactions

This dissertation addresses the safety issues of lithium-ion batteries under the application backgrounds of electric vehicles and electric energy storage.In particular,the work focuses on understanding the failure mechanism through side reactions on the anodic surface of lithium-ion batteries with an electrode of Li₄Ti₅O₁₂(LTO),Li Fe PO₄ or Li Ni_xMn_yCo_zO₂(NCM)and developing the methods for improving the performance of these batteries.This study provides a reference for the safe use of lithium-ion batteries and the development of new products.The main work and achievements are as follows.The gassing behavior of the LTO battery during 55?cycling and shelving was studied by using a self-devised in-situ measuring device.It is found that the gassing control step is from H₂ generation reaction to CO₂.A mixed gassing mechanism is proposed,which is mainly composed of H₂O decomposition at the initial stage of gassing,followed by electrolyte solvent decomposition.The main reason for the failure of the LTO battery is the side reaction of gas production at the interface between the anode and electrolyte.In the process of gassing,a solid electrolyte interface(SEI)film of 10-25 nm was gradually formed at the anode interface,which inhibited the continuous gassing.In this dissertation,the electrochemical properties of pouch LiFePO₄/C batteries with 10-year calendar life were studied at different temperatures and different current densities at-10?.After cycling under different conditions,the thermal safety of batteries was studied by an Accelerating Rate Calorimeter(ARC)and aging at elevated temperatures was performed to investigate the thermal abuse behavior of the batteries.The results show that the main reason for the decline of electrochemical performance is the increase of internal resistance and the formation of non-electrochemical active lithium dendrites on the anode surface during the low-temperature cycling.The growth of lithium dendrites destroys the thermal stability of SEI film.Lithium dendrites reduce the temperature of self-heating(T_onset)and the temperature of thermal runaway(T_TR)which is from 170? to about 100?.Exothermic reaction and gassing are the main ways for lithium dendrites to affect battery safety.The performance degradation of the NCM batteries in the whole life under four typical conditions of high temperatures,low temperatures,high current density and slightiy-overcharge was studied.A rapid degradation mechanism of the performance of the NCM battery is proposed,which is the dissolution of the positive transition metal ions and their deposition on the anode surface to catalyze the decomposition of the electrolyte solvent to gas.The coupling of gas production,internal resistance and temperature rise is strengthened.Under the conditions of low temperature,high current and micro-overcharge,lithium plating occurs on the anode surface,which reduces the capacity of the battery and participates in the gassing reaction.The failure mechanism of LTO battery and graphite-based lithium-ion battery was compared,and the improvement of battery performance was explored.The results show that PC based electrolyte containing 0.5%LiBOB can effectively inhibit the high-temperature gassing of LTO battery.SiO₂ coating on the anode surface of graphite-based lithium-ion battery can not only inhibit lithium plating,but also improve the thermal safety performance of the battery.