Capacity Fading Mechanisms And The Controlling Strategies For Lithium Ion Batteries

As high efficiency energy storage and conversion systems,lithium ion batteries?LIBs?have many advantages including high energy density,good cycling performance,low self-discharge rate and environmentally friendliness.With the development of electric transportation and large-scale energe storage systems,the demands of LIBs are dramatically increasing.Long cycle life is one of the prerequisites for large-scale applications of the state-of-the-art LIBs.Capacity degradation is not caused by any single factor,but a consquence of many electrochemical processes and the interaction.Therefore,it is of great importance to discover the key factors and mechanisms of the capacity deterioration.This enables us to take effective measurements to develop advanced LIBs with prolonged cycle life.In this work,long-term cycling behavior of commercial lithium iron phosphate?LiFePO₄?/graphite cells and LiNi_0.5Co_0.2Mn_0.3O₂?NCM523?/graphite cells under normal operation condition is investigated.The main factors reasonable for the capcity fading were quantitatively determined by using electrochemical and inductively coupled plasma-optical emission spectroscopy?ICP-OES?techniques.Lithium inventory loss is pecified to be the most important reason for the capacity decay of different lithium ion batteries.For suppressing capacity fading,carbon nanotubes?CNTs?was in-situ grown on graphite surface through chemical vapor deposition?CVD?method.Physical and electrochemical performances of the graphite@CNTs composite were investigated.Cycle life of the graphite electrode is significantly prolonged.LiFePO₄-based LIB is the first choice for large-scale energy storage systems.Commercial 18650 LiFePO₄/graphite cells were subject to deep charge-discharge cycling and constant voltage storage tests.The electrochemical analyses and ICP technique confirm that the capacity decay of the cells?5%,10%and 15%?under charge-discharge cycling is correlated with lithium deposition on the graphite surface.The results show that lithium inventory loss is the main cause for the capacity loss and the majority of active lithium loss can be found on the graphite surface due to the growth of the solid electrolyte interphase?SEI?.When the cells were held at different voltages?2V,3.45V and 3.9V?for 60 days,negligible capacity loss was obtained compared to that underwent deep charge-discharge cycling,implying that cell voltage is not an important factor affecting the SEI stability and lithium consumption.The damage and repair of SEI film is mainly resulted from the volume change of graphite particle due to lithium insertion and extraction.LiNi_xCo_yMn_zO₂?NCM?-based LIB is the most promising battery for electric vehicle applications.Quantitative analyses of the electrode surface evolution for commercial 18650LiNi_0.5Co_0.2Mn_0.3O₂?NCM523?/graphite cells during ca.3000 cycles under normal operation are presented.Electrochemical analyses and ICP technique confirm lithium inventory loss makes up for ca.60%of the cell's capacity loss.Electrochemical deterioration of the NCM523 cathode is identified to be another important factor,which accounts for more than 30%of the capacity decay.Irregular primary particle cracking due to the mechanical stress and the phase change aroused from Li-Ni mixing during repetitive cycles are identified to be the main contributors for the NCM cathode deterioration.The amount of transition metal dissolved into electrolyte is determined to be quite low and the resulted impedance rise after about 3000 cycles is obtained to be twice that of the reference cell,which are not very significant affecting the long-term cycling performance under normal operations.Hairy graphite is synthesized through a modified chemical vapor deposition?CVD?method,in which 2,3-dipicolinic acid and Ni?NO₃?₂ are employed as the carbon source and the catalyst precursor,respectively.Under Ar/H₂?95:5?reducing atmosphere at 900 ^oC,Ni?NO3?2 is reduced to nickel nanoparticles and 2,3-dipicolinic acid is pyrolyzed into carbon fragments.With the catalysis of nickel nanoparticles,carbon nanotubes?CNTs?are in-situ grown on the graphite surface through realignment of the carbon fragments.The hairy pattern of geometry of the graphite@CNT composite not only improves the mechanics and conductivity of the electrode,but also has more room to accommodate the volume change of the graphite particle upon prolonged electrochemical cycles.A superb rate performance and prolonged cycle life are obtained.The simple and effective strategy for integration of graphite with in-situ grown CNTs is very useful for boosting its electrochemical properties.