Zero-Volt Storage Of Li-Ion Batteries:Degradation Behavior And Tolerance Approaches

During the storage and shipment of lithium-ion batteries,pre-discharging each battery to zero-charge state(zero-volt state)enables them to avoid risks of thermal runaway.During zero-volt storage of batteries,the high zero-volt crossing potential(ZCP)causes oxidative decomposition of the solid electrolyte interface(SEI)film on anodes and corrosive dissolution of copper current collectors of anodes,further resulting in capacity degradation or even complete failure of batteries.The research of this dissertation is aimed at deeply understanding the capacity degradation mechanisms of lithium-ion batteries stored at zero volt,and developing approaches for lithium-ion batteries to tolerate zero voltage.Much literature has reported the corrosion resistance improvement of anode current collectors.But the research on improving the long-term stability of the SEI film on anodes at high potentials has been widely ignored.This dissertation proposes a method to improve the long-term stability of the SEI film on anodes at high potentials during the zero-volt storage of batteries.The method is adding lithium difluoro(oxalate)borate(Li DFOB)into electrolyte to optimize the proportion of SEI-film components on anodes.First,by means of the electrochemical method and the scanning electron microscope(SEM),we studied the effects of Li DFOB on the long-term stability of the SEI film on the mesocarbon-microbead(MCMB)anodes at high potentials.Then through the X-ray photoelectron spectroscopy(XPS)characterization,we analyzed the mechanism for the improved long-term tolerance of the anode SEI-film to high potentials by Li DFOB additive.The results show that Li DFOB additive changes the composition ratio of the SEI film.The proportion of components easily decomposed at high potentials,such as alkyl carbonates and phosphates,is reduced.Meanwhile the proportion of components stable at high potentials,such as carboxylates and Li F,is increased.Besides using Li DFOB additive,we employed the prelithiated anode to lower the ZCP,thus significantly improving the capacity recovery rate of the LiCoO₂/MCMB cell experienced the 7-week zero-volt storage from 15.7%to 93.3%.Because the electrochemical prelithiation process is hard to be applied in manufacture,this dissertation proposes adding Li₅FeO₄ to cathodes as a new method for the down-regulation of the ZCP.The plateau below 2.9 V vs.Li/Li⁺for Li⁺intercalation into the charging product from Li₅FeO₄ favors inhibiting an excess of lowering the ZCP,thereby avoiding the deleterious effects of the excessively low ZCP on the stability of cathode materials.By adding 23 cap%Li₅FeO₄ into the cathode,the ZCP of the LiCoO₂/MCMB cell was moderately lowered to 2.4 V vs.Li/Li⁺,thus avoiding the dissolution of the anode copper current collector and mitigating the decomposition of the anode SEI-film.Consequently,the capacity recovery rate of the cell stored at zero volt for 10 days is significantly increased from 37.6%to 95.5%.Through further adding2 wt%Li DFOB into the electrolyte,the capacity recovery rate of the cell stored at zero volt for 7 weeks is significantly increased from 15.7%to 95.4%.The ZCP of the LiCoO₂/MCMB cell with 23 cap%Li₅FeO₄ added into the cathode rises above 3.5 V vs.Li/Li⁺again after the cell capacity decreased by 30%with cycling.As a result,the cell degrades much.To solve this issue,this dissertation proposes the combined employment of Li₅FeO₄,Li DFOB and a copper dissolution inhibitor succinonitrile(SN).First,through polarization curves and the energy dispersive X-ray spectroscopy(EDS)characterization,we studied the effect of Li DFOB combined with SN on the dissolution suppression of copper current collectors.The combination of Li DFOB and SN also improves the long-term stability of the SEI film on MCMB anodes at high potentials.By the addition of 2 wt%Li DFOB and 3 wt%SN into the electrolyte of the LiCoO₂/MCMB cell with 23 cap%Li₅FeO₄ added,after the capacity of the cells decreased by 30%and 50%with cycling,their capacity recovery rates for the 10-day storage at zero volt are up to 104.0%and 101.8%,respectively.We employed the electrolyte additive vinylene carbonate(VC)for improving the cycling performance of Li Ni_0.8Co_0.15Al_0.05O₂(NCA)electrodes.VC can transform the discrete SEI film on the NCA particles into the continuous and uniform SEI film.The research of the electrochemical impedance spectroscopy(EIS)indicates that the VC additive is beneficial to lower the increase rate of the SEI impedance and the charge transfer impedance with cycling through inhibiting side reactions at the electrode/electrolyte interface.Because the anode potential of the NCA/MCMB cell rises more slowly than that of the LiCoO₂/MCMB cell during the over-discharge,no dissolution of the copper current collector of the anode occurred.The capacity recovery rate of the NCA/MCMB cell reaches 96.0%after over-discharged to zero volt and then charged and discharged immediately.This recovery rate is higher than that of the LiCoO₂/MCMB cell.In addition,through adding 13 cap%Li₅FeO₄ to the NCA cathode,the ZCP of the NCA/MCMB cell is lowered to 2.4 V vs.Li/Li⁺,thereby enhancing the capacity recovery rate of the cell stored at zero volt for 10 days from 63.0%to 97.2%.The approaches in this dissertation enable lithium-ion batteries to tolerate zero-volt storage.These approaches are expected to be applied in the battery systems of electric vehicles and spacecraft,and contribute to the safe transportation,storage and use of lithium-ion batteries.