Damage Suppression And Feigned Death Induced By Partial Delithiation In Silicon Composite Electrodes

Silicon has been regarded as one of the most promising negative electrode materials for new high-capacity lithium-ion batteries,but it undergoes huge volume change during charging and discharging,resulting in severe mechanical damage and rapid performance degradation to the electrode.By controlling the depth of delithiation,it can reduce the volume change of silicon and improve the cycle performance of silicon composite electrodes.However,since the silicon electrode is always at high depths of lithiation,the partial delithiation control of the electrode may associate with several mechanisms such us the phase transformation of lithiated silicon and voltage hysteresis.The impact of partial delithiation on the performance of silicon electrodes will be evaluated by experiments in the present study.In this paper,three upper cut-off voltages from high to low,namely 570m V,460m V and 320m V,were chosen,and they approximate 80%,60%and 30%of the initial delithiation capacity and the volume changes are 144%,108%and 64%respectively.The results of the charging-discharging cycle experiment of the silicon electrodes show that:Compared with the 10-1000m V,the partial delithiation of the silicon electrode under the voltage window of 10-570m V improves the electrode cycle performance without changing the battery material and treatment by reducing the volume change of silicon.By looking into the irreversibility,the crack,the elemental analysis and the thickness/weight of the silicon electrode,it is found that the 10-570m V case can suppress the coupled mechanical-electrochemical degradation and improve the long-term performance of silicon composite electrodes.However,the capacities of the partial delithiation strategies for the10-320/460m V cases fading rapidly at the very beginning of the testing cycles.Looking into the irreversibility,the crack,the elemental analysis and the thickness/weight of the electrode,it can be demonstrated that the 10-320/460m V cases experience less the coupled mechanical-electrochemical degradation.The reboot experiment（full charging and discharging,10-1000m V）suggests that the electrodes after hundreds of cycles with the upper cut-off voltages of 320/460m V are still close to fresh.To distinguish from the"true death"caused by degradation,the rapid capacity fading of 10-320/460m V cases have been named by"feigned death".To explore the feigned death and explain how it occurs,this paper further analyzes the d Q/d V profiles of silicon electrodes.According to the d Q/d V profiles,the asymmetric phase transformation of lithiated silicon(a-Li_xSi to c-Li_3.75Si and c-Li_3.75Si to a-Li_xSi)is recognized as a major reason for the feigned death.In combination with the c-Li_3.75Si occurs at high depth of lithiation and reverses at high depth of delithiation,the onset condition of the feigned death is identified.Namely,only if the lower cut-off voltage is lower than V_a-c（～60m V）and the upper cut-off voltage is lower than V_c-a（～440m V）,the feigned death can be triggered.Based on this mechanism,an effective way to identify and respond to the feigned death has also been provided in this work:the reboot,only adjusting the charging and discharging voltage window to determine whether feigned death occurs and to recover the initial cycle performance of the electrode.In general,the cyclic performance under different voltage windows for silicon electrodes has been evaluated in this work.It is found that the 10-570m V case can suppress the coupled mechanical-electrochemical degradation of the electrode and improve the cycle performance of the silicon electrode,while the 10-320/460m V cases induce feigned death.By analyzing the d Q/d V curve in the delithiation process of the silicon electrode,it is determined that the asymmetric phase transformation of lithiated silicon is the main reason for the feigned death.