Partial Lithiation Strategies For Suppressing Coupled Mechanical-Electrochemical Degradation Of Silicon Composite Electrodes

Silicon has been regarded as a promising candidate of active materials for next generation of lithium ion batteries because of its high theoretical capacity,abundant natural sources and low electrochemical potential.However,silicon commonly suffers from drastic volume changes(~300% increase upon full lithiation)during electrochemical cycling,leading to severe pulverization,electrical contact loss,unstable solid electrolyte interphase(SEI)as well as subsequent poor electrochemical performance.There have been a lot of attempts such as nanonization of silicon,selection of binder and fabrication of Si-based composites to mechanically stabilize the silicon electrode and improve its electrochemical performance.Most of these approaches involved new materials or complicated treatments.Long time consuming and high costs in producing electrodes make it difficult to widely apply silicon based electrodes.Therefore,development of a more facile and effective approach is in urgent demand.One possible approach is partial lithiation strategy of silicon.Two types of partial lithiation,i.e.voltage-and capacity-control strategies,have been investigated.In addition,a combined procedure that adding a phase of constant voltage to the capacity-control strategy have been investigated.The effects of different upper-limit voltage on the battery performance have been discussed.Through such simple modifications in the charge and discharge operations,the deformation of silicon particles can be controlled,and coupled mechanical-electrochemical degradation of silicon composite electrodes can be suppressed.Specifically,capacity loss and irreversibility of silicon electrodes are significantly suppressed by partial lithiation strategy without any complicated modifications or treatments.At an early stage the reversible capacity of partially lithiated electrodes is lower than that of fully lithiated one,but the capacitydecreases more slowly,the capacity of partially lithiated electrodes becomes higher after certain cycles.By partial lithiation strategies,the size of hysteresis loops significantly shrinks,indicating that the energy dissipation can be suppressed by partial lithiation method.It is known that the voltage hysteresis indicates energy dissipation which is a severe drawback of silicon electrodes and further leads to heat generation.The images of scanning electron microscope suggest that the mechanical damage generated in partially lithiated electrodes is less than that in fully lithiated one.When the lower limit cut-off voltage is 70 mV,the reversible capacity of silicon composite electrodes becomes and keeps being higher than that of the fully lithiated one after 77 cycles,and its capacity remains above 1000 mAh/g,as high as 1.53 times of the fully lithiated one at the 300 th cycle.When the lower limit cut-off voltage is 160 mV,the capacity of silicon electrodes is higher than that of the fully lithiated one after 298 cycles.The reversible capacity of the capacity-control strategy are more stable and barely change at the first stage.The electrode degradation cannot be characterized by the capacity loss but is reflected by the decrease in lower-limit voltage and the increase in irreversible capacity.After constant voltage operations were involved in capacity-control strategy,the first stage of silicon composite electrodes keeps longer,but in the second stage,the electrode degradation is severer.In general,the presented experimental results suggest that the partial lithiation strategy relieves the coupled mechanical-electrochemical degradation,reduces the deformation and damage of electrodes and is a promising approach for the long-term use of silicon composite electrodes.