Reasonable Design Of High-performance Binder Applied To Silicon-based Anode Batteries

High-capacity electrode materials play a vital role in high-energy-density lithium-ion batteries.Silicon（Si）material is considered to be a promising anode material due to its excellent theoretical capacity,but it has the problems of rapid capacity decay and short cycle life due to volume expansion.The binder plays the role of fixing the active material and the conductive agent on the current collector and maintaining the complete structure of the battery,which has proven to greatly improve the electrochemical performance of the Si anode.It is still a major challenge to adopt a reasonable binder strategy to achieve high-capacity Si anodes.The focus of this paper is to introduce super-elastic elastomer into the traditional strong binder as the binder of lithium Si battery,optimize and design the composite binder to achieve the purpose of stress dispersion of Si particles,and further Improve the electrochemical stability of Si negative electrode of lithium ion battery.The main research is as follows:（1）Super-elastic carboxyl butyronitrile（XNBR）latex is introduced into the rigid guar gum（GG）structure to prepare a composite binder that has both soft and hard properties.The hard GG can be used as a skeleton to support the entire electrode structure,and the elastic nitrile rubber can be used as a buffer medium.When the electrode undergoes large volume expansion,it can automatically restore the original structure of the electrode,maintain the integrity of the electrode,and improve the cycle life of the lithium Si battery.Our research shows that after 100 cycles,the X4G6@Si（Si anode with a mass ratio of XNBR to GG of 4:6）still has a reversible capacity of 1929mAh g^-1,and its capacity retention rate is as high as 65%.At the same time,under the same charge and discharge conditions,the specific discharge capacity of the Si-based negative electrode with a single XNBR as the binder after 100 cycles is only 810 mAh g^-1;the reversible capacity of the Si negative electrode with pure GG as the binder It is1078 mAh g^-1,and as a control group,the discharge capacity of the Si negative electrode with polyvinylidene fluoride（PVDF）as the binder is close to 0 mAh g^-1 after 100 cycles.Therefore,by using the optimally designed X4G6 binder,the electrochemical cycle stability of Si-based lithium-ion batteries can be greatly improved.Lithium-ion half-cells use Si as the working electrode,and pure lithium foil as the reference electrode is the most common configuration of half-cells in published studies.Although promising Si anode performance has been achieved in Si anode-to-lithium foil half-cells,these batteries are not practical systems.In addition,due to the limited supply of lithium by the positive electrode and the side reactions occurring on both the negative electrode and the positive electrode,the failure mechanism of the Si anode in a full battery may be different from that in a half battery.Here,we report the electrochemical behavior of X4G6 binder in Si/lithium iron phosphate（LFP）button batteries.Compare the results with the results of using PVDF as a binder.This work shows that the Si anode using LFP as the positive electrode and X4G6 as the binder can provide a reversible capacity of997 mAh g^-1 at a current density of 400 mA g^-1 in a full battery after 30 cycles.（2）In this study,water-based polyurethane（PU）was synthesized,and the electrochemical performance of the Si anode with PU and GG as the binder was explored.The results show that with the mass ratio of PU and GG is 1:1,the mechanical properties of the obtained polymer are moderate,and the balanced mechanical strength is proved to be more conducive to the stable cycle performance of the Si anode.After 100 cycles,the battery capacity retention rate of the cross-linked GG and PU binder is significantly higher than that of pure GG and PU.After 100 cycles,the G1P1 Si anode shows a significantly improved reversible capacity of 1913mAh g^-1.The capacity retention rate is 53%,which is attributed to the soft and hard binder effectively reducing the volume change of the active material during the cycle processing,which indicates that the cross-linking of the binder enhances the mechanical properties of the binder network to resist the huge volume change of the Si anode,and further improves the reversible capacity and coulomb efficiency of the Si anode.