Investigation On-NH₂ Grafted Polymer And Its Application In Solid Polymer Electrolytes/Binders For Lithium Metal Batteries

For the past fewer years,due to the widespread use of portable electronics and new energy vehicles,the demand for lithium-ion batteries,especially lithium batteries with high specific energy density,has continued to expand.However,traditional graphite based batteries are far from meeting the demand.Therefore,the development of materials with high specific capacity such as silicon,lithium,and other materials is considered a mainstream method to solve the problem of high-density rechargeable batteries.However,for lithium metal,the battery cycle has been faced with challenges such as common lithium dendrite growth,unstable SEI film and flammable organic electrolyte leakage.At the same time,the problems such as particle pulverization and electrode destruction caused by the huge volume expansion of the silicon anode greatly limit the commercial application.Polymers are regarded as crucial materials for developing polymer electrolytes and binders and constantly improve the performance of batteries.It can optimize and solve the lithium metal anode and silicon anode problems.For example,compared to liquid electrolytes,solid polymer electrolytes（SPEs）can avoid flammable organic electrolyte leakage,and have high safety,flexibility,and good interfacial compatibility.Although the binder occupies a small proportion in the whole battery system,it also plays a vital role in the battery performance.Therefore,it is of great practical value to optimize the composition and structure of polymer to solve the physical and electrochemical characteristics of polymer electrolyte and to alleviate the volume expansion caused by silicon anode materials during lithium intercalation.In this paper,polyethylenimide（PEI）rich in amino groups was selected to prepare-NH₂grafted solid polymer electrolyte PN-g-PHE（PVDF-HFP@PEI@Li TFSI）and three-dimensional cross-linked network polymer binder PN-g-PH（PVDF-HFP@PEI）.The results show that the grafted polymer can stabilize the interface between PN-g-PHE and electrode,restrain the growth of dendrites on the surface of lithium metal,inhibit the volume expansion of silicon anode,and improve the cycle stability of the battery.The main results obtained in this paper are as follows:（1）A solid polymer electrolyte PN-g-PHE was prepared,which was crosslinked with PEI and poly（vinylidene fluoride-co-hexafluoropropylene）（PVDF-HFP）grafted with-NH₂and composed of lithium bis（trifluoromethanesulphonyl）imide salt（Li TFSI）.This novel polymer can greatly improve the Li⁺migration number and stabilize the lithium metal/electrolyte interface,enabling the construction of high-performance solid LMBs.The experimental results showed that compared with PHE（PVDF-HFP@Li TFSi）without-NH₂ grafted solid polymer electrolyte,PN-g-PHE had a tensile strength of up to 6.73 MPa,higher ionic conductivity(2.14×10^-4 S/cm),Li⁺transference number（0.55）and wider electrochemical stability window（4.52 V）.Li/Li symmetrical cells can cycle stably for more than 1000 h at 0.1 m A/cm² and 0.2m A/cm² at 60℃.When the current density is further increased to 0.5 m A/cm²,it can still cycle stably for more than 300 h,especially the critical current density can reach 1m A/cm².In addition,Li/LFP full cells and multiplier performance are remarkable.It still maintains an ultra-high-capacity retention rate of 97%and coulomb efficiency of nearly 99%after 300 cycles at 0.2 C.In addition,the capacity retention of PN-g-PHE cell is still up to 84.9%after 300 cycles at 0.5 C.This work indicates that-NH₂ grafted polymer has a potential application in the development of solid lithium metal battery systems.（2）Based on the unique 3D network structure formed by PVDF-HFP and PEI crosslinking,we further designed the grafted-NH₂polymer binder PN-g-PH to inhibit the volume expansion of Si@C and enhance the cycle stability of the silicon-based negative electrode.The thermal stability,stripping test and mechanical properties of PVDF,PVDF-HFP and PN-g-PH binders were characterized.The results showed that PN-g-PH binders showed good thermal stability,strong bonding ability and excellent mechanical properties.In addition,Li/Si@C cells with PN-g-PH electrode has better cycle stability and rate performance.The PN-g-PH electrode retained a capacity of 438m Ah/g after 100 cycles with a capacity retention rate of 40.8%at 0.2 C（1 C=950 m A/g）current density,which was significantly higher than that of the PVDF electrode（21.7%）and PVDF-HFP electrode（11.4%）.The results of this study show that the enhanced mechanical properties of the unique cross-linked network structure can be used to alleviate the fatal problem of large volume expansion in the electrochemical process and improve the electrochemical performance of electrodes.