Lithiophilic Modulation Of Carbon Host Materials For The Protection Of Lithium Metal Anode

Lithium metal（Li）has been known as the "holy grail" of anode materials for secondary lithium batteries because of its ultra-high specific capacity（3860 mAh g-1）and extremely low redox potential（-3.04 V）.When Li metal is applied as anode for lithium batteries,including solid-state batteries,Li-S and Li-air batteries,greatly improved energy densities can be obtained.However,there are many scientistic and technolical challenges for Li metal anode before its commercial application,such as the growth of lithium dendrite,uncontrollable lithium deposition and infinite volume expansion,et al.These problems will lead to low Coulombic efficiency,worse charge-discharge cycling stability,and even safety risks.Therefore,it is important and necessary to solve these concerning issues.Various strategies have been propsaled to solve these problems,including optimizing electrolyte,fabricating artificial SEI,developing solid electrolyte and constructing host materials.Although the formal three strategies can effectively inhibit the growth of lithium dendrite,they cannot avoid the volume expansion of lithium metal during the repeatly Li deposition/stripping process.Currently,constructing 3D host materials is attracted much attention,because it can effectively reduce the local current density,adjust the Li deposition/stripping behaviors and limit the infinite volume expansion.Now,widely reported 3D host materials include carbon based,metal based,and multivoid ceramic based and other host materials.Among them,carbon based host material is one of researching hotspots due to excellent electrical conductivity and flexibility.Unfortunately,lithiophilic nature of normal carbon materilas is poor,which don’t benefit uniform Li nucleation and deposition.Here,this thesis focuses on improving the lipophilicity of 3D carbon based host materials to obtain efficient and stable Li metal anode.The specific content includes the following three aspects:（1）N,P co-doping to improve the lithium affinity of hollow carbon fibers host.We prepared free-standing N,P co-doped hollow carbon fibers（NPHCF）as efficient hosts for stable Li metal anodes with hollow polypyrrole（Ppy）fibers as template by simple filtration method and low temperature phosphatizing.The formation of pyrrole nitrogen and pyridine nitrogen enhanced the lipophilicity of carbon fiber,and the generation of P-C bond improved the electrical conductivity of the material.Benefitting from N,P co-doped,NPHCF hosts demonstrate improved lithiophilicity with the lower lithium nucleation overpotential.In addition,hollow fiber structure can not only reduce the local current density,but provide more space for Li deposition.As a result,an efficient and stable Li metal anode was obtained.Bsed on the as-prepared Li metal anode,the assembled symmetric battery can be stably cycled for over 250 cycles at 0.5 mA cm-2 and 0.5 mAh cm-2 without short circuit.The assembled LiFePO4‖NPHCF@Li full cell delivers a high initial discharge capacity of 164 mAh g-1,and successfully runs over 100 charge-discharge cycles at 0.2 C with a capacity retention rate of 72.13%.（2）Planting SnO₂ nanoseeds to improve the lithium affinity of carbon nanotube（CNT）hosts.In this chapter,we prepared SnO₂@CNT@SnO₂（SCS）host with sandwich structure to obtain stable lithium metal anode.Due to excellent electronic conductivity and hollow structure of carbon nanotubes,as well as the excellent lithium philicity of SnO₂ nanoseeds,SCS efficently reduce the lithium nucleation overpotential and increase the surface density of lithium deposition.The lithium nucleation overpotential of SCS was only 9 mV at 0.5 mA cm-2 and 0.5 mAh cm-2,Based on the as-prepared Li metal anode with 4 mAh cm-2 Li depositon,the assembled symmetrical battery was successfully operated for 300 cycles at 0.5 mA cm-2 and 0.5 mAh cm-2.The assembled LiFePO4‖SCS@Li full cell showed a capacity retention of 80%after 150 charge-discharge cycles at 0.5 C.（3）Introducing CoO,CoO/Co and Co nanoparticles assisting N-doping to enhance the lithiophilicity of acetylene black.In this chapter,three host materials,Co@N-C,Co/CoO@N-C and CoO@N-C,were prepared by one-pot water bath and high temperature calcination,respectively.The results show that Co@N-C exhibits the best lithiophilicity,which delivers a Coulomb efficiency as high as 98.6%,and a long-term cycling stabaility up to 200 h under the conditions of 1.0 mA cm-2 and 1.0 mAh cm-2 withouth short circuit.The symmetrical battery composed of 4 mAh cm-2 Li deposition can cycle stably for 1000 h at 0.5 mA cm-2 and 0.5 mAh cm-2.The assembled LiFePO4‖Co@N-C full cell showed a capacity retention of 84.65%after 130 charge-discharge cycles at 0.5 C.