Architecting High-performance Li-based Electrode Via Tuning Surface/interfacial Properties

Li-based electrodes with high energy/power densities have been attracting wide attention,and the surface/interface engineering can optimize the overall performance of rate,life and security.This thesis focuses on the surface/interface mechanisms,thereof appliable for understanding lithium dendrites and paving a new route for designing dendrite-free anode.Then originated from semiconductor-electrochemistry theory,a model has been proposed with insight into surface/interface of Li ion battery.Interfaces of substrate/active materials/electrolyte can be better understood,and there are two interphases: one is electronic transport in interface of substrate/active materials,and another is ionic transport in interface of active materials/electrolyte or inside active materials.By the way,the advanced PECVD(plasma enhanced chemical vapor deposition)technology was used for optimizing these interfaces of current collector/active materials and interfaces of active materials/electrolyte.(1)By insight into the mechanism of Li dendrites growth,a model based on nucleation of film growth has been put forward.Li dendrites are rooted from partial electron energy changing into surface energy of Li anode,then the surface of Li metal becomes more rough.Under electric field,there are plenty of charges on the surface of Li anode,which has been shifted into lower surface energy(a negative shift to initial surface energy).As a result,the Li anode film is intended for island nucleation,preferring for the formation of dendrites.Therefore,on one hand,the SEI film should be beneficial from the dynamics uniformity;on the other hand,the substrate also needs to be improved for thermodynamic capability of suppressing dendrite.(2)Based on the theory of homogenous nucleation,the Li anode was modified via mechanical pressing by Cu net,resulting in a net-like Li pits.The modified Li anode shows a stable reducing polarization and an optimizing rate performance.The SEM observation suggests there is no obvious dendrite for modified Li after cycles,and a zipper-like mechanism was used to explain this phenomenon: controlling SEI film broken regions to minimize releasing stress in terms of weaving lithium pits,leading to the cycling stability.(3)Based on the theory of semiconductor-electrochemisty,a model has been proposed to understand the interfaces of substrate/active materials/electrolyte.The interfacial reaction lies in band bending,originated from electronic thermodynamics and ionic kinetics.Thereof,PECVD was introduced to optimize the interfaces by two routes: one is wettability of substrate/active materials for large contact area;the other is high ionic transport of SEI film areas.(4)PECVD technology can optimize the interface between active materials and electrolyte.It is well accepted that carbon coating can improve rate and life of Li ion battery.However,amorphous carbon of traditional coating layer can only act as an insulated layer.Therefore,we re-define the vertical graphene/carbon nanowalls,and firstly propose anti-T-shaped graphene interface: "L" shaped graphenes are linked back-to-back forming ?-? bond offering with complete coating.The parallel layer acts as a protecting layer and the vertical layer enables a short and fast Li ion channels.Herein,anti-T-shaped graphene coating on the carbon fiber was prepared for Li storage materials.Benefited from the electronic and dimension-confined issues,as well as short Li ion migration distances,the anode affords well reversible Li ion storage performance.(5)PECVD technology can also contribute to the optimizing of the interface between active materials and current collector.We prepared vertical graphene nanowalls anchored on copper foil(VG)as current collector for graphite anode of Li ion battery via PECVD.Benefited from the wettability between current collector and active materials,and the properties of SEI film on the current collector,the half batteries of VG,commercial carbon coated copper(CC)and commercial copper(BC),show that at 3C,the discharge capacity of VG is 190 mAh/g,in contrast with 160 and 90 mAh/g for CC and BC,respectively.