Research On The Composite Structure To Relieve The Circulation Volume Effect Of Li Metal And Si Anode

With the development of science and technology,people have set much higher requirements for lithium battery system in the future,especially higher specific capacity density and safety requirements.In the case where the specific capacity of the existing positive electrode material gradually approaches its theoretical limit value,developing a new type anode material with high specific capacity density is one of the most effective methods for further increasing the specific capacity density of the battery.Among them,the lithium metal negative electrode(3860 mAh g^-1)and the silicon negative electrode(4200 mAh g^-1)are highly expected by people.However,these two kinds of negative electrode materials have serious circulation volume effect during charging and discharging process.The huge volume change will not only affect the specific capacity,but also bring some safety risks.The existence of the circulation volume effect greatly limits the application of lithium metal negative electrodes and silicon negative electrodes,and is an obstacle that must be overcomed for further development of lithium batteries.Therefore,this thesis propose a composite structure design for lithium metal negative electrode and silicon negative electrode respectively,expecting that the effect of the circulation volume can be effectively alleviated,and both the high specific capacity and high safety can be achieved.In order to effectively reduce the circulation volume effect of lithium metal,this paper presents a simple method that can be used to prepare a complex lithium metal negative electrode with a 3D framework inside it.By processing the carbon fiber structure obtained by electrospinning at a high temperature,the ordinary carbon matrix is converted into a surface graphitized carbon stent which has a good Li affinity.Then the stent is immersed in the molten lithium,and the molten lithium can be adsorbed easily into the stent under the capillary effect.Finally,a composite lithium metal negative electrode having a 3D skeleton inside is obtained.The existence of the 3D carbon skeleton greatly increases the electroactive surface area of the negative electrode,so that the nucleation sites can be increased a lot,and the effective current density of the electrode can be decreased.It is conducive to the uniform deposition of lithium ions,and the circulation volume effect of the lithium metal negative electrode can be effectively alleviated.Compared with the bare lithium negative electrode,the 3D lithium composite negative electrode has more stable lithium ion deposition-dissolution behavior,lower nucleation barrier,higher ion migration efficiency,and thus has higher cycle capacity and much better rate performance.In order to effectively reduce the circulation volume effect of the silicon negative electrode,this paper proposes a composite structure design that combines the advantages of a sandwich structure and a flexible structure.To obtain a flexible silicon negative electrode material with a sandwich composite structure,lithium phosphorus oxynitride?LiPON?,Si and Cu nano-films were sputtered on the separator in sequence by magnetron sputtering technology,Under the protection of the LiPON film,the negative electrode has a more stable SEI film.Meanwhile,the silicon negative electrode can effectively avoid the problem of stress concentration due to the elastic deformation of the flexible substrate?separator?,even after 1000 cycles,the electrode morphology still remains intact and stable.With the synergistic effect of the LiPON film and the flexible separator,the circulation volume effect of the silicon negative electrode can be effectively alleviated,and the battery also exhibits high reversible capacity,excellent cycle stability,and good rate performance.