Thin Layer Fluorine Rich Interphase Phase Stabilized Alkali Metal Anode Based On PVDF And High-Valent Metal Fluorides

Alkali metal anode represented by lithium and sodium has attracted more and more attention due to its high theoretical specific capacity（Li 3860 m Ah/g,Na:1166 m Ah/g）and low redox potential.However,there are some common problems of dendrite growth and side reaction in alkali metal anode,which lead to poor safety performance and short cycle life of secondary batteries constructed by alkali metal anode.The fundamental cause of the above problems is the existence of thermodynamic instabity between the alkali metal anode and the electrolyte,which leads to the spontaneous formation of unstable solid electrolyte interface phase（SEI）on the electrode surface.The heterogeneous,poor mechanical strength and low ionic conductivity of the primary SEI lead to uneven alkali metal deposition and dendrite growth during the cycle,further reducing the coulombic efficiency and accelerating the accumulation of by-products.Therefore,the design of uniform SEI with certain mechanical properties and ionic conductivity can improve its ion transport rate and stability,promote uniform deposition of alkali metal and inhibit dendrite growth,which will provide an effective way to solve the problem of poor stability of alkali metal anode cycle.Based on the above ideas,the main research content of this theiss is as follows:（1）In order to solve the problem of lithium dendrite growth and by-products accumulation caused by unstable SEI of lithium metal anode,we proposed to construct an artificial ultra-thin layer based on PVDF and high metal fluoride on copper surface to realize the construction of rich fluorinated SEI,so as to regulate the deposition and stripping behavior of lithium metal and reduce the side reaction between lithium metal anode and electrolyte.At the same time,SEI stability was effectively improved.The ultra-thin layer based on the PVDF flexibility and the rigidity of high price metal fluoride and the derived"rich fluorinated interphase"with high interfacial energy together act as the interfacial phase to inhibit the growth of dendrites and slow down the generation of by-products,thus achieving the improvement of the cycle stability of the lithium metal anode.We construct a uniform rich fluorinated ultra-thin layer（PVDF-Ti F₄layer is referred to as PT layer）was constructed on the surface of copper electrode by a simple two-dimensional capillary array method.Firstly,the effect of the ratio of PVDF to Ti F₄ on the ion transport rate and cycling stability in PT layer was studied.It was found that the ionic conductivity increased with the increase of Ti F₄ content.When the mass ratio of PVDF to Ti F₄ was 5:5,the PT layer had the highest ionic conductivity(2.99x10^-5 S cm^-1),showing the best cycling stability.Secondly,the influence of PT layer thickness on ion transport path and cycle stability was studied.It is found that the PT layer is too thick to promote the transmission of lithium ions,and the PT layer is too thin to act as an effective interface to promote the transmission of lithium ions.When the thickness of PT layer is 890 nm,the PT layer shortens the lithium ion transmission path,shows the lowest lithium nucleation and platform overpotential（40 m V,30 m V）,promotes the uniform deposition of lithium,and improves the cycle stability of lithium metal anode.The Li|Cu cells assembled with PT layer can cycle stably for 400 cycles,and the average coulombic efficiency is 96.6%,which is 2.5 times higher than that of the blank group.Further analysis of the chemical structure of the SEI shows that the PT layer derives abundant Li F,which enhances the stability of the SEI,regulates the lithium deposition and stripping behavior,inhibits the dendrite growth and by-products formation during the lithium metal anode cycle,and improves the cycle stability of the lithium anode.Finally,we assembled Anode-free lithium metal battery（N/P=0）using PT-protected copper foil.After 100 cycles,compared with the unprotected copper foil,the battery capacity retention rate increased from 21.1%to 46.7%,and the average coulombic efficiency increased from 98.4%to 98.8%.The Li Fe PO₄ full battery was assembled with lithium metal protected by PT layer.Under the condition of limited lithium（N/P=2.5）,the battery had a stable cycle of 600 cycles,the capacity retention rate was62.5%（initial capacity of 166.6 m Ah/g,104.1 m Ah/g after 600 cycles）,and the average coulombic efficiency was 99.5%.（2）On the basis of the above work,aiming at the problems of dendrite growth,dead sodium and other by-products accumulation caused by unstable SEI during the sodium metal anode cycle,the PT ultra-thin layer was extended to the protection of sodium metal anode.Firstly,the effect of PT ultrathin layer on the nucleation and growth process of sodium was studied.It was found that The introduction of PT layer reduces the nuclear potential of sodium metal（from 30 m V to 20 m V）and promoted the uniform deposition of sodium metal compared with the unmodified Cu electrode.Secondly,the PT layer can derive abundant Na F,and the Na F with low Na⁺diffusion energy barrier can enhance the ion transport rate of SEI,thus promoting the uniform deposition of sodium and inhibiting the growth of dendrites.Finally,compared with the unprotected electrode,the cycle stability time of the Na|Na cells assembled with the PT layer can be improved from 350 h to 1800 h,while the cycle stability of the Na|Cu cells is effectively improved from150 cycles to 1000 cycles,with an average coulomb efficiency of 99.6%.The anode-free Na|Na₃V₂（PO₄）₃cells assembled with PT layer protected copper foil has a high capacity retention rate.The Na|Na₃V₂（PO₄）₃cells assembled with PT layer has a capacity of 92.7 m Ah/g after 230 cycles,and the capacity retention rate is 91.7%.In contrast,the cycle stability of the blank Na|Na₃V₂（PO₄）₃cells decreases sharply after 100cycles.