The Influence Of Li~+ Transport On The Cycling Stability Of Lithium Metal Batteries

Lithium（Li）metal battery with high energy density is one of the important directions of Li batteries development.This is mainly due to the extremely high theoretical specific capacity（3860 m Ah/g）and the lowest potential（-3.04 V vs the standard hydrogen electrode）of the Li metal anode（LMA）.However,the unstable Li deposition behavior leads to the continuous growth of Li dendrites together with breakage/rebirth of solid electrolyte interphase（SEI）,the deterioration of cycling performance,and battery short circuit,which can be regarded as the obvious defect for LMA.In order to solve this problem,this dissertation focused the concentration distribution of lithium-ion（Li⁺）in nanochannels of anodized alumina oxide,the Li⁺distribution uniformity at the high temperature stable glass fiber based composite separator surface,the Li⁺transport continuity at the fluorinated composite solid electrolyte/Li metal interface and the Li⁺distribution/transport at the surface modified copper foam collector/Li metal interface,which can regulate the Li⁺transport,inhibit the growth of Li dendrites and achieve the stable cycle performance for Li metal batteries.The main researches of this dissertation are as follows:1.The influence of Li⁺concentration distribution in nanochannels on the cycling stability of Li metal batteries was studied.Anodized alumina oxide（AAO）with well-aligned nanochannels was used as the separator.The Li⁺transport in nanochannels was observably affected by different pore size AAO,which can lead the different Li⁺concentration distribution in nanochannels.As a result,Li⁺concentration distribution can regulate the Li deposition behavior and ultimately affect the cycling stability of Li metal batteries.Specifically,AAO with pore size of 20 nm（AAO-20）achieved the best transfer number and ionic conductivity in nanochannels,which can obtain higher Li⁺concentration near the deposition substrate,thus preventing the rapid Li⁺depletion with and inhibiting the growth of Li dendrites.Therefore,AAO-20 can help achieve uniform Li deposition behavior through the change of Li⁺concentration distribution.The Li-Li cell with ethers electrolyte achieved stable cycling performance under a low overpotential(<20 m V over 2400 h at 1 m A cm^-2&1 m Ah cm^-2).In addition,the Li Fe PO₄ coin full cell with AAO-20 also showed stable electrochemical performance,with higher capacity retention（over 95%）after 300 cycles at 0.5 C.2.The influence of Li⁺distribution uniformity at the high temperature stable glass fiber based composite separator surface on the cycling stability of Li metal batteries was studied.The polyvinylidene fluoride（PVDF）/Li_6.4La₃Zr_1.4Ta_0.6O₁₂（LLZTO）coating glass fiber（GF）membrane based composite separator by high temperature fluorination process achieved uniform Li⁺distribution at the surface of composite separator,which can help improve the Li deposition behavior,inhibit the growth of Li dendrites,and finally improve the cycling stability of Li metal batteries.Firstly,the PVDF/LLZTO membrane was introduced on the surface of GF（PL@GF）,where the LLZTO particles with uniform distribution can redistribute Li⁺.Secondly,the Li₂CO₃ impurities on the surface of LLZTO particles in PL@GF were converted to Li F by NH₄F high temperature fluorination process（NH₄F-PL@GF）,which can further improve the Li⁺transport in PVDF/LLZTO.With ethers electrolyte,under the condition of 1 m A cm^-2&1 m Ah cm^-2,the Li-Li cell kept stable for more than 2000 h and the average coulombic efficiency of the Li-Cu cell was stable above 97.5%after 400 cycles.Meanwhile,the coin full cell,which consists of NH₄F-PL@GF composite separator and cathodes(Li Fe PO₄ and Li Ni_0.8Co_0.1Mn_0.1O₂),also kept stable for 800 cycles at 1 C.3.The influence of Li⁺transport continuity at the fluorinated composite solid electrolyte/Li metal interface on the cycling stability of Li metal batteries was studied.The surface of three-dimensional Li_6.4La₃Zr_1.4Ta_0.6O₁₂ skeleton（3D-LLZTO）was modified by fluorination process to improve the Li⁺transport continuity at the solid electrolyte/Li metal interface,which can improve the cycling stability of Li metal batteries.NH₄F reacted with Li₂CO₃ impurities on 3D-LZZTO surface to form Li F,and then this skeleton constructed composite solid electrolyte by polymer phase.On the one hand,it can improve the ionic conductivity of the composite solid electrolyte.More importantly,the wettability of 3D-LZZTO surface can be improved to solve the poor contact problem at solid electrolyte/Li metal interface,which improve the Li⁺transport continuity at this interface,leading to uniform Li deposition behavior and dendrites-free.Li-Li cell with 50 m V overpotential achieved more than 1000 h stable cycling(0.1 m A cm^-2&0.1 m Ah cm^-2).In addition,the coin full cell,composed of fluorinated composite solid electrolyte and Li Fe PO₄,achieved the capacity retention of 76.7%after 200 cycles at 0.5 C.4.The influence of Li⁺distribution/transport at the surface modified copper foam collector/Li metal interface on the cycling stability of Li metal batteries was studied.The Li F-rich SEI layer was in-situ formed on copper foam collector having lithiophilic layer through fluorination process,which can solve the problems of uneven Li deposition and dendrite growth caused by uneven Li⁺distribution and poor Li⁺transport stability on the surface of copper foam collector.Firstly,Au and Zn O were introduced on the surface of collector by thin film deposition process.Then,some Zn O was converted to Zn F₂ by in-situ fluorination,where Zn F₂ can in-situ form Li F-rich SEI during electrochemical activation.The lithiophilic layer ensured the uniform Li⁺distribution at the collector/Li metal interface and regulated the uniform Li deposition.Moreover,the Li F-rich SEI layer ensured the stable Li⁺transport at the collector/Li metal interface during the cycling process,which can inhibit the growth of Li dendrites.With ethers electrolyte,the Li-Cu cell kept stable 500 cycles with the average coulomb efficiency of 97.74%(1 m A cm^-2&1 m Ah cm^-2).And the Li-Li cell kept stable for 2400 h with a low overpotential（10 m V）at 2 m A cm^-2&2 m Ah cm^-2.Besides,the coin/pouch full cells matched with cathodes(Li Fe PO₄,Li Co O₂ and Li Ni_0.8Co_0.1Mn_0.1O₂)and modified copper foam collector showed stable cycling performance and rate performance.