Beneficial Phase Construction Of Composite Solid Lithium Cathode For Long-Life Lithium Metal Batteries

As we strive to increase the energy density of secondary batteries,lithium-metal batteries（LMBs）have attracted wide attention due to the extremely low oxidation-reduction potential（compared to standard hydrogen electrode of-3.04 V）and high theoretical capacity(3862 m Ah g^-1)of their lithium negative electrode.However,during battery cycling,the high chemical reactivity of metallic lithium causes significant volume changes in the negative electrode during uneven lithium deposition/stripping.The brittle primary solid electrolyte interphase（SEI）cannot prevent further surface reactions of lithium,causing irreversible negative electrode electrochemistry and accumulation of dead lithium,ultimately leading to lithium dendrites that significantly reduce Coulombic efficiency and increase potential safety hazards,thus hindering the practical application of LMBs.Enhancing the stability of the negative electrode SEI layer is therefore crucial for achieving the commercialization of LMBs.In this study,two strategies were employed to enhance the strength of the SEI layer during battery charging and discharging by leveraging beneficial components for SEI construction and building stable composite lithium-negative electrode structures to suppress lithium dendrites and achieve long-term cycling of soft-packaged LMBs.（1）Using a simple and economical direct foaming technique,a foam silicon nitride ceramic（Si₃N₄）with ultrahigh porosity was prepared and combined with a cold rolling and heat treatment strategy to"infuse"molten lithium metal into the spherical ceramic substrate,successfully obtaining an intrinsic and stable lithium negative electrode with excellent SEI components to suppress the growth of lithium dendrites.In this method,the porous sphericalβ-Si₃N₄ foam ceramic prepared by foaming exhibited good mechanical properties and high porosity.The porosity of the silicon nitride foam ceramic ranged from 92.1%to96.8%.In the preparation process of the composite lithium negative electrode,the porous silicon nitride foam ceramic was uniformly spread on two layers of commercially available lithium foil with a thickness of 50μm,followed by lamination cold rolling and subsequent heat treatment which resulted in complete fusion and reaction of the porous silicon nitride ceramic and molten lithium,building a composite and stable lithium negative electrode.Through XRD and XPS analysis,it can be seen that the composite lithium metal not only contains excess silicon nitride precipitation,but also higher conductivity substances such as Li-Si-N and Li₃N.This protective layer can effectively promote uniform lithium ion deposition during battery charging and discharging,and enhance the beneficial components such as Li₃N in the formation of the SEI layer,thereby inhibiting the formation of lithium dendrites and improving battery cycle life.When the current density and capacitance density reach 3 m A cm^-2 and 3 m Ah cm^-2 respectively,a symmetrical Li||Li cell composed of this composite lithium film with multiple beneficial phases can stably cycle for more than 500 hours and exhibit ultra-long cycle stability.When the composite lithium negative electrode with the intrinsic beneficial layer is paired with a high-voltage positive electrode Li Ni_0.8Co_0.1Mn_0.1O₂,it exhibits a capacity retention rate of 94.33%after250 cycles under 2 C condition.（2）A simple hydrothermal method was used to prepare layered Bi₂O₂CO₃ and fluorinated Bi₂O₂CO₃（F-BOC）.After simple hydrothermal fluorination,the charge transfer and separation capability of F-BOC is significantly improved due to fluoride interlayer doping and fluoride surface adsorption.Considering its nano-layered structure and rich fluorinated component properties,this work continued to employ the cold-rolling and heating melt-composite method of lithium metal to construct a 3D highly stable composite lithium negative electrode that is rich in Li F and has good electrode kinetics.Specifically,F-BOC was used as a 3D lithium-friendly skeleton to infiltrate molten lithium foil,constructing a stable and fluorine-rich lithium negative electrode for high-performance lithium-metal batteries.In the electrochemical cycling process,the stereoscopic"host"material has a special thin-sheet structure that can accommodate lithium deposition grains in its pore structure and provide sufficient space to alleviate unrestricted volume expansion during lithium deposition.At the same time,Li F,as a special component of the SEI protective layer,can be strongly anchored on the entire surface of the 3D model during"infusion",effectively enhancing the SEI strength and suppressing the reaction between the lithium metal surface and the electrolyte,avoiding the production of heat and negative electrode powdering.With these advantages,the prepared 3D ultra-high stability Li electrode exhibited excellent electrochemical performance at room temperature.The corresponding symmetrical battery can cycle more than 100 hours at a current density of 5 m A cm^-2and an area capacity of 2.5 m Ah cm^-2,with low overvoltage（<100 m V）.Additionally,the F-BOC@Li||Li Ni_0.8Co_0.1Mn_0.1O₂ cell exhibits a capacity retention rate of 85.88%and Coulombic efficiency of approximately99.6%after 600 cycles at 1C rate.When its active material loading is as high as 20 mg cm^-2,its capacity retention rate reaches 90.3%after 120 cycles,demonstrating extremely excellent cycle performance.