Multifunctional Protected Strategy And Performance Research Of Lithium Metal Anodes

Metallic lithium(Li)is regarded to be an ideal anode material for the next generation of high-energy-density battery systems,which possess ultrahigh theoretical capacity(3860 mAh g^-1)and low redox potential(-3.04 V vs the standard hydrogen electrode).However,problems with the lithium metal anode(LMA),including inhomogeneous deposition of Li-ions(Li⁺)and an unstable interface,will lead to short cycle lifespans and potential safety risks in lithium metal batteries(LMBs).Especially in practical conditions,high areal capacity,lean electrolytes,and thin Li can occur simultaneously,and the actual cycle lifespan of LMB is extremely limited.A practical LMA needs to meet the requirements of uniform deposition kinetics,suppression of interfacial side reactions,and dynamic stability of the interface.Based on the above requirements,this dissertation develops a series of LMA-protected methods to improve the cycle performance of LMB in practical conditions.The specific research content is as follows:(1)A Li/Sn alloy-poly(tetramethylene ether glycol)(PTMEG)hybrid layer is constructed directly on the surface of lithium with a chemical-treatment method to achieve uniform deposition kinetics.Li/Sn alloy can provide fast Li⁺transportation channels,and regulate the deposition kinetics of metallic Li.The results show that treated Li remains dendrite-free for over 1000 h in a Li-Li symmetric cell and exhibits outstanding cycle performance in high-areal-loading Li-S batteries(5 mg cm^–2)and Li-LiFePO₄ full cells(17.4 mg cm^–2).Moreover,the treated Li retains good electrochemical activity after exposure to humid air,as it benefits from the hydrophobicity of the PTMEG.(2)Nitrofullerene(nitro-C₆₀)is introduced as a bifunctional electrolyte additive for LMAs to realize uniform deposition kinetics.Nitro-C₆₀ can gather on electrode protuberances via electrostatic interactions and can then be reduced to NO₂^-and insoluble C₆₀.Next,the C₆₀ anchors on the uneven grooves of the Li's surface,resulting in a homogeneous distribution of Li⁺.Also,NO₂^–anions react with metallic Li to form a stable protective layer.With a 5 m M nitro-C₆₀ additive,Li-Li symmetric cells show superior cycle stability in both carbonate and ether electrolytes.A high-areal-capacity Li-Li Ni_0.6Co_0.2Mn_0.2O₂ full cell(3.5 mAh cm^-2)using a carbonate electrolyte with the addition of nitro-C₆₀ also exhibits an outstanding cycle lifespan even under lean electrolyte conditions(3 g Ah^-1).(3)Electrolyte composition was optimized and a bifunctional diluted high-concentration electrolyte was obtained for practical lithium-sulfur(Li-S)batteries for the suppression of interfacial side reactions.Such an electrolyte can form an organic-inorganic hybrid layer both on the S cathode and Li anode,and a hybrid layer with fast Li⁺transport and high stability,improving the reversibility of the Li anode and ameliorative kinetics of the S cathode.Finally,a practical Li-S battery with both a high energy density of 325 Wh kg^–1 and stable cycling is successfully obtained.The practicability of such an electrolyte is further demonstrated in a 0.4 AhLi-S pouch cell,which presents no obvious fading of capacity with a steady Coulombic efficiency of99.6%.(4)Diluted high-concentration electrolytes(DHCE)can significantly inhibit interfacial side reactions.A novel fluorobenzene-diluted high-concentration electrolyte(FB-DHCE)is developed to further enhance the practicability of DHCE.FB has low cost and density compared to the widely used fluorinated ether diluents.The addition of FB can inhibit the decomposition of 1,2-dimethoxyethane(DME)by strengthening the interactions of 1,2-dimethoxyethane(DME)and bis(fluorosulfonyl)imide-ion(FSI^-)around Li⁺.Also,FB elevates the content of LiF in the solid-electrolyte interphase(SEI)via electrochemical reduction.FB-DHCE can deliver outstanding electrochemical performance in full cells in practical conditions,including low temperature(–20°C),high areal capacity(7.6 mAh cm^-2),high current density(3 m A cm^-2),thin Li(20?m Li),and a lean electrolyte(3 g Ah^-1),due to its unique solvation and interfacial chemistry.(5)An ultra-thick LMA with a stable interface was prepared by over-lithiating sulfurized polyacrylonitrile(SPAN)to reach dynamic stability of the interface.SPAN has a unique polymeric pyridine structure with outstanding Li⁺affinity,so it can act as a lithiophilic matrix.More importantly,an SEI with dynamic stability can be generated in situ on the surface of SPAN during the over-lithiation process.The synergistic effect of the lithiophilic matrix and robust SEI lead to a dense deposition of Li,which enables an ultra-thick LMA(159?m,30 mAh cm^–2)with high Coulombic efficiency(99.7%).Such an LMA paired with a sulfur cathode of high areal capacity(up to 16 mAh cm^–2)shows stable cycling under the practical conditions of a lean electrolyte and limited Li source,and the applicability of the ultra-thick LMA is further verified with a Li-S pouch cell.