Application Of Lithium Storage Organic Compounds In Li Batteries

Li batteries are widely utilized in portable electronic devices,electric vehicles,and energy storage systems due to their high specific energy,wide electrochemical window,long cycle life,and no memory effect.With the development of the country and society,the demand for high-performance power and energy storage devices is increasing.It requires Li batteries to develop in the direction of higher energy density,higher safety performance,higher efficiency and more stability.In order to meet above-mentioned demands,lithium metal anode,flexible composite polymer-ceramic electrolytes(CPEs)and high-votage nickel-rich LiNi0.8Co0.1Mn0.1O2(NCM811)cathode material are becoming research hotspots.However,there are many scientistic and technolical challenges for them before commercial applications:the uncontrollable lithium dendrite growth for lithium metal anode,which will result in short circuit even safety risks;the poor compatibility among different components for CPEs,which will cause insufficient ionic conductivity,low electrochemical window and interface issues with electrodes;the irreversible phase transition,secondary particle cracking and gas releasing side reaction during the repeated charge-discharge process for NCM811 cathode,which will lead to poor cycle stability and cause safety issues.Constracting a multifunctional interface is one of the effective strategies to solve these problems.In this thesis,we successfully constructed different multifunctional interfaces on the surface of lithium metal anode,solid state electrolyte and single-crystal NCM811 cathode by effectively utilize small-molecule lithium-storage organics,thereby improving the performance of Li batteries.The research includes the following three aspects:(1)Designing and fabricating a polymethyl methacrylate(PMMA)artificial SEI film to protect commercial Li metal anode by spin coating method.On the one hand,the as-prepared PMMA artificial SEI film can generate Li-O bonds with Li+ fixed in the PMMA segment,which benefits to regulate the distribution of Li+ and guide the continuous and uniform Li deposition/stripping to avoid Li dendrite and/or pulverization;on the other hand,the PMMA artificial SEI film can provide extral continuous and fast lithium ion transport pathways through its vast carboxyl groups and elastic microstructure to accommodate the volume expansion of Li metal during the repeated Li deposition/stripping process.As a result,commercial Li metal anode with PMMA artificial SEI film exhibits a cycling stability of over 180 h with a low polarization voltage of about 80 mV at 1 mA cm-2 and 1 mAh cm-2.In addition,the as-assembled PMMA-600r@Li‖LiFePO4 full cell delivers an initial specific discharge capacity of 152.2 mAh g-1,an excellent cycling stability with a discharge specific capacity retention rate of 65%after 500 cycles at 1 C.This chapter reveals the feasibility of constructing artificial SEI using carboxylic acid-based small-molecule lithium storage organics for the protection lithium metal anodes.(2)In chapter four,maleic acid(MA)is introduced into the PVDF-LLZTO composite polymer-ceramic electrolytes(PVDF-LLZTO CPEs)to optimize its poor components compatibility,insufficient ionic conductivity and low Li+transference number.On the one hand,benefiting the self-polymerization of MA,a nano-MA polymer shell layer is in situ formed on the surface of LLZTO ceramic particles,which can suppress side reactions of LLZTO and improve the compatibility of LLZTO with PVDF and other components;on the other hand,abundant carboxyl groups in MA provide additional Li+transport channels.The experimental result shows that the optimized 25MA-75PVDF-LLZTO CPEs demonstrate a high ionic conductivity of 1.15× 10-3 S cm-1 at 30℃,a wide electrochemical window<5.0 V(vs Li+/Li)and an enhanced Li+transfer number of 0.596.In addition,the 25MA-75PVDFLLZTO CPEs show excellent compatibility with the lithium metal anode.The Li‖25MA75PVDF-LLZTO CPEs‖Li symmetric battery presents no lithium dendrite or pulverization after charge-discharge cycling for 700 h.The as-prepared Li‖25MA-75PVDF-LLZTO CPEs‖LiFePO4 solid-state full battery delivers an initial discharge specific capacity of up to 170.5 mAh g-1 at 0.2 C,and a discharge specific capacity of 142.3 mAh g-1 after 180 cycles with a capacity retention rate of 83.5%.The research in this chapter provides a new strategy to optimize the compatibility between different components and improve the electrochemical performance of the CPEs by introducing unsaturated small organic molecules.(3)In chapter five,a uniform,dense and stable interfacial protective layer was constructed on the surface of single crystal NCM811 using the lithium storage organics of terephthalic acid(TPA).Compared with single crystal NCM811,the electrochemical performance of TPA-modified NCM811 is significantly improved.An initial discharge specific capacity as high as 170.1 mAh g-1 at 1C,and an enhanced cycling stability with a capacity retention rate of 78.4%after 100 cycles is achieved.The discharge specific capacity is 138.8 mAh g-1 at 2C,which is 47 mAh g-1 higher than that of pure NCM811.All research results suggest that:1.TPA can effectively utilize the residual lithium compounds on the surface of NCM811 to build a tightly bound cathodic protection layer,reducing the alkalinity of the surface of NCM811 and providing fast lithium ion transport channels;2.Dense and uniform TPA surface modification layer can effectively reduce the side reaction between the active material and the electrolyte;3.The TPA surface modification layer can effectively inhibit the irreversible phase transition and Li/Ni cation mixing of NCM811 during cycling,and utilize its excellent mechanical properties to accommodate the volume expansion of cathodic active material.