Modification And Electrochemical Performance Of Electrode Materials For High-Energy-Density Lithium-Metal Batteries

With the global needs of“Carbon neutrality”,it is a general trend to develop electric vehicles to replace the large number of fuel vehicles.As a result,many countries have issued policies to support the development of electric vehicles.At present,“Mileage anxiety”is the major obstacle for consumers to accept electric vehicles,and the lack of range has become the main crux of the development of new energy vehicle industry.As the main power source of electric vehicles,the energy density of lithium（Li）-ion battery directly determines its range and affects the vehicle performance.The theoretical energy density（260 Wh/kg）of current Li-ion battery with traditional graphite negative electrode is approaching the limit,and cannot meet the higher power demand of electric vehicles.Because of its light weight and low reduction potential,high-specific-energy Li metal anode,known as the“holy grail”,is considered as the ultimate choice of anode for Li-ion battery.More encouragingly,the energy density of Li-metal batteries（LMBs）using Li metal as the negative electrode and nickel-rich layered material as the positive electrode is expected to deliver a high energy density beyond 500 Wh/kg,fulfilling the energy density requirements of power batteries in the future.However,the uncontrolled growth of dendrite,the generation of“dead Li”and the huge volume change during cycling constitute the major obstacles for the development of Li metal anodes.For nickel-rich layered cathodes,the unstable surface and bulk structure,and mechanical failure of particles have severely restricted their electrochemical performance.To this end,we focus on Li metal anodes and nickel-rich layered cathodes in this dissertation.Different modification strategies are adopted to optimize their individual performance,in the hope of developing high-performance and compatible electrode materials to promote the commercial application of high-safety and high-energy-density LMBs.The specific content of the study is summarized below:（1）To resolve the issues of dendrite growth,dead Li and volume expansion during cycling,Li@Pb@CC composite Li metal anodes with different Li loads were successfully prepared by melt-pouring Li into the host of carbon cloth modified by lithiophilic nano-Pb particles.The uniformly distributed lithiophilic Pb particles in the carbon cloth host can not only help realize ultra-fast infusion of weight-controlled molten Li,but also serve as active sites to guide the uniform deposition of Li.Meanwhile,the carbon cloth is mechanically robust and exhibits a highly electron-conducting framework,effectively reducing the localized current density,mitigating dendrite growth and accommodating volume changes of the electrode.At the optimal Li loading of 10 mg,the Li@Pb@CC composite Li anodes demonstrated outstanding cyclability.Under the conditions of 1 m A/cm² and 1 m Ah/cm² using a traditional ester electrolyte,the corresponding symmetric cell delivers a low overpotential of≤50 m V and an ultra-long cycle life of 4648 h.The cycling and rate performances of Li Fe PO₄-and Li Ni_0.8Co_0.1Mn_0.1O₂（NCM811）-based full batteries are obviously superior to those with Li foil electrodes.This work provides a feasible idea for the development of dendritic-free,long-life and high-Li-utilization Li metal anodes and thus high-energy-density LMBs.（2）For NCM811 cathodes,the residual Li on the surface can result in surface degradation and structural instability.Here,we utilize the Li residual as the Li source and add La source and Mn precursors,thereby in-situ forming a mixed conductor Li_0.34La_0.55Mn O_3-x（LLMO）coating with high ionic and electronic conductivity on the surface of NCM811 via a simple heat treatment.The stable LLMO coating not only effectively utilizes the residual Li on NCM811 surface and reduces the side reactions between NCM811 to stabilize the interface,but also offers fast ionic and electronic conductivity to accelerate the interfacial charge transfer process.The in-situ LLMO-coated NCM811（I-LLMO-NCM811）exhibited a capacity retention rate of82.51%over 100 cycles at 1 C,while the pristine NCM811 only maintained 48.26%of its initial capacity.Furthermore,when combining I-LLMO-NCM811 and the abovementioned Li@Pb@CC composite Li metal anode in a full battery,a high capacity retention rate of 86.96%can be achieved after 200 cycles under the conditions of 1 C and 2.7-4.3 V,which was significantly better than that of the full cell with NCM811（42.24%）.This study provides a novel and comprehensive solution to stabilize nickel-rich layered cathode materials,and preliminarily verified the feasibility of Li@Pb@CC lithium metal anode and modified high nickel layered cathode materials in high energy density lithium metal batteries.（3）To handle the issue of serious Li-Ni intermixing and rapid capacity degradation of cobalt-free and Ni-rich materials,we successfully synthesized Zr-doped Li Ni_0.9Mn_0.1O₂（NM90）by a sol-gel method.It is found that a proper amount of Zr dopant can not only enlarge the interlayer spacing of NM90 thereby,accelerating the Li-ion transport in the lattice,but also effectively restrain the mixed arrangement of Li and Ni in NM90.Besides,Zr doping can improve the electronic conductivity of NM90.Benefiting from the above effects,Zr doping effectively improves the cycling and rate performance of NM90.The electrochemical test results show that 0.5%Zr-doped NM90 exhibits 66.51%capacity retention after 100 cycles at 0.5 C,while the undoped NM90 shows only 23.47%capacity retention.We further synthesized NM90 materials doped with Al,Zr,Ta,W,and co-doped with Al,Zr,and Ta,and the effects of the valence state of dops and co-substitution on the electrochemical properties are investigated.Among them,high-valence Ta and W doping as well as multi-metal（Al,Zr,Ta）co-doping show significant improvement in the cycling stability of NM90 materials.These studies provide a valuable reference for the development of cobalt-free and Ni-rich layered cathode materials.