| With the rapid development of society,great progress of science and technology,and increasing improvement of people’s living standards,the demand for energy becomes more and more urgent.Due to the gradual increasing intensity of exploitation,the reserves of traditional non-renewable fossil energy show a continuous downward trend and are facing the risk of depletion.In order to avoid this situation,environmentally-friendly new energy systems have attracted wide attention.Different from the traditional energy systems,the source of new energy systems can be obtained from renewable resources in nature,such as wind energy,solar energy,geothermal energy and tidal energy.Abundant energy can greatly promote industrial production and technological progress,and then improve people’s living standards.However,a large amount of energy is facing the problem that it is unable to be effectively stored,resulting in a great waste of resources and low economic benefits.In recent years,the development of some energy storage systems including rechargeable batteries and supercapacitors has gradually improved this problem.Lithium（Li）metal batteries（LMBs）,as the member of rechargeable batteries,have great advantages in energy storage and conversion for the high energy density.Especially,LMBs consisting of sulfur cathode and oxygen cathode exhibit the high specific energy densities of 2567 Wh kg-1 and 3582 Wh kg-1 respectively,which not only store more energy,but also enhance the cycling lifespan of electronic products.Despite these advantages,LMBs are still impeded by many unresolved issues for its practical application.As one of the core components of LMBs,Li metal anode is facing some serious problems,such as uncontrollable growth of Li dendrites,massive production of"dead Li",unstable solid electrolyte interface（SEI）,excessive consumption of electrolyte,low Coulombic efficiency（CE）and infinite volume expansion,which result in the decline of cycling stability and increased risks of battery safety.It is extremely significant to solve the above problems effectively for the practical application of safe and stable Li metal anode.Recently,some research results have demonstrated that the construction of three-dimensional（3D）porous current collector is beneficial to inhibiting the formation of dendrite and avoiding unnecessary consumption of electrolyte.The large specific surface area owing to 3D porous current collector helps reduce the local current density on the electrode surface and achieve stable Li nucleation.Moreover,abundant internal void space is able to alleviate the huge volume expansion caused by Li deposition.Based on these positive effects,high-efficiency and stable Li plating and stripping can be achieved on the 3D porous current collector.Based on the above research background,four different 3D lithiophilic current collectors with self-supporting structure have been constructed for dendrite-free Li deposition and to prolong the cycling lifespan of Li metal anode.There are many lithiophilic sites in these functional current collectors,which help disperse the distribution of Li ions and avoid the concentrated Li deposition.3D self-supporting lithiophilic current collectors with adequate pore space also exhibit superior ability of avoiding the substrate collapse caused by the volume expansion during Li deposition.In addition,electrolyte consumption would be reduced due to the formation of stable SEI on the Li metal anode,resulting in improvement of the cycling efficiency.The main research contents are as follows:（1）A composite polyacrylonitrile（PAN）nanofiber containing plenty of tin（Sn）nanoparticles is prepared by the electrospinning method.After the calcination process,a self-supporting and highly flexible 3D current collector made of carbon nanofibers（CNFs）conformally coated by continuous Sn layer is synthesized.Sn layer enables a lithiophilic surface of composite nanofiber skeleton and induces the formation of Li-Sn layer,which is beneficial to enhancing the electric contact between metallic Li and substrate.Moreover,uniform and continuous Li nucleation sites provided by Sn layer lead to unusual conformal Li plating behavior and effective inhibition of Li dendrites.The spatial confinement of Li plating mitigates the volume expansion and network distortion of CNFs.Meanwhile,the greatly suppressed formation of Li dendrites can be observed from ex-situ SEM images of CNF-Sn at different Li plating and stripping processes.The improved electric contact and homogenous Li nucleation achieve highly reversible Li stripping for more than 850 h for CNF-Sn@Li symmetric cell.The small nucleation overpotential（28 m V）and potential polarization（14 m V for symmetric cell）benefit from the low energy barrier of Li-Sn alloying and following Li nucleation on Li-Sn layer.For CNF-Sn@Li-Li Fe PO4 full cell,the capacity retention ratio is as high as 92.2%after150 cycles at 0.5 C and the reversible capacities are maintained at 134.3 and 106.7m Ah g-1at 2 C and 5 C respectively.（2）A sea cucumber-like lithiophilic composite skeleton（CLCS）is fabricated by a facile method of oxidation-immersion-reduction,in which pre-oxidized copper（Cu）foam is immersed in the PAN solution for coating and then annealed at the mixed atmosphere of H2 and Ar.Plenty of pyridinic nitrogen（N）,pyrrolic N and CuxN sites with excellent lithiophilicity work synergistically in distributing Li ions and suppressing the formation of Li dendrites.Strong interaction between these lithiophilic sites and Li ions can induce the stable Li nucleation.Moreover,stepwise Li deposition and stripping revealed by in-situ Raman spectra during galvanostatic Li charging/discharging process promotes the achievement of dendrite-free Li metal anode.Due to these positive effects,cells based on CLCS exhibit a high Coulombic efficiency of 97.3%for 700 cycles and improved lifespan of 2000 h for symmetric cell.When coupled with Li Fe PO4（LFP）and Se S2 cathodes,full cells with CLCS@Li anodes can still deliver high capacities of 110.1 m Ah g-1 after 600 cycles at 0.2 A g-1 in CLCS@Li|LFP and 491.8 m Ah g-1 after 500 cycles at 1 A g-1 in CLCS@Li|Se S2.（3）A unique lithiophilic vertical cactus-like framework（LVCF）derived from a Zn/Cu-based coordination polymer（CP）through the in situ chemical etching of Cu foam is proposed to enhance the safety and electrochemical performance and prolong the lifespan of Li metal anodes.When Cu foam is immersed in the alkaline growth solution of Zn/Cu-based CP,massive free Cu ions can be released from the corroded substrate to compete with Zn ions for the formation of Zn/Cu-based CP.As a result,the surface of Cu foam is tightly covered by dense Zn/Cu-based CP layer.After calcination process,a 3D self-supporting LVCF can be obtained.The coexistence of massive lithiophilic nitrogen-containing functional groups,Zn O quantum dots and in situ grown carbon nanotubes in the LVCF is beneficial to avoiding the generation of harmful Li dendrites,which is revealed by in situ FTIR spectra,in situ optical microscopy and electron microscopy.Benefiting from the positive effects of the improved lithiophilicity,decreased local current density and relieved volume expansion,LVCF delivers an ultrastable Coulombic efficiency of98.6%for 600 cycles at 1 m A cm-2 and an improved cycling lifespan of 1800 h for symmetric cells.Full cells comprising LVCF@Li anodes and Li Fe PO4cathodes can deliver an ultrahigh capacity of 101.8 m Ah g-1（capacity retention ratio:77.9%）after900 cycles at 1 C and excellent rate performance.（4）A Cu foam covered with dense and rigid organic supramolecular protective layer（OSPL）is successfully constructed by the polycondensation reaction of melamine and cyanuric acid.The lithiophilic OSPL works as an artificial SEI to protect Li metal anode.The self-supporting CF@OSPL with rearranged and defensive Li deposition is reported to effectively inhibit the formation of dendritic Li and suppress the depletion of Li metal and electrolyte,which is evidenced by optical/electron microscopy,in situ FTIR spectra,electrochemical probing and theoretical calculation.Specially,free Li ions can be strongly attracted and dispersed by massive polar functional groups（amino group,carbonyl and triazine）,leading to the avoidance of attack of concentrated Li ions and uniform Li deposition.Moreover,plated Li metal on the internal Cu foam is unable to destroy the artificial SEI due to the high mechanical modulus of OSPL,suppressing the appearance of Li dendrites.Due to these positive effects,the composite matrix performs high Coulombic efficiencies of 98%for 250 cycles at 1 m A cm-2 and prolonged cycling lifespan of1300 h with low potential polarization（20 m V）for symmetric cell.Moreover,small nucleation overpotential（27.1 m V）benefits from attracting Li ions by massive polar functional groups in the organic supramolecular structure.When coupled with Li Fe PO4,full cells display ultrahigh retention ratio（96.2%）and slow decay of specific capacity after 250 cycles at 0.5 C.Low voltage hysteresis reflects the improved kinetics and fast charge-transfer behavior during Li charging/discharging process. |
