| In recent years,with the increasing demand for portable electronic products,electric vehicles,and large-scale energy storage devices,high-energy-density rechargeable batteries have become an urgent requirement.Among them,lithium-ion battery（LIB）plays an increasingly significant role.However,the capacity of conventional LIBs is already close to its theoretical value.Therefore,it is particularly important to develop a new generation of energy storage devices with the aggressive energy density and long cycle life.Lithium metal as an anode makes lithium metal battery with high energy density and specific energy,because lithium metal has the low redox potential（-3.04 V vs.SHE）,extremely high theoretical specific capacity(3860 m Ah g-1),and low weigh density(0.53 g cm-3).Among them,lithium-sulfur batteries（LSBs）based on the lithium metal anode can achieve a discharge specific capacity as high as 1675 m Ah g-1 and an energy density as high as 500 Wh kg-1.Despite the promising application prospects of lithium metal batteries,there are still some problems related to lithium metal anodes that need to be solved before their large-scale commercialization.On the one hand,lithium metal easily reacts with electrolytes to bulid a heterogeneous solid electrolyte interphase（SEI）,leading to local fluctuation of Li+ion flux and current density,which results in the formation of lithium dendrites.The dendrites can pierce the separator and cause some safety problems such as:short circuit and even fire.On the other hand,large volume expansion during Li plating/stripping forms cracks in the brittle SEI,creats“dead Li”,and contributes to further electrode and electrolyte consumption.The above factors reduce the coulombic efficiency（CE）and destroy the cycle life of Li-metal anodes.Based on the above bachground,this dissertation focuses on the regulation of surface/interface chemical behavior of Li-metal ande,by constructing an in-situ SEI and an artificial polymeric SEI interphases,which can not only adjust the Li+ion flux and boost the uniform Li deposition,but also prvent the direct contact and relieve the continuus consumption of Li electrode and electrolyte as well as mitigate volume expansion of Li-metal anode,thus significantly improving the CE and cycling stability.The main research contents and results are displayed as follow:（1）Construction of an in-situ SEI layer based on the design of a localized high concentration electrolyte（LHCE）.The LHCE is designed using fluoromethyl 1,1,1,3,3,3-hexafluoroisopropyl ether（SFE）as an inert diluent in concentrated carbonate electrolyte.LHCE is optimized by varying the solvent（dimethyl carbonate,DMC）to diluent（SFE）ratio,but fixing the salt（lithium bis（fluorosulfonyl）imide,Li FSI）to DMC ratio.Comparing with the HCE（5 M Li FSI in DMC）,the wettability and ionic conductivity of the LHCEs were significantly improved.it is demonstrated that such diluent not only facilitated the formation of anion-derived Li F-rich SEI layer at the Li metal electrode,but also favored the formation of transitional metal-free SEI layer at the cathode side.Therefore,with the optimized LHCE-2 electrolyte（i.e.,1.86 M Li FSI in DMC/SFE,1:1 by molar）,extremely stable Li plating/stripping process of Li||Li symmetrical cell was obtained for 2130 h at 1 m A cm-2 and 2 m Ah cm-2;meantime,the corresponding Li||NMC811(Li Ni0.8Mn0.1Co0.1O2)cell reached high average coulombic efficiency of 99%and capacity retention of 84%for 300 cycles in the voltage range of 2.7～4.3 V.（2）Construction of a highly self-healing single-ion conductive artificial polymeric SEI（SS-ASEI）interphases.The SS-ASEI layer for stabilizing the Li metal anode is reported.This polymeric SS-ASEI layer is fabricated by dynamically crosslinking hydroxyl-terminated poly（dimethylsiloxane）and Li Al H4（noted as SS-PDMS）,and further optimized by adding Si O2 nanoparticles as reinforcement fillers（noted as SS-PDMS-Si O2）.The introduction of Si O2 was helpful for significantly improveing the ion conductivity and the mechanical properties.The crosslinking centers contributed to both its self-healing capability originated from the dynamic Al-O bonds and the Li-ion selectivity owing to the negatively charged Al（OR）4-groups.The optimized SS-ASEI film（with 7 wt%Si O2）achieved combined superior properties,including 96%self-healing efficiency,0.13×10-4 S cm-1ion conductivity,and 0.71 Li-ion transfer number.By applying such a SS-ASEI film on Li metal anode,extremely stable Li plating/stripping are achieved for 1340 h at 0.5 m A cm-2 and 1.0 m Ah cm-2cutting-off capacity.Higher rate capability and cycling performances of SS-ASEI modified anode than bare Li metal anode are further achieved in cells with commercial NCM811 cathode.（3）Construction of a rich-fluorine double network single-ion conductive artificial SEI（SFDN-ASEI）interphases.The SFDN-ASEI layer for stabilizing the Li metal anode is constructed.The polymeric SFDN-ASEI layer is synthesized by dynamically crosslinking 1H,1H,9H,9H-Tetradecafluorononane-1,9-diol（TDFND）and Li Al H4,PVDF-HFP is used as the film host,and further optimized by varying the PVDF-HFP and TDFND wight ratio.The crosslinking centers contributed to both its self-healing capability originated from the dynamic Al-O bonds and the Li-ion selectivity owing to the negatively charged Al（OR）4-groups.The optimized SFDN-ASEI layer（PVDF-HFP/TDFND=1:8,by wight）possesses combined superior properties,including 90%tensile deformation,1.115×10-4 S cm-1ion conductivity,and 0.70 Li-ion transfer number.By applying the SFDN-ASEI layer on Li metal anode,significantly stable Li plating/stripping are realized for 883 h at 0.5 m A cm-2 and 0.5 m Ah cm-2 cutting-off capacity.The SFDN-ASEI layer modified Li anode exhibits better electrochemical performance than bare Li in full cells with commercial NCM811 cathodes.（4）Construction of a high-density energy lithium-metal battery based on the Li-metal anode.Multifunctional（Co,Ni）9S8 nanoparticles evenly distributed on PVP-derived carbon nanofibers（（Co,Ni）9S8@CNFs）by an electrospinning method is proposed.Such structure offers several advantages for improving the performance of the（Co,Ni）9S8@CNFs electrocatalyst:（i）the small and uniformly distributed polar nanoparticles can maximize the chemical adsorption capability for solvable polysulfides;（ii）the pores produced by reaction process are favorable for the permeation of electrolyte and sulfur,which allows a high sulfur content up to 83.3%;（iii）the 1D nanofibers interlink into the network structure contributes to enhancing the electronic transport and preventing sulfur cathode volume expansion.Compared with CNFs/S and Co9S8@CNFs/S,the（Co,Ni）9S8@CNFs/S electrode achieves a high initial discharge capacity(1218 m Ah g-1 at 0.1 C),excellent rate capability(641 m Ah g-1 at 2.0 C)and long-term cycling stability(646 m Ah g-1 with an average CE of 99.7%and capacity decay per cycle of 0.037%after 1000 cycles at 1.0 C).Moreover,the（Co,Ni）9S8@CNFs electrode with the sulfur content of 83.3%and the high sulfur loading of 5.0 mg cm-2 as well as E/S ratio=20 ul mg-1 shows high capacities of 1056 and 824 m Ah g-1 after300 cycles at 0.1 and 1.0 C,demonstrating highly efficient sulfur utilization and remarkable potential for practical application. |
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