Improving Stability Of Lithium Metal Anodes Through Electrolyte Engineering For Rechargeable Lithium Metal Batteries

In the past decade,metallic lithium has been revived and studied widely in next-generation energy storage devices,particularly,lithium-oxygen（Li–O₂）batteries and lithium-sulfur（Li–S）batteries.This is significantly ascribed to the fact that the metallic lithium as an anode enables a high specific energy density(3860 mAh g^-1),being ten folds higher than that of commercial graphite(372 mAh g^-1)anode materials used in today’s lithium-ion batteries.In addition,among various metal materials,lithium metal shows the lightest density(0.59 g cm^-3)and the most negative electrochemical potential（-3.045 vs.standard hydrogen）.However,growth of lithium dendrites still occurs during repeated lithium plating/stripping in rechargeable lithium metal batteries（RLMBs）.The lithium dendrites may break away from metallic lithium matrix,resulting to dead dendrites.The formed dead dendrites in RLMBs,exhibiting low electrochemical activity and high chemical activity,not only critically decrease the coulombic efficiency（CE）of RLMBs,but also rapidly increase the parasitic reactions arising from lithium dendrites and electrolyte components（solvents,conducting salts and additives）,thus consuming active lithium metal and electrolyte components,ultimately causing poor cycle performances in RLMBs.Furthermore,the uncontrollable growth of dendrites may bridge the anode and cathode electrodes,finally causing internal short circuits in the RLMBs,and triggering safety issues,such as thermal run-away and exploding.Therefore,how to suppress the growth of lithium dendrites and improve the stability of lithium metal anode during cycling is one of the key issues in RLMBs for its potential application in the future.Engineering functionalized solid electrolyte interphases（SEIs）film on lithium metal anode through optimizing electrolyte components（e.g.,solvents,conducting salts and additives,etc.）is one of the effective strategies to improve its endurance during the repeated charge/discharge processes in the RLMBs.So far,fluorinated sulfonimide lithium salts(Li[(C_nF_2n+1SO₂)(C_mF_2m+1SO₂)N],n,m=0⁸)have been studied widely in the RLMBs,especially,lithium bis（trifluoromethanesulfonyl）imide（Li[（CF₃SO₂）₂N],LiTFSI）.This mainly owes to（1）its relatively high thermal and electrochemical stabilities;（2）the highly delocalized charge distribution of TFSI^-anions,which effectively weakens the interactions between Li⁺and TFSI^-ions,therefore enhancing ionic conductivities of the electrolytes.However,the LiTFSI-based electrolytes generally show relatively poor film-formation ability,and aluminum corrosion nature in the high potentials（3⁵ V vs.Li/Li⁺）.To effectively alleviate formation of lithium dendrites and improve the CE for the RLMBs,herein,we try to construct a functionalized SEI film on lithium metal anode through designing novel electrolyte components（e.g.,solvents,conducting salts and additives,etc.）.In detail,Li-oxygen（Li–O₂）batteries or lithium-lithium iron phosphate（Li|LiFePO₄）are employed as prototype cells to evaluate the effect of the chosed electrolyte on cycling performance of the RMBs.The main results of this work have been summarized as follow:1.To improve the interphase stability of lithium metal anode under oxygen atmosphere,anovelconductingsalt,namelylithium（trifluoromethanesulfonyl）（n-nonafluorobutanesulfonyl）imide（Li[（CF₃SO₂）（n-C₄F₉SO₂）N],LiTNFSI）,has been designed,prepared and applied into Li–O₂ batteries.It is found that,compared with LiTFSI-based electrolyte（1 M LiTFSI tetraethylene glycol dimethyl ether,TEGDME）,1 M LiTNFSI-TEGDME enables a stable,uniform,and O₂-resistive SEI film on lithium metal anode.As a result,the“cross-talk”between the lithium metal anode and dissolved oxygen shuttled from the oxygen cathode could be effectively inhibited in Li–O₂ batteries.In detail,the Li|Li symmetric cells based on 1 M LiTNFSI-TEGDME can operate for more than 1000 h with a low and stable polarization voltage of 0.05 mV at 0.2 mA cm^-2 under oxygen conditions.In addition,the cycle life of Li–O₂ batteries containing 1 M LiTNFSI-TEGDME is increased up to 49 cycles from 20 cycles（for that containing 1 M LiTFSI-TEGDME）.2.To deeply investigate the impact of F-rich SEI film formed on lithium metal anode and the structure of conducting salts on the cycle performances of Li–O₂ batteries,a highly fluorinated SEI film has been constructed by employing a salt concentrated electrolytes based on lithium（fluorosulfonyl）（n-nonafluorobutanesulfonyl）imide（Li[（FSO₂）（n-C₄F₉SO₂）N],LiFNFSI）.It is revealed by XPS that the participations of LiFNFSI in SEI formation have been successfully enhanced by increasing its concentration the electrolyte solutions.Additionally,it found that the F contents of SEI film formed on lithium metal anode cycled in 3 M LiFNFSI dimethoxyethane（DME）increases a high value as high as 34%compared with that of only13%formed in 1 M LiFNFSI-DME.Furthermore,with increasing Ar⁺etching time up to 40min（ca.30 nm in depth）,the F contents of SEI film in the concentrated electrolyte still remains up to 33%,being much higher than that formed in 1 M LiFNFSI-DME（ca.8%）.Because of the highly fluorinated SEI film formed on lithium metal anode in the concentrated LiFNFSI electrolyte,the Li|Cu cells display a relatively high average CE up to 95.6%and the Li|Li symmetric cells can also run smoothly more than 1000 h with a low polarization voltage of 18mV at 0.5 mA cm^-2 under Ar atmosphere.In 3 M LiFNFSI-DME,the immunity of metallic lithium towards dissolved oxygen can also be improved compared with that in the LiTFSI-based electrolyte,and the O₂ recovery efficiency and cycle life of Li–O₂ batteries increase up to 98.6%and 57 cycles,respectively,from 85%and 36 cycles for that soaked in LiTFSI-based one,respectively.3.To promote the rate-capability of the RLMBs and its long-term calendar life,an ionic liquid and ether double-solvent electrolyte,comprising 1 M LiTNFSI dissolved in N-propyl-N-methylpiperidinium bis（fluorosulfonyl）imide/1,3-dioxolane(PI₁₃FSI/DOL,1:1,v/v)is utilized.It is found that a highly ionic conductive,highly flexible SEI film including Li₃N(ionic conductivity:6×10^-3 S cm^-1)and oligomers（e.g.,（–CH₂CH₂OR–）_n and（–CH₂CH₂OCH₂O–）_n,etc.）has been formed on lithium metal anode.It is also revealed that,in this recipe,the Li|Li symmetric cells are able to run 1200 h with a low polarization potential of 18 mV under a high current density of 10 mA cm^-2,and that the Li|Cu cells can operate for ca.1300 h with an average CE up to 98.2%,and moreover,the Li|LiFePO₄ cells can run 1000cycles with a capacity fading less than 3%at 1 C.4.To improve the stability of lithium metal anode and enhance the cycling performances ofRLMBs,anovelorganicmolecularcompound,namely trimethylsilyl（fluorosulfonyl）（nonafluorobutanesulfonyl）imide（（CH₃）₃Si-[N（FSO₂）（n-C₄F₉SO₂）],TMS-FNFSI）,has been designed and prepared,and has been utilized as an electrolyte additive in the popular electrolyte（1 M LiTFSI-DOL/DME,1:1,v/v）for LMBs.It is demonstrated that TMS-FNFSI displays a relatively high reduction potential（peak potential:1.55 V vs.Li/Li⁺）compared with that of LiTFSI（1.25 V vs.Li/Li⁺）and the DOL/DME（0.35V vs.Li/Li⁺）,enabling its priority for SEI formation,resulting in a LiF-rich,organic F-and Si-containing SEI film.It is found that with 5 wt%TMS-FNFSI the Li|Li symmetric cells can operate stably for 1200 h with a polarization potential of 25 mV at the current density of 0.5mA cm^-2 and that the Li|Cu cells can run more than 100 cycles with an average CE up to 96.5%at current density and areal capacity of 1.0 mA cm^-2 and 2.0 mAh cm^-2.The Li|LiFePO₄ cells with TMS-FNFSI can operate steadily for 100 cycles with capacity retention of 92%at 0.2 C.