Study On The Interface Stability Of Lithium Metal Battery Electrode

Lithium metal batteries have a higher theoretical energy density than lithium ion batteries,and lithium metal anodes and intercalated positive batteries are regarded as the most promising next-generation batteries.The lithium metal anode is matched with the nickel-rich ternary material(NCM),which is expected to reach an energy density with 500Wh kg-1.However,for the commercial application of lithium metal batteries,there are still many problems to be solved urgently at the interface between the solid electrolyte interphase on the anode and the cathode electrolyte interface on the cathode.An unstable solid electrolyte interphase(SEI)and accompanying Li metal dendrites are the key impediments to commercialization of high-energy lithium metal batteries(LMBs).We employ a strontium fluoride(SrF2)microsphere coated polypropylene(PP)separator to stabilize the SEI and to prevent dendrites from growing.This approach is tested with LikCu half-cells,Li/Li symmetrical cells,and Li/NMC full LMBs,there being a major improvement in each case.The Li/Cu cell with SrF2 maintains a stable coulombic efficiency(CE)of 80%after 100 cycles,when tested at 0.25 mA cm-2 to a capacity of 0.5 mAh cm-2.By comparison,the uncoated PP reference has a CE of 10%in cycle 60.The Li/Li cell with SrF2 exhibits a markedly smaller voltage polarization and is able to stably cycle for approximately 340 h vs.the reference which begins to display severe voltage instability at 200 h.The Li/NMC full LMB with SrF2 shows an initial discharge capacity of 173 mA h g-1,with 167 mAh g-1(96.5%)being retained after 200 cycles at 200 mA g-1(1C rate).The SrF2 containing LMB also has a substantially improved rate capability over the reference,the difference being drastic even at the highest testing rate of 20C.First-principles calculations based on DFT indicate that lithium ions prefer to adsorb onto the SrF2 surface,which should create a more uniform ion flux and reduce the propensity for dendrite nucleation.In parallel,the SrF2 spheres bind with the SEI layer,creating a tough in situ formed composite membrane that mechanically stabilizes a planar metal interface.The interfaces stability between the cathode and anode with the electrolyte is the key to the performances of the lithium metal batteries(LMBs).A FEC optimized LiTFSI-LiBOB dual salt(Dual-salt+FEC)electrolyte is employed to systematically investigate its effects on the LMBs.The high nickel Li‖NCM full LMBs using the Dual-salt+FEC electrolyte presents significantly enhanced long-term cycling stability with 83.8%capacity retention after 500 cycles,which is much higher than 65.4%for LiPF6-based electrolyte.The improving mechanism has been studied using aberration-corrected high-angle annular dark-field scanning transmission electron microscopy(HAADF-STEM)imaging combined with the X-ray photoelectron spectroscopy(XPS)and electrochemical impedance spectroscopy(EIS).HAADF-STEM results firstly provide direct evidences in atomic scale that the corrosion and transition metal dissolution for the high nickel NCM cathode cycled in Dual-salt+FEC electrolyte could be effectively suppressed.Moreover,the HRTEM and XPS results further disclose that,a stable CEI on cathode and a thinner,smoother and strong adhesion of SEI on Li metal anode in the high nickel Li/NCM cell could be achieved using the Dual-salt+FEC electrolyte.This holistic analysis of the cathode and anode electrolyte interfaces will significantly advance the LMBs design principles through manipulation of the electrolyte chemistry.