Abstract:

The emerging market for consumer electronics and electric vehicles has stimulated intensive research on lithium metal batteries (LMBs) with high energy densities and large cycle lifetimes. A metallic Li anode has a high theoretical specific capacity of 3860 mAh·g^-1 and lowest redox potential of -3.04 V (vs. the standard hydrogen electrode) and is generally considered an ideal electrode for next-generation high-energy-density LMBs. However, their deployment in practical batteries is severely hindered by the formation of unsafe dendrites and fast capacity decay due to the uncontrollable formation of fragile solid electrolyte interfaces (SEIs). Herein, we describe the stable cycling of carbon paper (Cp)-supported Li-Sn alloy anodes in carbonate electrolytes modified with 1 mol·L^-1 bis(2, 2, 2-trifluorotoluene) carbonate (DTFEC). The molten Li-Sn alloy with 8% (mass fraction) Sn was synthesized through thermal treatment at 400 ℃ in an atmosphere of Ar. The Li-Sn-alloy-coated carbon paper (SnLi/Cp) was obtained after the molten alloy was conformally loaded onto the surface of a carbon paper under the action of capillarity. The as-synthesized interconnected SnLi/Cp composite was characterized by X-ray diffraction, energy-dispersive spectrometry, and scanning electron microscopy. The porous SnLi/Cp composite consisted of only Li and Sn₅Li₂₂ phases supported by the mechanically strong carbon paper with a good conductivity; no impurity was observed in the XRD results. The synergy of the DTFEC additive and alloying with Sn provided composite anodes with significantly improved rate capability and remarkable stability owing to the formation of a dense fluorinated SEI layer with high mechanical strength and ion penetration. Moreover, with the porous SnLi alloy covered by a fluorinated protection layer, lithium avoids the intrinsic issues of uncontrollable volume expansion and dendrite growth, which restrict the practical application of Li metal, exhibiting a stabilized over-potential of only 90 mV after 100 cycles at 8 mA·cm^-2. Notably, stable cycling with a 12 μL lean electrolyte was also observed at 5 mA·cm^-2. Overall, the prototype full cell assembled with the SnLi/Cp anode and NMC811 cathode exhibited a high Coulombic efficiency (98.1%) and remarkable cycling stability for 300 cycles at 1C (1.5 mA·cm^-2). The rate capability was evaluated at various rates of 0.5C to 5C. Compared to pure Li, the SnLi/Cp anode in the full cell exhibited a higher capacity, particularly at a high rate (~126 mAh·g^-1 at 5C). Our approach provides integrated Li metal electrodes with effectively improved cycle stabilities and is very attractive for practical high-energy-density lithium batteries.

Key words: 3D lithium alloy anode, Solid electrolyte interface, Additive, Fluorinated protection layer, Lean electrolyte