Lithium metal batteries (LMBs) are regarded as candidates for next-generation batteries due to their low electrochemical potential (i.e., -3.040 V vs. standard hydrogen electrode) and high theoretical specific capacity (i.e., 3 860 mA·h·g^-1). Lithium (Li) metal anode is susceptible to side reactions with organic electrolytes, leading to the uneven formation of the solid electrolyte interphase (SEI). The non-uniformity and fragility of the SEI can aggravate the growth of Li dendrites and excessive electrolyte consumption, ultimately resulting in a rapid attenuation of capacity and safety risks. These issues severely hinder the commercialization of LMBs. The development of an artificial SEI is crucial for protecting Li metal anode, suppressing Li dendrite growth, and accommodating volume changes during cycling. This advancement is essential for improving the stability and cycling life of LMBs.

In this paper, we synthesized an artificial elastic polymer SEI (EP-SEI) constructed on the surface of Li metal anode by an in-situ UV-curing method. The main composition of EP-SEI was poly (ethyleneglycol) diacrylate-co-vinylene carbonate (PEGDA-CO-VC) cross-linking polymer.

For the synthesis of an artificial elastic polymer SEI (EP-SEI) constructed on the surface of Li metal anode by an in-situ UV-curing method, polyethylene glycol diacrylate (PEGDA) and vinylene carbonate (VC) were mixed in a mass ratio of 4∶1 (i.e., 80 mg and 20 mg) under stirring. Afterwards, 2 mL of dimethoxyethane (DME) was added for dissolution. Subsequently, the mixture was dripped onto Li metal foils, followed by polymerization under a mercury lamp after the addition of 2-hydroxy-2-methylpropiophenone (HMPP). This method utilized UV light to rapidly excite the unsaturated bonds of polymer monomers, thereby shortening the polymerization reaction time and enhancing the preparation efficiency of the artificial SEI. This method was simple to operate, conducive to batch production, and aids in advancing the commercialization of LMB. The LMBs were assembled with LiFePO₄ (LFP) or LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) as a cathode material, Li metal foil as an anode, polypropylene (PP) as a separator, and 1 M LiPF₆ EC/DEC as an electrolyte. The batteries were assembled in a glove box filled with argon to ensure a moisture-free and oxygen-free environment during the process.

An artificial EP-SEI with PEGDA-co-VC was prepared on the surface of Li metal by an in-situ UV-curing polymerization method. The thickness of EP-SEI is 1.6 μm. The EP-SEI exhibits an uniform and dense morphology and tightly covers the surface of Li metal. In the results of electrochemical test, Li symmetric (Li || Li) cells assembled with EP-SEI protected Li exhibit highly reversible electrochemical behavior and faster charge transfer rates. The Li || Li cells with EP-SEI Li maintain stable cycling at a current density of 1 mA·cm^-2 and a capacity density of 1 mA·h·cm^-2 for over 300 h. And the impedance values of Li || Li cells with EP-SEI Li continuously decrease during the cycle, and eventually stabilize at 15 Ω. This indicates that EP-SEI exhibits a high stability during prolonged electrochemical reactions and a high lithium ion conductivity. In the results of Li ||Cu cells test, the low nucleation overpotential (μ_nuc) and plateau potential (μ_pla) indicate a reduction in the nucleation energy barrier for Li deposition. It is confirmed that EP-SEI can promote the uniform deposition of Li⁺ on the surface of Li metal. After cycling of Li ||Cu cell with EP-SEI Cu, the thickness of Li⁺ deposition approaches the theoretical thickness. The surface of EP-SEI Cu remains smooth and flat without the growth of Li dendrites. In the results of full cell test, the LFP || Li batteries with EP-SEI Li exhibit a discharge specific capacity of 164.1 mA·h·g^-1 and a capacity retention rate of 72% after 500 cycles with a coulombic efficiency of 98.5% at 0.5 C rate. The NCM811 || Li batteries with EP-SEI Li maintain a discharge specific capacity of 100 mA·h·g^-1 after 500 cycles. This demonstrates that EP-SEI effectively reduces electrolyte consumption and improves coulomb efficiency, cycle stability and safety of LMBs.

In this study, an elastic polymer artificial solid electrolyte interphase membrane (EP-SEI) was constructed on the Li metal anode by an in-situ UV-curing polymerization method. This EP-SEI exhibited excellent uniformity, stability and elasticity, promoting the uniform deposition of Li⁺ during cycling. These properties indicated that EP-SEI could inhibit Li dendrite formation, buffer the volume changes of the Li metal anode, and reduce electrolyte consumption. The LFP || Li and NCM811 || Li batteries assembled with EP-SEI-protected Li metal anodes achieved discharge specific capacities of 164.1 mA·h·g^-1 and 187.1 mA·h·g^-1 at 0.5 C rate, respectively. The LMBs maintained stable cycling for 500 cycles. The EP-SEI could effectively protect the Li metal anode, improve the coulombic efficiency and extend the cycling life of LMB. The EP-SEI protected Li metal anode provided the possibilities for the commercialization of LMBs.