柱状金属锂沉积物:电解液添加剂的影响

二次电池的能量密度已成为推动电动汽车和便携式电子产品技术向前发展的重要指标。使用石墨负极的锂离子电池正接近其理论能量密度的天花板,但仍难以满足高端储能设备的需求。金属锂负极因其极高的理论比容量和极低的电极电位,受到了广泛关注。然而,锂沉积过程中枝晶的生长会导致电池安全性差等问题。电解液对金属锂的沉积有着至关重要的影响。本文设计了一种独特的电解槽体系来进行柱状锂的沉积,研究了不同电解液体系(1mol·L^-1LiPF6-碳酸乙烯酯/碳酸二乙酯(EC/DEC,体积比为1:1)、1 mol·L^-1 LiPF6-氟代碳酸乙烯酯(FEC,体积分数5%)-EC/DEC (体积比为1:1))对金属锂沉积的影响。对两种电解液中金属锂沉积物长径比的研究表明,电解液的组分可以显著地影响金属锂的沉积形貌,在加入氟代碳酸乙烯酯(FEC)添加剂之后,柱状锂的直径从0.3–0.6μm增加到0.7–1.3μm,长径比从12.5下降到5.6。长径比的降低有助于减小金属锂和电解液的反应面积,提高金属锂负极的利用率和循环寿命。通过考察循环后锂片的表面化学性质,发现FEC的分解增加了锂表面固态电解质界面层中氟化锂(LiF)组分的比例,提高了界面层中锂离子的扩散速率,减少了锂的成核位点,从而给予锂核更大的生长空间,降低了沉积出的柱状锂的长径比。

Columnar Lithium Metal Deposits: the Role of Non-Aqueous Electrolyte Additive

With the booming growth market of electric vehicles and portable electronics, high-energy-density rechargeable lithium ion batteries are being extensively used to advance high-end devices. Lithium-ion batteries with graphite anodes approach the ceiling in energy density, but they cannot satisfy the current demand. Among the next-generation electrodes, lithium metal anodes are strong candidates because of their high theoretical capacity and the most negative electrochemical potential. However, lithium metal batteries have been abandoned because of their poor safety resulting from the growth of lithium dendrites during lithium deposition. Although several strategies have been proposed to suppress the generation of lithium dendrites as well as the side reactions between active lithium and the electrolyte, lithium metal anodes have not been practically applied so far. Various studies have been conducted on the factors influencing lithium deposition, with the aim of understanding the growth behavior of lithium dendrites. The electrolyte plays a crucial role in the performance of the working Li metal anode. In this study, a unique battery system is proposed to realize columnar lithium deposition, which is convenient for obtaining the length and diameter of lithium deposits. The influence of different electrolytes on lithium deposition was investigated by comparing the length-diameter (L/D) ratio of the lithium deposits in two kinds of electrolytes (1.0 mol·L^-1 LiPF₆-ethylene carbonate/diethyl carbonate (EC/DEC, 1 : 1 by volume) and 1.0 mol·L^-1 LiPF₆-5% (volume fraction) fluoroethylene carbonate (FEC)-EC/DEC (1 : 1 by volume)). The morphology of the lithium deposits was strongly affected by the electrolyte composition. In the electrolyte with the FEC additive, the diameter of columnar lithium increased from 0.3-0.6 μm to 0.7-1.3 μm, while the L/D ratio decreased from 12.5 to 5.6. The small L/D ratio can reduce the reactive area between the lithium metal anode and the electrolyte, which is beneficial for achieving high lithium utilization and a long lifespan. To probe the origin of this influence, the surface chemistry of the cycled lithium metal anode was investigated by X-ray photoelectron spectroscopy. The FEC additive can increase the proportion of lithium fluoride (LiF) in the solid electrolyte interphase, which is conducive for the rapid diffusion of lithium ions. As a result, fewer nucleation sites are formed, providing more space for the growth of lithium cores with a large diameter. Therefore, the addition of FEC leads to a decrease in the L/D ratio of columnar lithium.