Abstract:

Commercialization of high-energy rechargeable batteries can promote the rapid development of portable electronics and electric vehicles. Li metal batteries (LMBs) have attracted considerable attention owing to their high theoretical energy density. Li metal anodes (LMAs) used in LMBs suffer from the disadvantages of high reactivity, interface instability and dendrite growth, which impede the practical development of the LMBs. Coulombic efficiency (CE), which depends on the type of electrolyte used, is one of the key parameters for evaluating the reversibility of battery systems. Herein, we use atomic force microscopy (AFM) to study the initial plating stages and growth of the lithium metal in different electrolytes, such as 1 mol·L^-1 lithium hexafluorophosphate (LiPF₆)-ethylene carbonate/dimethyl carbonate (EC/DMC, 1 : 1, V/V), 1 mol·L^-1 LiPF₆-EC/DMC (1 : 1, V/V) + 5% (mass fraction, w) fluoroethylene carbonate (FEC), 1 mol·L^-1 lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)-1, 3-dioxolane/dimethoxyethane (DOL/DME, 1 : 1, V/V) + 2% (w) lithium nitrate (LiNO₃), and 4 mol·L^-1 lithium bis(fluorosulfonyl)imide (LiFSI)-DME, and further investigate the correlation between the CE of LMA and Li plating morphology. There are two types of Li morphologies in these electrolytes: strip-like and particle-like morphology. Since the specific surface area of particle-like deposits is much smaller than that of strip-like deposits, the particle-like morphology facilitates higher CE. (1) In the conventional carbonate electrolyte (1 mol·L^-1 LiPF₆-EC/DMC), Li predominantly forms strip-like deposits with large specific surface area, consuming much active Li (due to the side reaction between Li and the electrolyte). The dendrite morphology of the Li deposits lead to the formation of dead Li during the stripping process, which results in low CE. (2) FEC, an effective additive often used in carbonate electrolyte, can induce the transformation of Li plating morphology from strip-like to particle-like morphology. Therefore, the CE in FEC-containing electrolytes has been significantly improved with stable electrode/electrolyte interphase and small specific surface area of deposited Li. (3) In ether electrolytes, which have better compatibility with LMAs than carbonate electrolytes, Li metal exhibits a particle-like morphology and achieves high CE. (4) In the highly concentrated electrolyte (4 mol·L^-1 LiFSI-DME), Li metal grows into large particles without dendrite formation, which hampers the parasitic side reactions, and further enhances CE.

Key words: Li metal anode, Coulombic efficiency, Plating morphology, Electrolyte, Atomic force microscopy