An Effective Electrolyte Additive Achieving Dendrite-Free Lithium during Electrochemical Deposition

Lithium (Li) dendrite formation during repeated charging-discharging cycles is one of the critical challenges for the development of next-generation rechargeable Li metal batteries.^[1] The dendrites will penetrate through the separator, causing internal short circuit and even thermal runaway of the battery, which is not acceptable for commercial applications. To address this issue, a widely used approach is to create a strong interphase layer between the electrolyte and the Li metal anode in the form of a protective ion-permeable film, also known as a solid-electrolyte interphase (SEI) film.^[2] A stronger and more uniform SEI film makes ion conduction more homogeneous, which decreases protuberant tip formation and thus suppresses Li dendrite growth. These approaches include using different electrolyte solvents, salts and additives in electrolytes, which have been shown to effectively enhance the Li deposition behavior.^[3-6]

Here we introduce a new electrolyte additive A that acts as an effectively SEI formation agent to suppress the Li dendrite growth. This additive can be in-situ electrochemically reduced to form a proactive layer on Cu substrate at a high potential prior to Li deposition. This thin SEI layer can allow Li ion to pass while prevent the electrolyte erosion with the fresh deposited Li metal, thereby a dendrite-free Li film can be obtained.

Figure 1 gives the voltage-time profiles of Li deposition process in 1M LiPF₆/PC electrolyte without and with additive A. In the control electrolyte (black line), the voltage curve drops quickly from OCV to about 1.0 V vs. Li/Li⁺, followed by two short slopes between 1.0 V and 0 V which can be attributed to the reduction of PC solvent and salt anion on the substrate surface to form SEI film. Below 0 V, a long Li plating plateau occurs, corresponding to Li deposition reaction. In the electrolyte with additive A (red line), a high voltage plateau occurred at about 2.0 V, which can be ascribe to electrochemical reduction of the additive A to form a proactive layer on Cu substrate at this potential. This layer is supposed to favor the Li deposition of dendrite-free behavior. Figure 2 compares the morphologies (obtained by optical microscope) of the deposited Li films on Cu substrates using electrolytes without and with additive A. The Li film deposited in the control electrolyte is loose and dendritic (Figure 2a), and the Cu substrate surface is not completely covered by Li clusters. In contrast, the electrolyte with additive A shows significant improvement in the Li film surface quality with the elimination of dendrite formation. The Li deposition is smooth and dense, even reflecting blue metallic luminescence. Detailed electrochemical characterization and study of the working mechanism of this additive A will be discussed in the presentation.

Acknowledgements

This work was supported by the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the Basic Energy Sciences, Office of Science of the U.S. DOE.

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