Mechanism Explaining the Onset Time of Dendritic Lithium Electrodeposition via Considerations of the Li⁺Transport within the Solid Electrolyte Interphase

Abstract

A critical hurdle in realizing rechargeable lithium-metal batteries is the dendritic electrodeposition of lithium during the battery charging process. Here, we investigate the onset of dendritic morphology during galvanostatic lithium electrodeposition using electrochemical techniques combined with optical microscopy. We show that lithium dendrites initiate at the time when the surface overpotential during galvanostatic electrodeposition reaches a maximum value. This observation is explained using an analytical transport model wherein the Li + concentration within the solid electrolyte interphase (SEI) near the lithium metal surface decreases gradually as the SEI thickness and its transport resistance increases with time. At the dendrite onset time (τ onset ), the Li + concentration at the lithium-SEI interface approaches zero -a condition under which surface roughness on the lithium electrode amplifies, producing dendrites. Once dendrites form, they rupture the SEI, lowering the surface resistance for plating. Model predictions of how τ onset varies with current density and soak time are shown to be in qualitative agreement with experimental observations. Furthermore, pulsed currents are applied to mitigate Li + concentration depletion within the SEI, thereby delaying or preventing altogether the formation of lithium dendrites.

Suppressing Lithium Dendrite Growth via Sinusoidal Ripple Current Produced by Triboelectric Nanogenerators

“…Figure 1a shows the working mechanism of a R-TENG with radial grating electrodes. [32][33][34] Thus, we expect that charging Li metal batteries using sinusoidal ripple current may lead to the growth of densely packed Li with low surface area and hence suppress its reaction with electrolyte ( Figure 1c), in contrast to constant current charging that suffers from dendritic/mossy Li growth with severe electrolyte consumption ( Figure 1d). When a kapton film slides toward the right-hand Cu electrode, the positive charges will flow from the left Cu electrode to the right one through external load.…”

“…Previous reports indicated stable lithium metal anodes can be achieved by charging cells with high-frequency pulse current. [32][33][34] Thus, we expect that charging Li metal batteries using sinusoidal ripple current may lead to the growth of densely packed Li with low surface area and hence suppress its reaction with electrolyte ( Figure 1c), in contrast to constant current charging that suffers from dendritic/mossy Li growth with severe electrolyte consumption ( Figure 1d).…”

“…This improvement can be attributed to more uniform Li deposition using square wave current than constant current charging, as reported in the literature. [30][31][32][33][34] Charging with square wave current at different on/off time ratios (T on /T off ) suggested that a short rest period of 0.02 s between pulses (T on /T off = 0.5 s/0.02 s) is sufficient to prolong the cycling life of the Li/Li cells (Figures S8 and S9, Supporting Information).…”

“…[32] This is further confirmed by Hoffmann et al who demonstrated that 1 ms pulse charging can inhibit the dendrite propagation more effectively than 20 ms pulse or constant current charging. [34] The above simulation and experimental studies suggest that interval charging could improve homogeneous nucleation of Li by controlling the diffusion process, therefore high-frequency pulse charging is beneficial for suppressing the growth of Li dendrites.Recently, rotary triboelectric nanogenerators (R-TENGs) originating from contact electrification have been developed and successfully demonstrated as mechanical energy harvesters. [34] The above simulation and experimental studies suggest that interval charging could improve homogeneous nucleation of Li by controlling the diffusion process, therefore high-frequency pulse charging is beneficial for suppressing the growth of Li dendrites.…”

“…[33] A more recent work by Maraschky and Akolkar indicated that pulsed current may mitigate the concentration depletion of Li ions within the solid electrolyte interphase (SEI), because the diffusion time constant of Li ions has a value of 1 ms for a 10 nm thick SEI and thereby the concentration gradient of Li ions formed across the SEI during charging can be nullified within 1 ms of rest period. [34] The above simulation and experimental studies suggest that interval charging could improve homogeneous nucleation of Li by controlling the diffusion process, therefore high-frequency pulse charging is beneficial for suppressing the growth of Li dendrites.Recently, rotary triboelectric nanogenerators (R-TENGs) originating from contact electrification have been developed and successfully demonstrated as mechanical energy harvesters. [35][36][37] A unique character of R-TENGs is that their voltage and current outputs are high-frequency sinusoidal waveforms.…”