Novel ALD Chemistry Enabled Low-Temperature Synthesis of Lithium Fluoride Coatings for Durable Lithium Anodes

Abstract

Lithium metal anodes can largely enhance the energy density of rechargeable batteries because of the high theoretical capacity and the high negative potential. However, the problem of lithium dendrite formation and low Coulombic efficiency (CE) during electrochemical cycling must be solved before lithium anodes can be widely deployed. Herein, a new atomic layer deposition (ALD) chemistry to realize the low-temperature synthesis of homogeneous and stoichiometric lithium fluoride (LiF) is reported, which then for the first time, as far as we know, is deposited directly onto lithium metal. The LiF preparation is performed at 150 °C yielding 0.8 Å/cycle. The LiF films are found to be crystalline, highly conformal, and stoichiometric with purity levels >99%. Nanoindentation measurements demonstrate the LiF achieving a shear modulus of 58 GPa, 7 times higher than the sufficient value to resist lithium dendrites. When used as the protective coating on lithium, it enables a stable Coulombic efficiency as high as 99.5% for over 170 cycles, about 4 times longer than that of bare lithium anodes. The remarkable battery performance is attributed to the nanosized LiF that serves two critical functions simultaneously: (1) the high dielectric value creates a uniform current distribution for excellent lithium stripping/plating and ultrahigh mechanical strength to suppress lithium dendrites; (2) the great stability and electrolyte isolation by the pure LiF on lithium prevents parasitic reactions for a much improved CE. This new ALD chemistry for conformal LiF not only offers a promising avenue to implement lithium metal anodes for high-capacity batteries but also paves the way for future studies to investigate failure and evolution mechanisms of solid electrolyte interphase (SEI) using our LiF on anodes such as graphite, silicon, and lithium.

Building an Air Stable and Lithium Deposition Regulable Garnet Interface from Moderate‐Temperature Conversion Chemistry

“…Molten Li was applied onto the mLLZTOelectrolyte and spread out to cover the entire pellet surface (Figure 4a, compared with the pristine mLLZTO in Figure S17). Thei n situ formed fluorinated coating layer promotes the wetting of molten Li on garnet and creates an intimate contact at Li/ garnet interface (Figure 4b,c), which could be possibly due to the formation of non-stoichiometric LiF according to Chen et al [20] TOP-SIMS was employed to visualize the spatial distribution of chemical species at Li/garnet interface.T o enable easy detection of the interface,e xcessive Li was removed to ag reatest extent before the test. TheF À signal remains strong and stable in the initial 1200 s, and quickly diminishes as the sputter time increases (Figure 4d).…”

“…[21] Moreover,n on-stoichiometric LiF with Fv acancies,w hen contacted with Li metal, shows ah igh Li + conductivity as the local defects increases the concentration and mobility of charge carriers. [20] Step increased current density was applied to investigate the critical current density of LLZTOa nd mLLZTO.W hen the current density is below 0.5 mA cm À2 ,b oth symmetric cells show similar overpotentials,w hich increase with the increment of current density.T his phenomenon elucidates that the in situ formed LiF layer does not block Li + transport at the Li/mLLZTOinterface.Wealso calculated the exchange current density at the Li/LLZTOa nd Li/mLLZTOi nterface. [24a,c] Ther esistance (the slope of the fitted line) at unmodified Li/LLZTO interface (LLZTO@Li 2 CO 3 )i s 0.24 VmA À1 ,while the resistance at LiF-modified Li/LLZTO interface (LLZTO@LiF) is much smaller (0.19 VmA À1 ).…”

Key Aspects of Lithium Metal Anodes for Lithium Metal Batteries

“…However, the resistance of the bare Li anode was found to significantly increase after 240 cycles compared to MoS 2 ‐coated Li anode (Figure c) indicative of SEI failure (Figure d,e). Similarly, the Coulombic efficiency (ratio of irreversible capacity to the total reversible capacity) has become another important index for measuring the performance of an anode in a certain electrolyte . The average Coulombic efficiency of an electrode is closely associated with an SEI layer.…”

Towards high-performance lithium metal anodes via the modification of solid electrolyte interphases

“…High Li + diffusion conductivity is another important parameter for excellent artificial SEI as it can help to restrain the initiation of Li dendrites via homogenizing Li + flux. Thus, Li 3 N, LiF, 2D MoS 2 and Li 2 S that own both high mechanical modulus and fast Li + diffusion kinetic have been widely used to prolong the lifetime of Li metal anode [18,32,[98][99][100][101][102] . Li atoms can intercalate into atomic layers of 2D MoS 2 , in turn facilitating Li + diffuse into/out of bulk Li metal, which leads to stable Li deposition ( Fig.…”