Atomic and Molecular Layer Deposition for Superior Lithium‐Sulfur Batteries: Strategies, Performance, and Mechanisms

Abstract

sulfur (LiÀS) batteries hold great promise for powering future electric vehicles, given their high theoretical specific energy of 2600 Wh/kg and low cost. However, the commercialization of LiÀS batteries is being hindered by several serious technical challenges including the corrosion of lithium metal anodes, the formation of lithium dendrites, the shuttle effect of lithium polysulfides, the decomposition of electrolyte, and low conductivity of sulfur and lithium sulfide. In the past decade, atomic layer deposition (ALD) as a new research thrust has been demonstrated to be a very effective technique in dramatically improving the performance of lithium-ion batteries (LIBs), featuring many unique advantages in fabricating sub-nano to nanoscale inorganic films. Stimulated by ALD's benefits, recently more and more research efforts have been reported in ALD for addressing the technical issues of LiÀS batteries and show very promising outcomes. Analogous to ALD, molecular layer deposition (MLD) is a thin-film technique exclusively for polymeric films. Some of the latest studies have uncovered that MLD is an alternative tool for tackling the challenges of LiÀS batteries with exceptional effectiveness. In this review, for the first time, we systematically summarized the progresses of both ALD and MLD in developing superior LiÀS batteries, covering their technical strategies, resulting performance, and the underlying mechanisms. In terms of the functionalities of ALD and MLD in LiÀS batteries, we focus our discussion in two main complementary aspects: (i) surface coatings and (ii) electrode designs.[a] Q. polysulfide, Li 2 S 8 , and the subsequent reductions of Li 2 S 8 to low order polysulfides Li 2 S n (3 n 6), corresponding to the decreasing voltage slope from 2.2 V to~2.1 V. While the lower voltage plateau (1.9-2.1 V) is ascribed to the reductions from the soluble low-order polysulfides (Li 2 S n , 3 n 6) to the insoluble Li 2 S 2 or Li 2 S. During the final stage of a discharge process, the reduction from Li 2 S 2 to Li 2 S is the main reaction step. In a charging process, the discharge product, Li 2 S is oxidized and then finally changed into S 8 .However, LiÀS batteries suffer from a series of major challenges in practice. [20a,f,23] First, the lithium electrode is prone to present heterogeneous deposition and dissolution during charge-discharge processes. In addition, lithium is likely to grow into dendritic structures [24] and the resulting dendrites pose serious risks to the cell safety. The dendritic lithium is possible to penetrate the separator, reach the cathode side, and then short the cell. [24e,25] Second, lithium metal anodes tend to experience corrosion in contact with liquid organic electrolytes such as the widely applied mixture of dioxolane (DOL) and dimethoxyethane (DME), leading to the formation of some gaseous and solid by-products, i. e., solid electrolyte interphase (SEI). [26] The formation of SEI consumes lithium and electrolytes. This may increase cell impedance and lead to the final failu...

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