Solid-electrolyte interphase (SEI) is the key component that enables all advanced electrochemical devices, the best representative of which is Li-ion battery (LIB). It kinetically stabilizes electrolytes at potentials far beyond their thermodynamic stability limits, so that cell reactions could proceed reversibly. Its ad hoc chemistry and formation mechanism has been a topic under intensive investigation since the first commercialization of LIB 25 years ago. Traditionally SEI can only be formed in nonaqueous electrolytes. However, recent efforts successfully transplanted this concept into aqueous media, leading to significant expansion in the electrochemical stability window of aqueous electrolytes from 1.23 V to beyond 4.0 V. This not only made it possible to construct a series of high voltage/energy density aqueous LIBs with unprecedented safety, but also brought high flexibility and even "open configurations" that have been hitherto unavailable for any LIB chemistries. While this new class of aqueous electrolytes has been successfully demonstrated to support diversified battery chemistries, the chemistry and formation mechanism of the key component, an aqueous SEI, has remained virtually unknown. In this work, combining various spectroscopic, electrochemical and computational techniques, we rigorously examined this new interphase, and comprehensively characterized its chemical composition, microstructure and stability in battery environment. A dynamic picture obtained reveals how a dense and protective interphase forms on anode surface under competitive decompositions of salt anion, dissolved ambient gases and water molecule. By establishing basic laws governing the successful formation of an aqueous SEI, the in-depth understanding presented in this work will assist the efforts in tailor-designing better interphases that enable more energetic chemistries operating farther away from equilibria in aqueous media.

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The formation and continuous growth of a solid electrolyte interphase (SEI) layer are responsible for the irreversible capacity loss of batteries in the initial and subsequent cycles, respectively. In this article, the electron tunneling barriers from Li metal through three insulating SEI components, namely Li 2 CO 3 , LiF and Li 3 PO 4 , are computed using density function theory (DFT) calculations. Based on electron tunneling theory, it is estimated that ~nm of these components are sufficient to block electron tunneling. It is also found that the band gap decreases under tension while the work function remains the same, and thus the tunneling barrier decreases under tension and increases under compression. A new parameter, η, characterizing the average distances between anions, is proposed to unify the variation of band gap with strain under different loading conditions into a single linear function of η. An analytical model based on the tunneling results is developed to connect the irreversible capacity loss, due to the Li ions consumed in forming these SEI component layers on the surface of negative electrodes. The agreement between the model predictions and experimental results suggests that only the initial irreversible capacity loss is due to the self-limiting electron tunneling property of the SEI.

While high sulfur loading has been pursued as a key parameter to build realistic high-energy lithium-sulfur batteries, less attention has been paid to the cathode porosity, which is much higher in sulfur/carbon composite cathodes than in traditional lithium-ion battery electrodes. For high-energy lithium-sulfur batteries, a dense electrode with low porosity is desired to minimize electrolyte intake, parasitic weight, and cost. Here we report the profound impact on the discharge polarization, reversible capacity, and cell cycling life of lithium-sulfur batteries by decreasing cathode porosities from 70 to 40%. According to the developed mechanism-based analytical model, we demonstrate that sulfur utilization is limited by the solubility of lithium-polysulfides and further conversion from lithium-polysulfides to Li2S is limited by the electronically accessible surface area of the carbon matrix. Finally, we predict an optimized cathode porosity to maximize the cell level volumetric energy density without sacrificing the sulfur utilization.

The Solid electrolyte interphase (SEI), either naturally formed or artificially designed, plays a critical role in the stability and durability of Li-ion batteries (LIBs). It is even more important for high energy density electrodes such as Li metal anodes, which is subjected to large volumetric and interfacial variations due to Li deposition/stripping cycles during operation. Currently, there is a lack of understanding of the role of SEI/Li interfaces and their mechanical and electrochemical properties. In this paper, we present an interfacial study to evaluate the two major SEI components, LiF and Li 2 CO 3 , based on density functional theory (DFT) calculations. The calculated interfacial energy results show that the Li 2 CO 3 /Li interface has higher interfacial mechanical strength. To achieve the practical application of reversible high energy density Li metal electrode (3860 mAh g −1 ) for future "beyond Li-ion batteries", 1-3 one of the most significant challenges is to mitigate irreversible Li-dendrite formation.4-10 Dendrite formation is also often a concern for fast charging and low temperature operation in current Li-ion battery technology. Li dendrite formation and growth can be controlled by many factors. Intrinsically, dendrite morphology is determined by material properties, such as surface energy and growth anisotropy. However, it may also be influenced by other factors such as local current distribution 11,12 (due to electrode surface roughness and compositional inhomogeneity, etc.), operation voltage and charging rate, 13,14 as well as electrolyte composition. 7,15The dendrite growth process can also be strongly affected by the properties of solid electrolyte interphase (SEI), the passivation thin layer between electrolyte and electrode. Electrolyte solvents, such as ethylene carbonate (EC) and dimethyl carbonate (DMC) will naturally reduce and decompose at the low potential Li metal surface and spontaneously form an SEI layer. 16 First proposed by Peled in 1970s, 17 SEI remains "the most important but least understood" 18,19 in rechargeable LIBs due to its complicated content that is highly dependent on numerous factors such as electrolytes and additives, 20 electrode surface 21,22 and operating conditions. 23 Many recent efforts have been focused on using different electrolytes to alter the property of SEI or developing artificial coatings on the Li surface to mitigate dendrite growth. Therefore, it is important to understand how the SEI and coating properties may impact Li dendrite formation.After decades of debates, 4,17,24,25 there are some general agreements on the role of SEI in Li dendrites formation mechanisms ( Figure 1). Since Li metal oxidizes instantly when in contact with electrolyte, the only two probable Li plating sites are inside SEI and at Li anode surface, which is further determined by two processes: electron tunneling from anode to SEI and Li ion diffusion from electrolyte to anode. In most LIBs, where the well formed SEI is insulating enough to block electron tunneling f...