Reduction Mechanism of Solid Electrolyte Interphase Formation on Lithium Metal Anode: Fluorine-Rich Electrolyte

Metallic lithium is considered a promising anode that can significantly increase the energy density of rechargeable lithium-based batteries, but problems like uncontrollable growth of lithium dendrites and formation of dead lithium impede its application. Recently, a low-concentration single-salt two-solvent electrolyte, 1M LiTFSI/FDMA/FEC, has attracted attention because a high coulombic efficiency can be achieved even after many cycles owing to the formation of a robust solid electrolyte interface (SEI). However, the reaction mechanism and SEI structure remain unclear, posing significant challenges for further improvement. Here, a hybrid ab initio and reactive force field (HAIR) method revealed the underlying reaction mechanisms and detailed formation pathway. 1 ns HAIR simulation provides critical information on the initial reduction mechanism of solvent (FDMA and FEC) and salt (LiTFSI). FDMA and FEC quickly decompose to provide F- that builds LiF as the major component of the inner layer of inorganic SEI, which has been demonstrated to protect Li anode. Decomposition of FDMA also leads to a significant nitrogen-containing composition, producing Li-N-C, LixN, and other organic components that increase the conductivity of SEI to increase performance. XPS analysis confirms evolution of SEI morphology consistent with available experiments. These results provide atomic insight into SEI formation, which should be beneficial for the rational design of advanced electrolytes

Drag Reduction Mechanism of Viscoelastic Slick-Water Fracturing Fluid in Tortuous and Rough Fractures

Slick-water can effectively reduce the flow drag of fracturing fluid. Many studies have focused on the drag reduction performance of slick-water in wellbore and perforation, but there has been little research on drag reduction characteristics in fracture flow. In this paper, a new visualization experiment system is used to simulate real fracture. The fracture surface is produced through actual triaxial hydraulic fracturing and is copied by a three-dimensional printer using resin material to maintain its shape feature. In comparing the experimental results, it was found that the main factors affecting drag reduction in a fracture are the relative molecular weight and the added concentration. Unlike the flow rule of the drag reducer in a pipeline, when the concentration is greater than 0.10%, a negative DR effect begins to appear. The influence of molecular weight is related to the flow stage; the increasing of molecular weight causes a reduction in DR effect when the flow rate is 0.24 m/s. However, the flow rate exceeds 0.5 m/s; drag reducers with higher molecular weight demonstrate better drag reduction performance. The drag reduction mechanism analysis in fractures was obtained from visualization observations, and the flow characteristics of fluid were characterized by using tracking particles. Drag reduction effect occurs mainly on the surface of the fractures in contrast to near the centre of the flow channel. This research can provide a reference for the experimental study on drag reduction in fractures and is of great significance to the optimization and improvement of drag reducing agent.