The occurrence of Coulombic efficiency (CE) troughs in silicon (Si) anodes for lithium-ion batteries (LIBs) presents a critical yet overlooked concern that could lead to battery failure in full cells. Here we conduct a comprehensive investigation into this previously unreported phenomenon. Factors influencing CE trough occurrence and severity, including electrode thickness, Si particle size, cycling rate, electrolyte composition, and voltage window, are systematically examined. Experimental results demonstrate that thinner electrodes and slower cycling rates accelerate CE trough onset, whereas employing a THF-based electrolyte or a narrower voltage window (0.01-0.5 V) results in stable electrochemical performance without CE troughs, concurrently with the presence of LixSi. Structural analysis via HAADF-STEM and SEM reveals a close association between CE trough severity, electrode volume expansion, and delamination, influenced by the formation of a sponge-like structure and SEI stability. These findings yield valuable insights into CE trough mechanisms and provide guidance for mitigating their occurrence through electrode design, electrolyte selection, and operational parameters, thereby advancing high-performance LIB development. Future research directions involve exploring the role of SEI components and alternative electrolyte additives to enhance SEI stability and mitigate CE troughs.
Silicon-based anodes are considered a promising alternative for next-generation lithium-ion batteries (LIBs) due to their high theoretical capacity, which is significantly greater than that of traditional graphite anodes. However, the inherent challenge of the associated low initial Coulombic efficiency (ICE) due to irreversible lithium consumption limits their practical applications. Prelithiation techniques have emerged as a solution to compensate for this initial lithium loss, but current methods often face challenges such as high costs, incomplete lithiation, and complex setups. In this study, we present a novel modified direct contact prelithiation method utilizing a Li-ion-free biphenyl solution. This innovative approach integrates the advantages of both direct contact and wet chemical prelithiation, achieving fast, uniform, and cost-effective prelithiation of Si-based anodes. Electrochemical characterizations demonstrate that the method significantly enhances ICE, reaching from 66.7% to 115.4% after 10 minutes of prelithiation for SiOx anodes and from 91.4% to 100.5% after just 90 seconds of prelithiation for Si anodes, while also stabilizing open-circuit voltage. Furthermore, microstructural analyses reveal the formation of a distinct solid electrolyte interphase layer after prelithiation. XPS depth profiling confirms the progressive lithiation of Si-based anodes, highlighting the formation of lithium oxide and lithium silicate compounds at varying depths with extended prelithiation times. These findings demonstrate the effectiveness of the proposed integrated prelithiation method in enhancing the electrochemical performance of Si-based anodes, paving the way for the development of high-energy-density LIBs.
The development of high-energy-density and high-safety lithium-ion batteries requires advancements in electrolytes. The state-of-the-art carbonate electrolyte faces challenges for operation at high voltage and has low thermal stability. This study proposes an ionic liquid/ether composite high-entropy electrolyte that consists of N-propyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PMP–TFSI) ionic liquid, dimethoxymethane (DME), lithium difluoro(oxalato)borate (LiDFOB), fluoroethylene carbonate (FEC), and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE). In this electrolyte, a unique coordination structure forms, where Li+ is surrounded in a high-entropy environment consisting of DME, FEC, TTE, TFSI–, DFOB–, and PMP+. The effects of this solution structure on the solid-electrolyte interphase chemistry and Li+ desolvation kinetics are examined. The proposed electrolyte has low flammability, high thermal stability, negligible corrosivity toward an Al current collector, and ability to withstand a high potential of up to 5 V without showing a significant side reaction current. Importantly, this electrolyte is highly compatible with graphite and SiOx anodes, as well as a high-nickel LiNi0.8Co0.1Mn0.1O2 cathode. Operando X-ray diffraction data confirm that the co-intercalation of DME and PMP+ into the graphite lattice, a long-standing challenge, is eliminated with this electrolyte. Graphite, SiOx, and LiNi0.8Co0.1Mn0.1O2 electrodes all exhibit better rate capability and cycling stability in the proposed electrolyte compared to those measured in a conventional carbonate electrolyte. A 4.5-V LiNi0.8Co0.1Mn0.1O2//graphite full cell with the proposed high-entropy electrolyte is shown to have superior specific capacity, rate capability, and cycling stability, demonstrating the great potential of the proposed electrolyte for practical applications.
