Supercritical CO2 (SCCO2), characterized by gas‐like diffusivity, low surface tension, and excellent mass transfer properties, is applied to create a SiOx/carbon multi‐layer coating on Si particles. Interaction of SCCO2 with Si produces a continuous SiOx layer, which can buffer Si volume change during lithiation/delithiation. In addition, a conformal carbon film is deposited around the Si@SiOx core. Compared to the carbon film produced via a conventional wet‐chemical method, the SCCO2‐deposited carbon has significantly fewer oxygen‐containing functional groups and thus higher electronic conductivity. Three types of carbon precursors, namely, glucose, sucrose, and citric acid, in the SCCO2 syntheses are compared. An eco‐friendly, cost‐effective, and scalable SCCO2 process is thus developed for the single‐step production of a unique Si@SiOx@C anode for Li‐ion batteries. The sample prepared using the glucose precursor shows the highest tap density, the lowest charge transfer resistance, and the best Li+ transport kinetics among the electrodes, resulting in a high specific capacity of 918 mAh g−1 at 5 A g−1. After 300 charge–discharge cycles, the electrode retains its integrity and the accumulation of the solid electrolyte interphase is low. The great potential of the proposed SCCO2 synthesis and composite anode for Li‐ion battery applications is demonstrated.

Highly concentrated electrolytes, although promising, are of high cost and show high viscosity and unsatisfactory wettability toward electrodes and separators, making them unfavorable for practical applications. A more rational electrolyte design is thus needed. Here, we investigate moderately concentrated electrolytes and find that the lithium bis(fluorosulfonyl)imide (LiFSI) concentration effects on the capacity, rate capability, and cycling stability of Si anodes in an ethylene carbonate (EC)/diethyl carbonate (DEC) mixed electrolyte are opposite to those in a fluoroethylene carbonate (FEC) electrolyte. The reasons for these results are systematically examined using Raman spectroscopy, transmission electron microscopy, electrochemical impedance spectroscopy, and the galvanostatic intermittent titration technique. A detailed X-ray photoelectron spectroscopy analysis is performed to study the solid electrolyte interphase chemistry. Al corrosion that occurs with the EC/DEC-based electrolyte can be effectively suppressed with the FEC-based electrolyte if an adequate LiFSI concentration is used. In the proposed 2 mLiFSI/FEC electrolyte, the Si anode has reversible capacities of 2630 and 855 mA h g −1 at 0.2 and 5 A g −1 , respectively, and ∼75% capacity retention after 200 cycles (remarkably higher than that obtained with the EC/DEC-based electrolyte). This electrolyte also shows great compatibility with the high-energy-density LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NMC-811) cathode, allowing a stable charge−discharge of a Si//NMC-811 full cell.

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The lithiation/delithiation properties of 𝜶-Si 3 N 4 and 𝜷-Si 3 N 4 are compared and the carbon coating effects are examined. Then, 𝜷-Si 3 N 4 at various fractions is used as the secondary phase in a Si anode to modify the electrode properties. The incorporated 𝜷-Si 3 N 4 decreases the crystal size of Si and introduces a new N-Si-O species at the 𝜷-Si 3 N 4 /Si interface. The nitrogen from the milled 𝜷-Si 3 N 4 diffuses into the surface carbon coating during the carbonization heat treatment, forming pyrrolic nitrogen and C-N-O species. The synergistic effects of combining 𝜷-Si 3 N 4 and Si phases on the specific capacity are confirmed. The operando X-ray diffraction and X-ray photoelectron spectroscopy data indicate that 𝜷-Si 3 N 4 is partially consumed during lithiation to form a favorable Li 3 N species at the electrode. However, the crystalline structure of the hexagonal 𝜷-Si 3 N 4 is preserved after prolonged cycling, which prevents electrode agglomeration and performance deterioration. The carbon-coated 𝜷-Si 3 N 4 /Si composite anode shows specific capacities of 1068 and 480 mAh g −1 at 0.2 and 5 A g −1 , respectively. A full cell consisting of the carbon-coated 𝜷-Si 3 N 4 /Si anode and a LiNi 0.8 Co 0.1 Mn 0.1 O 2 cathode is constructed and its properties are evaluated. The potential of the proposed composite anodes for Li-ion battery applications is demonstrated.