Core-Shell Si@SiOC Particles Synthesized Using Supercritical Carbon Dioxide Fluid for Superior Li-Ion Storage Performance.

A supercritical carbon dioxide (SCCO2) fluid, characterized by gas-like diffusivity, near-zero surface tension, and excellent mass transfer properties, is used as a precursor to produce silicon oxycarbide (SiOC) coating. SCCO2 disperses and reacts with Si particles to form an interfacial layer consisting of Si, O, and C. After an 850 °C annealing process, a conformal SiOC coating layer forms, resulting in core-shell Si@SiOC particles. High-resolution transmission electron microscopy and its X-ray line-scan spectroscopy, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, and Raman spectroscopy, are used to examine the SiOC formation mechanism. Effects of SCCO2 interaction time on the SiOC properties are investigated. The SiOC layer connects the Si@SiOC particles, improving electron and Li+ transport. Cyclic voltammetry, galvanostatic intermittent titration technique, and electrochemical impedance spectroscopy are employed to examine the role of SiOC during charging/discharging. Operando X-ray diffraction data reveal that the SiOC coating reduces crystal size of the formed Li15Si4 and increases its formation/elimination reversibility during cycling. The Si@SiOC electrode shows a capacitiy of 2250 mAh g-1 at 0.2 A g-1. After 500 cycles, the capacity retention is 72% with Coulombic efficiency above 99.8%. A full cell consisting of Si@SiOC anode and LiNi0.8Co0.1Mn0.1O2 cathode is constructed, and its performance is evaluated.

Double Nitrogenation Layer Formed Using Nitric Oxide for Enhancing Li+ Storage Performance, Cycling Stability, and Safety of Si Electrodes.

To enhance Li storage properties, nitrogenation methods are developed for Si anodes. First, melamine, urea, and nitric oxide (NO) precursors are used to nitrogenize carbon-coated Si particles. The properties of the obtained particles are compared. It is found that the NO process can maximize the graphitic nitrogen (N) content and electronic conductivity of a sample. In addition, optimized N functional groups and O─C species on the electrode surface increase electrolyte wettability. However, with a carbon barrier layer, NO hardly nitrogenizes the Si cores. Therefore, bare Si particles are reacted with NO. Core-shell Si@amorphous SiNx particles are produced using a facile and scalable NO treatment route. The effects of the NO reaction time on the physicochemical properties and charge-discharge performance of the obtained materials are systematically examined. Finally, the Si@SiNx particles are coated with N-doped carbon. Superior capacities of 2435 and 1280 mAh g-1 are achieved at 0.2 and 5 A g-1, respectively. After 300 cycles, 90% of the initial capacity is retained. In addition, differential scanning calorimetry data indicate that the multiple nitrogenation layers formed by NO significantly suppress electrode exothermic reactions during thermal runaway.

Enhanced Electrochemical Performance of Ca-Doped Na3V2(PO4)2F3/C Cathode Materials for Sodium-Ion Batteries.

Na3V2(PO4)2F3 (NVPF) with a NASICON structure has garnered attention as a cathode material owing to its stable 3D structure, rapid ion diffusion channels, high operating voltage, and impressive cycling stability. Nevertheless, the low intrinsic electronic conductivity of the material leading to a poor rate capability presents a significant challenge for practical application. Herein, we develop a series of Ca-doped NVPF/C cathode materials with various Ca2+ doping levels using a simple sol-gel and carbon thermal reduction approach. X-ray diffraction analysis confirmed that the inclusion of Ca2+ does not alter the crystal structure of the parent material but instead expands the lattice spacing. Density functional theory calculations depict that substituting Ca2+ ions at the V3+ site reduces the band gap, leading to increased electronic conductivity. This substitution also enhanced the structural stability, preventing lattice distortion during the charge/discharge cycles. Furthermore, the presence of the Ca2+ ion introduces two localized states within the band gap, resulting in enhanced electrochemical performance compared to that of Mg-doped NVPF/C. The optimal NVPF-Ca-0.05/C cathode exhibits superior specific capacities of 124 and 86 mAh g-1 at 0.1 and 10 C, respectively. Additionally, the NVPF-Ca-0.05/C demonstrates satisfactory capacity retention of 70% after 1000 charge/discharge cycles at 10 C. These remarkable results can be attributed to the optimized particle size, excellent structural stability, and enhanced ionic and electronic conductivity induced by the Ca doping. Our findings provide valuable insight into the development of cathode material with desirable electrochemical properties.

In Situ Atomic-Scale Investigation of Structural Evolution During Sodiation/Desodiation Processes in Na3 V2 (PO4 )3 -Based All-Solid-State Sodium Batteries.

Recently, all-solid-state sodium batteries (Na-ASSBs) have received increased interest owing to their high safety and potential of high energy density. The potential of Na-ASSBs based on sodium superionic conductor (NASICON)-structured Na3 V2 (PO4 )3 (Na3 VP) cathodes have been proven by their high capacity and a long cycling stability closely related to the microstructural evolution. However, the detailed kinetics of the electrochemical processes in the cathodes is still unclear. In this work, the sodiation/desodiation process of Na3 VP is first investigated using in situ high-resolution transmission electron microscopy (HRTEM). The intermediate Na2 V2 (PO4 )3 (Na2 VP) phase with the P21 /c space group, which would be inhibited by constant electron beam irradiation, is observed at the atomic scale. With the calculated volume change and the electrode-electrolyte interface after cycling, it can be concluded that the  Na2 VP phase reduces the lattice mismatch between Na3 VP and NaV2 (PO4 )3 (NaVP), preventing structural collapse. Based on the density functional theory calculation (DFT), the Na+ ion migrates more rapidly in the Na2 VP structure, which facilitates the desodiation and sodiation processes. The formation of  Na2 VP phase lowers the formation energy of NaVP. This study demonstrates the dynamic evolution of the Na3 VP structure, paving the way for an in-depth understanding of electrode materials for energy-storage applications.

N-Containing Carbon-Coated ß-Si3 N4 Enhances Si Anodes for High-Performance Li-Ion Batteries.

The lithiation/delithiation properties of α-Si3 N4 and ß-Si3 N4 are compared and the carbon coating effects are examined. Then, ß-Si3 N4 at various fractions is used as the secondary phase in a Si anode to modify the electrode properties. The incorporated ß-Si3 N4 decreases the crystal size of Si and introduces a new NSiO species at the ß-Si3 N4 /Si interface. The nitrogen from the milled ß-Si3 N4 diffuses into the surface carbon coating during the carbonization heat treatment, forming pyrrolic nitrogen and CNO species. The synergistic effects of combining ß-Si3 N4 and Si phases on the specific capacity are confirmed. The operando X-ray diffraction and X-ray photoelectron spectroscopy data indicate that ß-Si3 N4 is partially consumed during lithiation to form a favorable Li3 N species at the electrode. However, the crystalline structure of the hexagonal ß-Si3 N4 is preserved after prolonged cycling, which prevents electrode agglomeration and performance deterioration. The carbon-coated ß-Si3 N4 /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 ß-Si3 N4 /Si anode and a LiNi0.8 Co0.1 Mn0.1 O2 cathode is constructed and its properties are evaluated. The potential of the proposed composite anodes for Li-ion battery applications is demonstrated.