Adsorption of high-temperature CO2 by Ca2+/Na+-doped lithium orthosilicate: characterization, kinetics, and recycle

High-temperature solid adsorbent Li4SiO4 has received broad attention due to its high theoretical adsorption capacity, high regeneration capacity, and wide range of raw materials for preparation. In this paper, a Li4SiO4 adsorbent was prepared by MCM-48 as the silica precursor and modified by doping with metal ions (Ca2+ and Na+) for high-temperature capture of low-concentration CO2. The results showed that the surface of the Ca-doped (or Na-doped) Li4SiO4 adsorbent developed some particles that are primarily composed by Li2CaSiO4 (or Li3NaSiO4). Furthermore, the grains of the adsorbents became finer, effectively increasing the specific surface area and enhancing adsorption performance. Under 15 vol% CO2, the maximum CO2 adsorption was 25.63 wt% and 32.86 wt% when the Ca2+ doping amount was 0.06 and the Na+ doping amount was 0.12, respectively. These values were both higher than the adsorption capacity before the metal ion doping. After 10 adsorption/desorption cycles, the adsorption capacity of Na-doped Li4SiO4 increased by 9.68 wt%, while that of Ca-doped Li4SiO4 decreased by 7.98 wt%. This difference could be attributed to the easy sintering of the Ca-containing adsorbent. Furthermore, a biexponential model was used to fit the CO2 adsorption curve of the adsorbent in order to study the adsorption kinetics. Compared to the conventional Li4SiO4, the Ca/Na-doped adsorbent offers several advantages, such as a high CO2 adsorption capacity and stable cycling ability.

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.