Preparation of Two−Dimensional Li₄SiO₄ from Attapulgite and Spent Lithium−Ion Batteries for CO₂ Capture

High−temperature CO₂ adsorbents play a key role in carbon capture and storage (CCS) technology. Among them, lithium orthosilicate (Li₄SiO₄) is promising due to its suitable operating temperature and good cyclic stability. However, the high cost of conventional precursors limits its practical application. In this study, a cost−effective strategy was developed to synthesize two−dimensional (2D) Li₄SiO₄ nanosheets using natural attapulgite (ATP) clay as the silicon source and spent lithium−ion battery cathode materials (LTCO) as the lithium source. Raw ATP was first acid−modified to remove most metal ions while preserving the Si–O tetrahedral framework. For lithium recovery, the spent LTCO was subjected to carbothermal reduction with freeze−dried onion powder as the biomass carbon source. The optimal conditions were identified as an LTCO−to−onion powder mass ratio of 1∶0.5 and a reduction temperature of 450 ℃, yielding Li₂CO₃ as the main product. The recovered Li₂CO₃ and the silica derived from acid−treated ATP were then used to prepare Li₄SiO₄ via a microwave−assisted hydrothermal method followed by calcination. Two reference samples were also prepared: one using analytical grade Li₂CO₃ (b−Li₄SiO₄) and another using commercial silica sol (c−Li₄SiO₄). a−Li₄SiO₄ exhibits a well−defined two−dimensional sheet−like morphology with a specific surface area of 213 m²/g, which is much higher than that of c−Li₄SiO₄ (63 m²/g). CO₂ adsorption performance was evaluated under 15% (vol) CO₂ at high temperatures. Dynamic adsorption–desorption tests show that the optimal adsorption temperature for a−Li₄SiO₄ is 625 ℃, lower than those of the reference samples. Isothermal adsorption measurements indicate that a−Li₄SiO₄ achieves a CO₂ uptake capacity of 0.307 g/g at 625 ℃, significantly outperforming b−Li₄SiO₄ (0.266 g/g) and c−Li₄SiO₄ (0.221 g/g). Furthermore, cyclic tests demonstrate that a−Li₄SiO₄ maintains excellent stability over multiple adsorption–desorption cycles.The superior performance of a−Li₄SiO₄ is attributed to its unique two−dimensional nanosheet architecture, which provides abundant active sites, facilitates CO₂ diffusion, and prevents particle agglomeration. This work therefore offers a synergistic approach for the high−value utilization of natural attapulgite and the resource recovery of spent lithium−ion batteries, paving the way for low−cost, high−performance CO₂ adsorbents.