Practical deployment of Li₄SiO₄ as a high-temperature CO₂ sorbent requires pelletization, which inevitably densifies the microstructure and imposes severe CO₂ diffusion limitations. Conventional sacrificial pore-forming agents address this issue but remain single-purpose, serving solely as structural templates without conferring chemical benefits. Here, we demonstrate that spent coffee grounds (SCG), an abundant food-industry waste, can serve as a single-source modifier that achieves three co-localized enhancements in Li₄SiO₄ pellets: hierarchical pore engineering, in-situ K-doping, and oxygen vacancy generation. The thermal decomposition of SCG creates an interconnected hierarchical macroporous network that effectively reduces intraparticle CO₂ diffusion resistance. Meanwhile, the mineral-rich SCG ash provides in-situ potassium doping, generating a localized eutectic molten carbonate phase that accelerates liquid-phase ion transport. Crucially, the transient reducing atmosphere during biomass combustion introduces oxygen vacancies into the silicate lattice; Density Functional Theory (DFT) calculations reveal that these vacancies serve as highly active CO₂ adsorption sites with a strongly exothermic adsorption energy of −0.914 eV. Benefiting from this triple-synergistic enhancement, the SCG-modified sorbent (LSO-50) achieves a CO₂ adsorption capacity of 0.275 g/g at 650 °C under 15 vol% CO₂, representing a more than fourfold improvement over unmodified pellets. When further combined with Na₂CO₃ co-doping to promote additional eutectic formation, the optimized sorbent (LSON-50) reaches 0.330 g/g, retains 0.284 g/g after 50 adsorption–desorption cycles, and exhibits robust mechanical stability (<10% attrition loss). By co-locating structural, chemical, and defect features within a single biomass-derived modifier, this work establishes a scalable waste-valorization route for high-performance, eco-friendly CO₂ capture.

The cold sintering process (CSP) is an advanced low-temperature sintering technology whose effectiveness is closely related to the selection of transient liquid phases (TLPs). While water serves as an ideal TLP for water-soluble ceramics, most water-insoluble materials necessitate acids, bases, or specialized solvents instead. This limitation has severely restricted the application of CSP, as many water-insoluble ceramics cannot be densified due to the lack of suitable TLPs. This study demonstrates a breakthrough approach that exploits nanoscale effects to enable water to act as an effective TLP for the densification of water-insoluble Li₂TiO₃ ceramics. A comparison of nano (19.71 nm) and microscale Li₂TiO₃ powders under identical sintering conditions revealed that despite the exceptionally low aqueous solubility of Li₂TiO₃, the nanopowders achieved 94.33% relative density at only 300 °C and 700 MPa, whereas the micropowders attained only 78% density. Further analysis revealed a distinctive densification mechanism that integrates dislocation-mediated plastic deformation with localized dissolution phenomena at nanoparticle interfaces. Compared with conventional sintering (1000 °C), the resulting nanoceramics exhibited superior Vickers hardness (905 HV) and enhanced electrical conductivity while maintaining a refined nanoscale grain structure (26.42 nm). This study established an effective strategy for the cold sintering of water-insoluble ceramics with layered structures using water as a TLP, significantly expanding the applicability of CSP technology and offering new pathways for the energy-efficient fabrication of advanced functional ceramics.