Optimal Operating Windows for CO₂ Capture in Lithium–Sodium Borates: Phase Transition and Composition Effects

Alkali metal borate molten salts have emerged as efficient high-temperature liquid CO2 sorbents, advancing carbon reduction for energy-intensive industrial chemical processes. This work investigated the relationship between the liquidus behavior and CO2 uptake characteristics of lithium–sodium borates, MxB1–xO1.5–x (M = Li0.5Na0.5), over a composition range of 0.50 ≤ x ≤ 0.80. Differential Scanning Calorimetry (DSC) measurements revealed detailed phase–transition profiles, with liquidus temperatures ranging from 500 to 650 °C. Composition-dependent liquidus behavior governs the CO2 sorption characteristics during the early sorption stages, transitioning from “solid-to-liquid” in low-alkali to “liquid-to-liquid” in high-alkali regions. Optimal working capacities and reaction rates consistently correspond to the liquidus transition range, minimizing energy demands for preserving molten state in the cyclic CO2 capture-release operations. These findings establish temperature–composition operating windows tailored to industrial needs, providing critical liquidus diagrams and demonstrating their potential as versatile sorbents for high-temperature CO2 capture.