Tri-lithium borate (Li₃BO₃) has emerged as a promising CO₂ sorbent for intermediate-high temperature applications, particularly in post-combustion carbon capture processes. Its high CO₂ uptake capacity, along with improved kinetic performance when modified with additives, makes it a compelling alternative to conventional lithium-based sorbents. Upon carbonation, Li₃BO₃ reacts to form lithium carbonate (Li₂CO₃) and lithium borates, such as Li₆B₄O₉ and LiBO₂, with reaction kinetics strongly dependent on CO₂ partial pressure and temperature. However, the fundamental high-temperature reaction dynamics governing Li₃BO₃ formation and carbonation remain largely unexplored. This study employs time-resolved in situ synchrotron X-ray diffraction (XRD) to unveil the phase transformations and reaction pathway occurring during Li₃BO₃ synthesis and its reaction with CO₂. Structural evolution during the heating of a mechanically milled H₃BO₃-Li₂CO₃ mixture from room temperature to 600 °C reveals a successful transformation to Li₃BO₃ via a multi-step mechanism. In situ high-temperature synchrotron XRD measurements coupled with Rietveld refinement demonstrate that the carbonation mechanism of Li₃BO₃ is significantly influenced by CO₂ partial pressure. Under 10% of CO₂, Li₆B₄O₉ and Li₂CO₃ are identified from 400 °C, followed by the formation of Li_(2+x)C_(1−x)B_xO₃ solid solution between 530 °C and 580 °C. In contrast, under pure CO₂, Li₆B₄O₉ forms at 350 °C, with LiBO₂ appearing at higher temperatures. Notably, the temperature range of Li_(2+x)C_(1−x)B_xO₃ stabilization shifts toward higher temperatures under pure CO₂ conditions. Understanding these transformations is essential for optimizing the performance and stability of Li₃BO₃ in CO₂ capture applications and serves as a basis for future additive optimization, contributing to the broader development of high-temperature sorbent materials.