Understanding the reversible charging/discharging mechanism of Li3BO3 for thermochemical energy storage: an experimental and computational study

Abstract

Desirable media for thermochemical energy storage (TCES) should possess stable reversibility without performance degradation. Herein, we demonstrate the potential CO₂-carrying capacity, heat storage density, and long-term cycling stability of Li₃BO₃, which exhibits a CO₂ uptake above 0.2 g_CO2·g_sorbent⁻¹ from 500 °C to 590 °C, reaching a maximum of ∼0.32 g_CO2·g_sorbent⁻¹ at 560 °C under 100% CO₂. The Li₃BO₃ + CO₂ system displayed a heat release of ∼909 J·g_sorbent⁻¹ during discharging and a heat absorption of ∼1110 J·g_sorbent⁻¹ during charging, without noticeable degradation over 30 cycles at 560 °C. The final-cycle heat storage capacity of ∼1146 J·g_sorbent⁻¹ was obtained, substantially outperforming those of previously reported CO₂ carriers, such as CaO, SrO, and Li₄SiO₄. The carbonation pathway of Li₃BO₃ was further elucidated by in situ X-ray diffraction measurements and pCO₂-T phase diagrams. Within the 400–580 °C window at pCO₂ = 1 bar, two pathways were observed: (1) direct carbonation via Li₃BO₃ + CO₂ → Li₄B₂O₅ + Li₂CO₃ and (2) stepwise carbonation via Li₃BO₃ + CO₂ → Li₄B₂O₅ + Li₂CO₃, followed by Li₄B₂O₅ + CO₂ → Li₆B₄O₉ + Li₂CO₃, wherein Li₄B₂O₅ was produced as a key intermediate in the 500–560 °C window. The beneficial effects of Li₄B₂O₅ were further explored via kinetic modeling and first-principles calculations. Li₄B₂O₅ exhibited a significantly slower carbonation rate and a higher ionic diffusion barrier than Li₃BO₃, providing rate regulation and structural buffering to the reversible reaction (4Li₃BO₃ + 3CO₂ ⇌ 3Li₂CO₃ + Li₆B₄O₉) in both the forward and backward directions, thus endowing the Li₃BO₃ + CO₂ system with long-term cyclic stability. The fundamental insights reported herein could provide guidance for the design of high-performing CO₂ carriers in broader industrial scenarios, such as TCES, flue gas CO₂ capture, and sorption-enhanced techniques.