Fig. 4: Sustainable lithium carbonate (Li₂CO₃) production from monovalent cation solution and waste CO₂.

a Schematic of Li⁺ recovery via the bipolar membrane in situ crystallization configuration and the mechanisms of Li₂CO₃ formation through utilization; b Operational principles of the bipolar membrane in situ crystallization system, with an inset showing the collected Li₂CO₃ powder (white); c SEM images of the Li₂CO₃ crystals (regular octahedral morphology, ~4 µm average diameter) and element mapping of C and O distribution; d XPS survey spectra of the Li₂CO₃ crystals, showing atomic percentages: Li (24.9%), O (45.1%), negligible Na (< 0.1%), and C (24.9%, with testing background); e XRD spectra of the synthesized crystals, confirming the crystallographic phase of Li₂CO₃. All diffraction peaks align with the standard reference pattern for lithium carbonate (PDF#22-1141), demonstrating high phase-purity and crystallinity.; f Temporal evolution of Li⁺ and Na⁺ concentrations in feed and recovery chambers under optimized current density (5 mA cm⁻²), Li⁺ depletion correlates with Li₂CO₃ crystallization, error bars denote the standard deviation of the means (n = 2); g Recovery rates (Li⁺: 98.0%; Na⁺: 97.5%) and product purity (Li₂CO₃: > 99.9%; NaOH: 95%); h The energy, economic and environmental analysis of the bipolar membrane in situ crystallization system: energy cost (1.10 $ kg⁻¹ Li₂CO₃), investment (2.11 $ kg⁻¹ Li₂CO₃), and profits (20.79 $ kg⁻¹ Li₂CO₃) relative to market price of Li₂CO₃ (24.00 $ kg⁻¹). Source data are provided as a Source data file.