Pressure-Induced Polymerization of CO₂ in Lithium-Carbon Dioxide Phases

We report the discovery of novel CO₂-based networks stabilized by Li⁺ cations in solid-state phases. Our exploration of the energy landscape of the Li-CO₂ binary phase diagram using ab initio evolutionary structure searches revealed that in addition to the well-characterized C₂O₄^2- oxalate in Li₂C₂O₄ viable covalent CO₂-based nets emerge upon compression within the 0-100 GPa pressure range. Novel molecular units are described, such as ethene-like C₂O₄^4- in C2/m Li₂(CO₂), finite C₄O₈^6- chains in P1̅ Li₃(CO₂)₂, one-dimensional polymeric forms based on 1,4-dioxane rings in P2/c LiCO₂, and the C(O^-)₂ moieties in Pnma Li₂CO₂. This investigation shows the oxalate motif is maintained when the concentration of lithium increases from 1 to 2 in Li_xCO₂; this interesting property may have potential in the development of renewable Li-ion batteries. Moreover, a variety of metastable phases were predicted, such as the covalent CO₂-based layer in P1̅ Li(CO₂)₂.