The 4th International Conference on Matter and Radiation at Extremes (ICMRE 2019)

532 / 2019-03-19 22:20:24

A Constrained Evolutionary Algorithm for the Crystal Structure Prediction of Novel Li-CO2 Based Materials

Crystal Structure Prediction,DFT,Carbon Dioxide,High Pressure,Polymerization

Plenary track > High Pressure Physics and Materials Science

In the context of CO2 capture and storage and utilization of CO2 as green renewable C1 building block in synthesis, one may think about the mixing of CO2 and s-block elements to activate the C=O double bonds by populating the antibonding CO levels. Moreover, applying external pressure may contribute to the polymerization of CO2 units and this conceptual process may lead to novel materials with interesting properties. In the present study, we explored the Li-CO2 phase diagrams at ambient and high pressures using evolutionary crystal structure searches and DFT calculations. By essence, an evolutionary algorithm leads to thermodynamic stable composition (Li2CO3 carbonate phases in Li-C-O ternary phase diagram). Therefore, we developed a constrained-evolutionary technic to avoid CO2 chemical decomposition, i.e. cleavage of carbon- oxygen bonds or formation of more stable Li-O and Li-C based structures. Our technics allow the exploration of a pseudo Li-CO2 binary phase diagram under pressures where the CO2 coordination mode is maintained. Thus, thermodynamical metastable Lix(CO2)y structures, but dynamical and mechanical stable ones, may be located in the Li-C-O ternary phase diagram. Our constrained search method has been implemented in a home-made version of the USPEX1 code which may be used in other crystal structure prediction studies dealing with metastable unsaturated molecules as components, e.g. CO, CS, CS2.
Here, we report the discovery of novel CO2-based networks stabilized by Li+ cations in solid-state phases.2 In addition to the well-characterized C2O42- oxalate in Li2C2O4—viable covalent CO2-based nets emerge upon compression within the 0-100 GPa pressure range (Figure 1a). Novel molecular units are described, such as ethene-like C2O44- in C2/m Li2(CO2), finite C4O86- chains in P-1 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 (Figure 1b). This investigation shows the oxalate motif is maintained when the concentration of lithium increases from 1 to 2 in LixCO2, 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 P-1 Li(CO2)2. Three Lix(CO2)y phases are viable at ambient conditions, being thermodynamically, dynamically and mechanically stable. Overall, these results indicate pressure may induce polymerization of carbon dioxide in binary Li-CO2 phases, and merit experimental confirmation.