Effect of Crystallinity on Li Adsorption in Polyethylene Oxide

Despite their numerous benefits, the use of solid-polymer electrolytes, such as polyethylene oxide (PEO), in next-generation Li-ion batteries is constrained by their lower ionic conductivity. To overcome this bottleneck and design materials with higher conductivity, it is important to elucidate the underlying atomistic mechanisms of Li-ion adsorption in such materials. Here, we have performed a comprehensive statistical analysis of the interaction of Li and Li+ at numerous locations in crystalline and amorphous PEO. Our in-depth analysis of the Li-O bonding environment using ab initio calculations reveals that Li/Li+ can bind with more number of oxygen atoms in amorphous PEO compared to crystalline case. The maximum value of coordination number, that is, the number of oxygen atoms bonded with Li/Li+ is 3 for crystalline PEO and 5 for amorphous PEO. This can be attributed to the access to more neighboring oxygen atoms in amorphous PEO. Binding energy calculations reveal that the interaction of Li and Li+ significantly depends on the degree of crystallinity. Li/Li+ adsorption is preferable in amorphous PEO and the average binding energy difference in amorphous and crystalline PEO is 3.36 eV for Li and 2 eV for Li+ because of greater free volume and higher coordination number in amorphous PEO. While the crystalline regions of PEO are required to provide robustness, amorphous regions have been found to facilitate Li+ ion adsorption providing higher coordination number and stronger binding energy.