Exploration of Organic Cathode Active Materials with High Energy Densities for Li-Ion Batteries via First-Principles Calculations

Developmentof cathode active materials (CAM) with higher energydensity has been desired in the development of next-generation batteries.In this respect, organic CAMs are probable candidates because of theirlightweight frameworks without heavy elements. Herein, we examinedthe characteristics of two possible organic crystals a naphthazarin(5,8-dihydroxy-1,4-naphthoquinone) lithium salt dimer fused by thedithiin ring (DNP) and phenazinetetrone (PTO)by density functionaltheory (DFT)-based first-principles calculations. Based on the pristinecrystals of fully oxidized molecules carrying no Li, we computationallyexplored the low-energy crystal structures at different states ofcharge (SOCs) during the lithiation (discharging), Li- n (DNP), and Li- n (PTO)(2) (n = 0-14), with the DFT energetics.We then calculated the voltage (Li vacancy formation energy) profileand confirmed that our calculations can reasonably explain the experimentaldischarge curve, indicating the validity of our structure models.It is also demonstrated that deeper discharge destabilizes the lithiatedstructure. Calculated unit cell shapes suggested that the volume changeis significant for early n (stage I), & SIM;10%for Li- n (DNP), and & SIM;15% for Li- n (PTO)(2). On the other hand, thevolume over this stage is almost kept by the adjustment of the DNP/PTOstacking manner according to the Li insertion, which is advantageousfor the usage as CAMs. Li in Li- n (DNP)mainly has fourfold coordination to the framework anions, while threefoldcoordination is dominant in Li- n (PTO)(2), implying that Li- n (DNP) is morestable than Li- n (PTO)(2). We alsoevaluated the electronic density of states and the partial electrondistributions of both materials with selected SOCs and demonstratedthat the electronic conductivities of both materials seem similar.On the other hand, the calculated migration barriers of Li indicatedthat the PTO crystal has faster Li migration and thus higher ratecapability than the DNP. These results suggest that Li- n (PTO)(2) exhibits better cathode performance.The present computational predictive approach reveals the voltageand structural as well as electronic characteristics of the potentialorganic CAMs, and suggests useful aspects for the material selection.