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

Development of cathode active materials (CAM) with higher energy density has been desired in the development of next-generation batteries. In this respect, organic CAMs are probable candidates because of their lightweight frameworks without heavy elements. Herein, we examined the characteristics of two possible organic crystals─a naphthazarin (5,8-dihydroxy-1,4-naphthoquinone) lithium salt dimer fused by the dithiin ring (DNP) and phenazinetetrone (PTO)─by density functional theory (DFT)-based first-principles calculations. Based on the pristine crystals of fully oxidized molecules carrying no Li, we computationally explored the low-energy crystal structures at different states of charge (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) profile and confirmed that our calculations can reasonably explain the experimental discharge curve, indicating the validity of our structure models. It is also demonstrated that deeper discharge destabilizes the lithiated structure. Calculated unit cell shapes suggested that the volume change is significant for early n (stage I), ∼10% for Li n (DNP), and ∼15% for Li n (PTO) 2 . On the other hand, the volume over this stage is almost kept by the adjustment of the DNP/PTO stacking manner according to the Li insertion, which is advantageous for the usage as CAMs. Li in Li n (DNP) mainly has fourfold coordination to the framework anions, while threefold coordination is dominant in Li n (PTO) 2, implying that Li n (DNP) is more stable than Li n (PTO) 2 . We also evaluated the electronic density of states and the partial electron distributions of both materials with selected SOCs and demonstrated that the electronic conductivities of both materials seem similar. On the other hand, the calculated migration barriers of Li indicated that the PTO crystal has faster Li migration and thus higher rate capability than the DNP. These results suggest that Li n (PTO) 2 exhibits better cathode performance. The present computational predictive approach reveals the voltage and structural as well as electronic characteristics of the potential organic CAMs, and suggests useful aspects for the material selection.