First-Principles Studies on Cation Dopants and ElectrolytelCathode Interphases for Lithium Garnets

Lithium garnet with the formula Li7La3Zr2O12 (LLZO) has many properties of an ideal electrolyte in all-solid state lithium batteries. However, internal resistance in batteries utilizing these electrolytes remains high. For widespread adoption, the LLZOs internal resistance must be lowered by increasing its bulk conductivity, reducing grain boundary resistance, and/or pairing it with an appropriate cathode to minimize interfacial resistance. Cation doping has been shown to be crucial in LLZO to stabilize the higher conductivity cubic structure, yet there is still little understanding about which cations have high solubility in LLZO. In this work, we apply density functional theory (DFT) to calculate the defect energies and site preference of all possible dopants in these materials. Our findings suggest several novel dopants such as Zn2+ and Mg2+ predicted to be stable on the Li- and Zr-sites, respectively. To understand the source of interfacial resistance between the electrolyte and the cathode, we investigate the thermodynamic stability of the electrolyte|cathode interphase, calculating the reaction energy for LLMO (M = Zr, Ta) against LiCoO2, LiMnO2, and LiFePO4 (LCO, LMO, and LFP, respectively) cathodes over the voltage range seen in lithium-ion battery operation. Our results suggest that, for LLZO, the LLZO|LCO is the most stable, showing only a low driving force for decomposition in the charged state into La2O3, La2Zr2O7, and Li2CoO3, while the LLZO|LFP appears to be the most reactive, forming Li3PO4, La2Zr2O7, LaFeO3, and Fe2O3. These results provide a reference for use by researchers interested in bonding these electrolytes to cathodes.