Evaluation of Alternative Lithium Salts for High-Voltage Lithium Ion Batteries: Higher Relevance of Plated Li Morphology Than the Amount of Electrode Crosstalk

Abstract

Increasing the upper cut-off voltage (UCV) enhances the specific energy of Li-ion batteries (LIBs), but is accompanied by higher capacity fade as a result of electrode cross-talk, i.e., transition metals (TM) dissolution from cathode and deposition on anode, finally triggering high surface area lithium (HSAL) formation due to locally enhanced resistance. Here, LiPF₆, LiBF₄, lithium difluoro(oxalate)borate (LiDFOB), lithium bis(oxalate)borate (LiBOB), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in carbonate-based solvents are investigated in LiNi_0.6Co_0.2Mn_0.2O₂ (NCM 622) || graphite pouch cells with 4.5 V UCV. Despite the lower oxidative stabilities of LiBF₄ and LiDFOB, thus enhanced HF formation, TM dissolution, and consequently electrode cross-talk, higher capacity retention is observed compared to the case of LiPF₆ electrolyte. Counterintuitively, it is not the TM deposit amount but rather the Li plating morphology that governs capacity fade, as these salts cause more uniform and compact lithium plating, i.e., lower surface area. In contrast, the dendritic HSAL with LiPF₆ has a higher surface area, and more parasitic reactions, thus active Li (“Li inventory”) losses and capacity fade. Although NCM initiates the failure cascade, the capacity losses and cycle life of high-voltage LIBs are predominantly determined by the anode, in particular the Li plating morphology and the corresponding surface area.