Investigation of Electrolyte Concentration Effects on the Performance of Lithium–Oxygen Batteries

A combined experimental and computational study has been performed in order to elucidate the effect of electrolyte salt concentration on the performance of Li–O₂ batteries. Four electrolyte solutions with varying lithium triflimide (LiTFSI) content in 1,2-dimethoxy­ethane (DME) were studied to identify principal failure mechanisms in Li–O₂ batteries for dilute and concentrated electrolytes (0.1 M to saturation) in cells cycled with high overpotentials and/or deep discharge. Quantitative ¹⁹F NMR was employed to determine that in 0.1 M electrolyte solutions salt decomposition can contribute to limitations in cell recycling arising from low ionic conductivity due to a decrease in available soluble Li⁺ over multiple cycles. In contrast, increased salt decomposition in high-concentration electrolytes can result in cathode passivation by insoluble Li salts that impact capacity by hindering Li₂O₂ production and further inhibiting electronic conductivity. By employing first-principles calculations, we modeled different pathways for the decomposition of the TFSI anion and found that it was particularly susceptible to decomposition in its neutral state, for example, if H⁺ is present and bound to the TFSI anion. The cumulative results suggest that employing low-concentration electrolytes with more stable lithium salts are ideal for better cell performance

Computational and Experimental Investigation of Ti Substitution in Li₁(Ni_<i>x</i>Mn_<i>x</i>Co_{1–2<i>x</i>–<i>y</i>}Ti_<i>y</i>)O₂ for Lithium Ion Batteries

Aliovalent substitutions in layered transition-metal cathode materials has been demonstrated to improve the energy densities of lithium ion batteries, with the mechanisms underlying such effects incompletely understood. Performance enhancement associated with Ti substitution of Co in the cathode material Li₁(Ni_<i>x</i>Mn_<i>x</i>Co_{1–2<i>x</i>})­O₂ were investigated using density functional theory calculations, including Hubbard-U corrections. An examination of the structural and electronic modifications revealed that Ti substitution reduces the structural distortions occurring during delithiation due to the larger cation radius of Ti⁴⁺ relative to Co³⁺ and the presence of an electron polaron on Mn cations induced by aliovalent Ti substitution. The structural differences were found to correlate with a decrease in the lithium intercalation voltage at lower lithium concentrations, which is consistent with quasi-equilibrium voltages obtained by integrating data from stepped potential experiments. Further, Ti is found to suppress the formation of a secondary rock salt phase at high voltage. Our results provide insights into how selective substitutions can enhance the performance of cathodes, maximizing the energy density and lifetime of current Li ion batteries