The interfacial (electro-)chemical reactions between electrode and electrolyte dictate the cycling stability of Li-ion batteries. Previous experimental and computational results have shown that replacing Mn and Co with Ni in layered LiNixMnyCo1-x-yO2 (NMC) positive electrodes promotes the dehydrogenation of carbonate-based electrolytes on the oxide surface, which generates protic species to decompose LiPF6 in the electrolyte. In this study, we utilized this understanding to stabilize LiNi0.8Mn0.1Co0.1O2 (NMC811) by decreasing free-solvent activity in the electrolyte through controlling salt concentration and salt dissociativity. Infrared spectroscopy revealed that highly concentrated electrolytes with low free-solvent activity had no dehydrogenation of ethylene carbonate, which could be attributed to slow kinetics of dissociative adsorption of Li +-coordinated solvents on oxide surfaces. The increased stability of the concentrated electrolyte against solvent dehydrogenation gave rise to high capacity retention of NMC811 with capacities greater than 150 mAhg-1 (77 % retention) after 500 cycles without oxide-coating, Ni-concentration gradients, or electrolyte additives.

Replacing graphite with alloying Al negative electrodes would allow for the development of high energy density Li-ion batteries. However, large volume changes associated with the alloying/dealloying process often result in pulverization of the electrode and rapid capacity fade during cycling due to the continuous formation of solid electrolyte interphase (SEI) layers and loss of electronic contact. In this study, we report that increasing salt concentration in the electrolyte to > 5 mol dm −3 led to enhanced capacity retention during cycling of Li-Al half-cells, which was accompanied by nearly constant impedance for the Al electrode in lithium bis(fluorosulfonyl)imide (LiFSI)/dimethyl carbonate (DMC) 1:1.1 (mol/mol) superconcentrated electrolyte. X-ray photoelectron spectroscopy (XPS) revealed that a potential hold in the superconcentrated electrolyte formed an SEI layer with a greater LiF concentration than in standard 1 mol dm −3 solution. This was supported by Raman spectroscopy of LiFSI solutions in DMC, supplemented with density functional theory calculations, which showed an increased driving force for the reduction of FSI − anions to form LiF from Li +-coordinated DMC complexes with increasing salt concentration. Therefore, the enhanced capacity retention and stability can be attributed to the stability of LiF-rich SEI layers which limit carbonate reduction and charge transfer impedance growth.

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