Nickel-rich layered oxide cathode materials are currently the material of choice for lithium-ion batteries that combine high energy density and reasonable costs. However, a high nickel content usually translates to high reactivity of these cathode materials, increasing the demands on the electrolyte. At the same time, the electrolyte should be non-flammable to improve the safety of the battery. Here, we will present our latest results on safe non-aqueous and aqueous electrolytes for Ni-rich NMC-based lithium-ion batteries. We render a state-of-the-art organic electrolyte, 1M LiPF<sub>6</sub> in ethylene carbonate (EC)/ethyl methyl carbonate (EMC) (3:7), non-flammable via addition of a fluorinated cyclophosphazene flame retardant. In addition to improving the safety of the electrolyte, the flame retardant also improves the cycling stability of Ni-rich NMC/graphite cells as demonstrated for NMC811/graphite pouch cells with a capacity of 1 Ah. Furthermore, we will present our latest results on the stability of NMC811 in two representative water-in-salt electrolytes, 21 mol/kg lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), the archetypical water-in-salt electrolyte, and a water-in-salt/ionic liquid hybrid electrolyte consisting of 40 mol/kg LiTFSI and 20 mol/kg 1-ethyl-3-methylimidazolium TFSI (EMImTFSI). The degradation of NMC811 in these electrolytes was studied via various techniques including galvanostatic cycling, (transmission) electron microscopy, and X-ray photoelectron spectroscopy. Finally, the results will be compared to the behavior of Ni-rich NMC in organic electrolytes.

Aqueous and non-aqueous electrolytes for Ni-rich NMC-based lithium-ion batteries

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