Since ether solvent typically decomposes as voltage exceeds 4.0 V (refs. , it is intriguing to unveil how such a small amount of NO3 -could boost the oxidation stability of the conventional dilute ether electrolyte at the cathode. Considering that the current common views of electrolyte design, especially with regard to the well-understood salt-concentrated electrolytes, mostly focus on passivating CEI or changing electrolyte ion solvation , similar mechanisms are worthy of being checked here. The first concern is whether a stable protective CEI film is formed in the NO3 -containing ether electrolyte. We probed the surface structure of the cycled LiCoO2 cathode in NO3 -containing ether electrolytes using the transmission electron microscope (TEM). Compared to the surface morphology of the bare LiCoO2 (Figure ), a clear and intact surface edge can be observed in the representative region of the cycled LiCoO2 (Figure ) -a selected region based on extensive TEM characterizations; while in some local regions, a nonuniform amorphous layer (2-10 nm) and gathered nanoscale crystalline grains (assigned to LiF) can be found (Figure ). The poor homogeneity cannot support the conclusion that an effective surface passivating layer forms in the NO3 --containing ether electrolyte.