Degradation Mechanism of LiNi_0.82Co_0.15Al_0.03O₂Positive Electrodes of a Lithium-Ion Battery by a Long-Term Cycling Test

Abstract

We conducted a long-term cycling test of a commercial 18650-type lithium-ion battery with a capacity of 3100 mA • h at room temperature and investigated the degradation mechanism of the battery's LiNi 0.82 Co 0.15 Al 0.03 O 2 positive electrode. Sphericalaberration-corrected scanning transmission electron microscopy (Cs-STEM) revealed the presence of a thick degraded surface layer on the surface of the positive active material after the cycling test, and electron energy loss spectroscopy (EELS) revealed that the degraded surface layer continuously evolved from a LiNiO 2 layered structure to a NiO structure, in the direction from the bulk toward the surface. Hard X-ray photoemission spectroscopy (HX-PES) indicated that the majority of low-valence Ni existed on the surface of the positive active material, which was charged after the cycling test, and that the degraded surface layer was inactive against charge reaction. The results suggest that the degraded surface layer was responsible for battery degradation during the cycling test. X-ray photoemission spectroscopy (XPS) indicated that Li 2 CO 3 increased on the surface of the positive electrode after the cycling test. The phenomena would contribute to the formation of the degraded surface layer on the surface of the positive active material.

The Impact of Electrolyte Additives and Upper Cut-off Voltage on the Formation of a Rocksalt Surface Layer in LiNi0.8Mn0.1Co0.1O2Electrodes

“…[6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]29,30,42 Such a layer is thought to be able to block diffusion paths for lithium and contribute to impedance growth in a cell.…”

“…This surface reconstruction phenomenon was also observed in many other reports about NMC and Li[Ni 0.80 Co 0.15 Al 0.05 ]O 2 positive electrode materials where the surface reconstruction was ascribed to be a result of interactions between the positive electrode surface and the electrolyte. [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] …”

Multi-scale 3D investigations of a commercial 18650 Li-ion battery with correlative electron- and X-ray microscopy

“…These high energy density cells contain a nickel-cobalt-aluminum oxide (NCA) positive electrode and have demonstrated applications for mobile electronics and electric vehicles [42].…”

“…The battery was discharged over a 5 Ω resistance for 2 days until the voltage read 0 V. A pipe cutter was used to open the steel casing and the interior spiral wound layers were carefully removed. After carefully unrolling the electrode layers, the negative electrode, positive electrode, and separator were isolated from each other, washed with dimethyl carbonate (DMC) to remove any residual electrolyte or Li salts, and then left to dry for several days [42].…”

“…The battery was discharged over a 5 Ω resistance for 2 days until the voltage read 0 V. A pipe cutter was used to open the steel casing and the interior spiral wound layers were carefully removed. After carefully unrolling the electrode layers, the negative electrode, positive electrode, and separator were isolated from each other, washed with dimethyl carbonate (DMC) to remove any residual electrolyte or Li salts, and then left to dry for several days [42].Subsequent imaging of the individual layers was carried out using several techniques, including submicron scale XRM, scanning electron microscopy (SEM), nanoscale XRM, focused-ion beam scanning electron microscopy (FIB-SEM), and energy dispersive spectroscopy (EDS). A 1 mm 1 mm section was cut from the positive electrode foil using a razor blade, to prepare the specimen for correlative XRM-SEM imaging.…”