The Role of Intragranular Nanopores in Capacity Fade of Nickel-Rich Layered Li(Ni1-x-yCoxMny)O2 Cathode Materials.

Abstract

Ni-rich layered LiNi1-x-yCoxMnyO2 (NCM, x + y ≤ 0.2) is an intensively studied class of cathode active materials for lithium-ion batteries, offering the advantage of high specific capacities. However, their reactivity is also one of the major issues limiting the lifetime of the batteries. NCM degradation, in literature, is mostly explained both by disintegration of secondary particles (large anisotropic volume changes during lithiation/delithiation) and by formation of rock-salt like phases at the grain surfaces at high potential with related oxygen loss. Here, we report the presence of intragranular nanopores in Li1+x(Ni0.85Co0.1Mn0.05)1-xO2 (NCM851005) and track their morphological evolution from pristine to cycled material (200 and 500 cycles) using aberration-corrected scanning transmission electron microscopy (STEM), electron energy loss spectroscopy, energy dispersive X-ray spectroscopy, and time-of-flight secondary ion mass spectrometry. Pores are already found in the primary particles of pristine material. Any potential effect of TEM sample preparation on the formation of nanopores is ruled out by performing thickness series measurements on the lamellae produced by focused ion beam milling. The presence of nanopores in pristine NCM851005 is in sharp contrast to previously observed pore formation during electrochemical cycling or heating. The intragranular pores have a diameter in the range between 10 and 50 nm with a distinct morphology that changes during cycling operation. A rock-salt like region is observed at the pore boundaries even in pristine material, and these regions grow with prolonged cycling. It is suggested that the presence of nanopores strongly affects the degradation of high-Ni NCM, as the pore surfaces apparently increase (i) oxygen loss, (ii) formation of rock-salt regions, and (iii) strain-induced effects within the primary grains. High-resolution STEM demonstrates that nanopores are a source of intragranular cracking during cycling.