Trapped O and the origin of voltage fade in layered Li-rich cathodes.

Oxygen redox cathodes, such as LiNiCoMnO, deliver higher energy densities than those based on transition metal redox alone. However, they commonly exhibit voltage fade, a gradually diminishing discharge voltage on extended cycling. Recent research has shown that, on the first charge, oxidation of O ions forms O molecules trapped in nano-sized voids within the structure, which can be fully reduced to O on the subsequent discharge. Here we show that the loss of O-redox capacity on cycling and therefore voltage fade arises from a combination of a reduction in the reversibility of the O/O redox process and O loss. The closed voids that trap O grow on cycling, rendering more of the trapped O electrochemically inactive. The size and density of voids leads to cracking of the particles and open voids at the surfaces, releasing O. Our findings implicate the thermodynamic driving force to form O as the root cause of transition metal migration, void formation and consequently voltage fade in Li-rich cathodes.