Trapped O 2 and the origin of voltage fade in layered Li-rich cathodes.

Oxygen redox cathodes, such as Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O 2 , 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 2- ions forms O 2 molecules trapped in nano-sized voids within the structure, which can be fully reduced to O 2- 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 2- /O 2 redox process and O 2 loss. The closed voids that trap O 2 grow on cycling, rendering more of the trapped O 2 electrochemically inactive. The size and density of voids leads to cracking of the particles and open voids at the surfaces, releasing O 2 . Our findings implicate the thermodynamic driving force to form O 2 as the root cause of transition metal migration, void formation and consequently voltage fade in Li-rich cathodes.