Trapped O₂ and the origin of voltage fade in layered Li-rich cathodes

Oxygen redox cathodes, such as Li_1.2Ni_0.13Co_0.13Mn_0.54O₂, 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 O₂ 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-/O₂ redox process and O₂ loss. The closed voids that trap O₂ grow on cycling, rendering more of the trapped O₂ electrochemically inactive. The size and density of voids leads to cracking of the particles and open voids at the surfaces, releasing O₂. Our findings implicate the thermodynamic driving force to form O₂ as the root cause of transition metal migration, void formation and consequently voltage fade in Li-rich cathodes.