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

A team of researchers from this department, the Faraday Institution and Diamond Light Source investigated 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. 

In this paper* the authors demonstrate 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.

The 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.

*'Trapped O₂ and the origin of voltage fade in layered Li-rich cathodes' published in Nature Materials.