(Invited) Superstructure Controls First Cycle Voltage Hysteresis in O-Redox Cathode Materials

Abstract

The cathode is a major constraint on the energy density of Li-ion batteries. State-of-the-art materials, such as Li(Mn0.8Ni0.1Co0.1)O2, are limited in their ability to store charge by the amount of available transition metal redox. It is now known these limits can be surpassed by storing charge on the oxide ions (oxygen redox) in so-called lithium-rich transition metal (TM) oxides, e.g. Li[Li0.2Ni0.2Mn0.6]O2. However, such O-redox cathodes suffer from several problems one of which is that, in almost all known cases, the high voltage plateau associated with oxide oxidation is followed by reduction at a much lower voltage (1st cycle hysteresis) leading to a substantial drop in energy density. Here, we discuss the origin of 1st cycle voltage hysteresis and show that it can be controlled by the superstructure ordering within the TM layer.We compare two layered Na-ion O-redox compounds as model systems with very similar compositions and structure but divergent 1st cycle voltage profiles. In P2-type Na0.75[Li0.25Mn0.75]O2 with honeycomb ordering of Li and Mn in the TM layer, present in almost all O-redox compounds, substantial voltage hysteresis is observed. Whereas, for P2-type Na0.6[Li0.2Mn0.8]O2 with unique ribbon ordering, reversible high voltage O-redox is realised. The origin of voltage hysteresis is shown to be rooted in the formation of molecular O2 on charge that is trapped in in the bulk of honeycomb ordered cathodes and which can be reduced on discharge. O2 formation can be suppressed with ribbon ordering in the TM layer. The implications for future O-redox materials will also be considered. Figure 1