Strain and collapse during lithiation of layered transition metal oxides: a unified picture

Abstract

Developing long-life, high-energy density materials such as the Ni-rich LiNi_xMn_yCo_zO₂ (NMCxyz) is needed to manufacture advanced Li-ion batteries. However, these compounds suffer from a capacity fade upon cycling, attributed to the H2–H3 phase transition and to the associated volume changes. Here, by applying operando XRD we show that the usually blamed H2-H3 transition is actually not taking place in Ni-rich layered transition metal oxides, except for the 100% nickel composition LNO. We find a universal mechanism instead, that only depends on the lithium content, irrespective of nickel content, applied charge rate or phase diagram of the bulk material. We discover that strain appears when the structure collapses, below 40% lithium in the layers. Below this critical lithium content, exponentially growing strain and large lattice distortions occur, which may favor irreversible cracks. Since lithium loss upon cycling induces a shift towards lower lithium content in positive active material, the consequences of repeated and irreversible stress are expected to be exacerbated on the long-term. Therefore, unifying structure, crystalline strain and cracks, our results reveal that the lithium content window is the main driver of degradation in layered transition metal oxides, rather than the potential window.