Competitive Ni/Mn Reduction and Microstrain-Coupled Negative Thermal Expansion in Delithiated Li-Rich Cathodes

The demand for high energy density in the field of Li-ion batteries has intensified interest in lithium-rich Mn-based layered oxide cathodes (LRLOs) owing to their high capacity and low cost. Nevertheless, the thermal runaway becomes an urgent concern because of the high-voltage operation (up to 4.8 V), and the structural evolution mechanism of delithiated LRLOs during heating remains unclear. Here, we combine in situ high-temperature X-ray diffraction and absorption spectroscopy to systematically investigate the structural and chemical evolution of Li_1.2Ni_0.2Mn_0.6O₂ (LLNMO) across distinct charge-discharge states. Interestingly, Ni is the first element to undergo thermally induced reduction in the charged state of LLNMO. With further increasing the temperature, Mn reduction sets in, coinciding with extensive lattice oxygen loss, and a phase transition from layered to disordered layered or Li-containing rock-salt-type phase occurs. More intriguingly, after the initial electrochemical cycle, LLNMO exhibits negative thermal expansion at low temperatures below 200 °C, which are attributed to the cycling-induced microstrain accumulation and long-range structural ordering. These findings provide a mechanistic insight into the state-of-charge-dependent thermal behavior of Li-rich layered materials and offer guidelines for designing safer, high-capacity battery materials.