(Invited) Thermal Properties of NMC Cathode Materials

Abstract

NMC (LiNixMnyCozO2; x+y+z»1) materials deliver high capacity and excellent performance when used as cathodes in lithium-ion batteries. Increasing demand for higher energy density and concerns about the cost and ethics of using cobalt have prompted battery manufacturers to use Ni-rich formulations such as LiNi0.6Mn0.2Co0.2O2 (NMC622) and LiNi0.8Mn0.1Co0.1O2 (NMC811). The thermal stability of (partially) delithiated NMCs is known to decrease with increasing Ni content, however, so that scenarios of fires and other safety events become more distinct possibilities. To investigate the thermal properties of Ni-rich NMCs, we prepared partially and fully delithiated Ni-rich NMCs corresponding to various states-of-charge by chemical methods. We then studied the bulk and surface properties of these materials as they were heated or after they were heated, using a variety of ex situ and in situ synchrotron methods to understand the chemical changes that occur. Of particular concern are reactions that result in release of oxygen. Heated materials progress through a series of bulk structural changes from layered to spinel to rock salt as temperatures rise. Transition temperatures depend on both lithium and nickel content, with bulk changes evident at temperatures as low as 150°C. Surface-sensitive techniques such as soft X-ray absorption spectroscopy (sXAS) indicate that subtle changes that lead to oxygen release can happen well below these temperatures. The thermal behavior of the Ni-rich materials is very complex, and involves lattice transformation, transition metal migration and valence change and lithium redistribution. Moreover, these changes are dependent upon primary particle size, with smaller particles reacting at lower temperatures than larger ones. These observations suggest that materials can be engineered (through primary particle size manipulation, for example) to improve thermal robustness, and therefore, safety and reliability of lithium-ion batteries.