Thermodynamic Stability of LixNiO2 and Polyvinylidene Difluoride Materials for Lithium+ Ion Batteries

Abstract

LixNiO2 (LNO), a layered Li-ion battery cathode has been investigated for decades. LNO has been shown to proceed through multiple phase transitions at different states of lithiation as well as at different temperatures. However, in these experiments it has always been paired with additional materials typically found in the creation of cathodes, such as carbon black, aluminum foil and polyvinylidene difluoride binder. This paper investigates the thermal stability of LNO as a stand-alone material as well as a component of a traditional cathode through performing thermal gravimetric analysis (TGA), variable temperature powdered X-ray diffraction (VT-PXRD) and solid-state nuclear magnetic resonance (ss-NMR) at different states of lithiation and high temperatures. The reduction to nickel metal was found at temperatures exceeding 500 °C for the traditional cathodes at all states of charge due to the creation of lithium fluoride from hydrogen fluoride gas being released as polyvinylidene difluoride degrades. Nickel metal was also identified for the stand-alone LNO at lithium concentrations greater than x = 0.32 at temperatures of 600 °C as the system becomes depleted of oxygen and lithium due to the creation of lithium oxide. These finding indicate that the thermal stability of LNO at temperatures over 200 °C is sensitive to the presence of fluorine as well as the state of charge of the battery. Typical batteries do not reach these temperatures during cycling, but in the case of thermal runaway and recycling it is important to know the chemical reactions occurring at these temperatures and concentrations as toxic and corrosive hydrogen fluoride gas is released. An investigation into less fluorinated binder options is desirable to reduce the addressed issues.