Controlled synthesis and interfacial properties of polyvinylidene fluoride based metal-fluoride surface treatments for high-nickel NCM cathodes

Abstract

Despite the advantage of reduced cobalt content, widely proposed Ni-rich cathode candidates face significant challenges related to thermal instability and structural deterioration under high-voltage conditions. In particular, when the Ni content exceeds 80 % of the total transition metal in LiNi_xCo_yMn_1-x-yO₂, the H2-H3 phase transition accelerates material degradation through irreversible structural evolutions during electrochemical cycling. A common irreversible reaction in Ni-rich cathodes arises from the weakening of Ni-O covalent bonds during Ni oxidation, which triggers oxygen reduction reactions and subsequent gas evolution, thereby further accelerating material degradation under high-voltage operation. As such degradation predominantly occurs at the particle surface, numerous studies have employed surface-coatings strategies based on metal cations that form strong bonds with oxygen in the host lattice and suppress oxygen release. However, coatings involving high-valence metal ions often reduce the initial capacity of the cathode material and hinder Li+ diffusion kinetics across the interphases. In this study, we present a straightforward surface-treatment strategy using fluorine anions derived from polyvinylidene fluoride (PVDF) to prevent gas evolution while maintaining both the capacity and lithium-ion diffusivity of a Ni-rich LiNi_0.96Co_0.035Mn_0.005O₂ cathode.