Effects of Surface Contaminants Inherited from Lithium Sources on the Electrochemical Performance of High-Nickel Positive Electrodes

Ultrahigh-Ni Li[Ni_xCo_yMn_z]O₂ (NCM) materials retain residual Li compounds derived from the Li source due to calcination temperatures below the melting point of lithium carbonate. Lithium hydroxide monohydrate and anhydrous lithium hydroxide contain different amounts of Li₂CO₃ impurity, and this difference is retained in the synthesized positive electrode materials. Specifically, the positive electrode material produced from Li sources with a higher Li₂CO₃ content shows limited formation of a positive electrode surface film during the formation cycle. Therefore, the electrode surface remains exposed to the electrolyte during cycling. This accelerates electrolyte decomposition, byproduct accumulation, structural degradation, and transition-metal dissolution. These processes destabilize the negative electrode through repeated solid electrolyte interphase (SEI) damage and reconstruction. In contrast, materials prepared from Li sources with lower Li₂CO₃ contents develop a positive electrode surface film that restricts direct electrode-electrolyte contact and suppresses subsequent degradation. These results demonstrate that the residual Li compounds present in the Li source determine the dominant degradation pathway in ultrahigh-Ni NCM materials. Controlling this parameter provides the basis for improving the performance of high-energy-density lithium-ion batteries.