Unraveling the lithium loss mechanisms during the high-temperature solid-state synthesis of ternary lithium-ion cathode materials

Abstract

Lithium losses during high-temperature synthesis of Nickel Manganese Cobalt (NMC) cathode materials are commonly attributed to lithium oxide (Li₂O) evaporation, based on early investigations of Li_xNi_1−xO solid solutions. A critical review of this literature reveals, however, that the reported vapor pressures of Li₂O and Li₂O₂ are far too low to account for the experimentally observed mass losses under typical sintering conditions, and that Li₂O₂ itself is thermally unstable above 400 °C. This work re-examines the origins of lithium losses through thermodynamic equilibrium calculations and targeted calcination experiments. Calculations in the Li–O system confirm that gaseous lithium species remain negligible below 1500 °C, though even trace moisture significantly enhances lithium volatility via LiOH(g) formation. Nevertheless, predicted evaporative losses under realistic synthesis conditions remain below 0.1 mol%, far less than the 1–10 mol% typically reported. Complementary calcination experiments demonstrate that the dominant source of lithium depletion is solid-state reaction between the cathode precursor and the crucible material. Lithium readily reacts with common oxidic substrates such as Al₂O₃ and SiO₂, with losses scaling with contact area, whereas chemically inert substrates (MgO, Au) effectively suppress depletion. These results demonstrate that the widely accepted attribution of lithium loss to Li₂O evaporation is incorrect: substrate reactivity, not volatilization, is the dominant loss mechanism during NMC cathode synthesis.