Comparative analysis of calcination strategies for NMC811 synthesized via oxalate coprecipitation

We report a controlled comparative study of four calcination strategies for LiNi0.8Mn0.1Co0.1O2 (NMC811) prepared from an oxalate co-precipitation precursor. Using identical precursor composition and lithiation, samples were produced with: pre-calcination + two-step sinter (CS1), pre-calcination + one-step sinter (CS2), direct lithiation + two-step sinter (CS3), and direct lithiation + one-step sinter (CS4). Materials were characterized by ICP-OES, high-energy XRD, SEM-EDX, BET, ATR-FTIR/Raman, XPS, granulometry, and electrochemical testing including long-term cycling and three-electrode EIS. All samples crystallized in the layered R3̄m NMC811 structure and exhibited similar bulk stoichiometry; however, the calcination route strongly influenced near-surface chemistry. CS1 and CS3 showed heterogeneous distributions of carbonate-containing species within the near-surface region together with sparse Li-rich surface domains, correlating with accelerated impedance growth and inferior capacity retention. In contrast, CS2 displayed the highest concentration of Li-rich species at the outermost surface by XPS but weak carbonate signatures in the near-surface volume, consistent with the formation of a thin, coherent passivation layer. This sample exhibited the lowest hysteresis growth, smallest impedance increase, and best capacity retention (∼72% after 200 cycles at 1C). The results demonstrate that the spatial distribution, rather than the total amount, of residual lithium compounds governs electrochemical performance in oxalate-derived NMC811.