Optimization of the N/P ratio in mixed NMC-NCA/graphite-N2 pouch cells for high-rate lithium-ion batteries

Optimizing the negative-to-positive capacity ratio (N/P) is essential for balancing energy density, rate capability, and interfacial stability in lithium-ion pouch cells. In this work, mixed Nickel-Manganese-Cobalt oxide (NMC) and Nickel-Cobalt-Aluminum oxide (NCA) / graphite-N 2 pouch cells were systematically investigated over an N/P range of 0.9–1.6 to determine the most favorable N/P ratio for high-rate applications. The cells were evaluated through calibrated 1C capacity measurement, rate capability testing from 0.5C to 12C, internal resistance analysis at different depths of discharge, self-discharge assessment, 100-cycle stability testing at 1C, polarization and power characterization, electrochemical impedance spectroscopy (EIS), and post-cycling SEM analysis. The results revealed a strong dependence of electrochemical behavior on N/P ratio. At 1C, the discharge capacity increased from 131.78 mAh g −1 for N/ P = 0.9 to 182.00 mAh g −1 for N/ P = 1.5, indicating improved cathode utilization with increasing anode excess. Under high-rate conditions, N/ P = 1.3 delivered the best overall performance, retaining 123.82 mAh g −1 at 12C with a 12C/1C retention ratio of 0.78. This composition also exhibited the minimum polarization-derived internal resistance (398.6 mΩ g), the highest peak power density (8327 mW g −1 ), and the lowest charge-transfer resistance (R ct = 1.38 Ω). In contrast, N/ P = 1.4 provided the best cycling stability, achieving 97.48% capacity retention after 100 cycles at 1C. Self-discharge behavior showed a U-shaped dependence on N/P ratio, with the lowest rates at N/ P = 1.2–1.3. SEM analysis confirmed that performance degradation was governed primarily by anode-side interfacial deterioration at low N/P and parasitic surface growth at high N/P. Overall, the findings demonstrate that N/P tuning is a decisive design parameter for high-rate NMC-NCA/graphite pouch cells, with N/P ≈ 1.3–1.4 offering the best balance between power capability, stability, and interfacial robustness.