image: Schematic illustration of inhibiting the release of O* and structural degradation of delithiated state Ni-rich layered cathodes by PN.
Credit: ©Science China Press
Effectiveness of PN in inhibiting crosstalk reactions
To evaluate the effectiveness of PN in enhancing the stability of higher-nickel-content layered oxides, 5 wt.% PN was incorporated into LiNi0.8Co0.1Mn0.1O2 (NCM811) and LiNi0.9Co0.05Mn0.05O2 (NCM90) based electrodes, which were subsequently tested using DSC in conjunction with the electrolyte. As result, the ΔH between d-PN@NCM811 and RCE is reduced from −577.2 to −302.2 J g−1, corresponding to a decrease of 47.6%. In the NCM90 system, the ΔH is reduced from −800.7 to −629.3 J g−1 and the reduction of ΔH is 21.4%. The above results indicate that PN universally inhibits O* in Ni-rich layered oxide cathodes.
In addition, the ΔH of the “PN@NCM811/LiC6+RCE” system is reduced from −1198.7 to −530.4 J g−1 representing a decrease of 55.8%. Furthermore, electrolyte with the functions of passivation and flame retardant (RCE-DHT) that we previously reported was also incorporated into LiC6 to investigate its synergistic effect. The suppression of exothermic side reactions between LiC6 and electrolyte led to a reduction in heat release between 125 and 150 °C, resulting in an overall decrease in ΔH to −406.5 J g−1.
Mechanisms for stabilizing lattice oxygen
The chelation of PN with Ni in NCM811, which effectively reduces the charge compensation from TM 2d to O 3p at elevated temperatures and in deep delithiated state, thereby inhibiting the oxidation of lattice oxygen. This process offers the advantage of mitigating the formation and diffusion of oxygen vacancies, slowing the transformation of the layered structure into the rock salt phase and inhibiting the extensive release of O*. The first-principles density functional theory (DFT) calculations indicate that the band center of the O 2p orbitals of PN@NCM811 (−3.872 eV) is located farther away from the Fermi level and fewer unoccupied states could be observed near the Fermi level, demonstrating its suppressed oxygen releasing activity within PN@NCM811.
Safety evaluation of pouch cell
In the NCM811 cell, as the temperature gradually increase, released O* react with electrolyte and LiC6. Under the dual crosstalk reactions involving O*, the TR trigger temperature (Ttr) of NCM811 cell is around 125.9 °C and the temperature climbs rapidly to the TR maximum temperature (Tmax) of 543.7 °C. In comparison, since PN in the PN@NCM811 sample effectively inhibits the release of O* and mitigates the dual crosstalk reactions, the Ttr is elevated to 184.8 °C, and the Tmax is reduced to 319.7 °C. Notably, the PN@NCM811 cell did not undergo TR upon heating to 125.9 °C (Ttr of NCM811 cell) and maintained for 20 minutes, indicating that at this temperature, the PN@NCM811 system does not release O* in significant quantities.
Evaluation of electrochemical performance
PN-modified NCM811 exhibits not only satisfactory safety properties but also favorable electrochemical performance. As the current density gradually increases from 0.1C to 2C, the specific discharge capacities of PN@NCM811 are 371.3, 367.4, 354.9, and 327.7 mAh g−1, respectively. Notably, PN@NCM811 exhibits slightly higher specific discharge capacities compared to NCM811. Additionally, cycling stability was evaluated for PN@NCM811. The capacity retention of PN@NCM811 at the current density of 1C is 85.4% after 300 cycles, which is an improvement over the 75.7% for NCM811.
Based on the mechanism that the PN protective layer can alleviate the charge compensation from TM 3d to O 2p. Therefore, it is theoretically expected that PN@NCM811 may demonstrate enhanced electrochemical performance at higher voltage. As expected, at the higher voltage of 4.6 V, the discharge capacity of NCM811 after 250 cycles is merely 60.8% of its initial capacity due to rapid structural degradation. Surprisingly, the capacity retention of PN@NCM811 is as high as 85.1% under the same conditions, remaining at 80.0% even after 700 cycles.