Interface Engineering of Magnetic Ni x Co3– x O4 Cathodes by Field Polarity for Li-O2 Batteries

Lithium-oxygen (Li-O2) batteries offer ultrahigh theoretical energy density, but sluggish cathodic oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) kinetics restrict practical application. Here, magnetically tunable NixCo3-xO4 (NCOx) spinel cathode catalysts were prepared by regulating the Ni:Co ratio to clarify the relationship among catalyst magnetization, magnetic-field polarity, and Li-O2 battery performance. Increasing Ni content raises the fraction of high-spin octahedral Ni2+, enhancing saturation magnetization; NCO3 (Ni:Co = 2:1) exhibits the highest Ms (28 emu/g) and strongest response under 300 mT. Electrochemical impedance spectroscopy reveals a polarity-dependent effect: an N-pole field decreases Rct from 27.42 to 19.74 Ω, whereas an S-pole field increases Rct, likely because it aggravates the spin mismatch between catalyst electrons and triplet-state O2, thereby increasing the interfacial charge-transfer barrier. Density functional theory (DFT) descriptor calculations show that the high-spin (HS)-initialized reference configuration strengthens O2 adsorption (-2.95 eV) and exhibits more favorable calculated ORR/OER energetics than the low-spin (LS)-initialized reference configuration; these calculations do not explicitly simulate the external magnetic field. Under optimal NCO3/N-pole/300 mT conditions, the discharge voltage increases by 107 mV and cycling life exceeds 990 h at 0.2 mA cm-2. Scanning electron microscopy (SEM) observations show that the N-pole-facing magnetic-field condition changes the discharge-product morphology from irregular agglomerates to more uniformly distributed platelet-like structures. Ex situ Li 1s X-ray photoelectron spectroscopy (XPS) is consistent with more extensive removal of Li2O2 containing products during recharge, although it does not by itself prove complete Li2O2 decomposition or suppression of Li2CO3 formation. This work establishes a cooperative strategy combining catalyst magnetic design and magnetic field regulation, providing insights into the rational design of magnetic field-assisted Li-O2 batteries.