Influence of gradient Ni incorporation on the structural, surface, and electrochemical properties of LiCoO2 thin-film cathodes.

Abstract

Layered LiCoO₂ thin-film (LCO) cathodes are attractive for lithium-ion microbatteries, but their use is constrained by structural instability, modest rate capability, and progressive capacity loss. In this work, we show that spatially graded Ni incorporation, achieved via controlled RF/DC co-sputtering, provides a promising strategy to tune structural and electrochemical limitations in layered LiNi _x Co_1-x O₂ (LNCO) thin film cathodes. Three architectures were developed: LCO, uniformly Ni-incorporated LNCO-0.25A, and gradient Ni-incorporated LNCO-G, the latter obtained by progressively increasing the Ni target current from 0.20 to 0.40 A during deposition, producing a depth-dependent Ni distribution. XRD and Raman analyses show that Ni incorporation is accompanied by sharper reflections and reduced Raman peak broadening, consistent with improved local structural order. Cross-sectional HAADF-STEM and STEM-EDS mapping confirm a dense microstructure and a continuous Ni gradient increasing progressively from the substrate interface toward the coating surface. Depth-resolved XPS shows that, although all films develop surface carbonate species upon air exposure, LNCO-G exhibits a reduced hydroxyl/carbonate contribution compared with LNCO-0.25A, which is consistent with a less hydrated near-surface chemistry. Electrochemically, LNCO-G delivers higher current densities (≈3-5× increase) and more defined high-voltage redox features than the uniformly incorporated counterpart. In a preliminary 10-cycle diagnostic test under an aggressive 1.0-4.9 V window, LNCO-G achieves an initial discharge capacity of ∼100-120 mAh g^-1 at 0.5C and retains ∼55% of its capacity. This short, aggressive protocol was intentionally chosen as a materials-level screening test to amplify differences in polarization and early degradation among the three architectures. Post-cycling EIS shows a 59% reduction in charge-transfer resistance (67.7 Ω vs. 164.3 Ω) and a lower Warburg coefficient (21.3 vs. 44.1 Ω s^-1/2) relative to LNCO-0.25A. These results demonstrate that compositional Ni grading is associated with a measurable performance advantage over the uniform incorporation strategy studied here, although the comparison is not composition-matched and therefore reflects combined effects of Ni content, distribution, and secondary-phase fraction. These findings identify spatial Ni grading as a promising route for tuning coupled structural, interfacial, and kinetic properties in sputtered LiCoO₂-based thin-film cathodes.