Structural and transport properties of P2-Type Na0.70Ni0.20Cu0.15Mn0.65O2 layered oxide

Abstract

P2-type Layered oxides have attracted increasing attention recently as the cathode materials for Na-ion batteries with promising cyclability and good specific capacity. In this work, structural, electrical, and electrochemical properties of P2-type Na0.70Ni0.20Cu0.15Mn0.65O2 (NNCM) ceramic fabricated via a sol-gel method were investigated. The Rietveld refinement of the room temperature XRD diffraction data confirmed the formation of a single P2-type phase with space group P63/mmc for the powder calcined at 850 °C. Complex impedance spectroscopy was used to deconvolute the contributions of grains and grain boundaries to the overall conduction inside the sample. The room temperature conductivity of the grains and grain boundaries calculated for the NNCM ceramic sintered at 950 °C were estimated to be (5.25 ± 0.03) × 10−5 Scm−1 and (4.70 ± 0.05) × 10−6 Scm−1, respectively. The respective activation energies for the grain and grain boundary conduction were 0.189 ± 0.008 eV and 0.22 ± 0.01 eV, respectively. Moreover, NNCM exhibited a sodium-ion transference number of ≈0.86, suggesting that the conduction in this material is dominated by the Na-ions. The conduction mechanisms and related relaxations were also investigated using the dielectric and ac conductivity formalisms. NNCM showed specific capacities of 99 mAh/g and 74 mAh/g at 0.1C and 1C discharge rates, respectively, between 2 V and 4.25 V (vs. Na/Na+) with 95% capacity retained after 300 cycles at 1C.