Effect of Sodium Content on the Reversible Lithium Intercalation into Sodium-Deficient Cobalt–Nickel–Manganese Oxides NaxCo1/3Ni1/3Mn1/3O2 (0.38 ≤ x ≤ 0.75) with a P3 Type of Structure

Abstract

Layered lithium transition metal oxides with optimized nickel–manganese content are nowadays of primary interest as electrode materials for lithium ion batteries, since they are able to deliver a high capacity at a low cost. Herein we report a new class of less expensive cathode materials, which comprise sodium-deficient cobalt–nickel–manganese oxides NaxCo1/3Ni1/3Mn1/3O2 characterized with a layered structure and broad concentration range of sodium solubility. NaxCo1/3Ni1/3Mn1/3O2 oxides are obtained by thermal decomposition of mixed acetate–oxalate precursors, followed by thermal annealing between 700 and 800 °C. In the concentration range of 0.33 < x ≤ 0.75, NaxCo1/3Ni1/3Mn1/3O2 oxides assume a layered structure with a three-layer stacking (i.e., P3 type of structure). Based on electron paramagnetic resonance spectroscopy operating in the X-band (9.4 GHz), it is found that the charge compensation of Na deficiency is achieved by preferential oxidation of Ni2+ to Ni3+ and Ni4+, while Co and Mn ions retain their oxidation state of 3+ and 4+ within the whole concentration range. The electrochemical performance of NaxCo1/3Ni1/3Mn1/3O2 in model lithium cells is simply controlled by the amount of sodium content in the pristine compositions: a higher reversible capacity is achieved for sodium-rich oxides (i.e., 0.75 ≥ x ≥ 0.67), while sodium-poor oxides (i.e., 0.38 ≤ x ≤ 0.50) display a lower reversible capacity and improved cycling stability. The mechanism of the lithium intercalation into NaxCo1/3Ni1/3Mn1/3O2 is discussed on the basis of ex situ XRD, HRTEM, and X-ray photoelectron spectroscopy analyses.