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

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 degrees 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.