Sodium deficient nickel–manganese oxides as intercalation electrodes in lithium ion batteries

Abstract

Sodium deficient nickel–manganese oxides Na_xNi_0.5Mn_0.5O₂ with a layered structure are of interest since they are capable of participating in reactions of intercalation of Li⁺ and exchange of Na⁺ with Li⁺. Taking into account the intercalation properties of these oxides, we provide new data on the direct use of Na_xNi_0.5Mn_0.5O₂ as low-cost electrode materials in lithium ion batteries instead of lithium analogues. Sodium deficient nickel–manganese oxides Na_xNi_0.5Mn_0.5O₂ are prepared at 700 °C from freeze-dried acetate precursors. The structure of Na_xNi_0.5Mn_0.5O₂ is analyzed by means of powder X-ray diffraction, SAED and HRTEM. The oxidation states of nickel and manganese ions are determined by X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance spectroscopy (EPR). Model lithium cells are used to monitor the lithium intercalation into Na_xNi_0.5Mn_0.5O₂. The surface and composition stability of Na_xNi_0.5Mn_0.5O₂ during the electrochemical reaction is monitored by using ex situ XPS and LA-ICPMS. Layered oxides Na_xNi_0.5Mn_0.5O₂ exhibit a P3-type of structure, in which the solubility of sodium is limited between 0.5 and 0.75. At 700 °C, Na_xNi_0.5Mn_0.5O₂ consists of thin well-crystallized nanoparticles; some of the particles have sizes higher than 100 nm, displaying a trigonal superstructure. For all oxides, manganese ions occur in the oxidation state of +4, while the oxidation state of nickel ions is higher than +2 and depends on the sodium content. The electrochemical reaction occurs within two potential ranges at 3.1 and 3.8 V due to the redox manganese and nickel couples, respectively. During the first discharge, Li⁺ intercalation and Li⁺/Na⁺ exchange reactions take place, while the consecutive charge process includes mainly Li⁺ and Na⁺ deintercalation. As a result, all oxides manifest a reversible capacity of about 120–130 mA h g⁻¹, corresponding to 0.5–0.6 moles of Li⁺. The formation of surface layers in the course of the electrochemical reaction is also discussed.