Impacts of Mg and Al Substitution on Na2/3Ni1/3Mn2/3O2 As Positive Electrode Materials for Na-Ion Batteries

Abstract

IntroductionRecently, layered Na x MeO2 (Me = metal element(s), x = ~1) compounds have been extensively explored as positive electrode materials for Na-ion batteries. From a viewpoint of materials abundance, Na-ion batteries are promising candidates as next generation energy storage devices. Average operating potential of Na x MeO2 is usually lower than that of Li x MeO2, resulting in lower energy density in Na-ion cells even though P2-type layered materials are considered to be potential candidates for high capacity positive electrode materials. Among the P2-type layered materials, Na2/3[Ni1/3Mn2/3]O2 performs at a relatively high operating voltage and delivers specific capacity of 150 mAh g−1 in the voltage range of 2.0–4.5 V in Na cells without compensation of initially deficient sodium in the structure.1 However, the reversible capacity of Na2/3[Ni1/3Mn2/3]O2 electrode is known to rapidly fade during cycles in the wide voltage range, probably due to a drastic lattice shrinkage of the fully charged state and electrolyte decomposition. In this study, we significantly improve the capacity retention of Na2/3Ni1/3Mn2/3O2 electrode by partial substitution of Mg and Al for the transition metals.ExperimentalNa2/3[Ni1/3 − x Mn2/3 − y Me x +y ]O2 (x = y = 1/36 for Me = Al, and x = 1/18, y = 0 for Me = Mg) was prepared by the calcination of stoichiometric mixtures of Mn2O3, Ni(OH)2, Na2CO3, Al(OH)3, and MgO with 5% excess sodium.2 The mixtures were pelletized and heated at 900 °C for 24 h in air and then quenched to room temperature. Crystal structures and morphology of the samples were examined by using powder X-ray diffraction (XRD) measurements and scanning electron microscopy (SEM). Electrochemical properties were tested using a coin-type cell. Positive electrodes consisted of the active material, acetylene black, and polyvinylidenefluoride (PVdF) with a gravimetric ratio of 80:10:10. Metallic sodium was used as a counter electrode. The electrolyte used was 1.0 mol dm-3 NaPF6 / PC.Results and discussionIn XRD patterns of synthesized samples, all diffraction lines can be assigned into the P2-type layered structure with space group P63/mmc without any crystalline impurities. Hexagonal platelet morphology for all the samples is found from SEM images. These particles have a diameter from 2 to 4 μm. Single phase P2-type layered samples modified with the Mg and Al substitution with similar particle morphology were successfully synthesized. The cycle stability of the sample electrodes operated at room temperature is shown in Fig. 1. The Mg and Al substitution effectively improves capacity retention. The substituted sample electrodes exhibit excellent cycle capability for 20 cycles. The capacity retention of the Al and Mg substituted samples is approximately 94 and 92%, respectively, after 20 cycles. Moreover, the Mg and Al substitution enhances rate capability. Although, at 2 C rate (about 260 mA g-1), Na2/3[Ni1/3Mn2/3]O2 delivers only 37% specific capacity of that at C / 20, the Al and Mg substituted samples show 78 and 77% of specific capacity of that at C / 20 rate. Influence of the Al and Mg substitution on electrochemical properties and structural changes induced by sodium extraction will be further discussed.