Abstract
Magnesium substituted P2-structure Na0.67Ni0.3Mn0.7O2 materials have been prepared by a facile solid-state method and investigated as cathodes in sodium-ion batteries. The Mg-doped materials described here were characterized by X-ray diffraction (XRD), 23Na solid-state nuclear magnetic resonance (SS-NMR), and scanning electron microscopy (SEM). The electrochemical performance of the samples was tested in half cells vs Na metal at room temperature. The Mg-doped materials operate at a high average voltage of ca. 3.3 V vs Na/Na+ delivering specific capacities of ∼120 mAh g–1, which remain stable up to 50 cycles. Mg doping suppresses the well-known P2–O2 phase transition observed in the undoped composition by stabilizing the reversible OP4 phase during charging (during Na removal). GITT measurements showed that the Na-ion mobility is improved by 2 orders of magnitude with respect to the parent P2–Na0.67Ni0.3Mn0.7O2 material. The fast Na-ion mobility may be the cause of the enhanced rate performance.
Highlights
Over the last two decades, most of the portable electronic market has been dominated by lithium-ion batteries (LIBs)
The Mg-doped materials described here were characterized by Xray diffraction (XRD), 23Na solid-state nuclear magnetic resonance (SS-NMR), and scanning electron microscopy (SEM)
Even though the high energy density of lithium-ion batteries makes them attractive for many applications, there is a demand for inexpensive technology for which the sources of the ores are more uniformly distributed across the globe
Summary
Over the last two decades, most of the portable electronic market has been dominated by lithium-ion batteries (LIBs). Article operating voltage greater than 3.5 V (vs Na/Na+), attributed to the Ni2+/Ni4+ redox couple.[10,11] The main drawback, is its poor cycle life, which has been attributed to the detrimental P2−O2 transformation that occurs at high voltages This transition is caused by gliding of the transition metal layers upon sodium removal.[8,9,12] In order to address the issue of capacity fade, doping of Na0.67Ni0.33Mn0.67O2 with ions such as Zn2+ or Ti4+ (substituting for Ni and Mn, respectively) has been adopted.[13,14] Doping with these cations increases the amount of sodium left in the structure at the end of charge, alleviating the layer gliding at the expense of capacity. Measurements were conducted in a 2-electrode cell (coin-cell type) with an AC signal amplitude of 10 mV
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