Earth-abundant Na _0.67 [Mn _1−x Mg _x ]O ₂ (0 ≤ x ≤ 0.2) cathode materials with the P2 structure have been synthesized as positive electrodes for sodium-ion batteries.

Sodium-ion batteries are a more sustainable alternative to the existing lithium-ion technology and could alleviate some of the stress on the global lithium market as a result of the growing electric car and portable electronics industries. Fundamental research focused on understanding the structural and electronic processes occurring on electrochemical cycling is key to devising rechargeable batteries with improved performance. We present an in-depth investigation of the effect of Mg doping on the electrochemical performance and structural stability of Na_2/3MnO₂ with a P2 layer stacking by comparing three compositions: Na_2/3Mn_1−yMg_yO₂ (y = 0.0, 0.05, 0.1). We show that Mg substitution leads to smoother electrochemistry, with fewer distinct electrochemical processes, improved rate performance and better capacity retention. These observations are attributed to the more gradual structural changes upon charge and discharge, as observed with synchrotron, powder X-ray, and neutron diffraction. Mg doping reduces the number of Mn³⁺ Jahn–Teller centers and delays the high voltage phase transition occurring in P2-Na_2/3MnO₂. The local structure is investigated using ²³Na solid-state nuclear magnetic resonance (ssNMR) spectroscopy. The ssNMR data provide direct evidence for fewer oxygen layer shearing events, leading to a stabilized P2 phase, and an enhanced Na-ion mobility up to 3.8 V vs. Na⁺/Na upon Mg doping. The 5% Mg-doped phase exhibits one of the best rate performances reported to date for sodium-ion cathodes with a P2 structure, with a reversible capacity of 106 mA h g⁻¹ at the very high discharge rate of 5000 mA g⁻¹. In addition, its structure is highly reversible and stable cycling is obtained between 1.5 and 4.0 V vs. Na⁺/Na, with a capacity of approximately 140 mA h g⁻¹ retained after 50 cycles at a rate of 1000 mA g⁻¹.

Magnesium substituted P2-structure Na_0.67Ni_0.3Mn_0.7O₂ 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–Na_0.67Ni_0.3Mn_0.7O₂ material. The fast Na-ion mobility may be the cause of the enhanced rate performance.

Polyoxyanion compounds, particularly the olivine-phosphate LiFePO₄, are receiving considerable attention as alternative cathodes for rechargeable lithium batteries. More recently, an entirely new class of polyoxyanion cathodes based on the orthosilicates, Li₂MSiO₄ (where M = Mn, Fe, and Co), has been attracting growing interest. In the case of Li₂FeSiO₄, iron and silicon are among the most abundant and lowest cost elements, and hence offer the tantalising prospect of preparing cheap and safe cathodes from rust and sand! This Highlight presents an overview of recent developments and future challenges of silicate cathode materials focusing on their structural polymorphs, electrochemical behaviour and nanomaterials chemistry.