Juliette Billaud

Na0.67Mn1−xMgxO2 (0 ≤ x ≤ 0.2): a high capacity cathode for sodium-ion batteries

Earth-abundant Na0.67[Mn1−xMgx]O2 (0 ≤ x ≤ 0.2) cathode materials with the P2 structure have been synthesized as positive electrodes for sodium-ion batteries. Na0.67MnO2 exhibits a capacity of 175 mA h g−1 with good capacity retention. A Mg content of 5% is sufficient to smooth the charge/discharge profiles without affecting the capacity, whilst further increasing the Mg content improves the cycling stability, but at the expense of a lower discharge capacity (∼150 mA h g−1 for Na0.67Mn0.8Mg0.2O2). It was observed that the cooling process during synthesis, as well as Mg content, have an influence on the structure.

β-NaMnO2: A High-Performance Cathode for Sodium-Ion Batteries

There is much interest in Na-ion batteries for grid storage because of the lower projected cost compared with Li-ion. Identifying Earth-abundant, low-cost, and safe materials that can function as intercalation cathodes in Na-ion batteries is an important challenge facing the field. Here we investigate such a material, β-NaMnO2, with a different structure from that of NaMnO2 polymorphs and other compounds studied extensively in the past. It exhibits a high capacity (of ca. 190 mA h g(-1) at a rate of C/20), along with a good rate capability (142 mA h g(-1) at a rate of 2C) and a good capacity retention (100 mA h g(-1)after 100 Na extraction/insertion cycles at a rate of 2C). Powder XRD, HRTEM, and (23)Na NMR studies revealed that this compound exhibits a complex structure consisting of intergrown regions of α-NaMnO2 and β-NaMnO2 domains. The collapse of the long-range structure at low Na content is expected to compromise the reversibility of the Na extraction and insertion processes occurring upon charge and discharge of the cathode material, respectively. Yet stable, reproducible, and reversible Na intercalation is observed.

Structurally stable Mg-doped P2-Na2/3Mn1−yMgyO2 sodium-ion battery cathodes with high rate performance: insights from electrochemical, NMR and diffraction studies

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 Na2/3MnO2 with a P2 layer stacking by comparing three compositions: Na2/3Mn1−yMgyO2 (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-Na2/3MnO2. 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.

In situ Fe K-edge X-ray absorption spectroscopy study during cycling of Li2FeSiO4 and Li2.2Fe0.9SiO4 Li ion battery materials

In situ X-ray Absorption Spectroscopy (XAS) results are presented for Li2FeSiO4 and Li2.2Fe0.9SiO4, promising cathode materials for lithium-ion batteries. The aims are to establish the valence and local structure of Fe during charge and discharge to understand if the Fe3+/Fe4+ redox pair can be reached in the current battery design. It is found that the valence state changes between Fe2+ and Fe3+, with no evidence of Fe4+ before the onset of electrolyte degradation. There is a reversible contraction and extension of the Fe–O bond lengths during cycling while the Fe–Si distance remains constant, which underlines the stability of the Li2FeSiO4 material. The same observations apply to Li2.2Fe0.9SiO4 cathode material indicating that changing the stoichiometry does not provide any additional structural stability.