Victor Duffort

The Emerging Chemistry of Sodium Ion Batteries for Electrochemical Energy Storage

Energy storage technology has received significant attention for portable electronic devices, electric vehicle propulsion, bulk electricity storage at power stations, and load leveling of renewable sources, such as solar energy and wind power. Lithium ion batteries have dominated most of the first two applications. For the last two cases, however, moving beyond lithium batteries to the element that lies below-sodium-is a sensible step that offers sustainability and cost-effectiveness. This requires an evaluation of the science underpinning these devices, including the discovery of new materials, their electrochemistry, and an increased understanding of ion mobility based on computational methods. The Review considers some of the current scientific issues underpinning sodium ion batteries.

Structure of the high voltage phase of layered P2-Na2/3−z[Mn1/2Fe1/2]O2 and the positive effect of Ni substitution on its stability

A combination of operando X-ray diffraction, pair distribution function (PDF) analysis coupled with electrochemical measurements and Mossbauer spectroscopy elucidates the nature of the phase transitions induced by insertion and extraction of sodium ions in P2-Na0.67[NiyMn0.5+yFe0.5−2y]O2 (y = 0, 0.10, 0.15). When phase transitions are avoided, the optimal cathode material – P2-Na0.67Fe0.2Mn0.65Ni0.15O2 – delivers 25% more energy than the unsubstituted material, sustaining high specific energy (350 Wh kg−1) at moderate rates and maintains 80% of the original energy density after 150 cycles – a significant improvement in performance vs. the unsubstituted analogue. The crystal structure of the high voltage phase is solved for the first time by X-ray PDF analysis of P2-Na0.67−zFe0.5Mn0.5O2 (where z ∼ 0.5), revealing that migration of the transition metals – particularly Fe3+ – into tetrahedral sites in the interlayer space occurs at high potential. This results in new short range order between two adjacent layers. Although the transition metal migration is reversible as proven by electrochemical performance, it induces a large disfavourable cell polarization. The deleterious high voltage transition is mitigated by substitution of Fe3+ by Mn4+/Ni2+, giving rise to better cycling performance. Moreover, as demonstrated by 57Fe Mossbauer spectroscopy, the much lower ratio of Fe4+O6 to Fe3+O6 observed systematically across the range of Ni content – compared to the values expected from a purely ionic model – suggests redox activity involves the O-2p orbitals owing to their overlap with the transition metal-3d orbitals.

A high capacity thiospinel cathode for Mg batteries

Magnesium batteries are energy storage systems that potentially offer high energy density owing to their ability to employ magnesium metal as a negative electrode. Their development, however, has been thwarted by a paucity of functional positive electrode materials after the seminal discovery of the Mo6S8 Chevrel phase over 15 years ago. Herein, we report the second such material – a thiospinel – and demonstrate fully reversible Mg2+ electrochemical cycling vs. a Mg anode, which is complemented by diffraction and first principles calculations. The capacity approaches 80% of the theoretical value at a practical rate (C/5) at 60 °C, and yields a specific energy of 230 Wh kg−1, twice that of the Chevrel benchmark. Our results emphasize the advantage in employing “soft” anions to achieve practical divalent cation mobility.

Prussian Blue MgLi Hybrid Batteries

The major advantage of Mg batteries relies on their promise of employing an Mg metal negative electrode, which offers much higher energy density compared to graphitic carbon. However, the strong coulombic interaction of Mg2+ ions with anions leads to their sluggish diffusion in the solid state, which along with a high desolvation energy, hinders the development of positive electrode materials. To circumvent this limitation, Mg metal negative electrodes can be used in hybrid systems by coupling an Li+ insertion cathode through a dual salt electrolyte. Two “high voltage” Prussian blue analogues (average 2.3 V vs Mg/Mg2+; 3.0 V vs Li/Li+) are investigated as cathode materials and the influence of structural water is shown. Their electrochemical profiles, presenting two voltage plateaus, are explained based on the two unique Fe bonding environments. Structural water has a beneficial impact on the cell voltage. Capacities of 125 mAh g−1 are obtained at a current density of 10 mA g−1 (≈C/10), while stable performance up to 300 cycles is demonstrated at 200 mA g−1 (≈2C). The hybrid cell design is a step toward building a safe and high density energy storage system.

Screening for positive electrodes for magnesium batteries: a protocol for studies at elevated temperatures

The well-known all phenyl complex (APC) electrolyte for magnesium batteries is studied for the first time at high temperature using tetraglyme as a solvent. Combined with a molybdenum current collector, this enables the examination of positive electrode materials for Mg batteries at temperatures as high as 180 °C and up to 2 V vs. Mg, allowing discovery of the auspicious properties of CuS as a conversion cathode.

A conditioning-free magnesium chloride complex electrolyte for rechargeable magnesium batteries

The dissolution of Mg metal in AlCl3/THF using CrCl3 as a “promoter” yields a magnesium aluminum chloride complex electrolyte which shares many common features with MACC, but does not require an onerous conditioning process. This crucial advantage originates from the very high Mg to Al ratio in the new electrolyte, “MaCC”, which promotes 100% coulombic efficiency for Mg in the first cycle.