Orthophosphates As Potential Electrode Materials for Li and Mg-Ion Batteries
Authors: Romio Martina , Cupid Damian Marlon , Surace Yuri , Kahr Jürgen , Hamid Raad , Jahn Marcus , Abrahams Isaac 
Published: May 2020

Batteries based on the magnesium-ion chemistry are considered as promising alternatives to lithium ion technologies due to the di-valent character of the Mg2+ cations, the high theoretical volumetric capacity of magnesium (3833 mA h cm-3 for the Mg-anode), as well as its low toxicity and natural abundance. Furthermore, since magnesium deposition takes place without dendrite formation on the Mg-metal anode surface, the Mg-cell is expected to be considerably safer than comparable cells based on lithium metal.1

Despite these positive attributes, the development of magnesium secondary batteries is hindered by the high charge density and the slow solid-state diffusion of Mg2+ ions into the cathode lattice, which lead to relatively low practical capacities, irreversible capacity losses and poor cyclic stability. In order to improve the slow kinetics of Mg2+ insertion-extraction during the electrochemical reaction, new electrode materials should be developed.2 Since phosphate-based compounds with NASICON and olivine-related structures exhibit open three-dimensional frameworks, structural stability and safety under oxidizing conditions, they are considered as suitable candidate electrode materials for multi-valent ion batteries.3,4 Therefore, in this work the NASICON-structured Mg0.5Ti2(PO4)3 and the novel olivine-related (Mg0.5Ni0.5)3(PO4)2 compounds were synthesised by the solid-state method and investigated as potential electrode materials.
Mg0.5Ti2(PO4)3 and (Mg0.5Ni0.5)3(PO4)2 were subjected to a comprehensive physico-chemical characterization (XRD, SEM, BET and DLS techniques) in order to optimize the synthesis conditions, determine the phase purities and assess the particle morphologies. Furthermore, the reaction mechanisms and electrochemical behaviour of the orthophosphate-based materials with monovalent Li+ and divalent Mg2+ ions were investigated using post-mortem analysis (XRD and IR) in order to establish the nature of the charge storage reaction mechanisms (i.e. intercalation or conversion).
In addition to the challenges associated with the development of novel electrode materials, it is well known that electrolytes, formed by ionic salts (i.e. Mg(ClO4)2) and aprotic organic solvents (i.e. acetonitrile), form cationic insulating layers on the surface of the Mg metal anode, leading to poor performance of the overall cell. In order to address this problem, two different approaches were adopted during the electrochemical testing. Firstly, sulfone and glyme-based solutions were tested as candidate electrolytes in order to investigate their suitability with the Mg anode by analysing the passivation of the metal surface via infrared spectroscopy measurements. In a second approach, activated carbon (AC) was used as the counter electrode to replace the metallic magnesium when using Mg(ClO4)2 in acetonitrile as the electrolyte, since AC does not participate in parasitic reactions.5 Moreover, due to its high surface area, AC stores charge in a double layer on its external surface. Therefore, the systems formed by coupling AC and a conventional working electrode exhibit hybrid capacitive behaviour. Despite this, the use of AC as a counter electrode allows the investigation of the insertion of Mg2+ cations from the electrolyte into the working electrode. This set up enables the assessment of the electrochemical behaviour of the candidate working material without passivation reactions which could occur at the electrode/electrolyte interface. In particular, promising results are achieved using Mg0.5Ti2(PO4)3 as working electrode material, even though only surface sites were occupied with Mg2+ cations due to the sluggish intercalation kinetics.

References

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