Deciphering the Origin of the High Capacities Seen in Magnesium Borate Polyanion Cathodes

Recent reviews have highlighted borate polyanion systems as promising high-voltage cathode candidates for rechargeable Mg-ion batteries (RMBs) [Coordination Chemistry Reviews, 427, 213551 (2021)]. However, evaluating the electrochemical performance of cathodes for Mg-ion batteries is challenging, with many reports relying on an observed electrochemical capacity rather than demonstrating Mg-ion (de)intercalation. To address these two points, we study three classes of borate polyanions: orthoborates M₃(BO₃)₂, ludwigites M₃BO₅, and pyroborates M₂B₂O₅ and use a suite of experimental techniques to investigate de-magnesiation on charging vs Li metal with a Li electrolyte. We select five representative materials Mg₂Mn(BO₃)₂, Mg₂Ni(BO₃)₂, Mg₂FeBO₅, MgFeB₂O₅ and MgFe_0.5Mn_0.5B₂O₅. Whilst promising first charge capacities up to 200 mAh g⁻¹ are observed for ball-milled cathodes cycled at 55°C in a Li containing electrolyte, extensive post-cycling analysis using ex-situ X-ray Photoelectron Spectroscopy (XPS) and ex-situ Synchrotron Powder X-ray Diffraction (SXRD), combined with operando X-ray Absorption Spectroscopy (XAS) and operando Online Electrochemical Mass Spectrometry (OEMS), show that the capacities obtained are not associated with Mg²⁺ mobility in the cathodes, de-magnesiation or transition-metal redox. The observed capacity originates from a process enhanced by ball-milling, which is common to all borate polyanions investigated in this work. This process is in part attributed to the irreversible reaction of an amorphous surface layer on the polycrystalline particle, rich in carbonate and glassy borate phases. Here we present the first systematic study of the viability of transition-metal borate polyanions as intercalation cathode materials for RMBs and conclude that, despite the promising electrochemistry, these materials do not de-magnesiate under our tested conditions.