Deciphering the origin of the high capacities seen in magnesium borate polyanion cathodes

DOI: 10.1039/D5TA07239E

Authors: Camilla Tacconis (University of Cambridge) , Sunita Dey (University of Cambridge; University of Aberdeen) , Carson D. Mclaughlin (University of Cambridge) , Debashis Tripathy (University of Cambridge) , Heather F. Greer (University of Cambridge) , Shaoliang Guan (University of Cambridge) , Iuliia Mikulska (Diamond Light Source) , Israel Temprano (University of Cambridge) , Clare P. Grey (University of Cambridge) , Sian E. Dutton (University of Cambridge)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Journal Of Materials Chemistry A

State: Published (Approved)
Published: September 2025
Diamond Proposal Number(s): 33172 , 14239

Abstract: 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 MgFe0.5Mn0.5B2O5. 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.

Diamond Keywords: Batteries; Magnesium-ion

Subject Areas: Chemistry, Materials, Energy


Instruments: B18-Core EXAFS , I11-High Resolution Powder Diffraction

Added On: 02/10/2025 08:54

Documents:
d5ta07239e.pdf

Discipline Tags:

Energy Storage Energy Physical Chemistry Energy Materials Chemistry Materials Science

Technical Tags:

Diffraction Spectroscopy X-ray Powder Diffraction X-ray Absorption Spectroscopy (XAS) X-ray Absorption Near Edge Structure (XANES)