Mg_xMn_2-xB₂O₅ Pyroborates (2/3 ≤ x ≤ 4/3): High Capacity and High Rate Cathodes for Li-Ion Batteries

MgMnB₂O₅, Mg_2/3Mn_4/3B₂O₅, and Mg_4/3Mn_2/3B₂O₅ pyroborates have been prepared via a ceramic method. When charging MgMnB₂O₅ vs Li, all of the Mg²⁺ can be removed, and with subsequent cycles, 1.4 Li ions, corresponding to a capacity of 250 mAhg^-1, can be reversibly intercalated. This is achieved at a C/25 rate with 99.6% Coulombic efficiency. Significant capacity is retained at high rates with 97 mAhg^-1 at a rate of 2C. Continuous cycling at moderate rates gradually improves performance leading to insertion of 1.8 Li, 314 mAhg^-1 with a specific energy of 802 Whkg^-1, after 1000 cycles at C/5. Ex situ X-ray and neutron diffraction demonstrate the retention of the pyroborate structure on cycling vs Li and a small volume change (1%) between the fully lithiated and delithiated structures. Mg_2/3Mn_4/3B₂O₅ and Mg_4/3Mn_2/3B₂O₅ are also shown to reversibly intercalate Li at 17.8 and 188.6 mAhg^-1, respectively, with Mn ions likely blocking Mg/Li transport in the Mg_2/3Mn_4/3B₂O₅ material. The electrochemical ion-exchange of polyanion materials with labile Mg ions could prove to be a route to high energy density Li-ion cathodes.