Electrochemical behavior of [{Mn(Bpy)}(VO3)2]≈(H2O)1.24 and [{Mn(Bpy)0.5}(VO3)2]≈(H2O)0.62 inorganic–organic Brannerites in lithium and sodium cells

The performance of MnV2O6 (MnV) and its [{Mn(Bpy)}(VO3)2]≈(H2O)1.16 (MnBpy) and [{Mn(Bpy)0.5}(VO3)2]≈(H2O)0.62 (MnBpy0.5) hybrid derivative compounds was investigated against sodium and lithium counter electrodes. For MnV2O6 stable capacities of 850 mAh/g were achieved in lithium cells, the best value reported so far. The whole capacity is ascribed to a conversion reaction in which the amorphization of the compounds takes place. No significant differences in the capacities for the inorganic compound and the hybrid ones were observed. Interestingly, the potential hysteresis decreases in the hybrid compounds. The difference between Li and Na cell capacity most probably comes from the difference of standard potential of the two redox couples Li+/Li and Na+/Na of about ca. 0.3 V leading to an incomplete conversion reaction and thus lowers capacity in the case of Na cells. The Raman and IR ex-situ experiments after cycling indicate that the bipyridine organic ligands are completely decomposed during the electrochemical testing. The IR studies in MnV inorganic and MnBpy and MnBpy0.5 hybrid electrodes after the electrochemical cycling, suggest that the SEI formation and bipyridine degradation give rise to different aliphatic compounds.