Fast Microwave-Assisted Hydrothermal Synthesis of Pure Layered δ-MnO2 for Multivalent Ion Intercalation

Abstract

This work reports on the synthesis of layered manganese oxides (δ-MnO2) and their possible application as cathode intercalation materials in Al-ion and Zn-ion batteries. By using a one-pot microwave-assisted synthesis route in 1.6 M KOH (MnVII:MnII = 0.33), a pure layered δ-MnO2 birnessite phase without any hausmannite traces was obtained after only a 14 h reaction time period at 110 °C. Attempts to enhance crystallinity level of as-prepared birnessite through increasing of reaction time up to 96 h in 1.6 M KOH failed and led to decreases in crystallinity and the emergence of an additional hausmannite phase. The influence of MnII:OH− ratio (1:2 to 1:10) on phase crystallinity and hausmannite phase formation for 96 h reaction time was investigated as well. By increasing alkalinity of the reaction mixture up to 2.5 M KOH, a slight increase in crystallinity of birnessite phase was achieved, but hausmannite formation couldn’t be inhibited as hoped. The as-prepared layered δ-MnO2 powder material was spray-coated on a carbon paper and tested in laboratory cells with Al or Zn as active materials. The Al-ion tests were carried out in EMIMCl/AlCl3 while the Zn-Ion experiments were performed in water containing choline acetate (ChAcO) or a ZnSO4 solution. Best performance in terms of capacity was yielded in the Zn-ion cell (200 mWh g−1 for 20 cycles) compared to about 3 mAh g−1 for the Al-ion cell. The poor activity of the latter system was attributed to low dissociation rate of tetrachloroaluminate ions (AlCl4−) in the EMIMCl/AlCl3 mixture into positive Al complexes which are needed for charge compensation of the oxide-based cathode during the discharge step.

δ‐MnO2 Nanofibers: A Promising Cathode Material for New Aluminum‐Ion Batteries

“…Remarkable results have been obtained both in coulombic efficiency (>99 %) and in specific capacity (37 mA h g −1 ). Thanks to this synthesis route and the proper preparation of the electrode, the capacity results obtained in this work are 10 times higher than those achieved previously for this material (∼3 mA h g −1 ) . Furthermore, these results are comparable to those obtained previously in expanded graphites .…”

“…The initial observed discharge capacity is 59 mA h g −1 , which stabilized at 37 mA h g −1 after 15 cycles and with a coulombic efficiency of 100 %. This means an outstanding improvement compared to the previous reported data . This improvement may be largely due to the synthesis route used.…”

“…There are currently several suggestions to describe the intercalation mechanisms in this type of batteries . In view of the results obtained, the model that best suits what is observed in our material could be given by the following equations [Eqs.…”

“…It has been observed that in order to achieve stabilization of this phase, the presence of K + ions and water molecules between the layers is indispensable . However, the results obtained previously in AIBs have given very poor results, not achieving capacities higher than 10 mA h g −1 even at very low charge/discharge speeds (2.5 mA g −1 ) …”

Electrophoresis Assembly of Novel Superhydrophobic Molybdenum Trioxide (MoO3) Films with Great Stability

“…Recently, hydrothermal synthesis method has been successfully used to grow oxides (R x O y , R = Zn, Ti, V, Mo etc.) [19,20,21,22,23,24,25], which is regarded as a high-efficient technique with the advantages of low-cost, straightforward operation, and high controllability of the microstructures of target powders or films.…”