Mn Binary Nano-Spinel Oxides Toward Mg Rechargeable Battery Cathode with Room-Temperature Operation

With the implementation of the decarbonization policies, the role and demand for storage batteries is significantly growing and changing. Therefore, further improvements for storage battery are required. Mg rechargeable batteries (MRBs), which use Mg metal as anode, are attracting attention as post-Li ion batteries. MRBs have been widely explored because of their high volumetric energy density, low cost and high safety. Regarding MRBs-cathode research, lots of issues such as low potential operation and low cyclability should be resolved. Recently, spinel oxide, MgM₂O₄(M = Cr, Mn, Fe, Co), has come to draw a lot of attention as a cathode material because of its high potential operation. In particular, MgMn₂O₄(MMO) is expected to be a high-energy cathode material as it can be used in both Mn²⁺/Mn³⁺ and Mn³⁺/Mn⁴⁺ redox reactions within the potential window of a standard electrolyte (&lt; 3.5 V vs. Mg/Mg²⁺). Nonetheless, oxide-type cathode materials show poor rate capabilities due to the sluggish diffusion rate of Mg-ion in solid derived from the large charge density of Mg²⁺. One effective solution is to make cathode materials nanosized to reduce the diffusion distance in the solid. In our previous work, a cubic nano-spinel MMO was synthesized via alcohol reduction (AR) process and demonstrated the room-temperature operation at full-cell test. However, the observed capacity (100 mAh g^-1) is not close to the theoretical capacity (270 mAh g^-1), implying that nanosizing cathode materials is not adequate for room-temperature operation. In this study, we focused on changing the structural stability of the redox phase and conductivity using a different transition metal instead of Mg in MMO toward high capacity at room temperature.<br/>Mn binary nano spinel oxides were synthesized by AR process. The appropriate amount of Bu₄NMnO₄ was added into the methanol dissolved with various transition metal salts and this reaction solution was vigorously stirred for an hour. After the reaction, the participants were washed with ethanol, dispersed into tert-butyl alcohol and freeze-dried. The crystalline phase and crystallite size of each spinel oxide were identified by X-ray diffraction and scanning transmission electron microscopy. The obtained sample was mixed with acetylene black and polytetrafluoroethylene at a weight ratio of 60/30/10. Then, these mixtures were pressed onto Al mesh to make a cathode. A coin-shaped full cell consisting of Mg[B(HFIP)₄]₂ / triglyme (HFIP: hexafluoroisopropyl) as electrolyte and a magnesium metal anode was prepared, and charge-discharge tests were conducted at room temperature.<br/>All the synthesized A-Mn spinel (A = Cu, Ni, Co) by AR method showed a single phase of cubic structure from XRD measurements, suggesting that Jahn-Teller effect of Mn³⁺ is suppressed and the metastable cubic structure is obtained. In addition, TEM images revealed that all the synthesized samples have 5 nm-primary particles and electron diffraction patterns attributed to the spinel structure were obtained through Selected Area Electron Diffraction measurements. Regarding charge-discharge tests, Cu-Mn spinel showed much higher discharge capacity than MMO, implying that Mg-ion was easier to be inserted at room temperature. Furthermore, Galvanostatic Intermittent Titration Technique revealed that Cu-Mn spinel has smaller overpotential during discharge than MMO, demonstrating smaller particle resistance.<br/>Therefore, we found that the cathode materials require not only nano-sizing but also smaller particle resistance toward room-temperature operation.