Electrochemical Behavior and Structural Change of Spinel-LiMn2-XNixO4 (x = 0, 0.5) Cathode in Potassium Cells

Abstract

On the recent research front for the next generation secondary batteries beyond Li, potassium insertion into graphite in non-aqueous cells has brought new insights into the electrochemical K+ intercalation behavior and introduced advantageous benefits from potassium. Since potassium resources are much abundant and the standard potential of K+/K is 0.13 V below that of Li+/Li, potassium-ion batteries can be regarded as an appealing alternative to LIBs to realize high voltage systems with low cost. The crucial issue is the development of cathode materials able to accommodate the large K-ion without displaying detrimental structural changes toward cycle life and rate capability performance. The 4.7 V LiMn1.5Ni0.5O4 spinel is already a promising cathode for the next generation of high voltage LIBs, and its host structure, λ-Mn0.75Ni0.25O2, could be of great interest for K-ion batteries.Here, we investigate for the first time the potassium insertion properties into electrochemically prepared λ-MnO2 (λ-MO) and λ-Mn0.75Ni0.25O2 (λ-MNO) spinels. Lithium ions in LiMn2O4 and LiMn1.5Ni0.5O4 samples are electrochemically extracted in lithium containing electrolyte solution, forming λ-Mn1- xNixO2 (x = 0, 0.25) by first oxidation process. Potassium ions are then inserted into the lithium extracted spinel phases. From structural analysis by X-ray diffraction and Raman spectroscopy, it is found that the original λ-Mn0.75Ni0.25O2 spinel phase converts into a layered phase, K~0.5Mn0.75Ni0.25O2. Then, K~0.5Mn0.75Ni0.25O2 can deliver a high reversible capacity of 90 mAh g-1 at C/20 in the 2 V- 4.6 V vs K+/K voltage range with 78% retention for 60 cycles at C/20 (Fig. 1). Promising rate performance are also evidenced with an initial capacity of 60 mAh g-1 at C/5 (Fig. 1), and 65% retention after 300 cycles at C/5. The origin of the attractive electrochemical performances of λ-MNO is investigated by XRD , Raman and EDS experiments after discharge and charge cycles. These results open the possibility of using inexpensive and high-capacity Mn-based active materials for potassium-ion batteries. Figure 1. Electrochemical properties of λ-Mn0.75Ni0.25O2 in a potassium cell. 1M KF6 /EC:PC 1:1 electrolyte, 2% vol. FEC. Voltage window 4.5 V- 2 V. Figure 1