Suppressing the Voltage Decay Based on a Distinct Stacking Sequence of Oxygen Atoms for Li-Rich Cathode Materials

Abstract

Li-rich cathode materials possess a much higher theoretical energy density than all intercalated cathode materials currently reported and thus are considered as the most promising candidate for next-generation high-energy density Li-ion batteries. However, the rapid voltage decay and the irreversible phase transition of O3-type Li-rich cathode materials often lessen their actual energy density and limit their practical applications, and thus, effectively suppressing the voltage decay of Li-rich cathodes becomes the hotspot of the current research. Herein, the F-doped O2-type Li-rich cathode materials Li 1.2 Mn 0.54 Ni 0.13 Co 0.13 O 2+δ−x F x (F-O2-LRO) are designed and prepared based on the P2-type sodium-ion cathode materials Na 5/6 Li 1/4 (Mn 0.54 Ni 0.13 Co 0.13 ) 3/4 -O 2+δ (Na-LRO) by ion exchange. It has been found that the asprepared F-O2-LRO exhibits excellent electrochemical performance, for example, a high discharge specific capacity of 280 mA h g −1 at 0.1 C with an initial Coulombic efficiency of 94.4%, which is obviously higher than the original LRO (77.2%). After 100 cycles, the F-O2-LRO cathode can still maintain a high capacity retention of 95% at a rate of 1 C, while the capacity retention of the original LRO is only 69.1% at the same current rate. Furthermore, the voltage difference (ΔV) of F-O2-LRO before and after cycling is only 0.268 V after 100 cycles at 1 C, which is less than that of the LRO cathode (0.681 V), indicating much lower polarization. Besides, even at a high current rate of 5 C, F-O2-LRO still displays a satisfactory discharge capacity of 210 mA h g −1 with a capacity retention of 90.1% after 100 cycles. Therefore, this work put forward a new strategy for the development and industrial application of Li-rich cathode materials in high-energy Li-ion batteries.

Multi-dimensional correlation of layered Li-rich Mn-based cathode materials

“…Mn 7+ was then reduced and returned to the original octahedral sites and there were also Mn 7+ ions in tetrahedral sites at the end of charging. Kleiner et al [95] quantified the migration track of TMs via synchrotron radiation XRD differential Fourier fitting. It was found that ~8% TMs occupied the tetrahedral sites in the Li layer during charging and ~2%-5% TMs occupied Li octahedral sites without the occupation of tetrahedral sites after discharge.…”

“…The O2type cathode material [96,97] then emerges and the special oxygen stack (ABCBA) enables TM ions to undergo , followed by the spontaneous formation of peroxide or trapped oxygen molecules accompanied by the reduction of Mn [94] . (B) TM migration in an LRMO from the octahedral TM sites via tetrahedral sites into octahedral Li sites [95] . (C) Comparison of crystal structures.…”

“…(C) Chargedischarge curves and phase structural evolution of LRMO during electrochemical cycling [105] . (D) Mean discharge/charge voltage of the two cells cycled simultaneously (cell 1/2: 22 °C, cycles 1-49; cell 3/4: 25 °C, cycles 1-49; 22 °C, cycles 49-105), with the error bars representing the standard deviation between the two cells [95] . (E) Contribution towards the discharge capacity from each element at various cycles (left).…”

“…Although a considerable amount of Mn will reversibly return to the original position after it migrates from its own octahedral position to the tetrahedral position of the Li layer in each charge and discharge process and the recurring spinel nanodomain proposed by Xiao et al [104] appears, there is still some Mn irreversibly migrated to the Li layer. This process continues to accumulate, from the first charge and discharge Li site octahedra are occupied by TMs (mainly Mn) at ~2%, which gradually increases to ~5% after 100 cycles [95] . In the wide voltage range of 2-4.8 V, the Mn ions undergo a profound valence state change.…”

“…Therefore, the reduction of the electrochemical reaction potential of the redox couples during cycling and the aforementioned kinetic factors are the direct causes of the voltage decay [15] . It is noteworthy that the charge/discharge average voltage changes during the cycle are inconsistent and the charge average voltage changes very little [95] compared with the serious attenuation of the discharge average voltage [Figure 9D], which can also be attributed to the combined effects of kinetics and thermodynamics. Because electrochemical polarization increases the potential during the charging process, the thermodynamically driven redox couples change will lower the electrode potential.…”