Structural Aspects of P2‐Type Na_0.67Mn_0.6Ni_0.2Li_0.2O₂ (MNL) Stabilization by Lithium Defects as a Cathode Material for Sodium‐Ion Batteries

Abstract

A known strategy for improving the properties of layered oxide electrodes in sodium-ion batteries is the partial substitution of transition metals by Li. Herein, the role of Li as a defect and its impact on sodium storage in P2-Na 0.67 Mn 0.6 Ni 0.2 Li 0.2 O 2 is discussed. In tandem with electrochemical studies, the electronic and atomic structure are studied using solid-state NMR, operando XRD, and density functional theory (DFT). For the as-synthesized material, Li is located in comparable amounts within the sodium and the transition metal oxide (TMO) layers. Desodiation leads to a redistribution of Li ions within the crystal lattice. During charging, Li ions from the Na layer first migrate to the TMO layer before reversing their course at low Na contents. There is little change in the lattice parameters during charging/ discharging, indicating stabilization of the P2 structure. This leads to a solidsolution type storage mechanism (sloping voltage profile) and hence excellent cycle life with a capacity of 110 mAh g -1 after 100 cycles. In contrast, the Li-free compositions Na 0.67 Mn 0.6 Ni 0.4 O 2 and Na 0.67 Mn 0.8 Ni 0.2 O 2 show phase transitions and a stair-case voltage profile. The capacity is found to originate from mainly Ni 3+ /Ni 4+ and O 2-/O 2-δ redox processes by DFT, although a small contribution from Mn 4+ /Mn 5+ to the capacity cannot be excluded.

Enabling an Excellent Ordering-Enhanced Electrochemistry and a Highly Reversible Whole-Voltage-Range Oxygen Anionic Chemistry for Sodium-Ion Batteries

“…Besides, though Na 0.75 Ca 0.04 [Li 0.1 Ni 0.2 Mn 0.67 ]O 2 and Na 0.67 [Ni 0.15 Zn 0.15 Mn 0.7 ]O 2 also exhibit excellent cycling stability after 200 cycles, both cathodes contain high amounts of Ni ions and the most specific capacity arises from the cationic Ni 2+ /Ni 4+ couples rather than the oxygen anionic reaction (less than 40 mA h g –1 ) in Figure a,b. , Oxygen anionic redox in Na 2/3 Mg 1/3 Ti 1/6 Mn 1/2 O 2 contributes a little higher specific capacity of ∼83 mA h g –1 than that of 79 mA h g –1 for the designed NMM; unfortunately, Na 2/3 Mg 1/3 Ti 1/6 Mn 1/2 O 2 still presents a large voltage hysteresis and a terrible cycling performance of ∼70% after only 50 cycles in Figure a,b . Therefore, the NMM cathode possesses the best comprehensive electrochemical performance including the high reversibility of the oxygen anionic reaction, no voltage hysteresis, and electrochemical performance among all the reported cathodes in Table S6 and Figure a,b. − ,,− ,, Therefore, these abovementioned features make the NMM cathode very promising as a low-cost and high-performance cathode of SIBs. The long-cycle stability at 1 C was also tested to evaluate the effect of a heavy dose of Mg-substitution, as shown in Figure c.…”

An orbital principle to design P2-NaxMO2 cathode materials for sodium-ion batteries

“…The structural changes in the electrode material during discharge process is one of the key factors to estimate the cycling performance of the corresponding material. 56,57 The more changes to the structure, the worse the cycling performance. In order to illustrate the structural changes, we calculated the rate of change in volume of the materials during the discharge process, according to the following formula:where V Na x MO 2 and V MO 2 are the volume of P2-Na x MO 2 after and before discharge, respectively.…”

Structural degradation mechanisms and modulation technologies of layered oxide cathodes for sodium‐ion batteries

“…Besides, Li + at the Na site enables movement of Li + to the TM layer upon charging and returns back to the Na site at lower Na contents, suppressing the P2−O2 phase transition. [ 71 ]…”

“…Therefore, the Na hopping prefers tetrahedral hopping rather than dumbbell hopping (Figure 6e). [70] Additionally, Li + , as a defect in [71] Doping/substitution at the TM ion site Doping/substitution of cations in the TM layer exerts significant effects on the stability of a layered cathode. For example, 4d TM cations with high oxidation states can be introduced into the TM layer to enhance the TM−O bond strength and thus the structural stability.…”