Role of Lithium Doping in P2-Na_0.67Ni_0.33Mn_0.67O₂ for Sodium-Ion Batteries

Abstract

P2-structured Na 0.67 Ni 0.33 Mn 0.67 O 2 (PNNMO) is a promising Na-ion battery cathode material, but its rapid capacity decay during cycling remains a hurdle. Li doping in layered transition-metal oxide (TMO) cathode materials is known to enhance their electrochemical properties. Nevertheless, the influence of Li at different locations in the structure has not been investigated. Here, the crystallographic role and electrochemical impact of lithium on different sites in PNNMO is investigated in Li x Na 0.67– y Ni 0.33 Mn 0.67 O 2+δ (0.00 ≤ x ≤ 0.2, y = 0, 0.1). Lithium occupancy on prismatic Na sites is promoted in Na-deficient (Na < 0.67) PNNMO, evidenced by ex situ and operando synchrotron X-ray diffraction, X-ray absorption spectroscopy, and 7 Li solid-state nuclear magnetic resonance. Partial substitution of Na with Li leads to enhanced stability and slightly increased specific capacity compared to PNNMO. In contrast, when lithium is located primarily on octahedral TM sites, capacity is increased but at the cost of stability.

Na2/3Li1/9[Ni2/9Li1/9Mn2/3]O2: A High-Performance Solid-Solution Reaction Layered Oxide Cathode Material for Sodium-Ion Batteries

“…33,34 Interestingly, the reduced TM layer space and interatomic distances of TM-Na/Li might be due to the higher bond energy of Li−O in comparison with the Ni−O interatomic distance. 25,35,36 SEM and EDS elementary mapping, as shown in Figure 3a, suggest that the particles exhibit the platelike morphology of the as-synthesized Li-NNMO material with the sizes ranging from 200 to 400 nm, and Na, Ni, Mn, and O elements are uniformly distributed in Li-NNMO material, as shown in Figure 3c−g. For comparison, the Li-free NNMO was also synthesized by the same experimental process.…”

“…The occupancy of Li ions in the two layers in this work confirmed by neutron diffraction is in line with those cathodes characterized by solid-state 7 Li NMR. 19,25,26 The similar layered feature between Na-based oxides and Li-based counterparts may offer the great possibility of Li incorporation, similar to the Li-rich layered cathode, as shown in Figure 1a. Figure 2c shows the structure evolution after Li incorporation.…”

“…The changes in lattice parameters do not agree with oxide cathodes with Li entering one of the two sites. 18,25 The NPD refinement result demonstrates that Li ions occupy the intrinsic Na-ion vacancies in the Na layer (Interslab) and TM sites in the metal layer (Slab), showing the successful material structural design by this idea. This is different from the reports stating that Li ions prefer to enter the TM layer rather than the Na layer.…”

Transition metal oxides as a cathode for indispensable Na-ion batteries

“…84 The stability is ascribed to the reversible migration of lithium while cycling and reduction of P2-O2 transformation. 85,90 The appropriate sodium content is important to achieve remarkable electrochemical performance. P2-type cathode-Na 0.85 Li 0.12 Ni 0.22 Mn 0.66 O 2 containing high sodium content exhibited plateau-free and fast Na + mobility with low volume expansion ($1.7%).…”

Spatial and Temporal Analysis of Sodium-Ion Batteries

“…In order to mitigate this problem, other elements have been adopted to substitute Na sites or TM sites and suppress the P2-O2 phase transition for better electrochemical performance . For instance, Xiong et al recently investigated the effect of Li doping on the electrochemical properties of P2-Na 0.67 Ni 0.33 ­Mn 0.67 O 2 cathode. It was found that Li occupancy on different sites (Na sites and/or TM sites) in the P2 structure has different effects on the crystallographic stability and electrochemical performance.…”