Abstract
AbstractA 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‐Na0.67Mn0.6Ni0.2Li0.2O2 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 solid‐solution 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 Na0.67Mn0.6Ni0.4O2 and Na0.67Mn0.8Ni0.2O2 show phase transitions and a stair‐case voltage profile. The capacity is found to originate from mainly Ni3+/Ni4+ and O2‐/O2‐δ redox processes by DFT, although a small contribution from Mn4+/Mn5+ to the capacity cannot be excluded.
Highlights
Quantitative analysis shows that around 46% of the lithium ions are located in the Na layer while around 54% are located in the transition metal oxide (TMO) layer
It is of note that this differs from our previous study on the compound Na0.8Mn0.6Fe0.2Li0.2O2 for which most of the Li+ was found to reside in the TMO layer.[28]
While the voltage profile for MN32 and MN41 showed several steps due to ordering phenomena and phase transitions, introducing lithium leads to a solid-solution type behavior in MNL and a much better cycle life
Summary
Similar stabilization effects were found in Li-substituted O3 materials.[21] Minimizing the number of phase transitions during charging/discharging is generally desired for achieving a better cycle life. We unravel the beneficial role of lithium for the compound Na0.67Mn0.6Ni0.2Li0.2O2 (MNL) that was synthesized using a previously reported sol-gel method followed by high temperature calcination.[5] In order to study the impact of Li-substitution at Mn or Ni sites on the structure and electrochemistry, materials with compositions of Na0.67Mn0.6Ni0.4O2 (MN32) and Na0.67Mn0.8Ni0.2O2 (MN41) were prepared, i.e., the compound contains either additional Mn or Ni instead of Li. X-ray diffraction (XRD), scanning electron microscopy (SEM) and solid-state nuclear magnetic resonance spectroscopy (ssNMR) were used to study the structure and the morphology of the compounds as well as the lattice occupation of these two alkali ions during cycling. The phase transition (P2 → O2) is mitigated, which leads to a solid-solution type mechanism for ion storage and improves cycle life
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