A Co‐ and Ni‐Free P2/O3 Biphasic Lithium Stabilized Layered Oxide for Sodium‐Ion Batteries and its Cycling Behavior

Abstract

Cobalt-and nickel-free cathode materials are desirable for developing low-cost sodium-ion batteries (SIBs). Compared to the single P-type and O-type structures, biphasic P/O structures become a topic of interest thanks to improved performance. However, the added complexity complicates the understanding of the storage mechanism and the phase behavior is still unclear, especially over consecutive cycling. Here, the properties of biphasic P2(34%)/O3(60%) Na 0.8 Li 0.2 Fe 0.2 Mn 0.6 O 2 and its behavior at different states of charge/discharge are reported on. The material is composed of single phase O3 and P2/O3 biphasic particles. Sodium occupies the alkali layers, whereas lithium predominantly (95%) is located in the transition metal layer. An initial reversible capacity of 174 mAh g-1 is delivered with a retention of 82% dominated by Fe 3+ /Fe 4+ along with contributions from oxygen and partial Mn 3+/4+ redox. Cycling leads to complex phase transitions and ion migration. The biphasic nature is nevertheless preserved, with lithium acting as the structure stabilizer.

Role of Lithium Doping in P2-Na0.67Ni0.33Mn0.67O2 for Sodium-Ion Batteries

“… 34 , 35 The resonance at ∼750 ppm was previously attributed to Li-ions in the Na-layer. 26 , 36 The resonance at ∼550 ppm stems from the minor spinel component, which agrees well with the XRD results, based on the previous 7 Li NMR study. 26 The relative amount of Li at different environments was quantified through the 7 Li solid-state NMR ( Figure S2 , Supporting Information).…”

Structural Aspects of P2‐Type Na0.67Mn0.6Ni0.2Li0.2O2 (MNL) Stabilization by Lithium Defects as a Cathode Material for Sodium‐Ion Batteries

“…It is of note that this differs from our previous study on the compound Na 0.8 Mn 0.6 Fe 0.2 Li 0.2 O 2 for which most of the Li + was found to reside in the TMO layer. [ 28 ] This might be due to the different Na contents (0.67 Na + per formula unit compared to 0.8), suggesting that decreasing the Na content leads to increased occupancy of the Na layer by Li + . At the moment, these multi‐element compounds are too complex to make more explicit statements on the ion occupancy.…”

23Na Solid‐State NMR Analyses for Na‐Ion Batteries and Materials

“…23 Na NMR clearly shows the slight difference in Na sites and distortion of the P2 structure. The effects of K‐, Rb‐, and Cs‐modification on the state of Na in P2−Na 0.7 Mn 0.8 Mg 0.2 O 2 , [11] substitution effects of Na 2/3 Mn 0.2 M 0.1 M′ 0.1 O 2 (M, M′=Fe 3+ , Al 3+ , Zn 2+ , Cu 2+ , Ti 4+ ), [12] and the Li‐stabilized compound (Na 0.8 Li 0.2 Fe 0.2 Mn 0.6 O 2 ) [13] have also been reported recently.…”