Tuning P2-Structured Cathode Material by Na-Site Mg Substitution for Na-Ion Batteries

Abstract

Most P2-type layered oxides suffer from multiple voltage plateaus, due to Na+/vacancy-order superstructures caused by strong interplay between Na–Na electrostatic interactions and charge ordering in the transition metal layers. Here, Mg ions are successfully introduced into Na sites in addition to the conventional transition metal sites in P2-type Na0.7[Mn0.6Ni0.4]­O2 as new cathode materials for sodium-ion batteries. Mg ions in the Na layer serve as “pillars” to stabilize the layered structure, especially for high-voltage charging, meanwhile Mg ions in the transition metal layer can destroy charge ordering. More importantly, Mg ion occupation in both sodium and transition metal layers will be able to create “Na–O–Mg” and “Mg–O–Mg” configurations in layered structures, resulting in ionic O 2p character, which allocates these O 2p states on top of those interacting with transition metals in the O-valence band, thus promoting reversible oxygen redox. This innovative design contributes smooth voltage profiles and high structural stability. Na0.7Mg0.05[Mn0.6Ni0.2Mg0.15]­O2 exhibits superior electrochemical performance, especially good capacity retention at high current rate under a high cutoff voltage (4.2 V). A new P2 phase is formed after charge, rather than an O2 phase for the unsubstituted material. Besides, multiple intermediate phases are observed during high-rate charging. Na-ion transport kinetics are mainly affected by elemental-related redox couples and structural reorganization. These findings will open new opportunities for designing and optimizing layer-structured cathodes for sodium-ion batteries.

O3‐Type Layered Ni‐Rich Oxide: A High‐Capacity and Superior‐Rate Cathode for Sodium‐Ion Batteries

“…Based on the fitted results and Randles–Sevcik formula (Equation (S1), Supporting Information), we have calculated the D Na+ value corresponding to A oxidation peak and C reduction peak to be 8.36 × 10 −12 and 3.56 × 10 −12 , respectively. Actually, Na + ions diffusion coefficient is mainly affected by redox couples and structural reorganization . Since CV measurement is sensitive to phase transition, this technique has a poor monitoring ability for the Na + diffusion kinetic in the single‐phase voltage region.…”

“…It is clearly demonstrated in Figure c,d that the GITT curves show smaller overpotential values between 2.6 and 3.5 V (P3 phase), indicating faster reaction kinetics than at low voltage region (2.0–2.6 V, O3 phase) and high voltage region (3.5–4.0 V, P″3 phase). The phase transition (O3‐P3 and P3‐P″3) indicated by voltage plateaus would result in the fluctuation of diffusion coefficients . Comparatively, the smooth voltage slope at 2.6–3.5 V is indicator of Na + fast extraction/insertion in the single phase (P3).…”

Unraveling the Cationic and Anionic Redox Reactions in a Conventional Layered Oxide Cathode

“…(Figure 2a, b), where the change of the half-edge energy (E 0.5 ) is deduced by the half-height method. [40][41][42] At pristine state, the E 0.5 of the Ni K-edge in LiNiO 2 corresponds to Ni 3+ and E 0.5 value increases to compensate for the Li + extraction from pristine material towards 4.8 V charge, with slight reduction at 5.0 V charge, which maybe related to the anionic oxygen redox according to the reductive coupling theory or Ni reduction on the surface upon oxygen release. 31,43 Moreover, Ni chemical state is characterized by soft XAS total fluorescence yield (TFY) and total electron yield (TEY) mode to reveal information close to bulk (~200 nm) and at the surface (~10 nm), respectively.…”

Core–Shell Structure and X‐Doped (X = Li, Zr) Comodified O3‐NaNi0.5Mn0.5O2: Excellent Electrochemical Performance as Cathode Materials of Sodium‐Ion Batteries

“…As Li + and Zr 4+ ions have smaller ionic radii (0.76 Å for Li + , 0.72 Å for Zr 4+ ) as the Na + ion (1.02 Å for Na + ), when sodium ions are replaced, there exhibits a decrease in the c parameter (15.99266 Å for CZS‐0.05, 15.96236 Å for CLS‐0.05, and 16.01565 Å for pristine). Zr 4+ ion are more positively charged than Na + ion meaning that Zr 4+ has a greater electrostatic attraction with nearby O layers …”