P2-Type Na2/3Ni1/3Mn2/3-xTixO2 as a 3.7 V Class Positive Electrode for Na-Ion Batteries

Introduction
Layered Na_xMeO₂ (Me = transition metal) compounds have been intensively studied as electrode materials for Na-ion batteries. Among them, P2-Na_2/3Fe_1/2Mn_1/2O₂ and O3-NaFe_0.5Co_0.5O₂ can deliver 200 and 160 mAh g^-1 of reversible capacity with relatively good capacity retention, respectively.^{1, 2} However, average operating potential of Na_xMeO₂ is usually lower than that of Li_xMeO₂, resulting in lower energy density of Na-ion cell. To increase the operating voltage of Na-ion batteries, we focus on P2-Na_2/3Ni_1/3Mn_2/3O₂ showing relatively high operating voltage based on a Ni²⁺/Ni⁴⁺ redox couple.³ The discharge capacity of Na//Na_2/3Ni_1/3Mn_2/3O₂ cell is, however, limited to only 80 mAh g^-1 because of rapid capacity decay during cycles by charge to 4.5 V.⁴ In this study, we present synthesis and improved electrode performance of titanium substituted Na_2/3Ni_1/3Mn_2/3-xTi_xO₂ as positive electrode materials.

Experimental
Na_2/3Ni_1/3Mn_2/3-xTi_xO₂ (x = 0, 1/6, 1/3, 2/3) samples were prepared by solid-state reaction from starting materials of Na₂CO₃, NiO, TiO₂, and Mn₂O₃. The mixture was pelletized and heated at 900-950 °C for 12 h in air. Crystal structures and morphology of the samples were examined by using powder X-ray diffraction (XRD) measurement and scanning electron microscopy (SEM). Electrochemical properties were tested using a coin-type cell. Positive electrodes consisted of the active material, acetylene black and polyvinylidenefluoride(PVdF) with a gravimetric ratio of 80:10:10. Metallic sodium was used as a negative electrode. The electrolyte used was 1.0 mol dm^-3 NaPF₆ / PC.

Results and discussion
All diffraction lines in XRD patterns of synthesized Na_2/3Ni_1/3Mn_2/3-xTi_xO₂ (x = 0, 1/6, 1/3, 2/3) samples can be assigned into the P2-type layered structure with space group P6₃/mmc without any impurities. The calculated lattice parameters linearly increase by the substitution of Ti for Mn, and its increase obeys Vegard’s law with the consideration of larger ionic size of Ti⁴⁺ than Mn⁴⁺, suggesting that a solid solution is successfully synthesized in the range of 0 ≤ x ≤ 2/3 in Na_2/3Ni_1/3Mn_2/3-xTi_xO₂ samples. The substitution of titanium effectively improves capacity retention even though reversible capacity decreases with the titanium substitution. Figure 1 shows charge/discharge curves of a Na//Na_2/3Ni_1/3Mn_1/2Ti_1/6O₂ cell. Among the substituted samples, the Na_2/3Ni_1/3Mn_1/2Ti_1/6O₂ particularly delivers 127 mAh g^-1 of reversible capacity with ca. 3.7 V of average discharge voltage for the initial cycle with relatively good capacity retention. Ex-situ XRD results reveal that volume shrinkage of the fully charged state is effectively reduced from 23% for Ti-free to 12-13% for Ti-substituted ones, which probably leads to the improved cycle stability. Estimated energy density of a Na//Na_2/3Ni_1/3Mn_1/2Ti_1/6O₂ cell reaches 470 Wh kg^-1 and that of a hard-carbon//Na_2/3Ni_1/3Mn_1/2Ti_1/6O₂ full cell achieves 300 Wh kg^-1, corresponding to approximately 80% of energy density for the graphite//LiCoO₂ system. Na_2/3Ni_1/3Mn_1/2Ti_1/6O₂ is one of the higher energy density materials for Na-ion batteries with good cyclability among layered oxides reported so far. Influence of the titanium substitution on structural changes during charge and electrochemical properties will be further discussed.

References
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