P2-Type Na2/3Ni1/3Mn2/3-

x
Ti
x
O2 as a 3.7 V Class Positive Electrode for Na-Ion Batteries
Authors: Kubota Kei , Yoshida Hiroaki , Yabuuchi Naoaki , Ikeuchi Issei , Garsuch Arnd , Schulz-Dobrick Martin , Komaba Shinichi 
Published: February 2020

Introduction
Layered Na
x
MeO2 (Me = transition metal) compounds have been intensively studied as electrode materials for Na-ion batteries. Among them, P2-Na2/3Fe1/2Mn1/2O2 and O3-NaFe0.5Co0.5O2 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
x
MeO2 is usually lower than that of Li
x
MeO2, resulting in lower energy density of Na-ion cell. To increase the operating voltage of Na-ion batteries, we focus on P2-Na2/3Ni1/3Mn2/3O2 showing relatively high operating voltage based on a Ni2+/Ni4+ redox couple.3 The discharge capacity of Na//Na2/3Ni1/3Mn2/3O2 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 Na2/3Ni1/3Mn2/3-x
Ti
x
O2 as positive electrode materials.
Experimental
Na2/3Ni1/3Mn2/3-x
Ti
x
O2 (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 Na2/3Ni1/3Mn2/3-x
Ti
x
O2 (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 Na2/3Ni1/3Mn2/3-x
Ti
x
O2 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//Na2/3Ni1/3Mn1/2Ti1/6O2 cell. Among the substituted samples, the Na2/3Ni1/3Mn1/2Ti1/6O2 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//Na2/3Ni1/3Mn1/2Ti1/6O2 cell reaches 470 Wh kg-1 and that of a hard-carbon//Na2/3Ni1/3Mn1/2Ti1/6O2 full cell achieves 300 Wh kg-1, corresponding to approximately 80% of energy density for the graphite//LiCoO2 system. Na2/3Ni1/3Mn1/2Ti1/6O2 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
[1] N. Yabuuchi, M. Kajiyama, J. Iwatate, H. Nishikawa, S. Hitomi, R. Okuyama, R. Usui, Y. Yamada and S. Komaba, Nat Mater, 11 (2012) 512.
[2] H. Yoshida, N. Yabuuchi and S. Komaba, Electrochemistry Communications, 34 (2013) 60.
[3] L. Zhonghua and J. R. Dahn, J Electrochem Soc, 148 (2001) A1225.
[4] D. H. Lee, J. Xu and Y. S. Meng, Phys. Chem. Chem. Phys., 15 (2013) 3304.