Impact of Sc Doping in Layered Nameo2 Oxides on Their Electrode Performance
Authors: Moriya Kodai , Kim Eunjeong , Tatara Ryoichi , Kumakura Shinichi , Komaba Shinichi 
Published: December 2024

In recent years, Na-ion batteries (SIBs) are expected to be an alternative to Li-ion batteries for certain applications[1]. A possible candidate of the cathode materials for SIBs is transition metal layered oxides Na
x
MeO2 (Me = metal element(s)). Among them, O3-type Na[Ni1/2Mn1/2]O2 is considered promising due to its high energy density[2]. In order to improve cycle performance, controlling the morphology and surface modification are known to be effective for improving cycle performance. Our group has studied improvement of cycle performance through a technique known as heteroatomic substitution, thus we here focus on Sc3+ which has not been a lot of attention. While Sc3+ is redox-inactive within the layered oxide[3], partial substation with inactive Mg2+ and Ti4+ improves cycle stability[4]. In this study, we synthesized O3-Na[(Ni1/2Mn1/2)1-xScx]O2 and studied the effect of Sc3+ substitution on the electrochemical performance of O3-Na[Ni1/2Mn1/2]O2 and compared with the case of the doping in other O3 and P2-type Mn based oxides.

O3-Na[(Ni1/2Mn1/2)1-xScx]O2 (x = 0, 0.08, 0.11) samples were synthesized using Na2CO3, Ni(OH)2, Mn2O3 and Sc2O3 as starting materials, based on the synthesis method of the non-doped O3-Na[Ni1/2Mn1/2]O2 as we previously reported[2]. To assess the electrochemical properties of P’2-Na2/3[Mn1-x
Me
x
]O2 samples, we employed a working electrode consisted of active materials, acetylene black, and PVdF in 80:10:10 wt.% on Al foil. Na metal served as a counter electrode, with a 1.0 mol dm-3 NaPF6 PC solution as an electrolyte.

XRD patterns of O3-Na[(Ni1/2Mn1/2)1-xScx]O2 are shown in Fig. 1 (a). Major diffraction peak can be indexed as rhombohedral Na[Ni1/2Mn1/2]O2 with space group of R-3m, confirming O3-type layered oxide. It also confirms that Sc3+ occupied the transition metal layer (Ni, Mn site) through the Rietveld refinements. At x = 0.11, other O3 phase with a different lattice parameter was identified as an impurity, indicating that Sc is substituted for Ni/Mn up to about 8% in O3-Na[Ni1/2Mn1/2]O2 despite being able to directly crystalize O3-NaScO2 via solid state reactions. Sc3+ (0.745 Å) which has a large ionic radius than Ni2+ (0.690 Å) and Mn4+ (0.645 Å), resulted in the enlarged lattice constant a (Fig. 1 (a), inset).

Fig. 1 (b) shows the cycle performance with initial charge/discharge curves of O3-Na[(Ni1/2Mn1/2)1-xScx]O2 electrode in Na cell. In the charge/discharge curve, in non-substituted x = 0 (NNMO), a stepwise curve was observed, whereas in Sc-substituted x = 0.08 (NNMSO8), curve shape changed smoothly. The capacity retention after 50 cycles was remarkably improved from 41.0% to 70.7% by the Sc doping. From analyses of the structural change during cycling, we found that the disappearance of Na/vacancy ordering and the reduction in volume change have led to enhance cycle stability of NNMO by Sc3+ doping.

Based on the results obtained so far, we attempted to compare the effects of further Sc3+ substitution into other materials, P’2/P2-Na2/3MnO2
[5]. From considering the characteristics of each non-doped materials, we verified how Sc3+ substitution affects the cycle performance for each layered oxide. We will further discuss the electrochemical performance when coating with scandium oxide like NaxScO2 is applied to the O3-Na[Ni1/2Mn1/2]O2, in order to understand the impact of Sc3+ on electrochemical properties of layered oxide.

References

[1] N. Yabuuchi, S. Komaba, et. al., Chem. Rev., 114, 11636 (2014).

[2] S. Komaba, et. al., Inorg. Chem., 51, 6211 (2012).

[3] K. Kubota, S. Komaba. et. al., Adv. Energy. Mater., 8, 1703415 (2018).

[4] K. Kubota, S. Komaba, et. al., J. Mater.
Chem. A., 9, 12830 (2021).

[5] K. Moriya, S. Komaba, et. al., ECS 244th Meet. Abstr., 4, 794 (2023).

Figure 1