P'2-Na2/3[Mn1-X
Sc
x
]O2 Electrode in Sodium-Batteries
Authors: Moriya Kodai , Miura Yusuke , Kim Eun Jeong , Hosaka Tomooki , Tatara Ryoichi , Kumakura Shinichi , Komaba Shinichi 
Published: February 2024

In recent years, Na-ion batteries (SIB) are expected to be an alternative to Li-ion batteries. One of the cathode materials for SIB is transition metal layered oxides Na
x
TMO2 (TM = transition metal). In order to improve their capacity, efforts have been made to utilize the redox process(es) of not only TM ions but also oxide ions (oxygen redox). In addition, partial replacement of TMs with electrochemically inactive elements has been investigated to enhance cycle stability. Our group reported P2-type Na
x
MnMeO2 (Me = Li, Mg) exhibit higher capacity than existing materials via using additional redox of oxide ions (1, 2). Another example delivering high capacity reported in our group is P’2-type Na2/3MnO2, where the average oxidation states of Mn is about +3.37 and a co-operative Jahn-Teller distortion (CJTD) of Mn3+ (t2g
3-eg
1) is shown (3). In this study, we synthesized P’2-Na2/3Mn1-x
Sc
x
O2 and studied the effect of Sc substitution on the electrochemical properties of P’2-Na2/3MnO2.
P’2-Na2/3Mn1-x
Sc
x
O2 (x = 0, 0.08, 0.11) samples were synthesized using Na2CO3, Mn2O3, and Sc2O3 as starting materials, based on the synthesis method of the non-doped P’2-Na2/3MnO2 as we previously reported (3). For galvanostatic cycling tests, a working electrode consisted of active materials, acetylene black, and PVdF in 80:10:10 wt.% on Al foil. Na metal was used as a counter electrode, and an electrolyte used was 1.0 mol dm-3 NaPF6 PC solution.
XRD patterns of P’2-Na2/3Mn1-x
Sc
x
O2 are shown in Fig. 1 (a). Major diffraction peaks can be indexed as orthorhombic Na2/3MnO2 with space group of Cmcm, confirming P’2-type layered oxides. Note that non-substituted Na2/3MnO2 shows a superstructure peak at 23.9º in 2θ attributed to in-plane superstructure (3). At x = 0.11, impurity of Sc2O3 is observed, indicating that Sc is substituted for Mn up to about 8% in P’2-Na2/3MnO2. Sc3+ ions which has a large ionic radius than Mn3+ ions and no Jahn-Teller distortion, resulted in the enlarged lattice constant a (Figure 1 (a), inset) and the reduced lattice distortion (4).
Fig. 1 (b) shows the discharge capacity and the coulombic efficiency of P’2-Na2/3Mn1-x
Sc
x
O2 Electrode in Na cell. The capacity retention after 50 cycles for the non-substituted x = 0 (non-sub) and Sc-substituted x = 0.08 (Sc-sub) in the voltage range of 1.5 – 4.4 V was 66.4% and 85.4%, respectively. In-situ XRD data can support better cycling life. The Sc-sub exhibited better rate characteristics than the non-sub, indicating that there is an optimal amount of Sc to improve the electrochemical properties.
Looking at the initial charge/discharge curves at different cutoff voltages (Fig. 1 (c)), setting the upper cutoff voltage beyond 3.47 V, causes a significant polarization due to the phase transition from P’2 to OP4. From the cycle stability tests at different upper cutoff voltages (Fig. 1 (d)), the upper cutoff limit of 4.0 V demonstrates notable capacity fade than that of 4.4 V. One possible reasons should be changing the surface chemistry of electrode by applying higher voltage than 4.0 V. We will further discuss the effects of Sc substitution and charge/discharge voltage range on the electrochemical properties.
References

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Yabuuchi, S. Komaba, et al., Adv Energy Mater., 4, 1301453 (2014).

Kumakura, S. Komaba, et al., Angew. Chem. Int. Ed., 55, 12760 (2016).

S. Kumakura, S. Komaba, et al., Chem Mater., 29, 8958 (2017).

Figure 1