The rapid development of renewable and sustainable energies requires large scale energy storage systems due to their characteristics of intermittence, randomness, and instability.^{[1, 2]} Sodium-ion batteries operated at room-temperature have been attracting increasing interest for such applications because of the abundance of Na resources and the potential low cost compared to lithium-ion batteries, which are more suitable for portable electronic devices and electric vehicles.^{[3– 6]} Up to now, numerous positive electrode materials with different crystal structures have been investigated for Na extraction/insertion.^{[7– 14]} Among them, layered metal oxides have long been exploited as an important class of hosts for rechargeable batteries.^{[13, 15– 35]} For instance, LiCoO₂ is one of the most famous and successful positive electrode materials for lithium-ion batteries.^[1] Since the discovery of Na_xCoO₂, which is able to store Na, ^[15] many other layered oxides have been extensively studied. However, in order to achieve better Na storage performance, most of them contain toxic and expensive transition metals of Ni and/or Co, ^{[15, 16, 18, 21, 23– 25, 28– 35]} which are also commonly and widely used for lithium-ion batteries. This will limit their large-scale application. Therefore, it is essential to explore new layered oxides with cheaper and environmentally-benign transition metals. For example, we recently found that the Cu²⁺ /Cu³⁺ redox couple is highly reversible in Na-containing layered oxides with high operation voltage.^[36] This important finding reveals that we can use cheaper, non-toxic Cu instead of Ni or Co as a building block for the layered oxides. In this work, we further demonstrate that we can incorporate Cu and Fe into an O3-type layered oxide to form quaternary Na_0.9Cu_1/4Fe_1/4Mn_1/4Ti_1/4O₂, which shows a good Na storage performance in terms of both high storage capacity and high operating voltage.

The Na_0.9Cu_1/4Fe_1/4Mn_1/4Ti_1/4O₂ material was synthesized by a simple solid-state reaction using precursors of Na₂CO₃ (99%), CuO (99.8%), Fe₂O₃ (99.5%), Mn₂O₃ (99%), and TiO₂ (99.8%). They were ground uniformly in stoichiometric proportion and heated in air at 850  ° C for 15  h to form the final products. Powder X-ray diffraction (XRD) was performed on a Bruker D8 Advance diffractometer equipped with a Cu Kα radiation source (λ ₁ = 1.54060  Å , λ ₂ = 1.54439  Å ). The morphologies and elemental mapping of the materials were investigated by a scanning electron microscope (SEM) (Hitachi S-4800).

The working electrode was prepared by spreading the slurry of the active material of Na_0.9Cu_1/4Fe_1/4Mn_1/4Ti_1/4O₂ (80  wt.%), acetylene black (15  wt.%), and the polyvinylidene fluoride (PVdF) (5  wt.%) binder on an Al foil. The working electrodes were dried at 110  ° C under a vacuum for 10  h. The electrolyte was 1  M NaClO₄ in propylene carbonate. The coin-type (CR2032) cells were assembled with a pure sodium foil as the counter electrode and a glass fiber as the separator in an argon-filled glove box. The charge and discharge measurements were carried out on a Land BT2000 battery test system (Wuhan, China) in the voltage range of 2.5– 4.0  V at room temperature.

Typically, layered metal oxides Na_xMO₂ (M: transition metal) can be mainly categorized as either O3 or P2 according to the definitions by Delmas et al.^[37] The letters O and P represent the Na coordination environments of octahedral and prismatic sites, while the numbers refer to the number of repeating MO₂ slabs in the unit cell. The formation of O3- or P2-type depends on the sodium content and the synthetic conditions. Figure  1 shows the XRD pattern of the Na_0.9Cu_1/4Fe_1/4Mn_1/4Ti_1/4O₂ sample synthesized at 850  ° C in air. For comparison, the standard XRD pattern of O3-NaFeO₂ is also displayed in Fig.  1. It can be seen that all of the diffraction peaks of the resulting sample are in good agreement with those of NaFeO₂, indicating a typical O3 phase material with no impurity. By careful examination, some peaks such as (003) and (006) are found to shift slightly to lower angles compared with those of NaFeO₂. This observation is related to the Na content in this Na-deficient material. In general, in NaMO₂, when a small amount of Na is extracted from the layered structure, the c axis expands slightly, leading to a downshift of the (00l) peaks.

Fig.  1.

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	Fig.  1. XRD pattern of as-synthesized Na0.9Cu1/4Fe1/4Mn1/4Ti1/4O2 sample and the standard JCPDS of α -NaFeO2.

Figure  2(a) shows a typical SEM image of the quaternary Na_0.9Cu_1/4Fe_1/4Mn_1/4Ti_1/4O₂ sample. The particle sizes are in the range from 2  μ m to 5  μ m. The surface morphology of the sample is revealed as steps in Fig.  2(b), which is a typical feature of the layered oxides. In order to check the rough distribution of four transition metals, energy dispersive spectroscopy (EDS) was performed. The EDS elemental mapping results in Fig.  3 reveal a homogeneous distribution of Cu, Fe, Mn, and Ti elements in the material. No aggregation of any single element was observed.

Fig.  2.

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	Fig.  2. Typical SEM images with different magnifications of the Na0.9Cu1/4Fe1/4Mn1/4Ti1/4O2 sample.

Fig.  3.

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	Fig.  3. EDS elemental mapping of the Na0.9Cu1/4Fe1/4Mn1/4Ti1/4O2 sample: (a) SEM image, (b)– (f) the corresponding elemental mapping of Na, Cu, Fe, Mn, and Ti, respectively.

To investigate the Na de-intercalation/intercalation behavior in the layered Na_0.9Cu_1/4Fe_1/4Mn_1/4Ti_1/4O₂ electrode, galvanostatic charge and discharge at a current rate of C/10 (10  mA/g) were carried out in sodium half cells. The results are shown in Fig.  4(a). In the first charge process, a short plateau at ca. 3.2  V was observed followed by a long slope until 4.0  V. This short plateau indicates a two-phase reaction with a phase transition from O3 to P3, which is a common characteristic of O3-type materials. The other feature, at the end of the discharge profile, is a short plateau at ca. 3.0  V, which is much higher than that of other O3-type materials such as O3-NaFe_1/3Ni_1/3Mn_1/3O₂, O3-NaNi_0.5Mn_0.5O₂, Na_xFe_0.5Mn_0.5O₂, Na[Ni_0.4 Fe_0.2Mn_0.2Ti_0.2]O₂.^{[24, 23, 26, 30, 33]} This could be due to the incorporation of Cu and/or Ti elements into the transition metal layer.

Fig.  4.

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	Fig.  4. (a) The typical first galvanostatic charge and discharge profiles of the Na0.9Cu1/4Fe1/4Mn1/4Ti1/4O2 electrode cycled between 2.5  V and 4.0  V at a current rate of C/10. (b) Cycle performance of the Na0.9Cu1/4Fe1/4Mn1/4Ti1/4O2 electrode cycled between 2.5  V and 4.0  V at a current rate of 1C.

The first charge and discharge capacities are 110  mAh/g and 94  mAh/g, respectively, corresponding to a Coulombic efficiency of 85%. The average discharge voltage is ca. 3.2  V. This high operation voltage could be attributed to the co-existence of Cu²⁺ /Cu³⁺ and Fe³⁺ /Fe⁴⁺ redox couples with a relatively high voltage in the transition metal layer. The resulting quaternary Na_0.9Cu_1/4Fe_1/4Mn_1/4Ti_1/4O₂ also exhibits moderate rate capability and cyclic performance as shown in Fig.  4(b). The charge and discharge capacities at a current rate of 1C are 95.5  mAh/g and 78  mAh/g, respectively. After 90 cycles, capacity retention of 60% was achieved.

In this work, following our first discovery of the high reversibility of the Cu²⁺ /Cu³⁺ redox couple in Na-containing layered positive electrode materials, we further demonstrate that Cu and Fe can be incorporated into an O3-type layered oxide to form Na_0.9Cu_1/4Fe_1/4Mn_1/4Ti_1/4O₂, which was prepared by a simple solid-state reaction method. The resulting material is able to deliver a reversible capacity of 94  mAh/g with moderate rate capability and cyclic stability. We believe that the Na storage performance can be further improved in this series of new layered oxides through tuning the compositions of Na_aCu_{1− x− y− z− d}Fe_xMn_yTi_zD_dO₂ (D: dopant, e.g., Li, Mg, Al, etc., 0< x< 1, 0< y< 1, 0 ≤ z< 1, 0 ≤ d< 1, 0.6< a ≤ 1) and controlling how oxygen stacks to form O3, P2, O'3, P3 , etc., which will show great promise for practical application in sodium-ion batteries.