A Honeycomb‐Layered Oxide Cathode for Sodium‐Ion Batteries with Suppressed P3–O1 Phase Transition

DOI: 10.1002/aenm.201601698 sliding and Na+/vacancy ordering during the Na+ insertion/ extraction process, accompanied by significant cell volume variations at each transition step (see Table 1, especially, the P3–O1 transition involves a volume variation of 98.7%). [10] Such a large volume variation makes the cathode structure easily collapse, deteriorating its cycling performance in SIBs. [11] To address the above problem, here we show a structure optimization of the Na3Ni2SbO6 cathode through a partial substitution of Ni2+ by Mg2+, an electrochemically inert cation with the same valence and an ionic radius (0.72 Å) close to that of Ni2+ (0.69 Å). The similarities between Mg2+ and Ni2+ ensure a homogeneous incorporation of Mg2+ into the TM layer without affecting its honeycomb-layered structure, yielding O3-type cathode materials with a formula of Na3Ni2−xMgxSbO6 (x = 0, 0.1, 0.3, and 0.5). Moreover, an electrochemical mechanism study combining galvanostatic intermittent titration technique (GITT) and ex situ X-ray diffraction (XRD) reveals an effective suppression of the P3–O1 phase transition at 3.64 V after the introduction of Mg, which establishes the linkage between the elimination of P3–O1 phase transition and a much improved cycling performance of the cathode. The Na3Ni2−xMgxSbO6 compounds were synthesized by a facile and scalable high-temperature solid-state approach as detailed in the Experimental Section. Figure S1 (Supporting Information) summarizes the XRD patterns of these compounds. All the samples show XRD peaks ascribable to a monoclinic lattice with the C2/m space group without any impurities, demonstrating a uniform incorporation of Mg2+ into the crystal lattice of the Na3Ni2SbO6. The Rietveld refinement of the XRD data in Figure 1 shows that Mg substitutes for Ni in the TM layer. According to the refined crystallographic data summarized in Tables S1 and S2 (Supporting Information), the Na3Ni2−xMgxSbO6 (x = 0.3) increases slightly the lattice parameters (a = 5.30812 Å, b = 9.19032 Å, c = 5.63888 Å, and β = 108.16°) compared with the unsubstituted ones (a = 5.30232 Å, b = 9.18028 Å, c = 5.63753 Å, and β = 108.40°), which can be attributed to a larger ionic radius of Mg2+ (0.72 Å) than that of Ni2+ (0.69 Å). The weak diffraction peaks at 2θ ≈ 20° in the XRD patterns (Figure 1a,b) represent stacking defects along the monoclinic c axis. Figure 1c shows the pristine structure of Na3Ni2SbO6, which is arranged in accordance with the typical O3 stacking sequence and consists of honeycomb-ordered TM layers with each SbO6 octahedron surrounded by six NiO6 octahedra. After Mg substitution, the honeycomb-layered structure is well maintained (Figure 1d), as Mg2+ takes the position of Ni2+ because of the size and valence similarities between the two cations. The morphologies of the prepared samples were visualized by scanning electron microscopy (SEM) as shown in Figure S2 (Supporting Information). All the Na3Ni2−xMgxSbO6 Recently, sodium-ion batteries (SIBs) have become an appealing choice for the next-generation sustainable energy storage due to a significant resource advantage of Na over Li. [1] In realizing key-enabling materials for SIBs, great efforts have been made onto the Na-intercalation cathode materials, among which the layered transitional-metal (TM) oxides with a general formula of NaxMO2 (M = Ni, Co, Mn, Fe, Cu, etc.) have attracted the most attention since the early 1980s. [2] According to Delmas et al.,[3] these layered metal oxides can be classified into P2 and O3 types based on the Na occupation sites (prismatic Na in P2 and octahedral Na in O3) in their crystal structure. Although the P2-type cathode material (Na/M < 0.67) delivers a high capacity in Na half-cells, its initial charge capacity is too low to meet the requirement of full-cells. [4] In comparison, the O3-type cathode materials (Na/M > 0.67) are an ideal Na+ reservoir to realize a practical Na-ion battery. [5] For example, Hu and co-workers demonstrated a 100 W h kg−1 practical prototype cell, employing air-stable O3-type Na0.9[Cu0.22Fe0.30Mn0.48]O2 cathode and a low-cost pyrolyzed anthracite anode. [1k,5d,e] As a representative O3 cathode material, NaNiO2 holds an acceptable capacity of 120 mA h g−1 at 3.75–1.25 V versus Na+/Na and an improved electrochemical stability compared with LiNiO2 due to the elimination of Ni/Na disordering, showing the potential to be employed in SIBs. [6] However, the discharge–charge process of NaNiO2 in SIBs involves an unfavorable transition between Ni3+ and Ni4+ (which may catalyze the electrolyte decomposition and irreversible formation of NiO) and a relatively low operating voltage. [7] Aiming to overcome these problems, researchers have tried to introduce cations with high valances, such as Sb5+, Te6+, and Bi5+, which gave rise to ordered cation arrangements in the TM layer to generate a honeycomb-layered structure. [8] For example, Na3Ni2SbO6, with each SbO6 octahedron surrounded by six NiO6 octahedrons to form the TM layer, shows significant electrochemical advantages over NaNiO2. First, the substitution of 1/3 of Ni by Sb could lower the valence of Ni to +2, which enables the Ni2+/Ni3+ redox couple to stabilize the Ni electrochemistry. On the other hand, the intense electrostatic repulsion between Ni2+ and Sb5+ results in a high voltage (an average value of 3.3 V vs Na+/Na) compared with NaNiO2 without sacrificing its specific capacity. Nevertheless, the honeycomb-layered Na3Ni2SbO6 cathode undergoes a complex O3–P3–O1 phase transition due to a continuous TM layer