English

• With the significant demand for rechargeable and clean-electrical energy storage, sodium-ion batteries (SIBs) are emerging as an alternative resource candidate to the grid, electric vehicles, and portable electronic devices because of their abundance, low cost as well as reliable sources [1-5]. The layered structures of Na_xTMO₂ (TM = consisting of one or more 3d orbital transition metal) are commonly made up of edge-sharing TMO₆ octahedra and sodium atoms with different sites, which can be mainly divided into P2-type and O3-type according to the local surroundings of Na⁺ (the prismatic or octahedral), stacking sequence of nonequivalent TMO₂ slabs, and difference of oxygen atoms [6,7]. P2-type oxides usually yield faster Na⁺ conductivity, high power density as well as good rate performances. Among them, manganese-based P2-type oxides also have attracted enormous attention due to their satisfactory electrochemical performance, low cost, and nontoxicity cathode systems for SIBs. Indelibly, influenced by Na⁺/vacancy ordering and gliding of the TMO₂ slabs, irreversible phase transitions were induced upon Na⁺ intercalation/deintercalation during the subsequent cycling. Slab gliding gratifies energetic balance, as increasing extraction of Na⁺, therefore arising octahedral vacancies and decreased interlayer spacing, forming the stacked fault OP4 (described as "Z" phase) or O2 phase [8,9]. Moreover, the notorious tetragonal Jahn-Teller distortion lowers the energy of a system by certain bond length distortions to break the local crystal symmetry and lift the degeneracy of the electronic system [10]. In which high spin Mn³⁺ (t_2g³e_g¹) inspires disproportionation from soluble Mn²⁺ and mutating of electrostatic interactions in adjoining atoms also leading to the grievous irreversible attenuation of capacity [11,12].

  Therefore, various perspectives have been devoted to the design of modification methods with enlarged structures, tailoring morphology, restraining vacancy ordering, complementary cations valence states, tuning sodium occupancy sites, and even theoretical predictions, etc. [13-21]. On the purpose of relieving volume changes and catering proper Na⁺ diffusion paths, a shale-liked Na_0.67MnO₂ via expanding the Na⁺ layer spacings to achieve near-zero-strain, which delivered a capacity of 95 mAh/g at 960 mA/g within the voltage range of 2.0–4.0 V [22]. Another strategy is to regulate the uniform distribution of Na⁺ in favor of P2-type Na_2/3Mn_1/2Ni_1/6Co_1/3O₂ cathode with the ultralow volume change of 1.9% in the voltage of 1.5–4.5 V [23]. Oxygen-redox chemistry and bulk oxygen vacancies also could effectively relieve the loss of O²⁻/Oⁿ⁻ redox and create Mn domains to resist the Jahn-Teller distortion [24,25]. Furthermore, ions substitution with various valence states and ion radii act as the pillars to support the robust framework among slabs. Such as redox active metal Ag⁺, Fe³⁺, Ce⁴⁺, Nb⁵⁺, Mo⁶⁺ as well as the electrochemical inert elements of Li⁺, Mg²⁺, Ca²⁺, Al³⁺, Si⁴⁺, F⁻, B⁻ and lanthanide, etc. They are also contributing to disrupting Na⁺/vacancy ordering, enlarging channels of Na⁺ migration, and efficaciously alleviation structure distortion, which is responsible for the improvement of electrochemical performances [26-39].

  Although ions substitute has been intensively investigated, where the contrast of the doped elements tends to be one or two types and chemical components also show differences. It should be pointed out that the different radii and valences render different performances. The TM elements of the subgroup of 3d orbital (e.g., Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn elements) with the same outer electronic configuration and a different number of electrons are possessed electrochemically active in layered oxides. Wherein, 3d orbital elements also have similar ionic radii and different redox potentials that could renew the TMO₂ slabs avoiding Jahn-Teller distortion and ordering of Na⁺/vacancy resulting in structural collapse. In addition, considering that doped Sc³⁺ has not been reported in SIBs, the counterparts of the LiNi_0.5Mn_1.5O₄ electrode were confirmed to enhance Li⁺ diffusion kinetics [40]. The foreign ions decided on local structure rather than all regions and guided by these, the effect mechanism between the doped element and structure evolving need to be clarified, which could be beneficial for the selection of doped ions of P2-type to come up with a robust skeleton and fulfill higher energy density [41].

  Herein, P2-type Na_0.50Li_0.08Mn_0.60Co_0.16Ni_0.16O₂ was adopted as a matrix material to study the impact of the changes of systematically doping Sc, Ti, V, Cr, Fe, Cu and Zn elements in the cathode materials. The strategy of compressing TM layered while broadening sodium layered in P2-type cathode materials could model a robust structure and expedite diffusion kinetics, indicating the smoothing processes of Na⁺ during the charge/discharge as well as improved electrochemical performance. Besides, the Rietveld refinements help to verify the hypothesis, in which the synergistic effect between the TMO₂ layer and d-spacing could enhance the structural stability and promote Na⁺ migration. Moreover, NCMSc_0.02, NCMFe_0.02, and NCMCu_0.02 electrodes suffer from reversible structure evolutions, which are reflected in the slight shift of (002), (004), (101), (106) as well as (012) diffraction peaks.

  The element quantitative analysis was confirmed by ICP-OES measurement for sintered samples listed in Table S1 (Supporting information), corresponding results of element composition are both consistent with the target designs. The powder XRD pattern of the precursor is shown in Fig. S1a (Supporting information), which is consistent with the Bragg positions for Mn₃O₄, Co₃O₄, Ni(OH)₂, Mn(OH)₂ and Co(OH)₂. Similarly, the sintered samples of NCM, NCMSc_0.02, NCMTi_0.02, NCMV_0.02, NCMCr_0.02, NCMFe_0.02 and NCMZn_0.02 are compared under the normalized method (Fig. S1b in Supporting information), confirming that all samples can be indexed into a major phase of the layered P2-type structure with P63/mmc (PDF#54–0894) space group. In detail, some additional Bragg peaks are arising from the superstructure due to the Na⁺/vacancy ordering, and the split peak is related to the appearance of the symmetry structure of the orthorhombic (with Pnma space group), as marked by stars and rounds separately, which also has two kinds of Na positions (share faces or edges), but provided fewer Na⁺ active sites than P2-type, specifically, the trigonal prism shares edges with TMO₆ is preferred for a broad range of Na contents (schematic of the crystal structure is shown in Fig. S2 in Supporting information) [42]. The cell parameters are further calculated by the GSAS software package (Fig. 1) [43,44]. From the structure refinements in Table S2 (Supporting information), the lattice parameters a/b, c, and V of doped samples are changed with various radii and valences comparing the prototype material of NCM (a = b = 2.85541 Å, c = 11.20723 Å and V = 79.1347 ų), affirming that the doped ions are successfully integrated into the crystal structure (Fig. 1i).

  Figure 1

  

  Figure 1.  The XRD patterns with corresponding Rietveld refinement plots of (a) NCM, (b) NCMSc_0.02, (c) NCMTi_0.02, (d) NCMV_0.02, (e) NCMCr_0.02, (f) NCMFe_0.02, (g) NCMCu_0.02 and (h) NCMZn_0.02 samples, respectively. (i) Comparison of refinement parameters for eight specimens.

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  The long-term cycling durability and rate performances are conducted to evaluate the positive electrode materials. The radius of seven ions is ranked as follows: Cr⁶⁺ < V⁴⁺ < Ti⁴⁺ < Fe³⁺ < Cu²⁺ < Zn²⁺ < Sc³⁺, while Figs. 2a and b show that the reversible discharge capacity of NCM, NCMSc_0.02, NCMTi_0.02, NCMV_0.02, NCMCr_0.02, NCMFe_0.02, NCMCu_0.02 and NCMZn_0.02 are 108, 104, 116, 86, 63, 112, 116 and 101 mAh/g at 0.5 C for 100 cycles, corresponding to the capacity retentions of ~84.2%, ~82.0%, ~85.9%, ~73.5%, ~56.3%, ~86.7%, ~89.4%, and ~85.5%, even the average discharge capacity are 49, 82, 78, 64, 10, 72, 86 and 82 mAh/g at 10 C, respectively. It should be noted that nearly all the composite phase exhibits fewer discharge capacities than the single P2-phase except for titanium doped, which is due to the decreased active sites in the orthorhombic phase. Whereas titanium doped could suppress the irreversible multiphase transformation accompanied by voltage increase by enlarging interslab distance, thus delivering higher reversible capacity for 100 cycles [45]. Conversely, the atomic coordination changed owing to the introduced chromium and vanadium ions, which higher valence and smaller radii than the matrix structure decided the local atomic distribution inclined to crystal lattice distortion. To meet potential battery applications, the energy densities of 274.9 Wh/kg in the half cell at 0.5 C for 100 cycles, after copper doping, which is higher than 252.7 Wh/kg for the prototype (Fig. S3 in Supporting information).

  Figure 2

  

  Figure 2.  (a) Cycling stability and (b) rate performances between 1.5 V and 4.2 V of NCM, NCMSc_0.02, NCMTi_0.02, NCMV_0.02, NCMCr_0.02, NCMFe_0.02, NCMCu_0.02 and NCMZn_0.02, respectively. (c-e) Schematic diagram of structures in various directions.

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  The galvanostatic charge/discharge and corresponding different capacity (dQ/dV) curves of NCM and other doped cathodes are characterized (Fig. S4 in Supporting information), which exhibits the voltage platform and reversible capacity at 0.5 C for 3^rd, 5^th, 10^th, 50^th and 100^th cycles. Moreover, there are about five couples of redox peaks at around 2.0 V, 3.36/3.28 V, 3.57/3.44 V, 3.93/3.81 V in dQ/dV curves and CV curves (Figs. S4a-h and S5 in Supporting information), corresponding to the redox reactions of Mn⁴⁺/Mn³⁺, Co³⁺/Co⁴⁺, Ni²⁺/Ni³⁺, Ni³⁺/Ni⁴⁺couples, respectively [46]. The dQ/dV curves delivered that the enhancement of cycle performance was related to the increase of structural reversibility. The redox peaks around 2.0 V are gradually absent in NCMTi_0.02, NCMV_0.02, NCMCr_0.02, and NCMZn_0.02 samples indicating that Mn²⁺ is dissolved in the electrolyte [47]. The superstructure materials would result in cracks, interface instability, and further consumption of electrolytes thus presenting a larger polarization along with local inactivation while cycling, which is consistent with electrochemical performances [48]. In comparison with counterparts, doped samples exhibit smoother galvanostatic curves and lower peaks intensities (Figs. S4i-p in Supporting information), especially for the NCMCu_0.02 electrode also delivered less voltage decrease along with the rising of cycles, which demonstrates that ions doped (Sc³⁺, Fe³⁺, and Cu²⁺) promote the disordering of Na⁺/vacancies [49]. Meanwhile, the dQ/dV curves for 100 cycles present good coincidence and demonstrate that Cu²⁺ doped decrement lattice distortion of Mn³⁺ and thereby strengthening the reversibility.

  Although the electrochemical performances are divergent, the refinement structure could reveal nuance (Figs. 2c-e). In P2-type materials, Na⁺ could occupy two different types of trigonal prismatic sites, where they either share faces (Na_f) or edges (Na_e) with TMO₆ octahedra because of the large Coulombic repulsion between the two adjacent Na⁺ [50]. The Na⁺ in the Na_e sites are usually via the Na_f site for extraction/insertion, the crystal structure thus suffers from a series of TMO₂ slabs gliding (uneven force along with the direction of the arrows) to form the octahedral site and stacking sequences of oxide layers (Figs. 2c-e) [51]. Nevertheless, the d-spacing, TMO₂ layer, as well as interlayer distance could help to research the interrelationships, which are contrasted in Table S3 (Supporting information). For example, the NCMCu_0.02 sample shows approximate TMO₂-layer yet larger d-spacing than NCMSc_0.02, while the NCMCr_0.02 and NCMV_0.02 samples show enlarged parameters due to the appearance of orthorhombic phase, which both agreed with former electrochemical performances. The results of the Rietveld refinements reveal that compressing the TMO₂ layer and interlayer distance in tandem with extending the d-spacing are rendered for a robust framework of P2-type structures and expedited diffusion channels, which is conducive to the extraction/embedding of Na⁺, structure stability as well as improving electrochemical performances. From the above results, we have concluded that the relatively higher ion radius attained good structure configuration, while the lower valence state could change intermolecular interaction within interslabs facile Na⁺ kinetics and enhanced the concentration of Mn⁴⁺ thus suppressing the cooperative Jahn-Teller distortion. Considering the phase composition, economic value, and research status, the NCM, NCMSc_0.02, NCMFe_0.02 and NCMCu_0.02 samples will be further discussed both from an electrochemical point of view and in situ XRD recorded during intercalation/deintercalation.

  The micro-morphology and element distribution were investigated by SEM and EDS mapping. In Fig. S6 (Supporting information), the typical uniformly nanosheet structure with a thickness range of 12.4–15.7 nm was synthesized via co-precipitation, and a thinner primary nanosheet with dispersal distribution could be conducive to mixing between carbonates and transition metal during the sintering process. After sintering (Fig. S7 in Supporting information), all of the as-prepared four samples exhibit micro-scale bulk characteristics, especially for the NCMCu_0.02 sample, which is made up of a series of thinner nanosheets stacked on top of each other. The corresponding EDS mapping for doped samples indicates that the doped elements of Sc, Fe and Cu are uniformly dispersed within the sintered samples (Fig. S8 in Supporting information), confirming that the Sc³⁺, Fe³⁺ and Cu²⁺ remained. TEM measurement was carried out to further confirm the lattice structure and micro-morphology, which displays a series of stacked structures with a side length of 1–3 µm in Figs. S7a₁-d₁. The d-spacing between the neighboring lattice fringes of four samples is about 0.25 nm and 0.56 nm, matching with the (100) and (002) planes of the P63/mmc space group. Meanwhile, the single P2 phase with a typical hexagonal lattice pattern is confirmed by SAED images, clearly showing the high ordering of the atomic arrangement, in which accordance with the XRD results.

  In order to verify the participation of transition metal elements and corresponding valences for four samples, the XPS analyses were performed (Fig. 3). It can be seen that all samples possess a similar survey spectrum (Fig. S9 in Supporting information). Two main peaks are located at ~642.1 eV and ~653.5 eV, indicating that Mn⁴⁺ and Mn³⁺ coexist in all samples (Fig. 3a) [52]. As compared with others, the feature of the slight split in Mn 2p_3/2 peak of the NCMCu_0.02 sample also demonstrates that more Mn⁴⁺ have existed after Cu²⁺ doped, which is beneficial for mitigating the Jahn-Teller effect and rendered for the stabilization of non-distortion P2 hexagonal crystal structure. The Co 2p_1/2 peak at ~795.4 eV and Co 2p_3/2 peak at ~780.4 eV as well as their corresponding satellite peaks can be ascribed to Co³⁺ and Co²⁺, respectively (Fig. 3b) [53]. The Ni 2p peak can be separated into four peaks, in which two distinct peaks appearing at ~872.1 eV and ~854.9 eV can be assigned to Ni 2p_1/2 and Ni 2p_3/2 respectively, another two satellite peaks also confirmed the existence of Ni²⁺ (Fig. 3c) [54]. The Ni²⁺ and Mn⁴⁺ are the Jahn-Teller inactive centers resulting in highly symmetric and stable crystal structures [55]. Moreover, the oxidation states of dopants are present in Figs. 3d-f. The peaks of Sc 2p_1/2 (401.8 eV) and Sc 2p_3/2 (405.7 eV) are assigned to Sc³⁺ [40]. By using a Gaussian fitting method, Fe 2p spectra can be assigned to the existence of Fe³⁺ [56]. The binding energies of 933.6 eV and 953.2 eV with corresponding satellite peaks have also confirmed the valence of Cu is +2 [57].

  Figure 3

  

  Figure 3.  XPS spectra of NCM, NCMSc_0.02, NCMFe_0.02 and NCMCu_0.02, respectively. (a) Mn 2p, (b) Co 2p, (c) Ni 2p, (d) Sc 2p, (e) Fe 2p and (f) Cu 2p.

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  Nyquist plots of four cathodes and corresponding fitting results via equivalent circuits were compared (Fig. S10 and Table S4 in Supporting information). The R_s was the internal resistance, CPE and R_ct are the double-layer capacitance and charge transfer resistance as well as R_f is regarded as the film resistance. The results of the NCMCu_0.02 electrode exhibit R_s and R_ct are 1.911 and 212.9 Ω, lower than other electrodes, indicating much faster kinetic and smaller polarization of Cu²⁺ doped cathode owing to the lighten of Na⁺/vacancy ordering.

  To obtain deeper insights into the structural evolution and behaviors of doped ions, operando XRD tests were carried out for NCMSc_0.02, NCMFe_0.02 and NCMCu_0.02 electrodes in the voltage of 1.5–4.5 V at 0.1 C. It can be seen from Fig. 4, all the samples exhibited the same trend of voltage platform upper 4.3 V and continuous peak shifts of the (002) and (004) diffraction peaks toward the lower angle. The results reveal that either interlayer distance along the c-axis is enlarged or compressed during Na⁺ extraction or insertion, which is owing to strengthened O—O electrostatic repulsion between the adjacent TMO₂ layer. Besides, the (101), (106) as well as (012) peaks shift toward higher angles due to the redox reactions of TM ions upon desodiation to form high-valence ions with smaller ionic radii. Upon Na⁺ intercalation, the structures almost follow a similar reverse evolution, contributing to the high reversibility of the structure for doped samples. According to Bragg's Law and refinement results, Cu²⁺ doped cathode delivered the modified distance in d-spacing and TMO₂ layer (along the c-axis) than the counterparts, which not only facilitate Na⁺ migrations from Na_e and Na_f position in the P2 phase but also synergistic alleviate the slab slide. The phenomena in operando XRD results are consistent with the theoretical calculations of TMO₂ layer, d-spacing, and interlayer distance by Rietveld refinements. The widened interlayer spacing could optimize and improve stability in the cathode structures, ultimately leading to the improvement of the electrochemical performances.

  Figure 4

  

  Figure 4.  The diffraction peaks of (002), (004), (101), (012) and (106) form operando XRD patterns of layered P2 phase during 1^st cycle with corresponding galvanostatic charge/discharge curves at 0.1C for NCMSc_0.02, NCMFe_0.02 and NCMCu_0.02 electrodes, respectively.

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  In summary, a series of hierarchical cathode materials were obtained by doping periodic elements such as Sc, Ti, V, Cr, Fe, Cu and Zn into the NCM by a facile sintered method. The results of operando XRD at high voltage (4.5 V) and Rietveld refinements delivered the collaborative strategy among the compressed TMO₂ layer, enlarged-spacing, as well as the changes of interlayer distance, which are closely related to the radii and valence states. Intermingle redox active metal ions (Sc³⁺, Fe³⁺ and Cu²⁺) into the TM sites could subdue the Na⁺/vacancy ordering and repress the expansion of the c axis during insertion/extraction, where the NCMCu_0.02 contained a higher ion radius and lower valence could further tune up the valence state of TM layers shows the best electrochemical performance. The fitted smaller R_s and R_ct via EIS test for NCMCu_0.02 also can expedite Na⁺ migration and heighten the high rate capability. This work provides a strategy to design higher energy densities for reversible cathode materials in SIBs.

  Declaration of competing interest

  The authors report no declarations of interest.

  Acknowledgments

  This work was financially supported by the National Natural Science Foundation of China (Nos. 52263010, 51902090), Henan Key Research Project Plan for Higher Education Institutions (No. 23A150038), 2023 Introduction of studying abroad talent program, "111" Project (No. D17007), Henan Provincial Key Scientific Research Project of Colleges and Universities (No. 23A150038), Key Scientific Research Project of Education Department of Henan Province (No. 22A150042), the National students' platform for innovation and entrepreneurship training program (No. 201910476010), the China Postdoctoral Science Foundation (No. 2019 M652546), and the Henan Province Postdoctoral StartUp Foundation (No. 1901017).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2023.108810.

  
  
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  Figure 1  The XRD patterns with corresponding Rietveld refinement plots of (a) NCM, (b) NCMSc_0.02, (c) NCMTi_0.02, (d) NCMV_0.02, (e) NCMCr_0.02, (f) NCMFe_0.02, (g) NCMCu_0.02 and (h) NCMZn_0.02 samples, respectively. (i) Comparison of refinement parameters for eight specimens.

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  Figure 2  (a) Cycling stability and (b) rate performances between 1.5 V and 4.2 V of NCM, NCMSc_0.02, NCMTi_0.02, NCMV_0.02, NCMCr_0.02, NCMFe_0.02, NCMCu_0.02 and NCMZn_0.02, respectively. (c-e) Schematic diagram of structures in various directions.

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  Figure 3  XPS spectra of NCM, NCMSc_0.02, NCMFe_0.02 and NCMCu_0.02, respectively. (a) Mn 2p, (b) Co 2p, (c) Ni 2p, (d) Sc 2p, (e) Fe 2p and (f) Cu 2p.

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  Figure 4  The diffraction peaks of (002), (004), (101), (012) and (106) form operando XRD patterns of layered P2 phase during 1^st cycle with corresponding galvanostatic charge/discharge curves at 0.1C for NCMSc_0.02, NCMFe_0.02 and NCMCu_0.02 electrodes, respectively.

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