English

• Lithium-ion batteries (LIBs) have been widely applied in large-scale energy storage systems (ESS) as a high-efficiency energy storage technology [1]. However, the sharply increasing demand for lithium leads to a drastic fluctuation in price and resource scarcity [2,3]. Therefore, sodium-ion batteries (SIBs) have regained great attention as one of the most promising candidates due to low cost and the abundant sodium reserves [4-7]. Besides, the components and electrical storage mechanisms of SIBs are essentially highly similar to those of LIBs. It means that the equipment and facilities for LIBs can be directly used in the large-scale production of SIBs. As the critical component determining the energy output and price of SIBs, cathode materials have attracted much interest [8], such as layered oxides [9-12], Prussian blue analogues [13-16], and polyanionic cathodes [17-21]. Among them, layered sodium transition metal oxides (Na_xMO₂, M = Mn, Fe, Co, Cr, Ni, etc.) are considered as the most competitive cathodes for SIBs for future large-scale cell assembly [22].

  Delmas et al. have defined the crystal structure of layered compounds based on the stacking order of alkali ions between layers [22,23]. It is mainly divided into the trigonal prismatic (P-type) and octahedral (O-type) according to the local environment formed by oxygen around alkali metal ions. The O3-type transition metal oxides possess not only high theoretical specific capacity, but also sufficient Na content. Particularly, NaCrO₂ (NCO), as a typical O3-type layered sodium transition metal oxide, has become the research focus because of its flat voltage curves, high initial Coulombic efficiency (CE), and excellent temperature adaptability. Nevertheless, NCO obstacles by a fast capacity fading resulted from the complex phase transitions and side reactions involving electrolyte decomposition during Na⁺ intercalation/deintercalation [24-26]. To date, several strategies have been explored to improve its electrochemical performance such as carbon coating [25,27], metal-ion doping [28,29], and morphology regulating strategy [30,31]. Doping inactive elements into the TM layer has been reported as an effective way to enhance the cycling performances [32,33]. Zhao et al. recently discussed that the doped Mg²⁺ ions in P2-Na_0.67Ni_0.18Mg_0.15Mn_0.67O₂ cathode material reduce the interlayer slip and inhibit the phase transition [34]. Mg-substituted Na_0.67[Mg_xMn_1-x]O₂ cathode has also been indicated with reduced phase transition and improved cycling stability. However, the effect of Mg doping on O3-type NCO cathode materials has not been investigated until now [35]. Noting that Ti ions have been used to improve the electrochemical properties of NCO, considering the similar Ti ionic radius to that of Cr and the stronger Ti-O bond than the Cr-O bond. Moreover, moderate Ti⁴⁺ doping can enhance the Cr³⁺/Cr⁴⁺ operating voltage, which can be attributed to the induction effect of Ti⁴⁺ [36,37]. A Ti-doped NCO layered oxide has been reported exhibiting a delayed O3 → P3 phase transition during discharge processes and an elevated average discharge voltage [26]. The Ti-doped Na_0.95Cr_0.95Ti_0.05O₂ cathode material provides a reversible discharge specific capacity of 96.7 mAh/g at 1 C with a capacity retention of 80.1% after 800 cycles in the voltage range of 2.3–3.6 V. Recently, Ca-Ti co-substitution strategy has been proposed to suppress the irreversible phase transitions and improve Na⁺ ion diffusion kinetics of NCO within a wide operating voltage window of 1.5–3.8 V [37], in which each doped element plays a distinct role leading to a synergistic effect on improving the structural stability of NaCrO₂ electrode material.

  Inspired by the previous works, a novel Mg-Ti co-doped O3-type layered Na_0.99Cr_0.95Mg_0.02Ti_0.03O₂ (NCO-MT) cathode material is designed, and the effect of Mg-Ti co-doping on structural and electrochemical characteristics is investigated in this paper. The NCO-MT delivers a superior cycling stability with a capacity retention of 71.6% after 2500 cycles at 5 C, which is mainly attributed to the high structural reversibility resulted from Mg-Ti co-doping. The work benefits the development of high-performance layered oxide cathode materials for advanced SIBs.

  The O3-type NCO-MT sample was synthesized by a facile solid-state reaction method. Stoichiometric amounts of Na₂CO₃ (Aladdin, 99.99%), Cr₂O₃ (Maclean, 99%), MgO (Aladdin, 98%), and TiO₂ (Aladdin, 99%) were mixed and grounded thoroughly in an agate mortar and then pressed into a pellet at a pressure of 15 MPa. Noting that an excess of 5 mol% Na₂CO₃ was added into the above mixture to compensate the Na loss during the high-temperature synthesis. The pressed pellet was calcined at 900 ℃ for 10 h under a flowing argon atmosphere with a ramping rate of 2 ℃/min, and then cooled naturally to room temperature. Afterwards, the resulted sample was immediately transferred to an argon-filled glove box for grinding and storage. For comparison, O3-type NCO samples were also synthesized by the same method only without MgO and TiO₂. The other information including materials characterization and electrochemical measurements were provided in Supporting information.

  As shown in Figs. 1a and b, all diffraction peaks of both samples can be well indexed by the hexagonal layered structure with a space group R3m (166) of PDF #25–1809. It indicates that Mg-Ti co-doping does not change the original crystal phase structure of NCO. And the refined results in Table S1 (Supporting information) summarize that the unit cell parameters of NCO-MT. The effect of Mg-Ti co-doping is clearly revealed by the XRD patterns of NCO-MT and NCO and the enlarged (003) diffraction peaks (Fig. S1 in Supporting information). The (003) peak of NCO-MT shifts to a lower angle compared to that of O3-NCO, indicating an increase in c-axis length, which may be attributed to the decreased Na content in NCO-MT. This increased lattice parameter c can facilitate the rapid intercalation/deintercalation of sodium ions in NCO-MT [27]. Furthermore, the stoichiometry of both samples is very close to the designed element ratios tested by ICP (Table S2 in Supporting information).

  Figure 1

  

  Figure 1.  Rietveld refinement for XRD patterns of (a) NCO-MT and (b) NCO.

  DownLoad: Full-Size Img PowerPoint

  From Figs. 2a and b, it can be seen that both samples are composed of secondary particles ranging from 0.5 µm to 1.0 µm including plate-like primary particles. Therefore, the morphology and particle size of NCO are well maintained after the Mg-Ti co-doping. The EDS results of NCO-MT illustrate the homogeneous distributions of Na, Cr, Mg, and Ti elements (Fig. 2c). As the survey spectrum in Fig. S2a (Supporting information) shows, Na, Cr, Mg, Ti and O elements are verified on the particle surface of NCO-MT, in accordance with EDS results. The Cr 2p spectrum in Fig. S2b (Supporting information) can be fitted with two peaks centered at 576.1 and 585.7 eV, indicating that the valance of Cr ion is +3. The Mg 1s peak located at 1303.6 eV is corresponded to Mg²⁺, and the Ti 2p_1/2 and Ti 2p_3/2 peaks appeared at 463.6 and 458.0 eV are assigned to Ti⁴⁺ (Figs. S2c and d in Supporting information). The oxidation state of Cr ions on the surface of NCO-MT closely resembles that of NCO, suggesting that charge compensation occurs primarily through sodium vacancies, rather than by altering the oxidation state at the CrO₆ octahedron site [38].

  Figure 2

  

  Figure 2.  SEM images of (a) NCO-MT and (b) NCO. (c) Elemental mappings of NCO-MT.

  DownLoad: Full-Size Img PowerPoint

  The electrochemical performance of NCO-MT and NCO cathode materials are comprehensively discussed using CR2032 coin cells. The comparison of the first desodiation/sodiation profiles at 0.1 C (1 C = 125 mAh/g) in Fig. 3a demonstrates that the initial specific capacities are 107 and 109 mAh/g with initial columbic efficiencies of 92.4% and 90.7% for NCO-MT and NCO, respectively. The slight capacity decay is assigned to the introduction of inactive Mg and Ti ions in NCO. Additionally, the average operating voltage for NCO-MT is slightly enhanced which can be attributed to the inductive effect from Ti⁴⁺ substitution of Cr³⁺ [39]. The broad and intense plateau at ~3.0 V (vs. Na/Na⁺) can be related to Cr³⁺/Cr⁴⁺ while the other additional small plateaus should be associated to the order/disorder phase transformations [40,41].

  Figure 3

  

  Figure 3.  Electrochemical properties of NCO-MT and NCO products in 2.3–3.6 V. (a) The initial charge-discharge curves at 0.1 C. (b) CV curves at a scan rate of 0.1 mV/s. (c) Charge-discharge curves up to the 80^th cycle of NCO-MT at 0.1 C. (d) Rate performance. (e) Cycling properties at 5 C.

  DownLoad: Full-Size Img PowerPoint

  In Fig. 3b, the redox peak at 3.08/2.74 V and 3.14/2.74 V in CV curves for NCO-MT and NCO can be related to Cr⁴⁺/Cr³⁺, which is in good agreement with the results in Fig. 3a, respectively. Besides, the other additional peaks should be due to a series of phase transformations during desodiation/sodiation process [42,43]. For NCO-MT, a lower polarization enabled by the Mg-Ti co-substitution is achieved, demonstrating the fast diffusion kinetics and decreased desodiation/sodiation resistance [30,44]. Representative charge-discharge profiles for different cycles for NCO-MT at 0.1 C (Fig. 3c) illustrate that the shape of the voltage curves remains almost the same after 80 cycles. The NCO-MT exhibits similar rate capability compared with NCO, and the discharge capacities of NCO-MT at 0.1, 0.5, 2, 10, 20, 30 and 50 C in Fig. 3d are 106.9, 99.6, 96.2, 90.9, 85.6, 78.4 and 66.1 mAh/g, respectively, and the capacity increases to 102.4 mAh/g when the current rate is reset to 0.1 C. The NCO-MT cathode manifests superior cycling stability with an initial discharge capacity of 101.8 mAh/g at 5 C and a capacity retention of 71.6% after 2500 cycles, compared to the corresponding 103.8 mAh/g and 45.4% for NCO (Fig. 3e). It is worth noting that the outstanding long cycling stability of NCO-MT materials is also superior to those of the reported Cr based layered oxides (Table S3 in Supporting information).

  To further explore the reasons for the differences in the cycling stability for NCO-MT and NCO, ex-situ XRD cycles were performed (Fig. 4). The (003) peaks of both samples after 2500 cycles shift to lower angles indicating an increase in c axis length, which is due to the increased repulsive force between oxygen layers with more Na⁺ vacancies in Na layers after long-term cycling [45]. For NCO-MT, it is clearly that all main diffraction peaks are basically coincident with those of the O3-type lattice, suggesting the excellent crystal structure reversibility during repeated insertion/extraction. However, the cycled NCO cannot recover to the initial O3 phase in a fully discharged state, which implies that some phase transitions have occurred. As shown in Figs. 4b and c, new reflections for the (003) and (006) crystal planes assigned to the O'3 phase appear at lower angles, companied with the weakened diffraction peaks for O3 phase, that is, NCO undergoes partially irreversible phase transformation from the O3 to O'3 phase during cycling. Therefore, the enhanced durable cycling ability for Mg-Ti doped NCO-MT cathode material in this work are primarily ascribed to the highly reversible crystal structure. Nyquist plots of the NCO-MT cathode in half-cells are shown in Fig. S3 (Supporting information). The Nyquist plots consist of a semicircle and a slop line before cycling, a new semicircle representing the resistant of the surface film (R_f) appears at the high frequency after 50 cycles. It demonstrates that the value of R_ct gradually increases after 50 cycles, which decreases the electron diffusion efficiency during charge and discharge leading to capacity decay.

  Figure 4

  

  Figure 4.  (a) Ex-situ XRD patterns of NCO-MT and NCO cathodes after 2500 cycles at 5 C. (b, c) Enlarged XRD patterns between 12°–21° and 32°–38°.

  DownLoad: Full-Size Img PowerPoint

  In summary, Mg-Ti co-doped NCO-MT was designed and synthesized as the cathode material for SIBs via a simple solid-state reaction method. NCO-MT cathode material provides a more open Na ions diffusion channel and highly reversible crystal structure evolution during desodiation/sodiation. Therefore, an improved capacity retention of 71.6% after 2500 cycles at 5 C is achieved for NCO-MT, with a capacity fade rate only 0.011% per cycle. The enhanced cycling stability compared to that of NCO can be attributed to the introduction of the inactive Mg and Ti ions. The Mg-Ti co-doping strategy benefits the long cycling performances of O3-type NCO and provides new clues for the development of cathode materials for practical SIBs.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Wenya Li: Writing – review & editing, Writing – original draft, Visualization, Data curation. Yuanqi Yang: Writing – original draft, Investigation, Formal analysis. Yuqing Yang: Writing – review & editing, Visualization. Min Liang: Visualization, Methodology, Investigation. Huizi Li: Visualization, Methodology, Investigation. Xi Ke: Resources, Conceptualization. Liying Liu: Supervision, Resources, Project administration, Methodology. Yan Sun: Supervision, Resources, Conceptualization. Chunsheng Li: Supervision, Resources, Project administration. Zhicong Shi: Supervision, Resources, Methodology. Su Ma: Resources, Project administration.

  Acknowledgments

  This work is financially supported by National Key Research and Development Program of China (No. 2022YFE0202400), the National Natural Science Foundation of China (No. 22379103), Natural Science Foundation of Guangdong Province of China (No. 2021A1515010388), the Science and Technology Projects of Suzhou City (No. SYC2022043), and the Qing Lan Project of Jiangsu Province (2022). We wish to thank the Analysis and Test Center, Guangdong University of Technology for the microscopy and micro-analysis of our specimens.

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110388.

  
  
• 1. [1]

     D. Zhang, L. Li, W. Zhang, et al., Chin. Chem. Lett. 34 (2023) 107122. doi: 10.1016/j.cclet.2022.01.015

  2. [2]

     A. Huang, Y. Ma, J. Peng, et al., eScience 1 (2021) 141–162. doi: 10.1016/j.esci.2021.11.006

  3. [3]

     J.M. Tarascon, Nat. Chem. 2 (2010) 510. doi: 10.1038/nchem.680

  4. [4]

     Q. Liu, Z. Hu, W. Li, et al., Energy Environ. Sci. 14 (2021) 158–179. doi: 10.1039/d0ee02997a

  5. [5]

     X. Jin, Y. Huang, M. Zhang, et al., Battery Energy 2 (2023) 20230029. doi: 10.1002/bte2.20230029

  6. [6]

     K. Wang, Z. Zhang, S. Cheng, et al., eScience 2 (2022) 529–536. doi: 10.1016/j.esci.2022.08.003

  7. [7]

     R. Chen, D.S. Butenko, S. Li, et al., Chin. Chem. Lett. 35 (2024) 108358. doi: 10.1016/j.cclet.2023.108358

  8. [8]

     Y. You, S. Xin, H.Y. Asl, et al., Chem 4 (2018) 2124–2139. doi: 10.1016/j.chempr.2018.05.018

  9. [9]

     M. Guignard, C. Didier, J. Darriet, et al., Nat. Mater. 12 (2013) 74–80. doi: 10.1038/nmat3478

  10. [10]

      J. Zhao, L. Zhao, K. Chihara, et al., J. Power Sources 244 (2013) 752–757. doi: 10.1016/j.jpowsour.2013.06.109

  11. [11]

      S. Komab, C. Takei, T. Nakayama, et al., Electrochem. Commun. 12 (2010) 355–358. doi: 10.1016/j.elecom.2009.12.033

  12. [12]

      E. Lee, J. Lu, Y. Ren, et al., Adv. Energy Mater. 4 (2014) 1400458. doi: 10.1002/aenm.201400458

  13. [13]

      J. Qian, M. Zhou, Y. Cao, et al., Adv. Energy Mater. 2 (2012) 410–414. doi: 10.1002/aenm.201100655

  14. [14]

      L. Xu, H. Li, T. Du, et al., Battery Energy 1 (2022) 20210003. doi: 10.1002/bte2.20210003

  15. [15]

      K. Hurlbutt, S. Wheeler, I. Capone, et al., Joule 2 (2018) 1950–1960.

  16. [16]

      B. Wang, Y. Han, X. Wang, et al., iScience 3 (2018) 110–133. doi: 10.1016/j.isci.2018.04.008

  17. [17]

      Q. Ni, Y. Bai, F. Wu, et al., Adv. Sci. 4 (2017) 1600275.

  18. [18]

      Z. Jian, X. Lu, H. Yang, et al., Adv. Energy Mater. 3 (2013) 156–160. doi: 10.1002/aenm.201200558

  19. [19]

      K. Saravanan, C.W. Mason, A. Rudola, et al., Adv. Energy Mater. 3 (2013) 444–450. doi: 10.1002/aenm.201200803

  20. [20]

      W. Li, J. Li, R. Li, et al., Battery Energy 2 (2023) 20220042.

  21. [21]

      K. Liang, D. Wu, Y. Ren, et al., Chin. Chem. Lett. 34 (2023) 107978. doi: 10.1016/j.cclet.2022.107978

  22. [22]

      C. Delmas, C. Fouassier, P. Hagenmuller, Phys. B+C 99 (1980) 81–85.

  23. [23]

      J.J. Braconnier, C. Delmas, C. Fouassier, et al., Mater. Res. Bull. 15 (1980) 1797–1804. doi: 10.1016/0025-5408(80)90199-3

  24. [24]

      K. Kubota, I. Ikeuchi, T. Nakayama, et al., J. Phys. Chem. C 119 (2014) 166–175.

  25. [25]

      C. Yu, J.S. Park, H.G. Jung, et al., Energy Environ. Sci. 8 (2015) 2019–2026.

  26. [26]

      W. Li, Y. Wang, G. Hu, et al., J. Alloy. Compd. 779 (2019) 147–155.

  27. [27]

      S. Wang, F. Chen, T. Zhu, et al., ACS Appl. Mater. Interfaces 12 (2020) 44671–44678. doi: 10.1021/acsami.0c11320

  28. [28]

      C. Ma, X. Li, X. Yue, et al., Chem. Eng. J. 432 (2022) 134305.

  29. [29]

      W. Li, M. Liang, Y. Yang, et al., Mater. Lett. 352 (2023) 135141.

  30. [30]

      L. Liang, W. Zhang, D.K. Denis, J. Mater. Chem. A 7 (2019) 11915–11927. doi: 10.1039/c9ta01540j

  31. [31]

      L. Liang, X. Sun, D.K. Denis, et al., ACS Appl. Mater. Interfaces 11 (2019) 4037–4046. doi: 10.1021/acsami.8b20149

  32. [32]

      T. Hwang, J.H. Lee, S.H. Choi, et al., ACS Appl. Mater. Interfaces 11 (2019) 30894–30901. doi: 10.1021/acsami.9b08987

  33. [33]

      H. Hou, J. Qiu, B. Li, et al., Chin. Chem. Lett. 34 (2023) 108810.

  34. [34]

      J. Feng, J. Wang, P. Li, et al., ACS Sustain. Chem. Eng. 10 (2022) 4994–5004. doi: 10.1021/acssuschemeng.2c00197

  35. [35]

      J. Billaud, G. Singh, A.R. Armstrong, et al., Energy Environ. Sci. 7 (2014) 1387–1391.

  36. [36]

      Y. Wang, R. Xiao, Y. Hu, et al., Nat. Commun. 6 (2015) 6954.

  37. [37]

      I. Lee, G. Oh, S. Lee, et al., Energy Storage Mater. 41 (2021) 183–195.

  38. [38]

      Y. Hu, Y. Lu, ACS Energy Lett. 4 (2019) 2689–2690. doi: 10.1021/acsenergylett.9b02190

  39. [39]

      Y. Tsuchiya, K. Takaanashi, T. Nishinobo, et al., Chem. Mater. 28 (2016) 7006–7016. doi: 10.1021/acs.chemmater.6b02814

  40. [40]

      J. Ding, Y. Zhou, Q. Sun, et al., Electrochem. Commun. 22 (2012) 85–88.

  41. [41]

      J. Liang, L. Liu, X. Liu, et al., ACS Appl. Mater. Interfaces 13 (2021) 22635–22645. doi: 10.1021/acsami.1c04997

  42. [42]

      Q. Liu, Z. Hu, M. Chen, et al., Small 15 (2019) e1805381.

  43. [43]

      Y. Wang, P. Cui, W. Zhu, et al., J. Power Sources 435 (2019) 226760.

  44. [44]

      C. Lin, X. Meng, M. Liang, et al., J. Mater. Chem. A 11 (2023) 68–76. doi: 10.1039/d2ta07413c

  45. [45]

      R.J. Clément, J. Billaud, A.R. Armstrong, et al., Energy Environ. Sci. 9 (2016) 3240–3251.

• 

  Figure 1  Rietveld refinement for XRD patterns of (a) NCO-MT and (b) NCO.

  下载: 全尺寸图片 幻灯片
  

  Figure 2  SEM images of (a) NCO-MT and (b) NCO. (c) Elemental mappings of NCO-MT.

  下载: 全尺寸图片 幻灯片
  

  Figure 3  Electrochemical properties of NCO-MT and NCO products in 2.3–3.6 V. (a) The initial charge-discharge curves at 0.1 C. (b) CV curves at a scan rate of 0.1 mV/s. (c) Charge-discharge curves up to the 80^th cycle of NCO-MT at 0.1 C. (d) Rate performance. (e) Cycling properties at 5 C.

  下载: 全尺寸图片 幻灯片
  

  Figure 4  (a) Ex-situ XRD patterns of NCO-MT and NCO cathodes after 2500 cycles at 5 C. (b, c) Enlarged XRD patterns between 12°–21° and 32°–38°.

  下载: 全尺寸图片 幻灯片
•