Chinese Chemical Letters  2021, Vol. 32 Issue (2): 849-853   PDF    
Trace Nb-doped Na0.7Ni0.3Co0.1Mn0.6O2 with suppressed voltage decay and enhanced low temperature performance
Ruyun Yuea, Fang Xiaa, Ruijuan Qic, Da Tiea, Shanshan Shia,b, Zhiping Lia, Yufeng Zhaoa,b,*, Jiujun Zhangb     
a Key Laboratory of Applied Chemistry, Yanshan University, Qinhuangdao 066004, China;
b Institute for Sustainable Energy & College of Sciences, Shanghai University, Shanghai 200444, China;
c Key Laboratory of Polar Materials and Devices (MOE), Department of Optoelectronics, East China Normal University, Shanghai 200241, China
Abstract: The P2-type manganese-based Na0.7MnO2 cathode materials attract great interest due to their high theoretical capacity. However, these materials suffer from rapid capacity fading, poor rate performance and severe voltage decay resulting from phase transition and sluggish reaction kinetics. In this work we report a novel Nb-doped Na0.7[Ni0.3Co0.1Mn0.6]1-xNbxO2 with significantly suppressed voltage decay and enhanced cycling stability. The strong Nb-O bond can efficiently stabilize the TMO framework, and the as prepared material demonstrates much lower discharge midpoint voltage decay (0.132 V) than that of pristine one (0.319 V) after 200 cycles. Consequently, a remarkably improved cycling performance with a capacity retention of 87.9% after 200 cycle at 0.5 C is achieved, showing a 2.4 fold improvement as compared to the control sample Na0.7Ni0.3Co0.1Mn0.6O2 (~37% rotation). Even at 2 C, a capacity retention of 68.4% is retained after 500 cycles. Remarkably, the as prepared material can be applied at low temperature of −20 ℃, showing a capacity retention of 81% as compared to that at room temperature.
Keywords: Manganese-based oxides    Sodium ion battery    Low temperature    Voltage decay    Cycling stability    

Increasing concerns about energy shortages and serious environmental problems in the modern society requires reasonable utilization sustainable energy sources, for which efficient large-scale electric energy storage technology is urgently demanded [1-6]. Owing to the low cost and abundant sodium resources in earth, sodium-ion batteries (SIBs) are considered as a promising candidate to substitute lithium-ion batteries (LIBs) for large-scale grid energy storage [7, 8]. However, the lack of high performance electrode materials, limits the practical application and commercialization of SIBs. In the past decades, various cathode materials have been intensively investigated [9-12]. Among them, layered sodium transition metal oxides NaxMO2 (M is a transition metal, Co, Mn, Fe, Ni, etc.), have been considered as one of the most potential cathode materials, which can be classified into P2 and O3 phases according to the Na occupation sites and repeated unit cell [13]. Especially, the manganese-based P2 layered structure (Na0.7MnO2) have attracted considerable attention due to their low cost, high theoretical capacity and environment friendliness of Mn [14-17]. Nevertheless, such structures usually undergo a structural degradation and serious voltage decay upon Na+ (de)intercalation, thus cannot fully satisfy the demands of advanced applications. Thus, voltage fade is the pivotal problem that urgently needs to be overcome.

As is generally accepted, the irreversible phase transition, Jahn–Teller lattice distortion of Mn(III) and Na+/vacancy ordering occurred in the electrochemical process are mainly responsible for the SIB fadings [18-20]. Various approaches including lattice doping and surface/interface modifications, have been intensively investigated to alleviate the structural change upon cycling. Cationic doping with heteroatoms, such as Li+, Mg2+, Al3+, Ti4+, has been proved as an effective method to promote the performance of cathode materials [21-25]. For instance, the incorporation of more Cu ions can suppress the valance change of Fe ions and voltage decay in the Fe-based layered oxides [26]. Li as an inert element, is also found can act as the structural support point to stabilize the crystal structure during Na+ intercalation/deintercalation [27]. Meanwhile, considering the cost of production in practice, trace amount doping with a heteroatom concentration generally two to three orders lower is placed on the agenda. Trace Ti-doping is recently reported to effectively modify the microstructure of LiCoO2 and stabilize the surface oxygen at high voltages, resulting in a promoted cycling stability at 4.6 V [28].

Despite high relevance in structure and electrochemistry, strong oxygen redox accompanied by the oxygen gas release from the Mn-based material was revealed, but none in the high-valent ion doped materials. Inspired by the above work, we report a novel low-concentration Nb doping of Na0.7Ni0.3Co0.1Mn0.6O2 as high performance SIB cathode material. Considering the relationship between transition-metal ion migration and voltage decay, the size of doped ions and the bond dissociation between metal and oxygen play an important role in suppressing voltage decay. Herein, 5d metal niobium has been incorporated into manganese-based layered oxides [29-32]. Particularly, the ionic radius of Nb5+ (0.64 Å) is close to that of Mn3+ (0.645 Å) and Co3+ (0.61 Å), ensuring the successful doping of Nb element in the layered structure. Besides, the Nb-O bonds generally demonstrate higher metal-oxygen bond energy [33-36], which is expected to enhance structural stability, and alleviate the structural degradation upon cycling. The improved electrochemical performance is resulted from the enhanced oxygen stability induced by local structural variation around doping Nb. Consequently, the as prepared Na0.7[Ni0.3Co0.1Mn0.6]0.98Nb0.02O2 demonstrates a high capacity retention of 87.9% after 200 cycle at 0.5 C, showing a 2.4 fold improvement as compared to that of the control sample Na0.7Ni0.3Co0.1Mn0.6O2 (37%). It is worth noting that, even in trace amounts, Nb-doping suppresses significantly the voltage decay of Na0.7Ni0.3Co0.1Mn0.6O2. Na0.7[Ni0.3Co0.1Mn0.6]0.98Nb0.02O2 shows much lower discharge midpoint voltage decay (0.132 V) than that of pristine one (0.319 V) after 200 cycles. Remarkably, the as prepared material also demonstrates good performance at low temperature (−20 ℃).

The Nb-doped materials with Na0.7[Ni0.3Co0.1Mn0.6]1-xNbxO2 were synthesized using two steps: co-precipitation and the solid phase sintering method. Nickel, cobalt and manganese acetate was dissolved in 30 mL deionized water according to the stoichiometric ratio, which corresponds to a concentration of 2 mol/L for the metal ions. Then, 90 mL Na2CO3 solution (2 mol/L) was added into the acetate solution. The above solution was stirred for 12 h, then filtered, washed and dried at 80 ℃ to obtain the carbonate precursor [Ni0.3Co0.1Mn0.6]CO3. Next, based on 0.01 mol of carbonate precursor, add a certain proportion of Na2CO3 and Nb2O5, and mix it thoroughly. The powder was calcined at 500 ℃ for 10 h, then sintered again at 900 ℃ for 15 h in air with heating rate of 5 ℃/min and cooled down to room temperature.

The diffraction data were collected by the powder X-ray diffraction (PXRD, Rigaku P/max 2200 VPC) equipped with Cu-Kα (λ1=0.15406 nm, λ2=0.154439 nm) radiation source within the range from 5° to 80° at a scan rate of 2°/min. The SEM (Zeiss SUPRA 55) and TEM (Hitachi HT-7700, 120 kV) were employed to observe the morphology of the cathode materials. X-ray photoelectron spectroscopy (XPS) performed on a Thermo Fisher Multi-element (K-alpha) high-transmission spectrometer input lens, with energy ranging from 200 eV to 3 keV.

The Nb-doped Na0.7[Ni0.3Co0.1Mn0.6]1-xNbxO2 (x=0, 0.02, denoted as NCM, NCMN) samples were successfully synthesized through a classical solid-state reaction. The inductively couple plasma (ICP) results (Table S1 in Supporting information) of NCMN confirm the ratio of 0.723:0.290:0.099:0.568:0.013 for Na: Ni: Co: Mn: Nb, which is very close to the expected stoichiometry. The XRD results of the obtained samples show all the diffraction peaks are well indexed to a typical P2-type structure with the P63/mmc space group (Fig. S1a in Supporting information). As shown in Fig. 1a and Table S2 (Supporting information), the Rietveld refinement of the NCMN gives the lattice parameters a=b=2.8771(6) Å, c=11.1332(5) Å and V=79.8143 Å3 with a reliability index (Rwp) of 4.53%. The molecular model structure diagram for the P2-NCMN is shown in inset of Fig. 1a, which is composed of alternating MnO6 slabs and Na layers. Besides, niobium ions occupy the transition metal sites in metal oxide octahedral. The refined XRD patterns of NCM sample is also displayed in Fig. S1b (Supporting information) and corresponding crystallographic parameters are listed in Table S3. After Nb-replacement, the (002) peak shifts to lower degree (Fig. 1b), which reveals that Nb has been successfully doped into Na0.7Ni0.3Co0.1Mn0.6O2 without affecting the hexagonal structure. The crystallographic lattice parameters after Rietveld refinement are summarized in Table S4 (Supporting information). The d-spacing of Na+ layers is decreased by 0.181 Å, whereas the slab thickness of TMO2 is increased 0.135 Å after Nb substitution. Thereby c parameter is generally decreased from 11.225 Å to 11.133 Å. The possible reason is the comprehensive effect of larger size and high valence state of Nb, which leads to the shrinkage of Na layer in c axis and expands TMO6 octahedron [37].

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Fig. 1. (a) Rietveld refinement plot of NCMN and inset of crystal structures of P2-phase. (b) Shift of the (002) peak of both materials. (c) TEM image. (d) HRTEM image. (e) The SAED pattern of NCMN. (f) SEM image and the corresponding elemental EDX mapping images of NCMN.

The transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images further confirm that the as-prepared sample are highly crystalline (Figs. 1c and d). The morphologies of NCM is shown in Fig. S2 (Supporting information). It is clear that there is no obvious change in morphology before and after doping. From the HRTEM image in Fig. 1d, the interplanar distance between neighboring lattice fringes is 0.565 nm, corresponding to the (002) planes of the layered structure. Furthermore, the selected area electron diffraction (SAED) patterns (Fig. 1e) are indexed to the P2-type crystalline structure viewed from the [001] direction, in accordance with the results of HRTEM images. The scanning electron microscopy (SEM) image shows that the morphology of the NCMN, which is an irregular layered structure in general with size ranging from 2 μm to 5 μm. The results of EDS mapping on the crystallites reveal that Na, Ni, Co, Mn, Nb and O elements are homogenously distributed in the materials (Fig. 1f).

X-ray photoelectron spectroscopy (XPS) was employed to investigate the oxidation states of elements in NCMN electrode (Fig. 2). Dominant peaks located at 854.90 and 872.15 eV are attributed to the peak of the Ni 2p line, indicating that the valence of nickel ion is +2 [38]. Two peaks of Co 2p3/2 and Co 2p1/2 respectively correspond to 780.29 and 795.46 eV, demonstrating the presence of Co3+. In addition, Mn 2p3/2 and Mn 2p1/2 peaks are located at 642.47 and 653.95 eV, respectively, which clearly reveals that the Mn3+ and Mn4+ coexist in the material [39]. The XPS peaks at 209.53 and 206.64 eV provide direct evidence that valence state of Nb is +5, in accord with previous report [40].

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Fig. 2. XPS spectra of the NCMN samples: (a) Ni 2p, (b) Co 2p, (c) Mn 2p and (d) Nb 3d.

In order to study the effect of Nb doping on the materials, electrochemical performance of both samples in Na half cells were tested in the voltage range of 2–4.25 V. As displayed in Fig. 3a, the first three cyclic voltammetry (CV) curves of the NCMN show three pairs of reversible cathodic/anodic peaks, which are located at 2.54/2.21 V, 3.37/3.21 V and 3.76/3.49 V. The peaks around 2.5 V are relative to the redox reaction of Mn4+/Mn3+, while these multiple peaks above 3.0 V could be attributed to the redox reactions of Ni and Co and the simultaneously occurred Na+/vacancy ordering [41-44]. The redox reactions of Ni and Co are the major contributors of the capacity in the charge/discharge cycles. Compared to the CV curves of the NCM (Fig. S3a in Supporting information), the sample NCMN exhibits smaller polarization and better reversibility, which may be attributed to the influence of Nb dopant. Figs. S3b and c (Supporting information) compare the typical charge/discharge voltage profiles for both cathode materials from 2 V to 4.25 V at a rate of 0.1 C (1 C=180 mA/g). Obviously, the charge/discharge curves of NCM and NCMN exhibit a long voltage platform over 3.5 V, then followed by a sloping region above 3.75 V accompanied with by the sequential oxidation of Ni and Co, which is consistent well with the CV results. The initial reversible discharge capacity of NCMN is estimated to be ≈ 84.2 mAh/g, the discharge curves of the first three cycles are almost identical, indicating that the material has good reversibility. Compared with NCM, the smoother charge/discharge curves are observed for the NCMN. This certifies that the substitution of Nb indeed greatly suppresses the Na+/vacancy ordering [45]. As shown in Fig. 3b, the electrodes were cycled at various current densities ranging from 0.1 C to 5 C. The reversible capacities of the NCMN are approximately 94, 87.1, 80.5, 76.9, 73.1 and 55.5 mAh/g, whereas the pristine delivers a discharge capacity of 94.4, 85.6, 77.1, 69.4, 50.6 and 0.1 mAh/g at 0.1 C, 0.2 C, 0.5 C, 1 C, 2 C and 5 C, respectively. Further compared with the rate performance of both electrodes (Fig. S4 in Supporting information), the galvanostatic charge/discharge curves show the slight increase of voltage polarization for NCMN at various rates. The excellent kinetic capability of Na+ extraction/insertion is also proved by small voltage polarization in the first charge/discharge galvanostatic intermittent titration technique (GITT) curves (Fig. 4a). The cycling performances for both the Na0.7[Ni0.3Co0.1Mn0.6]1-xNbxO2 electrodes at 0.5 C are compared in Figs. 3c and d. The initial discharge specific capacities of NCM and NCMN are 84.3 and 78.5 mAh/g, corresponding to the capacity retention of 37% and 87.9% after 200 cycles, respectively. Compared to the pristine NCM, although the initial capacity of the Nb-doped material slightly decreases, the presence of Nb leads to higher capacity retention.Fig. 3e shows the discharge midpoint voltage (MPV) of the pristine and Nb-doped electrode. It can be apparently seen that the discharge midpoint voltage decay of NCMN is 0.132 V after 200 cycles, whereas that of NCM is 0.319 V. Obviously, trace amount of Nb doping can enhance the cycling stability and mitigate the voltage decay. In addition to the superior rate capabilities and cycling stability of the as-prepared NCMN, noting that it displays outstanding low-temperature performance. The electrochemical performance at low temperature (−20 ℃) is also investigated and shown in Figs. 3f-g. A reversible capacity of 63.6 mAh/g is obtained at −20 ℃ at 0.5 C, which is 81% of that at room temperature. Significantly, at −20 ℃, all the charge-discharge curves give two evident voltage plateaus from 1st to 300th cycles indicating the very limited voltage decay upon cycling. The initial discharge specific capacities of NCMN are 78.5 and 63.6 mAh/g at 25 ℃and −20 ℃ respectively, corresponding to the capacity retention of 79.3% and 88.5% after 300 cycles (Fig. 3g). The higher capacity retention at −20 ℃ should be associated with the more sluggish Na-transport kinetics at low temperature. Besides, Fig. S5 shows the discharge capacity of NCMN remains 68.2 mAh/g at 2 C for 100 cycles with a high capacity retention of 94.8%, even achieving a capacity retention of 68.4% after 500 cycles.

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Fig. 3. (a) The cyclic voltammograms of NCMN for the first three cycles at a scan rate of 0.1 mV/s. (b) The rate capabilities of both electrodes. (c) Galvanostatic charge/discharge profiles of NCM and NCMN material in 2nd and 200th cycles at 0.5 C. (d) Cycling performance and (e) the discharge midpoint voltage of the pristine and Nb-doped electrodes at 0.5 C for 200 cycles. The low-temperature performance of NCMN: (f) galvanostatic charge/discharge profiles of NCMN cycled at 0.5 C between 2.0 V and 4.25 V at −20 ℃. (g) Long-term cycling performance of NCMN at 0.5 C for 300 cycles at 25 ℃ and −20 ℃.

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Fig. 4. (a) GITT curves of NCMN cathode material in the first cycle. (b) The chemical diffusion coefficient of Na+ ions as a function of voltage calculated from the GITT profile. (c) The EIS of the NCM and NCMN electrodes before cycle. (d) Relationship between real impedance with the low frequencies of the pristine (NCM) and Nb-doped (NCMN) electrodes.

To further explain the structural stability and internal mechanism, the morphology and electrochemical behavior of NCMN were examined before and after cycling, and the results are displayed in Fig. S6 (Supporting information). In terms of morphology, the main layered structure was still remained even after 200 cycles at 0.5 C as shown in Figs. S6a and b, suggesting the good structure stability for NCMN. Figs. S6c and d show the major oxidation/reduction peaks located at about 3.5 V is still sharp after 200 cycles, which further confirms that NCMN has high structure reversibility upon Na+ (de) intercalation. This is probably a result from reducing migration of Ni2+ and dissolution of Mn3+ in the transition metal layers, and to some extent, restraining the phase transition after Nb substitution [26, 28, 45, 46]. These results indicate that substitution of Nb can effectively stabilize the structure of the crystal and suppress voltage fading during the charging/discharging process, further improving the electrochemical properties of the materials.

The galvanostatic intermittent titration technique (GITT) and electrochemical impedance spectroscopy (EIS) are employed to further examine the effects of Nb-doping on kinetics behavior of Na+ ion. The test method was described in supporting information. Fig. 4b compares the diffusion coefficient of Na+ (DNa) calculated from the GITT curves. The values of DNa in the NCMN electrode vary from 7.29×10−12 cm2/s to 2.24×10-10 cm2/s, while NCM shows lower diffusion coefficients (8.97×10-13 cm2/s to 6.21×10-11 cm2/s). Although the trend of diffusion coefficients is similar, variation of DNa versus voltage plots in NCMN is slightly smaller than that of NCM. The slower diffusion rate corresponds to the charge plateaus, and sodium ions diffuse faster in the sloping region, which results from a phase transition or order-disorder transition during sodium intercalation/deintercalation in layered oxide cathode for SIBs [47]. Figs. 4c and d are electrochemical impedance spectroscopy (EIS) results of both samples at open circuit voltage (OCV), which provide further evidence for the better rate capability of the Nb-doped material. The DNa of NCMN determined from EIS test is 1.16×10−12 cm2/s larger than that of NCM (5.15×10-13 cm2/s), which is in good agreement with the GITT results. Both plots consist of two semicircles at the high frequency range, and a straight line at the low frequency range attributed to Warburg impedance of Na+ diffusion in the bulk material. The first semicircle reflects sodium ion migration through the interface between the surface of the particles and the electrolyte, the second semicircle could be regarded as the charge-transfer resistance. Obviously, the first semicircle of NCMN is smaller, which demonstrates that the Nb-doped samples show improved mobility of Na+ in bulk electrode materials than that of the pristine counterpart. In addition, total impedance of two parts decreases from 631.5 Ω (NCM) to 480.3 Ω (NCMN) after doping. As discussed above, it is plausible that the Nb-doping can increase the electronic conductivity and accelerate the migration rate of the sodium ions, thus enhancing the rate capability of cathode material [37]. The strong Nb-O ionic bond inhibits the variation of the TM-O bond or the distortion of the TMO6 octahedron, maintaining the integrity of the surface structure [47]. The incorporation of Nb into the transition metal layer and the Mn4+ oxidation state can improve the structural stability as well due to the presence of tough Nb-O bonds relative to Co/Mn/Ni-O bonds. These improvements are responsible for the promoted cycling and rate performances of NCMN.

In summary, we have successfully synthesized a novel trace Nb-doped cathode materials and explored the influence of Nb substitution on the electrochemical performance of P2-Na0.7Ni0.3Co0.1Mn0.6O2 composites. The NCMN electrodes deliver an initial discharge capacity of 78.5 mAh/g at 0.5 C (tested at 2~4.25 V), with retention of 87.9% and negligible voltage decay of 0.132 V after 200 cycles. Besides, it also keeps initial capacity of 63.6 mAh/g and retention of 88.5% at 0.5 C under −20 ℃ after 300 cycles. EIS and GITT tests demonstrate that the pronounced rate performance is attributed to high sodium diffusivity during the intercalation/deintercalation. The results indicate that Nb-doping has a positive effect on the improvement cycling stability and the sodium ion diffusion coefficient. In particular, it can dramatically suppress the discharge voltage decay during the cycling process. Nb-doping is a possible strategy to maintain structural stability for suppressing the detrimental voltage fade to improve energy density for practical applications.

Declaration of competing interest

The authors declare no competing interests.

Acknowledgments

We thank the financial supports from the National Natural Science Foundation of China (No. 51774251), Hebei Natural Science Foundation for Distinguished Young Scholars (No. B2017203313), Hundred Excellent Innovative Talents Support Program in Hebei Province (No. SLRC2017057), Talent Engineering Training Funds of Hebei Province (No. A201802001), and the Opening Project of the State Key Laboratory of Advanced Chemical Power Sources (No. SKL-ACPS-C-11).

Appendix A. Supplementary data

Supplementary material related to this article can be found, in the online version, at doi:

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