Open AccessRenewablesRESEARCH ARTICLES25 Jan 2023Ti, F Codoped Sodium Manganate of Layered P2-Na0.7MnO2.05 Cathode for High Capacity and Long-Life Sodium-Ion Battery Pengchao Wen, Haodong Shi, Dandan Guo, Aimin Wang, Yan Yu and Zhong-Shuai Wu Pengchao Wen State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023 Google Scholar More articles by this author , Haodong Shi State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023 Google Scholar More articles by this author , Dandan Guo Department of Material and Chemical Engineering, Yulin University, Yulin 719000 Google Scholar More articles by this author , Aimin Wang Department of Material and Chemical Engineering, Yulin University, Yulin 719000 Google Scholar More articles by this author , Yan Yu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Hefei National Laboratory for Physical Sciences at the Microscale, Department of Materials Science and Engineering, CAS Key Laboratory of Materials for Energy Conversion, University of Science and Technology of China, Hefei, Anhui 230026 Dalian National Laboratory for Clean Energy, Chinese Academy of Sciences, Dalian 116023 Google Scholar More articles by this author and Zhong-Shuai Wu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023 Dalian National Laboratory for Clean Energy, Chinese Academy of Sciences, Dalian 116023 Google Scholar More articles by this author https://doi.org/10.31635/renewables.023.202200012 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail The development of layered sodium manganese oxide cathode materials with high capacity and structural stability is one of the keys to boosting the performance of sodium-ion batteries (SIBs), but it remains a great challenge. Herein, a titanium and fluorine codoped P2 type sodium manganate of Na0.7MnO2.05 cathode material (NMO-0.1TF) is developed as a high-capacity and long-durable cathode for high-performance SIBs. Titanium and fluorine codoping significantly reduces the structural deformation, synergistically improves structural stabilization, inhibits the formation of irreversible phases, and enhances electrochemical kinetics. As a result, the NMO-0.1TF||Na battery working in the voltage ranges of 2.0–4.2 V exhibits a high specific capacity of 227 mAh g−1 at a current density of 20 mA g−1 and an excellent rate performance (76 mAh g−1 at a high current density of 3 A g−1). Such a battery still delivers an outstanding cycle stability, shows a high initial discharge capacity of 133 mAh g−1 at 1 A g−1, and maintains the initial capacity of 96.2% after 200 cycles. More importantly, the assembled full battery of NMO-0.1TF||hard carbon validates high capacity and impressive cyclability. Therefore, this codoped NMO-0.1TF cathode with high capacity and excellent stability represents a brilliantly practical application for developing high energy density SIBs. Download figure Download PowerPoint Introduction Sodium (Na) is widely available on the earth and its reserves are more than 420 times higher than those of lithium (Li). Thus, the sodium-ion battery (SIB) is considered a cost-effective alternative to the lithium-ion battery in applications for backup power supplies, electric bicycles, low-speed electric vehicles, and large-scale energy storage.1–6 However, owing to the large radius of the sodium ion element, SIBs generally display slow kinetic behavior, resulting in inferior electrochemical performance.7–9 To address this issue, development of novel electrode materials for SIBs is very critical. The cathode is a major component of these applications as the donor of the sodium source and its reversible capacity and working voltage plateau determine the performance of the full battery. Therefore, designing a cathode material with high capacity and long-term stability is one of the keys to creating superior SIBs. The layered transition metal oxides, NaxMO2 (M = Ni, Fe, Mn, Co, V, Cr, etc.), have attracted considerable attention to research about SIB cathodes due to their high theoretical specific capacity and low production cost.10–13 Among them, O3- and P2-phase NaxMO2 are the most common structural crystal types. In the O-type structure, sodium ions occupy octahedral positions and migrate from one vacancy to another through the interstitial tetrahedral position (i.e., a zigzag transfer diffusion pathway). The widely studied NaNi0.5Mn0.5O2 is a typical O3 cathode material. For instance, Komaba et al.14 reported that the O3-type NaNi0.5Mn0.5O2 has a higher reversible capacity (125 mAh g−1), but the cycle stability is poor, dropping quickly to about 100 mAh g−1 after 20 cycles. Element doping is a common method to improve its rate performance and cycle stability. Specifically, doping NaNi0.5Mn0.5O2 with Mg or Li can reduce the structural deformation, suppress the volume change of crystal lattices, and inhibit the formation of irreversible phases. Moreover, the surface coating can avoid transition metal ions dissolving from the positive electrode, thereby helping to improve the reversible capacity and cycle stability.11,15 For example, Sun et al.16 modified the NaNi0.5Mn0.5O2 cathode with MgO coating and Mg doping, achieving a specific capacity of up to 167 mAh g−1 and stable cycling (75% capacity retention after 100 cycles). Studies have also pointed out that optimizing the morphology and structure is very important for improving the electrochemical performance of cathode materials.17–19 Typically, Hao et al.20 synthesized O3-type microrod NaNi0.5Mn0.5O2 with hollow structure through an ethanol-mediated method, showing a capacity of 101 mAh g−1 and 70% of capacity retention after 500 cycles. Although the O3-type oxide cathode has a high reversible specific capacity, its air stability and cycle stability are very limited, making it difficult to apply on a large scale. The P2-type compound has better cycle stability and air stability due to its larger triangular prism position occupied by Na ions, which is conducive to the transport of Na ions. In this regard, the Na0.67Fe0.5Mn0.5O2 is a typical P2-type cathode material. Representatively, Komaba et al.13 synthesized P2-type Na2/3Fe1/2Mn1/2O2 by a solid-state method, and based on Fe3+/Fe4+ and Mn3+/Mn4+ redox couples, it exhibits an average voltage of 2.75 V, a capacity of 190 mAh g−1, and a corresponding energy density of 520 Wh kg−1. In addition, Cao et al.21 used the sol–gel method to prepare P2-type Na0.67Mn0.65Fe0.2Ni0.15O2, which shows a capacity of 208 mAh g−1 and a capacity retention rate of 71% after 50 cycles. However, layered transition metal oxides still present some defects that limit their application. First, the ionic radius of Na+ is large, so its dynamic performance is generally slow. Second, there are complex phase transitions and structural evolutions during the cycle, leading to some irreversible lattice deformations. Third, the stability in the air is poor, and layered transition metal oxides will react with water or carbon dioxide in the air to form NaOH or Na2CO3, resulting in the corrosion of the current collector. Finally, it is difficult to offer high specific capacity, cycle stability, and rate performance at the same time. To address the above problems and promote the application of layered transition metal oxide cathode materials, structural modification and valence control measures, such as doping modification and coating, are required to effectively improve the working voltage platform and ratio of layered transition metal oxide, which is critical to boosting the capacity, rate capability, and cycle life. Herein, we developed a facile solid-state synthesis of titanium and fluorine codoped P2 phase sodium manganate (P2-Na0.7MnO2.05) cathode materials (NMO-0.1TF) for high-performance SIBs. The uniform distribution of Ti and F in NMO-0.1TF inhibits the formation of irreversible phases. Benefiting from this merit, the NMO-0.1TF||Na battery shows excellent rate performance and long-term cycle stability, including the high-rate performance with a specific capacity of 76 mAh g−1 at the high current density of 3000 mA g−1 and high capacity retention rate of 96.2% after 200 cycles. Therefore, the NMO-0.1TF cathode material possesses practical application value for the development of high specific energy SIBs. Results and Discussion A series of titanium and fluorine codoped sodium manganate cathode materials (NMO-TF) were prepared by employing the dopant of NH4TiF6 with different ratios (x) of NH4TiF6/NMO (the corresponding sample is denoted as NMO-xTF). The X-ray diffraction (XRD) pattern of the as-prepared NMO-0.1TF shows a typical structure of P2-Na0.7MnO2.05 with a Na-deficient state (JCPDS No. 27-0751) (Figure 1a), which is the hexagonal space group of P63/mmc. Its crystal structure is schematically illustrated in Figure 1b, indicating that Ti and F elements are introduced into the crystal lattice without the appearance of other phases. The effect of different doping amounts of NH4TiF6 on the crystal structure of NMO was further studied ( Supporting Information Figure S1). The phase of pristine NMO is NaMnO2 (JCPDS No. 25-0844) and contains a small amount of Na0.7MnO2.05. For the doped NMO-0.05TF, its crystal phase is fully consistent with Na0.7MnO2.05. When the ratio of NH4TiF6/NMO is high (e.g., x = 0.15 and 0.2), the samples are the mixed phases of NaMnO2 and Na0.7MnO2.05. The scanning electron microscopy (SEM) image of NMO-0.1TF shows large chunks with the size of about 12 μm (Figure 1c). The energy-dispersive X-ray spectrum (EDS) images confirm the uniform distribution of Na, Mn, O, Ti, and F in NMO-0.1TF (Figure 1d–h). The high-resolution transmission electron microscopy (HRTEM) image (Figure 1i) displays the (004) plane of Na0.7MnO2.05 with d-spacing of ∼0.27 nm. The chemical composition and element state of NMO-0.1TF were further analyzed by X-ray photoelectron spectroscopy (XPS). The Ti 2p spectrum of NMO-0.1TF shows three peaks with the binding energy at 453.0, 457.9, and 463.6 eV (Figure 1j). Figure 1k shows the F 1s of NMO-0.1TF, which can be deconvoluted into two characteristic peaks at 685.2 and 687.5 eV. Figure 1 | Charaterization of NMO-0.1TF. (a) XRD pattern, (b) schematic diagram of the crystal structure, (c–h) SEM image and EDS element mappings, (i) HRTEM image, and (j and k) XPS spectra of (j) Ti 2p and (k) F 1s. Download figure Download PowerPoint Supporting Information Figure S2 compares the XPS changes of Ti and F elements in NMO cathode materials with different doping amounts of NH4TiF6. The peak intensity of Ti 2p increases significantly with the increase of NH4TiF6 doping content. The content of Ti in NMO-0.05TF, NMO-0.1TF, NMO-0.15TF, and NMO-0.2TF is calculated to be 1, 2, 5, and 7 at %, respectively. However, the F doping content shows a tendency to first increase and then decrease with the rising of used F amount ( Supporting Information Figure S2b), in which the optimal amount of F doped in NMO-0.1TF is ∼8 at %. To evaluate the performance, the pristine NMO and doped NMO cathodes were coupled with Na metal anode to assemble NMO-xTF||Na batteries. Figure 2a–c compare the rate performance of NMO-0.1TF||Na battery within different voltage windows. It can be seen that the NMO-0.1TF||Na battery has excellent rate performance in the voltage range of 1.5–4.5 V or 2.0–4.2 V. This battery shows high initial discharge-specific capacities of 240 and 227 mAh g−1 in the voltage ranges of 1.5–4.5 V and 2.0–4.2 V at current density of 20 mA g−1, respectively. Impressively, it is revealed that, in the voltage range of 1.5–4.5 V, the average specific discharge capacities of the NMO-0.1TF||Na battery are 207, 184, 167, 152, 137, 134, 116, and 102 mAh g−1 at current densities of 40, 100, 200, 400, 800, 1000, 2000, and 3000 mA g−1, respectively. The capacity can be restored to 205 mAh g−1 when the current density returns back to 20 mA g−1. Even when the voltage range is reduced to 2.0–4.2 V, the average specific discharge capacities of 116, 95, and 76 mAh g−1 are still obtained at current density of 1, 2, and 3 A g−1, respectively, demonstrative of excellent rate performance. The Ragone plot shows energy density and power density of the NMO-0.1TF||Na battery (Figure 2d). Notably, it offers a high energy density of 615 Wh kg−1 and a high-power density of 7381 W kg−1 (based on the mass of NMO-0.1TF cathode) in the voltage range of 1.5–4.5 V. Moreover, in a narrow voltage range of 2.0–4.2 V, the NMO-0.1TF||Na battery can still show impressive energy density of 607 Wh kg−1, and achieve a higher power density of 8198 W kg−1 owing to the elevated working voltage. Compared with the previously reported NMO-based cathodes (Figure 2e and Supporting Information Table S1), our NMO-0.1TF||Na battery achieves some of the best results in terms of specific discharge capacity and rate performance.16,20,22–34 Figure 2 | Electrochemical properties of NMO-0.1TF cathode for SIBs. (a and b) Galvanostatic charge and discharge profiles obtained at different rates within voltage ranges of (a) 1.5–4.5 V, (b) 2.0–4.2 V, and (c) rate performance. (d) Ragone plot, and (e) a comparison of electrochemical performance of our NMO-0.1TF cathode with the previously reported NMO-based cathodes for SIBs. (f) Cycling stability of NMO-0.1TF||Na and NMO||Na batteries at 200 mA g−1 within voltage ranges of 1.5–4.5 V and 2.0–4.2 V. (g) Cycling stability of NMO-0.1TF||Na battery at 1000 mA g−1 within voltage range of 2.0–4.2 V. Download figure Download PowerPoint The NMO-0.1TF||Na battery also displays excellent long-term cycling stability compared with the NMO||Na battery (Figure 2f and Supporting Information Figure S3). The first specific discharge capacity of the NMO-0.1TF||Na battery is 203 mAh g−1 at 200 mA g−1 in the voltage range of 1.5–4.5 V, which is decreased to 145 mAh g−1 after 100 cycles, showing a capacity retention rate of 71.4%. In sharp contrast, the first specific discharge capacity of the NMO||Na battery is only 148 mAh g−1, and the capacity decays rapidly with only 73.6 mAh g−1 after 100 cycles, corresponding to a capacity retention of only 49.7%. On the other hand, the cycle stability of the NMO-0.1TF||Na battery is significantly improved when the voltage range is narrowed to 2.0–4.2 V. Its initial discharge capacity is 169 mAh g−1 at 200 mA g−1, which is retained at 157 mAh g−1 after 100 cycles, displaying the enhanced capacity retention rate of 92.9%. For the NMO||Na battery, the capacity retention rate shows a slight increase to 54.2%, which however is still much lower than that of the NMO-0.1TF||Na battery. The electrochemical properties of NMO-xTF with different doping amounts of fluorine and titanium are compared in Figure 2f (voltage range of 2.0–4.2 V). It can be seen that with the increase of the doping amounts, the cycle stability of NMO-xTF has been significantly improved, but the specific discharge capacity shows a trend of first increasing and then decreasing. Apparently, the NMO-0.1TF||Na battery presents the highest discharge capacity. As shown in Figure 2g, the NMO-0.1TF||Na battery can still provide a high initial discharge capacity of 133 mAh g−1, even if the current density increases to 1000 mA g−1. The initial capacity of 96.2% is still maintained after 200 cycles, and the average Coulombic efficiency is as high as 99.8%. Supporting Information Figure S4 compares the cycle stability of NMO-xTF with different doping amounts of NH4TiF6 at a current density of 200 mA g−1. It can be observed that the cycle stability has been improved with the introduction of Ti and F elements. In particular, the NMO-0.1TF cathode exhibits the maximal specific discharge capacity when the doping ratio of 10% is optimized. The discharge-specific capacity shows a gradual decrease when the doping ratio is greater than 10%. To further explore the mechanism of Ti and F doping for improving electrochemical performance, we used the ex situ XRD patterns to elucidate the structural evolution of the NMO-0.1TF cathode during the charge and discharge processes (Figure 3a–d). It was revealed that during the charging process from open circuit voltage to 3.1 V, the (002) and (004) crystal planes only slightly shift to a low angle, implying a solid solution reaction. When the voltage is greater than 3.5 V, a new phase appears. After reaching to 4.2 V, the Na0.7MnO2.05 is fully transformed into the new phase. It is notable that the discharging process is almost reversible, in which there is only a phase until the voltage drops to 3.5 V, and two-phase transformation occurs at around 3.1 V. Further discharging to ∼2.8 V, the initial phase with a solid-solution reaction reappears. Figure 3 | Ex situ XRD patterns of NMO-0.1TF for SIBs during the charge–discharge process. (a) Galvanostatic charge and discharge profiles and (b) corresponding XRD profiles in the range of 10–60°. (c and d) The profile graphs obtained from the ex situ XRD patterns in the range of (c) 10–20° and (d) 24–34°. Download figure Download PowerPoint To verify the dynamic performance of the NMO-0.1TF||Na battery, the cyclic voltammetry (CV) curves were tested at different scan rates of 0.1–1.0 mV s−1 (Figure 4a and Supporting Information Figure S5). At a sweep rate of 0.1 mV s−1, it is disclosed that the capacity and redox peak positions of the first four cycles of the NMO-0.1TF||Na battery are relatively stable while the profiles of the NMO||Na battery change significantly ( Supporting Information Figure S5), proving the outstanding structural stability of NMO-0.1TF. On the other hand, the diffusion coefficient of Na+ (DNa) is an important kinetic index, which can be calculated according to the CV curves and Randles–Sevcik equation (eq 1):35–37 i p = 2.69 × 10 5 n 3 / 2 A D Na 1 / 2 C Na ν 1 / 2 (1)where ip is the peak current, n is the number of electrons transferred, A is the surface area of the electrode, CNa is the concentration of Na+, and ν is the scan rate. Based on the linear fitting graph of ip versus ν1/2 (Figure 4b), the diffusion coefficients DNa for peaks 1–4 are calculated to be 1.68 × 10−10, 8.17 × 10−11, 5.65 × 10−11, and 2.35 × 10−10 cm s−1, respectively. Therefore, it is revealed that Na ions can rapidly diffuse in the NMO-0.1TF||Na battery. Figure 4 | The kinetics performance of NMO-0.1TF||Na batteries. (a) The CV curves obtained at various sweep rates and (b) the linear fitting plots of ip versus ν1/2. (c) The linear fitting plots of log(ip) versus log(ν). (d) Quantitative analysis of capacitive contribution at a sweep rate of 0.4 mV s−1. (e) Contribution ratio of capacitive and diffusion-controlled charge at various sweep rates. (f and g) GITT curves of NMO-0.1TF||Na and NMO||Na batteries. (h and i) First-principles calculation of Na0.7MnO2.05 (P63/mmc) and NaMnO2 (Pmmn): schematic diagram of the migration of sodium ions in the crystal structure of (h) Na0.7MnO2.05 and (i) NaMnO2 as well as their migration energy. Download figure Download PowerPoint Furthermore, ip and ν also obey the power law i = aνb, where a and b are adjustable parameters, and the value of b is used to determine the type of electrochemical reaction (diffusion control or/and capacitance control mechanism). When the b value is close to 1, it is interpreted that the capacitive behavior accounts for a higher proportion, and the reaction kinetics are faster. According to the linear fitting diagram of log(ip) and log(ν) (Figure 4c), the calculated b values for peaks 1–4 are 0.71, 0.74, 0.80, and 1.00, respectively, indicating that the NMO-0.1TF||Na battery has fast reaction kinetics. The contribution rate of pseudocapacitance at a specific scan rate can also be calculated by eq (2): i ( V ) / v 1 / 2 = k 1 v 1 / 2 + k 2 (2)where i(V) is the current corresponding to a specific voltage, k1 and k2 are adjustable parameters, and the value of k1 is obtained by linear fitting of i(V)/ν1/2 versus ν1/2. It is calculated that the ratio of the pseudocapacitance contribution (green shade) at 0.4 mV s−1 is 79% (Figure 4d). And the proportions of pseudocapacitance contribution at different scan rates are summarized in Figure 4e. It can be seen that with the increase of scan rate, from 0.1 to 1 mV s−1, the proportion of pseudocapacitance gradually increases to 67%, 73%, 79%, 82%, and 87%. In addition, galvanostatic intermittent titration technique (GITT) was also used to analyze the Na+ diffusion coefficient in NMO-0.1TF||Na and NMO||Na batteries (Figure 4f,g). It is calculated that the Na+ diffusion coefficient range of the NMO||Na battery is 4.72 × 10−14 to 6.11 × 10−11 cm s−1 while that of NMO-0.1TF||Na battery is 2.31 × 10−12 to 3.66 × 10−10 cm s−1, further confirming the excellent dynamics of the NMO-0.1TF||Na battery. Through the first-principles calculations (Figure 4h,i), the migration energy of sodium ions in the crystal structure of Na0.7MnO2.05 (space group P63/mmc) is 0.80 eV while in the NaMnO2 crystal structure space group (Pmmn), the migration energy is as high as 1.26 eV, which also proves the fast kinetics of NMO-0.1TF (i.e., P2-Na0.7MnO2.05). For practical application, the NMO-0.1TF cathode and hard carbon (HC) anode were utilized to assemble the NMO-0.1TF||HC full battery (Figure 5a). The CV curves and charge/discharge profiles of the NMO-0.1TF cathode and HC anode half-cells (NMO-0.1TF||Na and HC||Na batteries) are shown in Figure 5b,c, respectively. The cycle stability of the NMO-0.1TF||HC battery is illustrated in Figure 5d. The initial discharge specific capacity is 140 mAh g−1 at a current density of 200 mA g−1 within the voltage range of 1.8–4.2 V, which maintains at about 111 mAh g−1 after 100 cycles and the corresponding capacity retention rate is 80%. Figure 5 | Electrochemical properties of NMO-0.1TF||HC full batteries. (a) Schematic illustration of NMO-0.1TF||HC battery. (b) The CV curves at 0.1 mV s−1 and (c) the charge/discharge profiles at 200 mA g−1of NMO-0.1TF||Na and HC||Na batteries. (d) Cycling stability of NMO-0.1TF||HC battery obtianed at 200 mA g−1. Download figure Download PowerPoint Conclusion In summary, a titanium and fluorine codoped sodium manganate cathode material of P2-Na0.7MnO2.05 has been demonstrated for high-capacity and long-life SIBs. The NMO-0.1TF||Na battery displays a high specific capacity of 227 mAh g−1 at 20 mA g−1, excellent rate capability (76 mAh g−1 at 3 A g−1), and a long-term cycle stability with a high capacity retention of 96.2% after 200 cycles at 1 A g−1. More importantly, the assembled NMO-0.1TF||HC full battery validates high capacity and impressive cyclability. Therefore, this proposed codoping strategy of a high-capacity and stable P2-Na0.7MnO2.05 cathode provides bright practical application prospects for the development of high energy density SIBs. Supporting Information Supporting Information is available and includes discussions of experimental details, XRD patterns, XPS spectra, figures of galvanostatic charge/discharge profiles, cycling performance, cyclic voltammograms, ex situ XRD patterns, and tables of comparison of the performances of NMO-0.1TF cathode with the reported NMO-based cathodes for SIBs. Conflict of Interest There is no conflict of interest to report. Funding Information This work was financially supported by the National Natural Science Foundation of China (grant nos. 22125903 and 51872283), the Dalian Innovation Support Plan for High Level Talents (grant no. 2019RT09), the Joint Fund of the Yulin University and the Dalian National Laboratory for Clean Energy (YLU-DNL Fund 2021002), the Dalian National Laboratory for Clean Energy (DNL), the CAS-DNL Cooperation Fund, CAS (grant nos. DNL201912, DNL201915, DNL202016, and DNL202019), and DICP (grant no. DICP I2020032). Preprint Statement Research presented in this article was posted on a preprint server prior to publication in Renewables. The corresponding preprint article can be found here: https://doi.org/10.31635/renewables.023.202200012.
Search from 250 M+ research papersSearch from 250 M+ research papers
Open Access
Feb 2, 2024
Abby R. Haworth + 8