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Article  |  Open Access  |  22 May 2024

Rational design of nitrogen-doping Ti3C2Txmicrospheres with enhanced polysulfide catalytic activity for lithium-sulfur batteries

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Energy Mater 2024;4:400051.
10.20517/energymater.2023.104 |  © The Author(s) 2024.
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Abstract

The primary challenges that impede the practical applications of lithium-sulfur batteries are the significant shuttle effect of polysulfides, huge volume expansion, and slow redox kinetics. In this work, three-dimensional nitrogen doping Ti3C2TxMXenes (3D N-Ti3C2Tx) were successfully synthesized by spray drying and subsequent annealing, and hydrochloric acid-treated melamine effectively reduces the oxidation of MXenes in these processes. The formation of a unique nanoflower-shaped microsphere endows 3D N-Ti3C2Tx with a significant specific surface area and pore volume. The combination of nitrogen doping and the large specific surface area increased adsorption capacity and catalytic conversion ability for polysulfide intermediates. Consequently, the obtained 3D N-Ti3C2Tx/S cathode exhibited high-capacity retention (578.5 mAh g-1 after 500 cycles at 0.5 C and 462.5 mAh g-1 after 1,000 cycles at 1 C), superior rate performance (651.2 mAh g-1 at 3 C), and excellent long-term cycling performance (capacity fading rate of 0.076% per cycle at 0.5 C and 0.046% per cycle at 1 C). This work expands the potential applications of MXenes for lithium-sulfur batteries.

Keywords

MXenes, spray drying, nitrogen doping, catalytic conversion, lithium-sulfur batteries

INTRODUCTION

With the rapid progress in energy storage technology, the demand for higher energy density is increasing. Lithium-sulfur (Li-S) batteries have shown tremendous potential because of their high theoretical energy density (2,600 Wh kg-1) and theoretical specific capacity (1,675 mAh g-1)[1-3]. However, their practical applications still face several challenges[4-6], including the poor electronic conductivity of sulfur and the discharge product lithium sulfide (Li2S), volume expansion during discharge, sluggish redox reaction kinetics, and the notorious shuttle-effects (polysulfide intermediates (LiPSs) dissolved in electrolyte and reacted with lithium anodes). These issues lead to low discharge capacity and rapid deterioration in performance[7,8].

Advanced cathode host has been developed to solve the aforementioned issues, including carbon and its derivate[9-11], metal (e.g., high-entropy alloy)[12,13], some conducting compounds or polymers[14,15], and so on. Among them, MXenes have received considerable interest. As first proposed by Naguib et al. in 2011, they are expressed as Mn+1XnTx (n = 1-3), where M represents transition metal; X stands for C or/and N elements; Txis the symbol of functional groups on the surface of MXenes[16]. It is impressive that MXenes have the following advantages for Li-S batteries compared to traditional carbon materials: (1) inherent metal conductivity[17,18]; (2) two-dimensional surface with high activity[19,20]; (3) large binding energy with LiPSs based on Lewis’s interaction to inhibit shuttle effect[21,22]; and (4) effective catalytic effect on the conversion of LiPSs achieved by abundant surface terminations[23,24]. It is these advantages that make MXenes also have excellent performance in other metal-sulfur batteries[25-27].

Zhao et al. selectively extracted Ti from the Ti2SC MAX phase to form carbon/sulfur nanolaminates, suggesting that MXene material is a promising positive carrier for Li-S batteries[28]. Liang et al. reported the Ti2C(OH)x/S cathode application, showing that lithium polysulfides have a strong interaction with the “acidic” Ti site and hydroxyl surface groups[29]. The electrochemical properties of MXenes can be further improved by stripping multi-layer MXenes into nanosheets with fewer layers[30,31]. However, severe agglomeration of MXene nanosheets happened through van der Waals forces and hydrogen bond interactions after stripping, limiting the accessibility of ions and preventing full utilization of functional surface terminations[32]. Doping of heterogeneous atoms, such as nitrogen atoms, can enlarge the interlayer spacing and prevent the agglomeration of MXene nanosheets[33,34]. For example, in 2017, Wen et al. obtained nitrogen-doped MXenes by heat treatment under an ammonia atmosphere[35], effectively expanding the c-lattice parameter of MXene sheets. In addition, introducing nitrogen doping into electrodes can effectively improve the adsorption capacity of LiPSs[36-38] and confer electrocatalytic activity on MXenes to accelerate the conversion of LiPSs[38]. Song et al. prepared porous nitrogen-doped Ti3C2 MXenes (P-NTC) using melamine-formaldehyde (MF) nanospheres as hard templates[39]. The negatively charged MXene nanosheets on the positively charged MF sphere surface through electrostatic self-assembly and the prepared P-NTC show an interconnect network with different pores, which greatly alleviate the agglomeration of the MXene nanosheets. Due to the synergistic effect of large specific surface area and nitrogen doping, P-NTC showed more effective adsorption and catalytic conversion ability for Li2S4 solution than MXenes (TC) without nitrogen doping.

Herein, three-dimensional nitrogen-doped porous MXene microspheres (3D N-Ti3C2Tx) were synthesized by spray drying and subsequent annealing as the host for sulfur. Firstly, a few layers of negatively charged MXene solution were mixed evenly with positively charged hydrochloric acid (HCl)-treated melamine, and the powder was collected by spray drying. Through sequential annealing, 3D N-Ti3C2Tx was obtained. The MXene nanosheets formed three-dimensional spherical nanoflower structure, effectively preventing the agglomeration of MXene nanosheets. During the heat treatment, the self-supported melamine decomposes, effectively increasing the surface area. Therefore, the synthesized 3D N-Ti3C2Txshows a large surface area, which enables the sulfur to be uniformly impregnated into the MXenes layer. Meanwhile, melamine acts as a nitrogen source to introduce nitrogen doping. Remarkably, due to the synergistic effect of large surface area and nitrogen doping, the 3D N-Ti3C2Tx/S cathode exhibits excellent electrochemical properties.

EXPERIMENTAL

Synthesis of multiple-layer MXene nanosheets

The accordion multiple-layer MXenes were obtained by hydrofluoric acid (HF) etching. The addition of 2 g Ti3AlC2 (MAX phase ) to 200 mL HF solution (40%) was conducted multiple times. Stir the above solution continuously at 35 °C for more than four days to remove the Al layer thoroughly. The powder was collected by centrifugation and washed with deionized (DI) water several times. After that, the multiple-layer Ti3C2Tx (MXenes) were obtained by drying the above powder at 70 °C under vacuum for 12 h.

Synthesis of few-layer MXene nanosheets

Then, 2 g Ti3C2Tx was added into 40 mL aqueous solution of tetramethylammonium hydroxide (TMAOH, 25 wt%) and stirred at 35 °C for 24 h. The samples were washed with DI water more than two times to remove TMAOH. Few-layer MXene suspension was obtained by sonicating for 1 h and subsequent centrifugation (3,500 r/min for 10 min). Repeat this step for several times.

Synthesis of positively charged HCl-treated melamine

Melamine exhibits low solubility in aqueous solutions. Adding HCl is an effective way to enhance its water solubility. We introduced 10 g of melamine into 150 mL of anhydrous ethanol and vigorously agitated the mixture for 4 h. Then, 10 mL HCl solution (37 wt%) was added and further stirred for 2 h. The melamine solution underwent centrifugation, followed by thrice washing with ethanol. Subsequently, the product was subjected to vacuum drying at 60 °C for 12 h. This culminated in the attainment of melamine that had undergone HCl treatment, rendering it soluble in water.

Synthesis of 3D N-Ti3C2Tx

First, 3.2 g HCl-treated melamine was poured into 200 mL of the aforementioned few-layered Ti3C2TxMXenes nanosheet solution and agitated for 24 h at room temperature. It is estimated that the mass ratio between the few-layered Ti3C2Tx nanosheets and the HCl-treated melamine is 1:16. Subsequently, the powder was obtained through spray drying and positioned within a tubular furnace enriched with an inert atmosphere (Anhui BEQ BTF. 1,200 C). Gradually raise the temperature at a rate of 5 °C per minute to 700 °C and then maintain it for 2 h. Finally, the three-dimensional nitrogen-doped porous Ti3C2TxMXene microspheres (noted as 3D N-Ti3C2Tx) can be collected after the furnace cools to ambient temperature.

Similarly, 3D Ti3C2Tx can be obtained, but the MXene solution was not mixed with HCl-treat melamine before spray drying.

Preparation of the 3D N-Ti3C2Tx/S cathode

The sulfur composite was prepared by traditional molten diffusion strategy: the prepared 3D N-Ti3C2Tx (or 3D Ti3C2Tx) and sulfur powder in a mass ratio of 3:7 were placed in an agate mortar for thorough grinding. Then, the mixture was heated at 155 °C for 18 h. After cooling to room temperature, 3D N-Ti3C2Tx/S (or 3D Ti3C2Tx/S) can be collected. The cathode was homogeneously blended with 3D N-Ti3C2Tx/S (or 3D Ti3C2Tx/S), Super P (SP), and polyvinylidene fluoride (PVDF) in N-Methyl-2-pyrrolidone (NMP) and coated on the carbon-coated aluminum foil. Ultimately, we dried it in vacuum at 60 °C for 12 h.

Physical characterizations

The X-ray diffraction (XRD) patterns were captured by a Rigaku MiniFlex 600 X-ray diffractometer utilizing a Cu Kα source within the range of 2θ = 2-80°. The field-emission scanning electron microscopy (FESEM, Hitachi, SU-8010), transmission electron microscopy (TEM, FEI Ltd, Tecnai F20), selected area electron diffraction (SAED), and high-resolution TEM (HRTEM) were employed to study the morphology, internal microstructure, and elemental mapping of the samples. The N2 adsorption/desorption isotherms were acquired through Micromeritics ASAP 2020 Plus HD88. The X-ray photoelectron spectroscopy (XPS) analysis was determined by Thermo Scientific K-Alpha to scrutinize the surface chemistry of the products. The thermogravimetric analysis (TGA) was conducted employing a Pyris diamond analyzer under an air atmosphere with a heating rate of 5 °C/min from 30 to 800 °C. Ultraviolet-visible (UV-vis) spectra were recorded using a Shimadzu UV-3150 UV-vis spectrometer.

Electrochemical measurements

We employed 3D N-Ti3C2Tx/S (or 3D Ti3C2Tx/S) as cathodes, with Li foil and Celgard 2500 as the anode and separator, respectively. The batteries were assembled in an Ar-filled glovebox, ensuring minimal exposure to O2 and H2O (both < 0.01 ppm). Additionally, 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in 1,3-dioxolane/1,2-dimethoxyethane (DOL/DME, 1:1 by volume) with 1 wt% LiNO3 was used as electrolyte. To evaluate the electrochemical performance, cyclic voltammetry (CV) measurements were conducted within a potential window of 1.8-2.8 V (vs. Li/Li+). Furthermore, electrochemical impedance spectroscopy (EIS) in the frequency range of 0.1 to 1,000 kHz was employed. The galvanostatic charge-discharge (GCD) curves were assessed using the LAND CT3001A system, with the voltage range set as 1.8-2.8 V (vs. Li/Li+).

RESULTS AND DISCUSSION

Figure 1 depicts the synthesizing process of 3D N-Ti3C2Tx/S. The Al layer in Ti3AlC2 (MAX phase) was etched by HF to acquire accordion-like multi-layer Ti3C2TxMXene nanosheets. Subsequently, few-layered MXenes can be synthesized by solution-phase flocculation strategy, according to our previous reports[40]. The amino group (-NH2) in melamine could capture protons (H+) from HCl solution, thus forming positively charged melamine through protonation, which can be easily dissolved in DI water[41]. Obviously, HCl-melamine can be fully dissolved in DI water [Supplementary Figure 1].

Rational design of nitrogen-doping Ti<sub>3</sub>C<sub>2</sub>T<i><sub>x</sub></i>microspheres with enhanced polysulfide catalytic activity for lithium-sulfur batteries

Figure 1. Schematic illustration of the synthesis process of 3D N-Ti3C2Tx and 3D N-Ti3C2Tx/S.

The aforementioned HCl-treated melamine was mixed with the few-layered Ti3C2Txsolution and stirred together. MXene nanosheets possess negative charges due to the presence of surface functional terminations (e.g., -O, -OH, and -F), allowing for the attachment of positively charged melamine onto the surface of Ti3C2Tx MXene nanosheets through electrostatic interactions[37,42]. The resultant powder was then collected via spray drying in a constant stirring, forming porous microspheres. The precursor underwent calcination at 700 °C in an inert atmosphere for 2 h, yielding flower-like porous nitrogen-doped MXene microspheres. Consequently, the synthesized 3D N-Ti3C2Tx demonstrates remarkable thermal stability, which aligns with previous reports that the tailoring or functionalization of Ti3C2 MXenes through polymers proficiently improves its stability[43,44]. As a comparative experiment, 3D Ti3C2Tx microspheres were obtained by directly spray drying a pure Ti3C2Tx solution, followed by calcination under identical conditions.

FESEM effectively captured the intricate morphology and microstructure. The precursor of Ti3AlC2 (MAX phase) presents an inherent structure characterized by a dense stratification, as illustrated in Supplementary Figure 2A. After HF etching, accordion-like MXenes can be obtained [Supplementary Figure 2B]. Then, few-layered MXenes can be synthesized by solution-phase flocculation strategy [Supplementary Figure 2C][40]. In Supplementary Figure 2D and E, 3D Ti3C2Tx maintains the lamellar morphology of MXenes, with its surface exhibiting nanoparticles, presumably attributable to the formation of TiO2[45]. Figure 2A illustrates the morphology of the HCl-treated melamine/Ti3C2Tx precursor. After spray drying, porous microspheres ranging from 2 to 10 μm formed. The 3D N-Ti3C2Tx exhibits a nano-flower-shaped morphology composed of MXene nanosheets after annealing at 700 °C [Figure 2B]. As can be seen in Supplementary Figure 2F, the surface of these Ti3C2Tx nanosheets is flat and smooth, with no observable TiO2 nanoparticles. This intriguing observation suggests that the positively charged HCl-treated melamine coated itself on the surface of Ti3C2Tx nanosheet layers through electrostatic self-assembly interaction, serving as a physical barrier to oxygen, effectively shielding the MXenes from oxidation during high-temperature processes. Consequently, the synthesized 3D N-Ti3C2Tx demonstrates exceptional thermal stability.

Rational design of nitrogen-doping Ti<sub>3</sub>C<sub>2</sub>T<i><sub>x</sub></i>microspheres with enhanced polysulfide catalytic activity for lithium-sulfur batteries

Figure 2. SEM images of (A) HCl-treated melamine/ Ti3C2Tx precursor and (B) 3D N-Ti3C2Tx, (C-F) TEM, and (G) STEM images of 3D N-Ti3C2Tx with corresponding elemental mappings of C, Ti, and N. Scale ba rs: 200 nm.

The morphology and microstructure were further investigated via TEM. As shown in Figure 2C-F, TEM images provide a clearer view of 3D N-Ti3C2Tx. Microspheres composed of MXene nanosheets can be observed. The lattice spacing corresponding to the MXenes (002) plane is approximately 1.5 nm [Figure 2E]. It is apparent that MXene nanosheets in 3D N-Ti3C2Tx feature nano-sized micropores and mesopores at higher magnification, which could be attributed to the decomposition of melamine. These pores introduce defects, exposing more active sites to constrain LiPSs. Furthermore, C, Ti, and N elements are uniformly distributed within 3D N-Ti3C2Tx, indicating the successful nitrogen doping into Ti3C2Tx introduced by melamine decomposition [Figure 2G].

However, as shown in Supplementary Figure 3, the TiO2 nanoparticle can be observed in TEM images, which demonstrates that Ti3C2Tx undergoes substantial oxidation during thermal processing. As for 3D N-Ti3C2Tx, melamine serves as a physical barrier to oxygen, effectively shielding the MXenes from oxidation.

After sulfur infusion, it is evident from the TEM and STEM images [Supplementary Figure 4] that sulfur has permeated uniformly into the layers of MXenes. Furthermore, the 3D N-Ti3C2Tx has maintained its original nano-flower spherical architecture, exhibiting remarkable structural stability.

In order to demonstrate the electrostatic self-assembly interaction between HCl-melamine and few-layers Ti3C2Tx, we tested the zeta potential of both [Figure 3A], and the results showed that they were positively charged and negatively charged, respectively, which was the origin of electrostatic self-assembly interaction.

Rational design of nitrogen-doping Ti<sub>3</sub>C<sub>2</sub>T<i><sub>x</sub></i>microspheres with enhanced polysulfide catalytic activity for lithium-sulfur batteries

Figure 3. (A) The zeta potential of HCl-treated melamine and 3D N-Ti3C2Tx. XRD patterns of Ti3AlC2, multi-layered Ti3C2Tx, few-layered Ti3C2Tx sheets and 3D N-Ti3C2Tx with (B) 2θ from 2° to 15°, (C) 2θ from 2° to 70°. (D) N2 adsorption-desorption isotherms and (E) pore-size distribution curves of 3D N-Ti3C2Tx and 3D Ti3C2Tx. XPS pattern of 3D N-Ti3C2Tx (F) C 1s, (G) Ti 2p, (H) O 1s and (I) N 1s.

XRD techniques investigated the structure of the synthesized 3D N-Ti3C2Tx. As shown in Figure 3B, compared to multilayered Ti3C2TxMXenes, the low-angle diffraction peak for few-layered MXenes shifted from 8.74° to 6.22°, indicative of an increased interlayer spacing of the Ti3C2TxMXenes (002) crystal plane. Furthermore, the XRD spectrum of 3D N-Ti3C2Tx exhibited a peak at 5.98° for the (002) crystal plane, a 0.24° low-angle shift relative to few-layered Ti3C2 MXenes (6.22°), which is attributed to nitrogen doping, as nitrogen atoms have a larger radius than carbon atoms, expanding interlayer spacing in MXenes[35]. Figure 3C illustrates the disappearance of the characteristic peak of the (104) crystal plane of Ti3AlC2 in the XRD spectrum of multilayered Ti3C2 MXenes, indicating the successful removal of aluminum atomic layers from Ti3AlC2. Without nitrogen doping, 3D Ti3C2Tx exhibited diffraction peaks at 25.3°, corresponding to the (101) crystal plane of anatase TiO2 (JCPDS#21-1272), consistent with the SEM and TEM images. This phenomenon indicates that oxidation occurs during the annealing, and on the contrary, the oxidation degree of 3D N-Ti3C2Tx is low, indicating the protective effect of melamine on MXenes. The XRD pattern of 3D N-Ti3C2Tx/S [Supplementary Figure 5] demonstrates the successful infusion of orthorhombic S8.

Nitrogen adsorption-desorption tests under isothermal conditions on 3D N-Ti3C2Tx, 3D Ti3C2Tx, 3D N-Ti3C2Tx/S, and the melamine/MXenes precursor were conducted. The obtained isothermal adsorption-desorption curves were analyzed to further elucidate the pore structure and surface characteristics of the samples [Figure 3D]. The N2 adsorption-desorption curve for 3D N-Ti3C2Tx exhibits a typical Type IV isotherm[46], indicating the presence of abundant mesopores in the material, as observed from the pore size distribution image [Figure 3E]. The Brunauer-Emmett-Teller (BET) specific surface areas revealed that 3D N-Ti3C2Tx possesses a substantial surface area of 247.4 m2g-1 and a pore volume of 0.83 cm3g-1, while the melamine/MXenes precursor exhibited a specific surface area of 22.2 m2g-1 and a pore volume of 0.09 cm3g-1 [Supplementary Figure 6A and B]. These results highlight an increased porosity in 3D N-Ti3C2Tx due to the melamine decomposition during annealing, which will facilitate the adsorption of LiPSs and the maintenance of discharge capacity[47]. The porous structure not only facilitates uniform sulfur loading in 3D N-Ti3C2Tx but also enhances electrolyte penetration and shortens ion/electron transport pathways. The specific surface area and pore volume of 3D N-Ti3C2Tx/S significantly decrease compared to 3D N-Ti3C2Tx, only 89.7 m2g-1 and 0.22 cm3g-1, respectively [Supplementary Figure 6C and D]. This indicates that sulfur has embedded itself into the pores after sulfur infusion. For comparison, 3D Ti3C2Tx displayed a lower specific surface area of 128.5 m2g-1 and a pore volume of 0.28 cm3g-1.

The thermal gravimetric analysis (TGA) curves of the HCl-treated melamine/MXenes revealed a sharp weight drop at the temperature range of 250-350 °C [Supplementary Figure 7], which can be attributed to the melamine decomposition[41]. During annealing, active functional groups on the Ti3C2Tx nanosheet surface, such as -F, -O, and -OH, are removed [Supplementary Figure 7], providing active sites suitable for nitrogen atom doping. The defects were introduced in the vicinity of doped nitrogen atoms[34]. Nitrogen-doped Ti3C2Tx nanosheets exhibit significantly improved surface activities, resulting in a hydrophilic surface and enhanced Lewis acid absorption capability[48].

Further analysis of the chemical composition and interactions was performed using XPS spectra. XPS spectra [Supplementary Figure 8A] of 3D Ti3C2Tx and 3D N-Ti3C2Txboth reveal the presence of C, O, and Ti elements, with the appearance of O elemental peaks attributed to surface-terminated functional groups (-O, -OH) on Ti3C2Tx. Figure 3F and Supplementary Figure 8B compare the C 1s spectra of 3D Ti3C2Tx and 3D N-Ti3C2Tx. The peaks at 281.8, 286.3, and 284.8 eV correspond to the C-Ti, C-O, and C-C bonds, respectively[32]. Furthermore, 3D N-Ti3C2Tx exhibits an additional peak at 285.7 eV corresponding to the C-N bond[30], and a noticeable decrease in peak intensity at 281.8 eV, possibly attributed to the removal of partial C-Ti bonds during nitrogen doping. In Ti 2p spectra, all two samples exhibit peaks associated with Ti-C bonds at 455.4 and 461.3 eV[49] and Ti-O bonds at 458.4 and 464.2 eV[50] [Figure 3G and Supplementary Figure 8C]. The Ti 2p spectrum of 3D N-Ti3C2Tx shows two additional peaks at 456.6 and 462.9 eV, corresponding to Ti-N bonds[49]. The O 1s high-resolution XPS spectra of 3D Ti3C2Tx and 3D N-Ti3C2Tx[Figure 3H and Supplementary Figure 8D] can be divided into characteristic peaks at 529.9 eV for Ti-O bonds and 531.5 eV for Ti-OH bonds[51], corresponding to common surface-terminated functional groups (-O and -OH) on the MXene surface. The strong Ti-O bond signals indicate the formation of TiO2 in 3D Ti3C2Tx.

Besides, nitrogen elemental peaks are observed in 3D N-Ti3C2Tx, indicating effective nitrogen doping. High-resolution N 1s XPS spectra of 3D N-Ti3C2Tx[Figure 3I] reveal the chemical bonding properties of nitrogen. Peaks at 398.5 and 400.3 eV correspond to pyridinic and pyrrolic nitrogen, while a peak at 396.3 eV corresponds to Ti-N, further confirming the successful incorporation of nitrogen into MXene nanosheets[49]. Nitrogen, especially pyrrolic and pyridinic nitrogen in carbon lattice, enhances the affinity and chemical binding energy between the polar Ti atoms and sulfur and polar lithium polysulfides.

To validate the adsorption efficiency of 3D N-Ti3C2Tx microspheres for LiPSs, visual adsorption experiment was applied to compare the adsorption capability of 3D N-Ti3C2Tx and 3D Ti3C2Tx to LiPSs. Figure 4A shows the color fading of Li2S6 solution with 3D N-Ti3C2Tx and 3D Ti3C2Tx, respectively. The Li2S6 solution containing 3D N-Ti3C2Txnearly faded completely, whereas the Li2S6 solution containing 3D Ti3C2Tx remains yellow. This conclusion was further confirmed by corresponding UV-Vis spectroscopy, which showed an intensity decrease of the Li2S6 solution peak within 3D N-Ti3C2Tx compared to the others. Therefore, it can be concluded that 3D N-Ti3C2Tx microspheres possess strong adsorption capabilities for LiPSs. This can be attributed to the enhanced polarity due to nitrogen doping and the higher specific surface area of 3D N-Ti3C2Tx, which provides more adsorption sites for LiPSs. XPS analyses were applied to understand the chemical interaction between LiPSs and 3D N-Ti3C2Tx. In the N 1s spectrum, the peaks shift to a higher binding energy after the adsorption of Li2S6, indicating the interaction between N and S atoms [Figure 4B][52]. Besides, the proportion of configuration nitrogen had changed after the adsorption of Li2S6, as seen in Supplementary Table 1. Hence, it can be inferred that the pyridinic N and Ti-N are vital in the adsorption of Li2S6, because their relative proportions showed a greater decline[53]. In terms of the S 2p spectrum [Figure 4C], it can be divided into S-O, S-S and S-Ti bonds. Notably, the strong peak located at 169.8 eV can be attributed to S-O bonds, indicating the robust chemical binding ability of 3D N-Ti3C2Tx to LiPSs[54].

Rational design of nitrogen-doping Ti<sub>3</sub>C<sub>2</sub>T<i><sub>x</sub></i>microspheres with enhanced polysulfide catalytic activity for lithium-sulfur batteries

Figure 4. (A) UV-vis absorption spectra of the supernatant of Li2S6 solution with 3D Ti3C2Tx and 3D N-Ti3C2Tx and the inset is a digital photograph. XPS pattern: (B) N 1s and (C) S 2p of 3D N-Ti3C2Tx after adsorption of Li2S6. The CV curves of symmetric cells with electrolytes containing Li2S6, (D) scan rate: 0.2 mV s-1 and (E) scan rate: 10 to 50 mV s-1 (3D N-Ti3C2Tx). (F) The potentiostatic discharging profiles of Li2S precipitation (at 2.05 V) for 3D N-Ti3C2Tx. (G) The potentiostatic discharging profiles of Li2S dissolution (at 2.35 V) for 3D N-Ti3C2Txand 3D Ti3C2Tx. (H) Tafel plot. (I) The EIS plot of 3D N-Ti3C2Tx symmetrical battery containing Li2S6 electrolytes at different temperatures.

Symmetric cells with electrolytes containing Li2S6 were assembled to evaluate the kinetics of LiPS conversion. The CV curves at 0.2 mV s-1 [Figure 4D] revealed that 3D N-Ti3C2Tx exhibited higher current response, more positive reduction peak potential, and more negative oxidation peak potential, indicating a larger surface area and nitrogen doping, providing additional polysulfide adsorption sites and faster kinetics for Li2S6 redox conversion. Both 3D N-Ti3C2Tx and 3D Ti3C2Tx electrodes displayed distinct oxidation-reduction peaks in CV curves at various scan rates (10~50 mV s-1) [Figure 4Eand Supplementary Figure 9A]. With increasing scan rate, the positions of the oxidation-reduction peaks slightly shifted, with the 3D N-Ti3C2Tx electrode exhibiting narrower peak separations, suggesting stronger catalytic activity in polysulfide conversion.

A potentiostatic Li2S precipitation experiment was conducted to evaluate the catalytic effect of 3D N-Ti3C2Tx and 3D-Ti3C2Tx on LiPS conversion. The results in Figure 4F and Supplementary Figure 9B show that 3D N-Ti3C2Tx displayed a higher peak current, earlier current response, and higher Li2S precipitation capacity (120.20 mAh g-1) than 3D-Ti3C2Tx. These findings indicate the high catalytic activity of 3D N-Ti3C2Tx for Li2S precipitation. Besides, we further investigated the impact of 3D N-Ti3C2Tx on the dissolution capacity of Li2S during the charging process. As depicted in Figure 4G, 3D N-Ti3C2Tx exhibits a quicker current response and contributes significantly to the dissolution of Li2S, which can be attributed to the superior electrical conductivity.

The enhanced redox kinetics from 3D N-Ti3C2Tx were further validated by exchanging current density [Figure 4H]. According to the fitting results of the Tafel plot, it exhibited a high exchange current density of 3.16 × 10-4 mA cm-2, remarkably larger than that of 3D Ti3C2Tx (1 × 10-4 mA cm-2), indicating a significant enhancement in redox kinetics.

In order to assess the performance of 3D N-Ti3C2Tx at both extremely high and low temperatures, we assembled a symmetrical battery containing Li2S6 electrolytes and conducted EIS analysis at a temperature range from -40 to 70 °C. As shown in Figure 4I, the battery displays its highest internal resistance (Rs) and charge transfer resistance (Rct) at the frigid temperature of -40 °C, primarily due to the slow Li+ transportation. As the temperature increases, the impedance progressively decreases, with the Rct plummeting to a mere 0.13 Ω at 70 °C, signaling 3D N-Ti3C2Tx facilitating charge transfer to LiPSs at high temperatures. Furthermore, an Arrhenius plot was constructed based on the Rct values of the cell at varying temperatures [Supplementary Figure 9C], unveiling a notably linear correlation. The activation energy of the Rct between 3D N-Ti3C2Tx and LiPSs was 0.396 eV.

The CR2032 coin cells with 3D N-Ti3C2Tx/S and 3D Ti3C2Tx/S cathodes were assembled to evaluate their electrochemical performance. TGA was also conducted on 3D N-Ti3C2Tx/S, 3D Ti3C2Tx/S, and pure sulfur to measure the sulfur content in the composites. As shown in Figure 5A, it is evident that all three samples exhibit significant mass loss at the temperature range of 200-350 °C, corresponding to the sublimation of sulfur under a nitrogen atmosphere. Accordingly, the sulfur content in 3D N-Ti3C2Tx/S and 3D Ti3C2Tx/S composites is measured as 72 and 66.4 wt%, respectively. CV tests were conducted in the voltage range of 1.8-2.8 V at a scan rate of 0.1 mV s-1 [Figure 5B]. Two cathodic peaks (2.28 and 2.05 V) correspond to the reduction of S8 to high-order LiPSs (Li2Sx, 4 ≤ x ≤ 8) and then to Li2S2/Li2S, which has been described previously[55]. The positive cathodic peaks of the 3D N-Ti3C2Tx/S electrode indicate the effective catalytic activity to the LiPS conversion. Besides, as evidenced by the larger peak currents, the 3D N-Ti3C2Tx/S electrode exhibits superior redox kinetics to LiPSs. To further assess the impact on the LiPS conversion redox kinetics and lithium-ion diffusion, CV measurements at progressively increasing scan rates ranging from 0.1 to 0.5 mV s-1 were conducted [Figure 5C and Supplementary Figure 10A]. As the scan rate increased, the cathodic peaks shifted towards negative potentials, indicating a growing polarization. A similar phenomenon can be observed at anodic peaks. The diffusion coefficient of lithium ions (DLi+) can be determined by calculating the slope of the current peaks against the square root of the scan rate, as given in the Randles-Sevcik equation[56]: $$ I_{p}=\left(2.69 \times 10^{5}\right) n^{1.5} A D_{L i^{+}}^{0.5} C_{L i}+v^{0.5} $$, where DLi+ represents the lithium-ion diffusion coefficient, which can be derived from the peak current (Ip). n is associated with the number of electrons transferred, A represents the electrode area, v denotes the sweeping rate, and CLi+ signifies the concentration of lithium ions. Notably, the cell with a 3D N-Ti3C2Tx/S cathode demonstrated the steepest slopes in both the reduction and oxidation processes [Figure 5D and E and Supplementary Figure 10B]. This observation clearly indicates that the DLi+ values of 3D N-Ti3C2Tx/S cathodes are significantly greater than those of 3D Ti3C2Tx/S cathodes.

Rational design of nitrogen-doping Ti<sub>3</sub>C<sub>2</sub>T<i><sub>x</sub></i>microspheres with enhanced polysulfide catalytic activity for lithium-sulfur batteries

Figure 5. (A) TGA curves in Ar atmosphere. (B) 3D N-Ti3C2Tx/S and 3D Ti3C2Tx/S cathodes at the scanning rate of 0.1 mV s-1. (C) The CV curves of 3D N-Ti3C2Tx/S cathodes at different scan rates. The plot of CV peak of (D) R1 (S8-Li2S4) and (E) R2 (Li2S4-Li2S) versus the square root of scan rates. (F) The GITT plot of the cell with 3D N-Ti3C2Tx/S.

To investigate the internal resistances of Li-S batteries with 3D N-Ti3C2Tx/S cathodes during the charging and discharging, the galvanostatic intermittent titration technique (GITT) was employed [Figure 5F]. The internal resistance associated with Li2S nucleation and Li2S activation was discernible through the depth of the dips observed in the charging and discharging profiles. Notably, the cell with a 3D N-Ti3C2Tx/S cathode shows a low discharge/charge polarization voltage plateau. The low potential variation between the points of Li2S nucleation and Li2S activation suggests the reduced polarization and enhanced kinetic efficiency.

Figure 6A presents the cycling performance of 3D N-Ti3C2Tx/S and 3D Ti3C2Tx/S electrodes at 0.2 C. The 3D N-Ti3C2Tx/S electrode exhibits an initial discharge capacity of 1,115 mAh g-1, maintained at 812.1 mAh g-1 even after 150 cycles, significantly higher than that of the 3D Ti3C2Tx/S. The long-term cycling performance at 0.5 and 1 C is illustrated in Figure 6B and C, respectively. The 3D N-Ti3C2Tx/S electrode delivered a high initial capacity of 937.0 and 853.7 mAh g-1, and maintained 578.5 mAh g-1 (500 cycles) and 462.5 mAh g-1 (1,000 cycles) at 0.5 and 1 C, corresponding to a low decay rate of 0.076% and 0.046% per cycle, respectively. Both are much better than that of 3D Ti3C2Tx/S electrodes. The 3D N-Ti3C2Tx/S electrode outperformed the 3D Ti3C2Tx/S electrode in terms of reversible capacity and cycling stability. These results collectively demonstrate that nitrogen doping improves the adsorption and sulfur conversion capabilities of 3D N-Ti3C2Tx, significantly enhancing the capacity and cycling stability of Li-S batteries.

Rational design of nitrogen-doping Ti<sub>3</sub>C<sub>2</sub>T<i><sub>x</sub></i>microspheres with enhanced polysulfide catalytic activity for lithium-sulfur batteries

Figure 6. Cycle performance of 3D Ti3C2Tx/S and 3D N-Ti3C2Tx/S cathodes at (A) 0.2 C, (B) 0.5 C, and (C) 1 C. (D) The galvanostatic charge-discharge (GCD) profiles based on 3D N-Ti3C2Tx/S and 3D Ti3C2Tx/S cathode at 0.1 C. (E) Rate performance of 3D Ti3C2Tx/S and 3D N-Ti3C2Tx/S cathodes. (F) The GCD profiles of 3D N-Ti3C2Tx/S at 0.1, 0.2, 0.5, 1, 2, and 3 C. Nyquist plots of 3D N-Ti3C2Tx/S and 3D Ti3C2Tx/S cathodes (G) before and (H) after cycles.

At 0.1 C, the battery demonstrates a distinctive charging plateau and two discharging plateaus [Figure 6D]. The charging plateau corresponds to the transformation of Li2S2/Li2S into S8. At the discharge process, the higher potential plateau is linked to the conversion of S8 into soluble high-order LiPSs, while the lower plateau is associated with the reduction of soluble LiPSs to Li2S2/Li2S, which is consistent with the discussion of CV curves. Due to enhanced affinity of 3D N-Ti3C2Tx to LiPSs and its catalytic performance, the kinetics of LiPS transformation have been markedly improved. Consequently, the polarization voltage (∆E) measures 125.3 mV, noteworthy smaller than that of 3D Ti3C2Tx.

The rate performance tests were conducted at current densities ranging from 0.1 to 3 C [Figure 6E]. The 3D N-Ti3C2Tx/S electrode achieved capacities of 1,274.1, 1,128.9, 876.8, 772.5, 694.3, and 651.2 mAh g-1 at 0.1, 0.2, 0.5, 1, 2, and 3 C, respectively. When the current density is back to 0.2 C, the discharge capacity of 1,070.6 mAh g-1 was achieved. In contrast, the 3D Ti3C2Tx/S electrode shows lower discharge specific capacities of 966.9, 833.5, 738.6, 675.8, 568.2, and 192.1 mAh g-1 at the same rates, which means lower utilization of active material at high-rate conditions.

Figure 6F and Supplementary Figure 11 depict the GCD curves of the 3D N-Ti3C2Tx/S and 3D Ti3C2Tx/S electrodes at different rates, respectively. The 3D N-Ti3C2Tx/S electrode displayed two typical discharge plateaus, corresponding to the conversion of S8 to long-chain LiPSs and the subsequent conversion to Li2S2/Li2S, respectively. All discharge curves of 3D N-Ti3C2Tx/S exhibited two characteristic plateaus even at a high current of 3 C. However, the discharge curves of 3D Ti3C2Tx/S show vague plateaus, indicating lower sulfur utilization than 3D N-Ti3C2Tx/S.

EIS tests further validated the improvement in electrochemical performance before and after 100 cycles at 0.5 C [Figure 6G and H]. Compared to the 3D Ti3C2Tx/S electrode, the pristine EIS curve of the 3D N-Ti3C2Tx/S electrode exhibits a smaller semicircle shape in the high-frequency region, indicating that nitrogen doping enhances the conductivity of Ti3C2TxMXenes. After 100 cycles, the Rct of the 3D N-Ti3C2Tx/S electrode is significantly lower than that of 3D Ti3C2Tx/S, which is attributed to the large surface area of 3D N-Ti3C2Tx for electrolyte infiltration and lithium-ion transport. Additionally, nitrogen doping promotes the formation and conversion of Li2S. This further underscores that the porous structure of 3D N-Ti3C2Tx can provide ample diffusion pathways, and nitrogen doping can effectively immobilize polysulfides, reducing irreversible consumption of sulfur and lithium passivation caused by shuttle effects, enabling the battery to maintain high electron/ion transfer rates, thereby enhancing the electrochemical performance. To explore the potential for practical application, the cells with high sulfur loadings (2.4 and 4.2 mgs cm-2) were assembled. As depicted in Supplementary Figure 12, the cell with 2.4 mgs cm-2 sulfur loading exhibits an areal capacity close to 1.8 mAh cm-2 at 2 C, and the cell with 4.2 mgs cm-2 sulfur loading can maintain a capacity of 3 mAh cm-2 at 1 C, experiencing a capacity reduction only at 2 C. This observation underscores the electrochemical performance of 3D N-Ti3C2Tx /S with a high sulfur loading.

The deterioration of lithium anodes is related to the severe shuttle effects[57]. Additionally, the degree of polysulfide dissolution can be evaluated by the simple optimal photograph of the separators after cycling. Supplementary Figure 13A depicts the scanning images of the disassembled lithium anode. It can be observed that the lithium anode surface is exceptionally smooth before cycling. After cycling, the lithium anode surface with 3D Ti3C2Tx/S exhibits pulverization and cracks, along with a significant accumulation of dead lithium [Supplementary Figure 13B], while the battery with 3D N-Ti3C2Tx/S is relatively smoother, and only a small amount of lithium dendrites can be observed [Supplementary Figure 13C]. As shown in Supplementary Figure 13D, the separator of the 3D Ti3C2Tx/S cell (left) is covered with a thick layer of dark yellow substance, corresponding to the LiPSs. However, only a small amount of yellow precipitate remains on the separator with the 3D N-Ti3C2Tx/S cathode (right). All the above results show that 3D N-Ti3C2Tx can effectively alleviate the shuttle effect in Li-S batteries, and excellent electrochemical performance can be achieved.

CONCLUSIONS

In summary, few-layered Ti3C2Txwas obtained, and 3D N-Ti3C2Tx was successfully synthesized by spray drying. Positively charged HCl-melamine is electrostatically attached to the surface of negatively charged MXenes, preventing the aggregation of MXene nanosheets and enhancing its thermal stability. Spray drying facilitates the transformation of unique nanoflower-shaped microspheres, endowing 3D N-Ti3C2Tx with a large specific surface area and pore volume. Due to the synergistic effect of the large specific surface area and nitrogen doping, MXenes exhibit increased adsorption capacity and catalytic conversion ability for LiPSs, effectively preventing the shuttle effect. Therefore, high-capacity retention (578.5 mAh g-1 after 500 cycles at 0.5 C, and 462.5 mAh g-1 after 1,000 cycles at 1 C remained), superior rate performance (651.2 mAh g-1 at 3 C), and excellent long-term cycling performance (capacity fading rate of 0.076% per cycle at 0.5 C and 0.046% per cycle at 1 C) have been achieved. This work broadens the application of MXenes and provides new insight into the practical use of Li-S batteries.

DECLARATIONS

Authors’ contributions

Formal analysis, investigation, writing - original draft: Cai L, Ying H

Formal analysis, writing - review: He C

Formal analysis, investigation, writing - review: Hui T

Gave advice and methodology to the research: Huang P, Han Q

Supervision, investigation, resources, writing - review and editing: Han WQ

Availability of data and materials

More data on our findings can be found in the Supplementary Material.

Financial support and sponsorship

This work is supported by the Natural Science Foundation of Zhejiang Province (No. LY23E020008), the Key Research and Development Program of Zhejiang Province (No. 2023C01127), the Technologies R&D Program of Huzhou City (No. 2022JB01), “Dongjiang Talent Program” of Qidong City, Highstar Corporation HSD20210118.

Conflicts of interest

All authors declared that there are no conflicts of interest.

Ethical approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Copyright

© The Author(s) 2024.

Supplementary Materials

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Cai L, Ying H, He C, Tan H, Huang P, Han Q, Han WQ. Rational design of nitrogen-doping Ti3C2Txmicrospheres with enhanced polysulfide catalytic activity for lithium-sulfur batteries. Energy Mater 2024;4:400051. http://dx.doi.org/10.20517/energymater.2023.104

AMA Style

Cai L, Ying H, He C, Tan H, Huang P, Han Q, Han WQ. Rational design of nitrogen-doping Ti3C2Txmicrospheres with enhanced polysulfide catalytic activity for lithium-sulfur batteries. Energy Materials. 2024; 4(5): 400051. http://dx.doi.org/10.20517/energymater.2023.104

Chicago/Turabian Style

Lucheng Cai, Hangjun Ying, Chaowei He, Hui Tan, Pengfei Huang, Qizhen Han, Wei-Qiang Han. 2024. "Rational design of nitrogen-doping Ti3C2Txmicrospheres with enhanced polysulfide catalytic activity for lithium-sulfur batteries" Energy Materials. 4, no.5: 400051. http://dx.doi.org/10.20517/energymater.2023.104

ACS Style

Cai, L.; Ying H.; He C.; Tan H.; Huang P.; Han Q.; Han W.Q. Rational design of nitrogen-doping Ti3C2Txmicrospheres with enhanced polysulfide catalytic activity for lithium-sulfur batteries. Energy Mater. 2024, 4, 400051. http://dx.doi.org/10.20517/energymater.2023.104

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