energies Article Tubular C3N4 Nanotubes as Metal-Free Sulfur Hosts toward Stable Lithium–Sulfur Batteries Chaoqi Zhang 1 , Ruifeng Du 1,2 , Sara Martí-Sánchez 3 , Ke Xiao 1,2 , Dawei Yang 1,2,4 , Chaoyue Zhang 1 , Canhuang Li 1,5 , Guifang Zeng 1,2 , Xingqi Chang 1,5 , Ren He 1,2 , Jordi Arbiol 3 , Junshan Li 6 , Jordi Jacas Biendicho 1, * and Andreu Cabot 1,7, * 1 Catalonia Institute for Energy Research–IREC, Sant Adrià de Besòs, 08930 Barcelona, Spain; czhang@irec.cat (C.Z.); kexiao@irec.cat (K.X.); czhang@irec-edu.cat (C.Z.); canhuangli2020@gmail.com (C.L.); zengguifangjennifer@gmail.com (G.Z.); xchan@irec.cat (X.C.); grenhe@irec-edu.cat (R.H.) 2 Department of Physics, Universitat de Barcelona, 08028 Barcelona, Spain 3 Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Spain; sara.marti@icn2.cat (S.M.-S.); arbiol@icrea.cat (J.A.) 4 Henan Key Laboratory of Photovoltaic Materials, Henan University, Kaifeng 475004, China 5 Department of Chemistry, Universitat de Barcelona, 08028 Barcelona, Spain 6 Institute of Advanced Study, Chengdu University, Chengdu 610106, China; lijunshan@foxmail.com 7 ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain * Correspondence: jjacas@irec.cat (J.J.B.); acabot@irec.cat (A.C.) Abstract: Lithium–sulfur batteries (LSBs) with high energy density have the potential to replace current commercial lithium-ion batteries. However, the shuttle effect and the low conversion kinetics of lithium polysulfide (LiPS) remain the main challenges in the development of LSBs. In this study, a metal-free and simple-to-prepare carbon nitride with a high surface area and tubular morphology (CN-nt) is used as the sulfur host for LSBs. Due to its unique nanostructure and rich active sites, it not only effectively disperses the active sulfur material and anchors soluble polysulfide species, but it also promotes the nucleation process of Li2 S, thus achieving fast and sustainable Li-S redox reactions. Experimental results show that the obtained S@CN-nt electrodes exhibit a high sulfur utilization of 1296.2 mAh g−1 at 0.1 C and a significant rate capability of 689.4 mAh g−1 at the high Citation: Zhang, C.; Du, R.; current rate of 3C. More importantly, the capacity retention reaches 87.7% after 500 cycles. This Martí-Sánchez, S.; Xiao, K.; Yang, D.; simple strategy of engineering unique carbon-based nanostructured hosts can inspire new ideas for Zhang, C.; Li, C.; Zeng, G.; Chang, X.; developing cost-effective and metal-free host materials for sulfur-based batteries. He, R.; et al. Tubular C3 N4 Nanotubes as Metal-Free Sulfur Keywords: carbon nitride; nanotube; nanosheet; lithium–sulfur battery; energy storage material Hosts toward Stable Lithium–Sulfur Batteries. Energies 2023, 16, 4545. https://doi.org/10.3390/en16124545 Academic Editor: Massimo Guarnieri 1. Introduction The existence of portable power sources mainly associated with the development of Received: 26 April 2023 lithium-ion batteries (LIBs) has brought great convenience to our daily life [1,2]. How- Revised: 27 May 2023 ever, after over 30 years of development, LIBs are experiencing a bottleneck in energy Accepted: 30 May 2023 density improvement (theoretical value 420 Wh kg−1 , based on graphite anodes and Published: 6 June 2023 LiNi1/3 Mn1/3 Co1/3 O2 cathodes) [3], and the rising raw material prices are increasing the cost pressure [4]. Lithium–sulfur batteries (LSBs), with high potential energy density (2500 Wh kg−1 ) and low-cost materials, can meet the rapidly growing demand in the fields Copyright: © 2023 by the authors. of electric vehicles and smart grids [5]. However, the practical application of Li-S bat- Licensee MDPI, Basel, Switzerland. teries is limited by several inherent challenges in the sulfur cathode [6]. Firstly, lithium This article is an open access article polysulfides (LiPSs) as intermediate products in the Li-S reaction process dissolve in the distributed under the terms and electrolyte and cause a shuttle effect, resulting in irreversible sulfur loss and negative elec- conditions of the Creative Commons trode corrosion [7]. Secondly, in the lithiation process, up to 80% volume expansion of Li2 S Attribution (CC BY) license (https:// compromises the structural stability, leading to electrode structure collapse and eventual creativecommons.org/licenses/by/ failure [8]. Thirdly, S/Li2 S with electronic/ion insulation characteristics cannot achieve 4.0/). Energies 2023, 16, 4545. https://doi.org/10.3390/en16124545 https://www.mdpi.com/journal/energies Energies 2023, 16, 4545 2 of 13 sufficient dispersion to obtain good utilization [9]. At the same time, the slow reaction kinetics in the Li-S multi-electron reaction process also needs to be improved urgently [10]. To overcome the above challenges, a series of strategies have been developed over the past decade, among which the use of sulfur hosts is the most extensively studied strat- egy [11]. Carbon materials, including mesoporous carbon, graphene, and carbon nanotubes, as the earliest studied hosts, could uniformly disperse sulfur-active materials through their high specific surface area and high electrical conductivity, ultimately improving sulfur utilization and mitigating volume expansion [12]. However, the weak physical interaction between polar LiPSs and non-polar carbon materials cannot suppress the severe shuttle effect of LiPSs. Although a series of transition metal compound materials has been inves- tigated in the past several years to utilize their polar surfaces to suppress shuttle effects, some drawbacks still cannot be avoided, such as lower specific surface area products and a complex preparation process, accompanied by the generation of toxic pollutants. Therefore, employing metal-free host materials through a simple preparation procedure has become a direction worthy of exploration to address the current challenges in LSBs. Efficient chemical adsorption of LiPSs can be achieved by heteroatom doping of carbon materials, which has been extensively studied in recent years [13]. Taking nitrogen as an example, the doped nitrogen atoms work as effective active sites to chemically adsorb Li atoms of LiPS through Lewis acid–base coupling, thereby inhibiting the shuttle effect of LiPS. However, even with various complex procedures, the nitrogen content in current nitrogen-doped carbon materials is still very low (less than 10 wt%). Therefore, further development is needed to achieve high-nitrogen-content carbon-based hosts through a simple method. Similar to graphene, graphitic carbon nitride (g-C3 N4 ) is another representative two- dimensional (2D) layered material [14]. It has attracted widespread attention due to its excellent chemical stability, ease of synthesis, and low cost. Theoretically, g-C3 N4 has a high nitrogen content of up to 60 wt%, including N-(C)3 and C=N-C (pyridinic-N) species [15]. Density functional theory (DFT) calculations have shown that pyridinic-N has a much stronger chemical interaction with LiPS compared to other forms of nitrogen [16,17]. However, it is worth noting that although g-C3 N4 theoretically possesses an ultra-high specific surface area similar to graphene, conventional synthesis methods cannot exploit this advantage. For example, with the most traditional calcination and liquid exfoliation method, the obtained carbon nitride nanosheets (CN-ns) suffer from severe Van der Waals forces-induced restacking, resulting in unsatisfactory specific surface area (even lower than 30 m2 g−1 ) [18]. By designing alternative nanostructures with more rigid architectures, such as nanotubes, the intrinsic structural rigidity can avoid the aggregation of layered materials, while the associated increase in pore volume can contribute to effective buffering of the electrode material volume expansion [19]. Additionally, an interconnected structure is also advantageous for the rapid diffusion of Li+ . Currently, the template method is an effective means of obtaining unique nanostructured g-C3 N4 [16]. However, the additional cost of the template material and subsequent removal steps undoubtedly increase the cost pressure, especially in a material-intensive industry such as batteries. In this paper, a simple one-step method is detailed for the preparation of carbon nitride nanotubes (CN-nt) with high specific surface area and porosity, which are applied as a metal-free sulfur-host material in LSBs. Unlike the conventionally prepared carbon nitride nanosheets (CN-ns), the CN-nt, characterized by higher surface areas and a tubular structure, improve the distribution of sulfur, effectively increasing the utilization of active materials. At the same time, the extensively exposed pyridinic-N active sites provide strong trapping of LiPS and accelerate the conversion process. Therefore, S@CN-nt cathodes exhibit excellent rate performance and cycling stability, demonstrating unique advantages in the application of Li-S batteries from the perspective of nanostructures. 2. Materials and Methods Preparing CN-nt and CN-ns. The synthesis of CN-nt was achieved via a one-step calcination method. In this process, 10 g of urea (99.5%, ACROS Organics, Geel, Belgium) Energies 2023, 16, 4545 3 of 13 and 1 g of melamine (99%, ACROS Organics) were thoroughly ground for 15 min using a mortar to create a white precursor mixture. The resulting mixture was then placed in a crucible covered loosely with a lid and annealed at 550 ◦ C for 4 h in a muffle furnace under a standing air atmosphere, with a heating rate of 5 ◦ C min−1 . Following natural cooling to room temperature, the resulting 0.72 g light-yellow powder was collected without further treatment (ca. 6.5% yield). CN-ns were also synthesized via the same annealing method, only using melamine as the precursor. Subsequently, the obtained yellow powder was ground into fine particles and subjected to 2 h of ultrasonic treatment to produce the CN-ns (ca. 16.2% yield). Synthesis of S@CN-nt, S@CN-ns. Typically, CN-nt and CN-ns are mixed with sulfur powder (Sigma Aldrich, Burlington, MA, USA, 99.98%) separately by the weight ratio of 1:3, and then the mixture is heated at 155 ◦ C overnight in an autoclave under Ar protection. 3. Results and Discussion A precursor regulation strategy is used for the preparation of different carbon nitrides. As shown in Figure 1a,b, scanning electron microscope (SEM) images indicate that CN-ns and CN-nt samples have different morphologies. The CN-ns sample is obtained by cal- cination of melamine precursor, followed by subsequent ultrasonic exfoliation, resulting in a stacked nanosheet morphology. In contrast, the CN-nt sample made by the precur- sor of urea and melamine mixture and subject to the same calcination step, without any subsequent treatment, resulted in a product with a hollow tube-shape morphology with a diameter of about 200–300 nm and a length of several micrometers. The formation mecha- nism of hollow g-C3 N4 nanotubes has been investigated before. In the pyrolysis process, urea is first converted to cyanuric acid at an initial rapid heating process. Then, through hydrogen bonding with melamine, supramolecular rod-shaped melamine–cyanuric acid is formed [20,21]. During the subsequent annealing process, the resulting collapse occurs from the center of the nanorod along the tube wall direction, thus forming hollow tubular CN-nt [22]. Energy dispersive spectrometry (EDS) compositional maps indicate that the C and N elements are evenly distributed both in CN-nt (Figure 1e) and CN-ns (Figure S1) samples. Transmission electron microscopy (TEM) images provide more detailed mor- phological information, as shown in Figure 1c, where the CN-nt exhibit a wrinkled outer wall. The internal hollow structure can be recognized by the cross-sectional TEM image in Figure 1c, which allows for calculating a tube wall thickness of about 30–40 nm. Fur- thermore, electron energy loss spectroscopy (EELS) analysis confirmed the homogeneous distribution of C and N (Figure 1f). X-ray diffraction (XRD) analysis shows that CN-ns and CN-nt samples exhibit identical diffraction peaks at 12.9◦ and 27.75◦ corresponding to the characteristic diffraction planes of the (100) and (002) of g-C3 N4 (Figure 1d) [23]. The nitrogen adsorption–desorption isotherms of CN-ns and CN-nt are displayed in Figure 1g. According to the Barrett–Joyner– Halenda model, CN-nt have a high specific surface area of 198.78 m2 g−1 , well above that of CN-ns at 33.27 m2 g−1 . Similarly, a significant difference in pore volume between the two samples can be found, with the CN-nt sample displaying a much larger pore volume of 0.89 cm3 g−1 than CN-ns (0.17 cm3 g−1 ). These results demonstrate that the unique hollow tubular structure of CN-nt helps to overcome the stacking limitation of CN-ns driven by Van der Waals forces. Table S1 shows the differences in the synthetic method, specific surface area, and pore volume of g-C3 N4 obtained by different methods. It is clear from the list that this one-step pyrolysis of urea and melamine mixture has unique advantages. The thermal stability of the CN-nt was also verified by conducting and analyzing thermogravimetric analysis (TGA) and derivative thermogravimetry (DTG) results, as illustrated in Figure 1h. The calcination process under N2 atmosphere showed that CN-nt were thermally stable at temperatures below 550 ◦ C. At this temperature, the weight started to slowly decrease, but the fastest weight loss occurred at around 725 ◦ C. CN-nt were completely decomposed at around 770 ◦ C. This result is consistent with previous reports about g-C3 N4 [24], proving that the hollow tubular structure does not decrease the thermal stability of g-C3 N4 . rgies 2023, 16, x FOR PEER REVIEW 4 of Energies 2023, 16, 4545 4 of 13 FigureFigure 1. (a)1. SEM image of CN-ns. (b) SEM image of CN-nt. Inset histograms show the nanotu (a) SEM image of CN-ns. (b) SEM image of CN-nt. Inset histograms show the nanotube diameter distributions. diameter (c)(c) distributions. HADDF-STEM HADDF-STEM imageimageofofCN-nt. CN-nt. (d)(d) XRDXRD pattern pattern of CN-ns of CN-ns and CN-nt. and CN-nt. SEM image and EDS mapping image of CN-nt. (f) HADDF-STEM image and (e) SEM image and EDS mapping image of CN-nt. (f) HADDF-STEM image and EELS compositional EELS compositio mapping of CN-nt. mapping of CN-nt. 2 adsorption–desorption (g)(g)NN isotherms 2 adsorption–desorption isotherms of CN-nt of CN-nt and Inset and CN-ns. CN-ns. Inset shows the shows specific surface specific area surface and area andpore porevolume volume ofofCN-nt CN-ntandand CN-ns. CN-ns. (h) and (h) TGA TGA and DTG DTG(i)curves. curves. (i) XPS spec XPS spectra of C1sofand C1s N1s. and N1s. XPS analyses were used to determine the surface composition and chemical states. X-ray diffraction (XRD) analysis shows that CN-ns and CN-nt samples exhibit ide Figure S2 shows that both CN-ns and CN-nt samples are mainly composed of C and N, tical with diffraction peaks of a small amount atO12.9° and with associated 27.75°the corresponding inevitable exposure totothe air. characteristic The proportion ofdiffracti planes of theelements nitrogen (100) and (002)and in CN-nt of g-C CN-ns3Nis4 (Figure 1d) [23]. close, at 50.5% The nitrogen and 56.3%, adsorption–deso respectively. Figure 1i exhibits the C1s and N1s spectra of CN-nt, and the C1s spectrum tion isotherms of CN-ns and CN-nt are displayed in Figure 1g. According to can be fitted with three the Barre peaks at binding energies of 284.6, 285.8, and 288.0 eV, which are attributed to pure graphitic2 −1 Joyner–Halenda model, CN-nt have a high specific surface area of 198.78 m g , w sites in the carbon nitride matrix (C–C), the sp2 -hybridized carbon atoms bonded to N in aboveanthat of CN-ns aromatic at 33.27and ring (C–N–C), m2 the g−1.spSimilarly, a significant difference in pore volume b 2 -hybridized carbon in the aromatic ring attached tweento the two the NH samples 2 group can [25]. (N-C=N) be found, The N1swith the CN-nt spectrum sample displaying can be deconvoluted a much larg into four peaks, ascribable to pyridinic-N (C=N-C) pore volume of 0.89 cm g than CN-ns (0.17 cm g 3 −1 at 398.3 eV, N–(C) 3 at 399.6 eV, N–H at 400.7 3 ). These results demonstrate −1 eV, and that t uniqueoxygen-N hollow at tubular 403.8 Ev [26]. Overall,of structure XPS analyses CN-nt showto helps that CN-nt contain overcome the astacking large number limitation of pyridinic-N species, which are expected to have a favorable effect on the confinement of CN-ns thedriven by Van LiPS shuttle effectder and Waals forces. thus favor stableTable S1 shows the differences in the synthe LSB performance. method, specific To prepare surface area,material, the cathode and pore volume sulfur of g-C3N4into was introduced obtained the CN-ntby different via a melt metho It is clear from diffusion the list method. The that this one-step XRD diffraction pyrolysis pattern in Figure S3 ofdemonstrates urea and melamine the presencemixture of h abundant sulfur unique advantages. The crystals in S@CN-nt, while the TGA curve shows a 73.7% thermal stability of the CN-nt was also verified by conducti mass loss in S@CN-nt before 350 ◦ C, corresponding to the mass fraction of S in the composite. SEM and analyzing thermogravimetric analysis (TGA) and derivative thermogravime images and the corresponding EDX elemental maps revealed that the tubular structure of (DTG) results, as illustrated in Figure 1h. The calcination process under N2 atmosphe showed that CN-nt were thermally stable at temperatures below 550 °C. At this tempe ture, the weight started to slowly decrease, but the fastest weight loss occurred at arou 725 °C. CN-nt were completely decomposed at around 770 °C. This result is consiste images and the corresponding EDX elemental maps revealed that the tubular structure of CN-nt was preserved in S@CN-nt, and the S element exhibited a homogeneous distribu- tion without independent S particles or major aggregates. This result demonstrates that CN-nt, with their high surface area and hollow tubular structure, serve as a sulfur host to Energies 2023, 16, 4545 5 of 13 achieve a uniform distribution. The same experimental data obtained for S@CN-ns (Figure S4) showed a less uniform dispersion of S in this sample. Visualizing the adsorption CN-nt was preservedbehavior of soluble in S@CN-nt, and theLiPS is beneficial S element exhibited afor understanding homogeneous distribution the adsorption capacity withoutof the host Smaterial independent particles or and validating major aggregates.its shuttle This effect inhibition. result demonstrates that CN-nt, Equal amounts of Super P, CN-ns, and CN-nt were immersed in the same amount oftoaachieve with their high surface area and hollow tubular structure, serve as a sulfur host 5 a uniform distribution. The same experimental data obtained for S@CN-ns (Figure S4) mM Li2S4 solution and left undisturbed overnight. After this time, compared to the brown- showed a less uniform dispersion of S in this sample. ish-red Li2S4 solution, CN-nt showed Visualizing a clear supernatant, the adsorption indicating behavior of soluble LiPS isthat soluble beneficial forLi 2S4 spe- understanding cies were adsorbedthe byadsorption the CN-nt material (Figure 2a). This high adsorption is capacity of the host material and validating its shuttle effect inhibition.related to Equal amounts the high specific surface area andofits Super polar P, nature CN-ns, and CN-nt were associated with immersed in the same the presence amount of a of nitrogen. 5 mM Li2 S4 solution and left undisturbed overnight. After this time, compared to the In particular, the presence of Lewis-base pyridine nitrogen in g-C3N4 helps to interact with brownish-red Li2 S4 solution, CN-nt showed a clear supernatant, indicating that soluble the strong Lewis-acidLi2 S4of terminal species were Li atomsby adsorbed inthe LiPS CN-nt[27]. In contrast, material a slightly (Figure 2a). This high yellowish adsorption is supernatant was observed related to thein the highCN-ns specific vial, surfacedemonstrating area and its polarthat theassociated nature CN-nt with a higher with the presence of specific surface area have a stronger adsorption capacity. The supernatant color of3 Super nitrogen. In particular, the presence of Lewis-base pyridine nitrogen in g-C N 4 helps to P is similar to thatinteract of LiPS, with the strong Lewis-acid indicating that Super of P terminal composed Li atoms of in LiPS [27]. In non-polar contrast, sur- graphite a slightly yellowish supernatant was observed in the CN-ns vial, demonstrating that the CN-nt with faces cannot work for LiPS capture [28]. As shown in the UV–vis absorption spectra of the a higher specific surface area have a stronger adsorption capacity. The supernatant color of supernatant (Figure 2b),P the Super 400–450 is similar to thatcm −1 band region is the fingerprint of the Li2S4 ab- of LiPS, indicating that Super P composed of non-polar graphite sorption spectra [29], in which surfaces cannotCN-nt work for exhibited LiPS capture the[28]. lowest intensity, As shown in theindicating UV–vis absorptionthe lowest spectra of the supernatant (Figure 2b), the 400–450 cm −1 band region is the fingerprint of the Li S content of Li2S4 in the supernatant and proving the best adsorption capacity. Furthermore, 2 4 absorption spectra [29], in which CN-nt exhibited the lowest intensity, indicating the lowest the XPS N1s spectra of the CN-nt sample were also compared and analyzed, both before content of Li2 S4 in the supernatant and proving the best adsorption capacity. Furthermore, and after adsorption theof XPS LiN1s 2S4.(Figure 2c), and it was found that the peak assigned to pyri- spectra of the CN-nt sample were also compared and analyzed, both before and dinic nitrogen shifted from 398.3ofeV after adsorption Li2to 398.45 2c), S4 .(Figure eV,and which is found it was attributed that thetopeak the assigned fact thattoLiPS pyridinic can anchor on active nitrogen nitrogenshiftedsites fromthrough 398.3 eV to 398.45 eV, whichinteractions, dipole–dipole is attributed toconsistent the fact thatwith LiPS can anchor previous reports [27]. on active nitrogen sites through dipole–dipole interactions, consistent with previous reports [27]. Figure 2. (a) Photograph Figureand 2. (a)(b) UV–vis spectra Photograph of thespectra and (b) UV–vis polysulfide solution after of the polysulfide exposure solution to theto the after exposure different adsorbers. (c) High-resolution different adsorbers. (c) XPS N1s spectra High-resolution XPSbefore and before N1s spectra after adsorption of Li2Sof and after adsorption 4. Li2 S4 . Considering the complexity of the multi-electron transfer process in the Li-S reaction, evaluating the electrocatalytic activity of host materials during the redox process can effec- tively forecast the performance of LSBs [30,31]. In this direction, symmetric cell tests based on different host materials were assembled and tested. As shown in Figures 3a and S5, in the symmetric cell test without polysulfide, CN-nt show a very low response current and the rectangular CV curve confirms that only double-layer adsorption occurs on their sur- face [32]. When adding Li2 S6 into the electrolyte, both CN-ns and CN-nt exhibit polysulfide oxidation-reduction signals in the CV curve. However, CN-nt show significantly higher oxidation-reduction current and more pronounced oxidation-reduction peaks, indicating that CN-nt have higher activity in promoting the Li-S reaction compared to CN-ns. S5, in the symmetric cell test without polysulfide, CN-nt show a very low response current and the rectangular CV curve confirms that only double-layer adsorption occurs on their surface [32]. When adding Li2S6 into the electrolyte, both CN-ns and CN-nt exhibit poly- sulfide oxidation-reduction signals in the CV curve. However, CN-nt show significantly higher oxidation-reduction current and more pronounced oxidation-reduction peaks, in- Energies 2023, 16, 4545 6 of 13 dicating that CN-nt have higher activity in promoting the Li-S reaction compared to CN- ns. a8 CN-nt b 2.09 V 2.06 V CN-ns 2.03 V 2.00 V 0.2 4 Current (mA) Deposition of Li2S Current (mA) −1 CN-nt: 760.2 mAh g 0 −1 CN-ns: 435.9 mAh g 0.1 -4 CN-nt w/o Li2S6 -8 0.0 -1.0 -0.5 0.0 0.5 1.0 0.0 3.0x10 4 6.0x10 4 9.0x10 4 1.2x10 5 c2 Potential (V) d6 Time (s) S@CN-ns Ⅲ −1 Current Density(A g ) 0.4 mV s III −1 Current Density (A g ) -1 S@CN-nt 0.3 mV s −1 1 3 −1 − 0.2 mV s −1 0.1 mV s 0 0 3 Ⅰ Ⅰ I Ⅱ -1 -3 0 Ⅲ II Ⅱ -3 0.010 0.015 0.020 -2 -6 1.6 2.0 2.4 2.8 2.0 2.4 2.8 3.2 Potential (V) Potential (V) (a)CV Figure 3. (a) CVprofiles profilesofof symmetrical symmetrical cells cells with with different different hosthost materials materials using using an electrolyte an electrolyte con- taining 0.5 0.5 containing molmol L−1 LLi−21S6Li . (b) S 2 6 PITT . (b) measurements PITT measurements of Li|Li of S Li|Li 2 8 cells S 2 8 with cells different with host different materials, host the materials, cells were discharged from 2.09 to 2.00 V with a step of 30 mV. (c) CV curves the cells were discharged from 2.09 to 2.00 V with a step of 30 mV. (c) CV curves of different of different cathodes in aa coin in coin cell. cell. (d) (d) CV CV curves curves of of S@CN-nt S@CN-nt cathode cathode under under various various scan scan rates. rates. The The inset inset plot plot is the CV is the CV peak current density (Y axial, A g−1) for peaks Ⅰ, Ⅱ, and Ⅲ versus the square root of the scan rates (X − 1 peak current density (Y axial, A g ) for peaks I, II, and III; versus the square root of the scan rates axial, (V s−1)0.5). (X axial, (V s−1 )0.5 ). In the In the discharge discharge process process of of an an LSB, LSB, the the theoretical theoretical contribution contribution of of the the transition transition from from liquid Li 2S 4 to solid Li 2S can account for 75% of the capacity. Therefore, liquid Li2 S4 to solid Li2 S can account for 75% of the capacity. Therefore, the potentiostatic the potentiostatic intermittent titration intermittent titration technique technique(PITT) (PITT)waswasused usedtotoquantitatively quantitatively study thethe study promoting promoting ef- fect ofofhost effect hostmaterials materialsininthethedeposition depositionprocess process[33,34]. [33,34]. During During thethe PITT PITT test, test, the cathode the cathode with an with an electrolyte electrolyte containing containing 0.25 0.25 MM LiLi22SS88 was was subjected subjected to pulse discharge to pulse discharge at at different different voltages, and voltages, and thethe corresponding corresponding response response current current isis shown shown in in Figure Figure 3b.3b. Due Due toto the the fast fast redox kinetics kinetics of LiPS in the solution state, a sharp current response response was observed in both CN-ns and CN-ns andCN-nt CN-ntcellscellsduring duringthe the2.09 2.09 VV potentiostatic potentiostatic process, process, followed followed by aby a rapid rapid de- decline clinegradual and and gradual stabilization. stabilization. In theInsubsequent the subsequent 2.06 2.06 V potentiostatic V potentiostatic process process related related to to the the electrochemical electrochemical phasephase transition transition reaction, reaction, bothshowed both cells cells showed a response a response currentcurrent peak peak except except for for therapid the initial initial rapid response response and decay, and decay, indicating indicating thatproducts that the solid the solidofproducts Li2 S/Li2 Sof2 Li2S/Li were 2S2 were precipitated precipitated from the containing from the electrolyte electrolyte LiPS containing with theLiPS with the prolongation prolongation of the constantof the constant voltage time voltage at 2.06 V.time at 2.06 V. The nucleation nucleation and and growth growth of ofLi Li22S/Li S/Li22SS22crystals crystalsare are closely closely related related to the inherent nucleation nucleation sitessitesononthe substrate the [35,36]. substrate DueDue [35,36]. to their higher to their specific higher surfacesurface specific area and unique area and tubular structure, CN-nt provide abundant nucleation sites, which effectively accelerates the nucleation kinetics of Li2 S/Li2 S2 . Consequently, compared with CN-ns, the CN-nt host exhibits a higher current response (approximately 50% increase in current density) at 2.06 V, as well as an earlier peak response time (approximately ahead 14,700 s). The insulating nature of Li2 S/Li2 S2 hinders the capacity to release during the discharge process. Therefore, based on Faraday’s law, the current vs. time curves were integrated for the phase transition discharge process from 2.09 to 2.00 V to quantitatively analyze the liquid–solid phase transformation capacity. The results show that the CN-nt-related cell can release a capacity of 760.2 mAh g−1 , while only 435.9 mAh g−1 in the CN-ns cell. These results demonstrate the promoting effect of CN-nt on the Li2 S nucleation process, which is beneficial for the performance of LSBs. Energies 2023, 16, 4545 7 of 13 Subsequently, coin-type LSBs were assembled using S@CN-ns and S@CN-nt cathodes. As shown in the cyclic voltammetry (CV) curves (Figure 3c), both S@CN-ns and S@CN-nt electrodes exhibit two reduction peaks, which correspond to the transformation of S8 into long-chain Li2 Sx (4 ≤ x ≤ 8) and subsequent conversion to insoluble Li2 S2 /Li2 S. The anodic peak during the oxidation process corresponded to the conversion of solid Li2 S/Li2 S2 to long-chain soluble LiPS and finally to S8 . By comparing the CV curves, it was evident that the S@CN-nt electrode exhibited a higher peak voltage than the S@CN-ns electrode during the reduction process. Peak I and II located at 2.290 and 1.992 V, respectively, were significantly higher than the 2.22 V and 1.919 V of S@CN-ns. In addition, compared to the 2.489 V of the S@CN-ns electrode, a significantly lower anodic peak voltage was shown at 2.438 V from the S@CN-nt electrode. Furthermore, the S@CN-nt electrode exhibited excellent current response, with a peak current density much higher than those of S@CN-ns at all three peaks. These results demonstrate that CN-nt, which have a higher specific surface area and porous structure, are more suitable as a sulfur host material than CN-ns and are therefore beneficial for improving the performance of LSBs. As the scan rate of the CV curve increases, a shift of the redox peaks and the response of the current density is generally observed. In Figure 3d, with the scan rate increasing from 0.1 to 0.4 mV s−1 , the reduction peak slightly shifts towards lower potential while the oxidation peak shifts towards higher potential, accompanied by an increasing current density. By fitting the current and square root of the scan rate, it can be found that the results show a good linear relationship, which is consistent with a diffusion limitation during the electrochemical test process [37,38]. Herein, the classical Randles–Sevchik equation for a semi-infinite diffusion can be applied [39,40], as shown below: Ip = (2.69 × 105 )n1.5 ADLi+ 0.5 CLi+ ν0.5 where Ip is the peak current (A), n is the charge-transfer number, S is the geometric area of the electrode (cm2 ), DLi+ is the chemical diffusion coefficient (cm2 s−1 ), CLi+ is the concentration of electrolyte (mol cm−3 ), and ν is the potential scan rate (V s−1 ). Based on this equation and the slope values from the inset plot of Figure 3d, the apparent diffusion coefficients for the peaks of I, II, and III are 1.55 × 10−7 , 3.52 × 10−7 , and 6.68 × 10−7 cm2 s−1 [41]. Subsequently, the assembled coin cells were subjected to electrochemical impedance spectroscopy (EIS) testing. The Nyquist plot from fresh cells consists of a semicircle in the high-frequency region and an oblique line in the low-frequency region, corresponding to the charge transfer resistance and the Li+ diffusion of the electrodes, respectively. It is clear that compared to the S@CN-ns electrode, the S@CN-nt electrode exhibits a smaller semicircle and a more sloping straight line, indicating a smaller charge transfer resistance and a faster Li+ diffusion process in the S@CN-nt electrode [42]. The smaller charge transfer resistance is due to the efficient dispersion of insulating sulfur by the high surface area of the CN-nt structure, while the fast Li+ diffusion process might be attributed to the fast Li+ transport in the tubular and interconnected structure [37,43]. Therefore, the EIS results suggest that the CN-nt host with unique architecture has a promoting effect on the performance of LSBs. The LSB performance was evaluated in a galvanostatic charge–discharge test at 0.1C. As shown in Figure 4a, both the S@CN-ns and S@CN-nt electrodes presented two discharge plateaus and one long charge plateau in the charge–discharge curves, corresponding to the two reduction peaks and one oxidation peak in the CV curves, respectively. Obviously, the S@CN-nt electrode exhibited a superior charge–discharge capacity, up to 1246 mAh g−1 , which was significantly higher than the 1040 mAh g−1 of the S@CN-ns electrode, proving the rationality of CN-nt material as a sulfur host. In addition, polarization voltage can be defined by the voltage difference at 50% discharge capacity to quantitatively evaluate the polarization situation of the cathode in the Li-S reaction [44,45]. The polarization voltage of the S@CN-nt electrode was 152 mV, much lower than the 223 mV of the S@CN-ns electrode, which could be explained by the uniform distribution of sulfur active materials tively. Obviously, the S@CN-nt electrode exhibited a superior charge–discharge capac- ity, up to 1246 mAh g−1, which was significantly higher than the 1040 mAh g−1 of the S@CN-ns electrode, proving the rationality of CN-nt material as a sulfur host. In addi- tion, polarization voltage can be defined by the voltage difference at 50% discharge ca- Energies 2023, 16, 4545 pacity to quantitatively evaluate the polarization situation of the cathode in the Li-S 8re- of 13 action [44,45]. The polarization voltage of the S@CN-nt electrode was 152 mV, much lower than the 223 mV of the S@CN-ns electrode, which could be explained by the uni- form distribution of sulfur active materials achieved by CN-nt and the abundant active achieved by CN-nt and the abundant active sites provided for the Li2 S conversion process, sites provided for the Li2S conversion process, thus effectively reducing the voltage po- thus effectively reducing the voltage polarization during the charge–discharge process. larization during the charge–discharge process. Moreover, during the discharge process, Moreover, during the discharge process, according to the electronic transfer theory, the according to the electronic transfer theory, the second plateau represented the Li2S4→ second plateau represented the Li2 S4 → Li2 S conversion process and released 75% of the Li 2S conversion process and released 75% of the capacity. Therefore, considering the in- capacity. Therefore, considering the inadequacy of the Li2 S nucleation process, the ratio of adequacy the secondofplateau the Li2Scapacity nucleation process, Q2 to the firstthe ratio of plateau the second capacity plateau Q1 can capacity Q2 quantitatively to describe the the conversion ability of LiPS to Li2 S [46,47]. According to the calculation, the valuetoof first plateau capacity Q1 can quantitatively describe the conversion ability of LiPS Li 2S [46,47]. According to the calculation, the value of Q2/Q1 corresponded to 2.31 in the Q2/Q1 corresponded to 2.31 in the S@CN-nt electrode, which was much higher than the S@CN-nt 2.02 of theelectrode, S@CN-nswhich was thus electrode, much higher than verifying the 2.02 of effect the promoting the S@CN-ns electrode, of the CN-nt host inthus the verifying the promoting Li S nucleation process. effect of the CN-nt host in the Li2S nucleation process. 2 a 600 S@CN-nt b 2.8 S@CN-ns c250 223 S@CN-ns 2.5 S@CN-ns S@CN-nt S@CN-nt 2.31 Potential (V) 400 Rct W 200 Ratio 2.4 −Z''(ohm) Rs 2.02 mV 2.0 CPE ΔE 152 200 2.0 150 0 1.6 100 1.5 0 100 200 300 400 0 400 800 1200 ΔE Q2/Q1 -1 Z'(ohm) Capacity (mAh g ) d1500 0.1C 0.2C 0.5C 1C 2C 3C 0.2C e Specific Capacity(mAh g ) 2.8 −1 Potential vs Li/Li (V) + 1000 2.4 3C, 2C, 1C, 0.5C, 0.2C, 0.1C 500 2.0 0 1.6 0 10 20 30 0 300 600 900 1200 1500 g Cycle Numbers -1 Capacity (mAh g ) f Specific Capacity (mAh g−1) 1200 1200 Specific Capacity (mAh g−1) 100 100 Coulombic Efficiency(%) Coulombic Efficiency(%) 900 Current Rate:3 C 900 Current Rate:0.5 C 75 75 600 600 50 50 87.8 % 74.8 % 300 S@CN-nt 25 300 S@CN-nt 25 −2 Charge −2 Charge S loading: 1.0 mg cm Discharge S loading: 3.5 mg cm Discharge 0 0 0 0 0 100 200 300 400 500 0 25 50 75 100 Cycle Numbers Cycle Numbers Figure Figure 4.4. (a) (a)EIS EISspectra spectraofof S@CN-ns S@CN-ns and S@CN-nt and S@CN-nt electrodes. The The electrodes. Nyquist curves Nyquist werewere curves fittedfitted con- sidering considering the equivalent circuits shown as inset, where Rs, Rct, and W stand for the resistance of the the equivalent circuits shown as inset, where Rs, Rct, and W stand for the resistance of the electrolyte, interfacial charge transportation, and semi-infinite Warburg diffusion, respectively; and electrolyte, interfacial charge transportation, and semi-infinite Warburg diffusion, respectively; and CPE stands for the corresponding capacitance. (b) Charge/discharge curves of different electrodes CPE stands for the corresponding capacitance. (b) Charge/discharge curves of different electrodes at a at a current rate of 0.1 C in second cycles. (c) The value of ΔE and Q2/Q1 obtained from charge/dis- current curves. charge rate of 0.1 (d)CRate in second cycles. capability of(c) theThe S@CN-ns ∆E and value ofand Q2/Q1 S@CN-nt obtainedatfrom cathodes charge/discharge various current rates curves. from 0.1(d) C Rate to 3 capability of the S@CN-nscurves C. (e) Charge/discharge and S@CN-nt cathodes of S@CN-nt at variouscurrent at different currentrates. rates from 0.1 C to (f) Capacity 3 C. (e) Charge/discharge curves of S@CN-nt at different current rates. (f) Capacity retention of S@CN- nt at 3 C for 500 cycles. (g) Cycling performance of electrodes with a 3.5 mg cm−2 sulfur loading. Figure 4d displays the rate performance of the different electrodes. S@CN-nt achieved an initial capacity of 1296.2 mAh g−1 at a current density of 0.1C, much higher than the 1098.9 mAh g−1 of the S@CN-ns electrode. In addition, even at a higher current density of 3C, S@CN-nt still maintained a high discharge capacity, reaching up to 689.4 mAh g−1 , while the S@CN-ns electrode could not release any capacity. The polarization situation at different current densities can be understood by analyzing the charge–discharge curves. The S@CN-nt electrode exhibited two plateaus in all discharge curves from 0.1C to 3C, although the voltage polarization increased with the enlarged current rates. This result was still significantly better than that of S@CN-ns. As an example, at a current rate of 1C, the second plateau of S@CN-ns disappeared due to severe polarization, and the capacity could Energies 2023, 16, 4545 9 of 13 not be released at 2C due to excessively high polarization (Figure S6). Therefore, it can be concluded that the S@CN-nt electrode effectively reduces the polarization voltage during the Li-S reaction, promoting the conversion process of LiPS. Cycle stability is an important indicator for evaluating the performance of LSB. A 500-cycle stability test was conducted on the CN-nt host under a high current rate of 3C. In addition, S@CN-nt were still able to release a capacity of 636.8 mAh g−1 , with a capacity retention of about 87.8% and an average Coulombic efficiency as high as 99.5%. Such excellent cycle stability can be attributed to the tubular hollow structure that cushions the volume expansion during Li-S conversion, as well as the high specific surface area and abundant pyrrolic-N active sites that anchor LiPS and suppress the shuttle effect. Table S2 in the Supporting Information displays a comparison of several parameters of state-of-the-art g-C3 N4 -based materials as cathode hosts for LSBs. It can be found that the CN-nt host presented here is characterized by competitive capacities and stabilities. To meet the future commercial demands of LSBs, it is necessary to detect the perfor- mance under higher areal sulfur loading conditions. Therefore, cathodes with a sulfur loading of up to 3.5 mg cm−2 were produced and electrochemically tested. As shown in Figure 4g, S@CN-nt exhibited an initial capacity of 805.9 mAh g−1 at a current rate of 0.5 C and achieved a cycle stability of 74.8% after 100 cycles. The charge–discharge curves at 0.5 C with low/high-sulfur loading conditions were compared, as shown in Figure S7. Higher voltage polarization is caused under high-sulfur loading conditions. This is mainly due to the challenge of electron transfer and ion transport caused by thickened electrodes [48–50]. At the same time, more polysulfide compounds will be produced on the cathode during the charging and discharging process, while higher current density on the anode side will exacerbate the growth and pulverization of lithium dendrites. Therefore, such a good cyclic stability of the S@CN-nt electrode under high-sulfur loading conditions demonstrates that CN-nt act as a host material that could promote the release of the capacity of sulfur active materials, inhibit shuttle effects, and maintain stability during cycling. Thus, it is a suitable metal-free host material for LSBs. 4. Conclusions In conclusion, this study presents a one-step method for obtaining tubular CN-nt through precursor modification. In comparison to conventionally obtained CN-ns, CN-nt offer distinct advantages in terms of specific surface area and pore volume. This leads to improved distribution of sulfur, enhancing the utilization of active materials and suppress- ing volume expansion during the Li-S reaction. The interconnected porous structure also facilitates the diffusion of Li+ ions. Additionally, the abundant pyridinic-N active sites play a crucial role in inhibiting the diffusion of LiPS, thereby ensuring a stable battery capacity. Leveraging these advantages, the S@CN-nt electrode exhibits robust performance, including high initial capacity and an ultra-low decay rate, surpassing that of the CN-ns host. This study highlights the effective regulation of g-C3 N4 nanostructures by controlling precursors, offering insights into the preparation of metal-free host materials for application in LSBs. Supplementary Materials: The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/en16124545/s1, Figure S1 SEM and EDS mapping images of CN-ns. Figure S2 XPS survey spectra of CN-ns and CN-nt. Figure S3 (a) XRD pattern of S@CN-nt. (b) TGA test under N2 atmosphere of S@CN-nt. (c) SEM and EDS mapping images of S@CN-nt. Figure S4 (a) XRD pattern of S@CN-ns. (b) TGA test under N2 atmosphere of S@CN-ns. (c) SEM and EDS mapping images of S@CN-ns. Figure S5 CV curves of CN-nt symmetrical cells without the presence of Li2 S6 . Figure S6 Charge/discharge curves of S@CN-ns at different current rates. Figure S7 Charge/discharge curves of S@CN-ns at 0.5 C with different sulfur loadings. Table S1. Comparison of precursor, specific surface area, and pore volume of CN-nt with previously reported g-C3 N4 materials obtained by different calcination methods. Table S2. Comparison of CN-nt electrochemical performance as host cathode for LSBs with state-of-the-art C3 N4 -based materials. References [51–67] are cited in the Supplementary Materials. Energies 2023, 16, 4545 10 of 13 Author Contributions: Conceptualization, C.Z. (Chaoqi Zhang), J.J.B., and A.C.; methodology, C.Z. (Chaoqi Zhang), R.D., and K.X.; software, C.Z. (Chaoqi Zhang); validation, C.Z. (Chaoqi Zhang), C.Z. (Chaoyue Zhang), R.H., and C.L.; formal analysis, C.Z. (Chaoqi Zhang), and S.M.-S.; investi- gation, C.Z. (Chaoqi Zhang), R.D., S.M.-S., K.X., D.Y., C.Z. (Chaoyue Zhang), C.L., G.Z., X.C., R.H., J.A. and J.L.; resources, J.J.B. and A.C.; writing—original draft preparation, C.Z. (Chaoqi Zhang); writing—review and editing, A.C., J.A., and J.J.B.; visualization, C.Z. (Chaoqi Zhang) and S.M.-S.; supervision, A.C., J.A., and J.J.B.; project administration, A.C. and J.J.B.; funding acquisition, A.C. and J.J.B. All authors have read and agreed to the published version of the manuscript. Funding: The authors thank the support from the project COMBENERGY, PID2019-105490RB-C32, from the Spanish Ministerio de Ciencia e Innovación. The authors acknowledge funding from Generalitat de Catalunya 2021 SGR 01581 and 2021 SGR 00457. ICN2 acknowledges the Severo Ochoa program from Spanish MINECO (Grant No. SEV-2017-0706). IREC and ICN2 are funded by the CERCA Programme from the Generalitat de Catalunya. ICN2 is supported by the Severo Ochoa program from Spanish MCIN/AEI (Grant No. CEX2021-001214-S). ICN2 acknowledges funding from Generalitat de Catalunya 2017 SGR 327. This study was supported by MCIN with funding from European Union NextGenerationEU (PRTR-C17.I1) and Generalitat de Catalunya. The authors thank the support from the project NANOGEN (PID2020-116093RB-C43), funded by MCIN/ AEI/10.13039/501100011033/ and by “ERDF A way of making Europe”, by the “European Union”. This work was supported by the Natural Science Foundation of Sichuan province (NSFSC) and funded by the Science and Technology Department of Sichuan Province (2022NSFSC1229). The authors acknowledge scholarship from the China Scholarship Council (201706650011). Data Availability Statement: Data are contained within the article or supplementary material. Conflicts of Interest: The authors declare no conflict of interest. References 1. Blomgren, G.E. The Development and Future of Lithium Ion Batteries. J. Electrochem. Soc. 2016, 164, A5019. [CrossRef] 2. Xie, J.; Lu, Y.-C. 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