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Article

Electrocatalytic and Conductive Vanadium Oxide on Carbonized Bacterial Cellulose Aerogel for the Sulfur Cathode in Li-S Batteries

by
Xueyan Lin
1,
Wenyue Li
2,
Xuan Pan
3,
Shu Wang
4 and
Zhaoyang Fan
2,*
1
School for Engineering of Matter, Transport & Energy, Arizona State University, Tempe, AZ 85281, USA
2
School of Electrical, Computer and Energy Engineering, Arizona State University, Tempe, AZ 85281, USA
3
Institutes of Science and Development, Chinese Academy of Sciences, Beijing 100190, China
4
College of Health Solutions, Arizona State University, Phoenix, AZ 85004, USA
*
Author to whom correspondence should be addressed.
Batteries 2023, 9(1), 14; https://doi.org/10.3390/batteries9010014
Submission received: 14 November 2022 / Revised: 15 December 2022 / Accepted: 22 December 2022 / Published: 26 December 2022
(This article belongs to the Special Issue Feature Papers to Celebrate the First Impact Factor of Batteries)
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Abstract

:
Many transition-metal-oxide-based catalysts have been investigated to chemically bind soluble lithium polysulfides and accelerate their redox kinetics in lithium-sulfur (Li-S) battery chemistry. However, the intrinsic poor electrical conductivities of these oxides restrict their catalytic performance, consequently limiting the sulfur utilization and the rate performance of Li-S batteries. Herein, we report a freestanding electrocatalytic sulfur host consisting of hydrogen-treated VO2 nanoparticles (H-VO2) anchored on nitrogen-doped carbonized bacterial cellulose aerogels (N-CBC). The hydrogen treatment enables the formation and stabilization of the rutile VO2(R) phase with metallic conductivity at room temperature, significantly enhancing its catalytic capability compared to the as-synthesized insulative VO2(M) phase. Several measurements characterize the electrocatalytic performance of this unique H-VO2@N-CBC structure. In particular, the two kinetic barriers between S8, polysulfides, and Li2S are largely reduced by 28.2 and 43.3 kJ/mol, respectively. Accordingly, the Li-S battery performance, in terms of sulfur utilization and charge/discharge rate, is greatly improved. This work suggests an effective strategy to develop conductive catalysts based on a typical transition metal oxide (VO2) for Li-S batteries.

1. Introduction

As one of the emerging energy storage devices, lithium–sulfur (Li-S) batteries are attracting considerable interest due to their high theoretical energy density (2600 Wh kg−1) and specific capacity (1672 mAh/g), as well as the abundance of low-cost sulfur sources [1]. However, the highly insulating nature of S and Li2S that causes poor sulfur utilization and large polarization, along with the severe shuttle effect of lithium polysulfides (LiPSs) that results in rapid capacity degradation and poor cycle life, critically impede the commercialization of Li-S batteries. Many studies have been conducted to construct conductive sulfur host materials and structures to resolve these problems [2]. Coupling sulfur with porous nanocarbons (e.g., aerogels, hollow spheres, graphene and carbon nanotubes) is a common strategy to simultaneously improve cathode electrical conductivity and physically confine dissolved LiPSs in the porous structure [3,4,5,6,7]. Furthermore, chemical modification of the nonpolar carbon host to form a polar surface by heteroatom doping vastly enhances its interactions with polar LiPSs, consequently alleviating the shuttle effect [8,9,10,11,12]. The sluggish redox reaction kinetics of LiPSs leads to their concertation accumulation in the electrolyte around the cathode region, accelerating the diffusion and migration of LiPSs toward the Li anode [13]. Therefore, electrocatalysts that capture and accelerate the redox reactions of LiPSs have attracted enormous attention. This category of materials consists of transition metal oxides, nitrides, and sulfides, among others [3,14,15,16,17,18,19,20].
Of the various metal compounds, oxides have a higher binding capability to LiPSs than their nitrides and sulfides counterparts but suffer from their more insulative nature that severely restricts their overall catalytic behavior [21,22]. For these oxides with low conductivity, the electrocatalytic reaction process of LiPSs involves three primary steps: adsorption on the electrocatalyst surface, diffusion to the neighboring conductive surface to accept (or donate) electrons and Li+ ions, and the subsequent redox conversion. The necessary strong binding between LiPSs and the catalyst essentially prevents LiPSs from diffusion on the catalyst surface and therefore suffocates the whole process [23]. An ideal polysulfide electrocatalyst must also be electronically conductive in addition to its appropriate adsorption capability to immobilize LiPSs and catalytic activity to accelerate polysulfide conversion [24]. A few conductive oxides, such as hydrogen-reduced TiO2-x [25] and Ti4O7 [26] and nitride/oxide composites [27], have been reported with excellent catalytic performance in Li-S chemistry.
Among the many polymorph phases of VO2, VO2(R) and VO2(M) are stable phases at high and low temperatures, respectively. The rutile VO2(R) phase is a metallic conductor with a conductivity of ~104 S/cm, rendering it a suitable electrocatalyst for polysulfides. However, this high-temperature stable phase experiences a metal-insulator transition at ~ 68 °C in the low-temperature monoclinic VO2(M) phase, which is electrically insulative with a conductivity of less than 0.1 S/cm−1 [28]. Fortunately, studies have shown that hydrogen doping stabilizes metallic VO2(R) below room temperatures [29,30,31,32]. Our previous study showed that changing the stoichiometry of VO2 or doping protons into VO2 using H2 treatment can substantially increase its electrical conductivity [33].
Herein, we report our study on conductive VO2 nanoparticles (NPs) as the electrocatalyst in Li-S chemistry. To our knowledge, this is the first report of exploiting the conductive rutile VO2(R) phase as the electrocatalyst to accelerate the polysulfide conversion reaction in Li-S batteries. We synthesized H2 thermal-treated VO2 NPs anchored on N-doped carbonized bacterial cellulose (H-VO2@N-CBC) to overcome the insulating problem of the oxide catalysts and to modify the carbon scaffold surface. N-doped carbonized bacterial cellulose has been proven to be an excellent freestanding sulfur host [34]. CBC surface modification by N-dopants and H-VO2 NPs can also improve electrolyte wettability, facilitating a better charge transfer [35]. N-dopants on the CBC surface could further serve as active sites for VO2 NP nucleation and growth [36]. The highly conductive H-VO2 NPs catalyze the Li-S redox reactions and regulate Li2S and S8 deposition. With this H-VO2@N-CBC structure as the sulfur host, we studied its electrocatalytic functions through several electrochemical characterizations. In particular, the two kinetic barriers between S8, polysulfides, and Li2S are significantly reduced by 28.2 and 43.3 kJ/mol, respectively, compared to the N-CBC-based structure. Accordingly, the Li-S battery performance, in terms of sulfur utilization and charge/discharge rate, is significantly improved.

2. Materials and Methods

2.1. Material Synthesis

The nitrogen-doped carbonized bacterial cellulose (N-CBC) matrix was prepared by carbonization of the freeze-dried BC aerogel at 900 °C under an Ar atmosphere with the assistance of urea. The freestanding VO2-NP-decorated N-doped CBC (VO2@N-CBC) composite was fabricated via a solvothermal method. In brief, 265 mg Vanadyl Acetylacetonate (VO(acac)2) was first dissolved in ethanol (95 mL) and kept stirring for 24 h, during which the V5+ was reduced to V4+. Afterward, the solution was transferred into a Teflon-lined stainless-steel autoclave with 50 mg N-CBC and 5 mL DI water added. After heating at 180 °C for 24 h, the obtained VO2@N-CBC was rinsed with DI water and ethanol several times and dried in an oven at 80 °C. The dried VO2@N-CBC sheet was treated at 450 °C for 2 h under a 5% H2/Ar mixed atmosphere to obtain the final H-VO2@N-CBC sample. Then, the samples were punched into a disk with a diameter of 12 mm, which served as the freestanding sulfur hosts.

2.2. Material Characterization

The scanning electron microscopy (SEM) images of the prepared nanostructured samples were acquired using Zeiss Auriga at an acceleration voltage of 5 kV. The powder X-ray diffraction (XRD) measurements were performed on an X-ray Diffractometer (Malvern PANalytical Aeris) with Cu Kα radiation (λ = 0.1541 nm) over the 2θ range of 10–80° with a scan speed of 2° min−1 at room temperature. The X-ray photoelectron spectroscopy (XPS) spectra were recorded by a Krator Axis Supra + apparatus using Al K-alpha (1486.6 eV) as the excitation light source.

2.3. Electrochemical Measurement

Lithium polysulfide adsorption tests: 0.25 M Li2S6 solution was first prepared by mixing sublimed sulfur powder and Li2S with a molar ratio of 5:1 in blank electrolyte, which consists of 1 M bis(trifluoroethanesulfonyl)imide lithium (LiTFSI), 2 wt% lithium nitrites (LiNO3) in 1.3-dioxolane (DOL) and 1.2-dimethoxy ethane (DME) (1:1, v/v) solution. The 0.25 M Li2S6 solution was then diluted to 0.05 M for the visual adsorption test. 1.13 m2 of each sample, H-VO2@N-CBC, VO2@N-CBC and N-CBC, was loaded into 4 mL 0.05 M Li2S6 solution to observe the color change. All samples were vacuum-dried at 80 °C overnight before testing.
Assembly of Li2S6 symmetric cells and their cyclic voltammogram (CV) measurement: Two identical electrodes with the same materials were used as working and counter electrodes, and 40 μL 0.25 M Li2S6 catholyte was dropped on each electrode. The CV of these symmetric cells was measured at a scan rate of 3 mV s−1 and 10 mV s−1 with a potential window between −1 and 1 V to evaluate the polysulfide conversion kinetics.
Li2S precipitation test: Nucleation and growth of Li2S on three different electrodes were conducted by dropping 20 μL 0.25 M Li2S8 catholyte on each electrode and another 20 μL blank electrolyte on the Li-chip anode side. The assembled cells were first to galvanostatically discharged at 0.1 mA/g to 2.06 V and then potentiostatically discharged at 2.05 V for Li2S precipitation and growth until the current dropped below 10−5 A.
Li-S coin cell battery assembly and electrochemical performance: The H-VO2@N-CBC/S, VO2@N-CBC/S, and N-CBC/S composite cathodes were prepared by dropping 60 μL 0.25 M Li2S6 catholyte on each electrode to obtain a sulfur loading of 2.5 mg cm−2. CR 2016 coin cells were assembled in an argon-filled glove box using the prepared H-VO2@N-CBC/S, VO2@N-CBC/S and N-CBC/S composite cathodes, Celgard 2400 membranes as the separators, and lithium chip as the anode. The galvanostatic charge–discharge curves of these Li-S cells were recorded on a LANDCT2001A battery test under different current rates at a voltage range from 1.7 to 2.8 V. Their CV profiles were collected on a Biologic SP-240 electrochemical workstation at a scan rate from 0.2 to 0.5 mV s−1. The electrochemical impedance spectrum (EIS) was collected in the range of 100 kHz–0.01 Hz with an AC voltage amplitude of 5 mV.

3. Results

3.1. Material Characterization

Figure 1a schematically shows the cathode scaffold fabrication process. BC aerogel, derived from BC hydrogel after freeze-drying, is carbonized with urea in an Ar environment to obtain N-doped carbon nanofiber aerogel (N-CBC). Through a solvothermal process, VO2 nanoparticles (NPs) are grown on N-CBC, which is further thermally treated in a H2/Ar environment to convert the insulating VO2 to its conductive form, called H-VO2 here. The SEM images in Figure 1b,c show the microscopic morphology of VO2@N-CBC and H-VO2@N-CBC, respectively, revealing that the oxide NPs are anchored on the carbon nanofibers. The whole structure is well connected to form a 3D scaffold for the rapid transportation of electrons and Li+ ions and for hosting sulfur species.
The crystal structures and phase transitions of the prepared samples were studied using the X-ray diffraction (XRD) technique, as presented in Figure 1d. The XRD patterns of N-CBC mainly display a broad peak in the range of 20–30°, indicating an amorphous nature of the carbonized aerogels [37] because the carbonization was conducted at a limited temperature of 900 °C. However, the prepared VO2@N-CBC presents two prominent diffraction peaks at 27.8° and 39.8°, corresponding to the (0 1 1) and (0 2 0) planes of the VO2(M) phase (JCPDS No. 074-1642), respectively. After hydrogen treatment, these two dominant VO2 characteristic peaks were retained at similar diffraction angles, but additional peaks appear at 24.3°, 33.0°, 36.3°, 41.2°, 49.9°, 53.9°, 63.2° and 65.2°, which are indexed to the (0 1 2), (1 0 4), (1 1 0), (1 1 3), (0 2 4), (1 1 6), (2 1 4) and (3 0 0) reflections of V2O3 (JCPDS No. 005-4280), respectively. The formation of V2O3 reveals that partial VO2(M) was reduced during the hydrogen annealing and carbothermal reduction processes [33]. It is noted that V2O3 has a conductivity of ~103 S/cm [38], while VO2(M) is insulative with a conductivity of ~0.1 S/cm.
A careful inspection reveals that the two preserved VO2-related peaks, in fact, slightly shift to a lower diffraction angle: from 27.8° to 27.6° (Figure 1e) and from 39.8° to 39.7°, respectively, and these two new peaks can be ascribed to the (1 1 0) and (2 0 0) planes of the VO2(R) phase [29,31,39]. It is well known that the high-temperature VO2(R) phase is a metallic conductor with a conductivity of ~104 S/cm, which experiences metal-insulator transition at ~68 °C to the low-temperature insulative VO2(M) phase [28]. However, studies have shown that hydrogen doping stabilizes metallic VO2(R) at low temperatures [29,30]. From the XRD peak position shift measured at room temperature and the appearance of the V2O3 phase, we consider that our H-VO2@N-CBC sample consists of metallic VO2(R) and V2O3 phases. During hydrogen thermal treatment, VO2 was in its metallic rutile phase. When the temperature cooled down, the metallic phase was maintained with hydrogen dopants with no transition to the insulating VO2(M) monoclinic phase. This inference was corroborated by the observation that the synthesized pure VO2 powder’s conductance measured at 100 °C is a few orders larger than that measured at room temperature. In contrast, after the hydrogen thermal treatment, its conductance at room temperature becomes similar to that at high temperatures. The resultant V2O3 /VO2(R) mixed phases possess metallic conductivity at room temperature compared to the initial insulating VO2(M) phase, which can remarkably facilitate electron and ion transport [31,33,40].
The chemical configuration and oxidation states of H-VO2@N-CBC were investigated using X-ray photoelectron spectroscopy (XPS). The broad survey shown in Figure 1f reveals the presence of C, N, V and O elements. The atomic percentages of N and V were detected to be 3.42 at % and 1.72 at %, respectively. The high-resolution V 2p spectra presented in Figure 1h show a typical doublet profile of V 2p3/2 and V 2p1/2, and each can be deconvoluted into three peaks. Two prominent peaks located at 517 and 524 V are ascribed to the V4+ oxidation states, whilst the two peaks at 515.8 and 522.8 V are assigned to the V3+ oxidation states, indicating the partial reduction from V4+ to V3+ [41]. The two small peaks corresponding to V5+ are due to unavoidable surface oxidation in the air [42]. In general, V4+ was found to be the dominating oxidation state in the vanadium oxide mixture, at least on the surface. Regarding the N 1s high-resolution spectra in Figure 1g, peaks at 398.6, 400.1, and 401.3 eV refer to pyridinic N, pyrrolic N, and graphitic N, respectively [43,44]. The existence of N species confirms that N dopants have been successfully incorporated into CBC nanofibers. Furthermore, the main graphitic-N species, which affect the electronic and geometric structure of carbon, can increase conductivity and promote electron transfer [45,46].

3.2. Polysulfide Binding and Li2S Precipitation

A strong binding to the electrocatalyst surface is essential for LiPSs to expedite their redox reactions. To evaluate the adsorption ability of H-VO2@N-CBC, VO2@N-CBC and N-CBC, the three freestanding scaffolds were placed into the diluted Li2S6 solution (0.005 M) and kept still for 12 h for observation as shown in Figure 2a. The dark yellow Li2S6 solution became completely colorless after 12 h for H-VO2@N-CBC. The visual test reveals that the polysulfide adsorption ability follows the sequence of H-VO2@N-CBC > VO2@N-CBC > N-CBC. In N-CBC, its N-dopants and porous structure contribute to the adsorption of polysulfides [34]. By incorporating a small amount of VO2 NPs in VO2@N-CBC, more polysulfides are absorbed due to the chemical capturing effect of VO2 NPs [47]. However, the color of the solutions only became clear for H-VO2@N-CBC. Oxygen vacancies in metal oxides have been regarded as the active sites for the chemical adsorption of polysulfide [48]. The more vital adsorption ability of H-VO2 than VO2 suggests that the hydrogen/carbon thermal treatment substantially promotes the generation of oxygen vacancies in H-VO2, facilitating its binding with polysulfides.
To assess the catalytic ability of the three types of sulfur host in promoting polysulfide conversion, we first characterized the Li2S6 symmetric cells using cyclic voltammetry (CV). When the symmetric cells were tested at the scan rate of 10 mV/s, as shown in Figure 2b, the H-VO2@N-CBC electrode displayed the smallest polarization (voltage gap between cathodic and anodic peaks) and the highest peak current density among the three samples for the polysulfide redox transformation. The CV curves of the VO2-based electrodes (H-VO2@N-CBC and VO2@N-CBC) with Li2S6 electrolyte clearly exhibited two pairs of reversible redox peaks after slowing down the scan rate to 3 mV/s as shown in Figure S1a,b, labeled as peaks a, b, c and d. At the same time, the N-CBC shown in Figure S1c still presents only one pair of redox peaks. It is generally considered that peak a represents the oxidation of Li2S to LiPSs, and peak b suggests the formation of S8 from LiPSs, while the other two peaks c and d are the reversible processes of peaks b and a, respectively [49]. Without Li2S6 presenting in the electrolyte, all these samples showed no apparent reactions. This result suggests that H-VO2@N-CBC had the best performance.
We then investigated the kinetics of Li2S nucleation and growth to further evaluate the polysulfide conversion ability of the three types of electrodes. For this measurement, the potentiostatic discharge profiles at 2.05 V of the cells for the three types of electrodes were collected. It was observed, according to Figure 2c–e, that the time–current curves first sharply decreased due to the reduction of Li2S8 followed by an increase in the current representing the onset of Li2S nucleation. Subsequently, the current reached its peak and then gradually dropped, corresponding to the impingement between Li2S nuclei that led to the ending of the Li2S phase growth. The background current intensity can be attributed to the reduction of Li2S6. By calculating the integrated area of the charge flow, the capacity released from Li2S precipitation can be obtained [50]. As shown in Figure 2c–e, the responsiveness of Li2S nucleation is earlier on H-VO2@N-CBC than on VO2@N-CBC and N-CBC. Moreover, H-VO2@N-CBC exhibited the highest current (0.195 mA) and capacity (277.3 mAh g−1) for the Li2S precipitation process compared to VO2@N-CBC (0.174 mA, 240.6 mAh g−1) and N-CBC (0.106 mA, 154.7 mAh g−1). Corroborating the above CV study, these results show that H-VO2@N-CBC features the best capability to promote Li2S precipitation and consequently enable the efficient utilization of lithium polysulfides [51].

3.3. Activation Energy Barriers

We assembled Li-S full cells using the three types of sulfur host and investigated their catalytic functions by measuring the reaction kinetic barriers. The CV profiles of these cells at a scan rate of 0.2 mV/s are presented in Figure 3a. In the cathodic scan, the first peak voltages of 2.295, 2.267 and 2.223 V were observed for the H-VO2@N-CBC, VO2@N-CBC and N-CBC electrodes, respectively, which corresponded to the reaction from S8 to polysulfide (S8 -> Li2Sn). Afterward, the second peak voltages referring to the conversion from polysulfide to Li2S (Li2Sn -> Li2S) in the cathodic scan were 1.977, 1.957 and 1.879 V for the H-VO2@N-CBC, VO2@N-CBC and N-CBC electrodes, respectively. Correspondingly, the anodic peaks can be attributed to the oxidation of Li2S toward soluble Li2Sn and sulfur. For H-VO2@N-CBC, the cathodic peaks shift to a higher potential, and the anodic peaks shift to a lower potential compared to the peaks of the VO2@N-CBC and N-CBC cathodes. The smallest peak voltage separation between the cathodic and anodic peaks as well as the highest exchange current density indicate the enhanced redox kinetics of H-VO2@N-CBC [52].
To quantitively verify the catalytic effect of H-VO2 and VO2 NPs, the relative activation energy for each reaction step in the discharge process was estimated using the equation E a = E a 0 − R T b   φ R e d i r   where b is the Tafel slopes and φ R e d i r   is the irreversible potential obtained from the CV curves (see details in Supplementary Materials) [53]. To obtain the Tafel slopes b, two linear sweep voltammetry curves (LSV) were obtained from the CV profiles for each reaction peak (Figure 3b,c). Then, the Tafel slopes were derived by converting the LSV curves into the form of E = b   l o g i + c where the E is the voltage and i is the exchange current density [54]. As shown in Figure 3e,f, H-VO2@N-CBC exhibits the lowest Tafel slopes in both cathodic peaks (55.76 and 66.74 mV dec−1) compared to VO2@N-CBC (62.24 and 113.66 mV dec−1) as well as N-CBC (74.63 and 163.75 mV dec−1). The lowest Tafel slope typically indicates the fastest redox kinetics [55]. Therefore, the difference in activation energy (Ea) in each cathode during the discharge process was estimated (Figure 3d). In the S8 to LiPSs conversion reaction, the Ea of N-CBC is 16.45 kJ mol−1 higher than that of VO2@N-CBC, while the latter is 11.73 kJ mol−1 higher than that of H-VO2@N-CBC. When soluble LiPSs are converted to solid Li2S, H-VO2@N-CBC still features the lowest activation energy, which is 28.74 kJ mol−1 lower than that of VO2@N-CBC and 43.34 kJ mol−1 lower than that of the N-CBC electrode. The lowest activation energy barrier in both reaction steps indicates the best catalytic capability of H-VO2@N-CBC among these three sulfur hosts [56].

3.4. Charge Transportation Evaluation

To further evaluate the charge transport characteristics, we measured the CVs of these cells with different scan rates ranging from 0.2 to 0.5 mV s−1. Upon increasing the scan rate, the anodic and cathodic peaks shifted positively and negatively, leading to a larger polarization (Figure 4a–c). The anodic and cathodic current peaks (IA, IC1, IC2) of all three electrodes feature a linear relationship with the square root of the scanning rates (Figure 4d–f), indicating that the ion diffusion process is the rate-determined step in these electrochemical reactions [57]. The lithium-ion diffusion coefficient (D) was derived using the Randles-Sevcik equation [58] I P = 2.69 × 10 5 n 3 / 2 A · C · D 1 / 2 v 1 / 2   where I P is the peak current, n is the number of electrons in the reaction, A is the electrode area, C is the lithium-ion concentration in the electrolyte, and v is the scan rate. The slopes can be derived from the linear relationship between the peak current I P and v 1 / 2 at each redox peak. H-VO2@N-CBC depicted considerably steeper IP/ν1/2 slopes for each redox reaction when compared with VO2@N-CBC and N-CBC, which suggests that H-VO2@N-CBC possess the highest lithium-ion diffusion coefficient. The higher slopes of VO2@N-CBC than N-CBC imply that the VO2 NPs are conducive to lithium-ion diffusion. The 3D porous framework as well as the formation of oxygen vacancies in H-VO2@N-CBC provide an efficient Li+-ions/electrons conduction pathway, which consequently facilitates more rapid charge transportation.

3.5. Li-S Battery Performance

Figure 5a–c displays the galvanostatic charge/discharge profiles of the H-VO2@N-CBC-, VO2@N-CBC- and N-CBC-based sulfur cathodes evaluated by stepwise increasing the rate to 0.1 C, 0.2 C, 0.3 C, 0.5 C and 1 C. When increasing the current density, the charge–discharge curves of H-VO2@N-CBC show a smaller polarization shift than those of VO2@N-CBC and N-CBC, implying the enhanced kinetics of the conversion reaction with the H-VO2@N-CBC cathode materials, which exhibit well-defined dual plateaus even at a high rate of 1 C. Figure 5d shows the overall rate performance of the three types of cathodes from 0.1 to 1 C per every 5 cycles and finally returning to 0.5 C. When cycled at rates of 0.1 C, 0.2 C, 0.3 C, 0.5 C and 1 C, the H-VO2@N-CBC/S cathode delivers the highest discharge capacities of 1347, 1123, 1085, 1044 and 957 mA h g−1, respectively. When the current density returns to 0.5 C, it restores a reversible discharge capacity of 1028 mA h g−1, demonstrating the superior electrochemical performance of the H-VO2@N-CBC cathode at varying rates.
The electrochemical impedance spectroscopy (EIS) curves of the H-VO2@N-CBC, VO2@N-CBC, and N-CBC cathodes were collected, as shown in Figure 5e. The high-frequency semicircles in the Nyquist plots correspond to the charge-transfer resistance (Rct) within the cathodes [59]. The H-VO2@N-CBC, VO2@N-CBC, and N-CBC cathodes exhibit Rct values of 36, 84 and 98 Ω, respectively, which indicates that even though the addition of VO2 results in a lower electron conductivity due to its intrinsic insulating property, hydrogen thermal treatment can significantly reduce the electrical resistance of VO2. The apparently reduced charge-transfer resistance validates the improved reaction kinetics and rate capabilities, which are consistent with the observations in Figure 5a–d. As shown in Figure 5f, the cathode based on H-VO2@N-CBC exhibits minimal polarization compared to those based on VO2@N-CBC and N-CBC, which can be attributed to less ohmic polarization than VO2@N-CBC and a lower activation polarization compared to N-CBC. The longer and flatter lower plateau (QL) during the discharge process suggests that H-VO2@N-CBC has a better affinity/absorption for LiPSs than N-CBC and improved conductivity of VO2 after hydrogen treatment to enable the effective participation of LiPSs in further conversion reactions after chemical capturing [60]. Figure 5g displays the cycling performance of the three cathodes at 1 C. After 300 cycles, the H-VO2@N-CBC cathode retains a remarkable discharge capacity of 758 mA h g−1 with a capacity decay of 0.139% per cycle. In contrast, the VO2@N-CBC and N-CBC cathodes after 300 cycles deliver capacities of 621 and 457 mA h g−1, respectively, owing to their inferior ability in catalyzing LiPS conversion. The outstanding electrochemical performance of H-VO2@N-CBC reveals enhanced polysulfide redox kinetics, which exploits the intrinsic merits of transition metal oxide while addressing its insulating drawbacks, enabling the synchronous capturing and accelerated conversion of LiPSs. In Table S1 in the Supplementary Materials, we compare our device performance with other recent works on catalysts used in Li-S batteries. Our battery cell exhibits a better rate capability as well as cycling stability at a relatively high sulfur loading.

3.6. Catalyst Stability

To observe the catalyst stability, the cells after 300 charge/discharge cycles were disassembled at the fully discharged status. The S-cathode morphology was observed using SEM imaging. With the assistance of H-VO2 (Figure 6a,b) and VO2 (Figure 6c,d) NP catalysts, the precipitation of Li2S was regulated, and fine Li2S particles were uniformly deposited along the surface of the carbon nanofibers. In contrast, without oxide catalysts, bulky Li2S clusters (Figure 6c,f) are found in the N-CBC matrix. After 300 cycles, although some Li2S agglomerations were observed in the H-VO2@N-CBC- and VO2@N-CBC-based sulfur electrodes, the Li2S reduction products were still firmly immobilized and well-dispersed on the carbon nanofibers, which suggests that the active VO2 and H-VO2 NPs were retained on the carbon nanofibers and verified the catalysts’ stability during battery cycling. It is worth noting that some Li2S clusters were formed in VO2@N-CBC in Figure 6e that were not observed in the H-VO2@N-CBC matrix. Therefore, H-VO2 NPs provide a better catalytic ability for converting LiPSs and regulating Li2S deposition than VO2 NPs.

4. Conclusions

In summary, a freestanding composite structure with catalytic H-VO2 NPs anchored on the N-doped CBC scaffold was designed and fabricated as a sulfur host for Li-S batteries. The hydrogen treatment stabilizes the rutile VO2(R) phase at room temperature and partially reduces it to V2O3. Converting the initial insulating VO2(M) phase into metallic conductive VO2(R)/V2O3 improves the cathode’s overall conductivity and overcomes the inherent insulating limitation of conventional transition metal oxide catalysts. This change boosts the redox conversion rates of polysulfides, demonstrated by the various electrochemical measurements. Accompanied by the merits of N-doped CBC, the sulfur cathode based on the H-VO2@N-CBC scaffold delivers a high initial specific capacity of 1347 mAh g−1 at 0.1 C and remarkable cycling stability with a capacity of 758 mAh g−1 after 300 cycles at 1 C. This work provides an effective strategy to design and fabricate conductive catalysts from a typical transition metal oxide (VO2) and subsequently promotes the electrochemical performance of Li-S batteries.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/batteries9010014/s1, Figure S1: The CV curves of H-VO2@N-CBC-, VO2@N-CBC- and N-CBC-based electrodes in a symmetric cell with Li2S6 as the electrolyte at the scan rate of 3 mV/s. The redox peaks are labeled as a, b, c, and d, respectively; Table S1: Comparison of the electrochemical performance of LSBs using different catalytic materials. References [61,62,63,64,65] are cited in the supplementary materials.

Author Contributions

Conceptualization, X.L. and Z.F.; methodology, X.L., W.L. and Z.F.; investigation, X.L.; resources, S.W. and Z.F.; writing—original draft preparation, X.L.; writing—review and editing, X.P., S.W. and Z.F.; supervision, Z.F.; funding acquisition, S.W. and Z.F. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Science Foundation of USA, grant number 2129983 and 2103582.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) Schematic illustration of the overall synthesis procedure of H-VO2@N-CBC; SEM images of (b) VO2@N-CBC and (c) H-VO2@N-CBC; (d) XRD pattern of H-VO2@N-CBC, VO2@N-CBC, and N-CBC, and (e) a magnified region of the XRD data in 26° ≤ 2θ ≤ 29°; (f) XPS broad survey and high-resolution (g) N 1 s and (h) V 2 p spectra of H-VO2@N-CBC.
Figure 1. (a) Schematic illustration of the overall synthesis procedure of H-VO2@N-CBC; SEM images of (b) VO2@N-CBC and (c) H-VO2@N-CBC; (d) XRD pattern of H-VO2@N-CBC, VO2@N-CBC, and N-CBC, and (e) a magnified region of the XRD data in 26° ≤ 2θ ≤ 29°; (f) XPS broad survey and high-resolution (g) N 1 s and (h) V 2 p spectra of H-VO2@N-CBC.
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Figure 2. (a) Photos of H-VO2@N-CBC, VO2@N-CBC, and N-CBC initially added in 5 mM Li2S6 solution and after aging for 12 h; (b) cyclic voltammetry (CV) curves of Li2S6 symmetric cells with H-VO2@N-CBC, VO2@N-CBC, and N-CBC electrodes at a scan rate of 10 mV/s; Li2S precipitation process of (c) H-VO2@N-CBC, (d) VO2@N-CBC, and (e) N-CBC under potentiostatic discharge conditions.
Figure 2. (a) Photos of H-VO2@N-CBC, VO2@N-CBC, and N-CBC initially added in 5 mM Li2S6 solution and after aging for 12 h; (b) cyclic voltammetry (CV) curves of Li2S6 symmetric cells with H-VO2@N-CBC, VO2@N-CBC, and N-CBC electrodes at a scan rate of 10 mV/s; Li2S precipitation process of (c) H-VO2@N-CBC, (d) VO2@N-CBC, and (e) N-CBC under potentiostatic discharge conditions.
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Figure 3. (a) Cyclic voltammetry (CV) profiles of the Li-S cells with H-VO2@N-CBC-, VO2@N-CBC- and N-CBC-based cathodes at a scan rate of 0.2 mV/s with the voltage range of 1.6~2.8 V; (b,c) linear sweep voltammetry (LSV) curves obtained from Figure 3a near Peak I and Peak II; (d) relative activation energy difference among three electrodes depending on a reaction coordinate; (e,f) the corresponding Tafel plot derived from the LSV curves in Figure 3b,c for Peak I and Peak II.
Figure 3. (a) Cyclic voltammetry (CV) profiles of the Li-S cells with H-VO2@N-CBC-, VO2@N-CBC- and N-CBC-based cathodes at a scan rate of 0.2 mV/s with the voltage range of 1.6~2.8 V; (b,c) linear sweep voltammetry (LSV) curves obtained from Figure 3a near Peak I and Peak II; (d) relative activation energy difference among three electrodes depending on a reaction coordinate; (e,f) the corresponding Tafel plot derived from the LSV curves in Figure 3b,c for Peak I and Peak II.
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Figure 4. Cyclic voltammetry (CV) profiles of Li-S full cells with (a) H-VO2@N-CBC, (b) VO2@N-CBC, and (c) N-CBC obtained at scan rates from 0.2 to 0.5 mV/s; (d–f) peak current density (Ip) versus square root of scan rates (ν0.5) with three different electrodes at redox peak A, C1, and C2.
Figure 4. Cyclic voltammetry (CV) profiles of Li-S full cells with (a) H-VO2@N-CBC, (b) VO2@N-CBC, and (c) N-CBC obtained at scan rates from 0.2 to 0.5 mV/s; (d–f) peak current density (Ip) versus square root of scan rates (ν0.5) with three different electrodes at redox peak A, C1, and C2.
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Figure 5. Galvanostatic charge/discharge profiles of (a) H-VO2@N-CBC-, (b) VO2@N-CBC-, and (c) N-CBC-based cathodes at current rates of 0.1, 0.2, 0.3, 0.5, and 1 C; (d) rate performance comparison from 0.1 to 1 C and (e) EIS curves comparison; (f) charge/discharge voltage profile comparison at 0.3 C; (g) long-term cycling stability comparison at 1 C.
Figure 5. Galvanostatic charge/discharge profiles of (a) H-VO2@N-CBC-, (b) VO2@N-CBC-, and (c) N-CBC-based cathodes at current rates of 0.1, 0.2, 0.3, 0.5, and 1 C; (d) rate performance comparison from 0.1 to 1 C and (e) EIS curves comparison; (f) charge/discharge voltage profile comparison at 0.3 C; (g) long-term cycling stability comparison at 1 C.
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Figure 6. SEM images of (a,d) H-VO2@N-CBC-, (b,e) VO2@N-CBC- and (c,f) N-CBC-based sulfur cathodes at the fully discharged state after initial and 300 cycles.
Figure 6. SEM images of (a,d) H-VO2@N-CBC-, (b,e) VO2@N-CBC- and (c,f) N-CBC-based sulfur cathodes at the fully discharged state after initial and 300 cycles.
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Lin, X.; Li, W.; Pan, X.; Wang, S.; Fan, Z. Electrocatalytic and Conductive Vanadium Oxide on Carbonized Bacterial Cellulose Aerogel for the Sulfur Cathode in Li-S Batteries. Batteries 2023, 9, 14. https://doi.org/10.3390/batteries9010014

AMA Style

Lin X, Li W, Pan X, Wang S, Fan Z. Electrocatalytic and Conductive Vanadium Oxide on Carbonized Bacterial Cellulose Aerogel for the Sulfur Cathode in Li-S Batteries. Batteries. 2023; 9(1):14. https://doi.org/10.3390/batteries9010014

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Lin, Xueyan, Wenyue Li, Xuan Pan, Shu Wang, and Zhaoyang Fan. 2023. "Electrocatalytic and Conductive Vanadium Oxide on Carbonized Bacterial Cellulose Aerogel for the Sulfur Cathode in Li-S Batteries" Batteries 9, no. 1: 14. https://doi.org/10.3390/batteries9010014

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