Titanium dioxide nanotubes modified with nickel oxide and nickel nanoparticles for improved polysulfide anchoring and redox kinetics in lithium–sulfur batteries

Abstract

This study systematically investigates and compares the roles of electronic conductivity, polysulfide chemisorption, and catalytic conversion in TiO₂ nanotube-based cathode hosts by evaluating three bifunctional additives: bare TiO₂ nanotubes, NiO-modified TiO₂ nanotubes (NiO/TiO₂), and Ni nanoparticle-modified TiO₂ nanotubes (Ni/TiO₂). Anatase phase TiO₂ nanotubes (∼18.3 nm) were synthesized via a hydrothermal method and integrated into carbon fibre paper to form TiO₂-CFP, NiO/TiO₂-CFP and Ni/TiO₂-CFP composite cathodes, which were evaluated at a high sulfur loading of 4 mg cm⁻². The cell with TiO₂-CFP exhibited moderate polysulfide adsorption but was constrained by poor conductivity and weak catalytic activity, delivering an initial capacity of 995.72 mA h g⁻¹ at 0.2C. The cell with NiO/TiO₂ improved chemisorption and redox conversion, achieving an initial capacity of 1196.4 mA h g⁻¹ at 0.2C. Among the tested electrodes, Ni/TiO₂-CFP delivered the best overall performance, exhibiting an initial specific capacity of 1285 mA h g⁻¹ at 0.2C and retaining ∼1095 mA h g⁻¹ after 100 cycles. Moreover, it showed excellent rate capability: 745.25 mA h g⁻¹, 659.03 mA h g⁻¹, and 381.52 mA h g⁻¹ at 0.5C, 1.0C and 2.0C, respectively, significantly outperforming cells with TiO₂-CFP and NiO/TiO₂-CFP. Ni/TiO₂-CFP further exhibited distinct charge–discharge plateaus with minimal polarization, the lowest charge transfer resistance (14 Ω) and the highest Li₂S nucleation capacity (746 mA h g⁻¹), confirming faster interfacial kinetics. These results establish that the metallic Ni modification of TiO₂ nanotubes most effectively balances polysulfide anchoring and catalytic conversion, providing a rational design pathway for high-loading Li–S battery cathodes.

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1 Introduction

The development of lithium-ion batteries (LIBs) has been crucial for the rise of modern, portable technology and electric mobility.^1,2 Generally, LIBs utilize cathode materials including intercalation-type transition metal oxides and phosphates. However, these materials inevitably limit the energy density of LIBs to below 400 W h kg⁻¹, a level that is not high enough for applications involving high energy storage requirements, such as grid-scale applications,³ electric vehicles,⁴ and advanced portable electronics.^5,6 Beyond energy limitations, the hazard potential and high cost of these cathode materials also underscore the paramount importance of new battery chemistries that are less hazardous, more environmentally friendly, and economically viable.⁷ Recent research continues to highlight these constraints, confirming that surpassing the intrinsic energy density limitations of conventional LIBs remains one of the most pressing challenges in modern energy storage.⁸ Therefore, it becomes crucial to investigate alternative battery materials that can meet the high demands of advanced technology while minimizing environmental harm.

Li–S batteries are seen to be among the most promising options for energy storage because of their high theoretical capacity, which is due to the redox reactions that occur between lithium and sulfur to form lithium sulfide (16 Li + S₈ = 8 Li₂S).^9,10 Despite the benefits of low cost, abundance, and low toxicity, sulfur's poor conductivity, massive volume growth during cycling, and dissolution of intermediate lithium polysulfide species (LiPSs) hinder the development of Li–S batteries.^11–14 A 2025 review underscored that despite significant recent progress, a substantial gap between fundamental Li–S research and practical commercialization persists, driven by slow cathode kinetics, electrolyte instability and anode interface challenges.¹⁵ Similarly, Fei and Li highlighted that low sulfur loading, excessive electrolyte use and an unlimited cathode sulfur fraction continue to significantly constrain the practical energy density of Li–S batteries.¹⁶ Hence, numerous host materials, such as carbon materials,^17,18 conductive polymers,¹⁹ metals,^20,21 etc., have been proposed to date to improve the performance of Li–S batteries. Carbon materials are frequently employed as host materials for Li–S batteries because of their strong electrical conductivity and tunable pore structure.²² Nevertheless, the nonpolar carbon material is physically unable to bind to polar LiPSs, which pass through the separator and reach the anode, causing an irreversible capacity loss during battery cycling; this phenomenon is known as the shuttle effect. The shuttle effect can be efficiently suppressed by using polar materials such as metal oxides,^23,24 metal sulfides,^25,26 metal carbides^27,28 and metal nitrides^29,30 based on Lewis's acid–base interaction or surface redox. Among these metal oxides, TiO₂ is a wide bandgap (3.2 eV) n-type semiconductor material,^31,32 which has been extensively reported as a promising electrode material for Li–S batteries.^33–35 Since interfacial phenomena (that is, the interactions occurring at the contact interface between LiPSs and host materials) play a key role in determining chemical interactions and affinities,³³ various structures with large specific surface areas, such as hollow nanosheets,³⁶ nanospheres,³⁷ nanoparticles,³⁸ and nanotubes,³⁹ have been developed to improve interfacial reactions.⁴⁰

Multiple TiO₂ morphologies have been studied and have shown promise, with TiO₂ nanotubes attracting particular attention.^41–44 TiO₂ nanotubes typically exhibit high specific surface areas, tunable tube or pore diameters in the tens of nanometers, and an open tubular architecture that promotes efficient electrolyte penetration and interfacial accessibility.^45,46 Recent developments in heterostructured systems, including anatase/bronze TiO₂ coupled with polypyrrole and TiO₂/GO-coated functional separators, have improved LiPS confinement and cycling stability in Li–S batteries.^47,48 More recently, TiO₂ nanotubes loaded with WS₂/MoS₂ heterostructures have been shown to construct adsorption–diffusion conversion channels for LiPSs, achieving strong cycling performance under sulfur loadings as high as 5.8 mg cm⁻², further demonstrating the promise of TiO₂ nanotube-based systems while simultaneously reinforcing that intrinsic conductivity limitations remain the central unresolved challenge.³⁹ Despite advances in TiO₂-based structures for LiPS control and structural stability, the practical application of TiO₂ remains constrained by its intrinsically low electronic conductivity. For pristine anatase TiO₂, a room-temperature resistivity of approximately 10¹² Ω cm, corresponding to a conductivity of approximately 10⁻¹² S cm⁻¹, has been reported.⁴⁹

To overcome the poor conductivity of TiO₂ while preserving its strong polysulfide-binding ability, several material modification strategies have been explored.^50–54 For instance, Pu et al. reported that Ru nanocluster-modified TiO₂ nanotubes created an active TiO₂ and Ru heterointerface that enhanced LiP adsorption and accelerated conversion kinetics and improved the cycling stability of Li–S batteries. Nickel nanoparticles have emerged as a particularly promising material due to their multifunctional properties in Li–S battery systems.⁵³ Moreover, a niobium nanoparticle catalyst loaded on TiO₂ nanotubes, according to Barlow et al., exhibits the combined effects of chemical adsorption, physical confinement, and catalytic conversion of polysulfides.⁵⁴ The natural polarity of TiO₂ and the similar ionic diameters of titanium and niobium, which result in numerous defects that enhance electrical conductivity and catalytic activity, facilitate catalyst infiltration into the TiO₂ lattice, leading to a three-dimensional hierarchical structure that effectively mitigates the polysulfide shuttle effect while accelerating conversion kinetics.⁵⁴

As nickel nanoparticles are added to metal oxide-based hosts in Li–S batteries, they give a boost to the catalytic and electrical conductivity of these materials. The interaction of nickel with the oxide host also helps to strongly anchor polysulfides to the cathode and promotes faster electron and ion transport.⁵⁵ Supporting this, a recent study demonstrated that Ni/NiO-embedded carbon nanofibers effectively reduced the polarization of Li–S, accelerated LiPS conversion kinetics, and improved active material utilization by leveraging the dual functionality of metallic Ni domains and NiO redox sites, validating the complementary role that Ni and NiO play when combined in a single host system.⁵⁶ Moreover, NiO exhibits complementary functionality: abundant redox-active Ni²⁺/Ni³⁺ sites for catalytic conversion of polysulfides, strong polysulfide binding (∼1.2 eV for Li₂S₄), and electrical conductivity several orders of magnitude higher than TiO₂ (10⁻³ to 10⁻¹ S cm⁻¹), depending on defect concentration.⁵⁷ In addition, the reduction of NiO nanoparticles forms metallic Ni domains, creating efficient electron transport pathways that enhance the system's conductivity and overall electrochemical performance.⁵⁵ Nickel-based composites with strong chemical trapping ability and multiple oxidation states have indeed shown significant promise for catalyzing LiPS conversion and inhibiting the shuttle effect in diverse host configurations.⁵⁸

Although TiO₂-based polysulfide trapping materials for Li–S batteries have been widely studied, direct comparisons of TiO₂ nanotubes, TiO₂ nanotubes modified with NiO, and TiO₂ nanotubes modified with Ni nanoparticles incorporated into carbon fibre paper remain limited. Moreover, despite growing recognition of the distinct but complementary roles of NiO and metallic Ni in polysulfide management, a systematic evaluation of these three cathode architectures under identical high sulfur loading conditions has also not been reported. As a result, their respective influence on polysulfide anchoring and redox kinetics has not yet been clearly established.

In this study, three anatase phase TiO₂ nanotube-based additives with an average nanotube diameter of approximately 18.3 nm were synthesized via a hydrothermal method. These materials were engineered as bifunctional additives to simultaneously anchor lithium polysulfides and catalyze their conversion at a high sulfur loading. For practical electrochemical evaluation, they were integrated into carbon fibre paper matrices to form composite cathodes, TiO₂-CFP, NiO/TiO₂-CFP, and Ni/TiO₂-CFP. Although the cell with TiO₂-CFP exhibited moderate polysulfide adsorption, its effectiveness was restricted by poor electrical conductivity and weak catalytic activity. By comparison, the cell with NiO/TiO₂-CFP exhibited enhanced chemisorption and facilitated redox conversion, whereas the cell with Ni/TiO₂-CFP markedly improved conductivity and accelerated lithium polysulfide reaction kinetics, leading to improved electrochemical performance in lithium–sulfur batteries. By directly comparing these three cathode systems under identical conditions at high sulfur loading, this study provides mechanistic clarity on the interplay between polysulfide anchoring, catalytic conversion, and electronic conductivity in TiO₂-based hosts, insights essential for guiding the rational design of next-generation Li–S battery cathodes towards practical energy density targets.

2 Experimental section

2.1 Preparation of TiO₂ nanotubes decorated with nickel oxide/nickel nanoparticles

Titanium(IV) dioxide (99.7% Sigma-Aldrich) nanotubes were synthesized by a one-step hydrothermal templating process.⁵⁹ In brief, 1.5 g of anatase TiO₂ powder (99.7%, Sigma-Aldrich, USA) was added to 100 ml of 10 M sodium hydroxide (NaOH, ≥98% Sigma-Aldrich) under magnetic stirring for 12 hours to produce a uniform solution. The solution was transferred to a 100 ml Teflon autoclave and heated at 130 °C in a muffle furnace for 24 hours. After cooling to room temperature, the resulting precipitate was collected via centrifugation. The precipitate was then washed several times with 0.5 M hydrochloric acid (HCl, 38%, Sigma-Aldrich) and deionized water until a neutral pH was achieved. Finally, the product (TiO₂ nanotubes) was air-dried in an oven at 60 °C overnight. The TiO₂ nanotubes in powder form were then calcined in a muffle furnace at 600 °C for 2 hours to improve crystallinity. Reaction temperature and time were found to influence the nanotube structure, wall thickness, and tube length. For surface decoration, 0.11 g of nickel(II) nitrate hexahydrate (Ni (NO₃)₂·6H₂O ≥97% Sigma-Aldrich, USA) was dissolved in 1.1 ml of deionized water and an alkaline polyethylene glycol (PEG) solution was prepared by dissolving 20 mg of PEG (MW = 10 000 g mol⁻¹, Sigma-Aldrich, Germany) in 50 ml deionized water, followed by adjustment of the alkalinity with NaOH to a final concentration of 50 mM. Subsequently, 0.3 g of synthesized TiO₂ nanotubes was added dropwise to the alkaline PEG solution. The solution was then stirred vigorously (1000 rpm) for 300 seconds at room temperature. The light-green precipitate was filtered and washed several times with deionized water and ethanol. Finally, the product (NiO/TiO₂) formed was air-dried at 50 °C for 24 hours. After calcination at 450 °C for 2 hours, NiO/TiO₂ nanotubes in powder form were synthesized with a TiO₂ to NiO molar ratio of 10 : 1.⁶⁰ To synthesize Ni/TiO₂, the NiO/TiO₂ powders were reduced in a N₂/H₂ atmosphere for 2 hours at 450 °C to produce TiO₂ nanotubes decorated with metallic Ni nanoparticles. The sample was cooled under inert conditions to avoid re-oxidation. Reaction time and temperature, along with molar ratios, were effectively regulated to tailor the structural and functional properties of the material. Fig. 1a illustrates the steps involved in the synthesis of TiO₂ nanotubes. The resulting nanotubes were subsequently decorated with NiO and Ni nanoparticles, as shown in Fig. 1b.

Fig. 1 Schematic illustration depicting the synthesis route of (a) TiO₂ nanotubes and (b) decoration of NiO and Ni nanoparticles on the nanotubes. (b) Created with Biorender.com.

2.2 Preparation of cathode materials

During the preparation of the slurry, 30 mg of TiO₂-based nanotubes, 20 mg of acetylene black (MTI Corporation, USA), 130 mg of carbon nanotubes (CNTs, ISOLAB, Germany) and 20 mg of polyvinylidene fluoride (PVDF, Sigma-Aldrich, France) were dispersed in 3.5 ml of N-methyl-2-pyrrolidone (NMP, ≥99%, Sigma-Aldrich, USA) using a high speed Thinky mixer. The prepared slurry was then cast on carbon fibre paper (current collector) to create cathodes for the cells. To ensure uniform dispersion, the mixture was homogenized for 300 seconds at 1200 rpm. An active material loading per disc was obtained by drop-casting approximately 30 µl of the slurry onto carbon fibre paper. Before use, the electrodes were dried in a vacuum oven at 70 °C for 24 hours.

2.3 Cell assembly

Using the prepared TiO₂-based cathodes, Celgard® 2500 as the separator, and a lithium chip as the anode, CR2032 coin cells were assembled in an MBRAUN argon-filled glovebox (H₂O < 0.1 ppm, O₂ < 0.1 ppm). After applying the polysulfide solution (catholyte) to the dried electrode, the electrolyte (1 M LiTFSI with 2 wt% LiNO₃ in DOL/DME, 1 : 1 v/v) was added. An additional amount of electrolyte was added to the anode side of the separator. A lithium chip, a spacer and a wavy spring were added to the cell stack, and the cell was sealed using a manual hydraulic crimper under a pressure of 100 kg cm⁻².

2.4 Materials characterization

The structure and morphological parameters of the synthesized materials were investigated using a scanning electron microscope (SEM), namely JEOL JSM-IT800, Japan, with Energy Dispersive X-ray Spectroscopy (EDS) and Cs-Corrected Scanning Transmission Electron Microscopy (JEOL(JEM-ARM200F)). X-ray diffraction (XRD, Rigaku SmartLab, Japan) in the angle range of 2θ between 10° and 90° was used for establishing the crystalline phase composition using Cu Kα radiation (λ = 1.54056 Å). Thermogravimetric analysis (TGA; an STA 449 F5 Jupiter coupled to a QMS 403 Aeolos Quadro, Germany) was employed to investigate the thermal behaviour of TiO₂, NiO/TiO₂, and Ni/TiO₂ nanotubes in powder form. This analysis was carried out by thermogravimetric analysis using a simultaneous thermal analyzer under argon gas (240.3 ml min⁻¹) from 25 to 650 °C at a rate of 5 K min⁻¹. Textural parameters, i.e., specific surface area and pore volume, were investigated based on nitrogen adsorption–desorption isotherms (Micromeritics TriStar II Plus, England) and pore size distribution determined by the Barrett–Joyner–Halenda (BJH) method. X-ray photoelectron spectroscopy (XPS, NEXSA, Thermo Scientific, USA) was employed for examining oxidation state changes and interactions with LiPS. Elemental composition and metal loading by metal oxide composites were also investigated by inductively coupled plasma optical emission spectroscopy (ICP-OES, iCAP 6300-Thermo Scientific, England).

2.5 Electrochemical characterization

Lithium metal was employed as the counter/reference electrode, with a 1 M Li₂S₆ solution serving as the catholyte and carbon fibre paper, CFP, serving as the cathode current collector. Carbon-coated separators were used for the assembly of the Li–S coin cells. For the two loading scenarios, 11.8 µL and 23.5 µL of Li₂S₆ were added, corresponding to sulfur loadings of 2.0 and 4.0 mg cm⁻², respectively. A strict electrolyte-to-sulfur, E/S, ratio of 10 : 1 was maintained by adding an additional electrolyte volume of 10.8 µL for the 2.0 mg cm⁻² cell and 21.7 µL for the 4.0 mg cm⁻² cell to the anode side. The electrolyte consisted of 1 M LiTFSI with 2% wt LiNO₃ in a 1 : 1 (v/v) mixture of DOL and DME. Galvanostatic charge–discharge measurements were performed using a Neware battery testing system within a voltage window of 1.7–2.8 V vs. Li/Li⁺. Cyclic voltammetry (CV) analyses were performed on a Biologic VMP3 potentiostat/galvanostat within the same potential range, while electrochemical impedance spectroscopy (EIS) was performed at an amplitude of 5 mV over a frequency range of 100 kHz to 1 MHz. The areal sulfur mass loading (s_m) was calculated using the following expression:
where s_m – the areal sulfur mass loading (mg cm⁻²), m_s – the sulfur mass on the CFP mat (mg), V – the catholyte volume (L), A – the CFP mat area (cm²), C – the Li₂S₆ molar concentration (mol L⁻¹), n – the number of sulfur atoms (for Li₂S₆, n = 6), and M_S – relative atomic mass of sulfur (32 g mol⁻¹).⁶¹

2.6 Lithium polysulfide binding analysis

The chemical binding behaviour of LiPS was assessed via the Ti 2p, Ni 2p, O 1s, and S 2p regions on the XPS spectra obtained from (Ø = 16 mm) CFP, TiO₂-CFP, NiO/TiO₂-CFP, and Ni/TiO₂-CFP electrodes. These electrodes were soaked in 2 ml of a 5 mM Li₂S₆ solution dissolved in 1,3-dioxolane and 1,2-dimethoxyethane (DOL/DME, ν = 1 : 1) for 24 h. The test was then conducted at room temperature inside an MBRAUN glovebox (H₂O < 0.1 ppm and O₂ < 0.1 ppm) with a duration of 24 h. Afterwards, XPS analysis was performed on the CFP, TiO₂-CFP, NiO/TiO₂-CFP, and Ni/TiO₂-CFP electrodes, before and after soaking in the Li₂S₆ solution.

2.7 Symmetric cell test

Symmetric cells were constructed by employing two identical synthesized electrodes, separated by a Celgard® 2400 membrane. The catholyte was prepared as a 0.3 M Li₂S₆ solution in a 1 : 1 (v/v) DOL/DME solvent system, and 25 µl was added to both the cathode and the separator. The electrochemical behaviour of polysulfides was investigated by cyclic voltammetry (CV) on a Biologic VMP3 potentiostat/galvanostat at scan rates of 20, 10, and 1 mV s⁻¹, respectively, within a potential window of −1.0 V to 1.0 V (vs. Li/Li⁺).

2.8 Nucleation test

The synthesized material was deposited on the electrode; thus, carbon fiber paper (CFP) served as the working electrode, while metallic lithium discs (Ø = 16 mm) functioned as both the counter and reference electrodes. A volume of 20 µl of 0.5 M Li₂S₆ solution in tetraglyme acted as the catholyte, whereas 20 µl of a 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) solution containing 2 wt% LiNO₃ prepared in a 1 : 1 (v/v) DOL/DME solvent mixture served as the anolyte. CR2032 coin cells with TiO₂-CFP, NiO/TiO₂-CFP and Ni/TiO₂-CFP electrodes were assembled separately and discharged under constant current to 2.06 V, followed by a potentiostatic hold at 2.05 V until the current decreased below 1 × 10⁻⁵ A to induce Li₂S₆ nucleation.

2.9 Beaker cell test

All cathodes with their respective additives (TiO₂, NiO/TiO₂, and Ni/TiO₂) were tested for Li₂S₆ adsorption to determine their adsorption capacities. The cathodes (TiO₂-CFP, NiO/TiO₂-CFP and Ni/TiO₂-CFP) and anode (lithium-ion metal disc) were submerged in a 10 ml solution of 3 µM Li₂S₆ solution. Electrochemical behaviour was investigated by cyclic voltammetry (CV) on a Biologic VMP3 potentiostat/galvanostat at a scan rate of 1 mV s⁻¹ within a potential window of −0.01 V to 0.01 V (vs. Li/Li⁺). Three time intervals, 3, 6 and 12 h, were monitored to provide sufficient time for polysulfide adsorption, leading to an observable colour change.

3 Results and discussion

3.1 Physical characterization of TiO₂ nanotube-based samples

In Fig. 2a, X-ray diffraction analysis of respective samples confirmed the anatase phase of TiO₂, as evidenced by the distinctive peaks at 2θ ≈ 25.3°, 37.8°, 48.0°, 55.1°, 63.1°, 70.9° and 75.4°, which can be assigned to the (101), (004), (200), (211), (204), (220), and (215) crystallographic planes corresponding to the anatase form of TiO₂ (JCPDS 21-1271).⁴⁰ In addition, NiO and Ni were identified by the characteristic (111) and (200) peaks near 2θ ≈ 44.5° and 43.3°, respectively.⁶² Moreover, in Fig. 2b, thermogravimetric analysis (TGA) results revealed that all samples exhibited a small initial weight loss at approximately 120 °C, which was attributed to the desorption of physically adsorbed water. A gradual decrease in weight was then observed up to approximately 450 °C, corresponding to dehydroxylation and the decomposition of organic residues remaining from the synthesis route. The highest total weight loss was observed for the pure TiO₂ sample. Lower weight losses were observed for NiO/TiO₂ and Ni/TiO₂, indicating enhanced thermal stability, which may be attributed to Ni decoration. No significant weight loss was observed for any of the three samples between approximately 450 °C and approximately 650 °C. Hence, in this current study, the optimal calcination temperature was fixed at 450 °C based on the TGA curves. The pore size distribution of the material was analyzed using the Barrett–Joyner and Halenda (BJH) method. The samples were further examined using nitrogen adsorption–desorption isotherms (Fig. 2c), which exhibited a type IV profile with H3-type hysteresis in the relative pressure range of 0.4 to 0.9 P/P_o, confirming the presence of mesopores and slit-like pores. The sharper uptake near 0.9 P/P_o indicates filling of large mesopores and interparticle voids, with the nickel-containing sample showing higher adsorption capacity, suggesting larger pore volume and surface area.⁶³ The pore size distribution (Fig. 2d) shows that Ni/TiO₂ exhibits a dominant mesoporous structure (3–5 nm) with a higher pore volume than NiO/TiO₂ and pure TiO₂, making it more suitable for applications requiring a large surface area and efficient mass transport. ICP-OES analysis (Fig. 2e) indicated a Ti : Ni molar ratio of approximately 10 : 1, consistent with the ratio used during the synthesis. Table S1 also reports the BET surface areas of the respective samples, with Ni/TiO₂ exhibiting the largest surface area, 87.71 m² g⁻¹. Moreover, the average particle size of nanotubes (Fig. S1) was determined to be 18.27 nm. Overall, the results confirm the successful formation of thermally stable, anatase-phase mesoporous Ni/TiO₂ nanotubes with an optimized composition, a large surface area, and a favourable pore structure.

Fig. 2 Physical characterization of TiO₂, NiO/TiO₂, and Ni/TiO₂ nanotubes: (a) XRD pattern, (b) TGA, (c) nitrogen adsorption–desorption isotherms, (d) pore size distribution and (e) ICP OES.

Fig. 3 presents comprehensive morphological characterization of TiO₂ nanotubes and TiO₂ nanotubes modified with NiO/Ni nanoparticles using SEM and TEM imaging across multiple scales. Fig. 3a shows TiO₂ nanotubes with vertically aligned, smooth-walled tubular morphology, consistent with hydrothermal treatment-derived structures. These structures are known to offer a high surface area and facilitate ion transport. Fig. 3b reveals NiO/TiO₂ with a visibly roughened surface and granular texture, resembling agglomerated NiO, which may be due to calcination. In Fig. 3c, TiO₂ shows a discrete nanoparticle dispersion across the TiO₂ framework with slightly shorter lengths of the nanotube. Low-resolution TEM images of Ni/TiO₂ at 50 nm and 100 nm are shown in Fig. 3d and e, respectively, revealing aggregated nanostructures and confirming the decoration of Ni within the TiO₂ matrix. An additional surface morphological investigation was conducted using TEM, as seen in Fig. 3f. The interplanar spacing of the (101) plane of the TiO₂ anatase phase was determined to be approximately 0.35 nm. For Ni nanoparticles, the interplanar spacings of the (111) and (200) planes were found to be 0.203 nm and 0.176 nm, respectively.

Fig. 3 SEM images of nanotubes: (a) TiO₂, (b) NiO/TiO₂, and (c) Ni/TiO₂; low-resolution TEM image of Ni/TiO₂ nanotubes at (d) 50 nm and (e) 100 nm; and (f) HRTEM images of Ni/TiO₂ (d-spacing).

Fig. 4(a–d) display Ni/TiO₂-CFP electrodes at increasing magnifications, revealing a hierarchically porous architecture with interconnected domain features and EDS maps indicating the presence of Ti and Ni, respectively. The purpose of this analysis was to determine the presence of TiO₂ nanotubes, NiO nanoparticles and Ni nanoparticles in the cathode material. Additionally, elemental mapping was performed to confirm the homogeneous distribution of these elements. It was observed that fragments were attached to single strands of carbon fibre in the cathode material, and each fragment was observed to contain Ti and Ni, as revealed by the EDS maps.

Fig. 4 SEM image of the Ni/TiO₂-CFP electrode at (a) 1 µm, (b) 0.5 µm, and (c) 100 nm; EDS maps of the Ni/TiO₂ electrode at (d) 2.5 µm.

Systematic XPS characterization revealing the surface chemical changes through all stages of material modification is shown in Fig. S2. Survey spectra (Fig. S2a) indicate that carbon fibre paper (CFP) contains C 1s and O 1s signatures, which agree with the presence of graphitic carbon and surface oxidic functionalities.⁶⁴ The TiO₂-CFP survey spectrum shows the presence of Ti 2p and F 1s peaks, where the appearance of the fluorine signal arises from the polymer binder used in fabricating the electrodes. Moreover, the appearance of Ni 2p features in the spectra of the electrodes containing NiO/TiO₂ and Ni/TiO₂ confirms the presence of nickel in the composite material. C 1s spectra (Fig. S2b) indicate three discernible carbon environments in all samples: C–C (∼284.8 eV), C–O (∼286.0 eV), and O=C–O (∼288.5–289.0 eV) corresponding to sp² carbon, ether/alcohol functionalities, and oxidized carbon, respectively.^65,66 The reduction in C 1s intensity with increasing surface coverage from CFP through the metal-decorated samples correlates with increasing TiO₂ and nickel coverage. Ni 2p spectra (Fig. S2c and f) distinguish nickel oxidation states. NiO/TiO₂-CFP presents a Ni 2p_3/2 peak at ∼856 eV and a Ni 2p_1/2 peak at ∼873.5 eV with high-intensity satellite bands, typical of Ni²⁺ in NiO. In contrast, Ni/TiO₂-CFP displays peaks at 852.8 eV and 857 eV, which correspond to Ni²⁺ and Ni³⁺, respectively.^40,56 The O 1s spectra (Fig. S2d) show three components, including lattice oxygen (Ti–O) at ∼530.0 eV, surface hydroxyls (Ti–OH) at ∼531.5 eV, and surface species at ∼533.0 eV, consistent with TiO₂ surface chemistry.⁵⁴ Ti 2p spectra (Fig. S3e) show a 2p doublet, namely Ti 2p_3/2 at ∼458.5 eV and Ti 2p_1/2 at ∼464.5 eV with ∼5.5 eV splitting, which confirms the Ti⁴⁺ oxidation state.^67–69 The XPS results validate the structural and chemical integrity of the materials before exposure to Li₂S₆ by confirming the presence of well-defined nickel and titanium oxidation states.

XPS characterization following Li₂S₆ soaking, as shown in Fig. 5 and S3, presents the polysulfide adsorption tendency along with chemical interactions in all the composite materials. Survey spectra (Fig. S3a) show that all electrodes (CFP, TiO₂-CFP, NiO/TiO₂-CFP, and Ni/TiO₂-CFP) show a distinct S 2p peak following Li₂S₆ exposure, verifying the successful polysulfide adsorption, with the accompanying retention of their intrinsic characteristic elemental signatures of C 1s, O 1s, and metal peaks (Ti 2p, Ni 2p, and F 1s where applicable). C 1s spectra (Fig. S3b) indicate that CFP after soaking has three carbon environments with C–C bonds at ∼284.8 eV, C–O bonds at ∼286 eV, and O=C–O bonds at ∼288–289 eV,⁶⁵ revealing that the carbon framework remains unaffected following polysulfide interaction across all the electrodes. Ti 2p spectra (Fig. 5a) recorded before and after Li₂S₆ soaking show that all TiO₂-containing electrodes maintain a Ti 2p_3/2 peak at 458.7 eV, characteristic of the Ti⁴⁺ oxidation state in TiO₂,^63,68 with the Ti 2p_1/2 peak appearing at ∼464.6 eV, resulting in a spin–orbit splitting of approximately 5.5 eV. After immersion in Li₂S₆, a Ti 2p shift of roughly 0.4 eV toward higher binding energy is observed, indicating a decrease in electron density around the metal centre.^70–72 O 1s spectra (Fig. 5b) after soaking reveal significant changes in oxygen chemical states: TiO₂-CFP shows increased contributions from Ti–OH (∼531.5 eV) and adsorbates (∼533 eV) compared to Ti–O lattice oxygen (∼530 eV),^67–69 indicating polysulfide interaction with surface hydroxyl groups. Ni 2p spectra (Fig. 5c) after Li₂S₆ soaking show that NiO/TiO₂-CFP maintains its Ni²⁺ character with Ni 2p_3/2 at ∼855 eV and characteristic satellite peaks at ∼862 eV typical of the NiO spectrum, while Ni/TiO₂ maintains its metallic Ni signature with Ni 2p_3/2 at ∼852.7 eV.^40,56 The notably increased noise and broader peaks may indicate surface oxidation or the formation of Ni–S interactions that enhance polysulfide trapping without complete oxidation of the metallic nickel core, thereby demonstrating that both nickel oxidation states contribute to polysulfide immobilization through distinct chemical mechanisms.^70,73 S 2p spectra (Fig. S3c) of the composites after soaking in lithium polysulfide show that the signals at 167.8 and 168.6 eV are attributed to thiosulfate and polythionate complexes. The newly emerging peaks at 164 and 162 eV are attributed to bridging sulfur (S_B⁰) and terminal sulfur (S_T⁻¹), respectively. The shuttle issue is mitigated by the polythionate species acting as mediators.⁷⁴ Furthermore, a significant chemical affinity between lithium polysulfides and nickel/titanium atoms is indicated by the development of S–Ti bonds (164.93 eV) and S–Ni bonds (162.08 eV).^74,75 Overall, XPS analysis indicates the successful modification of the materials and verifies the co-existence of the nickel and titanium oxidation states in the composites, as well as their interaction with Li₂S₆, indicating the adsorption of polysulfides and the role of nickel and titanium in counteracting the shuttle effect.

Fig. 5 XPS characterization of TiO₂-CFP, NiO/TiO₂-CFP, and Ni/TiO₂-CFP electrodes after soaking in Li₂S₆ solution: (a) Ti 2p, (b) O 1s, (c) Ni 2p (1) and Ni 2p (2).

A comparative study of polysulfide adsorption characteristics of different TiO₂-based electrodes as cathodes and a lithium chip as the anode was conducted using a beaker cell test. Fig. S4a shows the gradual decolourisation of the polysulfide solution after the insertion of TiO₂-CFP for 3 h, 6 h and 12 h, respectively. This suggests that TiO₂-CFP exhibited the strongest adsorption ability among the three electrodes. Fig. S4b shows increased colour fading for NiO/TiO₂-CFP, reflecting good adsorption arising from surface modification. Fig. S4c also indicates decolourisation for Ni/TiO₂-CFP, suggesting improved adsorption ability.

3.2 Effect on the kinetics of redox

The effect of TiO₂ nanotubes modified with NiO and Ni nanoparticles on the redox reaction kinetics in Li–S cells was investigated. Fig. 6 presents the cyclic voltammetry (CV) curves of cells with TiO₂-CFP, NiO/TiO₂-CFP, and Ni/TiO₂-CFP electrodes at 0.1 mV s⁻¹, respectively. The CV profiles of the three cells were compared to evaluate the catalytic influence of the TiO₂-based samples on the redox chemistry of sulfur species. Two pairs of reversible redox peaks were observed in the CV curves of all three TiO₂ based electrode cells. For each redox pair, the peaks were relatively sharp, suggesting faster kinetics and catalytic activity for the conversion of LiPSs.⁷⁶ The first pair of cathodic peaks found in the CV curve of the TiO₂ based electrode cells was assigned to the reduction of S₈ to Li₂S₄ (peak a) and the subsequent electrochemical reduction of Li₂S₄ to Li₂S (peak b). The corresponding anodic peaks were attributed to the electrochemical oxidation of Li₂S to Li₂S₄ (peak c) and the final oxidation of Li₂S₄ to S₈ (peak d), respectively.⁷⁷ The peak-to-peak separation (ΔE) between the anodic and cathodic peaks reflects the reversibility and the kinetic efficiency of the redox process.^78,79 Each pair of peaks (oxidation and reduction) shows similar redox conversion pathways during cycling, as shown in Fig. 6a–c. It is worth noting that their respective pair of peaks correspond to the chemical reactions associated with the electrochemical reduction from long-chain S₈ to Li₂S₄ and the electrochemical oxidation from Li₂S₄ to soluble high-order S₈, respectively.⁷⁷ However, their respective peak-to-peak separation varies due to chemical conversion rates. Therefore, peak-to-peak separation (ΔE) between pairs of peaks for cells with TiO₂-CFP, NiO/TiO₂-CFP and Ni/TiO₂-CFP electrodes are 0.24 V, 0.22 V and 0.20 V, respectively. These results indicated that the Ni/TiO₂-CFP electrode exhibited the lowest polarization and the fastest polysulfide conversion, while cells with the TiO₂-CFP and NiO/TiO₂-CFP electrodes showed relatively slower reaction kinetics and a slightly higher polarization.

Fig. 6 Comparative cyclic voltammetry (CV) curves at 0.1 mV s⁻¹ for all electrodes: (a) TiO₂-CFP, (b) NiO/TiO₂-CFP and (c) Ni/TiO₂-CFP.

Furthermore, CV profiles of all cells with TiO₂-based electrodes were recorded at various scan rates ranging from 0.1 to 0.5 mV s⁻¹, as shown in Fig. 7(a–c). The corresponding linear fittings of the peak currents versus the square root of the scan rates (ν^1/2) for peak A, peak B, and peak C were obtained from the CV curves in (a–c) as presented in Fig. 7(d–g). However, the purpose of the linear fittings was to determine the slopes used to calculate lithium-ion diffusion in the respective cathode materials. According to the Randles–Sevcik equation, there is a linear relationship between the redox peak current and the square root of the scan rate, which makes it possible to evaluate the diffusion of lithium ions within the electrodes:⁸⁰

where D_Li⁺ is the Li⁺ diffusion coefficient (cm² s⁻¹), C_Li⁺ is the concentration of lithium ions in the electrolyte (mol cm⁻³), v is the scan rate (V s⁻¹), I_p is the peak current (A), n is the number of electrons, and A is the electrode area (1.13097 cm²).

Fig. 7 CV profiles of electrodes recorded at various scan rates ranging from 0.1 to 0.5 mV s⁻¹: (a) TiO₂-CFP, (b) NiO/TiO₂-CFP, (c) Ni/TiO₂-CFP and (d–g) the linear fittings of the peak currents versus the square root of the scan rates (ν^1/2).

While the electrode area, the number of electrons (assumed to be 2 electrons), and the concentration of lithium ions in the electrolyte remained constant, the diffusion coefficient D_Li⁺ was determined from the slope of the curve (I_p/v^0.5), which reflected the diffusion rate of lithium ions, as shown in Table 1.⁷⁴ It was evident that the cell with the Ni/TiO₂-CFP electrode exhibited the highest lithium-ion diffusivity at peaks A and D, which mainly arose from improved lithium polysulfide adsorption and enhanced conversion of LiPSs to Li₂S. Diffusion coefficients (D) were deduced according to the Randles–Sevcik equation based on a 12 mm electrode surface and a 1 M polysulfide solution. From Table 1, peak A, corresponding to the fastest redox process, showed that the cell with the Ni/TiO₂-CFP electrode exhibited the highest value of D (5.75 × 10⁻⁶ cm² s⁻¹), indicating rapid ion transport and effective electrochemical conversion. By contrast, peak C exhibited intermediate diffusion behaviour with a value of 2.91 × 10⁻⁶ cm² s⁻¹ for Ni/TiO₂, suggesting a moderately rapid electrochemical process. For all peaks, the cell with the Ni/TiO₂ electrode exhibited the highest diffusion coefficients, followed by cells with NiO/TiO₂-CFP and TiO₂-CFP electrodes. Therefore, the cell with the Ni/TiO₂-CFP electrode showed better electrochemical kinetics for all redox peaks, indicating that this material is a promising electrode for improved rate performance in lithium–sulfur batteries. Generally, the best diffusion enhancement for cells with the Ni/TiO₂-CFP electrode can be attributed mainly to the presence of the metallic Ni phase, which provides better electronic conductivity, promotes the transport of Li⁺ and polysulfides, and facilitates rapid interfacial charge transfer. Moreover, this phenomenon suggests faster liquid–liquid conversion of LiPSs on the electrode surface.⁸¹

Table 1 Li⁺ diffusion coefficient (D_Li⁺) values of TiO₂, NiO/TiO₂ and Ni/TiO₂

Electrodes	Peak A	Peak B	Peak C	Peak D
TiO2-CFP	2.12 × 10^−6	3.56 × 10^−7	4.19 × 10^−7	6.15 × 10^−7
NiO/TiO2-CFP	4.14 × 10^−6	5.46 × 10^−7	5.77 × 10^−7	8.31 × 10^−7
Ni/TiO2-CFP	5.75 × 10^−6	8.31 × 10^−7	1.26 × 10^−6	2.91 × 10^−4

---

Cyclic voltammetry obtained from symmetric cells (Fig. 8) illustrates the conversion of lithium polysulfides. It can be observed from Fig. S5 that the TiO₂-CFP electrode's cyclic voltammetry for all scan rates exhibited very broad, weak peaks, indicating high resistance and sluggish reaction kinetics. In Fig. 8a, cells with Ni/TiO₂-CFP and NiO/TiO₂-CFP electrodes at a scan rate of 1 mV s⁻¹ showed moderate resistance, whereas the TiO₂-CFP electrode exhibited broad loops, indicating high resistance. Moreover, cells with Ni/TiO₂-CFP and NiO/TiO₂-CFP electrodes at a scan rate of 10 mV s⁻¹, as shown in Fig. 8b, showed more symmetric peaks and improved kinetics, while the thinner, more symmetric loops reflected lower polarization.^82,83 Cells with Ni/TiO₂-CFP and NiO/TiO₂-CFP electrodes at a scan rate of 20 mV s⁻¹ as shown in Fig. 8c displayed symmetric peaks, suggesting excellent reversibility and low resistance, as evidenced by thin and overlapping curves. With increasing scan rates, cells with Ni/TiO₂-CFP and NiO/TiO₂-CFP electrodes generally exhibited better kinetic performance and lower resistance, according to the symmetric cell results. The 20 mV s⁻¹ scan rate gave the most pronounced symmetric peaks and the best reversibility.

Fig. 8 CV curves of Li₂S₆ symmetric cells with NiO/TiO₂ and Ni/TiO₂ based electrodes at varying scan rates: (a) 1 mV s⁻¹, (b) 10 mV s⁻¹ and (c) 20 mV s⁻¹.

The potentiostatic charge–discharge behaviour of cells with TiO₂-based electrodes was evaluated using the Li₂S nucleation test. The cell with TiO₂-CFP reached the highest current at a time of ∼8370 s, as shown in Fig. 9a. This value is the highest when compared with the peak response time (∼7211 s) of the cell with the NiO/TiO₂-CFP electrode (Fig. 9b) and the peak response time (∼5970 s) of the cell with the Ni/TiO₂-CFP electrode (Fig. 9c), respectively. It is worth noting that the cell with Ni/TiO₂-CFP exhibited the best performance, because it reached the highest current after approximately 5970 s, suggesting accelerated Li₂S growth.⁵⁶ The maximum Li₂S deposition capacity was also determined for all TiO₂-based cells. The calculated Li₂S nucleation capacities of all TiO₂-based cells were 465 mA h g⁻¹, 600 mA h g⁻¹ and 746 mA h g⁻¹ for TiO₂-CFP, NiO/TiO₂-CFP and Ni/TiO₂-CFP electrodes, respectively. The cell with the Ni/TiO₂-CFP electrode exhibited the highest value and therefore showed excellent catalytic activity toward LiPSs, thereby accelerating Li₂S redox kinetics during the electrochemical process.

Fig. 9 Potentiostatic discharge of cells with (a) TiO₂-CFP, (b) NiO/TiO₂-CFP and (c) Ni/TiO₂-CFP.

The charge transfer resistance (R_ct) and the diffusion process associated with the Warburg impedance (Z_w) are shown by a semicircle in the high-frequency region and a sloping line in the low-frequency region of the Nyquist plots (Fig. 10). It is evident that the charge transfer resistance (R_ct) of cells with TiO₂-CFP, NiO/TiO₂-CFP and Ni/TiO₂-CFP electrodes decreased with decreasing semicircle diameter, indicating lower kinetic impedance. The charge transfer resistance values obtained for the respective cells with TiO₂ based electrodes were 46 Ω, 32 Ω and 14 Ω, respectively. The cell with the Ni/TiO₂-CFP electrode showed the smallest initial resistance (R_ct ≈ 14 Ω), reflecting enhanced kinetic activity, whereas cells with NiO/TiO₂-CFP (R_ct ≈ 32 Ω) and TiO₂-CFP (R_ct ≈ 46 Ω) electrodes showed higher R_ct values primarily owing to less efficient electron transport pathways. This kinetic facilitation is important because lower R_ct values reflect faster charge transport as well as accelerated redox reaction functions.⁸⁴

Fig. 10 Electrochemical impedance spectroscopy profiles for cells with TiO₂-based electrodes.

3.3 Cycling performance of modified TiO₂-based cathodes

The charge–discharge cycling characteristics of Li–S cells with TiO₂-based electrodes, namely, TiO₂-CFP, NiO/TiO₂-CFP, and Ni/TiO₂-CFP, at a sulfur loading of 2 mg cm⁻² were assessed as shown in Fig. 11a. At 0.5C, these cells exhibited promising initial capacities of 1053, 912, and 845 mA h g⁻¹, respectively. After 100 cycles, the cell with the TiO₂-CFP electrode showed the highest capacity retention of approximately 97%. In contrast, the retention for cells with NiO/TiO₂-CFP and Ni/TiO₂-CFP electrodes decreased to approximately 70% and 74%, respectively. The coulombic efficiencies (CEs) were approximately 90%, 91%, and 87%, respectively, for each cell. In cells with a TiO₂-CFP electrode, the capacity decreased steadily, with a sudden drop in capacity during initial cycles, after which relatively stable performance was observed. Nevertheless, this is expected to be due to the inefficiency of TiO₂ in holding the sulfur species, mainly due to the low conductivity of the material, which affects the electrochemical reactions. This behaviour is characteristic of TiO₂-based hosts, which have limited conductivity. As a result, the electrochemical cycling performance declines over time, leading to a gradual loss of capacity.^85,86 Similar to the cell with NiO/TiO₂-CFP, the capacity fading is evident in the first 15 cycles. However, it continues to decline steadily to 600 mA h g⁻¹ due to the lack of sufficient conductivity for long-term cycling.⁸⁷ In contrast, after Ni/TiO₂-CFP's initial discharge, the capacity remains relatively stable, showing minimal decline in the first 50 cycles. By the 100th cycle, the capacity remains around 730 mA h g⁻¹, confirming that the Ni-modified electrode has the best capacity retention among the three configurations. Adding the metal Ni enhances the conductivity of the material and provides more active catalytic sites for the conversion of lithium polysulfides. This enhances electrochemical reactions, keeping the sulfur species under control.^53,88 It is also worth noting that the specific capacity achieved for this rate and mass loading was higher than those of previously reported TiO₂-based electrodes.^89,90 The cycle stability with different cathode materials was investigated at 0.2C rates, as shown in Fig. 11b. Cells with TiO₂-CFP, NiO/TiO₂-CFP and Ni/TiO₂-CFP electrodes with areal sulfur mass loadings of 2 mg cm⁻² exhibited initial discharge capacities of 1192, 1121 and 895 mA h g⁻¹, respectively. After 100 cycles, the cell with the Ni/TiO₂-CFP electrode retained 92% of its initial capacity. In contrast, the cells with NiO/TiO₂-CFP and TiO₂-CFP electrodes retained only 67% and 55%, respectively. In cells with TiO₂-CFP, it can be observed that during the first 5 cycles, the capacity decreases sharply to about 600 mA h g⁻¹. After that, capacity further decreases steadily, reaching about 500 mA h g⁻¹ after the 100th cycle. This capacity trend is consistent with a TiO₂-based host, which provides polar adsorption sites for lithium polysulfides but has limited charge transport and reuse of dissolved intermediates. As the cycles progress, the active sulfur becomes less accessible. This is due to the well-known problem of lithium polysulfides, which dissolve and migrate during cycles, leading to a steady reduction of sulfur utilization and capacity fade.^91,92 For NiO/TiO₂-CFP, cycle 1 starts at a level above that of TiO₂-CFP, at 1121 mA h g⁻¹, and then it decreases gradually as the cycles progress, reaching a plateau at a level of 830–850 mA h g⁻¹. Thereafter, the capacity continues to slowly decrease with increasing cycles, reaching 720 to 750 mA h g⁻¹ at cycle 100. The presence of NiO creates polarity and chemical interaction that more effectively captures lithium polysulfides compared to TiO₂-CFP. However, the NiO material is not a highly conductive metallic phase, and the electrode does not fully eliminate slow kinetic limitations and gradual loss of accessible sulfur over many cycles.⁹³ However, Ni/TiO₂-CFP's first cycle begins at 1192 mA h g⁻¹. For the first few cycles, the capacity increases slightly before remaining nearly flat with a gradual capacity loss, still close to 1100 mA h g⁻¹ after 100 cycles. This is consistent with the presence of metallic Ni that enables better electronic conductivity and accelerates the rate of polysulfide conversion reactions to ensure that the sulfur redox reactions remain reversible and that more active material is electrochemically accessible after cycling.^94,95 This demonstrates that the cell with the Ni/TiO₂ electrode is most effective at trapping and reactivating LiPSs compared to other TiO₂-based electrodes reported in the literature.^53,54,89 Furthermore, the rate capability test of cells with TiO₂-CFP, NiO/TiO₂-CFP, and Ni/TiO₂-CFP electrodes was performed as illustrated in Fig. 11c. At current densities of 0.2, 0.5, 1, and 2C, the cells initially generated capacities of approximately 1122, 1013.61, and 887.65 mA h g⁻¹, respectively, at 0.2C. When the current density was reverted to 0.2C, their respective capacities recovered to 787.58, 700 and 670 mA h g⁻¹. This indicates that while some irreversible capacity loss occurred, the TiO₂-based host significantly aids cell performance by mitigating LiPS shuttling and accelerating the redox kinetics of sulfur and its discharge products. Moreover, the cycling performance of all three cells with TiO₂ based electrodes was evaluated at a high sulfur mass loading of 4.0 mg cm⁻² and a rate of 0.2C, as shown in Fig. 11d. The cell with the Ni/TiO₂-CFP electrode demonstrated superior performance with an initial specific capacity of approximately 1285 mA h g⁻¹ at 0.2C with a decay rate of only 0.1%, while reaching 85% of the theoretical capacity (1C = 1675 mA h g⁻¹). After 100 cycles, the cell maintained a capacity of approximately 1095 mA h g⁻¹, demonstrating excellent capacity retention. In contrast, the cells with the NiO/TiO₂-CFP and TiO₂-CFP electrodes exhibited comparatively lower initial specific capacities of approximately 1196.4 and 995.72 mA h g⁻¹, respectively. Cells with the TiO₂-CFP electrode show relatively good capacity retention in the initial cycles, but the capacity quickly stabilizes to a low value of approximately 700 mA h g⁻¹ by the 20th cycle. In contrast, cells with NiO/TiO₂-CFP electrodes demonstrated a rapid and significant capacity fade over the first 40–60 cycles. The capacity drops steadily to roughly 550 mA h g⁻¹ by the end of 100 cycles, indicating structural instability or poor suppression of the polysulfide shuttle effect.^87,92 The coulombic efficiencies of cells with TiO₂-CFP, NiO/TiO₂-CFP and Ni/TiO₂-CFP electrodes were determined to be 96%, 90% and 86% respectively. The superior specific capacity of the cell with the Ni/TiO₂-CFP electrode compared to the other electrodes can be attributed to its enhanced electrical conductivity and effective electron transport pathways, which accelerated the conversion of polysulfides into Li₂S.^53,93,94

Fig. 11 Charge–discharge of cells with TiO₂-CFP, NiO/TiO₂-CFP and Ni/TiO₂-CFP electrodes: (a and b) cycling performance at 0.5C and 0.2C, respectively, (c) rate capability test and (d) cycling performance (4 mg cm⁻²) at 0.2C.

Notably, cells with the Ni/TiO₂-CFP electrode outperformed those with other TiO₂-CFP electrodes with higher sulfur loadings with low current rates.^39,54 In summary, comprehensive electrochemical analysis reveals that the cells with the Ni/TiO₂-CFP electrode significantly outperform their TiO₂-CFP and NiO/TiO₂-CFP electrode counterparts in terms of capacity retention, reaction rate and cycling stability, particularly under challenging conditions of high sulfur loading (4.0 mg cm⁻²).

The galvanostatic charge–discharge profiles of Li–S cells within the voltage window of 1.6–2.8 V vs. Li⁺/Li at a current rate of 0.2C were assessed, as illustrated in Fig. 12. Every voltage profile shows a single charge plateau at about 2.4 V and two separate discharge plateaus at roughly 2.3 V and 2.1 V. These characteristics align with the typical redox reactions of sulfur species. In particular, the discharge plateaus show how elemental sulfur (S₈) is gradually reduced to long-chain LiPSs and then further reduced to short-chain LiPSs (Li₂S₂/Li₂S).^96–98 The reverse oxidation of LiPSs to elemental sulfur is reflected in the charge plateau at about 2.4 V. As indicated in Fig. 12a, the cell with the TiO₂-CFP electrode had the lowest specific capacity of 1091 mA h g⁻¹. In contrast, cells with NiO/TiO₂-CFP and Ni/TiO₂-CFP electrodes produced capacities of 1170 mA h g⁻¹ and 1283 mA h g⁻¹ at the 5th cycle. Moreover, cells with the Ni/TiO₂ electrode showed the lowest voltage hysteresis (ΔE = 0.25 V) compared with cells with NiO/TiO₂-CFP and TiO₂-CFP electrodes, indicating improved charge transfer. The galvanostatic charge–discharge profiles of the cells with the Ni/TiO₂-CFP electrode at different cycles were also assessed, as shown in Fig. 12b. This figure illustrates how cells with the Ni/TiO₂-CFP electrode performed when cycled under galvanostatic discharge and charge cycles, from the 2nd to the 100th cycle at 0.2C. Initially, the voltage plateau remains stable; however, from cycle number 50, there is already a noticeable gradual loss of capacity and an increase in the polarization gap (ΔE), which may be regarded as the onset of degradation. Subsequently, with increasing cycles, capacity loss becomes prominent, and discharge voltage moves lower, indicating irreversible loss and continued degradation. The increase in the polarization gap and reduction in plateau regions indicate that, with extensive cycling, structural changes in the modified material and an increase in irreversible resistance compromise its stability and electrochemical performance.

Fig. 12 Galvanostatic charge–discharge profiles of cells with TiO₂-CFP, NiO/TiO₂-CFP and Ni/TiO₂-CFP electrodes (a) at the 5th cycle and (b) Ni/TiO₂-CFP electrodes at the 2nd, 5th, 10th, 50th and 100th cycles, respectively.

To further evaluate the structural integrity of the Ni/TiO₂ electrode after cycling, post-cycling SEM analysis was performed, and the results are presented in Fig. S6. The low-magnification SEM image (Fig. S6a) confirms that the carbon fibre strand retains its structural stability, with the Ni/TiO₂ nanostructure coating remaining conformally adhered to the fibre surface after repeated charge–discharge cycling. At a similar magnification in Fig. S6b, the cycled electrode surface displays interconnected sheet-like and globular features, consistent with the deposition of Li₂S discharge products and residual active material, indicative of active polysulfide conversion during cycling. The high-magnification image (Fig. S6c) further reveals that the nanostructured surface morphology of the Ni/TiO₂ composite is largely preserved, with no evidence of structural collapse, consistent with the low-capacity decay rate per cycle observed electrochemically. EDS elemental mapping confirms the uniform spatial distribution of sulfur (S), titanium (Ti), and nickel (Ni) across the post-cycled surface (Fig. S6d), demonstrating effective polysulfide retention within the cathode and homogeneous dispersion of the Ni/TiO₂ active species. These observations collectively confirm the structural and chemical stability of the Ni/TiO₂-CFP electrode, providing direct morphological evidence for its superior long-term electrochemical performance.

As shown in Table 2, a detailed comparison of TiO₂ based nanotubes with varying average particle sizes and sulfur mass loadings is presented with their respective electrochemical performance in terms of specific capacities and cycling stability.

Table 2 Comparison of the electrochemical performance of TiO₂-based nanotubes

Cathode additive	Average particle size (nm)	Discharge capacity (mA h g^−1)	Long-term cycling performance	C-rate (C)	Sulfur loading (mg cm^−2)	Paper/study
Ni/TiO2 nanotubes	∼18.3	1053	100	0.5	2.0	This work
		1192	100	0.2	2.0
		1285	100	0.2	4.0
NiO/TiO2 nanotubes		912	100	0.5	2.0
		1121	100	0.2	2.0
		1136	100	0.2	4.0
TiO2 nanotubes		845	100	0.5	2.0
		895	100	0.2	2.0
		995	100	0.2	4.0
TiO2-supported Nb catalyst		575	350	3.0	1.0	54
		777	100	0.1	5.0
		966	100	0.2	1.0
TiO2-based nanotube	10.2	721	100	0.2	1.0
		484	350	3.0	1.0
		632	100	0.1	5.0
TiO2-Ru@S	3–4.5	827	200	0.5	Not reported	53
		710	700	2.0	Not reported
		1085	140	0.1	3.0
TiO2@S		1264	30	0.02	4.8
		471	200	0.5	Not reported
S@WS2–TiO2		560.9	1100	1	1.1	39
		455.1	100	0.2	5.878
S@MoS2–TiO2		634	1100	1	1.1
		503.8	100	0.2	5.867
S@TiO2		422.76	1100	1	1.1
TiO2(B)-NTs	7	1055	50	0.1	1.5–2.0	89
		805	80	1.0	1.5–2.0
		235	600	5.0	1.5–2.0
S-TNT@CNT		837	200	0.5	1.2–1.8
		510	80	1.0	1.2–1.8
TiO2/S 1 : 1	80–100	795	100	0.2	1.1	99
TiO2/S 1 : 2	∼110	913	100	0.2	1.1
TiO2 NTs-S	∼300	852	100	0.1	1.6	40
TiO2 NTs@Ni–S		1163	100	0.1	1.6
HGN@TiO2@C/S		626.8	200	1	Not reported	71

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4 Conclusion

In summary, this work presents the development and systematic electrochemical evaluation of anatase TiO₂ nanotubes (an average diameter of approximately 18.3 nm) synthesized via hydrothermal treatment. These nanotubes were decorated with Ni nanoparticles through hydrogen-assisted reduction, at a 10 : 1 molar ratio. ICP-OES analysis confirmed the molar ratio of bifunctional additives. These heterostructures were subsequently incorporated into carbon fibre paper (CFP) to form TiO₂-CFP, NiO/TiO₂-CFP and Ni/TiO₂-CFP composite cathodes, evaluated at a sulfur loading of 4 mg cm⁻². The use of these modified heterostructures significantly improved the electrochemical characteristics compared to the cell with bare TiO₂-CFP, as confirmed by galvanostatic charge–discharge tests, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The cell with Ni/TiO₂-CFP exhibited the best overall performance, delivering an initial specific capacity of 1285 mA h g⁻¹ at 0.2C with a capacity decay rate of only 0.1% per cycle, retaining approximately 1095 mA h g⁻¹ after 100 cycles at a coulombic efficiency of 96%. Even at higher current rates (2C), Ni/TiO₂-CFP (381.52 mA h g⁻¹) demonstrated superior rate capability, outperforming both cells with NiO/TiO₂-CFP (301.14 mA h g⁻¹) and TiO₂-CFP (230.27 mA h g⁻¹), respectively. The charge–discharge profiles of the cell with Ni/TiO₂-CFP showed minimal polarization, confirming stable lithium polysulfide retention and efficient electrocatalytic conversion. EIS measurements confirmed the lowest charge transfer resistance (14 Ω) for Ni/TiO₂, alongside the highest Li₂S nucleation capacity (746 mA h g⁻¹), collectively indicating significantly faster interfacial kinetics and superior ion transport. The NiO/TiO₂-CFP cell demonstrated intermediate characteristics, with an initial specific capacity of 1196.4 mA h g⁻¹ at 0.2C, attributed to enhanced polysulfide chemisorption through abundant Ni²⁺/Ni³⁺ redox active sites and improved chemisorption, but limited by lower long-term conductivity relative to metallic Ni. Meanwhile, the cell with TiO₂-CFP demonstrated the weakest performance, delivering an initial specific capacity of 995.72 mA h g⁻¹ at 0.2C, constrained by its intrinsically low electronic conductivity and weak electrocatalytic activity, despite enabling moderate polysulfide adsorption through its polar surface sites. Overall, the synergistic interaction of the conductive Ni nanoparticles and the stable, high surface area TiO₂ nanotube host resulted in improved electrocatalytic activity, enhanced charge transport, and superior interfacial stability, establishing Ni/TiO₂-CFP as a rational and scalable cathode architecture for high sulfur loading lithium–sulfur batteries. Future work could extend this strategy by systematically varying Ni loading and by evaluating the architecture under pouch cell conditions.

Author contributions

Emmanuel Siaw: writing – original draft, validation, methodology, investigation, and data curation. Dias Bekeshov: methodology and investigation. Alas Alaskhanov: methodology and investigation. Aishuak Konarov: methodology and validation. Zhumabay Bakenov: validation and supervision. Nurzhan Baikalov: writing – review & editing, supervision, methodology, investigation, and funding acquisition. Stavros Poulopoulos: writing – review & editing, validation, supervision, and funding acquisition.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

The authors confirm that any data that support the findings of this study are included within the article and included as a part of the supplementary information (SI). Upon reasonable request, raw data regarding this work can be requested from the corresponding author.

Supplementary information is available. See DOI: https://doi.org/10.1039/d6na00263c.

Acknowledgements

This work was funded by the Science Committee of the Ministry of Science and Higher Education of the Republic of Kazakhstan (grant No. AP23490764).

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