# Correlation of synthesis parameters and physicochemical properties of biomass-derived carbon-titania composites

Julia Wagner, Julien Jaxel, Katia Guérin, Sandrine Berthon-Fabry

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Julia Wagner, Julien Jaxel, Katia Guérin, Sandrine Berthon-Fabry. Correlation of synthesis parameters and physicochemical properties of biomass-derived carbon-titania composites. Journal of Materials Science, 2025, 60 (17), pp.7259-7272. 10.1007/s10853-025-10874-x. hal-05175725

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# Correlation of synthesis parameters and physicochemical properties of biomass-derived carbon-titania composites

Julia Wagner<fnref n="1" />,<fnref n="2" />, Julien Jaxel<fnref n="1" />, Katia Guérin<fnref n="1" />,<fnref n="2" />,* ID, and Sandrine Berthon-Fabry<fnref n="1" />,*, ID

Received: 28 November 2024

Accepted: 1 April 2025

Published online:

29 April 2025

© The Author(s), 2025

## ABSTRACT

The synthesis of an efficient lithium battery anode requires active and conductive materials. Composites are often used, but their preparation requires numerous parameters and attempts to maximize the crystalline phase or texture. To limit the number of syntheses, factor analysis is a useful tool for determining the critical parameters. This method has been applied to titania-based anodes. Biomass-derived carbon-titania composites were prepared through sol–gel synthesis followed by drying and pyrolysis under a nitrogen gas flow. Four synthesis parameters were investigated: the presence of kapok-based carbon fiber, the acid used as the sol–gel catalyst, the drying process, and the pyrolysis temperature. Owing to the Yates formula, the optimal conditions leading to an optimized specific surface area (SSA) enhancing lithium-ion diffusion are highlighted. Conclusively, whereas kapok-based carbon and supercritical drying are essential for achieving high SSAs, the effect of pyrolysis temperature on TiO₂ crystallinity depends on the acid-based sol–gel conditions, and to favor anatase instead of rutile, acetic acid should be used. As a result, this method of synthesis and implementation has made it possible to use the most suitable samples without extra conductive carbon as anodes in lithium-ion batteries and their electrochemical characteristics have been obtained using galvanostatic measurements over 40 cycles.

<footnote n="1">¹ MINES Paris, PSL University, Centre for Processes, Renewable Energy and Energy Systems (PERSEE), CS 10207 rue Claude Daunesse, 06904 Sophia Antipolis Cedex, France</footnote>

<footnote n="2">² Université Clermont Auvergne, CNRS, ICCF, 63000 Clermont-Ferrand, France</footnote>
GRAPHICAL ABSTRACT

<figure><img src="image_2.png" /></figure>

**Introduction**

For automobile applications, the lithium-ion battery market is presumed to have great potential for growth, as the EU projected to ban the sale of new personal vehicles with internal combustion engines in 2035 [1, 2]. Innovative materials are thus needed as part of the battery and particularly the anode, for which the most used materials are graphite and lithiated titanates that react with lithium by insertion mechanisms [3]. These materials have intrinsic properties that limit either the performance or the safety of the battery systems in which they can be used. The lithiation potential of graphite is very low (0.2 V vs Li⁺/Li), which can cause the formation of lithium dendrites at high current rates [4]. The titanate material Li₄Ti₅O₁₂ has a higher lithiation potential of 0.8 V vs Li⁺/Li, but its theoretical specific capacity is lower (175 mAh/g) [5, 6] than the maximum of 372 mAh/g for graphite in its most lithiated state [7]. These limitations led to the development of other insertion materials, such as titanium dioxide [8], which is abundant and relatively affordable, with only a 4% volume expansion upon lithiation and a high theoretical capacity of 335 mAh/g [9, 10]. However, titanium dioxide has the disadvantage of having a large bandgap (3.2 and 3.0 eV for anatase and rutile, respectively) and an electronic conductivity that could limit electron transfer in case of redox process [11]. Titania and titania-based composites have been widely studied, and their performance is correlated

with their physicochemical characteristics and the structure in which they are integrated. Two main synthesis approach strategies [12] were used: intrinsic (engineering of the physicochemical properties of titania, such as the particle size, material structure, facet, and electronic structure control) [13, 14] and extrinsic methods (combining titania with other materials by carbon coating, engineering of conductive substrates, or hybridization with a conductive agent). In the literature, many studies have addressed, in detail, one of these two synthesis approaches and their effects, but only a few systematic studies have been carried out.

The first important intrinsic property affecting the performance is the crystalline phase. Titania exists in many different phases, but the three phases that are most common are anatase, brookite, and rutile. These three structures are created by different arrangements of TiO₆ octahedra depending on the synthesis and post-treatment conditions. In the rutile and anatase phases, the octahedra are connected by one edge, while in the brookite phase, they are connected by two edges [15]. This simple but important structural distinction in brookite causes a slower diffusion of lithium ions and, consequently, a lower penetration depth inside the material compared to the rutile and anatase phases. Rutile and anatase phases are thus the most used phases for creating lithium-ion battery electrodes [12]. In the rutile phase, the insertion sites for Li⁺ ions are arranged in such a way that their migration is anisotropic, hosting smaller amounts of
lithium ions than in the anatase phase with “zigzag” channels that allow Li⁺ ions to migrate in the material following numerous pathways [16]. The insertion of Li⁺ ions in anatase occurs at approximately 1.78 V vs Li⁺/Li, creating lithiated phases up to Li₀.₆TiO₂. Earlier works from the literature also showed that LiTiO₂ can be formed, either at high temperatures or with nanosized particles [17, 18]. These differences explain why anatase is generally preferred as an active anode material. Thermal treatment allows amorphous titania to crystallize in the desired phase. Anatase is metastable under ambient conditions and is generally obtained by thermal treatment between 200 and 250 °C, while rutile is a thermodynamically stable phase that appears at approximately 650 °C. The anatase-to-rutile transition temperature widely depends on other parameters, such as particle size (caused by a difference in surface and bulk energy between anatase and rutile [19–21]). According to theoretical calculations, anatase is primarily obtained from particles under 11 nm, brookite is obtained from particles between 11 and 35 nm, and rutile is obtained from particles over 35 nm [20]. The synthesis of particles under 11 nm in size followed by thermal treatment is a simple step to obtain anatase, dopants (high-valence cations, e.g., Nb, Ta, Si or Zr [22–25]) help to inhibit the anatase-to-rutile transition, thermal treatment atmosphere [26], duration [27], and synthesis conditions (with acetic acid, the chelation of Ti by acetate ligands, forces the atoms to assemble into an anatase structure by steric effects [28], even at ambient temperatures by a sol–gel process [29]) and methods [14].

Another important intrinsic property of titania materials is the morphology of their particles, where the performance of TiO₂ strongly affects the development of nano- and microsized materials with various structural designs [8] to obtain a high contact area with the electrolyte. Two strategies are used to create a three-dimensional structure by using intertwined one- or two-dimensional nanostructures or directly synthesizing three-dimensional networks by forming porous or hierarchical morphologies [10, 30]. The desired morphologies can be obtained by a wide variety of synthesis processes with various levels of complexity of implementation. Hydrothermal or solvothermal syntheses usually lead to crystalline nanoparticles, nanowires, or nanosheets. Alternatively, a wide variety of morphologies can also be created by templating methods. One-dimensional materials

are obtained by electrospinning and calcination. Nanoparticles and porous structures are created by sol–gel methods followed by thermal treatment [8, 10, 31].

Finally, the extrinsic properties of TiO₂ can be changed by forming a titania-based composite. For commercial batteries, this step occurs during electrode formulation, where the active material is mixed with carbon black (to increase the electronic conductivity) and a polymeric binder. However, titania can be combined with other forms of carbon, such as graphene or carbon nanotubes [32, 33]. The intertwining of such one- and two-dimensional structures allows electrons to move around the entire composite structure. However, laboratory-made carbon nanomaterials are usually expensive and generally come from environmentally unfriendly synthesis processes. One solution to these problems is to use plant fibers as a replacement. Due to the large variety of natural compounds, a wide variety of morphologies are available, and after pyrolysis, the carbon fibers have the chosen morphology [34, 35].

This is the first study on the effects of both intrinsic and extrinsic strategies and their interactions in a composite made from an intertwined conducting material combined with a less conductive metal oxide. This configuration allows electrons coming from the current collector to easily flow toward the active material in all dimensions of space. In this study, an electrode material with a composite structure composed of bio-based carbon derived from kapok fibers and covered with TiO₂ was synthesized. The TiO₂ phase was obtained by a simple sol–gel method allowing numerous crystalline phases and morphologies to be formed by adjusting the intrinsic and extrinsic parameters. The carbon phase was obtained in one step by pyrolyzing the kapok fibers during the thermal treatment of TiO₂. The effects of four synthesis parameters were compared: The acid used as a sol–gel catalyst, the drying method, the pyrolysis temperature, and the amount of kapok fibers. A systematic study by factor analysis (limiting the number of experiments and allowing seeing the parameter interactions) was carried out to compare the effects of these intrinsic and extrinsic parameters on the specific surface area and the crystalline structure to select the best candidates to evaluate by galvanostatic cycling in half-cells. The simplicity of the synthesis method (directly adding organic fibers in the sol) and the electrode process (no extra carbon black added
for electronic conductivity limiting the inactive mass in the cell) make this method interesting and easily applicable at the industrial scale.

# Materials and methods

## Synthesis

Isopropanol (99.0%), acetic acid (99.9%), and titanium isopropoxide (TTIP, 97.0%) were purchased from Fisher Scientific and used as received. Nitric acid (2 M) was prepared from a 35 wt% solution. Kapok fibers were used without any treatment. Their characteristics before and after thermal treatment are presented in Fig. S1. The synthesis process and reagent quantities are presented in Fig. 1. For materials containing fibers, the number of added fibers was calculated at 2 wt% of the total weight of the gel (reagents + solvent). Typically, isopropanol, acid, and deionized water (sol A) were mixed with isopropanol and titanium isopropoxide (sol B). When kapok fibers were used, the mixed solution was poured onto the fibers. Two kinds of acids were evaluated: strong inorganic acid (nitric acid) and weak organic acid (acetic acid). For the

synthesis with nitric acid, gelation occurred 8 minutes after mixing the reagents, and for the synthesis with acetic acid, gelation occurred after only 2 minutes. All gels were aged for 2 days. Eight solvent exchanges with fresh isopropanol were carried out over 4 days to ensure that no remaining reagents were present for the drying step. The gels were either dried in supercritical CO₂ (SC) with the following drying conditions: pressure = 8.0 MPa and temperature = 37 °C, for 3 h at a flow rate of 50 mL/min and were depressurized at a rate of 0.4 MPa/h or dried in an oven at 60 °C overnight. The respective aerogels (SC) and xerogels (60 °C) were further pyrolyzed at either 550 or 700 °C for 1 h with a ramp rate of 5 °C/min and cooled by thermal inertia with a nitrogen flow of 1.5 L/min. This last step crystallizes TiO₂ and carbonizes the organic kapok fibers into carbon in a single step, resulting in an intimate TiO₂-carbon composite. The nomenclature is given in Table 1.

## Structural and textural characterization

The porosity of the TiO₂-C materials was studied by N₂ isotherms at -196 °C. The specific surface area (SSA) was determined by the

<figure><img src="image_3.png" /><figcaption>**Figure 1** Synthesis of TiO₂-C composites and quantities used.</figcaption></figure>
<table><caption>**Table 1** Sample identifications and their characteristics</caption><thead><tr><th rowspan="2"></th><th rowspan="2">Drying process<br>Pyrolysis temperature</th><th colspan="2">Ambient pressure (60 °C)</th><th colspan="2">SC</th></tr><tr><th>550 °C</th><th>700 °C</th><th>550 °C</th><th>700 °C</th></tr></thead><tbody><tr><td rowspan="2">Acetic acid</td><td>No fibers</td><td>A060550</td><td>A060700</td><td>A0SC550</td><td>A0SC700</td></tr><tr><td>Fibers</td><td>AK60550</td><td>AK60700</td><td>AKSC550</td><td>AKSC700</td></tr><tr><td rowspan="2">Nitric acid</td><td>No fibers</td><td>N060550</td><td>N060700</td><td>N0SC550</td><td>N0SC700</td></tr><tr><td>Fibers</td><td>NK60550</td><td>NK60700</td><td>NKSC550</td><td>NKSC700</td></tr></tbody></table>

Brunauer–Emmett–Teller (BET) method. The NLDFT method was used to calculate the pore volume. The geometric mean particle size $d_{geo}$ (nm) calculated with Eq. 1 formula, where $S_{BET}$ is the BET SSA (m$^2$/g): [36]

$$d_{geo} = \frac{6000}{\rho * S_{BET}} \qquad (1)$$

where $\rho$ is the phase density calculated from the relative intensities of anatase and rutile (Eq. 2) and their densities (fixed at 3.90 and 4.23 g/cm$^3$, respectively):

$$\rho = \%_{\text{anatase}} * \rho_{\text{anatase}} + \%_{\text{rutile}} * \rho_{\text{rutile}} \qquad (2)$$

The measurements were carried out with a Microtrac Belsorp mini X apparatus after 5 h of degassing under a primary vacuum at 100 °C. All the samples were manually ground for X-ray diffraction analysis. The analysis was performed on a PANalytical X'Pert Pro diffractometer equipped with a Cu Kα source operating at 45 kV and 30 mA. The measurements were carried out from 10 to 90° with a step size of 0.08°. The crystallite size $d_{cryst}$ was estimated with the well-known Scherrer equation.

The morphology of the materials was visualized with a Tescan MAIA3 XMU scanning electron microscope (SEM). The samples were coated with a 7 nm platinum layer with a Q150T Quarum rotating metallizer before observation. When particles were clearly visible on the SEM images, the particle sizes were estimated by averaging 30 particle diameter measurements.

The maximum amount of carbon contained in the samples was appreciated by weighing the samples before and after calcination in air at 800 °C, which was carried out in a Nabertherm LV 15/11/B180 muffle furnace.

## Electrochemical study

The electrochemical behavior was evaluated by galvanostatic cycling in half-cells in front of lithium as the anode. The electrodes were made by formulating an ink composed of 90% material and 10% PVDF dispersed in propylene carbonate. No carbon black was added to the ink because the composite materials already contained carbon from the pyrolysis of the kapok fibers and organic part of precursors mainly from titanium isopropoxide (FTIR, Fig. S2). The ink was deposited on a stainless-steel spacer which has been put inside a CR2032 coin cell with an electrolyte composed of 1 M LiPF$_6$ in ethylene carbonate–propylene carbonate–dimethyl carbonate (with a volume ratio of 1–1–3). The half-cells were then galvanostatically charged and discharged between 0.005 and 3 V vs Li$^+/Li$ at C/50 (calculated with C corresponding to 335 mA/g).

## Experimental design and factor analysis

Because of the number of parameters studied, we decided to optimize the experimental process by creating the experimental plan presented in Table S1. The samples are called as follows. The first letter (A or N) describes the acid used as a catalyst (acetic or nitric acid, respectively). The second symbol indicates whether the sample contains kapok fibers: 0 for no fibers and K for a sample containing fibers. The two following symbols correspond to the drying process: 60 for ambient pressure drying at 60 °C and SC for supercritical drying. The last three numbers represent the pyrolysis temperature. Therefore, for example, the sample called N060550 is synthesized with nitric acid and no fibers, is dried at 60 °C at ambient pressure, and is pyrolyzed at 550 °C.
The effect of each parameter was determined using experimental data as the system response with the Yates formula in Eq. 3 for the crystallinity response and Eq. 4 for the porosity response:

$$E_{\text{crystallinity}} = \frac{\sum_{k=1}^{2f} \pm 1 * I_k}{2^f} \qquad (3)$$

$$E_{\text{porosity}} = \frac{\sum_{k=1}^{2f} \pm 1 * SSA_k}{2^f} \qquad (4)$$

where f is the number of factors studied in the experiment (in our case, 4). The value of ±1 corresponds to the description of the sample in the Hadamard matrix describing the experimental design, which can be found in Table S1. In the crystallinity study, the experimental response $I_k$ is the relative intensity of the (101) anatase peak to the (110) rutile plus (101) anatase peaks for sample k. To determine the effects on the material porosity, the value of the response is the specific surface area $SSA_k$ of sample k obtained by the BET method. To study the effects of synthesis parameters on porosity, a 24 full factorial design with interactions was used. The corresponding matrix is

presented in Table S1, and the results of both factor analyses are presented in Table S2 in the supplementary information.

## Results and discussion

### Effects on crystallinity

The experimental data used for the factor analysis are presented in Table 2. For all the TiO$_2$-carbon composites, the table shows that anatase is the prevalent phase, even after thermal treatment at 700 °C, with a relative intensity ranging from 55 to 100%. All the samples contained residual carbon after pyrolysis at 800 °C in nitrogen (8–12 wt%) even without the addition of fibers, which originated from the residual organic compounds from the sol–gel reaction as shown in FTIR spectra (Fig. S2). Raman (Fig. S3) and XRD analyses showed that the carbon was poorly organized (Fig. 2), which is why it was not considered further in the crystallinity study.

The X-ray diffractograms obtained from the samples without fibers dried under subcritical conditions are presented in Fig. 2, which illustrates general

<table><caption>**Table 2** Physicochemical characteristics of the samples used for factor analysis (LD: limit of detection, —means the value cannot be determined, i.e., no SSA for geometric mean particle size and no visible particles defined by SEM)</caption><thead><tr><th rowspan="2">Sample identification</th><th rowspan="2">SSA<sub>BET</sub> (m<sup>2</sup>/g)</th><th rowspan="2">Pore volume (cm<sup>3</sup>/g)</th><th colspan="2">Anatase (101) peak</th><th colspan="2">Rutile (110) peak</th><th rowspan="2">Geometric mean particle size (nm)</th><th rowspan="2">SEM particle size (nm)</th></tr><tr><th>Relative intensity (%)</th><th>2θ (°)</th><th>Relative intensity (%)</th><th>2θ (°)</th></tr></thead><tbody><tr><td>A060550</td><td>7</td><td>3.52·10<sup>-2</sup></td><td>96</td><td>25.36</td><td>4%</td><td>27.32</td><td>219</td><td>15</td></tr><tr><td>AK60550</td><td>14</td><td>9.29·10<sup>-3</sup></td><td>93</td><td>25.33</td><td>7%</td><td>27.25</td><td>109</td><td>—</td></tr><tr><td>N060550</td><td>&lt;LD</td><td>—</td><td>100</td><td>25.37</td><td>&lt;LD</td><td>—</td><td>13</td><td>—</td></tr><tr><td>NK60550</td><td>15</td><td>1.35·10<sup>-2</sup></td><td>91</td><td>25.33</td><td>9%</td><td>27.70</td><td>102</td><td>—</td></tr><tr><td>A0SC550</td><td>115</td><td>8.11·10<sup>-1</sup></td><td>90</td><td>25.37</td><td>10%</td><td>27.45</td><td>13</td><td>18</td></tr><tr><td>AKSC550</td><td>178</td><td>2.92·10<sup>-1</sup></td><td>79</td><td>25.27</td><td>21%</td><td>26.88</td><td>8</td><td>26</td></tr><tr><td>N0SC550</td><td>174</td><td>7.94·10<sup>-1</sup></td><td>91</td><td>25.39</td><td>9%</td><td>27.50</td><td>9</td><td>12</td></tr><tr><td>NKSC550</td><td>198</td><td>1.15·10<sup>0</sup></td><td>92</td><td>25.37</td><td>8%</td><td>27.78</td><td>8</td><td>18</td></tr><tr><td>A060700</td><td>4</td><td>6.30·10<sup>-3</sup></td><td>96</td><td>25.37</td><td>4%</td><td>27.32</td><td>383</td><td>—</td></tr><tr><td>AK60700</td><td>14</td><td>1.22·10<sup>-2</sup></td><td>95</td><td>25.32</td><td>5%</td><td>27.40</td><td>109</td><td>—</td></tr><tr><td>N060700</td><td>&lt;LD</td><td>—</td><td>70</td><td>25.36</td><td>30%</td><td>27.48</td><td>—</td><td>—</td></tr><tr><td>NK60700</td><td>20</td><td>2.72·10<sup>-2</sup></td><td>96</td><td>25.27</td><td>4%</td><td>27.34</td><td>77</td><td>21</td></tr><tr><td>A0SC700</td><td>97</td><td>2.20·10<sup>-1</sup></td><td>76</td><td>25.30</td><td>24%</td><td>27.41</td><td>16</td><td>16</td></tr><tr><td>AKSC700</td><td>160</td><td>1.14·10<sup>0</sup></td><td>55</td><td>25.38</td><td>45%</td><td>27.48</td><td>9</td><td>15</td></tr><tr><td>N0SC700</td><td>92</td><td>4.83·10<sup>-1</sup></td><td>71</td><td>25.36</td><td>29%</td><td>28.54</td><td>16</td><td>16</td></tr><tr><td>NKSC700</td><td>220</td><td>8.57·10<sup>-1</sup></td><td>68</td><td>25.31</td><td>32%</td><td>27.41</td><td>7</td><td>12</td></tr></tbody></table>
<figure><img src="image_4.png" /><figcaption>**Figure 2** Evolution of the X-ray diffractograms of samples synthesized without fibers, with acetic acid (A060: A) and with nitric acid (N060: B) and dried at 60 °C according to thermal treatment with reference anatase (ICSD card no. 00-002-0387, dark blue) and rutile (ICSD card no. 00-001-1292, yellow).</figcaption></figure>

tendencies that can be observed in all diffractograms presented in Fig. S4. The (110) rutile peak is visible in all the diffractograms, even at 550 °C, but its intensity can vary. For the samples synthesized with weak organic acid (acetic acid), rutile formation is hindered, which is in accordance with the literature [37, 38]. Under these conditions, the intensity of the peak attributed to rutile did not seem to change significantly when the pyrolysis temperature increased from 550 to 700 °C, whereas an increase was observed when strong inorganic acid (nitric acid) was used as the catalyst. When nitric acid is used, the (110) rutile peak becomes more intense, and other rutile peaks appear for the sample pyrolyzed at 700 °C. Thus, these observations demonstrate that the acid impacts the crystalline phase transition when the pyrolysis temperature changes (Fig. 2).

However, factor analysis provides more information about the system than does diffractograms. The calculated effects for crystallinity presented in Table S2 have powers of magnitude ranging from 10⁻³ to 10⁻¹, which demonstrates that some of the following parameters—drying method, pyrolysis temperature, fiber content, and the acid used— have a greater impact on the prevalence of anatase than others. To summarize, the parameters “drying process” and “pyrolysis temperature” have effects of the order of magnitude of 10⁻¹, the parameter “fiber content” has an effect of the order of magnitude of 10⁻², and the parameter “acid used” has an effect of the order of magnitude of 10⁻³. Among these results, the most significant parameters are the parameters “drying process” and “pyrolysis temperature.” The negative sign for both parameters indicates that

ambient pressure drying and a pyrolysis temperature of 550 °C favor the desired prevalence of anatase. Experimentally, the effect of drying can be explained by what happens during both drying processes. During supercritical drying, the texture of the gel is preserved, while the porosity collapses during ambient pressure drying. Therefore, different textures and particle sizes are obtained when comparing aerogels and xerogels before pyrolysis. After thermal treatment, these particles lead to different phases of titania depending on their initial dimensions, as was studied in the literature [20]. The factor analysis also reflects what was observed in Fig. 2, where a lower pyrolysis temperature is favorable for the presence of anatase, which is in agreement with the literature [25]. The negative sign of the parameter “fiber content” effect indicates that samples containing no fibers are susceptible to containing more anatase. The carbon generated by the remaining organic groups of the titanium precursor and by the fibers is a well-known reductant that promotes the transformation to rutile through the formation of oxygen vacancies [25]. Finally, the effect of the parameter “acid used” is calculated at 10⁻³. However, most of the two-, three-, and four-factor interactions had significant calculated effects as well with an order of magnitude of 10⁻² (Table S2). Consequently, their effects are in a similar range as the sole effects of fiber content. This result means that all the parameters affect the crystallinity of the material, either by themselves or through multifactor interactions.

## Effects on material porosity

The calculated effects on the SSA obtained by the factor analysis are presented in Table S2. As expected, the factor that plays the greatest role in increasing the SSA is the “drying process” parameter, as it has the highest calculated effect, with an order of magnitude of 10², and is in favor of CO₂ supercritical drying. Supercritical CO₂ can preserve the porous network—due to its very low surface tension in the supercritical state—while readily extracting the solvent.

In contrast, ambient pressure drying causes the pore network to collapse with the agglomeration of the particles and the closing of some pores (Fig. 3). The second largest calculated effect is the parameter “fiber content,” with an order of magnitude of 10¹, which indicates that adding fibers increases the SSA. Characterization of the kapok fibers left in the same
<figure><img src="image_5.png" /><figcaption>**Figure 3** Nitrogen adsorption isotherms of AKSC550, NKSC700, and NK60700 at –196 °C (A), pore size distributions of AKSC550 and NKSC700 (B), and SEM micrographs of AKSC550 (C), NKSC700 (D), and NK60700 (E).</figcaption></figure>

acidic solutions as those used for sol–gel showed that pyrolyzed kapok has an SSA of 248 m²/g with acetic acid and 334 m²/g with nitric acid. Thus, the positive effect on the SSA highlighted by the factor analysis also comes from the effect of the presence of fibers in the final composite. Like the crystallinity factor analysis, several multiple-factor interactions have comparable values to single-factor effects: X₂X₄, X₃X₄, X₁X₂X₃, X₁X₂X₃X₄ and so on (see Table S2). Consequently, the four synthesis parameters play a role in the porosity due to single-factor and/or multiple-factor effects. In addition, a very large range of SSA values is obtained, contrary to the crystallinity factor analysis, where anatase is predominant in each case. Thus, porosity has a greater impact on the electrochemical properties than does the crystalline phase.

## Effect of the kapok addition

Electrical conductivity measurements were carried out on pyrolyzed kapok fibers at different temperatures, and on materials with and without fibers. It should be noted that good conductivity is observed even at 550 °C (6.9·10⁻⁷ S/cm and 0.18 S/cm for fibers pyrolyzed at 550 and 700 °C, respectively, with a 4-point method and an applied weight of 0.5 ton). The fiber amount inserted influences electrical conductivity. Without kapok, no conductivity was measured. On the contrary, we observed different conductivities in

composites synthesized under different conditions (9.2·10⁻⁸ S/cm for AKSC550, 1.1·10⁻³ S/cm for AKSC700, a value too low to be measured for NKSC550, and 2.2·10⁻⁴ S/cm for NKSC700).

## Electrochemical study

Finally, an electrochemical study was conducted on all the TiO₂-carbon composites. They are implemented in electrode to benefit from the advantages of the synthesis method, i.e., of the intimate TiO₂-carbon fiber composite prepared by simple sol–gel method, drying and pyrolysis, and limitation of the inactive mass without addition of conductive carbon black. The results of this characterization are presented in Fig. 4 and in Fig. S5. By comparing all samples, it is possible to deduce that without kapok fibers, the electrode cannot function properly. Moreover, materials dried at 60 °C were shown to reach lower capacities than those dried in supercritical CO₂. The best reproducibility was obtained for AKSC550 and NKSC700. Figure 4 shows their electrochemical results. The best properties among the series can be attributed to their high SSA, as already predicted and obtained through supercritical drying. Fiber addition also appears to be a common parameter for these two best samples, highlighting the necessity of conductive media. On the one hand, the addition of fibers favored the reaction of the active material since the carbon fibers were obtained after
<figure><img src="image_6.png" /><figcaption>**Figure 4** First cycle charge and discharge curves obtained at a rate of C/50 (A), capacity fade over 40 cycles at C/50 (B), and first cycle differential capacity analysis (C) of NKSC700 and</figcaption></figure>

AKSC550 (for clarity purposes, a multiplying factor of 3.5 was applied on the dQ/dV axis for the AKSC550 sample).

pyrolysis, and the carbon generated by the remaining organic groups of the titanium precursor created an intertwined conducting network that allowed electrons to travel in multiple dimensions. On the other hand, supercritical drying preserved the porosity and led to more open textures in the material, which facilitated the diffusion of lithium ions and increased the accessibility of the surface of the active material to the electrolyte. SEM and EDS (Fig. S6) observations of these materials showed that good TiO₂ coverage was present over the entire length of the carbon fibers. Interestingly, AKSC550 and NKSC700 were not synthesized using the same acid as a catalyst and had different pyrolysis temperatures. The fact that they led to the best results even if these parameters were different can be explained by the compensation of beneficial effects due to the factor interactions. AKSC550 has a slightly lower surface area but contains more anatase

(178 m²/g, anatase relative intensity of 79%), while NKSC700 has the highest surface area of all the samples but, in turn, contains less anatase than AKSC550 (220 m²/g, anatase relative intensity of 68%). Therefore, the properties of both samples seemingly tend to reach an optimal middle ground between parameters to obtain the most promising results. These results show that both intrinsic and extrinsic properties need to be considered.

Figure 4A shows the first charge and discharge curves of NKSC700 and AKSC550. Both samples have very similar curve shapes that are typical of anatase [10]. Since a small quantity of carbon is present in the material, only the electrochemical contribution of TiO₂ is considered. For both samples, the first part of the discharge curve appears to have a strong slope between 3 and 1.5 V, which can be associated with the weak insertion of Li ions in the
anatase phase. The slope is followed by a plateau at approximately 1.5 V, which is associated with the start of the embedding of Li⁺ into TiO₂ lattices. During that stage, two phases coexist: the Li-poor anatase phase and a Li-rich orthorhombic phase that is formed by lithium exchange with the anatase phase [39]. A similar plateau is observed in the charge curves at approximately 1.5 V, corresponding to a similar process [40]. Finally, the plateau progressively evolves into a slope. The plateau is slightly longer in AKSC550 than in NKSC700, and the difference can be attributed to the different SSAs of the materials since a higher electrode surface is accessible to lithium ions in NKSC700, allowing more active material to react. SEM observations also showed smaller average particle sizes in NKSC700, which also demonstrated that better contact with the electrolyte was achieved in this material. The same analysis can be applied to the first charge, where NKSC700 reaches a higher capacity than AKSC550.

Figure 4B shows that both materials have a good capacity retention rate with similar results after 40 cycles and a capacity retention of 80% after 40 cycles. The differential capacity analysis presented in Fig. 4C sheds light on the components of the material that contributes to the electrochemical properties. The two materials contain both anatase and rutile, with more rutile in NKSC700. For AKSC550, two different signals are visible and are superimposed: one that is associated with the reaction of lithium with anatase (1.5 and 2.07 V) and one that is associated with the reaction of rutile (1.7 and 2.2 V). For NKSC700, only one peak is registered, demonstrating the harmonization of all the redox centers. Moreover, this sample contains more rutile phase as proposed by Song et al. [41]. Therefore, better ion transport properties should be obtained. The high porosity resulting from supercritical drying obviously increases the properties, but the presence of rutile in the materials also seems to positively affect the properties, as shown by Wei et al. [42], who were able to reach higher capacities with rutile/ anatase composites than with anatase alone. After 40 cycles, the charge and discharge capacities of NKSC700 were 184 and 190 mAh/g, respectively, and 153 and 155 mAh/g for AKSC550. These values are in the mid-range values observed in the literature with our implementation that limits inactive mass (no conductive carbon addition). After 40 cycles, Yan et al. [32] reached 80 mAh/g while Trang et al. [33]

reached 270 mAh/g approximately. However, our results are in the same range as the values obtained in other anatase/rutile composites: After 40 cycles, Song et al. [41] attained a capacity of around 140 mAh/g, while Wei et al. [42] reached values between 160 and 200 mAh/g depending on the synthesis and tests conditions.

The results obtained in this study highlight the crucial role of synthesis parameters in tailoring the physicochemical properties and electrochemical performance of biomass-derived carbon-titania composites. This analysis covers both intrinsic and extrinsic parameters. While the factor analysis provided a structured approach to identify the most influential parameters, a broader discussion from a materials science perspective is necessary to contextualize these findings. The previous studies on TiO₂-based composites have demonstrated that crystallinity, porosity, and carbon content significantly influence lithium-ion storage capacity, cycling stability, and rate performance. For instance, the anatase-to-rutile transition [43, 44] has been extensively reported to impact ion mobility, with anatase offering better lithium insertion kinetics due to its open-framework structure, whereas rutile [45] contributes to higher electronic conductivity but lower insertion capacity [41]. Our results align with these observations, showing that supercritical drying and fiber addition led to high specific surface areas (up to 220 m²/g), which is comparable to previously reported TiO₂-carbon aerogels optimized for electrochemical applications [46].

Furthermore, the choice of acid catalyst in sol–gel synthesis has been reported to influence not only the crystallization pathway but also the morphology and interfacial properties of TiO₂ composites. Several studies have shown that organic acids, such as acetic acid, promote anatase formation, while inorganic acids, such as nitric acid, favor higher rutile content, which is consistent with our findings [47–49]. However, the interplay between porosity, phase composition, and electrochemical performance remains a complex phenomenon that requires further investigation. In this regard, the best-performing materials in our study—AKSC550 and NKSC700—demonstrated an optimal balance between porosity, anatase content, and electronic conductivity, a synergy that has also been reported in the literature for hierarchical TiO₂-carbon architectures [50].
## Conclusions

This study successfully demonstrated the synthesis of biomass-derived carbon–titania composites through a simple sol–gel approach, followed by controlled drying and thermal treatment. By employing a full factorial experimental design, we systematically analyzed the effects of four key synthesis parameters—acid catalyst, drying method, pyrolysis temperature, and fiber content—on the crystallinity and porosity of the final composite materials.

The factor analysis revealed that supercritical drying and the addition of kapok fibers were critical for achieving a high specific surface area (SSA), which is essential for optimizing lithium-ion diffusion. Conversely, the pyrolysis temperature and acid type significantly influenced the anatase-to-rutile phase transformation, with acetic acid favoring anatase formation over rutile. These interactions highlight the importance of multifactorial optimization in tailoring material properties for energy storage applications.

Electrochemical characterization confirmed that high SSA materials exhibited superior lithium-ion storage performance, particularly in composites containing kapok-derived carbon. Notably, the best-performing materials—AKSC550 and NKSC700—demonstrated stable cycling performance and an optimal balance between porosity, conductivity, and crystallinity. The presence of a synergistic anatase–rutile composite phase further contributed to enhanced electrochemical properties, reinforcing the role of structural and textural design in electrode material development.

Overall, this study provides valuable insights into the rational design of bio-based titania composites for lithium-ion battery anodes. The scalability and simplicity of the proposed synthesis route, combined with its reduced reliance on additional conductive additives, make it a promising approach for sustainable energy storage materials. Future research will focus on optimizing electrode formulations and exploring alternative biomass precursors to further enhance performance and industrial applicability.

## Acknowledgements

The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.

## Author contributions

Julia Wagner contributed to writing—original draft, drawing, samples synthesis and characterization, investigation, formal analysis, and data curation, Julien Jaxel helped in supercritical drying and SEM analysis, and writing—review & editing, Katia Guérin helped in conceptualization, formal analysis, data curation, and writing—review & editing. Sandrine Berthon-Fabry helped in conceptualization, formal analysis, data curation, and writing—review & editing. All authors have approved the final version of the manuscript.

## Funding

Open access funding provided by Mines Paris - PSL.

## Data availability

All the data are given in the article and in the supplementary information.

## Declarations

**Conflict of interest** The authors declare no competing interest and no conflict of interest.

**Supplementary Information** The online version contains supplementary material available at https://doi.org/10.1007/s10853-025-10874-x.

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