Introduction

Extensive use of renewable energy sources necessitates the development of low-cost and efficient energy storage systems. To address these needs, Li-ion battery systems (LIBs) have been extensively exploited in the last 3 decades. However, as the use of large-format Li batteries becomes widespread, the cost of the Li raw material has roughly doubled from the first application in 1991 to the present (Kim et al. 2012). Based on the wide availability, non-toxicity and low cost of sodium, ambient temperature sodium-based batteries have the potential for meeting large-scale grid energy storage needs.

Sodium-ion batteries (NIBs) have a working mechanism similar to that of LIBs. However, since the ionic radii of sodium and lithium are different, dissimilar thermodynamic and kinetic properties are expected between the individual cells. For instance, graphite, which is a commonly used anode material in Li-ion batteries, is considered nearly impossible for the application of NIBs since the ionic volume of sodium ions is almost double those of Li+ ions resulting in difficulties to intercalate between the interlayer of graphite (~0.34 nm) (Cao et al. 2012; Falco et al. 2013; Komaba et al. 2003; Palomares et al. 2012; Sangster 2007; Slater et al. 2013). This limitation can be circumvented by sodium ion co-intercalation into graphite by using diglyme as an electrolyte solvent (Jache and Adelhelm 2014).

In recent years, different types of materials including carbonaceous materials, sodium alloys and metal oxides have been investigated as anodes for NIBs. The electrochemical insertion of sodium ions into carbon was first reported by Doeff et al. (Kubo et al. 2013) with a reversible capacity of ∼85 mAh g−1. Disordered carbons including both hard and amorphous carbon are considered the dominant candidate materials for NIBs (Alcántara et al. 2005; Antonietti et al. 2007a, b; Cao et al. 2012; Komaba et al. 2011b; Kubo et al. 2013; Ponrouch et al. 2013; Soorholtz et al. 2013; Stevens and Dahn 2001; Tang et al. 2012; Wenzel et al. 2011; Xia and Dahn 2012; Zhao et al. 2013) because of the larger interlayer space between the graphene sheets. Stevens and Dahn reported the reversible sodium capacities of hard carbons derived from sugars achieving 300 mAh g−1 (Soorholtz et al. 2013). Tang et al. (2012) have also demonstrated the use of hollow carbon nanospheres as high rate anode materials for NIBs.

Although most studies focus on hard carbon materials for NIBs because of their large interlayer distance, alternative oxide anodes such as Na2Ti3O7 (Senguttuvan et al. 2011), Na2C8H4O4 (Zhao et al. 2013) and amorphous TiO2 (Xiong et al. 2011) nanotubes have also been investigated. However, the capacities of these materials toward sodium storage are still low. Tin dioxide (SnO2) is a promising anode material in LIBs because of its high theoretical capacity, although it suffers from an extremely large volume change during charge and discharge resulting in electrode degradation and fast capacity fading (Sevilla et al. 2012; Yu et al. 2013). SnO2 can exhibit a theoretical reversible specific capacity of 667 mAh g−1 (Park et al. 2014; Wang et al. 2013b, 2014). Wang’s group reported a SnO2@graphene nanocomposite demonstrating a capacity of about 302 mAh g−1 at 160 mA g−1 (Su et al. 2013). Wang’s group also reported the SnO2@MWCNT nanocomposite, which is capable of delivering about 71 % of its theoretical capacity toward sodium storage (Wang et al. 2013b). An SnO2-reduced graphene oxide (RGO) nanocomposite with 330 mAh g−1 capacities was obtained by Park and coworkers (Wang et al. 2014). In another study conducted by Ahn et al., the Na/SnO2 cell resulted in huge capacity fading after ten cycles (Park et al. 2014).

The most important focus in the advancement of lithium/sodium batteries is to enhance the lithium/sodium ion and electron transport kinetics in electrodes. To tackle this problem, embedding nanostructured active materials into a three-dimensional conductive porous network is generally suggested (Wang et al. 2013a).

Bacterial cellulose (BC) is an interesting low-cost, nontoxic and biocompatible extrapolymeric carbon material readily produced by various species of bacteria, such as those of the genera Gluconacetobacter (formerly Acetobacter), Agrobacterium, Aerobacter, Achromobacter, Azotobacter, Rhizobium, Sarcina and Salmonella (Shoda and Sugano 2005). BC is traditionally used to make the Filipino dessert Nata de coco, served as a gelatinous cube. In stationary culture conditions, these bacteria produce a thick pellicle of cellulose on the surface of the liquid medium. The isolation and purification of bacterial cellulose are relatively simple, not requiring energy- or chemical-intensive processes. Although plant cellulose contains impurities such as lignin, pectin and hemicellulose, BC is completely pure and highly crystalline (Guo and Catchmark 2012). BC has an ultrafine three-dimensional porous network composed of nanofibers with widths <100 nm, which are smaller than those of plant cellulose. This three-dimensional network provides unique properties to BC including a high tensile strength and high specific surface area. Therefore, BC has been investigated in different fields such as the food industry, medical purposes and additives in the production of paper (Barud et al. 2008; Czaja et al. 2007; Jonas and Farah 1998; Nge et al. 2010). Moreover, BC can also be considered an ideal hydrophilic matrix for metal incorporation to provide additional properties to BC (Shah et al. 2013; Wu et al. 2014; Zhang et al. 2013).

Lignocellulosic materials have gained ample attention because of their availability, low cost and porous structure, resulting in a high surface area of the carbon attained after the pyrolysis process for energy storage devices such as wood (Klijanienko et al. 2008), wheat straw (Li et al. 2010) and coconut husk (Tan et al. 2008). Recently, bacterial cellulose-derived electrically conducting anodic materials and the deposition of active battery components such as binder and electrode separators have shown promising potential in lithium storage capability (Hu et al. 2013; Leijonmarck et al. 2013; Nyström et al. 2009). The pyrolysis procedure compared with hydrothermal carbonization was found to be an easy and effective method for enhancement of surface areas and pore volumes because of the removal of water from cellulose resulting in the contraction of fibers during the process (Hu et al. 2013; Wang et al. 2013a; Wu et al. 2014). Previously, it was shown that the activation process could widen the preformed pores (Huidobro et al. 2001). It was also reported that the pyrolysis process could cause micropore and mesopore formation (Phan et al. 2006). Lee et al. (2013) pyrolyzed BC at 950 °C and physically activated with CO2 to obtain carbonaceous BC with a graphitic structure.

Herein, we report a simple, unique, low-cost and environmentally benign method to fabricate three-dimensional porous electrode structures, which are compatible with different nanostructured active anode materials. In this study, pyrolyzed bacterial celluloses (PBCs) with a microfibril network structure were used as anode electrodes. We investigated the electrochemical characteristics of microfibril network structure PBCs for NIBs at various heat treatment temperatures and SnO2 nanoparticle-decorated microfibril network structure PBCs. This is the first report based on the naturally occurring biomaterial BC with a microfibril network structure for sodium ion batteries.

Experimental

Preparation of the bacterial cellulose

Bacterial cellulose (BC) was produced by using Gluconacetobacter xylinus (strain BCRC 12334). For BC production, the G. xylinus strain was inoculated (2 %, v/v) into 100 ml of Hestrin and Schramm liquid medium (HS) containing 2 % glucose, 0.5 % yeast extract, 0.5 % peptone, 0.27 %. Na2HPO4 and 0.15 % citric acid (pH 4.5, Hestrin and Schramm, 1954) in a 500-ml flask and incubated at 30 °C and 50 rpm for 6 days. After incubation, the produced BC samples were washed with distilled water and dried at 60 °C for 4 h.

Synthesis of the materials

The obtained dried bacterial celluloses were pyrolyzed (carbonized) under argon atmosphere at three different temperatures including 500, 750 and 1000 °C. A heating rate of 2 °C min−1 was applied and kept for 6 h at the targeted temperature. Materials were described as PBC-500, PBC-750 and PBC-1000 according to their pyrolysis temperatures.

To obtain SnO2 nanoparticle-decorated PBC (labeled as SnO2@PBC), SnO2 nanoparticles were incorporated with BC during the preparation of BCs. First, SnO2 nanoparticles were synthesized using a hydrothermal method. In a typical synthesis, 2 g SnCl4·5H2O (Sigma-Aldrich, ≥98 %) was dissolved in 20 ml deionized water. After stirring until complete dissolution, it was heated to 180 °C in a Teflon-lined autoclave (50 ml in capacity) for 20 h. The precipitate was cooled to room temperature naturally and then collected and washed with distilled water and ethanol several times. After drying at 80 °C in a vacuum oven overnight, the final SnO2 products were obtained. For SnO2@PBC production, the G. xylinus strain was inoculated (2 %, v/v) along with the synthesized SnO2 nanoparticles (25 mg) into 100 ml of HS medium in a 500-ml flask and incubated at 30 °C and 50 rpm for 6 days. SnO2@BC nanoparticle composites were washed with distilled water (Wang et al. 2013a) and dried at 60 °C for 4 h (Scheme 1).

Scheme 1
scheme 1

The synthesis procedure for SnO2@PBC nanocomposites

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After successful covering of SnO2 nanoparticles by bacterial cellulose, the resulting products (quoted as SnO2@PBC) were pyrolyzed at 500 °C for 6 h with a heating rate of 2 °C min−1. Since the calcination temperature above ~600 °C causes thermal reduction of the composite and produces tin (Yu et al. 2013), careful temperature control was maintained during the course of the pyrolysis. After the pyrolysis, the white/yellowish product changed to a black color (Scheme 1).

Characterization of the materials

The phases were characterized by X-ray diffraction. The X-ray powder diffraction patterns of the SnO2 nanoparticles and SnO2@PBC were recorded on a Bruker D8 Advance diffractometer (θ–2θ mode, Cu Kα radiation λ = 1.5406 Å and a linear position-sensitive detector) at a scanning step of 0.02°s−1.

The microstructural and chemical analyses proceeded using a transmission electron microscope (TEM-FEI TECNAI F20 S-TWIN) fitted with a scanning mode and both high-angle annular dark-field (HAADF) detector and EDAX EDS as well as SEM (Philips XL30). The weight ratio between SnO2 and PBCs was determined by a thermogravimetric analyzer (TGA, PerkinElmer, TGA 4000) between 30 and 700 °C at a heating rate of 10 °C min−1 in oxygen atmosphere.

Micro Raman spectra were recorded on a Jobin Yvon LabRam spectrometer (excitation wavelength: 632.8 nm).

Electrochemical characterization

The electrochemical properties of PBC-500, PBC-750, PBC-1000 and SnO2@PBC materials were evaluated by assembling Swagelok cells in an argon-filled glove box (Innovative Technology, USA). The electrochemical tests were performed in a two-electrode configuration. Sodium metal foils (Sigma-Aldrich) were used as counter and reference electrodes. The working electrodes contained 80 wt% active materials (PBC-500, PBC-750, PBC-1000 and SnO2@PBC) and 20 wt% super P. The mass loading of the active material was approximately 4 mg cm−2. The electrolyte solution was 1M NaClO4 salt dissolved in a mixture of ethylene carbonate (EC) and propylene carbonate (PC) with a volume ratio of 1:1. The electrochemical performance was tested with a cutoff voltage range from 0.005 to 2 V (vs. Na/Na+) using a Biologic VMP-3 electrochemical workstation.

Results and discussion

SEM images of the bacterial cellulose (BC) before (Fig. 1a) and after being pyrolyzed at 500, 750 and 1000 °C are shown in Fig. 1b–d, respectively. Although the bacterial cellulose has dense and compact microstructural properties, after the pyrolysis procedure at different temperatures it shows a highly porous network structure constructed by numerous intertwined ultrathin nanofibrils. To further characterize the porous structure of the pyrolyzed bacterial celluloses (PBCs), Brunauer-Emmett-Teller nitrogen adsorption studies were performed. Figure S1 (Electronic supplementary material) depicts the adsorption isotherm of PBC-500, PBC-500 and PBC-1000 (Fig. S1a) and their resulting pore size distributions (Fig. S1b). PBC-500 has a very low surface area of 39.3 m2 g−1 possessing micropores with around 2 nm mean diameter. When the temperature is raised from 500 to 750 and 1000 °C, the surface areas increase dramatically, reaching to 331.3 m2 g−1 and 662.6 m2 g−1, respectively. Pyrolysis of the bacterial cellulose was further characterized by Raman spectroscopy. Figure 2a depicts the Raman spectra of the as-synthesized bacterial cellulose and after 750 °C heat treatment under argon atmosphere. The Raman spectrum of the PBC-750 nanocomposite contains the characteristic wide D and G bands around 1360 and 1590 cm−1, respectively, typical for amorphous carbon or disordered graphite, providing defects and a partially disordered nature of the graphitic layers within the PBC. The large intensity value I D/I G (~0.85) indicates the low degree of graphitization of the resulting pyrolyzed bacterial cellulose materials. In contrast, there is no obvious peak observed for the BC spectrum.

Fig. 1
figure 1

Morphological characterization of BC, PBCs and SnO2@PBC composite: SEM images of a BC before pyrolysis, b PBC-500, c PBC-750, d PBC-1000, e and f SnO2@PBC with different magnifications

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Fig. 2
figure 2

Structural characterization of samples: a Raman spectra of BC and PBC-500, b XRD patterns of SnO2 nanoparticles, PBC-500 and SnO2@PBC composite

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The XRD patterns of the pyrolyzed bacterial celluloses treated at 500 °C (PBC-500), SnO2 nanoparticles and SnO2@PBC composites are shown in Fig. 2b. Figure 2b shows broad particle size distributions without any aggregation of larger particles for the SnO2 nanoparticles. The crystallite dimension of SnO2 nanoparticles along (211) was calculated at around 5 nm by using the Scherrer equation. For the PBC-500, no diffraction peaks corresponding to graphitic carbon were observed in the XRD pattern, meaning that the carbon coating was amorphous. Bacterial cellulose in the presence of SnO2 nanoparticles was pyrolyzed at 500 °C to eliminate the reduction of SnO2 to Sn nanoparticles (Fig. 2b). We deduced crystallite dimensions of about 6 nm SnO2 nanoparticles along the (211) direction after pyrolysis, suggesting that the bacterial culture and temperature treatment do not change the crystallite size and nature of the SnO2 nanoparticles.

The microstructure of SnO2 nanoparticles and SnO2@PBC composite are also analyzed by TEM. The SnO2 nanoparticles show a rod-like structure with uniform lengths (Fig. 3a). In the SnO2@PBC composite, SnO2 particles are successfully surrounded by the bacterial cellulose and dispersed in the bacterial cellulose with some agglomerates. The direct addition of the nanoparticles along with the bacterial culture into the bacterial growth medium allows more interaction between the SnO2 nanoparticles. During the formation of the in situ coating of SnO2, PBC keeps its three-dimensional nanofibril porous network structure with obvious SnO2 nanoparticles (Fig. 1e, f). It was observed that SnO2 nanoparticles were homogeneously distributed during cellulose production. The reason could be the interaction of oxygen-containing moieties of BC nanofibrils such as hydroxyl groups with the as-synthesized SnO2 nanoparticles during cellulose production (Barud et al. 2011; Wang et al. 2013a). Indeed, the FT-IR spectrum of the materials (Fig. S2) reveals those oxygenated groups. The wide bands at 3500–3000 cm−1 indicate the existence of a large number of hydroxyl groups, which is much higher for the as-synthesized bacterial cellulose than that after being pyrolyzed at 500 °C. The bands at 1640 cm−1 can be attributed to the C=O for the BC samples whose intensity decreases upon pyrolysis. Instead, a visible band at 1570 cm−1 appears for the PBC samples, which can be attributed to the C=C vibrations, providing the presence of a higher degree of carbonaceous structure.

Fig. 3
figure 3

Morphological characterization of SnO2 nanoparticles and SnO2@PBC composite: TEM images of a SnO2 nanoparticles and b SnO2@PBC composite

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Production of BC and SnO2@BC under shaking growth conditions at 50 rpm allowed effective coating of SnO2 nanoparticles during the cellulose production. Moreover, the shaking incubation conditions led to nanoparticles being well dispersed on the BC bulk ultrafine reticulated structure. For the production of SnO2@BC nanocomposites, 25 or 50 mg of SnO2 nanoparticles was incorporated into 100 ml bacterial growth medium. The amount of cellulose production without addition of SnO2 nanocomposite particles was found as 160 mg dry weight per 100 ml medium after 6 days of incubation; 25 mg of SnO2 nanoparticles did not affect the growth of G. xylinus or the amount of cellulose production. However, the produced cellulose amounts decreased by 30 % (mg dry weight/100 ml medium) with incorporation of 50 mg nanoparticles, respectively, after 6 days. Therefore, the concentration of 25 mg of SnO2 nanoparticles per 100 ml of medium was used effective for SnO2@BC nanocomposite production. The weight percent of the SnO2 in SnO2@PBC was determined by thermogravimetric analysis (TGA) in O2 atmosphere indicating around 40 wt% SnO2 content (Fig. S3 of Electronic supplementary material).

Electrochemical performances of the pyrolyzed-BC electrodes prepared at three different temperatures (PBC-500, PBC-750 and PBC-1000) were first tested in the absence of SnO2 nanoparticles to understand the impact of the bacterial cellulose matrix. To do so, a comparative electrochemical study had been carried out with three different electrolyte compositions: 1M NaClO4 salt in PC solvent, 1M NaClO4 salt in PC solvent together with FEC additive and 1M NaClO4 salt in a binary mixture of EC:PC as the best solvent formulation and most attractive electrolyte for the development of Na-ion batteries (Ponrouch et al. 2012).

These three electrolytes were examined in Fig. 4 showing the discharge capacity as a function of cycle number for the PBC-500, PBC-750 and PBC-1000. The discharge capacities for all pyrolyzed BCs in the presence of PC electrolyte solution exhibited fast capacity fading. After adding 5 wt% FEC into the PC solution, the capacity degradation was sufficiently avoided because of the passivation and suppression of the side reactions between the Na metal and electrolyte. The thin passivation layer (SEI surface layer) that formed on the negative electrode was known to be modified by the FEC additive (Komaba et al. 2011a). Conversely, with the binary EC:PC solution containing NaClO4 salt, the PBCs showed better discharge capacity compared to the other electrolytes since the binary EC:PC electrolyte proved to be a much more thermally and electrochemically stable SEI layer with carbonaceous materials (Ponrouch et al. 2012). As can be seen from the voltage versus number of inserted electron curves for all of the combinations at the first cycle (Fig. S4), there was a huge irreversible capacity loss, which was attributed to the presence of the oxygenated group on the surface of the bacterial cellulose matrixes, which was supported above with the FT-IR analysis (Fig. S2), resulting in irreversible Na insertion and SEI formation. To sum up, Fig. 4d shows the discharge capacity of the PBC materials at three different temperatures in the presence of the electrolyte configuration of 1M NaClO4 salt in EC:PC solvent. The first discharge capacities of the PBC-500, PBC-750 and PBC-1000 materials were 194, 451 and 362 mAh g−1, respectively. When the pyrolysis temperature was increased up to 1000 °C, the structure of the materials became more ordered. Since the ordered carbon structure is not suitable for accommodating the larger ionic radius of sodium ions because of the insufficiently large interstitial space, the capacities of the PBC-1000 are much lower than those of PBC-750. We also tested the rate capabilities of the PBC-750 electrode at increasing current rates by gradationally increasing the current rate from 0.1 C up to 10 C (Fig. 4e) and returning the initial current to 0.1 C. At current rates of 0.1, 0.2, 0.5, 1, 2, 5 and 10 C, the reversible discharge capacities of the PBC-750 anode decreased to 161, 134, 88, 61, 44, 25 and 9 mAh g−1, respectively. The PBC-750 recovered almost 100 % of its initial capacity when returning to 0.1 C while presenting a long cycle life and good rate capability.

Fig. 4
figure 4

Cycling performances of PBCs and the rate performances of PBC-750: Effect of electrolytes on the discharge capacity of a PBC-500, b PBC-750 and c PBC-1000 at a C/10 rate, d effect of pyrolization temperature on the discharge capacity of PBCs in 1M NaClO4 containing in EC/PC (1:1 wt %) at a C/10 rate in the voltage range of 0.005–2 V (vs. Na/Na+), e rate performances of the PBC-750 electrode at various current rates ranging from 0.1 to 10 C in the voltage range of 0.005–2 V (vs. Na/Na+) and f comparison of PBC-750 and commercially available cellulose paper at a C/10 rate in the voltage range of 0.005–2 V (vs. Na/Na+)

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The sodium storage capacity of the PBC-750 electrode obtained from bacterial cellulose was sufficient, however, to determine the importance of the nanofibril porous structure and compare their performances over other types of cellulose paper (i.e., plant cellulose, commercially available cellulose paper). They were both carbonized at 750 °C for 6 h and tested in the same condition with a constant current rate of 0.1 C in the voltage range of 0.005–2 V (vs. Na/Na+). The cellulose paper shows (Fig. 4f) smooth and stabile discharge versus Na with a capacity of 50 mAh g−1 over 100 cycles; however, the electrochemical performance of the carbon material derived from bacterial cellulose (PBC-750) is almost three times higher than in the cellulose paper.

The number of inserted electron vs. voltage graphs and their discharge capacities are shown in Fig. 5a for the SnO2@PBC composite at a 0.1 C current density. The first discharge capacity of the SnO2@PBC composite was about 1100 mAh g−1, and they presented a very stable sodiation/desodiation process with a reversible capacity of about 400 mAh g−1. This could be attributed to the SnO2 nanoparticles being successfully confined in the carbon matrix derived from bacterial cellulose. The SnO2 nanoparticles, which were surrounded by pyrolyzed carbon, exhibited high conductivity, easy diffusion of Na+ ions and, most importantly, buffering of the large volumetric expansion during charge/discharge. To the best of our knowledge, the SnO2@bacterial cellulose composite has not yet been the subject for an anode material for sodium ion batteries. The specific capacity of SnO2 alone in the hybrid anode composite, in which both PBC-500 and SnO2 contributed concurrently to the entire capacity, was estimated by subtracting the contribution of PBC-500. Since the capacity given in Fig. 5a was based on the mass of SnO2, the entire capacity of the composite was calculated as 160 mAh g−1 (400 mAh g−1 × 0.4). After deducing the contribution of the PBC-500, which is around 30 mAh g−1 (50 mAh g−1 × 0.6), the rest of the capacity involvement (130 mAh g−1) was from SnO2 in the composite. Thus, the contribution of the SnO2 to the SnO2@PBC composite depicted in Fig. 5a was calculated as 325 mAh g−1 (130 mAh g−1/0.4). Figure 5b shows the rate capability of the SnO2@PBC composite by gradationally increasing the current rate from 0.1 C up to 10 C and returning to the initial current of 0.1 C. The SnO2@PBC composite electrode demonstrated a good rate capability. When the electrode was tested at 0.1 C, the capacity was around 400 mAh g−1, which then decreased to 50 mAh g−1 when the current rate was increased to 10 C. Conversely, the capacity was recovered when the rate was 0.1 C.

Fig. 5
figure 5

a Cycling and rate performance of the SnO2@PBC composite: a discharge (sodiation) capacity (inlet voltage vs. number of inserted electrons) of SnO2@PBC at a C/10 rate, in the voltage range of 0.005–2 V (vs. Na/Na+), b rate capability of the SnO2@PBC composite electrode at various current rates from 0.1 to 10 C in the voltage range of 0.005–2 V (vs. Na/Na+) (note: the specific capacity was calculated by using the mass of SnO2, given in mAh g−1)

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The SnO2@PBC composite anode was also compared to the SnO2 + (PBC-500) material for sodium ion batteries at a constant current rate of C/10 (Fig. 6). The SnO2 + (PBC-500) material was prepared by hand mixing of 40 wt% as-prepared SnO2 nanoparticles in a mortar together with the as-pyrolyzed bacterial cellulose (PBC-500) in order to clarify the effect of the nanofibril carbon support to the electrochemical performances of the SnO2@PBC composite. It is clear that the SnO2@PBC composite, which was prepared by in situ incorporation of the nanoparticles into the growth medium of G. xylinus, demonstrates quite good stabile capacity in comparison with the SnO2 + (PBC-500) material, demonstrating the strong interaction of oxygen-containing moieties of BC nanofibrils with the SnO2 nanoparticles during cellulose production, which is not the case for the SnO2 + (PBC-500) material. As can been seen from Fig. S5, since the SnO2 + (PBC-500) material was not covered with bacterial cellulose fibrils, their capacity fading was very dramatic because of the huge volume expansion of SnO2 nanoparticles, which was not compensated by the mechanically mixed PBC-500.

Fig. 6
figure 6

Capacity retention comparison of SnO2@PBC composite and SnO2 + (PBC-500) at a 0.1 C rate, in the voltage range of 0.005–2 V (vs. Na/Na+) (note: in both cases the materials contain 40 wt% SnO2 and the specific capacities were calculated by using the mass of SnO2, given in mAh g−1)

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Conclusions

In conclusion, the results of this study show that there is great potential in utilizing bacterial cellulose for the production of nanocomposites as anode materials for sodium-ion batteries. Simultaneous in situ incorporation of SnO2 nanoparticles with bacterial cellulose producing the G. xylinus strain into growth medium for the preparation of an SnO2@PBC composite enhanced the stabile capacity of the resulting modified BC. The higher number of SnO2 nanoparticles surrounding regions in the SnO2@PBC composite exhibited higher conductivity, easy diffusion of Na+ ions and buffering of a large volumetric expansion during charge/discharge, which could extend the potential applications of BC nanocomposites as anode materials for sodium-ion battery energy storage. Moreover, low-cost scaled-up production of bacterial cellulose for this purpose could be further investigated in future studies.