Carbon aerogels from bacterial nanocellulose as anodes for lithium ion batteries

Carbon aerogels with large open pores and high surface area are fabricated via pyrolysis of a readily available natural resource, e.g., bacterial nanocellulose (BNC) aerogels. Freeze-drying of the BNC hydrogels is used to preserve the 3D open network structure upon calcination whereas using Fe(III) improves the yield and H/ C ratio. These carbon aerogels are explored as anodes in lithium ion batteries where it is shown that they deliver superior capacity retention and rate performance compared to other carbon-based materials.


Introduction
The search for alternative energy sources to fossil fuels has increased in the recent years due to growing environmental concerns and an increasing oil price.Conventional non-fossil energy sources (such as wind, solar, and hydroelectric power) deliver energy continuously whether it is used or not.As such, it is advantageous to develop energy storage media which can also be light and easy to transport.][3][4] Carbon is the most commonly used anode material in commercial LIBs because of its low price, low lithium ion intercalation potential and good cyclic stability. 5Nevertheless, since carbon anodes may suffer of poor rate performance due to a slow lithium diffusivity, 5 intense efforts have been devoted to nding alternative anodes with different compositions, mostly oxides. 6However, the poor rate performance can be greatly improved by reducing the travel distance of the lithium ions within the carbon matrix.Mesoporous carbons have shown an improved rate performance and enhanced capacity, 7,8 particularly in combination with carbon nanotubes. 9Carbon materials derived from renewable natural, abundant nanomaterials, e.g., cellulose, chitin, collagen, and silk, may play an important role as they are available at a very low cost and readily available in developing countries. 10Moreover, these renewable carbon materials can be produced using much simpler methodologies, i.e. pyrolysis 11,12 and hydrothermal carbonization. 13,148][19][20][21] Particularly, bacterial nanocellulose (BNC), 15,16 which is readily obtained from cell cultures of G. xylinum and widely available as a dessert in South-East Asia, 22 has been suggested as a convenient scaffold for the preparation of functional materials with magnetic, 23 optical, 24 and mechanical properties due to their open network structure and low apparent density. 16,22][30] In this work, we study the behaviour of a carbon aerogel network obtained through the pyrolysis of BNC aerogels as anode material in LIBs.The pyrolyzed bres show a disordered structure with an enhanced surface area, resulting in anode materials with a good rate performance and capacity retention.Our results suggest that ordered graphitic layers are not a requirement for achieving high capacity, thus avoiding the use of high temperatures during the fabrication process.

Results and discussion
Fig. 1 shows freeze-dried BNC (FD-BNC, Fig. 1a-c) aerogel with a highly porous network and the preserved nanobrillar network aer pyrolysis at 900 C (FD-BNC-900, Fig. 1d-f).The ber thickness decreased from ca. 20-70 nm to about 20 nm.The original monolith reduced considerably its volume aer the pyrolysis mostly due to the loss of water and release of CO and CO 2 occurred around 300 C. 31 Normally a direct calcination of a gel body can lead a large shrinkage and collapse the network structure by capillary forces acting on the pore walls. 32However, although different drying techniques have been explored to maintained the porosity of nanocellulose-based materials, 33 the freeze-drying method can circumvent the drawbacks of direct thermal treatment and preserve the open porous networks. 19he obtained BNC-derived carbon (FD-BNC-900) was subsequently used as the anode in LIBs.The charge-discharge curves at a current density of 75 mA g À1 , namely 0.2 C (1 C ¼ 372 mA g À1 ), are shown in Fig. 2a.The initial discharge and charge capacity are 797 mA h g À1 and 386 mA h g À1 , respectively.The cycling performance and coulombic efficiency of the BNCderived material were also studied as shown in Fig. 2b.The initial charge capacity of 386 mA h g À1 decreased to 359 mA h g À1 aer 100 cycles.Aer an initial adjustment of the coulombic efficiency, probably due to stabilisation of the solid electrolyte interphase and the disordered structure. 8,34The capacity increases slightly during the cycling, likely due to the activation of the disordered structure. 34The capacity fade rate can be calculated to be 0.07% per cycle, which means that the capacity retention ability is very good.The capacity of the carbon aerogel as electrode is considerably better than that obtained for carbonized lms produced also from BNC, which was reported to be just about 100 mA h g À1 . 35The cellulose-derived carbon shows also a superior rate performance (Fig. 2c).The cell was directly discharged-charged at various current densities from 0.375 A g À1 (1 C) to 3.75 A g À1 (10 C) each for 10 cycles.The reversible capacities are 288, 228, 94, and 34 mA h g À1 at 0.375 A g À1 (1 C), 0.75 A g À1 (2 C), 1.875 A g À1 (5 C), and 3.75 A g À1 (10 C, namely 6 minutes to full charge), respectively.When the rate is tuned back to 0.375 A g À1 aer cycling at different current rates, the capacity can be recovered to 250 mA h g À1 , which evidences the stable cycling performance of the freeze-dried carbon aerogel.
Compared with mesoporous carbons 7,8 our sample exhibits a lower capacity but a much better capacity retention behavior at the same current rate of 0.2 C. The superior electrochemical performance of the carbon aerogel is attributed to its high surface area, open 3D network and the crosslinking between the carbon nanobres.The high surface area and open pore  structure offer a large electrode/electrolyte contact area, favorable for charge-transfer reaction, whereas the crosslinking of the nanobres ensures a continuous electron transport.Moreover, the thin bres with diameters of about 20 nm guarantee a very short diffusion distance for the lithium ion.All of these parameters are indispensable for the stable capacity and high rate performance.
In order to investigate the porosity of the FD-BNC and FD-BNC-900, N 2 sorption isotherms have been recorded (Fig. 3).The FD-BNC has a total pore volume of 0.23 cm 3 g À1 and a BET surface area of 109 m 2 g À1 , characteristic of materials without any apparent pores.Aer pyrolysis at 900 C in N 2 , the shape of the adsorption isotherm is maintained suggesting the maintenance of the structure and the presence of micropores.The BET surface area and the pore volume of FD-BNC-900 increased to 670 m 2 g À1 and 0.83 cm 3 g À1 , respectively.The enhancement of surface area and pore volume can be ascribed to removal of water from the cellulose structure and the severe contraction of the bres from about 50 nm to about 20 nm during pyrolysis.Compared with traditional carbon nanobres with high surface area normally obtained by chemical or physical activation with tedious work, our procedure is much easier and greener.
Fig. 4a and b show the XRD proles of FD-BNC and its carbon residue aer pyrolysis.The XRD pattern of FD-BNC aerogels were identied as cellulose I a (ref.16) and the characteristic peaks observed at 14.2 , 16.7 , and 22.6 were indexed as the (010), (001), and (011) reections of the triclinic unit cell, respectively. 36The characteristic sharp peaks indicate that the bacterial cellulose is highly crystalline, in agreement with other reports. 30,37The pyrolysis of BNC resulted in a graphitic-like structure with broad reection peaks at ca. 24 and 43 , which can be assigned to the (002) and (100) lattice planes of graphite,  respectively; in agreement with earlier reports. 38Interestingly, the relatively low pyrolysis temperature resulted in a disordered (and perhaps turbostratic) graphitic structure, as displayed in Fig. 4c.However, it has also been suggested that such relatively disordered structures may result in a decreased capacity, 4,39 these misaligned regions may be the origin of the increased surface area and higher rate performance. 4he overall yield, considering the carbon content of nanocellulose aer carbonization, is about 25%, which was improved when the BNC was soaked with iron(III) ion as dehydrating agent (FD-BNC-Fe1200).The elemental analysis of the samples is provided in Table 1.The analysis indicates that sample FD-BNC-900 has a nal carbon content is 90.96 wt% and a small amount of other elements, where most likely oxygen is still present in the bres probably as aldehyde groups, carbonates or carboxylates, in agreement with recent results. 30he sample soaked with iron shows a much lower hydrogen content, reecting a more efficient pyrolysis.
Fig. 5a shows the carbon bres decorated with 10-30 nm iron-based nanoparticles with a composition close to that of cementite (Fe 3 C).The bres retained the nanoscale dimensions and the BET surface area increased from 33 m 2 g À1 to 351 m 2 g À1 aer the pyrolysis process (Fig. 5b), which is expected for the pyrolysed bres decorated with ca.40 wt% nanoparticles.In comparison to the FD-BNC-900, the FD-BNC-Fe1200 has a slower performance but it has a remarkable stability.The rate performance of the iron doped carbon aerogel is stable and shows a recovery to 220 mA h g À1 aer 45 cycles at different current rates.Fig. 6 shows the Raman spectra for both FD-BNC-900 and FD-BNC-Fe1200.Albeit broad, both spectra display the D and G bands, characteristic for 6-membered aromatic rings, 40 at ca. 1340 and 1580 cm À1 , respectively.The presence of the bands and their relative ratio, I D /I G , indicates a graphitic correlation length of ca. 1 nm, 40 in agreement with the HRTEM images.In the case of FD-BNC-Fe1200, the bands are sharper and also the 2D overtone band (at ca.2680 cm À1 ) is clearly visible, indicating a somewhat higher degree of structural order.
A drawback observed in the electrochemical performance of the tested materials is the large irreversible capacity.This can be due to three different processes: the decomposition of electrolyte, formation of solid electrolyte interphase (SEI), as well as reaction of lithium with the hydrogen present in the sample (see Table 1).The common carbonate-based electrolyte solution (e.g.EC-DEC, EC-DMC) can easily have a reduction reaction below 1.0 V (versus Li + ) and an oxidation reaction above the voltage of 4.5 V (versus Li + ) to form an electronically insulating but lithium ion conductive lm on the electrode active material. 2he H/C molar ratio of the pyrolyzed aerogel is relatively low, i.e., H/C ¼ 0.215, but seems to inuence the electrochemical performance, being the effect particularly noticeable in the rst cycle, where the capacity is well beyond the theoretical limit of graphite.The capacity obtained in this rst cycle is indeed  matching that expected for one lithium atom being bound by one hydrogen atom, i.e., ca.800 mA h g À1 . 4However, the capacity and the shape of charge-discharge curves are quite similar aer the rst cycle, suggesting that the formation of the SEI stabilizes aer the initial cycle.Herein, we have to emphasize that if we expect to build a lithium ion cell, understanding and controlling the phenomena occurring during the initial cycles is always primordial independent of the electrode material with low operation voltage as anode or with high operation voltage as cathode.

Conclusions
In this study, the fabrication of a carbon aerogel composed of nanobres of about 20 nm in diameter was accomplished by pyrolysis of the readily available bacterial nanocellulose (BNC).The 3D open network of the BNC aerogel was preserved from capillary-induced shrinkage and collapse through a freeze-drying method before calcination.The carbon nanobres showed high surface area and pore volume which favors mass transportation and leads to a very good electrochemical performance in terms of both capacity retention as well as rate performance.The present work can possibly broaden the possibility of an economical and ecofriendly use of cellulose to produce 3D open networks of carbon nanobres not only for energy storage applications but also for other applications such as adsorption, (electro)catalysis, chromatography or even medical applications.We can already foresee that such an interesting and disordered carbon morphology will have a good performance in Na-ion batteries systems which is a new emerging technology giving the limited availability of lithium and the abundance and low cost of sodium around the globe.

Pyrolysis of BNC
Coconut gel cubes (ca. 1 Â 1 Â 1 cm 3 , Chaokoh, Thailand) were washed three times with 2 dm 3 of deionized water and stirred in 2 dm 3 of a 0.1 M sodium hydroxide solution for 48 hours to remove any adsorbed components.The materials were further washed with deionized water until the pH stabilized at around 7. The BNC cubes were frozen in liquid nitrogen (À196 C) and freeze-dried in a vacuum chamber at a temperature of À80 C. The freeze-dried BNC aerogels (labeled as FD-BNC) were then pyrolyzed under N 2 at 900 C for 2 hours using a heating rate of 30 C min À1 to obtain monolithic black carbon (denoted as FD-BNC-900).Alternatively, the freeze-dried BNC cubes (FD-BNC) were soaked with a 0.005 M FeCl 3 solution and subsequently freeze-dried (FD-BNC-Fe).The pyrolysis was carried out under argon atmosphere at 1200 C for 2 hours and a heating rate of 1 C min À1 (denoted as FD-BNC-Fe1200).

Characterisation
The size and morphology of all the samples were visualized using electron microscopy facilities (Gemini Leo-1550 SEM or Jeol JEM-7401 SEM and Omega 912 TEM).Nitrogen adsorption/ desorption isotherms were measured at 77 K using a Quantachrome Quadrasorb Adsorption Instrument or a Micromeritics ASAP 2020 where the BET method was used for surface area determination.Elemental composition was determined using a Vario E1 elemental analyzer.XRD patterns were recorded with a Bruker-D8 apparatus with Cu radiation.Each pattern was recorded with a step size of 0.03 .TGA was performed on a Perkin-Elmer Thermogravimetric Analyzer TGA7.The Raman spectra were recorded with a Horiba LabRAM HR 800 spectroscope using a 532 nm laser.

Electrochemical test
The electrode was prepared by mixing powder (80 wt%), carbon black (10 wt%), and polyvinylidene uoride (PVDF, 10 wt%) in N-methylpyrrolidone (NMP) to form a homogenous slurry.The slurry was then spread onto a copper foil and dried at 100 C for overnight in a vacuum oven.Foundation for the electron microscopy facilities, and Eva Björkman (Stockholm University) for fruitful discussions.

Fig. 1
Fig. 1 Morphology of bacterial nanocellulose and carbon aerogels.(a) Photograph of a freeze-dried bacterial nanocellulose aerogel (FD-BNC), (b and c) SEM images of FD-BNC aerogels at different magnifications, (d) photograph of a carbon aerogel obtained through pyrolysis of freeze-dried bacterial nanocellulose at 900 C (FD-BNC-900), (e and f) SEM micrograph of FD-BNC-900 aerogels at different magnifications.

Fig. 2
Fig. 2 Electrochemical performance of FD-BNC-900.(a) Chargedischarge curves at a current rate of 75 mA g À1 (0.2 C).(b) Capacity retention and coulombic efficiency as a function of cycle number and its (c) rate performance.

1 M
LiPF 6 in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC) (1 : 1 by volume) was used as the electrolyte.The assembly of the test cells was carried out in an argon-lled glove box.The batteries were charged and discharged at different rates from 0.2 C to 10 C between 0.0 and 3.0 V on a LAND CT2001A cell test apparatus (1 C corresponding to 372 mA g À1 of current rate).

Fig. 6
Fig. 6 Raman spectra of (bottom trace) FD-BNC-900 and (top trace) FD-BNC-Fe1200.The different bands are labelled and the I D /I G ratio is indicated.