Waste Biomass as in Situ Carbon Source for Sodium Vanadium

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Waste biomass as in-situ carbon source for sodium vanadium fluorophosphate/C cathodes for Na-ion batteries Verónica Palomares, Maitane Blas, Paula Serras, Amaia Iturrondobeitia, Alazne Peña, Alexander López Urionabarrenechea, Luis Lezama, and Teofilo Rojo ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03458 • Publication Date (Web): 22 Oct 2018 Downloaded from http://pubs.acs.org on October 28, 2018

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ACS Sustainable Chemistry & Engineering

Waste biomass as in-situ carbon source for sodium vanadium fluorophosphate/C cathodes for Na-ion batteries Verónica Palomares, [a,d] Maitane Blas, [a] Paula Serras, [b] Amaia Iturrondobeitia, [a] Alazne Peña, [a,d] Alexander Lopez-Urionabarrenechea, [c] Luis Lezama, [a,d] and Teófilo Rojo*[a,e]

[a] Departamento de Química Inorgánica, Universidad del País Vasco UPV/EHU, P.O. Box 644 48080, Bilbao, Spain. [b] Departamento de Ingeniería Nuclear y Mecánica de Fluidos, Universidad del País Vasco UPV/EHU, Plaza Ingeniero Torres Quevedo 1, 48013, Bilbao, Spain. [c] Departamento de Ingeniería Química y del Medio Ambiente, Universidad del País Vasco UPV/EHU, Plaza Ingeniero Torres Quevedo 1, 48013, Bilbao, Spain.

ACS Paragon Plus Environment

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[d] BCMaterials, Bld. Martina Casiano, 3rd. Floor, UPV/EHU Science Park, Bº Sarriena s/n, 48940 Leioa, Spain. [e] CIC ENERGIGUNE, Parque Tecnológico de Álava, Albert Einstein 48, ED. CIC, 01510, Miñano, Spain.

*Corresponding author: [email protected]; [email protected]

Abstract

This work presents a series of electrode materials constituted by a sodium vanadium fluorophosphate as electroactive phase and an in situ generated carbon derived from waste biomasses as carbon source. The waste biomasses consist on grounded vineshoots, eucalyptus wood, and sucrose as control sample. The materials were obtained by hydrothermal carbonization during the hydrothermal synthesis followed by a Flash Thermal Treatment (FTT). This way, carbonaceous matter decomposes into hydrochar


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while the electroactive phase is formed under autogenous pressure at 170 or 200º C. The electrochemical tests were performed by adjusting the electrode composition to a total carbon amount of a 20 wt. %, so that the suitability of the in situ carbon as conductive additive was evaluated. Electrochemical performance was significantly improved after an FTT treatment. Among the biomass-based materials, samples that had the highest amount of the in situ generated carbon, resulting from eucalyptus biomass, showed the best electrochemical performance. Thus, the quality of the in situ generated carbon derived from eucalyptus wood waste is good enough to partially replace the commercial conductive carbon. Therefore, using a residue as additive in the synthesis procedure allows fabricating environmentally friendly and well-performing cathodes for Na-ion batteries.

KEYWORDS: Na-ion batteries, cathodes, hydrothermal carbonization, biomass waste, valorization.


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Introduction

Battery research in the last 10-15 years has tried to contribute to solve some of the problems derived from the generalized use of fossil fuels (greenhouse effect, acid rain, smog, etc.) and also the arising issue of fossil fuel depletion. In this sense, batteries can provide solutions by allowing massive penetration of renewable energy sources in the electrical grid, so that the fossil fuel contribution to the electrical mix is deeply decreased. This can be done by setting up an energy buffer that modulates the energy received from renewable sources according to the needs of the society along the day. In this regard, sodium-ion batteries are an adequate technology that can offer great storage capacity with a moderate cost, due to the lower cost of sodium. The development of this technology implies finding cathode materials with a good electrochemical performance (high specific capacity at different cycling rates, long cycle life and high potential values) that can be prepared by low-cost and environmentally friendly synthesis methods.


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In the competition to produce the definite cathode material for commercial Na-ion batteries, it is obvious that active material properties are of maximum importance. This way, sodium vanadium fluorophosphates fulfill these criteria as they have high gravimetric and volumetric energy densities as well as elevated operating voltage values.

[1-6]

However, there are also other parameters that have great influence on the active material performance. In this regard, it has been demonstrated for Li-ion and Na-ion technology, that the existence of a conductive coating around the active material particles can make a difference in terms of electrochemical performance, specially when cycling at high rates. [7-8]

For this reason, several kinds of conductive coatings have been tested in different

cathode materials both, on Li-ion and Na-ion based systems.

[9-15]

These coatings may

have a carbonaceous nature, which could be derived from sucrose, citric acid, graphene, carbon nanotubes and electrochemical grade carbon

[7,16-25];

or a metallic nature, for

example when Ru is used. [26] All these coatings imply the use of different procedures and costs, depending on the starting materials and the processing involved. In this sense, the carbon coating can be grown in situ during the synthesis process by introducing a carbon source in the reaction,


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or ex situ by doing a post-synthetic treatment. The latter usually involves the suspension of the active material and the carbon source, followed by the spray pyrolysis or thermolysis of the mixture at high temperature (ca. 700ºC), mostly under inert gas flow, in order to transform the carbonaceous precursor into carbon. These multi-step routes are costly, energy consuming and may be difficult to upscale.

[27]

Moreover, electrode

preparation also involves the use of commercial carbon additives that contribute to higher costs in electrode fabrication. For this reason, this work proposes the use of a new method to use waste biomass as carbonaceous precursor in the preparation of sodium-vanadium fluorophosphates with the aim of creating an in situ carbon coating around the cathode material by hydrothermal carbonization. This in situ carbon could replace partially the commercial carbon additive used in electrode preparation. The biomasses that have been chosen are vine-shoots and eucalyptus wood, as representative of the Spanish agroforestry sector. Vine-shoot is the waste generated in the pruning of vine. Vine cultivation is the third most important agricultural crop in Spain in terms of hectares of cultivation, ranging almost 106 ha. such waste is estimated to be around 2 tons per hectare

[28]

The rate of production of

[29],

which means that almost


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2·106 tons of waste would be generated annually in Spain. Nowadays vine-shoots do not have any use and are usually left in the field or burnt in the vineyards. The eucalyptus sample is waste wood coming from thinning, which is a crop management operation consisting on the selective removal of low value trees from plantations, in order to stimulate tree growth by increasing tree-growing space and reduce fire danger. The

Eucalyptus genus covers approximately 14000 km2 of southern Europe, 5·105 ha in Spain, because it is a high-productivity crop extensively used in the paper industry. [30,31] In this case, the waste generation ratio of this activity is estimated around 0,7 tons per hectare. [32] Hydrothermal carbonization (HTC) is a common route used to prepare functional carbonaceous materials from biomass. In this process the biomass reacts inside a sealed container under a controlled temperature between 100 and 300º C by using water as solvent.

[33,34]

This synthesis process presents the advantages of high carbon efficiency

under mild conditions (≤ 300º C), and the use of water as solvent, which makes it environmentally friendly. During HTC, biomass is heated in the presence of subcritical water and autogenous pressure in the range of 2-10 MPa.

[35,36]

The resulting solid


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material is called hydrochar. This solid sometimes needs a post-treatment consisting on a heat treatment under inert atmosphere in order to favor carbon ordering as well as to control carbon surface functionality.

[37-39]

The starting biomass can be crude plant

materials coming from agricultural residues, wood and herbaceous energy crops or carbohydrates such as sugars, starch, cellulose [43]

[42],

[33,40,41]

hemicelluloses, or faecal sludge.

The great variety of starting biomass, the wide range of reaction temperatures and the

subsequent treatments, give rise to a huge extent of functional carbonaceous materials such as 3-dimensionally functionalized graphene carbon structures.

[45]

[44]

or more sophisticated inorganic-

These materials can be used in very different applications, which

include carbon fixation, chromatography, pollutant absorption in water

[41],

catalysts

supports and drug delivery. Additionally, it can also be applied in energy storage, for example, to produce electrodes for supercapacitors [46-48], or porous carbon as conductive additive for Li-S batteries

[49].

HTC process has also been used to prepare anode

materials for Li-ion batteries [38,50-54] but, up to our knowledge, it has not yet been applied to produce cathodic materials for application in batteries.


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As one of the main synthetic methods to produce sodium vanadium fluorophosphates is hydrothermal treatment, the purpose of this work is to use waste biomass during the synthesis of the electroactive material in order to both prepare the inorganic phase and to transform the waste organic biomass into carbon in situ. This way, the subsequent post-synthesis treatments that are usually needed to perform carbonaceous coatings would be avoided or minimized together with the time and economic savings that these processes involve by using a one-step process and a non-cost reactant. Additional savings in cathode preparation would be associated to the possibility of replacing some of the commercial carbon additive by in situ generated one. Experimental Section

Sample Preparation The materials with general formula Na3V2O2x(PO4)2F3-2x, where 0 < x < 1, have been synthesized by one step method under mild hydrothermal conditions (170/200°C and autogenous pressure), by mixing in an agate mortar VO2 (Sigma-Aldrich, 98% purity), NaH2PO4·H2O (Sigma-Aldrich, 98-102% purity) and NaF (Sigma-Aldrich, 99% purity) in a 1:2:1 molar proportion. To this mixture different amounts of biomass were added so as to


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get a final carbon amount between 5 and 10 wt.% in the samples before the thermal treatment. The initial amount of biomass or sucrose was determined in preliminary essays. The reaction mixture was sealed in a polytetrafuoroethylene (PTFE)-lined steel pressure vessel, which was maintained at 170 or 200 °C for 65 hours and thus, two different set of samples were obtained (total of six samples). The products were washed repeatedly with distilled water and acetone. Half of each sample was subjected to a Flash Thermal Treatment (FTT) process, maintaining the materials at 700 °C for 10 minutes under nitrogen inert atmosphere and then quenching them to room temperature in air. This way, the whole procedure led to a series of 12 powder samples (synthesized at 170 or 200º C, fresh and after an FTT) that were further characterized.

Characterization of the Biomass Samples The vine-shoots were received as they were generated in the field, so they were cleaned in order to eliminate the soil, and then milled to obtain a particle size smaller than 0.25 mm. Eucalyptus wood sample consisted of milled trunk wood, very homogenous and


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pure (no soil, leaves or stones), so it was only homogenized and ground to obtain a particle size smaller than 0.5 mm. Both biomass samples were provided by Befesa. Sucrose (Fluka, ≥99% purity), that is usually used to generate carbon coatings in batteries cathode materials was also used to produce a control sample with the aim of testing the hydrothermal carbonization method. The two biomass and the sucrose samples were characterized by proximate and elemental analysis. Proximate analysis was carried out in a LECO TGA-701 thermobalance, following the ASTM standards for the determination of moisture, volatile matter and ash in particulate wood fuels (ASTM E871, ASTM E872 and ASTM E1534 respectively). Ultimate analysis was carried out in an Eurovector 3000 equipment. The results are shown in supplementary information Table S1.

Characterization of the Synthesized Powder Samples Powder diffraction patterns were collected in a Philips X´Pert MPD diffractometer (using and angular intermission from 5 to 70 ° and a 0.026° pass) Bruker D8 Advance Vårio diffractometer working with Cu K- radiation at room temperature. Obtained X-ray diffractograms were fitted with the patternmatching option of Winplotr.

[55]

A Bruker


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ELEXSYS 500 spectrometer equipped with a super-high-Q resonator ER-4123-SHQ, operating at X band, was used to record the ESR polycrystalline spectra. The magnetic field was calibrated by a NMR probe and the frequency inside the cavity was determined with an integrated MW-frequency counter. Raman measurements were done by using a Renishaw InVia Raman spectrometer, joined to a Leica DMLM microscope at an excitation wavelength of 514 nm (ion-argon laser, Modu-Laser). For each spectrum 30 seconds were employed and 5 scans were accumulated with the 10% of the maximum power of the 514 nm laser in the spectral window from 1000 to 2000 cm-1. The morphological characterization was made by Scanning and Transmission Electron Microscopy (SEM and TEM). SEM images were taken in a JEOL JSM-7000F microscope with an applied acceleration tension of 10 kV and TEM micrographs were obtained in a Philips CM200 microscope. For SEM microscopy samples needed to be covered by a thin layer of chrome to obtain electronic conductive materials.

Nitrogen adsorption isotherms (77 K) were measured with a Quantachrome Autosorb-iQMP. Prior to the measurement, all samples were outgassed under vacuum at 423 K for 6


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h. The specific surface area was calculated from the adsorption branch in the relative pressure interval using the Brunauer–Emmett–Teller (BET) method.[56]

Electrochemical evaluation of the samples was made by galvanostatic cycling in a MTI Corp Battery cycler and a Biologic VMP3 Multichannel potentiostat-galvanostat. Swagelok cells were assembled vs. a sodium metal foil and electrolyte consisted on 1M solution of NaPF6 (Sigma-Aldrich, 98%) in (EC/DMC) diethyl carbonate and dimethyl carbonate in a 1:1 volume ratio and 2 wt% FEC (Alfa Aesar, 98%) fluoroethylene carbonate, impregnated on a porous fiber separator. The positive electrodes (slurry mixture) were manufactured with active material:carbon:binder in 70:20:10 proportions. The commercial conductive carbon (Super C65, Timcal) amount added to each of the active materials was adjusted to get a 20 wt% of total carbon in the electrode. This way, the conductive carbon in the cathodes comes from the combination of intrinsic carbon produced by in situ biomass decomposition during the synthesis process and the electrochemical grade carbon. A 5 wt% solution of polyvinylidenefluoride binder (PVDF 5230, Solvay) in N-methyl pyrrolidone, (NMP, Sigma-Aldrich, 99.5%) was used as binder.


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The use of 1 mL of anhydrous N-methylpyrrolidone (NMP, Sigma-Aldrich, 99.5%) was necessary in order to get a slurry with the adequate viscosity to prepare the laminate. The slurry was coated onto an aluminum foil using a doctor blade K Control Coater 101, Elcometer 4340 automatic Film Applicator. The electrode film was dried at 80 °C in a vacuum oven for 24 h. After cutting, the electrodes were pressed to 10 Ton and dried in a Büchi Glass Oven B-585 at 120 °C for 24 hours. Then, they were transferred to an Arfilled glovebox (MBraun Labstar) under Argon atmosphere and levels of water and oxygen under 0.1 ppm conditions. All the electrodes used in this study present a loading between 2.631 and 3.168 mg·cm-2. Three kinds of galvanostatic cyclings were made. First, a slow rate capability, that comprised repeated cycling at C/20, C/10, C/5, C/2 and 1C. Second, a high rate cycling at 1C, 2C, 5C, 10C, 15C and back to 1C. Third, cycle life comprising 2 cycles at C/10 and 1000 cycles at 1C. Electrochemical impedance spectroscopy measurements were made on Swagelok half-cell, in a Biologic VMP3 Multichannel potentiostat-galvanostat, from 1 MHz to 10 mHz. Results and Discussion


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A collection of twelve samples was prepared. Six of them were obtained directly from the hydrothermal process carried out at 170ºC or 200ºC (fresh samples). The other six were obtained by applying the FTT to the fresh materials (FTT samples). Accordingly, the samples were named with the following code: biomass source_synthesis temperature (where VS= vine-shoot, EU= eucalyptus and SUC= sucrose). In addition, the samples obtained via FTT are distinguished by having this same code at the beginning. Table 1. Amount of biomass used to prepare each sample, and the resulting carbon content, before and after FTT.

Fresh

Amount of Biomass (g) % wt. C

FTT samples

% wt. C

VS_170

1.98250

13.64

FTT_VS_170

5.35

VS_200

1.98200

5.65

FTT_VS_200

3.73

EU_170

3.3660

12.30

FTT_EU_170

9.09

EU_200

3.36716

11.17

FTT_EU_200

8.4

SUC_170

2.03286

10.65

FTT_SUC_170

2.36

SUC_200

2.03235

7.14

FTT_SUC_200

2.17

Samples


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The different amounts of the used biomass were based on preliminary essays with lower biomass contents and were adjusted in order to try to get a carbon content of 5-10 wt. % in each sample. As it can be seen, in all the hydrothermal samples the obtained carbon percentage was higher than the expected one, except for VS_200 and SUC_200. All the materials showed lower carbon content with the increasing of the synthesis temperature, which is the consequence of the greater decomposition of volatiles happening at higher temperatures.

[57]

In the case of vine-shoots samples, the carbon

content drops sharply from 13 to 5.6 %, whereas eucalyptus based materials only show a slight loss in carbon content. Sucrose-based materials show an intermediate carbon percentage loss when increasing the synthesis temperature from 170 to 200º C. These results show the different (hydro)thermal behavior for the three samples; in the case of vine-shoots, the rate of volatiles decomposition rises considerably between 170 and 200 ºC, that is, mild decomposition takes place at 170 ºC, while severe decomposition can be observed at 200 ºC. However, eucalyptus and sucrose based materials do not show such a big difference in that temperature range.


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FTT causes a steep carbon loss in sucrose whereas it causes a milder carbon loss in eucalyptus. Such a difference could be attributed to the fact that eucalyptus decomposes in a great extent under hydrothermal treatment, which explains the mild carbon loss in the FTT, while sucrose slightly decomposes under hydrothermal conditions, suffering a severe decomposition in the thermal treatment at 700 ºC. This carbon loss can be due to the rapid decomposition of the carbonaceous mater at high temperature (700 ºC). [7] The same argument could explain the carbon content of vine-shoots derived samples; hydrothermal treatment at 170 ºC slightly decomposes vine-shoots and therefore the main decomposition takes place when FTT is applied. On the contrary, hydrotreatment at 200 ºC produces a great decomposition of vine-shoots, which is somewhat increased under FTT conditions. In every sample, the majority of the volatile matter is decomposed after the thermal treatment at 700 ºC for 10 minutes, so that the remaining carbon would be mainly constituted by the fixed one coming from the biomass. Accordingly, eucalyptus, which possess the higher fixed carbon content in proximate analysis (19.4 wt.%), generates a greater amount of carbon than vine-shoots (fixed carbon 17.9 wt.%) and sucrose (fixed carbon 16.2 wt.%) (see proximate analysis in table S1).


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Figure 1 shows the diffraction patterns of the fresh materials obtained just after the hydrothermal process at 170 and 200º C (Fig. 1a) and the thermally treated ones (FTT materials, Fig. 1b). All the samples present the same main diffraction peaks characteristics of the family of Na3V2O2x(PO4)2F3-2x compounds.

[24]

A little amount of

unreacted VO2 appears as an impurity in all the hydrothermally prepared materials, but, as it can be seen in Figure 1b, the intensity of this peak decreases steeply after the FTT. Some of the fresh prepared materials show a second remarkable feature, the elevation of the diffractogram background around a 2  22º, typical for amorphous carbonaceous materials. This elevation is present in VS_170 (13.64 wt. % C) and EU_170 samples (12.30 wt. % C), that present the highest carbon content among all the prepared materials (see Table 1). However, the relative intensity of the background elevation is greater for EU_170 than for VS_170 material. Additionally, it is also remarkable that, despite the slight difference in carbon content between EU_170 (12.30 wt. % C) and SUC_170 (10.65 wt. % C), there is no sign of the background elevation in the sucrose-based sample. These facts indicate that there exist differences among the carbon that is generated


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depending on the used precursor. For FTT materials, no background elevation is appreciated due to the lower carbon proportion in the samples.

Figure 1. Comparison of fresh prepared samples (a) and thermally treated ones (b). Traces of amorphous carbon (grey zone) and remnants of VO2 starting material do not appear after FTT.

The diffractograms corresponding to fresh and FTT samples were fitted to Na3V2O2(PO4)2F (x=1) compound as main phase. The obtained unit cell parameters (Table S2, Figure S1) are in good agreement with the ones found in the literature for this phase.

[58]

Figure 2 shows the patternmatching fitting of the diffractograms of SUC_200

and FTT_SUC_200 materials as representative of all the samples. The comparison of the


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cell parameters for the fresh and FTT materials indicates that, in all the cases, there is a decrease in cell size after the thermal treatment.

Figure 2. Patternmatching of the fresh SUC_200 (a) and FTT_SUC_200 (b) samples, as representative of all the obtained materials.

In order to see the nature of the vanadium oxidation state in the samples, ESR measurements were carried out for the fresh materials. Figure 3 displays the ESR spectra of SC_170, VS_170 and EU_170 fresh samples. In the three cases a narrow signal is observed at 3350 Gauss with a g value of 2.003, that corresponds to the free electron. Thus, this narrow signal is caused by the free radicals of the carbon derived from sucrose, vine-shoots or eucalyptus wood. Additionally, all the samples present a signal centered at 3420 Gauss, which can be attributed to the vanadium in the fluorophosphate materials.


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This signal, related to the oxidation state of the vanadium, corresponds to a quasiisotropic signal instead of to an axial signal typical of vanadyl (VO2+) ion. The g-value is around 1.9612 and the ∆Hpp is 115 G for EU_170, to 169 G for SC_170 and to 210 G for VS_170 sample. The broadening of the signal, that slightly increases from EU to VS sample indicates in all cases the presence of a low amount of V3+ in the compound.

[24]

This fact indicates that V3+/V4+ mixed-valence phases with little variation in composition have been synthesized.

Figure 3. ESR spectra of sucrose, vine-shoots and eucaliptus fresh samples synthesized at 170º C.

The carbon present in all samples was analysed by Raman spectroscopy (Figure 4). As it can be seen, all the materials present the D and G bands, typical of amorphous


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carbon, at about 1360 and 1590 cm-1, respectively. The relative intensity of these bands (G/D ratio), which is shown in the legend of Figure 4, is usually taken as an indicator parameter of the degree of ordering of the carbonaceous structure. As it can be seen, the relative intensity of these bands varies with the synthesis temperature and the use of the FTT. This way, the G/D ratio increases with the synthesis temperature as well as with the application of the FTT. All the materials show this trend except the FTT_EU_200 sample, which has the lowest G/D ratio of all EU series, with a G/D value very close to that one of the VS_170 sample. From the G/D ratios, it can be deduced that the samples derived from eucalyptus have the highest degree of ordering (except FTT_EU_200) followed by those coming from sucrose and vine-shoots.


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Figure 4. Raman spectra of the samples produced from a) Vine-shoots; b) Eucaliptus wood and c) Sucrose (samples excited with 514 nm laser). G/D ratio is shown for each sample in the legend.

The morphology of the materials was observed by scanning electron microscopy (SEM). Thus, in the following paragraphs, the morphologic study of the synthesized material will be described as followed: VS derived materials, EU derived ones and SUC derived ones.


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Figure 5. SEM images of VS derived samples (Scalebar: 1m): a) VS_170, b) VS_200, c) FTT_VS_170, and d) FTT_VS_200.

Figure 5a and b show the micrographs of the VS_170 and VS_200 fresh materials before any thermal treatment. As it can be seen, vine-shoots have not completely been decomposed at 170º C under autogenous pressure in water, so that fluorophosphate crystals have grown on the surface of biomass remains. In the case of VS_200, Vineshoots biomass appears to have decomposed into porous sheets that are totally covered


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in fluorophosphate crystals. In both VS materials, fluorophosphate particles present two different morphologies: long prisms that have a length of 1 m and a width of 0.2 m, and cubes of ca. 1 m size. Moreover, desert-rose structures made of plates also appear in VS_170 sample, which could be related to a possible crystal growth mechanism for the fluorophosphate: initially plates are formed and afterwards are stacked so as to construct both, cubes or prisms. When the FTT is applied to VS_170 sample (Figure 5c), the material presents fluorophosphate prisms and cubes, as before the thermal treatment. Crystals seem to have agglomerated and widened but they have not grown in length. No desert-rose structures are present after the FTT, but a minor amount of nanometer sized crystals is observed. In the case of FTT_VS_200 (Figure 5d), two kinds of fluorophosphate crystals are present: 0.4-0.5 m average size prisms and nanometer-sized particles with no definite shape. The amount of nanosized fluorophosphate particles in FTT_VS_200 is the highest among the four VS-based materials. Regarding biomass, in both FTT_VS_170 and FTT_VS_200 materials, it has decomposed into carbonaceous matter and is present in the form of sponge-like carbonaceous aggregates of an average size of 2-4 m, which


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is smaller than those the biomass remains that have been observed for the fresh VS materials. It must be addressed that these carbonaceous aggregates present low densities as the fluorophosphate crystals that are underneath can be clearly seen through them. Transmission Electron microscopy confirms the microstructure observed by SEM, with carbon aggregates attached to the inorganic fluorophosphate crystals but not surrounding them (Figure S3a).


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Figure 6. SEM images of EU derived samples (Scalebar: 1m): a) EU_170, b) EU_200, c) FTT_EU_170, and d) FTT_EU_200.

In the case of EU_170 and EU_200 materials (Figure 6a and b), it can be seen that 170º C were not enough to totally decompose the eucalyptus biomass, so that wood remains are present in the sample and fluorophosphate crystals have grown on top of them. These crystals present two kinds of morphologies: elongated prisms and agglomerates made of tiny particles smaller than 1 m. The use of the FTT in EU materials (Figure 6c and d) causes the decomposition of the eucalyptus remains so that 1-2 m size aggregates that do not have a definite shape are formed. These big carbonaceous aggregates are surrounded by fluorophosphates crystals. Moreover, some tiny particles can be observed covering the fluorophosphates.


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These particles could come from the breakdown of the aggregates observed before. Both FTT_EU materials show these two kind of crystals, but a higher amount of tiny particles is observed for FTT_EU_200 sample. TEM images from FTT_EU_170 also show these tiny particles embedded in carbonaceous matter (Figure S3b). In general, it can be said that both, Vine-shoots and Eucaliptus wood need hydrothermal carbonization temperatures higher than 170º C to produce hydrochar and that this hydrochar decomposes to carbon during the FTT at 700º C in inert atmosphere. In spite of the initial objective of producing carbon coated sodium vanadium fluorophosphates, in all the cases the obtained materials comprise carbonaceous aggregates surrounded by fluorophosphates particles. In any case, there exists a close contact between the carbonaceous matter and the electroactive material. In terms of particle size, the FTT induces the breakdown of crystal aggregates so higher amounts of nanosized fluorophosphate particles are observed whereas bigger prisms do not change their size. The micrographs corresponding to the samples synthesized from sucrose, SUC_170 and SUC_200, are shown in Figure 7a and b. Two types of crystals can be distinguished in both samples: i) cubic or prismatic shaped large particles that have a width smaller


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than 1 μm and a length of 1- 2 μm , ii) smaller particles of less than 1 μm, that have an undefined shape and are homogeneously distributed all along the materials. In spite of the different synthesis temperatures that have been employed, the inorganic fraction of the material is similar for both materials. However, the physical appearance of the carbon derived from sucrose is changed with the synthesis temperature. This way, SUC_170 (10.65 wt.%) shows porous solid aggregates attached to the surface of the fluorophosphate crystals (Figure 7a), whereas SUC_200 (7.14%) sample presents spheres of ca. 1 m diameter that are not directly attached to the surface of the particles (figure 7b).


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Figure 7. SEM images of SUC derived samples (Scalebar: 1m): a) SUC_170, b) SUC_200, c) FTT_SUC_170, and d) FTT_SUC_200

When the FTT is applied on both sucrose based materials (Figure 7c and d), the inorganic particle size and the morphology are preserved, homogenizing the distribution of the particle size. To be more specific, for both samples can be appreciated a slight decrease in the size of the biggest particles compared to the non thermally treated ones, obtaining particles that have a maximum size of 1 μm. This size decrease is more remarkable for the FTT_SUC_200 material, and could be ascribed to the breakdown of the biggest crystals with the rapid heat treatment (heating and cooling).


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Figure 8. N2 physisorption isotherms (77 K) of FTT materials prepared at a) 170º C, and b) 200º C. Filled and open symbols stand for adsorption and desorption branches, respectively. The microstructure of FTT_VS and FTT_EU samples was further analyzed by adsorption isotherms of N2 measured at 77 K (Figure 8). As a result of the porosity arising from the interparticle space, all the adsorption curves feature an IUPAC type II/IV isotherm characteristic of macroporous materials with a mesoporous contribution

[59],

as

indicated by the thin but broad hysteretic desorption branch. Specific surface area values obtained fitting the adsorption data to Brunauer-Emmett-Teller (BET) equation are gathered in Table 2, together with hydrothermal synthesis temperature and carbon content. At first sight, the surface area is not directly related to the total carbon content but it is clearly influenced by the hydrothermal treatment conditions, as samples subjected


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to higher reaction temperatures exhibit higher surface area values while retaining a similar carbon content. It deserves to note that the sodium vanadium fluorophosphate phase consists of a mixture of micro- and nanoscopic crystals, and the fraction of the nanoscopic ones increases with the temperature (see Figures 5 and 6), which might explain the observed trend.

Table 2. Specific surface area values (SBET), hydrothermal synthesis temperature (TH) and carbon content.

Sample code

SBET

TH (°C)

C (%)

FTT_VS_170 18

170

5.35

FTT_EU_170 33

200

9.09

FTT_VS_170 16

170

3.73

FTT_EU_200 54

200

8.4

(m2g-1)


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Electrochemical characterization Electrochemical impedance spectra of FTT_VS and FTT_EU cathodes (Figure S2) indicate that EU-based materials present higher resistance values (between 500 and 800 ) compared to FTT_VS samples, which show a resistance value of about 400 . This difference can be related to the amount of in situ carbon in each cathode. FTT_EU_170 and FTT_EU_200 present the highest in situ carbon content (about 9 wt. %), so the amount of conductive additive in these cathodes is lower and overall resistance of the electrode increases. All the prepared materials were first cycled at slow and moderate rates in order to observe their general electrochemical behavior. The electrochemical curves of VS_170 and EU materials (fresh and FTT) at low rate (C/20) are depicted in Figure 9 as representative for all the samples. As it can be seen, the electrochemical signature of all the materials is the typical of the sodium vanadium fluorophosphate family, which consists of 2 pseudoplateaux at ca. 3.6 and 4.1 V vs. Na/Na+, without any significant variation among them apart from the specific capacity values, that are commented in this section.


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Figure 9. Charge and discharge curves at C/20 for the fresh and the FTT synthesized made from vine-shoots and eucalyptus wood at 170º C.

The specific capacity values obtained for the fresh and the FTT samples at slowmoderate rates, ranging from C/20 to C, are shown in Figure 10. For further comparison, sucrose-derived samples are shown in different graphs (Figure 10 c-d). As it can be seen, the specific capacity values steeply decrease with the increasing of the cycling rate in all the fresh samples. Capacity retention is very low for the biomass-based fresh materials, being about 16 and 35% for VS_170 and VS_200, respectively; and ca. 27 and 44% for EU_170 and EU_200. On the other hand, SUC_170 and SUC_200 samples preserve the 63% and the 32% of the initial specific capacity (Csp), respectively.


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A comparison of the rate capability of the fresh materials according to their preparation temperature shows that the samples prepared at 200º C perform better than the ones subjected to hydrothermal carbonization at 170º C (Figure 10a). The enhancement of the specific capacity observed seems to be higher for the VS based materials than for the EU ones. It must be also said that the coulombic efficiency is low during the first 15 cycles, which could be attributed to possible parallel reactions or to the decomposition of the electrolyte. [60-61]


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Figure 10. Rate capability at slow and moderate rates of VS, EU and SUC materials before (a, c) and after a Flash Thermal Treatment (b, d).

On the other hand, SUC_170 presents the highest specific capacity values among all the materials, whereas SUC_200 performance is similar to that observed for EU samples. It must be recalled that the sucrose-based samples have been prepared as control samples in order to test hydrothermal carbonization (combined or not with FTT) as a method to produce well-performing sodium vanadium fluorophosphate/C cathodes by using a known carbon source. The electrochemical behavior of SUC_170 and SUC_200, with a high amount of in situ carbon in electrode composition (about 11 and 7 wt. %, respectively) indicates that hydrothermal carbonization is a valid method to generate in

situ carbon during the synthesis of sodium vanadium fluorophosphates that can partially replace commercial conductive carbon in these cathodes.


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When the FTT is applied (Figure 10b and d) a great improvement is observed for all the materials, both, in terms of Csp values at each rate and specific capacity retention when increasing the cycling rate. This enhancement could be ascribed to the disappearance of the remaining wood in the fresh samples when performing the FTT process as it has been demonstrated by SEM measurements and also to the higher degree of order in carbonaceous matter after FTT that has been observed in Raman spectra. By using the FTT, specific capacity of the fresh VS_200 material evolves from ca. 40 mAh/g to 88 mAh/g at C/2, and from 30 mAh/g to 82 mAh/g at C. In the case of eucalyptus based materials, specific capacity of the fresh EU_200 rises from 49 to 86 mAh/g at C/2 and from 35 to 79 mAh/g at C only by subjecting the material to the FTT. Therefore, in most of the cases, the Csp value is doubled when the FTT is applied. In the fresh biomass samples, the electrodes made of EU present higher Csp values than those made of VS. These differences become slighter after the FTT. The electrochemical performance shown by the FTT_EU_200 sample is remarkable since this sample is the one with the highest content of the in situ generated carbon, which has replaced about half of the electrochemical grade carbon black that is used to fabricate the electrodes of the


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samples. This feature indicates that the quality of the generated carbon is good enough to successfully replace the commercial conductive carbon in a higher degree. The great improvement in the electrochemical performance after the FTT is also observed for the sucrose-based materials, although in this case the enhancement can be due to the low in situ carbon content in these materials, that have been prepared with the highest proportions of conductive commercial carbon. As it can be seen in Figure 10d, the FTT_SUC samples present very similar performance at medium rates (C/2 and 1C), but at low rates (C/20 and C/10) FTT_SUC_200 shows slightly higher Csp. Concerning specific capacity retention at higher rates, both of the FTT samples preserve a high percentage of the Csp values, being of 96 and 85% for FTT_170 and FTT_200 materials, respectively. The low coulombic efficiency (

Waste Biomass as in Situ Carbon Source for Sodium Vanadium

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