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1 In the format provided by the authors and unedited. DOI: /NMAT5029 A step toward safer and recyclable lithium-ion capacitors using sacrificial organic lithium salt Authors: P. Jeżowski 1, O. Crosnier 2,3, E. Deunf 2, P. Poizot 2,4, F. Béguin 1, T. Brousse 2,3* 1. Poznan University of Technology, Institute of Chemistry and Technical Electrochemistry, ul. Berdychowo 4, Poznan, Poland. 2. Institut des Matériaux Jean Rouxel, CNRS UMR 6502 Université de Nantes, 2 rue de la Houssinière BP32229, Nantes Cedex 3, France. 3. Réseau sur le Stockage Électrochimique de l Énergie, FR CNRS 3459, Amiens Cedex, France. 4. Institut Universitaire de France (IUF), 1 rue Descartes, Paris Cedex 05, France * thierry.brousse@univ-nantes.fr NATURE MATERIALS 1

2 DOI: /NMAT5029 S1. Characterisation of 3,4 dihydroxybenzonitrile dilithium salt (Li2DHBN) Fourier Transform Infra-Red spectrometry (FTIR): a Bruker Vertex 70 device was used in the wavenumber range from 4000 cm-1 to 400 cm-1. KBr pellets were prepared to record the KBr spectra. Supplementary figure 1 shows the IR spectra for the KBr pellet of as-prepared Li2DHBN after filtration (the dried Li2DHBN is extremely sensitive to moisture and reacted with the atmosphere during the recording of its IR spectrum), and of 3,4 dihydroxybenzonitrile for comparison purposes. Characteristic lines of tetrahydrofuran (THF) are easily identified in the spectrum of as-prepared Li2DHBN (Supplementary figure 1b) in the range from 2882 to 2979 cm-1 (C H stretching), and at 1041 cm-1 (C O stretching). The proof of lithiation in figure S1 b is evidenced by the disappearance of the characteristic O-H lines of 3,4 dihydroxybenzonitrile in the range from ca to 3500 cm-1 (Supplementary figure 1a). Interestingly, the spectrum of Li2DHBN shows a characteristic band at around 488 cm-1, which, according to Zhu et al. [S1], could be attributed to the Li O vibration. Supplementary figure 1 Infra-red spectra of a) 3,4 dihydroxybenzonitrile; and b) as-prepared Li2DHBN. 2 NATURE MATERIALS 2

3 Differential Scanning Calorimetry (DSC) was performed with a DSC204 Phoenix 204 (Netzsch) apparatus, where the protective gas was Nitrogen (N 2 ) of analytical purity. The material (4 mg) was placed in a closed aluminium crucible, and an empty crucible was used as a reference. The samples were heated/cooled at 10 C min -1 in the temperature range from 25 to 400 C. Dried Li 2 DHBN was introduced into the air-tight aluminium crucible in a glove box under argon atmosphere and analysed by DSC (Supplementary figure 2). The absence of any signal at temperatures lower than 100 C confirms that THF (and water) were thoroughly eliminated during drying. In the temperature range from 150 to 230 C, there is slight deviation of the baseline which is attributed to fast amorphisation, and is characteristic for lithiated organic compounds. Between 285 and 305 C, the Li 2 DHBN is exothermically decomposed with formation of Li 2 CO 3, as already observed for instance with tetrahydroxy-p-benzoquinone tetralithium salt (Li 4 C 6 O 6 ) [S2]. Supplementary figure 2 Differential scanning calorimetry thermogram of dried Li 2 DHBN at a heating rate of 10 C min -1. Nuclear Magnetic Resonance (NMR) spectra were recorded with a Bruker AVANCE 500 MHz spectrometer. Deuterated methanol (CD 3 OD) was used as the solvent and as the internal standard for chemical shifts. In the 13 C NMR spectrum of dried Li 2 DHBN (Supplementary figure 3), the peaks at 165 ppm and 156 ppm are attributed to the C OLi carbons in the 3rd and 4th positions of CN, respectively. The signals at 125 ppm and 117 ppm are related to the C H carbons of the aromatic ring. The shift at 124 ppm arises from the carbon atom of the CN group. The carbon NATURE MATERIALS 3

4 atom of the aromatic ring close to the C N group produces the signal at 95 ppm. The peak at 50 ppm is assigned to the deuterated solvent (CD 3 OD). Supplementary figure 3 13 C Nuclear magnetic resonance spectrum of dried Li 2 DHBN at 500 MHz. Deuterated methanol (CD 3 OD) was used as the solvent. The 1 H nuclear magnetic resonance (NMR) spectrum of dried Li 2 DHBN is presented in supplementary figure 4. The peak at 6.4 ppm, with an integration ratio of 1, characterises the single aromatic proton in the ortho position of CN. The multiplet at around 6.7 ppm, with an intensity ratio of 2, is attributed to the two other aromatic protons. The peak at 3.2 ppm is due to protons of the incompletely deuterated methanol. The peak at 4.8 ppm is characteristic of HDO and is attributed to the partial decomposition of Li 2 DHBN into 3,4 dihydroxybenzonitrile due to moisture present in the atmosphere or the deuterated methanol. NATURE MATERIALS 4

5 Supplementary figure 4 1 H nuclear magnetic resonance spectrum of dried Li 2 DHBN at 500 MHz. Deuterated methanol (CD 3 OD) was used as the solvent. NATURE MATERIALS 5

6 S2. Solubility measurement of Li 2 DHBN and electrochemical/thermal behaviours of both the positive and negative electrodes in an LIC cell Solubility evaluation of Li 2 DHBN in the relevant liquid electrolyte The supplementary figure 5 shows visual evidence of the poor solubility of Li 2 DHBN in EC DMC/LiPF 6 1 mol L -1 under real LIC cell conditions (i.e., 8 mg of Li 2 DHBN for 500 µl of electrolyte). Supplementary figure 5 Optical picture of powder of Li 2 DHBN in contact with EC-DMC/LiPF 6 1 mol L -1. The solubility of Li 2 DHBN in the electrolyte was then quantified by a simple spectrophotometric titration method performed on diluted solutions (aromatic compounds being efficient absorbers of UV radiation). The spectrophotometric data were recorded on a Cary UV-Vis spectrometer from Agilent with a 1 cm-length quartz cell at = 290 nm ( 290 = 6809 L mol -1 cm -1 ). Experimentally, a suspension of 8 mg of Li 2 DHBN with 500 µl of electrolyte (real LIC conditions) was prepared and stirred for a few hours. After decantation and filtration, the surnatant was diluted 100 times with the electrolyte solution prior to spectrophotometric measurements. The calibration curve was performed between 0.01 to 0.08 mmol L -1 (supplementary figure 6) and the resulting intrinsic solubility of Li 2 DHBN was found to be 0.95 g L -1 (= 950 ppm < 1%). NATURE MATERIALS 6

7 Supplementary figure 6 UV-Vis calibration line together with the corresponding value defining the intrinsic solubility of Li 2 DHBN in EC-DMC/LiPF 6 1 mol L -1 (red dot). Electrochemical behaviour A free-standing positive electrode material with PTFE as the binder was prepared inside an argon-filled glove box under the same conditions as described in the paragraph Methods of the article. In order to estimate the oxidation limit of the positive electrode material, the anodic polarisation of the composite electrode was prolonged to higher potentials. As evidenced in supplementary figure 7, after the first irreversible lithium extraction plateau, there is a potential rise followed by another endless irreversible plateau at a potential higher than 4.5 V vs. Li + /Li 0, which might be associated to electrolyte oxidation (Supplementary figure 7b). The exact value of irreversible lithium extraction capacity was thus determined by the position of the inflexion point, at ca. 360 mah g -1 in supplementary figure 7a, thereby demonstrating that all the lithium present in Li 2 DHBN (theoretical value of 365 mah g -1 ) is available for graphite lithiation. NATURE MATERIALS 7

8 Supplementary figure 7 Galvanostatic charge/discharge profiles of 3,4-dihydroxybenzonitrile dilithium salt at C/10. The lithium extraction takes place until ca. 4.0 V vs. Li + /Li 0. The second plateau at ca. 4.5 V vs. Li + /Li 0 is attributed to side reactions such as electrolyte oxidation. The electrode was composed of 65 wt.% Li 2 DHBN, 30 wt.% carbon black and 5 wt.% PTFE binder. The experiments were performed in a 1 mol L -1 LiPF 6 dissolved in EC: DMC (vol. ratio 1:1) electrolyte with metallic lithium as the counter/reference electrodes. Operando spectroelectrochemical measurements of pure Li 2 DHBN were also performed using a miniature fibre-optic spectrophotometer (FLAME-S-XR1-ES, Ocean Optics) in glove box. The sacrificial organic salt was deposited by dip-coating onto an Indium tin oxide (ITO) electrode using DMC as the dispersive solvent (supplementary figure 8 top), and then placed in a 1-cmlong quartz cell filled with EC-DMC/LiPF 6 1 mol L -1 (1.5 ml). The coated ITO electrode was then oxidized to 4.4 V vs. Li + /Li 0 using the Potentiostatic Intermittent Titration Technique (PITT) (supplementary figure 9), and removed from the electrolyte. No deposit is to be seen on the ITO electrode (supplementary figure 8 bottom), thus demonstrating the dissolution of the delithiated form of Li 2 DHBN (i.e., DOBN) in the electrolyte. The presence of dissolved molecules was further assessed by monitoring the UV-visible response (supplementary figure 10), which clearly shows a change in the electrolyte spectrum after first oxidation of the Li 2 DHBN-coated ITO electrode. NATURE MATERIALS 8

9 Supplementary figure 8 Optical pictures of the ITO electrode (0.5 cm wide) covered by Li 2 DHBN in the initial state (top) and after oxidation (bottom). Supplementary figure 9 PITT charge profile of pure Li 2 DHBN deposited onto the ITO electrode, measured during the in-operando spectroelectrochemical experiment in an argon-filled glove box: 1-cm-long quartz cell filled with EC-DMC/LiPF 6 1 mol L -1 (1.5 ml) and recorded versus lithium metal. NATURE MATERIALS 9

10 Supplementary figure 10 Corresponding UV-Vis response of the electrolyte after oxidation of Li 2 DHBN deposited onto the ITO electrode to 4.4 V vs. Li + /Li 0 by Potentiostatic Intermittent Titration Technique. The concentration of DOBN is so great that the measured signal is saturated in the UV region. The same experiment was carried out in a Swagelok cell with a custom-made Teflon ring (11 mm in diameter, 5 mm thick) where the amount of electrolyte was exactly 500 µl, which is the same amount as in the separator of a full cell. After oxidation of the composite electrode (40 wt.% AC, 40 wt.% Li 2 DHBN, 15 wt.% Super C65 and 5wt.% PTFE) at 4.5V vs. Li + /Li 0, the electrolyte was removed from the cavity inside the Swagelok cell. As can be seen in supplementary figure 11, the electrolyte turned a brownish colour. Supplementary figure 11 Optical pictures of the electrolyte after oxidation of a composite electrode (40 wt.% AC, 40 wt.% Li 2 DHBN, 15 wt.% Super C65 and 5wt.% of PTFE) at 4.5V vs Li + /Li 0 NATURE MATERIALS 10

11 The ionic conductivity of the electrolyte was measured after oxidation. No noticeable change was observed between the ionic conductivity of the electrolyte in contact with the Li 2 DHBN-loaded positive electrode, either at 293 K (10.4 ± 1.0 ms cm -1 ) or after oxidation (9.9 ± 1.0 ms cm -1 ). Thus the oxidized form of Li 2 DHBN (i.e., DOBN), although completely dissolved in the electrolyte, does not have significant influence on the ionic conductivity of the medium and subsequently will not affect the power capability of our LIC. Concomitantly, SEM images (supplementary figure 12) of electrodes, before a), b) and after extraction of lithium from the organic molecule c), d), show that after the prelithiation process using Li 2 DHBN (first charge) there are visible holes in surface of the electrode (one of the regions with visible holes is marked with a red circle in supplementary figure 12c). This observation is in agreement with the mass loss measured before and after lithium extraction (solubilized DOBN molecules). Higher magnifications (b and d) show that the texture of the electrode has changed after the lithium extraction process, including some holes and cracks, which are not, however, detrimental to the cycling ability. Supplementary figure 12 SEM images of composite positive electrodes (40 wt.% AC, 40 wt.% Li2DHBCN, 15 wt.% Super C65 and 5wt.% PTFE) before a), b) and after extraction of lithium from the organic molecule Li 2 DHBN, which is the part of the positive composite electrode c), d), at magnifications 100x a), c) and 200x b), d). NATURE MATERIALS 11

12 Self-discharge measurements were performed on a two-electrode LIC cell, where the positive electrode was loaded with Li 2 DHBN. A lithium reference electrode was added to monitor the behaviour of individual electrodes (supplementary figure 13). After the first oxidation of the composite positive electrode, the cell was polarized at 4 V for 2 hours, and then the open-circuit voltage and potential were recorded. Supplementary figure 13: Self-discharge plot of an LIC with a sacrificial composite positive electrode based on Li2DHBCN. The composition of the positive electrode is 40 wt.% AC, 40 wt.% Li2DHBCN, 15 wt.% Super C65 and 5wt.% PTFE. The electrolyte is 1 mol L -1 LiPF 6 in EC: DMC. The black solid line represents the cell voltage; the red and the blue lines are the potential profiles of the positive and negative electrodes, respectively. According to supplementary figure 13, the positive electrode is mainly responsible for the voltage decrease during the 20-hour OCV period. The potential fade of the positive electrode is equal to 16% after 20 hours, i.e. it drops from 4.0 V vs. ref. Li + /Li 0 to 3.6 V vs. ref. Li + /Li 0. Correspondingly, the potential of the negative electrode increased from 90 mv vs. ref. Li + /Li 0 to 135 mv vs. ref. Li + /Li 0 during the same OCV period. From the supplementary figure 13, it is clear that there is no redox shuttle effect due to the dissolved DOBN molecules. Indeed, a 0.4V loss over 20 hours is a commonly measured value for lithium-ion capacitors with an LP30 electrolyte [S3], and for symmetrical activated carbon electrochemical capacitors [S4]. NATURE MATERIALS 12

13 Thermal behaviour of the Li 2 DHBN/AC composite positive electrode in an LIC cell DSC measurements of both the positive and the negative electrodes of LIC cells were performed before (OCP state) and after the first charge (supplementary figure 14). These experiments were conducted using a Q20 DSC (TA instruments) heat-flux differential calorimeter at a heating rate of 10 K/min in a temperature range of C under a constant argon flow of 200 ml/min. Experimentally, the LIC cells were carefully dismantled in a glove box and the recovered samples (studied material with its electrolyte) were introduced into an aluminium crucible, which was then sealed. The loaded DSC crucibles were pierced prior to measurement. Supplementary figure 14 Galvanostatic profile upon the first charge of an LIC cell using 66 wt% Li 2 DHBN mixed with 33 wt% carbon black (Super C65, Imerys) as the composite positive electrode, and graphite (SLC 1512P, Superior Graphite) as the negative electrode, recorded at a cycling rate of C/10 using EC DMC/LiPF 6 1 mol L -1 as the electrolyte. NATURE MATERIALS 13

14 Supplementary figure 15 Typical DSC traces of the positive electrode (66 wt% Li 2 DHBN and 33wt% carbon black) and negative electrode (graphite), gathered a) before cycling the cell and b) after the first charge, without rinsing the electrodes after battery dismantling. All DSC measurements were recorded under Ar flow (200 ml min -1 ) in a pierced crucible. After the first charge, both electrodes exhibited a weak exothermic peak at 120 C and 150 C for the positive and negative electrodes, respectively (supplementary figure 15b). The heat generated at the charged positive electrode soaked in the electrolyte was a mere 63 J g -1. That at the charged negative electrode is in the same order of magnitude (79 J g -1 ). These values are well below those reported in literature for lithiated graphite electrodes in lithium-ion batteries using different graphite electrodes, where it was found that the exothermic peak corresponds to 725 J g - 1 [S5]. S3. Alternative prelithiation conditions of the graphite negative electrode Supplementary figure 16 shows the electrochemical prelithiation of graphite achieved by galvanostatic cycling, where the S.E.I. was formed at C, and intercalation at C/10 (C is the theoretical capacity of graphite, i.e. 372 mah g -1 ). The potential limits were fixed at 4.00 vs. Li + /Li 0 for the positive electrode and at 0.01 V vs. Li + /Li 0 for the negative one, in order to prevent electrolyte oxidation and lithium plating, respectively. When the potential of the negative electrode reached 0.20 V vs. Li + /Li 0 (at which point it can be considered that the S.E.I. is totally formed), the potential of the positive electrode was ca V vs. Li + /Li 0, which is very close to NATURE MATERIALS 14

15 the positive electrode limit. Therefore, at this point, the cell was left at open circuit potential for 2 h, after which the lithium was intercalated at C/10. These conditions were already optimised through our former research dedicated to the use of Li 5 ReO 6 as the irreversible lithium source for LICs [S6]. In supplementary figure 16 it can be seen that the total capacity is around 250 mah g -1, which is much lower than the expected 360 mah g -1 found in figure 3b, when the current is fixed at C/10. Hence, it seems that the presence of Li 2 DHBN modifies the optimal conditions of the prelithiation step. This thus implies that a certain amount of Li 2 DHBN might have been dissolved, probably due to the use of an excessively high current (C) during the S.E.I. formation. Supplementary figure 16 Profiles for the electrodes potential and cell voltage during galvanostatic S.E.I. formation and graphite lithiation facilitated by a composite positive electrode composed of 40 wt.% AC, 40 wt.% Li 2 DHBN, 15 wt.% carbon black and 5 wt.% PTFE. The S.E.I. was formed at C until reaching a potential of 0.2 V vs. Li + /Li 0 (area highlighted in green); then, after a rest period of 2 h, the lithium was intercalated into the graphite at C/10 (area highlighted in yellow). The potential limits for the positive and negative electrodes are 4.0 V and 0.01 V vs. Li + /Li 0, respectively. Colour of the curves: dashed red for the potential of the positive electrode; dotted blue for the potential of the negative electrode; black for the cell voltage profile. The electrolyte is 1 mol L -1 LiPF 6 in EC: DMC. NATURE MATERIALS 15

16 S4. Optimisation of cell voltage range for an LIC cell obtained after graphite prelithiation at C/10 In order to enhance the energy of the LIC based on the renewable lithium source, we tried slightly increasing the maximum voltage value to 4.1 V. The prelithiation was performed under the conditions shown in figure 3b. Unfortunately, as shown in supplementary figure 17, there is a slight but visible continuous capacitance decay, which might be connected to electrolyte oxidative decomposition, and this decay accelerates when the current is set to 0.65 A g -1 after 1350 cycles. Hence, to promote the stable operation of the LIC based on a positive electrode containing delithiated Li 2 DHBN, it is necessary to restrict the upper voltage limit to 4 V, as confirmed by supplementary figure 17. This value seems to be even slightly higher than that of 3.8 V put forward by JM Energy for the ULTIMO system [S7]. Supplementary figure 17 Cycle life of an LIC with a sacrificial composite positive electrode based on Li 2 DHBN where S.E.I.. formation and prelithiation was performed at C/10. The LIC was cycled at 0.25 A g -1 (blue squares), 0.50 A g -1 (green circles) and 0.65 A g -1 (red diamonds) in the voltage range from 2.2 to 4.1 V. The electrolyte was 1 mol L -1 LiPF 6 in EC:DMC. In order to avoid possible plating at higher currents, the upper voltage limit should be decreased, which can be achieved by using a system such as the ULTIMO (constructed by JM Energy) that is operated within a 2.2 to 3.8 V voltage range. Three separate two-electrode cells with reference electrode were assembled and tested in different voltage ranges seen in supplementary figure 18 a), b) V, c), d) V and e), f) V at current 0.5 A g -1. The plots a), c) and e) present the initial galvanostatic charge/discharge cycles, whereas plots b), d), f) show those recorded after 1000 cycles at corresponding voltage ranges. A noticeable difference was observed in the lowest values of potential for the negative electrode: 23 mv vs. ref. Li + /Li 0, 29 mv vs. ref. NATURE MATERIALS 16

17 Li + /Li 0 and 33 mv vs. ref. Li + /Li 0 when the systems reached their maximum voltage of 4.0 V, 3.9 and 3.8 V, respectively. This serves to demonstrate that it is possible to guard the LIC system against plating by adjusting the maximum voltage. The minimum potential of the negative electrode after 1000 cycles was 24 mv vs. ref. Li + /Li 0, 30mV vs. ref. Li + /Li 0 and 34 mv vs. ref. Li + /Li 0 for the respective voltages of 4.0 V, 3.9 V and 3.8 V. Supplementary figure 18 Galvanostatic charge/discharge profiles of a two-electrode LIC with reference electrode and with sacrificial composite positive electrode based on Li 2 DHBCN at 0.50 A g -1 (current per gram of graphite) in the voltage range of 2.2 V to 4.0 V a), b); to 3.9 V c), d); to 3.8 V e), f). The composition of the positive electrode is 40 wt.% AC, 40 wt.% Li 2 DHBCN, 15 wt.% Super C65 and 5wt.% PTFE. The electrolyte is 1 mol L -1 LiPF 6 in EC: DMC. The black NATURE MATERIALS 17

18 solid line represents the cell voltage; the red and the blue lines are the potential profiles of the positive and negative electrodes, respectively. For the purpose of monitoring the potential of each electrode, it was necessary to use a1550-μm-thick separator. References [S1] Cai, W., Wang, H., Sun, D., Zhang, Q., Yao, X. & Zhu, M. Destabilization of LiBH 4 dehydrogenation through H + H - interactions by cooperating with alkali metal hydroxides. RSC Adv. 4, (2014). [S2] Chen, H., Armand, M., Courty, M., Jiang, M., Grey, C.P., Dolhem, F., Tarascon, J.-M. & Poizot, P. Lithium salt of tetrahydroxybenzoquinone: toward the development of a sustainable Liion battery. J. Am. Chem. Soc. 131, (2009). [S3] Decaux, C., Lota, G., Raymundo-Piñero, E., Frackowiak, E. & Béguin F. Electrochemical performance of a hybrid lithium-ion capacitor with a graphite anode preloaded from lithium bis(trifluoromethane)sulfonimide-based electrolyte, Electrochim. Acta 86, (2012). [S4] Hao, C., Wang, X., Yin, Y., & You, Z. Analysis of Charge Redistribution During Selfdischarge of Double-Layer Supercapacitors, J. Electron. Mater. 45, (2016). [S5] Eshetu, G., Grugeon, S., Gachot, G., Mathiron, D., Armand, M., & Laruelle, S. LiFSI vs. LiPF 6 electrolytes in contact with lithiated graphite: Comparing thermal stabilities and identification of specific SEI-reinforcing additives. Electrochim. Acta 102, (2013). [S6] Jeżowski, P., Fic, K., Crosnier, O., Brousse, T. & Béguin, F. Lithium rhenium(vii) oxide as a novel material for graphite prelithiation in high performance lithium-ion capacitors. J. Mater. Chem. A 4, (2016). [S7] accessed January NATURE MATERIALS 18