Edited by: Federico Cesano, University of Turin, Italy
Reviewed by: Huan Pang, Yangzhou University, China; Qihui Wu, Jimei University, China; Liqiang Mai, Wuhan University of Technology, China
This article was submitted to Carbon-Based Materials, a section of the journal Frontiers in Materials
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Developing ‘carbon lithium sulfide composite (C-Li2S)’ cathode is a promising strategy for Li-S battery. Quite interestingly, when Li and S are caged in a heavily nitrogen-doped reduced graphene oxide (NDRGO) matrix derived from Tassar silk cocoon, the composite (NDRGO-Li-S) electrode behaves like a supercapacitor. In this work, we first optimized the concentrations of sulfur and then introduced molybdenum in the NDRGO matrix to develop a stable NDRGO-Mo-Li-2S (where 2 stands for 2M) composite electrode. The electrode design process utilized the concepts of “embedded redox couples” and “induced electron transfer”; a putative strategy to alter internal electron-shuttling kinetics for applications in various charge storage devices; where a time of electron-shuttling is the key. In NDRGO-Mo-Li-2S composite the charge transport occurs via “induced electron transfer,” where Li+, is an
A significant amount of research has undergone in Li-S battery; having sulfur as the cathode (positive electrode), lithium as the anode (negative electrode) and an aprotic organic solvent of lithium salt as an electrolyte (Rauh et al.,
The present work involves the design and development of stable cathode material for Li-S battery. During the last decade, two major approaches have been attempted to improve the sulfur cathode. In the first approach, the sulfur cathode is being stabilized by forming a carbon-sulfur composite (Ji et al.,
Li2S suffers from poor electrical conductivity and sluggish electrochemical performance. Li2S-carbon composite electrodes are currently being investigated to improve stability and electrochemical activity (Geng et al.,
In the earlier work, we used equimolar lithium and sulfur to develop the NDRGO-Li-S composite (Jangir et al.,
Since leaching of sulfur from both carbon-sulfur and Li2S-carbon cathode is a persistent problem; ‘
In the present work, we have addressed the
Overall experimental design and the strategies followed to develop a stable silk NDRGO, Li, S electrode. The heavily nitrogen doped reduced graphene oxide (NDRGO) derived from silk is used as a conductivity cage. Superscript 1 indicates NDRGO, a supercapacitor electrode. Superscript 2, stands for NDRGO-Li-S composite obtained from equimolar doping of Li and S on the NDRGO matrix. Superscript 3, stands for 1M Li and 2M S doping on the NDRGO matrix, i.e., NDRGO-Li-2S. Superscript 4, stands for 1M Li and 4M S doping on the NDRGO matrix, i.e., NDRGO-Li-4S. Superscript 5, stands for Mo, 1M Li and 2M S doping on the NDRGO matrix, i.e., MDRGO-Mo-Li-2S.
In the subsequent section, we will discuss each of these aspects and how these concepts assisted us in designing a stable Li2S-carbon composite electrode.
Electron shuttling in biological membranes is carried out by a series of embedded redox couples, mainly iron-sulfur (Fe-S), cytochromes, and copper proteins. These metalloproteins are used with different redox potentials to control the electron flow and sometimes the energy used in these steps are coupled to derive relevant biochemical reactions. The direction of electron flow is a function of the redox potential. The time an electron takes to travel from point A to point B in a chloroplast or mitochondrial membrane, though very fast, yet depends on the number of redox couples which it encounters during its journey. Thus, the capacitive discharge of such a membrane is a function of the number of redox couples present in the membrane (Stryer,
Quite interestingly, in such
Here, we have designed and engineered a bio-hybrid electrode by amalgamating these two above mentioned basic aspects of electron kinetics. The electrode consists of an embedded molybdenum-sulfur (Mo-S) redox couple; along with silk cocoon derived heavily nitrogen doped reduced graphene oxide (NDRGO) and lithium (Li), an electronegative and an electro-positive species, respectively. This electrode exhibits “
The work presented here is divided into three
Tassar silk cocoon were obtained from the forest reserves of the state of Chhattisgarh in India as described in our previous work (Roy et al.,
Around 200 mg of NDRGO was added in previously argon purged 15 ml of dimethylformamide (DMF) and continuously stirred at 50 °C for 40 min under argon. Then, 21 mg of Li metal was added into it and stirred for 20 min. To this, 100, 200 and 400 mg of sulfur powder, respectively, added for NDRGO-Li-S, NDRGO-Li-2S, NDRGO-Li-4S compositions, respectively. In each case the final ternary mixture (NDRGO, Li and S) was stirred for 2 h, to obtain the final product. Then, DMF was removed by washing with benzene and the sample was stored in oxygen and moisture free chamber. Based on the sulfur ratio used, we got three products as NDRGO-Li-S, NDRGO-Li-2S, and NDRGO-Li-4S. NDRO-Li-S synthesis has been discussed in detail in one of our previous publications (Jangir et al.,
As mentioned in the preceding paragraph, around 200 mg of NDRGO was added in previously argon purged 15 ml of dimethylformamide (DMF) and stirred at 50 °C for 40 min under argon. Forty milligram ammonium molybdate tetra hydrate was added to this mixture and further stirred in an argon environment for 20 min, while maintaining a temperature of 50 °C. Following this, 21 mg Li was added. The reaction was allowed to progress for next 20 min and after this 200 mg S was added, and the reaction was continued for next 2 h. The final product was washed with benzene and stored for further use in vacuum desiccator.
Electrode preparation: Electrode materials were coated on one side of the graphite sheet (current collector) of size 1.5 cm*1.5 cm. Only 5 mg of electrode material was coated.
Electrolyte preparation: 1 part of 1-butyl 3- methyl imidazolium tetrafluoroborate [BMIM]BF4 in 10 parts of acetonitrile (ACN) was used as electrolyte. This was prepared in argon atmosphere by gently dissolving in ACN for 30 min (Wang et al.,
Electrochemical characterization: Electrochemical analysis was performed using ZIVE SP1 single channel electrochemical workstation. We used three electrode configuration with platinum as counter electrode and a reference electrode. Two analyses were performed: cyclic voltammetry (CV) followed by charge-discharge study. CV was performed in the voltage range of −1.5 V to +1.5 V at a scan rate of 5 milliVolt per second (mV/s). Charge discharge analyses (CC/CC mode) were performed using a constant charging and discharging current of 5 milliampere (mA) in the voltage range of −1.5 V to +1.5 V. Long-term charge discharge analysis was performed for NDRGO-Mo-Li-2S at a constant charging and discharging current of 0.2 mA in a voltage range of −0.5 V to +1.0 V. In addition, for NDRGO-Mo-Li-2S, CV was performed at different scan rates of 100, 50, 5, and 1 mV/s. Also, charge discharge curves were recorded for 5, 1, and 0.2 mA constant current in a voltage window of −0.5 V to +1.0 V.
Field emission gun scanning electron microscopy (FE-SEM) was performed using JSM-7100F (JEOL Ltd.). Transmission electron micrographs were obtained from FEI Technai 20 U Twin Transmission Electron Microscope (TEM). X-ray photoelectron spectroscopy (XPS) was done using PHI 5000 Versa probe II FEI Inc. instrument. X-ray diffraction (XRD) pattern was obtained using PANanlyticalX’Pert instrument.
This results section has been divided into three subsections as discussed earlier in the “introduction.”
The electrode designing process was initiated from a naturally derived material of biological origin viz., Tassar silk cocoon. Bio-charred silk cocoon is a rich source of heavily nitrogen (~16%) doped reduced graphene oxide (NDRGO) matrix, exhibiting remarkable fluorescence, soft ferromagnetism, and supercapacitor properties (Roy et al.,
In the present work, we initially attempted to improve the electrochemical properties of NDRGO-Li-S complex, by increasing the S content from 1 to 2M and 4M, thus resulting in two new complexes NDRGO-Li-2S and NDRGO-Li-4S, respectively. The transmission electron micrographs of NDRGO-Li-S, NDRGO-Li-2S, and NDRGO-Li-4S composites are shown in
Representative TEM images:
XRD pattern of the synthesized composites.
Next, we performed CV analysis for NDRGO, NDRGO-Li-S, NDRGO-Li-2S, and NDRGO-Li-4S (
Electrochemical characterization of all the composites. CV was performed in the voltage range of −1.5 V to +1.5 V at a scan rate of 5 millivolt (mV) per second. Charge discharge analyses (CC/CC mode) were performed using a constant charging and discharging current of 5 milliampere (mA) in the voltage range of −1.5 V to +1.5 V.
While NDRGO-Li-2S performed better than NDRGO-Li-S, but to our surprise, NDRGO-Li-4S completely deviated from super capacitor behavior. But the unstable nature of the complex prevented us from exploring it further. Based on CV and charge-discharge results, we proceeded with NDRGO-Li-2S composite.
We further tried to improve the stability and supercapacitive property which we observed in NDRGO-Li-2S complex. Without altering the structural conformation, viz., layered sheet-like structure of this complex, we plan to introduce another 2-D sheet like matrix (MoS2) into it. Recently, to enhance the photo-catalytic activity of reduced graphene oxide (rGO), a similar approach has been reported, where a rGO-MoS2 composite has been prepared by Cravanzola et al. (
This newly synthesized NDRGO-Mo-Li-2S showed surprising deviation from super capacitor like properties in terms of cyclic voltammogram (
Next, we performed CV for NDRGO-Mo-Li-2S at different scan rate and further determined the normalized current density (
CV analysis of NDRGO-Mo-Li-2S at 1, 5, 50, 100 mV/s and normalized current density (As/Vcm2).
In the light of this result (
Charge discharge analyses (CC/CC mode) were performed for NDRGO-Mo-Li-2S using a constant charging and discharging current of 5, 1, and 0.2 mA in the voltage range of −0.5 V to +1.0 V.
Based on the electrochemical results, we further explored the structural characteristics of NDRGO-Mo-Li-2S (
Structural analysis of NDRGO-Mo-Li-2S.
In NDRGO-Mo-Li-2S, a weak 002 peak appears at 26.5 °C in its XRD, indicating significant deoxygenation and the presence of the nitrogen atoms in the crystal lattice of graphene. This intercalation of nitrogen, resulted in increased distance between the graphite layers (
In order to analyze the surface chemical features of NDRGO-Mo-Li-2S, we performed a detailed XPS.
The binding energies obtained from the narrow spectra for each constituting elements of NDRGO-Mo-Li-2S.
C1s | C-C | 284.0 |
CH3-Li | 282.4 | |
C-S2 | 287.0 | |
C5H5N/MoS2 | 286.0 | |
C6H5CN | 285.0 | |
N1s | C5H5N | 398.6 |
Mo-N | 397.95 | |
Li-N | 396.5 | |
C6H4CNSSH | 400.4 | |
Mo3d | Mo(0) | 226.23 |
Mo5/2(IV) | 230.3 | |
Mo(V) | 231.2 | |
Mo3/2(IV) | 233.9 | |
Mo(VI) | 232.9 | |
Li1s | Li | 53.0 |
Li-S | 54.3 | |
Li-N | 55.8 | |
S2p | MoS2 | 162.9 |
C-S | 161.7 | |
(Sn)2− | 166.2 | |
S-S | 160.84 | |
C6H5S(O)2NH(C4H2N(S))C6H5 | 168.00 |
The broad spectrum of XPS shows the presence N1s, C1s, Mo3d, S2p, Li1s (
XPS analysis of NDRGO-Mo-Li-2S.
The initial hypothesis we laid out in the synthesis of NDRGO-Mo-Li-2S, to our satisfaction corroborates with its structural and bonding characteristics. The TEM, FESEM, and AFM showed extensive stacking of sheets, this is in line with the characteristics of NDRGO (
The multivalency and the proposed atomic coordinating ability of Mo in NDRGO-Mo-Li-2S complex led us to study the life of the material at a lower current. The charge discharge experiment was tried to match the real life conditions, where, the NDRGO-Mo-Li-2S electrode (in three cell electrode configuration) was maintained under argon atmosphere continuously for 23 days. The time course of the experiment is illustrated in
The time course of charge-discharge cycles for NDRGO-Mo-Li-2S at 0.2 mA in the potential window, +1 V to −0.5 V.
I | 18 | ~65.0 | A |
II | 39 | ~130.0 | B,C |
III | 7 | ~19.4 | D |
IV | 28 | ~79.2 | E |
V | 6 | ~18 | – |
VI | 22 | ~72 | F |
VII | 6 | ~23 | – |
VIII | 7 | ~36 | G |
Long-term charge-discharge cycles for NDRGO-Mo-Li-2S.
One interesting aspect could be seen while carefully looking at the tendency of the charge discharge curves viz., the capacity decayed at the former cycle testing and increased at the latter cycle testing. It seems like the system is behaving like programmable matter in order to attain its most stable configuration. In this process, the entire quaternary mixture it slowly moving to a more stable and ordered state through several metastable states reflecting the transition from one type of electron transfer (supercapacitor like) to another kind of electron transfer (battery type). These transition classify the material into the range of cappatery like material.
This interesting behavior of NDRGO-Mo-Li-2S, prompted us to propose a putative model of electron transfer, where we attempted to address the following question: How introduction Mo changed the charge-discharge time and area within CV curve? These properties may be related to the stability of the charge and electron release processes which could be explained by introducing very unique “induced electron transfer” reactions (Taube,
Induced electron transfer in N-doped reduced graphene oxide-Mo-Li-S electrode; where m stands for number of electrons.
An important question which arises from this study is ‘
Further with reference to NDRGO-Mo-Li-2S complex, we can see that the capacitor capacity has been greatly improved after adding Mo. Among them, the composition of sulfur has obtained the optimum result. Is it feasible to optimize the concentration of Mo? From atom conservation the extra addition of Mo will not enhance the electron transfer behavior. This is simple for the reason that Mo can play multi electron donor and acceptor and therefore an optimal stoichiometry like 1 unit of Mo in the system is expected.
Here it is worth highlighting the exceptional electrochemical stability of the heavily nitrogen doped carbon derived from a natural protein polymer (silk cocoon) and its transition metal sulfide composites; as stability of such composite always remained a challenge for different energy applications (Cravanzola et al.,
In summary, we have successfully designed and engineered a stable bio-hybrid electrode material which integrates lithium (Li), sulfur (S), and molybdenum (Mo) in a matrix of biological origin viz., silk cocoon derived heavily nitrogen (16%) doped reduced graphene oxide (NDRGO); thus resulting in NDRGO-Mo-Li-2S. The electron shuttling on this electrode is govern by a lesser explored route viz., “induced electron transfer” regulated by an electropositive Li, electronegative NDRGO and an embedded redox couple of Mo-S. We believe that such a bio-hybrid approach is a step toward developing sustainable, eco-friendly high energy electrode materials.
All datasets generated for this study are included in the manuscript/supplementary files.
This work is part of HJ’s doctoral research. HJ and MD conceived the idea and designed the experiments. HJ developed the idea and conducted the synthesis, carried out the structural, chemical and electrochemical characterizations. AB conducted the synthesis and purification of the compounds. JR helped in TEM and AFM. SS and MD conceived the concept of electron transfer shuttle and role of Mo. HJ, SS, and MD wrote the manuscript.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Authors are thankful to the staff at Advance Centre for Material Science, Indian Institute of Technology Kanpur, India. We sincerely acknowledge the help offered to us by Mr. DD Pal for XPS, Mr. A Tiwari for XRD facility, and Mr. Mitesh for FESEM work. This work was an integral part of HJ’s doctoral thesis.