3-D vertically aligned few layer graphene – partially reduced graphene oxide/sulfur electrodes for high performance lithium–sulfur batteries
Summary (2 min read)
- As the next-generation energy storage materials, lithium–sulfur (Li–S) batteries have become increasingly attractive owing to their high gravimetric density (2600 W h kg 1) and specic capacity (1671 mA h g 1), in addition, sulfur is a highly costeffective and environmentally benign element.
- The overall performance of current Li–S batteries is impeded by inherently poor electronic and ionic conductivity of sulfur and the dissolution of higher-order polysulphides phases (Li2Sn (8$ n $ 2)) during potential cycling which cause irreversible loss of active material.
- (i) conning the insulating sulfur and polysulde shuttles in electronically conductive electrode matrix, (ii) facile ionic network around encapsulated sulfur formed by the liquid electrolyte in the pores of the composite electrode matrix and (iii) efficient charge transfer reaction between the liquid electrolyte and the active material, also known as 6,11 This requires.
- To this effect, various matrices including mesoporous carbons,11,12 microporous carbon spheres,13,14 nanotube/ bres,15,16 activated carbon;17 polar metal oxides such as TiO2, SiO2, Al2O3,18,19 have been investigated.
- This simple and facile method to fabricate the 3-D FLG/PrGO-S electrodes offers great exibility in controlling and tuning of electrode thickness and free volume based on the variation of growth parameters of the FLGs (see ESI, Fig. S2†).
2.1 Synthesis of partially reduced graphene oxide
- Graphene oxide was synthesised using a modied Hummers' method.
- As the reaction took place, the brown solution was no longer homogeneous and black precipitates started to appear.
- Once the samples were loaded, the chamber was pumped down to a base pressure of 2 10 3 Torr aer which nitrogen (N2) plasma pre-treatment of substrate was carried out at 700 W for certain duration of time.
- The vertical alignment of the resulting nanostructures is a unique feature of themicrowave plasma CVD route.
- It should be noted that the microwave plasma deposition route can be used for catalyst free growth of graphene nanoakes on any substrate which can sustain the high temperatures and plasma bombardment encountered during the growth process such as metallic foils, carbon cloth etc.
2.3 Preparation of vertically aligned FLG/PrGO-S electrodes
- The fabrication of vertically aligned FLG/PrGO-S electrode involves the following two steps.
- This was followed by melt inltration at 130 C under argon atmosphere with subsequent drop casting of PrGO/S on the vertically aligned FLGs to prepare the binder-free 3-D FLG/PrGO-S electrodes.
2.4 Microstructural characterisation
- Structural studies on the 3-D FLG/PrGO-S electrodes were performed using Titan Cubed Cs corrected electron microscope with a resolution of 0.08 nm.
- Inbuilt high resolution EDX was used for mapping the distribution of carbon and sulphur.
- XPS spectra were obtained with a spot size of 200 mm.
- High resolution scans of individual elements and the survey spectra were measured at a pass energy of 50 eV and 200 eV, respectively.
- The thermogravimetric analysis of the sulfur content in the PrGO/S electrodes was measured using TGA carried out on a Netzsch5 STA F3 system.
2.5 Electrochemical measurements
- The vertically aligned 3-D FLG/PrGO-S electrodes were tested within the voltage range of 1.8–2.6 V against lithium metal using Maccor Battery Tester.
- Prior to electrochemical testing, electrodes were dried in vacuum.
- Cells were assembled using a homemade vacuum ange type assembly in an argon-lled layers with a knife-edge like structure, (b and c) low and high magnig the diffraction pattern corresponding to crystalline nature of the nal SEM image of the 3-D FLG PrGO/S electrode, (e) dark-field STEM rresponding elemental mapping demonstrating the uniform coating of Fig. S4.†.
- The Royal Society of Chemistry 2017 glove box using polypropylene separator.
- To completely wet the electrode, approximately 100 ml of electrolyte was used.
3. Results and discussion
- Fig. 2(a) shows the transmission electron microscopy (TEM) image of the pristine FLGs, wherein the samples show a predominantly knife-edge structure with a thick (15–20 nm) base constantly narrowing down as it goes along the axial growth direction till it reaches the top with 1–3 layered graphene.
- On the contrary, reduced GO (RGO) or graphene with intrinsically higher electronic conductivity has also been employed to prepare RGO/S or G/S composites for improving rate performance.
- For the rst and third discharge cycles (Fig. 4), it can be observed that the voltage plateau and corresponding cathodic peak shis (from 2.08 V to 2.1 V and from 2.35 V to 2.38 V) towards equilibrium.
- Furthermore, with the on-going optimisation of height and porosity of the underlying FLG electrodes being currently undertaken, it is expected that the electrochemical performance of these 3D FLG/PrGO/S electrodes can be enhanced even further.
- In conclusion, hybrid 3-D electrode structures consisting of FLGs melt-impregnated with PrGO-sulfur nanocomposites were successfully prepared with a high sulphur loading of 80 wt%.
- The 1–3 layered FLGs with a predominantly knife-edge structure were synthesised using a microwave plasma technique and largely composed of carbon (98.8 at%) with a small amount of adsorbed oxygen (1.2 at%).
- The melt inltration process enabled deposition of a thin layer of sulfur nanoparticles as well as larger discrete particles embedded in the PrGO matrix as evident from the elemental mapping and EDX, which also conrmed the high sulfur/carbon ratio.
- This unique combination was able to facilitate electronic, ionic transport and ease of electrolyte accessibility along with connement of the polysulde shuttles within the matrix leading to excellent cycling capabilities.
- Thus, the current study highlights the importance of the electrode microstructure and the presence of surface functional groups to improve active material utilisation and charge discharge performance at high currents (C-rates).
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Frequently Asked Questions (23)
Q1. What are the contributions in this paper?
In this paper, a binder-free 3-dimensional ( 3-D ) aligned electrodes of few layered Graphene ( FLG ) nano-nodes with binder free 3-D nano-connections are presented.
Q2. What are the future works in this paper?
49 The presence of the –S–O– bonding was further corroborated by Fourier Transform Infra-red Spectroscopy ( FTIR ) studies, which revealed peak at 1030 cm 1 ( Fig. S5, ESI† ). 57 In their case, the 1520 | Sustainable Energy Fuels, 2017, 1, 1516–1523 PrGO shows a signicant oxygen content ( C: O ratio of 4. 6 ), as compared to the pristine GO ( C: O ratio of 2. 1 ) which provides the conductive, high surface area and alongside the FLGs, further decreases the need for a large amount of carbon to immobilize the high S-content. Aer a few charge–discharge cycles at C/20, the same 3-D PrGO/FLG-S electrode was further tested at different C rates ( C/ 10 to 5C ). 60 Interestingly, as the charge current density further increased to 2C and 5C ( constant discharge at C/5 ), the 3-D FLG/PrGO-S electrode continued to deliver high specic capacity of 830 mA h g 1 and 700 mA h g 1, respectively ( Fig. 5 ).
Q3. How many cycles did the 2-D planar geometry electrodes deliver?
In fact, as the charge current density increased from C/10 to 1C, the capacity continued to drop, and just aer 150 cycles, the 2-D planar geometry electrodes could deliver only 560 mA h g 1.
Q4. What is the role of the residual functional groups on the PrGO surface?
The residual functional groups on the PrGO surface especially C–O and –COOH can act as the reaction centres with Li+ ions by rapidly and reversibly capturing them through surface absorption and surface redox reaction.
Q5. What is the role of the polysulfide in the electrochemical performance of lithium?
During the continuous charge– discharge cycles, these polysuldes diffuse into liquid electrolyte and shuttle through the separator to lithium anode, and nally precipitate as an insulating layer (Li2S2 and/or Li2S) over the electrodes.
Q6. What is the role of the hydroxyl groups in the synthesis of the GO?
Recent reports have utilised the epoxide and hydroxyl groups on the basal plane of GO in addition to the edge dominant carbonyl and carboxyl groups for the immobilisation of sulfur and its discharge products.
Q7. What is the effect of epoxide groups on the surface of PrGO?
the epoxide groups on the PrGO surface further enhances the immobilization of sulfur and polysuldes during the discharge process.
Q8. How did the discharge capacity of the 3-D PrGO/S electrode change over time?
5. However, aer subsequent cycles, the capacity continued to drop, suggesting dissolution of soluble Lipolysuldes in liquid electrolyte, causing irreversible loss of active material from the cathode.
Q9. What is the effect of the presence of epoxide groups on the PrGO surface?
the presence of epoxide (C–O–C), hydroxyl (–OH) and carboxyl (–COOH) groups on the PrGO surface is expected to further enhance the sulfur and lithium polysulde immobilization during the discharge process.
Q10. How much sulphur was used in the preparation of the hybrid 3-D electrodes?
In conclusion, hybrid 3-D electrode structures consisting of FLGs melt-impregnated with PrGO-sulfur nanocomposites were successfully prepared with a high sulphur loading of 80 wt%.
Q11. What is the effect of the dissolution of the polysulfide phases on the battery?
the overall performance of current Li–S batteries is impeded by inherently poor electronic and ionic conductivity of sulfur and the dissolution of higher-order polysulphides phases (Li2Sn (8$ n $ 2)) during potential cycling which cause irreversible loss of active material.
Q12. What is the reason for the reduction of rst plateau?
This reduction of rst plateau may be because of the intermediate species (such as unstable polysuldes and radicals (S3c)) not getting enough time to evolve and actively participate in the electrochemical reaction.
Q13. What is the effect of the drop casting of PrGO on the FLG?
Drop casting of PrGO/S nanocomposite on the FLG is expected to have additional advantages; rstly, the large surface area of PrGO will act as a barrier layer between the electrode and electrolyte; with the functional groups such as epoxide, hydroxyl and carboxyl on the PrGO physically sandwiching the sulfur (and its polysuldes) within the micro void/channelled FLG structure.
Q14. What is the role of the residual groups in the charge-discharging process?
These residual groups have been previously suggested to immobilise sulfur and subsequently free lithium polysuldes during the charging–discharging process.
Q15. What is the effect of the presence of functional groups on the graphene matrix?
55 Considering the PrGO/FLG electrodes in this study have only 20% carbon content, the presence of this intermediate peak (at 2.15 V) even at low currents (C/20) is attributed to the enhanced sulfur immobilisation within the PrGO/FLG matrix due to the presence of oxygen functional (carbonyl and carboxyl) groups, which actively form S–O bonds facilitating the chemical adsorption of sulfur.
Q16. What is the role of the ionic network in the composition of the 3D electrodes?
6,11 This requires: (i) conning the insulating sulfur and polysulde shuttles in electronically conductive electrode matrix, (ii) facile ionic network around encapsulated sulfur formed by the liquid electrolyte in the pores of the composite electrode matrix and (iii) efficient charge transfer reaction between the liquid electrolyte and the active material.
Q17. What is the reason for the excellent rate capability of the 3-D electrodes?
Such excellent rate capability at high currents in 3-D electrodes can be attributed to low chargetransfer resistance (both ionic and electronic resistance) and low lithium polysulde dissolution.
Q18. What is the charge discharge capacity of the 2-D PrGO/S electrode?
The 2-D PrGO/S electrode showed an initial discharge capacity of 1080mA h g 1 at C/10 rate, as compared to an initial value of 1320 mA h g 1 for the 3D FLG/PrGO-S electrode, as shown in Fig.
Q19. What is the simplest method to fabricate graphene nanoakes?
The growth of few layered graphene (FLG) nanoakes was carried out in a 1.5 kW, 2.45 GHz SEKI microwave plasma enhanced chemical vapour deposition system.
Q20. What is the discharge performance of the planar 2-D PrGO/S electrodes?
The discharge performance of the planar 2-D PrGO/S composites with equivalent sulfur loading ( 80 wt%) was carried out using similar charge–discharge sequences as that for the 3-D FLG/PrGO/S electrodes.
Q21. What is the effect of the melt inltration process on the synthesis of the 3-?
Owing to the melt inltration process utilised for the 3-D FLG/PrGO-S synthesis, a thin layer of sulfur nanoparticles as well as larger discrete particles were observed embedded in the matrix as evident from the elemental mapping and energy dispersive X-ray (EDX) analysis, which further conrms the high sulfur/carbon ratio (see Fig. S4, ESI†).
Q22. What is the effect of the scan rate on the discharge of the Li-S battery?
63,64 Nonetheless, various complex processes are involved during charge–discharge of Li–S battery and are affected by factors such as scan rate, dissolution of active material in electrolyte, sulfur to carbon ratio, additives etcetera.
Q23. what is the role of surface functional groups in the electrode microstructure?
the current study highlights the importance of the electrode microstructure and the presence of surface functional groups to improve active material utilisation and charge discharge performance at high currents (C-rates).