Polar Bilayer Cathode for Advanced Lithium-Sulfur Battery: Synergy

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C: Energy Conversion and Storage; Energy and Charge Transport

Polar Bilayer Cathode for Advanced Lithium-Sulfur Battery: Synergy Between Polysulfide Conversion and Confinement Arpita Ghosh, Meenakshi Seshadhri Garapati, Ajay Piriya Vijaya Kumar Saroja, and Ramaprabhu Sundara J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01371 • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 15, 2019

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The Journal of Physical Chemistry

Polar Bilayer Cathode for Advanced Lithium-Sulfur Battery: Synergy Between Polysulfide Conversion and Confinement Arpita Ghosh a, Meenakshi Seshadhri Garapati a, Ajay Piriya V. K. S. a and Ramaprabhu Sundara

a ,*

Alternative Energy and Nanotechnology Laboratory (AENL), Nano Functional Materials Technology center (NFMTC), Department of Physics, Indian Institute of Technology Madras, Chennai 600036, India;

a ,* Corresponding

author. Tel: +91-44-22574862. E-mail: [email protected] (Sundara Ramaprabhu).

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Abstract Despite possessing five-fold higher specific energy density compared to commercial lithiumion batteries, insulating nature of sulfur and its reductive derivatives along with the uncontrollable migration of polysulfides hinder the commercialization of lithium-sulfur technology. Herein a bilayer cathode consists of nitrogen sulfur co-doped porous carbon network and titanium carbide has been introduced and investigated methodically. Porous sulfur host promotes uninterrupted diffusion of electrolyte and ions whereas titanium carbide acts as polysulfide trapping material. The superiority of this bilayer cathode over the conventional interlayer approach has been highlighted in terms of the diffusivity of lithium ions and the overall ohmic resistance. Variation in interfacial charge transfer resistance during charging and discharging has been investigated using dynamic electrochemical impedance spectroscopy. Discharge capacity reaches as high as 1300 mA h g-1 at 0.1 C with a coulombic efficiency of 99 %. Theoretical studies reveal polar nature and improved interfacial charge transfer between the TiC and polysulfides result in excellent binding strength and faster redox kinetics during operation respectively. This work provides an experimental as well as theoretical evidence of the bifunctional mechanism of TiC towards polysulfide confinement and conversion.

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Introduction Among all the rechargeable batteries, lithium sulfur (Li-S) batteries have been considered as one of the most promising candidates for next-generation energy storage device owing to high theoretical specific capacity (1672 mAh g-1) and energy density (2600 Wh kg-1) of sulfur.1,2 Abundance and non-toxicity of sulfur with enhanced energy density and specific capacity project Li-S batteries as an attractive storage device. To achieve high reversible capacity, complete electrochemical utilization of the active material at cathode is essential. But in a practical scenario, only a small fraction of the total active material is electrochemically accessible due to the insulating nature of sulfur and its reductive derivatives (Li2Sx). Additionally, the frequent change in phase from solid sulfur to liquid polysulfide to solid polysulfide (PS) leads to noticeable volume expansion (up to 79 %)3 during the operation of the cell and reduce the efficiency and longevity of the device. Another prominent difficulty associated with Li-S battery is the migration of the dissolved polysulfides, generated at the intermediate discharge stage, from cathode to anode which is known as so-called shuttle effect.1,4 This process not only reduces the active material utilization but also forms an insulating passivation layer over anode and further deteriorate the battery performance. Part of these species can further react with the incoming soluble species and again diffused back to the cathode.1 This uncontrollable shuttle mechanism consumes a huge part of the active material and thus plays a crucial role for irreversible capacity fading. Therefore, confining the dissolve polysulfides within the cathode matrix along with retrieving back the active material via electrochemical conversion are the key requisites for an efficient polysulfide trapping material.



Several strategies have been adopted in order to confine the generated polysulfides in the cathode matrix. It has been investigated extensively in recent years that with surface modification5–7 or doping with heteroatoms in conventional carbon substrates exhibit poor strength towards polysulfides binding.8 Except for carbon-based materials, several noble and non-noble materials have been studied extensively as PS trapping material.3,8–10 The most wellreported approach is to introduce an interlayer in between the cathode and separator to reduce the migration of the PS.11–15 Along with interlayer, several core-shell type cathodes also have been investigated.2 But introducing an additional component not only increase the overall resistance of the whole system but also affect the diffusion of the electrochemical species through the interlayer. Addressing all the above-mentioned concerns herein we propose a novel bilayer cathode fabrication consists of doped porous carbon-based sulfur host as the first and polar PS trapping material as the second layer. Porous nature of the sulfur host promotes the diffusion of electrolyte as well as Li+ in the cathode matrix, whereas, polar nature inhibited the migration of PS (which is also polar in nature) from cathode to anode. In the present work, we have chosen titanium carbide (TiC) as a PS trapping element. Previous studies shown confinement of PS can be achieved in conductive as well as insulating surfaces (mostly metal oxides).3,16–19 But the fundamental problem with insulating material is its inability to transport electrons. X. Tao et al. and the group has shown in their pioneering work that optimal balance between PS adsorption and diffusion on the surface of trapping material is highly required for the longevity of Li-S battery.20 The trapping material with weak PS adsorption strength can confine only a few PS, so the surface diffusion of PS leads to uncontrollable shuttling. On the other extreme, the anchoring material with strong binding strength without good diffusion hinders the further reduction and nucleation of PS, which results in an inferior specific capacity. Addressing the above-mentioned considerations an ideal PS trapping material should be ⅰ) polar in nature to bind the PS strongly, ⅱ) conductive to 4 ACS Paragon Plus Environment

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allow the charge transfer during the multistep reduction process and ⅲ) excellent catalyst towards PS conversion for complete utilization of the active material during cycling. In this work, TiC satisfies all the three above mentioned features. TiC not only confines PS also promotes the redox reaction involves reduction of sulfur to PS and further conversion of PS into sulfur. Superior electrochemical performance is supported with dynamic electrochemical impedance (DEIS) measurement. Comprehensive theoretical simulation has been carried out in order to estimate the binding strength of the PS on the TiC surface along with the interfacial charge transfer properties. Experimental Section Material synthesis The porous host material has been synthesized via a controlled heat treatment process reported earlier.21 Briefly, glucose (C6H12O6), thiourea (CH4N2S) and sodium bicarbonate (NaHCO3) were taken in 1:1:1 ration and mixed uniformly using mortar and pestle before loading in a tubular furnace. The mixture was pyrolyzed at 800 °C for 2 hrs at a heating rate of 5 °C min-1. Glucose and thiourea have been used as carbon and both nitrogen and sulfur source respectively. Post heat treatment, the sample was subjected to rigorous washing with deionized water and ethanol in order to get rid of all sodium impurity present in the sample and labeled as nitrogen and sulfur co-doped porous carbon (NSPC). Sulfur incorporation in NSPC has been carried out by conventional melt diffusion technique. Sublimed sulfur and NSPC were mixed uniformly in 5:1 ratio and the mixture was heated at 160 °C, above the melting point of sulfur where the viscosity is minimum, for 10 hrs to allow the sulfur to melt and diffuse through the mesopores of the carbon network under the action of capillary forces.22 Later the temperature was raised to 450 °C and held for another 4 h in order to vaporize the superficial sulfur outside



the carbon host. The final product has labeled as NSPC-S. This entire procedure has been carried out in a sealed glass tube. The most stable form of sulfur (α-S) in room temperature has a rhombic stacking of S8 which slowly converts to monoclinic sulfur (β-S) from 95.5 °C. The melting and boiling temperatures of monoclinic sulfur are around 119 °C and 444 °C respectively. Melted sulfur is primarily S8 in nature at the melting point. S8 rings undergo thermal scission at 159 °C. Between 159 °C and 444 °C, sulfur polymerizes and then depolymerizes, associated with a significant change in viscosity.23 Materials characterization Powder X-ray diffraction (XRD) patterns were recorded using Rigaku SmartLab X-Ray Diffractometer with Cu Kα radiation (λ = 1.5418 Å). All measurements were taken in the 2θ range of 5° to 90° with the step size of 0.02°. The scanning electron microscope (SEM) images and the energy dispersive X-ray spectra were taken in Quanta 200 (FEG) scanning electron microscope. Thermogravimetric analysis was carried out in SDTQ600 from TA instruments in air atmosphere from room temperature to 1000 °C at 20 °C min-1 heating rate and under 150 ml min-1 flow rate. Compositional analysis was carried out by analyzing X-ray photoelectron spectra (XPS) recorded by an instrument with Mg-Kα as the X-ray source and PHOIBOS 100MCD as the analyzer operated at ultra-high vacuum (10-9 mbar) from SPECS. All highresolution peaks are fitted using an iterated Shirley background. The line-shape used in every fit is a Doniach Sunjic form convoluted with a Gaussian/Lorentzian shape. Specific surface area measurement of all synthesized samples have been done by nitrogen adsorption-desorption isotherms at liquid nitrogen temperature of 77 K according to Brunauer– Emmett–Teller (BET) method using a Micromeritics ASAP 2020 instrument. Electrochemical measurements 6 ACS Paragon Plus Environment

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The electrode slurry has been prepared by mixing NSPC-S (for the 1st layer) and TiC (for the 2nd layer) as active material (75 %) with polyvinylidene fluoride (PVDF) as binder (15 %) and conductive carbon (10 %) thoroughly in N-methyl-2-pyrrolidone (NMP) solvent. This slurry was uniformly coated over the surface of the gas diffusion layer (GDL) current collector using doctor blade method. Post-coating, the GDL was dried at 60 °C for 8 hours in a vacuum oven. The electrodes with diameter 14 mm were cut from the coated GDL. The NSPC-S coated GDLs were recoated further with TiC as an active material following the same procedure explained above. For comparison, TiC coated GDL has been used as interlayer for NSPC-S coated GDL cathode. For all the fabricated cathodes, such as single layer (NSPC-S), bilayer (TiC/NSPC-S) and single layer cathode with TiC coated over the GDL as interlayer have the identical sulfur loading as the sulfur host in the cathode matrix is same in three cases. The diameter of the GDL current collector is 14 mm. Therefore the areal sulfur loading is 1.95 mg cm-2. CR2032 type coin cells were assembled in argon-filled glove box (mBraun; with controlled H2O and O2 < 0.1 ppm) using lithium foil (thickness 0.75 mm) as reference and counter electrode. Celgard 2400 microfiber dipped in electrolyte solution made of The electrolyte was 1.0 M lithium bis(trifluoromethane) sulfonamide (LiTFSI) dissolved in a mixture of 1,3-dioxolane (DOL) and 1,2-dimethoxymethane (DME) (1:1, v/v) and 1 wt% LiNO3 as an electrolyte additive. The total sulfur content (S) of each cathode is around 3 mg and the amount of electrolyte (E) used in each cell is 30 μL, hence the E/S is 10 μl mg-1. Optimum E/S is highly desirable in order to control the dissolution and diffusion of polysulfides in the electrolyte. A Biologic SP-300 electrochemical workstation was used to estimate the electrochemical performances. The galvanostatic charge/discharge has been carried out in the potential range of 1.5 V to 3.0 V with different C rates. All impedance spectra have been taken in the frequency range of 1 MHz to 10 mHz. All the measurements were carried out at room temperature. Dynamic response of impedance with state of charge/discharge was evaluated with dynamic electrochemical



impedance spectroscopy (DEIS) measurements in the above-mentioned potential and frequency range. To investigate the chemical composition of the electrodes after cycling, cycled electrodes were further de-crimped from 2032 coin cells and washed thoroughly with blank electrolyte and dried in vacuum for 12 h. Fabrication of bilayer cathode The slurry preparation methods for both NSPC-S and TiC are similar and discussed in electrochemical measurements section. The second layer of TiC acts as a shield or shell to encapsulate the sulfur core (NSPC-S). This layer hinders the diffusion of PS produced during the charge-discharge. Additionally, owing to its excellent polar nature, this second layer traps the generated PS chemically by means of electrostatic force. TiC also possesses superior electrocatalytic activity, which converts the trapped polysulfide back to the elemental sulfur, which can be reused again. Thus introducing the polysulfide trapping second layer improves the cyclic stability of the battery. Schematic of fabrication of the bilayer cathode has been shown in Figure 1. Computational methods Theoretical calculations based on DFT were carried out with the Gaussian 09 package.24 The geometry optimizations for TiC, NSPC, and different order PS (Li2Sx (1x8)) were performed using Becke's hybrid three-parameter nonlocal exchange functional combined with the LeeYang-Parr gradient corrected correlation functional (B3LYP).25,26 The Los Alamos LANL2DZ effective core pseudopotential (ECP) and the corresponding valence double- basis set was assigned for Ti atom whereas, the 6-31G(d,p) basis set was used for all Li, S, C and N atoms.27– 29

The optimized geometry for all the structures with the lowest energy was considered as the

ground state and was further used for PS adsorption studies. All calculations include Grimme’s dispersion corrections by using a locally modified version of the Gaussian 09 program


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(IOP(3/124=3).30–32 Further, in order to minimize the basis set superposition error (BSSE) for the multi-molecular aggregates, the counterpoise correction (CP) was performed.33 In order to understand the charge distribution over support materials as well as the PS molecules, the electron density mapping (with electrostatic potential (ESP)) were carried out.

Results and Discussion

Figure 1. Schematic illustration of fabrication of bilayer cathode consists of nitrogen and sulfur co-doped porous carbon (1st layer) and titanium carbide TiC (2nd layer) with illustrations of the binding and conversion mechanism in single layer and bilayer cathodes



Hydrogen bond interaction plays a key role in determining the porosity as well as the morphology of porous carbon. Interconnected thiourea molecules form zigzag ribbons via hydrogen bond interaction which is further cross-linked by (HCO3-)2 units (arising from bicarbonate salt). During carbonization, sodium bicarbonate and thiourea decomposed giving rise to abundant number of mesopores within interconnected host lattice due to the liberation of water and carbon dioxide molecules, which is ideal for the sulfur host in Li-S.21 Powder XRay diffraction pattern of elemental sulfur, NSPC, NSPC-S and TiC have been shown Figure S1. Thermogravimetric analysis for NSPC and NSPC-S and nitrogen adsorption along with BJH pore-size distribution curve of NSPC have also been included in Figure S1.

One of the primary limitations in Li-S battery is associated with incomplete utilization of active material (sulfur), as only a minor fraction of it is electrochemically accessible for the electrolyte and incoming lithium ions, which is just adjacent to conductive host. The lithiation process can only commence at the interface of sulfur, carbon, and electrolyte, as the diffusion paths for the charged species through the sulfur are practically nil. This limitation is taken care of by the high specific surface area and the porous structure of the host material, which encapsulates the sulfur in its abundant micro as well as mesopores upon sulfonization. The next constraint, which limits the performance and longevity of this battery, is the uncontrollable shuttling process of the dissolved polysulfides (PS) between anode and cathode. Several attempts have been made in order to confine the PS physically as well as chemically on the cathode to inhibit the shuttling process. One of the well-known approach to mitigate the PS shuttling is introducing an interlayer which mainly consists of some polar material (mainly metal oxides or surface functional groups) between the separator and cathode. Introduction of an additional component tends to increase the overall ohmic resistance of the cell and also affect the Li+ diffusion. Also, trapping insulating PS on insulating materials results in accumulation of PS and eventually make it electronically inactive region. Furthermore, the surface diffusion of PS, 10 ACS Paragon Plus Environment

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as well as the adsorption strength, play a crucial role in Li-S battery performance. In general, due to the poor conductivity of the metal oxides, the absorbed PS transferred from the surface of the oxide to the conductive carbon substrate to undergo further electrochemical reaction.20,34 Moreover, the weak binding strength for PS can facilitate the surface diffusion of the PS, resulting in uncontrollable shuttle effect and PS deposition.20 Therefore, maintaining synergy between PS adsorption and diffusion on the surface of trapping material is highly desirable in order to ensure the longevity of Li-S battery. Thus in the present work, we have attempted to immobilize the polysulfide shuttling physically as well as chemically with a novel two-layer cathode approach. The motivation of the present work along with the probable reaction paths has been shown in Figure 1. TiC is a polar conductive material with excellent catalytic activity towards PS redox reaction and fulfills all the essential criteria to employ as a cathode material for Li-S battery. Three cathode variations have been investigated in the present study, namely the single layer cathode consisting of sulfur incorporated nitrogen and sulfur co-doped porous carbon (NSPCS), the bilayer cathode consisting of an additional layer of titanium carbide (TiC) over NSPCS and the single layer cathode (NSPC-S) with TiC coated GDL as interlayer. In Figure 2a and 2b, two distinct reduction peaks at 2.36 V and 2.01 V for bilayer and at

2.36 V and 1.98 V

for single layer cathode reveal the two-step reduction mechanism. Superior binding strength is highly desirable for the electron transfer process in PS conversion. The strong interaction between the lithium polysulfides and TiC particles promotes faster electron transfer at TiC–PS interface. The increment in the interface reaction current further stimulates the conversion process. Evaluation of exchange current density from Tafel analysis of anodic and cathodic reaction for single layer, bilayer cathode and cathode with interlayer has been compared in Figure 2c and 2d respectively. Enhanced exchange current density indicates superior catalytic activity on charge-transfer kinetics during charging and discharging.9 The calculated exchange current densities (j0) of bilayer, interlayer and



single layer cathodes are 42.35 μA cm-2, 38.41 μA cm-2 and 31.28 μA cm−2 for the cathodic reaction and 12.65 μA cm-2, 10.11 μA cm-2 and 9.29 μA cm−2 for the anodic reaction, respectively. Improved j0 for bilayer cathode confirms the strong PS confinement and spontaneous conversion reactions. During the anodic scan in CV, the bilayer cathode exhibits an additional peak at unlike its

single layer counterpart (Figure 2a and 2b). The two cathodic peaks (at 2.34 V and 2.01 V for bilayer and at 2.35 V and 1.98 V for single layer cathode) correspond to the formation of soluble and insoluble polysulfides respectively.


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Figure 2. Cyclic voltammograms of (a) bilayer cathode (TiC/NSPC-S) and (b) single layer cathode (NSPC-S) with (c) anodic and (d) cathodic Tafel plots; Galvanostatic chargedischarge profile recorded for C rates ranging from 0.05C to 3C of (e) bilayer cathode and (f) single layer cathode. Soluble polysulfides are higher order polysulfides such as Li2S8, Li2S6, and Li2S4 and lower order polysulfides such as Li2S2 and Li2S are insoluble in nature. Subsequent oxidation peaks are associated with the reversible conversion of, Li2S2/Li2S to low-order polysulfides and then to high-order polysulfides and S8. In contrast to the only one anodic peak in the CV of the single layer cathode, the two separated oxidation peaks at 2.32 V and 2.39 V for bilayer cathode with slightly overlapping features imply reversible transitions between Li2S2/Li2S and highorder polysulfides during the charging process.35,36 This further reveals the superior catalytic activity of TiC towards recycling of S8 from the long order PS, corresponds to the second anodic peak at higher potential. The lower potential anodic peak, however, signifies the formation of middle order PS from short order PS. Unlike the case of NSPC-S, the additional layer of TiC promotes the faster reaction kinetics towards PS/S8 redox along with improved reversibility and utilization of the active material. Enhanced discharge capacity as well as the cyclic stability during operation, is consistent with the aforementioned argument (Figure 2e and 2f). Anodic and cathodic peak positions and currents have been denoted as Vpa, Vpc and Ipa, Ipc. The difference between Vpa and Vpc denotes the polarization of the cathodic half cells. For three different cathodes, all the values have been tabulated in Table 1. Table 1. Comparison of PS reduction (Vpc)/oxidation (Vpa) potential, the potential difference (ΔVp) between anodic and cathodic peaks, and peak current (ipc, ipa) for three different cathode configurations. Cathodes Vpa , Vpc ∆ Vp Ipa, Ipc (V) (mV) (mA) Bilayer Cathode 2.41, 2.34 70 2.04,-0.90 Cathode with interlayer 2.45, 2.34 110 1.08, -0.27 Single layer cathode 2.47, 2.27 200 3.65, -1.86



The potential difference between the anodic and cathodic peak (ΔVp) was found to be decreased from 200 mV (for single layer cathode) to 70 mV (for bilayer cathode). The cathode with an interlayer shows an intermediate polarization. This result implies the significance of electrocatalytic TiC surface on the S8-PS redox activity. Reducing the overpotential towards PS redox reaction is believed to be an efficient way to minimize the cell polarization and increase the rate capability as well as longevity.37 According to recent reports, platinum9, metal oxides6,18, metal nitrides3,38,39, metal sulfides18,40,41,37 metal carbides42,43including TiC16,19,44,45 have been employed as efficient electrocatalysts which promote the conversion of polysulfides, resulting in the improved reaction kinetics. In the present report, we have attempted to reveal the polysulfide anchoring mechanism dominated by electrostatic force owing to the polar nature of the polysulfides as well as TiC.

The galvanostatic charge-discharge was carried out in the specified potential window with double layered and single layered cathode. The discharge plateaus are well consistent with the reduction peaks observed in CV. The maximum discharge capacity of 1668 mA h gs-1 at C/20 has been achieved which dropped down to 515 mAh gs-1 at the 3C for double layer cathode. The charge-discharge profile of TiC/NSPC-S consists of two discharge plateaus at 2.37 V and 2.05 V, which is well consistent with the CV results. During discharge, the solid-state sulfur dissolves in the liquid electrolyte and form liquid-state sulfur, which reduced to Li2S8 upon Li+ uptake. Although the initial reduction of sulfur into PS is primarily considered a solid-state reaction, Harks et al. showed experimentally the significance of elemental sulfur dissolution into the electrolyte in liquid Li–S cells in their pioneering work.46 The solubility of sulfur in the mentioned electrolyte is 5 × 10-3 M S8 (= 0.13 wt% S8). According to this report, despite the low solubility of S8 in organic solvents, the replenishment of sulfur species in solution is fast enough to enable the electrochemical reactions during discharge the solid state sulfur firstly dissolves to form the liquid state sulfur and then subject to reduction in order to form the 14 ACS Paragon Plus Environment

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polysulfides. The concentration of Li2S8 increases with a continuous decrement in the voltage and subsequently forms other lower order dissolved PS, until the first reduction plateau. During this process viscosity of electrolyte attains a maximum, as a result, Li+ diffusion gets hindered and a small reverse peak can be observed in the starting of the second discharge plateau for initial cycles.1,47 In the second plateau the soluble PS species undergo further reduction to form insoluble Li2S2/Li2S. The solubility of both Li2S and Li2S2 in electrolyte is very low, which keeps the concentrations of Li2S2 and Li2S constant which results in the second plateau to stay a long time and contributes to the maximum capacity of Li-S cell. This process continues until all the active reaction sites are covered by the insulating Li2S2/Li2S, which increases the cell resistance and subsequently charge transfer reactions stop and a sharp drop in voltage can be seen.1,47 The sharp drop indicates the sluggishness of the conversion reaction of Li2S2 to Li2S. While charging, a stable plateau can be observed due to the oxidation of the Li2S2/Li2S to soluble long-chain PS species. A small peak during charging can be seen for both the cathode at a low current density (C/20) which indicated the reduced polarization due to the phase change from solid Li2S2/Li2S to dissolved long-chain PS.1,47 The decrement in the potential difference between charge and discharge plateaus at current density of C/5 in TiC/NSPC-S (160 mV) (Figure 2e) compared to only NSPC-S (180 mV) (Figure 2f) implies fast reaction kinetics during the cell operation. Discharge capacities correspond to the upper plateau (QH) reflect the polysulfide confinement capability and the redox accessibility of the Li-S cell. Theoretical values of QH is 419 mA h g-1 and.48 Initially, Bilayer cathode approaches high QH of 412 mA h g-1, signifying high active material utilization of 98 % (Figure 2e). Over cycling, with a high current density of 3C, it drops down to 51%. On the other hand, the single cathode commences with 95 % utilization and dropped down to 35 % at high current density of 3C (Figure 2f). Owing to the presence of TiC layer and its ability to confine the PS the active material utilization improved significantly even during the fast charge transfer process. This



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indicated the strong confinement and catalytic activity reduces the diffusion path-length and enhances the reaction kinetics of PS. Figure 3a shows excellent capacity retention of almost 87 % for bilayer cathode over its single layer counterpart. Further, the bilayer cathode shows excellent cyclic stability with a coulombic efficiency of greater than 98.8 % at C/2 for 500 cycles (Figure 3b). Extensive research has been carried out with a variety of materials as an interlayer between the cathode and the separator in order to trap the PS. Introducing an additional component as an interlayer not only increase the effective series and overall resistance of the whole system also may cause capacity fading due to the decrease in the diffusion coefficient of Li+ from anode to cathode. The diffusion coefficient of Li+ has been evaluated using Randles–Sevcik equation (eq 1). 𝑖𝑝 = 2.69 × 105𝑛3/2𝐴𝐷1/2ⅴ1/2𝐶

(1)

Where ip is the peak current (A), n is the number of electrons transfer (for Li+, n=1), A is the surface area of the carbon cloth electrode (1.54 cm-2), C is the concentration of Li+ ions, ν is the scan rate (V s-1) and D is the diffusion coefficient of Li+ (cm2 s-1) at the electrode. The calculated diffusion coefficient (Figure 3c) of Li+ for TiC used as an interlayer (Figure 3d) and TiC as bilayer (Figure 3e) are 5.15× 10-8 cm2 s-1 and 2.56× 10-8 cm2 s-1 respectively. From Figure 3e the dependence of the cathodic peak current on scan rate cannot be understood clearly, thus the enlarged version of cathodic peak current has been shown Figure S4. Hence the superiority of the bilayer cathode over the interlayer can be established by taking care of enhanced Li+ diffusion, specific capacity and overall ohmic resistance of the cell. In order to establish the superiority of bilayer cathode over conventional interlayer approach, galvanostatic charge-discharge has been carried out for NSPC-S as cathode with TiC coated over carbon cloth as an interlayer. Figure 3f shows the overall ohmic resistance is more for the cell with interlayer compared to single layer and bilayer cathodes. The charge-discharge profile


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has been shown in Figure S5. Introducing an additional component as an interlayer not only increase the effective series and overall resistance of the whole system also may cause capacity fading due to the decrease in the diffusion coefficient of Li+ from anode to cathode. The discharge capacities at different C rates for TiC coated GDL as interlayer has been compared with single layer and bilayer cathode (Figure 3g).

Figure 3. (a) Comparative cyclic performance of double layer and single layer cathode; (b) Evaluation of cyclic stability and coulombic efficiency carried out at 0.5 C for 500 cycles; (c) Estimation of diffusion coefficient of Li+ for TiC used as bilayer and TiC used as an interlayer using Randles–Sevcik equation; Cyclic voltammograms of cathodic half-cells with (d) TiC used as an interlayer and (e) TiC used as bilayer; (f) EIS spectra of single layer, bilayer and cell with interlayer; (g) Bar diagram of obtained maximum specific capacity with 17 ACS Paragon Plus Environment


C rates ranging from 0.05C to 3C of bilayer cathode, single layer cathode and cathode with interlayer.

In order to elucidate the dynamical variation of various impedances associated during charging and discharging, dynamical electrochemical impedance spectroscopy (DEIS) has been carried out with the cycled cathodic half-cells. In the DEIS technique, the variable frequency response of an AC signal is superimposed with a DC voltage in the same potential scan of the anodic half-cell. The superimposed DC potential on the corresponding AC amplitude holds the electrochemical state of the cell to a complete stationary state, which is advantageous over EIS. In order to have in-depth knowledge about the electrode reaction as a function of the state of charge (SOC), this technique is beneficial over EIS because it does not hold the potential at the requisite potential point leading to a non-stationary state.49 In Figure 4 two sets of spectra taken during charging and discharging process in the potential window of 1.5 V to 3.0 V at a regular potential interval of 0.1 V for bilayer cathode (Figure 4a and 4b) and single layer cathode (Figure 4e and 4f). The impedance spectra of both the cathodes mainly consist of two different kinds of profile either consists of two or one depressed semicircles followed by an inclines straight line (Figure S6a and S6b). All the circuits were fitted with the equivalent circuit shown in Figure 4. Where Rs and Rct correspond to the series resistance of the electrolyte and the charge transfer resistance respectively. The semicircles at the high frequency and mid frequency domain denote the formation of Li2S2/Li2S layer on the cathode and the charge transfer kinetics respectively.1,50 The inclined line in the low-frequency domain was modeled using a constant phase element (CPE) instead of Warburg (W) impedance, which signifies the diffusion of the ions in the cathode and provides an infinite length Warburg element. Besides, CPE is used instead of capacitor (C) to compensate for the non-ideal behavior of the porous sulfur host.50


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Figure 4. Dynamic electrochemical impedance spectroscopy (DEIS) taken and single layer cathode during discharging of (a) double layer cathode and (e) single layer cathode and (b) charging of (e) double layer cathode and (f) single layer cathode; All the EIS spectra have been fitted using the equivalent circuit shown. Variation of series resistance and charge transfer resistance of bilayer cathode ((c) during discharging and (d) during charging) and single layer cathode ((g) during discharging and (h) during charging)

Figure 4a and 4b depict with an increase in depth of discharge (DOD) the elemental sulfur transform to soluble long-chain PS results in an increase in the viscosity of the electrolyte and subsequent increase in the cathode resistance. During the formation of the Li2S2/Li2S only two semicircles can be seen (Figure S6a), the whole reaction was dominated by the charge transfer kinetics only. Variation of the series resistance (Rs) and the charge-transfer resistance (Rct) with the voltage during charging as well as discharging has been derived from the DEIS data (Figure 4c and 4d). During the commence of discharge, as the less concentration of soluble sulfur corresponds to insufficient active electrochemical species and thus corresponds to high charge transfer resistance. The higher order PS are soluble in the electrolyte, therefore, no significant increase in Rct can be observed. Elemental sulfur forms Li2S8 upon reacting with Li+, so the



active material present in the electrolyte was in the liquid state, which promotes the faster diffusion of Li+. Rct started increasing drastically after 2.2 V due to the increased concentration of insoluble and nonconductive PS. In contrast, during charging, Rct values decrease because of the conversion of the Li2S2/Li2S to Li2S8 and S8. The Rs remains almost constant as the temperature (room temperature) was constant during the cell operation. It should be noted that the diffusion tail attains a minimum of around 2.4 V during charging and discharging. This can be interpreted as follows: during discharging up to 2.4 V the reaction at cathode mostly influenced by the charge transfer kinetics, so more no of Li+ ion intake (faster diffusion of Li+ ion) takes place to produce the optimum concentration of soluble PS. The length of the diffusion tail starts decreasing 2.6 V onwards. After 2.4 V the viscosity of electrolyte starts increasing again and Li+ ion diffusion face hindrance, which results in the increase in diffusion tail. During charging Li2S2/Li2S converts to soluble long-chain PS. Starting from 2.2 V up to 2.4 V the dissolution takes place with a maximum number of Li+ diffusion. After this point viscosity of the electrolyte further enhances and owing to the catalytic activity of TiC further oxidized to solid elemental sulfur, as a result, the length of the diffusion tail starts increasing further. This observation is well consistent with the reported theoretical model.51 For single layer cathode (Figure 4e and 4f), DEIS profile follows the same pattern mentioned above. But the order of the Rct is several order high compared to TiC/NSPC-S counterpart. The reason behind this can be the conductive and polar nature of the TiC layer, which enhances the charge transfer, also prevents PS and converts PS to elemental sulfur. The electrochemical conversion of S8 from Li2S8 was absent in case of NSPC-S, due to which significant decrement in Rct value cannot be seen during charging (Figure 4h). Also during discharging the order of increase in Rct value is quite larger compared to the double-layered cathode (Figure 4g). Binding energy calculation has been carried out for all possible PS configurations generated during a different state of lithiation process. The PS-binding energy is defined in eq 2: 20 ACS Paragon Plus Environment

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EBE = – (ES-LP – ES – EPS)

(2)



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Where, ES, EPS and ES_PS are the ground state energies of trapping material (TiC and NSPC), PS molecule and the PS-adsorbed system respectively. According to the above definition, a negative EBE value is favorable. Owing to its polar nature TiC shows an excellent PS binding capacity. In Figure 5 the simulated binding energies of PS have been plotted at different lithiation stage. Negative values of binding energies ranging from -1.58 eV to -4.55 eV, indicating the favorable adsorption of PS on TiC surface. It is found that the after optimization the structures of PS underwent a huge transformation due to the interaction between PS and TiC surface. Whereas, in the case of NSPC, however, the binding energies are found to be in the range of -0.88 eV to 3.18 eV. Positive binding energies for most of the PS suggest the energetically unfavorable adsorption. In the case of TiC, the binding strength increases with increasing lithiation except for Li2S4 and Li2S6, which exhibits the least EBE value among all the Li2Sx species. The probable reason behind this peculiarity can be the change in phase from dissolved liquid state PS to solid bulk PS.8 In all optimized structures that, mainly lithium is attached to carbon

site whereas, sulfur is anchored by

the titanium site of TiC, which is the clear evidence of electrostatic attraction between electronrich and electron deficient sites of the TiC and PS. 22 ACS Paragon Plus Environment

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Figure 5. Binding energies for Li−S composites at various lithiation stages on TiC and NSPC surface. An ideal PS trapping material should possess prominent surface polarity. In order to visualize the charge polarization of the different order PS as well as TiC, surface electrostatic potentials plots have been investigated methodically in this study. In Figure 6a the electron density (mapped with electrostatic potential ESP) of TiC unit cell and Li2Sx (1 x 8) has been shown. Two distinct regions of the surface electrostatic potential can be observed for both the molecules. The blue and red regions depict the electron deficient and electron rich regions respectively.52,53 Electron enrich site in TiC (carbon atom) acts as an anchoring site to confine the electron deficient sites of PS under the electrostatic attraction. The similar study has been carried out with NSPC as well. Introducing N and S atoms within the carbon hexagonal framework induced surface polarity (Figure S7). But the strength of the positive and negative charge density is much weaker compared to TiC surface.

Figure 6. (a) Structure and surface electrostatic potential of the optimized structure of TiC and various polysulfides Li2Sx (1x8). (b) Charge transfers when (a) Li2S4 and (b) Li2S8 adsorbs on TiC. The charge transfer is the charge difference after and before Li−S cluster is put on TiC

In order to have insight knowledge about the charge transfer mechanism at PS-TiC interface, charge transfer analysis has been carried out with Li2S4 (generated at the mid-lithiation state) 23 ACS Paragon Plus Environment


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adsorbed TiC structure. Net charge transfer Δρ is defined as the total charge difference after and before Li2S4 adsorbs on TiC surface. The numerical expression of Δρ is given below. Δρ = ρ(TiC + Li2S4) − ρ(TiC) − ρ(Li2S4)

(3)

Where, ρ(TiC + Li2S4), ρ(TiC), and ρ(Li2S4) are the charge densities for adsorption system, TiC system, and Li2S4 respectively. Figure 6b shows the selected regions consist of the charge transfer in the adsorbed system. The blue region represents where the electrons are coming from, and the purple represents where the electrons are going. Here, the charge increases between Li and S atoms (purple region) whereas C (labeled as C1, C2) atoms in TiC are acted as electron donor species owing to its electronegativity. Accumulation and depletion of charge in the adsorbed system depict the strong chemical interaction between TiC and Li2S4. For freestanding Li2S4 (Figure 6b(ⅰ)), the sulfur atoms were bonded with the Li atom directly or via another S atom. After introducing TiC, the distance between S atoms and Li atom increases and their bonds disappear. Similarly, Li2S8 adsorbed TiC system has been investigated in order to evaluate the catalytic activity of TiC towards the conversion of Li2S8 to elemental sulfur. In the adsorbed system the charge depletion can be seen between the C-S (S1) and Li-S (Li1-S2, Li2-S3) bond (Figure 6b(ⅱ)) which promotes the utilization of the active material during the cell operation. Conclusions In conclusion, a novel bilayer cathode consists of nitrogen and sulfur co-doped highly porous carbon as sulfur host and TiC nanoparticles as an efficient polysulfide anchoring material has been used for lithium-sulfur battery. Introducing the second layer not only exhibits improved specific capacity and cyclic stability over the single layer cathode also shows excellent capacity retention. Additionally, the superior catalytic activity of TiC towards polysulfide-sulfur redox conversion increase the longevity of the cell. Variation in the interfacial charge transfer during 24 ACS Paragon Plus Environment

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charge-discharge has been evaluated dynamic electrochemical impedance spectra. Further, the working mechanism of polysulfide confinement has been analyzed theoretically by estimating the binding energy between the polysulfide and TiC revealing the polar nature of both. Furthermore, a complete computational analysis reveals the interfacial properties of the adsorbed system. Employing the unique bilayer cathode in lithium-sulfur battery provides a great lead for both experimental as well as theoretical investigation in the future. Acknowledgments The authors would like to thank RCI, Hyderabad for the support through a project and IIT Madras, India for supporting this work. One of the authors thanks the Department of Science and Technology (DST) for the financial support to establish the Nano Functional Materials Technology Centre (NFMTC) through SR/NM/NAT/02−2005 project. One of the authors would like to acknowledge the High-Performance Computing Environment (HPCE) facility of the Indian Institute of Technology Madras. Competing Interests The authors declare that they have no competing interests. Supplementary Information Detailed material characterization (XRD, BET, EDAX, XPS) along with electrochemical measurement details have been included. After cycling morphology analysis using SEM of cycled cells have been provided.



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Polar Bilayer Cathode for Advanced Lithium-Sulfur Battery: Synergy

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