Diusion mechanism and electrochemical investigation of 1T phase Al-MoS 2rGO nano-composite as a high-performance anode for sodium-ion batteries Manish Kr. Singh1Jayashree Pati1Deepak Seth2Jagdees Prasad1Manish

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Diﬀusion mechanism and electrochemical investigation of 1T phase Al-MoS2@rGO

nano-composite as a high-performance anode for sodium-ion batteries

Manish Kr. Singh,1, ∗Jayashree Pati,1, ∗Deepak Seth,2Jagdees Prasad,1Manish

Agarwal,3M. Ali Haider,2Jeng-Kuei Chang,4and Rajendra S. Dhaka1, †

1Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi-110016, India

2Renewable Energy and Chemicals Group, Department of Chemical Engineering,

Indian Institute of Technology Delhi, Hauz Khas, New Delhi-110016, India

3Computer Service Center, Indian Institute of Technology Delhi, Hauz Khas, New Delhi- 110016 India

4Department of Materials Science and Engineering, National Yang Ming Chiao Tung University, Hsinchu 30010 Taiwan

(Dated: October 14, 2022)

We report the electrochemical investigation of 5% Al doped MoS2@rGO composite as a high-

performance anode for sodium (Na)-ion batteries. The x-ray diﬀraction (XRD), Raman spectroscopy

and high-resolution transmission electron microscopy characterizations reveal that the Al doping

increase the interlayer spacing of (002) plane of MoS2nanosheets and form a stable 1T phase. The

galvanostatic charge-discharge measurements show the speciﬁc capacity stable around 450, 400,

350, 300 and 200 mAhg−1at current densities of 0.05, 0.1, 0.3, 0.5 and 1 Ag−1, respectively. Also,

we observe the capacity retentions of 86% and 66% at 0.1 and 0.3 Ag−1, respectively, over 200

cycles with a consistent Coulombic eﬃciency of nearly 100%. The cyclic voltammetry, galvanostatic

intermittent titration technique, and electrochemical impedance spectroscopy are used to ﬁnd the

kinetic behavior and the obtained value of diﬀusion coeﬃcient falls in the range of 10–10 to 10–12

cm2s–1. Intriguingly, the in-situ EIS also explains the electrochemical kinetics of the electrode at

diﬀerent charge-discharge states with the variation of charge transfer resistance. Moreover, the post

cycling investigation using ex-situ XRD and photoemission spectroscopy indicate the coexistence of

1T/2H phase and ﬁeld-emission scanning electron microscopy conﬁrm the stable morphology after

500 cycles. Also, the Na-ion transport properties are calculated for 1T Al–MoS2@rGO interface and

Al–MoS2–MoS2interlayer host structure by theoretical calculations using density functional theory.

Key words: sodium-ion batteries; anode materials; Al–MoS2@rGO; electrochemical performance.

I. INTRODUCTION

The gradual depletion of fossil fuels and emission of

greenhouse gases due to their combustion, pave the way

toward renewable energy resources like solar and wind

[1–3]. However, the conversion of energy through these

resources depend on several environmental factors and

not ample to meet the energy demands when required.

Therefore, the present research is geared towards energy

∗These authors contributed equally to this work

†rsdhaka@physics.iitd.ac.in

storage and conversion devices for the restoration of re-

newable energy. As we know, the lithium-ion batteries

(LIBs) are one of the most eﬃcient technology among

various energy storage devices on the subject of energy

density and power density [4–6]. The LIBs are virtually

the heart of all portable devices such as laptops, mobiles,

as well as the electric vehicles [7–10]. However, due to the

increasing demand with time, and the uneven geograph-

ical and limited distribution of lithium across the world,

the LIBs are becoming more expansive and not aﬀord-

able for common society [11, 12]. In this context, sodium

being widely and evenly distributed throughout as well

arXiv:2210.06735v1 [cond-mat.mtrl-sci] 13 Oct 2022

as its similar electrochemistry make the sodium-ion bat-

teries (SIBs) potential candidates for cost-eﬀective and

complementing the LIBs [13, 14]. However, it is not that

easy as there are great challenges, mainly coming from

the larger size of sodium (1.02 ˚

A), which need to be re-

solved in order to design eﬃcient electrode materials with

appealing characteristics for energy storage applications

[15–18]. Among the bottlenecks, searching the anode ma-

terials with high capacity, better rate capability, and ex-

cellent cycle life are major challenges, because graphite

cannot be used due to its reactivity/thermodynamic un-

stability with sodium [19, 20].

At present, the SIB anode materials based on alloying

and conversion are widely used because of their high the-

oretical capacity [21, 22]. On the other hand, the inser-

tion based materials provide excellent cycle life, but with

a low speciﬁc capacity in SIBs [23]. The sodium-ion up-

take is limited for insertion based compounds due to their

rigid frameworks [24]. However, the metals and metal-

loids based materials have the ability of multiple sodium-

ion uptakes per single atom resulting in capacities rang-

ing from 300 to 2000 mAhg−1with operational voltages

below 1 V vs Na/Na+as anode in SIBs. Mostly, group-15

(pnictogen), group-16 (chalcogen), and transition metals

are used for the production of alloying and conversion

based complexes [14]. Among these, the transition metal

dichalcogenides (TMDs) and particularly sulﬁdes are of

much importance because of their low cost, high capacity,

and environment friendly nature. Furthermore, the low

activation energy between transition metal and sulfur in

TMDs facilitates the Na-ion migration during charging-

discharging, while its high theoretical capacity oﬀers high

energy density for SIBs [25]. Particularly, the molyb-

denum disulﬁde (MoS2) with a two-dimensional open

framework has attracted considerable attention among

TMDs due to its structural ﬂexibility and high theoretical

capacity as an anode material for both LIBs and SIBs [26]

as well as Zn-ion batteries [27, 28]. The MoS2is found in

three phases namely 1T, 2H, and 3R where the 1T phase

shows metallic nature with better ion and electron trans-

port as compared to 3R and 2H phases because of two

reasons [29–36]: (i) it has a distorted octahedral coordi-

nation structure which results in high electronic conduc-

tivity as compared to 2H and 3R phases, (ii) another ad-

vantage is its high hydrophilic nature which enables aﬃn-

ity of electrolytes [37, 38]. In addition to these factors,

its revealing electrochemical active sites and large inter-

layer spacing (0.93 nm) with wide and fast ion-diﬀusion

channels make the 1T phase more suitable for Na+ion

storage [33].

In this line, one of the most eﬀective ways to enhance

the electrochemical activity of MoS2as an anode in SIBs,

is to fabricate nano-composite of MoS2particles through

diﬀerent synthesis routes to minimize the diﬀusion length

of Na-ion during sodiation/de-sodiation [39, 40]. Chen

et al. developed a scalable chemical vapor deposition

method to prepare MoS2deposited electrospun carbon

nanoﬁber hybrid to enhance the ionic conductivity, that

provides large contact area for electrolyte and prevent

the MoS2nanosheets agglomeration. This anode mate-

rial exhibited a reversible capacity of 198 mAhg−1after

500 cycles at a current density of 1 Ag−1[41]. In order to

study the eﬀect of conductive carbon matrices, Sahu et

al. synthesized modiﬁed 3-D framework of MoS2@rGO

hybrid through hydrothermal route, which was observed

to deliver high discharge capacities of 588 mAhg−1and

501 mAhg−1at current densities of 100 and 500 mAg−1

with capacity retention of 98% and 92.3% after 80 and

250 cycles, respectively [42]. Here, the rGO nano-sheets

act as backbone to hold the MoS2nano-plates during

cracking of the material owing to its high surface area and

mechanical strength [42]. Also, one way to enhance the

electronic conductivity and to adhere the volume expan-

sion, is to modify MoS2with diﬀerent conductive carbon

matrices like rGO, CNT, carbon-doped with nitrogen and

sulfur, etc. Additionally, the hetero-atom doping is also

another eﬃcient approach for improved structural stabil-

ity and boosting volume expansion in MoS2[36, 43, 44].

Li et al. proposed the improved cycling stability of Co-

doped 1T MoS2than the pristine MoS2, where the intro-

duction of Co mitigates the volume expansion up to 52%

during cycling [34]. Interestingly, a moderate Sn-doped

1T–2H MoS2anode was reported by Zeng et al., which

provides a signiﬁcant rate capability of 167 mAhg−1at

a current rate of 15 Ag−1with cycling retention of 320

mAhg−1after 500 cycles at 1 Ag−1[45]. Moreover, using

ﬁrst-principles calculations the sodium-ion intercalation

and diﬀusion mechanism are reported in diﬀerent phases

of MoS2/graphene layer [46].

Here, we have synthesized MoS2, MoS2@rGO, CNT-

MoS2@rGO, and 5% Al doped MoS2@rGO composites

by a simple hydrothermal method. We found that the

doping of Al ions enhance the interlayer spacing of (002)

crystal plane of pure MoS2@rGO nano-sheets and stabi-

lize in a stable 1T phase, as conﬁrmed by x-ray diﬀrac-

tion (XRD), Raman spectroscopy and transmission elec-

tron microscopy (TEM) measurements. Interestingly, we

observe signiﬁcant enhancement in the electrochemical

performance of Al doped MoS2@rGO, which exhibits the

discharge speciﬁc capacity of around 400 mAhg−1at a

current density of 100 mAg−1, which found to be highly

stable for more than 200 cycles even at faster charge-

discharge rates with about 100% Coulombic eﬃciency.

The diﬀusive behavior in the electrode material is also

investigated through detailed analysis of CV, EIS, and

GITT data, and the diﬀusion coeﬃcient was found to

be in the range of 10–10 to 10–12 cm2s–1. The post-

cycling analysis performed after 500 cycles at a current

density of 500 mAg−1through room temperature XRD

and FE-SEM measurements conﬁrm the structural evo-

lution and morphological stability of the anode material.

The photoemission results indicate the 1T phase of pris-

tine sample and coexistence of 1T/2H hetero-structure

phase for the cycled anode material. We have also em-

ployed density functional theory to understand the Na-

ion migration in the inter-layered structures of 1T phase

Al–MoS2@rGO.

II. METHODS

2.1.1 Synthesis of graphite oxide (GO): The GO

nano-sheets were synthesized by chemical oxidation of

bulk graphite powder using a slightly modiﬁed Stauden-

maier process [47]. In this typical procedure, 180 ml of

concentrated H2SO4was gradually combined with 90 ml

of HNO3in a 500 ml beaker while stirring with an ice

bath. We added 10 gms of graphite powder slowly into

the prepared solution with continuous stirring for 30 min-

utes. Then, 110 gms of KClO3was added for an extended

time of 2–2.5 hrs. The ice bath was withdrawn, and the

solution was left to stir at ambient temperature for the

next 5 days. The obtained GO solution was ﬁrst washed

7-8 times with DI water and then with 10% solution of

hydrochloric acid to remove (SO4)2−ions and other im-

purities. The resulting material was centrifuged 6-7 times

in puriﬁed DI water. The obtained GO powder was ﬁ-

nally vacuum dried overnight at 80◦C.

2.1.2 Synthesis of MoS2, MoS2@rGO, CNT-

MoS2@rGO: The pure MoS2, composite of MoS2@rGO

and CNT-MoS2@rGO were prepared by a simple one

step hydrothermal method. For the preparation of pure

MoS2nano-sheets, 30 mmol of thiourea [(NH2)2CS]

and 1 mmol of ammonium molybdate tetrahydrate

[(NH4)6Mo7O24·4H2O] were homogeneously dispersed

for 1 hr under the constant magnetic stirring in 40 ml of

DI water. The obtained black colored homogeneous solu-

tion was transferred into a 100 mL Teﬂon-lined stainless

steel autoclave and heated at a constant temperature of

220◦C for 24 hrs. It was followed by cooling the sys-

tem to room temperature naturally so that the obtained

black powder can be washed with absolute ethanol and

DI water several times. Then, the obtained precipitate

was dried in a vacuum oven at 65◦C for 12 hrs and the re-

sultant powder of MoS2was obtained. For the synthesis

of MoS2@rGO and CNT-MoS2@rGO composites, we use

140 mg of GO powder and 100 mg of CNT, which were

dispersed homogeneously in 40 ml of DI water with the

help of sonication for 1.5 hrs. After that, all the other

steps were similar to the formation of pure MoS2.

2.1.3 Synthesis of Al-doped MoS2@rGO: The

Al–MoS2@rGO nano-hybrids were synthesized using a

facile hydrothermal method. First we mix 140 mg (3.5

mg/mL) of GO powder and 12.70 mg of AlCl3.6H2O in

40 mL DI water under steady ultra-sonication for 2 hrs

at room temperature to make a homogenous suspended

solution. After that, 30 mmol (NH2)2CS and 1 mmol

(NH4)6Mo7O24·4H2O were slowly added to the above

formed suspension along with constant stirring for 1 hr.

A homogenous dark black solution was obtained, which

then placed into a Teﬂon-lined stainless-steel autoclave

(100 mL). A box furnace was used to heat the autoclave

at 220◦C for 24 hrs, and then cooled to room tempera-

ture when the reaction time was completed. To remove

the residual ions, the obtained black precipitate was cen-

trifuged at 3000 rpm for at least 4-5 times using DI water

and absolute ethanol. Finally, a black powder of Al–

MoS2@rGO was obtained by drying the washed product

in a vacuum oven at 65◦C for 12 hrs.

2.2 Structural and physical characterizations:

To investigate the crystallographic structure of the ac-

tive material, we perform the x-ray diﬀraction (XRD)

measurements (Panalytical Xpert 3) with CuKαradia-

tion (1.5406˚

A) in 2θrange of 5◦to 80◦. We use high-

resolution transmission electron microscopy (HR-TEM)

with a Tecnai G2-20 system, and ﬁeld emission scan-

ning electron microscope (FE-SEM) and energy disper-

sive x-ray (EDX) using the Quanta 200 FEG system to

study the morphology, structure, elemental composition,

and the distribution of elements across the sample. The

Raman spectra were recorded with 532 nm wavelength

laser using a Renishaw inVia confocal microscope having

2400 lines/mm grating and 10mW power on the sample.

The x-ray photoelectron spectroscopy (XPS) with Al Kα

source (1486.6 eV), from Thermo Fisher Scientiﬁc, was

used to investigate the core-level binding energy (BE) of

diﬀerent elements present in the sample before and after

cycling.

2.3 Coin-cell fabrication: The uniform slurry was

prepared by taking 5%Al–MoS2@rGO as an active ma-

terial, CNT as conductive carbon, and polyvinylchloride

(PVDF) binder in N-methylpyrrolidone (NMP) solvent

in a weight ratio of 8:1:1. The slurry was then coated

on a copper foil using a doctor blade method with active

material mass loading about 1 mg cm−2followed by vac-

uum drying at 120◦C overnight to evaporate the excess

solvent and moisture. The electrodes were cut and dried

in vacuum before inserting them into the glove box (Uni-

Lab Pro SP from MBraun). The CR2032 coin cells were

fabricated in Ar ﬁlled glove box under a controlled level of

O2and H2O (less than 0.1 ppm). The Na foil was used as

the counter and reference electrode.The electrolyte used

was 1 M NaPF6in a mixture of ethylene carbonate (EC)

and diethyl carbonate (DEC) 1:1 (vol %) with 5 wt %

ﬂuoroethylene carbonate (FEC) .

2.4 Electrochemical measurements: The galvano-

static charge-discharge (GCD) proﬁles were obtained by

using a Neware battery analyzer BTS400 in voltage win-

dow 0.005–2.5 V (vs Na+/Na) at diﬀerent current den-

sities. The cyclic voltammetry (CV) was conducted at

diﬀerent scan rates using a Biologic VMP-3 model in

the same voltage range as in GCD measurements. The

electrochemical impedance spectroscopy (EIS) measure-

ments were performed using a Biologic VMP-3 model in

the frequency range of 10 mHz to 100 kHz, and the ac

voltage amplitude was set to 10 mV at the open circuit

voltage (OCV) state of the cells.

2.5 Theoretical calculations:

A model MoS2@rGO interface is constructed with a

monolayer of MoS2in a supercell size of 5×4×1 and 20

formula units, which are stacked with a 5×5×1 supercell

of rGO layer. On geometry optimization of the structure,

a sodium atom was introduced at one of the stable sites.

Similarly, 5% Al doped MoS2–MoS2interlayer structure

is constructed. The transport of sodium-ion in these host

materials interlayer is simulated using plane wave basis

set code of DFT as implemented in vienna ab initio sim-

ulation package (VASP-6.2) [48]. A plane wave cut-oﬀ

energy of 500 eV is considered for the basis set expan-

sion. The electron–ion core interactions are described by

gradient-corrected projector augmented wave (PAW) [49]

pseudo potential using Perdew-Burke-Ernzeerhof (PBE)

exchange-correlation functional [50], to solve the Kohn-

Sham equations, which estimate the ground state energy

of the structures. A 2×2×2 gamma-centred k-point mesh

is used for Brillouin zone integration. The convergence

criteria for energy and force are kept at 10−6eV and 0.05

eV/˚

A, respectively, for structural optimization using the

conjugate gradient algorithm. To determine the mini-

mum energy path (MEP) for sodium-ion migration and

calculation of energy barriers, the Cl-NEB (climbing im-

age nudge elastic band) method is implemented [51, 52].

The optimized structure of Na-Al-MoS2@rGO is taken

as the initial state (IS) and corresponding structure after

sodium-ion migration to the next available site is taken

as the ﬁnal state (FS) respectively, for transition state

(TS) calculations. We use eight linearly interpolated im-

ages along the MEP between IS and FS. The convergence

criteria considered for TS calculations are 10−5eV and

0.05 eV/˚

A for energy and forces respectively. The activa-

tion energy (Eact) for sodium-ion migration is calculated

as the diﬀerence in energy of the TS and IS.

III. RESULTS AND DISCUSSION

3.1 Structural/morphological characterization:

First the crystal structure of as prepared MoS2,

MoS2@rGO, CNT-MoS2@rGO, and Al–MoS2@rGO

composites is investigated using the XRD measurements

at room temperature. The diﬀraction patterns of MoS2

MoS2@rGO and CNT-MoS2@rGO are appeared to be

very similar to each other having four major peaks lo-

cated at about 2θ= 14.14◦, 33.30◦, 39.34◦, and 59.06◦,

which are indexed to the lattice planes (002), (100),

(103), and (110) of the crystalline 2H phase of MoS2,

respectively [Fig. 1(a)]. The peak corresponding to

(002) with an interlayer spacing of about 6.3 ˚

A signiﬁes

the crystalline multilayers of pure MoS2with hexagonal

phase and the peak (100) depicts the stacking of Mo–S

edges in vertical planes as well as the number of S active

ions in the edge, as shown in Fig. 1(a). More importantly,

the Al–MoS2@rGO composite exhibits a new second or-

der diﬀraction peak (004) located at about 2θ= 17.06◦

and the (002) peaks shifted to lower 2θ= 9.1◦value

where the calculated dspacing found to be 5.2 and 9.3

A, respectively. This conﬁrms the transformation from

2H phase to 1T phase with Al doping in MoS2@rGO

composite. This new phase has an increased interlayer

spacing upon the intercalation of Al and NH3/NH4+

ions released as by-products of thiourea used as a re-

ductant in the hydrothermal reaction [53–55]. Notably,

the negatively charged surface of GO inhibits the ap-

proaching process of MoO42−precursors, which prevents

the nucleation of MoS2with proper spacing between in-

terlayers [42]. However, in the present case of Al doped

MoS2@rGO, the modiﬁcation through Al3+ dopant in-

creases the electrostatic interaction between MoO42−and

negatively charged GO surfaces. This phenomenon accel-

erates the MoS2nucleation and provides large space for

NH4+ions occupation at the proper sites, which increases

the interlayer distance along (002) direction [56]. For ex-

ample, the presence of NH4+ions between the layers is

evident from the increment in the interlayer distance by

about ∆d=3 ˚

A, which is comparable to the size of NH4+

ions (3.75 ˚

A) [56]. Additionally, no peaks corresponding

to any impurity were detected, which further indicating

the phase purity of the composites. We successfully pre-

pared 1T phase by doping 5% Al in MoS2@rGO com-

posite having d= 9.3 ˚

A, which is considered to be a

potential anode for sodium-ion batteries [57, 58]. The

Raman measurement are performed to further explore

the structure of all these samples, as shown in Fig. 1(b).

The spectra of MoS2, MoS2@rGO, and CNT-MoS2@rGO

display intense E2gand A1gpeaks at about 380 and 405

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Diusionmechanismandelectrochemicalinvestigationof1TphaseAl-MoS2@rGOnano-compositeasahigh-performanceanodeforsodium-ionbatteriesManishKr.Singh,1,JayashreePati,1,DeepakSeth,2JagdeesPrasad,1ManishAgarwal,3M.AliHaider,2Jeng-KueiChang,4andRajendraS.Dhaka1,y1DepartmentofPhysics,IndianInstituteofTechnol...

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Diusion mechanism and electrochemical investigation of 1T phase Al-MoS 2rGO nano-composite as a high-performance anode for sodium-ion batteries Manish Kr. Singh1Jayashree Pati1Deepak Seth2Jagdees Prasad1Manish.pdf

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Diusion mechanism and electrochemical investigation of 1T phase Al-MoS 2rGO nano-composite as a high-performance anode for sodium-ion batteries Manish Kr. Singh1Jayashree Pati1Deepak Seth2Jagdees Prasad1Manish

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