Controllable Pulse Reverse Electrodeposition of Mesoporous

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Controllable Pulse Reverse Electrodeposition of Mesoporous LixMnO2 Nano/Microstructures with Enhanced Electrochemical Performance for Li-ion Storage Sepideh Behboudi-Khiavi, Mehran Javanbakht, Sayed Ahmad Mozaffari, and Mehdi GHAEMI ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b05179 • Publication Date (Web): 24 May 2019 Downloaded from http://pubs.acs.org on May 24, 2019

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ACS Applied Materials & Interfaces

Controllable Pulse Reverse Electrodeposition of Mesoporous LixMnO2 Nano/Microstructures with Enhanced Electrochemical Performance for Li-ion Storage Sepideh Behboudi-Khiavi,

a,b

Mehran Javanbakht,

a,b*

Sayed Ahmad Mozaffari,

b,c

and Mehdi

Ghaemi b,d

a

b

Department of Chemistry, Amirkabir University of Technology, Tehran, Iran, 1591634311. Renewable Energy Research Center, Amirkabir University of Technology, Tehran, Iran,

1591634311. c

Department of Chemical Technologies, Iranian Research Organization for Science and

Technology (IROST), 3313193685. d

Department of Chemistry, Faculty of Science, Golestan University, Gorgan, Iran, 49138-15759.

*Corresponding author. Department of Chemistry, Amirkabir University of Technology, Tehran, Iran. 1591634311. E-mail address: [email protected] (M. Javanbakht). Tel: +982164545806.

Keywords: Pulse Reverse Current, Electrodeposition, LixMnO2, Hierarchical, urchin-like nanostructure, Cathode material, Li-ion battery.

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Abstract Given the ever growing demand of EVs and renewable energies, addressing the poor cyclic stability of lithium manganese dioxides is an urgent challenge. In this study, pulse reverse current as the driving force of a one-pot anodic electrodeposition was exploited to design the physicochemical and electrochemical characteristics of lithium manganese dioxides as cathode materials of Li-ion battery. The pulse reverse parameters including the span of anodic and cathodic current applying (ta and tc) and frequency (fʹ) were systematically modulated to determine the optimized values through the monitoring of the physicochemical properties using XRD, TGA/DSC, FE-SEM, TEM, EDS, Raman spectroscopy, N2 adsorption/desorption isotherms, ICP/OES as well as electrochemical properties using cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge-discharge at different currents. Based on the results, Li0.65MnO2 synthesized using ta = 95 ms, tc = 5 ms, fʹ = 8.33 Hz at the constant magnitude of anodic peak current density of 1 mA dm-2, was determined as the optimized sample. The optimized lithium manganese dioxide rendered the superior electrochemical performance delivering the initial discharge capacity of 283 mAh g-1 counted for 96.4% of the theoretical discharge capacity, preserving 88.3% of this capacity after 300 cycles at 0.1 C and in the meantime was able to release the discharge capacity of 115 mAh g-1 even after cycling at the higher current of 10 C. The superior electrochemical behavior of Li0.65MnO2 was attributed to the exclusive hierarchical urchin-like morphology as well as mesoporous nano/microstructures having notably high BET surface area of 320.12 m2 g-1 alongside mixed phase α/γ structure owing to the larger 2 × 2 tunnels which offer more facile Li+ diffusion.

1. Introduction 1 ACS Paragon Plus Environment

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Li-ion batteries owing to the tremendous advantages like long cycle life, light weight and low cost are being considered as an inevitable prerequisite of renewable power sources as well as electric vehicles deployment. However, providing cost-effective as well as environmentally friendly cathode materials without compromising the electrochemical performance, has still remained as a main challenge of the academic and industrial scientists 1,2. Lithium manganese dioxide cathode materials due to the low cost, abundant sources, environmental compatibility, higher safety in overcharge conditions, as well as higher discharge capacity are regarded as an attractive alternative to the conventional cathode materials such as LiCoO2 3–5. Numerous attempts have been made to develop a straightforward as well as economic synthesis approach to accomplish LiMnO2 with sophisticated electrochemical properties 6–10. The conventional solid-state, reverse micro-emulsion preparation, and conventional as well as microwave-assisted hydrothermal have been employed as the most common synthesis strategies of LiMnO2 7,10. It is well known, synthesis method could substantially determine the morphology, crystal size and structure of LiMnO2 which consequently affect the electrochemical properties

11.

In spite of the prosperity to produce LiMnO2 cathode

material, these methods are suffering from serious shortcoming such as a harsh reaction media (high temperature and pressure) 12, complex equipment and also multi-step procedures to prepare LixMnO2 from a variety of precursors such as Mn2O3, MnCO3, NaMnO2, and MnOOH 13. In this regard, the electrochemical synthesis as an alternative approach offering the simple, scalable and most importantly controllable procedure which would be able to address the drawbacks of the conventional synthesis methods of LiMnO2 14. Among the various electrodeposition techniques, reverse pulse electrodeposition (RPED) is a versatile and novel strategy to fabricate tailor-made functional nanostructures of metal, metal oxides and alloys with serving the capability of intentional adjustment of physicochemical and structural properties 15,16. Theoretically, RPED is a bipolar electrodeposition process where the direction of current is continuously changed as the anodic and cathodic periods 17. The schematic of reverse pulse current which has been employed to conduct the electrodepostioning of LixMnO2 is represented in Scheme 1. Referring to Scheme 1, ta, tc, tr, Ia, Ic and Īa stand for time of applying anodic pulse, time of applying cathodic pulse, current off-time, anodic peak current density, cathodic peak current density and average anodic current density, respectively. The reverse pulse current is mainly characterized with ta, tc, Ia and Ic as the independent variables and also duty cycle (θ) and frequency (fʹ) as the dependent variables. 2 ACS Paragon Plus Environment


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Scheme 1. Schematic illustration of applied pulse reverse current in the electrodeposition procedure of LixMnO2

The relationship of reverse pulse parameters may be expressed as the following Equations 1-3 17:

θa = Īa = f =

ta ta + tc + tr

(1)

× 100

Iata ― Ictc

(2)

ta + tc + tr 1

(3)

ta + tc + tr

As it has been previously demonstrated, modulating RPED parameters would profoundly affect physicochemical, structural and chemical properties of the electrodeposit as a consequence of interfering in the adsorption-desorption phenomena which leads to an altered thickness of the diffusion layer at the electrode-electrolyte interface

18.

Considering the immense potential of

PRED technique, a great deal of research have been devoted to achieve functional materials with superior structural and physicochemical behavior by exploiting this approach 19–21. Raj et al. have reported the in situ growth of spike-piece-structured Ni/Ni(OH)2 interlayer nano-plates by controlled reverse pulse modulated electrochemical approach which render a good capacitive 3 ACS Paragon Plus Environment

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behavior 22. Shinde et al. examined how modulating of RPED parameters influences the evolution of nanostructured α-Fe2O3 morphology and the resulting photoelectrochemical performance

23.

Pavithra et al. reported prosperity of the PRED approach to synthesize Cu/graphene composite foils via applying optimized values of the pulse reverse parameters which possess a high hardness of ~ 2.5 GPa without any compromising of electrical conductivity

24.

Lee et al. obtained fine

nanorod morphology of Mn2O3 with a high electrical conductivity via the RPED synthesis technique from manganese acetate precursor which render the excellent capacity of 448 F g-1 at 10 mV s-1 15. Herein, pursuing our previous work

25,26,

current

hierarchical

could

be

navigated

to

achieve

we have demonstrated how pulse reverse and

mesoporous

α/γ-LiMnO2

nano/microstructures with tuned chemical composition and superior electrochemical performance as a cathode material of Li-ion battery. To the best of our knowledge, though previous works have proved PRED as a highly susceptible method to obtain functional nanostructures with unique properties, there is no research work investigating this method to achieve engineered active materials of Li-ion batteries with a great potential of controlling the electrochemical properties through controlled modulating of PRED parameters.

2. Experimental 2.1.

Synthesis of LixMnO2

Pulse reverse current electrodeposition was carried out in a conventional 3-compartment Pyrex glass cell equipped with a thermostatic bath control apparatus. The cell temperature was maintained constant at 80 ºC during all the experiments. The LixMnO2 samples have been electrodeposited from the additive free bath containing aqueous acidic solution of Li2SO4 and MnSO4 with optimized Li+/Mn2+ ratio of 10 according to our previous research 25, which maintained constant throughout the electrodeposition reaction. The pH of the electrodeposition bath was adjusted to 6.0 throughout the synthesis by adding the appropriate amounts of H2SO4 as needed. The stainless steel sheet with 5 × 10 cm2 dimensions as a working electrode was located between a couple of lead sheets with the same dimensions as the counter electrodes. The distance of working electrode from each of the counter electrodes was fixed at 1.0 cm. The electrodeposition of LixMnO2 samples was carried out using pulse reverse current 4 ACS Paragon Plus Environment


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generator connected to a DC source under the constant peak current density of 1.0 mA dm-2. Also, the value of tr has been kept constant at 20 ms in the synthesis procedures of all the samples. Two series of samples have been synthesized with a systematic varying of the pulse reverse current parameters to comprehensive study of the individual pulse reverse current parameters as well as concurrent influence of those on the targeted properties of the LMO-R samples. Because introducing of tc leads to a stripping periods of the deposit, ta and tc should be adjusted within a given range to make whole process in favor of the anodic electrodeposition of lithium manganese dioxide on the working electrode. To examine the effect of ta, the first group of samples have been synthesized with varying the value of ta within the range of 15-95 ms at a constant magnitude of tc = 5 ms. Meanwhile, to study the effect of tc, The second group of samples have been synthesized with varying tc within the range of 5-15 ms at a constant value of ta = 20 ms. The elaborated conditions of each samples have been summarized in Table 1. Table 1. Detailed varied values of pulse reverse current parameters used in the electrodeposition of LixMnO2 samples. Sample

Formula

ta (ms)

tc (ms)

Li0.22MnO2

15

5

25.00 37.50

0.250

64.29%LMO-R Li0.36MnO2

45

5

14.29 64.29

0.570


95

5

8.33

79.17

0.750


20

5

22.22 44.44

0.333

40%LMO-R

Li0.52MnO2

20

10

20.00 40.00

0.200

36.36LMO-R

Li0.58MnO2

20

15

18.18 36.36

0.091

37.5%LMO-R

2.2.

fʹ (Hz)

θ (%)

Īa (mA dm-2)

Structural Characterization

The crystal structure of samples was characterized by XRD analysis using a Philips X՛ Pert MPD (Holland) diffractometer equipped with a Co kα radiation source (λ = 1.78897 Å) and operated at 40 kV and 30 mA. Raman spectra were recorded within the range of 400 – 1800 cm-1 via Bruker 5 ACS Paragon Plus Environment

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dispersive Raman microscope (SENTERRA, Germany, wavenuber of laser = 785 nm) at room temperature. Thermogravimetrical analysis and differential scanning calorimetery (TG/DSC) tests were conducted in the temperature range from 25 ºC to 600 ºC under the ambient atmosphere at heating rate of 10 ºC min-1 via METTLER TOLEDO DSC instrument, USA. The scan range was from 2θ = 10º to 80º with a step size of 0.02 º/s. Morphologies of samples were examined with field-emission scanning electron microscopy (FESEM, VEGA/TESCAN, Czech Republic) which also used to record EDX spectra. Transmission electron microscopy (TEM) images were prepared using Philips EM208S with an acceleration voltage of 100 kV. The Li+ and total manganese contents of prepared samples were determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES) (Varian VISTA-PRO). The tap density was measured according to the standard test method to determine tap density of metallic powders and compounds (ASTM B52706). The Brunauer-Emmett-Teller (BET) specific surface area was determined from nitrogen adsorption-desorption isotherm using Quantachrome Nova 1000 U.S.A. The pore size distribution was figured out using adsorption curve by Barret-Joyner-Halenda (BJH) method. 2.3.

Electrochemical characterization

The cathodes were prepared according to the following procedure: slurries of the active material powder (85%), graphite (10%) and polyvinylidene fluoride (PVDF) as a binder, (5%) were prepared in 1-methyl 2-pyrrolidone (NMP) solvent and coated on aluminum foil substrates. Subsequently, the electrodes were pressed and dried at 110 ºC for 12 hours under vacuum. Cointype cells were assembled in an argon-filled glove-box (with less than 1 ppm O2 and H2O levels) using lithium foil as anode separated by Celgard 2400 polypropylene porous film from the cathode. A 1.0 M solution of LiClO4 (Sigma-Aldrich with 99.99 purity) in propylene carbonate (PC) was used as the electrolyte. Galvanostatic charge-discharge performance was examined within a voltage window of 1.5 – 3.5 (vs. Li+/Li) using battery-tester instrument (4 channel KIMIASTAT). The cyclic voltammetry (CV) and the electrochemical impedance spectroscopy (EIS) were studied by Galvanostat/Potentiostat Autolab (PGSTAT302N). The cyclic voltammograms were recorded with lithium metal as both counter and reference electrode at a scan rate of 0.1 mV s-1 within the voltage range of 2.0 – 4.5 V (vs. Li+/Li). The EIS spectra were recorded from 100 kHz to 1 mHz with an amplitude of 5 mV. All electrochemical characterizations were performed at the ambient 6 ACS Paragon Plus Environment


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temperature. The reported experimental impedance data were analyzed using the NOVA 1.10.1.9 software.

3. Results and discussion 3.1. Mechanism of LixMnO2 electrodeposition using PRED method Exploiting PRED method leads to a total different electrocrystallization mechanism of LixMnO2 with disturbing the diffusion layer (σp) as well as adsorption-desorption of species at the electrodeelectrolyte interface. During anodic period of PRED, depletion of Mn2+ and Li+ species at the vicinity of the anode happens as a result of progressing anodic electrodeposition. Imposing tr, could substantially affect the ultimate physicochemical and chemical properties of deposit. During tr, replenishing of Mn2+ and Li+ species at the vicinity of anode happens as a result of diffusion which accompanied by simultaneous dissolving of MnOOH passive layers and discharging of the electrical double layer

27.

Also, imposing tr could contribute to a higher crystallinity and lower

residual stress of the synthesized LixMnO2 through triggering the recrystallization of electrdeposit as a result of Ostwald ripening

11,28.

Moreover, imposing tc immediately after tr, could make a

predominant difference in the electrodeposition mechanism through interfering with the growth mechanism of LixMnO2 crystallites and grain boundaries which may lead to the exclusive and unique morphologies. Thereof, a partial stripping of the deposit during tc, provides the possibility of constructing exclusive morphologies of LixMnO2 with controlled porosity by modulating the values of tc imposed on the anode 15,21,23. Hypothetically, exerting a given values of tc on the anode would stimulate the migration of lithium ions toward the working electrode with negative polarization leading to the higher Li+ contents in LixMnO2 samples 29,30. Comparing Li+ contents of LMO-R samples (summarized in Table 2) to those of LMO samples synthesized using pulse current electrodeposition reported in our previous work

26,

has definitely evidenced this

assumption. The mechanism of LixMnO2 electrodeposition using PRED has been schematically illustrated in Scheme 2.

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Scheme 2. Schematic mechanism of LixMnO2 electrodeposition using pulse reverse current.

According to Fig. 1a, the gradual increasing of fʹ at the constant values of tr and Ic = Ia, results in the decreased values of inserted Li+ within MnO2 lattice of LMO-R samples except in the case of 36.36%LMO-R and 40%LMO-R. As whole samples have been synthesized under constant values of tr, expectedly applying higher values of tc (i.e. 5 ms < tc

Controllable Pulse Reverse Electrodeposition of Mesoporous

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