Cost effective urea combustion derived mesoporous-Li₂MnSiO₄ as a novel material for supercapacitors

Abstract

Mesoporous-Li₂MnSiO₄ (LMS) having a surface area of 35 (±2) m² g⁻¹ is produced using a very simple, fast and cost effective urea combustion method for the first time and characterized by X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy and surface area analysis. Li₂MnSiO₄ is crystallized in the orthorhombic phase with space group – Pmn2₁. The lattice parameters: a = 6.317(10) Å, b = 5.385(9) Å, c = 4.988(8) Å are calculated by the Rietveld method. The urea combustion derived LMS is found to be thermally stable till 600 °C in air and thereafter disintegrates into Li₂SiO₃ (orthorhombic) and MnO₂ (cubic). The presence of a homogeneous mesoporous (<20 nm) framework with a high surface area shows orthorhombic Li₂MnSiO₄ to be promising as a novel supercapacitor electrode as it delivers a specific capacitance of 175 (±5) F g⁻¹ at 3 mV s⁻¹ in 2 M KOH solution. The supercapacitive performance was further examined by galvanostatic charge–discharge cycling and complimented by electrochemical impedance spectroscopy.

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Introduction

Due to the limited availability of fossil fuels and global environmental problems associated with pollution, utilization of renewable energy resources has received considerable attention.^1–4 The renewable energy sources such as solar, wind and hydro are intermittent power sources and cannot supply power whenever is required. Thus, energy storage devices need to be coupled with renewable energy resources to mitigate the above problem. In this context, research communities are rigorously engaged in developing energy storage devices to meet the requirement of futuristic high end applications (space craft, automobiles and power backups). Electrochemical capacitors have received considerable attention as energy storage devices due to their unique high power capability as compared to batteries and conventional capacitors.^1–3 Based on an energy storage mechanism, supercapacitors are divided mainly into two types viz. redox based capacitors and electrochemical double layer capacitors (EDLCs).^1–6 Transition metal oxides are widely investigated as active material for former class whereas carbon based materials are known to be used in EDLC. None of individual class of supercapacitor could meet the requirement of high end application where both high power as well as high energy density is required. To accomplish this, hybrid supercapacitor (HSC) is proposed to be a viable device which may deliver the power as of supercapacitor and energy density identical to batteries.^2,3,5 Currently, HSCs are made of battery-type electrode (mainly Li containing material) and a supercapacitor-type electrode.^2–6 Both the electrodes contribute to high and improved performance of HSC, therefore extensive research is being carried out to develop both the electrodes. For instance, a number of Li containing materials, LiTi₂(PO₄)₃,⁷ LiCoO₂,^8,9 LiMn₂O₄,^9,10 LiCo_1/3Ni_1/3Mn_1/3O₂,⁹ LiCoPO₄ (ref. 11) and Li_1.2(Mn_0.32Ni_0.32Fe_0.16)O₂,¹² have been examined as battery type materials. Following the same trend, Li₂MnSiO₄ containing 2 moles of Li is considered as a viable and prospective electrode for HSC and investigated as battery type material.^13–18 Further, it is structurally stable due to strong Si–O bonding, inexpensive and environmental benign. However, supercapacitive properties of LMS have not been carried out yet which is important to study prior to investigate the performance as a HSC electrode. Since mismatching in mass loading of two different electrodes used in HSC causes the ambiguity in rate performance estimation of HSC.^19,20

From literature, it is noticed that supercapacitive performance of any material extensively depends on its morphology where surface area, pore size and its distribution play an important role.^1–3 Hence, significant research is to be focused on synthesis of LMS by inexpensive, less time consuming and scalable method which could provide a material of appropriate morphology beneficial for improved supercapacitive performance. Thus, in the present work, mesoporous LMS having high surface area of 35 (±2) m² g⁻¹ is prepared by easy and cost effective urea combustion method for the first time and examined by XRD, TGA, FESEM, TEM and FTIR. LMS is found to be thermally stable upto 600 °C in air and afterward it disintegrate into Li₂SiO₃ (orthorhombic) and MnO₂ (cubic). Further, electrochemical characterization of LMS is carried out to examine its novel supercapacitive properties. These properties are found to be encouraging since a high value of specific capacitance (175 (±5) F g⁻¹) comparable to commercially available activated carbon justifies its suitability as supercapacitor type material of HSC. This improved performance of LMS is mainly attributed to mesoporous nano-LMS with high surface area which facilitates the ion exchange process between electrode and electrolyte ions.

Experimental

Material synthesis

Lithium nitrate, LiNO₃ (SRL, purity-99.5%), manganese(II) acetate tetrahydrate, Mn(CH₃COO)₂·4H₂O (Merck, purity-99.5%), tetraethyl orthosilicate, TEOS (Sigma Aldrich, purity-98%), urea, NH₂CONH₂ (Merck, purity-99.5%) and nitric acid, HNO₃ (Himedia, purity-69%) were used as starting materials without further purification.

In urea combustion method,^21–24 Mn(CH₃COO)₂·4H₂O (0.01 mol) was dissolved in minimum amount of HNO₃ to convert it into metal nitrate and then LiNO₃ (0.03 mol) and TEOS (0.01 mol) were added to manganese nitrate solution which was further kept at 80 °C for 15 min. While adding TEOS into resulting nitrate solution, the evolution of brown coloured gas (probably NO₂) is observed. Later, NH₂CONH₂ (0.15 mol) was added to this solution and allowed to stir for 15 min at 80 °C. This will turn the above final solution into transparent gel. Finally, the gel was kept into pre-heated muffle furnace at 350 °C in air. The ignition initiates reaction and smoke evolves quickly (within 10 min). This turns the gel into fluppy form of material. Later, this brown coloured material was ground using mortar and pestle and washed with distilled water several times to remove the unreacted residue and further dried at 90 °C for 6 h in air.

Material characterization

LMS powder was characterized by X-ray diffractometer (XRD, RIGAKU ULTIMA IV) employing CuK_α radiation. The thermal stability of material was investigated by thermo gravimetric analysis (TGA, SII 6300 EXSTAR A) in air with the ramping rate of 5 °C min⁻¹. Morphological study was carried out by field emission scanning electron microscope (FESEM, MIRA 3 TESCAN) with EDAX attachment and transmission electron microscope (TEM, FEI TECNAI G² 20 S-Twin). The Fourier transform infrared (FTIR) spectra was recorded with spectrometer (FTIR, PERKIN ELMER C91158) in the range of 1200–400 cm⁻¹ using KBr pellet method. Surface area was examined by Brunauer–Emmet–Teller (BET, QUANTA CHROME, ASIQWIN) technique by degassing the sample for 6 h at 120 °C. For conductivity measurement, the sample was made into pellet having thickness of 0.132 cm and flat surface area of 0.258 cm² at pressure level of 10 tons by using hydraulic press and then annealed at 350 °C for 12 h. The electrical contact on the flat surfaces was made by silver pasting and soaked at 60 °C for 6 h in air. DC conductivity of LMS pellet is studied at room temperature using I–V measurement setup. The conductivity (S cm⁻¹) is calculated by using following eqn (1)where, R is the resistance, t and A are the thickness and cross-sectional area of pellet.

Electrode preparation and electrochemical characterization

For electrochemical studies, three electrode system is used where LMS works as working electrode, platinum wire as counter electrode, and Ag/AgCl as reference electrode. An aqueous solution of 2 M KOH was used as electrolyte. Typically, active material, super P (conducting material) and polyvinylidene fluoride (PVDF) were mixed (70 : 15 : 15 wt%) using N-methyl-2-pyrrolidone (NMP) as solvent. Detailed description of electrode fabrication is described elsewhere.²⁴ The total weight of active material in the electrode is 2.0 mg. Electrochemical behavior of LMS was investigated by cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) in the potential range of 0 to 0.55 V and electrochemical impedance spectroscopy (EIS) using multichannel potentiostat/galvanostat (AUTOLAB, MAC-80039). The voltage of 3 mV is used for EIS measurement over the frequency range of 6 kHz to 10 mHz.

Results and discussion

Phase, structure and morphology

Lithium manganese silicate (Li₂MnSiO₄) isostructural to Li₃PO₄ is found to exhibit polymorphism. Theoretically, LMS crystallizes in different structures having the space groups of Pmn2₁, Pmnb, P2_1/n and Pn.^13,25 All these crystal structures contain distorted hexagonal close packed array of oxide ions with all cations in distorted corner sharing tetrahedra, but they differ in the orientation of these tetrahedra, particularly for the MnO₄–SiO₄ chains.^13,25 XRD pattern (Fig. 1) confirms the formation of LMS having orthorhombic structure with Pmn2₁ space group identified by JCPDS file no. 04-015-5273 with small unavoidable impurity of orthorhombic Li₂SiO₃ (JCPDS file no. 00-029-0829). The refinement of crystal structure is done using PDXL2 software (Rigaku). Refined lattice parameters are found to be a = 6.317(10) Å, b = 5.385(9) Å, c = 4.988(8) Å with unit cell volume of 169.68(5) ų. The refined lattice parameters are in close agreement with the parameters reported in literature.^25,26 The goodness of fit and other related fitting parameters are obtained within the limit of acceptance of crystal structure refinement. Atomic coordinates obtained are given in Table 1. The crystal structure of LMS generated from refined parameters is shown in Fig. 2, where Li, Si and Mn ions are surrounded by O ions to form LiO₄, SiO₄ and MnO₄ tetrahedras which results into the layered structure of LMS.^11,25 The average crystallite size is calculated to be 34 nm using Scherrer's formula d = kλ/β cos θ, where d is the mean crystallite size, k is the constant parameter of shape factor (0.89), λ is the wavelength of X-ray (0.154 nm), β is the full width of half maximum of peak corresponding to (011) plane and θ is the corresponding angle for Bragg diffraction. Further, novel combustion synthesis enables almost pure phase formation of LMS with small impurity of Li₂SiO₃. This impurity is found to be small as compared to the samples prepared by other methods in the past such as solid state, sol–gel, Pechini sol–gel, hydrothermal and solvothermal synthesis where additional secondary phases i.e. MnO and Mn₂SiO₄ have also been observed.^14–18 Moreover, it is worthwhile to mention that the synthesis of Li₂MnSiO₄ in pure phase (100%) is almost impossible.^15–17 Thus, LMS synthesized by combustion technique with minimum impurity is encouraging and novel.

Fig. 1 X-ray diffraction pattern of Li₂MnSiO₄ (inset) and its refinement plot.

Table 1 Refined crystallographic parameters for Li₂MnSiO₄, space group Pmn2₁ using Rietveld refinement software PDXL2 with a = 6.317(10) Å, b = 5.385(9) Å, c = 4.988(8) Å with combined parameters R_p = 3.3%, R_wp = 4.24%, S = 1.61 and χ² = 2.5

Element	x	y	z	Occupancy	Temperature factor
Li	0.174(5)	0.280(5)	0.838(5)	1.000	0.0(9)
Mn	0.500000	0.8505(13)	0.976400	1.000	14.3(5)
Si	0.000000	0.810(2)	−0.012(3)	1.000	5.2(4)
O	0.2339(19)	0.6552(12)	0.8859(19)	1.000	0.0(4)
O	0.000000	0.160(3)	0.970(5)	1.000	0.0(6)
O	0.500000	0.203(3)	0.822(3)	1.000	1.0(4)

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Fig. 2 Crystal structure generated for the synthesized Li₂MnSiO₄.

Thermal stability of LMS is quite important for its energy storage properties, therefore it is heat treated at different temperatures viz. 300 °C, 450 °C and 600 °C for the same period of 10 h and corresponding XRD patterns are shown in Fig. 3. As-prepared LMS (with small impurity of Li₂SiO₃) is stable upto 300 °C. Later, upon heating upto 450 °C, peaks corresponding to other secondary phase of MnO₂ are appeared. Simultaneously Li₂SiO₃ peaks become prominent. This leads to reduce the lattice parameters (a, b, c) of primary orthorhombic-LMS (Fig. 3b). Further heating of LMS at 600 °C causes complete disintegration of LMS into orthorhombic-Li₂SiO₃ and cubic-MnO₂. These observations are well supported by crystal structure refinement using RIR (relative intensity ratio) method by PDXL2. Table 2 displays the weight percentage of impurities determined from structural refinement. As can be noticed that the content of Li₂SiO₃ increases with temperature and LMS completely transforms into Li₂SiO₃ and MnO₂ at 600 °C. To verify further these results, TG analysis of LMS is carried out from room temperature to 700 °C in air with heating rate of 5 °C min⁻¹. The corresponding weight loss vs. temperature curve is shown in Fig. S1 (ESI†). The minute weight loss of ∼5% is occurred between room temperature to 600 °C attributed to moisture content and thereafter a nominal weight gain of the material is observed. This slight increase is ascribed to the decomposition of LMS into Li₂SiO₃ and MnO₂ following the eqn (2).

Fig. 3 (a) XRD patterns of Li₂MnSiO₄ synthesized at different temperatures, (b) variation of lattice parameters (a, b, c) with temperature.

Table 2 Weight percentage of impurities determined by Rietveld refinement

Sample	Weight (%)			Refined parameter
	Li2MnSiO4	Li2SiO3	MnO2	Rp (%)	S
LMS (90 °C)	90	10	—	3.3	1.6
LMS (300 °C)	87	13	—	3.3	1.3
LMS (450 °C)	31	56	13	7.1	4.1
LMS (600 °C)	—	75	25	5.8	3.1

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Agglomerated nano-sized particles along with pores/voids due to the evolution of gases during combustion are observed by FESEM and corresponding micrographs are shown in Fig. 4a. The morphological findings are further supported by transmission electron microscopy and corresponding results are shown in Fig. 4b. TEM image verifies the nano-sized particles of LMS in the range of 20–50 nm having good porosity. Further, the average crystallite size (34 nm) calculated by Scherrer's formula is found to be in good agreement with TEM observations. The elemental map of LMS is also shown in Fig. 5a–d where homogeneous dispersion of Mn, Si and O is seen. Fig. 5e shows the energy dispersive spectra indicating the presence of O, Mn and Si.

Fig. 4 (a) FESEM micrograph of LMS. Some of encircled regions show the pores of LMS. (b) TEM micrograph of LMS nanostructures.

Fig. 5 (a–d) Elemental mapping of LMS showing Si, Mn and O content. (e) Elemental peaks of Li₂MnSiO₄.

The FTIR spectrum of LMS was recorded from 1200–400 cm⁻¹ and the corresponding results are shown in Fig. S2 (ESI†). As discussed above, the crystal structure of orthosilicates-Li₂MnSiO₄ consists of LiO₄ and MnO₄ tetrahedra are linked to (SiO₄)⁴⁻ polyanions, therefore an IR vibrational spectrum is dominated by the fundamental vibrations of LiO₄, MnO₄ and SiO₄ tetrahedras.²⁷ More precisely, the intense absorption band observed at 912 and 886 cm⁻¹ are attributed to the stretching vibration of O–Si–O bond of SiO₄⁴⁻. Further, the absorption bands positioned at 580 and 532 cm⁻¹ are due to bending mode of SiO₄⁴⁻.^27–30 Li–O stretching mode in LiO₄ tetrahedra appears at ∼450 cm⁻¹.²⁸ A small kink observed at 739 cm⁻¹ may be an absorption band which is observed probably due to bending vibration of SiO₃²⁻ and confirms the unavoidable presence of impurity phase of Li₂SiO₃.³⁰

Morphological characterization, as shown in Fig. 4, demonstrates the voids in between individual particles which may provide large active sites along with good porosity. These features are extremely important to obtain improved supercapacitive performance. To check the surface area, pore size and its distribution nitrogen adsorption/desorption isotherm of LMS are recorded and results are shown in Fig. 6. The hysteresis is observed in the broad relative pressure region 0.2 < P/P₀ < 0.9, where P and P₀ are the equilibrium and saturation pressures. This shape of isotherm may be defined as type IV isotherm showing the mesopores.^31,32 Density functional theory (DFT) based pore size distribution curve shows various sharp peaks in the range of 3–10 nm along with broad regions within the range of 15–35 nm (inset). This small deviation in homogeneity of pores and their distribution may probably ascribed to the combustion synthesis where a fast exothermic reaction occurs that leads to evolution of different gaseous products which leaves behind the associated pores depending upon type of gas or availability of fuel (NH₂CONH₂). The specific surface area of the powder as obtained by BET analysis is 35 (±2) m² g⁻¹. The mesoporous nature with almost homogeneous pore distribution (3–10 nm) and high surface area may be beneficial for charge storage as a result of double layer storage and/or pseudocapacitance.

Fig. 6 Nitrogen adsorption–desorption isotherms and corresponding DFT pore size distribution plot for Li₂MnSiO₄ (inset).

Electrochemical properties

In order to investigate the supercapacitive performance of LMS, electrochemical characterization is further carried out by CV analysis in the potential range of 0 to 0.55 V using aqueous solution of 2 M KOH as the electrolyte. Fig. 7a shows the CV curves of LMS electrode at different potential scan rates ranging from 3 to 50 mV s⁻¹. The shape of the CV curves is almost rectangular with small kinks while oxidation and reduction, indicating the pseudo-capacitance behavior in addition to the electric double layer.^24,33–36 It is further noticed that highly accessible surface area and mesopores structure formed by the nano sized particles provide more active sites for effective redox reaction and channels for deep insertion and de-insertion of electrolyte ions (K⁺).^36,37 The specific capacitance (C_s) values are calculated using eqn (3)where, I is the average current (A) during the anodic and cathodic sweep, m is the mass of the active material (g) and V is the voltage sweep rate (V s⁻¹). The LMS electrode exhibits a specific capacitance of 175 (±5) F g⁻¹ at 3 mV s⁻¹ which may be considered reasonably good value. This C_s value arises mainly from surface adsorption of alkali ions (K⁺ in our case) to the porous and nano-crystalline LMS following eqn (4).^24,38

(Li₂MnSiO₄)_surface + K⁺ + e⁻ = [KLi₂MnSiO₄]_surface (4)

Fig. 7 (a) Cyclic voltammogram of the Li₂MnSiO₄ in 2 M KOH in the range of 0 to 0.55 V at different voltage scan rate (b) graph between specific capacitance and voltage scan rate, and graph between scan rate and average current (inset).

Initially, the C_s value decreases gradually with increasing the scan rate from 3 mV s⁻¹ to 20 mV s⁻¹ (Fig. 7b). However, specific capacitance remains almost constant at 120 (±10) F g⁻¹, even if scan rate is increased from 20 to 50 mV s⁻¹. This is attributed to increased electrolyte accessibility due to unique morphology of LMS (mainly pores and its homogeneous distribution). In general, accessible area of the active material is limited at higher scan rate, whereas at the lower scan rates, both inner as well as outer surface of LMS are used and exhibit good C_s value.^24,34–36 But in the present case, this high and stable value of 120 (±10) F g⁻¹ even at high scan rate demonstrates its enhanced capacitive properties. Further, improved rate capacitive performance is also verified by the quasi-linear dependence of the capacitive current density on the scan rate indicating the contribution of reversible and diffusion limited surface redox reactions (inset of Fig. 7b).^39–41

The stability of the electrode is tested by GCD analysis to explore the service life of LMS for its practical application as a supercapacitor. Almost triangular shaped charge–discharge profile (Fig. 8a) indicating good supercapacitive behavior of LMS is observed. The C_s is calculated using eqn (5) and found as 150 (±5) and 115 (±5) F g⁻¹ at current densities of 0.5 A g⁻¹ and 1.0 A g⁻¹, respectively.

where, I the applied current (A), Δt the discharging time (s), m the mass of active material (g), and ΔV the potential range (V). The cycling life of LMS is also tested over 500 cycles at different current densities and results are shown in Fig. 8b. Better C_s values stable upto atleast 500 cycles demonstrates the importance of mesoporous network of interconnected nanosized particles that provide the whole area (inner as well as outer) for energy storage.^24,36,37,40 It is worthwhile to mention that mesopores (3.5 nm) formed during the evolution of gas while combustion imposes less resistance to ion diffusion in electrode/electrolyte interface, and hence favorable for good supercapacitive properties. These results compliment the CV findings and emphasize on the necessity of pore size and its homogeneous distribution and good surface area for improved performance in supercapacitor. The observed value of C_s (175 (±5) F g⁻¹ at 3 mV s⁻¹ and 150 (±5) F g⁻¹ at 0.5 A g⁻¹) is found to be better than the commercially employed activated carbons in hybrid supercapacitor. The activated carbon of different morphology with varying surface area (700–2200 m² g⁻¹) commonly exhibit C_s values in the range of 70–200 F g⁻¹ in aqueous and 50–120 F g⁻¹ in organic electrolytes.⁶ Hence LMS prepared by simple and cost effective, and most importantly industrially scalable urea combustion method justify its viability as prospective electrode material for supercapacitor. However, further improvement in specific capacitance of LMS may be carried out by creating nanostructures of high surface area and/or increasing its intrinsic conductivity by doping, compositing with highly conducting material such as graphene, carbon nanotube/nanofiber.^{3,6,14,16–18} Further, supercapacitive performance of LMS may also be improved using different electrolyte of high ionic conductivity and of wider stable electrochemical window.²

Fig. 8 (a) Galvanostatic charge–discharge curves (first four cycles) of the Li₂MnSiO₄ recorded at current densities of 0.5 A g⁻¹ and 1.0 A g⁻¹ (b) specific capacitance of the Li₂MnSiO₄ in 2 M KOH at a current density of 0.5 A g⁻¹, 1.0 A g⁻¹, respectively.

The energy density (E) and power density (P) are also calculated to be 7 W h kg⁻¹ and 135 W kg⁻¹ employing the eqn (6) and (7), respectively.

where, C_s is the specific capacitance (F g⁻¹), ΔV is the potential window (V) and Δt is the discharge time (s).

To complement the CV and GCD studies, electrochemical impedance spectroscopy is also carried out. The Nyquist plot of LMS electrode (shown in inset of Fig. 9) display two regions; one is semicircle in the high frequency region and a slanted line in the low frequency region which indicate that the electrode process is controlled by the electrochemical reaction at high frequency and by mass transfer at low frequency.⁴² High frequency arc reveals the electronic resistance of supercapacitor electrode. At lower frequency, the slanted line indicates the faster ion transport in the electrolyte inside the mesoporous LMS structure. The origin of the semi-circle at higher frequency ranges is due to the ionic charge transfer resistance (R_ct) at the electrode/electrolyte interface. The internal resistance, R_s is the combination of ohmic resistance of electrolyte and internal resistance of the electrode material which determines the charge–discharge rate of the electrode. The value of indiscernible R_s + R_ct was estimated quantitatively from the fittings of the experimental impedance spectra and found to be 1.8 Ω (shown in Fig. 9). The charge transfer resistance (R_ct) is originated from the electrolyte accessible area of the electrode and is called the limiting factor for the specific power of any super capacitor.⁴² The small value of R_s + R_ct is ascribed to the lower intrinsic resistance of LMS due to its mesoporous structures with homogeneous pore size distribution which leads to improve charge–discharge rate and power density of supercapacitor. The fitting values of R_s + R_ct derived from the nyquist plot of cycled sample (500 cycles) is found to be 2.2 Ω. This negligible change in R_s + R_ct observed even after 500 cycles supports the durability as well as electrochemical stability of the LMS electrode.

Fig. 9 Nyquist plot of Li₂MnSiO₄ electrode as prepared and after 500 GCD charge–discharge cycling and equivalent circuit (inset).

Electrical properties

The supercapacitive performance of combustion routed mesoporous nano-LMS by CV and GCD analysis and supported by EIS technique is found to be novel and encouraging. Such a high value of C_s in LMS is surprising since LMS is reported to be highly insulating material (5 × 10⁻¹⁶ S cm⁻¹ at room temperature and 10⁻¹⁴ S cm⁻¹ at 60 °C).^43,44 Thus, it becomes necessary to investigate its electrical properties. I–V characteristics are shown in Fig. S3 (ESI†). The linear response observed in I–V graph indicates that the electronic transport in LMS occurs within the Ohmic region. The electrical conductivity of LMS is calculated to be 5.8 × 10⁻⁸ S cm⁻¹ using resistance of 9.89 × 10⁶ Ω (slope of the I–V curve). This electrical conductivity in LMS synthesized by urea combustion is found to be consistent (10⁻⁷ to 10⁻⁸ S cm⁻¹) examined by other supplementary techniques such as EIS, dielectric measurement (results are unpublished). Hence, the high conductivity, high surface area and mesopores in LMS are main factor by which high and stable value of C_s is observed.

Conclusions

Porous nano-sized Li₂MnSiO₄ was synthesized by simple and cost effective urea combustion method for the first time which was found to be orthorhombic with Pmn2₁ space group. A small unavoidable secondary phase of Li₂SiO₃ was observed. From cyclic voltammogram, high specific capacitance of 175 (±5) F g⁻¹ was obtained at voltage scan rate of 3 mV s⁻¹. The CV studies were well supported by GCD findings and EIS. A high and stable value of 150 F g⁻¹ atleast upto 500 cycles is observed. This improved supercapacitive performance of nano-sized porous LMS was attributed to uniform porosity with almost homogeneous distribution and high surface area of 35 (±2) m² g⁻¹. The conductivity of LMS (5.8 × 10⁻⁸ S cm⁻¹) was found to be eight order of magnitude higher than the solid state processed LMS. The energy density and power density was calculated to be 7 W h kg⁻¹ and 135 W kg⁻¹, respectively. Reasonable capacitance of Li₂MnSiO₄ processed by facile synthesis justifies its viability as counter electrode (supercapacitor type) in hybrid supercapacitor.

Acknowledgements

Council of Scientific and Industrial Research (CSIR), Government of India is gratefully acknowledged for its financial support under project no. 22(0658)/14/EMR-II. One of the author (P.C.) acknowledges Ministry of Human Resource Development (MHRD), India for research fellowship.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra02207j

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