- Email: [email protected]
Nano-composite LiMnPO4 as new insertion electrode for electrochemical supercapacitors
Accepted Manuscript Nano-composite LiMnPO4 as New Insertion Electrode for Electrochemical Supercapacitors S.R.S. Prabaharan, A. Anslin Star, Ajit R. Kulkarni, M.S. Michael PII:
S1567-1739(15)30074-2
DOI:
10.1016/j.cap.2015.09.009
Reference:
CAP 4072
To appear in:
Current Applied Physics
Received Date: 30 May 2015 Revised Date:
14 August 2015
Accepted Date: 10 September 2015
Please cite this article as: S.R.S. Prabaharan, A Anslin Star, A.R. Kulkarni, M.S. Michael, Nanocomposite LiMnPO4 as New Insertion Electrode for Electrochemical Supercapacitors, Current Applied Physics (2015), doi: 10.1016/j.cap.2015.09.009. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
RI PT
LiMnPO4 single crystallite
SC
Thin Porous carbon connectivity
600 400 200 0 -200 -400 -600 -800 0
0.5
M AN U
Capacitance (Fg-1)
800
1
1.5
2
Li2SO4 LiOH LiClO4/AN
2.5
TE D
Potential (V)
AC C
EP
AC//nC-LMP Asymmetric Hybrid Capacitor Voltammetric profile
3
ACCEPTED MANUSCRIPT
Nano-composite LiMnPO4 as New Insertion Electrode for Electrochemical Supercapacitors
RI PT
S.R.S. Prabaharan1)# A. Anslin Star1), Ajit R. Kulkarni2), M.S. Michael3)#, 1)
School of Electronics Engineering (SENSE), VIT University - Chennai Campus, Chennai, India
2)
Department of Metallurgical Engineering and Materials Science, IIT Bombay, Powai, Mumbai,
SC
400076, India. 3)
AC C
EP
TE D
M AN U
Chemical Sciences Research Lab, SSNCollege of Engineering, Chennai-603110, India
#
Corresponding authors M.S.Michael. Email: [email protected] Phone: +914427474844;
S.R.S. Prabaharan. Email: [email protected] Phone: +914439931187
1
ACCEPTED MANUSCRIPT
Abstract:
Nano-composite olivine LiMnPO4 (nC-LMP) was found to exhibit facile pseudo-capacitive characteristics in aqueous as well as non-aqueous electrolytes. We demonstrated employing the same as positive in hybrid electrochemical capacitors namely Li-Ion hybrid capacitors (LIC).
RI PT
Adapting a simple CVD technique, nano-crystallites of LiMnPO4 (LMP) were coated with carbon monolayers of ~2 nm thick to circumvent its poor intrinsic electronic conductivity. The novelty is that the single crystallites were intimately covered with carbon ring and networked to the neighboring crystallites via the continuous carbon wire-like connectivity as revealed from HRTEM
SC
analysis. Single electrode faradic capacitance of 3025 Fg-1 (versus standard calomel reference
M AN U
electrode) was deduced for carbon coated LMP, the highest reported hitherto in Li+ aqueous electrolytes. Employing nC-LMP as working electrode versus an activated carbon (AC) counter electrode, we obtained a high specific energy of 28.8Whkg-1 with appreciable stability in aqueous electrolytes whereas in nonaqueous electrolyte there is an obvious increase in energy density (35Whkg-1) due to larger potential window. That is, a full cell version of LIC, AC|Li+|LMP, was
TE D
fabricated and demonstrated its facile cycling characteristics via removal/insertion of Li+ within LiMnPO4 (positive electrode) and the electrosorption of Li+ into mesoporous carbon (AC) (negative Such cells ensured a typical battery-like charging and EDLC-like discharging
EP
electrode).
characteristics of LIC type electrochemical capacitors (ECs) desired to enhance safety and higher
AC C
energy densities.
2
ACCEPTED MANUSCRIPT
Introduction
Porous carbon materials exhibit non-faradaic type capacitance storing energy in the form of a simple electrostatic ion separation known as Electrical Double Layer Capacitors (EDLCs). Their specific capacitance values are lower than that of faradaic type electrode materials like metal oxides
RI PT
which store energy by virtue of surface redox reaction occurs at a depth of few nanometers from the surface. The charge transfer reaction is so fast that its charging characteristics are comparable with that of an EC exhibiting pseudocapacitive properties [1,2]. Different strategies were developed to
SC
improve the specific energy of ECs without compromising the specific power. One of the approaches recently being pursued is the hybrid capacitors (HCs) in view of obtaining both high
M AN U
specific energy (kWhkg-1) and high specific power (kWkg-1) in a single cell. Thus evolved a new approach by combining EDLC electrode (for Power) and an electrode exhibiting battery-like redox characteristics (pseudo-capacitance) forming asymmetric capacitor in which one of the electrodes acts as a cathode while the other electrode being an anode, extends potential window of the capacitor that resulted in substantial enhancement of specific energy [3-5].
Recently, active
TE D
research has been focused on a new concept of hybrid capacitor, known as Li-ion capacitors (LICs) which are identified as a new class of electrochemical capacitors (ECs) that take advantage of Li+
EP
as charge transport species between the positive and negative electrodes but at faster rates unlike LiIon batteries. The electrochemical redox reaction originates from the surface redox kinetics rather
AC C
than from the bulk of insertion type positive electrodes. Therefore, it is imperative that ionic and electronic conductivities of the positive electrode materials are rather crucial to realise specific capacity (mAhg-1) converted specific capacitance (Fg-1). Perhaps, most of the Li-intercalation electrodes itself behave like a redox capacitor exhibiting pseudocapacitance since they show steady increase/decrease of voltage during discharging/charging as happens in electrochemical capacitors (ECs) [6]. In these LICs, the source for charge storage is governed by means of Li+ ions supplied by insertion electrode itself that is, in its pre-lithiated form [7-10]. There are reports concentrating 3
ACCEPTED MANUSCRIPT
on non-lithiated electrodes stabilized with Li metal powder and/or pre-doped with lithium by electrochemical means [11-14]. By employing lithium containing nonaqueous electrolyte in LICs, it is possible to enhance the power and energy density of the capacitor due to high oxidative potential of the electrode materials ca. 3V - 5V vs. Li/Li+ [15-18]. The concept of LICs was first
RI PT
proposed by Amatucci et al. by fabricating a 2.8V hybrid capacitor involving Li4Ti5O12 and activated carbon (AC) electrodes [19]. Since then research has been intensified for the improvement of specific energy of hybrid capacitor using different types of Li-intercalation compounds and found significant enhancement in energy density [20-26]. Later, another concept was introduced
SC
with additive carbons as electrodes in conjunction with Li+ nonaqueous electrolyte achieving a high
M AN U
voltage (> 4V) Carbon/Carbon LICs [27]. Quite recently, a new pseudocapacitive material ca. Cu doped NiCo2O4 was reported in which very significant improvement in the capacitive performance of host NiCo2O4 using aqueous Li+ containing electrolytes was reported for the first time [28]. Aiming at introducing newer promising materials as electrodes for Li-Ion capacitor olivine type cathode-active material nC-LMP as Li+ ion source has been studied for the first time. The
TE D
present study demonstrates the stability of the cathode material in aqueous electrolytes. It is well known that phospho-olivine compounds have been studied extensively as positive electrode
EP
(cathode) materials for Li-ion batteries and these materials are proven to offer a variety of advantages for practical systems due to their low-cost, non-toxicity and outstanding thermal There have been number of reports on LiMnPO4 as one of the promising
AC C
stability [29-31].
candidate positive electrodes in advanced Li-Ion batteries owing to its high theoretical capacity (170 mAhg-1) and high redox potential. However, hitherto the achievement of its full theoretical capacity has not been realized due to its low electronic conductivity, small polaronic conduction of Jahn-Teller active Mn3+ and high surface energy barrier for Li+ diffusion at the surface [32-35]. Since the rate of charge storage in any intercalation electrode material is determined by the movement of electrons as well as the transfer of ions to the active sites, the structure of the material 4
ACCEPTED MANUSCRIPT
is crucial to enhance the performance of LIC. Several groups attempted different strategies to improve the electrical conductivity of LiMnPO4 and demonstrated that the intrinsic low rate capability can be overcome by nano-sizing the particles or carbon coating [36-40]. It is widely proven that nanoscale materials are beneficial for increasing electrode/electrolyte interface,
RI PT
shortening the diffusion length and easing the access of the ions to the surface intercalation sites [41-43]. Despite extensive research attempts hitherto particularly on LiMnPO4 to evaluate its suitability as a good cathode substitute for renowned LiCoO2 in lithium ion batteries, its potential as
SC
pseudo-capacitive material was not explored as yet to the best of our knowledge. In the present study, we report our success in making a single phase nanoscale crystallites of LiMnPO4
M AN U
synthesized via a simple solution synthesis protocol and subsequently an ex-situ carbon capping onto single crystallites of olivine LiMnPO4 facilitating a perfect carbon wiring between crystallites as well. This approach facilitated primarily the penetration of electrolyte solution into pores of porous carbon encircled around single crystallites. Thus, the pseudocapacitive electrode-active characteristics of nanoporous carbon encompassed single crystallite of LiMnPO4 were studied in
TE D
aqueous electrolyte media (neutral and alkaline solution) and its specific capacitance (Fg-1) was measured accordingly. Finally, a full cell version of a Li-Ion capacitor including a carbon
EP
encompassed LiMnPO4 namely nC-LMP as cathode against a (EDL) mesoporous activated carbon (AC) exhibiting electric double layer characteristics. In this paper, we report our new approach
AC C
towards developing such LICs employing monolayer carbon encompassed olivine type LiMnPO4 against mesoporous activated carbon and the results are reported based our systematic studies employing both aqueous and non-aqueous electrolytes.
5
ACCEPTED MANUSCRIPT
Experimental
Synthesis and characterization of carbon coated LiMnPO4 nanocrystallite Single phase LiMnPO4 powder sample was prepared by using a proprietary synthesis protocol involving lithium and manganese acetates and triethyl phosphate as starting materials. The
RI PT
stoichiometric quantities of the above starting materials were dissolved in appropriate quantity of de-ionized water/iso-propanol mixtures (4:1 ratio) separately and sonicated using an ultrasonic processor (UP200S 200 watts, 24kHz Hielscher Ultrasonics GmbH, Germany) for 10 min.
SC
Subsequently, 100 ml saturated solution of citric acid was added to the above mixture solution containing stoichiometric amounts of Li/Mn/P. The resulting dark colored clear solution was
M AN U
continuously stirred at 90 °C until a sticky paste-like substance was obtained. Using the following temperature programmed reaction (TPR) protocol, the dried precursor (as-prepared) powder was heated in a programmable tubular furnace (Carbolite, U.K) in three different steps firstly: at 350°C at a rate of 5oCmin-1 for 30 min, then to 450°C by 0.5oCmin-1 for 30 min and to 650°C by 2.5 o
Cmin-1 and finally the temperature was maintained at 650oC for 6 h under a flowing pure argon
TE D
atmosphere. When the programmed time was over, the sample was automatically cooled down to room temperature until then argon gas atmosphere was maintained throughout. During the entire
EP
heating process, the gas flow rate has been maintained at 1ml min-1 (rate of heating and cooling) to ensure uniform distribution of particles and to avoid surface cracking. This kind of synthesis allows
AC C
a better control of the morphology and texture of solid particles. In a distinct experiment, fine monolayer of carbon coating was accomplished employing an exsitu heating procedure in a CVD furnace by mixing the predetermined quantities of pure single phase nano-crystallites LiMnPO4 mixed with appropriate quantity of glucose and heat treated at 600°C (5oCmin-1) for 2 h under flowing argon/H2 gas atmosphere (flow rate was maintained at 1 mlmin-1). The reaction tube was pre-flushed with argon gas for 30 min to ensure O2 free atmosphere. The reacted sample was cooled down to room temperature under the same gaseous 6
ACCEPTED MANUSCRIPT
atmosphere before being submitted for other characterization.
Powder XRD was obtained using X’pert PRO MPD (Pan Analytical instrument, North Holland) equipped with CuKα radiation and the preliminary refinements of lattice parameters were performed using X'Pert High Score Plus software. The grain size, morphology and nanostructure
RI PT
were examined employing a field emission scanning electron microscopy (FESEM) (FEI Quanta 400F, The Netherlands) and FE-TEM analysis was performed using a FEI-TITAN machine (Titan G2 80-200 with Cs-corrector, 80kV) to observe thin carbon onto single crystallite LiMnPO4.
SC
This is the important observation of this work where a single crystallite has been found to be independently coated and linked or wired to other neighboring grains in the same manner. Electron
spherical crystallite from TEM Cu grid. Electrochemical characterization
M AN U
diffraction (ED) image has been obtained using the same machine by locating an individual
Slurry containing composite nC-LMP electrode was prepared by adding a solvent, N-methyl pyrrolidine (NMP) to the admixture of nC-LMP, PVDF binder and conductive additive carbon
TE D
(PRINTEX® L6, Degussa, Germany) in a weight ratio of 70:10:20. The slurry was then coated onto SS foil (Exmet, USA) uniformly by employing automatic film applicator with a bar coater (SHEEN,
EP
U.K). The carbon electrode was also prepared following the approach except that weight ratio of carbon composition with PVDF binder was 80:20. The coating was dried overnight under vacuum
AC C
at 110oC and ‘1cm x 1cm’ sized coated SS foil was duly cut with the loading mass of the active material varied from 15mgcm-2 to 20mgcm-2 for nC-LMP and 10mgcm-2 to 15 mgcm-2 for activated carbon (AC) electrodes. The pseudo-capacitive performance of nC-LMP was deduced using a conventional three electrode system having a saturated calomel electrode (SCE) and a platinum foil (~4 cm2) as reference and the counter electrode respectively.
Rate dependent cyclic
voltammogramms were performed in the range from 5.0 mVs-1 to 100 mVs-1. Cyclic voltammetry (CV) studies were carried out employing both aqueous and nonaqueous 7
ACCEPTED MANUSCRIPT
electrolytes containing Li+ using a Versastat3 potentiostat (AMETEK, USA). An another electrochemical workstation (Wonatech Multichannel system Model Zive MP5, South Korea) equipped with an impedance analyzer, was employed to study galvanostatic charge/discharge characterization, EIS analysis and rate dependent cyclic voltammetry (CV). An ac perturbation
RI PT
signal of 10mV peak-to-peak potential was applied in the frequency, range from 0.1Hz to 10 kHz for the electrochemical impedance spectroscopic (EIS) measurements.
We deduced the specific capacitance values of electrode material by integrating the area under CV curves divided by the specific current over the scan rate as per the formula, C= I/sm, where ‘C’ is
SC
the capacitance in ‘Fg-1’, ‘I’ is the average anodic/cathodic current in ‘A’, ‘s’ is the voltage sweep
M AN U
rate in ‘Vs-1 and ‘m’ is the weight of the active material in ‘g’ excluding the weight of coated carbon material. All electrochemical measurements were conducted in ambient air at room temperature. Aqueous Li-Ion Capacitor (LIC) cells were fabricated using a pseudo-capacitive nC-LMP as positive electrode and a porous EDL carbon (>3000 m2g-1) exhibiting double layer capacitive characteristics as a negative electrode in a two electrode cell made of 316L SS designed in-house. A
TE D
Whatman™ filter paper (grade 1) was used as a separator. The weight ratio of positive and negative electrode is optimized based on the specific capacitance values of both electrodes. On the other
EP
hand, LICs employing non-aqueous electrolyte were also assembled using 2032 coin cell hardware inside the Ar filled glove box. Celgard™ microporous polypropylene (2400) was used in this case
AC C
as separator.
Galvanostatic cycling at different current densities was used to calculate the specific power and specific energy of LICs using the formulae given below. Specific Power (P) = Specific Energy (E) =
Wkg-1 Whkg -1
………………
(equation 1)
………………
(equation 2)
Where ‘V’ is potential of the capacitor excluding ‘IR’ drop (ESR) in volt, ‘I ‘is current in ‘A’, ‘m’ is 8
ACCEPTED MANUSCRIPT
the total mass of active material in ‘kg’ and ‘t’ is the time in hour. The Equivalent Series Resistance (ESR) was obtained from EIS measurement at 1 kHz. Results and Discussion Synthesis of Single Phase nC-LMP
RI PT
The crystalline structure of pure and carbon embedded LiMnPO4 materials were studied using powder X-ray diffraction analysis. XRD refinement (Rietveld) pattern of as-prepared LiMnPO4 is shown in Fig. 1 prior to ex-situ carbon coating. Rietveld refinement ensures the pattern matched
SC
with calculated peaks although the difference is more between 20-40o 2θ angle, it is attributed obviously to the nanostructure nature of the powder samples as compared to the standard pattern.
M AN U
Hence such difference peaks are obvious. Otherwise, the pattern matches perfectly with standard
AC C
EP
TE D
XRD file of LiMnPO4 (98-007-6919 PDF file) with no additional peaks.
Figure 1 XRD refinement (Rietveld) pattern of as-prepared LiMnPO4 prior to ex-situ carbon coating; Indexed XRD patterns of a carbon composite LiMnPO4 is shown as inset. The background at low angle indicates the presence of amorphous carbon. (Pattern matched with 98-007- 6919 PDF file). HRTEM corroborates with this information. Single phase structure is clearly evident. The difference pattern as exemplified in blue color is due to nano-structure nature of the sample.
9
ACCEPTED MANUSCRIPT
Every peak in XRD profiles could be indexed in an orthorhombic structure with a space group Pnma. Cell parameters such as lattice constant, volume and space group are found to be matched the standard PDF XRD file of LiMnPO4 with no significant variations. Prior to carbon treatment, as-prepared LMP powders were examined using FESEM. It is evident
RI PT
from the SEM micrograph (see S1) that nanoscale loosely connected spherical grains having grain size ranging from 40-60 nm are distributed uniformly with good surface crystallite contacts. Such features are expected to enhance the intra surface crystallite-crystallite conductivity via surface
SC
coated monolayer carbon in turn to facilitating the electron transfer much easily. HRTEM analyses were carried out to confirm the carbon coating onto individual crystallites as shown in fig.2 (a-d).
M AN U
As evident from Fig 2(a) that individual crystallites of LMPs were encompassed with a fine layer of porous monolayer carbon exhibiting the wiring-like linkage between crystallite to crystallite. This indeed found to be essential for materials suffering from low intrinsic electronic conductivity. Each single crystallite is elegantly encompassed by a thin mono layer carbon structure as explicitly seen from HRTEM images as reproduced in Figs. 2(b & c). The porous nature of the
TE D
amorphous carbon matrix is also seen in high resolution images. Electron diffraction (ED) image show single nano-crystallite (Fig. 2d) single crystal diffraction features with bright spots depicting
EP
olivine crystalline order confirming the single crystal electron diffraction pattern. This shows the merit of our preparation technique adapted in the present work. Such feature is regarded imperative
AC C
for materials for electrochemical capacitor applications.
10
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Figure 2. Transmission Electron Microscopy images (a) FETEM high resolution images showing carbon wire-like connectivity with individual crystallites on the TEM grid (b) magnified high resolution images depicting the carbon monolayer deposition onto LiMnPO4 single crystallite (c) 1nm scale resolution at the edge of single crystallite LiMnPO4 showing explicitly the carbon enclosure onto LiMnPO4 single crystallite, (d) Electron diffraction pattern obtained on single crystallite (as shown in the inset as indicated by arrow) showing a perfect crystalline ordering at the single particle location confirming the single crystallite of LiMnPO4.
11
ACCEPTED MANUSCRIPT
Electrochemical Studies of single electrode nC-LMP
To deduce the single electrode capacitance, we examined nC-LMP as working electrode against a calomel reference electrode in different aqueous electrolytic solution (neutral and alkaline media). The influence of cations on the capacitive performance of LiMnPO4 was distinguished with respect the composition of
RI PT
to different alkali cations in alkaline and neutral electrolytes. Moreover,
different aqueous electrolytes (alkaline, neutral ) was optimized in different molar concentrations (2M LiOH, 2M NaOH, 2M KOH, 1M Li2SO4, 1M Na2SO4 and 1M K2SO4) as well as in
SC
nonaqueous Li+ containing electrolyte, 1M LiClO4 in acetonitrile (AN).
In order to rule out the influence of the current collector substance (Exmet SS mesh) used in the
M AN U
present investigation in three electrode cell measurements were carried out in all electrolytes at first before the actual measurements being carried out. It is inferred from Figs 3 (a&b) that the current collector does not contribute to any significant capacitance as exemplified from current-voltage response compared to nC-LMP within the potential window studied.
The typical rate dependent
current vs. voltage plots obtained by means of cyclic voltammetry in alkaline and neutral
AC C
EP
TE D
electrolytes are shown in figs 3(a&b).
12
SC
RI PT
ACCEPTED MANUSCRIPT
M AN U
Figure 3. Typical current versus voltage characteristics of Exmet (USA) SS current collector mesh in order to show the least influence on the capacitive properties of the actual electrode materials studied in alkaline and neutral electrolytic media. Capacitive Performance in Alkaline Electrolyte
It is well known that LiMnPO4 exhibits reversible Li+ kinetics as in Li-Ion battery cathode in Hitherto, the redox characteristics of olivine LiMnPO4 in aqueous
TE D
nonaqueous electrolytes.
electrolytes have not been investigated to the best of our knowledge. In this report, we have demonstrated the surface redox capacitive characteristics, the so-called pseudo-capacitance, of
EP
olivine LiMnPO4 involving alkali cations such as Na+ and K+ and proven that LMP can also be a nC-LMP showed pseudocapacitive performance prominently in
AC C
candidate for developing LICs.
alkaline electrolytes which is duly evident from the well defined anodic/cathodic peaks as shown in Fig. 4. It is obvious that the redox peak positions are systematically shifted in the following order K+>Na+>Li+ and the cause for such a shift is perhaps due to the size of the cations.
13
ACCEPTED MANUSCRIPT 2M NaOH
5000
2M KOH
2M LiOH
4000 3000 2000 1000 0 -1000 -2000 -3000 -4000 -0.8
-0.6
-0.4 -0.2 Potential (V)
0
0.2
0.4
0.6
SC
-1
RI PT
Specific Capacitance Fg-1
6000
M AN U
Figure 4 Pseudocapacitive performance of nC-LMP originated from faradic redox reaction in different alkaline electrolytes (scan rate 5mVs-1). The origin of redox peaks are attributed to surface oxidation/reduction of Mn2+/Mn3+ owing to the charge transfer across the electrode/electrolyte interface during insertion of Li+ into LiMnPO4. The quantum of charge transfer (one electron transfer) is construed as pseudocapacitance solely from the
TE D
intercalation of Li+ as olivine structure favors the reversible intercalation of Li+ cations [30]. The size of Na+ and K+ cations ruled out the possibility of their intercalation into olivine nC-LMP. Hence only Li+ extracted from olivine structure LiMnPO4 (during the first charge) acts as mere LiMnPO4 in these
EP
charge storage species that resulted in pseud capacitive performance of
electrolytes. Since the quantity of Li+ is limited (electrolyte does not contain Li+ ion) the
AC C
electrochemical performance of nC-LMP has been determined to be rather inferior in these two electrolytes. Indeed, the above explanation is duly supported by the magnitude of redox current profile in voltammograms. Thus, it is revealed from the rate dependent cyclic voltammogram that the specific capacitance of nC-LMP is higher in LiOH than that in NaOH and KOH as expected. Besides, the capacity loss is more pronounced in KOH and NaOH (20-30%) while it is less than 10% for LiOH during first 50 cycles which are found to be stabilized after 50 cycles and the capacitance values are summarized in table 1. 14
ACCEPTED MANUSCRIPT
Table.1 Specific capacitance values of nC-LMP in different aqueous electrolytes. Average Specific Capacitance (Fg-1) at different scan rate
Electrolyte
10mVs-1
5mVs-1
2M LiOH 2M NaOH
242 144
341 200
622 363
2M KOH 1M Li2SO4
135 184
194 321
317 472
1M Na2SO4 0.5M K2SO4
162 146
247 221
RI PT
100mVs-1
365 264
SC
To elucidate the cycling stability, the cell has been cycled for over 1000 cycles in 2M LiOH at 5
M AN U
mVs-1 (S2). Thus, the cycle stability of nC-LMP has been confirmed via refined one electron process redox peaks and proven the fact the olivine LMP retains an excellent stability in aqueous solutions and its facile Li+ kinetics is witnessed through multiple slow scan voltammograms for the first time.
Capacitive performance in sulphate containing neutral electrolytes
TE D
The capacitive behavior of nC-LMP has been evaluated in neutral electrolytes containing alkali cations with a common sulphate anion. It has been found that nC-LMP showed pseudocapacitive
AC C
shown in Fig.5.
EP
behavior in neutral electrolytes also that is evidenced from a pair of prominent redox peak as
15
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Figure 5: Pseudocapacitive performance of nC-LMP in neutral alkaline electrolytes (scan rate 5mVs-1); Inset shows that upon subsequent cycles the kink-like oxidation current profile disappeared as expected. A pair of faradic redox peak that has been observed may perhaps be attributed to intercalation of Li+
TE D
ions into LiMnPO4 as similar to alkaline electrolytes. As expected, the additional kink-like oxidation profiles observed between +0.1 to +0.5V vs SCE potential regime during the first charge disappeared upon subsequent cycling (See inset in Fig. 5).
This is most likely due to the
EP
irreversible adsorption of alkali cations at the surface of coated carbon onto LMP. The deduced
AC C
specific capacitance values are lesser in neutral electrolytes than that in alkaline electrolyte probably caused by its lower electrolytic conductivity of the former than the latter. Fig. 6 presents rate dependent cyclic voltammetry of nC-LMP in 1M Li2SO4 neutral medium. The effective specific capacitance (Fg-1) has been deduced based on calculating the area under the curve within the specific potential window fixed.
However, the highest specific capacitance of 3025 Fg-1
observed at the reduction peak position in LiOH solution. The high capacitance is due to one electron transfer (Mn2+/Mn3+) which is found to be higher than the capacitance values reported for
16
ACCEPTED MANUSCRIPT
different materials exhibiting pseudocapacitance involving Li+ intercalation mechanism so far [4448]. Nevertheless, when calculating the average discharge specific capacitance considering the potential window used, the capacitance value has been calculated to 622 Fg-1 as average. Thus, the accomplished nano-carbon coating over LMP in the present work has been proven to be an effective
RI PT
approach to overcome the inherent low electrical conductivity of olivine LiMnPO4. 0.5 0.4
SC
0.3
0.1
M AN U
Current (A)
0.2
0 -0.1 -0.2
-0.4 -0.5
-0.5
10mVs-1
5mVs-1
100mVs-1
0 Voltage (V)
0.5
1
EP
-1
TE D
-0.3
AC C
Figure 6: Rate dependent voltammograms of nC-LMP in 1M Li2SO4 neutral medium Since the full version of LICs involving AC as counter electrode, it is imperative to determine its specific capacitance in these electrolytes before being combined in a full cell. Therefore, we have estimated the specific capacitance as well as the potential window of the AC counter electrode. In a separate experiment, therefore, we have characterized electrochemical capacitive behavior of the porous carbon electrode in neutral (Li2SO4) and alkaline electrolytes (LiOH) using a three electrode cell. Obviously, the performance of activated carbon is greatly influenced by the pH of the electrolyte. That is, in neutral electrolyte, the activated carbon exhibited a perfect rectangular 17
ACCEPTED MANUSCRIPT
shaped voltammogram and stable for a wide potential range, from -1.0V to + 0.8V indicating nonfaradic charge transfer reaction the so-called electric double layer capacitance (EDLC) while in alkaline electrolyte for instance, 2M LiOH, it exhibited near ideal EDLC behavior and the hydrogen evolution reaction was noticed somewhere close to -1.2V vs SCE that limits its utilizable potential
RI PT
window between 0.0V to -1.1 V vs. SCE [not shown here]. Moreover, when compared to its specific capacitance values the performance of activated carbon was found to be good in alkaline electrolyte with specific capacitance as high as 1126 Fg-1 than that the neutral electrolyte ( 986 Fg-1
SC
at 5 mVs-1). Li-Ion Capacitor (LIC): nC-LMP ||AC
M AN U
To realize LICs, both battery-type electrode (positive) – working electrode and capacitor-type electrode (negative)- counter electrode are combined in the presence of Li+ electrolyte. By properly balancing the weight of both electrodes based on their specific capacitance it would be possible to achieve the theoretical capacitance of LICs and to avoid overshadow effect of one of the electrodes.
TE D
Estimation of Electrochemical Potential window
Since the potential window of AC has been found to be narrow in the alkaline electrolyte, the
AC C
in Fig.7.
EP
working potential of LIC is fixed between 0.0V to 1.4V invoking the stability of nC-LMP as shown
18
ACCEPTED MANUSCRIPT
Figure 7. Cyclic voltamogram of single electrode capacitive characteristics of AC (mesoporous carbon) and nC-LMP in LiOH alkaline electrolyte. (Scan rate used 5mVs-1)
On the other hand, AC as well as nC-LMP has exhibited better stability towards Li2SO4 electrolyte as shown in Fig 8. Thus, the working potential of the LIC as a full cell has been estimated to be
RI PT
within 0.0 V to +1.8V by combining the anodic and cathodic potentials of AC and nC-LMP
TE D
M AN U
SC
electrodes as shown in Fig 8.
EP
Figure 8. Cyclic voltamograms of single electrode capacitive characteristics of AC (mesoporous carbon) and nC-LMP in Li2SO4 neutral electrolyte. (Scan rate used 5mVs-1).
AC C
Considering the features of voltammograms, LIC employing LiOH alkaline electrolyte retains the faradic signatures as seen in the single electrode voltammetric profiles (see Figs. 4 and 5). Interestingly, as seen in Fig. 9, non-faradic feature is evident for LICs involving neutral electrolytes for instance, Li2SO4. The measured capacitance values of LIC are summarized in table 2. The capacitance values are found to be high at slow scan rate as expected since at higher current rate the concentration polarization causes disproportionation of diffusion coefficient of ions and their transport to the interface in both electrolytes. The dependence of capacitance values of LIC as a function of scan rate is more pronounced in the case of LiOH solution and the peak position has 19
ACCEPTED MANUSCRIPT
noticeably changed as well that corroborates with the observed deviation of AC from the ideal double layer capacitive behavior. In contrast, the LIC employing Li2SO4 could retain its shape and peak position with respect to change in current rates. Non-aqueous electrolyte of composition 1M LiClO4 in acetonitrile (AN) was used for high voltage
RI PT
hybrid capacitor. It was found to be stable up to 3.0V without significant loss in the initial capacity value on cycling. The appearance of pair of redox peaks was attributed to insertion of Li+ into nCLMP accompanied by Mn2+/Mn3+ oxidation. Figures 10 (a, b & c) describe the rate dependent
SC
voltammograms of LIC having the cell configurations, AC|LiOH|nC-LMP, AC|Li2SO4|nC-LMP
M AN U
and AC|1M LiClO4 in AN|nC-LMP.
800 600
200
TE D
Capacitance (Fg-1)
400
Li2SO4 LiOH LiClO4/AN
0 -200
EP
-400 -600
AC C
-800
0
0.5
1
1.5
2
2.5
3
Potential (V)
Figure 9. Cyclic voltammogram of Li-Ion Capacitor, nC-LMP//AC in alkaline, neutral and nonaqueous electrolytes. (Scan rate used 5mVs-1)
20
ACCEPTED MANUSCRIPT Table 2: Capacitance values of LIC of configuration electrolytes.
in different aqueous
Specific Capacitance / Capacitance (Fg-1) at different scan rate 100mVs-1 10mVs-1 5mVs-1 52.5 65 73
106 85 86
170 99 96
SC
nC-LMP|LiOH|AC nC-LMP|Li2SO4|AC nC-LMP|LiClO4 in AN|AC
RI PT
Electrolyte
M AN U
0.08 0.06 0.04 0.02
TE D
0
-0.02 -0.04 -0.06
0.5
1 Voltage (V)
100mVs-1 10mVs-1 5mVs-1
1.5
2
AC C
0
EP
Current (A)
nC-LMP||AC
Figure 10 (a). Rate dependent cyclic voltammograms of LIC, nC-LMP /LiOH/AC showing faradic-like capacitive properties.
21
ACCEPTED MANUSCRIPT
0.05
5mVs-1
0.04
10mVs-1
0.03
100mVs-1
0.02 0.01
RI PT
Current (A)
0.06
0
-0.01 -0.02 -0.04 0
0.5
1
SC
-0.03
1.5
2
M AN U
Voltage (V)
Figure 10(b). Rate dependent cyclic voltammograms of Li-Ion capacitor, AC /Li2SO4/ nC-LMP capacitor showing Non-faradic like capacitive behavior.
0.04 5mVs-1
TE D
0.03
10mVs-1
100mVs-1
0.01
EP
Current (A)
0.02
0
AC C
-0.01 -0.02 -0.03
0
0.5
1
1.5 Voltage (V)
2
2.5
3
Figure 10(c). Rate dependent cyclic voltammograms of LIC, nC-LMP /1MLiClO4/AC showing Non-faradic like capacitive behavior.
22
ACCEPTED MANUSCRIPT
Electrochemical Impedance Spectroscopy (EIS)
Impedance behavior of AC|Li2SO4|nC-LMP asymmetric capacitor before and after the cycling (scan rate used 5mVs-1) is described in Fig.11 and the ESR values measured at 1kHz before and after cycling
TE D
M AN U
SC
RI PT
are summarized in table 3.
EP
Figure 11. Electrochemical Impedance spectra of AC||nC-LMP (Inset shows the high to low frequency regime showing the Warburg diffusion behavior at the interface).
AC C
Table 3: Change in ESR values in different electrolytes on cycling nC-LMP||AC Electrolyte Fresh
ESR(ohm) After 1000cycles
2M LiOH
0.85
1.26
1M Li2SO4
1.16
1.43
1M LiClO4 (AN)
1.47
2.26
Like in the case of electric double layer capacitors (EDLCs), Fig 11 confirms that AC||nC-LMP, asymmetric capacitor exhibits true capacitive behavior as exemplified in Nyquist plot in which the 23
ACCEPTED MANUSCRIPT
capacitive characteristics are seen explicitly at low frequencies below 10Hz down to 10mHz. A small geometric bulk impedance seen with an indication of high to mid frequency contour-like (semicircle) frequency response probably attributed to bulk impedance behavior of the cell geometry and/or interfacial impedance due to current collector and electrode coating. A low frequency spike-like
RI PT
frequency response occurs at low frequency regime indicative of double layer capacitive behavior that can be attributed to pure capacitive behavior as normally being observed for Carbon/Carbon symmetric EDLCs. It is interesting to note that the ESR values do not show any remarkable variation upon cycling
SC
LIC (see table 3). In comparison, it is noteworthy that the change in ESR values are found to be less prominent in neutral electrolyte indicating the stability of both electrodes in neutral electrolyte and
M AN U
hence the cycle life. Moreover, in alkaline electrolytes the change is little more pronounced that reflects the inferior performance of activated carbon in terms of stability in that electrolyte. Galvanostatic charge/discharge
In order to complement the cycling characteristics, LICs were subjected to constant current cycling as well. Due to the integration of two different storage mechanisms in a cell, the shape of
TE D
charge/discharge curve is expected to be different from that of pure double layer capacitor. Both charge/discharge curves resemble those of capacitor-like behavior excepting that at low current
EP
rates (0.6 Ag-1) it resemble that of battery-like behavior with plateau region as shown in fig 12.
AC C
whereas at high current rates (3.0 Ag-1).
24
ACCEPTED MANUSCRIPT
3.5
LiOH
3 2.5
NEt4BF4/AN
2 1.5
RI PT
Potential (V)
Li2SO4
1
0 0
50
100
150
SC
0.5
200
300
350
M AN U
Time (s)
250
Figure 12. Typical constant current charge/discharge cycling for AC|nC-LMP at 0.6Ag-1 current rate for alkaline (1M LiOH), neutral (1M Li2SO4) and nonaqueous electrolytes (1M LiClO4 in AN)
TE D
Interestingly, the plateau region explains the improved energy storage capacity of the Li-ion capacitor at low current rates. Further, in order to study the cycle life and stability over cycling, these LIC cells were cycled over 1000 times as shown in Fig 13. The observed loss is found to be
EP
minimal in both electrolytes (8-12%). The achieved capacitance for LiOH based electrolyte is higher compared to Li2SO4 neutral electrolyte due to the synergetic effect of better capacitive
AC C
performance of both electrodes in LiOH. Even though the use of nonaqueous electrolyte enhances the working voltage of ~3V, the capacitance of LIC using nonaqueous electrolyte was found to be lower than that of aqueous electrolyte (LiOH) as expected and that might be attributed to its high ESR value originated from its two orders less conductivity yielding high IR drop.
25
ACCEPTED MANUSCRIPT
200
150
100
50
1M Li2SO4
RI PT
Capacitance(Fg-1)
250
1M LiClO4/AN
1M LiOH
0
200
400
600
SC
0 800
1000
1200
M AN U
Cell Cycle Stability
Figure 13. Cycling stability of Li-Ion capacitor nCLMP/Li+/AC with alkaline, neutral and nonaqueous electrolyte on galvanostatic cycling at the rate of 0.3Ag-1. The graphical correlation of energy and power densities of electrochemical power sources can be
TE D
depicted by Ragone plot as seen in fig.14. Accordingly, we have deduced the specific power and specific energy of AC|nC-LMP capacitor at different current rates using equations 1 and 2 and summarized in table 4. As for the specific power, the higher values obtained for LiOH for LIC is
EP
presumably due to synergic effect of high specific capacitance and low ESR value. Moreover, the higher specific energy observed in non aqueous electrolyte might be due to wide potential window
AC C
(0.0V-3.0V). Thus, the LIC system studied in neutral medium rendered a reasonably a high specific energy of 28.8Whkg-1 with a specific power of 2.5KWkg-1 while a high specific power of 12.6 KWkg-1 with a modest specific energy of 10.7 Whkg-1. It is noteworthy that the LICs studied in this work could maintain high specific energy and high specific power values even at high current rate as given in table 4.
26
ACCEPTED MANUSCRIPT
10
RI PT
Specific Energy (Whkg-1)
100
1 LiOH
SC
Li2SO4 LiClO4 in AN
0.1
1000 Specific Power (Wkg-1)
M AN U
100
10000
Figure 14. Ragone plot correlates specific energy and power of AC//nC-LMP Li Ion capacitors in Aqueous electrolytes (LiOH and Li2SO4) and Non-aqueous electrolyte 1MLiClO4/AN) at 0.6Ag-1.
Electrolyte/ current Rate
TE D
Table 4: Summary of Energy and Power density values measured at specific current densities 0.6Ag-1
LiOH
28.0 2.5
1MLiClO4 LiOH /AN 35.0 10.2
12.6
1MLiClO4 /AN 15.6
1.7
10.7
9.7
11.6
Li2SO4
AC C
EP
Specific Energy(Whkg- 16.5 1) Specific Power(kWkg-1) 3.3
Li2SO4
3Ag-1
The achieved specific energy and specific power of AC||nC-LMP is much higher than that of LiCoPO4 (2.9WhKg-1 and 0.19kWkg-1) using nonaqueous electrolyte working at high voltage >3V [47]. Such achievement is attributed to the novel method of preparation of nC-LMP which provides a very connecting path ways for low electronically connecting open framework olivine LiMnPO4.
27
ACCEPTED MANUSCRIPT
Conclusion
nC-LMP nano-composite electrode material has been found to exhibit facile pseudocapacitive behavior in Li+ containing aqueous and nonaqueous electrolytes and the effects of alkaline and neutral electrolyte in the context of capacitance, energy and power densities were distinguished. The
RI PT
surface insertion originated from the pseudocapacitive effect via Li+ deinsertion/insertion into olivine structure accompanied by the redox reaction owing to Mn2+/Mn3+ in LiMnPO4 leads to the capacitive behavior as observed. The high specific capacitance values of nC-LMP might possibly be attributed to high electrolytic conductivities of aqueous solutions (LiOH and Li2SO4) which
SC
favors the diffusion of Li+ into the olivine material, LiMnPO4 causing the pseudocapacitance. The
M AN U
novel method of single crystallite level intimate carbon enclosure has proven to be a viable approach to overcome the niche conductivity problems associated with olivine LMP that helped achieve a high specific capacitance of 622 Fg-1 with a peak profile capacitance of 3025 Fg-1 which is indeed a high measured value for such insertion type electrodes hitherto the best of our knowledge. Considering the working potential of the Li-ion capacitor, by combining the individual
TE D
potential window of both nC-LMP and AC as single electrodes, the potential window of LIC extends up to 1.8V in neutral aqueous electrolyte presumably due to the delayed onset of O2/H2
EP
evolution. Moreover, the latter exhibited more of non-faradic kind of capacitive characteristics. In contrast, in alkaline electrolytes, much pronounced faradic type capacitive profiles have been
AC C
observed. Because of the wide potential window, LIC with Li2SO4 rendered a specific energy as high as 28.8 Whkg-1. The highest specific power, 14.6 kWkg-1 obtained in LiOH electrolytes that presumably attributed to the cumulative effect of low ESR and high specific capacitance. Considering the performance of aqueous electrolytes with respect to capacitance properties of nCLMP, we have also studied the same with nonaqueous electrolyte (1MLiClO4 in AN) to understand how well aqueous counterpart performs.
Although its energy density has been remarkably
improved as obvious owing to its larger potential window in non-aqueous electrolytes, the 28
ACCEPTED MANUSCRIPT
capacitance values are found to be as good as aqueous electrolytes. Hence, considering stability of nC-LMP in aqueous electrolytes, it would be wonderful to develop hybrid capacitor without much stringent fabrication conditions for practical applications. Also, we conclude that the present study opens up new avenues for developing aqueous type Li-Ion batteries employing such carbon
RI PT
composite olivine cathodes which are plausibly evading away the safety concerns centered on LiIon batteries as well. Acknowledgement
SC
One of us (SRSP and AAS) would like to thank NRB, DRDO, India for the financial support through the project No: NRB-313/MAT/13-14 to carry out a portion of this work.
[1]
M AN U
References
B.E. Conway, Electrochemical Supercapacitors: Scientific fundamentals and technological applications, Kluwer Academic/Plenum: New York, 1999.
[2]
A.S. Arico, P. Bruce, B. Scroati, J-M. Tarascon, W. V. Schalkwijk, Nature Materials, 4 (2004) 366.
T. Brousse, D. Be’langer, Electrochem. Solid-State Lett. 6 (2003) A244.
[4]
V. Khomenko, E. Pinero, E.Frackowiak, F. Beguin, Appl. Phys. A: Mater. 82 (2006) 567.
[5]
S. Faraji, F.N. Ani, J. Power Sources, 263 (2014) 338.
[6]
D. Cericola, P. Novak, A. Wokaun, R. Kotz, J. Power Sources, 196 (2011) 10305.
[7]
B.E. Conway, W.G. Pell, J. Solid State Electrochem. 7 (2003) 637.
[8]
G.G. Amatucci, F. Badway, A.D. Pasquier, T. Zheng, J. Electrochem.Soc, 148 (2001) A930.
[9]
A.D. Pasquier, A. Laforgue, P. Simon, J. Power Sources, 125 (2004) 95.
AC C
EP
TE D
[3]
[10]
H. Li, L. Cheng, Y. Xia, Electrochem. Solid-State Lett. 8 (2005) A433.
[11]
T. Aida, I. Murayuma, K. Yamada, M. Morita, Electrochem. Solid-state Lett. 10 (2007)A93.
[12]
H. Li, L. Cheng, Y. Xia, Electrochem. Solid-State Lett. 8 (2005) A433.
[13]
S. Kumagai, T. Ishikawa, N. Sawa, J. Energy storage 2 (2015) 1 29
ACCEPTED MANUSCRIPT
[14]
W.J. Cao, J.P. Zheng, J.Power Sources 213 (2012) 182-185.
[15]
V. Khomenko, E. Raymundo-Pinero, F. Beguin, J. Power Sources 177 (2008) 643.
[16]
G. G. Amatucci, F. Badway, A. Du Pasquier, T. Zheng, J. Electrochem.Soc.148 (2001) A930. X. F. Luo, C. Wang, H. P. Yang, M. Yuan, ECS Trans. 59 ( 2014) 85.
[18]
C. Decaux, G. Lota, E. R. Pinero, E. E. Frackowiak, F. Beguin, Electrochim. Acta, 86
RI PT
[17]
(2012) 282.
G. G. Amatucci, F. Badway, A. Dupasquier, ECS Proc. 99 (2000) 344.
[20]
I. Plitz, A. Dupasquier, F. Badway, J. Gural, N. Pereira, A. Gmitter, G.G. Amatucci, Appl.
SC
[19]
[21]
N. Arun, A. Jain, V. Aravindan, S. Jayaraman, W. C. Ling, M. P. Srinivasan, S. Madhavi, Nano Energy 12 (2015) 69.
[22]
M AN U
Phys. A 82 (2006) 615.
L. Kavan, T. N. Murakami, P. Comte, M. Graetzel, Electrochem. Solid-state Lett, 10 (2007) A85.
P. C. Chen, G. Shen, Y. Shi, H. Chen, C. Zhou, ACS Nano 4 ( 2010) 5835.
[24]
Y.G. Wang, Y. Y. Xia, Electrochem. Commun. 7 (2005) 1138.
[25]
K. Naoi, S. Ishimoto, Y. Isobe, S. Aoyagi, J. Power Sources 195 (2010) 6250.
[26]
A. Laforgue, I. Plitz, J. Gural, S. Menocal, G.G. Amatucci, J. Power Sources 113 (2003) 62.
[27]
M.S. Michael, S.R.S. Prabaharan, J. Power Sources 136 (2004) 250.
[28]
M.S. Michael, Electrochim. Acta 120 (2014) 350.
[29]
F. Zhou, M. CoCoccioni, K. Kang, C. Cedar, Electrochem. Commun. 6 (2004) 1144.
[30]
A. K. Padhi, K. S. Nanjundaswamy, J. B. Goodenough, J. Electrochem. Soc. 144 (1997)
AC C
EP
TE D
[23]
1188. [31]
Z.X. Nie, C.Y.Ouyang, J.Z.Chen, Z.Y. Zhong,Y.L.Du, D.S.Liu, S.Q.Shi, M.S.Lei, Solid State. Commun.150(2010)40 30
ACCEPTED MANUSCRIPT
[32]
A. Yamada, Y. Kudo, K.Y. Liu, J. Electrochem. Soc. 148 (2001) A1153.
[33]
N. Meethong, H. Y. S. Huang, S.A. Seakmam, W. C. Carter, Y. M. Chiang, Adv. Funct. Mater. 17 (2007) 1115.
[34] S. K. Martha, B. Markovsky, J. Grinblat, Y. Gofer, O. Haik, E. Zinigrad, D. Aurbach , T.
[35]
RI PT
Drezen, D. Wang, G. Deghenghi, I. Exnar, J. Electrochem. Soc. 156 (2009) A541. M. Yonemura, A. Yamada, Y. Tkei, N. Sonoyama, R. Kanno, J. Electrochem. Soc. 151 (2004) A1352.
V. Aravindhan, J. Gnanaraj, Y. S. Lee, S. Madhavi, J. Mater.Chem.A,1(2013) 3518
[37]
D. Morgan, V. Van, G. Ceder, Electrochem. Solid-State Lett. 7 (2004) A30.
[38]
P. K. Ramesh, M. Venkateswarlu, M. Manjusri, K. M. Amar, N. Satyanarayana, J.
M AN U
SC
[36]
Electrochem. Soc. 158 (2011) A227. [39]
N.H. Kwon, T. Drezen, I. Exnar, I. Teerlinck, M. Isono, M. Graetzel, Electrochem. SolidStat. Lett. 9 (2006) A277.
G.Li, H. Azzuma, M. Tohda, Electrochem.Solid-state Lett. 5(2002)A135.
[41]
R. Dominko, M. Bele, M. Gaberscek, M. Remskar, D. Hanzel, J. M. Goupil, S. Pejovnik,
TE D
[40]
J. Jamnik, J. Power Sources 153 (2006) 274. C. Largeot, C. Portet, J. Chimiola, P. L. Tabema, T. Gogotsi, P. Simon, J. Am. Chem. Soc.
EP
[42]
130 (2008) 2730.
I. E. Rauda, V. Augustyn, B. Dunn, S. H. Tolbert, Acc. Chem. Research 46 (2013) 1113.
[44]
D. Cericola, R. Kotz, Electrochima. Acta. 72 (2012) 1.
[45]
Y. J. Hao, Q. Y. Lai, L. Wang, X. Y. Xu, H. Y. Chu, Synthetic Metals, 160 (2010) 669.
[46]
A. Yuan, Q. A. Zhang, Electrochem. Commun. 8 (2006) 1173.
[47]
V. Aravindan, J. Gnanaraj, Y-S. Lee, S. Madhavi, Chem. Rev. 114 (2014) 11619.
[48]
F. X. Wang, S.Y. Xiao, Y.S. Zhu, Z. Chang, C.L. Hu, Y. P. Wu, R. Holze, J. Power Sources
AC C
[43]
246 ( 2014) 19. 31
ACCEPTED MANUSCRIPT
Highlights
1) We report on carbon coating onto single crystallite LiMnPO4 via CVD approach.
RI PT
2) Nanocarbon composite of LiMnPO4 rendered highest capacitance of 3025 Fg-1. 3) Hybrid capacitor employing nano-composite LiMnPO4 is reported for the first time.
AC C
EP
TE D
M AN U
SC
4) AC// nC-LMP hybrid capacitor delivered a high specific energy of ~35Whkg-1.
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
1 µm
AC C
Figure 2. FESEM image showing the well connected spherical crystallites of as-prepared LiMnPO4 annealed at 400oC prior to carbon treatment.
ACCEPTED MANUSCRIPT
6000
RI PT
4000 3000
SC
2000
M AN U
1000 0 -1000 -2000 -3000 -4000 -0.8
-0.6
-0.4
-0.2
0
0.2
0.4
EP
-1
TE D
Specific Capacitance (Fg-1)
5000
AC C
Voltage (V)
Figure 5. Cycling stability of nC-LMP in 2M LiOH aqueous media at 5mVs-1showing the facile faradic redox peaks. One electron redox process is evident with a wide potential window in aqueous media (Typical plots are shown for clarity amongst 1000 cycles).
ACCEPTED MANUSCRIPT
RI PT
55
SC
50 45
M AN U
Type IV isotherm Capillarity condensation
40 35
TE D
30 25 20 0.2
0.4
0.6
0.8
1
AC C
0
EP
Quantity Adsorbed (mmolg-1)
60
Relative Pressure (p/p°)
Nitrogen Sorption Data showing the Type IV isotherm Capillarity condensation of Mesoporous carbon chosen for the present study for fabricating LIC ; BET surface area 3202 m²/g; BJH Pore width 2.7nm
S1567-1739(15)30074-2
DOI:
10.1016/j.cap.2015.09.009
Reference:
CAP 4072
To appear in:
Current Applied Physics
Received Date: 30 May 2015 Revised Date:
14 August 2015
Accepted Date: 10 September 2015
Please cite this article as: S.R.S. Prabaharan, A Anslin Star, A.R. Kulkarni, M.S. Michael, Nanocomposite LiMnPO4 as New Insertion Electrode for Electrochemical Supercapacitors, Current Applied Physics (2015), doi: 10.1016/j.cap.2015.09.009. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
RI PT
LiMnPO4 single crystallite
SC
Thin Porous carbon connectivity
600 400 200 0 -200 -400 -600 -800 0
0.5
M AN U
Capacitance (Fg-1)
800
1
1.5
2
Li2SO4 LiOH LiClO4/AN
2.5
TE D
Potential (V)
AC C
EP
AC//nC-LMP Asymmetric Hybrid Capacitor Voltammetric profile
3
ACCEPTED MANUSCRIPT
Nano-composite LiMnPO4 as New Insertion Electrode for Electrochemical Supercapacitors
RI PT
S.R.S. Prabaharan1)# A. Anslin Star1), Ajit R. Kulkarni2), M.S. Michael3)#, 1)
School of Electronics Engineering (SENSE), VIT University - Chennai Campus, Chennai, India
2)
Department of Metallurgical Engineering and Materials Science, IIT Bombay, Powai, Mumbai,
SC
400076, India. 3)
AC C
EP
TE D
M AN U
Chemical Sciences Research Lab, SSNCollege of Engineering, Chennai-603110, India
#
Corresponding authors M.S.Michael. Email: [email protected] Phone: +914427474844;
S.R.S. Prabaharan. Email: [email protected] Phone: +914439931187
1
ACCEPTED MANUSCRIPT
Abstract:
Nano-composite olivine LiMnPO4 (nC-LMP) was found to exhibit facile pseudo-capacitive characteristics in aqueous as well as non-aqueous electrolytes. We demonstrated employing the same as positive in hybrid electrochemical capacitors namely Li-Ion hybrid capacitors (LIC).
RI PT
Adapting a simple CVD technique, nano-crystallites of LiMnPO4 (LMP) were coated with carbon monolayers of ~2 nm thick to circumvent its poor intrinsic electronic conductivity. The novelty is that the single crystallites were intimately covered with carbon ring and networked to the neighboring crystallites via the continuous carbon wire-like connectivity as revealed from HRTEM
SC
analysis. Single electrode faradic capacitance of 3025 Fg-1 (versus standard calomel reference
M AN U
electrode) was deduced for carbon coated LMP, the highest reported hitherto in Li+ aqueous electrolytes. Employing nC-LMP as working electrode versus an activated carbon (AC) counter electrode, we obtained a high specific energy of 28.8Whkg-1 with appreciable stability in aqueous electrolytes whereas in nonaqueous electrolyte there is an obvious increase in energy density (35Whkg-1) due to larger potential window. That is, a full cell version of LIC, AC|Li+|LMP, was
TE D
fabricated and demonstrated its facile cycling characteristics via removal/insertion of Li+ within LiMnPO4 (positive electrode) and the electrosorption of Li+ into mesoporous carbon (AC) (negative Such cells ensured a typical battery-like charging and EDLC-like discharging
EP
electrode).
characteristics of LIC type electrochemical capacitors (ECs) desired to enhance safety and higher
AC C
energy densities.
2
ACCEPTED MANUSCRIPT
Introduction
Porous carbon materials exhibit non-faradaic type capacitance storing energy in the form of a simple electrostatic ion separation known as Electrical Double Layer Capacitors (EDLCs). Their specific capacitance values are lower than that of faradaic type electrode materials like metal oxides
RI PT
which store energy by virtue of surface redox reaction occurs at a depth of few nanometers from the surface. The charge transfer reaction is so fast that its charging characteristics are comparable with that of an EC exhibiting pseudocapacitive properties [1,2]. Different strategies were developed to
SC
improve the specific energy of ECs without compromising the specific power. One of the approaches recently being pursued is the hybrid capacitors (HCs) in view of obtaining both high
M AN U
specific energy (kWhkg-1) and high specific power (kWkg-1) in a single cell. Thus evolved a new approach by combining EDLC electrode (for Power) and an electrode exhibiting battery-like redox characteristics (pseudo-capacitance) forming asymmetric capacitor in which one of the electrodes acts as a cathode while the other electrode being an anode, extends potential window of the capacitor that resulted in substantial enhancement of specific energy [3-5].
Recently, active
TE D
research has been focused on a new concept of hybrid capacitor, known as Li-ion capacitors (LICs) which are identified as a new class of electrochemical capacitors (ECs) that take advantage of Li+
EP
as charge transport species between the positive and negative electrodes but at faster rates unlike LiIon batteries. The electrochemical redox reaction originates from the surface redox kinetics rather
AC C
than from the bulk of insertion type positive electrodes. Therefore, it is imperative that ionic and electronic conductivities of the positive electrode materials are rather crucial to realise specific capacity (mAhg-1) converted specific capacitance (Fg-1). Perhaps, most of the Li-intercalation electrodes itself behave like a redox capacitor exhibiting pseudocapacitance since they show steady increase/decrease of voltage during discharging/charging as happens in electrochemical capacitors (ECs) [6]. In these LICs, the source for charge storage is governed by means of Li+ ions supplied by insertion electrode itself that is, in its pre-lithiated form [7-10]. There are reports concentrating 3
ACCEPTED MANUSCRIPT
on non-lithiated electrodes stabilized with Li metal powder and/or pre-doped with lithium by electrochemical means [11-14]. By employing lithium containing nonaqueous electrolyte in LICs, it is possible to enhance the power and energy density of the capacitor due to high oxidative potential of the electrode materials ca. 3V - 5V vs. Li/Li+ [15-18]. The concept of LICs was first
RI PT
proposed by Amatucci et al. by fabricating a 2.8V hybrid capacitor involving Li4Ti5O12 and activated carbon (AC) electrodes [19]. Since then research has been intensified for the improvement of specific energy of hybrid capacitor using different types of Li-intercalation compounds and found significant enhancement in energy density [20-26]. Later, another concept was introduced
SC
with additive carbons as electrodes in conjunction with Li+ nonaqueous electrolyte achieving a high
M AN U
voltage (> 4V) Carbon/Carbon LICs [27]. Quite recently, a new pseudocapacitive material ca. Cu doped NiCo2O4 was reported in which very significant improvement in the capacitive performance of host NiCo2O4 using aqueous Li+ containing electrolytes was reported for the first time [28]. Aiming at introducing newer promising materials as electrodes for Li-Ion capacitor olivine type cathode-active material nC-LMP as Li+ ion source has been studied for the first time. The
TE D
present study demonstrates the stability of the cathode material in aqueous electrolytes. It is well known that phospho-olivine compounds have been studied extensively as positive electrode
EP
(cathode) materials for Li-ion batteries and these materials are proven to offer a variety of advantages for practical systems due to their low-cost, non-toxicity and outstanding thermal There have been number of reports on LiMnPO4 as one of the promising
AC C
stability [29-31].
candidate positive electrodes in advanced Li-Ion batteries owing to its high theoretical capacity (170 mAhg-1) and high redox potential. However, hitherto the achievement of its full theoretical capacity has not been realized due to its low electronic conductivity, small polaronic conduction of Jahn-Teller active Mn3+ and high surface energy barrier for Li+ diffusion at the surface [32-35]. Since the rate of charge storage in any intercalation electrode material is determined by the movement of electrons as well as the transfer of ions to the active sites, the structure of the material 4
ACCEPTED MANUSCRIPT
is crucial to enhance the performance of LIC. Several groups attempted different strategies to improve the electrical conductivity of LiMnPO4 and demonstrated that the intrinsic low rate capability can be overcome by nano-sizing the particles or carbon coating [36-40]. It is widely proven that nanoscale materials are beneficial for increasing electrode/electrolyte interface,
RI PT
shortening the diffusion length and easing the access of the ions to the surface intercalation sites [41-43]. Despite extensive research attempts hitherto particularly on LiMnPO4 to evaluate its suitability as a good cathode substitute for renowned LiCoO2 in lithium ion batteries, its potential as
SC
pseudo-capacitive material was not explored as yet to the best of our knowledge. In the present study, we report our success in making a single phase nanoscale crystallites of LiMnPO4
M AN U
synthesized via a simple solution synthesis protocol and subsequently an ex-situ carbon capping onto single crystallites of olivine LiMnPO4 facilitating a perfect carbon wiring between crystallites as well. This approach facilitated primarily the penetration of electrolyte solution into pores of porous carbon encircled around single crystallites. Thus, the pseudocapacitive electrode-active characteristics of nanoporous carbon encompassed single crystallite of LiMnPO4 were studied in
TE D
aqueous electrolyte media (neutral and alkaline solution) and its specific capacitance (Fg-1) was measured accordingly. Finally, a full cell version of a Li-Ion capacitor including a carbon
EP
encompassed LiMnPO4 namely nC-LMP as cathode against a (EDL) mesoporous activated carbon (AC) exhibiting electric double layer characteristics. In this paper, we report our new approach
AC C
towards developing such LICs employing monolayer carbon encompassed olivine type LiMnPO4 against mesoporous activated carbon and the results are reported based our systematic studies employing both aqueous and non-aqueous electrolytes.
5
ACCEPTED MANUSCRIPT
Experimental
Synthesis and characterization of carbon coated LiMnPO4 nanocrystallite Single phase LiMnPO4 powder sample was prepared by using a proprietary synthesis protocol involving lithium and manganese acetates and triethyl phosphate as starting materials. The
RI PT
stoichiometric quantities of the above starting materials were dissolved in appropriate quantity of de-ionized water/iso-propanol mixtures (4:1 ratio) separately and sonicated using an ultrasonic processor (UP200S 200 watts, 24kHz Hielscher Ultrasonics GmbH, Germany) for 10 min.
SC
Subsequently, 100 ml saturated solution of citric acid was added to the above mixture solution containing stoichiometric amounts of Li/Mn/P. The resulting dark colored clear solution was
M AN U
continuously stirred at 90 °C until a sticky paste-like substance was obtained. Using the following temperature programmed reaction (TPR) protocol, the dried precursor (as-prepared) powder was heated in a programmable tubular furnace (Carbolite, U.K) in three different steps firstly: at 350°C at a rate of 5oCmin-1 for 30 min, then to 450°C by 0.5oCmin-1 for 30 min and to 650°C by 2.5 o
Cmin-1 and finally the temperature was maintained at 650oC for 6 h under a flowing pure argon
TE D
atmosphere. When the programmed time was over, the sample was automatically cooled down to room temperature until then argon gas atmosphere was maintained throughout. During the entire
EP
heating process, the gas flow rate has been maintained at 1ml min-1 (rate of heating and cooling) to ensure uniform distribution of particles and to avoid surface cracking. This kind of synthesis allows
AC C
a better control of the morphology and texture of solid particles. In a distinct experiment, fine monolayer of carbon coating was accomplished employing an exsitu heating procedure in a CVD furnace by mixing the predetermined quantities of pure single phase nano-crystallites LiMnPO4 mixed with appropriate quantity of glucose and heat treated at 600°C (5oCmin-1) for 2 h under flowing argon/H2 gas atmosphere (flow rate was maintained at 1 mlmin-1). The reaction tube was pre-flushed with argon gas for 30 min to ensure O2 free atmosphere. The reacted sample was cooled down to room temperature under the same gaseous 6
ACCEPTED MANUSCRIPT
atmosphere before being submitted for other characterization.
Powder XRD was obtained using X’pert PRO MPD (Pan Analytical instrument, North Holland) equipped with CuKα radiation and the preliminary refinements of lattice parameters were performed using X'Pert High Score Plus software. The grain size, morphology and nanostructure
RI PT
were examined employing a field emission scanning electron microscopy (FESEM) (FEI Quanta 400F, The Netherlands) and FE-TEM analysis was performed using a FEI-TITAN machine (Titan G2 80-200 with Cs-corrector, 80kV) to observe thin carbon onto single crystallite LiMnPO4.
SC
This is the important observation of this work where a single crystallite has been found to be independently coated and linked or wired to other neighboring grains in the same manner. Electron
spherical crystallite from TEM Cu grid. Electrochemical characterization
M AN U
diffraction (ED) image has been obtained using the same machine by locating an individual
Slurry containing composite nC-LMP electrode was prepared by adding a solvent, N-methyl pyrrolidine (NMP) to the admixture of nC-LMP, PVDF binder and conductive additive carbon
TE D
(PRINTEX® L6, Degussa, Germany) in a weight ratio of 70:10:20. The slurry was then coated onto SS foil (Exmet, USA) uniformly by employing automatic film applicator with a bar coater (SHEEN,
EP
U.K). The carbon electrode was also prepared following the approach except that weight ratio of carbon composition with PVDF binder was 80:20. The coating was dried overnight under vacuum
AC C
at 110oC and ‘1cm x 1cm’ sized coated SS foil was duly cut with the loading mass of the active material varied from 15mgcm-2 to 20mgcm-2 for nC-LMP and 10mgcm-2 to 15 mgcm-2 for activated carbon (AC) electrodes. The pseudo-capacitive performance of nC-LMP was deduced using a conventional three electrode system having a saturated calomel electrode (SCE) and a platinum foil (~4 cm2) as reference and the counter electrode respectively.
Rate dependent cyclic
voltammogramms were performed in the range from 5.0 mVs-1 to 100 mVs-1. Cyclic voltammetry (CV) studies were carried out employing both aqueous and nonaqueous 7
ACCEPTED MANUSCRIPT
electrolytes containing Li+ using a Versastat3 potentiostat (AMETEK, USA). An another electrochemical workstation (Wonatech Multichannel system Model Zive MP5, South Korea) equipped with an impedance analyzer, was employed to study galvanostatic charge/discharge characterization, EIS analysis and rate dependent cyclic voltammetry (CV). An ac perturbation
RI PT
signal of 10mV peak-to-peak potential was applied in the frequency, range from 0.1Hz to 10 kHz for the electrochemical impedance spectroscopic (EIS) measurements.
We deduced the specific capacitance values of electrode material by integrating the area under CV curves divided by the specific current over the scan rate as per the formula, C= I/sm, where ‘C’ is
SC
the capacitance in ‘Fg-1’, ‘I’ is the average anodic/cathodic current in ‘A’, ‘s’ is the voltage sweep
M AN U
rate in ‘Vs-1 and ‘m’ is the weight of the active material in ‘g’ excluding the weight of coated carbon material. All electrochemical measurements were conducted in ambient air at room temperature. Aqueous Li-Ion Capacitor (LIC) cells were fabricated using a pseudo-capacitive nC-LMP as positive electrode and a porous EDL carbon (>3000 m2g-1) exhibiting double layer capacitive characteristics as a negative electrode in a two electrode cell made of 316L SS designed in-house. A
TE D
Whatman™ filter paper (grade 1) was used as a separator. The weight ratio of positive and negative electrode is optimized based on the specific capacitance values of both electrodes. On the other
EP
hand, LICs employing non-aqueous electrolyte were also assembled using 2032 coin cell hardware inside the Ar filled glove box. Celgard™ microporous polypropylene (2400) was used in this case
AC C
as separator.
Galvanostatic cycling at different current densities was used to calculate the specific power and specific energy of LICs using the formulae given below. Specific Power (P) = Specific Energy (E) =
Wkg-1 Whkg -1
………………
(equation 1)
………………
(equation 2)
Where ‘V’ is potential of the capacitor excluding ‘IR’ drop (ESR) in volt, ‘I ‘is current in ‘A’, ‘m’ is 8
ACCEPTED MANUSCRIPT
the total mass of active material in ‘kg’ and ‘t’ is the time in hour. The Equivalent Series Resistance (ESR) was obtained from EIS measurement at 1 kHz. Results and Discussion Synthesis of Single Phase nC-LMP
RI PT
The crystalline structure of pure and carbon embedded LiMnPO4 materials were studied using powder X-ray diffraction analysis. XRD refinement (Rietveld) pattern of as-prepared LiMnPO4 is shown in Fig. 1 prior to ex-situ carbon coating. Rietveld refinement ensures the pattern matched
SC
with calculated peaks although the difference is more between 20-40o 2θ angle, it is attributed obviously to the nanostructure nature of the powder samples as compared to the standard pattern.
M AN U
Hence such difference peaks are obvious. Otherwise, the pattern matches perfectly with standard
AC C
EP
TE D
XRD file of LiMnPO4 (98-007-6919 PDF file) with no additional peaks.
Figure 1 XRD refinement (Rietveld) pattern of as-prepared LiMnPO4 prior to ex-situ carbon coating; Indexed XRD patterns of a carbon composite LiMnPO4 is shown as inset. The background at low angle indicates the presence of amorphous carbon. (Pattern matched with 98-007- 6919 PDF file). HRTEM corroborates with this information. Single phase structure is clearly evident. The difference pattern as exemplified in blue color is due to nano-structure nature of the sample.
9
ACCEPTED MANUSCRIPT
Every peak in XRD profiles could be indexed in an orthorhombic structure with a space group Pnma. Cell parameters such as lattice constant, volume and space group are found to be matched the standard PDF XRD file of LiMnPO4 with no significant variations. Prior to carbon treatment, as-prepared LMP powders were examined using FESEM. It is evident
RI PT
from the SEM micrograph (see S1) that nanoscale loosely connected spherical grains having grain size ranging from 40-60 nm are distributed uniformly with good surface crystallite contacts. Such features are expected to enhance the intra surface crystallite-crystallite conductivity via surface
SC
coated monolayer carbon in turn to facilitating the electron transfer much easily. HRTEM analyses were carried out to confirm the carbon coating onto individual crystallites as shown in fig.2 (a-d).
M AN U
As evident from Fig 2(a) that individual crystallites of LMPs were encompassed with a fine layer of porous monolayer carbon exhibiting the wiring-like linkage between crystallite to crystallite. This indeed found to be essential for materials suffering from low intrinsic electronic conductivity. Each single crystallite is elegantly encompassed by a thin mono layer carbon structure as explicitly seen from HRTEM images as reproduced in Figs. 2(b & c). The porous nature of the
TE D
amorphous carbon matrix is also seen in high resolution images. Electron diffraction (ED) image show single nano-crystallite (Fig. 2d) single crystal diffraction features with bright spots depicting
EP
olivine crystalline order confirming the single crystal electron diffraction pattern. This shows the merit of our preparation technique adapted in the present work. Such feature is regarded imperative
AC C
for materials for electrochemical capacitor applications.
10
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Figure 2. Transmission Electron Microscopy images (a) FETEM high resolution images showing carbon wire-like connectivity with individual crystallites on the TEM grid (b) magnified high resolution images depicting the carbon monolayer deposition onto LiMnPO4 single crystallite (c) 1nm scale resolution at the edge of single crystallite LiMnPO4 showing explicitly the carbon enclosure onto LiMnPO4 single crystallite, (d) Electron diffraction pattern obtained on single crystallite (as shown in the inset as indicated by arrow) showing a perfect crystalline ordering at the single particle location confirming the single crystallite of LiMnPO4.
11
ACCEPTED MANUSCRIPT
Electrochemical Studies of single electrode nC-LMP
To deduce the single electrode capacitance, we examined nC-LMP as working electrode against a calomel reference electrode in different aqueous electrolytic solution (neutral and alkaline media). The influence of cations on the capacitive performance of LiMnPO4 was distinguished with respect the composition of
RI PT
to different alkali cations in alkaline and neutral electrolytes. Moreover,
different aqueous electrolytes (alkaline, neutral ) was optimized in different molar concentrations (2M LiOH, 2M NaOH, 2M KOH, 1M Li2SO4, 1M Na2SO4 and 1M K2SO4) as well as in
SC
nonaqueous Li+ containing electrolyte, 1M LiClO4 in acetonitrile (AN).
In order to rule out the influence of the current collector substance (Exmet SS mesh) used in the
M AN U
present investigation in three electrode cell measurements were carried out in all electrolytes at first before the actual measurements being carried out. It is inferred from Figs 3 (a&b) that the current collector does not contribute to any significant capacitance as exemplified from current-voltage response compared to nC-LMP within the potential window studied.
The typical rate dependent
current vs. voltage plots obtained by means of cyclic voltammetry in alkaline and neutral
AC C
EP
TE D
electrolytes are shown in figs 3(a&b).
12
SC
RI PT
ACCEPTED MANUSCRIPT
M AN U
Figure 3. Typical current versus voltage characteristics of Exmet (USA) SS current collector mesh in order to show the least influence on the capacitive properties of the actual electrode materials studied in alkaline and neutral electrolytic media. Capacitive Performance in Alkaline Electrolyte
It is well known that LiMnPO4 exhibits reversible Li+ kinetics as in Li-Ion battery cathode in Hitherto, the redox characteristics of olivine LiMnPO4 in aqueous
TE D
nonaqueous electrolytes.
electrolytes have not been investigated to the best of our knowledge. In this report, we have demonstrated the surface redox capacitive characteristics, the so-called pseudo-capacitance, of
EP
olivine LiMnPO4 involving alkali cations such as Na+ and K+ and proven that LMP can also be a nC-LMP showed pseudocapacitive performance prominently in
AC C
candidate for developing LICs.
alkaline electrolytes which is duly evident from the well defined anodic/cathodic peaks as shown in Fig. 4. It is obvious that the redox peak positions are systematically shifted in the following order K+>Na+>Li+ and the cause for such a shift is perhaps due to the size of the cations.
13
ACCEPTED MANUSCRIPT 2M NaOH
5000
2M KOH
2M LiOH
4000 3000 2000 1000 0 -1000 -2000 -3000 -4000 -0.8
-0.6
-0.4 -0.2 Potential (V)
0
0.2
0.4
0.6
SC
-1
RI PT
Specific Capacitance Fg-1
6000
M AN U
Figure 4 Pseudocapacitive performance of nC-LMP originated from faradic redox reaction in different alkaline electrolytes (scan rate 5mVs-1). The origin of redox peaks are attributed to surface oxidation/reduction of Mn2+/Mn3+ owing to the charge transfer across the electrode/electrolyte interface during insertion of Li+ into LiMnPO4. The quantum of charge transfer (one electron transfer) is construed as pseudocapacitance solely from the
TE D
intercalation of Li+ as olivine structure favors the reversible intercalation of Li+ cations [30]. The size of Na+ and K+ cations ruled out the possibility of their intercalation into olivine nC-LMP. Hence only Li+ extracted from olivine structure LiMnPO4 (during the first charge) acts as mere LiMnPO4 in these
EP
charge storage species that resulted in pseud capacitive performance of
electrolytes. Since the quantity of Li+ is limited (electrolyte does not contain Li+ ion) the
AC C
electrochemical performance of nC-LMP has been determined to be rather inferior in these two electrolytes. Indeed, the above explanation is duly supported by the magnitude of redox current profile in voltammograms. Thus, it is revealed from the rate dependent cyclic voltammogram that the specific capacitance of nC-LMP is higher in LiOH than that in NaOH and KOH as expected. Besides, the capacity loss is more pronounced in KOH and NaOH (20-30%) while it is less than 10% for LiOH during first 50 cycles which are found to be stabilized after 50 cycles and the capacitance values are summarized in table 1. 14
ACCEPTED MANUSCRIPT
Table.1 Specific capacitance values of nC-LMP in different aqueous electrolytes. Average Specific Capacitance (Fg-1) at different scan rate
Electrolyte
10mVs-1
5mVs-1
2M LiOH 2M NaOH
242 144
341 200
622 363
2M KOH 1M Li2SO4
135 184
194 321
317 472
1M Na2SO4 0.5M K2SO4
162 146
247 221
RI PT
100mVs-1
365 264
SC
To elucidate the cycling stability, the cell has been cycled for over 1000 cycles in 2M LiOH at 5
M AN U
mVs-1 (S2). Thus, the cycle stability of nC-LMP has been confirmed via refined one electron process redox peaks and proven the fact the olivine LMP retains an excellent stability in aqueous solutions and its facile Li+ kinetics is witnessed through multiple slow scan voltammograms for the first time.
Capacitive performance in sulphate containing neutral electrolytes
TE D
The capacitive behavior of nC-LMP has been evaluated in neutral electrolytes containing alkali cations with a common sulphate anion. It has been found that nC-LMP showed pseudocapacitive
AC C
shown in Fig.5.
EP
behavior in neutral electrolytes also that is evidenced from a pair of prominent redox peak as
15
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Figure 5: Pseudocapacitive performance of nC-LMP in neutral alkaline electrolytes (scan rate 5mVs-1); Inset shows that upon subsequent cycles the kink-like oxidation current profile disappeared as expected. A pair of faradic redox peak that has been observed may perhaps be attributed to intercalation of Li+
TE D
ions into LiMnPO4 as similar to alkaline electrolytes. As expected, the additional kink-like oxidation profiles observed between +0.1 to +0.5V vs SCE potential regime during the first charge disappeared upon subsequent cycling (See inset in Fig. 5).
This is most likely due to the
EP
irreversible adsorption of alkali cations at the surface of coated carbon onto LMP. The deduced
AC C
specific capacitance values are lesser in neutral electrolytes than that in alkaline electrolyte probably caused by its lower electrolytic conductivity of the former than the latter. Fig. 6 presents rate dependent cyclic voltammetry of nC-LMP in 1M Li2SO4 neutral medium. The effective specific capacitance (Fg-1) has been deduced based on calculating the area under the curve within the specific potential window fixed.
However, the highest specific capacitance of 3025 Fg-1
observed at the reduction peak position in LiOH solution. The high capacitance is due to one electron transfer (Mn2+/Mn3+) which is found to be higher than the capacitance values reported for
16
ACCEPTED MANUSCRIPT
different materials exhibiting pseudocapacitance involving Li+ intercalation mechanism so far [4448]. Nevertheless, when calculating the average discharge specific capacitance considering the potential window used, the capacitance value has been calculated to 622 Fg-1 as average. Thus, the accomplished nano-carbon coating over LMP in the present work has been proven to be an effective
RI PT
approach to overcome the inherent low electrical conductivity of olivine LiMnPO4. 0.5 0.4
SC
0.3
0.1
M AN U
Current (A)
0.2
0 -0.1 -0.2
-0.4 -0.5
-0.5
10mVs-1
5mVs-1
100mVs-1
0 Voltage (V)
0.5
1
EP
-1
TE D
-0.3
AC C
Figure 6: Rate dependent voltammograms of nC-LMP in 1M Li2SO4 neutral medium Since the full version of LICs involving AC as counter electrode, it is imperative to determine its specific capacitance in these electrolytes before being combined in a full cell. Therefore, we have estimated the specific capacitance as well as the potential window of the AC counter electrode. In a separate experiment, therefore, we have characterized electrochemical capacitive behavior of the porous carbon electrode in neutral (Li2SO4) and alkaline electrolytes (LiOH) using a three electrode cell. Obviously, the performance of activated carbon is greatly influenced by the pH of the electrolyte. That is, in neutral electrolyte, the activated carbon exhibited a perfect rectangular 17
ACCEPTED MANUSCRIPT
shaped voltammogram and stable for a wide potential range, from -1.0V to + 0.8V indicating nonfaradic charge transfer reaction the so-called electric double layer capacitance (EDLC) while in alkaline electrolyte for instance, 2M LiOH, it exhibited near ideal EDLC behavior and the hydrogen evolution reaction was noticed somewhere close to -1.2V vs SCE that limits its utilizable potential
RI PT
window between 0.0V to -1.1 V vs. SCE [not shown here]. Moreover, when compared to its specific capacitance values the performance of activated carbon was found to be good in alkaline electrolyte with specific capacitance as high as 1126 Fg-1 than that the neutral electrolyte ( 986 Fg-1
SC
at 5 mVs-1). Li-Ion Capacitor (LIC): nC-LMP ||AC
M AN U
To realize LICs, both battery-type electrode (positive) – working electrode and capacitor-type electrode (negative)- counter electrode are combined in the presence of Li+ electrolyte. By properly balancing the weight of both electrodes based on their specific capacitance it would be possible to achieve the theoretical capacitance of LICs and to avoid overshadow effect of one of the electrodes.
TE D
Estimation of Electrochemical Potential window
Since the potential window of AC has been found to be narrow in the alkaline electrolyte, the
AC C
in Fig.7.
EP
working potential of LIC is fixed between 0.0V to 1.4V invoking the stability of nC-LMP as shown
18
ACCEPTED MANUSCRIPT
Figure 7. Cyclic voltamogram of single electrode capacitive characteristics of AC (mesoporous carbon) and nC-LMP in LiOH alkaline electrolyte. (Scan rate used 5mVs-1)
On the other hand, AC as well as nC-LMP has exhibited better stability towards Li2SO4 electrolyte as shown in Fig 8. Thus, the working potential of the LIC as a full cell has been estimated to be
RI PT
within 0.0 V to +1.8V by combining the anodic and cathodic potentials of AC and nC-LMP
TE D
M AN U
SC
electrodes as shown in Fig 8.
EP
Figure 8. Cyclic voltamograms of single electrode capacitive characteristics of AC (mesoporous carbon) and nC-LMP in Li2SO4 neutral electrolyte. (Scan rate used 5mVs-1).
AC C
Considering the features of voltammograms, LIC employing LiOH alkaline electrolyte retains the faradic signatures as seen in the single electrode voltammetric profiles (see Figs. 4 and 5). Interestingly, as seen in Fig. 9, non-faradic feature is evident for LICs involving neutral electrolytes for instance, Li2SO4. The measured capacitance values of LIC are summarized in table 2. The capacitance values are found to be high at slow scan rate as expected since at higher current rate the concentration polarization causes disproportionation of diffusion coefficient of ions and their transport to the interface in both electrolytes. The dependence of capacitance values of LIC as a function of scan rate is more pronounced in the case of LiOH solution and the peak position has 19
ACCEPTED MANUSCRIPT
noticeably changed as well that corroborates with the observed deviation of AC from the ideal double layer capacitive behavior. In contrast, the LIC employing Li2SO4 could retain its shape and peak position with respect to change in current rates. Non-aqueous electrolyte of composition 1M LiClO4 in acetonitrile (AN) was used for high voltage
RI PT
hybrid capacitor. It was found to be stable up to 3.0V without significant loss in the initial capacity value on cycling. The appearance of pair of redox peaks was attributed to insertion of Li+ into nCLMP accompanied by Mn2+/Mn3+ oxidation. Figures 10 (a, b & c) describe the rate dependent
SC
voltammograms of LIC having the cell configurations, AC|LiOH|nC-LMP, AC|Li2SO4|nC-LMP
M AN U
and AC|1M LiClO4 in AN|nC-LMP.
800 600
200
TE D
Capacitance (Fg-1)
400
Li2SO4 LiOH LiClO4/AN
0 -200
EP
-400 -600
AC C
-800
0
0.5
1
1.5
2
2.5
3
Potential (V)
Figure 9. Cyclic voltammogram of Li-Ion Capacitor, nC-LMP//AC in alkaline, neutral and nonaqueous electrolytes. (Scan rate used 5mVs-1)
20
ACCEPTED MANUSCRIPT Table 2: Capacitance values of LIC of configuration electrolytes.
in different aqueous
Specific Capacitance / Capacitance (Fg-1) at different scan rate 100mVs-1 10mVs-1 5mVs-1 52.5 65 73
106 85 86
170 99 96
SC
nC-LMP|LiOH|AC nC-LMP|Li2SO4|AC nC-LMP|LiClO4 in AN|AC
RI PT
Electrolyte
M AN U
0.08 0.06 0.04 0.02
TE D
0
-0.02 -0.04 -0.06
0.5
1 Voltage (V)
100mVs-1 10mVs-1 5mVs-1
1.5
2
AC C
0
EP
Current (A)
nC-LMP||AC
Figure 10 (a). Rate dependent cyclic voltammograms of LIC, nC-LMP /LiOH/AC showing faradic-like capacitive properties.
21
ACCEPTED MANUSCRIPT
0.05
5mVs-1
0.04
10mVs-1
0.03
100mVs-1
0.02 0.01
RI PT
Current (A)
0.06
0
-0.01 -0.02 -0.04 0
0.5
1
SC
-0.03
1.5
2
M AN U
Voltage (V)
Figure 10(b). Rate dependent cyclic voltammograms of Li-Ion capacitor, AC /Li2SO4/ nC-LMP capacitor showing Non-faradic like capacitive behavior.
0.04 5mVs-1
TE D
0.03
10mVs-1
100mVs-1
0.01
EP
Current (A)
0.02
0
AC C
-0.01 -0.02 -0.03
0
0.5
1
1.5 Voltage (V)
2
2.5
3
Figure 10(c). Rate dependent cyclic voltammograms of LIC, nC-LMP /1MLiClO4/AC showing Non-faradic like capacitive behavior.
22
ACCEPTED MANUSCRIPT
Electrochemical Impedance Spectroscopy (EIS)
Impedance behavior of AC|Li2SO4|nC-LMP asymmetric capacitor before and after the cycling (scan rate used 5mVs-1) is described in Fig.11 and the ESR values measured at 1kHz before and after cycling
TE D
M AN U
SC
RI PT
are summarized in table 3.
EP
Figure 11. Electrochemical Impedance spectra of AC||nC-LMP (Inset shows the high to low frequency regime showing the Warburg diffusion behavior at the interface).
AC C
Table 3: Change in ESR values in different electrolytes on cycling nC-LMP||AC Electrolyte Fresh
ESR(ohm) After 1000cycles
2M LiOH
0.85
1.26
1M Li2SO4
1.16
1.43
1M LiClO4 (AN)
1.47
2.26
Like in the case of electric double layer capacitors (EDLCs), Fig 11 confirms that AC||nC-LMP, asymmetric capacitor exhibits true capacitive behavior as exemplified in Nyquist plot in which the 23
ACCEPTED MANUSCRIPT
capacitive characteristics are seen explicitly at low frequencies below 10Hz down to 10mHz. A small geometric bulk impedance seen with an indication of high to mid frequency contour-like (semicircle) frequency response probably attributed to bulk impedance behavior of the cell geometry and/or interfacial impedance due to current collector and electrode coating. A low frequency spike-like
RI PT
frequency response occurs at low frequency regime indicative of double layer capacitive behavior that can be attributed to pure capacitive behavior as normally being observed for Carbon/Carbon symmetric EDLCs. It is interesting to note that the ESR values do not show any remarkable variation upon cycling
SC
LIC (see table 3). In comparison, it is noteworthy that the change in ESR values are found to be less prominent in neutral electrolyte indicating the stability of both electrodes in neutral electrolyte and
M AN U
hence the cycle life. Moreover, in alkaline electrolytes the change is little more pronounced that reflects the inferior performance of activated carbon in terms of stability in that electrolyte. Galvanostatic charge/discharge
In order to complement the cycling characteristics, LICs were subjected to constant current cycling as well. Due to the integration of two different storage mechanisms in a cell, the shape of
TE D
charge/discharge curve is expected to be different from that of pure double layer capacitor. Both charge/discharge curves resemble those of capacitor-like behavior excepting that at low current
EP
rates (0.6 Ag-1) it resemble that of battery-like behavior with plateau region as shown in fig 12.
AC C
whereas at high current rates (3.0 Ag-1).
24
ACCEPTED MANUSCRIPT
3.5
LiOH
3 2.5
NEt4BF4/AN
2 1.5
RI PT
Potential (V)
Li2SO4
1
0 0
50
100
150
SC
0.5
200
300
350
M AN U
Time (s)
250
Figure 12. Typical constant current charge/discharge cycling for AC|nC-LMP at 0.6Ag-1 current rate for alkaline (1M LiOH), neutral (1M Li2SO4) and nonaqueous electrolytes (1M LiClO4 in AN)
TE D
Interestingly, the plateau region explains the improved energy storage capacity of the Li-ion capacitor at low current rates. Further, in order to study the cycle life and stability over cycling, these LIC cells were cycled over 1000 times as shown in Fig 13. The observed loss is found to be
EP
minimal in both electrolytes (8-12%). The achieved capacitance for LiOH based electrolyte is higher compared to Li2SO4 neutral electrolyte due to the synergetic effect of better capacitive
AC C
performance of both electrodes in LiOH. Even though the use of nonaqueous electrolyte enhances the working voltage of ~3V, the capacitance of LIC using nonaqueous electrolyte was found to be lower than that of aqueous electrolyte (LiOH) as expected and that might be attributed to its high ESR value originated from its two orders less conductivity yielding high IR drop.
25
ACCEPTED MANUSCRIPT
200
150
100
50
1M Li2SO4
RI PT
Capacitance(Fg-1)
250
1M LiClO4/AN
1M LiOH
0
200
400
600
SC
0 800
1000
1200
M AN U
Cell Cycle Stability
Figure 13. Cycling stability of Li-Ion capacitor nCLMP/Li+/AC with alkaline, neutral and nonaqueous electrolyte on galvanostatic cycling at the rate of 0.3Ag-1. The graphical correlation of energy and power densities of electrochemical power sources can be
TE D
depicted by Ragone plot as seen in fig.14. Accordingly, we have deduced the specific power and specific energy of AC|nC-LMP capacitor at different current rates using equations 1 and 2 and summarized in table 4. As for the specific power, the higher values obtained for LiOH for LIC is
EP
presumably due to synergic effect of high specific capacitance and low ESR value. Moreover, the higher specific energy observed in non aqueous electrolyte might be due to wide potential window
AC C
(0.0V-3.0V). Thus, the LIC system studied in neutral medium rendered a reasonably a high specific energy of 28.8Whkg-1 with a specific power of 2.5KWkg-1 while a high specific power of 12.6 KWkg-1 with a modest specific energy of 10.7 Whkg-1. It is noteworthy that the LICs studied in this work could maintain high specific energy and high specific power values even at high current rate as given in table 4.
26
ACCEPTED MANUSCRIPT
10
RI PT
Specific Energy (Whkg-1)
100
1 LiOH
SC
Li2SO4 LiClO4 in AN
0.1
1000 Specific Power (Wkg-1)
M AN U
100
10000
Figure 14. Ragone plot correlates specific energy and power of AC//nC-LMP Li Ion capacitors in Aqueous electrolytes (LiOH and Li2SO4) and Non-aqueous electrolyte 1MLiClO4/AN) at 0.6Ag-1.
Electrolyte/ current Rate
TE D
Table 4: Summary of Energy and Power density values measured at specific current densities 0.6Ag-1
LiOH
28.0 2.5
1MLiClO4 LiOH /AN 35.0 10.2
12.6
1MLiClO4 /AN 15.6
1.7
10.7
9.7
11.6
Li2SO4
AC C
EP
Specific Energy(Whkg- 16.5 1) Specific Power(kWkg-1) 3.3
Li2SO4
3Ag-1
The achieved specific energy and specific power of AC||nC-LMP is much higher than that of LiCoPO4 (2.9WhKg-1 and 0.19kWkg-1) using nonaqueous electrolyte working at high voltage >3V [47]. Such achievement is attributed to the novel method of preparation of nC-LMP which provides a very connecting path ways for low electronically connecting open framework olivine LiMnPO4.
27
ACCEPTED MANUSCRIPT
Conclusion
nC-LMP nano-composite electrode material has been found to exhibit facile pseudocapacitive behavior in Li+ containing aqueous and nonaqueous electrolytes and the effects of alkaline and neutral electrolyte in the context of capacitance, energy and power densities were distinguished. The
RI PT
surface insertion originated from the pseudocapacitive effect via Li+ deinsertion/insertion into olivine structure accompanied by the redox reaction owing to Mn2+/Mn3+ in LiMnPO4 leads to the capacitive behavior as observed. The high specific capacitance values of nC-LMP might possibly be attributed to high electrolytic conductivities of aqueous solutions (LiOH and Li2SO4) which
SC
favors the diffusion of Li+ into the olivine material, LiMnPO4 causing the pseudocapacitance. The
M AN U
novel method of single crystallite level intimate carbon enclosure has proven to be a viable approach to overcome the niche conductivity problems associated with olivine LMP that helped achieve a high specific capacitance of 622 Fg-1 with a peak profile capacitance of 3025 Fg-1 which is indeed a high measured value for such insertion type electrodes hitherto the best of our knowledge. Considering the working potential of the Li-ion capacitor, by combining the individual
TE D
potential window of both nC-LMP and AC as single electrodes, the potential window of LIC extends up to 1.8V in neutral aqueous electrolyte presumably due to the delayed onset of O2/H2
EP
evolution. Moreover, the latter exhibited more of non-faradic kind of capacitive characteristics. In contrast, in alkaline electrolytes, much pronounced faradic type capacitive profiles have been
AC C
observed. Because of the wide potential window, LIC with Li2SO4 rendered a specific energy as high as 28.8 Whkg-1. The highest specific power, 14.6 kWkg-1 obtained in LiOH electrolytes that presumably attributed to the cumulative effect of low ESR and high specific capacitance. Considering the performance of aqueous electrolytes with respect to capacitance properties of nCLMP, we have also studied the same with nonaqueous electrolyte (1MLiClO4 in AN) to understand how well aqueous counterpart performs.
Although its energy density has been remarkably
improved as obvious owing to its larger potential window in non-aqueous electrolytes, the 28
ACCEPTED MANUSCRIPT
capacitance values are found to be as good as aqueous electrolytes. Hence, considering stability of nC-LMP in aqueous electrolytes, it would be wonderful to develop hybrid capacitor without much stringent fabrication conditions for practical applications. Also, we conclude that the present study opens up new avenues for developing aqueous type Li-Ion batteries employing such carbon
RI PT
composite olivine cathodes which are plausibly evading away the safety concerns centered on LiIon batteries as well. Acknowledgement
SC
One of us (SRSP and AAS) would like to thank NRB, DRDO, India for the financial support through the project No: NRB-313/MAT/13-14 to carry out a portion of this work.
[1]
M AN U
References
B.E. Conway, Electrochemical Supercapacitors: Scientific fundamentals and technological applications, Kluwer Academic/Plenum: New York, 1999.
[2]
A.S. Arico, P. Bruce, B. Scroati, J-M. Tarascon, W. V. Schalkwijk, Nature Materials, 4 (2004) 366.
T. Brousse, D. Be’langer, Electrochem. Solid-State Lett. 6 (2003) A244.
[4]
V. Khomenko, E. Pinero, E.Frackowiak, F. Beguin, Appl. Phys. A: Mater. 82 (2006) 567.
[5]
S. Faraji, F.N. Ani, J. Power Sources, 263 (2014) 338.
[6]
D. Cericola, P. Novak, A. Wokaun, R. Kotz, J. Power Sources, 196 (2011) 10305.
[7]
B.E. Conway, W.G. Pell, J. Solid State Electrochem. 7 (2003) 637.
[8]
G.G. Amatucci, F. Badway, A.D. Pasquier, T. Zheng, J. Electrochem.Soc, 148 (2001) A930.
[9]
A.D. Pasquier, A. Laforgue, P. Simon, J. Power Sources, 125 (2004) 95.
AC C
EP
TE D
[3]
[10]
H. Li, L. Cheng, Y. Xia, Electrochem. Solid-State Lett. 8 (2005) A433.
[11]
T. Aida, I. Murayuma, K. Yamada, M. Morita, Electrochem. Solid-state Lett. 10 (2007)A93.
[12]
H. Li, L. Cheng, Y. Xia, Electrochem. Solid-State Lett. 8 (2005) A433.
[13]
S. Kumagai, T. Ishikawa, N. Sawa, J. Energy storage 2 (2015) 1 29
ACCEPTED MANUSCRIPT
[14]
W.J. Cao, J.P. Zheng, J.Power Sources 213 (2012) 182-185.
[15]
V. Khomenko, E. Raymundo-Pinero, F. Beguin, J. Power Sources 177 (2008) 643.
[16]
G. G. Amatucci, F. Badway, A. Du Pasquier, T. Zheng, J. Electrochem.Soc.148 (2001) A930. X. F. Luo, C. Wang, H. P. Yang, M. Yuan, ECS Trans. 59 ( 2014) 85.
[18]
C. Decaux, G. Lota, E. R. Pinero, E. E. Frackowiak, F. Beguin, Electrochim. Acta, 86
RI PT
[17]
(2012) 282.
G. G. Amatucci, F. Badway, A. Dupasquier, ECS Proc. 99 (2000) 344.
[20]
I. Plitz, A. Dupasquier, F. Badway, J. Gural, N. Pereira, A. Gmitter, G.G. Amatucci, Appl.
SC
[19]
[21]
N. Arun, A. Jain, V. Aravindan, S. Jayaraman, W. C. Ling, M. P. Srinivasan, S. Madhavi, Nano Energy 12 (2015) 69.
[22]
M AN U
Phys. A 82 (2006) 615.
L. Kavan, T. N. Murakami, P. Comte, M. Graetzel, Electrochem. Solid-state Lett, 10 (2007) A85.
P. C. Chen, G. Shen, Y. Shi, H. Chen, C. Zhou, ACS Nano 4 ( 2010) 5835.
[24]
Y.G. Wang, Y. Y. Xia, Electrochem. Commun. 7 (2005) 1138.
[25]
K. Naoi, S. Ishimoto, Y. Isobe, S. Aoyagi, J. Power Sources 195 (2010) 6250.
[26]
A. Laforgue, I. Plitz, J. Gural, S. Menocal, G.G. Amatucci, J. Power Sources 113 (2003) 62.
[27]
M.S. Michael, S.R.S. Prabaharan, J. Power Sources 136 (2004) 250.
[28]
M.S. Michael, Electrochim. Acta 120 (2014) 350.
[29]
F. Zhou, M. CoCoccioni, K. Kang, C. Cedar, Electrochem. Commun. 6 (2004) 1144.
[30]
A. K. Padhi, K. S. Nanjundaswamy, J. B. Goodenough, J. Electrochem. Soc. 144 (1997)
AC C
EP
TE D
[23]
1188. [31]
Z.X. Nie, C.Y.Ouyang, J.Z.Chen, Z.Y. Zhong,Y.L.Du, D.S.Liu, S.Q.Shi, M.S.Lei, Solid State. Commun.150(2010)40 30
ACCEPTED MANUSCRIPT
[32]
A. Yamada, Y. Kudo, K.Y. Liu, J. Electrochem. Soc. 148 (2001) A1153.
[33]
N. Meethong, H. Y. S. Huang, S.A. Seakmam, W. C. Carter, Y. M. Chiang, Adv. Funct. Mater. 17 (2007) 1115.
[34] S. K. Martha, B. Markovsky, J. Grinblat, Y. Gofer, O. Haik, E. Zinigrad, D. Aurbach , T.
[35]
RI PT
Drezen, D. Wang, G. Deghenghi, I. Exnar, J. Electrochem. Soc. 156 (2009) A541. M. Yonemura, A. Yamada, Y. Tkei, N. Sonoyama, R. Kanno, J. Electrochem. Soc. 151 (2004) A1352.
V. Aravindhan, J. Gnanaraj, Y. S. Lee, S. Madhavi, J. Mater.Chem.A,1(2013) 3518
[37]
D. Morgan, V. Van, G. Ceder, Electrochem. Solid-State Lett. 7 (2004) A30.
[38]
P. K. Ramesh, M. Venkateswarlu, M. Manjusri, K. M. Amar, N. Satyanarayana, J.
M AN U
SC
[36]
Electrochem. Soc. 158 (2011) A227. [39]
N.H. Kwon, T. Drezen, I. Exnar, I. Teerlinck, M. Isono, M. Graetzel, Electrochem. SolidStat. Lett. 9 (2006) A277.
G.Li, H. Azzuma, M. Tohda, Electrochem.Solid-state Lett. 5(2002)A135.
[41]
R. Dominko, M. Bele, M. Gaberscek, M. Remskar, D. Hanzel, J. M. Goupil, S. Pejovnik,
TE D
[40]
J. Jamnik, J. Power Sources 153 (2006) 274. C. Largeot, C. Portet, J. Chimiola, P. L. Tabema, T. Gogotsi, P. Simon, J. Am. Chem. Soc.
EP
[42]
130 (2008) 2730.
I. E. Rauda, V. Augustyn, B. Dunn, S. H. Tolbert, Acc. Chem. Research 46 (2013) 1113.
[44]
D. Cericola, R. Kotz, Electrochima. Acta. 72 (2012) 1.
[45]
Y. J. Hao, Q. Y. Lai, L. Wang, X. Y. Xu, H. Y. Chu, Synthetic Metals, 160 (2010) 669.
[46]
A. Yuan, Q. A. Zhang, Electrochem. Commun. 8 (2006) 1173.
[47]
V. Aravindan, J. Gnanaraj, Y-S. Lee, S. Madhavi, Chem. Rev. 114 (2014) 11619.
[48]
F. X. Wang, S.Y. Xiao, Y.S. Zhu, Z. Chang, C.L. Hu, Y. P. Wu, R. Holze, J. Power Sources
AC C
[43]
246 ( 2014) 19. 31
ACCEPTED MANUSCRIPT
Highlights
1) We report on carbon coating onto single crystallite LiMnPO4 via CVD approach.
RI PT
2) Nanocarbon composite of LiMnPO4 rendered highest capacitance of 3025 Fg-1. 3) Hybrid capacitor employing nano-composite LiMnPO4 is reported for the first time.
AC C
EP
TE D
M AN U
SC
4) AC// nC-LMP hybrid capacitor delivered a high specific energy of ~35Whkg-1.
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
1 µm
AC C
Figure 2. FESEM image showing the well connected spherical crystallites of as-prepared LiMnPO4 annealed at 400oC prior to carbon treatment.
ACCEPTED MANUSCRIPT
6000
RI PT
4000 3000
SC
2000
M AN U
1000 0 -1000 -2000 -3000 -4000 -0.8
-0.6
-0.4
-0.2
0
0.2
0.4
EP
-1
TE D
Specific Capacitance (Fg-1)
5000
AC C
Voltage (V)
Figure 5. Cycling stability of nC-LMP in 2M LiOH aqueous media at 5mVs-1showing the facile faradic redox peaks. One electron redox process is evident with a wide potential window in aqueous media (Typical plots are shown for clarity amongst 1000 cycles).
ACCEPTED MANUSCRIPT
RI PT
55
SC
50 45
M AN U
Type IV isotherm Capillarity condensation
40 35
TE D
30 25 20 0.2
0.4
0.6
0.8
1
AC C
0
EP
Quantity Adsorbed (mmolg-1)
60
Relative Pressure (p/p°)
Nitrogen Sorption Data showing the Type IV isotherm Capillarity condensation of Mesoporous carbon chosen for the present study for fabricating LIC ; BET surface area 3202 m²/g; BJH Pore width 2.7nm