One-pot solvothermal synthesis of Co1−xMnxC2O4 and their application as anode materials for lithium-ion batteries

Journal of Alloys and Compounds 638 (2015) 324–333

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One-pot solvothermal synthesis of Co1xMnxC2O4 and their application as anode materials for lithium-ion batteries Wei An Elijah Ang a,b, Yan Ling Cheah b, Chui Ling Wong b, Huey Hoon Hng a, Srinivasan Madhavi a,b,c,⇑ a

School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore Energy Research Institute @ NTU (ERI@N), Nanyang Technological University, 50 Nanyang Drive, Singapore 637553, Singapore c TUM CREATE Center for Electromobility, 1 CREATE Way, #10-02 CREATE Tower, Singapore 138602, Singapore b

a r t i c l e

i n f o

Article history: Received 30 December 2014 Received in revised form 26 February 2015 Accepted 27 February 2015 Available online 18 March 2015 Keywords: Lithium ion battery (LIB) Electrochemical performances Electrodes Metal oxalates Transition metals

a b s t r a c t A facile one-pot solvothermal route has been developed to synthesize phase pure MxC2O42H2O (M = Mn, Co; 0 < x 6 1) microstructures without employing any hard/soft template and their electrochemical performance in lithium-ion batteries has been systematically investigated. Morphology, microstructure and composition of the synthesized materials are characterized by field emission-scanning electron microscopy, X-ray diffraction and energy-dispersive X-ray spectroscopy. Anhydrous micron-sized MnC2O4 and CoC2O4 exhibits specific reversible discharge capacity of 800 and 950 mA h g1 respectively, at 1 C-rate. MnC2O4 exhibited good cycling stability while CoC2O4 showed severe capacity fading phenomenon after 40 cycles, thereafter attaining 400–600 mA h g1 for all C-rates. Interestingly, mixed solid solution having Co0.52Mn0.48C2O4 composition improved the specific reversible discharge capacity to a stable value of 1000 mA h g1 (1 C-rate), which is one of the highest reported values for such oxalates. The cycling stability of this mixed metal oxalate is remarkably better than its individual constituents at most C-rates. The Mn2+ substitution into CoC2O4 lattice has led to the synergistic modification of the electrochemical performances, thus making it a promising anode candidate for future LIBs. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction The field of energy storage has earned more and more interest in response to numerous practical demands of modern society and emerging environmental issues [1]. Currently, most commercial LIBs anode use carbonaceous materials (i.e. graphite, theoretical capacity of 372 mA h g1), which limits the possibility for further improvements [2]. The rapid development of high performance energy storage devices is urgently needed to overcome these challenges such as energy, power, cost, life and safety. The group of Tirado [3–8] is the pioneer to investigate the Li cyclability of metal carbonates and oxalates. It is reported that submicron MnCO3 particles showed a first charge capacity of 600 mA h g1 when cycled between 0 and 3 V at 0.25 C-rate. The capacity attained is higher than the theoretical value (466 mA h g1), assuming conversion reaction involving Mn and Li2CO3. Nevertheless, the experimental capacity gradually faded to 460 mA h g1 after 25 cycles [3]. In the work of metal oxalates, ⇑ Corresponding author at: School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore. Tel.: +65 6790 4606; fax: +65 6790 9081. E-mail address: [email protected] (S. Madhavi). http://dx.doi.org/10.1016/j.jallcom.2015.02.203 0925-8388/Ó 2015 Elsevier B.V. All rights reserved.

several transitional metal-based types are obtained by using reverse micelles technique. Notably, transition metal oxalates presented appealing performance on cycling as conversion anodes. Anhydrous CoC2O4 exhibited reversible capacities ranging from 400 to 800 mA h g1 between 10 and 80 cycles (in the range of 0.01 and 3 V vs. Li at 2 C-rate) [5]. On the other hand, Ang et al. [9] reported mesoporous cobalt oxalate nanostructures fabricated by chemical precipitation process attained higher reversible capacity, better rate capabilities and cycling stability than before. From Ref. [9], cobalt oxalate nanorods displayed good reversible capacities of 500 and 800 mA h g1 (at 8 and 1 C-rate respectively) even after 200 cycles. Nonetheless, the promising results on cobalt oxalate [9] are impaired by the toxicity, environmental and cost issues associated to cobalt (Co). A possible enhancement can be the partial replacement of Co by other transition elements such as manganese (Mn), which is less expensive and toxic than Co. In recent years, considerable research efforts on mixed transition metal oxalates [7,8] are also being studied as potential anode materials for LIBs. Leon et al. reported and evaluated two mixed transition metal oxalate series with M0 xM00 1xC2O4 (M0 M00 : Co–Mn and Fe–Mn) [8] nanoribbons, while Aragon et al. studied on Fe– Co [7] mixed metal oxalate system, of which all are synthesized by reverse micelles route. According to Ref. [8], intermediate

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compositions i.e. Co0.7Mn0.3C2O4 show a good compromise between reversible capacity values and capacity retention parameters, achieving capacities above 600 mA h g1 for more than 50 cycles at 2 C-rate. It is widely accepted that the morphology has major influence on the electrochemical performance of electrode material for LIBs [10–12]. To the best of our knowledge, there are only limited reports on the effect of morphologies and synthesis methodologies on the electrochemical performance of anhydrous metal oxalate anodes in LIBs. Most of the reported synthesis routes of metal oxalates for LIBs are by reverse micelles [4–8] and chemical precipitation [9,13] techniques. One of the most common softchemistry methods i.e. hydro-/solvothermal method is used to prepare high purity materials and to control the morphology of materials. Hydro-/solvothermal processes have many advantages such as fast reaction kinetics, high yield of homogenous particle products, are cost effective and environmentally benign over other synthetic routes [14,15]. To the best of our knowledge, only one instance on hydrated copper oxalate by hydro-/solvothermal for LIBs has been reported [16]. In this work, a facile, low cost and scalable solvothermal route to synthesize metal oxalates (MxC2O42H2O, M = Mn, Co; 0 < x 6 1) without the assistance of surfactant is presented with a view to their novel application in the anode of future LIBs. To overcome the high cost and noxious health issues associated with the use of Co, it is desirable to evaluate and find possible synergistic effect of Mn2+ substitution into CoC2O4 lattice for mixed metal oxalates via this simple method for the first time. Finally, the influence of morphology on synthesis parameters is unraveled and the effect of composition variations in mixed metal oxalates system to Li storage properties is also investigated in detail.

2. Experimental 2.1. Material synthesis All the chemicals (analytical grade) were used as received without further purification. The mixed solvent was prepared by mixing ethylene glycol (EG) and distilled water (DI) in various volume ratios (i.e. 3:1, 1:1, 1:3) for 10 min prior to the addition of H2C2O4 (2 mmol). The resultant solution was stirred for another 15 min prior to adding equimolar (2 mmol) of transition metal sulfates and followed by stirring for another 10 min at ambient conditions. Upon achieving a homogenous solution, the mixture (90% fill ratio) was tightly sealed in a 45 mL Teflon-lined stainless steel autoclave and heated up to 120 °C for 24 h in an oven. In this study, eleven samples were prepared with various EG:DI volume ratios and compositions (i.e. Co:Mn mole ratio) as listed in Table 1. The colored precipitate obtained is separated by centrifugation, washed with excess ethanol and dried under vacuum at 60 °C for 12 h. To get anhydrous MC2O4, MC2O42H2O samples were carefully dried under vacuum at 250 °C for 2 h and subsequently kept in dry box (20% relative humidity).

2.2. Characterization Morphology and microstructure of samples were observed by both field emission-scanning scanning electron microscopy (FE-SEM, JEOL 7600F) with an accelerating voltage of 15 kV and high resolution-transmission electron microscopy (HR-TEM, JEOL 2100F) at an accelerating voltage of 200 kV. Prior to record FE-SEM, the samples were placed on carbon tape and sputter coated with platinum (Pt) for 40 s. For TEM specimen preparation, the samples were prepared by dropping the dispersion of anhydrous MC2O4 samples in ethanol onto a carbon-coated copper grid and dried at room temperature. Crystallographic characterization was carried out via powder X-ray diffraction (XRD, Shimadzu XRD-6000) operating at 40 kV and 40 mA using Cu Ka1 radiation (k = 0.15406 nm) with the copper target and nickel filter. The samples were scanned between 10° to 80° (2h) at a scan rate of 1° min1. The lattice constants were determined using Rietveld refinement (TOPAS Version 3). Elemental compositions of samples were measured by energy dispersive X-ray spectroscopy (EDX) using INCA mapping attached with the FE-SEM. Quantitative EDX (with ZAF correction) was performed on uncoated samples for an acquisition time of 120 s at 15 kV for an average of 30 different spots. The specific surface area of the sample was measured with the Brunauer– Emmett–Teller (BET, Micromeritics ASAP-2020) method. Fourier transform-infrared spectroscopy (FT-IR, Perkin Elmer Spectrum GX) spectra were obtained for the samples in KBr pellets at a resolution of 1 cm1. Thermal behavior of samples was studied by thermo-gravimetric analysis (TGA, TA Instruments Q500) at a heating rate of 5 °C min1 from room temperature to 500 °C under air. 2.3. Electrochemical measurement The anode was prepared by mixing 60 wt.% anhydrous MC2O4, 30 wt.% conductive additive (Super P Li carbon, Timcal) and 10 wt.% PVDF binder (Kynar 2801) in N-methyl-2-pyrrolidone (NMP) as solvent for the binder to form the slurry. The resulting homogenous slurry was casted onto etched copper foil by doctor-blade technique followed by drying at 80 °C for 24 h under vacuum. The dried electrodes were pressed to enhance adhesion strength. Test cells, Li/MC2O4 were fabricated in coin cell configuration (CR2016) and electrochemical tests were conducted using a battery test system (XWJ Neware Tech. Co.) between 0 and 3 V at various C-rates (1–10 C-rates, 1 C = 1 Li h1 mol1 i.e. 0.2 mA cm2). 1 M LiPF6 in ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1 wt./wt., Danvec) was used as electrolyte and Celgard 2400 membrane as the separator. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were carried out on a Solartron (1470E and SI 1255B impedance/gain-phase analyzer coupled with a potentiostat) electrochemical workstation. CV was studied at 0.1 mV s1 in the voltage range of 0–3 V and EIS was studied by applying an A.C. perturbation of 10 mV over frequency range of 100 kHz–5 mHz at open circuit potential. The Nyquist plots were analyzed using Zplot and Zview programs (Version 2.2, Scribner Associates Inc.). All the test cells were fabricated in Ar-filled glove box (H2O, O2 <1 ppm, Mbraun, Unilab) using Li metal foil (0.59 mm thick, Hohsen Corporation) as counter and reference electrode. The cells were then aged for 24 h at ambient conditions prior to any measurements.

3. Results and discussion Fig. 1 shows the FE-SEM micrographs of various MnC2O4, CoC2O4 and mixed metal oxalate anhydrous samples as listed in Table 1. There are no considerable changes in the shape and size observed after dehydration. To better understand the role of the solvent, the morphologies of the metal oxalates are investigated

Table 1 Synthesis conditions of samples. Sample *

S1 S2* S3* S4* S5* S6* S7* S8* S9* S10* S11* ^ # *

Material types

EG:DI vol. ratio

M:H2C2O4 mole ratio^

MnC2O42H2O

3:1 1:1 1:3 3:1 1:1 1:3 3:1 1:1 1:3 1:1

1:1

CoC2O42H2O

Co0.77Mn0.23C2O42H2O

Co0.52Mn0.48C2O42H2O Co0.91Mn0.09C2O42H2O

M denotes the transitional metal/s being used for every 2 mmol of H2C2O4. As verified from EDX quantitative analysis of 30 scans. Denotes as-synthesized (hydrated) samples.

M = (Co:Mn) mole ratio# 0

1

3.35

1.08 10.11

Temperature (°C)/time (h) 120 °C/24 h

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Fig. 1. FE-SEM micrographs for MnC2O4: (a) Sample S1, (b) Sample S2 (c) Sample S3; CoC2O4: (d) Sample S4, (e) Sample S5, (f) Sample S6; mixed Co–Mn based: (g) Sample S7, (h) Sample S8, (i) Sample S9, (j) Sample S10 and (k) Sample S11.

from the mixed solutions with various EG:DI volume ratios. From Fig. 1(a)–(c), large micron-sized, irregular plate-like MnC2O4 particles became smaller in size with smooth texture having sharp-edged granules and eventually evolved into roughened surface texture of irregular cuboids and edged-granular particles with decreasing EG:DI volume ratio accordingly. Sharp-edged MnC2O4 granules (Sample S2) prepared in EG:DI volume ratio of 1:1 (Fig. 1(b)) exhibited particle size ranging from 2 to 6 lm. On the other hand, when CoSO47H2O is used as the precursor, edgedgranular particles are formed (Fig. 1(f)) with relatively smooth surface but become more rod-like and smaller in size with increasing EG:DI volume ratio. However, with the highest EG:DI volume ratio, the particles become more irregular with roughened surface (Fig. 1(d)). By comparison between the CoC2O4 series, Sample S5 (Fig. 1(e)) has the most promising structure. The square-cross sectional rods (Sample S5) have width and length in the range of 0.2–2 and 3–5 lm accordingly. The effect of solvent mixture (i.e. EG:DI volume ratio) on the morphology is clearly evident in all material types studied in this work. Oxalic acid dissociates to various degrees probably due to different solvent mixture properties such as solvent viscosity and ionization power. This resulted in distinct morphology variations. It is widely known that EG is a powerful chelating agent and coordinated with the transition metal cation to form a complex. The elevated temperature weakens the coordination ability of EG for

metallic ions, which leads to the precipitation of MC2O42H2O under such solvothermal conditions. The chemical reactions in the system can be represented as follows:

M2þ þ HO—CH2 CH2 —OH ! ðMHOCH2 CH2 OHÞ2þ complex

ð1Þ

H2 C2 O4 ! 2Hþ þ C2 O2 4

ð2Þ

ðMHOCH2 CH2 OHÞ2þ þ C2 O2 4 þ 2H2 O ! MC2 O4  2H2 O þ HO—CH2 CH2 —OH

ð3Þ

From the study, it is observed that both the EG:DI volume ratio, as well as the type of transitional metal, has an influence on the morphology as well. It is probably due to the different interaction between the solvent mixture and the transitional metal in which Co contributed to a more pronounced 1D growth than Mn precursor. In addition, it is observed that the morphological influence of Co is much greater than Mn as manifested in the mixed metal oxalate study. The predominantly rod-like with a near rectangularcross section morphology as revealed by the FE-SEM micrographs for Samples S8, S10 and S11 (Fig. 1(h), (j) and (k)) are having width and length in the range of 0.5–2.5 and 1–6 lm respectively for the same EG:DI volume ratio of 1:1. Comparing between Samples S10 and S11, the rods observed in Sample S11 are relatively longer than S10. While varying EG:DI volume ratio but keeping Mn:Co mole

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ratio constant, mixed metal oxalate sample evolved from mixture of edged-granules and cuboid (Fig. 1(i)) to rod-like (Fig. 1(h)) and irregular, roughened textured rods (Fig. 1(g)) with increasing EG:DI volume ratio. Samples S2, S5, S8, S10 and S11 are selected based on morphology for further material characterizations and electrochemical testing in this work. XRD patterns of as-synthesized and anhydrous pure metal oxalates and mixed metal oxalates are shown in Fig. 2. From Fig. 2(a), as-synthesized Sample S2⁄ can be indexed to single monoclinic a phase (space group: C2/c) of MnC2O42H2O (ICDD PDF No. 01-073-2578) with a = 12.0155 Å, b = 5.6323 Å and c = 9.9609 Å and b = 128.37°; while after dehydration, anhydrous Sample S2 obtained a completely different crystal structure, which is indexed in an orthorhombic b-phase of MnC2O4 (ICDD PDF No. 00-032-0646) with a = 7.146 Å, b = 5.398 Å and c = 5.398 Å and b = 113.62°. The change in crystal structure after drying of assynthesized sample is due to the removal of crystallized water molecules [13]. Similarly, the diffractograms are completely different between as-synthesized and anhydrous Co- and mixed Co–Mn-based metal oxalates as shown in Fig. 2(b) and (c). The patterns of all as-synthesized mixed metal oxalates (S8⁄, S10⁄ and S11⁄) and Sample S5⁄ presented similar reflections and are in good agreement with the reflections of single monoclinic a phase (space group: C2/c, Lindbergite) of CoC2O42H2O (ICDD PDF No. 00-0250251) with a = 11.775 Å, b = 5.416 Å and c = 9.859 Å and b = 127.9°. After heating (Fig. 2(c)), the broadened XRD peaks can be ascribed to the little crystal size and weak crystallinity which is matched to monoclinic a phase of CoC2O4 (ICDD PDF No. 00037-0719) with a = 11.775 Å, b = 5.416 Å and c = 9.859 Å and b = 127.9°. All mixed metal oxalates are single phase irrespective of the substitution degree. The variation of the lattice parameters with Mn2+ substitution degree according to Rietveld method is shown in Table 2. It is observed that lattice parameters a and b increase with Mn content and lattice parameter c does not change considerably. The overall linear increase in unit cell dimensions

Table 2 Unit cell parameters (Å) for MnxCo1xC2O42H2O. Sample ⁄

S5 S11⁄ S8⁄ S10⁄ S2⁄

x=

a

b

c

0 0.09 0.23 0.48 1

11.81(4) 11.83(3) 11.86(5) 11.90(9) 12.00(9)

5.43(1) 5.46(1) 5.48(1) 5.55(7) 5.65(1)

9.88(3) 9.90(7) 9.91(3) 9.93(3) 9.99(9)

with amount of Mn2+ substitution is in well agreement with the larger ionic radius of Mn2+ as compared to Co2+ and it obeyed Vegard’s law. It indicated homogenous solid solution formation for the mixed metal oxalates and no impurity phase is detected for all the samples. EDX spectra of anhydrous Sample S2, S5 and mixed metal oxalate series (Samples S8, S10 and S11) further reveal that the material is phase pure and is well supported by the XRD results (Fig. S1, the Supporting Information). An average of 30 different analysis spots are conducted for each mixed metal oxalate samples to verify its composition (see Table 1). In addition, FT-IR spectra analysis is conducted to ascertain the integrity of the oxalate structure in all the five samples (Fig. S2). The peak corresponding to 1320 cm1 is characteristic to the presence of oxalate ions. The data is in good agreement with earlier reports [5,9]. TEM/HR-TEM micrographs revealing the fine morphological details of anhydrous MnC2O4 (Sample S2), CoC2O4 (Sample S5) and Co0.52Mn0.48C2O4 (Sample S10) microstructures are shown in Fig. 3. It is clearly observed that Sample S2 (Fig. 3(a)) shows interior porous structure which may be generated due to release of crystallized water molecules during dehydration process. On the other hand, the pores observed in both Samples S5 and S10 (Fig. 3(b) and (c)) are smaller in size and less noticeable as compared to Sample S2. Fig. 3(d) shows the fine and intricate pore structures of Sample S10 at a higher magnification. From

Fig. 2. XRD patterns for (a) as-synthesized and anhydrous Mn metal oxalates, (b) as-synthesized and (c) anhydrous Co- and mixed Co–Mn metal oxalates (⁄denotes assynthesized).

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Fig. 3. TEM/HR-TEM micrographs: (a) Sample S2 (MnC2O4), (b) Sample S5 (CoC2O4) and (c) Sample S10 (Co0.52Mn0.48C2O4); Insets: illustrating lattice fringes respectively; (d) showing Sample S10 pore structures at higher magnification.

HR-TEM micrographs (Insets of Fig. 3(a)–(c)), all samples exhibited obvious lattice fringes, indicating crystalline nature and which is in good accordance with XRD observations. BET specific surface area for Samples S2, S5, S8, S10 and S11 is 21, 11, 12, 19 and 12 m2 g1 respectively. These anhydrous metal oxalate samples exhibit mesoporous nature (Fig. S3). BET measurements are consistent with the TEM micrographs. Thermal properties of both as-synthesized and anhydrous metal oxalates samples are studied by TGA. The as-synthesized metal oxalate microstructures contained two water molecules of crystallization (MC2O42H2O) and corroborates from previous works [8,9]. A gradual weight loss (20 wt.%) is observed for all as-synthesized samples from 100 to 200 °C (Fig. 4(a)) which is in good agreement with the theoretical weight loss according to Eq. (4) (water loss). At 260 °C, a drastic weight loss of 55.4 wt.% (theoretical weight loss: 56.1 wt.% according to Eq. (5)) is observed for Co- and mixed Co–Mn-based metal oxalates (Samples S5⁄, S8⁄, S10⁄ and S11⁄) but a gradual weight loss of 55.0 wt.% (theoretical weight loss: 55.9 wt.% according to Eq. (5)) for Mn metal oxalate (Sample S2⁄) is recorded in which all is ascribed to oxalate decomposition. After dehydration, there is no significant weight loss observed up to 260 °C for all anhydrous metal oxalates (Fig. 4(b)). A weight loss of 44.3–45.1 wt.% (theoretical weight loss: 44.8–45.4 wt.% according to Eq. (6)) is observed between 260 and 340 °C for all anhydrous metal oxalate samples. No significant weight loss is observed above 340 °C indicating complete decomposition of oxalate for all as-synthesized and anhydrous metal oxalate samples. The results indicate that at 250 °C, as-synthesized metal oxalates transform to anhydrous types by the removal of crystallized water

molecules and without breakage of the oxalate anion as shown in the following equations:

Dehydration : 100—200  C MnC2 O4  2H2 O ! MnC2 O4 þ 2H2 O " CoC2 O4  2H2 O ! CoC2 O4 þ 2H2 O "

ð4Þ

Mnx Co1x C2 O4  2H2 O ! Mnx Co1x C2 O4 þ 2H2 O " Decomposition :> 260  C MnC2 O4  2H2 O ! 0:5Mn2 O3 þ 0:5CO2 " þ1:5CO " þ2H2 O " CoC2 O4  2H2 O ! 0:3Co3 O4 þ 0:6CO2 " þ1:3CO " þ2H2 O " MC2 O4  2H2 O ! MO þ CO2 " þCO " þH2 O " ð5Þ MnC2 O4 ! 0:5Mn2 O3 þ 0:5CO2 " þ1:5CO " CoC2 O4 ! 0:3Co3 O4 þ 0:6CO2 " þ1:3CO "

ð6Þ

MC2 O4 ! MO þ CO2 " þCO " CV measurements were then conducted to elucidate the electrochemical process of anhydrous MC2O4 electrodes and its CV cycles are shown in Fig. 5. From Fig. 5(a), it can be seen that the first cathodic scan of Sample S2 (MnC2O4 electrodes) exhibited one small peak (0.67 V) which disappeared after second cycle and one high intensity peak (0.33 V) that shifted to higher potential (0.66 V) during subsequent cycles. It corresponds to the following reasons: (i) decomposition of electrolyte which results in the formation of an organic layer deposited on the surface of the

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Fig. 4. Thermo-gravimetric analysis (TGA) curves: (a) as-synthesized and (b) anhydrous metal and mixed metal oxalates (both at 5 °C min1 from room temperature to 500 °C under air).

Fig. 5. CV profiles between 0 and 3 V at a scan rate of 0.1 mV s1: (a) Sample S2 (MnC2O4), (b) Sample S5 (CoC2O4) and (c) Sample S10 (Co0.52Mn0.48C2O4).

particles [17–19] and (ii) a electrochemical conversion reaction of lithium-ion intercalation to form Li2C2O4 and metallic manganese (Mn0) particles as shown in a generalized Eq. (7). þ

MC2 O4 þ 2Li þ 2e ! M þ Li2 C2 O4

ð7Þ

Meanwhile, in the anodic cycle process, a peak at 1.22 V is recorded during the anodic process and is related to the reversible oxidation step of metallic Mn0 to Mn2+ accompanying with the decomposition of Li2C2O4 as seen in subsequent cycles. On the other hand, Sample S5 and representative of mixed Co–Mn type i.e. Sample S10 (CoC2O4 and Co0.52Mn0.48C2O4 electrodes) depicted similar CV profiles as shown in Fig. 5(b) and (c), respectively. Two cathodic peaks are observed in the first scan and only one reaction in the subsequent reduction process (after five scans) is recorded. This is probably attributed to the initial reduction of MC2O4, the

electrochemical formation of Li2C2O4 and a partially irreversible solid electrolyte interface (SEI) layer as discussed earlier. The CV results demonstrate an irreversible reaction present in the first electrochemical cycle, which accounts for the capacity loss of the anode materials accordingly. The CV agrees well with that previously reported for CoC2O4 [5,9]. To study lithium storage capacity of these anhydrous metal oxalates, galvanostatic charge–discharge performances were tested in the voltage window of 0–3 V in Li/MC2O4 test cells. Fig. 6 shows the voltage profile at a constant 1 C-rate and the cycling behavior of the samples at various C-rates. It can be observed that Sample S5 (CoC2O4 electrodes) exhibited the initial discharge potential plateau at around 1.2 V followed by weak slope corresponding to the electrochemical reaction of CoC2O4 with Li (Fig. 6(a)), which is in good accordance to previous reports [5,9]. Similarly, mixed MnxCo1xC2O4 types (i.e. Samples S8, S10 and

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Fig. 6. (a) Initial charge–discharge profile vs. specific capacity (1 C-rate). Inset: 100th cycle representation; galvanostatic cycling performance at various C-rates: (b) Sample S2, (c) Sample S5, (d) Sample S10, (e) Sample S8 and (f) Sample S11.

S11 electrodes) also display initial discharge voltage profile of close resemblance to Sample S5. However, it can be seen that Sample S2 (MnC2O4 electrodes) has a much lower initial discharge potential plateau (0.6 V) which is consistent with the CV studies and previously reported [6]. From Fig. 6(a), Sample S5 electrodes delivered an initial discharge capacity of 1487 mA h g1 and charge capacity of 966 mA h g1 with a Coulombic efficiency of 65%. This large irreversible capacity loss also observed in the rest of the samples could be considered as the nature of conversion reaction mechanism. In contrast, Sample S2 electrodes showed the lowest discharge and charge capacity of about 1106 mA h g1 and 589 mA h g1 respectively for the first cycle. This is mostly attributed to the intrinsic property of the Mn material itself (as compared to Co) and also morphological differences between Samples S2 and S5. The initial discharge capacity for all samples is observed to be larger than either the theoretical capacity of MnC2O4 or CoC2O4 (375 and 365 mA h g1 respectively based on Eq. (7)) which may be due to the decomposition of organic electrolyte during discharge [18]. It is clear that first discharge capacity is mostly due to faradaic contribution and irreversible. The MC2O4 charge–discharge profile is observed to be considerably different from other conventional oxide electrodes demonstrating the conversion reaction may be due to the lithium oxalate formation during the discharge instead

of lithium oxide [4,20]. From the 100th discharge profile (Fig. 6(a) Inset), one slight voltage plateau between 0.8 to 0.6 V corresponding to the reduction of Co2+ to Co0 and a discharge capacity of 506 mA h g1 is observed for Sample S5 (similar voltage profiles for the case of mixed Co–Mn-type i.e. Samples S8, S10 and S11). However, a defined discharge potential plateau at around 0.4 V can still be noticed for Sample S2 at the 100th cycle giving a discharge capacity of 850 mA h g1. Both Samples S2 and S5 attained specific discharge capacity which is much higher than commercial graphite (372 mA h g1) and those recently reported [6]. Fig. 6(b)–(f) depicts the rate capabilities and galvanostatic cycling performance of all five different anhydrous metal oxalates and mixed-metal oxalates. For CoC2O4 (Fig. 6(c)) and Co0.91Mn0.09C2O4 (Fig. 6(f)), high discharge capacity values ranging from 1020 to 1260 mA h g1 for 1C are observed between 2nd and 5th cycle but it rapidly drops, reaching a reversible capacity value of 506 and 481 mA h g1 at 100th cycle respectively. At 1 C-rate, Sample S5 (CoC2O4 electrode) has a capacity drop of 66% between 35th and 78th cycle while Sample S11 (Co0.91Mn0.09C2O4 electrode) show lesser capacity fading (59% between 23th and 65th cycle). The positive influence of Mn2+ substitution (even at small degree of substitution, x = 0.09) in improving cycling stability of CoC2O4 is clearly evident. It is well established that drastic volume changes

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from the reduced solids after discharge and the oxidized delithiated products in a conversion reaction is the main cause for capacity fading. The less severe volume change of MnC2O4 (as compared to CoC2O4) during conversion reaction would be the primary reason for good correspondence with the excellent capacity retention of MnC2O4 (as shown in Fig. 6(b)) and the mixed Co–Mn-metal oxalates (Fig. 6(d) and (e)). It is observed that Sample S5 (CoC2O4 electrode) has the best rate capability amongst the rest, achieving good reversible capacity between 400 and 600 mA h g1 at 5–10 Crates whilst not for Mn-containing samples. Taking a closer look at the electrochemical performances of Samples S2, S10 and S8 (Fig. 6(b), (d) and (e), respectively), the discharge capacity values experience a considerable decrease within first ten cycles and the capacity is recovered to a stable value of 842, 1072 and 903 mA h g1 (up to 100 cycle) at 1 C-rate for Samples S2, S10 and S8 respectively, which is more than two times its theoretical capacity (Eq. (7)). The phenomenon of initial capacity fading and then capacity rise could be a result of formation of a complex nano-composite whereby metal nano-particles interact between themselves and the oxalate matrix leading to the observed behavior. As the nano-composite stabilized (within first few cycles), the extra capacity (more than its theoretical) rises mainly due to the faradaic contribution that comes from the side reactions with the electrolyte and/or reversible formation of a gel-like layer as reported previously [21,22]. The extra capacity increases also by capacitive mechanism, especially at high rates (Fig. S4). From Fig. 6(b)–(f), it can be seen that as the current density increases, the reversible specific discharge capacities of the samples decreased evidently. The Coulombic efficiency of more than 98% was attained for all five samples at different C-rates after 10 cycles. In general, the presence of Co in the mixed metal oxalates is crucial to achieve high capacity values whereas Mn in the structure aids in the stability of the cell during cycling. Comparing between mixed metal oxalates – Samples S10 (Co0.52Mn0.48C2O4) and S8 (Co0.77Mn0.23C2O4), the outstanding achievement of S10 at 1 C-rate and comparable results at other C-rates make it the best composition mix for high capacity and stable cycling performances. A positive synergistic effect is observed for mixed metal oxalate of Sample S10, revealing good capacity values of Co nature and keeping good capacity retention upon cycling of Mn properties. The distinct electrochemical performance between Samples S10 and S8 is mainly attributed to the different degree of Mn2+ substitution. In addition, the highly porous nature of Sample S10 (as compared to Sample S8) is also one of the reasons for its excellent electrochemical performance. Compared to other oxalates studied [5–8], MnC2O4, CoC2O4 and mixed metal oxalate (Co0.52Mn0.48C2O4) prepared by solvothermal method showed higher capacity and excellent cycling performances. The total stored charge in a battery has two components: faradaic contribution (originating from Li conversion or insertion process, charge transfer with surface atoms and surface film formation) and non-faradaic contribution (double layer effect). These two components can be separated by analyzing CV at various sweep rates. The total current is separated into faradaic and non-faradaic components modeled and described by Brezesinski et al. [23] for TiO2 electrodes and are employed in metal oxalates LIBs [5–9,13]. By this method, faradaic and non-faradaic contributions have been determined (Fig. S4). It is observed that the main contribution is the faradaic component for MC2O4 at different sweep rates. This indicates that at low sweep rate the reaction mechanism is dominated by Li conversion process. The general behavior is the higher the voltage sweep rate, the higher the non-faradaic current contribution. For instance, the highest nonfaradaic current contribution amongst the other electrodes at all

sweep rates – Sample S10 (Co0.52Mn0.48C2O4) has increased from 11% to 28% with increased sweep rate (0.5–5 mV s1). In addition, the diffusion coefficients of Li ions (D+Li) in MC2O4 were determined by CV at several sweep rates for the first time (Figs. S5 and S6). If the charge transfer at the interface is rapid enough and the limiting process is the Li diffusion in electrode, the relationship of the peak current (Ip) and the CV sweep rate is:

Ip ¼ ð2:69  105 Þn3=2 AðDþLi Þ

1=2

C Li v 1=2

ð8Þ

where n is the number of electrons per molecules, A is the specific surface area, CLi is the bulk concentration of Li ions in electrode and v is the scan rate [24]. As shown in Supporting Information (Fig. S6), the values of D+Li are estimated to be in the range of 1010–108 cm2 s1 and undoubtedly allow fast diffusion of Li ions during discharge than charge processes for all the three MC2O4 electrodes. Based on the above results, the rate capability is influenced by both composition and morphological factors. Comparing Samples S5 and S10 (both of similar rod-like morphology), the substitution of Mn2+ into CoC2O4 has resulted in the decrease of D+Li from 3.67 to 0.363  108 cm2 s1; whilst Sample S2 (MnC2O4 electrode) of both different shape and composition has the lowest D+Li in the order of 1010 cm2 s1. In addition, Sample S10 (Co0.52Mn0.48C2O4) exhibits higher D+Li value than commercial graphite (apparent D+Li: 1010 at 20 °C) [25], representing enhanced rate performances. The findings are in well agreement with the EIS results (as discussed in the following section). Nevertheless, Mn2+ substitution can effectively improve the cycling stability of CoC2O4 but not its rate capability. Nyquist plots of MC2O4 electrodes are recorded for fresh cells (equilibrated overnight at room temperature) at open circuit potential, as well as for the cycled cells at 1 C-rate and shown in Fig. 7. An equivalent circuit (Fig. 7(f)) is constructed to evaluate the formation of surface film and charge-transfer process [13]. Typically in LIBs, the high-frequency semicircle (100 kHz–10 Hz) is related to the charge-transfer processes, while the spike at middle to low frequencies describes lithium diffusion kinetics [26]. The equivalent circuit of built-in series and parallel combinations of intrinsic resistance (Rs), surface and charge-transfer resistance (Rsf+ct), double layer capacitance across the surface of the electrode (CPEsf+dl) (with angle of distortion a) [13], Warburg impedance (Zw) and intercalation capacitance (CPEint) [27] enables description of the various processes occurring and the values of the elements via fitting are presented in Table 3. Comparing the fresh cells, it can be noted that Sample S10 (Co0.52Mn0.48C2O4) has the lowest intrinsic resistance at 0.04 X. This could indicate that the halfcell of Sample S10 enables the electrode to have good interaction with the electrolyte and separator, which possibly leads to improved electrochemical behavior. The double layer capacitance (CPEsf+dl) of Sample S10 is also the highest among all the samples at 2.05 lF, indicating that the formation of double layer at the surface of the electrode could contribute to improved cycling behavior as compared to the others. On the other hand, the Warburg impedance Zw is 780 X, Sample S10 could have lower lithium diffusion coefficient and this is also in agreement with the lower intercalation capacitance (CPEint) 1.38 mF. After cycling, the Rsf+ct of Sample S10 reduced from 290 to 21 X, indicating an improvement in charge-transfer related properties after electrochemical cycling [9]. As compared to the other cycled samples, its Rsf+ct is also the lowest. In terms of Warburg impedance, the drastic decrease to 190 X indicates improved ease of lithium diffusion after electrochemical cycling, which could be due to the interaction between the SEI layer and electrode surface. In Samples S2, S5 and S8, the increase in Zw indicates poorer lithium diffusion properties, indicated by the lower capacity

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Fig. 7. Nyquist plots and fitted data: (a) Sample S2, (b) Sample S5, (c) Sample S8, (d) Sample S10, (e) Sample S11 and (f) equivalent circuit.

Table 3 Fitting parameters of the cell with MC2O4 electrodes for the fresh cell equilibrated overnight and after 1 and 100 charge–discharge cycles at an open circuit potential for 1 C-rate. Active material

Cell configuration (tested)

Rsf+ct (X)

CPEsf+dl (lF)

a

Zw (X)

Sample S2 (MnC2O4)

Fresh cell 100th cycle

6.08 3.09

217.0 62.6

0.57 1.34

0.71 0.75

226 240

4.16 0.02

Sample S5 (CoC2O4)

Fresh cell 100th cycle

2.46 3.02

142.2 31.5

0.31 2.86

0.66 0.70

26 272

4.24 18.39

Sample S8 (Co0.77Mn0.23C2O4)

Fresh cell 100th cycle

2.85 10.0

45.5 35.0

0.50 2.05

0.68 0.66

28 77

3.73 7.53

Sample S10 (Co0.52Mn0.48C2O4)

Fresh cell 100th cycle

0.04 8.43

287 21

2.05 8.50

0.78 0.71

778 190

1.38 5.35

Sample S11 (Co0.91Mn0.09C2O4)

Fresh cell 100th cycle

8.74 19.5

115 149

0.69 0.005

0.69 0.48

78 45

1.79 1.50

Rs (X)

obtained as compared to Sample S10. As for Sample S11, the highest initial intrinsic resistance (Rs) could lead to poorer electrochemical properties. Upon electrochemical cycling, the reduced double layer capacitance and increasing charge transfer resistance led to large capacity fading. In Sample S11 (Co0.91Mn0.09C2O4), the high Co content of the electrode seems to affect the electrochemical behavior adversely. While the initial capacity values could be deemed to be similar to that of Sample S10 (Fig. 6(d)), the fading in specific capacity upon further cycling is evident. The CPEsf+dl of Sample S11 is observed to decrease drastically from 0.7 to

CPEint (mF)

0.005 lF upon further cycling, which could indicate that the formation of SEI layer acts as a barrier to charge transfer. This is further observed in the slight increase in the Rsf+ct upon cycling. This leads to relatively poorer conductivity, thus affecting the lithium diffusion properties. It can be observed in Sample S2 (MnC2O4) that the capacity increases continually after 100 cycles, which could be attributed to slow lithium diffusion, as noted by the decrease in the intercalation capacitance (CPEint) to 0.02 mF. Finally the intercalation capacitance of Sample S10 increased to 5 mF after cycling; contributing to the improved lithium diffusion properties,

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and in turn, stable capacity upon electrochemical cycling. The doping of Mn to the CoC2O4 structure seems to be optimized at the concentration of Co0.52Mn0.48C2O4, enabling high initial capacity of 1000 mA h g1 and stable cycling behavior for up to 100 cycles. 4. Conclusions Single phase granular MnC2O4, rod-liked Co1xMnxC2O4 (x = 0.09, 0.23 and 0.48) mixed metal oxalates and CoC2O4 have been successfully synthesized by solvothermal technique and investigated as a potential anode material in LIBs for the first time. By controlling the solvent mixtures and types of transition metal, differences in morphology can be achieved in this solvothermal synthesis. FE-SEM/TEM/HR-TEM characterizations showed both anhydrous metal oxalates and mixed metal oxalates are of micron-sized and mesoporous in structure. Electrochemical characterizations on both anhydrous metal oxalates and mixed metal oxalates showed superior performance as compared to earlier reports. The best electrochemical performance is achieved for mixed solid solution Co0.52Mn0.48C2O4 (Sample S10) where it delivers initial discharge capacity of 1389 mA h g1 and a high reversible capacity value of 1000 mA h g1 with excellent capacity retention (after 20 cycles) at 1 C-rate. Besides the high porosity nature, the positive synergistic effect of Co0.52Mn0.48C2O4 attributed to optimal Co:Mn mole ratio having desirable properties of high capacity values of Co and good cycling stability of Mn is a promising candidate to be employed as anode for LIBs. Acknowledgements Authors acknowledge Timcal for gratis providing Super P™ Li carbon black. The electron microscopy and XRD work were performed at the Facility for Analysis, Characterization, Testing and Simulation (FACTS) in Nanyang Technological University, Singapore. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jallcom.2015.02. 203.

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