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Comparative Study of the Cathode and Anode Performance of Li2MnSiO4 for Lithium-Ion Batteries
Accepted Manuscript Title: Comparative Study of the Cathode and Anode Performance of Li2 MnSiO4 for Lithium-Ion Batteries Author: Shuang-Shuang Liu Li-Jun Song Bao-Jun Yu Cheng-Yang Wang Ming-Wei Li PII: DOI: Reference:
S0013-4686(15)30916-6 http://dx.doi.org/doi:10.1016/j.electacta.2015.11.144 EA 26144
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
30-7-2015 28-11-2015 28-11-2015
Please cite this article as: Shuang-Shuang Liu, Li-Jun Song, Bao-Jun Yu, Cheng-Yang Wang, Ming-Wei Li, Comparative Study of the Cathode and Anode Performance of Li2MnSiO4 for Lithium-Ion Batteries, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2015.11.144 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.
Comparative Study of the Cathode and Anode Performance of Li2MnSiO4 for Lithium-Ion Batteries
Shuang-Shuang Liu a,c, Li-Jun Song a,c, Bao-Jun Yu b,c, Cheng-Yang Wang b,c, Ming-Wei Li a,c,*
a
Department of Chemistry, Tianjin University, Tianjin 300072, China
b
Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China
c
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin University, Tianjin 300072, China
*Corresponding author. Tel: 86 158-2229-5393. E-mail address: [email protected] (M.-W. Li).
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Abstract Carbon-coated Li2MnSiO4 nanocrystallites are synthesized via a solid state reaction, and their cathode and anode performance is comparatively studied. The Li2MnSiO4 cathode shows an initial charge/discharge capacity of 405/134 mAh g−1 at a current density of 16.6 mA g−1, and a low charge/discharge capacity of 93/66 mAh g−1 during the 20th cycle. Partial Li2MnSiO4 cathode decomposes at 4.56 V (vs Li+/Li) during the initial charge. Interestingly, the Li2MnSiO4 anode exhibits a high initial discharge/charge capacity of 658/388 mAh g−1 at a current density of 20 mA g−1, and a discharge/charge capacity of 459/456 mAh g−1 during the 50th cycle. Li2MnSiO4 anodes also have stable and good rate capabilities. The Li2MnSiO4 crystallites as anode decompose during cycling, and their capacity increases simultaneously. It is proposed that Li2MnSiO4 anode converts energy via a reversible conversion reaction.
Keywords Lithium manganese orthosilicate; Anode material; Cathode material; Lithium-ion battery
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1. Introduction As the dominant power sources of cell phones and notebook computers, rechargeable lithium-ion batteries (LIBs) have greatly promoted the development of portable electronic devices [1]. LiCoO2 is the first commercialized cathode material for LIBs in 1991, but its applications are severely limited due to its high cost and poor stability. The polyanionic compound LiFePO4 has advantages in intrinsic stability and natural abundance, and has been used as a commercial cathode material for LIBs [2]. The polyanionic orthosilicate Li2MSiO4 (M=Fe or/and Mn) cathodes have been widely studied mainly due to their high theoretical capacities [3−6]. If two lithium ions were extracted from per formula unit of Li2MnSiO4, Li2MnSiO4 should have a capacity of 332 mAh g−1. Whereas, low conductivity and slow kinetics induce the poor cycling stability and low rate capability of Li2MSiO4 cathodes. Hence, Some methods, including carbon coating [5−17], doping/substitution [5,15−17], and synthesizing nanoscale Li2MnSiO4 crystallites [5−18], have been adopted to enhance the electrochemical properties of Li2MSiO4. However, Li2MnSiO4 cathodes still suffer from the rapid capacity fading and the manganese dissolution during cycling [5,7,17,18]. Additionally, Li2MnSiO4 can be used as the negative material for supercapacitors, or as anode material for LIBs. As being used in a Li2MnSiO4/activated carbon hybrid supercapacitor, Li2MnSiO4 negative material exhibits a higher coulombic efficiency over 99% and 85% capacitance retention of its initial 43.2 F g−1 capacitance after 1000 cycles [19]. It was suggested that Li2MnSiO4 stores energy via the lithium extraction mechanism. Furthermore, Li2MnSiO4 as anode for LIBs has been investigated [20]. It delivers an initial discharge/charge capacity of 420/180 mAh g−1 at a current density of C/20. Similarly, some other conventional cathode materials have been used as anodes, and show high capacities, such as an initial discharge/charge capacity ~820/485 mAh g−1 for FePO4 [21], ~620/400 mAh g−1 for LiFePO4 [22], and 880/618
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mAh g−1 for Li2FeSiO4 [23]. Li2MnSiO4 shows rather different electrochemical behaviors when it is used as different electrodes. A comparative study on its cathode and anode performance may reveal some valuable information about its crystal conversion during cycling and the energy storage mechanisms. In this work, we synthesized carbon-coated Li2MnSiO4 crystallites, and respectively applied them as cathode or as anode for LIBs. The precursor of Li2MnSiO4 was prepared by a modified sol−gel method. Amphiphilic carbonaceous material (ACM) was used as the reducing agent to synthesize Li2MnSiO4 crystallites. ACM consists of aromatic molecules, and the excess ACM can favorably form carbon coating on the active crystallites to enhance the electrical conductivity of the composite [24]. It is found that the Li2MnSiO4 cathode partially decomposes at 4.56 V (vs Li+/Li) during the initial charge, and shows low charge/discharge capacities during cycling. It is interesting to find that Li2MnSiO4 anode exhibits good rate capability and increased capacities during cycling. Different delithiation/lithiation mechanisms are used to interpret the cathode and anode performance of Li2MnSiO4.
2. Experimental The precursor of Li2MnSiO4 was prepared via a modified sol−gel method. The main raw materials include CH3COOLi·2H2O, Mn(CH3COO)2·4H2O, and amorphous SiO2 powder (CAB-O-SIL, M-5). During the process, citric acid, ethylene glycol, and SiO2 in a molar ratio of 1/3/2 were well dissolved/dispersed in deionized water to form a mixture. An aqueous solution containing CH3COOLi·2H2O and Mn(CH3COO)2·4H2O was added into the mixture under stirring. The new mixture was stirred 24 h, and then heated at 80 oC under stirring until it formed
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a dried gel. Thermogravimetric analysis and differential scanning calorimetry (TG−DSC) of the dried gel were measured by a simultaneous thermal analyzer (STA 409 PC/PG, NETZSCH) from room temperature to 900 oC in nitrogen flow. The ground gel was ball-milled with ACM (20% of the theoretical mass of Li2MnSiO4) in ethanol for 6 h. The slurry was dried and ground into powder. According to the TG−DSC results, the powder was heated at 700 oC in nitrogen flow for 10 h to synthesize the carbon-coated Li2MnSiO4 crystallites. To estimate the carbon content in the carbon-coated Li2MnSiO4, a TG analysis for the synthetic Li2MnSiO4 material was carried out in air flow from room temperature to 900 oC. The synthetic material was identified by a powder X-ray diffractometer (XRD, PANalytical X’Pert Pro, Philips) with Cu Kα radiation (λ=0.15406 nm). Its morphologies and structures were detected by a field emission scanning electron microscope (SEM, S4800, Thermo Fisher) and a high-resolution transmission electron microscope (HRTEM, Tecnai G2 F20, FEI). The electrochemical properties of the synthetic Li2MnSiO4 were tested in CR2430-type coin cells with metallic lithium served as the counter electrodes. Li2MnSiO4 as electrochemical active material, Super P carbon black, and polyvinylidene fluoride were well mixed in a weight ratio of 8/1/1 using N-methyl-2-pyrrolidone as solvent. The viscous mixture was coated onto aluminum foil for cathodes (or copper foil for anodes), and then was vacuum-dried at 80 oC for 2 h. The coated aluminum/copper foil was punched into 13 mm diameter disks, which were used as the cathodes/anodes. 1.0 M LiPF6 in a mixture of ethylene carbonate and dimethyl carbonate (EC−DMC) in a volume ratio of 1/1 was used as the electrolyte. Microporous Cellgard 2400 films were used as the separators. The charge/discharge tests were performed on a battery testing system (CT2001A, LAND, China) in a voltage window between 1.5−4.8 V (vs Li+/Li) for Li2MnSiO4 cathodes, and 3.0−0 V (vs Li+/Li) for Li2MnSiO4 anodes. The cyclic voltammetry (CV) at a scanning rate of 0.1 mV s−1 was recorded by an electrochemical working station
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(CHI604A, CH Instruments, China). The capacities were calculated based on the mass of carbon-coated Li2MnSiO4 without Super P carbon black. To detect the crystal structures of the charged/discharged Li2MnSiO4, five electrodes from the cycled cells were analyzed by XRD. For the two cathodes charged/discharged during the initial cycle at 10 mA g−1, one is charged from 1.5 to 4.8 V, and the other is discharged from 4.8 to 1.5 V. For the two anodes charged/discharged during the initial cycle at 10 mA g−1, one is discharged from 3.0 to 0 V, and the other is charged from 0 to 3.0 V. The third anode is discharged after the 52th discharge at 100 mA g−1. The five Li2MnSiO4 electrodes were disassembled in an argon-filled glove box, and washed with DMC. After dried, the two cathodes and the powder samples scraped from the three anodes were held in argon-filled bottles and were quickly analyzed by XRD. Morphologies of powder samples scraped from two charged anodes were analyzed by HRTEM. The two anodes were charged after 61 and 100 cycles, respectively. The composition of the first sample was measured by an energy-dispersive X-ray spectroscopy (EDX).
3. Results and discussion Fig. 1a shows the TG−DSC results of the precursor of Li2MnSiO4. The total weight loss is about 59% below 660 °C. There is a rapid weight loss (~40 %) from 220 to 430 °C, which results from the decomposition of acetates and citric acid. The DSC curve implies that the Li2MnSiO4 crystallites form above 430 °C, which is in agreement with the literature [25]. Above 660 °C, the weight hardly changes. So the precursor was heated at 700 °C to form tiny Li2MnSiO4 crystallites. The residual carbon content in the synthetic material is below 13.4% based on the TG analysis in air. XRD pattern of the synthetic material is shown in Fig. 1b, most diffraction peaks are indexed
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to an orthorhombic structure with Pmn21 symmetry (ICSD-161305), which indicates the Li2MnSiO4 crystallites [26-29]. The lattice parameters are a=6.3158 Å, b=5.3882 Å, c=4.9796 Å, V=169.46 Å3, and Z=2. The average size of the Li2MnSiO4 crystallites is ~21 nm calculated by Scherrer’s formula. Besides the main Li2MnSiO4 phase, trace cubic MnSiO3 is found. There are no distinct diffraction peaks for the reported MnO, Li2SiO3, or Mn2SiO4 in literature [20,25]. The schematic unit cell of Li2MnSiO4 is inserted in Fig. 1b. It consists of alternating layers built by LiO4, SiO4, and MnO4 tetrahedra. The tetrahedra lie on the ab-planes and are perpendicular to the c axes. In each layer, the atoms of Li, Mn, and Si are connected by the O atoms between them. They are arranged in uneven honeycomb lattices. The number of the Li atoms equals to the total number of Mn and Si atoms. The alternating layers are connected by MO (M=Li, Mn, or Si) bonds along the c axes. The image of Li2MnSiO4 unit cell may be helpful to understand the structure changes of Li2MnSiO4 during cycling. As shown in Fig. 2a, the as-prepared sample consists of agglomerated particles with irregular shape. The HRTEM image (Fig. 2b) indicates that the particles consist of crystallites with a size range of ~10−40 nm, being consistent with the XRD result. There exist different lattice spaces of 0.36, 0.32, and 0.27 nm, respectively corresponding to the (011), (200), and (210) planes of the orthorhombic Li2MnSiO4. The crystallites are coated by a thin amorphous carbon layer, which originates from the carbonized ACM. Fig. 3 shows the charge/discharge curves of the Li2MnSiO4 electrodes. The Li2MnSiO4 cathode delivers an initial charge/discharge capacity of 405/134 mAh g−1. The initial capacity loss 271 mAh g−1 is involved to the formation of solid electrolyte interface (SEI) layers [30], or/and cathode decomposition and crystal structural rearrangement [31,32]. There is an approximate charge plateau at ~4.1 V, which is attributed to the lithium extraction from Li2MnSiO4. The charge plateau is in good agreement with the voltage for the oxidation of Mn2+
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[24]. It is predicted that the delithiated MnSiO4 polymorphs decompose into different phases, and the average lithium intercalation voltage for the Pmn21-Li2MnSiO4 is 4.18 V [33]. Herein, the initial charge capacity is higher, and is rather different from the subsequent charge ones. It is possible that partial Li2MnSiO4 cathode decomposed during the initial charge. Comparing with Li2MnSiO4 cathode, Li2MnSiO4 anode exhibits rather higher capacities as shown in Fig. 3b. It delivers an initial discharge/charge capacity of 658/388 mAh g−1. After the initial discharge, the subsequent charge/discharge curves show the similar profile, and almost overlap each other. It reflects the good anode stability during cycling. There are discharge plateaus at ~0.4 V, and charge plateaus at ~1.3 V. They are similar with the reported results of MnO as anode [34−38]. The Li2MnSiO4 anode shows a discharge/charge capacity of 459/456 mAh g−1 during the 50th cycle. Actually, partial capacity is attributed by the Super P carbon black, which is not calculated in the electrochemical active material mass. The capacity of coated carbon also disturbs the detection for the capacity of pure Li2MnSiO4 as anode. It has ever been reported that the discharge/charge capacity of a Super P anode is 610/518 mAh g−1 in a voltage window between 0.01–1.2 V (vs Li+/Li) [39]. Generally, hard carbons have anode capacities between 200 and 600 mAh g−1 between 0–1.5 V [40]. According to the contents and capacities of Super P carbon black and the coated carbon in the Li2MnSiO4 anode, the capacity of pure Li2MnSiO4 as anode should be higher than 400 mAh g−1. Fig. 4 exhibits the cycling performance of the Li2MnSiO4 electrodes. As shown in Fig. 4a, the discharge/charge capacity of the Li2MnSiO4 cathode gradually decreases from the initial 405/134 mAh g−1 to the 93/66 mAh g−1 during the 20th cycle. The low coulombic efficiency (<72%) reflects the continual capacity loss during cycling. It is interesting to find that the Li2MnSiO4 anode delivers increased discharge/charge
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capacities during cycling, as shown in Fig. 4b. The discharge/charge capacity gradually changed to 459/456 mAh g−1 from the initial 658/388 mAh g−1 after the 50th cycle. This phenomenon had ever been reported in the investigations on MnO anodes, and were attributed to the formation of SEI layers and/or high oxidation state products [34,36−38]. Herein, we attribute the most increased capacity to the enhanced lithium and electron transfer, which results from the formed many smaller electrochemical active particles due to the decomposition of Li2MnSiO4 crystallites during cycling. It also is facile for the small particles to form high oxidation state of manganese Mn3+. From the 6th cycle, the coulombic efficiencies jump to the high values (>95%), indicating a stable reversible lithiation/delithiation reaction. Fig. 5 shows the samples’ rate capability. The discharge capacity of Li2MnSiO4 cathode drops from 125 to 30 mAh g−1 with an increasing current density from 16.6 to 332 mA g−1. While the Li2MnSiO4 anode shows a higher discharge capacity drops from 431 to 162 mAh g−1 with an increasing current from 20 to 400 mA g−1. As the current density drops back to 50 mA g−1, the Li2MnSiO4 anode almost regains the capacity. CV spectra were measured to understand the energy storage mechanisms of Li2MnSiO4. As shown in Fig. 6a, during the initial charge, there are two distinct oxidation peaks at 4.11 and 4.56 V. It is proposed that they respectively reflect the extraction for the first and the second lithium ion from Li2MnSiO4 cathode. It almost coincides with the theoretic values of 4.13 and 4.43 V [41]. The oxidation peak at 4.56 V is similar with the experimental result of the Mn-contained lithium-rich oxide cathode, which decomposes and has a structural rearrangement at the charge voltage ~4.58 V [32]. In the sequent charges, the inconspicuous oxidation peaks indicate that at least partial Li2MnSiO4 crystallites had decomposed during the initial charge. It is in agreement with literature [8,31]. During the sequent cycles, there is a couple of oxidation/reduction peaks at ~4.2/~2.8 V. Due to the low discharge capacity (<166 mAh g−1), we suggest that the couple of
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oxidation/reduction peaks correspond to a lithium extraction/insertion reaction as follows. Li2MnSiO4 ↔ Li+ + LiMnSiO4 + e−
(1)
Comparing with Li2MnSiO4 cathode, Li2MnSiO4 anode shows rather different CV curves, as shown in Fig. 6b. In the initial discharge curve, the small reduction peak at 1.45 V is attributed to the conversions of some impurities, such as MnSiO3 [31,42]. The sharp reduction peak near 0 V is mainly attributed to the formation of SEI layers and the formation of metallic lithium [31,35]. Based on the experimental data shown in Figures 6b and 3b, we propose that the oxidation peaks ~1.3 and ~2.1 V during charging respectively reflect the conversions Mn0→Mn2+ and Mn2+→Mn3+. During discharging, the Mn3+ reduction conversion Mn3+→Mn2+ first happens, and the reduction peak ~0.3 V reflects the conversion Mn2+→Mn0. Fig. 7 shows the charged/discharged samples’ XRD patterns. Despite the intense disturbance of aluminum, the diffraction peaks for Li2MnSiO4 are observed in the cathodes’ XRD spectra (Fig. 7a). Noticeably, obvious diffraction peaks for MnO in both cathodes are found. They coincide with the peaks of the cycled MnO anodes in literature [34−38]. The broadened peaks at ~22.5o are attributed to amorphous SiO2 [42,43]. The MnO and amorphous SiO2 mainly result from the decomposition of Li2MnSiO4 during the initial charge. Three XRD spectra of the cycled anode powder are shown in Fig. 7b. After the initial discharge/charge, the two anode samples show well defined diffraction peaks. Most of the peaks are coincide with the diffraction peaks of the pristine Li2MnSiO4. The decreased peak number and the degressive peak intensities indicate the decomposition of Li2MnSiO4 crystallites. No distinct diffraction peaks for MnSiO3 crystal phase are found in the cycled anodes. There is the main diffraction peak (111) of copper, which is scraped from the current collectors. No obvious peaks for MnO and metallic Mn are found. After the 52th discharge, the anode shows a broad peak centered at ~22.5o and a little peak at 33.0o, which respectively are attributed to amorphous
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SiO2 and tiny Mn2O3 crystallites. Fig. 8 exhibits the HRTEM images of the anode powder scraped from the charged Li2MnSiO4 anodes. Fig. 8a shows the agglomerated particles. The EDX spectrum indicates that the atomic ratio of Mn/Si/O is 1.0/1.2/6.6 in the selected area. It means that the elements Mn and Si still are well-mixed after 61 cycles. The high carbon content 51.17% results from not only conductive carbon but also the SEI layers. Apart from forming Mn- and Si-contained oxides, partial oxygen also originates from the SEI layers. The lattice space of ~0.27 nm in Fig. 8c corresponds to the diffraction peak at ~33o, which is indexed to Mn2O3. As shown in the insert of Fig. 8d, many tiny particles formed in the powder after 100 cycles. The enlarged imagine exhibits that they are spherical crystallites with a size of ~3−5 nm. Their lattice space 0.21 nm corresponds to the main lattice plane of Mn or Cu. According to their regular shape and similar size, we think that they are metallic Mn. It is possible that XRD analysis cannot detect the metallic Mn with so small sizes and a dispersed state. As the above-mentioned results of XRD and HRTEM, there probably exist Mn2O3 and metallic Mn in the cycled anodes. According to the high capacities shown in Fig. 4b and the high oxygen content detected by EDX analysis, we propose that the anodes store energy via a reversible conversion reaction being simplified as follows. Mn + 3/2 Li2O ↔ 1/2 Mn2O3 + 3Li+ + 3e−
(2)
The nanosized electrochemical active particles also ensure the anodes’ nice rate/cycling capabilities. The proposed mechanism is similar with the energy storage mechanism of transition-metal oxide (MO, M=Mn, Fe, Co, Ni, Cu, etc) anodes [34−38,44]. Comparing with the binary oxides, polyanionic compound Li2MnSiO4 has more complicated structures. Li2MnSiO4 as anode has the acceptable capacities. It probably has small volume changes, and has shown the good
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cycling/rate capability in this research.
4. Conclusions Li2MnSiO4 nanocrystallites have been successfully synthesized via a solid state reaction at 700 °C. The synthetic Li2MnSiO4 has an orthorhombic structure with Pmn21 symmetry. It is respectively used as cathode and as anode for LIBs. It is found that the Li2MnSiO4 cathodes partially decompose during the initial charge at 4.56 V (vs Li+/Li). The Li2MnSiO4 cathode converts energy by the lithium extraction/insertion mechanism. However, the Li2MnSiO4 anode shows high cycling and rate capabilities. During cycling, Li2MnSiO4 crystallites as anode decompose into many smaller particles, which improve the capacities probably caused by the enhanced lithium and electron transfer. It is proposed that the Li2MnSiO4 anode transfers energy via a reversible conversion reaction.
Acknowledgments This research is financially supported by the National Nature Science Foundation of China (NSFC 5117260) and the Nature Science Foundation of Tianjin city (No. 14RCHZGX00859 and No. 14JCTPJC00484).
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Figure Captions
Fig. 1. (a) TG−DSC curves of the precursor of Li2MnSiO4, and (b) XRD pattern of the synthetic Li2MnSiO4 material. The insert is the schematic unit cell of Li2MnSiO4.
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Fig. 2. (a) SEM, and (b) HRTEM images of the synthetic Li2MnSiO4 material.
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Fig. 3. Charge/discharge curves of Li2MnSiO4: (a) as a cathode in a voltage range of 1.5−4.8 V at a current density of 16.6 mA g−1, and (b) as an anode in a voltage range of 3.0−0 V at a current density of 20 mA g−1.
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Fig. 4. Cycling performance of Li2MnSiO4: (a) as a cathode at a current density of 16.6 mA g−1, and (b) as an anode at a current density of 20 mA g−1.
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Fig. 5. Rate performance of Li2MnSiO4: (a) as a cathode at a current density increased from 16.6 to 332 mA g−1, i.e., 0.05 to 1 C (1 C=332 mA g−1), and (b) as an anode at a current density increased from 50 to 400 mA g−1.
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Fig. 6. CV spectra of Li2MnSiO4: (a) as a cathode, and (b) as an anode at a scanning rate of 0.1 mV s−1.
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Fig. 7. Ex-situ XRD patterns of the cycled Li2MnSiO4: (a) as two cathodes on the aluminum current collectors (one is charged to 4.8 V, and the other is discharged to 1.5 V at 10 mA g−1), and (b) as three anode powder samples scraped from the copper current collectors (the first one is initially discharged to 0 V, and the second one is initially charged to 3 V at 10 mA g−1. The third one is discharged to 0 V during the 52th discharge at 100 mA g−1). The standard XRD patterns of Mn2O3, MnO, Mn, and Cu are shown for comparison.
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Fig. 8. (a) and (c) HRTEM images of the powder scraped from the anode after the 61th charge, (b) EDX spectrum of the selected area, and (d) a HRTEM image of the powder scraped from the anode after the 100th charge. The insert in (d) shows the connected spherical tiny particles. The current density of the final charge is 20 mA g−1.
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S0013-4686(15)30916-6 http://dx.doi.org/doi:10.1016/j.electacta.2015.11.144 EA 26144
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
30-7-2015 28-11-2015 28-11-2015
Please cite this article as: Shuang-Shuang Liu, Li-Jun Song, Bao-Jun Yu, Cheng-Yang Wang, Ming-Wei Li, Comparative Study of the Cathode and Anode Performance of Li2MnSiO4 for Lithium-Ion Batteries, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2015.11.144 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.
Comparative Study of the Cathode and Anode Performance of Li2MnSiO4 for Lithium-Ion Batteries
Shuang-Shuang Liu a,c, Li-Jun Song a,c, Bao-Jun Yu b,c, Cheng-Yang Wang b,c, Ming-Wei Li a,c,*
a
Department of Chemistry, Tianjin University, Tianjin 300072, China
b
Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China
c
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin University, Tianjin 300072, China
*Corresponding author. Tel: 86 158-2229-5393. E-mail address: [email protected] (M.-W. Li).
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Abstract Carbon-coated Li2MnSiO4 nanocrystallites are synthesized via a solid state reaction, and their cathode and anode performance is comparatively studied. The Li2MnSiO4 cathode shows an initial charge/discharge capacity of 405/134 mAh g−1 at a current density of 16.6 mA g−1, and a low charge/discharge capacity of 93/66 mAh g−1 during the 20th cycle. Partial Li2MnSiO4 cathode decomposes at 4.56 V (vs Li+/Li) during the initial charge. Interestingly, the Li2MnSiO4 anode exhibits a high initial discharge/charge capacity of 658/388 mAh g−1 at a current density of 20 mA g−1, and a discharge/charge capacity of 459/456 mAh g−1 during the 50th cycle. Li2MnSiO4 anodes also have stable and good rate capabilities. The Li2MnSiO4 crystallites as anode decompose during cycling, and their capacity increases simultaneously. It is proposed that Li2MnSiO4 anode converts energy via a reversible conversion reaction.
Keywords Lithium manganese orthosilicate; Anode material; Cathode material; Lithium-ion battery
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1. Introduction As the dominant power sources of cell phones and notebook computers, rechargeable lithium-ion batteries (LIBs) have greatly promoted the development of portable electronic devices [1]. LiCoO2 is the first commercialized cathode material for LIBs in 1991, but its applications are severely limited due to its high cost and poor stability. The polyanionic compound LiFePO4 has advantages in intrinsic stability and natural abundance, and has been used as a commercial cathode material for LIBs [2]. The polyanionic orthosilicate Li2MSiO4 (M=Fe or/and Mn) cathodes have been widely studied mainly due to their high theoretical capacities [3−6]. If two lithium ions were extracted from per formula unit of Li2MnSiO4, Li2MnSiO4 should have a capacity of 332 mAh g−1. Whereas, low conductivity and slow kinetics induce the poor cycling stability and low rate capability of Li2MSiO4 cathodes. Hence, Some methods, including carbon coating [5−17], doping/substitution [5,15−17], and synthesizing nanoscale Li2MnSiO4 crystallites [5−18], have been adopted to enhance the electrochemical properties of Li2MSiO4. However, Li2MnSiO4 cathodes still suffer from the rapid capacity fading and the manganese dissolution during cycling [5,7,17,18]. Additionally, Li2MnSiO4 can be used as the negative material for supercapacitors, or as anode material for LIBs. As being used in a Li2MnSiO4/activated carbon hybrid supercapacitor, Li2MnSiO4 negative material exhibits a higher coulombic efficiency over 99% and 85% capacitance retention of its initial 43.2 F g−1 capacitance after 1000 cycles [19]. It was suggested that Li2MnSiO4 stores energy via the lithium extraction mechanism. Furthermore, Li2MnSiO4 as anode for LIBs has been investigated [20]. It delivers an initial discharge/charge capacity of 420/180 mAh g−1 at a current density of C/20. Similarly, some other conventional cathode materials have been used as anodes, and show high capacities, such as an initial discharge/charge capacity ~820/485 mAh g−1 for FePO4 [21], ~620/400 mAh g−1 for LiFePO4 [22], and 880/618
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mAh g−1 for Li2FeSiO4 [23]. Li2MnSiO4 shows rather different electrochemical behaviors when it is used as different electrodes. A comparative study on its cathode and anode performance may reveal some valuable information about its crystal conversion during cycling and the energy storage mechanisms. In this work, we synthesized carbon-coated Li2MnSiO4 crystallites, and respectively applied them as cathode or as anode for LIBs. The precursor of Li2MnSiO4 was prepared by a modified sol−gel method. Amphiphilic carbonaceous material (ACM) was used as the reducing agent to synthesize Li2MnSiO4 crystallites. ACM consists of aromatic molecules, and the excess ACM can favorably form carbon coating on the active crystallites to enhance the electrical conductivity of the composite [24]. It is found that the Li2MnSiO4 cathode partially decomposes at 4.56 V (vs Li+/Li) during the initial charge, and shows low charge/discharge capacities during cycling. It is interesting to find that Li2MnSiO4 anode exhibits good rate capability and increased capacities during cycling. Different delithiation/lithiation mechanisms are used to interpret the cathode and anode performance of Li2MnSiO4.
2. Experimental The precursor of Li2MnSiO4 was prepared via a modified sol−gel method. The main raw materials include CH3COOLi·2H2O, Mn(CH3COO)2·4H2O, and amorphous SiO2 powder (CAB-O-SIL, M-5). During the process, citric acid, ethylene glycol, and SiO2 in a molar ratio of 1/3/2 were well dissolved/dispersed in deionized water to form a mixture. An aqueous solution containing CH3COOLi·2H2O and Mn(CH3COO)2·4H2O was added into the mixture under stirring. The new mixture was stirred 24 h, and then heated at 80 oC under stirring until it formed
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a dried gel. Thermogravimetric analysis and differential scanning calorimetry (TG−DSC) of the dried gel were measured by a simultaneous thermal analyzer (STA 409 PC/PG, NETZSCH) from room temperature to 900 oC in nitrogen flow. The ground gel was ball-milled with ACM (20% of the theoretical mass of Li2MnSiO4) in ethanol for 6 h. The slurry was dried and ground into powder. According to the TG−DSC results, the powder was heated at 700 oC in nitrogen flow for 10 h to synthesize the carbon-coated Li2MnSiO4 crystallites. To estimate the carbon content in the carbon-coated Li2MnSiO4, a TG analysis for the synthetic Li2MnSiO4 material was carried out in air flow from room temperature to 900 oC. The synthetic material was identified by a powder X-ray diffractometer (XRD, PANalytical X’Pert Pro, Philips) with Cu Kα radiation (λ=0.15406 nm). Its morphologies and structures were detected by a field emission scanning electron microscope (SEM, S4800, Thermo Fisher) and a high-resolution transmission electron microscope (HRTEM, Tecnai G2 F20, FEI). The electrochemical properties of the synthetic Li2MnSiO4 were tested in CR2430-type coin cells with metallic lithium served as the counter electrodes. Li2MnSiO4 as electrochemical active material, Super P carbon black, and polyvinylidene fluoride were well mixed in a weight ratio of 8/1/1 using N-methyl-2-pyrrolidone as solvent. The viscous mixture was coated onto aluminum foil for cathodes (or copper foil for anodes), and then was vacuum-dried at 80 oC for 2 h. The coated aluminum/copper foil was punched into 13 mm diameter disks, which were used as the cathodes/anodes. 1.0 M LiPF6 in a mixture of ethylene carbonate and dimethyl carbonate (EC−DMC) in a volume ratio of 1/1 was used as the electrolyte. Microporous Cellgard 2400 films were used as the separators. The charge/discharge tests were performed on a battery testing system (CT2001A, LAND, China) in a voltage window between 1.5−4.8 V (vs Li+/Li) for Li2MnSiO4 cathodes, and 3.0−0 V (vs Li+/Li) for Li2MnSiO4 anodes. The cyclic voltammetry (CV) at a scanning rate of 0.1 mV s−1 was recorded by an electrochemical working station
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(CHI604A, CH Instruments, China). The capacities were calculated based on the mass of carbon-coated Li2MnSiO4 without Super P carbon black. To detect the crystal structures of the charged/discharged Li2MnSiO4, five electrodes from the cycled cells were analyzed by XRD. For the two cathodes charged/discharged during the initial cycle at 10 mA g−1, one is charged from 1.5 to 4.8 V, and the other is discharged from 4.8 to 1.5 V. For the two anodes charged/discharged during the initial cycle at 10 mA g−1, one is discharged from 3.0 to 0 V, and the other is charged from 0 to 3.0 V. The third anode is discharged after the 52th discharge at 100 mA g−1. The five Li2MnSiO4 electrodes were disassembled in an argon-filled glove box, and washed with DMC. After dried, the two cathodes and the powder samples scraped from the three anodes were held in argon-filled bottles and were quickly analyzed by XRD. Morphologies of powder samples scraped from two charged anodes were analyzed by HRTEM. The two anodes were charged after 61 and 100 cycles, respectively. The composition of the first sample was measured by an energy-dispersive X-ray spectroscopy (EDX).
3. Results and discussion Fig. 1a shows the TG−DSC results of the precursor of Li2MnSiO4. The total weight loss is about 59% below 660 °C. There is a rapid weight loss (~40 %) from 220 to 430 °C, which results from the decomposition of acetates and citric acid. The DSC curve implies that the Li2MnSiO4 crystallites form above 430 °C, which is in agreement with the literature [25]. Above 660 °C, the weight hardly changes. So the precursor was heated at 700 °C to form tiny Li2MnSiO4 crystallites. The residual carbon content in the synthetic material is below 13.4% based on the TG analysis in air. XRD pattern of the synthetic material is shown in Fig. 1b, most diffraction peaks are indexed
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to an orthorhombic structure with Pmn21 symmetry (ICSD-161305), which indicates the Li2MnSiO4 crystallites [26-29]. The lattice parameters are a=6.3158 Å, b=5.3882 Å, c=4.9796 Å, V=169.46 Å3, and Z=2. The average size of the Li2MnSiO4 crystallites is ~21 nm calculated by Scherrer’s formula. Besides the main Li2MnSiO4 phase, trace cubic MnSiO3 is found. There are no distinct diffraction peaks for the reported MnO, Li2SiO3, or Mn2SiO4 in literature [20,25]. The schematic unit cell of Li2MnSiO4 is inserted in Fig. 1b. It consists of alternating layers built by LiO4, SiO4, and MnO4 tetrahedra. The tetrahedra lie on the ab-planes and are perpendicular to the c axes. In each layer, the atoms of Li, Mn, and Si are connected by the O atoms between them. They are arranged in uneven honeycomb lattices. The number of the Li atoms equals to the total number of Mn and Si atoms. The alternating layers are connected by MO (M=Li, Mn, or Si) bonds along the c axes. The image of Li2MnSiO4 unit cell may be helpful to understand the structure changes of Li2MnSiO4 during cycling. As shown in Fig. 2a, the as-prepared sample consists of agglomerated particles with irregular shape. The HRTEM image (Fig. 2b) indicates that the particles consist of crystallites with a size range of ~10−40 nm, being consistent with the XRD result. There exist different lattice spaces of 0.36, 0.32, and 0.27 nm, respectively corresponding to the (011), (200), and (210) planes of the orthorhombic Li2MnSiO4. The crystallites are coated by a thin amorphous carbon layer, which originates from the carbonized ACM. Fig. 3 shows the charge/discharge curves of the Li2MnSiO4 electrodes. The Li2MnSiO4 cathode delivers an initial charge/discharge capacity of 405/134 mAh g−1. The initial capacity loss 271 mAh g−1 is involved to the formation of solid electrolyte interface (SEI) layers [30], or/and cathode decomposition and crystal structural rearrangement [31,32]. There is an approximate charge plateau at ~4.1 V, which is attributed to the lithium extraction from Li2MnSiO4. The charge plateau is in good agreement with the voltage for the oxidation of Mn2+
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[24]. It is predicted that the delithiated MnSiO4 polymorphs decompose into different phases, and the average lithium intercalation voltage for the Pmn21-Li2MnSiO4 is 4.18 V [33]. Herein, the initial charge capacity is higher, and is rather different from the subsequent charge ones. It is possible that partial Li2MnSiO4 cathode decomposed during the initial charge. Comparing with Li2MnSiO4 cathode, Li2MnSiO4 anode exhibits rather higher capacities as shown in Fig. 3b. It delivers an initial discharge/charge capacity of 658/388 mAh g−1. After the initial discharge, the subsequent charge/discharge curves show the similar profile, and almost overlap each other. It reflects the good anode stability during cycling. There are discharge plateaus at ~0.4 V, and charge plateaus at ~1.3 V. They are similar with the reported results of MnO as anode [34−38]. The Li2MnSiO4 anode shows a discharge/charge capacity of 459/456 mAh g−1 during the 50th cycle. Actually, partial capacity is attributed by the Super P carbon black, which is not calculated in the electrochemical active material mass. The capacity of coated carbon also disturbs the detection for the capacity of pure Li2MnSiO4 as anode. It has ever been reported that the discharge/charge capacity of a Super P anode is 610/518 mAh g−1 in a voltage window between 0.01–1.2 V (vs Li+/Li) [39]. Generally, hard carbons have anode capacities between 200 and 600 mAh g−1 between 0–1.5 V [40]. According to the contents and capacities of Super P carbon black and the coated carbon in the Li2MnSiO4 anode, the capacity of pure Li2MnSiO4 as anode should be higher than 400 mAh g−1. Fig. 4 exhibits the cycling performance of the Li2MnSiO4 electrodes. As shown in Fig. 4a, the discharge/charge capacity of the Li2MnSiO4 cathode gradually decreases from the initial 405/134 mAh g−1 to the 93/66 mAh g−1 during the 20th cycle. The low coulombic efficiency (<72%) reflects the continual capacity loss during cycling. It is interesting to find that the Li2MnSiO4 anode delivers increased discharge/charge
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capacities during cycling, as shown in Fig. 4b. The discharge/charge capacity gradually changed to 459/456 mAh g−1 from the initial 658/388 mAh g−1 after the 50th cycle. This phenomenon had ever been reported in the investigations on MnO anodes, and were attributed to the formation of SEI layers and/or high oxidation state products [34,36−38]. Herein, we attribute the most increased capacity to the enhanced lithium and electron transfer, which results from the formed many smaller electrochemical active particles due to the decomposition of Li2MnSiO4 crystallites during cycling. It also is facile for the small particles to form high oxidation state of manganese Mn3+. From the 6th cycle, the coulombic efficiencies jump to the high values (>95%), indicating a stable reversible lithiation/delithiation reaction. Fig. 5 shows the samples’ rate capability. The discharge capacity of Li2MnSiO4 cathode drops from 125 to 30 mAh g−1 with an increasing current density from 16.6 to 332 mA g−1. While the Li2MnSiO4 anode shows a higher discharge capacity drops from 431 to 162 mAh g−1 with an increasing current from 20 to 400 mA g−1. As the current density drops back to 50 mA g−1, the Li2MnSiO4 anode almost regains the capacity. CV spectra were measured to understand the energy storage mechanisms of Li2MnSiO4. As shown in Fig. 6a, during the initial charge, there are two distinct oxidation peaks at 4.11 and 4.56 V. It is proposed that they respectively reflect the extraction for the first and the second lithium ion from Li2MnSiO4 cathode. It almost coincides with the theoretic values of 4.13 and 4.43 V [41]. The oxidation peak at 4.56 V is similar with the experimental result of the Mn-contained lithium-rich oxide cathode, which decomposes and has a structural rearrangement at the charge voltage ~4.58 V [32]. In the sequent charges, the inconspicuous oxidation peaks indicate that at least partial Li2MnSiO4 crystallites had decomposed during the initial charge. It is in agreement with literature [8,31]. During the sequent cycles, there is a couple of oxidation/reduction peaks at ~4.2/~2.8 V. Due to the low discharge capacity (<166 mAh g−1), we suggest that the couple of
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oxidation/reduction peaks correspond to a lithium extraction/insertion reaction as follows. Li2MnSiO4 ↔ Li+ + LiMnSiO4 + e−
(1)
Comparing with Li2MnSiO4 cathode, Li2MnSiO4 anode shows rather different CV curves, as shown in Fig. 6b. In the initial discharge curve, the small reduction peak at 1.45 V is attributed to the conversions of some impurities, such as MnSiO3 [31,42]. The sharp reduction peak near 0 V is mainly attributed to the formation of SEI layers and the formation of metallic lithium [31,35]. Based on the experimental data shown in Figures 6b and 3b, we propose that the oxidation peaks ~1.3 and ~2.1 V during charging respectively reflect the conversions Mn0→Mn2+ and Mn2+→Mn3+. During discharging, the Mn3+ reduction conversion Mn3+→Mn2+ first happens, and the reduction peak ~0.3 V reflects the conversion Mn2+→Mn0. Fig. 7 shows the charged/discharged samples’ XRD patterns. Despite the intense disturbance of aluminum, the diffraction peaks for Li2MnSiO4 are observed in the cathodes’ XRD spectra (Fig. 7a). Noticeably, obvious diffraction peaks for MnO in both cathodes are found. They coincide with the peaks of the cycled MnO anodes in literature [34−38]. The broadened peaks at ~22.5o are attributed to amorphous SiO2 [42,43]. The MnO and amorphous SiO2 mainly result from the decomposition of Li2MnSiO4 during the initial charge. Three XRD spectra of the cycled anode powder are shown in Fig. 7b. After the initial discharge/charge, the two anode samples show well defined diffraction peaks. Most of the peaks are coincide with the diffraction peaks of the pristine Li2MnSiO4. The decreased peak number and the degressive peak intensities indicate the decomposition of Li2MnSiO4 crystallites. No distinct diffraction peaks for MnSiO3 crystal phase are found in the cycled anodes. There is the main diffraction peak (111) of copper, which is scraped from the current collectors. No obvious peaks for MnO and metallic Mn are found. After the 52th discharge, the anode shows a broad peak centered at ~22.5o and a little peak at 33.0o, which respectively are attributed to amorphous
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SiO2 and tiny Mn2O3 crystallites. Fig. 8 exhibits the HRTEM images of the anode powder scraped from the charged Li2MnSiO4 anodes. Fig. 8a shows the agglomerated particles. The EDX spectrum indicates that the atomic ratio of Mn/Si/O is 1.0/1.2/6.6 in the selected area. It means that the elements Mn and Si still are well-mixed after 61 cycles. The high carbon content 51.17% results from not only conductive carbon but also the SEI layers. Apart from forming Mn- and Si-contained oxides, partial oxygen also originates from the SEI layers. The lattice space of ~0.27 nm in Fig. 8c corresponds to the diffraction peak at ~33o, which is indexed to Mn2O3. As shown in the insert of Fig. 8d, many tiny particles formed in the powder after 100 cycles. The enlarged imagine exhibits that they are spherical crystallites with a size of ~3−5 nm. Their lattice space 0.21 nm corresponds to the main lattice plane of Mn or Cu. According to their regular shape and similar size, we think that they are metallic Mn. It is possible that XRD analysis cannot detect the metallic Mn with so small sizes and a dispersed state. As the above-mentioned results of XRD and HRTEM, there probably exist Mn2O3 and metallic Mn in the cycled anodes. According to the high capacities shown in Fig. 4b and the high oxygen content detected by EDX analysis, we propose that the anodes store energy via a reversible conversion reaction being simplified as follows. Mn + 3/2 Li2O ↔ 1/2 Mn2O3 + 3Li+ + 3e−
(2)
The nanosized electrochemical active particles also ensure the anodes’ nice rate/cycling capabilities. The proposed mechanism is similar with the energy storage mechanism of transition-metal oxide (MO, M=Mn, Fe, Co, Ni, Cu, etc) anodes [34−38,44]. Comparing with the binary oxides, polyanionic compound Li2MnSiO4 has more complicated structures. Li2MnSiO4 as anode has the acceptable capacities. It probably has small volume changes, and has shown the good
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cycling/rate capability in this research.
4. Conclusions Li2MnSiO4 nanocrystallites have been successfully synthesized via a solid state reaction at 700 °C. The synthetic Li2MnSiO4 has an orthorhombic structure with Pmn21 symmetry. It is respectively used as cathode and as anode for LIBs. It is found that the Li2MnSiO4 cathodes partially decompose during the initial charge at 4.56 V (vs Li+/Li). The Li2MnSiO4 cathode converts energy by the lithium extraction/insertion mechanism. However, the Li2MnSiO4 anode shows high cycling and rate capabilities. During cycling, Li2MnSiO4 crystallites as anode decompose into many smaller particles, which improve the capacities probably caused by the enhanced lithium and electron transfer. It is proposed that the Li2MnSiO4 anode transfers energy via a reversible conversion reaction.
Acknowledgments This research is financially supported by the National Nature Science Foundation of China (NSFC 5117260) and the Nature Science Foundation of Tianjin city (No. 14RCHZGX00859 and No. 14JCTPJC00484).
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Figure Captions
Fig. 1. (a) TG−DSC curves of the precursor of Li2MnSiO4, and (b) XRD pattern of the synthetic Li2MnSiO4 material. The insert is the schematic unit cell of Li2MnSiO4.
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Fig. 2. (a) SEM, and (b) HRTEM images of the synthetic Li2MnSiO4 material.
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Fig. 3. Charge/discharge curves of Li2MnSiO4: (a) as a cathode in a voltage range of 1.5−4.8 V at a current density of 16.6 mA g−1, and (b) as an anode in a voltage range of 3.0−0 V at a current density of 20 mA g−1.
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Fig. 4. Cycling performance of Li2MnSiO4: (a) as a cathode at a current density of 16.6 mA g−1, and (b) as an anode at a current density of 20 mA g−1.
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Fig. 5. Rate performance of Li2MnSiO4: (a) as a cathode at a current density increased from 16.6 to 332 mA g−1, i.e., 0.05 to 1 C (1 C=332 mA g−1), and (b) as an anode at a current density increased from 50 to 400 mA g−1.
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Fig. 6. CV spectra of Li2MnSiO4: (a) as a cathode, and (b) as an anode at a scanning rate of 0.1 mV s−1.
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Fig. 7. Ex-situ XRD patterns of the cycled Li2MnSiO4: (a) as two cathodes on the aluminum current collectors (one is charged to 4.8 V, and the other is discharged to 1.5 V at 10 mA g−1), and (b) as three anode powder samples scraped from the copper current collectors (the first one is initially discharged to 0 V, and the second one is initially charged to 3 V at 10 mA g−1. The third one is discharged to 0 V during the 52th discharge at 100 mA g−1). The standard XRD patterns of Mn2O3, MnO, Mn, and Cu are shown for comparison.
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Fig. 8. (a) and (c) HRTEM images of the powder scraped from the anode after the 61th charge, (b) EDX spectrum of the selected area, and (d) a HRTEM image of the powder scraped from the anode after the 100th charge. The insert in (d) shows the connected spherical tiny particles. The current density of the final charge is 20 mA g−1.
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