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Carbon-coated rhombohedral Li2NaV2(PO4)3 nanoflake cathode for Li-ion battery with excellent cycleability and rate capability
Accepted Manuscript Research paper Carbon-Coated Rhombohedral Li2NaV2(PO4)3 Nanoflake Cathode for Li-Ion Battery with Excellent Cycleability and Rate Capability Muhammad Hilmy Alfaruqi, Saiful Islam, Jinju Song, Sungjin Kim, Duong Tung Pham, Jeonggeun Jo, Seokhun Kim, Joseph Paul Baboo, Dimas Yunianto Putro, Vinod Mathew, Jaekook Kim PII: DOI: Reference:
S0009-2614(17)30495-5 http://dx.doi.org/10.1016/j.cplett.2017.05.047 CPLETT 34836
To appear in:
Chemical Physics Letters
Received Date: Accepted Date:
5 April 2017 16 May 2017
Please cite this article as: M. Hilmy Alfaruqi, S. Islam, J. Song, S. Kim, D. Tung Pham, J. Jo, S. Kim, J. Paul Baboo, D. Yunianto Putro, V. Mathew, J. Kim, Carbon-Coated Rhombohedral Li2NaV2(PO4)3 Nanoflake Cathode for LiIon Battery with Excellent Cycleability and Rate Capability, Chemical Physics Letters (2017), doi: http://dx.doi.org/ 10.1016/j.cplett.2017.05.047
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.
Carbon-Coated Rhombohedral Li2NaV2(PO4)3 Nanoflake Cathode for LiIon Battery with Excellent Cycleability and Rate Capability Muhammad Hilmy Alfaruqi1, Saiful Islam1, Jinju Song, Sungjin Kim, Duong Tung Pham, Jeonggeun Jo, Seokhun Kim, Joseph Paul Baboo, Dimas Yunianto Putro, Vinod Mathew, Jaekook Kim* Department of Materials Science and Engineering, Chonnam National University, 300 Yongbong-dong, Bukgu, Gwangju 61186, Republic of Korea
----------------------------------------------------------------------------------------------------------*
Corresponding author. Tel: +82-62-530-1703; Fax: +82-62-530-1699.
E–mail address: [email protected] (Jaekook Kim) 1
These authors contributed equally to this work.
1
Abstract
Rhombohedral Li2NaV2(PO4)3 is very attractive cathode material for lithium-ion battery (LIB) application due to its single voltage plateau at 3.7 V that provides a constant output power. Here, for the first time, we report a direct and simple synthesis of high performance carbon-coated rhombohedral Li2NaV2(PO4)3 (LNVP/C) nanoflake cathode using a pyrosynthesis technique. The cathode demonstrates long cycle stability (100% capacity retention over 300 cycles) and high rate capabilities (77 and 55 mAh g-1 at 6.4 and 12C, respectively). The present study may facilitate a simple and low-cost preparation technique towards high performance cathode materials for advanced LIB applications.
Keywords: Rhombohedral Li2NaV2(PO4)3; Pyro-synthesis; Electrochemical properties; Liion batteries
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1. Introduction Lithium-ion batteries (LIBs) are widely used to power various electronic devices, such as laptop computers, smart phones, digital cameras, and electric vehicles, owing to their high voltage, high energy density, light weight, and excellent life cycle [1-3]. LIBs are mainly composed of an anode (negative electrode), a cathode (positive electrode), and an electrolyte. In the early 1990s, SONY successfully fabricated commercial LIBs for the first time using the metal oxide cathode LiCoO2 (LCO), which adopts a layered α-NaFeO2 structure [4]. Although the LCO cathode has been widely used for years, it contains cobalt, which is toxic and expensive [3,5,6]. Significant efforts have been focused on exploring alternatives for the LCO cathode, with particular emphasis on phosphate-based compounds, which exhibit excellent thermal and electrochemical stability [5-10]. Among the various phosphate-based compounds, monoclinic Li3V2(PO4)3 (LVP) with Na Super Ionic Conductor (NASICON) structure has been proposed as a cathode for LIBs owing to its high voltage, high lithium-ion mobility, considerable capacity, and also thermal and structural stability [10-15]. Monoclinic LVP possesses three potential plateaus at around 4.04, 3.64, and 3.55 V [16,17]. However, electronic devices require a stable output power in order to run well [11-13]. In other word, the cathode must exhibit a single working potential. Interestingly, LVP also exists in rhombohedral structure, which exhibits a single voltage plateau during electrochemical charge/discharge [11-14]. Nonetheless, rhombohedral LVP (r-LVP) cannot be synthesized using direct reaction due to its poor structural stability compared to monoclinic LVP [14]. Therefore, in order to obtain the rhombohedral structure, several methods have been proposed, for example, oxidation of rhombohedral Na3V2(PO4)3 (NVP) followed by re-insertion of Li and ion exchange from NVP [18]. Gaubicher et al. found almost two Li-ions could be 3
extracted upon electrochemical extraction of r-LVP under slow current rate [18]. In r-LVP, only two Li-ions can be extracted from the structure because they are located at the tetrahedral, while the remaining one Li-ion cannot be extracted [18]. Considering these circumstances, rhombohedral Li2NaV2(PO4)3 (LNVP) is very attractive due to its single voltage plateau at approximately 3.7 V (corresponding to the V4+/V3+ redox couple) and two Li-ions can also be extracted form the structure [11,12]. However, only a few groups have reported the preparation of rhombohedral LNVP for LIB applications. Cushing et al. reported the synthesis of rhombohedral LNVP cathodes by a chemical ion exchange reaction from Na3V2(PO4)3 [11]. The cathode exhibited an initial discharge capacity of 96 mAh g-1 at a current density of 0.5 mA cm-2. However, a 10% decrease in discharge capacity was observed for this rhombohedral LNVP cathode after 50 cycles. To improve the electrochemical performance of rhombohedral LNVP, a few studies have examined the preparation of nanosized particles [12,20]. The high surface area of nanosized particles offers a larger contact area between the active materials and electrolyte, ensuring an increased amount of guest-ion insertion/extraction. In addition, nanosized particles also enhance the diffusion kinetics by reducing electronic and ionic transport pathways. Zhang et al. prepared rhombohedral LNVP with a porous nanosheet structure using facile sol-gel synthesis [12]. When tested in lithium cells, this LNVP cathode showed no significant capacity fading under long-term cycling (~93% of the initial discharge capacity is maintained in the 500th cycle). Recently, Li et al. reported the synthesis of rhombohedral LNVP with particle sizes of approximately 300–800 nm by solid-state reaction method [20]. The LNVP cathode showed 1st and 45th discharge capacities of 108 and 88 mAh g-1, respectively, when cycled at 0.3C. 4
In addition to the nanosizing strategy, it is well known that carbon coating of nanosized materials is very effective for improving the performance of LIB electrode. Carbon coatings improve the electrical conductivity and facilitate volume changes of the electrode during cycling, resulting in LIB electrodes with improved performance [21-22]. Therefore, a combined strategy employing both nanosizing and carbon coating rhombohedral LNVP, as well as a simple synthetic technique, will be beneficial for the development of high performance electrode materials. Previously, we have successfully demonstrated the preparation of high performance monoclinic LVP for LIB application using a simple pyrosynthesis technique [17]. Bearing in mind that only limited studies on nanosized rhombohedral LNVP, which applied complex preparation routes, in this study, we report for the first time a direct and simple pyro-synthesis method to prepare carbon-coated rhombohedral LNVP (LNVP/C) nanoflake. The pyro-synthesis method is a simple strategy for preparing nanosized materials in short reaction times [23-27]. Polyol not only acts as a reducing agent in the present synthesis, but also a carbon source, allowing the formation of a carbon coating on the LNVP particles. The prepared rhombohedral LNVP/C material demonstrated impressive electrochemical properties with high reversible capacity, rate capability, and long cycle life when tested as a cathode for LIB applications.
2. Experimental A pyro-synthesis technique was employed to obtain rhombohedral LNVP/C. Initially, CH3COOLi.2H2O (99%, Junsei Chemical, Japan), Na2C2O4 (99.5%, Sigma Aldrich), VCl3 (97%, Sigma Aldrich), and H3PO4 (85%, Daejung Chemicals & Metal, Rep. of Korea) were used as the initial precursors in the molar ratio of 2:1:2:3 (Li:N:V:P) at room temperature. All the precursors were dissolved in 80 ml of tetraethylene glycol (TEG, 99.5%, Acros Organic, 5
USA) and stirred at room temperature for 24 h. Subsequently, a 40 ml of liquid thinner was added to the homogenous solution, which was then stirred for 30 minutes. The final solution was transferred onto a hot plate with a surface temperature of 450 °C. The combustion reaction was induced by ignition with an electric torch lighter. Within a short time after the ignition, a precipitate of nanosized particles was formed. The as-prepared powder was obtained directly without any filtering or washing steps. Subsequently, the as-prepared powder was heated at 800 °C for 8 h under an Ar atmosphere to obtain the final product. To evaluate the crystalline nature of the rhombohedral LNVP/C powder, X-ray powder diffraction (XRD) measurements were performed using a Shimadzu X-ray diffractometer (Japan) with Cu-Kα radiation (λ = 1.5406 Å). Field-emission scanning electron microscopy (FE-SEM, Hitachi S-4700) and field-emission transmission electron microscopy (FE-TEM, Philips Tecnai F20 at 200 keV, KBSI, Chonnam National University, South Korea) analyses were conducted to examine the surface morphology and particles sizes of the samples. To study the elemental distribution of the sample, energy dispersive X-ray (EDX) elemental mapping was performed using an EDX analyzer (EMAX Energy EX-200, Horiba) attached to the F20 microscope. In order to further confirm the presence of carbon on the particles, CHN analysis was also performed. The vanadium oxidation state in the sample was examined using BL8C synchrotron X-ray absorption near edge structure (XANES), measured at the Pohang Light Source (PLS), a third-generation synchrotron radiation source, in the 2.5 GeV storage ring with a ring current of 120–180 mA. The spectrum was collected at room temperature in transmission mode. ATHENA software was used to process the obtained XANES spectrum. To investigate the electrochemical properties of the prepared rhombohedral LNVP/C 6
sample, lithium metal was used as the reference electrode in a coin-type (CR-2032) half-cell. The electrodes were prepared by pressing a mixture of 70 wt.% active materials, 20 wt.% Ketjen black, and 10 wt.% teflonated acetylene black (TAB) onto a stainless-steel mesh. A loading of 4.5 mg cm−2 as the active material was used. LiPF6 (1 M) in a mixture of ethylene carbonate and dimethyl carbonate (molar ratio 1:1) was used as the electrolyte. Before the electrochemical tests, the prepared cells were stored overnight at room temperature. The electrochemical charge/discharge measurements were performed using a BTS 2004H battery tester (Nagano Keiki Co. Ltd., Tokyo, Japan) at room temperature. The cells were cycled in the voltage range of 3.0–4.3 V vs. Li/Li+. Cyclic voltammetry (CV) measurements were carried using a VSP 1075 model (Bio Logic Science Instrument, Seyssinet-Pariset, France) at a scan rate of 0.5 mV s-1.
3. Results and discussion
CH3 COOLi.2H2 O
Na2 C2 O4
H3 PO4
VCl3
Precursors in TEG
800 oC Ar
Combustion reaction
Final product
Scheme 1. Schematic diagram of the pyro-synthesis of rhombohedral LNVP/C.
Rhombohedral LNVP is typically prepared by a complex process, whereas the present LNVP/C sample was prepared by a simple pyro-synthesis technique (Scheme 1). This pyro-synthesis technique involves the reaction of CH3COOLi, Na2C2O4, VCl3, and H3PO4 precursors in a polyol medium (TEG) to produce the carbon-containing LNVP/C sample. 7
This energetic process took place in a short reaction time. To obtain a highly crystalline sample, the as-prepared powder was heated at 800 °C for 8 h in an Ar atmosphere. Recently, Zhang et al. reported the preparation of rhombohedral LNVP via sol-gel synthesis [12]. However, compared with the present pyro-synthesis method, the sol-gel procedure requires several more steps, including drying and a two-step heating process. The XRD pattern of the prepared rhombohedral LNVP/C sample is shown in Fig. 1(a). All the diffraction peaks were indexed to pure rhombohedral LNVP (JCPDS card No. 54-0763) and were consistent with previous reports [11,12]. The XRD study also confirmed the high crystallinity of the present sample. The lattice constants of the LNVP/C sample were calculated to be a = 8.721 Å and c = 21.800 Å, which are in good agreement with the reported values. Rhombohedral LNVP is constructed from alkali ions, which occupy A(1) and A(2) sites, and [V2(PO4)3]3- units, which are formed by corner-shared VO6 octahedral and PO4 tetrahedral, as illustrated in Fig.1(b) [11,12].
8
Fig. 1. (a) XRD pattern of rhombohedral LNVP/C. (b) The crystal structure of rhombohedral LNVP. To identify the oxidation state of vanadium in the rhombohedral LNVP/C sample, XANES analysis was performed. The V K-edge spectra of the present sample and standard vanadium oxide are shown in the inset of Fig. 2. The V K-edge spectrum of the rhombohedral LNVP/C sample is closely situated to that of standard V 2O3, indicating that vanadium exists in the V3+ state in the present sample. The XANES analysis also confirmed that Li and Na are present in a 2:1 molar ratio in the present rhombohedral LNVP/C sample.
Fig. 2. Comparison of the normalized V K-edge XANES spectrum of rhombohedral LNVP/C with that of standard vanadium oxide (inset).
FE-SEM and FE-TEM were used to study the morphology of the rhombohedral LNVP/C powder, and the recorded images are shown in Fig. 3. Figs. 3(a) and (b) show the FE-SEM images of rhombohedral LNVP/C at different magnifications. The rhombohedral LNVP/C sample showed an agglomerated flake-like morphology with particle sizes of 9
approximately 250–400 nm, as also observed in the FE-TEM image (Fig. 3(c)). The TEM image in the inset of Fig. 3(c) suggests the formation of a carbon coating on the LNVP particles. The HR-TEM image in Fig. 3(d) shows a lattice spacing of 0.60 nm, which corresponds well with that of the (012) crystal plane of rhombohedral LNVP/C. The TEM images also suggested that the LNVP/C sample possessed highly crystalline structures.
Fig. 3. (a, b) FE-SEM, (c) FE-TEM, and (d) HR-TEM images of the rhombohedral LNVP/C sample.
To further investigate the distribution elements in the rhombohedral LNVP/C sample, EDX elemental mapping analysis was conducted, and the results are depicted in Fig. 4. The EDX elemental mapping analysis clearly suggested the presence of carbon along with Na, V, 10
P, and O. Moreover, carbon was uniformly spread over the selected area, and the carbon content measured by the EDX analysis was 5.8%. CHN analysis was also used to further confirm the presence of carbon on the particles. The carbon content was 5.6%, which is in good agreement with the EDX analysis.
Fig. 4. EDS elemental mapping distribution of the LNVP/C powder prepared by pyrosynthesis method.
CV and galvanostatic measurements were employed to evaluate the electrochemical properties of the rhombohedral LNVP/C sample as a cathode for LIB. Fig. 5(a) shows the 1st, 2nd, 5th, and 10th cycles of the rhombohedral LNVP/C cathode vs. Li/Li+ at a sweep rate of 0.5 mV s-1 in the voltage range of 3.0–4.3 V. During the first charge, one oxidation peak was clearly observed at approximately 3.9 V, which corresponds to the extraction of lithium from the rhombohedral LNVP/C host structure, accompanied by oxidation of vanadium from V3+ to V4+ [11-14]. In the subsequent cathodic scan, one reduction peak was seen at around 3.6 V, 11
corresponding to the insertion of lithium into the rhombohedral LNVP/C host structure. During the discharge cycle, vanadium was reduced from V4+ to V3+. The presence of only a single peak upon charging or discharging indicates the purity of the rhombohedral LNVP phase in the present sample. The CV curve of each cycle overlapped, indicating the consistent and stable electrochemical behavior of the rhombohedral LNVP/C sample [11,12].
Fig. 5. Electrochemical charge/discharge performance of LNVP/C electrode in lithium cell. (a) CV profiles at a scan rate of 0.5 mV s -1. (b) First charge/discharge profiles at 0.4C. (c) Cyclability between 3.0 and 4.3 V vs. Li+/Li at 0.4C. (d) C-rate capability at different current densities between 0.4 and 12C.
The rhombohedral LNVP/C cathode was subjected to galvanostatic tests in a lithium cell within the voltage range of 3.0–4.3 V at 0.4C. Fig. 5(b) presents the 1st, 2nd, and 10th 12
charge/discharge profiles of the rhombohedral LNVP/C cathode. During charging, one distinct plateau at approximately 3.8 V was observed. A single plateau was also observed during discharging at 3.7 V. These behaviors are in good agreement with the CV studies and correspond to the typical electrochemical characteristic of rhombohedral LNVP reported in the literature [11,12]. The first charge and discharge capacities of the present cathode at a current rate of 0.4C were 103 and 92 mAh g-1, respectively. As suggested in a previous study, during electrochemical charge/discharge, only lithium ions are extracted/inserted from/into the rhombohedral LNVP host structure [11]. The cycle performance of rhombohedral LNVP/C is shown in Fig. 5(c). The cathode was tested over 300 cycles at a current rate of 0.4C. It can be clearly seen that the cathode exhibited decent cycle life. During the 300th cycle, the cathode showed a discharge capacity of 92 mAh g-1, equals to 100% of the initial discharge capacity. The Coulombic efficiency at the first cycle was 89%, while under long-term cycling, the present cathode maintained approximately 99% of Coulombic efficiency. The stable electrochemical performance of the present cathode can be attributed to the combined effects of nanosized and carbon-coated particles in the present cathode. Nanosized particles with high surface areas facilitate the insertion/extraction of larger amounts of guest ions, whereas carbon-coated particles enhance the electric conductivity of the cathode. To date, studies on nanosized rhombohedral LNVP have been limited, and the present study offers a simple preparation method for nanosized carbon-coated rhombohedral LNVP with considerable electrochemical performance for LIB applications. The rate performance of the present rhombohedral LNVP/C cathode was also examined to assess the cathode performance at different current rates between 0.4 and 12C, 13
and the results are shown in Fig. 5(d). It can be clearly seen that the capacity of the present sample decreased with increasing current density. At current rates of 0.4, 0.8, 1.6, 3.2, 6.4, and 12C, the rhombohedral LNVP/C sample showed average discharge capacities of 97, 93, 90, 85, 77, and 55 mAh g -1. It is also interesting to note that after cycling at a high current rate of 12C, the discharge capacity of the cathode recovered to 95 mAh g -1 (nearly 99% of the initial discharge value) when the current rate was decreased to 0.4C. The electrochemical performance of the present cathode was comparable to that of previously reported rhombohedral LNVP synthesized by ion exchange or sol-gel synthesis methods, however, the synthesis technique proposed in the present study is simple and cost-effective [11,12].
4. Conclusions In summary, rhombohedral LNVP/C with nanosized particles was successfully prepared by a simple pyro-synthesis technique. This sample had a flake-like morphology with particles sizes of 250–400 nm, as observed from electron microscopy studies. After 300 cycles at a current rate of 0.4C, rhombohedral LNVP/C exhibited no capacity loss. The remarkable cycleability and rate capability of the present cathode can be ascribed to the combined effects of nanosized and carbon-coated particles. Considering that there are only a few reports on nanosized rhombohedral LNVP with complex synthesis strategies, the present study offers a simple and cost-effective preparation technique to obtain nanosized carboncoated rhombohedral LNVP with good electrochemical performance for LIB application.
Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) grant 14
funded by the Korea government (MSIP) (2014R1A2A1A10050821).
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Graphical Abstract
19
Research Highlights:
A simple pyro-synthesis is used to prepare a rhombohedral LNVP/C for LIB cathode.
The cathode shows cycling stability over 300 cycles with 100% capacity retention. The cathode exhibits high rate capability, delivering 55 mAh g-1 at 12C. The study offers opportunities to produce high performance LIB cathodes.
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S0009-2614(17)30495-5 http://dx.doi.org/10.1016/j.cplett.2017.05.047 CPLETT 34836
To appear in:
Chemical Physics Letters
Received Date: Accepted Date:
5 April 2017 16 May 2017
Please cite this article as: M. Hilmy Alfaruqi, S. Islam, J. Song, S. Kim, D. Tung Pham, J. Jo, S. Kim, J. Paul Baboo, D. Yunianto Putro, V. Mathew, J. Kim, Carbon-Coated Rhombohedral Li2NaV2(PO4)3 Nanoflake Cathode for LiIon Battery with Excellent Cycleability and Rate Capability, Chemical Physics Letters (2017), doi: http://dx.doi.org/ 10.1016/j.cplett.2017.05.047
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.
Carbon-Coated Rhombohedral Li2NaV2(PO4)3 Nanoflake Cathode for LiIon Battery with Excellent Cycleability and Rate Capability Muhammad Hilmy Alfaruqi1, Saiful Islam1, Jinju Song, Sungjin Kim, Duong Tung Pham, Jeonggeun Jo, Seokhun Kim, Joseph Paul Baboo, Dimas Yunianto Putro, Vinod Mathew, Jaekook Kim* Department of Materials Science and Engineering, Chonnam National University, 300 Yongbong-dong, Bukgu, Gwangju 61186, Republic of Korea
----------------------------------------------------------------------------------------------------------*
Corresponding author. Tel: +82-62-530-1703; Fax: +82-62-530-1699.
E–mail address: [email protected] (Jaekook Kim) 1
These authors contributed equally to this work.
1
Abstract
Rhombohedral Li2NaV2(PO4)3 is very attractive cathode material for lithium-ion battery (LIB) application due to its single voltage plateau at 3.7 V that provides a constant output power. Here, for the first time, we report a direct and simple synthesis of high performance carbon-coated rhombohedral Li2NaV2(PO4)3 (LNVP/C) nanoflake cathode using a pyrosynthesis technique. The cathode demonstrates long cycle stability (100% capacity retention over 300 cycles) and high rate capabilities (77 and 55 mAh g-1 at 6.4 and 12C, respectively). The present study may facilitate a simple and low-cost preparation technique towards high performance cathode materials for advanced LIB applications.
Keywords: Rhombohedral Li2NaV2(PO4)3; Pyro-synthesis; Electrochemical properties; Liion batteries
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1. Introduction Lithium-ion batteries (LIBs) are widely used to power various electronic devices, such as laptop computers, smart phones, digital cameras, and electric vehicles, owing to their high voltage, high energy density, light weight, and excellent life cycle [1-3]. LIBs are mainly composed of an anode (negative electrode), a cathode (positive electrode), and an electrolyte. In the early 1990s, SONY successfully fabricated commercial LIBs for the first time using the metal oxide cathode LiCoO2 (LCO), which adopts a layered α-NaFeO2 structure [4]. Although the LCO cathode has been widely used for years, it contains cobalt, which is toxic and expensive [3,5,6]. Significant efforts have been focused on exploring alternatives for the LCO cathode, with particular emphasis on phosphate-based compounds, which exhibit excellent thermal and electrochemical stability [5-10]. Among the various phosphate-based compounds, monoclinic Li3V2(PO4)3 (LVP) with Na Super Ionic Conductor (NASICON) structure has been proposed as a cathode for LIBs owing to its high voltage, high lithium-ion mobility, considerable capacity, and also thermal and structural stability [10-15]. Monoclinic LVP possesses three potential plateaus at around 4.04, 3.64, and 3.55 V [16,17]. However, electronic devices require a stable output power in order to run well [11-13]. In other word, the cathode must exhibit a single working potential. Interestingly, LVP also exists in rhombohedral structure, which exhibits a single voltage plateau during electrochemical charge/discharge [11-14]. Nonetheless, rhombohedral LVP (r-LVP) cannot be synthesized using direct reaction due to its poor structural stability compared to monoclinic LVP [14]. Therefore, in order to obtain the rhombohedral structure, several methods have been proposed, for example, oxidation of rhombohedral Na3V2(PO4)3 (NVP) followed by re-insertion of Li and ion exchange from NVP [18]. Gaubicher et al. found almost two Li-ions could be 3
extracted upon electrochemical extraction of r-LVP under slow current rate [18]. In r-LVP, only two Li-ions can be extracted from the structure because they are located at the tetrahedral, while the remaining one Li-ion cannot be extracted [18]. Considering these circumstances, rhombohedral Li2NaV2(PO4)3 (LNVP) is very attractive due to its single voltage plateau at approximately 3.7 V (corresponding to the V4+/V3+ redox couple) and two Li-ions can also be extracted form the structure [11,12]. However, only a few groups have reported the preparation of rhombohedral LNVP for LIB applications. Cushing et al. reported the synthesis of rhombohedral LNVP cathodes by a chemical ion exchange reaction from Na3V2(PO4)3 [11]. The cathode exhibited an initial discharge capacity of 96 mAh g-1 at a current density of 0.5 mA cm-2. However, a 10% decrease in discharge capacity was observed for this rhombohedral LNVP cathode after 50 cycles. To improve the electrochemical performance of rhombohedral LNVP, a few studies have examined the preparation of nanosized particles [12,20]. The high surface area of nanosized particles offers a larger contact area between the active materials and electrolyte, ensuring an increased amount of guest-ion insertion/extraction. In addition, nanosized particles also enhance the diffusion kinetics by reducing electronic and ionic transport pathways. Zhang et al. prepared rhombohedral LNVP with a porous nanosheet structure using facile sol-gel synthesis [12]. When tested in lithium cells, this LNVP cathode showed no significant capacity fading under long-term cycling (~93% of the initial discharge capacity is maintained in the 500th cycle). Recently, Li et al. reported the synthesis of rhombohedral LNVP with particle sizes of approximately 300–800 nm by solid-state reaction method [20]. The LNVP cathode showed 1st and 45th discharge capacities of 108 and 88 mAh g-1, respectively, when cycled at 0.3C. 4
In addition to the nanosizing strategy, it is well known that carbon coating of nanosized materials is very effective for improving the performance of LIB electrode. Carbon coatings improve the electrical conductivity and facilitate volume changes of the electrode during cycling, resulting in LIB electrodes with improved performance [21-22]. Therefore, a combined strategy employing both nanosizing and carbon coating rhombohedral LNVP, as well as a simple synthetic technique, will be beneficial for the development of high performance electrode materials. Previously, we have successfully demonstrated the preparation of high performance monoclinic LVP for LIB application using a simple pyrosynthesis technique [17]. Bearing in mind that only limited studies on nanosized rhombohedral LNVP, which applied complex preparation routes, in this study, we report for the first time a direct and simple pyro-synthesis method to prepare carbon-coated rhombohedral LNVP (LNVP/C) nanoflake. The pyro-synthesis method is a simple strategy for preparing nanosized materials in short reaction times [23-27]. Polyol not only acts as a reducing agent in the present synthesis, but also a carbon source, allowing the formation of a carbon coating on the LNVP particles. The prepared rhombohedral LNVP/C material demonstrated impressive electrochemical properties with high reversible capacity, rate capability, and long cycle life when tested as a cathode for LIB applications.
2. Experimental A pyro-synthesis technique was employed to obtain rhombohedral LNVP/C. Initially, CH3COOLi.2H2O (99%, Junsei Chemical, Japan), Na2C2O4 (99.5%, Sigma Aldrich), VCl3 (97%, Sigma Aldrich), and H3PO4 (85%, Daejung Chemicals & Metal, Rep. of Korea) were used as the initial precursors in the molar ratio of 2:1:2:3 (Li:N:V:P) at room temperature. All the precursors were dissolved in 80 ml of tetraethylene glycol (TEG, 99.5%, Acros Organic, 5
USA) and stirred at room temperature for 24 h. Subsequently, a 40 ml of liquid thinner was added to the homogenous solution, which was then stirred for 30 minutes. The final solution was transferred onto a hot plate with a surface temperature of 450 °C. The combustion reaction was induced by ignition with an electric torch lighter. Within a short time after the ignition, a precipitate of nanosized particles was formed. The as-prepared powder was obtained directly without any filtering or washing steps. Subsequently, the as-prepared powder was heated at 800 °C for 8 h under an Ar atmosphere to obtain the final product. To evaluate the crystalline nature of the rhombohedral LNVP/C powder, X-ray powder diffraction (XRD) measurements were performed using a Shimadzu X-ray diffractometer (Japan) with Cu-Kα radiation (λ = 1.5406 Å). Field-emission scanning electron microscopy (FE-SEM, Hitachi S-4700) and field-emission transmission electron microscopy (FE-TEM, Philips Tecnai F20 at 200 keV, KBSI, Chonnam National University, South Korea) analyses were conducted to examine the surface morphology and particles sizes of the samples. To study the elemental distribution of the sample, energy dispersive X-ray (EDX) elemental mapping was performed using an EDX analyzer (EMAX Energy EX-200, Horiba) attached to the F20 microscope. In order to further confirm the presence of carbon on the particles, CHN analysis was also performed. The vanadium oxidation state in the sample was examined using BL8C synchrotron X-ray absorption near edge structure (XANES), measured at the Pohang Light Source (PLS), a third-generation synchrotron radiation source, in the 2.5 GeV storage ring with a ring current of 120–180 mA. The spectrum was collected at room temperature in transmission mode. ATHENA software was used to process the obtained XANES spectrum. To investigate the electrochemical properties of the prepared rhombohedral LNVP/C 6
sample, lithium metal was used as the reference electrode in a coin-type (CR-2032) half-cell. The electrodes were prepared by pressing a mixture of 70 wt.% active materials, 20 wt.% Ketjen black, and 10 wt.% teflonated acetylene black (TAB) onto a stainless-steel mesh. A loading of 4.5 mg cm−2 as the active material was used. LiPF6 (1 M) in a mixture of ethylene carbonate and dimethyl carbonate (molar ratio 1:1) was used as the electrolyte. Before the electrochemical tests, the prepared cells were stored overnight at room temperature. The electrochemical charge/discharge measurements were performed using a BTS 2004H battery tester (Nagano Keiki Co. Ltd., Tokyo, Japan) at room temperature. The cells were cycled in the voltage range of 3.0–4.3 V vs. Li/Li+. Cyclic voltammetry (CV) measurements were carried using a VSP 1075 model (Bio Logic Science Instrument, Seyssinet-Pariset, France) at a scan rate of 0.5 mV s-1.
3. Results and discussion
CH3 COOLi.2H2 O
Na2 C2 O4
H3 PO4
VCl3
Precursors in TEG
800 oC Ar
Combustion reaction
Final product
Scheme 1. Schematic diagram of the pyro-synthesis of rhombohedral LNVP/C.
Rhombohedral LNVP is typically prepared by a complex process, whereas the present LNVP/C sample was prepared by a simple pyro-synthesis technique (Scheme 1). This pyro-synthesis technique involves the reaction of CH3COOLi, Na2C2O4, VCl3, and H3PO4 precursors in a polyol medium (TEG) to produce the carbon-containing LNVP/C sample. 7
This energetic process took place in a short reaction time. To obtain a highly crystalline sample, the as-prepared powder was heated at 800 °C for 8 h in an Ar atmosphere. Recently, Zhang et al. reported the preparation of rhombohedral LNVP via sol-gel synthesis [12]. However, compared with the present pyro-synthesis method, the sol-gel procedure requires several more steps, including drying and a two-step heating process. The XRD pattern of the prepared rhombohedral LNVP/C sample is shown in Fig. 1(a). All the diffraction peaks were indexed to pure rhombohedral LNVP (JCPDS card No. 54-0763) and were consistent with previous reports [11,12]. The XRD study also confirmed the high crystallinity of the present sample. The lattice constants of the LNVP/C sample were calculated to be a = 8.721 Å and c = 21.800 Å, which are in good agreement with the reported values. Rhombohedral LNVP is constructed from alkali ions, which occupy A(1) and A(2) sites, and [V2(PO4)3]3- units, which are formed by corner-shared VO6 octahedral and PO4 tetrahedral, as illustrated in Fig.1(b) [11,12].
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Fig. 1. (a) XRD pattern of rhombohedral LNVP/C. (b) The crystal structure of rhombohedral LNVP. To identify the oxidation state of vanadium in the rhombohedral LNVP/C sample, XANES analysis was performed. The V K-edge spectra of the present sample and standard vanadium oxide are shown in the inset of Fig. 2. The V K-edge spectrum of the rhombohedral LNVP/C sample is closely situated to that of standard V 2O3, indicating that vanadium exists in the V3+ state in the present sample. The XANES analysis also confirmed that Li and Na are present in a 2:1 molar ratio in the present rhombohedral LNVP/C sample.
Fig. 2. Comparison of the normalized V K-edge XANES spectrum of rhombohedral LNVP/C with that of standard vanadium oxide (inset).
FE-SEM and FE-TEM were used to study the morphology of the rhombohedral LNVP/C powder, and the recorded images are shown in Fig. 3. Figs. 3(a) and (b) show the FE-SEM images of rhombohedral LNVP/C at different magnifications. The rhombohedral LNVP/C sample showed an agglomerated flake-like morphology with particle sizes of 9
approximately 250–400 nm, as also observed in the FE-TEM image (Fig. 3(c)). The TEM image in the inset of Fig. 3(c) suggests the formation of a carbon coating on the LNVP particles. The HR-TEM image in Fig. 3(d) shows a lattice spacing of 0.60 nm, which corresponds well with that of the (012) crystal plane of rhombohedral LNVP/C. The TEM images also suggested that the LNVP/C sample possessed highly crystalline structures.
Fig. 3. (a, b) FE-SEM, (c) FE-TEM, and (d) HR-TEM images of the rhombohedral LNVP/C sample.
To further investigate the distribution elements in the rhombohedral LNVP/C sample, EDX elemental mapping analysis was conducted, and the results are depicted in Fig. 4. The EDX elemental mapping analysis clearly suggested the presence of carbon along with Na, V, 10
P, and O. Moreover, carbon was uniformly spread over the selected area, and the carbon content measured by the EDX analysis was 5.8%. CHN analysis was also used to further confirm the presence of carbon on the particles. The carbon content was 5.6%, which is in good agreement with the EDX analysis.
Fig. 4. EDS elemental mapping distribution of the LNVP/C powder prepared by pyrosynthesis method.
CV and galvanostatic measurements were employed to evaluate the electrochemical properties of the rhombohedral LNVP/C sample as a cathode for LIB. Fig. 5(a) shows the 1st, 2nd, 5th, and 10th cycles of the rhombohedral LNVP/C cathode vs. Li/Li+ at a sweep rate of 0.5 mV s-1 in the voltage range of 3.0–4.3 V. During the first charge, one oxidation peak was clearly observed at approximately 3.9 V, which corresponds to the extraction of lithium from the rhombohedral LNVP/C host structure, accompanied by oxidation of vanadium from V3+ to V4+ [11-14]. In the subsequent cathodic scan, one reduction peak was seen at around 3.6 V, 11
corresponding to the insertion of lithium into the rhombohedral LNVP/C host structure. During the discharge cycle, vanadium was reduced from V4+ to V3+. The presence of only a single peak upon charging or discharging indicates the purity of the rhombohedral LNVP phase in the present sample. The CV curve of each cycle overlapped, indicating the consistent and stable electrochemical behavior of the rhombohedral LNVP/C sample [11,12].
Fig. 5. Electrochemical charge/discharge performance of LNVP/C electrode in lithium cell. (a) CV profiles at a scan rate of 0.5 mV s -1. (b) First charge/discharge profiles at 0.4C. (c) Cyclability between 3.0 and 4.3 V vs. Li+/Li at 0.4C. (d) C-rate capability at different current densities between 0.4 and 12C.
The rhombohedral LNVP/C cathode was subjected to galvanostatic tests in a lithium cell within the voltage range of 3.0–4.3 V at 0.4C. Fig. 5(b) presents the 1st, 2nd, and 10th 12
charge/discharge profiles of the rhombohedral LNVP/C cathode. During charging, one distinct plateau at approximately 3.8 V was observed. A single plateau was also observed during discharging at 3.7 V. These behaviors are in good agreement with the CV studies and correspond to the typical electrochemical characteristic of rhombohedral LNVP reported in the literature [11,12]. The first charge and discharge capacities of the present cathode at a current rate of 0.4C were 103 and 92 mAh g-1, respectively. As suggested in a previous study, during electrochemical charge/discharge, only lithium ions are extracted/inserted from/into the rhombohedral LNVP host structure [11]. The cycle performance of rhombohedral LNVP/C is shown in Fig. 5(c). The cathode was tested over 300 cycles at a current rate of 0.4C. It can be clearly seen that the cathode exhibited decent cycle life. During the 300th cycle, the cathode showed a discharge capacity of 92 mAh g-1, equals to 100% of the initial discharge capacity. The Coulombic efficiency at the first cycle was 89%, while under long-term cycling, the present cathode maintained approximately 99% of Coulombic efficiency. The stable electrochemical performance of the present cathode can be attributed to the combined effects of nanosized and carbon-coated particles in the present cathode. Nanosized particles with high surface areas facilitate the insertion/extraction of larger amounts of guest ions, whereas carbon-coated particles enhance the electric conductivity of the cathode. To date, studies on nanosized rhombohedral LNVP have been limited, and the present study offers a simple preparation method for nanosized carbon-coated rhombohedral LNVP with considerable electrochemical performance for LIB applications. The rate performance of the present rhombohedral LNVP/C cathode was also examined to assess the cathode performance at different current rates between 0.4 and 12C, 13
and the results are shown in Fig. 5(d). It can be clearly seen that the capacity of the present sample decreased with increasing current density. At current rates of 0.4, 0.8, 1.6, 3.2, 6.4, and 12C, the rhombohedral LNVP/C sample showed average discharge capacities of 97, 93, 90, 85, 77, and 55 mAh g -1. It is also interesting to note that after cycling at a high current rate of 12C, the discharge capacity of the cathode recovered to 95 mAh g -1 (nearly 99% of the initial discharge value) when the current rate was decreased to 0.4C. The electrochemical performance of the present cathode was comparable to that of previously reported rhombohedral LNVP synthesized by ion exchange or sol-gel synthesis methods, however, the synthesis technique proposed in the present study is simple and cost-effective [11,12].
4. Conclusions In summary, rhombohedral LNVP/C with nanosized particles was successfully prepared by a simple pyro-synthesis technique. This sample had a flake-like morphology with particles sizes of 250–400 nm, as observed from electron microscopy studies. After 300 cycles at a current rate of 0.4C, rhombohedral LNVP/C exhibited no capacity loss. The remarkable cycleability and rate capability of the present cathode can be ascribed to the combined effects of nanosized and carbon-coated particles. Considering that there are only a few reports on nanosized rhombohedral LNVP with complex synthesis strategies, the present study offers a simple and cost-effective preparation technique to obtain nanosized carboncoated rhombohedral LNVP with good electrochemical performance for LIB application.
Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) grant 14
funded by the Korea government (MSIP) (2014R1A2A1A10050821).
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Graphical Abstract
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Research Highlights:
A simple pyro-synthesis is used to prepare a rhombohedral LNVP/C for LIB cathode.
The cathode shows cycling stability over 300 cycles with 100% capacity retention. The cathode exhibits high rate capability, delivering 55 mAh g-1 at 12C. The study offers opportunities to produce high performance LIB cathodes.
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