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3O2 cathode material
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CCLET 3364 1–6 Chinese Chemical Letters xxx (2015) xxx–xxx
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Original article
Electrochemical performance of a nano SnO2-modified LiNi1/3Co1/3Mn1/3O2 cathode material Q1 Zhi-Mei
Luo, Yan-Guang Sun, Hui-Yong Liu *
School of Chemical Engineering, Fuzhou University, Fuzhou 350116, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 29 April 2015 Received in revised form 1 June 2015 Accepted 8 June 2015 Available online xxx
The nano SnO2-modified LiNi1/3Co1/3Mn1/3O2 was successfully prepared by a carrier transfer method. The pristine and modified samples were characterized with various techniques such as XRD, SEM, XPS and EDS. The results showed that the SnO2 particles did not enter the crystal structure of LiNi1/3Co1/ 3Mn1/3O2, many nano SnO2 particles were uniformly covered on the surface of LiNi1/3Co1/3Mn1/3O2 and the modified thin layer could inhibit the dissolution of transition metal oxides. The electrochemical tests indicated that the existence of nano SnO2 could improve the discharge capacity and rate capability owing to the decreased interfacial polarization. The cycling stability was remarkably improved at room temperature and 55 8C. The XRD patterns of the fresh NCM electrode and after 50 cycles proved that the structural change of NCM was not so effective on the capacity fade. ß 2015 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Keywords: Carrier transfer method Nano SnO2 LiNi1/3Co1/3Mn1/3O2 Surface modification
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1. Introduction
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Lithium ion batteries have become the state-of-the-art power sources for portable appliances such as notebook computers, cellular phones, and digital cameras. In addition, the batteries are expected to be applied in hybrid electric vehicles (HEVs) and electric vehicles (EVs) [1,2]. Layered LiNi1/3Co1/3Mn1/3O2 as the cathode material of lithium ion batteries is now widely investigated for its low cost, high capability and safety, which is acknowledged as a candidate to replace LiCoO2 [3]. However, due to its poor cycling performance at high working voltage and high charge/discharge rate, this compound cannot completely substitute LiCoO2 as a cathode material [4]. In addition, more side reactions will take place between the active electrode and the electrolyte for cell and lead to the increase of the interfacial resistance and thus negatively influence the battery performance [5]. To overcome the deficiencies mentioned above, an effective approach to modify the surface of LiNi1/3Co1/3Mn1/3O2 by metal oxides is proposed. Some of the metal oxides reported as surface modifying candidates are: Al2O3, V2O5, ZrO2, TiO2, CeO2, etc. [6–12], which have recently been shown to improve both the thermal stability and the rate capability of LiNi1/3Mn1/3Co1/3O2. Surface modifying has been proved to be an
* Corresponding author. E-mail address: [email protected] (H.-Y. Liu).
effective method to scavenge HF species from the electrolyte and form stable metal fluoride layer, which can result in less decomposition of the cathode particles [13]. SnO2 can react with Li reversibly through a conversion reaction and has been successfully used to modify cathode materials such as LiCoO2 [14], LiFePO4 and LiMn2O4 [15,16]. The electrochemical properties of the modified cathode materials have been significantly improved without obvious capacity loss. The metal oxide coating layer acted as a solid electrolyte with a reasonable high Li ion conductivity. In this work, a nano SnO2-modified LiNi1/3Co1/ 3Mn1/3O2 sample was synthesized successfully by the carrier transfer method [17]. The effects of SnO2 modification on the structural and electrochemical properties of LiNi1/3Co1/3Mn1/3O2 were also investigated.
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2. Experimental
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Commercially available spherical LiNi1/3Co1/3Mn1/3O2 (NCM) powders (purchased from 3 M, American) were utilized as pristine sample. A certain amount of SnCl45H2O (AR, 99.0%) was dissolved in a hydrochloric acid solution and a solution of ammonia with proper concentration was added to promote the hydrolysis of Sn4+ under vigorous stirring conditions. The Sn(OH)4 collosol was formed after agitating for 12 h, then active carbon (AR, 99.0%) was added (in a weight ratio of SnO2:C = 3:7) with constant stirring for 5 h. After that, the resulting precipitate was washed several times with distilled water and dried at 100 8C overnight. Subsequently,
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http://dx.doi.org/10.1016/j.cclet.2015.06.007 1001-8417/ß 2015 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: Z.-M. Luo, et al., Electrochemical performance of a nano SnO2-modified LiNi1/3Co1/3Mn1/3O2 cathode material, Chin. Chem. Lett. (2015), http://dx.doi.org/10.1016/j.cclet.2015.06.007
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the precipitate was sintered in a tube furnace at 500 8C in argon atmosphere to obtain the SnO2/C powders. The Gravimetric method was used to determine the percentage of SnO2 in the SnO2/C, which was calcined in a furnace at 500 8C for 6 h under airflow. The active carbon generated CO2 and pure SnO2 was obtained. Based on the weight loss of the sample we could confirm a weight ratio of 3:7 between SnO2 and C. The obtained SnO2/C powders were calcined in a muffle furnace at 500 8C for 6 h in air and a stoichiometric amount of NCM was mixed thoroughly to obtain 2 wt% nano SnO2-modified NCM samples. The pristine and SnO2 modified samples were characterized by powder X-ray diffraction (D/MAX-2500, Rigaku, Japan) to identify the structure from 108 to 808 with a step size of 0.028. The surface morphologies of the samples were analyzed using a field emission scanning electron microscope (MIRA3XMH, TESCAN, Czech). Energy dispersive X-ray spectroscopy (EDS) was employed to analyze the surface composition and element distribution of the cathode materials. The surface chemical states of Ni, Co, Mn and Sn were investigated by X-ray photoelectron spectroscopy (XPS, VG Multilab 2000). The electrochemical properties of pristine and nano SnO2modified NCM were measured using a CR2025 coin cell. The working cathode films were consisted of 85 wt% cathode material, 5 wt% supper P, 5 wt% KS-6 and 5 wt% PVDF. Lithium foil was used as the counter electrode, 1 mol L1 LiPF6 in a EC:DMC = 1:1 (V/V) as the electrolyte, a polypropylene microporous film (Cellgard 2300) as the separator, and foam nickel as a filler material. The cells were assembled in an argon-filled glove box. The galvanostatic discharge–charge tests were performed on the LAND battery program-control test system (Wuhan, China) between 3.0 and 4.5 V (vs. Li/Li+) at a constant current density of 36 mA g1 (0.2 C) at room temperature and 55 8C, respectively. Cyclic voltammetry (CV) test was carried out on an electrochemical workstation (CHI660C, Shanghai, China) in the potential window of 2.7–4.5 V (vs. Li/Li+) at a scan rate of 0.1 mv s1.
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3. Results and discussion
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The XRD patterns of pristine and nano SnO2-modified NCM are shown in Fig. 1(a). The main phases belong to an a-NaFeO2 type layered hexagonal structure with a space group of R3m. No evident differences appear between the pristine and nano SnO2 modified samples, the lattice volume of nano SnO2-modified is very close to the pristine, which indicates that the layered structure of NCM is not affected. In order to confirm the phase of nano SnO2, which has
been obtained using the same procedure without adding layered NCM. The XRD patterns in Fig. 1(b) show that the main phase is a tetragonal structure with a space group of p42/mnm. However, the phases of nano SnO2 are not present in the patterns of modified NCM. It is believed that such a low mass (2 wt%) in nano SnO2 modified samples cannot be detected due to the detection limit of X-ray diffraction. Fig. 2 presents the SEM images of pristine and nano SnO2modified NCM. It can be seen that all samples are composed of many primary nanoparticles, these smaller primary particles were generated to form spherical second particles of 10 mm in diameter. The pristine samples show a very smooth and clean surface in Fig. 2(a) and (b). However, as shown in Fig. 2(c) and (d), the surface of nano SnO2-modified NCM are rough and some bright white spots are seen, owing to the partial coverage of its surface with nano-sized SnO2 particles. The TEM image of SnO2/C are given in Fig. 2(e), the darker area embedded within active carbon is called the nano SnO2, whose particle size distribution is uniform. EDS is also performed to confirm the distribution of SnO2 modified layer on the surface of modified NCM (Fig. 3). According to the mapping, Sn element in the sample is uniformly distributed on the NCM surface. The surface film composition and the oxidation state of the elements are analyzed by XPS. The XPS spectra of pristine NCM and nano SnO2-modified NCM are shown in Fig. 4. The binding energies obtained in the XPS analysis are corrected for specimen charging by referencing the C 1s line to 284.60 eV. It reveals that the modified sample has Mn 2p, Co 2p and Ni 2p peaks without remarkable chemical shift of binding energy compared to the pristine samples, indicating that their ion environments are not changed in the surface structure. This means that the modifying layer is only covered on the surface of NCM instead of entering NCM lattice. The 3d5/2 and 3d3/2 Sn peaks are found to appear at 485.95 and 494.39 eV, respectively, which are in good agreement with literature values reported for SnO2 [18]. It indicates that the valence of Sn is +4 and the Sn element on the material surface is in the form of SnO2. In addition, it is clear that the intensity of the Mn 2p, Co 2p and Ni 2p peaks decreased after modification due to the formation of the nano SnO2 layer on the particle surface. Fig. 5 shows the initial discharge curves of the pristine and nano SnO2-modified NCM electrodes at different discharge rates. The discharge capacities of 0.5, 1, 2, 5 and 10 C at the first cycle are 159.9, 156.9, 145.0, 138.8 and 116.5 mAh g1, respectively, which are only 98.1%, 90.9%, 86.8% and 72.9% of the discharge capacity at 0.5 C. However, the rate capability is improved dramatically by nano SnO2 modification. The modified sample presents a discharge
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Fig. 1. XRD of (a) pristine NCM and SnO2-modified NCM, (b) SnO2.
Please cite this article in press as: Z.-M. Luo, et al., Electrochemical performance of a nano SnO2-modified LiNi1/3Co1/3Mn1/3O2 cathode material, Chin. Chem. Lett. (2015), http://dx.doi.org/10.1016/j.cclet.2015.06.007
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Fig. 2. SEM of pristine NCM (a), (b) and SnO2-modified NCM (c), (d); TEM of SnO2/C sample (e).
Fig. 3. EDS maps of the SnO2-modified NCM.
capacity of 157.9, 154.3, 150.9 and 133.5 mAh g1 at 1, 2, 5 and 10 C, corresponding to 95.2%, 93.0%, 90.9% and 80.5% of its capacity of 165.9 mAh g1 at 0.5 C. The result indicates that nano SnO2 can improve initial discharge capacity and rate capability of NCM. It is well known that nano SnO2 possesses large effective specific surface area, the trace amount of FH in the electrolyte reacts with SnO2 primarily, which prevents the cathode material dissolution
leading to an increase of interfacial impedance and electrode polarization. We can conclude that the nano SnO2 modified layer suppresses the increase of interfacial polarization and therefore decrease the capacity fading [19]. The cycling performance of the two materials at 1 C over 3.0– 4.5 V is investigated (shown in Fig. 6). The capacity retention of SnO2-modified NCM is remarkably enhanced showing only a 14.0%
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Fig. 4. XPS spectra of pristine NCM and SnO2-modified NCM.
Please cite this article in press as: Z.-M. Luo, et al., Electrochemical performance of a nano SnO2-modified LiNi1/3Co1/3Mn1/3O2 cathode material, Chin. Chem. Lett. (2015), http://dx.doi.org/10.1016/j.cclet.2015.06.007
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Voltage (V) capacity loss after 50 cycles at room temperature, while the pristine sample suffers from a 19.3% capacity loss in Fig. 6(a). As observed in Fig. 6(b), the cycling performance at 55 8C is improved more significantly and the capacity loss of modified samples is only 7.8%. However, the capacity loss of pristine samples is very obvious (33.6%). This result is attributed to the fact that the cathode material suffers from serious corrosion due to the existing FH in the electrolyte at higher temperature. The role of SnO2 modifying layer is to consume the excess of FH [20], thus prevents the sidereactions between the electrode and electrolyte. Therefore, the nano SnO2 on the particle surface improves the stability of electrode against the HF attack and the cycling performance of the electrode. Cyclic voltammetry (CV) is an effective analytical technique to study the electrochemical performance and electrode kinetics of materials. The CV curves in a potential window of 2.7–4.5 V at a scan rate of 0.1 mV s1 at room temperature are presented in Fig. 7. The anodic peaks are located at around 3.87 V, and the cathodic peaks at about 3.69 V. These peaks correspond to the Ni2+ $ Ni4+ redox couples [21]. The absence of a reduction peak at 3.2 V indicates that there is no reduction of Mn3+/Mn4+, it is accepted
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that manganese with oxidation state 4+ is not active. Its role is only to increase the stability of the metal oxide lattice [22]. In addition, CV plots of the pristine and modified sample are similar, suggesting that the nano SnO2 does not enter into the lattice of the NCM and has no effects on the structures, which is in accordance with the XRD results. It should also be noted that, the CV curves of the nano SnO2-modified NCM composite exhibit more symmetrical and sharper anodic/cathodic peaks than the pristine sample. In addition, the anodic peaks for modified NCM shift left and the cathodic peaks shift right, suggesting that less polarization occurs in the nano SnO2-modified NCM [23]. Those phenomena indicate the existence of nano SnO2 improved the reversibility of Li+ deintercalation/intercalation during the charge/discharge procedure. The pristine and modified samples (each 2 g) are soaked in 20 mL electrolyte respectively for one month in an argon-filled glove box, the supernatant is tested by the ICP technique after high speed centrifugation to detect the activity of the Ni, Co and Mn. The content of three elements are listed in Table 1. The results show that the content of transitional metals in electrolyte soaking pristine are much higher than the modified material. This illustrates the pristine sample is far easier to be corroded by electrolyte. The SEM images of pristine and modified sample after immersed in electrolyte are showed Fig. 8. A distinct change is noticed on the surface of both samples. The surface of pristine sample is partly covered with some nanoparticles, while the surface of the modified material is fully covered with a thin layer composed of many nanoparticles. The possible source of the small particles dispersed on the cathode materials surface could be the dissolved active material from the cathode in electrolyte. We could speculate that the existence of thin layer may be an effective covered layer for the protection of cathode materials from the electrolyte.
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Please cite this article in press as: Z.-M. Luo, et al., Electrochemical performance of a nano SnO2-modified LiNi1/3Co1/3Mn1/3O2 cathode material, Chin. Chem. Lett. (2015), http://dx.doi.org/10.1016/j.cclet.2015.06.007
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with nano SnO2 is an effective way to enhance the electrochemical performance of LiNi1/3Co1/3Mn1/3O2 for lithium ion batteries.
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Acknowledgment
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This work was financially supported by National Science Q2244 Foundation for Fostering Talents in Basic Research of China (No. 245 J2013-002). 246
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Fig. 9. XRD patterns of the fresh and harvested NCM electrodes: (a) fresh electrode of pristine NCM; (b) the electrode of pristine NCM after 50 cycles; (c) fresh electrode of SnO2-modified NCM; (d) the electrode of SnO2-modified NCM after 50 cycles.
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In order to identify the mechanism for capacity fading of pristine and modified materials with increasing cycles, the halfcells, after 50 cycles at 1 C, were opened and subjected to post mortem analysis. Fig. 9 shows the XRD patterns of the fresh NCM electrode and after 50 cycles. The clear splitting of the reflections assigned to the Miller indices (0 0 6, 1 0 2) and (1 0 8, 1 1 0) for the pristine cathode indicates a well-layered structure. After 50 cycles, the typical 0 0 3, 1 0 1 and 1 0 8 peaks slightly shift to lower angles, indicating the lattice shrinkage along a, b directions and expansion along c direction. As it is believed, the disappearance of 0 0 6 peak should be associated with the mixing of Li with the transition metal ions. It is noted that both pristine and modified materials show much structural changes after 50 cycles compared with the fresh material. This means that structural change of NCM is not a major contributor to the capacity fade. The mechanism would have to be further studied. The conclusion is consistent with the literature reported by Zheng et al. [24].
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The nano SnO2-modified NCM samples have been successfully synthesized by the carrier transfer method. The modified layer cover is on the surface of samples instead of entering into the crystal lattice. The rate capacity of NCM cathode is enhanced because of the existence of nano SnO2, the initial discharge capacity increased by 3.75%, 0.96%, 6.41%, 8.71% and 14.59% at the rate of 0.5 C, 1 C, 2 C, 5 C and 10 C, respectively. The cycle performance is also improved, especially at high temperatures, the capacity loss of modified samples is only 7.8% while pristine samples is 33.6%. It can be concluded that surface modification
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Original article
Electrochemical performance of a nano SnO2-modified LiNi1/3Co1/3Mn1/3O2 cathode material Q1 Zhi-Mei
Luo, Yan-Guang Sun, Hui-Yong Liu *
School of Chemical Engineering, Fuzhou University, Fuzhou 350116, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 29 April 2015 Received in revised form 1 June 2015 Accepted 8 June 2015 Available online xxx
The nano SnO2-modified LiNi1/3Co1/3Mn1/3O2 was successfully prepared by a carrier transfer method. The pristine and modified samples were characterized with various techniques such as XRD, SEM, XPS and EDS. The results showed that the SnO2 particles did not enter the crystal structure of LiNi1/3Co1/ 3Mn1/3O2, many nano SnO2 particles were uniformly covered on the surface of LiNi1/3Co1/3Mn1/3O2 and the modified thin layer could inhibit the dissolution of transition metal oxides. The electrochemical tests indicated that the existence of nano SnO2 could improve the discharge capacity and rate capability owing to the decreased interfacial polarization. The cycling stability was remarkably improved at room temperature and 55 8C. The XRD patterns of the fresh NCM electrode and after 50 cycles proved that the structural change of NCM was not so effective on the capacity fade. ß 2015 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Keywords: Carrier transfer method Nano SnO2 LiNi1/3Co1/3Mn1/3O2 Surface modification
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1. Introduction
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Lithium ion batteries have become the state-of-the-art power sources for portable appliances such as notebook computers, cellular phones, and digital cameras. In addition, the batteries are expected to be applied in hybrid electric vehicles (HEVs) and electric vehicles (EVs) [1,2]. Layered LiNi1/3Co1/3Mn1/3O2 as the cathode material of lithium ion batteries is now widely investigated for its low cost, high capability and safety, which is acknowledged as a candidate to replace LiCoO2 [3]. However, due to its poor cycling performance at high working voltage and high charge/discharge rate, this compound cannot completely substitute LiCoO2 as a cathode material [4]. In addition, more side reactions will take place between the active electrode and the electrolyte for cell and lead to the increase of the interfacial resistance and thus negatively influence the battery performance [5]. To overcome the deficiencies mentioned above, an effective approach to modify the surface of LiNi1/3Co1/3Mn1/3O2 by metal oxides is proposed. Some of the metal oxides reported as surface modifying candidates are: Al2O3, V2O5, ZrO2, TiO2, CeO2, etc. [6–12], which have recently been shown to improve both the thermal stability and the rate capability of LiNi1/3Mn1/3Co1/3O2. Surface modifying has been proved to be an
* Corresponding author. E-mail address: [email protected] (H.-Y. Liu).
effective method to scavenge HF species from the electrolyte and form stable metal fluoride layer, which can result in less decomposition of the cathode particles [13]. SnO2 can react with Li reversibly through a conversion reaction and has been successfully used to modify cathode materials such as LiCoO2 [14], LiFePO4 and LiMn2O4 [15,16]. The electrochemical properties of the modified cathode materials have been significantly improved without obvious capacity loss. The metal oxide coating layer acted as a solid electrolyte with a reasonable high Li ion conductivity. In this work, a nano SnO2-modified LiNi1/3Co1/ 3Mn1/3O2 sample was synthesized successfully by the carrier transfer method [17]. The effects of SnO2 modification on the structural and electrochemical properties of LiNi1/3Co1/3Mn1/3O2 were also investigated.
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2. Experimental
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Commercially available spherical LiNi1/3Co1/3Mn1/3O2 (NCM) powders (purchased from 3 M, American) were utilized as pristine sample. A certain amount of SnCl45H2O (AR, 99.0%) was dissolved in a hydrochloric acid solution and a solution of ammonia with proper concentration was added to promote the hydrolysis of Sn4+ under vigorous stirring conditions. The Sn(OH)4 collosol was formed after agitating for 12 h, then active carbon (AR, 99.0%) was added (in a weight ratio of SnO2:C = 3:7) with constant stirring for 5 h. After that, the resulting precipitate was washed several times with distilled water and dried at 100 8C overnight. Subsequently,
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http://dx.doi.org/10.1016/j.cclet.2015.06.007 1001-8417/ß 2015 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: Z.-M. Luo, et al., Electrochemical performance of a nano SnO2-modified LiNi1/3Co1/3Mn1/3O2 cathode material, Chin. Chem. Lett. (2015), http://dx.doi.org/10.1016/j.cclet.2015.06.007
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the precipitate was sintered in a tube furnace at 500 8C in argon atmosphere to obtain the SnO2/C powders. The Gravimetric method was used to determine the percentage of SnO2 in the SnO2/C, which was calcined in a furnace at 500 8C for 6 h under airflow. The active carbon generated CO2 and pure SnO2 was obtained. Based on the weight loss of the sample we could confirm a weight ratio of 3:7 between SnO2 and C. The obtained SnO2/C powders were calcined in a muffle furnace at 500 8C for 6 h in air and a stoichiometric amount of NCM was mixed thoroughly to obtain 2 wt% nano SnO2-modified NCM samples. The pristine and SnO2 modified samples were characterized by powder X-ray diffraction (D/MAX-2500, Rigaku, Japan) to identify the structure from 108 to 808 with a step size of 0.028. The surface morphologies of the samples were analyzed using a field emission scanning electron microscope (MIRA3XMH, TESCAN, Czech). Energy dispersive X-ray spectroscopy (EDS) was employed to analyze the surface composition and element distribution of the cathode materials. The surface chemical states of Ni, Co, Mn and Sn were investigated by X-ray photoelectron spectroscopy (XPS, VG Multilab 2000). The electrochemical properties of pristine and nano SnO2modified NCM were measured using a CR2025 coin cell. The working cathode films were consisted of 85 wt% cathode material, 5 wt% supper P, 5 wt% KS-6 and 5 wt% PVDF. Lithium foil was used as the counter electrode, 1 mol L1 LiPF6 in a EC:DMC = 1:1 (V/V) as the electrolyte, a polypropylene microporous film (Cellgard 2300) as the separator, and foam nickel as a filler material. The cells were assembled in an argon-filled glove box. The galvanostatic discharge–charge tests were performed on the LAND battery program-control test system (Wuhan, China) between 3.0 and 4.5 V (vs. Li/Li+) at a constant current density of 36 mA g1 (0.2 C) at room temperature and 55 8C, respectively. Cyclic voltammetry (CV) test was carried out on an electrochemical workstation (CHI660C, Shanghai, China) in the potential window of 2.7–4.5 V (vs. Li/Li+) at a scan rate of 0.1 mv s1.
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3. Results and discussion
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The XRD patterns of pristine and nano SnO2-modified NCM are shown in Fig. 1(a). The main phases belong to an a-NaFeO2 type layered hexagonal structure with a space group of R3m. No evident differences appear between the pristine and nano SnO2 modified samples, the lattice volume of nano SnO2-modified is very close to the pristine, which indicates that the layered structure of NCM is not affected. In order to confirm the phase of nano SnO2, which has
been obtained using the same procedure without adding layered NCM. The XRD patterns in Fig. 1(b) show that the main phase is a tetragonal structure with a space group of p42/mnm. However, the phases of nano SnO2 are not present in the patterns of modified NCM. It is believed that such a low mass (2 wt%) in nano SnO2 modified samples cannot be detected due to the detection limit of X-ray diffraction. Fig. 2 presents the SEM images of pristine and nano SnO2modified NCM. It can be seen that all samples are composed of many primary nanoparticles, these smaller primary particles were generated to form spherical second particles of 10 mm in diameter. The pristine samples show a very smooth and clean surface in Fig. 2(a) and (b). However, as shown in Fig. 2(c) and (d), the surface of nano SnO2-modified NCM are rough and some bright white spots are seen, owing to the partial coverage of its surface with nano-sized SnO2 particles. The TEM image of SnO2/C are given in Fig. 2(e), the darker area embedded within active carbon is called the nano SnO2, whose particle size distribution is uniform. EDS is also performed to confirm the distribution of SnO2 modified layer on the surface of modified NCM (Fig. 3). According to the mapping, Sn element in the sample is uniformly distributed on the NCM surface. The surface film composition and the oxidation state of the elements are analyzed by XPS. The XPS spectra of pristine NCM and nano SnO2-modified NCM are shown in Fig. 4. The binding energies obtained in the XPS analysis are corrected for specimen charging by referencing the C 1s line to 284.60 eV. It reveals that the modified sample has Mn 2p, Co 2p and Ni 2p peaks without remarkable chemical shift of binding energy compared to the pristine samples, indicating that their ion environments are not changed in the surface structure. This means that the modifying layer is only covered on the surface of NCM instead of entering NCM lattice. The 3d5/2 and 3d3/2 Sn peaks are found to appear at 485.95 and 494.39 eV, respectively, which are in good agreement with literature values reported for SnO2 [18]. It indicates that the valence of Sn is +4 and the Sn element on the material surface is in the form of SnO2. In addition, it is clear that the intensity of the Mn 2p, Co 2p and Ni 2p peaks decreased after modification due to the formation of the nano SnO2 layer on the particle surface. Fig. 5 shows the initial discharge curves of the pristine and nano SnO2-modified NCM electrodes at different discharge rates. The discharge capacities of 0.5, 1, 2, 5 and 10 C at the first cycle are 159.9, 156.9, 145.0, 138.8 and 116.5 mAh g1, respectively, which are only 98.1%, 90.9%, 86.8% and 72.9% of the discharge capacity at 0.5 C. However, the rate capability is improved dramatically by nano SnO2 modification. The modified sample presents a discharge
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Fig. 1. XRD of (a) pristine NCM and SnO2-modified NCM, (b) SnO2.
Please cite this article in press as: Z.-M. Luo, et al., Electrochemical performance of a nano SnO2-modified LiNi1/3Co1/3Mn1/3O2 cathode material, Chin. Chem. Lett. (2015), http://dx.doi.org/10.1016/j.cclet.2015.06.007
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Fig. 2. SEM of pristine NCM (a), (b) and SnO2-modified NCM (c), (d); TEM of SnO2/C sample (e).
Fig. 3. EDS maps of the SnO2-modified NCM.
capacity of 157.9, 154.3, 150.9 and 133.5 mAh g1 at 1, 2, 5 and 10 C, corresponding to 95.2%, 93.0%, 90.9% and 80.5% of its capacity of 165.9 mAh g1 at 0.5 C. The result indicates that nano SnO2 can improve initial discharge capacity and rate capability of NCM. It is well known that nano SnO2 possesses large effective specific surface area, the trace amount of FH in the electrolyte reacts with SnO2 primarily, which prevents the cathode material dissolution
leading to an increase of interfacial impedance and electrode polarization. We can conclude that the nano SnO2 modified layer suppresses the increase of interfacial polarization and therefore decrease the capacity fading [19]. The cycling performance of the two materials at 1 C over 3.0– 4.5 V is investigated (shown in Fig. 6). The capacity retention of SnO2-modified NCM is remarkably enhanced showing only a 14.0%
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Fig. 4. XPS spectra of pristine NCM and SnO2-modified NCM.
Please cite this article in press as: Z.-M. Luo, et al., Electrochemical performance of a nano SnO2-modified LiNi1/3Co1/3Mn1/3O2 cathode material, Chin. Chem. Lett. (2015), http://dx.doi.org/10.1016/j.cclet.2015.06.007
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Voltage (V) capacity loss after 50 cycles at room temperature, while the pristine sample suffers from a 19.3% capacity loss in Fig. 6(a). As observed in Fig. 6(b), the cycling performance at 55 8C is improved more significantly and the capacity loss of modified samples is only 7.8%. However, the capacity loss of pristine samples is very obvious (33.6%). This result is attributed to the fact that the cathode material suffers from serious corrosion due to the existing FH in the electrolyte at higher temperature. The role of SnO2 modifying layer is to consume the excess of FH [20], thus prevents the sidereactions between the electrode and electrolyte. Therefore, the nano SnO2 on the particle surface improves the stability of electrode against the HF attack and the cycling performance of the electrode. Cyclic voltammetry (CV) is an effective analytical technique to study the electrochemical performance and electrode kinetics of materials. The CV curves in a potential window of 2.7–4.5 V at a scan rate of 0.1 mV s1 at room temperature are presented in Fig. 7. The anodic peaks are located at around 3.87 V, and the cathodic peaks at about 3.69 V. These peaks correspond to the Ni2+ $ Ni4+ redox couples [21]. The absence of a reduction peak at 3.2 V indicates that there is no reduction of Mn3+/Mn4+, it is accepted
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Fig. 7. CV curves of NCM and SnO2-modified NCM between 2.7 and 4.5 V at a scan rate of 0.1 mV s1.
that manganese with oxidation state 4+ is not active. Its role is only to increase the stability of the metal oxide lattice [22]. In addition, CV plots of the pristine and modified sample are similar, suggesting that the nano SnO2 does not enter into the lattice of the NCM and has no effects on the structures, which is in accordance with the XRD results. It should also be noted that, the CV curves of the nano SnO2-modified NCM composite exhibit more symmetrical and sharper anodic/cathodic peaks than the pristine sample. In addition, the anodic peaks for modified NCM shift left and the cathodic peaks shift right, suggesting that less polarization occurs in the nano SnO2-modified NCM [23]. Those phenomena indicate the existence of nano SnO2 improved the reversibility of Li+ deintercalation/intercalation during the charge/discharge procedure. The pristine and modified samples (each 2 g) are soaked in 20 mL electrolyte respectively for one month in an argon-filled glove box, the supernatant is tested by the ICP technique after high speed centrifugation to detect the activity of the Ni, Co and Mn. The content of three elements are listed in Table 1. The results show that the content of transitional metals in electrolyte soaking pristine are much higher than the modified material. This illustrates the pristine sample is far easier to be corroded by electrolyte. The SEM images of pristine and modified sample after immersed in electrolyte are showed Fig. 8. A distinct change is noticed on the surface of both samples. The surface of pristine sample is partly covered with some nanoparticles, while the surface of the modified material is fully covered with a thin layer composed of many nanoparticles. The possible source of the small particles dispersed on the cathode materials surface could be the dissolved active material from the cathode in electrolyte. We could speculate that the existence of thin layer may be an effective covered layer for the protection of cathode materials from the electrolyte.
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Table 1 The concentration of transition metal cations in electrolyte.
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Please cite this article in press as: Z.-M. Luo, et al., Electrochemical performance of a nano SnO2-modified LiNi1/3Co1/3Mn1/3O2 cathode material, Chin. Chem. Lett. (2015), http://dx.doi.org/10.1016/j.cclet.2015.06.007
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with nano SnO2 is an effective way to enhance the electrochemical performance of LiNi1/3Co1/3Mn1/3O2 for lithium ion batteries.
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Acknowledgment
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This work was financially supported by National Science Q2244 Foundation for Fostering Talents in Basic Research of China (No. 245 J2013-002). 246
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Fig. 9. XRD patterns of the fresh and harvested NCM electrodes: (a) fresh electrode of pristine NCM; (b) the electrode of pristine NCM after 50 cycles; (c) fresh electrode of SnO2-modified NCM; (d) the electrode of SnO2-modified NCM after 50 cycles.
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In order to identify the mechanism for capacity fading of pristine and modified materials with increasing cycles, the halfcells, after 50 cycles at 1 C, were opened and subjected to post mortem analysis. Fig. 9 shows the XRD patterns of the fresh NCM electrode and after 50 cycles. The clear splitting of the reflections assigned to the Miller indices (0 0 6, 1 0 2) and (1 0 8, 1 1 0) for the pristine cathode indicates a well-layered structure. After 50 cycles, the typical 0 0 3, 1 0 1 and 1 0 8 peaks slightly shift to lower angles, indicating the lattice shrinkage along a, b directions and expansion along c direction. As it is believed, the disappearance of 0 0 6 peak should be associated with the mixing of Li with the transition metal ions. It is noted that both pristine and modified materials show much structural changes after 50 cycles compared with the fresh material. This means that structural change of NCM is not a major contributor to the capacity fade. The mechanism would have to be further studied. The conclusion is consistent with the literature reported by Zheng et al. [24].
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The nano SnO2-modified NCM samples have been successfully synthesized by the carrier transfer method. The modified layer cover is on the surface of samples instead of entering into the crystal lattice. The rate capacity of NCM cathode is enhanced because of the existence of nano SnO2, the initial discharge capacity increased by 3.75%, 0.96%, 6.41%, 8.71% and 14.59% at the rate of 0.5 C, 1 C, 2 C, 5 C and 10 C, respectively. The cycle performance is also improved, especially at high temperatures, the capacity loss of modified samples is only 7.8% while pristine samples is 33.6%. It can be concluded that surface modification
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