Hai Zhua,
Xiaoling Ma*b,
Ling Zana and
Youxiang Zhang*a
aCollege of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China. E-mail: yxzhang04@whu.edu.cn
bCollege of Chemistry and Life Science, Hubei University of Education, Wuhan, 430205, China. E-mail: maxiaoling@hue.edu.cn
First published on 1st June 2015
The effects of vanadium pentoxide on the electrochemical properties of Li2MnSiO4 as a cathode material for lithium-ion batteries were tested by synthesizing a V2O5 nanowire-modified in situ carbon coated Li2MnSiO4 composite (LMS/C/V2O5) and comparing its performances with that of a Li2MnSiO4 composite without V2O5. In LMS/C/V2O5, the V2O5 nanowires, with diameters of around 10–20 nm and lengths up to tens of micrometers, entangled together and formed a 3D conductive network; the Li2MnSiO4 nanoparticles, with sizes around 30 nm, distributed uniformly in the network frame and tended to adhere to the V2O5 nanowires. In this structure, the LMS/C/V2O5 composite showed a superior performance as a cathode of lithium-ion batteries even with very low carbon content (3.4 wt%). Ex situ X-ray diffraction patterns, electrochemical impedance spectroscopies of the electrodes and the concentration of Mn ions in the electrolyte during the charge–discharge processes explained the effects of the V2O5 nanowires as an additive in the Li2MnSiO4 cathode material. The benefits of the nanowires include maintaining the crystal structure of Li2MnSiO4 during the charge–discharge cyclings, reducing the charge-transfer resistances at the solid–electrolyte interfaces, increasing the lithium ions diffusion coefficient in the cathode and alleviating the dissolution of manganese into the electrolyte of the batteries.
To resolve these problems, several approaches including metal ion doping,18,19 carbon coating20–22 and particle size reduction23 have been pursued. Metal ion doping can effectively stabilize the structure of Li2MnSiO4 and suppress the Jahn–Teller distortion. However, this strategy decreases the amount of Mn(II) ions in Li2MnSiO4 and thus reduces the capacity of the material. Meanwhile, the metal ion doping process may introduce undesired impurities in the product. Carbon coating is a suitable method to improve the performances of Li2MnSiO4. For example, Qu et al.20 reported a carbon coated Li2MnSiO4 composite with an initial discharge capacity of 240 mA h g−1 at a current density of 8 mA g−1 which was synthesized by a sol–gel method. Particle size reduction can also increase the capacity of the material. Kuezma et al.21 synthesized Li2MnSiO4 nanoparticles by a microwave-assisted solvothermal method which showed a discharge capacity of 250 mA h g−1 at 50 °C with a current density of 16 mA g−1. However, it has to be noted that this high capacity was obtained in the presence of a large amount of inactive materials in the electrode (the carbon content was as high as 31.7 wt% in the composite) and at low current density. At the same time, the capacity rapidly faded after several cycles.
Vanadium pentoxide (V2O5) was once an extensively studied cathode material for lithium batteries with high specific capacity.22–25 It has moderate electrical conductivity (10−2 to 10−3 S cm−1) and is a fast ionic conductor (LixV2−x/5O5) when the voltage is higher than 3.7 V. When used as coating material, it can act as a protective layer that depresses the dissolution of transition metal and reduces the side reactions between the electrode material and electrolyte.26,27 Kim et al.28 reported a V2O5 coated TiO2 that demonstrated a superior rate capability due to the better electrical conductivity and fast Li-ion diffusion. Lee et al.29 reported that a V2O5 coating layer on LiCoO2 improved the cyclability at a high charge cut-off voltage.
Herein, we report the electrochemical properties of a V2O5 nanowires-modified in situ carbon-coated Li2MnSiO4 composite (LMS/C/V2O5) and compare its performances with a Li2MnSiO4 composite without V2O5 nanowires (LMS/C) when used as cathode materials for lithium-ion batteries. With low carbon content (3.4 wt%), the LMS/C/V2O5 composite showed superior performances, especially at high current densities (800 mA g−1). The comparison between the two composites showed the benefits of the V2O5 nanowires as the additive in the cathode material. The ex situ XRD measurements during the charge–discharge process and the electrochemical impedance spectroscopies may account for the enhanced performances of the LMS/C/V2O5 composite.
The LMS/C composite was prepared by a facile sol–gel method. In a typical synthesis, 1 g of P123 was firstly dissolved in 20 mL absolute alcohol under vigorous magnetic stirring. After 4 mmol of CH3COOLi·2H2O, 2 mmol of Mn(CH3COO)2·4H2O and 2 mmol of tetraethyl orthosilicate (TEOS) were added and dissolved into the above P123 solution, a light pink solution was obtained. With continuous stirring, the pH of the solution was adjusted to <4 using CH3COOH. The solution was then evaporated at 60 °C to enable the formation of a dry gel. Finally the gel, after thorough grinding, was annealed in a furnace at 650 °C for 10 h in Ar atmosphere. When cooled to room temperature, the obtained product was LMS/C composite.
The LMS/C/V2O5 composite was fabricated via a simple vacuum filtration procedure. Specifically, 500 mg of the synthesized LMS/C composite and 26.3 mg of V2O5 nanowires (the mass of V2O5 nanowires was such chosen so that the V2O5 nanowires content was 5 wt% in the composite) were dispersed in water and mixed under ultrasonication for 30 min. After stirring for 2 h, the mixture was then filtered and the final freestanding paper of LMS/C/V2O5 composite was produced. The paper was transferred to a vacuum furnace and annealed at 200 °C for 2.5 h under Ar gas flow. After cooling to room temperature, product in black color was obtained.
![]() | ||
Fig. 1 The XRD pattern (a), SEM image (b), TEM image (c) and high-resolution TEM image (d) of the as-synthesized V2O5 nanowires. |
The XRD patterns of the as-synthesized LMS/C and LMS/C/V2O5 composites are shown in Fig. 2. The XRD pattern of the LMS/C composite can be well indexed to the orthorhombic Pmn21 structure of Li2MnSiO4 with no impurity peaks.6 No diffraction peaks of carbon can be found in the pattern, indicating that the carbon in the composite is amorphous or that the amount of carbon (3.6 wt%) is too small to be detected. The diffraction pattern of the LMS/C/V2O5 composite is the same as that of the LMS/C composite, indicating that the existence of V2O5 nanowires has not affected the crystal structure of Li2MnSiO4. The diffraction pattern of the V2O5 nanowires does not appear in the pattern of the composite, which may be due to the fact that the diffraction intensity of the nanoscale V2O5 wires, with a concentration of 5.0 wt%, is too low compared with the intensity of the Li2MnSiO4 in the composite.
The morphologies and micro-structures of the as-synthesized LMS/C and LMS/C/V2O5 composites were examined and the SEM and TEM images are presented in Fig. 1S† and 3. The SEM image of LMS/C (Fig. 1Sa†) shows that the nanoscale particles in the composite tend to assemble into microscale secondary particles. The TEM image of LMS/C (Fig. 1Sb†) shows clearly that the Li2MnSiO4 particles in the as-synthesized composite have pretty uniform sizes around 30 nm. Fig. 3a shows the SEM image of LMS/C/V2O5 after the carbon-coated Li2MnSiO4 nanoparticles were mixed with the V2O5 nanowires. The image shows that the nanoparticles are less aggregated to form secondary particles as they do in LMS/C when the V2O5 nanowires are present. Instead, it seems that the nanoparticles tend to adhere to the nanowires forming some quasi-1D structures. Fig. 3b and c show the TEM images of the LMS/C/V2O5 composite. In the images, the carbon-coated Li2MnSiO4 nanoparticles are dispersed uniformly in the cross-linked and interlaced V2O5 nanowires network. The high resolution TEM image is shown in Fig. 3d which focuses on one of the Li2MnSiO4 nanoparticles anchored on a V2O5 nanowire. The crystalline structure of the nanoparticle can be clearly seen in the inset of Fig. 3d. The distance between the crystalline planes is measured to be 0.236 nm corresponding to the (021) planes of the orthorhombic phase Li2MnSiO4. By comparing the morphologies and microstructures of the two composites with and without V2O5 nanowires, we are pretty sure that the existence of V2O5 nanowire will favor the electrical conductivity and the diffusion kinetics of lithium ions in the composite, thus enhancing the electrochemical properties of the composite as cathode materials for LIBs.
Galvanostatic charge–discharge measurements were carried out with lithium cells configuration in the voltage range of 1.5–4.8 V to evaluate the electrochemical properties of the as-synthesized composites at room temperature. Fig. 4 compares the charge–discharge profiles of the LMS/C/V2O5 and the LMS/C composites as cathodes at a current density of 0.1 C (1 C = 166 mA g−1) during the initial five cycles. It can be noted that the charge–discharge profiles of the two composites are similar qualitatively, indicating that the V2O5 nanowires in the composite did not change the intrinsic properties of the Li2MnSiO4. For both composites, the voltage profiles in the second charges are different from the long plateau-containing voltage profiles in the first charges. This phenomenon is ascribed to the structural rearrangement that occurs during the initial charge process.8,11,12 This structural rearrangement, including the formation of ammorphous phase in the material, during the first oxidation of Li2MnSiO4 is also shown in the cyclic voltammograms (CV) profiles of the LMS/C/V2O5 composite (Fig. 2S†). The anodic peak at 4.2 V in the first charge, corresponding to the simultaneously oxidation of Mn2+ and Mn3+ [ref. 12], moves to 3.5 V in the second charge. The broad cathodic peak at 2.9 V corresponds to the reductions of Mn4+ and Mn3+. While the potential interval of 0.6 V between the redox peaks at 3.5 V and 2.9 V is due to the polarities of the material, the movement of the anodic peak from being at 4.2 V in the first charge to being at 3.5 V in the second charge is due to the irreversible structural changes of Li2MnSiO4 occurring during the initial extraction of Li.
![]() | ||
Fig. 4 The galvanostatic charge–discharge curves of the LMS/C/V2O5 (a) and the LMS/C (b) composites in the voltage range of 1.5–4.8 V at the rate of 0.1 C for the initial 5 cycles. |
As seen from Fig. 4a, the LMS/C/V2O5 composite shows a high charge capacity (350 mA h g−1) in the 1st cycle, even higher than the theoretical capacity of Li2MnSiO4 (330 mA h g−1), which might be due to the formation of solid electrolyte interface (SEI) film or the decomposition of electrolyte at high voltage.33,34 The LMS/C/V2O5 composite delivers an initial discharge capacity of 277.0 mA h g−1, corresponding to the insertion of 1.66 Li+ ions per formula unit. Since V2O5 itself is a cathode material for lithium batteries, the capacities of the synthesized V2O5 nanowires were also measured. The charge–discharge profiles in the first two cycles were measured at a current density of 16 mA g−1 in a voltage window of 1.5–4.8 V vs. Li+/Li and are shown in Fig. 3S.† As seen from the profiles, the charge and discharge capacities of the V2O5 nanowires are 20 and 120 mA h g−1, respectively. With only 5 wt% concentration in the LMS/C/V2O5 composite, the contribution of the V2O5 nanowires to the capacities of the composite is negligible. For Li2MnSiO4, fast capacity fading is usual, especially when the capacity is very high, due to Mn dissolution during the charge–discharge process.15–17 However, the LMS/C/V2O5 composite delivers discharge capacities of 277.0, 274.8, 269.4, 262.6 and 258.4 mA h g−1, respectively, in the initial 5 cycles, showing very little capacity fading. For LMS/C (Fig. 4b), the discharge capacities are 205.5, 183.4, 172.1, 164.3, and 157.7 mA h g−1 for the initial 5 cycles, which are 74.2%, 66.7%, 63.9%, 62.6%, and 61.0%, respectively, of the discharge capacities of LMS/C/V2O5 in the corresponding cycles. It can be clearly seen that the LMS/C/V2O5 composite exhibits much higher capacities and much lower capacity losses than the LMS/C composite. This shows that the V2O5 nanowires additive in the composite improves the electrochemical performance of Li2MnSiO4 cathode materials.
To fully estimate the electrochemical performances of the composites as cathode materials for LIBs, galvanostatic cycling measurements were also performed at high current densities. Fig. 5 presents a comparison of the typical charge–discharge profiles (Fig. 5a and c) and the cycling performances (Fig. 5b and d) of the two composites. For composite LMS/C/V2O5, the initial discharge capacities are 179.4, 160.3, and 119.6 mA h g−1, respectively, at the rates of 1 C, 2 C, and 5 C. After 50 charge–discharge cycles, the capacities declined to 134.0, 123.6, and 91.5 mA h g−1, respectively, with capacity retentions of 74.7%, 77.1%, and 76.5%. For composite LMS/C, the initial discharge capacities are 162.7, 131.8, and 78.7 mA h g−1, respectively, at the rates of 1 C, 2 C, and 5 C. After 50 charge–discharge cycles, the capacities declined to 45.0, 36.6, and 23.0 mA h g−1, respectively, with capacity retentions of 27.6%, 27.8%, and 29.2%. We can clearly see that, like their behaviors at low rates, the LMS/C/V2O5 composite also shows much improved electrochemical performances than the LMS/C composite at high rates. When compared with the performances of the Li2MnSiO4 cathodes reported in literature, our LMS/C/V2O5 composite shows not only higher capacities but also better cyclabilities at all the 1 C, 2 C and 5 C rates. The comparisons of the performances between the previously reported results and our LMS/C/V2O5 composites are listed in Table 1.
![]() | ||
Fig. 5 The typical galvanostatic charge–discharge curves (a and c) and the cycling performances (b and d) of the LMS/C/V2O5 and LMS/C composites at rates of 1 C, 2 C and 5 C. |
The highest discharge capacities at different rates and the capacity retentions in different cycles | |||
---|---|---|---|
1 C | 2 C | 5 C | |
This work | 212.3 mA h g−1 | 185.6 mA h g−1 | 132.3 mA h g−1 |
102% (20th) | 102.3% (20th) | 197.6% (20th) | |
90.8% (30th) | 91.7% (30th) | 89.0% (30th) | |
74.7% (50th) | 77.1% (50th) | 76.5% (50th) | |
Qu et al.20 | 125 mA h g−1 | ∼85 mA h g−1 | |
67.8% (30th) | 68% (30th) | ||
Liu et al.33 | 193.1 mA h g−1 | 149.9 mA h g−1 | |
46.8% (20th) | 56.4% (20th) | ||
Hu et al.35 | 178.6 mA h g−1 | 103.4 mA h g−1 | |
81.2% (20th) | 74% (20th) |
To investigate the causes of the performances improvement of the composite LMS/C/V2O5, a series of experiments were carried out on the cycled electrodes. Ex situ XRD measurements were used to investigate the structural changes of the Li2MnSiO4 nanoparticles in the LMS/C/V2O5 and LMS/C composites after the composites were cycled at the rate of 1 C for 1, 15, 30 and 50 times, respectively, and the results are shown in Fig. 6. It can be observed from Fig. 6a and b that, for both composites, when the cycling numbers increase, the intensities of the diffraction peaks decrease, indicating that the crystallinity of the Li2MnSiO4 in the composites is decreased during the charge–discharge process. This crystallinity weakening was considered to be caused by irreversible structure distortion.8,15 Fig. 6a shows the ex situ XRD patterns of the LMS/C/V2O5 cathodes after different charge–discharge cycles. The peaks at 2θ = 16.4°and 49.6° corresponding to the (010) and (212) crystalline planes disappear after 50 cycles. The other peaks, although with decreased intensities, can still be obviously observed. From the ex situ XRD patterns of the LMS/C cathodes (Fig. 6b), we can see that the peak intensities decreased much faster than those of the LMS/C/V2O5 cathodes. After 30 charge–discharge cycles, the two diffraction peaks corresponding to the (010) and (212) crystalline planes disappear. Meanwhile the other peaks have lower intensities than those of the corresponding peaks of the LMS/C/V2O5 cathodes after they are cycled for 50 times. In view of these observations, we believe that the V2O5 nanowires modification can help the Li2MnSiO4 nanoparticles to retain its structure during insertion and extraction of lithium ions, and thus greatly improve its cycling performances.
![]() | ||
Fig. 6 The ex situ XRD patterns of LMS/C/V2O5 (a) and LMS/C (b) cathodes at the rate of 1 C after different charge–discharge cycles. |
The concentrations of Mn ions in electrolyte were measured after the two composites were charge–discharge for 50 times at the rate of 1 C. For the LMS/C cathode, the Mn ions concentration is 120.5 ppm. For the LMS/C/V2O5 cathode, the concentration is 63.6 ppm, almost half of the number for LMS/C. This shows that a much lower amount of Mn was dissolved into the electrolyte of the batteries at the presence of V2O5 nanowires. The Mn dissolution was supposed to be caused by the reaction of Li2MnSiO4 with HF generated from the hydrolysis of LiPF6-based electrolyte.16 The V2O5 nanowires modification, like the carbon coating layers on the surface of the Li2MnSiO4, although cannot prohibit the Jahn–Teller effect of the Mn ions, but is beneficial for protecting the cathode from the corrosive reactions with the electrolyte during the charge–discharge cyclings.
In order to further understand the improved electrochemical performances of the composites, electrochemical impedance spectroscopy (EIS) was also introduced to investigate their electrochemical kinetics and is shown in Fig. 7. Fig. 7a displays the Nyquist plots and the equivalent circuit (inset of Fig. 7a) of the two composites. The Nyquist plots are composed of a depressed semicircle in the high to medium frequency region followed by a slanted line in the low-frequency region.36 Whereas the former is related to the charge-transfer process at the electrode–electrolyte interfaces, the latter represents the Warburg impedance associated with Li diffusion in the cathode materials. As shown in Fig. 7a, the charge-transfer resistance (Rct) value of the LMS/C/V2O5 composite is 99.19 Ω which is much smaller than that of the LMS/C composite (191.76 Ω). This significantly reduced Rct indicates that the 3D conductive network formed by the long and entangled 1D conducting V2O5 nanowires improves the conductivity of the LMS/C/V2O5 composite. In addition to Rct, the apparent Li diffusion coefficient DLi+ also exhibits a noticeable synergistic effect. Fig. 7b displays the relationship between Zre and the reciprocal square root of the frequency (ω−1/2) in the low frequency region, with the slope of the fitting line as the Warburg coefficient σ. The σ can be obtained by eqn (1):36
Zre = Re + Rct + σω−1/2 | (1) |
D = R2T2/2A2n4F4C2σ2 | (2) |
![]() | ||
Fig. 7 The nyquist plots and equivalent circuit of composites LMS/C/V2O5 and LMS/C at room temperature (a) and the relationship between Zre and ω−1/2 in the low frequency region (b). |
In this equation, R is the gas constant, T is the absolute temperature, A is the surface area of the electrode, n is the number of electrons per molecule during oxidation, F is the Faraday constant, C is the concentration of lithium ion. As shown in Table 2, the calculated diffusion coefficients of lithium ions (DLi+) are 4.56 × 10−14 and 5.94 × 10−15 cm−2 s for the composites LMS/C/V2O5 and LMS/C, respectively. The larger Li+ diffusion coefficient in the composite of LMS/C/V2O5 indicates that the V2O5 nanowires in the composite not only can improve the electronic conductivity of the cathode but also is favorable for the Li diffusion during the electrochemical lithium insertion/extraction processes.
Samples | Re (Ω) | Rct (Ω) | DLi+ (cm−2 s) |
---|---|---|---|
LMS/C/V2O5 | 5.22 | 99.19 | 4.56 × 10−14 |
LMS/C | 6.83 | 191.76 | 5.94 × 10−15 |
Footnote |
† Electronic supplementary information (ESI) available: SEM and TEM images of LMS/C, CV curves of LMS/C/V2O5, and charge–discharge profiles of V2O5 nanowires. See DOI: 10.1039/c5ra07757e |
This journal is © The Royal Society of Chemistry 2015 |