Effects of V₂O₅ nanowires on the performances of Li₂MnSiO₄ as a cathode material for lithium-ion batteries

Abstract

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.

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1. Introduction

Rechargeable lithium-ion batteries (LIBs) have been extensively exploited as energy storage devices for electric, hybrid electric vehicles, and intermittent renewable energy sources because of their high energy and power densities and long cycle lifetime.^1–3 A variety of compounds have been tested, and some of them have been used commercially as cathode materials for the manufacture of LIBs. Among the compounds, lithium orthosilicate Li₂MSiO₄ (M = Fe, Mn and Co) based cathodes have attracted attention due to their overwhelming advantages such as high theoretical capacities (∼330 mA h g⁻¹ when extracting two Li⁺ ions per Li₂MSiO₄ formula unit), high thermal stabilities through strong Si–O bonding, safety, cost effectiveness, as well as being environmentally benign and easy to synthesize.^4–7 Especially, Li₂MnSiO₄ has attracted increasing attention as an alternative cathode material for LIBs. Since the redox reaction of the Mn³⁺/Mn⁴⁺ couple is much easier to realize than the reactions of the Fe³⁺/Fe⁴⁺ and Co³⁺/Co⁴⁺ couples within the present electrolytes potential ranges, the insertion/extraction of two Li⁺ ions per formula unit is much more feasible for Li₂MnSiO₄ when compared with Li₂FeSiO₄ and Li₂CoSiO₄.^8–10 Unfortunately, like most polyanion cathode materials, Li₂MnSiO₄ suffers from poor electrical conductivity (∼10⁻¹⁶ S cm⁻¹)^11,12 and low lithium ion diffusion coefficient (∼10⁻¹⁶ cm² s⁻¹).^13,14 Furthermore, the crystallinity of Li₂MnSiO₄ will be reduced during the charge–discharge process due to Jahn–Teller effects and the dissolution of Mn ions into the electrolyte.^15–17

To resolve these problems, several approaches including metal ion doping,^18,19 carbon coating^20–22 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 (Li_xV_2−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.

2. Experimental

2.1 Synthesis

The V₂O₅ nanowires were synthesized using a conventional hydrothermal reaction.³⁰ Specifically, 0.5 g of P123 (EO₂₀PO₇₀EO₂₀) and 0.3 g of ammonium metavanadate (NH₄VO₃) were first dissolved in a mixture of water (30 mL) and HCl (1.5 mL, 2 M). The solution was then transferred to a Teflon-lined autoclave and annealed at 120 °C for 24 h. The precipitates obtained were filtered and rinsed with water and acetone several times and then dried at 80 °C for 12 h under vacuum.

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.

2.2 Characterization

The crystal structural characterization of the samples was carried out on a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation (λ = 0.15406 nm). The morphologies of the materials (LMS/C/V₂O₅ and LMS/C) were investigated using field-emission scanning electron microscope (SEM, SIRION, USA) and transmission electron microscope (TEM, JEM 2010-FEF, JEOL Ltd., Japan). To determine the concentration of Mn in the electrolyte after the cathode materials were charge–discharged for different cycles, the coin cells were carefully disassembled in glove-box and the electrolytes were collected and examined by an inductively coupled plasma analyzer (ICP, IRIS Intrepid II XSP). The carbon content in the LMS/C composite was determined by VarioEL III elemental analyzer (Elementar Analysen System GmbH, Germany) and showed to be 3.6 wt%. With 5 wt% V₂O₅ nanowires in the composite, the carbon content in the LMS/C/V₂O₅ composite can thus be calculated to be 3.4 wt%.

2.3 Evaluation of electrochemical performances

The electrochemical measurements were carried out using CR2016 coin cells with lithium metal disks as the counter electrodes. The working electrodes were made by pressing mixtures of Li₂MnSiO₄-based composites, acetylene black, and polyvinylidene fluoride (PVDF) binder with a weight ratio 75 : 20 : 5 on stainless steel meshes which were used as the current collectors. The weight of active materials varied between 3.0 and 4.0 mg cm⁻² for each cell. The electrolyte was composed with 1 M LiPF₆ in ethylene carbonate/dimethyl carbonate (1 : 1 v/v) solvents and the separator was Celgard 2300 microporous film. The cell was assembled in a glovebox filled with high purity Ar gas. The electrochemical tests were performed galvanostatically at different current densities in a voltage window of 1.5–4.8 V on Neware battery test system (Shenzhen, China) at room temperature (20 °C). All the charge–discharge specific capacities were calculated on the net mass of Li₂MnSiO₄ excluding other materials contents. Cyclic voltammograms (CVs) were performed at a scan rate of 0.1 mV s⁻¹ between 1.5 and 4.8 V on a CHI760C electrochemistry workstation. Electrochemical impedance spectroscopies (EIS) were conducted using a CHI760C electrochemistry workstation. The AC amplitude was 5 mV, and the frequency range applied was 100 kHz to 0.01 Hz.

3. Results and discussion

Fig. 1 shows the XRD pattern, SEM, TEM and high resolution TEM images of the as-prepared V₂O₅ nanowires. The XRD pattern (Fig. 1a) shows the 001 reflections which is consistent with the layered structure of hydrated V₂O₅.^30–32 The characteristic peaks at 2θ = 8.2, 13.2, 25.0, 33.2, and 41.9° can be assigned to (001), (002), (003), (004), and (005) lattice planes, respectively. The d-spacing between the crystalline layers is calculated to be 0.98 nm using the Scherrer equation. The representative SEM (Fig. 1b) and TEM (Fig. 1c) images reveal that the V₂O₅ nanowires have smooth surfaces with lengths up to tens of micrometers. These nanowires entangle together, forming a 3D network frame. The high resolution TEM image (Fig. 1d) shows that the nanowires have diameters around 10–20 nm and the distances between the crystalline layers can be measured to be 0.96 nm which is consistent with the XRD results.

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.

Fig. 2 The XRD patterns of the LMS/C and LMS/C/V₂O₅ composites.

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.

Fig. 3 The SEM (a), TEM (b and c) and high-resolution TEM (d) images of the LMS/C/V₂O₅ composite.

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.

Table 1 Comparisons of the electrochemical performances of composite LMS/C/V₂O₅ and the previous reported Li₂MnSiO₄ composites

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% (20^th)	102.3% (20^th)	197.6% (20^th)
	90.8% (30^th)	91.7% (30^th)	89.0% (30^th)
	74.7% (50^th)	77.1% (50^th)	76.5% (50^th)
Qu et al.^20	125 mA h g^−1	∼85 mA h g^−1
	67.8% (30^th)	68% (30^th)
Liu et al.^33	193.1 mA h g^−1	149.9 mA h g^−1
	46.8% (20^th)	56.4% (20^th)
Hu et al.^35	178.6 mA h g^−1		103.4 mA h g^−1
	81.2% (20^th)		74% (20^th)

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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 (R_ct) 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 R_ct 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 R_ct, the apparent Li diffusion coefficient D_Li⁺ also exhibits a noticeable synergistic effect. Fig. 7b displays the relationship between Z_re 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

Z_re = R_e + R_ct + σω^−1/2 (1)

where ω is the angular frequency in the low frequency region, and both R_e and R_ct are kinetics parameters independent of frequency. Therefore, Z_re has a linear relationship with ω^−1/2. Consequently, using the resulting σ, the diffusion coefficient of the lithium ions (D_Li⁺) can be calculated based on eqn (2):³⁶

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 Z_re 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 (D_Li⁺) 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.

Table 2 Impedance parameters of the LMS/C/V₂O₅ and LMS/C composites

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

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4. Conclusions

In summary, we tested the effects of V₂O₅ nanowires on the performances of Li₂MnSiO₄ as the cathode material for lithium-ion batteries by synthesizing a Li₂MnSiO₄/C/V₂O₅ composite and comparing its electrochemical properties with a composite without V₂O₅ nanowires. In the composite, the Li₂MnSiO₄ nanoparticles were distributed uniformly in the 3D network formed by the long and entangled V₂O₅ nanowires. At the presence of this 5 wt% V₂O₅ additive, the composite showed superior performances as a cathode for Li-ion batteries even with a very low carbon content (3.4 wt%). Specific discharge capacities of 277.0, 212.3, 185.6 and 132.3 mA h g⁻¹ can be reached at the charge–discharge rates of 0.1 C, 1 C, 2 C, and 5 C, respectively. As regards the cycling properties, after being charge–discharged for 50 times at the rates of 1 C, 2 C, and 5 C, the capacity retentions of the composite can still maintain at 74.7%, 77.1% and 76.5%, respectively. After the electrochemical properties comparison with the composite without V₂O₅, the benefits of the V₂O₅ nanowires are obvious. First, the V₂O₅ nanowires can help the Li₂MnSiO₄ nanoparticles to retain its crystalline structure during the insertion/extraction of lithium ions and thus greatly improve its cycling performances. Secondly, to a certain extent the V₂O₅ nanowires in the composite can suppress the dissolution of manganese and protect Li₂MnSiO₄ from corrosive reactions with the electrolyte during the charge–discharge cyclings. Thirdly, V₂O₅ is not only a conductive material, but also a fast ionic conductor when the battery voltage is higher than 3.7 V, thus a 3D network formed by long and entangled V₂O₅ nanowires can not only improve the electronic conductivity of the composite but also is favorable for the Li diffusion between the composite and the electrolyte. In view of the working mechanism, our technique of the V₂O₅ nanowires modification could also be extended to other cathode materials, especially the polyanionic type materials, to improve their performances.

Acknowledgements

The authors thank the Center for Electron Microscopy at Wuhan University for helps in taking the TEM and high-resolution TEM images for the materials. This study was supported by the National Science Foundation of China (grants no. 20901062 and 21271145) and the Funds for Creative Research Groups of Hubei Province (2014CFA007).

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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

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