C composites as high capacity cathode materials for lithium-ion batteries

C composites as high capacity cathode materials for lithium-ion batteries

Accepted Manuscript 5LiFe0.9Mn0.1PO4∙4Li3V2(PO4)3/C composites capacity cathode materials for lithium-ion batteries as high Wei Zhao, Xunhui Xiong,...

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Accepted Manuscript 5LiFe0.9Mn0.1PO4∙4Li3V2(PO4)3/C composites capacity cathode materials for lithium-ion batteries

as

high

Wei Zhao, Xunhui Xiong, Yingbang Yao, Bo Liang, Ye Fan, Shengguo Lu, Tao Tao PII: DOI: Reference:

S0169-4332(19)31079-7 https://doi.org/10.1016/j.apsusc.2019.04.087 APSUSC 42395

To appear in:

Applied Surface Science

Received date: Revised date: Accepted date:

5 February 2019 7 March 2019 7 April 2019

Please cite this article as: W. Zhao, X. Xiong, Y. Yao, et al., 5LiFe0.9Mn0.1PO4∙4Li3V2(PO4)3/C composites as high capacity cathode materials for lithium-ion batteries, Applied Surface Science, https://doi.org/10.1016/ j.apsusc.2019.04.087

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ACCEPTED MANUSCRIPT 5LiFe0.9Mn0.1PO4∙4Li3V2(PO4)3/C composites as high capacity cathode materials for lithium-ion batteries Wei Zhaoa, Xunhui Xiongc, Yingbang Yaoa, Bo Lianga, Ye Fana, Shengguo Lua,b,Tao Taoa,b,* a School b

of Materials and Energy, Guangdong University of Technology, Guangzhou 510006, PR China

Dongguan South China Design Innovation Institute, Dongguan, 523808, PR China

c Guangzhou

Key Laboratory of Surface Chemistry of Energy Materials, New Energy Research Institute, School of

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Environment and Energy, South China University of Technology, Guangzhou 510006, PR China

Abstract:

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The construction of high capacity cathode materials consisting of two active components,

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LiFexMn1-xPO4 and Li3V2(PO4)3, is rarely reported. Herein, 5LiFe0.9Mn0.1PO4∙4Li3V2(PO4)3/C

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(LFMVP/C) composites are synthesized by a simple two-step process, including ball milling and heat-treatment. The electrochemical reactivity of the composites with lithium is evaluated. The

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cathode assembled with LFMVP/C shows better electrochemical performance than that of the single active component (LiFePO4, LiMnPO4, or Li3V2(PO4)3) during charging and discharging in

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the potential range of 2.4 and 4.8 V. It delivers the specific capacities of 195, 173, and 158 mAh∙g-1 at 0.1C, 0.5 C , and 2C, and retains 88% of the initial discharge capacity after100 cycles

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at 0.1 C. Compared with the single active component (LiFePO4, LiMnPO4, or Li3V2(PO4)3), the lithium-ion diffusion coefficient of the composite composed of two active components (LFMVP/C)

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is higher.

Keywords: Lithium-ion batteries; Cathode; Composites; Phosphates; Mechanism * Corresponding author, E-mail: [email protected]; Fax: +86 020 39322570 Tel: +86 020 39322571 Address: School of Materials and Energy, Guangdong University of Technology, Guangzhou 510006, China 1

ACCEPTED MANUSCRIPT 1. Introduction Rechargeable Li-ion batteries (LIBs) have been used as power sources for electric vehicles (EVs) and hybrid electric vehicles (HEVs) because of their relatively high cyclic stability and energy density [1]. The electrochemical performances of LIBs greatly rely on the number of

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exchanged lithium-ions and on the redox potential couples. To this regard, the cathode materials

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play a significant role in LIBs [2]. Among these materials, lithium transition metal phosphates,

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such as LiFePO4, LiMnPO4, and Li3V2(PO4)3, have attracted a wide range of attention in recent years due to their low cost, high energy density, long lifespan, and good safety [3-8].

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Olivine-type LiFePO4 was first identified as a cathode material for LIBs by Goodenough’s

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team in 1997 [9]. It has a relatively high theoretical capacity (170 mAh g-1), The suitable working potential (3.4 V of Fe3+/Fe2+ vs. Li/Li+) and high thermal stability. However, LiFePO4 cathode

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mainly suffers from low lithium-ion diffusion and poor electronic conductivity [10-12], which

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hinders its practical applications. To resolve these issues, the LiFe1-yMnyPO4 solid solution system has been developed [13-16], as it shows a high operating voltage (4.1V of Mn3+/Mn2+ vs. Li/Li+)

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arising from the Mn3+/Mn2+ redox couple, and improved redox kinetics[17]. Besides, a series of

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studies have demonstrated that the electrochemical performance of composites consisting of two active components (xLiFePO4·yLi3V2(PO4)3) is higher than that of LiFePO4[18-21]. Compared with LiFePO4, monoclinic Li3V2(PO4)3 possesses a higher theoretical capacity(197 mAh∙g-1), rapid ionic diffusion, and higher working potentials (4.1 V of V3+/V4+ and 4.6 V of V4+/V5+ vs. Li/Li+) [22]. Therefore, the electrochemical performance of LiFePO4 can be effectively enhanced by incorporating with Li3V2(PO4)3. Several

researchers

have

also

investigated 2

the

electrochemical

behavior

of

ACCEPTED MANUSCRIPT xLiMn0.9Fe0.1PO4·y Li3V2(PO4)3. Zhong et al. [23] reported the xLiMn0.9Fe0.1PO4·y Li3V2(PO4)3/C (x:y = 1:0, 9:1 5:1, 3:1, 1:1 and 0:1) cathode materials show better electrochemical properties than the

single

LiMn0.9Fe0.1PO4/C.

Yang

et

al.

[24]

demonstrated

that

0.95LiMn0.95Fe0.05PO4·0.05Li3V2(PO4)3 synthesized by sol-gel and heat-treatment delivers a

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higher capacity (176 mAh g-1) than LiMn0.9Fe0.05V0.05PO4 (115 mAh g-1). To further improve the

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electrochemical performance of lithium transition metal phosphates, it is desired to explore the

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composite system consisting of LiFexMn1-xPO4 and Li3V2(PO4)3.

In this work, we have synthesized a class of xLiFe0.9Mn0.1PO4∙yLi3V2(PO4)3/C composites

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via a solid-state method by using glucose as carbon sources. Among these samples,

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5LiFe0.9Mn0.1PO4∙4Li3V2(PO4)3/C (LFMVP/C) exhibits the best electrochemical performances and is different from the single active component (LiFePO4, LiMnPO4, or Li3V2(PO4)3). Possible Li

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ions extraction/insertion mechanism of composites is discussed on the basis of in-situ X-Ray

2. Experimental

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diffraction analysis and electrochemical tests.

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2.1. Preparation of xLiFe0.9Mn0.1PO4∙yLi3V2(PO4)3/C

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The cathode composite materials were synthesized by ball milling and heat-treatment. The stoichiometric amounts of Li2CO3(99.99% metals basis, Aladdin), NH4VO3(99.9% metals basis, Aladdin), NH4H2PO4(99.99% metals basis, Aladdin), FeC2O4∙2H2O(99% AR, Aladdin), MnCO3(99.95% metals basis, MACKLIN) and glucose(AR, Aladdin) were dispersed in ethanol, and then ball milled at rotation speed of 600 rpm for 8 h. After ball milling, the mixture slurry was dried at 60 ℃ for 6 h and then heated in two steps under argon atmosphere. First, the sample was heated at 350 ℃ for 6 h and then heated at 700 ℃ for 10 h. Finally, it was cooled down to room 3

ACCEPTED MANUSCRIPT temperature to obtain composite materials. The stoichiometric quantities of raw materials for producing the 5LiFe0.9Mn0.1PO4∙4Li3V2(PO4)3/C can be expressed as Li2CO3 1.1342g, NH4VO3 1.1465g, NH4H2PO4 3.5314g, FeC2O4∙2H2O 2.5904g, MnCO3 3.5314g, glucose 0.8586g.

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2.2. Sample characterization

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The crystalline phases of LFMVP/C were characterized by X-ray powder diffraction (XRD,

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D8 Advance with CuKα radiation (λ = 1.5418 Å) ), and the morphologies and microstructures were observed by a transmission electron microscopy (TEM, Talos F200S FEI) and a

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field-emission scanning electron microscopy (FESEM, Hitachi SV8220) equipped with

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energy-dispersive X-ray spectroscopy (EDS). Raman spectroscopy was performed with a Jobin-Yvon LabRAM HR-800 Evolution (an argon ion laser excited at 532 nm). X-ray

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photoelectron spectroscopy (XPS, Escalab 250xi) at a pass energy of 20 eV was performed to

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investigate the surface chemistry of samples. The cycled cells were disassembled in an Ar-filled glove box for preparing the samples, and the obtained samples were washed with dimethyl

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carbonate (DMC) before investigating.

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The detailed configuration and measurement description of the in situ XRD study can be found elsewhere [25]. The in-situ XRD measurement was performed by a home-design cell made of stainless steel and inset with an internal slot with 12-mm inner diameter. The electrode is prepared by mixing the polyvinylidene fluoride (PVDF) binder and active material with a ratio of 2:8, and the mixture was homogeneously dispersed in 1 mL N-methyl pyrrolidone (NMP) solvent. Then, the carbon paper was coated with the obtained slurry. Each XRD scan was performed in the step incremental (0.02°, between 2 θ = 10° and 60°) at a rate of 0.08° s-1. 4

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2.3. Electrochemistry measurement The electrochemical measurements of the cathode materials were carried out using the CR2025 coin cell. The slurry was prepared by mixing LFMVP/C, polyvinylidene fluoride (PVDF)

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and acetylene black with a weight ratio of 8:1:1 in N-methyl-2-pyrrolodinone(NMP) solvent. The

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obtained slurry was coated onto Al foil and dried at 120 ℃ under vacuum for 12 h. The loading

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density of active material on working electrodes was about 1.5~3.0 mg cm-2. Lithium metal was used as the counter and the reference electrode, 1 M LiPF6 dissolved in ethylene carbonate

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(EC)/dimethyl carbonate(DMC)/ethylene methyl carbonate (EMC) with the volume ratio of 1:1:1

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as an electrolyte, and porous polypropylene film as a separator. Cells were assembled in a glove box filled with pure Ar. The charge/discharge capacity and the cycle performance of cells were

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performed by a battery test system (LAND, CT-2001A) in the voltage range 2.4-4.2 V and 2.4-4.8

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V at room temperature, respectively. Cyclic voltammograms (CVs) were recorded in a potential range of 2.4-4.8 V (vs.Li/Li+) at different scan rates and electrochemical impedance spectroscopy

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to 10 mHz.

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(EIS) acquired by CHI660E electrochemical workstation with the frequency ranging from 10 KHz

3. Results and discussion Fig. 1a shows the XRD and Rietveld refined XRD patterns of the obtained cathode composite material. It indicates that the composite is mainly composed of orthorhombic LiFe0.9Mn0.1PO4 (JCPDS PD#29-0808) and monoclinic Li3V2(PO4)3 (JCPDS PD#47-0107). A Li0.6V1.67O3.67 (JCPDS PD#50-0270) impurity phase is also found in the composite, but its diffraction is very weak. To analyze the crystal structures and phases of the composite, XRD pattern is refined by the 5

ACCEPTED MANUSCRIPT Rietveld refinement method. The calculated XRD pattern of the LFMVP/C composites composed of three phases: LiFe0.9Mn0.1PO4, Li3V2(PO4)3, and Li0.6V1.67O3.67 matches well with the observed XRD pattern (Table 1). The mass percentage of these phases determined by the multiphase refinement is 30.71% , 63.52%, and 5.77%, respectively. Previous study have showed that

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Li0.6V1.67O3.67 has low activity, and its relatively small amounts has a very small effect on the

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electrochemical properties of the material[26].

Fig.1. (a) XRD pattern and Rietveld refinement XRD pattern, and (b)Raman spectrum of the 5LiFe0.9Mn0.1PO4∙4Li3V2(PO4)3/C sample. 6

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The Raman spectrum of the composite shows a D band(1350 cm-1)corresponding to disorder- induced phonon made and G band (1590cm-1) which is ascribed to graphite bond (Fig. 1b). The intensity ratio of ID/IG is 0.98, indicating the existence of a carbon layer in the

Table 1 Unit cell parameters and the standard data of samples. Phase content (%)

b(Å)

c(Å)

Li3V2(PO4)3

8.566

12.053

8.596

LiFe0.9Mn0.1PO4

5.916

10.026

4.796

Li0.6V1.67O3.67

3.692

3.692

6.804

63.52 30.71 5.77

9.97

PDF#

JCPDS PD#47-0107 JCPDS PD#29-0808 JCPDS PD#50-0270

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a(Å)

Rwp(%)

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Lattice parameters

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phase

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composites and a modest graphitization [27].

Fig. 2a shows an SEM image of the LFMVP/C composites, which consists of varied particles

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sizes and shapes. The elemental mapping of the composites for Fe, Mn, V, P, and O elements is shown in Fig. 2b-f, revealing that the elements are homogeneously distributed in the LFMVP/C

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composites. TEM images of the LFMVP/C composite are shown in Fig. 2g-i. It can be found that

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a thin carbon layer (about 3 nm) covers the surface of the particle (Fig. 2h). The interplanar distances (0.33 and 0.43 nm) between the clear lattice fringes agree well with the (122) and (020)

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planes of the monoclinic Li3V2(PO4)3 (Fig. 2h,i), and the lattice spacing of 0.40 nm could be

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indexed to (120) crystal plane of LiFe0.9Mn0.1PO4 . The results of TEM are consistent with the corresponding XRD observation, which indicates the LFMVP/C composite is composed of LiFe0.9Mn0.1PO4 and Li3V2(PO4)3.

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Fig.2. SEM image of LFMVP/C (a) and elemental mapping of (b) O, (c) P, (d) V, (e) Fe and (f) Mn, and TEM image of LFMVP/C(g) and (h, i) enlarged HRTEM images in (g).

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Fig. 3a shows the initial charge/discharge curves of samples at 0.1C (1C=170mA∙g-1). Among these samples, including LiFePO4/C (LFP/C), LiMnPO4/C (LMP/C), Li3V2(PO4)3/C (LVP/C), and

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5LiFe0.9Mn0.1PO4∙4Li3V2(PO4)3/C (LFMVP/C), the LFMVP/C exhibits the highest discharge capacity of 180m Ah∙g-1 in the voltage range of 2.4-4.8 V, and five distinct potential plateaus of

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about 3.45 V, 3.6 V, 3.7 V, 4.1 V, and 4.6 V were observed for LFMVP/C, corresponding to the Fe2+/Fe3+, Mn2+/Mn3+, and V3+/V4+/V5+ redox pairs, respectively. Also, the LFMVP/C cathode can deliver the discharge capacity of 152 mAh g-1 after 50 cycles in the potential range of 2.5-4.5 V at 0.1 C, and its capacity retention is about 96% (Fig. 3b). The cycling performance of the samples is shown in Fig. 3c. After 100 cycles at 0.1 C, the LFMVP/C delivers the highest discharge specific capacity of 165 mAh∙g-1 in the voltage range of 2.4-4.8 V and its capacity retention is 88%. The LVP/C exhibits the discharge specific capacity of 118 mAh∙g-1 in the voltage range of 2.4-4.8 V 8

ACCEPTED MANUSCRIPT and has the capacity retention of 78.6%. The LFP/C shows the discharge specific capacity of 145 mAh∙g-1 in the voltage range of 2.4-4.2 V, and the capacity retention of 98% can be achieved. The discharge specific capacity of the LMP/C is 38mAh∙g-1 in the voltage range of 2.4-4.5 V, and its capacity retention is 98%. Fig. 3d shows the rate performance of samples at C-rates ranging from

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0.1 C to 2 C. The LFMVP/C cathode show a higher electrochemical performance at high rates

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than the single active component (LiFePO4, LiMnPO4 or Li3V2(PO4)3).The discharge specific

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capacities of LFMVP/C are 189, 181, 175, 168, and 158 mAh g-1 at 0.1, 0.2, 0.5, 1, and 2 C,

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respectively.

Fig.3. The initial charge-discharge curves of samples at 0.1 C (a), (b) selected charge-discharge of LFMVP/C at 0.1C (2.4-4.5 V), (c) cycling performance and (d) rate capability of samples.

Even at a high current density of 2 C, The LFMVP/C cathode delivers a discharge specific capacity of 130 mAh∙g-1(capacity retention of 80%) in the voltage range of 2.4-4.8 V and a 9

ACCEPTED MANUSCRIPT discharge specific capacity of 112 mAh∙g-1(capacity retention of 87%) in the voltage range of 2.4-4.5 V (Fig. 4a) after 100 cycles. Cyclic voltammetry (CV) investigation was conducted at a scan rate of 0.1 mV s-1 in the voltage range of 2.4-4.8V (Fig. 4b). The CV curve of the LFMVP/C cathode presents the five anodic peaks (around 3.58 V, 3.63 V, 3.71 V, 4.13 V, and 4.58 V) and

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four cathodic peaks ( around 3.90 V, 3.62 V, 3.54 V, and 3.37 V). The pair of redox peaks around

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3.58/3.37 V of the LFMVP/C cathode is associated with the Fe2+/Fe3+ redox potential [28]. 4

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oxidation peaks (3.63 V, 3.71 V, 4.13 V, and 4.58 V) and 3 reduction peaks (3.90 V, 3.62 V, and 3.54 V) are ascribed to the Mn2+/Mn3+ and V3+/V4+/V5+ redox potentials [29, 30]. The CV data are

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consistent with the charge/discharge curves (Fig. 3a).

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Fig.4. (a) Cycling performance of LFMVP/C at 2 C (2.4-4.8 V) and (b) CV curve of LFMVP/C at scan rate of 0.1 mV s-1.

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In order to estimate the lithium diffusion in the cathodes, the CV curves are evaluated at scan rates of 0.1, 0.2, 0.4, 0.8 mV s-1 (Fig. 5). The variation of peak current (Ip) has a linear relationship with the square root of scan rate(V1/2) [31]. The Li-ion diffusion coefficients of electrodes can be calculated by the Randles-Sevik equation [32]:

I p  2.69  10 5 n 3 / 2 AD1/ 2V 1/ 2C0

(1)

The value of D (the diffusion coefficient of Li+ ions) calculated based on the Equation (1) and the slope (Ip versus v1/2 plots) for the LFMVP/C cathode is about 9.23×10-10 cm2 s−1 (Table 2), 11

ACCEPTED MANUSCRIPT which is higher than that of the single active component (LiFePO4, LiMnPO4 or Li3V2(PO4)3). In this equation, A represents the surface area of the electrode, C0 represents the effective concentration of Li+ ions in the electrode and n represents the number of electrons per species reaction.

7.69×10-3

LMP/C

7.69×10-3

LVP/C

3.7×10-3

LFMVP/C

6.65×10-3

1.63×10-13

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LFP/C

DLi+ (cm2 s-1)

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C0 (mol cm-3)

6.56×10-13 2.47×10-11 1.23×10-11

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Samples

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Table 2 The diffusion coefficients of Li+ ions in electrodes calculated from CV

Fig.5. CV curves of samples at various scan rates and the relationship between the peak current (Ip) and the square root of scan rate(v1/2) for (a) LFP/C, (b) LMP/C, (c) LVP/C and (d) LFMVP/C.

Electrical impedance spectroscopy(EIS) measurements of samples were carried out in the frequency range of 100 kHz to 10 mHz (Fig. 6). The Nyquist plots are composed of four parts: a small intercept in the highest frequency, two depressed semicircles at high and middle frequencies, 12

ACCEPTED MANUSCRIPT and a slanted line in the low frequency [33], which can be fitted with the equivalent circuit model

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proposed for the interpretation of the impedance spectra (Fig. 6b).

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Fig.6. Nyquist plots of samples (a), equivalent circuit model (b), and (c) Nyquist plots of LFMVP/C-1(before cycle) and LFMVP/C-2(after 100 cycles)

Rs is the solution resistance, Rsei represents Li+ migration resistance, Rct represents

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charge-transfer resistance and Zw represents the diffusion-controlled Warburg impedance. The simulated charge transfer resistances (Rct) of the LFMVP/C electrode is smaller than that of the single active component (LiFePO4, LiMnPO4, or Li3V2(PO4)3) (Table 3), indicating that LFMVP/C sample has the lower Li+ migration resistance and charge-transfer resistance. This can be attributed to the composite consisting of two phosphate phases, which enhances the charge transfer kinetics. After 100 cycles, the corresponding charge transfer resistances of the LFMVP/C electrode increase dramatically (Table 3), which is associated with the specific phenomenon of the 13

ACCEPTED MANUSCRIPT cell during the charge-discharge process, such as the slight dissolution of the active material, side reactions, and formation of a thick SEI [34]. Table 3 Electrochemical parameters for the EIS results. Rs(Ω)

Rsei(Ω)

Rct(Ω)

LFP/C LMP/C LVP/C LFMVP/C-1 LFMVP/C-2

12.33 12.67 13.08 12.55 30.68

30.6 28.7 18.9 12.4 39.8

134.4 448.2 193 76.8 115

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Sample

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To understand the Li+ ions extraction/insertion mechanisms of the LFMVP/C sample, in situ

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XRD has been carried out at the selected charge/discharge states in the first cycle in the potential range of 2.4 and 4.8V at 0.1C (Fig.7). In situ XRD measurement obviously exhibits Li+ ions

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extraction/insertion process for the LFMVP/C sample. After charging to 3.58 V, the main peaks of the LiFe0.9Mn0.1PO4 phase located at 20.49˚ (011), 23.26˚ (120), 30.17˚(200) and 32.6˚(031) are

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shifted towards the high 2θ range and become wider, demonstrating the Li+ ions extraction from

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the LiFe0.9Mn0.1PO4 [35,36]. With continuous Li+ extraction to 4.8 V, the two main peaks of phase Li3V2(PO4)3 locate at 33.9˚ and 35.8˚ is slightly shifted towards the high 2θ range [37,38], their

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intensity becomes weaker, and the peak at 35.8˚ almost disappears. When it is discharged to 3.37 V,

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the peaks of LiFe0.9Mn0.1PO4 and Li3V2(PO4)3 recover, indicating a reversible Li+ ions extraction/insertion in the LFMVP/C sample.

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Fig.7. In situ XRD pattern of LFMVP/C during the first cycle charge-discharge cycle at 0.1C.

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X-ray photoelectron spectroscopy (XPS) was performed to confirm the oxidation states of metal elements in the samples. The surface of the pristine LFMVP/C sample, the charged

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LFMVP/C sample and the charged-discharged LFMVP/C sample consists of Fe, Mn, V, P, O, and

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C according to the XPS spectra shown in Fig. 8a. The Fe2p spectrum of the pristine LFMVP/C sample shows peaks at 711.2 eV and 724 eV (Fig. 8b), are assigned to Fe2+ [39]. After charging,

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the binding energy of the Fe2p is shifted to 712 eV and 726 eV, which is consistent with the

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presence of Fe3+ [40], demonstrating that Fe2+ is oxidized to Fe3+. When it is discharged from 4.8V to 2.4V, the binding energy of the Fe2p returns back to 710.9 eV and 724 eV, indicating that Fe3+ is reduced to Fe2+ after discharging. Also, the binding energy of the V2p1/2 and the V2p3/2 change from 523.8 eV and 516. 6 eV (V3+) to 524.6 eV and 517.8 eV (V5+) [41-43],and return back to the initial state during charging and discharging process. It can be attributed to a redox reaction of the V element. Similarly, it can be concluded that Mn2+ is oxidized to Mn3+ and then Mn3+ is reduced to Mn2+ after charging and discharging according to the binding energy change of 15

ACCEPTED MANUSCRIPT the Mn2p1/2 and Mn2p3/2 ( from 653.4 eV and 641.2 eV to 654 eV and 642.3 eV )(Fig. 8d) [44,45].

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These match well with the results of CV and in situ XRD.

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Fig. 8. XPS spectra of (a) wide-scan, (b) Fe2p, (c) V2p and (d) Mn2p for the pristine, first charged and discharged samples.

Based on the above results, the LFMVP/C composites are mainly composed of two active

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components (LiFe0.9Mn0.1PO4 and Li3V2(PO4)3). A Li+ ions extraction/insertion mechanism for the LFMVP/C cathode can be proposed. During the charging process, the first Li+ is extracted from LiFe0.9Mn0.1PO4 component (Fe2+/Fe3+). The continuous Li+ extractions are related to the phase transitions of Li3V2(PO4)3→Li2.5V2(PO4)3→Li2V2(PO4)3→LiV2(PO4)3→V2(PO4)3 (V3+/V4+/V5+). Compared with the redox peaks of V in Li3V2(PO4)3, the redox peaks of Mn (Mn2+/Mn3+) in LiFe0.9Mn0.1PO4 are very weak, which could be attributed to its lower content and ionic conductivity in the LFMVP/C composite [46-48]. The reversible insertion of extracted 16

ACCEPTED MANUSCRIPT lithium-ions in the LFMVP/C composites during the subsequent discharge process is associated with the electrochemical reductions of LiFe0.9Mn0.1PO4 and Li3V2(PO4)3. Accordingly, V is reduced from V5+ to V3+, Mn from Mn3+ to Mn2+, and Fe from Fe3+ to Fe2+ on discharge. The disorder of lithium reinsertion during the initial discharge process of Li3V2(PO4)3 is attributed to a

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solid solution behavior [49].

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Although the electrolyte decomposition, the structural instability, and vanadium dissolution

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result in a low coulomb efficiency of the pure cathode for operation in a high potential (>4.5 V) [50, 51], the irreversible capacity of Li3V2(PO4)3 is clearly reduced, and the lithium-ion diffusion

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coefficient and conductivity of the single active component are improved by the incorporation of

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LiFe0.9Mn0.1PO4 and Li3V2(PO4)3 and mutual cross-doping between them. LiFe0.9Mn0.1PO4 acts as the component to support the structure crystal of delithiated Li3V2(PO4)3 with an electrolyte and

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protect the active materials from the HF attack for operation in a high voltage range. Mutual

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doping of metal ions may lower the band gap energy of cathode material, and improve its electric conductivity [51]. The synergistic effects between LiFe0.9Mn0.1PO4 and Li3V2(PO4)3 could play a

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key role in improving the electrochemical performance of cathodes. Considering to combine the

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advantages of the two single active materials, LiFe0.9Mn0.1PO4 and Li3V2(PO4)3, the LFMVP/C composites can be proposed as the optimized cathode for lithium-ion batteries.

4. Conclusions

5LiFe0.9Mn0.1PO4∙4Li3V2(PO4)3/C (LFMVP/C) composites were synthesized by wet ball milling and solid-state reaction. Compared with the single active component (LiFePO4, LiMnPO4 or Li3V2(PO4)3), the composites as a cathode for LIBs show an enhanced electrochemical performance. It exhibits a high discharge specific capacity of 180 mAh∙g-1 and good reversible 17

ACCEPTED MANUSCRIPT discharge capacity of 160 mAh∙g-1 after 100 cycles at 0.1C. At a high current density of 2 C, the LFMVP/C cathode even delivers a reversible capacity of 140 mAh∙g-1 after 100 cycles. A combined in situ XRD and XPS analysis reveals that a Li+ ions extraction/insertion mechanism of the LFMVP/C sample is associated with several conversion reactions between Li+ and metal

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elements ( Fe, Mn and V). The electrochemical performance improvement of the cathode

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materials can be attributed to incorporation of LiFe0.9Mn0.1PO4 and Li3V2(PO4)3 and a mutual

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cross-doping between them.

Acknowledgements

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The authors would like to acknowledge the financial support from Science and Technology

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Program of Guangzhou (Grant number 201607010110), Science and Technology Planning Project of Guangdong Province, China (Grant number 2016A010104014), and the NSFC-Guangdong

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Joint Fund (Grant number U1501246).

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References:

[1] S. Liang, J. Hu, Y. Zhang, Y. Wang, X. Cao, A. Pan, Facile synthesis of sandwich-structured Li3V2(PO4)3/carbon composite as cathodes for high performance lithium-ion batteries, J. Alloys Compd. 683 (2016) 178-185.

CE

[2] P. Rosaiah, O. M. Hussain, J. Zhu, Y. Qiu, Spectroscopic and electrochemical properties of lithium-rich LiFePO4 cathode synthesized by solid-state reaction, J. Electron. Mater. 46 (2017) 4865-4874.

AC

[3] J. Ma, B. Li, H. Du, C. Xu, F. Kang, Inorganic-based sol–gel synthesis of nano-structured LiFePO4/C composite materials for lithium ion batteries, J. Solid State Electrochem 16 (2012) 1353-1362.

[4] Y. Wang, Y. Wang, E. Hosono, K. Wang, H. Zhou, LiFePO 4/carbon nanocomposite with a core–shell structure and its synthesis by an in situ polymerization restriction method, Angew. Chem. 120 (2008) 7571-7575. [5] C. Delacourt, P. Poizot, M. Morcrette, J.-M. Tarascon, C. Masquelier, One-step low-temperature route for the preparation of electrochemically active LiMnPO4 powders, Chem. Mater 16 (2004) 93-99. [6] L. Zhang, Q. Qu, L. Zhang, J. Li, H. Zheng, A coordinatively cross-linked polymeric network as a functional binder for high-performance silicon submicro-particle anodes in lithium-ion batteries, J. Mater. Chem. A. 2 (2014) 711-719. 18

ACCEPTED MANUSCRIPT [7] X. Rui, Q. Yan, M. Skyllas-Kazacos, T. M. Lim, Li3V2(PO4)3 cathode materials for lithium-ion batteries: A review,

J. Power Sources 258 (2014) 19-38.

[8] Y. Liao, C. Li, X. Lou, X. Hu, Y. Ning, F. Yuan, B. Chen, M. Shen, B. Hu, Carbon-coated Li3V2(PO4)3 derived from metal-organic framework as cathode for lithium-ion batteries with high stability, Electrochimica. Acta 271 (2018) 608-616. [9] A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, Phospho-olivines as positive-electrode materials for rechargeable lithium batteries, J. Electrochem. Soc. 144 (1997) 1188-1194. [10] F. Sauvage, L. Laffont, J.-M. Tarascon, E. Baudrin, Factors affecting the electrochemical reactivity vs. lithium of carbon-free LiFePO4 thin films, J. Power Sources 175 (2008) 495-501.

PT

[11] F. Yu, L. Zhang, M. Zhu, Y. An, L. Xia, X. Wang, B. Dai, Overwhelming microwave irradiation assisted synthesis of olivine-structured LiMPO4(M=Fe, Mn, Co and Ni) for Li-ion batteries, Nano

RI

Energy 3(2014) 64-79.

[12] F. Yu, J. Zhang, Y. Yang, G. Song, Reaction mechanism and electrochemical performance of

SC

LiFePO4/C cathode materials synthesized by carbothermal method, Electrochimica Acta 54 (2009) 7389-7395

[13] M. Kope, A. Yamada, G. Kobayashi, S. Nishimura, R. Kanno, A. Mauger, F. Gendron, C.M. Julien, Power Sources 189 (2009) 1154-1163.

NU

Structural and magnetic properties of Lix(MnyFe1-y)PO4 electrode materials for Li-ion batteries, J. [14] D.-H. Baek, J.-K. Kim, Y.-J. Shin, G.S. Chauhan, J.-H. Ahn, K.-W. Kim, Effect of firing

MA

temperature on the electrochemical performance of LiMn0.4Fe0.6PO4/C materials prepared by mechanical activation, J. Power Sources 189 (2009) 59-65. [15] G.R. Gardiner, M.S. Islam, Anti-site defects and ion migration in the LiFe0.5Mn0.5PO4 mixed-metal cathode material, Chem. Mater. 22 (2009) 1242-1248.

D

[16] A. Yamada, Y. Kudo, K.Y. Liu, Reaction mechanism of the olivine-type Lix( Mn0.6Fe0.4 )PO4  ( 0 ⩽ x ⩽ 1 ) J. Electrochem. Soc. 148 (2001) A747-A754.

PT E

[17] S.M. Oh, S.W. Oh, C.S. Yoon, B. Scrosati, K. Amine, Y.K. Sun, High-performance carbon-LiMnPO4 nanocomposite cathode for lithium batteries, Adv. Funct. Mater 20 (2010) 3260-3265.

[18] J. F. Zhang, C. Shen, B. Zhang, J. C. Zheng, C.L. Peng, X.W. Wang, X. B. Yuan, H. Li, G. M.

CE

Chen, Synthesis and performances of 2LiFePO4·Li3V2(PO4)3/C cathode materials via spray drying method with double carbon sources, J. Power Sources 267 (2014) 227-234. [19] W. Cheng, L. Wang, Z. Sun, Z. Wang, Q. Zhang, D. Lv, W. Ren, L. Bian, J. Xu, A. Chang,

AC

Preparation and characterization of LiFePO4·xLi3V2(PO4)3 composites by two-step solid-state reaction method for lithium-ion batteries, Mater. Lett. 198 (2017) 172-175. [20]

S.

Zhong,

L.

Wu,

J.

Liu,

Sol–gel

synthesis

and

electrochemical

properties

of

9LiFePO4·Li3V2(PO4)3/C composite cathode material for lithium ion batteries, Electrochimica. Acta 74 (2012) 8-15. [21] F. Yu, P. Qi, Y. An, G. Wang, L. Xia, M. Zhu, B. Dai, Up-Scaled microspherical aggregates of LiFe0.4V0.4PO4/C nanocomposites as cathode materials for high-rate Li-ion batteries,Energy Technol.

3 (2015) 496-502.

[22] X. Rui, C. Li, C. Chen, Synthesis and characterization of carbon-coated Li3V2(PO4)3 cathode materials with different carbon sources, Electrochimica. Acta 54 (2009) 3374-3380.

19

ACCEPTED MANUSCRIPT [23] L. Wu, J. Lu, G. Wei, P. Wang, H. Ding, J. Zheng, X. Li, S. Zhong, Synthesis and electrochemical properties of xLiMn0.9Fe0.1PO4·yLi3V2(PO4)3/C composite cathode materials for lithium–ion batteries, Electrochimica. Acta 146 (2014) 288-294. [24] L. Chen, B. Yan, H. Wang, X. Jiang, G. Yang, Synthesis and characterization of 0.95LiMn0.95Fe0.05PO4·0.05Li3V2(PO4)3 nanocomposite by sol–gel method, J. Power Sources 287 (2015) 316-322. [25] X. Ou, X. Xiong, F. Zheng, C. Yang, Z. Lin, R. Hu, C. Jin, Y. Chen, M. Liu, In situ X-ray diffraction characterization of NbS2 nanosheets as the anode material for sodium ion batteries, J. Power Sources 325 (2016) 410-416.

PT

[26] B. Yan, P. Z. Gao, D. Y. Li, G. L. Tian, Influence of the preparation process on the electrochemical properties of xLiFePO4∙yLi3V2(PO4)3/C nano-sized composite cathode materials, Key Eng. Mater.

RI

680 (2016) 238-243.

[27] T. Jiang, W. Pan, J. Wang, X. Bie, F. Du, Y. Wei, C. Wang, G. Chen, Carbon coated Li3V2(PO4)3

SC

cathode material prepared by a PVA assisted sol-gel method, Electrochimica. Acta 55 (2010) 3864-3869.

[28] D. Shanmukaraj, G. Wang, R. Murugan, H.-K. Liu, Electrochemical studies on LiFe1composite cathode materials synthesized by citrate gel technique for lithium-ion

NU

xCoxPO4/carbon

batteries, J. Mater. Sci. Eng. B 149 (2008) 93-98.

[29] X. Wang, W. Tang, X. Hou, C. Gu, Z. Su, Synthesis and electrochemical properties of a composite

MA

LiMn0.63Fe0.37PO4 with Li3V2(PO4)3 for lithium-ion battery cathode materials, Ceram. Int. 42 (2016) 15798-15804.

[30] L. Qin, Y. Xia, B. Qiu, H. Cao, Y. Liu, Z. Liu, Synthesis and electrochemical performances of (1−x)LiMnPO4·xLi3V2(PO4)3/C composite cathode materials for lithium ion batteries, J. Power

D

Sources 239 (2013) 144-150.

[31] S. Liang, X. Cao, Y. Wang, Y. Hu, A. Pan, G. Cao, Uniform 8LiFePO 4·Li3V2(PO4)3/C nanoflakes

PT E

for high-performance Li-ion batteriesNano Energy 22 (2016) 48-58. [32] J. Wang, Z. Wang, X. Li, H. Guo, X. Wu, X. Zhang, W. Xiao, xLi3V2(PO4)3·LiVPO4F/C composite cathode materials for lithium ion batteries, Electrochimica. Acta 87 (2013) 224-229. [33] L. Gao, Z. Xu, S. Zhang, J. Xu, K. Tang, Enhanced electrochemical properties of LiFePO 4 cathode

CE

materials by Co and Zr multi-doping, Solid State Ionics 305 (2017) 52-56. [34]J. Huang, L. Yang, K. Liu, Y. Tang, Synthesis and characterization of Li 3V(2-2x/3)Mgx(PO4)3/C cathode material for lithium-ion batteries, J. Power Sources 195 (2010) 5013-5018.

AC

[35] H.C. Shin, K.W. Nam, W.Y. Chang, B.W. Cho, W.-S. Yoon, X.-Q. Yang, K.Y. Chung, Comparative studies on C-coated and uncoated LiFePO4 cycling at various rates and temperatures using synchrotron based in situ X-ray diffraction, Electrochimica. Acta 56 (2011) 1182-1189. [36] G. Chen, T. J. Richardson, Solid Solution Phases in the Olivine-Type LiMnPO4/MnPO4 System, J. Electrochem. Soc. 156 (2009) A756-A762. [37] J. Yoon, S. Muhammad, D. Jang, N. Sivakumar, J. Kim, W. H. Jang, Y. S. Lee, Y. U. Park, K. Kang, W. S. Yoon, Study on structure and electrochemical properties of carbon-coated monoclinic Li3V2(PO4)3 using synchrotron based in situ X-ray diffraction and absorption, J. Alloys compd. 569 (2013) 76-81. [38] M. Morcrette, J. Leriche, S. Patoux, C. Wurm, C. Masquelier, In Situ X-Ray Diffraction during Lithium Extraction from Rhombohedral and Monoclinic Li 3V2 ( PO4)3, Electrochem solid-state lett. 6 (2003) A80-A84. 20

ACCEPTED MANUSCRIPT [39] C. Nan, J. Lu, L. Li, L. Li, Q. Peng, Y. Li, Size and shape control of LiFePO 4 nanocrystals for better lithium ion battery cathode materials, Nano Research 6 (2013) 469-477. [40] L. Castro, R. Dedryvère, J.-B. Ledeuil, J. Bréger, C. Tessier, D. Gonbeau, Aging mechanisms of LiFePO4//graphite cells studied by XPS: redox reaction and electrode/electrolyte interfaces, J. Electrochem. Soc. 159 (2012) A357-A363. [41] Q. Ni, Y. Bai, Z. Yang, Y. Li, G. Chen, L. Ling, H. Ren, S. Chen, F. Wu, C. Wu, Wet-chemical coordination synthesized Li3V2(PO4)3/C for Li-ion battery cathodes, J. Alloys Compd. 729 (2017) 49-56. [42] K. Saravanan, C.W. Mason, A. Rudola, K.H. Wong, P. Balaya, The first report on excellent cycling

PT

stability and superior rate capability of Na3V2(PO4)3 for sodium ion batteries, Adv. Energy Mater. 3 (2013) 444-450.

RI

[43] M.C. Biesinger, L.W. Lau, A.R. Gerson, R.S.C. Smart, Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Sc, Ti, V, Cu and Zn, Appl. Surf. Sci.

SC

257 (2010) 887-898.

[44] J.-W. Lee, M.-S. Park, B. Anass, J.-H. Park, M.-S. Paik, S.-G. Doo, Electrochemical lithiation and delithiation of LiMnPO4: Effect of cation substitution, Electrochimica. Acta 55 (2010) 4162-4169.

NU

[45] M.C. Biesinger, B.P. Payne, A.P. Grosvenor, L.W. Lau, A.R. Gerson, R.S.C. Smart, Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Cr, Mn, Fe, Co and Ni, Appl. Surf. Sci. 257 (2011) 2717-2730.

MA

[46] C. Li, N. Hua, C. Wang, X. Kang, T. Wumair, Y. Han, Effect of Mn2+-doping in LiFePO4 and the low temperature electrochemical performances, J. Alloys Compd. 509 (2011) 1897-1900. [47] W. Cheng, L. Wang, Z. Sun, Z. Wang, Q. Zhang, D. Lv, W. Ren, L. Bian, J. Xu, A. Chang, Preparation and characterization of LiFePO4·xLi3V2(PO4)3 composites by two-step solid-state

D

reaction method for lithium-ion batteries, Mater. Lett. 198 (2017) 172-175. [48] P. Yang, E. Han, L. Zhu, Y. Han, X. Du, L. Li, L. Dou, T. Li, C. Feng, Synthesis and

PT E

electrochemical performance characterization of xLi3V2(PO4)3·yLiFe0.8Mn0.2PO4/C cathode materials for lithium-ion batteries, Ionics 24 (2018) 2945-1955. [49] X. Rui, Q. Yan, M. Skyllas-Kazacos, T. M. Lim, Li3V2(PO4)3 cathode materials for lithium-ion batteries: A review, J. Power Sources 258 (2014) 19-38.

CE

[50]A. R. Cho, J.N. Son, V. Aravindan, H. Kim, K.S. Kang, W.S. Yoon, W.S. Kim, Y.S. Lee, Carbon supported, Al doped-Li3V2(PO4)3 as a high rate cathode material for lithium-ion batteries, J. Mater. Chem. 22( 2012), 6556-6560.

AC

[51] J. S. Park, J. S. Kim, W. B. Park, Y. K. Sun, S. T. Myung, Effect of Mn in Li3V2–xMnx(PO4)3 as high capacity cathodes for lithium batteries, ACS Appl. Mater. Interfaces 9 (2017) 40307-40316.

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Graphical abstract

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ACCEPTED MANUSCRIPT Highlights 5LiFe0.9Mn0.1PO4∙4Li3V2(PO4)3/C composites were prepared via a two-step method. 5LiFe0.9Mn0.1PO4∙4Li3V2(PO4)3/C cathode has excellent electrochemical properties.

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Possible Li+ ions extraction/insertion mechanism of composites was investigated.

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