J Solid State Electrochem DOI 10.1007/s10008-017-3757-2

ORIGINAL PAPER

Improving the electrochemical properties of lithium iron(II) phosphate through surface modification with manganese ion(II) and reduced graphene oxide Yushuang Gao 1 & Lifeng Zhang 1 & Shuo Feng 2 & Wenzhuo Shen 2 & Shouwu Guo 1,2

Received: 3 July 2017 / Revised: 12 August 2017 / Accepted: 27 August 2017 # Springer-Verlag GmbH Germany 2017

Abstract Lithium iron(II) phosphate (LiFePO4) particles were simultaneously modified via reduced graphene oxide (rGO) and manganese ion(II) (Mn2+) through a facile onestep method. X-ray photoelectron spectroscopy unravels that the formation of Mn-O bond originated from Mn2+ ion and fringe oxygen atoms of LiFePO4, which is beneficial for the rate capability of cathode. As cathode for lithium-ion battery, the as-prepared rGO/Mn-LiFePO4 composite exhibits excellent electrochemical properties. Its discharge-specific capacity is 159 mAh g−1 at 1 C after 800 cycles with capacity retention of 92%. Even at a high rate of 10 C, the rGO/Mn-LiFePO4 composite is still capable of delivering 140 mAh g−1 of discharge-specific capacity, indicating its excellent rate capability and cycle stability. It was demonstrated that the simultaneous modification of Mn2+ and rGO does not destroy the olivine structure of LiFePO4, but it can stabilize the crystal structure, decrease the electrode polarization, enhance the electronic conductivity and Li+ diffusion coefficient, and thus improve its cycling and high-rate capability.

Keywords Lithium iron(II) phosphate . Reduced graphene oxide . Manganese ion . Electrochemical performance

* Wenzhuo Shen [email protected] * Shouwu Guo [email protected] 1

College of Materials Science and Engineering, Shaanxi University of Science and Technology, Xi’an 710021, Shaanxi, People’s Republic of China

2

Department of Electronic Engineering, School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China

Introduction The olivine-structured lithium iron(II) phosphate (LiFePO4) has been widely used as cathode materials for lithium-ion batteries due to its superior thermal safety, high reversibility, and acceptable operating voltage (3.45 V vs. Li/Li+) [1–3]. However, the low electronic conductivity (10 − 9 to 10−10 S cm−1) [4] and lithium-ion (Li+) diffusion coefficient (10−14 cm2 s−1) [5] seriously limit its practical applications in power batteries used for electric vehicles (EVs), plug-in hybrid electric vehicles (PHEVs), and hybrid electric vehicles (HEVs) [6]. To tackle these issues, many efforts have been devoted to compositing the LiFePO4 with various conductive carbon nanomaterials, such as graphite [7, 8], acetylene black [7–9], nanotubes [7, 10, 11], etc. Among them, graphene or reduced graphene oxide (rGO) has been considered as the ideal conductive carbon nanomaterials due to the excellent electronic conductivity and unique characteristics arising from the special two-dimensional atomic layered sheet structure [12, 13]. Hu et al. reported that the specific capacity of a graphenemodified LiFePO4 cathode is 208 mAh g−1, which is higher than the theoretical capacity of LiFePO4 (170 mAh g−1). They explained that the excess capacity is because of the graphene, although the detailed lithium storage mechanism is still unclear [14]. Recently, Huang et al. fabricated a binder/additive free composite electrode of LiFePO4/rGO using electrophoresis. At high rate of 2 C, the discharge capacity of 109.7 mAh g−1 is observed with little decay after 400 cycles [15]. Altough the capacity and cycling stability of LiFePO4 cathode have been identified to be significantly enhanced by the modification of graphene or rGO, no exceptional rate performance has been reported for LiFePO4/C cathode [16–19]. As mentioned by Ali Eftekhari, only carbon modification indeed improves the overall electrical conductivity of the

J Solid State Electrochem

LiFePO4 electrode, but it still cannot be a rate-determining process [20]. Scientists once added Au or Ag into the LiFePO4/C composite, resulting in enhanced rate performance for lithium-ion batteries [21, 22]. However, the increased cost evidently limits its practical applications. To searching for more inexpensive additive agent, Mg2+ has recently been reported to add into the LiFePO4/rGO composite, delivering an enhanced capacity of ca. 104 mAh g−1 at 10 C [23]. In addition, adding Ni2+ and Mn2+ into LiFePO4/C cathode has also brought significantly improved rate performance. Regrettably, the achievement of these results is at the cost of sacrificing cycling stability of electrodes, and few reports delivered the cycling numbers over 100 times. Moreover, the prepared procedures in previous reports are usually complicated [24–26]. In this work, we show that introducing Mn2+ and rGO to LiFePO4 together via simple and low-cost one-step method leads to largely improved rate performance of the cathode, while adding Mn2+ or rGO alone results in little or moderate performance improvement of the same cathode, respectively. It was demonstrated that the simultaneous modification of Mn2+ and rGO does not destroy the olivine structure of LiFePO4, but it can stabilize the crystal structure, decrease charge transfer resistance, enhance Li+ diffusion coefficient, and thus improve its cycling and high-rate capability of the LiFePO4 cathode.

Experimental Preparation of rGO/Mn-LiFePO4 The raw LiFePO4 powder used in the work was provided by General Lithium Corporation (Jiangsu, China). GO was prepared from graphite powder through a modified Hummers method that we reported previously [17, 27]. The rGO/MnLiFePO4 composite was prepared, in a typical experiment, by mixing LiFePO 4 and GO in deionized water, and then, MnSO4 and NaH2PO2 as reducers were added. The concentrations of MnSO4 and NaH2PO2 were adjusted to 0.06 and 0.1 mol L−1, and the mixture was magnetically stirred for Fig. 1 XRD patterns (a) and Raman spectra (b) of the samples

15 min at room temperature. After the reaction, the solid product was separated through filtration, washed thoroughly with deionized water, and finally dried in vacuum oven at 110 °C.

Morphology and structure characterization The morphologies of rGO/Mn-LiFePO4 composite were characterized using Ultra 55 field emission scanning electron microscope (FE-SEM) (Zeiss, Germany). The crystal structures of samples were examined using an X-ray diffraction (XRD) (Bruker, Germany). AXIS Ultra DLD X-ray photoelectron spectroscope (XPS) (Kratos Analytical, UK) was used to explore the composition of the LiFePO4 composite. Raman spectra were conducted on an Invia/Reflrx Lasser microRaman spectroscope (Renishaw, England) with an excitation laser beam wavelength of 514 nm. Determination of detailed element contents was also investigated by inductively coupled plasma optical emission spectrometer (ICP) (Thermo, USA).

Electrochemical measurements Electrochemical measurements were carried out between 2.5 and 4.2 V vs. Li+/Li on CR2025 coin cells. The cathodes for the coin cells were composed of 80 wt% as-prepared rGO/ Mn-LiFePO4 composite, 10 wt% polyvinylidene fluoride (PVDF) as a binder, and 10 wt% acetylene black as conductive agent. The metallic lithium foil was employed as anode which was separated from the cathode by polypropylene membrane. The electrolyte was 1 mol L−1 LiPF6 in a 4:3:3 mixture solvent of ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate (EC/EMC/DEC). The galvanostatic charge/discharge curves and cycle performance were recorded on the LAND CT2001A cell battery system (Wuhan, China) with current densities ranging from 0.2 to 10 C at room temperature. The potential static step charge measurement, the cyclic voltammetry (CV), and the electrochemical impedance spectroscopy (EIS) were carried out on Merohm electrochemical working station (PGSTAT 302N, Switzerland).

J Solid State Electrochem Table 1 Element contents of LiFePO 4 and rGO/Mn-LiFePO 4 determined by ICP

LiFePO4 rGO/Mn-LiFePO4

Fe (%)

Li (%)

Mn (%)

P (%)

32.20 32.54

3.90 3.82

0.00 0.22

18.10 18.02

Results and discussion XRD patterns of the LiFePO4, rGO/LiFePO4, Mn-LiFePO4, and rGO/Mn-LiFePO4 composites are shown in Fig. 1a. All of the diffraction peaks in these XRD patterns match well with the standard orthorhombic LiFePO4 (JCPDS No. 40-1499), revealing that the crystalline status of LiFePO4 is preserved well during the formation of the composites. The chemical composition of rGO/Mn-LiFePO4 was determined by ICP and compared with that of LiFePO4 in Table 1. The content of Mn in the composite is about 0.22% in mass. The existence of rGO was confirmed by the Raman spectroscopy analysis, and the results are depicted in Fig. 1b. The bands at 500–100 and 1120–520 cm−1 correspond to the Raman vibrations of Fe-O and PO43− in LiFePO4 [28]. There are two broad peaks appeared at around 1350 and 1600 cm−1, which can be assigned to the D and G band of rGO, respectively [29]. The intensity ratio of D/G bands of the composite (1.11) was higher than that of the raw GO (0.83), showing that the new domains of conjugated carbon atoms (bonded in sp2 hybridization) were formed accompanying the removal of the oxygen-containing groups of GO. These results demonstrate

Fig. 2 FE-SEM images of a LiFePO4, b rGO/Mn-LiFePO4. TEM (c) and HRTEM (d) images of rGO/Mn-LiFePO4

that the GO is reduced to the rGO by NaH2PO2, which should be beneficial to the electrical conductivity improvement of the rGO/Mn-LiFePO4 composite. Figure 2 shows the FE-SEM images of LiFePO4 and rGO/ Mn-LiFePO4 composites and the TEM images of rGO/MnLiFePO4 composite. As shown in Fig. 2a, the raw LiFePO4 microspheres prepared by spray drying consisted of olivinestructured LiFePO4 nanoparticles. Figure 2b presents that the surface of rGO/Mn-LiFePO4 composite is coated by a thin rGO layer with the crinkled edge. As illustrated in Fig. 2c, d, the TEM and HRTEM images show the detailed microstructure of rGO/Mn-LiFePO4 composite. The TEM image in Fig. 2c further confirms that the rGO wraps on the surface of rGO/ Mn-LiFePO4 composite. The regular lattice fringes with the spacing of 1.84 Å correspond to the (001) plane of orthorhombic LiFePO4 (Fig. 2d). The rGO surface coating could promote the surface charge transfer and lithium-ion diffusion. XPS was carried out to further investigate the surface composition and the chemical binding status of the rGO/MnLiFePO4 composite. As shown in Fig. 3a, the wide spectrum of rGO/Mn-LiFePO4 composite contains the elements of C, O, Mn, Li, Fe, and P. Figure 3b presents the high-resolution spectrum of Mn 2p; the peaks detected at ~ 653 and ~ 641 eV correspond to the Mn 2p1/2 and 2p3/2 of Mn2+ [30, 31]. The satellite peak at ∼647 eV is the characteristic peak of bivalent Mn [32, 33]. The C 1s peak (Fig. 3c) can be deconvoluted into C=C (284.6 eV), C-O (286.3 eV), and C=O (287.6 eV) [34]. The O 1s peak located at 531.6 eV corresponds to the lattice oxygen of LiFePO4 [35]. The fitting O 1s peaks located at 533.4 and 532.5 (Fig. 3d) are ascribed to C-O and C=O bands,

J Solid State Electrochem Fig. 3 XPS spectra of rGO/MnLiFePO4. a Wide XPS spectrum. b Mn 2p spectrum. c C 1s spectrum. d O 1s spectrum

respectively [36]. The peak at around 531.1 eV is related to the Mn-O band, which is attributed to the binding between Mn and fringe O of LiFePO4 [37]. In order to evaluate the rate performance of as-synthesized composites, the coin cells with the raw LiFePO4 particles, asprepared rGO/LiFePO4, Mn-LiFePO4, and rGO/Mn-LiFePO4 composites as cathodes were assembled. The electrochemical tests were carried out at various rates from 0.2 to 10 C at room temperature. As exhibited in Fig. 4a, the raw LiFePO4 cathode shows significantly low specific capacity since the rate increases to 5 C. By contrast, the rate performance of treated LiFePO4 composites is enhanced evidently. In particular, rGO/Mn-LiFePO4 cathode shows average discharge-specific capacities of 140 mAh g−1 at 10 C, while LiFePO4, rGO/ LiFePO4, and Mn-LiFePO4 cathodes deliver approximately 0, 5, and 18 mAh g−1 at the same rate. More importantly, the capacity of rGO/Mn-LiFePO4 composite is recovered to around 170 mAh g−1 when the current density is abruptly Fig. 4 The rate performance (a) and the cycle performance at 1 C (b) of the raw LiFePO4, rGO/ LiFePO4, Mn-LiFePO4, and rGO/ Mn-LiFePO4 composites as cathodes

adjusted from 10 to 0.2 C, demonstrating the excellent rate performance and high reversibility. The enhanced rate performance of the rGO/Mn-LiFePO4 composite is attributed to the simultaneous modification of rGO and Mn2+, which both contribute to the improvement of the electronic conductivity and Li+ diffusion coefficient of the LiFePO4 particles. Moreover, Mn2+ facilitates the intimate integration of rGO on the surface of LiFePO4 particles, stabilizing the crystal structure and enhancing the capacity of rGO/Mn-LiFePO4 composite especially at high rate. Figure 4b presents the cycle performance of LiFePO4, rGO/LiFePO4, Mn-LiFePO4, and rGO/MnLiFePO4 cathodes at the rate of 1 C for 800 cycles. The specific capacity of the raw LiFePO4 cathode shows considerable falloff during repetitive cycles. Nevertheless, the treated LiFePO4 composite cathodes display the superior cycle performance. It is noteworthy that the rGO/Mn-LiFePO4 cathode exhibits a specific capacity of 159 mAh g−1 with the capacity retention of 92% after 800 cycles.

J Solid State Electrochem Fig. 5 The galvanostatic charge/ discharge profiles of a the raw LiFePO4, b rGO/LiFePO4, c MnLiFePO4, and d rGO/MnLiFePO4 at different rates

The voltage profiles of LiFePO4 particles, rGO/LiFePO4, Mn-LiFePO4, and rGO/Mn-LiFePO4 composites are exposed in Fig. 5. The prepared cells were tested in the voltage range of 2.5–4.2 V at various C rates. As is expected, the raw LiFePO4

and the treated LiFePO4 composites all present a smooth and steady voltage plateaus at around 3.4 V, which is related to redox reaction of Fe2+/Fe3+. It is observed that the interval between the charge and discharge voltage plateaus gradually enlarges with the increase of the C rate. At high C rate, the plateaus almost disappear for raw LiFePO4, rGO/LiFePO4, and Mn-LiFePO4. By contrast, the plateaus of rGO/MnLiFePO4 are steady even at 10 C. This result indicates that the electrochemical polarization of LiFePO4 at high C rate is efficiently reduced via the simultaneous surface treatment of rGO and Mn2+, thus improving the charge-discharge reversibility and cycle stability. The AC impedance analysis (Nyquist plots) of the raw LiFePO 4 , rGO/LiFePO 4 , Mn-LiFePO 4 , and rGO/MnLiFePO4 composites is drawn in Fig. 6a. The experiment was carried out by applying a perturbation voltage of 5 mV in a frequency range of 100 mHz to 100 kHz. In general, the Nyquist plot comprises a compressed semicircle in the highfrequency to medium-frequency range, which is described by Table 2 Electrokinetic performances of LiFePO4, rGO/LiFePO4, MnLiFePO4, and rGO/Mn-LiFePO4 cathodes

Fig. 6 a The Nyquist plots of the raw LiFePO4, rGO/LiFePO4, MnLiFePO4, and rGO/Mn-LiFePO4 composites. b The equivalent circuit

LiFePO4 Mn-LiFePO4 rGO/LiFePO4 rGO/Mn-LiFePO4

Re (Ω)

Rct (Ω)

D (×10−11 cm2 s−1)

3.58 4.12 3.09 3.17

429 207 260 168

4.75 7.09 8.20 12.30

J Solid State Electrochem Fig. 7 The relationship between the cathodic current and charge time at + 500 mV potential steps (a) and CV curves at the scanning rate of 0.5 mV s−1 (b) of the raw LiFePO4, rGO/LiFePO4, Mn/ LiFePO4, and rGO/Mn-LiFePO4 cathodes

resistance owing to electrolyte (Re) and the charge transfer resistance (Rct). An approximately 45° inclined line in the low-frequency range is attributed to the Warburg impedance (Zw) related to the Li+ diffusion in the bulk [38]. As displayed in Fig. 6a, the diameters of the semicircles for all treated LiFePO4 composite cathodes are much smaller than that of the raw LiFePO4 cathode. In particular, the diameter of the semicircle of the rGO/Mn-LiFePO4 cathode is smallest corresponding to the parameters of the equivalent circuit (Fig. 6b) in Table 2. Moreover, the result of Warburg impedance (Zw) analysis also reveals that the rGO/Mn-LiFePO4 cathode has the highest and is similar with that of Rct. The above results indicate that the simultaneous modification of Mn2+ and rGO can enhance Li+ diffusion coefficient and reduce charge transfer resistance. Figure 7a shows the relationship between the cathodic current and charge time of the raw LiFePO4, rGO/LiFePO4, Mn/ LiFePO4, and rGO/Mn-LiFePO4 cathodes at + 500 mV potential steps for 1000 s. The logarithmic value of the cathodic currents (ln i) drops quickly at the beginning, while the curves tend to be flat after 600 s for all cathodes. This phenomenon indicates that the Li+ diffusion begins from the surface of the cathodes and completes after 600-s discharging. Subsequently, the electrode reaction will be controlled by Li+ diffusion inside the cathode material [16]. The certain Li+ diffusion coefficient of the cathode can be acquired by the formula [39, 40] as shown in Eq. (1). ln½iðt Þ ¼ ln


 =r ðC ∞ −C 0 Þ−πDt 4r2

2nFAD

ð1Þ

The active materials of LiFePO4, rGO/LiFePO4, Mn/ LiFePO4, and rGO/Mn-LiFePO4 cathodes should be regarded as the sphere with a radius of r. And i, t, F, A, D, C∞, and C0 in the formula is the representative of cathode current density, charge time, Faraday constant, total surface area of electrode, Li+ diffusion coefficient, surface Li+ concentration, and initial Li+ concentration in the bulk of the electrode material. Using the formula, the Li+ diffusion coefficients (D) are calculated

and presented in Table 2. The rGO/Mn-LiFePO4 composite as cathode exhibits the highest D value, indicating that the Li+ diffusion coefficient is significantly enhanced by the simultaneous modification of Mn2+ and rGO. To investigate the kinetics of the electrode processes, CVs were measured at the scanning rate of 0.5 mV s−1 between 2.5 and 4.2 V. The CV curves reveal the Fe2+/Fe3+ redox peaks during the charge-discharge reactions, which are presented in Fig. 7b. The rGO/Mn-LiFePO4 electrode has higher anodic (charge) and cathodic (discharge) peak current compared to those of LiFePO 4 , rGO/LiFePO 4 , and Mn-LiFePO 4 . Meanwhile, the potential interval between the redox peaks for rGO/Mn-LiFePO4 cathode is obviously smaller than that for other cathodes, reflecting its small polarization, high Liion diffusion rate, and low inner resistance [6]. This result is in accordance with the results of EIS and static potential step measurement.

Conclusion We found that introducing Mn2+ and rGO to LiFePO4 together via a facile one-step method leads to largely improved rate performance of the cathode, while adding Mn2+ or rGO alone results in little or moderate performance improvement of the same cathode, respectively. The rGO/Mn-LiFePO4 composite delivers 159 mAh g−1 of discharge-specific capacity at 1 C after 800 cycles with capacity retention of 92%. Furthermore, the composite is capable of delivering 140 mAh g−1 of discharge-specific capacity at a high rate of 10 C. The high reversible capacity, excellent rate capability, and cycle stability of the rGO/Mn-LiFePO4 composite are attributed to the enhanced electronic conductivity and Li+ diffusion coefficient through the simultaneous modification of rGO and Mn2+. The simple fabrication process provides a simple and effective mean to improve the rate performance of the LiFePO 4 cathode. Acknowledgements This work was supported by 973 Special Preliminary Study Plan (Nos. 2014CB260411 and 2015CB931801), the

J Solid State Electrochem National Science Foundation of China (No. 11374205), Scientific Research Fund of San-Qin Scholar (BJ11-26) and Scientific Research Fund of Shaanxi University of Science & Technology (XSG(4)006), National Natural Science Foundations of China (No. 21203116), and China Postdoctoral Science Foundation-funded project (No. 2014M562514XB).

References 1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12. 13.

14.

15.

16.

17.

Padhi AK, Nanjundaswamy KS, Goodenough JB (1997) Phosphoolivines as positive-electrode materials for rechargeable lithium batteries. J Electrochem Soc 144(4):1188–1194 Wang JJ, Sun XL (2015) Olivine LiFePO4: the remaining challenges for future energy storage. Energ. Environ Sci 8(4):1110– 1138 Giorgetti M, Berrettoni M, Scaccia S, Passerini S (2006) Characterization of sol-gel-synthesized LiFePO4 by multiple scattering XAFS. Inorg Chem 45(6):2750–2757 Chung SY, Chiang YM (2003) Microscale measurements of the electrical conductivity of doped LiFePO4. Electrochem SolidState Lett 6(12):278–281 Prosini PP, Lisi M, Zane D, Pasquali M (2002) Determination of the chemical diffusion coefficient of lithium in LiFePO4. Solid State Ionics 148(1):45–51 Lei XL, Zhang HY, Chen YM, Wang WG, Ye YP, Zheng CC, Deng P, Shi ZC (2015) A three-dimensional LiFePO4/carbon nanotubes/ graphene composite as a cathode material for lithium-ion batteries with superior high-rate performance. J Alloys and Compds 626: 280–286 Zhao D, Feng YL, Wang YG, Xia YY (2013) Electrochemical performance comparison of LiFePO4 supported by various carbon materials. Electrochim Acta 88(2):632–638 Shin HC, Cho WI, Jang H (2006) Electrochemical properties of carbon-coated LiFePO4 cathode using graphite, carbon black, and acetylene black. Electrochim Acta 52(4):1472–1476 Chen TK, Qi X, Chen CY, Chang CC, Chang WC (2013) Maleicanhydride-grafted ketjen black as the alternative carbon additive for LiFePO4 cathode. Electrochim Acta 107(10):503–508 Zhang H, Chen Y, Zheng C, Zhang D, He C (2015) Enhancement of the electrochemical performance of LiFePO4/carbon nanotubes composite electrode for Li-ion batteries. Ion 21(7):1813–1818 Qin G, Wu Q, Zhao J, Ma Q, Wang C (2014) C/LiFePO4/multiwalled carbon nanotube cathode material with enhanced electrochemical performance for lithium-ion batteries. J Power Sources 248:588–595 Geim AK (2009) Graphene: status and prospects. Science 324(5934):1530–1534 Stankovich S, Dikin DA, Dommett GH, Kohlhaas KM, Zimney EJ, Stach EA, Piner RD, Nguyen ST, Ruoff SS (2006) Graphene-based composite materials. Nature 442(7100):282–286 Hu LH, Wu FY, Lin CT, Khlobystov AN, Li JY (2013) Graphenemodified LiFePO4 cathode for lithium ion battery beyond theoretical capacity. Nat Comms 4(2):1687–1694 Huang Y, Liu H, YC L, Hou YL, Li Q (2015) Electrophoretic lithium iron phosphate/reduced graphene oxide composite for lithium ion battery cathode application. J Power Sources 284:236–244 Croce F, Epifanio AD, Hassoun J, Deptula A, Olczacb T, Scrosati B (2002) A novel concept for the synthesis of an improved LiFePO4 lithium battery cathode. Electrochem Solid-State Lett 5(3):47–50 Shen WZ, Wang YM, Yan J, HX W, Guo SW (2015) Enhanced electrochemical performance of lithium iron(II) phosphate modified cooperatively via chemically reduced graphene oxide and polyaniline. Electrochim Acta 173:310–315

18.

Yang CC, Jang JH, Jiang JR (2016) Study of electrochemical performances of lithium titanium oxide-coated LiFePO4/C cathode composite at low and high temperatures. Appl Energy 162:1419– 1427 19. RF W, Xia GF, Shen SY, Zhu FJ, Jiang FJ, Zhang JL (2015) In-situ growth of LiFePO 4 nanocrystals on interconnected carbon nanotubes/mesoporous carbon nanosheets for high-performance lithium ion batteries. Electrochim Acta 153:334–342 20. Eftekhari A (2017) LiFePO4/C nanocomposites for lithium-ion batteries. J Power Sources 343:395–411 21. Morales J, Trócoli R, Rodríguez-Castellón E, Franger S, SantosPeña J (2009) Effect of C and Au additives produced by simple coaters on the surface and the electrochemical properties of nanosized LiFePO4. J Electroanal Chem 631(1):29–35 22. Shao D, Wang J, Dong X, Yu W, Liu G (2013) Coaxial electrospinning fabrication and electrochemical properties of LiFePO4/C/Ag composite hollow nanofibers. J Mater Sci-Mater El 24(12):4718–4724 23. Huang Y, Liu H, Gong L, Hou Y, Li Q (2017) A simple route to improve rate performance of LiFePO4/reduced graphene oxide composite cathode by adding Mg2+ via mechanical mixing. J Power Sources 347:29–36 24. Mi YY, Yang CK, Zuo ZC, Qi LY, Tang CX, Zhang WD, Zhou HH (2015) Positive effect of minor manganese doping on the electrochemical performance of LiFePO4/C under extreme conditions. Electrochim Acta 176:642–648 25. Yuan H, Wang XY, Wu Q, Shu HB, Yang XK (2016) Effects of Ni and Mn doping on physicochemical and electrochemical performances of LiFePO4/C. J Alloy Compd 675:187–194 26. Shu HB, Wang XY, Wu Q, BA H, Yang XK, Wei QL, Liang QQ, Bai YS, Zhou M, Wu C, Chen MF, Wang AW, Jiang LL (2013) Improved electrochemical performance of LiFePO4/C cathode via Ni and Mn co-doping for lithium-ion batteries. J Power Sources 237:149–155 27. Zhang JL, Yang HJ, Shen GX, Cheng P, Zhang JY, Guo SW (2010) Reduction of graphene oxide via L-ascorbic acid. Chem Commun 46(7):1112–1114 28. Shi Y, Chou SL, Wang JZ, Wexler D, Li HJ, Liu HK, Wu YP (2012) Graphene wrapped LiFePO4/C composites as cathode materials for Li-ion batteries with enhanced rate capability. J Mater Chem 22: 16465–16470 29. Fan Q, Lei LX, XY X, Yin G, Sun YM (2014) Direct growth of FePO4/graphene and LiFePO4/graphene hybrids for high rate Liion batteries. J Power Sources 257:65–69 30. Kou LQ, Chen FJ, Tao F, Dong Y, Chen L (2015) High rate capability and cycle performance of Ce-doped LiMnPO4/C via an efficient solvothermal synthesis in water/diethylene glycol system. Electrochim Acta 173:721–727 31. Huang QY, Wu Z, Su J, Long YF, Lv XY, Wen YX (2016) Synthesis and electrochemical performance of Ti-Fe co-doped LiMnPO4/C as cathode material for lithium-ion batteries. Ceram Int 42(9):11348–11354 32. Zhao H, Lin M, Fang K, Zhou J, Sun YH (2016) Preparation and evaluation of Cu-Mn/Ca-Zr catalyst for methyl formate synthesis from syngas. Appl Catal A. G E N 514:276–283 33. Liu LF, SS W, Dong YB, Lü SL (2016) Effects of alloyed Mn on oxidation behaviour of a Co-Ni-Cr-Fe alloy between 1050 and 1250 °C. Corros Sci 104:236–247 34. LH H, FY W, Lin CT, Khlobystov AN, Li LJ (2013) Graphenemodified LiFePO4 cathode for lithium ion battery beyond theoretical capacity. Nat Commun 4(2):1687 35. Ramana CV, AitSalah A, Utsunomiya S, Morhange JF, Mauger A, Gendron F, Julien CM (2006) Spectroscopic and chemical imaging analysis of lithium iron triphosphate. J Phys Chem C 111(2):226– 233

J Solid State Electrochem 36.

37.

Zhao CX, Zhang Y, Deng SZ, Xu NS, Chen J (2016) Surface nitrogen functionality for the enhanced field emission of freestanding few-layer graphene nanowalls. J Alloys Compds 672: 433–439 Tan BJ, Klabunde KJ, Sherwood PMA (1991) XPS studies of solvated metal atom dispersed catalysts: evidence for layered cobaltmanganese particles on alumina and silica. J Am Chem Soc 113: 855–861

38.

Lim J, Gim J, Song J, Nguyen DT, Kim SJ, Jo J, Mathew V, Kim J (2016) Direct formation of LiFePO 4/graphene composite via microwave-assisted polyol process. J Power Sources 304:354–359 39. Miao S, Gao S, Shu J, Zheng WD, Xu D, Chen LL, Feng L, Ren YL (2012) LiNi1/3Co1/3Mn1/3O2 cathode materials for LIB prepared by spray pyrolysis. II. Li+ diffusion kinetics. Ion 19(1):47–52 40. MacArthur DM (1970) The proton diffusion coefficient for the nickel hydroxide electrode. J Electrochem Soc 117:729–733