Improved cycling performance and rate stability of ITO-compounded Li2MnSiO4 for lithium-ion batteries

Li2MnSiO4 compounded with indium tin oxide (ITO) was successfully synthesized through a sol–gel method. The structure and morphology characterization of Li2MnSiO4/ITO nanocomposite are demonstrated by XRD, SEM, TEM, EDS and XPS. Galvanostatic charge–discharge tests, EIS and CV are employed to examine the electrochemical performance of the composite. From those results, it could be observed that the electrochemical performance of Li2MnSiO4 cathode material has been significantly improved due to the introducing of indium tin oxide. The 3 wt% ITO-compounded sample displayed a discharge specific capacity around 141 mA h g−1 at 0.05C, 134.4 mA h g−1 at 0.1C, 132.9 mA h g−1 at 0.2C and 127.4 mA h g−1 at 0.5C in the first cycle, which is much higher than the pristine sample.


Introduction
With the development of the economy and the wide application of portable electronic devices, energy storage equipment is extensively utilized. Owing to their high energy density, long cycle life, being naturally abundant and environment-friendly and having low self-discharge, lithium-ion batteries dominate the main position of batteries. Polyanion compounds are a series of materials with tetrahedral or octahedral polyanion structure units (XO m ) nÀ (X ¼ Si, P or As). Due to the strong X-O covalent bond in the middle of polyanion, this kind of cathode material can withstand over-charge or over-discharge and possess higher thermal stability than conventional cathode materials. 1,2 Therefore, the lithium orthosilicate family, Li 2 -MSiO 4 (M ¼ Fe, Mn, Co and Ni) have drawn widespread attention.
As for Li 2 FeSiO 4 , the potential window at 4.85 V (Fe 3+ /Fe 4+ ) 3 is too high to reach, the capacity was only conned to 166 mA h g À1 . 4 In comparison, it is quite possible to fully deintercalate two lithium ions from Li 2 MnSiO 4 in principle, its theoretical specic capacity reaches 330 mA h g À1 . However, low electronic conductivity and lithium-ion diffusivity hinder its commercial application. Since Li 2 MnSiO 4 was synthesized for the rst time in 2007 by Dominko, 5 many researchers had made a great effort to enhance its electrochemical performances in the last decade.
Researches on computation and mechanism of Li 2 MnSiO 4 had been conducted by many scientists. Li 2 MnSiO 4 can be synthesized in different space groups, such as Pmnb, Pmn2 1 , Pn and P2 1 /n, and the orthorhombic Pmn2 1 was reported most popularly due to its facile. Furthermore, density functional theory (DFT) calculations performed by M. M. Kalantarian et al. 6 demonstrated that Pmn2 1 was the best polymorph for using as a cathode material in lithium-ion batteries. Dompablo et al. 7 quantify the inductive effect and calculate the linear dependence between voltage plateau of polyoxianionic compounds and the Mulliken X electronegativity.
Ion doping can improve the cycle stability due to its function of stabilizing the crystalline of Li 2  Surface modifying restrains the growth of particle size during the annealing process and protect the dissolution of Li 2 MnSiO 4 from HF in the electrolyte. As for Li 2 MnSiO 4 , many articles about coating have been published. Coating with carbon or metal oxide will help materials obtain superior cyclic performance, especially rate capability. Some metal oxide coatings are expected to neutralize the HF arise in the electrolyte. 11  Indium tin oxide (ITO, SnIn 2 O 3 ) is an n-type transparent conductive semiconductor material. 15 Because of excellent optical and electrical properties, it is widely applied in a variety of optoelectronic devices. While indium tin oxide was used as a compound material for Li 2 MnSiO 4 has not been reported up to now. In this paper, we prepared Li 2 MnSiO 4 /ITO by sol-gel method, and compared physical and electrochemical characterization with the "standard" Li 2 MnSiO 4 . The relationship between the electrochemical performance and the ITO compounding on Li 2 MnSiO 4 was explored.

Synthesis of indium tin oxide (ITO)
The indium tin oxide (ITO) was synthesized by co-precipitation method. In order to obtain correct weight percent of ITO (In 2 -O 3 : SnO 2 ¼ 9 : 1), indium(III) chloride and tin(IV) chloride are mixed in the beaker on the basis of calculation. Slight polyvinyl pyrrolidone was added to alleviate particles agglomeration. Aer dissolving completely with deionized water, NH 3 $H 2 O was dripped into the solution to adjust the pH range between 7 and 9. Magnetic stir thermostatically the solution at 60 C for 2 h, and vacuum dry it to obtain a dry precursor. Then the prematerial is calcined at 400 C for 5 h in atmosphere.

Synthesis of pristine and ITO-compounded Li 2 MnSiO 4
To prepared the pristine and the ITO-compounded Li 2 MnSiO 4 , a stoichiometric amount (the molar ratio of Li : Mn is 2 : 1) of LiCH 3 COO$2H 2 O and Mn(CH 3 COO) 2 $4H 2 O are dissolved with deionized water in weighing bottles and citric acid is used as complexant to assist the synthesis of the composite. For modied materials, ITO (wt ¼ 2, 3, 4, 5%) is added in and then ethanol is added to disperse Si(OC 2 H 5 ) 4 (TEOS). Magnetic stir thermostatically the solution at 65 C for 10 h to form gel, and vacuum dry it for 20 h to obtain the precursor. Then the prematerial is calcined at 750 C for 10 h in nitrogen atmosphere.

Structure and morphology characterization
The crystal structures of the synthesized materials were measured by D8 Advance X-ray diffractometer of Bruker. The samples were scanned from 10 to 80 at the rate of 6 min À1 with Cu Ka as the radiation source (l ¼ 0.15418 nm). The morphologies were recorded by a eld emission scanning electron microscope (Hitachi, S-520) at 2 kV and transmission electron microscope (PHILIPS, FEGCM200).

Cells assemble and electrochemical performance characterization
The cells are assembled according to the following procedure: acetylene black as conductive carbon and PVDF as a binder are mixed with active material powder at the ratio of 1 : 1 : 8 by weight. Aer stirring for 2 hours, the slurry was coated rmly on aluminum foil and then evaporated for 8 hours. The electrolyte was 1 M LiPF 6 in a mixture (1 : 1 : 1 volume ratio) of ethylene carbonate (EC), ethylmethyl carbonate (EMC), and dimethyl carbonate (DMC), meanwhile polypropylene microporous membrane was used as the separator. The CR2016 type coin cells were assembled in Ar-lled glove box. Galvanostatic charge and discharge tests were conducted on Land CT2001A battery test systems (Wuhan Jinnuo Electronics Co. Ltd., China) with the potential range between 1.5 V and 4.8 V at room temperature (293.15 K). Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) measurements were carried out using an electrochemical workstation CHI 660E (Shanghai Chenhua, China).

Result and discussion
3.1 Structure and morphology of ITO-compounded Li 2 MnSiO 4 Fig. 1a is the powder X-ray diffraction pattern of the synthesized ITO. All the characteristic peaks are indexed to In 2 O 3 (JCPDS#71-2195) without any trace of tin or other tin compounds, which means that tin atoms have doped into In 2 O 3 lattices. 16 Fig. 1b reveals the XRD patterns of pristine sample and ITO-compounded Li 2 MnSiO 4 materials. Because the differences in formation energies of those phases are very small, the pristine sample was considered to be polymorphs mixture of Pmn2 1 and Pmnb due to the characteristic Pmnb peak (301) at 30.5 . 17 While, the synthesized composites Li 2 MnSiO 4 /ITO (w ¼ 3, 4, 5%) were identied as orthorhombic crystal structure with a space group of Pmn2 1 , and the characteristic peaks are in good agreement with articles. 9,18-20 Besides, the major characteristic peaks of modied materials are sharp, showing a good crystallinity. The results indicated that ITO adding will inhibit the formation of Pmnb in Li 2 MnSiO 4 . As impurities, MnO and Mn 2 SiO 4 were allowed in some samples, which have been reported in previous literatures. 18,20 There is no evidence of diffraction peaks for ITO in the XRD patterns of Li 2 MnSiO 4 /ITO (w ¼ 3, 4%), may due to its low content. 14 However, in the XRD patterns of Li 2 MnSiO 4 /ITO (w ¼ 5%) sample, a diffraction peak at 30.58 could be assigned to the most strong peak of ITO (Fig. 1a) due to a higher content.
The X-ray photoelectron spectroscopy was used to analyze the oxidation state of main elements in samples, and the survey spectrum was presented in Fig. 2a, the obtained binding energy (BE) had been calibrated by the BE of C 1s at 284.5 eV. The binding energy of Mn 2p 3/2 (642.07 eV) and Mn 2p 1/2 (653.77 eV) correspond to the divalent state of Mn. 21 Besides, the shoulder satellite peak at 646.5 eV is a rigorous proof for Mn 2+ . 22 In the narrow scan of In, the BE of 445.27 eV and 452.77 eV belong to In 3+ . Fig. 2d showed the Sn 3d XPS spectrum, where the peaks locate at 487.27 eV and 495.67 eV corresponding to Sn 3d 5/2 and Sn 3d 3/2 respectively. The peaks position demonstrated the quadrivalent state of Sn well. 15 The sample was etched with Ar for 0 s, 5 s, 10 s and 15 s, the etched depth was 2 nm s À1 . The Height CPS and atomic ratio of elements in Li 2 MnSiO 4 /ITO with Ar etched were presented in Table 1. The Height CPS represents photoelectron intensity. Although the sample was etched for different time, the atomic ratio of metal elements has not varied in large range. The ratios of In and Sn maintain in 0.63% and 0.3% indicate that In and Sn appear not only on the surface but also in the interior of Li 2 MnSiO 4 /ITO, and indium tin oxide (ITO, SnIn 2 O 3 ) compounded with Li 2 MnSiO 4 uniformly.
The morphology of Li 2 MnSiO 4 and Li 2 MnSiO 4 /ITO (w ¼ 3%) composites were presented in Fig. 3. Fig. 3a exhibits the pristine sample composed of nanoscale particles, and Fig. 3b shows the modied material. Compared with the pristine, the surface morphology of modied sample did not change visibly aer compounding with 3 wt% ITO, however, the particle size varying from 20 nm to 50 nm was smaller than the pristine. Due to the small particle size, great surface tension and high surface free energy, agglomeration phenomenon may easily occurs. 23 Transmission electron microscopy pictures of Li 2 MnSiO 4 / ITO nanoparticles were shown in Fig. 3c and d. Due to the samples were prepared via citric acid assisted sol-gel method, a thin carbon layer can be observed in Fig. 3d. 24 The particle agglomerated slightly, lattice fringe can be seen from HR-TEM   Fig. 5 shows the charge/discharge proles of LMS/ITO. Capacity of modied-LMS has promoted signicantly compared with the pristine. It could be seen from Fig. 5a that the material compounded with 3 wt% ITO displays the best electrochemical performance. The Li 2 MnSiO 4 /ITO (w ¼ 3%) sample delivered a discharge specic capacity around 141 mA h g À1 at 0.05C in the rst cycle which is much higher than that of LMS (68.1 mA h g À1 ). It may be attributed to that ITO stabilize the structure of material and suppresses the amorphization of Li 2 -MnSiO 4 during the cycle. And the degradation in discharge capacity during cycles has been claimed to be due to the mixed valence state of Mn and the Jahn-Teller effect in LMS materials. 25 While the decreased capacities of 4 wt% and 5 wt% samples reect that there is an appropriate amount for compounding and excess amount of ITO will result in negative effect on the performance of the Li 2 MnSiO 4 /ITO. Excessive ITO may hinder the transportation of Li + , hence a large number of experiments were carried to pick out the optimal quantity. ITO-introducing not only raises the specic capacity but also enhances the stability of Li 2 MnSiO 4 . Even at the rate of 0.1C, 0.2C and 0.5C in Fig. 5b, the initial discharge capacity reach  134.4 mA h g À1 , 132.9 mA h g À1 , 127.4 mA h g À1 separately. Compared with some reported results, 12,14 the synthetic material in this research also obtain high rate discharge ability. For comparison, Fig. 5c and d present the initial cycle curves at 0.05C and 0.1C of pristine and 3 wt% ITO-modied samples. The discharge capacities of modied samples are obviously higher than those of pristine sample. In Fig. 5e, the LMS/ITO cathode shows better rate stability than the LMS cathode. It comes to a conclusion that the better electrochemical performance of LMS/ITO can be attributed to indium tin oxide.
The cyclic voltammograms (CV) were measured at a scan rate of 0.1 mV s À1 ranging in potential windows from 1.5 V to 4.8 V.
As it is depicted in Fig. 6, both LMS and LMS/ITO have different initial cycle with the following two CV curves, which illustrates that the structure of LMS and LMS/ITO may change to an amorphous state on the rst charge-discharge process and a little part of LMS crystallites had decomposed. 26,27 In the following cycles, LMS and LMS/ITO showed highly overlap curves, which mean better reversibility. In Fig. 6a, there is an oxidation peak around 4.24 V, corresponding to the extraction of lithium ion from Li 2 MnSiO 4 cathode and a reduction peak at 2.81 V refers to the insertion of lithium ion. 27 In Fig. 6b, the oxidation/reduction peaks at 4.18 V/3.91 V and 3.46 V/2.84 V respectively are ascribed to the Mn 3+ /Mn 4+ and Mn 2+ /Mn 3+ redox processes. 28 Compared with the pristine, the oxidation peaks of LMS/ITO shied to low potential and the reduction peaks moved higher and the oxidation/reduction current peaks of the LMS/ITO are larger. All above data illustrate that LMS/ITO has higher electrochemical activity than LMS, and the electrochemical reactions in LMS/ITO is more easily to occur. As for peaks area, LMS/ITO is larger than LMS, which reduces the polarization of electrode. Fig. 7 exhibits the electrochemical impedance spectrum (EIS) of the samples, which is benecial to analyze the kinetic properties. The equivalent circuit model, R e and R ct are calculated by Zview soware. The diffusion coefficient is calculated as the following equations. 29 where R is the gas constant (8.314 J mol À1 K À1 ), T is the room temperature (298.15 K), A is the surface area of the electrode, F is the Faraday's constant (96 485 C mol À1 ), C is the molar concentration of Li + in materials and s is Warburg factor, the slope of Z w vs. u À1/2 and u ¼ 2pf. R e is the ohmic resistance, representing the resistance of the electrolyte. R ct is the charge transfer resistance of electrochemical reaction. The straight line at the low frequency corresponds to the Warburg impedance which is attributed to the diffusion of the Li-ion into the bulk of the electrode material. Table 2 lists the calculation for both samples. Compared with the uncompounded material (306.9 U and 7.84 Â 10 À17 cm 2 s À1 ), the modied sample with smaller R ct (93.67 U) and larger D Li + (3.67 Â 10 À16 cm 2 s À1 ) has more efficient electron conductivity and lithium ion insertion/extraction.
For root reason, as a dopant a modicum of Sn caused defects in the microstructure and then change the grain boundaries of ITO. 30 The carrier concentration of the sample increases and the Hall mobility decreases with the ITO compounded, which lead to smaller resistivity. 16 It indicates that the inclusion of ITO in the Li 2 MnSiO 4 increase the Li-ion diffusion kinetics and offer lower charge transfer resistance, all of these are consistent with the charge/discharge performance of Li 2 MnSiO 4 /ITO.

Conclusion
Li 2 MnSiO 4 /ITO nanocomposites were successfully prepared via sol-gel method. The material compounded with 3 wt% ITO showed the best electrochemical performance. The initial discharge capacity achieved 141 mA h g À1 , 134.4 mA h g À1 , 132.9 mA h g À1 and 127.4 mA h g À1 separately at 0.05C, 0.1C, 0.2C and 0.5C. Compounding with ITO is an effective method to reduce the resistance and enhance the inherent conductivity, which lead to superior electrochemical performance.

Conflicts of interest
There are no conicts to declare.