LiMnPO4/graphene nanocomposites with high electrochemical performance for lithium-ion batteries

ZHAO Bing, WANG Zhixuan, CHEN Lu, YANG Yaqing, CHEN Fang, GAO Yang, JIANG Yong

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LiMnPO4/graphene nanocomposites with high electrochemical performance for lithium-ion batteries

ZHAO Bing, WANG Zhixuan, CHEN Lu, YANG Yaqing, CHEN Fang, GAO Yang, JIANG Yong

(Institute of Environmental and Chemical Engineering, Shanghai University, Shanghai 200444, China)

A high performance LiMnPO4/graphene nanocomposite as cathode material for lithium-ion batteries was preparedsurface modification of 3-aminopropyltrimethoxysilane (APS) on LiMnPO4nanoparticles and electrostatic self-assembly of positively charged APS-LiMnPO4nanoparticles and negatively charged graphene oxide. Successful APS modification on LiMnPO4was demonstrated by the existence of 3-aminopropyl and SiOC groups in FTIR spectra. LiMnPO4nanoparticles (. 25 nm) were found uniformly distributed on the surface of graphene sheets. The intimate contact of LiMnPO4nanoparticles with graphene conductive network allows achieving fast electron transfer between the active material and charge collector and accommodating volume expansion/contraction of LiMnPO4nanoparticles during electric discharge/charge process. The nanocomposite cathode material could deliver an initial capacity of 142.2 mA·h·g-1at 0.05 C and maintain 90.5% capacity after 50 cycles, which were significant better than no APS-modified counterpart.

lithium manganese phosphate; aminopropyltrimethoxysilane modification; nanomaterials; composites; electrochemistry

Introduction

Interest in lithium-ion batteries is driven by increased market demand for portable electronics, transportation and energy storage. Research efforts have been focused on developing new cathode materials to replace current LiCoO2, which constitutes nearly half the cost of a Li-ion cell.

Since the pioneer work of Goodenough’s group[1], olivine structured lithium transition-metal phosphates LiMPO4(MFe, Mn, Co, Ni, V) have been received much attention as promising cathode materials for next generation of lithium-ion batteries due to their large theoretical capacities as well as chemical and thermal stability[2]. Among them, LiCoPO4and LiNiPO4are more challenging for developing stable electrolytes, because of their higher voltage (4.8 and 5.1 V)Li/Li+[3]. LiFePO4is now considered as a practical cathode material for its excellent rate capability achieved through particle size control and a coating of conductive layer[4-5]. Compared to other positive electrode materials, LiFePO4has a lower working voltage with the Fe2+/Fe3+redox reaction at 3.4 VLi/Li+. LiMnPO4has been proposed as a candidate material for positive electrode because high redox potential (4.1 VLi/Li+), nearly0.65 V higher than LiFePO4, makes theoretical energy density of LiMnPO4(701 W·h·kg-1) about 1.2 times larger than that of LiFePO4(586 W·h·kg-1). Theoretical energy density is defined as the maximum energy density practically achievable within stability window of carbonate ester-based electrolytes[6]. Unfortunately, poor electrochemical performance is most often observed on LiMnPO4because of slow lithium diffusion within crystals and very low intrinsic electronic conductivity which is about five orders of magnitude lower than that of LiFePO4[7-8]. Other debatable rate limiting factors may include passivation phenomenon upon delithiation, Jahn-Teller anisotropic lattice distortion in Mn3+, interface strain by large volume change between LiMnPO4and MnPO4, and metastable nature of delithiated MnPO4phase.

Considerable effort has been made in recent years to enhance electrochemical properties of LiMnPO4by particle size reduction[9-11], cation doping[12-15], carbon coating[9,16-19]or synthesis of LiMnPO4/C composites[7,11-15,20-25]. Among them, particle size reduction and carbon coating are main pathways. Despite of those tremendous efforts, the electrochemicalperformances are not sufficient for commercial application. Large charging polarization seems significant for achieving reasonable rate, besides crystallinity and particle size of LiMnPO4. The general liquid-phase routes for nanomaterial synthesis, which avoid grain growth and agglomeration, may lead to Mn2+disorder on Li+sites in LiMnPO4and thus limit electrochemical activity. Additionally, a high sintering temperature is required to form thermodynamically stable materials for high-potential cathode. It is of great interest to create a relatively low-temperature route to prepare well-dispersed uniform nanoparticles for crystalline LiMnPO4.

Graphene is a promising material in Li ion battery application, because of the high conductivity and structural flexibility. Graphene modified materials were reported to have improvement on specific capacity and cycling stability of LiMPO4(MFe, V, Mn) and as metal oxide anode materials[26-28]. Graphene- modified LiFePO4and Li3V2(PO4)3composites were successfully prepared by spray-drying process in our laboratory[29-30]. Olivine primary nanoparticles with sizes of 20—50 nm were wrapped loosely by multilayergraphene films, which could supply a three-dimensional conductive framework in the composites, and facilitate electron migration and Li+diffusion throughout micron-sized spherical secondary particles. Compared to pyrolytic carbon based composites, these composites had some improvement in electrochemical properties. Good dispersion and uniform chemical bonding between components were difficult to achieve for these transition metal phosphates/graphene composites. The non-intimate contact between graphene layers and active nanoparticles lead to aggregation of transition metal phosphate nanoparticles during cycle processes, which in turn caused low discharge capacity and rapid capacity decay in most cases.

Self-assembly of charged nanomaterialselectrostatic interactions in liquid phase environment is a controllable route for synthesis of stable hybrid materials that could be promising to design and synthesize robust electrode materials. In this work, a novel fabrication strategy for LiMnPO4/graphene nanocomposite was developed by co-assembly of positively charged APS-modified LiMnPO4and negatively charged graphite oxide, in which flexible graphene and LiMnPO4primary nanoparticles contactedstrongly and interlaced closely with each other. Compared to LiMnPO4/graphene without surface modification, the APS-LiMnPO4/graphene nanocomposite showed higher specific capacity and better cycling stability.

1 Experimental

1.1 Materials and chemicals

All chemicals were analytical grade (Sinopharm Chemical Reagent Co., Ltd.) and used as received without any purification. Graphite oxide (GO) sheets were prepared from natural graphite powdera modified Hummer’s method as described elsewhere[31-32].

1.2 Sample preparation

(1) Synthesis of pristine LiMnPO4

0.06 mol manganese acetate tetrahydrate (Mn(CH3COO)2·4H2O) dissolved in 30 ml deionized water was poured into a three-neck round-bottom flask containing 200 ml diethylene glycol (DEG). This vigorously stirred solution was heated to 100℃ and kept for 1 h. 30 ml of 2 mol·L-1lithium dihydrogen phosphate (LiH2PO4) aqueous solution was added dropwise at a speed of 1 ml·min-1. After kept for more than 4 h at this temperature and cooled down to room temperature, LiMnPO4precipitates were separated by centrifuge and washed three times with ethanol to remove DEG residual and other organic remnants. The obtained material was dried in oven at 120℃ overnight.

(2) Synthesis of LiMnPO4/graphene composite

As illustrated in Fig.1, surface of pristine LiMnPO4nanoparticles was modified by reacting with NH2groups of APS. Pristine LiMnPO4(0.25 g), APS (0.5 ml), and toluene (80 ml) were added successively into a 250 ml round-bottom flask. The reaction mixture was stirred and refluxed at 100℃ under N2for 12 h. Then APS-LiMnPO4nanoparticles were collected after ethanol washing and drying at 60℃.

Fig.1 Illustration of preparation process and microscale structure of APS-LiMnPO4/graphene nanocomposite (APS=(HC3O)3Si(CH2)2NH2)

Self-assembly of APS-LiMnPO4and GO was carried out by mixing 30 ml of APS-LiMnPO4nanoparticle dispersion (3.33 mg·ml-1) with 30 ml of GO dispersion (1.5 mg·ml-1) under mild magnetic stirring. The pH of suspension mixture was adjusted to around 3.0 by dropwise adding phosphoric acid (1 mol·L-1). After stirred continuously at room temperature for 1 h, the mixture was spray-dried at 200℃ to form a solid APS-LiMnPO4/graphite oxide nanocomposite, which was heated at a rate of 5℃·min-1at 200℃ for 2 h and then at 550℃ for another 4 h under N2atmosphere.

As a comparison, a mixture of 0.1 g pristine LiMnPO4in 30 ml of deionized water and 30 ml of GO dispersion (1.5 mg·ml-1) was spray-dried and exposed to the same thermal treatment as that of APS-LiMnPO4/graphene composite. This material without APS was denoted as LiMnPO4/graphene. Both composites contained about 12.5% (mass) graphene in the final cathode material.

1.3 Structural characterization

Morphology and microscopic structure of the nanocomposites were characterized by X-ray power diffraction (XRD, Rigaku D/max-2500, CuKαradiation,0.150405 nm), field-emission scanning electron microscopy (FE-SEM, JEOL JSM-6700F) and transmission electron microscopy (TEM, JEOL 200CX). Fourier transform infrared spectroscopy (FTIR) measurements on LiMnPO4nanoparticles were performed with a Bio-Rad, FTS 165 spectrometer. Carbon contents in the nanocomposites were determined by Carbon & Sulfur Determinator (C S444LSc).

1.4 Electrochemical measurements

Electrochemical study was carried out in 2016 coin-type cells. Working electrodes were prepared by mixing 85% (mass) active materials, 10% (mass) acetylene black and 5% (mass) polytetrafluoroethylene (PTFE). The anode was lithium metal foil and the electrolyte was a solution of 1 mol·L-1LiPF6in dimethyl carbonate (DMC) and ethylene carbonate (EC) (1:1 by mass). The coin-cells were assembled in argon-filled glove box. Galvanostatic charge and discharge in the voltage range of 2.5—4.4 V was performed on a LAND CT2001A cell test system with various current densities at room temperature.

2 Results and discussion

2.1 FTIR analysis for APS modification

Fig.2 showed FTIR spectra of APS-LiMnPO4and pristine LiMnPO4. Pristine LiMnPO4had weak bands at 1150—900 cm-1by PO stretching vibrations and bands at 628, 547 and 445 cm-1, which was assigned to(POMn),(PO4), and(MnOP), respectively[33-35]. Besides these bands, APS-LiMnPO4exhibited several additional peaks. The peak around 3350 cm-1was assigned to the NH stretching and small peaks at about 2930 and 2860 cm-1were assigned to asymmetric and symmetricCH2stretching. The absorption band from 1450 to 1600 cm-1was enhanced due to overlapping ofCH2bending and NH bending, indicating presence of 3-aminopropyl group in APS. Several strong SiO stretching adsorptions from 900—1100 cm-1were observed in spectrum b, which two peaks at 1070 and 963 cm-1were characteristic to SiOC in APS. These results suggested that silane-coupling reaction on nanoparticle surface created chemical bonding between APS molecule and surface of LiMnPO4nanoparticles.

Fig.2 FTIR spectra of pristine LiMnPO4 (a) and APS-modified LiMnPO4 (b)

This modification rendered positively charged LiMnPO4which could then assemble with negatively charged GO by electrostatic interactions. The oxygen containing functional groups and surface defects of GO were anchor sites for homogeneous distribution of LiMnPO4nanoparticles on curly surface of GO nanosheet. Under optimal condition (pH3.0), almost all GO and APS-LiMnPO4nanoparticles co-assembled into a stable suspension.

2.2 XRD

XRD patterns of pristine LiMnPO4, LiMnPO4/ graphene and APS-LiMnPO4/graphene composites were shown in Fig.3 with all diffraction peaks indexed to olivine LiMnPO4phase with space group of(JCPDS No.: 01-072-7844). No featured peaks of other possible impurities such as Li3PO4, Mn2P2O7or Mn2P were observed as result of low calcination temperature (550℃). No evidence of graphene diffraction peaks in LiMnPO4/graphene or APS-LiMnPO4/graphene nanocomposite indicated that regular lamellar graphene was broken and exfoliated graphene monolayer was formed[14]. Thedomain size calculated by Scherer’s formula was 25 nm for APS-LiMnPO4/graphene, much lower than that of LiMnPO4/graphene (41 nm). Thesmallparticle size was caused by efficient self-assembly between the positively charged LiMnPO4nanoparticles and negatively charged GO that confined crystal growth by chemical bonding and close contact.

Fig.3 XRD patterns of pristine LiMnPO4, LiMnPO4/graphene and APS-LiMnPO4/graphene

2.3 SEM

Fig.4 showed SEM photos of pristine LiMnPO4,LiMnPO4/graphene and APS-LiMnPO4/graphene composites. Pristine LiMnPO4possesses a platelet structure with width of 0.5—1 μm and thickness of 20—30 nm (Fig.4(a)). The platelet structure of LiMnPO4diminished slightly and interweaved with flexible graphene sheets (Fig.4(b)), generating a novel LiMnPO4/graphene composite with particles of ～1 μm wide. By contrast, the APS-LiMnPO4/graphene nanocomposites consisted of randomly aggregated, thin graphene sheets which crumpled closely and formed quasi-spherical microparticles with size of 3—4 μm (Fig.4(c)). Multilayer wrapping graphenes might be formed by stacking or folding of flexible GO sheets during spray-drying. SEM image under higher magnification (Fig.4(d)) showed that spherical LiMnPO4nanoparticles were uniformly distributed on curly graphene nanosheets. The small particle size of LiMnPO4, comparable to the value obtained from XRD calculation, was most likely due to confining effect of disordered graphene nanosheets.

Fig.4 SEM photos of pristine LiMnPO4 (a), LiMnPO4/graphene (b) and APS-LiMnPO4/graphene composites [(c), (d)]

2.4 TEM

The TEM images in Fig.5(a) further showed a platelet structure of LiMnPO4particles in LiMnPO4/graphene composite. Some platelets were stacked together without contact to graphene while other platelets were shattered outside graphene. The crinkled and rough texture in Fig.5(b) was observed for APS-LiMnPO4/graphene nanocomposite, which was associated to strong electrostatic interactions between LiMnPO4and graphene. Bright LiMnPO4nanoparticles were probably covered by a voile-like graphene sheet. LiMnPO4nanoparticles appeared on both sides of graphene nanosheets that some located above it while others lay on back of it. The interaction between LiMnPO4and graphene formed by self-assembly was so strong that the components were firmly attached even after ultrasonication used to disperse nanocomposites for TEM characterization.

Fig.5 TEM photos of LiMnPO4/graphene (a) and APS-LiMnPO4/graphene composites (b)

2.5 Electrochemical properties

Fig.6 showed initial charge-discharge curves of LiMnPO4/graphene and APS-LiMnPO4/graphene composites at 0.05 C (1 C171 mA·h·g-1) in voltage range of 2.5—4.4 V. Both composites exhibited a reversible plateau around 4.1 VLi/Li+corresponding totheredoxofMn3+/Mn2+thatwasaccompaniedby lithium ion extraction and insertion in LiMnPO4. The voltage difference between charge and discharge plateaus for APS-LiMnPO4/graphene was smaller than that of LiMnPO4/graphene, indicating less polarizationin APS-LiMnPO4/graphene. A much flattened dischargecurve meant more effective energy use in APS-LiMnPO4/ graphene composite[36]. The initial discharge specific capacity ofAPS-LiMnPO4/graphene was 142.2 mA·h·g-1with Coulombic efficiency of about 92.4%, which were higher than those obtained on LiMnPO4/graphene composite at 91.4 mA·h·g-1and 82.6%, respectively.

Fig.6 Initial charge-discharge curves of LiMnPO4/graphene and APS-LiMnPO4/graphene composites

As shown in Fig.7 for rate capabilities of LiMnPO4/graphene and APS-LiMnPO4/graphene composites at different charge/discharge rate, the specific capacities decreased gradually with the increase of discharge rate from 0.05 C to 1 C, which could be ascribed to limit control of Li+diffusion at interface between LiMnPO4and MnPO4[36]. APS-LiMnPO4/ graphene composite had rather good rate capability withadischargecapacityof126.4mA·h·g-1at0.1 C and 75.6 mA·h·g-1at 1 C, respectively, whereas LiMnPO4/graphene composite had limited charge capacities of 79.4 and 42.4 mA·h·g-1at 0.1 C and 1 C, respectively. Apparently, the rate performance and charge-discharge efficiency of APS-LiMnPO4/ graphene increased dramatically in comparison with those of LiMnPO4/graphene.

Fig.7 Charge-discharge curves of LiMnPO4/graphene and APS-LiMnPO4/graphene composites at different discharge rate

Fig.8 exhibited cycle performances of LiMnPO4/graphene and APS-LiMnPO4/graphene composites at 0.05 C in voltage range of 2.5—4.4 V.For LiMnPO4/graphene, the discharge specific capacity faded gradually down to 70.7 mA·h·g-1after 50 cycleswith only 75.5% capacity retention. Contrarily,APS-LiMnPO4/graphene showed much better cycling stability with a discharge capacity of 128.7 mA·h·g-1and capacity retention of 90.5% after 50 cycles.

Fig.8 Cycling performances of LiMnPO4/graphene and APS-LiMnPO4/graphene composites at 0.05 C rate

The intimate contact between LiMnPO4particles and graphene sheetselectrostatic interactions and the confinement of flexible graphene sheets obviously played an important role on such performance enhancement. When APS-modified LiMnPO4nanoparticle dispersion was added into negatively charged GO solution, abundant oxygen containing functional groups on GO could act as anchoring sites for LiMnPO4and consequently created uniform distribution of LiMnPO4nanoparticles on the surface and defects of GO, allowing small size of particles and intimate contact. LiMnPO4nanoparticles with high surface area could decrease transport length of Li ions and electrons as well as might ease strain associated with two phases of LiMnPO4and MnPO4. After calcination, graphene nanosheets acted as conducting routes between LiMnPO4nanoparticles and contact resistance of the whole electrode was reduced[14,19]. Moreover, graphene could accommodate strain induced by volume change of LiMnPO4and maintain electrode integrity during charge/discharge process, which is responsible for the good cycling stability and rate capability.

Electrochemical impedance was measured to understand electrochemical performance of APS-LiMnPO4/graphene composite for lithium storage (Fig.9). The charge transfer resistance (ct) and surface film resistance (f) of APS-LiMnPO4/ graphene electrode were 23.91 and 118.8 Ω, respectively, which were significantly lower than those of LiMnPO4/graphene electrode (81.41 and 165.38 Ω). Therefore, the close contact between flexible graphene and LiMnPO4primary nanoparticles in APS-LiMnPO4/ graphene composite was confirmed by increase in electrical conductivity and electron transfer.

Fig.9 Nyquist plot of LiMnPO4/graphene and APS-LiMnPO4/graphene electrodes at the fifth cycle(Inset was an equivalent circuit model of electrode)

3 Conclusions

APS-LiMnPO4/graphene nanocomposite was successfully prepared by co-assembly of negatively charged graphite oxide and positively charged LiMnPO4nanoparticles. The close contact and interlace of flexible graphene and LiMnPO4primary nanoparticles facilitated fast electron migration and Li+diffusion throughout the quasi-spherical microparticles. Compared to none surface-modified LiMnPO4/graphene, the APS-LiMnPO4/graphene composite showed higher specific capacity and more excellent cycling stability.

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利用表面改性制備磷酸錳鋰/石墨烯鋰離子電池復合材料

趙兵，王志軒，陳盧，楊雅晴，陳芳，高陽，蔣永

（上海大學環(huán)境與化學工程學院，上海 200444）

3-氨丙基三甲氧硅烷（APS）改性的磷酸錳鋰納米片與氧化石墨烯通過靜電自組裝，經(jīng)噴霧干燥和高溫煅燒，得到磷酸錳鋰/石墨烯復合材料。APS修飾后的磷酸錳鋰帶正電荷，并可通過紅外光譜中3-氨丙基和SiOC官能團的存在證明磷酸錳鋰成功被APS修飾，使得其與帶負電荷的氧化石墨烯自組裝形成磷酸錳鋰/石墨烯復合材料。測試結(jié)果表明約25 nm的磷酸錳鋰納米顆粒均勻負載在石墨烯表面，石墨烯片層充當導電網(wǎng)絡，提高了材料的電子電導率和鋰離子擴散速率，緩解了LiMnPO4在充放電過程中的體積變化。電性能測試發(fā)現(xiàn)，該材料的首次放電比容量為142.2 mA·h·g-1，50個循環(huán)后容量保持率達到90.5%，較未經(jīng)APS修飾的磷酸錳鋰/石墨烯材料有大幅提高。

磷酸錳鋰；3-氨丙基三甲氧硅烷改性；納米材料；復合材料；電化學

O 646

A

0438—1157（2016）11—4779—08

趙兵（1971—），男，博士，研究員。

國家自然科學基金項目（21501119, 11575105）；上海市科委技術標準項目（15DZ0501402）。

10.11949/j.issn.0438-1157.20160651

2016-05-11.

JIANG Yong, jiangyong@shu.edu.cn

supported by the National Natural Science Foundation of China (21501119, 11575105) and the Science and Technology Committee of Shanghai (15DZ0501402).

2016-05-11收到初稿，2016-07-21收到修改稿。

聯(lián)系人：蔣永。