CERAMICSINTERNATIONAL

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http://dx.doi.org/0272-8842/& 20

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(2014) 14391–14395

Ceramics International 40 www.elsevier.com/locate/ceramint

Extremely rapid synthesis of disordered LiNi0.5Mn1.5O4 with excellentelectrochemical performance

Guiyang Liun, Xin Kong, Hongyan Sun, Baosen Wang

Lab of New Materials for Power Sources, College of Science, Honghe University, Mengzi, Yunnan 661199, China

Received 8 May 2014; received in revised form 4 June 2014; accepted 5 June 2014Available online 12 June 2014

Abstract

Single phase LiNi0.5Mn1.5O4 spinel with excellent electrochemical performance has been synthesized by a novel solution combustion synthesismethod at 700 1C for 30 min. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscope (SEM)have been used to investigate the phase structure and micro-morphology of the product. Charge–discharge measurement, cyclic voltammetry(CV) and electrochemical impedance spectroscopy (EIS) have been used to study the electrochemical performance. The results indicate that theas-prepared product is a single phase LiNi0.5Mn1.5O4 spinel with a disordered Fd�3m space group. The LiNi0.5Mn1.5O4 exhibits excellentelectrochemical performance including cycling stability and rate capability due to its high cystallinity, large Liþ diffusion coefficient and lowcharge transfer resistance. It can deliver reversible capacities of 130, 113 and 104 mAh/g at 1 C, 10 C and 20 C current rates, respectively. Thecapacity retentions of the product at 1 C at room temperature, at 10 C at room temperature and at 1 C at 55 1C after 100 cycles are 97.5%, 99.7%and 94.2%, respectively.& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Lithium ion batteries; Solution combustion synthesis method; LiNi0.5Mn1.5O4 spinel

1. Introduction

Nowadays, rechargeable lithium batteries can offer thehighest energy density of any battery technology [1]. Althoughpowering most of today's portable electronic devices, they stillsuffer some limitations for new practical applications such aselectric vehicles (EVs) and hybrid electric vehicles (HEVs),which require high power density and high energy density [2].Thus, electrode materials with large reversible capacities andhigh rate capabilities are needed to produce batteries whichcould satisfy the high power and high energy demands.LiNi0.5Mn1.5O4 spinels, which possess high average dischargevoltage (around 4.7 V vs. Liþ /Li couple), fast Liþ diffusionwithin the three-dimensional spinel structure, low cost andenvironmental friendliness, have been considered as one of themost prospective candidates for high power and high energy

10.1016/j.ceramint.2014.06.03214 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

g author. Tel.: þ86 8733694922; fax: þ86 8733694923.ss: liuguiyang@tsinghua.org.cn (G. Liu).

cathode materials for lithium ion batteries [3]. There are twotypes of LiNi0.5Mn1.5O4 spinels with different space groups,disordered Fd�3m or ordered P4332 [4]. In the disorderedLiNi0.5Mn1.5O4�x, accompanied by the loss of oxygen, part ofthe inactive Mn4þ ions are reduced to Mn3þ due to the chargeneutrality. It has been reported that the disorderedLiNi0.5Mn1.5O4�x, which possesses an appropriate amount ofMn3þ ions, can exhibit better electrochemical performancethan that of the ordered one [5].Various synthesis methods including solid-state method [6],

sol–gel method [7] or polymer-pyrolysis method [8] producingLiNi0.5Mn1.5O4 with different amounts of Mn3þ ions, degreeof structural ordering and morphologies have been reported.Although solid-state method is simple, the mixture of rawmaterials is non-homogeneous. The non-homogeneous pre-cursor results in a high synthetic temperature and a longsynthetic time. The high temperature leads to increased grainsize and undesirable impurities such as NiO or LixNi1�xO inthe final product, which could result in rate capability and

G. Liu et al. / Ceramics International 40 (2014) 14391–1439514392

capacity fading [6]. Other methods such as the sol–gel method,polymer-pyrolysis method and co-precipitation method couldobtain homogenous precursors, but the processes of them arecomplicated, resulting in the limitation in practical manufac-turing applications.

In order to obtain a simple and effective method to rapidlysynthesize single phase LiNi0.5Mn1.5O4 with excellent electro-chemical performance, in this paper, an optimized solutioncombustion synthesis method has been introduced. We havepreviously found that based on our previous works [9,10], weoptimized the experimental condition of the solution combustionsynthesis method. As a result, a single phase LiNi0.5Mn1.5O4

spinel with excellent electrochemical performance has beensynthesized by the method at 700 1C for only 30 min. The phasestructure and the electrochemical performances including cyclingstability and rate capability of the LiNi0.5Mn1.5O4 have beeninvestigated in details.

2. Experimental

2.1. Preparation

About 10 g of raw materials of LiNO3, CH3COOLi, Mn(NO3)2, (CH3COO)2Mn, Ni(NO3)2 and (CH3COO)2Ni (AR,99%, Aladdin) with the mole ratio of 0.5:0.5:0.75:0.75:0.25:0.25 were firstly dissolved in 5 ml distilled water toobtain a solution. Then, the solution was directly put into amuffle furnace which was preset at 700 1C. After combustionand reaction for 30 min, the product was taken out and cooleddown to room temperature in air, and the final product wasobtained.

2.2. Characterization

The phase structure of the product was ascertained by X-raydiffraction (XRD, D/max-rB, Cu-Kα radiation) and Fouriertransform infrared spectroscopy (FTIR, Perkin Elmer, withKBr pellets). The morphology of the product was observedby scanning electron microscope (SEM, XL30ESEM-TMP,Philips).

Fig. 1. XRD pattern and FTI

2.3. Elctrochemical performance

The electrochemical characterization was performed byusing CR2032 coin-type cell. For LiNi0.5Mn1.5O4 electrodefabrication, the as-prepared LiNi0.5Mn1.5O4 powders weremixed with 12 wt% of carbon black and 8 wt% of polyviny-lidene fluoride in N-methyl pyrrolidinone until slurry wasobtained. The active material loading on the electrode is about3 mg/cm2. Then, the blended slurries were pasted onto analuminum current collector, and the electrode was dried at120 1C for 12 h in vacuum. The test cell consisted of theLiNi0.5Mn1.5O4 electrode as cathode electrode, lithium foil asanode electrode, a porous polypropylene film as a separatorand 1 M LiPF6 in EC/EMC/DMC (1:1:1 in volume) as anelectrolyte. The cells were assembled in an argon-filled glovebox and cycled at room temperature. The electrochemicalperformances of the cells were evaluated upon cycling in the3.5–5.0 V vs. Li/Liþ electrode at different C rate. Here1 C=150 mA/g. Cyclic voltammograms (CV) were carriedout by an electrochemical workstation (Chenhua, CHI650B,China) with a scan rate of 0.1–0.5 mV/s between 3.5 and5.0 V. The CV test was carried out by a two-electrode mode, inwhich the active material was used as work electrode, andmetal Li was used as both counter and reference electrodes.Electrochemical impedance spectroscopy (EIS) was collectedat 100% state of charge with an AC amplitude of 5 mV in thefrequency range of 10 kHz to 0.1 Hz. Li foil was used as bothcounter and reference electrodes.

3. Results and discussion

3.1. Phase structure and micro-morphology

The XRD and FTIR patterns of the product are shown inFig. 1. As shown in Fig. 1a, all peaks of the productcorrespond to LiNi0.5Mn1.5O4 (JCPDS 80-2162) and no otherimpurity peaks can be found, suggesting that a single phaseLiNi0.5Mn1.5O4 spinel was obtained. However, the structuraldifferences between the ordered and the disordered spacegroups are hardly to be directly distinguished by normal

R spectra of the product.

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X-ray diffraction because of the similar scattering factors of Niand Mn [11,12]. FTIR spectroscopy has been proved to be aneffective technique in qualitatively resolving the cation order-ing [13]. Characteristic infrared vibration bands of the M–Obonds of the sample between 700 cm�1 and 400 cm�1 wereused to examine the structural ordering in LiNi0.5Mn1.5O4.With increasing the degree of lattice ordering, the intensity ofNi–O band at about 588 cm�1 increases while the intensity ofMn–O band at 620 cm�1 decreases [14]. The product exhibitslower band intensity at 586 cm�1 than that at 620 cm�1 (asshown in Fig. 1b), suggesting that the product has disorderedphase structure with space group of Fd�3m [14,15].

Fig. 2. SEM imagine of the product.

Fig. 3. Electrochemical performance of the product. (a) Cycling performances at roocurves at 1 C rate at room temperature, (c) variation of the discharge capacity at d

Fig. 2 shows the scanning electron microscopy (SEM)image of the LiNi0.5Mn1.5O4. It can be found that the grainsof the product are well developed and the grain sizes are 300–500 nm. The surface facets of the grains are very clear,indicating that the product is well crystallized.

3.2. Electrochemical performance

Cycling performance is of great importance for lithium ionbatteries. The cycling performances of the product are shownin Fig. 3a. The capacity retentions after 100 cycles of theproduct at 1 C at room temperature and at 55 1C are 97.5% and94.2%, respectively, and at 10 C at room temperature, thecapacity retention is as high as 99.7%. The product exhibitsexcellent cycle stabilities at room temperature, elevatedtemperature or at high C rate.The voltage profiles of the 1st and the 100th cycle of the

product at 1 C rate are shown in Fig. 3b. It can be found fromFig. 3b that the charge/discharge curves show two mainvoltage plateaus in 4.7 V and 4.0 V regions, respectively.The long plateau at 4.7 V is ascribed to the two-step (Ni4þ /Ni3þ , Ni3þ /Ni2þ ) redox reactions, and the short plateau at4.0 V reflects the redox reaction between the Mn3þ and Mn4þ

couple [16]. The existence of voltage plateau in the 4.0 Vregion of the product indicates that there are certain amount ofMn3þ ions in the product [5]. The percentages of capacitycontribution from the 4.0 V plateau for the product can be

m temperature and 55 1C, (b) the comparison of the 1st and the 100th dischargeifferent C rate, and (d) discharge curves at different rates.

Fig. 4. (a) Cyclic voltammogram (CV) of the electrode made with the product at different scan rates. The inset in (a) is the ip vs. the square root of the scan rates(v1/2) of the product. (b) EIS spectra of the product in the frequency range between 0.1 Hz and 100 kHz after 5th and 100th cycles.

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calculated from the capacity between 3.80 and 4.25 V dividedby the total discharge capacity and can be qualitatively used toevaluate the relative concentration of Mn3þ ions in the spinels[5]. For the product in this paper, about 23.1% of the totalcapacity is from the 4.0 V plateau. The value is similar as areported LiNi0.5Mn1.5O4 with excellent electrochemical per-formance [17]. It reveals that the amount of Mn3þ ions in theproduct is appropriate, and also indicates that the product is anon-stoichiometric LiNi0.5Mn1.5O4�x [5]. Moreover, fromFig. 3b, it also can be seen that after 100 cycles at 1 Cdischarge rate, the plateau voltage in the discharge curvechanges little (as seen in Fig. 3b), suggesting that the producthas negligible polarization and capacity fading during cycling.

Rate capability is another important factor for lithium ionbatteries. Fig. 3c clearly shows the capacity changing of theproduct with increasing C rate. A slow charge rate of 0.5 Cwas used in Fig. 3c to effectively evaluate the ability of thehost spinel structures to accommodate Liþ ions when dis-charge rate is Z0.5 C. Here, the initial discharge capacity ofthe electrode at 0.2 C is used as 100% for comparison. Whendischarged at 0.5, 1, 2, 5, 10 C and 20 C, it is calculated thatthe capacity of the LiNi0.5Mn1.5O4 remains 100%, 100%,97.8%, 93.2%, 87.5% and 80.6% of the initial capacity at0.2 C, respectively. The product exhibits remarkably superiorcapacity retention even at high rate. Fig. 3d shows thedischarge curves of the product at different rates. FromFig. 3d, it can be found that the polarizations of the productfrom 0.2 C to 20 C are small. At 5 C, the operation voltage ofthe products only drops to 4.65 V and still maintains a capacityclose to 120 mAh/g. Even at 20 C, the capacity is around104 mAh/g with a still-observable plateau at 4.4 V. It isreported that an appropriate amount of Mn3þ ions and highcrystallinity can efficiently improve the electrochemical per-formance of LiNi0.5Mn1.5O4 [5,17], the product in this paperpossessing an appropriate amount of Mn3þ ions and highcrystallinity therefore exhibits excellent cycling stability andrate capability. Compared with many LiNi0.5Mn1.5O4 spinelsprepared by the conventional solution combustion synthesis[18], polymer-pyrolysis method [19] or solid-state method[11], which took sintering time more than 10 h at 700 1C or

higher temperature, the product reported in this paper presentssignificantly enhanced cycle stability and rate capability.Notably, the as-prepared product took only 30 min at700 1C, suggesting that the method adopted in this paper ismore effective and promising for synthesis of LiNi0.5Mn1.5O4

spinels.A more detailed analysis of the electrochemical behavior is

further investigated by CV and EIS. Fig. 4a shows the CVs ofthe product recorded at 0.1–0.5 mV/s. The redox peaks mainlylocated near 4.7 V are ascribed to the two-step oxidation/reduction of Ni2þ /Ni4þ or Ni2þ /Ni3þ and Ni3þ /Ni4þ [20].From Fig. 4a, it can be found that there are evident redoxpeaks appeared near 4.0 V, which are attributed to the redoxreaction of Mn3þ /Mn4þ couples. It suggests the existence ofMn3þ ions in the product [5]. With increasing scan rate from0.1 to 0.5 mV/s, the potential between the anodic and cathodicpeaks and the shapes of the CVs change a little, suggesting thelithium insertion/extraction kinetics in the product is fast sothat the polarization of the product is low [6]. Fig. 4a alsoshows that when the scan rate (v) increases, the peak current(ip) increases. The inset in Fig. 4a shows the ip vs. the squareroot of the scan rates (v1/2) of the product. It displays a linearincrease, suggesting that the intercalation reaction is controlledby solid-state diffusion of lithium-ion [21]. The dependence ofip on v1/2 can be applied to approximately determine thediffusion coefficient of Liþ (DLi) on the basis of the followingequation [22]:

ip ¼ 2:69� 105n3=2ACLiD1=2Li υ

1=2 ð1Þwhere n is the number of electrons per reaction species (for

lithium-ion n¼1), A is the total surface area of the electrode(2 cm2 in this case), and CLi is the bulk concentration of Liþ

in the electrode (given as 0.02378 mol/cm3) [20,23]. From theslope of linear fit of ip vs. v

1/2 of the inset in Fig. 4a, the DLi ofthe product could be calculated [20]. The value is as large as1.08� 10�11 cm2/s, which presents 1–2 orders of magnitudelarger than that of the ordered LiNi0.5Mn1.5O4 and is in accordwith the values of the disordered ones [24,25].Fig. 4b shows the EIS of the battery after 5th and 100th

cycles. The EIS spectra are combination of the depressed

G. Liu et al. / Ceramics International 40 (2014) 14391–14395 14395

semicircle at high-to-middle frequency region and an inclinedline in the low frequency region. The intercept at the Z0 axiscorresponds to the ohmic resistance (Rs), the semicircle isrelated closely to the lithium-ion migration resistance (Rf)through the multilayer surface films and the charge transferresistance (Rct) in high-to-middle frequency range, and theinclined line in the low frequency region is the Warburgimpedance of solid phase diffusion (σw), which is related to thelithium-ion diffusion in the spinel particles [26]. It can be seenfrom Fig. 4b that the Rct of the product is small and changeslittle during cycles, indicating good conductivity and goodcycling stability of the product [4]. Lower Rct value means alower electrochemical polarization, and can lead to highercycling stability and rate capability [27]. The as-preparedproduct presenting small Rct therefore exhibits excellent ratecapability and cycle stability, which are in good agreementwith the C-rate capacity results.

4. Conclusion

Single phase LiNi0.5Mn1.5O4 spinel with excellent electro-chemical performance has been synthesized by an optimizedsolution combustion synthesis method at 700 1C within 30 min.The LiNi0.5Mn1.5O4 exhibits disordered Fd�3m structure andexcellent electrochemical performance including cycling stabi-lity and rate capability. It can deliver the capacities of 130, 113and 104 mAh/g at 1, 10 and 20 C, respectively. The capacityretentions at 1 C at room temperature and at 55 1C after 100cycles are 97.5% and 94.2%, respectively and at 10 C at roomtemperature is 99.7%.

Acknowledgments

The present work was supported by the National NaturalScience Foundation of China (No. 51362012), Natural ScienceFoundation of Yunnan Province (2012FB173) and the KeyConstruction Disciplines of Chemistry for Master DegreeProgram in Yunnan.

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