LiNi0.5Mn1.5O4 with significantly improved rate capability synthesizedby a facile template method using pine wood as a bio-template

Guiyang Liu n, Yannan Li, Baosen WangLab of New Materials for Power Sources, College of Science, Honghe University, Mengzi, Yunnan 661199, China

a r t i c l e i n f o

Article history:Received 15 July 2014Accepted 25 October 2014Available online 4 November 2014

Keywords:Lithium ion batteriesTemplate methodPine woodSpinelParticlesEnergy storage and conversion

a b s t r a c t

A porous LiNi0.5Mn1.5O4 spinel with significantly improved rate capability has been synthesized by afacile template method using pine wood as a bio-template. The templated LiNi0.5Mn1.5O4 (T-LNMO) hasdisordered Fd-3m phase, and consists of sub-micrometric and highly dispersed particles. It exhibitssignificantly improved rate capability. The capacity of T-LNMO at 10 C is 97 mAh/g and retains 90% after100 cycles. Contrastively, the non-templated LiNi0.5Mn1.5O4 (NT-LNMO) cannot deliver any capacity at10 C, and at 5 C rate after 100 cycles, the capacity retention is only 22%.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

Although powering most of today's portable electronics, recharge-able lithium ion batteries (LIB) still suffer some limitations in newapplications such as electric vehicles (EVs) and hybrid electric vehicles(HEVs), which require high power density [1]. LIBs with high powerdensity require the electrode materials possessing high rate capability.Porous electrode materials for LIB can efficiently improve their ratecapabilities [2]. Template method has been proved to be an efficientroute to synthesize porous materials. Recently, porous electrodematerials such as meso-porous Co3O4 [3] and LiMn2O4 nano-tube [4]have successfully synthesized by a hard template method using siliconnano-rod, KIT-6 or anodic aluminum oxide (AAO) membrane astemplates. However, the removal of the commonly used hard tem-plates is difficult and the improvement in the electrochemical perfor-mance of the templated materials is not significant.

As alternative templates to synthesize porous materials, woodshave been attracted more attentions [5]. Woods are mainly com-posed of cellulose, hemicellulose and lignin, forming a cellularmicrostructure with high porosity and interconnectivity [6]. Thetissues of woods can be transformed into carbon when treatedat high temperatures in an inert atmosphere, and they can beremoved by oxygen or air treatment at high temperatures, makingthem useful templates. In this paper, a pine wood was used as a bio-template to originally synthesize porous LiNi0.5Mn1.5O4 by a faciletemplate method. Owing to the porous structure and highly

dispersed particles, this bio-templated LiNi0.5Mn1.5O4 exhibits sig-nificantly improved rate capability.

2. Experimental

Preparation: Pine wood (Yunnan province, China) was used asthe bio-template. Firstly, small blocks (e.g. 10 mm�5 mm�2 mm)of pine wood were boiled in 25% ammonia at 90 1C for 1 h toremove the extractive organic compounds and then were washedby distilled water. Then they were dried in air at 60 1C for 12 h. Asstarting chemicals, about 10 g mixtures of LiNO3 (AR, 99%),CH3COOLi (AR, 99%), Mn(NO3)2 (AR, 99%), (CH3COO)2Mn (AR,99%), Ni(NO3)2 (AR, 99%) and (CH3COO)2Ni (AR, 99%) with themole ratio of 0.5:0.5:0.75:0.75:0.25:0.25 were dissolved in 15 mldistilled water to form a solution. And then, the templates wereput into the solution and immersed for 10 h. After filtration, thesample was firstly heated at 60 1C for 5 h and then heated at250 1C for 2 h in a drying oven in air. Finally, the sample wascalcined at 800 1C for 10 min in air in a muffle furnace. After coolingdown to room temperature in the furnace, the final product wasobtained. For comparison, a non-templated LiNi0.5Mn1.5O4 (NT-LNMO) was synthesized at the same starting materials and processwithout the template.

Characterization: The phase composition of the products wasascertained by X-ray diffraction (XRD, D/max-rB, Cu-Kα, λ¼1.5406 Å)and Fourier transform infrared spectroscopy (FTIR, Perkin-Elmer IRspectrometer). The morphologies of the products were observed byscanning electron microscope (SEM, XL30ESEM-TMP, Philips).

Contents lists available at ScienceDirect

journal homepage: www.elsevier.com/locate/matlet

Materials Letters

http://dx.doi.org/10.1016/j.matlet.2014.10.1370167-577X/& 2014 Elsevier B.V. All rights reserved.

n Corresponding author. Tel.: þ86 8733694922; fax: þ86 8733694923.E-mail address: liuguiyang@tsinghua.org.cn (G. Liu).

Materials Letters 139 (2015) 385–388

Elctrochemical measurement: The electrochemical characteriza-tions were performed by using CR2032 coin-type cell. The cathodewas prepared by spreading a mixture of 80 wt% active material,10 wt% of carbon black and 10 wt% of polyvinylidene fluoride ontoan Al foil current collector. The obtained electrodes were dried at120 1C for 12 h in vacuum. Lithium foil was used as anode andCelgard 2400 membrane as separator. The electrolyte was 1 MLiPF6 dissolved in a mixture of ethylene carbonate (EC)/dimethyl-carbonate (DMC) (1:1 in volume). The cells were galvanostaticallycharged and discharged in the voltage range of 3.5–5.0 V on abattery test system (LAND-200 T, Land Electronic Co., Ltd.,Wuhan).1 C rate is equivalent to 150 mA/g in our definition.

3. Results and discussion

Phase composition and micro morphologies: Fig. 1a shows theXRD patterns of T-LNMO and NT-LNMO. As shown in Fig. 1a, themain peaks of the two products correspond to LiNi0.5Mn1.5O4

(JCPDS 80-2162), which suggests that the main phase of the twoproducts is LiNi0.5Mn1.5O4. However, some very weak LixNi1�xO(JCPDS 47-0835) peaks can be found in the products, which areattributed to the high preparation temperature [7,8] and thedifference in the oxidation states of Li, Ni and Mn. Fig. 1b showsthe FTIR spectra of the products. The band peaks at 588 cm�1 ofthe two products are distinct, but the intensity of the peak at

Fig. 1. XRD patterns (a) and FTIR spectra (b) of the products.

Fig. 2. SEM images of (a) pine wood, (b) and (c) T-LNMO, and (d) NT-LNMO.

G. Liu et al. / Materials Letters 139 (2015) 385–388386

588 cm�1 is lower than the peak at 620 cm�1, suggesting that thephase structures of the products are disordered and the spacegroup of the products are Fd-3m [9].

Fig. 2 shows the SEM images of the template and the products.T-LNMO exhibits a porous aspect (Fig. 2b), which is similar as thetemplate (Fig. 2a). The particles of T-LNMO with �100–200 nm insize are well dispersed (Fig. 2c). In contrary, the particles of NT-LNMO (Fig. 3d) are badly agglomerated. The BET specific surfacearea of T-LNMO is 35 m2/g, which is much higher than the 7 m2/gof NT-LNMO (as seen in Supplementary materials of this paper).The increased BET surface area may be arisen from the moreporous structure and more dispersed particles of LNMO-T thanthat of LNMO (as seen in Fig. 2c and d).

Electrochemical performance: The electrochemical performancesof the products were tested by using coin-type Li cells, whoseschematic image is shown in Fig. 3e. Typical galvanostatic dis-charge profiles are shown in Fig. 3a and b. The charge/dischargecurves of the two products show two flat voltage plateaus in the4.7 V and 4.0 V region respectively. The small plateau in 4.0 Vindicates that the redox reaction between the Mn3þ and Mn4þ

couples is not obvious [8]. At 0.2 C, the initial capacities of T-LNMOand NT-LNMO are 129 mAh/g and 119 mAh/g, respectively. At 5 C,T-LNMO can deliver a capacity close to 117 mAh/g. Even at 10 C,the capacity of T-LNMO is still 97 mAh/g. However, for NT-LNMO,at 5 C, the capacity is only 91 mAh/g, and at 10 C, the capacity isnearly 0 mAh/g. Moreover, with increasing C rate, the voltage ofNT-LNMO drops much larger than that of T-LNMO. These suggestthat T-LNMO exhibits much better rate capability than that of NT-LNMO. The cycles at different rates of the products are shown inFig. 3c. Evidently, T-LNMO exhibits significantly improved ratecapability, especially at higher rate.

Cycling performances of the products are shown in Fig. 3d.Although the capacity retentions of T-LNMO and NT-LNMO aresimilar at 1 C after 100 cycles, at higher rate, T-LNMO displaysmuch better cycle stability than that of NT-LNMO. After 100 cyclesat 10 C, the capacity retention of T-LNMO is still 90%. In contrary,after 100 cycles at 5 C, the capacity retention of NT-LNMO is only22%. Since the two products have same composition and sametreatments, the remarkable improvement in rate capability ofT-LNMO is mainly attributed to the higher BET surface area and

Fig. 3. Discharge curves of (a) T-LNMO and (b) NT-LNMO at different C rates. (c) Cycles at different rates of the products. (d) Cycling performance of the products at differentrates. (e) Schematic diagram of a constructed cell.

G. Liu et al. / Materials Letters 139 (2015) 385–388 387

more dispersed particles. Higher BET surface area and moredispersed particles could easily form short ion diffusion channelsand provide better penetration of electrolyte thereby lead to abetter rate performance [10,11]. The specific capacity and capacityretention at high rate of T-LNMO is also superior to some pristineLiNi0.5Mn1.5O4 prepared by sol–gel method [12] or self-sacrificetemplate method [13]. Noticeably, the calcination time of NT-LNMO is only 10 min, suggesting the adopted method in this paperis suitable for synthesis of LiNi0.5Mn1.5O4 with good performance.

4. Conclusion

A porous LiNi0.5Mn1.5O4 spinel has been synthesized by a facilebio-template method using pine wood as a bio-template. Theproduct has a disordered phase structure and consists of sub-micrometric and highly dispersed particles. It exhibits significantlyimproved rate capability. It can deliver a specific capacity 124 mAhg�1 up to 2 C rate. At 10 C rate, the capacity is still 97 mAh g�1 andretains 90% after 100 cycles.

Acknowledgment

The present work was supported by the National NaturalScience Foundation of China (No. 51362012), Natural ScienceFoundation of Yunnan Province (2012FB173).

Appendix A. Supporting information

Supplementary data associated with this article can be foundin the online version at http://dx.doi.org/10.1016/j.matlet.2014.10.137.

References

[1] Li H, Wang Z, Chen L, Huang X. Adv Mater 2009;21:4593–607.[2] Vu A, Qian YQ, Stein A. Adv Energy Mater 2012;2:1056–85.[3] Tian B, Liu X, Solovyov LA, Liu Z, Yang H, Zhang Z, et al. J Am Chem Soc

2004;126:865–75.[4] Nishizawa M, Mukai K, Kuwabata S, Martin CR, Yoneyama H. J Electrochem Soc

1997;144:1923–7.[5] Zhou H, Fan T, Zhang D. ChemSusChem 20111344–87.[6] Sieber H, Hoffmann C, Kaindl A, Greil P. Adv Eng Mater 2000;2:105–9.[7] Liu Z, Fan T, Zhang D. J Am Ceram Soc 2006;89:662–5.[8] Xiao J, Chen X, Sushko PV, Sushko ML, Kovarik L, Feng J, et al. Adv Mater

2012;24:2109–16.[9] Patoux S, Daniel L, Bourbon C, Lignier H, Pagano C, Cras FL, et al. Power

Sources 2009;189:344–52.[10] Myung S, Komab S, Kumagai N, Yashiro H, Chung H, Cho T. Electrochim Acta

2002;47:2543–9.[11] Xi LJ, Wang HE, Lu ZG, Yang SL, Ma RG, Deng JQ, et al. Power Sources

2012;198:251–7.[12] Yi T, Chen B, Zhu YR, Li XY, Zhu RS J. Power Sources 2014;247:778–85.[13] Lee HW, Muralidharan P, Mari CM, Ruffo R, Kim DK. J Power Sources

2011;196:10712–6.

G. Liu et al. / Materials Letters 139 (2015) 385–388388