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Journal of Power Sources 276 (2015) 247e254

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Journal of Power Sources

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Highly-crystalline ultrathin Li4Ti5O12 nanosheets decorated with silvernanocrystals as a high-performance anode material for lithium ionbatteries

G.B. Xu a, W. Li b, L.W. Yang a, b, *, X.L. Wei a, J.W. Ding a, J.X. Zhong a, Paul K. Chu b, **

a Hunan Key Laboratory of Micro-Nano Energy Materials and Devices, School of Physics and Optoelectronics, Xiangtan University, Hunan 411105, Chinab Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China

h i g h l i g h t s

� A novel composite of ultrathin Li4Ti5O12 nanosheets and Ag nanocrystals is prepared.� The LTO nanosheets have single-crystal nature with a thickness of about 10 nm.� The composite shows low polarization of the voltage difference.� The composite has high electrical conductivity and large Liþ diffusion coefficient.� The composite demonstrates superior lithium storage performance.

a r t i c l e i n f o

Article history:Received 3 October 2014Received in revised form21 November 2014Accepted 24 November 2014Available online 25 November 2014

Keywords:Lithium ion batteryAnode materialCompositeLithium titanium oxideNanosheetsSilver nanocrystals

* Corresponding author. Hunan Key Laboratory of Mand Devices, School of Physics and Optoelectronics,411105, China.** Corresponding author.

E-mail addresses: ylwxtu@xtu.edu.cn (L.W. Y(P.K. Chu).

http://dx.doi.org/10.1016/j.jpowsour.2014.11.1080378-7753/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

A novel composite of highly-crystalline ultrathin Li4Ti5O12 (LTO) nanosheets and Ag nanocrystals(denoted as LTO NSs/Ag) as an anode material for Li-ion batteries (LIBs) is prepared by hydrothermalsynthesis, post calcination and electroless deposition. The characterizations of structure and morphologyreveal that the LTO nanosheets have single-crystal nature with a thickness of about 10 nm and highlydispersed Ag nanocrystals have an average diameter of 5.8 nm. The designed LTO NSs/Ag composite takesadvantage of both components, thereby providing large contact area between the electrolyte and elec-trode, low polarization of voltage difference, high electrical conductivity and lithium ion diffusion co-efficient during electrochemical processes. The evaluation of its electrochemical performancedemonstrates that the prepared LTO NSs/Ag composite has superior lithium storage performance. Moreimportantly, this unique composite has an ability to deliver high reversible capacities with superlativecyclic capacity retention at different current rates, and exhibit excellent high-rate performance at acurrent rate as high as 30 C. Our results improve the current performance of LTO based anode materialfor LIBs.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Lithium ion batteries (LIBs) are widely used as power sources inportable electronics due to the high energy density, long lifespan,and environmental benignity and also considered a candidate to

icro-Nano Energy MaterialsXiangtan University, Hunan

ang), paul.chu@cityu.edu.hk

power electrical and hybrid electrical vehicles. However, theirperformance is approaching the achievable limit of currentlycommercial graphite electrodes [1e3] and there is increasing in-terest in identifying new electrode materials such as silicon [4], Ti-based compound [5e7], transition metal oxides [8e10], etc. thatcan store and deliver energy more efficiently with better safety.Among various materials, spinel Li4Ti5O12 (LTO) is promising inhigh-performance LIBs because of its extreme flat charge/dischargeplateau with a high potential at 1.55 V vs. Li/Liþ, zero-strain featuretoward lithium insertion/extraction, and environmental friendli-ness [11e14]. Unfortunately, application of spinel LTO to hybridelectrical vehicles and large-scale energy storage is hampered

G.B. Xu et al. / Journal of Power Sources 276 (2015) 247e254248

because the high-rate capability is unsatisfactory and cannot meetthe demand for a high power density due to problems related to thepoor electrical conductivity (ca.10�13 S cm�1) and small lithiumdiffusion coefficient (ca.10�9e10�13cm2 s�1) [15,16]. Hence, mucheffort has been made to ameliorate the rate capability of LTO, forinstance, developing various nano/microstructured materials[15,17e26], elemental doping [27e33], and surface modificationwith highly conductive additives [15,16,31,34e47]. In particular,LTO nanosheets (NSs) have been demonstrated to have a shortertransport distance for both lithium ions and electrons [17,37,48,49].It is also believed that LTO NSs possess a pseudocapacitive effect asthe interaction takes place on the surface [50], thus leading toimproved rate capability. Modification with noble metal nano-crystals can also increase the conductivity of the electrode greatlythereby improving the high-rate discharge capacity and cyclingstability while not decreasing the volumetric power density[35,39,46,47]. Despite recent research on LTO NSs and noble metalmodified LTO nanocomposites, there have been few studies on thenanocomposite consisting of ultrathin LTO NSs and highlydispersed noble metal nanocrystals. Compared to the conventionalsolid-state reaction, hydrothermal, or solegel method to prepareLTO nano/microstructures, large scale synthesis of highly-crystalline ultrathin LTO NSs with a thickness of 10 nm is quitechallenging. In previous studies [35,46,47], noble metal modifiedLTO nanocomposites were obtained by direct mixing or thermaldecomposition of precursors to induce growth or agglomeration ofnoble metal particles. In this work, highly-crystalline ultrathin LTONSs with thickness of about 10 nm are prepared by a hydrothermalmethod followed by calcinations. Subsequently, a simple and lowtemperature electroless deposition method was used to deposithighly dispersed small-sized Ag nanocrystals on highly-crystallineultrathin LTO NSs. The LTO NSs/Ag composite exhibits highreversible capacity and high-rate capability with good cyclingperformance and is desirable anode materials in LIBs.

2. Experimental details

2.1. Synthesis of highly-crystalline ultrathin Li4Ti5O12 nanosheets

Highly-crystalline ultrathin LTO NSs were prepared using amodified hydrothermal method followed by calcination reportedbyWang et al. [15] In the typical synthesis procedure,1.7ml (5 mM)of tetrabutyl titanate, 0.03 g of GdCl3$6H2O, and 0.189 g ofLiOH$H2O were thoroughly mixed in 20 ml of ethanol at roomtemperature. The solution was mixed completely with a magneticstirrer in a closed container for 24 h and then, 25 ml of deionizedwater were added to the container. After stirring for 0.5 h, the so-lution was transferred to a 50 ml Teflon-lined stainless autoclaveand placed in an oven at 180 �C for 36 h. The white powder on thebottom of the reactor was collected, washed with ethanol 3 times,and dried at 80 �C for 6 h. Finally, the white hydrothermal productwas heated at 700 �C for 6 h in a horizontal tube furnace in air toobtain the ultrathin LTO NSs.

Fig. 1. (a)e(c) XRD, EDS, and Raman spectra of the LTO NSs/Ag and LTO NSs. The insetin (b) shows the EDS spectrum of the LTO NSs.

2.2. Synthesis of Li4Ti5O12 nanosheets/Ag composite

The small Ag nanocryatals were deposited on ultrathin LTO NSsby electroless deposition [39]. Diluted NH4OH was dropped into anAgNO3 solution (0.01 mol/L) until the white deposit disappeared.The solution turned transparent completely due to the followingreaction:

Agþ þ NH3 / Ag(NH3)þ (1)

Ag(NH3)þ þ NH3 / [Ag(NH3)2]þ (2)

Then, the ultrathin LTONSs and diluted HCHOwere added to theabove solution under vigorous magnetic stirring for 1.5 h at roomtemperature. During this process, Ag nanoparticles were formedaccording to following reaction:

2[Ag(NH3)2]þ þ 2OH� þ CH3CHO / 2AgY þ 3NH3[ þ CH3COO� þH2O þ NH4

þ (3)

Finally, the precipitate was collected, washed with deionizedwater, and dried at 60 �C in air for 24 h. The weight ratio of AgNO3and LTO NSs was estimated to be 6:94, corresponding to nominal3.6 wt% of Ag in the final LTO NSs/Ag composite.

G.B. Xu et al. / Journal of Power Sources 276 (2015) 247e254 249

2.3. Materials characterization

The crystal structure of the synthesized samples was deter-mined by powder X-ray diffraction (XRD, Rigaku, D/MAX 2500)using a copper Ka radiation source (l ¼ 0.154 nm). X-ray photo-electron spectroscopy (XPS) measurements were performed usingan Al Ka source (Kratos Analytical Ltd., UK) and the binding energyof 284.8 eV for C 1 s was used as calibration. The morphology andmicrostructure of the samples were characterized by field-emissionscanning electron microscopy (FE-SEM, Hitachi, S4800) equippedwith energy dispersive spectroscopy (EDS) and transmission elec-tron microscopy (TEM, JEOL 2100F) equipped with selected areaelectron diffraction (SAED). The Raman spectra were recorded on aRenishaw InVia system with the excitation laser at l ¼ 532.

2.4. Electrochemical characterization

The electrochemical tests were conducted on the two-electrodeCR2032 type coin cells. The working electrodes were prepared bypasting a mixture of the active materials (LTO NSs or LTO NSs/Ag),carbon black, and polyvinylidene fluoride (PVDF) at a weight ratioof 80:10:10 onto a Cu foil which acted as a current collector. Theelectrodes were dried at 80 �C for 6 h in air and then at 120 �C invacuum for another 12 h followed by pressed. Every electrode wasweighed accurately on an electronic balance and the weight of theactive materials was controlled to be 1e2 mg. The coin cells wereassembled in an argon-filled gloved box with a metallic lithium foilas the counter electrode, 1 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DEC) (1:1, volume ratio) as the electrolyte, andCelgard 2400 polypropylene as the separator. The charge/dischargemeasurements were performed at various current densities over avoltage range of 1e2.5 V (vs Li/Liþ) using a multi-channel batterytest system (NEWARE BTS-610). Cyclic voltammetry (CV) was car-ried out on an electrochemical workstation (CHI660D) in thevoltage range of 1e2.5 V (vs Li/Liþ) at different scanning rates.Electrochemical impedance spectroscopy (EIS) was carried out byapplying a perturbation voltage of 10 mV in a frequency range of100 kHz to 10 mHz on a CHI660D electrochemical workstation.

3. Results and discussion

In our previous reports, we found that the doping of rare earthions had obvious effect on the size and morphology of SnO2nanocrystals [51]. Herein, we attempt to verify similar effect on LTONSs by doping Gd3þ ions with appropriate content. The phasestructure of the LTO NSs and LTO NSs/Ag samples is firstly charac-terized by XRD. As shown in Fig. 1a, all the peaks can be indexed tothe spinel structure (JCPDS Card No. 49-0207, S.G.: Fd3m)) of bareLTO NSs. The sharp and well-defined peaks suggest highly crys-tallinity. No other peaks such as TiO2 can be detected indicatinghigh purity. These results confirm that hydrothermal productindexed by orthorhombic Li1.81H0.19Ti2O5$xH2O (see Fig. S1)completely transforms into the cubic Li4Ti5O12 phase. The result isconsistent with those reported before [15, 49]. Compared to bareLTO NSs, the XRD spectrum of the LTO NSs/Ag composite exhibitstwo broad and weak peaks at 38.1� and 44.3� ascribed to the (111)and (200) peaks of cubic silver, respectively, in addition to the mainspinel structure. The results indicate that Ag nanocrystals arecoated on the surface of the LTO NSs. EDS (Fig. 1b) shows the ex-istence of Ag on the surface of LTO NSs in agreement with XRD.Quantitative EDS reveals that theweight ratio of Ag is about 3.3%. Infact, loading content of silver in the material is an importantparameter that affects the electrochemical performance of theobtained LTO NSs/Ag composite. With different loading contents ofsilver, the cycling performances of the obtained LTO NSs/Ag

composites are different. As shown in Fig. S2, the loading content ofsilver with nominal 3.6 wt% is an optimized value. The detectableGd3þ, which is below the detection limitation of EDS measurementand can be confirmed by high resolution XPS (see Fig. S3) is farbelow the nominal content, implying that it is difficult to dope Gd3þ

into LTO host due to large difference of ionic diameters betweenGd3þ and Ti4þ. The chemical composition of the samples is studiedby Raman scattering. As shown in Fig. 1c, the Raman spectrum ofLTO NSs shows typical Li4/3Me5/3O4 spinel features with all fiveactive Raman phonon modes (A1g þ Egþ3F2g) indicative of cubicspinel LTO [47]. The strong band at 678 cm�1 (A1g) with a shoulderat 754 cm�1 is assigned to the stretching vibrational mode of TieOcovalent bonding in TiO6 octahedra. The stretching vibrationalmode of LieO ionic bonds located in the LiO4 tetrahedra (Eg) ispresent at 431 cm�1. Three bands (F2g) at 340 cm�1, 268 cm�1 and234 cm�1 originate from the vibration of lithium, which isoctahedrally-coordinated by oxygen. Similar Raman modes can beobserved from the LTO NSs/Ag composite but the intensity isdepressed and it is consistent with that reported by Krajewski et al.[47]. The results provide evidence of the existence of Agnanocrystals.

The morphology and structure are then determined by SEM,TEM and high-resolution TEM (HR-TEM). Representative SEM im-age (see Fig. S4) at a low magnification of Li1.81H0.19Ti2O5$xH2Oprecursor indicates that the main product consists of two dimen-sional nanosheets. Fig. 2a depicts a typical SEM image at a lowmagnification of the as-obtained LTO NSs, which indicates that thesample maintained the original sheet-like morphology of the pre-cursor. Fig. 2b shows a typical TEM image of as-obtained LTO NSs.The thickness of a typical vertical nanosheet with about 9 nm canbe observed. Atomic force microscopy analyses also (see Fig. 2c)display the 2D sheet-like feature of single LTO NSs with a thicknessabout 10 nm. The inset in Fig. 2b is the representative SAED patterntaken from an individual sheet side suggesting single crystallinity.As shown in Fig. 2d, the crystalline region with clear lattice fringeshas an inter-planar spacing of approximately 0.48 nm, corre-sponding to the (111) atomic planes of spinel LTO and in goodagreement with the SAED patterns. The SEAD and HR-TEM resultsalso indicate that LTO NSs grow along the ð011Þ plane. The thick-ness of LTO NSs in our experiment is thinner than the LTO samplewithout the introduction of Gd3þ (see Fig. S5) and those reported inRefs. [15,49] prepared under high calcination temperature of700 �C. The results imply that the minute content of Gd3þ likelyaffect the growth behavior of LTO NSs. When Gd3þ ions are intro-duced into LTO host, they occupy Ti4þ sites and generate negativecharges at the center of the nanosheets due to the lower valence ofGd3þ than Ti4þ. In order to establish charge balance, extra positiveions have to be introduced into nanosheet surface, which formtransient electric dipole with the direction pointing from the centerto surface [51,52]. These surface electric dipoles likely benefit tostabilize the surface of LTO and preserve the precursor's ultrathinsheet morphology [49]. Further origins for the formation of thinnerLTO NSs are in progress. The thinner NSs with high-quality not onlypromote uniform deliveries of electrons and ions due to theirsingle-crystal nature, but also improve the kinetics of delithiationand lithiation due to short Li ion diffusion distances.

Fig. 3a shows typical TEM image of the LTO-NSs/Ag composite,indicating that the LTO NSs maintain the original sheet-likemorphology without destruction by electroless deposition of Agnanocrystals. The spherical silver nanoparticles are highlydispersed on the surface of LTO NSs. From the high-angle annulardark-field (HAADF) scanning TEM image (see Fig. 3b), the dispersedAg nanocrystals are distinguished from the LTO NSs by their brightcontrast. The average size of the Ag nanocrystals is determined tobe 5.8 nm from statistical histogram, which also shows a narrow

Fig. 2. (a) and (b) Typical SEM and TEM images of the LTO NSs. The inset in (b) shows the SAED patterns of a single LTO nanosheet. (c) Representative AFM image (see inset) of singleLTO nanosheet and its corresponding thickness analysis taken along the marked line. (d) Typical HRTEM image of LTO NSs.

Fig. 3. (a) and (b) Typical TEM and high-angle annular dark-field (HAADF) scanning TEM images of the LTO NSs/Ag composite; (c) Statistical histogram of the Ag nanocrystals sizedistribution; (d) typical HRTEM image of the LTO NSs/Ag composite.

G.B. Xu et al. / Journal of Power Sources 276 (2015) 247e254250

size distribution and that the size of the majority of Ag nanocrystalsis 3e9 nm (Fig. 3c). Fig. 3d depicts the typical HR-TEM image of theLTO NSs/Ag composite revealing that Ag nanocrystals are in closecontact with highly-crystalline LTO NSs. The lattice arrangement inthe Ag nanocrystals is clearly visible indicative of a highly crystal-line nature. The distance between two fringes is 0.24 nm which

corresponds to the distance of the (111) plane of standard cubic Ag.The surface chemical composition and electronic states of the LTONSs/Ag composite are assessed by XPS. The survey XPS spectrum(not shown) reveals the presence of Li, Ti, O, and Ag in goodagreement with the nominal atomic composition of the LTO NSs/Agcomposite beside minute content of Gd3þ. The high-resolution Li

G.B. Xu et al. / Journal of Power Sources 276 (2015) 247e254 251

1 s XPS spectrum in the low binding energy region in Fig. 4a showsa weak Li 1 s peak at 54.6 eV from the LieO bond in the spinel LTOstructure. The peak at 61.6 eV originates from the Ti 3S band. Fig. 4bshows the high-resolution O 1 s XPS spectrum. The asymmetric O1 s peak displays two dominant components, one at 530.3 eV andthe other at 532.2 eV, corresponding to TieO bonding in the spinelLi4Ti5O12 structure and chemisorbed oxygen on the surface ofspinel crystallites, respectively. Fig. 4c presents the high-resolutionXPS spectra of Ti 2p. Besides one satellite peak at 460.8 eV, twopeaks are observed at 458.7 and 464.2 eV assigned to Ti 2p3/2 and2p1/2, respectively, from titanium in the IV oxidation state. Fig. 4dshows the high-resolution Ag3d binding energy region which canbe fitted well with two spineorbit doublets by Gaussian fittingmethod. The peaks at 367.38 and 373.19 eV correspond to thebinding energies of Ag3d5/2 and Ag3d3/2, respectively. The spinenergy separation is 5.81 eV indicates that the silver nanocrystals inthe LTO NSs/Ag composite have a metallic nature [39,47]. In addi-tion, the binding energy and spin energy separation of Ag nano-crystals are smaller than those reported previously [39], thusindicating electron coupling between LTO NSs and deposited Agnanocrystals.

The electrochemical performance of the LTO NSs/Ag compositeis systematically evaluated in a half-cell in which metallic lithiumacts as the counter and reference electrodes and the results arecompared with those of the LTO NSs. In this work, we focus on theeffect of silver nanocrystals on the electrochemical performance ofLTO NSs. The influence of Gd3þ doping on the morphology, size andelectrochemical performance is in progress. Fig. 5a depicts thechargeedischarge voltage profiles of the LTO NSs/Ag and LTO NSselectrodes for the 1st, 10th, and 100th cycles at a current rate of 1 Cwithin a cut off window of 1.0e2.5 V. The LTO NSs and LTO NSs/Agelectrodes have similar charge and discharge flat plateaus at around1.5 V resulting from a two-phase reaction during electrochemicallithium insertioneextraction according to the following equation[31,53,54]:

Fig. 4. High-resolution XPS of the LTO-NSs/Ag compo

Li4Ti5O12 þ xLiþþxe� 4 Li4þxTi5O12 (0 < x < 3) (4)

Addition of Ag nanocrystals does not alter the electrochemicalreaction of LTO. Fig. 5b shows the cycling performance of the LTONSs and LTO NSs/Ag composite at the standard dischargeechargerate of 1 C. The initial discharge capacities are 178.3 mAh g�1 and188.2 mAh g�1 for the LTO NSs and LTO NSs/Ag composite,respectively, which show higher than theoretical specific capacityat the first discharge. The high discharge capacities can beexplained by possible occupation of the 8a sites in nanoscale LTO byLiþ when the storage limit at the 16c sites of spinel structure isexceeded [31]. Co-occupation of the 8a and 16c sites in the surfaceregion is expected to be more significant in ultrathin NSs where thespecific surface area is larger, resulting in storage propertiesdifferent from those of the bulk. With regard to the LTO NSs/Agcomposite, the stable reversible capacity of electrode after the 20thcycle is 174.1 mAh g�1 and can be retained at 167.8 mAh g�1 after100 cycles with a retention of 96.3.1%. They are larger than those ofthe LTO NSs with stable reversible capacity and retention of172.5 mAh g�1 and 95.1%, respectively, when tested at the samecurrent density. The results indicate that the addition of Ag nano-crystals has a positive impact on the charge/discharge capacity ofLTO NSs and the reversibility is improved. To evaluate the high-rateperformance of the LTO NSs/Ag composite and LTO NSs, the ratecapability is determined. The electrode is initially cycled at 1 Cwhere the stable charge capacity is about 185 mAh g�1 in the first20 cycles. The rate increases stepwise to 5, 10, 20 and 30 C in suc-cessionwith each rate stage cycled for 20 times. As shown in Fig. 5c,the specific capacity of the LTO NSs/Ag composite is superior to thatof LTO NSs at all charge/discharge rates. The relative increase in thespecific capacity is larger at higher rates. For example, at a rate of30 C, the specific capacity of the LTO NSs/Ag composite is140.1 mAh g�1, which is greater than that of LTO NSs(126.4 mAh g�1). The results imply highly efficient solid-statediffusion of lithium in the LTO NSs/Ag composite. More

site: (a) Li 1 s, (b) Ti 2p, (c) O 1 s and (d) Ag 3d.

Fig. 5. (a) Chargeedischarge voltage profiles of the LTO NSs/Ag and LTO NSs electrodes for the 1st, 10th, and 100th cycles at a current rate of 1 C; (b) Cycling performance of the LTONSs and LTO NSs/Ag electrodes at the standard dischargeecharge rate of 1 C; (c) Cycling performance at different current rates from 1 C to 30 C of the LTO NSs and LTO NSs/Agelectrodes; (d) Coulombic efficiency versus cycle number of the LTO NSs and LTO NSs/Ag electrodes at different current rates (1e30 C).

G.B. Xu et al. / Journal of Power Sources 276 (2015) 247e254252

importantly, a stable capacity of 172 mAh g�1 can be retained whenthe current rate is returned to 1 C after 120 cycles and it is retainedfor the next 20 cycles with negligible loss. Furthermore, thecoulombic efficiencies of the LTO NSs/Ag composite at high charge/discharge rates approach 100% for each cycle (see Fig. 5d). Both therate capability and cycling performance of the LTO-NSs/Ag com-posite are improved when compared with previous results of LTOmaterials modified with noble metals [35,39,46,47] and N dopedLTO NSs [55]. The results demonstrate excellent rate capability thusmaking the LTO NSs/Ag composite suitable for high power LIBs.

The superior electrochemical performance of the LTO NSs/Agcomposite can be attributed to the enhanced transport kinetics ofthe electrode. Owing to the unique two-phase reaction duringelectrochemical lithium insertioneextraction shown in Eq. (4), LTOmaterials with an excellent rate capability should possess both highelectrical conductivity and lithium ion diffusion [54]. Electro-chemical polarization is one of the most important factors, whichassociates with electrical conductivity of the electrode [37,54].Fig. 6a shows the magnified image of the region marked by ellipsein chargeedischarge voltage profiles (see Fig. 5a) of the LTO NSs/Agand LTO NSs electrodes for the 1st and 100th cycles at a current rateof 1 C. Notably, the LTO NSs/Ag composite presents a flatter profilethan LTO-NSs. The polarizations between the charge and dischargeplateau are 70 and 50 mV at the 1st and 100th cycles, respectively,for LTO NSs/Ag composite, which is lower than the values of 80 and90 mV for LTO NSs. This lower polarization implies improvedelectrical conductivity due to the modification of the Ag nano-crystals [54,49].

To investigate the synergetic enhancement of transport kinetics,CV measurements are conducted on the LTO NSs/Ag and LTO NSselectrodes at various scanning rates. The anodic peaks at about1.70 V (vs. Li/Liþ) and the cathodic peaks at around 1.4 V (vs. Li/Liþ)are correspond to the processes of Li deintercalation and interca-lation. The voltage differences between anodic and cathodic peaks

reflect the polarization degree of the electrode. As shown in Fig. 6band c, for all scanning rates, the potential difference betweencathodic and anodic peaks of the LTO NSs/Ag (for example, 0.25 V(see the inset of Fig. 6c)) is smaller than that of the LTO NSs (0.31 V),which suggests that the modification of Ag nanocrystals is favor-able for reducing the electrode polarization. Usually, the change inthe cathodic and anodic peaks with sweeping rates reflects thekinetics of lithium insertion/extraction at the electrode/electrolyteinterface and lithium diffusion in the film. From 6b and 6c, one canfind that the peak currents of the LTO NSs/Ag electrodes are higherthan those of the LTO NSs electrodes, indicating that modificationwith Ag nanocrystals yields much faster lithium diffusion andhigher lithium storage capacity. In addition, as the scanning ratesincrease, the cathodic and anodic peaks move to lower and higherpotential, respectively, with increasing peak currents. As shown inFig. 6d and S7, the currents (Ip) of the anodic and cathodic peakexhibits a linear relationship with the square root of the scanningrate (v1/2 V1/2), thus indicating a diffusion-controlled reactionrather the surface control [54,56]. Especially, the slopes for anodicand cathodic peak on the LTO NSs/Ag are higher than those for LTONSs. This result implies that the LTO NSs/Ag has improved Li-ionchemical diffusion, resulting in enhanced electrochemical perfor-mance in lithium storage.

To further confirm that the incorporation of Ag nanocrystals intoultrathin LTO NSs speeds up transport kinetics of the electrode, themeasurement of EIS for the LTO NSs/Ag and LTO NSs electrodes areconducted. The resulting Nyquist plots (see Fig. 7a) showa commonfeature of a purely resistive response at the high frequency endattributed to the ohmic resistance (Rs) of electrode and electrolyte,a semicircle attributed to the charge-transfer impedance (Rct) onthe electrodeeelectrolyte interface in the high-to-middle fre-quency region, and an inclined straight line ascribed to the War-burg impedance (Zw) in the low frequency region. The obtainedEIS spectra were well simulated by Z-view software using inset

Fig. 6. (a) The magnified region marked by ellipse in chargeedischarge voltage profiles (see Fig. 5a) of the LTO NSs/Ag and LTO NSs electrodes for the 1st and 100th cycles at acurrent rate of 1 C; (b) and (c) CV plots of the LTO NSs and LTO NSs/Ag at various scanning rates from 0.1 to 2.0 mV s�1. The inset of (c) show the comparison of CV plots of the LTONSs and LTO NSs/Ag electrodes at a scan rate of 0.1 mV s�1; (d) Linear relationship of anodic peak current (Ip) in (b) and (c) versus square root of scanning rate (v1/2 V1/2).

Fig. 7. (a) Nyquist plots of the LTO NSs and LTO NSs/Ag electrodes. The inset show theequivalent circuit used to fit the EIS. (b) Graph of Z

0plotted against 6�1=2 at low fre-

quency region (10e0.1 Hz) for the LTO NSs and LTO NSs/Ag electrodes.

G.B. Xu et al. / Journal of Power Sources 276 (2015) 247e254 253

equivalent circuit model and the corresponding simulation pa-rameters are presented in Table 1. From EIS spectra, one can findthat the semicircular arc of the LTO NSs/Ag electrode is muchsmaller than that of the LTO NSs electrode. The smaller semicirculararc is an indication of an overall smaller charge transfer resistanceor more facile charge transfer process at the electrode/electrolyteinterface for the LTO NSs/Ag electrode [54,57], which is consistentwith the simulation results shown in Table 1. This indicates thatmodification by Ag nanocrystals improves the electrical conduc-tivity of the LTO NSs/Ag composite and relieves the polarization ofthe electrode, thereby enhancing the rate capability. The lithiumion diffusion coefficient (DLi) can also be evaluated from EIS ac-cording to the following equations [49]:

DLi ¼R2T2

2A2n4F4C2s2(5)

Z0 ¼ Rs þ Rct þ su�1=2 (6)

Where R is the gas constant, T is the absolute temperature, A is thesurface area of the electrode, n is the number of electrons trans-ferred in the half reaction for the redox couple, F is the Faradayconstant, C is the concentration of lithium ions and s is theWarburgfactor. The value for s can be obtained from the slope of the linesbetween Z0 and6�1=2 as shown in Fig. 7b. Then, theDLi values of theelectrodes can be calculated and the results are presented inTable 1. The DLi value for the LTO NSs/Ag electrodes is

Table 1Rs, Rct, s and Di values for the LTO NSs and LTO NSs/Ag electrodes according to Fig. 7aand b.

Electrode RS (U) Rct(U) s Di(cm2 s�1)

LTO NSs/Ag 3.823 22.67 3.31 9.658 � 10�11

LTO NSs 3.997 49.99 6.35 2.637 � 10�11

G.B. Xu et al. / Journal of Power Sources 276 (2015) 247e254254

9.658 � 10�11 cm2 s�1, which is much higher than the value of2.637 � 10�11 cm2 s�1 of the LTO NSs electrodes. The results indi-cate that the modification by Ag nanocrystals enables fast migra-tion of lithium ions to reach the interior of LTO NSs, enhancing thefull utilization of the active materials. Consequently, superiorelectrochemical performances are demonstrated in the LTO NSs/Agcomposite.

4. Conclusion

In summary, we have prepared a novel composite consisting ofultrathin single-crystal LTO NSs and highly dispersed Ag nano-crystals via a three-step synthesis of hydrothermal reaction, postcalcination and electroless deposition. Due to synergetic effect ofultrathin highly-crystalline LTO NSs and ultra-small dispersed Agnanocrystals, the LTO NSs/Ag composite show lower polarization ofthe voltage difference, higher electrical conductivity, and largerlithium ion diffusion coefficient, resulting in improved lithiumstorage performance compared to pure LTO NSs as anode materialsin LIBs. This unique hybrid structure has an ability to deliver highreversible capacities with excellent cyclic capacity retention atdifferent current rates as well as high-rate performance at a currentrate as high as 30 C in comparison with previous results of LTOmaterials modified with noble metals and N doped LTO NSs.

The combination of highly-crystalline ultrathin LTO NSs andhighly dispersed Ag nanocrystals is an effective strategy to improvethe performance of LTO-based anode materials in LIBs.

Acknowledgments

This workwas financially supported by the Grants fromNationalNatural Science Foundation of China (Nos. 51272220, 11374252,11474242 and 51472209), the Program for Changjiang Scholars andInnovative Research Team in University (IRT13093), and City Uni-versity of Hong Kong Applied Research Grant No. 9667085.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.jpowsour.2014.11.108.

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Electronic Supplementary Information

Highly-crystalline ultrathin Li4Ti5O12 nanosheets decorated with silver nanocrystals as

a high-performance anode material for lithium ion batteries

G. B. Xua, W. Lib, L. W. Yanga,b, X. L. Weia, J. W. Dinga, J. X. Zhonga, Paul K. Chub

a Hunan Key Laboratory of Micro-Nano Energy Materials and Devices, Faculty of

Materials and Optoelectronic Physics, Xiangtan University, Hunan 411105,

China

b Department of Physics and Materials Science, City University of Hong Kong, Tat

Chee Avenue, Kowloon, Hong Kong, China

Corresponding authors: ylwxtu@xtu.edu.cn (LW Yang)；paul.chu@cityu.edu.hk (PK

Chu)

Figure S1 XRD pattern of as prepared hydrothermal product.

0 20 40 60 80 100 120100

120

140

160

180

200

1C

30C

20C

10C

5C

Ca

pa

city(m

Ah

-1)

Cycle number

LTO NSs/Ag(1.8%)(Discharge)

LTO NSs/Ag(1.8%)(Discharge)

LTO NSs/Ag(3.6%)(Discharge)

LTO NSs/Ag(3.6%)(Discharge)

LTO NSs/Ag(5.4%)(Discharge)

LTO NSs/Ag(5.4%)(Discharge)

LTO NSs

LTO NSs

1C

Figure S2 Cycling performance at different current rates from 1 C to 30 C of LTO

NSs and LTO NSs/Ag composites with different loading contents of silver

Figure S3 High-resolution Gd 3d XPS of the LTO-NSs/Ag composite.

Figure S4 Typical SEM image of as prepared hydrothermal product.

1180 1200 1220 1240

Gd 3d

Binding Energy (eV)

Inte

nsity (

a.

u.)

Figure S5 Typical TEM image of as prepared LTO sample without the introduction of

Gd3+.

Figure S6 Typical high-angle annular dark-field (HAADF) scanning TEM image (a)

and spot-profile EDX spectra (b) of LTO NSs/Ag composite

Figure S7 Linear relationship of anodic peak current ( pI ) in (a) and (b) versus square

root of scanning rate ( 2/1v V1/2).

0.0 0.5 1.0 1.5 2.0 2.50

2

4

6

8

10

12

peak 2 (LTO NSs/Ag)

peak 2 (LTO NSs)

peak 2 fitting

Lower 2 fitting

Cu

rre

nt (m

A)

v1/2

(mV/s)1/2

0 2 4 6 8 10

0

50

100

150

Cu

Cu

Ti

Ti

Cu

O

Ti

Energy (KeV)

Ag

Inte

nsity (

a.

u.)

(a)

(b)