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Enhanced Conductivity and Structure Stability of Ti Doped LiVO as Anodes for Lithium-Ion Batteries 4+

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Chaonan Mu, Kaixiang Lei, Haixia Li, Fujun Li, and Jun Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08197 • Publication Date (Web): 09 Nov 2017 Downloaded from http://pubs.acs.org on November 10, 2017

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

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Enhanced Conductivity and Structure Stability of

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Ti4+ Doped Li3VO4 as Anodes for Lithium-Ion

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Batteries

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Chaonan Mu, Kaixiang Lei, Haixia Li, Fujun Li*, Jun Chen

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Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of

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Chemistry, Nankai University, and Collaborative Innovation Center of Chemical Science and

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Engineering, Tianjin 300071, China

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ACS Paragon Plus Environment

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ABSTRACT: Tunnel-type Li3VO4 (LVO) has become of great interest as a promising anode

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material for lithium-ion batteries owing to its large capacity, low cost, and safe potentials.

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However, the poor conductivity of LVO limits its wide applications. Here, LVO was doped with

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0.05 unit of Ti4+ in each formula (LTVO) via a solid-state synthetic method to increase its

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conductivity and structural stability. The partial replacement of V5+ with Ti4+ leads to formation

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of additional interstitial Li+ ions in the octahedrons between MO4 (M = Li, V) tetrahedrons, and

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hence improves Li+ ion diffusion kinetics. The resultant LTVO electrode exhibits a high

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discharge/charge capacity of ~376 mAh g-1 at 20 mA g-1. Good cycling stability over 500 cycles

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with small polarization is achieved at 1000 mA g-1. This makes it a promising anode for Li-ion

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batteries.

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INTRODUCTION

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Lithium-ion batteries (LIBs) are playing an increasingly vital role in portable electronic

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devices and electric vehicles.1,2 However, the safety concern is a great challenge for LIBs due to

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formation of Li dendrites at low Li deposition potentials, especially at high current loads.3,4

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Graphite is a dominant anode in commercial LIBs and is reversibly intercalated with Li+ ions at

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~0.1 V versus Li+/Li to form LiC6 with a theoretical capacity of 372 mAh g-1.5,6 The low

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operating potential of graphite is close to the plating potential of Li and favors formation of Li

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dendrites.7 Replacing the graphite anodes with alternatives, which possess Li+ intercalation

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potentials of ˃0.3 V, has been suggested for the application of LIBs in high-power devices. In

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line with this point, the representative of Li4Ti5O12 with working potentials at around 1.5 V vs.

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Li+/Li was attempted to against LiFePO4 in a full cell and exhibited good electrochemical

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performance.8,9 However, the high working potential of Li4Ti5O12 compromised the voltage of

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the full cell and thus energy density.10-12 Therefore, searching for suitable anodes with both safe

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operating potentials and large specific capacity will be urgent for the development of LIBs.

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β-Li3VO4 (LVO) has a potential slope between 0.5 and 1.0 V and high theoretical capacity

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(~394 mAh g-1), and attracts much attention.13-15 However, LVO suffers low electrical

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conductivity and sluggish reaction kinetic, which severely affect its electrochemical

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performance.16,17 Lots of efforts have been made to address the problems, including decreasing

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particle size,13 carbon-coating,18,19 designing rational micro/nanostructures.20-22 These methods

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fail to improve the intrinsic electronic conductivity and diffusion coefficient of Li+ ions.

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Considering the drawbacks, aliovalent doping was applied as an effective strategy to tune the

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atomic arrangement and electronic structure.23,24 Massarotti et al. found that the Li3VO4 doped

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with Cr3+ facilitated formation of Li vacancies and resulted in enhanced ionic conductivity.24


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Cao’s group reported Ni-doped LVO via a co-precipitation process, exhibiting small

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overpotential and fast reaction kinetics. This was attributed to the increased surface energy of Ni-

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LVO that favored intercalation reaction at the interface.25 In addition, Ti was reported to stabilize

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the layered structure and enhance electrochemical performance of transition metal oxides for

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sodium storage.26 Therefore, doping Ti into LVO will be of great interest and significance for

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LIBs.

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LVO is tunnel-structured and built by the corner-sharing LiO4 and VO4 tetrahedrons. With a

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similar radius and electronegativity to V5+, Ti4+ can be introduced into the VO4 tetrahedron to

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take the place of V5+. This aliovalent substitution generates additional interstitial Li+ cations for

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charge compensation, which leads to a higher conductivity.27-29 Herein, Ti is doped into LVO to

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enhance Li+ diffusion kinetics and electronic conductivity and to improve the electrochemical

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performance. These are attributed to the presence of interstitial Li+ ions upon the substitution of

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V5+ by Ti4+. A high capacity of ~376 mAh g-1 at 20 mA g-1 and good cycling stability have been

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achieved, which are of great significance for high performance LIBs.

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2. EXPERIMENTAL SECTION

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Material Synthesis. Li3+xV1-xTixO4 (x=0.0 and 0.05) was synthesized via a high-temperature

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solid-state reaction. Based on the doping levels of Ti in LVO, the corresponding samples are

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named as LVO and LTVO, respectively. Briefly, an appropriate ratio of Li2CO3 (Aladdin, ＞

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99%), V2O5 (Energy chemical, 99%), and TiO2 (Aladdin, 99.8%) were mixed by ball-milling at

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300 rpm for 24 h. An excess of 10 wt.% Li2CO3 was added to compensate lithium loss in the

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high-temperature calcination process. The obtained mixture was pressed into pellets under a


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pressure of 20 MPa for 15 minutes. Finally, the pellets were calcined in an alumina crucible at

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600 oC for 5 h and then at 700 oC for 24 h in air. The heating rate was 5 oC min-1.

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Characterization. The phase composition and crystallographic structures of the as-synthesized

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samples and intermediates at different discharge and charge states were analyzed by powder X-

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ray diffraction (XRD) on a Rigaku X-2500 diffractometer using Cu Kα radiation. Rietveld

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refinement was conducted to determine the lattice parameters by using the GSAS/EXPGUI

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package. The morphologies and microstructures of LVO and its derivatives were examined by

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scanning electron microscopy (SEM, JEOL JSM-7500F) and transmission electron microscopy

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(TEM, Tecnai F20). Energy Dispersive X-ray spectroscopy (EDX, JEOL JSM-7500F) and

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inductively coupled plasma test (ICP-9000 (N+M) USA Thermo Jarrell-Ash Corp) were used to

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analyze the composition of elements in samples. Further investigation on the valence of V and Ti

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in LVO and LTVO was performed on X-ray photoelectron spectroscopy (XPS, Perkin Elmer

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PHI 1600 ESCA system).

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Electrochemical Measurements. The CR2032 coin cells, which were assembled in an argon-

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filled glovebox, were used to investigate the electrochemical performance of LVO and LTVO.

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The working electrode was prepared by mixing active material, Super P, and polyvinylidene

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fluoride (PVdF) with a ratio of 7:2:1 in N-methyl-2-pyrrolidone (NMP). The obtained slurry was

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pasted onto a Cu foil and then dried in a vacuum oven at 110 oC for 10 h. The loading of active

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material in each electrode is about 1.0 mg cm-2. 1.0 M of LiPF6 in ethylene carbonate (EC)/ethyl

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methyl carbonate (EMC)/dimethyl carbonate (DMC) (1:1:1 in V/V/V) was used as electrolyte,

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and 60 μL of the electrolyte was used in each coin cell. The separator was Celgard 2400 porous

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polypropylene. Galvanostatic charging/discharging measurements and galvanostatic intermittent

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titration technique (GITT) were conducted in a voltage range of 0.2 to 3.0 V (vs. Li/Li+) on Land


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CT2001A battery instruments. Cyclic voltammetry (CV) was carried out on a Parstat 263A

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electrochemical workstation at different scan rates. Electrochemical impedance spectroscopies

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(EIS) were tested on a Parstat 2273 electrochemical workstation (AMETEK). The AC

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perturbation amplitude was 5 mV with a frequency ranged from 100 mHz to 100 kHz. To

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investigate the structure and morphology evolution of the electrode during cycling, the electrode

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was taken out from the cycled cell in the glovebox and washed with dimethyl carbonate (DMC).

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3. RESULTS AND DISCUSSION

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XRD patterns and the corresponding Rietveld refinement of LVO and LTVO are shown in

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Figure 1a and b, respectively. They match well with the standard (Joint Committee on Powder

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Diffraction Standards, JCPDS No. 38-1247), and confirm the orthorhombic phase of high purity.

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Rietveld refinements on both LVO and LTVO reveal that Ti doping in LVO maintains the

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orthorhombic tunnel model of space group Pmn21. The derived crystal lattice parameters have a

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slight increase in LTVO (a=6.35 Å, b=5.47 Å, c= 4.97 Å and V= 172.88 Å3 from the pure LVO

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(a=6.33 Å, b=5.45 Å, c= 4.95 Å and V= 170.54 Å). This is due to the slightly larger ionic radius

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of Ti4+ (0.42 Å) than V5+ (0.36 Å). The detailed structural parameters of LVO and LTVO are

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summarized in Table S1 and S2. The V and Ti ions occupy the tetrahedral 2a Wyckoff positions,

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suggesting uniform incorporation of Ti4+ into the tetrahedrons of transition-metal oxides.30 As

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depicted in Figure 1c, the tunnel-structured LVO is constructed with alternating tetrahedrons of

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LiO4 and VO4 via corner-sharing. Upon Ti substitution, the subtle tilting of MO4 tetrahedrons (M

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= Li or V) is accompanied by the appearance of Ti-O bonds and a simultaneous increase in bond

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length of V-O bonds from 1.721, 1.720, and 1.714 Å to 1.812, 1.728, and 1.797 Å, respectively,

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in Figure 1d. It is clear that Ti4+ in the place of V5+ in LTVO retains the tunnel structure and


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creates more Li+ ions sitting in octahedral sites between MO4 tetrahedrons for charge

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compensation, which may favor Li+ ion migration in the three-dimensional channels of LTVO.

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Figure 1. Experimental XRD patterns, Rietveld-refined profiles, and the corresponding crystal

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structure of LVO (a, c) and LTVO (b, d).

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SEM images of LVO and LTVO in Figure 2a and b exhibit irregular shapes with particle sizes

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ranging from hundreds of nanometers to micrometers. Uniform distribution of Ti and V in

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LTVO is revealed by EDX mappings in Figure S2. The molar ratio of Li:Ti:V is calculated to be

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3.051:0.046:0.952 based on ICP measurement, which is consistent with the EDX and the

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stoichiometric composition of LTVO (Li3.05Ti0.05V0.95O4). The crystal structure of LVO and


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LTVO was characterized by HRTEM in Figure 2c. The clear lattice fringes display the

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interplanar distance of 0.372 nm, which is in accordance with the d-spacing of the (011) crystal

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planes of LVO. The interlayer spacing of 0.378 nm in Figure 2d corresponds to the (011) crystal

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plane of LTVO, indicating a slight expansion upon Ti doping. This is in good agreement with the

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refined structural parameters of LTVO in Figure 1b and Table S2.

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The chemical valance states of V and Ti in LVO and LTVO were analyzed by applying XPS.

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As shown in Figure 2e, two characteristic peaks at 524.2 and 516.9 eV in LVO can be assigned

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to 2p1/2 and 2p2/3 of V5+, respectively. After introducing Ti, the V2p spectrum of LTVO presents

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a similar peak profile with a slight negatively shifted binding energy, implying the distorted

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chemical environment of V with the addition of Ti and its valence state of +5. The Ti XPS

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spectrum is displayed in Figure 2f with two typical peaks at binding energies of 463.4 and 457.9

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eV, corresponding to 2p1/2 and 2p2/3, respectively. They are associated with Ti 2p1/2 and 2p2/3 in

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TiO2,31 suggesting the valance state of +4 of Ti in LTVO. Therefore, it can be speculated that for

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charge compensation interstitial Li+ ions are created in the octahedral sites between MO4

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tetrahedrons with the partial replacement of V5+ by Ti4+ in LVO.

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Figure 2. SEM and HRTEM images of LVO (a, c) and LTVO (b, d). XPS spectra of V2p (e)

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and Ti2p (f).

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Electrochemical performance of LVO and LTVO was evaluated in coin cells against Li metal

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in a voltage window from 0.2 to 3.0 V. Figure 3a shows the CV curves of LVO and LTVO in the

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third cycle at a scan rate of 0.1 mV s-1. In a cathodic process, LVO exhibits two obvious

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reduction peaks at 0.91 and 0.51 V, corresponding to insertion of Li+ into LVO and gradual


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reduction of V5+ to V4+ and V3+, respectively. In a following anodic process, reversible oxidation

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of V3+ occurs with two oxidation peaks at 0.85 and 1.27 V, respectively. For LTVO, the typical

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reduction and oxidation peaks related to V are observed, but the characteristic redox peak of Ti4+

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is not obvious due to its limited amount. Remarkably, the potential differences of the two

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couples of redox peaks of LTVO (0.30 and 0.25 V) are smaller than those of the pure LVO (0.36

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and 0.34 V), indicative of a better reversibility. This is ascribed to the enhanced kinetics of Li+

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ion diffusion upon introduction of Ti4+.

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Galvanostatic discharge and charge profiles of LVO and LTVO at 20 mA g-1 are presented in

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Figure 3b. LTVO displays a discharge/charge capacity of 376 mAh g-1, which is higher than that

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of LVO (316 mAh g-1). Particularly, the voltage gap between discharge and charge of LTVO is

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smaller than that of LVO, in good accordance with CV curves in Figure 3a. Cycle performance

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of LTVO is comparatively presented with LVO in Figure 3c at 1000 mA g-1. Obviously, LTVO

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maintains stable discharge capacities of 328.3 mAh g-1 with Coulombic efficiencies of almost

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100% after initial cycles. This is in contrast with the decaying capacities of LVO at the same

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testing condition. The irreversible capacity in initial cycles is resulted from formation of solid

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electrolyte interface (SEI).32 In Figure 3d, LTVO presents good rate capability at current density

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from 20 to 1000 mA g-1, and the achievable discharge capacities of LTVO are 395.7, 378.8,

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362.5, 348.2, 322.8, 300.1 mAh g-1, respectively. When the current density is switched back to

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20 from 1000 mA g-1, the discharge capacity of LTVO is recovered to 362.8 mAh g-1. A similar

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trend is found for LVO with smaller discharge capacities in Figure 3d. Therefore, the

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incorporation of Ti into LVO can efficiently reduce the electrochemical polarization and enhance

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the reversible capacity, cyclic stability and rate performance.


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Figure 3. Electrochemical performance of LTVO and LVO in the potential window from 0.2 to

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3.0 V: CV curves of the 3rd cycle at 0.1 mV s-1 (a); galvanostatic charge-discharge profile of the

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3rd cycle at 20 mA g-1 (b); cycle stability at 1000 mA g-1 (c) and rate capability (d).

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To further investigate the kinetics in both LTVO and LVO, galvanostatic intermittent titration

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technique (GITT) is employed. The applied current pulse is 20 mA g-1 for 0.5 h, and between

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each the battery is relaxed for 2 h to reach a quasi-equilibrium state. Figure 4a presents the

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typical GITT profiles of LTVO and LVO, indicative of small overpotentials during discharge

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and charge. Then, the diffusion coefficient (DLi+) can be calculated according to Fick’s second

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law. For a sufficient time interval, the equation for the DLi+ could be written as Equation 1:

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D Li+ =

D Es 4 m B Vm 2 ( ) ( )2 π M BS τ (dEτ / d τ )

(τ ≪

L2 ), D Li+


(1)

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Where, DLi+ (cm2 s-1) is chemical diffusion coefficient of Li+ ions; Vm (cm3 mol-1) is molar

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volume of compound, which is deduced from the crystallographic data; mB (g) and MB (g mol-1)

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are the active mass and molecular weight of LTVO or LVO; S (cm2) is the geometric area of

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electrode; L (cm) is the thickness of electrode; τ (s) is the relaxation time between each current

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pulse; ΔEτ (V) is the total change in cell voltage during the current flux, ΔEs (V) is the diﬀerence

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in open circuit voltage measured over the relaxation period. If Eτ is linearly proportional to τ1/2,

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the equation can be further simplified as Equation 2:

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D Li+ =

4 m B Vm 2 D E s 2 ( ) ( ) πτ M BS D Eτ

(τ ≪

L2 ). D Li+

(2)

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The DLi+ of LTVO and LVO at each potential in the discharging and charging processes is

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obtained and shown in Figure 4b and Figure S2, respectively. Remarkably, LTVO displays

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higher DLi+ than LVO in both discharging and charging processes. This is attributed to the

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additional interstitial Li+ ions upon the substitution of V5+ by Ti4+ in LTVO. CV measurement at

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different scan rates from 0.2 to 1.2 mV s-1 was also performed to investigate the Li+ ion diffusion,

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as shown in Figure S3. According to the Randles-Sevcik equation, the DLi+ in LVO and LTVO

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are estimated to be ~5.44×10-12 and ~1.36×10-11 cm2 s-1, respectively. This is in agreement with

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the enhanced Li+ ion diffusion kinetics as revealed by GITT in Figure 4b, and guarantees

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excellent rate performance of LTVO.


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Figure 4. (a) GITT pofiles of LTVO and LVO in the 3rd cycle at 20 mA g-1; (b) The

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correspongding diffusion coefficients calculated from GITT profiles; EIS spectra of LVO (c) and

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LTVO (d) before test and after the 1st, 2nd and 10th charge, and the enlarged regions (insets).

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Electrochemical impedance spectroscopy (EIS) was conducted from 100 kHz to 0.1 Hz to get

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more insights into the interfacial property between the electrode and electrolyte. As shown in

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Figure 4c and d, all the Nyquist plots of LTVO and LVO present a compressed semicircle in the

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high frequency region and an inclined line in the low frequency region. This indicates that the

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electrochemical process is determined by charge transfer in the electrode/electrolyte interface

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and Li+ ion diffusion. Before cycling, a smaller semicircle in the high frequency region is

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obviously observed on LTVO than that on LVO, manifesting its greatly lowered charge-transfer


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resistance (Rct). The enhanced conductivity in LTVO is probably derived from the formation of

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interstitial Li+ cations and distorted chemical environment of V.25 For LVO electrodes after 1st,

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2nd and 50th charging, the increased Rct after initial activation indicates sluggish charge transfer

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kinetics. In contrast, LTVO exhibits gradually stabilizing Rct in the initial 10 cycles, suggesting

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the enhanced electron transport and structural stability by Ti doping, while LVO presents

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increased Rct after initial activation. This is correlated with the demonstrated electrochemical

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performance.

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The reaction mechanism and structural evolution of LTVO upon Li+ insertion/extraction were

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investigated by using ex situ XRD. The discharge/charge curves of LTVO at 20 mA g-1 in the 3rd

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and 20th cycles are displayed in Figure S4a and b, respectively. Upon both discharging and

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charging, all the diffraction peaks of LTVO are preserved and assigned to the LTVO except the

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two peaks at 26.0o and 28.5o from adhesive tapes, which are used to prevent exposure of LTVO

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to air. This suggests that the crystalline structure of LTVO is maintained during

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interaction/deintercalation of Li+ ions. In particular, the selected diffraction patterns of LTVO in

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the 3rd and 20th cycles are displayed in Figure 5b and c, respectively. It can be found that the

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diffraction peaks of (101), (011) and (210) of LTVO located at 22.4o, 23.9o and 32.5o are slightly

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shifted to lower angles during discharge, which indicates expansion of crystal lattices upon Li+

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insertion. These diffraction peaks are gradually recovered back to their original locations. Even

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after 500 cycles, the reversible characteristic diffraction peaks can still be maintained at the

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discharge and charge states of LTVO. These confirm good structural stability and reversibility of

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LTVO upon the repeated Li+ insertion and extraction.


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Figure 5. (a) Discharge and charge profiles of LTVO; XRD patterns with selected diffraction

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angles at various discharge/charge states as indicated in (a): (b) in the 3rd cycle and (c) after 20th

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cycles, respectively.

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4. CONCLUSIONS

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Li3.05Ti0.05V0.95O4 (LTVO) has been successfully synthesized by a solid-state synthetic method.

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It exhibits smaller polarization and a higher reversible capacity of ~376.0 mAh g-1 at 20 mA g-1

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than the pristine LVO. It also presents good rate performance with high capacities of 395.7 and

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300.1 mAh g-1 at 0.02 and 1 A g-1, respectively, while LVO exhibits decaying capacities from

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342.8 to 218.5 mAh g-1. At 1000 mA g-1, LTVO presents stable discharge capacities of ~328.3

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mAh g-1 with Coulombic efficiencies of ~100% during 500 cycles. The improved

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electrochemical performance can be endowed by the partial replacement of V5+ with Ti4+, which

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introduces additional interstitial Li+ ions in octahedrons between MO4 (M = Li, V) tetrahedrons

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and contributes to faster kinetics, higher conductivities and enhanced structural reversibility.


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This strategy will encourage more investigations on design of electrode materials for enhanced

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battery performance.

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ASSOCIATED CONTENT

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Supporting Information

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The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.

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The detailed Refined structural parameters, SEM images, diffusion coefficients, CV curves and

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ex situ XRD patterns, Figures S1 to S5 and Tables S1 to S2. (PDF)

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AUTHOR INFORMATION

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Corresponding Author

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*E-mail: [email protected]

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ORCID

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Fujun Li: 0000-0002-1298-0267

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Jun Chen: 0000-0001-8604-9689

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ACKNOWLEDGMENT

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Financial supports from National Key R&D Program of China (2017YFA0206700), NSFC with

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grant No. 21603108 and 51671107, 111 project of B12015, and Natural Science Foundation of

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Tianjin (grant No. 17JCQNJC06200) are acknowledged.

19

REFERENCES


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