Enhanced Conductivity and Structure Stability of Ti4+ Doped Li3VO4

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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

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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

7

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

8

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

9

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

12

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

5

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

8

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]

12

ORCID

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Fujun Li: 0000-0002-1298-0267

14

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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Enhanced Conductivity and Structure Stability of Ti4+ Doped Li3VO4

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