Facile Lithium Ion Transport through Superionic Pathways Formed on

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Facile Lithium Ion Transport through Super-ionic Pathways Formed on the Surface of Li3V2(PO4)3/C for High Power Li Ion Battery Dongwook Han, Sung-Jin Lim, Yong-Il Kim, Seung Ho Kang, Yoon Cheol Lee, and Yong-Mook Kang Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 05 Jun 2014 Downloaded from http://pubs.acs.org on June 6, 2014

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Chemistry of Materials

Facile Lithium Ion Transport through Super-ionic Pathways Formed on the Surface of Li3V2(PO4)3/C for High Power Li Ion Battery Dong-Wook Han,† Sung-Jin Lim,† Yong-Il Kim,‡ Seung Ho Kang,§ Yoon Cheol Lee,§ and Yong-Mook Kang*,§ †

Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Yuseong, Daejeon 305-701, Republic of Korea

‡

Korea Research Institute of Standards and Science, P.O. Box102, Yuseong, Daejeon 305-340, Republic of Korea

§

Department of Energy and Materials Engineering, Dongguk University-Seoul, Seoul 100-715, Republic of Korea

ABSTRACT: We report a new discovery for enhancing Li ion transport at the surface of Li 3V2(PO4)3 particles through super-ionic pathways built along an ionic conductor. The Li3V1.95Zr0.05(PO4)3/C composite has much higher initial discharge capacity, superior rate-capability, and excellent cycling performance when compared with pristine Li 3V2(PO4)3/C. This is partly due to the occupation of vanadium sites by Zr4+ ions in the Li3V2(PO4)3 host crystals and facile Li ion migration through a LiZr2(PO4)3-like secondary phase that forms on the surface of the Li3V1.95Zr0.05(PO4)3 particles. Our findings about high Li ion transport and structure stabilization induced by Zr incorporation suggests a breakthrough strategy for achieving high-power Li rechargeable batteries using NASICON-structured cathode materials in combination with nanoarchitecture tailoring.

INTRODUCTION Lithium rechargeable batteries have emerged as the overwhelming choice for portable power sources in the modern world, due to their high energy density and sustainable chemical architecture after long-term repeated use.1-3 However, serious concerns are now being raised regarding the practicality of lithium rechargeable batteries for electric vehicles, electric power storage systems, smart grids, etc. These large-scale appliances need tremendously high energy density and excellent thermal stability at elevated temperatures, but the current lithium rechargeable battery systems cannot totally fulfill all of these requirements because of the limited energy/power density of the current cathode materials, which are typically layered-LiMO2(M ≡Co, Mn, Ni),4-6 spinel-LiMn2O4,710 and olivine-LiFePO4.11-14 In this respect, new cathode materials such as Li2MPO4F,15,16 Li2MSiO4,17,18 and Li3M2(PO4)3 have attracted substantial attention because more than two formula units of Li-ions can possibly deintercalate/intercalate from/into their host crystal structure with a moderate operating voltage. In particular, NASICON-type Li3V2(PO4)3 (LVP) is favored among these materials due to its high theoretical capacity (197 mA h g-1 when all three

Li atoms are reversibly transferred) and average redox voltage (4.0 V vs. Li+/Li). However, several drawbacks still exist that prevent the use of LVP as a commercial cathode material, including its poor kinetic properties caused by its intrinsic low electronic (2 x 10-8 S cm-1) and ionic (10-9 to 10-10 cm2 s-1) conductivity.19,20 Approaches such as conductive carbon coating,21,22 nano-tailoring,23-25 and heterogeneous atom doping26-29 have been attempted in order to overcome the state-of-the-art problems of LVP, but these traditional strategies have not proven particularly effective. New strategies are therefore required so that breakthroughs can be made in improving the electrochemical properties of LVP. In this paper, we describe the use of a LVP/C composite merged with a NASICON-structured LiZr2(PO4)3 phase, which is evolved by incorporating Zr into LVP (Figure 1). The resulting material has higher discharge capacity and greatly improved rate capability when compared with pristine LVP/C. Since the electronic conductivity of LVP can be readily enhanced to the practical level by the simple methods including carbon coating, the critical focus should be now on improving its Li ionic conductivity. Actually, this argument can be supported by a recent review pointing out that none of the particle size reduction to nano-scale, amorphous phase formation, and surface

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coating have scarcely made a breakthrough in ionic conductivity enhancement, even for the cathode materials having 3D channels for Li diffusion.30 Meanwhile, the LiZr2(PO4)3 engaged in NASICON structure has found the use as a Li ionic conductor in solid electrolytes due to its excellent Li ion conductivity.31,32 Our experiment confirmed that if the proper amount of LiZr2(PO4)3 phase can be generated on the surface of LVP particles, the ionic conductivity of LVP might be maximized with a minimal decrease in the electronic conductivity. In addition, the structural stability of LVP at high voltage (> 4.3 V) could be also improved if the average oxidation number of the centered vanadium in LVP is decreased below +3 by Zr4+ doping, because the repetitive transition between V4+ and V5+ from 4.3 to 4.8 V is believed to seriously deteriorate the crystal structure of LVP upon cycling. Finally, Zrdoped LVP/C partially composed of the impregnated LiZr2(PO4)3 with high ionic conductivity exhibited greatly enhanced cycling stability and rate capability in spite of its micro-scale particle size and relatively small amount of incorporated carbon (< 1 wt%), providing a concrete solution for the limited kinetics of LVP.24 Herein, the effects of Zr doping and the simultaneous formation of NASICON-type LiZr2(PO4)3 on the structure and electrochemical performance of LVP/C composites will be elucidated in details.

EXPERIMETAL SECTION Li3V2-xZrx(PO4)3/C (LVZrP/C, x = 0, 0.05, 0.1) composites were prepared by the solid state reaction of Li2CO3, V2O5, Zr(OH)4, and NH4H2PO4. First, stoichiometric amounts of precursors were dispersed in a deionized (DI) water/ethanol mixture containing the dissolved sucrose (3 wt % carbon). This mixture was stirred for 6 h and then dried at 120 °C in a conventional oven to vaporize the mixing solvent. The final product, LVZrP/C, was obtained by calcination at 750 °C for 10 h in a mixed gas flow of 5 wt.% H2+Ar. The actual Li/V and Li/Zr ratio of the final product was examined by the inductively coupled plasmaatomic emission spectroscopy (ICP-AES). Its morphology was observed by scanning electron microscopy (SEM, Philips, XL30SFEG). Various phases present in the LVZrP/C were identified by X-ray powder diffraction (XRD, Rigaku D/MAX-IIIC, 3 kW) measurements and Rietveld refinements were conducted for analyzing XRD patterns. The residual carbon content and the electronic conductivity of the LVZrP/C were measured with an element analyzer (EA, CE Instruments, EA1110-FISONS) and 4-point probe electrical measurement system, respectively. The V 2p core binding energy of this material was verified by X-ray photoelectron spectroscopy (XPS, Thermo, MultiLab 2000). The XPS depth profile and electron energy-loss spectroscopy (TEM-EELS)/energy dispersive spectroscopy (EDS) analyses were commonly conducted to confirm the atomic distribution of Zr inside LVZrP particle.

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Figure 1. Schematic illustration of the Zr-incorporated Li3V2(PO4)3/C composite merged with a NASICONLiZr2(PO4)3 secondary phase. The electrode was fabricated by adding a mixture of 75 wt.% of each active material and 17 wt.% acetylene black to N-methyl-2-pyrrolidene (NMP) solvent containing 8 wt.% polyvinylidene fluoride (PVdF). The resulting slurry was pasted onto an Al foil current collector and dried at 120 °C for 6 h in a vacuum oven. After pressing, the dried electrode was punched into a disc (1.3 cm diameter). The electrochemical properties of the fabricated electrodes were evaluated using 2032 coin-type cells assembled in an Ar-filled glove box. Li-metal foil (Cyprus Foote Mineral, 99.98%, USA) was used as the counter electrode, and 1M LiPF6 dissolved in 1:1 (v/v) ethylene carbonate (EC) and dimethyl carbonate (DMC) was adopted as the electrolyte. The charge/discharge characteristics of the assembled cells (Loading level: 0.25 mg cm-2) were examined at 25 °C using a battery cycler (WBCS3000S, WonaTech). For the first two cycles, the cells were charged at a constant current density of 0.1 C to 4.3 V or 4.8 V (vs. Li+/Li) and were held continuously at a constant voltage of 4.3 V or 4.8 V until the current density dropped below a capacity corresponding to 0.05 C. They were then discharged at a constant current density of 0.1 C to 3.0 V. From the third cycle onward, the cells were charged and discharged galvanostatically in a potential range of 3.0-4.3 V or 3.0-4.8 V (vs. Li+/Li). Cyclic voltammetry (CV) measurements were carried out between 3.0 and 4.8 V (vs. Li+/Li) at a scan rate of 0.1 mV s-1. The activated two electrode cells (Working: LVZrP/C, Counter & Reference: Li metal) were subjected to electrochemical impedance spectroscopy (EIS) after the first two cycles, with a sinusoidal voltage signal (10 mV) over a frequency range from 1000 kHz to 100 mHz. The electrodes were washed with DMC to remove residual Li salts from the electrolyte before the ex situ XRD analysis.

RESULTS AND DISCUSSION The X-ray diffraction (XRD) patterns of Li3V2-xZrx(PO4)3/C (LVZrP/C, x = 0, 0.05, 0.1) composites are shown in Figure 2. The XRD pattern for the pristine Li3V2(PO4)3/C (LVP/C) coincides with that for the reference LVP19, confirming the absence of any secondary phases. However, the increase of incorporated Zr results in the appearance of


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0.05, 0.1) composites obtained from X-ray Rietveld refinement and (b) Actual Li/V and Li/Zr ratio of the materials examined by the inductively coupled plasma-atomic emission spectroscopy (ICP-AES) (a) Lattice parameters [Å] Compounds a

b

c

Li3V2(PO4)3/C

8.6117(2)

12.0483(2)

8.5993(2)

Li3V1.95Zr0.05(PO4)3/C

8.6142(1)

12.0505(2)

8.6008(2)

Li3V1.9Zr0.1(PO4)3/C

8.6162(2)

12.0517(2)

8.6023(2)

(b)

Figure 2. (a) X-ray diffraction (XRD) patterns for the Li3V2-xZrx(PO4)3/C (x = 0, 0.05, 0.1) composites and (b) Rietveld refinement patterns of Li3V1.95Zr0.05(PO4)3 using X-ray powder diffraction data. Plus marks (+) represent the observed intensities, and the red solid line is calculated result. The difference plot (blue) is shown at the bottom. Tick marks above the difference plot indicate the reflection positions (top: Li3V2(PO4)3, bottom: LiZr2(PO4)3) additional peaks, which well match to a LiZr2(PO4)3-like phase reported by Petit et. al.33 Herein, Li3V1.95Zr0.05(PO4)3/C seems to almost exist as a single phase, but a little amount of the LiZr2(PO4)3-like phase is also clearly observed. More minute survey revealed that as more Zr is incorporated, the peak shifting to left side becomes larger, implying that the unit cell of LVP crystals expands somewhat as a result of Zr incorporation.34,35 This observation is well supported by Rietveld refinement results for the XRD patterns of LVZrP/C (Table 1a). The amount of the impregnated LiZr2(PO4)3 in Li3V1.95Zr0.05(PO4)3 and Li3V1.9Zr0.1(PO4)3, estimated by the Rietveld refinement, was ~4.15 and ~4.48%, respectively. When considering very small discrepancy between Li3V1.95Zr0.05(PO4)3 and Li3V1.9Zr0.1(PO4)3 in the amount of LiZr2(PO4)3, the formation of LiZr2(PO4)3 by Zr incorporation seems to be dominant until the contents of Table 1. (a) Lattice parameters of the Li3V2-xZrx(PO4)3/C (x = 0,

Compounds

Li [% wt]

V

Zr

Li/V [%at/at]

Li3V2(PO4)3/C

5.16

25.52

-

1.48

-

Li3V1.95Zr0.05(PO4)3/C

5.12

24.53

0.97

1.53

69.4

Li3V1.9Zr0.1(PO4)3/C

5.18

24.44

1.84

1.56

37.0

Li/Zr

Zr reaches 0.05 prior to Zr doping on bulk. All information on the crystal structure of LVZrP/C involving space group, lattice parameters, and the location of atom, derived from the Rietveld refinement are tabulated in Table S1 and S2. The actual Li/V and Li/Zr ratio of LVZrP/C measured by the ICP-AES is given in Table 1b. The X-ray photoelectron spectroscopy (XPS) spectra of the LVZrP/C composites shown in Figure 3a demonstrates that the V 2p XPS core peak is shifted to the right side as a result of Zr incorporation, which indicates that the V 2p core level of the Zr-containing LVP/C has lower binding energy for electrons than that of pristine LVP/C. Herein, the P 2p and O 1s XPS spectra for the pristine LVP/C (Figure S1), compensated by the peak binding energy for the measured C 1s XPS spectra, coincide with the previously referred Li3V2(PO4)3/C,36 proving that both the V 2p XPS spectra for the LVZrP/C and the results obtained by deconvoluting their V 2p XPS spectra are very reliable. Because the binding energy for electrons is closely linked to the oxidation number of the cation of interest, the shifting of the V 2p XPS core peak by Zr incorporation might stem from the reduction in the average oxidation number of vanadium (V). In details, the V 2p XPS core levels of all LVPs with or without Zr4+ roughly match to that observed in V2O3 (V3+, 517.4 eV),36 and the V2+/V3+ ratios obtained by deconvoluting their XPS peaks proved the higher V2+/V3+ ratio for Li3V1.9Zr0.1(PO4)3/C than Li3V1.95Zr0.05(PO4)3/C, which corresponds to ~0.10 and ~0.06, respectively. The X-ray Rietveld refinement results also support this, as minor Zr4+ ions doped on V3+ sites in



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Figure 3. (a) V 2p X-ray photoelectron spectroscopy (XPS) spectra for the Li3V2-xZrx (PO4)3/C (x = 0, 0.05, 0.1) composite and (b) XPS depth profiles of Zr and V in the Li3V2xZrx (PO4)3/C (x = 0.05, 0.1). LVP/C possibly decrease the average oxidation number of V to less than +3. The location where the LiZr2(PO4)3-like phase is formed inside LVZrP particle was validated by electron energyloss spectroscopy analysis and cross-checked with energy dispersive spectroscopy (EDS) for Li3V1.95Zr0.05(PO4)3/C and Li3V1.9Zr0.1(PO4)3/C as described in Figure S2. Zr ions were only found on the surface of LVZrP particles, while there was no trace of Zr ions in the bulk of LVZrP. Hence, we believe that the LiZr2(PO4)3-like secondary phase is primarily formed near the surface of LVZrP particles in great agreement with the previously discussed crystallographic analysis. In addition, from the transmission electron microscopy (TEM) images of the Li3V1.95Zr0.05(PO4)3/C and its selected area electron diffraction (SAED) patterns (Figure S3), we found that a single crystal LVP was formed all over LVZrP particle while LiZr2(PO4)3-like phase was only observed between bulk LVZrP and surface amorphous carbon. Further evidence for the existence of the secondary phase on the surface of LVP could be provided by the XPS depth profiles of Zr and V in Li3V2-xZrx (PO4)3/C (x = 0.05, 0.1), as shown in Figure 3b. The Zr content in the vicinity of LVZrP particle surface is relatively high, but it gradually decreases as the sputter depth into

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the interior of particle increases, in good agreement with the TEM-EELS/EDS results (Figure S2). Unlike the variation in Zr content, the amount of V is lowest at the surface and becomes almost constant from approximately 75 nm. These results offer novel phase separation on the surface of NASICON-structured transition metal phosphates caused by the inhomogeneity in chemical composition, which differs from the grain boundary segregation that has been well understood, mostly observed at an internal interface in binary metallic alloys. We consider that our results are also supported by a previous research group who showed a LiFePO4 with high lithium bulk mobility induced by a fast Li ion-conducting surface phase (Fe3+-containing Li4P2O7-like phase) created through the controlled off-stoichiometry.37 Figure 4a and 4b show the initial galvanostatic (0.2 Crate) voltage profiles of the LVZrP/C composites, in the range of 3.0-4.8 and 3.0-4.3 V (vs. Li+/Li), respectively. Regardless of Zr content (x), the charge-discharge curve of the LVZrP/C agrees well with the previously described charge-discharge behavior of LVP/C,21 implying that the LiZr2(PO4)3-like phase in LVZrP doesn’t undergo any reduction or oxidation. During charging to 4.8 V, all three Li atoms look like being extracted from LVZrP/C, making four distinct voltage plateaus appear at 3.60, 3.68, 4.08, and 4.52 V, as depicted in Figure 4a. Each voltage plateau corresponds to the phase transition between two adjacent single phases of Li3-yV2-xZrx(PO4)3 (y = 0, 0.5, 1.0, 2.0, and 3).21,26 The voltage plateau at 4.52 V, which relates to the phase transition between LiV2-xZrx(PO4)3 and V2-xZrx(PO4)3, can be avoided if the upper limiting voltage is set to 4.3 V. Differing from the discharge voltage profile of LVZrP/C charged to 4.3 V, the profile to 4.8 V features a sloping shape that covers a wide capacity range up to two mole of Li ions followed by two voltage plateaus near 3.6 V. The higher initial discharge capacity (162 and 118 mA h g-1, charged to 4.8 and 4.3 V, respectively) of the Li3V1.95Zr0.05(PO4)3/C compared with that (143 and 105 mA h g-1) of the pristine LVP/C can probably be attributed to not only the doped Zr ions but also the ionic channels through LiZr2(PO4)3-like secondary phase that formed on the surface of LVP particles, as described by the TEMEELS/EDS and XPS analyses. The similarity between LVP/C and LVZrP/C in the average particle size (1-2 μm) and distribution (Figure S4) gives us more confidence that this capacity discrepancy is not induced by a morphological difference but primarily associated with the change of crystal structure through Zr incorporation. Herein, the doped Zr4+ decreases the average oxidation number of vanadium in LVP crystals, resultantly increasing both charge capacity and discharge capacity in the V3+ ↔ V4+ transition region, but has no noticeable effect on the charge capacity during the V4+ ↔ V5+ transition above 4.5 V (Figure 4a). In addition, because the ionic conductivity of the LiZr2(PO4)3 near R.T. is ~10-5 S cm-1, it is natural that its Li diffusivity (DLi+),


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a

b

c

d

e

f

Figure 4. Electrochemical performance of the Li3V2-xZrx(PO4)3/C (x = 0, 0.05, 0.1) composites. (a) and (d) Initial galvanostatic (0.2 C, 1 C = 197 mA h g-1) voltage profiles and cycling performance of the Li3V2-xZrx(PO4)3/C at a cut-off voltage of 34.8 V. (b) and (e) Initial galvanostatic (0.2 C, 1 C = 133 mA h g -1) voltage profiles and cycling performance of the Li3V2xZrx(PO4)3/C at a cut-off voltage of 3-4.3 V. (c) Rate-capability for the Li3V2-xZrx(PO4)3/C and (f) Discharge profiles of the Li3V1.95Zr0.05(PO4)3/C at different discharge rates with a constant charge at 0.1 C. estimated by combining the noted ionic conductivity value and Nernst-Einstein equation, is at least several orders higher than that of LVP (10-9 to 10-10 cm2 s-1). Hence, the enhanced ionic conductivity of LVP provided by the impregnated LiZr2(PO4)3 can be regarded as the most crucial factor for the improved electrochemical performance of LVZrP/C. Meanwhile, Li3V1.9Zr0.1(PO4)3/C displays lower initial discharge capacity than the pristine LVP/C probably due to the overdose of Zr content responsible for the abrupt drop of electronic conductivity induced by the presence of large amount of the LiZr2(PO4)3-like ionic conductor (Table S3). The difference between targeted (3 wt %) and actual (0.3-0.6 wt %) carbon content in LVZrP/C here can be attributed to the loss of carbons through the carbothermal reaction at 750 °C to calcine LVP. Taking large size of LVZrP particles on microscale and its extremely low residual carbon content into account, the high discharge capacity of the Li3V1.95Zr0.05(PO4)3/C is even more remarkable. These observations well explain the rate-capability results of the LVZrP/C composites shown in Figure 4c. At the low current density of 0.1 and 0.2 C, the discharge capacity degradation of the LVZrP/C seems to have nothing to do with Zr incorporation. However, the capacity loss at the high current densities above 0.5 C is significantly suppressed by Zr incorporation, indicative of facile

Li+ ion insertion/extraction into/out of the LVZrP particles. The superior rate-capability of the Li3V1.9Zr0.1(PO4)3/C to the pristine LVP/C demonstrates that the ratedetermining step of discharge reaction is not electron conduction on the LVP surface but Li ion transport through the elecrode/electrolyte interface, which clearly verifies the important role of the impregnated LiZr2(PO4)3. Figure 4f shows that two voltage plateaus of Li3V1.95Zr0.05(PO4)3/C at the end of discharge (near 3.6 V) vanish with an increase in the applied current density. Despite its great kinetic enhancement, Li3V1.95Zr0.05(PO4)3/C just delivers a discharge capacity of 74 mA h g-1 at the high current density of 10 C. However, we are convinced that if a valid nanostructure with kinetic superiority is established with a proper size control for LVZrP/C composites, their rate-capability can be maximized for the commercialization. The cycling performances of the LVZrP/C composites, measured in the range of 3.0-4.8 and 3.0-4.3 V (vs. Li+/Li), are given in Figure 4d and 4e. In addition, Figure S5 exhibits the galvanostatic (0.2 C, 1 C = 197 mA h g-1) voltage profiles of the LVZrP/C measured after 1st, 10th, 20th, 30th, 40th, and 50th cycles. After 50 cycles between 3.0 and 4.8 V, Li3V1.95Zr0.05(PO4)3/C exhibits a high discharge capacity of 138 mA h g-1, corresponding to 85.4 % of its initial discharge capacity (162 mA h g-1). In contrast, the pristine LVP/C and Li3V1.9Zr0.1(PO4)3/C each show the



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discharge capacities of 112 and 107 mA h g-1 corresponding to 78.8 and 82.7 % of their initial discharge capacities (143 and 130 mA h g-1). Interestingly, Li3V1.9Zr0.1(PO4)3/C as well as Li3V1.95Zr0.05(PO4)3/C also shows better capacity retention than the pristine LVP/C, which implies that the LiZr2(PO4)3-like phase can yield some positive effects on the structural stability of LVP host crystals, thereby mitigating the cyclic degradation of LVP/C. The ex-situ XRD patterns of LVP/C and LVZrP/C composites in Figure S6 show that all samples retain their original crystal structure even after 50 cycles, regardless of the amount of the impregnated LiZr2(PO4)3. In particular, the mass fraction of LVP and LiZr2(PO4)3 in Li3V1.95Zr0.05(PO4)3/C was ~95.5 and ~4.5%, respectively (Table S4). Because the contents of LiZr2(PO4)3 are significantly increased after cycling, the Zr4+ ions may tend to thoroughly occupy the site of V to generate LiZr2(PO4)3 during cycling. This novel phenomenon, not reported for other cathode material systems such as LiFePO4/Fe2P,38,39 Li2FeSiO4/Li2SiO3,17 etc., mainly results from the structural analogy between NASICON-type LVP and LiZr2(PO4)3. When cycled between 3.0 and 4.3 V, Li3V1.95Zr0.05(PO4)3/C exhibits incomparably superb cyclic retention coming up to 99% of the initial capacity after 50 cycles. It should be noted that Li3V1.95Zr0.05(PO4)3/C proposed in the present study displayed comparable or better electrochemical performances in comparison with Co, Mn, Cr, or Sc doped LVP reported in literatures.35,40-42 The electrochemical reactions and their kinetics during the initial charge-discharge process were identified for the LVZrP/C composites by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements as shown in Figure 5. Regardless of the amount of incorporated Zr, LVZrP/C composites commonly displays four anodic current peaks and three cathodic current peaks and the peak potential difference between each redox reaction was diminished by Zr4+ doping. This feature strongly agrees with the results derived from the initial charge-discharge profiles in Figure 4a, which indicated that the voltage deviation from the equilibrium potential is lower for Li3V1.95Zr0.05(PO4)3/C than for the others. The superior kinetic properties of Li3V1.95Zr0.05(PO4)3/C can be further understood by EIS analysis showing all sorts of resistances linked to charge transfer at electrode/electrolyte interface or inside electrode. As a result, the charge transfer resistance (Rct,LVP) of Li3V2(PO4)3 decreased from 641.4 to 439.4 Ω cm2 with an increase in Zr content. However, the Rct,LVP rather increased to 567.9 Ω cm2 when the amount of Zr (x) exceeded its optimum value (x = 0.05), as shown in Table S5. In our EIS spectra, the charge transfer resistance estimated from the semicircle in high-middle frequency region is attributed to the mixed Li ionic and electronic conduction through the interface. Thus, the reduced

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Figure 5. (a) Cyclic voltammograms for the first cycle after cell formation and Electrochemical impedance spectroscopy (EIS) spectra of the Li3V2-xZrx(PO4)3/C (x = 0, 0.05, 0.1) composites measured (b) before cycling and (c) after cell formation. charge transfer resistance of LVP/C by Zr incorporation is associated with a facile Li ion migration through it because the electronic conductivity of LVP/C is decreased by the increase of Zr content due to the formation of the ionic conducting LiZr2(PO4)3 as already explained. Hence, the reason why Li3V1.95Zr0.05(PO4)3/C has the lowest resistance may come from its optimum LiZr2(PO4)3. Even though Zr-containing LVP/C (Li3V1.95Zr0.05(PO4)3/C) is favorably influenced by Zr4+ doping as well as the formation of a LiZr2(PO4)3-like secondary phase, Zr should be optimally incorporated for the minimum expense in terms of theoretical capacity loss, electronic conductivity drop, and structural collapse. We believe that these findings including high Li ion transport and structure stabilization induced by Zr incorporation suggests a totally brand-new strategy for achieving high-power Li rechargeable batteries using NASICONstructured cathode materials in combination with nanoarchitecture tailoring, thereby solving general issues in NASICON-structured phosphates applicable to electric vehicles and energy storage systems for renewable energy in the field of energy storage and conversion materials, as well as it lays a fundamental ground for tailoring lattice structure of materials for various energy devices such as capacitors, lithium rechargeable batteries, solar cells, and etc. as well as diverse semiconducting devices based on metal oxides.

CONCLUSION ACS Paragon Plus Environment

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In summary, we succeeded in realizing Li3V2-xZrx(PO4)3/C (x = 0.05, 0.1) composites that are merged with a NASICON-LiZr2(PO4)3 phase by Zr incorporation into Li3V2(PO4)3, and investigated their electrochemical behaviors when the Zr content is varied. Rietveld refinement results for the XRD patterns, TEM-EELS/EDS, and XPS depth profiles for the Li3V2-xZrx(PO4)3/C confirmed that a LiZr2(PO4)3-like secondary phase was formed locally on the surface of Li3V2(PO4)3 particles by the incorporated Zr ions and simultaneously the excess Zr ions were doped on the vanadium sites in Li3V2(PO4)3 crystals. The high initial discharge capacity and superior rate-capability of the Li3V1.95Zr0.05(PO4)3/C was attributed to the facile Li ion conduction through super-ionic channels built along the LiZr2(PO4)3-like ionic conductors. In addition, the improved cycling performance of the Li3V1.95Zr0.05(PO4)3/C resulted from the enhanced structural stability imparted by doped Zr ions and/or the generated secondary phase. Thus, the high Li ion transport channel and structure stabilization induced by Zr incorporation may indicate a promising method for achieving high-power Li rechargeable batteries using NASICON-structured cathode materials.

ASSOCIATED CONTENT Supporting Information.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

ACKNOWLEDGMENT This research was supported by (NRF-2010-C1AAA0010029018), the Basic Science Research Program (S-2013A0434-00024), and the Converging Research Center Program (2012K001240) through the Ministry of Education, Science and Technology.

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Facile Lithium Ion Transport through Superionic Pathways Formed on

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