Stable Nanostructured Cathode with ... - ACS Publications

The lithium-ion battery, a major renewable power source, has been widely applied in portable .... Journal of Materials Chemistry A 2015 3 (30), 15523-...
3 downloads 0 Views 2MB Size
Download PDF
Letter pubs.acs.org/NanoLett

Stable Nanostructured Cathode with Polycrystalline Li-Deficient Li0.28Co0.29Ni0.30Mn0.20O2 for Lithium-Ion Batteries Feng Wu,†,‡,⊥ Guoqiang Tan,†,⊥ Jun Lu,§,⊥ Renjie Chen,*,†,‡ Li Li,*,†,‡ and Khalil Amine*,§,∥ †

School of Chemical Engineering and the Environment, Beijing Key Laboratory of Environmental Science and Engineering, Beijing Institute of Technology, Beijing 100081, China ‡ National Development Center of High Technology Green Materials, Beijing 100081, China § Chemical Science and Engineering Division, Argonne National Laboratory, 9700 S. Cass Avenue, Lemont, Illinois 60439, United States ∥ Faculty of Science, Chemistry Department, King Abdulaziz University, 80203 Jeddah, Saudi Arabia S Supporting Information *

ABSTRACT: The lithium-ion battery, a major renewable power source, has been widely applied in portable electronic devices and extended to hybrid electric vehicles and all-electric vehicles. One of the main issues for the transportation application is the need to develop high-performance cathode materials. Here we report a novel nanostructured cathode material based on air-stable polycrystalline Li0.28Co0.29Ni0.30Mn0.20O2 thin film with lithium deficiency for high-energy density lithium-ion batteries. This film is prepared via a method combining radio frequency magnetron sputtering and annealing using a crystalline and stoichiometric LiCo1/3Ni1/3Mn1/3O2 target. This lithium-deficient Li0.28Co0.29Ni0.30Mn0.20O2 thin film has a polycrystalline nanostructure, high tap density, and higher energy and power density compared to the initial stoichiometric LiCo1/3Ni1/3Mn1/3O2. Such a material is a promising cathode candidate for high-energy lithium-ion batteries, especially thin-film batteries. KEYWORDS: Li-deficient, Li-ion battery, nanostructure, thin film, radio-frequency magnetron sputtering, polycrystalline

G

generally exhibited excellent electrochemical properties.12−15 Yet to be investigated are the properties of Li-deficient cathode materials. In general, however, such Li-deficient compounds are only stable under the electrochemical test condition. Therefore, the biggest challenge to make the Li-deficient materials be used in rechargeable LIBs is whether they can be fabricated with stable structure under normal condition. In this paper, we report a stable, high energy density thin-film cathode with polycrystalline Li-deficient Li−Co−Ni−Mn−O prepared by radio frequency (RF) magnetron sputtering combined with annealing using a crystalline LiCo1/3Ni1/3Mn1/3O2 target. To our best knowledge, this is the first time to report an air-stable Li-deficient Li−Co−Ni−Mn− O compound. This material has the chemical composition of Li0.28Co0.29Ni0.30Mn0.20O2 with a polycrystalline nanostructure, which demonstrates a much higher reversible capacity than that of the initial LiCo1/3Ni1/3Mn1/3O2. The improvement on the capacity of this material is mainly attributed to the fact that disordered (polycrystalline film) cathodes often show better properties than crystalline cathodes of the same material.16

reen technologies are critical for the realization of global sustainable development. In the past decades, lithium-ion batteries (LIBs) have become the main and most promising renewable power source for electronic devices and electric vehicles. 1,2 Worldwide effort is being directed toward developing high-performance LIBs for the transportation application, which has resulted in increasing interest in the search for alternative cathode materials for the next generation LIB.3,4 The 3d transition metal oxides, including layered LiCoO2, LiNiO2, and Li(Ni,Co)O2, spinel LiMn2O4, and olivine LiFePO4 exhibit good rechargeability5−7 but relatively low specific capacity (120−160 mAh/g at 2.7−4.2 V) and instability at high voltage (>4.3 V). The current generation of LIBs using such conventional cathodes does not meet the stringent requirements of practical applications for high efficiency and energy saving. In 2001, Ohzuku and Makimura reported a layered compound, LiCo1/3Ni1/3Mn1/3O2, which exhibits high reversible capacity and excellent cycle performance.8 After several theoretical analyses and practical trials, this material has been proposed as a promising cathode candidate that may be able to deliver a high capacity (200 mAh/g) with a wider range for cutoff voltage (2.5−4.6 V).9−11 Currently, Li-enriched cathode materials have been widely researched and have © 2014 American Chemical Society

Received: November 14, 2013 Revised: February 5, 2014 Published: February 14, 2014 1281

dx.doi.org/10.1021/nl404215h | Nano Lett. 2014, 14, 1281−1287

Nano Letters

Letter

after 4 h sputtering was approximately 2.2 μm with a deposition rate of 9.2 nm/min. After annealing at 800 °C, the surface of the LTF became smoother and denser, and the spherical particles on the surface were obvious decreased. This change occurred because the annealing provided higher energies to facilitate chemical interactions and rearrangement of the deposition atoms to form a polycrystalline structure, decreasing the content of free Li atoms in the LTF. The annealed LTF (PLTF) was thus more stable in air. A cross section of the PLTF exhibited a cliff structure with obvious edges and corners, indicating a good crystallization of the PLTF. The surface elemental analysis of both LTFs by EDX revealed an approximate Mn/Co/Ni molar ratio of 9/9.5/10. The EDX elemental mapping of the PLTF is shown in Figure 1, which indicated a uniform distribution of all the elements on the surface of PLFT. The surface topographies of both LTFs were further characterized by atomic force microscopy (AFM), as presented in Supporting Information Figures S2 and S3. The results show that the PLTF has relatively smaller grains and a slightly smoother surface than the ALTF. The crystallinity of the LTFs was supported by highresolution TEM. Figure 1E,F presents TEM images along with SAED patterns of the LTFs. In agreement with the XRD results discussed below, the ALTF was primarily amorphous. However, there were a few crystalline zones dispersed in amorphous matrices, identified by the circle in Figure 1e, owing to the increased temperature caused by the 100-W RF power. The SAED pattern in this phase was made up of two blurry rings, indicating the amorphous bulk in ALTF. Comparatively, the TEM image of the PLTF showed many more clear stripes, meaning that crystalline particles in PLTF were well-oriented. The corresponding SAED pattern showed clear rings consisting of discrete spots, confirming the polycrystalline nature of the PLTF. The structural and chemical characteristics of the LTFs were evaluated by XRD, Raman spectroscopy, Fourier transform infrared (FTIR), and atomic emission spectroscopy (AES), as shown in Figure 2. In Figure 2A, the XRD pattern of the target possessed very clear diffraction patterns consistent with the well-crystallized hexagonal α-NaFeO2 layered structure of LiCo1/3Ni1/3Mn1/3O2.19 In contrast, the XRD pattern of the ALTF was devoid of dominant diffraction peaks, except a broad (104) preferred orientation at 44°, indicating a primarily amorphous structure. The XRD pattern of the PLTF showed several depressed diffraction peaks from (003), (101), (102), (104), (107), and (108) reflections for the lithium deficient LixCo1/3Ni1/3Mn1/3O2 (0 < x < 1) layered structure observed during the in situ XRD measurement.20,21 The weak intensity and broad full width of half-maximum (fwhm) of these diffraction peaks were due to the decrease of lithium content in the LTF bulk, corresponding to a polycrystalline structure. It is found that the (003), (006), and (108) peaks shift toward lower angles, which correspond to an expansion along the c-axis of the hexagonal unit cell. While the shift of (110) peak to higher angle contributes to the contractions along the a- and baxis of the hexagonal unit cell.20 Previous researches indicated that Li-deficient Li1−xCo1/3Ni1/3Mn1/3O2 materials exhibited good crystal structural stability because of its relatively small change in unit cell volume compared to the normal layered LiCo1/3Ni1/3Mn1/3O2 .20,22 It should be also noted that a quite weak diffraction peak at 2θ = 31°, attributed to the spinel structure, was due to the suppression of active oxygen gas evolution by annealing.23−25 The LTF exhibited improved

More importantly, the dense nanostructure and thin film design reduce the Li+ ionic and electronic transfer path lengths either between active particles or across the solid−electrolyte interface, increase electroactive zones, and facilitate transfer kinetics.17 As a consequence, this thin film cathode exhibits a high reversible capacity, good cycle stability, and excellent rate performance. Thus, the present polycrystalline Li-deficient Li0.28Co0.29Ni0.30Mn0.20O2 thin film shows the great potential to be a candidate of the cathode materials for high energy density LIBs, especially, for thin-film LIBs. For simplicity, hereafter we designate these special thin films with the acronyms ALTF for amorphous Li−Co−Ni−Mn−O thin film (as-deposited) and PLTF for polycrystalline Li−Co−Ni−Mn−O thin film (annealed). We prepared LTFs by RF magnetron sputtering using a LiCo1/3Ni1/3Mn1/3O2 target, and the mechanism of LTF deposition is illustrated in Scheme 1. Ionized Ar ions were Scheme 1. Schematic Diagram Showing Process for Li−Co− Ni−Mn−O Thin Film Deposition

generated in glow discharge plasma and then accelerated by an electric field to bombard the target. This bombardment caused the removal of target atoms, which condensed onto the substrate as a thin film. Secondary electrons were also generated as a result of the ion bombardment and were constrained near the target by magnetrons to maintain the plasma. This step increased the Ar ionization efficiency and, in turn, increased the ion bombardment toward the target. The target atoms reassembled on the substrate in a disordered arrangement, forming an ALTF. The ALTF was then annealed at 800 °C for 5 h, which promoted chemical interactions and rearrangement of deposited atoms to form a PLTF structure. The morphologies of the obtained LTFs were characterized by field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), and selected area electron diffraction (SAED), as shown in Figure 1. In the SEM images, the ALTF showed a fairly coarse surface with numerous similar spherical particles, the formation of which was due to the reaction of the free Li-rich LTF with air, indicating that the ALTF is not air-stable.18 The particle sizes were generally less than 200 nm, and they were also composed of several smaller particles. The SEM images of the ALTF cross section showed that the material was dense, uniform, and crack free with a homogeneous and glassy texture. The thickness of the ALTF 1282

dx.doi.org/10.1021/nl404215h | Nano Lett. 2014, 14, 1281−1287

Nano Letters

Letter

Figure 1. FE-SEM images of the (A,a) surface and (B,b) cross-section of the ALTF and (C,c) surface and (D,d) cross-section of the PLTF; EDX elemental mappings of Co, Ni, Mn, and O of the (C1−C4) surface and (D1−D4) cross-section for the PLTF; TEM images and SAED patterns of the (E) ALTF and (F) PLTF.

Figure 2C shows the FTIR spectra of the LTFs. For the ALTF, the bands at 472, 556, 589, and 590−650 cm−1 were attributed to the O−M−O bonds, and the bands at 862 and 1133 cm−1 were attributed to the M−O bonds.28 The bands at 1281, 1432, and 1556 cm−1 have been reported to correspond to Li2CO3 formed by reaction of the Li-rich thin film with CO2, indicating its instability in air.28 For the PLTF, most of the Li2CO3 bands disappeared, which is a clear evidence showing that the PLTF is air-stable. Meanwhile, the bands at 450−650 cm−1 corresponding to the O−M−O bonds in PLTF became stronger and shifted to lower wave numbers, a feature for the Li-deficient structure. AES depth profiling was employed to identify the elements in the bulk LTFs. In Figure 2D for the PLTF, the atomic concentration of each element was generally constant during the AES depth sputtering. The plot indicates a uniform distribution of elements regardless of location in the PLTF bulk

crystallinity upon annealing, but its lithium content decreased in the bulk that leads to a Li-deficient structure. It is also believed that the annealing provided more energy for the chemical interaction and rearrangement of atoms to form a polycrystalline structure. The Raman spectrum also reflects the crystallinity of the LTFs. In Figure 2B, the Raman spectra for the target showed obvious scattering bands at 380, 470, and 583 cm−1, indicating that the target was well crystallized. The bands at 470 and 583 cm−1 were due to Eg and A1g modes originating from O−M−O bending and M−O symmetrical stretching vibrations (M = Co, Ni, Mn), respectively.26 However, the bands of the ALTF were broad, suggesting that it was nearly amorphous. The bands of the PLTF were restored, indicating its improved crystallinity. Note that the small peak at 313 cm−1 in the PLFT sample was attributed to the spinel phase,27 which is consistent with the XRD observation. 1283

dx.doi.org/10.1021/nl404215h | Nano Lett. 2014, 14, 1281−1287

Nano Letters

Letter

Figure 2. (A) XRD patterns of the (a) target, (b) SS substrate, (c) ALTF, and (d) PLTF. (B) Raman spectra of the target and LTFs. (C) FTIR spectra of the LTFs. (D) AES depth profiles and Auger spectra (inset) of the PLTF.

recovered to 194 mAh/g when the current density is returned to its initial value. These results revealed that the PLTF electrode showed good charge transfer kinetics and stable structural integrity. For comparison, the electrochemical properties of the ALTF and the target LiCo1/3Ni1/3Mn1/3O2 were also evaluated under the same tested condition as that for PLTF, and the results are shown in Supporting Information Figure S4 and S5. It is clearly from these results that, indeed, the PLTF demonstrate much superior performance, especially in terms of capacity and rate capability. The charge-transfer kinetics was analyzed by using the CV and EIS results in Figure 3D,E, respectively. In Figure 3D, the first anodic peak at 4.55 V was due to the oxidation of Co3+/ Co4+; the subsequent cathodic peaks at 4.38 and 3.55 V were ascribed to the reduction of Co4+/Co3+ and Ni4+/Ni2+, respectively.22,30 The peak at 4.55 V was not present in the subsequent cycles, indicating that the redox couple Co3+/Co4+ was only active during the first cycle. During the second cycle, two anodic peaks at 4.02 and 4.14 V together with corresponding cathodic peaks at 3.55 and 3.86 V were ascribed to the Ni2+/Ni4+ redox reaction. The weak peaks at 4.14/3.86 V were due to the order/disorder phase transitions involving a partial interchange of occupancy of Li ion and transition metal ion (M = Co, Ni, Mn), called “cation mixing”.31 This pair of peaks almost disappeared in the subsequent cycles, indicating a more stable bulk material structure after the initial cycles. With the increased cycles, the anodic peak decreased from 4.55 V to the lower value and stabilized at 3.90 V; this behavior also indicated that the bulk material structure and/or the electrode− electrolyte interface were modified after the initial cycles.32,33 The CV curves of the cell after 100 cycles in Figure 3D (insert) still showed a sharp redox peak, which also indicated excellent kinetics behavior of the PLTF electrode, while the CV curves for ALTF and target LiCo1/3Ni1/3Mn1/3O2 (Supporting Information Figures S4 and S5) showed different behaviors to the PLTF, which is mainly attributed to different composition and structure of these cathode materials.

material. The stoichiometric formula for the ALTF and PLTF was calculated as Li 0 . 9 2 Co 0 . 3 0 Ni 0 . 3 2 Mn 0 . 2 5 O 2 an d Li0.28Co0.29Ni0.30Mn0.20O2, respectively. The low Li content in bulk film confirmed that the PLTF had been delithiated. The molar ratio of Mn/Co/Ni in the film was almost consistent with the EDX result, which is most likely caused by the high mobility of Mn ions during annealing process.25 On the basis of all the above results, we concluded that the as-prepared PLTF has the features of Li-deficiency, polycrystalline nanostructure, high density, and air stability. Such features are beneficial to increase the electroactive zones and facilitate charge transfer kinetics. Consequently, the electrochemical performance of the PLTF is expected to be improved in terms of the capacity and rate capability, as demonstrated next in the tests with Li-ion cells. The electrochemical properties of the LTFs were evaluated by CV, EIS, and galvanostatic charge−discharge tests. Figure 3 shows the electrochemical properties of the PLTF. The charge−discharge profiles and cycle performance of a Li/ PLTF cell are shown in Figure 3A,B, respectively. The cell showed an initial charge capacity of 158 mAh/g, corresponding to the extraction of Li+ ions from the PLTF. The subsequent discharge delivered a higher capacity of 202 mAh/g. This is characteristic of a Li-deficient cathode material.25,29 The second cycle delivered an even higher charge capacity of 216 mAh/g−1 with the second discharge capacity of 203 mAh/g. Although the capacity gradually decreased and the polarization between charge and discharge plateaus gradually increased with cycle number thereafter, a discharge capacity of 138 mAh/g was still maintained after 100 cycles with average Coulombic efficiency for the 100 cycles being 98.4%, which indicated that the cell exhibited high reversible capacity and good cycle performance. Showing in Figure 3C is the rate performance of the cell. Although the capacity decreased with increased rate, the cell exhibited a good rate capacity. A discharge capacity of 168 mAh/g was achieved at 1 C; even at the highest rate of 5 C, the capacity was still maintained at about 86 mAh/g, which can be 1284

dx.doi.org/10.1021/nl404215h | Nano Lett. 2014, 14, 1281−1287

Nano Letters

Letter

Figure 3. Electrochemical performance of Li/PLTF half cell: (A) charge−discharge profiles, (B) cycle performance, (C) rate performance, (D) cyclic voltammograms, (E) electrochemical impedance spectra profiles, and (F) equivalent model fits to the impedance spectra.

by SEM and AES analyses, respectively, as shown in Figure 4. Figure 4A shows the PLFT electrode after the cell had been operated for 100 cycles at the charge state (to 4.5 V). The PLTF electrode showed a relatively rough surface with nanosized island structures. This morphology change was likely caused by the volume contraction of nanoparticles corresponding to Li ion extraction from the PLTF bulk material. However, after the cell was discharged to 3.0 V and even to 2.8 V again, the nanoparticles restored to the initial size, and the electrode surface became smoother, as shown in Figure 4B,C, respectively. Such restorable structure indicated that the PLTF electrode had a stable structural integrity. The nanoparticle structures of the PLTF may have reduced the stress that arises from volume contraction/expansion during cycling.16 Figure 4A1,C1 illustrates the elemental composition of the PLTF electrodes at 4.5 and 2.8 V, respectively. The corresponding stoichiometric formula was calculated to be Li0.10Co0.30Ni0.31Mn0.25O2 and Li0.87Co0.32Ni0.34Mn0.25O2, respectively. As shown in above, the PLTF exhibited higher reversible capacity and better rate capability than the LiCo1/3Ni1/3Mn1/3O2 and ALTF. We believe that several unique features of the PLTF are responsible for the improvement of the electrochemical performance, including its lithium deficiency, air-stability of polycrystalline structure, nanostruc-

Figure 3E shows the EIS profiles of the PLTF electrode discharged to 2.5 V after 3, 30, 50, and 100 cycles. The impedance spectra consist of a depressed semicircle at high frequency and a straight line at low frequency. An intercept at the Z′real axis at high frequency indicated the ohmic resistance of the electrolyte (Rs); the depressed semicircle corresponded to the charge transfer resistance (Rct) and double-layer capacitance (Cdl); and the inclined line represented the Warburg impedance (ZW) associated with lithium-ion diffusion in the PLTF bulk material. Rs is referred to as the “ohmic impedance”, and the combination of Rct and ZW is called the “faradaic impedance”, which reflects the kinetics of the cell reactions.34 The Rct value of the PLTF after three cycles is about 130 Ω, and it is gradually increased with the increasing cycles due to the electrode polarization. The slow increase in Rct upon cycling indicates a relatively stable interface between the PLTF electrode and electrolyte and therefore good cycle performance of the PLTF cathode. Figure 3F illustrates the typical equivalent model fits to the impedance spectra for the PLTF electrode after 100 cycles. The Rs is 19.3 Ω, and the Rct is about 324 Ω, corresponding to a low polarization voltage of about 0.0114 V. Therefore, the cell can still be operated well. To investigate the interface between the PLTF and electrolyte, we determined the surface morphology and chemical composition of the PLTF at different states of charge 1285

dx.doi.org/10.1021/nl404215h | Nano Lett. 2014, 14, 1281−1287

Nano Letters

Letter

Figure 4. SEM images of the PLTF electrode (after 100 cycles) at different states of charge: (A) charged to 4.5 V, (B) discharged to 3.0 V, and (C) discharged to 2.8 V; (D) the corresponding charge/discharge schematic diagram; (A1) and (C1) AES depth profiles of PLTF electrodes for A and C, respectively.

■

ture, and thin film design. First, Li-deficient cathode material normally provide more Li+ vacancy in the cathode,29 which leads to the high reversible capacity. Second, the disordered state of the stable polycrystalline structure resulted in a high chemical diffusion coefficiency, thereby inducing the excellent charge transfer kinetics.15 Third, the PLTF consists of nanosized particles that reduces the path lengths for Li+ and electron transfer between active particles, significantly increasing the electroactive zone. Last, the micrometer size of designed thin film also reduced the path lengths of the Li+ ionic transfer in the bulk and electron transfer to the current collector. These three features are significantly beneficial to the high rate performance of the PLTF. In conclusion, we prepared air-stable Li-deficient polycrystalline Li0.28Co0.29Ni0.30Mn0.20O2 thin film with nanostructure and high tap density by a method combining RF magnetron sputtering and annealing. The unique features of PLFT led us to believe that it would be a potential cathode material for high energy density LIBs, especially as thin film LIBs. Additional work will be carried out to understand the mechanism of lithium insertion/extraction of the PLTF and to further improve the properties of these PLTFs, especially in terms of cycle performance.

■

AUTHOR INFORMATION

Corresponding Authors

*(R.C) E-mail: [email protected]. Phone: +86-10-6891-2508. *(L.L.) E-mail: [email protected]. Phone: +86-10-6891-2508. *(K.A.) E-mail: [email protected]. Phone: 1-630-252-3838. Author Contributions ⊥

F.W., G.T., and J.L. contributed equally to this work.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Key Program for Basic Research of China (No. 2009CB220100), National 863 Program (2011AA11A256), the International S&T Cooperation Program of China (2010DFB63370), New Century Educational Talents Plan of Chinese Education Ministry (NCET-12-0050), and Beijing Novel Program (2010B018). This work was also supported by the U.S. Department of Energy under Contract DE-AC0206CH11357 with the main support provided by the Vehicle Technologies Office, Department of Energy (DOE) Office of Energy Efficiency and Renewable Energy (EERE). J.L. was supported by the Department of Energy (DOE) Office of Energy Efficiency and Renewable Energy (EERE) Postdoctoral Research Award under the EERE Vehicles Technology Program administered by the Oak Ridge Institute for Science and Education (ORISE) for the DOE. This work especially thanks to U.S.-China Electric Vehicle and Battery Technology collaboration between Argonne National Laboratory and Beijing Institute of Technology.

ASSOCIATED CONTENT

S Supporting Information *

Experimental detail of the synthetic route for Li−Co−Ni− Mn−O thin film cathodes; AFM images for ALTF and PLTF; electrochemical test of ALTF and target LiCo1/3Ni1/3Mn1/3O2 cathode materials. This material is available free of charge via the Internet at http://pubs.acs.org.

■

REFERENCES

(1) Armand, M.; Tarascon, J.-M. Nature 2008, 451, 652−657.

1286

dx.doi.org/10.1021/nl404215h | Nano Lett. 2014, 14, 1281−1287

Nano Letters

Letter

(2) Tollefson, J. Nature 2008, 456, 436−440. (3) Yamada, A.; Iwane, N.; Harada, Y.; Nishimura, S.-I.; Koyama, Y.; Tanaka, I. Adv. Mater. 2010, 22, 3583−3587. (4) Oh, S.-M.; Myung, S.-T.; Park, J. B.; Scrosati, B.; Amine, K.; Sun, Y.-K. Angew. Chem., Int. Ed. 2012, 51, 1853−1856. (5) Li, H.; Wang, Z.; Chen, L.; Huang, X. Adv. Mater. 2009, 21, 4593−4607. (6) Ellis, B. L.; Lee, K. T.; Nazar, L. F. Chem. Mater. 2010, 22, 691− 714. (7) Cheng, F.; Liang, J.; Tao, Z.; Chen, J. Adv. Mater. 2011, 23, 1695−1715. (8) Ohzuku, T.; Makimura, Y. Chem. Lett. 2001, 642−643. (9) Yabuuchi, N.; Ohzuku, T. J. Power Sources 2003, 119−121, 171− 174. (10) Shaju, K. M.; Bruce, P. G. Adv. Mater. 2006, 18, 2330−2334. (11) Lu, C.-H.; Lin, Y.-K. J. Power Sources 2009, 189, 40−44. (12) Chen, Z.; Lee, D.-J.; Sun, Y.-K.; Amine, K. MRS. Bull. 2011, 36, 498−505. (13) Yabuuchi, N.; Yoshii, K.; Myung, S.-T.; Nakai, I.; Komaba, S. J. Am. Chem. Soc. 2011, 133, 4404−4419. (14) Wu, F.; Li, N.; Su, Y.; Lu, H.; Zhang, L.; An, R.; Wang, Z.; Bao, L.; Chen, S. J. Mater. Chem. 2012, 22, 1489−1497. (15) Lee, M.-H.; Kang, Y.-J.; Myuang, S.-T.; Sun, Y.-K. Electrochim. Acta 2004, 50, 939−948. (16) Julien, C. Ionics 1996, 2, 169−178. (17) Lee, K. T.; Cho, J. Nano Today 2011, 6, 28−41. (18) Xie, J.; Imanishi, N.; Zhang, T.; Hirano, A.; Takeda, Y.; Yamamoto, O. J. Power Sources 2010, 195, 5780−5783. (19) Wu, F.; Wang, M.; Su, Y.; Bao, L.; Chen, S. J. Power Sources 2010, 195, 2362−2367. (20) Yoon, W.-S.; Chung, K. Y.; McBreen, J.; Yang, X.-Q. Electrochem. Commun. 2006, 8, 1257−1262. (21) Li, J.; Zhang, Z. R.; Guo, X. J.; Yang, Y. Solid State Ionics 2006, 117, 1509−1516. (22) Koyama, Y.; Tanaka, I.; Adachi, H.; Makimura, Y.; Ohzuku, T. J. Power Sources 2003, 119−121, 644−648. (23) Guilmard, M.; Croguennec, L.; Delmas, C. Chem. Mater. 2003, 15, 4484−4493. (24) Fujii, Y.; Miura, H.; Suzuki, N.; Shoji, T.; Nakayama, N. J. Power Sources 2007, 171, 894−903. (25) Yoshizawa, H.; Ohzuku, T. J. Power Sources 2007, 174, 813−817. (26) Zhang, X.; Mauger, A.; Lu, Q.; Groult, H.; Perrigaud, L.; Gendron, F.; Julien, C. M. Electrochim. Acta 2010, 55, 6440−6449. (27) Julien, C.; Massot, M. Solid State Ionics 2002, 148, 53−59. (28) Kim, H. U.; Mumm, D. R.; Park, H. R.; Song, M. Y. Electron. Mater. Lett. 2010, 6, 91−95. (29) Shaju, K. M.; Rao, G. V. S.; Chowdari, B. V. R. Electrochem. Commun. 2002, 4, 633−638. (30) Yin, S.-C.; Rho, Y.-H.; Swainson, I.; Nazar, L. F. Chem. Mater. 2006, 18, 1901−1910. (31) Wu, F.; Wang, M.; Su, Y.; Bao, L.; Chen, S. J. Power Sources 2010, 195, 2900−2904. (32) Johnson, C. S.; Li, N.; Lefief, C.; Vaughey, J. T.; Thackeray, M. M. Chem. Mater. 2008, 20, 6095−6106. (33) Koenig, G. M.; Belharouak, I.; Wu, H.; Amine, K. Electrochim. Acta 2011, 56, 1426−1431. (34) Tan, G.; Wu, F.; Li, L.; Chen, R.; Chen, S. J. Phys. Chem. C 2013, 117, 6013−6021.

1287

dx.doi.org/10.1021/nl404215h | Nano Lett. 2014, 14, 1281−1287