Mo6+ for

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Letter

Reversible Three-Electron Redox Reaction of Mo3+/Mo6+ for Rechargeable Lithium Batteries Satoshi Hoshino, Alexey M. Glushenkov, Shinnosuke Ichikawa, Tetsuya Ozaki, Tokuo Inamasu, and Naoaki Yabuuchi ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00037 • Publication Date (Web): 27 Feb 2017 Downloaded from http://pubs.acs.org on March 1, 2017

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Reversible Three-Electron Redox Reaction of Mo3+/Mo6+ for Rechargeable Lithium Batteries Satoshi Hoshino,1 Alexey M. Glushenkov,2 Shinnosuke Ichikawa,3 Tetsuya Ozaki,3 Tokuo Inamasu,3 and Naoaki Yabuuchi*1 1

Department of Green and Sustainable Chemistry, Tokyo Denki University, 5 Senju Asahi-Cho,

Adachi, Tokyo 120-8551, Japan 2

Institute for Frontier Materials, Deakin University, 75 Pigdons Road, Waurn Ponds, Geelong,

Victoria 3216, Australia 3

R&D Center, GS Yuasa International Ltd., 1 Inobanba-cho, Nishinosho, Kisshoin, Minami-ku,

Kyoto 601-8520, Japan

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

ABSTRACT

To increase the energy density of lithium batteries, the development of high-capacity positive electrode materials is essential. Herein, we propose the use of a three-electron redox reaction of Mo3+/Mo6+ for a new series of high-capacity lithium insertion materials. In this study, a binary

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system of LiMoO2 – Li3NbO4 is targeted, and nanosize and metastable Li9/7Nb2/7Mo3/7O2 is successfully prepared by a mechanical milling process. The sample delivers a large reversible capacity of ~280 mAh g-1 in a Li cell with good capacity retention. On the basis of these results, the future possibility of high-capacity electrode materials with three-electron Mo3+/Mo6+ redox reaction is discussed.

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The development of convenient portable electronic devices and electric vehicles has contributed to innovate modern society through the technological progress of state-of-the-art rechargeable Li batteries as power sources. Nevertheless, there is an ever-increasing demand on energy density of lithium batteries. Lithium batteries consist of two lithium insertion materials, i.e., positive and negative electrodes, and the energy density is currently restricted by the lack of advanced high-capacity positive electrode materials.

As positive electrode materials, many

oxide- and phosphate-based materials have been extensively studied in the past three decades.1 Recently, lithium-excess manganese layered oxides, Li2MnO3–based materials, have been especially attractive to researchers in the field of high-capacity positive electrode materials.2-6 Recent experimental7-9 and theoretical10 studies for the Li-excess system have revealed that oxide ions as anionic species can also participate in the charge compensation process. However, irreversible phase transitions on electrochemical cycles, which are associated with partial oxygen loss on charge, hinder its use for battery applications without further breakthroughs in materials engineering.11 The theoretical capacities of lithium insertion materials are decided by both total extractable lithium and electron contents in the structure of positive electrodes.12 Therefore, in addition to the lithium-enrichment in the structure, the use of multi-electron redox processes in transition metals is expected to be an important strategy to increase the energy density of positive electrode materials further and also minimize the amount of transition metal ions in their structures. Among multi-electron redox systems, two-electron redox of Ni2+/Ni4+ has been the most widely studied for lithium batteries, e.g., LiNi1/2Mn1/2O213 and LiNi1/2Mn3/2O4.14 V3+/V5+ two-electron redox is also a highly reversible process.15-18 Historically, three-electron redox reaction of transition metals is reported for Cr3+/Cr6+, e.g., Li1.2Cr0.4Mn0.4O2.19

However, chemical


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compounds containing Cr6+ are toxic and restricted in use. In this study, three-electron redox reaction of Mo3+/Mo6+ is, therefore, targeted for a new series of electrode materials. Since a conventional layered system, LiMo3+O2, has one mole of Li in the formula unit, only oneelectron redox of Mo3+/Mo4+ is used.20 As a proof of concept, Mo3+ is diluted in lithium-excess oxides, and in this study, Li3NbO4 is used as a model material to form a binary system with the chemical formula of x LiMoO2 – (1 – x) Li3NbO4. The crystal structures of LiMoO220 and Li3NbO49 are classified as rocksalt-type superstructures, indicating that both oxides consist of a common cubic close-packed (ccp) oxygen lattice, and a difference is found only in cation distribution in octahedral sites. Therefore, the formation of solid solution samples is anticipated in the binary system, and the highest reversible capacity (based on Mo3+/Mo6+ redox) is expected when x = 0.6 (reformulated as Li9/7Nb2/7Mo3/7O2). If all Li ions are reversibly extracted/reinserted from/into the crystal lattice with the three-electron redox of Mo, the theoretical capacity reaches 317 mAh g-1.

Nevertheless, all our trials to synthesize samples by conventional

calcination failed (Supporting Figure S1 – S3). Phase segregation into Li3NbO4 and LiMoO2 has been evidenced, and a narrow solid solution range is anticipated in this binary system. Li9/7Nb2/7Mo3/7O2 was not identified found as a thermodynamically stable phase. Therefore, an alternate route was selected, i.e., synthesis of a metastable phase, and mechanical milling has been chosen in this study. The structural analysis was carried out using RIETAN- FP,21 and schematic illustrations of crystal structures were drawn using VESTA.22 Figure 1a shows X-ray diffraction patterns of the samples before and after the mechanical milling. The diffraction lines of LiMoO2 and Li3NbO4 in a 2θ range of 15 – 37o are gradually disappeared by mechanical milling, and new peaks are appeared at 38, 42, and 63o, which are assigned as a cation-disordered rocksalt structure. This fact indicates that a mixture of Li3NbO4


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and LiMoO2 is gradually changed into the cation-disordered rocksalt structure, in which all cations are located in the same octahedral sites of the ccp lattice. A single phase sample is obtained after milling for 36 h. Color of the powder also changes from gray into black after the mechanical milling. The heating of the rocksalt sample results in the phase segregation into Li3NbO4 and LiMoO2 phase used in the precursor (Supporting Figure S4), indicating that this rocksalt phase is a metastable phase.

The successful synthesis of the metastable phase,

Li9/7Nb2/7Mo3/7O2, is further supported by the results of scanning electron microscopy (SEM) accompanied by energy-dispersive X-ray (EDX) elemental mapping (Supporting Figure S3b). SEM/EDX spectroscopy is also used for the analysis of sample chemical compositions. Chemical compositions after mechanical milling are consistent with the nominal composition as summarized in Supporting Table 1. The electrode performances in Li cells are compared for the samples before and after the mechanical milling in Figure 1b. Although Li9/7Nb2/7Mo3/7O2 prepared by mechanical milling delivers a larger reversible capacity, the observed capacity is limited to 150 mAh g-1, which corresponds to a redox reaction involving 1.5 electrons. To improve the electrode performance, the sample was thoroughly mixed with acetylene black (10 wt%) by ball milling at 300 rpm (hereafter, denoted as a carbon composite sample). XRD patterns of the sample before and after ball milling are shown in Supporting Figure S5. Electrode performance is drastically improved by the preparation of the carbon composite sample and over 280 mAh g-1 of reversible capacity is obtained with high reversibility. No capacity degradation is observed for continuous 5 cycles. The observed reversible capacity is much larger or comparable to those of molybdenum-based oxides.16,20,23,24 Note that the carbon content in the composite electrode is slightly higher for the


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carbon composite sample, but such large capacity is obtained by ball milling and nano-scale integration of Li9/7Nb2/7Mo3/7O2 and carbon (Supporting Figure S6). To examine the morphology of the carbon composite sample, transmission electron microscopy (TEM) was utilized. A bright-field image of a typical area in the carbon composite sample (Figure 1c) demonstrates nanocrystalline aggregates of oxide nanoparticles intermixed with smaller number of acetylene black.

The nanocrystalline aggregates consist of small

individual nanoparticles/crystallites with a typical size of 5 – 20 nm. High-resolution TEM imaging was further performed to visualize these crystallites in a direct manner, and the representative images are shown in Figure 1d. The outcomes of EDX mapping of the carbon composite sample conducted by scanning transmission electron microscopy (STEM) are shown in Figure 1c. Mapping of Nb and Mo elements is generally problematic as their EDX signature peaks overlap (Supporting Figure S7).

To solve this problem, fine-tuned X-ray energy

windows were used for the mapping. EDX spectra of Nb and Mo, and the selected X-ray energy windows are highlighted in Supporting Figure S6. Using these settings, accurate mapping of Nb and Mo was successfully obtained as revealed in maps acquired from a test sample (a simple powder mixture of Li3NbO4 and LiMoO2). The mapping of the carbon composite sample obtained by the mechanical milling was conducted using the same settings, and it can be observed that the maps of Nb and Mo are effectively identical (Figure 1d), indicating homogeneous distribution of Nb and Mo in the sample, at least at the level of the resolution of STEM scanning (~1 – 2 nm). Moreover, nanosize acetylene black is integrated with nanosized Li9/7Nb2/7Mo3/7O2, leading to high reversible capacity. In addition, synthesis by the mechanical milling is extended to other lithium-excess materials, i.e., Li2TiO325 and Li4MoO5,26,27 with crystal structures also classified as cation-


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ordered rocksalt phases. Nanosize and metastable Li6/5Ti2/5Mo2/5O2 and Li4/3Mo6+2/9Mo3+4/9O2 were also successfully prepared by the mechanical milling with the same experimental condition used for the synthesis of Li9/7Nb2/7Mo3/7O2. The crystal structures and electrochemical properties of the three different metastable samples are compared in Figure 2. Theoretical capacities reach 317 – 340 mAh g-1 when all lithium ions are extracted from these samples with Mo3+/Mo6+ redox. The samples obtained by the mechanical milling deliver large reversible capacities of approximately 270 mAh g-1 for the Ti and Nb samples, and 320 mAh g-1 for the Mo sample after the preparation of carbon composite samples. The observed reversible capacities correspond to more than 80 % of the theoretical capacities based on three-electron redox of Mo ions with acceptable capacity retention. Li4/3Mo6+2/9Mo3+4/9O2 delivers 95 % of the theoretical capacity in the Li cell. Herein, the theoretical capacity of Li4/3Mo6+2/9Mo3+4/9O2 was calculated based on the number of octahedral sites for Li ions. Among the three samples, this sample has the highest Li content, and thus the highest probability is expected for the formation of percolating Li migration paths.24 However, the charge capacities for the initial cycle are lower than those of the initial discharge capacities for the three samples. This fact probably suggests that some oxidation of Mo and the formation of a Li-deficient/off-stoichiometric sample by the mechanical milling cannot be eliminated.

Good cyclabillity as electrode materials is further supported by an

accelerated cycle test at 100 mA g-1 for Li9/7Nb2/7Mo3/7O2. The sample delivers approximately 200 mAh g-1 after 60 cycles as shown in Figure 2d. To examine charge compensation mechanisms, ex-situ XRD and X-ray absorption spectroscopy (XAS) are utilized for Li9/7

– yNb2/7Mo3/7O2

(Figure 3). Since oxidation of the

sample is noted for the as-prepared sample, measurement was carried out for the second charge process. Ex-situ XRD studies have revealed that topotactic reaction occurs on charge/discharge


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processes with a very small volume change (only 1.4 %). On the fully charged state, peak broadening is observed for diffraction lines, and no phase transition to other phases is noted. In general, volume “shrinkage” associated with the oxidation of transition metal ions occurs on charge, and such an unexpectedly small volume change combined with a large reversible capacity is hypothesized to originate from migration of small Mo6+ from octahedral to tetrahedral sites, similar to the vanadium system.18 The small volume change is expected to be beneficial to maintain good cyclability as shown in Figure 2c. Figure 3b compares Mo K-edge XAS spectra of Li9/7 – yNb2/7Mo3/7O2 after electrochemical cycling. Mo K-edge XAS spectra of the reference samples (LiMoO2 and MoO3) are shown in Figure 3b. The oxidation state of Mo for the fully discharged state is found to be almost the same with that of LiMoO2, as expected. The XAS spectra shift to a higher energy region on charge as increase in charge capacities, and systematic changes in the peak top energy are noted for differential curves of the spectra. The spectrum of the fully charged sample resembles that of MoO3, and the results are consistent with the large reversible capacity in Li cells. Moreover, no change is found for Nb K-edge on charge/discharge processes, as shown in Supporting Figure S8, indicating that the reversible three-electron Mo3+/Mo6+ redox is effectively utilized in Li9/7 – yNb2/7Mo3/7O2.

However, the mechanical milling process results in the formation of nanosized

particles, and the samples contain many grain boundaries. Therefore, contributions of interfacial charge storage28 and surface conversion29 to reversible capacities cannot be eliminated. Further studies are in progress in our group. In summary, metastable lithium-excess cation-disordered rocksalt oxides with Mo3+ have been successfully prepared by the mechanical milling. Li9/7Nb2/7Mo3/7O2, used as a model material, shows a large reversible capacity with a small volume change, which is associated with a highly


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reversible Mo3+/Mo6+ three-electron redox reaction. Moreover, the expensive Nb element is easily replaced to other non-expensive elements in the structure as shown in this study. The use of Mo3+/Mo6+ three-electron redox is beneficial for battery applications with long cycle life, and further systematic studies will contribute to the development of high-energy rechargeable Li batteries in the future.

Experimental Section Synthesis of Li9/7Nb2/7Mo3/7O2: Li9/7Nb2/7Mo3/7O2 as a metastable phase was prepared by the mechanical milling with a planetary ball mill (PULVERISETTE 7; FRITSCH). A mixture of Li3NbO4 and LiMoO2 was used as a precursor for mechanical milling. 0.7 g of Li3NbO4 and 0.8 g of LiMoO2 were mixed by using a zirconia pot (45 mL) and zirconia balls (15.5 g) at 600 rpm for 12 h. After milling for 12 h, the mixture was taken out from the container and mixed using a mortar and pestle. The mixture was again milled using the zirconia pot and balls at 600 rpm for 12 h. Overall, this process was performed for three times, and the total milling time was 36 h. Li3NbO4 was prepared from Li2CO3 (98.5 %; Kanto Kagaku Co., Inc.) and Nb2O5 (99.9 %; Wako Pure Chemical Industries, Ltd.) at 950 oC for 24 h in air. LiMoO2 was synthesized from Li2CO3 (98.5%; Kanto Kagaku Co., Inc.) and MoO3 (99.5 %; Kanto Kagaku Co., Inc.) at 800 oC in Ar. Acetylene black (HS-100; Denka Co., Ltd.) was also added to reduce Mo ions. After the preparation, LiMoO2 was stored in an Ar-filled glove box. The sample handling after the mechanical milling was also carried out in the Ar-filled glove box without exposure to moist air.


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Electrochemical measurement: The electrode performance of the samples was examined for the as-prepared sample and carbon composite sample prepared by ball milling. The as-prepared Li9/7Nb2/7Mo3/7O2 sample (80 wt%) was mixed with 10 wt% acetylene black (HS-100; Denka

Corp., Ltd., 10 wt% polyvinylidene fluoride (KF 1100; Kureha Co.

Ltd.), and pasted on

aluminum foil as a current collector. Thus prepared electrode was used for the electrochemical characterization of the as-prepared sample. To prepare a carbon composite sample, the asprepared Li9/7Nb2/7Mo3/7O2 sample was mixed with acetylene black (Li9/7Nb2/7Mo3/7O2 : AB = 90 : 10 wt%) by using the planetary ball mill at 300 rpm for 12 h with a zirconia pot and balls. For the preparation of electrodes, 85 wt% of the cabon composite sample, 5wt% of additional AB, and 10 wt% of PVdF were mixed and then pasted on Al foil. The final composition of the electrode was Li9/7Nb2/7Mo3/7O2 : AB : PVdF = 76.5 : 13.5 : 10 in wt%. The electrodes were dried at 80 °C for 2 h in vacuum, and then heated at 120 oC for 2 h. Metallic lithium (Honjo Metal Co., Ltd.) was used as a negative electrode. The electrolyte solution used was 1.0 mol dm−3 LiPF6 dissolved in in ethylene carbonate : dimethyl carbonate (3:7 by volume) (battery grade; Kishida Chemical Corp., Ltd.). A polyolefin microporous membrane was used as a separator. Two-electrode cells (TJ-AC, Tomcell Japan) were assembled in the Ar-filled glovebox. The cells were cycled at a rate of 10 mA g−1 at room temperature.

Characterization of Li9/7Nb2/7Mo3/7O2: X-ray diffraction (XRD) patterns of samples were collected using an X-ray diffractometer (D2 PHASER, Bruker Corp., Ltd.) equipped with a one dimensional X-ray detector using Cu Kα radiation generated at 300 W (30 kV and 10 mA) with a Ni filter. An airtight sample holder was used for the XRD measurement.


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The particle morphology of the samples was observed using a scanning electron microscope (JCM-6000, JEOL Ltd.) with an acceleration voltage of 15 keV. Elemental mapping of the samples was also collected using an energy-dispersive X-ray (EDX) spectrometer. Transmission electron microscopy (TEM) examination was conducted on a JEOL JEM 2100F instrument operated under 200 kV. A bright-field scanning TEM (STEM) image as well as EDX spectra and maps were acquired in STEM mode. In addition, bright-field images of the material and high resolution images of individual crystallites were acquired in the parallel beam mode. Hard X-ray absorption spectroscopy (XAS) at the Mo K-edge was performed at beamline BL-12C of the Photon Factory Synchrotron Source in Japan. The XAS spectra were collected with a silicon monochromator in a transmission mode. The intensity of incident and transmitted X-ray was measured using an ionization chamber at room temperature. Composite electrode samples were prepared using the two-electrode cells at a rate of 10 mA g−1. The composite electrodes were rinsed with dimethyl carbonate and sealed in a water-resistant polymer film in the Ar-filled glovebox. Normalization of the XAS spectra was carried out using the program code IFEFFIT.30 The post-edge background was determined using a cubic spline procedure.

ACKNOWLEDGMENTS The synchrotron X-ray absorption work was done under the approval of the Photon Factory Program Advisory Committee (Proposal No. 2015G529). TEM and STEM work was carried out with the support from Deakin Advanced Characterization Facility.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Summary of chemical analysis of the samples, synthesis of Li9/7Nb2/7Mo3/7O2 by calcination, electrode performance of Li9/7Nb2/7Mo3/7O2 prepared by calcination, SEM/EDX spectra of Li9/7Nb2/7Mo3/7O2 prepared by different methods, structural evolution of Li9/7Nb2/7Mo3/7O2 on heating, XRD patterns of Li9/7Nb2/7Mo3/7O2 before and after ball milling with AB, charge/discharge curves of Li9/7Nb2/7Mo3/7O2 before ball milling with AB,

fine tuning EDX

spectra for Nb/Mo mapping, Nb K-edge spectra of Li9/7–xNb2/7Mo3/7O2 (PDF)

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Figure Captions Figure 1. Synthesis and characterization of Li9/7Nb2/7Mo3/7O2; (a) X-ray diffraction patterns of a mixture of Li3NbO4 and LiMoO2 before/after the mechanical milling. Photographs of powders are also shown. (b) Charge/discharge curves of Li9/7Nb2/7Mo3/7O2 before and after the mechanical milling at 10 mA g-1 at room temperature in the voltage range of 1.0 – 4.4 V vs. Li (4.0 V cut-off for the sample before ball milling).

Electrochemical properties of

Li9/7Nb2/7Mo3/7O2 are further increased by the preparation of a carbon composite sample. Transmission electron microscopy characterization of the sample: (c) a bright-field TEM image and the results of STEM/EDX mapping, including a bright-field STEM image (bottom row), and (d) high-resolution TEM images of Li9/7Nb2/7Mo3/7O2. Clear lattice fringes are noted, indicating the formation of nanosize crystallite samples.


15

ACS Energy Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 20

Figure 2. (a) XRD patterns and (b) electrochemical properties of the three different metastable cation-disordered rocksalt phases prepared by the mechanical milling. Electrode performance is tested after the preparation of carbon composite samples in the voltage range of 1.0 – 4.0 V vs. Li at a rate of 10 mA g-1. Capacity retention of the samples is compared in (c). A result of an accelerated cycle test at 100 mA g-1 for Li9/7Nb2/7Mo3/7O2 is shown in (d).

Figure 3. Changes in (a) ex-situ XRD patterns and (b) XAS spectra of Li9/7 – yNb2/7Mo3/7O2 on the second charge process. Highlighted XRD patterns and the points where data were collected are also shown in the inset. Differential curves of the XAS spectra are plotted in (b). XAS spectra of LiMoO2 and MoO3 as reference samples are also shown in (b).


16

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ACS Energy Letters Mo

(a) ○

○

Nb

LiMoO2

■

Li3NbO4

(c)

Gray

th

st

Voltage / V

th

st

220

111

200

Mechanical milling

1 Li ○ 2 ■ ○ ■ Before mechanical milling ■ ■ ○ ○ ■ ○■○ 3 4 LiMoO2 5 + 300 rpm, 12 h 6Li3NbO4 7 450 rpm, 12 h 8 9 10Cation600 rpm, 12 h Disordered Black 11 Single Phase Rocksalt 12 600 rpm, 36 h 13 14 20 40 60 80 15 16 2/ deg. (Cu K) Li, Nb, and Mo 17 (d) 18 (b) Theoretical Mo3+/Mo4+ Mo3+/Mo5+ Mo3+/Mo6+ 19 Capacities 105 mAh g-1 211 mAh g-1 317 mAh g-1 5.0 20 1 5 -2 Before Mechanical Milling 4.0 21 3.0 22 2.0 23 1.0 24 5 -1 5.0 1 25 5 -2 After Mechanical Milling 4.0 26 (600 rpm, 36 h) 3.0 27 2.0 28 1.0 5 -1 29 5.0 1 5 -2 30 4.0 31 3.0 32 2.0 33 1.0 Carbon Composite Sample 5 -1 34 0.0 0 50 100 150 200 250 300 350 35 -1 Capacity / mAh g 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 ACS Paragon Plus Environment 56 57 58 nd

st

nd

th

th

st

st

th

th

Revised

nd

st

Figure 1

100 nm

50 nm

ACS Energy Letters

220

Page 18 of 20 3+ 6+ Mo /Mo -based

5.0 3.0 2.0

Li6/5Ti2/5Mo2/5O2

Voltage / V

311 222 222

Li4/3Mo6+2/9Mo3+4/9O2 311

111

220

200

220

200

1.0

Li9/7Nb2/7Mo3/7O2

60

80

5 -2

1

5.0

st

5th-1

nd

1st

4.0

5th- 2

3.0 2.0

Li9/7Nb2/7Mo3/7O2

1.0 5.0

st

5th- 1

1st

4.0

40

nd

th

st

4.0

311 222

111 111

Li6/5Ti2/5Mo2/5O2

5th- 2nd

3.0

100

2/ deg. (Cu K)

2.0

Li4/3Mo6+2/9Mo3+4/9O2

1.0

st

5th- 1

0.0 0

Li6/5Ti2/5Mo2/5O2 Li9/7Nb2/7Mo3/7O2 Li4/3Mo6+2/9Mo3+4/9O2

Revised

10

20

100

200

300

Capacity / mAh g

(d) Capacity / mAh g-1

Capacity / mAh g-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 20 15 16 17(c) 18 400 19 20 21 22 300 23 24 25 200 26 27 28 100 29 30 31 0 32 0 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58

(b)

200

(a)

400

-1

300

200

100

at 100 mA g-1 Li/Li9/7Nb2/7Mo3/7O2cell

0

30

0

Cycle Number

20

40

Cycle Number

Figure 2 ACS Paragon Plus Environment

60

Page 19 of 20

ΔV =1.4 %

a = 4.202 Å V = 74.19 Å3

2nd full charge

Theoretical

Mo3+/Mo4+

Mo3+/Mo5+

Mo3+/Mo6+

capacity

105 mAh g-1

211 mAh g-1

317 mAh g-1

5

2nd charge 211 mAh g

a = 4.182 Å V = 73.14 Å3


a = 4.186 Å V = 73.35 Å3

-1

Voltage / V

a = 4.194 Å V = 73.77 Å3

-1

40

42

44

2 105 mAh g-1

46

60

80

1st full discharge

0

2/ deg. (Cu K) 40

2nd full charge

211 mAh g-1

3

1

1st full discharge

20

2nd cycle 10 mA g-1 at RT

4

50

100 150 200 250 300 350

Capacity / mAh g-1

100

2/ deg. (Cu K)

(b)

2nd charge 105 mAh g-1

-1 2nd charge 105 mAh g-1 2nd charge 211 mAh g -1

1st full discharge


1st full discharge

2nd full charge

2nd full charge

LiMoO2

dx / dE

(a)

Normalized Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58

ACS Energy Letters

MoO3

Differential Curves

Mo K-edge 19980

20000

20020

20040

20060

20080

19980

19990

20000

Energy / eV

Figure 3 Revised

20010

Energy / eV


20020

20030

20040

ACS Energy Letters

TOC

Mo3+/Mo6+-based

5.0

nd

5th- 2

st

1

4.0 3.0 2.0

Li6/5Ti2/5Mo2/5O2

1.0

Voltage / V

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58

Page 20 of 20

5.0

st

5th-1

st

nd

th

st

5 -2

1

4.0

th

3.0 2.0

100 nm

Li9/7Nb2/7Mo3/7O2

1.0 5.0

5 -1

1st

4.0

5th- 2nd

3.0 2.0

Li4/3Mo6+2/9Mo3+4/9O2

1.0

st

5th- 1

0.0 0

100

200

300

400

Capacity / mAh g-1


Mo6+ for

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