In Situ TEM Observation of Local Phase Transformation in a

Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology, Umezono 1-1-1, Tsukuba 305-8568, Japan...
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In Situ TEM Observation of Local Phase Transformation in a Rechargeable LiMn2O4 Nanowire Battery Soyeon Lee,†,‡ Yoshifumi Oshima,*,‡,§ Eiji Hosono,∥ Haoshen Zhou,*,∥ Kyungsu Kim,⊥ Hansen M. Chang,⊥ Ryoji Kanno,⊥ and Kunio Takayanagi†,‡ †

Department of Material Science and Engineering, Tokyo Institute of Technology, 2-12-1-H-51 Oh-okayama, Meguro-ku, Tokyo 152-8551, Japan ‡ JST−CREST, 7-Gobancho, Chiyoda-ku, Tokyo 102-0075, Japan § Research Center for Ultra HVEM, Osaka University, 7-1 Mihokaoka, Ibaraki 567-0047, Japan ∥ Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology, Umezono 1-1-1, Tsukuba 305-8568, Japan ⊥ Department of Material Science and Engineering, Tokyo Institute of Technology, G1-1 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan S Supporting Information *

ABSTRACT: We developed a “nanobattery” consisting of LiMn2O4 nanowires, ionic liquid electrolyte, and Li4Ti5O12 crystals and performed in situ transmission electron microscope observation during charge and discharge cycles. The LiMn2O4 nanowire was found to be changed into the tetragonal phase at the interface region with electrolyte during the discharge process of the 4 V reaction (vs Li/Li+). This result is explained by the lithium accumulation due to the different local lithium diffusion rates: the lithium ions were inserted into the nanowire at a higher rate than they diffused toward the bulk side of the nanowire. We also found that the LiMn2O4 nanowire cathode was restored to be cubic phase without any fracture in the charge process. No fracture during the cubic-tetragonal transition is promising for the long-lifetime battery with LiMn2O4 nanowire cathode.

structure change of germanium,16 aluminum,17 and SnO12 nanowire anodes during lithiation and delithiation cycles. However, electrochemical measurement such as cyclic voltammetry has not been performed, which is important to understand the battery performance. In this study, for clarifying the deterioration mechanism of LiMn2O4 cathode during the 4 V reaction, we developed a nanowire-based battery for TEM observation. The nanowire battery consists of LiMn2O4 nanowires, ionic liquid electrolyte (ILE), and Li4Ti5O12 crystals. In situ TEM observation revealed that the tetragonal phase appeared at the interface region of LiMn2O4 nanowires with the electrolyte at early discharge stage, 3.9 V vs Li/Li+. Furthermore, the tetragonal phase was reversibly restored to the cubic phase without fracture of the nanowires during the charge process.

1. INTRODUCTION A spinel lithium manganese oxide (LiMn2O4) crystal is one of the most promising cathode materials for lithium ion batteries. It has advantages of environmental harmlessness, great natural abundance, good safety, and low cost.1−3 However, it has been pointed out that the capacity of LiMn2O4 crystal fades out during 4 V battery reaction.4,5 Mn ion dissolution,6,7 instability of highly charged phase,8 and microcrack formation have been discussed as the reasons for the capacity fading out.9 As another reason, Thackeray et al. have found that the tetragonal phase appeared at the surface (about a few tenths of nanometers in depth) of discharged LiMn2O4 crystal even above 3.3 V vs Li/Li+ by transmission electron microscopy (TEM) observation,10 although the phase transformation from cubic spinel to tetragonal rock-salt structure has been expected to occur at 2.96 V due to the Jahn−Teller (JT) effect.10 They concluded that the lattice mismatch between the cubic and tetragonal phase (tetragonality: c/a ≈ 1.16) may cause physical fracture and capacity fading during 4 V reaction.10,11 In situ TEM observation is a powerful tool to understand local structure change of cathode and/or anode materials during battery reaction.12−21 Huang and co-workers revealed anisotropic swelling of Si nanowire anode during lithiation by in situ TEM observation.14,15 They also investigated the local © 2013 American Chemical Society

2. EXPERIMENTAL SECTION The nanowire battery was fabricated on a small stage of ∼3 mm in radius. First, the platform, which consists of ionic liquid electrolyte (ILE)/Li4Ti5O12 (anode)/Cu (current collector) Received: September 10, 2013 Revised: October 23, 2013 Published: October 23, 2013 24236

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was prepared. Lithium N,N-bis(trifluoromethane sulfonyl)imide (Li-TFSI) in N-methylpropylpiperidinium bis(trifluoromethanesulfonyl)imide (PP13TFSI) was used as electrolyte because of its low vapor pressure. Li4Ti5O12 crystals were used as an anode because of their high structural stability and no volume change during charge−discharge cycles. The prepared platform was set on the stage, and platinum current collector was set on the opposite site of the stage. Then, LiMn2O4 nanowire bundles constructed by connection of 2−10 short LiMn2O4 nanowires of 100−200 nm in diameter were placed between the platinum and platform using micromanipulator. The total volume ratio of the LiMn2O4 nanowire cathode to Li4Ti5O12 crystal anode (∼1/1 000 000) was extremely small to keep the voltage of the Li4Ti5O12 crystal anode during the charge and discharge processes. LiMn2O4 nanowires were obtained by the method previous reported.22 All electrochemical measurements were performed with the nanowire batteries (two-electrode cell). The cell voltage was converted into voltage vs Li/Li+ for convenience in this paper. In situ TEM observation was carried out using our homemade electrical biasing double-tilt TEM holder. Cyclic voltammetry was performed by scanning voltage from 3.50 to 5.50 V vs Li/Li+ at 0.55 mV/s. The measurement was performed by source-measurement unit, Keithley 2635A. During charge/discharge cycles, crystalline LiMn2O4 nanowires in the nanowire battery were observed by an aberrationcorrected TEM, R005 at 300 kV.23 The diffraction spots of the transmission electron diffraction (TED) patterns were analyzed by comparing with the TED patterns of silicon crystal viewed from the [110] and [111] directions, which were obtained under the same experimental conditions with the present study.

Figure 1. (a) Schematic illustration of nanowire battery. A LiMn2O4 nanowire bundle is bridged between Pt current collector (left) and ionic liquid electrolyte (ILE). Li4Ti5O12 crystals were used as an anode. At the vicinity of ILE, the nanowire is covered by a thin ILE layer for a few micrometers. (b) Typical TEM image of “nanowire battery”. Two LiMn2O4 nanowire bundles are contacted with ILE (right bottom). (c) Cyclic voltammogram of a nanowire battery with a single LiMn2O4 nanowire bundle obtained by ex situ measurement. Cyclic voltammetry was performed between 3.0 and 5.0 V vs Li/Li+ at a scan rate of 2 mV/s.

3. RESULT AND DISCUSSION Figure 1a shows the illustration of our developed nanowire battery. The nanowire batteries consist of LiMn2O4 nanowire cathode, ionic liquid electrolyte (ILE), and Li4Ti5O12 crystal anode. Figure 1b shows a typical TEM image of the nanowire batteries. Between the gap (50−100 μm) of Pt current collector and ILE, LiMn2O4 nanowire bundles were suspended as shown in Figure 1b. The bundles consist of 2−20 short LiMn2O4 nanowires of 100−200 nm in diameter by connecting each other and become 100−400 nm in diameter. At the contact area with ILE, sides of LiMn2O4 nanowires were covered by a thin ILE layer due to capillary phenomenon for 2−3 μm distance from ILE. Because of the high lithium diffusion coefficient in liquid,24,25 this covered area acts as the actual interface where lithium ions are inserted into or extracted from the nanowire. Figure 1c shows the cyclic voltammogram (CV) of the nanowire battery with a single LiMn2O4 nanowire bundle. The voltage was scanned in the range from 3.0 to 5.0 V vs Li/Li+ at 2 mV/s (the cell voltage is converted into voltage vs Li/Li+ for convenience). The pair of double anodic/cathodic current peaks is clearly visible, which is characteristic of the 4 V reaction of LiMn2O4 crystals. For more than 20 cycles, the positions of these current peaks are almost the same in spite of pA-order current level. It shows that the nanowire battery works as a rechargeable battery with the stable cell potential. For in situ TEM observation of the nanowire cathode−ILE interface with electrochemical measurement, we fabricated nanowire batteries with a thicker cathode, i.e., ∼10 bundles of LiMn2O4 nanowires. Figure 2 shows the CV of the nanowire battery during TEM observation (Figure3). The voltage was

Figure 2. Cyclic voltammogram of the nanowire battery with 10 LiMn2O4 nanowire bundles obtained during in situ TEM observation. Cyclic voltammetry was performed between 3.5 and 5.5 V vs Li/Li+ at a scan rate of 0.55 mV/s. Black arrowheads indicate a pair of doublecurrent (anodic/cathodic) peaks. Blue, black, and green colored lines correspond to 1st, 2nd, and 3rd cycles, respectively. Transmission electron diffraction patterns were obtained following the order of ①-②-a-b-c-d-③-④, which are shown in Figures 3 and 4.

scanned from 3.5 to 5.5 V vs Li/Li+ at 0.55 mV/s. A pair of double-current peaks is visible: two anodic current (Li deinsertion) peaks appear at 4.1 and 4.7 V vs Li/Li+, and two cathodic current (Li insertion) peaks appear at 3.9 and 3.6 V (marked by arrowheads in Figure 2). The two anodic/cathodic current peaks look broad compared with those of the single24237

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cathode was estimated to react at least for the charge/discharge process. Transmission electron diffraction (TED) patterns were observed simultaneously with measuring the CV. The results for Figure 2 are shown in Figure 3 (the lattice parameters for each case are shown in Table S1 of the Supporting Information). The TED patterns were taken at the area marked by a red circle in Figure 3a where the side surfaces of LiMn2O4 nanowires were covered by a thin ILE layer. The wire axis of the nanowire was the [110] direction of the spinel cubic structure. The LiMn2O4 nanowire had the cubic phase before the charge process (①) as shown in Figure 3b. The cubic phase was maintained during the charge process (① to ② of Figure 3b). In the discharge process, the cubic structure was found to change into the tetragonal phase (② to ③ of Figure 3b). In the next charge process, the tetragonal phase was recovered into the cubic phase again (③ to ④ of Figure 3b). The LiMn2O4 nanowires changed the structure between the cubic and tetragonal phases reversibly during further charge and discharge processes. Figure 4 shows details of the cubic-tetragonal phase change during the discharge process; a series of TED patterns was taken at 4.70, 4.25, 4.12, and 3.90 V, respectively (as indicated by red bars on the CV curve in Figure 2). At 4.70 V (Figure 4a), the nanowire cathode had a cubic structure. At 4.25 V (Figure 4b), which corresponds to the beginning of discharge process, the diffraction spots started being split along the [1− 10]cub direction (marked by arrows), while they did not split along the [110]cub direction of the wire axis. The TED pattern (Figure 4b) means coexistence of the original cubic structure and an orthorhombic structure at 4.25 V. Also, the orthorhombic phase has larger lattice spacing than the cubic phase in the normal direction to the nanowire surfaces, while it keeps the same lattice spacing with the cubic phase in the wire axis. By further discharge, the lattice spacing of the orthorhombic phase increased along the normal direction to the surfaces. Even the cubic phase started to change into orthorhombic structure. In Figure 4c (at 4.12 V), the spots for the cubic phase ([1−10]cub spot) were displaced from the original positions toward the orthorhombic ones (the ratio of the lattice spacing of 4−40 spot to 440 spot is shown in Figure S1 of the Supporting Information). Finally, the tetragonal phase (a = 5.57, c = 8.71 c/√2a ≈ 1.11, where a is the lattice parameter in the tetragonal structure) appeared as shown in Figure 4d (at 3.9 V). The appearance is during the first cathodic double peaks in the CV curve. After the present in situ observations demonstrated in Figures 2−4, we consider the observed cubic-tetragonal

Figure 3. (a) TEM image of a single LiMn2O4 nanowire that contacts ionic liquid electrolyte (right side). The observation area is marked by a red circle. The observed area was far from ILE by about two micrometers and was covered by thin ILE layer. (b) Change of the transmission electron diffraction (TED) pattern of the LiMn2O4 nanowire during charge/discharge cycles. TED patterns ①, ②, ③, and ④ were obtained at spots marked by the same numerical symbols in Figure 2. The horizontal direction of the TED patterns is the projected nanowire axis (white arrow).

bundle battery in Figure 1c. This is because each bundle of the nanowires had different impedance and/or contact resistance with the electrolyte to scatter the current peaks. The anodic/ cathodic peak currents in Figure 2 were ∼10 times higher than those in Figure 1c. Because these double-current peaks correspond to the 4 V reaction, the concentration of lithium ions, x, of LixMn2O4 crystal is expected to change between 0 and 1. The cell capacity during charge was estimated to be ∼30 nC (8.6 × 10−9 mAh) from the CV of Figure 2. This amount corresponds to about half of the lithium ions contained in the whole LiMn2O4 nanowire cathode. The nanowire cathode, bridged between Pt current collector and ILE, had 10 LiMn2O4 bundles of 100 μm in length and 200 nm in diameter. Thus, the lithium ions moved not only from the area of LiMn2O4−ILE interface but also from inside of the LiMn2O4 nanowires. In the present nanowire battery reactions, half of the nanowire

Figure 4. (a−d) Series of TED patterns taken during the discharge process at 4.70, 4.25, 4.12, and 3.90 V, respectively, of CV curve as indicated by red bars in Figure 2. The split diffraction spots are marked by white arrows. The horizontal direction of TED patterns is the projected nanowire axis. 24238

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changed even after 12 h of electron irradiation under the same illumination condition with this study. A possible scenario explaining our tetragonal transition, then, is different local lithium diffusion rates, which results in the inhomogeneous lithium distribution in a LiMn2O4 nanowire. Figure 5 schematically illustrates the distribution of the lithium ions in the LiMn2O4 nanowire during the discharging process. After the charge process, the LiMn2O4 nanowire has low lithium concentration (shown as the blue area in Figure 5a), having cubic phase (shown in the top of Figure 5b). At the beginning of the discharge process, lithium ions are accumulated at the interface with ILE, or surface region of the nanowire, due to fast lithium insertion. Then, the lithium ions are gradually diffused toward the core region of the nanowire. The lattice spacing of the lithium-rich surface layer expands along the normal direction to the surface, while keeping a coherent interface with the cubic core region along the wire axis (orthorhombic phase shown in the middle of Figure 5b). As the discharge proceeds, the boundary of the orthorhombic surface propagates toward the cubic core region of the nanowire. When local valence of Mn ions goes below the critical value27 due to the lithium accumulation, the lithium-rich region transforms into the tetragonal phase (shown in the bottom of Figure 5b) by Jahn−Teller effect. Finally, the lithium ions are diffused along the wire axis through the tetragonal phase. In this study, we found that the LiMn2O4 nanowire was completely restored into the cubic phase from the tetragonal phase in the charge process without fracture. No crack formation was confirmed also by TEM observation (not shown). It is different from the LiMn2O4 particles, which have been pointed out to be damaged due to the tetragonal formation.11 It might be interesting to consider some mechanisms to avoid fracture. We consider first geometric relationships between the cubic and tetragonal phases in the LiMn2O4 nanowire cathode. For cubic-tetragonal transformation in Figure 3, the lattice spacing along the wire axis was conserved. The tetragonal (c/a = 1.11)/cubic geometry is [001]tet//[010]cub and (224)tet//(110)cub for the [110]cub wire axis. In another case, however, the lattice spacing expanded by 6% in the wire direction and 11% in the normal direction to the surface. The tetragonal (c/a = 1.17)/cubic geometry is [001]tet//[001]cub and (100)tet//(110)cub for the [110]cub wire axis. Thus, tetragonal phases grew in the orientation that minimizes the lattice dilatation along the wire axis, taking advantage of the free surface. In addition, the fact that the orthorhombic layer formed along with the nanowire surface covered by ILE must be important, because one mediates between the tetragonal and cubic phases. Nothing has such one-dimensional characteristic but the nanowire. The observed reversible phase transformation without fracture supports that LiMn2O4 nanowires are promising as a cathode material improving the lifetime of LiMn2O4 crystal-based batteries.

transformation of the nanowire cathode during the 4 V reaction. As seen in the TED patterns of Figure 3, the tetragonal phase is obviously nucleated at 3.90 V during the 4 V reaction. However, it has been known by previous X-ray diffraction studies5,26 that charged and discharged LixMn2O4 crystals during the 4 V reaction (0 < x ≤ 1) have only cubic phases. In addition, our CV curve (Figure 3) shows that the concentration of lithium ions at 3.90 V is below x = 0.5, being judged from the position of the cathodic double peaks (arrowheads in Figure 2). A key observation that gave us an understanding of tetragonal phase formation is the TED patterns in Figure 4. At 4.25 V, the orthorhombic phase appears with the cubic phase. The lattice spacing of both orthorhombic and coexisting cubic phase gradually expands along the normal direction to the surfaces by further discharge process until sudden structure transformation into the tetragonal phase occurs at 3.90 V. The process of this phase transformation indicates that the orthorhombic phase is a precursor of the tetragonal transition, and the gradual lattice expansion indicates the local concentration increase of lithium ions in correlation with the discharge process. We thus consider that a lithium-rich region generated locally, at least in the TED observation area, promoted the observed phase transition from the cubic to the tetragonal via orthorhombic phase. As a candidate of the highly lithium-rich region, we consider the surface region of the nanowire cathode, more precisely, the interface between the nanowire and ILE (see Figure 1a and Figure 5). Of course, low

Figure 5. (a) Schematic illustration of lithium concentration (x) change in a LixMn2O4 nanowire during discharge. Color indicates the amount of Li concentration. Red capital letters of the color scale bar correspond to each phase in (b) during the discharge process (not equilibrium phase). (b) Atomic structure models of cubic, orthorhombic, and tetragonal phases observed. In orthorhombic phase, the lattice spacing is expanded along the normal direction to the surface, as indicated by the gray arrow.

lithium diffusion rate inside the LiMn2O4 nanowire causes the lithium-rich interface region. The phase transformation was not detected by CV. It can be explained by the fact that the portion of the observed tetragonal phase is too small in comparison with the reaction region of the nanowire (about half of the nanowire cathode, as mentioned above). We think that the observed tetragonal transformation is not caused by oxygen deficiencies, although tetragonal formation due to oxygen defects27 must be considered. Our observed values of tetragonality (c/a ≈ 1.11 and 1.17) were larger than the one due to the oxygen defect (c/a ≈ 1.07). We also confirmed the tetragonal formation is not affected by imaging electrons. The cubic spinel structure of the LiMn2O4 nanowire was not

4. CONCLUSION In conclusion, we developed a nanowire-based battery consisting of a single LiMn2O4 nanowire cathode, ionic liquid electrolyte, and Li4Ti5O12 anode, which works as a rechargeable 4 V battery. We directly observed the structure change of LiMn2O4 nanowire cathode, in situ by electron microscopy and diffraction, during the charge and discharge processes while measuring the cyclic voltammetry. We found that cubictetragonal phase transformation occurs at the interface area of 24239

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LiMn2O4 nanowire cathode with ionic liquid electrolyte during the early discharge stage, 3.90 V, but no fracture of the nanowire occurs during the 4 V reaction. In the discharge processes, first the cubic phase of the nanowire transforms into the orthorhombic phase at the side surfaces covered by ILE, and the second orthorhombic phase transforms into the tetragonal phase. The appearance of tetragonal phase is understood by local lithium accumulation. An orthorhombic phase that mediates cubic-tetragonal transformation and strain release via the surface layer avoids the fracture.

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ASSOCIATED CONTENT

S Supporting Information *

Lattice parameters obtained by each TED pattern, instructions for how to estimate the amount of lithium ions in the whole nanowire, and instructions for how to estimate the charge from the CV curve. This information is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*Y.O.: Research Center for Ultra HVEM, Osaka University, 7-1 Mihokaoka, Ibaraki 567-0047, Japan. Tel.: +81-6-6879-7941. Email: [email protected]. *H.Z.: Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology, 1-11 Umezono, Tsukuba 305-8568, Japan. Tel.: +81-29-861-5648. E-mail: [email protected].

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ACKNOWLEDGMENTS This work was supported by the Japan Science and Technology Agency (JST) under the CREST project.

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REFERENCES

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