Effect of AlF3 Coating on Thermal Behavior of Chemically

Feb 19, 2010 - Seung-Taek Myung,*,† Ki-Soo Lee,‡ Chong Seung Yoon,§ Yang-Kook Sun,*,‡ Khalil Amine,| and Hitoshi Yashiro†. Department of Chem...
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J. Phys. Chem. C 2010, 114, 4710–4718

Effect of AlF3 Coating on Thermal Behavior of Chemically Delithiated Li0.35[Ni1/3Co1/3Mn1/3]O2 Seung-Taek Myung,*,† Ki-Soo Lee,‡ Chong Seung Yoon,§ Yang-Kook Sun,*,‡ Khalil Amine,| and Hitoshi Yashiro† Department of Chemical Engineering, Iwate UniVersity, 4-3-5 Ueda, Morioka, Iwate 020-8551, Japan; Department of Chemical Engineering, Hanyang UniVersity, Seungdong-Gu, Seoul 133-791, South Korea; Department of Materials Science and Engineering, Hanyang UniVersity, Seungdong-Gu, Seoul 133-791, South Korea; and Electrochemical Technology Program, Chemical Sciences and Engineering DiVision, Argonne National Laboratory, 9700 South Cass AVenue, Argonne, Illinois 60439, United States of America ReceiVed: August 26, 2009; ReVised Manuscript ReceiVed: February 01, 2010

The thermal behavior of chemically delithiated Li0.35[Ni1/3Co1/3Mn1/3]O2 and the AlF3-coated material was studied in the temperature range from room temperature to 600 °C. Thermogravimetric analysis results showed that the uncoated and the AlF3-coated Li0.35[Ni1/3Co1/3Mn1/3]O2 powders experienced distinct weight loss with increasing temperature, of which the weight loss was ascribed to oxygen release from the active materials. The released oxygen amount was less for the AlF3-coated Li0.35[Ni1/3Co1/3Mn1/3]O2 than the uncoated material, probably due to the blocking of the oxygen evolution by the AlF3 coating. The weight loss was associated with the irreversible phase transformation from a rhombohedral layer (R3m j ) structure to a cubic spinel (Fd3m) structure, as confirmed by in situ high-temperature X-ray diffraction. The reduced oxygen release brought about by the AlF3 coating delayed the phase transformation to the cubic spinel structure. This entailed shifts of the main exothermic reactions to higher temperatures for the active material in the presence of an electrolyte. The AlF3 coating remained on the surface of the active material to 300 °C. Thereafter, the layer changed to Li-Al-O with increasing temperature, as observed by the time-of-flight secondary ion mass spectroscopy. The improved thermal properties of the chemically delithiated AlF3-coated Li0.35[Ni1/3Co1/3Mn1/3]O2 were ascribed to the suppression of oxygen release from the active material, and this, in turn, retarded the formation of the cubic spinel phase. Introduction Recently, many researchers have reported that Li[Ni1/3Co1/ 3Mn1/3]O2

has considerable promise as the positive electrode material for lithium batteries due to its outstanding reversible high capacity and improved thermal properties over the conventional LiCoO2.1-5 This active material

is composed of divalent Ni, trivalent Co, and tetravalent Mn.6 The unique redox couple of Ni2+/4+ and Co3+/4+ provides a high reversible capacity, approximately 280 mAh (g-oxide)-1, when lithium ions are fully deintercalated from the structure. The tetravalent Mn, which is electrochemically inactive, improves the thermal properties during cycling.7 Use of surface coatings or modifications with electrochemically inactive materials (Al2O3, ZnO, ZrO2, TiO2, etc.) on the surface of positive electrode materials is a common way to improve their electrochemical and thermal properties.8-28 Myung et al.29 found that a Al2O3 coating on Li[Li0.05Ni0.4Co0.15Mn0.4]O2 particles gradually transformed to AlF3 through an intermediate Al-O-F compound, as confirmed by ToF-SIMS. When other oxides were coated on the active material surface, the coating layer also transformed to stable metal fluorides.30 The newly * To whom correspondence should be addressed. Tel: +81 19 621 6345. Fax: +81 19 621 6345. E-mail: [email protected] (S. Myung). Tel: +82 2 2220 0524. Fax: +82 2 2282 7329. E-mail: [email protected] (Y. Sun). † Department of Chemical Engineering, Iwate University. ‡ Department of Chemical Engineering, Hanyang University. § Department of Materials Science and Engineering, Hanyang University. | Electrochemical Technology Program, Chemical Sciences and Engineering Division, Argonne National Laboratory.

formed metal fluoride layers protect the active material from the direct contact of HF that originates from the decomposition of the electrolyte salt, LiPF6. For this reason, we applied a protective AlF3 nanolayer on the surface of various active materials, such as LiCoO2, Li[Ni0.8Co0.15Al0.05]O2,Li[Ni1/3Co1/3Mn1/3]O2,andLi[Ni1/2Mn1/2]O2.31-34 In cell tests with these positive electrodes, the capacity, capacity retention, and rate capability were improved primarily due to the stabilization of the interface between the positive electrode and electrolyte. In all cases, the exothermic reactions shifted to higher temperature, and the generated heat for the AlF3-coated active materials was always reduced in DSC studies. We speculated that the thin insulating AlF3 coating suppressed the exothermic reaction with the liquid electrolyte.31-34 However, we do not understand the detailed chemistry of the AlF3 coating on the active materials. Here, we investigate again the AlF3coated Li[Ni1/3Co1/3Mn1/3]O2 to better understand its thermal behavior. The AlF3-coated Li[Ni1/3Co1/3Mn1/3]O2 was chemically delithiated to Li0.35[Ni1/3Co1/3Mn1/3]O2, which corresponds to an electrochemical capacity of 180.6 mAh (g-oxide)-1. In this work, we report the thermal properties of the chemically delithiated AlF3-coated Li0.35[Ni1/3Co1/3Mn1/3]O2 as a function of temperature to 600 °C. Experimental Section Spherical Li[Ni1/3Co1/3Mn1/3]O2 was synthesized via coprecipitation. Details of the synthetic process was reported in a previous article.35 To prepare AlF3-coated Li[Ni1/3Co1/3Mn1/3]O2,

10.1021/jp9082322  2010 American Chemical Society Published on Web 02/19/2010

Effect of AlF3 Coating on Thermal Behavior ammonium fluoride (Aldrich) and aluminum nitrate nonahydrate (Aldrich) were separately dissolved in distilled water. Then, the Li[Ni1/3Co1/3Mn1/3]O2 powders were immersed in the aluminum nitrate solution, and the ammonium fluoride solution was slowly added to adjust the pH during precipitation of AlF3 powder.31-34 The solution containing the active material was constantly stirred at 80 °C for 5 h. After filtering, the as-received materials were washed with deionized water and dried at 120 °C. The resulting Li[Ni1/3Co1/3Mn1/3]O2 powders were heated at 400 °C for 5 h under a nitrogen atmosphere to avoid the formation of Al2O3. Chemical extraction of lithium from the synthesized materials was carried out by NO2BF4 (2-fold excess versus active material) in acetonitrile for several days under an Ar atmosphere in a glovebox to produce Li0.35[Ni1/3Co1/3Mn1/3]O2 and AlF3-coated Li0.35[Ni1/3Co1/3Mn1/3]O2. The reacted products were washed several times with acetonitrile to remove LiBF4 and dried under Ar. The chemical compositions of the resulting powders were determined by atomic absorption spectroscopy (Analyst 300, Perkin-Elmer). The prepared materials were subjected to various analyses: X-ray diffraction (XRD), thermogravimetric analysis (TGA), high-temperature X-ray diffraction (HT-XRD), transmission electron microscopy (TEM), time-of-flight secondary ion mass spectroscopy (ToF-SIMS), and differential scanning calorimetry (DSC). The crystalline phase of the synthesized products was characterized by powder XRD (Rint-2000, Rigaku) using Cu-KR radiation. The FULLPROF Rietveld program was used to analyze the powder diffraction patterns.36 The chemically delithiated powders were subjected to TGA (loaded sample amount: 10 mg, DTG-60, SHIMADZU, Japan) combined with in situ HT-XRD (Rint-2200 and PTC-30, Rigaku, Japan). The HT-XRD patterns were collected on a Pt heating strip in air. For the TGA, samples were thermally cycled from room temperature to 600 °C at a heating and cooling rate of 1 °C min-1 and were held at selected temperatures for 10 min prior to data collection. The XRD data were obtained at 2θ ) 10-80°, with a step size of 0.03° and a count time of 0.5 s during each temperature increment. The lattice parameters were calculated by the following method: the positions of the individual peaks were fitted with a pseudo-Voigt or Lorentz function, and typically, peak positions were input to minimize the least-squares difference between the calculated and measured peak positions by adjusting the lattice constant and the vertical displacement of the sample. We employed TEM (JEOL, 2010) to observe the phase evolution of the chemically delithiated and AlF3coated Li0.35[Ni1/3Co1/3Mn1/3]O2. For the TEM measurement, chemically delithiated uncoated and AlF3-coated Li0.35[Ni1/3Co1/ 3Mn1/3]O2 were heated at 200-300 °C in air for 1 h and quenched to room temperature. To investigate the temperature dependence of the AlF3 coating layer, the chemically delithiated AlF3-coated Li0.35[Ni1/3Co1/ 3Mn1/3]O2 was examined by ToF-SIMS (ULVAC-PHI TFS2000, Perkin-Elmer), equipped with a liquid Ga+ ion source and pulse electron flooding and operated at 10-9 torr. The samples were heated in the temperature range of 200-600 °C in air for 1 h and were quenched to room temperature. During the analysis, the targets were bombarded by pulsed 15 keV Ga+ beams over a 12 × 12 µm2 area. In a DSC experiment, the chemically delithiated AlF3-coated Li0.35[Ni1/3Co1/3Mn1/3]O2 powders (3-5 mg) were loaded in a stainless-steel sealed pan with a gold-plated copper seal (which can withstand 150 atm of pressure before rupturing and has a capacity of 30 µL). These measurements were carried out in a

J. Phys. Chem. C, Vol. 114, No. 10, 2010 4711 Pyris 1 calorimeter (Perkin-Elmer) under a temperature scan rate of 1 °C min-1. The weight was constant in all cases, indicating that no leaks occurred during the experiments. Results and Discussion Material Characteristics. Figure 1 presents Rietveld refinement results of XRD data for the chemically delithiated uncoated and AlF3-coated Li0.35[Ni1/3Co1/3Mn1/3]O2. The chemically delithiated amount of Li ingredient was controlled to 0.65 mol, which corresponds to ∼180 mAh g-1, since a nominal cutoff charge voltage versus Li0 is usually 4.3 V (resulting charge capacity of about ∼180 mAh g-1), where the Li[Ni1/3Co1/3Mn1/ 3]O2 shows good reversibility. The advantage for chemical delithiation is that the pseudocapacity resulting from electrolytic decomposition can be eliminated. Refinements were carried out based on the rhombohedral (R3m j space group) crystal system and the chemical composition by atomic absorption spectroscopy. Because the ionic radii of Li+ (0.76 Å37) and Ni2+ (0.69 Å37) are similar, site exchange for both elements between Li and transition metal layers was allowed for the refinement. Isotropic displacement parameters for the chemically delithiated samples were referred from the fully lithiated results shown in Tables 1 and 2. Chemically delithiated uncoated and AlF3-coated material presented a clear peak separation of the (108) and (110) doublets (Figure 1, parts a and b, respectively), in contrast to the lithiated materials (Supporting Information Figure 1). The observed patterns matched well with the calculated ones in Figure 1a,b. Compared with the fully lithiated samples in Supporting Information Figure 1, the site exchange between Li+ and Ni2+ (2.9 wt %) was slightly progressed by the chemical delithiation, which is commonly observed for delithiated samples.38,39 Increase of oxidation states for the transition metals by the chemical delithiation led to decrease in the a-axis and interatomic distance (M-O) values in Table 3. These parameters for the delithiated samples were lower than those of the lithiated samples (Supporting Information Table 3). The c-axes parameters were greater than those of the lithiated samples due to the stronger oxygen-oxygen electrostatic repulsions causing increase in the Li-O distance (Table 3) through the interslab space when the Li+ ions were removed. This further confirms that Li+ ions were extracted from the structure. The calculated lattice constants and interatomic distances (Li-O and M-O) were quite close for both delithiated samples (Table 3), indicating that the same amount of Li+ ions was extracted from the structure by the chemical delithiation. Cho et al.8,9 proposed that surface coating of positive electrode materials suppressed the change in lattice with Li+ extraction. By contrast, Dahn et al.14 and Myung et al.29 suggested that, even though active materials were modified by thin coating layers, the structural parameters of the active materials varied with Li+ deintercalation, as proved with in situ XRD studies. Similarly, the AlF3coated Li[Ni1/3Co1/3Mn1/3]O2 also underwent structural change by Li+ delithiation. Figure 1c,d displays bright-field TEM images of the chemically delithiated uncoated and AlF3-coated Li0.35[Ni1/3Co1/3Mn1/ 3]O2. The pristine Li[Ni1/3Co1/3Mn1/3]O2 showed a smooth edge line (Supporting Information Figure 2a). After the chemical delithiation, the particle morphology of the uncoated Li0.35[Ni1/ 3Co1/3Mn1/3]O2 was degraded, and the resulting surface appeared rough (Figure 1c). The AlF3-coated Li[Ni1/3Co1/3Mn1/3]O2 was covered with a layer of 10-nm thickness (Supporting Information Figure 2b). The original surface was well preserved with the AlF3 coating layer even after the Li+ extraction (Figure 1d).

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Myung et al.

Figure 1. Rietveld refinement patterns of XRD data for the chemically delithiated (a) uncoated Li0.35[Ni1/3Co1/3Mn1/3]O2 and (b) AlF3-coated Li0.35[Ni1/3Co1/3Mn1/3]O2; corresponding TEM bright-field images of (c) chemically delithiated uncoated Li0.35[Ni1/3Co1/3Mn1/3]O2 and (d) AlF3coated Li0.35[Ni1/3Co1/3Mn1/3]O2.

TABLE 1: Rietveld Refinement Results of XRD Data of Chemically Delithiated Uncoated Li0.35[Ni1/3Co1/3Mn1/3]O2 formula crystal system space group atom site Li1 3a Ni2 3a Li2 3b Ni1 3b Mn 3b Co 3b O 6c Rwp (%) Rp (%)

Li0.35[Ni1/3Co1/3Mn1/3]O2 rhombohedral R3m j x 0 0 0 0 0 0 0

y 0 0 0 0 0 0 0

z 1/2 1/2 0 0 0 0 0.266(2)

g 0.321(1) 0.029(1) 0.029 0.305 0.333 0.333 1 12.6 11.6

B (Å2) 1.2 1.2 0.9 0.9 0.9 0.9 0.8

TABLE 2: Rietveld Refinement Results of XRD Data of Chemically Delithiated AlF3-Coated Li0.35[Ni1/3Co1/3Mn1/3]O2 formula crystal system space group atom site Li1 3a Ni2 3a Li2 3b Ni1 3b Mn 3b Co 3b O 6c Rwp (%) Rp (%)

AlF3-coated Li0.35[Ni1/3Co1/3Mn1/3]O2 rhombohedral R3jm x 0 0 0 0 0 0 0

y 0 0 0 0 0 0 0

z 1/2 1/2 0 0 0 0 0.265(2)

g 0.321(1) 0.029(1) 0.029 0.305 0.333 0.333 1 12.3 11.7

B (Å2) 1.2 1.2 0.9 0.9 0.9 0.9 0.8

We believe that the AlF3 coating is necessary to preserve the original particle morphology. Thermal Gravimetric Behavior. The TGA behavior of the chemically delithiated uncoated Li0.35[Ni1/3Co1/3Mn1/3]O2 and

the chemically delithiated AlF3-coated material is shown in Figure 2. Distinct changes in weight for both samples were observed at 50-450 °C and 450-600-650 °C. Though the reaction temperatures were different, as reported by Belharouak et al.,40 the thermal behavior in weight change seems to be similar to the delithiated Li0.55[Ni1/3Co1/3Mn1/3]O2 in TGA. The uncoated Li0.35[Ni1/3Co1/3Mn1/3]O2 underwent a gradual weight loss (∼4.7 wt %) from 50 to 450 °C. This weight loss is mainly associated with oxygen release from the Li0.35[Ni1/3Co1/3Mn1/ 3]O2.41 Oxygen loss from the structure further progressed to around 600 °C (∼4.2 wt %). The total weight change was ∼8.9 wt % to 600 °C. For the AlF3-coated Li0.35[Ni1/3Co1/3Mn1/3]O2, heating from 50 to 500 °C resulted in a weight loss of 2.9 wt %. Taking into account that the uncoated Li0.35[Ni1/3Co1/3Mn1/3]O2 exhibited a weight loss of 4.7 wt % to 450 °C, the oxygen release occurred more slowly and the amount was smaller for the AlF3-coated material. It is thought that the AlF3 layer resides on the surface of the Li0.35[Ni1/3Co1/3Mn1/3]O2 and impedes oxygen evolution. The coated material underwent a similar drastic weight loss (∼3.7 wt %) from 500 to 650 °C, and the upper temperature was slightly higher, by about 50 °C, compared with the uncoated material. The total change in the weight for the AlF3-coated material was approximately 6.6 wt % to 650 °C. Thus, even though similar reactions occurred during heating, the weight loss was smaller, and the decomposition reactions progressed more slowly for the AlF3-coated Li0.35[Ni1/3Co1/3Mn1/3]O2 than the uncoated material. Structural Changes in Uncoated Material. We followed the structural changes for the chemically delithiated uncoated and AlF3-coated Li0.35[Ni1/3Co1/3Mn1/3]O2 with heating. Figures 3 and 4 show the in situ HT-XRD patterns obtained from room

Effect of AlF3 Coating on Thermal Behavior

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TABLE 3: Structural Parameters Obtained from Rietveld Refinement of XRD Data of Chemically Delithiated Uncoated Li0.35[Ni1/3Co1/3Mn1/3]O2 and AlF3-Coated Li0.35[Ni1/3Co1/3Mn1/3]O2 Li0.35[Ni1/3Co1/3Mn1/3]O2 AlF3-coated Li0.35[Ni1/3Co1/3Mn1/3]O2

a-axis (Å)

c-axis (Å)

Li-O (Å)

M-O (Å)

2.827(1) 2.827(1)

14.429(6) 14.428(6)

2.173(19) 2.170(18)

1.899(15) 1.902(14)

Figure 2. TGA curves of the chemically delithiated uncoated Li0.35[Ni1/ and AlF3-coated Li0.35[Ni1/3Co1/3Mn1/3]O2.

3Co1/3Mn1/3]O2

Figure 4. Highlighted in situ HT-XRD patterns of the chemically delithiated (a) uncoated Li0.35[Ni1/3Co1/3Mn1/3]O2 and (b) AlF3-coated Li0.35[Ni1/3Co1/3Mn1/3]O2 in temperature range 150-300 °C.

Figure 3. In situ HT-XRD patterns of the chemically delithiated (a) uncoated Li0.35[Ni1/3Co1/3Mn1/3]O2 and (b) AlF3-coated Li0.35[Ni1/3Co1/ 3Mn1/3]O2 in temperature range 25-600 °C.

temperature to 600 °C. Peaks marked by circles at 39.5°, 44.8°, and 67.5° (2θ) indicate the Pt sample holder. This section deals with the results for the uncoated material. In the highlighted diffraction patterns of the (003) peaks (Figure 4a), advent of the cubic spinel (111) peak occurred at 200 °C for the uncoated Li0.35[Ni1/3Co1/3Mn1/3]O2. The TEM images obtained from the quenched powders at 200 °C (Figure 5a) indicated multiple spinel particles in the [211] zone that nucleated within a single-layer structured particle, as indicated by the arrows. An electron diffraction pattern provided further evidence of the coexistence of the rhombohedral layer structure with the newly formed spinel phase (Figure 5b), which is in a good agreement with the in situ HT-XRD results (Figure 4a).

Figure 5. TEM bright-field image of (a) the chemically delithiated uncoated Li0.35[Ni1/3Co1/3Mn1/3]O2 quenched at 200 °C and (b) the corresponding electron diffraction taken in the rhombohedral layer structure [210] zone and the cubic spinel structure [211] zone, where “l” indicates rhombohedral layer structure and “s” denotes the cubic spinel structure; TEM bright-field image of (c) the chemically delithiated AlF3-coated Li0.35[Ni1/3Co1/3Mn1/3]O2 quenched at 200 °C and (d) the corresponding electron diffraction taken in the rhombohedral layer structure [110] zone.

The separated (108) and (110) doublets started to merge together from 250 °C (Figure 3a). On the basis of the XRD data, we assumed that a relatively large portion of the layered structure is being transformed to the cubic spinel structure at 280 °C. It is thought that cation migration occurred progressively between the octahedral sites in the slab and those in the interslab space via tetrahedral sites and formed the cubic spinel phase M3O4.30,31 Beyond this temperature the relative diffraction intensities of the (003) peaks gradually decreased due to the

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Myung et al. lower 2θ angle in the temperature range of 450 - 600 °C because of the thermal expansion of the spinel lattice. In this step, the weight change is primarily due to oxygen loss (4.2 wt %) from the newly formed spinel ∼ [00.18Li0.75M0.07]8a[M2]16dO4. On the basis of the TGA and HT-XRD results, this can be expressed as follows:

Li0.75M2.07O4 f Li0.75M2.07O3.941 + 0.059O2v (∼[00.14Li0.76M0.10]8a[M2]16dO4)

Figure 6. TEM bright-field image of (a) the chemically delithiated uncoated Li0.35[Ni1/3Co1/3Mn1/3]O2 quenched at 300 °C, where a wellcrystallized spinel particle starts to develop facets due to the heat treatment, (b) the corresponding electron diffraction in the cubic spinel phase [111], (c) bright-field image of the chemically delithiated AlF3coated Li0.35[Ni1/3Co1/3Mn1/3]O2 quenched at 300 °C, and (d) the corresponding electron diffraction taken in the cubic spinel structure [211] zone.

progressive formation of the spinel structure. Reorganization of the structure is driven by oxygen evaporation from the structure. At 300 °C, the uncoated Li0.35[Ni1/3Co1/3Mn1/3]O2 particle started to develop facets, as shown in Figure 6a. The strong diffraction spots in the [111] zone (Figure 6b) imply that the resulting crystallinity of the spinel phase was improved at that temperature, compared with that at 200 °C. Further increasing the temperature to 450 °C did not significantly change the diffraction patterns in Figure 3a, except for the complete consolidation of the (108) and (110) doublets at 300 °C. Beyond that temperature (450 °C), the fingerprint peaks of the rhombohedral structure were not detected in the in situ HT-XRD patterns. The total amount of oxygen released to complete the phase transition to the cubic spinel phase was about 4.7 wt % to 450 °C in TGA result. Combining the TGA and HT-XRD results, we hypothesized that the phase transition from the rhombohedral (R3m j ) to the cubic spinel-type structure in Li0.35[Ni1/3Co1/3Mn1/3]O2 can be written as follow:

Li0.35MO2 f Li0.7M2O1.933 + 0.067O2v (∼[00.18Li0.75M0.07]8a[M2]16dO4, Li0.75M2.07O4)

(1)

where M represents Ni, Co, and Mn transition metals. The initial site exchange between Li+ and Ni2+ was not considered for this reaction. On the basis of the cation migration between the octahedral sites in the slab and those in the interslab space via the tetrahedral sites to form the spinel phase, we calculated that approximately 0.07 mol of transition metal elements occupied the 8a tetrahedral site of the spinel structure, ∼[00.18Li0.75M0.07]8a[M2]16dO4. The newly formed cubic spinel phase was maintained to 600 °C, showing better crystallinity than at 450 °C in Figure 3a. The diffraction peaks shifted to a

(2)

The oxygen loss at 450-600 °C gave rise to the further movement of transition metals to the tetrahedral sites. The spinel phase remained during cooling (Figure 3a). Structural Changes in Coated Material. Figures 3b and 4b present in situ HT-XRD patterns for the AlF3-coated Li0.35[Ni1/3Co1/3Mn1/3]O2. The phase evolution with increased temperature was similar to that of the uncoated Li0.35[Ni1/3Co1/ 3Mn1/3]O2 shown in Figure 3a. However, the (111) spinel peak that was seen at 200 °C for the uncoated material was not found at that temperature for the AlF3-coated material (Figure 4b). Also, no spinel structure was observed in the particle with the layered structure in the [110] zone for the AlF3-coated material (Figure 5c,d). However, the splitting of the 11j2 peaks and streaks in the diffraction pattern suggest that domains and planar defects are being generated in the particle. We concluded that the spinel structure did not appear at 200 °C in the particle with the layered structure in the [110] zone, which is consistent with the in situ HT-XRD patterns (Figure 4b). The onset temperature of the spinel phase formation seemed to be 260 °C, in which the small (111) spinel peak was marked by an arrow (in Figure 4b). The merger of the (108) and (110) doublets began at 280 °C, as shown in Figure 3b. Note that those peaks started to merge from 250 °C for the uncoated material (Figure 3a). The presence of AlF3 on the surface of the Li0.35[Ni1/3Co1/3Mn1/3]O2 particle retarded the formation of the cubic spinel structure. An individual spinel particle for the AlF3-coated Li0.35[Ni1/3Co1/3Mn1/3]O2 quenched at 300 °C shows that the original layer structure was almost transformed to the cubic spinel phase (Figure 6d). Beyond 300 °C, no significant changes in the diffraction patterns were seen to 500 °C. The (108) and (110) doublets were not apparent in this temperature range. Also, the spinel (440) peak was evident in Figure 3b, whereas it was at 450 °C for the uncoated one (Figure 3a), implying that phase transformation to the cubic spinel phase terminated at higher temperature for the coated than the uncoated material. Though the monotonic weight loss continued to 500 °C, the released oxygen amount was smaller for the AlF3-coated Li0.35[Ni1/3Co1/3Mn1/3]O2. The AlF3 coating of the active material clearly suppressed oxygen evolution from the oxide lattice so that it raised the transformation temperature of the spinel structure. The phase transition from the rhombohedral to the cubic spinel structure for the AlF3coated Li0.35[Ni1/3Co1/3Mn1/3]O2 can be represented as follows:

AlF3-coated Li0.35[Ni1/3Co1/3Mn1/3]O2 f Li0.7M2O1.958 + 0.042O2 v . (∼[00.242Li0.715M0.043]8a[M2]16dO4, Li0.715M2.043O4) (3) In this thermal process, the decreased oxygen loss from the original rhombohedral structure retards the formation and completion of the spinel structure. The amount of transition metal in the 8a tetrahedral site that migrated from the 16d octahedral site was reduced compared with the uncoated material.

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Figure 7. (a) Phase evolution of the chemically delithiated uncoated Li0.35[Ni1/3Co1/3Mn1/3]O2 and AlF3-coated Li0.35[Ni1/3Co1/3Mn1/3]O2 with temperature summarized from Figure 3; (b) change in the lattice parameters of ah-axis, (c) ch-axis, (d) ch/ah ratio, and (e) ac-axis for the chemically delithiated uncoated Li0.35[Ni1/3Co1/3Mn1/3]O2 and AlF3coated Li0.35[Ni1/3Co1/3Mn1/3]O2 as function of temperature. The structural parameters were calculated from the in situ XRD patterns of Figure 3.

After completion of the phase transformation, the diffraction peaks in Figure 3b shifted to lower 2θ angle at 500-600 °C because of thermal expansion. A possible thermal process is as follows:

AlF3-coated Li0.715M2.043O4 f Li0.715M2.043O3.948 + 0.052O2 v . (∼[00.206Li0.724M0.07]8a[M2]16dO4, Li0.724M2.07O4)

(4)

The evolved oxygen amount in the 500-600 °C range was ∼3.7 wt %, which is slightly lower than that of the uncoated material. As a result, the cation migration to the tetrahedral sites would be restricted due to the reduced oxygen evaporation from the structure. The formed spinel structure remained after cooling to room temperature (Figure 3b). Summary of Structural Changes. On the basis of the in situ HT-XRD patterns, we summarize the phase evolution of the chemically delithiated uncoated and AlF3-coated Li0.35[Ni1/ 3Co1/3Mn1/3]O2 in Figure 7a. The cubic spinel phase (Fd3m) emerged at 200 °C for the chemically delithiated uncoated material. Alternatively, the presence of AlF3 on the surface of the material increased the onset spinel phase formation temperature to 260 °C. Further oxygen loss from the uncoated Li0.35[Ni1/3Co1/3Mn1/3]O2 resulted in completion of the phase transformation at 450 °C. For the AlF3-coated Li0.35[Ni1/3Co1/ 3Mn1/3]O2, meanwhile, the layer structure completely extinguished at 500 °C. The reduced oxygen evolution derived by the AlF3 coating on the active material contributed to delaying the phase transition temperature to the cubic spinel structure.

From the in situ HT-XRD patterns for the chemically delithiated uncoated and the AlF3-coated Li0.35[Ni1/3Co1/3Mn1/ 3]O2, the resulting lattice parameters were calculated by a leastsquares method (Figure 7b-e). For the uncoated material, slight increases in the ah- and ch-axes were observed to 150 °C. Those changes in the lattice were ascribed to a simple thermal expansion of the oxide lattice. The ah parameter monotonically increased over the temperature range 150-280 °C, while the ch value considerably decreased over the same temperature range. The change in the lattice resulted in a sharp decrease in the ch/ah ratio above 150 °C, which approached almost 4.93 at 280 °C. This decrease implies that the cubic close-packed oxygen array of the rhombohedral structure derived from the gradual cation migration is being transformed to cubic symmetry, which has a ch/ah ratio of 4.9. For the AlF3-coated Li0.35[Ni1/3Co1/3Mn1/3]O2, the ah- and chaxis values increased to 210 °C, which is considerably higher (about 60 °C) than that of the uncoated material (Figure 7b,c). From 220 to 320 °C, the ah-axis value became gradually higher, while the ch constant fell drastically (from above 14.4 Å to 14.1 Å). The ch/ah ratio decreased sharply from 5.10 at room temperature to 4.93 in the temperature range, indicating that the rhombohedral structure is being transformed to the cubic spinel structure. It is likely that the majority of the phase is cubic spinel at 320 °C. It is notable that the uncoated material exhibited the same ah- and ch-axis values and ch/ah ratio at 280 °C. This consistency indicates that the structural evolution from the rhombohedral to cubic spinel phase is progressing at elevated temperature for the AlF3-coated material. In the temperature range of 330-450 °C, the structure did not change significantly. The cubic lattice parameter, ac, started to increase from 500 °C as a consequence of the merger of the (108)h and (110)h doublets. The phase transformation to the cubic spinel phase for the AlF3-coated material ended at 500 °C, which is about 50 °C higher than that of the uncoated material. Thermal expansion of the cubic lattice induced an increase in the lattice parameters to 600 °C. On the basis of the in situ HT-XRD, TEM, and TGA results, we believe that the presence of the AlF3 coating hinders the oxygen evolution from the active material with increasing temperature, and the phase transition from the rhombohedral to the cubic spinel structure is simultaneously delayed to a higher temperature. Thermal Stability of Coating Medium AlF3. To investigate the thermal stability of the AlF3 coating layer, the AlF3-coated Li0.35[Ni1/3Co1/3Mn1/3]O2 material was analyzed by ToF-SIMS at temperatures of 200-600 °C. The ToF-SIMS spectra were then compared with TGA results of the as-synthesized AlF3. As shown in Figure 8a,b, the observed spectra were almost identical at 200 and 250 °C. The AlF+ signals were strong, whereas the AlO+ signals were almost negligible. Surprisingly, LiAlF+ (49.99 amu) and LiAlO+ (59.99 amu) fragments were detected (Figure 8, parts a-2 and b-2) instead of an AlO+ signal. At 300 °C, the intensities of the AlF+ and LiAlF+ fragments were quite reduced (Figure 8, parts c-1 and c-2), which was ascribed to the partial evaporation of F2 (Figure 9). At 400 °C, the AlF+ and LiAlF+ fragments were hardly visible at 45.98 and 52.99 amu, respectively (Figure 8, parts d-1 and d-2). Indeed, we hypothesized that decomposition of the AlF3 in air gives rise to the formation of Al2O3 via the following reaction:

4AlF3+3O2 f 2Al2O3+6F2v

(5)

As confirmed in the TGA curve (Figure 9), the weight loss is primarily due to the F2 evolution from the AlF3. Taking into account the above reaction, we concluded that the Al-O based oxide layer is preferentially formed in the outer surface of the

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Figure 8. ToF-SIMS results of the chemically delithiated AlF3-coated Li0.35[Ni1/3Co1/3Mn1/3]O2 quenched at (a) 200 °C, (b) 250 °C, (c) 300 °C, (d) 400 °C, and (e) 600 °C. Fragment information: AlO+ (42.98 amu), AlF+ (45.98 amu), LiAlO+ (49.99 amu), and LiAlF+ (52.99 amu).

Figure 9. TGA curve of AlF3 in nitrogen atmosphere.

AlF3 layer after the partial F2 loss, and the inner part of the layer retains the AlF3 compound, meaning the two compounds coexist on the surface of the Li0.35[Ni1/3Co1/3Mn1/3]O2. The Li ingredient from the Li0.35[Ni1/3Co1/3Mn1/3]O2 inner particle appears to diffuse to the particle surface with increasing temperature. Then, the diffused Li element simultaneously reacts with the inner AlF3 layer and outer oxide layer, leading to the formation of a Li-Al-F complex in the inner particle and a Li-Al-O complex in the coating. Thus, the LiAlF+ and LiAlO+ fragments likely appear as the consequence of the Li diffusion to the outer surface from the Li0.35[Ni1/3Co1/3Mn1/3]O2 particle inside, stimulated by the heat treatment. The Fcontaining compounds were barely detected at 400 °C (Figure 8, parts d-1 and d-2), whereas the LiAlO+ fragment remained. This finding indicates that almost all of the fluorine from the original AlF3 layer had evaporated, and the layer was simultaneously transformed to the Li-Al-O compound at 400 °C. This compound was still present at 600 °C (Figure 8e-2). Thermal Behavior in Presence of Electrolyte. Figure 10 represents DSC data for the electrochemically and the chemically delithiated uncoated and AlF3-coated Li0.35[Ni1/3Co1/3Mn1/

Figure 10. Comparison of DSC traces; (a) the electrochemically delithiated uncoated Li0.35[Ni1/3Co1/3Mn1/3]O2 and AlF3-coated

Li0.35[Ni1/3Co1/3Mn1/3]O2; (b) the chemically delithiated uncoated Li0.35[Ni1/3Co1/3Mn1/3]O2 and AlF3-coated Li0.35[Ni1/3Co1/3Mn1/ 3]O2.

3]O2 in the presence of 1 M LiPF6 in ethylene carbonate/diethyl carbonate (EC/DEC) electrolyte (1:1 ratio by volume). It is confirmed in Figure 10a that, even though the both electrochemically uncoated and AlF3-coated Li0.35[Ni1/3Co1/3Mn1/3]O2 are compared, there is no significant difference in the exothermic

Effect of AlF3 Coating on Thermal Behavior

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Figure 11. Schematic illustration of thermal processes for chemically delithiated (a) uncoated Li0.35[Ni1/3Co1/3Mn1/3]O2 and (b) AlF3-coated Li0.35[Ni1/ with temperature.

3Co1/3Mn1/3]O2

temperature and heat generation in Figure 10a,b, implying that the chemical delithiation enables to simulate the real environment of lithium batteries. The slight increases in the exothermic temperature and heat are probably ascribed to the presence of electrolyte, HF, surface film, carbonate-based byproducts adhere on the surface of electrode derived from electrolyte decomposition. During heating, the rhombohedral layer structure undergoes oxygen release from the oxide lattice, and this process simultaneously accompanies the cation migration to form the cubic spinel structure. The temperature range in Figure 10b is close to the onset temperatures of the cubic spinel phase for both delithiated powders (Figure 7a). The oxygen release likely results in the first small exothermic reactions at 240 °C for the uncoated material and at 265 °C for the AlF3-coated material. At these temperatures, fluorine evolution from the AlF3 and diffusion of the Li ingredient from the inner Li0.35[Ni1/3Co1/ 3Mn1/3]O2 particle to the surface leave the partial formation of Li-Al-F and Li-Al-O complexes as a part of the decomposition of the AlF3 coating. Further temperature increase causes aggressive oxygen release from the oxide matrix in the temperature range of 250-300 °C for the uncoated material and 300-350 °C for the AlF3-coated material (Figure 2). The accumulated oxygen in the stainless steel pan could have caused a major exothermic reaction with the organic electrolyte.42 However, the oxygen loss was relatively small for the AlF3coated sample. We believe that the coating layer blocked the oxygen release from the active material and it in turn gave rise to the shift of the main exothermic reaction to higher temperature. Thus, the major thermal event occurred at a higher temperature (315 °C) for the AlF3-coated material. As shown in Figure 10b, the generated heat for the exothermic reactions was smaller for the AlF3-coated sample (∼774.9 J g-1) than the uncoated one (∼900.6 J g-1), even though the temperature was higher. The majority of the resulting phase is composed of the cubic spinel phase at 290 °C for the uncoated material and

at 315 °C for the AlF3-coated material. Those temperatures of the main exothermic reactions coincide well with the temperatures at which the ch/ah ratio leveled at 4.93. As indicated by the ToF-SIMS (Figure 9), furthermore, the AlF3, Li-Al-F, and Li-Al-O complexes layers remained on the surface of the active material in the main exothermic temperature range. This condition also would contribute to the reduced evolution of oxygen from the active material, thereby retarding formation and delaying completion of the cubic spinel phase. Therefore, the presence of the AlF3 coating on the surface of the delithiated Li0.35[Ni1/3Co1/3Mn1/3]O2 is responsible for the improved thermal properties. Conclusions The thermal behavior of chemically delithiated AlF3-coated Li0.35[Ni1/3Co1/3Mn1/3]O2 was investigated by comparing it with chemically delithiated uncoated Li0.35[Ni1/3Co1/3Mn1/3]O2. Figure 11 summarizes the thermal processes that were deduced for the two materials. We concluded that the thermal properties of the Li0.35[Ni1/3Co1/3Mn1/3]O2 were improved by the protection of the active material from the oxygen loss by the AlF3 coating and the sacrificing of the AlF3 coating as a result of the formation of the Li-Al-O and Li-Al-F complexes on the surface, where the rhombohedral structure was actively transformed to the cubic spinel structure in the presence of the AlF3 coating. Therefore, the mechanism behind the improved thermal stability of the AlF3-coated Li0.35[Ni1/3Co1/3Mn1/3]O2 positive electrode material is comprehensively understood. We also concluded that surface modifications of positive active materials with stable materials can significantly improve the thermal and safety properties of rechargeable lithium-ion batteries. Acknowledgment. The authors thank Mr. S. Takahashi and N. Takahashi, Iwate University, for his helpful assistance in

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