Electrochemical effect of lithium tungsten oxide modification on LiCoO2 thin film electrode

Journal of Power Sources 285 (2015) 559e567

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Electrochemical effect of lithium tungsten oxide modification on LiCoO2 thin film electrode Tetsutaro Hayashi a, b, *, Jiro Okada c, Eiji Toda a, Ryuichi Kuzuo a, Yasutaka Matsuda b, Naoaki Kuwata b, Junichi Kawamura b a b c

Sumitomo Metal Mining Co., Ltd, Battery Research Laboratories, 17-3, Isoura-cho, Nihama, Ehime, 792-008, Japan Tohoku University Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai, 980-8577, Japan Sumitomo Metal Mining Co., Ltd, Ichikawa Research Laboratories, Ichikawa, Chiba, 272-8588, Japan

h i g h l i g h t s
Lithium tungsten oxide (LWO)-modified LiCoO2 (LCO) electrode is developed.
The mechanism for lowered resistance is investigated.
LWO decreases lithium ion transfer resistance at the interface and increases frequency factor.
LWO prevents the surface of the LCO particle from accumulating phosphate.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 December 2014 Received in revised form 28 February 2015 Accepted 17 March 2015 Available online 18 March 2015

We fabricated a lithium tungsten oxide (LWO)-modified LiCoO2 (LCO) thin film electrode by pulsed laser deposition and investigated the reason for its lower resistance as compared with a bare LCO electrode. Xray diffraction revealed that the LWO layer has a randomly oriented Li2WO4 structure with tetragonal symmetry. Scanning electron microscopy and energy-dispersive X-ray spectroscopy (EDX) indicated that the LWO modification changes the LCO particle surface, and the electrochemical impedance spectroscopy demonstrated that the LWO modification on LCO decreases the lithium ion transfer resistance at the interface between the positive electrode and the liquid electrolyte and increases the frequency factor at the interface. X-ray photoemission spectroscopy, EDX, and electron energy loss spectroscopy (EELS) indicated the presence of phosphate on the surface of the unmodified LCO electrode after electrochemical tests, but EDX and EELS did not indicate the presence of phosphate in the LWO-modified LCO electrode. The absence of phosphates apparently alleviates the hindrance of Liþ ion diffusion and increases the frequency factor in LCO, resulting in lowered Liþ ion transfer resistance at the interface. © 2015 Elsevier B.V. All rights reserved.

Keywords: Positive electrode Lithium-ion battery Surface modification Low resistance

1. Introduction Portable rechargeable lithium ion batteries (LIBs) exhibit excellent performance, such as high energy density, limited memory effect, long lifecycle, and high voltage, compared with nickelecadmium and nickelemetal hydride batteries. In addition to their wide use in mobile phones and portable personal computers, LIBs have recently been used in hybrid electric vehicles and electric vehicles. In particular, vehicles require high-power LIBs, which

* Corresponding author. Sumitomo Metal Mining Co., Ltd, Battery Research Laboratories, 17-3, Isoura-cho, Nihama, Ehime, 792-008, Japan. E-mail address: [email protected] (T. Hayashi). http://dx.doi.org/10.1016/j.jpowsour.2015.03.108 0378-7753/© 2015 Elsevier B.V. All rights reserved.

combine low internal resistance with high durability, because these batteries need to provide a large amount of energy immediately after the vehicles start to ensure smooth motion and acceleration. Ogumi [1] has reported that Liþ ion transfer at the interface between the positive electrode and electrolyte plays an important role in LIB reactions. Some battery performance problems reportedly stem from the hindered Liþ ion diffusion at the interface between the positive electrode and electrolyte [2e4]. Surface modifications of the positive active material have been recently pursued to solve this issue [5e11]. Oxides, such as MgO [5e7], ZrO2 [8,9], TiO2 [10], and Al2O3 [11] have been used to coat the positive active material, resulting in an enhanced battery durability performance [5e11]. One such oxide is a lithium tungsten oxide (LWO) [12e14].

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LWO deposited on positive active materials reportedly reduces the charge transfer resistance of LIBs [13,14]. Understanding the mechanism for this lowered resistance, the cause of which remains unclear, is extremely important for developing high-performance LIBs; however, it is difficult to understand the above mechanism with composite electrodes due to the complicated morphology of the electrodes, which consist of positive active materials, conductive additives, and binders. Thin film electrodes fabricated by pulsed laser deposition (PLD) or RF sputtering [15] are useful for avoiding the above problem, because conductive additives and binders are absent. In this study, thin film electrodes were fabricated with PLD, with which we previously fabricated thin films of various inorganic oxide lithium conductors [16,17]. Bare and LWO-modified LCO thin film electrodes were fabricated by PLD, and the mechanism behind the lowered resistance with LWO modification was studied by a digital still camera (DSC), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), spherical-aberration-corrected scanning transmission electron microscopy (STEM), electron energy loss spectroscopy (EELS), X-ray photoemission spectroscopy (XPS), and electrochemical techniques. 2. Experimental 2.1. Fabrication of bare and LWO-modified LCO LCO powder was synthesized at 980  C for 10 h under O2 atmosphere by solid-state reaction of Li2CO3 and Co3O4. The LCO

powder was compressed into cylindrical disks at appropriate pressure and calcined at 1000  C for 24 h in order to produce LCO target material, the composition of which has a Li/Co mole ratio of 1.02. Bare LCO thin film was deposited onto a Pt/Cr/SiO2 substrate at 10 mm  10 mm  0.5 mm. Bare LCO was grown by PLD using the fourth harmonic of a Nd:YAG laser (l ¼ 266 nm) on the LCO target material. The films were developed following the literature [17]. The substrate temperature was 500  C under O2 atmosphere at 20 Pa. The thickness of the bare LCO was 200 nm. A LiOHeWO3 mixture was prepared at a mole ratio of 4:1 in an appropriate amount of water and then dried at 500  C for 10 h under O2 atmosphere in order to give LWO powder. The LWO powder was compressed into cylindrical disks at appropriate pressure and calcined at 650  C for 24 h in order to produce LWO target material. LWO was deposited onto the bare LCO thin film electrode by PLD using an ArF excimer laser (l ¼ 193 nm) on the LWO target material. The substrate temperature was 500  C under O2 atmosphere at 20 Pa. The thickness of the LWO was 200 nm. 2.2. DSC, SEM, and XRD measurements Bare and LWO-modified LCO were characterized by DSC, SEM, and XRD. The electrodes were observed using a DSC (WG-2, Pentax). Surfaces of both thin film electrodes were examined using an SEM apparatus (S4700, Hitachi) at an acceleration voltage of 5 kV. EDX spectra were acquired by coupling the SEM apparatus with an energy-dispersive X-ray detector (GENESIS, EDAX). Crystalline phases were identified using an XRD instrument (X'Pert PRO MPD, PANalytical) equipped with a Cu Ka source.

Fig. 1. DSC images of (a) bare LCO and (b) LWO-modified LCO. SEM images of (c) bare LCO and (d) LWO-modified LCO. EDX spectra of (e) bare LCO and (f) LWO-modified LCO.

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2.3. Electrochemical techniques Electrochemical properties of bare and LWO-modified LCO were tested in coin cells equipped with a lithium metal negative electrode. The electrolyte consisted of 2:2:6 mixture of 1.2 M LiPF6ethylene carbonate, ethylmethyl carbonate, and dimethyl carbonate (v/v/v). A Celgard® 2400 microporous polypropylene membrane was used for the separator. Coin cells were assembled in an argonfilled glove box. Chargeedischarge tests were performed between 3.0 and 4.2 V at a rate of 3.3 mA cm2. Subsequently, the assembled coin cells were charged up to 4.0 V at a charging rate of 3.3 mA cm2 and characterized by electrochemical impedance spectroscopy (EIS). The amplitude voltage was 10 mV and the frequency range was from 0.05 Hz to 100,000 Hz. 2.4. XPS measurement After electrochemical testing, the thin film electrode was recovered from the disassembled coin cell, rinsed with dimethyl carbonate (DMC) to remove residual LiPF6, and used as a sample for XPS analysis. A fresh thin film electrode that had not undergone any electrochemical testing was also tested by XPS analysis for comparison. The two samples were transferred from the glove box to the XPS apparatus (PHI 5000 Versa Probe II, ULVAC-PHI) via a transfer vessel filled with argon. XPS spectra were acquired using a monochromatic Al Ka source. 2.5. STEM, EDX, and EELS measurements The recovered thin film electrodes, after the electrochemical tests, were rinsed with DMC. A protective film was coated on the recovered thin film electrodes that were sliced to less than 100 nm thick using a focused ion beam (FIB, FB-2100, Hitachi) before STEM, EDX, and EELS analyses. Cross sections of bare and LWO-modified LCO were observed using an STEM apparatus (JEM-ARM200F, JEOL) operated at an acceleration voltage of 200 kV. EDX spectra were acquired by coupling the STEM apparatus with an EDX analyzer (NORAN System 7, Thermo Scientific). EELS spectra were acquired using an energy filter (GIF Quantum, Gatan) attached to the STEM apparatus. The EDX and EELS analyses were conducted at the same time. The collection time per spectrum was 2 s. 3. Results and discussion 3.1. Structural characterization of LWO-modified LCO Fig. 1(a) and (b) show the DSC images of bare and LWO-modified LCO surfaces that were deposited onto 8 mm  8 mm Pt/Cr/SiO2 substrates. No obvious differences can be discerned. The surfaces of thin films were analyzed by SEM and EDX. Fig. 1(c) and (d) show the SEM images of the bare and LWO-modified LCO, respectively. Numerous particles are clearly observable in the bare LCO [Fig. 1(c)], but not in the LWO-modified LCO [Fig. 1(d)]. On the other hand, coated film is clearly observable on the LWO-modified LCO [Fig. 1(d)]. Fig. 1(e) and (f) show the EDX spectra for the bare and LWO-modified LCO, respectively. Co is found on the bare LCO thin film surface [Fig. 1(e)] but not on the LWO-modified LCO thin film surface [Fig. 1(f)]. On the other hand, W is found on the LWOmodified LCO surface [Fig. 1(f)], suggesting that the W compound is modified on the surface of the LCO thin film. Fig. 2(a) and (b) show the XRD patterns of bare and LWO-modified LCO, respectively. As shown Fig. 2(a) for bare LCO, only one peak around 2q ¼ 19.0 is strongly observed. This peak can be attributed to the reflection of the (003) plane of the hexagonal LiCoO2 structure (ICSD #51182), indicating that bare LCO is a c-axis-oriented film.

Fig. 2. XRD patterns of (a) bare LCO and (b) LWO-modified LCO.

Other peaks are attributed to Pt in the substrate. As shown in Fig. 2(b) for LWO-modified LCO, there are peaks similar to those for bare LCO [Fig. 2(a)], but numerous additional peaks are consistent with randomly oriented Li2WO4 with tetragonal symmetry (ICSD #10479), which suggests that the W compound adopts a randomly oriented tetragonal LWO phase.

Fig. 3. Chargeedischarge curves of bare and LWO-modified LCO.

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Fig. 6. Effects of temperature on the interfacial lithium ion conductance of (a) bare LCO and (b) LWO-modified LCO. Fig. 4. dQ/dV versus voltage derived from the chargeedischarge curves of bare and LWO-modified LCO.

3.2. Electrochemical properties of LCO before and after LWO modification Fig. 3 shows the chargeedischarge curves of assembled coin cells at 25  C using bare and LWO-modified LCO electrodes. Fig. 4 shows the differential capacity (dQ/dV) versus voltage derived from the chargeedischarge curves. No distinct difference is observable between the bare and LWO-modified LCO in either Fig. 3 or 4. The major peak in the dQ/dV curves at about 3.9 V, corresponding to the long potential plateau in the chargeedischarge curves, is attributed to a first-order phase transition between two different hexagonal phases. Two minor peaks in the dQ/dV curves at about 4.07 and 4.18 V, corresponding to the two small continuous shoulders in the chargeedischarge curves, are due to the secondorder transition between order and disorder phases, consistent with LiCoO2-type positive active materials [18]. Bare and LWOmodified LCO electrodes display discharge capacities of 8.13 and 8.49 mAh cm2, respectively, confirming that LWO modification does not largely affect the charge and discharge behaviors of the positive active materials. 3.3. Electrochemical effects of the LWO modification

Fig. 5. (a) Alternate current impedance spectra of bare and LWO-modified LCO. (b) Equivalent circuit for the impedance spectra of bare LCO and LWO-modified LCO. Rs corresponds to the high-frequency intercept of the semicircle with the horizontal axis, R1 is the first semicircle in the high-frequency range, and Rct is the charge transfer resistance at the positive electrodeeliquid electrolyte interface as represented by the second semicircle in the intermediate-frequency range. CPE1 and CPE2 are constant phase elements. W is the Warburg element, which corresponds to the straight line in the low-frequency range. (c) Contributions of Rs, R1, and Rct in bare and LWO-modified LCO.

The electrochemical effects of the LWO modification on LCO were evaluated by EIS. Fig. 5 shows the impedance spectra of bare and LWO-modified LCO. Clear differences can be observed between them. Impedance spectra show two semicircles in the high- and intermediate-frequency ranges and a straight line at low frequency. An equivalent circuit [Fig. 5(b)] [19] is proposed to analyze these impedance spectra. In this circuit, the ohmic resistance Rs, such as the electrolyte resistance [9], corresponds to the high-frequency intercept of the semicircle with the horizontal axis; the resistance R1 of the first semicircle corresponds to the surface film resistance [20] in the high-frequency range; and the Liþ ion transfer resistance Rct is the charge-transfer resistance at the positive electrode interface [4] represented by the second semicircle in the intermediate frequency range. While CPE1 and CPE2 are constant phase elements, the Warburg element W corresponds to the straight line in the lowfrequency range. Fig. 5(c) shows the Rs, R1, and Rct contributions in cells with bare and LWO-modified LCO positive electrodes. These Table 1 Activation energy (Ea) and frequency factor (A) for bare and LWO-modified LCO.

LWO-modified LCO Bare LCO

Ea (kJ mol1)

A

51 49

3.4  106 6.9  105

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(a)

563

(b)

Fig. 7. (a) O1s core-level XPS spectra of bare LCO before and after electrochemical test. (b) P2p core-level XPS spectra of bare LCO after electrochemical test. Each spectrum was normalized by its maximum intensity.

graphs reveal a high contribution of Rct to the internal resistance (Rs þ R1 þ Rct), especially for bare LCO, in which it is as high as 82%. This contribution decreases by one third with LWO modification, suggesting that LWO modification decreases the interfacial Liþ ion transfer resistance and allows Liþ ions to smoothly intercalate and de-intercalate at the positive electrode interface. Fig. 6 shows the effects of temperature from 25  C to 50  C on the interfacial Liþ ion conductance (1/Rct) of bare and LWOmodified LCO. LWO-modified LCO exhibits a higher interfacial conductance than its bare counterpart. As shown in Fig. 6, 1/Rct versus absolute temperature (T) exhibits the Arrhenius law behavior [1]:

1=Rct ¼ A expð  Ea =RTÞ;

(1)

where A is the frequency factor, Ea is the activation energy of the interfacial Liþ ion transfer, and R is the gas constant. Table 1 lists Ea and A calculated using Equation (1). The estimated Ea for LWOmodified LCO is almost the same as that for bare LCO, in contrast with a previous report by Iriyama et al. [6], which states that the MgO coating of LCO surfaces decreases Ea and promotes interfacial Liþ ion transfer. Conversely, LWO-modified LCO displays a much higher A than bare LCO (Table 1), suggesting that the lowering of the interfacial resistance with LWO modification is due to the changes in frequency factor (A) rather than activation energy (Ea)

Fig. 8. (a) STEM image of the cross section of a bare LCO primary particle after the electrochemical test. (b) Magnified STEM image of the selected region of the particle surface in (a). (c) Magnified STEM image of the selected region in (b) of the interface between the LCO particle surface and the deposited layer.

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for Li ion transfer. We observe that the electrode surface area of bare LCO is larger than that of LWO-modified LCO (Fig. 1). However, we cannot explain the reason for low resistance by the difference of the electrode surface area because the interfacial resistance of LWO-modified LCO is lower than that of bare LCO. Thus, the differences of the frequency factor play an important role in the reduction of the interfacial resistance. Ogumi has proposed that the frequency factor depends on the number of the reaction sites for Liþ ion transfer at the positive electrode interface [1]. In the case of LWO-modified LCO, Liþ ion transfer process proceeds at the following two interfaces: between LWO and LCO, and between liquid electrolyte and LWO. In this study, we performed an analysis focusing on the interface between LWO and LCO because only LCO causes battery reaction in both LWO-modified LCO and bare LCO (Figs. 3 and 4). Therefore, we analyzed the surfaces of LCO, the interface between the surface of bare LCO primary particles and the liquid electrolyte in the bare LCO thin film, and the interface between the LCO primary particles and the LWO layer in LWO-modified LCO thin film. 3.4. Interfacial analysis of bare LCO The surfaces of the LCO particles in bare LCO thin films before and after the electrochemical tests were analyzed by XPS in order to investigate the possibility of the decomposition of the electrolyte during tests. As shown in Fig. 7, lithium transition metal oxides (LMO) such as LCO in the 529.3 eV energy region [21] are detected on the surface of the bare LCO before the electrochemical tests. On the other hand, the peak corresponding to LCO is not detected, and phosphates such as PxOy at the 532.2 eV and 135.5 eV energy regions [22e24] are detected on the surface of the electrode after electrochemical test. The results suggest that phosphate accumulates on the surface of the LCO electrode during the test. To investigate the interface between the surface of the LCO particle and the electrolyte, the cross section of a bare LCO primary particle after electrochemical testing was analyzed by STEM. As shown in Fig. 8(a), a deposited layer is observed between the LCO primary particles and the protective film for FIB fabrication. As shown in the close-up of the primary particle in Fig. 8(c), the bright layers of Co alternate with the dark layers of Li, consistent with the (003) plane in LCO. This image of the bare LCO primary particle after electrochemical testing indicates that the transition metals are arrayed in layered LiCoO2 and the structure is intact in these regions. On the other hand, near the surface of the LCO particle, disordered layers, as indicated by the arrow in Fig. 8(c), are observed, and the thickness is about 3 nm. The results suggest that the LCO structure stays almost completely intact, and the degree of degradation of the LCO primary particle is small. As shown in Fig. 8(c), in the interface between the surface of the LCO and the deposited layer, shown in black, a foggy layer is observed. This interfacial region was analyzed in more detail by EDX and EELS line analysis. EDX and EELS spectra were acquired in the same region simultaneously. Fig. 9(a) shows EDX line profiles for Co, P, and F for the primary particle interfacial region scanning from the intact LCO through the deposited layer (0e34 nm), Fig. 9(b) shows the EELS spectra results of the OeK edge, and Fig. 9(c) shows the EELS spectra results of the CoeL edge. As shown in EDX profile [Fig. 9(a)], in the intact LCO (0e18 nm), a long plateau of high concentration for Co is observed but P or F is not found. On the other hand, in the interfacial region corresponding to the foggy layer (18e24 nm), Co concentration decreases gradually, P concentration increases gradually, and F is barely seen. In the deposited layer region (24e34 nm), there are a high concentration plateau for P, a lower concentration plateau for Co, and no F. These results

Fig. 9. (a) EDX line profiles for Co, P, and F for the bare LCO particle interface corresponding to the blue squared area in Fig. 8(b). Distances from the intact LCO particle surface to the deposited layer (0e34 nm) are indicated by color coding along the right side of the graph. (b) EELS spectra for the oxygen K edge for the interfacial region corresponding to the blue squared area in Fig. 8(b). Distances from the intact LCO particle surface to the deposited layer (2e34 nm) are indicated by color coding along the right side of the graph. (c) EELS spectra for the cobalt L edge for the interfacial region corresponding to the blue squared area in Fig. 8(b). Distances from the intact LCO particle surface to the deposited layer (2e34 nm) are indicated by color coding along the right side of the graph. Each spectrum was normalized by its maximum intensity. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

suggest that Co dissolves gradually from the LCO particle surface due to contacting liquid electrolyte or acids, such as phosphoric acid caused by decomposition of the LiPF6 electrolyte, and phosphate accumulates on the surface of the LCO particles during the test, which is consistent with the XPS results (Fig. 7).

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Fig. 10. (a) STEM image of the cross section of an LWO-modified LCO primary particle after electrochemical test. (b) Magnified STEM image of the selected region in (a) of the LCO particle surface. (c) Magnified STEM image of the selected region in (b) of the interface between the LCO particle and LWO layer.

As shown by the EELS spectra for the OeK edge [Fig. 9(b)], in the intact LCO (0e18 nm), a peak labeled A in the vicinity of 530 eV is observed. Graetz [25] reports that in the OeK edge spectra of LiCoO2, the peak A corresponds to O 2p and Co 3d orbitals, and the peak is largely influenced by covalent bonding between Co and O. In the intact LCO (0e18 nm), alterations of the OeK edge and CoeL edge are not noticeable, which are consistent with intact LCO. In the interfacial region corresponding to the foggy layer (18e24 nm), the peak A of the OeK edge decreases, and the peak corresponding to CoeL shifts to a lower energy compared with the intact region (0e18 nm), which indicates that the LCO structure is altered, and the Co reduces to a lower valence (II). These results agree with the EDX profiles, which indicate Co dissolution from the surface of the LCO primary particle [Fig. 9(a)]. Moreover, in the deposited-layer region (24e34 nm), the peak A reflecting the covalent bonding between Co and O disappears, and the appearance of the peak labeled B in the vicinity of 536 eV reflects the covalent bonding between P and O rather than Co and O, considering the XPS and EDX results of detecting phosphate [Figs. 7 and 9(a)]. In the deposited region, spectra for CoeL corresponding to Co2þ are observed. Thus, the results suggest that cobalt phosphate forms on the surface of the bare LCO primary particles after the electrochemical test, and it is formed from the reaction between Co dissolved from the surface of the LCO particle and phosphoric acid from the decomposed LiPF6 electrolyte. 3.5. Interfacial analysis of LWO-modified LCO To investigate the interface between the surface of the LCO particles and the LWO layer, the cross section of an LWO-modified LCO primary particle after the electrochemical test was analyzed by STEM. As shown in Fig. 10(c), the bright layers of Co alternate with the dark layers of Li in the region of the primary particle, consistent with the (003) plane in LCO. This image of the LCO primary particle

after the test indicates the transition metals arrayed in layered LiCoO2 and that the structure is intact. On the other hand, near the surface of the particle in Fig. 10(c), a thin disordered layer indicated by the arrow is observed, and the thickness is about 4 nm. Results suggest that the LCO structure stays almost completely intact throughout the particle, and the degree of degradation of the LCO primary particle is small. As shown in Fig. 10(c), in the interface between the surface of the LCO and the LWO layer (in white), an interfacial layer (between the white dotted lines) is observed. This interfacial layer was analyzed in more detail by EDX and EELS line analysis. Fig. 11(a) shows EDX line profiles for Co, W, P, and F along this interface from the intact LCO to the LWO layer (0e34 nm); Fig. 11(b) shows the EELS spectra results for the OeK edge; and Fig. 11(c) shows the EELS spectra results for the CoeL edge. As shown in the EDX profile [Fig. 11(a)], the P concentration in the interfacial region is much lower for the LWO-modified LCO than bare LCO [Fig. 9(a)]. The little phosphate in this interfacial region suggests that LWO modification prevents LCO from reacting with the electrolyte. On the other hand, as shown in Fig. 11(a), Co concentration decreases gradually and W concentration increases gradually from the intact LCO to the LWO layer, and in the interfacial region between the LCO particle surface and the LWO layer, narrow concentration plateaus for W and Co are observed. The results suggest that some intermediate layer composed of LWO and LCO is formed in the interfacial region. As shown in the EELS spectra of the OeK edge shown in Fig. 11(b), in the intact LCO (0e14 nm), there is little alteration in the OeK edge and CoeL edge spectra, which confirms that LCO is intact in this region. In the interfacial region between the LCO particle surface and the LWO layer (14e22 nm), the peak labeled A in the OeK edge spectrum, corresponding to O 2p and Co 3d orbitals, decreases and shifts to a slightly higher energy peak position labeled B, and the peak corresponding to CoeL shifts to a lower

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Fig. 11. (a) EDX line profiles for Co, W, P, and F for the LWO-modified LCO particle surface corresponding to the blue squared region in Fig. 10(b). Distances from the intact LCO particle surface through to the LWO layer (0e34 nm) are indicated by color coding along the right side of the graph. (b) EELS spectra of the OeK edge for the particle interface corresponding to the blue squared region in Fig. 10(b). Distances from the intact LCO particle surface through to the LWO layer (2e34 nm) are indicated by color coding along the right side of the graph. (c) EELS spectra of the CoeL edge for the particle interface corresponding to the blue squared area in Fig. 10(b). Distances from the intact LCO region to the LWO layer (2e34 nm) are indicated by color coding along the right side of the graph. Each spectrum was normalized by its maximum intensity. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

energy compared with the intact region (0e14 nm). The change in peak A differs from the change in A for the bare LCO interface [Fig. 9(b)]. These results suggest that the intermediate layer between LCO and the LWO layer is formed by the mutual diffusion of Co from LCO and W from LWO, in consideration of the Co and W results in the EDX line profiles [Fig. 11(a)].

Moreover, in the LWO layer region (24e34 nm), the spectrum of the oxygen K edge changes greatly, reflecting the covalent bonding of W and O rather than Co and O, as suggested by the large amount of W in the LWO region, as shown in Fig. 11(a). In this LWO region, the CoeL edge is observed, but the Co concentration decreases gradually from the intermediate layer toward the LWO layer, as shown in Fig. 11(a). All these results suggest that the LWO layer region is basically Li2WO4, in consideration of the XRD result [Fig. 2(b)], and in the vicinity of the interface, Co slightly dissolves into LWO. Orikasa et al. [7] have reported that the structural degradation of LCO in chargeedischarge cycle is suppressed by the MgO coating of LCO, the mechanism of which is that Mg2þ ions migrating into vacant Liþ sites in LCO form strong MgeO bonds and stabilize the layered structure of the surface of LCO, resulting in the reduction of the interfacial resistance. In the present study, the thickness of the degraded layer at the surface of the LCO primary particles is almost the same for bare LCO as for the LWO-modified LCO [Figs. 8(c) and 10(c)], which causes the same degree of irreversible capacity at the chargeedischarge curves (Fig. 3). This result suggests that W6þ ions rarely migrate into vacant Liþ sites; this is different from Mg2þ ions, which may be influenced by the difference between the high valence (VI) of W6þ and the low valence (II) of Mg2þ in consideration of charge compensation. Thus, in case of LWO-modified LCO, the differences in degraded layer thickness are not responsible for the differences in the interfacial resistance. In the present study, Co dissolves from the surface of bare LCO toward the electrolyte in an LIB (Fig. 9). On the other hand, in LWOmodified LCO, there is a large decrease in the interfacial resistance with LWO modification (Fig. 5), and Co diffusion from the LCO is slightly detected in the LWO layer (Fig. 11). The results suggest that LWO modification plays an important role as a protective layer that suppresses Co diffusion toward the electrolyte and hardly impede Liþ diffusion near the interface; this is different from Mg2þ that easily migrates into vacant Liþ sites. Moreover, in bare LCO electrodes, the phosphate that forms from decomposed LiPF6 and Co dissolved from LCO largely accumulates on the surface of the LCO (Figs. 7 and 9). On the other hand, in LWO-modified LCO, little evidence for phosphate is found in the interfacial layer between LCO and LWO (Fig. 11). Thus, we suggest that in bare LCO, the accumulated phosphate hinders Liþ transfer at the interface and decreases the number of the reaction sites for Liþ transfer at the LCO particle interface, which decreases the frequency factor (Table 1). On the other hand, LWO modification suppresses the accumulation of phosphate at the interface by preventing the surface of the LCO from reacting with the electrolyte, which results in the prevention of the decrease in the number of reaction sites for Liþ transfer. Thus, LWO modification suppresses the decrease in the frequency factor (A) found for bare LCO, resulting in the reduction of interfacial Liþ ion transfer resistance. LWO itself possibly has another contribution to the lowered resistance in addition to the role of a protective layer, because LWO is a lithium ion conductive material [12]; this is different from electrochemically inactive MgO [6]. Further study is necessary to clarify such LWO's contribution. The analysis of the surface between liquid electrolyte and LWO will also be performed, in addition to the present analysis of the LWOeLCO interface, in our future work. 4. Conclusion An LWO-modified LCO electrode was fabricated, and the mechanism with which LWO lowers resistance was investigated by DSC, SEM, XRD, STEM, EDX, EELS, and electrochemical techniques. Combined with SEM and EDX, XRD reveals that LWO is modified on

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the surface of the LCO and adopts a randomly oriented Li2WO4 structure with tetragonal symmetry. EIS measurements demonstrate that the LWO modification of LCO decreases the interfacial Liþ ion transfer resistance between the positive electrode and liquid electrolyte and increases A at the interface. XPS, EDX, and EELS indicate that phosphate accumulates on the surface of unmodified LCO after electrochemical test, but EDX and EELS indicates little evidence for phosphate at the surface of an LCO particle modified with LWO. The results indicate that LWO modification improves Liþ ion diffusion and suppresses the low A found for bare LCO; thus, LWO lowers Liþ ion transfer resistance at the interface. References [1] Z. Ogumi, Electrochemistry 78 (2010) 319e324. [2] K. Amine, C.H. Chen, J. Liu, M. Hammond, A. Jansen, D. Dees, I. Bloom, D. Vissers, G. Henriksen, J. Power Sources 97 (2001) 684e687. [3] D.P. Abraham, R.D. Twesten, M. Balasubramanian, I. Petrov, J. McBreen, K. Amine, Electrochem Commun. 4 (2002) 620e625. [4] T. Hayashi, J. Okada, E. Toda, R. Kuzuo, N. Oshimura, N. Kuwata, J. Kawamura, J. Electrochem. Soc. 161 (2014) A1007eA1011. [5] Z. Wang, X. Huang, L. Chen, J. Electrochem. Soc. 150 (2003) A199eA208. [6] Y. Iriyama, H. Kurita, I. Yamada, T. Abe, Z. Ogumi, J. Power Sources 172 (2004) 111e116. [7] Y. Orikasa, D. Takamatsu, K. Yamamoto, Y. Koyama, S. Mori, T. Masese, T. Mori, T. Minato, H. Tanida, T. Uruga, Z. Ogumi, Y. Uchimoto, Adv. Mater. Interfaces 1

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