Extending the High-Voltage Capacity of LiCoO - ACS Publications

Dec 16, 2015 - electrode. The carbonate coating layer is deposited on LCO ... As a well-known cathode material for lithium ion batteries. (LIBs), laye...
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Extending the High-Voltage Capacity of LiCoO2 Cathode by Direct Coating of the Composite Electrode with Li2CO3 via Magnetron Sputtering Xinyi Dai, Aijun Zhou,* Jin Xu, Yanting Lu, Liping Wang, Cong Fan, and Jingze Li* State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Microelectronics and Solid-State Electronics, University of Electronic Science and Technology of China, Chengdu 610054, China S Supporting Information *

ABSTRACT: Surface coating of composite electrode has recently received increasing attention and has been demonstrated to be effective in enhancing the electrochemical performance of lithium ion battery (LIB) materials. In this work, an electronic-insulating but ionic-conductive lithium carbonate (Li2CO3) is rationally selected as the unique coating material for commercial LiCoO2 (LCO) cathode. Li2CO3 is a well-known constitute in conventional solid electrolyte interface (SEI) layer, which can electrochemically protect the electrode. The carbonate coating layer is deposited on LCO composite electrodes via a facial magnetron sputtering approach. The sputtered Li2CO3 layer serves as an artificial SEI layer between the active material and electrolyte and can impede the formation of the primary SEI layer, which will permanently consume Li+ and reduce the reversible capacity of the electrode. After a 10 min Li2CO3 coating, the capacity retention of the composite electrode is improved from 64.4% to 87.8% when cycled at room temperature in the potential range of 3.0−4.5 V vs Li/Li+ for 60 cycles. The obtained discharge capacity is extended to 161 mAh g−1, which is 36% higher than the uncoated one (118 mAh g−1). When further increasing the charging potential up to 4.7 V, or elevating the operation temperature to 55 °C, the Li2CO3-coated LCO electrodes still display remarkably improved cycling stability.

1. INTRODUCTION As a well-known cathode material for lithium ion batteries (LIBs), layered LiCoO2 (LCO) has been widely used in smallformat portable electronic devices such as laptops, mobile phones, and tablet intelligent kits.1−3 LCO has a high theoretical capacity of 274 mAh g−1, but in fact only 50% of the theoretical Li can be extracted (∼140 mAh g−1) when charged to 4.2 V vs Li/Li+, above which the LCO structure will become unstable.4,5 However, with the fast development of electronic devices, the LIBs with higher energy and power density are extremely desirable.6−8 Although several ternary and Li-rich compounds have emerged as promising cathodes for LIBs with higher gravimetric capacities than LCO,9−11 further improvement of the traditional LCO cathodes, e.g., by elevating the upper cutoff potential (UCP), has deserved equal attention in industry due to the large tap density (4.1 g cm−3) of LCO. Recently, the commercial LCO-based LIBs can achieve a high voltage up to 4.4 V and deliver a quite stable capacity of ∼150 mAh g−1. However, further increase of the UCP will lead to a rapid capacity decay. Several reasons including lattice defects, oxygen loss, transition metal dissolution, and structural degradation, which are associated with intensified electrolyte decomposition and electrode−electrolyte side reactions, are © 2015 American Chemical Society

believed to account for the poor performance of LCO at high voltages.12−16 Lattice doping and surface modification are two major strategies to overcome these issues.17,18 As the electrode− electrolyte side reactions take place on the surface of the active materials,19,20 appropriate surface modifications should provide solutions to suppress serious surface passivation and capacity loss of LCO. So far, various materials have been coated on LCO via different methods. These materials can be categorized into three groups in term of their different conduction capability: (1) electronic-conductive materials such as C21,22 and metals (e.g., Ag23), with the possibility to enhance the electronic conductivity of the electrode; (2) conductive or nonconductive metal oxides (e.g., Al2O3,24 SnO2,25 TiO2,26 and ZrO227), which can consume detrimental electrolyte decomposition byproducts (e.g., HF and LiF) and form a more stable interface on the electrode surface by electrochemical reactions; (3) ionic-conductive materials such as phosphates (e.g., AlPO328 and LiPON29) and fluorides (e.g., AlF3,30 MgF2,31 and LaF332), which can enhance the ionic transfer at the Received: November 1, 2015 Revised: December 14, 2015 Published: December 16, 2015 422

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Pa, respectively. The target-to-substrate distance was set to be 5.0 cm, and the substrate temperature was maintained at 120 °C. During sputtering, the electrode (as a substrate) was rotated to guarantee a uniform coating on the whole electrode. In addition, Li2CO3 was also sputtered on silicon wafers in order to determine the sputtering rate, which is estimated to be 2 nm/min by thickness measurements. The surface-coated electrodes are labeled as LCO/Li2CO3-x (x denotes the sputtering time in minutes) in the following text. 2.2. Materials Characterization and Electrochemical Test. The morphology of the electrodes was observed by using a field emission scanning electron microscopy (FESEM, Hitachi S3400N). X-ray diffraction (XRD) measurement was performed on a Panalytical X’Pert Pro MPD using Cu Kα radiation (λ = 1.540 56 Å). The components were analyzed with a Raman spectroscope (Renishaw, inVia Reflex) and a Fourier transform infrared spectrometer (FTIR, IRrestige-21). Electrochemical measurements were conducted using two-electrode half-cells. The composite electrodes were prepared by spreading well-mixed commercial LCO powders (active materials, 80 wt %), acetylene black (conducting additive, 10 wt %), and poly(vinylidene fluoride) (PVDF, binder, 10 wt %) on a piece of Al foil. The electrodes and separators (polypropylene, Celgard 2400) were dried in a vacuum overnight at 110 and 50 °C, respectively. LCO/Li half-cells were assembled in a glovebox under an argon atmosphere. 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and propylene carbonate (PC) (1:1:1:1, v/v) was used as the liquid electrolyte. The galvanostatic charge/discharge tests were performed between 3.0 and 4.5, 4.6, and 4.7 V (vs Li/Li+) at 0.2 C (1 C = 140 mA g−1) using a CT2001A cell test instrument (LAND Electronic Co.). Cyclic voltammetry (CV) tests were carried out using a Solartron SI1287 equipment. Electrochemical impedance spectroscopy (EIS) measurements were performed on charged half-cells (4.5 V) using an electrochemical workstation (CHI660B) in the frequency range from 105 to 10−2 Hz.

interface and meanwhile serve as a chemically stable barrier. Besides coating on the surface of the electrode precursor powders, the composite electrode coating is becoming more and more popular,33−35 which can preserve the electronic conductivity of the electrode if the coating material is a poor electronic conductor. For example, Li et al.34 reported an effective coating of a solid-state electrolyte, LiTaO3, on LiNi1/3Co1/3Mn1/3O2 cathode by atomic layer deposition technique which allows a conformal coating with atomic-level thickness accuracy. Recently, we reported an inferiorly conductive ZnO coating36 and a superiorly conductive AZO coating37 on the surface of LCO composite electrodes; both are effective in improving the electrochemical performance of LCO at high voltages. The magnetron sputtering technique was used in these studies for the surface coating due to its facial operation and superior controllability in coating thickness in comparison with conventional wet-chemical and atmosphereannealing methods.38,39 It is widely accepted that self-passivation of electrode surface takes place since the first charge of the battery, forming a complex solid electrolyte interface (SEI) layer on the electrode surface.40−42 Although being able to act as a barrier to suppress further surface side reactions,43 the chemically in situ formed SEI is not stable and propagates rapidly at high voltage or high temperature. Therefore, it will be interesting to make a more stable protection layer on the electrode surface to prevent disastrous corrosion of LCO surface under severe conditions. Previous studies showed that Li 2 CO 3 is a beneficial composition of SEI.44−46 It is electronically insulating44,47 and ionically conductive (with a Li+ conductivity up to 10−8 S cm−147,48). Furthermore, it is also insoluble and electrochemically stable in the electrolyte. Bhattacharya et al.49 studied Li2CO3 coating on graphite anode and found that the Li2CO3enriched SEI could reduce the electrode surface damage and facilitate the Li+ diffusion. Kim et al.50 claimed that Li2CO3 coating can effectively suppress the decomposition of the electrolyte. Zhang et al.51 reported a method of Li2CO3 coating by annealing LCO powders in CO2 atmosphere and demonstrated a suppressed augment of impedance during long-term cycling. However, it is to note that Li2CO3 is a poor electronic conductor, and the overall coating of the powder material with Li2CO3 on individual grain of LCO will intrinsically block the electronic transport in the whole electrode. Therefore, it may be more preferable to coat the surface of the as-fabricated LCO composite electrode using the ionic-conductive but electronic-nonconductive Li2CO3. In this work, Li2CO3 is for the first time coated on the asfabricated LCO composite electrode via a facial radio-frequency (RF) magnetron sputtering deposition. The focus of this work is to examine the effect of the Li2CO3 sputtering coating on the high-voltage and high-temperature cycling performance of LCO. Because of the excellent chemical resistance and ionic transfer capability of the carbonate coating layer, the Li2CO3coated LCO electrodes exhibit improved electrochemical performance at both high voltage and elevated temperature.

3. RESULTS AND DISCUSSION The surface FESEM images of the bare and Li2CO3-coated LCO electrodes are shown in Figure 1. For the bare electrode (Figure 1a), a porous and loose surface microstructure is observed. In this case, it is expected that side reactions will seriously take place on the surface of LCO particles during electrochemical cycling because of the large and unprotected electrode−electrolyte contact area. For the Li2CO3-coated electrodes (Figure 1b,c), a glassy coating layer is clearly observed to cover on the electrode surface, and the morphology of the electrode surface is directly influenced by the thickness (or sputtering time) of the coating film. As the sputtering time is increased, the electrode surface is smoother and the pore size of the electrode becomes smaller due to the filling of sputtered matters. When the sputtering time is long enough (e.g., 60 min), the coating layer is quite thick and the pores between the LCO particles can no longer be observed clearly (Figure 1d). It is expected that with an optimum coating on the porous electrode a favorable surface network can be obtained which could both passivate the LCO surface and facilitate Li+ transfer while also preserve efficient pervasion and wetting of the electrolyte. The phase structure and constitute of the composite electrodes were analyzed by XRD and Raman spectroscopy, as shown in Figure 2. In the XRD patterns (Figure 2a), all of

2. EXPERIMENTAL SECTION 2.1. Magnetron Sputtering Coating. Li2CO3 was deposited on as-fabricated LCO composite electrodes by RF magnetron sputtering using a commercial Li2CO3 target (99.9%). The sputtering was performed at different times (5, 10, 20, 40, and 60 min) to control the coating thickness, while the sputtering power and pressure were fixed at 120 W and 1.0 423

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distinguish the vibrations of C−O (near 1400 cm−1) and the CO32− bend (near 876 cm−1) of Li2CO3 from the C−H vibrations of PVDF (near 872 cm−1) and other carbon composition. This is expected for us because the amount of Li2CO3 in the whole electrode is very small, and in addition, the sample for the FTIR measurement is again considerably diluted by mixing with KBr. Nevertheless, the Raman spectra (Figure 2b) collected on the surface of the electrodes are able to evidence the existence of Li2CO3. The two characteristic peaks centered at 482 and 593 cm−1 can be ascribed to the Eg (O− Co−O bending) and A1g (Co−O stretching) Raman-active modes of the HT-LCO phase,36,52 while the additional peak near 1090 cm−1 for the sputtering-coated electrode (LCO/ Li2CO3-60) can be been assigned to Li2CO3 according to previous studies.53−55 Figure 3 displays the cycling performance of the composite electrodes tested at room temperature in the range of 3.0−4.5 V at 0.2 C. As expected, the cycling performance is remarkably improved by the Li2CO3 sputtering coating (Figure 3a). For the initial cycle, the bare and Li2CO3-coated LCO electrodes show little difference in their discharge capacity. During subsequent cycles, however, the bare LCO degrades very rapidly due to several reasons related with surface side reactions. After Li2CO3 coating, different results are obtained for different sputtering time. It is demonstrated that the capacity retention of the composite electrode first increases and then decreases with the augment of the deposition time, as shown in Figure 3b. In comparison with the capacity retention (64.4%) of the bare electrode at the 60th cycle, both the LCO/Li2CO3-5 and LCO/ Li2CO3-10 electrodes exhibit improved capacity retention to 81.3% and 87.8%, respectively. However, a further increase of the sputtering time leads to undesired decay of the discharge capacity. This unwanted effect induced by overthick coatings has been also observed in previous studies using ZnO,36 AZO,37 and LiTaO334 as coating layers. In this work, the optimum result is obtained with a sputtering time of 10 min (corresponding to ∼20 nm Li2CO3 coating), where the protecting effect (high chemical stability) and the impeding effect (low electronic conductivity) of Li2CO3 reach the best coupling. It is remarkable that the thicker Li2CO3 coatings will again reduce the battery performance due to increased diffusion length of Li+ in the surface, as the ionic conductivity of Li2CO3 is several orders of magnitude lower than that of the liquid electrolyte, although it can be higher than LCO and the chemically formed SEI layer. In addition, the electrolyte wetting can be worse for the thicker coatings, which can be reflected by a relatively lower initial discharge capacity, e.g., of LCO/ Li2CO3-60. Figures 3c and 3d display the charge−discharge profiles of the bare LCO, LCO/Li2CO3-10, and LCO/Li2CO340 electrodes. For the initial cycle (Figure 3c), the Li2CO3coated electrodes show a slightly larger polarization than the bare LCO electrode. However, after 20 cycles (Figure 3d), the polarization of the Li2CO3-coated electrodes is even smaller than that of the bare one. The LCO/Li2CO3-10 electrode shows the smallest polarization, further indicating that a properly thick Li2CO3 coating is important to keep a balance between surface protection and Li+ diffusion. Furthermore, the electrode with the optimum thick Li2CO3 coating exhibited the best rate performance, as illustrated in Figure S2. The discharge capacity of LCO/Li2CO3-10 electrode at 4 C can reach 125 mAh g−1, which is 110% and 117% of the capacity for the LCO/Li2CO3-40 and bare LCO electrodes, respectively.

Figure 1. Top-view FESEM images of the LCO electrodes: (a) bare LCO, (b) LCO/Li2CO3-10, (c) LCO/Li2CO3-20, and (d) LCO/ Li2CO3-60.

Figure 2. (a) XRD patterns and (b) Raman spectra of the bare and Li2CO3-coated LCO electrodes.

the samples display clear diffraction peaks, which match well with the α-NaFeO2 structure of LCO (space group: R3m). This means the Li2CO3 sputtering coating does not cause any noticeable change in the crystal structure of LCO. Furthermore, there is no diffraction peak corresponding to the Li2CO3 phase, even in LCO/Li2CO3-60 with the greatest coating thickness, suggesting that the sputtered Li2CO3 is not well crystallized. Combining with the glassy morphology of the coating layer in Figure 1b−d, we can infer that the sputtered film on the electrodes is in an amorphous state. FTIR spectra of the bare LCO and LCO/Li2CO3-x electrodes are collected, as shown in Figure S1 (Supporting Information). However, it is hard to 424

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Figure 3. (a) Cycling performance of the bare and Li2CO3-coated LCO electrodes tested at room temperature in the range of 3.0−4.5 V at 0.2 C. (b) Capacity retention of the electrodes with different sputtering time at the 60th cycle. (c) The initial and (d) the 20th charge−discharge profiles of the bare LCO, LCO/Li2CO3-10, and LCO/Li2CO3-40 electrodes.

cathodic peak denotes the Li+ insertion process. We can see that LCO/Li2 CO 3 -10 has a higher reversibility for Li + extraction/insertion from its better peak symmetry. The peak currents of LCO/Li2CO3-10 are also more stable than those of the bare LCO, which is in consonance with the high capacity retention of LCO/Li2CO3-10. The potential differences between the anodic and cathodic peaks (ΔV) in the CV curves are summarized in Table 1. It is noteworthy that LCO/Li2CO3-

In order to elucidate the reasons for the improved electrochemical performance of the Li2CO3-coated electrodes, CV tests were carried out between 3.0 and 4.5 V. Figure 4 shows the CV profiles of the bare LCO (Figure 4a) and LCO/ Li2CO3-10 (Figure 4b) electrodes for the first four cycles at a sweeping rate of 0.05 mV s−1. In the CV curves, the major anodic peak is a result of the Li+ extraction process while the

Table 1. Potential Differences (ΔV) between the Anodic and Cathodic Peaks in the CV Profiles of Bare LCO and LCO/ Li2CO3-10 Electrodes

a

samples

ΔV1a (V)

ΔV2 (V)

ΔV3 (V)

ΔV4 (V)

bare LCO LCO/Li2CO3-10

0.1522 0.2840

0.1239 0.2473

0.1337 0.2308

0.1412 0.2276

The suffix denotes the cycle number.

10 has a much larger ΔV1 than the bare LCO, indicating that the coated sample is initially more polarized than the bare one. The increased polarization should be related to the impeding effect of the Li2CO3 coating layer, which reduces the contact area between LCO and the liquid electrolyte. From the second to the fourth cycle, the bare LCO shows an increased polarization as seen from its increased ΔV. In contrast, LCO/Li2CO3-10 exhibits a continuously decreasing ΔV (smaller polarization) from the first to the fourth cycle, which means that the physicochemical protection of the Li2CO3 coating layer on LCO electrode is effective. The protection mechanism of the sputtering coated Li2CO3 is schematically illustrated in Scheme 1. If the bare LCO electrode is cycled at high voltage, significant parasitic reactions with the electrolyte will occur, leading to the formation of SEI layer on the electrode surface as well as structural degradation (phase

Figure 4. CV profiles of (a) bare LCO and (b) LCO/Li2CO3-10 electrodes at a sweeping rate of 0.05 mV s−1 between 3.0 and 4.5 V. 425

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Table 2. Resistance Parameters Fitted from EIS Spectra bare LCO

LCO/Li2CO3-10

cycle

Rsa (Ω)

Rfb (Ω)

Rctc (Ω)

Rs (Ω)

Rf (Ω)

Rct (Ω)

1st 5th 10th 50th

1.3 1.4 2.8 6.5

144.8 92.6 118.2 205.8

164.0 160.4 256.4 522.6

2.1 2.1 5.1 2.2

184.3 93.3 97.2 163.5

115.0 107.5 145.0 176.1

a c

Rs: ionic resistance of the electrolyte. bRf: surface layer resistance. Rct: charge transfer resistance.

parameters. After the first charge (Figure 5a), LCO/Li2CO3-10 exhibits a slightly larger Rf but smaller Rct than the bare one, which means the Li2CO3 coating can facilitate Li+ transfer (due to effective surface stabilization) although it is an electronically insulating material. With the increase of cycle number, the measured impedances (both Rf and Rct) for all samples first show a decreasing trend (until the fifth charge, Figure 5b), due to the improved electrolyte wetting and electrode activation, but then increase again at prolonged cycles (10th charge, Figure 5c), which is caused by intensified surface passivation and structural degradation of LCO.40 However, as shown in Table 2, the increase of both Rf and Rct in LCO/Li2CO3-10 is significantly less than the bare LCO. After the 50th charge (Figure 5d), the Rf and Rct values of LCO/Li2CO3-10 are 163.5 and 176.1 Ω, respectively, which are much smaller than those of the uncoated electrode (Rf = 205.8 Ω and Rct = 522.6 Ω), especially for Rct. The impedance results are in accordance with our inference that the artificial SEI layer of Li2CO3 can suppress the formation of the passivation layer on the electrode surface and slow the degradation of the LCO structure during longterm cycling (Scheme 1). From the impedance results, it is evident that an appropriate Li2CO3 coating on the LCO electrode surface can enhance the kinetics of Li+ and electron

change or cracking of large grains) of the electrode material. After sputtering coating, the surface of the LCO electrode is protected by a thin but stable Li2CO3 layer instead of forming a thick primary SEI layer which may propagate in subsequent cycles. Therefore, the detrimental side reactions on the LCO electrode surface and the structural degradation of LCO are minimized by the Li2CO3-coating which ensure a better cycling performance at high voltage. Figure 5 illustrates the impedance spectra (Nyquist plots) of the bare LCO and LCO/Li2CO3-10 after the 1st, 5th, 10th, and 50th charge. All the spectra consist of two well-defined semicircles and then followed by a linear portion. An equivalent circuit in the inset of Figure 5a is used to fit the spectra. Here, Rs, Rf, and Rct represent the ionic resistance of the electrolyte, the surface layer resistance, and the charge-transfer resistance, respectively. Table 2 lists the values of the fitted resistance

Figure 5. EIS Nyquist plots of the bare LCO and LCO/Li2CO3-10 measured after (a) 1st, (b) 5th, (c) 10th, and (d) 50th charge (to 4.5 V) at room temperature. The inset shows the equivalent circuit model for fitting. 426

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where n is the charge transfer number, F is the Faraday constant, CLi is the Li+ concentration in the electrolyte (0.051 mol cm−3),36 S is the area of the working electrode with contact of the electrolyte, R is the gas constant, T is the absolute temperature, and v is the sweeping rate. The apparent DLi of LCO/Li2CO3-10 is calculated to be 8.7 × 10−12 cm2 s−1, which is 4 times higher than that of the bare LCO (1.9 × 10−12 cm2 s−1). The enhanced Li+ transfer in the Li2CO3-coated LCO electrode is believed to be a consequence of the stabilized surface and structure by Li2CO3 coating as discussed above, which is responsible for the superior cycling performance of LCO/Li2CO3-10. To further confirm the effect of Li2CO3 coating on the highvoltage performance of LCO electrode, the half-cells were subjected to cycling tests with intentionally higher UCP of 4.6 and 4.7 V, which were previously reported to cause a serious structural degradation.58 The cycling performance of the bare and coated LCO electrodes charged at elevated UCP is demonstrated in Figure 7. As expected, the inferior perform-

transport at the interface, which is very important for obtaining a long-term stability for cycling and a high capacity retention. To investigate the influence of the Li2CO3 coating on the kinetic performance of LCO electrode, CV experiments were carried out with various sweeping rates from 0.1 to 0.8 mV s−1 after a formation cycle. Figure 6 shows the dynamic CV profiles

Figure 6. CV profiles of (a) bare LCO and (b) LCO/Li2CO3-10 between 3.0 and 4.5 V with various sweeping rates from 0.1 to 0.8 mV s−1. (c) Ip−v1/2 relationships for both electrodes.

Figure 7. Cycling performance of the bare and Li2CO3-coated LCO electrodes at 0.2 C in the potential range of (a) 3.0−4.6 V and (b) 3.0−4.7 V at room temperature.

for the bare LCO (Figure 6a) and LCO/Li2CO3-10 (Figure 6b) electrode. It is obvious that the cathodic and anodic peaks show both increasing current values and are departing away from each other as the sweeping rate increases. Moreover, LCO/ Li2CO3-10 shows a smaller ΔV (potential difference between the anodic and cathodic peaks at a certain rate) than the bare LCO, indicating smaller polarization of the sputter-coated electrode, which is complies with the EIS results discussed earlier. Plotting the cathodic current (Ip) as a function of the square root of sweeping rate (v1/2), a perfect linear relationship is obtained in Figure 6c, which matches well with the typical diffusion-controlled electrochemical behavior for LIBs. According to the Randles−Sevcik equation, the Li+ diffusion coefficient (DLi) can be calculated with the following relationship:56,57

ance of the bare LCO electrode is observed, especially for UCP = 4.7 V. The capacity retention of the bare LCO at the 50th cycle is quickly reduced from 68% (4.5 V) to 59.3% (4.6 V, Figure 7a) and 36.3% (4.7 V, Figure 7b). For LCO/Li2CO3-x, the high-voltage performances with UCP of 4.6 and 4.7 V are both improved compared to the uncoated electrode, with a capacity retention of 70.2% and 45.6%, respectively. This result further confirms the positive role of the Li2CO3 coating in enhancing the structural stability of LCO. In this case, the 10 and 20 min coated samples have almost the same optimal performance. Although the performance of long-term cycling is still below the acceptable level for practical application, the concept of using Li2CO3 sputtering coating to improve the electrode performance of LIB materials is very promising.

Ip = 0.4463n3/2F 3/2C LiSR−1/2T −1/2D̃ Li1/2 ν1/2 427

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profiles of the coated electrodes become similar to the roomtemperature one. The 10 and 20 min coated electrodes exhibit larger initial Coulombic efficiency (76.4 and 77.1%, Figure 8b) than the uncoated one (60.9%) at 55 °C, along with the improved cyclability (Figure 8c). The above results indicate that the protective role of Li2CO3 is still effective at the elevated temperature. In contrast with the low capacity retention of the bare LCO (35%), LCO/Li2CO3-10 can retain 66.5% of the initial capacity after 60 cycles at 55 °C. In order to further elucidate the effect of Li2CO3 coating on LCO electrode, the top-view FESEM images of bare LCO and LCO/Li2CO3-10 electrodes after 60 cycles at 55 °C are illustrated in Figure 9. It is obvious that a mass of amorphous

Self-heating is an unavoidable problem in LIBs which will in turn influence the capacity and lifetime of the battery. Previous studies claimed that the cyclability of cathodes at elevated temperature is limited due to the increased surface phenomena of the cathode electrodes during cycling process.59 Therefore, the high-temperature tolerability of electrode materials is very important for practical use. While the Li2CO3 coating has significantly improved the room-temperature performance of LCO electrode, whether the effect is still available at elevated temperature will be of great interest. In consequence, the electrochemical performance of the LCO electrodes at 55 °C (3.0−4.5 V) is tested and then demonstrated in Figure 8.

Figure 9. Top-view FESEM images of (a) bare LCO and (b) LCO/ Li2CO3-10 electrodes after 60 cycles between 3.0 and 4.5 V at 55 °C.

SEI layer is formed on the bare LCO electrode after multiple cycles (Figure 9a). The as-formed amorphous SEI layer looks quite thick which may explain the most significant increase of the impedance for the bare LCO electrode. In contrast, the SEI layer formed on the LCO/Li2CO3-10 electrode is quite thin, and the integrity of the LCO grains is much better preserved (Figure 9b). This result further confirms that the Li2CO3 coating on LCO electrode can effectively suppress the formation of the primary SEI layer as well as improve the integrity of LCO grains.

Figure 8. (a) Initial charge−discharge profiles, (b) initial Coulombic efficiency, and (c) cycling performance of bare LCO and LCO/ Li2CO3-x electrodes at 55 °C in the potential range of 3.0−4.5 V at 0.2 C. RT denotes room temperature; S1, S2, S3, and S4 denote bare LCO (RT), bare LCO (55 °C), LCO/Li2CO3-10 (55 °C), and LCO/ Li2CO3-20 (55 °C), respectively.

4. CONCLUSIONS In this study, we rationally investigated the effect of sputter coating of Li2CO3 on the electrochemical performance of LCO composite electrodes. By coating the electrode with an artificial Li2CO3 layer, which is chemically stable against electrolyte and ionically conductive for Li+ diffusion, the formation of the primary SEI layer on the LCO surface can be effectively suppressed. The sputtered Li2CO3 is uniformly coated on the surface of the composite electrode rather than wrapping each particle of the active materials, which can enhance ionic conduction at the electrode−electrolyte interface while not

Figure 8a displays the initial charge−discharge profiles of the bare and Li2CO3-coated electrodes. It is clear that the charge− discharge profile of bare LCO at 55 °C deviates greatly from the room-temperature one. Furthermore, an extremely high initial charge capacity of 306 mAh g−1, even over the theoretical limit, is obtained at 55 °C, which is ascribed to the hightemperature induced electrochemical parasitic reactions in the electrolyte or on the electrode surface. Fortunately, these parasitic reactions can be effectively suppressed by Li2CO3 coating (as an artificial SEI layer), and the charge−discharge 428

DOI: 10.1021/acs.jpcc.5b10677 J. Phys. Chem. C 2016, 120, 422−430

Article

The Journal of Physical Chemistry C

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deteriorate the electronic conduction in the inner part of the electrode. The LCO electrode coated with 10 min Li2CO3 shows the maximum discharge capacity of 161 mAh g−1 at the 60th cycle between 3.0 and 4.5 V at room temperature, which is 36% higher than that of the uncoated electrode. With further increased charging potential up to 4.7 V, or at elevated temperature of 55 °C, the Li2CO3 sputtering coating shows the expected effects on the battery performance of LCO. This improved electrochemical performance is ascribed to the more stable electrode structure and interface as well as the lower charge transfer resistance benefiting from the physicochemical protection of Li2CO3.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b10677. FTIR spectra of the bare LCO and LCO/Li2CO3-x electrodes; rate performance of the bare LCO and Li2CO3 coated electrodes at different charge−discharge rates in the range of 3.0−4.5 V at room temperature; the complete author list of ref 54 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel +86 28 83207620; Fax +86 28 83202569; e-mail lijingze@ uestc.edu.cn (J.Z.L.). *Tel +86 28 83207620; Fax +86 28 83202569; e-mail zhouaj@ uestc.edu.cn (A.J.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21073029, 11234013, and 21473022), the Science and Technology Bureau of Sichuan Province of China (2015HH0033), and Fundamental Research Funds for the Central Universities, China (ZYGX2012Z003).

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REFERENCES

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DOI: 10.1021/acs.jpcc.5b10677 J. Phys. Chem. C 2016, 120, 422−430

Article

The Journal of Physical Chemistry C

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DOI: 10.1021/acs.jpcc.5b10677 J. Phys. Chem. C 2016, 120, 422−430