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Enhanced interfacial kinetics and high-voltage/high-rate performance of LiCoO cathode by controlled sputter-coating with a nanoscale LiTiO ionic conductor 4

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Aijun Zhou, Xinyi Dai, Yanting Lu, Qingji Wang, Maosen Fu, and Jingze Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11630 • Publication Date (Web): 17 Nov 2016 Downloaded from http://pubs.acs.org on November 22, 2016

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ACS Applied Materials & Interfaces

Enhanced interfacial kinetics and high-voltage/highrate performance of LiCoO2 cathode by controlled sputter-coating with a nanoscale Li4Ti5O12 ionic conductor Aijun Zhou†, Xinyi Dai†,‡,*,Yanting Lu†, Qingji Wang†, Maosen Fu§, 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 ‡

§

College of Materials and Metallurgy, Guizhou University, Guiyang 550025, China State Key Laboratory of Solidification Processing, Northwestern Polytechnical University,

Xi'an 710072, China

ACS Paragon Plus Environment

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KEYWORDS: surface coating, ionic conductor, LiCoO2, magnetron sputtering, Li4Ti5O12, lithium-ion battery

ABSTRACT: The selection and optimization of coating material/approach for electrode materials have being under intensive pursuit to address the high-voltage induced degradation of lithium-ion batteries. Herein, we demonstrate an efficient way to enhance the high-voltage electrochemical performance of LiCoO2 cathode by post-coating of its composite electrode with Li4Ti5O12 (LTO) via magnetron sputtering. With a nanoscale (~ 25 nm) LTO-coating, the reversible capacity of LiCoO2 after 60 cycles is significantly increased by 40 % (to 170 mAh g-1) at room temperature and by 118 % (to 139 mAh g-1) at 55 °C. Meanwhile, the electrode’s rate capability is also greatly improved, which should be associated with the high Li+ diffusivity of the LTO surface layer while the bulk electronic conductivity of the electrode is unaffected. At 12 C, the capacity of the coated electrode reaches 113 mAh g-1, being 70 % larger than that of the uncoated one. The surface interaction between LTO and LiCoO2 is supposed to reduce the spacecharge layer at the LiCoO2-electrolyte interface, which makes the Li+ diffusion much easier as evidenced by the largely enhanced diffusion coefficient of the coated electrode (an order of magnitude improvement). In addition, the LTO coating layer which is electrochemically and structurally stable in the applied potential range plays the role of an passivation layer or an artificial and friendly solid electrolyte interface (SEI) layer on the electrode surface. Such protection is able to impede propagation of the in-situ formed irreversible SEI and thus guarantee a high initial columbic efficiency and superior cycling stability at high voltage.


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1. INTRODUCTION As the market of portable electronics flourishes, lithium-ion batteries (LIBs) have entered their golden age in the last decade. The next-generation LIBs are expected to be more stable, safe and possess higher energy density to meet the increasing demand.1-3 As the energy density (Wh L-1 or Wh kg-1) is calculated by multiplying the specific capacity (Ah/L or Ah/kg) and voltage (V), one way to enhance the energy density of the battery is to increase the voltage.4 However, higher voltages bring also problems such as electrolyte decomposition, side reactions and degradation of active materials. For example, LiCoO2,5 which is an industrial cathode material with the greatest share of market, undergoes a fast decay of its cyclability and rate capability as the charging potential is elevated from 4.2 V to 4.5 V vs. Li/Li+, although its initial capacity can be improved by 30–40 %.6,7 This is ascribed partially to the reduced stability of the layered structure above 4.2 V.8 On the other side, the active material is prone to irreversible capacity loss due to acidic dissolution of metal ions and propagation of the in-situ formed solid electrolyte interface (SEI), which kinetically hinders Li+ diffusion through the electrode-electrolyte interface.9,10 Furthermore, the increase of the working temperature was found to accelerate the degradation of the battery due to intensification of such unwanted side reactions.9,11,12 To address these issues, surface coating of the electrode materials has been extensively employed.13 Generally, the interface kinetics and electrochemical performance of surface-coated electrodes are highly sensitive to two factors: i) the way of coating and ii) the nature and amount of the coating material. Scheme 1 illustrates two major coating approaches used in literature and their effects on the electrode kinetics during cycling. In most studies, the “powder-coating” approach was employed (Scheme 1a), typically through mechanical mixing,14 solution coating,1517

and vapor deposition.18-20 In these ways, all the powders of the active material are coated with


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Scheme 1. Illustration of (a) the “powder-coating” approach, (b) “electrode-coating” approach and (c) the high-throughput sputter machine used in the present work; (d) Illustration of the changes of the electrodes coated by different ways before and after electrochemical cycling. a particular material prior to the fabrication of composite electrodes. Another approach developed in recent years is post-coating of the as-fabricated composite electrode (Scheme 1b), which can be achieved by thin film depositions like atomic layer deposition,21-24 or the more economic magnetron sputtering6,25-28 that has been widely used in diverse fields.29,30 By the facial sputtering, the target material is coated mainly on the outer surface of the electrode rather than conformably on all powders, which can reduce the risk of electron-blocking effect of the coating material if it is electrically insulative (Scheme 1d). In addition, it is highly flexible to select the


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species of target and control the amount of the coating material by sputtering, which makes this method increasingly attractive. As to the materials used for coating, there have been numerous reports so far. Great attention has been paid to metal oxides (ZnO,25 MgO,31 Al2O3,32 SnO2,33 ZrO234,35) and fluorides (MgF236 and LaF337), which are able to scavenge the detrimental components of HF and LiF generated by decomposition of conventional LiPF6-based electrolyte.38,39 Beside these materials, some ionic-type Li-salts such as Li3PO4,11 LiPON,40 Li2CO3,27 and Li3xLa(2/3)−x□(1/3)−2xTiO341 have also shown promising effects as coating materials due to their high electrochemical stability and ion-conducting feature. As a well-known anode material for LIBs, Li4Ti5O12 (LTO)18,42-44 seems to satisfy the basic requirements of an ionic-type coating material. LTO has a transfer number of 0.9945 and a high Li+ diffusivity (10-6 cm2 s-1).46 It is also electrochemically stable above its intercalation plateaus (1.5 V), which makes it possible to be used as a coating material for cathodes. The first attempt of using LTO in this scope was reported by Takada et al.47 in a LiCoO2-based solid-state battery. It was found that a nanoscale LTO interposition between LiCoO2 and the Li4GeS4–Li3PS4 solid electrolyte was able to reduce the double layer impedance of the battery and greatly enhanced its rate performance. Later, LTO was coated by sol-gel method on LiCoO2 powders, and the cyclability of the liquid-electrolyte based battery was improved.14,48,49 Through atomic-level observations, Shim et al.50 revealed recently that the structure of the Li4Ti5O12 coating layer was indeed stable during charge and discharge of LiCoO2. However, in these reports employing the powder-coating approach, the LTO-coated LiCoO2 did not show very competitive high-voltage performance, which is probably associated with the poor electronic conductivity of LTO and the difficulty of precise control of the wet-chemical coating.


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In this work, LTO is for the first time coated by magnetron sputtering on LiCoO2 composite electrodes. Due to combined merits of the coating material and the coating method, the surfacemodified LiCoO2 exhibits remarkably improved high-voltage (4.5 V) cycling stability and rate capability far above the reported performance of the powder-coated LiCoO2 with LTO. It is inspiring that the interfacial kinetics of the electrode is considerably enhanced by an order of magnitude with the nanoscale LTO-coating. In addition, by comparative investigation, the mechanism of some reported materials used for sputter-coating is discussed in view of their different chemical and conduction properties. 2.

EXPERIMENTAL SECTION 2.1 Fabrication of composite electrode. LiCoO2 composite electrodes were prepared by

spreading well-mixed commercial LiCoO2 powders (active material, 80 wt. %), acetylene black (conducting additive, 10 wt. %), polyvinylidene fluoride (PVDF, binder, 10 wt. %) and N-methyl pyrrolidone (NMP) onto a piece of Al foil, followed by vacuum-drying at 110 °C for 12 h. 2.2 High-throughput sputter-coating. The coating of the electrode was performed by magnetron sputtering of a commercial Li4Ti5O12 target. A mixture of Ar (40 sccm) and O2 (10 sccm) was used as the working gas. The working pressure and sputtering power were set as 1.0 Pa and 120 W, respectively. The deposition rate of LTO is evaluated to be 2.5 nm min-1 by measuring the thickness of a LTO film deposited simultaneously on a silicon wafer, as shown in the Supporting Information (SI, Figure S1). A time-efficient high-throughput sputtering method was employed in this work (Scheme 1c). A motor-controlled mask that can shield one part of the electrode was moved along one direction during sputtering. In this way, a gradient coating time/thickness could be obtained in different positions of the electrode in one experiment,


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which ensures high consistency and efficiency of the experiment. For simplicity of description, the sputter-coated LiCoO2 electrodes are labeled as LiCoO2/LTO-x in the following text, where x denotes the average sputtering time. 2.3 Cell assembly. Selected regions of the electrode with different average sputtering time (5, 10, 15, 20 and 30 min) were used to assemble LiCoO2/Li half cells in a glove-box filled with Ar. Prior to the cell assembly, the electrodes and separators (polypropylene, Celgard 2400) were dried in vacuum overnight at 110 °C and 50 °C, respectively. 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC), diethyl carbonate (DEC) and dimethyl carbonate (DMC) (1:1:1, v/v) was used as the liquid electrolyte. 2.4 Characterizations and electrochemical performance tests. The microstructure of the electrodes was observed with a field emission scanning electron microscope (FESEM, Hitachi S3400N) and high-resolution transmission electron microscope (HRTEM, FEI Tecnai G2 F30). Energy-dispersive X-ray spectroscopy (EDS) and Raman spectroscopy (Renishaw in Via Reflex) and X-ray photoelectron spectroscopy (XPS, Kratos XSAM800) wer used to analyze the surface of the electrodes. For XPS studies, Al-Kα (1486.6 eV) radiation was used as the primary excitation source operated at 150 W and the binding energies were calibrated using the C 1s level (284.6 eV) as an internal reference. The phase structure of the electrode was determined by Xray diffraction (XRD, Panalytical X’Pert Pro MPD) using Cu Kα radiation (λ= 1.54056 Å). Galvanostatic charge/discharge tests of the cells were performed using a CT2001A cell test instrument (LAND Electronic Co.) between 3.0 V and 4.5 V vs. Li/Li+ at 0.2 C (1 C = 140 mA g1

) at room temperature (RT) and at 55 °C. Cyclic voltammetry (CV) tests were carried out using

a Solartron SI1287 equipment with a sweeping rate varying from 0.1 mV s-1 to 1.0 mV s-1. Electrochemical impedance spectroscopy (EIS) measurements were performed using an


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electrochemical workstation (CHI660B) in the frequency range from 105 to 10-2 Hz with a perturbation voltage of 10 mV. 3. RESUILS AND DISCUSSION The microstructure and surface information of the composite electrodes are investigated by a series of characterizations, as displayed in Figure 1. The FESEM images of the electrodes show that the uncoated electrode have a relatively porous microstructure with plenty of micro LiCoO2 grains and nanoparticle additives (Figure 1a). After 10 min sputter-coating, the morphology of LiCoO2/LTO-10 (Figure 1b) does not change significantly, but it is noticeable that the surface has been coated by a particular material. As the sputtering time increases to 30 min, the coating layer appears to be thick enough to cover all LiCoO2 grains (Figure 1c). Nevertheless, the coated electrodes look still porous due to the large surface roughness. With increase of sputtering time, the pore numbers can be reduced to a certain extent. It is expected that the LTO coating can reduce the direct contact area between LiCoO2 and electrolyte, which may affect the wetting of the electrode, but is not supposed to completely and densely cover the whole electrode at a moderate coating thickness like LiCoO2/LTO-10. However, even after 30 min coating, no secondary phase related to the coating material can be detected by XRD (SI, Figure S2), indicating that the amount of the coating material is very samll as compared to the active material. Nevertheless, the EDS result of LiCoO2/LTO-30 clearly implies the presence of Ti element on the electrode surface (Figure 1d). The cross-sectional FSEM images and EDS mapping of a thickly-coated electrode (SI, Figure S3) are able to confirm that the sputtered LTO is distributed predominantly on the surface part of the electrode. Furthermore, the Raman spectra of the LTO-coated electrodes show an additional band at ~680 cm-1 (Figure 1e), the intensity of which increases with increasing sputtering time. This band can be indexed to the strongest


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characteristic band (A1g mode) of spinel Li4Ti5O12.51 For detailed study, some powders on the surface of LCO/LTO-10 were carefully scratched out and dispersed in ethanol by ultrasonic treatment for TEM observation. The presence of Ti is again evidenced by EDS mapping collected in the TEM (SI, Figure S4). From the HRTEM image (Figure 1f), a nanoscale coating layer is observed. The measured fringe distances of the coating layer are consistent with the spacing of the (331) and (400) planes of Li4Ti5O12. From these results, it is to believed that by using the facial magnetron sputtering, the target material has been successfully coated on the LiCoO2 composite electrode with basically unchanged phase of Li4Ti5O12.

Figure 1. (a-c) FESEM images of LiCoO2/LTO-x electrodes; (d) EDS spectra of bare LiCoO2 and LiCoO2/LTO-30; (e) Raman spectra of the bare and LTO-coated electrodes; (f) HRTEM image of LiCoO2/LTO-10.


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The benefit of LTO sputter-coating is demonstrated by the remarkable improvement of the room-temperature electrochemical performances of the electrode as shown in Figure 2. It is to see that the reversible capacity and the capacity retention of the electrode show first an increase and then a decrease with increasing coating time. The optimum performance is obtained in LiCoO2/LTO-10 (corresponds to a coating thickness of ~ 25 nm), which can deliver 90 % (170 mAh g-1) of its initial capacity after 60 cycles. In comparison, the uncoated electrode has a retention of only 64.4 % (121 mAh g-1) after the same number of cycles. Even for the thickestcoated electrode (LiCoO2/LTO-30), the cycling stability is still superior to that of the bare one. As to the rate capability, the LTO-coated electrodes are also superior to the bare one. At the

Figure 2. (a) Cycling performance of the bare and LTO-coated LiCoO2 electrodes at 0.2 C within 3.0–4.5 V vs. Li/Li+ at RT. The inset shows the capacity retention after 60 cycles; (b) Rate capability of the electrodes at RT.


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largest rate of 12 C, the reversible capacity of LiCoO2/LTO-10 reaches 113 mAh g-1, which is 70 % higher than that of the uncoated electrode (Figure 2b). This improvement implies that the kinetics of the composite electrode is also significantly enhanced by LTO-coating. Similar to the effect on the cycling performance, over-coating (30 min of sputtering) is not able to achieve continuous improvement of the rate capability, which again emphasizes the importance of thickness control of the coating layer. The surface morphologies of the electrodes after 50 cycles are displayed in Figure 3. For the bare electrode, typical morphology of a thick glassy SEI being ambiguous in appearance is observed after cycling, which is quite different with its original morphology. In contrast, the surface of the cycled electrode with LTO-coating is much clearer, indicating less formation of the glassy SEI, which will be further evidenced by the XPS results provided later. This observation suggests that the sputter-coated LTO can serve as a favorable interface layer between LiCoO2 and the liquid electrolyte, which can, to a great extent, impede irreversible side reactions taking place on the electrode surface and save the electrode from fast capacity loss. This effect can be further supported by the evident increase of the initial coulombic efficiency of the LiCoO2 electrode with LTO-coatings (SI, Figure S5).

Figure 3. FESEM images of (a) bare LiCoO2 and (b) LiCoO2/LTO-10 after 50 cycles at RT. To further examine the role of LTO-coating, the electrodes were cycled at 55 °C while other conditions were unchanged. As shown in Figure 4a, the bare LiCoO2 electrode undergoes even


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faster degradation at the elevated temperature, delivering only 35 % of the initial capacity after 60 cycles. This is caused by intensified electrolyte decomposition and interfacial side reactions at high temperature. Surprisingly, the LTO-coated electrode (LiCoO2/LTO-10) exhibits a capacity retention as high as 72.6 %, which further proves the excellent physio-chemical protection effect of the LTO coating layer. From the initial charge-discharge profiles (Figure 4b), we can see that the LTO-coated electrode is less polarized, and its initial capacity loss (49 mAh g-1) is much smaller than the bare one (120 mAh g-1), leading to a higher coulombic efficiency (80 % vs. 61 %). From the high-temperature cycling performance of bare LiCoO2, it is to see that the

Figure 4. (a) Cycling performance of bare LiCoO2 and LiCoO2/LTO-10 at 55 °C in comparison with the performance at RT; (b) Initial charge-discharge profiles of bare LiCoO2 and LiCoO2/LTO-10.


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electrode’s capacity loss during the first few cycles contributes the biggest part to the total loss, because the interfacial side reactions are most active in this period. With a nanoscale coating of LTO, which is electrochemically stable in the applied potential range, such harmful interfacial reactions can be effectively mitigated even though the temperature is increased. Table 1. Fitted resistances from the EIS data. Rs: ionic resistance of the electrolyte, Rf: surface layer resistance, Rct: charge transfer resistance. bare LiCoO2

LiCoO2/LTO-10

Cycle Rs(Ω)

Rf(Ω)

Rct(Ω)

Rs(Ω)

Rf(Ω)

Rct(Ω)

10th

2.8

118.2

256.4

5.0

112.8

105.3

50th

6.5

205.8

522.6

4.9

121.2

148.7

Increase ratio

-

74 %

104 %

-

7%

41 %

Figure 5. EIS Nyquist plots of bare LiCoO2 and LiCoO2/LTO-10 (a) before cycling and after (b) 10 and (c) 50 cycles (charged to 4.5 V). The inset shows the equivalent circuit model for data fitting.


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EIS tests were performed to reveal the kinetics change of the composite electrode after LTO coating. Figure 5 shows the Nyquist plots of bare LiCoO2 and LiCoO2/LTO-10 after 10 and 50 cycles. From the diameter of the semicircle of the uncycled electrodes (Figure 5a), it is to see that the overall impedance of the electrode is obviously reduced by LTO-coating, which is similar to the trend observed in the LiCoO2/ LTO/Li4GeS4–Li3PS4 solid state battery.47 It was believed that the LTO coating was able to suppress the development of the space-charge layer at the solid-solid interface due to transition of the chemical composition and chemical potential. In this work, we think the LTO-coating on LiCoO2 has analogous effect in reducing the spacecharge layer formed between LiCoO2 and the liquid interface, which was previously observed on LiCoO2 thin films adsorbed with DEC.52 After a few cycles (Figure 5b and 5c), the big semicircles are divided into two well-defined semicircles, which correspond respectively to the surface layer resistance (Rf) and the interfacial charge transfer resistance (Rct), both are closely related with the SEI grown on the electrode surface. A Voigt-type equivalent circuit is applied to fit the plots and the obtained resistances are listed in Table 1. It is found that as the cycle number increases from 10 to 50, the bare electrode shows remarkable increase of Rf and Rct by 74 % and 100 %, respectively. However, such increases are much smaller for LiCoO2/LTO-10, being only 7 % and 40 %, respectively. This result implies two facts: i) the SEI propagation on the electrode surface is much slower after LTO-coating (smaller Rf) and ii) the Li+ transfer at the LTOmodified interface is much easier (smaller Rct). From analysis of the Warburg impedance (the linear part of the Nyquist plot), it can be qualitatively revealed that the Li+ diffusion in the LTOcoated electrode is faster than in the bare one (SI, Figure S6). In addition to that, CV measurements with varing sweeping rates were carried out, which allows quantitative determination of the apparent Li+ diffusion coefficient (DLi+) of the electrode by using the


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Randles-Sevcik equation.25,53 Figure 6 shows the CV profiles and the calculated DLi+. Inspiringly, we find that the Li+ diffusion coefficient is enhanced by an order of magnitude from 5.27 ×10-12 cm2 s-1 to 3.90 ×10-11 cm2 s-1 after 10 min of LTO sputter-coating. Such a great improvement of DLi+ is believed to account for the significantly enhanced rate capability of LiCoO2/LTO-10.

Figure 6. CV profiles of (a) bare LiCoO2 and (b) LiCoO2/LTO-10 with varying sweeping rates; (c) Ip–v1/2 plots and the calculated DLi+.

Figure. 7. XPS spectra of bare LiCoO2 and LiCoO2/LTO-10 after 50 cycles. (a) full spectra; (b) Ti 2p spectrum; (c) Co 2p spectra; (d) F 1s spectra; (e) O 1s spectra; (f) C 1s spectra.


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For further elucidation of the coating effect of LTO, the electrodes after 50 cycles were subjected to XPS studies and the obtained spectra are displayed in Figure 7. It is very clear that the signals from Ti and O are much stronger on the cycled LiCoO2/LTO-10, and the chemical state of Ti is confirmed to be Ti4+ being fully consistent with the Ti spectrum of pure Li4Ti5O12.64,65 This suggests that the LTO coating layer is chemically stable after certain cycles of high-voltage charge/discharge, which basically agrees with the HRTEM observations reported by Shim et al.50 Due to the presence of the LTO cover layer, the amount of Co and F, which should come from the matrix electrode, is less detected on the coated electrode. In the O 1s spectra, a broad shoulder on the high binding energy side is found, which can be attributed to O– H bonds from hydroxyl groups or water molecules adsorbed on the electrode surface.66 Meanwhile, the C 1s spectrum of the coated electrode is also much simpler than that of the bare one, with less C-F related peaks. From the XPS results, one can elucidate that after 50 cycles, the coated electrode experiences no dramatic changes on the surface, with the LTO being the major constitute. In contrast, a plenty of organics components, either form the pristine composite electrode (PVDF) or as new side-reaction products (SEI), are existing on the surface of the cycled bare electrode. From this point of view, the sputter-coated LTO in this work can serve as a friendly artificial SEI being advantageous for Li+ transfer, and meanwhile is effective to suppress the in-situ formed thick SEI, which are attributed to the enhancement of the cycling and rate performances of LiCoO2 electrode. Figure 8 summarizes the effects of some reported coatings on LiCoO2, which can be compared and discussed to get more understanding. For the use of LTO, post-coating of the composite electrode presented in this work seems to be more effective than the so-gel powder-


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Figure 8. Comparison of (a) cycling performance and (b) rate capability of LiCoO2 electrodes with different coating materials or methods. The inset shows the comparison of apparent Li+ diffusion coefficient (DLi+) of the electrode after each coating. coating reported previously.50 It is to admit that the source of active material and electrolyte composition used are different, which can cause differences of the electrochemical performance. However, considering the fact that capacity of the powder-coated electrode at > 3 C is lower than that of the sputter-coated one although it is higher at smaller rates, it is to infer that the powdercoated LiCoO2 electrode may have poor electronic conductivity due to the electron-blocking effect of LTO, as schematically illustrated in Scheme 1d. On the other hand, the post sputtercoating on the composite electrode allows optimized control of the coating position/thickness of LTO and avoids the impeding effect of LTO on electron transfer, which is responsible to the better cycling stability and high-rate capability obtained in this work. It is also to see from Figure 7 that when the same approach (sputtering) is employed for LiCoO2 coating, the use of LTO is


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Table 2. Comparison of the intrinsic conductivity and Li+ diffusivity of some materials. Li+ diffusivity Material

Conduction type

-1

Conductivity (S cm ) (cm2 s-1)

ZnO

electronic

50 54

10-14–10-12 55

AZO

electronic

2 × 103 54

N/A

Li2CO3

ionic

10-13–10-12 56

~ 10-8 57

Li4Ti5O12

ionic

10-13–10-8 58,59

~ 10-6 46

LiCoO2

mixed

10-4–10-1 60,61

10-13–10-8 61-63

very promising in comparison with the use of ZnO, Al2O3-doped ZnO (AZO) and Li2CO3, especially for the high-rate performance. This comparison is very interesting because these materials have quite distinct conduction properties, as shown in Table 2, and the associated mechanisms are somehow different. Oxide materials (ZnO and AZO) are well-known for the HF-scavenging ability.39,67 They can partially with the trace amount of HF in the electrolyte and form a chemically more stable metal fluoride layer (ZnF2 or AlOxFy) on the surface,6 being able to mitigate further side reactions and prevent dissolution of the active material. ZnO and AZO are predominantly electronic conductors. Although the Li+ diffusivity in pure ZnO is not very high (10-14 – 10-12 cm2 s-1),55 wich should be similar for the case of AZO, the sputter-coated layer has a nanoscale thickness and the as-formed metal fluorides are ionically more conductive than the oxides. In electronic conduction, AZO is obviously superior to ZnO.54 In addition, the Al in AZO can be partially doped into LiCoO2 forming a LiAlxCo1-xO2 solid solution, which can


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increase the structural stability of the active material under high voltage.68,69 These multiple reasons together can explain the best performance of LiCoO2 coated with AZO. As to Li2CO3 and Li4Ti5O12, they are similar ionic conductors whose electronic conduction part is extremely low.45 Li2CO3 is known as a major component of the in-situ formed SEI layer,70 while Li4Ti5O12 itself is an well-known “zero-stain” anode material, on which no SEI is supposed to form above 1.5 V.71 Therefore, both Li2CO3 and Li4Ti5O12 are supposed to be structurally and electrochemically stable in the electrolyte, and the benefits of Li2CO3 and Li4Ti5O12 coating rest mainly with their own ability to suppress undesired redox reactions and SEI propagation on the electrode surface. On the other side, the rate capability of the electrode is determined by the interfacial kinetics of the electrode. As seen from Table 2, Li4Ti5O12 shows a much higher intrinsic Li+ diffusivity than Li2CO3 and ZnO. As a result, the LiCoO2 electrode coated with LTO exhibits the largest DLi+ among all the electrodes (inset of Figure 7). This should be the major reason for the outstanding high-rate (12 C) performance of the LTO-coated electrode, which is comparable to the AZO-coated one. However, the coating of Li2CO3 and ZnO whose electronic and ionic conductivity are both poor brought about less improved high-rate performance of LiCoO2 than the use of LTO coating. 4. CONCLUSIONS An ionic conductor, Li4Ti5O12, is coated on LiCoO2 composite electrode by facial magnetron sputtering. The high-voltage (3.0–4.5 V) cycling stability and rate-capability of the LTO-coated electrode are remarkably improved due to high electrochemical stability and high diffusivity of Li+ in LTO. These two merits of LTO can respectively limit the unwanted interfacial side reactions between LiCoO2 and liquid electrolyte, and enhance the interfacial kinetics of the electrode. The electrode performance of the LTO sputter-coated LiCoO2 is superior to that of the


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“powder-coated” electrode reported previously. In addition, with the same sputter-coating approach, the LTO-coated electrode also shows outstanding high-voltage electrochemical performance close to the AZO-coated one. Our work suggests that the material’s ionic diffusivity is a key factor which should be considered when selecting the coating material. The combined use of the electrode post-coating approach and an efficient ionic coating material like LTO is very promising to address the high-voltage degradation of LiCoO2, which may be also applicable to other cathode materials. ASSOCIATED CONTENT Supporting Information. FESEM image of LTO deposited on Si substrate, XRD patterns of LiCoO2 composite electrodes, Cross-sectional FESEM images and EDS mapping for LiCoO2/LTO-60, HRTEM image of LiCoO2/LTO-10 and the EDS mapping result, the initial discharge-charge profiles and coulombic efficiency data, and the analysis of the Warburg impedance. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * Email: [email protected] (J.Z. Li). * Email: [email protected] (X.Y. Dai) Author Contributions Aijun Zhou and Xinyi Dai contributed equally to this work.


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is supported by the National Science Foundation of China (11234013, 21473022, 51502032, 21673033, 51603028), the Science and Technology Bureau of Sichuan Province of China (2015HH0033), the Fundamental Research Funds for the Central Universities of China (ZYGX2015J027) and the Opening Project of State Key Laboratory of Polymer Materials Engineering (Sichuan University) (Grant No. SKLPME 2016-4-23)


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