Conductive Polymers Encapsulation To Enhance Electrochemical

Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 18270−18280

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Conductive Polymers Encapsulation To Enhance Electrochemical Performance of Ni-Rich Cathode Materials for Li-Ion Batteries Yanbing Cao,† Xianyue Qi,† Kaihua Hu, Yong Wang, Zhanggen Gan, Ying Li, Guorong Hu, Zhongdong Peng, and Ke Du* School of Metallurgy and Environment, Central South University, Changsha 410083, China S Supporting Information *

ABSTRACT: Ni-rich cathode materials have drawn lots of attention owing to its high discharge specific capacity and low cost. Nevertheless, there are still some inherent problems that desiderate to be settled, such as cycling stability and rate properties as well as thermal stability. In this article, the conductive polymers that integrate the excellent electronic conductivity of polyaniline (PANI) and the high ionic conductivity of poly(ethylene glycol) (PEG) are designed for the surface modification of LiNi0.8Co0.1Mn0.1O2 cathode materials. Besides, the PANI−PEG polymers with elasticity and flexibility play a significant role in alleviating the volume contraction or expansion of the host materials during cycling. A diversity of characterization methods including scanning electron microscopy, energy-dispersive X-ray spectrometer, transmission electron microscopy, thermogravimetric analysis, Fourier transform infrared have demonstrated that LiNi0.8Co0.1Mn0.1O2 cathode materials is covered with a homogeneous and thorough PANI−PEG polymers. As a result, the surface-modified LiNi0.8Co0.1Mn0.1O2 delivers high discharge specific capacity, excellent rate properties, and outstanding cycling performance. KEYWORDS: Ni-rich cathode materials, conductive polymers, surface modification, rate properties, cycle performance

1. INTRODUCTION The development of new energy systems and clean electric vehicles is the inevitable choice of energy institution reform in the world, such as lithium ion,1 sodium ion,2 and potassium ion.3 Layered Ni-rich (Ni content > 0.6) cathode materials for lithiumion batteries are by far have the most potential to be applied to battery systems for electric vehicles owing to faster charge rate, high discharge specific capacity, low cost, etc.4−7 Nevertheless, there still exist some inherent questions desiderate to be settled for the Ni-rich cathode materials. First, high content of residual alkali compounds on the surface of Ni-rich materials is a prominent problem in practical application.8,9 That is because the residual lithium, such as Li2O/LiOH, on the surface of the Nirich materials will absorb H2O/CO2 and thereby form a LiOH/ Li2CO3 layer, whereas LiOH will react with LiPF6 in the electrolyte. Besides, the decomposition of Li2CO3 at high voltage is one of the main reasons for the battery flatulence. Second, transition metals in the Ni-rich materials, such as Mn4+/Ni4+, will generate severe side reactions with electrolyte, accompanied by the release of oxygen and heat, which leads to a deterioration in thermal stability and a latent risk of thermal runaway.10,11 Third, the anisotropic lattice volume shrinkage or dilation of the primary particles under electrochemical measurement will bring about cracks, resulting in poor cycle properties.12 Surface modification is widely recognized to be an expedient strategy to solve these problems. Metal oxides (Al2O3,13 CeO2,14 MgO,15 SiO216), fluorides (LiF,17 AlF3,18), and phosphates (AlPO4,19 Li3PO420) are generally introduced onto the surface of the host materials to enhance the electrochemical properties. © 2018 American Chemical Society

Nevertheless, a majority of coating materials have drawbacks of low conductivity for ions and/or electrons, which decrease the rate properties and increase the irreversible capacity of the host materials to a certain extent. Conductive polymers, such as polypyrrole (PPy),21,22 polyacene,23 poly(3,4-ethylenedioxythiophene),24,25 and polyaniline (PANI),26−28 have attracted great attention of researchers as surface modification materials because of their high electronic conductivity and electrochemical activity. Among these conductive polymers, polyaniline (PANI) stands out for its easy preparation, controllable conductivity, excellent environment stability, and cheap raw materials.29−31 In addition, the layered Ni-rich materials can be uniformly coated because the conductive polyaniline has a nanowire-like structure, and polyaniline exhibits a compatible polarity with the electrolyte through the H bonds. Nevertheless, PANI is a good electronconducting polymer, but not an ion-conducting polymer. To compensate for the disadvantages of PANI cladding layer and improve the charge transfer speed on the surface of electrode materials, it is essential to select a kind of conductive polymer to transport lithium ions. Poly(ethylene glycol) (PEG) is a good ionic conductor for the polymer as well as a superduper lithium salt solvent.32,33 In this article, surface modification for LiNi0.8Co0.1Mn0.1O2 materials with the conductive PANI−PEG polymers was proposed for the first time. The hybrid polymers cladding layer could be described as a “freeway” to transfer Received: February 8, 2018 Accepted: May 7, 2018 Published: May 7, 2018 18270

DOI: 10.1021/acsami.8b02396 ACS Appl. Mater. Interfaces 2018, 10, 18270−18280

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ACS Applied Materials & Interfaces Scheme 1. Schematic Illustration of the Synthesis of NCM@PANI−PEG Composites

2.3. Electrochemical Evaluation. The cathodes were fabricated by pasting the mixture of the active materials, acetylene black and poly(vinylidene fluoride) binder (8:1:1 in weight). The mixture was dispersed in N-methylpyrrolidinone (NMP) solvent and the obtained slurry was coated onto an Al foil. Then, the cathode electrode film was dried overnight in the vacuum oven at 120 °C. The half-cells (CR2025) were fabricated in a glovebox full of high pure Ar. A lithium foil was adopted as negative electrode, a Celgard2400 porous polypropylene film was introduced as a separator and 1 M LiPF6 dissolved in ethyl methyl carbonate/dimethyl carbonate/ethylene carbonate (1:1:1 in volume) was employed as an electrolyte. The mass loading of active materials was 4 mg and the amount of electrolyte was 0.25 mL. The assembled halfcells were cycled between 2.8 and 4.3 V for the first five cycles at 0.2C rate and followed by 1C rate for 100 cycles under 25 and 55 °C, respectively. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry tested were carried out on an electrochemical workstation (CHI660e, CH Instruments).

electrons and lithium ions between cathode materials and electrolyte. The rate properties and cycling stability of LiNi0.8Co0.1Mn0.1O2 materials were effectively enhanced via a conductive PANI−PEG polymers coating layer, and a detailed investigation of the mechanism for the enhancement in electrochemical properties is shown below.

2. EXPERIMENTAL SECTION 2.1. Synthesis of the Materials. LiNi0.8Co0.1Mn0.1O2 (NCM) was synthesized via mixing the commercial Ni0.8Co0.1Mn0.1(OH)2 precursor (Guoguang, Guangzhou) and LiOH·H2O (Battery Grade, Ganfeng, Jiangxi) in a molar ratio of 1:1.04. The compound was preheated for 6 h at 550 °C and then sintered for 15 h at 750 °C under an O2 atmosphere. To prepare NCM@PANI−PEG composites, a wet-coating method was applied. First, 1.5 g PANI (Doped by HCl, Jiaying, Shandong) and 1.5 g PEG (1500 Mw, Guoyao, Shanghai) polymers were dispersed in Nmethyl pyrrolidone (NMP) (AR, Kemiou, Tianjin) solvent. Then, 10 g NCM sample was immersed in the above dispersion and stirred continuously for an hour at 50 °C. After filtering the mixed solution, the resulting powders were dried in the vacuum oven under 120 °C for 12 h, from which the surface treatment for LiNi0.8Co0.1Mn0.1O2 materials with PANI−PEG polymers was obtained. A schematic illustration for the preparation of NCM@PANI−PEG composites is displayed in Scheme 1. 2.2. Material Characterizations. The phase identification of the samples was employed by X-ray diffraction (XRD, Rigaku TTRIII) with Cu Kα1 radiation under a scan speed of 10° min−1. The surface morphology of the as-synthesized materials was observed by using scanning electron microscopy (SEM, Quanta 650 FEG) with an acceleration voltage of 20 kV. The surface compositional characterization of the materials was analyzed by energy-dispersive X-ray (EDX) spectrometer (JEOLJSM-6100LV). The crystal structure of the samples was performed by transmission electron microscopy (TEM, TECNAI G2 F20) and high-resolution transmission electron microscopy (HRTEM). Thermogravimetric analysis (TGA) was performed on a TG209F1 instrument with air condition in the temperature range of 25−700 °C under a heating speed of 10 °C min−1 to estimate the polymers content in the composites. The electronic conductivities of all the samples were characterized by four-probe technique (ST2253). The chemical structures analysis of all the samples was performed by Fourier transform infrared (FT-IR, Thermo, Nicolet iS50). The assembled halfcells were charged to 4.3 V and disassembled in a glovebox full of high pure Ar. The obtained cathodes were stored in the electrolyte at 55 °C for different time and the dissolved amounts of transition metal ions from electrodes into electrolyte were measured with inductively coupled plasma optical emission spectrometer (SPECTRO BLUE SOP). The method for determining the pH values of different samples is shown below: add 5 g of sample into the deionized water of 50 mL under continuous stirring for 30 min and then allow to stand for 30 min. Finally, the pH values of solutions were measured with a pH meter (DELTA 320).

3. RESULTS AND DISCUSSION The crystal structure of the bare NCM and NCM@PANI−PEG powders were characterized by X-ray diffraction (XRD), as compared in Figure 1. The XRD patterns of both powders

Figure 1. XRD patterns of (a) NCM and (b) NCM@PANI−PEG.

revealed that all of the diffraction patterns could be well matched with a layered hexagonal α-NaFeO2 structure (R3/m). As observed from Figure 1, both (006)/(102) and (108)/(110) diffraction patterns were clearly split, which meant that the powders exhibited a well-organized crystal structure.34,35 No additional diffraction peaks from PANI−PEG polymers were presented in the XRD pattern of NCM@PANI−PEG powder 18271


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Figure 2. SEM images of (a, b) NCM, (c, d) NCM@PANI, and (e, f) NCM@PANI−PEG samples.

owing to the amorphous state and low content of the PANI− PEG polymers. Furthermore, no obvious differences in the diffraction peaks between pristine NCM and NCM@PANI− PEG powders were exhibited, which signified that the surface treatment of LiNi0.8Co0.1Mn0.1O2 materials with PANI−PEG polymers had hardly any effect on the crystal structure. The surface morphologies of the bare NCM, NCM@PANI, and NCM@PANI−PEG powders were investigated by SEM measurement. As observed from Figure 2, all the powders were typical secondary spherical particles consisting of small primary particles, which meant that the PNAI or PANI−PEG polymers coating layer did not cause damage the gross morphologies of the host material. As displayed in Figure 2a,b, the primary particles of the bare NCM were clearly visible. In comparison with pristine NCM powder, the surface of the NCM@PANI in Figure 2c,d showed a blurred edge. It can be conspicuously noticed in Figure 2e,f that the surface of NCM@PANI−PEG was encapsulated with a thin and continuous film. To investigate the elements distribution on the surface of NCM@PANI−PEG sample, the EDX detection was adopted. As displayed in Figure 3, the distribution of Ni, Co, Mn (from NCM), and N (from PANI−PEG) elements was all fairly homogeneous on the surface of NCM@PANI−PEG material,

indicating that pristine NCM was uniformly coated with PANI− PEG polymers and consistent with the SEM result. Figure 4 exhibits the TEM images of NCM, NCM@PANI, and NCM@PANI−PEG materials. As shown in Figure 4a, the surface of pristine NCM was relatively smooth as expected. By comparison, on the surface of NCM@PANI and NCM@PANI− PEG particles, we can distinctly observe homogeneous and amorphous films with a thickness of 25−30 and 30−35 nm, respectively. The electronic conductivities of all the samples were characterized by a four-probe technique, and the electronic conductivity of pristine NCM was tested to be 1.74 × 10−5 S cm−1, much lower than that of NCM@PANI and NCM@ PANI−PEG (3.83 × 10−2 and 2.85 × 10−2 S cm−1, respectively). The existence of PANI−PEG polymers on the surface of NCM material was further verified by FT-IR. Figure 5 displays the result of FT-IR for (a) PANI, (b) PEG, (c) PANI−PEG, (d) NCM, and (e) NCM@PANI−PEG. It can be observed from the spectra of Figure 5e that the characteristic peaks of 1124 and 1593 cm−1 were assigned to the C−H bending vibration of the benzenoid ring and the CN stretching vibration of the quinoid ring in PANI, respectively.36,37 Besides, the typical bands located at 1486, 2912, and 3423 cm−1 can be attributed to the bending vibration of −CH2−, the symmetric stretching vibration of 18272


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Figure 3. (a) SEM image of NCM@PANI−PEG sample and the corresponding EDX mappings of (b) Ni, (c) Co, (d) Mn, and (e) N elements.

−CH2−, and the stretching vibration of OH− in PEG.38,39 This result demonstrated that LiNi0.8Co0.1Mn0.1O2 material was composed of PANI−PEG polymers. To figure out the amount of PANI−PEG polymers in the NCM@PANI−PEG composite, TGA measurement was performed with air flow condition. Figure 6 displays the TGA curve of NCM@PANI−PEG powders. It can be observed from Figure 6 that the weight content of the PANI−PEG polymers introduced onto the surface of pristine NCM particles was estimated to be about 3.37 wt %. To research the electrochemical performance of the cathodes, a galvanostatic charge−discharge measurement was employed. Figure 7a exhibits the initial charge−discharge plots of the bare NCM, NCM@PANI, and NCM@PANI−PEG within the potential range of 2.8−4.3 V under 0.2C rate and 25 °C. The initial discharge-specific capacities of the three electrodes were 194.7, 199.4, and 201.6 mAh g−1, respectively, and the corresponding Coulombic efficiencies were 85.7, 87.6, and 88.4%, respectively. It showed that both initial discharge-specific capacity and Coulombic efficiency of NCM@PANI−PEG were highest. This result can be attributed to PANI−PEG polymers promoting the transmission of electrons and lithium ions at the

interface of electrode/electrolyte, thereby decreasing the loss of initial irreversible capacity. As exhibited in Figure 7b, the discharge-specific capacity of the bare NCM declined quickly from 181.6 to 151.5 mAh g−1 during 100 cycles, with a capacity retention of 83.4%, whereas the capacity retention of NCM@ PANI and NCM@PANI−PEG cathodes was 89.6 and 93.4%, respectively. The improvement in cycle performance was due to the existence of a conductive polymer, which prevented the active materials from being eroded by the hydrofluoric acid (HF) and provided a physical barrier to suppress side reactions during cycling. It was noteworthy that the capacity retention of NCM@ PANI−PEG was better than that of NCM@PANI because the elasticity of the PANI−PEG polymers involving flexible PEG played an important role in alleviating the volume contraction or the expansion of the host material during cycling.40,41 In addition, the introduced polymers can reduce the residual alkali on the surface of the bare NCM. As exhibited in Table 1, the bare NCM showed a higher pH value in contrast to the NCM@PANI and NCM@PANI−PEG samples (11.94 vs 11.47, 11.28), which can be assigned to the formation of a continuous and thorough polymers protective layer. It can be clearly observed from Figure 7c that the rate performance of NCM@PANI and NCM@ 18273


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Figure 4. TEM images of (a) NCM, (b) NCM@PANI, and (c) NCM@PANI−PEG samples.

Figure 6. TGA plot of NCM@PANI−PEG powders under the heating rate of 10 °C min−1 in air condition.

Figure 5. FT-IR spectra of (a) PANI, (b) PEG, (c) PANI−PEG, (d) NCM, and (e) NCM@PANI−PEG.

diffusion pathway for lithium ions and thereby improved the rate properties. Figure 7d exhibits the cycle performance of NCM, NCM@PANI, and NCM@PANI−PEG samples at 55 °C. As displayed in Figure 7b,d, the initial discharge-specific capacities of the three materials at 55 °C were higher than those at 25 °C owing to the improvement in temperature accelerating the charge transfer kinetics.42 In addition, the fading rate at the elevated temperature of 55 °C was faster than that at the lower temperature of 25 °C in the discharge−charge process because

PANI−PEG was better than that of the bare NCM, especially at high rates. For instance, the discharge-specific capacity of pristine NCM was 139.4 mAh g−1 under 10C rate, whereas NCM@PANI and NCM@PANI−PEG electrodes delivered the dischargespecific capacities of 151.0 and 156.7 mAh g−1 under the same rate, respectively. This was because the PANI coating layer enhanced the electronic conductivity of the host material and then promoted the charge transfer. In particular, the existence of PEG in the PANI−PEG polymers provided an easily accessible 18274


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Figure 7. Electrochemical performance of NCM, NCM@PANI, and NCM@PANI−PEG. (a) Initial charge and discharge curves. (b) Cycle performance at 25 °C. (c) Rate performance. (d) Cycle performance at 55 °C.

properties at 55 °C. It is observed from Figure 8 that the dissolved amounts of transition metals from all the charged NCM, NCM@PANI, and NCM@PANI−PEG electrodes into the electrolyte continuously increased with time. The dissolution of transition metals was caused by the erosion of HF that was produced by electrolyte decomposition, which had an adverse influence on the electrochemical properties. For instance, the transference of dissolved Mn element to the anode electrode surface led to the solid electrolyte interphase (SEI) film regeneration and lithium consumption, thereby driving a reduction of reversible capacity, especially at elevated temperature.45,46 As displayed in Figure 8, the dissolved amounts of Ni, Co and Mn transition metals from the surface-modified electrodes were all lower than that of the bare electrodes at the same storage time. The PANI−PEG clapping layer introduced onto the surface of NCM particles provided a physical barrier to prevent the cathode materials from being eroded by HF and therefore suppressed the transition metals dissolution, resulting in enhancement of the cycle performance of the host materials at elevated temperature. In particular, the NCM@PANI−PEG electrodes exhibited the lowest dissolution amounts of the transition metals at the same storage time, which may be because PEG was present as a kind of Lewis base in the PEG−PANI polymers, whereas PF5 produced by the thermal dissociation of LiPF6 behaved as a Lewis acid. Therefore, PEG can complex with PF5 available to prevent the PF5 from being hydrolyzed to generate HF. Abraham47 found that the Lewis basic additives

Table 1. pH Values of NCM, NCM@PANI, and NCM@ PANI−PEG Powders samples

pH value

pristine NCM NCM@PANI NCM@PANI−PEG

11.94 11.47 11.28

more of the transition metal dissolved by HF erosion and electrolyte decomposition caused by high oxidation.43 The NCM cathode materials suffered the erosion of HF that was produced by electrolyte decomposition and some side reactions occurred at the interface between the electrode and the electrolyte during cycling, which had a negative influence on the electrochemical performance of the host materials. The side reactions as shown:8,44 LiPF6 → PF5 + LiF, PF5 + H2O → POF3 + HF, POF3 + Li2O → LixPOFy + LiF, and Li2O/LiOH + HF → H2O + 2LiF. As displayed in Figure 7d, the surface-modified samples have a superior capacity retention than the bare NCM during 100 cycles at 1C (81.4, 77.1 vs 53.6%). This may be attributed the conductive polymer film forming a protective layer to protect the bulk materials from HF corroding and to suppress the side reactions. As a result, pristine NCM that was surface treated with the conductive polymer displayed a better cycle performance at 55 °C. The dissolved amounts of transition metals were measured to further research the reasons for the enhancement of cycle 18275


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Figure 8. Dissolved amounts of transition metals of (a) Ni, (b) Co, and (c) Mn under different storage times at 55 °C.

Figure 9. EIS plots (a) without cycle and (b) after 100 cycles of NCM, NCM@PANI, and NCM@PANI−PEG electrodes.

To further explore the inherent origin of the improvement in electrochemical properties, the EIS analysis was applied. Figure 9 exhibits the EIS plots of pristine NCM, NCM@PANI, and NCM@PANI−PEG electrodes without cycle and after 100 cycles. As displayed in Figure 9a, the plots of electrodes without

significantly improved the thermal stability of organic electrolyte. It was necessary to point out that the amount of electrolyte in Figure 8 was higher than that in Figure 7, resulting in the dissolved amounts of transition metals being higher than the data in Figure 7. 18276


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It was observed that the Rct data of all the electrodes increased with the increase in the cycle numbers, indicating more side reactions were produced at the electrode/electrolyte interface. However, the Rct data of pristine NCM were much higher than that of the surface-modified electrodes, suggesting that the conductive polymer (PANI or PANI−PEG) introduced onto the surface of NCM can promote the transfer of electrons or electrons/ions, as well as protect the active materials from direct contact with the electrolyte, which, in turn, improved the conductivity of NCM and alleviated the severe side reactions with the electrolyte. To calculate the Li+ diffusion coefficient of NCM and NCM@ PANI−PEG, the cyclic voltammetry curves of the two electrodes at different scan rates (0.1, 0.2, 0.4, and 0.8 mV s−1) under 2.8− 4.3 V are shown in Figure S3. The calculation method is described as follows

cycle consisted of a depressed semicircle and a slope, whereas the plots of electrodes after 100 cycles consisted of two semicircles and a slope as shown in Figures 9b and S1. Here, Rs represents the electrolyte resistance corresponding to the highest frequency. Rf is assigned to the surface film resistance in the high frequency. The medium−high-frequency semicircle was ascribed to the charge transfer resistance Rct at the interface between the electrode and the electrolyte, and the slope corresponding to the low frequency was ascribed to the Li-ion diffusion in the bulk materials (Wo, Warburg impedance). Table 2 exhibits the EIS Table 2. EIS Data of NCM, NCM@PANI, and NCM@PANI− PEG Electrodes samples

cycle

Rs (Ω)

pristine NCM

0 100 0 100 0 100

3.437 4.635 3.044 3.661 2.816 3.394

NCM@PANI NCM@PANI−PEG

Rf (Ω) 23.82 15.64 18.37

Rct (Ω) 47.64 480.7 29.89 293.4 21.31 232.6

i p = 2.69 × 105n3/2AD Li+1/2 C Li+ν1/2

(1)

where ip is the peak current (A) of the cyclic voltammetry test, n is the number of electrons transferred per reaction species (for Li+ insertion or extraction, n = 1), A is the contact area of the electrode (1.77 cm2), DLi+ (cm2 s−1) is the Li+ diffusion coefficient, CLi+ (mol cm−3) is the concentration of lithium ion (there is one lithium ion occupying an average unit cell, and Table S1 lists the unit cell parameter, CLi+ = n/Na/V), and ν (V s−1) is the scan rate. The linear fitting curves between the peak current (ip) and the square root of the scan rate (ν)1/2 is shown in Figure S3b,d, and the results are displayed in Table S2. It can be observed that the lithium-ion diffusion coefficient of NCM@ PANI−PEG was higher than that of pristine NCM. To further reveal the inherent origin of the improvement in the electrochemical performance of the surface-modified electrode, XRD detection of bare NCM and NCM@PANI−PEG after 100 cycles was carried out, as displayed in Figure S2. It can be seen

data of bare NCM, NCM@PANI, and NCM@PANI−PEG electrodes at various cycles. As observed from Table 2, the Rs data of all the electrodes remained small and relatively stable as the number of cycles increased because the electrolyte system tended to be stable when the electrodes were cycled several times. The Rf data for pristine NCM electrode was higher than that of NCM@ PANI and NCM@PANI−PEG electrodes, meaning that the SEI film regeneration was suppressed. The data of charge transfer resistance Rct for the bare NCM was increased drastically from 47.64 to 470.7 Ω after 100 cycles in accordance with its relatively poor electrochemical performance. As a contrast, the Rct data of NCM@PANI and NCM@PANI−PEG electrodes only increased from 29.89 to 293.4 and 21.31 to 212.6 Ω, respectively.

Figure 10. SEM images of (a, c) NCM and (b, d) NCM@PANI−PEG electrodes after 100 cycles. 18277


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ACS Applied Materials & Interfaces from Figure S2a that both samples had a certain degree of decline in the peak intensities after 100 cycles, indicating that the crystalline structures of both samples were sustained some degree of destruction. The fluctuation range of (003), which stood for the variation in c-axis,48 was enlarged in Figure S2b. Obviously, both (003) diffraction peaks were shifted toward a lower angle, meaning that the crystal structures of both samples suffered damage. Although the PANI−PEG coating layer cannot thoroughly protect the structure from collapse, it suppressed the shift of (003) peak in comparison with that of pristine NCM. Therefore, the conductive polymers coating layer was beneficial to alleviate the structural destruction of the host. Figures 10 and S4 exhibit the SEM and HRTEM images of bare NCM and NCM@PANI−PEG electrodes after 100 cycles, respectively. As displayed in Figure 10a,c, the morphology of bare NCM was destroyed with extensive cracks and rusty surface mainly owing to the direct contact between the electrode and the electrolyte. The electrolyte would penetrate into the bulk material along these cracks, promoting the structural degradation and more adverse reaction, leading to the deterioration of the electrochemical performance. In contrast, the surface-modified NCM retained well-defined spherical shape and a visible film on the host material was still observed. The cladding layer could protect the host materials from HF attacking, and the elasticity of polymers played a significant role in alleviating the volume contraction or expansion of the host material during cycling. As observed from Figure S4a, the surface of pristine NCM was covered with an insulative layer compared with Figure 4a, this was due to the accumulation of side-reaction byproducts, such as LiF and LixPOFy, which was disadvantageous for lithium-ion transfer. Furthermore, a large proportion of the disordered phase was also observed in Figure S4a, which was a great responsibility for the sharp drop in the capacity of pristine NCM. In contrast, the cycled NCM@PANI−PEG cathode materials still maintained a good layer structure and almost no visible disordered phase can be observed in Figure S4b, which was consistent with the superior electrochemical performance.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xianyue Qi: 0000-0002-8267-5794 Author Contributions †

Y.C. and X.Q. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Nature Science Foundation of China (Grant No. 51602352) and the Fundamental Research Funds for the Central Universities of Central South University (Grant No. 2018zzts424).

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REFERENCES

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4. CONCLUSIONS In summary, a simple strategy for the preparation of surfacemodified LiNi0.8Co0.1Mn0.1O2 (NCM) materials with PANI− PEG polymers was designed in this article. The surface-modified NCM materials displayed better cycle stability and lower transition metals dissolution amounts than that of pristine NCM, which, owing to the existence of polymers, can serve as a physical barrier to prevent the cathode materials from being eroded by HF. The elastic and flexible polymers played a significant role in alleviating the volume contraction or expansion of the host materials during cycling. Besides, the conductive polymers coating layer facilitated the transfer of electrons and lithium ions, enhancing the rate properties and reducing the electrochemical impedance. Therefore, the simple strategy of surface modification in the work presented here could lead to the extension to other high-performance cathode materials for lithium-ion batteries.

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fitting curves; HRTEM images; lattice parameters; data from cyclic voltammetry curves (PDF)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b02396. EIS plots and amplified curves; XRD patterns and magnified curves; cyclic voltammetry plots and linear 18278


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ACS Applied Materials & Interfaces Cycling Stability and Rate Capacity toward Li-Ion Batteries. ACS Appl. Mater. Interfaces 2018, 10, 5498−5510.

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Conductive Polymers Encapsulation To Enhance Electrochemical

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