Conductive Polymers Encapsulating to Enhance Electrochemical

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Conductive Polymers Encapsulating 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 ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02396 • Publication Date (Web): 07 May 2018 Downloaded from http://pubs.acs.org on May 7, 2018

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ACS Applied Materials & Interfaces

Conductive Polymers Encapsulating 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, Ke Du * School of Metallurgy and Environment, Central South University, Changsha City, 410083, China.

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 which desiderate to be settled such as cycling stability and rate properties as well as thermal stability. In this paper, the conductive polymers which integrates the excellent electronic conductivity of polyaniline (PANI) and the high ionic conductivity of polyethylene glycol (PEG) is designed for the surface modification of LiNi0.8Co0.1Mn0.1O2 cathode materials. Besides, the PANI-PEG polymers with elasticity and flexibility plays 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 1 ACS Paragon Plus Environment

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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-ion2 and potassium-ion.3 Layered Ni-rich (Ni content>0.6) cathode materials for lithium-ion batteries are by far the most potential to be applied to battery systems for electric vehicles owing to faster charge rate, high discharge specific capacity and low cost, etc.4-7 Nevertheless, there still exist some inherent questions desiderate to be settled for the Ni-rich cathode materials. Firstly, high content of residual alkali compounds on the surface of Ni-rich materials is a prominent problem in practical application.8-9 That’s because the residual lithium, such as Li2O/LiOH, on the surface of the Ni-rich materials will absorb H2O/CO2 and thereby form a LiOH/Li2CO3 layer, while 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. Secondly, 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 Thirdly, 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 2 ACS Paragon Plus Environment

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to enhance the electrochemical properties. 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

(PAS),23

poly

(3,

4-ethylenedioxythiophene) (PEDOT)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 since 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 electron conducting polymer, but not an ionic 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. Polyethylene glycol (PEG) is a good ionic conductor for the polymer as well as a superduper lithium salt solvent.32-33 In this paper, 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 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 3 ACS Paragon Plus Environment


electrochemical properties was shown below.

2. EXPERIMENTAL 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) with a molar ratio of 1: 1.04. The compound was preheated for 6 hours at 550 °C, and then sintered for 15 hours at 750°C under an O2 atmosphere. To prepare NCM@PANI-PEG composites, a wet-coating method was applied. Firstly, 1.5 g PANI (Doped by HCl, Jiaying, Shandong) and 1.5 g PEG (1500 Mw, Guoyao, Shanghai) polymers were dispersed in N-Methyl pyrrolidone (NMP) (AR, Kemiou, Tianjin) solvent. Then, 10 g NCM sample was immersed in the above dispersion, and kept stirring for an hour at 50 ℃. The acquired solution was filtered and then 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 was displayed in Scheme 1. 2.2. Material Characterizations. The phase identification of the samples was employed by X-ray diffraction (XRD, Rigaku TTRШ) with Cu Kα1 radiation under a scan speed of 10° min-1. The surface morphology of the as- synthesized materials were 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 spectrometer (EDX, JEOLJSM-6100LV). The crystal structure of the samples was performed by transmission electron microscopy (TEM, TECNAI G2 F20) and 4 ACS Paragon Plus Environment

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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 ℃ under a heating speed of 10 ℃ 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 half-cells were charged to 4.3 V and disassembled in a glove box 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 (ICP-OES, SPECTRO BLUE SOP). The method for determining the pH values of different samples is shown below: Adding 5 g of sample into the deionized water of 50 ml under continuous stirring for 30 min, and then allowed to stand for 30 min. Finally, the pH values of solutions were measured with a pH Meter (DELTA 320). 2.3. Electrochemical Evaluation. The cathodes were fabricated via pasting the mixture of the active materials, acetylene black and poly(vinylidene fluoride) (PVDF) binder (8:1:1 in weight). The mixture was dispersed in N-methylpyrrolidinone (NMP) solvent, and the obtained slurry was coated onto 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 glove box full of high pure Ar. A lithium foil was adopted as negative electrode, a Celgard2400 porous polypropylene film was introduced as separator and 1 M LiPF6 dissolved in ethyl methyl carbonate (EMC)/dimethyl carbonate (DMC)/ethylene carbonate (EC) (1:1:1 in volume) was employed as electrolyte. The mass loading of active materials was 4 mg and the amount of 5 ACS Paragon Plus Environment


electrolyte was 0.25 ml. The assembled half-cells were cycled between 2.8 and 4.3 V for the first five cycles at 0.2 C rate and followed by 1C rate for 100 cycles under 25 °C and 55 °C, respectively. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) tested were carried out on an electrochemical workstation (CHI660e, CH Instruments).

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 revealed that all of the diffraction patterns could be well matched with a layered hexagonal α-NaFeO2 structure (R3/m). As observed from the Figure 1, both of the (006) / (102) and (108) / (110) diffraction patterns were clear splitting, 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 owing to the amorphous state and low content of PANI-PEG polymers. Furthermore, no obvious differences of the diffraction peaks between the 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 consisted of small primary particles, which meant that the PNAI or PANI-PEG polymers coating layer didn't cause damage to the gross morphologies of the host material. As displayed in Figure 2(a, b), the primary particles of the 6 ACS Paragon Plus Environment

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bare NCM were clearly visible. In comparison with the pristine NCM powder, the surface of the NCM@PANI in Figure 2(c, d) showed a blurred edge, it can be conspicuously noticed in Figure 2(e, 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 means 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 the pristine NCM was uniformly coated with PANI-PEG polymers and was consistent with the SEM result. Figure 4 exhibited the TEM images of NCM, NCM@PANI and NCM@PANI-PEG materials. As exhibited in the 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 observed the homogeneous and amorphous films with the thickness of 25-30 and 30–35 nm, respectively. The electronic conductivities of all the samples were characterized by four-probe technique, and the electronic conductivity of the pristine NCM was tested to be 1.74×10−5 S•cm−1, which was 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 displayed 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 C=N stretching vibration of the quinoid ring in 7 ACS Paragon Plus Environment


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 -CH2- and the stretching vibration of OH- in PEG.38-39 This result demonstrated that LiNi0.8Co0.1Mn0.1O2 material was composited with 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 displayed 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 the pristine NCM particles was estimated to be about 3.37 wt%. In order to research the electrochemical performance of the cathodes, a galvanostatic charge-discharge measurement was employed. Figure 7a exhibited 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 0.2 C rate and 25 ℃. 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, which had a capacity retention of 83.4%, while the capacity retention of NCM@PANI and NCM@PANI-PEG cathodes were 89.6% and 93.4%, respectively. The improvement of cycle performance was 8 ACS Paragon Plus Environment

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owing to the existence of conductive polymer, which prevented the active materials from being eroded by the 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, which was because the elasticity of the PANI-PEG polymers involving flexible PEG played an important role in alleviating the volume contraction or 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@PANI-PEG was better than that of the bare NCM, especially at high rates. For instance, the discharge specific capacity of the pristine NCM was 139.4 mAh•g-1 under 10 C rate, whereas NCM@PANI and NCM@PANI-PEG electrodes delivered discharge specific capacities of 151.0 and 156.7 mAh•g-1 under the same rate, respectively. This was due to the PANI coating layer enhanced 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 diffusion pathway for lithium ions and thereby improved the rate properties. Figure 7d exhibited 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 of 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 9 ACS Paragon Plus Environment


temperature of 25 °C in the discharging-charging process due to 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 electrode and electrolyte during cycling, which had a negative influence on the electrochemical performance of the host materials. The side reactions as shown below:8, 44 LiPF6→PF5+LiF, PF5+H2O→POF3+HF, POF3+Li2O→LixPOFy+LiF, Li2O/LiOH+HF→H2O+2LiF. As displayed in Figure 7d, the surface-modified samples have superior capacity retention than that of the bare NCM during 100 cycles at 1C (81.4, 77.1 vs. 53.6%). This may be attributed to that the conductive polymer film as a protective layer to protect the bulk materials from HF corroding and suppress the side reactions. As a result, the pristine NCM that was surface treated with the conductive polymer displayed better cycle performance at 55 °C. The dissolved amounts of transition metals were measured to further research the reasons for the enhancement of cycle properties at 55 °C. It was observed from Figure 8 that the dissolved amounts of transition metals from all the charged NCM, NCM@PANI and NCM@PANI-PEG electrodes into electrolyte were continuously increased with time. The dissolved transition metals were 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 SEI film regeneration and the lithium consumption, thereby drove a reduction of reversible capacity, especially at the elevated temperature.45-46 As displayed in Figure 8, the dissolved transition metals from the surface-modified electrodes were all lower than the bare electrodes 10 ACS Paragon Plus Environment

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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 wherefore suppressed the transition metals dissolution, resulting in enhancing the cycle performance of the host materials at the 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 due to PEG was present as a kind of Lewis base in the PEG-PANI polymers while the PF5 produced by thermal dissociation of LiPF6 belonged to the Lewis acid. Therefore, PEG can complex with PF5 availably and so as to prevent the PF5 from being hydrolyzed to generate HF. Abraham47 found that the Lewis basic additives made the thermal stability of organic electrolyte significantly improved. It was necessary to point out that the amount of electrolyte in Figure 8 was higher than Figure 7, which led to the dissolved amounts of transition metals were bigger than data in Figure 7. To further explore the inherent origin for the improvement of electrochemical properties, EIS analysis was applied. Figure 9 exhibited the EIS plots of the 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 cycle was consisted of a depressed semicircle and a slope, meanwhile the plots of electrodes after 100 cycles was consisted of two semicircles and a slope as shown in Figure 9b and Figure S1. Here, Rs represented the electrolyte resistance corresponding to the highest frequency. Rf was 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 electrode and electrolyte, and the slope corresponding to the low frequency was ascribed to the Li-ion diffusion in the bulk materials (Wo, Warburg 11 ACS Paragon Plus Environment


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impedance). Table 2 exhibited the EIS data of the bare NCM, NCM@PANI and NCM@PANI-PEG electrodes at various cycles. As observed from Table 2, Rs data of all the electrodes were small and relatively stable as the number of cycles increased, this was due to electrolyte system tended to be stable when the electrodes were cycled several times. The Rf data for the 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 Ω and 21.31 to 212.6 Ω, respectively. It was observed that the Rct data of all the electrodes increased with the increase of the cycle numbers, indicating more side reactions produced at the electrode/electrolyte interface. However, Rct data of the pristine NCM was much higher than that of the surface-modified electrodes, which suggested 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 electrolyte, thereby improved the conductivity of NCM and alleviated the severe side reactions with electrolyte. In order 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, 0.8 mV• s-1) under 2.8-4.3V were shown in Figure S3. The calculation method was described below. ip = 2.69×105 n3/2ADLi+1/2CLi+v1/2

(1)

where ip is the peak current (A) of the cyclic voltammetry test, n is the number of electrons 12 ACS Paragon Plus Environment

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transfer 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 in an average unit cell, and Table S1 lists the unit cell parameter, CLi+=n/Na/V ), v (V•s-1) is the scan rate. The linear fitting curves between the peak current (ip) and the square root of the scan rate (v)

1/2

was

shown in Figure S3 (b, d), and the results were displayed in Table S2. It can be observed that the lithium ion diffusion coefficient of NCM@PANI-PEG was higher than that of the pristine NCM. To further reveal the inherent origin of the improvement for the electrochemical performance of surface-modified electrode, XRD detection of the bare NCM and NCM@ PANI-PEG after 100 cycles was carried out, as displayed in Figure S2. It can be seen from Figure S2 (a) 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 of c axis,48 were enlarged in Figure S2 (b). Obviously, both of the (003) diffraction peaks were shifted towards a lower angle, which meant that the crystal structures of both samples have been suffered damage. Although PANI-PEG coating layer cannot thoroughly protect the structure from collapse, it suppressed the shift of (003) peak in comparison with that of the pristine NCM. Therefore, the conductive polymers coating layer was beneficial to alleviate structural destruction the host structure. Figure 10 and Figure S4 exhibited the SEM and HRTEM images of the bare NCM and NCM@PANI-PEG electrodes after100 cycles. As displayed in Figure 10 (a, c), the 13 ACS Paragon Plus Environment


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morphology of the bare NCM was destroyed with extensive cracks and rusty surface mainly owing to the direct contact between electrode and electrolyte. The electrolyte would penetrate into the bulk material along these cracks, promoting the structural degradation and more adverse reaction came about, which led to the deterioration of 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 S4 (a), the surface of the pristine NCM was cover 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 S4 (a), which was a great responsibility for the sharp drop in the capacity of the pristine NCM. In contrast, the cycled NCM@PANI-PEG cathode materials still maintained good layer structure and almost no visible disordered phase can be observed in Figure S4 (b), which was consistent with the superior electrochemical performance.

4. CONCLUSIONS In

summary,

a

simple

strategy

for

the

preparation

of

surface-modified

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 the pristine NCM, which owing to the existence of 14 ACS Paragon Plus Environment

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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 rate properties and reducing 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.

Scheme 1. Schematic illustration of the synthesis of NCM@PANI-PEG composites.

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


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



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.


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



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


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Figure 6. TGA plot of NCM@PANI-PEG powders under a heating rate of 10 ℃/min with air condition.



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.


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Figure 8. The dissolved amounts of transition metals of (a) Ni, (b) Co, and (c) Mn under different storage time at 55 ℃.



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

Figure 10. SEM images of (a, c) NCM and (b, d) NCM@PANI-PEG electrodes after100 cycles.


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Table 1. pH values of NCM, NCM@PANI and NCM@PANI-PEG Powders. Samples

pH value

Pristine NCM

11.94

NCM@PANI

11.47

NCM@PANI-PEG

11.28

Table 2. The EIS Data of NCM, NCM@PANI and NCM@PANI-PEG Electrodes. Samples Pristine NCM

NCM@PANI

NCM@PANI-PEG

Cycle

Rs (Ω)

Rf (Ω)

Rct (Ω)

0

3.437

--

47.64

100

4.635

23.82

480.7

0

3.044

--

29.89

100

3.661

15.64

293.4

0

2.816

--

21.31

100

3.394

18.37

232.6

Table of Contents (TOC) graphic



AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] Author Contributions ‡These authors contributed equally to this work. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the Nature Science Foundation of China (Grant No. 51602352). Supporting Information EIS plots and amplified curves; XRD patterns and magnified curves; Cyclic voltammetry plots and linear fitting curves; HRTEM images; Lattice parameters; Data from cyclic voltammetry curves.

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