Enhancing the Electrochemical Performance of Ni-Rich Layered Oxide

Apr 17, 2019 - To further explore the inherent origin of the improvement of lithium ion/electron properties, the EIS measurement was performed. Figure...
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Enhancing the Electrochemical Performance of Ni-Rich Layered Oxide Cathodes by Combination of the Gradient Doping and DualConductive Layers Coating Qiwen Ran,*,† Hongyuan Zhao,*,†,‡ Xiaohui Shu,† Youzuo Hu,† Shuai Hao,† Qianqian Shen,§ Wei Liu,∥ Jintao Liu,† Meiling Zhang,† Hao Li,† and Xingquan Liu*,†

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†

R&D Center for New Energy Materials and Integrated Energy Devices, School of Materials and Energy, University of Electronic Science and Technology of China, Chengdu 610054, P. R. China ‡ Research Laboratory for Advanced Materials and Electrochemical Technology, Henan Institute of Science and Technology, Xinxiang 453003, P. R. China § College of Materials Science and Engineering and ∥Department of Prosthodontics, State Key Laboratory of Oral Diseases, West China Hospital of Stomatology, Sichuan University, Chengdu 610065, P. R. China S Supporting Information *

ABSTRACT: Currently, there is an urgent demand for Ni-rich cathode materials with excellent electrochemical properties under harsh conditions; however, obtaining such materials is very challenging. Here, we propose an innovative modification strategy that combines gradient phosphate polyanion doping and dual-conductive layer (Li3PO4-PANI) coating. The phosphate polyanion gradient doping can be described as acting in a “support role” to optimize the crystal structure. Moreover, the dual-conductive (Li3PO4-PANI) layers can be described as acting in a “palisade role” to inhibit side reactions and enhance the ionic/electronic conductivity of the NCM cathode. For the NCM cathode, this strategy synergistically achieves three main objectives: enhancement of structure stability, improvement of the ionic/electronic conductivity of the interface, and reduction of residual lithium salts. The modified NCM cathode delivers superior cycling stability, with 81.4% capacity retention after 100 cycles (4.5 V/55 °C), whereas the original NCM shows only a quite low capacity retention (57.7%). Moreover, this strategy also significantly improves the rate performance of the NCM cathode. These results indicate that this innovative modification strategy can be utilized to enhance the electrochemical performance of the NCM cathode at 4.5 V and 55 °C. KEYWORDS: gradient doping, dual-conductive layer, LiNi0.6Co0.2Mn0.2O2, cycling stability, rate performance

1. INTRODUCTION

NCM crystal structure will deteriorate to the spinel phase and then convert into to the rock salt phase during the lithiation process.3,9,14 More importantly, this phase transformation leads to asymmetric contraction and expansion of the cell (anisotropy), leading to reversible capacity fading.15 Furthermore, it should also be noted that the side reactions between the electrolyte and the active substance are another factor that gives rise to a considerable degradation in the electrochemical properties of the NCM cathode.16−18 Furthermore, a large amount of residual lithium salts not only increases the electrochemical impedance but also accelerates the side reactions.19 All of the above-mentioned problems become particularly severe for operation at 4.5 V and 55 °C.20,21 Therefore, it is urgent and practical to develop innovative and

Because of the widespread application of electric energy, rechargeable lithium-ion batteries (LIBs) have been widely used in modern society as energy storage devices due to their high energy density and high power.1−4 Among a large number of candidate materials, layered LiNi0.6Co0.2Mn0.2O2 (NCM) has attracted intense attention as a cathode material because of its well-balanced properties of battery discharge capacity and low cost.5,6 However, with the increasing requirements for improved energy density, the poor electrochemical performance of the NCM cathode at a high cutoff voltage and high temperature limits its further application for EVs.7,8 In addition, unsatisfactory high temperature performance also limits its suitability for the operation of new energy vehicles (EVs) in tropical climates.9,10 Previous studies have shown that the structural degradation and side reactions are two important factors that give rise to the fast fading of the discharge capacity of the NCM cathode.11−13 Generally, the layered phase of the © XXXX American Chemical Society

Received: December 4, 2018 Accepted: April 17, 2019 Published: April 17, 2019 A

DOI: 10.1021/acsaem.8b02112 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Figure 1. Schematic illustration of the preparation process of the P-NCM@Li3PO4-PANI sample.

many conductive polymers, polyaniline (PANI) is notable for its high electronic conduction, easy preparation, and cheap raw materials.21 Nevertheless, a single polyanion gradient doping or a single conducting polymer can hardly meet the requirements of crystal structural stability, ionic conductivity, and electrical mobility at the same time.19,35 On the basis of the above analysis, it is necessary to exploit a novel modification strategy to combine the advantages of the previously used modification methods such as bulk doping, fast ionic conductor coating, and fast electronic conductor coating to further meet the demands for the electrochemical properties of the Ni-rich cathode (4.5 V/55 °C). To date, little research has been done in this area, inspiring us to combine the polyanion gradient doping and Li3PO4-PANI coating to further promote the electrochemical properties of the NCM cathode. Currently, this strategy has been largely unexplored, motivating us to apply a polyanion gradient doping combined with Li3PO4-PANI coating to the NCM cathode. In this work, we proposed a novel modification strategy combining gradient phosphate polyanion doping and PANI coating to modify the NCM cathode by a solid-state reaction and wet-coating method. The phosphate polyanion gradient introduces the advantages of internal structure enhancement and surface Li3PO4 coating layer. The PANI coating layer can further enhance the electronic conductivity at the material’s interface. Moreover, the dual-conductive (Li3PO4-PANI) layers can be described as playing a “palisade role” to inhibit the side reactions and promote the ionic/electronic conductivity of the NCM cathode. The objective of this work is to enhance the cycling performance and rate performance of Nirich cathode at 4.5 V and 55 °C.

practical methods for the modification of NCM cathodes to substantially enhance their electrochemical performance at 4.5 V and 55 °C. To date, many experimental strategies for controlling the electrochemical performance of Ni-rich cathode materials such as bulk phase doping and surface modification/coating have been investigated.5,7,8,22,23 Bulk phase doping has attracted extensive attention as an effective modification method for the optimization of the crystal structure stability by introducing other cations such as Na+,24 Mg2+,25 and Al3+.18 Moreover, surface modification/coating strategies have been widely demonstrated to be effective for suppressing the side reactions at the interface, and the corresponding coating materials mainly include metal phosphates,26,27 metal oxides,15,28 fast ion conductors,29 and conducting polymers.21 Researchers have recognized that the strategy of coating with fast ionic conductor/fast electronic conductors can improve the ion/ electron transport properties at the interface.30 Unfortunately, both bulk doping and surface coating can only regulate the electrochemical performance in a single aspect (structure stability or side reactions).31−33 Therefore, it is essential to exploit an efficient modification method that can simultaneously promote the stability of the crystal structure and ionic/ electronic conductivity of Ni-rich cathode materials. Large tetrahedral PO43− polyanions are typically used to stabilized the local structure for achieving the stable energy density of the cathode.34 Zhang et al. reported that the introduction of large tetrahedral PO43− polyanions into the Lirich layered oxide stabilizes the structure of the material.34 Following PO43− polyanions doping, the modified material shows improved electrochemical performance during long cycling. Lee et al.27,51 obtained a nanoscale layer of Li3PO4coated LiNi0.6Co0.2Mn0.2O2 cathode material by the citric acidassisted sol−gel method, realizing a substantial improvement in the electrochemical performance of the LiNi0.6Co0.2Mn0.2O2 cathode material. The novel strategy of gradient doping can synergistically realize surface coating and bulk doping.9,14 Thus, the electrochemical properties of NCM cathode materials can be improved by gradient doping.9,34 Among

2. EXPERIMENTAL SECTION 2.1. Preparation of Materials. The specific details of the LiNi0.6Co0.2Mn0.2O2 (NCM) and LiNi0.6Co0.2Mn0.2O2−x(PO4)x@ Li3PO4 (P-NCM) synthesis process are described in our previous reports.52 To synthesize the LiNi0.6Co0.2Mn0.2O2−x(PO4)x@Li3PO4PANI (P-NCM@Li3PO4-PANI) sample, 0.5 g of commercial PANI (Jiaying, Shandong) and 5 g of P-NCM sample were dispersed in NB

DOI: 10.1021/acsaem.8b02112 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Figure 2. (a) XRD patterns of the as-synthesized samples. Rietveld refinement of XRD patterns for (b) NCM, (c) P-NCM, and (d) P-NCM@ Li3PO4-PANI. (e) Gradient phosphate polyanion doping schematic diagram and structure model for (f) pristine NCM and (g) P-NCM@Li3PO4PANI. methylpyrrolidone (NMP) solvent. Then, the solution was continuously stirred at 50 °C for 2 h and fully dried in a vacuum at 105 °C overnight. 2.2. Characterization of Materials. X-ray diffraction (XRD) measurements were performed for phase identification using a Bruker DX-1000 diffractometer (Cu Kα radiation, 1.54198 Å, 2θ range 10°− 80°). The Rietveld refinement data were acquired using the GSAS package. Fourier transform infrared spectroscopy (FT-IR, 400− 2000 cm−1, 8400S) was used to identify the polymer and phosphate polyanions. Thermogravimetric analysis (TGA, TG209F1, 10 °C min−1) was conducted in air. Field emission scanning electron microscopy (FESEM, FEI INSPECT-F) and transmission electron microscopy (TEM, Tecnai G2-F20) were used to observe the morphological features of the prepared samples. The surface chemical element state and distribution of the prepared samples were characterized by X-ray photoelectron spectroscopy (XPS, ULVACPHL, Inc., Al Kα radiation) and energy dispersive spectrometry (EDS, INCA Energy 300) mapping measurements. Inductively coupled plasma mass spectrometry (ICP-MS) was used to determine the transition metal ions content (Ni, Co, and Mn) in the organic electrolyte. PHS-3C was performed to obtain the pH of the prepared samples. 2.3. Electrochemical Evaluation. To evaluate the electrochemical performance characteristics of the prepared samples, 2025 type coin cells were assembled in an argon-filled glovebox. The positive electrodes were fabricated by pasting the mixture of the active materials, acetylene black, and poly(vinylidene fluoride) (PVDF) binder (8:1:1 in weight) in NMP solvent. Then, the obtained slurry was coated onto an Al foil and fully dried at 105 °C overnight under vacuum. The corresponding active materials with the mass loading of ≈4 mg cm−2 were punched into 1.27 cm−2 disks. A carbonate-based electrolyte (1 M LiPF6/EC+DMC) was used as the organic electrolyte. The cycling performance and galvanostatic discharge−

charge thermodynamic equilibrium potential (GITT) measurements were performed using a testing system (CT2001A, Land, 1 C = 180 mA g−1). The cyclic voltammetry (CV, 0.1 mV s−1) curves and electrochemical impedance spectroscopy (EIS, 100 kHz−0.01 Hz) data were obtained by using an electrochemical workstation (model CHI660E).

3. RESULTS AND DISCUSSION A schematic illustration for the preparation of P-NCM@ Li3PO4-PANI sample is shown in Figure 1. In the first process, some of the PO43− ions were gradient doped into the crystal lattice, while the rest of the PO43− ions were fixed on the surface and reacted with lithium residue salts to obtain Li3PO4.19 The reaction between NH4H2PO4 and lithium residues is described by eq 1:27,34 NH4H 2PO4 + 3LiOH/Li 2CO3 = Li3PO4 + NH3 ↑+ H 2O/CO2 ↑

(1)

However, it difficult to form a continuous and uniform Li3PO4 coating layer due to factors such as the uneven dispersion of the residual lithium salts and the small coating amount. In the second process, PANI acting as the conducting polymer is either coated on the Li3PO4 layer or directly coated on the NCM cathode surface to further isolate the contact between the active material and electrolyte. In Figure 2a, the XRD patterns of the as-synthesized powders are indexed to the layered α-NaFeO2 structure (R3m).36,37 It is observed that both (006)/(102) and (108)/ (110) diffraction peaks are distinctly split, indicating that all of these samples developed a well-ordered crystal structure C

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peaks of C−H and −CH2− bonds for the P-NCM@Li3PO4PANI sample are located at 1135 and 1480 cm −1 , respectively.19 These typical peaks correspond well to some characteristic peaks of the FT-IR spectrum of PANI. This result confirms that PANI polymer was successfully coated on the surface of the P-NCM@Li3PO4-PANI material. In addition, TGA measurements were performed to determine the PANI polymer content. Figure S2 shows that the weight content of PANI was estimated to be ∼1.87 wt %. Figure 4 shows the XPS images of the pristine NCM and PNCM@Li3PO4-PANI powders. According to Figure 4a, there was no obvious difference in the Ni 2p spectra. The oxidation state of Ni is a mixture of Ni2+ and Ni3+, which is consistent with previous reports.1,2,33 After surface modification, more Ni3+ appeared in the Ni 2p1/2 split peaks, which is believed to be conducive to the improvement in the structural stability of the NCM cathode. For the C 1s spectrum shown in Figure 4b, two peaks arise due to the hydrocarbon pollutants in the chamber and the carbonate residues in the synthesis process.9 Clearly, the content of carbonate on the surface of the PNCM@Li3PO4-PANI powder is lower than that of the pristine NCM powder to some extent. This result further proves that lithium residues are inhibited effectively and their content is reduced.11 In addition, the P 2p and N 1s peaks are located at 134.0 and 399.7 eV, respectively.19 On the basis of these analyses, it is inferred that Li3PO4 and PANI were successfully coated on the surface of P-NCM@Li3PO4-PANI powder. The elemental (Ni, Co, Mn, P, and N) distributions in the P-NCM@Li3PO4-PANI sample were also investigated by EDS mapping. As shown in Figure S3, the element (P and N) distributions are all fairly uniform on the surface of P-NCM@ Li3PO4-PANI powder, illustrating that the Li3PO4-PANI composite layers were successfully coated on the NCM cathode, which is consistent with the results of the FT-IR and XPS analyses.42,43 At the same time, the pH values of PNCM and P-NCM@Li3PO4-PANI powders are relatively low (Table S2), which agrees with the FT-IR and XPS analyses.19,35 Therefore, the interfacial properties of the NCM cathode have been optimized effectively by reducing the pH. According to the previous reports, lithium residue salts can increase the surface alkalinity of Ni-rich cathode materials and induce a series of undesirable side reactions.7,8 It is expected that the modified NCM cathode may exhibit excellent electrochemical properties. FESEM images reveal the surface morphologies of all of the prepared samples, as shown in Figure S4. All of the prepared powders are typically spherical (5−6 μm), demonstrating that the gradient doping and PANI coating layer caused little damage to the secondary particles of the NCM cathode. Moreover, the particles of the NCM sample have rough surfaces and a large amount of residual lithium salts (Figure S4b).4,39 In contrast, the surfaces of P-NCM and P-NCM@ Li3PO4-PANI are extremely smooth, implying that formation of lithium residue salts was greatly inhibited (Figure S4c,d). Notably, the surface of the P-NCM@Li3PO4-PANI sample is encapsulated with a thin and continuous film, corresponding to the PANI polymer. To confirm the presence of the coating layers on the surface of P-NCM@Li3PO4-PANI samples, HRTEM and SAED measurements were performed. Figure 5 shows the HRTEM and SAED results obtained for all of the prepared powders. According to Figure 5a, the pristine NCM has a smooth surface without any coating layer. In contrast, a clear coating

(Figure 2f).38 In addition, no impurity peaks are found in the XRD patterns of the P-NCM@Li3PO4-PANI material. This phenomenon can be summarized as follows: (1) some of the phosphate atoms have entered the lattice structure; (2) because of the lack of XRD sensitivity, the low content of Li3PO4 cannot be observed by XRD.33,39 This means that the gradient doping and surface modification have little effect on the crystal structure.9,40 Rietveld refinement was used to obtain the lattice parameters (Figure 2b−d). The values of Rwp and Rp are low, which show that the results are reliable (Table S1).41 The enlarged c lattice parameter and unit volume demonstrate the expansion of the layered structure (Figure 2f,g).34,52 According to previous reports, for layered materials, the value of the c lattice parameter is crucial for the diffusion of lithium ions.34,24 Thus, the increase in the c value strongly contributes to the diffusion of lithium ions. We note that the widened Li+ diffusion channel is critical for the extraction and intercalation of Li+.24 Moreover, the increase in the cell volume is also beneficial for the improvement of the electrochemical properties of the NCM cathode. This is mainly because the ionic radius of O2− anions (140 nm) is smaller than that of PO43− polyanions (thermochemical radius of 238 nm). This result is consistent with the previous reports (Figure 2g).8,34 Because of its strong covalent P−O bonds and excellent structural stability, the gradient doped phosphate polyanions can realize the stabilization of the oxygen layer of the NCM cathode.9,34 In addition, the Ar ion depth etching profile measurement for P-NCM@Li3PO4-PANI sample was performed to analyze the atomic concentration of the elements. As shown in Figure S1, the concentration of P decreases with increasing depth, indicating that P atoms are successfully introduced into the NCM cathode in the form of a gradient distribution.35,52 To verify the presence of the PO43− polyanions and PANI polymer, the P-NCM@Li3PO4-PANI powder was examined by FT-IR spectroscopy. As displayed in Figure 3, the FT-IR spectra of the P-NCM and P-NCM@Li3PO4-PANI samples are different from that of the NCM sample. The peaks related to the asymmetric stretching P−O vibrations in PO43− ions are located at 1035.5 and 1085.7 cm−1, respectively.2,34 The typical

Figure 3. FT-IR spectra of all of the prepared samples. D

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Figure 4. XPS spectra of (a) Ni, (b) C, (c) N, and (d) P for the NCM and P-NCM@Li3PO4-PANI samples.

Figure 5. TEM images of (a) pristine NCM, (b) P-NCM, and (c) P-NCM@Li3PO4-PANI samples. HRTEM of (d) NCM and (e) Li3PO4 layer. SAED images of (f) PANI coating layer, (g) Li3PO4 coating layer, and (h) NCM.

prefers the amorphous state (Figure 5f).21 On the other hand, the SAED image of the inner coating layer shows a large number of spots in an irregular pattern, suggesting that the Li3PO4 coating layer is more inclined to be crystalline (Figure 5g).21 According to the SAED analysis, the parent material exhibits a typical single-crystal structure (Figure 5h).2 On the basis of the results of the TEM and SAED analyses, we can draw an accurate conclusion that the NCM cathode is covered by dual-conductive layers (Li3PO4-PANI) that not only protect the NCM cathode from corrosion by organic electrolyte but also improve the ionic/electronic conductivity of the NCM cathode. To better prove the positive effect of the PANI conductive polymer on NCM conductivity, the four-probe

layer can be clearly observed for the P-NCM sample (Figure 5b). For the P-NCM@Li3PO4-PANI sample (Figure 5c), a Li3PO4 nanolayer partially covered the surface of NCM cathode, and then PANI electronic conductive layer was either coated on the Li3PO4 layer or directly coated on the NCM cathode surface. Moreover, the lattice fringes of the pristine NCM have an interplanar distance of 0.478 nm, corresponding to the (003) planes of the NCM cathode materials (Figure 5d).13,14 According to Figure 5e, the Li3PO4 layer shows ambiguous lattice fringes that have an interplanar distance of 0.355 nm corresponding to the (011) planes of Li3PO4. In addition, the SAED result corresponding to the PANI layer exhibits few spot patterns, indicating that the PANI layer E

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Figure 6. (a) Initial charge/discharge profiles of the as-synthesized powders (0.2 C, 2.7−4.5 V). (b) Comparison of rate performance (2.7−4.5 V). (c) Cycling performance at 0.5 C and 25 °C.

Figure 7. (a) Cycling performance at 0.5 C and 55 °C over 2.7−4.5 V. (b) Cycling performance at 5 C and 25 °C over 2.7−4.5 V.

To study the electrochemical properties of the assynthesized samples, galvanostatic charge−discharge measurement were performed in the range of 2.7−4.5 V vs Li+/Li. Figure 6a shows the initial discharge−charge curves of all of the samples. All of these samples have similar charging curves

technique was also used to characterize the electronic conductivity of all of the prepared powders. It was found that the electronic conductivity values for the pristine NCM, PNCM, and P-NCM@Li3PO4-PANI samples are 1.53 × 10−5, 2.1 × 10−5, and 4.7 × 10−4 S cm−1, respectively. F

DOI: 10.1021/acsaem.8b02112 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials and same voltage plateaus, confirming that the electrochemical process of the NCM cathode is not affected by the comodification of gradient doping and surface coating.37,44 We note that the Coulombic efficiencies of the pristine NCM, P-NCM, and P-NCM@Li3PO4-PANI samples are 81.4%, 75.6%, and 77.1%, respectively. This result can be attributed to following two aspects: (i) the relative content of the active material decreases, resulting in the reduction in the Coulombic efficiency, and (2) the promotion effect of Li3PO4-PANI on the transmission of ions/electrons at the electrode/electrolyte interface inhibits the reduction of Coulombic efficiency to some extent.38,45 As shown in Figure 6b, the rate performance of P-NCM@Li3PO4-PANI is better than that of pristine NCM and P-NCM. This is because the Li3PO4 layer promotes the diffusion of lithium ions, and the PANI layer enhances the charge transfer, so Li3PO4-PANI layers synergistically improve the rate performance of the NCM cathode.46 In particular, the discharge capacity of pristine NCM decreases rapidly from 175.7 to 119.8 mAh g−1 after 100 cycles at 0.5 C/25 °C with low capacity retention of 68.1%, as shown in Figure 6c. Such poor cycling performance of NCM is attributed primarily to the decrease in the structural stability and severe side reactions at the electrode/electrolyte interface.47 In contrast, the PNCM and P-NCM@Li3PO4-PANI samples show better cycling performance, with the capacity retention values of 76.6% and 86.6%, respectively (Figure 6c). The largest improvement in cycling performance of the P-NCM@ Li3PO4-PANI powder is due mainly to the enhancement in the stability of oxygen layers by gradient phosphate polyanion doping and the promotion of ionic/electronic conductivity by the dual-conductive layers (Li3PO4-PANI) coating.19 In addition, the introduction of dual-conductive layers can reduce the amount of residual lithium salts on the surface of NCM cathode, leading to the suppression of side reactions at the electrode/electrolyte interface to some extent.21,48 To further investigate the improvement in cycling stability, we cycled the cells with pristine NCM and P-NCM@Li3PO4PANI at high temperature (55 °C/0.5 C) and large current (25 °C/5 C), respectively. As shown in Figure 7a, the discharge capacity of pristine NCM fades rapidly from 171.5 to 99.1 mA g−1 after 100 cycles at 55 °C/0.5 C, and the capacity retention is only 57.7%. By contrast, the discharge capacity retention of P-NCM@Li3PO4-PANI powder is 81.4% after 100 cycles at 55 °C/0.5 C. This result indicates that the comodification of lattice stability and interface performance can greatly improve the cycling stability of the NCM cathode at high temperature. Moreover, the cycling performance of the NCM cathode at high current (5 C) was also significantly optimized (Figure 7b). This is attributed mainly to the effective modification of the interface properties such as ionic conductivity/electronic conductivity by the dual-conductive layers.27 According to the Figure 8a,b, the surface morphology of the NCM sample was greatly damaged after 100 cycles at 55 °C/ 0.5 C. However, the P-NCM@Li3PO4-PANI sample remained highly spherical after 100 cycles at 55 °C/0.5 C (Figure 8c,d). Previous reports in the literature have pointed out that the volume change caused by the changes in the lattice parameters (a/c) during the charging/discharging process is an important factor for the decline in the electrochemical properties, particularly at (4.5 V/55 °C).34 Furthermore, the breakdown of secondary particles generates continuous side reactions of the active substances, and the dispersed fragments gradually lose their electrochemical activity.13

Figure 8. FESEM images of electrode plates before cycles (a) NCM and (c) P-NCM@Li3PO4-PANI as well as electrode plates after 100 cycles at 55 °C/0.5 C (b) pristine NCM and (d) P-NCM@Li3PO4PANI.

To provide further evidence for the effective inhibition of side reactions by the dual-conductive layers (Li3PO4-PANI), a reasonable ICP-MS method was used to study the transition metal ions (Ni, Co, and Mn) in the electrolyte after 100 cycles (55 °C/0.5 C). Generally, the LiPF6-based electrolyte is unstable at (4.5 V/55 °C) and may decompose with the accompanying generation of HF as shown below:5,7,19 LiPF6 → PF5 + LiF↓

(2)

H 2O + PF5 → POF3 + 2HF

(3)

−

Li 2O + POF3 → LixPOFy + LiF↓

(4)

Li 2O(LiOH) + 2HF → H 2O + 2LiF↓

(5)

HF + MOx → MFx + H 2O

(6)

According to the chemical equations (2−6), residual lithium salts accelerate the hydrolysis of LiPF6 and produce more LixPOFy-type compounds such as LiF, greatly reducing the interfacial activity and increasing the electrochemical impedance.49 More importantly, the transition metal ions (Ni, Co, and Mn) will be subjected to chemical attack from HF, leading to a reduction in chemical reversibility of the material.3,9 It is clear from Figure S5 that the content of transition metals (Ni, Co, and Mn) in the organic electrolyte of the P-NCM@ Li3PO4-PANI sample is significantly lower than that of the NCM sample. This analysis clearly confirms that the dualconductive layers (Li3PO4-PANI) effectively reduce the amount of residual lithium salts and inhibits the side reactions to some extent, resulting in the distinct improvement of interface stability and cycling stability.30 Therefore, ICP analysis and FESEM images further demonstrate that the dual-conductive layers (Li3PO4-PANI) effectively suppress the side reactions, while the gradient doping greatly enhances the lattice stability. GITT is a useful method for determining the electrochemical polarization based on chronopotentiometry at nearly thermodynamic equilibrium conditions.33,35 The GITT analysis demonstrates that the electrochemical polarization of PNCM@Li3PO4-PANI electrode is much smaller than that of G

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Figure 9. (a) Nyquist plots after 100 cycles at 0.5 C/25 °C. (b) Z′−ω−0.5 plots and linear fitting of low-frequency regions obtained from EIS.

the NCM electrode (Figure S6a,b). In addition, Figure S6 depicts the CV curves at a scan rate of 0.1 mV s−1 over the range of 2.7−4.5 V. All of the three sample indicate the presence of a redox couple of extraction and insertion of lithium ions.31,50 The initial oxidation peak and reduction peak of NCM sample are located at 3.993 and 3.641 V, respectively (ΔE1 = 0.352 V), while ΔE2 and ΔE3 are 0.261 and 0.190 V (Figure S6c). By contrast, the P-NCM@Li3PO4-PANI sample (Figure S6e) shows the smallest polarization, and the corresponding ΔE1, ΔE2, and ΔE3 are 0.215, 0.173, and 0.171 V, respectively.39 Moreover, it should be noted that the P-NCM@Li3PO4-PANI shows the optimal electrochemical reversibility.12 All of these findings prove that a combination of gradient doping and dual-conductive layers (Li3PO4-PANI) coating is an effective strategy for reducing the electrochemical polarization of electrode materials. In fact, gradient doping can contribute to the improvement in lattice stability and the dualconductive layers (Li3PO4-PANI) can synergistically promote the ionic/electronic transport of the NCM cathode. Therefore, the small electrochemical polarization of P-NCM@Li3PO4PANI is justifiable. To further explore the inherent origin of the improvement of lithium ion/electron properties, the EIS measurement was performed. Figure 9a shows the EIS plots of the pristine NCM, P-NCM, and P-NCM@Li3PO4-PANI electrodes after 100 cycles at 0.5 C/25 °C. As shown in Figure 9a, the Nyquist plots of all of the powders consist of two semicircles and a slope. Rs is ascribed to the electrolyte resistance,11 Rsf is the contribution of the solid electrolyte interface resistance,14 and Rct is the charge transfer resistance in the high-frequency region.15 The Warburg impedance (W0) is located in the low frequency region and is related to the solid diffusion of Li ions.42 Table 1 shows the fitting results of Rs, Rsf, and Rct for all of the electrodes. The Rs values of all of the samples are small and have similar values. The Rsf value of pristine NCM (60.95 Ω) is higher than that of P-NCM (59.76 Ω) and P-NCM@ Li3PO4-PANI (7.65 Ω), demonstrating that the SEI film regeneration is suppressed. We note that the Rct value of pristine NCM sample is 462.75 Ω, while the corresponding values of the P-NCM and P-NCM@Li3PO4-PANI samples are 113.1 and 52.35 Ω, respectively. These results can be attributed to the fact that the dual-conductive layers increase the ionic conductivity and the electronic conductively to some extent, promoting interface reactivity and hindering the side reactions between the electrolyte and the electrode material.

Table 1. Impedance Data for Pristine NCM, P-NCM, and PNCM@Li3PO4-PANI Samples after 100 Cycles at 0.5 C/25 °C samples

Rs (Ω)

fitting errors (%)

Rsf (Ω)

fitting errors (%)

Rct (Ω)

fitting errors (%)

pristine NCM P-NCM P-NCM@ Li3PO4PANI

5.458

0.6257

60.95

0.3959

462.75

0.6989

9.744 3.293

0.5528 1.0589

56.92 7.65

0.4765 0.9653

113.1 52.35

0.7825 0.9892

To determine the kinetics of Li+ intercalation/deintercalation in the electrode, the EIS plots of three electrodes after 100 cycles at 0.5 C/25 °C were obtained. The straight line in the low frequency region is related to solid-state diffusion of Li+ in the bulk of the electrode. Thus, according to Table 2, the Li+ Table 2. Supplementary Information for Eq 79 symbol

value

R T A n F σ

gas constant absolute temperature electrode plate area number of electrons transferred Faraday constant Warburg factor

diffusion coefficient (DLi+) can be calculated as shown in eq 7:19 R2T 2 (7) 2A n F c σ where σ is related to the value of the lithium ion diffusion coefficient.19 According to the fitting results, the σ values for pristine NCM, P-NCM, and P-NCM@Li3PO4−PANI were 18.30(5), 5.40(6), and 2.54(5), respectively. Thus, the corresponding results for the Li+ diffusion coefficient for the pristine NCM, P-NCM, and P-NCM@Li3PO4−PANI samples were 3.4 × 10−15, 3.9 × 10−14, and 1.7 × 10−13 cm2 s−1, respectively. This improvement can be attributed to two aspects: on the one hand, the dual-conductive (Li3PO4-PANI) coating layers effectively inhibit side reactions to alleviate the irreversible phase transition. Therefore, the dissolution of D Li+ =

H

2 4 4 2 2

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the remaining PO43− ions were fixed on the surface and reacted with lithium residue salts to obtain Li3PO4. This effectively improves the stability of the crystal structure and reduces the amount of lithium residue salts. In the second process, the dual-conductive layers (Li3PO4-PANI) effectively protect the interface of the NCM electron plate from chemical corrosion by HF and promote the Li+/e− conductivity of the interface. This synergistic strategy can also be extended to other candidate materials.

transition metal ions is greatly inhibited and the interfacial stability of the materials is improved. As a result, the excellent interfacial stability alleviates the decrease in the diffusion coefficient of the lithium ions in the bulk phase to some extent; on the other hand, the introduction of Li3PO4 and PANI successfully improves the ionic conductivity and electronic conductivity at the electrode/electrolyte interface. Z′ = R ct + R e + σω−0.5

(8)

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All of the findings on the comodification by combining gradient doping and dual-conductive layers coating are shown in Figure 10. The improvement in the interface stability and

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.8b02112. Compositional change for the transition metal ions and Zr as a function of the etched depth of P-NCM@ Li3PO4-PANI sample; TGA spectra; EDs mapping images; FESEM pictures; Rietveld refinement results for all samples; pH value of all samples; ICP results after cycling test; GITT curves (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (Q.R.). *E-mail [email protected] (H.Z.). *E-mail [email protected] (X.L.). ORCID

Qiwen Ran: 0000-0002-1658-7016 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The National Natural Science Foundation of China (No. 21071026) and the Outstanding Talent Introduction Project of University of Electronic Science and Technology of China (No. 08JC00303) supported our work.

Figure 10. Schematic diagram of interface/bulk modification.

■

lattice stability are attributed to the following effects: (i) More stable lattice structure. The stability of oxygen layer is greatly enhanced by gradient doping, leading to the improvement in lattice stability. In addition, after gradient doping, the c lattice parameter and unit volume increase, demonstrating that the lithium ion diffusion channel expands. (ii) Excellent interfacial properties. Dual-conductive layers (Li3PO4-PANI) not only reduce the amount of residual lithium salts but also inhibit the side reactions at the interface between the active materials and electrolyte. Therefore, the interfacial properties of NCM cathode are greatly enhanced by dual-conductive layers coating. (iii) Superior ionic/electronic conductivity. Li3PO4PANI layers acting as fast ion conductors and fast electron conductors enhance the interfacial proton/electron transport properties of the NCM cathode.

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4. CONCLUSION In our research, we have successfully synthesized the layered LiNi 0.6 Co 0.2 Mn 0.2 O 2−x (PO 4 ) x @Li 3 PO 4 -PANI (P-NCM@ Li3PO4-PANI) cathode material by a combination of gradient doping and dual-conductive layers (Li3PO4-PA NI) coating. The modified LiNi0.6Co0.2Mn0.2O2 cathode showed better cyclic stability and rate performance than the LiNi0.6Co0.2Mn0.2O2 cathode. In the first process, some of PO43− ions were gradient doped into the crystal lattice, while I

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K

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