Fluorine-doped carbon surface modification of Li-rich layered oxide

Research Article pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Fluorine-Doped Carbon Surface Modification of Li-Rich Layered Oxide Composite Cathodes for High Performance Lithium-Ion Batteries Fenghua Zheng,†,‡ Qiang Deng,†,‡ Wentao Zhong,†,‡ Xing Ou,†,‡ Qichang Pan,†,‡ Yanzhen Liu,†,‡ Xunhui Xiong,†,‡ Chenghao Yang,*,†,‡ Yu Chen,§ and Meilin Liu†,‡,§

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Guangzhou Key Laboratory for Surface Chemistry of Energy Materials, New Energy Research Institute, School of Environment and Energy, South China University of Technology, Guangzhou 510006, China ‡ Guangdong Engineering and Technology Research Center for Surface Chemistry of Energy Materials, New Energy Research Institute, School of Environment and Energy, South China University of Technology, Guangzhou 510006, PR China § School of Materials Science & Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, United States S Supporting Information *

ABSTRACT: Fluorine (F)-doped carbon modified lithium rich layered oxides Li1.2Mn0.54Ni0.13Co0.13O2 (LMCNO@C-F) are synthesized by a facile sol−gel process. In this constructed architecture, the F-doped carbon surface modification layers can not only enhance the electronic conductivity of the overall electrode but also avoid the direct exposure of LNMCO to the electrolyte. As a result, The LMCNO@C-F sample exhibits a high reversible capacity (289.5 mA h g−1 at 0.1 C), excellent rate capability (263.6, 218.9, 182.9, and 108.6 mA h g−1 at 0.5, 1, 5, and 10 C, respectively), and superior cycling stability (with a high capacity retention of 88.5% at 5 C after 500 cycles). The enhanced performance is ascribed to the formed metal fluorides (Mn−F bond) and a strong electronic coupling between F-doped carbon and bulk LMNCO, which can greatly enhance the structure stability and electronic conductivity of LMNCO cathode materials. KEYWORDS: Lithium-ion battery, Cathode materials, Li1.2Mn0.54Ni0.13Co0.13O2, Fluorine-doped carbon, Electrochemistry performance

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INTRODUCTION Lithium ion batteries (LIBs) have been considered as promising power sources for electric vehicles (EVs), hybrid electric vehicles (HEVs), and portable electronic devices.1−3 The energy storage mechanism of LIBs is based on the extraction/insertion of Li+ from/into the electrode materials. Therefore, both cathode and cathode materials are vital to the electrochemical performance of whole LIBs.4−6 Currently, the specific capacity of commercial graphite anode is 372 mA h g−1. But specific capacity of conventional cathode materials (e.g., LiFePO4, LiMn2O4, and LiNixCoyMn1−x−yO2 (0 ≤ x ≤ 1, 0 ≤ y ≤ 1)) is much lower than that of graphite anode, which becomes the bottleneck for fabrication of high performance LIBs. Therefore, it is important to select a suitable cathode material with high specific capacity.7−12 Li-rich layered oxides (0.5Li2MnO3− 0.5LiMn0.33Ni0.33Co0.33O2) have been considered as promising cathode materials for LIBs, as they have high operation voltage (4.8 V) and can deliver a much higher specific capacity up to 250 mA h g−1.13 The Li-rich layered oxides are composed of layered LiMO2 and Li2MnO3, resulting in an overall Li+ ions to transition metal ions stoichiometry greater than unity. The initial charge curve of Li-rich layered oxides usually shows a © XXXX American Chemical Society

long and high voltage plateau at 4.5 V corresponding to the oxidation of O2− to O2 due to the reaction of Li2MnO3 → 2Li + MnO2 + 0.5O2, which is named as the oxygen releasing plateau and ascribed to be responsible for its high rechargeable capacity.14 However, Li-rich layered oxides often show the drawback of high irreversible capacity in the initial charge/ discharge process, and it greatly hampers their practical applications.16 On the other hand, the low electronic/ionic conductivity of Li2MnO3 and side reaction between electrolyte and electrode at higher operation potential (4.8 V) result in the poor rate and cycling performance.15,16 Many strategies have been proposed to improve the initial Coulombic efficiency, rate capability, and cycling stability, for example, cationic doping17−19 and surface coating.20−23 Cationic doping is a novel method to improve the structure stability of cathode materials (e.g., Mg, Al, and Y),24−26 and surface coating (e.g., AlF3, Al2O3, ZrO2, TiO2, and AlPO4 et al.) has been considered as an effective method to eliminate side reaction between electrolyte and electrode materials.27,28 Received: July 19, 2018 Revised: September 26, 2018 Published: October 26, 2018 A

DOI: 10.1021/acssuschemeng.8b03442 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

For TEM test of the samples after cycling, the positive electrodes after 500 cycles at 5 C were rinsed using dimethyl carbonate (DMC) to remove the electrolyte from the surfaces of electrodes in a glovebox, and then dried to remove the residual DMC. After that, the obtained electrode dispersed in ethanol solution and ultrasonicated for 2 h. Finally, the obtained solution drop into the copper mesh and dried for TEM test. Electrochemical Evaluation. The electrochemical performances of the samples were evaluated by using 2032 type coin cells. A cathode slurry was prepared by mixing the active materials, acetylene black, and PVDF in a weight ratio of 8:1:1 in N-methyl-2-pyrrolidene (NMP) solvent. The slurry was coated on the Al foil and dried at 120 °C overnight in a vacuum oven. Then it was punched into 12 mm diameter disks with the loading of active cathode mass in the range of 3.2−3.5 mg cm−2. The CR2032-type half coin cells were assembled in an Ar-filled glovebox (Etelux LAB2000) using the prepared samples as cathode, lithium metal as the counter electrode, Celgards 2200 separator, and 1 M LiPF6 dissolved in EC/EMC/DEC (1:1:1 in volume) as the electrolyte. Galvanostatic charge/discharge measurements were carried out using LAND CT2001A battery testing system (Wuhan, China) within the voltage range of 2.0−4.8 V vs Li+/Li under different current rates at 25 °C. Cyclic voltammetry (CV) measurements were carried out on IM6 electrochemical workstation at a scan rate of 0.1 mV s−1. Electrochemical impedance spectroscopy (EIS) of the cell was measured by using IM6 electrochemical workstation, with 5 mV amplitude of AC signal at the frequency range between 100 kHz and 0.01 Hz.

Either cationic doping or surface coating is a novel approach to enhance the structure and cycling stability of lithium-rich layered oxides. However, the advanced structural and cycling stability come at the cost of specific capacity of the cathodes, since some of the cationic dopant or surface coating layers are electrochemically inactive.29−33 Recently, nanoscale LiFePO4 surface modified Li-rich layered oxides reported by us previously have also been regarded as a promising approach to enhance electrochemical performance, as they combines the benefits of surface doping and surface coating.34 Moreover, nonmetal ion (N, S, and B et al.) doped carbon as an anode material has shown excellent electrochemical performance for the high electronic/ionic conductivity.35,36 Nonmetal ion doped carbon surface coated cathode materials can effectively enhance electrochemical performance, e.g. the B-doped carbon coated Li3V2(PO4)3 reported by Wang et al. showed excellent rate performance and cycling stability.37 N-doped carbon coated LiFePO4 synthesized by Wang et al. exhibited supervisor rate and cycling performance.38 Herein, fluorine (F)-doped carbon surface modified Li1.2Mn0.54Ni0.13Co0.13O2 (LMNCO) has been prepared by a facile sol−gel process. The effect of the F-doped carbon surface modification on the initial Coulombic efficiency, rate performance, and cyclic stability of LMNCO will be discussed in detail.

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EXPERIMENTAL SECTION

RESULTS AND DISCUSSION Scheme S1 illustrates the detailed fabrication process of LMCNO@C-F. First, pure LMNCO particles are synthesized by a sol−gel method. Then, pure LMNCO particles are mixed with Tween80 and PVDF powders by ball milling. During the ball milling process, Tween80 is adsorbed on the surface of LMNCO particles, while PVDF particles are adsorbed on the outside surface of LMNCO@Tween80 composites. Subsequently, the LMNCO/Tween80/PVDF composites are sintered at 400 °C in Ar atmosphere, and after cooling to room temperature, a thin film of F-doped carbon surface modified LMNCO particles are obtained. To explain the effect of Tween80 and PVDF on the microstructure of F-doped carbon, FTIR analysis has been carried out. Figure S2 exhibits the IR spectra of LMNCO mixed with Tween80 and PVDF. As seen in Figure S2, the characteristic peaks for functional groups of Tween80 and PVDF simultaneously appeared in the IR spectra of the LMNCO/Tween80/PVDF mixture. The characteristic bending modes for CO and C−O functional groups of Tween80 in the composite carbon sources (Tween80 and PVDF) shift to lower wavenumbers compared to pure Tween80. However, compared to pure PVDF, the characteristic bending modes for C−F functional groups of PVDF in the composite carbon sources (Tween80 and PVDF) do not vary. These testing results suggest that Tween80 interacts with pure LMNCO, and PVDF does not interact with LMNCO directly, due to the Tween 80 surfactant containing hydrophobic and hydrophilic groups. Therefore, the Tween 80 surfactant could uniformly mix with materials in aqueous solution and adsorb on the particle surface to form a carbon layer during heat treatment, which can coat tightly on the surface of the LMNCO particles. This indicates that Tween80 can be absorbed and completely cover the surface of LMNCO particles, and PVDF only exists between LMNCO@Tween80 particles. After sintering, Tween80 can form the fluorine doped carbon coating layer

Preparation of Materials. Li1.2Mn0.54Ni0.13Co0.13O2 (LMNCO) powders were prepared via a sol−gel method using citric acid as a chelating agent. A stoichiometric amount of Li(CH3COO)·2H2O, Mn(CH3COO)2·4H2O, Ni(CH3COO)2·4H2O, and Co(CH3COO)2· 4H2O were dissolved in deionized (DI) water together with citric acid under vigorous stirring. The molar ratio of citric acid to LMNCO was set to 1:1. The mixed solution was condensed to a viscous gel by evaporation at 80 °C on a hot plate. The mixture was dried in an oven at 120 °C for 12 h and then decomposed at 550 °C for 5 h and sintered at 850 °C for 15 h in air to get the final products. After cooling down room temperature, as prepared LMNCO was mixed with Tween 80 and polyvinylidene fluoride (PVDF) (2:0.75, w/w) in DI water by ball milling for 8 h, and the mixed compound was dried at 100 °C for 12 h. Finally, the obtained powders were heated at 400 °C for 1 h under a N2 atmosphere to get F-doped carbon surface modified LMNCO (LMCNO@C-F) powders. The carbon surface coated LMNCO (LMNCO@C) layer was fabricated by similar procedures in the absence of PVDF. Characterization. X-ray diffraction (XRD: Rigaku, D/max 2500v/pc, Cu Kα radiation) was carried out to analyze crystal structure of the samples. Scanning electron microscopy (SEM: Philips, FEI Quanta 200 FEG) and transmission electron microscopy (TEM: JEM-2010, JEOL) were used to study the morphology and microstructure of the samples. Fourier transform infrared (FTIR) spectra were recorded with an AVATAR370 spectrometer. Thermogravimetric analysis (TGA) was determined by a thermal analyzer (SDTO600) in air flow with a temperature increase of 5 °C min−1. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Phi X-Tool XPS system. For in situ XRD measurements, the configuration of the in situ battery cell is shown in Figure S1. A 12 mm inner diameter electrochemical cell was assembled in an Ar-filled glovebox using a Be window for X-ray transmission, carbon paper as a current collector, LMCNO@C-F as a cathode, lithium metal as the counter electrode, Celgards 2200 separator, 1 M LiPF6 dissolved in ethylene carbonate (EC)/ethyl methyl carbonate (EMC)/dimethyl carbonate (DEC) (1:1:1 in volume) as the electrolyte, respectively. Each scan was collected in 0.02° increments between 10° and 50° at a scanning speed of 0.04° s−1. The time interval for each scan was set for 180 s to ensure enough time to trace the reaction during the charge/discharge process. B


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Figure 1. XPS patterns of (A) survey spectrum of LMCNO@C-F, (B) Ni 2p, (C) Co 2p, (D) Mn 2p in all samples ((B1/C1/D1) LMNCO, (B2/ C2/D2) LMNCO@C, and (B2/C2/D2) LMCNO@C-F), (E) C 1s, and (F) F 1s of LMCNO@C-F.

on the surface of LMNCO, and PVDF can form the fluorine doped carbon network between the LMNCO particles. XRD patterns of LMNCO, LMCNO@C, and LMCNO@CF are shown in Figure S4A; all of the three samples show the sharp diffraction peaks, suggesting that these samples have high crystallinity. The XRD patterns demonstrate that all of the three samples belong to the hexagonal structure (α-NaFeO2) with R3̅m space group symmetry. Clear separation of adjacent peaks of (006)/(012) and (108)/(110) can be seen in the XRD patterns, indicating that all the three samples have typical layered structure.39 No other characteristic diffraction peaks have been observed in LMCNO@C and LMCNO@C-F, which reveals that carbon and F-doped carbon in the samples are amorphous carbon. Additionally, the lattice parameters of all samples are calculated and listed in Table S1. As seen from the table, the c-lattice parameter and c/a ratio do not vary much after carbon and F-doped carbon surface modification, due to F only occupying the oxygen vacancy from minor Orelease of the Li2MnO3 surface and not being doped into the O site of the LiTMO2 component TM = Mn, Ni, and Co). The above results indicate that the carbon and F-doped carbon surface modifications have little effect on the materials’ crystal

structure. The content fractions of carbon and F-doped carbon are analyzed by TGA, and the carbon and F-doped carbon in the samples are about 6.2% and 5.5%, respectively (Figure S3). In order to analyze the effect of the F-doped coating on the chemical composition of LMNCO, stoichiometric amounts of metal element in pure LMNCO, LMNCO@C, and LMCNO@C-F have been determined by ICP analysis, and the results are listed in Table S2. As seen in Table S2, the molar ratio of Li:Ni:Co:Mn in pure LMNCO is 1.212:0.134:0.133:0.545, which is close to the theoretical ratio of 1.2:0.133:0.133:0.54. Moreover, the molar ratio of Li:Ni:Co:Mn of LMNCO@C and LMCNO@C-F does not vary, indicating that the carbon and F-doped carbon surface modifications do not affect the chemical composition of LMNCO. Figure S4B shows Raman spectra of LMNCO, LMCNCO@ C, and LMCNO@C-F. The Raman spectra of all the three samples consist of three bands at 422, 470, and 592 cm−1, which are attributed to characteristic peaks of LMNCO. The peak at 470 cm−1 is attributed to the symmetrical deformation (Eg) of TM−O, while the peak at 592 cm−1 is assigned to the symmetrical stretching (A1g) of TM−O (TM = Mn, Ni, and C


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Figure 2. Illustration of the strong interaction between LMNCO particles and F-doped carbon.

Figure 3. SEM images of pure LMNCO (A1 and A2), LMNCO@C (B1 and B2), LMCNO@C-F (C1 and C2), and corresponding EDX elemental mapping images of Mn, Co, Ni, F, and C (D1, D2, D3, D4, D5, and D6).

Co). 40 And these two vibrational modes belong to rhombohedral lattice of R3̅ m symmetry of the LiTMO2 component. Furthermore, a peak at 423 cm−1 corresponds to the fingerprint vibration of Li2MnO3.41 On the other hand, for LMNCO@C, the intensity of the Raman peak at 422 cm−1 decreases compared to pure LMNCO, due to secondary sintering in the presence of Tween 80 causing changes in the surface of LMNCO, resulting in minor Li leaching and O-

release, formation of oxygen-deficient intermediate structure, and decreasing the content of the Li2MnO3 component.42 But for LMCNO@C-F, the intensity of the peak does not vary much compared to pure LMNCO, which may be due to F occupying the oxygen vacancy from O2− elimination on the particle surface, thus stabilizing the structure of Li2MnO3. In addition, from the XPS results, the electron clouds of F-doped carbon bias to bulk LMNCO, which could suppress the oxygen D


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Figure 4. TEM images and HRTEM images of pure LMNCO (A1 and A2), LMNCO@C (B1 and B2), and LMCNO@C-F (C1 and C2).

increase the electron cloud density on the surface of LMNCO. As illustrated in Figure 2, the F-doped carbon has more electrons than does pure carbon, due to fluorine having more electrons compared to carbon. Considering that secondary sintering in the presence of Tween 80 lead to minor O-release, oxygen vacancies are formed on the on the particles surface. Meanwhile, the oxygen vacancies on the particle surface have positive charge. Therefore, the electron clouds of F-doped carbon are attracted to bulk LMNCO, resulting in a strong electronic coupling between F-doped carbon and LMNCO.47−49 Morphology and microstructure of the three samples have been further studied by SEM and TEM. Figure 3A−C shows the SEM images of LMCNO, LMNCO@C, and LMCNO@CF, respectively. LMNCO exhibits diamond-like primary particles with diameters of 200−400 nm whose surfaces are very smooth. After surface modification with carbon and Fdoped carbon, the diamond shape and particle size of LMNCO are similar to that of pure LMNCO, indicating that the surface modification exerts no influence on the morphology of LMNCO. In addition, the tap density of pure LMNCO, LMNCO@C, and LMCNO@C-F are 1.93, 1.91, and 1.89 g cm−3 (Table S3), respectively, suggesting that little effect on tap density after carbon and F-doped carbon surface modification for LMNCO. Figure 3 shows the EDX elemental mapping images of LMCNO@C-F. As exhibited in the figures, Ni, Co, Mn, C, and F elements are uniformly distributed in the sample. The TEM image shown in Figure 4A1 also indicates that pure LMNCO particles have diameters of 200−400 nm, which is consistent with the SEM results. The LMNCO particles exhibit a smooth edge, and no other coating layer on the surface has been observed (Figure 4A2). TEM and HRTEM images of LMNCO@C and LMCNO@C-F are shown in Figure 4B and C, respectively. The particle size of LMNCO@ C and LMCNO@C-F is similar to that of LMNCO, but a thin layer of carbon with the thickness of 2−5 nm covered on LMNCO surface has been observed. Meanwhile, as demonstrated in Figure 4A2, B2, and C2, the d-spacing of lattice fringes for all the three samples are 0.47 nm, which is related to the (003) fringe of the layered structured LMMCO. It indicates that the carbon and F doped carbon surface modification have no effect the crystal structure of LMNCO materials.

vacancy loss, thus hindering Li leaching from the Li2MnO3 and also keeping the structure stable. In the end, the Eg peak for LMCNO@C-F is weaker than that of pure LMNCO and LMNCO@C, suggesting that a strong electronic coupling between F-doped carbon and LMNCO suppressed the symmetrical stretching of TM−O. Above, the results suggest that the F-doped carbon surface modification could effectively stabilize the particle surface of LMNCO. Moreover, two intense bands located at 1350 and 1605 cm−1 are observed for both LMNCO@C and LMCNO@C-F, which are associated with the D and G bands of carbon, respectively. The D and G bands are ascribed to defects carbon atom and the sp2 graphitebased structure, respectively. The relative intensity ratio of ID/ IG is associated with disorder of graphitic materials.43 The result shows that ID/IG of LMCNO@C-F (1.15) is higher than that of LMNCO@C (1.03), demonstrating the presence of more disorder structured carbon in LMCNO@C-F. The surface chemistries for all three samples have been analyzed by XPS, and the results are illustrated in Figure 1. The C 1s high resolution XPS spectrum can be fitted into three peaks centered at 284.6, 286.4, and 288.8 eV (Figure 1E), corresponding to the C−C (sp3−C), C−O, and C−F bonds, respectively.44 For high revolution XPS spectrum of F 1s for LMCNO@C-F (Figure 1F), two peaks located at 687.0 and 684.7 eV are observed, which are ascribed to F chemically bonded with C and Mn, respectively.45 This suggests that the F element not only combined with carbon but also successfully doped into the bulk LMNCO materials. The main binding energies of Ni (2p3/2 and 2p1/2), Co (2p3/2 and 2p1/2), and Mn (2p3/2 and 2p1/2) of pure LMNCO are 854.63 and 871.91 eV, 779.92 and 794.89.03 eV, and 642.03 and 653.85 eV, respectively.46 Meanwhile, the main binding energies of Ni, Co, and Mn of LMNCO@C are similar to that of pure LMNCO. However, compared to pure LMNCO, the main peaks of Ni, Co, and Mn for LMCNO@C-F shift to the low binding energy, owing to the increase in the electron cloud density around the surface of LMNCO. The F− may be doped into the O site of the Li2MnO3 surface and not the LiTMO2 component (TM = Mn, Ni, and Co) according to the lattice parameter calculated results. Therefore, the introduced low F with higher electronegativity cannot change the electron cloud density of Ni/Co/Mn cations in the LiTMO2 component but can reduce the electron cloud density of Mn cations in the Li2MnO3 component. Meanwhile, the F-doped carbon could E


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Figure 5. (A) Contour plots of in situ XRD results and (B) corresponding in situ XRD patterns of the pure LMNCO electrode against the voltage profile during the initial cycle at a cutoff voltage of 2.0−4.8 V. (C) Contour plots of in situ XRD results and (D) corresponding in situ XRD patterns of an LMCNO@C-F electrode against the voltage profile during the initial cycle at a cutoff voltage of 2.0−4.8 V.

carbon, the peaks at 38.7°, 41.4°, and 44.1° are related to BeO, and the peak at 45.2° is derived from the Be window. The peaks at 18.7°, 36.9°, 58.7°, 64.5°, 65.5°, and 68.9° are from the LMNCO cathode, corresponding to the (003), (010), (107), (018), (110), and (113) planes of LMNCO, respectively. Based on the reaction sequence revealed in the XRD testing results, a 5-stage scheme is proposed to illustrate the structural evolution and reaction mechanisms undertaken

To understand the reaction mechanism of as prepared LMNCO, an in situ XRD test has been carried out by a special in situ cell containing a LMNCO cathode operated in potential range of 2.0−4.8 V at current density of 0.1 A g−1. The corresponding delithiation/lithiation process to each phase composition is color-marked in Figure 5. The electrochemical voltage profile during the initial cycles is located in the left to show different states in initial charge/discharge process. It needs to be pointed out that the peak at 26.5° corresponds to F


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Figure 6. Schematic of change in crystal structure of LMNCO in the initial charge/discharge process. (Li lithium; TM transition metal).

in the charge/discharge process in LMNCO cathode (Figure 6). In stage I (OCV-4.4 V), the rapid increase in the potential corresponds well with the (003) plane of LMNCO shifts to lower 2θ angle in Figure 5B. In this region, Li+ ions extraction from lithium layer eventually increases the electrostatic repulsion between the oxygen layers and causes the unit cell expansion along the c-axis, which is ascribed to responsible for the left shifts (Figure 6B). In stage II (4.4−4.6 V), the (003) plane continue shifts to lower 2θ angle, indicating a further lattice expansion of LMNCO during the Li+ intercalation (Figure 6C). In stage III (4.6−4.8 V), the (003) plane moves to the higher angle, indicating that the lattice parameter decreases along the c-axis. In this voltage region, Li2MnO3 was activated, and Li+ ions are extracted from transition metal layers since the energy required to remove Li+ ions from transition metal layers is much higher than that from the lithium layers (Figure 6D). In whole initial charging process, (018) and (110) planes separate clearly with increasing voltage. The (110) peak shift to higher 2θ angle, and the (018) peak moved to lower 2θ angle, indicating a−b plane shrinks. As the LMNCO cathode was discharged to 3.5 V in stage IV, the (003) peak shifts toward the lower 2θ angle. This indicates the expansion of lattice parameters along the c-axis during Li+ ions insertion into the transition metal layers in this voltage range (Figure 6E). When the LMNCO cathode was fully discharged to 2.0 V in stage V, the (003) peak recovered back to its initial position again, demonstrating the Li+ ions insertion into the lithium layers. Therefore, the reaction mechanism of Li1.2Mn0.54Ni0.13Co0.13O2 during charge/discharge process in range of 2.0−4.8 V is proposed as the following equation. During the charge process

0.5Li 2MnO3 ·0.5Mn 0.33Ni 0.17Co0.25O2 → 0.5MnO2 ·0.5Mn 0.33Ni 0.17Co0.25O2 + Li+ + O2 − (4.6−4.8V)

During the discharge process Stage IV 0.5MnO2 ·0.5Mn 0.33Ni 0.17Co0.25O2 + 0.5Li+ + 0.5e− → 0.5LiMnO2 · 0.5Mn 0.33Ni 0.17Co0.25O2 (4.8−3.5V)

Stage V 0.5LiMnO2 · 0.5Mn 0.33Ni 0.17Co0.25O2 + 0.5Li+ + 0.5e → 0.5LiMnO2 · 0.5LiMn 0.33Ni 0.33Co0.33O2 (3.5−2.0V)

This result indicates that the phase transformation between LMNCO and MnO2, Mn0.33Ni0.33Co0.33O2 oxides occurs during charge/discharge process. MnO2 and Mn0.33Ni0.33Co0.33O2 oxides show lower electronic conductivity, resulting in a low electrochemical reaction kinetic as well poor rate performance. In addition, the MnO2 can produce Mn3+ ions during electrochemical lithiation processes, and Mn3+ in a spinel-like phase can cause a Jahn−Teller distortion effect, which will accelerate the manganese dissolution.50,51 In the end, pure LMNCO occurs volume expansion/contraction during charge/discharge process. To solve this problem, nanoscale F-doped carbon surface modified LMNCO is used to increase the electronic conductivity and suppresse the dissolution of manganese metal ions and the volume expansion/contraction. Furthermore, the in situ XRD test for LMCNO@C-F have been carried out, and the results are shown in Figure 5C and D. The results indicate that the structural evolution of LMCNO@C-F is similar to that of pure LMNCO during charge/discharge processes. The diffraction peak shifts as the voltage increases and eventually returns to its original position as the voltage decreases. Nevertheless, the variations of diffraction peak positions for LMCNO@C-F are smaller than that of pure LMNCO, which suggests that the Fdoped carbon surface modification could effectively suppress the volume expansion/contraction, thus stabilizing the structure of LMNCO. The initial charge/discharge curves for all the three samples between 2.0 and 4.8 V at 0.1 C are displayed in Figure S5A.

Stage I and II 0.5Li 2MnO3 ·0.5LiMn 0.33Ni 0.33Co0.33O2 → 0.5Li 2MnO3 ·0.5Mn 0.33Ni 0.17Co0.25O2 + 0.5Li+ + 0.5e− (OCV − 4.6V)

Stage III: G


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Figure 7. Rate performance of (A) pure LMNCO, (B) LMNCO@C, and (C) LMCNO@C-F and cycling performance of (D1 and E1) pure LMNCO, (D2 and E2) LMNCO@C, and (D3 and E3) LMCNO@C-F at 0.5 C rate after (D) and 5 C rate (E).

between F-doped carbon and LMNCO, thus stabilizing the interfaces and the structure of Li2MnO3 in the bulk LMNCO. To study the electrochemical reaction kinetic, cyclic voltammograms (CVs) of LMNCO, LMNCO@C, and LMCNO@C-F at different scanning rates between 2.0 and 4.8 V have been carried out, and the results are shown in Figure S5B−D. As shown in the figures, as the scanning rate increases, the potential intervals (ΔV) also increase between the corresponding anodic and cathodic peaks for the three samples, because of the electrochemical reaction kinetic limitations. But the ΔV of LMCNO@C-F at different sweeping rates is much lower than that of LMNCO and LMNCO@C. Moreover, when the redox peaks of all the three samples are enhanced, the redox peak current value of all samples also increases. The redox peak current of LMCNO@ C-F is much higher than those of LMNCO and LMNCO@C. All of these results confirm that LMCNO@C-F has better ion transportation ability and higher electronic conductivity than that of LMNCO and LMNCO@C. Figure 7A−C exhibits the rate performance of all three samples. As shown in Figure 7C, LMCNO@C-F delivers a high specific discharge capacity of 289.5, 263.6, 218.9, 182.9,

There are two plateaus for LMNCO, LMNCO@C, and LMCNO@C-F; the first one located at 3.7−4.5 V is ascribed to the reversible extraction of Li+ ions from layered LiMO2 (M = Ni, Co) phase, accompanied by the oxidation from Ni2+ to Ni4+ and Co3+ to Co4+.52−54 The second plateau located at 4.5 V is associated with the Li+ ions extraction from Li2MnO3 component.55,56 The above two charge plateaus of LMCNO@ C-F are longer than those of the LMNCO/C and pure LMNCO, suggesting that more lithium ion extract from the LiMO2 (M = Ni, Co) phase and Li2MnO3 component. Therefore, the initial charge capacity of LMCNO@C-F reaches to 360.4 mA h g−1 at 0.1 C which is higher than that of pure LMNCO (349.3 mA h g−1) and LMNCO@C (358.2 mA h g−1). It is attributed to fluorine doped carbon increasing the number of electrons around the surface of bulk LMNCO, resulting in more active sites for electrons and Li+ ion diffusion and transportation. In addition, the LMCNO@CF composite shows a high initial Coulombic efficiency of 82.4%. However, the initial Coulombic efficiency of pure LMNCO and LMNCO@C are only 73.9% and 77.9%, respectively. This may be associated with incorporating fluorine in bulk LMNCO and a strong electronic coupling H


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Figure 8. (A) Nyquist plots of all samples ((A1) LMNCO, (A2) LMNCO@C, and (A3) LMCNO@C-F) before cycling, (B) pure LMNCO electrodes, (C) LMNCO@C, and (D) LMCNO@C-F after cycling at discharge rate of 5 C. (E) Equivalent circuit models.

and 108.6 mA h g−1 under the current rate of 0.1, 0.5, 1, 5, and 10 C, respectively. While the specific discharge capacities of LMNCO (Figure 7A) and LMNCO@C (Figure 7B) are only 254.9, 209.2, 143.1, 80.2, 33.3 mA h g−1 and 278.5, 245.3 192.3, 120.2, 62.7 mA h g−1 at 0.1, 0.5, 1, 5, and 10 C, respectively. The excellent rate performance of LMCNO@C-F originates from the formed metal fluorides (Mn−F bond) and F-doped carbon, which promote Li+ ion transportation of the samples. Figure 7D and E show cycling stability of LMNCO, LMNCO@C, and LMCNO@C-F between 2.0 and 4.8 V at 0.5 and 1 C, respectively. As seen Figure 7D and E, all sample show stable Coulombic efficiencies during cycling at 0.5 and 5 C. Pure LMNCO delivers a discharge capacity of 170.6 mA h g−1 with capacity retention of 81.6% after 100 cycles at 0.5 C. Meanwhile, pure LMNCO shows a much lower capacity retention of 56.9% at 5 C after 500 cycles. The poor cycling performance of LMNCO is attributed to the fact that the organic electrolyte can react with electrode and form an inactive interface coating layer. It results in a rapid increase of the interfacial resistance and collapse of LMNCO microstructure. While LMNCO@C exhibits much better cycling stability than LMNCO, LMNCO@C exhibits a high capacity retention of 88.7% and 80.2% after 100 cycles at 0.5 C and 500 cycles at 5 C, respectively. Compared to those of pure LMNCO and LMNCO@C, LMCNO@C-F exhibits an outstanding cycling stability. LMCNO@C-F can deliver a discharge capacity of 246.7 mA h g−1 at 0.5 C after 100 cycles and with high capacity retention of 94.6% (Figure 7D). More importantly, a high reversible capacity of 162.2 mA h g−1 can

be achieved even over 500 cycles at 5 C high-rate (Figure 7E) with a capacity retention of 88.5%. Such improved cycling stability is attributed to the formation of Mn−F bond and a strong electronic coupling between F-doped carbon layer and bulk material. It can increase the bonding force between the Fdoped carbon layer and bulk LMNCO, and thus effectively suppress the reaction between electrolyte and electrode. As we all know, surface corrosion of the sample can accelerate the dissolution of transition metals from LMNCO, which will cause quick capacity fading. The F-doped carbon surface modified LMNCO could effectively suppress Mn dissolution (Table S4), stabilize its surface structure, and mitigate the capacity fades during cycling. Furthermore, compared to that of most reported Li-rich layered oxide cathodes, the LMCNO@C-F fabricated in this work shows an excellent electrochemical performance (see Table S5).57−71 Especially, the rate performance and cycling performance of the LMCNO@C-F are better than the above-reported LMNCO materials. The results suggest that F-doped carbon surface modified LMNCO is a promising cathode materials for fabrication of high performance LIBs. TEM images of all the three sample after cycling at 5 C are shown in Figure S5. As seen in Figure S5A1 and SA2, the particles of pure LMNCO are corroded severely after cycling tests, which is attributed to side reactions between the electrolyte and electrode. Meanwhile, the carbon layer coated on the LMNCO@C particle surface only becomes thinner after 300 cycles (Figure S5B1). But the carbon coating layers are completely peeled off from LMNCO particles and I



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CONCLUSIONS In summary, the F-doped carbon modified LMNCO has been successfully prepared via a simple wet chemical method. The F-doped carbon homogeneously coated on the surface of LMNCO particles, and a strong electronic coupling was formed between F-doped carbon and bulk LMNCO, which can greatly enhance the LMNCO crystal structure stability and electronic conductivity. Moreover, the F-doped carbon surface modification layer can effectively suppress the reaction between the electrode and electrolyte. Therefore, the LMCNO@C-F has high initial Coulombic efficiency (82.4%), excellent rate capability (263.6, 218.9, 182.9, and 108.6 mA h g−1 at 0.5, 1, 5, and 10 C, respectively), and high cycling stability (the capacity retention is 88.5% after 500 cycles at 5 C). This evidence shows that F-doped carbon surface modified Li1.2Mn0.54Ni0.13Co0.13O2 could be an advanced cathode materials for LIBs.

LMNCO particles are etched after 500 cycles (Figure S5B2). The peeling off of the carbon coating layer will lead to the decrease of LMNCO@C electrical conductivity and exposure of LMNCO to the electrolyte and accelerate the side reactions between organic electrolyte and electrode. It is important to note that the carbon layer is still uniformly coated on LMNCO particles surface in LMCNO@C-F sample after 300 and 500 cycles (Figure S5C). The strong bonding force between Fdoped carbon layer and LMNCO is contributed to suppress the delamination of F-doped carbon layer during charge/ discharge process and enhance the structure stability of LMCNO@C-F. Figure 8 shows the Nyquist plots of LMNCO, LMNCO@C, and LMCNO@C-F before and after cycling. As seen from the figures, the intercept at high frequency relates to the ohmic resistance (RΩ), the semicircle at high frequency and the sloping line in the low frequency are attributed to the charge transfer resistance (Rct), and the Warburg impedance (W), respectively.72,73 The equivalent circuit model is constructed to analyze these impedance spectra, and corresponding parameters are exhibited in Tables S6 and S7. The Rct value of LMCNO@C-F is 125.3 Ω before cycling, while Rct values for LMNCO and LMNCO@C are 450.2 and 210.5 Ω before cycling, respectively. The LMCNO@C-F sample shows the lowest Rct value in the three samples. This suggests that Fdoped carbon can greatly enhance the charge transport coefficiency. The Li+ ion diffusion coefficiency (DLi+) is calculated by the following equations:74−77 R2T 2 2A n F C Li2 σ2

(1)

Z′ = R Ω + R ct σω−1/2

(2)

D Li+ =

2 4 4

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b03442.

where T, R, A, F, n, C, ω represent constant temperature, gas constant, surface area of the LMNCO cathode, Faraday constant, number of involved electrons, Li+ ions concentration, and angular frequency in the low frequency region, respectively. While, σ is the warburg coefficient, it is associated with Z′ against ω−1/2. As seen Table S6, the DLi+ value of LMCNO@C-F before cycling is much higher than that of pure LMNCO and LMNCO@C. The Li+ diffusion coefficient of LMCNO@C-F is 3.95 × 10−11 cm2 s−1, while, those of LMNCO and LMNCO@C are 1.1 × 10−11 and 2.35 × 10−11 cm2 s−1, respectively. This result suggests that LMCNO@C-F has the highest DLi+. The F-doped carbon surface modification of LMNCO will greatly enhance its electronic conductivity and DLi+, which eventually result in a lower Rct. Figure 8B−D show the Nyquist plots of pure LMNCO, LMNCO@C, and LMCNO@C-F at 5 C after the first, 300th, and 500th, respectively. RΩ and Rct values of LMNCO increase from 5.14 and 513.2 Ω after the first cycle to 9.24 and 1132.1 Ω after the 500th cycle, respectively. Meanwhile, only a slight increase of the RΩ and Rct values for LMCNO@C-F have been observed during cycling. Moreover, LMCNO@C-F shows the highest DLi+ value among these three samples. It confirms that the F-doping carbon layer surface modification of LMNCO can effectively enhance the electronic and ionic conductivity and mitigate the structure evolution of LMNCO during cycling.

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Assembly drawing of the in situ XRD cell; schematic illustration of the fabrication of LMCNO@C-F; infrared (IR) spectra of PVDF, Tween 80 and LMNCO mixed with composite carbon; TGA curves of LMNCO@C and LMCNO@C-F; XRD patterns, Raman patterns, crystallographic parameters, ICP analysis, tap density, initial charge/discharge, and CV curves for pure LMNCO, LMNCO@C, and LMCNO@C-F; TEM images and dissolution of Mn for pure LMNCO, LMNCO@C, and LMCNO@C-F after cycling; electrochemical performances of various LMNCO for lithium batteries; impedance parameters of pure LMNCO, LMNCO@C, and LMCNO@C-F before and after cycling; carbon content, fluorine content, rate performance, and cycling capability of different LMCNO@C-Fs (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail address: [email protected] (C.Y.). ORCID

Xunhui Xiong: 0000-0002-5858-9247 Chenghao Yang: 0000-0002-3214-328X Meilin Liu: 0000-0002-6188-2372 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the Science and Technology Planning Project of Guangdong Province, China (No. 2017B090916002), National Natural Science Foundation of China (51872098), Guangdong Natural Science Funds for Distinguished Young Scholar (2016A030306010), Guangdong Innovative and Entrepreneurial Research Team Program (2014ZT05N200), Fundamental Research Funds for Central Universities, China (2017ZX010), China Postdoctoral Science Foundation (2017M622675), and J


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Fundamental Research Funds for Central Universities, China (2018MS87).

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Fluorine-doped carbon surface modification of Li-rich layered oxide

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