Enhanced Electrochemical Performance of Layered Lithium-Rich

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Enhanced Electrochemical Performance of Layered Lithium-Rich Cathode Materials by Constructing Spinel-Structure Skin and Ferric Oxide Islands Shi Chen,†,‡,§,¶ Yu Zheng,†,§,¶ Yun Lu,*,†,§ Yuefeng Su,*,†,‡,§ Liying Bao,†,§ Ning Li,† Yitong Li,† Jing Wang,†,‡,§ Renjie Chen,†,‡,§ and Feng Wu*,†,‡,§ †

School of Material Science and Engineering, Beijing Key Laboratory of Environmental Science and Engineering, Beijing Institute of Technology, Beijing 100081, China ‡ Collaborative Innovation Center for Electric Vehicles in Beijing, Beijing 100081, China § National Development Center of High Technology Green Materials, Beijing 100081, China S Supporting Information *

ABSTRACT: Layered lithium-rich cathode materials have been considered as competitive candidates for advanced lithium-ion batteries because they are environmentally benign, high capacity (more than 250 mAh·g−1), and low cost. However, they still suffer from poor rate capability and modest cycling performance. To address these issues, we have proposed and constructed a spinel-structure skin and ferric oxide islands on the surface of layered lithium-rich cathode materials through a facile wet chemical method. During the surface modification, Li ions in the surface area of pristine particles could be partially extracted by H+, along with the depositing process of ferric hydrogen. After calcination, the surface structure transformed to spinel structure, and ferric hydrogen was oxidized to ferric oxide. The as-designed surface structure was verified by EDX, HRTEM, XPS, and CV. The experimental results demonstrated that the rate performance and capacity retentions were significantly enhanced after such surface modification. The modified sample displayed a high discharge capacity of 166 mAh·g−1 at a current density of 1250 mA·g−1 and much more stable capacity retention of 84.0% after 50 cycles at 0.1C rate in contrast to 60.6% for pristine material. Our surface modification strategy, which combines the advantages of spinel structure and chemically inert ferric oxide nanoparticles, has been shown to be effective for realizing the layered lithium-rich cathodes with surface construction of fast ion diffusing capability as well as robust electrolyte corroding durability. KEYWORDS: lithium-ion batteries, layered lithium-rich cathode material, spinel structure, ferric oxide, surface modification capability,11,12 continuous decrease of average voltage,13,14 and severe capacity fading.15 Many strategies have been adopted to address these problems.16−28Among these methods, surface coating is the most commonly used and effective method to suppress the capacity degradation of layered lithiumrich cathode material.29 Recently, various studies have focused on enhancing the rate performance of layered Li-rich cathode materials with spinel structure.26,30−32 For instance, our group has proposed two reasonable designs, spinel/layered heterostructure30 and ultrathin spinel membrane,33 to improve the rate performance of layered Li-rich cathode materials. The better rate performances are believed to be due to the high Li ion conductivity of spinel structure and their structure compatibility with layered lithium−metal oxides. However, the spinel structure itself suffers from poor cycling performance

1. INTRODUCTION Lithium-ion batteries are a big success in modern society. They power a myriad of portable electronic devices and play an increasingly important role in electric vehicles and energystorage systems.1 Currently, the high-energy and long-life lithium-ion batteries are urgently demanded. The energy density of the commercialized cathode materials, such as LiCoO2, LiNixCoyMnzO2 and LiFePO4, continuously increases through various modifications.2 However, their practical capacities are inferior to those of layered lithium-rich cathode materials, and hence, the latter has drawn much attention as one of the most promising candidate cathode materials for the next-generation Li-ion batteries.3 In addition to their high specific capacity of 250 mAh·g−1, Lirich layered oxides (LLOs),4−7 xLi2MnO3·(1−x)LiMO2 (M = transition metal), are low cost and environmentally benign because of their high abundance and the low toxicity of manganese. However, they suffer from some intrinsic defects, such as low initial Coulombic efficiency,8−10 poor rate © 2017 American Chemical Society

Received: November 19, 2016 Accepted: February 20, 2017 Published: February 20, 2017 8669

DOI: 10.1021/acsami.6b14862 ACS Appl. Mater. Interfaces 2017, 9, 8669−8678

Research Article

ACS Applied Materials & Interfaces Scheme 1. Schematic Illustration Showing the Strategy of the Surface Modification

First, 0.1616 g of Fe(NO3)3·9H20 and 0.046 g of NH4H2PO4 were dissolved in 50 mL of distilled water, respectively. Owing to the ionization effect of H2PO4−, the NH4H2PO4 solution is weakly acidic. Then, 2 g of as-prepared pristine powders were added into the ferric nitrate solution and ultrasonically dispersed for 30 min. Next, the NH4H2PO4 solution was slowly dropped into the suspension. The resulting suspension solution was continuously stirred for 3 h. During the process, the Li ion in the surface of pristine material was extracted through Li+/H+ ion exchange. In the meantime, Fe(OH)3 gradually deposited on the surface because of its smaller Ksp(Fe(OH)3) than Ksp(FePO4). Thus, the surface component of pristine material transformed to Li1.2−xHxMn0.6Ni0.2O2 and Fe(OH)3 particles were attached like islands on the surface. The corresponding reactions are as follows:

associated with the dissolution of Mn. In order to suppress the dissolution of Mn, nanosize oxide particles have been reported to serve as a HF scavenger.34 HF has been blamed for dissolution of the transition metals and the surface corrosion of cathode materials,35 and it could be neutralized by the added HF scavenger. In light of above research, here we propose a construction of spinel-structure skin and nanosize ferric oxide islands on LLOs, so as to exploit the advantages of both spinel structure and nanosize oxide particles. For simplicity, we label the sample with such surface construction as layer@spinel@oxide (LSO). In our approach, spinel structure provides a fast lithium ion diffusion channel and nanosize oxide particles help to stable the surface structure. As a consequence, LSO demonstrates enhanced electrochemical performance in terms of rate capability and cycling performance.

NH4H 2PO4 ↔ NH+4 + H 2PO−4 H 2PO−4 ↔ H+ + HPO24 −

Li1.2Mn 0.6Ni 0.2O2 + x H+ ↔ Li1.2 − xHxMn 0.6Ni 0.2O2 + x Li+

2. EXPERIMENTAL SECTION

Fe3 + + 3OH− ↔ Fe(OH)3

2.1. Synthesis of Pristine Material. Nanomaterials have larger specific surface area thereby the side reactions on which are more serious. Here, we chose to test the modification effects on nanomaterials. Pristine layered lithium-rich cathode materials, Li1.2Mn0.6Ni0.2O2, were synthesized by sol−gel method. The detailed processes are as follows. First, stoichiometric amounts of Ni(CH3COO)2·4H2O, Mn(CH3COO)2·4H2O and CH3COOLi·2H2O were dissolved together in distilled water. A 5% excess of lithium salt was used to offset lithium evaporative loss during the high-temperature reaction. Citric acid as a chelating agent was added dropwise into the salt solution after dissolution. The pH value of the result solution was adjusted to 7 using ammonia. Then, the solution was evaporated at 75 °C with continuous stirring until a gel was obtained. The obtained gel was dried in vacuum oven at 120 °C for 12 h. The dried precursors were preliminary calcined at 450 °C for 5 in air. Afterward, the obtained powders were pressed into pellets and calcined at 900 °C for 12 h in air. After cooling, the pristine material was obtained. In this work, all the chemicals were of analytical grade and used asreceived without further purification. 2.2. Procedures of Surface Construction. The surface modification strategy is composed of partial extraction of Li+ through Li+/H+ ion exchange, deposition of ferric hydroxide, and calcination process. The scheme is illustrated in Scheme 1, and a detailed description is as follows.

Thereafter, the solution was filtered, and the obtained powders were dried in a vacuum oven at 80 °C for 12 h. Finally, they were calcined at 450 °C for 5 h, during which the surface atoms were rearranged to form a defect-spinel structure and Fe(OH)3 was oxidized to Fe2O3. After they were allowed to cool, the construction was completed. 2.3. Preparation of Reference Samples. Preparing Fe2O3 nanoparticles: 1.616 g of Fe(NO3)3·9H2O was dissolved in 80 mL of deionized water with continuous stirring. Then, 0.46 g of NH4H2PO4 was added in. The resulting solution was transparent. Thereafter, pH was adjusted with NH4OH, and red-brown precipitate appeared immediately. The precipitate was filtered and washed, and it was dried at 80 °C before it was sintered at 450 °C for 5 h. Here, the as-prepared ferric oxide appeared as microsize particles, as observed by SEM. After the microsize particles were subjected to ball milling for 6 h, they became nanoparticles. Finally, 0.028 g of as-prepared Fe2O3 nanoparticles was mixed with 1.6 g of LLOs for 1 h using agate mortar. In order to construct a surface spinel layer, 0.046 g of NH4H2PO4 was dissolved in 50 mL of distilled water. Then, 2 g of as-prepared pristine powders was added. The resulting suspension solution was continuously stirred for 3 h. Thereafter, the particles were filtered, washed, and dried at 80 °C before they were sintered at 450 °C for 5 h. 2.4. Characterization Methods. Morphology and Structure Characterization. Power X-ray diffraction (XRD) was performed on 8670


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ACS Applied Materials & Interfaces

Figure 1. SEM images and energy-dispersive X-ray (EDX) spectroscopy: (a) SEM image and (b) EDX spectroscopy of LLOs; (c) SEM image, (d) EDX spectroscopy, and (e−g) EDX mapping of LSO. Rigaku UlTIMA IV−185 with Cu Kα radiation from 2θ = 10°∼ 90° at a scan rate of 8° per minute. The morphological studies were observed by FEI QUANTA 6000 scanning electron microscope (SEM) and JEOL JEM-1200EX transmission electron microscope. The X-ray photoelectron spectroscopy (XPS) was conducted on PHI Quantera II and analyzed by the software of XPSpeak. The energy of the spectra was calibrated by the binding energy of the hydrocarbon (C−H) at 284.6 eV. Raman spectroscopies were carried out on the Renishaw RM2000 with RL 633 nm laser. Electrochemical Performance Tests. Electrochemical performances of all samples were tested with coin-type half cell (2025) at 30 °C. The positive electrode was made of 80 wt % as-synthesized cathode materials, 10 wt % acetylene black, and 10 wt % polyvinylidene difluoride. The electrolyte was 1 M LiPF6 dissolved in mixed solvents of ethylene carbonate (EC) and dimethyl carbonate (DMC) with a volume ratio of 1:1. Circular lithium metal was used as negative electrode, and Celgard 2400 membrane was used as the separator. Galvaostatic charge/discharge test, rate performance, and galvanostatic intermittent titration technique (GITT) were performed by Land battery test system (Wuhan, China) in the voltage range of 2.0− 4.8 V. The current density of 250 mA·g−1 was defined as 1C rate during test. The cells were charged and discharged for one cycle at 0.1C rate before they were used to measure the performance cycled at 1C rate. During rate performance test, all charge current densities were 0.1C rate. In the GITT test, the cells were charged at 0.1C rate for an interval of 1 h followed by an open circuit stand for 2 h to allow the cell voltage to relax to the ready-state value. The procedure was repeated until the cell potential reached the set cutoff value. Cyclic voltammetry (CV) measurement and electrochemical impedance spectra (EIS) were conducted on the CHI660 electrochemical workstation. The potential window was 2−4.8 V, and the scan rates were 0.1 mV·s−1 in the CV test. EIS measurement was performed at frequencies from 100 kHz to 0.01 Hz with an AC perturbation signal of 5 mV.

primary particle sizes are between 100−200 nm. After surface modification, many additional nanosize particles emerge on the surface of original particles (some of which are marked with a circle in Figure 1c). Although some of the additional nanoparticles agglomerate, the particle size is much smaller than that of the primary particles. In order to identify the chemical composition of the nanoparticles, the ICP measurement has been carried out to determine the contents of Li, Mn, Ni, Fe, and P (normalized to the Ni content), and the results are shown in Table 1. The Table 1. Chemical Compositions of LLOs and LSO sample

Li

Mn

Ni

Fe

P

LLOs LSO

5.906 5.340

3.010 3.024

1 1

0.155

0.026

chemical formula of LLOs is close to the designed Li1.2Ni0.2Mn0.6O2 chemical stoichiometry. After surface modification, the Li content of LSO decreases significantly, as expected with the consideration of the Li+/H+ ion exchange. In the process of modification, a tiny amount of FePO4 is inevitably produced because of the presence of phosphate, which is detected by ICP for LSO. However, the molar ratio of detected P to Fe is only 16.7%, which confirms that the main coating agent is iron oxide rather than phosphate. The small amount of P compared to other elements also explains the experiment results that the EDX spectrum shows no trace of element P and that the signal of P 2p in the XPS spectra (Figure S1) for LSO is also negligible. Thus, combining the results of EDX, ICP, and XPS, it is reasonable to ascribe the coating nanoparticles to iron oxide. The crystal structures of both samples have been characterized by XRD as shown in Figure 2. The two collected patterns are nearly the same. The main patterns can be well indexed with α-NaFeO2 layered structure (R3̅m symmetry).21,36−38 The weak superlattice reflections that emerge

3. RESULTS AND DISCUSSION 3.1. Characterization of the Morphology and Structure. SEM images and energy dispersive X-ray (EDX) results of LLOs and LSO are shown in Figure 1. Clearly, the asprepared particles are irregular convex polyhedrons, and 8671


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Figure 2. Powder XRD patterns of the two samples and some Miller indices of the main peaks; (b,c) are enlarged view in (a).

Figure 3. HRTEM images of both LLOs and LSO: (a) TEM image of the PL particle; (b) HRTEM image in (a); (c) the SAED of (a); (d) The TEM image of the LSO particle; (e,f) the HRTEM image in (d).

Figure 4. XPS and Raman spectra of LLOs and LSO: (a) Fe 2p region for LSO; (b) Mn 2p region for LLOs and LSO; (c) Raman spectroscopies of both samples.

modification. From the enlarged view in Figure 2b,c, no signal from the spinel phase is found, which may be due to its tiny amount. In order to identify the phase of iron oxide, we prepared Fe2O3 nanoparticles under similar conditions. The XRD pattern of as-prepared Fe2O3 nanoparticles (seen in Figure S2) shows no sharp peaks, indicating the decomposition product of ferric hydroxide at 450 °C is amorphous. To further identify the microstructure, we have observed LLOs and LSO under high-resolution transmission electron microscope (HRTEM), and the typical images are shown in Figure 3. As for LLOs, the measured interlayer spacing of ca. 0.472 nm in Figure 3b corresponds to (003) fringes of typical layered structure.16,19 The corresponding selected area electron diffraction (SAED) patterns in Figure 3c present the reflections of (003) plane and (110) plane expected from rhombohedral α-NaFeO2 structure.19,45,46

around 2θ = 20−23° correspond to the Li2MnO3 component (C/2m symmetry), indicating the LiMn6 cation arrangement in the transition-metal layers.4,39,40 The clear splits of (006), (102) and (108), (110) peaks signify a well-layered structure.17,41,42 The calculated lattice parameters are listed in Table S1. Both lattice parameters a and c of LSO are larger than that of LLOs, which indicates unit cell expansion in a and c directions. The increased a-parameter is attributed to the decrease in average metal−metal distance due to the reduction of some Mn4+ during the process of heat treatment of surface modification.43 The extraction of Li ions during the process of Li+/H+ ion exchange increases the electrostatic repulsion between the oxygen layers, which leads to the lattice expansion along c-axis. Both the ratios of I(003)/I(104) (integrated intensity ratio) are greater than 1.2, which suggest low cation disorder.44 Meanwhile, the intensity of peaks displays a slight decrease caused by the process of Li+/H+ ion exchange after surface 8672


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Figure 5. Electrochemical performances of the two samples: (a) initial charging and discharging curves; (b) maximum discharge capacity curves at different rates; discharge capacity vs cycle number when cycled at 0.1C (c) and 1C (d) rate.

whereas an additional shoulder band emerging around 670 nm−1 for LSO, as indicated by the arrow, suggests the existence of spinel Li1+xMn2O4.7,33 3.2. Characterization of Electrochemical Performance. To explore the advantages of our approach, we have investigated the electrochemical performance as illustrated in Figure 5, and the specific values are summarized in Table S2. It is demonstrated that both the rate performance and the capacity retentions of LSO are significantly enhanced compared to those of LLOs. As shown in Figure 5a, the two samples deliver a similar initial discharge capacity at 0.1C rate. However, when testing the high-rate performance, LSO shows much superior performance thanks to the surface spinel structure. The maximum discharge capacity curves at different rates are compared in Figure 5b. The discharge capacity falls below 200 mAh·g−1 for LLOs when discharged at a 1C rate, whereas that of LSO is ca. 220 mAh·g−1 and its discharge capacity is still beyond 200 mAh·g−1 even at 2C rate. When discharged at 5C rate, the maximum discharge capacity can achieve as high as ca. 166 mAh·g−1 for LSO. When cycled at 0.1C rate in Figure 5c, LLOs suffers sever capacity fading due to the nanometre size effect and unprotected surface. Its discharge capacity fades rapidly from 275.9 mAh·g−1 to 167.2 mAh·g−1 after 50 cycles, with a capacity retention of 60.6%. Owing to the protection of ferric oxide nanoparticles, the capacity retention of LSO is improved to 84.0% at the same condition. In addition, when cycled at 1C rate in Figure 5d, LSO not only delivers a superior discharge capacity of 230.6 mAh·g−1 but also displays better capacity retention of 71.7% compared to the capacity retention of 60.6% for pristine material after 150 cycles. The Coulombic efficiency at 1C rate is slight higher than that at 0.1C rate. When testing at 0.1C, the smaller charging and discharging current will lead the electrolyte to be exposed to high voltage region for a longer

After surface modification, the bulk layered structure is preserved, and the measured bulk interplanar spacing of ca. 0.42 nm in Figure 3e is indexed with the (020) plane of C/2m symmetry. Different from the PL particle, the fringes of LSO are not stretched to the surface and display distinct characteristics. In the surface area, lattice fringes with an interplanar spacing of ca. 0.248 and 0.294 nm are observed, which could be indexed with the (311) plane and the (220) plane of the cubic spinel (Fd3̅m) structure. However, on the surface, there is a tiny additional nanoparticle adhered to the surface of this particle in Figure 3d, as indicated by the green rectangle. The measured lattice fringes, with interplanar spacing of ca. 0.223 nm and ca. 0.271 nm, are quite indexed with the (113) plane and the (104) plane of R3̅c symmetry, which are expected from ferric oxide. Thereby, the microstructures observed by HRTEM clearly confirm the as-design of spinel-structure skin and nanoparticle island, which reveals our strategy is carried out successfully. XPS as a powerful tool has been also applied to characterize surface chemical properties of both samples, as shown in Figure 4. It is demonstrated that there are two peaks at ca. 723.7 eV and ca. 710.8 eV and a satellite peak at ca. 719.2 eV in the Fe 2p spectra, which are assigned to Fe2O3.47 The Mn 2p3/2 spectra obtained from LLOs and LSO are compared in Figure 4b. The Mn 2p3/2 peak of LLOs is located at 641.8 eV. After surface modification, the Mn 2p3/2 peak of LSO become broader and a peak with lower binding energy can be clearly observed. This indicates the local environment change originating from the formation of spinel structure.48 Thus, the XPS results further confirm the existences of ferric oxide and the formation of spinel structure. To further verify the transformed spinel structure, Raman spectroscopy as another powerful tool of determining the shortrange local structure has been applied. Both samples share the typical characterizations of lithium-rich layered materials, 8673


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Figure 6. Compared electrochemical performances: (a) the cycling stability at 1C rate and (b) rate performance of the four samples; the rate performance was tested with a charging current of 0.1C.

charging/discharging curves in Figure 5a, is attributed to the reaction involving the removal of lithium and oxygen from Li2MnO3.50−53 After careful examination of the profile, a weak oxidation peak around 3.2 V is found for LSO, and it could be observed more clearly in the corresponding dQ/dV plots of the first charging curve (seen in Figure S3), which results from the existence of the spinel structure. The whole oxidation area for LSO is smaller than that for pristine material, indicating less charge capacity. Given the fact of less Li content and the reduced charge capacities in both regions (below and at ∼4.5 V plateau, as seen in Figure 5a) for LSO, we attribute the decline of the first charge capacity to two reasons: (1) Some Li ion is lost during the process of ion exchange, which indicates the reduction of available Li source; (2) Ferric oxide as electrochemically inert cathode material adds extra mass, thus lowering the specific charge capacity. During the discharging, apart from the two reduction peaks above 3.5 V corresponding to the reduction of Ni4+ for both samples and the special reduction peak at ca. 2.9 V relating to spinel structure in the surface of LSO, there are peaks below 3.5 V that signify the partial reduction of Mn4+ in the bulk.5,39,54 Differential capacity (dQ/dV) plot as useful tool to observe the capacity change at various voltages has been applied to investigate reason for the different capacity retentions. Figure 8 shows the dQ/dV plots of the two samples for the 2nd, 18th, 34th, and 50th cycle when cycling at a current density of 25 mA·g−1. It is demonstrated that the cathodic peak intensities above 3.5 V decrease upon cycling while the cathodic peak around 3.3 V shifts to ca. 2.9 V after 50 cycles. This phenomenon signifies the phase transformation from layered to spinel.14,55,56 Compared to the continuously decreased intensity of the cathodic peaks below 3.3 V for LLOs, LSO retains considerable discharge capacity originated from the reduction of Mn4+. Obviously, although LSO could not suppress the phase transformation from layered to spinel, the deterioration of transformed spinel structure is suppressed. We believe that is the primary reason for the better capacity retention for LSO. In order to explore the reason for the larger amount electrochemical reversible manganese of LSO, GITT test has been carried out to assess the overpotentials at various stages. As shown in Figure 9, during the initial charging, the overpotentials are limited until period 5 where the cells are charged to 4.5 V. This phenomenon implies that the process associated with lithium ion extraction from the LiMn0.5Ni0.5O2 component is highly reversible, whereas the increasing

time in each cycle. It has been shown that alkyl-carbonate-based electrolytes undergo oxidation at potentials in excess of 4.2 to 4.4 V vs Li+ leading to H+ generation.49 The longer residence time under high voltage, the more side reactions. Thus, LLOs shows better cycle performance at 1C rate than 0.1C rate. After surface coating, the Coulombic efficiency at 0.1C rate is slight improved. Consequently, we could conclude that our approach shows a great effect on enhancing the rate capability and cycling performance of the layered lithium-rich cathode materials. To further confirm the effect of spinel-structure skin and ferric oxide nanoparticles, we have prepared the sample of L@ O (LLOs + Fe2O3 nanoparticles) and L@S (LLOs with surface spinel structure) for comparison. We tested their cycling stability at 1C rate and rate performance, as compared in Figure 6. All the other samples show improved electrochemical performance compared with LLOs. Specially, the samples with Fe2O3 nanoparticles exhibit superior cycling performance, whereas the samples with surface spinel structure deliver better rate performance. These results strongly demonstrate the roles of Fe2O3 nanoparticles as HF scavenger and spinel structure as fast lithium ion diffusion channel. 3.3. Discussion. In order to achieve a more detailed understanding of electrochemical behaviors, we have carried out further characterization. First, the CV profiles of the two samples for the first cycle are displayed in Figure 7. The observed two oxidation peaks below 4.5 V result from the oxidation of Ni2+/Ni3+ and Ni3+/Ni4+. The strong peak at ca. 4.5 V, corresponding to the long plateau around 4.5 V in initial

Figure 7. CV profiles of the two samples at a scan of 0.1 mV·s−1 between the voltage range of 2−4.8 V. 8674


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Figure 8. dQ/dV curves of LLOs (a) and LSO (b) for the 2nd, 18th, 34th and 50th cycle with a 25 mA·g−1 current density between 2 and 4.8 V.

Figure 9. GITT curves of the two samples for the first cycle at a charge/discharge current density of 25 mA·g−1: (a) GITT curves; inset: the corresponding OCVs; (b) calculated specific overpotential at each stage.

overpotentials starting from period 6 to 15 suggest that the activation process of the Li2MnO3 component is kinetically limited.12,46,57 During the first discharge process, the equivalent open circle voltages (OCVs) of the two samples are nearly the same, but the overpotential of pristine material at the end of discharging increases more rapidly. During period j, the voltage of pristine material reaches to the lower cutoff voltage quickly while there is another period k for LSO. Definitely, it is the larger overpotential below 3.5 V (marked with background pattern) during discharging that limits the amount of reduced Mn4+ for LLOs. Namely, the surface modification endows LSO with a higher discharge capacity by reducing its overpotential and leveraging more Mn4+/Mn3+ couples. This reduced overpotential could be also attributed to a better condition of the surface structure, which is protected by ferric oxide from the eroding of HF. To further prove the protection effect, EIS technique has been introduced to assess the change of the interface resistance. Figure 10 shows the Nyquist plots of the two samples in the 2nd and 50th charges when cycled at a current density of 500 mA·g−1. All of the Nyquist plots show two semicircles and one slope. Given evidence from previous research on the same type of layered cathode materials,58−60 the semicircle in the high frequency is related to the lithium ion diffusion through the solid−electrolyte interface (SEI), whereas the second semicircle in the middle frequency is associated with the charge-transfer reaction between the surface film and the active cathode mass. The diameter of the semicircle represents the charge-transfer resistance (Rct). Accordingly, the fitted plots are shown in

Figure 10. Nyquist plots of the two samples in the 2nd and 50th charges. The data were acquired over 100 kHz ∼ 0.01 Hz frequency range after the cells charging to 4.0 V at a current density of 25 mA· g−1.

Figure S4 as well as discharge capacity versus cycle number. The corresponding impedance parameters are listed in Table S3. In the second cycle, the measured resistances for the two samples are similar, and the Rct of LLOs is even smaller than that of LSO. However, the charge-transfer resistances of both samples increase sharply after 50 cycles, which implies the interface structure suffering from damage. Obviously, the Rct of 8675



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LSO is much lower than that of the pristine material after 50 cycles. The protection of the ferric oxide nanoparticles from the eroding of HF in the electrolyte is believed to be responsible for the lower surface charge-transfer resistance of LSO.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b14862. XPS spectra of P 2p region for LSO, XRD pattern of Fe2O3 nanoparticle, the lattice parameters of the two samples, the detailed values of electrochemical performances for the two samples, the dQ/dV plots of the two samples corresponding to their initial charge/discharge curves, the fitted Nyquist curves, and the impedance parameters of two samples in the 2nd and 50th charges at the voltage of 4.0 V (PDF)

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REFERENCES

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4. CONCLUSIONS In summary, we have proposed and constructed a spinelstructure skin and ferric oxide islands on the surface of layered lithium-rich cathode materials through a facile wet chemical method. This symbiotic surface structure was verified by EDX, HRTEM, XPS, and CV. Through this strategy, the spinel structure provides a fast lithium ion diffusion channel, and ferric oxide nanoparticles help to stabilize the surface structure by neutralizing HF in the electrolyte. It is demonstrated that the rate performance and capacity retentions are significantly enhanced after such surface modification. The GITT and EIS results confirm the lower surface charge-transfer resistance in the cycling, which should result from the better surface condition for LSO. Our modification strategy is able to modify the subsurface structure and surface environment of layered lithium−metal oxides particles, simultaneously. We anticipate this design should inspire the development of advance intercalation materials and energy materials.

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Research Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail for Y.L.: [email protected]. Tel: 86-010-68918099. *E-mail for Y.S.: [email protected]. *E-mail for F.W.: [email protected]. ORCID

Yu Zheng: 0000-0001-7808-8004 Yuefeng Su: 0000-0002-5144-2832 Renjie Chen: 0000-0002-7001-2926 Author Contributions ¶

S.C. and Y.Z. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the National Natural Science Foundation of China (51472032, 21573017), National Key Research and Development Program (2016YFB0100301), Program for New Century Excellent Talents in University (NCET-13−0044) and Major Achievements Transformation Project for Central University in Beijing. 8676


Research Article

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