Surface phase transformation and CaF2 coating for enhanced electrochemical performance of Li-rich Mn-based cathodes

Electrochimica Acta 163 (2015) 82–92

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Surface phase transformation and CaF2 coating for enhanced electrochemical performance of Li-rich Mn-based cathodes Xiaoyu Liu, Tao Huang, Aishui Yu * Department of Chemistry, Collaborative Innovation Center of Chemistry for Energy Materials, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Institute of New Energy, Fudan University, Shanghai 200438, China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 10 October 2014 Received in revised form 17 February 2015 Accepted 17 February 2015 Available online 18 February 2015

To overcome the voltage decay upon cycling and increase the initial coulombic efficiency of the layered Li-rich Mn-based oxides, the double modification combining Na2S2O8 treatment with CaF2 coating has been first proposed in this study. The precondition Na2S2O8 treatment activates the Li2MnO3 phase gently and generates a stabilized three-dimensional spinel structure on the surface of particles, leading to a suppression of surface reaction and structure conversion during the subsequent electrochemical process. The mitigation of phase transformation for Na2S2O8-treated Li1.2Mn0.54Ni0.13Co0.13O2 alleviates the voltage decay and energy density degradation upon long-term charge-discharge cycling. In order to further restrain the capacity loss derived from the HF attack and manganese dissolution, 40 wt% Na2S2O8 treated-sample has been modified by an amorphous CaF2 layer with nano-scale thickness. The firstreported CaF2-coated/40 wt% Na2S2O8 treated-Li1.2Mn0.54Ni0.13Co0.13O2 presents excellent electrochemical properties with a high initial coulombic efficiency of 99.2%, a capacity retention rate of 89.2% after 200 cycles and a high-rate capability of 152.1 mAh g
1 at 3 C. The double surface modification offers a smart design concept for Li-rich Mn-based oxides to meet the practical requirements for advanced lithium ion batteries in electric vehicles. ã 2015 Elsevier Ltd. All rights reserved.

Keywords: Lithium-ion battery Li-rich cathode material Surface modification Phase transformation

1. Introduction Due to high energy density and high power density, lithium ion battery has been considered as the promising power sources for electric vehicles (EV) and hybrid electric vehicles (HEVs) [1,2]. However, it falls short of meeting the demands of new markets in these areas of EVs or HEVs because of insufficient specific capacity when using the conventional cathode materials, such as LiCoO2, LiMn2O4 and LiFePO4. Recently, Li-rich Mn-based layered solidsolution system Li2MnO3-LiMO2 (M¼Co, Ni, Mn1/2Ni1/2, Mn1/3Ni1/ 3Co1/3) has drawn much attention for its higher capacity over 250 mAh g
1 with significantly reduced cost and toxicity compared to the LiCoO2 cathode [3,4]. However, the extensive commercial application of Li-rich Mn-based cathode materials is hindered by several deficiencies. The first problem is the low initial coulombic efficiency which is attributed to the extraction of Li2O in the initial charge process and the subsequent elimination of Li+ insertionextraction sites [5]. Second, poor cyclic performance and rate capability due to complicated structural evolution and formation of solid–electrolyte interfacial (SEI) layer at higher potential are

* Corresponding author. Tel.: +86 21 51630320. E-mail address: [email protected] (A. Yu). http://dx.doi.org/10.1016/j.electacta.2015.02.155 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.

also practical problems to solve urgently. Furthermore, severe discharge voltage decay due to the structural conversion during high voltage cycling leads to the continual decrease in energy density of the batteries [6]. Owing to surface-sensitivity for Li-rich Mn-based cathode materials, many efforts have been made to improve the electrochemical properties through surface modification. One effective strategy is to achieve the formation of spinel structure in the surface regions by chemical treatment and post-annealing. The enhanced initial coulombic efficiency and high-rate capability of Li-rich Mn-based layered cathode materials have been accomplished by inducing the surface spinel transformation after surface treatment of acidic aqueous solution [7,8], persulfate [9], (NH4)2SO4 [10], AlF3 [11] and Super P (carbon black) [12]. However, the structural evolution of treated cathode materials and electrochemical mechanism of this surface modification upon cycling are barely discussed. What is more, the cyclic stability of treated sample still needs further improvement for practical applications. Here, our goal is to enhance electrochemical performance of layered Li-rich Li1.2Mn0.54Ni0.13Co0.13O2 through surface modification. Firstly, Na2S2O8 was used to modify the Li1.2Mn0.54Ni0.13Co0.13O2 particle surface. The effects of surface treatment with different amounts of Na2S2O8 on the structure and electrochemical

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Fig. 1. SEM images of as-prepared Li1.2Mn0.54Ni0.13Co0.13O2 at different magnification.

properties were systematically investigated. And the other objective of our research is to attempt to understand the improvement mechanism for Na2S2O8-modification through investigating the structure transformation of both pristine and treated materials upon electrochemical cycling. Furthermore, an amorphous CaF2 layer with nano-scale thickness was coated on the surface of Na2S2O8-treated Li1.2Mn0.54Ni0.13Co0.13O2 particles to stabilize the electrolyte-electrode interface and then further enhance the cycling stability upon long-term cycling. As cathode candidate of advanced lithium ion battery, the first-reported CaF2coated/Na2S2O8-treated Li1.2Mn0.54Ni0.13Co0.13O2 exhibits excellent electrochemical properties.

450  C in the flowing nitrogen for 4 h to obtain the CaF2-coated/ 40 wt% Na2S2O8-treated Li1.2Mn0.54Ni0.13Co0.13O2. 2.2. Characterization The crystalline structure and morphology were characterized by X-ray diffraction (XRD, Bruker D8 Advance, Cu Ka radiation,

2. Experimental 2.1. Preparation The layered Li-rich Li1.2Mn0.54Ni0.13Co0.13O2 was synthesized by a precipitation method. Ni(CH3COO)24H2O, Co(NO3)39H2O and Mn(CH3COO)24H2O with a desired stochiometric ratio (54:13:13) and Na2CO3 solution were added simultaneously to agitating aqueous solution to form the carbonate precipitate MCO3. During this process, NH3H2O was added to control the pH of the solution within 7–8. The obtained MCO3 and Li2CO3 (5% excess) were mixed and calcined in air for 5 h at 500  C and then for 20 h at 900  C. The prepared active materials were immersed in Na2S2O8 aqueous solution. The amount of Na2S2O8 used relative to the weight of the active materials was 20, 40 and 60 wt% respectively. The mixed solution was stirred and then fully dried at 80  C. The resulting powder was annealed at 350  C for 5 h. Finally, the materials were dispersed, washed and centrifuged in distilled water four times to remove the soluble impurities from the active materials. The obtained powders were then dried in a vacuum oven at 100  C for 12 h. The subsequent CaF2 coating was obtained by a chemical deposition method. The 40 wt% Na2S2O8-treated Li1.2Mn0.54Ni0.13Co0.13O2 powders were immersed in the Ca(NO3)2 dilute aqueous solution. Then the solution was heated to 80  C and stirred vigorously. NH4F dilute solution was then added into the solution drop by drop. The molar ratio of Ca to F was controlled to be 1:2, and the designed amount of CaF2 was 2 wt% of the active materials. Continuous stirring was performed till the mixed solution was evaporated to dryness. The obtained powders were heated at

Fig. 2. XRD patterns (a) and Raman spectra (b) of Li1.2Mn0.54Ni0.13Co0.13O2 treated with different amount of Na2S2O8.

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Fig. 3. XPS spectra of Mn 2p, Ni 2p and Co 2p for pristine (a) and 60 wt% Na2S2O8-treated Li1.2Mn0.54Ni0.13Co0.13O2 (b).

Fig. 4. Initial charge-discharge curves (a) and corresponding dQ/dV plots of the first charge process (b) and the first discharge process (c) for Li1.2Mn0.54Ni0.13Co0.13O2 treated with different amount of Na2S2O8.

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Table 1 Electrochemical properties of Li1.2Mn0.54Ni0.13Co0.13O2 treated with different Na2S2O8 amount during the initial cycle. Sample

1st charge capacity/mAh g
1

1st discharge capacity/mAh g
1

1st coulombic efficiency/%

Pristine 20 wt% Na2S2O8 treated 40 wt% Na2S2O8 treated 60 wt% Na2S2O8 treated

330.4 284.4 265.8 233.4

262.6 255.7 261.7 251.4

79.5 89.9 98.4 107.7

Fig. 5. Cyclic stability of Li1.2Mn0.54Ni0.13Co0.13O2 treated with different amount of Na2S2O8 at 0.1 C (a) and 1 C (b) and EIS plots of these samples with the equivalent circuit in the inset and the fitting results in the form of lines (c).

l = 1.5406 Å), scanning electron microscopy (SEM, JEOL JSM-6390), energy-dispersive X-ray spectrometer (EDX, FE-SEM, Hitachi S-4800), transmission electron microscopy (TEM, JEM
2100 F) and selected area electron diffraction Table 2 The simulated data from EIS spectra using the equivalent circuit shown in the inset of Fig. 5(c). Sample

Rs/ohm

Rct/ohm

Pristine 20 wt% Na2S2O8 treated 40 wt% Na2S2O8 treated 60 wt% Na2S2O8 treated

3.320 3.549 3.522 3.096

163.4 139.8 112.1 104.3

(SAED, TEM, JEM-2100F). Furthermore, X-ray photoelectron spectroscopy (XPS) experiments were carried out on a RBD upgraded PHI-5000C ESCA system (Perkin Elmer) with Al Karadiation (hy = 1486.6 eV). Binding energies were calibrated using the containment carbon (C1s = 284.6 eV). Raman spectra were recorded on a Raman spectrometer (Renishaw inVia Reflex) coupled with microscope in a reflectance mode with a 514.5 nm excitation laser source. 2.3. Electrochemical measurements The working electrodes were consisted of 80 wt% as-prepared powders, 10 wt% carbon conductive agents (super P) and 10 wt%

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polytetrafluoroethylene (PTFE) and compressed onto the aluminum nets. The total mass of the active electrode material is 4–5 mg and the electrode surface area is 0.785 cm2 (F10 mm). The electrodes were dried overnight at 80  C in a vacuum oven prior to use. A metallic lithium foil was used as an anode, 1 M LiPF6 in ethylene carbonate (EC)-dimethyl carbonate (DMC)- diethyl carbonate (DEC) (1:1:1 in volume) was used as the electrolyte, and a polypropylene micro-porous film (Cellgard 2300) served as the separator. Coin-type (CR2016) half-cells were assembled in an argon-filled glove box (Mikarouna, Superstar 1220/750/900). The galvanostatic discharge-charge measurements were performed on the battery test system (Land CT2001A, Wuhan Jinnuo Electronic Co. Ltd.) between 2.0 and 4.8 V (vs. Li/Li+) at different rates at room temperature. Electrochemical impedance spectroscopy (EIS) measurements were performed on an electrochemical workstation (Zahner IM6e). 3. Results and discussion 3.1. Morphology and structure of the Na2S2O8-treated materials The SEM images of as-prepared Li1.2Mn0.54Ni0.13Co0.13O2 cathode materials are shown in Fig. 1. The samples exist as loose spherical aggregates with a size of several microns, and the aggregates are assembled from the primary particles with a diameter of 100–200 nm. The small particle size is expected to facilitate lithium ion transport by shortening the diffusion lengths and providing a larger interface area between the electrode materials and the electrolyte.

Fig. 2(a) shows XRD patterns of pristine and Na2S2O8-treated Li1.2Mn0.54Ni0.13Co0.13O2. As can be seen, all the diffraction peaks of both samples, except weak peaks between 2u = 20–25 , are indexed to a-NaFeO2 hexagonal type structure with a space group symmetry of R

3

m. Clear separation of adjacent peaks of

(006)/(012) and (108)/(110) indicates that the samples have a well crystalline layered structure. Weak peaks located between 2u = 20–25  are attributed to the short-range Li-Mn cation ordering in the transition metal (TM) layers [13,14], showing the presence of monoclinic Li2MnO3 phase with a space group symmetry of C2/m. As the amount of treating agent increases, the intensity of these weak peaks appears lower, resulting from the chemical dilithiation during the Na2S2O8 treatment process. The Li2MnO3 phase acts as the Li+ reservoir and the Li+ in the TM layers can migrate to the Li layer when Li+ ions in the Li layers are removed [15]. Thus the chemical dilithiation in the surface region during the Na2S2O8 treatment process induces partial disappearance of Li—Mn cation ordering in the TM layers. Additionally, from the results of the Raman spectroscopy of pristine and Na2S2O8treated Li1.2Mn0.54Ni0.13Co0.13O2 presented in Fig. 2(b), we can deduce the surface structure change of the samples. The pristine Li1.2Mn0.54Ni0.13Co0.13O2 shows two major Raman peaks at 495 cm
1 and 598 cm
1, indicating the M-O stretching n1 (A1 g) and the O—M—O bending n2 (Eg) of layered LiMO2 compound.9 After the Na2S2O8 treatment, an emergence of a new peak at about 648 cm
1 and gradual disappearance of the peaks at 495 cm
1 and 598 cm
1 suggest the structural transformation from the layered structure to the spinel structure on the surface [9,16]. The surface spinel structure can be attributed to the lithium and oxygen

Fig. 6. dQ/dV plots of the 2nd (solid) and 100th (dash) cycle for pristine (a) and 40 wt% Na2S2O8-treated (b) Li1.2Mn0.54Ni0.13Co0.13O2and continuous charge/discharge curves of pristine (c) and 40 wt% Na2S2O8 treated (d) Li1.2Mn0.54Ni0.13Co0.13O2.

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Fig. 7. XRD patterns (a) and Raman spectra (b) of pristine and 40 wt% Na2S2O8treated Li1.2Mn0.54Ni0.13Co0.13O2 electrode after 100 cycles at 0.1 C.

removal and subsequent cations rearrangement during the chemical treatment process, resembling the electrochemical activation process in the first charge process for the layered Lirich cathode materials. In order to further determine the surface transformations of the cathode after the most severe chemical treatment, the XPS measurements were conducted, as exhibited in Fig. 3. It is shown that the valences of Mn, Co and Ni did not change after the precondition process, which is supported by the evidences of the Ni2+, Co3+, Mn4+ 2P3/2 peaks remain at their original binding energies after 60 wt% Na2S2O8 treatment. 3.2. Electrochemical behaviors of the Na2S2O8-treated materials Galvanostatic charge/discharge experiments were used to investigate the effects of surface treatment with different amounts of Na2S2O8 on the electrochemical properties. Fig. 4(a) compares the performance of the initial charge/discharge curves of Li1.2Mn0.54Ni0.13Co0.13O2 treated with different amounts of Na2S2O8 at the current rate of 0.1 C (1 C = 250 mA g
1). It can be seen that the pristine sample displays a long plateau at around 4.5 V during the first charge process, which is related to removal of oxygen from the surface with further delithiation [17–19]. As the Na2S2O8 amount increases during the precondition process, this plateau is obviously suppressed, leading to smaller initial charge capacity and larger initial coulombic efficiency (shown in Table 1). Moreover, there is

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also a significant change in the profile of the discharge curves with Na2S2O8 treatment. An additional plateau below 3.0 V emerges at the end of the discharge curves after Na2S2O8 treatment, which is characteristic of spinel structure [9–12] and consistent with Raman results in Fig. 2(b). These changes in the initial charge/ discharge curves are supported by the corresponding dQ/dV plots of the first charge/discharge process in Fig. 4(b) and (c). An obvious decrease in the intensity and area of the oxidation peak at 4.5 V, as well as a gradual increase in the intensity and area of the reduction peak at 2.9 V is observed. These findings suggest that a certain amount of oxygen ions in the lattice have been leached out and the surface structure has transformed from the layered phase to the spinel phase to some extent after Na2S2O8 treatment. Therefore, the Na2S2O8 treatment is similar to traditional electrochemical activation for Li-rich cathode with the removal of lithium and oxygen, resulting in significantly enhanced initial coulombic efficiency for Na2S2O8-treated samples, particularly closed to 100% for 40 wt% Na2S2O8-treated sample. The heating process can accelerate the phase transformation and generate a stable spinel structure with three-dimensional interstitial space for Li+ ion diffusion, which has a great effect on the electrochemical behaviors. From the results of cyclic performances at a low current rate of 0.1 C for Li1.2Mn0.54Ni0.13Co0.13O2 cathode materials treated with different Na2S2O8 amounts as demonstrated in Fig. 5(a), it can be found that all the samples show an obvious capacity fading, with the capacity retention of 65.0%, 73.8%, 72.5% and 59.4% after 100 cycles respectively for the pristine, 20 wt%, 40 wt% and 60 wt% Na2S2O8-treated Li1.2Mn0.54Ni0.13Co0.13O2 sample. However, at a high current rate of 1 C, there is a significant capacity increase after Na2S2O8 treatment as displayed in Fig. 5(b). As can be seen, the initial discharge capacity at 1 C reaches 208 mAh g
1 and remains 170 mAh g
1 after 100 cycles for 40 wt% Na2S2O8-treated Li1.2Mn0.54Ni0.13Co0.13O2, which is clearly superior to pristine Li1.2Mn0.54Ni0.13Co0.13O2. In order to investigate the origin of significantly improved high-rate capability of Na2S2O8-treated samples, the EIS measurements of the pristine and treated samples were carried out at the open circuit status. The experimental results were fitted with the equivalent circuits inset in Fig. 5(c) and the fitting results are listed in Table 2, where Rct is the charge transfer resistance in the electrode/electrolyte interface and Rs is the resistance of the solution. A huge reduction in Rct from 163.4 V to 104.3 V with the increase of the treating Na2S2O8 amount is observed. The lower Rct can facilitate Li+ transfer on the electrode/ electrolyte interface, leading to obviously enhanced hig-rate capability. The decrease of Rct value may result from the formation of ordered spinel structure in the surface region of particles during the treatment, supported by the above Raman results and the initial discharge curves. Nevertheless, for 60 wt% Na2S2O8-treated Li1.2Mn0.54Ni0.13Co0.13O2, in spite of the lowest Rct value and highest initial discharge capacity at the 1C-rate, the deterioration of cyclic performance demonstrates that the bulk structure of Li-rich cathode material has been damaged by excess Na2S2O8-treatment. From all the above results, 40 wt% Na2S2O8-treated Li1.2Mn0.54Ni0.13Co0.13O2 was selected as the target for further investigation and modification due to its superior electrochemical properties. 3.3. Investigation on the effects of Na2S2O8-treatment It is generally accepted that voltage decay is one of the major challenges for the Li-rich Mn-base cathode materials, which is attributed to the structure conversion from the layered phase to the spinel-like phase upon high-voltage cycling [6,20,21]. Thus we have studied the profiles of dQ/dV plots before and after long-term cycling as well as continuous charge/discharge curves for pristine and treated samples. As depicted in the dQ/dV plots in Fig 6(a), the

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Fig. 8. HR-TEM images of pristine (a) and 40 wt% Na2S2O8-treated Li1.2Mn0.54Ni0.13Co0.13O2 (b) before electrochemical process and pristine (c) and 40 wt% Na2S2O8-treated Li1.2Mn0.54Ni0.13Co0.13O2 (d) after 100 cycles at 0.1 C.

Mn4+ reduction peak shifted from 3.2 V to 2.8 V after 100 cycles, indicating the rapidly increasing formation of the spinel-like phase for pristine Li1.2Mn0.54Ni0.13Co0.13O2 during cycling. By contrast, the dQ/dV plots in Fig 6(b) display a broad Mn4+ reduction peak with a swell at 2.9 V before long-term cycling, corresponding to the coexistence of the surface spinel structure and the bulk layered structure. After long-term cycling, the layered-to-spinel transformation was also observed according to the evolution of Mn4+ reduction peak. However, the evolution was evidently mitigated and the Mn4+ reduction peak related to the layered structure at 3.2 V still remained after 100 cycles in spite of the low intensity. According to the above results and discussion, we can conclude that the phase conversion upon long-term cycling has been suppressed by Na2S2O8-treatment, associating with the chemical activation accompanied with the Li2O removal and the spinel formation during the precondition process and giving rise to the mitigation of voltage decay for 40 wt% Na2S2O8-treated sample compared with the pristine one, as shown in Fig 6(c) and (d). To further investigate the possible explanations for enhanced electrochemical performance after Na2S2O8-treatment, ex-situ XRD patterns of cycled samples for both cathode materials were utilized to try to provide further evidence to the structural conversion, as described in Fig 7(a). It is obvious that the superstructure peaks in the 2u range of 20–25  have disappeared in the cycled samples, indicating that Li—Mn cation ordering in the TM layers has been removed during long-term high-voltage cycling. Most diffraction peaks of both samples are still assigned to a-NaFeO2 hexagonal type structure, except a trace of spinel-related shoulder peaks appearing

at 36.5 , suggesting a layer-to-spinel phase transformations [11,12,22]. For the cycled sample of pristine Li1.2Mn0.54Ni0.13Co0.13O2, the split and broadening of peak (015) at 48.2 and peak (107) at 57.6  are attributed to the deterioration of layered structure due to the formation of disordered spinel-like domains. Comparatively, the cycled sample of 40 wt.% Na2S2O8-treated Li1.2Mn0.54Ni0.13Co0.13O2 exhibits a more robust structure, attributed to the restrain of structure conversion upon cycling. With the aim to examine the surface structure evolution of both cathode materials, the Raman spectroscopy measurements of the cycled samples were carried out. Both samples display similar Raman spectroscopy patterns with two main peaks at 498 cm
1 and 648 cm
1. As mentioned above, the emergence of a new peak at about 648 cm
1 is attributed to the formation of the spinel structure on the surface. Thus the formation of an independent peak at 648 cm
1, accompanied with the complete disappearance of pristine peak at 598 cm
1 corresponding to the layered structure, illustrates the large-scale occupation of spinel or spinel-like structure in the surface regions. The peak at 648 cm
1 for 40 wt% Na2S2O8-treated Li1.2Mn0.54Ni0.13Co0.13O2 after cycles is much sharper. It may be attributed to the good crystalline of the spinel structure generated in the post-annealing process of Na2S2O8-treatment in contrast with the disordered spinel-like structure created upon high-voltage cycling. Explicit evidence for surface and structure transformation of both samples is provided by comparative TEM images offered in Fig. 8. Before the electrochemical process, the pristine Li1.2Mn0.54Ni0.13Co0.13O2 exhibits a hexagonal layered structure while 40 wt%

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Na2S2O8-treated Li1.2Mn0.54Ni0.13Co0.13O2 is composed of cubic spinel structure and hexagonal layered structure, as shown in Fig. 8(a) and (b). The similarity in symmetry of the two phase results in good structural compatibility of the two components [12]. The newly-formed spinel phase during the Na2S2O8 treatment process is distributed on the surface regions, as presented in the inset of Fig. 8(b), and consistent with the Raman results of Fig. 2(b). After 100 cycles at a low current rate of 0.1 C, a great amount of distortions formed in the surface regions of pristine Li1.2Mn0.54Ni0.13Co0.13O2 particles, as circled by red lines in Fig. 8(c). These domains with typical dislocation structure suggest the collapse of the partial layered matrix and the formation of disordered spinellike nano-domains. And many micro-cracks are observed at the surface of the particles, originating from the large lattice strain due to the extensive oxygen removal and the instability of defect spinel-like structure [23,24]. Furthermore, an amorphous layer without fringe contrast formed on the grain surface and it could be the product of electrolyte oxidation caused by active oxygen species during the electrochemical activation process [19,22]. These evolutions hinder the lithium ion pathway in the electrodeelectrolyte interface, leading to the deterioration of electrochemical properties during long-term cycling. Fig. 8(d) presents the images of 40 wt% Na2S2O8-treated Li1.2Mn0.54Ni0.13Co0.13O2 after 100 cycles. The well-crystallized grain with a small amount of disorder in the crystal periodicity of surface regions indicates the more robust structure with less phase transformation upon longterm cycling. The absence of the amorphous layer can be attributed to the suppression of electrochemical oxygen activation during the initial several cycles. Interestingly, we observed obvious corrosion

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on the surface of these particles. The corrosion holes generated upon long-term cycling could result from the etching by the acidic species [25–27] and the dissolution of manganese [28–30], widely reported for spinel cathode materials. This phenomenon may explain the capacity fading for 40 wt.% Na2S2O8-treated Li1.2Mn0.54Ni0.13Co0.13O2, especially at a low current rate. According to the experimental results presented above, a probable mechanism for the structural evolution of pristine and Na2S2O8-treated Li-rich Mn-based cathode materials upon highvoltage cycling is illustrated in Fig. 9. For untreated Li-rich Mnbased cathode materials, O2
in the surface region is oxidized to O2 during the electrochemical activation process above 4.5 V in the initial charge process. Meanwhile, part of active intermediate oxygen species reacts with electrolyte at the high potential, leading to the formation of an amorphous surface layer. Then oxygen removal triggers the phase transformation from the pristine layered phase to a defect spinel-like phase in the surface regions [31,32] and the scope of defect spinel-like structure expands inwards continuously during repeated electrochemical cycles, resulting in obvious voltage decay. After long-term cycling, the large lattice strain, induced by extensive oxygen removal and spinel-like structure expansion, causes the appearance of micro-cracks at the particle surface and collapse of lithium ion transport channel in the interface. On the other hand, for 40 wt% Na2S2O8-treated Li-rich Mn-based cathode materials, the Li2MnO3 phase is pre-activated mildly and stable three-dimensional spinel phase is generated in the surface region of particles during the post-annealing process of chemical pretreatment. Therefore, the formation of oxygen

Fig. 9. Schematic showing the proposed mechanism for the structural evolution of pristine and 40 wt% Na2S2O8-treated Li1.2Mn0.54Ni0.13Co0.13O2 upon high-voltage cycling.

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Fig. 10. SEM image (a) and EDX patterns (b) and HRTEM images (c) of CaF2-coated/40 wt% Na2S2O8-treated Li1.2Mn0.54Ni0.13Co0.13O2.

vacancies and the following phase transformation are significantly suppressed, contributing to the mitigation of voltage decay upon cycling. For the same reason, micro-cracks and amorphous layer at the surface are also prevented effectively, leading to a relatively stable electrolyte/electrode interface. Unfortunately, corrosion derived from HF attack and manganese dissolution, which causes continuous capacity fading, cannot be avoided by Na2S2O8 pretreatment alone. Thus further improvement of cycling performance of Na2S2O8-treated Li-rich Mn-based cathode materials could be achieved via surface coating. Recently, CaF2 has been reported to exhibit good thermal stability and inert behavior in the electrolyte [33,34]. In the present paper, CaF2 was chosen as the coating agent on the 40 wt% Na2S2O8-treated Li1.2Mn0.54Ni0.13Co0.13O2 to obtain the stable electrolyte/electrode interfacial structure and enhanced electrochemical performance. 3.4. Further CaF2-coating on the Na2S2O8-treated Li1.2Mn0.54Ni0.13Co0.13O2 Fig. 10 displays the morphology of as-prepared CaF2-coated/ 40 wt% Na2S2O8-treated Li1.2Mn0.54Ni0.13Co0.13O2 cathode materials. The high-magnification SEM image in Fig. 10(a) clearly illustrates that the surface of CaF2-coated samples becomes quite rough. In order to prove the homogeneity of the coating layer, EDX test was carried out on CaF2-coated samples and the results in Fig. 10(b) show the existence of Ca and F elements besides pristine O, Mn, Ni, Co elements. Meanwhile, as exhibited in the TEM image of Fig. 10(c), a clear boundary separates the 5–8 nm thick amorphous coating layer from the bulk crystalline phase,

indicating that CaF2 coating has been successfully achieved on the surface of Na2S2O8-treated Li1.2Mn0.54Ni0.13Co0.13O2. The obtained CaF2-coated/40 wt% Na2S2O8-treated Li1.2Mn0.54Ni0.13Co0.13O2 cathode materials deliver attractive electrochemical behaviors, as shown in Fig. 11. The initial charge and discharge capacities at 0.1 C are 265.5 mAh g
1 and 262.7 mAh g
1, with an initial coulombic efficiency as high as 99.2%. The suppression of side reaction at the high voltage due to CaF2 coating helps to further enhance the initial coulombic efficiency. After 100 cycles, the capacity retention rate still maintains a high value of 90.2%.The stable cyclic performance at a low current rate of 0.1 C exhibited in Fig. 11(b) should be attributed to robust interfacial structure upon long-term cycling. The rate capability further highlights the advantage of the CaF2 coating, as presented in Fig. 11(c). When the charge/discharge rate is increased to 1 C, 2 C and 3 C, the discharge capacities are measured at 216.5, 178.0 and 152.1 mAh g
1, respectively. Moreover, as the charge/discharge rate is decreased back to 0.1 C, the high specific capacity can be reversibly recovered, demonstrating that the surface modified sample is quite durable to higher charge/discharge current. To get insight into the origin of the improvement in cycling stability after CaF2 surface modification, EIS patterns of the pristine, 40 wt% Na2S2O8-treated and CaF2-coated/40 wt% Na2S2O8-treated Li1.2Mn0.54Ni0.13Co0.13O2 cathode were measured after 100 charge/discharge cycles at 0.1 C and presented in Fig. 12. The experimental results were fitted with the equivalent circuits inset in Fig. 12 and the fitting results are listed in Table 3. An additional high-frequency semicircle ascribed to the formation of the passivating surface film upon cycling is observed and represented as Rsl. The cycled CaF2-coated/40 wt% Na2S2O8treated Li1.2Mn0.54Ni0.13Co0.13O2 cathode shows the lowest Rct and

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Fig. 11. Initial charge-discharge curves (a), cyclic stability at 0.1 C (b) and rate capability (c) for CaF2-coated/40 wt% Na2S2O8-treated Li1.2Mn0.54Ni0.13Co0.13O2.

Table 3 The simulated data from EIS spectra using the equivalent circuit shown in the inset of Fig. 12. Sample

Rs/ohm

Rsl/ohm

Rct/ohm

Pristine 40 wt% Na2S2O8-treated CaF2-coated/40 wt% Na2S2O8-treated

6.217 6.682 4.125

23.37 11.44 9.53

80.29 65.27 31.52

Rsl values, demonstrating that the electrolyte oxidation and the electrolyte/electrode interface degradation have been alleviated effectively by CaF2 coating, which leads to the obviously improved cyclic stability. 4. Conclusions

Fig. 12. Electrochemical impedance spectroscopy (EIS) and the used equivalent circuit (inset) of pristine, 40 wt% Na2S2O8-treated and CaF2-coated/40 wt% Na2S2O8-treated Li1.2Mn0.54Ni0.13Co0.13O2 after 100 cycles at 0.1 C. The symbols and lines correspond to the experimental and simulated data, respectively.

In the present paper, different amounts of Na2S2O8 are used to modify the layered Li-rich Li1.2Mn0.54Ni0.13Co0.13O2 particle surface. Among them, 40 wt% Na2S2O8 treated-Li1.2Mn0.54Ni0.13Co0.13O2 exhibits the most outstanding electrochemical performance with a high initial coulombic efficiency of 98.4% and a superior high-rate capability. The mild chemical activation of the Li2MnO3 phase through the Na2S2O8 treatment alleviates the surface deterioration

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resulted from extensive oxygen removal whlie the formation of stable surface spinel structure mitigate the phase conversion and thus the voltage decay upon cycling. To further improve the cyclic stability, an amorphous CaF2 layer with a thickness of 5–8 nm is successfully coated on the surface of 40 wt% Na2S2O8 treatedsample. The CaF2-coated/40 wt% Na2S2O8-treated Li1.2Mn0.54Ni0.13Co0.13O2 cathode displays excellent electrochemical properties, due to the suppression of corrosion from the HF in the electrolyte. The results presented here illustrate the benefits of double surface modification including appropriate Na2S2O8 pretreatment and subsequent CaF2 coating for the Li1.2Mn0.54Ni0.13Co0.13O2 cathode materials. Therefore, we believe that this simple and straightforward method provides an opportunity to achieve large-scale commercial application for Li-rich Mn-based layered oxides as appealing cathode materials of high-energy lithium ion batteries as power sources of electric vehicles. Acknowledgements The authors acknowledge funding supports from Program (2013CB934103), and Science & Technology Commission of Shanghai Municipality (12dz1200402 & 08DZ2270500), China. References [1] J.M. Chen, C.H. Hsu, Y.R. Lin, M.H. Hsiao, T.K. Fey, High-power LiFePO4 cathode materials with a continuous nano carbon network for lithium-ion batteries, J. Power Sources 184 (2008) 498. [2] B. Huang, P.F. Shi, Z.C. Liang, M. Chen, Y.F. Guan, Effects of sintering on the performance of hydrogen storage alloy electrode for high-power Ni/MH batteries, J. Alloys Compd. 394 (2005) 303. [3] Z. Lu, D.D. Macneil, J.R. Dahn, Layered cathode materials Li[NixLi(1/3–2x/3)Mn(2/ 3–x/3)]O2 for lithium-ion batteries, Electrochem. Solid-State Lett. 34 (2001) 191. [4] Z. Lu, J.R. Dahn, Understanding the anomalous capacity of Li/Li[NixLi(1/3–2x/ 3)Mn(2/3–x/3)]O2 cells using in situ X-ray diffraction and electrochemical studies, J. Electrochem. Soc. 49 (2002) 815. [5] Y. Wu, A. Manthiram, Surface-modified layered Li[Li(1-x)/3Mn(2-x)/3Ni x/3Co x/ 3]O2 cathodes with low irreversible capacity loss, Electrochem. Solid-State Lett. 9 (2006) 221. [6] J.R. Croy, D. Kim, M. Balasubramanian, K. Gallagher, S.H. Kang, M.M. Thackeray, Countering the voltage decay in high capacity xLi2MnO3  (1–x)LiMO2 electrodes (MMn Ni, Co) for Li+-ion batteries, J. Electrochem. Soc. 159 (2012) 781. [7] M.M. Thackeray, C.S. Johnson, J.T. Vaughey, N. Li, S.A. Hackney, Advances in manganese-oxide ‘composite’ electrodes for lithium-ion batteries, J. Mater. Chem. 15 (2005) 2257. [8] S.H. Kang, C.S. Johnson, J.T. Vaughey, K. Amine, M.M. Thackeray, The effects of acid treatment on the electrochemical properties of 0.5 Li2MnO30.5 LiNi0.44Co0.25Mn0.31O2 electrodesin lithium cells, J. Electrochem. Soc. 153 (2006) 1186. [9] J. Zheng, S. Deng, Z. Shi, H. Xu, H. Xu, Y. Deng, Z. Zhang, G. Chen, The effects of persulfate treatment on the electrochemical properties of Li [Li0.2Mn0.54Ni0.13Co0.13]O2 cathode material, J. Power Sources 221 (2013) 108. [10] D.Y.W. Yu, K. Yanagida, H. Nakamura, Surface modification of Li-excess Mnbased cathode materials, J. Electrochem. Soc. 157 (2010) 1177. [11] Y.K. Sun, M.J. Lee, C.S. Yoon, J. Hassoun, K. Amine, B. Scrosati, The role of AlF3 coatings in improving electrochemical cycling of Li-enriched nickelmanganese oxide electrodes for Li-Ion batteries, Adv. Mater. 24 (2012) 1192.

[12] B. Song, H. Liu, Z. Liu, P. Xiao, M.O. Lai, L. Lu, High rate capability caused by surface cubic spinels in Li-rich layer-structured cathodes for Li-ion batteries, Sci. Rep. 3 (2013) 3094. [13] Y. Wu, A. Manthiram, Structural stability of chemically delithiated layered (1-z)Li[Li1/3Mn2/3]O2-zLi[Mn0.5-yNi0.5-yCo2y]O2 solid solution cathodes, J. Power Sources 183 (2008) 749. [14] Y.S. Meng, G. Ceder, C.P. Grey, W.S. Yoon, S.H. Yang, Understanding the crystal structure of layered LiNi0.5Mn0.5O2 by electron diffraction and powder diffraction simulation, Electrochem. Solid-State Lett. 7 (2004) 155. [15] M.M. Thackeray, S.H. Kang, C.S. Johnson, J.T. Vaughey, R. Benedek, S.A. Hackney, Li2MnO3-stabilized LiMO2 (M;Mn, Ni, Co) electrodes for lithium-ion batteries, J. Mater. Chem. 17 (2007) 3112. [16] R. Baddour-Hadjean, J. Pereira-Ramos, Raman microspectrometry applied to the study of electrode materials for lithium batteries, Chem. Rev. 110 (2010) 1278. [17] A.R. Armstrong, M. Holzapfel, P. Novak, C.S. Johnson, S.H. Kang, M.M. Thackeray, P.G. Bruce, Demonstrating oxygen loss and associated structural reorganization in the lithium battery cathode Li[Ni0.2Li0.2Mn0.6]O2, J. Am. Chem. Soc. 128 (2006) 8694. [18] J. Kikkawa, T. Akita, M. Tabuchi, K. Tatsumi, M. Kohyama, Participation of oxygen in charge/discharge reactions in Li1.2Mn0.4Fe0.4O2: evidence of removal/reinsertion of oxide ions, J. Electrochem. Soc. 158 (2011) 760. [19] N. Yabuuchi, K. Yoshii, S.K. Myung, I. Nakai, S. Komaba, Detailed studies of a high-capacity electrode material for rechargeable batteries Li2MnO3-LiCo1/ 3Ni1/3Mn1/3O2, J. Am. Chem. Soc. 133 (2011) 4404. [20] M. Bettge, Y. Li, K. Gallagher, Y. Zhu, Q. Wu, W. Lu, I. Bloom, D.P. Abraham, Voltage fade of layered oxides: Its measurement and impact on energy density, J. Electrochem. Soc. 160 (2013) 2046. [21] Y. Li, M. Bettge, B. Polzin, Y. Zhu, M. Balasubramanian, D.P. Abraham, Understanding long-term cycling performance of Li1.2Ni0.15Mn0.55Co0.1O2graphite lithium-ion cells, J. Electrochem. Soc. 160 (2013) 3006. [22] X. Liu, T. Huang, A. Yu, CaF2-coated Li1.2Mn0.54Ni0.13Co0.13O2 as cathode materials for Li-ion batteries, Electrochim. Acta 133 (2014) 555. [23] A. Ito, D. Li, Y. Sato, M. Arao, M. Watanabe, M. Hatano, H. Horie, Y. Ohsawa, Cyclic deterioration and its improvement for Li-rich layered cathode material Li[Ni0.17Li0.2Co0.07Mn0.56]O2, J. Power Sources 195 (2010) 567. [24] J. Liu, J. Liu, R. Wang, Y. Xia, Degradation and structural evolution of xLi2MnO3(1-x)LiMn1/3Ni1/3Co1/3O2 during cycling, J. Electrochem. Soc. 161 (2014) 160. [25] J. Zheng, M. Gu, J. Xiao, P. Zuo, C. Wang, J.G. Zhang, Corrosion/fragmentation of layered composite cathode and related capacity/voltage fading during cycling process, Nano Lett. 13 (2013) 3824. [26] D.H. Jang, Y.J. Shin, S.M. Oh, Dissolution of spinel oxides and capacity losses in 4 V Li/LixMn2O4 cells, J. Electrochem. Soc. 143 (1996) 2204. [27] A.D. Robertson, P.G. Bruce, Mechanism of electrochemical activity in Li2MnO3, Chem. Mater. 15 (2003) 1984. [28] K.J. Carroll, D. Qian, C. Fell, S. Calvin, G.M. Veith, M. Chi, L. Baggetto, Y.S. Meng, Probing the electrode/electrolyte interface in the lithium excess layered oxide Li1.2Ni0.2Mn0.6O2, Phys. Chem. Chem. Phys. 15 (2013) 11128. [29] R.A. Quinlan, Y.C. Lu, S.H. Yang, A.N. Mansour, XPS studies of surface chemistry changes of LiNi0.5Mn0.5O2 Electrodes during high-voltage cycling, J. Electrochem. Soc. 160 (2013) 669. [30] Y. Xia, Y. Zhou, M. Yoshio, Capacity fading on cycling of 4 V Li/LiMn2O4 cells, J. Electrochem. Soc. 144 (1997) 2593. [31] C.R. Fell, D. Qian, K.J. Carroll, M. Chi, J.L. Jones, Y.S. Meng, Correlation between oxygen vacancy, microstrain, and cation distribution in lithium-excess layered oxides during the first electrochemical cycle, Chem. Mater. 25 (2013) 1621. [32] A. Boulineau, L. Simonin, J.F. Colin, C. Bourbon, S. Patoux, First evidence of manganese-nickel segregation and densification upon cycling in Li-rich layered oxides for lithium batteries, Nano Lett. 13 (2013) 3857. [33] K. Xu, Z. Jie, R. Li, Z. Chen, S. Wu, J. Gu, J. Chen, Synthesis and electrochemical properties of CaF2-coated for long-cycling Li[Mn1/3Co1/3Ni1/3]O2 cathode materials, Electrochim. Acta 60 (2012) 130. [34] S.J. Shi, J.P. Tu, Y.J. Mai, Y.A. Zhang, X.L. Wang, Structure and electrochemical performance of CaF2 coated LiMn1/3Ni1/3Co1/3O2 cathode material for Li-ion batteries, Electrochim. Acta 83 (2012) 105.