Insights into the Electrochemical Reaction Mechanism of a Novel

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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 3522−3529

Insights into the Electrochemical Reaction Mechanism of a Novel Cathode Material CuNi2(PO4)2/C for Li-Ion Batteries Wengao Zhao,† Guiming Zhong,‡,⊥ Jian Zheng,§ Jianming Zheng,∥ Junhua Song,∥ Zhengliang Gong,† Zheng Chen,† Guorui Zheng,‡ Zheng Jiang,# and Yong Yang*,†,‡

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†

School of Energy Research and ‡State Key Lab of Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials and Department of Chemistry, Xiamen University, Xiamen, Fujian 361005, China § Institute for Integrated Catalysis, and Fundamental and Computational Science Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352, United States ∥ Energy and Environmental Directorate, Pacific Northwest National Laboratory, 902 Battelle Boulevard, Richland, Washington 99354, United States ⊥ Xiamen Institute of Rare Earth Materials, Haixi Institutes, Chinese Academy of Sciences, Xiamen 361021, China # Shanghai Institute of Space Power-Sources, No. 2965 Dongchuan Road, Shanghai 200245, China S Supporting Information *

ABSTRACT: In this work, we first report the composite of CuNi2(PO4)2/C (CNP/C) can be employed as the highcapacity conversion-type cathode material for rechargeable Liion batteries (LIBs), delivering a reversible capacity as high as 306 mA h g−1 at a current density of 20 mA g−1. Furthermore, CNP/C also presents good rate performance and reasonable cycling stability based on a nontraditional conversion reaction mode. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) characterizations show that CNP is reduced to form Cu/Ni and Li3PO4 during the discharging process, which is reversed in the following charging process, demonstrating that a reversible conversion reaction mechanism occurs. X-ray absorption spectroscopy (XAS) discloses that Ni2+/Ni0 exhibits a better reversibility compared to Cu2+/Cu during the conversion reaction process, while Cu0 is more difficult to be reoxidized during the recharge process, leading to capacity loss as a consequence. The fundamental understanding obtained in this work provides some important clues to explore the high-capacity conversion-type cathode materials for rechargeable LIBs. KEYWORDS: CuNi2(PO4)2/C, high capacity, conversion reaction, XAS, reversibility

1. INTRODUCTION Rechargeable lithium-ion batteries (LIBs) are regarded as an attractive power device for a series of cell-driven vehicles and large-scale energy storage applications, such as smart grids and renewable solar and wind energy.1−5 Especially, to realize the vehicle electrification, there is an urgent need to further improve the energy density of LIBs. Recently, researchers from all over the world are devoting much effort on the development of alternative anode materials, especially in high-capacity silicon-based anode and the lithium-metal anode.6,7 However, the growth of alternative cathode materials with high capacity and high operating voltage is still an important way to further enhance the energy density of LIBs. Current cathode materials for LIBs are mainly based on layered, spinel, and polyaniontype compounds.8−12 Although conventional cathode materials exhibit excellent long-term cycling stability in LIBs, their reversible capacities and energy densities are limited by the intrinsic one-electron transfer during repeated charge and discharge processes. © 2017 American Chemical Society

Electrochemical conversion reactions appear to be a promising approach to enhance energy density of cathode materials because of the full utilization of all the charge of a multivalent transition-metal compound.13,14 Recently, much effort has been devoted to the research of transition-metal oxides and fluorides (MaXb, X = O, F), which are carried out by forming metal (M) and Li2O or LiF compounds during discharging and reconverting to the pristine MaXb phase upon charging according to the conversion reaction shown as follows:15 MaX b + nb Li ⇋ b Li nX + a M

Because of the multivalent oxidation state change of the metal ion, Li incorporation is associated with more than one electron transfer, leading to higher capacities than conventional insertion Received: October 5, 2017 Accepted: December 29, 2017 Published: December 29, 2017 3522

DOI: 10.1021/acsami.7b15086 ACS Appl. Mater. Interfaces 2018, 10, 3522−3529

Research Article

ACS Applied Materials & Interfaces

glovebox using a CNP/C composite electrode, fresh Li metal, separator (Celgard 2400), and as-prepared electrolyte (1 M LiPF6 and EC:DMC = 4:6 wt). The electrochemical performance of cells were carried out on the Land-CT2001A battery tester under 30 °C, and the voltage range was controlled between 1.5 and 4.5 V under 10− 200 mA g−1 current densities. CHI660D electrochemical station were employed to test the reduction/oxidation behavior from 1.5 to 4.5 V under 0.05 mV s−1.

compounds such as LiCoO2 and LiFePO4. To date, among the MxNy family of conversion materials (M = Fe, Co, Ni, Cu, etc.; N = N, O, F, P, S, etc.), only the MFx family and Cu3(PO4)2 could afford sufficiently high operating voltages during conversion reactions, validating them as appealing candidates for constructing high-energy-density LIBs.14,16−26 It has been reported earlier that Cu3(PO4)2/C composite could be used as novel cathode materials for LIBs, which exhibited a high rechargeable capacity up to 280 mA h g−1 and displayed two discharge plateaus at 2.7 and 2.1 V, respectively. However, the crystalline Cu3(PO4)2shows limited cycling performance because of the aggregation of Cu particles and dissolution of Cu+1. In this work, to further improve the cycling capability, Ni2+modified CuNi2(PO4)2 (CNP) compound was synthesized and employed as a reliable cathode for rechargeable LIBs for the first time. It is demonstrated that the CNP/C composite is able to deliver a high specific discharge capacity of 400 mA h g−1 with a voltage plateau of 2.3 V, implying an energy density of 920 W h kg−1, which is one of the highest values ever reported for cathode materials. Furthermore, the CNP/C also demonstrates a good rate performance, confirming the 329 mA h g−1 even under the current density of 200 mA g−1 during the first discharge process. In addition, the material also exhibits an acceptable cycling performance that 200 mA h g−1 can be retained after 30 cycles at a current density of 200 mA g−1. A variety of characterization techniques, such as ex situ X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and ex situ X-ray absorption spectroscopy (XAS) were employed to disclose the detailed reversible electrochemical reaction mechanism of rechargeable CNP/C cathode and a profound understanding of the reaction and capacity fading mechanism was obtained.

3. RESULTS AND DISCUSSION Synthesis and Characterization of CNP/C. XRD patterns of CNP synthesized at different temperatures (700−850 °C) are depicted in Figure 1. At 700 °C, some peaks of impurities

Figure 1. XRD patterns of CNP materials that were calcined at different temperatures for 6 h.

are obviously observed at 2θ = 15°−40°. With the temperature being increased to 750 °C, the impurity peaks disappear, and all the XRD reflections of the obtained CNP matches well with diffraction peaks of pure phase (P1̅ space group, JCPDS 01089-6558),27 indicating the high purity of CNP. However, when the temperature is further increased, the impurity peaks appear again under 800 °C at 2θ = 26.12 and 2θ = 32.79, which become much stronger at 850 °C and an additional impurity peak appears at 2θ = 36.10. On the basis of the phase equilibrium of the reaction product (CuO−NiO−P4O10) during the synthesis process, the impurity peaks may belong to the Cu2NiO(PO4)2 or CuxNi2−xP2O7 compounds.28,29 The above result indicates that the optimal calcination temperature for the preparation of pure CNP is 750 °C. Figure2A presents SEM image of the CNP material; the sizes of the particles range from 400 to 800 nm. The carbon coating is achieved by exfoliation of graphite particles through highenergy ball milling with CuNi2(PO4)2.30,31 After high energy ball milling with carbon (10 wt % C), forming the carboncoated (CNP/C) composite material, the sizes of the particle decreases to 100−300 nm, as displayed in Figure 2B. The corresponding EDS (Figure 2C) data strongly suggest the existence of Cu/Ni/P/O and C elements in CNP/C composite.Figure2D further compares the XRD patterns of CNP, CNP/C, and the standard PDF card of CNP. Before ball milling, the XRD pattern of CNP shows sharp peaks with high intensity, indicating its decent crystallinity and large primary particle size. After ball milling with carbon, the diffraction peaks of CNP/C obviously become weaker and broader. The broadened diffraction peaks demonstrate a loss of long-range ordering during the mechanic-chemical synthesis, indicating a

2. EXPERIMENTAL SECTION 2.1. Materials Preparation. Crystalline CNP material was prepared with a solid-state reaction method byhomogeneously mixing the stoichiometric amounts of (NH4)2HPO4, CuO, and NiO, and then the mixture was calcined under different temperatures (700−850 °C) for 6 h. The composite of CNP/C was prepared through high-energy ball milling of the CNP particles and carbon black. Either 1 or 2 g of a mixture composed of 90 wt % CNP and 10 wt % carbon black was loaded into a zirconia milling jar (with two zirconia balls) in a glovebox filled with Ar gas. The jar was sealed up before transferring to the ball mill, which was programmed to run at 500 rpm for 60 and 90 min. 2.2. Materials Characterization. SEM (Hitachi S-4800) equipped with energy-dispersive X-ray spectroscopy (EDS) were employed to investigate the morphology and confirm the elements of the CuNi2(PO4)2 particles.The cycled electrodes were washed using dimethyl carbonate (DMC) and packed with a Mylar membrane and Kapton tape for preventing H2O and O2 contamination before examination by ex situ XRD, HRTEM, and XAS. The diffraction pattern of the samples were collected by XRD equipment (PANalytical, Netherlands). The Mylar membrane produced a strong peak at 24°−28°, which however did not affect the signal collection from the electrodes. HRTEM images were collected with a Tecnai F20 STTEM operating at 200 kV. Ex situ XAS was operated at beamline BL141W at the Shanghai Synchrotron Radiation Facility (SSRF).The XAS signal was extracted in a standard way using Athena software packages. The oscillations were weighed with k2 and Fourier transformed within the limit k = 1.8−14 Å−1. 2.3. Electrochemical Characterization. The electrodes were prepared using CNP/C, super C, and poly(vinylidene difluoride) (PVDF) by the mass ratio of 8:1:1, and the loading of cathode material was about 2.5 mg cm−2. Coin cells of CNP/C || Li were assembled in a 3523


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Figure 2. (A) SEM images of CNP; (B) SEM and (C) EDS data of CNP/C; (D) XRD patterns corresponding to CuNi2(PO4)2, CuNi2(PO4)2/C, and a standard PDF card.

Figure 3. Electrochemical performance of CNP/C. (A) Charge/discharge profile under the current density of 40 mA g−1; (B) cycle performance under different current densities; (C) cycle performance and Coulombic efficiency data at a current density of 200 mA g−1; (D) CV curves of CNP/ C at a scan rate of 0.05 mV s−1. The voltage range is between 1.5 and 4.5 V.

shorten the Li+ diffusion distance and enhance the interfacial reaction kinetics, which is beneficial for improving the

visible decrease in the particle size and crystallinity of CNP material. The formation of smaller particles could largely 3524


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Figure 4. Ex situ XRD patterns of CNP/C electrode under different discharged/charged states. (A) Discharged/charged states of one and a half cycle process; (B) enlarged view of different discharged states of the first cycle.

strating the new cathode material has a potential to be a promising candidate for conversion reaction. To further confirm the electrochemical capability of CNP/C-based cell under high current density, its capacity retention and Columbic efficiency have been investigated after 30 cycles at 200 mA g−1. Figure3C shows the cycling performance of CNP/C vs Li+/Li under the current density of 200 mA g−1. The initial capacities of discharge/charge can maintain 329 and 232 mA h g−1, respectively. The capacity retention is 80.92% after 30 cycles, and the CE maintains at 99.2% over 30 cycles. The result shows CNP has reasonable cycle performance and good capacity retention. The cyclic voltammogram (CV) curve was collected with a scan rate of 0.05 mV s−1 in Figure3D. There are two welldefined lithiation peaks and three delithiation peaks, indicating the transformation among Cu2+ to Cu+ and then to Cu, and Ni2+ to Ni. During the second discharge process, the discharge plateaus increased from 1.55 and 1.72 V to 1.70 and 2.08 V. These higher discharge plateaus are beneficial for the improvement of energy density. The CNP almost completely transformed into smaller Cu nanoparticles, Ni nanoparticles, and Li3PO4 after the first discharge, so the improved contact between smaller particles may enhance the electronic conductivity as well as shorten the diffusion path of Li+ ion, raising the discharge plateau.37,38 Structural Evolution during Reversible Charging− Discharging Process. XRD is employed for investigating crystalline-phase changes during discharge/charge process. We mark the rechargeable process combining cycle number and capacity, such as first DC means the full discharge at first cycle and second C200 indicate the capacity holds 200 mA h g−1 during the second discharge process. Figure4A depicts the XRD patterns of CNP/C at different lithiation/delithiation states. To analyze the reversible conversion reaction mechanism, Figure 4B shows a clearer phase transformation of CNP system during the first discharge. Obviously, the intensities of the XRD peaks of CNP gradually decrease with the increase of the state of discharge (SOD), and a new and broad Bragg peak appears at 43−45° under first discharge (DC), second DC 200, and second DC, indicative of the decomposition of CNP and the formation of metallic Cu (111) and Ni (111) during discharge.39−41 Some weak diffraction peaks are also observed at 22−24°, further confirming the formation of Li3PO4 discharge product. In addition, there is some residual CNP

material’s charge/discharge property, especially the rate performance.32−34 Electrochemical Performance. Figure 3A demonstrates the voltage profiles of the CNP/C at different stages of cycling under specific current of 40 mA g−1. At the initial discharge from 3.0 to 1.5 V, the discharge capacity of CNP/C electrode can reach up to 400 mA h g−1, which is very close to the theoretical capacity of CNP (423 mA h g−1), indicating that CNP particles are almost completely electrochemically reduced to metallic Cu and Ni as the discharge proceeds to 1.5 V.14,34−36 After the formation cycle, the electrode still demonstrates a reversible capacity of 306 mA h g−1, associating with a 72.4% utilization of the theoretical capacity (423 mA h g−1). The loss of capacity may be attributed to the following two reasons: first, the CNP cannot be fully converted to metallic Cu and Ni even at cutoff voltage of 1.5 V, ascribed to the deposition of Cu causing a large polarization and thus the incomplete conversion. Second, the dissolution of Cu+ is also an important factor accounting for the decreased capacity during subsequent discharge/charge process.13,14 The discharge plateau (2.3 V) in the second cycle is higher than that of the first cycle (2.1 V), which can be attributed to the following two reasons: (1) the decreased particle sizes of the materials shorten the Li+ ion diffusion route; (2) the incomplete reaction of metallic Cu and Ni particles increased the electronic conductivity of CNP/C composite. To assess the performance of the cathode material for LIBs, the electrochemical performances of CNP/C were tested under different current densities at room temperature. Figure3B compares the capacities and Coulombic efficiencies (CEs) of the CNP/C composite at various current densities ranging from 20 to 200 mA g−1. The capacity of CNP/C declined slightly from 400 to 374 mA h g−1 and then to 338 mA h g−1, while the current density increased successively from 20 to 40 mA g−1 and further to 100 mA g−1, respectively. Even at the current density of 200 mA g−1, the composite cathode still delivers the capacity of 329 mA h g−1, showing a good rate performance. At the second cycle, the CNP/C-based cell exhibited the discharge capacity of 232 mA h g−1, and it still delivers 200 mA h g−1 after 30 cycles. Moreover, the CNP/C all shows reasonable first CE and high CE during cycling performance under different currents. The good rate performance, high Coulombic efficiency, and reasonable cycling capability were observed for the CNP/C composite, demon3525


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CNP is again converted to Cu/Ni and Li3PO4 during the second discharge. Therefore, in good accordance with the ex situ XRD results, the HRTEM results further confirm the reversible conversion mechanism of the CNP/C cathode material. Reaction Mechanism of Ni and Cu in CNP/C during Reversible Cycling. With the aim to further confirm the evolution of reduction/oxidation of CNP/C cathode, the valence state and local coordination of Cu and Ni on CNP/C electrodes was further tracked by X-ray adsorption spectra (XANES and EXAFS of Ni and Cu K-edge). To precisely analyze the transformation during the rechargeable process, the XANES and EXAFS spectroscopy of Ni0, Ni2, Cu0, Cu1+, and Cu2+ are marked in Figure6 and Figure7. As we can see, Figure6A shows the process of Ni2+ transformed into Ni0 during the first discharge. The visible small peak at 833 eV in Figure 6A−C belongs to the pre-edge signal of Ni2+.42 It is found that the signal of Ni2+ gradually decreases with the increasing of depth of discharge until almost all of Ni2+ was reduced to Ni0. Meanwhile, Figure6D shows a similar process that the bulk Cu2+ was transformed into Cu0. The result is also very consistent with ex situ XRD and HRTEM results. Figure 6B,E shows the evolution of Cu and Ni during the second charge process. The large part of Ni0 was transformed into Ni2+ when the electrode was fully charged to 4.5 V. However, there is only a small fraction of Cu0 which is transformed into Cu2+, due to the fact that Ni has much lower oxidation potential than Cu, and therefore Cu0 is not easily oxidized to Cu2+ when it coexists with Ni.43,44 Figure6C,F shows a similar phenomenon/ result to Figure6A,D during the second discharge process that almost all of Ni2+ and Cu2+ are converted into Ni0 and Cu0. It is concluded that Ni0/Ni2+ is more electrochemically active in the reversible conversion reactions, while Cu/Cu2+ redox couple is less reversible in this system which may be ascribed to the competitive reaction between the oxidation of Ni0 and Cu0.44 The similar results in normalized Cu and Ni K-edge XANES spectra (Figures S1 and S2) indicatethat almost all of Cu2+ and Ni2+ were transformed into metallic forms after the first discharge cycle, and the main capacity comes from the transformation of Ni during rechargeable process. So the main reason for capacity loss is due to the irreversible conversion of Cu0 to Cu2+. The finding is different from early works that the capacity loss of Cu3(PO4)2 mainly derives from the formation of isolated Cu particles which cannot actively become involved in the rechargeable process.13,14,34 Figure7 shows Fourier-transformed, k2-weighted Cu K-edge and Ni K-edge EXAFS spectra over the range of 0−5 Å. Figure 7A exhibits the evolution process between Ni−O and Ni−Ni. In the pristine state, no Ni−Ni scattering is observed. With the increasing of depth of discharge from first DC 100 to first DC, the intensity of Ni−O interaction decreases and the Ni−Ni bond increases. Finally, the Ni−O bond almost fully disappears and that of the Ni−Ni bond reaches the maximum. Figure7D shows a similar process where most Cu−O bond disappears, simultaneously forming Cu−Cu bond, although there is some residual Cu−O features after the first full discharge process.45 The EXAFS result is very consistent with HRTEM in Figure 5C,D. Figure7B,E shows transformation from the metallic Ni and Cu species to corresponding oxides. With the enhancing of depth of charge, the bulk of metallic Ni converts into Ni oxide. However, there is only a small amount of metallic Cu involved in the conversion reaction between Cu and Cu2+. The approximate amount of Ni and Cu species changed during

((131) and (310)) identified in the XRD patterns, which means CNP does not fully involve in conversion reaction. This could explain why the actual discharge capacity is lower than the theoretical value of CuNi2 (PO4)2 as presented Figure3A. During charging, the broad peak around 43.5° disappears, suggesting that the crystalline Cu and Ni were reoxidized during the charging process. During the second discharge process, the broad Ni (111) and Cu (111) diffraction peaks reappear, confirming the reversibility of the charge/discharge processes. To further analyze the rechargeable reaction mechanism, Figure4B shows a more accurate analysis of the CNP system during the first discharge. It can be clearly observed XRD peaks at 2θ ≈ 10−60 correspond to CNP with the P1̅ space group. The main structure of CNP stays intact until full discharge proceeds, which is destroyed during full discharge, concurrently with large phase changes. Furthermore, no new Bragg peaks are visible except the broad peak at 43−45° (Cu(111) and Ni (111)) and some weak peaks at 22−24° which belongs to Li3PO4, indicating that Li3PO4 and any other products formed during discharge are almost amorphous. Thus, the ex situ XRD technology just confirms that CNP is converted into Ni and Cu metal after the first cycle discharge process, while metallic Ni and Cu are reoxidized during the rechargeable process.14,34 HRTEM was employed to explore the nanostructure evolution of CNP/C during the cycling process. Figure5A

Figure 5. HRTEM images of pristine CNP and different discharged/ charged states of CNP/C. (A) Pristine; (B) the first cycle discharge; (C) the second cycle charge; (D) the second cycle discharge.

shows that the pristine CNP consists of well-crystallized particles, and the CNP (−101) exhibits interplanar crystal spacing of 0.38 nm.Figure 4B demonstrates an HRTEM image of CNP after first discharge, in which the phases of CNP (011) and Li3PO4 (101) are clearly observed, indicating that CNP has not been fully transformed into Li3PO4. The HRTEM result matches well with XRD data, and is also consistent with the electrochemical capability that the initial discharge capacity of CNP (400 mA h g−1) is subordinate to its theoretical capacity (423 mA h g−1) (Figure 3A).Figure5C shows the phase structure after the second charge, where the crystalline CNP (021) and CNP (200) can be clearly identified, demonstrating the conversion reaction of Cu with surrounding Li3PO4. After the second full discharge, areas of Cu/Ni (111), Cu/Ni (200), and Li3PO4 (120) can be seen in Figure5D, indicating that 3526


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Figure 6. First derivative curves of the XANES spectra from crystalline CNP during the rechargeable process. (A) Ni first discharge; (B) Ni second charge; (C) Ni second discharge; (D) Cu first discharge; (E) Cu second charge; (F) Cu second discharge.

Figure 7. R range curves of the EXAFS spectra from crystalline CNP during the rechargeable process. (A) Ni first discharge; (B) Ni second charge; (C) Ni second discharge; (D) Cu first discharge; (E) Cu second charge; (F) Cu second discharge.

summarized in Figure 8. During the first discharge cycle, an ideal carbon-coated CNP (CNP/C) composite model shows a high capacity of 400 mA h g−1, and combined with the XRD and XAS results confirm that almost all of CNP converted into LixPO4 and nanoscale of Ni/Cu. During the charge process, the CNP/C electrode demonstrates impressive rechargeable capacity ranging to 306 mA h g−1 and the existence of incomplete phase of Cu0 and LixPO4, as shown in the scheme. The capacity loss as disclosed by XAS is attributed to the poor reversibility of Cu/Cu2+ during the conversion process, which is not comparable to the excellent reversibility of Ni/Ni2+.

the charge and discharge processes are determined via linear combination fitting and the detailed results are listed in Tables S1−S6 in the Supporting Information. Figure7C,F shows a similar reaction pathway with the first discharge process that almost all of Ni and Cu oxides were reduced to metallic forms. The above results substantiate that the irreversible capacity loss mainly comes from the Cu particles formed in the first discharge process, which is not fully oxidized to Cu2+ during subsequent charge process. Scheme of Reaction Mechanisms of CNP/C during Rechargeable Process. As discussed above, the electrochemical reaction mechanisms of CNP/C||Li cells are 3527


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Figure 8. Scheme of reaction mechanisms of CNP/C during rechargeable process.

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4. CONCLUSION CNP/C nanocomposite has been successfully synthesized and adopted as a promising cathode for rechargeable LIBs. It is shown that CNP/C cathode delivers a high specific capacity (rechargeable capacity of 306 mA h g−1 at 20 mA g−1), a decent cycling performance as well as relatively good rate performance, validating it a potential cathode candidate for next-generation rechargeable LIBs. The reaction mechanism study indicates that Ni2+ and Cu2+ were almost fully transformed into Ni0 and Cu0 during the first discharge process. Furthermore, the evolution between Ni0 and Ni2+ is reversible during the second charge and second discharge process. However, the majority of Cu0 could not be reconverted to Cu2+ in the second charge process, which is considered to be the primary reason for capacity degradation during the initial cycles. The revealed reaction mechanism of CNP/C cathode will be beneficial for future development of high-performance Cu-based/Ni-based conversion-type cathode materials for LIBs.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b15086. Ni/Cu K-edge XANES spectra; tables of Ni/Cu content (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Wengao Zhao: 0000-0003-0868-8404 Guiming Zhong: 0000-0003-2313-4741 Jian Zheng: 0000-0003-2054-9482 Jianming Zheng: 0000-0002-4928-8194 Zheng Jiang: 0000-0002-0132-0319 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful for the support of the National Natural Science Foundation of China (Grants 21233004, 21473148, 21428303, and 21303147), and National Key Research and Development Program of China (Grant 2016YFB0901500). 3528


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