Investigation on the electrochemical properties and stabilized surface

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Investigation on the electrochemical properties and stabilized surface/interface of Nano-AlPO4 coated Li1.15Ni0.17Co0.11Mn0.57O2 as the cathode for lithium-ion batteries Jinhua Song, Yong Wang, Zhenhe Feng, Xinghao Zhang, Ke Wang, Haitao Gu, and Jingying Xie ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06670 • Publication Date (Web): 20 Jul 2018 Downloaded from http://pubs.acs.org on July 20, 2018

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ACS Applied Materials & Interfaces

Investigation on the electrochemical properties and stabilized surface/interface of Nano-AlPO4 coated Li1.15Ni0.17Co0.11Mn0.57O2 as the cathode for lithium-ion batteries Jinhua Song ‡,† , Yong Wang ‡,†, Zhenhe Feng ‡, Xinghao Zhang ‡, Ke Wang ‡, Haitao Gu ‡,*, Jingying Xie ‡,* ‡

State Key Laboratory of Space Power-Sources Technology, Shanghai Institute of

Space Power-Sources, Shanghai 200245, China †

This author contributes equally.

*Corresponding author: [email protected] (J. Y. Xie) and [email protected] (H. T. Gu) TOC Graphic

Abstract Being

considered

as

one

of

the

most

potential

cathode

materials,

Li1.15Ni0.17Co0.11Mn0.57O2 draws plenty of attentions towards its optimization on the cyclic and rate performance. Surface coating process provides a longer cycling life

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and better rate performance for the cathodes. A systematic investigation has been carried out upon the nano-AlPO4 coating layer for the Li1.15Ni0.17Co0.11Mn0.57O2 cathode material through a facile in-situ dispersion process. The 1% coated cathode material can hold about 90% capacity retention after 100 cycles. Besides, the surface coating enhances the rate ability of Li1.15Ni0.17Co0.11Mn0.57O2, which owns a reversible capacity of 202.3 mAh g-1 at the rate of 1 C. Surface information is collected during the cycling, which reveals that the less side reactions happen on the electrode-electrolyte interface after the coating process for the improved cyclic and rate performance

Key Words Li-rich cathode; nano-AlPO4; in-situ coating; surface modification; less side products

Introduction Lithium ion batteries have sprung up to one of the most excellent secondary batteries for its commercialization due to the high energy density, ultra long cycling life and environmentally friendliness 1-2. As the most extensively applied cathode, LiCoO2 suffered from its relatively low specific capacity and high cost which was caused by the expensive element of Co 3-6. The Mn-based Li-rich cathode materials, which are formed by Li2MnO3 and Li[NixCoyMn(1-x-y)]O2, have been paid significant attention among the researchers because of the brilliant electrochemical properties(>250 mAh g-1)

7-13

. However, what hindered the Mn-based Li-rich cathode materials into

application can be listed as following: (1) There are palpable irreversible capacity loss in the 1st lithiated/delithiated process which resulted in a low coulombic efficiency comparing to those of other materials

14

; (2) O2- releasing from Li2MnO3 when

charging over 4.5 V could have a bad effect upon the safety issues and cycling performance

15

; (3) Side reactions under high potential between the material and

electrolyte will thicken the SEI layer, which blocks the Li+ diffusion and causes a unfavorable rate performance 16-17. Several efforts have been attempted towards aforementioned problems. Among them, building a coating layer upon the surface of the material was commonly tried. The


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most common coating material can be listed as carbon material, oxides, fluorides and phosphates. Carbon based coating layer was mostly considered due to its good electronic conductivity. A. Manthiram et al. 18 presented a thermal evaporation method to obtain a carbon coating layer upon the surface of the Li-rich material. With the modification, a high capacity layered Li[Li0.2Mn0.54Ni0.13Co0.13]O2 with graphite rod coating was obtained and performed over 180 mAh g-1 at the rate of 2 C. A. Manthiram et al.

19

performed another investigation of Li-rich material

Li1.17Mn0.56Ni0.135Co0.135O2 with Al2O3 coating and the coated material exhibited better properties under the low temperature of 0 ℃. A phosphate like AlPO4 has been introduced to cathode materials for many times. For example, AlPO4@LiCoO2 has been successfully synthesized and applied as a cathode in lithium ion batteries 20. Better cyclic performance and smaller polarization were obtained. Also, it was believed that a LiAlxCo(1-x)O2 solid solution interphase would be formed on the surface with the AlPO4 coating, which could lower the side reactions of the degradation of electrolyte species. In this work, a series of nano-AlPO4 coated Li1.15Ni0.17Co0.11Mn0.57O2

materials

were

synthesized

via

a

facile

PAAM

(Polyacrylamide) assisted in-situ dispersion-coating process, which was the first attempt, to the best of our knowledge, that PAAM assisted in-situ dispersion-coating process was applied to Li-rich cathodes. Suitable coating thickness and temperature for heat treatment were investigated. It was found that 1% coated sample with 400 ℃ heat treatment delivered the highest capacity of 236.7 mAh g-1 after 100 cycles. Moreover, interface properties were investigated after 50 cycles via XPS and SEM methods.

Experimental Section Material preparation Nano-AlPO4 coated Li1.15Ni0.17Co0.11Mn0.57O2 materials were prepared via an in-situ dispersion process. The active material was added into PAAM solution and dispersed uniformly Then, the corresponding amount of Al(NO3)3 solution with stoichiometric amount of (NH4)2HPO4 was added under the constant stirring. Filtering, drying and



heat treatment were preceded after reaction of 24 hours. The TD-DSC curve were listed as Figure S1. Electrochemical measurements The working electrodes were composed of as-prepared samples, Super P and polyvinylidenefluoride (PVDF) with a mass ratio of 8:1:1 on Al foil. The loading amount of active materials was about 5 mg cm-2. Coin cells (CR2016) were assembled in an Ar-filled glovebox. 1.20 M LiPF6 in the non-aqueous solution of ethylene carbonate (EC) and dimethyl carbonate (DMC) (volume ratio: 3:7) was used as the electrolyte. Celgard 2325 was used as the separator and pure lithium foil was used as the counter electrode (anode). Galvanostatic charge-discharge measurements were carried out at 25 ℃ with a LAND battery test system. And the current density was calculated based on the specific capacity and the weight of active material. The cyclic voltammetry (CV) tests were carried out at 25 ℃ with a Princeton Applied Research electrochemical workstation at the corresponding scan rate. In this work, the specific current at a rate of 0.1 C is equivalent to 25 mA g-1. Properties characterization The morphology of the product was characterized by field emission scanning electron microscopy (FE-SEM, Hitachi S-4800) operated at an accelerating voltage of 10 kV. High resolution transmission electron microscopy (HRTEM) was examined for the structure of the as-prepared sample. Powder X-ray diffraction (XRD) patterns were collected on an X-ray diffractometer (Rigaku 2600/pc) with Cu-Kα radiation at 40 kV, 40 mA. Data were obtained over the 2θ range of 10-90° with a scan rate of 10° min-1. The High-resolution X-ray photoelectron spectroscopy (XPS) spectra were tested on a Thermo ESCALAB250Xi. Mn K-edge XAS spectra were collected in transmission mode at beamline BL14W1 of the Shanghai synchrotron radiation facility in China. The XAS data were processed using Athena and Artemis software packages.

Results and Discussion


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Figure 1 (a) XRD patterns of 1% nano-AlPO4 coated materials after different heat treatment; (b) EDS-Mapping image of A-400; cycle (c) and rate (d) performance of 1% nano-AlPO4 coated materials after different heat treatment;

1% nano-AlPO4 coated Li1.15Ni0.17Co0.11Mn0.57O2 materials were prepared via an in-situ dispersion process. With different heat treatment temperature after the coated process, a series of coated materials were obtained and marked as A-X (where X referred to the heat temperature). The XRD patterns were collected in Figure 1a, and it was clear that both the coating process and the heat treatment had no obvious effect upon the bulk material. EDS Mapping results in Figure 1b showed the existence of element Al and P upon the surface of the coated material. Furthermore, for the best heat temperature, the cyclic performance and rate performance were evaluated and listed in Figure 1c-d. All coated samples owned the better coulombic efficiency in the 1st cycle than the pristine sample, and all coated samples except for the A-700 exhibited the higher capacity as well as the better capacity retention. Among the samples, A-400 behaved best. When it came to the rate properties, A-400 showed the excellent performance as well. At the rate of 2 C, it held 65.1% capacity of that at the ACS Paragon Plus Environment


rate of 0.1 C. To take the cycle and rate performance into consideration, 400℃ was chosen as the key heat temperature in the following work.

Figure 2 (a) XRD patterns of various nano-AlPO4 coating amount of Li1.15Ni0.17Co0.11Mn0.57O2 materials; corresponding SEM images of pristine material (b), 0.5% (c), 1% (d), 2% (e), 4% (f) nano-AlPO4 coated material.

Once chosen a suitable heat temperature, a series amount of nano-AlPO4 coated materials were synthesized. As shown in Figure 2a, similarly to the results mentioned above, all peaks of various coating amount samples were well indexed to a typical hexagonal α-NaFeO2 with a space group of R3m. Typical peaks were kept well and no clear peak of AlPO4 was observed even at the 4% coating amount. The SEM images ACS Paragon Plus Environment

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showed that the materials remained unchanged after the coated process, as shown in Figure 2b-f. The particle sizes were kept well with the pristine sample. The EDS results verified the existence of element Al and P on the surface of the 2% coated sample, as shown in Figure S2. Precise amount of element Al and P were tested via ICP method and the results were listed in Table 1, which revealed a nice consistency between the calculation and reality of element Al. However, palpable difference could be observed between the calculation and reality of element P, and it probably owed to the great loss of PO43- during the preparation. Therefore, further investigation would be done to identify the component of the coating layer in the following. Table 1 ICP results of element Al and P of various nano-AlPO4 coating amount of Li1.15Ni0.17Co0.11Mn0.57O2 materials

Al (µg g-1)

P (µg g-1)

Calculation

Reality

Calculation

Reality

0.5% coated

1105

826

1269

30.2

1% coated

2208

2423

2535

49.7

2% coated

4407

3314

5056

1107

4% coated

8775

9974

10062

4806

Figure 3 Corresponding TEM images of 0.5% (a), 1% (b), 2% (c), 4% (d) nano-AlPO4



coated materials.

As shown in Figure 3, amorphous layers of 5-15 nm were observed on the surface of materials with different coating amounts (after heat treatment of 400℃). And the thickness was increased with the increasing coating amount. Combing with the XRD results, it was clear that the nano-AlPO4 layer was amorphous and incontinuous.

Figure 4 XPS spectra of various nano-AlPO4 coating amount of Li1.15Ni0.17Co0.11Mn0.57O2 materials: Al 2p (a) and P 2p (b); XAFS (c) and first two peaks of the Fourier transformed spectra (d) of Mn element before/after coating process.

For further identification of the coating layer component, XPS measurements were carried out. Peaks were observed at 73.8 eV, which could not be indexed to AlPO4 (75.3 eV). As the peak of Al 2p located at 73.3 eV could be divided into LiAlO2 21. As shown in Figure 4a and Figure S3, it was proposed that Al3+ had been reacted with the pristine material and formed a phase of Li-Al-M-O (in which M was referred to metal such as Ni, Co and Mn). And when it came to P 2p spectra in Figure 4b, the peaks locating at 133.2 eV were identical with the one of Li3PO4 22, which indicated the existed Li3PO4 in the coating layer. Valence states of Ni, Co and Mn were indentified ACS Paragon Plus Environment

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via XPS and XAFS method. As shown in Figure S4a-b, peaks of Ni and Co exhibited no difference after coating, and the intensity decreased with the increasing coating amount, which indicated the thicker coating layer on the other side. Similarly, no clear difference could be observed from the Mn 2p spectra in Figure S5. XAFS studies were investigated for further identification of the valence of Mn. As listed in Figure 4c, same lines at the energy of 6550.4 eV were observed, indicating the little valance variation during the coating process. The detailed structural parameters could be obtained by fitting the first two peaks of the Fourier transformed Mn XAFS spectra and were shown in Figure 4d. It was depicted in Figure 4d that intensity of Mn-O bond had been decreased while that of Mn-M bond had been increased, which indicated the existence of Mn3+ during the coating process causing the variation of Mn-O and Mn-M bond.

Figure 5 1st (a) and 50th (b) charge-discharge curves; cycle (c, 0.1 C) and rate (d) performance of various nano-AlPO4 coating amount of Li1.15Ni0.17Co0.11Mn0.57O2 materials.

The electrochemical properties were evaluated by coin cell between the potential of 2 - 4.8 V at the rate of 0.1 C. As depicted in Figure 5a, with the thicker coating layer,



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the coulombic efficiency (C.E.) of 1st cycle became greater. However, 4% nano-AlPO4 coated sample owned the smallest capacity, which might owe to the thick coating layer. Cyclic performance was shown in Figure 5b-c, the capacity retention had been improved after coating. Sample with 1% nano-AlPO4 coating obtained reversible capacity of 236.7 mAh g-1 after 100 cycles, comparing with the pristine of 208.4 mAh·g-1. Rate behavior was examined and listed in Figure 5d. Sample with 1% nano-AlPO4 coating performed best at all rates, which owned 202.3 mAh g-1 at the rate of 1 C. It was believed that suitable coating layer could protect the cathode material from

the

side

reaction

with

the

electrolyte

and

improve

the

cathode-electrolyte interface performance.

Figure 6 SEM image of pristine electrode (a) and 1% nano-AlPO4 coated sample after 400℃ ℃ heat treatment electrode (b) after 50 cycles (0.1 C) with the inset of corresponding uncycled electrode; F 1s XPS spectra of 1% coated after different heat treatment (c) and various amount nano-AlPO4 coated (d) electrodes after 50 cycles (0.1 C).

For identification of the interface properties mentioned above, characterizations upon electrodes after 50 cycles were performed. Electrodes of different coating amount looked similar before cycling, as seen in inset images of Figure 6a-b. However, the


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electrodes after 50 cycles exhibited great difference. Plenty of side products were assembled upon the particles without coating, which looked like cracked after 50 cycles. While the one with 1% nano-AlPO4 coated kept a similar looking during cycling. It was known that the products of reaction between cathode and electrolyte could be listed as followings: LiF, LixPOyFz and so on 23. Considering that contents of PVDF was almost the same on every electrode, the relative value of peak intensity of F 1s could be mainly considered as the effects of side products. On the other hand, XPS was a method for the external information, which was mostly affected by the SEI layer on the electrode after cycle. Hence, these products could be examined by measuring the F element on the electrode. Based as mentioned above, XPS spectra of electrodes after 50 cycles were collected and listed in Figure 6c-d. A-400 showed smallest intensity of LiF and LixPOyFz located at 685.3 and 687.8 eV, indicating least side products upon the electrode after 50 cycles, which in accord with the electrochemical performance in Figure 1. Similar results were observed in Figure 6d, which revealed nice protection of the nano-AlPO4 coating layer towards the cathode material.

Conclusion In summary, a series of nano-AlPO4 coated Li1.15Ni0.17Co0.11Mn0.57O2 materials were synthesized via an in-situ dispersion process. Electrochemical performance showed the best heat treatment temperature of 400 ℃. After that, different amounts of nano-AlPO4 coating were compared for a best coating thickness. It was found that the sample with 1% coated behaved best, which performed about 90% capacity retention after 100 cycles at the rate of 0.1 C. Meanwhile, it delivered 257.2, 244.1, 236.2, 222.9, 202.3 and 159.3mAh g-1 at the rate of 0.1, 0.2, 0.3, 0.5, 1 and 2 C. Surface investigations were carried out on the electrodes after 50 cycles, which revealed the effective protection of nano-AlPO4 upon the material.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XX-XXXX.



Materials and methods and supplemental figures and tables, including the TG-DSC, EDS images, XPS patterns, XRD patterns.

Corresponding Authors *E-mail: [email protected] (J. Y. Xie) and [email protected] (H. T. Gu)

Author Contributions †

J. H. S. and Y. W. contributed equally. The manuscript was written through

contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

Acknowledgement This work was supported by the National Natural Science Foundation of China (Grant No. U1637202 and No. U163720101), the Shanghai Science and Technology Commision Project (Grant No. 16DZ1120700) and the Shanghai Sailing Program (No.18YF1417000). The authors also acknowledge beamline BL14W1 of the Shanghai Synchrotron Radiation Facility (SSRF).

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Investigation on the electrochemical properties and stabilized surface

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