In Situ Encapsulation of the Nanoscale Er2O3 Phase To Drastically

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In-situ Encapsulation of Nanoscale Er2O3 Phase to Drastically Suppress Voltage Fading and Capacity Degradation of Liand Mn-rich Layered Oxide Cathode for Lithium-Ion Batteries Shiming Zhang, Haitao Gu, Tian Tang, Wubin Du, Mingxia Gao, Yongfeng Liu, Dechao Jian, and Hongge Pan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09002 • Publication Date (Web): 11 Sep 2017 Downloaded from http://pubs.acs.org on September 13, 2017

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ACS Applied Materials & Interfaces

InIn-situ Encapsulation of Nanoscale Er2O3 Phase to Drastically Suppress Voltage Fading and Capacity Degradation of LiLi- and MnMn-rich Layered Oxide Cathode for LithiumLithium-Ion Batteries Shiming Zhang, † Haitao Gu, Tian Tang, Wubin Du, † Mingxia Gao, *† Yongfeng Liu, † and Dechao Jian, ‡Hongge Pan, *† ‡

‡

†

State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027, P.R. China. ‡

State Key Laboratory of Space Power Technology, Shanghai Institute of Space Power-sources, 2965 Dongchuang Road, Shanghai, 200245, P.R. China. Supporting Information Placeholder than 20–30% of the charge capacity.25 Moreover, the voltage and capacity fading of the LMRO cathode also can be mainly ascribed to the dissolution of the transition metal (TM) elements into the electrolyte.26-27The particle surface of the cathode material directly contacts with the electrolyte to make it easy to be corroded by the acidic species (such as hydrofluoric acid (HF)) from the electrolyte and consequently resulting in capacity losses. 28-29 The corrosion mechanism of oxide cathodes can be expressed as follows:30

ABSTRACT: A novel strategy of in-situ precipitation and encapsulation of Er2O3 phase on Li(Li0.2Ni0.13Co0.13Mn0.54)O2 (LNCMO) cathode material for lithium-ion batteries is proposed for the first time. The Er2O3 phase is precipitated from the bulk of the LNCMO material and encapsulated onto its entire surface during calcining process. Electrochemicial performance is investigated by a galvanostatic charge and discharge test. Structure and morphology are characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electronic microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). Results show that an about 10 nm Er2O3 layer is successfully encapsulated onto the entire surface of the LNCMO matrix material. This unique nanoscale Er2O3 encapsulation can significantly prevent the LNCMO cathode material to be corroded by electrolyte and stabilize the crystal structure of the LNCMO cathode during cycling. Therefore, the prepared Er2O3 coated LNCMO composite exhibits excellent cycling performace and high initial coulombic efficiency.

LiMO2 +2xHF→2xLiF+Li1-2x MO2-x +xH2 O Li1-x Mn2 O4 +21-xHF→

(1)

3+x 1-x λ-Mn2 O4 +1-xLiF+1-xH2 O+ MnF2 4 2

The HF in the electrolyte mainly comes from the reaction of the LiPF6 with traces of water:30 LiPF6 +H2 O→2HF+LiF+POF3 3

Keywords: In-situ precipitation and encapsulation, Er2O3, Voltage fading, Capacity degradation, Li- and Mn-rich layered oxide cathode material, Lithium-ion batteries

One way to overcome this problem is to modify the cathode surface by coating it with inert oxides and thus minimize the cathode/electrolyte interfacial reactions.31 Recently, surface coating of the LMRO cathodes with MnOx,32-33 Al2O3,34-35 MoO3,36 TiO2,37 ZrO2,38 AlPO4,39-40 AlF341-42 inert phases has been reported to be an effective way in stabilizing the layered oxide surface from the electrolyte and improving the cycling ability to higher cut off charge voltages, which can prevent the electrode from etching by acidic species and suppress the dissolution of TM elements into the electrolyte and consequntly to significantly improve its initial coulombic efficiency and cycling stability.

Introduction Lithium-ion batteries (LIBs) have been widely used as power sources of electric vehicles (EV), 3C (computer, communication and consumer electronic) products, and energy storage devices for renewable energy and smart grid.1 However, the energy density of commercialized LIBs cannot meet the ever-growing requirements for practical applications. Success in this field will dominantly depend on further developing new electrode materials with higher energy density and other overall performance such as long cycle life and high rate capability.2-5 Li- and Mn-rich layered oxide (LMRO) has been considered as one of the promising candidates for the next generation cathode material of lithium-ion batteries due to its high energy density over 1000 Wh kg-1.6-9 However, there are several challenges for the LMRO cathode:10-14 high irreversible capacity loss (ICL) in the first cycle,15-16 fast capacity/voltage fading during cycling,17-19 and poor rate capability.20-21

Nevertheless, the traditional coating method generally includes two steps, illustrated in Figure1(a).43-47 Firstly, the LMRO material was prepared by various methods; Secondly, the coating and matrix materials were mechanically mixed and then calcined at a certain temperature. However, these coating approaches have some drawbacks: (1) They cannot establish a uniform, complete, robust and controllable coating layer on the surface of the matrix material. Specifically, it is difficult to build a complete coating layer for every particle of the matrix material with mechanical mixing methods; (2) The severe agglomeration of the powder particles of the matrix material during synthesis at the first step leads to the random and inhomogeneous distribution of the coating materials.

The ICL of the LMRO cathode mainly originates from the corrosion of the cathode materials.22-23 The high ICL can be attributed to a side reaction on the interphase between the cathode surface and the electrolyte, especially at high cut off charge voltages over 4.6 V,24 which causes the discharge capacity less

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In order to overcome these disadvantages and toward better surface-coated cathode material, we propose a novel one step encapsulation strategy by in-situ precipitating nano-scale oxide and encapsulating onto the entire surface of the LRMO material. This method is quite different from the traditonal two step coating appoaches. The encapsulating oxide phase were in situ precipitated on the surfaces of the particles in the high temperature calcining process, which make it more robust,

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complete, and homogeneous. In this work, Er2O3 is considered as the encapsulating material due to its large difference in chemical properties with the Ni, Co, Mn and Li, which can not occupy the crystal sites of the LRMO material but precipitate on the particle surfaces of the matrix materials.

Figure 1. (a) Schematic diagram of the two-step approach for LMRO encapsulation; (b) Schematic diagram of the in-situ precipitation approach for encapsulation Er2O3 onto the surface of LNCMO material.

The prepared mixed solution was then placed in a water bath kept at 80 ℃ stirring with an electric blender at 500 r/min for 1 h to form the uniform spray solution, and then the spray solution was sent to the pulverizer with 200℃ high press gas by a peristaltic pump to obtain precursor powders. The collected precursor powders were then calcined at 900℃ to obtain the final samples, which were named as ErO0, ErO4, ErO8 and ErO10, respectively, according to the encapsulation content of Er2O3.

Experimental Section Synthesis of samples In this work, the Er2O3 is considered as the coating material due to its large difference in chemical properties with the Ni, Co, Mn and Li, so it could not substitute their sites but precipitate on the surface of the crystal, and the schemical route is displayed as Figure 1(b). Firstly, the origin materials of the Li, Ni, Co, Mn, and Er were dissolved in water to formation uniform solution, and then this solution was sprayed to obtain precursors, where, the Li, Ni, Co, Mn, and Er also distribute uniformly. After that, the precursors were calcined at a high temperature.

Material Characterizations Crystal structures of the Er2O3 coated LNCMO powders were analyzed by X-ray diffraction (XRD) measurements (X’Pert PRO, PANalytical) using Kα radiation with 2θ ranging from 10° to 120°at a scan rate of 0.08°/min. Structural refinement was performed using General Structure Analysis System (GSAS). Scanning electronic microscopy (SEM, S-4800, Hitachi) and high resolution transmission electron microscopy (HRTEM) (TEM, Techai G2 F30, FEI company) were used to investigate the morphology, size and micro-crystal structure. X-ray photoelectron spectroscopy (XPS) measurements were performed using an Xray photoelectron spectrometer (ESCALAB 250Xi, Thermo Scientific) equipped with an Al Kα X-ray radiation source (photon energy 1486.6 eV).

The detail experiment process is as follows: Er2O3 coated LNCMO samples were synthesized by a spray pyrolysis followed by high temperature calcining approach. Lithium acetate (LiAC, AR), manganese acetate (MnAC, AR), cobalt acetate (CoAC, AR) and nickel acetate (NiAC, AR) (a molar ratio of Li: Mn: Ni: Co= 1.2:0.54:0.13:0.13) were added to a 5000 mL deionized water with a concentration of 0.3 mol/L, and then 0.4 mol/L citric acid was added to this solution as a complex reagent to inhibit the flocculation of metal salt. Erbium acetate (ErAC, AR) with different encapsulation contents on the LNCMO matrix material (weight ratio=0%, 4%, 8%, 10%) were added into the solution.


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ACS Applied Materials & Interfaces In order to demonstrate the structures and morphologies of the Er2O3 encapsulating LNCMO materials by this in-situ precipitation strategy, the X-ray diffraction (XRD), scanning electronic microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM) were performed. Figure 2 shows that the XRD patterns of the ErO0, ErO4, ErO8 and ErO10 materials are readily indexed as layered structures of LMO2(M=Ni, Co, Mn, etc.) and Li2MnO3 with the space group R-3m (PDF#85-1966) and space group C/2m(PDF#84-1634), respectively. Furthermore, it also clearly displays the peaks of the Er2O3 phase with Ia3 space group(PDF#74-1983) and its intensity increases with the rise of the Er2O3 encapsulation content. Compared with the pristine material, the peaks of the Er2O3 encapsulating materials do not show obvious shift, which indicates that the Er element cannot enter into the lattice of the LNCMO crystal structure but precipitates onto the surface by formation of the independent Er2O3 phase. Moreover, the lattice parameters of the pristine material and materials with different Er2O3 coating content were calculated by GSAS refinement, listed in Table S1. The results of the Rietveld refinement of the XRD data of the ErO0, ErO4, ErO8, and ErO10 samples further confirm that the Er didn’t dope into the layered structure lattice.

Preparation of Electrodes Firstly, active materials (1.6 g) and conductive agent super P(TIMCAL) (0.2 g) were added to a stainless-steel ball mill jar (120 mL) with a weight ratio of 8:1, and then WC ball (108 g) and ethanol (80 mL) were also added. Secondly, mixed materials were prepared via a high-energy ball-milling route with 400 r/min for 6 h. Lastly, the obtained mixed materials were dried at 80 ℃ in an air-circulation oven for 12 h. Then obtained mixed materials and binder sodium carboxymethyl cellulose (CMC) with weight ratio of 95:5 were mixed with magnetic stirring for 4 h and then pasted onto aluminum foil to obtain electrodes. After that, prepared electrodes were dried at 80 ℃ in vacuum for 12 h to remove the solvent. Electrochemical Measurements Electrochemical performance were examined by 2025 coin-type cells. The cells were assembled in an Ar-filled glove box (water content < 0.1 ppm) with lithium metal foil as the counter electrode, and Celgard-2400 membrane as the separator. Electrolyte was consisted of 1 M LiPF6 dissolved in ethylenecarbonate (EC), dimethycarbon (DMC) and diethycarbonate (DEC) with a volume ratio of 1:1:1. Galvanostatic charge/discharge tests were performed on a multichannel battery testing system (NEWARE BST-610, China) at different currents with a voltage window of 2.0-4.8 V vs. Li+/Li at 30 ℃. The electrochemical impedance spectra (EIS) was carried out by a VMP system (Biologic, English) with an AC amplitude of 5mV in the frequency range from 0.01 Hz to 100000 Hz at 30 ℃.

SEM images (Figure 3(a, b)) display that the pristine and encapsulated LNCMO powders are comprised of homogeneous particles with diameters in the range of 100 to 200 nm. It should be noted that the surface of the pristine particle is very smooth but the encapsulated particles exhibit a rough surface, which suggests that the Er2O3 was precipitated on the surface of the LNCMO particles. The low magnification TEM images also reveal that there exists an obvious and homogeneous encapsulation layer on the surface of the LNCMO matrix material (Figure 3(c, d). As illustrated in Figure 3(e, f), the HRTEM images further demonstrates that a thickness of about 10 nm encapsulation layer exists on the surface of the matrix material to form a core-shell Er2O3 coated LNCMO composite. The corresponding fast Fourier transformation (FFT) images (Figure 3(g, h, i)) also confirm the matrix material is the LNCMO and the encapsulating layer is the Er2O3 phase.

Results and Discussion

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Figure 3. SEM images of the Er2O3 coated LNCMO composites: (a)ErO0, and(b) ErO8 materials; Low magnification TEM bright field images: (c)ErO0, and (d)ErO8; HRTEM images: (e) ErO0, and (f)ErO8; Fast Flourier transformation (FFT): (g) From the (e), and (h, i) corresponding to the A and B areas of the (f), respectively. at 20 mA g-1. However, the ErO4, ErO8 and ErO10 electrodes are only capable of providing the discharge capacity of 283, 280, 277 mAh g-1, respectively. It clearly shows that the specific capacities of the electrodes decrease with the increase of the Er2O3 encapsulation content. According to the result of the EIS (Figure S1), the decrease of the electrochemical capacity of the LNCMO electrode with Er2O3 coating may also be due to the Er2O3 coating layer increasing the resistance of the electrode. Moreover, Figure 4(c-f) demonstrates the ErO0, ErO4, ErO8 and ErO10 electrodes deliver the first irreversible capacities of 64, 55, 46 and 39 mAh g-1, respectively, and their corresponding first coulombic efficiencies are 80.7, 83.1, 87.0 and 87.8%, respectively (Figure S2). The dQ/dv curves (Figure 4(b)) suggests that there is no obvious difference for the first charge and discharge process of the LNCMO cathode with different content of Er2O3 encapsulation.

Obviously Improving First Coulombic Efficiency of LNCMO Cathode The first charge/discharge profiles of the ErO0, ErO4, ErO8 and ErO10 electrodes are represented in Figure 4(a). It is clear that all curves display a smooth voltage ramp in the range from 3.7 V to 4.4 V along with a plateau between 4.4 V and 4.6V. The corresponding differential capacity-versus-voltage (dQ/dV) profiles are shown in Figure 4(b). They show that there are two oxide peaks at 3.95 V and 4.50V, respectively, during the charge. The first oxide peak related with the smooth voltage ramp can be ascribed to the Li+ de-intercalation from the layered structure with the oxide of Ni2+/Ni4+ and Co3+/Co4+ and the second oxide peak differentiated from the long plateau can be attributed to the activation of the Li2MnO3 component.48 The results exhibit that the pristine ErO0 cathode delivers discharge capacity of 290 mAh g-1



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4.0 46 mAh g

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Figure 4. (a, c-f) Initial charge and discharge curves of the ErO0, ErO4, ErO8, Er10 electrodes at 20 mA g-1; (b) dQ/dV profiles of the ErO0, ErO4, ErO8, ErO10 electrodes at the first cycle at 20 mA g-1 in the potential range of 2.0 to 4.8 V vs. Li/Li+.

Figure 5 displays the X-ray photoelectron spectroscopy (XPS) spectra of the ErO0 and ErO8 electrodes charged up to 4.8V at the first cycle. It clearly shows that the contents of the Ni, Co and Mn on the surface of the ErO0 electrode are lower than that of ErO8 when they are charged up to 4.8 V. The lower contents of the Ni, Co and Mn on the surface of the ErO0 electrode can be reasonably explained by the corrosion of the HF or other acid species during charge process.49 Another reason that should be taken into account is the aggressive side reactions occurring at high voltages (>4.5 V), and the resulting formation of thick SEI layer on the cathode particle. The above results indicate that the Er2O3 encapsulation improves the first coulombic efficiency of the LNCMO cathode mainly due to the Er2O3 encapsulation layer preventing the LNCMO cathode material to be corroded by the HF or other acid species. Compared with other ways to improve the first cou-

lombic efficiency of LMRO cathodes, this encapsulation strategy has several advantages over the other methods. Song et al.50 reported using Super P to facilitate the phase transformation from Li2MnO3-active to corresponding spinel-active component in Li(Li0.2Mn0.54Ni0.13Co0.13)O2 cathode to reduce its ICL. However, this way will decrease the discharge voltage of the matrix material because of the spinel-active component formation. In addition, the carbon materials coating can decrease the tap density of electrode materials. The other way to low the ICL is acid treatment by preactivating LMRO cathodes, which also decreases the discharge voltage of the matrix materials because this acid treatment process will produce many spinel like components.51-52 The Er2O3 encapsulation in this work does not change the crystal structure of the matrix material but can effectively improve its first coulombic efficiency.


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Figure 5. XPS spectra of the ErO0 and ErO8 electrodes charged up to 4.8V at the first cycle, which were collected after the Ar+ etching for 10 nm(surface), 100 nm respectively: (a, d) Ni 2p; (b, e) Co2p; (c, f) Mn 2p.

stability compared with the pristine and the other cathodes. However, the LNCMO cathode with 8 wt% Er2O3 encapsulation not only demonstrates the similar excellent cycling stability but also delivers the higher discharge specific capacity when compared with that of the LNCMO cathode with 10 wt% Er2O3 encapsulation. Therefore, the LNCMO cathode with 8 wt% Er2O3 encapsulation shows the best overall electrochemical performance. Therefore, this in-situ precipitating Er2O3 encapsulation on LNCMO cathode can significantly enhance its cycling stability with a slight decrease of the discharge capacity due to the existence of the inactive Er2O3 component. The inactive oxides coating on LMRO cathodes strategies have been reported in previous works. Ghanty et al.45 reported the LMRO cathode coated with ZrO2 by a twostep heat-treatment showed the best capacity retention of only 86% at shorter 50 cycles. Guo et al.53 demonstrated that the Li1.2Mn0.567Ni0.167Co0.066O2 coated with MnO2 delivered capacity retention 93% after 50 cycles, which is the relatively better results compared with other similar reports. In contrast, in this work, the cycling performance was performed over 300 cycles, in this prolonged cycling condition, the Er2O3 coated LNCMO cathode still

Drastically Suppressing Capacity Degradation and Voltage fading In order to investigate the cycling stability of the Er2O3 coated LNCMO composite cathodes. The galvanostatic charge/discharge test with a prolonged cycling over 300 cycles was performed at 200 mA g-1. The prolonged cycling performance of the pristine ErO0 and coated ErO4, ErO8 and ErO10 cathodes are compared in Figure 6(a), and the corresponding capacity retentions are plotted in Figure 6(c). The results demonstrate that the pristine LNCMO cathode (ErO0) displays the discharge specific capacity of 226 mAh g-1 at the first cycle with the capacity retention of 84% after 300 cycles compared with initial discharge specific capacity. By contrast, the coated ErO4, ErO8 and ErO10 cathodes exhibit discharge specific capacities of 220, 216, and 205 mAh g-1 with the capacity retentions of 89%, 99%, and 101%, respectively, indicating the Er2O3 encapsulation can effectively improve the cycling stability of the LNCMO cathodes and the LNCMO cathode with 10 wt% Er2O3 encapsulation delivers the best cycling



tion layer indicates a strong interaction between the Er2O3 layer and the LNCMO matrix material, which is essential for its prolonged structural stability. Therefore, enhanced cycling stability of the Er2O3 coated LNCMO composite with this in-situ precipitating encapsulation is expected.

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exhibits the high capacity retention over 100%. It can be explained by this in-situ precipitating Er2O3 encapsulation strategy on the LNCMO materials to a uniform, complete, and robust Er2O3 coated LNCMO composite structure compared with the traditional two-steps encapsulation routes. This unique encapsula-

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ErO10

Figure 6. (a) Cycling performance of the ErO0 and ErO8 electrodes at 200 mA g-1; (b) Voltage fading profiles of the ErO0 and ErO8 electrodes at 200 mA g-1; (c) Mid-point discharge voltage retentions, capacity retentions of the ErO0 and ErO8 electrodes at 200 mA g-1.

Figure 6(b) shows the voltage fading curves of the ErO0, ErO4, ErO8 and ErO10 electrodes during cycling and Figure 6(c) shows that the relationship between the mid-voltage retentions and the Er2O3 encapsulation content. It can be seen that the ErO10 electrode delivers the best mid-point voltage retention 84% after 300 cycles at 200 mA g-1 compared with the 78% of the ErO0 electrode. Figure 7(a, b) shows the differences of the charge/discharge curves between the ErO0 and ErO8 electrodes at different cycles. It is obvious that the ErO0 electrode delivers different voltage plateaus and faster voltage fading after 100 cycles compared with the ErO8 electrode indicating that the ErO0 electrode suffered from the severe phase transformation processes during cycling. The dQ/dV plots calculated from the discharge curves of various cycles also further reveals the phase transformation processes as displayed in Figure 7(c, d). According to the plots, there are mainly three electrochemical reductions, noted as Re1 around 3.75V, Re2 at 3.32V, and Re3 lower than 3.1V, respectively. Re1 can be ascribed to Li de-intercalation from layered structure accompanied by of reduction of the Ni4+/3+2+ and Co4+/3+. Re2 is related to the reduction of Mn4+/3+ in the layered structure. Re3 is associated with the reduction of the Ni4+/3+2+, Co4+/3+ and Mn4+/3+in the spinel structure.48 It can be seen that the Re1 and Re2 peaks gradually decrease and transform to Re3 peaks, which clearly suggest that the layered structure of the

LNCMO cathode is gradually decayed and transformed to the spinel-like phase during cycling. In comparison of the Figure 7(c) and Figure 7(d), it clearly shows that the Er2O3 encapsulation can effectively suppress the decay of the layered structure of the LNCMO cathode to inhibit the formation of the spinel like phase. The dQ/dV plots of different cycles are compared in Figure S3 for the ErO0 and ErO8 electrodes. It can be seen that, at the 2nd cycle (Figure S3(a)), there are two electrochemical reductions of Re1 and Re2 for ErO0 and ErO8 electrodes. After 10 cycles (Figure S3(b)), the Re1 peak remarkably declines for the ErO0 electrode but it is still obvious for the ErO8 electrode. To 50th cycle (Figure S3(c)), the Re2 peaks both the ErO0 and the ErO8 electrodes have gradually shifted to lower potentials. Meantime, it clearly shows that the Re1 peak of the ErO0 electrode almost disappears and transforms to Re2, but the Re1 only shows a slight decrease for the ErO8 electrode. From the 100th cycle to the 300th cycle (Figure S3(d-f)), the Re2 almost transforms to Re3 for the ErO0, but for the ErO8 electrode, the Re1 peak is also obvious and there are a large of Re2 does not transform to the Re3 compared to the ErO0 electrode. We also analyze the contributions of the Re1, Re2, and Re3 to the discharge specific capacities of the ErO0, ErO4, ErO8 and ErO10 electrodes during cycling, and the (Re1+Re2)/total capacity is also calculated, which are summarized in Figure S4. The results exhibit the specific capacity con-


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tributions from the Re1 and Re2 reductions for the ErO0 electrode decrease faster than that from the ErO8 electrode. The ratio of the (Re1+Re2)/total capacity for the ErO8 electrode is also larger than

that of the ErO0 electrode. These results further confirm the Er2O3 encapsulation can effectively suppress the destruction of layered structure of the LNCMO cathode.

(a)

(b)

5.0

5.0 ErO0

ErO8

10st 50st

3.5

100st

3.0

200st 300st

2.5 2.0

4.0

30

50st

3.5

100st 200st

3.0

300st

2.5

60 90 120 150 180 210 240 270 -1 Specific capacity (mAh g )

0

100 ErO0 -1 -1

dQ/dV (mAh g v )

0

Re1

-200

Re2 3.40V 3.75 V

-300

2nd 10st 50st 100st 200st

-400

90

120 150 180 210 240 270

-500

(d) ErO8

-100 Re2 3.32V

-200

2nd

Re1 3.74V

-300

10st 50st 100st 200st

-400

300st

Re3

60

-1

(c)

-100

30


0 -1 -1

10st

2.0 0

100

2nd

+

2nd

4.0


4.5

+


4.5

dQ/dV (mAh g v )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Re3

300st

-500 2.1 2.4 2.7 3.0 3.3 3.6 3.9 4.2 4.5 4.8

2.1 2.4 2.7 3.0 3.3 3.6 3.9 4.2 4.5 4.8 +

+



Figure 7. (a, b) Charge and discharge curves of the ErO0 and ErO8 electrodes at different cycles at 200 mA g-1 in the potential range of 2.0 to 4.8 V vs. Li/Li+; (c, d) dQ/dV profiles of the ErO0 and ErO8 electrode materials at different cycles at 200 mA g-1 in the potential range of 2.0 to 4.8 V vs. Li/Li+.

The XRD patterns and the corresponding refined results by General Structure Analysis System (GSAS) software package of the pristine ErO0 and ErO8 electrodes before cycling and after 300 cycles are displayed in Figure 8, and the refined cell parameters are summarized in Table S2. According to the XRD patterns (in Figure 8(a)), it clearly demonstrates that the peaks related to (003)R(001)M, (101)R(-202)M and (104)R(202)M are all shifted to a lower angle for pristine and coated LNCMO cathode materials after 300 cycles. However, in comparison with the shift of the coated ErO8 cathode, the pristine ErO0 cathode exhibits a larger shift, which means a larger structure change. The cell parameters of a(b) and c (in Table S2) of the ErO0 material after 300 cycles increase from 2.854 Å to 2.888 Å, and from 14.227 Å to 14.416 Å, respectively, in comparison with that of before cycling, and its cell volume is up to 104.184 Å3 from 100.368 Å3. However, for the ErO8 cathode material, the cell parameters of a(b) increase from 2.853 Å to 2.864 Å, and c rise from 14.215 Å to 14.304 Å, and cell volumes are up from100.229 Å3 to 101.641 Å3, respectively. In order to better understand these changes, the ratio of the cell parameters after 300 cycles to that of before cycling for the pristine ErO0 and ErO8 electrodes are plotted in Figure 8(b). It clearly shows that the pristine ErO0 material displays a larger cell

parameter change than that of the ErO8 cathode material. In comparison with a(b), the c delivers a larger increase. From above results, it can be concluded that the Er2O3 encapsulation can stabilize the crystal structure of the LNCMO materials by preventing the dissolution of TM elements during cycling. The XPS spectra of these electrodes after 300 cycles was conducted to explore the dissolution of TM elements of the LNCMO cathode materials. In this study, Ar+ ions were used to consecutively etch the electrodes from the 10 nm to 50 nm and 100 nm, respectively. As illustrated in Figure 9, all spectra with respect to Ni 2p, Co 2p and Mn 2p orbital have been compared in order to demonstrate the variations in chemical state and content as a function of depth. Figure 9(a-c) shows the Ni content on the surface (10 nm) of the ErO0 electrodes is much lower than that of 50 nm and 100 nm below the surface and almost no Co and Mn elements are found on the surface (10 nm) for the ErO0 electrode compared with that of the 50 nm and 100 nm. At the same time, it is also clear that the Co and Mn elements of the 50 nm below the surface are less than that of the 100 nm below the surface. The results demonstrate that the LNCMO cathode material suffered from the dissolution of the TM elements from the surface gradually pene-


ACS Applied Materials & Interfaces trating into the bulk of the electrode material during cycling. Surprisingly as showing as Figure 9(d-e), it is apparent that the Ni, Co, Mn contents of the ErO8 electrode on the surface (10 nm), 50 nm and 100 nm below the surface have no obvious change. These

results clearly demonstrate that the Er2O3 coating on the surface of the LNCMO cathode material can effectively prevent the dissolution of the TM elements during cycling.

♣ ♣

♦

♦ Er8-300 cycles

♣ ♠ ♠

15

20

(020)M

Er0-300 cycles ♥

10

(104)R(202)M

♣ Er2O3

♦

(101)R(-201)M

♦

Expansion ratio to pristine (%)

(b) (003)R(001)M

(a)

Intensity /(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 17

25

Er0-before cycling

30 35 40 45 2-Theta (°)

50

55

60

104.0 103.5 103.0

ErO0-300 ErO8-300

102.5 102.0 101.5 101.0 100.5 a

b c Cell parameters

vol.

Figure 8. (a) XRD patterns of the ErO0 and ErO8 electrode materials after 300 cycles; (b) The ratios of the cell parameters of the Er2O3 coated LNCMO cathodes after 300 cycles to the pristine cathode, which are refined by GSAS.

when cycled in the electrolytes with addition of the 0 wt% and 0.1 wt% H2O after prolong cycles. When cycled in the electrolytes with addition of the 0.5 wt% H2O, the cycling stability of the ErO8 electrode is still well before 200 cycles and then decreases at the following cycles. These results clearly demonstrate that the Er2O3 coating can effectively prevent the LNCMO cathode material to be etched by the acidic species during cycling.

The dissolution of TM elements into the electrolyte is further confirmed by TEM. According to the HRTEM lattice images (Figure 10(a, b)) and their corresponding electron diffraction patterns shown in Figure 10(c, d), it can be seen that the pristine ErO0 electrode material cycled after 300 cycles demonstrates irregular lattice fringes with much lattice disorder and dislocations, and obvious polycrystalline rings. In contrast, the coated ErO8 electrode material exhibits relative regular lattice fringes and better electron diffraction patterns. Furthermore, the enlarged HRTEM images (Figure 10(e, f)) from the A and B regions of the Figure 10(a, b) images, respectively, illustrate that the lattice of the pristine ErO0 electrode material shows much more defects than that of the coated ErO8 electrode materials. The HRTEM results reveal that the stabilized crystal for the coated LNCMO cathode is due to the Er2O3 encapsulation layer to prevent the dissolution of TM elements during cycling.

The Er2O3 coating can effectively suppress the corrosion of the LNCMO cathode material be by the electrolyte and the dissolution of the TM ions during cycling to significantly improve the overall electrochemical performance, but it needs to consider how does Li transport for getting into and out of the cathode? We believe that maybe the Er2O3 can react with Li to form LixEr2O3. Some related works have been reported that the binary compound aluminum oxide (Al2O3) is one of the most effective coating materials for LIBs and has been extensively utilized for various positive and negative electrodes. Liu et al.57 monitored the morphological changes of Al nanowires surrounded by Al2O3 surface layers during lithiation–delithiation cycles using in situ transmission electron microscopy (TEM) technique. The authors observed that the lithiation first takes place in the Al2O3 layer by forming a stable Li–Al–O glass tube, and once the volume expansion of LixAl2O3 reaches a certain level, subsequent lithiation into the inner Al core is followed. Jung et al.58 showed, via first-principles calculations, that the observed Li–Al–O tube contains 3.4 Li atoms per each Al2O3. They suggested that the lithiation of the Al2O3 layer continues until it reaches the thermodynamically stable phase (Li3.4Al2O3) and extra Li+ ions then overflow into the adjacent active electrode.

It is generally accepted that the particle surface of the cathode material directly contacts with the electrolyte to make it easy to be etched by the acidic species in the electrolyte, such as HF produced mainly by reaction between the electrolyte and the residual H2O.28-29 The attack by acidic species aggravates the dissolution of TM elements, and consequently resulting in the capacity/voltage fading.54-56 Therefore, in this work, the Ero0 and ErO8 electrodes were cycled in the different electrolytes with adding different H2O to investigate its effect on the cycling stability. The results show that the cycling stability of the ErO0 electrode decreases with the increase of the added H2O content in the electrolyte (Figure S5(a)). Most importantly, displayed as Figure S5(b), the cycling stability of the ErO8 electrode is very well


(a)

Ni 2p1/2

Ni 2p3/2

(d)

ErO0-Ni-300 cycles

Ni 2p3/2

ErO8-Ni-500 cycles Ni 2p1/2 100nm

Intensity(a.u.)

100nm

50nm

50nm

10nm

10nm

(d) 845

850

(b)

855

860 865 BE(eV)

870

875

845

880

850

(e)

ErO0-Co-300 cycles

Co 2p3/2

855

860 865 BE(eV)

870

875

880

ErO8-Co-300 cycles

Co 2p3/2

Co 2p1/2 100nm

Intensity(a.u.)

100nm

Intensity(a.u.)

Co 2p1/2

50nm

50nm

10nm

10nm

770 775 780 785 790 795 800 805 810 BE(eV)

770 775 780 785 790 795 800 805 810 BE(eV)

(c)

Mn 2p3/2

ErO0-Mn-300 cycles

(f)

Mn 2p3/2

ErO8-Mn-300 cycles Mn 2p1/2

Mn 2p1/2

100nm

Intensity(a.u.)

Intensity(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Intensity(a.u.)

Page 11 of 17

100nm

50nm

50nm

10nm 10nm

635

640

645 650 BE(eV)

655

660

635

640

645 650 655 BE(eV) BE(eV)

660

Figure 9. XPS spectra of the ErO0 and ErO8 electrodes after 300 cycles at 200 mA g-1 in the potential window of 2.0 to 4.8 V vs. Li/Li+ which were collected after the Ar+ etching for 10 nm, 50 nm, 100 nm respectively: (a, d) Ni 2p; (b, e) Co2p; (c, f) Mn 2p.



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Page 12 of 17

Figure 10. (a, b) HRTEM images of the ErO0 and ErO8 electrode materials after 300 cycles; (c, d) Electron diffraction patterns of the ErO0 and ErO8 electrode materials after 300 cycles; (e, f) Enlarged HRTEM images from the A and B regions of the (a, b).

Conclusion

Supporting Information Initial irreversible capacities and coulombic efficiencies, comparisons of the dQ/dV profiles, comparisons of the discharge specific capacities contributions, cycling performance of the ErO0 and ErO8 electrodes under different H2O contents, comparison of the cell parameters of the Er2O3 coated LNCMO cathode materials before cycling and after 300 cycles.

In conclusion, a novel encapsulation strategy of in-situ precipitating nanoscale Er2O3 encapsulation on the LNCMO cathode material is proposed for the first time. The XRD and HRTEM results confirm that an about 10 nm Er2O3 layer was successfully coated onto the surface of the LNCMO material. The results suggest that the in-situ precipitating Er2O3 encapsulation onto the surface of the LNCMO material forms a uniform, complete, and robust Er2O3 coated LNCMO composite structure. It clearly demonstrates this unique Er2O3 encapsulation can effectively increase the initial coulombic efficiency and improve the cycling stability of the LNCMO cathode with the initial coulombic efficiency of 87.8% and high capacity retention over 100% after 300 cycles, respectively. Moreover, this unique Er2O3 encapsulation can also obviously inhibit the voltage fading of the LNCMO cathode during cycling. This unique Er2O3 encapsulation can effectively suppress the dissolution of TM elements to suppress the decay of the structure of the LNCMO cathode.

AUTHOR INFORMATION Corresponding Author E-mail: [email protected]; Tel./Fax: +86-571-87952615

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of PR China (No. 51571178), Aerospace Science and Technology Innovation Fund of CASC, and National Materials Genome Project (2016YFB0700600).

ASSOCIATED CONTENT


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Table of Contents

A novel strategy of in-situ precipitation and encapsulation of Er2O3 phase on Li(Li0.2Ni0.13Co0.13Mn0.54)O2 (LNCMO) cathode material for lithium-ion batteries is proposed for the first time. The Er2O3 phase is precipitated from the bulk of the LNCMO material and encapsulated onto its entire surface during calcining process. This unique nanoscale Er2O3 encapsulation can significantly prevent the LNCMO cathode material to be corroded by electrolyte and stabilize the crystal structure of the LNCMO cathode during cycling. Therefore, the prepared Er2O3 coated LNCMO composite exhibits excellent cycling performace and high initial coulombic efficiency.


In Situ Encapsulation of the Nanoscale Er2O3 Phase To Drastically

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