A Synthesis Method for Long Cycle Life Lithium-Ion Cathode Material

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A Synthesis Method for Long Cycle Life Lithium-Ion Cathode Material: Ni-Rich Core-Shell LiNi0.8Co0.1Mn0.1O2 Qi Li, Rongbin Dang, Minmin Chen, Yulin Lee, Zhongbo Hu, and Xiaoling Xiao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02000 • Publication Date (Web): 07 May 2018 Downloaded from http://pubs.acs.org on May 7, 2018

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ACS Applied Materials & Interfaces

A Synthesis Method for Long Cycle Life Lithium-Ion Cathode Material: Ni-Rich Core-Shell LiNi0.8Co0.1Mn0.1O2 Qi Li†, Rongbin Dang†, Minmin Chen†, Yulin Lee‡, Zhongbo Hu†, Xiaoling Xiao†* †

College of Materials Science and Opto-electronic Technology, University of Chinese Academy of Sciences, Beijing 100049, P. R. China. ‡ Department of Materials, Imperial College London, Royal School of Mines, Exhibition Road, London SW7 2AZ, UK. KEYWORDS: high-nickel materials, core-shell structure, hydrothermal synthesis, high-thermal performance, long cycle life Abstract High-nickel materials with core-shell structures, whose bulk is rich in nickel content and the outer shell is rich in manganese content, have been demonstrated to improve cycle stability. The high-nickel cathode material LiNi0.8Co0.1Mn0.1O2 is a very promising material for lithium-ion batteries; however, its low rate performance and especially cycle performance currently hampers further commercialization. This study presents

a

new

synthesis

method

to

prepare

this

core-shell

material

(LiNi0.8Co0.1Mn0.1O2@x[Li-Mn-O], x=0.01, 0.03, 0.06). Electrochemical data shows that LiNi0.8Co0.1Mn0.1O2@x[Li-Mn-O] (x=0.03, CS-0.03) exhibits the best high-rate performance, cycle stability and thermal stability. The initial discharge capacity of core-shell sample CS-0.03 is 118 mAh g-1, which is almost the same as the discharge capacity of pristine LiNi0.8Mn0.1Co0.1O2 (117 mAh g-1) at the rate of 10 C in the voltage range of 3.0-4.3 V. Notably the capacity decay of CS-0.03 is 18.4% after 200 cycles compared to 27% decay in capacity of the pristine. Furthermore, CS-0.03 1 ACS Paragon Plus Environment

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exhibits better thermal cycling stability. The capacity retention of the CS-0.03 sample reached 65.1% which is over 1.3 times than that of the pristine, whose capacity retention is 49.2% after 105 cycles (55 °C). Evidently, the core-shell structured CS-0.03 sample has excellent cycle stability and this synthesis method can be applied to other cathode materials. 1. Introduction Lithium-ion battery has penetrated into all aspects of our daily lives, portable

electronic

devices

and

electric

vehicles.

High-nickel

1

such as oxides

（ LiNi1-x-yCoxMnyO2 ） have attracted extensive interests for applications in lithium-ion batteries owing to their high discharge capacities and low costs.2-3Though high-nickel materials have satisfactory specific discharge capacities, the capacity retention reported so far are very low.4 For example, the discharge capacity of LiNi0.8Co0.1Mn0.1O2 was 144 mAh g-1 at the first cycle at the rate of 1 C (25 °C), but capacity retention was only 82.20% after 200 cycles. Due to the severe side reactions at elevated temperatures, the cycle stability of LiNi0.8Co0.1Mn0.1O2 was worse at higher temperatures. At 55°C, the initial discharge capacity of LiNi0.8Co0.1Mn0.1O2 was 175 mAh g-1 and then capacity decayed significantly to 131.57 mAh g-1 after 100 cycles, corresponding to a capacity retention of 74.9% at 2 C (1 C = 200 mA g-1).5 As it can be seen from the above examples, high-nickel materials showed terrible cycle stability, especially high temperature cycling stability. Some studies have proven that surface coating is an effective method for improving the cycle stability and thermal stability of high-nickel materials.6-9 For 2 ACS Paragon Plus Environment

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example, fluorides, phosphates, metal oxides and polymers have been applied to modify the surfaces of high-nickel materials and successfully enhance their electrochemical performances.10-18 The mechanisms behind the improvements mainly stem from the following aspects:(1)electron-conducting media can promote electron transfer at the surface of the cathodes; (2) modification of cathode surfaces enhances their structural stability by impeding side reactions and reducing the dissolution of metal into electrolyte.19Among them, CeO2@Li[Ni0.5Co0.2Mn0.3]O2 is a good example. The 2 wt % CeO2-coated LiNi0.5Mn0.2Co0.3O2 had a higher capacity of 168 mAh g-1 compared to 152 mAh g-1 for the pristine uncoated LiNi0.5Mn0.2Co0.3O2 at the first cycle. After the 100th cycle, a discharge capacity of 97.4 mAh g-1 was retained for the CeO2-coated Li[Ni0.5Co0.2Mn0.3]O2 which was much higher than the retained discharge capacity of the pristine (23.5 mAh g-1). The corresponding capacity retention of CeO2@Li[Ni0.5Co0.2Mn0.3]O2 and Li[Ni0.5Co0.2Mn0.3]O2 were 57.7% and 15.4% respectively (at 10 C over 2.8-4.6 V at 55 °C, 1 C = 180 mA g-1).20 Results from these studies demonstrate that surface coating can effectively improve cycle stability. In addition to surface coating, the core-shell structure is another effective method to significantly improve the cycle stability of high-nickel cathode materials.21Sun et al. have done much outstanding work and pioneered a new field of research on high-nickel cathode materials with core-shell structures. Core-shell materials have common features: the core is composed of a high-Nickel material, providing high capacity; the shell contains higher concentrations of Mn4+ in its composition, ensuring 3 ACS Paragon Plus Environment


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cycle stability and thermal stability at highly delithiated states.22-23 A number of core-shell structure cathode materials have been synthesized successfully such as Li[(Ni0.8Co0.1Mn0.1)0.8(Ni0.5Mn0.5)0.2]O2 Li[Ni0.83Co0.07Mn0.1]O2

consists

of

and a

uniform

Li[Ni0.83Co0.07Mn0.1]O2. concentration

core

of

Li[Ni0.9Co0.05Mn0.05]O2 and a concentration-gradient outer layer24-25.The capacity retention of Li[Ni0.9Co0.05Mn0.05]O2and Li[Ni0.83Co0.07Mn0.1]O2 were reported to be 72.9% and 96.9% respectively after 50 cycles at the rate of 0.2 C (40 mA g-1) at 25 °C. Capacity decay was only 7.9% for the concentration-gradient Li[Ni0.83Co0.07Mn0.1]O2 and 35.4% for the uniform concentration core Li[Ni0.9Co0.05Mn0.05]O2 at 55 °C after 50 cycles. Core-shell materials are more ideal choices to improve cycle stability compared to surface coating. In this study a novel and simple method is presented for the preparation of high-nickel cathode materials with shell-core structures. The core-shell structure synthesized

by

this

method

exhibited

excellent

cycle

performance,

and

high-temperature cycle performance. The reversible capacity of CS-0.03 decreased from 118 mAh g-1 to 96 mAh g-1 at 10C in the voltage range of 3.0-4.3 V (25 °C) after 200 cycles with a capacity retention of 81.6%. The discharge capacity of the pristine changed from 117 mAh g-1 to 85 mAh g-1 with a capacity retention of 73.1%. In addition, CS-0.03 had better thermal stability than that of the pristine. The capacity retention of the CS-0.03 sample reached 65.1% which was over 1.3 times than that of the pristine, whose capacity retention was 49.2% at 55 °C after 105 cycles at 1 C. The excellent electrochemical performances originate from the novel core-shell structure. 4 ACS Paragon Plus Environment

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The central bulk that is rich in Ni provides a high discharge capacity. The shell, in which nickel ions were replaced with manganese ions partly, offers outstanding cycle life and safety. In addition, the shell is spinel structure, which has three-dimensional lithium ion diffusion channels, and can improve the rate performance. Compared with the reported core-shell structures, our advantages are: (1) An ultra-thin shell, which ensures that the core of high-Nickel occupies a relatively large proportion, offering high capacity; (2) Atomic-level transitions between core and shell parts. This method is also an effective strategy for improving cycle stability as well as thermal stability of other high-Nickel cathode materials. 2. EXPERIMENTAL SECTION

Scheme 1 Schematic view of the synthesis process of core-shell structure. The preparation process of samples is shown in Scheme 1. Core-shell high-nickel cathode materials were synthesized through three steps. Step 1: NiSO4, CoSO4, and 5 ACS Paragon Plus Environment


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MnSO4 were dissolved in deionized water at a stoichiometric ratio to form a uniform solution; Step 2: Melamine and manganese acetate were dissolved in mixed solvents of ethanol and glycerin where melamine and manganese acetate were complexed; Step

3:

Melamines

complexed

with

different

amounts

of

manganese

acetate(n(manganese acetate):n(Ni0.8Co0.1Mn0.1·2H2O)=0.01,0.03,0.06) were bonded to crystalline water on the oxalate surface by hydrogen bonding under hydrothermal conditions, and the core-shell precursors were obtained as a result. Finally, the precursors were grinded with LiOH·H2O to obtain the pristine and core-shell materials (LiNi0.8Co0.1Mn0.1O2@x[Li-Mn-O], x=0.01, 0.03, 0.06), and samples were marked as pristine, CS-0.01, CS-0.03, and CS-0.06. This core-shell structure is annotated as LiNi0.8Co0.1Mn0.1O2@Li-Mn-O: the core consists of LiNi0.8Co0.1Mn0.1O2; the shell Li-Mn-O contains Mn4+, with higher concentration than that of the core. Detailed synthesis steps are as follows. 2.1 Materials Preparation 2.1.1. Synthesis of Ni0.8Co0.1Mn0.1C2O4·xH2O Precursor The oxalate precursor was synthesized by a co-precipitation method, as follows: NiSO4·6H2O, CoSO4·7H2O, and MnSO4·5H2O were weighed according to the molar stoichiometric ratio of 8:1:1 and then dissolved into deionized water with the concentration of 2 M. 2 M of H2C2O4 water/ethanol solution was dropped into the above solution. After violently stirring overnight, the obtained precursor was washed with deionized water and ethanol for three times respectively and then dried overnight at 80 °C. 6 ACS Paragon Plus Environment

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2.1.2. Synthesis of Ni0.8Co0.1Mn0.1C2O4·xH2O @Mn2+Precursor

A solvothermal process was carried out to prepare Ni0.8Co0.1Mn0.1C2O4·xH2O @Mn2+. 1g Ni0.8Co0.1Mn0.1C2O4·xH2O precursor was dissolved into 15 ml ethanol. Another aqueous was prepared by adding C3N6H6 (n (C3N6H6): n (Mn2+) = 4:1) and Mn(CH3COO)2·4H2O into the mixed solution of 5 ml ethanol and 4 ml C3H8O3 under magnetic stirring until the solutions were transparent. Next the two above homogeneous solutions were mixed under rigorous stirring to form a clear mixed solution. Then the mixed solution was transferred into a Teflon-lined stainless-steel autoclave and reacted at 150 °C for 5 hours. In order to rule out the impact of glycerol and melamine, oxalate precursor treated with melamine and glycerol by the hydrothermal

method

was

obtained

and

named

cheated

pristine.

The

Ni0.8Co0.1Mn0.1C2O4·xH2O@Mn2+ precursors and the cheated precursor were obtained.

2.1.3. Synthesis of Pristine and Core-Shell Cathode Materials LiNi0.8Co0.1Mn0.1O2@x[Li-Mn-O], (x=0.01, 0.03, 0.06) were obtained by grinding the LiOH·H2O and Ni0.8Co0.1Mn0.1C2O4·xH2O@Mn2+ precursor uniformly, n (Li)：n (transition metals)=1.05.

The mixtures were calcined at 400 °C for 5 h and

then 800 °C for 12 h in pure oxygen atmosphere. Similarly, the pristine and cheated pristine were synthesized under the same conditions. 2.3 Materials Characterization The crystal structure of the samples were tested by X-ray diffractometer (XRD,

7 ACS Paragon Plus Environment


Rigaku, Cu Ka, 1.544 Å) scanning in the 2θ range of 10° to 80° at a step width of 0.01° and scan speed of 10° min-1. 2.4 Electrochemical Measurements The electrochemical performances of the samples were tested in CR2016 coin cells. The positive electrodes were the mixture of active materials (80 wt %), carbon black (10 wt %), and poly(vinylidene fluoride) (10 wt %) assembled in a glove box filled with high purity argon. Galvanostatic charge−discharge cycling was performed between 3.0 and 4.3 V (vs Li/Li+) using automatic galvanostat (NEWARE) at different current rates. The cyclic voltammogram (CV) measurements was tested at a scan rate of 0.5 mV s−1 between 3.0 and 4.3 V (vs Li/Li+) and electrochemical impedance spectroscopy (EIS) was also carried out on an Electrochemical Workstation (Metrohm-Autolab, PGSTAT 302N) instrument. 3. RESULTS AND DISCUSSION

Figure 1. (a) Corresponding diffraction peaks of (003).（b）XRD patterns of LiNi0.8Co0.1Mn0.1O2 and core-shell materials. (c) Corresponding diffraction peaks of (108) and (110).


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Table 1. The molar ratios of n (Mn):n (Ni+Co+Mn) and n (Mn):n (Co) for the pristine, cheated pristine and CS-0.03 by analyzing ICP results. samples pristine Cheated pristine CS-0.03

n (Mn):n (Ni+Co+Mn)(%) 8.28 8.1 10.96

n (Mn):n (Co)(%) 78.8 74.8 108.93

XRD patterns of the pristine and core-shell materials are shown in Figure 1. As shown in Figure 1(b), no impurity existed and all peaks are well indexed to the R-3m group.26Two pairs of splitting peaks-(006/102) and (108/110)-in the XRD patterns are typical structural features of hexagonal layered materials. No peaks corresponding to crystallized spinel Li-Mn-O were observed in the samples indicating low amounts of Li-Mn-O or the shell layer being beyond XRD resolution.27 As shown in Figure 1(a), as the concentration of Mn4+ increased, the (003) peaks shift to the right. This is due to the fact that the radius of Mn4+ is smaller than that of Ni2+, Ni3+ and Co3+, which achieves a similar doping effect in the transition region between the spinel and layered parts. This is an evidence of the atomic-level transitions between the spinel and layered structures. Figure 1(c) shows the split peaks of (108) and (110), which proves that the CS-0.03 crystal structure was not affected by the Li-Mn-O shell. The ratio of I (003)/I (104) belonging to the CS-0.03 sample is 1.309, nearly the same as the pristine (1.299), hence we can conclude that Li+/Ni2+ cation mixing was suppressed slightly. As shown in Table 1，ICP results show that the actual chemical formulas for the pristine, cheated pristine, and CS-0.03 are LiNi0.81Co0.11Mn0.08O2, Li0.81Co0.11Mn0.08O2

and

LiNi0.79Co0.1Mn0.11O2

respectively.

concentrations of elements matched expected values. 9 ACS Paragon Plus Environment

The

relative


Figure 2. (a) Discharge curves for samples at different current densities for the pristine and CS-0.01, CS-0.03 and CS-0.06. (b) Corresponding cycling performance at a current density of 10 C in the voltage range of 3.0-4.3 V. 1 C = 160 mA g-1 Subsequently, the electrochemical performances of the pristine and core-shell structure samples were studied. As shown in Figure 2(a), the rate capability of LiNi0.8Co0.1Mn0.1O2 and core-shell materials were tested under different current densities (0.2 C, 0.5 C, 1 C, 3 C, 5 C, 10 C) for every five cycles in the voltage range of 3.0-4.3 V. At low current densities of 0.2 C and 0.5 C, the improvement on discharge capacities for the core-shell materials was not obvious. However, with the increase of current density, the rate-performances of core-shell materials were 10 ACS Paragon Plus Environment

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obviously better than that of the pristine. With a current rate of 10 C，CS-0.01 and CS-0.03 delivered similar discharge capacities of approximately 120 mAh g-1 and 116 mAh g-1, both of which were higher than that of the pristine (105 mAh g-1). In addition, as shown in Figure S1, the cheated pristine shows the worst rate performance and cycle performance at room temperature. Evidently the positive electrode material treated with melamine and glycerol cannot improve the electrochemical performance at room temperature. It can further be shown that the improvements in electrochemical performances of the core-shell materials are due to the increase in surface manganese concentrations. Furthermore, we investigated the cycle stability at 10 C of the pristine and core-shell structure samples. As shown in Figure 2 (b), in the first few cycles, CS-0.01 and CS-0.03 both showed lower specific capacities than that of the pristine. While the discharge capacities of CS-0.01 and CS-0.03 exceeded that of the pristine on the 21st and 30th cycle respectively. CS-0.01 and CS-0.03 exhibited better cycle stability than the pristine. The capacity retention ratios of the pristine, CS-0.01, and CS-0.03 samples were 73.1%, 73.7%, and 81.6% respectively after 200 cycles, which demonstrates that the CS-0.03 sample material had higher cycle stability than CS-0.01 and

the

pristine.

Therefore,

the

most

optimal

LiNi0.8Co0.1Mn0.1O2@x[Li-Mn-O], (x=0.01, 0.03, 0.06) is 0.03.


amount

of


Figure 3. (a) Cycling performance at a current density 1 C in the voltage of 3.0-4.3 V at 55 ℃. Discharge curve of different cycles at 1 C for pristine (b) and for CS-0.03 (c). (d) Charge−discharge curves of different cycles for samples in a voltage window of 3.0-4.3 V at 1 C. (1 C = 160 mA g-1). Then we further explored the thermal stability (55 ℃) of the pristine and LiNi0.8Co0.1Mn0.1O2@x[Li-Mn-O], (x=0.01, 0.03, 0.06) in the voltage range of 3.0-4.3 V at 1 C after activation at 0.2 C for 4 cycles. At the first few cycles, since Mn4+ is electrochemically inactive, CS-0.03 material showed a relatively lower discharge capacity than the pristine. But with increasing Mn4+ concentration, Mn4+ can play a role in stabilizing the structure. Hence with the increase of charge/discharge cycles, the core-shell materials showed more excellent cycle stability compared to the pristine. 12 ACS Paragon Plus Environment

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As shown in Figure 3(a), the initial capacities delivered by the pristine (188 mAh g-1) was a little higher than the CS-0.03(181 mAh g-1) after the activation of 0.2 C for 4 cycles, while the discharge capacities of the pristine and CS-0.03 were 93 mAh g-1 and 118 mAh g-1 respectively after 105 cycles at 1 C. The corresponding capacity retention of the CS-0.03 sample reached 65.1%, which was over 1.3 times that of the pristine sample’s (49.2%). The discharge capacity of the cheated pristine was lower than that of the CS-0.03 sample, as shown in Figure S2. As such, core-shell structured CS-0.03 also showed more excellent thermal stability. Figure 3(b) and (c) compare the charge−discharge curves of different cycles at a current density of 1 C in a voltage range of 3.0 to 4.3 V at 55 ℃. The thermal stability of core-shell materials is attributed to the increase of Mn4+ concentration in the shell. The initial discharge capacity was 210 mAh g-1 for the pristine, little higher than the CS-0.03 sample (192 mAh g-1) at the activation of 0.2 C. After the 85th cycle at 1 C, the discharge capacity of CS-0.03 was 134.5 mAh g-1 while the pristine was just 107.3 mAh g-1. The core-shell materials exhibited excellent cycle stability at high temperatures. Figure 3(d) shows the difference between charge and discharge voltage platforms. It can be seen that the charging and discharging platforms show basically no difference between the pristine and the CS-0.03 samples on the 6th cycle. However, on the 105th cycles, the platform gap for the CS-0.03 sample was much less than that of the pristine. This indicates that the polarization of the CS-0.03 sample had been narrowed.11,28 This may be attributed to the high-manganese concentration on the 13 ACS Paragon Plus Environment


surface, which can effectively inhibit the dissolution of positive electrode materials into the electrolyte. The stability of the interface between the positive electrode material and the electrolyte is improved as a result.

Figure 4. Corresponding dQ/dV curves for the pristine (a) and CS-0.03 (b). The dQ/dV charging/discharging curves of 6th, 25th, 55th, 85th cycles at 1 C in the voltage of 3.0-4.5 V (55 °C) are supplied and shown in Figure 4. The reducing value of voltage is a criterion for evaluating the effect of polarization. As the cycling proceeded, the decrease of reduction peaks the CS-0.03 decreases is slower than that of the pristine, which demonstrates that CS-0.03 has a lower polarization than that of the pristine. 14 ACS Paragon Plus Environment

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Table 2. The electrochemical performance comparison between our prepared samples (LiNi0.8Co0.1Mn0.1O2 and CS-0.03) and other reported results (Ni-rich materials and modified Ni-rich materials) electrochemical

temperature

window [V]

(°C)

samples

capacity

initial

retention

capacity

[%]

[mAh

ref

g−1] LiNi0.8Co0.1Mn0.1O2

45.5 % (at 2 C and

2.8-4.3 V

-

[17]

100

60

cycles) Polypyrrole (PPy) coating

69.8 %(at

LiNi0.8Co0.1Mn0.1O2

2 C and

-

100 cycles) 2.7-4.5 V 55

CC

51% (at

205 mAh

(LiNi0.62Co0.14Mn0.24O2)

0.2 C and

g-1

[29]

50 cycles) CG-Al2O3

89% (at

205 mAh

(Al2O3-coated

0.2 C and

g-1

concentration-gradient

50 cycles)

Li[Ni0.8Co0.2]0.7[Ni0.2Mn0.8]0.3O2) LiNi0.8Co0.1Mn0.1O2 2.7-4.3 V

72.4% (at

178.7

1 C and

mAh g−1

[30]

100

40

cycles) LiAlO2-coated

87.4% (at

180.9

LiNi0.8Co0.1Mn0.1O2

1 C and

mAh g−1

100 cycles) LiNi0.8Co0.15Al0.05O2

69.78 (at 1

[31]

C and 150 2.8-4.3 V

55

cycles) SDCNCA2

85.14 (at 1

(Al2O3@ LiNi0.8Co0.15Al0.05O2)

C and 150 cycles)

3.0-4.3 V

Pristine

49.2% (at

188.8

LiNi0.8Co0.1Mn0.1O2

1 C and

mAh g-1

100

55

work

cycles) CS-0.03


65.1% (at

Our

181


Page 16 of 35

1 C and

mAh g-1

100 cycles

As shown in Table 2, the capacity retention improves in a larger degree for CS-0.03 compared with most reported results.17 The capacity retention of our prepared LiNi0.8Co0.1Mn0.1O2 is 49.2% at 1 C after 100 cycles while 51% for reported LiNi0.62Co0.14Mn0.24O2 at 0.2 C after 50 cycles.29 Furthermore, the capacity retention of the CS-0.03 sample reaches 65.1%, which is over 1.3 times than that of the pristine sample (49.2%), higher than 1.2 times for LiAlO2-coated LiNi0.8Co0.1Mn0.1O2 and 1.22 for SDCNCA2.30-31


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Figure 5. Cyclic voltammograms with a scanning rate of 0.1 mV s-1 (a) pristine and (b) CS-0.03. (c) pristine and (d) CS-0.03 at 0.5 mV s-1. (e) Nyquist plots of the pristine and CS-0.03. (f) The mathematical relationship between Z’ and W-1/2. Cyclic voltammetry (CV) is an effective tool to investigate the electrochemical irreversibility and phase transformations during electrochemical reactions.3 Figure 17 ACS Paragon Plus Environment


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5(a)-(d) show the CV curves of pristine and CS-0.03 cathode materials which were measured between 3.0-4.3 V (vs Li+/Li). The electrodes for Figure 5(a) and (b) were measured at a scan rate of 0.1 mV s−1 which were charged at the rate of 10 C after having been cycled 300 cycles already. Those two curves show the unique redox couples corresponding to LiNi0.8Co0.1Mn0.1O2 according to previous reports. Three peaks on both the anodic (3.77, 4.04, and 4.23 V) and cathodic (3.68, 3.96 and 4.15 V) curves can be observed, which correspond to Ni2+/Ni3+, Ni3+/Ni4+, and Co3+/Co4+. There is no cathodic peak near the 3 V region in both samples which indicates there was no reduction of Mn3+/Mn4+ (Mn3+/Mn4+ redox reactions occur at about 2.9 V).32 This demonstrates that Mn-ions were electrochemically inactive and were present in the +4 oxidation state. In addition, R1, R2, and R3 represent phase transitions of -

-

-

rhombohedral (R 3 m), spinel-like phase(Fd 3 m), and NiO-like rocksalt (Fm 3 m) respectively.33 In order to further study the effects of high-rate performance, we measured the CV on the electrodes without charging at a scan rate of 0.5 mV s−1. As shown in Figure 5 (c) and (d), the anodic peaks of the pristine and CS-0.03 were 4.05 V and 4.02 V at the first cycle. And the corresponding anodic peaks shift to 3.91 and 3.89 V on subsequent cycles. The potential intervals (△V) between the anodic and cathodic peaks indicate the reversibility of Li+ (de)intercalation and electrode polarization.34 As shown in Figure 5(c) and (d), the △V of the main peak (Ni3+/Ni4+) of the CS-0.03 is 0.22 V, smaller than that of the pristine (0.29 V), which suggests that increasing the surface concentration of Mn4+ can suppress potential difference. The electrochemical reversibility of core-shell materials was improved as a result. 18 ACS Paragon Plus Environment

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Electrochemical impedance spectra (EIS) can measure the interface conditions between the cathode surface and electrolyte, and calculate lithium-ion transfer resistances. Figure 5(e) depicts the EIS results of the pristine and CS-0.03 samples without charging. The Nyquist plot consists of a semicircle at high frequencies and the linear part at low frequencies.35 The EIS data was then simulated with an equivalent circuit [R(RQ)WC] explicated in Figure S3, giving quantitative results to interpret the measured data. The total impedances can be regarded as the electrolyte resistance Rele, the charge transfer resistance Rct, and double-layer capacitance Cdl. W is the Warburg impedance that reflects the diffusion of lithium-ion in the solid. 36Cl is the intercalation capacitance. Through circuit simulation, we obtained the charge-transfer resistance Rct of the pristine as 483 Ω, twice as much as the CS-0.03 sample (245 Ω) shown in Table S1, which indicates that the Li-Mn-O shell layer can reduce the electrodes' resistances and improve their kinetics diffusion coefficients. Diffusion coefficient and exchange current density satisfied the following equations:

j0 =

DLi+ =

i0 = A

RT (1) nFR ct A

R 2T 2 (2) 2 A 2 n 4 F 4 C 2σ 2

Z’ = Rele + Rct + σW-1/2 (3) According to equation (2), D (Li+) is inversely proportional to σ which is the slope of the straight line in Figure 5(f). As can be seen from Figure 5(f), σ (pristine) > σ (CS-0.03), so it can be inferred that the diffusion coefficient of Li+ (D (Li+) ) in the



CS-0.03 is larger than that in the pristine cathode material. All of the above can result in improvements of rate performance.

Figure 6. Differential scanning calorimetry traces showing heat flow from the reaction of the electrolyte with LiNi0.8Co0.1Mn0.1O2 and CS-0.03 charged to 4.3 V. The thermal stability performances and safety of cathode materials are important metrics for practical applications such as P-HEVs. Figure 6 shows differential scanning calorimetry curves of LiNi0.8Co0.1Mn0.1O2 and core-shell cathode materials LiNi0.8Co0.1Mn0.1O2@x[Li-Mn-O], (x=0.01, 0.03, 0.06) charged to 4.3 V. For LiNi0.8Co0.1Mn0.1O2, the onset temperature of the exothermic peak was 200.96 °C, with the exothermic peak at 248.29 °C for core-shell materials, the onset temperature of the exothermic peaks shifted right compared to that of the pristine (202.57 °C for CS-0.01, 219.13 °C for CS-0.03, 212.15 °C for CS-0.06). Although the onset of exothermic peaks of CS-0.06 was lower than for CS-0.03, the exothermic peak of CS-0.06 was 258.91 °C which was higher than that of CS-0.03 (249.82 °C). In addition, the reduced heat generation of CS-0.03 was 51.7 J g-1, lower than 78.98 J g-1 of the pristine. We can conclude that core-shell materials possess better thermal


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stability.37 As the data have shown, the CS-0.03 material had better thermal stability.

Figure 7. XPS spectra of Ni 2p for the pristine (a), and cheated pristine (b), and CS-0.03 (c). (d) Mn 2p for the pristine, and cheated pristine, and CS-0.03 respectively.

Table 3. Binding energies (eV) and relative atomic concentrations for the pristine, cheated pristine and CS-0.03. Samples

Pristine

Cheated pristine

CS-0.03

Ni2+ 2p3/2 (eV)

854.83

854.74

854.72

Ni3+ 2p3/2 (eV)

855.98

855.8

855.69

X-ray photoelectron spectroscopy measurements (XPS) were carried out to



examine the valence states of Ni, Co and Mn. Figure 7 (a)-(c) show the Ni 2p of the pristine, cheated pristine, and CS-0.03: the most intense peak (Ni 2p2/3) and a satellite peak.38-40 The most intense peaks Ni 2p3/2 shifted from a higher binding energy of 855.65 (pristine) to 855.54 eV (CS-0.03), which indicates the amount of Ni2+ in the pristine and CS-0.03 samples progressively increased. This may be attributed to the increase in amount of Mn4+ in the shell. Ni 2p3/2 peaks are well fitted in two sub-signals (Ni2+ 2p3/2 and Ni3+ 2p3/2) at 854.83, 854.8 for the pristine, 854.74, 855.8 for cheated pristine, 854.72, 855.69 eV for CS-0.03 respectively shown in Table 3, which is characteristic of high-nickel materials.41 The XPS plot of the Mn 2p spectra consists of two peaks (2p3/2 and 2p1/2) and an overlap with the Ni Auger spectrum.42 The binding energies of Mn 2p3/2 were 642.46, 642.41, and 642.7 eV for the pristine, cheated pristine, and CS-0.03 respectively, consistent with the peak location of Mn4+. It can be concluded that Mn-ions in the samples were preferably in 4+ states. Mn4+ did not participate in electrochemical reactions which enhanced the stability of the structure effectively. As shown from Figure 7(d), the area of Mn 2p for the pristine is smaller than that of CS-0.03, which illustrates that the amount of Mn4+ in the shell was higher compared to the pristine. Metal ions were rearranged to keep the balance of valence, which leads to in situ layered-to-spinel structural transformation and a hetero-structure was obtained.43 In addition, as shown in Table 1, the CS-0.03 had a higher concentration of Mn-ions, the value of n (Mn):n (Ni+Co+Mn) for CS-0.03 was 1.32 times as much as that for the pristine.


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Figure 8. SEM morphology images of (a) the pristine, (b) cheated pristine, (c) CS-0.03. HRTEM images of samples (d) pristine, (e) cheated pristine, (f) CS-0.03. (d1)-(f2) are corresponding FFTs of (d)-(f). Figure 8(a)-(c) show the morphologies of the pristine, cheated pristine, and CS-0.03. As indicated in the SEM images, the particle morphologies have been changed with the treatment of melamine, glycerol, and manganese acetate. The trend of appearances transform from rectangles to squares, from sharp to smooth.44 More detailed structural features of the pristine, cheated pristine, and CS-0.03 were



Page 24 of 35

explored by high-resolution TEM. Figure 8 (d-f) are the corresponding high-resolved lattice fringes of Figure 8 (a-c). Figure 8 (d1-f2) are the Fourier transform images corresponding to the symbols in Figure 8 (d-f). Figure 8 (a) and (b) indicate that the inner and outer parts possess the same crystal planes—(104) and (101)—with lattice spacings of 0.2 and 0.24 nm (R-3m) respectively. While Figure 8(f) shows two groups of vertical crystal planes with lattice spacings of 0.47 and 0.25 nm, corresponding to (003) (R-3m) and (311) (Fd-3m).45-46

The results from the FFT images are

coincidence with the lattice spacings.39, 47 From the HRTEM images it can be seen that after the treatment of melamine, glycerol, and manganese acetate, a novel layered@spinel hetero-structure was formed.48 This structure may be due to the increase of the surface manganese concentrations. Table 1 shows that the relative concentrations of n (Mn):n (Ni + Co + Mn) in the core-shell structure increased from 8.28% to 10.96% compared to that of the pristine.

Table 4. The amount of dissolved Ni、Co and Mn ions of pristine, cheated pristine and CS-0.03 (stored in fresh electrolyte for 10 days) . The electrodes were charging to 4.3 V at 10 C. Storage time 10 days (25 ml)

Samples

Ni (mg/Kg)

Co (mg/Kg)

Mn (mg/Kg)

pristine

0.29

0.78

0.98

CS-0.03

0.12

0.45

0.13

To further demonstrate the positive electrode material's resistance to electrolyte attack and to study the surface stability of the positive electrode material. We charged the electrode to 4.3 V at the rate of 10 C (1 C = 160 mA g-1) after activation of 0.2 C for 2 cycles, then the electrodes were stored in fresh electrolyte at 25 °C for 10 days.


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ICP was used to measure the concentration of dissolved transition metal ions into the electrolyte with respect to storage time. As shown in Table 4, the leaching of metal ions from the particle for the pristine were 0.29, 0.78 and 0.98 mg/Kg, higher concentrations than for the CS-0.03 sample (0.12, 0.45, and 0.13 mg/kg). Thus, CS-0.03 can suppress the dissolution of the transition metal and protect the electrode from structural degradation, and thereby improve the cycle stability and rate performance.49

Figure 9. Schematic view of the interface between electrode and electrolyte Figure

9

exhibits

the

electrode

structure

diagram

of

the

pristine

LiNi0.8Co0.1Mn0.1O2 and core-shell structure CS-0.03. The following two aspects could explain the mechanisms behind the improved performances: (1) The bulk is a high-Nickel material, guaranteeing high discharge capacity; (2) At the same time, a high-manganese concentration in the surface spinel structure serves a protective function which can effectively resist the erosion of the electrolyte, which in turn ensures excellent cycle stability. Therefore, the core-shell CS-0.03 sample exhibited not only high discharge capacity but also excellent cycle stability.

3. CONCLUSIONS In conclusion, we have successfully designed a novel and simple method for



the preparation of high-Nickel cathode materials with shell-core structures. The core-shell materials prepared by this method not only exhibited high discharge capacities of up to 118 mAh g-1 attributed to the high-Nickel content LiNi0.8Co0.1Mn0.1O2 core but also excellent cycle stability and thermal stability due to shells with relative high concentrations of Mn4+.50 In addition, the spinel shell can provide three-dimensional lithium-ion diffusion channels, which improve rate performances. This work demonstrates a novel method to prepare controllable and ultra-thin shells with relatively high concentrations of Mn4+. It presents a novel opportunity for building high performance cathode materials for LIBs.

AUTHOR INFORMATION Corresponding Author *(X. L.) E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEGEMENTS This work was supported by the Beijing Nova Program (Z141103001814065), the Youth Innovation Promotion Association CAS (2016152), and the Scientific Instrument Developing Project of the Chinese Academy of Sciences (Grant ZDKYYQ20170001)

SUPPORTING INFORMATION The electrochemical performance of the pristine, cheated pristine, and CS-0.03 at room temperature and high-temperature (55 ℃ ), equivalent circuit of EIS, The


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simulation results of Rele and Rct.

ABBREVIATIONS CS-0.01, LiNi0.8Co0.1Mn0.1O2@x[Li-Mn-O], x=0.01; CS-0.03, LiNi0.8Co0.1Mn0.1O2@x[Li-Mn-O], x=0.03; CS-0.06, LiNi0.8Co0.1Mn0.1O2@x[Li-Mn-O], x=0.06; EIS, Electrochemical impedance spectra; CV, Cyclic voltammetry; XPS, X-ray photoelectron spectroscopy measurements.

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