Surfactant-Assisted Synthesis of High Energy {010} Facets Beneficial

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Surfactant-Assisted Synthesis of High Energy {010} Facets Benefiting to Li Ion Transport Kinetics with Layered LiNi0.6Co0.2Mn0.2O2 Xiaokang Ju, Hui Huang, Wei He, Hongfei Zheng, Pan Deng, Shengyang Li, Baihua Qu, and Taihong Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00126 • Publication Date (Web): 02 Apr 2018 Downloaded from http://pubs.acs.org on April 3, 2018

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ACS Sustainable Chemistry & Engineering

Surfactant-assisted Synthesis of High Energy {010} facets benefiting to Li ion transport kinetics with layered LiNi0.6Co0.2Mn0.2O2

Xiaokang Ju†, Hui Huang†, Wei He‡, Hongfei Zheng‡, Pan Deng†, Shengyang Li†, Baihua Qu†*, and Taihong Wang†* †

Pen-Tung Sah Institute of Micro-Nano Science and Technology, Center for Sensing and Aero-

propulsion, Xiamen University, No. 422, Siming South Road, Xiamen, Fujian, 361005 PR China ‡

Department of Materials Science and Engineering, College of Materials, Xiamen University,

No. 422, Siming South Road, Xiamen, Fujian, 361005 PR China *

To

whom

correspondence

should

be

addressed,

E-mail:

[email protected], Tel.:+86-0592-2183063; Fax: +86-0592-2187196

1 Environment ACS Paragon Plus

[email protected],

ACS Sustainable Chemistry & Engineering 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

KEYWORDS Lithium-ion batteries; 3D hierarchical flower; Surfactant; High energy facets; High-rate ABSTRACT High energy {010} facets are favorable for Li+ transport in layered Ni-rich LiNi0.6Co0.2Mn0.2O2 cathode through two-dimensional channels that are perpendicular to c axis. However, those planes can hardly be maintained during the synthesis of layered cathodes. Therefore, we provide a strategy to use appropriate surface active agents which can alter the surface free energy by reducing surface tension directly. Here, a novel self-assembled 3D flowerlike hierarchical LiNi0.6Co0.2Mn0.2O2 is formed with the help of sodium-dodecyl-sulphate (SDS) and those high energy facets are preserved. Due to the unique surface architectures which would lead to the fast ion transport kinetics as current expands to 100 times （from 0.1 C to 10 C）, the capacity decay only about 23.4 %. Furthermore, full cells assembled against Li4Ti5O12 are constructed with a capacity retention of 80.61 % at 1 C charge/discharge. This study could show a promising material model for the preferred orientation active planes and higher Li+ transport kinetics Introduction In face of growing requirements for large-scale applications in portable storage devices, electric vehicles (EVs) and hybrid electric vehicles (HEVs), many efforts have been focused on identifying and developing green energy storage devices such as Lithium-ion batteries (LIBs) with high energy density, long cruising autonomy, superior safety as well as low cost. 1-3 And in the perspective of material evolution perspective, more progresses have been made in the anode than in the cathode active materials.

4, 5

Therefore, the cathode materials are considered as the


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key to determining the performance of LIBs. So far, the most widely-used commercial and favorable LIBs are based on layered LiMO2 (M=Ni, Co, Mn et al). So it’s necessary to achieve high rate property in order to meet the demands of power batteries. 6, 7 On the other hand, the rate performance of the electrodes depends on the density of the lithium ion transport channels (d), the diffusion coefficient in the electrode (D) and the diffusion length (L). That is：

Kinetics ∝

஽ ௅మ

݀

(1)

It can be seen that, the surface structure, especially the exposed density (d) of surface transmission channels can be a greatly crucial factor that determines the rate of Li+ ion deintercalation/intercalation. Hence, the design of the structure and Li-ion diffusion channels of LiMO2 can provide an effective route to enhance the electrochemical performance. The layered crystal structure of α-NaFeO2 is shown in Figure 1. It has been extensively established that the diffusion of lithium ions in the LiMO2 cathode occurs along a twodimensional (2D) interstitial space, which is considered as a pathway that parallels to the (001) planes (along the a and b anises).

8

That is (010), (01ത0), (100), (1ത00), (11ത0) and (110) planes.

They are denoted as {010} facets. Contrarily, the layers which are perpendicular to c axis are indexed as {001} planes, that hinder Li ion diffusion due to the composed NiO6, CoO6, and MnO6 octahedral.

9-11

Therefore, the rate performance combined stable cycle of layered LiMO2

cathodes are strongly dependent on the density of open structure -- exposed {010} active facets. Many efforts have been devoted to increasing the percentage of exposed active facets for typical layered cathode.

12

For example, Wang. et al have acquired exposed {010} planes in

Li1.2Mn0.54Ni0.13Co0.13O2

cathodes

and

these

cathode

materials

display

outstanding

electrochemical properties. 3 As we know, the rate property is strongly related to the high energy



{010} facets. However, it is still a huge challenge to preparing layered cathodes with high density of exposed {010} facets, as these high energy facets tend to disappear according to minimization of energy. Moreover, the synthesized active facets are prone to be stacked, leading to reduced surface area of {010} planes.

10

Therefore, the design of a unique structure with

highly density of {010} planes is strongly desired for high-performance layered LiMO2 cathodes.

Figure 1：：The crystal structure of layered LiMO2 cathodes. Besides, we consider that surface active agents can reduce the Gibbs surface energy significantly, as the reducing of surface tension can alter the surface free energy directly (according to the definition of surfactant). It means those high energy {010} facets would be preserved comparing with reaction system without surfactant that the resulting crystal would be dominated by low energy {001} facets. And some previous studies have adopted polyvinylpyrrolidone (PVP)

13

to form the high energy facets. However, most of those resulting

cathodes are in nano level (nano-plates) and the nano-materials tend to have low packing density, 4 Environment ACS Paragon Plus

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agglomerate and thermodynamically unstable

14

. Hence, the cycle stability of nano-materials

may not be satisfying. Especially for Ni-rich layered material, the terrible side reaction between electrode and electrolyte would lead to poor electrochemical cycle performance.

15, 16

Notably,

the development of Ni-rich layered cathodes must be one of the most important tendencies in LIBs field, 17-20 as layered Ni-rich materials like LiNi0.6Co0.2Mn0.2O2 can provided higher special capacity (∼180 mAh g-1) than LiNi1/3Co1/3Mn1/3O2 (~150 mAh g-1), LiMn2O4 (148 mAh g-1), and LiFePO4 (170 mAh g-1) , 21, 22 which can meet the rapid development of EVs and HEVs. Herein, we need to look for another alternative surface active agent that can provide not only more active high energy facets, but also stable cycle performance for layered Ni-rich cathode LiNi0.6Co0.2Mn0.2O2. On the basis of this and after many careful experiments, we decided to choose sodium-dodecyl-sulphate (SDS) as a substitute surfactant. On one hand, most of the precursors for layered LiMO2 cathodes are synthesized in an alkaline condition, and the anionic surfactants like SDS can standardize alkaline condition and be more effective to decrease the surface energy of high-energy surfaces, promoting the growth of the exposed {010} active planes. On the other hand, the novel self-assembled 3D flower-like hierarchical with an elegant micro/nano morphology for LiNi0.6Co0.2Mn0.2O2 can be obtained with a simple-operated SDSassisted hydrothermal process, resulting in improving cycle stability and superior Li ion transport kinetics prosperity. 23-31 Urea is used as a precipitant. According to the report of Li et al, 32, 33 the polar group of –NH2 in urea could also play a role of coordination on crystal nucleation. After the formation of the self-assembled 3D flower precursors, lithiation reaction combined with a stepwise calcinations process is adapted to develop the final LiNi0.6Co0.2Mn0.2O2, promoting phase transformation from precursors to layered cathodes while preserving high energy planes from disappearance and maintaining the novel hierarchical flower-like surface structures. This



structural analysis of TEM (shown in the following section) indicates that the lateral planes of nano-flowers are electrochemically active {010} planes. The 3D diffusiont paths spanning from the center of flower to the surface could be built up for Li+ transport, exhibiting higher rate property. Compared with sphere-like LiNi0.6Co0.2Mn0.2O2 (SL622), our novel 3D hierarchical flower-like LiNi0.6Co0.2Mn0.2O2 (FL622) can offer higher surface energy with more active exposed planes, which result in the enhanced Li ion transport kinetics. We anticipate that the concept of designing self-assembled 3D flower hierarchical micro/nano architecture with active {010} planes will prove an interesting model for the fundamental relationship between the structures and electrochemical properties in LIBs. In addition, the improved performance is instructive for the synthesis of other materials in the energy store fields. Experimental Section Material synthesis Hydrothermal method can improve the elemental homogeneity, which may result in superior performance. 9 Hence, we take the method of hydrothermal in this article. In this work, all the chemicals are of analytical grade and used without further purification. In a typical synthesis, NiCl2·6H2O， MnCl2·4H2O， CoCl2·6H2O purchased from Sinopharm (Ni: Co: Mn =6:2:2 molar ratio) are chosen as the starting materials and the transition metal ions are 5 mmol in total. The urea (urea/transition metal ion molar ratio is 1.2 : 1) is used as a precipitant. In addition, the donor of –NH2 polar group in urea combined with appropriate anionic surfactants SDS (~ 0.2 g) is adopted to control the surface Gibbs energy. After being stirred for 15 minutes, the reactant solution is transferred into a Teflon-lined stainless steel autoclave. The autoclave is sealed and maintained at 150 ℃ for 5 h in an oven. Once the reaction is complete, the precursors are filtered, washed and dried at 80 ℃ for 24 h. Then the prepared precursors are mixed


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thoroughly with Li2CO3 in a molar ratio of transition metal ion : Li = 1:1.05, and subsequently annealed at 350 ℃ to remove the residue water. Afterwards, the temperature is controlled by every 200 ℃ to the final temperature 750 ℃ in air. Each temperature ramp and stepwise temperature are taken for 2 h. The final temperature will last for 6 h. The cathode is denoted as FL622. For the comparison, a sphere-like LiNi0.6Co0.2Mn0.2O2 (SL622) material is synthesized in the same condition without SDS. Characterization The powder X-ray diffraction (XRD) patterns are recorded on a Panalytical X–pert diffract meter with Cu Kα radiation at 250 mA and 40 kV. The morphology and crystal structure are observed by scanning electron microscopy (SEM, Hitachi S4800) and high-resolution transmission electron microscopy (HRTEM, JEM–2100) with an acceleration voltage of 200 kV. The valence states of transition metal ions of the cathode are investigated via X-ray photoelectron spectroscopy (XPS) measurements with a Mg Kα source operating at 14.0 kV and 25 mA. Electrochemical measurements The electrochemical tests are performed by using a CR2032-type coin cell. The counter electrodes are used with pure lithium foils. The active materials are mixed with carbon conductive agent (acetylene black) and polyvinylidene fluoride (PVDF) at weight ratios of 80: 10: 10. The mixed slurry is tape casted on Al foil and is dried at 80 ℃ for 12 h under blast oven then transforming to vacuum. The dried foil is cut into circular disks and is used as working electrode. The loading density of active material in the working electrode is about 1.2-2.0 mg cm2

. The electrolyte solution is 1 M LiPF6 in ethylene carbonate (EC) and dimethyl carbonate

(DMC) (1: 1, in weight percent). A Celgard 2400 micro porous polypropylene membrane is used



as a separator. The coin-type cells are assembled in an argon filled glove box with water and oxygen contents of less than 1 ppm. Galvanostatic charge-discharge cycling performance is carried out by Land CT 2001 battery tester in the voltage range of 4.3 V-2.8 V (1 C=180 mA g-1). The electrochemical impedance spectroscopy (EIS) is carried out on an electrochemical workstation (CHI660E) by using a three electrode cell. A full cell with the commercial Li4Ti5O12 (LTO) anode (provided by BTR Company, Shenzhen, China) is constructed within the voltage ranging from 2.75 V to 1.25 V. Results and discussions Structure and morphology The formation of precursors is critical to the final product. As we know, the low energy {001} facets are thermodynamic equilibrium products if the synthesis reactions are under hydrothermal conditions for a long time.

34

Therefore, the reaction time must be taken into

consideration in order to obtain more active high energy plates. In general, the time need to be shorter. However, too short reaction time is not conducive to the later sintering process. Here, the influences of reaction time on the particle size and morphology are investigated according to the SEM images and schematic illustration in Figure 2a combined with SDS. Obviously, the morphology evolution is strongly dependent on the synthesis time. Typically after the reaction time of 30 mins, the produced crystal nuclei of the both intermediate products are similar to each other in Figure 2b and 2f. This phenomenon means the surfactant has little effect on the nucleation stage of the intermediate product. Then at the time of 100 mins, the SDS may play an important role in the flower-like architecture. Followed by the increasing and growth of nuclei, the irregular nanosheets are formed with the help of SDS in Figure 2c, while a sphere-like product in Figure 2g can be achieved as a control experiment. With increasing reaction time


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continually, the aggregation procedure of intermediate product is affected through diffusion which could result in different fractal structures like Figure 2d and 2h of 200 mins. When the reaction time is maintained to 300 mins, the complex flower-like architecture precursor with an average thickness of nanoplates around 190 nm (Figure 2e) could be finally obtained. It looks like a really blooming flower in Figure 2a. The low magnified image of the self-assembled precursor is inserted in Figure 2e. As stressed by Yu et.al that different self-assembled surface structures would lead to various ratio of exposed {010} facets, offer higher surface energy than bulk material.

25

12

the microflower structure can

In contrast to the precursor of the FL622,

Figure 2i provides an image of larger sphere shape precursor of the reaction product of SL622. It should be noted that surfactant is also critical in the preparation of FL622. The further explanation for formation of flower like structure using SDS is been investigated with SEM images (Figure S1) and discussed in the supporting information. These results provide evidence that the nanosheet of a microﬂower may be evolved from a nanowire “weaving” process with the hydrophilic group of SDS. Besides the reduced surface energy with SDS may slow down the disappearance rate of {010} facets, making the high-energy surface easier to synthesize in a short time. Likewise, the 3D micro/nano hierarchical structure can be more effective to ensure cycle stability according to the reports in decades.



Figure 2 : (a) Schematic illustration for the formation of precursors of FL622 at different stages of the hydrothermal reaction. (b-e) SEM images of FL622 precursors at 30 mins, 100 mins, 200 mins, 300 mins, respectively. (f-i) SEM images of SL622 precursors at 30 mins, 100 mins, 200 mins, 300 mins, respectively

Figure 3a and 3b show the SEM images of the FL622 at different magnified levels. The morphology is well inherited from the precursor. Noting that without the stepwise sintering process, we can hardly maintain the surface state, though we have controlled the growth of precursor to a certain degree. The morphology of SL622 is provided in the supporting information of Figure S2a and 2b. In addition, the Energy Dispersive Spectrometers (EDS) are carried out to detect the chemical compositions of both samples. The results are provided in the supporting information of Figure S3. It seems that the chemical compositions of Ni, Co and Mn are similar to each other, suggesting the chemical compositions may not sufficient to affect the electrochemical differences between the both samples.


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Structural information for as-prepared FL622 is identified by using X-ray diffraction (XRD), which is shown in Figure 3c. The observed diffraction peaks all index to a main phase of hexagonal α-NaFeO2 with space group of R3ത m. As we know, the XRD spectrum is an effective method to count the crystal plane via the intensity of the diffraction peak. Obviously, the I(110)/I(018) is about 1.42 for sample FL622 (Figure 3c). For sample SL622 (Figure S4), the I(110)/I(018) is about 1.06. This phenomenon has been reported in previous literatures. 10,11 As the (110) planes belong to {010}, which means an open channel for Li+ transport, while (108) planes are densely packed. The higher value of I(110)/I(018) implies the enhanced growth of the high energy planes. Table S1 lists the lattice parameters and the ratios of c/a, I(003)/I(104) refined from the XRD data. It is clearly seen that the crystal lattice of FL622 and SL622 are similar to each other, suggesting other factors of structures may have little effect on the performance of the cathodes. Just XRD data may not be sufficient to give evidence that the FL622 sample has more active planes. Furthermore, TEM analysis is a sensitive tool that can provide direct evidence of the exposure of active planes though it’s difficult to quantify the exposed active facets. Figure 3d is a typical magnified region of FL622. Notably, the HRTEM images taken from the selected square region marked in Figure 3d are also been exhibited in Figure 3e and 3f. The Figure .3e is the yellow region marked in the FL622 cathode. The lattice spacing of 0.247 nm that indexes with the (010) planes and the Fast Fourier Transform (FFT) pattern which reveals these nanoplates are hexagonal symmetry of layered materials. The HRTEM image and the FFT pattern of the lateral plane (Figure 3f) show an inter planar distance of 0.477 nm assigned to the (003) planes of LiNi0.6Co0.2Mn0.2O2 of R3തm structure (the red area in Figure 3d), suggesting that the presented image planes of the nanoplates may belong to (010), (01ത0), (100), (1ത00), (11ത0) or



(110) planes. This structural analysis indicates that the lateral planes of flower-like cathode are electrochemically active {010} planes. The surface of the flower-like structure will be dominated by {010} planes, affording faster rate property. While the surface of SL622 is covered by the low energy facets (Figure S5) The schematic illustration of SL622 and FL622 is been exhibited in Figure S6, which can express our design vividly.

Figure 3: (a and b) SEM images of the FL622 at different magnified level. (c) The XRD pattern of FL622 cathode. (d) A typical magnified region of FL622 cathode. (e and f) The HRTEM images taken from the selected square region marked in (d) and corresponding FFT analysis


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In order to better understand the oxidation states of transition metals, X-ray photoelectron spectroscopy (XPS) is performed. Spectrums are obtained and spectral analysis is carried out using Gaussian curves to fit the experimental date. The dominant peak with respect to Ni 2p is found at around 856.70 eV for both samples (Figure 4a). According to those reportes on Ni2+, most intense Ni 2p3/2 peak is located at 854.7 eV.

35

Obviously, the peak shifting to higher

binding energies implies an increase in the oxidation state of the Ni ions in LiNi0.6Co0.2Mn0.2O2. It is likely to affect the degree of cation mixing, because only Ni2+ ions can move to the Li+ slab owing to the similar ionic sizes of Ni2+ (0.69 Å) and Li+ (0.76 Å) in the high-Ni based layered structure. The Ni3+/Ni2+ ratios of FL622 and SL622 are about 0.78 and 0.70 respectively. The Co 2p spectrum in Figure 4b shows two major peaks of Co 2p1/2 and Co 2p3/2 with binding energy at around 780.0 and 795.0 eV followed by two weak shakeup satellites at 789.3 and 804.5 eV, confirming the existence of the Co3+. The Mn 2p spectra (Figure 4c) exhibits two main peaks of 2p3/2 at 642.6 eV and 2p1/2 at 654.4 eV with separation of 11.6 eV, demonstrating that the main chemical state of Mn is +4 valence. 36 Overall, the observed XPS peaks of FL622 and SL622 are identical with each other, indicating the oxidation states of transition metals would have little effect on the electrochemical differences between that two electrodes.

Figure 4: (a- c) XPS spectra of Ni 2p, Mn 2p, and Co 2p for both cathodes, respectively`



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Electrochemical properties Cycling performances of the LiNi0.6Co0.2Mn0.2O2 /Li half cells are evaluated in the voltage range of 2.8 to 4.3 V. Figure 5a presents the initial charge and discharge curves of the cells with FL622 and SL622 cathode materials at a rate of 0.1 C respectively (0.1 C= 18 mA g-1). The FL622 active cathode with more transport channels delivers a slightly higher discharge capacity (180.7 mAh g-1) and coulombic efficiency (85.8%) than the SL622 cathode material. This is reasonable

since

the

exposed

facets

are

favorable

for

the

reversibility

of

Li+

deintercalation/intercalation. A weak peak that locates at around 3.6 V of the charge curve is been marked with circle, resulting from the remaining Li2CO3 and Li(OH) on the surface of high-Ni cathode. After active process at 0.1 C rate, cycling tests of both samples are performed at a constant charge-discharge rate at 1 C, and the results are compared in Figure 5b. The discharge capacity of FL622 sample maintains at 149.2 mAh g-1 (91.87 % capacity retention) after 100 cycles. Compared with that of SL622 material (only 41.50 % capacity retention after 100 cycles), the FL622 shows a promising capacity retention. This implies that the as-prepared FL622 has higher rate potential as cathode material. The FL622 electrode after cycling is shown in Figure S7. Obviously, the FL622 electrode is well maintained. The residue may be acetylene black and PVDF. Maintenance of the flower-like porous structure ensures that the stability of cycle. The rate property of the electrodes have been further determined in Figure 5c and 5d. (the corresponding discharge profiles of SL622 are shown in Figure S8) The discharge capacities of FL622 at 0.1 C, 0.2 C, 0.5 C, 1 C, 2 C, 5 C, 10 C are around 178.3, 175.6, 170.8 167.3, 164.1, 151.4, 136.5 mAh g-1, respectively. When the rate is expanded to 100 times (from 0.1 C to 10 C), the specific capacity of 178.3 would fall to 136.5 mAh g-1, which means the capacity decay is


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about 23.4 % for FL622. It's clear that FL622 exhibits much higher discharge capacities than SL622 (the capacity decay for SL622 is about 51.3 %). When the rate returns to 0.1 C, the capacity of FL622 recovers from 136.5 to 175.8 mAh g-1. (98.6 % of the initial discharge capacity at 0.1 C).

Figure 5: (a) The initial charge and discharge curves of the cells with FL622 and SL622 cathode materials at a rate of 0.1 C. (b) Cycling tests of both samples at a constant charge-discharge rate at 1 C. (c) The rate property of the FL622 and SL622 electrodes, respectively. (d) the corresponding discharge profiles of FL622 at different rates



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Those mentioned above all indicate that the high energy planes could have fast charge and discharge potential for layered cathode material. However, the effect of the hierarchical structures hasn’t been shown directly. Hence, we have destroyed the flower-like structure by ultrasound in cell crusher for a long time. Then followed by a heat treatment process at 500 ℃ for 2 hours to remove the residual Li2CO3 and LiOH of high-Ni materials. The SEM images of the broken FL622 samples are shown in the supporting information of Figure S9. It's easy to discover that the 3D hierarchical surface structures have been broken. For comparison, the tests at different rates and cycle at 1 C are also applied for the materials suffered from ultrasonic treatment (Figure S10). As mentioned above, FL622 electrode displays excellent rate ability that, after 10 C cycles, the capacity decay is about 23.4% and when returning back to 0.1 C, there are few capacities loss of FL622 electrode. The discharge capacity of FL622 sample maintains at 149.2 mAh g-1 (91.87 % capacity retention) after 100 cycles at 1 C. In contrast, after ultrasonic treatment, though the electrode still exhibits a charming rate property when current expands to 100 times (the specific capacities drop from 177.1 mAh g-1 to 105.7 mAh g-1), the capacity recovery is not satisfied. In addition, the broken sample shows a larger rapid capacity after 45 cycles and fades from 163.3 mAh g-1 to 123.1 mAh g-1 after 100 cycles. The different rate and cycling performance before and after ultrasonic could demonstrate that, the unique structure may play an important role in achieving cyclic stability. In fact, it is the synergy that improves the overall performance of high-nickel layered cathode. As discussed before, enhancing density of these active planes should be conducive to the rate performance. Combination of the other inheritance properties from 3D flower-like hierarchical architecture like easy insertion extraction of lithium ions and more stability than nano-sized material,


15

a great enhancement of

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electrochemical kinetic characteristics has been achieved from the as prepared FL622. It could hunt for integral performance of layered LiNi0.6Co0.2Mn0.2O2. Without ion doping or surface coating in this work, therefore, both samples could have similar phase transitions process. Taking FL622 electrode as an example. Figure 6 shows the corresponding differential capacity versus voltage (dQ/dV) curves of the first, 20 th, 40 th, 60 th, 80 th and 100 th cycles charged to 4.3 V at 1 C. According to the reports of Xia et.al,

18

the

hexagonal phase (R1) to monoclinic (M) phase transition would occur at around 3.68 V (peak I) in the initial charge curve, which means the released Li+ in Li1-xNi0.6Co0.2Mn0.2O2 is about 0< x< 0.25. Followed by the increasing of potential, the M phase to rhombohedra phase (R2) transition locates at around 3.77 V (peak II). The X in Li1-xNi0.6Co0.2Mn0.2O2 ranges from 0.25 to 0.55 58. The occurrence of phase transitions is related to the amount of lithium ions released in the highNi based electrode materials. The Peak I at around 3.68 V is not obvious in the 1st cycle, which may be attributed to the electrode activation process. Followed by the electrochemical cycling, the redox peaks changed to a more polarized state and shifted farther apart. Hence, the oxidation peak at 3.68 V became sharp and moved to a higher potential. According to the report by Y.-K. Sun et.al, 37 the sharp and broad peak I in the later cycles indicates the two-phase coexistence of hexagonal and monoclinic. Therefore, peak I seems more dominant in the later cycles. With the constant charge and discharge, the dQ/dV curves of those peaks are compact to each other, which means the crystal structure of the electrode material is relatively stable.



Figure 6: Differential capacity versus voltage (dQ/dV) curves of the first, 20 th, 40th, 60 th, 80 th and 100 th cycles charged to 4.3 V at 1 C for FL622

The good rate capability of the FL622 electrodes is still confirmed by calculating corresponding Li+ diffusion coefficient (DLi+) with Electrochemical Impedance Spectroscopy (EIS). The EIS tests are performed after the first charge state to 3.8 V at 0.1 C. The Nyquist plots of two samples are shown in Figure 7a. The insert is the equivalent circuit. Generally, high frequency area is ascribed to the resistance of electrolyte (Rs), which is about 1.88 and 1.61 Ω cm2. The semicircle in middle-high frequency region can be put down to the charge transfer impedance (Rct). The fitting results of Rct are about 516 and 348 Ω cm2 for SL622 and FL622, respectively. Obviously, the FL622 sample exhibits the smaller impedance. The straight line in the low frequency region is linked to Warburg diffusion, which is related to the solid-state diffusion of Li+ in the electrode materials. 38 18 Environment ACS Paragon Plus

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ୖ మ ୘మ

‫୐ܦ‬୧ = ଶେమ ୅మσమ ୊ర ୬ర

(2)

where R is the ideal gas constant (R=8.314 J mol-1k-1); T is the absolute temperature (T= 298K); n is the number of electrons per molecule oxidized (n=1);. F is the Faraday constant (F= 96500 C mol-1); A is the surface area of electrode; C is the concentration of Li+ in the cathode, which can be obtained from XRD data with the refinement of cell volume. Here, taking FL622 as an example to introduce the calculation of concentration of Li+ ion in the layered cathode. The LiNi0.6Co0.2Mn0.2O2 have the similar structure with LiCoO2 and there are three lithium ions in a unit cell. 39 Therefore, the C=

ଷ

(3)

ே஺∗௏

where NA is Avogadro constant, V is the cell volume. (a=b=2.87018 Å, c=14.19973 Å, α=β=90 ゜ ,γ=120 ゜ , V=101.40 Å3, C = ଺.଴ଶ∗ଵ଴మయ

ଷ ∗ଵ଴ଵ.ସ଴∗ଵ଴షమర

= 0.049 mol cm-3 ) and σ is the

Warburg factor which is related to Zre as follows: Z୰ୣ = R ୈ + R ୐ + σω

ିଵൗ ଶ

(4)

Figure 7b shows the relationship between the Zre and the square root of frequency (ω-1/2) in low frequency region. The calculated lithium diffusion coefficient D (cm2s−1) of FL622 and SL622 are about 1.35 ×10-12 and 6.09 ×10-13 cm2s−1 in the state of charging to 3.8 V, respectively. It is further illustrated that the samples with increasing of the exposure of active planes have a greater lithium ion diffusion coefficient, which may account for better electrochemical performance.



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Figure 7 (a) The Nyquist plots of two samples (the insert is the equivalent circuit). (b) the relationship between the Zre and the square root of frequency (ω-1/2) in low frequency region.

In order to further demonstrate the FL622 with the unique surface structure linked to high energy {010} facets as a superior cathode material for practical LIBs applications, a full cell with the commercial Li4Ti5O12 (LTO) anode (provided by BTR Company, Shenzhen, China) is constructed. The SEM image and corresponding initial charge-discharge profile at 0.1 C of LTO half cell are shown in Figure S11 and S12. Commercial spinel LTO anode has a specific capacity of 163.4 mAh g-1 within the voltage ranging from 2.5 V to 1.0 V. The LTO could exhibit excellent reversibility due to its zero volume change during charge/discharge cycles and the surface of LTO can hardly be covered by the thick solid electrolyte interface (SEI) film,

40

which makes this anode a charming material that can assemble against LiNi0.6Co0.2Mn0.2O2 cathode. The calculated capacity of full cell is based on cathode material. Different potential windows have been investigated in Figure S13 with (a) 2.75-1.05 V, (b) 2.75-1.15 V, (c) 2.751.25 V, (d) 2.85-1.25 V and (e) 2.95-1.25 V, respectively. Those cells are all been tested at 1 C ( 1C= 180 mA g-1) for 100 cycles. Considering the voltage decay and capacity decay of the full 20 Environment ACS Paragon Plus

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batteries, we think the appropriate potential window is 2.75 V- 1.25 V. Figure 8a exhibits the first charge-discharge curves of FL622/LTO full cell with voltage range of 2.75 V-1.25 V at 0.1 C. It can be clearly observed that FL622/LTO full cell shows a satisfying initial discharge capacity (163.6 mAh g-1) and coulombic efficiency (83.38 %). After 100 charge/discharge cycles at 1 C (Figure 8b), FL622/LTO still maintains a discharge capacity of 121.6 mAh g-1 corresponding to a capacity retention of 80.61 % (comparing with the initial specific capacity at 1 C charge/discharge). In contrast, after 100 cycle, the full cell constructed by SL622/LTO can just maintain the capacity of 56.8 mAh g-1. The fact that the capacity decay of full cells worse than half cell may be ascribed to the following two elements: (1) The voltage of the cathode and the anode can be hardly controlled separately. Electrodes may be overcharged or deeply discharged inevitably and the cycling stability of the full cell will deteriorate. (2) The supply of lithium is limited. Though full cells suffer from worse capacity fade than half cells, it is evident that FL622/LTO exhibits a better cycle life combined with higher rate than SL622/LTO cells. In addition, the superior electrochemical performance of SL622 further illustrates that sample with orientation growth of the active {010} facets and novel surface architectures has a prominent amelioration in Li+ intercalation and de-intercalation kinetics.



Figure 8: (a) First charge-discharge curves of FL622/LTO full cell with the voltage range of 2.75 V-1.25 V at 0.1 C (1 C=180 mA g-1) (b) Cycling performance of SL622/LTO and FL622/LTO full cells at current density of 1 C.

Conclusion In conclusion, surfactant may enhance the growth of high energy facets which can supply more Li+ diffusion pathway. The novel self-assembled 3D flower-like hierarchical architecture with an elegant micro/nano morphology for LiNi0.6Co0.2Mn0.2O2 can be obtained by a simpleoperated SDS-assisted hydrothermal process, resulting in faster Li ion transport kinetics prosperity and improved cycle stability. The discharge capacity of FL622 sample maintains at 149.2 mAh g-1 (91.87 % capacity retention) after 100 cycles at 1C charge/discharge. In addition, the specific capacity of 136.5 mAh g-1 can be obtained at 10 C. In order to further demonstrate the FL622 with high energy {010} facets could be a superior cathode material for practical LIBs applications. Full cells with Li4Ti5O12 are constructed and exhibit satisfying cycle stability at 1C charge/discharge with appropriate potential window (capacity retention of 80.61 %). Hence, FL622 has the merits of stable cycling performance and high rate capability. The work inspires us to use surfactants appropriately to improve electrode performance, however, it’s notable that the volumetric energy and tap densities haven’t been improved to a higher level. Hence, we intend to find a more suitable surfactant to increase not only the exposure of active high energy planes, but also the volumetric energy and packing densities and those work will be carried out and reported in the future.


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ASSOCIATED CONTENT Supporting Information Available: The lattice parameters and the ratios of c/a, I(003)/I(104) for all samples refined from the XRD data is shown Table S1. Some parts of the materials and characterization related to electrochemical studies are available in the Supporting Information section contain Figure. S1 to Figure. S13. Corresponding Author * To whom correspondence should be addressed, E-mail: [email protected] (Baihua Qu); [email protected] (Taihong Wang) Present Addresses †

Pen-Tung Sah Institute of Micro-Nano Science and Technology, Center for Sensing and Aero-

propulsion, Xiamen University, No. 422, Siming South Road, Xiamen, Fujian, 361005 PR China ‡

Department of Materials Science and Engineering, College of Materials, Xiamen University,

No. 422, Siming South Road, Xiamen, Fujian, 361005 PR China Funding Sources This work is financially supported by the National Natural Science Foundation of China (grant no. 21503178), and the Natural Science Foundation of Fujian Province of China (No. 2016J05138). Notes The authors declare that they have no competing financial interest. ACKNOWLEDGMENT



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For Table of Contents Use Only

The surfactant has been proposed to design Layered LiNi0.6Co0.2Mn0.2O2 with high energy planes to hunt for high rate and cycle stability.


Surfactant-Assisted Synthesis of High Energy {010} Facets Beneficial

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