3D Reticular Li1.2Ni0.2Mn0.6O2 Cathode Material for Lithium-Ion

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3D-Reticular Li1.2Ni0.2Mn0.6O2 Cathode Material for Lithium-Ion Batteries Li Li, Lecai Wang, Xiaoxiao Zhang, Qing Xue, Lei Wei, Feng Wu, and Renjie Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13229 • Publication Date (Web): 27 Dec 2016 Downloaded from http://pubs.acs.org on December 29, 2016

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ACS Applied Materials & Interfaces

3D-Reticular Li1.2Ni0.2Mn0.6O2 Cathode Material for Lithium-Ion Batteries Li Lia,b, Lecai Wanga, Xiaoxiao Zhanga, Qing Xuea, Lei Weia, Feng Wua,b, Renjie Chena,b* a. School of Materials Science & Engineering, Beijing Key Laboratory of Environmental Science and Engineering, Beijing Institute of Technology, Beijing 100081, China b. Collaborative Innovation Center of Electric Vehicles in Beijing, Beijing 100081, China Corresponding author [email protected] (R.Chen) Tel: +86 10 68912508; Fax: +86 10 68451429

Keywords: 3D-reticular, Li1.2Ni0.2Mn0.6O2, Cathode, Lithium-Ion Battery, Hierarchy

Abstract: In this study, a hard-templating route is developed to synthesize 3D-reticular Li1.2Ni0.2Mn0.6O2 cathode material using ordered mesoporous silica as the hard template. The synthesized 3D-reticular Li1.2Ni0.2Mn0.6O2 microparticles consisted of two interlaced 3D nano-networks and a mesopore-channel system. When used as the cathode material in a lithium-ion battery, the as-synthesized 3D-reticular Li1.2Ni0.2Mn0.6O2 exhibited remarkably enhanced electrochemical performance, namely superior rate capability and better cycling stability than those of its bulk counterpart. Specifically, a high discharge capacity of 195.6 mA h g−1 at 1 C with 95.6 % capacity retention after 50 cycles was achieved with the 3D-reticular Li1.2Ni0.2Mn0.6O2. A high discharge capacity of 135.7 mA h g−1 even at a high current of 1000 mA g−1 was also obtained. This excellent electrochemical performance of the 3D-reticular Li1.2Ni0.2Mn0.6O2 is attributed to its designed structure, which provided nanoscale lithium pathways, large specific surface area, good thermal and mechanical stability, and easy access to the material center. ACS Paragon Plus Environment


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1. Introduction

Cathode material is one of the major components of modern lithium-ion batteries, and directly influences the performance and price of the final battery pack. The high cost of current state-of-the-art commercialized lithium-ion batteries is still the main barrier to the storage of energy on the electrical grid to improve grid efficiency and stability while enabling the integration of intermittent renewable energy technologies (such as wind and solar) into the baseload supply.1 Furthermore, user experiences of electric vehicles and portable electronics are rarely free from battery life anxiety. Recently, the lithium-rich cathode material Li1.2Ni0.2Mn0.6O2 has attracted much attention for its high specific capacity (ca. 250 mA h g−1), a promising factor for long battery life.2-13 It is believed that this layered oxide belongs to the xLi2MnO3·(1−x)LiMO2 (M = Mn, Co, Ni) family, which is composed of two layered α-NaFeO2-type structures.12 One of the most interesting features of these lithium-rich materials is that except for the active LiMO2 (M = Mn, Co, Ni) phase, they can be charged to a high potential (> 4.5 V vs Li), resulting in the electrochemical activation of the Li2MnO3 component, and a high capacity. Notably, Li1.2Ni0.2Mn0.6O2 is also relatively cheap and has a low environmental toxicity owing to the absence of cobalt. Along with its intrinsic high capacity, however, Li1.2Ni0.2Mn0.6O2 suffers from poor rate capacity and modest cycle stability, mainly because of structural rearrangement at its surface during cycling.12 Specifically, lithium extraction of lithium-rich materials is accompanied with the oxidation of the O2p band (Li2O) during the initial charge, followed by the irreversible migration of a significant amount of transition metal ions into the lithium sites.6 Many strategies, such as element doping and surface coating, have been adopted to improve the electrochemical performance of lithium-rich layered oxides.3-4, 14-15 These methods can mitigate the poor electrochemical performance fairly. The recently reported gradient surface Na+ doping method showed a ACS Paragon Plus Environment

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sort of insight by enhancing the kinetics.16 Meanwhile, reducing the particle size to nanoscale levels has also been proved to be an effective way to improve the rate capability of layered oxides.13, 17-18 The use of nanosized cathode materials shortens lithium-ion diffusion pathways and increases the size of the active surfaces, mainly the 010 lattice planes in the case of α-NaFeO2 structure layered oxides like Li1.2Ni0.2Mn0.6O2, so electrode kinetic issues can be circumvented by switching to nanomaterials. This is an approach with high potential; scientists have been successfully controlling and expanding the properties of materials through nanotechnology in many fields for decades.19-24 Research also indicates that nanomaterials are usually thermodynamically unstable and tend to agglomerate, which increases the resistance of the final electrode. Also, a high specific surface area tends to promote side reactions between the electrode and electrolyte, leading to inferior cycle performance and poor safety. To make the nano strategy work, the better option is to form a functional structure by artificially arranging and stabilizing nanosized constructions in the structural hierarchy of the material in a controlled and expected way. This is more than only seemingly reasonable. Indeed, Geoffrey A. Ozin used a whole book to introduce the building of good structure-function relationships using chemistry a number of years ago.25 Meanwhile, a category of successful designs for cathode materials in which primary and secondary particles are clearly defined in a nano/micro structure hierarchy has acted as a demonstration, including those in our 2015 study.11, 13, 17-18, 26 Herein, we adopt a hard-templating method to synthesize 3D-reticular Li1.2Ni0.2Mn0.6O2. In the hierarchy of 3D-reticular Li1.2Ni0.2Mn0.6O2, microscopic particles of the material are constructed from gyroid-like nano-3D networks inherited from a 3D-reticular Ni0.75Mn2.25O4 precursor synthesized using mesoporous silica (KIT-6) as a template. Besides controlling the morphology of synthesized samples, the template also filled a certain amount of space in the particles. When the template was etched away, a ACS Paragon Plus Environment


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mesopore-channel system was created in each individual particle. As a cathode material, 3D-reticular Li1.2Ni0.2Mn0.6O2 showed a significantly improvement in electrochemical performance compared with that of the bulk counterpart.

2. Experimental

2.1 Preparation of materials

Mn(NO3)2·4H2O (99%), Ni(NO3)2·6H2O (99%), NaOH (99.3%), LiOH·H2O (99%), 1-butanol (99.4%), ethanol (99.0%), aqueous ammonium hydroxide (28-30%), and tetraethyl orthosilicate (98%) were purchased from Aladdin (Shanghai, China). Pluronic P123 (Mn = 5800) was purchased from Sigma-Aldrich (St. Gallen, USA). To synthesize the 3D-reticular Ni0.75Mn2.25O4 precursor, 1.5 g of the two metal nitrates in a molar ratio of 1:3 was dissolved in 20 mL of ethanol followed by the addition of 1 g mesoporous silica (KIT-6) synthesized according to the literature.27-28 After stirring the mixture at room temperature until almost dry, the sample was heated slowly to 700°C and then held at that temperature for 5 h. The resulting material was treated three times with a hot solution of 2.0 M NaOH to remove the silica template. After centrifugation and washing several times with water and ethanol, the sample was dried at 60°C and labeled as r-NMO. To prepare 3D-reticular Li1.2Ni0.2Mn0.6O2, 0.2 g of r-NMO was mixed with excess LiOH·H2O (7 %) in 20 mL ethanol. The mixture was dried at room temperature with stirring, and then heated to 800°C at 2°C min−1 and calcined for 5 h. The final product was labeled as r-LNMO. Bulk Li1.2Ni0.2Mn0.6O2 (b-LNMO) was prepared according to the literature, with only a change in the final calcination conditions to 800°C for 5 h.29 Ultrasound-treated r-LNMO (ur-LNMO) was also prepared for a thermal ACS Paragon Plus Environment

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stability test.

2.2 Characterization

Crystallographic analysis of the prepared samples was carried out on an X-ray diffractometer (XRD; Rigaku Ultima IV-185) with a Cu Kα radiation source. Field emission scanning electron microscopy (FESEM; FEI, Quanta 200f), specific surface area and pore size analysis (ASIQM000-1-MP, Thermo Fisher Scientific Inc.), transmission electron microscopy (TEM; JEM-2100f), and small angle X-ray scattering (SAXS; Rigaku SmartLab 2) were used to characterize the morphology of the prepared samples. Elemental mappings were carried out with an energy dispersive X-ray detector (EDX). X-ray photoelectron spectroscopy (XPS; VGESCA-LABMK II) was used to determine the ion valence states in the metal oxide. Differential scanning calorimetry (DSC; Mettler-Toledo DSC 1) was used to evaluate the thermodynamic stability of the samples.

2.3 Electrochemical tests

Electrochemical tests were performed on two-electrode coin cells (type: 2025) at ambient temperature. In this case, lithium metal served as both the reference electrode and the counter electrode. The working electrode was composed of 80 wt% active material, 10 wt% conductive additive (acetylene black), and 10 wt% binder (polyvinylidene difluoride, PVDF), with an aluminum foil current collector. A lithium salt solution, 1 M LiPF6 in a mixture of equal volumes of dimethyl carbonate (DMC), ethylene carbonate (EC), and diethyl carbonate (DEC) was used as the electrolyte. Cell assembly was carried out in a glovebox filled with argon, the oxygen and moisture contents of which were maintained below 1 ppm. Galvanostatic charge/discharge tests were implemented on a battery tester (LAND-CT2001A) with a voltage window of 2.0−4.8 V at selected current rates. Cells were cycled under 0.1 C for three ACS Paragon Plus Environment


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cycles before continuous high rate (e.g. 1 C, 5 C) tests. Cyclic voltammetry (CV) tests were carried out on a CHI660d electrochemical workstation with the same voltage window. Electrochemical impedance spectroscopy (EIS) tests were also carried out on the CHI660d in the frequency range of 0.1 MHz to 0.01 Hz with an AC perturbation signal of 5 mV.

3. Result and discussion

3.1 Composition and morphology analysis of r-LNMO

A 3D-reticular Li1.2Ni0.2Mn0.6O2 (r-LNMO) particle consists of two interlaced 3D networks and a mesopore-channel system. Ideally, the interface of the particle should be a double-gyroid surface with the 3 space group. In practice, the obtained structure is slightly rough, but still provides excellent performance for electrochemical applications. Scheme 1 briefly shows an r-LNMO electrode and the hard-templating process used to synthesize r-LNMO. XRD patterns were collected to investigate the crystallographic characteristics of the as-prepared r-LNMO, as shown in Figure 1. Most peaks in the XRD pattern of r-LNMO could be indexed to the layered rock-salt form with hexagonal α-NaFeO2-type structure (3 space group). The low-intensity peaks within the range of 20−25° (dashed circle) corresponded to Li2MnO3 with C/2m space group, which is characteristic of Li-rich Mn-based materials.30 The clear (006/102) and (018/110) split peaks were evidence of a high degree of crystallization. Meanwhile, no distinct reflection peaks corresponding to impurity phases were observed. In contrast, the as-synthesized Ni0.75Mn2.25O4 precursor inherited the cubic spinel structure (space group 3), with interconnecting O6 octahedra as alternate layers along the [111] direction. According to the literature, the similarity between the radius of Li+ and Ni2+ plays a crucial role in making cubic-to-layered topotaxy on the basis of [0001]h/[111]c possible.31 During the ACS Paragon Plus Environment

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transformation, the O6 octahedra stacking layers function as the fundamental framework. The structural diagrams presented in Figure 1(b, c) provide a more vivid demonstration of this topotaxy. XPS measurements were performed to learn more about the valence state of the transition metals in r-LNMO. The results were calibrated with reference to the C 1s binding energy at 284.5 eV, and are presented in Figure 2. The dominant peaks at 854.5 and 642.0 eV are attributed to Ni2+, and Mn4+, respectively.32 As shown in Figure 2(b), the intense Ni 2p3/2 peak at about 854.5 eV was accompanied by a shake-up peak at about 861.0 eV. This particular pattern is characteristic of Ni2+. Meanwhile, a less prominent peak at about 855.8 eV was also observed, indicating the existence of Ni3+. In the Mn 2p spectrum (Figure 2(c)), two main spin-orbit lines at about 642.0 eV for 2p3/2 and 653.7 eV for 2p1/2 with a separation of 11.7 eV indicated a majority of Mn4+. However, a pair of less prominent peaks at about 643.4 and 655.1 eV revealed the presence of Mn3+. According to the literature, minor contributions of Ni3+ and Mn3+ are inevitable owing to valence degeneracy.33 The O 1s spectrum (Figure 2(d)) of r-LNMO contained two peaks. The peak at 529.2 eV is typical of metal oxygen bonds, while that at 531.5eV is associated with low coordinative oxygen ions at the surface of the material.34 Brunauer–Emmett–Teller (BET) analysis (Figure 3(a)) showed that r-LNMO possessed a specific surface area as large as 61.695 m2 g−1. A large specific area directly enlarges the solid-liquid interface in the final electrodes, and simultaneously promotes rate performance. The average pore width of r-LNMO was 5.872 nm, which may promise a performance of r-LNMO similar to that of other mesoporous cathode materials.13,

28, 35

Higher capacity is one of the effects of mesopores.13 The 3D-reticular

morphology of r-LNMO was first confirmed by the obtained SAXS data. As shown in Figure 3(b), the relatively sharp diffraction peaks in the SAXS pattern could be indexed to the corresponding reflections in the cubic 3 space group, which indicated the presence of a double-gyroid morphology as ACS Paragon Plus Environment


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intended.36 The SEM image (Figure 3(c)) confirmed that the r-LNMO particles were microscale. Indications of networks and the mesopore-channel system were found on the surface of the material in the high magnification SEM image (Figure 3(c)). There 3D-reticular structure could be easily identified in TEM images (Figure 3(e)). A honeycomb-like structure along the [111] direction was clearly observed. Ideally, electrolyte cloud flux through the whole particle along this direction. The r-LNMO was further analyzed by EDX analysis (Figure 3(f)). According to the results, the ratio of the transition metals in the synthesized sample was very close to stoichometric, and both the elements were uniformly distributed in the particle.

3.2 Electrochemical performance of r-LNMO

Galvanostatic charge/discharge curves measured for r-LNMO at 0.1 C rate (defined as 20 mA g−1) between 2.0 and 4.8 V are displayed in Figure 4(a). The r-LNMO material delivered initial charge/discharge capacities of 364.55 mA h g−1 and 259.14 mA h g−1, respectively. As mentioned before, the typical high charge capacity of r-LNMO in the first cycle is caused by the electrochemical activation of the Li2MnO3 component when charged over 4.5 V. Much higher charge/discharge capacities than those of conventional cathode materials (e.g., LiCoO2 and LiFePO4) were still maintained during the successive cycles, which is an important factor of a higher energy density for r-LNMO. The discharge capacity retention after 25 cycles was 93 %, which is among the best in previous reports.2, 4, 6, 9, 37 Cyclic voltammetry curves of a typical r-LNMO electrode measured between 2.0 V and 4.8 V with a scanning rate of 0.1 mV s−1 are presented in Figure 4(b), and give a more detailed illustration of the electrochemical behavior of r-LNMO. More than one oxidation or reduction reactions occurred during cycling. In the first cycle, the first anodic peak around 3.7 V is associated with Ni oxidation from Ni2+ to ACS Paragon Plus Environment

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Ni4+, while the second anodic peak at a higher potential (~4.7 V) is attributed to irreversible Li2O extraction from the Li2MnO3 component to form MnO2. The reduction reactions remained almost the same. There were three distinguishable reduction peaks, at ~4.3, ~3.7, and ~3.3 V. The rather inconspicuous reduction peak at ~4.3 and the distinct reduction peak ~3.7 V are associated with the reduction of Ni4+, while the peak at ~3.3 V can be assigned to the reduction of Mn4+. Hence, the reversible oxidation/reduction reactions of r-LNMO involved both Ni and Mn. The changes in the cycle profile from the 2nd cycle onwards were quite smooth, indicating a good reversibility of r-LNMO. The contrasting electrochemical performances of r-LNMO and b-LNMO are shown in Figure 5. As shown in Figure 5(a), the discharge capacity of r-LNMO at 1 C rate was 187.0 mA h g−1 after 50 cycles while that of b-LNMO was 117.6 mA h g−1, representing 95.6 % and 64.5 % capacity retention, respectively. Meanwhile, the initial discharge capacity of r-LNMO was a little higher than that of b-LNMO. Thus, r-LNMO possessed both better cycle stability and higher capacity. Similar superiorities of r-LNMO was also observed in the test under 0.2 C (Figure 5(b)). Meanwhile, Coulombic efficiency of r-LNMO was higher in both conditions. Besides high specific capacity and good cycling stability, good rate capability is also crucial for cathode materials. Figure 5(c) presents the rate performance of both r-LNMO and b-LNMO at a charge rate of 0.1 C and various discharge rates (0.1–5 C). The discharge capacity of the r-LNMO electrode was 261.2, 225.6, 214.4, 188.5, 166.9, and 135.4 mA h g−1 at a current rate of 0.1 C, 0.2 C, 0.5 C, 1 C, 2 C, and 5 C, respectively. Notably, the electrode could still deliver a capacity of more than 135.4 mA h g−1 even at 5 C. Additionally, the discharge capacity of r-LNMO recovered to 250.6 mA h g−1 when discharge rate was dropped back down to 0.1 C, close to its initial discharge capacity under 0.1 C rate. In contrast, the capacity drop observed for b-LNMO was much more drastic when the current rate was increased. The unique 3D-reticular structure of r-LNMO is ACS Paragon Plus Environment


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considered to be responsible for its superior electrochemical performance, namely high capacity and excellent cycle stability and rate capability. A quick high temperature test under 60 degrees was also performed (Figure 5(d)). It is feasible to deduce that r-LNMO possesses similar beneficial features to those of other kinds of nano/micro hierarchical cathode materials, because r-LNMO provides no less fine characteristics on the basis of its particle and sub-particle structure. Specifically, the 3D-reticular structure can increase the utilization of the cathode material and introduces new lithium storage space in its mesopores, leading to high capacity. The presence of mesopores and nanosized material dimensions can shorten the Li+ ion diffusion path and provide a larger electrode–electrolyte interface for Li+ to flux across, which results in rapid lithium-ion intercalation/deintercalation and fast electron transfer. Ideally, the mesopore-channel system in this structure provides room for the electrolyte and conductive additives, allowing easy access to the center of the material, and thus may ensure that Li+, electrons, and the active material are in the same place at the same time. Therefore, both a good working voltage and rate capability are guaranteed. A well-designed hierarchical structure in cathode materials can slow down structural breakage by buffering the mechanical strain introduced by volume change during cycling to slow down aging, and can also relax side reactions which often trouble thermodynamically unstable isolated nanoparticles with the same Li+ diffusion distance. Hence, the cycle stability of r-LNMO is also improved. Electrochemical impedance spectroscopy (EIS) of r-LNMO further consolidates the above-mentioned deduction, because rapid lithium-ion intercalation/deintercalation and fast electron transfer also means small impedance. As shown in Figure 5(e), the impedance spectra of both r-LNMO and b-LNMO electrodes contained a partial semicircle in the high to medium frequency region and an inclined line in the low-frequency region. The semicircle in the high to medium frequency region is usually assigned to ACS Paragon Plus Environment

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the charge transfer impedance (Rct), which is related to charge transfer through the electrode/electrolyte interface. Meanwhile, the quasi-straight line in the low frequency region represents the Warburg impedance, which reflects the solid-state diffusion of Li in electrode materials. According to Figure 5(e), r-LNMO (Rct = 59.57 Ω) possessed an obviously lower charge transfer resistance than that of b-LNMO (Rct = 102.40 Ω). To be concise, a comparison table (Figure 5(f)) was appended to the end of Figure 5 to summarize the above four paragraphs and the upcoming thermal stability test.

3.3 Thermal stability of r-LNMO

Thermal stability is an important safety related property for cathode materials considered for applications in commercial energy storage systems. Figure 6 shows the differential scanning calorimetry (DSC) curves of a r-LNMO electrode and an electrode constructed with ultrasound-treated r-LNMO (ur-LNMO). The major exothermal peak of the r-LNMO electrode emerged at 241.5°C, about 10°C higher than that of the ur-LNMO electrode. The high exothermal temperature (good thermal stability) indicates that the r-LMNO had good structural stability and safety characteristics. The designed 3D-reticular structure wins isolated thermodynamically unstable nano particles by combining them. Likely, this novel structure reduced the amount of side reactions between the active material and electrolyte just as seen for other kinds of effective hierarchical structures, improving its safety.6

4. Conclusion

In summary, we successfully adopted a hard-templating route to synthesize 3D-reticular Li1.2Ni0.2Mn0.6O2. The as-synthesized 3D-reticular Li1.2Ni0.2Mn0.6O2 delivered high capacity, excellent cycle performance, and good rate capability as a lithium-ion battery cathode material. The excellent ACS Paragon Plus Environment


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performance of the material is mainly ascribed to its function-promoting structure based on the nano 3D-network and mesopore-channel system combination. Simply put, this particular structure provides nanoscale lithium pathways, large specific surface area, good thermal and mechanical stability, and easy access to the center of the material. Based on this work, 3D-reticular Li1.2Ni0.2Mn0.6O2 can be considered to be a potential cathode candidate for the development of future lithium-ion batteries. More importantly, the hard-templating strategy verified in this work may also provide an effective general approach to improve the cycle stability and rate capability of cathode materials for lithium-ion batteries.

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AUTHOR INFORMATION

Corresponding Author

E-mail address: [email protected] (R.Chen)

Author Contributions

L. L, L. W and R.Chen contributed equally to this work.

Notes:

The authors declare no competing financial interest.

ACKNOWLEDGMENT The experimental work of this study was supported by the Chinese National 973 Program (2015CB251106), the Joint Funds of the National Natural Science Foundation of China (U1564206), Major achievements Transformation Project for Central University in Beijing and Beijing Science and Technology Project (D151100003015001).



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Figures and Tables

Scheme 1. Brief illustration for the designed route for synthesizing 3D-reticular Li1.2Ni0.2Mn0.6O2 and the morphological effects of the as-synthesized material.


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Figure 1. XRD pattern of the 3D-reticular Li1.2Ni0.2Mn0.6O2 sample (a); Crystallographic structure of spinel Ni0.75Mn2.25O4 (b) and layered Li1.2Ni0.2Mn0.6O2 (c).



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Figure 2. XPS spectra of (a) C 1s, (b) O 1s, (c) Ni 2p, and (d) Mn 2p for 3D-reticular Li1.2Ni0.2Mn0.6O2 (r-LNMO).


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Figure 3. Nitrogen adsorption/desorption isotherms and pore size distribution of 3D-reticular Li1.2Ni0.2Mn0.6O2 (r-LNMO) (a); Small Angle X-ray Scattering (SAXS) curve of r-LNMO (b); SEM images of r-LNMO (c, d); TEM image of r-LNMO (e); Energy dispersive X-ray Analysis (EDX) results of r-LNMO (f).



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Figure 4. Charge/discharge curves of a 3D-reticular Li1.2Ni0.2Mn0.6O2 (r-LNMO) electrode at 0.1 C in the voltage range between 2.0–4.8 V (a); Cyclic voltammetry (CV) curves of a r-LNMO electrode in the voltage range of 2.0–4.8 V at the scan rate of 0.1 mV s-1 (b).


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Figure 5. Cycle performances for 3D-reticular and bulk Li1.2Ni0.2Mn0.6O2 (r-LNMO and b-LNMO, respectively) electrodes at 1 C and 0.2 C rate (a, b); Rate performance of r-LNMO and b-LNMO electrodes (c); High temperature performance of r-LNMO (d); Nyquist plots of r-LNMO and b-LNMO electrodes in the frequency range from 100 kHz to 0.1 Hz (e). Performance comparison table of prepared samples (f).



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Figure 6. Differential scanning calorimetry (DSC) profile of 3D-reticular Li1.2Ni0.2Mn0.6O2 (r-LNMO) electrodes and electrodes constructed with ultrasound-treated r-LNMO (ur-LNMO).


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TOC


3D Reticular Li1.2Ni0.2Mn0.6O2 Cathode Material for Lithium-Ion

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