Three-Dimensional Honeycomb-Structural LiAlO2-Modified LiMnPO4

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Three-dimension Honeycomb Structural LiAlO2 Modified LiMnPO4 Composite with Superior High-rate Capability as Li-ion Battery Cathodes Junzhe Li, Shaohua Luo, Xueyong Ding, Qing Wang, and Ping He ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17597 • Publication Date (Web): 12 Mar 2018 Downloaded from http://pubs.acs.org on March 12, 2018

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ACS Applied Materials & Interfaces

Three-dimension Honeycomb Structural LiAlO2 Modified LiMnPO4 Composite with Superior High-rate Capability as Li-ion Battery Cathodes Junzhe Li a,d,e, Shaohua Luo b,c,d,*, Xueyong Dinga, Qing Wang b,c,d, Ping Hee a

School of Metallurgy, Northeastern University, Shenyang 110819, People’s Republic of China b

School of Resources and Materials, Northeastern University at Qinhuangdao, Qinhuangdao 066004, People’s Republic of China

c

School of Materials Science and Engineering, Northeastern University, Shenyang 110819, People’s Republic of China

d

Hebei Key Laboratory of Dielectric and Electrolyte Functional Material, Qinhuangdao 066004, People’s Republic of China

e

Metallurgical Technology Research Department, Central Iron & Steel Research Institute, Beijing 100081, People’s Republic of China

Keywords lithium ion batteries, three-dimension configuration, core-shell architectures, LiAlO2 templates, LiMnPO4

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Abstract In the efforts towards the rapidly increasing demands for high-power application, the cathode materials with three-dimensional (3D) architectures have been proposed. Here, we report the construction of the 3D LiAlO2-LiMnPO4/C cathode materials for lithium ion batteries in an innovation way. The as-prepared 3D active materials LiMnPO4/C and the honeycomb-like Li-ion conductor LiAlO2 framework are used as working electrode directly without additional usage of polymeric binder. The electrochemical performance has been improved significantly due to the special designed core–shell architectures of LiMnPO4/C@LiAlO2. The 3D binder-free electrode exhibits high rate capability as well as superior cycling stability with a capability of ~105 mAh g−1 and 98.4% capacity retention after 100 cycles at a high discharge rate of 10 C. Such synthesis method adopted in our work can be further extended to other promising candidates, and would also inspire new avenues of the development of the 3D materials for lithium ion batteries. 1. Introduction Lithium-ion batteries (LIBs) are one of the most widely used energy storage devices used in portable electronic devices and electric vehicles.1-3 However, the sluggish intrinsic properties of LIBs often lead to unsatisfactory battery performances or even complete electrochemical inactivity.4 Significant effort has been focused on design and fabrication of suitable electrode materials for the next generation cathode materials of LIBs. Three-dimensional (3D) architecture possesses high specific surface areas with a large quantity of active sites, connected networks of crystalline 2


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nanoparticles to maintain short transport pathways for ions and electrons, which could maximize power and energy densities.4-8 Unfortunately, until now, little has been done with LIBs due to the rigid 3D electrode framework, which plays a key role on improving the electrochemical performance. These advantages and challenges inspire us to develop novel electrode design on LIBs. To realize the novel 3D architecture, a facile and versatile technique is using template-assisted synthesis route. As known, anodic aluminum oxide (AAO) membranes have been widely used as a template to fabricate regular nanoarrays.4, 9 However, the AAO templates have to be removed in alkaline solution finally as it’s complete electrochemial inactivity. Without the structural support from the template, the arrays tend to collapse and aggregate, resulting in a deterioration of the rate capability when using as the active materials for electrodes.10 However, from our previous studies, we find that the AAO templates can be transformed to α-LiAlO2 (LAO) templates with a stable fast ion transport conducting properties by in-situ hydrothermal synthesis.11 The reason for LAO serves as a fast lithium ions diffusion channels is that Li-ion can pass through the lattice via vacancies and the interstitial Li in the lattice, meanwhile, the low migration energy barriers for Li binds to its neighboring

atoms,

making

LAO

a

promising

lithium-conducting

solid

electrolyte.12-15 Besides, many literatures report that LAO has been widely used as a surface modified material to increase cathode performance in LIBs.16-18 Moreover, the LAO template has 3D honeycomb-like structure, which can act as the templates for nanostructured cathode materials growth. 3



LIBs cathode material LiMnPO4 (LMP) is selected to construct the 3D electrodes together with LAO templates, due to its environmental friendliness, thermal stability, long-term cycle life, and high theoretical energy density (701 Wh kg-1).19-25 Specifically, the LMP gel is drown into the channels of LAO templates with negative pressure to form the 3D core-shell structures. The LMP core part acts as the active materials, while the LAO channels act as the Li-ion transport conductor. Compared to the conventional electrode, the core-shell nanostructured 3D electrode can keep all the advantages including the larger interfacial area, fast electron transport efficiency and high ion accessibility, which is essential to the improvement of the electrochemical performances of LIBs, especially the high rate capability.8, 26, 27 In this work, 3D LiAlO2-LiMnPO4/C core-shell architecture (LAO-LMP/C, hereafter) is first successfully fabricated by using the LAO templates. The LAO-LMP/C electrodes exhibit a high initial discharge capacity of 156.4 mAh g−1 at 0.05 C discharge rate and of ~105 mAh g−1 even at a high discharge rate of 10 C. Superior cycling stability with negligible capacity fading after 100 cycles at 10 C is also obtained. The excellent electrochemical performances are believed to relate with the novel designed 3D core-shell structures. Our findings will open up access to further explorations in the design of the active materials for LIBs with superb electrochemical performances. 2. Experimental Section All the chemical reagents used in our experiments were of analytical grade and were used directly without further purification. A schematic illustration of the

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microstructure and fabrication process of the LAO-LMP/C architecture is shown in Scheme 1.

Scheme 1. Schematic illustration of the 3D nanostructured configuration and fabrication process of the LAO-LMP/C electrode. A) The preparation of through-hole AAO template by anodization and chemical etching. B) Synthesis of LAO template by hydrothermal reaction and subsequently calcining process. C, D) The preparation of LMP/C precursors in LAO template channels by sol-gel method and following vacuum impregnation procedure. E) The preparation of 3D LAO-LMP/C by calcination at high temperature. F) The preparation of 3D LAO-LMP/C electrode by physical vapor deposition. Preparation of the AAO template: The through-hole AAO templates were synthesized by a two-step anodization route.28 Briefly, the high-purity aluminum foils 5



(thickness 0.3 mm, purity 99.99%) were annealed at 600 °C in Ar atmosphere to release the strain followed by electrochemically polish in a mixture solution of perchloric acid and ethanol (1:4, v:v) at a voltage of 15 V. The first anodization was conducted in a 0.3 M H3PO4 solution with the voltage of 160 V for 1 h at 5 °C. Subsequently, the chemical etching was performed to remove irregular oxide in a mixture of 6 wt.% H3PO4 and 1.8 wt.% H2CrO4 at 60 °C for 3 h. The second anodizing procedure was then conducted under the same condition as that of the first anodization with an extended anodization duration of 3 h. Subsequently, excess aluminum was stripped from the porous aluminum oxide film by immersing in an aqueous saturated HgCl2 solution and then the oxide film was punched into round pieces with a diameter of 10 mm. The obtained AAO templates were further immerged in a 5 wt.% H3PO4 solution at 30 °C for about 1 h to remove the alumina barrier layer and widen the pore diameter, as shown in step A in Scheme 1. Preparation of the LAO template: The LAO templates were prepared by the hydrothermal reaction and the subsequent calcining process, as described in our previous work.11 The AAO template and well-mixed Li2CO3 solution (20 mmol Li2CO3 dissolved in 40 mL deionized water) were added to a 50 mL Teflon autoclave vessel. Then, the autoclave was sealed and maintained at 220 °C under autogenous pressure for 60 h. The collected products were washed by distilled water for several times, dried under vacuum at 80 °C overnight, and calcined at 700 °C for 5 h in a tubular electric furnace to obtain the final LAO templates as indicated in step B in Scheme 1. 6


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Preparation of the 3D LAO-LMP/C configuration: LMP precursor was synthesized by a modified sol-gel method. First, 20 mmol LiH2PO4 was dissolved in 24 mL mixed solution of ethylene glycol ((CH2OH)2) and deionized water (the volume ratio of ethylene glycol and deionized water was 1:1). Next, 20 mmol MnSO4•4H2O and 20 mmol citric acid (C6H8O7) dissolved in the same 24 mL mixed solution as the above ratio and then dropped into the above solution. The resulting mixture was kept at 140 °C for enough time during vigorous magnetic stirring in dimethyl silicon oil bath until forming the wet gel, which is the LMP/C precursor. Then, one piece LAO template was put into the wet gel and kept for 2 h in vacuum to ensure the sufficient LMP/C precursor gel could infiltrate into the channels of the LAO template as demonstrated in step C and D in Scheme 1. The two purple layers formed on the top and bottom of the LAO template in Scheme 1D are symbolized of the LMP/C precursors. After then, excess gel on the surface of LAO template was carefully wiped off using the tissues. The LAO template together with the gel in the channels was then annealed at 550 °C for 5 h in an argon atmosphere to obtain the 3D LAO-LMP/C, as illustrated in step E in Scheme 1. Physical vapor deposition method was further adopted to deposite the Au film on the 3D LAO-LMP/C electrode to form the current-carrying substrate as shown in step F in Scheme 1. For comparison, pristine LMP/C composite without the LAO template was prepared using the same sol-gel method described above. Structural and Morphological Characterization: The synthesized crystal structures were examined by X-ray diffraction (XRD.DX-2500) analysis with nickel-filtered Cu 7



Kα radiation (k = 1.5418 Å) over the 2θ range from 10° to 80°. A ZEISS SUPRA55 scanning electron microscope (SEM) and a JEOL JEM-2100F transmission electron microscope (TEM) with an accelerating voltage of 200 kV were used to observe the morphologies. The specific surface area was calculated by the Brunauer–Emmett– Teller (BET.SSA-4300) multiple points method and the pore size distribution was estimated based on the nitrogen gas adsorption isotherm (77 K). Electrochemical Measurements: The electrochemical tests were carried out under ambient temperature using a coin-type half cell configuration, CR2032, with a diameter and thickness of 20 mm and 3.2 mm, respectively. Highly pure Li foil served as the counter/reference electrodes was separated from the working electrode using a Teflon Celgard separator (#2400). The LAO-LMP/C electrode was directly used as the working electrode without adding extra conductive additive and polymeric binder. The mass of the electrode was weighed using a microbalance (METTLER TOLERDO, XSE-105) with the weight of 3.50 mg. The as-prepared working electrode area was ~ 0.785 cm2 and the active materials loading of the 3D LAO-LMP/C configuration was 1.10 mg. The capacity of the 3D LAO-LMP/C cathode was calculated based on the mass of LMP/C. For comparison, the pristine LMP/C electrode slurry was obtained by dispersing 80 wt.% active materials, 10 wt.% acetylene black and 10 wt.% polyvinylidene fluoride (PVDF) binder in N-methyl-pyrrolidone (NMP) solvent with magnetic stirring for 3 h. Subsequently, the acquired slurry was pasted on an Al foil and dried in a vacuum oven at 120 °C for 8 h. After drying, the electrode foil was pressed and punched into a disc type with a diameter of 10 mm to yield the working 8


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electrode. The amount of active materials loading on the Al substrate is 1.22 mg. The cell assembly was performed in an argon-filled glove box with both oxygen and moisture concentrations below 0.1 ppm. The electrolyte solution was 1 M LiPF6 in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) at a 1:1:1 volume ratio. The charge/discharge and cycling performance tests were conducted in a constant current-constant voltage mode using a battery testing system (Land CT2001A) in a potential range from 2.5 to 4.5 V at various current rates (1 C = 170 mA g-1). The electrochemical impedance spectro (EIS) was evaluated on a Solartron 1260 + 1287 electrochemical workstation in the frequency range from 100 kHz to 100 mHz with a voltage amplitude of 10 mV. 3. Results and Discussion

Figure 1. a1, a2) SEM images of the AAO template. a1) The top-view image of the AAO template and the inset in (a1) showing the photograph of the AAO template. a2) The bottom-up view image of the AAO template after removing the barrier layer and the inset in (a2) showing the cross-sectional image of the through-hole AAO template. 9



b1, b2) SEM images of LAO template. b1) The top-view image of the LAO template. b2) The cross-sectional image of the LAO template. The insets in (b1) and (b2) are the corresponding enlarged SEM images. c1, c2) SEM images of the 3D LAO-LMP/C configuration. c1) The top-view image of the LAO-LMP/C configuration and the inset in (c1) showing the photograph of the LAO-LMP/C electrode. c2) The cross-sectional image of the LAO-LMP/C configuration. The insets in (c2) is the corresponding enlarged SEM images. Figure 1a1 and a2 show the representative top-view and bottom-up view of the as-prepared AAO template, respectively. As one can see, the AAO template with a diameter of 10 mm (Figure 1a1 inset) is composed of the honeycomb-shaped pores. The average diameter of the pores in the top part is about 300 nm (Figure 1a1), which is slight larger than that in the bottom part of about 200 nm (Figure 1a2). Such a difference in pore diameter is due to the pore in the bottom is produced by chemical etching of the alumina barrier layer by phosphoric acid in the pore widening process (Figure S1, Supporting Information). The clear pore structures can be well recognized from the top-view and the bottom-up view, which demonstrates the as-prepared AAO templates are of through-hole structure. The depth of the AAO template is estimated to be about 20 um, as shown in the inset of Figure 1a2. In this study, we choose phosphoric acid as the electrolyte, because the pore size obtained with phosphoric acid can be relatively larger than other electrolytes such as sulphuric acid and oxalic acid although the self-ordered level is relatively lower (Figure S2, Supporting Information).29 The larger pore diameter can ensure that much more active materials 10


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filled into the channels and improve the loading amounts of electrode materials, which in turn enhance the volumetric energy density of electrode materials. Figure 1b1 and b2 show the SEM images of the top view and side view of the LAO template. The honeycomb-like structure of the AAO template is well preserved after the hydrothermal reaction and calcination processes (Figure 1b1). The channels are parallel to each other and the LAO nanoparticles growing along the surface and channels of the LAO template are well dispersive and neat (Figure 1b2). The parallel LAO wall can effectively prevent the formation of larger LMP/C crystallites and alleviate the agglomeration of LMP/C by separating them from each other. Figure 1c1 and c2 show the representative images of the surface and the cross-section of the 3D LAO-LMP/C structures. We can see that the LMP/C cathode materials are drawn into the LAO pore channel successfully and the primary LMP/C pillars parallel aligned to form an ordered array. In contrast, the pristine LMP/C sample shows significant agglomeration since it has no structural support deriving from the 3D LAO framework (Figure S3a, Supporting Information). The LAO-LMP/C electrode has a diameter of 10 mm (Figure 1c1, inset) and a thickness of about 20 um which is equal to the thickness of the AAO template as shown in Figure S4a (Supporting Information). In addition, the LMP/C pillars are composed of little LMP/C grains and they inherit the shape from the LAO pores with a typical diameter of about 270 nm (Figure S4b, Supporting Information), which constitute the core-shell structure of LMP/C@LAO as shown in Figures 1c2 and Figure S4b, c in Supporting Information. Specifically, there are a certain amount of LMP/C pillars 11



cross-connected with each other highlighted by yellow arrows in Figure S4b and c in Supporting Information, resulting from the broken of cell walls of AAO template during the pore widen process. The stagger structure can improve the porosity and provide enough inner space for the electrolyte penetration.

Figure 2. a) XRD patterns of LAO template (a1), the pristine LMP/C (a2) and LAO-LMP/C (a3). b) SEM image and line profiles across the cross-section of LAO-LMP/C. XRD analysis was carried out to illustrate the phase evolution during the synthesis processes, as shown in Figure 2. After the hydrothermal reaction and calcination processes, the characteristic peaks of hexagonal (R-3M) LAO phase (Figure 2a1) with lattice constants a = 2.800 Å and c = 14.216 Å (JCPDS No. 74-2232) are clearly observed. No diffraction peaks of Al2O3 phase are detected, suggesting the obtained LAO templates are of high purity. Figure 2a2 and a3 show the XRD patterns of the pristine LMP/C and LAO-LMP/C samples. For the pristine sample (Figure 2a2), all the diffraction peaks can be assigned undisputedly to the orthorhombic olivine structure with a pnma space group (JCPDS No. 74-0375), indicating the high purity of the sample. Interestingly, the characteristic peak intensity ratio between the (020) and 12


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(200) peaks, I200/I020 = 4.33, for the pristine LMP/C sample is higher than 2.64 for the standard JCPDS card. In consideration of the olivine structure, the peak intensity ratio of I200/I020 is recognized as an important characteristic indicating an oriented growth in the [010] direction of the obtained crystal.30, 31 This is favorable for Li-ion diffusion, because the Li-ion diffusion pathway in the olivine LMP structure is known to be along the 1D [010] direction.23 This orientation means that considerable electrochemical active facets exposing to the ac-plane, which will provide abundant active sites of Li+ insertion/extraction, accelerate the exchange rate of Li+ between LMP and the electrolyte, improve the rate capability of the electrode. Because the obtained LAO-LMP/C sample has the same synthesis procedure as the pristine LMP/C, the LMP/C within the 3D LAO framework has the same phase composition and crystal orientation showing in Figure 2a3. Although the background diffraction peaks of LAO template are very strong, the major characteristic peaks of LMP are still observed, which is corresponding to (021), (200) and (131) planes of olivine structure LMP. The elemental distribution across the cross-section of the 3D configuration was analysized by energy dispersive spectrometer (EDS) line scan, as shown in Figure 2b. The Al, Mn, O, P and C elements can be well identified. The elemental distribution of Al shows four peaks located at the LAO position with four valleys of Mn, P and C at the corresponding position, which means the content of Al is the main phase at the LAO wall position. Clearly, a small amount of Al3+ ion distributes between the neighbor LAO wall. This phenomenon may be ascribed to a small amount of Al3+ 13



ions accumulated on the surface of LMP during calcination.16 Meanwhile, the intensity of Mn, P and C is relatively stronger between the neighbor LAO wall, suggesting the LMP/C are the main phase and the formation of carbon layer on the LMP grains, while element O distributes relatively uniform along the cross-section. Accordingly, the distance between the neighbor Al peaks is about 270 nm which is equal to the pore diameter of LAO template indicating the LMP/C have been drew into the LAO channel successfully. The line profile data is in good agreement with the target configuration, further confirming its 3D core-shell structure of LMP/C@LAO.

Figure 3. TEM, HRTEM and SAED images of LAO-LMP/C. a) TEM image of a single unit of the 3D LAO-LMP/C pillar. b, c) HRTEM images of the selected area b and c highlighted by yellow dash boxes in (a), along with the corresponding SAED. d)

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HRTEM image of the carbon coating LMP and the inset showing the representative carbon coating layer. Figure 3 shows the TEM images of the as-prepared LAO-LMP/C structures, in which the formation of the core-shell structure is evidenced. The inner darker region (Figure 3a) is composed of numerous interconnected LMP/C grains while the brighter region is the LAO pore channel. The width of the LAO pore channel is about 270 nm, as indicated by the white arrows, which is consistent with our previous SEM observations. The dispersive LMP/C nanoparticles within the LAO framework is beneficial for the electrode materials penetration by the electrolyte. In contrast, the pristine LMP/C sample showing in Figure S3b (Supporting Information) exhibits distinctly agglomeration, which will weak the effective wetting with the electrolyte. It can be further confirmed by nitrogen adsorption–desorption isotherms and the corresponding Barrett–Joyner–Halenda (BJH) pore size distributions measurements shown in Figure S5 in Supporting Information. The specific surface areas is 79.88 m2 g-1 and 58.73 m2 g-1 for LAO-LMP/C and LMP/C, respectively, indicating the more dispersive grains of the 3D electrode than that of the pristine sample. Besides, The LAO-LMP/C shows a narrow pore size distribution of ~30 nm calculated by BJH pore distribution plot (Figure S5a inset, Supporting Information). In contrast, the pristine LMP/C sample presents a broad pore size distribution in Figure S5b inset in Supporting Information. Figure 3b and c are the corresponding high-resolution transmission electron microscopy (HRTEM) images collected from the regions highlighted by yellow 15



dashed boxes in Figure 3a. Clearly lattice fringes depicted in Figure 3b indicate the high crystallinity of LAO. The interplanar distances of 2 and 0.255 nm match well with (003) and (101) crystal planes of LAO. The corresponding selected area electron diffraction (SAED) pattern indicates the polycrystalline nature of LAO and the first five concentric rings from the center can be indexed to (101), (104), (107), (018), and (113) planes in order, which agrees well with the XRD data. Figure 3c shows the representative HRTEM image of LMP/C sample, the observed interplanar distances between the clear lattice fringes are corresponding well to (002), (200), and (101) planes of the olivine-type structural LMP, respectively. Besides, the angel between the (101) and (200) facets is measured to be ~55°, which agrees well with the olivine-type structure of LMP. In light of the crystal structure of LMP, it can be concluded that the LMP/C nanoplate possess a large (010) facet, which is consistent with Bao’s and Xu’s work.32, 33 To better interpret the crystal orientation of the sample, the SAED image is presented in Figure. 3c inset. Additionally, the SAED pattern is taken to the [010] zone axis from the reciprocal lattice vectors of facets (001) and (100). In other words, the largest exposed facets of the crystal belong to (010) planes. The result matches well with the corresponding XRD observation. For comparison, the pristine LMP/C has the same crystal orientation since the similar synthetic approach, as the related analysis shown in Figure S3 in Supporting Information. Moreover, there are obvious lattice fringes separated by the surrounded coating layers, as shown in Figure 3d. This phenomenon reveals a thin amorphous carbon 16


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layer covers on the small LMP grains, and the grains are interconnected with each other. Notably, we can see that the carbon layers with a uniform thickness of about 3 nm cohering tightly on the surface of the particle (Figure.3d inset). The uniform carbon-coating layer was synthesized by the two steps. Firstly, the polyester network were initially formed and wrapped the LMP precursor through the esterification reaction during the sol-gel process. Subsequently, the polyester network were further transformed into the uniform carbon layer coated on the surface of LMP grains in the following high temperature calcination process. The thin carbon layer can guide more electrons around the active materials and make sure that electrons directly reach the positions Li-ion insertion/extraction reactions taking place.

Figure 4. Electrochemical performance of the two samples. a) The initial charge and discharge profiles of LAO-LMP/C and LMP/C at 0.05 C. b) The discharge profiles of 17



the LAO-LMP/C and LMP/C at various rates from 0.1 C to 10 C. c) The rate performance of the as-prepared LAO-LMP/C and the pristine LMP/C from 0.05 C to 10 C. d) The discharge capacity vs. cycling number for LAO-LMP/C and LMP/C at the high rate of 10 C. Figure 4a shows the initial charge/discharge curves of the 3D LAO-LMP/C and the pristine LMP/C. The cells were first charged to 4.5 V (vs. Li/Li+) at a 0.05 C rate (1 C = 170 mA g−1), kept at 4.5 V for 2 h, and then discharged to 2.5 V at 0.05 C. A typical reversible voltage at ~ 4.2 V and ~ 4.1 V for LAO-LMP/C and LMP/C, respectively, is observed, corresponding to the Mn3+/Mn2+ redox couple accompanied by Li-ion extraction/insertion process. The potential gap between the initial charge and discharge plateaus reduces from 0.215 V for LMP/C to 0.106 V for LAO-LMP/C, suggesting the lower polarization and the better kinetic properties of LAO-LMP/C electrode. The initial discharge capacities are 156.4 and 151.5 mAh g-1 for LAO-LMP/C and LMP/C, respectively, which is comparable to the previous reports.34-36 Moreover, the high voltage discharge capacity above 4.0 V of LAO-LMP/C is 112 mAh g-1, which is higher than that of LMP/C of 80 mAh g-1, demonstrating the novel 3D LAO-LMP/C electrode can dramatically reduce the polarization and improve the discharge capability at high voltage. The discharge performance of these two samples are investigated at 0.05 C charge rate and various discharge rates ranging from 0.05 to 10 C. As illustrated in Figure 4b, the 3D LAO-LMP/C electrode exhibits superior discharge capacity than that of the pristine LMP/C electrode at all discharge rates. Remarkably, the discharge capacity of 18


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these two samples is rough the same in the 0.05 C-0.5 C range, for instance, 156.4 mAh g-1 at 0.05 C and 145.3 mAh g-1 at 0.5 C for LAO-LMP/C and 151.5 mAh g-1 at 0.05 C and 136.2 mAh g-1 at 0.5 C for LMP/C, respectively. However, the difference in the rate performance between these two samples becomes distinct when the discharge rate over 1 C. The discharge capacity at the rates of 2, 5, and 10 C is 127.1, 113.3 and 98.8 mAh g-1, respectively, for LAO-LMP/C sample, which is larger than that of 108.0, 86.6 and 67.5 mAh g-1, respectively, for LMP/C sample. Besides, it is noted that the 3D LAO-LMP/C sample presents a more stable and long voltage plateau during the discharge process and the potential drop trend is a curve at 10 C rate rather than a skew line(e.g. LMP/C sample) as occurred in many literatures, indicating the excellent high rate performance of the 3D LAO-LMP/C sample.23, 36 However, for the LMP/C sample, the discharge potential plateau decline and shorten significantly and the discharge specific capacity decreases quickly with the increasing current rate. The rate performance of the electrode is a very crucial factor for lithium ion batteries, especially for high-power application in electrical vehicles (EVs) and hybrid electrical vehicles (HEVs). Figure 4c exhibits the rate capability of these two samples cycling at 0.05 C charge rate and different discharge rates sequentially from 0.05 C to 10 C for every 5 cycles. It is noted that the rate capability of LAO-LMP/C is superior than that of LMP/C at various current density. Even at the high discharge rate of 10 C, a capacity of ~100 mAh g−1 is still obtained in 3D LAO-LMP/C, demonstrating that the as-synthesized LAO-LMP/C electrode can endure high rate discharge. What’s 19



more, a high capacity of ~150 mAh g−1 can be recovered rapidly after 40 cycles under various rates. The almost complete recovery of the capacity indicates the electrode maintains the good integrity and outstanding reversibility during the increasing rate currents. However, the pristine LMP/C exhibits a weak rate performance with an inferior stability. At the low current densities, the difference between the two electrodes is not evident. When the current rate increases above 1 C, the discharge performance and the stability are immediately deteriorated, only a value of ~70 mAh g−1 is maintained at 10 C. When the current rate goes back to 0.05 C, the discharge capacity does not fully recover as the 3D LAO-LMP/C electrode. To explore the cycle life of the 3D LAO-LMP/C electrode at the high rate, the cycling tests were conducted at a high current rate of 10 C, as shown in Figure.4d. It can be seen that the capacity of the 3D LAO-LMP/C electrode exhibits a trivial decrease during the first 5 cycles, followed by a slow increase in the next 9 cycles before stabilizing at around 105 mAh g−1 and no distinct capacity fading is observed in the subsequent cycles. Besides, the charge and discharge capacity profiles are almost overlapping, which demonstrates the ultra-high Coulombic efficiency. After 100 cycles, it can still deliver a capacity of 97.2 mAh g−1 (about 98.4% capacity retention), suggesting the superior cycling stability of the 3D sample. The reason for the capacity recession and the low Coulombic efficiency in the first few cycles could be attributed to the irreversible reactions involved in the formation of the solid electrolyte interface (SEI) layer and the structure change of electrode from Li+ extraction, which is common in many cathode materials.37, 38 While for the capacity 20


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increase in the following several cycles, it may be ascribed to the activation effect stemming from the gradual infiltration of the electrolyte into the electrode. By contrast, the pristine LMP/C sample exhibits quite a different situation. It exhibits the same cycling performance tendency of the 3D LAO-LMP/C electrode in the first 20 cycles except for a lower capacity. However, a significant capacity fading is appeared in the following 80 cycles and the cycling performance of only 58 mAh g−1 is retained at the 100th cycle. The reasons of the excellent high rate capability for the 3D LAO-LMP/C sample are manifold. First, the 3D LAO-LMP/C electrode has the 3D LAO framework acting as the fast Li-ion conductor. Besides, the 3D structure of the LAO-LMP/C electrode possesses more exposed active sites deriving from the alleviative agglomeration, which could provide sufficient contact areas for rapid Li-ion insertion/extraction. Moreover, the larger specific surface area makes the better storage of electrolyte within the 3D architecture and the cathode materials will be infiltrated sufficiently, which could reduce concentration polarization resistance and improve the specific discharge capacity. What’s more, the binder-free LAO-LMP/C electrode effectively reduces the side-effect of the polymeric binder and could directly adhere to the Au film current collector tightly, which will maintain good integrity and stability of the electrode during the long-term charge/discharge cycles. While the binder of the pristine LMP/C electrode may enfold a certain amount of the active materials, which may keep them from contacting with the electrolyte. Finally, the large exposed (010)

21



crystal face and the uniform carbon coating provide the facile transport paths for Li-ion and electron transport.

Figure 5. CV profiles of the LAO-LMP/C (a) and LMP/C (b) at various scan rates of 0.1, 0.3, 0.5, 0.7, 0.9 mV s-1. c) CV profiles of the LAO-LMP/C and LMP/C electrodes at a scan rate of 0.9 mV s-1. d) Linear relation of the anodic peak current (Ip) as a function of the square root of the scan rate ( γ

1 2

) for both LAO-LMP/C and

LMP/C electrodes. To get further insight into the superior electrochemical characteristics of the LAO-LMP/C, the cyclic voltammetry (CV) measurements were performed as illustrated in Figure 5. Figure 5a and b show the CV profiles of the 3D LAO-LMP/C and the LMP/C electrodes at the scanning rates ranging from 0.1 to 0.9 mV s-1 between 2.5 and 4.5 V. All the curves consist of oxidation and reduction peaks in the 22


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vicinity of 4.1 V, which is corresponding to the respective charge and discharge process as described above. Besides, with the increase of the scanning rate, both of these two samples show an increasing peak current and area of each redox peaks couple. Strikingly, the redox peaks of 3D LAO-LMP/C is sharper and more symmetric than that of LMP/C, indicating the superior redox kinetic properties and cyclic reversibility of the 3D LAO-LMP/C. Figure 5c compares the curves of these two samples at the high scanning rate of 0.9 mV s-1. It is clear that the 3D LAO-LMP/C exhibits the oxidation and reduction peaks even at the high scanning rate of 0.9 mV s-1 and the potential interval between the oxidation and reduction peak of 0.38 V is smaller than that of 0.48 V of LMP/C electrode. However, the LMP/C sample only shows the realtively ambiguous redox peaks due to the aggravated electrochemical polarization. These results demonstrate the novel 3D electrode could maintain high ion accessibility and fast electron transport during fast charge and discharge process. Figure 5d exhibits the peak current (Ip) has linear relationship versus the square root of the scan rate ( γ

1 2

). The fitting results show good linearity with R2 value (the

correlation coefficient, the closer to the value 1, the better of the fitting result) close to 1, indicating the electrochemical reaction is kinetically controlled by Li-ion diffusion.39-41 Therefore, the apparent diffusion coefficients of Li-ion can be calculated based on the Randles-Sevcik equation (1): 1

3

I P = 2.69 × 105 ACD 2 n 2 γ

1

2

(1)

23



where Ip is the peak current, n is the number of electrons transferred in the half-reaction for the redox process (for LAO-LMP/C and LMP/C, n = 1), A is the contact area of the electroactive material, D is the Li-ion diffusion coefficient in LMP at 298 K, C is the molar concentration of Li-ion in LMP (0.0223 mol cm−3), and γ is the scan rate. According to the slope of Ip versus γ

1 2

and the equation (1), the Li-ion

diffusion coefficients of the LAO-LMP/C and LMP/C are calculated to be 2.48ൈ10-11 and 1.51ൈ10-11 cm2 s−1, respectively, comparable to the results previously reported by Cao et al. 39

Figure 6. Nyquist plots of LAO-LMP/C and LMP/C electrodes with the inset showing the corresponding equivalent circuit. The electrochemical impedance spectroscopy (EIS) measurements of the two samples were carried out after CV tests, which is an important tool to analyze the electrochemical kinetic of Li insertion electrodes.42 Figure 6 shows Nyquist plots of these two samples after 20 cycles. It can be seen that each EIS spectrum is composed of a compressed semicircle in the high-to-medium frequency range and an oblique 24


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line in the low frequency range. The semicircle usually relates to the combination resistance (Rct) of the Li-ion diffusion at the electrolyte/electrode interface through the SEI and the charge transfer at the electrode surface. The oblique line is Warburg impedance attributed to the Li-ion diffusion in the bulk of the active materials.42, 43 As can be seen, the 3D LAO-LMP/C sample possesses a lower diameter and larger slope than that of the LMP/C sample, indicating the novel structure is favorable for overcoming the kinetics restrictions. This is also the main reason that the 3D LAO-LMP/C electrode has long high discharge voltage plateau and excellent high rate capability. Based on the modified Randles equivalent circuits in the inset, the Nyquist plots are fitted well with the experiment data, and the fitting results are listed in Table S1 in Supporting Information. In details, the values of Rct for the LAO-LMP/C and LMP/C electrodes are calculated to be 45.56 Ω and 62.59 Ω, respectively. In addition, the exchange current density ( ‫ܫ‬଴ ) is another crucial parameter to describe kinetics properties, which can be used to illustrate the electrochemical activity of the electrodes.38, 44 The ‫ܫ‬଴ of 3D LAO-LMP/C (0.56 mA cm-2) is larger than that of LMP/C (0.41 mA cm-2), according to the following equation (2):

I 0 = RT

(2)

nR ct F

where R is the ideal gas constant (8.314 J mol-1 k-1), T is the absolute temperature (298 K), F is the Faraday constant (96485 C mol-1).

25



Figure 7. The morphology of the LAO-LMP/C electrode after 100 cycles under 10 C. a) The SEM image of the cross-sectional LAO-LMP/C. b,c) TEM image of a single 3D LAO-LMP/C pillar. Figure 7 shows the morphology of the LAO-LMP/C electrode after 100 cycles at 10 C between 2.5 V to 4.5 V. Based on Figure 7a, the vertical alignment of the LMP/C pillars is well maintained, demonstrating the good electrode integrity after the long-term cycling. In contrast, the structures of the pristine LMP/C electrode have agglomerated severely after the cycling process (Figure S6, Supporting Information). TEM analyses in Figure 7b and c further reveal the 3D LAO-LMP/C electrode has the similar morphologies before and after the cycling process. The observations make us believe that the superior cycling stability of the as-prepared LAO-LMP/C electrodes is mainly due to the unique 3D architecture. 4. Conclusions In conclusion, a novel 3D core-shell architecture LAO-LMP/C electrode has been successfully fabricated. The binder-free LAO-LMP/C electrode exhibits superior high rate capability with an excellent cycleability and other satisfactory electrochemical performances due to the special designed core-shell configuration, associated with the 3D LAO Li-ion conductivity framework, exposed (010) facet, uniform carbon coating and weaker agglomeration. Through material architecture design, the unique synthetic 26


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method may shed light on ways to design other advanced electrode materials to alleviate the reliance on the materials inherent defects of poorly conducting for LIBs.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at… SEM image of AAO template from the cross-sectional view with the alumina barrier layer (Figure S1); Surface SEM images of AAO templates. a, b) Anodization was conducted in sulfuric acid. c, d) Anodization was conducted in oxalic acid (Figure S2); SEM, TEM and HRTEM images of the pristine LMP/C and the corresponding SAED (Figure S3); SEM images of the 3D LAO-LMP/C configuration viewed from the cross-sectional (Figure S4); Nitrogen adsorption–desorption isotherms for the LAO-LMP/C and the pristine LMP/C, the insets are the corresponding pore size distribution (Figure S5); SEM images of the pristine LMP/C electrode after 100 cycles under 10 C at different magnification (Figure S6); The primary simulation results of LAO-LMP/C and LMP/C electrodes (Table S1).

AUTHOR INFORMATION Corresponding authors: *E-mail：[email protected] (S.L) ORCID Junzhe Li: 0000-0002-5225-5238 27



Shaohua Luo: 0000-0002-4517-3728

Notes The authors declare no conflict of interest.

Acknowledgements This work was supported by National Natural Science Foundation of China (Nos. 51374056, 51404055, 51571054, 51674068), Natural Science Foundation of Hebei Province (No. E2014501143), and the Science and Technology Research Project of Higher Education of Hebei Province (No.QN2017103).

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Table of Contents

The 3D nanostructured configuration and fabrication process of the LAO-LMP/C electrode.

35


Three-Dimensional Honeycomb-Structural LiAlO2-Modified LiMnPO4

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