Assembly of LiMnPO4 Nanoplates into Microclusters as a High

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Assembly of LiMnPO Nanoplates into Microcluster as a High Performance Cathode in Lithium-Ion Battery Chao Wang, Shiheng Li, Yuyao Han, and Zhenda Lu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b05868 • Publication Date (Web): 03 Aug 2017 Downloaded from http://pubs.acs.org on August 3, 2017

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ACS Applied Materials & Interfaces

Assembly

of

LiMnPO4

Nanoplates

into

Microcluster as a High Performance Cathode in Lithium-Ion Battery Chao Wang, Shiheng Li, Yuyao Han, and Zhenda Lu* National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China KEYWORDS:

LiMnPO4

cathode,

self-assembly,

microcluster,

improved

electronic

conductivity, carbon coating, lithium ion battery ABSTRACT: A novel structure of carbon-coated LiMnPO4 microcluster through emulsionbased self-assembly has been fabricated to yield high-performance battery cathode. In this rational design, nanosized LiMnPO4 plates are assembled to microclusters to achieve the dense packing and robust interparticle contact. In addition, the conductive carbon framework wrapping on these clusters functions as fast electron highway, ensuring the high utilization of the active materials. The designed structure demonstrates enhanced specific capacity and cycling stability in lithium ion battery, delivering discharge capacity of 120 mAh g-1 after 200 cycles at 0.2 C. It also performs a superior rate capability with discharge capacity of 139.7 mAh g-1 at 0.05 C, 131.7 mAh g-1 at 0.1 C, and 99.2 mAh g-1 at 1 C at room temperature.

ACS Paragon Plus Environment


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■ INTRODUCTION Recently, the ordered olivine-type lithium transition-metal phosphates (LiMPO4, M= Fe, Mn, Co or Ni) have attracted increased interest as potential Li-ion battery (LIB) cathode materials due to their low cost and good thermal stability compared to conventional cathode materials such as LiCoO2, LiNiO2 and LiMn2O4.1-6 Among them, LiFePO4 (LFP) cathode materials have been successfully commercialized, encouraging the later investigation of LiMPO4 olivine family.7-11 LiMnPO4 (LMP) with high working voltage (4.1 V vs. Li/Li+, 0.65 V higher than LFP) and high theoretical energy density (701 W h kg-1, 1.2 times of LFP) is considered as one of the most promising candidates of cathode materials in LIB.10,12-15 Unfortunately, the intrinsic high electronic resistance of LMP results in an extremely poor electrochemical activity, which severely limits its practical applications.1,7,14,16 Different methods like particle nanosizing,5,17 conductive surface coating,18,19 and substitutional doping20,21 have been developed to improve the electronic conductivity. These new LMP cathode materials exhibit greatly improved performance in lithium ion battery as expected.22-28 However, it is still a big challenge for the nanoscale electrode materials to maintain robust electronic and ionic connections between neighboring nanoparticles during the charge/discharge cycles.29 The fragile interparticle contact usually causes significant internal resistance and therefore a poor battery performance as results.22,29-32 In this work, we report the self-assembly of porous LiMnPO4 microclusters (LMP-MC) composed of thousands of close-packed LiMnPO4 nanoplates (LMP-NP) with conformal carbon coating layer. Such microcluster structure offers multiple advantages for LMP cathode materials: (1) the primary nanosized plate effectively reduces the diffusion path length of lithium ions during charge/discharge process; (2) LMP nanoplates are assembled to microcluster achieving


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the robust interparticle contact, and the gaps formed during the microcluster assembly are expected to facilitate electrolyte penetration into the active materials compared to randomly packed LMP-NPs; (3) the interconnected carbon framework wrapping on the microcluster functions as electron highway, ensuring the good electrochemical activity of all nanoplates. In a word, our wonderful design combines the benefits of nanoplates and microclusters, which is expected to achieve a superior electrode by the high electrochemical activity of NPs and stable contact between each of them. ■ EXPERIMENTAL SECTION Reagent and materials Manganese acetate tetra hydrate (Mn(CH3COO)2·4H2O, 99%, Xilong Science Co., Ltd), diethylene glycol (DEG, 99%, Aladdin), polyvinylpyrrolidone (PVP, MW 10000, Aladdin), lithium dihydrogen phosphate (LiH2PO4, 99%, Aladdin); 1-octadecene (ODE, 90%, Aladdin), amphiphilic block copolymer (Hypermer 2524, Croda, USA), cyclohexane (Sinopharm Chemical Reagent Co., Ltd); cetyl trimethylammonium bromide (CTAB, 99%, Aladdin), ammonia (NH3·H2O, 28%, Sinopharm Chemical Reagent Co., Ltd), ethanol (99.7%, Sinopharm Chemical Reagent Co., Ltd), resorcinol (99%, Sinopharm Chemical Reagent Co., Ltd), formaldehyde solution (HCHO, 37 wt%, Xilong Science Co., Ltd). Synthesis of LiMnPO4 nanoplates LiMnPO4 nanoplates were synthesized via a polyol method with some modification. Here, PVP was functioned as surfactant. 3.712 g Mn(CH3COO)2·4H2O and 2 g PVP were dissolved into 50 mL DEG in a 100 mL three-neck round-bottom flask, the DEG solution was heated to 100 oC under vigorous stirring, kept for 30 min. And then 7.5 mL 2 M LiH2PO4 aqueous solution was injected with a rate of 0.5 mL min-1, the suspension mixture was kept for 3 h at 100 oC.



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After cooled down to room temperature, the LiMnPO4 material was collected by centrifugation and washed three times with ethanol. Finally, the material was dried in an oven at 90 oC overnight. Fabrication of carbon-coated LiMnPO4 microclusters The LiMnPO4 microclusters were obtained by an emulsion-based assembly process. The above resulting LiMnPO4 compounds were grinded into powder. Next, 0.5 g LiMnPO4 powder was dispersed in 6 mL deionized water, and the obtained water dispersion of LiMnPO4 (~ 6 mL) was added into 24 mL ODE solution containing 0.3 wt% Hypermer 2524 and homogenized at 8000 rpm for 10 min to yield uniformly water-in-oil (W/O) emulsion system. The emulsion mixture was heated at 98 oC for 2 h. After evaporation of water, the LiMnPO4 microclusters were collected by centrifugation (8000 rpm) and washed twice using cyclohexane, dried at 90 oC overnight. At last, the powders were calcined at 500 oC for 2 h to remove the organics. 0.5 g LiMnPO4 microclusters (or LiMnPO4 nanoplates) were dispersed in 150 mL deionized water, 1 mL ammonia and 10 mL 0.01 M CTAB aqueous solution were successively added and vigorously stirred for 30 min to ensure the surface adsorption of CTAB. Next, 0.2 g resorcinol and 280 µL formaldehyde solution were added and stirred 4 h to form resorcinol-formaldehyde resin (RF) shell. Then, the RF coated LiMnPO4 microclusters were collected by centrifugation and washed with ethanol three times, dried at 90 oC for 6 h. Finally, the carbon framework was formed through carbonizing RF shell under Ar atmosphere at 700 oC for 2 h with a heating rate of 5 oC min-1. Characterizations The morphologies and structure details of our products were examined by field-emission scanning electron microscopy (FESEM, Ultra 55, Zeiss, Germany) and transmission electron


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microscopy (TEM, Tecnai G2 F20 X-TWIN, FEI, USA). To directly observe the carbon outer framework, the carbon-coated LiMnPO4 microclusters were etched away by hydrochloric acid (HCl, 2 M) remaining the outer shell. X-ray diffraction was carried out to verify the crystal structures on an X-ray diffractometer (XRD, Ultima III, Rigaku, Japan) using Kα radiation (40 kV, 40 mA). The carbon-coated uniformity of as-prepared samples was determined by X-ray photoelectron spectroscopy (XPS, K-Alpha, Thermo Scientific, UK). The weight percentage of LiMnPO4 and carbon in the final carbon-coated products was determined from the weight loss curves measured under simulated air atmosphere (25% O2 + 75% N2, both are ultra-purity grade gases) on a TG/DTA instrument (Netzsch STA 409) with a heating rate of 10 oC min-1. LiMnPO4 is thermo-stabilized under this condition and does not cause mass change, while carbon is oxidized to gaseous and causes mass decrease. Electrochemical measurements Electrochemical measurements were carried out using LIR 2032-type coin cells. The carboncoated LiMnPO4 microclusters, acetylene black, and polyvinylidene fluoride (PVDF) binder at a mass ratio of 65: 20: 15 were dispersed in NMP to form slurry. The resulting slurry was uniformly cast onto aluminum foil and dried at 90 oC under vacuum overnight to yield the working electrode. Then the resulting electrodes were punched into disks and pressed to obtain the final cathodes with an active material loading of 0.3-1.0 mg cm-2. The assembly of the test cells was performed in an Ar-filled glove box using lithium foil as the counter electrode and Celgard 2300 film as separator. The electrolyte was 1.0 M LiPF6 dissolved in ethylene (EC)/dimethyl carbonate (DMC) (1:1 by volume). Galvanostatic charge-discharge measurements were performed between 2.7 and 4.4 V at various constant rates on a cell test instrument (LAND CT2001A, Wuhan, China). All specific



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capacities are calculated based on the total mass of LiMnPO4/C composites. And electrochemical impedance spectroscopy (EIS) measurements were carried out on an electrochemical workstation (Bio-logic VMP3, USA) over the frequency from 100 kHz to 10 mHz with an alternating voltage of 5 mV. All electrochemical measurements were carried out at room temperature. ■ RESULTS AND DISCUSSION A bottom-up microemulsion-based approach is employed here to fabricate well-designed LMP microclusters as a high performance battery cathode. As depicted in Figure 1, the primary LMPNPs were first synthesized via a modified polyol method.33 Then, a water-in-oil microemulsion system is formed by mixing the LMP-NPs aqueous suspension with ODE oil phase. After water evaporation at moderate speed, the primary LMP-NPs densely packed into microscale LMP clusters. Finally, a resorcinol-formaldehyde resin (RF) layer was coated on the clusters, which was subsequently converted into a carbon layer under argon atmosphere at 700 oC. The obtained carbon-coated LMP cluster was used as a cathode material for a lithium half-cell. Electrodes composed by nanostructured materials are difficult to maintain a stable interparticle contact during cycling, and thus leading to poor electron transport and resulting in poor electrochemical activity. Conversely, in our unique cluster structure, the interconnected conductive carbon framework enables reliable electrical connections between LMP-NPs and serves as fast electron transport pathways. For a clear comparison, we prepared three samples: as-synthesized LiMnPO4 nanoplates, carbon-coated LiMnPO4 nanoplates, and carbon-coated LiMnPO4 microclusters, simplified as LMP-NP, LMP-NP@C, and LMP-MC@C, respectively. X-ray diffraction (XRD) measurements (Figure 2a) indicate that the synthesized LMP-NPs have a pure lithium manganese phosphate phase (JCPDS card No. 74-0375). The XRD patterns of LMP-NP@C and LMP-MC@C samples


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confirm that the RF shell carbonization process (Ar atmosphere at 700 oC) does not change the LMP crystal phase. X-ray photoelectron spectroscopy (XPS, Figure 2b) measurements demonstrate the coexistence of lithium (Li), manganese (Mn), oxygen (O), phosphorus (P) and carbon (C) signals in all the samples, but with obviously different surface ratios. For the LMPMC@C materials, the negligible signal of Mn2p1/2 or Mn2p3/2 in XPS analysis compared with the carbon shows a very low surface percentage of LMP, clearly indicating that the carbon coating is conformal to completely seal the LMP-MCs. Figure 3a-c show the typical scanning electron microscopy (SEM) images of the LMP samples under different magnifications. The morphology of primary building blocks, LMP nanoplates is demonstrated in Figure 3a with a thickness around dozens of nanometers. The as-fabricated LMP microclusters ranging from 1 to 10 µm in diameter shows a spherical overall morphology, composed of thousands of primary nanoplates (Figure 3b). These nanoplates are densely packed in the cluster during the assembly process. One can easily observe the porous nature of LMPMCs in the magnified SEM image, as shown in the inset. The existence of gaps formed by nanoplate packing is crucial for the later matter transfer. Here, the RF precursors penetrate into these interior gaps, polymerize in the presence of ammonia, and then generate a RF layer. After calcined at 700 °C under Ar atmosphere, the RF layer is converted to conductive carbon shell to wrap the LMP clusters. The retained spherical morphology of carbon coated LMP microclusters (Figure 3c-d) confirms the structure integrity after RF coating and carbonization. The highmagnification TEM image (Figure 3d inset) clearly shows the carbon layer wrapping around LMP NPs is only a few nanometers. The mass percentage of carbon in the LMP-MC structures found to be 5% through thermogravimetric analysis (TGA, Figure 3f), while the percentage increases to 15% in normal LMP-NP@C structures. We also obtained the carbon percentage by



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measuring the mass loss of the samples after calcined at 500 oC for 2 h in the air. The resulting data is consistent with TGA analysis. The carbon framework on LMP-MC structures, although with very low mass percentage and only a few nanometers thick, keeps intact even after etching away LMP using hydrochloric acid (2 M), as shown in Figure 3e. This confirms that the carbon framework can support the whole microcluster and functions as an electrical highway as well. Electrochemical properties of the carbon-coated LiMnPO4 microcluster (LMP-MC@C) cathodes were evaluated by galvanostatic cycling test and rate capability test (Figure 4a-d). LIR 2032-type coin half-cells were assembled in the glove box with Li foil as the counter electrode and LMP-MC@C as the working electrode. For comparison, the LMP-NP and LMP-NP@C two control samples were also tested under identical conditions. As shown in Figure 4a, two test stages were applied. The cells were charged and discharged at a rate of 0.1 C in the first 50 cycles, and 0.2 C in the following 150 cycles at RT. LMP-MC@C material delivered the initial discharge capacity of 132.6 mAh g-1 at 0.1 C with a coulombic efficiency (CE) of 81.6%, and 99% of the capacity was maintained after 50 cycles. If not mentioned, all reported capacities are based on the total mass of LiMnPO4/C composites. From the 51st to 200th cycle at the rate of 0.2 C, the capacity remains in a tight range around 120 mAh g-1. Under the same condition, the control sample LMP-NP@C delivered a lower initial discharge capacity of 106.4 mAh g-1 at 0.1 C with a CE of 72.7%, and with even lower capacity of 84.9 mAh g-1 at 0.2 C. The distinct capacity differences should be suffered from worse electron transport ascribed to the weak contact between individual carbon-coated LMP nanoplates and the high carbon content in LMPNP@C structures (from 15% to 5% compared with LMP-MC@C). The as-synthesized LMP-NP sample presented the lowest specific discharge capacity (initial 65.5 mAh g-1 at 0.1 C, and 55.0 mAh g-1 at 0.2 C), which mainly results from the inherently poor electronic conductivity of LMP


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materials. Figure 4b shows the initial charge/discharge curves of three samples at 0.1 C. All the voltage profiles display a reversible plateau around 4.1 V vs. Li/Li+, which is the typical redox potential of Mn2+/Mn3+ in olivine LiMnPO4. Note that from the initial charge/discharge plateaus, our LMP-MC@C structure exhibited the smallest polarization among the three cathode materials. The LMP-MC@C structure also exhibited higher rate performance in comparison to LMP nanoplates and their carbon-coated products (Figure 4c). It yielded a reversible discharge capacity of 139.7, 131.7, 121.8, 109.9, and 99.2 mAh g-1 at 0.05, 0.1, 0.2, 0.5, and 1 C, respectively. Even at comparatively high rate of 1.5 C, it could still deliver an acceptable capacity of 92.6 mAh g-1. Moreover, this LMP-MC@C cathode achieved superior discharge capacities of 123.3 and 133.7 mAh g-1 when the charge/discharge rates switched back to 0.1 C and 0.2 C. These excellent rate capabilities indicate fast kinetics induced by nanosized primary LMP-NP and the great electronic conductivity originated from complete 3D carbon framework.29,36 Voltage profiles of LMP-MC@C cathode at different current rates are shown in Figure 4d. The redox plateau of Mn2+/Mn3+ are clearly observed at ~4 V and ~4.2 V during discharge and charge process, respectively. The flat discharge voltage plateau declines and shortens with the increase of the current rates, which is ascribed to the polarization of the cell induced by the internal resistance of the cell.34,35 A considerable plateau is maintained at around 4.0 V under 1.5 C, implying the relatively low internal resistance of our LMP-MC@C structure.36 Electrochemical impedance spectroscopy (EIS) measurements were employed to further investigate the essence of electrochemical properties of three LMP materials. Figure 4e shows typical Nyquist plots of three samples obtained after cycling (2 cycles at 0.1 C), with the



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corresponding equivalent circuit inserted. Each spectrum displays a similar depressed semicircle in the high-to-medium frequency range and an inclined line in the low frequency range. The semicircle in medium frequency range represents the charge transfer resistance (Rct) related to the electrode/electrolyte interface.25,37 From the fitting results based on the equivalent circuit, the carbon-coated LMP microcluster shows the lowest charge transfer resistance of 276 Ω (2339 Ω for LMP-NP and 474 Ω for LMP-NP@C), indicting excellent electronic conductivity achieved by our rational design. This result was in accordance with the superior electrical highway enabled by the 3D carbon framework, which wrapped and connected each primary LMP nanoplate in the microcluster. Moreover, since the inclined line in low frequency range (Warburg region) is corresponding to Li+ diffusion within the electrode, it is possible to further estimate the Li+ diffusion coefficients (DLi+) through Warburg impedance analysis using the following equation (eq) 1,25,36-40 DLi +

R 2T 2 = 4 4 2 2 2 2n F A C σ

(1)

where R is the gas constant, T is the absolute temperature, n is the number of electrons transferred in the redox process, F is the Faraday constant, A is the surface area of the LMP electrodes, C is the Li+ concentration and σ is the Warburg factor. The value of σ is determined by the linear slope between the real components of the impedance (Zreal) versus the reciprocal of the square root of the angular frequency (ω-1/2) according to the following eq 2: Z real = Rct + Rs + σω −1 / 2

( 2)

where Rs is the internal resistance. As shown in Figure 4f, the lowest Warburg factor σ of LMPMC@C sample demonstrates the largest Li+ diffusion coefficient as well as the fastest Li+ kinetics. This exciting result further confirms that the robust 3D carbon framework encapsulated


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the close-packed LMP NPs not only benefits for superior electronic conductivity, but also improves the Li+ diffusion rate. ■ CONCLUSION In summary, we have successfully designed a novel structure of carbon-coated LiMnPO4 cluster and demonstrated its application as a cathode in LIBs. In this design, the primary LiMnPO4 plates with nanoscale diameter reduce the diffusion path length of lithium ions, and then they are assembled to microclusters achieving dense packing and robust contact. Meanwhile, the conformal carbon framework wraps on these microcluster structures to provide fast electron transport pathways, leading to improved electronic conductivity and therefore superior battery performance. As a result, the obtained cathode delivered a stabilized reversible discharge specific capacity of over 120 mAh g-1 after 200 cycles at the current rate of 0.2 C, which is much better than normal carbon-coated LMP nanoplates. Meanwhile, the cathode exhibits a superior rate performance with discharge capacity of 139.7 mAh g-1 at 0.05 C, 131.7 mAh g-1 at 0.1 C, and 99.2 mAh g-1 at 1 C at RT. This assembly of nanoplates into microclusters is a very promising strategy to combine the nanoscale and microscale benefits, which could be adopted for other LIB anode and cathode materials. ■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Zhenda Lu: 0000-0002-9616-8814 Chao Wang: 0000-0001-6418-0504



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Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS This work was supported by National Key R&D Program of China (2016YFA0201100), Thousand Talents Program for Young Researchers, National Natural Science Foundation of China (Grant No. 21601083), Natural Science Foundation of Jiangsu Province (Grant No. BK20160614). ■ REFERENCES (1) Padhi, A. K.; Nanjundaswamy, K. S.; Goodenough, J. B. Phospho-Olivines as PositiveElectrode Materials for Rechargeable Lithium Batteries. J. Electrochem. Soc. 1997, 144, 11881194. (2) Whittingham, M. S. Lithium Batteries and Cathode Materials. Chem. Rev. 2004, 104, 42714302. (3) Herle, P. S.; Ellis, B.; Coombs, N.; Nazar, L. F. Nano-Network Electronic Conduction in Iron and Nickel Olivine Phosphates. Nat. Mater. 2004, 3, 147-152. (4) Tarascon, J. M.; Armand, M. Issues and Challenges Facing Rechargeable Lithium Batteries. Nature 2001, 414, 359-367. (5) Yamada, A.; Chung, S. C.; Hinokuma , K. Optimized LiFePO4 for Lithium Battery Cathodes. J. Electrochem. Soc. 2001, 148, A224-A229. (6) Oh, S. M.; Myung, S. T.; Sun, Y. K. Olivine LiCoPO4-Carbon Composite Showing High Rechargeable Capacity. J. Mater. Chem. 2012, 22, 14932-14937. (7) Okada, S.; Sawa, S.; Egashira, M.; Yamaki, J. I.; Tabuchi, M.; Kageyama, H.; Konishi, T.; Yoshino, A. Cathode Properties of Phospho-Olivine LiMPO4 for Lithium Secondary Batteries. J. Power Sources 2001, 97, 430-432. (8) Zhou, F.; Cococcioni, M.; Kang, K.; Ceder, G. The Li Intercalation Potential of LiMPO4 and LiMSiO4 Olivines with M = Fe, Mn, Co, Ni. Electrochem. Commun. 2004, 6, 1144-1148. (9) Wolfenstine, J.; Allen, J. LiNiPO4-LiCoPO4 Solid Solutions as Cathodes. J. Power Sources 2004, 136, 150-153.


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(23) Delacourt, C.; Laffont, L.; Bouchet, R.; Wurm, C.; Leriche, J. B.; Morcrette, M.; Tarascon, J. M.; Masquelier, C. Toward Understanding of Electrical Limitations (Electronic, Ionic) in LiMPO4 (M = Fe , Mn) Electrode Materials. J. Electrochem. Soc. 2005, 152, A913-A921. (24) Drezen, T.; Kwon, N. H.; Bowen, P.; Teerlinck, I.; Isono, M.; Exnar, I. Effect of Particle Size on LiMnPO4 Cathodes. J. Power Sources 2007, 174, 949-953. (25) Chen, L.; Dilena, E.; Paolella, A.; Bertoni, G.; Ansaldo, A.; Colombo, M.; Marras, S.; Scrosati, B.; Manna, L.; Monaco, S. Relevance of LiPF6 as Etching Agent of LiMnPO4 Colloidal Nanocrystals for High Rate Performing Li-ion Battery Cathodes. ACS Appl. Mater. Interfaces 2016, 8, 4069-4075. (26) Wang, K.; Wang, Y.; Wang, C.; Xia, Y. Graphene Oxide Assisted Solvothermal Synthesis of LiMnPO4 Nanoplates Cathode Materials for Lithium Ion Batteries. Electrochim. Acta 2014, 146, 8-14. (27) Sun, Y. K.; Oh, S. M.; Park, H. K.; Scrosati, B. Micrometer-Sized, Nanoporous, HighVolumetric-Capacity LiMn0.85Fe0.15PO4 Cathode Material for Rechargeable Lithium-Ion Batteries. Adv. Mater. 2011, 23, 5050-5054. (28) Oh, S. M.; Oh, S. W.; Yoon, C. S.; Scrosati, B.; Amine, K.; Sun, Y. K. High-performance Carbon-LiMnPO4 Nanocomposite Cathode for Lithium Batteries. Adv. Funct. Mater. 2010, 20, 3260-3265. (29) Li, W.; Liang, Z.; Lu, Z.; Yao, H.; Seh, Z. W.; Yan, K.; Zheng, G.; Cui, Y. A Sulfur Cathode with Pomegranate-Like Cluster Structure. Adv. Energy Mater. 2015, 5, 1500211. (30) Liu, N.; Lu, Z.; Zhao, J.; McDowell, M. T.; Lee, H. W.; Zhao, W.; Cui, Y. A PomegranateInspired Nanoscale Design for Large-Volume-Change Lithium Battery Anodes. Nat. Nanotechnol. 2014, 9, 187-192. (31) Xia, Q.; Liu, T.; Xu, J.; Cheng, X.; Lu, W.; Wu, X. High Performance Porous LiMnPO4 Nanoflakes: Synthesis from a Novel Nanosheet Precursor. J. Mater. Chem. A 2015, 3, 63016305. (32) Sun, C.; Rajasekhara, S.; Goodenough, J. B.; Zhou, F. Monodisperse Porous LiFePO4 Microspheres for a High Power Li-Ion Battery Cathode. J. Am. Chem. Soc. 2011, 133, 21322135.


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Figure 1. Schematic of the fabrication process for carbon-coated LiMnPO4 microclusters. The red arrows between the nanoplates represent the electron transport.


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Figure 2. (a) XRD patterns, (b) XPS survey spectra of as-prepared different LMP samples.



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Figure 3. Characterization of different LMP samples: (a) overview and magnified SEM image of as-synthesized LiMnPO4 nanoplates; (b), (c) overall morphology of microclusters before and after carbon coating. The insets are the corresponding enlarged SEM image of the individual microcluster. (d) Typical and high-magnification TEM images of an individual carbon-coated microcluster. (e) TEM image of the carbon framework after etching away LiMnPO4 using hydrochloric acid. (f) Thermogravimetric (TG) profiles of carbon-coated microclusters and nanoplates.


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Figure 4. Electrochemical characterization of LMP cathodes. All specific capacities are based on the total mass of LiMnPO4/C composites. (a) Capacity retention and corresponding Coulombic efficiency (CE) of LMP-NP, LMP-NP@C, and LMP-MC@C samples for 200 galvanostatic cycles tested under the same conditions, the rate was 0.1 C for the first 50 cycles and 0.2 C for



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the later cycles (1 C = 0.17 A g-1 LMP). (b) The first cycle charge/discharge voltage profiles of three samples at the current rate of 0.1 C. (c) Rate capabilities of three different LMP samples. (d) Charge/discharge voltage profiles of LMP-MC@C at different rates. (e) Nyquist plots of three samples after 2 cycles at 0.1 C. Inset is the corresponding equivalent circuit that better fits the above impedance data. (f) Real components of the impedance (Zreal) vs. the reciprocal of the square root of the angular frequency (ω-1/2) for three LMP samples. The corresponding slope of the curve represents the Warburg factors, σ. All electrochemical measurements were carried out at RT.


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Table of Contents (ToC) graphic


Assembly of LiMnPO4 Nanoplates into Microclusters as a High

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