Developing a Diamine-Assisted Polymerization Method To Synthesize

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Developing a novel diamine-assisted polymerization method to synthesize nano-LiMnPO4 with N-doped carbon from polyamides for high-performance Li-ion batteries Hao Yang, Yan Wang, and Jenq-Gong Duh ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02868 • Publication Date (Web): 01 Sep 2018 Downloaded from http://pubs.acs.org on September 3, 2018

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ACS Sustainable Chemistry & Engineering

Developing a novel diamine-assisted polymerization method to synthesize nano-LiMnPO4 with N-doped carbon from polyamides for high-performance Liion batteries Hao Yang1, Yan Wang1,2, and Jenq-Gong Duh1,* 1.

Department of Materials Science and Engineering, National Tsing Hua University, No. 101,

Section 2, Kuang-Fu Road, Hsinchu, Taiwan 2.

School of Chemical Engineering and Pharmacy, Wuhan Institute of Technology, No. 206,

Guanggu 1st road, Wuhan, China

E-mail: [email protected] (Jenq-Gong Duh), [email protected] (Hao Yang), [email protected] (Yan Wang)

KEYWORDS. Cathode, LiMnPO4, N-doped carbon, polymerization, and polyamide

ACS Paragon Plus Environment

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Abstract

A diamine-assisted polymerization method is invented to synthesize nano-LiMnPO4 coated with highly homogenous polyamides. The additive, p-phenylenediamine with a diamine and an aromatic group, enters the whole reaction, effectively adsorbed on the LiMnPO4. pphenylenediamine suppresses the particle growth and maintains the reaction pH value that promotes the formation of impurity-free LiMnPO4. When carbon is prepared with sucrose, the LiMnPO4/C prepared with large amounts of p-phenylenediamine exhibits a capacity of 134 mAhg-1 at 0.1 C. To further synthesize a more homogenous and conductive carbon, pphenylenediamine and acyl chloride are in-situ polymerized to two types of polyamide, aromatic (known as, aramid) and semi-aliphatic polyamide. N-doped carbon pyrolyzed from the polyamide allows a fast Li-ion migration into the LiMnPO4. Li-ions are favorable for being adsorbed/ desorbed on the N-doped carbon as compared with the non-doped carbon. It is demonstrated that N is bonding with P and Mn on the LiMnPO4 surface, decreasing the contact resistance of carbon. Thus, LiMnPO4/N-doped C exhibits better cycling performance and rate capability than the LiMnPO4/C prepared with sucrose.

Introduction Olivine LiMPO4 (M= Fe, Mn, Co, and Ni) has attracted much attention as cathode material for decades since their superiority in thermal stability, cost efficiency, and cycle life.1, 2 LiFePO4 has already been widely studied and used in electric products whereas it exhibits low operating voltage (3.4 V vs. Li) and theoretical energy density (578 Whkg-1). Among other olivines, LiMnPO4 provides 20 % higher energy density (701 Whkg-1) than LiFePO4. Its


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moderate voltage (4.1 V vs. Li) prevents the severe decomposition of conventional carbonate electrolyte. However, LiMnPO4 suffers from poor electric conductivity (< 10-9 Scm-1), and Liion diffusivity (< 10-14 cm2s-1).3,

4

These arise from the separated arrangement of MnO6

octahedron, Jahn-Teller distortion in Mn3+, the large interfacial strain between MnPO4/LiMnPO4, and the unstable delithiated phase.5, 6 Several strategies are adapted to improve the drawbacks mentioned above, including a carbon coating, particle size reduction, and cation doping.7-11 The morphology and size of LiMnPO4 depend on several parameters, such as solvent, additive, pressure, and temperature etc.2, 12 Polymers, and aliphatic surfactants can act as capping agents to prevent grain growth and aggregation, but they easily remain in the products, influencing the adhesion of carbon sources.13-15 Small molecules with high electronegativity, like ethylene glycol and nitrogenous compounds, are capable of bonding with phosphates.16, 17 Ammonium and ethylene diamine can stack

layer-by-layer

between

manganese

phosphate,

forming

NH4MnPO4

and

(C2N2H10)Mn2(PO4)2·2H2O.18 Troung et al. also observed that some amines would be adsorbed on specific facets of phosphate during the supercritical fluid process.19, 20 Nano-sized LiMnPO4 coated with homogenous carbon is an essential strategy for improving electrochemical performance.21-23 The few nm thick carbon with partial 2D structures can provide high electrical conductivity, Li-insertion defects as well as avoid particle aggregation.24-26 Carbohydrate materials are used to be ex-situ coated on oxide materials by ball milling, evaporation, and hydrothermal method.27 Oxide materials with vacancies and functional groups are easily bonding with carbohydrate.2 However, P-O covalent bonds in LiMnPO4 are too strong to tolerate oxygen vacancies. The weak bonding between carbohydrates and LiMnPO4 will make the coating become only a physical blending, whereas carbohydrates cannot permeate


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into agglomerated nanoparticles. The inhomogeneity will increase the diffusion length and interfacial resistance.13,

23, 28

Some researchers are devoted to synthesizing carbon-coated

LiFePO4, LiFexMn1-xPO4, and Li3V2(PO4)3 by a polymerization process, but none of them execute size reduction and in-situ polymerization on LiMnPO4 in one synthesis process.29-33 Herein, the diamine-assisted polymerization method is invented to synthesize LiMnPO4 and polyamide-coated LiMnPO4. The additive, p-phenylenediamine (PPD), controls particle sizes and becomes one of the monomers for polymerization. The para-substitutional amine will create hydrogen bonds with LiMnPO4 and prevent the aggregation of particles.19, 20 The aromatic group provides steric hindrance, preventing stacking with manganese phosphates. It is found that the Li-ion concentration during the synthesis process can change the Li content of LiMnPO4. The assynthesized LiMnPO4 follows two routes to coat carbon: sucrose-coating process obtains the LiMnPO4/C, while in-situ polymerization process obtains the LiMnPO4/N-doped C. The Ndoped carbon is pyrolyzed from the poly(p-phenylene terephthalamide) (PPTA) and poly(pphenylene decanamide) (PPDA), as illustrated in Fig. s1(a) (supporting information). The Ndoped carbon is tightly coated on the LiMnPO4 surface. It is demonstrated that the conductivity, contact ability and surface capacitance of LiMnPO4 are improved by the N-doped carbon.

Experimental Section Chemicals. Ethylene glycol (EG, Scharlau, 99.5 %), 1,2-dichloroethane (DCE, TEDIA, 99 %), p-phenylenediamine (PPD, Alfa Aesar, 97 %), terephthaloyl chloride (TCL, ACROS, 99 %), Sebacoyl chloride (SC, Alfa Aesar, 97 %), LiOH·H2O (Baker, 100 %), MnCl2·4H2O (Baker, 98 %) and H3PO4 (SHOWA, 85 %)


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The preparation of LiMnPO4/C. LiMnPO4 was synthesized by a diamine-assisted method. Initially, 0.01-0.03 mol PPD was dissolved into 50 ml EG. A 0.02 mol LiOH (dissolving in H2O to form 3.3 M solution), 0.01 mol H3PO4 and 0.01 mol MnCl2 (dissolving in H2O to form 4 M solution) were added into the EG in the order. The molar ratio of Li: Mn: P was 2: 1: 1. The solution was placed in a 3-neck flask and heated at 120 °C for 2.5 hr under reflux in the N2 atmosphere. After cooling, the obtained LiMnPO4 was filtered and dried at 60 °C in the air. To coat the carbon layer, LiMnPO4 was ground with sucrose solution in a mortar. When the slurry was dried, it was calcined at 600 °C for 2 hr in N2/ 5%H2 atmosphere to obtain LiMnPO4/C. The stoichiometric LiMnPO4 was synthesized by enhancing the LiOH aqueous solution to 9 ml. The preparation of LiMnPO4/polyamide and LiMnPO4/N-doped C. The synthesis of LiMnPO4/PPTA or PPDA was held after the LiMnPO4 formation reaction was done. As the reflux reaction was complete, the flask was cooled down to 4 °C. The monomers (0.002 mol TCL or 0.004 mol SC) were dissolved into 5 ml DCE. After the pH value was enhanced from 6.5 to 7.6 by LiOH, the monomer was dripped into the flask and was stirred for 2 hrs. The reaction was held at 80 °C for 1 hr in N2 to complete the amorphization reaction. The as-synthesized LiMnPO4/PPTA or PPDA was calcined at 600 °C for 2 hr to obtain LiMnPO4/N-doped C. Electrochemical measurements. The electrode was prepared with the water-soluble binder (Sodium Alginate, ACROS). The active material, carbon black, and sodium alginate were blended with H2O at the weight ratio of 80: 13: 7. After coating on Al foil, the slurry was dried at 60 oC in an oven. Electrochemical tests were performed using 2032 type coin cells. The reference was metallic Li. Electrolytes are composed of 1.0 M LiPF6 in EC/DMC (1:1, vol.%). The assembled cells were aged for 12 hr to ensure electrodes completely soaked in electrolyte. Cycling tests were performed by an Arbin battery tester. In this work, 1 C was defined as 171


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mAg-1. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry were carried out by an electrochemical workstation (Ametek 263A, USA). The Nyquist plot was tested in the range of 10 mHz-100 kHz. Materials characterization. Samples were identified by in-house X-ray (XRD, Bruker D2phaser, Cu Kα) and synchrotron X-ray (18 kV, λ= 0.68892nm). Morphology and microstructure were characterized by ﬁeld-emission SEM (JSM-7600F, JEOL) and spherical-aberration corrected field emission TEM (JEM-ARM200F, JEOL, 80 and 200 kV). The composition was analyzed by an inductively coupled plasma-optical spectrometer (Agilent 725 ICP-OES). Highresolution

X-ray

photoelectron

spectroscopy

(PHI-5000

Versaprobe-II,

ULVAC-PHI,

monochromatic Al kα source) was used to derive the oxidation states of LiMnPO4/C.

Results and discussion LiMnPO4 was prepared with different amounts of PPD (LMPx representing PPD: LiMnPO4= x: 1). Fig. 1 shows the SEM images and XRD patterns of as-synthesized LiMnPO4. The morphology of LMP3 is more spherical and smaller (~ 30 nm) than that of LMP1. The particle growth is restricted by PPD which creates N-H⋯O-P hydrogen bonds with phosphate. The X-ray diffractions (Fig. 1(g)-(i)) of LMP1, LMP2, and LMP3 are all indexed to olivine phase (space group: Pnma, COD# 9011037). In the absence of PPD, particles will be bulky and be indexed to the phase of Mn5(HPO4)2(PO4)2·(H2O)4. The reaction without PPD is found to be pH= 3.7. It was reported that LiMnPO4 was thermodynamically stable at pH= 10.7.12 Lower pH environment (pH= 9.5) would precipitate the intermediates of Mn5(HPO4)2(PO4)2 ·(H2O)4 and Mn3(PO4)2 ·


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3H2O at 100 °C. Therefore, PO43- will be protonated much easier when pH< 9.5, making Mn5(HPO4)2(PO4)2·(H2O)4 permanently exist rather than Mn3(PO4)2·3H2O. In opposite, the pH value is neutralized and buffered by PPD, as illustrated in Fig. 1(k). The neutral environment makes the reaction between Mn3(PO4)2·3H2O and Li3PO4 happen. ICP-OES analysis (Table 1) reveals that the as-synthesized LiMnPO4 has 10 % of Li deficiencies due to the low LiOH concentration of the reaction (Li: Mn: P=2: 1: 1). Meanwhile, stoichiometric LiMnPO4 (denoted as SLMP) is synthesized with enhanced LiOH concentration. It is known that the pure nonstoichiometric LiMnPO4 is very rare since the reaction with excess LiOH will precipitate Li3PO4, and the reaction with low LiOH generally has impurities, like Mn2P2O7 and MnHPO4.34-37 Therefore, LiMnPO4 was analyzed by synchrotron X-ray to ensure the quantification reliable, as shown in Fig. 2. Both XRD patterns do not reveal Li3PO4 at 10.26°. The SLMP/C exhibits higher crystallinity and smaller crystallite size than the LMP3/C. Rietveld refinement (Table 2) reveals that lattice parameters are identical. However, Li deficiencies imply a possible presence of Mn∎ anti-site defects. The refinement reveals that the occupancy of Li□ is well matched with the ICPOES analysis and only a few higher Mn∎ antisite defects exist in the LMP3/C (3.2 %) than the SLMP/C (2.3 %). The in-situ polymerization process is executed after the LiMnPO4 colloidal solution was completely formed, as illustrated in Fig. s1(b). The zeta potential analysis (Fig. s2) shows that the colloidal solution has a positive surface charge which indicates the adsorption of -NH3+ on the LiMnPO4. The post-addition LiOH changes the value to negative. Since the polymerization will be initialized immediately by acyl chlorides, the negative zeta potential will promote the adsorption of dissociated acyl chlorides, preventing the polymerization randomly initiate in the solution. It is found that the DCE solvent stabilizes acyl chlorides well, whereas solvents without


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chloride groups, like ethylene glycol and toluene, will lead to inhomogeneous polyamide coating. The as-synthesized LiMnPO4/PPTA and LiMnPO4/PPDA are coated with yellow and pink polyamide. They are pyrolyzed to LiMnPO4/N-doped C (denoted as LMPT and LMPD) after calcined at 600 °C. The microstructures of LiMnPO4/C prepared with different carbon sources are shown in TEM images (Fig. 3). The LMP3 particle exposes (111) and (020) facets in HRTEM images, whereas the particles in Fig. s3 show other facets, demonstrating that PPD does not induce preferred orientation. Since the grain size (43 nm) estimated by the Rietveld refinement is close to the observed particle sizes, the particle is considered to be single grain. All LiMnPO4 samples with identical d-spacing in HR-TEM images are further proved by XRD patterns (Fig. s4) which show the same diffraction locations. Additionally, all LiMnPO4 samples have 1-2 nm carbon layers. In the LMP3/C, carbon prepared with sucrose can be coated on the particle, whereas the carbon (circled by the dashed line) is more randomly distributed around particles than the N-doped carbon.23, 28 The C distribution of LMP3/C in elemental mapping (Fig. s5) is inhomogeneous, but the C distribution in LMPT and LMPD is evenly distributed along the particles. The inhomogeneous carbon usually occurs in ex-situ coating process, and even though it can create consecutively conductive networks, some space within agglomerated LiMnPO4 will not have enough carbon. In opposite, bright-field TEM images of LMPT and LMPD (Fig. 3(d) and (e)) reveal that the particles, including the space within particles, are entirely coated with Ndoped carbon, which means a much complete conductive network in LiMnPO4/N-doped C.23 Moreover, the thickness of carbon in LMPT is more uniform than that in LMPD which might influence their electrochemical performances. ICP-OES analysis shows that Li deficiency is alleviated in LMPT and LMPD. To realize this, another LMPT was prepared without post-


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addition LiOH (denoted as LMPT-1). The LMPT-1 still has lower Li deficiency than the LMP3. It is indicated that some Li-ion will involve into the polymerization. FTIR spectra (Fig. 4) was conducted to identify the functional groups of LiMnPO4 composites. In LiMnPO4/PPTA and LiMnPO4/PPDA, strong absorption bands between 800 and 1200 belong to P-O stretching vibrations, splitting due to the non-identical PO4 tetrahedron in the lattice.38, 39 The absorption bands above 1200 cm-1 coincided with the corresponding polyamides belong to the coating layer. The number of absorptions bands at ~3300 cm-1 is used to identify the level of amine groups. The only existed one narrow band demonstrates that the primary amine of PPD has successfully polymerized to the secondary amine of polyamide. Meanwhile, the amide group comprised of C=O, N-H, and C-N bonds should appear at ~1645, 1540 and 1250 cm-1 which are classified into the groups of Amide I, II and III, respectively.40, 41 Except for Amide I belonging to the C=O stretching vibration, Amide II and III are the coupling of several absorption bands due to the similar bond strength, as listed in Table s1. The benzenes effected by neighboring substituents and stereochemistry result in four C=C absorption bands, located around 1610, 1513, 1317, and 1118 cm-1. Most functional groups will decompose after calcined at 600 °C. Fig. 4b shows the residual functional groups of as-synthesized LiMnPO4 and LiMnPO4/C. The assynthesized LiMnPO4 is the absence of functional groups. Only H2O is adsorbed, displaying absorption bands at 1635 and 3400 cm-1. The polyamide is assumed to pyrolyze to N-doped carbon with C-N bonds after calcined at 600°C. The carbon matrix makes the H2O bending vibration shift to 1611 cm-1, indicating the presence of C=C vibration. For the N-doped carbon, the secondary amine disappears since the amine might transform into the tertiary amine or incorporate into the carbon matrix. Although carbon matrix is believed to be weak dipole moment (weak IR-active), C-N stretching vibration is still observed at ~1260 cm-1. The other


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absorption bands between 1200 and 1600 cm-1 are the presence of residual C-H bonds. The sp3 C-H vibration (2851 and 2930 cm-1) induced by the PPDA blueshifts to the sp2 C-H vibration after calcination. This indicates that the alkyl chain has pyrolyzed to the sp2 carbon matrix. It was reported that amines might coordinate or cover on LiMnPO4.14, 20 FTIR spectra (Fig. s6) show that the P-O stretching vibration around 960 cm-1 will shift when the as-synthesized LiMnPO4 is mixing with PPD and EG. Since PO43- is sensitive to the structural symmetry, the hydrogen bonds created by PPD might change the P-O bonding length.17 The redshifts of other bands, such as C-NH2 and C-O vibration from PPD and EG, imply the presence of hydrogen bonds between PPD and EG as well as EG and LiMnPO4. In Fig. 4b, the present of benzene vibration (~1513 cm-1) in the as-synthesized LiMnPO4 indicates the adsorption of PPD. The absence of N-H vibration might be ascribed to the oxidation of PPD when the product was exposed in the air. Therefore, XPS measurements analyzed the LiMnPO4 surface for its sensitivity to the surface chemistry. In N1s spectra (Fig. 5(a)), the as-synthesized LiMnPO4 shows a small peak at ~400 eV. Unlike alkyl amines, the small aromatic amine is adsorbed only a monolayer on LiMnPO4 since the N content is close to 0 %.13,

14

The N-C bonding in

LiMnPO4/N-doped C is also realized by XPS. N locations are classified into pyridinic (398.3 eV), pyrrolic (399.8 eV), graphitic (400.7 eV), and oxidized N (402.8 eV). Pyridinic and pyrrolic N is located at edge or defects. Lone pairs of pyridinic N and defects are favorable for Li-ion migration and storage.42, 43 The graphitic N located at center or edge can introduce extra electron into delocalized π -system.42 The electrical conductivity of carbon enhances with more delocalized electrons.42 The N content in carbon matrix is estimated to be 7.0 atom% in LMPD and 9.3 atom% in LMPT. The values are comparable with the N-doped graphene and CNT in the literature.42


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It’s important to realize if C-N improves the contact with LiMnPO4. N with high electronegativity (3.04) is considered to electrostatically bond with Mn (1.55) or P (2.19). Therefore, the coated layer can cause a chemical shift when it contains dipolar elements.44 Fig 5(c) shows the binding energy of the LMP3/C, LMPT, and LMPD surface. Ag particles are the internal standard to ensure the chemical bonding being right. The Mn oxidation state is determined by Mn3s since Mn2p has a multiplet splitting.45 The splitting of Mn3s with 6.3 eV indicates an oxidation state of 2+. XPS reveals that the Mn3s, P2p, and O1s representing Mn-O and P-O bonds in the LMPT and LMPD shift to the lower binding energy. A slight chemical shift implies the polarity change of the corresponding ligand.44 The electrostatic attraction between N, Mn, and P will not break the bonding but elongated Mn-O and P-O bonds. The P-O seems to be more attracted by N since the chemical shifts in P2p and O1s are large. The existence of electrostatic attraction might eliminate the interfacial resistance between N-doped carbon and LiMnPO4, improving the electron migration. The carbon content was tested by TGA, as presented in Fig. 5(d). The LMP3/C, LMPD, and LMPT show the 5.0, 5.5 and 5.9 % carbon loss around 320 and 520 °C, respectively. Owing to The higher carbon content and less aggregated LiMnPO4 particles, the specific surface area of LMPT (65 m2g-1) and LMPD (59 m2g-1) is higher than that of LMP3/C (55 m2g-1). The LMPD has a lower surface area since the aliphatic chains easily intertwine and agglomerate some particles, as observed in Fig. 3(c). The disorder of carbon was identified by Raman spectra, as shown in Fig. 5(e). The ID/IG ratio represents the degree of disorder, and both N-doped carbons have more disorder structure than non-doped carbon. Disorder carbon arisen from structural defects is correlated with the stacking of polyamides. PPTA with totally aromatic backbones stacks compactly. It can pyrolyze to graphite without much structural rearrangement.


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The electrochemical performances of LiMnPO4/C prepared with different amounts of PPD are shown in Fig. 6. Cells were charged and held at 4.5 V till the current was below 0.01 C (the CCCV mode). Fig. 6(a) exhibits the cycling performance of LMPx/C. The discharge capacities of LMP1/C, LMP2/C, and LMP3/C at 0.1 C are 108, 126 and 134 mAhg-1, respectively. The chargedischarge curves exhibit plateaus around 4.2/ 4.0 V, representing a Mn2+/3+ redox reaction, as shown in Fig. 6c. The LMP3/C shows the best rate capability from 0.1 to 20 C, delivering 134, 132, 130, 129, 120, 100, 85, and 75 mAhg-1. Also, the LMP3/C delivers more stable and higher capacity than the others at 0.1 C, maintaining 135 mAhg-1 after 50 cycles. The superior performance is ascribed to the fine particles and the morphology of LMP3/C. The onedimensional Li-ion diffusion length is shortened by fine particles. Owing to a large volume change (~11%) between LiMnPO4 and MnPO4, the spherical particles with short phasetransformation interface will release the large elastic strain. It is found that the good cycling retention of LMP2/C and LMP3/C is accompanied by low Coulomb efficiency (< 95 %) before the 25th cycle. In opposite, the LMP1/C with higher coulomb efficiency (98.5 %) shows capacity decay. The gradual decline in capacity is the phenomenon of Mn3+ dissolution induced by HF attack which can be avoided by homogenous surface coating.28 The charge-discharge curves of LMP3/C in Fig. 6(d) reveals that the low coulomb efficiency is attributed to a prolong plateau above 4.3 V. The side reaction above 4.3 V is the catalytic reaction within the electrolyte and electrode.46 To reduce irreversibility, the cut-off current was enhanced to 0.02 C. Fig. 6(e) shows the cycling performance of stoichiometric and non-stoichiometric LiMnPO4/C at 0.01 and 0.02 cut-off current. The LMP3/C has high coulomb efficiencies at 0.02 cut-off current, whereas the capacity decays after the 25th cycle. LiMnPO4 that can be quickly activated in the 1st cycle is because sustaining at 4.5 V for a period will construct the conductive networks and forcibly


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extract Li-ion from LiMnPO4. By comparing the charge curves within 50 cycles (Fig. s7a and b), the solid-solution reaction below 4.2 V increases at 0.01 cut-off current. The solid-solution reaction that related to the surface compound on LiMnPO4 is accompanied by the low Coulomb efficiencies. This indicates that the side reaction above 4.3 V within the electrolyte and electrode will deposit some Li-contained components on the LiMnPO4 surface. Since the LiMnPO4 surface is less reactive than other cathode materials, the solid-solution capacitance might arise from the sodium alginate binder or the carbon.46 A passivated SEI layer, like LiF, ROCO2Li, and polycarbonates might also exist, but it would increase the resistance and cannot store capacity which is not observed in this study.46 The carboxylic groups on sodium alginate can interact with Li-ion and will store Li-ions after cycling.47 Therefore, the excellent performance of LMP3/C at 0.01 cut-off current is due to the complete Li extraction and the protection by the solid solution. Fig. 6(e) also reveals the cycling performance of SLMP/C. It exhibits slightly higher initial discharge capacity than the LMP3/C regardless of the cut-off current. Since both particle sizes and phase are the same, the capacity difference is ascribed to the concentration of Mn∎ anti-site defects. The anti-site defects will block some Li-diffusion channels.48 The SLMP/C also undergoes a capacity decay but a much severe at 0.01 C cut-off current. Since the decline is highly relevant to the surface condition, XPS analysis was investigated in more detail, as shown in Fig. s8. There is no difference between Mn3s, P2p, and O1s. A shift of Li1s (55.2 eV) in SLMP/C is close to the binding energy of Li2CO3.49 This implies that small amounts of amorphous Li2CO3 precipitates from a high LiOH environment and covers the LiMnPO4 surface. Li2CO3 would react to electrolytes, forming passivated SEI layer as cell operating above 4.5 V for a period.50 Therefore, the reaction of Li2CO3 is more severe in low cut-off current and high current density. As shown in the rate performance (Fig. s9(a)), Li2CO3 suppresses the rate


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capability of SLMP/C in both 0.01 C and 0.02 C cut-off current. Overall, species over the particle surface are key factors to influence the cycling retention. LiMnPO4/C without impurity will exhibit the best performance at 0.01 C cut-off current. A more homogenous carbon is essential to prevent Mn dissolution if cells work at the cut-off current> 0.02 C. The electrochemical performance of LiMnPO4/N-doped C is shown in Fig. 7(a) and (b) which cut-off current was altered to 0.02 C to reveal the real performance of LiMnPO4. Both LMPD and LMPT exhibit highest capacities of 142 and 145 mAhg-1 as compared to the LMP3/C of 132 mAhg-1 at 0.1C. The LMPT exhibits the highest rate capability (Fig. 7(b)) between 0.1 and 20 C, delivering 145, 142. 134, 126, 116, 102, 89, and 73 mAhg-1, respectively. Moreover, the LMPT shows higher capacity than the one prepared without post-addition LiOH (LMPT-1), exhibiting less polarization in charge-discharge curves (Fig. 7(c)). Since both have the same carbon content, it demonstrates that modifying the surface charge will change the contact of polyamides. It is noted that, as shown in Fig. s9(b), all LiMnPO4/N-doped C samples exhibits better rate capability than the SLMP/C above 5 C due to the absence of Li2CO3. Table. s2 also compares the electrochemical performance of LMP3/C and LMPT with other studies. It is assumed that Ndoped carbon facilitates the Li-ion transportation but does not influence the Li-ion diffusivity inside LiMnPO4. However, the charge-discharge curves depict that the additional capacities majorly come from the prolonged redox plateau. The enhanced redox behavior in LiMnPO4 reveals that the interfacial resistance on LiMnPO4 is decreased by improving the conductivity and the contact of N-doped carbon. Moreover, carbon with defects or edges can accommodate and transport Li-ion. It was reported that N-doped carbon materials, like N-doped graphene, deliver higher capacity than pristine graphene regardless being anodes or cathodes.51 Pyridinic and pyrrolic N is energetically favorable to accommodate Li-ion at the nearby defect.43 The


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energy storage mechanism between 1.5 and 4.5 V is demonstrated to be a surface capacitance including double-layer capacitance and pseudocapacitance. In Fig. 7(c), charge capacities below 4.2 V and discharge capacities below 3.3 V in LMPD and LMPT are slightly higher than the LMP3/C. It should be noted that these capacities do not come from the side reaction because it is only the 2nd cycle. The two slope regions might be related to the surface behavior. Therefore, the cyclic voltammetry is tested between 2.0 and 4.5 V (Fig. 7(d)) to reveal undetected electrochemical reactions. The sharp peaks at 4.3/3.9 V are the Mn2+/3+ redox reaction which shows polarization of 0.46, 0.39, and 0.39 V for the LMP3/C, LMPD, and LMPT, respectively. The LiMnPO4/N-doped C with a large area between 2.5 and 4.0 V reveal that Li-ion is favorable for adsorbing/desorbing on N-doped carbon. The peaks found at 3.0 V might belong to Li-ions extracted from the pyridinic and pyrrolic N.43 The surface capacitance contributed from the LiMnPO4 can be neglected since the LiMnPO4 surface is not electrochemically active. TEM images in Fig. s10 show the morphology of LiMnPO4 after few cycles. The particles remain crystallinity, whereas the N-doped carbon in LMPT is more tightly coated on particles than the others. As regarding the long cycling stability, the gradual capacity decay in LMPT seems originated from the post-addition Li-ion which possibly remains in the carbon matrix. Since the coulomb efficiency of both LMPT and LMPD is 98 %, lower than the LMPT-1 of almost 100 % coulomb efficiencies, Li-ions from N-doped carbon might induce some irreversible reaction, such as the defect formation, and the increase of interfacial resistance after several cycles. Therefore, replacing LiOH with NH3 would be a solution. Additionally, the LMPD exhibits comparable capacity to the LMPT, yet it decays after 25 cycles. The severe decay is not the same as LMPT but due to the losing conductive network. The agglomerated particles in LMPD will lose contact with carbon when LiMnPO4 undergoes a continuous cycling


15


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deformation. Therefore, polymers with alkyl chain need more content than the one with the aromatic chain. EIS measurement (Fig. 8) was carried out to reveal the Li-ion migration behaviors. The equivalent circuit is composed of an electrolyte resistance ( ), constant phase ( /CPE), and Warburg impedance () unit which relates to a Li-ion diffusivity ( ). The calculation is according to the following equations: = + + σ .

= 0.5 # !"

$# %&

'

(1)

(

(2)

where σ is the Warburg impedance coefficient, ) is the surface area of the electrode, * is the number of Li transferred in the redox reaction, and + is the Li-ion concentration of LiMnPO4 equal to the ratio of tap density to molecular weight. The calculation (Table 3) reveals that slightly increases and largely reduces due to the N-doped carbon. The N-doped carbon with Li-ion insertion/extraction behavior increases the Li-ion diffusivity. is the summation of charge-transfer resistance at the interface. It is found that the interfacial resistance is large before cycling (Fig. s11), whereas the LMPT still has the lowest . Based on the above discussion, the low interfacial resistance in the LMPT is ascribed to several improvements, such as the enhanced carbon conductivity, the low contact resistance between LiMnPO4 and carbon, as well as the fast Li-ion migration inside N-doped carbon. The slightly larger interfacial resistance in the LMPD is attributed to the effects of low N content and severe agglomerated particles, which influences the conductive networks of electrode. Overall, these advantages demonstrate that the N-doped


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carbon prepared with PPTA is a feasible and outstanding coating that can provide LiMnPO4 to operate under high voltage.

Conclusions Nano-sized LiMnPO4 is successfully prepared with p-phenylenediamine (PPD) via the diamine-assisted method. PPD provides a neutral pH environment for synthesizing nonstoichiometric and impurity-free LiMnPO4. It effectively reduces the Li-ion diffusion length of LiMnPO4 which exhibits a capacity of 134 mAhg-1 at 0.1 C when the PPD: LiMnPO4 ratio is 3: 1. It is found that the LiMnPO4 surface is sensitive to the cut-off current. The 0.01 C cut-off current will induce the solid solution to protect the LiMnPO4 surface. In opposite, the capacity is severe decay at 0.02 C cut-off current due to the unprotected surface. The LiMnPO4/N-doped C prepared with PPTA restrains the decline, exhibiting 145 mAhg-1 at 0.1 C. The advantages of insitu polymerization are realized as the homogenous carbon distribution, less agglomeration between particles, and the high N content. The N-doped carbon enhances the electrostatic attraction with Mn and P, improving the contact with LiMnPO4. Li-ion is favorable for adsorbing on N-doped carbon and inserting/ extracting through the pyridinic and pyrrolic N. Therefore, the N-doped carbon layer from polyamide is reliable to be operated under high voltage, which provides the feasibility and potential to design high power Li-ion battery in the future.


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Acknowledgments This work was supported by the Cheng Li Company, and the Ministry of Science and Technology under contract No. MOST-106-2811-E-007-012-. Technical support from the National Synchrotron Radiation Research Center (NSRRC, Hsinchu, Taiwan) is gratefully acknowledged. Supporting Information. Zeta-potential measurement, TEM images, XRD patterns, list of stretching vibrations, FTIR spectra, galvanostatic curves, XPS analysis, additional rate performance, comparison of electrochemical performance, and Nyquist plots are included.

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50. Bi, Y.; Wang, T.; Liu, M.; Du, R.; Yang, W.; Liu, Z.; Peng, Z.; Liu, Y.; Wang, D.; Sun, X. Stability of Li2CO3 in cathode of lithium ion battery and its influence on electrochemical performance. RSC Adv. 2016, 6 (23), 19233-19237, DOI 10.1039/c6ra00648e. 51. Xiong, D.; Li, X.; Bai, Z.; Shan, H.; Fan, L.; Wu, C.; Li, D.; Lu, S. Superior Cathode Performance of Nitrogen-Doped Graphene Frameworks for Lithium Ion Batteries. ACS Appl Mater Interfaces 2017, 9 (12), 10643-10651, DOI 10.1021/acsami.6b15872.


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Table of content

A facile method is invented to synthesize nano-LiMnPO4/N-doped carbon for developing highsafety and non-toxic Li-ion batteries for future sustainability


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Graphic

Figure 1. SEM images of LiMnPO4 prepared with different amounts of PPD. PPD: LiMnPO4= (a) 0:1, (b) 1:1, (c) 2:1, and (d) 3:1. The corresponding XRD patterns for (e) 0:1, (g) 1:1, (h) 2:1, and (i) 3:1. XRD patterns of the intermediate product prepared (f) without PPD, and (j) with PPD before heated at 120 °C. (k) Formation schematic of LiMnPO4.


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Table 1. ICP-OES data of as-synthesized LiMnPO4 and LiMnPO4/polyamide. Sample

composition*

LMP1 (PPD: LiMnPO4= 1:1)

Li0.89Mn1.04PO4


Li0.90Mn1.05PO4


Li0.87Mn1.09PO4

SLMP (stoichometric LiMnPO4)

Li0.97Mn1.05PO4

LMPD

Li0.95Mn1.04PO4

LMPT

Li1.02Mn1.02PO4

LMPT-1 (preparing PPTA without using postaddition LiOH)**

Li0.95Mn1.01PO4

*Error in ICP-OES are 1 %. \\

f

Figure 2. Rietveld refinements of (a) LMP3/C and (b) SLMP/C. The sample was filled in a Kapton tube and tested by the synchrotron X-ray at an operating voltage of 18 keV (λ= 0.68892 Å). The inset shows the enlarged patterns to point out the location of Li3PO4.


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Table 2. Lattice parameters of the stoichiometric and non-stoichiometric LiMnPO4/C a(Å)

b(Å)

c(Å)

V (Å3)

Cry Size (nm)

□ ,- *

∎ /* *

LMP3/C

10.454

6.104

4.749

303.08

43.4

0.902

0.032

SLMP/C

10.455

6.104

4.749

303.12

37.2

1.003

0.023

* The elemental occupancy in the lattice

Figure 3. TEM images of (a) LMP3/C, (b) LMPT, and (c) LMPD. The lattice fringes are identified with Fast-Fourier transform images. Bright-field TEM images of (d) LMPT and (e) LMPD reveal the uniformity of N-doped carbon.


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Figure 4. FTIR spectra of (a) pure PPDA, PPTA, and LiMnPO4/polyamides, and (b) the assynthesized LiMnPO4, LiMnPO4/C, and LiMnPO4/N-doped C.


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Figure 5. (a) XPS spectra of N1s, (b) schematic of N-doped carbon and the atomic concentration of LiMnPO4/N-doped C calculated by the XPS spectra, (c) XPS spectra of Mn3s, P2p and O1s, (d) TGA analysis operated at a heating rate of 5oC/ mins in the air atmosphere, and (e) Raman analysis.


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Figure 6. Electrochemical performance of LiMnPO4/C prepared with different amounts of PPD, charged using the CC-CV mode (cut-off current= 0.01 C): (a) cycling performance at 0.1 C, (b) rate capability, (c) the charge-discharge curves at 0.1 C (d) the charge-discharge curves of LMP3/C, and (e) cycling performance of LMP3/C and SLMP/C at the 0.01 and 0.02 C cut-off current.


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Figure 7. Electrochemical performance of the LMP3/C, LMPD, LMPT, and LMPT-1, charge using the CC-CV mode (cut-off current= 0.02 C): (a) cycling performance at 0.1 C, (b) rate capability, (c) charge-discharge curves at 0.1 C, and (d) cyclic voltammetry at 0.1 mVs-1 (cells have been cycled for 5 rounds).


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70

Rs

CPE

W

Rct

60 -Z" / ohm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


50 40 30 LMP3/C LMPD LMPT LMPT-1

20 10 0 0

10 20 30 40 50 60 70 80 90 100

Z' / ohm Figure 8. Nyquist plots of the LMP3/C, LMPD, LMPT, and LMPT-1, obtained from the thermodynamic equilibrium of a fully charged cell (4.5 V). Dots represent the raw data and lines represent the fitting curves. (cells have been cycled for 5 rounds)

Table 3. The value of equivalent circuit

Sample

/ohm

/ohm

/cm ·s

LMP3/C

9.73

26.05

8.86*10-14

LMPD

8.90

18.62

2.38*10-13

LMPT

6.47

5.51

4.62*10-13

LMPT-1

6.51

13.99

1.25*10-13

2

-1


31

Developing a Diamine-Assisted Polymerization Method To Synthesize

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