Synthesis and Characterization of Alginate-Based Sol-Gel Synthesis

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Synthesis and Characterization of Alginate-Based Sol−Gel Synthesis of Lithium Nickel Phosphate with Surface Area Control Ying Jie Thong,† Jeng Hua Beh,‡ Jau Choy Lai,‡ and Teck Hock Lim*,† †

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Department of Physical Science, Faculty of Applied Sciences, Tunku Abdul Rahman University College, Jalan Genting Kelang, Setapak, Kuala Lumpur 53300, Malaysia ‡ Department of Bioprocess and Polymer Engineering, School of Chemical and Energy Engineering, Universiti Teknologi Malaysia, Skudai 81310, Malaysia S Supporting Information *

ABSTRACT: A sol−gel method in which alginate was used to help prepare carbon-coated LiNiPO4 nanocrystal aggregates with controllable surface area is reported. Nickel alginate prepared from NiCl2 and sodium alginate was blended with stoichiometric LiOH and H3PO4 to produce a single-source precursor gel. Calcining the gel in air at 400−800 °C produced LiNiPO4 nanocrystal aggregates with different levels of crystallinity, sizes, and surface areas. Powder X-ray diffraction, fieldemission scanning electron microscopy, high-resolution transmission electron microscopy, and Brunauer−Emmett−Teller analyses collectively showed that 600 °C was the optimal calcination temperature that gave LiNiPO4 with an average crystallite size of 43 nm, a surface carbon coating of 2−4 nm, and a surface area of 27.47 m2/g. The method allows simultaneous achievement of four features, namely, surface carbon coating, high crystallinity, control of primary particle size, and high surface area. Simple modification of the method would allow the production of a wider range of LiMPO4 (M = Fe, Co, Mn).

1. INTRODUCTION

A LiCoO2-based rechargeable battery has a higher risk of thermal runaways, during which the release of oxygen and intense heat may cause the electrolyte to catch fire. In 2014, Golubkov et al. reported that when paired with a graphite anode, a LiFePO4-based cathode outperformed LiCoO2 in terms of fire safety, with significantly higher onset temperature, ∼3-fold less temperature increase after onset, ∼5-fold less total gas production, and 7-fold less toxic CO.9 Such a performance in safety is in part due to the strong covalent P−O bond, which stabilizes the oxygen during thermal runaways and reduces the release of reactive oxygen species. While the LiMPO4 family performs better in safety measures, in general they suffer from poorer ionic (i.e., the Li+ ion) and electronic conductivity, in comparison to a

With the arrival of the information age, the ever-increasing demand of portable gadgets, and the rising environmental issues due to the burning of fossil fuels for automobiles and electricity generation, efficient rechargeable battery technologies as a means for electrical energy storage are becoming more essential, especially if the human race is to sustain our powerhungry lifestyle while being able to reduce the environmental damage caused by anthropogenic activities.1,2 Such is reflected in the growing demand for lithium-based rechargeable batteries, which has been reported to have almost quintupled since 2005 to 140000 tons in 2015.3 Among the cathode materials suitable for lithium batteries are the olivine-type lithium transition-metal orthophosphates with the general formula of LiMPO4 (M = Co, Mn, Ni, Fe, and others). LiMPO4-based materials are considered to be attractive cathode materials based on their ability to provide voltages close to that of lead-acid battery while being safer and highly stable during charging−discharging cycles when compared to the commercialized LiCoO2-based battery.4−18 © XXXX American Chemical Society

Special Issue: 2018 International Conference of Chemical Engineering & Industrial Biotechnology Received: Revised: Accepted: Published: A

July 26, 2018 November 2, 2018 November 7, 2018 November 7, 2018 DOI: 10.1021/acs.iecr.8b03468 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research LiCoO2-based system.5 To improve both conductivities, a reduction in the particle size and the formation of a surface conducting coating such as carbon have been proven to work.8,15 Other effective means to improve the conductivity is by producing LiMPO4 in various forms inclusive of nanosheets, nanoflakes, and nanorods via high-pressure high temperature exfoliation, chemical etching, and a supercritical system, respectively.19−21 A reduction in the particle size (such as nanosized primary particles organized into a micron-sized aggregate) shortens the Li+ diffusion pathway, thus allowing an increase in the overall conductivity.18 Reducing the particle size also helps to improve the cycle life of the batteries because of the fact that smaller particles could better accommodate volume changes during charging−discharging cycles. The particle size, however, was recommended by Garcher et al. in 2013 to be between 40 and 50 nm, at least in the case of LiFePO4. With particles of 700 °C, as previously reported by Rahayu et al. for the C-LiFePO4 system.33 Additionally, the near-complete oxidative loss of surface carbon after 1 h of annealing at 800 °C, as

observed in TGA, likely facilitated the event of interparticle contact formation, which, in turn, enhanced the migration of materials, leading to severe densification and the significantly low surface area observed for S800. Such an observation is congruent with the classical sintering theory, where sintering occurred via a bulk transport mechanism, causing materials to migrate from inside the particles to the surface and resulting in contact flattening and densification, specifically when the surface carbon boundary was removed by oxidation.34 Figure 3 shows the diffractograms of S600 and S800. Both diffractograms were matched to orthorhombic LiNiPO4 under

Figure 3. PXRD diffractograms of S600 and S800 matched to the orthorhombic LiNiPO4 phase. The impurity phase (labeled with ●) observed for both S600 and S800 was identified as Li4P2O7, a material known to enhance the rate capability of rechargeable batteries.35 The extra impurity, observed only in S600 (labeled with *), was found to be a carbazite (SiO2) contaminant from the sample preparation (Figures S4 and S5 in Supporting Information).

space group Pnma (No. 62; JCPSD 00-032-0578 and 01-0727846). S400 was mostly amorphous carbon based on the TEM study; thus, no PXRD analysis was performed (see the Supporting Information). The minor phase observed in both S600 and S800 with signals marked with the symbol ● was identified to be tetralithium diphosphate of the formula Li4P2O7 (JCPDS 98024-6859). The presence of Li4P2O7 on the surface of LiFePO4 has previously been reported as beneficial in that it significantly improved the rate capability (i.e., faster charging−discharging) of lithium rechargeable batteries.35 The presence of this phase therefore should be advantageous when the LiNiPO4 produced is studied in an actual rechargeable battery system. By using the Debye−Scherrer equation and using the full width at half-maximum of the strongest diffraction from (311) at a 2θ of 36.3 °, the average crystalline sizes for S600 and S800 were found to be 43 and 47 nm, respectively, using a shape D

DOI: 10.1021/acs.iecr.8b03468 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research factor of 0.94 for the spheroid. These particles are within the size range (40−50 nm) recommended by Garcher et al. in 2013 for a LiFePO4-based rechargeable battery.22 Bigger average crystallites observed for S800 are expected because it was produced at a higher temperature, where the coalescence and densification processes were enhanced. According to Minakshi et al., stringent synthetic conditions are often required in order to obtain single-phase-pure LiNiPO4.36 They found that a post-synthesis sintering at 800 °C for 24 h was necessary for the production of phase-pure LiNiPO4, after an initial solid-state synthesis.37,38 The sintering step led to the densification of crystals and produced micronsized LiNiPO4. From PXRD analyses, it was found that the S600 sample, apart from the carbazite contaminant, was composed of highly crystalline LiNiPO4 with a small amount of Li4P2O7. The S800 sample exhibited similar features with a larger average crystallite size. The single-source-precursor method reported here therefore could produce LiNiPO4 of purity similar to that reported by Minakshi et al. but with the average crystallite size maintained between 40 and 50 nm instead of micron-sized crystals.37,38 3.4. Morphological and Structural Analysis. On the basis of BET and PXRD analyses, S600 was judged to be a better potential cathode material because of its relatively high surface area, high crystallinity, and smaller average crystallite size among the three samples produced. Parts a and b of Figure 4 represent the images taken for S600 and S800 on a field-emission scanning electron microscope under secondary electron imaging (SEI) mode. S600 was

observed to have certain porous characteristics and be voluminous. This is congruent with the relatively high surface area observed for S600. S800 appeared to be more sintered, which explained its significantly lower surface area, which was around ∼2% that of S600. The cause of the lower surface was due to a higher degree of sintering and densification, as discussed in section 3.3. HRTEM analyses of S600 showed highly crystalline nanoparticles as the primary particles interconnected in a submicron aggregate (Figure 5a). Particle size analysis showed that the primary particle size ranged from 10 to 60 nm. This is in line with the average crystallite size of 43 nm estimated using the Scherrer equation. The interconnected network of the primary LiNiPO4 nanocrystals with plenty of grain boundaries was considered to be beneficial because it resembled a LiFePO4 nano-network reported in Nature Materials by Herle et al., where the noncarbonaceous-network grain-boundary-conduction phenomenon allowed an enhancement in the ionic and electronic conductivities of both doped and nondoped LiFePO4.39 In Figure 5b, an amorphous carbon coating covering a group of crystalline primary LiNiPO4 nanoparticles and the grain boundaries between the nanoparticles may be readily observed. Figure 5c shows evidence of an amorphous carbon coating on a LiNiPO4 nanocrystal with the lattice fringes clearly resolved. The thickness of the surface carbon coating was measured as 2−4 nm. While the coating thickness may not be highly uniform, this should not constitute an issue because the homogeneity of the carbon coating was previously reported to be a nondeterminant parameter.5 It is more critical to increase the coverage of LiMPO4 crystals by carbon. The controlled carbonization of alginate in this report helps make the coverage possible. The lattice fringe resolved along the direction of the arrow in Figure 5c is close to 2.94 Å, which may be attributed to the (211) lattice plane with a d spacing of 2.95 Å. However, the crystal image was not in the “in-zone-condition” (i.e., the electron beam being parallel to the zone axis) during imaging; therefore, further analysis is required to confirm the plane being (211) unambiguously. The phase-pure LiNiPO4 produced by Minakshi et al. after a 24-h at 800 °C sintering process, as discussed in section 3.3, appeared under TEM as fused aggregates of highly crystalline LiNiPO4 circa 0.5 μm in size. Because of the thickness (microns thick), lattice fringes were not resolved, but selectedarea electron diffraction was used to confirm the highly crystalline nature of the product.36

4. CONCLUSION A nickel alginate gel was successfully prepared from nickel chloride and sodium alginate. The gel was homogenized with stoichiometric LiOH and H3PO4 to produce a single-source precursor, after the amount of Ni2+ in the gel was determined by finding the number of moles of residue aqueous Ni2+ using UV−Vis spectroscopy. By controlling the calcination temperature of the precursor gel, LiNiPO4 nanocrystals of different sizes, surface areas, and surface carbon coating could be produced. 600 °C was identified as the optimized calcination temperature, which allowed the formation of crystalline LiNiPO4 with an average crystallite size of 43 nm and a surface area of 27.47 m2/g within 1 h. Crystalline LiNiPO4 particles with an average crystallite size of 47 nm were produced at 800 °C with a significantly lower surface area of

Figure 4. SEI images of S600 and S800 taken by a FESEM. (a) At a lower calcination temperature, S600 shows a relatively voluminous morphology, which is congruent with the BET surface area measurement. (b) The coarser primary particles observed for S800 are a reflection of the extended coalescence and sintering at 800 °C. E

DOI: 10.1021/acs.iecr.8b03468 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 5. Bright-field TEM images of S600. (a) Nanosized primary particles agglomerated into a submicron-sized aggregate. (b) An aggregate made of individual nanocrystals with the grain boundaries and surface carbon coating shown. (c) HRTEM image showing the amorphous carbon coating between 2 and 4 nm in thickness and the grain boundary between two LiNiPO4 primary nanocrystals.

0.57 m2/g due to enhanced sintering and densification. The sol−gel method reported herein represents a promising option for the design and preparation of porous LiMPO4. With the capability of controlling the aggregate surface area and surface carbon coating coverage, this method would allow the fabrication of LiMPO4-based cathodes and lithium batteries with improved electrical performance including the favored LiFePO4 in the near future.



the manuscript. T.H.L. and Y.J.T. conceived the plan. Y.J.T. and J.H.B. performed the experiments. Y.J.T., J.H.B., and T.H.L. performed data analysis. T.H.L. and J.C.L. wrote the paper. Notes

The authors declare no competing financial interest.



ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.8b03468. UV−Vis absorption calibration curve of NiCl2 in water (Figure S1), photographs of the samples and singlesource precursor (Figure S2), bright-field TEM images of S400 (sample produced at 400 °C; Figure S3), the impurity phase identified by PXRD as Li4P2O7 (Figure S4), and the impurity phase in S600 identified by PXRD (Figure S5) (PDF)



ACKNOWLEDGMENTS

The authors express their gratitude to the Ministry of Education Malaysia for funding this work through the Fundamental Research Grant Scheme (Grant Number 4F827).



ABBREVIATIONS BET = Brunauer−Emmett−Teller FESEM = field-emission scanning electron microscopy HRTEM = high-resolution transmission electron microscopy PXRD = Powder X-ray diffraction



AUTHOR INFORMATION

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Corresponding Author

*Tel: (+60)3-41450123. E-mail: [email protected]. ORCID

Teck Hock Lim: 0000-0001-9397-5146 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of F

DOI: 10.1021/acs.iecr.8b03468 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.iecr.8b03468 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX