Tailored Solution Combustion Synthesis of High Performance

Article

Tailored Solution Combustion Synthesis of High Performance ZnCoO Anode Materials for Lithium-ion Batteries 2

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Ryan A Adams, Vilas G. Pol, and Arvind Varma Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 01 Jun 2017 Downloaded from http://pubs.acs.org on June 6, 2017

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Industrial & Engineering Chemistry Research

Tailored Solution Combustion Synthesis of High Performance ZnCo2O4 Anode Materials for Lithiumion Batteries Ryan A. Adams, Vilas G. Pol and Arvind Varma* School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907, USA *Corresponding Author E-mail: [email protected] Telephone: (765) 494-8484; Fax: (765) 494-0805

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Abstract: A promising Li-ion battery anode material, nanostructured ZnCo2O4 spinel, is tailored by solution combustion synthesis to explore how reaction conditions can be tuned to enhance electrochemical performance. A strategy of using glycine and citric acid as fuels, ammonium nitrate as gas generating agent, and optimized fuel to oxidizer ratio results in a mild volume combustion mode, with significant weight loss by gas evolution occurring during calcination to mitigate particle sintering. This yields a mesoporous product structure with a tap density of 1.48 g cm-3, which accommodates volumetric changes during lithiation, resulting in a high stable capacity of 1000 mAh g-1 (C/2 rate) and 950 mAh g-1 (1C rate) at 22°C after initial formation cycles. This study demonstrates that with the use of mixed fuels, gas generating additives, and appropriate fuel to oxidizer ratio, ZnCo2O4 material synthesized by the efficient, one-step solution combustion synthesis method can be tuned to provide excellent electrochemical performance.

Keywords: solution combustion synthesis, Li-ion battery, ZnCo2O4 anode, galvanostatic cycling

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1. Introduction With increasing demand of lithium ion batteries (LIBs) for high performance energy storage applications, such as portable electronics, stationary storage, electric vehicles (EVs) and hybrid electric vehicles (HEVs), alternative electrode materials are being investigated to overcome the inherent capacity, cycling, and safety limitations of the traditional graphite/LiCoO2 chemistry.1–4 On the anode side, nanostructured transition metal oxides have shown promise due to reversible conversion reactions, where Li+ ions react with metal oxides, reducing them to metals and delivering higher capacities.5,6 To mitigate sluggish Li+ diffusion and volumetric expansion from Li alloying, nanostructures are required for stable electrode performance.7 Among the investigated materials, cobalt based oxides have a high theoretical capacity of 890 mAh g-1 and have shown stable cycling.8–10 The toxicity and high cost of cobalt, however, has led to substitution of the metal with cheaper and environmentally friendly metals. ZnCo2O4 (900 mAh g-1) is a ternary cobalt-based material isostructural to the Co3O4 spinel and has recently been investigated as an alternative. The advantages of improved stability and capacity have been demonstrated for the material in a wide variety of nanostructures such as nanoparticles, nanorods, nanosheets and micro cubes synthesized by different methods (sol gel, hydrothermal, electrospinning, etc.).11–15 A combination of micron sized secondary structures composed of nanoparticles exhibits higher tap density while containing channels that accommodate volumetric expansion and Li+ diffusion.14 This work focuses on the solution combustion synthesis method, exploring the reaction mechanism and introducing an optimization strategy that can be extended to other electrode materials for lithium and sodium ion batteries. Solution combustion synthesis (SCS) is a facile and versatile technique used primarily to produce nanostructured metal oxides, in addition to metal alloys, sulfides, phosphates, silicates, 3



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and borates.16–19 It developed from the solid state combustion synthesis technique, where combining oxidizer and fuel with application of some heat ignites the mixture.20 It allows for overall low energy input as the exothermic reaction leads to self-sustained synthesis of the product.19 In SCS, the precursors are dissolved in a solvent (typically water) to ensure molecular level homogenization and produce a uniform product. SCS typically utilizes metal nitrates as oxidizer, along with fuels such as glycine, urea, and citric acid. The SCS products can be tailored by adjusting the fuel/oxidizer ratio, type of fuel, adding gas generating agents, and salt as a template, etc.16–19,21 Due to the generation of gases such as CO2, N2, and H2O during combustion, high surface area and porous nanomaterials can be formed and tuned by varying reaction conditions. Combinations of fuels can lead to improvements such as smaller particle size and moderate surface areas, and may alter the reaction mechanism and combustion temperature.22–24 Due to its one step process that requires minimal energy/time input, possible scalability has been highlighted for this synthesis method with a continuous reactor design proposed.18 For LIBs, SCS has been used to synthesize both cathode and anode materials with promising performances. Typically, the synthesis uses a single fuel with a specific fuel/oxidizer ratio.25–31 Materials such as LiFePO4, LiCoO2, LiMn2O4, Mn3O4, etc., have been synthesized by this and related techniques, utilizing glycine, urea, or sucrose as the fuel.25,26,29,30,32–34 We are aware of two prior studies, where ZnCo2O4 was produced by SCS, using either urea (fuel/oxidizer ratio = 2) or graphene additive with urea (fuel/oxidizer ratio = 5/3).27,31 In these studies, exploration of the parameter space of SCS and development of the relation between synthesis conditions, material properties and electrochemical performance of produced electrode was limited. To our knowledge, this is the first study that explores the combination of fuels and ammonium nitrate as gas generating agent for SCS of electrode materials. The reaction characteristics were monitored 4


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by measuring temperature-time profiles during SCS and conducting thermogravimetric analyses and differential scanning calorimetry to understand reaction mechanisms. The material properties were analyzed by characterizing crystallinity, surface area and morphology. With the optimized mixture of glycine, citric acid and ammonium nitrate, the mesoporous ZnCo2O4 offers excellent lithiation storage and rate capabilities, with the advantage of a facile and energy efficient process. The systematic exploration of the reaction enabled significant improvements in electrochemical performance over prior studies, demonstrating the advantage of using SCS for advanced electrodes. 2. Experimental Section 2.1. Chemicals Metal nitrates (MN), Zn(NO3)2·6H2O (98%) and Co(NO3)2·6H2O (>98%) from Sigma Aldrich, were used as metal precursors in all reactions without further treatment. Glycine (G, 99.5%), citric acid monohydrate (CA, 99.5%) and ammonium nitrate (AN, >95%) were supplied by Alfa Aesar. 2.2. Materials Synthesis For a typical synthesis, the metal precursors Zn(NO3)2·6H2O and Co(NO3)2·6H2O were measured in a 1:2 molar ratio with 2 mmol of Zn(NO3)2·6H2O and 4 mmol of Co(NO3)2·6H2O as the basis. For each fuel type, the amount was determined by the fuel to oxidizer ratio (Φ), where the combustion equations are given in the Results section. For systems containing ammonium nitrate or multiple fuels, the method of Jain et al. was used to define the molar ratio of reducing elements to oxidizing elements (Ψ).35 This value enables comparisons between the various systems and the corresponding equation is given in the Results section. For ease of comparison in cases where ammonium nitrate was used, the molar ratio of ammonium nitrate to metal nitrate 5



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(AN/MN) was also defined. The precursors were dissolved in deionized water with continuous stirring until a clear transparent solution was obtained. The solution was heated ~5°C min-1 on a hot plate until boiling began. Excess water was evaporated off until the homogeneous solution became viscous and gel-like. The temperature continued to increase until ignition, which occurred at ~150°C depending on the reaction conditions. Temperature of the system over the reaction period was measured by a type S thermocouple (0.150 mm diameter) at 100 Hz sampling rate. Depending on the reaction conditions, the gel either ignited with a flame that propagated in a front across the bottom of the vessel (self-propagating synthesis), or the gel uniformly ignited (volume synthesis) and emitted large volumes of gas.21 Once ignition occurred, the reaction vessel was removed from the hot plate and allowed to cool. The product powder was transferred into alumina crucibles and placed in a high temperature tube furnace (MTI OTF-1200X). The samples were calcined in air flow at 400°C or 600°C for 5 hours with heating and cooling rates of 10°C min-1 to form fine black powders. 2.3. Materials Characterization Powder x-ray diffraction was utilized to characterize the crystalline phase via the BraggBrentano method (Rigaku SmartLab X-Ray Diffractometer). A Cu−Kα X-ray source (λ = 0.154 184 nm) was used to obtain the X-ray diffraction (XRD) patterns (2θ = 10−80°) of ZnCo2O4 at a scanning rate of 5° min-1. Thermogravimetric analysis (Instrument Specialists Incorporated TGA i-1000) was conducted on precursors and product material prior to calcination to determine reaction mechanisms and phase changes. The range of 25°C to 800°C was used with 10°C min-1 heating rate and 10 cc min-1 of air and 10 cc min-1 of argon flow rates. Differential scanning calorimetry was performed using a Q-2000 instrument in the range of 40°C to 500°C with a 6


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10°C min-1 heating rate. Surface area analysis (Quantachrome NOVA 2200e) was conducted using nitrogen adsorption/desorption isotherm measurements at 77 K. Samples were degassed for 12 hours at 300°C prior to measurements. Multipoint specific surface area calculations were performed using the linear portion (P/P0 = 0.05−0.30) of the Brunauer−Emmett−Teller (BET) model. Pore size and volume distribution curves were generated using Barrett-Joyner-Halenda (BJH) Pore Size and Volume Analysis calculations for the N2 desorption curve. Transmission electron microscope images (TEM and HRTEM) were taken using the FETEM mode of a Titan 80-300 kV Environmental Transmission Electron Microscope. Samples were prepared through dispersion of solid powders in ethanol via bath sonication for 2 minutes. A few drops of the suspension were loaded onto a carbon-coated 400-mesh copper grid (Ted Pella) and dried at room temperature prior to loading into the sample holder. Scanning electron microscopy (FESEM) images of the ZnCo2O4 samples were recorded using a Hitachi S-4800 microscope. Raman spectra were collected with a 532 nm laser utilizing a Thermo Scientific DXR Raman Microscope. 2.4. Electrochemical Characterization Electrodes were prepared by taking a ratio of 70 wt. % active material (ZnCo2O4), 15 wt. % conductive additive (Timcal Super C65), and 15 wt. % polymer binder (Carboxymethyl Cellulose and Styrene-Butadiene Rubber). Using water as the solvent, the slurry was mixed for 30 minutes and coated onto copper foil using a doctor blade. The laminate was dried in a vacuum oven at 80°C for 12 hours, and then 12 mm diameter electrodes were punched out with an active material density of ~1.5 mg cm-2. Electrochemical tests and cycling were performed in a cointype 2032 half-cell with lithium metal as the counter electrode. Celgard 2500 polypropylene was used as separator and 1 M LiPF6 in 1:1:1 mixture of ethylene carbonate/dimethyl carbonate 7



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/diethyl carbonate with 3% added volume fluoroethylene carbonate was utilized as electrolyte. Cells were assembled in a high-purity glovebox (99.998% Argon) with oxygen and water sensors ensuring O2 and H2O concentrations 1 = fuel rich). The citric acid system generally results in more gas production as explained by the stoichiometry and has a less vigorous reaction.37 To calculate Ψ, the ratio of the total valance of the fuels (citric acid and glycine) and oxidizers (Zn(NO3)2·6H2O, Co(NO3)2·6H2O and NH4NO3) were taken, resulting in the following expression, 35

Ψ=

%&∙() (∙(* (∙+, -(.&∙() (∙(* (/0 (∙+, 1&/23 (∙/0 (∙+, -(4&)5 (∙/0 (∙+, -(6&∙(* (∙/0 (∙+, -

(3)

where n, m, x, y, and z represent the moles of citric acid, glycine, Zn(NO3)2∙6H2O, Co(NO3)2∙6H2O and ammonium nitrate, respectively. Similar to the combustion equations, Ψ = 1 is the stoichiometric condition, while Ψ > 1 is fuel rich. The various systems explored are summarized in Table 1, with compositional parameters and calcination temperature described.

Table 1. Systems Investigated Along with Sample Surface Areas and Pore Volumes Ammonium Nitrate (AN/MN)

Ψ (Overall Calcinatio Fuel: n Temp. Oxidizer (°C) Ratio)

BET BJH Pore Surface Volume Area (m2 (cc g-1) g-1)

System

Φ (G)

Φ (CA )

G: Φ = 2

2

-

-

2.31

400

31.9

0.072

G: Φ = 2

2

-

-

2.31

600

22.2

0.058

G: Φ = 3

3

-

-

2.87

400

25.5

0.076

G: Φ = 3

3

-

-

2.87

600

13.0

0.059

G: Φ = 4

4

-

-

3.43

400

23.6

0.038

G: Φ = 4

4

-

-

3.43

600

11.7

0.032

CA

-

4

-

5.41

400

24.9

0.079 9



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G+AN

4

-

0.5

2.50

400

23.5

0.060

G+CA+A N

3

2

0.5

4.30

400

29.0

0.075

Figure 1. TGA and DSC measurements of precursor fuels and metal nitrates. (a) TGA/DSC of glycine. (b) TGA/DSC of citric acid. (c) TGA of Co(NO3)26H2O and Zn(NO3)26H2O with decomposition intermediates shown at corresponding weight % loss and the ignition temperature range. (d) TGA/DSC of ammonium nitrate. To elucidate the reaction mechanism, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed on precursors in air atmosphere. Figure 1a shows TGA/DSC for glycine, where 60% weight loss occurs at 250°C along with an endothermic peak 10


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as the fuel decomposes. In Figure 1b, citric acid undergoes several phase transitions, but the significant weight loss and endothermic peak occur at 220°C after the decomposition product beings to melt.38 The enthalpy of reaction for glycine is an order of magnitude larger than for citric acid (20 W g-1 vs. 2 W g-1), and the sharper reactivity can be attributed to the amine group ( -NH2 > -OH > -COOH).37 The metal nitrate precursors were analyzed by TGA as shown in Figure 1c. Based on the weight %, heat differentials, and prior studies, the decomposition of metal (M) nitrates is proposed as follows: +89 :

+8>:?

MNO 6H O ;

Tailored Solution Combustion Synthesis of High Performance

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