Silver-mediated calcium terephthalate with enhanced electronic conductivity as an organic anode for efficient Li-ion batteries

Abstract

Ag particles were selectively added into the organic anode material of calcium terephthalate (CaTP) to improve its electronic conductivity, because Ag metal was the most conductive one and easily accessible from the decomposition of unstable AgNO₃ at certain temperatures tolerated by organics. The reduced size of CaTP particles observed after adding Ag particles could increase the specific surface area and shorten the Li⁺ ion diffusion pathway. Consequently, the resulting CaTP/Ag anode exhibited an enhanced and reversible 92 mA h g⁻¹ capacity under a 240 mA g⁻¹ current density, which was four times higher than that of pristine CaTP under the same conditions. In addition, the cycling performance at 120 mA g⁻¹ current density was also improved in the cycle range of 21^st to 130^th, with a ∼113 mA h g⁻¹ discharge capacity value, which was also higher than that of pristine CaTP (∼26 mA h g⁻¹).

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1. Introduction

In the past two decades, Li-ion batteries have dominated the commercial market for portable electronic devices such as laptops, mobile phones, and camcorders.^1–6 Currently, the electrode materials in Li-ion batteries are mainly focused on inorganic materials (carbons, transition metal oxides, fluorides and intermetallic compounds).^7–11 For example, lithium metal oxides of LiCoO₂ and Li₄Ti₅O₁₂ are widely employed as the cathode and anode materials, respectively. Despite that Li₄Ti₅O₁₂ as the anode material displays good Li-ion intercalation/de-intercalation reversibility and stable structure during long charge–discharge cycling, the pristine Li₄Ti₅O₁₂ exhibits poor lithium storage property at high current density, due to its quite low electronic conductivity (ca. 10⁻¹³ S cm⁻¹).¹² To overcome the issue, one of the useful strategies is to add conductive metals to produce Li₄Ti₅O₁₂–metal composites. As the three most conductive metals, Ag-, Au- and Cu-added Li₄Ti₅O₁₂ composites as the modified anodes have been reported, wherein improved rate performance was achieved.^13–15

Despite the advancements, it is noteworthy that the synthesis of inorganic electrode materials usually involves energy-demanding ceramic processes, as well as a variety of harmful effects to the environment. Nowadays, to answer for the urgent request of sustainable development, it is extremely desirable to develop new species of electrode materials for the next generation Li-ion batteries.^16–20 Expectably, organic electrode materials, which are structurally contrivable, abundant on earth and environmentally benign, have gradually attracted academic attention as promising alternatives.^21–25 As one of the examples, calcium terephthalate (CaTP, CaC₈H₄O₄), which is constructed by the typical metal cation and electroactive terephthalate anion, has been recently reported as a very promising organic anode material by our group.²⁶ The electrochemical process of the terephthalate anion is shown in Scheme 1. The terephthalate anion has two conjugated carboxylate groups in the para-position of benzene backbone, which has the capability to gain two electrons in the charge process and then neutralizes with two coming Li⁺ ions and vice versa. Consequently, the theoretical specific capacity of CaTP is calculated to be 263 mA h g⁻¹.

Scheme 1 The electrochemical process of the terephthalate anion.

However, CaTP inherently possesses poor electronic conductivity like other organic materials, which limits its rate performance and cycling stability.²⁴ In our previous study, we added conductive graphite into CaTP and meanwhile reduced the particle size by ball-milling. Satisfactory results were achieved.²⁷ Currently, we notice that few studies about the addition of conductive metals into organic electrode materials were reported. Herein, with the main intention to enhance the CaTP electronic conductivity along with inspiration from the metal-added cases for inorganic electrodes, we report a modified CaTP anode by taking the method of adding Ag particles as the electro-conductive additive. The rational selection of Ag particles was based on that Ag possesses slightly better electronic conductivity (6.14 × 10⁵ S cm⁻¹) than Cu and Au (5.80 and 4.44 × 10⁵ S cm⁻¹), respectively.²⁸ Moreover, the required calcination temperature for obtaining Ag particles, which always originates from the thermal decomposition of silver nitrate (AgNO₃) for its chemical unstability, is relatively lower than the cases of generating Cu and Au particles.^13–15 The benefit of low sintering temperature is to decrease the decomposition risk of CaTP, because organic frameworks are usually more thermally fragile than their inorganic counterparts.²⁹ Moreover, unlike the cases using graphite as the additive, wherein the electroactive graphite with the amount more than 10 wt% will greatly reduce volumetric energy density,^27,30 the Ag particles have no obvious impact on the volumetric energy density for its high density. Indeed, with the addition of Ag particles, the charge transfer resistance of the as-prepared CaTP/Ag composites was reduced compared to pristine CaTP. Furthermore, with the optimized 5 wt% additive of AgNO₃, the resulting CaTP/Ag-5 composite displayed the best 92 mA h g⁻¹ discharge capacity at the high current density of 240 mA g⁻¹, which was four times higher than the pristine CaTP under the same conditions.

2. Experimental section

2.1 Synthesis of CaTP/Ag-x composites

CaTP was obtained by the same procedures reported by our group.²⁹ Subsequently, the AgNO₃ aqueous solution (2, 5, 10 wt% amount to CaTP, respectively) was added to the CaTP suspension (500 mg CaTP in 75 mL H₂O). The resulting mixture was stirred for 1 h and then dried out with constantly stirring. The obtained powders were sent to an agate mortar for grinding. Finally, the ground powders were sintered in a tube furnace under an argon atmosphere at 450 °C within 1 h for reducing Ag⁺ to metallic Ag. The as-prepared composite powders are named as CaTP/Ag-x (x = 2, 5, 10). In the following main text, CaTP/Ag-0 denotes the calcined CaTP without AgNO₃, while CaTP means the pristine one with grinding but without calcining treatment.

2.2 Materials characterizations

X-ray powder diffraction (XRD) patterns of CaTP/Ag-x were obtained by using Cu K_α radiation (λ = 1.5405 Å, X'Pert Pro MPD) with a 0.06° s⁻¹ scanning rate in the 5–60° 2θ range. Fourier transform infrared spectroscopy (FT-IR, Shimadzu, IR Prestige-21) analysis was operated in the wavenumber range of 400–2000 cm⁻¹. Morphology characterization was carried out by field-emission scanning electron microscopy (FE-SEM) using a Hitachi S3400N instrument. The element distribution analysis was determined by an energy dispersive X-ray spectroscopy instrument (EDX, Oxford INCA PentaFET-x3).

2.3 Cell assembly and electrochemical measurements

The two-electrode half cells were assembled in an argon-filled glove box. Similar to our previous study, the working electrode is prepared by 60 wt% active material, 30 wt% acetylene black and 10 wt% polyvinylidene fluoride (PVDF).^26,27,29,31 All the constituents were uniformly mixed in N-methyl-2-pyrrolidone (NMP) solvent and pasted onto a thin copper foil. The counter electrode was a lithium metal foil. The separator was a polypropylene membrane. The electrolyte was 1 M LiPF₆ solution and the related solvents were mixed by a set volume ratio (1 : 1 : 1) of ethylene carbonate (EC), diethyl carbonate (DEC) and dimethyl carbonate (DMC).

The galvanostatic discharge–charge tests were operated by a CT2001A cell test instrument (LAND Electronic Co.) at room temperature. The cyclic voltammograms (CVs) were collected from 0.05 to 2.5 V using a Solartron SI1287 instrument. The electrochemical impedance spectra (EIS) were obtained after the CVs by a Solartron Analytical apparatus in the frequency range of 10⁻¹ to 10⁵ Hz, where R_s represents the resistance of the electrolyte; R_ct is the charge transfer resistance; CPE is the constant phase elements; and W_o is the Warburg impedance.

3. Results and discussion

3.1 Synthesis and characterization

The CaTP/Ag composites were easily obtained as follows: CaTP and AgNO₃ were physically mixed with water and then the resulting solid mixture was ground and sintered under an argon atmosphere to form the final products by the decomposition of AgNO₃ (see details in the Experimental section). Noticeably, the temperature for sintering was 450 °C, lower than the critical temperatures for the thermally-reduced Cu and Au particles (600 and 550 °C) reported in the inorganic cases^13,15 and also lower than the decomposition temperature (T_d ∼ 600 °C) of CaTP.²⁹

To confirm the integrity of CaTP after calcination, the compositions of the as-prepared mixtures were characterized by FT-IR and XRD. Fig. 1a displays the FT-IR spectra. All the CaTP/Ag-2, -5, and -10 composites exhibited two distinctive C–O vibration bands at 1364 and 1605 cm⁻¹ from ν_as(COO⁻) and ν_s(COO⁻), respectively, which were similar to the C–O vibration bands of CaTP/Ag-0, CaTP and Li₂TP (Fig. 1a). Moreover, the O–Li vibration band (534 cm⁻¹) disappeared for all the CaTP/Ag-0, -2, -5, and -10 composites, whereas the new O–Ca vibration bands appeared at the wavenumbers of 505 and 530 cm⁻¹. The XRD patterns of the CaTP/Ag-0, -2, -5, and -10 composites as well as the pristine CaTP and metallic Ag (from PDF card 87-0717) are shown in Fig. 1b. There was no diffraction signal originating from AgNO₃, indicating that the thermal decomposition was completed. At the same time, it was easy to observe that all the CaTP/Ag composites displayed nearly the same diffraction patterns as the pristine CaTP, which suggested that the physical mixture of Ag particles could not change the crystal structure of CaTP. However, it was noticeable that the diffraction intensity of the characteristic peaks (2θ = 9.3° and 18.8°) from pristine CaTP gradually decreased when the Ag particles were gradually added, which might illustrate that the additives changed the preferred orientation of the CaTP lattice plane. On the other hand, it was obvious that the intensity of the characteristic peaks (2θ = 38.1° and 44.3°) from Ag metal gradually increased with the augmented amount of AgNO₃. Therefore, the abovementioned phenomena confirmed the integrity of CaTP after the thermal treatment and the co-existence of Ag particles.

Fig. 1 (a) FT-IR spectra of CaTP/Ag-0, -2, -5, -10, CaTP and Li₂TP; (b) XRD patterns of CaTP/Ag-0, -2, -5, and -10 (pristine CaTP and metallic Ag are depicted for comparison).

The morphology of the composites (CaTP/Ag-0, -2, -5, and -10) prepared after thermal treatment was observed by SEM. As shown in Fig. 2a, without the addition of Ag, the CaTP/Ag-0 exhibited wide distribution of particle sizes, ranging from micrometer to nanometer scales. However, when the content of AgNO₃ precursor started to increase, the particle size of the resulting composites became smaller and decreased in the order of CaTP/Ag-2 (Fig. 2b) > CaTP/Ag-5 (Fig. 2c) = CaTP/Ag-10 (Fig. 2d). The CaTP/Ag-10 composite exhibited the most uniform particle size dispersion. The apparent particle size reduction was probably ascribed to the thermal decomposition of NO₃⁻ ions. As a result, the augmented surface area without further ball-milling was advantageous for providing better contact with the electrolyte and offering more reactive sites, which assisted the Li⁺ ion transfer between electrolyte and CaTP.

Fig. 2 SEM images of (a) CaTP/Ag-0; (b) CaTP/Ag-2; (c) CaTP/Ag-5; and (d) CaTP/Ag-10 under the same treatments (scale bar is 5 μm in the I-series and 1 μm in the II-series).

Furthermore, to identify the element distribution of the calcined composites, the CaTP/Ag-5 composite was selected to map the distribution of its C, Ag and Ca elements by energy dispersive X-ray spectroscopy (EDX). As depicted in Fig. 3, the Ag element was well distributed and consistent with the distribution of C and Ca elements. The distribution uniformity of the Ag element on CaTP was supposed to be good for electron transportation.

Fig. 3 EDX mapping images for C, Ag and Ca elements in the CaTP/Ag-5 composite.

3.2 Electrochemical performance

The galvanostatic discharge–charge properties and the rate performance of the half cells based on CaTP/Ag-2, -5, and -10 composites are displayed in Fig. 4. For comparison, the half-cell based on pristine CaTP is also shown. As shown in Fig. 4a, the rate performance of all the half-cells based on CaTP/Ag composites was enhanced when compared to pristine CaTP, because CaTP is a well-known insulating material and electrons cannot be easily transported during the electrochemical process. However, the CaTP/Ag composites with Ag assistance could accelerate the reaction kinetics. Indeed, under different current densities, all the three half-cells of CaTP/Ag-2, -5, and -10 displayed superior performance compared to the pristine CaTP. For example, under a low 15 mA g⁻¹ current density the discharge capacities were 138, 153 and 130 mA h g⁻¹ for CaTP/Ag-2, -5 and -10, respectively, higher than 125 mA h g⁻¹ of pristine CaTP (Fig. 4b). Moreover, at the high 240 mA g⁻¹ current density, these values (31, 92 and 73 mA h g⁻¹) were still better than pristine CaTP (20 mA h g⁻¹) (Fig. 4c). At the same time, the discharge capacity of pristine CaTP was quickly reduced to 75 mA h g⁻¹ when the current density was back to 15 mA g⁻¹ at the 19^th cycle, while these data remained to be 120, 155 and 130 mA h g⁻¹ for CaTP/Ag-2, -5 and -10, respectively, which was probably due to the suppressed polarization with the Ag presence. Regretfully, although the aforementioned trend was clearly positive, we have to admit that the currently-obtained data are behind the theoretical specific capacity of CaTP even after several repeated experiments, which was probably due to the newly-employed electrolyte system.^8,26

Fig. 4 (a) The rate performance of CaTP/Ag-2, -5, and -10; the selected charge–discharge curves of CaTP/Ag-2, -5, -10 at different current densities of (b) 15 mA g⁻¹, (c) 240 mA g⁻¹ and (d) 120 mA g⁻¹.

On the other hand, it is notable that CaTP/Ag-5 displayed the best rate performance among the three composites (CaTP/Ag-2, -5, and -10). For example, CaTP/Ag-5 exhibited a stable discharge capacity (average 114 mA h g⁻¹) during the cycle range of 20–130^th under a 120 mA g⁻¹ current density, while the average data were 38 and 88 mA h g⁻¹ for CaTP/Ag-2 and CaTP/Ag-10, respectively (Fig. 4d). It seems that more Ag content in CaTP/Ag-10 does not bring in more advancement. How could this be? We notice that Ag metal is a good conductor for electrons but not good for Li⁺ ion transportation. Therefore, it was suspected that the existence of more Ag particles could probably surround around CaTP and then hamper the Li⁺ ion transfer. To confirm our hypothesis, the Li⁺ ion diffusion kinetic information of CaTP/Ag-5 and CaTP/Ag-10 was checked in the following CV experiments, as well as CaTP/Ag-0 for comparison. The related Li⁺ ion diffusion data (D_R) could be estimated according to the equation as follows:³²

i_p = 269An^3/2D_R^1/2C^*_Rv^1/2,

where i_p is the peak current, A is the electrode area, n is the number of transfer electrons, D_R is the Li⁺ ion diffusion coefficient, C^*_R is the concentration of the reactant and v is the variable scan rate. The apparent Li-ion diffusion coefficient (D_R) was proportional to the slope of the linear evolution of the peak current (i_p) versus the square root of the scanning rate (v^1/2), if the other variables (A, n and C^*_R) remain the same. Indeed, after collecting the CV data (Fig. 5a–c) under the same conditions and plotting the resulting curves in Fig. 5d, the as-obtained slope is in the order of CaTP/Ag-5 > CaTP/Ag-10 > CaTP/Ag-0 and consequently signifies the apparent D_R value in CaTP/Ag-5 is bigger than CaTP/Ag-10, verifying the faster Li⁺ ion transfer in the CaTP/Ag-5 composite.

Fig. 5 CV curves of CaTP/Ag-0 (a), -5 (b) and -10 (c) at different scan rates; (d) plots of peak current density (i_p) versus square root of scanning rate (v^1/2) obtained in the CV experiments.

In a further step, the whole resistance of the half cells based on CaTP/Ag-0, -5 and -10 was investigated by electrochemical impedance spectroscopy (EIS). As depicted in Fig. 6, they are composed of a depressed-semicircle ranging from high to intermediate frequency, as well as a linear part at low frequencies. It is straightforward that the resistance values of the electrolyte (R_s in Table 1) for CaTP/Ag-5 (3.3 Ω) and CaTP/Ag-10 (3.6 Ω) composites were similar to the CaTP/Ag-0 (6.0 Ω), but the charge transfer resistance (R_ct) in CaTP/Ag-5 (186.5 Ω) and CaTP/Ag-10 (297.6 Ω) composites exhibited magnificently lower values than CaTP/Ag-0 (492.1 Ω), which was well coincided with the abovementioned results that the CaTP/Ag composites were better for acceleration of the electrochemical reaction. Especially, the lowest R_ct data (186.5 Ω) in CaTP/Ag-5 truly highlights that the more balanced transportation between electrons and Li⁺ ions is more favourable to obtain high rate performance, which agrees well with the phenomena reported for the surface-coated Li₂TP and Li₄Ti₅O₁₂.^31,33

Fig. 6 EIS plots for CaTP/Ag-0, -5 and -10 (inset is equivalent circuit from impedance spectra by Zview software).

Table 1 Impedance parameters fit by the equivalent circuit model

Composites	Rs^a (Ω)	Rct^b (Ω)
a Rs represents the solution resistance.b Rct is the charge transfer resistance.
CaTP/Ag-0	6.0	492.1
CaTP/Ag-5	3.3	186.5
CaTP/Ag-10	3.6	297.6

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4. Conclusions

In summary, we initially and rationally added Ag particles into organic calcium terephthalate to improve its electronic conductivity. The expected better rate performance was successfully achieved. Our study unveiled that AgNO₃ is an effective and easily-available precursor for introducing electro-conductive metal into organic electrode materials to achieve better electronic conductivity.

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

This study was supported by the Startup Grant of UESTC (No. ZYGX2015KYQD058); the National Science Foundation of China (201073029, 11234013, 21473022); the Science and Technology Bureau of Sichuan Province of China (no. 2015HH0033); and Fundamental Research Funds for the Central Universities.

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