Rate-capability response of graphite anode materials in advanced energy storage systems: a structural comparison

Abstract

The work presented in this report was a detailed comparative study of the electrochemical response exhibited by graphite anodes in Li-ion batteries having different physical features. A comprehensive morphological and physical characterization was carried out for these graphite samples via X-ray diffraction and scanning electron microscopy. Later, the electrochemical performance was analyzed using galvanostatic charge/discharge testing and the galvanostatic intermittent titration technique for these graphite samples as negative electrode materials in battery operation. The results demonstrated that a material having a higher crystalline order exhibits enhanced electrochemical properties when evaluated in terms of rate-capability performance. All these materials were investigated at high C-rates ranging from 0.1C up to 10C. Such improved response was attributed to the crystalline morphology providing short layers, which facilitate rapid Li⁺ ions diffusivity and electron transport during the course of battery operation. The values obtained for the electrical conductivity of these graphite anodes support this possible explanation.

1. Introduction

At present, the total amount of human power consumption is around 14 terawatts and that is expected to increase up to three times by 2050 [1]. Oil is currently the major source of energy that is accountable for CO2 emission, and it is considered to be a major cause of geopolitical instability. Since oil is the main fuel used to run automotive applications, a necessary transition to electrified transportation systems is of utmost urgency [2]. Hybrid electric vehicles have been commercialized that are accelerating as plug-in hybrid vehicles, and finally, electric vehicles are on the verge of entering their commercialization phase. The greatest challenges to the secure establishment of the electrified automobile industry are insufficient storage capacity and the low power density of current battery systems [3-4]. Liion batteries (LIBs) are currently used in portable electronic devices because they offer high energy density for application in small mobile devices [5-7]. However, they are difficult to use in electric vehicles because they need better energy storage systems with high rate-capability. To address the increased energy demand for LIBs with higher power density, intensive research has been carried out to design novel and efficient electrode materials structures [8]. Efforts have been made to introduce transition metal oxides, silicon, and several other materials as anodes for high power LIBs, but the electrochemical results have revealed no major success [9-13].

Carbon-based materials have already received much attention as negative electrode materials for LIBs because of their excellent features, such as low cost as well as chemical and thermal stability [14-16]. Many carbon based materials, such as hard carbon, carbon nanotubes, carbon composites, and disordered carbons have been widely studied as anode materials [17-20]. However, various explanations have been suggested for the storage mechanism of lithium ions for various samples. Among all carbonaceous materials, graphite has shown the greatest potential because of its stable and favorable physical structure [21-22]. In this work, a comprehensive comparative study was conducted to examine the effects of various physical properties on the electrochemical performance of graphitic anodes. This study revealed that the physical features of graphitic anodes have a definite role in Li+ ion diffusion and the high-rate capability performance of LIBs.

 

2. Experimental

Various kinds of artificial graphite samples were received from three different companies (A, B, and C). These samples were named A1, B1, B2, C1, and C2 for later discussion in this report. First, physical characterization was performed via field emission scanning electron microscopy (Hitachi FE-SEM S4800; Hitachi, Tokyo, Japan). X-ray diffraction (XRD) was performed on an X’pert Philips PMD (Philips) with a panalytical X’celerator detector (Panalytical, Almelo, The Netherlands) using graphite monochromized Cu Kα radiation (wavelength = 1.54056 Å). To investigate electrochemical charge/discharge performance, working electrodes were prepared by respectively mixing the various graphite sample powders, carbon black (Super P, M.M.M. Carbon, Brussels, Belgium), and binder (polyvinylidene fluoride) in the ratio of 90:5:5. Then, each mixture was compressed onto a copper foil and dried at 100℃ for 1 h. Lithium foil (purity 99.9%) was used as a counter electrode, polypropylene membrane separator (Celgard 2400; Celgard, Charlotte, NC, USA) and 1.2 MLiPF6 in EC/EMC (1:1 v/v) + 2 wt% VC as the electrolyte. Coin cells were assembled in a dry room at room temperature. Rate capability experiments were performed using a multi-channel battery tester (Toscat-3100U; Toyo System, Fukushima, Japan). The potential range was kept between 0.005 to 1.5 V, and the current density was varied between 0.1C to 10C in the experiments. The electronic conductivity of these samples was measured using impedance spectroscopy. The galvanostatic intermittent titration technique (GITT) was used to calculate the diffusion coefficient. Values were recorded after 2 cycles at 0.5C, and a constant current pulse was provided for 5 min with a rest time of 4 h at 20% depth of discharge (DOD).

 

3. Results and Discussion

XRD is a fundamental characterization technique that has been established for the determination of structural parameters. The XRD pattern presented in Fig. 1 shows that all the reflections of 002, 100, 004, and 110 can be indexed to graphite in all of the five samples [23]. The XRD patterns of these graphite samples show a common diffraction peak, which is attributed to disordered carbon structure. Crystallite size was calculated based on Sherrer’s equation:

Fig. 1X-ray diffraction patterns of various artificial graphite samples.

where K is a dimensionless shape factor (typical value = 0.9), λ is the X-ray wavelength, β is the line broadening at full width at half maximum intensity (FWHM), and θ is the Bragg angle. In Fig. 2a the inter-layer spacing (d-spacingd002) values of various samples are presented. Sample B1 had the highest value of d002, while sample C2 exhibited the lowest values. The lateral size (La) and the stacking height (Lc) were also calculated by Sherrer’s equation, and the values obtained are presented in Fig. 2b. The highest values of La and Lc were recorded for the C2 sample.

Fig. 2(a) d002 values for all graphite materials and (b) La, Lc, and Lc/La values calculated by Sherrer’s equations. La, lateral size; Lc, stacking height.

Fig. 3a compares the particle size distributions of various graphite materials. The average particle size for most of the samples was calculated to be around 20 μm. Brunauer, Emmett, and Teller (BET) theory was applied to measure the surface area of all samples. The results in Fig. 3b show that sample C2 had a larger surface area, which is likely to have a more positive impact on the electrochemical performance of the material as an anode since there would be a larger reaction area. SEM images were taken to investigate other physical features of the microstructures. Fig. 4 presents SEM images of all the graphite samples. Samples C1 and C2 have relatively non-uniform particle sizes and elliptic or plate-like shapes, while samples of the B1 type have uniform particle distribution among all samples. Differences in size and shape of these graphite samples are related to the manufacturing processes employed by the companies that produced them.

Fig. 3(a) Particle size analysis of graphite materials, (b) specific surface area calculated by Brunauer, Emmett, and Teller (BET) theory for all graphite materials.

Fig. 4Scanning electron microscopy micrographs for all graphite samples.

Fig. 5 presents the conductivity features of particles that showed the lowest resistance values for C2. All the obtained values for these samples presented in Fig. 5 suggest that electrical conductivity is inversely proportional to the d002 spacing of graphite materials [24]. Higher electronic conductivity is one of the key characteristics of high-performance battery systems in terms of rate capability and power density [25, 26].

Fig. 5Electrical conductivity of graphite samples as function of pressure.

The GITT is a useful tool to obtain information about the diffusion coefficient of electrode materials during delithiation. To calculate the diffusion coefficients of the samples, the following equation was used:

here τ is the constant current pulse time (s), mB is the mass of the insertion electrode material (g), VM is the molar volume of the electrode material (cm3 mol−1), MB is the molar mass of the insertion electrode material (g mol−1), S is the area of the electrodeelectrolyte interface (cm2), ΔEs is the change of the steady-state voltage during a single-step GITT experiment (V), and ΔEt is the total change of cell voltage during a constant current pulse τ of a single-step GITT experiment neglecting the IR-drop (V). Table 1 show diffusion coefficient values obtained through careful calculations for these samples. In Fig. 6, diffusion coefficient results are presented for all the samples calculated under the same experimental conditions. These calculations were carried out during delithiation when the DOD was 20%. The results clearly demonstrate that the C2 sample had the highest diffusion coefficient values during delithiation because this material has low specific resistance and high electronic conductivity.

Table 1GITT, galvanostatic intermittent titration technique.

Fig. 6(a) Potential vs. time and (b) diffusion coefficient values for all the graphite samples during delithiation at depth of discharge 20%.

To investigate the electrochemical response of these materials, rate-capability testing was performed at room temperature. These samples were tested at variable C-rates ranging from 0.1C to 10C. The results presented in Fig. 7 reveal that the best performance was achieved by the C2 sample. The capacity shown by the sample at 0.1C was considered standard, and it was compared with the capacities observed at 3C, 5C, and 10C. The C2 sample showed remarkably good electrochemical performance in terms of high rate-capability. The capacity recorded for this sample at 0.1C was around 360 mAh·g−1 (consider to be 100%), and it decreased to 89.45% at 3C, 65.23% at 5C, and 28.10% at 10C. This high electrochemical performance can be attributed to the high diffusion coefficient and electrical conductivity of the material that was observed during physical characterization. This high crystalline order of the material enhanced its electronic and ionic conductivity, which in turn improved the rate-capability of material. These high values facilitated the Li-ions to intercalate during the course of battery cycling, which enhanced the performance of battery.

Fig. 7(a) Rate capability of graphite samples and (b) delithiation profiles of C2 at various C-rates.

 

4. Conclusions

To conclude this work, a comprehensive comparative study among the physical properties and electrochemical responses of five different graphite anodes was performed. The C2 sample, which was more crystalline in nature, as proven by XRD analysis, exhibited enhanced electrochemical performance in terms of high rate capability. The strong performance shown by this particular material can be attributed to the high diffusion coefficient that was measured by GITT. Moreover, improved ionic and electronic conductivity was observed in this sample, which played a key role in enhancing electrochemical performance. Improved crystallinity and suitable morphology of a material are the main characteristics that can provide high rate capability in a battery system.