Electrochemical Characterization of High Energy Density Graphite

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Cite This: ACS Appl. Energy Mater. 2018, 1, 4976−4981

Electrochemical Characterization of High Energy Density Graphite Electrodes Made by Freeze-Casting Ruhul Amin,†,§ Benjamin Delattre,†,‡ Antoni P. Tomsia,‡ and Yet-Ming Chiang*,† †

ACS Appl. Energy Mater. 2018.1:4976-4981. Downloaded from pubs.acs.org by DURHAM UNIV on 10/02/18. For personal use only.

Department of Materials Science and Engineering, Massachusetts Institute of Technology (MIT), Cambridge, Massachusetts 02139, United States ‡ Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States § Qatar Environment and Energy Research Institute, Qatar Foundation, Doha 34110, Qatar ABSTRACT: Despite extensive and continuous research to find alternatives, the Li-ion battery community still relies on graphite to manufacture most negative electrodes. Although graphite has many advantages, its relatively low volumetric energy density remains a limiting factor for many heavy-duty applications, such as electric vehicles (EVs) and grid storage. With the aim of increasing cells level energy density and decreasing cost by building thicker electrodes, freeze-casting, a shaping technique able to produce lowtortuosity structures by using ice crystals as a pore-forming agent, is used here to produce porous graphite anodes. The electrochemical performance is assessed using galvanostatic constant current (GCC) charge−discharge as well as hybrid pulse power characterization (HPPC) techniques. The obtained GCC discharge capacities of ∼18, ∼14, and ∼7 mAh/cm2 at C/10, C/5, and 1C, respectively, show a 5-fold enhancement compared with conventional composite electrodes. Finally, our freeze-cast electrodes also meet the pulse power requierements (1C rate pulses) embodied in the HPPC test protocol. KEYWORDS: carbon, anode, electrode, lithium battery, capacity, freeze-cast, interfacial resistance

1. INTRODUCTION Due to the ever-increasing demand for mobile devices and applications, many novel energy storage systems have been developed over the past decades. Although the design of lowcost and lightweight batteries has been successfully achieved for devices such as cellular phones, tablets, and laptops, larger battery systems for high power demanding applications did not storm the market yet, primarily because of their current high cost of production. Batteries for electric vehicles (EVs) fall into this category, requiring additional characteristics: they should ensure fast and short charges/discharges processes that come either from braking events or charging stations. Carbon graphite is today the most commonly used anode material for commercialized LIBs. In addition to its low cost, its advantages include a very low volume expansion coefficient as well as excellent electrochemical stability compared with alternative negative electrodes. The capacity of conventional graphite anodes is nevertheless relatively low (372 mAh/g),1 and current electrode designs cannot efficiently afford the energy density that many emerging applications require (e.g., EVs and grid-level energy storage). Extensive research is currently being carried out to improve the electrochemical energy storage performances of graphite alternatives. This includes other types of carbon-based anodes such as carbon nanotubes,2,3 nanofibers,4 hollow nanospheres,5 graphene,6,7 porous carbon nanofibers,8 and their hybrids. On the other © 2018 American Chemical Society

hand, many metals can form alloys with lithium metal at low potential, resulting in very high specific capacities ranging from 1000 to 4000 mAh/g, as measured for instance for Sn and Si, respectively.9 However, major drawbacks, such as a large volume change during alloying/dealloying with lithium, fast capacity fading, and ultimately unstable electrochemical performances, are still today important limiting factors for the development of alloying electrodes, making them currently not competitive with graphite negative electrodes.9−11 Typical carbon anodes are fabricated by mixing graphite powder with at least 10 vol % binder diluted in an inorganic solvent. The slurry is then cast on a copper foil before being dried and calendered at high pressures to densify the electrode and promote adhesion to the metal current collector. Such technique produces highly tortuous electrodes with a disordered porosity which usually restrains the thicknesses of commercial electrodes to about 100 μm before the ionic transport becomes a limiting factor.12 Recently, several designs13,14 and strategies15,16 have been proposed to reduce electrodes tortuosity in the direction normal to the electrode plane. Bae et al.14 demonstrated that coextruded LiCoO2 cathodes with dual-scale porosity, partitioned between a fineReceived: June 13, 2018 Accepted: July 20, 2018 Published: July 20, 2018 4976

DOI: 10.1021/acsaem.8b00962 ACS Appl. Energy Mater. 2018, 1, 4976−4981

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ACS Applied Energy Materials scale porous matrix and large-scale low-tortuosity channels, could greatly improve electrode kinetics. Billaud et al.15 used a magnetic alignment approach to align graphite flakes functionalized with Fe3O4 nanoparticles. Compared to a reference electrode with an isotropic porosity, the oriented electrodes showed a 3-fold improvement of their specific capacity when cycled at 1C in a lithium half-cell configuration. Sander et al.16 also used a magnetic field to manufacture low-tortuosity electrodes by aligning LiCoO2 particles suspended in an emulsion containing a ferrofluid. These electrodes exhibited more than twice the area capacity of a reference electrode with the same porosity in model electric vehicle drive cycles. Overall, these studies demonstrated that a structural alignment and a tortuosity reduction greatly improved electrochemical performance. In this work, we report on the development of unsintered graphite anodes made by freeze-casting, a recent shaping technique which allows producing anisotropic, interconnected and highly aligned porous structure, using ice crystals as a pore-forming agent.17,18 To the best of our knowledge, this is the first time the freeze-casting technique has been applied to align the pore structure of a carbon electrode. The technique enabled engineering of thick low-tortuosity electrodes with dual-scale architecture. This specific design led to a dramatic improvement of the volumetric energy density compared with conventional cells. In addition, the as-prepared anodes demonstrated very promising results when cycled galvanostatically and when tested under hybrid pulse power sequences, filling the needs of high energy density anodes and making them excellent candidates for EV duty cycles and applications.

Figure 1. Schematic of electrochemical cell with aligned pore structure of carbon electrode. Because of the large size change of the lithium electrode thickness that occurs during the charge−discharge process, a combination of Whatman glass wool separator (Fisher Scientific, U.K.) and Celgard separator (Charlotte, NC, USA) was used to separate the electrodes, thus preventing the shorting of the cell in the case of dendrite formation. Multiple charging rates, ranging from C/10 to 1C rate, were applied using a VMP3 Biologic SA France Model (Claix, France). The current was applied at constant current mode during charge and discharge. Hybrid pulse power characterization (HPPC) measurements were conducted with half-cells charged at C/5 rate. Cells were then sequentially discharged at 1C rate for 10% of state of charge (SOC), followed by 2C pulse discharges for 30 s. The sequence was repeated after 1 h rest until 90% SOC was reached.

3. RESULTS AND DISCUSSION Freeze-casting was used as a shaping technique to fabricate porous graphite electrodes. As illustrated in Figure 2, the

2. EXPERIMENTAL SECTION 2.1. Slurry Preparation. Graphite powder (density = 1.52 g/cm3; Kureha, Japan) was first planetary-milled for 6 h to decrease the average particle size and avoid settling during the freezing process. The slurry loading was set at 32 vol %, and the dry powder was mixed with distilled water, 2 wt % Darvan CN (R. T. Vanderbilt Co., USA), and 5 wt % sodium carboxymethyl cellulose (Sigma, USA) with a molecular weight of 90 kDa. Finally, the slurry was ball-milled for 20 h. 2.2. Freeze-Casting. The cooling stage, whose details are reported in previous studies,19,20 is composed of a liquid nitrogen tank and a copper rod coiled with heating elements. The slurry is poured into a Teflon mold placed on top of the copper rod. A control unit, linked to the heating elements, is used to control the cooling rate during the solidification process. When completely frozen, the sample is tapped out of the mold and freeze-dried for more than 48 h at −50 °C under a pressure of 0.035 mbar (Freeze-dryer 8, Labconco, USA). 2.3. Characterization of Structural Features. After being cut and polished, the scaffold microstructure was analyzed using fieldemission scanning electron microscopy (SEM; JSM-5700F, JEOL, Japan) at an acceleration voltage of 5 kV, without any additional conductive coating. Structural parameters such as the pore size and the lamellae thickness were determined from SEM images by averaging 50 measurements using ImageJ. 2.4. Electrochemical Test. To be used as working electrodes, the carbon scaffolds were first polished down to thicknesses ranging between 575 and 800 μm and then assembled in lithium half-cells (Swagelok type) with their macroporosity oriented normal to the electrode plane. The schematic of cell assembly is displayed in Figure 1 for better understanding. A lithium metal foil acted as the counter electrode, and a liquid mixture containing 1 M LiPF6 in 1:1 (mol) ethylene carbonate/diethyl carbonate was used as the electrolyte. The carbon anodes were electrically connected to the copper current collector using carbon paste.

Figure 2. Fabrication of porous graphite electrodes. (1) Graphite particles are homogeneously dispersed in a water-based suspension. (2) Because of the temperature gradient, ice crystals grow preferentially vertically, forcing the particles to be expelled and entrapped between the ice lamellae. (3) Ice is removed from the frozen structure by sublimation. As a result, an interconnected, highly aligned porous structure is obtained.

process first consists of freezing unidirectionally a water-based particles suspension (here graphite particles). During the solidification process, ice crystals preferentially grow in the vertical direction, aligned with the temperature gradient, expelling and entrapping the particles between them. Ice is then removed by a sublimation process. Although it can be further consolidated and densified by sintering, the highly aligned, interconnected structure was here used as-is. This versatile technique has already been successfully applied to a wide range of applications, including gas separation devices,21 4977

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ACS Applied Energy Materials scaffolds for tissue engineering,22−24 solid oxide fuel cells electrodes,25,26 composite materials,27,28 supercapacitor,29 oxide anode material,30 and others.17 Owing to the ice growth anisotropy, the freeze-cast electrodes showed a lamellar porous structure with a longrange order parallel to the freezing direction (Figure. 3). We

exhibited reasonable resistance compared with composite electrodes.33 Although the values slightly increased after cycling (approximately from 95 to 100 Ω/cm2), no significant difference was found between the electrodes with the two different thicknesses. The electrochemical measurements were performed using a half-cell configuration (details provided in the Experimental Section). Figure 5a shows the charge−discharge profile of the cell under constant current mode. The cell was galvanostatically charged at C/10 rate and galvanostatically discharged at 1C, C/5, and C/10 rates (the C rate being given by C/n where n is the charge or discharge time in hours). With a value of approximately 14 mAh/cm2, the area-specific capacity showed 5-fold increase compared with composite carbon electrodes.33 At least four cells were measured in order to ascertain the data reproducibility. Representative results are plotted in Figure 5b as the voltage versus the discharge area capacity (mAh/cm2). At C/10 rate, the measured specific capacity is equal to 312 mAh/g, which corresponds to a typical value of ∼18 mAh/cm2 over the voltage window tested (0.05−3.00 V).34 Thus, the electrode yields almost 83% of the active materials capacity at low rates. Unlike the NCA electrodes fabricated using the same freeze-casting technique,20 the decrease in capacity (from ∼18 to ∼14 and to ∼7 mAh/cm2 at C/10, C/5, and 1C, respectively) with increasing C rate appears here to be limited by the electrolyte diffusivity rather than the electronic conductivity of the graphite electrode. Figure 5c shows the discharge profile of the 575 and 800 μm thick electrodes. At 1C rate, no significant difference was observed between the two electrodes. Figure 5d displays the cycling stability as a function of area-specific capacity. It is seen from Figure 5d that the capacity slowly decreases after 40 cycles. However, Coulombic efficiency is near about one. Considering the high electronic conductivity of carbon, this might indicate that the cell capacity is here limited either by the effective lithium ion diffusion in the electrolyte at higher C rates or charge transfer resistance at the electrode/electrolyte interface. It is perceived from Figure 5 that at higher C rate, the limiting factors may either be ionic diffusivity of the electrolyte solution, ionic diffusion through porous electrode, the charge transfer resistance at the electrode/electrolyte interface, or the electronic conductivity of a long porous electrode. It would be more evidence if we can estimate the characteristic times, ts, for ion diffusion, cell voltage polarization due to electronic conductivity, and interfacial charge transfer kinetics at the electrode/electrolyte interface (Figure 4). The cell voltage polarization due to the electronic conductivity of porous electrode is negligible as demonstrated in Figure 6a. The voltage polarization at the porous electrode was calculated using the following equation:

Figure 3. SEM images of a freeze-cast carbon microstructure: crosssections (a, b) perpendicular to and (c, d) parallel with the freezing direction.

note here that the walls also have an intrinsic porosity. This specific dual-scale architecture, with good connectivity and low tortuosity, was recently found to dramatically enhance ion transport and thus electrochemical performances.14−16,20,28,31 In addition to the total porosity, which can be easily and precisely controlled by the slurry loading, the size of structural features can be further tailored by properly controlling the cooling rate. Based on our previous studies on other systems,20,28,32 a cooling rate of 7.5 °C/min was selected here. This resulted in an average wall (lamellae) thickness of 16.62 ± 3.31 μm and an interlamellar distance (pores) of 8.75 ± 1.39 μm. More importantly, only a small amount of binder (around 5 wt %) was necessary to obtain self-standing porous scaffolds that could be handled and cut before being used without any additional thermal treatment. As a result, such a decrease of the amount of inactive material potentially results in an increase of the cell energy density along with the possibility to increase the electrode thickness while preserving sufficient transport properties. Figure 4 shows the impedance spectra of the half-cell assembly for two different electrodes’ thickness (575 and 800 μm). These data revealed a good contact between the carbon anode and the copper current collector. In addition, our cells

1 ΔV = t 2 ρ(1 − p)Cc σ

(1)

where t = thickness of electrode, c = capacity (372 mAh/g), C = cycling rate, P = electrode porosity (0.5), ρ = density of carbon (2.266 g/cm3), and σ = electronic conductivity of porous carbon electrode (333 S cm−1) It is seen from Figure 6a that at 1C rate cell voltage polarization is less than 10 mV for the electrode thickness up to 800 μm. tS for an ion to diffuse throughout one electrode to another via electrolyte solution and through the pore aligned carbon

Figure 4. Normalized impedance spectra of battery cells using freezecast carbon anodes 575 and 800 μm thick. 4978

DOI: 10.1021/acsaem.8b00962 ACS Appl. Energy Mater. 2018, 1, 4976−4981

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Figure 5. (a) Charge−discharge profile of a 800 μm thick freeze-cast carbon electrode. (b) Discharge area-specific capacity of a 800 μm thick freeze-cast carbon electrode. (c) Comparison of the discharge capacity and cell polarization at 1C rate between two freeze-cast anodes of different thicknesses and (d) cycling stability as a function of area-specific capacity at 1C rate.

Figure 6. (a) Cell voltage polarization as a function of electrode thickness at three different C rates and (b) ionic diffusion time through electrolyte solution and electrode as a function of electrode thickness.

electrode soaked by electrolyte solution with the diffusion length, LS, is given by tS =

LS 2 DS

It is interesting to note that the same electrode yielded only around 7 mAh/cm2 under a continuous 1C discharge, while the HPPC sequence allowed delivering up to 14 mAh/cm2 before hitting the lower cutoff voltage. As a result, we believe that for the thick, high area capacity, low-tortuosity electrodes being fabricated in this work, continuous discharge tests underestimate to a large extent the energy that can be delivered in EV-type duty cycles. The electrochemical performance of freeze-cast electrodes could be compared to the ones obtained for conventional electrodes, whose typical thickness is restricted to 100 μm and which deliver area-specific capacities of about 2 mAh/cm2 at best. Although the practical capacity at high C rate achieved here is far away from the theoretical value, it remains significantly higher than the conventional composite electrode. In particular, Figure 5b illustrates that the areaspecific capacity obtained for a continuous discharge at 1C rate is still above 7 mAh/cm2. Overall, the results presented in this work clearly highlight the advantages of electrodes with controlled porosity over conventional composite electrodes and open up the possibility of using such an approach to fabricate thicker electrodes with the aim of increasing cells level energy density and decreasing cells cost.

(2)

where DS is the ionic diffusivity of electrolyte solution (5 × 10−7 cm2/s) and ionic diffusion of the pore aligned electrode soaked with electrolyte (2 × 10−7 cm2/s).20 tS and Ls for the electrolyte solution and electrode are presented in Figure 6b. From these estimated results, it appears that the ionic diffusion of pore aligned electrode is the limiting factor at higher C rate for electrode with thickness above 400 μm, whereas at lower C rate (C/10) electrode with the thickness up to 800 μm can be charged and discharged. The interfacial charge transfer resistance is around 95−100 Ω/cm2 (cf. Figure. 4). At higher C rate interfacial resistance might also play a decisive role in the rate limiting factor. HPPC tests were also conducted using half-cells charged at C/5 rate and sequentially discharged at 1C rate for 10% SOC, followed by 2C pulse discharge for 30 s. Between each sequence, the cells were submitted to 1 h rest until 90% SOC was reached. Resulting data are plotted in Figure 7a,b as the voltage versus time and as the voltage versus the area-specific cumulative capacity, respectively. 4979

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nologies of the US Department of Energy under Contract no. DEAC02- 05CH11231, Subcontract no. 7056592 under the under the Advanced Battery Materials Research (ABMR) Program. B.D. was later supported by the Skoltech-MIT Center for Electrochemical Energy Storage. B.D. thanks Wallonie- Bruxelles International for a WBI World grant. The authors thank Dr. Jonathan Sander, Dr. Hao Bai, Ms. Grace Lau, Dr. Sebastian Behr, Mr. James Wu, and Mr. Tushar Swamy for their kind help with experiments and discussions.

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Figure 7. (a) HPPC test for a freeze-cast carbon electrode 800 μm thick. The cell was first charged at C/5 followed by an hour rest. The cell was then sequentially discharged at 1C rate for 10% of SOC, followed by 2C pulse discharges for 30 s. The sequence was repeated after 1 h rest until 90% SOC was reached. (b) Voltage versus cumulative area-specific discharge capacity in the HPPC measurements.

4. CONCLUSIONS In summary, this work demonstrates the great potential of using freeze-casting as a shaping technique to manufacture advanced graphite anodes for lithium-ion battery applications and provides a pathway to engineer thick porous anodes with a low-tortuosity and dual-scale architecture. In addition, the technique allowed decreasing the amount of inactive materials in the cell compared with conventional composite anodes. Altogether, this resulted in a significant improvement of the anode electrochemical performance: under galvanostatic discharge, the area-specific capacity was increased by a factor of 5 compared with conventional electrodes. Practical issues, such as the low mechanical strength of the created electrodes, however, have yet to be solved before practical use can be considered. The optimal structural parameterssuch as pore and wall size, the total sample porosity, and the intrinsic lamellae porosityassociated with the maximum areal energy density have also to be determined. Finally, our as-prepared anodes showed very promising results when tested under hybrid pulse power sequences, therefore meeting the power requirements for EV duty cycles and applications.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ruhul Amin: 0000-0002-0054-3510 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Tech4980

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DOI: 10.1021/acsaem.8b00962 ACS Appl. Energy Mater. 2018, 1, 4976−4981