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Letter Cite This: J. Phys. Chem. Lett. 2018, 9, 5100−5104

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Revealing the Rate-Limiting Li-Ion Diffusion Pathway in Ultrathick Electrodes for Li-Ion Batteries Han Gao,† Qiang Wu,‡ Yixin Hu,§ Jim P. Zheng,*,‡ Khalil Amine,*,†,∥ and Zonghai Chen*,† †

Chemical Science and Engineering Division, Argonne National Laboratory, Lemont, Illinois 60439, United States Department of Electrical and Computer Engineering, Florida A&M University-Florida State University College of Engineering, Florida State University, Tallahassee, Florida 32310, United States § Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27514, United States ∥ Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States Downloaded via KAOHSIUNG MEDICAL UNIV on August 25, 2018 at 03:20:44 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: Increasing the loading of active materials by thickening the battery electrode coating can enhance the energy density of a Li-ion cell, but the trade-off is the much reduced Li+ transport kinetics. To reach the optimum energy and power density for thick electrodes, the effective chemical diffusion coefficient of Li+ (DLi) must be maximized. However, the diffusion of Li+ inside an electrode is a complex process involving both microscopic and macroscopic processes. Fundamental understandings are needed on the rate-limiting process that governs the diffusion kinetics of Li+ to minimize the negative impact of the large electrode thickness on their electrochemical performance. In this work, lithium Ni−Mn−Co oxide (NMC) cathodes of various thicknesses ranging from 100 to 300 μm were used as a model system to study the rate-limiting diffusion process during charge/discharge. The rate-limiting diffusion coefficient of Li+ was investigated and quantified, which was correlated to the electrochemical performance degradation of thick electrodes. It is revealed here that the under-utilization of the active material was caused by the limited diffusion of Li+ inside the porous electrode, leading to a critical electrode thickness, beyond which the specific capacity was significantly reduced.

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liquid and solid phases. From micro- to macroscopic point of view, the diffusion pathways can be categorized into the following: (1) solid-phase diffusion within the crystal structures of active material (i.e., intragranular diffusion), (2) solid-phase diffusion within the primary particles (i.e., intergranular diffusion), (3) liquid-phase diffusion within the secondary particles (i.e., diffusion from one primary particle to another), (4) liquidphase diffusion within the porous electrode (i.e., diffusion from one secondary particle to another), and finally (5) liquid-phase diffusion in the bulk electrolyte. It can be seen from Figure 1 that the diffusion of Li+ inside an electrode is such a complex process, and it is critical to determine the rate-limiting step among the various contributors toward DLi during charge/discharge. In this work, we will emphasize on the rate-limiting process that governs the diffusion kinetics of Li+ and its impact on the electrochemical performance of electrodes with large thicknesses. Thick electrodes (∼100 to 300 μm) prepared using lithium Ni−Mn−Co oxide (NMC) cathode material will be

arge battery applications, such as electrical vehicles, require higher charge storage capacity and lower battery volume and weight.1−5 Thickening the electrode (>100 μm) with higher active material loadings not only can achieve this goal but also can reduce the battery pack cost.6 Much progress have been accomplished toward the fabrication of thick electrodes to eliminate the issues associated with the conventional wet coating methods.7−11 Binder-free electrodes prepared via different methods including sputtering,12−15 spraying,16,17 deposition,18 and sintering19−21 with the potential application for thick electrodes are also demonstrated. In addition to these fabrication methods, investigations on the fundamental cause behind the electrochemical performance decay with increasing electrode thickness are another critical topic for the development of ultrathick electrodes. In general, the sluggish Li+ transport kinetics and poor rate capability of thick electrodes are the key obstacles to their commercial utilization.22,23 To reach the optimum energy and power density for the thick electrodes and thus battery performance, the effect of various parameters on the reaction kinetics must be considered during the design of thick electrodes. One of the most important parameters is the effective chemical diffusion coefficient of Li+ (DLi). Figure 1 shows a schematic picture summarizing the different Li+ diffusion pathway in both © XXXX American Chemical Society

Received: July 17, 2018 Accepted: August 21, 2018 Published: August 21, 2018 5100

DOI: 10.1021/acs.jpclett.8b02229 J. Phys. Chem. Lett. 2018, 9, 5100−5104

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The Journal of Physical Chemistry Letters

(PTFE, DuPont) binder, and carbon filler. Then, the mixed powder was fed into a two-roll mill to form freestanding electrode sheet by passing through the gap between the rolls with controlled temperature. The thickness of electrode sheets was adjusted by controlling the roll gap, roll temperature, and number of pressings. Finally, the electrode sheets with desired thickness and aluminum foils were passed through the mill again to form electrode laminates. Figure S1 shows a schematic diagram of this dry electrode-making process. Experimental details are included in the Supporting Information. The loadings of the NMC 622 active materials in the prepared electrodes showed a linear dependent on the electrode thickness, as shown in Figure 2a. In our case, the thickest electrode had an NMC loading of >110 mg (or ∼70 mg cm−2). Before investigating the effect of electrode thickness on the Li+ diffusion, the basic electrochemical properties of the prepared electrodes were first examined using half-cell configurations. During the formation cycles up to 4.4 V vs Li/Li+ with a C/50 current, the thick electrodes exhibited characteristic voltage profiles (Figure 2b and Figure S2a) and differential capacity (dQ/dV) profiles (Figure 2c and Figure S2b) as typical NMC 622. The gain in areal capacity showed a linear trend with the increase in electrode thickness (Figure S2c). In addition, a continuous shifting of the peaks in the dQ/dV plots implies an increase in the electrode resistance with increasing thickness. These resistance values were quantified based on the iR drop between the charge and discharge steps from Figure S2a, and thicker electrodes showed larger resistance values (Figure S2d). Figure 2d shows the dependence of first-cycle coulombic efficiency (CE) and irreversible capacity as a function of coating thickness. The half-cells with thicker electrodes exhibited lower CE and higher capacity loss. This agrees with the cycling performance of the half-cells (Figure S3). The half-cells

Figure 1. Schematic diagram showing the diffusion pathway of Li+ from micro- to macroscopic pictures: (1) diffusion within the crystal structures, (2) diffusion within the primary particles, (3) diffusion within the secondary particles, (4) diffusion within the porous electrode, and (5) diffusion in the bulk electrolyte.

used as a model system to validate our hypothesis. The effect of increasing electrode thickness on the active material utilization, capacity, and resistance will be investigated. More importantly, this paper can provide fundamental understandings on the diffusion of Li+ inside thick electrodes so that we can quantify the rationale behind the performance degradation and provide a design guideline for preparing proper thick electrodes for the next-generation Li-ion cells. LiNi0.6Mn0.2Co0.2O2 (NMC 622, secured from an industrial partner) working electrodes ranging from 100 to 300 μm were prepared by mixing NMC 622, polytetrafluoroethylene

Figure 2. (a) Loading of NMC 622 on the electrode as a function of electrode thickness. First cycle (b) voltage profiles, (c) differential capacity (dQ/dV) profiles, and (d) coulombic efficiency values and irreversible capacity values of the NMC/Li half-cells containing electrodes with different thickness. 5101

DOI: 10.1021/acs.jpclett.8b02229 J. Phys. Chem. Lett. 2018, 9, 5100−5104

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Figure S4a shows the voltage profiles of the half-cells during the GITT measurement. The chemical diffusion coefficient of Li+ can be determined from GITT using eq 124

with thicker electrodes suffered from a faster capacity fade. In summary, the above data clearly demonstrated that the thick cathodes prepared can be successfully applied in Li-ion cells, and the thickness of the electrodes showed clear influence on their electrochemical performance. To understand the effect of thickness on the specific capacity of the prepared electrodes, the specific discharge capacity values obtained from first and fourth formation cycles (current = C/50) were normalized by their coating thickness, as shown in Figure 3a

DLiGITT =

2 4 ij nmVm yz ijj ΔEs yzz jj zz jj z πτ k S { jk ΔEt zz{

2

(1)

where τ is the time during which constant current is applied, nm is the number of moles of the electrode, Vm is the molar volume of the electrode, S is the electrode/electrolyte contact area, and ΔEs and ΔEt are the change in voltage at each step and under constant current conditions, respectively (Figure S4b). DLi values were calculated for both charging and discharging steps. For simplicity, the following discussion will focus on the Li+ extraction processes because DLi values calculated from charging and discharging steps showed identical behaviors (see Figure 3c and Figure S5a). Figure 3b shows the variation of DLi values as a function of potential for electrodes with increasing thickness. Here we first focused on the common trend of DLi as a function of potential. In general, three different potential regions can be identified in Figure 3b. A quick reduction in DLi was observed in region 1, while a significant downward peak was observed in region 2. On the contrary, DLi demonstrated a relatively flat tail with higher values in region 3. From this Figure, the limiting DLi values were observed in region 2. Interestingly, the position of the downward peak roughly overlapped with the position of the peaks in the dQ/dV plots. However, previous studies on the solid-state diffusion of Li+ utilizing thin-film electrodes25,26 and kinetic Monte Carlo simulations27,28 have all revealed maximum DLi values during charge and discharge of layered cathode materials. This contradicts with our observations and suggests the obtained DLi values in Figure 3c were not reflecting the nature of the solid-state diffusion. Instead, they resulted from the influence of the electrodes’ porous nature, especially with thicker electrodes. This agrees well with our hypothesis that the liquid-phase diffusion within the pores of the thick electrodes actually limits the transport kinetics of Li+. With this in mind, the obtained values in Figure 3b were compared to the liquid-phase diffusion coefficient of Li+ in the bulk LiPF6-based nonaqueous electrolyte (10−6 to 10−5 cm2s−1),29,30 which showed two to five times larger values than what we observed. This is well-expected because the effective diffusion coefficient of Li+ is much smaller in a porous medium. Commonly, this effective diffusion coefficient of Li+ can be represented by eq 231 ε DLi = D0 (2) τ

Figure 3. (a) First cycle thickness-normalized specific capacity as a function of electrode thickness, (b) variation in the chemical diffusion coefficient of Li+ (DLi) and areal differential capacity as a function of potential during Li+ extraction, and (c) extracted DLi value at 3.74 V vs Li/Li+ as a function of electrode thickness.

and Figure S3b. The thickness-normalized capacity decreased with the larger thickness, indicating an under-utilization of active materials and loss of capacity even with an extremely small current. Interestingly, a critical thickness of ∼200 μm occurred, above which a much faster reduction in capacity was observed. We believe this reduction in the active material utilization with larger electrode thickness was caused by the inefficient Li+ transport within the system with lower effective Li+ diffusivity (DLi). With increasing the electrode thickness, we speculate that the liquid-phase diffusion within the porous electrode would be the rate-limiting mechanism that governs the diffusion kinetics of Li+ during charge/discharge. Therefore, it is of our great interest to quantify DLi and to obtain fundamental understandings behind the trend in Figure 3a and Figure S3b. It is presented above that the electrochemical performance of the prepared electrodes is significantly influenced by their thickness. Both galvanostatic intermittent titration technique (GITT) and electrochemical impedance spectroscopy (EIS) were employed to validate our hypothesis and investigate the Li+ transport kinetics for the ultrathick electrodes. Because the diffusion pathway of Li+ is complex, as shown in Figure 1, the obtained DLi values from both methods reflect the rate-limiting diffusion process. However, we can isolate and solely focus on the effect of electrode thickness because the other diffusion pathways are associated with the intrinsic properties of the NMC material and electrolyte chemistry.

where D0 is the concentration-dependent intrinsic liquid-phase diffusion coefficient of Li+, ε is the porosity of the electrode (always ≤1), and τ is the tortuosity of the electrode (always ≥1). Therefore, we believe the large amount of Li+ extracted in region 2 can oversaturate the local Li+ concentration in the liquid phase and lead to a substantial reduction in D0 and ultimately, DLi. On the contrary, the DLi values were much higher in regions 3 and 1 because the Li+ concentration was not oversaturated in either of the two regions. However, the insufficient Li+ concentration in region 1 could lead to a slightly lower D0, and thus DLi, when comparing with the values in region 3. Because the Li+ transport is mostly limited by the low DLi values observed in region 2, these limiting DLi values were extracted plotted as a function of electrode thickness, as shown 5102

DOI: 10.1021/acs.jpclett.8b02229 J. Phys. Chem. Lett. 2018, 9, 5100−5104

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The Journal of Physical Chemistry Letters in Figure 3c. Interestingly, a critical thickness at ca. 200 μm also occurred, which was identical to the critical thickness observed in Figure 3a and Figure S3b. This confirmed our hypothesis that the rapid loss of capacity and under-utilization of active materials are well correlated with the limitations in the effective Li+ diffusivity (DLi). This observation still holds true during the Li insertion (i.e., discharge) process, as shown in Figure S5b. In that case, the quick reduction of DLi with increasing electrode thickness was caused by Li+ overdepletion instead of oversaturation during Li insertion (i.e., discharging) process. EIS was used to cross-validate the trend of DLi obtained from GITT. Figure 4a shows the overlaid Nyquist plots of the

where L is the effective diffusion thickness and τ is the diffusion time. Here we estimated the value of L to be half of the average primary particle size. Scanning electron microscopy (SEM) images of the electrode at higher magnifications suggest the particle size was ∼1 μm (Figures S5c,d). Although it is difficult to verify this assumption of the effective diffusion length of Li+, the trend of DLi obtained from EIS was very similar to the one obtained from GITT: The reduction in DLi with increasing electrode thickness became severe at an electrode thickness beyond 200 μm (Figure 4c). It is of interest to evaluate the rate performance of the thick electrode. The cells were cycled for three cycles under different current densities, and the capacity from the third cycle of each C-rate was normalized with the formation capacity (Figure 4d). Indeed, a general trend of a reduction in rate capability with thicker electrodes was observed. More importantly, these cells can be divided into two groups, especially when comparing their capacity retention at currents higher than 0.1 C. Once again, the critical thickness of the half-cell separating the two groups was ∼200 μm, which agrees well with critical thickness obtained from the extraction of both DLi and specific capacity. In summary, we demonstrated that the capacity loss and the under-utilization of the active materials with thicker electrodes agree well with the trend of the limiting DLi values. The more rapid decrease in DLi for the thicker electrodes was caused by their localized Li+ oversaturation (during Li extraction) or overdepletion (during Li insertion), affecting the transport of Li+. The trend of DLi also revealed a critical electrode thickness, beyond which the specific capacity and rate performance were all significantly reduced. Further optimizations on the design parameters, such as porosity and tortuosity, of the ultrathick electrodes are needed to push DLi toward higher values. For example, structuring of electrodes comprising different morphologies can greatly improve the ion transport kinetics.33 Directional freeze-casting34 and coextrusion35 are novel ways to create corrugated electrode structures that can provide faster ionic transport. These approaches, together with fundamental understandings on Li+ transport kinetics, can ultimately lead to faster charging or discharging of the thick electrodes for the next-generation Li-ion batteries.

Figure 4. (a) Overlays of Nyquist plots of the NMC/Li half-cells containing electrodes with different thickness, (b) extracted characteristic diffusion time from EIS data fitting as a function of electrode thickness, (c) overlays of the chemical diffusion coefficient of Li+ (DLi) from both EIS and GITT calculations, and (d) capacity retention of the NMC/Li half-cells as a function of current density.

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half-cells at 3.7 V vs Li/Li (i.e., the potential exhibiting the limiting DLi in Figure 3b). The impedance spectra showed multiple overlapping semicircles and a “tail”. Similar to the dc resistance (Figure S2d), the total equivalent series resistance (ESR) of the cells increased with their working electrode thickness, as evidenced by the shifting of the Nyquist plots. More importantly, we focused our analysis on the “tail” at the low-frequency region because it is affected by the Li+ diffusion. A constant-phase element (CPE) model was used instead of a finite-length Warburg (FLW) model because the data do not extend to low enough frequencies to demonstrate the finite (low-frequency) character of the Warburg element. By using the CPE model, a more generalized solution to the diffusion equation can be obtained.32 Figure S6 shows three examples of the fitted impedance spectra and the equivalent circuit model used to extract the diffusion time (τ) from the “tails”, while Figure 4b summarizes the τ values as a function of electrode thickness. DLi can be estimated according to eq 3 L2 DLi = τ

ASSOCIATED CONTENT

S Supporting Information *

+

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.8b02229. Experimental details; schematic diagram showing the process making the ultrathick electrodes; voltage profiles, dQ/dV profiles, areal capacity, and resistance values of the NMC half-cells; cycling performance of the NMC half-cells; thickness-normalized specific capacity values; details of the GITT cycle; DLi values during Li+ insertion; high-magnification SEM images; and examples of fitted impedance spectra for NMC/Li half-cells. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*J.P.Z.: Email: [email protected]. *K.A.: E-mail: [email protected]. *Z.C.: E-mail: [email protected]. ORCID

Khalil Amine: 0000-0001-9206-3719 Zonghai Chen: 0000-0001-5371-9463

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DOI: 10.1021/acs.jpclett.8b02229 J. Phys. Chem. Lett. 2018, 9, 5100−5104

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

Research at the Argonne National Laboratory was supported by the U.S. Department of Energy (DOE), Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Office. Argonne National Laboratory is operated for DOE Office of Science by UChicago Argonne, LLC, under contract number DE-AC02-06CH11357. Support from Tien Duong of the U.S. DOE ’s Office of Vehicle Technologies Program is gratefully acknowledged. We also acknowledge the use of the Center for Nanoscale Materials, an Office of Science user facility, which was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. Research at the Florida State University was supported by the U.S. Army Power Division under grant number GTS-S-17-356. H.G. acknowledges the NSERC Canada Postdoctoral Fellowships Program.

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