Kinetics Tuning the Electrochemistry of Lithium Dendrites Formation in

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Kinetics Tuning the Electrochemistry of Lithium Dendrites Formation in Lithium Batteries through Electrolytes Ran Tao,†,‡ Xuanxuan Bi,†,§,⊥ Shu Li,‡ Ying Yao,‡,∥ Feng Wu,‡,∥ Qian Wang,# Cunzhong Zhang,*,‡,∥,¶ and Jun Lu*,§ ‡

School of Materials Science & Engineering, Beijing Institute of Technology, Beijing 100081, China Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, Illinois 60439, United States ⊥ Department of Chemistry and Biochemistry, Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210, United States ∥ The National High Technology Development Center of Green Materials, Beijing 100081, China # Beijing Key Laboratory of Ionic Liquids Clean Process, Key Laboratory of Green Process and Engineering, State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, and ¶Institute of Physics, Chinese Academy of Sciences, Beijing 100190, P.R. China §

S Supporting Information *

ABSTRACT: Lithium batteries are one of the most advance energy storage devices in the world and have attracted extensive research interests. However, lithium dendrite growth was a safety issue which handicapped the application of pure lithium metal in the negative electrode. In this investigation, two solvents, propylene carbonate (PC) and 2-methyltetrahydrofuran (2MeTHF), and four Li+ salts, LiPF6, LiAsF6, LiBF4 and LiClO4 were investigated in terms of their effects on the kinetics of lithium dendrite formation in eight electrolyte solutions. The kinetic parameters of charge transfer step (exchange current density, j0, transfer coefficient, α) of Li+/Li redox system, the mass transfer parameters of Li+ (transfer number of Li+, tLi+, diffusion coefficient of Li+, DLi+), and the conductivity (κ) of each electrolyte were studied separately. The results demonstrate that the solvents play a critical role in the measured j0, tLi+, DLi+, and κ of the electrolyte, while the choice of Li+ salts only slightly affect the measured parameters. The understanding of the kinetics will gain insight into the mechanism of lithium dendrite formation and provide guidelines to the future application of lithium metal. KEYWORDS: lithium dendrites, propylene carbonate, 2-methyl-tetrahydrofuran, lithium battery, coulombic efficiency

1. INTRODUCTION Because of the lowest standard potential (−3.045 V vs E0H+/H2) and ultrahigh specific capacity (3860 mAh/g), metallic lithium has been widely implanted in secondary lithium batteries1−3 as a lithium source, such as lithium ion (Li-ion), lithium air/ oxygen (Li−O2) and lithium sulfur (Li−S) batteries. However, compared to the graphite negative electrode used in commercialized secondary Li-ion batteries, the safety, cycle life, and Coulombic efficiency of pure lithium metal-based secondary batteries handicapped its application. In addition to the drawbacks plaguing the oxygen4,5 and sulfur positive electrodes,6 several critical issues are also responsible for the poor performance of metallic lithium negative electrode, which include the formation and growth of metal lithium dendrite, dead lithium, and formation of SEI7 on lithium metal surface.8−10 Different from the framework-based electrode such as MesoCarbon MicroBeads (MCMB), Li4Ti5O12, and Fe3O4, lithium metal has more freedom to undergo drastic change in structure, size, and morphology upon charge/ discharge cycles. In addition, the SEI formed on lithium is not as stable as graphite. Under cycling, it could crack and then © XXXX American Chemical Society

reform leading to an uneven electrode surface, which ends up to dendrite growth. Recent advances report that electrolytes used in metallic lithium-based batteries play an important role in adjusting the electrochemical performance such as cyclability, Coulombic efficiency and capacity retention.11−13 For instance, in ether-based electrolyte, metallic lithium exhibits much higher Coulombic efficiency than in carbonate-based electrolyte. In situ micro-optical analysis indicates that the lithium dendrite growth could be suppressed in ether-based electrolyte, whereas it is not well alleviated in a carbonate environment.14−17 The conducting salts did not always effectively inhibit the dendrite formation because of the volumetric evolution of metallic lithium and the vulnerability of SEI under mechanical deformation upon cycling, although a few functional salts may to some extent.8,9 Nevertheless, the above results still suggested that the choice of solvents/salts, which acted as the Received: October 30, 2016 Accepted: December 14, 2016

A

DOI: 10.1021/acsami.6b13859 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

in propylene carbonate (PC) and 2-methyl-tetrahydrofuran (2MeTHF) are investigated by microelectrode technique. PC and 2MeTHF are chosen as the electrolytes because they are shown to exhibit obvious discrepancy in charge/discharge cyclic performance.14−16 Through this study, we aim to provide clear kinetic mechanism and its possible effects on the performance of secondary lithium batteries, and more importantly, to explore universal electrochemical criteria for the preparation, selection, and optimization of electrolytes toward high battery performance.

second-staged protection against Li dendrite growth, is of great importance once the SEI as the first-staged protection failed. It is well-known that the performance of batteries is largely influenced by not only electrolyte ingredients but also polarization conditions. As such, it is critical that both universal kinetic mechanism degrading the performance of lithium negative electrode and formation of lithium dendrites could be explored in all kinds of electrolytes. Meanwhile, previous studies8,18 indicated that the plating/stripping process of lithium on the negative electrode surface is a very complicate process involving some basic elementary steps. From the electrochemical point of view, the kinetic mechanisms and related kinetic parameters in this process could not be investigated precisely using any conventional electrodes of large size. For example, in situ micro-optical analyses suffer from the inaccuracy caused by the low mass transfer rate, long response time, low single/noise (S/N) ratio of such large sized electrodes, although the morphology evolution of lithium dendrite has been observed in various electrolytes using such technique. In terms of the lithium dendrite growth, previous studies19−23 indicated that lithium clusters formed at the interface of “Li/SEI” at the very beginning of electrodeposition could penetrate through the SEI layer and then directly stretch into liquid electrolyte. Therefore, the protective effect of SEI is invalid at this condition, which consequently leads to the formation of microinterfaces of “Li/electrolyte”, as shown in Figure 1. This type of microinterface could appear at the beginning of Li plating regardless of the plated morphologies (moss, whisker, or dendrite).

2. EXPERIMENTAL SECTION 2.1. Reagents and Electrolytes. PC and 2MeTHF were used as solvents, whereas LiPF6, LiAsF6, LiBF4, and LiClO4 were independently added as Li+ salts in an Ar-saturated glovebox (Mbraun, H2O < 5 ppm and O2 < 3 ppm). The concentration of Li+ salts was controlled at 1 mol L−1. All of used reagents (Aldrich) were analytically pure and used as received without further purification. 2.2. Electrochemical Device and Measurement. The compartment cell is assembled by using Ni microdisc (100 μm in diameter) as the working electrode and a pure lithium disc (12 mm in diameter) as the reference electrode/counter electrode.26 The linear voltammetry analysis and cyclic voltammetry analysis were carried out by electrochemical workstation-CHI660E. The transfer number of Li+, tLi+, and the conductivity (κ) of each electrolyte were measured in a symmetric sandwiched electrochemical cell (Li/electrolyte/Li). The AC impedance measurement was carried out on an Autolab impedance analyzer with an amplitude of 5 mV sine excitation signal. The transfer number of Li+, tLi+, was measured based on the d.c. polarization technique in each electrolyte. All of electrochemical measurements were carried out at a temperature of 295 K ± 1 K.

3. RESULTS AND DISCUSSION 3.1. Electrochemical Performance and Parameters (Exchange Current Density, j0, and Transfer Coefficient, α) of the Charge Transfer Step. The typical cyclic voltammograms of the Ni microdisc electrode immersed in PC and 2MeTHF containing 1 mol L−1 LiPF6 were shown in Figure 2. On the potential-decreasing scan (from left to right), the current density was maintained at very low value until a potential was reached where nucleation of metallic lithium occurred on the nickel surface in both electrolytes. The positive current represents the electrodeposition (reduction) process of the working electrode. As can be seen, the onset potential for

Figure 1. Schematic of the formation and growth of the lithium cluster with the mass transfer in vicinity electrolyte and interface charge transfer of the microinterface of “Li/electrolyte”.

Because of its advantages of very high mass transfer rate, low iR drop, short response time, and high S/N ratio, the microelectrode technique has been used for the analysis of complex electrochemical reactions and energy storage kinetics.24,25 In this work, we explored this technique to analyze the complicated electrochemical kinetics during lithium plating and stripping in different types of electrolytes. Specifically, the relationships among charge transfer, mass transfer, and performance of metallic lithium negative electrode

Figure 2. Cyclic voltammetry for Ni microdisc electrode in (a) PC and (b) 2MeTHFcontaining 1 mol L−1 LiPF6 with a scan rate of 20 mV s−l.The arrows indicate scan direction. Insert is the Allen-Hickling plots of Li deposition/stripping in PC (red) and 2MeTHF (blue). B

DOI: 10.1021/acsami.6b13859 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces the lithium deposition starts around −0.07 V for both electrolytes and the current density increases rapidly to the limit of the electrochemical windows. For the reversing scan, the removal of lithium on the nickel surface occurs below 0.0 V (vs Li+/Li). The lithium stripping peaks are observed at 0.16 and 0.22 V in PC-based electrolyte and 2MeTHF-based electrolyte, respectively. In order to extract the kinetic parameters of this charge transfer step, exchange current density j0, transfer coefficient α, and the weak polarization region under the potential-increasing scan were calculated according to the Allen-Hickling eq 1. Four different conducting salts were employed in PC and 2MeTHF. The corresponding results were listed in the inset of Figure 2. log

j nF η RT

( )−1

exp

= log j0 −

(1 − α)nF η 2.3RT

transfer parameter for the selection of electrolytes and polarized degree parameter aiming to inhibit the lithium dendrite formation and improve the safety and cyclic Coulombic efficiency in practical charge/discharge operation. The results indicated that the values of transfer coefficient, α, were not affected distinctly by solvents and Li+ salts in investigated electrolyte samples. According to the Marcus theory, the value of α could be described as following equation α=

(1)

Table 1. Values of Exchange Current Densities (j0 /mA cm−2) and Transfer Coefficient (α) in PC-Based Electrolytes and 2MeTHF-Based Electrolytes PC

2MeTHF

LiPF6 LiAsF6 LiBF4 LiClO4 LiPF6 LiAsF6 LiBF4 LiClO4

0.43 0.39 0.45 0.39 0.37 0.43 0.43 0.42

± ± ± ± ± ± ± ±

j0 (mA cm−2) 0.05 0.02 0.01 0.03 0.02 0.01 0.03 0.04

2.17 6.20 7.51 4.62 1.40 3.56 2.49 2.16

± ± ± ± ± ± ± ±

(2)

where λ represents the reorganization energy which is the different of energy of solvated lithium ion and lithium atom in lattice of bulk lithium metal; F(E − E0) is Gibbs free energy, wO and wR are work terms. In Marcus theory, wO and wR are defined as the work required to establish the reactive position from the average environment of reactants and products in the medium. Because the pure charge transfer step occurs between Li+ and metal lithium only, the wO and wR could be accepted as the solvation energy of Li+ in each investigated electrolyte and lattice energy of metal lithium, respectively. In this investigation, the wR could be accepted as a constant due to the lithium metal was an exclusive product. wO is determined by the employed solvents and the coordination number of solvated Li+. In addition, investigated potential scope is closed to equilibrium potential of Li+/Li couple, the contribution of free energy is very low. Hence, we could estimate that the values of wO of Li+ in all of investigated electrolyte samples were similar. 3.2. Electrochemical Performance and Parameters (transference Number of Li + , t Li+ , and Diffusion Coefficient of Li+, DLi+) of the Mass Transfer Step. Previous investigations stated that there was a certain relationship between formation and growth of dendrite and appearance of concentration gradient in electrolyte layers that are close to the interface of “Li/electrolyte”.29,33,35,36 It was known that the appearance of concentration gradient should be ascribed to the diffusion of Li+ rather than its migration. In order to illuminate the individual effect of diffusion and migration of Li+ on the formation of dendrite, the tLi+ and DLi+ were measured separately in each electrolyte. The value of tLi+ was obtained from polarization measurement according to the method mentioned by Bruce.37−39 Typical chronoamperometric curves and AC-impedance curves obtained for PC-based electrolytes containing 1 mol L−1 Li+ salts are given in Figure 3. On the basis of the parameters

The relation of j0 and α in all the investigated electrolytes are extracted and summarized in Table 1.

α

F(E − E 0) − (wO − wR ) 1 + 2 2λ

0.50 0.18 0.35 0.42 0.41 0.51 0.24 0.27

The results indicate that j0 obtained from PC-based electrolytes are 2.1−3.0 times larger than the one obtained from 2MeTHF-based electrolytes. Meanwhile, the value of j0 is not affected much by the species of Li+ salts dissolved in the solution. It is well-known that a larger j0 implies a faster charge exchange rate for the charge transfer step. On the microlevel, results of in situ optical microscopy18,27−30 confirms that the prerequisite for formation of lithium dendrites is the appearance of limitation concentration gradient of Li+ in thin electrolyte layers that are close to the microinterface of “Li/ electrolyte”. In addition, thicker and longer lithium dendrites are much easier to form in PC-based electrolytes.14,31 On the macro-level, previous reports demonstrated poor performance of the lithium negative electrode in PC-based electrolyte.28 Moreover, the inhibition on formation of lithium dendrite8 with lower values of j032 was also found to be more effective in the polymer electrolyte. Results in various electrolytes reveal an obvious correlation between value of j0 and the inhibition effect on lithium dendrite formation. Although these results do not show a direct correlation between j0 and the length of lithium dendrite, previous results on in situ optical microscopy18,26−29,31 and model simulation33 indicate that the parameters related to lithium dendrite, such as growth rate, length and thickness, are all controlled by the ratio of applied current density to j0, namely the polarized degree. Moreover, the appropriate size of lithium dendrite could be formed26,28,34 with a better Coulombic efficiency26 as long as the polarized degree could be mediated properly. Based on all of reported results, j0 could be accepted as a critical charge

Figure 3. (A) Chronoamperometry of the cell with PC-based electrolytes containing 1 mol L−1 Li-salt, V = 0.005 V; (B) ACimpedance spectra (from 0.1 Hz to 100 kHz) for same electrolyte at (a) t = 0 s and (b) t = t s. C

DOI: 10.1021/acsami.6b13859 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces obtained from chronoamperometry and AC-impedance, tLi+ was calculated by the following equation t Li + =

Table 2. Values of Transference Number, Diffusion Coefficient of Li+, and Conductivity of Each Electrolyte

I ssR bss(ΔV − I0R el0) I 0R b0(ΔV − IssR elss)

PC

(3)

In eq 3, the superscripts “0’ and ‘ss” indicate initial values and steady-state values, respectively. Rb is the bulk resistance of the electrolyte, Rel is the resistance of electrode, ΔV is the applied voltage, and I is the applied current. To have an accurate value of I0, Rb is obtained from the results of AC-impedance and conductivity was recalculated from Rb based on the follow equation

κ=

l 1 A Rb

2MeTHF

Cs = C0 −

+

Figure 4. Steady-state polarization curve of Li /Li couple in PC, containing 1.0 mol L−1: (a) LiAsF6, (b) LiBF4, (c) LiClO4, and (d) LiPF6 on Ni microdisc electrode. Arrows indicate scan direction, scan rate: 20 mV s−1 (LiPF6 10 mV s−1).

0.19 0.15 0.25 0.31 0.51 0.44 0.40 0.56

± ± ± ± ± ± ± ±

0.05 0.02 0.05 0.05 0.08 0.02 0.02 0.03

6.75 2.04 1.96 1.26 8.07 5.75

× × × × × ×

10 10−5 10−5 10−5 10−6 10−5

6.15 × 10−6

κ (S/m) 0.71 0.73 0.42 0.61 0.28 0.43 0.01 0.08

2j(1 − t Li +) t nF πDLi +

(6)

Here, Cs, C0, tLi+ and DLi+ stand for the surface concentration, bulk concentration, transference number and diffusion coefficient of Li+, respectively. j is the employed polarization current density and t is the polarization time. According to eq 6 and our current results, the formation of concentration gradient is easier to develop in PC-based electrolyte than in 2MeTHFbased electrolyte. This is in agreement with the easy degree during the formation of dendrite in PC-based electrolyte. In addition to these electrolytes with lower viscosity, the relationship between inhibition performance on formation of dendrite and mass transfer parameters, tLi+ and DLi+ could also be obtained by careful comparison of reported results in electrolyte possessing higher viscosity, such as polymer electrolytes34−36 and solvent-in-salt electrolyte.42 So, simply improving the conductivity value was not very suitable for the inhibition of lithium dendrite formation when the value of tLi+ is low. Increasing the value of tLi+ and decreasing the value of DLi+ are more effective toward the improvement of lithium negative electrode and the inhibition of dendrite formation. Therefore, in addition to the conductivity, κ, tLi+, and DLi+ deserve significant emphasis and attention as they are two critical transfer parameters for the selection and preparation of electrolyte, and the evaluation of the performance of secondary lithium secondary.

exhibited in Figure 4. The steady-state limited mass transfer current density, could be described by eq 5 4nFDLi +c Li + πr(1 − t Li +)

LiPF6 LiAsF6 LiBF4 LiClO4 LiPF6 LiAsF6 LiBF4 LiClO4

−6

2MeTHF exhibit significant effect on the values of tLi+ and DLi+, respectively. Specifically, in Table 2, the value of DLi+ in 2MeTHF containing 1 mol L−1 LiBF4 is not available due to the absent of isslim. The fact implies that the primary mass transfer pattern of Li+ is diffusion-type in PC-based electrolyte, and migration-type in 2MeTHF-based electrolyte. In general, the diffusion is shape-dependent mass transfer considering that the limitation diffusion current density is inversely proportional to the radius of microelectrode.41 Meanwhile, the migration current density is not affected by the microtopography of the bulk lithium surface. As such, the distribution discrepancy of local current densities and protrusion degree of local microtip at different position on the bulk lithium surface could be enlarged due to the present of diffusion pattern. Consequently, the surface of lithium negative electrode would become rougher in PC-based electrolyte because of the lower value of tLi+. Moreover, Nishida et al. stated28,31 that the formation of lithium dendrite was controlled by not only migration but also diffusion, and that the effects of migration and diffusion could be described by eq 6

(4)

In eq 4, l/A is the cell constant and Rb is bulk resistance of electrolyte. The values of DLi+ were measured in all electrolytes based on the reported method.40 The typical linear voltammetry curves of the Ni microdisc electrode immersed in PC-based electrolytes containing different 1 mol L−1 Li+ salts were

ss = jlim

DLi+ (cm2/s)

tLi+

(5)

jsslim

This equation indicates that the value of is determined by both the migration current and the diffusion current because the unique Li+ salt is applied in each electrolyte. In addition, a larger potential scope of is found in the electrolyte containing LiAsF6 and LiClO4, while it is significantly smaller when the dissolved salts become LiPF6 and LiBF4 with the solvent unchanged. In addition, the steady-state mass transfer current could be found on most investigated electrolytes except for 2MeTHF containing 1 mol L−1 LiBF4. The underlying reason was not clear and is worth intensive study in the future. Introducing tLi+ into eq 5, the values of DLi+ could be obtained. The values of tLi+, DLi+, and each electrolyte were summarized in Table 2. The results indicate that the values of κ in 2MeTHF-based electrolyte are about 12.5−50% of those in PC-based electrolyte. Such a phenomenon is caused by the choice of solvent rather than Li+ salts. Moreover, PC and D

DOI: 10.1021/acsami.6b13859 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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4. CONCLUSION The performance and relevant kinetic parameters of two basic steps of electrochemical redox process in Li+/Li system were tested and measured using the technique of microelectrode. Two electrolytes exhibiting different effects on inhibiting lithium dendrite formation are investigated and compared with various Li+-containing salts dissolved inside. The results confirm a strong relationship between the appearance performance of metallic lithium electrode and kinetic parameters (j0, tLi+, DLi+) of mass transfer step and charge transfer step. With the value of tLi+ increased and the values of j0 and DLi+ decreased, the electrolyte exhibits better Coulombic efficiency at the metal lithium electrode and better inhibition effect on lithium dendrite formation. The results also indicate that the choice of solvent affects the intrinsic kinetics more than the choice of Li ion salts does. Our work confirms the kinetic parameters as the key criteria in the selection and evaluation of electrolyte for improvement of rechargeable lithium batteries, in addition to the conventional important parameters, such as electrochemical window and conductivity. The technique of microelectrode is an important tool to select and evaluate electrolyte due to its instinct advantage on the analysis of electrochemical reaction mechanism and measurement of relevant parameters during basic kinetic steps. Moreover, it could be utilized as a tool to reveal essential and universal kinetic laws and to account for the appearance performance of secondary batteries.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b13859. Diagrammatic cross-section of two-electrode electrochemical device and electrochemical principle of microelectrode technique (PDF)

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Research Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Ying Yao: 0000-0002-0472-0852 Jun Lu: 0000-0003-0858-8577 Author Contributions †

R.T. and X.B. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial supports from the NSFC project (21473011 and 51402018) and National Basic Research Program of China (2014CB932300, 2015CB251100) are appreciated. J.L. acknowledges the financial support from the U.S. Department of Energy under Contract DE-AC02-06CH11357 from the Vehicle Technologies Office, Department of Energy, Office of Energy Efficiency and Renewable Energy. Q.W. gratefully acknowledges the financial support from the General Program Youth of National Natural Science Foundation of China (51404230). E

DOI: 10.1021/acsami.6b13859 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsami.6b13859 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX