Identifying Active Sites for Parasitic Reactions at the Cathode

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Identifying Active Sites for Parasitic Reactions at Cathode Electrolyte Interface Yingying Xie, Han Gao, Jihyeon Gim, Anh T. Ngo, ZiFeng Ma, and Zonghai Chen J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on January 22, 2019

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The Journal of Physical Chemistry Letters

Identifying Active Sites for Parasitic Reactions at Cathode Electrolyte Interface Yingying Xie, †§ Han Gao,† Jihyeon Gim,† Anh T. Ngo, ‡* Zi-Feng Ma, § Zonghai Chen†* † Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, IL 60439, USA § Department of Chemical Engineering, Shanghai Electrochemical Energy Devices Research Center, Shanghai Jiao Tong University, Shanghai 200240, China ‡ Materials Science Division, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, IL 60439, USA

Corresponding Author *Email: [email protected] (ATN); [email protected] (ZC)

ACS Paragon Plus Environment

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ABSTRACT: Nickel-rich transition metal oxides are the most promising high voltage and high capacity cathode materials for high energy density lithium batteries.

Improving the

chemical/electrochemical stability of the cathode electrolyte interface has been the major technical focus to enable this class of cathode materials. In this work, LiCoO2 is adopted as the model cathode material to investigate the active sites for parasitic reactions between the delithiated cathode and the non-aqueous electrolyte. Both ab initio calculations and experimental results clearly show that the partially coordinated transition metal atoms at the surface is responsible for the parasitic reactions at the cathode electrolyte interface. This finding lays out a fundamental support for rational interfacial engineering to further improve the life and safety characteristics of nickel-rich cathode materials.

TOC GRAPHICS

Lithium Cobalt Oxygen Carbon Hydrogen

1 2

Electron transfer

+

3

Deprotonation

KEYWORDS: parasitic reaction, cathode electrolyte interface, lithium battery, surface chemistry


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It has been a global effort to develop high energy density, and low cost lithium batteries to meet the demanding requirements for emerging applications in electric vehicles and grid energy storage. Among all possible solutions, nickel-rich cathode materials are considered most promising strategy because of their high working potential, high specific capacity and relatively low cost through the reduction of the utilization of expensive cobalt. However, these advantages can only be materialized with an expense on the life and safety characteristics of the nickel-rich cathode materials

1-2.

It has been commonly accepted that the major issues of nickel-rich cathodes are

originated from the instability of cathode electrolyte interface, especially at a relatively high working potential. Therefore, lots of cutting edge characterization techniques, such as high resolution transmission electron microscopy (HRTEM) (Cryo-SEM)

5-6,

3-4,

X-ray photoelectron spectroscopy (XPS)

Cryo scanning electron microscopy

7-8,

synchrotron probes

9-10,

have been

deployed to investigate the chemical composition and spatial distribution of chemical species inside the interfacial layer and the outer layer of the cathode materials. Even with the massive amount of knowledge collected from the above diagnosis effort, a comprehensive physical image of a good cathode electrolyte interface has not fully established.

Hence, the technology

development to stabilize the cathode electrolyte interface is still based on the trial-and-error approach. On the other hand, the ultimate functionality of a good cathode electrolyte interface is to eliminate/suppress the electron transfer reaction between the cathode material and the electrolyte, which is the major component of parasitic reaction at the interface. Therefore, high precision electrochemical measurement approaches

11-18

were recently proposed to quantify the electron

transfer reactions at the interface. Dahn et al proposed the concept of the high precision columbic (HPC) efficiency measurement as a quantitative indicator of battery performance 14-15, 17-18. Instead


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of relying on the measurement of reversible capacity during long-term cycling of a lithium-ion cell, Dahn et al built a protocol to accurately measure the columbic efficiency, or in other words the loss of reversible capacity per cycle, during the low-rate cycling of lithium-ion batteries 17. The positive validation between the HPC measurement and the real electrochemical performance using a traditional lengthy charge/discharge cycling has been successfully established

14, 16.

The

columbic efficiency discussed above has the contribution from parasitic reactions at both the positive electrode and the negative electrode. Accurate measurement of columbic efficiency is an excellent practical approach for the rapid evaluation of the life of a lithium-ion cell. However, it has a limited capability in identifying the relative contribution between the positive electrode and the negative electrode, and hence, empirical interpretation is needed to gain insight on the failure mechanism of the investigated lithium-ion cell

17.

Having the same desire to quantitatively

investigate parasitic reactions inside lithium batteries, we developed a prototype high precision leakage current measuring system to measure the rate of electron transfer reaction across the solidelectrolyte interface 19-21. With the help of the prototype system, we successfully demonstrated that the parasitic reactions include at least one chemical reaction and at least one electrochemical reaction 20-22. At the cathode side, the parasitic reactions are dominated by the chemical reaction between the delithiated cathode and ethylene carbonate at a potential below 4.5 V, beyond which the electrochemical oxidation of ethylene carbonate makes the dominant contribution 20. In this work, LiCoO2 was selected as the model cathode material to identify the active chemical sites that promote the electron transfer reaction between the delithiated cathode and ethylene carbonate. Figure 1a schematically shows the measuring principles our prototype high precision electrochemical system 20, 22 using LiCoO2 as an exemplary cathode material. When charging the half-cell comprising LiCoO2 as the working electrode, lithium was removed from the working


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electrode, and deposited onto the lithium counter electrode. The current pass through the external circuit included two components: (1) an actual charging current that increases the state of charge (SOC) of the working electrode; and (2) a leakage current resulted from the chemical/electrochemical oxidation of active species in the electrolyte. In general, the leakage current is only a small fraction of the actual charging current, less than 0.1 %. When the cell was constant-voltage charged to a specific voltage (i.e. 4.35 V) for an extended period (i.e. 20 hours), the current measured by the external circuit decayed exponentially with the holding time (see Figure 1b). The decay of the current shown in Figure 1b was originated from the relaxation of the concentration gradient of active species, like Li+, in both liquid and solid phases during the charging process.

After constant-voltage holding for more than 5 hours, the measured current

decayed to a steady value, small but no zero, which was solely contributed from the electron transfer reaction between the working electrode and the active specie in the electrolyte, specifically ethylene carbonate 22. Therefore, we can use the steady leakage current as a quantitative index for the rate of the electron transfer reaction at the cathode electrolyte interface (see Figure 1a), which is the first step of a series of parasitic reactions. By holding the voltage of the cell at different values in sequence, we were able to measure the rate of the electron transfer reaction at different working potential as shown in Figure 1c. It is clear that the rate of the electron transfer reaction increased monotonically with a sharp increase at about 4.5 V vs. Li+/Li. The trend agrees well with previously reported for LiNi0.Mn0.2Co0.2O2 cathode

20, 23-24.

The kinetic investigation in the

previous work confirmed that the steady leakage current at a potential below 4.4 V was dominated by the chemical reaction between the delithiated cathode and ethylene carbonate, the rate of which reaction is primarily attenuated by the concentration of active intermediate phase on the interface.


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Meanwhile, the steady leakage current at a higher potential (>4.5 V) is dominated by the direct electrochemical oxidation of ethylene carbonate 22. The chemical reaction within the low potential range is of more practical importance since all cathode materials will bear such electron transfer reaction during the normal charge/discharge process. Different from the direct electrochemical reaction, the chemical reaction involves the coupling of atomic/molecular orbitals between the electron donor and electron acceptor; the coupling of atomic/molecular orbitals will form a weak chemical bond to facilitate the electron transfer reaction. In order to gain more chemical insight of the electron transfer reaction at low potential, a series of the spin polarized Density Functional Theory (DFT) calculations were carried out to investigate the interaction between the ethylene carbonate (an electron donor) and the delithiated LiCoO2 (an electron acceptor). DFT calculations were carried out with the Vienna ab initio simulation package (VASP) code

25-26,

with core electrons described by the projected

augmented wave method (PAW) 27-28. Exchange-correlation was treated in the PBE, generalized Gradient Approximation (GGA) 29. The plane wave basis was expanded to a cutoff of 600 eV. The layered LiCoO2 (10-14) surface 30 was modeled by four-layer slabs with a vacuum space of 30 angstroms contain 144 atoms. The electrolyte molecule ethylene carbonate (EC) was then deposited on top of the LCO surface. The DFT calculation clearly shows that ethylene carbonate tends to be chemically absorbed on the surface of LiCoO2 by forming a weak bond between the interfacial Co atoms and the carbonyl group of ethylene carbonate (see Figure 2). It is also shown that the bonding strength increases with the degree of the delithiation of LiCoO2 (see Figure 2a), and that the bond length between the oxygen (in ethylene carbonate) and cobalt (in LiCoO2) monotonically decreases (see Figure 2b). At the fully discharged state when lithium sites are fully occupied (SOC=0%), ethylene carbonate shows a weak bonding to cobalt atoms at the surface of


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LiCoO2 with a bonding energy of about 0.25 eV (see Figure 2c). The length of O-Co (O in EC and Co in LiCoO2) is about 2.10 Å. When LiCoO2 is slightly charged by introducing 25% lithium vacancies into the super cell, ethylene carbonate shows a stronger bonding to the cobalt atoms at the surface with an increased bonding energy of 0.66 eV (see Figure 2d). In this case, the O-Co bond length reduces from 2.10 Å to 2.19 Å. This trend continues when more lithium is removed from the super cell (see Figures 2a and 2b). This suggests that the chemical bond is formed through the coupling of 2p orbital in ethylene carbonate and 3d orbital of cobalt in LiCoO2 (p- coupling). The removal of lithium from LiCoO2 cause the oxidation of cobalt atoms and a reduction on the electron density in theirs 3d orbitals, making them a better electron acceptor to promote the electron transfer from ethylene carbonate molecules. The DFT calculation also implies that poisoning the cobalt atom at the surface of LiCoO2 be an effective approach to suppress the electron transfer reaction between ethylene carbonate and delithiated LiCoO2 to enhance the interfacial stability of the cathode. Here, two surface modification agents were investigate to confirm the reaction mechanism between ethylene carbonate and delithiated LiCoO2. Figure 3a shows a direct comparison of the measured steady state leakage current for Li/LiCoO2 using different surface modification agents, demonstrating the positive impact of cyanide in suppressing the electron transfer reaction. It is well known that cyanide is a group of chemicals that can effectively coordinate with transition metals through - coupling. To demonstrate the concept, heptyl cyanide (CH3(CH2)6CN) was used as the surface modification agent to poison the cobalt atoms at the surface of LiCoO2 (see Figure 3b). To have a fair estimate on the alkyl group introduced in heptyl cyanide, trimethoxy(octyl)silane, CH3(CH2)7Si(OCH3)3) was used as the control surface modification agent to bond to the surficial oxygen atoms adjunct to cobalt atoms (see Figure 3c). Figure 3a shows a consistent trend that the


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steady leakage current increased with the increase of working potential of Li/LiCoO2 cells regardless of the surface modification agents used. In addition, the cell comprising the pristine LiCoO2 without surface modification showed a steady leakage current at about 0.05 A/mg (LiCoO2) and higher. When heptyl cyanide was introduced as the surface modification agent, the leakage current was dramatically suppressed to about 0.02 A/mg (LiCoO2). However, the introduction of trimethoxy(octyl) silane had no significant impact on the steady leakage current (see Figure 3a). This suggests that the poison effect of the surface modification agent was not contributed from the coverage of active sites by introducing additional alkyl groups, but from the coordination between cobalt atoms and cyanides, leaving less active sites for bonding with ethylene carbonate molecules. Figure 4a shows the initial voltage profiles of Li/LiCoO2 with and without surface modification; the cells were cycled between 3.0 V and 4.4 V using a constant current of C/10 (~18 mA/g). The initial discharge capacity of the pristine cell was about 171.8 mAh/g. When the heptyl cyanide (cyanide) was introduced as the modification agent for cobalt atoms at the surface, the reversible capacity was improved to about 176.3 mAh/g. The cell with trimethoxy(octyl) silane (silane) delivered a reversible capacity of about 174.2 mAh/g. Figure 4b shows a similar capacity retention for all cells, but the cells with surface modification agents maintained theirs advantages on higher reversible capacity during the course of cycling test. Combing the results shown in Figures 3 and 4, one can conclude that the heptyl cyanide, a surface modification agent, can effectively poison the active sites for parasitic reactions, and hence is beneficial to suppress the rate of the parasitic reactions, resulting in less undesired side products deposited on the surface of the working electrode. Therefore, an improvement on the electrochemical performance can be achieved. On the other hand, silane based surface modification is not able to reduce the rate of parasitic reactions,


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but still acts as an physical protection layer on the surface of LiCoO2 to enhance its tolerance against the deposited side products generated from the parasitic reactions. In our previous work, we demonstrated that ethylene carbonate is responsible for the oxidation reaction at the surface of the working electrodes 22, which is followed by a deprotonation reaction to generate high concentration protons at the surface of the working electrode. In addition, we also illustrated that the parasitic reactions at a potential lower than 4.4 V is primarily dominated by a chemical reaction between the working electrode and the electrolyte 20. In this work, we can further conclude that the active sites for the chemical reaction is the cobalt atoms (see Figure 4c for a schematic illustration), transition metals for the case of NMC cathodes. In principle, the carbonyl group of ethylene carbonate first chemically absorbs on surface cobalt atoms. The charging process will cause the reduction of the electron density 3d band of cobalt atoms in LiCoO2. The p- interaction between O atoms and Co atoms provide an energetically preferred pathway to facilitate the electron transfer between ethylene carbonate and LiCoO2. At the same time, the depleting 3d orbital of Co during charging reduces the energy gap between O atoms and Co atoms for actual electron transfer. Figure 4c implies that a slow electron transfer reaction, which is highly desired for high performance cathodes, can be achieved by surface modification either to reduce the rate of parasitic reactions or to enhance the tolerance of the material towards side products of parasitic reactions.


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(a)

e-

LixCoO2 e-

Source-meter

Li electrode

Self-discharge current (A/mg)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Self-discharge current (A/mg)


1.2 1.0

(c)

Page 10 of 17

(b)

0.2

Exerimental Expontial decay fit

0.1

0.0

-0.1

0

5

10

15

20

Time (hr) Electrochemical

Chemical Reaction

Reaction

0.8 0.6 0.4 0.2 0.0 3.8

3.9

4.0

4.1

4.2

4.3

4.4 +

4.5

4.6

4.7

4.8

Voltage (V) vs. Li/Li

Figure 1 (a) schematics showing the measuring principle of the prototype high precision electrochemical system; (b) a typical raw data collected to extract the steady state leakage current by holding the voltage of the cell at 4.35 V for 20 hours; (c) dependence of the steady state leakage current on the working potential of LiCoO2.


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2.2

0.0

(b) -0.5

(a)

Co-O bond length, Å

Binding energy, eV

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-1.0

2.1

2.0

1.9

-1.5

0

0.25

0.5

1.8

0.75

0

E=-0.25eV

0.5

0.75

x in Li1-xCoO2

x in Li1-xCoO2

(c)

0.25

C Co H Li

(d) E=-0.66eV

O

Figure 2 DFT results showing the dependence of (a) bonding strength between the Co and carbonyl group, and (b) the bond length between oxygen atom in carbonate and Co in LiCoO2; and illustration of bonding between ethylene carbonate and Li1-xCoO2 for (c) x=0; and (d) x=0.25.


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0.10

Pristine Cyanide modified Silane modified

0.08 0.06 0.04 0.02 0.00

3.9

4.0

4.1

4.2

4.3

TM O Li

R

R

R

R

R

R

R

R

Si

Si

Si

Si

Si

Si

Si

(c)

Si

R NC

R NC

NC

R

Voltage (V) R

(b)

Page 12 of 17

(a)

NC

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Selfdischarge current (A/mg)


TM O Li

Figure 3 (a) Steady state leakage current of LiCoO2/Li cells showing the impact of the surface modification using (b) heptyl cyanide, and (c) trimethoxy(octyl) silane.


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4.6

180

(a)

4.4

Specific capacity (mAh/g)

Voltage (V) vs.Li/Li+

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4.2 4.0 3.8 3.6 3.4


3.2 3.0 2.8

0

20

40

60

80 100 120 140 160 180

Specific capacity (mAh/g)

(b)

175 170 165 160


155 150

0

4

8

12

Cycle number

16

20

Lithium Cobalt Oxygen Carbon Hydrogen

(c) 1 2

Electron transfer

+

3

Deprotonation

Figure 4 (a) Initial cycling voltage profile of Li/LiCoO2 cells, and (b) Cycling performance of Li/LiCoO2 cells showing the impact of surface modification on the electrochemical performance LiCoO2 cathode; (c) proposed reaction mechanism of electron transfer reaction between ethylene carbonate and delithiated LiCoO2 using surficial cobalt atoms as the active reaction sites.


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AUTHOR INFORMATION Corresponding author: [email protected] (ATN); [email protected] (ZC) ORCID: Zi-Feng Ma, 0000-0001-5002-9766

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT Research was funded by U.S. Department of Energy (DOE), Vehicle Technologies Office. Support from David Howell and Tien Duong of the U.S. DOE’s Office of Vehicle Technologies Program is gratefully acknowledged. Argonne National Laboratory is operated for the US Department of Energy by UChicago Argonne, LLC, under contract DE-AC02-06CH11357. This work was partially supported by the Natural Science Foundation of China (21336003, 21676165 and 21506123).

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Identifying Active Sites for Parasitic Reactions at the Cathode

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