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Master's Thesis

Functional electrolytes to improve fast charging

performances of lithium-ion batteries

Hyebin Son

Department of Energy Engineering

(Battery Science and Technology)

Graduate School of UNIST

2018

1



Hyebin Son

Department of Energy Engineering

(Battery Science and Technology)

Graduate School of UNIST

2



A thesis/dissertation

submitted to the Graduate School of UNIST

in partial fulfillment of the

requirements for the degree of

Master of Science

Hyebin Son

01. 11. 2018 of submission

3



Hyebin Son

This certifies that the thesis/dissertation of Hyebin Son is approved.

01/11/2018

4

Abstract

Lithium-ion batteries have an outstanding property of high energy and power density, making it

the useful tools of choice for portable electronics, energy storage system (ESS), and EVs. If EVs

replace the majority petroleum transportations, it can reduce the greenhouse gas emissions

significantly. According to environment problem and resource depletion, many countries are

developing EVs even more. In EVs market, there are many important issues such as long cycle

life of the battery, affordable price, safety and charging time. At this present, the gasoline is

refueled in the car around 2min. But in the current circumstance, we still take a long time to

charge the EVs. So, we must improve the charging performance for our convenience and saving

time.

There have been no studies so far in terms of electrolytes for fast charging enhancement. In

this study, I demonstrated the dominating factors of the fast charging system. The first factor is

the anode interface stabilization. The most important problem at the fast charging is the lithium

plating, so anode additives should be applied to stabilize the negative electrode interface. I

adopted the EC, VC, FEC as additives. I analyzed SEI components by XPS. Also, I discovered

the difference of fast charging characteristic through the EIS and XRD measurements. The second

factor is the cathode interface stabilization. The unstable interface will result in overpotential of

the cell. Finally, the ionic conductivity of the electrolyte is one of the key factors in fast charging.

The solvents in the electrolytes serve to transport lithium ions, and low viscosity and high

transference number can make lithium ions move faster. It examined by means of GITT, rate

capability test.

Among these factors, I will talk about the effects of anode interface modification during fast

charging and ionic conductivity depending on the solvent composition, which determines fast

charging property by affecting lithium ion migration on the cathode side rather than on the anode

side.

6

Contents

1. INTRODUCTION

1.1 Lithium-ion battery

1.1.1. Lithium–ion batteries (LIBs) application in electric vehicles(EVs) market….…….....…...11

1.2. Importance of fast charging…………………………….……………………………..…...….....13

1.3. Challenges of fast charging….………………………………………..…….….….……....……..14

1.3.1. Lithium plating on the graphite…….……………………………….………….….……...14

1.3.2. Harmful effect of lithium plating….……………………………….…………….….........14

1.4. Electrolyte materials development for fast charging….………………….……….………….…..17

1.4.1 Inhibiting lithium plating…………………………………………….…….…………........17

1.4.2. Improving fast charge rate capability………....…………………….….….………………18

1.4.3. Ionic conductivity of electrolyte…………………………………….…….………………20

2.EXPERIMENTAL DETAILS

2.1 Materials and electrolytes………………………………………………….…….…………….…21

2.2 Electrochemical measurements………………………………………….….………………..…..22

2.3 Characterization……………….……………………………………….………….….…….….…23

3. RESULTS AND DISCUUSION

3.1 Dominating factors of fast charging…………………………………….…..…….……….……...24

3.2 Fast charging effect of anode additives……………………………………..….………….……...26

3.2.1. Electrochemical performances of EC, VC, FEC additives on the

Li[Ni0.6Co0.2Mn0.2]O2/graphite full cells………………….………………………..…........26

3.2.2 Surface chemistry and morphology of Li[Ni0.6Co0.2Mn0.2]O2/graphite full cells…..…........32

3.3 Fast charging effect of solvent composition……………………………………………...............40

7

3.3.1. Surface chemistry and morphology of Li[Ni0.6Co0.2Mn0.2]O2/graphite full cells................42

3.3.2. Structure of cycled Li[Ni0.6Co0.2Mn0.2]O2 from Li[Ni0.6Co0.2Mn0.2]O2/graphite full cells...45

3.3.3. Cause of the difference of fast charging performances..........................................................48

4. CONCLUSION..............................................................................................................................53

5. REFERENCES..............................................................................................................................54

8

List of figures

Figure 1. Schematic illustration of the Lithium-ion battery.

Figure 2. Graph of EVs market analysis.

Figure 3. Comparing gasoline vehicles and EVs with a photograph for fast charging.

Figure 4. SEM images of metal plating on the graphite anode.

Figure 5. A simplified model of the lithium plating-stripping process at different SOC levels: (a)

plating and (b) stripping at low SOC; (c) plating and (d) stripping at medium SOC; (e) plating and (f)

stripping at high SOC.

Figure 6. Cathode and anode potentials in MCMB-NCO Li-ion test cells with electrolytes containing

1 M LiPF6 in EC:DMC:DEC:EMC (1:1:1:4) without VC and with VC additive during charging at –

20°C at 50 mA to 4.1 V followed by a taper.

Figure 7. Rate capability of graphite/Li half cells at a) various charge (lithiation) rates.

Figure 8. Reversible capacity of natural graphite in the two electrolytes at various C-rates and 25 °C.

Charge and discharge were conducted at the same C-rate without using a constant-voltage mode at

both ends of charge and discharge, and the charge (lithium deintercalation) capacity was plotted.

(b)compared with 4.5M LiFSA/AN (c) 3.6M LiFSA/DME.

Figure 9. The Ionic conductivity of electrolyte and charge time of cells based on LiPF6 concentration.

Figure 10. (a) Voltage profiles of the NCM/graphite full cells with different additives (EC, VC, FEC)

during precycling between 3.0 and 4.2 V at a rate of C/10 (b) dQ/dV plot.

Figure 11. Electrochemical impedance spectroscopy (EIS) results of NCM/graphite full cells with

different additives after precycling (SOC=100%).

Figure 12. Fast charging performance and coulombic efficiency of NCM/graphite full cells at various

charge C - rates.

Figure 13. (a) Cycle stability and (b) coulombic efficiency of NCM/graphite full cells at low charge C-

rate (0.5C=1.5mAh/cm2) with difference reductive additive (EC, VC, FEC).

Figure 14. (a) The general voltage profile of three electrodes with pouch full cells system at 0.5C

(1.3mAh/cm2). (b) Anode voltage profile versus Li/Li+ at fast charging (26mAh/cm2).

Figure 15. Top view (in the red box), cross-sectional SEM images and EDS mapping of graphite anodes

from NCM/graphite full cells after 10cycles and 100cycles (Argon ion milling) : (a), (b), (c)after

9

10cycles, 100cycles, EDS image (O element) of 100cycles of EC additive. (d), (e), (f) after 10cycles,

100cycles, EDS image (O element) of 100cycles of VC additive. (g), (h), (i) after 10cycles, 100cycles,

EDS image (O element) of 100cycles of FEC additive.

Figure 16. XRD patterns and digital images after charging (in half cells) at different C –rate (time cut :

2h, 30min, 20min, 12min) of (a) EC additive (c) VC additive (e) FEC additive.

Figure 17. Comparison of XPS spectra of graphite from NCM /graphite full cells after precycling at

different additives. (a), (b), (c) C 1s spectra (d), (e), (f) O 1s spectra (g), (h), (i) F 1s spectra.


charge C - rates. XPS patterns of NCM cathode from NCM/graphite full cells after precycling at

different solvents.

Figure 19. (a) Longterm and fast charging (charge : 2C, discharge : 1C) performance and (b) coulombic

efficiency of NCM/graphite full cells with fixing reductive additive (FEC) in different solvents.

Figure 20. (a) Voltage profiles of the NCM/graphite full cells with fixing reductive additive(FEC)

during precycling between 3.0 and 4.2 V in different solvents. (b) dQ/dV plot.

Figure 21. EIS results of NCM/graphite full cells with different solvents after precycling (SOC=100%).

Figure 22. XPS spectra of graphite from NCM/graphite full cells after precycling with fixing reductive

additive (FEC) in different solvents. (a) C 1s spectra (b) O 1S spectra.

Figure 23. XPS spectra of NCM from NCM/graphite full cells after precycling with fixing reductive

additive (FEC) in different solvents. (a) C 1s spectra (b) O 1s spectra.

Figure 24. (a), (b) Top view, cross-sectional SEM images of NCM cathode from NCM/graphite full

cells after 100cycles (charge : 2C, discharge : 1C) with FEC/DMC (30/70) electrolytes. (c), (d) with

FEC/EMC (30/70).

Figure 25. XRD patterns of NCM cathode from NCM/graphite full cells after 100 cycles (charge : 2C,

discharge : 1C) with different solvent compositions.

Figure 26. Charge rate capability of graphite half cells with fixing reductive additive (FEC) in different

solvent composition (discharge : 0.5C).

Figure 27. Charge rate capability of NCM half cells with fixing reductive additive (FEC) in different


Figure 28. NCM/graphite full cells with fixing reductive additive(FEC) in different solvent composition

10

(discharge : 0.5C).

Figure 29. (a) IR drop (b) Overpotential from GITT test as a function of the SOC during fast charging

(5C) at NCM half cells.

Figure 30. Viscosity and ionic conductivity of 1.15M 30/70 (FEC/X) (X = DMC, EMC).

Scheme 1. Scheme of lithium plating process.

Scheme 2. Three kinds of factors for fast charging property from electrolytes view points

Scheme 3. Possible electrochemical reduction of (a) EC, (b) VC, (c) FEC.

Table 1. Previous study about lithium plating.

11

1.INTRODUCTION

1.1. Lithium-ion battery

1.1.1. Lithium–ion batteries (LIBs) application in electric vehicles(EVs) market

Lithium-ion batteries have an outstanding property of high energy and power density, making it the

useful tools of choice for portable electronics, power tools, energy storage system (ESS), and EVs. In

commercialized LIBs system, it consists of four kinds of materials such as cathode, anode, separator,

and electrolytes (Fig. 1).1 Both electrodes are able of reversible lithium insertion. The transfer of lithium

ions from the anode to cathode stores the energy whereas the reverse lithium transfer consumes the

energy. Nowadays, the usage of LIBs is increasing every year in various fields. Nowadays, the usage

of LIBs is increasing every year in various fields. Also, global warming came to the fore because of

carbon dioxide (CO2), many climate agreements were reached. If EVs replace the majority of petroleum

transportations, it can reduce the greenhouse gas emissions significantly. According to environment

problem and resource depletion, many countries are developing EVs even more. Also demands are

increasing (Fig. 2).2 It is important to develop the batteries for EVs market.

12

Figure 1. Schematic illustration of the Lithium-ion battery.1

Figure 2. Graph of EVs market analysis.2

13

1.2. Importance of fast charging

Recently, electrical vehicles becoming more important every year. In EVs market, there are many

important issues such as long cycle life of the battery, affordable price, safety and charging time. At this

present, the gasoline is refueled in the car around 2min. But in the current circumstance, we still take a

long time to charge the EVs (Fig. 3).3 So, we must improve the charging performance for our

convenience and saving time.

Portable Applications_Telecommunications Energy Conference, 2004. INTELEC 2004. 26th Annual

International

Figure 3. Comparing gasoline vehicles and EVs with a photograph for fast charging.

14

1.3. Challenges of fast charging

1.3.1. Lithium plating on the graphite

Lithium plating, so-called lithium deposition, on the graphite anode is one of the most problems for

degradation (Fig. 4).4-7,11 Generally, chemical reactions occur on the graphite like scheme 1-(1). If the

lithium ion current exceeds the intercalation current, lithium plaiting occurs the way of scheme 1-(2).8

Some researchers report the physical reason of lithium plating related to the charge transfer limitation

and the solid lithium diffusion limitation.9,10 These reactions take place in many environments. First,

charging at low temperature. Second, charging at high current rates. Third, the high polarization (poor

electrode kinetics, cell design, highly resistive solid electrolyte interphase(SEI)). The disordered carbon

and high N/P ratio make less lithium plating.12 These situations make the graphite (in full cell) reaches

the 0V vs Li+/Li. It can be detected by many ways such as differential analysis (dV/dQ), in-situ optical

test-cell, disassembling cell, or three-electrode measurements.7,8,13

1.3.2. Harmful effect of lithium plating

Lithium plating on the graphite causes a serious problem. Lithium metal on the graphite reacts with

electrolyte tremendously. Stacked lithium metal enlarges the active sites, which in the electrolyte reacts

with the metal significantly. It leads to loss of reversible lithium ion, and decomposed electrolyte covers

the top of the graphite anode. Continuously, the electrical contact surface which can offer the lithium

ion between electrode and electrolyte is decreasing. Therefore, it causes increasing the internal

resistance and the capacity degradation more accelerates. Also, plated lithium metal makes the severe

problem about the safety issue including internal short circuits or thermal runaway.23,24 For these reasons,

it is important to avoid the lithium metal plating for fast charging system.22

15

Scheme 1. Scheme of lithium plating process.8

Figure 4. SEM images of metal plating on the graphite anode.4

Figure 5. Simplified model of the lithium plating-stripping process at different SOC levels: (a) plating

and (b) stripping at low SOC; (c) plating and (d) stripping at medium SOC; (e) plating and (f)

stripping at high SOC.7

16

1.4. Development of electrolyte materials for fast charging

In the point of view electrolytes, a few methods for improving fast charging performance are reported.

1.4.1. Inhibiting lithium plating

In the previous part, it is mentioned that the lithium plating occurs at a high current density or low

temperature. So, we can improve the fast charging performance by inhibiting metal plating on the

graphite with less resistive SEI layer. Some researchers studied the optimization of electrolyte

composition in low temperature not in high charge rates. The ethylene carbonate (EC) contents are high

in low temperature, it causes much lithium metal plating. Also, vinylene carbonate (VC) is well known

for stabilizing the SEI layer.21 But in low temperature, With VC electrolytes show higher polarization

than without VC additive. It leads to lithium metal plating on the graphite due to kinetic hinderance of

lithium ion movement (Fig. 6).13

17

Electrode Comparing amount of lithium plating Condition

Ref

13MCMB/NCO

* High EC : 1.0M LiPF6 5:3:2 (EC : DMC : EMC)

> Low EC: 1.0M LiPF6 1:1:1:4

(EC : DMC : DEC : EMC)

* LOW EC + 1.5% VC > LOW EC

-20℃

0.1C Charge

2.75V

-.4.1V

Ref

14Graphite/NCA

REF : 1.0M LiPF6 2:2:6 (EC:EMC:MP)

* REF+ 0.1M LiBOB > REF + 2% VC > REF + 0.1M

LiDFOB

-20℃

0.5C Charge

2.75V

-.4.1V

Ref

15

Graphite/Lithiu

m

REF :1.3M LiPF6 3:2:5 (EC:EMC:DEC)

* REF > REF +2% AS

-30℃

10C Charge

Relaxation

test

Ref

16

Graphite/NMC

111

REF :1.0M LiPF6 3:7 (EC:EMC)

* REF 2% VC + 1% ES > REF + 0.25% TTSPi

(TMSPi) > REF > REF + 1% TAP

20℃

1C Charge

2.8V-4.1V

Table 1. Previous study about lithium plating.

Figure 6. Cathode and anode potentials in MCMB-NCO Li-ion test cells with electrolytes containing

1 M LiPF6 in EC:DMC:DEC:EMC (1:1:1:4) without VC and with VC additive during charging at –

20℃ at 50 mA to 4.1 V followed by taper.13

18

1.4.2. Improving fast charge rate capability

Choi et al. improved the charge rate capability with a combination of VC and LiDFP (Fig. 7). The XPS

measurement was conducted, the P – O moieties generated by LiDFP enhanced the kinetic of the SEI

layers. The polymer - based moieties generated by VC enhanced the stability of SEI layers.17 Amada et

al. reported superconcentrated electrolytes for fast charging. They used the high LiFSA concentration

for lithium salt, and dissolved in 1,2dimethoxyethane and acetonitrile (Fig. 8). It has little free solvents,

and shows the different Li+-conduction mechanism. It can be easily de-complexation of lithium salt in

less free solvents, so it shows ultrafast Li+ intercalation.18,19

19

Figure 7. Rate capability of graphite/Li half cells at various charge (lithiation) rates.17

Figure 8. Reversible capacity of natural graphite in the two electrolytes at various C-rates and 25 °C.

Charge and discharge were conducted at the same C-rate without using a constant-voltage mode at

both ends of charge and discharge, and the charge (lithium deintercalation) capacity was plotted.

(a)compared with 4.5M LiFSA/AN (b) 3.6M LiFSA/DME.18,19

(a) Cy

(b) Cy

20

1.4.3. Ionic conductivity of electrolytes

The ionic conductivity of electrolytes also related to the fast charging properties. Only the C.-K. Park

et al. compared the charging time in different salt concentration (x LiPF6 (x = 8% to 20%) in

EC/DEC/EMC (30/30/40 wt.%)). It can be shown in inverse proportion to each other (Fig. 9). The

charging time was reduced, when ionic conductivity increases. It explains that the fast lithium ion

moving helps the improving charging performance.20

Figure 9. Ionic conductivity of electrolyte and charge time of cells based on LiPF6 concentration.20

21

2. Experimental Details

2.1 Materials and electrolytes

In this study, I used a Li[Ni0.6Co0.2Mn0.2]O2 (NCM) positive electrode and an artificial graphite

negative electrode for the full cells. The cathode was fabricated by mixing 92.5 : 3 : 1.5 : 3 (w/w/w)

ratio of Li[Ni0.6Co0.2Mn0.2]O2 as anactive material (NCM, L&F Co.), carbon black, SFG6 as conducting

materials, and polyvinylidene fluoride (PVDF) as a binder. The anode was fabricated by mixing 96.6 :

1 : 1.4 : 1 (w/w/w) ratio of artificial graphite as an active material, SFG6L as a conducting material,

styrene–butadiene rubber, sodium carboxymethyl cellulose as binders. The loading density of cathode

and anode is 18mg/cm-2 and 8.3mg/cm-2, respectively. The specific capacity of the NCM/graphite full

cell is 3.5mAh/cm-2, which has high current density.

The microporous membrane (polyethylene film, SK Innovation Co., Ltd.) was used as a separator.

Thickness and porosity of a microporous polyethylene film were 20 μm and 38%, respectively.

The reference electrolyte(Ref) was composed of EC/DMC (5 : 95 vol.%) with 1.15M lithium

hexafluoro phosphate (LiPF6) (Soulbrain Co ., Ltd). To discuss the effect for fast charging, vinylene

carbonate(VC) and fluoroethylene carbonate(FEC) (Enchem Co., Ltd.) content of the cells, 5 wt.% of

anode additives were added instead of EC in the reference electrolyte. Then, to discuss the effect of

solvent, The second reference electrolyte(Ref_2) was composed of FEC/DMC (30 : 70 vol.%) and

FEC/EMC (30 : 70 col.%) with 1.15M lithium hexafluoro phosphate (LiPF6) (Soulbrain Co ., Ltd). The

water content in the electrolytes was removed from calcium hydride (CaH2) by stirring 1h. Coin-type

full cells (2032) with a LiNi0.6Co0.2Mn0.2O2 (NCM) cathode and a graphite anode assembled in an argon-

filled glove box with less than 1 ppm of both oxygen and moisture. A half cell also used in this study

was composed of each electrode of full cell and a Li metal electrode

22

2.2 Electrochemical measurements

The electrochemical performance of full cells was tested on a battery tester of WonATech WBCS 3000

at 30℃. Precycling was performed at C/10 in the voltage range of 3.0 to 4.2V (1 C = 175 mA g−1), then

the constant voltage condition was applied to the end of the charge of precycling until the current was

below C/50. Standard cycling was performed at C/5 during 3 cycles for the stabilization of full cells. To

investigate the fast charging cycle performance of the full cells, the high charge current (350mA g-1)

was applied, and then discharged at 1C. The cells used for the charge rate capability experiments were

cycled successively at fixed discharge C rate (C/5) and various charging C rates (C/5, 1C, 2C, 3C, 5C,

10C, C/5). It also was charged at a constant current (CC) - constant voltage (CV) mode, and it was

controlled by time cut off (1C = 1h cut off). Galvanostatic intermittent titration technique (GITT) test

also was performed by fast charging the cell for 1min 12s at a constant current density of 875mA g-1

and by relaxing for 2h to investigate the solvent behavior at the cathode side.

The dQ/dV graphs were obtained by computing the differential capacity versus the potential of cells

during precycling.

The electrochemical impedance spectroscopy (EIS) measurements for full cells were performed by

an IVIUM frequency response analyzer at SOC of 100%. The frequency range selected was between

1MHz and 10mHz. The potentiostatic signal amplitude was 5 mV.

23

2.3 Characterization

The density and viscosity of the solutions were evaluated with a Brookfield viscometer (LVDV-ll+P)

and an Oakton CON 11 standard conductivity meter, respectively. To investigate the degree of charge

state according to a different component of solid electrolyte interphase (SEI) at a different charge rate,

the X-ray diffraction (XRD) data for charged electrodes at different charge rate (0.5C, 2C, 3C, 5C) were

obtained using a Bruker D2 Phaser powder diffractometer equipped with a Cu Ka radiation source (λ =

1.54184 Å). The samples were scanned over the 2θ range of 10−80° at 1.5°/min. To retrieve the cycled

electrodes for surface chemistry and morphological analyses, the full cells were disassembled in a globe

box. Each of electrodes was washed with DMC to remove residual electrolyte components. The SEM

morphology analysis of anode surface in the cycled full cells was conducted by field-emission (FE)

SEM (JSM-6700F, JEOL) in a high-vacuum environment. The ion milling (HITACHI IM4000) was

performed with Ar+ ions at beam angles of incidence ranging from 0° to 60° from normal incidence to

observe the cross section of graphite side. The ex – situ X-ray photoelectron spectroscopy (XPS)

measurement was performed with a Scientific K-Alpha system, Thermo Scientific, (Al Ka radiation, hν

= 1486.6eV) under high vacuum. All XPS spectra were calibrated by the hydrocarbon peak at a binding

energy of 284.4 eV as a reference. Samples were prepared in a glove box and sealed with an aluminum

pouch film under a vacuum before analysis of surface. Then, samples were rapidly transferred into a

chamber of XPS or FE-SEM instrument to minimize any possible contamination.

24

3. Results and discussion

3.1 Dominating factors of fast charging

Scheme 2 shows the dominating factors of the fast charging system. The first factor is the anode

interface stabilization. The most important problem at the fast charging is the lithium plating, so anode

additives should be applied to stabilize the negative electrode interface. Based on the EC, I evaluated

the additives of VC and FEC, which are typical additives having cyclic structure at the anode. VC and

FEC have been reported many times in the literature. Generally, VC forms the poly(VC) – based

elements at the anode and shows improved thermal stability of interfacial.25-27 FEC composes the

interphase with the LiF component, it also increases the strength of SEI at the silicon anode.28-32 It will

be mentioned later how these additives have an effect on fast charging. The second factor is the cathode

interface stabilization. The unstable interface will result in associated overpotential of the cell.

Furthermore, metal dissolution of the cathode materials can be generated under a tensile stress of NCM

structure and dissolved metal ion can deposit at the anode. It promotes the lithium metal plating,33,34 so

it is important to stabilize the cathode interphase, too. Finally, the solvent composition of the electrolyte

is one of key factors in fast charging. The solvent in the electrolytes serves to transport lithium ions,35

and low viscosity and high transference number can make lithium ions move faster.36,37 In some papers,

the breakup of Li+ solvation sheath during intercalation process is defined as a rate determining step,

and the energy barrier is associated with the solvent composition.38-41 However, there are limited that

the experiments were carried out only in graphite half cells. In addition, it has been reported that the ion

mobility is improved when the ion conductivity is high, thereby reducing the charging time.20 It related

to the solvent composition for the fast charging.

Among these factors, I will talk about the effects of interface modification on the charging

characteristics and ionic conductivity depending on the solvent composition, which determines fast

charging property by affecting fast lithium ion migration on the cathode side rather than on the anode

side.

25

Scheme 2. Three kinds of factors for fast charging property from electrolytes view points

26

3.2. Fast charging effect of anode additives

3.2.1. Electrochemical performances of EC, VC, FEC additives on the NCM/graphite full cells

Fig. 10(a) shows the voltage profiles of NCM/graphite full cells with various additives during

precycling at 25°. The full cells without additive displays reduced charge capacity (163.4mAh g-1)

compared to the VC (172.1mAh g-1), FEC (172.3mAh g-1). Also, the EC - based electrolyte shows the

low initial coulombic efficiency (ICE = 85.2%). Fig. 10(b) provides differential capacity profiles

associated with the formation of SEI layer. The peak in the 2.6V - 2.7V range corresponds to the early

reduction of FEC, then VC is decomposed at 2.9V.42,44,45 Commonly, the EC is reduced at around 3.2V.

This suggests that VC and FEC additives inhibit the capacity loss making stable SEI layer and it

correlates with the initial coulombic efficiency. The EC-based electrolyte makes the unstable SEI layer

trapping more lithium ion. To explore the resistance of each SEI layer for Li diffusion at charge state, I

measured the electrochemical impedance spectroscopy (EIS) spectra of the full cells (SOC=100%) as

shown in Fig. 11. Surprisingly, the VC - based electrolyte makes stable SEI layer but shows the highest

value of resistance at charge state because of the thick layer.43 On the contrary, the FEC – based

electrolyte shows lowest value of impedance due to thin and stable SEI layer.46,47

27

Figure 10. (a) Voltage profiles of the NCM/graphite full cells with different additives (EC, VC, FEC)

during precycling between 3.0 and 4.2 V at a rate of C/10 (b) dQ/dV plot.

Specific Capacity(mAh g-1)

0 50 100 150 200

Vo

ltag

e (

V)

0

1

2

3

4

5

5/95 (EC/DMC)5/95 (VC/DMC)

5/95 (FEC/DMC)

Voltage (V)

2.4 2.6 2.8 3.0 3.2

dQ

/dV

0.0000

0.0002

0.0004

0.0006

0.0008

0.00105/95 (EC/DMC)5/95 (VC/DMC)5/95 (FEC/DMC)

28

Figure 11. Electrochemical impedance spectroscopy (EIS) results of NCM/graphite full cells with

different additives after precycling (SOC=100%).

Z' (ohm)

0 5 10 15 20

-Z"

(oh

m)

0

5

10

15

205/95 EC/DMC5/95 VC/DMC5/95 FEC/DMC

29

Fig. 12 presents the discharge capacities of NCM/graphite full cells. The preceding 100 cycles were

charged at a rate of 2C (6mAh/cm2) and the next 100 cycles were charged at a rate of 3C (8.8mAh/cm2).

Interestingly, the VC – based electrolyte with the highest ICE shows the lowest capacity retention (61%).

The FEC-based electrolyte has a retention of 90% at a current density of 8.8mA/cm2. The 0.5C charging

evaluation was performed to investigate whether the SEI characteristics were the same at low speed.

Contrary to the fast charging, Fig. 13(a) presents that the EC – based electrolytes with an unstable film

show the lowest discharge capacity and coulombic efficiency in accordance with the ICE. So, it is

thought that the cell performances at the fast charging can be significantly changed according to the SEI

property of the anode. Fig. 14(a) was indicated the voltage profile of NCM vs Li+/Li, graphite vs Li+/Li

in the three-electrode system at 0.5C. However, in case of charging at high current density, the anode

voltage drops below 0V compared to the reference electrode due to the overvoltage, especially in the

VC-based electrolyte (Fig 14(b)).


charge C - rates.

Cycle Number

0 50 100 150 200

Sp

ec

ific

Ca

pa

cit

y (

mA

h g

-1)

0

50

100

150

200

Co

ulo

mb

ic E

ffic

ien

cy

(%

)

0

20

40

60

80

100

5/95 (EC/DMC)5/95 (VC/DMC)5/95 (FEC/DMC)

6mAh/cm2

8.8mAh/cm2

30

Figure 13. (a) Cycle stability and (b) coulombic efficiency of NCM/graphite full cells at low charge C-

rate (0.5C=1.5mAh/cm2) with difference reductive additive (EC, VC, FEC).

Cycle Number

0 50 100 150 200 250 300

Sp

ec

ific

Ca

pa

cit

y (

mA

h g

-1)

0

50

100

150

200


Cycle Number

0 50 100 150 200 250 300

Co

ulo

mb

ic E

ffic

ien

cy (

%)

80

85

90

95

100

105


31

Figure 14. (a) The general voltage profile of three electrodes with pouch full cells system at 0.5C

(1.3mAh/cm2). (b) Anode voltage profile versus Li/Li+ at fast charging (26mAh/cm2).

Specific capacity (mAh/g)

0 50 100 150 200

Vo

lta

ge

(V

)

0

1

2

3

4

50.5C

Specific capacity (mAh/g)

0 50 100 150 200

Vo

lta

ge

(V

)

-0.2

0.0

0.2

0.4

0.6

0.8

1.00.5CEC 10CVC 10CFEC 10C

(a) Cy

(b) Cy

32

3.2.2. Surface chemistry and morphology of NCM/graphite full cells

In Fig. (12), there is a difference of capacity fading after 100 cycles. It was confirmed by SEM and

EDS images. There is no change of electrode surface between the compositions after 10 cycles, as

shown in Fig. 15(a), (d), (g). However, I can investigate the difference in surface morphology after

100cycles. Comparing Fig. 15(b), (e) and (h), the VC-based anode is covered with the thickest

decomposition of the electrolyte. It confirmed that the electrolyte was decomposed on the anode through

the EDS of O element (Fig. 15(c), (f), (i)). The reason for lithium plating is that the large resistance due

to the thick SEI layer of poly(VC) as evidence by the EIS measurement. Therefore, the cause of the

capacity fading is the loss of reversible lithium because of its high reactivity with the electrolytes. In

the red box in Fig. 15(e), I can see the dendritic lithium metal, it also has adverse effects on the safety

issue.8,9

33

Figure 15. Top view (in the red box), cross-sectional SEM images and EDS mapping of graphite anodes

from NCM/graphite full cells after 10cycles and 100cycles (Argon ion milling) : (a), (b), (c)after

10cycles, 100cycles, EDS image (O element) of 100cycles of EC additive. (d), (e), (f) after 10cycles,

100cycles, EDS image (O element) of 100cycles of VC additive. (g), (h), (i) after 10cycles, 100cycles,

EDS image (O element) of 100cycles of FEC additive.

(a) Cy

(d) Cy

(b) Cy

(c) Cy

(e) Cy

(f) Cy

(g) Cy

(h) Cy

(i) Cy

34

XRD analysis was performed to observe the depth of charging at the anode according to the surface

components (Fig. (16)). The pristine peak located at 26.5° is attributed to the (002) graphitic plane, and

a phase transition to LiC6 and LiC12 is can be seen at various C - rate.48,49 Comparing Fig. 16(a) and (b),

the intensity of LiC6 and LiC12 is very similar, but there is a clear difference in 5C charging. Fig. 16(c)

represents that the FEC -based interface enables rapid lithium ion insertion. I can also check the

difference in digital images after charging. Graphite becomes gold color when lithium is inserted.49,50

In case of FEC - based electrolyte, slight gold color occurs from 3C and VC – based electrolyte appears

similar color to pristine color at 5C. It means that the when the SEI components are different, even if

they are charged at the same current density for the same time, the insertion speed of lithium can be

changed.

35

Figure 16. XRD patterns and digital images after charging (in half cells) at different C –rate (time cut :

2h, 30min, 20min, 12min) of (a) EC additive (c) VC additive (e) FEC additive.

36

In Fig. 17, I measured the composition of SEI layer by using XPS after formation cycling. In Fig.

17(b), two new peaks appear at 291.1eV and 287.1eV.27,29 It corresponds to CO3, C-O of poly (VC).

The FEC- based electrolytes show the large FEC- based CO3 at 290eV than EC - based CO3.51,52

Furthermore, this result suggests that EC-based electrolytes form the general SEI component like the

lithium ethylene dicarbonate (LEDC).54 The poly (VC) composition also can be observed at O 1s

(535.1eV) XPS data in Fig. 17(a), (b), (c). Comparing Fig. 17(g) and (i), the intensity of Li-F bond is

larger than EC based electrolyte’s at 684eV but remaining intensity of P-F bond at 686.5eV. It means

that the decomposed of FEC cause the formation of Li-F components not from LiPF6 decomposition.

The VC - based electrolyte attributes the thick and resistive SEI layer with poly (VC),17,25 and the FEC

- based electrolyte modifies the combined SEI compositions of Li-F and FEC-based CO3, which induce

lithium ion to move fast. All these formation mechanisms are presented in scheme 3, and it matches

well with the XPS data.

37

Binding energy (eV)

279282285288291294297

Inte

ns

ity

(a

.u.)

0

10000

20000

30000

40000

500005/95 (EC/DMC)

Li2CO3

C-O-C

C-H

CO3

C 1s

Binding Energy (eV)

528532536540

Inte

ns

ity

(a

.u.)

0

10000

20000

30000

400005/95 (EC/DMC)

Li2CO3

C=O

C-O-C

O 1s

72

4.5

5

16.5

2

Binding energy (eV)

279282285288291294297

Inte

ns

ity

(a

.u.)

0

10000

20000

30000

40000

500005/95 (VC/DMC)

Poly VC1Li2CO3

Poly VC2

C-O-C

C-HC 1s

Binding Energy (eV)

528532536540

Inte

ns

ity (

a.u

.)

0

10000

20000

30000

400005/95 (VC/DMC)

Li2CO3

C=O

C-O-C

Poly VC

O 1s

Binding energy (eV)

279282285288291294297

Inte

ns

ity

(a

.u.)

0

10000

20000

30000

40000

500005/95 FEC/DMC

Li2CO3

C-O-C

C-H

C-F

F basedCO3

C 1s

Binding Energy (eV)

528532536540

Inte

nsit

y (

a.u

.)

0

10000

20000

30000

400005/95 (FEC/DMC)

Li2CO3

C=O

C-O-C

O 1s

(a)c

(b)c

(c)c

(f)

c

(e)

c

(d)

1s

27

45

42

71

55

3121

38

Figure 17. Comparison of XPS spectra of graphite from NCM /graphite full cells after precycling at

different additives. (a), (b), (c) C 1s spectra (d), (e), (f) O 1s spectra (g), (h), (i) F 1s spectra.

Binding energy (eV)

678681684687690693696

Inte

ns

ity

(a

.u.)

0

5000

10000

15000

200005/95 (EC/DMC) F 1s

P-F

Li-FC-F

Binding energy (eV)

678681684687690693696

Inte

ns

ity

(a

.u.)

0

5000

10000

15000

200005/95 (VC/DMC) F 1s

P-F

Li-FC-F

Binding energy (eV)

678681684687690693696

Inte

ns

ity

(a

.u.)

0

5000

10000

15000

200005/95 (FEC/DMC) F 1s

P-F

Li-F

C-F

(g)

(h)

(i)

39

Scheme 3. Possible electrochemical reduction of (a) EC57, (b) VC25, (c) FEC.30,55,56

(a)

Cy

40

3.3. Fast charging effect of solvent composition

I have confirmed that the FEC produces stable SEI layer at the anode that can inhibit the lithium plating,

and then FEC was fixed to change the solvent composition. Fig. 18 presents the discharge capacity of

NCM/graphite full cells with different solvents at fast charging condition. As a result, fast charging

performances are improved when using DMC than EMC. At a current density of 8.8mAh/cm2, the result

shows an improved capacity retention of 93% for FEC/DMC over FEC/EMC (60%). Additionally, it

delivers also a high capacity retention of 70% under the harder condition at a current density of

15mAh/cm2 (5C). The long cycle capacity retention of 80% is obtained for the full cell with the DMC -

based electrolyte (Fig. 19(a), (b)). It will be discussed why this difference is occurred due to the type of

linear carbonate, even though the anode interface additive is fixed.

41


charge C – rates (6, 8.8, 15mAh/cm2).

Figure 19. (a) Longterm and fast charging (charge : 2C, discharge : 1C) performance and (b) coulombic

efficiency of NCM/graphite full cells with fixing reductive additive (FEC) in different solvents.

Cycle Number

0 50 100 150 200 250 300

Sp

ec

ific

Ca

pa

cit

y (

mA

h g

-1)

0

50

100

150

200

Co

ulo

mb

ic E

ffic

ien

cy (

%)

0

20

40

60

80

100

30/70 (FEC/DMC)30/70 (FEC/EMC)

Cycle number

0 200 400 600 800 1000

Co

ulo

mb

ic e

ffic

ien

cy (

%)

80

85

90

95

100

105

30/70 FEC/DMC30/70 FEC/EMC

Cycle number

0 200 400 600 800 1000

Sp

ec

ific

cap

ac

ity

(m

Ah

/g)

0

50

100

150

200


(b) Cy

6mAh/cm2

(a) Cy

8.8mAh/cm2

15mAh/cm2

6mAh/cm2

42

3.3.1. Surface chemistry and morphology of NCM/graphite full cell

The electrochemical performances and interface properties of full cells were analyzed. In the initial

cycling, both compositions have a 171mAh/g and an ICE of 90% (Fig. 20(a)). The dQ/dV plots in Fig.

20(b) present the same decomposition voltages of FEC in electrolytes, and the peaks occurred at a

voltage lower than the 5/95(FEC/DMC) composition due to the higher FEC contents. Additionally, the

two impedance spectra have similar values. The semicircle is an indication of SEI resistance and charges

transfer resistance (Fig. 21).

I measured the composition of SEI layer of anode and cathode by using XPS after formation cycling

as well. It can be thought that the formation of CO3 or C=O because of the reductive decomposition of

FEC (Fig. 22). There is not a big difference of C 1s or O 1s peak intensity in cathode part (Fig. 23).58

Therefore, the distinction in fast charging performances due to solvent composition is not from the

interface characteristics.

43

Figure 20. (a) Voltage profiles of the NCM/graphite full cells with fixing reductive additive(FEC)

during precycling between 3.0 and 4.2 V in different solvents. (b) dQ/dV plot.

Figure 21. EIS results of NCM/graphite full cells with different solvents after precycling (SOC=100%).

Specific Capacity(mAh g-1

)

0 50 100 150 200

Vo

lta

ge

(V

)

0

1

2

3

4

5


Voltage(V)

2.4 2.6 2.8 3.0 3.2

dQ

/dV

0.0000

0.0002

0.0004

0.0006

0.0008

0.001030/70 (FEC/DMC)

30/70 (FEC/EMC)

Z' (ohm)

0 3 6 9 12 15

-Z"

(oh

m)

0

3

6

9

12


(a) Cy

(b) Cy

44

Figure 22. XPS spectra of graphite from NCM/graphite full cells after precycling with fixing reductive

additive (FEC) in different solvents. (a) C 1s spectra (b) O 1S spectra.

Figure 23. XPS spectra of NCM from NCM/graphite full cells after precycling with fixing reductive

additive (FEC) in different solvents. (a) C 1s spectra (b) O 1s spectra.

Binding Energy (eV)

279282285288291294297

Inte

ns

ity

(a

.u.)

0.0

2.0e+4

4.0e+4

6.0e+4

8.0e+4

1.0e+5

1.2e+530/70 (FEC/DMC)30/70 (FEC/EMC)pristine

Li2CO3

C-O-CC-H

F- basedCO3

Binding Energy (eV)

528532536540

Inte

ns

ity

(a.u

.)

0

10000

20000

30000

40000

50000

6000030/70 (FEC/DMC)30/70 (FEC/EMC)pristine Li2CO3

C=O

C-O-C

Binding Energy (eV)

279282285288291294297

Inte

ns

ity

(a

.u.)

0

20000

40000

6000030/70 (FEC/DMC)30/70 (FEC/EMC)pristine

CH2-CF2

(Binder)

CH2-CF2

(Binder)

C-H

Binding Energy (eV)

528532536540

Inte

ns

ity

(a.u

.)

0

5000

10000

15000

20000

25000

3000030/70 (FEC/DMC)30/70 (FEC/EMC)pristine

M-O

C=O

C-O-C

(a) (b)

(b) Cy

(c) (d)

45

3.3.2. Structure of cycled NCM form NCM/graphite full cells

SEM analysis was performed to confirm if the capacity fading in Fig. 17 was due to the structural

collapse of the cathode. As a result of cross-sectional SEM image, an intragranular crack formed by

tensile stress was not observed in both DMC – based (Fig. 24(a)) and EMC – based (Fig. 24(b))

compositions. 62 After 100 cycles, NCM structure obtained from NCM/graphite full cells was measured

with XRD measurement. Another structural collapse was not observed except the gap of the peak (108)

and (110) due to open circuit voltage. Both samples possess a single phase with layered structure (α-

NaFeO2 type structure space group, R3m). The intensity of I(003)/I(104) also similar with each other

(Fig. 25).63 Resultingly, the capacity fading after 100 cycles seems not to be related to the structural

collapse of the cathode or instability of the initial cathode interface.

46

DMC

EMC

Figure 24. (a), (b) Top view, cross-sectional SEM images of NCM cathode from NCM/graphite full

cells after 100cycles (charge : 2C, discharge : 1C) with FEC/DMC (30/70) electrolytes. (c), (d) with

FEC/EMC (30/70).

(a) (b)

(b)

(c) (b)

(d) (b)

47

Figure 25. XRD patterns of NCM cathode from NCM/graphite full cells after 100 cycles (charge : 2C,

discharge : 1C) with different solvent compositions.

2q (degree)

10 20 30 40 50 60 70 80

Inte

ns

ity


00

3

10

10

06

/10

2 104

10

5

107

10

8/1

10

113

003

101

00

6/1

02

104

10

5

10

7

10

8/1

10

11

3

48

3.3.3. Cause analysis of the difference between fast charging performances

In the previous section, I have shown that there is no difference in the initial interfacial components,

so it can be considered that there is an effect on the delivery rate of lithium ions due to the difference in

solvent composition. Existing researches have reported the difference in activation energy depending

on the solvent when lithium ions are inserted into the anode half cell. When EC content is less than

20%, DMC is the main mediator of lithium ion transfer. DMC has a higher dielectric constant (3.1) than

EMC (2.9), so it has a stronger binding with lithium ion and increases the activation energy required

for lithium insertion, but when the EC content is more than 20%, EMC requires more activation

energy.40,41 It is also reported that stabilization of the interface as well as solvent composition can reduce

the activation energy required for insertion.39 However, there is a limitation that it is calculated through

the interface resistance using a three-electrode tool instead of the actual full-cell system. In the actual

full-cell system, as a cathode can also affect fast charging.

Fig. 26 and Fig. 27 presented the rate capability of each half cells. Interestingly, there is almost no

specific capacity difference in the anode half cell. However, in the cathode half cell data, the DMC

composition showed an improved rate characteristic, which was similar to that of the full cell system.

(Fig. 28). I thought to be that the DMC – based electrolyte can receive the lithium ion rapidly from the

cathode and can move fast towards the anode when charging at high current density.

49

Figure 26. Charge rate capability of graphite half cells with fixing reductive additive (FEC) in different


Figure 27. Charge rate capability of NCM half cells with fixing reductive additive (FEC) in different


Figure 28. NCM/graphite full cells with fixing reductive additive(FEC) in different solvent composition

(discharge : 0.5C).

50

To investigate the characteristics of the electrolyte at the cathode interface, the GITT test was carried

out at 5C. As a result, DMC – based electrolyte showed the lower values in both overpotential and IR

drop, as shown in Fig 29(a), (b). Assuming that the cathode material and the SEI component are same,

the IR drop after applying the instantaneous voltage can represent the property showing the rapid

movement of the electrolytes, the overpotential can represent the transport property between electrode

and SEI layer.59,60 The DMC – based electrolyte can provide a fast migration of lithium ions at the

cathode interface. In Fig. 30, the ionic conductivity and viscosity of the electrolyte are shown, and this

property seems to be able to alleviate the concentration gradient in the full cell.61 It is thought to be that

the degradation of EMC – based electrolyte is not related to the structural collapse or SEI components.

As a result of GITT of rate test, the delay of lithium ion transport at cathode lead to the increasing

overall cell overpotential, and occur the lithium plating in the full cell. So, it is essential for increasing

the ionic conductivity of electrolytes for fast charging.

51

Figure 29. (a) IR drop (b) Overpotential from GITT test as a function of the SOC during fast charging

(5C) at NCM half cells.

State of the Charge (%)

0 20 40 60 80 100

IR d

rop

(V

)

0.0

0.1

0.2

0.3

0.4

0.530/70 (FEC/DMC)30/70 (FEC/EMC)

State of the Charge (%)

0 20 40 60 80 100

Ov

erp

ote

nti

al (V

)

0.0

0.1

0.2

0.3

0.4

0.5


(a) (b)

(b) (b)

52

Figure 30. Viscosity and ionic conductivity of 1.15M 30/70 (FEC/X) (X = DMC, EMC).

53

4. Conclusions

In this study, I investigated the dominating factors of the fast charging system. I discovered that the

FEC as an anode additive improves the fast charging performance. In solvent composition, DMC-based

electrolyte which has higher ionic conductivity also improves the fast charging performance.

Generally, VC forms the poly(VC) – based elements at the anode and shows the improved thermal

stability of interfacial. But in the fast charging, the poly(VC) causes large resistance due to the thick

SEI layer as evidence by the EIS measurement. Then, the cell polarization leads to the lithium plating.

Finally, the capacity fading occurs due to the loss of reversible lithium because of its high reactivity

with the electrolytes. I can see the dendritic lithium metal with SEM images, it also has adverse effects

on the safety issue. However, the FEC - based electrolyte modifies the combined SEI compositions of

Li-F and FEC-based CO3, which induce lithium ion to move fast. I examined that the FEC -based

interface enables rapid lithium ion insertion by XRD. I can also check the difference in digital images

after charging. Graphite becomes gold color when lithium ion is inserted. In case of FEC - based

electrolyte, slight gold color occurs from 3C.

Then, the FEC was fixed to change the solvent composition. In the cathode half cell test, the DMC

composition shows an improved rate characteristic, which was similar to that of the full cell system. I

thought to be that the DMC – based electrolyte can receive the lithium ion rapidly from the cathode and

can move fast towards the anode when charging at high current density. The GITT revealed that the

DMC – based electrolyte shows the lower values in both overpotential and IR drop. Also, the ionic

conductivity and viscosity of the electrolyte are shown, and this property seems to be able to alleviate

the concentration gradient in the full cell.

54

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