Lithium Dendrite Prevention for Lithium-Ion Batteries€¦ · Lithium Dendrite Prevention for...

Lithium Dendrite Prevention for Lithium-Ion Batteries

Wu Xu and Ji-Guang Zhang

Pacific Northwest National Laboratory

2016 DOE Vehicle Technologies Program ReviewJune 6-10, 2016

This presentation does not contain any proprietary, confidential, or otherwise restricted information

Project ID #ES27511

2

OverviewTimeline• Start date: Oct. 2012

• End date: Sept. 2017

• Percent complete: 80%

Budget• Project funding

– DOE share 100%• Funding received in FY15:

$400k• Funding for FY16: $400k

Barriers addressed• Growth of lithium dendrites

• Low Coulombic efficiency

• Low charge current density

Partners• Argonne National Laboratory

• Lawrence Berkeley National

Laboratory

• U.S. Army Research Laboratory

2

3

Relevance/Objectives Prevent lithium (Li) dendrite formation on Li-metal anodes

in Li-metal batteries and on carbon anodes in Li-ion batteries.

Enable Li metal to be used as an effective anode in rechargeable Li-metal batteries for long cycle life at a reasonably high current density.

Explore various factors that affect the morphology of Li deposition, especially under high current density conditions.

3

4

MilestonesFY16 Develop mixed salts electrolytes to

protect Al substrate and Li metal anode, and to maintain Li Coulombic efficiency over 98%. (Dec. 2015) Completed

Demonstrate over 300 cycles for Li||LFP cells with high LFP loading and at high current density cycling. (March 2016) Completed

Demonstrate over 100 cycles for 4-V Li-metal batteries with high cathode loading and at high current density cycling. (June 2016) Ongoing

Achieve over 500 cycles for 4-V Li-metal batteries with high cathode loading and at high current density cycling. (Sept. 2016) Ongoing

FY15 Develop electrolytes to suppress Li

dendrite growth on Li metal and graphite anode. (Dec. 2014) Completed

Protect graphite electrode in PC-EC-based carbonate electrolytes with electrolyte additives. (March 2015) Completed

Demonstrate over 300 cycles for 4-V Li-metal batteries without internal short circuiting, through optimized electrolyte formulation. (June 2015) Completed

Achieve over 300 cycles for 4-V Li-metal batteries without internal short circuiting, through optimized electrolyte formulation. (Sept. 2015) Completed

4

Approach1. Use Cs+ additive and EC-PC-based electrolytes to form

high quality SEI layer to suppress Li dendrites on surfaces of Li-metal and graphite anodes while maintaining long-term operation of batteries.

2. Use optimized charge/discharge protocol to form a transient high-concentration electrolyte layer on Li surface so as to protect Li metal anode and to enable sustainable operation of Li metal batteries.

3. Use dual-salt electrolytes to form highly conductive SEI, protect Li metal, enable long cycle life and fast chargeability of Li metal batteries.

5

Performances of graphite||NCA full cells in a wide temperature range

Technical Accomplishments

0 30 60 90 120 150 180 2100

1

2

3

4

5

E17 E17Cs

Volta

ge (V

vs.

Li/L

i+ )

Capacity (mAh g-1)

CE: 79%84%

E17 E17Cs

RT

0 20 40 60 80 1000

20

40

60

80

100

120

140

160

180

Cap

acity

(mA

h g-1

)

Cycle number

E17 E17Cs

E17Cs

E17

60oC

-40 -30 -20 -10 0 10 20 300

20

40

60

80

100

120

Cap

acity

rete

ntio

n (%

)

Temperature (°C)

E17Cs E2

0 20 40 60 80 100 120 140 1602.42.73.03.33.63.94.2

-40oC -30oC -20oC 0oC RT

E17Cs

Volta

ge (V

) E2

RT-40oC -30oC -20oC 0oC

0 20 40 60 80 100 120 140 1602.42.73.03.33.63.94.2

Volta

ge (V

)

Discharge capacity (mAh g-1)

Electrolytes with Cs+ additive significantly enhance the cycling stability at elevated temperature and the low-temperature discharge performances of graphite||NCA full cells.

E17: 1.0 M LiPF6 in EC-PC-EMC (2:1:7 by wt.)

E17Cs: E17 + 0.04 M CsPF6

E2: 1.0 M LiPF6 in EC/EMC (3:7 by vol.)

6

Characterization of graphite anodes after tested in full cells for 100 cycles at 60°C

7


E1Cs E1FEC

SEM

TEM

The graphite anode from E1Cs appears to be clean (a), and the SEI layer is rather uniform with a thickness of <2 nm (c). This is almost the same as the

SEI thickness generated at 0.3 V following the first charge, indicating a stable and uniform interphasial morphology even at high temperature for long-term cycling.

The SEI layer on the graphite tested in E1FEC contains many particles and the thickness is uneven (b), varying between 11 and 28 nm (d).

E1: 1.0 M LiPF6 in EC-PC-EMC (5:2:3 by wt.)E1Cs: E1 + 0.05 M CsPF6E1FEC: E1 + 2% FEC

7

Electrochemical analysis on Cs+-additive in Li||graphite cells

8


0 200 400 600 800 10000.0

0.5

1.0

1.5

2.0

2.5

Capacity (mAh g-1)

Volta

ge (V

vs.

Li/L

i+ )

(A)

CE: 88%

E1 E1Cs E1FEC E2

CE: 84% CE: 67%

CE: 30%

0.0 0.5 1.0 1.5 2.0

-6000

-4000

-2000

0

2000

4000

6000

E1 E1Cs E1FEC

(B)

dQ/d

V (m

Ah g

-1 V

-1)

Voltage (V vs. Li/Li+)0.4 0.5 0.6 0.7 0.8 0.9 1.0

-70

-60

-50

-40

-30

-20

-10

0

dQ/d

V ( m

Ah g

-1 V

-1)

Voltage (V vs. Li/Li+)

E2 E2Cs

E2

E2Cs

(C)

0.4 0.6 0.8 1.0 1.2 1.4

-600

-500

-400

-300

-200

-100

0


dQ/d

V (m

Ah g

-1 V

-1)

in EC+PC+EMC (5:2:3, wt.)

E8

E3

E9

E7

E7: 0.95M LiTFSI + 0.05M CsTFSI E8: 0.9M LiTFSI + 0.1M CsTFSI E9: 0.8M LiTFSI + 0.2M CsTFSI E3: 0.5M LiTFSI + 0.5M CsTFSI

(B)

0.4 0.6 0.8 1.0 1.2 1.4

-200

-150

-100

-50

0

50


E10: LiTFSI-CsTFSI/EC+PC+EMC (1:2:7) E11: LiTFSI-CsTFSI/EC+PC+EMC (2:2:6) E12: LiTFSI-CsTFSI/EC+PC+EMC (3:2:5) E8: LiTFSI-CsTFSI/EC+PC+EMC (5:2:3)

E10

E11E12

E8

(C)

dQ/d

V (m

Ah g

-1 V

-1)

0 20 40 60 80 100 120 140 400 5000.0

0.5

1.0

1.5

2.0

2.5

Volta

ge (V

vs.

Li/L

i+ )

Capacity (mAh g-1)

E3 LiTFSI-CsTFSI in EC-PC-EMC E4 LiTFSI-CsTFSI in EC-EMC E5 LiTFSI-CsTFSI in PC E6 LiTFSI in EC-EMC

E3E4

E5

E6E6 E4 E3

(A)

Reduction potentials: FEC ~0.95 V, EC 0.75 V, PC 0.7–0.5 V Cs+ induces slightly more EC decomposition.

Increasing Cs+ content or EC content shifts the peak at 0.75 V to 0.8 V and increases the peak intensity.

EC reduction enhanced by Cs+ is responsible for the SEI formation.8

Protection mechanism of Cs+-additive on graphite in PC-containing electrolytes

9


Cs+-(EC)m (m=1,2) solvates have planar structure, indicating possibly fast mobility and easy intercalation to graphite layers to decompose.

Cs+-(EC)m (m=1,2) solvates reduce earlier than Li+-(PC)n (n=3,4). Cs+ at 0.05 M has an effective redox potential of -3.103 V vs. SHE, lower

than that of Li+ at 1.0 M (-3.040 V vs. SHE). The formed SEI suppresses further solvent decomposition Thin SEI

layer.9

Effect of PC content on graphite in half cells using electrolytes with and without Cs+ additive

10


Cs+ has no effect in sole PC electrolyte. In EC-PC-EMC with 30 wt% EC and 20-

30% PC, Cs+ significantly suppresses PC co-intercalation.

Without Cs+ additive, the SEI is thick. Addition of Cs+ significantly reduces SEI

thickness.10

Effect of Cs+-additive on Li dendrite formation on graphite anode at various current densities

11


1.5 mAh/cm2 graphite electrode E-control has slightly higher

polarization than E20PC-Cs. For E-control, some Li dendrites

form at lithiation current density of 0.5 mA/cm2, and more Li dendrites form at higher current densities.

For E20PC-Cs, no Li dendrites are found at lithiation current densities of 0.2 and 0.5 mA/cm2, and much less Li dendrites are observed at current densities of 1.0 and 2.0 mA/cm2.

Cs+ additive can suppress Li dendrite formation on graphite anode surface even at relatively high lithiation current densities.

11

Electrochemical performances of graphite||NCA full cells using Cs+-electrolytes with various PC contents

12


1C rate = 1.5 mA/cm2

E20PC-Cs shows better cycling stability at RT and 60°C and low-temperature discharge than E-control.

Both E20PC-Cs and E-control show nearly the same rate capability. E20PC-Cs can replace the conventional electrolyte for Li-ion batteries.12

Effect of discharge rate on electrochemical performances of Li||NMC cells at 30°C

13


NMC areal loading is 2.0 mAh/cm2 so 1C rate = 2.0 mA/cm2

Conventional 1.0 M LiPF6/EC-DMC (1:2 v/v) electrolyte Discharge rate has significant effect on cycling stability of Li metal cells. At the same charge rate, certain discharge rates (1C~2C or 2.0~4.0 mA/cm2) lead to

stable and long cycling performance of Li metal batteries.

13

Effect of discharge rate on Li anode morphology and cell impedance of Li||NMC cells after 150 cycles at 30°C

14


Lower discharge rate results in more serious corrosion to Li metal anode, thicker SEI and higher cell impedance.

Discharge at 1C and 2C leads to much less Li corrosion, nearly no change in Li thickness and low cell impedance.14

Ab initio MD simulations of electrolyte conditions at Li anode surface and schematic illustration of SEI evolution

15


Fast discharge results in high concentration of Li+ at Li surface which reduces free solvent molecules in contact with Li metal Alleviate parasitic reactions between Li and solvents and salt anion Form stable SEI and protect Li metal anode during long-term cycling.

At C/3 charge and 1C discharge rate (2 mA cm-2), Li||NMC cell has 81.6% capacity retention after 500 cycles and the Coulombic efficiency as high as 99.8%.

15

Effect of LiTFSI-LiBOB dual-salt/carbonate solvent electrolyte on fast chargeability of Li||NCA cells

16


LiTFSI-LiBOB electrolyte outperforms LiPF6 electrolyte.

o Higher 1st cycle Coulombic efficiency (88.7% vs. 86.7%).

o Much better cycling stability at 1C charging and discharging.

o Much slower capacity fading.o Higher capacity retention after

100 cycles (80% vs. 15%).o Lower polarization between

charging and discharging.

NCA loading 1.5 mAh/cm2


After 2 formation cycles at C/10, the cells were cycled at 1C charging and discharging.

16

Analyses of cell impedance and Li morphology after cycling in LiPF6 and LiTFSI-LiBOB electrolytes

17


Bulk electrolyte resistance (Rb) for LiPF6 cell increases after 100 cycles while that for LiTFSI-LiBOB keeps stable The dual-salt forms compact SEI and prevent continuous electrolyte decomposition.

LiTFSI-LiBOB electrolyte leads to much smaller Rsfand Rct More ionic conductive SEI formed.

LiTFSI-LiBOB electrolyte forms much thinner SEI layer on Li anode.

0 50 100 150 200 250 300 3500

50

100

150

200

250

300

Rct

RctRsfRb

2nd

100th

(a)LiPF6

-Z" (

ohm

)

Z' (ohm)

(a)

200µm

(b)

50µm

LiPF6 LiTFSI-LiBOB

SEM images of Li metal anodes after 100 cycles

17

Dual-salt electrolytes for long-term cycling of Li||LFP cells with a high areal loading LFP cathode at 60°C

18


High areal loading LFP electrode of 3.0 mAh/cm2 from LBNL. The dual-salt electrolytes show slightly higher capacity than the LiPF6-

based control electrolyte (E256). E319 electrolyte shows better cycling stability than other electrolytes at high

charging rate (1.5 mAh/cm2 ) and at elevated temperatures in high loading Li||LFP cells 70% capacity retention after 500 cycles.

0 1 2 32.5

3.0

3.5

4.0

Formation at C/20

RT

Vol

tage

(V v

s. L

i/Li+ )

Areal capacity (mAh cm-2)

E256 E319 E319D E320D E321D

0 100 200 300 400 5000

1

2

3

4@HT60oC

C/20 C/2, 1C

E256 E319 E319D E320D E321D

Are

al c

apac

ity (m

Ah

cm-2)

Cycle number0 1 2 3

2.5

3.0

3.5

4.0

charge at C/2

discharge at 1C

E319 at 60oC

Volta

ge (V

vs.

Li/L

i+ )

Areal capacity (mAh cm-2)

1st 3rd 50th 100th 200th 300th 400th 500th

1st at C/20

18

Collaboration and Coordination with Other Institutions

Partners:Argonne National Laboratory: Provided coated NCA

cathode and graphite anode sheets for testing.

Lawrence Berkeley National Laboratory: Provided coated LFP cathode for testing.

Army Research Laboratory: Conducted EFM and ESI-MS analyses.

19

Low Coulombic efficiency of Li metal anode during cycling.

Cycling stability of Li metal batteries with high loading cathodes.

Li metal dimension or volume change during charging and discharging cycles.

Remaining Challenges and Barriers

20

Future Work - FY2016/17 Continue to evaluate electrolytes in 4-V Li-metal batteries

with high cathode loading and at high current density cycling to achieve over 500 cycles.

Develop ionic conductive protection layer on Li metal anode to reduce parasitic reactions between Li metal and electrolytes and improve Li Coulombic efficiency.

Develop 3D substrates to maintain Li metal dimension during charging and discharging cycles.

21

Summary1. Investigated the mechanism of Cs+/PC on improved

performances of graphite anode and Li dendrite suppression- Cs+ additive and appropriate PC content in electrolytes have synergistic

effects to form an ultrathin, uniform, compact and robust SEI layer on graphite surface.

- This high quality SEI layer suppresses Li dendrites on graphite anode and enhances long-term operation of Li-ion full cells.

2. Optimized the charge/discharge protocol for the improved performance of Li metal batteries- The optimized cycling protocol of slow charge and fast discharge can form a

transient high-concentration electrolyte layer on Li surface to protect Li metal anode and to enable sustainable operation of Li metal batteries.

3. Developed dual-salt electrolytes to improve charge rate and long-term cycling stability of Li metal batteries

- LiTFSI-LiBOB and other dual-salt electrolytes with or without additives can form highly conductive SEI, protect Li metal, enable long cycle life and fast chargeability of Li metal batteries.

22

Acknowledgments

Support from the DOE/OVT/BMR program is greatly appreciated.

Team Members: Hongfa Xiang, Jianming Zheng, Donghai Mei, Pengfei Yan, Liang Xiao, Ruiguo Cao, Jiangfeng Qian, Priyanka Bhattacharya, Chongmin Wang, Mark Bowden, Mark Engelhard, Sarah Burton, Zihua Zhu

23

24

Technical Backup Slides

Electrochemical performances of graphite||NCA full cells with various electrolytes


Cs+ additive enhances the performances of graphite||NCA full cells even with PC in electrolyte.

0 100 200 300 400 5000

40

80

120

160

200

Dis

char

ge c

apac

ity (m

Ah

g-1)

Cycle number

E1Cs E1VC E2

E2

E1Cs

E1VC

RT

0 20 40 60 80 1000

40

80

120

160

200

60oC

Dis

char

ge c

apac

ity (m

Ah

g-1)

Cycle number

E1Cs E1VC E2

0 50 100 150 200 2500

1

2

3

4

5

Volta

ge (V

vs.

Li/L

i+ )

Capacity (mAh g-1)

E1 E1Cs E1FEC

CE:30%84%80%RT

0 100 200 300 400 5000

50

100

150

200

250

Cap

acity

(mA

h g-1

)Cycle number

E1 E1Cs E1FEC

Two formation cycles @ C/20RT

0 20 40 60 80 1000

20

40

60

80

100

120

140

160

Cap

acity

(mA

h g-1

)

Cycle number

E1Cs E1FEC

60oC

0 5 10 15 20 25 300

30

60

90

120

150

180

Dis

char

ge c

apac

ity (m

Ah

g-1)

Cycle number

E1Cs E1FEC

3C

2C1C

C/2

5C

C/5

RT

E1 = 1.0 M LiPF6 in EC-PC-EMC (5:2:3 by wt.) E2 = 1.0 M LiPF6 in EC-EMC (3:7 by vol.)

25

Characterization of graphite anodes from half cells after charging to 0.3 V at the 1st formation cycle

26


2nm

(A) E1

5nm

(B) E1FEC

5nm

(C) E1Cs

Li O F C0

10

20

30

40

50

60

70

80

A

tom

ic ra

tio (%

)

Element

Pristine E1 E1FEC E1Cs

(D)

>6 nm

<2 nm

2~4 nm

500nm 500nm

500nm 500nm

(A) Pristine (B) E1

(C) E1Cs (D) E1FEC

20 40 60 80 100 120 140 160

(A)

(101) (110)

(103)(004)

(102)(100)

Inte

nsity

(a.u

.)

2 Theta (deg.)

Pristine

E1

E1Cs

E1FEC

(002) (B) (C)E1 E1• Broad peak below 20° in XRD

for graphite from E1 suggests partial exfoliation of graphite at the edges of the particles.

• Yellow arrows in SEM and TEM also indicate partial exfoliation of graphite at the edges.

Cs+-containing electrolyte E1 forms an ultrathin and uniform SEI on graphite, which contains more organic species.

Electrolytes without additive or with FEC form thicker SEI with small sports.

26

17O NMR characterization of Li+ and Cs+ solvation in electrolytes

27


Cs+ causes much less displacement in the C=O chemical shift (Δδ = 1.1 ppm) than does Li+(Δδ = 11.6 ppm).

Cs+ acts as a much weaker Lewis acid, and its solvation number with cyclic carbonate solvents should be lower than that of Li+,

Consistent with literature reports that the coordination number for Li+ in nonaqueous solvents is normally ∼4 and for Cs+ is <2.

Li+ is more like to be solvated by cyclic carbonate molecules than Cs+.

330 300 270 240 210 180 150 120 90 60

¦¤= 5.3 ppm

¦¤= 1.1 ppm

E3: LiTFSI-CsTFSI/EC-PC-EMC

E15: CsTFSI/EC-PC-EMC

E16: LiTFSI/EC-PC-EMC

Chemical shift (ppm)

EC

PC

EMC

EC-PC-EMC (5:2:3)

C=OTFSI-

C-O-C(A)

¦¤= 11.6 ppm

27

ESI-MS characterization of Li+ and Cs+ solvation in electrolytes

28


• ESI-MS: Electrospray ionization mass spectroscopy.

Literature reports that Li+ prefers to coordinate with PC over EC. Cs+ has the same trend.

In E1Cs, Cs+-(EC)1 of m/z 221 exists but no Cs+-(PC)1 and Cs+-(PC)2 are present.

PC molecules are scavenged by Li+coordination.

Cs+ is passively “preferably” solvated by EC molecules to for Cs+-(EC)1 and Cs+-(EC)2.

0 100 200 300 400 500 6000

50

100

150

200

(B)

Cs+(PC)2

Cs+(EC)1(PC)1

337323

235Cs+(PC)1

Cs+(EC)1PC+

221

Inte

nsity

(x10

00)

E14: 0.06M CsPF6/PC+EC

m/z

103

0 100 200 300 400 500 6000

50

100

150

200

250

300

Cs+(PC)2

Cs+(PC)1

E13: 0.06M CsPF6/PC

Inte

nsity

(x10

00)

m/z

235

337

(A)

0 100 200 300 400 500 6000

50

100

150

105285

271313

299

221

211

197

183

103

89

Inte

nsity

(X10

00)

m/z

E1Cs(C)

28

Computational calculations on molecular energies and structures of Li+- and Cs+-solvates

29


Cs+-(EC)1 and Cs+-(EC)2 have planar structure. Li+-(PC)3 and Li+-(PC)4 have tetrahedron structure. Cs+-(EC)1 and Cs+-(EC)2 have lower LUMO

energies than Li+-(PC)3 and Li+-(PC)4. Cs+-(EC)m (m=1,2) solvates reduce earlier than Li+-

(PC)n (n=3,4)

0 1 2 3 4 50

-50

-100

-150

-200

∆G (k

J m

ol-1)

Coordination number (n)

Li+(EC)n

Li+(PC)n

Cs+(EC)n

Cs+(PC)n

(A)

3.01 Å2.88 Å

1.94 Å

9.26 Å 9.98 Å

1.91 Å

(B)

(a) (b)

(c)

Li+-(EC)3Li+-(EC)4

Cs+-(EC)2

29

XPS analysis on cycled Li anode surfaces from Li||NMC cells discharged at two rates

30


The SEI formed at slow discharge rate (C/10) contains mainly lithium alkylcarbonate, Li2O, LiF and LiPFx.

The SEI formed at fast discharge rate (2C) mainly contains poly(ethylene carbonate) from EC polymerization and less C/F/P contents (i.e. less electrolyte decompositions.

30

Effect of PC content on graphite in half cells using electrolytes with Cs+ and various PC contents

31


20% PC in solvent mixture is the optimal content in EC-PC-EMC with 30 wt% EC.

E20PC-Cs allows graphite anode to show the best RT cycling in Li||graphite half cells.

E20PC-Cs (with 30EC-20PC-50EMC) leads graphite anode to cycle stably in Li||graphite half cells.

31

Effect of discharge rate on electrochemical performances of Li||NMC cells at other temperatures

32


Same as cycling at 30°C, discharge at 1C~2C rate also results in stable and long cycling performance of Li metal batteries at other temperatures from 10 to 60°C.

1C discharge rate is good for discharge at elevated temperatures. Increasing temperature leads to shorter cycle life due to increased side reactions.32

Long-term cycling stability of Li||NMC442 cells using different electrolytes

33


Two control electrolytes (E004 and E256) and two studied electrolytes (E446 and E448) show stable cycling for 500 cycles before the capacity retention reaches 80%.

Nearly all of these electrolytes have Coulombic efficiencies above 98% after the first two formation cycles.

Areal loading of NMC442 electrode is 2.0 mAh/cm2 –medium high


Control electrolytes – E004, E256 and E448 based on LiPF6 in EC/EMC

E432, E440, E444 and E446 are LiPF6 + CsPF6 in EC/PC/EMC

E319, E319D and E456 are based on dual-salts or other additives

33

Characterization of SEI layers on cycled Li metal anodes from two electrolytes by XPS and FTIR

34


298 296 294 292 290 288 286 284 282

5.0k

10.0k

15.0k

20.0k

25.0kC 1s

C-O

C-HC-C

C=OLi2CO3

CF3

Binder Energy (eV)

Coun

ts (s

)

LiPF6

LiTFSI-LiBOB

(a)

700 698 696 694 692 690 688 686 684 682

5.0k

10.0k

15.0k

20.0k

25.0k

30.0k

35.0k

40.0kF 1s

(b)LiF

CF3

LiPF6

LiTFSI-LiBOB

Binding Energy (eV)

Coun

ts (s

)

546 544 542 540 538 536 534 532 530 5280

10k

20k

30k

40k

50k

60k

70kO 1s

LiPF6

LiTFSI-LiBOB

(c)

Binding Energy (eV)

Coun

ts (s

)

C-O

C=O

64 62 60 58 56 54 52200.0

400.0

600.0

800.0

1.0k

1.2k

1.4k(d)

LiPF6

LiTFSI-LiBOB

Binding Energy (eV)

Coun

ts (s

)

Li 1s

1200 1000 800 600 400 200 00

100k200k300k400k500k600k

LiTFSI-LiBOB

Coun

ts (s

)

Binding Energy (eV)

LiPF6

0100k200k300k400k500k600k700k

S2pN1s

P2p Li1s

Li1s

C1s

C1s

O1s

O1sF1s

F1s

B1s

1800 1600 1400 1200 1000 800

ROCO2LiROCO2Li

ROCO2Li

-SO2

Li2CO3

Tran

smitt

ance

(a.u

.)

Wavenumber (cm-1)

LiTFSI-LiBOB

LiPF6

CF3

Li2CO3

-SO2

ROCO2Li The SEI from LiTFSI-LiBOB electrolyte contains

different components than that from LiPF6. Mainly from the decompositions of the salt anions

BOB− and TFSI−.34

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