Development of Electrolytes for Lithium-ion...

Development of Electrolytes for Lithium-ion Batteries

Brett LuchtUniversity of Rhode Island

June 8th 2010ES067

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

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• 04/01/2009• 12/31/2113• 23 Percent complete

• Barriers addressed

– Calendar Life (40 oC for 15 years)– Cycle life (5000 cycles)– Survival Temp Range (-46 to 66 oC)

• Total project funding– DOE share $731 K

• Funding received in FY09 -$145 K

• Funding for FY10 - $146 K

Timeline

Budget

Barriers

• D. Abraham (ANL)

• M. Smart (NASA JPL)

• W. Li (S. China Univ. Tech.)

• V. Battaglia(LBNL)

• M. Payne (Novolyte)

• F. Puglia (Yardney)

• W. Henderson (N.C. State)

Partners

Overview

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Develop novel electrolytes for lithium ion batteries that improve the performance to meet or exceed DOE goals.

• Develop electrolytes with improved thermal stability and superior calendar life performance.

• Develop novel electrolytes or electrolyte/additive combinations that will facilitate a more stable SEI on the anode.

• Develop additives that allow for formation of protective coatings on the cathode, i.e., a cathode SEI, and enhance electrochemical stability above 4.5 V.

• The development of improved electrolytes is of critical importance for meeting the DOE goals for cycle life, calendar life, temperature of performance, and capacity loss.

Objective

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Milestones• FY09• (a) Investigate LiPF4(C2O4) in small cells. (Aug. 09) Completed• (b) Produce 100 g LiPF4(C2O4) for testing. (Sep. 09) Completed• (c) Investigate combinations of additives for LiPF6 electrolytes. (Sep. 09)

Completed

• FY10• (a) Develop cathode film forming additives for high voltage (>4.5 V vs Li)

cathode materials. (March 10) Completed• (b) Investigate cell performance upon accelerated aging of

graphite/LiNixCo1-2xMnxO2 cells with LiPF4(C2O4) electrolytes compared to LiPF6 electrolytes. (March 10) Completed

• (c) Investigate cell performance of LiPF4(C2O4) compared to LiPF6 in small cells with new chemistries graphite/LiMn2O4,LiNixCo1-2xMnxO2 cells, or with PC electrolytes. (Sept 10) On Schedule

• (d) Develop commercially viable synthesis for LiPF4(C2O4). (Sept 10) On Schedule

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• Develop novel additives and additive combinations to improve the performance of LiPF6 electrolytes.

• Investigate properties of LiPF4(C2O4) /carbonate electrolytes in small Li-ion cells with multiple cell chemistries.

• Develop commercially viable low-cost synthetic method for production of LiPF4(C2O4) and produce high purity LiPF4(C2O4) for additional testing.

• Investigate novel cathode film forming additives, aided by computational methods, to improve performance of high voltage (> 4.5 V) cathode materials.

• Investigate the surface of cathodes and anodes cycled with novel electrolytes, with or without additives, to develop a mechanistic understanding of interface formation and degradation for standard and novel electrolytes.

Approach

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• Developed a mechanistic understanding of the thermal decomposition of LiPF6/carbonate electrolytes

• Developed Lewis Basic additives that inhibit the thermal decomposition of LiPF6 /carbonate electrolytes

• Developed integrated method of electrode surface analysis to understand how changes to electrolyte formulations effect the surfaces of the electrodes and related electrochemical performance of the cells

Previous Technical Accomplishments

OP

FF F

O

RO OR

O

O ORPO

FF

CO2ORPO

FF

RF

PF5

RF

LiF + PF5

HF + RF

F P FF

F

OR

OP

FF F

ROHLiPF6

FP F

FFF

OP

ORFF

The auto-catalytic decomposition of LiPF6 is initiated by trace impurities of water and can be inhibited by Lewis basic additives.

FY09 Technical Accomplishments –Electrolyte with Mixed Additives

• We have conducted an investigation of the effect of thermal stabilizing additives, including dimethyl acetamide (DMAc), vinylene carbonate (VC), and lithium bis(oxalato) borate (LiBOB), on the performance of lithium ion batteries after accelerated aging experiments.

• Cells containing mixtures of two or more electrolytes have superior performance after accelerated aging than cells containing a single additive or standard electrolyte.

• Additive with different mechanisms of improving thermal stability can work cooperatively.

• Surface analysis of the electrodes provides support for cooperative surface film structures along with a better understaniding of the source of performance differences

7

0 2 4 6 8 10 12 14 16 18 20 22 248

9

10

11

12

13

14

15

30 day storage at 70oC

20oC

20oC

30oC20oC10oC

20oC

10oC

30oC

0oC

0oC

Formation at 25oC

Disc

harg

e Ca

paci

rt (A

h)

Cycle No.

STD 2.0% LiBOB 0.5% DMAc 3% VC 2.0% LiBOB & 1.0% DMAc 1.0% DMAc & 1.5% VC 2.0% LiBOB, 1.0 DMAc & 1.5% VC

Discharge capacities vs. cycle number for 12 Ah cells (MCMB/LiNi0.8Co0.2O2) with and without electrolyte additives. The cells were cycled at 25oC, 0oC, 10oC, 20oC & 30oC temperatures before and after high temperature storage at 70 oC for 30 days.

FY 09 Technical Accomplishments –Investigation of LiPF4(C2O4)

• We have been investigating the novel lithium salt LiPF4(C2O4) for use in lithium ion battery electrolytes.

• We have synthesized ~100 g of salt for initial investigations.

• The salt has been investigated in our laboratory and samples have been investigated by F. Puglia (Yardney Technical Products), V. Battaglia (LBNL, BATT program), and two other companies under confidential non-disclosure agreements.

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O

O

OLi

OLiLi++ 2 x PF5 + LiPF6PO

O FF

F

FO

O

LiPF4(C2O4) is synthesized via the reaction of PF5 with lithium oxalate. The reaction is conducted in organic solvents as opposed to HF (as conducted for LiPF6). We are developing a commercially viable route (in collaboration with a commercial partner) that will likely be cost competitive to the production of LiPF6.

FY09 Technical Accomplishments –Investigation of LiPF4(C2O4)

99

-40 -20 0 20 40 60

0

2

4

6

8

10

12

14

16

18

κ,m

s/cm

t,oC

LiFOP LiPF6Investigation of the solution

conductivity of LiPF4(C2O4) over a broad temperature range (-30 to + 50 oC) suggests very similar conductivity in carbonate solution (EC:DEC:DMC, 1:1:1) to LiPF6.

FY09 Technical Accomplishments –Investigation of LiPF4(C2O4)

10

0 10 20 30 40 500.0

1.5

3.0

4.5

0 10 20 30 40 50

0.75

0.80

0.85

0.90

0.95

1.00

Effic

ienc

y

Capa

city,

mAh

Cycle Number

LiPF6 charge LiPF6 discharge LiPF4(C2O4) charge LiPF4(C2O4) discharge

b

a

LiPF6 LiPF4(C2O4)

0 20 40 60 80 100 1200.0

0.5

1.0

1.5

2.0

0 2 4 60.3

0.6

0.9

1.2

1.5

1.8

Volta

ge /

V

Time / h

The lower initial reversible capacity for cells containing LiPF4(C2O4) is due to reduction of oxalate impurities (1.8 V), however cycling efficiency is comparable after formation cycles.

Similar results have been observed for graphite/ LiNi1/3Mn1/3Co1/3O2 cells.

Room temperature cycling behavior of LiPF4(C2O4)/carbonate and LiPF6/carbonate electrolytes (MCMB/LiNi08Co0.2O2) cells

FY10 Technical Accomplishments –Investigation of LiPF4(C2O4) – High Temperature Stability

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0 100 200 300 400 500 6000

20

40

60

80

100

21

Wei

ght P

erce

ntag

e / %

Temperature / ℃

1. LiPF4(C2O4)2. LiPF6

The thermal stability of LiPF4(C2O4) and LiPF6 were analyzed in the solid state by TGA.

The thermal stabilities of the solids are very similar.

FY09 Technical Accomplishments –Investigation of LiPF4(C2O4) – High Temperature Stability

12

100 80 60 40 20 0 -20 -40

19F NMR

δ,ppm50 25 0 -25 -50 -75 -100 -125 -150 -175

31P NMR

δ,ppm

100 80 60 40 20 0 -20 -40

19F NMR

δ,ppm50 25 0 -25 -50 -75 -100 -125 -150 -175

31P NMR

δ,ppm

Analysis of the thermal stability of LiPF4(C2O4) in solution {EC:DEC:DMC (1:1:1)} was conducted by multi-nuclear (19Fand 31 P) NMR spectroscopy. The LiPF4(C2O4) liquid electrolyte stable for 1 month at 85 oC with only low concentrations of impurities.

Similar conditions quantitatively degrade LiPF6

LiPF4(C2O4) is also more stable to low concentrations of H20While the thermal stability of solid LiPF6 and LiPF4(C2O4) are very similar the stability of the liquid

electrolytes are very different, suggesting that the inherent stability of the solid may not be a good predictor of the stability of the electrolyte

LiPF4(C2O4) liquid electrolyte before storage

LiPF4(C2O4) liquid electrolyte after storage at 85 oC for 1 month

FY10 Technical Accomplishments –Investigation of LiPF4(C2O4) – Accelerated Aging

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0 10 20 30 40 500

1

2

3

4

5

60

70

80

90

100

Capa

city,

mAh

Cycle Number

LiPF6 LiPF4(C2O4)

initial formation cycles

Effic

ienc

y, %

The cycling performance of cells containing LiPF4(C2O4) electrolyte after accelerated aging (storage at 65 oC for two weeks) was conducted.

While cells containing LiPF4(C2O4) have lower initial capacity, after accelerated aging cells containing LiPF4(C2O4) have superior cycling performance.

Similar results have been observed for graphite/LiNi1/3Mn1/3Co1/3O2cells.

Cycling behavior of LiPF4(C2O4) and LiPF6 in 1:1:1 (EC:DEC:DMC) in MCMB/LiNi0.8Co0.2O2 cells before and after storage at 65 oC for two weeks.


LixPOyFz

P 2pLiF

PVDF

F 1s

C-O

C=O

O 1s

PVDF,C=O PVDF,C-O

C-C, C-HC 1sBefore

In order to develop and understanding of the superior

li f f

LiPF6

cycling performance after accelerated aging experiments, post-mortem analysis of the electrodes was conducted and cells before and after accelerated aging.

LiPF4(C2O4)

LixPOyFz

C O

C=O

Li2CO

3LixPyOFz

LiFoxalate

C-O

PEO

C-C,C-H

P 2pO 1s F 1sC 1s

140 138 136 134 132 130 128Binding Energy (eV)

692 690 688 686 684 682 680 678Binding Energy (eV)

538 536 534 532 530 528 526Binding Energy (eV)

296 294 292 290 288 286 284 282 280Binding Energy (eV)

AfterSmaller changes to Anode SEI were observed for cells containing LiPF4(C2O4)

Anodes form cells cycled with LiPF6

C-OC=O

yLiPF4(C2O4) have higher concentration of oxalate (286-288 eV ) and a higher concentration of LiF (684.5 eV).

LiPF4(C2O4)

140 138 136 134 132 130 128 126

Binding Energy (eV)

538 536 534 532 530 528 526

Binding Energy (eV)690 688 686 684 682 680 678

Binding Energy (eV)

292 290 288 286 284 282 280

Binding Energy (eV)

XPS analysis of the anodes before and after accelerated aging experiments containing LiPF4(C2O4) and LIPF6 in

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EC:DMC:DEC (1:1:1).


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IR-ATR analysis of the anodes after accelerated aging experiments containing LiPF4(C2O4) and LIPF6 in EC:DMC:DEC (1:1:1).

2400 2200 2000 1800 1600 1400 1200 1000 800 600

0.000

0.004

0.008

0.012

0.016

0.020

0.024

0.028

0.032

859

*

1645

** *

14421062

1800 1198

719

Abso

rban

ce, a

.u.

Wavenumbers, cm-1

LiPF6 LiPF4(C2O4)

Most of the IR absorptions present on both anodes are similar and consistent with the presence of the EC/Li solvate.

Cells containing LiPF4(C2O4) have additional absorption at 1645 cm-1

consistent with the presence of oxalates.

FY10 Technical Accomplishments - Investigation of LiPF4(C2O4) – Accelerated Aging

BeforeLixPOyFz

P 2pF 1s

LiF

PVDF

O 1s

C-OC=O

LiNi0.8Co0.2O2

C 1s

PVDF,C-O

C-C, C-H

PVDF,C=O

The XPS data suggests that

LiPF6

suggests that the surface film on the cathode is thinner with LiPF4(C2O4).

LiPF4(C2O4)

P-FC=O

C O

Li2CO3LixPyFOz

PVDF

C=O C-O

PVDFC-C,C-H

P 2pO 1s F 1sC 1s

142 140 138 136 134 132 130 128

Binding Energy (eV)

694 692 690 688 686 684 682 680

Binding Energy (eV)540 538 536 534 532 530 528 526

Binding Energy (eV)

294 292 290 288 286 284 282 280Binding Energy (eV)

After

LiPF4(C2O4).

The cell containing LiPF4(C2O4) has C-O

LiMO2

LiFPVDF

LiPF4(C2O4) has more surface metal oxide (529 eV ) and less LiF (684.5 eV).

LiPF6

142 140 138 136 134 132 130 128

Binding Energy (eV)540 538 536 534 532 530 528

Binding Energy (eV)

694 692 690 688 686 684 682

Binding Energy (eV)294 292 290 288 286 284 282 280

Binding Energy (eV)

LiF (684.5 eV). LiPF4(C2O4)

XPS analysis of the cathodes before and after accelerated aging

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XPS analysis of the cathodes before and after accelerated aging experiments containing LiPF4(C2O4) and LIPF6 in EC:DMC:DEC (1:1:1)


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Most of the absorptions are associated with PVDF.

The cell with LiPF4(C2O4) has less polycarbonate (1776 cm -1) some oxalate (1645 cm-1).

The superior cycling behavior of cells containing LiPF4(C2O4) electrolyte can be attributed to differences in the electrode surface films.

2400 2200 2000 1800 1600 1400 1200 1000 800 600

0.000

0.004

0.008

0.012

0.016

1645

*

1776

1271

1400

1170

1070877

840

LiPF6 LiPF4(C2O4)

Abso

rban

ce, a

.u.

Wavenumbers, cm-1

IR-ATR analysis of the anodes after accelerated aging experiments containing LiPF4(C2O4) and LIPF6 in EC:DMC:DEC (1:1:1)

FY10 Technical Accomplishments –Cathode film forming additives

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• It has been previously reported that the cathode SEI responsible for increased impedance of aged cells.

• While there is significant interest in the use of higher voltage cathodes (>4.5 V vs Li), cathode electrolyte reactions limit voltage.

• Several additives/salts have been reported to react on the cathode surface.

• We are interested in the development of novel additives that will generate cathode surface passivation films which improve cycling performance to higher voltages (> 4.5V).

Abraham et.al J. Electrochem. Soc. 149, A1358 (2002)Wu et. al. Electrochem. Comm. 11, 526 (2009)Xu J. Electrochem. Soc. 155, A733 (2008)


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0

1

2

3

4

0.01 0.1 1 10 100 1000time, hr

Q, m

Ah

0

1

2

3

4

I, mA

Q, 4.0 VQ, 4.3 VQ, 4.7 VQ, 5.0 VQ, 5.3 VI, 4.0 VI. 4.3 VI, 4.7 VI, 5.0 VI, 5.3 V

Oxidation of the electrolyte solution (1 M LiPF6 in 1:1:1 (EC:DEC:DMC) at various potentials with a LiNi0.5Mn1.5O4/Li half cell.

In order to develop cathode film forming additives to improve the performance of cathodes at high voltage (>4.5 V vs Li), we initially investigated the reactions of the electrolyte with the cathode surface.

LNMS/Li cells were stored at various voltages between 4.0 and 5.3 V vs Li for one week.

At low voltages, below 4.7 V, there is very little current flow through the cells. As the voltage surpasses 4.7 V vs Li there is a significant increase in charge flow through the cells.

The increased charge flow is consistent with electrolyte oxidation on the surface of the cathode at high potential.


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140 138 136 134 132 130

692 690 688 686 684 682

538 536 534 532 530 528

Metal OxideC-O

C=O

O=C

O-C

292 290 288 286 284 282 280

4.4 V

4.7 V

5.0 V

5.3 V

4.2 V

P2pF1sO1sC1s

XPS spectra of cathodes with Ethylene Propylene Diene Monomer (EPDM) as the binder after storage for one week at various voltages.

After retention at the desired voltages for one week the LNMS/Li cells were disassembled and the surface of the cathodes were analyzed by XPS and FTIR-ATR.

EPDM binder was used to avoid overlap of the peaks associated with PVDF binder. Similar results were observed for cells with PVDF binder.

At low voltages (< 4.7 V vs Li) the surface contains EPDM, metal oxide, and LixPFyOz.

As the potential is raised above 4.7 V vs Li the relative intensity of the C-O and C=O peaks increase while the metal oxide peaks decrease.

The changes in the surface are consistent with oxidation of the electrolyte to generate poly(ethylene carbonate) on the surface.


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3200 3000 2800 2600 2400 2200 2000 1800 1600 1400 1200 1000 800

LixPFy

LixPFyOz

PEC

EPDM

Wavenumber (cm-1)

EPDM 4.0V EPDM 4.3V EPDM 5.0V EPDM 5.3V EPDM, EC free electrolyte

IR-ATR spectra of cathodes with Ethylene Propylene Diene Monomer (EPDM) as the binder after storage for one week at various voltages.

Electrodes stored at lower potentials (4.0 and 4.3 V) contain absorptions characteristic of EPDM including absorptions at 2920 cm-1 and 2850 cm-1 corresponding C–H stretching and 1300 – 1500 cm-1 characteristic of C-H bending.

At higher cathode potentials (5.0 and 5.3 V), the relative intensity of the C-H absorptions are decreased while absorptions at 1750 cm-1 and 1250 cm-1, characteristic of the C=O and C-O absorptions, respectively, of poly(ethylene carbonate) (PEC) are increased.

In an effort to confirm that EC is the source of the PEC on the surface cells containing EC free electrolyte (1 M LiPF6 in DMC/DEC/EMC) were prepared and retained at 5.0 V. No PEC was observed


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3.0 3.5 4.0 4.5 5.0 5.5 6.0-2.1x10-5

-1.8x10-5

-1.5x10-5

-1.2x10-5

-9.0x10-6

-6.0x10-6

-3.0x10-6

0.0

3.0x10-6

Curre

nt (A

)

Voltage (V)

Standard 2% 2,5-DHF 2% GBL

Initial experiments to determine if sacrificial additives can be incorporated into electrolytes to generate a cathode passivation layer.

The addition of 2,5-DHF has lower voltage oxidation on first cycle, but higher stability on later cycles.

GBL reduces the decomposition current.

The additives generate a stable passivation layer which inhibits electrolyte oxidation.

O

O O

2,5-DHF

GBL

Cyclic voltammetry of 1 M LiPF6 in 1:1:1 EC/DEC/DMC with/without additives vs glassy carbon electrode.


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The cycling performance of Li1.17Mn0.58Ni0.25O2 half cells charged to 4.9 V vs Li confirm superior cycling performance with additives.

Similar results have been observed with LiNi1/3Co1/3Mn1/3O2 charged to 4.8 V vs Li.

Incorporation of LiBOB and Additive X provide the best performance.

The decreased initial capacity loss for LiBOB and Additive X is likely due to inhibition of oxygen evolution at the cathode surface.

0 10 20 30 40 50 600.0007

0.0008

0.0009

0.0010

0.0011

0.0012

0.0013

0.0014

0.0015

0.0016

0.0017

0.0018

Standard 1% LiBOB 0.5% 2,5-DHF Additive X

Capa

city

(Ah)

Cycling Number


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The incorporation of additives results in a greater retention of surface metal oxide (529.5 eV) and less PEC C-O (533.5 eV) and C=O (531.8 eV).

Cathode film forming additives result in thinner surface films on Li1.17Mn0.58Ni0.25O2 cathodes.

Cells cycled with LiBOB contain B on the surface of the cathode confirming reaction of LiBOB on the surface.

Similar results have been observed with LiNi1/3Co1/3Mn1/3O2 charged to 4.8 V vs Li.

XPS analysis of cathodes extracted from Li1.17Mn0.58Ni0.25O2 half cells cycled to 4.9 V vs Li.

Collaborations

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• D. Abraham (ANL, National Lab, ABRT Program): Collaborations on the investigation of novel salts, solvents and additives in lithium ion battery electrolytes.

• M. Smart (NASA JPL, National Lab, BATT Program): Collaborations on the investigation of novel salts, solvents and additives in lithium ion battery electrolytes.

• W. Li (S. China Univ. Tech., Academic): Collaboration on the investigation of LiPF4(C2O4) and computational investigations of additives for cathode and anode film formation.

• V. Battaglia (LBNL, National Lab, BATT Program): Collaboration on performance testing of LiPF4(C2O4) electrolytes in BATT program cells.

• M. Payne (Novolyte, Industrial): Collaboration on the scale-up and commercialization of novel electrolytes.

• F. Puglia, J. Gnanaraj, and B. Ravdel (Yardney, Industrial): Collaboration on testing novel electrolytes in large format cells (7 – 12 Ah).

• W. Henderson (N.C. State, Academic, BATT Program): Collaborations on the development of novel salts for lithium ion battery electrolytes.

Proposed Future Work FY 10 – FY 11

• Investigate the cycling behavior of LiPF4(C2O4) with Propylene Carbonate (PC) electrolytes to determine if novel SEI allows efficient cycling of PC with graphite anode.

• Investigate the cycling behavior of LiPF4(C2O4) other cathodes LiMn2O4 or LiFePO4 or novel cathode materials being developed as part of the BATT program.

• Develop commercially viable synthesis of LiPF4(C2O4) .

• Optimize novel cathode film forming additives for different cell chemistries (LiMn2O4 or LiFePO4 or novel cathode materials being developed as part of the BATT program).

• Develop novel anode film forming additives for new anode materials being developed as part of BATT program.

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• The investigation of combinations of different types of thermal stabilizing additives provides a cooperative beneficial effect to lithium ion batteries.

• A novel lithium salt LiPF4(C2O4) has been developed and investigated in lithium ion cells.

• Cells containing LiPF4(C2O4) electrolytes have greater initial capacity loss, which is likely due to oxalate impurities, but otherwise comparable room temperature performance to LiPF6.

• LiPF4(C2O4) has superior performance to LiPF6 after accelerated aging.• Larger quantities of LiPF4(C2O4) (100 g) have been prepared and provided to

collaborators for independent confirmation. Similar results have been obtained.

• The oxidation reactions of electrolytes on the surface of cathodes retained at high voltage (>4.5 V vs Li) have been investigated.

• Cathode film forming additives have been developed, with the assistance of computational chemistry, which improve the cycling performance of lithium ion cells at high voltage (>4.5 V vs Li)

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