Winter-PSI Symposium 2015.pptx [Schreibgeschützt] · From 1791 Until Today From Aqueous to...

“The dose makes the poison.“‐‐‐Paracelsus (Philippus Theophrastus Aureolus Bombastus von Hohenheim) (Swiss Medic, Alchemist, Theologist, Astrologist and Philosopher, 1493 ‐ 1541)

Electrolytes as Key to New Battery Technologies

D. R. Gallus, R. Wagner, J. Kasnatscheew, M. Amereller, B. Streipert, A. Reyes Jiménez, S. Roeser, R. Kloepsch, B. Hoffmann, R. Schmitz,

R. Schmitz, A. Berken, V. Kraft, W. Weber, S. Wiemers-Meyer,S. Nowak, I. Cekic-Laskovic and M. Winter

MEET Battery Research Center, Institute of Physical Chemistry, University of Muenster, GER

&Helmholtz Institute Münster (HI MS), GER

31st PSI Electrochemistry Symposium - Electrochemical Energy Storage: A Key for Future Energy Systems, Villigen, CH, 6 May 2015

Funding:

BMW Group

German Ministry of Education and Research (BMBF) within the project “Electrolyte Lab 4E”

German Research Foundation (DFG) within the project “High Performance Batteries”

Material support by BASF

Acknowledgments

The medic Luigi Galvani (IT) generated electricity using a “galvanic cell” with a frog leg as electrolyte between two metal electrodesThe physicist Allesandro Volta (IT) built the 1st battery (“Volta pile”), using Zn and Cu/air as electrodes and saltwater/cardboard as electrolyte/separator system. The failure of the Volta‐Pile was related to the electrolyte: Drying out, because of H2O evaporationThe chemist Johann Wilhelm Ritter (GER) constructed a rechargeable battery, where the electrolyte also participated in the cell reaction: via electrolysis of water during charge; and recombination of H2 and O2 during discharge

From the Beginning:Electrolytes Enable Batteries

Ritter (1776‐1810)

Volta (1745‐1827)

Galvani(1737‐1798)

Cell Pile

Electrolytes are decisive for power, life and safety of the battery cellIn many cases electrolytes are (co‐) determining the energy, as well

Electrolyte in the Center of Electrochemical Energy Storage

Electrolyte is a system component, that is in contact with many other cell components

Electrolyte is a system consisting out of components

From 1791 Until TodayFrom Aqueous to Non‐Aqueous ElectrolytesFrom 1 V to >5 V Batteries

1.1 VZn-O2(Volta)

1791

Primary

2 VPb-Acid

(Sin-steden,Planté)

1854/59

Recharge-able

1 VH2-O2(Ritter)

1803

Recharge-able

1 VAqu. IonTransferSystem(Rüdorff,

Hoffmann)

1938

Recharge-able

3 VLi-Ion

TransferSystems

1980ies

Recharge-able

Up to 4.2 VLi-Ion

Systems

1990ies

Recharge-able

>4.2VLi-Ion

Systems

Today &Tomorrow

Recharge-able

1800 1900 2000 Today +

3 VLi-MetalSystems

1960ies

Primary

High Voltage Cathodes:‘High Voltage’ is Relative

Static electricity: Several 10,000 VoltsHumans: Up to 30.000 Volts

High voltage grid: Several 100.000 Volts

Lightning: Several 10.000.000 Volts

‐4.1 ‐3.040 0.0 3.070* 3.27* E / V vs. SHE

Sr/Sr+ Li/Li+ NHE F2/F‐ KrF2/KrF+Batteries: Possible: <8VPractical: ≤5 VTypical: 1.2 – 4VThe challenge: >4.3V*in acidic solution

Non‐Aqueous Liquid Electrolyte Economics• Standard non‐aqueous electrolytes are blends of linear and cyclic carbonates

High permittivity cyclic carbonate Low viscosity linear carbonates

• Blends are formulated to achieve specific physical & chemical electrolyte properties• Typical range of Li salts, such as LiPF6 is 0.8 to 1.2 molar (10 ‐ 15% by weight)• Production of nonaqueus electrolytes in 2013: ca. 30,000 tons• With ca. 12$/kg, electrolyte contributes ca. 6‐8 % of the lithium ion battery material costs• With a mass fraction of <15%, the salt costs are up to 90% of the electrolyte costs

depending on the salt concentration, purity, and supplier and use of electrolyte additives • Bulk solvents add only 5 to 15% of the total cost. Additives (0.1 – 5wt.‐%) increase costs• Moderate changes in solvent and additive chemistry are tolerable in view of costs

Lower salt content and lower purity requirements can reduce costs

Towards High Energy Density LIBwith High Voltage (HV) Cathodes

• Energy density can be elevated by:higher specific capacity and higher cell voltage (via cathode potential increase)

• The use of high voltage cathodes materials presents a major challenge to the oxidation stability of the electrolytee.g., organic carbonate solvents:> 4.2 ‐ 4.3 V

E = C · V

GraphiteAnode

Cathode

Cel

lVol

tage

0 V

NMC

1 V

2 V

3 V

4 V

5 V

Pote

ntia

l vs.

Li/L

i+

SE

IE

lect

roly

teS

tabi

lity

HVCathodes?

NMCat HV

Cel

lVol

tage

Cathode

• NMC can be charged to different upper cut‐off potentials

• Higher cut‐off potential HV application higher specific energy

LiNi0.33Mn0.33Co0.33O2 (“1/3‐NMC”) at HV:Enhanced Potential and Capacity

B. Xu, D. Qian, Z. Wang, Y.S. Meng, Materials Science and Engineering: R: Reports 2012, 73, 51-65

Li layermetal oxide

layer

48%Li+

68%Li+

B. Xu, D. Qian, Z. Wang, Y.S. Meng, Materials Science and Engineering: R: Reports 2012, 73, 51-65

LithiumTransition-metal oxide

NMC:Failure Mechanisms at 4.6 V

NMC de‐gradationat 4.6 V

Overall cell impedance

rise

Oxidative electrolyte degradation

Irreversible phase changes

Transition metal

dissolution

Oxygen release fromcathode

[1] H. Zheng, Q. Sun, G. Liu, X. Song, V.S. Battaglia, J Power Sources, 207 (2012) 134-140.[2] V.A. Godbole, J.-F. Colin, P. Novák, J Electrochem Soc, 158 (2011) A1005-A1010.[3] G. Yan, X. Li, Z. Wang, H. Guo, C. Wang, J Power Sources, 248 (2014) 1306-1311.[4] A. Riley, et al., J Power Sources, 196 (2011) 3317-3324.

3.0 3.5 4.0 4.5

0

500

1000

1500

2000

Con

cent

ratio

n /

g L-1

Electrode potential vs. Li/Li+ / V

Ni Co Mn

• Enhanced average discharge potential

• Higher specific capacity

• Lower Coulombic Efficiency

• Insufficient cycle life

High Voltage Application of NMCUse of LiPF6 in Electrolyte at HV Metal Dissolution (Promoted by HF)

D.R. Gallus, R. Schmitz, R. Wagner, B. Hoffmann, S. Nowak, I. Cekic-Laskovic, R.W. Schmitz, M. Winter, Electrochim Acta, 134 (2014) 393-398.

• Electrolyte: 1M LiPF6 in EC/DMC (1:1)

• Ni, Co, and Mn dissolution

• Large dissolution at 4.6 V vs. Li/Li+

NMC storage in electrolytefor 28 days

WE: NMC, CE, RE: Li

LiPF6

0 10 20 30 40 5080

100

120

140

160

180

Upper cut-off potential vs. Li/Li+

4.2 V4.4 V4.6 V

Spe

cific

cap

acity

/ m

Ah

g-1

Cycle number

Protection of the Cathode vs. Dissolution

HF Scavenger

HF Scavenger

Alternative Conducting

Salt


Salt

Salt Stabilizerto avoid

HF


HF

CathodeMaterialCoating/Doping


CEI* Forming Electrolyte Additive


HF

*CEI: Cathode Electrolyte Interphase

3.0 3.5 4.0 4.5

0

100

200

300

400

Con

cent

ratio

n /

g L-1

Electrode potential vs. Li/Li+ / V

Ni Co Mn

0 10 20 30 40 5040

60

80

100

120

140

160

180

Upper cut-off potential vs. Li/Li+

4.2 V4.6 V

Spe

cific

cap

acity

/ m

Ah

g-1

Cycle number

Alternative Conducting Salt: LiBF4 Less Sensitive to HF formation Enables HV application

• Higher capacity at 4.6 V vs. Li/Li+

• Enhanced cycling stability

• Enables HV application

• Electrolyte: 1M LiBF4 in EC/DMC (1:1)• Metal dissolution in the presence of LiBF4

5 times lower than with LiPF6

LiBF4

D.R. Gallus, R. Schmitz, R. Wagner, B. Hoffmann, S. Nowak, I. Cekic-Laskovic, R.W. Schmitz, M. Winter, Electrochim Acta, 134 (2014) 393-398.

WE: NMC, CE, RE: Li

NMC storage in electrolytefor 28 days

5xlower

HF Scavenger

HF Scavenger


Salt


Salt


HF


HF



CEI*FormingElectrolyte Additive

CEI*FormingElectrolyte Additive

HF



3.0 3.5 4.0 4.5 5.0 5.5 6.0

0.0

0.2

0.4

0.6

Cur

rent

den

sity

/ m

A c

m-2

Potential vs. Li/Li+ / V

1 M LiPF6 in EC/DMC (1/1) + 1 wt% TMS diethylamine

0

400

800

1200

1600

Mn

conc

entra

tion

g L-1

NMC storage at 4.6Vvs. Li/Li+ for 28 days

1M LiPF6 in EC/DMC + 1wt% TMS diethylamine

WE: LMO; CE, RE: LiScan rate: 0.1 mV s‐1

• Patent Claim by Saidi et al.*: TMS diethylamine can reduce HF induced transition metal dissolution*

• Proposal of mechanism by Zhang**• Diethylamine = Leaving group (LG)

• However: Not stable at high cathode potentials

New HF (and H2O) Scavenging Electrolyte Additives: TMS (Trimethylsilyl‐) Based

Si N

HF

H2O

Si F

Si OH

+

+

NH

NH

**Mechanism S.S. Zhang, J Power Sources, 162 (2006)1379-1394

*M.Y. Saidi, F. Gao, J. Barker, C. Scordilis‐Kelley, U.S. Patent 5,846,673 (1998)

• Si‐F bond is one of the strongest bonds

• Si is a much better electrophile for F‐ than carbon TMS group offers a more attractive electrophilic center

than organic carbonate solvents

• TMS based additives are known to enhance the effectiveness of the CEI[1‐3]

• Structural modification of TMS diethyl amine by variation of the Leaving Group (LG)to achieve better oxidation stability:

Or Or

Why Using a TMS Group in the Additive?

Si LGLG

Single bond energy with F /

kJ mol-1

H B C Si N P O S

570 646 489 595 278 496 214 368

Si LGLG

[1] X. Liao, Q. Huang, S. Mai, X. Wang, M. Xu, L. Xing, Y. Liao, W. Li, J Power Sources, 272 (2014) 501-507.[2] Y. Liu, L. Tan, L. Li, J Power Sources, 221 (2013) 90-96.[3] H. Rong, M. Xu, B. Xie, X. Liao, W. Huang, L. Xing, W. Li, Electrochim. Acta, 147 (2014) 31-39.

• Good LGs stabilize the additional negative charge before (in the activated complex) and after heterolytic fission of the Si‐LG‐bond.

• This is indicated by a low pKa of the corresponding acid of the LG

• Charge stabilization supports the F‐‐scavenging capability of the TMS‐LG additive → “LG Ability”

Design of the HF ScavengerThe Choice of the Leaving Group LG by pKa Considerations

• Ka: Acid strength

• The smaller the pKa value, the higher the acidity of HA and the higher the affinity of A‐ to attract negative charge

• A low pKa value is proportional to a low HOMO energy level**,as both are related to a high electron affinity.

• Only an approximate, but still useful correlation: The interaction of the LG with the TMS group is unattended by the pkA.

• pKa values are listed in tables and unlike HOMO energies do not have to be calculated.

Correlation of pka with HOMO Energy**Oxidation Stability and LG Ability Can Be Estimated by pKa Value

HOMO* energy pKa value

~

*Highest Occupied Molecular Orbital

**H.J. Soscún Machado, A. Hinchliffe, Journal of Molecular Structure: THEOCHEM, 339 (1995) 255-258.

TMS diethyl amine:

• Better capacity retention

• Low Coulombic efficiency

• Oxid. decomposition during cycling

0 10 20 30 40 500

40

80

120

160

200

240

1M LiPF6 in EC/DMC (1/1)+ 1wt-% TMS diethylamine+ 1wt-% TMS trifluoroacetate

Spe

cific

cap

acity

/ m

Ah

g-1

Cycle number0 10 20 30 40 50

80

85

90

95

100

105

1M LiPF6 in EC/DMC (1/1)+ 1wt-% TMS diethylamine+ 1wt-% TMS trifluoroacetate

Cou

lom

bic

effic

ienc

y / %

Cycle number

TMS trifluoro acetate:

• Better capacity retention

• Higher Coulombic efficiency

• Enables HV application

Effect of TMS Additives on NMC Cycling at HV

WE: NMC; CE, RE: Li; 3.0-4.6V vs. Li1st -3rd cycle: 0.2C; 4th-50th cycle: 1C

-10 0 10 2020

40

60

80

100

1 TMS diethylamine 4 TMS trifluoroacetate 7 N,O-Bis(TMS)trifluoroacetamide2 TMS isocyanate 5 Methoxy TMS 8 TMS methanesulfonate3 TMS acetate 6 N,O-Bis(TMS)acetamide 9 TMS TFSI

96

3

4

5

8

72

1

Cap

acity

rete

ntio

n / %

pKa value of the leaving group

Lower pKa of the LG Better Capacity Retention

Ox. stability / HF‐scavenger / More effective CEI

Capacity retention: Discharge capacity 5th/50thcycle

Based on an initial capacity of ca. 180 mAh g‐1

-15 -10 -5 0 5 10 15 200

100

200

300

400

5001 TMS diethylamine 6 N,O-Bis(TMS)acetamide 2 TMS isocyanate 7 N,O-Bis(TMS)trifluoroacetamide3 TMS acetate 8 TMS methanesulfonate4 TMS trifluoroacetate 9 TMS TFSI 5 Methoxy TMS

7

6

532

4

1

98C

um. i

rr. c

ap. l

oss

/ mA

h g-1

pKa value of the leaving group-13.0 -12.5 -12.0 -11.5 -11.0 -10.5 -10.0 -9.50

100

200

300

400

5001 TMS diethylamine 6 N,O-Bis(TMS)acetamide 2 TMS isocyanate 7 N,O-Bis(TMS)trifluoroacetamide3 TMS acetate 8 TMS methanesulfonate4 TMS trifluoroacetate 9 TMS TFSI 5 Methoxy TMS

7

65

3 2

4

1

98C

um. i

rr. c

ap. l

oss

/ mA

h g-1

HOMO energy level / eV

• Irreversible capacity losses accumulated in the 4th – 50th cycle:

• Are an indication for the oxidation stability of the electrolyte and/or the effectiveness of the CEI.

• Can be correlated with the pKa value and the HOMO energy level.

• Better correlation with the HOMO energy, as TMS group is considered.

Cumulated Irreversible Capacity Losses vs.pKa Value and HOMO Energy

• Higher Coulombic efficiency with TMS additive may be a hint for a change of the properties of the CEI film.

• LiF is known to increase the charge transfer resistance. [1]

• TMS trifluoro acetate leads to a decrease of charge transfer resistance (3rd → 50th cycle) → Less LiF?

CEI Film Formation?Reverted Trend of Impedance Development During Cycling after Addition of the Additive

[1] X. Zuo, C. Fan, J. Liu, X. Xiao, J. Wu, J. Nan, J Power Sources, 229 (2013) 308-312.

WE: NMC; CE, RE: Li1M LiPF6 in EC/DMC (1:1)

+ 1 wt‐% TMS trifluoroacetate With additive

Storage of the electrolytes w/wo

additives @60°C, 7 days

Methods to Understand the Mechanisms

19F & 29Si NMR SEMGC-MS

Post mortem ana‐lysis of the electro‐lyte after CC cycling

Storage of a water‐doped electrolyte @ 60°C, 7 days

Post mortem ana‐lysis of the electro‐des after CC cycling

Determination of the HF content

Determination of reaction products of TMS additives

Study of the elec‐trode surface/CEI

Determination of reaction products of

TMS additives

• HF scavenging route predominant in LIB, due to higher HF concentration and higher Si affinity towards HF (compared to H2O)

• Products after “H2O scavenging“ below the limit of detection (LOD) in the post mortem analysis data.

Si

Si

H2O

HF F

Si OH +

+ HO

HO

Condensation

Si O SiHF

Si F Si OH+

main routeO

CF3

O

CF3

OCF3

O

Proposal for the Mechanismfor Reaction of TMS Trifluoro Acetate

GC-MS

GC-MS

NMR

Post mortemNMR

GC-MS Storageexperiment

• Compatible with graphite

• No negative influence on capacity retention

• Slightly enhanced Coulombic efficiency

TMS Trifluoroacetate Performance with Graphite Anode

• Sufficient amounts of HF in the electrolyte are beneficial in order to passivate the Al current collector*:

Al2O3 + 2HF 2AlOF + H2O

2AlOF + 2HF Al2OF4 + H2O

Al2OF4 + 2HF 2AlF3 + H2O

Al Passivation in TMS Electrolyte at Smaller HF Amount

SEM of Al foils after polarization to 4.6 V vs. Li/Li+ for 24 h a.) 1M LiPF6 in EC/DMC, b.) + 1 wt.‐% TMS trifluoro acetate, c.) 1M LiTFSI in EC/DMC

Al : Constant voltage@ 4.6 V vs. Li/Li+, 24 h

a b c

*Krämer, et al. J. Electrochem. Soc. 2013, 160 (2), A356-A360Krämer, et al. Electrochem. Lett., 2012, 1(5), C1 - C3.


HF Scavenger

HF Scavenger


Salt


Salt


HF


HF





HF


Metal dissolution or addition of metal salts

Transition metals are incorporated in the SEI

Impedance rise at the anode

Capacity fading

Previous Reports [1‐3]: Negative Influence of Metal Cations on Cycling Performance

[1] H. Zheng, Q. Sun, G. Liu, X. Song, V.S. Battaglia, J Power Sources, 207 (2012) 134-140.[2] S. Komaba, N. Kumagai, Y. Kataoka, Electrochim Acta, 47 (2002) 1229-1239.[3] J.C. Burns, A. Kassam, N.N. Sinha, L.E. Downie, L. Solnickova, B.M. Way, J.R. Dahn, J Electrochem Soc, 160 (2013) A1451-A1456.

Previous Reports [1]: Positive Influence of Metal Cations on Cycling Performance on Graphite

[1] S. Takeuchi, S. Yano, T. Fukutsuka, K. Miyazaki, T. Abe, J ElectrochemSoc , 159 (2012), A2089.

Addition ofCa(TFSA)2

1 M LiTFSA in EC:DEC:TMP (3:3:4, by vol.)

TFSA- = TFSI-

Summary of the Investigated Additives

Additive Supplier, Purity5th cycle

Discharge Cap. [mAh g‐1]

50th cycleDischarge Cap.

[mAh g‐1]

Cap. Retention [50th/5th]

[%]No additive 167.9 146.4 87.2LiTFSI 3M, battery grade 157.1 139.2 88.6NaTFSI Solvionic, 99.5% 156.3 136.4 87.3Mg(TFSI)2 Solvionic, 99.5% 144.5 149.8 103.6Al(OTf)3 Aldrich, 99.9% 121.0 114.8 94.9Ca(TFSI)2 Solvionic, 99.5% 149.4 119.6 80.0Sc(TFSI)3 abcr, 99% 168.9 150.7 89.2Cr(TFSI)3 Synthesis: In‐house 143.3 141.0 98.4Ni(TFSI)2 Solvionic, 99.5% 165.2 146.8 88.8Cu(TFSI)2 Synthesis: In‐house 167.1 158.1 94.6Zn(TFSI)2 Solvionic, 99.5% 161.9 142.6 88.1

Electrolyte: 1M LiPF6 in EC:EMC (1:1, by wt.), WE: NMC, 3-electrode cell with Li as CE and RE

3 formation cycles with C/5 and 50 cycles with 1C, Cut-off: 3.0 V to 4.6 V vs. Li/Li+

Effect of Mg(TFSI)2 on NMC Cycling Performance in a Full Cell

• 2‐Electrode Cell: NMC/MAGD2015% Anode ExcessCut‐off: 2.8 – 4.5VEnd of life (EOL) criterion, which indicates 80% of the initial cell capacity after formation, is reached:

after 250 cycles without Mg2+

after 750 cycles with Mg2+

• Coulombic efficiencies in 1. and 5. cycle

Electrolyte 1st 5th

LP50 (1 M LiPF6 inEC:EMC, 1:1) 78.5% 99.1%

LP50 + 1% Mg(TFSI)2

87.9% 99.7%

0 100 200 300 400 500 600 700 800 900 10000

20

40

60

80

100

120

140

160

180

200 1 M LiPF6 in EC:EMC (1:1) 1 M LiPF6 in EC:EMC (1:1) + 1% Mg(TFSI)2

Dis

char

ge c

apac

ity /

mA

h g-1

Cycle no.3 formation cycles: 15 mA g‐1 (C/10) and CV step at 4.5 V down to C ≤ 0.02C

1000 cycles: 150 mA g‐1 (1C) and CV step at 4.5 V down to C ≤ 0.2C

• Reduced self‐discharge with Mg(TFSI)2

WE: NMC; CE, RE: Li; Cut‐off 4.6 V vs. Li/Li+3 formation cycles at C/5120h storage at OCP

Effect of Mg(TFSI)2 on NMC Self‐Discharge

0 20 40 60 80 100 1204.2

4.3

4.4

4.5

4.6

4.7

1 M LiPF6 in EC:EMC (1:1) 1 M LiPF6 in EC:EMC (1:1) + 1% Mg(TFSI)2

Pot

entia

l vs.

(Li/L

i+ ) / V

Time / h

Mg(TFSI)2 asHF Scavenger? 19F‐NMR

19F‐NMR spectroscopy after storage at 60 °Cunder inert atmosphere in NMR tubes for upto 12 days.

• HF concentration in the case of the benchmark electrolyte is increasing during storage.

• With addition of Mg(TFSI)2, the HF concentration remains below the limit of detection (LOD).

Mg(TFSI)2 is an effective HF‐scavenger

(no intuitive mechanism)orprevents HF formation

0 2 4 6 8 10 12

0

2

4

6

8

10

Con

cent

ratio

n / m

mol

time / days


• We assume that the Mg(TFSI)2 additive has the following functions:

Mechanism?

Mg(TFSI)2

CEI‐film‐forming additive

Stabilization of the host structure

?

HF‐ScavengerHF‐Prevention

Reduced HF formation: MgF2 formation

Formation of a Mg‐containing film on the surface or sub‐surface by in situ coating: EIS, XPS, SEM, TOF‐SIMS, LA‐ICP‐MS

Reduced Li/Ni mixing: XRD, Rietveld refinement

Very probable: no bulk‐doping of NMC

Investigations on Capacity Fading by EIS:Reverted Trend of Impedance Development During Cycling after Mg2+ Addition

• Impedance increase in Rct during cycling

Growth of a passivation layer on the cathode by electrolyte decomposition?

• With Mg2+: Higher Rct after 5 cycles,

But decrease in Rct during cycling

0 100 200 300 400

0

50

100

150

2001 M LiPF6 in EC:EMC (1:1) + 1% Mg(TFSI)2

after 5 cycles after 30 cycles after 50 cycles

-Zim

ag /

ohm

Zreal / ohm

0 200 400 600 800

0

50

100

150

200

250

300

1 M LiPF6 in EC:EMC (1:1) after 5 cycles after 30 cycles after 50 cycles

-Zim

ag /

ohm

Zreal / ohm

WE: NMC; CE, RE: Li

0 10 20 30 40 50 60 70 80 90 100 110 120

4.2

4.4

4.6

4.8

5.0

Pot

entia

l vs.

(Li/L

i+ ) / V

time / h


0 20 40 60 80 100 120 140 1600

102030405060708090

100110120

1C

Dis

char

ge c

apac

ity /

mA

h g-1

Cycle no.

1CC/5D-rate testrecovery

1 M LiPF6 in EC:EMC (1:1) + 0.1% Mg(TFSI)2 + 0.2% Al(Otf)3

• Better mechanistic elucidation

• Not optimized with regard to additive amount and additive combination

• Use with of other cathode materials, e.g. LiNi0.5Mn1.5O2 (LMNO)

Future Work

WE: LMNO; CE, RE: Li; Cut‐off: 4.95 V vs. Li/Li+3 formation cycles: C/5120h storage at OCP

Cycling Self‐discharge

WE: LMNO; CE, RE: Li;Cut‐off: 4.95 – 3.0 V vs. Li/Li+

3 formation cycles at C/5, then C/2D‐rate tests at: D/5, D/3, D/2, 1D, 2D, 3D und 5D

0 2 4 6 50 52 54100

120

140

160

180

200

220

0.2 C0.2 D

1.0 C1.0 D

Charge Discharge

Spec

ific

capa

city

/ m

Ah

g-1

Cycle No.

0.2 C0.2 D

Capacity loss (CL) in the 1st cycle and after rate increase in the 4th cycle

Rate switch in cycle No. 54 results in a partial capacity regain

Explanation: Lithiation (discharge) of NMC slower than de‐lithiation (charge)

NMC: Lithiation vs. De‐lithiation Kinetics: Effect of Rate Switching During Cycling

WE: NMC; CE, Re: Li1M LiPF6 in EC/EMC (1/1)Cut‐off: 4.6–3.0 V vs. Li/Li+Cycles 1‐3: 0.2C, 0.2D Cycles 4‐53: 1.0C, 1.0DCycle 54: 0.2C, 0.2D

16.5% CL

0 40 80 120 160 200 240

3.0

3.5

4.0

4.5

Pote

ntia

l vs.

Li/L

i+ / V

Specific Capacity / mAh g-1

0.2 C - X D 0.2 0.01

Facilitating Lithiation (I): Lowering the Discharge Rate to 0.01 D

D‐rate decrease gains 12.8 mAh g‐1 in the 1st cycle

Evidence for kinetics effect Achievable discharge capacity and Coulombic efficiency (CE) depend on D‐rate Lithiation (= discharge) of NMC slower than de‐lithiation (= charge)

1. Charge capacity: 222.7 mAh g‐12. Discharge capacity

(0.2 D): 185.9 mAh g‐13. Discharge capacity

(0.01 D): 198.7 mAh g‐14. Coulombic Eff: 83.5 % 89.2 %

WE: NMC; CE, Re: Li1M LiPF6 in EC/EMC (1/1)Cut‐off: 4.6–3.0V vs. Li/Li+Rate: 0.2 C, ?D

1st Cycle

0 40 80 120 160 200

3.0

3.3

3.6

3.9

4.2

4.5

Pote

ntia

l vs

Li/L

i+ / V


Facilitating Lithiation (II): Constant Potential Holding Step during Discharge at 3.0 V vs. Li/Li+


(without CP step): 184.9 mAh g‐13. Discharge capacity

(with CP step): 206.2 mAh g‐14. Coulombic Eff.: 83.5 % 93.0 %

Evidence for kinetics effect Lithiation (= discharge) of NMC slower than de‐lithiation (= charge)

CP step at 3.0V vs. Li/Li+ gains extra 21.3 mAh g‐1 in the 1stcycle

WE: NMC; CE, Re: Li1M LiPF6 in EC/EMC (1/1)Cut‐off: 4.6–3.0 V vs. Li/Li+Constant Potential Step: 3.0 V vs. Li/Li+ for 24 hRate: 0.2C, 0.2D

1st Cycle

0 40 80 120 160 2002.0

2.5

3.0

3.5

4.0

4.5

Pote

ntia

l vs

Li/L

i+ / V


Facilitating Lithiation (III): Lowering the Cut‐off Down to 2.0 V vs Li/Li+

Cut‐off potential decrease gains 2.3 mAh g‐1 in the 1st cycle

Achievable discharge capacity depends also on discharge cut‐off potential!!! Be careful in full cell: At (too) low cell voltages, SEI and/or Cu oxidation may take place at the anode

1. Charge capacity: 222.7 mAh g‐12. Discharge [email protected] vs Li/Li+:

185.9 mAh g‐13. Discharge [email protected] vs Li/Li+:

188.2 mAh g‐14. Coulombic Eff: 83.5 % 84.5 %

WE: NMC; CE, Re: Li1M LiPF6 in EC/EMC (1/1)Cut‐off: 4.6–2.0V vs. Li/Li+Rate: 0.2C, 0.2D

1st Cycle

0 40 80 120 160 2002.0

2.5

3.0

3.5

4.0

4.5

Pote

ntia

l vs

Li/L

i+ / V


CP step at 2.0 V vs Li/Li+gains 21.7 mAh g‐1




In summary 24 mAh g‐1 (2.3 mAh g‐1 + 21.7 mAh g‐1) of total capacity loss of 36.8 mAh g‐1 in the first cycle could be regenerated by simple kinetic measures 12.8 mAh g‐1 irr. capacity loss

Residual capacity loss of 12.8 mAh g‐1 attributable to electrolyte oxidation?

Facilitating Lithiation (IV): Constant Potential (CP) Holding Step in Discharge at 2.0 V vs. Li/Li+

WE: NMC; CE, Re: Li1M LiPF6 in EC/EMC (1/1)Cut‐off: 4.6–2.0V vs. Li/Li+Constant Potential Step: 2.0V vs. Li/Li+ for 24 hRate: 0.2C, 0.2D

Cut‐off potential decrease gains 2.3 mAh g‐1

1st Cycle

1st Cycle Capacity Losses at DifferentUpper Cut‐Off Potentials

Total capacity loss constant up to 4.3 V vs. Li/Li+After CP step: Irr. capacity loss: constant up to 4.3 V vs. Li/Li+

Intrinsic Material problem, e.g., Li/Ni Mixing? Intrinsic Irreversible Capacity Loss: ca. 5 mAh g‐1

WE: NMC; CE, Re: Li1M LiPF6 in EC/EMC (1/1)Cut‐off: ?V–3.0 V vs. Li/Li+Constant Potential Step: 3.0 V vs. Li/Li+ for 24 hRate: 0.2C, 0.2D

3.9 4.0 4.1 4.2 4.3 4.4 4.5 4.60

10

20

30

40

Cap

acity

loss

/ m

Ah

g-1

Potential / V vs. Li/Li+

Total capacity loss Irr. capacity loss

0 40 80 120 160

3.0

3.2

3.4

3.6

3.8

4.0

4.2

4.4

Pote

ntia

l vs.

Li/L

i+ / V


3.9 V 4.0 V 4.3 V

Stable electrolyteenvironment

4.3 V vs. Li/Li+

Total capacity loss

Effect of Ni2+ (Trapped on Li+ Sites) on Capacity Loss

Labrini et al.: Irreversible oxidation of the “trapped” Ni2+ to Ni3+ during charge (= delithiation)1

5 mAh g‐1 (observed by us) can be assigned to 1.8% of Li/Ni mixing

Literature reports 1‐6% Li/Ni mixing for NMC2

M. Hu et al. / Journal of Power Sources 237 (2013) 229-242

1Labrini, M. Electrochim Acta 2013, 111, 5672Ellis, B. L.; Lee, K. T.; Nazar, L. F. Chem Mater 2010, 22, 691.

Li+/Ni2+ mixing is well known for Ni based layered oxide cathodes.

Determination of the Irrev. Charge Loss in the Overall Charge Loss, e.g.: in the 1st Cycle

0 10 20 30 40 50 60

3.0

3.2

3.4

3.6

3.8

Pote

ntia

l vs

Li/L

i+ / V


36.5 mAh g‐1 initial charge loss

21.3 mAh g‐1 kinetics effect

Ca. 5 mAh g‐1 intrinsic material loss

10 mAh g‐1 parasitic reactions, such as electrolyte oxidation

WE: NMC; CE, Re: Li1M LiPF6 in EC/EMC (1/1)Cut‐off: 4.6–3.0 V vs. Li/Li+Constant Potential Step: 3.0V vs. Li/Li+ for 24 hRate: 0.2C, 0.2D

Note: For a full cell: Apparent “Li loss” at the NMC cathode happens in parallel to Li loss in the SEI at the graphite anode

Charge

Discharge

0 40 80 120 160 2000

1

2

3

4

0

1

2

3

4

Pote

ntia

l vs.

Li/L

i+ / V

Volta

ge /

V


Cell voltage Graphite Potential NMC Potential

Full Cells with NMC/MCMB

CV step at 2.4 Vgains 15.1 mAh g‐1

1. Charge capacity: 222.7 mAh g‐12. Discharge capacity @ 2.0 V: 186.3 mAh g‐13. (with CP step): 201.4 mAh g‐14. Coulombic Eff.: 83.7 % 90.4 %

In summary 15.1 mAh g‐1 of an overall capacity loss of 36.4 mAh g‐1 in the first cycle could be regenerated using a CV step

In full cells, the max. possible capacity regain is limited by the CE of the specific graphite.

“Extra” Li (capacity regain) serves as reservoir for SEI repair and other Li loss mechanisms.

WE: NMC; CE: MCMB (30 % oversized); Re: Li1M LiPF6 in EC/EMC (1/1)Cut‐off: 4.55–2.0 V Constant Voltage Step@Discharge cut‐off voltage for 24 hRate: 0.2C, 0.2D

3.9 4.60

10

20

30

40

Spec

ific

capa

city

/ m

Ah

g-1

Potential vs. Li/Li+

Specific capacity losses: Parasitic reactions Kinetic inhibition Intrinsic NCM loss

Parasitic reactions increase the kinetic inhibition probably due to the formation of passivation layer caused by electrolyte oxidation

Influence of a Higher Cut‐off Potential on Apparent (Kinetic) Capacity Loss of NMC

5.0 mAh g-1 5.0 mAh g-1

17.1 mAh g-1

10.2 mAh g-1

( 4.2 mAh g-1)

21.3 mAh g-1

22.1 mAh g-1

36.5 mAh g-1

Parasitic reactionslead to larger

kinetic inhibition(= kinetic capacity loss)

3.9 4.0 4.1 4.2 4.3 4.4 4.5 4.670

80

90

100

Cou

lom

bic

Effic

ienc

y / %

Potential / V vs. Li/Li+

CE (conventional) CE (after CP-step)

1st Cycle Coulombic Efficiencies (CE) at Different Upper Cut‐Off Potentials

Highest possible 1st cyle CE (97%) for NMC at a potential of 4.4 V vs. Li/Li+

WE: NMC; CE, Re: Li1M LiPF6 in EC/EMC (1/1)Cut‐off: ?V–3.0 V vs. Li/Li+Constant Potential Step: 3.0 V vs. Li/Li+ for 24 hRate: 0.2C, 0.2D

CE (after CP step): 97%

CE (conventional): 87%

+10%IntrinsicNMC loss

Add. parasiticreactions

High C‐rate: low irr./total capacity loss

Low C‐rate: high irr./total capacity lossWhy?

WE: NMC; CE, Re: Li1M LiPF6 in EC/EMC (1/1)Cut‐off: 4.6–3.0 V vs. Li/Li++ Constant Potential Step: 3.0 V vs. Li/Li+ for 24 hRate: XC, 0.2D

0.01 0.1 10

15

30

45

60

Cap

acity

loss

/ m

Ah

g-1

C-rate

Total capacity loss Irr. capacity loss

Irreversible and Total Capacity Losses in the 1st Cycle vs. C‐Rate

High C‐rate: Shorter exposition in HV region

Low C‐rate: Longer exposition in HV region

0 4 8 12 163.6

3.8

4.0

4.2

4.4

4.6

Pote

ntia

l vs.

Li/L

i+ / V

Time / h

1.0 C 0.5 C 0.2 C 0.1 C

C‐Rate Decrease Longer Operation Time in Unstable Potential Environment

WE: NMC; CE, Re: Li1M LiPF6 in EC/EMC (1/1)Cut‐off: 4.6 V vs. Li/Li+Rate: ?C

Unstable electrolyteenvironment

3.5 h1.7 h0.6 h0.3 h

More parasitic reactions in HV region

0.01 0.1 1150

165

180

195D

isch

arge

cap

acity

/ m

Ah

g-1

C-rate

Discharge Capacity in the 1st Cycle vs. the C‐Rate (= Rate during Charge)

High C‐rate: Low parasitic reactions and low additional kinetic hindrancevs. a low charge capacity

Low C‐rate: More parasitic reactions and higher additional kinetic hindrance vs. a high charge capacity Compromise:

0.1 – 0.5 C

WE: NMC; CE, Re: Li1M LiPF6 in EC/EMC (1/1)Cut‐off: 4.6–3.0 V vs. Li/Li+Rate: ?C, 0.2D

0 40 80 120 160 200 240

3.0

3.5

4.0

4.5

Pote

ntia

l vs.

Li/L

i+ / V


Minimize Effects of Impedance on Capacity loss: CP Step During Charge AND Discharge

Using a CP step during charge AND discharge discharge capacity 213 mAh g‐1 !Li1‐xNi1/3Co1/3Mn1/3O2 x = 0.78 (at 4.6 V vs. Li/Li+)

WE: NMC; CE, Re: Li1M LiPF6 in EC/EMC (1/1)Cut‐off: 4.6–3.0 V vs. Li/Li+Constant Potential Step: 4.6 V vs. Li/Li+ for 5 h3.0 V vs. Li/Li+ for 24 hRate: 0.2 C, 0.2 D

CP step at 3.0V vs. Li/Li+ gains extra 24.8 mAh g‐1

CP step at 4.6V vs. Li/Li+ gains extra 13.3 mAh g‐1

1. Discharge capacity (No charge CP step): 206.2 mAh g‐1

2. Discharge capacity (Charge CP step): 213 mAh g‐1

3. Irreversible capacity (No charge step): 15.2 mAh g‐1

4. Irreversible capacity (Charge CP step): 21.7 mAh g‐1

+6.8 mAh g‐1(Rev. Li extraction)

‐6.5 mAh g‐1(EL oxidation)

‐6.5 mAh g‐1

+6.8 mAh g‐1

Additional Li extraction “Extra” capacity May lead to phase changes in NMC* Future Research *S. C. Yin, Y. H. Rho, I. Swainson, L. F. Nazar, Chemistry of Materials 2006, 18, 1901.

Comparison of Specific Energies With and Without CP Step at Various Cut‐Off Potentialsand Temperatures

WE: NMC; CE, Re: Li1M LiPF6 in EC/EMC (1/1)Cut‐off: ?V vs. Li/Li+Constant Potential Step:3.0 V vs. Li/Li+ for 24 hRate: 0.2C, 0.2D

Operation temperatures: 25 °C and 60°C

Ca. 18% energy loss due to kinetic effects

54%Li+

68%Li+

75%Li+

48%Li+

4.2 4.2 (CP) 4.6 4.6 (CP) 4.6(CP;60°C)0

200

400

600

800

1000

Upper cut-off potential / V vs. Li/Li+

54%Li+

68%Li+

75%Li+

48%Li+

80%Li+

0 40 80 120 160

3.0

3.2

3.4

3.6

3.8

4.0

4.2

4.4

Pote

ntia

l vs.

Li/L

i+ / V


For Comparison:Capacity Loss in LiCoO2, LCO (at 4.3 V)




2.3 mAh g‐1 irr. cap. loss!

CP step at 3.0V vs. Li/Li+ gains extra 4.8 mAh g‐1 in the 1st cycle

WE: LFP; CE, Re: Li1M LiPF6 in EC/EMC (1/1)Cut‐off: 4.3–3.0 V vs. Li/Li+Constant Potential Step: 3.0 V vs. Li/Li+ for 24 hSp. current: 30 mA g‐1

161.4 mAh g‐1 of possible 276 mAh g‐1 were extracted (58%)

0 40 80 120 160

3.0

3.5

4.0

4.5

5.0

Pote

ntia

l vs.

Li/L

i+ / V


For Comparison:Capacity Loss in LiNi0.5Mn1.5O2, LNMO






WE: LNMO; CE, Re: Li1M LiPF6 in EC/EMC (1/1)Cut‐off: 4.95–3.0 V vs. Li/Li+Constant Potential Step: 3.0 V vs. Li/Li+ for 24 hRate: 0.2C, 0.2D

Comparison to NMC:Despite higher charging potentials (!) lower irr. capacity loss (3.8 vs. 15.2 mAh g‐1 )

0 40 80 120 160

3.0

3.2

3.4

3.6

3.8

4.0

4.2

Pote

ntia

l vs.

Li/L

i+ / V


For Comparison:Capacity Loss in LiFePO4, LFPO






WE: LFP; CE, Re: Li1M LiPF6 in EC/EMC (1/1)Cut‐off: 4.1–3.0 V vs. Li/Li+Constant Potential Step: 3.0 V vs. Li/Li+ for 24 hSpec. current: 30 mA g‐1

0

5

10

15

20

25

1 D 2 D 3 D 2D?

Capa

city

Loss / m

Ah g

‐1

Li Transport in Cathode Structure

Kinetic cap. lossIrr. cap. loss

Capacity Loss: NMC is an Exception

LFP LCO LNMO

4.1 V4.3 V

4.95 V

3.9‐4.3 V

NMC

• “1/3 NMC” has been used as model electrode for investigations on HV electrolytes using novel classes of electrolyte additives:

Trimethylsilyl (TMS)‐ based additives can be selected according to pKa considerations.

Additives with inorganic metal cations, such as Mg2+, Al3+, etc.

• The additives are enabling NMC operation at 4.6 V and have HF‐preventing properties, probably due to formation of an effective protecting CEI layer on the surface or in the sub‐surface of NMC (maybe also by an additional HF‐scavenging effect)

• The CEI formed in the presence of the additives shows a higher impedance at the beginning of cycling, but a lower impedance at higher cycle number (cf. reference electrolyte). So far, we do not see a bulk effect.

• Kinetics affect capacity loss and Coulombic efficiency Capacity regains

• For NMC, the contribution of irreversible reactions such as electrolyte oxidation to the “irreversible charge loss” at 4.6 V in the 1st cycle can be estimated to be less than 30%.

• NMC shows a “unique“ capacity loss behavior campared to LFP, LCO and LNMO

Conclusions

“God made the bulk; surfaces were invented by the devil.”‐‐‐Wolfgang Pauli (Austrian‐Swiss Physicist, 1900‐1958)

Back-Up

0 10 20 30 40 50 60100

120

140

160

180

200

220 Charge Discharge

Spec

ific

capa

city

/ m

Ah

g-1

Cycle No.

0.2 C0.2 D

1.0 C1.0 D

0.2 C0.2 D

NMC at 4.6V vs. Li/Li+: Capacity Fade vs. Power Fade During Cycling

Only 83.5 % of initial capacity could be achieved in the 53rd cycle

Material degradation: Capacity fade Impedance increase: Power fade

Most of the “lost capacity” can be re‐gained at low rates (in the 54th cycle)

184 mAh g‐1 could be achieved in cycle 54, despite previous significant “capacity fade” Power fade is dominant, not capacity fade!

WE: NMC; CE, Re: Li1M LiPF6 in EC/EMC (1/1)Cut‐off: 4.6–3.0 V vs. Li/Li+Cycles 1‐3: 0.2C, 0.2D Cycles 4‐53: 1.0C, 1.0DCycle 54: 0.2C, 0.2D(with Constant Potential Stepat 3.0V vs. Li/Li+ down to <0.2 C)

0 50 100 150 200 2502.8

3.2

3.6

4.0

4.4

4.8

Pote

ntia

l vs.

Li/L

i+ / V


20°C 40°C 60°C

Facilitating Lithiation (IV): Increasing the Temperature

WE: NMC; CE, Re: Li1M LiPF6 in EC/EMC (1/1)Cut‐off: 4.6–3.0 V vs. Li/Li+Rate: 0.2 C, 0.2 D

NMC Investigations | Johannes Kasnatscheew |

Electrochemical performance strongly depends on temperature

Temperature increase in 1st cycle1. Lowers Polarizations2. Increase charge capacity3. Increase discharge capacity4. Lowers total and irr. capacity loss5. Increase Coulombic efficiency

Winter-PSI Symposium 2015.pptx [Schreibgeschützt] · From 1791 Until Today From Aqueous to...

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