“The dose makes the poison.“
‐‐‐Paracelsus (Philippus Theophrastus Aureolus Bombastus von Hohenheim) (Swiss Medic, Alchemist, Theologist, Astrologist and Philosopher, 1493 ‐ 1541) 31st PSI Electrochemistry Symposium - Electrochemical Energy Storage:
A Key for Future Energy Systems, Villigen, CH, 6 May 2015
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
Acknowledgments
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
From the Beginning:
Electrolytes Enable Batteries The medic Luigi Galvani (IT) generated electricity using a “galvanic cell” with a frog leg as electrolyte between two metal electrodes
Galvani(1737‐1798)
Volta (1745‐1827)
Ritter (1776‐1810)
Cell Pile
The 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 evaporation
The 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
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
Electrolytes are decisive for power, life and safety of the battery cell
In many cases electrolytes are (co‐) determining the energy, as well
From 1791 Until Today
From Aqueous to Non‐Aqueous Electrolytes
From 1 V to >5 V Batteries 1800
1900
1.1 V
Zn-O2
(Volta)
1 V
H2-O2
(Ritter)
2V
Pb-Acid
(Sinsteden,
Planté)
1791
1803
1854/59
Primary Recharge- Rechargeable
able
2000
1 V
3 V
Aqu. Ion Li-Metal
Transfer Systems
System
(Rüdorff,
Hoffmann)
1938
1960ies
Rechargeable
Primary
Today +
Up to 4.2 V
3 V
Li-Ion
Li-Ion
Transfer Systems
Systems
1980ies
1990ies
Recharge- Rechargeable
able
>4.2V
Li-Ion
Systems
Today &
Tomorrow
Rechargeable
High Voltage Cathodes:
‘High Voltage’ is Relative
Lightning: Several 10.000.000 Volts
High voltage grid: Several 100.000 Volts
Static electricity: Several 10,000 Volts
Humans: Up to 30.000 Volts
Sr/Sr+ Li/Li+
NHE F2/F‐ KrF2/KrF+
‐4.1 ‐3.040 0.0 3.070* 3.27* E / V vs. SHE *in acidic solution
Batteries:
Possible: <8V
Practical: ≤5 V
Typical: 1.2 – 4V
The challenge: >4.3V
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 LIB
with High Voltage (HV) Cathodes
E=C·V
2V
1V
0V
HV
Cathodes
NMC
at
HV
NMC
Cathode
•
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 electrolyte
e.g., organic carbonate solvents:
> 4.2 ‐ 4.3 V
Cathode
CellCell
Voltage
Voltage
3V
Electrolyte Stability
4V
?
SEI
Potential vs. Li/Li+
5V
Graphite
Anode
LiNi0.33Mn0.33Co0.33O2 (“1/3‐NMC”) at HV:
Enhanced Potential and Capacity
• NMC can be charged to different upper cut‐off potentials Lithium
Li layer
• Higher cut‐off potential  HV application
 higher specific energy Transitionmetal
oxide
metal oxide
layer
B. Xu, D. Qian, Z. Wang, Y.S. Meng, Materials Science and Engineering: R: Reports 2012, 73, 51-65
48%
Li+
68%
Li+
NMC:
Failure Mechanisms at 4.6 V
Overall cell impedance rise
Oxygen release from
cathode
Transition metal dissolution
NMC de‐
gradation
at 4.6 V
Oxidative electrolyte degradation
Irreversible phase changes
[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.
NMC:
Failure Mechanisms at 4.6 V
Overall cell impedance rise
Oxygen release from
cathode
Transition metal dissolution
NMC de‐
gradation
at 4.6 V
Oxidative electrolyte degradation
Irreversible phase changes
[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.
High Voltage Application of NMC
Use of LiPF6 in Electrolyte at HV  Metal Dissolution (Promoted by HF)
180
-1
WE: NMC, CE, RE: Li
LiPF6
160
1000
140
120
Upper cut-off potential vs. Li/Li
4.2 V
4.4 V
4.6 V
100
80
1500
Concentration / g L
Specific capacity / mAh g
-1
2000
0
10
20
30
Cycle number
+
500
Ni
Co
Mn
NMC storage in
electrolyte
for 28 days
0
40
50
3.0
3.5
4.0
4.5
+
Electrode potential vs. Li/Li / V
•
Enhanced average discharge potential
•
Electrolyte: 1M LiPF6 in EC/DMC (1:1)
•
Higher specific capacity
•
Ni, Co, and Mn dissolution
•
Lower Coulombic Efficiency
•
Large dissolution at 4.6 V vs. Li/Li+
•
Insufficient cycle life
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.
Protection of the Cathode vs. Dissolution
HF
HF Scavenger
CEI* Forming Electrolyte Additive
Alternative Conducting Salt
Cathode
Material
Coating/
Doping
Salt Stabilizer
to avoid
HF
*CEI: Cathode Electrolyte Interphase
Alternative Conducting Salt: LiBF4 Less Sensitive to HF formation  Enables HV application
160
-1
LiBF4
Concentration / g L
Specific capacity / mAh g
-1
400
180
WE: NMC, CE, RE: Li
140
120
100
Upper cut-off potential vs. Li/Li
4.2 V
4.6 V
80
60
40
0
10
20
30
Cycle number
+
40
•
Higher capacity at 4.6 V vs. Li/Li+
•
Enhanced cycling stability
•
Enables HV application
Ni
Co
Mn
300
200
100
NMC storage in
electrolyte
for 28 days
5
x
l
o
w
e
r
0
50
3.0
3.5
4.0
+
4.5
Electrode potential vs. Li/Li / V
•
Electrolyte: 1M LiBF4 in EC/DMC (1:1)
•
Metal dissolution in the presence of LiBF4
5 times lower than with LiPF6
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.
Protection of the Cathode vs. Dissolution
HF
HF Scavenger
CEI*
Forming
Electrolyte Additive
Alternative Conducting Salt
Cathode
Material
Coating/
Doping
Salt Stabilizer
to avoid
HF
*CEI: Cathode Electrolyte Interphase
New HF (and H2O) Scavenging Electrolyte Additives: TMS (Trimethylsilyl‐) Based
•
HF
Si F
+
NH
Si N
H2O
Patent Claim by Saidi et al.*: TMS diethylamine can reduce HF induced transition metal dissolution*
Proposal of mechanism by Zhang**
Diethylamine = Leaving group (LG)
Si OH + NH
•
•
•
However: Not stable at high cathode potentials
-2
1600
NMC storage at 4.6V
vs. Li/Li+ for 28 days
1200
800
400
0
1M LiPF6 in EC/DMC + 1wt% TMS diethylamine
Current density / mA cm
Mn concentration g L-1
**Mechanism S.S. Zhang, J Power Sources, 162 (2006)
1379-1394
1 M LiPF6 in EC/DMC (1/1)
0.6
+ 1 wt% TMS diethylamine
0.4
0.2
WE: LMO; CE, RE: Li
Scan rate: 0.1 mV s‐1
0.0
3.0
3.5
4.0
4.5
5.0
5.5
+
Potential vs. Li/Li / V
*M.Y. Saidi, F. Gao, J. Barker, C. Scordilis‐Kelley, U.S. Patent 5,846,673 (1998)
6.0
Why Using a TMS Group in the Additive?
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‐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:
Si
LG
Or Or
LG
Si
[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.
Design of the HF Scavenger
The 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
•
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”
Correlation of pka with HOMO Energy**
Oxidation Stability and LG Ability Can Be Estimated by pKa Value
HOMO* energy
pKa value
~
•
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.
*Highest Occupied Molecular Orbital
**H.J. Soscún Machado, A. Hinchliffe, Journal of Molecular Structure: THEOCHEM, 339 (1995) 255-258.
Effect of TMS Additives on NMC Cycling at HV
105
WE: NMC; CE, RE: Li; 3.0-4.6V vs. Li
1st -3rd cycle: 0.2C; 4th-50th cycle: 1C
200
Coulombic efficiency / %
Specific capacity / mAh g
-1
240
100
160
120
80
1M LiPF6 in EC/DMC (1/1)
+ 1wt-% TMS diethylamine
+ 1wt-% TMS trifluoroacetate
40
0
0
10
20
30
Cycle number
40
50
95
90
1M LiPF6 in EC/DMC (1/1)
85
80
+ 1wt-% TMS diethylamine
+ 1wt-% TMS trifluoroacetate
0
10
20
30
Cycle number
TMS diethyl amine:
TMS trifluoro acetate:
•
Better capacity retention
•
Better capacity retention
•
Low Coulombic efficiency
•
Higher Coulombic efficiency
•
Oxid. decomposition during cycling
•
Enables HV application
40
50
Capacity retention / %
Lower pKa of the LG  Better Capacity Retention
100
8
9
4
7 6
80
1
2
3
5
60
Capacity retention: Discharge capacity 5th/50th
cycle
Ox. stability / HF‐scavenger / More effective CEI
40
20
1 TMS diethylamine
2 TMS isocyanate
3 TMS acetate
4 TMS trifluoroacetate
5 Methoxy TMS
6 N,O-Bis(TMS)acetamide
-10
0
7 N,O-Bis(TMS)trifluoroacetamide
8 TMS methanesulfonate
9 TMS TFSI
10
pKa value of the leaving group
Based on an initial capacity of ca. 180 mAh g‐1
20
500
1 TMS diethylamine
2 TMS isocyanate
3 TMS acetate
4 TMS trifluoroacetate
5 Methoxy TMS
400
Cum. irr. cap. loss / mAh g-1
Cum. irr. cap. loss / mAh g-1
Cumulated Irreversible Capacity Losses vs.
pKa Value and HOMO Energy
6 N,O-Bis(TMS)acetamide
7 N,O-Bis(TMS)trifluoroacetamide
8 TMS methanesulfonate
9 TMS TFSI
300
1
200
7
5
100
9
8
0
-15
-10
-5
6 4
0
23
5
10
15
pKa value of the leaving group
•
•
20
500
400
1 TMS diethylamine
2 TMS isocyanate
3 TMS acetate
4 TMS trifluoroacetate
5 Methoxy TMS
6 N,O-Bis(TMS)acetamide
7 N,O-Bis(TMS)trifluoroacetamide
8 TMS methanesulfonate
9 TMS TFSI
300
1
200
7
3
100
2
4
5
9
6
8
0
-13.0
-12.5
-12.0
-11.5
-11.0
-10.5
-10.0
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.
-9.5
CEI Film Formation?
Reverted Trend of Impedance Development During Cycling after Addition of the Additive
WE: NMC; CE, RE: Li
1M LiPF6 in EC/DMC (1:1)
•
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]
+ 1 wt‐% TMS trifluoroacetate
With additive
•
TMS trifluoro acetate leads to a decrease of charge transfer resistance (3rd → 50th cycle) → Less LiF?
[1] X. Zuo, C. Fan, J. Liu, X. Xiao, J. Wu, J. Nan, J Power Sources, 229 (2013) 308-312.
Methods to Understand the Mechanisms
19F
& 29Si NMR
Storage of the electrolytes w/wo additives @60°C, 7 days
Determination of the HF content
Determination of reaction products of TMS additives
GC-MS
SEM
Post mortem ana‐
lysis of the electro‐
lyte after CC cycling
Post mortem ana‐
lysis of the electro‐
des after CC cycling
Storage of a water‐
doped electrolyte @ 60°C, 7 days
Determination of reaction products of TMS additives
Study of the elec‐
trode surface/CEI
Proposal for the Mechanism
for Reaction of TMS Trifluoro Acetate
GC-MS
HF
Si F
Si O Si
+
Si OH
Condensation
GC-MS
Storage
experiment
O
H2O
O
Si O
CF3
main
r
Si OH + HO
NMR
oute
HF
Si F
+ HO
CF3
Post
mortem
O
CF3
NMR
GC-MS
•
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.
TMS Trifluoroacetate Performance with Graphite Anode
•
Compatible with graphite
•
No negative influence on capacity retention
•
Slightly enhanced Coulombic efficiency
Al Passivation in TMS Electrolyte at Smaller HF Amount
•
Sufficient amounts of HF in the electrolyte are beneficial in order to passivate the Al current collector*:
a
Al2O3 + 2HF
2AlOF + H2O
2AlOF + 2HF
Al2OF4 + H2O
Al2OF4 + 2HF
2AlF3 + H2O
b
Al : Constant voltage
@ 4.6 V vs. Li/Li+, 24 h
c
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
*Krämer, et al. J. Electrochem. Soc. 2013, 160 (2), A356-A360
Krämer, et al. Electrochem. Lett., 2012, 1(5), C1 - C3.
Protection of the Cathode vs. Dissolution
HF
HF Scavenger
CEI* Forming Electrolyte Additive
Cathode
Material
Coating/
Doping
Alternative Conducting Salt
Salt Stabilizer
to avoid
HF
*CEI: Cathode Electrolyte Interphase
Previous Reports [1‐3]: Negative Influence of Metal Cations on Cycling Performance
Metal dissolution or addition of metal salts
Transition metals are incorporated in the SEI
Impedance rise at the anode
Capacity fading
[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 M LiTFSA in EC:DEC:TMP (3:3:4, by vol.) Addition of
Ca(TFSA)2
[1] S. Takeuchi, S. Yano, T. Fukutsuka, K. Miyazaki, T. Abe, J Electrochem
Soc , 159 (2012), A2089.
TFSA- = TFSI-
Summary of the Investigated Additives
Additive
No additive
LiTFSI
NaTFSI
Mg(TFSI)2
Al(OTf)3
Ca(TFSI)2
Sc(TFSI)3
Cr(TFSI)3
Ni(TFSI)2
Cu(TFSI)2
Zn(TFSI)2
Supplier, Purity
3M, battery grade
Solvionic, 99.5%
Solvionic, 99.5%
Aldrich, 99.9%
Solvionic, 99.5%
abcr, 99%
Synthesis: In‐house
Solvionic, 99.5%
Synthesis: In‐house
Solvionic, 99.5%
5th cycle
Discharge Cap. [mAh g‐1]
167.9
157.1
156.3
144.5
121.0
149.4
168.9
143.3
165.2
167.1
161.9
50th cycle
Discharge Cap.
[mAh g‐1]
146.4
139.2
136.4
149.8
114.8
119.6
150.7
141.0
146.8
158.1
142.6
Cap. Retention [50th/5th]
[%]
87.2
88.6
87.3
103.6
94.9
80.0
89.2
98.4
88.8
94.6
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
Discharge capacity / mAh g-1
200
180
1 M LiPF6 in EC:EMC (1:1)
160
1 M LiPF6 in EC:EMC (1:1) + 1% Mg(TFSI)2
140
120
100
 after 250 cycles without Mg2+
80
 after 750 cycles with Mg2+
60
• Coulombic efficiencies in 1. and 5. cycle
40
Electrolyte
20
0
• 2‐Electrode Cell: NMC/MAGD20
15% Anode Excess
Cut‐off: 2.8 – 4.5V
End of life (EOL) criterion, which indicates 80% of the initial cell capacity after formation, is reached:
0
LP50 (1 M LiPF6 in
100 200 300 400 500 600 700 800 900 1000 EC:EMC, 1:1)
Cycle no.
LP50 + 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
1% Mg(TFSI)2
1st
5th
78.5%
99.1%
87.9%
99.7%
Effect of Mg(TFSI)2 on NMC Self‐Discharge
Potential vs. (Li/Li+) / V
4.7
WE: NMC; CE, RE: Li; Cut‐off 4.6 V vs. Li/Li+
3 formation cycles at C/5
120h storage at OCP
4.6
4.5
4.4
4.3
4.2
1 M LiPF6 in EC:EMC (1:1)
1 M LiPF6 in EC:EMC (1:1) + 1% Mg(TFSI)2
0
20
40
60
80
Time / h
• Reduced self‐discharge with Mg(TFSI)2
100
120
Mg(TFSI)2 as HF Scavenger? 19F‐NMR 10
spectroscopy after storage at 60 °C
under inert atmosphere in NMR tubes for up
to 12 days.
19F‐NMR
1 M LiPF6 in EC:EMC (1:1)
1 M LiPF6 in EC:EMC (1:1) + 2% Mg(TFSI)2
Concentration / mmol
8
6
• HF concentration in the case of the benchmark electrolyte is increasing during storage.
4
• With addition of Mg(TFSI)2, the HF concentration remains below the limit of detection (LOD).
2
0
0
2
4
6
time / days
8
10
12
 Mg(TFSI)2 is an effective HF‐scavenger
 (no intuitive mechanism)
or
prevents HF formation
Mechanism? • We assume that the Mg(TFSI)2 additive has the following functions:
Mg(TFSI)2
HF‐Scavenger
HF‐Prevention 
CEI‐film‐
forming additive
Stabilization of the host structure
?

 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
200
300
1 M LiPF6 in EC:EMC (1:1) + 1% Mg(TFSI)2
after 5 cycles
after 30 cycles
after 50 cycles
150
200
-Zimag / ohm
-Zimag / ohm
250
150
100
1 M LiPF6 in EC:EMC (1:1)
50
after 5 cycles
after 30 cycles
after 50 cycles
0
0
200
400
600
800
Zreal / ohm
100
50
WE: NMC; CE, RE: Li
0
0
100
200
300
400
Zreal / ohm
• Impedance increase in Rct during cycling • With Mg2+: Higher Rct after 5 cycles,  Growth of a passivation layer on the cathode by electrolyte decomposition?
But decrease in Rct during cycling
Future Work
• 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)
1C
1C
5.0
Cycling
110
Self‐discharge
100
Potential vs. (Li/Li ) / V
90
4.8
+
Discharge capacity / mAh g
-1
C/5
120
D-rate test
recovery
80
70
WE: LMNO; CE, RE: Li;
Cut‐off: 4.95 – 3.0 V vs. Li/Li+
60
50
40
1 M LiPF6 in EC:EMC (1:1)
30
20
+ 0.1% Mg(TFSI)2
10
+ 0.2% Al(Otf)3
20
40
60
WE: LMNO; CE, RE: Li; Cut‐off: 4.95 V vs. Li/Li+
3 formation cycles: C/5
120h storage at OCP
4.4
1 M LiPF6 in EC:EMC (1:1)
4.2
1 M LiPF6 in EC:EMC (1:1) + 1% Mg(TFSI)2
0
0
4.6
80
100
120
140
Cycle no.
3 formation cycles at C/5, then C/2
D‐rate tests at: D/5, D/3, D/2, 1D, 2D, 3D und 5D
160
0
10
20
30
40
50
60
70
time / h
80
90 100 110 120
NMC: Lithiation vs. De‐lithiation Kinetics: Effect of Rate Switching During Cycling
Charge
Discharge
Specific capacity / mAh g-1
220
16.5% CL
200
180
160
140
120
100
0
2
0.2 C
0.2 D
1.0 C
1.0 D
0.2 C
0.2 D
4
6
50
52
WE: NMC; CE, Re: Li
1M 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, 1.0D
Cycle 54: 0.2C, 0.2D 54
Cycle No.
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)
Facilitating Lithiation (I): Lowering the Discharge Rate to 0.01 D
1st Cycle
Potential vs. Li/Li+ / V
4.5
WE: NMC; CE, Re: Li
1M LiPF6 in EC/EMC (1/1)
Cut‐off: 4.6–3.0V vs. Li/Li+
Rate: 0.2 C, ?D
0.2 C - X D
0.2
0.01
4.0
3.5
D‐rate decrease gains 12.8 mAh g‐1 in the 1st cycle
3.0
0
40
80
120
160
200
-1
Specific Capacity / mAh g
240
1. Charge capacity: 222.7 mAh g‐1
2. Discharge capacity (0.2 D): 185.9 mAh g‐1
3. Discharge capacity (0.01 D): 198.7 mAh g‐1
4. Coulombic Eff: 83.5 %  89.2 %
 Evidence for kinetics effect
 Achievable discharge capacity and Coulombic efficiency (CE) depend on D‐rate
 Lithiation (= discharge) of NMC slower than de‐lithiation (= charge)
Facilitating Lithiation (II): Constant Potential Holding Step during Discharge at 3.0 V vs. Li/Li+
Potential vs Li/Li+ / V
4.5
WE: NMC; CE, Re: Li
1M 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 h
Rate: 0.2C, 0.2D 1st Cycle
4.2
3.9
3.6
CP step at 3.0V vs. Li/Li+ gains extra 21.3 mAh g‐1 in the 1st
cycle
3.3
3.0
0
40
80
120
160
1. Charge capacity: 221.4 mAh g‐1
2. Discharge capacity (without CP step): 184.9 mAh g‐1
3. Discharge capacity (with CP step): 206.2 mAh g‐1
4. Coulombic Eff.: 83.5 %  93.0 %
200
Specific Capacity / mAh g-1
 Evidence for kinetics effect
 Lithiation (= discharge) of NMC slower than de‐lithiation (= charge) Facilitating Lithiation (III): Lowering the Cut‐off Down to 2.0 V vs Li/Li+
Potential vs Li/Li+ / V
4.5
1st Cycle
WE: NMC; CE, Re: Li
1M LiPF6 in EC/EMC (1/1)
Cut‐off: 4.6–2.0V vs. Li/Li+
Rate: 0.2C, 0.2D 4.0
3.5
Cut‐off potential decrease gains 2.3 mAh g‐1 in the 1st cycle
3.0
2.5
2.0
0
40
80
120
160
Specific Capacity / mAh g-1
200
1. Charge capacity: 222.7 mAh g‐1
2. Discharge [email protected] vs Li/Li+: 185.9 mAh g‐1
3. Discharge [email protected] vs Li/Li+: 188.2 mAh g‐1
4. Coulombic Eff: 83.5 %  84.5 %
 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
Facilitating Lithiation (IV): Constant Potential (CP) Holding Step in Discharge at 2.0 V vs. Li/Li+
Potential vs Li/Li+ / V
4.5
1st Cycle
WE: NMC; CE, Re: Li
1M 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 h
Rate: 0.2C, 0.2D 4.0
3.5
 Cut‐off potential decrease gains 2.3 mAh g‐1
3.0
2.5
 CP step at 2.0 V vs Li/Li+
gains 21.7 mAh g‐1
2.0
0
40
80
120
160
Specific Capacity / mAh g-1
1. Charge capacity: 222.7 mAh g‐1
2. Discharge capacity (without CP step): 188.2 mAh g‐1
3. Discharge capacity (with CP step): 209.9 mAh g‐1
4. Coulombic Eff.: 84.5 %  94.3 %
200
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?
1st Cycle Capacity Losses at Different
Upper Cut‐Off Potentials
4.3 V
vs. Li/Li+
WE: NMC; CE, Re: Li
1M 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 h
Rate: 0.2C, 0.2D 4.4
Stable electrolyte
environment
4.0
40
3.8
3.6
3.9 V
4.0 V
4.3 V
3.4
3.2
Total capacity loss
3.0
0
40
80
120
Specific Capacity / mAh g-1
160
Capacity loss / mAh g-1
Potential vs. Li/Li+ / V
4.2
Total capacity loss
Irr. capacity loss
30
20
10
0
Total capacity loss constant up to 4.3 V vs. Li/Li+
3.9 4.0 4.1 4.2 4.3 4.4 4.5 4.6
After CP step: Irr. capacity loss: constant up to 4.3 V vs. Li/Li+
+
Potential / V vs. Li/Li
Intrinsic Material problem, e.g., Li/Ni Mixing?
 Intrinsic Irreversible Capacity Loss: ca. 5 mAh g‐1
Effect of Ni2+ (Trapped on Li+ Sites) on Capacity Loss
Li+/Ni2+ mixing is well known for Ni based layered oxide cathodes.
M. Hu et al. / Journal of Power Sources 237 (2013) 229-242
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
1Labrini, M. Electrochim
Acta 2013, 111, 567
Mater 2010, 22, 691. 2Ellis, B. L.; Lee, K. T.; Nazar, L. F. Chem
Determination of the Irrev. Charge Loss in the Overall Charge Loss, e.g.: in the 1st Cycle
Charge
Potential vs Li/Li+ / V
3.8
WE: NMC; CE, Re: Li
1M 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 h
Rate: 0.2C, 0.2D Discharge
3.6
3.4
10 mAh g‐1 parasitic reactions, such as electrolyte oxidation
Ca. 5 mAh g‐1 intrinsic material loss
3.2
21.3 mAh g‐1 kinetics effect
36.5 mAh g‐1 initial charge loss
3.0
0
10
20
30
40
50
Specific Capacity / mAh g-1
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 60
4
4
3
3
 CV step at 2.4 V
gains 15.1 mAh g‐1
2
2
Cell voltage
Graphite Potential
NMC Potential
1
0
1
Potential vs. Li/Li+ / V
Voltage / V
Full Cells with NMC/MCMB
WE: NMC; CE: MCMB (30 % oversized); Re: Li
1M LiPF6 in EC/EMC (1/1)
Cut‐off: 4.55–2.0 V Constant Voltage Step
@Discharge cut‐off voltage for 24 h
Rate: 0.2C, 0.2D 1.
2.
3.
4.
Charge capacity: 222.7 mAh g‐1
Discharge capacity @ 2.0 V: 186.3 mAh g‐1
(with CP step): 201.4 mAh g‐1
Coulombic Eff.: 83.7 %  90.4 %
0
0
40
80
120
160
Specific Capacity / mAh g-1
200
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.
Influence of a Higher Cut‐off Potential on Apparent (Kinetic) Capacity Loss of NMC
Specific capacity / mAh g-1
40
30
Specific capacity losses:
Parasitic reactions
Kinetic inhibition
Intrinsic NCM loss
36.5 mAh g-1
10.2 mAh g-1
22.1 mAh g-1
( 4.2 mAh g-1)
17.1 mAh g-1
21.3 mAh g-1
5.0 mAh g-1
5.0 mAh g-1
3.9
4.6
20
Parasitic reactions
lead to larger
kinetic inhibition
(= kinetic capacity loss)
10
0
Potential vs. Li/Li+
Parasitic reactions increase the kinetic inhibition  probably due to the formation of passivation layer caused by electrolyte oxidation
1st Cycle Coulombic Efficiencies (CE) at Different Upper Cut‐Off Potentials
Coulombic Efficiency / %
100
 CE (after CP step): 97%
Intrinsic
NMC loss
90
Add. parasitic
reactions
80
70
4.0
4.1
4.2
 CE (conventional): 87%
WE: NMC; CE, Re: Li
1M 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 h
Rate: 0.2C, 0.2D CE (conventional)
CE (after CP-step)
3.9
+10%
4.3
4.4
4.5
4.6
Potential / V vs. Li/Li+
 Highest possible 1st cyle CE (97%) for NMC at a potential of 4.4 V vs. Li/Li+ Capacity loss / mAh g-1
Irreversible and Total Capacity Losses in the 1st Cycle vs. C‐Rate
60
Total capacity loss
Irr. capacity loss
45
WE: NMC; CE, Re: Li
1M 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 h
Rate: XC, 0.2D 30
15
0
0.01
0.1
1
C-rate
High C‐rate: low irr./total capacity loss
Low C‐rate: high irr./total capacity loss
Why?
Potential vs. Li/Li+ / V
C‐Rate Decrease  Longer Operation Time in Unstable Potential Environment
4.6
0.3 h 0.6 h
1.7 h
3.5 h
Unstable electrolyte
environment
4.4
4.2
4.0
1.0 C
0.5 C
0.2 C
0.1 C
3.8
3.6
0
4
8
12
Time / h
High C‐rate: Shorter exposition in HV region
WE: NMC; CE, Re: Li
1M LiPF6 in EC/EMC (1/1)
Cut‐off: 4.6 V vs. Li/Li+
Rate: ?C
16
Low C‐rate: Longer exposition in HV region
More parasitic reactions in HV region
Discharge capacity / mAh g-1
Discharge Capacity in the 1st Cycle vs. the C‐Rate (= Rate during Charge)
195
WE: NMC; CE, Re: Li
1M LiPF6 in EC/EMC (1/1)
Cut‐off: 4.6–3.0 V vs. Li/Li+
Rate: ?C, 0.2D 180
165
150
0.01
0.1
1
C-rate
High C‐rate: Low parasitic reactions and low additional kinetic hindrance
vs. 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
Minimize Effects of Impedance on Capacity loss: CP Step During Charge AND Discharge
CP step at 4.6V vs. Li/Li+ gains extra Potential vs. Li/Li+ / V
4.5
4.0
13.3 mAh g‐1
+6.8 mAh g‐1
‐6.5 mAh g‐1
(Rev. Li extraction)
(EL oxidation)
3.5
3.0
0
WE: NMC; CE, Re: Li
1M 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 h
3.0 V vs. Li/Li+ for 24 h
Rate: 0.2 C, 0.2 D 1. Discharge capacity (No charge CP step): 206.2 mAh g‐1
+6.8 2. Discharge capacity ‐1
mAh g
(Charge CP step): 213 mAh g‐1
CP step at 3.0V vs. Li/Li+ gains extra 3. Irreversible capacity 24.8 mAh g‐1
(No charge step): 15.2 mAh g‐1
‐6.5 4. Irreversible capacity 80
120
160
200
240
mAh g‐1
‐1
(Charge CP step): 21.7 mAh g
-1
40
Specific Capacity / mAh g
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+)
Additional Li extraction  “Extra” capacity  May lead to phase changes in NMC*
*S. C. Yin, Y. H. Rho, I. Swainson, L. F. Nazar, Chemistry of Materials 2006, 18, 1901.
 Future Research
Comparison of Specific Energies With and Without CP Step at Various Cut‐Off Potentials
and Temperatures WE: NMC; CE, Re: Li
1M LiPF6 in EC/EMC (1/1)
Cut‐off: ?V vs. Li/Li+
Constant Potential Step:
3.0 V vs. Li/Li+ for 24 h
Rate: 0.2C, 0.2D 1000
800
600
Operation temperatures: 25 °C and 60°C
400
200
0
48% 48%54%
+
Li+
Li+ Li
4.2
4.2 (CP)
68%
54%
+ +
LiLi
4.6
75%
68%
+ +
LiLi
80%75%
Li+ Li+
4.6 (CP) 4.6(CP;60°C)
Upper cut-off potential / V vs. Li/Li+
Ca. 18% energy loss due to kinetic effects
For Comparison:
Capacity Loss in LiCoO2, LCO (at 4.3 V)
4.4
WE: LFP; CE, Re: Li
1M 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 h
Sp. current: 30 mA g‐1
Potential vs. Li/Li+ / V
4.2
4.0
1. Charge capacity: 161.4 mAh g‐1
2. Discharge capacity (without CP step): 154.3 mAh g‐1
3. Discharge capacity (with CP step): 159.1 mAh g‐1
4. Coulombic Eff.: 95.6 %  98.6 %
3.8
3.6
3.4
CP step at 3.0V vs. Li/Li+ gains extra 4.8 mAh g‐1 in the 1st cycle
3.2
 2.3 mAh g‐1 irr. cap. loss!
3.0
0
40
80
120
Specific Capacity / mAh g-1
160
161.4 mAh g‐1 of possible 276 mAh g‐1 were extracted (58%) For Comparison:
Capacity Loss in LiNi0.5Mn1.5O2, LNMO
WE: LNMO; CE, Re: Li
1M 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 h
Rate: 0.2C, 0.2D Potential vs. Li/Li+ / V
5.0
4.5
1. Charge capacity: 143.9 mAh g‐1
2. Discharge capacity (without CP step): 138.3 mAh g‐1
3. Discharge capacity (with CP step): 140.1 mAh g‐1
4. Coulombic Eff.: 96.1 %  97.4 %
4.0
3.5
CP step at 3.0V vs. Li/Li+ gains extra 1.8 mAh g‐1 in the 1st cycle
3.0
 3.8 mAh g‐1 irr. cap. loss!
0
40
80
120
Specific Capacity / mAh g-1
160
Comparison to NMC:
Despite higher charging potentials (!) lower irr. capacity loss (3.8 vs. 15.2 mAh g‐1 )
For Comparison:
Capacity Loss in LiFePO4, LFPO
Potential vs. Li/Li+ / V
4.2
WE: LFP; CE, Re: Li
1M 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 h
Spec. current: 30 mA g‐1
4.0
3.8
3.6
3.4
CP step at 3.0V vs. Li/Li+ gains extra 5.9 mAh g‐1 in the 1st cycle
3.2
1. Charge capacity: 163.1 mAh g‐1
2. Discharge capacity (without CP step): 155.0 mAh g‐1
3. Discharge capacity (with CP step): 160.9 mAh g‐1
4. Coulombic Eff.: 95.0 %  98.7 %
 2.2 mAh g‐1 irr. cap. loss!
3.0
0
40
80
120
Specific Capacity / mAh g-1
160
Capacity Loss: NMC is an Exception
Capacity Loss / mAh g‐1
25
3.9‐4.3 V
20
15
10
4.1 V
4.3 V
5
0
LFP
1D
LCO
Kinetic cap. loss
Irr. cap. loss
4.95 V
LNMO
NMC
2D
3D
2D?
Li Transport in Cathode Structure
Conclusions
• “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 “God made the bulk; surfaces were invented by the devil.”
‐‐‐Wolfgang Pauli (Austrian‐Swiss Physicist, 1900‐1958)
Back-Up
NMC at 4.6V vs. Li/Li+: Capacity Fade vs. Power Fade During Cycling
Charge
Discharge
220
Specific capacity / mAh g-1
WE: NMC; CE, Re: Li
1M 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, 1.0D
Cycle 54: 0.2C, 0.2D
(with Constant Potential Step
at 3.0V vs. Li/Li+ down to <0.2 C) 200
180
160
Only 83.5 % of initial capacity could be achieved in the 53rd cycle
140
120
100
0
0.2 C
0.2 D
1.0 C
1.0 D
0.2 C
0.2 D
10
20
30
Cycle No.
40
50
60
 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!
Facilitating Lithiation (IV): Increasing the Temperature
Potential vs. Li/Li+ / V
4.8
WE: NMC; CE, Re: Li
1M LiPF6 in EC/EMC (1/1)
Cut‐off: 4.6–3.0 V vs. Li/Li+
Rate: 0.2 C, 0.2 D 4.4
4.0
Temperature increase in 1st cycle
1. Lowers Polarizations
2. Increase charge capacity
3. Increase discharge capacity
4. Lowers total and irr. capacity loss
5. Increase Coulombic efficiency
3.6
20°C
40°C
60°C
3.2
2.8
0
50
100
150
200
-1
Specific Capacity / mAh g
250
Electrochemical performance strongly depends on temperature
NMC Investigations | Johannes Kasnatscheew |
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