International Battery Seminar | Fort Lauderdale, FL | Mar 22, 2017

Feng LinDepartment of Chemistry

Virginia Tech

Controlling the Surface Chemistry of Cathode Materials for Manufacturing High-Energy Rechargeable Batteries

CHARGING UP THE WORLD

2

Lemon Battery

Approximately 6, 000, 000 lemons to give a power of a car batteryApproximately 5, 000, 000, 000 lemons to power a Tesla Model S at its acceleration

Zn anode Cu anode

_ +Separator

𝐸𝐸𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 = 𝐸𝐸°[0.337 𝑉𝑉] + 𝑅𝑅𝑅𝑅𝑧𝑧𝑧𝑧

ln 𝑐𝑐𝑜𝑜𝑜𝑜𝑐𝑐𝑟𝑟𝑟𝑟𝑟𝑟

𝐶𝐶𝐶𝐶2+(𝑎𝑎𝑎𝑎) + 2𝑒𝑒− → 𝐶𝐶𝐶𝐶 (𝑠𝑠)

𝑍𝑍𝑍𝑍 (𝑠𝑠) → 𝑍𝑍𝑍𝑍2+(𝑎𝑎𝑎𝑎) + 2𝑒𝑒−

𝐸𝐸𝑐𝑐𝑎𝑎𝑐𝑐𝑐𝑐𝑐𝑐 = 𝐸𝐸°[−0.762 𝑉𝑉] + 𝑅𝑅𝑅𝑅𝑧𝑧𝑧𝑧

ln 𝑐𝑐𝑜𝑜𝑜𝑜𝑐𝑐𝑟𝑟𝑟𝑟𝑟𝑟

𝐸𝐸𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 = 𝐸𝐸𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 − 𝐸𝐸𝑐𝑐𝑎𝑎𝑐𝑐𝑐𝑐𝑐𝑐 ≈ 0.7 𝑉𝑉

iPhone 7 plus11.1 Wh

2015 Mac Air (13 inch)54 Wh

Tesla Modal S 60-100 kWh

Energy storage plantMWh level1 mile lightning flash

300,000 Bacon Cheeseburger Deluxe~ 10 days of lunch to feed VT campus

Get to Know the Scale of Battery Energy

3

Battery Science and Advanced Characterization at Multiple Length Scales

Lin et al Chem Rev (under revision)

Materials Synthesis

Advanced Characterization

Electrode Formulation

Electrochemistry

Our research activity at Virginia Tech 4

Challengeso Poor first-cycle Coulombic

efficiencyo Fast capacity fadingo Inferior rate capabilityo Potentially release oxygen and

induce thermal runaway

High Voltage Failure of NMC Materials

ca. 87% Li deintercalation

Upper cutoff: 4.7 V vs Li+/Li, C rate: C/20

1st cycle20th cycle

Nat. Commun. 5, 3529 (2014)

Upper cutoff: 4.7 V vs Li+/Li, C rate: C/50

After 20 cycles and significant capacity fading, 20% discharge capacity is recovered under extremely slow cycling rate

5

ca. 64% Li deintercalation

Upper cutoff 4.3 V vs Li+/Li

Pristine4.3 V for 20 cycles4.7 V for20 cycles

Less capacity fading at 4.3 V cutoff

Build-up of reduced Mn increased by elevating voltage

Tradeoff In the real world, battery is operated to guarantee that the voltage, temperature, etc do not exceed the safety limit. However, by doing so the energy density is compromised

High Voltage Failure of NMC Materials

6

Common Rocking-Chair Battery Configuration

−eVoc = μLi(C) − μLi(A) = Δμe + ΔμLi+

Open circuit voltage

Chemical potential of cathode as a function of Li content

Chemical potential of anode as a function of Li content

Potential change of charge carriers

7

Schematic Open-Circuit Energy Diagram of a Cell

Anode

Energy

SEI

SEI

LUMO

HOMO

Eg

CathodeμLi(C)

μLi(A)

Voc

Φ(C)

Φ(A)

Oxidant

Electrolyte

Reductant

Cathode work function

Anode work function

SEI: solid-electrolyte interphaseLUMO: lowest unoccupied molecular orbitalHOMO: highest occupied molecular orbital

Electrode-electrolyte interfacial problem is a “1 + 1 > 2” challenge !

Challenges:Electrode materialsElectrolyte

8

T.M.LiO

3a

3b/6c

(R-3m) α-NaFeO2 type

LiNi1-x-yMnxCoyO2 Material: Crystal and Electronic Structures

o Reduced cost?o Enhanced safety?o Higher energy density?

Sci. Rep. 4, 5694, (2014) 9

What happens at the cathode surfaces?

i ii iii

Reaction layer100 nm 50 nm 50 nm

Surface reaction layer (cathode-electrolyte interphase, CEI)

Nat. Commun. 5, 3529 (2014) 10

NMC

Electrolyte exposure

Fully charged

Pristine NMC

What happens at the cathode surfaces?

Surface reaction layer (cathode-electrolyte interphase, CEI)

O K-edge

F K-edge

Mn L-edge Co L-edge

Ni L-edge

SRL

NMC

10 nm

Energy & Environmental Science 7, 3077 (2014) 11

Challenges in Oxide Cathode Materials: Surface Reactivity and Phase Change

Xu, Fell, Chi & Meng. Energy Environ. Sci. 4, 2223 (2011)

Li[NixLi1/3−2x/3Mn2/3−x/3]O2

5 nm

Lin, Markus, Nordlund, Weng, Asta, Xin & Doeff. Nat. Commun. 5, 3529 (2014)Lin, Nordlund, Markus, Weng, Xin & Doeff. Energy Environ. Sci. 7, 3077 (2014)Lin, Nordlund, Pan, Markus, Weng, Xin & Doeff. J. Mater. Chem. A 2, 19833 (2014)Markus, Lin, Kam, Asta & Doeff. J Phys. Chem. Lett. 5, 3649 (2014)Lin, Markus, Doeff & Xin. Sci. Rep. 4, 5694, (2014)

Li[NixMnyCo1-x-y]O2

Hwang, Chang, Kim, Su, Kim, Lee, Chung & Stach. Chem. Mater. 26, 1084 (2014)

Li[Ni0.5Mn1.5]O4

unpublished result

x in Li1-xTMO2

ΔH

for c

lose

-pac

ked

phas

eΔH = 0

Layered phase

closed packed phase

12

dz2 dx2-y2

Tokura & Nagaosa. Science 288, 462 (2000)

dzx dyz dxy

eg orbitals

t2g orbitals

Crystal field theory d orbitals

13

LiNi1-x-yMnxCoyO2 Material: Energy vs Density of States upon Charging

N (E)

O2- : 2p6

Co3+/4+ : 3d-t2g

Ni3+/4+ : 3d-eg

Ni: 3d-t2g

EF1

Ene

rgy

Ni2+/3+ : 3d-eg

Mn3+/4+ : 3d-eg

Co: 3d-eg

Mn: 3d-t2g

Discharged stateLi1Ni1-x-yMnxCoyO2

N (E)

O2- : 2p6

Ni3+/4+: 3d-eg

Ni: 3d-t2g

Ni2+/3+ : 3d-eg

Mn3+/4+ : 3d-eg

Co: 3d-eg

Mn: 3d-t2g

Co3+/4+ : 3d-t2g

Ene

rgy

EF3

Discharged stateLi0Ni1-x-yMnxCoyO2

During the charging process (cathode under oxidative condition), lithium is deintercalated fromlattice, and charge compensation occurs at transition metal (TM) 3d orbitals, namely, 3d electronoccupancy decreases and Fermi level is lowered. Oxygen participation is expected because of theTM3d-O2p hybridization in TM-O6 octahedral unit.

N (E)

O2- : 2p6

Co3+/4+ : 3d-t2g

Ni3+/4+ : 3d-eg

Ni: 3d-t2g

Ni2+/3+ : 3d-eg

Mn3+/4+ : 3d-eg

Co: 3d-eg

Mn: 3d-t2g

Ene

rgy

EF2

Discharged stateLi0.5Ni1-x-yMnxCoyO2

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Lin, Nordlund, Weng, Moore, Gillaspie, Dillon, Richards & Engtrakul. ACS Appl. Mater. Interfaces 5, 301 (2013)Ratcliff, Meyer, Steirer, Garcia, Berry, Ginley, Olson, Kahn & Armstrong. Chem. Mater. 23, 4988 (2011)Lin, Nordlund, Weng, Sokaras, Jones, Reed, Gillaspie, Weir, Moore, Dillon, Richards & Engtrakul. ACS Appl. Mater. Interfaces 5, 3643 (2013)Lin, Nordlund, Weng, Zhu, Ban, Richards & Xin. Nat. Commun. 5, 3358 (2014)

Soft X-ray Absorption Spectroscopy (Sensitivity to Surface and Orbital Occupancy)

Synchrotron X-ray

SnO2:F (TCO)

NiOx

Glass

O3

O3

O3

O3

UV Ozone Processing

O3O3

Ni2+ Ni3+o Peak ratio (e.g., L3/L2, a/b)o Properly calibrated absolute peak energy

L3

Ni L3,2 XAS (Ni2p to 3d unoccupied state)

L2

Ni0 [Ar]4s23d8

Ni2+ [Ar]3d8

Ni3+ [Ar]3d7

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Changes of Ni3d Occupancy during Charging and Discharging

dz2 dx2-y2

Tokura & Nagaosa. Science 288, 462 (2000)

dzx dyz dxy

eg orbitals

t2g orbitals

According to the crystal-field theory, the unpaired eg electrons make the high valency nickelions (e.g., Ni3+/Ni4+) thermodynamically unstable in the octahedral unit (in particular when incontact with electrolytic solution)

Crystal field theory

Li1Ni0.4Mn0.4Co0.2O2

Li0.6Ni0.4Mn0.4Co0.2O2

Li0.2Ni0.4Mn0.4Co0.2O2 (ca. 220 mAh/g)

Li0.7Ni0.4Mn0.4Co0.2O2

Li0.9Ni0.4Mn0.4Co0.2O2

t2g eg

Energy Environ. Sci. 7, 3077 (2014)

Ni L-edge

16

2 nm

Li+ pathway

R[100]

(00-3)(01-2)

(011)

R3m_

F[110]

(1-11)(1-1-1)

(002)

Fm3m_

Surface Phase Transition after Cycling

Nat. Commun. 5, 3529 (2014)Phys. Chem. Chem. Phys. 17, 21778 (2015)

“Rock-salt”“Layered”

1 cycle

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“Layered”

Ni2+, Mn4+, Co3+

“Rock-salt”

Ni2+, Mn2+, Co2+

Stable R-3m“Layered”

Stable Fm-3m“Rock-salt”

Thermodynamic DFT Calculation: Undesired Phase Transition

2Lix(Ni1/3Mn1/3Co1/3)O2 2(Ni1/3Mn1/3Co1/3)O + O2 + 2xLi+ + 2e−

“Layered” “Rock-salt”

ca. 188 mAh/g

Nat. Commun. 5, 3529 (2014)J Phys. Chem. Lett. 5, 3649 (2014) 18

Nat. Commun. 5, 3529 (2014)Nat. Commun. 5, 3358 (2014)

Surface Phase Transition Grows as Cycle Number Increases

I = I0 exp (-Ax)

Mn2+ rich

Mn4+ rich

Soft XAS (surface sensitive, ~10 nm)

o Pristine electrode showed exclusively Mn4+ (“layered”)o Mn2+ dominated surface layer (“rock-salt”) continued to

grow as cycle number increasedo Mn2+ can be dissolved in electrolyte, migrate to anode and

interrupt SEI

Cyc

le n

umbe

r

19

2 nm

Lin, Markus, Asta & Doeff et al. unpublished result

Surface Oxygen Activity: Chemical Side Reaction

Surface oxygen is more reactive

Energy Environ. Sci. 7, 3077 (2014)Nat. Commun. 5, 3529 (2014)

The higher state of charge, the higher possibility of side reactions, due to the activation of oxygen 2p orbitals

20

J Phys. Chem. Lett. 5, 3649 (2014)Nat. Commun. 5, 3529 (2014)

Stable R3m“layered”

Stable Fm3m“Rock-salt”

Pristine NMC

“Substituted” NMC

LiNixMnyCo1-x-y-zTizO2

_

_

Partial Aliovalent Substitution to Expand Thermodynamic Window

21

Li

OCoMn

Mn

Polaron state

J Phys. Chem. Lett. 5, 3649 (2014)Nat. Commun. 5, 3529 (2014)

Partial Aliovalent Substitution to Improve Electronic Conductivity

22

Electrolyte Development for Li/Mn rich NMC Materials

Tris(2,2,2-trifluoroethyl) borate (TTFEB)

Ma, Y et al. Chem. Mater. 29, 2141–2149 (2017)

Li1.16Ni0.2Co0.1Mn0.54O2

23

XPS Surface Analysis of Electrolyte Decomposition Layer

24

Less surface electrolyte decomposition layer on the cathode with the TTFEB additive

Ma, Y et al. Chem. Mater. 29, 2141–2149 (2017)

XAS Surface Analysis of Transition Metal Reduction Layer

25

Less surface TM reduction layer on the cathode with the TTFEB additiveSurface of cathode becomes more stable with the TTFEB additive

Ma, Y et al. Chem. Mater. 29, 2141–2149 (2017)

XAS Surface Analysis of Al2O3 Surface Protection for LNMO Spinel

Xin, F et al. Adv. Funct. Mater. 27, 1602873 (2017) 26

More stable surface with Al2O3 surface protection

Conclusion

Pristine

Build up

Roc

k-Sa

lt Fo

rmat

ion

Anode

SEI

SEI

LUMO

HOMO

Eg

CathodeμLi(C)

μLi(A)

Voc

Φ(C)

Φ(A)

Oxidant

Electrolyte

Reductant

• Electrochemical interface and interphase in batteries• Cathode surface chemistry and failure mechanism• Methods of stabilizing surfaces: (1) Surface engineering

(2) aliovalent substitution(3) optimization of electrolyte 27

Acknowledgement

Dr. T.-C. Weng, Dr. D. Nordlund, Dr. D. Sokaras, Dr. Y. Liu, Dr. H. Xin

Office of Vehicle Technologies

Dr. Marca Doeff

28

Xin FangYulin MaChongwu ZhouIsaac MarkusMark Asta

Thank You

Feng LinDepartments of Chemistry and MSEVirginia Tech900 West Campus DriveBlacksburg, VA 24061Office: Hahn Hall North 306-XYZPhone: (540) 231-4067E-mail: fenglin@vt.edu