Computation Accelerated Design of Materials and Interfaces for All-Solid-State Li-ion Batteries

Yifei Mo

Department of Materials Science and EngineeringMaryland Energy Innovation Institute

University of Maryland, College Park, MD

Funding support: DOE, VTO, BMR programDOE-EERE DE-EE0006860, DE-EE0007807

Opportunities and Challenges: All-Solid-State Li-ion Batteries

2

Challenges : • Li solid electrolyte with high ionic conductivity, good stability, etc. • Interfaces between electrolyte and electrodes.

Interfacial resistance

Interfacial resistance

Opportunities and potentials: • Improved safety : non-flammable ceramic electrolyte• High energy density : Li metal anode and/or high-voltage cathode• High power, long cycle life, wide temperature range …

Fast Li+ transport in solid electrolyte

Our goal: Use first principles computation to achieve: • fundamental understanding• accelerated design of materials and interfaces.

What makes super-ionic conductor – the key enabler ?

3Shao-horn et al. Chem. Rev. (2016)

Li9.54Si1.74P1.44S11.7Cl0.3

Crucial to understand universal features among super-ionic conductors and to rationally design new conductors

Ion Diffusion in Solids – Classical Model

Nernst-Einstein equation:

Energy landscape

Atomistic diffusion in solid is mediated by vacancy or interstitial as carrier hopping among lattice sites.

! = #$%& '()*

+(-./0)Ea : Activation energy

n : mobile carrier concentration

Ea

Ionic conductivity

To achieve high ionic conductivity, needslow activation energy Ea + high carrier concentration n

Concerted migration mechanism dominates in super-ionic conductors

5

Ab initio molecular dynamics (AIMD) simulations to observe real-time ion migration mechanism. Multiple Li ions hop simultaneously in a concerted migration mechanismStrong concerted migration is dominant in LGPS, garnet LLZO, NASICON LATP, as a general phenomena of super-ionic conductors.

LGPSLLZO LATP

How concerted migration happens?

0.20 eV 0.26 eV 0.27 eV

0.47 eV 0.58 eV0.49 eV

LLZO LATPLGPS

Barrier of concerted migration

Energy landscapeof single Li+ migration

Contradiction: How multiple ions migrating together can lead to a lower barrier ?

Why concerted migration has lower barrier?

0.58 eV

Position along migration path

Energy landscape

of single Li+migration

(eV)

Position along migration path (Å)

0.0

0.3

0.6Energy

landscapeof single Li+migration

(eV)

High-energy sites occupancyLi-Li Coulomb interactionTet TetOct

e.g. In LLZO

Position along migration path (Å)

0.0

0.3

0.6

0.3 eV

Energy landscape 0.6 eV

Concerted migration barrier 0.3 eV

During concerted migration, the down-hill migration of high-energy ions cancels out a part of hill-climbing migration barrier.

Migration path

Energy barrier

Energy landscape

Single ion migration in typical solids

Low-barrier concerted migration in super-ionic conductors

X. He, Y. Zhu and Y. Mo, Nat. Commun., 2017, 8, 15893.

8

How to design super-ionic conductor ?

• Strong coulomb interaction• Occupied high-energy site• Flat landscape at high-energy site

Design strategy for super-ionic conductor : Tailor mobile ion configuration to activate low-barrier concerted migration.

Mechanistic origin: High-energy site Li+ migrate downhill, canceling out a part of the energy barrier felt by other uphill-climbing ions.

Design and discover new super-ionic conductor: Li1+xTa1-xZrxSiO5

AB

C

A A

Position along migration path

Energy landscape

(eV)

X. He, Y. Zhu and Y. Mo, Nat. Commun., 2017, 8, 15893.

LiTaSiO5Not been studied for Li diffusion. B

CA

Li1.25Ta0.75Zr0.25SiO5

9

TaO6

SiO4

Li

Ea = 0.73 eV! (300 K) = 2.8 ´ 10-7 mS/cm

Ea = 0.23 eV" (300 K) = 4.3 mS/cm

Zr4+ à Ta5+ Demonstrated our design strategy in discover and design new super-ionic conductors

Interfaces in All-Solid-State Li-ion Batteries

10

Interfacial resistance

Interfacial resistance

Fast Li+ transport in solid electrolyte

Significant amount of solid-solid interfaces in solid-state batteries: • Formation of SEI ? • Interface compatibility & stability.

(coulombic efficiency, cycle life)• Interfacial ionic transport. (Rate

performance)

• What are the fundamental limitations at the interfaces in all-solid-state batteries? • What are the general design principles for interfaces with good battery performance?

Thermodynamics indicate that the interface may degrade and an interphase layer may form due to: 1. The reduction / oxidation of the solid electrolyte materials at applied voltage. 2. Chemical reaction between solid electrolyte and electrodes. 3. Electrochemical reaction (during cycling voltage) between solid electrolyte and electrodes.

Zhu, He, Mo, J. Mater. Chem. A, 2016,4, 3253-3266

LiFLiCl

LiI

LGPS

LiPONLLZOLLTOLATPLAGP

LISICON

0 1 2 3 4 5 6

0 -1 -2 -3 -4 -5 -6Li Chemical potential(eV)

Potential Ref. to Li/Li+ (V)

Li3N

Li2O

Li3PLi2S

Li6PS5ClLi7P2S8I

Li3PS4

Phase equilibria with Li metal LGPS Li15Ge4, Li3P, Li2S Li3PS4 Li3P, Li2S

Li6PS5Cl Li3P, Li2S, LiCl Li7P2S8I Li3P, Li2S, LiI LiPON Li3P, Li3N, Li2O LLZO Zr (or Zr3O), La2O3, Li2O LLTO Ti6O, La2O3, Li2O LATP Ti3P, TiAl, Li3P, Li2O LAGP Li9Al4, Li15Ge4, Li3P, Li2O

LISICON Li15Ge4, LiZn, Li2O

(Thermodynamic Intrinsic) Electrochemical window of solid electrolytes

Electrochemical window

Solid electrolyteLi metal

Li

Zhu, He, Mo, ACS Appl. Mater. Interfaces, 2015, 7 (42), 23685–23693

Thermodynamic equilibrium potential is

Li-LGPS : Interphase layer formation and IncompatibilityLi10GeP2S12

(LGPS)

Li metal Red

uctio

n

Lithiation

Li10GeP2S12 + 23.75 Li è12 Li2S + 2 Li3P + 0.25 Li15Ge4(ΔH = -31.3 eV / -3019 kJ/mol)

Wenzel, Randau, Leichtweiß, Weber, Sann, Zeier, Janek, Chemistry of Materials (2016)

First principles calculations

Mixed Ionic & electronic conducting (MIEC) interphase layer formation:

- Thick interphase layers- High interfacial resistance. - Incompatible !

Phase equilibria with Li metal LGPS Li15Ge4, Li3P, Li2S Li3PS4 Li3P, Li2S

Li6PS5Cl Li3P, Li2S, LiCl Li7P2S8I Li3P, Li2S, LiI LiPON Li3P, Li3N, Li2O LLZO Zr (or Zr3O), La2O3, Li2O LLTO Ti6O, La2O3, Li2O LATP Ti3P, TiAl, Li3P, Li2O LAGP Li9Al4, Li15Ge4, Li3P, Li2O

LISICON Li15Ge4, LiZn, Li2O

Oxides have better stability than sulfides ?

Electrochemical window

Solid electrolyteLi metal

Li

Zhu, He, Mo, ACS Appl. Mater. Interfaces, 2015, 7 (42), 23685–23693

Thermodynamic equilibrium potential is LiF

LiClLiI

LGPS

LiPONLLZOLLTOLATPLAGP

LISICON

0 1 2 3 4 5 6

0 -1 -2 -3 -4 -5 -6Li Chemical potential(eV)

Potential Ref. to Li/Li+ (V)

Li3N

Li2O

Li3PLi2S

Li6PS5ClLi7P2S8I

Li3PS4

Formation of SEI enables compatible solid-solid interface

Li3N

Li2Ox

Li3P

LiPO

N

LiLi

3N,L

i 3P, L

i 2O

Schwobel et al. Solid State Ionic (2016)

Li2O Li3N Li3P

Lith

iatio

n

pristine pristine

In-situ XPS also observed Li reduction of LiPON

² LiPON is well demonstrated for its Li metal compatibility in thin-film batteries. Why?

² Form SEI-like layer, Li3P, Li3N, Li2O, ion conducting but electronic insulatingpassivates the solid electrolytes.

Thermodynamics also shows Li reduction is energetically favorable

Formation of MIEC interphase layers in sulfide SE.

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LiCoO2 LiCoO2Sulfide electrolyte,

e.g.LGPS

Sulfide electrolyte,

e.g.LGPS

Origin of poor interfacial performance: • Poor electrochemical/chemical stability • Formation of MIEC interphase--> high resistive interphase layers

Zhu, He, Mo, J. Mater. Chem. A, 2016,4, 3253-3266

Incompatible interface between sulfide SE-LiCoO2

Interphase Layer

Co9S8, etc.

Oxi

de c

oatin

g

Solution: Converting to SEI interface by coating

Oxide coating layer, (e.g. LiNbO3, Li3PO4, etc. )

serves as artificial SEI

Li10GeP2S12 + LiCoO2 è Co9S8 + Li2S + Li2SO4 + Li3PO4+ Li4GeO4 (ΔH = -0.35 eV/atom)

Li10GeP2S12 + Li0.5CoO2 è Co9S8 + Li2S + Li2SO4 + Li3PO4+ Li4GeO4 (ΔH = -0.53 eV/atom)

Interface reactions for LiPON – Cathode interfaceY. S. Meng et al. Nano Letters (2016)

LiCoO2LiPON

Zhu, He, Mo, J. Mater. Chem. A, 2016,4, 3253-3266

Electron energy loss (eV)

For LiPON-LiCoO2 interface

Electronic insulating but ion conducting interphase (like SEI) formed to stabilize the interface -> Self-Limiting decomposition-> Form SEI-like passivation-> Decent interfacial Li+ transport.

Li3PO4, Li2O, Li2O2, CoNx, CoOx, LixCoOy

LiPON+ LiCoO2 è Li3PO4 + Li2O + CoN (ΔH = -0.1 eV/atom)

At applied voltage of 4.2V to 5.0V, ΔH = -0.17 to -0.48 eV/atom

Resolving interface compatibility in all-solid-state batteryCurrent success cases:

Strategies for resolving interface issues: • Optimize and design SE to form stable SEI (e.g. LiPON) – good

interfacial compatibility. • Applying coating layer as artificial SEI (e.g. LiNbO3 coating on

sulfide SE)• Novel interfacial engineering to spontaneously form stable SEI.

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Computation can rapidly narrow down chemistry space.

LiPO

N

Li

LCO

/ Hi

gh-V

olta

geCa

thod

e

LPS

Sulfid

e

Car

bon

anod

e

LCO

Cath

ode

Spontaneously formed stable SEI layer

formed SEI layer

Artificial coating layer

Can we have materials stable against Li metal ? Electrochemical stability window of example Li-M-X compounds.

General trend of cathode limit

Cations lead to Li reduction and MIEC interphase (non-

compatible! )

Nitride < Oxide ~ Sulfide < Fluoride

M = Al, Zr, Si ,Ge, PX = N, S, O, F

Nitrides like Li3AlN2, Li2ZrN2 are Li metal stable and electronic insulating. 18

NitrideSulfide Oxide

Fluoride

Zhu, He, Mo, Advanced Science (2017) 1600517

Nitrides have unique thermodynamic stability against Li metal

Zhu, He, Mo, Advanced Science (2017) 1600517

OxideSulfide

Nitride

FluorideLi + LixMFy -> LizM + LiFLi + LixMOy -> LizM + Li2O

Strategies using Nitride to stabilize Li metal

Form Li-stable SEI at interface• High nitrogen doping at interfaces • Li3N to react with SE to from stable SEI. • N-rich salt doping

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Coat Li-stable artificial SEI• Li-stable nitride.

Solid

El

ectro

lyte

Li

Spontaneously formed stable SEI layer

Solid

El

ectro

lyte

Li

Li+

Apply artificial SEI layer as coating

Li-stable nitrideN rich

Zhu, He, Mo, Advanced Science (2017) 1600517

Conclusions

• Developed first principles computation techniques based on materials database to 1) design novel solid electrolytes and to 2) evaluate the thermodynamic equilibria of solid interfaces.

• The computation framework can be transferable to any materials and interfaces.

First principles Computation Methods

• Unique insights for super-ionic conductors obtained through atomistic modeling. • First principles computation is demonstrated to discover and design new Li ion conductor

materials.

Materials Design and Discovery

• The interphase layers play a crucial role in the performance of solid-state batteries, and are likely an origin of high interfacial resistance.

• Interface engineering is the key to achieve good performance: 1) develop compatible electrolyte and electrode; 2) apply coating layer and novel interfacial engineering.

Implications for all-solid-state battery

He, Zhu, Mo, Nature Communications 2017, 8, 15893Zhu, He, Mo, ACS Appl. Mater. Inter. 2015, 7 (42), 23685 Zhu, He, Mo, J. Mater. Chem. A 2016,4, 3253-3266Han, Zhu, He, Mo, Wang, Adv. Energy Mater. 2016, 1501590. Zhu, He, Mo, Advanced Science 2017, 1600517

AcknowledgementFunding support:

BMR program, VTO, EEREDOE-EERE DE-EE0006860, DE-EE0007807

Collaborations at University of Maryland• Prof. Chunsheng Wang• Prof. Liangbing Hu • Prof. Eric Wachsman

Materials Project

Computational resources: • XSEDE: NSF TG-DMR130142, TG-DMR150038 • University of Maryland supercomputers • Maryland Advanced Research Computing Center (MARCC) 22