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
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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