Computation-Guided Understanding and Design of Interfaces in All-Solid-State Li-ion Batteries. Yifei Mo

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1 Computation-Guided Understanding and Design of Interfaces in All-Solid-State Li-ion Batteries Yifei Mo Assistant Professor Department of Materials Science and Engineering University of Maryland Energy Research Center University of Maryland, College Park, MD

2 Interfaces play crucial roles in batteries Solid-State battery The SEI does the magic in enabling commercial Li-ion batteries Janek, Zeier, Nature Energy (2016) Critical roles of interfaces in battery performance: Formation of SEI? Interface stability. (coulombic efficiency, cycle life) 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? 2

3 Interfaces in batteries are yet to understood In all-solid-state batteries, interfaces form between two chemically highly distinctive materials: e.g., Li LiPON, Li 3 PS 4 LiCoO 2, or LLZO NMC, etc. These interfaces are under repeated cycling of a wide voltage window. Solid electrolyte: Li ionic conductor Anode Coating layer Cathode Li+ transport How does interface degrade over time or during cycling? Does SEI layer form? How to select coating/buffer/barrier materials for improving interfacial properties and battery performance? 3

4 What happens at the electrolyte-electrode interface? - Thinking from first principles : Thermodynamics can tell a lot Anode Li ionic conductor Solid electrolyte Cathode?? 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. First principles computation to investigate such thermodynamics of interfaces in solid-state batteries. Zhu, He, Mo, J. Mater. Chem. A, 2016,4,

5 What thermodynamics tell about potential degradation of the solid electrolyte during cycling voltage 1. The reduction / oxidation of the solid electrolyte materials at applied voltage? V Anode Li source High μ Li Reduction Solid Electrolytes (LGPS) Oxidation Cathode Li sink Low μ Li Lithiation Delithiation Does the solid electrolyte get reduced at the anode side (low potential)? Does the solid electrolyte get oxidized at the cathode side (high potential)? Electrochemical Stability can be evaluated as phase stability with a Li reservoir à Li grand potential phase diagram

6 Evaluating Electrochemical Stability with First Principles Calculation Case Study: Li 10 GeP 2 S 12 (LGPS) super-ionic conductor Anode Li source Reduction Solid Electrolytes (LGPS) Oxidation Cathode Li sink Lithiation Delithiation At 0V At 5V Li 10 GeP 2 S Li è12 Li 2 S + 2 Li 3 P Li 15 Ge 4 (ΔH = ev / kj/mol) Li 10 GeP 2 S 12 è 5 S + P 2 S 5 + GeS Li + 10e*(-5 V) (ΔH = ev / kj/mol) Strong thermodynamic driving force for the reduction at 0 V and oxidation at 5 V. Y. Mo, S. P. Ong, G. Ceder, Chem. Mater. 2012,

7 LGPS decomposition as a function of voltage Thermodynamics suggests a limited electrochemical window of LGPS. S oxidation (Courtesy of CS UMD) Li-P alloying Li-Ge alloying Can we observe the lithiation/delithiation of LGPS? Han, Zhu, He, Mo, Wang, Adv. Energy Mater. (2016) Experiments agree with first principles calculations! 7

8 Interphase layer formation due to the reaction of Solid Electrolyte Li metal Lithiation Reduction Li 10 GeP 2 S 12 (LGPS) Our first principles calculations Li 10 GeP 2 S Li è12 Li 2 S + 2 Li 3 P Li 15 Ge 4 (ΔH = ev / kj/mol) Lithiation In-situ Characterization agrees with the first principles calculations! Wenzel, Randau, Leichtweiß, Weber, Sann, Zeier, Janek, Chemistry of Materials (2016)

9 Interphase layer formation due to the reaction of Solid Electrolyte Li metal Lithiation Reduction Li 10 GeP 2 S 12 (LGPS) Our first principles calculations Li 10 GeP 2 S Li è12 Li 2 S + 2 Li 3 P Li 15 Ge 4 (ΔH = ev / kj/mol) Outcome of interphase layer formation: - Thick interphase layers - High interfacial resistance. Interfacial resistance increase Thickness increase of interphase layers Wenzel, Randau, Leichtweiß, Weber, Sann, Zeier, Janek, Chemistry of Materials (2016)

10 Stability of solid electrolytes against Li metal Li metal LGPS Li 3 PS 4 Li 6 PS 5 Cl Li 7 P 2 S 8 I LiPON LLZO LLTO LATP LAGP LISICON Li Solid electrolyte Phase equilibria with Li metal Li 15 Ge 4, Li 3 P, Li 2 S Li 3 P, Li 2 S Li 3 P, Li 2 S, LiCl Li 3 P, Li 2 S, LiI Li 3 P, Li 3 N, Li 2 O Zr (or Zr 3 O), La 2 O 3, Li 2 O Ti 6 O, La 2 O 3, Li 2 O Ti 3 P, TiAl, Li 3 P, Li 2 O Li 9 Al 4, Li 15 Ge 4, Li 3 P, Li 2 O Li 15 Ge 4, LiZn, Li 2 O Zhu, He, Mo, ACS Appl. Mater. Interfaces, 2015, 7 (42), LiF LiCl LiI Li2O Li2S Li3P Li3N LGPS Li3PS4 Li6PS5Cl Li7P2S8I LiPON LLZO LLTO LATP LAGP LISICON Potential Ref. to Li/Li + (V) Electrochemical window Li Chemical potential(ev)

11 Interphase formation: Solid electrolytes against Li metal Li metal LGPS Li 3 PS 4 Li 6 PS 5 Cl Li 7 P 2 S 8 I LiPON LLZO LLTO LATP LAGP LISICON Li Solid electrolyte Phase equilibria with Li metal Li 15 Ge 4, Li 3 P, Li 2 S Li 3 P, Li 2 S Li 3 P, Li 2 S, LiCl Li 3 P, Li 2 S, LiI Li 3 P, Li 3 N, Li 2 O Zr (or Zr 3 O), La 2 O 3, Li 2 O Ti 6 O, La 2 O 3, Li 2 O Ti 3 P, TiAl, Li 3 P, Li 2 O Li 9 Al 4, Li 15 Ge 4, Li 3 P, Li 2 O Li 15 Ge 4, LiZn, Li 2 O Zhu, He, Mo, ACS Appl. Mater. Interfaces, 2015, 7 (42), LiF LiCl LiI Li2O Li2S Li3P Li3N LGPS Li3PS4 Li6PS5Cl Li7P2S8I LiPON LLZO LLTO LATP LAGP LISICON Potential Ref. to Li/Li + (V) Electrochemical window Li Chemical potential(ev)

12 What happens when putting LiPON against Li metal? Li Li 3 N,Li 3 P, Li 2 O LiPON x Li 2 O Li 3 N Li 3 P LiPON is well demonstrated for its Li metal compatibility in thin-film batteries. However, Li reduction is thermodynamically favorable Li reduction of LiPON is also observed in in-situ experiments Schwobel et al. Solid State Ionic (2016) Li 2 O Li 3 N Li 3 P Lithiation pristine pristine But LiPON is experimentally demonstrated to work with Li metal for thousands of cycles. Why? 12

13 WHY some Solid Electrolytes are compatible with Li metal? Li Li+ e - LiPON Li 3 N, Li 3 P Li 2 O ² Why some SEs are stable against Li metal? E.g. Li LiPON interphases: Li 3 P, Li 3 N, Li 2 O ² Form electronic insulating interphase layer Mitigate high μ Li from Li metal by blocking e - transport Form passivation layer Limit continuous decomposition into bulk ² The SEI-like layer passivates the solid electrolytes Li Li+ e- LLTO Li titanate w/ Ti 3+ or lower ² Why some SEs are reduced by Li metal? E.g. Li LLTO: titanate with Ti 3+ or lower valence state Similar for LATP, LAGP, LGPS, etc. ² Form mixed ionic-electronic conductors (MIEC) interphase Simultaneous transport of both Li + and e -. Thermodynamic favorable decomposition will continue into the bulk. ² MIEC type interphase layers cannot provide passivation for solid electrolytes.

14 Guide for interfacial engineering - Types of interfaces Type 1. Stable interfaces w/ no decomposition or interphase layers Type 2. Interfaces formed interphase layer that is mixed ionic and electronic conductor (MIEC) Type 3. Interfaces formed with stable solid-electrolyte interphase (SEI) Li+ e.g. Li+ e.g. Li+ Li Li LLTO Li LiPON No interphase layer Maybe in Li-LLZO e- MIEC interphase e.g. Li-LLTO Li-LATP Stable SEI e.g. Li-LiPON Desired but mostly not available Avoid! The properties of the SEI are critical. Wenzel, Leichtweiss, Kruger, Sann, Janek, Solid State Ionics, 2015, 278, Zhu, He, Mo, J. Mater. Chem. A, 2016,4,

15 What materials are thermodynamically stabilize against Li metal? Li + Li x MF y -> Li z M + LiF Li + Li x MO y -> Li z M + Li 2 O Fluoride Oxide Sulfide Nitride Most Li-M-X oxides, sulfides, halides are reduced by Li metal. Most form MIEC interphase due to the reduction of metal cations M. (Not passivate) Nitrides have unique thermodynamic stability against Li metal. Computation predicted nitrides chemistry space to ultimately stabilize Li metal Zhu, He, Mo, Advanced Science (2017)

16 The formation of interphase layers is thermodynamically favorable V Anode Li ionic conductor Solid electrolyte Cathode Strong thermodynamic driving force for the reduction of solid electrolyte, leading to the formation of interphase layers? How about the SE-cathode interfaces? 16

17 Evaluating interface stability between two materials Chemical reaction between solid electrolyte and electrodes. Electrochemical reaction (during cycling voltage) between solid electrolyte and electrodes. Solid electrolyte, e.g. LGPS Cathode, e.g. LiCoO 2 Zhu, He, Mo, J. Mater. Chem. A, 2016,4, Phase Equilibria Chemical stability: there is an exothermic reaction to form a new phase (interphase layer). Electrochemical stability: there is an exothermic reaction at applied potential/voltage We evaluate the chemical and electrochemical stabilities of interfaces using first principles calculations. Zhu, He, Mo, J. Mater. Chem. A, 2016,4,

18 Formation of interphase layers is highly favorable for sulfide SE. For LGPS-LiCoO 2 interface Sakuda et al. Chem. Mat. 22, 949 (2010) Li 10 GeP 2 S 12 LiCoO 2 Interphase formation? Li 10 GeP 2 S 12 + LiCoO 2 è Co 9 S 8 + Li 2 S + Li 2 SO 4 + Li 3 PO 4 + Li 4 GeO 4 (ΔH = ev/atom) Li 10 GeP 2 S 12 + Li 0.5 CoO 2 è Co 9 S 8 + Li 2 S + Li 2 SO 4 + Li 3 PO 4 + Li 4 GeO 4 (ΔH = ev/atom) The formation of interphase layers is highly favorable at the sulfide SE-LCO interfaces. The electronic conductive Co x S y may cause forming thick interphase layer. Thick resistive interphase layer may be the origin of high interfacial resistance, and quick interface degradation. Zhu, He, Mo, J. Mater. Chem. A, 2016,4,

19 Interface reactions for LiPON solid electrolytes For LiPON-LiCoO 2 interface Y. S. Meng et al. Nano Letters (2016) LiPON LiCoO 2 Li 3 PO 4, Li 2 O, Li 2 O 2, CoN x, CoO x, Li x CoO y Electron energy loss (ev) LiPON+ LiCoO 2 è Li 3 PO 4 + Li 2 O + CoN (ΔH = -0.1 ev/atom) At applied voltage of 4.2V to 5.0V, ΔH = to ev/atom Electronic insulating but ion conducting interphase formed to stabilize the interface -> Self-Limiting decomposition -> Form SEI-like passivation -> Decent interfacial Li+ transport. Zhu, He, Mo, J. Mater. Chem. A, 2016,4,

20 Interface reactions for Garnet solid electrolytes Kim et al. J. Power Source (2011) Garnet electrolyte, e.g. LLZO LiCoO 2 La 2 O 3, La 2 Zr 2 O 7, Li 6 Zr 2 O 7, Li x CoO y Still favorable reaction but much better than other SEs. May observe after high temperature sintering. Against LiCoO 2 ΔH = ev/atom At applied voltage > V, ΔH = to ev/atom Zhu, He, Mo, J. Mater. Chem. A, 2016,4,

21 Why Interfaces become limiting factors for performance? Guide for interfacial engineering Type 2. MIEC interphase Sulfide SE-LCO Type 3. Stable SEI LiPON-LCO e.g. Li+ Li+ Sulfide SE LiCoO 2 LiPON LiCoO 2 e- Co 9 S 8, etc. Electronic conductive interphase formed -> Sustained decomposition. -> Thick interphase layer. -> High interfacial resistance. Li 3 PO 4, etc. Electronic insulating interphase formed -> Limited decomposition -> Form SEI-like passivation -> Decent interfacial Li+ transport. Can we fix Type 2 interface? Zhu, He, Mo, J. Mater. Chem. A, 2016,4,

22 How to Fix a Type 2 (MIEC) Interface? Type 2 interface as a result of poorly compatible between sulfide-licoo 2 Solution: Converting to Type 3 by proper coating Sulfide electrolyte, e.g. LGPS Interphase Layer Co 9 S 8, etc. Oxide coating LiCoO 2 Sulfide electrolyte, LiCoO 2 e.g. LGPS E decomp ~= mev/atom Origin of poor interfacial performance: Oxidation of SSE at high voltage. Chemical reaction Enhanced reaction at applied voltage. --> Formation of high resistive interphase layers Coating layer, as artificial SEI, serves a passivation What are the design principles for good coating layer materials? Zhu, He, Mo, J. Mater. Chem. A, 2016,4,

23 Why these coating layers work? Demonstrated coating materials: LiNbO 3 Li 2 SiO 3 LiTaO 3 Li 3 PO 4 Li 4 Ti 5 O 12 Sulfide electrolyte, e.g. LGPS Oxide coating LiCoO 2 Takada et al. Effects of coating on LiCoO 2 Electronic insulating (Type 3) Has Li+ carriers Chemically compatible with both SSE and cathode Small E decomp LiCoO 2 Electrochemically stable at cycling voltage (a) 23

24 Interface is the key in enabling all-solid-state battery Thermodynamically favorable interphase formation due to the limited electrochemical stability of SE and poor incompatibility between SE and electrodes. Formed interphase layers have critical effects on battery performance. ϕ=5v / μ Li =-5eV Higher ϕ / Lower μ Li Cathode Oxidation potential Interphase /coating Low μ Li ϕ=0v / μ Li =0eV Intrinsic Electrochemical Window Anode High μ Li Interphase Solid electrolyte Reduction potential Extended Electrochemical Window Y. Zhu, X. He, Y. Mo, ACS Appl Mater & Interfaces, 7, (2015)

25 Resolving interface issues in all-solid-state battery Current success cases: Li LiPON High-Voltage Cathode Carbon anode LPS Sulfide Oxide Cathode Spontaneously formed stable SEI layer formed SEI layer Artificial coating layer Strategies for resolving interface issues: Optimize and design SE to form stable SEI (e.g. LiPON) good interfacial compatibility. Applying thin coating layer as artificial SEI (e.g. LiNbO 3 coating on sulfide SE) Novel interfacial engineering to spontaneously form stable SEI. Computation can rapidly narrow down chemistry space. 25

26 Conclusions First principles Computation Methods First principles computation techniques were developed to evaluate the interfacial equilibria of solid materials. The computation framework can be transferable to any heterogeneous interfaces. Electrochemical stability of solid electrolyte Most solid electrolyte materials have limited electrochemical window, and are not thermodynamically stable against Li metal, cathode, or at high voltages. The decomposition and reactions of solid electrolyte form interphase layers between the solid electrolyte and the electrode. The interphase layers play a key role in passivating the solid electrolyte. The interphase layer is likely an origin of high interfacial resistance. Implications for all-solid-state battery Interface engineering is the key to achieve good performance. Develop compatible materials combinations to optimize interfaces. Apply thin coating layers to construct desired interfaces. Zhu, He, Mo, ACS Appl. Mater. Interfaces, 2015, 7 (42), Zhu, He, Mo, J. Mater. Chem. A, 2016,4, Han, Zhu, He, Mo, Wang, Adv. Energy Mater., 2016, Zhu, He, Mo, Advanced Science, 2017,

27 Acknowledgement Funding support: BMR program DOE-EERE DE-EE , DE-EE Collaborations at University of Maryland Prof. Chunsheng Wang Prof. Eric Wachsman Prof. Liangbing Hu Materials Project Computational resources: XSEDE: NSF TG-DMR130142, TG-DMR University of Maryland supercomputers Maryland Advanced Research Computing Center (MARCC) 27