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Simulated electrolyte-metal interfaces -- γ-Li3PO4 and LiXiao Xu , Yaojun Du and N.A.W. Holzwarth
• Introduction to Li-ion Batteries
• Project Motivation
• Model and Method of Calculation
• Results for geometry optimization and densities of states
• Conclusions and future work
Supported by NSF grants DMR-0465456 + 0427055
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Discharge operation of Li ion battery
Cathode Electrolyte Anode
Li+ e
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Cathode materials Old technology: LiCoO2
LiMn2O4
LiNiO2
New technology : LiFePO4
Store Li+ ions and electrons in discharge mode
Electrolyte materials
Liquid solvent , gel, polymer
And LiPF6 or LiClO4
Solid : LiPON , γ-Li3PO4
Transport Li+ ions
Exclude electrons
Anode materials Li Al alloy
Li intercalated graphite
Metal Li
provide source of Li+ ions
Make stable interface and electrons in discharge mode.
Li ion battery components
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Cathode Electrolyte Anode
Li+ e
This talk : What the interface would look like ?
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Next talk : How Li would migrate with in the electrolyte
Cathode Electrolyte Anode
Li+ e
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Motivation & Questions
• Motivation– LiPON1 And Li3PO4
– Why crystal ?
• Questions – What are the possible structures of an ideal Li3PO4 – Li metal interface
– Are the interfaces physically and chemically stable ?
1 LiPON materials are developed at Oak Ridge National Lab
vacuumLi3PO4Li
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Model & Method Of Calculation
Model• Started with ideal γ-Li3PO4 crystal • Constructed an ideal surface plane, assuming charge neutrality and keep all PO4 bonds.• Relax surface in vacuum • Deposit a few layers of Li between electrolyte surface and vacuum • Relax the structure
Method of Caculation• Plan wave basis with soft pseudo potentials and PAW (PWscf1 code and PWPAW2 code ) • |k + K|2 30 Ryd ≦• Atomic positions relaxed until force components less than 3 ×10 -4 Ry/Bohr
1 www.pwscf.org
2 pwpaw.wfu.edu
Li3PO4 Li3PO4 vacuum Li3PO4 vacuumLi
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Crystal structure of γ-Li3PO4 (Pnma)
a
2c
2b
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Pure Crystal Partial DOS
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Li γ-Li3PO4 interface a-direction
2b
a
Relaxed Structure of γ-Li3PO4 with vacuum
Converged structure of Li- γ-Li3PO4 interface
2c
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Interface a-direction Partial Density Of States
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Interface a-direction Partial Density Of States
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Interface a-direction Partial Density Of States
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Li-Li3PO4 interface b-direction
Relaxed Structure of γ-Li3PO4 with vacuum
2b
a
2c
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Interface b-direction Partial Density Of States
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Interface b-direction Partial Density Of States
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Interface b-direction Partial Density Of States
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Li-Li3PO4 interface c-direction
2c
2b
2a
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Interface c-direction Partial Density Of States
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Interface c-direction Partial Density Of States
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Interface c-direction Partial Density Of States
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Comparing Pure Crystal with Interface
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Simplified DOS model
+Strong interaction
Weak interaction
Or
Pure crystal Metallic Li
Our results
DO
S DO
S
E E
DO
S
DO
S
E
E
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Conclusion and future work
• We constructed 3 different interfaces on a , b and c planes, with Li metal on Li3PO4
• We found plausible structures with well-defined electrolyte boundary
• From the Partial DOS plots, we found an energy gap between electrolyte and metal states.
• On the presence of Li metal, electrolyte is physically and chemically stable.
• We plan to study Li-ion diffusion across these interfaces
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Other possible structures two a-direction interfaces
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Other possible structures two b-direction interfaces