diffusivity and solvation of alkali metal ions in solid and aqueous electrolytes
TRANSCRIPT
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Diffusivity of Alkali Metal Ions in Solid and Aqueous Electrolytes
Student: Bhavin ShahTeacher: Jeffrey SapinMentors: Michiel Niesen & the Miller Group
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Chemistry Background
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The Big Idea - Batteries
Depicted above are (a) a typical smartphone battery, (b) a Tesla Model S battery pack, and (c) Tesla Powerwall, a home battery pack.
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A Molecular Insight
Electrolyte
Anode
Cat
hode
Li+
A lithium battery has two conducting plates
known as electrodes separated by a
conductive medium called an electrolyte.
E
In a lithium-ion battery, electrons split from lithium and
carry charge through the circuit, while the remaining
lithium ions (cations) move to the cathode.
Li+ Li+
Li+
E
The electrons from the anode then flow through the circuit
Electrodes are generally made of lithium salts, such
as LiFePO4
Image Credits: Islam, M. Saiful, and Craig A. J. Fisher. "Lithium and Sodium Battery Cathode Materials: Computational Insights into Voltage, Diffusion and Nanostructural Properties." Chem. Soc. Rev. 43.1 (2014)
Discharge of a Li-ion Battery
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Liquid Electrolytes
● Widely used for their high ionic conductivitiesDownsides:
1) Are safety hazards, especially in vehicles such as cars (Leaking, evaporating, overheating)
2) Are chemically unstableImage Credit: Josie Garthwaite, for National Geographic, December 06, 2013. "While U.S. Probes Tesla, What You Should Know About Car Fires." National Geographic. National Geographic Society
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A New Battery Electrolyte: Polymers
● Polymers are solid plastics composed of long molecular chains● Benefits: Won’t leak, lightweight, safe, long-lasting
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Movers of Charge: IonsHighly
Reactive{ Highly Reactive
{
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The Big Questions
(1) What trends can we observe in the behavior of alkali, alkaline earth, and halogen ions in different electrolytes?
(2) Do solid polymer electrolytes offer significant benefits over liquid electrolytes?
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Technical Overview
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Computational Chemistry
★ Use of computational tools to model molecular systems
★ Employs Molecular Dynamics○ Molecular Dynamics is the process of computing the
motion trajectories for molecules
t = 1
t = 2t = 4
θ
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Tools of Molecular Dynamics: LAMMPS & VMD
LAMMPS● Molecular Dynamics Simulator● Continuously solving F=ma for all particles● Takes coordinates of atoms and, using various
equations, distributions, and parameters, produces a trajectory of the atoms’ motion
VMD● Visualizes the trajectory from LAMMPS● Used to render snapshots and videos
To the right is a portion of a LAMMPS input script used in our simulations
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How MD Works
Start
Temperature
Temperature - the average kinetic motion of particles
Kinetic Energy =
Newton’s Second Law is at the heart of how simulations calculate motion trajectories for particles
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VMD Movie
Temperature
Density
Volume
Lennard-Jones
Bonds Angles
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Objectives
Using MD Simulations, we want to:● Determine the coordination effects of each
electrolyte structure on various ions● Determine the diffusivity, or mobility, of
various ions in both a polymer and liquid electrolyte
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Simulating Ions in Electrolytes
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Electrolyte Materials: PEO & Dimethyl Ether
● PEO - is a leading candidate for a polymer electrolyte○ Hindered by low conductivity at ambient temperatures
● Dimethyl Ether (DME) is a segment of PEO ○ Set as a liquid electrolyte for simulations
LIQUID
SOLID
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Simulation - Setup
1
2
5
4
3
Chlorine
Lithium
Magnesium
Potassium
Sodium
PEO Box at 400 K with fixed density
DME Box at 400 K with fixed density
NOTE: Running both materials at the same density gives us a basis for
comparison
On the right is a box of PEO
On the right is a box of Dimethyl Ether
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Simulation Analysis
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Objective #1
Using MD Simulations, we want to:● Determine the coordination effects of each
electrolyte structure
● Determine the diffusivity, or mobility, of various ions in both a polymer and liquid electrolyte
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Solvation Shells:Ion Coordination
● Particles surrounding an ion are known as solvation shells● Solvation Shells contribute to how ions move through a material:
aka coordination
This is the first solvation shell surrounding this atom. The
shell is said to be ‘coordinating’ the ion by
keeping it trapped within the ring-like structure.
This is the second solvation shell surrounding this atom. The shell
is actually made up of atoms bonded to the first shell. It lies
further from the atom because it does not coordinate the atom as
well as the first shell.
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RDF - PEO
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O
Solvation Structure - PEO
This Potassium ion in PEO is surrounded by oxygen atoms,
which form its first solvation shell.
O
O
O
O
O
O
O
Ion Predicted 1st Solvation
Simulated 1st Solvation
Li (+)
Mg (+2)
K (+)
Na (+)
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Solvation Structure - DME
Oxygen AtomsSodium Ion
Ion Predicted 1st Solvation
Simulated 1st Solvation
K (+)
Na (+)
Cl (-)
O O
O O
CH2 CH2
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Objective #2
Using MD Simulations, we want to:● Determine the coordination effects of each
electrolyte structure
● Determine the diffusivity, or mobility, of various ions in both a polymer and liquid electrolyte
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MSD & the Diffusion Coefficient
● MSD is the displacement of an atom from a its starting point● Greater MSD means that atoms are diffusing freely in a given material
Dimethyl Ether MSD - 400 K Origin
1
4 2
3
MSD Visualized
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Ion Diffusivity in PEOMost Diffusive
Least Diffusive
Cl-1
K+1
Na+1
Li+1
Mg+2 log t (Picoseconds)
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Ion Diffusivity in DME
Cl-1
K+1
Na+1
Most Diffusive
Least Diffusive
log
MS
D (A
ngst
rom
s)
log t (Picoseconds)
Mean-Square-Displacement for Dimethyl Ether
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Conclusions
The Big Answers
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The Big Answers(1) What trends can we observe in the behavior of alkaline, alkali earth,
and halogens in different electrolytes?
IncreasedDiffusivity
IncreasedOxygen Coordination
IncreasedDiffusivity
IncreasedOxygen Coordination
Predicted Trend
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The Big Answers(2) Do polymer electrolytes offer significant benefits over liquid electrolytes?
● Diffusivity of ions was approximately 50% as strong in polymer electrolyte compared to liquids
● Trends suggest study of new ions may be appropriate for polymer electrolytes ○ Fluorine (F-)○ Calcium (Ca+2)○ Chlorine (Cl-)
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Next Steps
● Further simulations of Potassium & Calcium cations, Chlorine & Fluorine anions in polymers
● Look at new, synthetic polymers● Conduct feasibility studies of other ions for battery applications
Image Credit: “Polymers.” http://www.survivalworld.com/science/polymers N.p., n.d. Web. 30 July 2015
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Computational Chemistry in the Classroom
From the Lab: Demos From Caltech: Programming
Observations and Analysis of various phenomena in physics, biology, and chemistry
Scripting: The new language requirement
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Acknowledgements Michiel Niesen, Brett Savoie, Michael Webb, and the Miller Group @ Caltech
James Maloney and Julius Su, Program Directors
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Questions?