ab initio li-s battery calculations
TRANSCRIPT
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Ab initio Li-S battery calculations
Lin-Wang Wang
Guoping Gao, Swastika Banerjee
Materials Sciences Division
Lawrence Berkeley National Laboratory, Berkeley, CA
BMR theory deep dive, Sept.25-26.
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The projects
Li-S battery cathode design
• “2D framework C2N as a potential cathode for lihitum-Sulfur batteries: an ab initio density functional study”, J. Wu, L.W. Wang, J. Mat. Chem. A, 2018, 6, 2984.
• “Phosphorene oxides as a promising cathode material for sealed non-aqueous Li-oxygen batteries”, Y. Li, F. Ma, L.W. Wang, J. Mat. Chem. A, 2018, 6, 7815.
• “Theoretical investigation of 2D conductive microporous coordination polymersas Li-S battery cathode with ultrahigh energy density”, G. Gao, F. Zheng, F. Pan, L.W. Wang, Advanced Energy Mat. https://doi.org/10.1002/aenm.201801823
Li Solid electrolyte design and mechanistic studies
• “Design principles for oxysufilde class of Lithium superionic conductors”, S. Banerjee, X. Zhang, L.W. Wang, Environ. Energy Science (in review)
• “Cooperative transport enabling fast Li-ion diffusion in thio-LISICON Li10SiP2S12 solid electrolyte”, B. Zhang, L. Yang, L.W. Wang, F. Pang (submitted)
• “Finding Li ion diffusion pathway in typical solid state electrolytes and electrode materials by BV-Ewald method”
D. Chen, M. Weng, J. Jie, S. Li, D. Chen, F. Pan, L.W. Wang (submitted)
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Li/S cathode electrode design principles
Prevent Li2Sn dissolution Electric conductive Mechanical and redox stable Proper binding energy (high capacity) Light weight Dense in volume
Use upper-right corner elements Using 2D materials Curve up the 2D into amorphous
structures
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The first system, C2N as cathode
2Li 3Li 4Li 5Li
6Li 7Li 8Li
Using genetic algorithm to search the structure (fix thesubstrate).
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J. Wu, L.W. Wang, J. Mater. Chem. A, 2018, 6, 2984
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Oxidized black phosphorene can be used as a Li battery electrode
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Energy density and voltage for phosphorene oxides as Li-cathodes
Y. Li, F. Ma, L.W. Wang, J. Mat. Chem. A2018, 6, 7815
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(a) Li1P4O1; (b) Li2P4O2; (c) Li3P4O3; (d) Li4P4O4
The conductivities of the electrodes
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Structural stability of the system: N_Li must be < N_O
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Experimental realized, and electronic properties is tuneable
through transition metal. (J. Am. Chem. Soc. 2017, 139, 19)
2D Coordination Polymers
Conductive
Transition metal: Adsorption sites for terminated S.
Nitrogen: Adsorption sites for terminated Li.
Porous:
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Charge polarized solvent model for Li2Sn in electrolyte
ref our result
S8→Li2S8 2.39 2.39
Li2S8-Li2S6 2.37 1.92
Li2S6-Li2S4 2.24 1.36
Li2S4-Li2S2 2.20 2.18
Li2S2-Li2S 2.15 2.33
S8+Li-Li2S 2.20 2.21
Table 1 Experiment voltage vs calculatedvoltage (eV)
The experimental Formation energy of bulk Li2S is obtained by the α-octasulfur,
and Li crystal structures
Li2S2 should be crystal or larger cluster
Li2S4, Li2S6, Li2S8 with solvent effect fit very well with experimental value
Combine experiment and theory, better reliability
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The adsorption energy of isolated S8, and Li2Sy molecule on Mn-HAB-CP in the vacuum (bars without pattern) and solvent (bars with net pattern).
The absorption (anchoring) of Li2Sn on Mn-HAB-CP
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Optimized Stable Structures
7
isolated
linear
meshed
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The genetic algorithm to search the LinSm structure on a substrate
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Energy Density
8
The lithiation formation energy is almost a straight line of the number of Li atoms,
indicating a constant voltage of about 1.74 V for the whole charge/discharge
process
The final energy density for the Mn-HAB-CP-Li20S8-Li16S8 final product is 1395
Wh/Kg
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Stablitity
Li10S8 Li9S4+1/2Li2S8 Li9S5+1/2Li2S6 Li9S6+1/2Li2S4 Whether dissolved
0 0.59 -0.23 -0.49 Partially
Li20S8 Li19S4+1/2Li2S8 Li19S5+1/2Li2S6 Li19S6+1/2Li2S4 Whether dissolved
0 13.86 10.31 6.24 No
Table 2 The separation energies of Mn-HAB-CP-LixSy into dissolved Li2Sy and the remaining Mn-HAB-CP bounded cluster.
9
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Conductivity
10
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Volumetric Expansion
10
only 2.7% lattice constant increases in the z-direction, and is similar to that of the
LIB battery.
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Optimized Stable Structures
7
No significant volume expansion (lattice constant increases in the z-direction)
during the charge process.
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Challenges For Diffusion Rate
12
Nudged elastic band to calculate barrier Only when pathway is simply and well defined system
MetadynamicsNeeds a well defined reaction coordinate
Higher temperatureThe sulfur will be melt !
How about the diffusion in the complex materials, especially in liquid or with liquid involved?
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Assumption
13
Our way: Forces are applied to targeted atom (Li atoms) in Molecular Dynamics to drive the diffusion. Then the minimum external force and the diffusion velocity are chosen as new parameters of diffusion ability.
Fexternal
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Our method vs NEB
15
The barrier obtained from the
potential in our method is consistent
with the barrier obtained by NEB
calculation. Therefore, our method
is robust.
Time (fs)
Time (fs)
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Applied on Li-S battery
15
F = 0.55 eV/Å in X direction
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Cut off
15
If di < r, Pot =σ𝑝𝑜𝑡𝑖, if di ≥ r, Pot =σ𝑝𝑜𝑡𝑖 × 𝑒(𝑑−𝑟)2/2.5
F = 0.55 eV/Å in X direction
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Applied on Li-S battery
15
F =
0.5
0 e
V/Å
in Z
dir
ecti
on
The diffusion is much easier in the Z direction than that in the X direction
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Improve the moisture stability of LiGePS solid electrolyte
M- S + H2O Decomposed product + H2S
Li10GeP2S12 has a high Li-ion conductivity (~ 20 mS cm-1)
But it is moisture unstable:
The problem is at the PS4 motif
Can we selectively replace PS4 to PO4 ?
Change Li10GeP2S12 to Li10GeP2S4O8
Solid electrolyte
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From Li10GeP2S12 to Li10GeP2S4O8
PS4 + 4H2O PO4 + 4H2S
Moisture instability happens at PS4 tetrahedron
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The stability of the quinary compound decomposition to quaternary compounds
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Stability against all the other decompositions
Use a linear program to test all other possible decomposition (~100 possible compounds) Like LiGePO, it is unstable against decompose into ternary compounds (but can have
kinetic barrier) Stable against decomposition into any binary (gas) compounds It is moisture stable
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The Li-ion conductivity
The path has changed, instead of a 1D path, it is more a 3D path
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The mobility is even higher than pure LiGePS !
More distorted system More possible path (3D instead of 1D) Li goes through LiSxO4-x tetrahedrons, the transition point has lower barrier
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Redox stability window
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Future plans and challenges (from a methodology point of view)
Large scale linear scaling calculations for solid-liquid, SEI, or solid electrolyte/Li-metal interface, or grain boundary?
Direct ab initio liquid electrolyte chemical free energy calculations?
Machine learning force field?
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Linear scaling 3 dimensional fragment (LS3DF)
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LS3DF DFT calculations for >10,000 atom systems
It works for metal !
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A bi-canonical AIMD for thermal dynamic integration
It is very important to calculate the free energy of Li2Sn in different electrolytes, and to study their configurations (dissociated, or cluster)?
But there is a lack of classical potential to describe the interaction accurately, and it is difficult to calculate them using ab initio method
A recent bi-canonical AIMD method (Frenzel eta, JCTC, 13, 3455, 2017)might make it possible
We will like to implement this method in our code.
It allows thermal dynamic integration (to calculate the free energy) using ab initio method
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The bi-canonical AIMD calculation
At least it works for simple ion and water
Does it work for more complicated solvate and solvent ?
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Our new algorithm
Old method(the Ei for each atom is not known, only the total energy is known).
New method(Ei is calculated directly from DFT, significantly simplify the training, increase the dataset).
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The NNFF accuracy compared to DFT
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Acknowledgment
Thanks Tien for the funding !
Thanks the BMR program !