Non-Aqueous Redox Flow Batteries: Challenges & Opportunities
11/18/14
Fikile Brushett Department of Chemical Engineering
Massachusetts of Institute of Technology, Cambridge, MA
2nd Annual MRES Conference, Northeastern University, Boston MA August 19th, 2014
Need for Distributed Grid Energy
2
Solar Wind Tidal
Power control & generation
Storage
Community Power grid
How can excess electricity be
utilized?
Batteries are <1% of Grid-scale EES
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DOE, Report on the First Quadrennial Technology Review, 2012
“To accelerate widespread adoption…installed cost of electrochemical grid storage systems fall to the low $100’s/kWh range.” (US Dept. of Energy)
Redox Flow Batteries for Grid Storage
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Xn+ + e- ↔ X(n-1)+ Y(m-1)+ ↔ Ym+ + e-
Xn+
X(n-1)+ Ym+
Y(m-1)+ Y(m-1)+
Ym+
Xn+
X(n-1)+ C+
e-
Charge – solid line Discharge – dotted line
only charge shown
11/18/14
May contain trade secrets or commercial or financial informa7on that is privileged or confiden7al and exempt from public disclosure.
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Why Non-Aqueous Flow Batteries? Benefits: (+) Expanded voltage window may enable higher energy (and power)
densities and increased round-trip energy efficiency
(+) Broad design space facilitated by wide range of electrolyte options
(+) A greater selection of redox couples available based on solvent choice and/or wider voltage window
(+) Smaller footprint may enable specific high-value urban applications
Challenges: (-) Higher solvent costs may hamper cost-competitiveness
(-) Reduced electrolyte conductivity may decrease performance and limit cell stack design options
(-) Undesirable solvent properties (e.g., volatility, flammability, toxicity)
(-) Unknowns (not a well-studied area)
Towards Cost-effective Energy Storage
11/18/14
May contain trade secrets or commercial or financial informa7on that is privileged or confiden7al and exempt from public disclosure.
6
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Power Energy All Else
( ) 1,0
−+++= ∑ ddbopaddi
iima tEccmcAcP
Price = area + materials + overhead + system
• 5 h storage • $120/kWh*
*includes inverter
• 7000 cycles • 20 year life
Linking performance & cost (as simply as possible):
In collaboration with R. Darling (UTRC) & K. Gallagher(ANL)
Flow Battery Component Cost Inputs
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Component Year 2014 Cost, $/m2
Ref. Future State Cost, $/m2
Ref.
Graphite flow field plate 55 1 25-35 1
Stainless-steel flow field plate 40 2 10-20 2
Carbon fiber felt / paper electrode
70 1 10-30 2
Fluorinated ion-exchange membrane
500 1 25-75 3
Polyolefin microporous separator
10 1 1-3 Est.
Frames, seals, and manifolds 6 Est. 1-3 Est.
May contain trade secrets or commercial or financial informa7on that is privileged or confiden7al and exempt from public disclosure.
1. V. Viswanathan et al., J. Power Sources, 2014
2. B.D. James and A.B. Spisak, Report from Strategic Analysis Inc., October 2012
3. M. Mathias et al., The Electrochemical Society Interface, Fall 2005
$120/kWh Flow Battery Designs (includes inverter)
11/18/14
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8
Both Nonaqueous & Aqueous Flow Batteries have the potential for $120/kWh!
Assumes: § 75% roundtrip
efficiency
§ 5 hr discharge
§ 10% - 90% SOC
§ MW = 100 g/mol
§ Specific volume = 0.1 L/kg
reactor cost
actives, salts,
solvents, & storage vessels
What might success look like?
11/18/14
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§ Tailored redox molecules are an attractive pathway if…
Properties Non-aqueous Aqueous Uavg. (V) 3 1.5
ASR (Ω-cm2) 5 0.5
Capacity (g/mol*e-) 150 (~180 mAh/g) 150 (~180 mAh/g)
Solubility (kg/kg) 0.8 0.05
Concentration (mol/L)* ~4-5 ~1-2
Material Cost ($/kg) 5 5
Electrolyte Cost ($/kg) 5 0.1
*Assuming 1 g/mL electrolyte density
§ Metal anodes are promising but cycle life (> 7000 cycles with 20% irreversible capacity fade) and charging current may limit adoption.
Non-Aqueous Flow Battery Materials
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Redox-Active Organic Molecules
Brushett et al., Adv. Energy Mater., 2012
DBBB
Shinkle et al., J. Power Sources, 2012
Metal-centered Coordination Complexes Metal-centered Ionic Liquids
Anderson et al., Dalton Trans., 2010
Semi-solid Slurry Suspension
Duduta et al., Adv. Energy Mater., 2011
Why Redox Active Organic Molecules?
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Benefits: (+) High storage capacity (low MWs & multi e-)
(+) Structural tunability (e.g., activity, solubility)
(+) Cheap & abundant raw materials
(+) Potential low “CO2” footprint
Challenges: (-) Low mass density (e.g., 1-2 g/cm3)
(-) Low electronic conductivity
(-) Solubility in organic electrolytes
(-) Unproven long term stability
Armand & Tarascon, Nature, 2008
Overcharge Protection Materials
Zhang et al., LIBs– New Developments, 2012
A Wide Range of Candidate Materials
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Reductive Decomposition
Oxidative Decomposition
~4.0 V
Non-Aqueous Window
Aqueous Window
~1.23 V
A Wide Range of Candidate Materials
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~180 mAh/g
overcharge materials
discovery materials
goal
goal: validate chemistry
DBBB (Overcharge Material)
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2,5-di-tert-butyl-1,4-bis(2-methoxyethoxy)benzene (DBBB)
MW: 338.5 g/mol electrolyte A: 1.2 M LiPF6 in EC:EMC (3:7 by vol)
Brushett et al., Adv. Energy Mater., 2012; Zhang et al., Energy & Environ. Sci., 2012; Zhang et al., J. Power Sources, 2010
A successful overcharge protection material developed which has shown remarkable reversibility, stability, & longevity in LIB applications
- e-
+ e-
Molecular Accounting: Addition by Subtraction
11/18/14
May contain trade secrets or commercial or financial informa7on that is privileged or confiden7al and exempt from public disclosure.
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n = 1 electron MW: 338.5 g/mol
Density: 0.9 g/cm3
Capacity: 79 mAh/g [DBBB]neat = 2.7 M
Do we need the long ether chains?
Redox-active core
Do we need tert-butyl protective group?
Can this core support multi-electron transfer?
Would methyl groups suffice?
Can we maintain solubility without it?
𝑞= 𝑛𝐹/3.6∗𝑀𝑊
n – # of electron transfer F – Faraday constant MW – molecular weight
Derivatization of Substituted Alkoxybenzenes
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DBBB: ~79 mAh/g
O
O
O
asymmetric structures
O
O
P
O
P
O
alternate substituents
O
O
simpler structures
O
O
other structures
In collaboration with J. Huang & L. Zhang (ANL)
Just a few examples…
Towards higher capacity materials…
11/18/14
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𝑞= 𝒏𝑭/3.6∗𝑀𝑊 What are the next steps…
-0.8
-0.4
0
0.4
0.8
3.5 3.7 3.9 4.1 4.3
Cur
rent
(mA/
cm2 )
Potential (V vs. Li/Li+)
1005020105
scan rate(mV/s)
oxidation
reduction
Experimental: 0.01 M redox species / 1 M LiClO4 / PC, GCE/Au/Li
-1
-0.5
0
0.5
1
3.5 3.7 3.9 4.1 4.3
Cur
rent
(mA/
cm2 )
Potential (V vs. Li/Li+)
1005020105
scan rate(mV/s)
oxidation
reduction
~79 mAh/g ~162 mAh/g
Quinoxalines (Discovery Material)
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Substituent groups can impart favorable and unfavorable traits on the base molecule
Can we validate the observed chemistry and better understand how the value proposition of this discovery material
MW: 130.15 g/mol Solubility: ~7 M
Capacity: 412 mAh/g
Effect of Substituent Group on Redox Behavior
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5 mM redox, 0.2 M LiBF4/PC in Pt/Li/Li cell, 20 mV/s
- 2e-
+ 2e-
Compounds Lower Redox (V vs. Li/Li+)
Upper Redox (V vs. Li/Li+)
Quinoxaline 2.649 ± 0.005 3.07 ± 0.02
2-methylquinoxaline 2.609 ± 0.006 2.94 ± 0.02
2,3-dimethylquinoxaline 2.525 ± 0.007 2.85 ± 0.02
2,3,6-trimethylquinoxaline 2.484 ± 0.008 2.80 ± 0.02
Brushett et al., Adv. Energy Mater., 2012
Unusual Redox Behavior of Quinoxalines
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Barqawi & Atfah, Electrochim. Acta, 1987
quinoxaline
2,3,6,7-tetramethylquinoxaline
cyclopentano{b}quinoxaline
Prior reports show different CV behavior Unexpected Anion Sensitivity
A Quick Detour into Li-ion Battery Degradation
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Xu, Chem. Soc. Rev., 2004
§ At moderate temperatures (> 55°C), capacity fades after a few months
§ Decomposition primarily driven by Lewis acid equilibrium products
LiPF6 ↔ LiF + PF5
§ Mitigation strategy: Bind with a Lewis base (e.g. pyridine)
What can this tell us about Quinoxaline activity? Li et al., J. Electrochem. Soc., 2005
Quantum Calculations to Understand Redox Potential
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LiBF4 Li+ + BF4- + LiF + BF3 Salt dissociation:
In collaboration with R. Assary & L. Curtiss (ANL)
11/18/14
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Validation of BF3-enabled Redox Behavior
Pre-synthesized Adduct Direct spike of BF3 into the electrolyte
In collaboration with C. Diesendruck & J. Moore (UIUC)
§ Leads to an order of magnitude increase in the observed current § Activation in previously inert electrolytes § Complex electrochemical behavior (still under investigation…)
DFT Prediction
DFT Prediction
Concluding Remarks
11/18/14
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§ Non-aqueous (and aqueous) flow batteries have the potential to meet $100/kWh grid storage targets
§ Non-aqueous solvents loosen one constraint (voltage) but take on another (solubility from electrolyte cost)
§ Organic molecules offer unique design opportunities through substituent modification and electrolyte interactions
§ A flow battery is more than the redox materials (electrolyte, membranes, electrodes) all of which need developmental work
Acknowledgements
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Funding:
Not shown: Kyler Carroll (PD), Apurba Sakti (PD), John Barton (GS), Steven Brown (GS), Jeff Kowalski (GS), Jarrod Milshtein (GS)
T-E Modeling: § Rob Darling (UTRC) § Seungbum Ha, Kevin Gallagher (ANL)
Organic Synthesis: § Jinhua Huang, Lu Zhang (ANL) § Charles Diesendruck, Jeff Moore (UIUC)
Quantum Calculations: § Rajeev Assary, Larry Curtiss (ANL)
Flow Cell Testing: § Larry Pederson, Xiaoliang Wei, Wei
Wang (PNNL)
We gratefully acknowledge the Materials Engineering Research Facility and the ABR program providing scaled-up materials.
BACK-UP SLIDES
11/18/14
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Range of Grid Storage Options & Applications
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Adapted from the original figure in Sandia National Laboratory report, SAND 2013-5131, 2013
UPS-Power Quality T & D Grid Support – Load Shifting Bulk Power Mgmt.
Pumped Hydro
Flow Batteries
NaS Battery
Li-ion Battery
Lead-Acid Battery
High-Power Flywheels
High-Power Supercapacitors SMES
Advanced Pb-Acid battery High-Energy Supercapacitors
1 kW 1 GW 100 MW 10 MW 1 MW 100 kW 10 kW
Seco
nds
Min
utes
H
ours
Dis
char
ge T
ime
at R
ated
Pow
er
System Power Ratings, Module Size
Molten Salt
CAES
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3.8
4
4.2
4.4
4.6
0 1 2 3 4 5 6 7 8 9 10
E0' (V
vs.
Li/L
i+ )
Redox Potential
0.0
1.0
2.0
3.0
0 1 2 3 4 5 6 7 8 9 10
k0(×10
-4cm/s) Rate Constant
MW 338 248 402 346 294 338 427 166 166
0
0.5
1
1.5
2
0 1 2 3 4 5 6 7 8 9 10
D0(×10
-6cm2/s) Diffusion
1 M LiClO4 / PC (Similar trends observed in LiTFSI/DME)
May contain trade secrets or commercial or financial informa7on that is privileged or confiden7al and exempt from public disclosure.
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(But high cost of inac7ve materials and manufacturing, >40%)
V-‐redox
Target range
Goal: Lower cost chemistries at higher concentra7ons and higher voltage
Flow BaUeries: Cost of energy ($/kWh) is func7on of cost of materials ($/kg) and concentra7on of redox-‐ac7ve species (M) and hardware cost*
(Plot assumes 40% of system cost is hardware)
JCESR goal
Li-‐ion electrodes
Non-Aqueous vs. Aqueous Power Performance
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Interesting Trends as Function of Concentration
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Peak currents were obtained from CV measurements taken in a GC / Pt / Ag/Ag+ cell at a scan rate of 10 mV/s. PC was used as a solvent.
§ For 0.005 M → 0.05 M 2,3,6-TMQ, leads to a 2-fold increase in peak current
§ Increasing the [LiBF4] is directly proportional to increased peak current
§ Similar results observed with quinoxaline (Q)
An All-‐Organic Non-‐Aqueous Redox Couple
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V
2DBBB
2[DBBB+A-‐]
TMQ
TMQ2-‐ [C+]2
Note: Only showing charging
2e-‐
2C+
Can we transition this electrochemical redox couple to a prototype flow cell?
2e-‐
JCESR’s 10 cm2 Flow Cell Prototype
11/18/14
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Test Conditions in Ar-glovebox: Electrolyte = 0.1 M DBBB & 0.2 M TMQ / 0.6 M LiBF4 / PC Electrolyte Volume = 10 mL each Flow rate = 40 mL/min Li+-Nafion 212 membrane ic/id = 0.0625 mA/cm2, 0.2 – 2.5 V
0th order Energy Density based on Li metal
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Based only on the capacity of the
organic material