1. Reliable Electrochemical Energy Storage for Alternative Energy
Craig B. Arnold
Department of Mechanical and Aerospace Engineering
Princeton Institute for Science and Technology of Materials
Princeton University
2500 mm

2. We don’t necessarily generate power where or when we need it
Introduction
We don’t necessarily generate power where or when we need it
Alternative energy, non-constant energy generation  solar, wind load leveling
Excess energy is needed to meet an unexpected demand  ramping
Energy demand requires greater regulation of characteristics  frequency regulation
Energy needs to be portable  transportation, small applications
Novel systems require novel solutions  Flexible, long life, lightweight, fast recharge, etc.
Energy storage is one of the key challenges we face in the 21st century

3. Magic Storage Device would have:
Why is this a problem?
Why can’t we just invent a giant energy storage device to solve the storage problem?
Maximum power capabilities
Maximum energy storage capabilities
Insensitive to charging/discharging parameters
Instant response
No internal impedance
Long life without degradation of properties
Portable
Lightweight
Small footprint/Volume
Magic Storage Device would have:
Portable vs. Stationary
Required time range
Usage profile
Obviously we cannot get all of these things in a single device
But we can make tradeoffs to optimize performance for a given application and we can continue to make innovative breakthroughs

4. Project Outline
Assessing and optimizing the integration of hybrid energy storage with alternative energy
Improving lifetime and capacity fade in secondary batteries through improved mechanics

5. Electrochemical Energy Storage
Batteries are a compact method of converting chemical energy into electrical energy
Other methods, fuel cell, photovoltaic cell, electrochemical capacitors etc.
Primary: Non-rechargeable
Secondary: rechargeable
Voltage Potential difference between anode and cathode. Related to energy of reactions
Capacity amount of charge stored (usually given per unit mass or volume) e-
Anode
Cathode
Electrolyte/Separator
Current
Collectors
Anode (Oxidation):
Zn + 2 OH-  Zn(OH)2 + 2e-
E = 1.25 V
Ag2O + H2O + 2e-  2 Ag + 2 OH-
E = 0.34 V
Cathode (Reduction):
C-rate  charging/discharging rate, 1C is current needed to discharge in 1 hour
All work the same, but the details are different

6. Specific power increases  specific energy decreases
Battery Limitations
Electrochemical energy storage such as batteries or supercapacitors provide unique properties for the energy storage portfolio but they have some limitations
E.g. Ragone Relation
Specific power increases  specific energy decreases
Corollaries:
capacity is lower at higher discharge/charging rates
Some systems charge fast some slow
Each system has a sweet-spot for energy/power capacity
But, different battery chemistries and technologies have different characteristic regimes

7. Case Study: Wind Power
P. Denholm, G. L. Kulcinski, and T. Holloway, "Emissions and energy efficiency assessment of baseload wind energy systems," Environmental Science and Technology, vol. 39, pp , 2005.
Fluctuations occur over many different time periods

8. What to do about it Assessment
Our approach to this challenge is to integrate and optimize multiple types of energy storage devices into a single system  Hybrid Energy Storage System
We can try to match a combination of batteries to the fluctuating system where each battery is optimized for a particular time scale
Assessment
Assess existing battery technology for charge storage efficiency as a function of rate and state of charge
Using laboratory scale wind turbine, test different batteries under simulated wind spectrum
Design circuitry/systems to incorporate multiple types of batteries in a single system
Optimization (work done in collaboration with W. Powell, ORFE)
Given the random fluctuations, and performance metrics, develop models to determine when and how to charge/discharge the system for optimal performance

9. Improving Cycle Life and Capacity Fade
Common misunderstanding  Most failure in batteries happens because of mechanics
Clearly this is true for flexible but also fixed
Discharge: Li1-xCoO2+xe-+xLi+→LiCoO2
In Lithium Batteries, the ions have to ‘intercalate’ into the host lattice e-
Li+
Understanding relation between mechanics and electrochemistry  improved Lifetime and lower fade
Very large strains can be achieved > 7% !
Cathode Material

10. Mechanical Properties
In real battery systems, applied stresses can be quite large
Flexible batteries →tensile, compressive, and bending stresses
Traditional batteries also subject to applied compressive stresses
Compression testing of batteries will advance understanding of electrochemical/mechanical interaction
Fatigue
Stress
Strain
Cycle life
Energy density
Power density

11. Mechanics
As the batteries are charged and discharged, they expand and contract
T. Chin et. al., Electrochem. Sol. State Lett. (2006)
But more importantly, the properties change in time as the internal materials change in response to the forces

12. Creep Behavior
Static load testing confirms viscous flow behavior
Application of a 3 parameter model provides information about elastic and viscosity parameters
Fully Discharged (3.0V)
The 3 parameter model for viscoelastic polymer behavior accurately describes the strain response of the battery s h1 h2 E
Partially Charged (3.5V)
Fully Charged (4.1V)

13. Conductivity Measurements
Does the effect of Creep make any difference?
Compressed systems show a decrease in conductivity  Increased internal resistance, capacity fade

14. Why?
The pores begin to close in samples that have experienced creep

15. Conclusions
Assessment and Optimization of hybrid systems can provide a pathway for electrochemical energy storage in alternative energy applications
By studying the mechanics of the electrochemical systems, we can understand limitations to capacity and cycle life and develop pathways to improvement

16. Acknowledgement Matt Brown Nick Kattamis Elena Kreiger
Christina Peabody
Guodan Wei
Ashwin Atre
Paul Rosa
Jonathan Scholl
Karl Suabedissan

17. Research Projects Batteries Supercapacitors Integration/Systems
Relation between mechanical and electrochemical properties
Fabrication and design of flexible platforms
Fabrication and design of microbatteries
Advanced laser processing and embedding of microbatteries
Batteries
Small, Long lasting, Advanced applications
Optimizing nanoscale architecture for optimized capacity
Laser modification of nanoscale materials for improved performance
Advanced laser methods of fabricating small scale supercapacitors
Supercapacitors
Control of nanoscale structures, High power, Novel applications
Integration/Systems
How to integrate storage with alternative energy
Hybrid systems for small scale applications

18. SEM II
Similar result in other Celgard materials