This document discusses the challenges of energy storage for alternative energy sources like solar and wind power. It notes that no single device can provide all the ideal properties of maximum power, energy storage, fast response, long life, and portability. The author proposes using hybrid energy storage systems combining different device types optimized for different timescales. The document outlines projects to assess hybrid systems integration with alternative energy and improve battery lifetime through better understanding the relationship between electrochemical and mechanical properties like stress and strain during charging/discharging. Understanding how mechanics impacts capacity fade and cycle life could lead to improved battery design and performance.

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 µm

2. Introduction
• 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
We don’t necessarily generate power
where or when we need it

3. Why is this a problem?
Why can’t we just invent a giant energy storage device to solve the
storage problem?
Magic Storage Device
would have:
• 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
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. Batteries are a compact method of converting chemical energy
into electrical energy
Electrochemical Energy Storage
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):
e-
e-
e-
e-
e-
e-
Anode
Cathode
Electrolyte/Separator
Current
Collectors
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)
All work the same, but the details are different
C-rate  charging/discharging
rate, 1C is current needed to
discharge in 1 hour

6. Battery Limitations
Electrochemical energy storage such as batteries or supercapacitors provide
unique properties for the energy storage portfolio but they have some limitations
http://www.powerstream.comz/ragone.gif
E.g. Ragone Relation
Specific power increases  specific
energy decreases
• 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
Corollaries:

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. 1903-1911, 2005.
Fluctuations occur over many different time periods

8. What to do about it
Our approach to this challenge is to integrate and optimize
multiple types of energy storage devices into a single system 
Hybrid Energy Storage 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
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
We can try to match a combination of batteries to the fluctuating
system where each battery is optimized for a particular time scale

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

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

11. Mechanics
T. Chin et. al., Electrochem. Sol. State Lett. (2006)
As the batteries are charged and discharged, they expand and contract
0 0.05 0.1 0.15 0.2
0
5
10
15
20
25
30
35
Strain (mm/mm)
Stress(MPa)
But more importantly, the properties change in time as the internal
materials change in response to the forces

12. •Static load testing confirms viscous flow behavior
•Application of a 3 parameter model provides
information about elastic and viscosity parameters
σ
η1
η2E
t
tE
E
t
E
12
2
1
exp1)(
η
σ
η
σ
ε
ε
η
ε
η
σ
+












−
−=
+= 
The 3 parameter model for viscoelastic
polymer behavior accurately describes
the strain response of the battery
0 1000 2000 3000
0
0.005
0.01
0.015
0.02
Test Time (s)
Strain(mm/mm)
Measured Strain
3 Parameter Fit
Partially Charged (3.5V) Fully Charged (4.1V)
0 1000 2000 3000
0
0.005
0.01
0.015
0.02
0.025
Test Time (s)
Strain(mm/mm)
Measured Strain
3 Parameter Fit
Fully Discharged (3.0V)
0 1000 2000 3000
0
0.005
0.01
0.015
0.02
Test Time (s)
Strain(mm/mm)
Measured Strain
3 Parameter Fit
Creep Behavior

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
• Optimizing nanoscale architecture for optimized capacity
• Laser modification of nanoscale materials for improved performance
• Advanced laser methods of fabricating small scale supercapacitors
Small, Long lasting, Advanced applications
How to integrate storage with alternative energy
Hybrid systems for small scale applications
Control of nanoscale structures, High power, Novel applications

18. SEM II
Similar result in other Celgard materials