Reliable Electrochemical Energy Storage for Alternative Energy

Craig B. Arnold

Department of Mechanical and Aerospace EngineeringPrinceton Institute for Science and Technology of Materials

Princeton University

2500 m

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

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

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

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 VCathode (Reduction):

e-

e-

e-

e-

e-

e-

Anode

Cathode

Electrolyte/SeparatorCurrent

Collectors

Other methods, fuel cell, photovoltaic cell, electrochemical capacitors etc.

Primary: Non-rechargeableSecondary: 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

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:

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

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

AssessmentAssess existing battery technology for charge storage efficiency as a function of rate and state of chargeUsing laboratory scale wind turbine, test different batteries under simulated wind spectrumDesign 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

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

•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

FatigueStressStrain

Cycle lifeEnergy densityPower density

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 0.20

5

10

15

20

25

30

35

Strain (mm/mm)

Str

ess

(MP

a)

But more importantly, the properties change in time as the internal materials change in response to the forces

•Static load testing confirms viscous flow behavior

•Application of a 3 parameter model provides information about elastic and viscosity parameters

E

ttE

Et

E

12

2

1

exp1)(

The 3 parameter model for viscoelastic polymer behavior accurately describes

the strain response of the battery

0 1000 2000 30000

0.005

0.01

0.015

0.02

Test Time (s)

Str

ain

(mm

/mm

)

Measured Strain3 Parameter Fit

Partially Charged (3.5V) Fully Charged (4.1V)

0 1000 2000 30000

0.005

0.01

0.015

0.02

0.025

Test Time (s)

Str

ain

(mm

/mm

)

Measured Strain3 Parameter Fit

Fully Discharged (3.0V)

0 1000 2000 30000

0.005

0.01

0.015

0.02

Test Time (s)

Str

ain

(mm

/mm

)

Measured Strain3 Parameter Fit

Creep Behavior

Conductivity Measurements

Does the effect of Creep make any difference?

Compressed systems show a decrease in conductivity Increased internal resistance, capacity fade

Why?

The pores begin to close in samples that have experienced creep

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

Acknowledgement

Matt BrownNick KattamisElena KreigerChristina PeabodyGuodan WeiAshwin AtrePaul RosaJonathan SchollKarl Suabedissan

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 energyHybrid systems for small scale applications

Control of nanoscale structures, High power, Novel applications

SEM II

Similar result in other Celgard materials