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

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

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

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

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

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 http://www.powerstream.comz/ragone.gif 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

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 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

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

Mechanical Properties In real battery systems, applied stresses can be quite large www.powerstream.com 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

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

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)

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 Brown Nick Kattamis Elena Kreiger Christina Peabody Guodan Wei Ashwin Atre Paul Rosa Jonathan Scholl Karl 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 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

SEM II Similar result in other Celgard materials