PECS Talk -- Batteries

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  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 21 st 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 – Why charging is different than discharging Improving lifetime and capacity fade in secondary batteries through improved mechanics

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

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 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

Charging versus Discharging We all like to think about batteries like cups of coffee. We simply fill it up until it is filled and empty it out until it is empty But in reality, batteries are not that simple  the rate matters

Charging is different than discharging

So What does all this mean ? Although Batteries are charge storage devices, we typically talk about them in terms of their discharge Charging and discharging behave differently in a battery For grid level storage or other places where the rates are non constant, one has to account for changes in current This can be accounted for with the right models

Motivation Batteries are electrochemically and mechanically active devices How are these stresses and strains accommodated in a battery cell and how do they affect the electrochemical performance of the system? Charging Li + Li + Li + Li + Intercalation strains Thermal expansion effects SEI growth Internal Sources Applied stack pressure Designed mechanical loadings Unintentional mechanical loadings External Sources

Mechanical Degradation Much work has been done looking at mechanical degradation to electrodes after electrochemically cycling  cracking, delamination, etc. However, these types of studies typically ignore effects to the rest of the system Physical constraints can lead to elevated stress states throughout the battery Stress source Electrode Stress State Separator Stress state Electrode Expansion Tensile Compressive SEI Formation Compressive/Neutral Compressive External Compression Compressive Compressive External Bending Tensile/compressive Tensile/compressive

Separator Compression Regardless of most internal or external sources, the separator will experience compressive stress But how large is the stress ? Also notice that during rest step, the stress begins to decline J. Cannarella and CBA, in preparation Li + pouch cell (LiCoO 2 cathode) Cycle electrochemically while directly measuring the stress of the system Max stress is 1.3 MPa Average pinch stress for adult is ~1 MPa Mathiowetz et al., Arch Phys Med Rehabil , 66, p.69 (1985)

Experiment A=1.837cm 2 t=3.800 mm Compressive force applied by Instron electromechanical test machine Strain measured with a differential variable reluctance transducer (DVRT) www.instron.com www.powerstream.com 90 mAh prismatic lithium pouch cell Separator/Foil Jellyroll & Separator Only Jellyroll A~1.50 cm 2 t~5.0 mm Celgard® 2320, 2340, 3501 F F

SEM Images of Compressed Battery Unstressed Stressed Electrodes are unchanged, most deformation occurs in separator 30 MPa for 3 hours This is expected as the polymer separator is significantly weaker than the electrode material

Electrodes cycled in t-cell vs. Li metal counter-electrodes show no capacity loss Electrochemical Evaluation of Battery Shift at high frequency -> change in separator impedance Further verifies role of separator under stress

Mechanical Creep Battery mechanical response can be modeled using only electrochemically passive components Battery and separator/foil creep responses are nearly identical In the separator and separator/foil samples the viscoelastic creep can only result from the polymer separator Current collector foils reinforce jellyroll structure, decreasing creep strain C. Peabody and C.B. Arnold, J. Power Sources 196, 8147 (2011) Recall the stress relaxation upon holding  Indication of creep Permanent deformation over time at stress below yield strength 30MPa Peak Stress

Creep Strain C. Peabody and CBA, in preparation As applied stress is increased the total permanent strain continues to increases

Magnitude of Stress But wait a minute … I said that the average internal stress is only ~ 1-2 MPa under cycling So why are we using the stresses we are using ? External stresses could be higher We want to see a wide range By modeling the creep at higher stress, we can determine the functional dependence of strain rate versus applied stress in this system High stress  larger strain, shorter time Low stress  smaller strain, longer time This is effectively an accelerated testing strategy that allows us to get meaningful deformation data in a reasonable time h 1 h 2 E 1 E 2 s Maxwell K-V

Mechanism of Relaxation 0 MPa 10 MPa 30 MPa Viscoelastic creep causes pore closure in the polymer separator A 30MPa compressive stress for 3 hours decreases pore volume by almost 60% C. Peabody and C.B. Arnold, J. Power Sources 196, 8147 (2011) Pore volume decrease appears linear with respect to permanent strain

EIS Characterization of Separators Pore closure impedes ion transport through the separator C. Peabody and C.B. Arnold, J. Power Sources 196, 8147 (2011) Permanent strains as small as 10% can significantly reduce the capacity by a factor 2-3

Other Separators Higher porosity corresponds to larger creep strains at same stress state Similar effects occur for other separator materials Composition Porosity (%) Avg Pore Diam ( μ m) Thickness ( μ m) 2320 PP/PE/PP 39 0.027 20 2340 PP/PE/PP 45 0.035 40 3501 PP 55 0.064 25

2320 has lowest initial porosity and smallest average pore size-> increased porosity loss at lower strains 3501 is more porous than 2340-> similar amounts of pore closure because PP is more creep resistant than PE Differing Response to Creep Smaller pores and less porosity-> more conductivity loss due to viscoelastic creep

Electrochemical Performance The electrochemical effects of stress induced pore closure in the separator manifest as capacity fade C. Peabody and C.B. Arnold, J. Power Sources 196, 8147 (2011) Cycle separator in t-cell using commercial cathode and Li metal anode Charge/Discharge current constant 0.5 mA

Capacity Fade As a function of pore volume, we see a linear decrease in capacity The discharge capacity decreases with permanent strain As permanent strain reaches ~15%, the capacity of the cell has decreased by a factor of 2

Relevance Are these values relevant or way out of the realm of a real system ? In a typical Li + ion battery the electrodes may expand up to 10% Since the electrodes are typically 2-3 times thicker than the separator, such expansion if totally accommodated by the separator, could lead to strain values as high as 40-50 % They are very relevant Consider: Mechanical stress on the separator can be an important factor in battery design e.g. if T electrodes = 80, T sep = 20  T electrodes = 8 Then  sep = 40% !

Electrochemically inactive materials can have a significant impact on electrochemical performance through their mechanical behavior Compressive stresses cause viscoelastic creep to occur in polymer separator membranes for lithium-ion batteries Stress induced viscoelastic creep leads to pore closure in the separator Permanent separator strains as low as 10-15% can cause significant degradation to the electrochemical performance of the cells Small stresses acting over long times can lead to these levels of total strain Summary Stress induced pore closure in the polymer separator is a potentially important aging mechanism in lithium-ion batteries

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

In ancient times, the generation of electricity was purely accidental. But how did we get here ? For today, we’ll focus on batteries for portable energy storage We still use this method today: Van de Graff generators Drag feet on carpet Pet a cat Take off a sweater By rubbing certain materials together, static charges can be accumulated Ancient Greeks rubbed amber on fur to generate electricity. In fact, the word elektron comes from the greek word for amber By the mid 1600’s, static electricity could be readily generated by rubbing insulating materials together: fur/cloth, sulfur, amber, etc.

But in the 1600’s, scientists did not really know much about electricity or how to use it. The spark generators were mostly used by scientists to study the nature of the sparks In 1745, scientists (Musschenbroek and Cunaeus) noticed that one could “charge” up a glass filled with water and get a shock by touching a metal nail Shortly thereafter, this was simplified to just metal foil wrapped around the inside and outside of a jar with a chain connecting the inner layer.  Leyden Jar We know these devices as capacitors, but they work by storing charge ELECTROSTATICALLY

Although they still didn’t know all that much about electricity, they now had methods of storing and generating electricity, but it was still a research tool (and a parlor trick) In fact, this enabled many important experiments of the time 1746: Nollet assembled a line of 200 monks each holding the end of a wire to test if electricity can travel faster than human communication. Without warning he connected a Leyden Jar to the ends … But Cavendish did not publish all that much and these discoveries were rediscovered years later by Faraday, Ohm, Coulomb, Maxwell 1747-1753 Cavendish used Leyden Jars to discover many of the fundamental physics laws of electricity  Inverse square law for force, electric potential, capacitance, resistance

1752: Ben Franklin and his famous kite experiment Showed that lightening is the same as electricity stored in Leyden Jar Franklin’s other main contributions to the field include the concept of current as the flow of positive charges, and the term battery We later found out he was very wrong, but unfortunately it was too late. This is why current goes in the opposite direction of electron flow Two disadvantages of the Leyden Jar are that it doesn’t store charge all that long (This is true in general for electrostatic storage) and it doesn’t store all that much energy

1786: Galvani’s famous experiments on frog legs He took two dissimilar metals (Zn, Cu) and touched them to the ends of a dead frog’s leg Surprisingly, the leg moved and Galvani attributed this to bioelectricity

But Volta did not believe that the electricity came from the frog. He believed the electricity came from the metals In 1799, he showed that by combining different metals that are separated by a salt or acidic solution it was possible to generate electricity  VOLTA PILE First commercially available battery An enabling techno-logy for the telegraph Side note: Galvani died one year earlier and never knowing the answer

Chemical Energy is Converted to Electrical Energy Through Oxidation and Reduction reactions But, if reactions are spontaneous … … how do we harness them? How does all of this work ? It’s all electrochemistry!! Voltaic(Galvanic) Spontaneous Generates energy Electrolytic Non-spontaneous Needs energy to occur  Batteries, Fuel Cells, corrosion  Electrodeposition, electrowinning, polishing

Separate reactions into half-cells Salt bridge: allows ions to move between cells Oxidation occurs at anode Material gives up electrons i.e. Zn  Zn +2 + 2 e - Reduction occurs at cathode Material takes in electrons i.e. Cu +2 + 2e -  Cu Voltaic or Galvanic Cells

M  M n+ + ne - Oxidation Reduction M n+ + e -  M (n-1)+ Oxidation is loss of electrons Reduction gains electrons Electrochemistry: chemical reactions require charge transfer This occurs through redox reactions Oxidation occurs at the anode  anodic reaction Reduction occurs at the cathode  cathodic reaction Both need to happen… electron is generated at the anode and must be consumed at the cathode so net charge is conserved in overall process Ions dissolve into solution Ions deposited from solution These are called Half-Cell reactions We need a salt bridge to complete the circuit so that the charge remains balanced  otherwise, the charge would build up and the battery would stop working

In the process of oxidation and reduction, energy is converted from chemical into electrical i.e. Electrons are free to run through the circuit and do work Voltage of the cell is determined by the oxidized and reduced species and related to the change in free energy But they must go through the external wires Can think of this as analogous to water flowing downhill where the voltage is the height of the hill Zn can lower its energy by giving up electrons and dissolving into solution Cu + can lower its energy by capturing electron and ‘plating’ out on electrode In previous example, Zn  Zn +2 + 2 e - Cu +2 + 2e -  Cu +0.763 V +0.337 V 1.10 V

The important thing here is that every material has a slightly different potential

Just because a reaction is energetically favorable, that doesn’t mean we know how fast it will occur. The rate depends on kinetics It’s got to depend on: Temperature Voltage Concentration or pH Let’s consider a single electrode of Zn We know that oxidation and reduction can occur according to Zn  Zn 2+ + 2 e - Furthermore, we know that if this is in equilibrium, then the forward and back reactions must be occurring at the same rate Lets define a current density i = I/A

Then, i f = i b = i o Where i o is called the exchange current density , sometimes denoted i e If we apply a voltage to this electrode, we will shift the reaction one direction or the other i f – i b = i  0 This condition can be described by the Butler-Volmer equation Where  a and  c are called the anodic and cathodic transfer coefficents  s is called the surface overpotential or polarization  s = V-V 0 * This expression for the B-V equation assumes a one electron process. Typically it is safe to use this equation since most reactions involving multiple electrons also involve multiple intermediary steps which involve a single electron and for current, we only have to worry about the slowest reaction. The details of this is way beyond the scope of this introductory course. When we don’t know, we usually assume  a and  c ~ 0.5 The exchange current tells us how fast a reaction can occur There are other prefactors that I have omitted

We notice the similarity of this equation to that for diffusion Also, note the sign convention that the forward current direction denotes an anodic reaction i.e. i > 0 oxidation is occurring (corrosion) i < 0, reduction is occurring (deposition)  a =  c = 0.5  a >  c

Overpotential is a term that tells us how far from equilibrium we are Activation Overpotential Tafel (1905) observed:  =a+b log i If we do some math on the B-V equation, we can see where this comes from Consider larger overpotentials so that one term dominates the B-V equation (lets choose anodic term) It’s a driving force for transport (just like chemical potential) So we can interpret the B-V equation as telling us the amount of current we get from a reaction given a certain electrode potential

So, we can plot this in a different way to find  , i o , and V 0 Rearranging, we find or If we extrapolate the linear regime for both anodic and cathodic, the intersection will be at the equilibrium voltage and exchange current density Tafel Plot Voltage (V) Ln | i | (mA/cm 2 ) V 0 I o anodic cathodic RT/  a F RT/  c F

Other Overpotentials In reality, the Tafel equations are only valid in a small regime of potential At potentials close to the equilibrium potential, the slope decreases as reverse reactions become important At potentials much higher, the current becomes limited as the transport properties in the electrolyte limit the reaction  Concentration overpotential The limiting current depends on the concentration in solution and could be increased significantly by mixing electrolyte

Fig 17.5W from Callister So depending on the overpotential, we may need to take concentration into account Also, don’t forget about resistive losses in the system as well

Electrochemical Energy Storage Batteries are a compact method of converting chemical energy into electrical energy Other methods, fuel cell, photovoltaic cell, electrochemical capacitors etc. All work the same, but the details are different C-rate  charging/discharging rate, 1C is current needed to discharge in 1 hour Anode (Oxidation): Zn + 2 OH -  Zn(OH) 2 + 2e - E = 1.25 V Ag 2 O + H 2 O + 2e -  2 Ag + 2 OH - E = 0.34 V Cathode (Reduction): e - e - e - e - e - e - Anode Cathode Electrolyte/Separator Current Collectors 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)

So this raises a potential problem with the Volta Pile If Zn is the anode and H is the cathode, there is an evolution of hydrogen gas that could passivate the electrode or generate a large resistance to flow Solution is to use a wet cell that has the proper ions around (1836) Daniel Cell: Cu/Zn in sulfate solutions (1859) Plante: Pb/PbSO 4 cell Wet cells have an obvious disadvantage for transport, but they do work well

But actually, a wet cell battery may have existed well before Volta Archeologists found this clay pot in the Baghdad area in the 1930’s Carbon dating places it ~250 BC How does this work?

Leclanche in 1866 developed another kind of wet cell that had a better shelf life and was less reactive with the environment This battery was constructed with Zn as the anode and MnO 2 + C as the cathode The cathode was mixed into a paste and placed in a porous pot The Zn anode was immersed in a chloride electrolyte Very popular with the new Telegraph !!

Other variations came about as well Different materials: Ni/Cd (Junger) 1899 Different electrolyte: Alkaline (Edison) 1911, “dry” cell (Gassner) 1888 They all pretty much looked about the same, but some were better than others Lifetime Capacity Marketing Convenience Rechargeability Edison Cell Ni/Fe in KOH electrolyte Edison was actually trying to make a battery for the Automobile, but gas engines were too good

Modern Batteries Modern Button Cell

Battery Types Battery chemistry has not changed much since 1800’s Leclanché Alkaline Silver-Oxide Rechargeable Alkaline 1866, 1888 1949 1950 1978 Zn-MnO 2 Zn-MnO 2 Zn-Ag 2 O Zn-MnO 2 Lead-acid 1859 Pb-PbSO 4 Ni- based NiCad Metal Hydride 1899 1990 Cd-NiOOH MH-NiOOH Li- based Lithium-Iodine Lithium ion Plastic/Polymer 1968 1991 1995 Li-I 2 Li-LiCoO 2 Li-LiCoO 2

How does a Li-ion battery work? Li ions intercalate into the crystal structure of the electrode materials But Li is very reactive (high V) which causes side reactions and passivation layers (solid electrolyte interphase)  Critical to the proper functioning of these cells More advanced topic

So what makes a battery rechargeable ? In reality, as long as the electrochemical reaction is reversible, the battery should be rechargeable However, other effects are important Decay of structural properties as ions move in and out of electrodes Growth of metal on electrodes Decay/contamination of electrolyte Cost Power signature Lead-acid works well

Impedance Matching So faced with all these choices, how do we choose an appropriate power source? The optimal power transfer to the load will occur when the impedance (resistance) is equal to the internal resistance Can derive this from Ohm’s laws However, we would not typically run a battery at this current as it would heat up too much

Ragone relation Energy density  Energy per unit area/volume Specific Energy  Energy per unit mass Other issues such as total weight, size, voltage, environmental concerns can limit our selection process In general, electrochemical systems always show the characteristically downward curved plots High power  Low energy High energy  Low power One might think that it is best to just push the limits of energy or power http://www.powerstream.comz/ragone.gif

Rate effects Another major factor in optimizing energy storage is the rate at which the energy is needed As with all batteries, there is a decrease in capacity (efficiency) as the power is increased But many application for batteries do not require extended periods of high power draw Energy demands differ from application to application Main limiting factor is rate of ion movement across electrolyte Duracell ‘D’ cell

Although standard specs can be reported, there are a variety of important issues that ultimately affect the lifetime and performance of a battery system for a desired application Current Drain: Different batteries respond differently to current In general, as current is increased, the available voltage and capacity decrease Peukert’s equation: I n x t = C I is current in A t is time in hr C is rated capacity Modified Peukert’s law But since C depends on rate, one must correct for this Where H is the hours for the rated capacity Handbook of Batteries 3e, Eds Linden and Reddy Rate effects

Different chemistries will have different voltages and different characteristic discharge curves  Appropriate choice will depend on application limitations For instance, Silver Oxide batteries have a very flat discharge at 1.5 volts compared to Li-ion batteries with a sloping curve around 3 V Cut-off voltage of the device is an important parameter as it will determine the actual capacity that can be used Handbook of Batteries 3e, Eds Linden and Reddy Discharge Characteristics

Batteries operated in pulsed applications will last longer than constant discharge at same current During rest time, battery recovers voltage  more of the theoretical capacity can be used Factors such as maximum current and duty cycle will have a profound effect on this issue Pulsing between high and low current (e.g. transmit/receive operations) will have a similar effect  voltage will oscillate Voltage response to pulse will vary with chemistry Alkaline Zn-C High current pulsed can lead to catastrophic failure Handbook of Batteries 3e, Eds Linden and Reddy Pulsed Discharges

Effects of temperature Discharge mode (constant power, current, load) Self discharge Depth of discharge (for secondary) We mentioned Current drain Shape of discharge curve Pulsed operation Other considerations include: Handbook of Batteries 3e, Eds Linden and Reddy Other Considerations

An ultracapacitor has properties of both battery and capacitor It has a high power density and can be cycled like a capacitor But it also has a significant energy density like a battery Different Flavors: carbon, transition metal oxides Ionic conduction in liquid or solid-state electrolyte Electronically conductive electrodes Electrochemical capacitors Supercapacitors pseudocapacitors First ultracapacitor was patented in 1957 using porous carbon electrodes Require high surface area electrodes to achieve large capacitance Supercapacitors Double Layer: Charge stored in double layer at interface between electrolyte and electrode Pseudocapacitance: Faradaic charge storage at electrode surface ultracapacitors Capacitors Batteries Electrode Electrode Electrolyte

One could envision a supercapacitor for transmitting information, but one must take care not to have too large a duty cycle or it will not sufficiently recharge Supercapacitors have certain advantages over traditional batteries for tracking applications Red line shows results from our work Supercapacitors Lower energy density Shorter shelf life Higher self-discharge Cost Cons Greater cycle life Better reversibility  low capacity fade Higher power Rapid charging discharging Pros

PECS Talk -- Batteries

More Related Content

What's hot

Viewers also liked

Similar to PECS Talk -- Batteries

More from Princeton University Energy & Climate Scholars

Recently uploaded

PECS Talk -- Batteries

Editor's Notes