1 Battery Technology and Markets, Spring January 2010 Lecture 1: Introduction to Electrochemistry

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1 1 Battery Technology and Markets, Sprng 2010 Lecture 1: Introducton to Electrochemstry 1. Defnton of battery 2. Energy storage devce: voltage and capacty 3. Descrpton of electrochemcal cell and standard cell potental 5. Calculatng capacty- Faraday s Law Approach to ths part of the course In the frst part of ths course, about four weeks, we ll be studyng and learnng about basc concepts nvolved n electrochemstry. We recognze that people are comng from very dfferent backgrounds n ths course, and so what s already famlar to some students may be very new to others. The dea s that n ths part of the course, you ll become famlar wth the basc physcs nvolved n batteres. If you ever are havng problems wth the materal because we are assumng background knowledge that you don t have, come talk to us. If you fnd that you are nterested n ths subject and want to learn n much more detal, we hope that ths class wll functon as a good prmer for Professor Newman s class n the fall. 1. Defnton of batteres The frst thng we re gong to do s to defne a battery as an electrochemcal storage devce- somethng that stores electrcty n chemcal bonds. Ths s a functonal defnton- you can have batteres made of many dfferent systems because lke the battery s defned n terms of what t does and not the materals that make t up. Ths means you encounter many dfferent materal systems (Lead oxde / Lead sulfate, Graphte / Lthum Iron Phosphate, Metal hydrde / Nckel hydroxde). A smlar example s a solar cell. A solar cell captures sunlght and turns t nto electrcty, and can be made any of a number of materals- slcon or organcs, GaAs. Man dea of electrochemcal devces s that chemcal energy s converted drectly to electrcal energy. Chemcal energy can be converted to other forms as well; for example, n combuston processes, the chemcal energy of combuston s converted to mechancal energy by an engne. Or, n nuclear power plants, the chemcal energy of nuclear fsson s converted to heat and then to electrcty. Electrochemcal devces are unque n that they convert chemcal energy drectly to electrcal energy. Because electrochemcal processes do not nvolve the transfer of heat, Carnot lmtatons are avoded and processes can be very effcent. For example, the round-trp

2 2 Battery Technology and Markets, Sprng 2010 energy effcency (for example, the amount of energy you get out of battery compared to the amount of energy used to charge t) for a battery s typcally greater than 85%. 2. The energy storage devce- some basc defntons Batteres are energy storage devces. What physcal propertes do we care about for energy storage? Energy, power, mass, and volume. Energy s a fundamental quantty and has unts of Joules. Power s energy/tme, and has unts of Watts, whch s Joules / second 1 Wh = 3600 J. Mass and volume usually, but not always, go together (thnk: automotve versus mcrodevce applcatons. Are mass and volume equally mportant?). We use specfc energy for energy per unt mass and energy densty for energy per unt volume. We can talk about power densty and specfc power as well. These four propertes are not specfc to batteres, but rather general for all energy storage systems: lqud fuels, thermal, etc. Let s do an example of how to calculate the energy that s contaned n a consumer battery. Crack open the cell phone you have n your pocket what are the two numbers that are gven on the battery? Voltage and capacty. These propertes are specfc to electrochemcal energy storage systems (.e. batteres). Voltage s the electrc potental energy, and has unts of Volts. A volt s equvalent to J/C. Capacty s a measure of an amount of electrc charge, and has unts of Coulombs. In battery lterature, we often use a dfferent unt of Amp-hours nstead of Coulombs. 1 Ah 1 C/s*3600 s/hr = 3600C. Despte the smlar-soundng name, capacty s NOT the same thng as capactance. Fuel cells and batteres dffer greatly n that capacty s specfed for a battery but not for a fuel cell. Why s that? Quck example: f you are gven the voltage and capacty of a battery, how would you calculate the energy of the battery? Say you have a battery wth 3.7 V and 1000 mah. 3.7 J/C * 3600 C = J = 13.3 kj = 3.7 Wh How much does your battery wegh and cost? For comparson, the combuston of a gallon of gasolne releases kwh; you would need around 10,000 of those batteres to match the energy n a gallon of gasolne.

3 3 Battery Technology and Markets, Sprng 2010 What s the power of your battery? From basc crcuts: P = IV. Why sn t the current wrtten on the battery? It s complcated, but the short answer s that I = V/R, and nether of those s constant- they depend on rate, temperature, hstory, and other factors. 3. Descrpton of the electrochemcal cell, wth examples So we ve just seen the propertes we care about n our battery- power and energy, whch are determned by voltage and capacty. How are those two numbers determned for any gven system? The basc framework for any battery s the electrochemcal cell. Every electrochemcal cell s made up of the followng components: Anode, cathode, electrolyte, separator, external crcut. Typcally a cell also ncludes a current collector. [go to Danell cell handout, drawng] Ths s an mage of the Danell cell. The Danell cell was nvented n the 1830s by the Brtsh chemst Danell, and had some commercal success n telegraph systems. An electrochemcal cell s a set of reactons, each nvolvng an electron, that are separated by an electroncally nsulatng layer. Ths s really mportant- you cannot measure a voltage wthout havng two reactons nvolvng electrons. So f you are ever dong any problem nvolvng electrochemstry, your frst step n understandng what s gong on s almost always to defne the reactons. Current flows n the form of electrons through an external crcut, and n the form of ons through the separator. Therefore, the current must change phases as t moves through the electrodes. Current s movement of charge, not electrons, lke we re used to thnkng of- moble ons or protons can be charge carrers nstead. For every electron that goes through the external crcut, we need to balance wth onc charge that goes through the battery. Ths wll mean understandng: thermodynamcs, for the drvng force behnd the reactons; transport, for the movement of ons nsde the battery; and knetcs, for the rates of reactons that occur at the electrodes. The smultaneous presence of all these phenomena s what makes batteres so complcated, and, for some people, so nterestng. Here are some mportant defntons:

4 4 Battery Technology and Markets, Sprng 2010 Anode: an electrode at whch oxdaton occurs; electrons are produced. We also use the phrase anodc reacton. Cathode: an electrode at whch reducton occurs; electrons are consumed. We also use the phrase cathodc reacton. Electrolyte: a phase that has moble ons to carry current. An electrolyte typcally contans a solvent (such as water) n addton to a salt (such as KOH). An excepton to the solvent/salt system s somethng lke Nafon. Separator: a regon that separates the two electrodes, and whch s electroncally nsulatng. A separator may also help provde mechancal rgdty to a cell. Postve electrode: Electrode that s at a more postve electrc potental. Electrons move spontaneously towards more postve potentals. Negatve electrode: Electrode that s at a more negatve electrc potental. Galvanc cell s one n whch energy s spontaneously produced by the reactons. (Dschargng a battery) Electrolytc cell s one n whch energy s consumed to drve the reactons. (Chargng a battery) We also have an external crcut, and a current collector. Sometmes the current collectors are separate from the electrode, but here they are the electrodes. Practce queston: When swtch from chargng to dschargng a battery, what changes? The locaton of the anode and cathode, or the locaton of the postve and negatve electrodes? Why don t electrons always flow towards the postve electrode? Lttle bt of nomenclature about batteres and cells- battery s techncally made up of several electrochemcal cells. Applcable- many batteres you buy are more than one cell wred n ether seres or parallel (e.g. car, laptop, 9V). For our purposes, we talk about batteres as the applcaton (and thus busness, polcy, manufacturng), cells when referrng to the unversal physcs. If you go nto other areas of electrochemstry, you ll fnd electrochemcal cells used for all knds of thngs besdes energy storage. Trva fact- nventor of nomenclature was Ben Frankln, n analogy to a battery of cannons. What s the voltage of the cell? It wll depend on the dentty of the reactons. In the cell above we have the reactons

5 5 Battery Technology and Markets, Sprng 2010 Zn Zn Cu e e Cu Dependng on whch drecton the reacton goes, t can be ether anodc or cathodc. Note that f we add these reactons together we get Zn + CuSO 4 Cu + ZnSO 4 Ths shows that the overall reacton s chemcal, and that electrons serve as an ntermedary n the process. Every reacton has a potental assocated wth t, whch s a measure of the energy of the change that occurs when the reacton proceeds. We ll dscuss more about ths next lecture. A voltage really measures an energy dfference; the reactons n tables of standard reducton potentals are typcally gven relatve to a standard hydrogen electrode (SHE), whch s an acdc soluton at unt concentraton at 25 C and 1 atm. The potental of a cell s defned as E 0 cell = E 0 cathode E 0 anode The symbol 0 refers to the standard condtons at whch these measurements are made; these are 25 C and 1 atm. We can look up the potental of ths reacton n a table that gves standard potentals. E 0 cell = 0.34 ( 0.76) = 1.10V We can get these values from Appendx B of Lnden. Note that the values n ths table are equlbrum potentals. Roughly speakng, an equlbrum potental s the potental at whch the rate of the forward and reverse reactons are equal. As wrtten n Appendx B of Lnden, f the actual potental s hgher than the equlbrum potental, the reverse reacton wll be favored, whle f the actual potental s lower than the equlbrum potental, the forward reacton wll be favored. Knowng how to look up the standard electrode potental s an mportant skll. Let s do another example here. What are the reactons n a lthum (not lthum-on) battery? L L + + e LCoO e L 0.5CoO + 0.5L E 0 cell = 0.7 ( 3.01) = 3.71V

6 6 Battery Technology and Markets, Sprng Calculatng the capacty of a battery So far we ve covered half of what you ll fnd on your cell phone battery: the voltage. What about the other part, the capacty of the battery? In partcular, how do we relate the sze of the batteres and the chemcals that are nsde t wth the amount of charge that t can store? One Ah s defned as one amp that s passed for one hour. An Ah has the same unts as the Coulomb, the fundamental unt of electrcal charge. One Coulomb s the charge of 6.24 x electrons. 1 Ah = 1 Amp*hour = 1 C/s *3600s = 3600C Note that the amount of electrcal charged that s passed can be drectly related to the change n the amount of chemcals n the system by the use of Faraday s law. N = s It nf Here are defntons: N = Number of moles of speces s = stochometrc coeffcent of speces (see conventon below) I = current t = tme n = number of electrons nvolved n the reacton F = Faraday s constant, wth value C/mole. Equal to Avogadro s number * charge of electron. The conventon that we use to defne the value of the stochometrc coeffcents s s M z ne Here, M refers to the dentty of speces, and z s the charge number of that speces. Example: Zn Zn e s Zn = 1, z Zn = 0, s Zn 2+ = -1, z Zn 2+ = 2, n = 2 Another example s water splttng. + O 2 + 4H + 4e 2H 2O s O2 = -1, z O2 = 0, s H+ = -4, z H+ = 1, S H2O = 2, z H2O = 0, n = 4

7 7 Battery Technology and Markets, Sprng 2010 Note that Faraday s law s nothng more than a relaton between the number of electrons and the number of moles n an electrochemcal reacton: f a certan number of electrons have been passed, that means a certan number of moles have passed. Let s do an example wth our Danell cell. Assume we have 1 gm each of znc. What s the capacty of the znc electrode? 1 gm Zn = mol Zn nfn It = s Zn C mol = mol = 0. C 3600 Ah Zn 82 Ah What s the specfc capacty of the znc electrode? 0.82 Ah/ 1gm = 820 mah/gm. If we dd somethng mpossble that halved the molecular of znc so that only 0.5 gm were needed for 0.82 Ah, what would happen to the specfc capacty of the entre battery? What does that mply for research on hgh energy-densty battery materals? You should be able to use ths procedure to calculate all the capacty values n Table 1.1 n Lnden. So, to summarze: we talked about the defnton of a battery, that t s defned not by what t s made of but by what t does, store electrcty va chemcal reactons. Two key propertes for energy storage are voltage and capacty. To fgure out how those are determned for a battery, we ntroduced the electrochemcal cell and some terms for t- anode, cathode, etc. Voltage- comes from what the reactons are (whch chemcals), more next lecture. Capacty- comes from reacton rate, and the formula used for ths s Faraday s Law.