电化学Chapter5(1)

1、 A battery is a device which can store chemical energy and, on demand, convert it into electrical energy to drive an external circuit. The electrical energy results from a spontaneous chemical change within the battery. Development of batteries: 1800 Volta cell 1836 Daniell cell Generator 5.1 Introd

2、uction 5.1.1 Classification of chemical power sources (1)Primarycell It ban not be recharged. The cell reactions are irreversible. e.g. Zn NH4Cl, ZnCl2 MnO2 Zn KOH HgO Zn KOH Ag2O (2)Secondarycell/battery Cell reactions are reversible and can be recharged.e.g. Pb H2SO4 PbO2 Cd KOH NiOOH Zn KOH Ag2O

3、(3)Reservecell/battery stored dry and activated before usee.g. Mg MgCl2 AgCl Zn KOH Ag2O (4)Fuelcell Active mass are not stored within the cell. H2 KOH O2 N2H4 KOH O2 5.1.2 Battery components The performance of the battery will depend on the cell geometry and on the design and composition of all the

4、 components of the battery, in addition to the choice of electrode reactions and their kinetics. 1. The container Requirements: Resistant to corrosion (stable) The required mechanical strength Cheap and light Simple of sealing For alkaline batteries: steel For systems with an acid electrolyte: polyp

5、ropylene 2. SeparatorsRequirements for separators: (1) stable chemically to the electrolyte and to the active materials (2) low resistance to the transport of ions (3) mechanical strength, flexibility, wetability (4) cheap and readily available Materials: such as plastic pins, nylon or cellulose-bas

6、ed felt, ion exchange membranes etc. Form: microporous (pore size 0.01-10 um) or macroporous (30-70um pores) polymer sheets 3. Current collectors Function: a) provide a conducting path through the paste and thereby minimize the resistance of the battery b) act as a physical support for the active ma

7、ss which otherwise would be a very brittle structure. Form: plate , closely spaced grid, thin metal sheet, expanded metal Corrosion of the collector and shedding of the active paste are two major causes of battery failure. 4. Electrolyte The selection of the electrolyte is determined by the electrod

8、e reactions and its concentration is also important. Temperature has a great effect on electrolyte properties. The weight of the electrolyte is a major contribution to that of the complete battery and, hence, must be minimized. 5. Active materialsRequirements: a) high electromotive force of the cell

9、 b) more active electrochemical reaction c) large specific capacity d) stable chemically to electrolyte e) good electronic conductivity f) cheap and readily available Porous electrodes are commonly used to increase the surface area between the solid reactants and the electro-lyte. The porous electro

10、de is a mass of particulate re-actants with many random and tortuous electrolyte channels between. The sizes of the particles and the pores (i.e. the porosity of the paste) are important in determining the performance of the battery. In practice, porosity should be about 50%. 5.2 Battery characteris

11、tics 5.2.1 Voltage 1. Terminal voltage and electromotive force Electromotive force AeCeCELLEEE Open circuit voltage Vopen ECELL Operation voltage (discharge voltage) VCELL VCELLVopen The terminal voltage of the battery will depend on the free-energy change in the overall cell reaction and, hence, th

12、e choice of electrode reactions, the kinetics of the electrode reactions .CELLACAeCeCELLiREEV The cell voltage, excluding the iRCELL of a practical battery, can be estimated from i-E data for the anode and cathode processes. 2. Cell resistanceOperation voltage can be expressed as: )(fCELLRRiEiREVwhe

13、re RCELL is the total electric resistance of the battery, R，Rf are the ohm resistance and polarization resistance respectively. The battery designers concern is to make the voltage large and positive. How to do? (1) the selection of electrode reactions which lead to an overall cell reaction with a h

14、igh negative free energy change; (2) electrode reactions without large overpotentials in the range of current to be drawn from the cell (i.e. the electrode reaction should be fast); (3) designing the cell with a low resistance, i.e. with a high conductivity electrolyte, low resistance separator and

15、small interelectrode gap. The major contribution will normally be from the charge transfer overpotential since mass transport control has a catastrophic effect on the battery voltage and one would not normally design a battery to operate in such conditions. Examples of nucleation and passivation ove

16、rpotentials do occur. The nucleation overpotential is normally a transitory phenomenon since, once nuclei of the new phase are present in numbers, the overpotential will disappear. The nucleation overpotential therefore occurs as a dip on the discharge curve at the commencement of discharge. 3. Disc

17、harge system (放电制度放电制度) Discharge conditions: Stoppingvoltage(终止或截止电压终止或截止电压): the minimum voltage Modesofdischarge: a) Discharge into fixed resistance b) Discharge (and charge) at constant currents c) Discharge at constant wattages continuous discharge and intermittent discharge po5.2.2 Current and

18、 discharge rate Current is a measure of the rate at which the battery is discharging. The ability to deliver a high current without an excessive voltage penalty is dependent on rapid electron transfer reactions and correct design of the active material to ensure a plentiful supply of electroactive s

19、pecies to the site where the electron transfer is occurring. The discharge rate is, as is current, a measure of the rate at which charge is drawn from the cell. It is normally quoted as the C/n or n-hourrate, which is the current to discharge the nominal capacity C of the battery in n hours. 倍率指电池在规

20、定的时间内放出其额定容量时所输出的电流值，它在数值上等于额定容量的倍数。例如，2倍率(2C)放电，则表示放电电流的数值是额定容量的2倍。 e.g. C=3Ah 2倍率放电，i=236A. hour rate: 3Ah/6A=0.5 hour rate 5.2.3 Capacity The capacity is the charge that may be obtained from the battery. It is usually quoted in ampere hours and clearly depends on the size of the battery. Nominal

21、capacity （额定容量） 1.TheoreticalcapacityCo The theoretical capacity of each electrode may be calculated from the weight w of active material via Faradays law Co=wnF/M=w/K where M is the molar mass of the active material. The capacity of the battery will be determined by the electrode with lower capacit

22、y. 2.PracticalcapacityC Discharge at constant currentC=i Discharge into fixed resistance 001VdtRidtC The practical capacity is determined by the quantity and use factor/utilization (利用率) of the active materials. RC/Co Certainly the capacity will depend on the discharge conditions (e.g. current) and

23、commonly it is measured by monitoring voltage vs. time during a fixed current discharge. The capacity is then i where is the time at the fixed current i for the voltage to reach a value Vmin where the battery is no longer useful. The use factor of active mass (current efficiency) R3.Specificcapacity

24、/capacitydensityC C=C/W or C=C/Vwhere W, V are the total weight and volume of a battery respectively.A high storage density depends on good design (minimizing the weight of all subsidiary elements) and also the correct selection of electrode reactions. The popularity in recent years of lithium anode

25、s is due partly to the low weight of a mole of the metal. 5.2.4 Energy density Energy density is the energy obtainable per unit weight of battery, (in, for example, kilowatt hours per kilogram). 1.TheoreticalenergydensitySEo SEo=CoE/W=E/Ke.g. Lead-acid battery, the cell reaction: Pb + PbO2 + 2H2SO4

26、2PbSO4 + 2H2O Electrochemical equivalent K:Total K=KPb + KPbO + KH2SO4 =3.866+4.463+3.659=12 gAh-1Electromotive force E=2.044Vso SEo=2.044/0.012=170.5 Wh/kg 2.PracticalenergydensitySE Practical specific energy is the energy obtainable per unit weight of battery. SE=CV/W where V is the average voltag

27、e of the battery. SE=SEoVRw (1) Voltage efficiencyV V=V/E (2) Use factor (or reaction efficiency)R (3) Weight efficiencyw The weight of active materials per unit weight of battery.In general, SE/SEo=1/31/5High energy cell: SE80 Wh/kg 5.2.5 Power density Power density (specific power) is the capabili

28、ty to deliver power per unit weight of battery, W/kg (or W/L). 1.TheoreticalpowerdensitySPo SPo=SEo/t=CoE/Wt=iE/W 2.PracticalpowerdensitySP The power of battery P=iV=i(E-iRcell)=iE-i2Rcell We can get the conditions to deliver the maximum power from above equation dP/di=0 Rin.=Rout. The power density

29、 is a measure of the ability for battery to discharge with large current. 5.2.6 Cycle life and shelf life 1.Cyclelife The cycle life is the number of charge/discharge cycles that are possible before failure occurs. The most common forms of failure include: a) corrosion of the current collectors or c

30、ontacts; b) shedding of the active material from the plates; c) shorting due to dendrites growing between the electrodes; d) changes in morphology 2.Shelflife for primary cells selfdischarge: reactions between the anode and cathode active materials or between either and the solvent electrolyte. 5.2.

31、7 Energy efficiency energy released on discharge% energy efficiency= energy required for charge This will depend on the current efficiency of the electrode processes and the overpotential involved in both discharge and charge reactions as well as the battery resistance. Again, it will depend on the

32、rate of charge and discharge. 5.2.8 Others tolerance to service conditions reliability economic factors 课程作业：文献综述或总结课程作业：文献综述或总结题目：自选（与电池有关内容）题目：自选（与电池有关内容）要求：要求：1、英文、英文 2、Word文档（文档（3000字以上）字以上） 3、PowerPoint（30张以上）张以上）时间：时间：16周以前周以前5.3 Present battery systems 5.3.1 Zinc-manganese cells 锌锰电池型号表示方法锌锰电

33、池型号表示方法: 根据一次电池型号命名方法根据一次电池型号命名方法 R圆形圆形 S方形方形 F扁形扁形 同一外型不同规格者编成相应的序号，以阿拉伯数字表示。同一外型不同规格者编成相应的序号，以阿拉伯数字表示。 如如R20，其，其R表示圆筒形锌锰电池，表示圆筒形锌锰电池，20表示电池大小的顺序，表示电池大小的顺序，其尺寸可查表。其尺寸可查表。 电池的外形尺寸越大，序号就越大。电池的外形尺寸越大，序号就越大。 1. Leclanche cells (1) Battery components (-) ZnNH4Cl, ZnCl2MnO2, C (+) Positive electrode (cat

34、hode): MnO2 pasteNegative electrode (anode): ZnElectrolyte: moist NH4Cl+ZnCl2+ MnO2+C powder Current collectors: Graphite and Zn (2) The cell reactions Despite the long history of manufacture its detailed electrochemistry is again far from understood. (+) MnO2 (s)+ 2H2O + e MnO(OH) (s) + OH- The cur

35、rent-voltage characteristics depend on the source of the MnO2 and more directly the exact oxidation state of the manganese, the density of the lattice imperfections, the crystallite size and the extent of hydration. (-) Zn Zn2+ + 2e 2NH4Cl + Zn2+ Zn(NH3)2Cl2 + 2H+ The overall cell reaction: 2MnO2 (s

36、)+ 2NH4Cl + Zn 2MnO(OH) (s) + Zn(NH3)2Cl2 The open circuit potential measured for the completed cell is frequently higher than that estimated on the basis of the cell reactions, and, hence, it is clear that they do not faithfully represent the reaction which occurs. (3) The performance of Leclanche

37、cell A)Operationvoltage The open circuit voltageVopen=E+-E- For zinc electrode, E=-0.8V For MnO2 electrode, E=0.71.0V In general, Vopen=1.51.8V (higher than the estimated electromotive force E=1.55V) The operation voltage V is related to the discharge system. I， V Discussion: a) The diffusion of H+

38、is low in the solid, so as to increase the concentration polarization, hence, V b) At the negative, the product Zn2+ react with OH- to produce insoluble Zn(OH)2, that increase the resistance. B)Capacityandenergydensity The nominal capacity of dry Leclanche cells is commonly 0.05500Ah. Practical capa

39、city: high current discharge, 20% C Low current discharge C If E=1.5V, we get SEo=232Wh/kg SE=5570Wh/kg C)OthersSerious self-dischargelow SPBad low temperature performance2. Alkaline battery (-) Zn KOH MnO2 (+)Positive electrode: MnO2 powderNegative electrode: zinc powder amalgamated with mercury (H

40、gO 14%, CMC 1%) (-) Zn + 2OH- Zn(OH)2 + 2e(+) MnO2 + H2O + e MnO(OH) + OH-Zn + 2MnO2 + 2H2O Zn(OH)2 + MnO(OH) The cell reactions: 5.3.2 Lead-acid batteries 1. Battery components and cell reactions (1) Battery components (-) Pb H2SO4 PbO2 (+) Positive active material: PbO2Negative active material: sp

41、ongy lead Electrolyte: aqueous H2SO4 (1.251.28g/cm3) Current collectors: both Pb Reversible cell potential=2.05V (2) Cell reactions (+) (cathode): PbO2 + 4H+ + SO42- + 2e 2H2O + PbSO4 (-) (anode): Pb + SO42- PbSO4 + 2e Overall reaction: PbO2 + Pb + 4H+ + SO42- 2H2O + 2PbSO42. Performance of lead-aci

42、d battery The performance of SLI (starting, lighting and ignition) batteries has been characterized extensively. The capacity-discharge rate characteristics will be known as a function of temperature, plate thickness, type of separator and sulphuric acid concentration as will the variation of voltag

43、e and power as a function of current at various stages of charge. Overcharge: water electrolysis Nominal voltage of unit battery: 2.0V Energy density: 2040wh/kg energy efficiency: 7080% Cycle life: 250400 e.g. Car batteries A starting, lighting and ignition battery will be a 6 or 12V (three or six c

44、ells in series) with the following characteristics: (1) a capacity of 100Ah at the 20-h rate; (2) a high pulse capability to permit engine starting, typically 400-500A for 30s without the voltage dropping below 7.2V; (3) the ability to provide a low current for an extended period (e.g. 25A for 3h wi

45、thout the voltage dropping below 10.5V); (4) multiple charge/discharge cycles 3. Preparation of lead-acid batteries (1) The current collections are die-cast grids made from an alloy of lead usually Pb-5% Sb. The alloying elements is present to improve the die casting and mechanical properties of the

46、 metal. (2) The electroactive pastes are prepared from pure lead. Lead ingots are air oxidized under controlled conditions to give a powder mixture of lead oxide and lead (approximately 50:50). (3) To the paste to be used for the negative electrode (spongy lead) are added various additives, most com

47、monly carbon powder (0.25%), lignin sulphonates (0.2%) and barium sulphate (0.35%). The carbon improves the conductivity of the final plate while the latter compounds prevent the reduction in surface area of the lead and other changes in morphology of the plate, i.e. they are expanders. (4) Traction

48、 batteries are constructed with both the flat plate and the tubular positive plate design. The advantage of the tubular anode is that shedding of the positive active material is not possible and, hence, the cycle life is likely to be improved. 4. Maintenance-free sealed batteries This requires prote

49、ction against overcharge; on overcharge, water electrolysis will occur and oxygen and hydrogen will be formed. The approach is to build a catalyst for the recombination of hydrogen and oxygen into the cell. 5.3.3 Nickel-cadmium batteries Development of Ni-Cd batteries:1899 JungnerBefore 1950s pocket

50、-plate 1950s-1960s sintered-plateAfter 1960s sealed batteries NiCd batteries show the outstanding features of long durability and excellent high power performance, and very quick discharging and charging characteristic. Another advantage of NiCd batteries is the wide variety of the product line up w

51、hich permits a diversity of applications. Moreover, various kinds of purpose-specific batteries have been developed, each of which features high performance optimized for a focused application. 1. Battery components and cell reactions Battery components (-) Cd KOH (or NaOH) NiOOH (+) Negative electr

52、ode: sponge cadmium Positive electrode: Nickel oxide Electrolyte: aqueous KOH Current collectors: Ni and CdCell reactions:+(cathode) NiO(OH) + H2O + e Ni(OH)2 + OH-(anode) Cd + 2OH- Cd(OH)2 + 2eOverall reaction 2NiO(OH) + Cd + 2H2O 2Ni(OH)2 + Cd(OH)2 Reversible cell potential = 1.48V In the nickel o

53、xide paste, the oxidation state of the oxidized nickel species is uncertain and varies between +2 and +4; both the oxidized and reduced species exist in several crystal modifications and the important of water and potassium ions are not included in above equations. Two different kinds of electrode :

54、 sinterednon-sintered. The sintered electrode has a nickel substrate involving innumerable fine pores (several mm diameter), which are filled with an active material composed of mostly Ni(OH)2 or Cd(OH)2 prepared by chemical or electrochemical conversion from aqueous nitrate solutions. Owing to its

55、high electrical conductivity, the sintered electrode can display a large current discharge characteristic and quick charge characteristic. The nonsintered positive electrode is manufactured by filling directly a foam-type nickel substrate with an active material that is granular Ni(OH)2 powder. The

56、non-sintered negative electrode is obtained by coating a Ni-plated pierced steel sheet with an active material paste composed mostly of cadmium oxide. The non-sintered electrode is advantageous in attaining a higher capacity, but is inferior to the sintered electrode in terms of electrical conductiv

57、ity. 2.The performance of the pocket-plate Ni-Cd batteries Pocket-plate designs are known for their reliability and very long shelf life (20 yr) without any significant maintenance; hence, they are ideal for emergency power supplies and are also used for train lighting, switchgear and engine startin

58、g. Where there is direct overlap in applications with Pb-acid batteries, the Ni-Cd alternative often gives better performance but is also more expensive. The chief advantages of the pocket-plate batteries: (1) the ability to retain charge during long periods of storage; (2) to deliver all their rate

59、d charge; (3) to maintain a steady voltage over a wide range of discharge rates and temperatures. In addition they have long cycle lives. The energy density is, however, only moderate and lies in the range 15-25 Wh/kg. (1) Voltage Vopen =1.401.35V (2) Reaction efficiency R Theoretical capacity Co =1

60、61.6Ah/kg In general, (+) R =70% (-) R=7585% Influencing factors: design of the cell structure of the electrodes composition of electrolyte Additive of electrolyte: LiOH (3) Self-dischargeSelf-discharge of Ni-Cd battery is small. Why? Cd electrode : stable in the electrolyte3. Production of Ni-Cd ba