15. Energy Applications I: Batteries

What are Batteries, Fuel Cells, and Supercapacitors, Chem Rev, 2004, 104, 4245, Martin Winter and Ralph J. Brodd Battery types: Primary Battery: Non reversible chemical reactions (no recharge) Secondary Battery: Rechargeable Common characteristics Electrode complex coposite of powders of active material and conductive diluent, polymer matrix to bind the mix typically 30% porosity, with complex surface throughout the material allows current production to be uniform in the structure Current distribution primary – cell geometry secondary – production sites within the porous electrode parameters affecting the secondarycurrent distribution are conductivity of diluent (matrix) electrolyte conductivity, exchange current diffusion characteristics of reactants and products total current flow porosity, pore size, and tortuosisity

What are Batteries, Fuel Cells, and Supercapacitors, Chem Rev, 2004, 104, 4245, Martin Winter and Ralph J. Brodd We will briefly look at: Lead and Lithium insertion

What are Batteries, Fuel Cells, and Supercapacitors, Chem Rev, 2004, 104, 4245, Martin Winter and Ralph J. Brodd

Require very good conductivity Throughout the system Which tends to lower the energy Content of the system In the lead acid system a significant amount Of the weight Is in the grids required To hold the paste

Equivalent Circuit for a Battery Terminals, Resistance To current flow of, R M External Resistance, R ext Internal Discharge Rate (e.t.) Capacitance of electrode Resistance of electrolyte

Lead Acid Battery Basic requirements for a battery 1.chemical energy stored near the electrode ( if too far away current will be controlled by time to get to electrode) 2.The chemical form coating the electrode must allow ion transport, or better yet, electronic conduction 3.The chemical form of the energy must be mechanically robust 4.The chemical form of the energy should generate a large voltage

Fitch lead book Support grids The capacity of the battery depends on The type of material present.

One possible mechanism:.simultaneous dissolution of PbO2 and introduction of 2e Requires electronic conductivity of PbO2 and pore space for motion of water 1.Add e, H+ and OH- to PbO2 2.Add 2 nd e to reduce valence of Pb 3.Add 3 rd e to reduce valence while removing OH- for charge nuetrality 4.PbO is more soluble than PbO2 so it dissolves and reacts with sulfate to 5.Initiate formation of PbSO4, nucleation rate rises with lg conc. Sulfate, which reduces growth of large sized crystals 6.PbSO4 structure is rhombic which matches the PbO2 so it can easily attach 7.Therefore need to control the alletropes of PbO2 and PbO

Beta PbO2 is formed under acid and can be compressed to shorten bonds overlap induces semiconductor behavior which increases the performance Of the battery Alpha forms when Pb metal Corrodes – reduces lifetime of Battery, is more compressible. Add antiomony To drive reaction To beta phase

Lead Acid battery a.What is the potential associated with a lead acid battery with the overall reaction: at the following concentration: [H 2 SO 4 ] = 4.5 M

-0.35 VoVo (-0.35)

Lead Acid battery energy

c.What is the free energy associated with the lead acid battery?

Dendrites are Good: porous (makes more Of possible energy available) Bad: fragile, break and fall from underlying electrode = NO CURRENT e No e

The type of structure that forms depends upon the rate of crystallization which Depends upon rate of reaction which depends upon: Loss/production of products (current) Which depends also upon the rate constant (potential dependent)

One way to “image” the various processes described above is by an Equivalent Circuit

In a simplified system As the battery is discharged the discharge voltage is the Difference between what we started with and the remaining Voltage in the battery

Lead acid batteries can be valve regulated to control the pressure associated With 1.29 V 1.38 V No pressure pressurized Lower CT resistance Under pressure Suggests higher Degree of interparticle Contact under pressure

Insulating layer which can conduct only protons and lead Solubility Diffusion Et at conducting PbO2

Solubility Diffusion Et at conducting PbO2 Modeled effect of diffusion

Solubility Diffusion Et at conducting PbO2 Modeled effect of proton conc

Solubility Diffusion Et at conducting PbO2 Different magnitude of discharge Changes the solubility and proton conc As well as the conductivity of the film

Based on V. S. Bagotsky text, Fundamentals of Electrochemistry

For the simplified model

Monitor structural changes at electrode as a function of the discharge power

High charge transfer Resistance due to insulating PbSO4 layer Charge transfer resistance Decreases due formation of more porous PbO2 Small diameter Of impedance Circle here indicates The fast et kinetics of O2 reaction. Increasing Charge transfer Resistance due To layer of PbSO4

ReactionV o Li + +eLi-3.0 K + + e K-2.95 Na + + eNa-2.71 NCl 3 _4H + + 6e 3Cl - + NH H 2 O + 2e H 2 + 2OH Fe eFe-0.44 Pb ePb H + + 2e H 2(gas) 0 N 2 (g) + 8H + + 6e2NH Cu e Cu0.34 O 2 + 2H 2 O + 4e4OH O 2 + 2H + + 2eH 2 O Ag + + e Ag0.799 NO H + + 3e NO(g) +2H 2 O0.957 Br 2 + 2e2Br NO H eN 2 (g) +6H 2 O1.246 Cl 2 + 2e2Cl Au + + eAu1.83 F 2 + 2e2F g/mol 207g/mol

Lithium oxidation proceeds a little too uncontrollably Lithium reduction does not not result in good attachment back to the lithium metal Forms dendrites which can grow to Short circuit Lithium intercalated in graphite is close to metallic, formal potential differs by only 0.1 to.3 V = -2.7 to -2.9V

Anode – Solid electroactive metal salt (Can change overall charge so that it can electrostatically stabilize & localize Li + ) Potential should be very positive (far from -2.5 V for Li/C reaction Solid should conduct charge throughout Solid should allow ion motion Should have fast kinetics (open and porous) Should be stable (does not convert to alleotropes) Low cost Environmentally benign M. Stanley Whittingham, Lithium Batteries and Cathode Materials, Chem. Rev. 2004, 104,

Group I Group II Group III Spinels

M. Stanley Whittingham, Lithium Batteries and Cathode Materials, Chem. Rev. 2004, 104, Smooth galvanostatic curve indicates That there are no sites nucleating Alleotropes of the compound. Allotropes would alter the structure, Porosity, and the ease of intercalation, Potential, and conductivity Went to market In the late 1970s Single phase Light weight Conducting, but not Reactive (oxidised or reduced) Li ion intercalates in response to double layer charging

M. Stanley Whittingham, Lithium Batteries and Cathode Materials, Chem. Rev. 2004, 104, Indicates various crystal forms Lithium ion inserts in response To reduction of vanadium Different phases of VSe2 have similar structures So the distortion is not great octahedral 2 nd is tetrahedral

M. Stanley Whittingham, Lithium Batteries and Cathode Materials, Chem. Rev. 2004, 104, Group II

Major phase changes in Li x V 2 O 5  (x<0.01) is well ordered Є ( 0.35<x<0.7)is more puckered  (x=1) shifting of layers  (x>1) results in permanent structural change ω (x>>1) is a rock salt form

Sol gel processes of the V 2 O 5 materials

M. Stanley Whittingham, Lithium Batteries and Cathode Materials, Chem. Rev. 2004, 104, Group III Spinels These materials have a major change in Unit cell dimensions when Mn changes Oxidation state (see B). Need to keep the Lattice parameter less than 8.23 A for good Cycling, which 1.Keeps Mn in higher oxidation state, therefore less soluble 2.Prevents distortion in the coordination of oxygen (Jahn-Teller) around the manganese. These distortions will alter the oxidation and reduction potential as seen in the next slide

M. Stanley Whittingham, Lithium Batteries and Cathode Materials, Chem. Rev. 2004, 104,