Ch8: Battery Types

Battery Classification (Primary vs. Secondary)

• Primary Batteries
  • One-time use; reaction proceeds only in the spontaneous direction.
  • Called non-rechargeable.
  • Example: Alkaline cells.
• Secondary Batteries
  • Reversible; can be driven backwards by an external energy source.
  • Called rechargeable.
  • Examples: Nickel–Cadmium (NiCd), Lead–Acid, Lithium-Ion.
• Key idea: To reverse a non-spontaneous redox reaction you must supply energy (plugging a charger into the wall).

Alkaline Batteries (Primary)

• Most common household battery format (AA, AAA, C, D, 9 V, etc.).
• Electrochemistry
  • Anode (oxidation): $\text{Zn (s)} + 2\text{OH}^- \rightarrow \text{Zn(OH)}_2 \text{ (s)} + 2e^-$
  • Cathode (reduction): $2\text{MnO}<em>2 \text{ (s)} + \text{H}</em>2\text{O} + 2e^- \rightarrow \text{Mn}<em>2\text{O}</em>3 \text{ (s)} + 2\text{OH}^-$
• Overall: transfer of electrons from zinc surface through external circuit to manganese oxide rod.
• Parts & roles
  • Zinc powder or plated can = anode shell.
  • Electrolyte: $\text{KOH}$ or $\text{Ca(OH)}_2$ provides $\text{OH}^-$ ions.
  • Manganese dioxide packed around a central carbon rod = cathode mix.
• Practical points
  • High energy density for a primary cell; inexpensive; long shelf life.
  • Single use \rightarrow disposal issues (Zn, Mn compounds are comparatively benign but mass-produced).

Nickel–Cadmium Batteries (Secondary)

• Abbrev. NiCd; once popular in cordless tools, early laptops, emergency lighting.
• Electrodes
  • Anode: Cadmium metal; oxidation releases electrons.
    $\text{Cd (s)} + 2\text{OH}^- \rightarrow \text{Cd(OH)}_2 \text{ (s)} + 2e^-$
  • Cathode: Nickel(III) oxyhydroxide; accepts electrons.
    $2\text{NiO(OH) (s)} + 2\text{H}<em>2\text{O} + 2e^- \rightarrow 2\text{Ni(OH)}</em>2 \text{ (s)} + 2\text{OH}^-$
  • Overall: $\text{Cd (s)} + 2\text{NiO(OH) (s)} + 2\text{H}<em>2\text{O} \rightleftharpoons \text{Cd(OH)}</em>2 \text{ (s)} + 2\text{Ni(OH)}_2 \text{ (s)}$
• Construction tricks
  • Alternating rolled layers ("jelly-roll") of blue anode and red cathode increase surface area \rightarrow higher current.
• Charging vs. discharging
  • Discharge = spontaneous; electrons flow from Cd to NiO(OH).
  • Recharge = non-spontaneous; external power source forces electrons backward (analogy: pumping water up a hill).
• Limitations & concerns
  • Cadmium is toxic \rightarrow strict recycling requirements.
  • Memory effect (voltage depression) if not fully cycled.

Lead–Acid Storage Batteries (Secondary)

• Classic car battery; delivers very high surge currents for engine starters.
• Discharge reaction
  $\text{Pb (s)} + \text{PbO}<em>2 \text{ (s)} + 2\text{H}</em>2\text{SO}<em>4 \text{ (aq)} \rightarrow 2\text{PbSO}</em>4 \text{ (s)} + 2\text{H}_2\text{O} \text{ (l)}$
• Oxidation-state analysis
  • Pb(0) \rightarrow Pb(+2): oxidation.
  • Pb(+4) \rightarrow Pb(+2): reduction.
  • Same element participates on both sides in different compounds.
• Design features
  • Multiple $2\text{ V}$ cells in series ($12\text{ V}$ total in most cars).
  • Spongy Pb plates (anode) & PbO₂ plates (cathode) immersed in $\text{H}<em>2\text{SO}</em>4$ .
• Real-world anecdote
  • Misdiagnosed car transmission failure was actually a simple battery fault (avoided a $20,000$ USD repair bill) \rightarrow shows critical role of healthy battery.
• Pros/cons
  • Pros: Low cost, recyclability, high current delivery.
  • Cons: Heavy, contains toxic lead & corrosive acid, limited cycle life.

Lithium-Ion Batteries (Secondary, High-Performance)

• Power source for phones, laptops, electric vehicles (e.g., Tesla).
• Principle: Shuttling of $\text{Li}^+$ ions between two host materials through a separator; electrons travel through external circuit.
  • Charging: $\text{Li}^+$ forced into anode (typically graphite).
  • Discharge: $\text{Li}^+$ migrate to cathode (various metal oxides), releasing electrons.
• Key advantages
  • Highest energy density among commercial batteries.
  • No lead or cadmium \rightarrow fewer toxic issues.
  • Low self-discharge, no memory effect (with modern chemistries).
• Ongoing research
  • Collaboration across chemistry, physics, materials science, even biology.
  • Goals: higher capacity, faster charging, improved safety, lower cost.

Capacitors, Supercapacitors & Comparison with Batteries

• Capacitor
  • Stores energy as a static electric field between two conductive plates separated by a dielectric.
  • Instant charge/discharge; limited total energy.
  • Everyday example: Static electricity from walking on carpet discharging on a doorknob.
  • On computer motherboards (cylindrical cans) they filter voltage spikes and ride-through brief power losses.
• Supercapacitor (Electrochemical Double-Layer Capacitor)
  • Adds electrolyte to greatly enlarge effective plate area \rightarrow far higher capacitance & energy than conventional capacitors.
  • Charges in seconds–minutes; delivers bursts of power (regenerative braking, UPS systems).
• Battery vs. (Super)Capacitor
  • Battery: high energy, slower power delivery (chemical processes).
  • Supercapacitor: lower energy, very high power, rapid charge.
  • Emerging idea: Hybrid systems combining both for electric vehicles.

General Themes, Analogies & Practical Implications

• Water-on-a-hill analogy for reversible cells
  • Pumping water up = charging (requires work).
  • Water flowing down = discharging (spontaneous energy release).
• Environmental & Ethical considerations
  • Toxic metals (Pb, Cd) demand careful recycling.
  • Lithium mining vs. fossil fuel reduction; still an environmental trade-off.
• Technological appreciation
  • Compact electrochemistry underlies modern mobility and electronics.
  • Nobel Prize–winning breakthroughs (lithium-ion) illustrate importance of fundamental chemistry.
• Interdisciplinary future
  • Advanced storage requires synergy among chemists, physicists, material scientists, engineers, and even biologists.
  • Ongoing exploration of solid-state batteries, sodium-ion, metal–air, and next-gen supercapacitors.