Battery Types

Why This Matters

Batteries are the bridge between chemistry and practical electrical power—and on the AP Chemistry exam, you're being tested on your understanding of electrochemical cells, redox reactions, and thermodynamic favorability. Every battery type represents a different solution to the same fundamental challenge: how do we harness spontaneous electron transfer to do useful work? The exam expects you to connect electrode materials, electrolyte composition, and cell potential to real-world performance characteristics like energy density, cycle life, and safety.

Understanding battery chemistry also reinforces core concepts you'll see throughout electrochemistry: standard reduction potentials, half-reactions, Gibbs free energy, and the relationship between cell voltage and spontaneity. When you study these battery types, don't just memorize which metals go where—know why certain electrode combinations produce higher voltages, how electrolyte choice affects ion transport, and what limits a battery's lifespan. This conceptual grounding will serve you well on both multiple-choice questions and FRQs that ask you to analyze unfamiliar electrochemical systems.

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Aqueous Electrolyte Systems

These batteries use water-based electrolyte solutions, which limits their voltage (water electrolyzes above ~2V) but offers advantages in cost and established manufacturing. The aqueous environment enables fast ion transport but restricts the electrochemical window.

Lead-Acid Batteries

• Electrodes are lead dioxide ($PbO_2$) and sponge lead ($Pb$)—both convert to lead sulfate ($PbSO_4$) during discharge in a classic double-sulfate reaction
• Sulfuric acid electrolyte ($H_2SO_4$) participates directly in the cell reaction, meaning electrolyte concentration changes with state of charge
• High surge current capability makes them ideal for automotive starting applications despite their low energy density and heavy weight

Alkaline Batteries

• Zinc anode and manganese dioxide cathode ($Zn/MnO_2$)—the zinc oxidizes while $MnO_2$ undergoes reduction in potassium hydroxide electrolyte
• Non-rechargeable (primary cell) because the discharge products don't readily reform into reactants under reverse current
• Higher energy density than zinc-carbon due to more efficient electrode utilization and better ionic conductivity in alkaline medium

Zinc-Carbon Batteries

• Oldest commercial battery design—zinc casing serves as both anode and container, with a carbon rod current collector at the cathode
• Acidic electrolyte ($NH_4Cl$ or $ZnCl_2$ paste) limits performance compared to alkaline systems due to lower ionic conductivity
• Rapid voltage drop under load results from slow diffusion kinetics and increasing internal resistance as discharge proceeds

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Alkaline Electrolyte Rechargeable Systems

These batteries use potassium hydroxide ($KOH$) as the electrolyte, enabling reversible electrode reactions. The alkaline environment supports different electrode chemistries than acidic systems and generally offers good cycle stability.

Nickel-Cadmium (NiCd) Batteries

• Nickel oxide hydroxide cathode and cadmium anode—the $Cd/Cd(OH)_2$ and $NiOOH/Ni(OH)_2$ couples provide ~1.2V nominal voltage
• Memory effect occurs when repeated partial discharges cause crystalline growth that reduces effective electrode surface area
• Cadmium toxicity has led to environmental regulations phasing out NiCd in favor of alternatives despite excellent temperature tolerance

Nickel-Metal Hydride (NiMH) Batteries

• Hydrogen-absorbing alloy anode replaces toxic cadmium—metal hydride ($MH$) stores hydrogen atoms that release electrons during discharge
• Higher energy density than NiCd because the metal hydride electrode packs more electrochemical capacity per unit mass
• Reduced memory effect compared to NiCd, though still sensitive to overcharging which can cause hydrogen gas evolution

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Lithium-Based Systems

Lithium's position as the most electropositive metal ($E° = -3.04V$) enables the highest cell voltages and energy densities. These systems require non-aqueous electrolytes because lithium reacts violently with water.

Lithium-Ion Batteries

• Intercalation mechanism—lithium ions shuttle between layered electrode structures (typically graphite anode, metal oxide cathode) without forming metallic lithium
• Organic solvent electrolyte with lithium salts (e.g., $LiPF_6$) enables ion transport while maintaining stability across the ~3.7V operating range
• Protection circuits required because overcharging can cause lithium plating, thermal runaway, and potential fire hazards from flammable electrolyte

Solid-State Batteries

• Solid electrolyte replaces flammable liquid—ceramic or polymer materials conduct lithium ions while eliminating leakage and dendrite penetration risks
• Higher theoretical energy density because solid electrolytes may enable lithium metal anodes, which have ~10× the capacity of graphite
• Manufacturing challenges currently limit commercialization—achieving good electrode-electrolyte contact without liquid is technically demanding

Sodium-Ion Batteries

• Sodium replaces lithium ($Na^+$ instead of $Li^+$)—similar intercalation chemistry but with earth-abundant, lower-cost materials
• Slightly lower energy density because sodium ions are larger and heavier than lithium ions, reducing gravimetric capacity
• Promising for grid storage where weight matters less than cost and material sustainability

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Continuous-Feed and Flow Systems

Unlike sealed batteries, these systems separate energy storage from power generation, allowing independent scaling. Chemical fuel is supplied externally, so capacity depends on tank size rather than electrode mass.

Fuel Cells

• Direct conversion of chemical to electrical energy—hydrogen oxidation at anode ($H_2 \rightarrow 2H^+ + 2e^-$) and oxygen reduction at cathode produce electricity continuously
• Water is the only byproduct when using pure hydrogen, making fuel cells attractive for zero-emission applications
• Not technically batteries because they don't store energy internally—they're electrochemical converters requiring external fuel supply and infrastructure

Flow Batteries

• Electrolytes stored in external tanks—vanadium or other redox-active species flow through the cell stack where electron transfer occurs
• Decoupled energy and power—capacity scales with tank size while power scales with electrode area, enabling flexible system design
• Extremely long cycle life because electrode degradation is minimized when active materials remain in solution rather than undergoing solid-state transformations

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Quick Reference Table

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Self-Check Questions

1. Which two battery types use the same cathode material (nickel oxide hydroxide) but differ in their anode chemistry, and what advantage does this substitution provide?

2. Why must lithium-ion batteries use non-aqueous electrolytes while lead-acid batteries can use aqueous sulfuric acid? Connect your answer to standard reduction potentials.

3. Compare the "memory effect" in NiCd batteries to the capacity fade mechanisms in lithium-ion batteries—what electrode-level phenomena cause each?

4. If an FRQ asked you to design a battery system for grid-scale renewable energy storage where cost matters more than weight, which battery type would you recommend and why?

5. Both fuel cells and conventional batteries convert chemical energy to electrical energy through redox reactions. What fundamental difference in design allows fuel cells to operate continuously while batteries require recharging?