Batteries are the unsung heroes of our electronic world. They power everything from smartphones to electric cars, relying on clever chemistry to store and release energy. Understanding how they work is key to improving their performance and developing new technologies.
At their core, batteries consist of two electrodes and an electrolyte. During discharge, the anode releases electrons while the cathode accepts them, creating an electric current. The electrolyte allows ions to flow, maintaining charge balance. This simple setup enables the storage and release of electrical energy on demand.
Battery Fundamentals
Components of battery cells
- Anode (negative electrode)
- Undergoes oxidation during discharge releasing electrons to the external circuit
- Typically made of materials with low reduction potential (lithium, zinc, lead) that readily give up electrons
- Cathode (positive electrode)
- Undergoes reduction during discharge accepting electrons from the external circuit
- Typically made of materials with high reduction potential (lithium cobalt oxide, lead dioxide) that readily accept electrons
- Electrolyte
- Ionically conductive medium allowing charge transfer between electrodes
- Can be liquid (aqueous solutions like sulfuric acid or organic solvents like ethylene carbonate), gel, or solid (polymer like lithium salt in polyethylene oxide)
- Separator
- Porous membrane physically separating electrodes to prevent short circuit
- Allows ion transport through the electrolyte maintaining ionic conductivity
- Current collectors (metal foils or grids) attached to electrodes enable external electrical connection
- Cell casing (cylindrical, prismatic, or pouch) contains and seals the cell components
Electrochemical processes in batteries
- Discharge process
- Anode oxidizes releasing electrons to the external circuit
- Cathode reduces accepting electrons from the external circuit
- Ions migrate through the electrolyte maintaining charge balance (In a lithium-ion battery, Li+ ions move from anode to cathode)
- Charge process reverses the discharge process
- External power source forces electrons to flow from cathode to anode
- Ions migrate back to their original positions (In a lithium-ion battery, Li+ ions move from cathode to anode)
- Redox reactions driven by difference in reduction potential between anode and cathode
- Reduction potential quantifies tendency of a species to gain electrons and reduce
- Standard electrode potential ($E^0$) measured relative to standard hydrogen electrode (SHE)
- Nernst equation relates electrode potential to standard potential and concentrations of oxidized and reduced species: $E = E^0 - \frac{RT}{nF} \ln \frac{[Red]}{[Ox]}$
Roles of battery components
- Anode (negative electrode)
- Serves as electron source during discharge undergoing oxidation
- Has lower reduction potential compared to cathode
- Cathode (positive electrode)
- Accepts electrons during discharge undergoing reduction
- Has higher reduction potential compared to anode
- Electrolyte
- Provides medium for ion transport between electrodes
- Electrically insulating but ionically conductive
- Maintains charge balance in the cell
- Composition influences cell performance and safety
- Electrode materials
- Anode materials with high specific capacity and low reduction potential (lithium, silicon) maximize energy density
- Cathode materials with high specific capacity and high reduction potential (lithium cobalt oxide, lithium iron phosphate) maximize cell voltage
- Material properties affect capacity, voltage, and cycle life
- Electrolyte composition
- Solvent affects ion mobility, conductivity, and electrochemical stability
- Salt concentration influences ionic conductivity and mass transport
- Additives can improve performance and safety (flame retardants, overcharge protectors)
- Cell design parameters influence ion and electron transport
- Electrode thickness and porosity
- Separator properties (porosity, wettability)
- Current collector materials and design affect electronic conductivity and cell resistance
- Cell packaging (cylindrical, prismatic, pouch) impacts energy density and mechanical stability
- Operating conditions affect cell performance
- Temperature influences reaction kinetics, ion mobility, and material stability
- Charge/discharge rate (C-rate) affects realized capacity and efficiency
- Depth of discharge (DoD) and state of charge (SoC) impact lifetime