🔌Electrochemistry Unit 8 – Electrochemical Energy Storage: Batteries

Key Concepts and Terminology

• Electrochemistry studies the interrelation of electrical and chemical changes in a system
• Redox reactions involve the transfer of electrons between species
  • Reduction occurs when a species gains electrons and its oxidation state decreases
  • Oxidation occurs when a species loses electrons and its oxidation state increases
• Electrodes are conductors where reduction and oxidation half-reactions take place
  • Anode is the electrode where oxidation occurs (negative electrode)
  • Cathode is the electrode where reduction occurs (positive electrode)
• Electrolyte is a substance that conducts ions between electrodes in a battery
• Voltage ($V$) is the potential difference between two points in an electrical circuit
• Capacity ($Ah$) represents the total charge a battery can store and deliver

Fundamentals of Electrochemistry

• Electrochemical cells convert chemical energy into electrical energy (batteries) or vice versa (electrolysis)
• Galvanic cells spontaneously generate electricity through redox reactions (batteries)
• Electrolytic cells use an external power source to drive non-spontaneous redox reactions (electrolysis)
• Nernst equation relates the potential of an electrochemical cell to the standard electrode potential and concentrations of reactants and products: $E = E^0 - \frac{RT}{nF} \ln Q$
  • $E$ is the cell potential at non-standard conditions
  • $E^0$ is the standard cell potential
  • $R$ is the universal gas constant (8.314 J/mol·K)
  • $T$ is the absolute temperature (K)
  • $n$ is the number of electrons transferred in the redox reaction
  • $F$ is Faraday's constant (96,485 C/mol)
  • $Q$ is the reaction quotient, which relates the concentrations of products and reactants
• Faraday's laws of electrolysis relate the amount of substance produced or consumed in an electrolytic cell to the quantity of electricity passed

Types of Batteries

• Primary batteries are single-use and cannot be recharged (alkaline, zinc-carbon)
• Secondary batteries are rechargeable and can be used multiple times (lithium-ion, lead-acid, nickel-cadmium)
• Flow batteries store energy in external tanks containing liquid electrolytes (vanadium redox, zinc-bromine)
• Thermal batteries use molten salts as electrolytes and operate at high temperatures (sodium-sulfur)
• Thin-film batteries have a compact design with thin layers of electrodes and electrolytes (lithium-polymer)
• Reserve batteries are inactive until activated by adding an electrolyte or other component (water-activated)

Battery Components and Materials

• Current collectors are conductive materials that facilitate electron transfer between the electrodes and external circuit (aluminum, copper)
• Separators are porous insulators that prevent physical contact between electrodes while allowing ion transport (polyethylene, polypropylene)
• Electrolytes are ionically conductive materials that enable ion transfer between electrodes
  • Liquid electrolytes are solutions of salts in water or organic solvents (aqueous, non-aqueous)
  • Solid electrolytes are ion-conducting materials in the solid state (polymer, ceramic)
• Electrode materials determine the specific redox reactions and performance characteristics of a battery
  • Anodes typically use materials with low reduction potentials (lithium, zinc, graphite)
  • Cathodes typically use materials with high reduction potentials (metal oxides, sulfides, phosphates)
• Binders help to maintain the structural integrity of electrodes (PVDF, CMC)
• Additives enhance specific properties of battery components (conductive additives, flame retardants)

Electrochemical Reactions in Batteries

• Discharging a battery involves spontaneous redox reactions that convert chemical energy into electrical energy
  • Anode undergoes oxidation, releasing electrons to the external circuit
  • Cathode undergoes reduction, accepting electrons from the external circuit
  • Ions migrate through the electrolyte to maintain charge balance
• Charging a battery (for secondary batteries) involves non-spontaneous redox reactions driven by an external power source
  • Anode undergoes reduction, accepting electrons from the external circuit
  • Cathode undergoes oxidation, releasing electrons to the external circuit
  • Ions migrate through the electrolyte in the opposite direction compared to discharging
• Specific redox reactions depend on the chemistry of the battery system
  • Lithium-ion batteries: $LiMO_2 + C \leftrightarrow Li_{1-x}MO_2 + Li_xC$
  • Lead-acid batteries: $Pb + PbO_2 + 2H_2SO_4 \leftrightarrow 2PbSO_4 + 2H_2O$
• Side reactions can occur alongside the primary redox reactions, affecting battery performance and lifetime (electrolyte decomposition, corrosion)

Battery Performance Metrics

• Open-circuit voltage (OCV) is the voltage of a battery when no current is flowing
• Closed-circuit voltage (CCV) is the voltage of a battery under load conditions
• Capacity is the total amount of charge a battery can store and deliver
  • Theoretical capacity is determined by the amount of active materials in the electrodes
  • Practical capacity is lower than theoretical capacity due to various losses and inefficiencies
• Energy density is the amount of energy stored per unit volume (Wh/L) or mass (Wh/kg)
• Power density is the amount of power delivered per unit volume (W/L) or mass (W/kg)
• Cycle life is the number of charge-discharge cycles a battery can undergo before its capacity falls below a specified threshold (typically 80% of initial capacity)
• Coulombic efficiency is the ratio of the charge extracted from a battery during discharging to the charge input during charging
• Self-discharge is the gradual loss of capacity when a battery is not in use

Applications and Real-World Uses

• Portable electronics rely on compact, high-energy-density batteries (smartphones, laptops)
• Electric vehicles (EVs) require batteries with high energy density, power density, and cycle life
• Grid-scale energy storage uses large-scale battery systems to balance supply and demand (load leveling, frequency regulation)
• Aerospace applications demand lightweight, reliable, and safe batteries (satellites, spacecraft)
• Medical devices employ batteries with long shelf life and high reliability (pacemakers, implantable defibrillators)
• Wearable technology incorporates flexible, thin-film batteries (smartwatches, fitness trackers)

Challenges and Future Developments

• Improving energy density to increase the storage capacity and runtime of batteries
• Enhancing power density to enable faster charging and high-power applications
• Extending cycle life and calendar life to reduce the need for battery replacement
• Ensuring safety by mitigating risks associated with thermal runaway, short circuits, and mechanical damage
• Reducing cost through materials optimization, manufacturing advancements, and economies of scale
• Developing sustainable and environmentally friendly battery technologies (recycling, bio-based materials)
• Exploring new battery chemistries and architectures (solid-state, lithium-sulfur, lithium-air)
• Integrating batteries with other energy storage technologies (supercapacitors, fuel cells) for hybrid systems
• Implementing smart battery management systems (BMS) for optimal performance and longevity
• Addressing the challenges of large-scale production and supply chain management for widespread adoption