In electrochemical systems, voltage serves as a fundamental indicator of energy potential and state transitions during operation. Two critical voltage concepts govern battery behavior: open-circuit voltage (OCV) and working voltage. These parameters differ fundamentally in measurement conditions, thermodynamic basis, and operational relevance.
Open-circuit voltage represents the potential difference between a battery's terminals when no external current flows. It reflects equilibrium conditions where electrochemical reactions reach a steady state without kinetic overpotentials. OCV originates from the Gibbs free energy difference between anode and cathode materials, following the Nernst equation:
E = E° - (RT/nF) * ln(Q)
Here, E° denotes standard electrode potential, R the gas constant, T temperature, n electrons transferred, F Faraday's constant, and Q reaction quotient. For lithium-ion cells, OCV typically ranges between 2.5-4.2V depending on chemistry, with lithium cobalt oxide (LCO) systems showing higher values than lithium iron phosphate (LFP).
Working voltage, conversely, emerges under current flow during charge/discharge cycles. It deviates from OCV due to polarization effects:
- Ohmic losses from ionic/electronic resistance
- Activation polarization at electrode interfaces
- Concentration gradients in electrolytes
The voltage gap between OCV and working potential widens with increasing current rates. A 1C discharge might create 100-300mV deviation in commercial cells, while 5C rates can exceed 500mV. This relationship follows:
V_working = OCV - η_ohmic - η_activation - η_concentration
Voltage profiles during cycling reveal state-of-charge (SOC) through characteristic plateaus and slopes. Different electrode materials produce distinct signatures:
- Layered oxides (NMC, LCO) show sloping voltage-SOC curves
- Spinel materials (LMO) display flat mid-range plateaus
- Phosphate compounds (LFP) exhibit extremely flat voltage profiles
The Nernst equation explains these variations through thermodynamic activity changes. For lithium intercalation cathodes:
E = E° + (RT/F) * ln[(1-θ)/θ] + (RT/F) * ln(a_Li+)
Where θ represents lithium site occupancy and a_Li+ electrolyte activity. Phase transitions during (de)intercalation create voltage plateaus as the system maintains constant chemical potential during two-phase coexistence. Graphite anodes demonstrate staged intercalation with multiple plateaus between 0.01-0.2V vs Li/Li+.
Three regions typically appear in complete charge/discharge curves:
1. Initial steep slope: Corresponds to surface reactions and double-layer charging
2. Main plateau/gradual slope: Bulk phase transformations occur
3. Terminal voltage drop: Approaches complete (de)lithiation
The voltage hysteresis between charge and discharge curves stems from:
- Different reaction pathways during insertion/extraction
- Structural irreversibilities
- Interface reconstruction
- Local stress accumulation
Hysteresis magnitude varies by chemistry:
- Silicon anodes show >200mV hysteresis
- Titanate anodes exhibit <50mV
- Sulfur cathodes demonstrate >300mV
Temperature significantly impacts voltage profiles through:
- Nernst equation temperature dependence
- Ionic conductivity changes
- Reaction kinetics alteration
A 10°C decrease typically raises LFP cell overpotential by 15-20mV at 1C.
Advanced analysis employs differential voltage (dV/dQ) to identify phase transitions more clearly than raw voltage curves. Peaks in dV/dQ plots correspond to:
- New phase nucleation
- Interface movement completion
- Crystallographic structure changes
Practical battery management utilizes voltage-SOC relationships while accounting for:
- Current-induced polarization
- Temperature compensation
- Aging effects on electrode materials
- Relaxation time for OCV stabilization
The voltage profile evolution over cycles provides degradation insights:
- Phase transition peak broadening indicates particle cracking
- Capacity loss with maintained voltage features suggests lithium inventory loss
- Voltage curve compression implies active material detachment
Understanding these voltage characteristics enables proper battery utilization without requiring direct SOC estimation algorithms or energy density measurements. The fundamental thermodynamic principles governing OCV and kinetic factors determining working voltage collectively shape observable battery behavior across applications.