The voltage of a lithium-ion battery is essentially determined by the electrochemical potential difference between the positive and negative electrode materials. The two key values of 3.2V and 3.6V~3.7V correspond to the two most mainstream cathode materials: LFP and NCM/NCA, respectively.
I. How is voltage determined? (Physical and Chemical Principles)
1.Electrode Potential (Reduction Potential): Every material has its inherent “electrochemical potential.” In a battery, we use lithium metal as a reference (its potential is defined as 0 V).
Negative Electrode (Anode): Typically graphite, its potential is very close to that of lithium metal, approximately 0.01V ~ 0.1V.
Positive Electrode (Cathode): Various transition metal oxides, with higher potentials.
2. Open Circuit Voltage (OCV) Formula:
Because the voltage of the negative electrode graphite is almost constant and extremely low, the battery’s terminal voltage mainly depends on the voltage of the positive electrode material.
3. Changes during charging and discharging:
Voltage is not constant. As lithium ions are extracted (charged) or inserted (discharged) from the positive electrode, the chemical environment of the positive electrode material changes, causing voltage fluctuations. The 3.2V or 3.6V we usually refer to refers to the most stable plateau voltage during battery discharge.
II. Why 3.2V? (LFP)
3.2V is the standard discharge plateau voltage for lithium iron phosphate (LiFePO₄).
Material Characteristics: Lithium iron phosphate has a very stable crystal structure. During lithium-ion insertion/extraction, a unique two-phase reaction occurs (FePO₄ and LiFePO₄ coexist).
Stable Plateau: This two-phase coexistence state ensures that the battery voltage remains stable at around 3.2V for most of the discharge time, only dropping sharply when the battery is nearly depleted.
Voltage Range: Approximately 3.65V at full charge (100%), and approximately 2.5V at 0% discharge.
Why not use a higher voltage? This is determined by the chemical bond energy within the lithium iron phosphate molecule. Forcing a charge above 3.65V would damage its olivine structure, leading to battery failure or even danger.
III. Why 3.6V/3.7V? (NCM/NCA)
3.6V~3.7V is the average operating voltage of NCM or lithium nickel cobalt aluminum oxide (NCA).
Material Characteristics: Ternary materials have a layered structure. Compared to lithium iron phosphate, the voltage decreases slowly and linearly as the charge decreases.
Source of Values: Fully charged: Approximately 4.2V (This is a hard upper limit; higher values can easily lead to oxygen evolution and thermal runaway).
Discharged: Approximately 2.8V~3.0V.
Nominal Voltage: The industry typically uses the median or average value of commonly used cutoff voltages, defined as 3.6V or 3.7V (e.g., 3.7V is the standard for many mobile phone batteries).
Composition Influence: The higher the proportion of Ni in ternary materials, the higher the voltage plateau usually is.
The 3.6V/3.7V for NCM/NCA is not an absolutely fixed “step,” but rather a statistically significant average or characteristic voltage. This is fundamentally different from the physicochemical structure of the LFP’s 3.2V platform, which is as flat as if cut with a knife. Its theoretical basis mainly involves the Nernst Equation, Solid Solution Reaction, and changes in the Fermi Level.
