I. Primary Cells and Electrolytes: The "Two-Way Factory" of Lithium-ion Batteries
A lithium battery is like a factory that can operate in two directions: when discharging, it is a primary cell (chemical energy → electrical energy), and when charging, it transforms into an electrolytic cell (electrical energy → chemical energy). This switching between the two states constitutes the complete life cycle of the battery.
1. Primary cell: "Energy output mode" during discharge.
The core of a primary battery is a spontaneous redox reaction, much like water flowing from a high place to a low place, releasing energy without external intervention. During the discharge of a lithium battery:
Negative electrode (oxidation reaction): Lithium atoms lose electrons (Li → Li + e), and the electrons flow to the positive electrode through the external circuit (wire), forming an electric current (this is the electricity obtained by mobile phones or motors).
Positive electrode (reduction reaction): Transition metal ions (such as Co³) in the positive electrode material "accept" electrons and become a lower valence state (Co³ + e → Co²), while Li migrates to the positive electrode through the electrolyte to maintain charge balance.
This process is like "two teams of athletes passing a ball": the negative electrode is the "serving side" (losing electrons), and the positive electrode is the "receiving side" (gaining electrons). The directional movement of electrons forms an electric current, while the migration of Li ensures the overall electrical neutrality of the "playing field." A fully charged lithium battery is like a reservoir filled with water, and the discharge process is like the water flow driving a turbine to generate electricity.
2. Electrolytic cell: The "energy storage mode" during charging.
When a battery is charged, the external power source (such as a charger) "forces a reversal" of the reaction direction, at which point the battery becomes an electrolytic cell—a non-spontaneous reaction that occurs under the drive of electrical energy, much like using a pump to pump water back into a reservoir.
Positive electrode (oxidation reaction): Under the action of an external power source, low-valence metal ions (such as Co²) are forced to lose electrons and change back to a high-valence state (Co² → Co³ + e), and the electrons are "pushed back" to the negative electrode.
Negative electrode (reduction reaction): Li migrates to the negative electrode in the electrolyte, combines with electrons to become lithium atoms again (Li + e → Li), and is stored in the negative electrode material (such as graphite).
The key to this process is that the voltage of the external power supply must be higher than the battery's own electromotive force, just like a water pump must have sufficient power to pump water from a low point to a high point. The core of fast charging technology is to accelerate this "reverse reaction" process by increasing the charging voltage (such as an 800V high-voltage platform).
3. The "role reversal" of positive and negative poles
Interestingly, the positive and negative electrodes of a lithium battery "switch roles" during charging and discharging:
During discharge, the negative electrode acts as a "reducing agent" (loses electrons), and the positive electrode acts as an "oxidizing agent" (gains electrons).
During charging, the positive electrode becomes the "anode" (loses electrons, oxidation reaction) and the negative electrode becomes the "cathode" (gains electrons, reduction reaction).
This "reversal" is like a reversible gear set, which changes direction by external energy to achieve a "storage-release" cycle of energy.
II. Electrode Potential and Electromotive Force: The "Voltage Code" of a Battery
Why is the voltage of lithium batteries typically 3.7V? Why can some batteries output higher voltages? The answer lies in two core concepts: electrode potential and electromotive force—which determine the battery's "energy level" and output capability.
1. Electrode potential: The "energy level" of a single electrode.
Each electrode has its own "potential" (which can be understood as an "energy level"), which measures the electrode's ability to lose or gain electrons:
The lower the potential (e.g., lithium electrode, standard potential -3.04V), the easier it is to lose electrons (stronger oxidizing ability);
The higher the potential (such as the Co³/Co² electrode, the standard potential is about 3.7V), the easier it is to gain electrons (strong reduction ability).
This "water level difference" is the driving force behind the flow of electrons. Just as water flows from high to low, electrons flow from the low-potential electrode (negative electrode) to the high-potential electrode (positive electrode). The "water level difference" between the negative electrode (graphite lithium-intercalated electrode, potential approximately -0.1V) and the positive electrode (NCM electrode, potential approximately 3.6V) of a lithium battery is the battery's output voltage (around 3.7V).
2. Electromotive force: The battery's "total voltage potential"
Electromotive force (E) is the difference between the potentials of the positive and negative electrodes (E = positive electrode potential - negative electrode potential), and it represents the "theoretically maximum voltage that the battery can output." For example:
The electromotive force of a ternary lithium battery (NCM) is approximately 3.7V, which is the "rated voltage" marked on mobile phones and electric vehicles;
The electromotive force of lithium iron phosphate (LFP) batteries is about 3.2V, which is slightly lower but more stable.
The electromotive force is determined by the chemical properties of the electrode material itself, much like the "innate genes" of a battery: different combinations of materials (such as high-nickel ternary vs. lithium iron phosphate) result in different electromotive forces, and the energy density will also change accordingly (high electromotive force usually corresponds to high energy density).
3. Nernst Equation: The "Fine-tuning" of voltage by concentration
In actual use, battery voltage will change with the amount of charge (e.g., a mobile phone drops from 4.2V when fully charged to 3.0V when depleted). This is because the ion concentration near the electrodes changes the electrode potential, which conforms to the Nernst equation:
E = E + (RT/nF) × ln (Oxidized ion concentration / Reduced ion concentration)
Simply put:
During charging, the concentration of high-valence ions (such as Co³) near the positive electrode increases, the potential rises, and the battery voltage increases (e.g., to 4.2V).
During discharge, the concentration of low-valence ions (such as Co²) increases, the potential decreases, and the battery voltage drops (e.g., to 3.0V).
This is similar to how the water level in a reservoir changes with the amount of water: the more water (the higher the ion concentration), the higher the water level (electric potential), and the greater the output pressure (voltage). The "battery percentage" displayed on a mobile phone is essentially a result of the change in ion concentration inferred from the change in voltage.
III. Electrochemical Kinetics: The "Invisible Driver" of Reaction Rate
Even with a high electromotive force, a battery cannot function efficiently if the reaction is too slow. Electrochemical kinetics studies the issue of "reaction rate"—why do some batteries charge quickly while others charge slowly? Why does battery performance degrade at low temperatures?
1. Electrode reaction rate: "The bottleneck to reaction speed"
Electrode reactions involve two key steps, much like the "two processes" in a factory:
Charge transfer steps: Electrons are transferred between the electrode and ions (e.g., Li gains electrons and becomes Li). This step requires overcoming the "activation energy" (similar to the strength needed to climb a mountain).
Diffusion step: Ions migrate from the electrolyte to the electrode surface (e.g., Li diffuses from the electrolyte to the graphite surface), and this step depends on the ion migration speed.
Slowing down any step will slow down the entire reaction. For example:
During fast charging, the charge transfer process may not keep up, leading to heat generation (activation energy is converted into heat energy).
At low temperatures, the viscosity of the electrolyte increases, ion diffusion slows down, and the charging speed is forced to decrease (just like engine oil thickens in winter, making it difficult for the engine to run).
2. Overpotential: "The extra energy required to accelerate a reaction"
To accelerate the reaction, an "extra voltage" is needed to overcome the activation energy and diffusion resistance; this extra voltage is called the overpotential (η). For example:
During fast charging, the voltage provided by the charger (e.g., 5V) is higher than the battery's electromotive force (3.7V). The extra 1.3V is used to overcome the overpotential, accelerating the "push" reaction.
Overpotential is converted into heat (Joule heating), which is why batteries get hot during fast charging.
Engineers can reduce overpotential by optimizing electrode materials (such as using nanostructures to shorten the diffusion distance) and adding electrolyte additives (to lower the activation energy), making fast charging more efficient and safer.
3. Exchange current density: the material's "inherent responsiveness"
Different electrode materials have different inherent reaction rates, which are measured by exchange current density (i): the larger the i, the easier the reaction proceeds. For example:
Graphite anodes have high exchange current density and fast Li insertion/extraction speed, making them suitable for fast charging;
Some high-capacity alloy anodes (such as silicon-based anodes) have low exchange current density and slow response, requiring special design to achieve fast charging.
This is similar to an athlete's "natural speed": some people are born fast (high i) and can achieve results with a little training; others need more skills (material modification) to increase their speed.
From the bidirectional conversion between galvanic and electrolytic cells, to the voltage characteristics determined by electrode potential, and the kinetic-controlled reaction rate, these electrochemical principles collectively constitute the "operating manual" of lithium batteries. Understanding these principles allows one to comprehend:
Why do high-nickel ternary batteries have high energy density (high electromotive force)?
Why does fast charging require high voltage (to overcome overpotential)?
Why does low temperature lead to a decrease in capacity (slower diffusion kinetics)?
