Usually when we talk about lithium deposition, we assume that it is the negative electrode that deposits lithium, so why doesn’t the positive electrode deposit lithium? Today, I will analyze it from three aspects: thermodynamic potential limitation, electrochemical reaction direction, and material structure stability, revealing that its essence is derived from high potential characteristics (>3V) and oxidation reaction mechanism, and clarifying the priority of metal deposition under over-discharge conditions.
Thermodynamic constraints (potential not satisfied)
The essence of lithium precipitation is that lithium ions gain electrons and are reduced to metallic lithium. This reaction must occur when the potential is lower than 0 V (vs. Li⁺/Li).
The cathode electrode materials (such as LCO, NCM, LFP) are in a high potential state during charging (still higher than 3 V when completely de-lithiated), which is much higher than the 0 V required for lithium deposition. For example:
- The potential of lithium cobalt oxide (LCO) after delithiation is > 3.9 V
- The potential of lithium iron phosphate (LFP) after delithiation is > 3.4 V
- Even under high-rate discharge (such as 12C) or over-discharge conditions, the positive electrode potential cannot drop below 0 V.
Electrochemical reactions are in opposite directions
- During charging: the positive electrode undergoes an oxidation reaction (lithium removal and electron loss), and the electrons flow to the negative electrode through the external circuit. The positive electrode itself cannot obtain electrons to reduce and precipitate lithium.
- During discharge: lithium ions are removed from the negative electrode and embedded in the positive electrode, and the positive electrode undergoes a reduction reaction, but this process is the lithium ions embedded in the lattice rather than surface deposition.
Other metals are preferentially precipitated during over-discharge
In case of severe overdischarge, if the negative electrode has a higher first efficiency than the positive electrode, residual lithium ions may accumulate on the positive electrode surface. However, the deposition potential of lithium (0 V) is much lower than that of copper (3.35 V) or aluminum (1.35 V), so the positive electrode current collector (copper/aluminum foil) will first dissolve and precipitate, rather than lithium.
Material structure stability
The positive electrode materials (such as NCM and LFP) still maintain a stable crystal structure in the delithiation state, and the lithium ions are stored in the crystal lattice in an embedded form, with no physical space for metallic lithium deposition.
Additional Notes
The relationship between lithium plating and negative electrode:
The lithium insertion potential of negative electrode graphite is close to 0 V (65–200 mV vs. Li⁺/Li), and it is easy to reach the lithium deposition potential at low temperature, fast charging or overcharging. The high potential characteristics of the positive electrode material fundamentally eliminate the possibility of lithium deposition.
Summary
Due to the high potential characteristics (>3 V) and the nature of the oxidation reaction, the positive electrode cannot meet the thermodynamic conditions for lithium precipitation (potential <0 V). Even under extreme conditions, only the current collector may dissolve, rather than lithium precipitation.