No Problems with the N/P Ratio

An explanation of the N to P ratio, and what the implications are of having an N/P less than 1 versus greater than 1 in lithium-ion batteries.

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What is it?

The N/P ratio is the ratio of anode (negative electrode) capacity to cathode (positive electrode) capacity, where electrode capacity is defined by the number of ions that can be reversibly hosted per unit area or volume. It is calculated by multiplying the electrode area density by the ratio of active material in the electrode, and by the active material specific discharge capacity. This is done for both anode and cathode and then the anode capacity is divided by the cathode capacity

The N/P ratio is typically majorly classified as either less than 1 (<1) or greater than 1 (>1)

• N/P < 1: The cathode has a higher capacity than the anode

• N/P > 1: The anode has a higher capacity than the cathode

Here are the implications of both situations:

1. N/P ratio <1

   In this situation, the cathode provides more lithium ions than the anode can accommodate. When there are not enough sites on the anode to host the ions, they just build up on the anode surface, lowering the potential, and leading to lithium plating and possibly dendrite growth.

   
   

   An example of where an N/P ratio lower than 1 is often used is with LTO anodes. These have a spinel structure which undergoes very minimal volume change during cycling. As such, the anode is very stable and doesn’t degrade easily. It additionally operates at about 1.55V vs. Li/Li+ which is much higher than the Li plating potential of 0 V vs. Li/Li⁺. Resultantly, lithium plating is not a concern with LTO anodes. Instead, the cathode is the limiting electrode, and more prone to degradation from electrolyte oxidation at high states of charge. Using N/P < 1 ensures the cathode has excess capacity, preventing overcharge and enhancing overall cycle life.

2. N/P ratio >1

   In this situation, there are more ion hosting sites on the anode than there are Li ions stored in the cathode. This design is typically used as a preventive measure against lithium plating because it attempts to eliminate the risk of lithium ions building up on the anode surface and lowering the potential to lithium plating potential. Commercial lithium ion batteries typically employ ratios between 1 and 1.2 due to their high propensity for lithium plating and dendrite related failures [3].

   
   

   However, if plating is not a concern, excess anode capacity leads to underutilization, adding unnecessary volume and mass that do not contribute to cell capacity. This reduces energy density of the battery. 

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References

[3]J. Teng, X. Tang, M. Tang, Q. Wu, and J. Li, “Failure mechanism and voltage regulation strategy of low N/P ratio lithium iron phosphate battery,” J. Energy Storage, vol. 51, p. 104588, Jul. 2022, doi: 10.1016/j.est.2022.104588.