Why is there one more negative electrode sheet than the positive electrode sheet in lithium battery design?
April 22, 2025
During the manufacturing process of lithium batteries, a crucial design detail is that the number of negative electrode sheets (anodes) is always one more than that of the positive electrode sheets (cathodes). This seemingly simple numerical difference actually embodies profound electrochemical principles and engineering considerations.
Core Reason: Completeness of Electrochemical Reactions
The essence of lithium battery operation is the shuttling of lithium ions between the positive and negative electrodes:
During charging: Lithium ions are extracted from the positive electrode, pass through the electrolyte, and are intercalated into the negative electrode.
During discharging: Lithium ions are extracted from the negative electrode and return to the positive electrode.
To achieve 100% utilization of active materials, it is necessary to ensure that:
1. Each side of every positive electrode sheet corresponds to negative electrode material.
2. The edge regions of the positive electrode are completely covered by the negative electrode.
If the number of electrode sheets were the same, the outermost positive electrode sheet would have one side directly exposed to the electrolyte, resulting in:
The active material on that side being unable to participate in the reaction.
Abnormal local current density.
Possible side reactions such as electrolyte decomposition.
Three Major Engineering Considerations
1. Current Distribution Optimization
The design of having one more negative electrode sheet makes each positive electrode sheet in a "sandwich" state:
Positive electrode sheet: Surrounded by negative electrodes on both sides.
Negative electrode sheet: Only one side has a positive electrode (the outermost negative electrode).
This layout ensures:
Symmetrical current paths.
Uniform electron conduction.
Minimization of polarization phenomena.
2. Safety Protection Mechanism
The key role of the extra negative electrode sheet is to:
Fully encapsulate the active material of the positive electrode.
Prevent lithium dendrite growth caused by edge effects.
Provide buffer space for expansion (silicon-based negative electrodes can expand by up to 300%).
Experimental data shows that batteries with an N+1 electrode sheet design can have their cycle life increased by 15-20%.
3. Manufacturing Process Adaptation
For wound batteries, the negative electrode naturally forms a "one more" structure as the outer layer. For example:
18650 cylindrical battery: 4 layers of positive electrode → 5 layers of negative electrode.
Stacked batteries: Achieve N+1 pairing through precise counting.
Stacking process: By stacking in a "book page" manner, it is ensured that the sequence is positive electrode ↔ negative electrode ↔ positive electrode ↔ negative electrode... always ending with a negative electrode.
Special Case Analysis
1. Solid-State Battery Differences
Some solid-state electrolyte batteries may use an equal number of electrode sheets because:
Solid-state electrolytes have high mechanical strength, which can suppress dendrites.
The ion conduction path is different from that of liquid batteries.
However, industrialized products still mostly retain the N+1 design.
2. Laboratory Test Batteries
Symmetric batteries used for research (such as Li-Li batteries) break this rule, but:
They cannot be applied in practice.
They are only used for fundamental research.
Summary:
The design of having "one more negative electrode sheet" in lithium batteries is:
✓ A necessity for electrochemical reactions
✓ A manifestation of engineering optimization
✓ A crucial safeguard for safety