The negative/positive ratio of lithium-ion batteries is a key design parameter, which refers to the capacity ratio of the negative electrode material relative to the positive electrode material in battery capacity design.
1 Definition and Calculation Method
The N/P ratio is usually defined as the ratio of the negative electrode capacity to the positive electrode capacity, used to ensure that the negative electrode material has sufficient capacity to accept the lithium ions released by the positive electrode material during charging. The calculation formula is:
N/P=negative electrode active material gram capacity x negative electrode surface density x negative electrode active material content ratio ÷ (positive electrode active material gram capacity x positive electrode surface density x positive electrode active material content ratio)
2 Influencing factors
First effect: refers to the first coulombic efficiency, which affects the actual capacity of the positive and negative electrodes. The gram capacity data obtained from material suppliers often only considers the half electric gram capacity of the active substance, while the actual full cell gram capacity is also affected by factors such as conductive agents, adhesives, current collectors, separators, electrolytes, etc. Therefore, there is a difference between the actual full cell gram capacity and the design gram capacity.
Assembly process: There are differences in the N/P ratio design between cylindrical and square batteries, mainly due to the tightness of the contact between the positive and negative electrode plates.
Chemical conversion process: The chemical conversion process also affects the N/P ratio, as it affects the first effect and thus affects the performance of the gram capacity.
Cycle performance: Cycle life is one of the important indicators for measuring battery performance. If the positive electrode decays quickly, the N/P ratio should be designed to be lower, allowing the positive electrode to be in a shallow charge discharge state; On the contrary, if the negative electrode decays quickly, the N/P ratio should be designed higher to keep the negative electrode in a shallow charge discharge state.
Security: Security is a more important indicator than loop performance. The design of N/P ratio should not only consider the safety performance of the finished product, but also avoid the occurrence of lithium deposition and heat generation in the battery cells during pre charging.
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3 The impact on battery performance
N/P ratio too high:
Excessive negative electrode discharge can cause shallow charging and discharging of the negative electrode and deep charging and discharging of the positive electrode.
The fully charged negative electrode is less prone to lithium deposition (some materials such as soft and hard carbon and LTO materials also do not undergo lithium deposition), making it safer.
But the increase in oxidation state of the positive electrode actually increases safety hazards.
Due to the unchanged initial effect of the negative electrode, the more part needs to be reacted, and at the same time, due to the influence of kinetics, the positive electrode capacity will be lower.
The increase in oxidation state of the positive electrode material may lead to detachment of the positive electrode powder from the foil, yellowing of the separator, increased internal resistance, and cyclic failure under high temperature conditions.
N/P ratio too low:
Insufficient negative electrode capacity may cause excess lithium ions released from the positive electrode during charging to precipitate on the negative electrode surface and form lithium dendrites, which can easily cause internal short circuits in the battery and affect its safety performance.
The excess Li released provides a Li source for the deposition of lithium salts on the negative electrode surface, and the continuous deposition of lithium salts leads to cycling failure.
4 Considerations in practical applications
Graphite negative electrode batteries: The N/P ratio is generally greater than 1.0, usually controlled between 1.04-1.20, mainly for safety design to prevent lithium deposition from the negative electrode. When designing, process capability should be considered, such as coating deviation.
Lithium titanate negative electrode battery: adopting a positive electrode excess design, the battery capacity is determined by the capacity of the lithium titanate negative electrode. The excessive design of the positive electrode is beneficial for improving the high-temperature performance of the battery, as the high-temperature gas mainly comes from the negative electrode. When the positive electrode is excessively designed, the negative electrode potential is lower, making it easier to form an SEI film on the surface of lithium titanate.
5 Dynamic variability
The N/P ratio is a dynamically changing parameter. During the charging/discharging cycle of the battery, changes in the reversible specific capacity of the positive and negative electrodes and/or the negative mass load of the electrode active material can lead to continuous variations in the N/P ratio. In addition, factors such as the initial irreversible capacity of the entire battery, charging cut-off voltage, current density, ambient temperature, and battery aging can also affect changes in the N/P ratio.
The N/P ratio of lithium-ion batteries is an important and complex parameter in battery design. Before designing the N/P ratio of the entire battery, it is necessary to fully understand the characteristics of the cathode and anode materials, as well as various factors that affect the N/P ratio, to ensure the performance and safety of the battery.