Lithium Battery Design: The cell design table is one of the instruments required for cell product development. Although the format of the design table varies from company to company and even within a single organization, the fundamentals remain the same. Some students who recently received the design table can still thoroughly consider the formula by their own reasoning, even without the oral instruction of their predecessors. According to what we understand, the design table comprises three main equations: the volume equation, the volume equation, and the N/P ratio equation. Customers or processes define the capacity and volume. The following is an explanation of the N/P ratio equation:
Definition of N/P ratio
The residual of the negative capacity on the opponent at the same stage and under the same circumstances is known as the N/P Ratio (Negative/Positive). In actuality, CB is another saying (cell Balance).
N/P calculation formula:
N/P = negative active substance g capacity × negative surface density × negative active substance content ratio ÷ (positive active substance g volume × positive surface density × positive active substance content ratio).
Identical stage: Lithium batteries can be charged and discharged in two stages, each with a different weight capacity. The first charging stage and the discharge stage are respectively represented by the (first) charging N/P ratio and the discharge N/P ratio.
We know that the first effect of lithium materials is the first (Cullen) efficiency, that is, the first charge and discharge capacity ratio. In the process of initial charging, an SEI membrane is formed on the surface of the material, the defective position of the material is reflected, and the impurities in the material are also reflected, etc., resulting in the first charging capacity > the first discharge capacity > discharge capacity after aging. Although the discharge capacity is still attenuated after aging and subsequent charging and discharge cycles, a large number of reactions have been completed in the early stage. There is a difference in the gram capacity of the two stages. One is the first charging capacity, and the other is multiplied by the gram capacity after the first effect. If mixed, it will cause the design to fail.
The calculation of calorie capacity is similarly affected by the same issue. The same test condition, such as temperature, magnification, voltage range, etc., is referred to as this condition. It will also result in design failure if the conditions for positive and negative pole capacity tests are different and are employed in the same formula.
Lithium Battery Design Opposite: We need to calculate the surface density, which means positively opposite. But what if the shape of the pole is curved? That is, when the outer ring shrinks and the inner ring expands, we need to use curvature to correct the value of surface density, which is why there is a yin-yang surface in the coating process of cylindrical batteries.
Factors to be taken into account when designing the N/P ratio
Lithium Battery Design factors
First Lithium Battery Design factor: consider all substances that have reactions, including conductive agents, adhesives, collectors, diaphragms, and electrolytes. However, the gram capacity data we get from the material supplier often only examines the half-gram capacity of the active substance, which is why there is a difference between the actual full battery gram capacity and the design gram capacity.
Second Lithium Battery Design factor, assembly process: There is a difference in the N/P ratio design of cylindrical batteries to square batteries, mainly caused by the elasticity of positive and negative electrode contact. We also regard the combination of powder and collector as an assembly. The direct contact between the powder and the collector and the contact between the powder are also one of the factors that affect the gram capacity and thus the N/P ratio.
Third Lithium Battery Design factor, the transformation process: the transformation process is different, which also has an impact on the N/P ratio. The transformation process also affects the first effect, which in turn affects the performance of gram capacity. Therefore, when we design the N/P ratio, the transformation process should also be discussed. The impact of the specification process will be explained in subsequent articles.
Performance factors
Fourth Lithium Battery Design factor, cycle: Cycle life is one of the most important indicators to measure battery performance. If the positive pole attenuates faster, then N/P is lower than the design, so that the positive pole is in a shallow charging state. Conversely, if the negative pole attenuates faster, then the N/P ratio is higher so that the negative pole is in a shallow charge state.
Fifth Lithium Battery Design factor, safety: More significant than circulation is the indicator of safety. It affects the completed product’s safety performance in addition to some cells that analyze lithium heating being recharged.
What are the effects of N/P ratios on lithium batteries?
Typically, we believe that a large negative pole will result in a shallow recharge of the negative pole and a deep positive pole if the N/P ratio is too large. Although it is safer to analyze lithium using a full electric negative electrode (certain materials, such as soft and hard carbon, and LTO materials, won’t precipitate lithium), the potential for safety risks grows as the positive oxidation state increases. The first negative effect still exists, so more pieces are required to reflect it. The capacity of the positive pole will also be limited due to the influence of kinetics, but when N/P is somewhat deficient, the positive pole cannot be fully utilized, which will also have an impact on the performance of the unit capacity.
Batteries using graphite anodes should have an N/P ratio of more than 1.0, typically 1.04 to 1.20. The major purpose of this is safety design, namely to stop negative lithium variation. When designing, process capability such as coating variation should be taken into account. The irreversible capacity of the battery is lost when N/P is too high, resulting in a low battery capacity and lower battery energy density.
The capacity of the lithium titanate negative electrode, which determines the battery’s capacity, is used in the positive electrode overload design for the lithium titanate negative electrode. The battery’s high-temperature performance is enhanced by the positive excess design since high-temperature gas primarily originates from the negative electrode. The negative potential is low in the positive electrode overload design, making it simpler to create an SEI membrane on the surface of lithium titanate.
How to set the N/P ratio when the battery design is carried out for the first time? After calculating the theoretical value, the gradient experiment is carried out and then evaluated through low-temperature discharge, gram capacity, cycle life, safety test, etc.
The influence of the N/P ratio on the positive pole
Excessive N/P ratio will lead to an increase in the oxidation state of cathode materials. In addition to causing safety problems, what are the hidden dangers? Here is only an example of ternary/graphite materials.
First of all, clarify two concepts:
Concept 1: First of all, it is necessary to clarify the different positions of the pole, even the reactions in different positions of the particles are uneven, which involves the problem of potential difference in the direction of the thickness of a pole.
Concept 2: Ni3+/4+ and Co3+/4+ overlap with O, and O will be free radical-formed from the lattice, which is highly oxidizing.
The mechanism of yellowing diaphragm oxidation has been very clear. It has been reported in the literature [1] that PS and other easily oxidized protective additives are added to the electrolyte, which can relieve diaphragm oxidation.
It has been reported in the literature [2] In the negative MCMB material because the interface potential of the negative powder and the collector is the most negative, lithium salt deposition first occurs in the contact position between the negative powder and the collector. The cross-section SEM diagram of MCMB material clearly observes the presence of lithium salt deposition at the contact interface between the negative material and the collector, but No graphite materials have been observed. However, there is little literature on the cathode SEI membrane. Because the cathode powder is in contact with the collector at a high potential and has high oxidation, it is assumed that a layer of cathode lithium salt sediment will be formed (the reaction is accelerated at high temperature), which hinders the contact between the cathode powder and the collector, resulting in cathode powder. Positive peeling increases the internal resistance and directly leads to the failure of circulation under high-temperature conditions.
The influence of the N/P ratio on the negative electrode
The extra Li will provide a Li source for the deposition of lithium salts on the negative surface [2], and the continuous deposition of lithium salt leads to the failure of the cycle. Therefore, too low an N/P ratio will increase this risk. But let’s discuss what may happen in another dimension. What happens if the N/P ratio is too high? The same positive electrode is used here, which causes a difference in the N/P ratio by adjusting the negative pole dosage. At the end of the discharge, the N/P ratio is lower than the positive and negative voltages, the positive electrode is deep, and the negative electrode is shallow. At the charging end, the positive and negative voltages with also N/P ratios are low, and the negative electrode is deeply charged and the positive electrode shallow.
It should be noted that:
- One potential curve in the figure represents the two processes of charging and discharging, which can be regarded as the potential of the equilibrium state.
- The first effect of the positive pole causes capacity attenuation to be ignored here. Even after the first effect loss, the N/P ratio corresponds to the same positive curve compared to different negative poles. It is believed that the positive first effect loss is only caused at the starting end of charging, and the film at the charging end is ignored here due to oxidation. In fact, only with the progress of the cycle, oxidation into film will affect capacity.
- The negative first-effect ratio is considered to have nothing to do with the N/P ratio. It is a constant, with a lot of negative effects, and the capacity lost through the first effect is also large. The reaction stage is also at the beginning of charging.
- The positive and negative potentials are free, and the only limitation is the voltage of the whole battery, that is, the blue vertical double arrow. The two double arrows at the discharge end and the charging end are equal in length.
- The difference between the two red dotted lines, that is, the difference in potential, shows the depth of the corresponding electrode charge and discharge.
Because the proportion of negative electrodes is the same in the first reaction, and the total amount of negative electrodes are different, the negative and discharge curves with many negative electrodes and very few negative electrodes correspond to the phase difference in the same positive electrode charging and discharge curve.
Because the positive potential gradually decreases with the increase of the voltage of embedded lithium (discharge process), in the process of the rise of the negative de-de Li/negative voltage, the use position of the positive discharge curve corresponding to the end of the negative discharge curve of the more negative electrode and the small negative electrode discharge curve is different, and the positive voltage corresponding to the end of the negative electrode discharge end of the very small negative electrode is even lower.
In order to achieve the same full battery voltage, the very little negative voltage rises to low, which also avoids the excessive level of negative detachment Li. Too much negative delux Li will cause damage and reorganization of the SEI membrane, which will cause circulatory failure. This analysis method can also be applied to the charging end, and it is concluded that the positive electrode is shallowly charged and the negative electrode is deeply charged in the case of positive extreme excess.
Summary: Batteries with smaller N/P ratios, that is, batteries with insufficient negative poles, can reach a shallow charge and deep discharge state in the cycle, and the negative electrode is deep charging shallow. The opposite is true.
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