Careful inventory of four key parameters of lithium batteries

——Lithium battery cathode materials NMP solvent supplier

With the development and growth of the lithium battery industry, various preparation data of lithium batteries have been exposed one after another. Today, the editor has carefully sorted out the four key parameters of lithium batteries. Although we are the production and supplier of NMP (N-methylpyrrolidone) solvent for the cathode material of lithium batteries, we are also very concerned about the preparation process and various parameters of lithium batteries. Lithium batteries are mainly composed of positive electrodes, negative electrodes, electrolytes and separators. It seems simple to say, but in fact every parameter of each part affects the overall performance of the lithium battery.

[Energy Density] Batteries in different application directions have different performance requirements. However, energy density is the most important parameter to measure the ability of a battery system to store electrical energy, and the formula is fixed. Energy density is divided into mass energy density and volume energy density, which are respectively applied to different scenarios.

Mass energy density is defined as the battery energy density per unit mass: εM=ΔrGθ/ΣM

The volume energy density is defined as the battery energy density per unit volume: εv=ΔrGθ /Σ VM

According to the above formula and the Nernst equation, the specific energy and specific capacity of the battery can be calculated by the following formulas:

Specific energy: W=n·F·E*·1000/M (W h kg-1)

Specific capacity: Capacity=nF/3.6M

[What do these letters represent? 】

n is the number of alkali metal ions participating in the electrochemical reaction, F is the Faraday constant, E* is the average working voltage of the battery, and M is the sum of the relative molecular weights of the positive and negative materials of the battery.

It can be seen from the above equation that the energy density of the battery system is proportional to the voltage and specific capacity of the system. Therefore, the two main factors to improve the energy density are to increase the output voltage and specific capacity of the battery.

So how to increase the output voltage of the full battery? Usually, it is necessary to select positive electrode materials with higher working voltage (that is, those raw materials generally used in conjunction with NMP (N-methylpyrrolidone) solvent), and negative electrode materials with lower working voltage. However, at the same time, the improvement of the working voltage is also limited by the electrolyte system, and the current commercial electrolyte voltage range is usually below 4.5V, and it is difficult to obtain further improvement.

Therefore, another factor is particularly important: increase the specific capacity! To increase the specific capacity in a battery system, the electrode material should have a lower relative molar mass and have more electron reactions: let M decrease and n increase.

[The initial coulombic efficiency] This concept is more complicated, and people who have no basic knowledge may not understand it. Simply put, the initial coulombic efficiency (ICE) is a performance indicator used to quantify the negative electrode material of lithium-ion batteries.

The specific definition is: the ratio of the discharge capacity to the charge capacity of the lithium-ion battery in the first charge-discharge cycle. With the progress of the battery charge and discharge cycle, the battery power has been attenuated; that is to say, the initial coulombic efficiency corresponds to the “life peak” of the lithium battery!

The irreversible capacity that leads to the reduction of the initial coulombic efficiency mainly comes from the decomposition of the electrolyte that occurs when the material is discharged to a low voltage during the first discharge, and the process of producing the SEI film, as well as some related side reactions. The first irreversible reaction will reduce the initial coulombic efficiency of the material and affect the matching of the full cell. This phenomenon is more pronounced in nanomaterials.

[Power density] This is another important parameter that reflects the battery’s charge and discharge capability. It represents the speed at which the battery outputs energy during the discharge process, and corresponds to the battery’s rate performance.

The formula for power density: P=I(V-IR int)

Among them, I is the charge and discharge current of the battery, V represents the voltage, and Rint corresponds to the internal resistance of the battery. It can be seen that the smaller the internal resistance of the battery, the greater the power density. The internal resistance is mainly related to the ionic conductivity of the electrolyte, the ionic conductivity of the electrode material, the electronic conductivity, the charge transfer kinetics, the storage mechanism of the battery and other factors; it is also related to the interface resistance between the electrode and the electrolyte ( Rct) related. This is also an important reference factor for the selection of electrode materials.

[Battery charge and discharge rate] The charge and discharge rate refers to the current value required to discharge its rated capacity (Q) within a specified time, which is numerically equal to the multiple of the battery’s rated capacity. That is, charge and discharge current (A) / rated capacity (Ah), and its unit is generally C (abbreviation of C-rate), such as 0.5C, 1C, 5C, etc.

For example, for a 24Ah battery:

With 48A discharge, the discharge rate is 2C, conversely speaking, 2C discharge, the discharge current is 48A, and the discharge is completed in 0.5 hours;

When charging with 12A, the charging rate is 0.5C. Conversely, when charging at 0.5C, the charging current is 12A, and the charging is completed in 2 hours;

The charge and discharge rate of the battery determines how fast we can store a certain amount of energy into the battery, or how fast we can release the energy in the battery.

The above is the concept analysis and formula description of the four key parameters of lithium batteries. If you have more information, please continue to leave a message for discussion.

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