1. Basic concepts and units of energy density
Energy density is a key indicator for measuring the performance of lithium batteries. It reflects the amount of energy that a battery can store per unit volume or per unit mass. The energy density per unit volume is called volume energy density, and its unit is watt-hour per liter (Wh/L); the energy density per unit mass is called mass energy density, and its unit is watt-hour per kilogram (Wh/kg). Mass energy density can intuitively represent the amount of electrical energy that a battery can provide under a certain weight, which is very important for applications that are sensitive to weight (such as electric vehicles, aerospace equipment, etc.). Volume energy density considers more the ability of a battery to store energy in a limited space, and is a key performance indicator for some space-constrained devices (such as mobile phones, laptops, etc.).
2. Calculation of theoretical energy density
In theory, the energy density of a lithium battery can be calculated based on the chemical reaction formula of the battery. Taking lithium-ion batteries as an example, their energy density is related to the electrochemical potential difference between the positive and negative electrode materials and the number of electrons transferred in the battery reaction. Through theoretical tools such as the Nernst equation, the theoretical voltage of the battery can be calculated. Combined with the theoretical specific capacity of the active material in the battery (i.e., the amount of electricity that can be provided by the unit mass of the active material), the theoretical energy density can be obtained according to the formula energy density = voltage × specific capacity. However, the theoretical energy density is the result of calculation under ideal conditions. In actual situations, due to the incomplete utilization of electrode materials, the existence of battery internal resistance, and other side reactions, the actual energy density will be lower than the theoretical value.
3. Measurement method of actual energy density
In actual measurement, for mass energy density, it is necessary to accurately measure the mass of the lithium battery (including the total mass of all components such as the shell, electrodes, and electrolyte), and then perform a complete charge and discharge test on the battery through the charge and discharge equipment, and record the amount of electricity charged or discharged by the battery (in watt-hours). Finally, divide the amount of electricity by the mass of the battery to obtain the actual mass energy density. For the measurement of volume energy density, it is necessary to accurately measure the volume of the battery (including the volume of the shell). After the charge and discharge test, the amount of electricity divided by the volume is the volume energy density. During the measurement process, it is necessary to ensure that the charging and discharging conditions (such as charging and discharging current, temperature, etc.) comply with the relevant standards to ensure the accuracy and comparability of the measurement results.
4. Factors affecting the measurement results of energy density
There are multiple factors that affect the measurement results of the energy density of Lithium Battery. The first is the manufacturing process and quality of the battery. Different manufacturing processes may lead to differences in the density of electrode materials and the compactness of the internal structure of the battery, thereby affecting the measured value of energy density. Secondly, the charging and discharging conditions also have a great impact on the measurement results. For example, excessive charging and discharging currents may cause the polarization of the battery to increase, making it impossible for the battery to be fully charged and discharged, thereby reducing the measured energy density.
