1. Voltage resolution of lithium-ion batteries
(1) Open circuit voltage: refers to the voltage of the lithium-ion battery when it is not in working condition. In this state, there is no current flowing inside the battery, and its voltage is represented by the potential difference between the positive and negative electrodes. When the battery is fully charged, the open-circuit voltage is typically around 3.7V and can be as high as 3.8V in some cases.
(2) Working voltage: Compared to the open circuit voltage, it is the voltage of the lithium-ion battery in the working state. At this time, there is current flowing through the battery, and when the current passes through, it will be hindered by internal resistance, so the operating voltage is always lower than the open circuit voltage when fully charged.
(3) Termination voltage: that is, the critical voltage at which the battery should not continue to discharge after it is discharged to a certain voltage value. This voltage value is determined by the lithium-ion battery's own structure, and under the action of the protection plate, the voltage of the battery usually stabilizes at around 2.95V when the discharge is terminated.
(4) Standard voltage: From the principle level, standard voltage is also known as rated voltage, which is the standard value of the potential difference generated by the chemical reaction of the positive and negative electrode materials of the battery. The rated voltage of lithium-ion batteries is 3.7V, which shows that the standard voltage is actually the operating voltage in the standard state.
Judging from the four lithium-ion battery voltages mentioned earlier, the voltages involved in the working state are standard voltage and working voltage; When not in the working state, the voltage is reflected as the open circuit voltage and the end voltage. Because the chemistry of lithium-ion batteries is repeatable, they need to be charged promptly when the battery voltage drops to the termination voltage. If it is not charged for a long time, it will lead to a significant reduction in battery life, and in severe cases, it may even be scrapped.
During the entire discharge process, the voltage curve of the lithium-ion battery can be divided into three stages:
2. The voltage of lithium batteries is closely related to the electrode potential of the positive and negative electrode materials of the battery
The voltage of lithium batteries varies depending on the material, mainly for the following reasons:
(1) The influence of the chemical properties of electrode materials
The charging and discharging process of lithium batteries is essentially the process of lithium ions migrating between positive and negative electrodes, and the chemical properties of electrode materials are the core factors that determine the battery voltage. Taking common cathode materials as an example, the cobalt element in lithium cobalt oxide (LiCoO₂) has a high redox potential, which makes it easier to release lithium ions and output electrons when working. When paired with a graphite anode, the resulting battery voltage can reach around 3.7V. The lithium iron phosphate (LiFePO₄) cathode material, because the redox potential of iron is lower than that of cobalt, the voltage of the battery composed of graphite anode is usually stable at about 3.2V. The root cause of this voltage difference lies in the difference in the distribution and chemical structure of the electron cloud of different elements, which in turn leads to differences in their ability to gain and lose electrons and release lithium ions.
(2) Voltage changes caused by crystal structure differences
The influence of the material's crystal structure on lithium battery voltage is equally important. Ternary materials (Li (NiCoMn) O₂) are typical representatives, and the three elements of nickel, cobalt and manganese optimize the crystal structure of the material through synergistic action, so that the diffusion path of lithium ions is smoother, and the embedding and escape processes are smoother. When matched with a suitable negative electrode, a higher voltage platform can be formed, typically between 3.6-3.7V. Looking at lithium manganese oxide (LiMn₂O₄), its spinel structure has the problem of manganese ion dissolution during charging and discharging, which will hinder the diffusion of lithium ions, resulting in a relatively low battery voltage of about 3.0V. Obviously, differences in crystal structure can significantly affect the transport performance of lithium ions in the material, which in turn has an impact on the battery voltage.
(3) The relationship between energy density and voltage
There is a strong correlation between the energy density of the electrode material and the battery voltage. Materials with high energy density are able to store more energy per unit mass or volume, which usually corresponds to higher voltages. For example, with the increase of nickel content, the energy density of the material increases, and the battery voltage will also increase. This not only improves the overall performance of the battery, but also meets some application scenarios that require high energy. However, early lithium battery materials, due to their low energy density, could not store enough energy in the unit, and the corresponding voltage was also low, making it difficult to meet the needs of modern equipment for high energy and high voltage.
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