2025 button cell battery

Why can't the maximum voltage of 2025 button cell battery exceed 4.2V?

The parameter that describes the energy storage capacity of 2025 button cell battery is energy density, which is approximately equivalent to the product of voltage and lithium battery capacity. In order to effectively increase the storage capacity of lithium batteries, people generally use the method of increasing battery capacity to achieve the goal. However, due to the nature of the raw materials used, the capacity increase is always limited, so increasing the voltage value becomes another way to improve the storage capacity of lithium batteries. As we all know, the nominal voltage of lithium batteries is 3.6V or 3.7V, and the maximum voltage is 4.2V. So why can't the voltage of lithium batteries achieve a greater breakthrough? In the final analysis, this is also determined by the material and structural properties of lithium batteries.

The voltage of lithium batteries is determined by the electrode potential. Voltage is also called potential difference or potential difference, which is a physical quantity that measures the energy difference generated by the different potentials of charges in an electrostatic field. The electrode potential of lithium ions is about 3V, and the voltage of lithium batteries varies with different materials. For example, the rated voltage of a general lithium-ion battery is 3.7V, and the fully charged voltage is 4.2V; while the rated voltage of a lithium iron phosphate battery is 3.2V, and the fully charged voltage is 3.65V. In other words, the potential difference between the positive and negative electrodes of a practical lithium-ion battery cannot exceed 4.2V, which is a requirement based on materials and safety of use.

If the Li/Li+ electrode is used as the reference potential, μA is the relative electrochemical potential of the negative electrode material, μC is the relative electrochemical potential of the positive electrode material, and the electrolyte potential interval Eg is the difference between the lowest electron unoccupied energy level and the highest electron occupied energy level of the electrolyte. Then, the three factors that determine the maximum voltage value of a lithium battery are μA, μC, and Eg.

The difference between μA and μC is the open circuit voltage (maximum voltage value) of a lithium-ion battery. When this voltage value is within the Eg range, the electrolyte can be guaranteed to work normally. "Normal operation" means that the lithium-ion battery moves back and forth between the positive and negative electrodes through the electrolyte, but does not undergo redox reactions with the electrolyte, thereby ensuring the stability of the battery structure. There are two forms of abnormal electrolyte operation caused by the electrochemical potential of the positive and negative electrode materials:

1. When the electrochemical potential of the negative electrode is higher than the lowest unoccupied energy level of the electrolyte, the electrons of the negative electrode will be captured by the electrolyte, so the electrolyte is oxidized, and the reaction products form a "solid-liquid interface layer" on the surface of the negative electrode material particles, which may cause the negative electrode to be damaged.

2. When the electrochemical potential of the positive electrode is lower than the highest electron-occupied energy level of the electrolyte, the electrons in the electrolyte will be captured by the positive electrode, and thus oxidized by the electrolyte, and the reaction products form a "solid-liquid interface layer" on the surface of the positive electrode material particles, which may cause the positive electrode to be damaged.

However, the possibility of damage to the positive or negative electrode is prevented by the existence of the "solid-liquid interface layer", which prevents the further movement of electrons between the electrolyte and the positive and negative electrodes, and protects the electrode materials instead. That is to say, the "solid-liquid interface layer" with a lighter degree is "protective". The premise of this protectiveness is that the electrochemical potential of the positive and negative electrodes can slightly exceed the Eg interval, but not too much. For example, the reason why most of the negative electrode materials of 2025 button cell battery are graphite is that the electrochemical potential of graphite relative to the Li/Li+ electrode is about 0.2V, which slightly exceeds the Eg range (1V~4.5V), but because of the "protective" "solid-liquid interface layer", the electrolyte is not further reduced, thus stopping the continued development of the polarization reaction. However, the 5V high-voltage positive electrode material exceeds the Eg range of the current commercial organic electrolyte by too much, so it is very easy to be oxidized during the charge and discharge process. As the number of charge and discharge increases, the capacity decreases and the life is shortened.

Now it is understood that the open circuit voltage of 2025 button cell battery is selected as 4.2V because the Eg range of the existing commercial lithium battery electrolyte is 1V~4.5V. If the open circuit voltage is set to 4.5V, it may increase the power output of the lithium battery, but it also increases the risk of overcharging the battery. There is a lot of information on the harm of overcharging, so I won't say more here.

According to the above principle, if people want to increase the energy density of lithium batteries by increasing the voltage value, there are only two ways to go. One is to find an electrolyte that can match the high-voltage positive electrode material, and the other is to perform protective surface modification on the battery.

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