The parameter that describes the size of lithium-ion battery energy storage is energy density, which is approximately equivalent to the product of voltage and lithium battery capacity. In order to effectively improve 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, capacity improvement 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 greater breakthroughs? Ultimately, this is also determined by the material and structural properties of lithium batteries.
The voltage of a lithium battery is determined by the electrode potential. Voltage, also known as potential difference or potential difference, is a physical quantity that measures the energy difference generated by charges in an electrostatic field due to different potentials. The electrode potential of lithium ions is about 3V, and the voltage of lithium batteries varies with different materials. For example, a typical lithium-ion battery has a rated voltage of 3.7V and a full charge voltage of 4.2V; The rated voltage of lithium iron phosphate batteries is 3.2V, and the full charge voltage is 3.65V. In other words, the potential difference between the positive and negative electrodes of practical lithium-ion batteries cannot exceed 4.2V, which is a requirement based on material and usage safety.
If Li/Li+electrode is used as the reference potential, let μ A is the relative electrochemical potential of the negative electrode material, μ C is the relative electrochemical potential of the positive electrode material, and the electrolyte potential range Eg is the difference between the lowest electron unoccupied energy level and the highest electron occupied energy level in the electrolyte. So, what determines the highest voltage value of a lithium battery is μ A μ C. The three factors of Eg.
μ A and μ The difference in C is the open circuit voltage (highest voltage value) of lithium-ion batteries. When this voltage value is within the Eg range, it can ensure the normal operation of the electrolyte. The meaning of "normal operation" is that lithium-ion batteries move back and forth between the positive and negative electrodes through the electrolyte, but do 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 positive and negative electrode materials:
1. When the electrochemical potential of the negative electrode is higher than the lowest electron unoccupied energy level of the electrolyte, the electrons of the negative electrode will be taken away by the electrolyte, resulting in the oxidation of the electrolyte. 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 occupancy energy level of the electrolyte, electrons in the electrolyte will be taken away by the positive electrode, which will be oxidized by the electrolyte. The reaction products will 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 hindered by the presence of a solid-liquid interface layer, which prevents further movement of electrons between the electrolyte and the positive and negative electrodes, and instead protects the electrode material. This means that the lighter degree of solid-liquid interface layer is protective. The premise of this protection is that the positive and negative electrochemical potentials can slightly exceed the Eg range, but not too much. For example, the reason why graphite is mostly used as the negative electrode material for lithium-ion batteries nowadays is because the electrochemical potential of graphite relative to Li/Li+electrodes is about 0.2V, slightly exceeding the Eg range (1V~4.5V). However, due to the "protective" solid-liquid interface layer, the electrolyte is not further reduced, thus stopping the further development of polarization reaction. However, the 5V high-voltage cathode material exceeds the Eg range of commercial organic electrolytes by too much, making it highly susceptible to oxidation during charging and discharging. As the number of charging and discharging increases, the capacity decreases and the lifespan decreases.
Now I understand that the reason why the open circuit voltage of lithium-ion batteries is chosen as 4.2V is because the Eg range of existing commercial lithium battery electrolytes is 1V~4.5V. If the open circuit voltage is set to 4.5V, it may increase the output energy of the lithium battery, but it also increases the risk of overcharging. There is a considerable amount of information on the harm of overcharging, so I won't go into more detail here.
According to the above principles, there are only two ways for people to improve the energy density of lithium batteries by increasing the voltage value. 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.