There is three-stage charging strategy for Li-ion batteries: soft start for wake-up, constant current fast-charge and constant voltage fully charge. So why are there three charging stages? More info shown as below.
To charge Li-ion batteries, we know the higher the charging cut-off voltage, the shorter the cycle life and the faster the capacity drop. The larger the charging rate, the faster the capacity attenuation speed.
Based on Li-ion batteries' chemical properties, during the charging process, the Li+ protrudes from the positive electrode into the electrolyte and moves to the negative electrode, and then enters the negative electrode composed of graphite. If the charging speed is too fast, it will make it too late for Li+ to enter the negative electrode grid, and Li+ will gather in the electrolyte near the negative electrode. These Li+ close to the negative electrode are likely to capture an electron from the negative electrode and become a metal Li. Continuous lithium metal generation will accumulate near the negative electrode and grow into dendrites. The growth of dendrites will eventually pierce the diaphragm between the positive and negative stages, forming a short circuit.
In another case, as the negative electrode is filled to a higher degree, there are fewer spaces left in the LiC lattice, the chance of Li+ moving from the positive electrode to find the space is getting smaller. And the time required is getting longer. If the charging speed remains the same, it's also possible to form a local accumulation of Li+ on the negative electrode. Therefore, the charging current must be gradually reduced in the second half of charging.
To charge Li-ion batteries, we know the higher the charging cut-off voltage, the shorter the cycle life and the faster the capacity drop. The larger the charging rate, the faster the capacity attenuation speed.
Based on Li-ion batteries' chemical properties, during the charging process, the Li+ protrudes from the positive electrode into the electrolyte and moves to the negative electrode, and then enters the negative electrode composed of graphite. If the charging speed is too fast, it will make it too late for Li+ to enter the negative electrode grid, and Li+ will gather in the electrolyte near the negative electrode. These Li+ close to the negative electrode are likely to capture an electron from the negative electrode and become a metal Li. Continuous lithium metal generation will accumulate near the negative electrode and grow into dendrites. The growth of dendrites will eventually pierce the diaphragm between the positive and negative stages, forming a short circuit.
In another case, as the negative electrode is filled to a higher degree, there are fewer spaces left in the LiC lattice, the chance of Li+ moving from the positive electrode to find the space is getting smaller. And the time required is getting longer. If the charging speed remains the same, it's also possible to form a local accumulation of Li+ on the negative electrode. Therefore, the charging current must be gradually reduced in the second half of charging.
