Examining the negative reactions that occur in lithium batteries during cycling, we can summarize the effects of these reactions into three major battery degradation scenarios and observe the impact of solid electrolytes on the degradation phenomena.
During the cycle, due to the volume expansion or contraction of the positive and negative electrodes, the SEI film will undergo fission and continue to grow. The growth process of the SEI film will consume active lithium, resulting in a decrease in the overall capacity of the battery and an increase in internal resistance; in addition, when charging , the positive electrode is in a highly oxidized state, and the reduction phase transition is prone to occur. The transition metals in the framework, such as cobalt ions, precipitate into the electrolyte and diffuse to the negative electrode, catalyzing the further growth of the SEI film, resulting in the consumption of active lithium. At the same time, due to the structure of the positive electrode When the negative electrode is damaged, the potential of the negative electrode becomes lower during charging, and Li+ diffuses from the positive electrode and intercalates into the negative electrode. When the temperature is too low or the charging current is too high, the intercalation speed of metal lithium is reduced, and it is directly precipitated out of the negative electrode. On the surface, the polarization effect is more severe. In addition to causing the loss of active lithium and increasing internal resistance, it will also form fatal lithium dendrites, which will cause internal short circuits in the long run.
Theoretically, the ions themselves do not move when the all-solid-state battery works, so the irreversible reactions will be reduced. If a solid-state electrolyte that is electrochemically stable with lithium is used, problems such as SEI and electrolyte degradation can also be slowed down, which can effectively reduce the consumption of lithium ions during charging and discharging. The magnitude of the capacity decline can reduce or inhibit the generation of lithium dendrites. For example, lithium lanthanum zirconium oxide (LLZO) with a garnet structure in oxide electrolytes has excellent chemical stability, while solid polymer electrolytes are still It is composed of lithium salt and polymer matrix, so its chemical stability is not much different from that of liquid polymer electrolytes.
The increase in volume is mainly due to the high oxidation state of the positive electrode during charging. The free oxygen in the crystal lattice is easy to precipitate and then oxidizes with the electrolyte to generate carbon dioxide and oxygen, which gradually cause swelling during charge and discharge cycles. The decomposition of the electrolyte is accelerated when the voltage is higher than 4.35V (ternary system) or in a high temperature environment, resulting in continuous expansion of the battery cell, which will affect the configuration of components in the device at least, and cause damage to the structure of the battery cell and cause fire and explosion.
The solid electrolyte is not easy to oxidize with the positive electrode due to the aforementioned chemical stability, which can slow down the rate of electrolyte decomposition and gasification, and greatly reduce the degree of volume expansion. In addition, the solid electrolyte can withstand voltages exceeding 5V without decomposition , so that the internal series technology is no longer out of reach. In fact, the increase of single-cell voltage can save part of the BMS and shunt, and greatly improve the energy density and cost of the module. It has already attracted Nissan and other companies to invest in research and development for more than ten years, but it has been unable to overcome the problem of electrolyte decomposition under high pressure.
Thermal runaway is the most harmful and unpredictable risk of lithium batteries. When the battery core is damaged by external force and causes a short circuit or internal short circuit or overcharge, the temperature inside the battery core will rise accordingly. Once it rises to 130 ° C, the SEI film It begins to disintegrate, and causes the organic electrolyte to directly contact the highly active positive and negative electrodes, so a large number of decomposition and exothermic reactions occur, resulting in a rapid increase in temperature and internal pressure, and a large amount of gas is generated to cause rapid expansion of the battery. After reaching the critical temperature, the positive electrode disintegrates, releasing more heat energy and oxygen, and the superposition of many factors causes the chain reaction of heating up, decomposition, and heat release to intensify, and finally ignites and explodes.
If the polymer electrolyte and separator, which originally started to generate a large amount of flammable gas and heat at around 150°C, are replaced with a solid electrolyte that vaporizes slowly at high temperatures and is non-flammable, the chain reaction of thermal runaway can be blocked. Avoiding fire and explosion accidents is like drawing salary from the bottom of the pot. However, there is a big gap in the thermal stability of different electrolyte systems. For example, the ignition point of oxide ceramic electrolytes is above 1,000 degrees, which can completely block thermal runaway reactions; and solid polymer electrolytes It will start to disintegrate at about 280°C, and has the worst thermal stability. So far, there has been no test record of solid polymer batteries maintaining stability above 300°C.