Lithium-ion battery safety has always been a concern in the industry. Due to increasingly stringent energy density requirements from both application and policy perspectives, the trend of ternary lithium batteries becoming the mainstream technology is irreversible. However, to this day, the safety concerns plaguing ternary lithium batteries remain largely unresolved. Even Tesla, which boasts the world's best BMS (Battery Management System), has experienced numerous safety incidents; in 2017 alone, two Model S vehicles in China experienced serious fires. The safety of ternary lithium batteries continues to be questioned.
With the development of new energy vehicles, high-energy-density and high-safety batteries have become a fiercely contested market objective. Some experts believe that using solid-state electrolytes to replace traditional electrolytes is the only way to fundamentally improve the safety of lithium batteries.
All-solid-state lithium-ion batteries use solid electrolytes instead of traditional organic liquid electrolytes, which is expected to fundamentally solve battery safety issues and is an ideal chemical power source for electric vehicles and large-scale energy storage. The structure of an all-solid-state lithium-ion battery includes a positive electrode, an electrolyte, and a negative electrode, all composed of solid materials.
Advantages compared to traditional electrolyte lithium-ion batteries
Solid-state batteries completely eliminate the safety hazards of electrolyte corrosion and leakage, and offer higher thermal stability. Because liquid electrolytes contain flammable organic solutions, they are prone to combustion and explosion during short-circuit temperature spikes, requiring the installation of safety devices to withstand temperature rises and prevent short circuits. Solid-state electrolytes, on the other hand, are non-flammable, non-corrosive, non-volatile, and do not leak, and they also overcome the lithium dendrite phenomenon. Therefore, all-solid-state batteries possess extremely high safety.
It eliminates the need for liquid encapsulation, supports serial stacking and bipolar structures, and improves production efficiency;
Due to the solid-state nature of solid electrolytes, multiple electrodes can be stacked.
It has a wide electrochemical stability window (up to 5V and above), making it compatible with high-voltage electrode materials.
Solid electrolytes are generally single-ion conductors, with almost no side reactions and a longer service life.
Relatively lighter. Compared to liquid batteries, solid-state electrolyte batteries are relatively lighter for the same capacity. For example, the ternary lithium battery packs produced by Tesla-Panasonic weigh 900kg, while the same capacity battery pack produced by solid-state battery startup SeeoInc weighs only 323kg, nearly one-third of the former.
However, solid-state batteries also have drawbacks. The generally low conductivity of solid-state electrolytes leads to lower rate performance, higher internal resistance, slower charging speeds, and higher overall costs. Currently, solid-state batteries do not have a significant advantage in competing with conventional lithium-ion batteries in the traditional market. Therefore, leveraging the high safety, high-temperature stability, and potential flexibility of solid-state batteries to compete in a differentiated market may be the most promising direction for a market breakthrough in the near future.
Polymer solid electrolyte (SPE)
Composition: It consists of a polymer matrix (such as polyester, polyenzyme and polyamine) and lithium salts (such as LiClO4, LiAsF4, LiPF6, LiBF4).
Features: Lightweight, good viscoelasticity, and excellent machinability.
Main categories: polyethylene oxide (PEO), polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF), polymethyl methacrylate (PMMA), polypropylene oxide (PPO), polyvinylidene chloride (PVDC), and single-ion polymer electrolytes.
Mechanism: Ion transport in solid polymer electrolytes mainly occurs in the amorphous region. However, unmodified PEO at room temperature has high crystallinity, resulting in low ionic conductivity and severely affecting high-current charge-discharge capability. Researchers have improved the conductivity of the system by reducing crystallinity to enhance the mobility of PEO chain segments. The simplest and most effective method is to hybridize the polymer matrix with inorganic particles. Currently, the most studied inorganic fillers include metal oxide nanoparticles such as MgO, Al2O3, and SiO2, as well as zeolites and montmorillonite. The addition of these inorganic particles disrupts the orderliness of polymer chain segments in the matrix, reduces its crystallinity, and the interactions between the polymer, lithium salt, and inorganic particles increase lithium-ion transport channels, thereby improving conductivity and ion transference number. Inorganic fillers can also adsorb trace impurities (such as moisture) in the composite electrolyte and improve mechanical properties.
Overall, the manufacturing technology for solid-state batteries still needs further development, and only a limited number of companies can achieve large-scale production. Many difficulties remain to be overcome in scaling up production, and the technology is still in its early stages of promotion and development. However, it is expected that with continuous advancements in research and industrial technology, the scientific and technological challenges of all-solid-state batteries will gradually be alleviated, and the market for solid-state battery products will experience rapid growth in the coming years.
