A semi-solid-state battery (also formally known as a quasi-solid-state battery, QSSB) is a type of rechargeable battery that serves as an intermediate technology between conventional lithium-ion batteries (LIB) with liquid electrolytes and all-solid-state batteries (ASSB) using a hybrid solid-liquid semi-solid-state electrolyte. [1]
The primary goal of this technology is to improve battery safety by reducing the amount of flammable liquid electrolyte, thereby mitigating the risk of fire and dendrite growth associated with traditional LIBs. At the same time, it aims to overcome the significant challenges facing the ASSBs, such as high interfacial resistance and poor contact between the solid electrolyte and the electrodes. [2] [3] As of 2023, commercially available batteries marketed as "solid-state" were in fact quasi-solid-state batteries. [1]
The development of semi-solid-state batteries is driven by the distinct limitations of both liquid and all-solid-state electrolytes.
Semi-solid-state batteries are designed as a compromise to balance these factors. By using a small amount of liquid or a gel-like substance within a solid matrix, they maintain good interfacial contact and high ionic conductivity while significantly improving safety compared to traditional LIBs. [2]
The core of a semi-solid-state battery is the semi-solid-state electrolyte, which is typically a gel polymer electrolyte (GPE). A GPE consists of a solid polymer matrix that traps a liquid component (a plasticizer, such as a conventional liquid electrolyte). [2] This structure provides the mechanical stability of a solid while retaining the high ionic conductivity of a liquid. The properties of the GPE are largely determined by the choice of polymer matrix.
Another common approach is to create a composite electrolyte by dispersing inorganic ceramic fillers (such as LLZO, SiO₂, or BaTiO₃) into a polymer matrix. These ceramic fillers can enhance ionic conductivity and improve the mechanical and thermal stability of the electrolyte. This design also prevents the direct and often reactive contact between the ceramic material and the electrodes. [5]
The fabrication method for the gel polymer electrolyte is critical to the battery's final performance, particularly its interfacial resistance.
In the ex-situ method, a free-standing polymer membrane is prepared first through techniques like solution-casting or electrospinning. This membrane is then placed inside the battery cell during assembly and swollen with a liquid electrolyte to form the GPE. A significant drawback of this method is the potential for poor physical contact between the pre-formed membrane and the electrodes, which can result in high interfacial resistance. [11]
The in-situ method involves injecting a liquid precursor—a mixture of monomers, a lithium salt, and an initiator—directly into the assembled battery cell. The liquid fully penetrates the pores of the electrodes and then undergoes polymerization (triggered by heat or UV light) to form the GPE in place. This process creates an intimate and seamless interface between the electrolyte and the electrodes, significantly lowering interfacial resistance and improving overall battery performance. This method is considered more conducive to commercialization as it aligns well with existing battery manufacturing processes. [12]
While semi-solid-state batteries are significantly safer than conventional liquid-electrolyte batteries, they are not inherently immune to failure. The presence of even a small amount of liquid or gel plasticizer means that they still contain a flammable component. [13]
Comparative safety tests have shown that under external heating, QSSBs can still undergo thermal runaway, though the reaction may be initiated at a higher temperature and be slightly less energetic than in a traditional LIB. For failures caused by an internal short circuit (e.g., from dendrite penetration), the heat released can be comparable to that of a liquid-electrolyte battery. Therefore, while safety is enhanced, it is not absolute, and thermal management and battery protection systems remain crucial. [4]