Transference number, denoted as ‘t+’, is a dimensionless quantity representing the fraction of the total current carried by lithium ions in an electrolyte. A transference number close to 1 indicates that the majority of the current is carried by lithium ions, which is desirable for efficient energy transfer and high performance in lithium-ion batteries. The transference number influences various performance metrics of batteries, including their charge-discharge efficiency, rate capability, and cycle life.
Battery separators, which are thin layers between the anode and cathode, are vital for preventing electrical shorts while facilitating ion transport. The separator’s material and structure should be meticulously selected to ensure it supports a high transference number. For example, a separator with high ionic conductivity allows for efficient lithium-ion movement, while a low transference number might result in inadequate ion transport, ultimately undermining battery performance.
Materials commonly used for separators include polyethylene (PE), polypropylene (PP), and their composites, which offer satisfactory mechanical strength, thermal stability, and ionic conductivity. However, the pursuit of enhanced battery performance has necessitated the development of separators that can sustain higher transference numbers.
Recent advancements in the separators have yielded innovative materials and designs that improves the transference number. Ceramic-coated separators exhibit superior ionic conductivity and thermal stability compared to conventional polymer-based separators.
Another significant breakthrough is the introduction of single-ion conductive polymers as separator materials. These polymers facilitate the movement of lithium ions exclusively, resulting in a transference number approaching unity. Single-ion conductive polymers have emerged as promising materials for crafting high-performance lithium-ion battery separators owing to their potential to substantially increase energy efficiency and cycle life.
It is important to consider the transference number when evaluating and selecting separators for lithium-ion batteries, given its decisive impact on battery performance.