Hybrid Ion Conductor: Polysulfide Exclusion for Advanced Lithium Sulfur Batteries

Today’s lithium ion batteries, which have more than twice energy of those first released 25 years ago, power most of mobile electronic devices. However, their energy density is not high enough to provide electric vehicles and drones with mobility freedom. To make electric vehicles and drones more affordable, one needs batteries that can offer a longer cruise range with a lower cost. In this regard, lithium–sulfur (Li–S) battery is now considered a promising candidate to succeed lithium-ion batteries owing to its high theoretical energy density of 2500 Wh∙kg-1, non-toxic nature, low cost, and natural abundance. Despite such promises, the commercialization of Li–S batteries has yet to succeed even after sustained effort spanning several decades. Significant problems encountered have included low sulfur utilization, short cycle life, low cycling efficiency, and high self-discharge rate. These are mainly attributed to a process known as the polysulfide (PS) shuttle; PS chains dissolved in the electrolyte diffuse to the Li anode where they directly react with the Li metal to produce lower order PS species, which diffuse back to the sulfur cathode to regenerate higher PS forms. The PS shuttle leads to incomplete charging of the sulfur electrode, corrosion of the Li electrode, and formation of electrochemically inactive lithium sulfates (LixSOy) on the sulfur electrode, thus resulting in poor battery performance. Prevention of the PS shuttle is therefore extremely important for the practical use of Li–S batteries. In this talk, we present a single ion conductor which effectively rejects PS when used as an electrolyte medium for lithium sulfur battery and demonstrate a quasi-solid state lithium sulfur battery employing it. The ion conductor features a perfluorinated lithium sulfonate polymer swollen with organic polar solvents. For the ion conduction, Li+ is the sole charge carrier, because the SO3 - groups attached to the polymer chain are immobilized and the ion conductor does not include any bi-ionic lithium salt. The solvents selected from an intensive screening process dissociate polymeric lithium salts and form a 5~6 nm-sized Li+ conducting channels. As a result, the ion conductivity of the ion conductor is as high as 10-4 S cm-1 in its quasi-solid state. To our interest, the PS solubility of the hybrid ion conductor is quite low although the solvents have high PS solubility. This behavior originates from Donnan exclusion principle; the fixed negative charges decrease the Donnan potential of the conductor, with lowering its equilibrium PS concentration. Such Donnan exclusion effect is more intensified in the absence of bi-ionic lithium salt, therefore, to strengthen PS rejection function, the hybrid ion conductor is designed without any lithium salts. The sulfur battery based on the hybrid ion conductor has highly interesting features. The composite sulfur cathode comprising of sulfur/carbon composite and the hybrid ion conductor effectively confines PS in the vicinity of the carbon matrix owing to the nearby PS-rejecting hybrid ion conductors. Moreover, the polymer electrolyte membrane between the sulfur cathode and lithium further blocks PS passage. Owing to its quasi-solid state nature, the hybrid ion conductor allows the design of bipolar stack-type lithium sulfur battery. Because the electrolyte phases of each cell are spatially separated in bipolar configuration, shunt current, which is unavoidable for liquid electrolyte based batteries, can be eliminated. The electrochemical characteristics and performances of the quasi-solid state lithium sulfur battery are presented and the underlying physics is discussed.

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