Chuang Sun,Xinlei Shi,Yabo Zhang,Jiajie Liang,Jie Qu,Chao Lai
Lithium-iodine (Li-I2) batteries are promising candidates for next-generation electrochemical energy storage systems due to their high energy density and the excellent kinetic rates of I2 cathodes. However, dissolution of iodine and iodide has hindered their widespread adoption for practical applications. Herein, a Ti3C2Tx MXene foam with a three-dimensional hierarchical porous architecture is proposed as a cathode-electrolyte interface layer in Li-I2 batteries, enabling high-rate and ultrastable cycling performance at a high iodine content and loading mass. Theoretical calculations and empirical characterizations indicate that Ti3C2Tx MXene sheets with high metallic conductivity not only provide strong chemical binding with iodine species to suppress the shuttle effect but also facilitate fast redox reactions during cell cycling. As a result, the Li-I2 battery using a cathode with 70 wt % I2 cycled stably for over 1000 cycles at a rate of 2 C, even at an ultrahigh loading mass of 5.2 mg cm–2. To the best of the authors’ knowledge, this is the highest reported loading at such a high iodine content. This work suggests that using a Ti3C2Tx MXene interface layer can enable the design and application of high-energy Li-I2 batteries.
The high reduction potential and the abundance of iodine have prompted its use in the positive electrolytes of aqueous flow batteries, where the transition between highly soluble iodide (I−) and triiodide (I3−) may give rise to superior rate-performance. Yet, the operating currents of Zn–I2 flow batteries remain low, for which the elusive kinetics of the poorly soluble intermediate, iodine (I2), is responsible. With cyclic voltammetry and chronoamperometry, we confirm that solid iodine passivates the carbon electrode, leading to an oscillating current even in a highly concentrated I− solution. UV-Vis spectroscopy and an interface-controlled dissolution model are used to measure the rate constant of the transition from I2 to I3− to be ∼10−6 mol (cm−2 s−1), which agrees with the steady-state current revealed by chronoamperometry and corroborates the current-limiting kinetics. To boost the kinetics, we screen common organic solvents for a cosolvent that can better solvate I2 and find that the addition of 7.5 vol% acetonitrile increases the rate constant by an order of magnitude and the current by more than six times. Calculations using density functional theory reveal the role of the cosolvent in weakening the interaction between iodine and the electrode surface. When employed in a Zn–I2 flow battery, the cosolvent enables a high charging current of over 100 mA cm−2, which is stable for more than 170 cycles, and is promising for high-performance energy storage at an affordable cost.
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