Modulation of Surface Chemistry to Overcome the Challenges in Zinc Ion Batteries

Rechargeable aqueous zinc ion batteries (ZIBs) are considered promising candidates for large-scale energy storage systems by virtue of their high safety and low cost. Nevertheless, the application of ZIB has been hesitant because of the low cyclability caused by cathode dissolution, non-uniform Zn deposition, and unwanted hydrogen evolution reaction (HER). In this presentation, we present strategies to overcome the above-mentioned problems in ZIBs via modulating surface chemistry of electrode materials.<br/>Manganese oxide (typically MnO₂) is acknowledged as one of the most promising cathode materials in ZIB due to its high voltage and high specific capacity. However, Mn²⁺ continues to dissolve from MnO₂, resulting in low capacity retention of MnO₂. The problematic Mn²⁺ dissolution originates from the charge disproportionation (CD) reaction of Mn³⁺. Electrically degenerate Mn³⁺ has labile energy states and therefore participates in the relaxation process (CD reaction) producing non-degenerate Mn²⁺ and Mn⁴⁺. Several works have notably improved the capacity retention of MnO₂ by coating the surface with physical barriers to prevent Mn²⁺ dissolution. However, these approaches do not address the fundamental cause of the Mn²⁺ formation and only physically suppress the Mn²⁺ dissolution. In order to properly target the underlying cause of Mn²⁺ dissolution, we propose a dicyandiamide-induced cooperative Jahn-Teller distortion (JTD) of Mn⁴⁺. Triggering the cooperative JTD of Mn⁴⁺ alleviates lattice distortion and impedes the CD reaction of Mn³⁺. Thus, the cycling stability of MnO₂ is improved by more than 170% compared to pristine MnO₂, and exhibits a 99.9% Coulombic efficiency at 1 C rate condition.<br/>With the increasing demand on high current density and high areal capacity, several issues with negative electrode is also revealed to impinge the cycle life of ZIB. This is particularly true of uneven Zn deposition/dissolution reaction and H₂O decomposition reaction considering that these problems are aggravated under harsh operating conditions, consequently deteriorating the reversibility of Zn electrodes. Although numerous reports have challenged these issues by adopting various strategies, the positive effects have been limited to low current densities, low areal capacities, and low Zn utilizations. Here, we report a polymer of intrinsic microporosity (PIM-1) as an interface modulator and ion regulating layer, which reduces the HER and promotes a uniform Zn deposition/dissolution process. Experimental and computational methods are used to demonstrate that PIM-1 improves the reaction kinetics of Zn metal electrodes by altering the solvation structure of Zn²⁺ ions and increasing the work function of Zn surface. As a result, the PIM-1 coated Zn (PIM-Zn) electrode shows significantly improved cycling stability compared to the bare Zn electrode (1700 h and 340 h at 0.5 mA cm^-2, respectively). Moreover, PIM-Zn can operate for more than 200 h at 70% Zn utilization even under 10 mA cm^-2 and 110 h at 95% Zn utilization of Zn metal electrode. The cycle life of the Zn||V₂O₅ full cell is also extended by more than 7 times compared to the cell using bare Zn by adopting the PIM-Zn.<br/>Aqueous ZIB with high safety is an attractive battery system, and there is a persistent consensus that it can partake in the field of future energy storage system. However, it is true that the current technology level of ZIB is far below the level required for commercialization. We believe it is necessary to modulate the surface chemistry of the positive and negative electrode to build better ZIBs.