Viologen Covalent Organic Frameworks and their Self-Exfoliated Nanosheets Cathodes for Sodium-Ion Batteries

The proposed project plans to carry out a systematic investigation on the physicochemical and electrochemical properties of viologen-based organic (VCOF) and hybrid (VCOF-MXN) materials for SIBs correlating their structure-property-performance relationship. Considering the theoretical capacity and voltage output of the VCOF cathode, the work expects an energy density of ≈ 100 Wh kg-1 in the full-cell (state-of-the-art SIB prototypes ≈ 150 Wh kg-1). With the incorporation of MXene, the motive is to extend the full-cell cycling stability (> 1000 cycles) and capacity retention (> 90%) in VCOF-MXN. The SIB full-cell in pouch cell configuration with a high areal capacity (1-2 mAh cm-2) will be achievable since MXN offsets the requirement of additional conducting additives in the electrode. It is also highly possible to process the VCON/MXN hybrid material into self-standing electrodes owing to the 2D layer-by-layer assembly, avoiding the need for the current collector and polymeric binders to improve energy density further. The volumetric capacity will also be evaluated and optimized during the project, e.g., by pressing the electrodes.
This proposal discusses a new concept of ‘beyond-intercalation’ organic electrodes for SIBs based on 2D COF possessing electrochemically active viologen linkers (VCOF) for the first time. The insights on the redox reactions in VCOF involving both anions and cations (a dual-ion mechanism) offer an exciting scope to design a new battery configuration based on a ‘two-in-one’ electrode. The ordered porous structure of VCOF will be advantageous for easy electrolyte infiltration and fast ion (cation and anions) diffusion kinetics, essential for achieving the high power capability of SIBs. The concept of integrating VCONs and MXNs (MXene nanosheets) is also innovative and rational, exploiting the counteracting properties (opposite surface charges) of these two materials mutually beneficial to each other. The electrically conducting MXNs in the hybrid material will prevent the restacking of the VCONs, help avoid additional conducting carbon additives, and maximize the active material utilization, thereby augmenting the gravimetric/volumetric capacity further. As a first attempt to utilize the VCOF and its hybrid as the electrode materials, the outcomes of this proposal will therefore be relevant in the context of other battery chemistries based on mono- and divalent ions.