High-Capacity CuSi₂P₃-Based Semisolid Anolyte for Redox Flow Batteries

Redox flow batteries (RFBs) have attracted more attention due to their ability of decoupling energy and power, but their low energy density has greatly restricted their applications. Semisolid flow batteries (SSFBs) are a kind of RFBs, but they have high energy density. However, there is a lack of research on semisolid anolytes, and thus the application of SSFBs is still in its infancy. In this work, a low-potential (0.6 V vs Li/Li⁺) CuSi₂P₃@C-LiPAA composite is synthesized through a simple high-energy mechanical ball milling and impregnation method based on the CuSi₂P₃ (CSP) compound; then, it is used to prepare a semisolid anolyte, which is able to achieve a high volumetric capacity of 400 Ah L^-1 in static mode and 320 Ah L^-1 in intermittent-flow mode. This is the highest volumetric capacity of anolyte so far. The effect of adding binder to a composite is also discussed for the first time, which makes the connection between the composite particles closer and the semisolid suspension more uniform so as to obtain stable electrochemical performance. At the same time, through pairing respectively with two types of catholytes, liquid 10-methylphenothiazine (MPT) and semisolid LiFePO₄ (LFP), a single-cell voltage of 3 V and more than 100 stable cycles with the Coulombic efficiency of 99% have been achieved by CSP-MPT and CSP-LFP full-cell systems. The result fully demonstrates the applicability of the prepared CSP semisolid anolyte. The synthesis method of adding a binder to the composite in this work also provides a direction for optimizing the suspension for other active materials to be applied to SSFBs in the future.