Integrating Dually Encapsulated Si Architecture and Dense Structural Engineering for Ultrahigh Volumetric and Areal Capacity of Lithium Storage

High-theoretical-capacity silicon anodes hold promise in lithium-ion batteries (LIBs). Nevertheless, their huge volume expansion (∼300%) and poor conductivity show the need for the simultaneous introduction of low-density conductive carbon and nanosized Si to conquer the above issues, yet they result in low volumetric performance. Herein, we develop an integration strategy of a dually encapsulated Si structure and dense structural engineering to fabricate a three-dimensional (3D) highly dense Ti3C2Tx MXene and graphene dual-encapsulated Si monolith architecture (HD-Si@Ti3C2Tx@G). Because of its high density (1.6 g cm-3), high conductivity (151 S m-1), and 3D dense dual-encapsulated Si architecture, the resultant HD-Si@Ti3C2Tx@G monolith anode displays an ultrahigh volumetric capacity of 5206 mAh cm-3 (gravimetric capacity: 2892 mAh g-1) at 0.1 A g-1 and a superior long lifespan of 800 cycles at 1.0 A g-1. Notably, the thick and dense monolithic anode presents a large areal capacity of 17.9 mAh cm-2. In-situ TEM and ex-situ SEM techniques, and systematic kinetics and structural stability analysis during cycling demonstrate that such superior volumetric and areal performances stem from its dual-encapsulated Si architecture by the 3D conductive and elastic networks of MXene and graphene, which can provide fast electron and ion transfer, effective volume buffer, and good electrolyte permeability even with a thick electrode, whereas the dense structure results in a large volumetric performance. This work offers a simple and feasible strategy to greatly improve the volumetric and areal capacity of alloy-based anodes for large-scale applications via integrating a dual-encapsulated strategy and dense-structure engineering.