▎ 摘　　要

Tin-based composites hold promise as anodes for highcapacity lithium/sodium-ion batteries (LIBs/SIBs); however, it is necessary to use carbon coated nanosized tin to solve the issues related to large volume changes during electrochemical cycling, thus leading to the low volumetric capacity for tin-based composites due to their low packing density. Herein, we design a highly dense graphene-encapsulated nitrogen-doped carbon@Sn (HD N-C@Sn/G) compact monolith with Sn nanoparticles double-encapsulated by N-C and graphene, which exhibits a high density of 2.6 g cm(-3) and a high conductivity of 212 S-1. The as-obtained HD N-C@Sn/G monolith anode exhibits ultrahigh and durable volumetric lithium/sodium storage. Specifically, it delivers a high volumetric capacity of 2692 mAh cm(-3) after 100 cycles at 0.1 A g(-1) and an ultralong cycling stability exceeding 1500 cycles at 1.0 A g(-1) with only 0.019% capacity decay per cycle in lithium-ion batteries. Besides, in situ TEM and ex situ SEM have revealed that the unique double-encapsulated structure effectively mitigates drastic volume variation of the tin nanoparticles during electrode cycling. Furthermore, the full cell using HD N-C@Sn/G as an anode and LiCoO2 as a cathode displays a superior cycling stability. This work provides a new avenue and deep insight into the design of high-volumetric- capacity alloy-based anodes with ultralong cycle life.