# Core‐Shell: Resolving the Dilemma of Hard Carbon Anodes by Sealing Nanoporous Particles With Semi‐Permeable Coatings

## TL;DR

This paper introduces a core-shell design for hard carbon anodes that reduces first-cycle losses while maintaining high storage capacity for sodium-ion batteries.

## Contribution

A novel core-shell structure with semi-permeable shells is developed to separate SEI formation from Na storage in nanoporous cores.

## Key findings

- Core-shell anodes achieved 400 ± 24 mAh g−1 with 82 ± 2% first-cycle reversibility.
- Diethyl carbonate sorption analysis better predicts first-cycle efficiency than N2 or CO2 sorption.
- The design compensates for sodium's larger size compared to lithium in graphite anodes.

## Abstract

A core‐shell strategy is introduced to overcome the dilemma of common non‐graphitic hard carbon anodes, linking high reversible storage capacity to practically unacceptable irreversible losses in the first cycle(s). Just as graphite homogeneously combines effective lithium storage with an electrolyte solvent‐sieving function, we show that both of these functions could be strategically integrated into non‐graphitic carbons in a heterogeneous structure. Highly porous activated carbons are sealed by kinetically tuned gas‐phase deposition of non‐graphitic carbon to form a functional core‐shell structure. Gas sorption porosimetry on core, shell, core–shell, and cracked core‐shell particles confirms preserved core porosity and a semi‐permeable shell. Diethyl carbonate sorption analysis is introduced as a more suitable probe than N2 or CO2 sorption, linking first‐cycle losses to the liquid–solid interface of carbon anodes. The functional core‐shell particles with much reduced diethyl carbonate uptake allow for high storage capacity and reduced first cycle losses. Delivering 400 ± 24 mAh g−
1 with 82 ± 2% first‐cycle reversibility, it is shown that three‐stage Na storage in designed core‐shell anodes can compensate for the larger size of sodium compared to lithium stored in graphite anodes (372 mAh g−1). The designed core‐shell anodes show state‐of‐the‐art performance with commercial promise.

A core–shell strategy is presented to overcome the high first‐cycle losses of non‐graphitic hard carbon anodes. Activated carbon is sealed by gas‐phase carbon deposition to create semi‐permeable shells that spatially separate SEI formation from Na storage in a retained nanoporous core. The resulting anode delivers 400 ± 24 mAh g−1 with an ICE of 82 ± 2%. Diethyl carbonate vapor sorption quantifies the solid‐liquid interface to predict first‐cycle efficiency for adsorption‐based Na storage.