Yolk-Shell Germanium@Polypyrrole Architecture with Precision Expansion Void Control for Lithium Ion Batteries.

The key properties of yolk-shell architecture in improving electrochemical performance lies in its uniformity and the appropriate void space, which can expand/contract freely upon lithium alloying and leaching without damaging the outer shell, while being achievable with minimal sacrifice of volumetric energy density. Therefore, we developed a highly controllable strategy to fabricate a uniform porous germanium@polypyrrole (PGe@PPy) yolk-shell architecture with conformal AlO sacrificial layer by atomic layer deposition (ALD) process. The PGe@PPy yolk-shell anode fabricated with 300 ALD cycles delivers excellent electrochemical performance: high reversible capacity (1,220 mA hr g), long cycle performance (>95% capacity retention after 1,000 cycles), and excellent rate capability (>750 mA hr g at 32 A g). Electrodes with high areal capacity and current density were also successfully fabricated, opening a new pathway to develop high-capacity electrode materials with large volume expansion.