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

摘要

class="head no_bottom_margin" id="sec1title">IntroductionThere has been a rapid increase in the demand for rechargeable lithium-ion batteries (LIBs) with long cycling lifetimes and high energy and power densities for next-generation advanced energy storage devices such as advanced portable electronics and fast-developing electric vehicles (, ). Owing to the limitation of the commercial graphite anode (theoretical capacity of only 372 mA hr g⁻¹), the demands for next-generation batteries with high energy and power densities cannot be met (, ). Toward this aim, a large variety of anode materials for rechargeable batteries have been explored, mainly including oxides (e.g., Fe3O4, GeO2, and SiOx) and alloys (e.g., Sb, Sn, and Ge), owing to their high theoretical capacities and safe operation potential (, , , , , ). Among these materials, germanium (Ge) is a potential candidate to replace the commercial graphite anode for LIBs because of its high gravimetric and volumetric capacities (1,626 mA hr g⁻¹ and 7,360 mA hr cm⁻³), low electrochemical potential of Li insertion/extraction (<0.5 V vs Li⁺/Li), excellent lithium diffusivity (400 times faster than in Si), and higher intrinsic electrical conductivity compared with Si. Despite these promising characteristics, the inferior structure stability due to the huge volume expansion (ca. 300% volume change for fully lithiated Ge) would result in the rapid capacity fade of the Ge-based electrode accompanied by a large irreversible capacity similar to Si, hindering the practical implementation of Ge anodes in future technological applications (, , ).To tackle this problem, two main strategies have been used to stabilizing the material structure and improving the electrochemical performance of Ge anodes by design of various nanostructures and electronically conductive coatings. Recently, the design of various nanostructures, such as nanowires (, , href="#bib11" rid="bib11" class=" bibr popnode">Kennedy et al., 2015), nanotubes (href="#bib22" rid="bib22" class=" bibr popnode">Liu et al., 2015a, href="#bib23" rid="bib23" class=" bibr popnode">Liu et al., 2015b, href="#bib31" rid="bib31" class=" bibr popnode">Song et al., 2012, href="#bib43" rid="bib43" class=" bibr popnode">Xiao et al., 2016), and porous structures (href="#bib27" rid="bib27" class=" bibr popnode">Park et al., 2010, href="#bib22" rid="bib22" class=" bibr popnode">Liu et al., 2015a, href="#bib23" rid="bib23" class=" bibr popnode">Liu et al., 2015b), has attracted great attention. Particularly, Park and co-workers rationally designed a porous germanium architecture, which induces only a 2% capacity decrease after 100 cycles (href="#bib27" rid="bib27" class=" bibr popnode">Park et al., 2010). The porous nanostructure materials show excellent cycle stability and rate capability because the uniform pores also act as a buffer to effectively alleviate the volume expansion and provide favorable structural stability during the lithiation/delithiation process (href="#bib27" rid="bib27" class=" bibr popnode">Park et al., 2010). Besides design of nanostructure strategies, electronically conductive coatings on Ge anode structures have been explored (href="#bib17" rid="bib17" class=" bibr popnode">Li et al., 2014, href="#bib24" rid="bib24" class=" bibr popnode">Ngo et al., 2014, href="#bib25" rid="bib25" class=" bibr popnode">Ngo et al., 2015, href="#bib38" rid="bib38" class=" bibr popnode">Wang et al., 2016, href="#bib29" rid="bib29" class=" bibr popnode">Seng et al., 2013). In this respect, Park and co-workers developed a facile method for the synthesis of Ge interconnected by a carbon buffer layer, which works as a channel for the supply of lithium during the charge-discharge process (href="#bib25" rid="bib25" class=" bibr popnode">Ngo et al., 2015). However, this strategy does not provide appropriate void space to alleviate the huge volume changes during lithium alloying and leaching and results in the pulverization, exfoliation, and aggregation of electrode materials. Very recently, yolk-shell architecture has attracted much attention in many fields, particularly in the field of energy storage (href="#bib47" rid="bib47" class=" bibr popnode">Zhang et al., 2016, href="#bib19" rid="bib19" class=" bibr popnode">Liu et al., 2012, href="#bib3" rid="bib3" class=" bibr popnode">Cai et al., 2015, href="#bib8" rid="bib8" class=" bibr popnode">Hong et al., 2013). The key design of the yolk-shell architecture in improving electrochemical performance lies in the ideal void space, which would be expanding/contracting freely upon lithium alloying and leaching without damaging the outer shell and, more importantly, be achieved with a minimal sacrifice of volumetric energy density. It is noteworthy that the volumetric capacity is an important indicator for the commercialization of electrode materials. If the void space is too much, even in the fully lithiated state, the core will not touch the shell, leading to the decrease of the volumetric capacity of the electrode. In addition, electrically conducting polymers such as polypyrrole may form a conducting elastic matrix, which offers a conducting backbone for the electrode material, and it could also be used as a flexible host matrix of electrode material to alleviate the huge volume changes during lithium alloying and leaching. Therefore, it remains a challenge to develop a facile approach for the synthesis of uniform yolk-shell architecture with the incorporation of the ideal void space and robust conducting polymer coating for the high volumetric capacity and long-cycle LIBs.It is noted that a conformal, homogeneous, and controllable nature of the sacrificial coating layer is crucial for appropriate void space of the yolk-shell architecture. Atomic layer deposition (ALD) is a technique to apply conformal, homogeneous, and controllable coating on high-surface nanostructures as the sacrificial layer by sequential, self-limiting surface reactions (href="#bib6" rid="bib6" class=" bibr popnode">George et al., 1996, href="#bib5" rid="bib5" class=" bibr popnode">George, 2010). Here, we developed a facile and highly controllable approach to uniform porous germanium@polypyrrole (PGe@PPy) yolk-shell architecture with conformal and controllable Al2O3 sacrificial layer by the ALD technique. There are several advantages of the yolk-shell architecture as anode material: (1) The presence of appropriate void space to alleviate the huge volume changes during lithium alloying and leaching, thus maintaining the structural stability of the outer shell to avoid the pulverization of electrode materials, and also achieved with a minimal sacrifice of volumetric capacity. (2) Recently, the inner relationship between the stability of the solid/electrolyte interphase (SEI) and the electrochemical properties of LIBs has been studied (href="#bib41" rid="bib41" class=" bibr popnode">Wu et al., 2012), which indicated that the SEI formed around the PPy shell during the cycles can be stable because the fully lithiated state of Ge core cannot damage the PPy outer shell. (3) The conductivity and stability can be improved for the yolk-shell architecture because the PPy outer shell has outstanding properties, including excellent chemical stability, electronic conductivity, and structural flexibility. Insight gained from this study can be applied to other high-capacity electrode materials, particularly those that suffer from huge volume expansion, offering a route for the future development of the promising electrode material for practical applications.