Nanonetwork-structured yolk-shell FeS₂@C as high-performance cathode materials for Li-ion batteries

Abstract

Pyrite FeS₂ is a promising alternative to typical intercalation cathodes for rechargeable lithium-ion batteries (LIBs) by virtue of its extremely high theoretical capacity. However, the inferior rate capability and fast capacity degradation caused by the sluggish reaction kinetics and large volume expansion upon lithiation greatly hinder its practical application. Herein, a chemical crosslinking strategy is developed for the synthesis of the yolk-shell pyrite FeS₂@porosity-rich sulfur-doped carbon nanonetworks (FeS₂@C NNs) as cathode materials for high-performance LIBs. The 3D nanonetwork structure constructed by tight covalent connection of carbon shells can act as highways to facilitate the electron transport kinetics, while the well-orchestrated internal cavities of the yolk-shell nanostructure provide large void space to accommodate the volume expansion of pyrite FeS₂. In addition, the porosity-rich characteristic of carbon shells ensures fast pathways for the Li⁺ diffusion across the shells. As a result, the yolk-shell FeS₂@C nanonetworks exhibit excellent high-rate capability (353 mAh g⁻¹ at 10 C) and exceptionally long lifespan of 1000 cycles with a high capacity of 435 mAh g⁻¹ at a large current density of 5 C, which is by far the best of pyrite FeS₂-based cathodes for LIBs.

Introduction

Rechargeable lithium-ion batteries (LIBs) represent an appealing power source for fulfilling the demands of portable electronics and low-emission electric vehicles, owing to the myriad merits such as high energy density, long cycling life, and environmental benignity [[1], [2], [3], [4], [5]]. Nevertheless, the energy density of commercial LIBs is nearly approaching the upper limitation, mainly due to the low theoretical capacity of typical LiMO₂ (M = transition metal) cathodes [[6], [7], [8], [9]]. In this context, extensive efforts have been devoted to searching for advanced high-capacity cathode materials. As one kind of conversion-type cathode material, pyrite FeS₂ has attracted great attention as a promising alternative to typical LiMO₂ intercalation cathodes because of the extremely high theoretical energy density [[10], [11], [12], [13]]. The four electron reduction of pyrite FeS₂ by lithium (FeS₂ + 4Li⁺ + 4e⁻ → Fe + 2Li₂S) provides a theoretical specific capacity of 894 mAh g⁻¹, much higher than that of the best LiNi_xCo_yMn_zO₂ intercalation-type cathodes which only deliver less than 300 mAh g⁻¹ [14,15]. In addition, the inexpensive, earth-abundant, and environmentally benign features of pyrite FeS₂ bring extra benefits to the market potential. However, the pyrite FeS₂ usually suffers from large volume expansion (164%) upon lithiation, intrinsic low charge/ionic conductivity, and severe dissolution of polysulfides converted from the in situ generated elemental sulfur [[16], [17], [18], [19]]. These drawbacks greatly hamper the operational performance of pyrite FeS₂ cathodes, rendering inferior rate capabilities and fast capacity fading during prolonged cycles. Therefore, the rational design of advanced pyrite FeS₂ cathodes to achieve superior electrochemical performance is highly desirable for their practical application.

It is well known that the ideal electrode materials should have the following merits: (i) the required free space and good structural stability for alleviating the structural strain of volume change during the repeated Li⁺ insertion/extraction processes; (ii) short ionic diffusion pathways with low resistance to promote the reaction kinetics; and (iii) smooth charge transfer network within the whole electrode matrix for improving the utilization of active materials and the rate response [[20], [21], [22], [23], [24], [25]]. The construction of well-organized nanocomposites by embedding nanostructured pyrite FeS₂ in a porous carbon matrix can not only shorten the pathways for Li⁺ diffusion but also promote the facile access of electrons throughout the active material nanoparticles. In addition, the robust carbon matrix can alleviate the pulverization of pyrite FeS₂ nanoparticles to some extent, thus leading to enhanced reversibility and rate capability [12,16,26]. However, due to the lack of required free space for accommodating the volume expansion of pyrite FeS₂ nanoparticles, the cycling performance of current pyrite FeS₂ cathodes is still unsatisfactory (typically ≤ 100 cycles), which is far away from practical use.

Recently, yolk-shell nanostructures with high-capacity active material yolks and good-conductivity carbon shells have received great attention because of their unique structural advantages. The carbon shells can provide the nanostructures with good conductivity and excellent structural stability [[27], [28], [29], [30]]. Moreover, the interior void space between the yolks and shells can provide required space for efficiently accommodating the large volume change during repeated discharge/charge processes [31,32]. Traditionally, most previous studies of yolk-shell electrode materials have been focused on developing highly dispersed nanoparticles. However, these isolated nanoparticles inevitably present loose physical aggregation between each other, resulting in large interfacial contact resistances and low tap densities (Fig. 1a) [21]. In addition, the pore structures of the carbon shells in these yolk-shell electrode materials are usually poorly developed, and thus the ionic diffusion into the internal void spaces is greatly limited, causing sluggish reaction kinetics of the active material yolks with unsatisfactory performance [33]. Herein, we demonstrate a chemical crosslinking strategy for the synthesis of covalently connected yolk-shell pyrite FeS₂@porosity-rich sulfur-doped carbon nanonetworks (FeS₂@C NNs), with the aim of realizing an ideal nanostructure for high-performance LIBs. Such a design integrates several advantages: (i) the yolk-shell structured network units provide large internal void spaces to accommodate the volume expansion of active materials; (ii) the 3D network structure constructed by tight covalent connection of carbon shells can accelerate the charge transfer; and (iii) the porosity-rich characteristic of carbon shells affords fast pathways for the Li⁺ diffusion across the shells (Fig. 1b). As a result, the yolk-shell FeS₂@C NNs cathode demonstrates exceptional rate performance with capacities of 353–943 mA h g⁻¹ at varied current rates of 0.2–10 C. Even when cycled at a very high current rate of 5 C, the yolk-shell FeS₂@C NNs cathode can still deliver a high capacity of 435 mAh g⁻¹ after 1000 cycles. To our knowledge, a pyrite FeS₂ cathode with such superior rate performance and ultralong cycle life has not been reported before.