Hollow carbon sphere with open pore encapsulated MnO₂ nanosheets as high-performance anode materials for lithium ion batteries

Abstract

A new structured hollow carbon spheres with an open pore (HCSO) were synthesized by introducing a pore-forming agent PEG. Unlike the conventional hollow particles, the void space is fully utilized due to the presence of the open pore. As a proof-of-concept, MnO₂ nanosheets are in-situ grown on both the outer shell and the inner cavity of HCSO forming sandwich structure via a facile redox method, named MnO₂@HCSO composite. Meanwhile, the distance for lithium ion diffusion greatly reduces. When tested as an anode material for lithium ion batteries, MnO₂@HCSO composite exhibits increased performance compared to MnO₂/HCS composites which use traditional closed HCS as carbon matrix. It can still deliver a specific capacity of 398 mAh g⁻¹ based on the whole mass of composite even when the current density was increased to 5 A g⁻¹. This special designed structure would be extended to different fields, such as sensors and catalyst.

Introduction

Carbon based composites have been perused in different areas. Normally, the structure of carbon supporters, such as size, shape and pore structure, has a direct effect on the performance of composites [1], [2], [3], [4]. As one important carbon material, hollow carbon spheres (HCS) are a desirable choice for conductive framework material with unique spherality, porosity, and high specific surface area [5], [6], [7]. When used in lithium/sodium ion batteries, the carbon shell not only increases the conductivity, but prevents the aggregation of active materials. While the conductive shell is produced thinly, it could provide enough space to accommodate volume expansion for Li⁺/Na⁺ intercalation [8], [9], [10], [11]. However, traditional HCS structures have closed surface, which makes it difficult to utilize the hollow interior space, in effect, decreasing the volumetric energy density of the whole batteries [12].

Lithium-ion batteries (LIBs), one of the most concerned energy storage systems, have been implemented in different areas [13], [14]. Because of relative low theoretical capacity (372 mAh g⁻¹), commercialized graphite anode is insufficient for practical applications in electric vehicles (EVs). Transition metal oxides (TMOs), firstly reported by Poizot et al., in 2000, have been regarded as the promising anode materials due to their high theoretical capacities compared to graphite [15], [16]. Additionally, they are safe for commercial applications because of the limited formation of lithium dendrite [17]. In this regard, manganese dioxide (MnO₂) has been considered as an attractive candidate by its resource abundance, environmental friendliness and modest discharge potential (∼0.5–0.6 V vs. Li/Li⁺). If MnO₂ follows the conversion reaction with 4e process when used as an anode material, the theoretical capacity is high up to 1233 mAh g⁻¹, which is top among most of TMOs [18], [19], [20], [21], [22], [23]. However, the development of practical MnO₂ anodes is still hindered by the low electronic conductivity and poor structure stability of MnO₂ materials. The large volume change can cause severe cracking and disintegration of the electrode and lead to significant capacity loss. To overcome these shortcomings, building nanostructured materials is an effective strategy for TMOs to enhance their rate performance and cyclability. Smaller particle size means shorter length for lithium ion and electronic transport [24], [25], [26]. Introducing conductive matrix, for example, carbon nanotubes, graphene and carbon aerogel is another favourable strategy. These conductive matrices could not only improve the conductivity, but accommodate the large volume change during discharge/charge [27], [28], [29], [30], [31]. Therefore, a definable composite in which the TMOs were highly dispersed on conductive matrix with a stable structure and morphology will be an effective way to handle these issues.

Herein, to make better utilization of the inner hollow cavity and speed up the mass transfer rate, a new hollow carbon sphere structure with an open pore (HCSO) was designed and synthesized to encapsulate MnO₂ nanosheets (MnO₂@HCSO). MnO₂ nanosheets are in-situ grown on both the outer shell and the inner cavity of hollow carbon spheres via a facile redox method. The prepared MnO₂@HCSO highlights all the advantages of hollow sphere structures, such as stable composite structure, high utilization, accommodating volume expansion during cycling, etc. Unlike the conventional hollow particles, the presence of an open pore reduces the distance for lithium ion diffusion and utilizes the void space, resulting in an increased rate and cycling performance than the MnO₂/HCS composites which use traditional closed HCS as carbon matrix [32].