The keys for effective distribution of intergranular voids of peapod-like MnO@C core-shell for lithium ion batteries

Highlights

• We elucidated the structure-properties relationship in peapod-like MnO@C nanowires for Li-ion batteries.

• The morphology of the particles inside the carbon layer could be controlled by adjusting the parameters for carbonization.

• The optimized composites exhibited excellent cycling performance and rate properties.

• The optimized structure can maximize the advantages such as structural durability against the stress involving huge volume changes.

Abstract

Conversion reaction-based transition metal oxides have shown high reversible capacity compared to conventional intercalation reaction-based materials. However, their practical applications have been impeded by a poor cycle life resulted from their low electrical conductivity and huge volume changes. During the past decade, remarkable advances have been achieved in the preparation of nanostructured transition metal oxides for conversion reaction anodes. Among the various shaped nanomaterials, core-shell structure with a carbon shell showed excellent electrochemical performance. Herein, we prepared peapod-like MnO@C nanowires as one special type of core-shell structure, and we elucidated the structure-properties relationship in peapod-like MnO@C nanowires for Li-ion batteries. The morphology of the manganese oxide particles inside the carbon layer could be simply controlled by adjusting the parameters in the carbonization process. The optimized composites exhibited excellent cycling performance without decreasing the capacity, and outstanding rate properties. The optimized structure can maximize the advantages such as structural durability against the stress involving huge volume changes, as well as minimize the side reaction at the surface.

Introduction

Lithium-ion batteries (LIBs) have been considered as a main power source for portable devices during the past 30 years because of their high energy density and long lifespan, and they are becoming more widely used including in a new, emerging industry such as grid storage, hybrid electric vehicles, and electronic vehicles (EVs) [[1], [2], [3]]. To meet the increasing demands for the new technical leap in cathodes and anodes, the search for high-performance anode materials becomes very urgent [[2], [3], [4]]. Transition metal oxides such as iron oxide [[5], [6], [7], [8]], cobalt oxide [[9], [10], [11]], nickel oxide [12], and manganese oxide [[13], [14], [15], [16]] have higher theoretical capacities than graphite, so they have been intensively studied as alternative anode materials of LIBs. Manganese oxides, as one of transition metal oxides, have a high theoretical specific capacity, and they can operate at low conversion potential. Also, they are abundant, inexpensive and environmental benign. Therefore, they are very promising candidate for the electrode materials of LIBs [17]. However, like most of other transition metal oxides, low electrical conductivity of MnO makes it suffer from poor rate capability. Furthermore, the biggest challenge of MnO materials for LIBs application is drastic volume change and severe capacity fading after only a few discharge/charge cycles [[17], [18], [19]].

Incorporation of MnO materials with carbonaceous nanomaterials provides an excellent avenue for enhancing the power and energy densities and improving cyclic stability. The carbon nanomaterials can significantly enhance the conductivity of MnO-based active materials, and effectively relieve the strain caused by the volume change during lithiation/delithiation process, improving their cycling stability. So far, various MnO/C composites such as MnO/C core/shell nanorods [20,21], MnO NPs attached to graphene [14,15,22] and hollow porous MnO/C microsphere [16,23] exhibited improved electrochemical performance. Among them, peapod-like nanowires with carbon layer showed excellent performance [25,[28], [29], [30], [31], [32]]. A peapod-like nanowire structure has multiple advantages such as large surface, buffering the internal stress, stable SEI layer formation, and high electron conductivity [[28], [29], [30], [31], [32]]. However, the performance of reported peapod-like core-shell nanowires significantly depends on the distribution and morphology of the internal oxide particles [[30], [31], [32]]. Therefore, it is essential to control the structure of the inner-core oxide particles for achieving high-performance anodes.

The peapod-like nanowire structures synthesized in the previous study were prepared by heat treatment of the carbon precursor coated nanowire oxides in an inert atmosphere to obtain a carbon coating layer. During the process, oxygen-rich oxides release oxygen and undergo phase changes accompanied by nucleation and particle sintering [25,[28], [29], [30], [31], [32]]. The equation related to the nucleation rate is proposed as follows [26]:

$$\frac{dN}{dt}=A\exp \left(-\frac{16\text{π}{\gamma }^3{v}^2}{{3k_B^3{T}^3\left(\text{ln}S\right)}^2}\right)$$

where N is nucleation particle number, γ is the surface energy, k_B is Boltzmann’s constant, S is supersaturation of the solution, and ν is molar volume of S. From the equation, three experimental parameters can be variable: supersaturation, temperature, and surface free energy. Supersaturation concentration increases with an increase in the heating rate [26]. Supposing that supersaturation represents the degree of deviation from equilibrium, the equation is applicable even if it is not a solution system. Sintering is the process that agglomerates fines into a porous mass. The driving force for sintering comes from surface energy [27]. Sintering models include parameters such as particle size, surface area, temperature, time, pressure, and atmosphere.

In this study, we elucidated the structure-properties relationship in peapod-like MnO@C nanowires for Li-ion batteries. Structural optimization inner MnO nanoparticles, which was overlooked in the previous studies, was achieved through a controlled carbonization process. Furthermore, we compared the electrochemical performance of the materials synthesized under different conditions.