Surface modification of coordination polymers to enable the construction of CoP/N,P-codoped carbon nanowires towards high-performance lithium storage

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Abstract

A one-dimensional hybrid with N,P co-doped carbon nanowires threaded CoP nanoparticles is rationally fabricated by employing surface modified coordination polymers as a precursor. Ultrasmall CoP nanoparticles are well encapsulated in N,P co-doped carbon nanowires, which can effectively buffer the volume expansion of active CoP and facilitate fast lithium-ion/electron transfer during charge/discharge processes. Moreover, N,P co-doped carbon with high defect density and graphitic-N content are obtained, which facilitates high lithium storage capacity and fast electron transfer. As a result, attractive lithium storage properties are gained by employing this unique architecture as an anode material for lithium-ion batteries, including high reversible charge/discharge capacities, good rate capability, and excellent long-term cycling stability. Kinetic investigation shows that the fast lithium ion uptake/release is related to the remarkable capacitive contribution. This work may offer an effective way for design well-defined transition metal phosphide-based anodes for advanced lithium-ion batteries.

Introduction

Lithium-ion batteries (LIBs) are the most widely used rechargeable batteries in our daily life, mainly because of their high energy and power densities, long cycle performance and no memory effect. Currently, the increasing request for LIBs with higher energy densities, longer cycle life and lower cost drives the exploration of new electrode materials [1], [2]. However, the graphite anode used in traditional LIBs exhibits a relative low theoretical capacity (~372 mAh g−1). Thus, developing anode materials with high capacities is more and more important for the next generation of LIBs [3], [4], [5].

Recently, transition metal phosphides (TMPs) have gained great attentions for their high theoretical capacities (500–1800 mAh g−1) [6], [7], [8], [9]. The storage of lithium-ions in TMPs is realized by a conversion reaction, which enables more than one lithium-ion transfer during the intercalation/deintercalation processes [7], [8], [9]. Usually, TMPs have a lower polarization (~0.4 V) than other conversion-type anode materials such as transition metal oxides (~0.9 V) and sulfides (~0.7 V). Moreover, the discharge product of TMPs (Li3P) shows much higher conductivity (>1 × 10−4 S cm−1) than that of transition metal oxides (Li2O, 5 × 10−8 S cm−1), which is conducive to achieving faster lithium-ion/electron transport. However, two major drawbacks limit the practical application of TMPs anode [7], [8], [9]. One is the low intrinsic electronic conductivity, making the poor electrochemical kinetic. Another is the huge volume variation during charge/discharging, resulting in the limited cycle life and capacity retention.

Encapsulating nanosized TMPs into a carbonaceous matrix is an effectively way to solve these problems, which may shorten the diffusion pathway of lithium-ion, relieve the volume change during cycling, and enhance the electronic conductivity [10], [11], [12], [13], [14], [15], [16]. For example, monodispersed carbon-coated cubic NiP2 nanoparticles anchored on carbon nanotubes reported by Cui and Guo et al. [14] showed excellent cycling stability with high reversible capacities of 816.0 and 654.5 mAh g−1 at 1.3 A g−1 after 1200 cycles and at 5 A g−1 after 1500 cycles, respectively. In this respect, well-designed and nanostructured TMP/carbon composites have been proved to have good lithium storage performance, because their large surface-area-to-volume-ratio can effectively buffer the drastic volume change of TMPs and ensure fast lithium-ion/electron transfer during charge/discharge processes [17], [18], [19], [20]. For example, well-dispersed and porous FeP/C nanoplates reported by Li et al. [17] exhibited stable and ultrafast lithium storage performance. Even so, the controllable fabrication of well-defined TMP/carbon composites such as nanowires is still a challenging task.

Coordination polymers (CPs) are a class of crystalline materials constructed from coordinate bonds between metal ions/clusters and multidentate organic ligands [21], [22], [23]. The compositions and structures of CPs can be tuned by metal centers, organic ligands and their bridging modes. In addition, the morphology of CPs can be regulated by controlling the growth process. Recently, CPs have been recognized as promising precursors for the preparation of carbon-based composites via pyrolysis in an inert atmosphere [24], [25], [26]. On the one hand, the diversity of metal ions/clusters and organic ligands result in a large variety of CPs with designable structures, morphologies and sizes, which will be inherited by their derived carbon-based composites. On the other hand, the periodic arrangement of metal ions and organic ligands in CPs will generate homogeneous carbon coating and prevent the agglomeration of metal or metal compounds under thermal treatment. The synthesis of TMP/carbon composites from CPs have also been reported [27], [28], [29]. It always includes two steps. The first step is carbonization to obtain transition metal/carbon or transition metal oxide/carbon composites and the second step is phosphorization to transform the intermediates into TMP/carbon composites. However, there is a certain volume change in the conversion process of intermediates to TMP/carbon composites, which may lead to the collapse of nanostructures. Thus, the controlling of transformation of selected CPs into TMP/carbon composites is still worth noticing and exploring [27], [28], [29].

In this work, a novel one-dimensional (1D) hybrid architecture based on N,P co-doped carbon nanowires threaded CoP nanoparticles is developed for high-performance lithium storage. 1D Co-based coordination polymers coated with glucose-derived carbon are served as a precursor, and are transformed into the final CoP/N,P co-doped carbon nanowires through carbonization and phosphorization. Ultrasmall CoP nanoparticles are uniformly embedded in N,P co-doped carbon nanowires. This unique hybrid shows excellent lithium storage performance when used as an anode material for LIBs. It delivers reversible charge/discharge capacities of 634/640 mAh g−1 at a current density of 0.2 A g−1 after 200 cycles. Moreover, the specific discharge capacities of 629, 508, 434, 378, 318 mAh g−1 are gained at the current densities of 0.1, 0.2, 0.5, 1.0 and 2.0 A g−1, respectively. In addition, an ultrastable cycling performance is observed with a reversible discharge capacity of 483 mAh g−1 after 550 cycles at a high current density of 1.0 A g−1.

Section snippets

Material preparation

Co-NTA nanowires were prepared as precursors according to the reported method [30], and the original synthesis was partly modified. In a typical procedure, 1.5 g of CoCl2·6H2O (Alfa Aesar, 98%) and 0.6 g of nitrilotriacetic acid (NTA, Alfa Aesar, 98.0%) were added to 40 ml mixed solvent of 20 ml deionized water and 20 ml isopropyl alcohol (IPA, Sinopharm Chemical Reagent Co. LTD, AR) with vigorous stirring. The suspension was transferred into a 50 ml teflon-lined autoclave, sealed and

Results and discussion

The synthesis of CoP/N,P co-doped carbon nanowires is schematically illustrated in Fig. 1. Dozens of Co2+ from dissolved CoCl2·6H2O are coordinated with NTA to generate wire-like Co-NTA coordination polymers (Co-NTA). When Co-NTA are directly carbonized, Co/N-doped carbon nanowires (Co/N-C) are obtained. However, these nanowires collapse into CoP/N,P co-doped carbon nanoparticles (Co/N,P-C) under the following phosphidation process, which may result from the volume expansion of Co to CoP. If

Conclusions

In summary, a 1D nanocomposite consisted of N,P co-doped carbon nanowires decorated CoP nanoparticles is synthesized from glucose-derived carbon coated Co-NTA nanowires. The introduced glucose plays a key role in maintaining the stability of 1D nanostructure through carbonization and phosphorization, which ensures the fast lithium-ion/electron transport. In addition, N,P co-doped carbon with high defect density and graphitic-N content are obtained, which facilitates high lithium storage

CRediT authorship contribution statement

Huanhuan Li: Conceptualization, Formal analysis, Methodology, Investigation, Writing - original draft. Yuqiang Zhu: Data curation, Formal analysis, Validation, Writing - review & editing. Kangjia Zhao: Data curation, Formal analysis, Validation, Writing - review & editing. Qi Fu: Data curation, Investigation, Validation, Writing - review & editing. Kui Wang: Investigation, Validation. Yaping Wang: Conceptualization, Data curation, Funding acquisition, Methodology, Supervision, Writing - review

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

The authors are sincerely grateful to NSFC (21501071, 51805219, 51902139), Six Talents Peak Project of Jiangsu Province (2016-XNYQC-003, 2015-XNYQC-008), Transformation of Scientific and Technology Achievements in Jiangsu Province (BA2016162) and China Postdoctoral Science Foundation (2019TQ0126).

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