Electrospinning-derived ${ZnCo}_{2} O_{4} @$ carbon nanofiber composite for high-performance lithium ion storage
用于高性能锂离子存储的电纺丝衍生 ${ZnCo}_{2} O_{4} @$ 碳纳米纤维复合材料

Hong Tang $^{a, b}$ , Mengjin Jiang $^{c}$ , Wenhao Bai $^{a, b}$ , Ce Cui $^{a, b}$ , Hongyan Xiao $^{a, b}$ , Erhui Ren $^{a, b}$ ,
唐红 $^{a, b}$ 、蒋梦瑾 $^{c}$ 、白文豪 $^{a, b}$ 、崔策 $^{a, b}$ 、肖红艳 $^{a, b}$ 、任尔辉 $^{a, b}$ 、Mi Zhou $^{a}$ , Jinwei Zhang $^{a}$ , Cheng Cheng $^{d}$ , Ronghui Guo $^{a, b,^{*}}$
Mi Zhou $^{a}$ 、Jinwei Zhang $^{a}$ 、Cheng Cheng $^{d}$ 、Ronghui Guo $^{a, b,^{*}}$ $^{a}$ College of Biomass Science and Engineering, Sichuan University, Chengdu, 610065, China
$^{a}$ 四川大学生物质科学与工程学院，中国成都，610065 $^{b}$ Yibin Industrial Technology Research Institute of Sichuan University, Yibin, Sichuan, China
$^{b}$ 四川大学宜宾产业技术研究院，中国四川宜宾 $^{c}$ College of Polymer Science & Engineering and State Key Lab Polymer Material & Engineering, Sichuan University, Chengdu, 610065, China
$^{c}$ 四川大学高分子科学与工程学院、高分子材料与工程国家重点实验室，成都，610065 $^{d}$ School of Chemical and Process Engineering, University of Leeds, Leeds, United Kingdom
$^{d}$ 利兹大学化学与工艺工程学院，英国，利兹

A R TICLE IN F O
文章

Handling Editor: Dr P. Vincenzini
责任编辑：P. Vincenzini 博士P. Vincenzini 博士

Keywords: 关键词：

Electrospinning 电纺丝

{ZnCo}_{2} O_{4}

Carbon nanofiber 纳米碳纤维
Long-cycle performance 长周期性能
Lithium ion storage 锂离子存储器

Abstract 摘要

A B S T R A C T The degradation of the cobalt-zinc oxide structure and its poor conductivity during the charge and discharge limit their further applications for lithium ion storage. Herein, ${ZnCo}_{2} O_{4} @$ carbon nanofiber composite with nanofibrous structure is obtained by electrospinning, annealing in argon and low-temperature oxidation to effectively overcome the above issue. The active sites of ${ZnCo}_{2} O_{4}$ are evenly dispersed inside the carbon nanofibers, which can effectively avoid its aggregation and improve electrical conductivity. Additionally, the stable nanofibrous structure can maintain structural stability. The composite exhibits superior lithium ion storage capacity when being served as anode electrode. The ${ZnCo}_{2} O_{4} @$ carbon nanofiber electrode possesses a high capacity of 1071
A B S T R A C T 氧化钴锌结构的降解及其在充放电过程中的不良导电性限制了其在锂离子存储领域的进一步应用。在此，通过电纺丝、氩气退火和低温氧化得到了具有纳米纤维结构的 ${ZnCo}_{2} O_{4} @$ 碳纳米纤维复合材料，有效地克服了上述问题。 ${ZnCo}_{2} O_{4}$ 的活性位点均匀地分散在碳纳米纤维内部，可有效避免其聚集，提高导电性。此外，稳定的纳米纤维结构还能保持结构的稳定性。该复合材料在用作阳极电极时表现出卓越的锂离子存储能力。 ${ZnCo}_{2} O_{4} @$ 碳纳米纤维电极的容量高达 1071

maintain $714 mA h g^{- 1}$ after 250 cycles when current density is adjusted to $0.2 A g^{- 1}$ again. Additionally, the electrode has an outstanding long-cycle performance, which remains a capacity of $447.165 mA h^{- 1}$ at $0.5 A g^{- 1}$ after 500 cycles and $421.477 mA h g^{- 1}$ at $1 A g^{- 1}$ after 518 cycles. This result demonstrates that ${ZnCo}_{2} O_{4} @$ carbon nanofiber composite has potential application prospects in the fields of advanced energy storage.
当电流密度再次调整到 $0.2 A g^{- 1}$ 时，250 次循环后仍能保持 $714 mA h g^{- 1}$ 的容量。此外，该电极还具有出色的长循环性能，在 500 次循环后， $0.5 A g^{- 1}$ 的容量仍为 $447.165 mA h^{- 1}$ ，在 518 次循环后， $1 A g^{- 1}$ 的容量仍为 $421.477 mA h g^{- 1}$ 。这一结果表明， ${ZnCo}_{2} O_{4} @$ 碳纳米纤维复合材料在先进储能领域具有潜在的应用前景。

1. Introduction 1.导言

Lithium-ion batteries, as the most important rechargeable batteries in modern commercial batteries, have been widely used in energy storage systems and portable electronic devices [1]. However, lithium-ion batteries urgently need to improve energy density, long life as well as low cost when lithium-ion batteries are further applied in the fields of power store systems, electric cars and aerospace [2-4]. The endurance capacity of lithium-ion batteries usually depends on the capacity of anode material due to the capacity limitation of cathode material [5], nevertheless, the conventional graphite anodes is far from being able to satisfy the ever-increasing energy demand due to its low capacity [6]. Therefore, it is becoming more and more urgent to explore lithium ion storage materials with high capacity.
锂离子电池作为现代商用电池中最重要的可充电电池，已被广泛应用于储能系统和便携式电子设备中[1]。然而，当锂离子电池进一步应用于储能系统、电动汽车和航空航天领域时，锂离子电池亟需提高能量密度、延长使用寿命并降低成本[2-4]。由于正极材料的容量限制，锂离子电池的续航能力通常取决于负极材料的容量[5]，然而传统的石墨负极由于容量低，远远不能满足日益增长的能源需求[6]。因此，探索高容量锂离子存储材料变得越来越迫切。

Among the alternatives of various anode electrode materials, metal oxides own high theoretical capacity as result of the conversion reaction and the alloying reaction of partial metal oxides during lithium ion storage, which make it received a lot of attention [7,8]. As a typical transition metal oxide, the bimetallic oxide

{ZnCo}_{2} O_{4}

has a promising application prospect when being used as a new generation of lithium ion storage material. However, when

{ZnCo}_{2} O_{4}

reacts with lithium ions, the

{ZnCo}_{2} O_{4}

undergoes a conversion reaction to produce metal Co and Zn , and then the Zn alloying with

{Li}^{+}

to form

{Li}_{2} Zn

alloy, which always causes large volume fluctuations of the structure and the repeated insertion and extract of lithium ions will lead to structure of material become unstable and even cause collapse [9], thereby cause severe decline in capacity and a lower service life. In addition, the

{ZnCo}_{2} O_{4}

has poor electrical conductivity and inferior electron mobility, which makes it difficult for some active sites to intercalate and deintercalate with lithium ions, resulting in poor rate performance and reaction kinetics.
在各种正极电极材料中，金属氧化物因其在锂离子存储过程中的转化反应和部分金属氧化物的合金化反应而具有较高的理论容量，因而受到广泛关注[7,8]。作为一种典型的过渡金属氧化物，双金属氧化物

{ZnCo}_{2} O_{4}

作为新一代锂离子存储材料具有广阔的应用前景。然而，当

{ZnCo}_{2} O_{4}

与锂离子反应时，

{ZnCo}_{2} O_{4}

会发生转化反应生成金属Co和Zn，然后Zn与

{Li}^{+}

合金化形成

{Li}_{2} Zn

合金，这总会引起结构体积的大幅波动，锂离子的反复插入和提取会导致材料结构变得不稳定，甚至造成坍塌[9]，从而导致容量严重下降，使用寿命降低。此外，

{ZnCo}_{2} O_{4}

导电性差，电子迁移率低，一些活性位点难以与锂离子发生插层和脱插层反应，导致速率性能和反应动力学性能不佳。

In order to solve this problem, many efforts have used to regulate the microstructure of

{ZnCo}_{2} O_{4}

with stable structure, thereby avoiding structural collapse and aggregation of active sites in the process of charging and discharging, which in turn enables more active sites reacts with lithium ions and reduces the capacity attenuation caused by the degradation of unstable structure [10,11].

{ZnCo}_{2} O_{4}

-based composite
为了解决这一问题，许多人致力于用稳定的结构来调节

{ZnCo}_{2} O_{4}

的微观结构，从而避免在充放电过程中出现结构坍塌和活性位点聚集，进而使更多的活性位点与锂离子发生反应，减少因结构不稳定而导致的容量衰减 [10,11]。基于

{ZnCo}_{2} O_{4}

的复合材料

with various structures have been regulated toward improve lithium ion storage capacity such as PAN (Polyacrylonitrile) -based carbon fiber/

{ZnCo}_{2} O_{4}

[12],

{ZnCo}_{2} O_{4}

hierarchical nanocubes [13],

{ZnCo}_{2} O_{4}

Nanowire [14],

{ZnCo}_{2} O_{4} / ZnO / C

microcubes [15], nano-

{ZnCo}_{2} O_{4}

/porous rGO [16], yolk-shell structured

{ZnCo}_{2} O_{4}

spheres [17] and hierarchical porous

{ZnCo}_{2} O_{4} @ NiO /

nickel foam [18]. The stability of

{ZnCo}_{2} O_{4}

and the lithium ion storage performance were improved. Nevertheless, this synthesis processes are often complicated, poorly controllable and cannot maintain consistency well. It is a very valuable question to prepare composite with porosity, good conductivity and stable structure through simple preparation processes. Compared with this complex structure, the preparation of composite with nanofiber structure by electrospinning is simple and easy controllable, which is conducive to mass production and has greater application prospects [19]. The active sites are uniformly distributed in the nanofibers, which is beneficial to participate in the electrochemical reaction during charge and discharge.
为提高锂离子存储容量，对各种结构的碳纤维进行了调节，如基于 PAN（聚丙烯腈）的碳纤维/

{ZnCo}_{2} O_{4}

[12]、

{ZnCo}_{2} O_{4}

分层纳米立方体[13]、

{ZnCo}_{2} O_{4}

纳米线 [14]、

{ZnCo}_{2} O_{4} / ZnO / C

微立方体 [15]、纳米

{ZnCo}_{2} O_{4}

/多孔 rGO [16]、卵黄壳结构

{ZnCo}_{2} O_{4}

球 [17]和分层多孔

{ZnCo}_{2} O_{4} @ NiO /

泡沫镍 [18]。

{ZnCo}_{2} O_{4}

的稳定性和锂离子存储性能都得到了提高。然而，这种合成工艺往往比较复杂，可控性差，不能很好地保持一致性。如何通过简单的制备工艺制备出具有多孔性、良好导电性和稳定结构的复合材料是一个非常有价值的问题。与这种复杂的结构相比，电纺丝法制备纳米纤维结构的复合材料工艺简单，易于控制，有利于大规模生产，具有更大的应用前景[19]。活性位点均匀分布在纳米纤维中，有利于在充放电过程中参与电化学反应。

In this work, the transition metal oxide composite with fibrous structure were rationally designed by using electrospinning. The

{Zn}^{2 +} @ {Co}^{2 +} @ P A N

nanofibers were prepared by electrospinning, and then carbonized at high temperature to form Zn@Co@carbon nanofiber composite, so that it can effectively maintain the fibrous structure. Afterwards, the composite was oxidized in the air to obtain

{ZnCo}_{2} O_{4} @

carbon nanofiber composite, which is beneficial to avoid the degradation of the fiber structure caused by the direct oxidation of

{Zn}^{2 +} @ {Co}^{2 +} @ P A N

nanofibers in air. In the composite, the presence of a large number of heteroatoms N come from PAN helps to improving conductivity and affinity of lithium ion. In addition, the large specific surface area of

{ZnCo}_{2} O_{4} @

carbon nanofiber composite allows it to give many active sites for the insertion/extraction of lithium ions. Benefited from these advantages, the

{ZnCo}_{2} O_{4} @

carbon nanofiber electrode exhibits a high capacity of

1053.88 mA h g^{- 1}

when the current density is

0.1 A g^{- 1}

. In addition, the electrode possesses outstanding rate perfor-
本研究利用电纺丝技术合理设计了具有纤维结构的过渡金属氧化物复合材料。通过电纺丝制备出

{Zn}^{2 +} @ {Co}^{2 +} @ P A N

纳米纤维，然后在高温下碳化形成Zn@Co@碳纳米纤维复合材料，使其能有效保持纤维状结构。之后，将复合材料在空气中氧化，得到

{ZnCo}_{2} O_{4} @

碳纳米纤维复合材料，有利于避免

{Zn}^{2 +} @ {Co}^{2 +} @ P A N

纳米纤维在空气中直接氧化造成的纤维结构退化。复合材料中存在大量来自 PAN 的杂原子 N，有助于提高锂离子的导电性和亲和性。此外，

{ZnCo}_{2} O_{4} @

碳纳米纤维复合材料的大比表面积使其能够为锂离子的插入/萃取提供许多活性位点。得益于这些优点，当电流密度为

0.1 A g^{- 1}

时，

{ZnCo}_{2} O_{4} @

碳纳米纤维电极显示出

1053.88 mA h g^{- 1}

的高容量。此外，该电极还具有出色的速率性能。

946.998 mA h g^{- 1}

when current density returns to

0.1 A g^{- 1}

again. Moreover, the electrode has excellent recyclable performance, which delivers

635.175 mA h g^{- 1}

0.5 {Ag}^{- 1}

over 135 cycles and maintains

424.615 mA h g^{- 1}

1 {Ag}^{- 1}

over 300 cycles.

946.998 mA h g^{- 1}

当电流密度再次恢复到

0.1 A g^{- 1}

时。此外，该电极还具有出色的可回收性能，可在 135 次循环中以

0.5 {Ag}^{- 1}

提供

635.175 mA h g^{- 1}

，并在 300 次循环中以

1 {Ag}^{- 1}

保持

424.615 mA h g^{- 1}

。

2. Experimentals 2.试验品

2.1. Preparation of metal ion ${Zn}^{2 +}$ @Co $^{2 +}$ @PAN@nanofibers
2.1.制备金属离子 ${Zn}^{2 +}$ @Co $^{2 +}$ @PAN@ 纳米纤维

219 mg Co {({CH}_{3} COO)}_{2} \cdot 4 H_{2} O, 498 mg Zn {({CH}_{3} COO)}_{2} \cdot 2 H_{2} O

and 1.0 g polyacrylonitrile (PAN) were dissolved in

10 mL N, N - 2

methyl formamide (DMF) under the condition of magnetic stirring at room temperature to become a uniformly dispersed solution for electrospinning. The solution was then transferred to a syringe and fix it on syringe pump. A positive voltage of 20 kV was used for spinneret, and delivery rate of

0.5 mL / \min