Boosting cyclability performance of GeP anode via in-situ generation of free expansion volume

Highlights

• An effective way in preparing GeP/C by dissolving raw materials→heat treatment→phosphatization is innovatively proposed.

• A facile ball milling-assisted in-situ generation of free expansion volume is being achieved upon NaCl as grinding agent.

• Free volume is incorporated into GeP matrix to address the severe volumetric change of GeP electrode for the first time.

Abstract

Among the germanium-based composites, germanium phosphide (GeP) is a promising anode material for lithium-ion batteries (LiBs) because of its good electronic conductivity, low binding energy and high theoretical energy density. Nevertheless, the severe volume variation and sluggish reaction kinetics hindered its applications. To solve the above issues, this work come up with a killer idea that inspired by the free volume theory of polymers, which was mainly involved creation of free volume into GeP matrix. Specifically, structural design that elaborately deals with severe aggregation while helps alleviating volume expansion and contraction of GeP have being made by a facile ball milling-assisted in-situ generation of free expansion volume. Compared with pristine GeP and direct ball milled GeP, the addition of NaCl as grinding agent plays a very positive role on generating free volume space in flake-like GeP particles, which was assembled by numerous amorphous GeP granules. In this way, the as obtained GeP particles exhibited enhanced reversible capacity, improved cyclability and rate performance (883 mAh g⁻¹ at 0.2 C after 100th cycle, and 680 mAh g⁻¹ at 2 C) when served as anode material in LiBs. This study highlights a highly efficient and cost effective approach in advancing the high performance GeP towards practical applications for energy storage.

Introduction

With the development of electronic product science and technology, the national economy and high technology call for the miniaturization and high specific capacity of lithium-ion batteries (LiBs) [1], [2]. Graphite has dominated the anode electrode market for LiBs over the past 30 years [3]. Nevertheless, the graphite negative electrode is insufficient in meeting these high demand for practical use, i.e., unmanned aerial vehicle with high mobility and long endurance. Therefore, it is meaningful to develop an anode electrode with high specific capacity cost effectively.

Up to now, six types of anodes that different from graphite have set off a frenzy of research towards LiBs: (1) Surface absorption of lithium storage anode, such as Mxene, graphene materials [4], [5], [6], [7]; (2) Conversion reaction storage lithium type anode electrode, such as transition metal oxygen/sulfur/selenium/telluride [8], [9], [10], [11]; (3) Alloying/de-alloying reaction lithium storage type anode electrode, such as Si, Ge, Sn, Sb, Bi elementary substances [12], [13], [14]; (4) Intercalation/microporous absorption lithium storage type anode, such as soft/hard carbon materials anode [15], [16]; (5) Alloy reaction/transformation reaction type anode electrode, such as Sn-, Ge- and Sb- based phosphorus/sulfur/selenium/telluride negative electrode [17], [18], [19]; (6) A small number of anode electrodes different from the above five categories, such as C₃N, polyaniline, etc [20], [21], [22]. In general, the alloy reaction/transformation reaction type anode materials featured higher theoretical specific capacity and lower redox voltage, which is more advantageous in the above several kinds of anode materials. In addition, the concept of alloy reaction/transformation reaction type anode is to have different onset potentials such that one component is lithiated while the other one buffered the tension to alleviate the volume change [23]. Germanium phosphide (GeP) it is a promising anode for LiBs, because of it has the advantages of high theoretical specific capacity, good electronic conductivity and low operating voltage (usually lower than 1.5 V) [24], [25]. Except that, the GeP has the lowest binding energy, which indicated that it has better reversibility when served as anode electrodes.

However, the high volume expansion ratio of the GeP after insertion of Li⁺ in the practical application usually lead to inevitable pulverization of anode coating, then result in falling of active material. Two methods have been reported so far to solve the above problems of GeP-based anode: (1) Exfoliated the block GeP to few layer GeP in order to reduce the absolute volume expansion ratio during the repeated discharge/charge cycling [26]. However, the efficiency of sample preparation by exfoliation method is typically very low, and also comes with the usage of extremely dangerous raw materials such as butyl lithium, which is not conducive to practical production; (2) Prepared the GeP/Carbon (GeP/C) composite by heat treatment of ball milling Ge, P, and carbon powder [25], [27]. The purpose of ball milling was mixing the Ge and P complex powder as homogeneously as possible, and heat treatment was applied to compound the Ge and P to the formation of GeP. The carbon served as dispersing agent during the GeP sample preparation. The as-obtained GeP/C composite have structural reversibility as lithium/sodium anode. It was no doubt that ball milling is a good method to produce a large number of products in a short time, however, the ball milling process will have various influences on the properties of the anode. For instance, ball milling firstly, followed by heat treatment are ease of causing serious product agglomeration and resulting large grain size, which is unfavorable to GeP anode material with large volume change before and after charge/discharge process. Further, the crystallized GeP is insufficient to relax expansion stress that caused by volume expansion during lithiation process. Recent studies have demonstrated that the density of amorphous materials are usually lower than crystallized counterparts, and the absolute volume expansion ratio of the corresponding amorphous anode materials was lower than that of crystallized anode materials when applied to LIBs [28], [29], [30], [31], [32].

To address the severe volume change problem while improving the electrochemical performance of GeP-based anode, we supposed that if the GeP/C was synthesized firstly, followed by ball milling the GeP/C composites, the obtained GeP would be in amorphous state, which is very beneficial to improve the charge/discharge stability of the LiBs as discussed above. The binding energy of GeP is very low (0.08 eV/atom) [25], the local high temperature generated by the instantaneous impact force of the ball milling will cause the decomposition of GeP. In order to avoid the decomposition of GeP during the ball milling process, it is of importance to add some grinding agent to dilute the impact force and reduce the local temperature. Accordingly, it is assumed that the ball milling process can be made as simple as possible by using grinding agent with low cost and ease of removal properties. Considering that the NaCl has extraordinary high stability and is very easy to be removed by water washing, it was chosen as the grinding agent in this study to investigate its influences on final integral electrochemical property of GeP. The integral electrochemical performance of NaCl-assisted milled GeP/C were improved by a large margin compared to the directly milled or un-milled samples. This study shed some lights on preparation of other types of high performance anodes.