Elsevier

Applied Surface Science

Volume 542, 15 March 2021, 148663
Applied Surface Science

Full Length Article
A scalable snowballing strategy to construct uniform rGO-wrapped LiNi0.8Co0.1Mn0.1O2 with enhanced processability and electrochemical performance

https://doi.org/10.1016/j.apsusc.2020.148663Get rights and content

Highlights

  • RGO uniform wrapped LiNi0.8Co0.1Mn0.1O2 was prepared with the help of a semi-conductive PTCDA layer.

  • The firmness of graphene coating and its effect on the slurry have been revealed.

  • The enhanced cycling stability and excellent rate performances have been achieved with uniform rGO-wrapped NCM cathode.

Abstract

Graphene is considered to be a desirable coating material to enhance the performance of Ni-rich cathodes. However, there are few facile methods to form a uniform graphene coating layer. Here, we propose a scalable snowballing strategy to prepare uniform rGO-wrapped LiNi0.8Co0.1Mn0.1O2 (PG-NCM) through convenient physical mixing with the help of a semi-conductive Perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) layer, which can ensure the adhesion of graphene on the particle surface. Most importantly, verified by rheological tests, the PTCDA layer also plays a significant role in the homogeneous coating of graphene layer, suppressing the gelation phenomenon in slurry. While using PG-NCM as cathode materials, the synergistic effect of rGO nanosheets and PTCDA can provide better conductivity and more stable electrode–electrolyte interphase. In particular, the P1G1-NCM (only 1.0 wt% additives) performs best among all samples. It reaches a high discharge capacity of 194.1 mAh g−1 at 1C, 92.8% capacity retention (100 cycles, 1C) and enhanced rate performance (122.1 mAh g−1 at 10C). With these results, this strategy is likely to be a practical technology in mass production of modified cathodes in Li-ion battery for large-scale production and cost-effective operability.

Introduction

Benefitting from high energy density and long cycle life, Li-ion battery has been regarded as a crucial part of the modern energy industry [1]. In order to achieve higher energy density to unlock the full potential of electric vehicles, the employment of ternary cathode materials (LiNixCoyMn1-x-y) has been a certain trend [2]. High Ni content can greatly increase the specific capacity of ternary cathodes but also lead to some intractable problems in practical application [3], [4], [5]. One of the most important issues is the surface damage caused by the parasitic reaction between the cathode material and the organic electrolytes [6]. The resulting microcracks along the grain boundaries lead to the increase of internal polarization, rapid capacity decay and the penetration of liquid electrolytes [6], [7], [8]. Another serious problem points to the residual lithium compounds (RLCs) generated both in synthesis and storage process in Ni-rich cathodes. The harmful RLCs induce gelation during the pulping process, that is, affect the processability of the slurry [4].

To overcome the aforementioned issues, surface coating, as an effective strategy among various modification methods, has been investigated [9]. An ideal coating layer can avoid severe side reactions with electrolyte without inhibiting the electron–ion transport [10]. Methods used to construct coating layers can be divided into two categories: wet coating and dry coating. Wet coating is a frequently-used method that can achieve a homogeneous coating layer. However, almost all liquid phase methods require a long period of stirring and ultrasonication, which may exacerbate the dissolution of lithium ions and generate undesirable impurities. Dry coating, commonly represented by mechanical ball-milling, is a direct method to modify the raw materials without above-mentioned issues [11]. However, with inferior uniformity, it failed to be a widely adopted method. Recently, various coating materials were used in Ni-rich cathodes, such as Al2O3 [12], SiO2 [13], AlF3 [14] and Li3PO4 [15]. Although these coating layers work as an obstacle to shelter from bulk damage, some of their natural drawbacks (low conductivity, complicated preparation, low productivity for example) may limit their commercial application [16], [17], [18].

Recently, graphene, with good electrical conductivity, large specific surface area and stable chemical properties, is considered as a favorable material for the modification of electrode materials [19], [20], [21]. However, there are still some problems to be tackled. On the one hand, the two-dimensional planar structure of graphene makes it tough to be evenly coated on the surface of the spherical electrode materials [9], [16]; On the other hand, the effect of the addition of graphene on the viscosity of slurry cannot be ignored but this issue has received little attention. Results from the difficulty in the dispersion of graphene, gelation will be more vexing after adding graphene to the slurry, which deteriorate the leveling during the coating process. Aiming at realizing a uniform and tight graphene coating, scientists have done a lot of related work commonly with wet coating strategies, such as LiNi0.6Co0.2Mn0.2O2 @ graphene aerogel composite prepared by hydrothermal method [22] and graphene modified LiNi0.8Co0.1Mn0.1O2 formed with the assistance of epoxy-functionalized silane in liquid phase [16]. These attempts exhibit improvements in electrochemical performances but the inherent shortcomings of liquid-phase preparation limit the whole potential of cathodes. In the field of dry coating, graphene is technically difficult to be firmly fixed on the surface when the external effect is not strong enough, but in the meantime, violent mixing will cause the secondary particles to be severely damaged. Besides, few papers attach importance to the effect of adding graphene on the rheological behavior of slurry.

Perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) is an N-type organic semiconductor with excellent chemical stability [23], [24]. The high electron affinity of PTCDA (about 3.6 eV) [25] leads to easy stabilization of additional negative charges, providing a theoretical basis for the adhesion of graphene (with a large number of delocalized electrons) [26]. From a structural perspective, with two dimensional conjugated π-electron systems [24], the benzene rings carried by PTCDA molecules are consistent with the hexagonal structure of graphene. And the anhydride bonds of PTCDA result good affinity with electrode materials (especially metal oxides). In addition, compared to inorganic materials, the flexibility of organic semiconductors [27] makes PTCDA more suitable for encapsulation. Integrating these features, we think it has the feasibility of being an excellent coupling agent in large scale electrode modification with graphene.

Herein, we advocate a snowballing strategy that can accomplish uniform coating of graphene, where PTCDA is introduced as an electron-absorbing bridge between graphene and LiNi0.8Co0.1Mn0.1O2 (NCM). As shown in Fig. 1, only a few minutes of two-step mixing can achieve fast and efficient coating. Except as some morphological and structural characterizations, the rarely used rheological tests were performed to evaluate the uniformity of graphene coating. Restricted by the limited inhibition of side reactions by single layer coating, neither of these two can perform ideally when used alone [28]. But when a double-layer structure was built, the synergistic effect of the two ingredients can provide a more robust barrier thus show better electrochemical performance. The brand-new coating method with features of energy-saving and low cost can provide us a large-scale cathode protection for Li-ion battery industry.

Section snippets

Synthesis of PTCDA/Graphene-LiNi0.8Co0.1Mn0.1O2 composites

The synthesis procedure for graphene oxide preparation was followed by a modified Hummer’s method. After freeze drying process and calcination, high-quality reduced graphene oxide (rGO) was obtained. The preparation process of coated NCM composites can be concluded as a two-step grinding to mix different ingredients together. The synthesis process was illustrated in Fig. 1. As Fig. 1shows, several small particles adhere to the surface of NCM after the addition of PTCDA, and then some multilayer

Results and discussion

Fig. 2(a) shows the results of Rietveld refinement of XRD for NCM and P1G1-NCM. As shown in the diffraction patterns, peaks of NCM and P1G1-NCM prove that the samples are in accordance with the hexagonal α-NaFeO2 crystal structure (space group: R-3 m) [30]. Heterogeneous peaks cannot be seen in P1G1-NCM, indicating that the introduction of PTCDA and rGO coating layers did not affect the original layered structure of pristine NCM [31]. Well-defined layered structures can be easily concluded from

Conclusion

In summary, a scalable snowballing strategy has been proposed to prepare rGO uniform wrapped LiNi0.8Co0.1Mn0.1O2 by a two-step mixing method. Several experiments have been used to prove the effectiveness and uniformity of this method. In addition, the decreased viscosities and increased solid contents have also been demonstrated by rheological tests. In particular, the P1G1-NCM sample exhibits significant improvements with a specific capacity of 194.1 mAh g−1 and capacity retention of 92.8%

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.

Acknowledgements

The authors acknowledge the financial support provided by the Science, Technology, and Innovation Commission of Shenzhen Municipality (JCYJ20180508151856806 and JCYJ20180306171355233), the National Natural Science Foundation of China (51974256), the Outstanding Young Scholars of Shaanxi (2019JC-12), the Key R&D Program of Shanxi (No.2019ZDLGY04-05), the National Natural Science Foundation of Shaanxi (No.2019JLZ-01; No.2019JLM-29 and No.2020JQ-189), the Research Fund of the State Key Laboratory

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