Elsevier

Thin Solid Films

Volume 701, 1 May 2020, 137925
Thin Solid Films

Analytical study of dark spots on anode surface in high energy-density lithium-ion cells

https://doi.org/10.1016/j.tsf.2020.137925Get rights and content

Highlights

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    The occurrence of dark spots is caused by deformed solid electrolyte interface.

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    Worse structural stability for dark spot than normal anode.

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    Dark spots, lower capacity and poorer safety are related.

Abstract

There are three types of visible surface stains on the lithium-ion cell graphite anode. These are dark spots, bubble spots, and lithium plating. Dark spots are formed by a different mechanism from the other two types of stains, while bubble-spots and lithium-plating can be well demonstrated. X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, nuclear magnetic resonance, and Raman spectroscopy used to analyze dark spots and normal anode regions. These analyses indicate that the dark spots are mainly caused by imperfect solid electrolyte interface formation, which affects the structural stability of the graphite resulting in cell capacity and safety.

Introduction

It is well known that lithium-ion cell show the highest energy density of any commercially available secondary battery chemistry. Today, lithium-ion cells are used for automotive applications in plug-in hybrid and battery electric vehicles. The energy density of a lithium-ion cell strongly depends on the choice of an appropriate anode material [1], [2], [3], [4]. There are many anode materials including graphite, amorphous carbons, graphene, silicon, tin, transition metal oxides and lithium metal for lithium-ion cell. All of these anode materials have been extensively studied for use in next-generation batteries. Significant results have been achieved for many potential lithium-ion cell anode materials at the lab scale. Artificial graphite (A.G) is currently the most promising candidate for lithium-ion cell applications such as electric vehicles due to its high capacity, outstanding rate capability, long cycle life and acceptable safety performance [5], [6], [7], [8].

Despite the high performance, instability of Nickel-enriched + A.G chemistry system can cause parasitic side reactions that lead to rapid performance degradation. When higher cut-off voltages and heavy electrode packing densities are pursued, anode defects often show up in mass-production at the beginning of lithium-ion cell life. Sometimes these defects are seen as stains on the surface of the anode electrode, such as dark spots, bubble spots and lithium plating [9,10]. Lithium plating and bubble spots can be well understood and defined, but dark spots are formed differently which needs to be explained by different mechanism.

In this paper, dark spots which were encountered in a commercial research project were analyzed, and the cause of this defect was confirmed. The analysis indicates that imperfect solid electrolyte interface (SEI) formation causes dark spots to develop.

Section snippets

Sample preparation

A commercialized lithium-ion cell designed for electric vehicles (50 Ah = 1C) with dark spots in the anode was disassembled and analyzed. The cathode electrode was made using Li[Ni0.6Co0.2Mn0.2]O2 with poly (vinylidene fluoride) as the binder, acetylene black as a conductive and aluminum foil as the current collector. The graphite anode consisted of 95.2 wt% commercialized artificial graphite (BTR New Energy Materials Co., Ltd.), 0.8 wt% C65 (Timcal) as the conductive additive, and 4.0 wt% of

Results and discussion

In this study, the dark spot problem encountered in the pouch cell development process was analyzed. The following images are representative of common dark spots in fully charged (100% SOC) anode electrode.

Fig. 1 shows the anode electrode surfaces of batteries at 100% SOC. As seen in Fig. 1a, when the graphite anode is fully charged, the charged region changes color from black-gray to golden-yellow. Uncharged regions, i.e. edge of electrode, retain the black-gray color. Unfortunately, many

Conclusions

The dark spot defects observed in pouch cell development have been analyzed. Dark spot defects mainly form during room temperature pre-charge. It appears that SEI quality is the key contributor to dark spot generation, as determined by comparing the graphite structure and chemical composition of dark spots to normal regions within the anode. In the dark spots, the content of Li, O, and F in the SEI are elevated, chemical composition is abnormal, and graphite is floating from the electrode

CRediT authorship contribution statement

Xiao Han: Conceptualization, Methodology, Writing - original draft, Visualization. Saisai Xia: Validation, Data curation. Ming-gong Chen: Supervision, Writing - review & editing.

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.

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