Application of Field Emission Scanning Electron Microscope in Lithium-ion Diaphragm Inspection

I. Lithium-ion battery

The lithium-ion battery is a secondary battery, which mainly relies on lithium ions moving between the positive and negative electrodes to work. During the charging and discharging process, lithium ions are embedded and de-embedded back and forth between the two electrodes through the diaphragm, and the storage and release of lithium-ion energy are achieved through the redox reaction of the electrode material.

Lithium-ion battery mainly consists of positive electrode material, diaphragm, negative electrode material, electrolyte, and other materials. Among them, the diaphragm in the lithium-ion battery plays a role in preventing direct contact between the positive and negative electrodes, and allows the free passage of lithium ions in the electrolyte, providing a microporous channel for lithium ion transport.

The pore size, degree of porosity, uniformity of distribution, and thickness of the lithium-ion battery diaphragm directly affect the diffusion rate and safety of the electrolyte, which has a great impact on the performance of the battery. If the pore size of the diaphragm is too small, the permeability of lithium ions is limited, affecting the transfer performance of lithium ions in the battery, and making the battery resistance increases. If the aperture is too large, the growth of lithium dendrites may pierce the diaphragm, causing accidents such as short circuits or explosions.

Ⅱ. The application of field emission scanning electron microscopy in the detection of lithium diaphragm

The use of scanning electron microscopy can observe the pore size and distribution uniformity of the diaphragm, but also on the multi-layer and coated diaphragm cross-section to measure the thickness of the diaphragm. Conventional commercial diaphragm materials are mostly microporous films prepared from polyolefin materials, including polyethylene (PE), polypropylene (PP) single-layer films, and PP/PE/PP three-layer composite films. Polyolefin polymer materials are insulating and non-conductive, and are very sensitive to electron beams, which can lead to charging effects when observed under high voltage, and the fine structure of polymer diaphragms can be damaged by electron beams. The SEM5000 field emission scanning electron microscope, which is independently developed by GSI, has the capability of low voltage and high resolution, and can directly observe the fine structure of the diaphragm surface at low voltage without damaging the diaphragm.

The diaphragm preparation process is mainly divided into two types of dry and wet methods. The dry method is the melt stretching method, including the unidirectional stretching process and bidirectional stretching process, the process is simple, has low manufacturing costs, and is a common method of lithium-ion battery diaphragm production. The diaphragm prepared by the dry method has flat and long microporous (Figure 1), but the prepared diaphragm is thicker, the microporous uniformity is poor, the pore size and porosity are difficult to control, the assembled battery energy density is low, mainly used in low-end lithium-ion batteries.

Figure 1 Dry stretch diaphragm/0.5KV/Inlens

The wet process, i.e., thermogenic phase separation, involves the mixing and melting of polymers with high-boiling solvents, etc., and the production of microporous membranes through the process of cooling phase separation, stretching, extraction and drying, and heat treatment and shaping. Compared with the dry process, the wet process is stable and controllable, resulting in thin diaphragm thickness, high mechanical strength, uniform pore size distribution and interpenetration (Figure 2). Although the cost of the diaphragm made by the wet process is higher than that of the dry process, the assembled battery has high energy density and good charging and discharging performance, and is mostly used in mid- to high-end lithium-ion batteries. Combined with the pore size analysis system independently developed by GSI, the pore size and porosity of the diaphragm can be analyzed quickly and automatically (Figure 3).

Figure 2 Wet stretch diaphragm/1KV/Inlens

Figure 3 Diaphragm pore size analysis/1KV/Inlens

Although polyolefin-based diaphragms are widely used in lithium-ion batteries, they are limited by the mechanical properties, heat resistance, and surface inertness of the material itself, and simple polyolefin diaphragms cannot meet the requirements of high safety and high performance of lithium-ion batteries. For this reason, surface modification of polyolefin diaphragms is needed to improve their mechanical properties, heat resistance, and affinity with electrolytes. One of the most commonly used methods is the physical coating of the surface of the diaphragm. Inorganic ceramic materials (Figure 4) are characterized by good heat resistance, high chemical stability, and polar functional groups on the surface to improve the wettability of the polyolefin diaphragm to the electrolyte, so they are often used as coated particles to enhance the heat resistance and electrochemical properties of the diaphragm. Figure 5 shows the surface morphology of the ceramic surface of the diaphragm after coating with inorganic ceramic particles.

Figure 4 Alumina ceramic powder/5KV/BSED

Figure 5 Ceramic coated diaphragm/1KV/Inlens

1. III. Field Emission Scanning Electron Microscope SEM5000

SEM5000 is a high-resolution, feature-rich field emission scanning electron microscope with advanced barrel design, high-voltage tunneling technology, and low aberration non-leakage magnetic objective lens design, to achieve low-voltage high-resolution imaging. Its operating software is equipped with optical navigation to optimize the operation and usage process. Users, whether experienced or not, can quickly get started and complete high-resolution shooting tasks.