Evolution of the morphology and structure of graphite anodes during long-term battery cycles

In the current lithium-ion batteries, the commonly used anode materials for lithium batteries are mainly divided into carbon-based anode materials, lithium titanate, and silicon-based composite materials. Due to the energy density limitation of lithium titanate, the expansion and inferiority of silicon-based composite materials has been It has not been well resolved, and carbon-based anode materials still occupy the main part of lithium battery anodes.

Carbon anode materials are mainly composed of graphite, hard carbon, and soft carbon anodes. Graphite is a commonly used anode material. Graphite has the advantages of high electronic conductivity, large lithium ion diffusion coefficient, small volume change before and after lithium insertion, high lithium insertion capacity and low lithium insertion potential, etc., and has become the current mainstream commercial lithium-ion battery anode material. Everyone knows that a lithium battery is a secondary battery that works in a "rocking chair" style. If lithium ions shuttle back and forth between the graphite negative electrode and the positive electrode material without loss, then this will be the most ideal state, but the fact is that it is affected by the graphite layer. Lithium batteries will gradually attenuate and deteriorate during use due to the influence of multiple factors such as the shape structure, the crystal structure of the cathode material, the ion conductivity of the electrolyte, and the temperature, until it becomes invalid.

In the process of long-term cycling, how will the morphology and structure of the graphite anode of lithium batteries change? The positive electrode material is lithium cobalt oxide, and the negative electrode material is graphite. After the lithium battery is prepared, a long-term cycle test is performed on it, and samples are taken for detection and analysis at different cycle nodes.

1. The evolution of the morphology of graphite anode during long-term cycling

The lithium battery cycle test was carried out for 1000 weeks, respectively unassembled (a), activated (b), 600 cycles (c), 700 cycles (d), 800 cycles (e), 900 cycles (f), 1000 cycles ( g) The negative pole piece is analyzed by SEM, and the result is shown in Figure 1:

Figure 1. SEM image of graphite anode after different cycles (5000 times)

It can be seen that graphite materials, whether unassembled, activated, or recycled, are composed of particles ranging from hundreds of nanometers to tens of micrometers, and the particle size distribution is not uniform, and no graphite material is found in the enlarged image of 5000 times. The appearance changes. In the 50,000 times magnified image (Figure 2), the unassembled graphite has a clean surface, and only the activated graphite surface begins to show film-like substances, and these film-like substances also exist on the graphite surface during the following charge and discharge cycles. substance. After EDS test and analysis, it was found that the unassembled graphite electrode contained only C element. However, in addition to the C element, the O element appeared in the graphite electrode after only activation and different cycles. This result shows that only the activated and cycled graphite electrode generates O-containing material, which proves that the film-like material is an SEI film.

Figure 2. SEM images of graphite anode after different cycles (50,000 times)

2. The evolution of the structure of graphite anode during long-term cycling

The possible changes of graphite anode during long-term cycling are mainly reflected in the graphite layer glass and the increase of the layer spacing. XRD tests were performed on the unassembled graphite negative electrodes after 600, 700, 800, 900 and 1000 cycles, and the results are shown in Figure 3. According to Bragg's equation and Scherrer's formula, the interlayer spacing d002, the degree of graphitization, the grain size Lc, and the grain size La of the graphite material in the direction of the (002) crystal plane can be calculated.

Figure 3. XRD patterns of graphite anode after different cycles

Figure 4 shows the curve of d002 and graphitization degree of graphite electrode with the number of cycles. During the entire 1000 charge-discharge cycles, the d002 and graphitization degree of the graphite electrode material changed very little, but d002 showed an increasing trend and the graphitization degree showed a decreasing trend.

Figure 4. Graphite d002 and graphitization degree change with the number of cycles

Figure 5 is the graph of the crystal grain size Lc and La of the graphite electrode material as a function of the number of cycles. The Lc in the process of not recirculating to 1000 times shows a gradual decrease trend, La has no obvious change rule, and its value fluctuates in the range of 47~49 nm

Figure 5. Graphite grain size Lc and La changes with the number of cycles

The morphology of the graphite negative pole piece during long-term cycling was observed, and the result is shown in Figure 6. The activated graphite negative electrode is well bonded and the surface condition is normal, but the electrode material gradually appears at the edge and winding crease of the graphite negative electrode after 100 and 1000 cycles. Since the reaction activity at the end of the graphite edge is higher than that at the base plane, the side reaction at the end of the edge is more intense, making the graphite material more likely to fall off. During the entire long-term charge and discharge cycle, the Lc value of the graphite material shows a decreasing trend, and d002 shows an increasing trend. The Lc value is the product of d002 and the number of graphite flakes in the grain, so the number of graphite flakes in the grain shows a decreasing trend. Such structural changes are macroscopically manifested as the shedding of graphite material.

Figure 6. Digital photo of graphite anode after activation only, 100 cycles and 1000 cycles

During the use of lithium batteries, the capacity decay often occurs faster, and the structural change of the graphite anode is one of its main factors. We can also judge the reasonable cycle life of the lithium battery by analyzing the changes in the structure and morphology of the graphite negative electrode. Stop using it when it is close to this parameter to prevent the negative electrode graphite from peeling off the copper foil and causing safety hazards.