Modern lithium ion batteries is established on the basis of the graphite anode, in the process of charging Li + emerge from the positive lattice structure, through the spread to the surface of the graphite anode, electrolyte and then embedded in the graphite structure, X-ray diffraction, neutron diffraction methods are shown, along with the increase in the number of Li + embedded in graphite structure, can form LiC12 compounds, Li concentration continued to increase, the final formation of LiC6 compounds, the whole process is divided into a lot of steps to implement, the present study showed complete LiC6 embedded process can be divided into four stages or eight stages.



Recently from the German Otto Schott materials institute of Martin Dru ̈ e, XRD and metallographic microscope technology of graphite anode in the intercalated-li and is embedded in the process of phase change has carried on the detailed research, study shows that graphite anode in the process of lithium and embedded to take off the embedded with different reaction mechanism, LiC12 is a kind of solid solution of stoichiometric ratio of compound.



Experiment Martin Dru ̈ e graphite sheet (SGL Carbon Sigrafine7500) and lithium metal powder synthesized by golden LiC6 under 330 ℃, graphite samples of different embedded lithium state is by putting a LiC6 roasting in a vacuum, give Li volatilization. Martin Dru ̈ e also with Li concentration gradient distribution of graphite were synthesized, the following figure b.

Samples of XRD diffraction curve as shown in the figure below, the figure for a graphite flake XRD figure, can see from the figure, (002) diffraction peak to the low Angle has a small mobile, showed that C axis crystal package parameters, (100) and (101) diffraction peak width, and the (102) diffraction peak disappear completely, this indicates that graphite flake is a disorder configuration, this may be a graphite powder rolling into graphite pieces of production process, material of LiC6 XRD figure shows the chaos of graphite in no order of configuration characteristics after the intercalated-li be preserved.

The following figure shows the original LiC6 material after polishing and heat treatment, and the XRD chart of graphite material. From the figure, we can see that the diffraction peak of the original LiC6 material is 24.5°, and the diffraction peak is relatively wide, which may be due to the poor crystallinity of LiC6 material, or due to the considerable amount of LiC12.



After polishing, the diffraction peak of LiC12 (002) appears in the diffraction peak of (001), which may be because the structure is slightly changed due to the loss of Li in the polishing process. Graphite materials with different amount of lithium embedded can be obtained after treatment at 330℃ for different times. After 72h treatment, the diffraction peak of LiC12 (002) is more obvious, and the diffraction peak of LiC6 (001) begins to decline, indicating that the proportion of LiC12 begins to increase due to the decrease of the quantity of Li, and the proportion of LiC6 begins to decline. After 96h treatment, the XRD pattern no longer contains (001) diffraction peak, and the position of (002) diffraction peak also moves to a larger Angle, indicating that LiC6 is no longer contained in the sample at this time. After treatment for 146h, the XRD pattern of the samples is almost unchanged compared with that of the samples treated for 96h. After 240h heat treatment, the diffraction peak of (002) was significantly shifted to a larger Angle (29°), indicating that the c axis lattice parameters of LiC12 decreased significantly, while the diffraction peak of (101) and diffraction peak of (004) of graphite showed that Li in the sample was unevenly distributed in the graphite lattice.

The figure below shows the change data of c-axis lattice parameters of graphite samples with different lithium embedding. It can be seen from the figure that the c axis lattice parameters of LiC6 components are very stable until the decomposition disappears, while the c axis lattice parameters of LiC12 components decrease with the decrease of Li content, so it is likely that the formation of a solid solution structure in the graphite structure.

The following figure is a metallographic microscope image of graphite with different lithium embedded samples. Figure a shows the polished LiC6 sample, which contains golden particles with diameters of 10-20um (LiC6), surrounded by dark red phases (LiC12). Figure b shows the sample treated for 72h at 330℃. Compared with the sample without heat treatment, the phase color of the sample is darker and there are more red areas. Figure c is the sample treated for 96h at 330℃. It can be seen that only a few red areas have been transformed into blue and purple in the image. FIG. D shows the original phase structure of graphite. There is only one phase and some microholes in the whole image area.

Martin Dru ̈ e Li with gradient concentration samples for metallographic analysis, analysis of the sample is divided into seven areas, as shown in the figure below, the metallographic in area 1 in figure a, figure shows bright red yellow area and the surrounding area, as the concentration of Li (diagram b, c, d), yellow area gradually disappear, the red area around also transformed into violet, eventually turned into dark blue and grey (figure e, f).



This kind of direct access to different concentration of Li Li sample of graphite samples and after roasting of XRD diffraction data has a very significant difference, for example in roasting obtained samples, LiC6 composition of c axis crystal package parameters changes with the change of the content of Li, not in the direct synthesis of Li gradient concentration samples, with the decreasing concentration of Li, LiC6 c axis crystal package parameters increase. In addition, region 3 of the Li gradient concentration sample (FIG. C) has the same composition as the sample obtained after 72h roasting, but the phase composition is obviously different. The main phase of both samples is blue-purple, but the samples obtained by roasting still contain a considerable amount of LiC6 components. This indicates that Li has different reaction mechanisms in the process of embedding and removing graphite structures.

According to the experimental results and reference other research results, Martin Dru ̈ e thought graphite in embedded lithium and lithium in the process of phase change as shown in the figure below, the intercalated-li process (figure a), first at lower concentration of Li formed under low concentration stage 2 phase, with the increase of the number of Li, began to change for the orderly LiC12, then with the increase of Li phase shift for high-density LiC12 mixed with pseudo phase 1 phase structure, finally into super density LiC12 structure and LiC6 hybrid structure.



In Li emerge in the process of the graphite structure (figure b), first by the LiC6 and super density phase 2 phase transition to low concentration of Li lamellar phase 1 and orderly LiC12 mixed structure, and then with the further out of Li, into an orderly LiC12, finally as Li concentration further reduced, eventually transformed into low concentration stage 1 phase.

Martin Dru ̈ e study in a high concentration of Li LiC6 observed in samples of the two phase, a bright yellow LiC6. One is a dark red LiC12, which is quite different from our general understanding. Martin Dru ̈ think LiC12 e is a kind of with different content of Li, and the lattice structure is in a very wide range of Li can have a continuous change of crystal structure, indicates that it is more like a solid solution structure. At the same time, the study also shows that Li embedded in and out of graphite, has a different reaction mechanism, in the process of embedding can form a pseudo phase 1 phase, under the low concentration of Li instead have larger lattice parameters, in the process of embedded off the lattice parameters of LiC6 hardly changes with the concentration of Li, show that Li and graphite stack structure did not change, but formed a stage 1 phase of low concentration of Li.