Interfacial Reaction of Graphite (Carbon Material) and Influence of Additives

Interfacial Reaction of Graphite (Carbon Material) and Influence of Additives

  1. Interfacial reaction of graphite (carbon material)

Similar to the SEI film formed by the reductive decomposition reaction of the organic solvent and lithium salt on the surface of the lithium metal-electrolyte, the interfacial reaction also occurs at the interface between the graphite and the electrolyte. However, due to the different surface properties, the interfacial reaction between graphite and the electrolyte surface is different from that on the lithium-electrolyte interface.

Depending on the production process, different types of graphite differ in surface structure, chemical composition, particle size and morphology, pore size and distribution, open porosity, surface area, and types of impurities. When carbon materials are used as anodes, a fact that cannot be ignored is that the first irreversible capacity decay is larger. During the charging process, the transfer of electrons from graphite to the electrolyte causes the decomposition of the electrolyte, and this decomposition promotes the generation of SEI. In addition to the capacity attenuation, the consumption of electrons is caused by the intercalation reaction of lithium ions in the graphite. Also one of the reasons. Because of this, high specific surface area carbon materials will increase the carbon material-electrolyte reaction interface, so the progress leads to the first irreversible capacity fading.

Ideally, Li ions dissolved in the electrolyte in the solvent-free state would permeate the SEI membrane and intercalate into the carbon material, thus suppressing additional reactions induced by organic solvents. However, PC intercalates into the crystalline structure of graphite together with lithium ions, which destroys the layered structure of graphite and causes material exfoliation.

With the progress of the decomposition reaction of the electrolyte, the SEI film began to form on the surface of graphite at potentials of 1.7~0.5 V and close to 0.0 V. The potential generated by the SEI film varies with the graphite lattice plane, basal plane-to-end plane ratio, temperature, type of electrolyte solvent, concentration of lithium salt, and applied charge density. In order to maximize the performance of the battery, these factors should be considered.

The semi-circle in the high-frequency region represents the resistance of lithium ion penetration in the surface layer, and the semi-circle in the middle represents the resistance of charge transfer at the surface layer-graphite interface. The Warburg characteristic in the low frequency region is the diffusion impedance of lithium ions in graphite, and the impedance in the low frequency range represents the accumulation of lithium ions in the bulk phase. The impedance map illustrates the entire process of Li ion intercalation in the graphite anode.

  1. Thickness of SEI film

It is very difficult to measure the thickness of the SEI film at the electrode-electrolyte interface. Figure 1 is the result of measuring the thickness of the SEI film by an electrochemical method. The CV curve was measured using the electrolyte and the innate metal electrode system. The positive charge is provided by the SEI layer on the metal surface. The tests are performed under the assumption that the SEI is composed of a single compound such as lithium ethylene carbonate.

Figure 1 - Cyclic voltammetry of electrolyte/gold electrodes to measure SIE layer thickness (reproduced with permission from ACS, copyright 2005)
Figure 1 – Cyclic voltammetry of electrolyte/gold electrodes to measure SIE layer thickness (reproduced with permission from ACS, copyright 2005)

As can be seen from Figure 1, from the reduction peak of CV (1.7V), it can be seen that the lithium sodium bicarbonate of vinyl leads to the reduction of 0.01 C/cm2 of electricity. For this case, it can be known that the transfer of two electrons takes place for each EC molecule. Assuming a transfer of four electrons per dimer, this can be expressed as 1.56 × 1016 per dimer/cm2. If vinyl sodium bicarbonate is a cylindrical shape of 3 (diameter) x 20 (length) on the electrode surface, this number becomes 1.67 x 1014 (dimer/cm2)/layer, and the thickness of the surface layer is 300 (=30 nm).

Recent technological developments have made it possible to directly measure the thickness of the SEI layer using in situ AFM techniques and observe the change in grain size on the surface of the electrode produced by the SEI layer. As can be seen from Figure 2, the thickness of the SEI layer is determined by observing the changes on the AFM map. In this case, the thickness of the SEI film was 40 nm after the first cycle and grew to 70 nm after the second cycle.

Figure 2-AFM method to measure the thickness of the SIE layer
Figure 2-AFM method to measure the thickness of the SIE layer

The above measurement results are consistent with the results of the SEI layer thickness measured by other different methods. The thickness of the SEI film was 10–40 nm after the first cycle, and a surface layer of similar thickness was added to the surface after the second cycle.

  1. The effect of additives

The use of PC as an electrolyte is limited because PC intercalates into graphite together with lithium ions and causes electrode exfoliation during lithium ion extraction. Using EC instead of PC to generate SEI film can solve these problems, but its ionic conductivity will decrease at low temperature. Another way to prevent graphite exfoliation is to add a small amount of VC or lithium bis-oxalate borate (LiBOB) to the PC.

If PC alone is used as the solvent, the decomposition of LiBOB caused by the single electron transfer reaction can generate the SEI film. The resultant containing hydroxyl and oxalate forms a passivation layer on the graphite surface, which protects the graphite electrode in the PC-graphite interfacial reaction and inhibits electrode exfoliation.

The addition of LiBOB accelerated the decomposition of the electrolyte and formed an SEI layer on the surface. These SEI layers are mainly composed of compounds (semicarbonates) similar to 4,4-phenolsulfonamido compounds. in LiPF. Adding 1~5mol% LiBOB to the electrolyte can solve the problem of early exfoliation on the graphite surface.

VC is also one of the most commonly used additives. Adding 1% VC to PC can change the SEI properties of carbon electrodes, and 67% of the reversible capacity can be obtained during the first cycle, and 93% to 95% of the reversible capacity can be obtained in the following cycles [30]. Compared with the first reversible capacity of only 12% in the electrolyte without VC, the addition of VC is significantly improved. The decomposition of VC starts at a potential of 1.2 V for Li, which is earlier than the intercalation reaction of Li ions. The potential of the carbon electrode to decompose to form a passivation film at high potential is higher than that of PC and other organic solvents. The exfoliation of the carbon material can be prevented by blocking the intercalation path of PC and lithium ions.

According to thermal analysis and spectroscopic analysis, the components of the SEI film layer are mainly polymers, such as VC multipolymer, VC oligomer, VC open-chain polymer and other high molecular polymers and for example vinylidene lithium carbonate (CHOCO2Li)2, diene pentacyclic sodium bicarbonate lithium salt (CH=HOCO2Li)2, (CH=HOLi)2, hydroxy acid lithium salt (RCO2Li) [4].
As shown below, the decomposition of EC/DMC electrolyte produces ethylene, CO, and methane gas, and the decomposition of VC produces acetylene and CO gas.

The decomposition of EC/DMC electrolyte produces ethylene, CO and methane gas, and the decomposition of VC produces acetylene and CO gas.
The decomposition of EC/DMC electrolyte produces ethylene, CO and methane gas, and the decomposition of VC produces acetylene and CO gas.

VC preferentially decomposes EC at a potential of 1.0 V to lithium, and can also increase the reduction potential of EC by 0.8 V from 0.7 V, while contributing to the chain-opening reaction. Therefore, the SEI film can be effectively formed on the graphite surface, and the performance of the battery at high temperature is also improved.

Related Posts