Cathode-electrolyte interfacial reaction

Cathode-electrolyte interfacial reaction

Nowadays, the formation of SEI films on the anode-electrolyte surface has been extensively studied, while the research on the cathode-electrolyte interfacial reaction is seldom studied. This is because the cathode material maintains a stable lattice structure during the entire intercalation and deintercalation process of lithium ions. However, the intercalation of lithium ions in the electrolyte leads to exfoliation of the graphite electrodes. However, the oxidation reaction of the electrolyte may cause the formation of the positive electrode passivation layer. This is similar to the formation of SEI films on Li metal and graphite surfaces through reduction reactions, the only difference being the oxidation reaction of the electrolyte. In order to control and improve the performance of lithium batteries, it is important to form a stable negative SEI film.

  1. Intrinsic surface layer of oxide cathode

Before detecting the formation of SEI on the positive electrode, in order to distinguish the surface layer components formed by electrochemical and chemical reactions from the SEI layer components, the intrinsic surface layer present on the positive electrode surface must be considered. Conventional cathode materials, such as Li1-xNi1+xO2, LiCoO2 and LiMn2O4, are found to have Li2CO3 on their surfaces. The formation of Li2CO3 is the result of the reaction with CO2 in the air during the synthesis of the electrode material. It dissolves in the electrolyte and forms an intrinsic layer on the surface of the negative electrode. LiCO3 is formed according to the following equation with a thickness of 10 nm.

When the surface of the positive electrode is covered with an insulating layer of Li2CO3, some positive particles may be electrically insulating, which reduces the output and capacity of the battery.
When the surface of the positive electrode is placed in a non-aqueous pure solvent, the Li2CO3 on the surface will remain stable, but in the electrolyte containing HF, the Li2CO3 will begin to dissolve. This is due to the acidic character of lithium salts such as LiPF6 and LiAsF6, and a small amount of water in the electrolyte reacts to generate HF acid.

  1. SEI film of oxide cathode

Compared with the graphite electrode, the first irreversible capacity of the positive electrode is relatively small, and the EIS analysis confirmed the existence of the SEI film on the surface of the positive electrode. The lithium battery with Li1-xCoO2 as the positive electrode material gradually increases the thickness of the positive electrode surface and the internal impedance of the battery during the cycling process. The highly oxidizing Co4+ is formed on the surface of the Li1-xCoO2 cathode at about 4.5 V. As the PC and Li1-xCoO2 particles mix, electron transfer occurs between the Co4+ electrolytes. This reaction promotes the oxidative decomposition of PC and forms a dense polymer surface layer. Figure 1 shows the Randles equivalent circuit of the anode-electrolyte interface in the presence of an SEI film.

Figure 1 - Randles Equivalent Circuit of Anode-Electrolyte Interface with SEI Film
Figure 1 – Randles Equivalent Circuit of Anode-Electrolyte Interface with SEI Film

Similar to the negative electrode, when lithium ions pass through the first SEI film and come into contact with the surface of the positive electrode, an electron transfer reaction occurs, generating an additional SEI layer outside the first SEI film layer. Repeated cycling will increase the battery’s internal resistance, potentially affecting battery performance, battery life, and thermal stability.

  1. Interfacial reactions of oxide cathodes

LiMn2O4 or other cathode materials do not initiate electrochemical reactions, and these cathode materials undergo chemical reactions upon contact with the electrolyte to easily produce SEI films. The HOMO and LUMO energy levels of the oxide will be changed after the contact between the oxide and the electrolyte, similar to that of the negative electrode. Electron transfer occurs from the HOMO level of the electrolyte to the conduction band of the oxide, resulting in a negative charge on the oxide surface and a positive charge on the electrolyte surface. The oxidatively decomposed compounds of the electrolyte are deposited on the surface of the positive electrode to form a passivation film. When a lithium battery is charged, lithium ions are deintercalated from the positive electrode and transferred to the negative electrode through an external circuit. The valence band energy level will decrease and be close to the HOMO energy level, which will make it easier for electrons to transfer from the electrolyte to the cathode, and accelerate the oxidative decomposition of the electrolyte and the formation of the SEI film. The SEI films formed by the chemical reaction of the electrolyte are mainly polyoxyethylene, alkyl carbonate, LiF, LixPFy and LixPyFz.

Generally, the irreversible capacity of the battery decreases by 10% for the first time due to the formation of the SEI film. In addition, during the charging and discharging process, the formation of the SEI film will also increase the internal resistance of the battery. As the cycle progresses, the internal resistance of the battery increases while the capacity continues to decrease. As shown in Figure 2, the semicircle in the high frequency range corresponds to the passivation film and the interface impedance, and the interface internal resistance has been increasing until the voltage rises to 3.8V, but the internal resistance decreases until 4.3V. This indicates that the formation of the SEI film changes with the charging voltage.

The SEI film on the surface of LiMn2O4 is more unstable than that on the surface of the graphite anode. The stability of LiMn2O4 will affect the interfacial reaction and make Mn3+ disproportionate in LiMn2O4. If LiMn2O4 is placed in pure

In DMC solvent, DMC will be oxidized by LiMn2O4, and part of LiMn2O4 will be reduced to Mn2O3. The reaction formula is as follows:

Reaction with DMC produces intermediates that initiate polymerization to produce lithium alkoxides, and DMC is continuously consumed.

When LiMn2O4 oxidizes the electrolyte solvent, the resultant from the reduction reaction will decompose to form MnO and Li2MnO3, MnO, Li2O or -MnO2. Due to the porosity of the SEI film, -MnO may dissolve into the electrolyte. In practice, dissolution of -MnO can be seen at temperatures above 60°C. The reaction formula is as follows:

Besides chemical reactions leading to SEI film formation and interfacial reactions on the cathode, electrochemical reactions also have a great impact on battery performance. As mentioned above, PC undergoes an epoxidation reaction [40], and electrochemical oxidation occurs at 4.1 V. It will be deposited on the surface of LiCoO2 in the form of organic compounds containing carboxyl, dicarboxylate, -CH2-, -CH3 functional groups [44]. This effect is more obvious in LiMn2O4. Cycling of Li-ion batteries in the range of 3.8–4.3 V will seriously reduce the reversible capacity of the batteries. This is because the oxidative decomposition of the electrolyte will generate an SEI film on the surface of the positive electrode, which increases the internal resistance of the battery [45]. When the oxidative decomposition occurs, the electrolyte transfers electrons to the electrode to participate in the oxidative decomposition, which will cause the occurrence of self-discharge, and lithium ions will intercalate into LiMn2O4 to balance the charge. The decomposition products of the electrolyte will be deposited on the surface of LiMn2O4 to form an SEI film. As shown in the following equation:

Similar reactions for the formation of SEI films on LiMn2O4 also occur in Li1-xNi1+xO2 and LiCoO2. As shown in Figure 3, the SEI film contains polycarbonate, ROCO2Li, ROLi, LiF and P-F/As-F functional groups. The interfacial reaction between metal oxide and organic solvent is shown in the following equation [46].

Figure 3-FTIR spectrum of the surface passivation layer of LiMn2O4 cathode
Figure 3-FTIR spectrum of the surface passivation layer of LiMn2O4 cathode

In the equation below, EC undergoes a nucleophilic reaction to form alkyllithium carbonate, and alkyllithium carbonate undergoes a polymerization reaction to form polycarbonate [6].
LiF is often found in SEI films, because a small amount of water will react with LiPF6 to generate HF, and HF will react with lithium ions on the electrolyte or electrode surface to generate LiF. The reaction formula is as follows:

Since LiF is an insulator, an increase in LiF concentration leads to a concomitant increase in cathode resistance. In addition to the formation of SEI film, Mn3+ in LiMn2O4 also undergoes disproportionation reaction [47].

Both the electrolyte salt and the solvent must be carefully chosen because disproportionation and electrolyte acidity are closely related. For lithium salts, the degree of decomposition due to manganese disproportionation is as follows:

In addition to the disproportionation reaction, due to the transfer of electrons from the electrolyte to LiMn2O4. Mn3+/4+ in LiMn2O4 will be reduced to Mn2+. Subsequent dissolution of Mn2+ occurs. This not only affects the structure of the positive electrode but also greatly affects the performance of the negative electrode, because Mn2+ will pass through the electrolyte and then adsorb in the negative electrode in the form of metallic Mn.

The oxidative decomposition of the electrolyte and the potential for various reactions vary with the composition of the electrolyte and the type of positive electrode. When using PC, LiNiO2 outgassed at 4.2 V, while LiCoO2 and LiMn2O4 only started outgassing at 4.8 V. Whereas when PC/DMC solvent is used, only LiNiO2 releases CO2 at 4.2 V [49].
Li intercalation and deintercalation stabilize the cathode structure, which is observed in Li1-xNi1+xO2. Due to the mixed arrangement of cations and atoms in the Li and Ni layers, the non-stoichiometric ratio of Li1-xNi1+xO2 exists in a half-layer state. When the voltage is charged to 4.2V (or more than 0.6 lithium ions are intercalated), cation mixing will occur in the Li1-xNi1+xO2 positive electrode (Ni2+ will be generated), and the positive electrode will release unstable oxides and cause electrolyte. Oxidative decomposition occurs. SEI films are composed of organic substances such as dicarbonyl anhydrides and polyesters [37]. From Figure 4 we can see that the first irreversible capacity decay occurs with irreversible structural changes and the formation of the SEI film.

During the first cycle, the active cation mixes lead to a decrease in Ni3+/4+ concentration, an increase in Ni2+ concentration, and the continuous release of oxygen to maintain the charge balance. The evolution of oxygen, in turn, generates heat and may cause ignition of other substances (such as electrolytes, binders and organic substances). Since these interfacial reactions are caused by electron transfer or oxygen transfer, the changes in the electronic structure of Ni in the cathode are directly related to the oxidative decomposition of the electrolyte and the formation of SEI. Therefore, the first irreversible capacity fading observed in Li1-xNi1+xO2 can be traced to the structural change of the cathode as well as the interfacial reaction of the electrolyte [51].

For a more accurate understanding of the electrode interface in lithium batteries, we need to focus on the surface of the metal foil as the current collector. Since the positive electrode current collector is made of metal Al, which is easily oxidized, the metal surface is usually coated with Al2O3. As shown in Fig. 5, the surface of commercial metal Al is coated with a layer of polyamide or other substances to prevent metal oxidation [37].

When performing FTIR detection and organic analysis on SEI films, the current collector coating and binder polymers need to be analyzed in advance to obtain more accurate information about the SEI composition.

  1. Interfacial Reaction of Phosphate Cathode

LiFePO4 is being researched as a new cathode material. Due to the low conductivity of LiFePO4, it is often coated with a layer of carbon. And the low operating voltage results in different interfacial properties with other metal oxides. Although Li2CO3 appears intrinsically on the surface of other cathode materials, it is not found in LiFePO4. This suggests that phosphate functional groups do not react in air [50-54]. Conversely, lithium iron oxide (LixFeyOz) is generated during the synthesis process, which exists in a small amount (<2wt%) on the surface of LiFePO4 [50, 53]. This LiFeO on the surface releases Li during the first charge and increases the charging capacity of LiFePO4. Since the above reaction is an irreversible reaction and reduces the activity of iron oxide, the discharge capacity can only be obtained from LiFePO4. Therefore, the first irreversible charge capacity will be greatly reduced. Meanwhile, LiFePO4 synthesized by the sol-gel method may be contaminated by surface impurities such as FeP and Li3PO4, which may be generated during thermal treatment [55]. The type and concentration of compounds will vary with the synthesis method, thus affecting the electrochemical cycling performance of LiFePO4.

Trace amounts of water in the electrolyte may generate hydrochloric acid and hinder the performance of LiFePO4 electrodes, as is the case with other lithium metal oxides. For example, LiPF6 reacts with water and decomposes to form HF, and when HF contacts LiFePO4, Fe will be dissolved out. The dissolution of Fe not only reduces the irreversible capacity of LiFePO4, but also Fe may be adsorbed on the surface of the negative electrode, resulting in the loss of the irreversible capacity of the negative electrode.

Read more: What are the preparation methods of polymer electrolytes?

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