Materials for Surface Modification
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Introduction
Since the commercialization of lithium batteries, the cathodes of commercially available batteries are now made of graphitic carbon, while the batteries are usually identified by the chemical active element of the positive electrode, such as the anode of lithium batteries, which is lithium. One of the routes that have been explored to improve the performance of electrode materials is surface modification of electrode materials. In general, the idea is to reduce the interfacial resistance by crystallization of the surface layer or by suspending something on the particles to prevent their side reactions with the electrolyte, to improve structural stability, to avoid the loss of transition metal ions or oxygen, and to reduce ionic disorder without changing the electron and ion conductivity. This route has been shown to be more effective than doping processes in improving the electrochemical performance and safety of batteries. In commercial batteries, the negative electrode is usually graphitic carbon. Graphite is a good conductor and has twice the specific capacity of the positive element. On the other hand, it has some well-known limitations, such as the need to stabilize the solid-electrolyte interface, which requires surface modification.
Cathode electrodes
For cathode materials such as LiCoO2, LiNiO2 and LiMn2O4, the surface coating with oxides of electroactive cathode materials such as MgO and Al2O3 can prevent direct contact between electrolyte and cathode material, improve structural stability and inhibit phase transition. In the case of some coatings, the conductivity can be improved. As a result, improvements in capacity retention, multiplicative performance and electrochemical behavior at high temperatures have been achieved. Other cathode materials, such as the rhombohedral system LiMnO2 and aerogel V2O5, can also exhibit better electrochemical performance by coating.
Anode electrode
In the case of graphitic carbon anode materials, the electrochemical properties were significantly improved after modification of their surface structure by mild oxidation, deposition of metals or metal oxides, polymer or other kinds of carbon coatings. For example, graphite can be modified by using polythiophene coated on its surface. The polymer coating not only reduces the contact of the active sites of graphite with the electrolyte and the consequent irreversible capacity, but also acts as a conductor, forming a composite material that becomes a good electronic network. Moreover, it has a good elasticity and binder role, so there is no need to add additional good binder, insulating fluoropolymers such as poly(difluoroethylene), to achieve good and stable contact of the active material particles during cycling process. Different surface coatings play different roles. Other types of surface modifications, such as a combination of high-temperature gas treatment and methylsilylation and by co-blending of coated polymers can also be used to achieve some improvements. For other negative electrode materials, a progressive understanding of the surface structure will lead to great advances in their performance.
Reference
- Mauger, A., and C. Julien. Surface modifications of electrode materials for lithium-ion batteries: status and trends. Ionics, 2014, 20.6: 751-787.