Plasma effects refer to the impacts of plasma on the microstructures and surface chemical properties of materials through its physical and chemical actions during the preparation and modification of cathode materials. These effects originate from high-energy electrons, ions, excited atoms, free radicals, and electromagnetic fields within the plasma.

During material preparation and modification, high-energy ions or neutral atoms excited by plasma bombard the material surface, regulating its microstructure and surface chemical properties to achieve etching, exfoliation, or vacancy creation. Active species in the plasma can induce surface chemical reactions such as surface functionalization, doping, or deposition. In addition, the thermal effect generated by plasma can induce material phase transitions or structural reconstruction.

Phase Transition Induction

High-energy particles and free radicals excited by plasma can induce chemical reactions and phase transitions in materials. High-energy electrons, ions, free radicals, and photons in plasma destroy the original lattice structure of materials via momentum transfer or direct reactions, promoting the formation of new phases.

Plasma can regulate the interfaces between different phases on or inside cathode materials, facilitating grain reconstruction. The preparation of cathode materials via plasma can stabilize intermediate phases or suppress detrimental phase transitions (e.g., transformation from a high-voltage active phase to a low-activity phase), thereby improving the cycle life and energy density of batteries. Plasma-induced phase transitions feature high speed, low processing temperature, and high selectivity.

Etching

Plasma etching uses high-energy particles in plasma to bombard the material surface, causing sputtering of surface atoms, or employs active species such as free radicals and ions excited by plasma to react chemically with the material surface to form volatile products, thereby achieving an etching effect.

High-energy active species generated by plasma accelerate the reaction process. Compared with wet and dry etching processes, plasma etching offers higher efficiency. Furthermore, selective high-precision etching can be realized by controlling the type and flow rate of discharge gases, as well as etching duration.

Exfoliation

When high-energy particles excited by plasma interact with the material surface via momentum transfer, surface atoms and molecules are activated to overcome weak intermolecular forces such as van der Waals forces or hydrogen bonds, completing surface layer exfoliation and realizing surface modification.

Thus, two-dimensional layered materials including graphene and layered hydroxides can be exfoliated by plasma. Plasma exfoliation is not only highly efficient but also free of toxic or environmentally unfriendly chemicals. For cathode materials, plasma exfoliation removes surface contaminants or oxide layers for surface cleaning, alters surface composition and structure, and increases surface roughness.

Doping

Plasma doping introduces dopant elements into materials through physical or chemical reactions between active species excited by plasma and the target material.

Plasma doping of non-metallic elements (e.g., N, O, S, P, B, F), metallic elements (e.g., Mg, Ti, Fe), and dual/triple elements (e.g., NS, NP, NSP) has been widely used to regulate the physical and chemical properties of materials. Doping with specific elements improves electronic conductivity and ion diffusion rate, enhances structural stability, reduces lattice deformation during battery charge-discharge cycles, and boosts cycling stability.

Deposition

Plasma deposition involves reactions between high-energy active species excited by plasma and gas-phase precursors. The resulting active intermediates adsorb onto the substrate surface and react with its chemically active sites, forming a uniform thin film.

Plasma can deposit carbon, metal oxides, nitrides, and other materials. Compared with traditional high-temperature deposition, plasma deposition operates at lower temperatures, avoiding structural damage caused by high heat. By adjusting parameters such as gas flow rate, precursor type, equipment power, and deposition time, the thickness and composition of the deposited layer can be controlled to modify the cathode material surface.

Vacancy Creation

High-energy active species (e.g., electrons, ions, and neutral particles) excited by plasma collide with the material surface and transfer momentum to surface atoms. When the transferred energy exceeds the bond energy of atoms in the material, these atoms are sputtered out of the original lattice, creating vacancies inside the material.

Due to plasma’s high energy density and reactivity, plasma treatment generates a large number of vacancies in a short time, enabling uniform modification of the cathode material surface. Currently, plasma-generated vacancies include cation vacancies (e.g., Co and Fe vacancies), anion vacancies (e.g., O, S, and N vacancies), and multiple vacancies (e.g., cation and O vacancies).

Surface Functionalization

Plasma is commonly used for surface functionalization such as oxidation, nitridation, sulfidation, and phosphating, or to introduce specific chemical functional groups, dopant elements, or protective layers onto electrode materials, thereby improving conductivity and cycle life.

For example, reactive oxygen species (O, O⁺, O⁻, O₂⁻, O₃, etc.) generated by oxygen plasma readily react with organometallic compounds to form oxides, accomplishing surface coating of cathode materials and enhancing cycle life. Similarly, nitrogen plasma, sulfur plasma, and phosphorus plasma react with metal ions to form nitrides, sulfides, and phosphides, respectively, during material modification.