The energy crisis and environmental pollution have put forward an urgent need for sustainable energy. Research has shown that defects typically have a significant impact on the electrical, thermal, optical, magnetic, acoustic, and mechanical properties of materials. With the development of solid defect research, some basic theories of solid defect chemistry have gradually been established, and defect engineering has been widely applied in functional material research, especially in cutting-edge fields such as optoelectronics, catalysis, and energy storage. As an effective defect control method, plasma modification technology has received widespread attention from researchers.

Plasma is known as the fourth state of matter, composed of electrons, ions, molecules, free radicals, photons, and other excited species. In 1928, American scientist Irving Langmuir first introduced the term "plasma" into physics to describe the collective behavior of charged particles. Plasma can be generated by ionizing gas, and ionization can occur when sufficient energy is provided to trigger collisions between gas molecules and electrons. Plasma can be divided into high-temperature plasma and low-temperature plasma according to temperature. According to the thermodynamic equilibrium properties, low-temperature plasmas can be further divided into thermal plasmas with local thermodynamic equilibrium and non thermal plasmas (NTP) with non thermodynamic equilibrium. The electron temperature (Te) and gas temperature (Tg) of hot plasma are approximately equal (<2eV, 1eV=1165K), and NTP has high Te (up to several eV, 1eV=1165K) and low Tg (close to room temperature). The high-energy particle bombardment process in NTP can effectively promote the occurrence of thermodynamically unfavorable reactions under mild conditions. More importantly, a large number of high momentum active species interact with materials through physical and chemical effects, such as causing atomic rearrangement, defect formation, and amorphization on the material surface, providing necessary conditions for in-situ generation of new materials with controllable defects (active sites). In recent years, plasma technology has become an important means of modifying electrode materials due to its advantages of easy operation, high efficiency, and environmental friendliness.

Plasma material surface interaction

The interaction process between plasma and materials has a critical impact on the efficiency and selectivity of induced effects. This effect determines the material and energy flow transferred from the plasma body to the material surface, and its properties and flux directly determine the effectiveness of surface modification. In plasma, free electrons collide with gaseous species, producing ions, neutral particles, electrons, and photons. Inside the plasma body, the total amount of positive and negative charges is basically equal, exhibiting a "quasi electric neutrality". However, when the plasma comes into contact with the material surface, due to the extremely small electron mass and fast thermal motion, they will reach and impact the surface at a very high speed, causing the surface to accumulate negative charges and present a negative potential relative to the plasma body. This negative potential will repel subsequent electrons while strongly attracting positively charged ions to accelerate towards it. Finally, a non electrically neutral thin layer region is formed near the material surface, where the concentration of positive ions is much higher than that of electrons, known as the plasma sheath (Figure 1 (a)). The strong electric field existing within the sheath is the key to accelerating ions, endowing them with directional kinetic energy, and thereby regulating the energy and material flow transferred from the plasma body to the surface, providing an effective mechanism for regulating plasma surface interface reactions across multiple spatiotemporal scales. For example, the vertical orientation of carbon nanotubes (CNTs) and graphene is significantly influenced by the direction of the plasma sheath. As shown in Figure 1 (b), the incident active particles in the plasma undergo momentum transfer with the material surface, which may cause surface atoms or ions to be sputtered, thereby forming vacancy defects in the lattice. During this process, the energy of ions and active neutral particles is usually higher than 5eV and between 0. 025-0. Between 05eV. The degree of sputtering is controlled by the type, energy, and direction of the incident particles. The currently reported types of vacancies include cation vacancies, anion vacancies, and multiple vacancies. In addition to physical sputtering, excited/charged particles in plasma have high chemical activity and can promote chemical reactions (such as reactive ion etching) to form volatile products. These physical and chemical reactions collectively lead to surface etching effects. Vacancy defects refer to the absence of one or a few atoms, while pore defects involve the absence of a large range of atoms, forming a hierarchical pore structure containing micropores, mesopores, and macropores (Figure 1 (c)). The electric field strength formed within the sheath is an effective means of regulating energy and material flow. The formation of the sheath layer improves the surface modification efficiency, as shown in Figure 1 (d). When high-energy dopant ions collide with surface atoms of the material, if the energy transmitted to the collided atom exceeds its displacement threshold, the atom will be knocked away from its original lattice position, and dopant ions will remain on the material surface, thereby forming doping defects.

Figure 1 (a) Schematic diagram of plasma sheath, (b) vacancy defects, (c) pore defects, and (d) doping defects

Non thermal plasma, as a fast, environmentally friendly, and efficient method, can efficiently introduce various defects such as vacancies, pore structures, and heteroatom doping under mild conditions, significantly improving the specific surface area, conductivity, reactivity, and stability of materials, and showing broad application prospects in electrode material modification.