Research On Plasma Modulation And Electrochemical Performance Of Anode Materials For Lithium Ion Batteries

Lithium-ion battery is one of the most widely used energy storage devices,but their low energy density,Coulomb efficiency and cycling stability have become bottlenecks in the rapid development of areas such as large-scale energy storage and power batteries.Lithium titanate(Li₄Ti₅O₁₂)and titanium dioxide（TiO₂）with excellent rate performance and stability and silicon（Si）with ultra-high specific capacity are regarded as new anode materials for lithium-ion batteries with great application prospects,but low electron/ion conductivity,cyclic volume expansion and pulverization,and unstable solid-state electrolyte interphase（SEI）have seriously restricted their development.As the fourth state of matter,plasma has rich physical and chemical effects with materials,such as defect modulation,heteroatom doping and surface deposition,and related technologies are gaining more and more attention and applications in the field of electrode materials,but the research and applications for the above materials are still weak.In this paper,the plasma technology is used for the regulation and preparation of TiO₂,Li₄Ti₅O₁₂ and Si-related anode materials,and the mechanism of plasma effect on vacancy generation,morphology regulation and compounding are investigated,and the laws and mechanisms of the effect on conductivity,specific capacity,impedance and cycling performance are discussed.The specific studies are as follows.（1）TiO₂/C nanocomposites with high defect concentration are prepared using a combined strategy of MOF-derived nano/microstructure and plasma defect engineering.It is found that the introduction of defects including oxygen vacancies,Ti³⁺ions and lattice distortion in TiO₂ can significantly increase its intrinsic conductivity,which is confirmed by DFT calculations and GITT tests.In addition,the specific surface area of TiO₂/C can be increased by controlling the power of the plasma without destroying the nano/microstructure of the MOF.The treated samples possess ultra-high reversible capacity(316.9 m Ah·g^-1 at 0.5 A·g^-1),excellent multiplicative performance(186.1m Ah·g^-1 at 10A·g^-1)and long cycle life(219.3 m Ah·g^-1 after 2500 cycles at 5 A·g^-1)when assembled into half-cells.（2）Li₄Ti₅O₁₂/CuO heterojunctions are constructed by depositing CuO ultrafine nanoparticles on Li₄Ti₅O₁₂ nanosheets using active screen plasma technique.It is found that the rich interface and built-in electric field provided by the heterojunction enhances the electron conductivity and ionic conductivity,and DFT calculations confirm that the heterojunction reduces the band gap and enhances the adsorption energy for lithium ions;in the lithium storage process,the conversion reaction between CuO and lithium ions not only provides additional capacity,but also the product Cu facilitates electron transport and accelerates the electrochemical process;In addition,the increase in vacancies and specific surface area due to high-energy particle bombardment contributes to the increase in conductivity and the number of active sites,and the treated sample exhibits excellent reversible capacity(182.6 m Ah·g^-1 at 1C),high rate performance(170.6 m Ah·g^-1 at 30C)and long cycle stability(136.7 m Ah·g^-1 after 4500cycles at 20C)in the half-cell,and the full cell assembled with NCM811 maintained227.4 m Ah·g^-1 after 700 cycles at 1C.（3）Three common carbon nanomaterials（carbon nanofiber CNF,graphene oxide GO,and activated carbon AC）are used as carriers on which Si O_x is deposited using active screen plasma technique to obtain silicon carbon anodes.The results reveal that Si O_x is uniformly deposited on the surface of GO,while the deposited particles on CNF and AC are clustered and inhomogeneously distributed.The deposition amounts of Si O_xon CNF,GO and AC are 43.4%,49.3%and 42.3%,respectively.The morphological and structural analysis demonstrates that the two-dimensional structure of GO is favorable for Si O_x nucleation and growth,and the structural stability of GO is better under the same etching conditions.The electrochemical analysis shows that Si O_x/GO exhibits relatively high initial capacity and initial coulomb efficiency（ICE）,because the uniformly deposited Si O_x/GO can form a stable SEI interface and reduce the contact between Si O_x and electrolyte inside the pores,while the mechanical stability of GO can suppress the volume expansion during the reaction.With the coupling effect of high Si O_x deposition and excellent structural stability,the prepared Si O_x/GO composite anode can reach a capacity of 820.8 m Ah·g^-1 after 30 cycles at 0.1A·g^-1,with a capacity retention rate of 59.3%.However,the ability of the Si O_x/GO negative electrode to suppress the volume expansion of Si O_x at high current densities is still somewhat limited.The above results demonstrate that the silicon carbon anode can be prepared by active screen plasma,but the good matching with carbon matrix and the construction of stable structure are also the keys to the preparation of high-performance silicon carbon anode by active screen plasma.（4）Using the high specific surface area and porous three-dimensional structure of ordered mesoporous carbon CMK-3,a Si O_x/CMK-3 composite with a novel hybrid structure is successfully prepared.The morphological and structural analysis indicate that the interior of Si O_x/CMK-3 maintains the ordered mesoporous channels of CMK-3,the middle is a porous structure with uniform distribution of nano-Si O_x,and the exterior is a continuous shell layer of Si O_x.With the increase of deposition temperature,the rod-like structure of Si O_x/CMK-3 gradually disperses into a fibrous structure.The TG reveals that the Si O_x content is 46.1%,61.7%and 90.1%under the treatment conditions of 300,400 and 500°C,respectively.The electrochemical analysis shows that the Si O_x/CMK-3 deposited at 400°C has good ICE and cycling stability.This is due to the appropriate content to balance the capacity loss with volume expansion.The porous structure and Si O_x shell layer can not only reduce the contact between Si O_x and electrolyte to reduce the capacity loss to enhance the ICE,but also can mitigate the volume expansion during cycling;the nanosized Si O_x inside the mesopores reduces stress concentration and provides high reversible capacity;the carbon matrix with internal mesoporous channels can enhance electron conductivity and diffusion kinetics.In addition,active screen plasma is able to introduce oxygen vacancies to enhance lithium storage sites and conductivity.With the help of suitable content and unique structure,the Si O_x/CMK-3 deposited at 400°C exhibits excellent reversible capacity(1870.3 m Ah·g^-1 at 0.1 A·g^-1),rate performance(389.1 m Ah·g^-1 at 5A·g^-1)and ultra-long cycle stability(618.9 m Ah·g^-1 after 4000 cycles at 1A·g^-1)when assembling half-cells.