Researches On Introducing Diamond In Anode And Electrolyte For Improving Battery Performances

Lithium-ion batteries（LIBs）,as one of the foremost components for energy conversion and storage,have been widely applied in electronic products and electric vehicles.In order to meet the huge energy demand,improving energy density,stability as well as safety is the development trend for the next generation of LIBs.Anode material and electrolyte are the two important components in a whole structure of LIBs.Currently,the most widely studied anode materials mainly include graphite,silicon carbon and metal oxides,among which,graphite with low cost and simple production process has become the most commercial anode material for LIBs.However,the theoretical capacity of graphite is only 372 m A h g^-1,which is difficult to meet the growing energy demand,and the low Li⁺insertion potential（～0.1 V versus Li/Li⁺）would lead to the formation of lithium dendrites,thus causing safety risks.Electrolyte is generally thought of the“blood”for LIBs,and is of great significance for the energy density,power density,application temperature,cycling life and safety of LIBs.Moreover,the composition of electrolyte can directly affect the formation of solid electrolyte interface（SEI）,which possesses ionic conductivity and electronic insulation to prevent the continuous consumption of the electrolyte and electrode materials.However,the formation of SEI consumes the lithium ions in the electrolyte,forming irreversible products and resulting in the waste of lithium resources to reduce the power density of LIBs.Especially when the electrolyte contains trace water（H2O）,the high sensitivity of electrolyte and graphite anode to H2O could lead to the hydrolysis of lithium salts,large resistance of the solid-liquid interface,and the degradation of electrode structure and battery performances.Hence,the evolution of composition and structure for graphite anode,electrolyte and SEI is worthy of further investigation.In this work,nanodiamonds（NDs）were selected as the research objective,and different modified strategies such as multiphase composite structure design and electrolyte modification have been implemented to optimize the performance of LIBs.In detail,as a transition metal oxide,titanium dioxide（TiO2）has many outstanding properties including stable structure,low polarization,and high Li⁺insertion voltage（～1.7 V versus Li/Li⁺）,making it an promising anode material for LIBs.In addition,the NDs have been paid much attention in the field of LIBs due to its unique properties such as superhard,chemical inertness,large specific surface area,abundant surface groups and strong lithium ion adsorption capacity.The introduction of NDs into anode materials could provide more adsorption sites for Li⁺,further improving the reversible capacity and rate performance of LIBs.Moreover,introducing NDs into the electrolyte could facilitate the formation of a stable SEI rich in NDs（ND-SEI）on the surface of graphite anode,which possesses low interface resistance,fast ion transport and could provide additional Li⁺storage sites,further improving the capacity,rate performance and cycle stability of LIBs.The specific research contents and results are listed as follows:1.A hybrid structure composed of commercial ND and TiO2 particles coated with synthetic graphite（TiO2/G/ND,TGD）was prepared by microwave treatment and carbonization process to improve the performance of TiO2-based LIBs.The TGD composites were used as anode material to assemble the lithium-ion half batteries.The electrochemical performance results show that the hybrid TGD anode exhibits high capacities of 540 m A h g^-1 at 0.5 C after 100 cycles and 300 m A h g^-1 at 5 C after 1000cycles,which are significantly higher than the data of sole TiO2(168 m A h g^-1)and graphite anodes(372 m A h g^-1).The high capacity and excellent cycling stability of LIBs can be attributed to the synergistic effect of the NDs（larger surface area,low expansion,high Li⁺adsorption capacity）,TiO2（high Li⁺insertion voltage,stable structure,low volume expansion）and the synthetic graphite（high conductivity）.2.The NDs with average size of 5 nm were introduced into commercial Li PF6electrolyte to prepare ND electrolyte with ND concentration of 0.8 g L^-1.During the cycling（charge/discharge）processes,the Li⁺from electrolyte and cathode adsorbed on ND particles surface and migrated to graphite anode under the action of electric field force,and eventually forming a ND-SEI on the surface of graphite anode.The ND-SEI has low interface resistance,high Li⁺diffusion coefficient and could provide abundant storage sites for Li⁺to significantly enhance the capacity and rate performance of the battery.The half-cells with ND electrolyte exhibit high capacities of 700 m A h g^-1 after100 cycles at 0.2 C,and 400 m A h g^-1 after 1000 cycles at 5 C,which are almost twice that of the original electrolyte without NDs under the same processing conditions.Further considering the compatibility with industrial processes,the strategy of forming ND-SEI facilitates the manufacture of high-performance LIBs.3.Under the protection of argon atmosphere in the glove box,adding a certain amount of deionized water（H2O）into the ND electrolyte to prepare H2O and ND mixed electrolyte.When the H2O concentration in electrolyte reaches 3000 ppm or above,the electrolyte is partially decomposed,and its ionic conductivity and wettability to polypropylene separators（PP separator）are greatly improved.The electrochemical tests show that the high concentration of H2O in the electrolyte could improve the performance of LIBs with the assistance of NDs.At the current density of 0.1 A g^-1,a reversible capacity of 900 m A h g^-1 can be achieved after 100 cycles,and even at a high current density of 5 A g^-1,a reversible capacity of 130 m A h g^-1 is maintained after10000 cycles.The high capacity and excellent cycling stability can be attributed to the ND particles in the SEI,providing more storage sites for Li⁺and improving the rapid adsorption/desorption capacity of Li⁺.Meanwhile,the ND particles can effectively inhibit the intercalation of H2O molecules from the electrolyte into the graphite anode,thus protecting the structural stability of the anode material.This work provides new insights to address electrolyte,SEI,and anode stability challenges,especially for batteries operating in wet environments.The design of multiphase composite anode materials and high quality SEI have significant research significance in improving the comprehensive performance of LIBs.In this paper,nanodiamonds were introduced into anode material and electrolyte to increase the Li⁺storage sites and enhance the solid phase diffusion coefficient of Li⁺,thus realizing substantial improvements in the capacity,rate performance and cycle stability of LIBs.The ND-related electrode preparation and electrolyte modification in this paper have the advantages of simple process,environmental friendly and low cost,providing ideal insights in the design and manufacture of energy storage devices and components for industrial applications.