Performance Study Of Carbon Silicon Based Composites And Commercial Lithium Cobaltate Modified Materials For Lithium Ion Battery Applications

Since the last century,the global issue of clean energy storage has grown in importance.With the introduction of the’double carbon’target,the use of fossil fuels has been restricted by governments and they have had an irreversible impact on environmental pollution and global warming.Lithium-ion batteries（LIBs）are a promising source of clean energy due to their high output voltage,long cycle life,wide operating temperature range,low self-discharge rate and lack of memory effect,and are widely used in portable devices,photovoltaic energy storage and new energy vehicles.However,as the demand for long range applications increases,the task of increasing the energy density of battery systems is critical.Therefore,this paper aims to design Si-based composite anode materials with high capacity and long-cycle stability and cathode materials with high capacity,high cut-off voltage and long-cycle stability of Li Co O₂,and to complete the preparation of highly conductive carbon and silicon composites and the modification of commercial lithium cobaltate materials by carbon cladding and oxide cladding,respectively.The morphology,structure,elemental composition and electrochemical properties of the two different cathode and anode materials were investigated through physical characterisation and electrochemical performance tests as follows:1.Silicon has a high theoretical capacity and is a promising anode material for lithium-ion batteries.However,the high bulk variability of silicon has led to severe chalking and rapid capacity decay of the electrode material.To ameliorate this type of problem,this paper uses glucose and phytic acid as carbon sources to prepare a carbon-coated silicon-based material with a 3D conductive network structure,which was tested by N₂adsorption/desorption to have a specific surface area of 49.8 m²g^-1and an ID/IG value of 0.97.The results show that the carbon coating formed by glucose pyrolysis effectively suppressed the volume variability of silicon during charge and discharge.The carbon skeleton formed by the pyrolysis of phytate improves the charge transfer between silicon nanoparticles,and the carbon skeleton structure combined with the carbon coating has a dual inhibitory effect on the volume expansion of silicon,which further enhances the electron conduction and improves the Li⁺diffusion coefficient,and the addition of the dual carbon sources exhibits excellent synergy in terms of structure and performance.When used as an anode material for Li-ion batteries,this unique silica-based composite exhibits an initial reversible capacity of 1612m Ah g^-1at a current density of 0.1 A g^-1,maintains a capacity of 600 m Ah g^-1after 200 cycles,and a capacity of 503 m Ah g^-1at 5 A g^-1In addition,a full cell assembled from the prepared silicon-carbon anode material and a commercial lithium cobaltate cathode material exhibited an excellent capacity(135 m Ah g^-1)at a current density of 0.2 C.2.Li Co O₂,as the earliest commercially available cathode material for lithium-ion batteries,is still highly competitive in today’s lithium-ion battery cathode market.However,the upper charge as of voltage for the current commercial Li Co O₂cathode material is 4.3 V（graphite as the negative electrode）,resulting in its actual capacity being much lower than its theoretical capacity.Increasing the operating voltage will result in an irreversible hexagonal-monoclinic-hexagonal phase transition of Li Co O₂,leading to severe structural changes,generating serious surface side reactions and intensifying the formation of SEI films,resulting in an extremely rapid decrease in electrode performance.In order to improve these problems,a Co₃O₄-coated Li Co O₂modified material was prepared using cobalt nitrate and citric acid by sol-gel method.The modified Li Co O₂cathode material exhibits an initial reversible capacity of 197 m Ah g^-1at 1C and maintains a capacity of 100 m Ah g^-1after 200cycles.In addition,full batteries assembled with the modified Li Co O₂cathode material and commercial graphite cathode material have excellent capacity(139 m Ah g^-1)at 1 C.