Design Of Carbon Dots-based Composite Electrolytes And Their Application In Solid-state Lithium Batteries

The development of lithium-ion battery systems based on liquid electrolytes is limited by both the difficulty of energy density enhancement and high safety hazards.The development of solid-state lithium batteries is one of the important directions for the future,which can achieve high energy density and safety.Polymer electrolytes have attracted much attention due to their good interfacial contact,compatibility with large-scale roll-to-roll manufacturing processes,and good flexibility.However,issues such as slow ion transport and interfacial instability are the key points that need to be improved.The construction of composite electrolytes is a common strategy,and the choice of filler has a great impact on the performance of the electrolytes.Carbon dots,as a new type of carbon-based materials,have the advantages of small size,tunable compatibility,and abundant surface functional groups,which have great potential in regulating the performance of solid electrolytes.Based on the above considerations,a variety of carbon dots with different structures and surface states were designed and synthesized in this thesis to control the ionic dynamics of the electrolyte and the stability of the electrode/electrolyte interface,and a high-performance carbon dot-based composite electrolyte was successfully constructed.Through the functional design of carbon dots,the internal relationship between carbon dot material design and electrolyte performance is analyzed.The main research contents of this thesis are as follows:（1）Aiming at the issues of low ion dynamics in polymer electrolytes,three different carbon dots（undoped carbon dots（W-CD）,nitrogen-doped carbon dots（N-CD）,nitrogen-sulfur co-doped carbon dots（NS-CD））were synthesized for construction of composite electrolytes with fast ion transport.The effects of three carbon dots on the performance of composite electrolytes were systematically studied.Due to the enhanced polymer chain ion transport capacity and Li ion freedom,all the carbon dots-based composite electrolytes achieve improved ionic conductivity.Among them,the edge nitrogen/sulfur at the surface of NS-CD has a strong interaction with Li⁺,which can weaken the coordination between Li⁺and PEO chains and release the maximum degree of freedom.Therefore,the ionic conductivity of the NS-CD based composite electrolyte reaches 2.10×10^-4S cm^-1（room temperature）,which is higher than that of the W-CD system(8.60×10^-5 S cm^-1)and the N-CD system electrolyte(9.95×10^-5 S cm^-1).Furthermore,the stability of the lithium/electrolyte interface is enhanced due to the enhanced mechanical properties,high lithium ion migration number,and fast ion transport in the composite electrolyte.The PEO/NS-CD-based lithium symmetric battery remains stable cycling for over 1200hours at a current density of 0.5 m A cm^-2.When using Li Fe PO₄ or Li Ni_0.5Co_0.2Mn_0.3O₂ as the cathode,the corresponding solid-state lithium battery exhibits stable long cycle life and excellent rate performance.（2）Aiming at the issues of low ionic conductivity and poor selective Li⁺transport in polymer electrolytes,lithiated carbon dots（CD-Li）were designed and synthesized as lithium ion conductors.The introduction of CD-Li effectively enhanced the ion transport of the electrolyte.On the one hand,by destroying the crystallinity of the polymer chain,it improves the mobility of the segment,thereby accelerating the ion transport.On the other hand,as a lithium ion conductor,CD-Li has a large anion volume and is difficult to move,which provides more free lithium ions to the system and achieves a higher lithium ion migration number.At the same time,the heteroatoms on the surface of the carbon dots will form a fast ion transport interfacial layer containing Li F on the Li metal surface.The composite electrolyte-based Li Fe PO₄/Li battery can operate stably for 300 cycles（50°C）,providing a reversible capacity of 137.6 m Ah g^-1 at a current density of 0.5 C(1 C=170 m A g^-1),with capacity retention of 95.88%.（3）In order to further construct the Li F-rich fast ion transport interface layer and regulate the electrolyte ion transport,high fluorine-doped carbon dots F-CD were designed and synthesized.By changing the precursor of the aldol condensation reaction and using p-fluorobenzaldehyde as the fluorine-doped precursor,high-fluorine-doped carbon dots（the atomic ratio of fluorine reached 8.16%）was successfully synthesized.In the composite electrolyte system,F-CD can destroy the crystallinity of PEO chains and generate more amorphous regions,which can effectively promote the transport of lithium ions.At the same time,F-CD can interact with perchlorate to improve the selective transport of lithium ions in the system.Moreover,the F atoms will react with the strongly reducing Li metal to in-situ generate a Li F-rich interface layer.The fast transport of ions in the composite electrolyte and stable interfacial properties enable stable cycling of solid-state lithium batteries.