Preparation And Modification Of TiNb₂O₇ Anode Materials For Lithium Ion Batteries

The development of lithium storage electrode materials with high capacity,high rate,and high stability is required to fulfill the pressing requirement for fast-charging lithium-ion batteries with high specific energy,high safety,and long life.TiNb₂O₇（TNO）is a promising anode material for lithium-ion batteries due to its high theoretical specific capacity(387.6 mA h g^-1),stable lattice structure during charging and discharging,and lack of lithium dendrite development during fast charging.Unfortunately,the development of TNO’s practical applications is severely constrained by its poor electronic and ionic conductivity.TNO materials were synthesized using a high-temperature solid-phase approach,followed by TiNb₂O₇@C（TNO@C）,TiNb₂O₇@MXene（TNO@MXene）,and Cr_0.02Ti_0.96Nb_2.02O₇(Cr_0.02-TNO),three high-performance anode materials for Li-ion batteries.The primary studies are as follows:（1）The TNO materials were prepared by a simple high-temperature solidphase method and modified by carbon coating with polydopamine as the carbon source.The TNO material was prepared by calcination at 1200 °C for 20 h under nitrogen atmosphere with a smooth surface and a uniform particle size of 1-2μm.The capacity retention after 100 cycles was 98.0%.The TNO@C material was generated by high temperature carbonisation and was further coated on the surface of TNO particles using the self-polymerisation property of dopamine.The carbon layer on the surface of the resulting TNO@C particles with a carbon content of 5.2% was uniformly thick（50 nm）,and the carbon layer efficiently promoted rapid electron transport,boosting the electrical conductivity of the TNO material from 8.91 × 10-12 S cm^-1 to 3.16 × 10-6 S cm^-1.The reversible specific capacities are 294.6 and 177.4 mA h g^-1 at 0.1,10 C.The reversible specific capacity has a capacity retention of 90.9% after 400 cycles,showing good rate performance and cycling stability.（2）The TNO@MXene composite was constructed by electrostatic selfassembly of MXene nanosheets with high electrical conductivity and TNO particles.The MXene nanosheets with metal-level electrical conductivity encapsulate the TNO particles to form a three-dimensional conductive network and enable the transfer of electrons from the MXene nanosheets to the TNO particles through the TNO@MXene interface,which effectively promotes the rapid migration of electrons and ions,resulting in a high electronic conductivity(1.78 × 10^-4 S cm^-1)and lithium ion diffusion coefficient of the TNO@MXene composite.As a result,the TNO@MXene composite has faster electrochemical reaction kinetics compared to TNO,exhibiting reversible specific capacities of up to 198.0 and 166.0 mA h g^-1 at 10 C and 20 C,and a capacity retention of92.3% after 500 cycling at 10 C.（3）The Cr-doped single-crystal structure Cr_xTi_1-2x Nb_2+x O₇ material was prepared by high-temperature solid-phase method,which has fewer grain boundaries in the single-crystal structure and can effectively improve the Li⁺transport rate,the Cr doping increases the TNO cell volume,which provides a wider diffusion channel for Li⁺ transport,decreasing the Li⁺ migration energy barrier from 0.97 eV to 0.63 eV and reduces the band gap of TNO from 1.92 eV to 0.75 eV,which facilitates the rapid conduction of Li-ions and electrons.The Cr_0.02-TNO with 2 at% Cr doping has a reversible specific capacity of 126.6 mA h g^-1 at 20 C and capacity retention of 97.0% after 500 cycles at 50 C.The reversible specific capacity of Cr_0.02-TNO at-40 °C is 188.3 mA h g^-1 at 1 C,which may reach 74.7% of the reversible specific capacity at room temperature.The Li Co O₂||Cr_0.02-TNO full cell made by matching with Li Co O₂ exhibits reversible specific capacities of 238.3,227.0,and 154.9 mA h g^-1 at 0.1,1,and10 C,with a capacity retention of 81.3% after 300 cycles at 1 C,confirming its promising practical application possibilities.