Synthesis And Lithium-ion Storage Performance Of Titanium-based Oxides Anchored On Multi-dimensional Carbon-based Arrays

The titanium-based oxides have low volume expansion during insertion/extraction process of lithium ion and the high voltage platform suppresses the formation of lithium dendrite and solid electrolyte interface（SEI）film.Therefore,they exhibit excellent cycling stability and safety performance at large currents,and is suitable for high-rate lithium-ion batteries.However,titanium-based oxides(Li₄Ti₅O₁₂（LTO）and Ti₂Nb₁₀O₂₉（TNO）)suffer low electron/ion conductivity that affects the high rate performance of electrodes.In our research,a series of multi-dimensional carbon arrays are designed as conductive skeleton to offer fast electron transfer channels and stable structure;atomic layer deposition（ALD）,hydrothermal and solvothermal methods are used to load nanoscale oxides to shorten the ion/electron transport paths;ion-doping is carried out to improve the intrinsic electronic conductivity and ion diffusion efficiency of titanium-based oxides.The synergistic effects of various modification methods have effectively improved the electrochemical performance of titanium-based oxides.Meanwhile,we explore the mechanism of the enhanced electrochemical performance of titanium-based oxides through electron paramagnetic resonance spectroscopy and ultraviolet-visible spectroscopy.In addition,the application potential of Li₄Ti₅O₁₂//LiFePO₄（LTO//LFP）and Ti₂Nb₁₀O₂₉//LiFePO₄（TNO//LFP）full cells is also studied in our research.The main works and results can be summarized as follows:（1）Carbon nanotubes（CNTs）/LTO core/shell arrays on carbon cloth as integrated high-quality anode are constructed via a facile combined chemical vapor deposition（CVD）-atomic layer deposition（ALD）method.ALD-synthesized LTO is strongly anchored on the CNTs skeleton forming core/shell structure with diameters of 70-80 nm.Combined advantages including highly conductive network,large surface area and strong adhesion are obtained in the CNTs/LTO core/shell arrays.The electrochemical performance of the CNTs/LTO electrode is completely studied as anode of LIBs,which shows noticeable high-rate capability(a capacity of 102mAh g^-1 at 30 C),and a stable cycle life with a capacity retention of 86%after 5000cycles at 10 C.Meanwhile,LTO//LFP full cells deliver a capacity of 91 mAh g^-1 at20 C and could maintain 87%capacity retention after 1500 cycles at 10 C.（2）Hybrid vertical graphene（VG）/lithium titanate-CNTs arrays on carbon cloth substrate are fabricated via CVD in combination with ALD-chemical lithiated.A novel array architecture is formed where active lithium titanate is uniformly sandwiched by vertical graphene backbone and interconnected CNTs shell.The dual role of VG and CNTs provides an omnibearing porous conductive network for LTO,which enables the electrode to exhibit excellent ultra-high rate performance(a capacity of 131 mAh g^-1 at 100 C)and ultra-stable cycling life up to 10,000 cycles at a high rate of 20 C.（3）Hydrogenated Li₄Ti₅O₁₂（H-LTO）nanoparticles are combined with N-doped carbon fibers（NCFs）to form integrated H-LTO@NCFs arrays.The NCFs are synthesized by the carbonization of polypyrrole wires and used as the conductive support to grow LTO nanoparticles via the combination of ALD and chemical lithiation.Binder-free conductive NCFs skeletons are used as strong support for H-LTO,in which Ti³⁺is self-doped along with oxygen vacancies in LTO lattice to realize enhanced intrinsic conductivity.Positive advantages including large surface area,boosted conductivity and structural stability are obtained in the designed H-LTO@NCFs electrode,which is demonstrated with preeminent high-rate capability(128 mAh g^-1 at 50 C)and long cycling life up to 10000 cycles.In addition,the LTO//LFP full batteries can maintain a capacity of 100 mAh g^-1 after2000 cycles at 10 C,and the capacity retention is 85%.（4）NH₃ thermal treated N-doped Li₄Ti₅O₁₂（N-LTO）is combined with highly conductive TiC/C skeleton on Ti6Al4V mesh to realize enhanced ultra-fast Li ion storage.Interlinked hydrothermal method-synthesized N-LTO nanosheets are homogeneously decorated on the CVD-derived TiC/C nanowires forming binder-free N-LTO@TiC/C core-branch arrays.Positive advantages including large surface area,strong mechanical stability,and enhanced electronic/ionic conductivity are obtained in the designed integrated arrays and rooted upon synergistic TiC/C matrix and N doping.The above appealing features can effectively boost kinetic properties throughout the N-LTO@TiC/C electrodes to realize outstanding high-rate capability at different working temperatures(143 mAh g^-1/10 C at 25°C and 122 mAh g^-1/50C at 50°C)and notable cycling stability with a capacity retention of 99.3%after10000 cycles at 10 C.Moreover,the LTO//LFP full batteries can still exhibit 124mAh g^-1 at a high rate of 20 C.（5）Omnibearing conductive networks composed of CVD-derived TiC/C arrays core and N-doped carbon（NC）shell on Ti6Al4V mesh are constructed to sandwich Ti₂Nb₁₀O₂₉（TNO）nanoparticles forming integrated NC-TNO@TiC/C core/shell arrays.TNO nanoparticles are synthesized via solvothermal method coated with NC shell which is fabricated by the carbonization of polydopamine.Except for good electronic conductivity and high rigidity from TiC/C arrays skeleton,lower energy barrier of Li ion is obtained via the N-doped carbon layer facilitating the ion/electron transport kinetics according to density functional theory（DFT）results.Accordingly,the NC-TNO@TiC/C electrode shows preeminent high-rate capacities(318 mAh g^-1at 1 C and 202 mAh g^-1 at 50 C)and a long cycle life with a capacity retention of 85%after 10000 cycles at 10 C.In addition,the TNO//LFP full batteries can still exhibit141 mAh g^-1 at a high rate of 10 C.（6）Integrated N-doped TNO@TiC/C-NC hierarchical electrodes are constructed on Ti6Al4V substrate,in which novel designed TiC/C-NC core-branch skeleton is used to load N-doped TNO（N-TNO）nanoparticles with symbiotic oxygen vacancy.The solvothermal-derived TNO is doped by the decomposition of melamine and the CVD-derived TiC/C nanowires are coated with hollow carbon sheet by combining electrodeposition and CVD to form TiC/C-NC.The conductive hierarchical TiC/C-C core-branch skeleton can not only enhance the electronic conductivity of the whole electrode,but also provide more active loading sites and ensure the structure stability.Moreover,the oxygen defect and N-doping in TNO could narrow the band-gap and enlarge the lattice,both improving the intrinsic electronic and ionic conductivity.The above internal&external synergistic strategies work together to accelerate reaction kinetics of N-TNO@TiC/C-C to realize a long cycle life（up to 10000 cycles）and preeminent high-rate capability both in liquid batteries(165 mAh g^-1 at 100 C)and solid-state batteries（108 mAh g^-11 at 40 C）.Furthermore,the TNO//LFP full batteries have also confirmed their excellent high-rate performance(a high specific capacity of 118 mAh g^-1 at a high rate of 50 C).