Structural Design Of Black Titania-based Anode Materials And Their Improved Electrochemical Performance

Lithium-ion batteries,as a green energy source,have become an indispensable part of nowadays daily life.However,to meet the needs of new markets such as electric vehicles and smart grids,new generations of lithium batteries are required with increased energy and power density,improved safety.The electrochemical performance of lithium-ion batteries is highly dependent on the electrode material,so the design and modification of the electrode material are particularly important.The most commonly used anode material for commercial lithium-ion batteries is graphite,which has the merits of abundant resources and low price,but the rate performance is difficult to improve.In addition,due to its low voltage window,it can cause the growth of lithium dendrites,which leads to safety issues.TiO2 is effective in preventing lithium deposition due to its higher oxidation-reduction potential and is expected to be a safer anode material.Additionally,TiO2 exhibits excellent structural stability during lithium ion intercalation-deintercalation reaction with low volume expansion(4%),which enables TiO2 to be functionally combined with alloying type or conversion reaction type anode materials to increase specific capacity.However,the poor electronic conductivity(10-9 S cm-1)and Li+ diffusion coefficient(10-11-10-12 cm2 s-1)of titania restrict the rate performance and hinder its practical application in lithium-ion batteries.The relatively low theoretical specific capacity of TiO2(340 mA h g-1)is still unsatisfying to fulfill the increasing requirement of the energy density of lithium-ion batteries.To address aforementioned challenges,in this dissertation,surface modification(reduction,pre-lithiation)was introduced to improve the intrinsic electron conductivity and lithium ion diffusion coefficient of black titania,and further increased its specific capacity through functional composite modification(SnO2,NiO).The main research contents and results are as follows:(1)The controllable magnesium reduction method was first proposed to prepare black titania,and the intrinsic electronic conductivity of black titania was improved via reduction,resulting in enhanced cycling stability of lithium-ion batternes.A systematic study of black titania prepared by metal-reduction methods was provided and found out the best controllable magnesium reduction method.In addition,two commonly used TiO2 phases,i.e.anatase and rutile,were prepared to explore the effect of crystal phase during the reduction process.It was found that nano rutile phase is preferentially reduced over anatase under the same reduction conditions.The color,absorption properties,concentrations of Ti3+,and electrical conductivities of the reduced samples can be easily tuned by the dosage of Mg metals,wherein the anatase phase sample(A-3)showed the highest conductivity(236.3(μS cm-1).When used as anode material,A-3 delivered a high reversible capacity of 180 mA h g-1 at 1C even after 100 cycles.(2)Rationally designed and created a fast ion-conducting surface layer of mesoporous titania,addressing the limited lithium diffusion across the interface between the electrolyte and the active electrode materials,and thus achieved superior high-rate performance.Via a facile molten-salt-assisted lithiation process,an amorphous lithiated surface layer was created and a mesoporous structure was formed simultaneously.The obtained lithiated surface layer with open structure resulted in higher Li+ ion migration with lower activation energy.Optimization between the electron conductivity and ion conduction was achieved by tuning the reaction temperature.The optimized Li-TiO2-x-300 sample showed high electrical conductivity(198.2 pS cm-1),high surface area(259 m2 g-1)with mesoporous structure,and high lithium diffusion coefficient.When used as anode material,Li-TiO2-x-300 delivered remarkable reversible capacities as high as 114 mA h g-1,93 mA h g-1 at high rates of 50 C and 100 C even after 1000 cycles.Further kinetic analysis found that the contribution of pseudocapacitance to the specific capacity of the electrochemical reaction increased with increasing current density.(3)Based on the concept of functional composite modification,black rutile(Sn,Ti)O2 was prepared,in which SnO2 contributes high specific capacity,TiO2 provides structural support,and the electronic conductivity was further enhanced via reduction,thereby achieving high specific capacity and excellent rate performance.Black(Sn,Ti)O2 with a core-shell structure was prepared through a facile co-precipitation followed by hydrogen plasma reduction,and its electrical conductivity was as high as 35.7 μS cm-1.The conductive surface layer acted as highways for electron transfer to promote an isotropic electrochemical reaction.The rutile solid solution with a homogenous mixing of Sn and Ti helps to form a uniform distribution of Sn nanodots in an amorphous lithiated titania matrix after lithiation,and subsequently maintains a sub 10 nm scale nanostructure even after long-term cycling.The lithiated titania matrix can prevent the aggregation of tin nanodots,accommodate the volume change,and provide a stable conductive network for ion kinetics,which consequently results in excellent lithium-ion battery performance.Ultimately,the black(Sn,Ti)O2 anode harvested significantly enhanced reversible capacity(583 mA h g-1 after 100 cycles at 0.2 A g-1),high rate performance(419 mA h g-1 at 2 A g-1 and 335 mA h g-1 at 5 A g-1)and superior cycling stability.(4)Based on the concept of functional composite modification,NiTiO3(NiO+TiO2)nanorods were prepared,and in-situ grown graphene was further introduced by plasma-enhanced chemical vapor deposition(PECVD)to improve the electronic conductivity and structural stability,resulting in excellent cycling stability and rate performance.A two-step seeded growth procedure was developed to grow micrometer-sized single crystals of nickel titanium glycolate complex,leading to an unambiguous solution of crystal structure and precise refinement,i.e.NiTi(OCH2CH2OH)6.Under the reductive plasma enhanced CVD atmosphere,partially reduced Ni served as self-catalysis substrates for in-situ graphene growth,enabling the perfect encapsulation of NiTiO3 nanorods with few-layer graphene.The electrical conductivity of NiTiO3@Graphene sample was 5.7 mS cm-1,over four orders of magnitude improved over 0.2 μS cm-1 of the bare NiTiO3 sample.The graphene coating also helps to retain the electrical connectivity and suppress the pulverization caused by volume expansion during cycling.The NiTiO3@Graphene anode ultimately harvested significantly enhanced reversible capacity(556 mA h g-1 after 500 cycles at 0.2 A g-1)and superior cycling stability.As a result,a general strategy for in-situ growth of a conductive graphene shell on MTiO3(M=Ni,Co,Fe)materials through a facile PECVD route to obtain stable lithium-ion batteries performance was proposed.