Solid-phase Synthesis And Electrochemical Performance Of Metal-ion Doped Na₂Li₂Ti₆O₁₄ Anode Material

In view of the advantages of high discharge voltage, power density and environmental friendliness, Na₂Li₂Ti₆O₁₄ has attracted the interest of researchers for its inventiveness. However, low electronic conductivity and poor lithium ion diffusion limit its capacity and rate performance as titanate. Thus, to research and improve rate capability and cycling performance of Na₂Li₂Ti₆O₁₄, the material and its doped products are successfully synthesized by solid state method.Firstly, Na₂Li₂Ti₆O₁₄ and its Ti-site substitution Na₂Li₂Ti_5.9M_0.1O₁₄(M = Al³⁺, Zr⁴⁺, V⁵⁺ are prepared by a solid-state reaction method and used as anode materials for lithium-ion batteries. It is found that metal doping can effectively enhance the electronic conductivity and ionic diffusion coefficient of Na₂Li₂Ti₆O₁₄. Especially for Na₂Li₂Ti_5.9Al_0.1O₁₄, it reveals the highest electronic conductivity（1.02×10-9·S cm-1） and lithium ion diffusion coefficient（8.38×10-15 cm2·s-1） among all the samples. As a result, Na₂Li₂Ti_5.9Al_0.1O₁₄ reveals the best electrochemical performance. It can deliver a charge specific capacity of 270.3 mAh·g-1 at 50 mA·g-1. Even cycled at 1000 mA·g-1, it still can present a charge capacity of 180.7 mAh·g-1. All these enhanced lithium storage capabilities of Na₂Li₂Ti_5.9Al_0.1O₁₄ should be attributed to the increased electronic/ionic conductivities and the decreased charge transfer resistance induced by Al³⁺ doping. Besides, in-situ X-ray diffration observation also confirms that the structural change of Na₂Li₂Ti_5.9Al_0.1O₁₄ is highly reversible process for lithium storage.Secondly, Na₂Li₂Ti₆O₁₄ and its Li-site substitution Na2Li1.9M0.1Ti₆O₁₄(Mn⁺= Na⁺, Mg²⁺, Cr³⁺, Ti⁴⁺, V⁵⁺ samples are synthesized by a simple solid state reaction route and evaluated as anode materials for lithium-ion batteries. Their crystal structures and ion doping behaviors are described and verified by Rietveld refinement. Electrochemical results exhibit that Na⁺, Mg²⁺ and Cr³⁺ dopings can effectively improve the lithium storage capability of Na₂Li₂Ti₆O₁₄. Especially for Na2Li1.9Cr0.1Ti₆O₁₄, it shows the best cycling and rate properties among all the as-prepared samples, with a cycling reversible capacity of 262.2 mAh·g-1 at 100 mA·g-1 and a rate charge capacity of 233.3 mAh·g-1 at 700 mA·g-1. The enhanced electrochemical properties are contributed to the reduced particle size, decreased charge transfer resistance and improved ionic diffusion coefficient of Na₂Li₂Ti₆O₁₄ via Cr³⁺ doping. Furthermore, the zero-strain characteristic should also be responsible for the outstanding lithium storage capability of Na2Li1.9Cr0.1Ti₆O₁₄. Besides, in-situ X-ray diffraction also reveals that Na2Li1.9Cr0.1Ti₆O₁₄ has high structural stability and reversibility during charge-discharge process. Therefore, Na2Li1.9Cr0.1Ti₆O₁₄ may be a probable high performance anode material for lithium-ion batteries.Finally, via Li⁺, Cu²⁺, Y³⁺, Ce⁴⁺ and Nb⁵⁺ dopings, a series of Na-site substitution Na_1.9M_0.1Li₂Ti₆O₁₄ are prepared and evaluated as lithium storage host materials. Structural and electrochemical analyses suggest that Na-site substitution by high-valent metal ions can effectively enhance the ionic and electronic conductivities of Na₂Li₂Ti₆O₁₄. As a result, Cu²⁺, Y³⁺, Ce⁴⁺ and Nb⁵⁺-doped samples reveal better electrochemical performance than bare Na₂Li₂Ti₆O₁₄. Especially for Na1.9Nb0.1Li2Ti₆O₁₄, it can deliver the highest reversible charge capacity of 259.4 mAh g-1 at 100 mA g-1 among all samples. Even cycled at higher rates, Na1.9Nb0.1Li2Ti₆O₁₄ still can maintain excellent lithium storage capability with the reversible charge capacities of 242.9 mAh·g-1 at 700 mA·g-1, 216.4 mAh·g-1 at 900 mA·g-1 and 190.5 mAh·g-1 at 1100 mA·g-1. In addition, ex-situ and in-situ observations demonstrate that the zero-strain characteristic should also be responsible for the outstanding lithium storage capability of Na1.9Nb0.1Li2Ti₆O₁₄. All these evidences indicate that Na1.9Nb0.1Li2Ti₆O₁₄ is a high performance anode material for rechargeable lithium-ion batteries.