Novel Potassium Titanate As Potential Anodes for Sodium Ion Battery

Abstract

Sodium ion batteries (SIBs) have been intensively investigated as a low cost alternative for lithium ion batteries due to the abundance of sodium ion on the earth and its similar chemical properties to lithium.[1] Taking low cost, wide abundance and high security into consideration, titanium based intercalation materials should be the most promising choice for their low redox potential (Ti3+/Ti4+) relative to the Na+/Na redox potential and abundant reserves among various sodium storage anode materials.[2] In the past few years, researchers have explored many titanium-based materials as potential anode materials for SIBs, such as various TiO2 polymorphs, including anatase, rutile, TiO2-B. Ternary sodium titanates-based materials with related crystal structures also greatly enrich the kinds of anode materials. While, the practical application of Ti-based anodes in SIBs is severely hindered by their inferior rate capabilities and relatively low capacity resulting from both extremely poor electronic conductivity and sluggish sodium ion diffusion. Here, we proposed K2Ti6O13 with an analogue tunnel structure with Na2Ti6O13 as a new anode for sodium ion batteries.[3] And by a soft chemistry method, we can successfully get ultrafine K2Ti6O13 nanowires with a diameter of only 5-10 nm, which grow perpendicularly to the tunnel direction. The ultrafine nanowires greatly shorten the diffusion path length of Na-ion and endow K2Ti6O13 a remarkably improved reactivity as shown in Fig.1. This new titanium based anode material exhibits a high charge capacity of 186 mAh g-1 at 20 mA g-1 and excellent rate performance of 61 mAh g-1 at 1000 mA g-1. The capacity is nearly 3 times that of the bulk phase. This suggests that shortening the diffusion length especially in the diffusion direction by design nanostructure is very important to promote the reactivity of Ti-based material for sodium storage. Furthermore, we introduce Ti3+ into this Ti-based material to enhance the inherent electronic conductivity. We synthesized the hollandite-type KxTiO2 nanorod by a facile carbothermal reduction method and used it as a new sodium ion anode. The as-prepared hollandite-type KxTiO2 shows uniform nanorod of 50 nm with a carbon coating. The carbon can not only partially reduce the Ti (IV) of the precursor to Ti (III), but also limit the particle-size growth to some extent during the heat treatment process. Morever, the carbon coating could also efficiently improve the conductivity of the active materials. This new material can deliver about 135 mAh g-1 at 20 mA g-1 and exhibit great rate capability with 67 mAh g-1 at 1000 mA g-1. It is obvious that these new potassium titanates with the high capacity and excellent rate performance can serve as potential sodium ion anodes.