The Journal of Physical Chemistry C - ACS Publications - American

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Article pubs.acs.org/JPCC

Sodium Storage and Pseudocapacitive Charge in Textured Li4Ti5O12 Thin Films Pengfei Yu, Chilin Li,* and Xiangxin Guo* State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Ding Xi Road, Shanghai 200050, China S Supporting Information *

ABSTRACT: Phase transformation reactions including alloying or conversion ones have often been utilized recently to improve the capacity performance of Na-ion battery anodes. However, they tend to induce larger volume change and more sluggish Na-ion transport at multiphase solid interfaces than for Li-ion batteries, leading to inefficiency of mixed conductive networks and thus degradation of reversibility, polarization, or rate performance. In this work, we use a structurally stable Li4Ti5O12 spinel thin film as insertion-type model material to investigate its intrinsic Na-ion transport kinetics and coupled pseudocapacitive charging. It is found that the latter effect is remarkably activated by the nanocrystalline microstructure full of defect-rich surface, which can simultaneously promote Na-ion and electron accessibility to the surface/subsurface. It is proposed that the extra pseudocapacitive charge storage is a potential solution to the high-capacity and high-rate insertion anodes without tradeoff of serious phase transformation or structural collapse. Therefore, a highly reversible charge capacity of 225 mAh g−1 (exceeding the theoretical value 175 mAh g−1 based on insertion reaction) at 1C is achievable.

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INTRODUCTION The energy-storage systems beyond lithium ion batteries (LIBs) are attracting increasing attention in view of not only the limited abundance and uneven distribution of lithium resources in the earth’s crust but also the requirement on largescale stationary energy storage and the popularization of electric vehicles.1 These lead to an urgent demand on decreasing the cost, which encourages us to develop alternative technologies to enrich the existing energy-storage market. Room temperature sodium ion batteries (NIBs) are thought to be one of the most promising systems due to the fourth most abundant reserve of sodium in nature and its low cost.2,3 Because the ion radius (1.02 Å) of Na+ is 34% larger than that (0.76 Å) of Li+,4 Na transport and storage are quite sluggish in most of the Li-insertable host structures. Recently open framework strategy has proved to be successful to construct kinetically favorable Na-ion channels with increased size and dimension and is especially suitable for cathodes of NIBs.5−7 However, from the anode aspect, very few materials have been demonstrated to be promising in view of the more serious volume expansion and more sluggish mass transport at multiphase interfaces when reaction with Na.8 During the early stage, nongraphitic carbonaceous materials were investigated as anode candidates because graphite, the most commonly used anode in LIBs, cannot allow Na-ion intercalation into the graphitic interlayers.9 However, their reaction potentials are very close to 0 V versus Na+/Na, likely resulting in sodium plating and thus the safety issue. The © 2014 American Chemical Society

alloying materials (e.g., Sn, Sb, P) appear to present the highest capacity for Na-storage so far but accompany the potential degradation of mixed conductive networks and even pulverization/delamination of electrodes.10−12 The reactivity of conversion anodes (e.g., FeOx) with Na is disappointingly poor when compared with their electrochemistry with Li, leading to large polarization, poor capacity, and rate performance.13 The insertion anodes with flexible but robust frameworks for Na-storage are highly desired to avoid drastic collapse/ rearrangement of phase structures and to improve the cycIeability of NIBs. Ti-based oxides, for example, Nacontained layered Na2Ti3O7, Na0.66[Li0.22Ti0.78]O2, and NaTi3O6(OH)·2H2O, displayed a typical solid-solution or biphasic reaction behavior at low voltages below 1 V.14−16 However, the reversible capacity is very limited (