© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 www.advmat.de www.MaterialsViews.com wileyonlinelibrary.com COMMUNICATION Formation of ZnMn 2 O 4 Ball-in-Ball Hollow Microspheres as a High-Performance Anode for Lithium-Ion Batteries Genqiang Zhang, Le Yu, Hao Bin Wu, Harry E. Hoster, and Xiong Wen (David) Lou* Dr. G. Q. Zhang, Prof. H. E. Hoster, Prof. X. W. Lou TUM-CREATE Centre for Electromobility 62 Nanyang Drive, Singapore, 637459 Webpage: http://www.ntu.edu.sg/home/xwlou E-mail: xwlou@ntu.edu.sg Dr. G. Q. Zhang, L. Yu, H. B. Wu, Prof. X. W. Lou School of Chemical and Biomedical Engineering Nanyang Technological University 70 Nanyang Drive, Singapore 637457 DOI: 10.1002/adma.201201779 Hollow micro-/nanostructures with controllable size, shape, composition and interior architecture are attracting fast growing interests due to their intriguing properties and wide- spread applications in various areas including catalysis, drug delivery, gas sensor, energy conversion and storage systems, and many others. [1–8] Numerous efforts have been devoted to develop methods for rational synthesis of various hollow struc- tures with great progress achieved. With that, researchers are now more focused on design and synthesis of complex hollow structures, such as multi-shelled, yolk-shell type and hollow structures with controlled interior functionalization as these structures are shown to exhibit enhanced performance in var- ious applications. [9–14] Till now, most complex hollow structures are synthesized through templating strategies using different colloidal templates, which are generally quite tedious. [10,15,16] For example, Wang and coworkers successfully developed a general route for the synthesis of several oxide multi-shelled hollow microspheres using carbonaceous microspheres as the sacrificial templates, which exhibit enhanced gas sensing prop- erties compared with simple hollow microspheres. [10] Lou and coworkers have previously synthesized double-shelled SnO 2 hollow spheres using silica spheres as templates combining with designed procedures. [17,18] Despite these great successes, it is generally observed that the current methods are mostly based on hard templates that usually require multiple steps and post- treatment. As a result, these methods might not be very suitable for cost-effective large-scale production. Another observation is that it remains as a great challenge to develop template-free methodologies that enable facile synthesis of complex hollow structures, especially for multi-component compounds. Searching for new electrode materials is one urgent task in building next-generation high-performance lithium-ion bat- teries (LIBs) in order to fulfill the ever-growing demand in various consumer electronic devices. Transition metal oxides (TMOs) have been widely investigated as high-capacity anodes for LIBs in view of their high theoretical capacities. [19] Various TMO hollow structures have also been fabricated with var- ious methods in order to improve the capacity retention. For example, Lou and coworkers synthesized hierarchical α-Fe 2 O 3 hollow spheres with high capacity and stable cycling ability through a quasi-emulsion mechanism using a simple hydro- thermal method. [20] Besides these simple binary TMOs, ternary ZnMn 2 O 4 has been proposed as one interesting material in view of its low price, abundance and environmental friendliness, and more importantly, the possible synergetic enhancement of the components in such mixed metal oxides. [21–24] However, it is far more challenging to synthesize complex hollow structures of mixed metal oxides. [21–24] Herein, we report a facile and scalable polyol method combined with a simple post-annealing process to synthesize a novel ball-in-ball hollow structure of ternary ZnMn 2 O 4 with high capacity, enhanced cycling stability and rate performance as negative electrode materials for LIBs. The novel ZnMn 2 O 4 ball-in-ball hollow structure is formed via a thermally driven contraction process. Schematically illus- trated in Figure 1, the formation process mainly consists of two steps. When a certain amount of polyvinyl pyrrolidone (PVP, M w ∼ 58000) is dissolved in ethylene glycol (EG), the micro-emul- sion might be formed and acts as the soft template, which has also been demonstrated in previous literature. [13] After that, the precursors, Zn(CH 3 COO) 2 ·2H 2 O and Mn(CH 3 COO) 2 ·4H 2 O, are dissolved into the PVP-EG solution. Due to the strong coordination ability of PVP to metal ions through the–N and/ or C =O functional groups, the assembled molecular structure shown as (I) of Figure 1 will form. The solution is then refluxed at 170 °C for 90 min. During the reaction, the solution gradu- ally turns into cloudy with a light brown color, which indicates the formation of ZnMn-glycolate hollow spheres shown as (II) in Figure 1. The ball-in-ball hollow structure is formed after the post annealing treatment, which could be described as a ther- mally driven contraction process. More specifically, the ZnMn- glycolate hollow spheres contain a large fraction of organic spe- cies including for example PVP, CH 3 COO - and partially polym- erized EG. Indeed, the thermogravimetric analysis (TGA) (see Supporting Information, Figure S1) indicates a large weight loss of around 44.2%. This means that the ZnMn-glycolates are uniformly dispersed in a carbon-rich matrix. During the heat treatment at high temperature, there will be a large contraction force (F c ) induced by the oxidative degradation of the organic molecules. [10,12] The contraction effect can be demonstrated by annealing the sample in pure nitrogen at 500 °C for 4 h (see Supporting Information, Figure S2). As can be seen, the hollow feature of the ZnMn-glycolate microspheres disappears and only solid spheres are obtained due to the large contraction effect during the annealing. Meanwhile, besides the contraction force, there is also an opposite adhesion force (F a ) which could be ascribed to the formation of ZnMn 2 O 4 crystallites and the release of CO 2 gas from the decomposition of organic species. The large contraction force together with the adhesion force will Adv. Mater. 2012, DOI: 10.1002/adma.201201779