rXXXX American Chemical Society 1874 dx.doi.org/10.1021/jz2008209 | J. Phys. Chem. Lett. 2011, 2, 1874–1878 LETTER pubs.acs.org/JPCL In Situ Hydrothermal Synthesis of LiFePO 4 Studied by Synchrotron X-ray Diffraction Jiajun Chen,* ,† Jianming Bai, ‡ Haiyan Chen, § and Jason Graetz † † Sustainable Energy Technologies Department, Brookhaven National Laboratory, Upton, New York 11973, United States ‡ Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States § Physics Department, New Jersey Institute of Technology, Newark, New Jersey 07102, United States b S Supporting Information C oncerns over global climate change and energy indepen- dence have motivated an interest in improved battery materials, driven by a change toward alternative forms of trans- portation, such as plug-in hybrid electric vehicles (PHEVs) and all electric vehicles (EVs), along with renewable (often inter- mittent) sources of energy (e.g., solar and wind). 1,2 Present-day Li-ion battery technology offers promise for meeting the elec- trical energy storage demands for both mobile and stationary applications. A key hurdle for the widespread commercialization of many candidate electrode materials lies in developing an eco- nomical manufacturing process. A low temperature, soft chem- istry route, such as hydrothermal, is efficient, inexpensive, and sufficiently flexible so that the material’s properties (e.g., cation distribution, particle size, and morphology) can be generally tailored by the synthesis conditions. However, little is known about the overall reaction pathways, the formation of intermedi- ate phases, and exactly how the material properties are affected by the synthesis conditions. Thus, optimizing the hydrothermal pro- cedure rests upon a tedious, time-consuming Edisonian process. Conventional hydrothermal synthesis is carried out in a sealed reactor; hence, phase identification can only take place after the reactor has cooled to room temperature and the contents are removed. Furthermore, the product must be recovered from solution, washed to remove any surface impurities, and filtered; thereafter, the filtrate cake must be dried for several hours in an oven. These ex situ experiments are not only tedious, but also make it difficult to identify the real composition of any inter- mediates that form during the reaction. Recent advances in transmission electron microscopy (TEM) allow some in situ characterization of the electrode materials formed via solid-state synthesis 3 or simplified electrochemical cycling. 4 However, electron microscopy-based techniques are best suited for investigations of very localized regions of a sample, and issues with beam damage and the high vacuum requirements make it difficult to use these techniques to study wet-chemistry processes. X-ray diffraction (XRD) has proven to be a powerful technique for acquiring a fundamental understanding of struc- ture and phase transformations. The high energy-flux of synchro- tron radiation offers opportunities for studying changes in the structure and properties of complex systems in real time, e.g., in a pressurized hydrothermal environment. Due to the complexity of in situ hydrothermal configurations, only a few wet-chemistry synchrotron studies have been performed, and most have focused on phase transformations. However, the low signal-to-noise ratio and poor resolution in these studies made it difficult to obtain detailed crystallographic information. 5 Hydrothermally prepared LiFePO 4 is typically plagued by a high concentration (5À7%) of antisite defects (e.g., Fe on Li sites). Received: June 17, 2011 Accepted: July 12, 2011 ABSTRACT: The development of high capacity, safe lithium battery materials requires new tools to better understand how reaction conditions affect nucleation and crystal- lization, particle size, morphology, and defects. We present a general approach for studying the synthesis of Li battery electrode materials in real time. The formation of LiFePO 4 was investigated by time-resolved in situ synchrotron X-ray diffraction under hydrothermal conditions, and the reaction kinetics were determined by changes of the Bragg reflections. We provide the first evidence in support of a dissolutionÀreprecipitation process for the formation of LiFePO 4 , which occurs at temperatures as low as 105 °C and appears to be a three-dimensional diffusion-controlled process. Lattice parameters and their evolution were monitored in situ, as well as the formation of antisite defects and their subsequent elimination under various synthesis conditions. The ability to characterize and tailor synthesis reactions in situ is essential for rapid optimization of the synthesis procedures and, ultimately, the development of new battery electrodes. SECTION: Energy Conversion and Storage