Recent Progress on Spray Pyrolysis for High Performance Electrode Materials in Lithium and Sodium Rechargeable Batteries

Since the introduction to market by Sony in the early 1990s,[1] lithium-ion batteries (LIBs) have become dominate power sources for consumer electronics and have been recognized as a critical enabling technology to make electric vehicles (EVs)/ hybrid electric vehicles (HEVs) a success.[2] They are also being used to store carbon-free and renewable energy sources, such as solar, wind, and ocean (wave, currents, and tides), because these renewable energy sources are seasonal and intermittent. Compared with other battery systems, such as lead-acid, Ni-Cd, and Ni-MH batteries, LIBs present much higher energy densities (both gravimetric and volumetric), and also feature no memory effects or slow self-discharge, which make them popular in many applications.[3] The 1st generation commercial LIBs utilized LiCoO2 and graphite for the cathode and anode, respectively. The energy density of LIBs is mainly limited by the capacities of electrode materials. To further increase the energy density of LIBs to meet the requirements of EVs and other high energy applications, considerable effort has been made to seek cathode and anode materials with higher capacities over the past decades.[4–6] Several candidates have demonstrated significant advantages over LiCoO2 cathode and graphite anode used in the 1st generation commercial LIBs.[7–9] The electrochemical performance of a LIB is mainly determined by properties of its anode and cathode. These properties can be roughly classified into: intrinsic material properties (such as theoretical capacity, ion conductivities, volume changes upon charge-discharge, and voltage with respect to lithium metal) and extrinsic structure properties (for example active materials’ particle size, morphology, porosity, and spatial distribution). While it is relatively difficult to tune the intrinsic natures of electrode materials,[10] appropriate synthetic methods offer the possibility to manipulate the electrode microstructure/ architecture to improve electrochemical performance. There are numerous examples demonstrating how the performance is highly dependent on electrode structure.[11–14] Advanced electrode materials have been intensively explored for next-generation lithium-ion batteries (LIBs), and great progresses have been achieved for many potential candidates at the lab-scale. To realize the commercialization of these materials, industrially-viable synthetic approaches are urgently needed. Spray pyrolysis (SP), which is highly scalable and compatible with on-line continuous production processes, offers great fidelity in synthesis of electrode materials with complex architectures and chemistries. In this review, motivated by the rapid advancement of the given technology in the battery area, we have summarized the recent progress on SP for preparing a great variety of anode and cathode materials of LIBs with emphasis on their unique structures generated by SP and how the structures enhanced the electrochemical performance of various electrode materials. Considering the emerging popularity of sodium-ion batteries (SIBs), recent electrode materials for SIBs produced by SP will also be discussed. Finally, the powerfulness and limitation along with future research efforts of SP on preparing electrode materials are concisely provided. Given current worldwide interests on LIBs and SIBs, we hope this review will greatly stimulate the collaborative efforts among different communities to optimize existing approaches and to develop innovative processes for preparing electrode materials.