(Invited) Deciphering the Interplay between Thermodynamics and Kinetics during Cathode Calcination

Synthesis of the cathode materials used in lithium ion batteries involve the two steps, coprecipitation and calcination, where in the first step cathode precursors are formed, and later in the second step lithium ions are inserted into these cathode particles through chemical routes. During calcination, the cathode precursors obtained from the coprecipitation is mixed with lithium salt (LiOH or Li 2 CO 3 ) and heated at high temperatures (~ 650°C – 1000°C) where oxidation and lithiation of the cathode particles take place. Calcination is considered to be of major significance in determining the final state of the cathode particles because it not only determines the extent of oxidation and lithiation experienced by the cathodes, but also substantially influences its morphology (for example, primary particle size, internal porosity, etc.). Significant surface modification of the cathode particles is also possible during calcination, which can impact its charge transfer resistance. Before the oxidation and lithiation of the cathode precursors, removal of water or CO 2 is observed. All these chemical changes within the cathode precursors lead to change in its lattice volume, which is reflected as variation in the size of the primary particles. Apart from this, sintering induced change in particle size is also observed during the calcination process. In order to decipher the structural and morphological variations that occur during the calcination of cathode precursors, detailed experimental characterization is conducted in the present context using in situ X-ray diffraction (XRD) and thermogravimetric analysis (TGA) techniques. Atomistic simulations are conducted to decipher the feasibility of different chemical reactions that can possibly occur under elevated temperatures. Impact of reaction rate constant and mass transport in determining the extent of the oxidation and lithiation experienced by the cathode particles is analyzed by using a mesoscale level computational technique. Sintering induced grain growth experienced by the cathode primary particles is also investigated at the mesoscale level, which is demonstrated in Figure 1. Overall, a thorough understanding of the various physicochemical phenomena that occur during the calcination of cathode precursors has been developed using a combination of experimental and computational techniques, which will be discussed in detail. Figure 1