Molten Salt Synthesis of High-Performance Cobalt Free Lithium Excess Cathodes
Authors: Burke Sven Anders , Whitacre Jay 
Published: November 2022

The cobalt free lithium excess chemistry cathode material of the compositional series Li[NixLi(1/3-2x/3)Mn(2/3-x/3)]O2 despite its high capacity has many shortcomings, such as voltage fade, poor materials stability, and poor rate capability 1–10. Our past investigations, among others, have highlighted the importance of the synthetic route on the on the performance of this material 6,11–13. This investigation found a novel synthesis route for Li[NixLi(1/3-2x/3)Mn(2/3-x/3)]O2 (x = 0.25) involving the use of molten salts during the 900 °C synthesis step would reduce the degree of voltage fade seen in materials cycling, and increase the overall stability of the material over the course of cycling. Our samples we prepared using a standard sol-gel approach where the post 500 °C precursor oxide was mixed with NaCl, Li2SO4, or LiNO3, before the final 900 °C calcination, all samples where then rapidly water quenched to stabilize the meta-stable materials phase, but also to dissolve any residual salts and wash the powders before use. These salts were selected due to their different chemistries and molten ranges. NaCl is thermodynamically and chemically stable enough where it likely functioned as an observer species, and Li2SO4 and LiNO3 could have served as additional lithiation sources. Thermogravimetric analysis was used to help determine the behaviors of these salts through range for each salts’ molten regime as well as to help determine any salt behaviors up to 900 °C; compared to the control sample NaCl sample was found to have rapid mass lost starting at 850 °C; Li2SO4 was found to cause less mass loss than the control sample; LiNO3 had rapid mass lost starting at 600 °C when compared to the control sample, suggesting a shorter window of molten-salt synthesis environment. Galvanostatic testing revealed that each molten salt sample had distinct electrochemical behaviors, and many had enhanced voltage and capacity stability. Analysis of the samples surfaces through the use of high-resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy (STEM), energy dispersive spectroscopy (EDS), suggest the formation of doped surface facets, and changes in the preferred facet orientation of samples. These methods also revealed that the molten salt synthesis conditions can lower the diffusion barrier between active material particles encouraging Ostwald ripening, as well as increased batch homogeneity.
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