(Student Battery Slam Best Presentation Award Winner) Investigation of Tunable Properties in Single-Crystal LiNi_0.5Mn_1.5O₄ Cathode Materials

The commercialization of lithium-ion batteries has played a pivotal role in the development of consumer electronics and electric vehicles. In recent years, much research has focused on the development and modification of the active materials of electrodes to obtain higher energies for a broader range of applications. High-voltage spinel materials including LiNi_0.5Mn_1.5O_4-δ (LNMO) have been considered as promising cathode materials to address the increasing demands for improved battery performance due to their high operating potential, high energy density, and stable cycling lifetimes.

Spinel LNMO can adopt two crystallographic structures depending on the synthesis conditions: an ordered P4₃32 structure which contains Ni and Mn on separate octahedral sites and a disordered Fdm structure which contains Ni and Mn in a random arrangement on octahedral sites. The disordered spinel phase requires higher synthesis temperatures than the ordered phase to form and is often associated with the loss of oxygen from the material lattice, as well as the reduction of a small amount of electrochemically inactive Mn⁴⁺ to active Mn³⁺ for overall charge compensation. The differences in structure and Mn oxidation state of LNMO cathode materials can influence the electrochemical performance. Increased structural disorder can increase Li⁺ transport properties through reducing two-phase transformation domains, leading to improved rate performances. Through manipulation of synthetic parameters, the structural and electronic properties of LNMO can be well controlled during formation. The molten salt synthesis method has become one of the most prominent techniques to synthesize single-crystal layered and spinel materials.

In this work, the molten salt synthesis method is used as a technique to tune both the morphology and Mn³⁺ content of high-voltage LNMO cathodes. The resulting materials are thoroughly characterized by a suite of analytical techniques, including synchrotron X-ray core-level spectroscopy, which are sensitive to the materials properties at multiple length scales. The multidimensional characterization allows us to build a materials library according to the molten salt phase diagram as well as to establish the relationship between synthesis, materials properties, and battery performance. Results of this work show that Mn³⁺ content is primarily dependent on the synthesis temperature and increases as temperature is increased. Particle morphology is mostly dependent on the composition of the molten salt flux, which can be tailored to obtain well-defined octahedrons enclosed by (111) facets, plates with predominant (11) facets, irregularly shaped particles, or mixtures of these. The electrochemical measurements indicate that the Mn³⁺ content has a larger contribution to the battery performance of LNMO than morphological characteristics and that a significant amount of Mn³⁺ could become detrimental to the battery performance. However, with similar Mn³⁺ contents, morphology still plays a role in influencing the battery cycle life and rate performance.

We also employ high spatial resolution, synchrotron X-ray nanodiffraction techniques to identify the presence and quantify the percentage of the ordered versus disordered spinel phases at the individual particle level as LNMO single-crystal particles likely contain local regions of both ordered and disordered regimes which standard lab X-ray diffractometers may be unable to distinguish. The relationship between structural ordering and Mn³⁺ content is critical to the understanding of the intrinsic chemomechanical and electrochemical properties of LNMO. The insights of molten salt synthesis parameters on the formation and performance of LNMO, with deconvolution of the roles of Mn³⁺, crystal structure, and morphology, are crucial for continuing studies in the rational design of LNMO cathode materials for high energy Li batteries.