Studies of Synthesis Temperature Effects on Cubic Lithated-Spinel to Layered Phase Transformation and Electrochemical Performance of LiMn0.5Ni0.5O2 Cathode Materials
Authors: Shi Boyu , Gim Jihyeon , Li Linze , Wang Chongmin , Vu Anh , Croy Jason R. , Thackeray Michael , Lee Eungje 
Published: November 2022

Recently, our group has reported a novel Li-ion battery cathode, ‘LT-LiMn0.5Ni0.5O2’, where LT refers to its relatively low synthesis temperature (400 °C). By collecting high energy X-ray diffraction and high-angle, annular-dark-field (HAADF) scanning-transmission electron microscopy (STEM) data, we confirm that the LT-LiMn0.5Ni0.5O2 features a unique partially disordered rock salt structure with predominant lithiated-spinel-like character. The electrochemical data indicates that the Li/LT-LiMn0.5Ni0.5O2 cells shows good cycle stability when operate between 2.5-5.0V. The 1st cycle specific discharge capacity was determined to be 225mAh/g. Two distinct voltage plateaus can be identified at approximately at ~4.6 V and ~3.5 V during discharge. The ~1 voltage separation of the plateaus is caused by the octahedral and tetrahedral site energy difference during lithium extraction and insertion.1 The discovery of LT-LiMn0.5Ni0.5O2 not only expand the compositional space for the known lithated-spinel family but also provide a practical way to increase the operational capacity of traditional spinels such like LiMn2O4 and LiMn1.5Ni0.5O2.2-5

It is well known that when synthesized at 900°C, LiMn0.5Ni0.5O2 features a layered structure.6 The existence of lithated-spinel like LiMn0.5Ni0.5O2 at low temperature (400°C) immediately raise another question: How does the synthesis temperature affect the structure and electrochemical performance of LiMn0.5Ni0.5O2? We synthesized a series of different temperature of LiMn0.5Ni0.5O2 powders. By collecting high energy X-ray diffraction and high-resolution transmission electron microscopy (HR-TEM) data, we confirm that, the LiMn0.5Ni0.5O2 exhibits a lithated spinel → disordered layered → ordered layered structure transformation as temperature increase from 400-900 °C. The Rietveld refinements results also reveals a gradual decrease of the Li/Ni exchange ratio accompanied by the synthesis temperature changes. The evolution of specific discharge capacity for different temperature LiMn0.5Ni0.5O2 shows a parabola-like curve behavior with the lowest discharge capacity at 700 °C. This unique behavior is likely to be related to the diffusion channels reconstruction along with the structure changes. 3D diffusion channels are incorporated with 2D diffusion channels at low temperature (400°C) for lithated-spinel LiMn0.5Ni0.5O2. Within the middle temperature range (500°C-700°C), the 3D channels destroy rapidly while 2D diffusion channels are not well established due to the highly disorder ratio resulting in a decrease of capacity. At higher temperature (800-900°C), the 2D diffusion channels are more well established enabling an increase of the capacity.

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