Recent Advances in Cathode Materials for Li-Ion Batteries
Authors: Aurbach Doron , Markovsky Boris , Erickson Evan Mark , Schipper Florian , Bouzaglo Hana , Srur-Lavi Onit , Sclar Hadar , Ghanty Chandan , Amalraj Francis , Major Dan Thomas , Dixit Mudit , Nayak Prasant , Park Kang-Joon , Lim Byung-Beom , Sun Yang-Kook 
Published: February 2020

In this presentation, we report on studies of novel positive electrode materials for lithium-ion batteries with the main emphasis on their structural and surface modifications by cation doping and coating. We have chosen three families of materials: Li- and Mn-rich high-energy density and high capacity (HE-NCM)(1), Ni-rich layered materials Li[NixCoMn]O2 (x>0.5),(2) and gradient materials of the general formulae of LiNi0.65Co0.08Mn0.27O2
. Al3+ and other cations as dopants(3) and salts such as AlF3 as  coating materials were studied(4). We have demonstrated the impact of a minor level of Al-doping on the electrochemical characteristics of LiNi0.5Co0.2Mn0.3O2 electrodes and on the interfacial reactions.(4) We propose that the lower capacity fading of the Al-doped electrodes upon aging of the cells in a charged state (4.3 V) at 60 0C in comparison with their undoped counterparts, as well as more stable mean voltage behavior, are likely due to the chemical and structural modifications of the electrode/solution interface. The lower electrochemical impedance of Al-doped LiNi0.5Co0.2Mn0.3O2 electrodes can be explained by more stable surface chemistry developed on the doped particles due to the interfacial reactions of the dopant in Al3+ enriched surface layer (“segregated” aluminum) with an EC-EMC/LiPF6 solution. The modified interface on the Al-doped particles is less resistive and comprises the Li+-ion conducting nano-sized centers like AlF3, which promote Li+ ionic transport to the bulk.

            Furthermore, we present our recent results on the study of Ni-rich, layered-structure LiNi0.65Co0.08Mn0.27O2 cathode materials and compare their electrochemical performance with materials of the same overall composition, but with a concentration gradient throughout the particles. The gradient was organized as follows: the Ni concentration is higher at the center of the particles but lower at surface, while the Mn concentration is higher at the surface and lower at the center. The synthesis parameters of the co-precipitation method were optimized comparing annealing periods, followed by electrochemical testing. Three different sets of gradient and standard non-gradient materials were explored, and all gradient materials provided superior capacity and rate capability than their respective non-gradient materials. The reasons for the improved discharge capacity of the gradient materials at moderate temperatures and cut-off potentials were explored through impedance spectroscopy and post-mortem characterization. The Mn-rich surface of the gradient material limits the growth of too resistive surface films during cycling, even at extreme temperatures and potentials, improving stability of these cathode materials. The evolution of the gradient structure was examined via TEM and EDX of FIB-produced particle cross-sections. We have established that prolonged cycling, even at elevated temperatures, did not change the initial concentration profiles determined by the synthesis. Transition metal ion dissolution from the cathode was confirmed via ICP of dissolved Li metal anodes, showing a greater degree of Mn dissolution from the non-gradient materials, possibly due to nickel-manganese segregation tendencies. This greater degree of Mn dissolution from non-gradient materials was confirmed in EDX of cycled particles. Electron diffraction measurements of these cycled particles show that spinel formation during cycling of the gradient materials is limited or even eliminated likely due to the higher concentration of Ni in the bulk of the gradient materials opposed to the non-gradient materials. Finally, we demonstrate long-term, (>1000 cycles) experiments of the gradient material electrodes vs. graphite electrodes in full cells that were performed in order to explore the practical advantage of these materials.

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