Phase Engineering via Aluminum Doping Enhances the Electrochemical Stability of Lithium-Rich Cobalt-Free Layered Oxides for Lithium-Ion Batteries.

Lithium-rich, cobalt-free oxides are promising potential positive electrode materials for lithium-ion batteries because of their high energy density, lower cost, and reduced environmental and ethical concerns. However, their commercial breakthrough is hindered because of their subpar electrochemical stability. This work studies the effect of aluminum doping on Li1.26 Ni0.15 Mn0.61 O2 as a lithium-rich, cobalt-free layered oxide. Al doping suppresses voltage fade and improves the capacity retention from 46% for Li1.26 Ni0.15 Mn0.61 O2 to 67% for Li1.26 Ni0.15 Mn0.56 Al0.05 O2 after 250 cycles at 0.2 C. The undoped material has a monoclinic Li2 MnO3 -type structure with spinel on the particle edges. In contrast, Al-doped materials (Li1.26 Ni0.15 Mn0.61-x Alx O2 ) consist of a more stable rhombohedral phase at the particle edges, with a monoclinic phase core. For this core-shell structure, the formation of Mn3+ is suppressed along with the material's decomposition to a disordered spinel, and the amount of the rhombohedral phase content increases during galvanostatic cycling. Whereas previous studies generally provided qualitative insight into the degradation mechanisms during electrochemical cycling, this work provides quantitative information on the stabilizing effect of the rhombohedral shell in the doped sample. As such, this study provides fundamental insight into the mechanisms through which Al doping increases the electrochemical stability of lithium-rich cobalt-free layered oxides.

Polymeric Backbone Eutectogel Electrolytes for High-Energy Lithium-Ion Batteries.

This work introduces a polymeric backbone eutectogel (P-ETG) hybrid solid-state electrolyte with an N-isopropylacrylamide (NIPAM) backbone for high-energy lithium-ion batteries (LIBs). The NIPAM-based P-ETG is (electro)chemically compatible with commercially relevant positive electrode materials such as the nickel-rich layered oxide LiNi0.6Mn0.2Co0.2O2 (NMC622). The chemical compatibility was demonstrated through (physico)chemical characterization methods. The nonexistence (within detection limits) of interfacial reactions between the electrolyte and the positive electrode, the unchanged bulk crystallographic composition, and the absence of transition metal ions leaching from the positive electrode in contact with the electrolyte were demonstrated by Fourier transform infrared spectroscopy, powder X-ray diffraction, and elemental analysis, respectively. Moreover, the NIPAM-based P-ETG demonstrates a wide electrochemical stability window (1.5-5.0 V vs Li+/Li) and a reasonably high ionic conductivity at room temperature (0.82 mS cm-1). The electrochemical compatibility of a high-potential NMC622-containing positive electrode and the P-ETG is further demonstrated in Li|P-ETG|NMC622 cells, which deliver a discharge capacity of 134, 110, and 97 mAh g-1 at C/5, C/2, and 1C, respectively, after 90 cycles. The Coulombic efficiency is >95% at C/5, C/2, and 1C. Hence, gaining scientific insights into the compatibility of the electrolytes with positive electrode materials that are relevant to the commercial market, like NMC622, is important because this requires going beyond the electrolyte design itself, which is essential to their practical applications.