Accumulated Lattice Strain as an Intrinsic Trigger for the First-Cycle Voltage Decay in Li-Rich 3d Layered Oxides

Abstract

Li-and Mn-rich layered oxides (LMLOs) are promising cathode materials for Li-ion batteries (LIBs) owing to their high discharge capacity of above 250 mA h g −1 . A high voltage plateau related to the oxidation of lattice oxygen appears upon the first charge, but it cannot be recovered during discharge, resulting in the so-called voltage decay. Disappearance of the honeycomb superstructure of the layered structure at a slow C-rate (e.g., 0.1 C) has been proposed to cause the first-cycle voltage decay. By comparing the structural evolution of Li[Li 0.2 Ni 0.2 Mn 0.6 ]O 2 (LLNMO) at various current densities, the operando synchrotron-based X-ray diffraction results show that the lattice strain in bulk LLNMO is continuously increased over cycling, resulting in the first-cycle voltage loss upon Li-ion insertion. Unlike the LLNMO, the accumulated average lattice strain of LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM811) and LiNi 0.6 Co 0.2 Mn 0.2 O 2 (NCM622) from the open-circuit voltage to 4.8 V could be released on discharge. These findings help to gain a deep understanding of the voltage decay in LMLOs.

Suppressing Voltage Decay in O2-Type Li-Rich Layered Cathode Materials through Microstrain Alleviation

“…In order to figure out the structural evolution mechanism of LNMO-HT during the Li + extraction/insertion process, in situ X-ray diffraction (XRD) experiments were carried out during the first cycle within the voltage range of 2.0–4.8 V at 0.1 C. As exhibited in Figure a, with the extraction of Li ions, the 002 reflection removes to higher scattering angles, indicating a contraction of the c axis, which can be ascribed to the shrinkage of TM layers induced by the oxidation of Ni 2+ and/or lattice oxygen. , Simultaneously, Figure c demonstrates the progressive reduction of lattice parameter c (the average TM–TM interplane distance) during the charge process. Distinctly, the changes are different from those observed in most O3-type Li-rich layered oxide materials. , Meanwhile, the 110 reflection shifts toward higher two theta angles, suggesting a shrinkage in the ab plane due to the oxidation of Ni cations (Figure b). Furthermore, as the voltage increases to 4.5 V, lattice parameter a (representing the average TM–TM intraplane distance) gradually decreases from 2.84(5) to 2.83(5) Å and then remains nearly constant (2.83 ± 0.02 Å) within the voltage range of 4.5–4.8 V. During the discharge process, both 002 and 110 reflections tend to move back to their primary positions.…”

“…Distinctly, the changes are different from those observed in most O3-type Li-rich layered oxide materials. 50,51 Meanwhile, the 110 reflection shifts toward higher two theta angles, suggesting a shrinkage in the ab plane due to the oxidation of Ni cations (Figure 5b). Furthermore, as the voltage increases to 4.5 V, lattice parameter a (representing the average TM− TM intraplane distance) gradually decreases from 2.84(5) to 2.83(5) Å and then remains nearly constant (2.83 ± 0.02 Å) within the voltage range of 4.5−4.8 V. During the discharge process, both 002 and 110 reflections tend to move back to their primary positions.…”

Microwave-assisted synthesis of Co-free Li[Li0.2Ni0.2Mn0.6]O2 cathodes with a spinel-layered coherent structure for high-power Li-ion batteries

“…Given the rapid expansion of renewable energy technologies, the imperative to optimize the electrochemical performance of cathode materials is paramount for the advancement of next-generation Li-ion batteries (LIBs). 1–4 Li- and Mn-rich layered oxides (LMLOs), denoted as Li[Li,Mn,TM]O 2 (with TM representing a transition metal), have garnered widespread attention as promising candidates for advanced LIB cathodes. 5–7 Thanks to the high specific capacity (>250 mA h g −1 ) and an average discharge voltage of around 3.5 V, the energy density of LMLOs boosts an energy density potential of up to 900 W h kg −1 .…”