Structural Origin of Suppressed Voltage Decay in Single‐Crystalline Li‐Rich Layered Li[Li_0.2Ni_0.2Mn_0.6]O₂ Cathodes

Abstract

electric vehicles (HEVs), next-generation lithium-ion batteries (LIBs) that utilize high-energy cathode materials are crucially needed. [1][2][3] Among various types of cathode materials, Li-and Mn-rich layered oxides (LMLOs) are regarded as one of the most promising cathode candidates owing to their high capacity (≥250 mAh g −1 ) and low cost. [4][5][6][7] The high capacity of LMLOs is widely believed to originate predominantly from the reversible cationic and anionic redox activities, [8][9][10] which remarkably overcome the capacity limitations of conventional cathode materials (<200 mAh g −1 ) like olivine LiFePO 4 (space group: Pnma), [11,12] spinel LiMn 2 O 4 (Fd3m), [13,14] and layered Li[Ni,Co,Mn]O 2 (NCM, square brackets represent transition metal ions located on octahedral positions, R3m) [15][16][17][18] because of the sole transition metal (TM) redox activity in these cathodes. Nevertheless, LMLOs always undergo a severe voltage decay upon cycling, [19,20] which seriously hinders the practical application of LMLOs. Over the last two decades, a series of attempts have been made to reveal the underlying structural degradation mechanism and mitigate the voltage decay during extended cycling.Lithium-and manganese-rich layered oxides (LMLOs, ≥ 250 mAh g −1 ) with polycrystalline morphology always suffer from severe voltage decay upon cycling because of the anisotropic lattice strain and oxygen release induced chemo-mechanical breakdown. Herein, a Co-free single-crystalline LMLO, that is, Li[Li 0.2 Ni 0.2 Mn 0.6 ]O 2 (LLNMO-SC), is prepared via a Li + /Na + ionexchange reaction. In situ synchrotron-based X-ray diffraction (sXRD) results demonstrate that relatively small changes in lattice parameters and reduced average micro-strain are observed in LLNMO-SC compared to its polycrystalline counterpart (LLNMO-PC) during the charge-discharge process. Specifically, the as-synthesized LLNMO-SC exhibits a unit cell volume change as low as 1.1% during electrochemical cycling. Such low strain characteristics ensure a stable framework for Li-ion insertion/extraction, which considerably enhances the structural stability of LLNMO during long-term cycling. Due to these peculiar benefits, the average discharge voltage of LLNMO-SC decreases by only ≈0.2 V after 100 cycles at 28 mA g −1 between 2.0 and 4.8 V, which is much lower than that of LLNMO-PC (≈0.5 V). Such a single-crystalline strategy offers a promising solution to constructing stable high-energy lithium-ion batteries (LIBs).

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

“…The Fourier transform (FT R -space) magnitudes of k 2 weighted χ­( k ) extended X-ray absorption fine structure (EXAFS) spectra at Mn and Ni K-edges are displayed in Figure d. There are two distinct peaks in the range of 1–3 Å in the FT spectra for both Ni and Mn ions, which correspond to a TM–O first coordination and a TM–TM second coordination. , Apparently, the intensities of both Ni–TM and Mn–TM peaks decline after the first cycle, revealing a relatively high degree of local structural disordering in the TM–TM interactions of cycled LLNMO. , …”

“…There are two distinct peaks in the range of 1−3 Å in the FT spectra for both Ni and Mn ions, which correspond to a TM−O first coordination and a TM−TM second coordination. 42,43 Apparently, the intensities of both Ni−TM and Mn−TM peaks decline after the first cycle, revealing a relatively high degree of local structural disordering in the TM−TM interactions of cycled LLNMO. 44,45 Although small changes of lattice parameters are involved in the LLNMO cathode during the first charge−discharge cycle at 0.2 C, the oxygen-involved voltage plateau is also disappeared during the first discharge.…”

Design of Gradient Ti Reconstituted Fe2O3 Anodes with Enhanced Lithium Affinity Modulated Electronic Structures: First-Principles Calculations and Experiment Verification

“…After 1200 cycles, the Ti-doped Fe 2 O 3 electrode (Figure e) maintains a reversible specific capacity of 1001.9 mAh g –1 , while Fe 2 O 3 maintains a specific capacity of only 503.9 mAh g –1 . The upward trend in capacity during cycling is mainly attributed to the activation of active components, a phenomenon that has been widely observed. − To highlight the electrochemical superiority of Ti-doped Fe 2 O 3 , the cycling stability at a higher current density (2 A g –1 , Figure g) was further evaluated. After 2000 cycles, no significant decay in specific capacity was observed, and the reversible capacity (343.7 mAh g –1 ) remained comparable to the theoretical capacity of graphite electrodes (372 mAh g –1 ), indicating the potential application of Ti concentration gradient doped Fe 2 O 3 .…”

Computational Understandings of Cation Configuration-Dependent Redox Activity and Oxygen Dimerization in Lithium-Rich Manganese-Based Layered Cathodes

“…Traditionally, Li-rich Mn-based layered Li 1.2 Ni 0.2 Mn 0.6 O 2 material is regarded as the two-phase composite model , or the single-phase solid solution model , of R 3̅ m -LiNi 0.5 Mn 0.5 O 2 and C 2/ m -Li 2 MnO 3 . In a massive previous work, X-ray diffraction (XRD) patterns of Li 1.2 Ni 0.2 Mn 0.6 O 2 were usually matched to R 3̅ m -LiNi 0.5 Mn 0.5 O 2 and C 2/ m -Li 2 MnO 3 , especially for the (003) peak of R 3̅ m -LiNi 0.5 Mn 0.5 O 2 , the (020) and (110) peaks of C 2/ m -Li 2 MnO 3 . − On the other hand, it is usually believed that layered Li 1.2 Ni 0.2 Mn 0.6 O 2 is composed of pure lithium layers fully filled with lithium ions and Li–Ni–Mn mixed atomic layers with a molar ratio of 1:1:3, , as shown in Figure S1.…”