Reversible anionic redox chemistry in layered Li4/7[□1/7Mn6/7]O2 enabled by stable Li–O-vacancy configuration

•Design the Li–O-vacancy configuration to trigger oxygen redox reactions•Restrain the irreversible oxygen release by means of the stable Mn vacancy•Quantify the approximate capacity distributions of anionic/cationic redox reactions High-energy-density cathode materials for Li-ion batteries are developed along the avenue of configuration transition from conventional Li–O-transition metal (TM) to typical Li–O–Li. It achieved a great increase in energy density because anionic redox activities can provide additional capacities. However, the utilization of anionic redox chemistry based on a typical Li–O–Li configuration always suffers from inherent issues such as serious oxygen release and induced structural distortion, further resulting in the rapid decay of energy density upon cycling. To address these problems, new configurations should be designed to achieve additional capacity and restrain oxygen release, which is significant for the commercialization of high-energy-density Li-rich cathode materials. Besides, the discovery of new configurations will undoubtedly excite the immediate interest of a wide audience of chemistry scientists to develop various advanced cathode materials. The combination of anionic and cationic activities within Li-rich materials breaks through the traditional capacity limitation and achieves high-energy-density batteries. However, the utilization of anionic oxygen redox reactions always leads to detrimental lattice oxygen release, which accelerates structural distortion and electrochemical performance deterioration. In contrast to the typical Li–O–Li configuration in Li-rich layered oxides, not only can oxygen redox behaviors be triggered within layered Li4/7[□1/7Mn6/7]O2 (□: Mn vacancy) with Li–O-vacancy configuration, but lattice oxygen loss can be effectively suppressed. Upon Li+ (de)intercalations, Mn vacancy within the TM layer also enables reversible structural evolution and Li migration processes, further boosting high output capacity and long-term cycling stability. Besides, not only can the irreversible/reversible anionic/cationic redox reactions be clearly unraveled, but their capacity distributions can be roughly quantified upon cycling. Overall, our findings demonstrate that the introduction of Mn vacancy provides a promising configuration to achieve high-capacity cathode candidates for next-generation Li-ion batteries. The combination of anionic and cationic activities within Li-rich materials breaks through the traditional capacity limitation and achieves high-energy-density batteries. However, the utilization of anionic oxygen redox reactions always leads to detrimental lattice oxygen release, which accelerates structural distortion and electrochemical performance deterioration. In contrast to the typical Li–O–Li configuration in Li-rich layered oxides, not only can oxygen redox behaviors be triggered within layered Li4/7[□1/7Mn6/7]O2 (□: Mn vacancy) with Li–O-vacancy configuration, but lattice oxygen loss can be effectively suppressed. Upon Li+ (de)intercalations, Mn vacancy within the TM layer also enables reversible structural evolution and Li migration processes, further boosting high output capacity and long-term cycling stability. Besides, not only can the irreversible/reversible anionic/cationic redox reactions be clearly unraveled, but their capacity distributions can be roughly quantified upon cycling. Overall, our findings demonstrate that the introduction of Mn vacancy provides a promising configuration to achieve high-capacity cathode candidates for next-generation Li-ion batteries. Rechargeable lithium-ion batteries can be regarded as efficient devices to store energy because of their relatively high-energy density and long-cycling life.1Xie J. Lu Y.C. A retrospective on lithium-ion batteries.Nat. Commun. 2020; 11: 2499Crossref PubMed Scopus (349) Google Scholar With the evolution of application scenarios from mobile phones to large electric vehicles, there is an increasing demand for high-energy-density batteries for existing lithium-ion battery systems.2Choi J.W. Aurbach D. Promise and reality of post-lithium-ion batteries with high energy densities.Nat. Rev. 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Highly reversible oxygen redox chemistry at 4.1 V in Na4/7−x[□1/7Mn6/7]O2 (□: Mn vacancy).Adv. Energy Mater. 2018; 8: 1800409Crossref Scopus (138) Google Scholar However, there is a lack of similar structural configuration in activating oxygen redox activities within Li-based oxides. Therefore, it is important to enrich the structural configurations in Li-based cathodes to unlock the stable oxygen redox chemistry. Moreover, Li-rich oxide cathodes generally suffer from severe lattice oxygen release because of excessive oxidation of lattice oxide ions to molecular oxygen.17Rana J. Papp J.K. Lebens-Higgins Z. Zuba M. Kaufman L.A. Goel A. Schmuch R. Winter M. Whittingham M.S. Yang W. et al.Quantifying the capacity contributions during activation of Li2MnO3.ACS Energy Lett. 2020; 5: 634-641Crossref Google Scholar,18Papp J.K. Li N. Kaufman L.A. Naylor A.J. Younesi R. Tong W. Mccloskey B.D. A comparison of high voltage outgassing of LiCoO2, LiNiO2, and Li2MnO3 layered Li-ion cathode materials.Electrochim. Acta. 2021; 368: 137505Crossref Scopus (31) Google Scholar The formation of oxygen molecules induces serious capacity loss because it cannot be effectively reduced upon the discharge process.19Shimoda K. Minato T. Nakanishi K. Komatsu H. Matsunaga T. Tanida H. Arai H. Ukyo Y. Uchimoto Y. Ogumi Z. Oxidation behaviour of lattice oxygen in Li-rich manganese-based layered oxide studied by hard X-ray photoelectron spectroscopy.J. Mater. Chem. A. 2016; 4: 5909-5916Crossref Google Scholar,20Sudayama T. Uehara K. Mukai T. Asakura D. Shi X.-M. Tsuchimoto A. de Boisse B.M.D. Shimada T. Watanabe E. Harada Y. et al.Multiorbital bond formation for stable oxygen-redox reaction in battery electrodes.Energy Environ. Sci. 2020; 13: 1492-1500Crossref Google Scholar What is worse, the Li–O–Li configuration cannot be well preserved during subsequent cycles since the Li-ions migration process is irreversible between the alkali metal (AM) and the TM layer.21Dogan F. Croy J.R. Balasubramanian M. Slater M.D. Iddir H. Johnson C.S. Vaughey J.T. Key B. Solid state NMR studies of Li2MnO3 and Li-rich cathode materials: proton insertion, local structure, and voltage fade.J. Electrochem. Soc. 2014; 162: A235-A243Crossref Scopus (69) Google Scholar It not only decreases the output capacity triggered by oxygen redox reaction but also accelerates the irreversible TM migration.22Amalraj S.F. Burlaka L. Julien C.M. Mauger A. Kovacheva D. Talianker M. Markovsky B. Aurbach D. Phase transitions in Li2MnO3 electrodes at various states-of-charge.Electrochim. 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Mater. 2019; 18: 496-502Crossref PubMed Scopus (261) Google Scholar Thus, it is pivotal to stabilize the oxygen-related and TM redox reactions and quantify corresponding capacity contributions, which is beneficial for developing next-generation high-energy-density cathode materials based on stable anionic and cationic redox chemistry. The high-energy-density cathode materials for Li-ion batteries are developed along the avenue of configuration transition from conventional Li–O-TM to typical Li–O–Li (first-generation design, Figure 1). It achieved a great increase in energy density because anionic redox activities also can be triggered to provide additional capacities, as typical Li-rich/excess cathode materials. However, the utilization of anionic redox chemistry based on a typical Li–O–Li configuration always suffers from inherent issues such as serious oxygen release and induced structural distortion, further resulting in the rapid decay of energy density upon cycling. To address these problems, the second-generation configuration design of the Li–O-vacancy configuration was proposed to achieve additional capacity and restrain oxygen release the first time (Figure 1). In this study, we demonstrate that oxygen redox reactions can be triggered in Li-based oxides by means of Li–O-vacancy configuration, and the irreversible lattice oxygen loss can be effectively restrained during oxygen activation processes. Herein, a novel Co/Ni-free layered oxide cathode, Li4/7[□1/7Mn6/7]O2, was developed by grafting intrinsic vacancies from Na2Mn3O7. Coupling anionic oxygen-related with cationic Mn redox activities, Li4/7[□1/7Mn6/7]O2 harvests a high output capacity of 312 mAh g−1 and energy density of 970 Wh Kg−1. Benefiting from the stable vacancies within the TM layer, Li4/7[□1/7Mn6/7]O2 exhibits a reversible structural evolution and Li migration process, which provides a solid foundation of excellent structural and electrochemical stability upon Li+ (de)intercalation, resulting in limited capacity loss (0.076% per cycle upon 500 cycles) during long-term cycling. Moreover, the irreversible/reversible oxygen-centered and Mn-based redox reactions are comprehensively assigned by in situ/ex situ characterizations, further unraveling the charge compensation mechanisms of anionic and cationic redox chemistry. Besides, with the aid of theory calculation, it indicates that the strong Mn–O interactions in Li4/7[□1/7Mn6/7]O2 are beneficial for suppressing lattice oxygen release compared with Li2MnO3, further facilitating the reversibility of oxygen redox during cycling. The layered Li4/7[□1/7Mn6/7]O2 was developed via the chemical ion-exchange method from Na2Mn3O7 (Na4/7[□1/7Mn6/7]O2). The Na-based precursor was prepared via solid-state reaction, which displayed the ordered vacancy/Mn arrangement in the TM layer and reversible oxygen-related plateaus with limited voltage hysteresis.16Mortemard de Boisse B.M.D. Nishimura S.I. Watanabe E. Lander L. Tsuchimoto A. Kikkawa J. Kobayashi E. Asakura D. Okubo M. Yamada A. Highly reversible oxygen redox chemistry at 4.1 V in Na4/7−x[□1/7Mn6/7]O2 (□: Mn vacancy).Adv. Energy Mater. 2018; 8: 1800409Crossref Scopus (138) Google Scholar,24Tsuchimoto A. Shi X.M. Kawai K. Mortemard De Boisse B. Kikkawa J. Asakura D. Okubo M. Yamada A. Nonpolarizing oxygen-redox capacity without O-O dimerization in Na2Mn3O7.Nat. Commun. 2021; 12: 631Crossref PubMed Scopus (38) Google Scholar,25Abate I.I. Pemmaraju C.D. Kim S.Y. Hsu K.H. Sainio S. Moritz B. Vinson J. Toney M.F. Yang W. Gent W.E. et al.Coulombically-stabilized oxygen hole polarons enable fully reversible oxygen redox.Energy Environ. Sci. 2021; 14: 4858-4867Crossref Google Scholar The XRD pattern and its Rietveld refinement (Figure S1A; Table S1) as well as elemental analysis by inductively coupled plasma (ICP) (Table S2) verified the formation of layered Na2Mn3O7 with a space group of triclinic P1¯.26Adamczyk E. Pralong V. Na2Mn3O7: A suitable electrode material for Na-ion batteries?.Chem. Mater. 2017; 29: 4645-4648Crossref Scopus (64) Google Scholar The sodium coordination environment can be prismatic or octahedral depending on the specific site in the AM layer, and the TM layer exhibits an Mn-vacancy ordering arrangement (Figure S1B).27Song B. Tang M. Hu E. Borkiewicz O.J. Wiaderek K.M. Zhang Y. Phillip N.D. Liu X. Shadike Z. Li C. et al.Understanding the low voltage hysteresis of anionic redox in Na2Mn3O7.Chem. Mater. 2019; 31: 3756-3765Crossref Scopus (91) Google Scholar Moreover, scanning electron microscopy (SEM) (Figure S2A) and transmission electron microscopy (TEM) (Figure S2B) images show that the morphology of Na2Mn3O7, a secondary particle constructed by primary nanoparticles, is irregular. Furthermore, a high-resolution TEM image reveals a highly crystalline layered structure, where the interlayer spacing of approximately 0.55 nm fits well with the (11¯ 0) plane. In addition, the electrochemical behaviors of Na-based Na2Mn3O7 are also shown (Figure S3), representing a pair of reversible plateaus emerged at <4.2 V with relatively small voltage hysteresis, which suggests reversible anionic redox activities can be obtained because of the formation of □-O–Na configuration along c axis direction.16Mortemard de Boisse B.M.D. Nishimura S.I. Watanabe E. Lander L. Tsuchimoto A. Kikkawa J. Kobayashi E. Asakura D. Okubo M. Yamada A. Highly reversible oxygen redox chemistry at 4.1 V in Na4/7−x[□1/7Mn6/7]O2 (□: Mn vacancy).Adv. Energy Mater. 2018; 8: 1800409Crossref Scopus (138) Google Scholar,25Abate I.I. Pemmaraju C.D. Kim S.Y. Hsu K.H. Sainio S. Moritz B. Vinson J. Toney M.F. Yang W. Gent W.E. et al.Coulombically-stabilized oxygen hole polarons enable fully reversible oxygen redox.Energy Environ. Sci. 2021; 14: 4858-4867Crossref Google Scholar,27Song B. Tang M. Hu E. Borkiewicz O.J. Wiaderek K.M. Zhang Y. Phillip N.D. Liu X. Shadike Z. Li C. et al.Understanding the low voltage hysteresis of anionic redox in Na2Mn3O7.Chem. Mater. 2019; 31: 3756-3765Crossref Scopus (91) Google Scholar During the chemical ion-exchange process, the stored mobile ions (Na+) in the AM layer of Na2Mn3O7 can be replaced by equivalent target species (Li+ ions from molten salts), whereas the compositions and arrangements of the TM layer can be maintained (Figure S4).28Cao X. Qiao Y. Jia M. He P. Zhou H. Ion-exchange: A promising strategy to design Li-Rich and Li-excess layered cathode materials for Li-ion batteries.Adv. Energy Mater. 2022; 12: 2003972Crossref Scopus (28) Google Scholar Thus, the as-prepared product is expected to inherit layered structure and intrinsic vacancies form Na2Mn3O7, displaying a new chemical formula of Li4/7[□1/7Mn6/7]O2 with an ordering Mn-vacancy arrangement in the TM layer (Figure 2A), which cannot be obtained by traditional solid-state reactions. Laboratory XRD measurements (λ = 1.54059 Å) and the corresponding Rietveld refinement results prove that the Li4/7[□1/7Mn6/7]O2 can be indexed as the O3-type layered structure with the space group of hexagonal R3¯m (Figure 2B; Table S3). The calculated lattice parameters are a = b = 2.84(4) Å, c = 14.52(2) Å, α = β = 90°, and γ = 120° with goodness-of-fitting parameters of χ2 (1.78) and Rwp (2.67%). In the range of 20°–30°, the superlattice peaks generated by ordering Mn-vacancy arrangement in the TM layer cannot be detected, which might be caused by the low resolution of laboratory XRD measurement. Besides, the formation of stacking faults and defects during the ion-exchange process also decreased the intensity of diffraction peaks.29Raekelboom E.A. Hector A.L. Owen J. Vitins G. Weller M.T. Syntheses, structures, and preliminary electrochemistry of the layered lithium and sodium manganese (IV).Chem. Mater. 2001; 13: 4618-4623Crossref Scopus (31) Google Scholar Thus, the intensity of all diffraction peaks is relatively low and the superlattice peaks are absent at the detection conditions. Instead, synchrotron XRD (λ = 0.457944 Å) was characterized to detect any fine structure, demonstrating the Li4/7[□1/7Mn6/7]O2 has three characteristic peaks, which can be indexed as the space groups of hexagonal R3¯m, monoclinic C2/m and triclinic P1¯, respectively (Figure 2C). The characteristic peaks of C2/m space groups further prove that Li4/7[□1/7Mn6/7]O2 possesses a layered structure with an ordered arrangement within the TM layer, which is similar to typical Li-rich layered oxides such as Li[Li0.2Ni0.2Mn0.6]O2.30Lei C.H. Bareño J. Wen J.G. Petrov I. Kang S.-H. Abraham D.P. Local structure and composition studies of Li1.2Ni0.2Mn0.6O2 by analytical electron microscopy.J. Power Sources. 2008; 178: 422-433Crossref Scopus (142) Google Scholar Besides, a trace amount of triclinic P1¯ phase can be detected due to the residual Na+ ions in the AM layer after the ion-exchange process, which is consistent with ICP result with Na+ ions residue (Table S4). The Na-based content can be neglected because the Li/Mn ratio (1:1.48) calculated by the ICP result approaches the stoichiometric value (1:1.5). By employment of 7Li solid-state nuclear magnetic resonance (ssNMR), the Li environments within Li4/7[□1/7Mn6/7]O2 can be clearly identified (Figure 2D).31Li X. Tang M. Feng X. Hung I. Rose A. Chien P.-H. Gan Z. Hu Y.-Y. Lithiation and delithiation dynamics of different Li sites in Li-rich battery cathodes studied by operando nuclear magnetic resonance.Chem. Mater. 2017; 29: 8282-8291Crossref Scopus (36) Google Scholar At various frequency conditions (25 and 30 KHz), the 7Li ssNMR spectra suggest that all Li+ ions reside in AM layers, whereas the signal of Li in TM layers cannot be clearly detected. And the projection magic-angle turning phase-adjusted sideband separation (pjMATPASS) spectrum provides an individual signal of Li located in the AM layer with the absence of the Li signal located in the TM layer, suggesting the superlattice peaks originate from the ordered arrangement of vacancy/Mn in the TM layer. Altogether, this evidence demonstrates that layered Li4/7[□1/7Mn6/7]O2 shows the ordered □/Mn arrangement in the TM layer, and all Li+ ions were located in the AM layers, indicating the generation of Li–O-vacancy configurations along the c axis direction. In addition, the morphology of Li4/7[□1/7Mn6/7]O2 was also investigated in which both SEM (Figure S5A) and TEM (Figure S5B) images exhibit a similar morphology compared with the Na-based precursor Na2Mn3O7, proving the chemical ion exchange is effective to prepare Li-based layered materials with various structures.28Cao X. Qiao Y. Jia M. He P. Zhou H. Ion-exchange: A promising strategy to design Li-Rich and Li-excess layered cathode materials for Li-ion batteries.Adv. Energy Mater. 2022; 12: 2003972Crossref Scopus (28) Google Scholar Moreover, both high-resolution TEM and scanning transmission electron microscope (STEM) images reveal Li4/7[□1/7Mn6/7]O2 displays an ordered layered structure, where the interlayer spacing of about 0.48 nm fits well with the (003) plane, indicating sodium ions within the AM layer have been successfully replaced by lithium ions after the ion exchange (Figures S5C and S5D). Therefore, not only can the layered structure ordering arrangement in the TM layer be obtained within Li4/7[□1/7Mn6/7]O2, but also the morphology is inherited from its precursor. The electrochemical behavior of Li4/7[□1/7Mn6/7]O2 as cathode material was investigated in galvanostatic mode with a voltage window from 2.0–4.8 V at 10 mA/g (Figure 2E). The first charge profile exhibits a long high-voltage plateau at around 4.65 V (versus Li/Li+), delivering an initial charge capacity of approximately 123 mAh g−1 (corresponds to deintercalation of 0.38 mol Li+ ions). The high-voltage plateau can be attributed to a process purely centered at the oxygen ligands triggered by the Li–O-vacancy configuration in layered Li4/7[□1/7Mn6/7]O2 because Mn4+ cannot be further oxidized in this structure. The typical Li-rich Li[Li1/3Mn2/3]O2 cathode with Li–O–Li configuration also displayed the oxygen-related plateau upon the first charge process.32Robertson A.D. Bruce P.G. Mechanism of electrochemical activity in Li2MnO3.Chem. Mater. 2003; 15: 1984-1992Crossref Scopus (470) Google Scholar Upon the subsequent discharge, about 0.8 mol Li+ ions inserted into the layered structure with the formal composition of Li0.99Mn0.857O2, corresponds to the combination of oxygen and Mn reduction reactions during Li+ intercalation process. When recharged to 4.8 V, two plateaus occurred at around 3.05 and 4.65 V, delivering a high charge capacity of 313 mAh g−1 (corresponds to deintercalation of approximately 0.97 mol Li+ ions). Furthermore, the chemical formula of Li0.99Mn0.857O2 can be obtained after ≈0.97 mol Li+ ions insertion upon the second discharge, suggesting excellent electrochemical reversibility can be achieved in the electrode with intrinsic Mn vacancies. Notably, both a high output capacity of 312 mAh g−1 and a high energy density of 970 Wh kg−1 can be harvested upon this discharge process. Furthermore, the electrode also harvests a high output capacity of 252 mAh g−1 even after 20 cycles at a low current density of 10 mA g−1 with a voltage window from 2.0 to 4.8 V (Figure 2E, insert). The dQ/dV profiles of Li4/7[□1/7Mn6/7]O2 and Li-rich Li[Li1/3Mn2/3]O2 were compared (Figure S6). A drastic downshift of voltage was observed for Li[Li1/3Mn2/3]O2 upon 20 cycles. The voltage decay can be attributed to the structural distortion such as layered/spinel phase transition upon cycling, which plagues the practical applications of Li-rich layered cathode materials. On the contrary, outstanding voltage retention can be obtained with Li4/7[□1/7Mn6/7]O2, indicating the voltage decay caused by structural rearrangement is suppressed. Moreover, the Raman spectrum of Li4/7[□1/7Mn6/7]O2 after 100 cycles presented the absence of the spinel phase formation (Figure S7), which is consistent with dQ/dV results. Furthermore, at a high current density of 300 mA g−1, capacity loss also can be restrained with a limited capacity drop of 0.076% after 500 cycles (Figure 2F), delivering the output capacities are 175 and 111 mAh g−1 after 5 and 500 cycles, respectively. And the electrode also delivered the discharge capacity of 95 mAh g−1 even after 700 cycles (Figure S8). Besides, the ICP result proposed the capacity loss during cycling can be attributed to Mn dissolution (Figure S9). These electrochemical results suggest that a favorable oxygen and Mn redox process and superior structural stability can be obtained with Li4/7[□1/7Mn6/7]O2 during long-term cycling. The combination of operando XRD and ssNMR measurements was conducted to evaluate the structural evolution and Li migration upon the initial two delithiation/lithiation processes (Figure 3).33Cao X. Li H. Qiao Y. Jia M. Li X. Cabana J. Zhou H. Stabilizing anionic redox chemistry in a Mn-based layered oxide cathode constructed by Li-deficient pristine state.Adv. Mater. 2021; 33: e2004280Crossref PubMed Scopus (52) Google Scholar Because of the structural similarity between the layered LiMnO2 with the space group of R3¯m and the layered Li2MnO3 with the space group of C2/m, operando XRD patterns can be analyzed and fitted to the unit cell of hexagonal LiMnO2 for convenience. During the initial two cycles, the shifts of (003) and (101) peaks are reversible concomitant with Li+ ions (de)intercalation (Figures 3 and S10). Upon first charging, the (003) peak gradually shifted to high diffraction angle, whereas the (101) peak almost have no change in position, which is similar to the first Li+ deintercalation process in Li-rich Li[Li1/3Mn2/3]O2.34Kan Y. Hu Y. Croy J. Ren Y. Sun C.-J. Heald S.M. Bareño J. Bloom I. Chen Z. Formation of Li2MnO3 investigated by in situ synchrotron probes.J. Power Sources. 2014; 266: 341-346Crossref Scopus (20) Google Scholar Upon the first discharge process, the (003) peak shifted to a low diffraction angle, whereas the (101) peak almost remained constant at the initial stage and then shifted to low diffraction angle. Notably, the (003) peak during the second charge shifted to a higher location compared with that of the first charge, which suggests more Li+ ions can be exserted from the layered structure, delivering a higher charge capacity upon the second charge process. After the second discharge, the locations of both (003) and (101) peaks shifted back to the same locations compared with that after the first cycle, indicating the reversible structural evolution processes can be realized during the initial two cycles. Besides, in situ synchrotron XRD patterns and corresponding contour figures (Figure S11) displayed the evolutions of (003), (104), (201¯), and (204¯) peaks during Li+ ions (de)intercalation, which is consistent with the operando laboratory XRD results, further demonstrating the reversible structural evolution without phase transition can be achieved upon initial two cycles. To clearly clarify the structural evolution upon Li+ (de)intercalation processes, the unit cell parameters of a-lattice, c-lattice, and volume can be obtained by XRD refinement. The a-lattice parameter almost kept constant during the first charge and then rapidly decreased at the end of discharging. In contrast, c-lattice parameter decreased at the beginning of the first charging process, whereas it displayed a slight change until discharged to 3 V and then increased at the end of discharging. During the second cycle, the evolutions of the a-lattice, c-lattice, and cell volume display the analogous processes compared with the first cycle, indicating excellent reversibility of structural change has been realized within Li4/7[□1/7Mn6/7]O2 electrode upon lithiation and delithiat