05-acsenergylett-2019-low-ratio

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Though Li2MnO3 was originally considered to be electrochemically inert, its observed activation has spawned a new class of Li-rich layered compounds that deliver capacities beyond the traditional transition-metal redox limit. Despite progress in our understanding of oxygen redox in Li-rich compounds, the underlying origin of the initial charge capacity of Li2MnO3 remains hotly contested. To resolve this issue, we review all possible charge compensation mechanisms including bulk oxygen redox, oxidation of Mn 4+ , and surface degradation for Li2MnO3 cathodes displaying capacities exceeding 350 mAh g -1 . Using elemental and orbital selective X-ray spectroscopy techniques, we rule out oxidation of Mn 4+ and bulk oxygen redox during activation of Li2MnO3. Quantitative gas-evolution and titration studies reveal that O2 and CO2 release accounted for a large fraction of the observed capacity

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• p1 scientific_body coverage=0.0: Quantifying the Capacity Contributions during Activation of Li2MnO3 Jatinkumar Rana,¶ Joseph K. Papp,¶ Zachary Lebens-Higgins, Mateusz Zuba, Lori A. Kaufman, Anshika Goel, Richard Schmuch, Martin Winter, M. Stanley Whittingham, Wanli Yang, Bryan D. McCloskey,
• p1 front_summary coverage=0.0: While Li2MnO3 is regarded as a model compound for describing bulk oxygen redox activity in LR-NMCs, recent RIXS studies did not detect similar spectroscopic signatures of oxidized lattice oxygen in Li2MnO3.30 Additionally, an alternative scenario explaining th
• p1 scientific_body coverage=0.0: Understanding the underlying charge compensation mecha- nism in Li2MnO3 is critical for progressing the development of LR-NMCs. This requires answering two fundamental ques- tions: (1) should Li2MnO3 be regarded as a model compound describing anionic redox act
• p1 front_summary coverage=0.0: Li2MnO3 can deliver substantial capacity during charge, as demonstrated by Kalyani et al.2 Later, Robertson and Bruce3 revealed how Li2MnO3 could be activated through the use of nanosized particles. Indeed, capacities exceeding 300 mAh g−1 have been reported d
• p2 scientific_body coverage=0.032: To resolve these issues, we considered all possible charge compensation mechanisms including bulk oxygen redox, Mn4+/Mn7+ redox, and surface degradation. Our investigation employed a combination of techniques sensitive to oxygen oxidation (O K-edge RIXS) Mn ox
• p2 scientific_body coverage=0.0: 20 to 34°. However, these superlattice reflections appear convoluted into a broad asymmetric peak as shown in Figure 1a. Previously, the intensity and asymmetry of these super- lattice reflections are correlated with the degree of disorder in the stacking sequen
• p3 scientific_body coverage=0.0: first charge and the bottom of the first discharge. Here we have found that these titrations demonstrate minimal contributions from bulk oxygen redox (only 10 mAh g−1), which is in agreement with the lack of spectroscopic feature corresponding to oxidized oxygen
• p3 scientific_body coverage=0.0: charge compensation mechanisms, such as oxidation of Mn4+ and/or lattice oxygen redox need to be considered to fully account for the observed total capacity.23 We first employed O K-edge XAS and RIXS studies to probe bulk redox activity of oxygen anions in Li2M
• p3 scientific_body coverage=0.0: We then turn to operando Mn K-edge XAS to probe bulk Mn redox activity in Li2MnO3 involving Mn4+/Mn7+ redox as proposed by Radin et al.23 However, the experimental verification of the proposed Mn4+/Mn7+ redox is considered to be extremely challenging due to fra
• p3 scientific_body coverage=0.0: These results are further complemented by a quantitative measure of oxide oxidation using an acid titration of extracted Li2MnO3 cathodes. Previous studies on NMC cathode materials have found that O2 evolves from partially delithiated cathodes when exposed to
• p3 scientific_body coverage=0.0: Parts d and e of Figure 2 show operando Mn K-edge X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) data of Li2MnO3. At 5.0 V, no clear shift of the main edge beyond that of the pristine state is observed, which r
• p4 scientific_body coverage=0.0: quickly rose to a sharp peak in evolution rate. As the voltage plateau region was reached, oxygen gas became the dominant evolution product, though CO2 continued to evolve at a lower rate. Total gas evolved across the first charge summed to be 38 mmol of CO2 pe
• p4 scientific_body coverage=0.0: increase in the intensity of the pre-edge region can now be correlated to major restructuring during activation. This restructuring also likely accounts for the observed broadening of the O K-edge spectral features in Figure 2a−c.41,42 Upon discharge to 2.0 V,
• p4 scientific_body coverage=0.0: O2 evolution from the oxide lattice is a 4e−process, such that the irreversible oxygen contribution to capacity can be calculated from the total oxygen evolved as 125 mAh g−1, or 71% of the total first charge capacity. This oxygen evolved on the first charge acc
• p5 scientific_body coverage=0.0: (O2−/O−) revealed about 6% contribution to the total charge capacity. We suspect the remaining 14% of the first charge capacity (Table 1) results from a combination of processes including electrolyte decomposition given the high cutoff voltage, as well as the ox
• p5 scientific_body coverage=0.0: We note that the CO2 evolution is remarkably high for a transition metal oxide material. The large quantity of CO2 evolved (Figure 3c) could originate from a variety of mechanisms, including electrolyte reaction with generated singlet oxygen,43 surface peroxo
• p5 scientific_body coverage=0.0: In summary, using the combination of operando Mn K-edge XAS, O K-edge RIXS, XPS/HAXPES, and DEMS, we interpret and quantify the capacity contributions observed during electrochemical activation of Li2MnO3. Taken together, the first charge capacity of Li2MnO3 or
• p5 scientific_body coverage=0.0: The first charge capacity for Li2MnO3 as observed during the DEMS measurement can now be analyzed in terms of the contributions from the processes examined in this study (Table 1). O2 evolution originating from oxygen oxidation accounts for roughly 70% of the t
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• p6 back_matter coverage=0.018: Author Contributions ¶J.R. and J.K.P. had equal contributions. Notes The authors declare no competing financial interest. ■ACKNOWLEDGMENTS This work was supported as part of the NorthEast Center for Chemical Energy Storage (NECCES), an Energy Frontier Research
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