Quantifying the Capacity Contributions during Activation of Li₂MnO₃

Abstract

Though Li 2 MnO 3 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 Li 2 MnO 3 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 Li 2 MnO 3 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 Li 2 MnO 3 . Quantitative gas-evolution and titration studies reveal that O 2 and CO 2 release accounted for a large fraction of the observed capacity during activation with minor contributions from reduced Mn species on the surface. These studies reveal that, although Li 2 MnO 3 is considered critical for promoting bulk anionic redox in Li-rich layered oxides, Li 2 MnO 3 by itself does not exhibit bulk oxygen redox or manganese oxidation beyond its initial Mn 4+ valence.

Dislocation and oxygen-release driven delithiation in Li2MnO3

“…However, the present delithiation is rather slow chemical reaction (<10 nm thick with 16 h), and hence the surface damage could be sufficiently suppressed. Moreover, the observed oxygen release and cation mixing in Li 2 MnO 3 have also been reported by the electrochemical reaction process 13 – 16 , which strongly supports the validity of the present study. We therefore conclude that the observed delithiation process with our chemical method could be well equivalent to the electrochemical reaction process.…”

“…Normally, the charge variation introduced by a moderate delithiation in a reversible charge/discharge range is compensated by the oxidation of transition metal in the other system such as LiCoO 2 . However, in the present case, the oxidation state of Mn is rather reduced in the delithiated region and therefore, the formation of anion redox such as O 2 n-(n ≤ 3) or oxygen release should be required for the charge neutrality in the system 16,27 . This situation is directly confirmed by the upward shift of O-K edge near the surface as indicated by the arrow in Fig.…”

“…It is therefore important to understand in detail the delithiation process of Li 2 MnO 3 . Previous studies have proposed that Li 2 MnO 3 may undergo two types of structural modifications during charge/discharge cycles: (i) oxygen release (the loss of oxygen in a sample), (ii) Mn migrations into Li sites that results in a Mn/Li disordered rocksalt-like structure, which have been extensively investigated by various methods 13 – 16 . To elucidate the relationship between such modifications and lattice defects, it has been investigated by scanning transmission electron microscopy (STEM) 13 , 14 , 17 , 18 .…”

How Bulk Sensitive is Hard X-ray Photoelectron Spectroscopy: Accounting for the Cathode–Electrolyte Interface when Addressing Oxygen Redox

“…This conclusion is supported by complementary HAXPES of the Mn 2p core region ( Figure S1). These results are consistent with previous observations of extensive oxygen loss 30,49 and surface reduction 30,39 for Li 2 MnO 3 .…”

Universal solid-state oxygen redox in antifluorite lithium oxidesviatransition metal doping

“…Enhancing the energy densities of the cathodes in LIBs by utilizing the redox reactions of oxide ions in cathode materials has recently attracted extensive attention. [1][2][3][4][5] For instance, Li-rich layered rocksalts (Li 2 MnO 3 , [6][7][8][9][10][11][12] Li 2 RuO 3 , [13][14][15] Li 2 IrO 3 , [16][17][18] and their solid solutions with LiTMO 2 (TM: transition metal) [19][20][21][22][23] ), disordered rocksalts (Li 3 NbO 4 -LiTMO 2 , 24 Li 2 TiO 3 -LiTMO 2 , 25 and Li 4 Mn 2 O 5 26,27 ), and antifluorites (Li 5 FeO 4 , [28][29][30][31][32][33][34][35][36][37] Li 6 CoO 4 , 28,38 and Li 5 AlO 4 39,40 ) facilitate an increase in capacity by using partial oxygen redox along with transition metal redox. Among them, antifluorite Li 2 O exhibits the highest possible theoretical capacity of 897 mA h g À1 , which is based on the solid-state oxygen redox reaction given by…”