Deconvolution of intermixed redox processes in Ni-based cation-disordered Li-excess cathodes

Abstract

Cation-disordered rock-salt transition-metal oxides and oxyfluorides (DRX) have emerged as promising cathode materials for Li-ion batteries due to their potential to reach high energy densities and accommodate diverse, lower cost...

An Overview of Cation-Disordered Lithium-Excess Rocksalt Cathodes

“…A better understanding of the oxygen redox processes and their interplay is therefore crucial for further DRX development. Up to now, operando differential electrochemical mass spectroscopy (DEMS), capable of quantifying evolved O 2 gas, has been extensively used to investigate processes associated with pathway (1). − Figure a shows an example of using operando DEMS in correlating charge/discharge voltage profiles and oxygen gas evolution in a Li 1.3 Nb 0.3 Mn 0.4 O 2 cathode . O 2 release was detected only during the first charge, when the voltage reached above 4.2 V. Gas evolution continued during the voltage plateau between 4.2 and 4.8 V, confirming that the charging plateau is associated with oxygen redox activities.…”

“…In addition, titration mass spectrometry (TiMS) has been used to quantify the amount of peroxo-like oxygen dimers formed in the bulk, , complementing RIXS analysis of oxidized oxygen species. Crafton et al recently applied TiMS to detect the formation of peroxide-like oxygen species in Li 1.2 Mn 0.6 Nb 0.2 O 2 and their contribution to the charge storage capacity .…”

Titration Mass Spectroscopy (TMS): A Quantitative Analytical Technology for Rechargeable Batteries

“…Based on the TMS results collected on each charged state, obvious O 2 evolution observed on the delithiation Li-rich cathode indicates the oxidized lattice oxygen, while the non-ARR spinel cathode does not present any O 2 evolution (upper section, Figure d), which indicates that high-valence Ni 3/4+ and/or Mn 4+ in charged electrodes would not react with H 2 SO 4 to generate O 2 (e.g., potential water splitting). Moreover, based on TMS quantification of O 2 evolution, the charge compensation contributed from the oxidized lattice oxygen (O 2– to O n – ) presents as 75.8 mAh/g during initial charging, and the TMS-related reaction pathway is shown below: false[ normalO ··· normalO false] 2 italicn − ( aq ) + 2 italicn normalH + → 2 − italicn 2 normalO 2 postfixfalse↑ + italicn normalH 2 normalO …”

Interplay between Cation and Anion Redox in Ni-Based Disordered Rocksalt Cathodes

“…20−22 Ni is principally an attractive redox center for the design of DRX cathodes owing to the following three advantages: (1) high theoretical capacity associated with Ni 2+ /Ni 4+ double redox; 10,23 (2) relatively high operating potential of Ni 2+ /Ni 4+ redox; 24 (3) facile synthesis of Ni 2+ based DRX materials in air. 25,26 However, the desired electrochemical attributes have yet to be fully achieved in the Ni 2+ -based DRX systems. In contrast, the Ni-based DRX cathodes often exhibit the following characteristics: (1) incomplete Ni oxidation with the final oxidation state around 3+, though a high charge capacity is obtained with the complementary O oxidation; 25,27,28 (2) strong voltage hysteresis at the low-voltage plateau (∼2 V) upon high-voltage cycling.…”

“…Since then, a number of DRX compounds with an exceptionally high capacity of 250–300 mAh g –1 have been successfully developed . This large capacity is often achieved through the combined cationic transition metal (TM) and anionic O redox by tuning the accessible Li , and TM redox center − via the high-valence TM charge compensation and/or bulk fluorination. − Ni is principally an attractive redox center for the design of DRX cathodes owing to the following three advantages: (1) high theoretical capacity associated with Ni 2+ /Ni 4+ double redox; , (2) relatively high operating potential of Ni 2+ /Ni 4+ redox; (3) facile synthesis of Ni 2+ -based DRX materials in air. , However, the desired electrochemical attributes have yet to be fully achieved in the Ni 2+ -based DRX systems. In contrast, the Ni-based DRX cathodes often exhibit the following characteristics: (1) incomplete Ni oxidation with the final oxidation state around 3+, though a high charge capacity is obtained with the complementary O oxidation; ,, (2) strong voltage hysteresis at the low-voltage plateau (∼2 V) upon high-voltage cycling. − These features are manifested by a comparable charge energy density but lower discharge energy density, making the Ni redox center less favorable compared to other analogues ( e.g.…”