Combining Acid Titration and Mass Spectrometry to Decouple Mixed Redox Processes in Cation-Disordered Li-Excess Cathodes
Authors: Huang Tzu-Yang , Yue Yuan , Tong Wei , McCloskey Bryan D. 
Published: December 2020

In recent years, cation-disordered rock-salt transition-metal oxides (DRX) have emerged as promising cathode materials for Li-ion batteries. With sufficient extent of lithium excess in DRX, lithium ions can diffuse and percolate through lattice tetrahedral holes having low energy barriers. In certain compositions, over 0.7 Li per unit formula (Li1+

x
TM1

−

x
O2, where TM represents transition metal ions) are electrochemically accessible, which leads to exceptionally high capacity that can surpass 300 mAh g
−

1. In addition, the limitation of having specific transition metal ions (Mn, Co, and Ni) to form a stable layered structure is relaxed for DRX materials.1 Accommodating various transition metal ions into the DRX lattice enlarges the domain of design and optimization. Thus, DRX material’s potential in reaching a high energy density and increasing the design flexibility makes it a prominent candidate for next-generation cathodes in Li-ion batteries.
However, the diverse metal-oxide (or even metal-oxyfluoride) chemistry in DRX also leads to its complex local coordination environment. The inherent cation-mixing and Li-excess nature of the DRX cathode has been argued as the major cause of oxygen redox, which is an atypical redox process in Li-stoichiometric layered materials. Overlapped TM-dominated orbitals and unhybridized O-dominated orbitals in the DRX electronic configuration result in parallel mechanisms of transition-metal redox and oxygen redox.1,2 While X-ray spectroscopic research has elucidated oxygen redox process in Li-excess layered oxides, few studies can successfully resolve and quantify each redox process’s contribution to capacity individually.3 Without quantitative information of the interwoven transition-metal and oxygen redox processes, designing and optimizing DRX materials could be impeded. For example, understanding which redox process dominates the capacity loss may be critical to rationally-design a DRX with a long cycle life. Therefore, the aim of this study is to develop a chemical approach to deconvolute the mixed redox processes in the DRX.
In this research, a DRX material, Li1

.

15Ni0

.

45Ti0

.

3Mo0

.

1O1

.

85F0

.

15 (NTMF-DRX), was first isotopically enriched with 18O. Electrodes comprising the 18O-enriched DRX were charged to various cut-off voltages. Meanwhile, the O2 loss from the DRX lattice was monitored and quantified by differential electrochemical mass spectrometry (DEMS). Furthermore, each extracted electrode was then titrated by a concentrated acid solution. When the DRX was etched and dissolved by the acid, O2 evolved from two reactions: 1) disproportionation of oxidized oxygen dissolved from the 18O-enriched DRX lattice; 2) water splitting by oxidized nickel ions (Ni3+ or Ni4+). With the known 18O enrichment (%) in DRX, two sources of O2 can be distinguished and quantified individually via mass spectrometry. Finally, the amount of O2 evolution from two reactions can be used to backcalculate the electrochemical capacity contributed from the nickel redox and the oxygen redox, respectively.
The results of this research generally agree with the predictions from ab initio calculations.2 Within the NTMF-DRX material, the nickel redox predominates the charge transfer at low states of charge (below 4.1 V vs. Li/Li+). The oxygen redox occurs and competes with the nickel redox between 4.1 V and 4.5 V, where 39.5% of the charge capacity is supplied by oxidizing oxygen. Moreover, the DEMS analysis provides quantitative outgassing information that is usually not captured by other approaches. At the high voltage ranging from 4.5 V to 4.7 V, the activated formation and further oxidation of dimerized lattice oxygen atoms results in irreversible gaseous O2 loss, which is accounted for 77.7% of the net oxygen redox capacity. On the whole, adding up each component capacity of various redox processes measured using our analytical techniques matches the net charge transfer measured by the potentiostat, with a difference of ≤10 mAh g
−

1 at any given potential. To conclude, this study presents a new route to characterize the intermixed redox mechanisms in the DRX. The deconvoluted redox capacity can be useful in developing DRX cathode materials for Li-ion batteries with the supreme performance.

References

(1) Clément, R. J.; Lun, Z.; Ceder, G. Cation-disordered rocksalt transition metal oxides and oxyfluorides for high energy lithium-ion cathodes. Energy & Environmental Science
2020, 345–373.
(2) Seo, D. H.; Lee, J.; Urban, A.; Malik, R.; Kang, S.; Ceder, G. The structural and chemical origin of the oxygen redox activity in layered and cation-disordered Li-excess cathode materials. Nature Chemistry
2016, 8, 692–697.
(3) Assat, G.; Tarascon, J. M. Fundamental understanding and practical challenges of anionic redox activity in Li-ion batteries. Nature Energy
2018, 3, 373–386.