(Invited) In Operando NMR/EPR Studies of High-Performance Transition Metal Oxides for Electrochemical Energy Storage
Authors: Hu Yan-Yan 
Published: February 2020

High-voltage cathodes made of transition metal oxides have been the leading candidates for rechargeable Li and Na ion batteries. Continuous effort for further improvement in the energy density and stability of available oxides and for discovering new materials is ongoing. Understanding the Li extraction and insertion dynamics, the involved anionic redox chemistry and its coupling with transition metal redox, and the role that Li plays in stabilizing Na transition metal oxides, is important for making the effort more efficient and effective.

Li in Li-rich cathodes mostly resides at octahedral sites in both Li layers (LiLi) and transition metal layers (LiTM). Extraction and insertion of LiLi and LiTM are strongly influenced by surrounding transition metals. pjMATPASS and operando Li nuclear magnetic resonance are combined to achieve both high spectral and temporal resolution for quantitative real time monitoring of lithiation and delithiation at LiLi and LiTM sites in Li2MnO3, Li1.2Ni0.2Mn0.6O2, and Li1.2Ni0.13Mn0.54Co0.13O2cathodes. The results have revealed that LiTM are preferentially extracted for the first 20% of charge and then LiLi and LiTM are removed at the same rate. No preferential insertion or extraction of LiLi and LiTM is observed beyond the first charge. Ni and Co promote faster and more complete removal of LiTM. The recovery of the removed Li is <60% for LiTM and >80% for LiLi upon first discharge. The study sheds light on the activity of LiLi and LiTM during electrochemical processes as well as their respective contributions to cathode capacity.

Anionic redox chemistry offers a transformative approach for significantly increasing specific energy capacities of cathodes for rechargeable Li-ion batteries. This study employs operando electron paramagnetic resonance (EPR) to simultaneously monitor the evolution of both transition metal and oxygen redox reactions, as well as their intertwined couplings in Li2MnO3, Li1.2Ni0.2Mn0.6O2, and Li1.2Ni0.13Mn0.54Co0.13O2 cathodes. Reversible O2–/O2

n

– redox takes place above 3.0 V, which is clearly distinguished from transition metal redox in the operando EPR on Li2MnO3 cathodes. O2–/O2

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– redox is also observed in Li1.2Ni0.2Mn0.6O2, and Li1.2Ni0.13Mn0.54Co0.13O2 cathodes, albeit its overlapping potential ranges with Ni redox. This study further reveals the stabilization of the reversible O redox by Mn and e– hole delocalization within the Mn–O complex. The interactions within the cation–anion pairs are essential for preventing O2

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– from recombination into gaseous O2 and prove to activate Mn for its increasing participation in redox reactions. Operando EPR helps to establish a fundamental understanding of reversible anionic redox chemistry. The gained insights will support the search for structural factors that promote desirable O redox reactions. Further efforts have been invested on monitoring the evolution of O species in Li-rich transition metal oxides using both ex and in situ 17O solid-state NMR spectroscopy.

Sodium manganese oxides with different compositions and crystal structures have attracted much attention because of their high capacity and low cost. We have investigated a group of promising lithium doped sodium manganese oxide cathode materials with exceptionally high initial capacity of ∼223 mAh g−1 and excellent capacity retentions, attributed primarily to the absence of phase transformation in a wide potential range of electrochemical cycling. Solid-state 7Li and 23Na NMR characterizations have been employed to follow the evolution of these active ions, in order to understand the roles that they play in stabilizing the structures over electrochemical cycling. The systematic study of structural evolution and the correlation with the electrochemical behavior of the doped cathode materials provides new insights into rational design of high-performance intercalation compounds.

Figure 1