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Voltage decay,i.e., the voltage decrease during electrochemical cycling, has been a decade-long challenge for lithium-ion batteries. This issue not only leads to a substantial loss of energy density, but also raises challenges for the battery management system, hindering the commercial application of high capacity lithium-rich oxide. Here, we show that through a combination of electrochemical conditioning and thermal treatment, Li2MnO3, the parent compound of lithium-rich oxide, which typically displays severe voltage and capacity decay, could be converted into a new phase that essentially suppresses the voltage decay with improved capacity retention and rate performance. By combining atomic-sensitive nuclear magnetic resonance, differential electrochemical mass spectrometry and synchrotron-based resonant inelastic X-ray scattering, we disclose that treatment triggers the formation of three-coexisting phases,i.e., the lithium-rich layered, spinel and defect spinel phases, which enables improved reversibility of the oxygen redox activity and enhanced manganese redox reactions in the initial cycle. Our findings suggest the key role of the local structure in the voltage decay problem and provide insights for material optimizations towards lithium- and manganese-rich cathodes without the voltage decay.

Voltage decay, i.e., the voltage decrease during electrochemical cycling, has been a decade-long challenge for lithium-ion batteries. This issue not only leads to a substantial loss of energy density, but also raises challenges for the battery management system, hindering the commercial application of high capacity lithium-rich oxide. Here, we show that through a combination of electrochemical conditioning and thermal treatment, Li2MnO3, the parent compound of lithium-rich oxide, which typically displays severe voltage and capacity decay, could be converted into a new phase that essentially suppresses the voltage decay with improved capacity retention and rate performance. By combining atomic-sensitive nuclear magnetic resonance, differential electrochemical mass spectrometry and synchrotron-based resonant inelastic X-ray scattering, we disclose that treatment triggers the formation of three-coexisting phases, i.e., the lithium-rich layered, spinel and defect spinel phases, which enables improved reversibility of the oxygen redox activity and enhanced manganese redox reactions in the initial cycle. Our findings suggest the key role of the local structure in the voltage decay problem and provide insights for material optimizations towards lithium- and manganese-rich cathodes without the voltage decay. [ABSTRACT FROM AUTHOR]

Li-excess cation-disordered cathodes for Li-ion batteries have attracted much interest due to their high capacity. However, issues regarding anionic redox activity and capacity degradation still need to be clarified for their performance improvement. In this work, a newly designed cation-disordered Li1.25Nb0.25Fe0.50O2/C cathode is investigated. The mapping of resonant inelastic X-ray scattering (mRIXS) reveals direct evidence of reversible anionic redox during cycling for the first time and defines the voltage range. Furthermore, differential electrochemical mass spectroscopy (DEMS) finds that oxygen release is involved during high voltage charging with electrolyte decomposition. A surface densified layer induced by excessive oxygen oxidation is also identified by in/ex situ X-ray diffraction (XRD). In the densified layer, lithium ion diffusion is blocked due to the reduced amount of percolating 0-TM (transition metal) network. Besides, irreversible oxygen loss during cycling leads to the sustained growth of the densified layer and results in larger lattice volume variations. Therefore, the performance degradation of the material is largely due to oxygen loss, surface densification, and large volume variation upon electrochemical operation. [ABSTRACT FROM AUTHOR]