Abstract
Historically long accepted to be the singular root cause of capacity fading, transition metal dissolution has been reported to severely degrade the anode. However, its impact on the cathode behavior remains poorly understood. Here we show the correlation between capacity fading and phase/surface stability of an LiMn2O4 cathode. It is revealed that a combination of structural transformation and transition metal dissolution dominates the cathode capacity fading. LiMn2O4 exhibits irreversible phase transitions driven by manganese(III) disproportionation and Jahn-Teller distortion, which in conjunction with particle cracks results in serious manganese dissolution. Meanwhile, fast manganese dissolution in turn triggers irreversible structural evolution, and as such, forms a detrimental cycle constantly consuming active cathode components. Furthermore, lithium-rich LiMn2O4 with lithium/manganese disorder and surface reconstruction could effectively suppress the irreversible phase transition and manganese dissolution. These findings close the loop of understanding capacity fading mechanisms and allow for development of longer life batteries.
Highlights
Long accepted to be the singular root cause of capacity fading, transition metal dissolution has been reported to severely degrade the anode
We employ spinel LiMn2O4 as a model system to explicitly demonstrate what cathode structural evolutions are involved in charge/discharge processes and how structural evolutions interact with Mn dissolution
Through advanced X-ray techniques, including X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), and X-ray fluorescence (XRF), together with scanning transmission electron microscopy (STEM), it is evident that LiMn2O4 suffers from severe irreversible phase evolutions
Summary
Long accepted to be the singular root cause of capacity fading, transition metal dissolution has been reported to severely degrade the anode. In current commercial LIBs (lithium/transition metal (TM) oxides or polyanionic compounds), TM ions function as redox centers that facilitate rapid electron exchange and accompanying reversible structural evolution[15,16,17] Their effects for bulk structural stability and surface chemical stability are crucial to the electrochemical performance of LIBs. almost all cathode TM ions have been observed to suffer from pronounced dissolution[18,19,20] that subsequently induces a negative effect on the anode. Through advanced X-ray techniques, including X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), and X-ray fluorescence (XRF), together with scanning transmission electron microscopy (STEM), it is evident that LiMn2O4 suffers from severe irreversible phase evolutions These transitions produce an unexpected, soluble Mn3O4 phase driven by Mn(III) disproportionation during charging, an over-lithiated Li2Mn2O4 at high discharge potential (3.4 V), and particle cracks during cycling. The insight into cathode capacity decay can serve as design principles to facilitate future discovery of improved, structurally stable cathode materials in LIBs
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