ABSTRACT
Cation-disordered rocksalt (DRX) oxides are a promising new class of high-energy-density cathode materials for next-generation Li-ion batteries. However, their capacity fade presents a major challenge. Partial fluorine (F) substitution into the oxygen (O) lattice appears to be an effective strategy for improving the cycling stability, but the underlying atomistic mechanism remains elusive. Here, using a combination of advanced transmission electron microscopy based imaging and spectroscopy techniques, the structural and chemical evolution upon cycling of Mn-based DRX cathodes with an increasing F content (Li-Mn-Nb-O-Fx , x = 0, 0.05, 0.2) are probed. The atomic origin behind the beneficial effect of high-level fluorination for enhancing the surface stability of the DRX is revealed. It is discovered that, due to the reduced O redox activity while with increasing F concentration, F in the DRX lattice mitigates the formation of an O-deficient surface layer upon cycling. For low F-substituted DRX, the O loss near the surface results in the formation of an amorphous cathode-electrolyte interphase layer and nanoscale voids after extended cycling. Increased F concentration in the DRX lattice minimizes both O loss and the interfacial reactions between DRX and the liquid electrolyte, enhancing the surface stability of DRX. These results provide guidance on the development of next-generation cathode materials through anion substitution.
ABSTRACT
Battery performance, such as the rate capability and cycle stability of lithium transition metal oxides, is strongly correlated with the surface properties of active particles. For lithium-rich layered oxides, transition metal segregation in the initial state and migration upon cycling leads to a significant structural rearrangement, which eventually degrades the electrode performance. Here, we show that a fine-tuning of surface chemistry on the particular crystal facet can facilitate ionic diffusion and thus improve the rate capability dramatically, delivering a specific capacity of â¼110 mAh g-1 at 30C. This high rate performance is realized by creating a nanoscale zirconium-abundant rock-salt-like surface phase epitaxially grown on the layered bulk. This surface layer is spontaneously formed on the Li+-diffusive crystallographic facets during the synthesis and is also durable upon electrochemical cycling. As a result, Li-ions can move rapidly through this nanoscale surface layer over hundreds of cycles. This study provides a promising new strategy for designing and preparing a high-performance lithium-rich layered oxide cathode material.
