Boosting the voltage/capacity stability of O2-type Li-rich layered cathodes by tailoring transition metal distribution for Li-ion batteries

Transition metal (TM) migration into Li vacancies induces a phase transition from a layered to spinel and/ or rock-salt structure in traditional O3-type Li-rich layered oxides (LLOs). O2-type LLOs with a special oxygen stacking sequence of ABAC can restrict irreversible TM migration and avoid the phase transition, but the melting chemical reaction leads to the valence reduction of Mn ions and the aggravated Jahn- Teller (J-T) effect. Here, the TM distribution in an O2-type Mn-based LLO is rationally tailored, in which the Mn fraction decreases linearly while the Ni content increases continually from the inside to the outside of secondary particles, forming a compositionally graded structure. The divalent Ni-enriched surface remarkably decreases the content of high-spin Mn3+ ions on the particle surface and further reduces disproportionation accompanied by Mn2+ dissolution upon cycling. Besides, it is found that the reversibility of the oxygen-anionic redox couple is also improved in this graded O2-type LLO. Consequently, the graded LLO shows much-enhanced cycling stability, maintaining a good voltage/ capacity retention of 95.1%/86.7% after 100 cycles at 0.1C. The graded O2-type Mn-based LLOs with alle-viated disproportionation and enhanced oxygen-anionic redox stability offer an alternative cathode for high energy density (>350 W h kg(-1)) Li-ion batteries.

Automated summary

Transition metal migration induces phase transition in Li-rich layered oxides (LLOs). O2-type LLOs with a unique stacking sequence can restrict irreversible TM migration but face valence reduction and J-T effect. Here, a graded structure of O2-type Mn-based LLO was designed with decreased Mn fraction and increased Ni content, which significantly reduced disproportionation and enhanced stability. The graded LLO showed improved cycling stability and reversible oxygen-anionic redox, making it a promising cathode material for high energy density Li-ion batteries.