Improved cycle life and Li-ion transport parameters at low temperature in doped Ni-rich NMC cathodes

Ni-rich layered oxides such as (LiNi _x Mn _y Co_1-x-y O₂ (x ≥ 0.6)) exhibit structural degradation, surface instability, and poor lithium ion transport, particularly under extreme temperature conditions, limiting their viability for next generation high energy batteries. This work demonstrates that low-level boron (B25) and tin-boron codoping (SB25) enhance the structural resilience and electrochemical performance of LiNi_0.9Mn_0.05Co_0.05O₂ (NMC955) cathodes across a range of temperatures: -5 °C, 25 °C, and 45 °C. Both dopants integrate into the layered α-NaFeO₂ structure, expanding lattice parameters and reducing cation mixing, while preserving particle morphology. At sub-ambient temperatures (-5 °C) where slow Li-ion transport is the primary limitation, Sn-B codoping delivers a 25% improvement in specific capacity at 500 mA g^-1 relative to pristine NMC955, suppresses the emergence of a second high resistance charge transfer (R _CT reduces from 717 Ω to 71.4 Ω), and maintains the highest exchange current densities, 0.3 A m^-2. At 25 °C R _CT is reduced, from 10.34 Ω in pristine NMC955 to 8.79 Ω, and the effective diffusion coefficient increases, from 1.5 to 1.6 × 10^-12 cm^-2 s^-1, demonstrating enhanced low temperature transport kinetics. Long-term cycling at approximately 1C shows improved capacity retentions of 92.7% (B25) and 88.7% (SB25) after 100 cycles versus 78% for undoped NMC. Postmortem XPS/XAS confirm that codoping suppresses electrolyte-induced transition metal fluorination and CEI thickening, with Sn-B showing the smallest change in Ni oxidation state and local coordination after 200 cycles. Together, these results establish Sn-B co-doping as a scalable and effective strategy to simultaneously enhance the structural stability, interfacial chemistry, and low-temperature transport kinetics of Ni-rich NMC cathodes for demanding lithium-ion battery applications.