Mitigating carbothermal reduction in disordered rock-salt cathodes via direct electrode-slurry carbon mixing

Abstract

Disordered rock-salt (DRX) cathodes have emerged as promising Ni- and Co-free alternatives for lithium-ion batteries, combining the cost advantages of LiFePO₄ with the high energy density of layered oxides. Composed of earth-abundant elements such as Mn and Ti, they deliver specific energies above 900 Wh kg⁻¹ and capacities over 250 mAh g⁻¹, rivaling or surpassing conventional layered oxides. A critical challenge, however, is their intrinsically low electronic conductivity (10⁻¹⁰–10⁻⁸ S cm⁻¹), which makes electrochemical performance highly dependent on the type of conductive additives and how they are processed. This need is typically met through mechanical mixing of carbon via ball milling, but the consequences of this process on material stability and electrode performance remain insufficiently understood. Using Li_1.2Mn_0.6Nb_0.2O₂ as a model DRX system, we reveal that excessive mechanical carbon mixing induces carbothermal reduction (CTR), producing the more soluble Mn²⁺ from Mn³⁺ and converting conductive carbon into insulating lithium carbonate surface species, both of which severely degrade electrochemical performance. These reactions prevent the intended benefits of carbon mixing, leading to increased overpotential, reduced discharge voltage, and accelerated capacity fade. To overcome this challenge, we introduce a direct electrode-slurry mixing method using acid-treated multi-walled carbon nanotubes, which avoids high-energy milling, ensures uniform dispersion, and mitigates CTR-related degradation. This approach enhances cycling stability and voltage retention while remaining compatible with industry-standard slurry processing. Our findings provide mechanistic insights into processing-induced degradation in DRX cathodes and establish a practical pathway toward their scalable, high-performance application in next-generation lithium-ion batteries.