Revealing Dynamic Electrochemical Interface of LiNi_0.5Mn_1.5O₄ Spinel Cathode for Realizing Simultaneous Mn and Ni Redox for Cost-Effect High-Energy Rechargeable Lithium Batteries

The exploding electric-vehicle market requires cost-effective high-energy materials for rechargeable lithium batteries (RLBs), including Li-ion and Li-metal systems. For a decade, substituting cobalt with nickel in cathodes has been an effective strategy to reduce the materials cost for RLB production while increasing the energy density. However, the rapid spread of EVs has consumed a tremendous amount of metal resources, eventually triggering a price surge of nickel and lithium. In contrast, the price of manganese has remained incomparably low. Consequently, the use of Mn-rich chemistry has attracted attention from researchers as a promising option to realize cathode materials with high economic sustainability and reasonable energy density.<br/>Among the Mn-rich cathode materials, spinel-type LiNi_0.5Mn_1.5O₄ (LNMO) is an attractive alternative to the Ni-rich layered oxides. LNMO is renowned for the high potential of the Ni²⁺/Ni⁴⁺ redox couple, 4.7 V (vs. Li), offering a theoretical energy density of 690 Wh kg⁻¹, within a conventional voltage range of 3.5–4.9 V. A more exciting feature of LNMO is its capability to store more energy using an additional Mn³⁺/Mn⁴⁺ redox reaction at 2.7 V, exhibiting a capacity greater than 200 mAh g^-1. The simultaneous use of Ni and Mn multi-cation redox reactions in LNMO enables an energy density exceeding 860 Wh kg⁻¹. Combining the high energy density with low materials cost, LNMO exhibits a high energy-to-cost ratio of 89.7 Wh USD⁻¹, corresponding to 547% and 279% of that of LiCoO₂ and LiNi_0.8Mn_0.1Co_0.1O₂, respectively, when cycled over a wide voltage range.<br/>Despite this benefit, a long-established hypothesis has restricted the use of multi-cation redox reactions in LNMO: the reduction of Mn⁴⁺ to Mn³⁺ inevitably results in poor capacity retention.¹ The Jahn-Teller distortion of the Mn³⁺ octahedral sites causes the phase transition from cubic (<i>Fd</i><i>m</i>) to tetragonal (<i>I4₁/amd</i>) phases, inducing an extra anisotropic lattice strain. Furthermore, trivalent Mn is considered labile to the disproportionation into Mn⁴⁺ and Mn²⁺, which has a high solubility in organic electrolytes. The reversible use of the Mn redox reaction, however, has been successfully demonstrated in many other cathode materials, such as Li-rich Mn-rich layered oxides and disordered Li-rich rocksalt oxides. The extended cycle life of Mn-based cathodes confirms that the formation of divalent and trivalent manganese cations does not necessarily deteriorate the battery performance. Such inconsistency requires a precise understanding of the degradation mechanism to facilitate the stable use of Mn redox chemistry in LNMO in addition to the conventional Ni redox.<br/>Herein, for the first time, we demonstrate the excellent reversibility of Mn³⁺/Mn⁴⁺ redox within 2.3–4.3 V, leading us to revisit the conventional theory of LNMO degradation. LNMO loses capacity only when cycled within a wide voltage range of 2.3–4.9 V, simultaneously employing Mn³⁺/Mn⁴⁺ and Ni²⁺/Ni⁴⁺ redox. Using surface-sensitive structural and chemical probes, we reveal that a dynamic evolution of the electrochemical interface, e.g., potential-driven rocksalt phase formation and decomposition, repeatedly occurs during cycling. The interfacial evolution induces electrolyte degradation and surface passivation, impeding the charge-transfer reactions. We further demonstrate that stabilizing the interface by electrolyte modification (e.g., EC exclusion) enables LNMO to possess a capacity retention of 91.0% after 100 cycles, an energy density of 715 Wh kg⁻¹, and a power density of 588 W kg⁻¹ within the voltage range of 2.3–4.9 V. Our discovery of the prominent role of electrochemical interfaces on the cycling of LNMO proposes modulating the CEI stability as a promising strategy to develop high-energy and long-cycle-life RLB cathode materials of Mn-rich chemistries.<br/><br/>Reference<br/>G. Lim, D. Shin, K. H. Chae, M. K. Cho, C. Kim, S. S. Sohn, M. Lee, J. Hong*, <i>Advanced Energy Materials</i> (2022), 2202049.