Surface reconstruction via rapid solution quenching to enhance structural stability of lithium-rich layered cathodes

Abstract

Lithium-rich layered oxides are prospective materials for future-generation cathodes attributable to their high specific capacity. However, significant surface instability, particularly under high-voltage operating conditions, leads to substantial voltage decay and dramatic capacity degradation during long-term cycling, severely limiting their widespread application. In this study, we developed a universal brine quenching strategy to construct a stabilized composite surface structure for lithium-rich layered oxides. This structure comprises an inner surface layer with a Y-doped layered structure and an outermost layer featuring a disordered rock-salt structure. Doping in the layered structure strengthens the Y–O bonds, raises the energy barrier for oxygen evolution, and significantly increases the stability of the lattice oxygen. Additionally, the disordered rock-salt surface structure reduces oxygen release during the charge and discharge cycles. Consequently, this well-designed surface structure significantly boosts the structural stability of the lithium-rich layered oxide surface, suppresses structural degradation during long-term cycling, and facilitates Li+ diffusion kinetics. The improved redox activity, combined with superior structural stability, contributes to an outstanding electrochemical performance. For instance, the Y-quenched Li1.2Mn0.54Ni0.13Co0.13O2 (LLO) cathode exhibited an improved discharge capacity of 283 mAh·g−1 at 0.1 C and 223 mAh·g−1 at 1 C, along with remarkable cyclic stability retaining 91.2% of its capacity after 300 cycles at 1 C, and a reduced voltage decay of 0.76 mV per cycle (compared to 1.16 mV per cycle for pristine LLO). This research provides valuable insights into the design and synthesis of high-energy-density lithium-rich layered oxides through a simple and cost-effective strategy.

1. Introduction

Lithium-ion batteries (LIBs) are primarily used in portable electronics, electric vehicles (EVs), and grid-scale energy storage systems owing to their high energy density, cost-effectiveness, and sustained cycling lifespan [1–3]. As the industry advances, there is an increasing demand for LIBs with significantly higher energy densities to optimize energy storage within constrained spaces, a goal that critically depends on cathode performance [4–6]. However, current commercial cathodes (e.g., LiCoO2, LiFePO4, and LiMn1/3Co1/3Ni1/3O2) face inherent limitations in energy density due to their heavy reliance on transition metal (TM) cationic redox chemistry, which does not meet the requirements for next-generation LIBs aiming for energy densities exceeding 350 Wh·kg−1 [7–8].

Lithium-rich layered oxides, represented as xLi2MnO3·(1−x)LiTMO2 (TM = Mn, Ni, and Co, etc.), have emerged as one of the most potential front-runners for high-capacity cathodes owing to their exceptional specific capacity (>250 mAh·g−1) and ultrahigh energy density (>1000 Wh·kg−1) [9–10]. The ultra-high specific capacity of lithium-rich layered oxides originates from TM cationic redox reactions and anionic oxygen redox activity under high-voltage conditions. The oxygen redox mechanism contributes additional capacity through reversible bulk redox processes (O2−/Ox−, x < 2) and irreversible oxygen evolution (O2−/O2) at the surface [11–12].

Unfortunately, the unique anionic oxygen redox also imposes significant challenges to the structural stability of lithium-rich layered oxides [13–14]. During high-voltage operation (i.e., >4.5 V vs. Li+/Li), irreversible oxygen evolution at the surface triggers the migration of TMs from octahedral sites in the TM layer to octahedral sites in the Li layer [15–17]. This migration progressively degrades the layered structure, resulting in irreversible phase transformations, such as transitions from layered structures to spinel or rock-salt phases [18–19]. O2 released from the surface, along with unstable TM ions, synergistically exacerbate the electrolyte decomposition. This promotes the parasitic side reactions and leads to the formation of an unstable cathode electrolyte interphase (CEI) film [20–22]. Consequently, the poor surface stability of lithium-rich layered oxides under high-voltage conditions triggers reduced initial Coulombic efficiency (ICE), significant capacity degradation, and rapid voltage decay [23–24]. To date, enhancing surface stability and suppressing irreversible oxygen release at the cathode surface remain the primary challenges for the sustainable development of lithium-rich layered oxides [25–26].