Iron phosphide stabilization strategy enables long-cycling Co-free lithium-rich manganese-based cathode materials

Abstract

Co-free lithium-rich manganese-based oxides (LRMOs), which offer energy densities over 1000 Wh·kg−1 and low raw material cost, are attractive cathode candidates for next generation high-energy density lithium-ion batteries (LIBs). Nonetheless, their practical application is hindered by their high initial irreversible capacity, capacity and voltage decay, and voltage hysteresis. Herein, a novel iron phosphide modification strategy is presented, where Fe3P is incorporated into the bulk phase of the Li1.2Ni0.2Mn0.6O2 (LNMO) cathode material during its fabrication process of high-temperature calcination of the precursor after spray drying. This regulation stabilizes the crystal lattice of LNMO, promotes the formation of a robust cathode–electrolyte interphase, and mitigates decomposition of the electrolyte, thereby significantly enhancing the cycling stability and rate capability. Consequently, the modified LNMO achieves a capacity of 179 mAh·g−1 (98% capacity retention) after 450 cycles at 1 C (1 C = 200 mA·g−1), and 82% capacity retention after 1000 cycles at 5 C. The regulatory strategy is facile and straightforward contributes superior electrochemical performance for LNMO cathode materials, which has potential for wide-ranging applications.

1. Introduction

Rising to the forefront of global energy transformation, electrochemical power storage is progressively displacing fossil fuel-driven propulsion, with the automotive industry leading the way in this transition. In parallel, solid-state hydrides is emerging as a complementary vector for long-duration, high-specific-energy applications across mobility deployments [1–4]. Together, the increasing markets of the electric vehicle and grid-scale energy storage intensify the demand of developing lithium-ion batteries with high energy density and long-term cycle life. This growing demand, in turn, drives the exploration of advanced cathode materials with superior electrochemical performances. However, conventional cathode materials such as LiCoO2, LiMn2O4, LiFePO4, and nickel-rich layered oxides are approaching their theoretical specific capacity limits [5–7]. Impressively, lithium-rich manganese-based oxides, with the general formula of xLi2MnO3·(1−x)LiTMO2 (TM = Co, Ni, Mn; 0 < x < 1), offer specific capacities over 240 mAh·g−1 and energy densities over 1000 Wh·kg−1, positioning them as highly promising cathodes for high-energy-density lithium-ion batteries (LIBs) [8]. Unfortunately, lithium-rich manganese-based oxides (LRMOs) suffer from severe irreversible capacity loss in the initial cycle, rapid capacity and voltage fading during cycling as well as sluggish rate performance, all of which critically impede their commercial deployment [9–10].

Large-scale researches have sought to identify the root causes of capacity degradation during cycling thus to find a way to mitigate the degradation. The main factors for capacity degradation are the irreversible surface oxygen evolution, side reactions at the electrode–electrolyte interface, and structural phase transitions driven by transition-metal (TM) migration from TM layers into adjacent Li layers [11–12]. In particular, the high-voltage electrochemical window required to access the high-energy density of LRMOs accelerates the electrolyte decomposition, which induces cathode surface degradation of TM dissolution into the electrolyte, and further leads to lattice distortion within the cathode materials [13–14]. Furthermore, during oxygen-redox reactions, TM ions inevitably migrate from the octahedral sites in the TM layers into the octahedral sites in the Li layers, triggering a layered-to-spinel-like phase transition that blocks Li-ion transport pathways and also lowers the operation potential [15–16].

Regarding the mechanism of irreversible oxygen release, one prevailing explanation is that electrochemical oxidation generates O− species that subsequently disproportionate into O0 and O2−. While O2− stabilizes through bonding with TM layers, O0 can migrate into the vacancies created by TM migration, which is ultimately released at the surface [17–18]. The release of lattice oxygen further accelerates the irreversible phase transitions within the crystal structure, ultimately resulting in capacity and voltage decay [19–20]. Extensive strategies have been developed to mitigate the capacity and voltage decay as well as voltage hysteresis, including surface modifications, ion doping, defect engineering, and morphological control [21–24].