Construction of the Na_0.92Li_0.40Ni_0.73Mn_0.24Co_0.12O₂ sodium-ion cathode with balanced high-power/energy-densities

Abstract

Layered P2 type transition metal oxides (TMOs) are considered as the promising cathode candidates for the sodium ion batteries (SIBs). However, the high operating voltage of the P2 cathodes always involves the irreversible phasic transition, which thus compromises the structural stability and practical applications. Through the sustainable recycling of biomass carbon as the sacrificial precursor framework, herein, a Na_0.92Li_0.40Ni_0.73Mn_0.24Co_0.12O₂ cathode with the coexisting P2/O3 phases is reported. By the aid of transmission-mode operando X-ray diffraction, the real-time phasic transition upon the solid-state reaction is precisely tracked. Furthermore, a full cell prototype by pairing the as-fabricated cathode with the anode that developed via a similar sacrificial templating strategy is established. The full cell model renders the simultaneous robust stability, the high energy density of ~218.5 ​Wh kg⁻¹ at a power density of 83 ​W ​kg⁻¹ (0.5C). This biomass-templated strategy demonstrates a precise control over the structural and compositional features of electrodes for the SIBs.

Introduction

To cater to the market needs of the emerging electric vehicles and utility-scale energy storage applications, the key performance metrics of the rechargeable batteries, for instance, the raw material cost, energy/power densities and operation safety become increasingly demanding that far beyond current lithium ion batteries (LIBs) technologies [[1], [2], [3], [4], [5], [6]]. Besides, the finite lithium reserve and its spatially localized distribution mismatch with the rapid growing requirements for the sustainable deployment of energy storage systems [7]. In this context, the development of the alternative sodium ion batteries (SIBs), attracts more attention due to the merits of unlimited sodium resources, low environment foot-print and the low operating voltage (−2.71 ​V vs. standard hydrogen electrode (SHE)) [[8], [9], [10], [11], [12], [13], [14]]. Among the cathode candidates for SIBs, transitional metal oxides (TMOs) with layered host for sodium ions, such as the binary (NaNi^xMn^yO₂, NaFe^xMn^yO₂) and ternary layered derivatives, i.e. NaNi^xMn^yCo^zO₂ (NaNMC) have been extensively investigated due to the high gravimetric capacities of ~120–130 mAh g⁻¹ and the possible operating voltage up to 4.5 ​V, which render the SIBs comparable energy density of ~250–300 ​Wh/kg as LIBs [1,12,15,16]. However, the performance optimization of the layered TMOs cathodes is rather challenging, especially in consideration of the precise control over the compositional and structural parameters, i.e. crystallinity, particle size, cation distribution as well as the synergistic coupling of coexisting phases.

The Delmas’ notation designates two main types of polymorph for the TMOs derivatives as SIBs cathodes: The O3 type structure arranges the crystal layers in a manner of ABCABC stacking, which possesses the higher reversible capacity upon the high voltage charge. However, the Na⁺ migration in the O3 structure requires more energy during the high voltage range, which unfortunately induces the retarded Na⁺ migration and the compromised rate behaviors. While P2 type structure possesses the close-packed oxygen atoms with ABBA stacking, the enlarged interplanar spacing of which favors the fast diffusion of sodium in the structures with a limited reversible capacity [10,[17], [18], [19], [20], [21]]. For instance, M.Sathiya et al. reported that O3–NaNi_1/3Mn_1/3Co_1/3O₂ cathode delivered a high capacity ~120 mAh g⁻¹ ​at 0.1C, yet only ~80 mAh g⁻¹ ​at 1C [22]. The P2–Na_2/3Ni_1/3Mn_2/3O₂ layered cathode, as reported in pioneer studies by Li. Y et al. and Liu. Y et al., delivered an enhanced level of rate behaviors ~100 mAh g⁻¹ ​at 1C and ~120 mAh g⁻¹ ​at 0.2C within the voltage charge range of 2.0–4.4 ​V [23,24]. However, the complete extraction of the Na ions during the high voltage range would induce the lattice distortion of P2 structure and thus the irreversible phasic transition to O2 phase, preventing the reinsertion of Na⁺ with the deteriorated cycling reversibility [17,[24], [25], [26], [27], [28]]. Thus the rational design of cathode materials and the facile electrode kinetics of the P2 type composites will be the feasible route to realize the simultaneous high energy/power densities and long cycle life, especially upon the high operating voltage. Although the similar design rationale of the intergrowth biphasic structure has been proposed, the precise compositional manipulation of the layered cathodes as well as their practical use in the full cell models are yet to be further explored.

The elucidation of the dynamic phasic transition of the NaNMC phase is considered as the prerequisite for the rational optimization of the layered cathodes. Thus far, pioneer studies have investigated the feasible modification strategies to improve the reversibility of the sodiation process. Chen et al. introduced the Cu doping to enhance the lattice spacing, thereby reducing the internal resistance of the electrode and improving the cycle performance within the voltage range of 1.5–4.0 ​V [29]. Dang et al. investigated the suppression effect of the oxide coating layer which effectively avoided the unfavorable side reaction and the exfoliation of the transition metal (TM) layers [25]. Luo et al. proposed the quasi-solid-solution reaction for the sodium extraction process within the voltage range of 2.2–3.9 ​V [30]. Xu et al. investigated the effect of lithium substitution of Mn sites in the P2 structure of Na_0.80[Li_0.12Ni_0.22Mn_0.66]O₂, stabilizing the P2 structure through inhibition of the Mn dissolution above 4.0 ​V [31]. However, the dynamic phasic transition of the ternary oxide derivatives, especially during the high operating voltage, is yet to be further elucidated, so as to simultaneously realize the high operating voltage and structural stabilization.

Herein, we propose a coherent modification strategy to ameliorate the electrochemical performance of the layered P2/O3 NaLiNMC structure at the high operating voltage range. The deliberate control over the compositional stoichiometry (Li occupancy) and microstructural features (P2/O3 mixture nanocrystallines with optimal particle size) was realized via the sustainable recycling of the biomass as the sacrificial framework. The transmission-mode operando X-ray diffraction (XRD) documented the dynamic phasic transformation in the voltage region of 2.5–4.4 ​V, suggesting the trace lithium doping has introduced the intergrowth O3 phase, and thus enhanced the reversibility of the P2–O2 phasic transition upon the continued cycling. The as-fabricated Fatsia Japonica- Na_0.92Li_0.40Ni_0.73Mn_0.24Co_0.12O₂ (FJ-NaLiNMC, the default calcining time set as 10h) cathode exhibited a reversible capacity of ~120.6 mAh g⁻¹ at 0.5C, high rate behavior and average coulombic efficiency (CE) of ~99% for 100 cycles. To deliver a more informative demonstration, the FJ-NaLiNMC cathode was paired with an as-developed ternary metal oxide anode and assembled into a full cell prototype: a stable reversible capacity, high energy density of ~152.65 ​Wh kg⁻¹ at a power density of ~827 ​W ​kg⁻¹ (5C) could be realized. Noted that the biomass carbon (Fatsia Japonica) was introduced as a generic sacrificial framework for both the cathode and anode synthesis with the precise control over the stoichiometric ratio, particle size, phase purity, uniform cation distribution and microstructure. We hope this synthetic strategy will promote the low cost, scalable production of multinary oxide electrode materials in the practical application of SIBs.