ion battery research has been evolving as an indigenous solution to electrochemical energy storage applications. [1][2][3][4] This is in light of the high availability and inexpensiveness of Na resources, sustainability, and redox chemistry similar to lithium-ion batteries. [5] Amidst the various cathode materials being explored for Na-ion batteries, the P2-type Na 2/3 Ni 1/3 Mn 2/3 O 2 is a preeminent one, [6] owing to the environmental friendliness, [7] facile synthesis, [8] superior specific capacity (theoretical capacity, 173 m Ah g −1 ), [9] higher operating voltage (ideally ≈3.7 V), [10] higher energy density, [11,12] open prismatic framework for sodium-ion diffusion with a low diffusion barrier, [13] good structural integrity [14] cobalt free composition, and air-moisture insensitiveness. [12,15] Challenges that inhibit the peak utilization of the P2 type Na x Ni 1/3 Mn 2/3 O 2 (0 ≤ x ≤ 2/3) cathode for sodium-ion battery are; rapid capacity decay when operated at a higher voltage (>4.2 V), [16,17] large volume change in the structure due to a phase transition from P2 to O2 type, inducing particle cracks and exfoliation. [18] The phase transition from P2 to O2 type is thermodynamically favorable when x < 1/3, where the stacking fault of the O2-type structure co-exists with the P2-type. [19] However, the incapability of the O2-type structure to convert back to P2-type causes an irreversible capacity loss. [20,21] The P2-O2 phase transition during desodiation also causes severe volume change (≈23%). This is due to the lower formation energy density of the O2 type than the P2 type phase at higher voltage, inducing exfoliation and cracking in the crystal. [22] Various strategies such as limiting the cut-off potential have been employed, [23] but the operating voltage, capacity, and energy density were sacrificed. [23,24] Efforts have also been devoted to improving the charge capacity by synthesizing high sodium content P2-type layered oxide cathode materials. It helps in avoiding the structural transition by ensuring electrostatic repulsions between the transition metal oxide slabs. [25] Simultaneous surface coating and Ni doping to subdue the Mn-ions dissolution and the orthorhombic distortion, thereby enhancing the structural stability have also been adopted. [26] P2-type Na 2/3 Ni 1/3 Mn 1/2 Ti 1/6 O 2 (NMTNO) cathode is a preeminent electrode material for Na-ion batteries owing to its open prismatic framework, air-moisture stability, inexpensiveness, appealing capacity, environmental benignity, and Co-free composition. However, the poor cycling stability, sluggish Na-ion kinetics induced in bulk-sized cathode particles, cracking, and exfoliation in the crystallites remain a setback. To outmaneuver these, a designing strategy of a mechanically robust, hexagonal nano-crystallites of P2-type Na 2/3 Ni 1/3 Mn 1/2 Ti 1/6 O 2 (NMTNO nano ) electrode via quick, energy-efficient, and low-cost microwave-irradiated synthesis is proposed. For the first time, employing a unified experimental and theoretical approach w...
In this work, a strategy is introduced wherein without keeping any excess cathode, a practical full-cell sodium-ion battery has been demonstrated by utilizing a hard carbon (HC) anode and sodium vanadium fluorophosphate and carbon nanotube composite (NVPF@C@CNT) cathode. A thin, robust, and durable solid electrolyte interface (SEI) is created on the surface of HC through its incubation wetted with a fluoroethylene carbonate (FEC)-rich warm electrolyte in direct contact with Na metal. During the incubation, the HC anode is partially sodiated and passivated with a thin SEI layer. The sodium-ion full cell fabricated while maintaining N/P ∼1.1 showed the first cycle Coulombic efficiency of ∼97% and delivered a stable areal capacity of 1.4 mAh cm–2 at a current rate of 0.1 mA cm–2 realized for the first time to the best of our knowledge. The full cell also showed a good rate capability, retaining 1.18 mAh cm–2 of its initial capacity even at a high current rate of 0.5 mA cm–2, and excellent cycling stability, giving a capacity of ∼1.0 mAh cm–2 after 500 cycles. The current strategy presents a practical way to make a sodium-ion full cell, utilizing no excess cathode material, significantly saving cost and time.
Remarkable efforts have been put forth for the development of environmentally benign, abundant, low-cost and high-performance cathode materials for Li-ion batteries. Spinel LiMn2O4 has been considered as the most promising cathode among the many candidates for next-generation energy storage systems due to afore-mentioned advantages[1]. However, there are some challenges associated to this cathode that inhibit its practical usage for future applications. During cycling, the structural change from cubic to tetragonal takes place, in the Mn3+ configuration due to Jahn-teller distortion followed by Mn2+ dissolution under the influence of acidic electrolyte which destabilises the cathode structure[2]. These changes lead to cathode’s structural transformation and particle cracking resulting in active material loss and capacity fading. The deposition of soluble Mn2+ on anode's SEI results in increased cell’s impedance[3]. Moreover, severe capacity fading at elevated temperatures is still an issue hindering its full-scale commercial development due surface manganese dissolution in the electrolyte at those temperatures[4]. Ever since the emergence of LiMn2O4, various strategies have been employed to exploit its potential. Among many techniques, surface-coating, material composites and doping have been widely used[5]. Most of these techniques focus either on the restriction of direct contact between the active material and electrolyte via coating or the structural enhancement to accommodate for the anisotropic volume changes caused due to disproportionation reaction. Although, these strategies have shown some positive impact on the cycle life of the LMO based Li-ion battery but, in most of the cases the protection from the acidic electrolyte and structural strengthening of LMO has not been addressed simultaneously. Therefore, In order to develop a practical LMO based Li-ion battery, the active material requires a robust protection from the acidic electrolyte contact via a continuous, uniform and Li-ion permeable coating with minimum mismatch while enhancing the structural stability simultaneously to mitigate the irreversible phase transitions. Herein, we report a novel heterostructure design; NMC layered Li-ion permeable phase grown on the surface of Lithium-rich LiMn2O4 octahedra. The layered surface phase protects the host spinel from being directly exposed to the acidic electrolyte during electrochemical cycling. In addition, it provides an efficient path for the ionic and electronic mobility resulting in improved kinetics due to its Li-ion permeability. On the other hand, the excess Li in LMO contributes to the structural enhancement during cycling to accommodate anisotropic volume changes, thus resulting in a robust cathode for high-voltage Li-ion batteries. In comparison to spinel LMO, the newly modified LMO displays an enhanced cyclic performance with superior charge-discharge rate capability. The uniquely developed LiMn2O4 phase surface coated with layered structure demonstrated discharge capacity of 120 mAh g-1 at 20 ℃ temperature while retaining >97% of its initial capacity after 300 cycles at 0.5C. Further, The cathode was tested at elevated temperatures of 60 °C, showing stabilised reversible specific capacity of 113 m Ah g-1 at 0.2 C-rate ensuring energy density of 452 W h kg-1. The full-cell utilising MCMB anode and newly modified LMO cathode showed the areal capacity of 1.22 m Ah cm-2, after 100 cycles with a capacity retention of 95.3% at 0.44 mA cm-2 while maintaining its trend till 300 cycles. References: [1] J. Gummow, A. de Kock, M. M. Thackeray, Solid State Ionics 69 (1994) 59. [2] C. Hunter, J. Solid State Chem. 39 (1981) 142. [3] Blyr, A. D. Pasquier, G. Amatucci, J. M. Tarascon, Ionics 09 (1997) 321. [4] M. Thackeray, J. Cho, J. Electrochem. Soc. 146 (1999) 3577. [5] Xiao, D. Ahn, Z. Liu, J-H. Kim, P. Lu. Electrochem. Commun. 32 (2013) 31-34.
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