Dopant Segregation Boosting High‐Voltage Cyclability of Layered Cathode for Sodium Ion Batteries

Abstract

PIBs). [7,8] Either in the form of O3 structure, P2 structure or other else structures, their common similarity is the layer by layer stacking sequence of transition metal ions, oxygen ions, and alkaline ions. [9,10] Such a layered structure not only enables high-specific capacity but also favors the fast migration of alkaline ions due to the unique 2D diffusion pathway. Thus, layer structured LiCoO 2 , ternary NMC (LiNi x Mn y Co z O 2 , x + y + z = 1), and NCA (LiNi 0.85 Co 0.1 Al 0.05 O 2 ) have achieved great commercial success as high-performance cathode materials for LIBs. [11][12][13] Other layered cathode materials, such as Ni-rich NMC for LIBs [14,15] and Na-NMC layered cathodes for SIBs, are gaining increasing attentions. [12,16,17] One of the primary efforts on layered cathodes is further unlocking its capacity potential by narrowing the gap between the practical capacity and theoretical capacity. However, increasing the utilization of alkaline ions, usually by elevating the high-charge cutoff voltage, results in structural instability of the layered cathodes, due to aggravated surface/ interface chemical degradations and bulk degradations. [18][19][20] The surface/interface degradations are attributed to the chemical reactions between cathode and electrolyte, which leads to cathode surface phase transition, [21,22] active material dissolution, [23] passivation layer formation, [24] electrolyte consumption, [25] and so on. For bulk degradations, irreversible bulk phase transition and cracking are the two main failure mechanisms. At high state of charge, phase transition not only destructs the active layered structure but also introduces substantial volume changes causing mechanical failures. A direct correlation between phase transition and cracking has been established in the P2 layered cathode for SIBs. [26] Cleavage along layered planes leads to the typical intragranular cracking for many layered cathodes, which has been frequently observed after high-voltage cycling. [26][27][28] The detrimental effects of cracking include fracture caused disintegration, [28] which leads to poor electronic conduction [29,30] and loss of active materials, and new surfaces exposure to electrolyte which results in cathode surface degradation [31][32][33] and electrolyte consumption. In addition, high density of intragranular cracks also plagues battery thermal stability and safety. [27] Doping electrochemically inactive elements has been verified as an effective method to improve the electrochemical performance of layered cathodes. [34,35] Dopants can play multiroles in engineering bulk material, such as eliminating unwanted As a widely used approach to modify a material's bulk properties, doping can effectively improve electrochemical properties and structural stability of various cathodes for rechargeable batteries, which usually empirically favors a uniform distribution of dopants. It is reported that dopant aggregation effectively boosts the cyclability of a Mg-doped P2-type layered cathode (Na 0.67 Ni 0.33 Mn 0.6...

Shape‐Induced Kinetics Enhancement in Layered P2‐Na0.67Ni0.33Mn0.67O2 Porous Microcuboids Enables High Energy/Power Sodium‐Ion Full Battery

“…The cycling performance of P2‐NNMO‐PMCs is demonstrated via long‐term cycling measurement at 2 C and 5 C, as shown in Figure 2D and E, respectively. As depicted, it can deliver an initial specific capacity of 83.2 mAh g −1 with a capacity retention of 98.3 % after 300 cycles and could achieve a capacity retention of 94.6 % after cycling 1500 cycles at 5 C. The cycling performance of P2‐NNMO‐PMCs is superior when compared with other reported layered oxide cathodes (Table S3) [19,24, 43–50] . The calculated energy densities for P2‐NNMO‐PMCs electrode also exhibit an overwhelming advantage compared to P2‐NNMO‐B (Figure S11), which could be critical for full cell device.…”

“…As depicted, it can deliver an initial specific capacity of 83.2 mAh g À 1 with a capacity retention of 98.3 % after 300 cycles and could achieve a capacity retention of 94.6 % after cycling 1500 cycles at 5 C. The cycling performance of P2-NNMO-PMCs is superior when compared with other reported layered oxide cathodes (Table S3). [19,24,[43][44][45][46][47][48][49][50] The calculated energy densities for P2-NNMO-PMCs electrode also exhibit an overwhelming advantage compared to P2-NNMO-B ( Figure S11), which could be critical for full cell device. In addition, the testing potential range is further extended to 1.5-4.0 V to further investigate the effect of structural design on our P2-NNMO-PMCs.…”

P3/O3 Integrated Layered Oxide as High‐Power and Long‐Life Cathode toward Na‐Ion Batteries

“…[ 7–10 ] As one type of promising cathode materials for NIBs, layered transition metal oxides Na x TMO 2 (0 < x ≤ 1; TM = Fe, Co, Ni, and Mn) have been intensively investigated because of possessing high capacity, appropriate operating potentials, and facile synthesis. [ 11–14 ] Typically, Na x TMO 2 is classified into two main types of phases: O3 and P2, [ 15–17 ] where O represents the octahedral environment for Na site, P represents the prismatic environment for Na site, and the number 2 or 3 means the minimum number of transition metal layers in a single cell unit. O3‐phase cathode with sufficient Na content delivers a high reversible capacity, however, suffers from the limited cycle stability and poor rate capability owing to one and more complex phase transformations resulting from the Na ions diffuse through the face‐shared tetrahedral sites (intermediate site) during sodiation/desodiation process.…”

Pinning Effect Enhanced Structural Stability toward a Zero‐Strain Layered Cathode for Sodium‐Ion Batteries

“…As expected, recent research results indicate that TM accidentally occupied in the sodium layer does improve the structural stability of the layered cathode. [8] Our recent work revealed that part of the TM irreversibly occupied from the TM layer to the Na layer during the first de-sodiation process,a nd the unfavorable phase transformation also disappeared during the subsequent sodiation and de-sodiation process. [8b] But, it should be noted that too much TM in sodium layer would severely block the diffusion of sodiumions,w hile insufficient pinned TM would not form the rocksteady structure.T herefore,i tr emains ah uge challenge to intelligently control the pinned TM in sodium layer to enhance the thermodynamic stability without affecting the normal Na-ion diffusion.…”