High-voltage Cycling Stability Research Articles

Ti-Modification of Grain Boundaries for Enhanced High Voltage Cycling Stability of NCM811

Ti-Modification of Grain Boundaries for Enhanced High Voltage Cycling Stability of NCM811

A Dielectric MXene-Induced Self-Built Electric Field in Polymer Electrolyte Triggering Fast Lithium-Ion Transport and High-Voltage Cycling Stability.

Quasi-solid polymer electrolyte (QPE) lithium (Li)-metal battery holds significant promise in the application of high-energy-density batteries, yet it suffers from low ionic conductivity and poor oxidation stability. Herein, a novel self-built electric field (SBEF) strategy is proposed to enhance Li+ transportation and accelerate the degradation dynamics of carbon-fluorine bond cleavage in LiTFSI by optimizing the termination of MXene. Among them, the SBEF induced by dielectric Nb4C3F2 MXene effectively constructs highly conductive LiF-enriched SEI and CEI stable interfaces, moreover, enhances the electrochemical performance of the QPE. The related Li-ion transfer mechanism and dual-reinforced stable interface are thoroughly investigated using ab initio molecular dynamics, COMSOL, XPS depth profiling, and ToF-SIMS. This comprehensive approach results in a high conductivity of 1.34 mS cm-1, leading to a small polarization of approximately 25 mV for Li//Li symmetric cell after 6000 h. Furthermore, it enables a prolonged cycle life at a high voltage of up to 4.6 V. Overall, this work not only broadens the application of MXene for QPE but also inspires the great potential of the self-built electric field in QPE-based high-voltage batteries.

Achieving a Deeply Desodiated Stabilized Cathode Material by the High Entropy Strategy for Sodium-ion Batteries.

Manganese-based layered oxides are currently of significant interest as cathode materials for sodium-ion batteries due to their low toxicity and high specific capacity. However, the practical applications are impeded by sluggish intrinsic Na+ migration and poor structure stability as a result of Jahn-Teller distortion and complicated phase transition. In this study, a high-entropy strategy is proposed to enhance the high-voltage capacity and cycling stability. The designed P2-Na0.67Mn0.6Cu0.08Ni0.09Fe0.18Ti0.05O2 achieves a deeply desodiation and delivers charging capacity of 158.1 mA h g-1 corresponding to 0.61 Na with a high initial Coulombic efficiency of 98.2%. The charge compensation is attributed to the cationic and anionic redox reactions conjunctively. Moreover, the crystal structure is effectively stabilized, leading to a slight variation of lattice parameters. This research carries implications for the expedited development of low-cost, high-energy-density cathode materials for sodium-ion batteries.

MXene-Stabilized VS2 Nanostructures for High-Performance Aqueous Zinc Ion Storage.

Aqueous zinc-ion batteries (AZIBs) based on vanadium oxides or sulfides are promising candidates for large-scale rechargeable energy storage due to their ease of fabrication, low cost, and high safety. However, the commercial application of vanadium-based electrode materials has been hindered by challenging problems such as poor cyclability and low-rate performance. To this regard, sophisticated nanostructure engineering technology is used to adeptly incorporate VS2 nanosheets into the MXene interlayers to create a stable 2D heterogeneous layered structure. The MXene nanosheets exhibit stable interactions with VS2 nanosheets, while intercalation between nanosheets effectively increases the interlayer spacing, further enhancing their stability in AZIBs. Benefiting from the heterogeneous layered structure with high conductivity, excellent electron/ion transport, and abundant reactive sites, the free-standing VS2/Ti3C2Tz composite film can be used as both the cathode and the anode of AZIBs. Specifically, the VS2/Ti3C2Tz cathode presents a high specific capacity of 285 mAh g-1 at 0.2 A g-1. Furthermore, the flexible Zn-metal free in-plane VS2/Ti3C2Tz//MnO2/CNT AZIBs deliver high operation voltage (2.0 V) and impressive long-term cycling stability (with a capacity retention of 97% after 5000 cycles) which outperforms almost all reported Vanadium-based electrodes for AZIBs. The effective modulation of the material structure through nanocomposite engineering effectively enhances the stability of VS2, which shows great potential in Zn2+ storage. This work will hasten and stimulate further development of such composite material in the direction of energy storage.

All‐Fluorinated Low‐Solvation Electrolytes for High‐Voltage Sodium Metal Batteries with Appealing Stability

AbstractHigh‐voltage sodium‐metal batteries (SMBs) are considered a promising battery system due to their abundant sodium source and high energy density. However, the instability of electrolytes, notorious reactivity of Na metal, and severe dissolution of solid electrolyte interface (SEI) film impede their long lifespan and high safety. Herein, a low‐solvation modulating strategy is proposed to design an all‐fluorinated electrolyte consisting of 1 m NaPF6 in fluoroethylene carbonate (FEC), methyl 2, 2, 2‐trifluoroethyl carbonate (FEMC), and lithium difluoro(oxalato)borate (LiDFOB), realizing high working voltage and excellent cycling stability of SMBs. Theoretical calculations and experimental characterizations confirm that the presence of FEMC significantly improves the antioxidant property of the electrolyte and contributes to the formation of high‐quality interfacial phases on both anode and cathode. While LiDFOB benefits to optimize the energy bandgap and the solvated structure and stabilizing the FEMC. Consequently, excellent oxidative stability up to 5 V and ultra‐stable cycling of Na||Na3(VOPO4)2F battery (with a capacity retention of 82.9% after 1800 cycles at 4.4 V) are obtained, exhibiting superior stability to its counterparts. The proposed strategy provides a new insight to design SMBs operated at high voltages.

Regulating atomic Fe/Cu dual sites with unsymmetrical Fe-N6 and Cu-N1S2 coordination for promoting bifunctional oxygen electrocatalysis in advanced zinc-air batteries

Dual-metal single-atom catalysts with heteronuclear active sites exhibit superior oxygen catalysis performance than their single-atom counterparts, facilitating the practical applications in advanced electrochemical energy devices. Herein, a Fe-Cu dual-metal single-atom catalyst with Fe-N6 and Cu-N1S2 coordination environment (FeCu-DSAs/NSC) is reported. Due to the synergism of bimetallic active sites and asymmetric heteroatom coordination, the resultant FeCu-DSAs/NSC displays better bifunctional catalytic performance (a small potential gap between the Eη10 and E1/2, ΔE=0.647 V) when compared with the counterparts (Pt/C+IrO2 and Cu-ISAs/NSC), as well as surpassing most of bifunctional metal atom-based catalysts reported to date. As expected, FeCu-DSAs/NSC based rechargeable zinc-air battery shows remarkable superiority than commercial Pt/C+IrO2 benchmark, resulting in a high peak power density (230.66 mW cm−2), open-circuit voltage (1.464 V) and long-term cycling stability (up to 350 h at 10 mA cm−2).

A high-voltage and low-solvating electrolyte towards promising micro-Si/Ni-rich NMC full cells

A full cell via pairing high-capacity silicon anode with high-voltage NMC cathode (Li(Ni0.83Co0.12Mn0.05)O2) holds great promise as a high-energy battery system for practical applications. Nevertheless, this potential has been hindered by challenges in terms of the pulverization of Si particles, unstable electrode/electrolyte interface, and severe dissolution of transition metals at high voltage. Herein, we present a novel high-voltage and weakly solvating electrolyte, consisted of lithium bis(fluorosulfonyl)imide in mixed solvent of diethyl carbonate and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, which effectively enables the remarkable cycling stability of both micron silicon anode (a high reversible capacity of 1667 mAh g−1 after 200 cycles) and Ni-rich cathode (a notable capacity retention of 83% after 140 cycles at an ultrahigh cut-off voltage of 4.9 V). It is revealed that the tailored electrolyte leads to the formation of a robust and reinforced concrete-like solid electrolyte interphase, characterized by a distinctive vertical side-by-side columnar structure comprised of LiF and sulfide on the micro-silicon anode. Meanwhile, a uniform and thin cathode-electrolyte interphase is observed on the surface of NMC particles when cycled in the designed electrolyte. Furthermore, coupled with high-capacity micro-Si and high-voltage NMC, this designed electrolyte endows a remarkable full-cell with high energy density (475 Wh kg−1), excellent rate capability, and high-voltage cycling stability. This research contributes a pioneering approach to enabling the utilization of micron-sized silicon anodes and Ni-rich cathodes for next-generation high-energy lithium-ion batteries.

High Voltage Cycling Stability of LiF-Coated NMC811 Electrode.

The development of LiNi0.8Mn0.1Co0.1O2 (NMC811) as a cathode material for high-energy-density lithium-ion batteries (LIBs) intends to address the driving limitations of electric vehicles. However, the commercialization of this technology has been hindered by poor cycling stability at high cutoff voltages. The potential instability and drastic capacity fade stem from irreversible parasitic side reactions at the electrode-electrolyte interface. To address these issues, a stable nanoscale lithium fluoride (LiF) coating is deposited on the NMC811 electrode via atomic layer deposition. The nanoscale LiF coating diminishes the direct contact between NMC811 and the electrolyte, suppressing the detrimental parasitic reactions. LiF-NMC811 delivers cycling stability superior to uncoated NMC811 with high cutoff voltage for half-cell (3.0-4.6 V vs Li/Li+) and full-cell (2.8-4.5 V vs graphite) configurations. The structural, morphological, and chemical analyses of the electrodes after cycling show that capacity decline fundamentally arises from the electrode-electrolyte interface growth, irreversible phase transformation, transition metal dissolution and crossover, and particle cracking. Overall, this work demonstrates that LiF is an effective electrode coating for high-voltage cycling without compromising rate performance, even at high discharge rates. The findings of this work highlight the need to stabilize the electrode-electrolyte interface to fully utilize the high-capacity performance of NMC811.

In situ formation of partially carbonized polyvinylpyrrolidone as a green binder for high-voltage Li-ion battery cathode with improved electrochemical performance

We propose the in situ formation of partially carbonized polyvinylpyrrolidone (PC-PVP) as a green binder for high-voltage LiNi0.5Mn1.5O4 (LNMO) cathodes. A mixture of water and ethanol with an equal volume ratio is shown to enable a more uniform dispersion of LNMO particles in the active material matrix with fewer crevices rather than pure water or pure ethanol. Conformal coverage of LNMO with PC-PVP can be achieved through a stepwise thermal process, which not only enhances the electrical conductance of the cathode material but also protects the electrode surface from overreaction with the electrolyte at high voltages. This can be attributed to the coral reef-like structure of the PC-PVP coating layer, which offers abundant electrical contact without sacrificing ion transport pathways. In addition, the specific nitrogen-containing groups of PC-PVP promote ion transport within the electrode. Furthermore, a carbon paper (CP) substrate is introduced as a current collector to support the PC-PVP/LNMO material layer and to establish a good interconnection. Owing to these features, the PC-PVP/LNMO/CP electrode outperforms the PVDF/LNMO/Al counterpart in terms of high-voltage cycling stability and rate capability. The improved electrode performance is supported by the reduced cathode–electrolyte interphase (CEI) resistance, which corresponds to mitigated electrolyte decomposition and stable CEI formation.

LiMgPO4 -Coating-Induced Phosphate Shell and Bulk Mg-Doping Enables Stable Ultra-High-Voltage Cycling of LiCoO2 Cathode.

Stable cycling of LiCoO2 (LCO) cathode at high voltage is extremely challenging due to the notable structural instability in deeply delithiated states. Here, using the sol-gel coating method, LCO materials (LMP-LCO) are obtained with bulk Mg-doping and surface LiMgPO4 /Li3 PO4 (LMP/LPO) coating. The experimental results suggest that the simultaneous modification in the bulk and at the surface is demonstrated to be highly effective in improving the high-voltage performance of LCO. LMP-LCO cathodes deliver 149.8mAhg-1 @4.60 V and 146.1mAhg-1 @4.65 V after 200 cycles at 1C. For higher cut-off voltages, 4.70 and 4.80V, LMP-LCO cathodes still achieve 144.9mAhg-1 after 150 cycles and 136.8mAhg-1 after 100 cycles at 1C, respectively. Bulk Mg-dopants enhance the ionicity of CoO bond by tailoring the band centers of Co 3d and O 2p, promoting stable redox on O2- , and thus enhancing stable cycling at high cut-off voltages. Meanwhile, LMP/LPO surface coating suppresses detrimental surface side reactions while allowing facile Li-ion diffusion. The mechanism of high-voltage cycling stability is investigated by combining experimental characterizations and theoretical calculations. This study proposes a strategy of surface-to-bulk simultaneous modification to achieve superior structural stability at high voltages.

Dielectric polarization in MgFe2O4 coating and bulk doping to enhance high-voltage cycling stability of Na2/3Ni1/3Mn2/3O2 cathode material

Charging P2-Na2/3Ni1/3Mn2/3O2 to 4.5 V for higher capacity is enticing. However, it leads to severe capacity fading, ascribing to the lattice oxygen evolution and the P2-O2 phase transformation. Here, the MgFe2O4 coating and Mg, Fe co-doping were constructed simultaneously by Mg, Fe surface treatment to suppress lattice oxygen evolution and P2-O2 phase transformation of P2-Na2/3Ni1/3Mn2/3O2 at deep charging. Through ex-situ X-ray diffraction (XRD) tests, we found that the Mg, Fe bulk co-doping could reduce the repulsion between transition metals and Na+/vacancies ordering, thus inhibiting the P2-O2 phase transition and significantly reducing the irreversible volume change of the material. Meanwhile, the internal electric field formed by the dielectric polarization of MgFe2O4 effectively inhibits the outward migration of oxidized Oα− (α < 2), thereby suppressing the lattice oxygen evolution at deep charging, confirmed by in situ Raman and ex situ XPS techniques. P2-NaNM@MF-3 shows enhanced high-voltage cycling performance with capacity retentions of 84.8% and 81.3% at 0.1 and 1 C after cycles. This work sheds light on regulating the surface chemistry for Na-layered oxide materials to enhance the high-voltage performance of Na-ion batteries.

Enhancing Thermal and High-Voltage Cycling Stability of Ni-Rich Layered Cathodes through a Ti-Doping-Induced Surface-Disordered Structure

Due to the high-energy density and potential cost advantage, Ni-rich layered oxide cathodes have attracted attention in electric vehicles. However, the inferior thermal stability and long-term cycling stability in a deeply charged state seriously restrict their large-scale applications. The oxygen topotactic lattice structure degradation caused by the irreversible anionic redox reaction has been considered one of the main reasons that cause the abovementioned electrochemical issues. Herein, we propose a modification strategy by constructing a surface-disordered structure to improve the thermal and high-voltage oxygen lattice structure stability of Ni-rich layered cathodes. Density functional theory calculations indicate that Ti4+ with a d0 electronic structure can be used as the inducer of surface-disordered structures. We directly demonstrate the successful design of a surface-disordered structure on the LiNi0.8Mn0.1Co0.1O2 model compound through neutron diffraction and scanning transmission electron microscopy measurements. The constructed surface-disordered structure can enhance the thermal and high-voltage cycling stability of LiNi0.8Mn0.1Co0.1O2. Furthermore, in situ X-ray diffraction and surface structure analyses indicate that this surface-disordered structure can improve the reversibility of bulk structure transition and stability of the formed cathode electrolyte interphase. This facile construction method of the surface-disordered structure provides a reference for industry and other layered oxide cathodes.

Nanoscale surface modification to suppress capacity fade of Ni-Rich layered oxide material at high cut-off voltage

Ni-rich layered oxide is a promising cathode material for future advanced lithium-ion batteries (LIBs). However, the nickel-rich layered cathode material exhibits insufficient high-voltage cycling stability ascribing to the phase transformation, surface side reactions, and lattice oxygen evolution. This work developed an effective inorganic coating strategy to construct LiNiO2/Na1-xNi1-yPO4 surface hybrid coating layer and Na bulk doping simultaneously by a simple wet-chemical method, improving the high-voltage performance of LiNi0.8Co0.1Mn0.1O2 (NCM) effectively. It is found that the surface oxygen vacancy and surface residual lithium on NCM reacts with NaNiPO4 precursors at 500 °C to form LiNiO2 inner layer in addition to the Na1-xNi1-yPO4 outer layer, which is structurally coherent to the layered lattice of NCM. In the constructed hybrid layer, layered phase LiNiO2 firmly fixes on the NCM surface. At the same time, highly chemical stable Na1-xNi1-yPO4 with lattice tunnels can enable lithium-ion transport and act as a protecting layer to inhibit surface side reactions. In addition, Na bulk doping could enlarge lithium-ion diffusion channels and stabilize lattice structure. All these advantages contribute to improving the electrochemical performance of NCM. The results show that the surface treatment NCM cathode exhibit excellent cycling stability with a higher capacity retention of 88 % after 200 cycles under 25 °C and 86 % after 100 cycles under 55 °C, both at 1C. This method provides a feasible strategy to promote the high-voltage cycling performance of electrode material, thus improving the service life, energy density, and safety performance of Li-ion batteries.

Application of Tetraethylsulfamide (TES) As a Cathode Additive in Cylindrical Cells

Recently, sulfonamides have been shown to be promising electrolyte components due to their high chemical and electrochemical stability in lithium batteries [1, 2]. The electrolyte stability becomes critical when applying high voltage and/or utilizing Ni-rich layered oxides in high energy density lithium-ion batteries. Another approach to successful Ni-rich cathode performance is to develop a stable and effective cathode electrolyte interphase (CEI). Given the success of sultones and sulfates in this regard [3, 4], it is hypothesized that nitrogen analogs, like sulfonamides, could be tailored to provide a similar benefit. Indeed, Yim et al. [5, 6] have shown that N,N,N’,N’-tetraethylsulfamide (TES) forms a CEI on NMC811 that imparts high voltage cycling stability and less cathode corrosion. Our earlier studies of TES with Ni-rich NCA also formed a favorable CEI and these results are the topic of this presentation.Herein, we examine the performance of 0 - 4 wt.% TES in our commercially available, high power INR18650-P28A. These cells contain a composite SiO/graphite anode in addition to a Ni-rich cathode. As shown in Fig 1, TES significantly decreased the impedance of the cathode interface after conditioning compared to the control electrolyte. Thereafter, cells containing up to 2%TES show improved capacity retention during long-term high-rate cycling (+1C/-80W). Part of this success was due to a suppression of resistance growth during cycling by TES. Fast charge cycling (+3C/-2C), however, was moderately impaired with increased TES. Considering the largely reduced impedance of the cathode, fast-charge performance may have suffered due to anode rate limitations. These results will be discussed as well as gas generation, storage performance, and additional rate and cycling tests.[1] Shuting Feng, Mingjun Huang, Jessica R. Lamb, Wenxu Zhang, Ryoichi Tatara, Yirui Zhang, Yun Guang Zhu, Collin F. Perkinson, Jeremiah A. Johnson, Yang Shao-Horn. Chem, 5, 2630-2641 (2019)[2] Weijiang Xue, Mingjun Huang, Yutao Li, Yun Guang Zhu, Rui Gao, Xianghui Xiao, Wenxu Zhang, Sipei Li, Guiyin Xu, Yang Yu, Peng Li, Jeffrey Lopez, Daiwei Yu, Yanhao Dong, Weiwei Fan, Zhe Shi, Rui Xiong, Cheng-Jun Sun, Inhui Hwang, Wah-Keat Lee, Yang Shao-Horn, Jeremiah A. Johnson, Ju Li. Nature Energy, 6, 495-505 (2021)[3] Koji Abe, Manuel Colera, Kei Shimamoto, Masahide Kondo, Kazuhiro Miyoshi. Journal of Electrochemical Society, 161 (6) A863-A870 (2014)[4] Jian Xia, N. N. Sinha, L. P. Chen, J. R. Dahn. Journal of Electrochemical Society, 161 (3) A264-A274 (2014)[5] Kwangeun Jung, Taeeun Yim. Journal of Alloys and Compounds, 834,155155 (2020)[6] Ji Won Kim, Kwangeun Jung, Taeeun Yim. Journal of Mater. Sci & Tech. 86, 70-76 (2021) Figure 1

Layered over-lithiated oxide coating for reviving spent LiCoO2 cathode for stable high-voltage lithium-ion battery

Layered lithium-rich manganese-based materials with high voltage stability and high energy density are used to modify the surface of spent LiCoO2 materials (LCO) for designing high-performance Li+-storage structures with high specific capacity and high voltage cycling stability. In this typical regeneration process, Mn2+, Co2+, Ni2+, and Li+ acetates were introduced as raw materials for coating Li1.20Mn0.54Co0.13Ni0.13O2 (LLO) on the surface of spent LCO particles uniformly via a sol-gel method. The structural, morphological, and elemental-chemical-state characterisation results indicate that the recycled [email protected] materials exhibit a typical core-shell structure, with a layer-structured-Li1.20Mn0.54Co0.13Ni0.13O2 layer ~ 10 nm thick as the high-voltage-stable-shell and a well-ordered layer-LiCoO2 as the high-capacity-core. As expected, the regenerated [email protected] composites show an upgraded Li+-storage performance compared to bare LCO. The optimised [email protected] materials show an initial capacity of 197.1 mAh g−1 at 0.1 C, and ~ 95.8% retention after 100 cycles under 3.0–4.5 V at a rate of 1 C. All results indicate that this highly efficient, LLO assisted modification regenerate strategy can be easily extended to regenerate other spent cathodes to synthesise advanced energy storage materials with high voltage cycle stability and high specific capacity.

Artificial cathode electrolyte interphase for improving high voltage cycling stability of thick electrode with Co-free 5 V spinel oxides

Spinel-type cathode LiNi0.5Mn1.5O4 (LNMO) has intrigued the transportation industry due to its high operating voltage and total elimination of the expensive cobalt element. However, LNMO cathode with high mass loading (> 3 mAh/cm2 in areal capacity) has suffered from excessive capacity degradation upon long cycling. Here, a robust Al2O3 surface layer is introduced to the thick LNMO electrode via atomic layer deposition (ALD). The capacity retention in full cells with the graphite anode is improved from 46.3% to 75.3% after 300 cycles with cutoff voltage up to 4.85 V, while enabling average Coulombic efficiency of 99.9% during the cycling. The post-mortem analyses reveal that the Al2O3 surface layer would convert to Al-O-F /Al-F species upon cycling, offering stable interphase to protect the cathode material. These results demonstrate the significance of surface modification enabling high voltage cathode for next-generation LIBs.

P2‐Na2/3Ni2/3Te1/3O2 Cathode for Na‐ion Batteries with High Voltage and Excellent Stability

Air‐stable layered structured cathodes with high voltage and good cycling stability are highly desired for the practical application of Na‐ion batteries. Herein, we report a P2‐Na2/3Ni2/3Te1/3O2 cathode that is stable in ambient air with an average operating voltage of ~3.8 V, demonstrating excellent cycling stability with a capacity retention of more than 92.7% after 500 cycles at 20 mA g−1 and good rate capability with 91.9% capacity utilization at 500 mA g−1 with respect to capacity at 5 mA g−1 between 2.0 and 4.0 V. When the upper cutoff voltage is increased to 4.4 V, P2‐Na2/3Ni2/3Te1/3O2 delivers a reversible capacity of 71.9 mAh g−1 and retains 91.8% of the capacity after 100 cycles at 20 mA g−1. The charge compensation during charge/discharge is mainly due to the redox couple of Ni2+/Ni3+ in the host with a small amount of contribution from oxygen. The stable structure of the material without phase transformation and with small volume change during charge‐discharge allows it to give excellent cycle performance especially when the upper cutoff voltage is not higher than 4.2 V.

Enhanced rate capability and high-voltage cycling stability of single-crystal nickel-rich cathode by surface anchoring dielectric BaTiO3

The single-crystal Ni-rich Li(NixCoyMn1−x−y)O2 cathode (NCM) demonstrates better cycle performance, enhanced tap density and improved mechanical structure stability, compared with polycrystalline NCM.However, limited Li+ transports, (003) plane slips and microcracks in large single particles hinder rate capability and cycle performance. To overcome these shortcomings,single-crystal NCM cathodes have been modified by nanosized tetragonal BaTiO3. Due to the dielectric properties, BaTiO3 particles induce electric field concentration at the BaTiO3-NCM-electrolyte interface. Thus, a large amount of lithium vacancies can be formed, providing sufficient sites for the hopping diffusion of lithium ions, thereby significantly enhancing the diffusion coefficient of Li+. Moreover, the redistribution of charges can inhibit the formation and accumulation of cathode-electrolyte-interface. Owing to the synergetic effect of BaTiO3, the BT-modified single-crystal NCM with the optimized loading shows a remarkable initial discharge capacity of 138.5 mAh g−1 and maintains 53.8% of its initial discharge capacity after 100 cycles under 5C at 4.5 V cut-off voltage. Overall, the proposed dielectric cathode-electrolyte-interface strategy can enhance Li+ ion transport and stabilize the interface structure, leading to improved rate performance. Meanwhile, the diffusion-induced state of charge gradient can also be inhibited, resulting in high structure stability of single-crystal NCMs under high rate and cut-off voltage cycling.

Artificial Cathode Electrolyte Interphase for Improving High Voltage Cycling Stability of Thick Electrode with Co-Free 5 V Spinel Oxides

Artificial Cathode Electrolyte Interphase for Improving High Voltage Cycling Stability of Thick Electrode with Co-Free 5 V Spinel Oxides

Probing the Limiting Mechanism of Sodium-Ion Extraction in the Na5V(PO4)2F2 Cathode

Layered Na5V(PO4)2F2 has been recently identified as a cathode material for sodium-ion batteries due to its high voltage and excellent cycling stability. However, the capacity loss is a main challenge for the application of Na5V(PO4)2F2. Here, by using first-principles calculations and AIMD simulations based on density functional theory, we explored the structural evolution, structural stability, electronic properties, and sodium-ion diffusion of Na5V(PO4)2F2 during the desodiation process. After the removal of one sodium ion, the migration barrier of the sodium ion is 0.57 eV, which is significantly increased compared to Na5V(PO4)2F2. When two sodium ions are removed, the Na3V(PO4)2F2 will undergo a lattice deformation and decompose into VOPO4, NaF, and NaPO3 under standard conditions. The decreasing diffusivity will affect the rate capacity, while the decomposition of the structure during the deep sodium extraction may cause the irreversible capacity of the Na5V(PO4)2F2. In addition, density of states and spin moments analysis demonstrated that V participated in the redox reaction.