High-Entropy Cathode Materials for Sodium-Ion Batteries

Improved high-rate performance of Na3V2(PO4)3 with an atomic layer deposition-generated Al2O3 layer as a cathode material for sodium-ion batteries

Electrochemical performance and structural evolution of layered oxide cathodes materials for sodium-ion batteries: A review

Energy storage systems made from abundant materials are essential for the transition to a more sustainable economy. Although today lithium-ion batteries (LIBs) are the most popular battery technology, the growing demand and low availability of lithium, as well as the use of cobalt and other rare metals raise questions about the sustainability and long-term viability of LIB as the only energy storage solution. The high abundance of sodium content and relative similarity to LIBs, allows the sodium ion batteries (SIBs) to be considered as alternative for stationary energy storage [1]. However, the widespread adoption of SIB technology is hampered by many challenges, including the relatively low energy density compared to LIB. Lower energy density electrodes, such as Na2FeP2O7, are generally stable during cycling [2], while many higher energy density electrodes, such as NaxMnO2, have had a shorter cycle life [3]. In this work we show several possible solutions how to improve the electrochemical properties of the SIBs made of these cathode materials.The promising cathode material Na2FeP2O7 was studied to improve its electrical conductivity, which is often low in the case of sodium pyrophosphates. Solution synthesis was used to prepare pristine Na2FeP2O7 and Na2FeP2O7/C composite cathode materials for sodium-ion batteries, using glucose as a carbon source. While the pristine Na2FeP2O7 displays capacity of only 45 mAh/g due to the relatively large grain size, the addition of carbon increases the capacity to up to 92 mAh/g (95% of the theoretical 97 mAh/g capacity) with excellent rate capability, as 44 mAh/g capacity is still retained even at 20 C (1.94 A/g) current. The optimal content of carbon was found to be 4.8%. The initial capacity of 81 mAh/g is fully retained after 500 cycles at 1 C, indicating excellent cycle life of Na2FeP2O7/C. Electrochemical measurements were carried out in 1 M NaClO4 salt in propylene carbonate as electrolyte and show that the addition of 5 wt.% fluoroethylene carbonate solid electrolyte interphase stabilizing additive greatly benefits the rate and cycling performance of Na2FeP2O7/C as measured in half-cells [4].Na0,67MnO2 is another compound that is widely studied as cathode materials in sodium ion batteries. Currently polyvinylidene fluoride (PVDF) is the most popular binder choice. In our study, a novel tetrabutylammonium (TBA) alginate binder is used to prepare a Na0,67MnO2 electrode for sodium-ion batteries with improved electrochemical performance. The ageing of the electrodes has been characterized. TBA alginate-based electrodes are compared to PVDF and Na alginate-based electrodes and show favorable electrochemical performance, with gravimetric capacity values of up to 164 mAh/g, which is 6% higher than measured for the electrode prepared with PVDF binder. TBA alginate-based Na0,67MnO2 electrodes also display good rate capability and improved cyclability and their solid–electrolyte interface is similar to that of PVDF-based electrodes. As the only salt of alginic acid soluble in non-aqueous solvents, TBA alginate emerges as a good alternative to PVDF binder in battery applications where the water-based processing of electrode slurries is not feasible, such as the demonstrated case with Na0,67MnO2 [5].Overall, we have shown that binder and electrolyte selection can significantly improve the electrochemical properties of electrode materials for SIBs.The financial support of projects No. 1.1.1.2/VIAA/1/16/166 “Advanced materials for sodium Ion batteries” and No. lzp-2020/1-0391 “Advanced polymer – ionic liquid composites for sodium-ion polymer batteries” is greatly acknowledged. Institute of Solid-State Physics, University of Latvia as the Center of Excellence has received funding from the European Union's Horizon 2020 Framework Program H2020-WIDESPREAD-01–2016-2017-Teaming Phase 2 under grant agreement No. 739508, project CAMART2. Vaalma, C.; Buchholz, D.; Weil, M.; Passerini, S. A cost and resource analysis of sodium-ion batteries. Nat. Rev. Mater. 2018, 3, 18013.Jin, T.; Li, H.; Zhu, K.; Wang, P.-F.; Liu, P.; Jiao, L. Polyanion-type cathode materials for sodium-ion batteries. Chem. Soc. Rev. 2020, 49, 2342.Lyu, Y.; Liu, Y.; Yu, Z.-E.; Su, N.; Liu, Y.; Li, W.; Li, Q.; Guo, B.; Liu, B. Recent advances in high energy-density cathode materials for sodium-ion batteries. Sustain. Mater. Technol. 2019, 21, e00098.Kucinskis, G.; Nesterova, I.; Sarakovskis, A.; Bikse, L.; Hodakovska, J.; Bajars, G. Electrochemical performance of Na2FeP2O7/C cathode for sodium-ion batteries in electrolyte with fluoroethylene carbonate additive. J. Alloys Compd. 2022, 895, 162656.Kucinskis, G.; Kruze, B.; Korde, P.; Sarakovskis, A.; Viksna, A.; Hodakovska, J.; Bajars, G. Enhanced Electrochemical Properties of Na67MnO2 Cathode for Na-Ion Batteries Prepared with Novel Tetrabutylammonium Alginate Binder. Batteries 2022, 8, 6. Figure 1

Development of novel cathode materials for sodium-ion batteries with high capacity and excellent cyclic performance is an exciting and demanding research direction. Herein, we demonstrate the synthesis of NaV3O8 via a rheological phase reaction method. The crystal structure and morphology of synthesized NaV3O8 were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The NaV3O8 powder, calcined at moderate temperature (350 °C) with more uniform and smaller nanorod/nanoplate morphology, and larger d001 spacing, exhibited excellent electrochemical performance as cathode material in sodium ion batteries. A specific discharge capacity of 120 mAh g−1 was achieved at the current density of 120 mA g−1, with exceptional cyclic performance (discharge capacity of 95 mAh g−1 at the 500th cycle). In addition, the NaV3O8 cathode demonstrated excellent rate capability and delivered specific capacity of 80.8 mAh g−1 at current density of 300 mA g−1. The superior electrochemical performance corresponds to the structural stability and faster ionic diffusion. The preliminary results indicate that NaV3O8 can be an alternative cathode material for high-performance sodium-ion batteries.

NASICON-type Na3V2(PO4)3 with a three-dimensional open framework structure has attracted wide attention, and it is regarded as one of the most promising cathode material for sodium-ion batteries. However, the low electronic conductivity restricts its charge–discharge capacity and electrochemical performance. With the purpose to solve this problem, polystyrene microspheres are applied in the preparation of cathode materials for sodium-ion batteries. Particular porous-structured Na3V2(PO4)3 composing of interlaced nanosheets is obtained samples by a simple hydrothermal-assisted sol–gel method via a self-sacrificed template (polystyrene microsphere). As expected, the as-prepared porous sample delivers a reversible capacity of 109.2 mAh g−1 at 0.2 C, an excellent rate performance (89.6 mAh g−1 at 50 C) and superior cyclic stability (retention of 94% over 500 cycles at 50 C). The outstanding rate and cyclic performance are attributed to its unique porous structure which is conducive to improve electron conductivity and facilitate the diffusion of sodium ions.

Cu-substituted Prussian white with low crystal defects as high-energy cathode materials for sodium-ion batteries

Alluaudite-type Na2+2xFe2-x(SO4)3 has been a promising cathode material for sodium-ion batteries (SIBs) due to its high operating voltage and stable structure. However, its actual electrochemical performance suffers from intrinsic sluggish kinetics and poor electronic conductivity. In this work, for the first time, we propose a Na2.48(Fe0.89Mg0.03Sn0.04)1.76(SO4)3 cathode material prepared via a Mg/Sn co-doping strategy. Inactive Mg2+ stabilizes the structure, while Sn4+ inhibits the decomposition of electrolytes under high voltage. The Mg/Sn co-doping strategy enhances the kinetics of sodium ion diffusion reactions, leading to improved electrochemical properties, especially at low temperatures. The optimal NFMS/C-Sn0.03 cathode exhibits a long-term capacity retention of 91.6% after 1500 cycles at 5C and outstanding reversible capacities of 74.3 and 58.3 mAh g-1 at 10C and even at 50C, respectively. Furthermore, the NFMS/C-Sn0.03 cathode demonstrates a high capacity retention of 95.5% at -5 °C and 88.4% at -15 °C, with a remarkable capacity retention of 93.9% after 1000 cycles at room temperature and 85.5% after 700 cycles at -15 °C, respectively. Electron paramagnetic resonance (EPR) and atomic force microscopy (AFM) techniques confirmed that the presence of unpaired electrons and enhanced electronic conductivity could be attributed to the Mg/Sn co-doping. This work provides a feasible approach for designing low-cost, durable, low-temperature, and high-performance cathode materials for SIBs.

Prussian white (PW) is considered one of the most promising cathode materials for sodium-ion batteries because of its large ion diffusion channels, low lattice strain, facile preparation, nontoxicity, and low cost. At present, research on PW mainly focuses on optimizing the material's structures for the ambient environment yet less on its practical application under extreme temperatures. In this Spotlight, we intend to offer progress we have made in developing PW cathode materials working over wide temperatures in terms of intrinsic feasibility and development prospects. These findings provide a direction to promote the practical viability of PW under extreme conditions.

NaV6O15 nanoplates are successfully prepared by a facile and low-cost self-combustion method, which can be considered as promising cathode materials for high capacity sodium-ion batteries (NIBs). Morphology analysis suggests that the self-combustion method leads to the NaV6O15 nanoplates possessing uniform size, with an average length of 400 nm and width of 100 nm. As the cathode materials, the NaV6O15 nanoplates exhibit a high initial discharge specific capacity of 149.48 mAh g−1 at a current density of 20 mA g−1 and remains 81.99 mAh g−1 after 30 cycles. The volume change is as little as 6.4%, which is demonstrated by the first-principles calculation. To understand the diffusion performance of sodium ion in NaV6O15 crystalline lattice, the diffusion coefficients of sodium ion are investigated by the electrochemical impedance spectroscopy (EIS) method and the first-principles calculation. The vacancy-hopping diffusion mechanism is proposed based on a quasi-2D energy favorable trajectory, which relieves the sodium ions diffuse along b-axis in NaV6O15 lattice with desirable activation energy of 0.481 eV.

Sodium iron phosphate (NaFePO₄) has emerged as a promising cathode material for sodium-ion batteries (SIBs) due to its cost-effectiveness, environmental sustainability, and structural similarity to the well-established lithium iron phosphate (LiFePO₄) used in commercial lithium-ion batteries. The triphylite phase of NaFePO₄ offers a theoretical capacity of 154 mAh/g, making it an attractive candidate for large-scale energy storage applications. This study presents a scalable and economical synthesis route for producing triphylite NaFePO₄ through a two-step conversion process involving chemical delithiation of commercial LiFePO₄ followed by sodiation. Structural and morphological characterizations using X-ray diffraction (XRD), attenuated total reflectance (ATR) spectroscopy, and field-emission scanning electron microscopy (FESEM) confirmed the successful formation of phase-pure triphylite NaFePO₄ with an average crystallite size of 25 nm and flake-like morphology (~60 nm thickness). Electrochemical performance evaluation in half-cell configurations demonstrated a reversible capacity of 42 mAh/g after 100 cycles at 100 mA/g, with 91% capacity retention and near-100% Coulombic efficiency. Rate capability tests revealed stable performance across varying current densities (50–2000 mA/g), with capacity recovery to 91% upon returning to 50 mA/g. The low charge transfer resistance and structural stability of NaFePO₄ underscore its suitability for SIB applications. This work highlights a facile, scalable synthesis method that leverages existing LiFePO₄ infrastructure, offering a viable pathway for commercialization. The findings contribute to advancing sustainable and cost-efficient cathode materials for next-generation sodium-ion batteries.

An O3-type NaNi0.5Mn0.3Ti0.2O2 compound as new cathode material for room-temperature sodium-ion batteries

NASICON-structured Na3V2(PO4)3 (NVP) is one of the most promising cathode material for rechargeable sodium-ion batteries. NVP is characterized by a robust 3D structural framework and a high operating potential; these properties have enabled it to be widely studied as a stable and high-energy density cathode material for sodium-ion batteries (SIBs).1-4 In the present study, we designed a Na3V1.6Cr0.4(PO4)3/C (NVCrP@C) cathode by implanting Cr into the crystal structure of NVP and simultaneously coating the surface of NVP with carbon for realizing high power density SIBs. NVP is fabricated using a low-cost pyro synthesis technique with the advantage of self-carbon-coating and nano sized particles are gained through this facile technique. The substitution of Cr with vanadium in the NVP structure significantly enhanced the structural stability of the electrode while the uniform and thin carbon layer improved the electrical conductivity. Interestingly, the NVCrP@C cathode showed high electrochemical activities with multiple V3+/4+/5+ redox reactions triggered by Cr3+ substitution in a wide voltage range (2.5–4.1 V). Consequently, the NVCrP@C cathode delivered excellent cycling stability over 500 cycles even at 15 C-rate and power capability up to 70 C-rate. References Y. Cao, L. Xiao, W. Wang, D. Choi, Z. Nie, J. Yu, L. V. Saraf, Z. Yang and J. Liu, Mater., 2011, 23, 3155-3160.M. Giot, L. C. Chapon, J. Androulakis, M. A. Green, P. G. Radaelli and A. Lappas, Rev. Lett., 2007, 99, 247211 M. H. Han, E. Gonzalo, G. Singh and T. Rojo, Energy Environ. Sci., 2015, 8, 81-102. H. Liu, J. Xu, C. Ma and Y. S. Meng, Chem. Comm., 2015, 51, 4693-4696

P2-type Na0.66Ni0.33–xZnxMn0.67O2 as new high-voltage cathode materials for sodium-ion batteries

P2-Na2/3[Fe1/2Mn1/2]O2 layered oxide is a promising high energy density cathode material for sodium-ion batteries. However, one of its drawbacks is the poor long-term stability in the operating voltage window of 1.5–4.25 V vs Na+/Na that prevents its commercialization. In this work, additional light is shed on the origin of capacity fading, which has been analyzed using a combination of experimental techniques and theoretical methods. Electrochemical impedance spectroscopy has been performed on P2-Na2/3[Fe1/2Mn1/2]O2 half-cells operating in two different working voltage windows, one allowing and one preventing the high voltage phase transition occurring in P2-Na2/3[Fe1/2Mn1/2]O2 above 4.0 V vs Na+/Na; so as to unveil the transport properties at different states of charge and correlate them with the existing phases in P2-Na2/3[Fe1/2Mn1/2]O2. Supporting X-ray photoelectron spectroscopy experiments to elucidate the surface properties along with theoretical calculations have concluded that the formed electrode-electrolyte interphase is very thin and stable, mainly composed by inorganic species, and reveal that the structural phase transition at high voltage from P2- to “Z”/OP4-oxygen stacking is associated with a drastic increased in the bulk electronic resistance of P2-Na2/3[Fe1/2Mn1/2]O2 electrodes which is one of the causes of the observed capacity fading.

Nanoscale surface modification of P2-type Na0.65[Mn0.70Ni0.16Co0.14]O2 cathode material for high-performance sodium-ion batteries