High‐power and low‐cost sodium‐ion batteries with a wide operation temperature from −70 °C to 130 °C

Low‐cost sodium‐ion batteries (SIBs) are promising candidates for grid‐scale energy‐storage systems, and the wide‐temperature operations of SIBs are highly demanded to accommodate extreme weather. Herein, a low‐cost SIB is fabricated with a Na4Fe3(PO4)2P2O7 (NFPP) cathode, a natural graphite (NG) anode, and an ether‐based electrolyte. The prepared NG//NFPP batteries deliver a long lifespan of 1000 cycles, high‐power density of 5938 W/kg, and remarkable rate performance of 10 A/g with a high capacity retention of 60%. Benefiting from the solvent co‐intercalation process of the NG anode and the high Na+ diffusion rate of the NFPP cathode, the NG//NFPP battery displays outstanding performance at −40 °C and even can work at an ultralow temperature of −70 °C. Furthermore, the high boiling point of the electrolytes and high thermal stability of the electrode materials also enable the high‐temperature operation of the full battery up to 130 °C. This work will guide the design of the wide‐temperature SIBs.


| INTRODUCTION
2][3][4][5] However, the limited reserves of lithium hinder the development of the LIBs.7][18][19][20][21][22][23][24] Moreover, to integrate the intermittent renewables into the electric grid, the SIBs with high power and low cost are highly demanded. 25In addition, the electrochemical performance of SIBs including capacity, efficiency, and energy/power density will deteriorate obviously when the temperature decreases/increases. [26][27][28] Accordingly, the wide-temperature operation of SIBs is essential for satisfying the demand for practical applications in extreme weather and different regions (e.g., high-altitude and tropical zones). 29,302][33][34][35][36] Disappointingly, the exploration of full batteries with outstanding widetemperature operation is rarely reported up to the present.
Inevitably, the operation temperature significantly impacts the electrochemical performance of the SIBs.When the operation temperature decreases below 0°C, the battery suffers from a slower Na + diffusion rate in the electrode materials and a sluggish desolvation process of solvated-Na + at the electrode/electrolyte interface, which results in serious polarization of the electrode reactions.2][43][44] In hightemperature conditions, the ion diffusion rate increases, which enables fast reaction kinetics and excellent rate performance.However, the thermal stability of the electrolytes, the electrode materials, the solid electrolyte interphase (SEI) and cathode electrolyte interphase (CEI) poses a huge challenge for the exploration of high-temperature operation.6][47] Therefore, great efforts should be devoted to explore the SIBs, which can be operated in a wide temperature range.
It is well known that graphite is the most common anode material for LIBs with the advantages of low cost and abundant resources.In an ether-based electrolyte, the natural graphite (NG) anode experiences a co-intercalation of Na + and solvent molecules without the desolvation process, [48][49][50][51] which not only enables fast reaction kinetics but also shows the potential for low-temperature operation.At the same time, for Na + storage, the graphite anode delivers an average discharge platform of about 0.6 V versus Na/Na + , which can effectively avoid the risk of Na-plating with the increasing polarization at low-temperature conditions.Moreover, the excellent thermal stability of graphite materials is beneficial for the high-temperature operation. 52The polyanionic Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 (NFPP) with 3D Na + diffusion pathways and excellent cycle stability is considered the promising cathode materials. 53The Fe-based NFPP cathode materials also possess the low-cost feature.In previous work, we have proved the high Na + diffusion coefficients of the NFPP cathode at low-temperature conditions. 54However, the high-temperature behavior of the NFPP cathode is required to explore in depth.
In this study, we successfully fabricated a highpower sodium-ion full battery with low-cost NFPP and NG as the cathode and anode, respectively.It is demonstrated that both the NFPP cathode and NG anode exhibit fast reaction kinetics and long-term cycle stability.The prepared NG//NFPP batteries display long-term cycle stability for 1000 cycles with a highcapacity retention of 81% and outstanding rate performance.The excellent cycle stability of the NG// NFPP battery can be maintained even with high massloaded electrodes (16 mg NFPP /cm and 9 mg NG /cm).Moreover, the desolvation-free process of the NG anode and high Na + diffusion coefficients of the NFPP cathode enable the excellent low-temperature performance of the NG//NFPP battery.Especially, combined with the selected electrolyte (1 mol/L NaPF6 in diglyme) with a high boiling point (162°C), the highest operating temperature of the NG//NFPP battery can reach 130°C.Differential scanning calorimetry (DSC) and X-ray photoelectron spectroscopy (XPS) have also been used to explore the reasons for cyclic instability at high temperatures.This provides a reference for the design of a high-temperature battery.

| RESULT AND DISCUSSION
We synthesized carbon-coated NFPP modified by carbon nanotubes using a conventional sol-gel method.The XRD pattern, FT-IR spectrum, and TG curve of the NFPP are shown in Supporting Information: Figure S1.The carbon content of NFPP is 13 wt%.The NFPP displays a microsphere-like morphology, which is composed of many stacked microsheets (Supporting Information: S2A-C).The high-resolution transmission electron microscopy image of Supporting Information: Figure S2D shows clear lattice fringes of d = 0.33 nm, which corresponds to the (202) crystal planes of NFPP.In Supporting Information: Figure S3, the NG shows a flake structure of 1-4 μm.Before assembling the full battery, the electrochemical performance of the cathode and anode was first investigated by a half battery (Na//NFPP and Na// NG) in an ether-based electrolyte (i.e., 1 mol/L NaPF 6 dissolved in diglyme).As shown in Figure 1A, the Na// NFPP battery displays two obvious charge/discharge platforms and the discharge capacity of 89.5 mAh/g at 200 mA/g.The Na//NFPP battery delivers a reversible discharge capacity of around 86 mAh/g at 200 mA/g for 200 cycles (Figure 1B).In addition, the Na//NFPP battery shows impressive rate performance from 0.05 to 10 A/g as given in Figure 1C and Supporting Information: Figure S4.What's more, NG was selected as the anode materials.In the first cycle, the Na//NG battery shows an average discharge voltage of 0.6 V and a coulombic efficiency of 76.9% (Figure 1D).The NG anode presents a discharge capacity of around 136 mAh/g at 200 mA/g and remarkable stability for 200 cycles (Figure 1E).Even at a high current density of 15 A/g, the NG anode delivers a discharge capacity of 110.1 mAh/g and ultrahigh capacity retention of 77% from 0.05 to 15 A/g, which indicates the outstanding rate property of the NG anode (Figure 1F).After the examination of the half battery, we fabricated the full battery (NG//NFPP) with an NFPP cathode, an NG anode, and the ether-based electrolyte (1 mol/L NaPF 6 in diglyme).The capacity and current density of the full battery are calculated according to the mass of NG in the anode.The cycle stability of the NG//NFPP battery was evaluated at the current density of 1.0 A/g.It can be detected from Figure 2A that the NG//NFPP battery can obtain a high-capacity retention of 81% after 1000 cycles.The remarkable stability benefits from the high stability of both the NFPP cathode and NG anode.The rate property of the NG//NFPP battery is investigated from 0.2 to 10 A/g (Figure 2B,C).The NG// NFPP battery delivers a discharge capacity of 153.2 mAh/g at 0.2 A/g.Even at an extremely high current density of 10 A/g, a discharge capacity of 90.8 mAh/g can be obtained with a capacity retention of 60%.Especially, only 32.6 s is required for the charging process to reach 60% capacity.The NG// NFPP battery exhibits extraordinary rate performance, which is ascribed to the 3D Na + diffusion pathways of the NFPP cathode and the Na + -solvent co-intercalation process of the NG anode enabling the fast reaction kinetics of both electrodes.Based on the data given in Figure 2B,C, the energy/power density of the NG//NFPP battery is calculated based on the total mass of NG and NFPP.As the Ragone plot presents in Supporting Information: Figure S7, the fabricated NG//NFPP battery delivers a considerable power density of 5938 W/kg at an energy density of 54 Wh/kg.And a high energy density of 119 Wh/kg is obtained at a power density of 153 W/kg.To investigate the practical application potential, we assembled the NG//NFPP battery with a high mass loading of 16 mg NFPP /cm and 9 mg NG /cm.At a current density of 200 mA/g, the high mass loaded NG//NFPP battery exhibits a long lifespan for 250 cycles with a high capacity retention of 86%.The corresponding charge/ discharge curves are shown in Supporting Information: Figure S8.This high mass loaded full battery satisfies the requirement of large energy storage.In the full battery, the N/P value is 0.8.Graphite is a great host for sodium ions in ether electrolytes.Excess sodium ions are also stored by graphite, so the full batteries exhibit a higher capacity than the half batteries.
In ether-based electrolytes, the solvent co-intercalation process of Na + in graphite electrodes has been verified in the previous reports, [48][49][50][51] which benefits the low-temperature operation of the graphite electrode.The low-temperature performance of the NFPP cathode has been verified in our previous work. 54In combination with the ether-based electrolyte with a low freezing point, we speculated that the NG//NFPP battery can exhibit excellent performance at low temperatures.We investigated the boiling point (162°C) and freezing point (−71°C) of the selected electrolyte by DSC measurement (Figure 3A).The exothermic peak near −100°C is a supercooled peak caused by rapid cooling during DSC measurements.Moreover, the ionic conductivities of the ether-based electrolyte are calculated to be 6.87, 3.16, 0.42, and 0.11 mS/cm at 25°C, −20°C, −40°C, and −60°C, respectively (Figure 3B and Supporting Information: Figure S9).These results indicate that the NaPF 6 -diglyme electrolyte is beneficial for the low-temperature operation of the full battery.The low-temperature performance of the NG//NFPP battery was investigated at different temperatures at a low rate of 10 mA/g.It can be clearly observed from Figure 3C   respectively.When the operation temperature decreases to −60°C (Figure 3D), the full battery can still maintain a discharge capacity of 94.4 mAh/g, around 60% of the roomtemperature capacity (Figure 3D and Supporting Information: S10).Especially, the NG//NFPP battery can work well with a discharge capacity of 61.9 mAh/g even at an ultralow temperature of −70°C (Figure 3D).It can be noticed that the charge/discharge curves for the NG//NFPP battery show obvious changes at low-temperature conditions.As shown in Supporting Information: Figure S11, at low temperatures, the charge/discharge curves of the anode change significantly leading to this phenomenon.We attempted to discern the contributions from intercalation and capacitive reactions in Na storage using cyclic voltammetry data at various scan rates from 0.2 to 1 mV/s (Supporting Information: Figure S12) through the following power-law relationship: low temperatures.Furthermore, the rate performance and cycle stability of the NG//NFPP battery at lowtemperature conditions were explored at −40°C.At −40°C, the NG//NFPP battery displays exceptional cycle stability within 120 cycles with a high-capacity retention of 92% (Figure 3E).Moreover, the excellent rate property of the NG//NFPP battery at −40°C can be observed in Figure 3F at current densities from 10 to 150 mA/g.The low-temperature performance of the NG//NFPP battery has been demonstrated, and then, the hightemperature operation of the NG/NFPP battery requires further investigation.First, as mentioned above, the selected NaPF 6 -diglyme electrolyte also shows a high boiling point of up to 162°C.Second, Figure 4A shows the DSC profiles of the NFPP cathode at the charged states (3.8 V) in the NaPF 6 -diglyme electrolyte.Significant exothermic reactions occur when the temperature is raised up to 169°C.The thermal stability of the NG anode at the discharged states (0 V) is shown in Figure 4B, and the Na-intercalated graphite starts to show a lot of exotherm at 197°C.The above results show that both cathode and anode materials are suitable for high-temperature operation.The high-temperature performance of the full battery was examined at a current density of 2 A/g. Figure 4C presents the galvanostatic charge/discharge curves of the NG//NFPP battery at 60°C and 80°C, in which it delivers discharge capacities of 144.8 and 145.1 mAh/g.When increasing to 100°C, this battery can maintain a discharge capacity of 141.6 mAh/g (Figure 4D).Even at an ultrahigh temperature of up to 130°C, the NG//NFPP battery can work efficiently with a discharge capacity of 129.8 mAh/g and a coulombic efficiency of 93%.The lower columbic efficiency is ascribed to the partial decomposition of the SEI/CEI and electrolyte at 130°C.In addition, the electrochemical behaviors of the NG//NFPP batteries were further investigated at 80°C.At 3 A/g, the full battery can cycle stably for 200 cycles (Figure 4E).The rate performance of the NG//NFPP battery at high-temperature conditions was verified at current densities from 2 to 40 A/g (Figure 4F and Supporting Information: Figure S13).The full battery delivers discharge capacities of 143.2, 116.7, 98.8, and 87.7 mAh/g at 2, 10, 20, and 30 A/g, respectively.Even at an ultrahigh rate of 40 A/g, it can maintain a capacity of 78.9 mAh/g, which indicates the outstanding rate behavior at high-temperature conditions.Accordingly, the NG//NFPP battery displays a considerable power density of 22,045.4W/ kg at an energy density of 44.1 Wh/kg (Supporting Information: Figure S14).Therefore, the fabricated NG// NFPP battery exhibits remarkable performance in a wideoperation temperature, which is ascribed to the fast reaction kinetics, the high stability of the electrodes, and the electrolyte with a high boiling point and low freezing point.[57][58][59][60][61][62][63][64][65][66] The DSC curves indicate that both cathode and anode materials are suitable for high-temperature operation.However, Figure 4E shows that the cycling stability of the full battery is not satisfactory.Therefore, we further investigated the state of the solid electrolyte interphase (CEI/SEI) on the electrode surface at different temperatures.Full batteries cycled twice at different temperatures were disassembled and the electrodes were used for XPS measurement.As shown in Figure 5A, the organic component content of CEI gradually increases during the temperature increase, which means the decomposition of the electrolyte solvent.When the temperature reaches 100°C, the inorganic component signal in the CEI is significantly decreased, which indicates the violent decomposition of the solvent.Since the electrode used for the XPS measurement was cycled only twice at the current temperature, the potential problems of solvent decomposition are not seriously accentuated.So, solvent decomposition may be the reason for the inferior cycling stability at 80°C.When the temperature reaches 130°C, the content of inorganic components (C=O) in CEI increases significantly, which implies the decomposition of organic components in CEI.The percentage of F-atoms on the electrode surface can also directly indicate the change in the content of inorganic components in CEI. Figure 5C shows the statistical results of F-atom content on the cathode surface (The corresponding F 1s spectra are shown in Supporting Information: Figure S15).The increased solvent decomposition with increasing temperature leads to a decrease in the relative content of F atoms in CEI.The relative content of F atoms in the CEI increases significantly when the temperature is increased to 130°C, which indicates the decomposition of organic components in the CEI.The above results maintain consistency with the results of O 1s spectra.In Figure 5B,D, the SEI on the anode surface shows a similar pattern.The corresponding F 1s spectra of Figure 5D are shown in Supporting Information: Figure S16.The above research shows the difficulties of designing high-temperature batteries, which include how to design a stable CEI/SEI in addition to considering the electrolyte boiling point and the stability of electrode materials at high temperatures.

| CONCLUSION
In summary, a low-cost sodium-ion full battery with an NFPP cathode, an NG anode, and a NaPF 6 -diglyme electrolyte has been successfully prepared to operate in a wide temperature range.The fast reaction kinetics and stability of both the NFPP cathode and NG anode enable the outstanding performance of the NG//NFPP battery, which displays a long lifespan of 1000 cycles, remarkable rate property of 10 A/g (60% capacity can be charged within only 32.6 s), and a high-power density of 5938 W/kg.In addition, the NaPF 6 -diglyme electrolyte possesses a high boiling point of 162°C and a low freezing point of −72°C.Benefitting from the solvent co-intercalation charge storage mechanism of the NG anode and the high Na + diffusion rate of the NFPP cathode, the NG//NFPP full batteries deliver excellent cycle and rate performance at −40°C and can work well at an ultralow temperature of −70°C.Moreover, the NG//NFPP full battery can operate at 80°C with a high-power density of 22045.4W/kg and a lifespan of 200 cycles.The full battery can also operate at an ultrahigh temperature of 130°C.Therefore, the full battery can be operated in a wide temperature range from −70°C to 130°C.This work provides a reference for the design of the low-cost sodium-ion full battery for wide-temperature operation.
The corresponding galvanostatic charge/discharge curves are shown in Supporting Information: Figure S5.The low resistance of the NG anode shown in Supporting Information: Figure S6 reflects the fast redox reaction at the surface of the NG electrode and the fast intercalation kinetics of the NG electrode in SIBs.
that the NG//NFPP battery delivers discharge capacities of 148.7 and 130.3 mAh/g at −20°C and −40°C,

F
I G U R E 2 (A) Cycle stability of the NG//NFPP battery at 1 A/g.(B) Galvanostatic charge/discharge curves of the NG//NFPP battery at various current densities.(C) Rate performance of the NG//NFPP battery.(D) Cycle stability of the NG//NFPP battery with high mass loading at 200 mA/g.NG, natural graphite; NFPP, Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 .
the current (A), ν is the scan rate (mV/s), and a and b are the adjustable values.The b value of C3 is significantly lower than those of C1, C2, and C4, indicating that the reaction is controlled by diffusion.The different b values imply differences in the reaction kinetics, which would lead to differences in capacity decay at low temperatures.This could be the reason for the change in the charge/discharge curves of the anode at F I G U R E 3 (A) DSC measurement of the electrolyte (1 mol/L NaPF 6 in diglyme).(B) Calculated temperature-dependent ionic conductivities of the electrolyte.Galvanostatic charge/discharge curves of the NG//NFPP battery at (C) −20°C, −40°C and (D) −60°C, −70°C at 10 mA/g.(E) Cycle stability and (F) rate performance of the NG//NFPP battery at −40°C.DSC, differential scanning calorimetry; NG, natural graphite; NFPP, Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 .

F
I G U R E 5 XPS measurements.The O 1s spectra of (A) NFPP electrodes and (B) NG electrodes after cycling at different temperatures.F-atom content on the surface of (C) NFPP electrodes and (D) NG electrodes.NG, natural graphite; NFPP, Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 ; XPS, X-ray photoelectron spectroscopy.