Sodium Niobate with a Large Interlayer Spacing: A Fast‐Charging, Long‐Life, and Low‐Temperature Friendly Lithium‐Storage Material

Abstract Niobate Li+‐storage anode materials with shear ReO3 crystal structures have attracted intensive attention due to their inherent safety and large capacities. However, they generally suffer from limited rate performance, cyclic stability, and temperature adaptability, which are rooted in their insufficient interlayer spacings. Here, sodium niobate (NaNb13O33) micron‐sized particles are developed as a new anode material owning the largest interlayer spacing among the known shear ReO3‐type niobates. The large interlayer spacing of NaNb13O33 enables very fast Li+ diffusivity, remarkably contributing to its superior rate performance with a 2500 to 125 mA g−1 capacity percentage of 63.2%. Moreover, its large interlayer spacing increases the volume‐accommodation capability during lithiation, allowing small unit‐cell‐volume variations (maximum 6.02%), which leads to its outstanding cyclic stability with 87.9% capacity retention after as long as 5000 cycles at 2500 mA g−1. Its cyclic stability is the best in the research field of niobate micron‐sized particles, and comparable to that of “zero‐strain” Li4Ti5O12. At a low temperature of −10 °C, it also exhibits high rate performance with a 1250 to 125 mA g−1 capacity percentage of 65.6%, and even better cyclic stability with 105.4% capacity retention after 5000 cycles at 1250 mA g−1. These comprehensively good electrochemical results pave the way for the practical application of NaNb13O33 in high‐performance Li+ storage.


Introduction
Lithium-ion batteries (LIBs) have been widely used for various devices and equipment, such as battery electric vehicles (BEVs), [1] which require higherperformance electrode materials. [2] The traditional and cheap graphite anode material shows a large theoretical capacity of 372 mAh g −1 and practical capacity of 300-360 mAh g −1 . [3] However, its wide applications are limited by its unsatisfactory rate and safety performance. [4] The popular Li 4 Ti 5 O 12 with a plateau at 1.55 V (vs Li/Li + ) serves as a safe anode material, but suffers from its small capacity (theoretically 175 mAh g −1 and practically ≈170 mAh g −1 ). [5] In addition, both materials deliver poor low-temperature electrochemical properties, limiting their applications in high latitudes, high altitudes, and winter periods. [6] Thus, it becomes highly desirable to explore new anode materials with comprehensively good electrochemical properties and broad temperature adaptability. Recently, niobates have been regarded as promising anode materials with high performance. [7] Nb is not a rare metal, and its amount in the earth's crust is comparable to that of Li and Pb. [8] The active Nb 4+ /Nb 5+ and Nb 3+ /Nb 4+ redox couples enable not only safe operating potentials but also large theoretical capacities. Moreover, the large anion versus cation ratios in niobates lead to open crystal structures, such as shear ReO 3 crystal structures. The structural units of the shear ReO 3 structures are generally constructed by corner-sharing octahedron-blocks, which are connected to neighboring blocks via octahedron edgesharing. [9] Such edge-sharing can stabilize the formed A-B-A interlayer structure during Li + insertion. However, the interlayer spacings of previously-reported shear ReO 3 -type niobates are insufficient. For instance, the popular TiNb 2 O 7 has an interlayer spacing of only 3.7989 Å, which limits its Li + transport and thus rate performance. [10] In addition, the inferior volume-buffering capability rooted in its smaller interlayer spacing leads to relatively large unit-cell-volume expansion (7.22%) after lithiation to 1.0 V and thus insufficient cyclic stability. [11] These issues hinder its Li + -storage applications, especially when its electrode loading is large. The popular nanostructure construction is an extrinsic solution that can tackle the above two issues, because nanomaterials have not only small primary particles with small Li + transport lengths but also abundant nanopores for accommodating primary-particle expansion. [12] Unfortunately, nanomaterials are generally of high production cost and low tap densities. Clearly, a better and intrinsic solution is to explore new niobates with large interlayer spacings.
Here, we design and explore sodium niobate (NaNb 13 O 33 , theoretical capacity: 396 mAh g −1 based on Nb 5+ ↔Nb 3+ ) as a new niobate for Li + storage, which owns the largest interlayer spacing (3.8484 Å) among those of previously-reported shear ReO 3type niobates. Thus, the Li + diffusivity can be enhanced and the maximum unit-cell-volume variation can be decreased, [13] benefiting not only the rate performance and cyclic stability but also the low-temperature electrochemical properties. In this work, we successfully prepare pure NaNb 13 O 33 micron-sized particles through a two-step solid-state reaction method. The Li + -storage properties and mechanisms of NaNb 13 O 33 at room and low temperatures are intensively studied. At 25°C, NaNb 13 O 33 delivers very fast Li + diffusivity, remarkably contributing to its outstanding rate performance with a 1250 mA g −1 /2500 mA g −1 to 125 mA g −1 capacity percentage of 71.8%/63.2%. Very excitingly, NaNb 13 O 33 exhibits superior cyclic stability with 87.9% capacity retention after as long as 5000 cycles at 2500 mA g −1 owing to its maximum unit-cell-volume variation of only 6.02% at 0.8 V. At −10°C, NaNb 13 O 33 also delivers high rate performance with a 1250 to 125 mA g −1 capacity percentage of 65.6%, and superior cyclic stability with 105.4% capacity retention after 5000 cycles at 1250 mA g −1 . Hence, NaNb 13 O 33 can be an ideal anode material for LIBs of BEVs, even when its particles are micron-sized.

Physico-Chemical Characterizations
The Rietveld-refined X-ray diffraction (XRD) pattern (Figure 1a) matches well with monoclinic NaNb 13  ac-planes. [14] The interlayers share both octahedron corners and edges. The interlayers are connected through sharing octahedron edges. Such octahedron arrangement guarantees an A-B-A interlayered structure with high stability. Interestingly, this special arrangement is regularly interrupted by Na + . The NaO 12 decahedra serve as anchors connecting shear ac-planes by edgesharing, which further enhances the structural stability. The refinement results (Tables S1 and S2, Supporting Information) reveal the lattice constants of a = 22.49614(68) Å, b = 3.84842(10) Å, c = 15.42272(42) Å, and V = 1334.800(83) Å 3 . Very excitingly, NaNb 13 O 33 exhibits the largest b value among the known shear ReO 3 -type niobate materials ( Figure 1c; Table S3, Supporting Information), indicating its largest interlayer spacing (3.8484 Å), which is significantly larger than that of graphite (3.35 Å). [15] As a result, very fast Li + transport within the NaNb 13 O 33 lattice can be achieved since it is known that even tiny lattice enlargement in intercalating electrode materials can obviously increase their Li + diffusion coefficients. [16] The NaNb 13 O 33 sample shows micron-sized particles (1-6 μm, Figure 1d) and a small specific surface area (0.444 m 2 g −1 based on the Brunauer−Emmett−Teller (BET) model, Figure  S1, Supporting Information). Interestingly, there are abundant nanopores exist within the particles ( Figure S2, Supporting Information). The lattice-fringe spacing of NaNb 13 O 33 is determined to be 0.378 nm (Figure 1e), corresponding to its (110) plane. Its large interlayer spacing is directly revealed by its high-angle annular dark-field (HAADF)-scanning transmission electron microscopy (STEM) image ( Figure S3, Supporting Information). Its regular electron-diffraction spots (Figure 1e inset) match with its (110), (111), and (221) planes, which verifies its monoclinic structure and space group of C2/m. The element distributions of Na, Nb, and O in NaNb 13 O 33 very homogeneous ( Figure S4, Supporting Information), confirming the high purity of NaNb 13 O 33 . The Na:Nb molar ratio in NaNb 13 O 33 is determined to be 0.96:13 by the X-ray fluorescence (XRF) test, which matches well with its theoretical ratio (1:13).

Temperature-Dependent Li + -Storage Properties
At 25°C, the galvanostatic charge-discharge (GCD) curves of NaNb 13 O 33 reveal a large first-cycle discharge/charge capacity of 238.5/221.6 mAh g −1 with a high Coulombic efficiency of 92.9% at 25 mA g −1 (Figure 2a). The average operating potential is ≈1.52 V during lithiation-delithiation, which is slightly lower than that of the popular Li 4 Ti 5 O 12 (1.55 V). [5] When increasing the current rate to 125, 250, 500, 1250, and 2500 mA g −1 , NaNb 13 O 33 is capable of retaining large reversible capacities of 190.1, 175.0, 159.0, 136.4, and 120.1 mAh g −1 , respectively (Figure 2b,c), indicating its outstanding rate performance with a large 1250 mA g −1 /2500 mA g −1 to 125 mA g −1 capacity percentage of 71.8%/63.2%. When turning the current rate back to 125 mA g −1 , the capacity has no fading. NaNb 13 O 33 exhibits good cyclic stability with 92.0% capacity retention after 200 cycles at 250 mA g −1 ( Figure S5, Supporting Information). Ultra-longterm cycling tests are further performed at 2500 mA g −1 , showing 87.9% retention after 5000 cycles (Figure 2d), which is the best result obtained from the known shear ReO 3 -type niobates (Table  S4, Supporting Information).
At a low temperature of −10°C, the average operating potential of NaNb 13 O 33 slightly increases to 1.59 V (Figure 2e). Its reversible capacity at 25 mA g −1 is 176.1 mAh g −1 (Figure 2e), retaining up to 79.5% of that at 25°C. The first-cycle Coulombic efficiency obviously increases to 96.1% (Figure 2e), which is originated from the formation of thinner solid-electrolyte interphase (SEI) films at −10°C ( Figure S6, Supporting Information). [17] At 125, 250, 500, and 1250 mA g −1 , 151.0, 138.7, 124.8, and 99.0 mAh g −1 remain, respectively (Figure 2f,g). The 1250 to 125 mA g −1 capacity percentage at −10°C reaches 65.6%, only slightly smaller than that at 25°C (71.8%). This rate performance is remarkably higher than that of graphite with 5C to 0.5C capacity percentages of only 27.8% at −10°C, [13] and that of Li 4 Ti 5 O 12 nano-sized particles with no capacity at 5C and −10°C. [18] Moreover, NaNb 13 O 33 still delivers excellent cyclic stability at −10°C without any capacity decay (105.4% capacity retention) after 5000 cycles at 1250 mA g −1 (Figure 2h). This slight capacity increase could be rooted in the gradual NaNb 13 O 33 activation because the full electrolyte soaking needs relatively long time at the low temperature. [19] To demonstrate the practicability of NaNb 13 O 33 , the NaNb 13 O 33 working electrode is coupled with a LiFePO 4 cathode. At 25 mA g −1 , the LiFePO 4 /NaNb 13 O 33 full cell shows a reversible capacity of 186.8 mAh g −1 (Figure 2i). At 125, 250, 500, and 1250 mA g −1 , 154.4, 134.6, 110.3, and 75.0 mAh g −1 remain, respectively (Figure 2j,k). It exhibits 95.3% capacity retention over 1000 cycles at 500 mA g −1 (Figure 2l). All the half-and fullcell electrochemical data clearly reveal that NaNb 13 O 33 can be a practical Li + -storage material with a proper operating potential, high first-cycle Coulombic efficiency, large reversible capacity, outstanding rate performance, excellent cyclic stability, and good low-temperature electrochemical properties, which is especially suitable for the high-performance LIBs used in electric logistics vehicles.
The Cyclic voltammogram (CV) experiments at different temperatures are performed on the half cells, revealing the redox mechanism of NaNb 13 O 33 . At 25°C and 0.2 mV s −1 (Figure 3d), the first-cycle CV curves are slightly different from that in the subsequent cycle, which can be rooted in the formation of thin SEI films and the irreversible polarization during the first cycle. [21] After the first cycle, however, the CV curves display good repeatability. The second cycle exist three obvious CV-peak pairs centered at 1.62/1.73, 1.55/1.67, and 1.36/1.47 V, respectively. Both the first and second pairs could be assigned to the Nb 4+ /Nb 5+ redox reaction. [22] This peak split for Nb 4+ /Nb 5+ could be ascribed to the Li + insertion into different lattice sites. The third pair could correspond to the Nb 3+ /Nb 4+ redox couple. [22] At −10°C (Figure 3g), the positions of the cathodic/anodic peaks for Nb 4+ /Nb 5+ shift to 1.29/1.91 V in the first cycle, indicating increased electrode polarization than that at 25°C, as expected. The high-potential peaks do not obviously split and the low-potential peaks become weak, which could be rooted in the smaller capacity at the low temperature.
At different sweep rates and temperatures (Figure 3e,h), the CV-peak current (I) keeps increasing with the sweep rate (v), which can conform to the equation of I = av b , [23] in which a and b (0.5≤b≤1) are adjustable parameters. A larger b value indicates more significant capacitive behavior, which enables faster charge transport because the capacitive behavior is not determined by solid-state diffusion. [24] The b values for the intensive anodic and cathodic peaks are respectively determined to be 0.582 and 0.662 at 25°C, and those at −10°C remarkably increase to 0.839 and 0.842 (Figure 3f), suggesting that the intercalationpseudocapacitive behavior in NaNb 13 O 33 is significant at 25°C and becomes much more significant at −10°C due to the undoubtedly slower low-temperature Li + diffusivity (as described below). This phenomenon can be attributed to its very large in-terlayer spacing, [16a] similar to the case of T-Nb 2 O 5 . [23b] Undoubtedly, the existence of the intercalation-pseudocapacitive behavior in NaNb 13 O 33 benefits its electrochemical kinetics and Li +storage properties (especially at the low temperature).
The Li + apparent diffusion coefficients (D Li ) of NaNb 13 O 33 at different temperatures are calculated through different www.advancedsciencenews.com www.advancedscience.com methods. [25] At 25°C, its average D Li values calculated from the galvanostatic intermittent titration technique (GITT) data reach 6.00 × 10 −11 and 5.70 × 10 −11 cm 2 s −1 during lithiation and delithiation, respectively (Figure 3i, calculation details in Supporting Information), which are among the best results in the previously-reported shear ReO 3 -type niobates (Table S5, Supporting Information). Although the temperature significantly drops to −10°C, its D Li values still retain 1.60 × 10 (lithiation) and 2.30 × 10 −11 cm 2 s −1 (delithiation). It is noteworthy that this Li + diffusivity of NaNb 13 O 33 at such low temperature is even faster than that of most shear ReO 3 -type niobates at 25°C (Table S5, Supporting Information). Similar Li + -diffusivity results are achieved through the CV method ( Figure S9, calculation details in Supporting Information). The DFT calculations reveal 3D Li + transport pathways for fast Li + transport with a maximum energy barrier of only ≈0.5 eV ( Figure S10, calculation details in Supporting Information). Clearly, the very fast Li + diffusivity in NaNb 13 O 33 undoubtedly arises from its open shear ReO 3 -type interlayered structure with the very large interlayer spacing. [16a] In summary, the intrinsically fast Li + diffusivity and significant intercalation-pseudocapacitive behavior in NaNb 13 O 33 work together, achieving its prominent rate performance at 25 and −10°C.
With increasing the active-material loading of the working electrodes respectively by two and four times, the reversible capacity of NaNb 13 O 33 is not obviously decreased at both low (125 mA g −1 ) and high (1250 mA g −1 ) current rates (Figure 3j). The thick electrode (5.0 mg cm −2 ) shows much higher rate performance than commercial graphite ( Figure S11, Supporting Information), and is still capable of delivering excellent cyclic stability with 88.1% capacity retention at 1250 mA g −1 after 500 cycles (Figure 3k), even though the particle sizes of NaNb 13 O 33 are on the order of micrometers. Surprisingly, this retention percentage of NaNb 13 O 33 is significantly higher than that of niobates with similar active-material loadings and micron-sized particles ( In addition, this cyclic stability of NaNb 13 O 33 is also better than that of T-Nb 2 O 5 ( Figure S13, Supporting Information), and even comparable to that of "zero-strain" Li 4 Ti 5 O 12 ( Figure S14, Supporting Information). Therefore, the superior kinetics and practicability of NaNb 13 O 33 are further confirmed. Figure 4a shows the first-four-cycle pristine and contour in situ XRD patterns collected at 25°C and 125 mA g −1 . During the first lithiation, NaNb 13 O 33 undergoes three obvious phase transformations when lithiation to ≈1.6, ≈1.5, and ≈1.3 V. Before the first phase transformation, the (110), (204), and (602) peaks shift toward smaller angles, and their intensity changes are not obvious. During the first phase transformation, the (110) peak shifts from 23.3°to 23.2°, whereas the (204) and (602) peaks shift toward larger angles, and their peak intensities significantly decrease. Between the first and second phase transformations, the (110) peak continues shifting toward smaller angles, whereas the (204) and (602) peaks continue shifting toward larger angles, and their peak intensities slowly decrease. During the second phase transformation, the (110) peak shifts from 22.6°to 22.2°a nd the (602) peak shifts from 26.2°to 26.4°, whereas the (204) peak shifts slightly toward larger angles, and their peak intensities significantly decrease. Between the second and third phase transformations, the (110) and (204) peaks shift toward smaller angles, whereas the (602) peak slowly shifts toward larger angles, and their peak intensities first increase and then decrease gradually. During the third phase transformation, the (110) and (204) peaks continue shifting toward smaller angles, whereas the (602) peak continues shifting toward larger angles, and their peak widths and intensities significantly increase and decrease, respectively. [26] These complicated evolutions of the peaks significantly reverse during the subsequent delithiation. However, the delithiation and lithiation processes are not completely reversible (dotted squares in Figure S15, Supporting Information) because it is possible that the Li + -extraction sequence may not fully reverse the Li + -insertion one. The (110) peak-intensity change in the second lithiation is slightly different from that in the first lithiation during the first phase transformation (dotted circles in Figure  S15, Supporting Information), which matches with the CV results. The peak evolutions in the following cycles, however, are almost the same as those in the second cycle, confirming the superior crystal-structure stability of the intercalating NaNb 13 O 33 .

Crystal-Structure Evolutions
During lithiation, the Rietveld-refined a, b, c, and values of NaNb 13 O 33 gradually increase before the first phase transformation (Figure 4b). As a result, the V value increases by 2.68%. The first transformed phase reveals slightly smaller a, c, and V (0.41% shrinkage) values, and slightly larger b and values. Between the first and second phase transformations, the a and c values decrease gradually, and the b and values increase gradually. Thus, the V value increases by 1.63%. The second transformed phase exhibits slightly smaller a, c, , and V (0.76% shrinkage) values, and a slightly larger b value. Between the second and third phase transformations, the a, b, and c values increase gradually, and the value decreases gradually. Consequently, the V value increases by 1.15%. The third transformed phase shows slightly larger a, b, and c values, and a slightly smaller value. Therefore, the V value continues increasing until 0.8 V (1.73%). Importantly, the lattice-constant variations during each phase transformation are small, thereby allowing the achievement of the high rate performance, which is similar to the case of Li 4 Ti 5 O 12 with Li 4 Ti 5 O 12 ↔Li 7 Ti 5 O 12 phase transformation. [5] The lattice-constant variations during delithiation do not fully reverse those during lithiation, and the a-, c-, and -value variations during the first phase transformation in the last three lithiation processes are slightly different from those in the first lithiation process, matching well with the peak variations. The total V-value variation of NaNb 13 O 33 is limited to 6.02% at 0.8 V. These lattice-constant variations are verified by the ex situ highresolution transmission electron microscopy (HRTEM) results ( Figure S16, Supporting Information). This maximum unit-cellvolume variation of NaNb 13 O 33 is remarkably smaller than that of the previously-reported shear ReO 3 -type niobates (Table S6, Supporting Information), undoubtedly leading to the excellent cyclic stability of NaNb 13 O 33 .
At −10°C, however, NaNb 13 O 33 only undergoes two obvious phase transformations respectively at ≈1.4 and ≈1.3 V during  lithiation (Figure 4c). The third phase transformation does not occur (dotted circles in Figure S17, Supporting Information), attributed to the smaller inserted-Li + amount at −10°C. In addition, the second phase transformation process is prolonged, as revealed by the dotted rectangles in Figure S17 (Supporting Information), which could be ascribed to the slower Li + diffusivity at −10°C. The a-, c-, and -value variations are similar to those at 25°C, whereas the b-and V-value changes are significantly smaller (Figure 4d). The maximum V-value variation of NaNb 13 O 33 is significantly limited to only 4.12% at −10°C. This V-value decrease is reasonable owing to the smaller Li + -storage amount at −10°C, and undoubted results in the even better cyclic stability at −10°C.
The HAADF-STEM image (Figure 5a) demonstrates that the synthesized NaNb 13 O 33 owns excellent structural stability during lithiation. The integrated differential phase contrast (iDPC)-STEM image (Figure 5b) of the lithiated NaNb 13 O 33 reveals the inserted Li + ions, as highlighted by green balls. Additionally, the in situ XRD results (Figure 4b,d) indicate that the V-value variation is mainly determined by the b-value variation. It can therefore be concluded that the external Li + ions mainly insert into the interstices between the interlayers of NaNb 13 O 33 .
To directly observe the lithiation process of NaNb 13 O 33 , in situ transmission electron microscopy (in situ TEM) is used based on a micron-sized solid-state cell, [27] which is built by sandwiching the Li 6. 4

Conclusion
NaNb 13 O 33 micron-sized particles are developed as a practical anode material with comprehensively good electrochemical properties and broad temperature adaptability. The largest interlayer spacing (3.8484 Å) in this new shear ReO 3 -type niobate greatly benefits its rate performance, cyclic stability, and lowtemperature electrochemical properties. The active Nb 5+ ↔Nb 3+ redox reaction in NaNb 13 O 33 is highly reversible, allowing its large reversible capacities of 221.6 (25°C) and 176.1 mAh g −1 (−10°C), high first-cycle Coulombic efficiencies of 92.9% (25°C) and 96.1% (−10°C), and safe operating potentials of ≈1.52 (25°C) and ≈1.59 V (−10°C) at 25 mA g −1 . Its large interlayer spacing allows notable intercalation-pseudocapacitive behavior and very fast Li + diffusivity. These advantages are significantly beneficial for its rate performance, which shows a large www.advancedsciencenews.com www.advancedscience.com 1250 to 125 mA g −1 capacity percentage of 71.8% and 2500 to 1250 mA g −1 percentage of 63.2% at 25°C, and a 1250 to 125 mA g −1 percentage of 65.6% at −10°C. The stable shear ReO 3 -type structure with the large interlayer spacing limits the maximum unit-cell-volume expansions to 6.02% (25°C) and 4.12% (−10°C) after the majority of the external Li + ions insert into the interstices between the interlayers, enabling desirable cyclic stability with high capacity retentions of 87.9% (25°C and 2500 mA g −1 ) and 105.4% (−10°C and 1250 mA g −1 ) after 5000 cycles. The insight gained here can promote the exploration of fast-charging, long-life, and low-temperature friendly energystorage materials.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.