3.1. Oxide Compounds
Layered LiMO2 compounds have been extensively investigated as Li-intercalation materials. Hence, it is no surprise that similar NaMO2 compounds have been targeted as Na-intercalation electrodes.19, 60–63 Layered NaxMO2 possesses various crystal structures. In the notation of Delmas et al.,64 On (n = 1, 2, 3, etc.) refers to structures in which Na is octahedrally coordinated by oxygen, and n refers to the repeat period of the transition metal stacking. Na atoms can also occupy trigonal prismatic sites and these structures are noted as Pn.65 Phase transitions between Pn and On occur via the gliding of MO2 sheets. Such gliding of oxygen layers with respect to each other has been observed at room temperature. For example, O3-NaCoO2 can transform into a P'3 structure during electrochemical desodiation.60 However, phase transitions between Pn and Om (m ≠ n) need to break M-O bonds, and are only possible at high temperatures.66 Due to this competition between P and O type arrangements the desodiation behavior of layered Na-compounds is in many cases not fully topotactic. Very little is understood how these gliding transitions occur and what their effect on the kinetics of sodiation/desodiation is. As Li atoms usually do not occupy trigonal prismatic sites, these gliding transitions are not an issue in layered Li-intercalation compounds. But Na-compounds may also have advantages. In layered LixMO2 compounds the competition usually exists with structures in which the Li ion is tetrahedrally coordinated leading to the formation of spinel-like structures with accompanying loss in capacity.18, 67–69 Such a driving force towards spinel formation will less likely be present for Na-intercalation compounds, making it possible that they are stable over a larger concentration range of alkali.
In 1981 Delmas et al. showed that the phase transition of various NaxCoO2 structures occurred reversibly in electrochemical Na-cells, demonstrating the feasibility of NaxCoO2 as a positive electrode material.60 Various crystal structures of NaxCoO2 compounds are illustrated in Figure 7a–c. Among those, P2-Na0.70CoO2 showed the largest energy density (∼260 Wh kg−1) in NIB. Recently, P2-NaxCoO2 has been reinvestigated,70, 71 and found to reversibly operate between 0.45 ≤ x ≤ 0.90, when Na0.74CoO2 was used as the starting material. The existence of several well defined steps in the voltage profile indicated that various structures were formed during the cycling (Figure 8a). The formation of Na-vacancy ordered layer-structures was confirmed via in-situ X-ray diffraction study.70, 72 The feasibility of using P2-NaxCoO2 as the positive electrode in an all solid-state NIB was also recently demonstrated.71
Figure 7. Crystal structures of various NaxMOy: (a) P2-NaxCoO2, (b) O3-NaxCoO2, (c) P3-NaxCoO2, (d) Na0.44MnO2, and (e) Na0.33V2O5 (Na: yellow, Co/Mn/V: blue, O: red).
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Figure 8. Charge-discharge profiles of various oxide compounds: (a) (black) P2-NaxCoO2 and (blue) P2-NaxCo2/3Mn1/3O2, (Reproduced with permission.73 Copyright 2011, Royal Society of Chemistry.), (b) NaCrO2, (c) LiCrO2 (Reproduced with permission.18 Copyright 2010, Elsevier.) (d) Na1.0Li0.2Ni0.25Mn0.75O2.35 (Reproduced with permission.80), (e) Na0.44MnO2 (Reproduced with permission.62 Copyright 2007, American Chemical Society.), (f) NaMnO2, (Reproduced with permission.15 Copyright 2011, The Electrochemical Society.) (g) NaVO2, (h) Na0.7VO2 (Reproduced with permission.44 Copyright 2011, Elsevier.) and (i) Na0.33V2O5 (Reproduced with permission.96 Copyright 2010, Elsevier).
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Analogues of P2-NaxCoO2 were suggested including Mn-substituted P2-Na2/3Co2/3Mn1/3O2, in which Co3+ and Mn4+ coexist.73 While P2-Na2/3CoO2 showed several voltage steps in discharge, the Mn-substituted P2-Na2/3Co2/3Mn1/3O2 electrode showed only one step at Na1/2Co2/3Mn1/3O2 composition (Figure 8a). Similar to P2-Na2/3CoO2, P2-Na0.6MnO2, exhibited several voltage steps upon cycling.74 These P2-Na2/3Co1-xMnxO2 compounds all retained the P2 framework upon the cycling. On the other hand, P2-Na2/3Ni1/3Mn2/3O2 undergoes different phase transition.75 During Na extraction, O2-type stackings form in the P2-NaxNi1/3Mn2/3O2 when x reaches ∼1/3. P2-Na1/3Ni1/3Mn2/3O2 (with some O2-type stackings) and O2-Ni1/3Mn2/3O2 phases coexist when x is lower than 1/3. The original P2 phase was reversibly recovered after the discharge. All of these P2 compounds could deliver about 0.5 Na ions, resulting in a specific capacity of 120–150 mAh g−1.
Layered materials with composition NaMO2 seem to be more promising. NaMO2 compounds generally form an O3 structure as illustrated in Figure 7b.21, 61, 76 Early work on NaMO2 reported that only a small amount of Na ions (∼0.2) participated in the electrochemical reaction for NaCrO2 and NaNiO2 electrodes resulting in low specific capacities.61 However, when NaCrO2 was recently revisited, ∼120 mAh g−1 (∼0.5 Na) of capacity near 3 V was obtained (Figure 8b).18, 63 It is interesting that NaCrO2 shows much better performance than LiCrO2 in (<10 mAh g−1) as shown in Figure 8b-c. The authors of Ref 18 speculate that bonding distances strongly affect the reactivity of the two compounds. The size of the inter-slab interstitial tetrahedral sites in LiCrO2 lattice is well matched to the stable configuration of Cr4+O42− tetrahedral, hence Cr4+, formed during charge, easily migrates and gets trapped in the tetrahedral sites. However, the interstitial sites in NaCrO2 lattice cannot stabilize the Cr4+ due to large inter-slab distance and oxygen-oxygen bonding length preventing the migration of Cr4+.
In Li-cells, LiNi0.5Mn0.5O2 can deliver over 200 mAh g−1 at low rate,77, 78 and high rate LiNi0.5Mn0.5O2 can be made by ion-exchange from NaNi0.5Mn0.5O2.79 When directly testing in Na cells, the layered NaNi0.5Mn0.5O2 delivered 130 mAh g−1.38, 63 Almost no alkali-Ni disordering occurs in the Na version of the compound, in contrast to Ni-containing layered Li compounds which have considerable disorder. The better ordering of the Na compound is due to its larger ionic size.79 Electrochemical desodiation and subsequent lithiation of NaFeO2 was observed in a hybrid Li-cell.21 The reaction was reversible until charged to Na0.5FeO2, but irreversible reaction occurred with further electrochemical oxidation.
Recently, Kim et al. suggested Li-substituted Na1.0Li0.2Ni0.25Mn0.75O2.35 (or Na0.85Li0.17Ni0.21Mn0.64O2) as the positive electrode material.80 In this composition, the (Ni+Mn)/(Na+Li) ratio was 1/1.2, and thus, this compound can be regarded in the family of alkali-excess materials like Li1.2Ni0.2Mn0.6O2.67, 81, 82 Due to the difference in ionic radius of Na and Li, Na ions are located in the inter-slab spaces, while the excess-Li ions are located in the transition metal layers. The electrochemical reaction in these materials is fully supported by the two-electron reaction of the Ni2+/4+ redox couple with the oxidation state of Mn ions (+4) remaining unchanged which imparts stability to these materials. This behavior is frequently observed in Ni2+-Mn4+ containing compounds such as LiNi0.5Mn0.5O2 and Li1.2Ni0.2Mn0.6O2 electrodes in LIB.79, 83, 84 The specific capacity and average voltage was approximately 100 mAh g−1 and 3.4 V, respectively (Figure 8d). It was claimed that the excess Li ions stabilize the crystal structure upon cycling.
There exists a variety of stable manganese-based oxides, which are composed of one-, two-, or three-dimensionally connected sites for alkali metal intercalations.85 Tarascon et al. reported electrochemical Na intercalation into spinel λ-MnO2.86 When Na ions were inserted into λ-MnO2 at the first discharge, the spinel phase transformed to a new layered structure. After this irreversible phase transition, almost 0.6 Na ions could reversibly cycle. Co-substituted Mn2.2Co0.27O4 also showed electrochemical activity with up to one Na ion transfer.87 Orthorhombic Na0.44MnO2 (or Na4Mn9O18) possesses a 3D tunnel structure as illustrated in Figure 7d.62, 88–90 Figure 8e shows that reversible Na storage reaction takes place for 0.18 ≤ x ≤ 0.64 (∼140 mAh g−1) in NaxMnO2.62 By nanosizing Na0.44MnO2, promising electrochemical properties were obtained in both a Na-cell and full-cell with pyrolyzed carbon as a negative electrode.80 Stable electrochemical reaction in an aqueous electrolyte has also been demonstrated for this material.91 Recent results showed that O3-NaMnO2 exhibits 185 mAh g−1 discharge capacity for the first cycle at C/10 rate with 132 mAh g−1 remaining after 20 cycles (Figure 8f).15 O3-NaMnO2 shows remarkably different electrochemical properties from O3-LiMnO2, which transforms into a spinel-like structure with a rapid capacity decay during cycling.17, 92
O3-NaVO2 electrodes could deliver a specific capacity of ca. 120 mAh g−1 (∼0.5 Na) reversibly at 1.4–2.5 V.19 When approximately 0.9 Na was extracted with higher voltage (∼3 V), irreversible phase transition occurred and only a small amount of Na (∼0.4 Na) was re-inserted at subsequent discharge. Hamani et al. reported that Na0.7VO2 showed much less polarization than NaVO2 as shown in Figure 8g and h.44 The original structure of P2-Na0.7VO2 electrode was well retained during cycling without drastic structural change. High electronic conductivity of P2-Na0.7VO2 is believed as the origin of the reduced polarization while O3-NaVO2 is electronically insulating. NaxVO2 with low redox potential (<2.5 V) is sensitive to air exposure. NaVO2 readily decomposes to NaxVO2 and Na2O when exposed to air. However, V compounds with higher oxidation state (V4+/5+) such as V2O5, NaxV2O5, and Na1+xV3O8 are stable in air and can be operated above 3 V.93–96 Among those, Na0.33V2O5 (in Figure 7e) nanorods showed good electrochemical behavior with 142 mAh g−1 discharge capacity at 1.5–4.0 V range (Figure 8i).96
3.2. Polyanion Compounds
In the past decade, extensive research efforts have been focused on polyanion compounds for positive electrode materials in LIB.4, 97 Polyanion compounds provide some advantages.97 Various crystal structures with open channels for Na and Li ions are available.97 Since the operation voltage is influenced by local environments of polyanions, the operation voltage of a specific redox couple can be tailored.98 In addition, the polyanion compounds are believed to have high thermal stabilities due to strong covalent bonding of oxygen atom in the polyanion polyhedra, though this may not be universally true.99, 100 A recent high-throughput computational study evaluated on a large scale the properties of phosphate compounds for LIB.101 Some Na compounds with the polyanion frameworks have been studied to be used as structural motifs and starting materials for hybrid Li-cells, or to compare the electrochemical properties with isostructural Li compounds. Crystal structures of various polyanion compounds are described in Figure 9.
Figure 9. Crystal structures of various polyanion compounds: (a) Na3V2(PO4)3, (b) NaFePO4, (c) Na2FePO4F, and (d) NaFeSO4F (Na: yellow, V/Fe: blue, O: red, F: gray).
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NASICON, Na super-ionic conductor, whose general formula is AxMM’(XO4)3, was originally studied as a solid electrolytes that allows fast Na ion conduction through the empty space in its crystal structure.102–104 Corner-shared MO6 (or M'O6) and XO4 polyhedra form a framework with large Na diffusion channels. NASICON compounds were studied as solid electrolytes in Na-S batteries for a long time. In 1987 and 1988, Delmas et al. demonstrated that NASICON-type compounds, NaTi2(PO4)3, can be electrochemically active with Na in a reversible manner.102, 104 Since then, NASICON-type compounds (transition metal = V, Fe, Ti, and etc) were investigated for both LIB and NIB, however, most work was focused on LIB due to poor cell performance in NIB.105–107 Recently, carbon-coated Na3V2(PO4)3 (structure illustrated in Figure 9a) was investigated and showed promising performance as a NIB electrode material. In the electrochemical profiles of Na3V2(PO4)3 shown in Figure 10a-b, two distinct plateaus near 1.63 and 3.40 V were identified, which are related to the V2+/3+ and V3+/4+ redox couple, respectively. Using a voltage range of 2.7–3.8 V, a reversible capacity of 93 mAh g−1 was obtained with excellent cyclability. The lower voltage region (1.0–3.0 V) delivered about 59 mAh g−1 capacity with good cycle performance. Taking advantage of the big voltage difference between the plateaus, a symmetric cell using Na3V2(PO4)3 as both negative and positive electrodes was designed with an ionic liquid electrolyte.108 The symmetric cell reversibly delivered 64 mAh g−1. F-containing V-based polyanion compounds such as NaVPO4F, Na3V2(PO4)2F3, and Na1.5VOPO4F0.5 have also been investigated in NIB. The crystal structures of F- containing compounds are comparable to those of the NASICON structure.109–111 Na1.5VOPO4F0.5 delivered about 0.56 Na in Na-cells with two voltage plateaus near 3.6 and 4.0 V.111 A full-cell composed of hard carbon and NaVPO4F exhibited reversible capacity about 80 mAh g−1 with 3.7 V average voltage.109
Figure 10. Electrochemical profiles of various polyanion compounds in Na-cells: (a) Na3V2(PO4)3 at high voltage region, (b) Na3V2(PO4)3 at low voltage region (Reproduced with permission.141 Copyright 2011, Elsevier.), (c) NaFePO4 (Reproduced with permission.116 Copyright 2010, American Chemical Society.), (d) NaMn0.5Fe0.5PO4 (Reproduced with permission.118 Copyright 2011, American Chemical Society.), (e) Na2FePO4F, (Reproduced with permission.126 Copyright 2011, Elsevier.), and (f) NaFeSO4F. (Reproduced with permission.123 Copyright 2010, American Chemical Society.)
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LiMPO4 (M = Fe, Mn, Co, Ni) is the most common polyanion electrode in LIB.112–114 Conversely, only a few reports were published for electrochemical characterization of NaMPO4 for NIB because maricite NaMPO4, which is thermodynamically more stable than olivine NaMPO4, is almost inactive with Na deintercalation due to the absence of adequate Na diffusion channel.115–117 Meta-stable olivine NaFePO4 phase (Figure 9b) could be obtained by delithiation and subsequent sodiation of the olivine LiFePO4.116, 118 NaMn1-xMxPO4 (M = Fe, Ca, Mg) can be synthesized via topochemical synthesis starting from NH4Mn1-xMxPO4·H2O.118 It is well-known that de/intercalation of Li in olivine compounds occurs via two-phase reaction between Li-deficient LixMPO4 and Li-rich Li1-xMPO4 (x∼0), except for some special cases.98, 119–121 Interestingly, de/intercalation of Na in olivine NaFePO4 and NaMn0.5Fe0.5PO4 did not follow the conventional behavior (Figure 10c-d).116, 118 When discharging olivine FePO4, a distinct step in the discharge profile was observed near 2.95 V, which corresponds to the formation of a Na0.7FePO4 intermediate phase (Figure 10c).116 The intermediate phase was also identified after chemical desodiation of NaFePO4.118 Approximately 0.9 Na ions were seen to reversibly react with an average voltage near 2.92 V in NaFePO4. NaMn0.5Fe0.5PO4, on the other hand, showed a single-phase reaction for all Na compositions118 and displayed a sloping discharge profile without any voltage plateau (∼0.6 Na ion reaction). Strain energy between the Na-rich and Na-deficient phases due to the large size of Na ion was speculated to be the origin of this behavior. Shiratsuchi et al. investigated electrochemical activity of FePO4 electrodes with an amorphous or trigonal structure.122 Amorphous FePO4 electrodes (∼120 mAh g−1) showed better performance compared to the one with trigonal structure.
Recently, various (PO4F)4− and (SO4F)3− based Na compounds have been investigated in searching for new positive electrode materials in LIB.123–130 While these Na compounds were mostly considered as structural motifs for Li compounds, some can be used as active electrodes for NIB. Na2FePO4F possesses a layered-like two-dimensional framework of Fe2O7F2 bioctahedra connected by PO4 tetrahedra with Na ions located in the inter-layer space as illustrated in Figure 9c. Ellis et al. confirmed that desodiation from Na2FePO4F to NaFePO4F did take place successfully via chemical oxidation, however, electrochemical properties in Na-cell were not shown in this study.16 Recham et al. further investigated Na2Fe1-xMnxPO4F (0 ≤ x ≤ 1) in Na-cell.131 While Na2FePO4F showed fair electrochemical activity to Na (∼100 mAh g−1 with 3 V operation), Na2MnPO4F was almost inactive. Furthermore, the electrochemical activity of Na2Fe1-xMnxPO4F electrodes drastically decreased with increasing Mn content. This is coincident with a crystal structure change in Na2Fe1-xMnxPO4F depending on the Mn content. At∼25% of Mn, the crystal structure of Na2Fe1-xMnxPO4F changes to a Na2MnPO4F-like structure, which is believed to be disadvantageous for Na ion conduction.124, 131 A Na2FePO4F electrode could deliver 110 mAh g−1 at 6.2 mA g−1, but the capacity decayed drastically with an increase in the current density (60 mAh g−1 at 124 mA g−1) as shown in Figure 10e. Recently, electrochemically active Na2MnPO4F was reported by size reduction and conductive carbon coating, however, this work was performed in a hybrid Li-cell.132 Most work on Na2MPO4F has been done with hybrid Li-cells to show the possibilities of Li2MPO4F electrodes, hence, a clear understanding of the electrochemical activity of sodiation and desodiation in these materials is still missing.
Tavorite LiFeSO4F was reported to be a promising positive electrode material for LIB due to the multi-channel Li ion conduction.125 NaFeSO4F is composed of almost same polyanion framework with LiFeSO4F, but with slight difference in Li and Na positions.123, 129 While there are two distinguishable Li ion sites in LiFeSO4F, only one Na ion site is available for NaFeSO4F (Figure 9d). Hence, NaFeSO4F (P21/c) has higher symmetry compared to LiFeSO4F (P-1). Barpanda et al. argued that NaFeSO4F (∼7.14 × 10−7 S·m2) had higher ionic mobility compared to LiFeSO4F (∼7.0 × 10−11 S·m2) through impedance spectroscopy on pellets.123 A NaFeSO4F electrode was electrochemically active in a hybrid Li-cell, but showed little electrochemical activity (∼0.08 Na ion transfer) in a Na-cell up to 4.2 V (Figure 10f).123, 125 Electrochemical Na intercalation into delithiated FeSO4F was not successful either. It was claimed that limited electronic conduction and large volume difference between NaFeSO4F and FeSO4F phases (∼16%) might be the reason for poor electrochemical activity. A computational study on ionic diffusivity in both phases by Tripathi et al. suggested that LiFeSO4F has lower activation energy for the diffusion (∼400 meV) in three-dimensional diffusion paths.125 In contrast, only one-dimensional zigzag path for Na diffusion was possible for NaFeSO4F with relatively high activation energy of 600 meV. First principles calculations on LiFeSO4F indicate that the structure has a one-dimensional diffusion path for Li with very low activation barrier, but that diffusion in large particles may require access to a defect cross over mechanism between these one-dimensional paths, resulting in a higher activation energy.130 This is similar to the situation in LiFePO4.133
3.3. Other Compounds
Early studies in chalcogenide compounds showed that Na can also be intercalated in compounds such as TiS2, TaS2, MoSe2, and SnSeyS2-y.134–136 Na ions can occupy the inter-slab vacant sites in these materials. However, no extensive studies have been performed on these materials due to their poor electrochemical characteristics. Recently, Yamaki et al. suggested NaMF3 as the positive electrode in NIB.137–139 It was claimed that NaMF3 can provide high safety as there is no risk of oxygen release during battery operation.137 Various Na-free MF3 (M = Fe, Ti, Co, Mn) compounds were electrochemically characterized.138 Mössbauer spectroscopy before and after the discharge revealed that the Fe2+/3+ redox couple was active in FeF3-C electrodes giving a specific capacity of 150 mAh g−1 in NIB. Na-intercalated MF3 compounds, NaMF3 (M = Fe, Mn, Ni), was also directly synthesized via mechanochemical reaction and liquid-based synthesis.137, 139 Nanosized NaFeF3 electrode could deliver 180 mAh g−1 at 1.5–4.5 V.