Manganese-based polyanionic cathodes for sodium-ion batteries

3.1 Phosphates

Phosphates are by far the most studied class of Mn-based polyanionic cathodes, and NaMnPO₄ is one of the earliest. Unlike LiMnPO₄, two polymorphs of this material exist, namely, olivine and maricite. The former is a thermodynamically substable and electrochemically active phase, while the latter is a thermodynamically stable and electrochemically inert phase^{[82, 83]}. Specifically, Na⁺ and Mn²⁺ occupy the Mn1 (4a) and Mn2 (4c) positions, respectively, in the olivine structure, while the opposite is true in maricite. Previous theoretical calculations^[84] have shown that doping of As, N, and Sb at the P-site in olivine-type NaMnPO₄ reduces the band gap and the ionic character of Na–O bond, thereby achieving a simultaneous increase in electronic and ionic conductivity, with the best effect being achieved by doping Sb. NaMnPO₄ prefers to form an electrochemically inert maricite phase lacking ion diffusion paths under conventional synthesis conditions. To obtain highly active materials, complex electrochemical transformations, chemical substitutions, and so on are more effective^[85-88]. A detailed study of the structural stability and ion transport properties of olivine and maricite NaMPO₄ (M = V–Ni) using atomic simulation techniques carried out by Zhu et al^[89] is a report with reference value. The breakthrough study shows that the nanoscale maricite NaMPO₄ converts to the amorphous phase MnPO₄ during the electrochemical process, thus exhibiting sodium storage reversibility. Moreover, replacing part of Mn with Fe is a development direction for this material^{[25, 90, 91]}.

Mn-based phosphates based on NASICON-type Na₃V₂(PO₄)₃ derivatives are one of the current research hotspots. The highly open skeleton structure of NASICON provides three-dimensional diffusion channels and a large migration gap for ions resulting in a diffusion coefficient of sodium ions in such materials of up to 10⁻⁸ cm² s⁻¹. The design idea is substitution and the basic principle is electroneutrality. In the extreme case, two Mn³⁺ are used to completely replace V³⁺ to form Na₃Mn₂(PO₄)₃. Theoretical calculations suggest that it can reach an average voltage of 3.64 V, but no relevant experimental electrochemical data have been reported^[92]. Comparatively, phosphates based on Mn²⁺ are the mainstay of research, and the design ideas of the other polyanionic compounds mentioned later are similar. One typical representative is Na₃MnTi(PO₄)₃^{[41, 42, 93-96]}. There are two pairs of oxidation/reduction peaks located at 3.73/3.43 and 4.14/3.94 V (Figure 4a) between a voltage window of 2.5 and 4.2 V corresponding to Mn^3+/2+ and Mn^4+/3+ redox couples, respectively^[41]. At 0.2 C, it can deliver a capacity of 114 mAh g⁻¹ and in situ X-ray powder diffraction (XRD) analysis shows its sodium storage involving both solid solution and two-phase reactions (Figure 4b).

Further lowering the discharge cutoff voltage to 1.5 V activates Ti^4+/3+ redox couple, allowing a further increase in capacity of ~170 mAh g⁻¹ at 0.1 C^{[42, 96]}. In addition, Na₃MnZr(PO₄)₃ generated by the complete substitution of Ti⁴⁺ with Zr⁴⁺ has similar electrochemical properties^[40]. Interestingly, the presence of Na⁺/Mn²⁺ cation mixing in Na₃MnTi(PO₄)₃ leads to a significant voltage hysteresis and significantly reduces the capacity contributed by the Mn³⁺/Mn²⁺ redox couple, which can be alleviated by adding an excess of sodium during synthesis process to return the position occupied by Mn²⁺ to Na⁺ (Figure 4c,d)^[95].

Another typical representative is Na₄MnV(PO₄)₃ with a theoretical capacity of 110 mAh g⁻¹ corresponding to two redox pairs of V^4+/3+ and Mn^{3+/2+[27, 28, 99-103]}. As already reported^[97], the extraction and insertion of sodium in Na₄MnV(PO₄)₃ proceeds via two steps between a voltage window of 2.5–3.8 V, with one sloping and one flat plateau at ~3.4 and 3.6 V. Cycling within this potential range is carried out by solid solution and two-phase processes (Figure 4e,f). If the cutoff voltage is broadened, an additional voltage plateau at ~3.9 V is observed, being associated with the unlocking of the Na1 site in the rhombohedral phase. Meantime, the reverse insertion of sodium ions proceeds through the entire solid-solution region (Figure 4g,h). This means that activation of more electron reactions (e.g., V^5+/4+) can yield higher capacity and energy density, but often leads to irreversible structural evolution and changes in the storage mechanism^[103]. In addition, it is worth mentioning that the sodium storage mechanism is also related to the V/Mn ratio in Na_3+xMn_xV_2−x(PO₄)₃ (Figure 4i)^{[98, 104]}.

Further complete substitution of V³⁺ with other covalent transition metal ions can be further derived from Na₄MnCr(PO₄)₃^[29] and Na₄MnAl(PO₄)₃^{[31, 105]}. In these interesting systems, Mn^4/3+ redox couple is easily activated below 4.3 V. As with the activation of V^5/4+ redox couple in other V-based polyanions^[106], Cr³⁺ and Al³⁺ can stabilize the structure like pillars, and Cr³⁺ can also contribute to the capacity via Cr^4/3+ redox couple above 4.6 V^{[30, 107]}. On the basis of these materials, doping or substitution of covalent or heterovalent ions such as Cu^2+[33], Ca^2+[108], Fe^{2+[109, 110]}, Co^2+[45], Mg^{2+[32, 37]}, Al^3+[26], Ce^3+[34], Cr^3+[44], V^{3+[48, 98]}, Zr^4+[46], and so on, has also been extensively explored, some of which have beneficial effects in activating high-valent redox pairs or enhancing structural stability. Overall, these designs greatly expand the selection of Mn-based polyanionic cathodes and overcome the potential resource shortage and environmental pollution of V, providing new opportunities for large-scale applications in SIBs.

3.2 Fluorophosphates

The introduction of fluorine with greater electronegativity to enhance the inductive effect of anionic groups to obtain higher electrochemical potentials is one of the important research directions, which is mostly seen in V-based polyanionic materials^[106], such as Na₃V₂(PO₄)₂F₃, NaVPO₄F, and Na₃V₂O₂(PO₄)₂F. The current Mn-based fluorophosphate cathodes mainly include Na₂MnPO₄F and its partially Fe-substituted derivatives^{[59, 60, 68, 111-113]}. Unlike Na₂FePO₄F with a two-dimensional layered structure, Na₂MnPO₄F presents a three-dimensional channel structure, whereas its ion diffusion coefficient is still as low as 10⁻¹⁶ cm² s⁻¹. At the theoretical level, Na₂MnPO₄F can achieve a two-electron reaction to reach a capacity of ~250 mAh g⁻¹, corresponding to two operating voltages of about ~3.66 and ~4.67 V. Optimizing the synthesis temperature can increase the capacity from less than 50 mAh g⁻¹ at 750°C to greater than 100 mAh g⁻¹ at 650°C accompanied by Mn^3+/2+ redox couple (Figure 5a). Comparatively, its rate performance is poor, only 26.7 mAh g⁻¹ at 1 C^[59], which may be related to the sluggish conductivity and severe Jahn–Teller effect^[68].

At the expense of the average voltage, the electrochemical activity can be gradually enhanced by Fe substitution. For instance, when 0.5 Mn²⁺ is replaced by Fe²⁺, the resulting Na₂Fe_0.5Mn_0.5PO₄F can deliver a capacity of 107 mAh g⁻¹ at 0.1 C with two distinct voltage plateaus at 2.9 and 3.5 V associated with Fe³⁺/²⁺ and Mn³⁺/²⁺ redox reactions sequentially^[68]. A capacity of around 70 mAh g⁻¹ can still be obtained at 1 C but almost originates from the contribution of the Fe³⁺/²⁺ redox couple, further indicating the impoverished reaction kinetics of the Mn³⁺/²⁺ redox couple. In situ XRD analysis showed that similar to Na₂MnPO₄F, Na₂Fe_0.5Mn_0.5PO₄F does not display new peaks throughout the charging and discharging process (Figure 5b), indicating that a single-phase solid solution reaction mechanism is followed, which differs from Na_2–xFePO₄F system that forms intermediates.

In addition, Mn can also be seen in V-based fluorophosphates, and studies have shown that it demonstrates decent effects on capacity, cycling, and so on. At trace substitution (doping, <0.1), like Na₃V_1.95Mn_0.05O₂(PO₄)₂F^[56] and Na₃V_1.95Mn_0.05(PO₄)₂F₃^[57], Mn²⁺ does not change the charge/discharge characteristics of the parent material, while its effect cannot be underestimated when the substitution is larger. Instead of the galvanostatic profiles with two pronounced plateaus observed in Na₃V₂(PO₄)₂F₃, the Mn-substituted samples, such as Na_3.6V_1.4Mn_0.6(PO₄)₂F₃ and Na_3.4V_1.7Mn_0.3(PO₄)₂F₃ (Figure 5c), exhibit sloping profiles consisting of several pseudoplateaus^[58]. An invisible benefit is perhaps to improve the ease of battery management based on V-based phosphate or fluorophosphate cathodes.

3.3 Pyrophosphates

Since the electronegativity of pyrophosphate is slightly higher than that of phosphate, pyrophosphate cathodes exhibit higher redox potentials. Such materials can be represented by the general formula Na_4−2xM_2+x(P₂O₇)₂. When M = Mn and x = 0, a typical Na₂MnP₂O₇ is formed^{[62, 114-118]}. The current research is dominated by the triclinic-type β-phase with three-dimensional ion diffusion channels^[117]. Unlike most Mn-based cathode materials that suffer severely from sluggish kinetics, this material abnormally exhibits a capacity of up to about 90 mAh g⁻¹ at 0.05 C with an average voltage of ~3.8 V when used as a cathode accompanied by Mn^3+/2+ redox couple (Figure 5d). Moreover, it also shows remarkable cycling and rate performance, that is, 96% capacity retention after 30 cycles and 70% capacity retention at a c-rate increase from 0.05 to 1 C. This significantly enhanced kinetics is mainly ascribed to the locally flexible accommodation of Jahn–Teller distortions aided by the corner-sharing crystal structure. During charging, sodium ions are extracted first from half of the Na1 sites, immediately followed by all of the Na3 sites, and finally from half of the Na4 sites. That is, the occupancy of sodium in the four crystallographic sodium sites (Na1–Na4) is 0.5, 1, 0, and 0.5, respectively^[114].

In addition to the β-phase, Na₂MnP₂O₇ also exists in a layered phase considered to possess superior electronic conductivity and can be irreversibly transformed to the β-phase when heated to 600°C (Figure 5e). Although its electrochemical properties are still lacking (~20 mAh g⁻¹ at 0.1 C), such concerns may be improved by optimization of grain size or carbon coating processes^[115]. Using the Na₂MnP₂O₇ as the parent, partial substitutions of Fe and Co have also been studied for sodium storage^{[61, 119-122]}.

Similar to Fe-based pyrophosphates, Mn-based pyrophosphates also exist in a series of off-stoichiometric structures, and only Na_3.12Mn_2.44(P₂O₇)₂^[63] and Na_3.12Mn_1.22Fe_1.22(P₂O₇)₂^[123] have been reported so far. In Na_3.12Mn_2.44(P₂O₇)₂, three pairs of redox peaks (located at 3.42/3.25, 3.81/3.63, and 4.00/3.80 V) can be identified. This off-stoichiometric structure follows the solid solution reaction mechanism and has two one-dimensional ion diffusion channels, which exhibit ion diffusion coefficients of up to 10⁻¹¹ S cm^−2[63], probably due to the increased carrier concentration. Following our previous experience^[124-126], it seems that the theoretical capacity of such materials can also be maximized by modulating the Na/Mn ratio under the premise of electroneutrality (Figure 5f). Whether the rate capability or stability can still be reconciled remains to be supported by more experimental data.

3.4 Sulfates

According to the Pauling electronegativity rule, the bonding of S–O is stronger than that of P–O. It means that the inductive effect of sulfate is stronger than those of phosphate and pyrophosphate, and therefore the energy level resulting from the hybridization of transition metal d orbitals with O–2p orbitals in sulfates will undergo greater cleavage, thus showing higher redox potential. From the present status, there are few experimental reports on Mn-based sulfates, and only two systems, Na₂FeMn(SO₄)₃^[65] and Na_2.5(Fe_1−yMn_y)_1.75(SO₄)₃^[66], have been evaluated exactly for sodium storage performance. Such sulfates with alluaudite structure can be represented by the general formula AA′BM₂(SO₄)₃ (M = Fe, Mn, Ni, Co), in which the sodium ions are in three different crystallographic positions, which are partially occupied A and A′ and fully occupied B, while the M sites are filled by transition metals. The presence of characteristically open channels along the [001] crystallographic directions in the formed three-dimensional network makes it possible to achieve diffusion of sodium ions at low activation energies. It was shown that Na₂FeMn(SO₄)₃ can deliver a capacity close to 110 mAh g⁻¹ at 0.1 C with an average voltage of 3.6 V and is generally between FeFe and FeNi analogs (Figure 6a). In a series of off-stoichiometric Na_2.5(Fe_1−yMn_y)_1.75(SO₄)₃ solid solutions, the average operating voltage gradually increases with increasing Mn substitution, yet it is accompanied by a rapid capacity decay, that is, from 110 mAh g⁻¹ without Mn to about 0 mAh g⁻¹ with Mn only (Figure 6b), which is mainly attributed to the entire Mn²⁺ inactivity during cycling and charge compensation originates only from Fe^3+/2+ redox couple.

As for other materials such as Na₂Mn₃(SO₄)₄^{[128, 129]}, Na_xFe_0.5Mn_0.5(SO₄)₂^[130], Na_2+dMn_2−d/2(SO₄)₃^{[131, 132]}, Na_2.44Mn_1.79(SO₄)₃^[133], Na₂Mn(SO₄)₂·nH₂O^[134], K_0.5Na_1.5Mn_1.5Mg_0.5(SO₄)₃^[135], and so on, they are still only available at the level of theoretical studies or experimental synthesis. Generally, sulfates can be synthesized below 450°C by low-temperature solid-state, ionothermal or solution processes. However, what is often obtained are some off-stoichiometric phases containing impurities, so it is urgent to improve their crystallinity and purity in addition to addressing thermal stability. A more successful experience is the addition of excess sodium or organic carbon sources during the synthesis, for example, when 25 mol% of CH₃COONa together with 15 mol% of glucose was used, the amount of Na₂Fe₂(SO₄)₃ in the final product can be significantly increased from 76 to 98 wt% without additional additive (Figure 6c).

3.5 Silicates

Silicate polyanionic materials having the advantages of low cost and high elemental abundance play a delightful role in lithium-ion batteries (LIBs). In SIBs, two types of Mn-based silicates have been reported, one is Na₂Mn₂Si₂O₇^{[69, 127]}. Unlike edge-sharing connectivity of Mn–O polyhedra in Li₂Mn₂Si₂O₇, corner-sharing in Na₂Mn₂Si₂O₇ is apparent, and this structure theoretically enables the strain introduced by the Jahn–Teller distortion of Mn³⁺ to be relieved by taking only smaller atomic rearrangements. It can deliver a theoretical capacity of 165 mAh g⁻¹, but even after carbon coating, only about 25% is actually released with an average discharge voltage of only 2.2 V^[127]. Such sluggish kinetics may be related to the rearrangement of the Mn coordination accompanied by a substantial volume contraction (Figure 6d)^[127] or possibly to the huge migration coefficient change of the sodium ions during cycling^[69].

The other one is Na₂MnSiO₄ with three-dimensional ion diffusion channels^{[67, 136-139]}, which has a much higher diffusion coefficient than the lithium analog due to a larger spacing between atomic layers and to the nondirectional bonding between sodium and the neighboring atoms^{[138, 139]}. Moreover, the doping of other ions such as Al³⁺ at the Si site is expected to further optimize its ion diffusion properties^[137]. It was shown that by adding 5% VC to a conventional organic liquid electrolyte, the capacity of Na₂MnSiO₄ can be significantly increased from 129 to 155 mAh g⁻¹ in the first cycle and further to 210 mAh g⁻¹ after 10 cycles at 0.1 C (Figure 6e)^[67]. Such a high capacity contribution is attributed to the two-electron reaction and the structural stability of the active material itself (no amorphization like Li₂MnSiO₄) as well as the overall electrode during charge and discharge.

3.6 Mixed polyanions

Such electrode materials usually contain two or more polyanionic units, and the pairing of different anion groups can broaden the structural and redox potential diversity. For Mn-based materials, Na₃MnPO₄CO₃^{[79, 140-146]}, Na₄Mn_3−xM_x(PO₄)₂P₂O₇ (M is one or more of Fe, Co and Ni, 0 ≤ x < 3)^{[70-75, 147-149]}, Na₃Mn_2−xFe_xPO₄P₂O₇ (0 ≤ x < 2)^{[76-78, 150]}, and Na_2+2zMn_2−z(SO₄)_3−x(SeO₄)_x^[151] have been reported so far.

Na₃MnPO₄CO₃ belongs to the category of sidorenkite that have rarely been explored and was used as a sodium storage cathode until 2013 by Ceder's group^[146]. It has a peculiar structure in which the MnO₆ octahedra are connected by PO₄ tetrahedra to form bilayers, while the CO₃ groups share an oxygen edge with the MnO₆ octahedra and point toward a bilayer. The sodium atoms are located in two different interstitial sites, namely, Na1 and Na2 (Figure 7a), and theoretical studies^[145] have shown that these sodium ions can diffuse in three directions with low activation energy barriers. During the electrochemical process, the sodium ions at the Na2 site are extracted first, and then 1/2 of the sodium ions at the Na1 site are extracted (Figure 7b), along with a two-electron reaction resulting in a theoretical capacity of up to 191 mAh g⁻¹, which is rare among the mixed polyanions. The current better-optimized sample, graphene-based composite, can achieve a capacity of 160 mAh g⁻¹ at 0.01 C between the voltage limits of 2.0 and 4.4 V accompanied by two plateaus at 3.9 and 3.3 V that can be attributed to Mn^4+/3+ and Mn^3+/2+ redox couples, respectively (Figure 7c)^[144]. The main problem of this material is that the electronic conductivity is so poor that its rate capability is not desirable. To stimulate its potential, excessive carbonaceous additions once became necessary^[141]. Moreover, the sluggish kinetics may also be related to the crystallinity of the sample, after all, its synthesis is usually employed by low-temperature hydrothermal or ball milling methods.

Na₄Mn₃(PO₄)₂P₂O₇ can be obtained when x = 0 in Na₄Mn_3−xM_x(PO₄)₂P₂O₇ (M is one or more of Fe, Co, and Ni, 0 ≤ x < 3). This material with a three-dimensional open structure containing four crystallographic sodium sites is currently one of the most deserving of attention among the mixed polyanions. Upon charging, the Na2 site preferentially extracts sodium ions and follows a single-phase reaction (below ~3.51 V) until one sodium ion is completely extracted, then immediately transfers to a two-phase reaction process and extracts another 0.25, 0.25, and 0.5 sodium at the Na1, Na3, and Na4 sites, respectively (Figure 8a). The state of the art dates back to 6 years ago, that is, a capacity of 80 mAh g⁻¹ at 5 C accompanied with Mn^3+/2+ redox coupled with an average voltage of 3.8 V (Figure 8b). After 500 cycles, its capacity has almost no decay^[71]. It is common knowledge that the Mn³⁺ suffers from a severe Jahn–Teller effect, which is often one of the main factors that are detrimental to the electrochemical performance. However, this effect is rather anomalous in this material, meaning that the distortion of the MnO₆ octahedral instead promotes its electrochemical kinetic behavior. If the Mn²⁺ is partially substituted with Fe²⁺, Co²⁺, or Ni²⁺ to form binary or ternary phases, the electrochemical properties such as voltage, capacity, and cycle life will be modulated^{[70, 72-75]}.

Na₃Mn₂PO₄P₂O₇ is a new paradigm recently reported with a theoretical capacity of 119 mAh g⁻¹ slightly lower than that of Na₄Mn₃(PO₄)₂P₂O₇. It can deliver a capacity of 101 mAh g⁻¹ at 0.1 C with an average operating voltage of 3.6 V, corresponding to the redox reaction of the Mn^3+/2+ redox couple (Figure 8c). Further studies show that the material contains three crystallographic sodium sites, where sodium ions in the Na1 and Na3 sites are more easily extracted during charge, and follows only the solid-solution reaction mechanism throughout the cycle (Figure 8d)^[76]. It is worth mentioning that the volume change of the material is as low as 0.87% during desodiation/sodiation, which can essentially be considered as a zero-strain material. Partial substitution of Mn²⁺ with Fe²⁺ is beneficial to yield a highly active material but sacrifices part of the voltage strength. Optimally, acceptable results can be obtained when the Mn/Fe ratio is 1:1 (Figure 8e)^[78]. Moreover, these resulting binary phases follow the same single-phase reaction mechanism as Na₃Mn₂PO₄P₂O₇^{[78, 150]}.

For Na_2+2zMn_2−z(SO₄)_3−x(SeO₄)_x using monoclinic C2/c alluaudite-type structure, it can be regarded as a derivative obtained by partial substitution of SO₄²⁻ in Na₂Mn₂(SO₄)₃ with SeO₄²⁻. As the selenate content increase, the electrochemical activity of such mixed alluaudites gradually increases accompanied by a decrease in the average voltage (Figure 9a), in agreement with the lower electronegativity of Se compared to that of S. Only a minor part of the total electrochemical activity could be attributed to Mn^3+/2+ redox couple and the additional capacity may originate from the Se^6+/4+ couple (~2.25 V)^[151]. Studies on the sodium storage behavior of such materials are relatively poor, and the only data available (60 mAh g⁻¹ at 0.05 C) do not show any competitive advantage in terms of electrochemical performance compared to other polyanions.

3.7 Other polyanions

Apart from the above-mentioned categories, there are some special Mn-based polyanionic cathodes worth mentioning. For example, Na_2.67Mn_1.67(MoO₄)₃, which belongs to the alluaudite family, sodium ions can migrate along c-axis and b-axis in its three-dimensional framework structure, while in Na_2.76Mn_1.78(SO₄)₃, sodium ions can only migrate along the c-axis, implying a higher mobility. When used as a cathode, it can liberate a capacity of 92 mAh g⁻¹ at 0.1 C with an average discharge voltage of about 3.45 V (Figure 9b)^[80]. It is demonstrated that the capacity contribution mainly originates from the Mn^3+/2+ redox couple (~3.5 V), while a small portion comes from the Mo^6+/5+ redox couple (~2.4 V), for which further verification is needed. Following Na₂Mn₂Fe(VO₄)₃ with low capacity, a new alluaudite, Na₂Mn_3−xAl_x(VO₄)₃ (x = 0, 0.05, and 0.2), has recently emerged. With increasing Al³⁺ content, the electrochemical activity of the as-prepared material at 0.05 C rises significantly from 35 mAh g⁻¹ at x = 0 to 75 mAh g⁻¹ at x = 0.2 (Figure 9c). This positive effect is attributed, on the one hand, to the greater ionicity of the V–O bond optimizing electron transport and, on the other hand, to the Al³⁺ alleviating the Jahn–Teller distortion. Another example is NaFe_1−xMn_x(PO₃)₃ (x = 0–1)^[152], but its electrochemical details published so far are only for x = 0, that is, materials containing Mn have yet to be investigated.