XPS profiles of non-stoichiometric sodium iron pyrophosphate with composition Na3.42Fe2.44(P2O7)2.05 in Fe2p. Rate performance of non-stoichiometric Na3,32Fe2,34(P2O7)2 by varying levels of C upon discharge (sodium). Rietveld data on the purification of chemically desodiated sodium ferric pyrophosphates with a theoretical equivalent amount at (a) SOC 50% and (b) SOC 100% (fully charged).
Nyquist plots of Na3.12Co2.44(P2O7)2 (violet) and Na2CoP2O7 (blue) tested with the sodium and lithium half-cells with (inset) scale enlarged a. Crystallographic information for each atomic site of the partially deiodinated sodium iron pyrophosphate with a theoretical equivalent amount at SOC 50%.
Introduction
- Lithium-ion batteries
- Sodium-ion batteries
- Cathode materials for lithium- and sodium-ion batteries
- Phosphate-based cathode materials
- Pyrophosphate-based cathode materials
While the lithium normally resides in the octahedral sites and tetrahedral sites on migration (REFs), the sodium ion is more likely to reside in the prismatic sites due to the larger ion size, especially in the sodium deficient states.61-70 However, there are many There are also O3-type layered oxides known as O3-NaFeO2.71 O'3-NaMnO2.72 O'3-NaNiO2.73 O3-NaVO2.74 O3-NaCoO2.75 and O3-Na[Ni1/2Mn1/2 ]O2.76 . Although their electrochemical performance is improved, oxygen gas often evolves at higher temperatures.77 The crucial point is that this can easily happen even in our real lives. Therefore, they now face challenges to ensure safety. Single-phase reaction has the advantage in kinetics because the diffusion of lithium ions can occur in any dimension, while two-phase reactions are found only at the phase boundaries.
Indeed, the particle size affects the phase transition so that the slope appears in the voltage profile with nanoscale particles.88, 89. The reversible capacity of 110 mAh g-1, corresponding to an electron transfer (Fe2+/ 3+), was exposed to the Galvanostatic test at a speed of 0.05 C (Figure 7).
Experimental methods
- Synthesis method
- Structural analysis
- Electrochemical measurement
- Chemical de/sodiation processes
To find the cause, X-ray photoelectron spectroscopy (XPS) analysis was performed to understand the surface environment of the active material, Na3,12Fe2,44(P2O7)2 (Figure 15). The powder X-ray diffraction pattern of non-stoichiometric sodium iron pyrophosphate shows exactly the same peaks with the usual peaks of Na3,12Fe2,44(P2O7)2 and small additional impurities of maricite NaFePO4 (Figure 17). The intermediate structure between Na3,12Fe2,44(P2O7)2 and Na3,12Mn2,44(P2O7)2, with the composition Na3,12Fe1,22Mn1,22(P2O7)2, is analyzed by Rietveld refinement to understand the structural changes with the insertion of manganese atoms to traditional iron sites (Figure 28a).
Rietveld accuracy of triclinic Na3,12Co2,44(P2O7)2 with the XRD pattern indicates that it has a slightly distorted structure than Na3,12Fe2,44(P2O7)2 with a smaller ionic size of the transition metal (Figure 32). The XRD peak of the powder of the latter is mixed with Na3,12Fe2,44(P2O7)2 and a small amount of maricite NaFePO4.
Results and Discussion
Sodium iron pyrophosphate
- Synthesis and morphological interpretation of sodium iron pyrophosphate
- Electrochemical performances of sodium iron pyrophosphate
- Off-stoichiometric sodium iron pyrophosphate
- Electrochemical mechanism for sodium iron pyrophosphate
First, half-cells in the coin type were assembled for the Galvanostatic electrochemical tests with Na3.12Fe2.44(P2O7)2 as the cathode material and sodium metal as the anode. Theoretically and mathematically, Na3.12Fe2.44(P2O7)2 can be oxidized by releasing 2.44 electrons, after redox reaction from Fe2+ to Fe3+. Indeed, during the 1st charging and discharging process, Na3.12Fe2.44(P2O7)2 delivered about 85 mA h g-1 of reversible capacity corresponding to the amount of 1.76 electron transferred (Figure 14).
Unfortunately, peaks indexed as sodium carbonate certainly appear in Na1s and C1s, as do peaks for Na3.12Fe2.44(P2O7)2. Also, the peak of Na2CO3 is clearly eliminated by an additional etching process for 2650 s, so that Na2CO3 is only present on the surface of Na3.12Fe2.44(P2O7)2 particles. Washing the final product of Na3.12Fe2.44(P2O7)2 with deionized water was considered as the next step to predissolve Na2CO3 and inhibit the irreversible reaction during the charging process.
The same synthesis method and conditions are applied to the non-stoichiometric method, except its composition to Na3.42Fe2.44(P2O7)2.05, with the addition of a small additional amount of sodium and phosphate precursors. By the way, the specific situation of a system with a continuous solid solution was already known for Na3.12Fe2.44(P2O7)2, taking the example of the binary system between Na4P2O7 and Mg2P2O7, where the sodium iron pyrophosphate can exist in continuous form. of Na4-αFe2+α/2(P2O7)2 (2/3 ≤ α ≤ 1), where α means the occupancy factor of the Na(3) site.105 Therefore, the overall composition of the sodium iron pyrophosphate can be determined in the manner the Na(3) site is largely occupied. Adjusting this factor, the synthesized non-stoichiometric sodium iron pyrophosphate mathematically consists of NaFePO4 with Na3.32Fe2.34(P2O7)2 (α = 0.68).
As a result, Na3.32Fe2.34(P2O7)2 delivered 85 mA h g-1 of the initial reversible capacity without any irreversible capacity during charging unlike the test with Na3.12Fe2.44(P2O7)2 thanks to the pre-suppression of the formation of Na2CO3. Phase transition of the sodium iron pyrophosphate during the electrochemical reaction was analyzed by chemical de/sodiation processes. Powder XRD patterns of the sodium iron pyrophosphates after chemical de/sodiation processes along (a) 10 to 26° and (b) enlarged ranges.
Rietveld refinement data of the chemically desodium iron pyrophosphates with a theoretically equivalent amount at (a) SOC 50%. Crystallographic information for each atomic location of the fully desodicated sodium iron pyrophosphate with a theoretically equivalent amount at SOC 100%.
Sodium manganese pyrophosphate
- Synthesis and structural differences with addition of manganese precursor to sodium
- Differences in electrochemical performances with addition of manganese precursor to
Compared to Fig. 12a, it has a slightly distorted unit due to the manganese atoms, but the overall structure is very similar to Na3.12Fe2.44(P2O7)2. This situation is also followed by Table 4, where there are no changes in the sites, except that a certain part of the iron sites is replaced by manganese atoms. Essentially, the occupancy factor for the transition metal sites is equally divided for manganese and iron because they have an exact molar ratio of 0.5 : 0.5 during synthesis, and the M(1) site is the same as the Na(4) coordination site as in the case of iron , so three atoms coexist at this point.
Differences in electrochemical performances with the addition of manganese precursor to sodium iron pyrophosphate, sodium iron pyrophosphate. Considering that these compounds have theoretically higher redox potential (Mn2+/3+) unlike Na3.12Fe2.44(P2O7)2, their breakdown voltages were also modified to 4.5 V vs. This means that these two equilibrium potentials are still from four iron sites, exhibiting a reversible capacity that is almost half that of sodium iron pyrophosphate (Figure 30).
This indicates that it is not just the electrolyte problem with the high voltage one, but the manganese itself. Some cases of phosphate-based active materials which have manganese as a transition metal suffer very poor cycling performance due to the electrochemical activity of manganese.91.
Sodium cobalt pyrophosphates
- Synthesis and structural examination toward triclinic and orthorhombic sodium cobalt
- Electrochemical performances for sodium cobalt pyrophosphates
- Evaluation in mass and charge transfer for sodium cobalt pyrophosphates
However, the basic structure is preserved consisting of two centrosymmetric crowns of Co2P4O22 and Co2P4O20. In addition, lithium half-cells were also assembled towards these two types of sodium cobalt pyrophosphate to check the possibility of being used as lithium de/interference host at this time (Figure 37). However, it generally shows worse results than those in sodium cells that give similar or less reversible capacity with larger polarizations.
Plus, the dQ/dV plots certainly show that the Co(3) site can rarely act as a redox center at this time (Figure 38), unlike the situation in sodium cells, given that discharge occurred at about 4.5 V vs . small amount of reversible capacity compared to their theoretical capacity (2.4 Na should be transferred to Na3.12Co2.44(P2O7)2 at most, and 1 Na to Na2CoP2O7), the profiles themselves are not as bad as the case of sodium manganese. pyrophosphate because they show precise plateaus and electrochemical mechanisms which are expected similarly to sodium iron pyrophosphates. First, the diffusion abilities between sodium cobalt pyrophosphates were evaluated by obtaining data of diffusion coefficients.
Potentiostatic intermittent titration technique (PITT) was introduced to calculate solid mass transfer diffusion coefficients at every five voltage steps of these compounds. Diffusion coefficient is easily calculated at finite diffusion, thus applied with a long time, because semi-finite diffusion region at short time is seriously affected by surface roughness of the material particle.113 Taking this into account, the logarithm term of the current ( ln (i) )) should have a linear relationship corresponding to time, with a slope of -π2 D / a2. The diffusion coefficients are naturally reduced during charging because the repulsive force between sodium or lithium ions already inserted would be increased, and minimized at the redox potential.114 Figure 40 finally shows calculated diffusion coefficients, and the value of Na2CoP2O7 is slightly greater than that of Na3.12Co2.44(P2O7)2 at the main redox potential different from their voltage profiles.
The trend is exactly consistent with the galvanostatic tests in Figures 35 and 37, with Na3.12Co2.44(P2O7)2 tested with the sodium half-cell producing the least charge transfer resistance. In detail, Na3.12Co2.44(P2O7)2 always shows less resistance than Na2CoP2O7 in both sodium and lithium half-cell tests. Furthermore, the resistances obtained from the sodium half-cell tests are smaller than those from the lithium cases, indicating that both pyrophosphates are more favorable as sodium hosts than lithium hosts in the kinetics with smaller polarization.
Conclusion
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![Figure 12. Structure model of Na 3.12 Fe 2.44 (P 2 O 7 ) 2 view along (a) [100], (b) [010], and (c) [101] directions](https://thumb-ap.123doks.com/thumbv2/123dokinfo/10541905.0/34.892.277.624.186.926/figure-structure-model-na-fe-view-100-directions.webp)
