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Introduction: (There is a benefit at the end of the article, Faraday company patent collection download link) and lithium-ion batteries are similar, sodium-ion batteries are also a rechargeable battery. However, he used sodium ions as charge carriers. Aside from using sodium compounds instead of lithium compounds, his battery structure and working principle are almost indistinguishable from lithium-ion batteries.
Sodium-ion batteries have aroused widespread concern from the mainland people from july last year, the world's largest manufacturer of lithium-ion power batteries - CATL times announced the development of sodium-ion batteries. In fact, in the field of sodium-ion batteries in the world, the most famous company is faradion in the United Kingdom.
Faraday is headquartered in Sheffield, England, a city on the mainland known for hosting the Annual World Billiards Championship in Crucible. Founded in 2011, he is the world's first non-aqueous commercial sodium-ion battery company. Late last year, the world's leading sodium-ion battery company was acquired by India's Reliance New Energy Solar Ltd (RNESL) for £100 million, and RNESL will also invest £25 million as growth capital to accelerate commercial rollout.
This article will detail Faraday's process of developing cathode materials for sodium-ion batteries. This article explains some of the battery processes, electrochemical concepts, calculation of key parameters, and battery process techniques, and strives to use concise and clear language. This gives the reader a clear and general concept.
(a) prerequisites for potential cathode materials
1. Cost
Recently, CATL has been dynamically raising the price of lithium-ion power batteries it manufactures to cope with the cost pressure brought about by the rise in upstream raw materials. Sodium-ion batteries are positioned as the ideal middle ground between the high performance, low cost and long-term sustainability offered by modern lithium-ion batteries. In the Earth's crust, the content of lithium is only 0.01%, while sodium reaches 2.83%. That is to say, the sodium content is nearly 300 times that of lithium. The abundance of sodium on Earth and the ease with which he extracted it determined that his price was cheaper than lithium.
Therefore, in the selection of cathode materials, it is necessary to use very rich elements, otherwise it will offset the low cost of using sodium resources. This would immediately eliminate cobalt, which is expensive and faces some humanitarian problems in his main African producing countries. Some transition metals such as vanadium (V) and nickel (Ni) are ideal, these transition metals on Earth are not as high as sodium, ideally, the cathode material can contain more than these transition metals, which can reduce costs.
Therefore, our consideration is this: nickel is the first choice, compared to cobalt, it is much cheaper, has a good geographical distribution, and is easily available, the supply chain is relatively complete, nickel is also used in large quantities in steel smelting, so the source is much easier than cobalt. Electrode materials based on two other transition metals, iron and manganese, are also considered for design.
2. High coulomb efficiency
After the positive electrode material and the negative electrode material electrolyte form a full battery, it is necessary to have a high coulomb efficiency. Coulombic efficiency, also known as Faraday efficiency or current efficiency, describes the efficiency with which electrons are transferred in a battery. He represents the ratio of the total charge obtained to the total charge input over a full cycle. In practical applications, it is embodied in a charge-discharge cycle, the ratio of discharge capacity to charging capacity. Usually a score less than 1.
3. High vibration density (Tap Density)
High vibration density is a key factor in battery design. The vibration density is the ratio of the powder mass to the volume of the powder after being vibrated for a certain period of time. The vibration density of the powder represents its unceremonious accumulation.
Before being shaken, the density of the powder is called Bulk Density. Also known as apparent density or bulk density. Bulk density is the total mass of the powder of many particles divided by their total volume. This total volume includes particle volume, interparticle void volume, and internal pore volume.
(ii) Faraday cathode material roadmap
- Polyanion materials
The first material developed and researched by Faraday was polyanion materials, so what is a polyanionic material?
Polyanionic electrode materials can be classified as a class of compounds containing a series of tetrahedral anionic units (XO4)n– or derivatives thereof (XmO3m+ 1)n– (a class of compounds such as X=S, P, Si, As, Mo, or W) and MOx polyhedra (M stands for transition metals) bound by strong covalent bonds.
Faraday began developing polyanionic materials based on this knowledge: Valence Technology has demonstrated that this material is rich in sodium ion chemistry. Moreover, polyanionic materials are characterized by: high average discharge voltage, good cycle stability and magnification performance, high safety, but its reversible capacity is generally less than 100mAh/g.
Faraday developed polyanionic materials and applied for two patents in September 2011 and February 2012. The first is the "Condensed polyanion electrode" with patent number "US9608269". The second item is "Sulfate electrodes" (Sulfate electrodes), patent number is "WO2013114102".
These patents cover some of today's popular sodium ion cathodes, such as Na4M3(PO4)2(P2O7), Na7V4(P2O7)4(PO4) or Na2Fe2(SO4)3 for the transition metal manganese (Mn) or iron (Fe).
Figure 1(a)First cycle of Na4Fe3 (PO4)2(P2O7)/Na battery (b)Na7V4(P2O7)4(PO4)//Hard carbon full battery cycle 16th cycle between 4-2V
These polyanionic materials operate at high operating voltages, such as Na7V4 (P2O7)4 (PO4)//hard carbon full batteries with an average discharge voltage of 3.6V. However, its reversible capacity is only 55mA h g-1.
In order to increase the reversible capacity, Faraday began the development of a new material.
2. Layered oxide cathode material (oxide cathode material)
Because lithium cobalt oxide LiCoO2, a layered material, is so successful in lithium-ion batteries, it is widely used in 3C products. When Faraday studied oxide materials, it was natural to focus on layered oxides.
In sodium-ion batteries, the general formula of this layered material is NaxMO2 (M is a transition metal, such as Ni, Co, Mn, Fe, Cr), note that this general formula can also be written as NaxM1-aM'aO2. Where M and M' are transition metals, the sum of their metrology numbers is 1. Of course, this general formula can also be extended to include more transition metals. Just make sure that the sum of the stoichiometric numbers of these transition metals is 1.
If x< = 0.8, it is generally the P2 phase, and if x > 0.8, it is generally the O3 phase. For example, Na0.68Cu0.34Mn0.66O2 is the P2 phase and NaCu0.67Sb0.33O2 is the O3 phase. We can find that in the first compound, the sum of the stoichiometric numbers of the transition metal Cu and Mn is 1, while the content of Na is less than 0.8, so it is the P2 phase. The stoichiometric number of the second compound, the transition metal Cu and Sb, is also 1, and the content of Na is 1, which is the O3 phase.
Faraday filed a patent in March 2012 for a number of layered oxide cathode materials under the name "Metallic Acid Electrode" and the patent number "US10115966". Faraday had initial success in designing high-capacity sodium ion cathodes using layered oxide cathode materials, so it further explored a variety of different layered oxides.
2.1 Detailed explanations of O3, P2 and P3
Of course, these oxides also belong to the O3, P2 and P3 phases. So, what exactly are these phases? How are they classified? What is their structure?
We know that there are 230 groups of space in the crystal. The O3, P2 and P3 phases also belong to it. But these concepts were defined in Delmas's 1980 article "Structural classification and properties of the layered oxides."
The general formula of the layered oxides of the O3, P2 and P3 phases can be written as AxMO2. A is a basic metal, M can be one or several transition metals, in a variety of oxidation states. Figure 2 is a schematic diagram of these phases.
Figure 2: Schematic diagram of O3, O2, P2, and P3
- O3 phase
The A, B and C layers are all formed by the close arrangement of O atoms, taking the O3 phase as an example, between the A and B layers is the M element, they form a hexahedron with 6 O's, and M is in the middle of this hexahedron.
Figure 3: Layers A and B are tightly arranged
From Figure 3, you can see more clearly how the A and B layers are arranged. Note that these elements represent O. We next stack layer C on layer B, where the O atoms of layer C are themselves closely arranged, and the position of his stacking starts at the white point position in Figure 3 (the white dot that can be seen between the A and B layers actually represents a gap), as shown in Figure 4.
Figure 4: Stacking of layers A, B, and C
The 6 O atoms of layers B and C form an 8-sided body, and the alkaline metal A is located in this 8-sided body, as shown in the 8-sided figure at the left end of Fig. 2. This is the composition of the O3 phase, whose representation of the spatial group is R3 ̄m. For example, the common α-NaFeO2 is the O3 phase.
Figure 5: α-NaFeO2 (O: red, Na: purple, Fe: orange)
- P2 phase
In Figure 3, if you continue to stack the same B layer on layer B as him, a triangular prism will be formed, and the alkaline metal is located in this triangular prism. This can be seen from the far right end of Figure 2. This is the composition of the P2 phase, and his representation of the spatial group is P63/mmc.
- P3 phase
As shown in Figure 3: Continue stacking in BCCA order on layer B, and we will find that the alkaline metal is always located in two layers of the same structure, that is, the alkaline metal is always located in two A or two B or two C layers. Therefore, also as in the P2 phase, the alkaline metal is located in a triangular prism. This is the composition of the P3 phase, and the representation of his spatial group is R3m.
Faraday conducted research on layered oxide cathode active materials of a variety of O3, P2 and P3 phases. Among them, the layered oxide cathode active material of the O3 phase exhibits the highest energy density. This is because in the NaxMO2 formula, the value of x in the O3 phase is the highest, which necessarily makes the energy density of the O3 phase higher.
Faraday also noted that the electrolyte was stable in the range of redox potentials in the cathode material of the O3 phase. Therefore, in order to maximize energy density and minimize costs, Faraday doubled the number of Na+ moles per mole of Ni2+ in Ni2+ by using a small amount of dual electron variation in the starting material.
Some readers may not know much about the above passage. In fact, in sodium ion batteries, the de-embedding and embedding of sodium ions is carried out in the form of sodium ions, which will inevitably bring about the problem of electrical neutralization of the active substance of the electrode. In order to achieve electrical neutralization, the electrode must also gain or lose electrons. These electrons are associated with the redox of transition metals. For example, Ni4+ gets two electrons and reduces to Ni2+. For the redox of molar Ni ions, there must be a movement of 2 moles of electrons, which means de-embedding and embedding of 2 molars of Na+.
Faraday also noted that some layered oxides were able to exceed their expected theoretical charge energies. That is, when charging, we can know that all the Na+ goes from the positive to the negative, but it is also possible to continue charging. The main reason for this is the redox of O in the positive electrode and the irreversible oxidation of O2-.
This ability to get additional charge doesn't make any sense for commercial use, unless it's also possible to get extra charge on the discharge charge. Faraday sought to exploit this phenomenon by initially adding redox pairs present in a high oxidation state. For example, mn in a high oxidation state is added, which can be activated during the first discharge or during the cycle.
With this concept in mind, Faraday designed the first cathode material that could be mass-produced to the kilogram level, which was also known as the first generation of materials. The stoichiometric formula of this material is Na0950Ni0317Mn0317Mg0158Ti0208O2, which is typical of O3 phases.
Figure 6: Faraday's first generation of cathode materials. (a) The XRD plot confirms the typical O3 phase. (b) The first cycle of an all-sodium-ion battery paired with commercial hard carbon. (c) The second cycle of this sodium-ion battery shows a smooth charge-discharge curve. (d) Cyclic stability over 200 cycles at ±C/6 magnification. Faraday filed a patent for this material under the name "Doped Nickelate Compounds" under patent number "US9774035".
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