(Report Producer/Author: CITIC Securities, Zhu Yue, Zhang Yichi, Ma Tianyi)
1. When we talk about secondary batteries, what is the exciting "shine point"
1. Demand-oriented, complex storage Energy index system
Energy storage technology is ultimately a demand-oriented technology. Its evaluation index system covers energy index , power index, scale index, life index, Efficiency indicators, self-release rate indicators, cost indicators, environmental impact indicators, etc. Depending on the application scenarios, energy storage technology also has different demand types and weights for indicators.
Take consumer batteries, power batteries, and energy storage batteries as examples. Consumer batteries tend to have higher volume, mass energy density, and higher charging rates; power batteries require balance and high cost weight; energy storage batteries have high energy and power Relevant indicators can be relaxed appropriately, but the requirements for life and cost-related indicators are very high.
Among various energy storage technologies, secondary batteries ( electrochemical rechargeable batteries) are a very critical component. It has a wide range of applications and has a strong ability to connect with renewable energy. From the perspective of applicable energy and power range, various secondary batteries cover the technical requirements of most energy storage applications, with lithium-ion battery being the most universal. Generally speaking, energy-related indicators are of greatest importance at the technical level of secondary batteries, and high-specific energy (i.e. mass energy density) secondary batteries have the most application scenarios, especially cutting-edge potential application scenarios.
2. Considering the supply, the electrochemical reaction of the secondary battery corresponds to the core reversible electrochemical reaction of the carrier
electrochemical rechargeability of the secondary battery. During the reaction process, the movement of carriers (ions or ion groups) is required to provide charge balance for the entire circuit.
The oxidant-oxidation product in the reversible electrochemical reaction constitutes the active material of the positive electrode of the battery, and the reducing agent-reduction product constitutes the active material of the negative electrode of the battery. The higher the specific capacity of the electrochemical reaction corresponding to the electrode and the greater the potential difference between the electrodes, which means the higher the reactivity of the electrode, the greater the specific energy of the battery. At the same time, the electrode material also needs to have structural stability (corresponding to the charged state and discharge state) and charge-discharge reversibility, interface stability (and electrolyte), chemical stability, thermal stability, relatively stable voltage platform, and high electronic conductivity. and high carrier conductance, and these properties are best maintained effectively over a wide temperature range.
For carriers, we need them to have good diffusion and migration capabilities in the electrolyte or solid electrolyte (preferably they can be effectively maintained within a wide temperature range), and have a high charge-to-mass ratio (or a low (mass-to-charge ratio), with a moderate ionic radius (too large a radius will lead to excessive volume changes in the matrix due to embedding and deintercalation in solid materials, and too small a radius will lead to difficulties in desolvation and charge balance). At the same time, the corresponding electrolyte or solid electrolyte also needs to have a wide electrochemical window, chemical stability, thermal stability, interface stability, etc. Therefore, there are only a few common types of carriers so far.
Of course, we also hope that secondary batteries that can be applied on a large scale have low costs. This corresponds to the best abundance of the elements used, the basic material system is cheap, and the battery production process is simple.
3. If we work on high-priced metal carriers
in the field of multi-electronic systems, the advantages and disadvantages are very obvious. The abundance of metals such as magnesium and aluminum in the earth's crust is much higher than that of lithium. Although the corresponding elemental electrode potential is higher than lithium and the specific capacity is lower than lithium, the capacity density (capacity per unit volume) is high; the zinc metal abundance is slightly higher than that of lithium, and the capacity density is high. In short, high-priced metals have the potential to be used to build low-cost, high volumetric energy density, and higher mass energy density batteries.
But on the other hand, high-valent metal carriers have high charge, small ionic radius, strong solvation, and are not easy to efficiently embed into solid materials to achieve charge balance.This makes the construction of electrodes (if the negative electrode is directly the corresponding metal, the electrode refers to the positive electrode) and electrolyte material systems of high-priced metal carrier batteries more difficult than alkali metals such as lithium and sodium. So, where have magnesium, aluminum, and zinc secondary batteries come to?
2. Magnesium secondary battery: expectations of magnesium metal anode
1. Construction of material system, starting from high-definition magnesium metal anode
As we know, the material system of secondary batteries needs to consider carriers, active materials and Auxiliary components. For magnesium battery , the magnesium metal negative electrode is the "breaking point" of the entire battery material system.
Researchers concluded that in addition to the natural difference in element abundance in the earth's crust, the capacity density of magnesium metal (3833Ah/L) is greater than that of lithium metal (2046Ah/L), which is much greater than the capacity density of graphite negative electrode for lithium storage, which is beneficial to Construct a battery system with high volumetric energy density; although the specific capacity of magnesium metal (2205Ah/kg) It is not as good as lithium metal (3862mAh/g), but it is also much larger than the specific capacity of graphite negative electrode for lithium storage. The standard electrode potential of magnesium redox couple (-2.37V) is higher than that of lithium (-3V), but it is also a lower metal. ; Magnesium is stable in air (due to surface oxidation), and dendrites are not produced during magnesium metal deposition (unlike lithium metal).
2. Magnesium battery electrolyte, defining milestone
Magnesium battery electrolyte must not only ensure stability to magnesium, but also ensure a smooth process of magnesium ions embedding/extracting from the cathode material. Of course, indicators such as ion conduction, electronic insulation, wide operating temperature range, and high safety are also required. Based on this, researchers have carried out research on magnesium battery electrolyte material systems from two technical routes: electrolyte and solid electrolyte. The earliest exploration of magnesium electrolytes using the strongly reducing Grignard reagent RMgX with diethyl ether solvent can be traced back to the 1920s. As we know, the reducing power of the Format reagent is too strong and the electrochemical window is narrow, making it difficult to find a suitable cathode material to match it; at the same time, the dissociation degree of the Format reagent in ether is low, so the electrolyte The ionic conductivity of is also low. This system has not been put into practical use.
Starting in the 1980s, the "first milestone" in the practical exploration of magnesium secondary batteries was the electrolyte system constructed of magnesium organic borate- tetrahydrofuran . The reducing property of magnesium salt is significantly weakened compared to the Format reagent, but the problem of too narrow electrochemical window (2V for magnesium voltage) has not yet been solved. Since then, researchers have discovered that alkyl magnesium- alkyl aluminum chloride-diethyl ether solution can be stable at a magnesium voltage of 2.1V. This "second milestone" led to the birth of the first usable magnesium secondary battery.
As a recent "milestone", the paper Solvation sheath reorganization enables divalent metal batteries with fast interfacial charge transfer kinetics published in Science in the fall of 2021 discusses the use of methoxyamine solvent additives to chelate magnesium ions (also including calcium ions), and use solvents to chelate magnesium ions (including calcium ions). The method of shell reorganization improves the electrode-electrolyte interface dynamics and suppresses interface side reactions, and finally obtains a high energy density battery (this system magnesium Secondary battery theoretical energy density 412Wh/kg) method. Researchers believe that the affinity of methoxyethylamine chelating agent for magnesium ions and calcium ions is 6-41 times that of traditional ether electrolytes. The methoxyamine solvation shell formed around magnesium ions also ensures Highly reversible cycling of magnesium and calcium ions on the negative electrode surface, and rapid intercalation/extraction (relatively) The ability of high-capacity cathodes; the reorganization of the solvation shell improves the desolvation process of magnesium ions on the electrolyte-electrode surface, reduces overvoltage, and also suppresses interface side reactions. Based on this, use magnesium metal (foil material, 100nm thick) negative electrode, Mg0.15MnO (magnesium manganate) 2 positive electrode, 0.5mol/L Mg(TFSI)2-DME ( glycol dimethyl ether ) and add Magnesium secondary battery composed of 1-methoxy-2-propylamine electrolyte, 0.5C rate cycle at 2.0-3.3V voltage 200 times, reflecting a capacity of 190mAh/g.
It can be seen from the electrochemical performance test results of the battery that the solvation shell formed by the chelating agent has a very obvious effect.The main capacity degradation process of the battery is basically completed within the first 20 cycles, and the subsequent Coulombic efficiency is close to 100%; the positive electrode steady-state capacity is still about 190mAh/g after 200 cycles. Of course, on the other hand, there is still a certain gap between the voltage curves during charging and discharging, indicating that overvoltage still exists to a certain extent; the theoretical energy density of the battery is only calculated by the positive electrode capacity and the average battery discharge voltage, and actually takes into account the negative electrode, electrolyte After being mixed with packaging materials, there will still be a considerable degree of attenuation. In summary, electrolyte-based magnesium-ion batteries are gradually making progress over the course of about 100 years. The solid electrolytes used in magnesium solid-state batteries are basically divided into organic polymer systems (adding magnesium salts and possibly inorganic fillers) and inorganic systems ( phosphates , borohydride, chalcogenide compounds, metal organic framework materials), etc.
The exploration of organic polymer-based magnesium solid electrolytes began in the 1980s. The initial material composition was PEO (polyethylene oxide)-magnesium chloride. The electrical conductivity is only an almost negligible 10-9S/cm at room temperature, and it only rises to 10-5S/cm at 80 degrees. If magnesium chloride is replaced by magnesium perchlorate, the room temperature ionic conductivity can be increased to 10-5S/cm. Similar to the polymer electrolyte system used in solid-state lithium batteries, some inorganic fillers, such as silica, magnesium, alumina, titanium, etc., can improve electrolyte performance. Other polymer matrices such as PVDF (polyvinylidene fluoride), PAN (polyacrylonitrile), etc. are also in the research process, and their comprehensive performance is expected to be close to the corresponding polymer electrolyte for lithium solid-state batteries.
3. Magnesium battery cathode, searching up and down so far
Although we have a relatively certain negative electrode and an electrolyte that has achieved initial results (mainly the electrolyte system), the main difficulty of magnesium secondary batteries - the cathode - has not yet made a satisfactory breakthrough. . According to the different principles of magnesium storage, we can divide magnesium battery cathodes into two categories: intercalation materials and phase change materials (such as ternary/iron lithium cathodes and sulfur cathodes for lithium batteries), and each category has subdivided technical paths. Among the intercalation materials, the Chevrel phase material MgxMo6S8-Mo6S8 is the first cathode to demonstrate magnesium storage capacity and has a three-dimensional magnesium ion diffusion channel. The researchers believe that the highly reversible deintercalation of magnesium ions is because the Mo6S8 cluster can effectively balance the charge of divalent magnesium. Of course, the voltage of this cathode material against magnesium is not high (the voltage platform is about 1.2V), the capacity is not large (only 122mAh/g when x=2), and the molybdenum in the frame structure is not cheap and easy to obtain, so the practical application value is not big.
The layered structure cathode also has subdivided paths of oxide/complex anions, such as magnesium vanadate, fluorine-doped magnesium molybdate cathode, etc. However, the capacity and voltage performance of this type of positive electrode are also average.
In addition, open frame structures such as iron magnesium silicate, magnesium-based Prussian blue, etc. have large vacancies that can accommodate negative effects such as polarization caused by magnesium. However, the energy density of the battery corresponding to this type of electrode material is also very low. Phase change materials serve as magnesium battery cathodes, the most typical example being α-manganese dioxide. When the magnesium content embedded in it is not high (no more than 0.25 mol per mol), the diffusion energy barrier of magnesium is only 0.3-0.6eV, which is roughly the same as the diffusion barrier of lithium ion in typical cathode materials; but the magnesium content Once increased, the structure of α-manganese dioxide is no longer stable, and the magnesium diffusion barrier increases sharply. The positive electrode used in the aforementioned chelating agent electrolyte system research is Mg0.15MnO (magnesium manganate) positive electrode.
Therefore, considering the aspects of capacity, average voltage, rate performance, applicable temperature, overvoltage, cycle life and cost, the cathode material of magnesium battery has not yet found its own "iron lithium", its own " lithium cobalt oxide ", "Ternary" even has its own " lithium manganate ". The intercalation materials are "wary" of divalent magnesium, and the phase change material is "not adaptable" to various dynamics caused by phase changes, which makes magnesium (secondary) batteries still not out of the laboratory and ushering in an explosion of industrial applications. brilliant dawn.
3. Aluminum secondary battery: Let a hundred flowers blossom and thousands of horses bloom together.
1, aluminum battery system also starts from the aluminum metal negative electrode.
is similar to other secondary battery material systems. Researchers also need to find active materials and auxiliary materials with aluminum as the core. component. Considering our preference for the capacity and electrode potential of aluminum metal, choosing aluminum as the negative electrode of aluminum secondary batteries is the mainstream research idea, and the entire material system is built on this basis.
Aluminum metal is used as the negative electrode, and its advantages and disadvantages are very distinct. In addition to capacity, voltage, and element abundance, the advantages also include higher safety (compared to lithium-embedded graphite anodes, let alone lithium metal, aluminum metal can almost be called an "absolutely safe anode"). The main disadvantages include that under normal conditions, the aluminum metal surface will be passivated to form aluminum oxide (resulting in the actual value of the electrode potential being much lower than the theoretical value, and the voltage behavior is lagging). In many electrolyte systems, the fresh aluminum surface will Corrosion/aluminum dendrite growth occurs (some alloy systems are trying to solve this problem, but charge alloying is also a problem); and the kinetic difficulty of effective deposition/stripping of +3-valent aluminum ions is relatively more difficult Higher (whose derived atomic groups as "real carriers" are trying to mitigate this effect).
It can even be said that the electrochemical performance of aluminum in a water system (when there is no external voltage, the neutral environment is effectively passivated, the acidic environment severely generates hydrogen, and the strong alkaline environment also generates hydrogen; during charging, the decomposition of water is earlier than the deposition of aluminum ) directly hindered the invention of aqueous aluminum secondary batteries.
2, aluminum battery electrolyte, may be the real " tram problem "
Since it is difficult to obtain effective aluminum secondary batteries in aqueous systems, the focus of research work will be carried out in three aspects. The first aspect is to find a suitable molten salt system (it can be said that aluminum is obtained from the cryolite solution of electrolytic alumina). The second aspect is similar to liquid lithium-ion batteries, looking for a suitable organic electrolyte system; the third aspect, similar to solid-state lithium-ion batteries, looking for a suitable solid electrolyte; of course, any system needs to meet the requirements of containing aluminum or dissolved Aluminum salt can effectively migrate aluminum ions and is the basic condition for charging and depositing metallic aluminum. Aluminum chloride - The combination of other metal chloride salts is the first generation of molten salts, but has a very high melting point. Switching to a combination of aluminum chloride-inorganic metal and organic chloride salts can lower the melting point of the electrolyte system to the room temperature range and achieve effective aluminum conduction (of course, the carriers at this time are chloroaluminate ions instead of trisulfide). Valent aluminum ions; the organic chloride salts in the system are the corresponding ionic liquids), and the pH of the system can be controlled to a certain extent by adjusting the proportion of salts.
If a researcher develops methylimidazolium chloride ionic liquid, when combined with aluminum chloride, the overall performance of the battery will be relatively high (from the perspective of the electrolyte, the conductivity of aluminum ions is as high as 10E-2 S/cm The cycle life is as high as thousands of times, and the battery rate performance is as high as tens of C), which can be regarded as a pioneering research work on aluminum secondary batteries. The advantage of
aluminum chloride system ionic liquid is that it can effectively solve the problem of aluminum surface passivation, and it is not easy to produce aluminum dendrites during the circulation process. However, it is highly corrosive and easily hygroscopic, making synthesis, storage, and transportation demanding on the environment. Researchers are also developing a chlorine-free ionic liquid electrolytic liquid system. Of course, ionic liquids are expensive, and the co-embedding of aluminum and anions into the cathode also affects capacity and energy density.
For electrolyte systems similar to traditional lithium-ion batteries, the main problem in current research work is that a suitable aluminum salt-solvent system has not yet been found that can simultaneously have high solubility , high aluminum ion dissociation ability, and passivation film Properties such as removal capabilities. The solid electrolyte conducts aluminum. Since aluminum ions can be embedded in certain cathode materials alone, it is feasible in principle. However, the aluminum ion has a small radius and high charge, making it difficult to embed and extract from the inorganic solid electrolyte. The polymer electrolyte needs to match the aluminum salt, which faces the same problem as the electrolyte (or prepare a gel ionic liquid electrolyte, the aluminum conductivity is the same as the aforementioned ionic liquid).Research in this area is still in the early exploratory stage.
3. Aluminum battery cathodes have a long way to go.
Aluminum battery cathodes can also be divided into two categories: intercalation materials and phase change materials. Each category has subdivided technical paths. The aforementioned graphite materials can effectively embed chloroaluminate ions and combine with electrons to form Cn[AlCl4]. Its actual capacity is about 100mAh/g, and its voltage against aluminum is about 2V. The rate performance of different types of graphite varies greatly, and various types of graphene perform better in laboratory research work.
Some transition metal oxides have demonstrated the ability to store aluminum. Among them, vanadium pentoxide has a very high theoretical capacity (more than 400mAh/g), and the actual value is also close to 300mAh/g. Lower voltage and poor cycle life are its main disadvantages. In addition to vanadium pentoxide, titanium oxide, tin oxide , ferric oxide , etc., have also shown aluminum storage capabilities to varying degrees.
In short, the overall performance of the positive electrode of aluminum secondary batteries is even weaker than that of magnesium secondary batteries. Aluminum ions with small radius and high valence can be said to be a well-deserved fierce horse, which requires great efforts to tame.
4. If someone says, I have a high-performance aluminum secondary battery
At the end of 2021, Saturnose, a company supported by Saudi Arabia ’s Dana Venture Fund, provided two rounds of seed funding, announced that its aluminum ion battery has a power of more than 1500Wh/L The volume energy density exceeds 550Wh/kg, the cycle life exceeds 30,000 times, and the energy density is still There is a lot of room for improvement.
But when we review the "problem set" of aluminum-ion batteries, it is obvious that the birth of high-performance aluminum-ion batteries requires almost simultaneous, all-round technological and even scientific breakthroughs. Estimates over a ten-year period are probably very optimistic.
4. Zinc secondary battery: efforts focusing on water system
1. Zinc battery system, zinc metal anode is still the preferred choice
Zinc ions are heavier than lithium, sodium, magnesium, and aluminum ions, so researchers do not have very high energy density of the corresponding battery. expectations. There are many choices for its material system, but the negative electrode is relatively consistent, and zinc metal sheets are the mainstream application direction.
Since the distant past, zinc negative electrodes have been used as the first choice for disposable batteries. zinc-air batteries , zinc-silver button batteries, and zinc-manganese dry batteries have all gone through more than half a century or even a century of application. Many methods for constructing zinc secondary battery material systems are also inspired by the pros and cons of zinc-manganese dry batteries.
2. Zinc battery electrolyte, water system is the mainstream.
Zinc is less reducible than aluminum, and its passivation effect is also weaker, which makes zinc secondary batteries more suitable for the use of water systems. Zinc metal has poor stability in strong acids and easily forms dendrites and insoluble products in strong alkalis. It is relatively better to use a neutral (weak acid) aqueous solution with zinc salt as the electrolyte.
Researchers have found that zinc aqueous solutions represented by zinc sulfate , zinc nitrate , zinc chloride , etc. have high zinc ion conductivity (more than 1 S/cm). Taking into account the oxidation potential of chloride ions and the passivation effect of nitrate radicals, it is a better system construction strategy to use zinc sulfate as the main salt and add some inorganic/organic group auxiliary salts to suppress side reactions. Of course, the voltage of aqueous zinc-ion batteries is within 2V, which makes it difficult to significantly increase its energy density.
3. Zinc battery cathode: manganese, vanadium and other
The cathode system of zinc secondary batteries is mainly divided into phase change type-based manganese-based ( manganese dioxide ) materials, intercalation-type vanadium-based (vanadium pentoxide ), spinel phase composite oxide, etc. Manganese dioxide cathodes have various structural types, including tunnel structures (αβγ and rhombohedral structure R-type), layered structures (δ-type) and spinel structures (λ-type), which are composed of manganese-oxygen octahedrons passing through common edges. Or the vertex angles are connected in an orderly manner. Some manganese dioxide cathodes can fully store/dezincify zinc and show a large capacity.However, during the electrochemical cycle, manganese dioxide structures with different crystal configurations are easy to transform into each other, and the accompanying stress will cause the destruction of the crystal structure , thereby causing the battery capacity to decay during long cycles. In addition, the divalent manganese obtained at a reduced price after zinc storage is easily soluble in water. The poor conductivity of manganese dioxide also affects ion diffusion and the overall electrochemical performance of the electrode. In addition, some systems have co-intercalation of hydrogen ions and zinc ions .
4, then look forward to the breakthrough of the low-cost route
Although the specific capacity and voltage of the positive electrode limit the energy density of the battery, the progress of zinc secondary batteries is generally ahead of magnesium and aluminum batteries. If effective progress is made in research on bulk materials and interfaces, low-cost zinc secondary batteries may achieve certain achievements.
5. The double edge of the sword! Looking back at the growth of lithium batteries
1, the meeting of the times, the construction of lithium battery material system Looking back at
Looking back at the development history of lithium ion batteries, if we count from the discovery of metallic lithium, it has been about 200 years to now; if we count from the construction of the electrolyte system, it has also More than half a century.
It can be seen that there seems to be a significant scale of lithium-ion batteries at present, and the construction of its basic material system has also gone through a long process, and there are many wonderful things in it.
2, consumption - power - energy storage, the deterministic big market
The building of the battery material system is beginning to take shape, coinciding with the explosion of consumer battery demand. The cost tolerance and performance requirements in the era of mobile PCs and smartphones have given rise to the large-scale commercialization of lithium cobalt oxide batteries . In the more than ten years after 2010, electric vehicles have promoted ternary and iron-lithium batteries, and the demand for energy storage has also emerged.
Looking into the future, by 2030, global consumption, power, and energy storage demand may exceed 3TWh. As the energy revolution advances, the scale of the secondary battery market will inevitably expand further.
3. Expensive lithium carbonate! Single resource dependence
From the second half of 2018 to 2020, lithium resources experienced the bottom of the cycle, and the supply side expanded cautiously. Since the fourth quarter of 2020, terminal demand has increased sharply, midstream expansion has accelerated, lithium resource expansion has lagged behind, there has been an obvious supply gap, and prices have risen sharply to record highs. In the future, certainty in consumption, power, energy storage and other fields will create strong long-term demand for lithium resources.
The price of lithium carbonate has risen sharply, which has affected the process of reducing the cost of lithium batteries. If we try to complete the energy revolution at a low cost, high-quality and low-cost secondary batteries are indispensable. Then, on the one hand, the industry should develop lithium resources, and on the other hand, it must also actively develop competitive technology routes.
Therefore, the development of sodium-ion batteries is necessary; even if high-priced carrier batteries have not yet emerged, they still deserve long-term attention.
(This article is for reference only and does not represent any investment advice on our part. If you need to use relevant information, please refer to the original text of the report.)
Selected report source: [Future Think Tank]. Future Think Tank - Official website
