Analysis of key materials for all-solid-state lithium-ion batteries

The all-solid-state LI-ion battery replaces the traditional organic liquid electrolyte with a solid electrolyte, and is expected to solve the battery safety problem fundamentally. It is an ideal chemical power source for electric vehicles and large-scale energy storage.

The key factors include the preparation of solid electrolytes with high room temperature conductivity and electrochemical stability, high energy electrode materials suitable for all solid state LI ion batteries, and improved electrode/solid electrolyte interfacial compatibility.

The structure of the all-solid-state LI ion battery includes a positive electrode, an electrolyte, and a negative electrode, all of which are composed of a solid material, and has advantages over the conventional electrolyte LI ion battery:

1 completely eliminates the safety hazard of electrolyte corrosion and leakage, and has higher thermal stability;

2 It is not necessary to package liquid, support serial superposition arrangement and bipolar structure, and improve production efficiency;

3 due to the solid state characteristics of the solid electrolyte, a plurality of electrodes may be stacked;

4 electrochemical stable window width (up to 5V or more), can match high voltage electrode materials;

5 The solid electrolyte is generally a single-ion conductor with almost no side reactions and a longer service life.

Solid electrolyte

Polymer solid electrolyte

Polymer solid electrolyte (SPE) consists of polymer matrix (such as polyester, polymerase and polyamine) and LI salt (such as LiClO4, LiAsF4, LiPF6, LiBF4, etc.) because of its light weight and good viscoelasticity. The mechanical processing performance is excellent and has received extensive attention. Up to now, common SPEs include polyepoxy wan (PEO), polyacrylonitrile (PAN), poly-p-acetyl (PVDF), polymethyl methacrylate (PMMA), polyepoxy wan (Other systems such as PPO), polyvinylidene LV (PVDC), and single ion polymer electrolytes.

At present, the mainstream SPE matrix is still the earliest proposed PEO and its derivatives, mainly due to the stability of PEO to metal LI and better dissociation of LI salt. However, since the ion transport in the solid polymer electrolyte mainly occurs in the amorphous region, the crystallinity of the unmodified PEO at room temperature is high, resulting in low ionic conductivity, which seriously affects the high current charge and discharge capability.

The researchers improved the conductivity of the PEO segment by reducing the crystallinity, thereby increasing the electrical conductivity of the system. The most simple and effective method is to carry out the hybridization of the inorganic particles on the polymer matrix. At present, many inorganic packing include MgO, Al2O3, SiO2 and other metal oxide nano particles as well as zeolite, montmorillonite, etc. The addition of these inorganic particles disturbs the order of the polymer segments in the matrix and reduces its crystallinity. The interaction between the polymer, the LI salt, and the inorganic particles increases the LI ion transport channel and increases the conductivity and ion mobility. The inorganic filler can also act to adsorb trace impurities (such as moisture) in the composite electrolyte and improve mechanical properties.

To further improve performance, the researchers have developed new types of fillers in which the transition metal ions of the unsaturated coordination sites and the organic linking chains (generally rigid) self-assemble to form a metal organic framework (MOF) due to its porosity. And high stability has attracted attention.

Oxide solid electrolyte

According to the material structure, the oxide solid electrolyte can be classified into two types: crystalline state and glassy state (amorphous state), wherein the crystalline electrolyte includes a perovskite type, a NASICON type, a LISICON type, and a garnet type, and the glassy oxide electrolyte The research hotspot is the LIPON type electrolyte used in thin film batteries.

Oxide crystalline solid electrolyte

The oxide crystalline solid electrolyte has high chemical stability and can exist stably in the atmospheric environment, which is beneficial to the large-scale production of all-solid-state batteries. The current research hotspot is to improve the room temperature ionic conductivity and its compatibility with the electrode. The current methods for improving conductivity are mainly element replacement and doping of heterovalent elements. In addition, compatibility with electrodes is also an important issue that restricts its application.

LIPON electrolyte

In 1992, the American Oak Ridge National Laboratory (ORNL) used a radio frequency magnetron sputtering device to sputter a high purity Li3P04 target in a high purity nitrogen atmosphere to prepare a LI phosphorus oxide (LIPON) electrolyte film.

The material has excellent comprehensive performance, room temperature ionic conductivity is 2.3x10-6S/cm, electrochemical window is 5.5V (vs. Li/Li+), thermal stability is good, and positive electrode with LiCoO2, LiMn2O4 and metal The negative electrode such as LI and LI alloy has good compatibility. The ionic conductivity of the LIPON film depends on the amorphous structure and the content of N in the film material, and the increase in the N content can increase the ionic conductivity. It is widely believed that LIPON is a standard electrolyte material for all-solid-state thin film batteries and has been commercialized.

The method of RF magnetron sputtering can produce a large-area and uniform surface film, but at the same time it is difficult to control the composition of the film and the deposition rate is small. Therefore, researchers have tried other methods to prepare LIPON films, such as pulsed laser deposition, electron beam evaporation and ion beam assisted vacuum thermal evaporation.

In addition to changes in the preparation methods, the element replacement and partial substitution methods have also been used by researchers to prepare a variety of LIPON-type amorphous electrolytes having superior properties.

LIU compound crystalline solid electrolyte

The most typical LIU compound crystalline solid electrolyte is THIO-LISICON. It was first discovered in the Li2S-GeS2-P2S system by Professor KANNO of Tokyo Institute of Technology. The chemical composition is Li4-xGe1-xPxS4, and the room temperature ionic conductivity is up to 2.2x10-3S/cm (where x=0.75) and the electronic conductivity is negligible. The chemical formula of THIO-LISICON is Li4-xGe1-xPxS4 (A=GE, Si, etc., B=P, A1, Zn, etc.).

LIU compound glass and glass ceramic solid electrolyte

The glassy electrolyte is usually composed of a network forming body such as P2S5, SiS2, B2S3 and a network modified body Li2S. The system mainly includes Li2S-P2S5, Li2S-SiS2, Li2S-B2S3, and has a wide composition variation range and high room temperature ionic conductivity. It has the characteristics of high heat stability, good safety performance, and wide electrochemical window width (up to 5V). It has outstanding advantages in high-power and high-temperature solid-state batteries, and is a potential solid-state battery electrolyte material.

Professor TATSUMISAGO of Osaka Prefecture University in Japan is at the forefront of the research on Li2S-P2S5 electrolytes. They first discovered that Li2S-P2S5 glass is subjected to high temperature treatment to partially crystallize it to form glass ceramics. The crystal phase deposited in the glass matrix makes the electrolyte The conductivity is greatly improved.

All solid state battery electrode material

Although the interface between the solid electrolyte and the electrode material is substantially free of side reactions of solid electrolyte decomposition, the solid characteristics make the electrode/electrolyte interface poorly compatible, and the interface impedance is too high, which seriously affects the ion transport, and ultimately leads to a low cycle life of the solid state battery. , the rate performance is poor.

In addition, the energy density cannot meet the requirements of large batteries. Research on electrode materials has focused on two main areas:

First, the electrode material and its interface are modified to improve the electrode/electrolyte interface compatibility;

The second is to develop new electrode materials to further improve the electrochemical performance of solid-state batteries.

Cathode material

The solid-state battery positive electrode generally adopts a composite electrode, and includes a solid electrolyte and a conductive agent in addition to the electrode active material, and functions to transport ions and electrons in the electrode. Oxide positive electrodes such as LiCoO2, LiFePO4, and LiMn2O4 are commonly used in all solid state batteries.

When the electrolyte is a LIU compound, due to the large difference in chemical potential phase, the attraction of the oxide positive electrode to Li+ is much stronger than that of the LIU compound electrolyte, causing a large amount of Li+ to move toward the positive electrode and the interface electrolyte to be lean LI. If the oxide positive electrode is an ionic conductor, a space charge layer is also formed at the positive electrode, but if the positive electrode is a mixed conductor (such as LiCoO2 or the like is both an ionic conductor and an electron conductor), the Li+ concentration at the oxide is diluted by electronic conduction, and space The charge layer disappears, at which point Li+ at the LIU compound electrolyte moves again toward the positive electrode, and the space charge layer at the electrolyte further increases, thereby producing a very large interface impedance that affects battery performance.

The addition of only the ion conductive oxide layer between the positive electrode and the electrolyte can effectively suppress the generation of the space charge layer and reduce the interface impedance. In addition, improving the ionic conductivity of the positive electrode material itself can achieve the purpose of optimizing battery performance and increasing energy density.

In order to further improve the energy density and electrochemical performance of all-solid-state batteries, people are also actively researching and developing new high-energy cathodes, including high-capacity ternary cathode materials and 5V high-voltage materials. Typical representatives of ternary materials are LiNi1-x-yCoxMnyO2 (NCM) and LiNi1-x-yCoxA1yO2 (NCA), both of which have a layered structure and a high theoretical specific capacity.

Compared with the spinel LiMn2O4, the 5V spinel LiNi0.5Mn1.5O4 has a higher discharge platform voltage (4.7V) and rate performance, and thus becomes a powerful candidate for the solid state battery positive electrode.

In addition to the oxide positive electrode, the positive electrode of the LIU compound is also an important component of the positive electrode material of the all-solid battery. Such materials generally have a high theoretical specific capacity, several times or even an order of magnitude higher than the oxide positive electrode, and a solid conductive solid state. When the electrolyte is matched, since the chemical potential is close, it will not cause a serious space charge layer effect, and the obtained all-solid battery is expected to achieve the real-life requirement of high capacity and long life. However, there is still a problem of poor contact, high impedance, and inability to charge and discharge of the solidified interface between the positive electrode of the LIU compound and the electrolyte.

Anode material

Metal Li anode material

Because of its high capacity and low potential, it is one of the most important anode materials for all-solid-state batteries. However, the production of LI dendrites during the process of metal Li will not only reduce the amount of LI available for insertion/desorption, but also seriously, it can cause safety problems such as short circuits. In addition, the metal Li is very active, easily reacts with oxygen and moisture in the air. And the metal Li cannot withstand high temperatures, which makes it difficult to assemble and apply the battery.

Adding other metals and LI alloys is one of the main methods to solve the above problems. These alloy materials generally have high theoretical capacity, and the activity of metal LI is reduced by the addition of other metals, which can effectively control the formation of L dendrites. The occurrence of electrochemical side reactions promotes interface stability. The general formula of the LI alloy is LixM, wherein M may be In, B, Al, Ga, Sn, Si, Ge, Pb, As, Bi, Sb, Cu, Ag, Zn, or the like.

However, there are some obvious defects in the anode of the LI alloy, mainly due to the large volume change of the electrode during the cycle. In severe cases, the electrode powder is invalidated and the cycle performance is greatly reduced. At the same time, since LI is still an electrode active material, the corresponding Security risks still exist.

At present, methods that can improve these problems mainly include the synthesis of new alloy materials, the preparation of ultrafine nano-alloys and composite alloy systems (such as active/inactive, active/cleanness, carbon-based composites, and porous structures).

Carbon family anode material

The carbon-based, silicon-based, and tin-based materials of the carbon group are another important negative electrode material for all-solid-state batteries. Carbon-based is typical of graphite materials. Graphite carbon has a layered structure suitable for LI ion intercalation and deintercalation. It has a good voltage platform and has a charge and discharge efficiency of over 90%. However, the theoretical capacity is low (only 372mAh/g). ) is the biggest deficiency of this type of material, and the current practical application has basically reached the theoretical limit and cannot meet the demand of high energy density. Recently, nanocarbons such as graphite xi and carbon nano tubes have appeared on the market as new carbon materials, which can expand the battery capacity to 2-3 times.

Oxide anode material

It mainly includes metal oxides, metal-based composite oxides and other oxides. Typical fireworks non-negative materials are: TiO2, MoO2, In2O3, Al2O3, Cu2O, VO2, SnOx, SiOx, Ga2O3, Sb2O5, BiO5, etc. These oxides all have higher theoretical specific capacity, but are replaced by oxides. In the process of elemental metal, a large amount of Li is consumed, causing a huge capacity loss, and a large volume change accompanying the cycle, causing battery failure, which can be improved by compounding with a carbon-based material.

Conclusion

The solid electrolyte materials currently most likely to be applied to all solid state LI ion batteries include PEO based polymer electrolytes, NASICON type and garnet oxide electrolytes, and LIU compound electrolytes.

In terms of electrodes, in addition to the traditional transition metal oxide positive electrode, metal LI and graphite negative electrode, a series of high-performance positive and negative electrode materials are also being developed, including high-voltage oxide positive electrode, high-capacity LIU compound positive electrode and compound negative electrode with good stability.

But there are still problems to be solved:

(1) The conductivity of PEO-based polymer electrolytes is still low, resulting in poor battery rate and low-temperature performance, and poor compatibility with high-voltage positive electrodes. New polymer electrolytes with high electrical conductivity and high pressure resistance are yet to be developed;

(2) In order to achieve high energy storage and long life of all-solid-state batteries, it is imperative to develop new high-energy, high-stability positive and negative materials. The best combination and safety of high-energy electrode materials and solid electrolytes need to be confirmed. .

(3) There are always serious problems in the electrode/electrolyte solid-solid interface in all-solid-state batteries, including large interface impedance, poor interface stability, and interface stress changes, which directly affect the performance of the battery.

Although there are many problems, in general, the development prospect of all-solid-state batteries is very bright, and it is also an irresistible trend to replace existing LI-ion batteries into mainstream energy storage power sources in the future.

The page contains the contents of the machine translation.