Describe the key materials of all solid lithium ion batteries

All-solid lithium ion battery adopts solid electrolyte to replace traditional organic liquid electrolyte, which is expected to fundamentally solve the problem of battery safety and is an ideal chemical power source for electric vehicles and large-scale energy storage.

The key technologies include preparation of solid electrolytes with high room temperature conductivity and electrochemical stability, high-energy electrode materials for all-solid lithium ion batteries, and improvement of electrode/solid electrolyte interface compatibility.

The structure of all-solid lithium ion battery includes positive electrode, electrolyte and negative electrode, all composed of solid materials. Compared with traditional electrolyte lithium ion battery, it has the following advantages:

(1) completely eliminate the electrolyte corrosion and leakage of safety hazards, thermal stability is higher;

(2) without packaging liquid, support serial superposition arrangement and bipolar structure, improve production efficiency;

(3) because of the solid state characteristics of solid electrolyte, can stack multiple electrodes;

(4) electrochemical stability window width (up to 5V), can match the high voltage electrode material;

(5) solid electrolyte is generally a single ion conductor, almost no side reactions, longer service life.

1. Solid electrolyte

Polymer solid electrolyte

Polymer solid electrolyte (SPE) is composed of polymer matrix (such as polyester, polyenzyme and polyamine) and lithium (such as LiClO4, LiAsF4, LiPF6 and LiBF4).

Up to now, common spes include polyethylene oxide (PEO), polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF), polymethyl methacrylate (PMMA), polypropylene oxide (PPO), polyvinylidene chloride (PVDC) and other single-ion polymer electrolytes.

At present, the mainstream SPE matrix is still the PEO and its derivatives that were first proposed, mainly because PEO is stable to lithium metal and can better dissociate lithium salts.

However, the ion transport in solid polymer electrolyte mainly occurs in amorphous region, and the high crystallinity of unmodified PEO at room temperature leads to low ionic conductivity, which seriously affects the ability of large current charging and discharging.

The researchers improved the movement ability of PEO chain segment by reducing the crystallinity, so as to improve the electrical conductivity of the system. The simplest and most effective method is to conduct inorganic particle hybridization treatment on the polymer matrix.

More inorganic fillers including current studies of MgO style, Al2O3, SiO2 of metal oxide nanoparticles and zeolite, montmorillonite, etc., these inorganic particles in disturbed the matrix of polymer chain segment of the order, reduce the degree of crystallinity, polymer, lithium salt and interaction between the inorganic particles to produce increased the lithium ion transport channels, improve the conductivity and ionic migration. Inorganic fillers can also adsorb trace impurities (such as water) in composite electrolytes and improve mechanical properties.

To further improve performance, researchers have developed novel fillers that self-assemble from unsaturated ligand transition metal ions and organic linkage chains (typically rigid), forming a metal-organic framework (MOF) that is of interest due to its porosity and high stability.

Oxide solid electrolyte

According to the material structure, the oxide solid electrolyte can be divided into crystal state and glass state (amorphous), among which the crystal electrolyte includes perovskite type, NASICON type, LISICON type and garnet type, etc. The research hotspot of glass oxide electrolyte is LiPON type electrolyte used in thin film batteries.

Oxide crystalline solid electrolyte

The oxide crystalline solid electrolyte has high chemical stability and can be stable in the atmospheric environment, which is conducive to the large-scale production of all-solid battery. The current research focus is on improving the room temperature ionic conductivity and its compatibility with the electrode. At present, the main methods to improve the conductivity are element substitution and heterovalent element doping. In addition, the compatibility with the electrode is also an important problem restricting its application.

LiPON electrolyte

In 1992, the American oak ridge national laboratory (ORNL) prepared LiPON electrolyte thin films by sputtering high purity Li3P04 target in high purity nitrogen atmosphere with rf magnetron sputtering device.

This material has excellent comprehensive properties, with room temperature ionic conductivity of 2.3 x10-6s /cm, electrochemical window of 5.5V(vs.Li/Li+), good thermal stability, and good compatibility with positive poles such as LiCoO2 and LiMn2O4, as well as negative poles such as lithium metal and lithium alloy. The ionic conductivity of LiPON films depends on the amorphous structure of the film material and the content of N, and the increase of N content can improve the ionic conductivity.

LiPON is widely regarded as the standard electrolyte material for all-solid thin-film batteries and has been applied commercially.

The rf magnetron sputtering method can produce large-area films with uniform surface, but it also has the disadvantages of small film composition and low deposition rate, so the researchers try to use other methods to prepare LiPON films, such as pulsed laser deposition, electron beam evaporation and ion beam assisted vacuum thermal evaporation.

In addition to the changes in preparation methods, element substitution and partial substitution methods have also been used by researchers to prepare LiPON type amorphous electrolytes with better performance.

Sulfide crystalline solid electrolyte

The most typical sulfide crystalline solid electrolyte is thio-lisicon, which was first discovered in the li2s-ges2-p2s system by professor KANNO of Tokyo polytechnic university. 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 general chemical formula of thio-lisicon is li4-xge1-xpxs4 (A=Ge, Si, etc.,B=P, A1, Zn, etc.).

Sulfide glass and glass ceramic solid electrolyte

Glassy state electrolyte consists of P2S5, usually SiS2, B2S3 network forming and network modification of Li2S, system mainly includes Li2S - P2S5, Li2S SiS2, Li2S B2S3, composition range wide, high ionic conductivity at room temperature, at the same time with high thermal stability, safety performance is good, wide electrochemical stability window (more than 5 v), the characteristics of advantages in terms of high power and high temperature solid-state batteries is outstanding, is a potential solid-state batteries electrolyte materials.

Professor TATSUMISAGO of Osaka prefecture university in Japan has been at the forefront of the research on the electrolyte of li2s-p2s5 in the world. They were the first to find that the partial crystallization of li2s-p2s5 glass by high temperature treatment resulted in the formation of glass ceramics, and the crystal phase deposited in the glass matrix greatly improved the electrolyte conductivity.

All solid state battery electrode material

Although the interface between solid electrolyte and electrode material basically has no side reaction of decomposition of solid electrolyte, the interface compatibility between electrode and electrolyte is not good due to solid characteristics, and the interface impedance is too high, which seriously affects the transmission of ions, and finally leads to the low cycle life and poor magnification performance of solid battery. In addition, the energy density can not meet the requirements of large batteries. The research on electrode materials mainly focuses on two aspects: first, modification of electrode materials and their interfaces to improve the interface compatibility of electrodes and electrolytes; The second is to develop new electrode materials to further improve the electrochemical properties of solid-state batteries.

2. Anode materials

The positive electrode of all-solid-state battery generally adopts composite electrode, which includes solid electrolyte and conductive agent in addition to electrode active material, and plays the role of transporting ions and electrons in the electrode. Oxide positive electrodes such as LiCoO2, LiFePO4 and LiMn2O4 are widely used in all solid state batteries.

When the electrolyte is sulfide, due to the large difference in chemical potential, the attraction of the oxide positive electrode to Li+ is much stronger than that of the sulfide electrolyte, resulting in a large number of Li+ moving to the positive electrode, and the interface electrolyte is deficient in lithium.

If the positive ion conductor oxide, is desperately also can form the space charge layer, but if you are extremely mixed conductor (such as LiCoO2 is both ionic conductors and electronic conductor), Li + concentration diluted by electrically conductive oxide, space charge layer disappears, the sulfide electrolyte of Li + again to move to the anode, electrolyte of space charge layer increases further, the resulting affect battery performance very large interfacial impedance.

Adding only ionic conductive oxide layer between the anode and electrolyte can effectively inhibit the generation of space charge layer and reduce the interface impedance. In addition, improving the ionic conductivity of the anode material itself can optimize the battery performance and improve the energy density.

In order to further improve the energy density and electrochemical properties of solid-state batteries, people are also actively researching and developing new high-energy anode materials, including high-capacity ternary anode materials and 5V high-voltage materials.

Typical ternary materials are lini1-x-ycoxmnyo2 (NCM) and lini1-x-ycoxa1yo2 (NCA), both of which have layered structure and high theoretical capacity.

Compared with spinel LiMn2O4, 5V spinel lini0.5mn1.5o4 has higher discharge platform voltage (4.7v) and multiplier performance, so it is a strong candidate material for all-solid state batteries.

In addition to the oxide anode, sulfide cathode is an important part of solid-state batteries battery anode materials, this kind of material has high theoretical specific capacity generally, several times higher than the oxide anode even an order of magnitude, good match sulphide solid electrolyte, with electrical conductivity due to the chemical potential, will not cause serious effects of space charge layer, solid-state batteries are expected to achieve high capacity and long life of solid weeks requirements.

However, there are still some problems such as bad contact, high impedance and failure to charge and discharge in the solid-solid interface between the positive electrode of sulfide and electrolyte.

Negative material

Metal Li anode material

Due to its advantages of high capacity and low potential, it has become one of the most important negative electrode materials for all-solid-state batteries. However, lithium dendrites will be generated in the recycling process of metal Li, which not only reduces the amount of lithium that can be embedded/removed, but also causes safety problems such as short circuit.

In addition, metal Li is very lively and easy to react with oxygen and water in the air. Moreover, metal Li is not able to withstand high temperature, which brings difficulties to the assembly and application of batteries. The addition of other metal and lithium alloy is one of the main methods to solve the above problems. These alloy materials generally have high theoretical capacity, and the activity of lithium metal decreases with the addition of other metals, which can effectively control the formation of lithium dendrites and the occurrence of electrochemical side reactions, thus promoting the interface stability. The general formula of lithium alloy is LixM, In which M can be In, B, Al, Ga, Sn, Si, Ge, Pb, As, Bi, Sb, Cu, Ag, Zn and so on.

However, there are some obvious defects in the negative electrode of lithium alloy, mainly due to the large volume change of the electrode in the cycling process, which will lead to the electrode powder failure and the significant decline in the cycling performance. At the same time, because lithium is still the active material of the electrode, the corresponding safety risks still exist.

At present, methods to improve these problems mainly include the synthesis of new alloy materials, the preparation of superfine nano-alloy and composite alloy systems (such as active/inactive, active/clean, carbon-based composite and porous structure).

Carbon negative material

The carbon, silicon and tin - based materials of carbon group are another important anode materials for all - solid battery. Carbon is typical representative with graphite materials, graphite carbon is suitable for lithium ion embedding and emergence of layered structure, has a good platform for the voltage, charge and discharge efficiency over 90%, however the theoretical capacity is low (only 372 mah/g) is one of the largest, this kind of material and the practical application has been the basic of theoretical limit, cannot meet the needs of high energy density.

Recently, graphene, carbon nanotubes and other nanocarbons have appeared on the market as new carbon materials, which can increase the battery capacity by 2-3 times.

Oxide anode material

It mainly includes metal oxides, metal matrix composite oxides and other oxides. Typical fireworks no anode materials are: TiO2, MoO2, In2O3, Al2O3, Cu2O, VO2, SnOx, SiOx, Ga2O3, Sb2O5, BiO5 etc, these oxide has high theoretical specific capacity, but in the process of replacement of metal from oxide, a large number of Li is consumed, the huge capacity loss, and with the huge volume change during the process of circulation, causing the failure of the battery, through composite with carbon materials can improve the problem.

conclusion

Solid electrolyte materials most likely to be used in all-solid lithium-ion batteries include peo-based polymer electrolytes, NASICON and garnet oxide and sulfide electrolytes.

In terms of electrodes, in addition to the traditional transition metal oxide positive electrode, lithium metal 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 sulfide positive electrode and composite negative electrode with good stability.

But there are still problems to be solved:

1) the electrical conductivity of peo-based polymer electrolyte is still low, resulting in poor battery multiplier and low temperature performance. In addition, it has poor compatibility with high voltage positive electrode, and a new type of polymer electrolyte with high electrical conductivity and high voltage resistance needs to be developed;

2) in order to achieve the high energy storage and long life of all-solid-state batteries, the development of new high-energy, high-stability positive and negative electrode materials is imperative, and the optimal combination and safety of high-energy electrode materials and solid electrolyte need to be confirmed.

3) in all solid state batteries, there are always serious problems in the interface between electrodes and electrolytes, including high interface impedance, poor interface stability, and changes in interface stress, which directly affect the performance of the battery.

Despite many problems, all-solid-state batteries have a bright future in general, and it is an inevitable trend to replace existing lithium-ion batteries as the mainstream energy storage power supply in the future.

The page contains the contents of the machine translation.