Analysis of key materials for all-solid lithium ion batteries

All solid state lithium ion batteries with the replacement of traditional organic liquid electrolyte and solid electrolyte is expected to be fundamentally solved battery security issues, is the ideal chemical power electric cars and large-scale energy storage.

The key of the solid state battery technology mainly includes the preparation of high room temperature conductivity and electrochemical stability of solid electrolyte and used throughout the high energy of solid state lithium ion battery electrode materials, the improvement of electrode/solid electrolyte interface compatibility.

All solid state lithium ion battery structure including the anode, electrolyte and cathode, all is composed of solid materials, compared with the traditional electrolyte of lithium ion batteries have the advantages are:

1) Completely eliminates the electrolyte corrosion and leakage of safe hidden trouble, higher thermal stability;

(2) Don't have to be encapsulated liquid, support serial stack arrangement and bipolar structure, improve the production efficiency;

(3) As a result of the solid electrolyte solid features, can be superimposed multiple electrode;

(4) The electrochemical stability window width up to 5 v (above), can match the high voltage electrode materials;

(5) Solid electrolyte are generally single ion conductor, almost no side effects, longer service life.

Solid electrolyte

Polymer solid electrolyte

Polymer solid electrolyte (SPE), by the polymer substrate (such as polyester, enzyme and polyamine, etc.) and lithium (e.g., LiClO4, LiPF6, LiBF4, etc.), because of its lighter, the viscoelasticity of high quality and good performance characteristics such as mechanical processing and has received the widespread attention.

Development so far, the common SPE includes polyethylene oxide (PEO), polyacrylonitrile (PAN), poly (vinylidene fluoride) (PVDF), polymethyl methacrylate (PMMA), poly (propylene oxide (PPO), poly (vinylidene chloride (PVDC) and single ion polymer electrolyte and other system.

At present, is still the mainstream of the SPE matrix was first proposed by the PEO and its derivatives, mainly thanks to the PEO of metallic lithium is stable and can better solution from lithium salt.

However, due to the solid polymer electrolyte ion transport mainly occurred in the amorphous region, and the room temperature without modification, the crystallinity of the PEO is high, leading to a lower ionic conductivity, seriously affect the large current charge and discharge capacity.

Researchers improve the PEO chains by using the method of reduce crystallinity of sports ability, so as to improve the conductivity of system, one of the most simple and effective method is to inorganic polymeric matrix particle hybrid processing.

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 packing also can rise to the adsorption of trace impurities in the electrolyte composite (such as water), enhance the role of mechanical properties.

In order to further improve performance, researchers have developed some new type of packing, including by unsaturated locus of transition metal ions and organic connection chains self-assembly (rigid), the formation of metal organic framework (MOF) attention due to its porosity and high stability.

Oxide solid electrolyte

According to the oxide solid electrolyte material structure can be divided into crystalline state and the glassy state (amorphous), and amorphous electrolyte perovskite and NASICON, LISICON and garnet, research focus in the glassy state oxide electrolyte is used in film type Lipton, electrolyte in the battery.

The oxide and amorphous solid electrolyte

Oxide and amorphous solid electrolyte chemistry stability is high, can exist in atmospheric environment stable, scale production of solid-state batteries, the current research focus is to improve the ionic conductivity at room temperature and its compatibility with electrodes. The way to improve electrical conductivity is mainly element doping element and different price. In addition, the compatibility with electrodes is also the important problem of its application.

Lipton type electrolyte

In 1992, the United States, oak ridge national laboratory (ORNL) in high purity nitrogen atmosphere by RF magnetron sputtering device sputtering target preparation of high purity Li3P04 lithium phosphate oxygen nitrogen (Lipton) electrolyte membrane.

The material has excellent comprehensive performance, room temperature ionic conductivity is 2.3 x10-6 s/cm, electrochemical window of 5.5 V (vs Li/Li +), better thermal stability, and with the positive and metal such as LiCoO2, LiMn2O4 lithium, lithium alloy anode good compatibility, etc. Lipton, membrane ionic conductivity depends on the size of the China-Africa and amorphous thin film material structure and the content of N, N content increase can improve the ionic conductivity.

Is generally believed that Lipton, are the standard of all solid state thin film battery electrolyte materials, and have got the commercial application.

Rf magnetron sputtering method with large area and the surface is bright and uniform thin film can be prepared, but at the same time there are more difficult to control, thin-film, the deposition rate of small shortcoming, therefore, the researchers try to use other method preparation of Lipton, thin films, such as pulsed laser deposition, electron beam evaporation and ion beam assisted vacuum thermal evaporation, etc.

In addition to the change of the preparation methods, element and the method of partly replacing researchers have also been used to the preparation of a variety of excellent performance of Lipton type amorphous electrolyte.

Sulfide amorphous solid electrolyte

Is the most typical sulfide amorphous solid electrolyte thio - LISICON, by KANNO professor at the university of Tokyo industry first in Li2S - GeS2 - P2S, found in the system, the chemical composition of Li4 - xGe1 - xPxS4, ionic conductivity at room temperature up to 2.2 x10 ~ 3 s/cm (x = 0.75), and electrical conductivity can be neglected. Thio - chemical general formula for Li4 LISICON - xGe1 - xPxS4 (Ge, Si, etc., A = B = P, A1, zinc, etc.).

Sulfide glass and glass-ceramic solid electrolytes

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.

Japan Osaka prefecture university professor TATSUMISAGO study of Li2S - P2S5 electrolyte is a world leading edge position, they are the first to find the Li2S - P2S5 glass heat treatment to make it part of crystallization of glass ceramic, deposited on glass substrate of the crystal phase of electrolyte conductivity improved.

All solid state battery electrode materials

Although a solid electrolyte and electrode material interface basic does not exist solid electrolyte decomposition side effects, but solid feature makes the electrode/electrolyte interface compatibility, interface has been affected by the impedance is high ion transport, leads to the low cycle life, ratio of solid state battery performance is poor. In addition, the energy density also can't meet the requirements of large batteries. On the study of electrode materials are mainly concentrated in two aspects: one is to modification of electrode materials and its interface, improve the electrode/electrolyte interface compatibility; The second is to develop new electrode material, so as to further enhance the electrochemical properties of the solid-state batteries.

The anode material

Solid-state batteries positive generally adopts the composite electrode, in addition to the electrode active material also includes a solid electrolyte and conductive agent, the electrode in the transport of ions and electrons.LiCoO2, LiFePO4, LiMn2O4 oxide anode used in solid-state batteries are common.

When the electrolyte is sulfide, due to the chemical potential is large, the oxide anode attract much stronger than the sulfide electrolyte of Li + Li + a lot of moved to the anode, electrolyte interface in poor 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.

Between the anode and electrolyte increased only ion conductive oxide layer, can effectively restrain the generation of space charge layer, reduce the interfacial impedance. In addition, the ionic conductivity increase the positive material itself, can achieve the goal of optimization of battery performance, improve the energy density.

In order to further improve the energy density of the solid-state batteries and electrochemical properties, and people in the positive, active research and development of new type high energy mainly includes three yuan high capacity of anode material and high 5 v voltage, etc.

Is a typical representative of the ternary material LiNi1 - x - yCoxMnyO2 (sliding) and LiNi1 - x - yCoxA1yO2 (NCA), are layered structure, and high theoretical specific capacity.

Compared with the spinel LiMn2O4, 5 V spinel LiNi0.5 Mn1.5 m1 has higher voltage (4.7 V) and discharge platform ratio performance, thus become the candidate of the solid-state batteries positive materials.

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.

Sulfide, however, the anode with solid electrolyte interface there are still problems such as poor contact, high impedance, unable to charge and discharge.

Anode materials

Li metal anode materials

Because of its high capacity and low potential advantages into solid-state batteries one of the main anode materials, but Li metal in the process of circulation will be the production of lithium dendrite, not only will make available to reduce the amount of lithium the embedded/take off, more serious is that can cause safety problem such as short circuit.

In addition, the metal Li is very lively, easy to react with oxygen in the air and water, etc., and high temperature metal Li didn't ability, brings to the battery assembly and application of the difficulties. Join other of lithium metal and alloy is one of the main methods to solve these problems, the alloy material are usually has high theoretical capacity, and the activity of metallic lithium and decreased by the participation of other metals, can effectively control the production of lithium dendrite and the electrochemical reaction, thus promotes the interface stability. Lithium alloy is the general formula of LixM, M can be In, B, Al, Ga, Sn, Si, Ge, Pb, As, Bi, Sb, Cu, Ag, zinc, etc.

Lithium alloy anode, however, there are some obvious flaws, mainly is the electrode in the process of circulation volume change is big, will lead to serious electrode pulverization, cycle performance fell sharply, at the same time, due to the lithium is still an electrode active material, so the corresponding safety problems still exist.

At present, can improve these problems mainly include the synthesis of new alloy materials, the preparation of ultrafine nano alloy and composite alloy system (such as active/inactive, active/clean sex, carbon composite and porous structure), etc.

Carbon anode materials

Carbon group of carbon, silicon and tin based material is another important group of all solid state battery anode materials. 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, as a new type of carbon nano carbon materials such as appeared on the market, can make the battery capacity to expand to 2-3 times before.

The oxide anode materials

Mainly include metal oxide, metal matrix composite oxide 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: at present, the most likely to be applied to all solid state lithium ion battery of the solid electrolyte materials including the PEO base polymer electrolyte, NASICON and garnet oxide electrolytes and sulfide electrolyte.

In terms of electrode, in addition to the traditional transition metal oxide anode, metal lithium, graphite anode, a series of high-performance anode materials is, has been developed, including sulfide anode oxide anode high voltage, high capacity, good stability of composite cathode, etc.

But there is still a problem to be solved:

1) the PEO base polymer electrolyte conductivity is still low, leading to poor battery ratio and low temperature performance, in addition, poor compatibility with high voltage anode new polymer electrolytes with high conductivity and resistance to high pressure remains to be developed;

2) in order to implement solid-state batteries of long life and high energy storage, the new type of high energy, high stability, it is imperative to the development of the cathode materials, high-energy electrode materials and the best combination of solid electrolyte and security need to confirm.

3) Solid-state batteries in the electrode/electrolyte interface of solid solid there has been a serious problem, including interface impedance, bad stability, interface stress changes of interface, a direct impact on the performance of the battery.

While there are many problems, in general, solid-state batteries development prospects are very bright, replace existing lithium ion battery become mainstream in the future energy storage power supply is also the trend of The Times.

The page contains the contents of the machine translation.