All-solid-state battery cathodes generally use composite electrodes, which include solid electrolytes and conductive agents in addition to the electrode active materials, which play the role of transporting ions and electrons in the electrodes. Oxide cathodes such as LiCoO2 and LiMn2O4 are commonly used in all-solid-state batteries. When the electrolyte is sulfide, due to the large difference in chemical potential, the oxide positive electrode attracts Li much stronger than the sulfide electrolyte, causing a large amount of Li+ to move to the positive electrode, and the interface electrolyte is poor in lithium.
If the oxide positive electrode is an ionic conductor, the space charge layer will also be formed at the positive electrode, but if the positive electrode is a mixed conductor (such as LiCoO2 is both an ionic conductor and an electronic conductor), the concentration of Li+ at the oxide will be diluted by electronic conduction, the space charge layer disappears. At this time, the Li+ at the sulfide electrolyte moves to the positive electrode again, and the space charge layer at the electrolyte further increases, resulting in a very large interface impedance that affects the battery performance. Adding only an ion-conducting oxide layer between the positive electrode and the electrolyte can effectively inhibit the generation of a space charge layer and reduce the interface impedance. In addition, increasing the ionic conductivity of the positive electrode material 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, which mainly include high-capacity ternary cathode materials and 5V high-voltage materials. Typical representatives of ternary materials are LiNi1-x-yCoxMnyO2 (NCM) and LiNi1-x-yCoxAlyO2 (NCA), both of which have a layered structure and a high theoretical specific capacity. Compared with spinel LiMn2O4, 5V spinel LiNi0.5Mn1.5O4 has a higher discharge plateau voltage (4.7V) and rate performance, so it becomes a powerful candidate material for all-solid-state battery cathodes. In addition to oxide cathodes, sulfide cathodes are also an important part of all-solid-state battery cathode materials. Such materials generally have a high theoretical specific capacity, which is several times or even an order of magnitude higher than that of oxide cathodes. When matched with a sulfide solid electrolyte with good conductivity, due to the similar chemical potential, no serious space charge layer effect will be caused, and the obtained all-solid-state battery is expected to achieve the real cycle requirements of high capacity and long life. However, the solid/solid interface between the sulfide positive electrode and the electrolyte still has problems such as poor contact, high impedance, and inability to charge and discharge.
The anode materials mainly include metallic Li anode materials, Li alloy anode materials, carbon group anode materials and oxide anode materials. The metal Li anode material has become one of the most important anode materials for all-solid-state batteries due to its high capacity and low potential. However, metal Li will produce lithium dendrites during the cycle, which will not only reduce the amount of lithium that can be inserted/desorbed, but also cause safety problems such as short circuits. In addition, metallic Li is very active and easily reacts with oxygen and moisture in the air, and metallic Li cannot withstand high temperatures, which brings difficulties to the assembly and application of batteries. Adding other metals and lithium to form an alloy is one of the main methods to solve the above problems. These alloy materials generally have a high theoretical capacity, and the activity of metallic lithium is reduced by the addition of other metals, which can effectively control the formation of lithium dendrites and the occurrence of electrochemical side reactions, thereby promoting interface stability. The general formula of lithium alloy is LixM, where M can be In, B, Al, Ga, Sn, Si, Ge, Pb, As, Bi, Sb, Cu, Ag, Zn, etc. However, the lithium alloy negative electrode has some obvious defects, mainly due to the large change in the electrode volume during the cycle. In severe cases, the electrode powder will fail and the cycle performance will be greatly reduced; at the same time, since lithium is still the active material of the electrode, corresponding safety hazards still exist. At present, the methods that can improve these problems mainly include the synthesis of new alloy materials, the preparation of ultra-fine nano alloys and composite alloy systems (such as active/inactive, active/cleanness, carbon-based composite and porous structure). Carbon-based, silicon-based and tin-based materials of the carbon group are another important negative electrode material for all-solid-state batteries. Recently, nanocarbons such as graphene and carbon nanotubes have appeared on the market as new types of carbon materials, which can expand the battery capacity by 2 to 3 times. Oxide anode materials mainly include metal oxides, metal-based composite oxides and other oxides. Typical oxide anode materials are TiO2, MoO2, In2O3, Al2O3, Cu2O, VO2, SnOx, SiOx, Ga2O3, Sb2O5, BiO5, etc. These oxides all have a higher theoretical specific capacity. However, in the process of replacing the elemental metal from the oxide, a large amount of Li is consumed, causing a huge loss of capacity, and a huge volume change is accompanied by a huge volume change during the cycle, which causes the battery to fail. This problem can be improved by compounding with carbon-based materials.