Technical Development of Electrode Materials for Lithium Ion Batteries
1. Anode materials for lithium-ion batteries
The early anode materials used metal lithium directly, but during the charging and discharging process, dendritic lithium would pierce the diaphragm and cause short circuits, leakage and even explosion. The use of aluminum-lithium alloy can solve the problem of dendritic lithium, but serious volume expansion and even powdering will occur after several cycles. The concept of the rocking chair battery solves this problem. It uses non-metallic materials with a layered structure such as graphite to store lithium to avoid the generation of dendritic lithium, thereby greatly improving the safety of the battery.
Currently, the research on anode materials for lithium-ion batteries with practical value or application prospects mainly focuses on four aspects: (1) carbon materials; (2) metal oxides; (3) metal nitrides; (4) Nano silicon. At present, only carbon materials can be widely used as anode materials for commercial lithium-ion batteries. Choose carbon material as the anode of the battery, which is compatible with the high-performance cathode materials LiCoO2, LilNiO2, and manganese-containing compounds of today's lithium-ion batteries. During the production process, the cathode and anode materials are in a discharged state, and the battery needs to be activated through the initial charge and discharge.
(1) Carbon material
The weak intermolecular interaction force between the graphite middle layer and the layer is conducive to lithium intercalation and deintercalation. Lithium is inserted into the carbon layer to form a lithium-intercalated graphite compound with a maximum theoretical capacity of 372mAh.g1. Carbon materials can be divided into natural carbon materials and artificial materials. Natural graphite materials have a high degree of graphitization, complete crystallization, many embedding positions, and large capacity, but they are more sensitive to electrolyte and have poor cycle stability. Artificial carbon materials include soft carbon materials and hard carbon materials. Soft carbon materials can be graphitized, have certain impurities, and are difficult to prepare with high purity, but they have abundant resources and low prices. Hard carbon materials are obtained by high-temperature pyrolysis of various high-molecular polymers, are not easy to graphitize, have a highly disordered and irregular structure, and have a capacity of more than 1000mAh.gl. However, there is a larger irreversible capacity in hard carbon materials.
Incorporating potassium and boron into carbon materials and plating a layer of Ag, Zn, and Sn on the surface of carbon fibers can effectively improve the capacity and charge-discharge efficiency of the material.
(2) Metal oxide
In order to solve the problem of metal powdering, Idota proposed to use metal oxide such as SnO2 instead of pure metal as anode material. In the process of lithium insertion, it first undergoes an irreversible reaction, namely SnO2+4Liu003dSn+2Lo, and the generated nano-elemental tin is uniformly dispersed in the crystal lattice formed by lithium oxide. Then the inserted lithium and tin form a lithium-tin alloy Sn+4.4Liu003dLg4Sn. This process is a reversible process, that is, lithium can be reversibly inserted and released in the lithium-tin alloy.
The lattice volume of LiTisO12+3LiLiTis012 is basically unchanged during the lithium insertion and removal of LaTisO12, and the material has good cycle stability. The capacity of metal oxide Mo (Mu003dCo, Cu, Ni, Fe, etc.) nanomaterials can still be maintained at 700mAh.g1 after 100 cycles. In addition, other metal oxides such as InVO4, FeVO4, MnV206, and TiO2 also have a large lithium storage capacity, but a large irreversible capacity.
(3) Metal nitrides
Recently, it has been discovered that some transition metal nitrides Li3.xMsN (M:Co, Ni, Cu) have good electrochemical stability and very High reversible inventory, charge and discharge capacity up to 760mAh.glL2.6CooN capacity up to 900mAh.g1 and can be used to improve the electrochemical performance of SnO. Because SnO's first irreversible capacity is too high and its application is limited, compounding with L2.6Coo4N can effectively reduce the excessively high first irreversible capacity of Sno to improve the cycle performance of the material. Research on the lithium insertion function found that the material will transform from the hexagonal phase to the amorphous phase after the first lithium removal, and the amorphous phase can insert a large amount of lithium ions.
(4) Nano silicon
Nano silicon also has a high lithium storage capacity, which is also a current research hotspot. The thin film silicon made by uniformly dispersing nano-Si in the electrochemically inert TiN lattice and depositing silicon on the porous nickel substrate can obtain higher capacity. By using chemical vapor deposition method to compound some nano-silicon in the carbon material, the capacity of the material can be obviously improved, and the capacity of silicon coated with carbon can reach 1200mAh.gl.
II. Cathode materials for lithium-ion batteries
(1) LiCoO2 The cathode materials for lithium battery pack products currently on the market are mainly used LiCoO2, because of its simple production process, good material stability, and the number of cycles can reach more than a thousand times. However, LiCoO2 has many shortcomings: expensive, polluting the environment, poor safety performance, low specific energy, about 140mAh.g1. Partial replacement of Co with Ni or Mn on the one hand can reduce costs, reduce pollution, and can also improve the reversible capacity and cycle stability of the material.
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