The biggest difference between lithium-ion batteries and rechargeable lithium batteries is that the former uses lithium intercalation compounds instead of metallic lithium as the negative electrode. Therefore, the research and development of lithium-ion batteries is largely the research and development of negative electrode lithium intercalation compounds.
The necessary conditions for a negative electrode material of lithium-ion batteries are: (1) low electrochemical equivalent; (2) easy and highly reversible insertion and extraction of lithium ions; (3) large diffusion coefficient of Li+; (4) good electronic conductivity; (5) good thermal stability and electrolyte compatibility, making it easy to make suitable electrodes.
Currently, the negative electrode materials for lithium-ion batteries are mainly divided into two categories: carbon materials and non-carbon materials. The negative electrode materials that have been practically used in lithium-ion batteries are basically carbon materials, such as artificial graphite, natural graphite, mesophase carbon microspheres (MCMB), petroleum coke, carbon fiber, pyrolytic resin carbon, etc. In addition, people are also actively researching and developing non-carbon negative electrode materials.
1. Carbon anode materials
Carbon materials can be divided into two categories based on their structural characteristics: easily graphitized carbon and difficult-to-graphitize carbon, which are commonly referred to as soft carbon and hard carbon materials. Hard carbon typically has smaller grains, irregular grain orientation, lower density, porous surface, and larger interplanar spacing (d002), generally between 0.35 and 0.40 nm, while soft carbon has a spacing of around 0.35 nm.
Soft carbon mainly includes carbon fibers, carbon microspheres, and petroleum coke. Among these, ordinary petroleum coke has a relatively low specific capacity, approximately 160 mAh·g⁻¹, and poor cycle performance. However, by modifying domestically produced petroleum coke, the specific capacity can be increased to 250 mAh·g⁻¹, and better cycle performance can be achieved. Hard carbon mainly includes resin carbon, pyrolytic carbon from organic polymers (PVA, PVC, PVDF, PAN, etc.), and carbon black (such as acetylene black).
Compared with non-graphitized carbon materials, graphite has better electrical conductivity, higher crystallinity, and a good layered structure, making it more suitable for the insertion/extraction of Li ions, forming LiC6 lithium-graphite intercalation compounds Li-GIC.
Graphite materials are mainly divided into two categories: artificial graphite and natural graphite. Artificial graphite is produced by graphitizing easily graphitizable carbon (soft carbon) at high temperatures. Artificial graphite materials used as anode materials in lithium-ion batteries mainly include graphitized mesophase carbon microspheres, graphite fibers, and various other graphitized carbons.
2. Non-carbon anode materials
Lithium-containing transition metal nitrides were developed based on the research of lithium nitride (Li3N), a high-ionic conductor material (with a conductivity of 102 cm⁻¹). They can be divided into two types: anti-CaF₂ and Li3N, with representative materials being Li₃-xCoxN and Li₇MnN₄, respectively. Li₃-xCoxN belongs to the Li₃N-type lithium transition metal nitride structure (its general formula is Li₃-xMxN, where M is Co, Ni, Cu, etc.). This material has a high specific capacity, reaching 900 mAh·g⁻¹, no irreversible capacity, and an average charge-discharge voltage of about 0.6V. It can also be matched with cathode materials that cannot provide lithium to form batteries.
Li7MnN4 is a lithium transition metal nitride with an anti-CaF2 structure (its general formula is Li2n-1MNn, where M represents a transition metal). It has a low specific capacity of about 200 mAh·g-1, but good cycle performance, flat charge and discharge voltage, and no irreversible capacity. In particular, when this material is used as the negative electrode of a lithium-ion battery, it can be matched with positive electrode materials that cannot provide lithium sources to form a battery.
Sulfides such as TiS2 and MoS2 can also be used as negative electrode materials for lithium-ion batteries, and can be matched with 4V-level positive electrode materials such as LiCoO2, LiNiO2, and LiMn2O4 to form batteries. These batteries have a lower voltage; for example, if TiS2 is used as the negative electrode and LiCoO2 is used as the positive electrode, the voltage is around 2V, and its cycle performance is good, reaching 500 cycles.
