1. Electrode materials
cathode materials
(1) Based on traditional cathode materials (LiCoO2, LiMn2O4, LiFePO4, etc.), various related derivative materials are developed. Through doping, coating, adjusting microstructure, controlling material morphology, size distribution, specific surface area, impurity content and other technical means, their specific capacity, rate capability, cycle performance, compaction density, electrochemical, chemical and thermal stability are comprehensively improved.
(2) Ternary materials (LiNixCoyMn1-xy) and lithium-rich materials (Mn-based and V-based) have significant development and technological research potential and broad application prospects. Therefore, nickel-cobalt-manganese ternary materials, lithium-rich manganese-based and vanadium-based materials, high-performance composite cathode materials, and high-efficiency and energy-saving polyanionic cathode materials are the mainstream cathode materials for future lithium-ion batteries; developing more efficient and energy-saving new cathode materials to overcome and replace existing defective cathode materials is also a research hotspot.
(3) A series of transition metal fluorides, oxides, sulfides, and nitrides have been shown to achieve multi-electron transfer and thus very high capacity. Electrode materials that achieve lithium storage based on conversion reaction mechanisms have a specific capacity that is 2 to 4 times higher than that of traditional lithium-ion battery electrode materials based on lithium-ion insertion and extraction mechanisms. However, many problems still need to be solved, and research on these materials is relatively limited, with many aspects of the mechanism still unclear.
(4) According to the literature, some people have also worked on organic cathode materials, which are mainly divided into conductive polymers, sulfur-containing compounds, nitrogen oxide free radical compounds and carbonyl compounds, etc. (I don't know much about the specifics. If anyone knows, please add to this information).
Where P1 and P2 are organic electrode materials (which can be small molecules or polymers), and M+ and A+ are doped positive and negative ions, typically Li+, Na+, (C4H9)4N+, Cl-, CICV, PF6-, etc. P1-M+, P2+A-, P1+A-, and P2-M+ are the doped organic electrode materials.
Anode material
(1) Carbon-based materials
Key future developments will focus on high-power graphite-based anodes and non-graphite high-capacity carbon anodes (soft carbon, hard carbon, etc.) to meet the demands of future power and high-energy batteries. Novel carbon materials, such as carbon nanotubes (CNTs) and graphene, possess unique one-dimensional and two-dimensional flexible structures, excellent thermal and electrical conductivity, and their cost is being reduced towards higher energy density, longer cycle life, and lower costs.
(2) Non-carbon materials
LTO can be compared to carbon-based materials. Metals or semiconductors such as Fe, Ge, Sn, and Si are currently hot research topics. Research focuses on coating, surface modification, nano-sizing, and composite materials to reduce volume expansion and form a stable SEI film. These metal materials, especially Si, have very high specific capacity and should be ideal cathode materials for next-generation lithium-ion batteries. However, the problems of volume expansion and SEI instability have not yet been well solved, which has restricted their development to some extent. In particular, the volumetric energy density advantage of graphite anodes is far less than theoretical calculations show, so it is not an absolute advantage in applications. Ultimately, the anode material for lithium-ion batteries will likely return to elemental Li. New types of batteries such as rechargeable lithium-ion batteries, all-solid-state lithium-ion batteries, lithium-sulfur batteries, and lithium-air batteries are being extensively researched.
2. Electrolyte materials
The key considerations are to improve the voltage window of the electrolyte, reduce costs, expand the applicable temperature range of the electrolyte, increase the ionic conductivity of the solid electrolyte, and control the formation of a stable SEI film.
Liquid electrolyte:
Currently, LiPF6/EC with one or more linear carbonates is generally used as a solvent. Various types and applications are tested by adding different additives, using different solvents, and replacing different lithium salts, because the operating temperature range of the LIPF6/EC:DMC electrolyte system is -20 to 50°C. There are also many attempts to use ionic liquids, which have a wider temperature range, lower vapor pressure, better electrochemical performance, and greater electrochemical stability, but are very expensive (Professor Dai Hongjie's aluminum-ion battery in Nature uses an ionic liquid). Another approach is to develop gel/solid electrolytes. High-voltage electrolytes are developed by purifying solvents, using ionic liquids, fluorinated carbonates, and adding additives to the positive electrode surface film. Similarly, developing solid electrolytes can significantly improve the voltage range.
Gel electrolyte
Commonly used gel-type polymer electrolyte matrices include polyacrylonitrile (PAN), polyethylene oxide (PEO), polymethyl methacrylate (PMMA), and polyvinylidene fluoride (PVDF). Gel-type polymer electrolytes have lower environmental pollution and better safety performance, making them highly favored in the battery market. In recent years, the development trend has been to obtain electrolyte membranes with high porosity, low electrical resistance, high tear strength, good acid and alkali resistance, and good elasticity through modification copolymerization or blending with nanoparticles (common inorganic fillers include SiO2 and Al2O3).
solid electrolyte
Solid electrolytes, also known as fast ion conductors, are required to have high ionic conductivity, low electronic conductivity, and low activation energy. Frankly, I think solid electrolytes are the ultimate solution for lithium-ion electrolytes. Their purpose is to solve all the current problems with lithium-ion electrolytes, so their development goal is to fundamentally address the safety issues of currently used lithium-ion batteries, improve energy density, cycle life, service life, and reduce battery costs, among other things.
3. Development and Prospects
Once issues related to lithium dendrite formation and safety are resolved, lithium metal is likely to become the ultimate negative electrode material for lithium-ion batteries. The following diagram, presented in a paper, shows a theoretical calculation-based development plan for lithium-ion batteries, progressing from lithium-ion batteries to lithium metal batteries, and finally to lithium fuel cell batteries.
Therefore, based on this, considering the year-on-year increase in energy density, the future development trend of rechargeable lithium-ion batteries may be:
A new generation of lithium-ion batteries employs high-capacity positive electrodes, high-voltage positive electrodes, and high-capacity negative electrodes, such as lithium-ion batteries using LiNi1/2Mn3/2O4,xLi2MnO3(1–x)LiNi1/3Co1/3Mn1/3O2 as the positive electrode and high-capacity Si-based materials as the negative electrode.
Rechargeable lithium-ion batteries using lithium metal as the negative electrode. Fluorinated graphite (CF)n operates at 2.9V and has a lithium storage capacity of 800mAh/g. Li/(CF)n batteries have high gravimetric energy density, but currently cannot be cycled. Other lithium-ion batteries, such as Li/FeF3, Li/MnO2, and Li/FeS2 batteries, do not yet fully meet the overall performance requirements of applications in terms of cycle life and safety.
It is expected that the first product to be realized will be a rechargeable lithium-ion battery that uses metallic lithium as the negative electrode and existing lithium-ion battery positive electrode materials.
The ultimate high-energy-density battery should be a rechargeable lithium-ion battery with lithium metal as the negative electrode and O2, H2O, CO2, and S as the positive electrode.
