1. All-solid-state lithium-ion battery
Currently, commercially available lithium-ion batteries use liquid electrolytes, hence they are also called liquid lithium-ion batteries. Simply put, an all-solid-state lithium-ion battery refers to a battery structure where all components exist in a solid state, replacing the liquid electrolyte and separator of traditional lithium-ion batteries with a solid electrolyte.
Compared to liquid lithium-ion batteries, all-solid-state electrolytes have several advantages: high safety and excellent thermal stability, capable of long-term normal operation at 60-120℃; a wide electrochemical window, reaching above 5V, allowing for compatibility with high-voltage materials; conduction of only lithium ions and not electrons; simple cooling system and high energy density; and applicability in ultra-thin flexible batteries. However, they also have significant disadvantages: lower ionic conductivity per unit area, resulting in poor specific power at room temperature; extremely high cost; and significant challenges in the industrial production of large-capacity batteries.
The performance of electrolyte materials largely determines the power density, cycle stability, safety performance, high and low temperature performance, and lifespan of all-solid-state lithium-ion batteries. Solid-state electrolytes can be divided into two main categories: polymer electrolytes (generally using a mixture of PEO and lithium salts such as LiTFSI as the electrolyte substrate) and inorganic electrolytes (such as oxides and sulfides). All-solid-state battery technology is widely recognized as a key next-generation innovative battery technology, and it is believed that as the technology matures in the near future, these problems will be easily solved.
2. High-energy-density ternary lithium battery
With the increasing pursuit of higher battery energy density, ternary cathode materials are attracting more and more attention. Ternary cathode materials offer advantages such as high specific capacity, good cycle performance, and low cost, and generally refer to layered lithium nickel cobalt manganese oxide materials. Increasing the battery voltage and the nickel content in the material can effectively improve the energy density of ternary cathode materials.
Theoretically, ternary materials inherently possess the advantage of high voltage: the standard half-cell test voltage for ternary cathode materials is 4.35V, at which ordinary ternary materials exhibit good cycle performance; increasing the charging voltage to 4.5V, the capacity of symmetrical materials (333 and 442) can reach 190, with decent cycle performance, while 532 exhibits slightly worse cycle performance; however, at 4.6V, the cycle performance of ternary materials begins to decline, and the gas expansion phenomenon becomes increasingly severe. Currently, the factor restricting the practical application of high-voltage ternary cathode materials is the difficulty in finding a matching high-voltage electrolyte.
Another way to improve the energy density of ternary materials is to increase the nickel content. Generally, high-nickel ternary cathode materials refer to materials with a nickel molar fraction greater than 0.6%. Such ternary materials have the characteristics of high specific capacity and low cost, but their capacity retention is low and their thermal stability is poor. The performance of such materials can be effectively improved by improving the preparation process. The micro-nano size and morphology have a significant impact on the performance of high-nickel ternary cathode materials. Therefore, most of the preparation methods currently used focus on uniform dispersion to obtain small-sized spherical particles with a large specific surface area.
Among numerous preparation methods, the combination of co-precipitation and high-temperature solid-state methods is the mainstream approach. First, co-precipitation is used to obtain a precursor with uniformly mixed raw materials and uniform particle size. Then, high-temperature calcination yields a ternary material with regular surface morphology and easily controlled process, which is currently the main method used in industrial production. Spray drying is simpler and faster than co-precipitation, and the resulting material morphology is not inferior to that of co-precipitation, making it a promising method for further research. The drawbacks of high-nickel ternary cathode materials, such as cation mixing and phase transitions during charge and discharge, can be effectively improved through doping and coating modifications. Improving conductivity, cycle performance, rate performance, storage performance, and high-temperature, high-pressure performance while suppressing side reactions and stabilizing the structure will remain a research hotspot.
3. High-capacity silicon-carbon anode
As a crucial component of lithium-ion batteries, the anode material directly impacts key performance indicators such as energy density, cycle life, and safety. Silicon currently boasts the highest known specific capacity (4200 mAh/g) for lithium-ion batteries. However, due to its volume effect exceeding 300%, silicon electrode materials pulverize and peel off from the current collector during charging and discharging. This results in a loss of electrical contact between active materials and between the active material and the current collector, while simultaneously forming a new solid electrolyte interphase (SEI), ultimately leading to deterioration of electrochemical performance. To address this issue, researchers have conducted extensive explorations and experiments, among which silicon-carbon composite materials represent a promising candidate for application.
Carbon materials, as anode materials for lithium-ion batteries, exhibit minimal volume change during charge and discharge, demonstrating excellent cycle stability and conductivity. Therefore, they are frequently used in composites with silicon. Carbon-silicon composite anode materials can be categorized into two types based on the type of carbon material: composites of silicon with traditional carbon materials and composites of silicon with novel carbon materials. Traditional carbon materials primarily include graphite, mesophase microspheres, carbon black, and amorphous carbon; novel carbon materials mainly include carbon nanotubes, carbon nanowires, carbon gels, and graphene. By employing silicon-carbon composites, the porous nature of the carbon material constrains and buffers the volume expansion of the silicon active centers, preventing particle aggregation and electrolyte penetration into the centers, thus maintaining the stability of the interface and the SEI film.
Many companies around the world have begun to dedicate themselves to this new type of anode material. For example, Shenzhen BTR and Jiangxi Zichen have taken the lead in launching a number of silicon-carbon anode material products. Shanghai Shanshan is in the process of industrializing silicon-carbon anode materials, and Xingcheng Graphite has taken silicon-carbon new anode materials as the future product development direction.
4. High-voltage, high-capacity lithium-rich materials
Lithium-rich manganese-based materials (xLi[Li1/3-Mn2/3]O2;(1–x)LiMO2, where M is a transition metal (0≤x≤1) and the structure is similar to LiCoO2) exhibit very high discharge specific capacity, approximately twice that of currently used cathode materials, and are therefore widely studied for use in lithium-ion battery materials. Furthermore, due to the large amount of Mn in the material, it is more environmentally friendly, safer, and cheaper than LiCoO2 and the ternary material Li[Ni1/3Mn1/3Co1/3]O2. Therefore, xLi[Li1/3-Mn2/3]O2;(1–x)LiMO2 is considered by many researchers to be an ideal choice for next-generation lithium-ion battery cathode materials.
Currently, the main method for preparing lithium-rich manganese-based materials is co-precipitation. Some researchers have also used sol-gel, solid-state, combustion, and hydrothermal methods, but the resulting materials are less stable than those prepared by co-precipitation. While these materials exhibit high specific capacity, several issues remain for practical applications: irreversible capacity during the first cycle is as high as 40–100 mAh/g; rate performance is poor, with 1C capacity below 200 mAh/g; high charging voltage causes electrolyte decomposition, resulting in suboptimal cycle performance; and there are safety concerns. These problems can be effectively addressed by employing metal oxide coating, compositing with other cathode materials, surface treatment, constructing special structures, and low upper voltage pre-charge/discharge treatment.
In 2013, the Ningbo Institute of Materials Technology and Engineering developed a novel gas-solid interface modification technology that creates uniform oxygen vacancies on the surface of lithium-rich manganese-based cathode material particles, thereby greatly improving the material's initial charge-discharge efficiency, discharge specific capacity, and cycle stability, and powerfully promoting the practical application of lithium-rich manganese-based cathode materials.
5. High voltage withstand electrolyte
Although high-voltage lithium-ion battery materials are receiving increasing attention, these high-voltage cathode materials still cannot achieve satisfactory results in practical production applications. The biggest limiting factor is the low electrochemical stability window of carbonate-based electrolytes. When the battery voltage reaches around 4.5V (vs. Li/Li+), the electrolyte begins to undergo severe oxidative decomposition, preventing the lithium intercalation/deintercalation reaction from proceeding normally. Developing electrolyte systems that can withstand high voltages has become a crucial step in promoting the practical application of this new material.
The biggest advantage of lithium-sulfur batteries lies in their high theoretical specific capacity (1672 mAh/g) and specific energy (2600 Wh/kg), far exceeding other types of lithium-ion batteries widely used in the market. Furthermore, the abundance of elemental sulfur makes these batteries inexpensive and environmentally friendly. However, lithium-sulfur batteries also have some disadvantages: elemental sulfur has poor electronic and ionic conductivity; intermediate discharge products dissolve into the organic electrolyte; polysulfide ions can migrate between the positive and negative electrodes, leading to the loss of active materials; the lithium metal anode undergoes volume changes during charge and discharge and is prone to dendrite formation; and the sulfur cathode experiences up to 79% volume expansion/contraction during charge and discharge.
The main solutions to the above problems generally focus on two aspects: electrolyte and cathode material. For the electrolyte, ether-based electrolytes are mainly used as the battery electrolyte. Adding some additives to the electrolyte can effectively alleviate the dissolution problem of lithium polysulfide compounds. For the cathode material, the main approach is to combine sulfur with carbon materials or sulfur with organic matter, which can solve the problems of sulfur's non-conductivity and volume expansion.
Lithium-sulfur batteries are currently in the laboratory research and development stage. The Chinese Academy of Sciences, Nanyang Technological University, Stanford University, the National Institute of Advanced Industrial Science and Technology (AIST) of Japan, and the University of Tsukuba are leading the research. SionPower has already conducted significant application trials in the fields of laptops and drones.
8. Lithium-air batteries
The lithium-air battery is a new type of high-capacity lithium-ion battery jointly developed by the National Institute of Advanced Industrial Science and Technology (AIST) and the Japan Society for the Promotion of Science (JSPS). The battery uses metallic lithium as the negative electrode and oxygen from the air as the positive electrode, separated by a solid electrolyte. The negative electrode uses an organic electrolyte, while the positive electrode uses an aqueous electrolyte.
During discharge, lithium ions dissolve in the organic electrolyte at the negative electrode and then migrate through the solid electrolyte to the aqueous electrolyte at the positive electrode. Electrons are transferred to the positive electrode via wires, where oxygen and water from the air react on the surface of the micronized carbon to generate hydroxide ions, which combine with lithium ions in the aqueous electrolyte at the positive electrode to form water-soluble lithium hydroxide. During charging, electrons are transferred to the negative electrode via wires, and lithium ions travel from the aqueous electrolyte at the positive electrode through the solid electrolyte to the surface of the negative electrode, where they react to form metallic lithium. Hydroxides at the positive electrode lose electrons to generate oxygen.
Lithium-air batteries can operate without recharging by replacing the positive electrolyte and the negative lithium electrode, boasting a discharge capacity of up to 50,000 mAh/g and high energy density. Theoretically, 30 kg of metallic lithium releases the same amount of energy as 40 liters of gasoline. The byproduct, lithium hydroxide, is easily recyclable and environmentally friendly. However, its cycle stability, conversion efficiency, and rate performance are its shortcomings.
In 2015, Cambridge University's Greg developed a high-energy-density lithium-air battery with "more than 2,000 charge cycles" and a theoretical energy efficiency exceeding 90%, taking another step forward in the practical application of lithium-air batteries. As early as 2009, IBM launched a sustainable transportation project to develop a lithium-air battery suitable for home electric vehicles, hoping to achieve a range of approximately 500 miles on a single charge. Recently, Asahi Kasei and Chuo Glass of Japan have also joined this project. The research and development of lithium-air batteries by scientific institutions and well-known companies will undoubtedly greatly promote the application of this battery technology.
