Currently, vigorously developing new energy vehicles has become a consensus among countries to achieve energy conservation, emission reduction, and address climate change. Many countries have even elevated the development of new energy vehicles to a national strategic level. Major automotive groups in the United States, Europe, Japan, and other countries have launched their own development plans. For example, Volkswagen proposed the "2025 Strategy," which aims to launch more than 30 electric vehicles by 2025, striving to achieve sales of 3 million units. Especially since 2016, major automotive powers have further increased their support for the new energy vehicle industry.
However, the large-scale use of electric vehicles is still constrained by factors such as driving range, safety, and cost. For example, simply increasing the number of batteries would increase the vehicle's weight, leading to a significant increase in energy consumption per 100 kilometers. This, in turn, would increase carbon emissions over the entire lifecycle and raise the vehicle's price. Therefore, the fundamental solution still requires a substantial improvement in all aspects of battery performance. For instance, Tesla's Model S electric vehicle, designed to address "range anxiety," uses nearly 7,000 3.1Ah 18650 lithium-ion batteries, achieving a range of over 400km. However, its battery weight reaches 500kg, and the car's price is as high as $79,000, which has hindered its market adoption to some extent.
my country adopts pure electric drive as its technological route, which requires higher battery power and places more stringent demands on battery energy density and safety. Therefore, there is an urgent need to develop high-energy-density and high-safety power lithium-ion batteries, while also taking into account other performance characteristics such as power characteristics, cycle life and cost.
Every significant improvement in battery performance is essentially a major revolution in battery material systems. From the first generation of nickel-metal hydride and lithium manganese oxide batteries, to the second generation of lithium iron phosphate batteries, and now to the widely used third generation of ternary lithium batteries, which are expected to remain so until around 2020, their energy density and cost have shown distinct trends of increasing and decreasing, respectively. Therefore, the choice of battery system for the next generation of automotive batteries is crucial to achieving the battery goals for 2020-2025.
Among the various new battery systems currently available, solid-state batteries use a novel solid electrolyte to replace the current organic electrolyte and separator. They offer high safety and high volumetric energy density, and their broad compatibility with different novel high-energy-density electrode systems (such as lithium-sulfur systems and metal-air systems) can further improve their mass energy density. They are expected to become the ultimate solution for the next generation of power lithium-ion batteries, attracting widespread attention from numerous research institutions, startups, and some automakers in Japan, the United States, Germany, and other countries.
1. Overview of Solid-State Batteries
Traditional lithium-ion batteries use organic liquid electrolytes. Under abnormal conditions such as overcharging or internal short circuits, the batteries are prone to overheating, causing electrolyte expansion, spontaneous combustion, or even explosion, posing serious safety hazards. All-solid-state lithium-ion batteries, developed in the 1950s based on solid electrolytes, completely eliminate the safety hazards of battery smoke and fire caused by leakage because they use solid electrolytes and contain no flammable or volatile components. They are considered the safest battery system.
Regarding energy density, the governments of China, the United States, and Japan hope to develop prototype devices with a capacity of 400–500 Wh/kg by 2020 and achieve mass production by 2025–2030. To achieve this goal, the most widely accepted approach is the use of lithium metal anodes. Traditional liquid lithium-ion batteries face numerous technical challenges with lithium metal, including dendrite formation, pulverization, instability of the SEI (solid electrolyte interphase), and numerous surface side reactions. However, the compatibility between solid electrolytes and lithium metal makes it possible to use lithium as an anode, thereby significantly improving energy density.
Based on its own characteristics, the solid-state battery system presents possible solutions for each of the expected requirements for automotive battery use.
Battery usage requirements and solid-state battery system processing ideas
2. Research Progress of Solid-State Electrolytes, a Core Component of Solid-State Batteries
Regarding solid-state batteries, the solid electrolyte is the core component that distinguishes them from other battery systems. An ideal solid-state electrolyte should possess the following characteristics:
It maintains high lithium-ion conductivity over its operating temperature range (especially at room temperature);
Negligible or non-existent grain boundary impedance;
Matching the coefficient of thermal expansion of the electrode material;
During battery charging and discharging, the positive and negative electrode materials maintain good chemical stability, especially the lithium metal or lithium alloy negative electrode;
The electrochemical aperture is wide, and the analytical voltage is high (5.5V vs. Li/Li+).
It does not easily absorb moisture, is inexpensive, and has a simple preparation process;
Environmentally friendly.
The following section will provide a detailed discussion of the composition, basic properties, current technological status, existing problems, and modification strategies of different types of solid electrolytes that are currently the focus of research.
2.1 Polymer Solid Electrolytes
Polymer solid electrolytes are a type of lithium-ion conductor composed of organic polymers and lithium salts. They possess characteristics such as light weight, ease of film formation, and good viscoelasticity. When used in lithium-ion batteries, they can achieve high specific energy, high power, and long cycle life over a wide operating temperature range. Furthermore, the batteries can be fabricated into various shapes, making full use of the available space in electrochemical devices. Polymer lithium-ion batteries can withstand compression, impacts, and changes in internal temperature and shape during assembly, use, and transportation.
In addition to its own lithium-ion transport function, polymer electrolytes also act as separators, isolating the positive and negative electrodes, compensating for volume changes in electrode materials during battery charging and discharging, and maintaining close contact between the electrodes and electrolyte. Polymer electrolytes can also, to some extent, inhibit lithium dendrite growth, reduce the reactivity between the electrolyte and electrode materials, and improve battery safety. Furthermore, polymer electrolytes facilitate large-scale roll-to-roll battery processing, potentially reducing processing costs. Currently, commercially available polymer lithium-ion batteries are increasingly used in electronic devices such as mobile phones, laptops, and power banks.
Solid-state polymer batteries can be approximated as a solid solution system formed by directly dissolving a salt in a polymer. Their key performance characteristics are determined by the polymer, lithium salt, and various additives. The choice of lithium salt is essentially the choice of anion. In aprotic polymer solvents with low dielectric constants, the charge density and alkalinity of the anion play a crucial role in the formation of the polymer electrolyte.
The ability to form polymer electrolytes depends on the relative magnitudes of the solvation energy of the cation and the lattice energy of the salt. A higher lattice energy results in a weaker ability to form polymer electrolytes. The upper limit of the lattice energy of lithium salts is generally considered to be 850 J/mol. Different lithium salts have different lattice energies, with the common lithium salt lattice energy ranking as follows: F-Cl-br-I-SCN-ClO4-～CF3SO3-bF4-～6AsF6-. Besides lattice energy and the charge density distribution of anions, the dissociation constant of lithium salts also has a certain influence.
Poly(ethylene oxide) (PEO) is a typical polymeric electrolyte composed of -CH2CH2O- and -CH2CH2CH2O- units. The optimal distribution of ether oxygen atoms in PEO allows it to form complexes with various lithium salts, leading to the widespread research and use of PEO-based polymeric electrolytes. Regarding inorganic additives, chemically inert inorganic fillers with high specific surface areas can improve the thermal stability of polymeric electrolytes, inhibit the formation of passivation layers at electrode interfaces, and contribute to electrolyte conductivity and cation transference numbers. Commonly used inorganic additives include SiO2, Al2O3, MgO, ZrO2, TiO2, LiTaO3, Li3N, and LiAlO2.
Currently, polymer electrolytes offer a significant improvement in safety compared to liquid electrolytes, but further improvements are still needed to enhance the lithium-ion conductivity of the electrolyte and maintain the mechanical and chemical stability of the polymer.
2.2 Inorganic Solid Electrolytes
Inorganic solid electrolytes, leveraging their advantages of single-ion conduction and high stability, are used in all-solid-state lithium-ion batteries. They possess advantages such as high thermal stability, non-flammability and explosion resistance, environmental friendliness, high cycle stability, and strong impact resistance, attracting widespread attention. They also hold promise for use in novel lithium-ion batteries such as lithium-sulfur batteries and lithium-air batteries, representing a crucial direction for future electrolyte development.
Based on their material structure, inorganic solid electrolytes can be divided into two main categories: crystalline and amorphous (glassy). Each category can be further divided into oxides and sulfides according to their elemental composition.
2.3 Amorphous (Glassy) Inorganic Electrolytes
Glassy inorganic solid electrolytes exhibit a wide range of compositional variations, isotropic ion conductivity, relatively low interfacial impedance, and ease of film formation, making them promising candidates for use in all-solid-state batteries. Based on composition, they can be classified into oxide-based glass electrolytes and sulfide-based glass electrolytes. Oxide glass electrolytes demonstrate good electrochemical and thermal stability but relatively low ionic conductivity, while sulfide glass electrolytes, although possessing high ionic conductivity, suffer from poor electrochemical stability and are difficult to prepare.
Oxide glass electrolyte systems consist of network-forming oxides (such as SiO2, b2O3, P2O5, etc.) and network modifiers (such as Li2O). The network-forming oxides are interconnected by covalent bonds to form a glass network, while the network-modifying oxides break the oxygen bridges in the network, allowing lithium ions to migrate between the networks. Improving the conductivity of oxide glass electrolyte systems can be achieved through several methods:
First, the content of the network modifier can be appropriately increased. Increasing the Li₂O content appropriately leads to an increase in the conductivity of the oxide glass electrolyte. However, if the Li₂O content increases to a certain extent, it will lead to an increase in the number of non-oxygen bridge atoms. These non-oxygen bridge atoms can capture lithium ions, thereby reducing the conductivity of the oxide glass. Hybrid networks can be used to form oxides. Using binary or multi-component networks to form oxides will result in a hybrid network effect, adding defect structures to the network, improving transport bottlenecks in lithium ion conduction channels, and enhancing lithium ion conduction. For example, in the Li₂O-P₂O₅-b₂O₃ ternary system glass, when the lithium ion concentration is 5 mol%, the conductivity is 9 × 10⁻⁵ S/cm.
