Fundamentals and Recent Advancements in Li-Ion Batteries

Abstract

In the last 10 years, lithium-ion batteries have been applied to a variety of electronic devices. In recent years, electronic equipment has been rapidly developed to meet high-performance requirements, thus opening new markets. However, due to the low theoretical energy density of existing commercial electrode materials, lithium-ion batteries have limited applications. Therefore, many researchers explore different methods to improve the performance of the battery to adapt to the needs of practical applications. This chapter describes the research work related to the development of lithium-ion batteries, which has shown excellent performance in electrochemical tests. More importantly, the evaluation and testing methods of full batteries are critical to the development of new electrode materials for practical applications, which will also help meet the growing demand for high-performance lithium-ion batteries in emerging markets. The purpose of this chapter is to organize relevant information about new electrode materials and advanced nanotechnology to promote the development of lithium-ion batteries. In addition, it also outlines the challenges encountered in the research of high-performance batteries and the prospects of the next generation of lithium-ion batteries.

Introduction

Due to environmental protection, high energy density, high power density, long cycle life, low cost, and low self-discharge characteristics, lithium ion batteries (LIBs) are widely used as power sources for various applications (Choi and Aurbach 2016; Wu et al. 2020a). In recent years, with the rapid development of hybrid electric vehicles (HEV), electric vehicles (EV), and aerospace applications, the higher requirements have been based on the performance of LIBs (Chae et al. 2020). However, the current commercial cathode materials for LIBs (such as LiCoO₂, LiMn₂O₄, and LiFePO₄) are based on the Li + intercalation mechanism and therefore cannot meet the high charging capacity (>200 mAh g⁻¹) required for specific applications (Manthiram 2020; Li et al. 2021). In addition, the development of anode materials, electrolytes, and electrolytes is very important for enhancing the electrochemical properties of the full battery. Therefore, researchers have carried out a lot of research work on high energy density battery technology, especially for high-capacity cathode materials, high-rate anode materials, electrolytes and electrolytes, lithium sulfur batteries, lithium carbon dioxide, and lithium air batteries have shown great interest. Compared with the current commercial LIBs, they not only have high energy density and lower cost, but also have lower toxicity. However, in order to achieve commercialization, they still have many key issues that need to be solved urgently, such as batch stability of active material preparation, deactivation of electrolyte, volume change and/or surface reconstruction of electrode materials, and failure mechanism of battery during long cycle (Guan et al. 2020; Heiskanen et al. 2019).

In this chapter, we will systematically introduce the current progress of LIBs in terms of electrode materials, electrolyte, and electrolyte. Further discussion will focus on some of the most advanced electrode preparation and material characterization techniques in LIBs. In short, we have demonstrated the evolution of LIBs (working electrodes) from traditional battery technology to advanced battery technology. The purpose of this chapter is to provide some future development directions for the design and development of the next generation of high-performance LIBs. This allows next-generation LIBs to better meet the needs of new markets (such as wearable devices, energy harvesting smart grids, HEV, EV, aerospace applications, and personal mobile devices).

The Fundamentals in Lithium-Ion Batteries

As mentioned earlier, compared with other energy storage systems, LIBs is the most used energy storage device in the world because of their high capacity and versatility (Diouf and Pode 2015). The working principle of these energy storage devices is to convert chemical energy into electrical energy. They exhibit high energy density and thus have broad application potential in smaller devices (Yang et al. 2021). However, in order for LIB technology to be fully optimized, some key problems related to gradual degradation of components, cost, and the necessity of keeping current and voltage within a safe range need to be resolved.

Lithium-ion technology was invented in the second half of the twentieth century, so it is considered a new technology. There are some milestones in the development of LIBs. As first in 1977, Professor Stanley Whittingham successfully developed the first device (Whittingham 1976). Later in the 1980s, Professor John Goodenough developed new carbonaceous anode materials and cathode active materials to avoid the use of lithium metal anodes, thereby solving the safety problems related to lithium dendritic growth (Mizushima et al. 1980). Next, in 1991, Akira Yoshino from Sony developed the first commercial lithium-ion battery and successfully marketed it as a product (Nishi 2001). Due to the outstanding contributions of the above three researchers in the development of LIBs, they won the Nobel Prize in Chemistry in 2019. In order to make LIBs safer, researchers have been working hard to develop solid-state battery technology, the purpose of which is to avoid the use of liquid components in the battery structure, although so far no consistent results have been obtained (Takada 2013).

The working principle of LIBs is mainly based on redox reaction (Barbosa et al. 2021). During the charging process of a LIB, electrons flow from the anode to the cathode to provide energy for the system, and along with lithium ions migrate from the anode to the cathode through the electrolyte. The difference in charge is compensated by the equivalent transfer of electrons and lithium ions. During the discharge process of a LIB, electrons and lithium ions undergo opposite transfers. In this process, lithium ions return to their original state and can release corresponding energy. This flow of electrons and lithium ions is described by Eq. (1):

$$ \mathrm{L} iAM\leftrightarrow AM+\mathrm{x}{L\mathrm{i}}^{+}+\mathrm{x}{e}^{-} $$

(1)

where e⁻ is an electron, Li⁺is a lithium ion, and AM is the active material.

The main components of LIBs are divided into cathode, anode, separator, and electrolyte. In terms of cathode materials, the most commonly used LIBs are metal oxides, such as LiCoO₂, LiFePO₄, or LiMnO₂. These cathode materials have different working voltages and specific capacities, so they can meet different application requirements. In terms of anode materials, carbon-based materials are currently the most commonly used in LIBs, such as graphite and silicon-carbon materials. In terms of this separator, polymers are currently commonly used in LIBs. It is worth noting here that when choosing a polymer as a separator, the porosity, mechanical properties, heat resistance, electrochemical stability and electrolyte wettability of the material need to be considered comprehensively. The membranes that have been successfully applied include poly(propylene) (PP), poly(ethylene) (PE), or poly(vinylidene fluoride) (PVDF) (Zhang et al. 2018). In terms of electrolyte, the electrolyte currently used in commercial LIBs is mainly composed of lithium salt and lithium hexafluorophosphate (LiPF₆), which are soluble in organic carbonates such as diethyl carbonate (DEC) and ethylene carbonate (EC) or dimethyl carbonate (DMC) (Zhang et al. 2018). In addition to the electrolytes mentioned above, the current focus is on solid electrolytes, which can be organic or inorganic. In terms of organic matter, solid electrolytes usually contain a polymer matrix and one or more fillers incorporated in its structure. Such solid polymer electrolytes exhibit excellent thermal stability and high mechanical properties, but there are still problems such as low ionic conductivity (Wu et al. 2020b). In terms of inorganics, solid electrolytes are usually composed of ceramic crystal materials with high ionic conductivity, such as NASICON, LISICON, or perovskite, but there are still many problems in interface compatibility that need to be solved urgently (Heiskanen et al. 2019).

Recent Advancements in Li-Ion Batteries

Cathode

In 1991, Goodenough and his colleagues successfully invented lithium cobalt oxide (LiCoO₂, LCO) layered cathode material, which was also the first cathode material to be developed and is still used in most commercial LIBs. With the development of cathode materials for LIBs, lithium iron phosphate (LiFePO₄, LFP) with an olivine structure and lithium manganese oxide (LiMn₂O₄, LMO) with a spinel structure have also been successfully used in commercial LIBs. Nowadays, as the demand for various electronic devices has increased significantly, higher requirements have been put forward on the performance of LIBs. In order to meet this increase in demand, it is necessary to develop high-performance LIBs, focusing on relatively low self-discharge characteristics, high operating voltage, high volumetric and gravimetric capacity, and stable long-cycle performance (Wu et al. 2020a). In addition, to achieve the purpose of high-energy LIBs, the newly developed cathode material needs to have a high tap density. Although nanotechnology has made a lot of research progress in the preparation of cathode materials, it is not suitable from the perspective of practical applications. In order to better develop the practicability of high-performance cathode materials, many new research methods have been reported. These new research methods include surface modification of traditional cathode materials (for example, LMO spinel and LCO layered), control of the ratio of nickel or lithium metal in the crystal architecture, and preparation of new composition transition metal oxide cathodes (such as lithium nickel cobalt aluminum oxide and lithium nickel cobalt manganese oxide materials) (Guan et al. 2020). After recent years of research, the newly developed cathode material exhibits excellent electrochemical performance. Here, we focus on the main points of the technology and novel scientific strategies for achieving excellent electrochemical performance in full battery testing.

Surface Modification of LCO Cathode Materials

Surface modification of cathode materials is an effective strategy to enhance electrochemical properties. Many development strategies have been reported so far, mainly including the use of some synthesis methods (such as radio frequency magnetron sputtering system, wet chemical reaction method, physical adsorption method, and chemical polymerization method) to dope heterogeneous elements in the LCO crystal structure or to coat the surface of the LCO with inorganic materials (for example, metal phosphate, metal oxide, and metal fluoride) (Cho et al. 2001; Cho et al. 2003). Therefore, in order to improve the development progress of LIBs and achieve excellent electrochemical performance, it is necessary to analyze the electrochemical phenomena on the cathode and adjust the interface reaction between electrolyte and cathode. It is worth noting here that the selection of metal oxides for surface modification is an effective strategy to protect the original cathode under high voltage conditions (Cho et al. 2003; Lee et al. 2014; Kalluri et al. 2017). Specifically, the metal oxide coating modification on the surface of the cathode can not only effectively inhibit the unstable interface reaction between electrolyte and cathode, but the coating material can also act as a passivation layer to prevent structural degradation. Especially under the condition of high voltage, it can show excellent electrochemical performance. More importantly, the surface coating cathode material formed through the new strategy still exhibits excellent battery performance during the full battery test, which is crucial for practical applications.

In order to achieve stable operation in the high voltage range, recent research work reported that an antimony tin oxide (ATO) layer was doped in the LCO cathode material. This layer is composed of colloidal ATO nanoparticles, which aggregate to form a conductive network (Lee et al. 2014). Compared with the original LCO cathode material, the doping modification of ATO nanoparticles improves the electrical conductivity of the LCO cathode to ≈2.93 × 10⁻⁶ S cm⁻¹, resulting in a high-mass-loading cathode in LIBs. The ATO-doped LCO cathode can effectively inhibit the dissolution of cobalt ions in the electrolyte and the generation of structural degradation under high voltage conditions, thereby alleviating the adverse side reactions of the cathode material under high voltage conditions. It is worth noting that the full battery was assembled with a commercially available natural graphite (NG) anode, and the electrochemical performance of the full battery was tested. After 100 charge and discharge tests, it exhibited excellent cycle stability. The research work also forms a full battery with ATO-coated high-capacity silicon anodes. After 100 charge-discharge cycle tests, the capacity retention rate of the full battery was as high as 83.9%, and the volume discharge capacity was as high as 274 mAh cm⁻³. The doping modification of ATO can effectively improve its electrochemical performance, and matching high-voltage cathodes and high-capacity anodes can achieve high-energy LIBs.

In addition, to promote the practical application of new cathode materials, it not only needs to achieve high performance, but also needs to pay attention to safety issues. In particular, LCO cathodes are prone to thermal runaway caused by electrochemical or thermal decomposition under high temperature conditions. In order to overcome the above-mentioned problems, researchers have proved that the surface of LCO particles is doped with Mg and P elements through the use of wet chemical methods and calcination processes (MP-LCO) (Kalluri et al. 2017). Compared with bare LCO, the doping of P and Mg elements may be used to reduce the exothermic energy to 50% of heat generation using differential scanning calorimetry. Assembling MP-LCO/NG into a full battery for electrochemical testing showed high rate capability and high energy density. It also exhibited the outstanding cycle performance and thermal stability. The capacity retention rate of MP-LCO cathode over 200 charge-discharge cycles under 60 °C was 51%, which was about twice that of bare LCO (28%). And the researchers also prepared a layered LiNi_0.5Mn_0.5O₂ coated LCO cathode material, and assembled it into a full battery to study its electrochemical performance under high-quality load (18 mg cm⁻²). It showed the outstanding cycle performance and thermal stability. The full battery not only has a capacity retention rate of 84% over 100 cycles of charge and discharge at 25 °C, but also has a capacity retention rate of 83% over 50 cycles under 45 °C. In summary, the research results show that the surface modification of cathode materials could greatly improve their electrochemical performance, which is an effective way to realize practical high-energy LIB.

Transition Metal Oxide Cathode Materials

Compared with the traditional LCO cathode, LiNi_xCo_yMn_zO₂ (NCM) not only has a similar crystal structure, but also has a similar specific capacity and working voltage. The difference is that the NCM cathode reduces the cobalt content in the element composition, thereby reducing the cost (Kim et al. 2015; Cho et al. 2013). As the most common composition of NCM, LiNi_0.33Co_0.33Mn_0.33O₂ has been widely used as commercial LIBs. In recent years, many research works have been carried out on NCM cathode materials, such as the design of NCM nanomaterials with porous architecture, which showed excellent cycle stability under high temperatures of 50 °C and high reversible specific capacity of 234 mAh g⁻¹ (Kim et al. 2015; Cho et al. 2013).

In order to further improve their electrochemical performance, researchers have proposed a new strategy for nanostructure and surface modification. The strategy is mainly to design a core-shell structure. Specifically, the outer layer of NCM chooses manganese and cobalt to replace NCM to obtain long-cycle stability and high safety, which is mainly attributed to the high nickel content that is beneficial to the rapid delithiation without damaging the structure. The core of NCM is composed of nickel-rich oxide with high energy density and high power density (Lee et al. 2016; Chae et al. 2014; Oh et al. 2014; Oh et al. 2016). The development of new synthesis technology to prepare the cathode materials can effectively improve its electrochemical performance (Lee et al. 2016). Researchers have successfully prepared a two-sloped full concentration gradient Li[Ni_0.85Co_0.05Mn_0.10]O₂ structure, which is mainly composed of manganese-rich outer layers and nickel-rich core sites. Used as a cathode material, it exhibited high specific capacity and excellent thermal stability. And it was assembled with the CNT-Si composite anode to form a full battery for electrochemical testing. The full battery exhibited a highly reversible capacity of 213.0 mAh g⁻¹. The full battery also exhibited outstanding rate capability. The experimental results exhibit that its capacity retention rate is as high as 82.7% under 5C, and the capacity retention rate can still reach 77.5% even under a rate of 10C. In addition, the coulombic efficiency and capacity retention rate of the full battery are as high as 99.8% and 81%, respectively, over 500 cycles under 1 C. It is worth noting here that electrochemical testing of a full battery is very important for commercial applications.

Recently, their research team chose the co-precipitation method to successfully synthesize Li[Ni_0.75Co_0.1Mn_0.15]O₂ with optimized structure (Chae et al. 2014). The NCM cathode material has a core-shell structure, which is also composed of a nickel-poor shell and a nickel-rich core. It should be noted that a gradual concentration is formed between the shell and the core, which is named full concentration gradient NCM cathode material. This material as a cathode shows a high reversible capacity of about 200 mAh g⁻¹. Assembling the cathode into a full battery has an energy density of up to 240 W h kg⁻¹, and can achieve 750 charge-discharge cycles under a voltage range of 2.7–4.2 V at 1 C. The experimental results show that the design of gradient concentration of NCM material can effectively improve its cycle performance and reversible capacity.

In addition to the above strategies, researchers have recently proposed a new surface modification strategy on the surface of the original cathode material. The coating layer was mainly composed of chemically activated cathode materials and reduced graphene oxide (rGO) (Oh et al. 2014). Specifically, a small amount of GO layer was coated on the surface of NCM cathodes, and then a chemical activator was selected to treat the surface of the NCM cathode into a Li₂MnO₃ phase. The surface-treated NCM cathode has a specific capacity as high as 250 mAh g⁻¹, and the coulombic efficiency was as high as 99.5% in the initial cycle under 0.1 C. In addition, the capacity retention rate of the cathode material can reach 94.6% over 100 charge-discharge cycles under 1 C conditions, and the capacity retention rate can reach 60% under the voltage range of 2.0–4.6 V under 12 C. Recently, other research groups reported a heterogeneous structure of NCM cathode, which is composed of lithium-rich Li_1.2-xNi_0.2Mn_0.6O₂ as the shell and nickel-rich LiNi_0.7Co_0.15Mn_0.15O₂ as the core and AlF₃ as the coating layer (Oh et al. 2016). When it was assembled and tested in a full battery, the capacity retention rate of this heterogeneous cathode was as high as 82% over 600 charge-discharge cycles at 1 C. The experimental results exhibit that the core-shell structure combines the chemical stability of the shell and the structural stability of the core, thus exhibiting excellent electrochemical performance.

Lithium-Rich Cathode Materials

To develop advanced LIBs technology with high energy density, it is very important to design new cathode materials with high theoretical capacity. Recently, over-lithiated oxides (OLOs) as cathode materials have aroused widespread interest among researchers due to their high theoretical capacity. Each OLO is based on the composition of its lithium metal oxide, which is composed of various chemical formulas xLi₂MnO₃-(1-x)LiMO₂. As a cathode material, OLO has an extremely high theoretical specific capacity of 250 mAh g⁻¹ under high voltage conditions of over 4.5 V (Zheng et al. 2014a). The OLO cathode materials exhibit high capacity in the high voltage range, which could effectively improve the energy density of LIBs. However, undesirable interface reactions may occur under high voltage conditions, for example, electrolyte decomposition may occur when the high voltage is cut off during the charging process. The OLO cathode materials have irreversible initial capacity loss, which is mainly attributed to the formation of Li₂O in the first cycle. In addition, it shows poor cycle stability in the charge-discharge cycle test. The main reason for the analysis is that manganese is easily reduced to its trivalent Mn³⁺ ion, and the decomposition reaction of Mn³⁺ ion accelerates the dissolution reaction of manganese, which leads to the phenomenon of structural degradation caused through Jahn-Teller distortion (Sun et al. 2012).

Although the OLO cathode has the problem of structural instability, its high specific capacity still arouses wide interest among researchers. To overcome this problem, researchers developed a strategy for surface modification of the cathode (Mun et al. 2014; Liu et al. 2015; Park et al. 2017; Chen et al. 2016). In order to overcome the problems of poor conductivity and surface degradation, it is an effective method to choose conventional carbon materials (such as hard carbon, carbon fibers, carbon nanotubes, and graphene) to make a protective layer coating on the surface of cathode material. However, due to the phenomenon that the thermal reduction mechanism easily degrades the oxide structure, it is difficult to apply the carbon material coating process to metal oxide-based cathodes under high temperature conditions. Researchers have developed a new type of cathode material with multiwalled carbon nanotubes (MWCNT) coated with OLO (Li_1.17Ni_0.17Co_0.1Mn_0.56O₂) (Mun et al. 2014). In order to avoid damage to the crystal structure and morphology, a method of applying shear stress was used during the synthesis of MWCNT-coated OLOs cathodes. Compared with the original OLOs, the electronic conductivity of MWCNT-coated OLOs has increased by more than 40 times, which enables the MWCNT-coated OLOs cathode to achieve higher coulombic efficiency and reversible capacity. The MWCNT-coated OLO was assembled into a full battery for electrochemical performance testing, which showed a high discharge capacity of more than 150 mAh g⁻¹ and outstanding cycle performance.

Furthermore, the researchers also designed a fiber-type OLOs cathode material, which is prepared by electrodeposition and hydrothermal reaction to deposit on carbon fiber (Liu et al. 2015). The OLOs layer synthesized on the carbon fiber is composed of particles, which are determined by X-ray diffraction analysis that it is composed of 5.2 wt% of a small amount of Li₄Mn₅O₁₂ phase and 94.8 wt% Li₂MnO₃ primary phase. The irreversible capacity of the OLO cathode material during the initial charge-discharge cycle could be used to compensate for the loss of lithium ions caused by the formation of the intermediate phase of the solid electrolyte from the matched anode material. Therefore, the full battery assembled with OLO cathode can achieve normal operation without providing additional lithium ions. More importantly, the electrode structure exhibits high Coulomb efficiency and excellent rate characteristics, which is attributed to one-dimensional architecture of carbon fiber with large surface area and high electrical conductivity. From the research results, it can be concluded that the fiber-type OLO cathode has a unique one-dimensional structure that is conducive to conduction and rapid ion diffusion.

In recent years, researchers from Samsung Electronics have prepared spinel-embedded lithium-rich oxides and systematically analyzed their structural phases (Park et al. 2017). Through research on various composite materials made of different spinel phases, it is found that OLO embedded with LiCo_0.5Mn_1.5O₄ as a cathode exhibited the most excellent electrochemical properties, which is attributed to high structural stability and no parasitic phases. It is worth noting that the 18,650-type full battery is assembled from the positive electrode material and tested for electrochemical performance, which is of great significance for commercial applications. Compared with the original sample, the OLO cathode doped with LiCo_0.5Mn_1.5O₄ exhibited a higher capacity retention rate of 86.2%. Due to the doping of the spinel phase in the OLO, the voltage attenuation of the OLO doped with LiCo_0.5Mn_1.5O₄ is also smaller. Furthermore, Chen et al. chose the wet chemical method to prepare other doped spinel OLO (layered-Li_1.2Mn_0.54Ni_0.13Co_0.13O₂) cathodes and conducted electrochemical tests (Chen et al. 2016). The cathode exhibited excellent electrochemical performance, especially the capacity retention rate over 100 cycles under a high rate of 4 C is 92%. Matched and assembled with graphite anode to obtain a full battery, the full battery can exhibit a high reversible capacity of 253 mAh g⁻¹ and a high energy density of 801 W h kg⁻¹ under 0.1 C. Compared with other battery results reported in the literature, this battery exhibited more excellent electrochemical performance.

Anode

An ideal anode for LIBs needs to have excellent comprehensive characteristics, such as low cost; low delithiation potential; high reversible volumetric and gravimetric capacity; long cycle stability; high rate performance; and environmental friendliness. Due to its light weight and high specific capacity, pure metal lithium is considered to be the best anode material, which has aroused widespread interest. However, pure metals have serious safety problems, which are mainly attributed to the internal short circuit caused by the electroplating of dendritic lithium during the charging process. In addition, researchers have conducted a lot of research on non-carbon and carbon materials as high-performance anodes, including, germanium, silicon monoxide and silicon, tin, phosphides, nitrides, sulfide transition metal oxides, and carbon nanofibers, carbon nanotubes, porous carbon, graphene, etc. It is worth noting here that choosing a suitable anode material is very important to improve the power density, energy density, and cycle life of LIBs.

Alloying Anode

As an important part of anode materials, alloyed anode materials have aroused widespread interest among researchers. The reaction equation of the alloying mechanism is xLi⁺+xe⁻ + M → Li_xM, where M represents the alloy negative electrode, such as Ge, Si, P, Sn (McDowell et al. 2013; Obrovac and Chevrier 2014). Generally speaking, the lithium ion storage capacity of these materials can be several times that of graphite. As a representative alloy-based anode material, Si has a high specific capacity of 4200 mA h g⁻¹, which is about 11 times that of graphite (McDowell et al. 2013; Obrovac and Chevrier 2014). From the perspective of portable and electric vehicle applications, not only the gravimetric capacity of the material needs to be considered, but also the volumetric capacity of the material under the expanded (lithiation) state. In addition to the above parameters, the delithiation potential is also an important parameter. In order to increase the discharge voltage of the full battery, the anode material needs to have a low delithiation potential. It is worth noting that the delithiation potential is more important for the full battery than the lithiation potential. As classic alloy materials, Si, Sn, and P exhibit low delithiation potentials, which are 0.6, 0.45, and 0.9 V, respectively.

However, anode materials based on alloying mechanisms also have some serious problems. The first is volume expansion and rupture (McDowell et al. 2013; Obrovac and Chevrier 2014). The alloy-based negative electrode material has a large-capacity lithium storage capacity inevitably leading to large volume expansion during the lithiation process; for example, the volume expansion rate of Si is 400%. The large volume expansion causes capacity decay and loss of electrical contact, which is attributed to the occurrence of mechanical fracture in a single particle. The second is swelling after being made into an electrode. The electrode is composed of a single particle, and the volume expansion of a single particle will cause the expansion of the entire electrode, which brings challenges to battery manufacturing. The last one is the instability of the solid electrolyte interphase (SEI) (McDowell et al. 2013; Obrovac and Chevrier 2014). The main reason for the analysis is that the expansion and contraction of the particle volume will be found in the process of deintercalating lithium, which will cause the movement of the boundary between the electrolyte and the particle.

In order to solve the above problems, the researchers designed the nanostructure of the Si anode, specifically; it is divided into four directions: the first is the solid silicon nanostructure. The design focus of the researchers through solid nanostructures is their small size, which is smaller than the critical fracture size of Si. It is worth noting that solid Si nanostructures have a variety of forms, including nanoparticles, nanowires, and carbon-Si composites. In the design of solid nanostructures, core-shell nanostructures show excellent performance. In this structure, Si as a shell material can store lithium ions, and the core material provides effective electron transmission and stable mechanical support. The second is the design of hollow Si nanostructures. Compared to solid Si structures, this hollow Si nanostructure can provide volumetric strain, which will avoid the problem of fracture of the Si anode during charging and discharging process (Ren et al. 2019). The third is to constrain the hollow Si. During the long charge and discharge cycle, the nano-Si structure continuously undergoes volume expansion and contraction, which is not conducive to the construction of a stable SEI. This problem can be overcome by designing a mechanical confinement layer on the hollow Si structure. This is an innovative Si-based structure design concept. The researchers successfully designed a double-walled nanotube structure with SiO₂ as the external mechanical confinement layer and Si as the inner tube (Wu et al. 2012). In this structure, the outer layer of SiO₂ can allow lithium ions to diffuse to cause an electrochemical reaction with Si. More importantly, the outer layer of SiO₂ has good mechanical strength during the lithiation process to force the volume to expand to the internal space. Such a structural design can form a stable SEI during charging and discharging. The experimental results prove that the structure has excellent electrochemical performance; especially after 6000 charge and discharge cycles, it can still maintain a high capacity retention rate. Other researchers have also carried out S-TiO₂, Si-C, Al-TiO₂ egg yolk-shell structure and permanganate-like Si-C structure design, thus further confirming that this constrained hollow structure can effectively improve electrochemical properties (Liu et al. 2012; Liu et al. 2014). More importantly, the constrained hollow structure reserves space for internal volume expansion, which can avoid the problem of electrode expansion. For example, the volume change of the permanganate-like structure at the electrode level is negligible. The last one is to explore polymer binders. The design of the polymer binder can play the role of cohesion of Si particles and maintain electrical connection when huge volume changes occur. At present, there are mainly two types of polymer binders. The first is to build a strong binding force between the polymer and the surface of the Si particles, such as carboxymethyl cellulose, CMC, and alginate, which can maintain the integrity of the electrode during the charge and discharge process. The second one is self-healing polymers. After a long cycle test, the polymer is prone to rupture due to volume expansion. Self-healing can be achieved through dynamic hydrogen bond design, which is an exciting adhesive (Wang et al. 2013).

Intercalation Anode: Graphite and Graphene

Due to their excellent chemical and physical properties, carbon-based materials with various forms have long been considered as promising anode materials. Among them, graphite has excellent thermal conductivity (~ 3000 W/mK) and electrical conductivity (~ 10⁻⁴ S/cm), so it has been widely used as an anode material for commercial LIBs (Dahn et al. 1995; Yoo et al. 2008). In addition, Dahn et al. conducted in-depth research on the intercalation of lithium in carbonaceous materials (Dahn et al. 1995). Some breakthrough research results have been made mainly on the lithium intercalation mechanism of ordered and disordered carbon-based materials in LIBs. In recent years, as a representative of carbon materials, graphene derived from exfoliated graphite exhibits excellent physical and chemical properties due to its unique two-dimensional structure, which has attracted wide interest from researchers (Yoo et al. 2008).

Graphene is composed of carbon atoms with sp² crystal structure, which are arranged into a two-dimensional sheet with a honeycomb network structure with a thickness of one atom. Due to this unique structure, it exhibits good mechanical strength, large surface area, outstanding electrical conductivity and high charge mobility. The excellent properties of graphene make it an ideal anode for LIBs, which has been extensively studied in recent years (Mo et al. 2019; Zhou et al. 2012). Research results showed that graphene sheets (multilayers) obtained by graphite exfoliation can effectively increase the storage capacity of lithium ions, which can be attributed to improving electrolyte penetration and shortening the ion diffusion distance in the electrode material (Mo et al. 2019). In graphene with a complete crystal structure, there are three active sites that can be used to absorb lithium ions, which are top sites, hollow sites (in the center of the carbon hexagonal ring), and bridge sites. According to the first-principles calculation results, the hollow position seems to be more advantageous in terms of energy, but it is still unclear how to realize the storage and arrangement of lithium ions on graphene with a complete crystal structure (Zhou et al. 2012).

Researchers have found that if lithium ions are located in the hollow positions of graphene, stoichiometric Li₃C₆ compounds can be formed, so the theoretical specific capacity can be as high as 1116 mAhg⁻¹ (Yoo et al. 2008). Through theoretical calculations, other research groups have concluded that both sides of graphene can absorb lithium ions, one of which is on top of a carbon atom, and the other forms a stoichiometric Li₂C₆ under different carbon atoms in the original unit cell. According to the above theoretical calculation results, it can be concluded that its specific capacity can be as high as about 780 mAhg⁻¹ (Yang 2009). Recent studies have shown that lithium ions are more likely to form clusters rather than uniformly distributed on the surface of graphene (Mo et al. 2019). Obviously, compared with the theoretical capacity of graphite, the lithium storage mechanism of graphene is still unclear, so there are still many controversies about the theoretical capacity of graphene. This requires researchers to conduct in-depth and systematic research on the mechanism of lithium ion storage in graphene.

Although the lithium ion storage mechanism of graphene remains to be further studied, researchers have done a lot of research work to increase the capacity of graphene. Among them, constructing spacers to expand the interlayer distance of graphene sheets by reassembling the so-called pseudo-graphite materials is an effective strategy to improve the lithium storage capacity of graphene. The results of You et al. showed that by adding nano-carbon macromolecules (such as fullerene and carbon nanotubes) to graphene, the specific capacity of graphene sheets could be increased from 540 to 700 mA h g⁻¹ (Yoo et al. 2008). The research results exhibited that the reversible capacity of graphene-based materials can be effectively improved by carefully adjusting and controlling the d-spacing between the recombined graphene sheets. The d-spacing of the graphene carbon layer in the graphene sheet could be extended to 0.40 nm, and the reversible lithium storage capacity of graphene could reach 784 mAh g⁻¹. Furthermore, researchers have also developed some innovative strategies for preparing graphene paper by filtering and reducing prefabricated graphene oxide paper or using graphene sheets functionalized by oxalic acid molecules. These strategies can also increase the d-spacing of the graphene layer, thereby increasing the capacity of the graphene. However, such graphene materials still have many problems in their practical applications, such as poor cycle performance and larger irreversible capacity loss in the initial charge-discharge cycle.

Although it is found through research results that graphene-based materials have great potential as the anode of next-generation LIBs, they still have many problems that need to be solved urgently. Graphene is prone to re-stacking, which is attributed to the van der Waals force between adjacent layers. This stacking phenomenon greatly reduces the specific surface area of the material, thereby reducing its specific capacity. The introduction of porosity into graphene-based materials seems to be an effective strategy to solve this problem. The increase in the porosity of graphene materials can accelerate the transport of lithium ions in graphene (Mo et al. 2019). Specifically, holes could be introduced as interstitial spaces between adjacent sheets or as in-plane holes in a single sheet. The electrochemical performance can be improved by constructing various types of pore structures in the graphene material. Another problem of graphene-based anode materials for LIBs is the low coulombic efficiency, which is attributed to the relatively high irreversible capacity loss after the initial charge-discharge cycle. Analyze the reason for this is related to the high surface area of graphene. To protect the lithiated graphene from further reaction with the electrolyte during charging and discharging process, a solid electrolyte interphase (SEI) layer is formed on the graphene anode (Mo et al. 2019). In response to the above problems, researchers have developed some strategies to try to solve these problems, but there are still some problems that need to be solved urgently.

Intercalation Anode: Li₄Ti₅O₁₂ and TiO₂

Titanium-based oxides, including Li₄Ti₅O₁₂ (LTO) and TiO₂, can be charged and discharged at a voltage higher than 0.8 V. This allows Li⁺/Li to form SEI by eliminating the reduction of the electrolyte and avoiding its deposition on the anode surface. Therefore, compared with other anode materials, titanium-based oxide has better safety performance, making it a candidate for the next generation of anode materials for high-safety LIBs (Wang et al. 2019a). In addition to the above advantages, the titanium-based anode material also has the characteristics of low toxicity, low cost, small volumetric strain during charging and discharging, outstanding cycle stability, and excellent rate performance. However, such materials have the disadvantages of low conductivity of bulk materials and low inherent theoretical capacity (in the range of 175–330 mAh g⁻¹) with micron-sized particles. Their electrochemical performance in batteries largely depends on the morphology, structure, and size of titanium-based oxides. Although there are still many problems to be solved, the titanium-based oxides with various allotrope forms have attracted wide interest from researchers as potential anode materials for LIBs.

As a representative of titanium-based oxide anode materials, Li₄Ti₅O₁₂ has a spinel structure, which can be described as Li[Li_1/3Ti_5/3]O₄ with a cubic space group Fd3m. This Li₄Ti₅O₁₂ material has a theoretical capacity of 175 mAhg⁻¹ during the lithiation process under a voltage of 1.55 V (relative to Li⁺/Li) (Wang et al. 2019a). It is also important that Li₄Ti₅O₁₂ is a well-known zero-strain lithium insertion material, so the rate of volume expansion during charging and discharging process is only 0.2%. Due to its excellent structural stability, it exhibits an excellent long cycle stability during charge and discharge process. Lu et al. studied the microstructure evolution of LTO materials during charging and discharging at the atomic level, which exhibited that Ti-O bonds have obvious changes in shrinkage/tension under different state of charge (Lu et al. 2015). This structural torque plays a key role in the formation of trapping centers of electron/hole pairs in titanium-based oxide materials.

However, LTO as anode for LIBs has two disadvantages.On the one hand, its electronic conductivity is very low (~10⁻¹³ S/cm), on the other hand, it easily reacts with the electrolyte interface to cause the release of undesirable gases, which limited its application in commercial electrochemical energy storage devices. The first problem could be solved through reducing LTO to nanoscale or by surface treatment of LTO to increase the diffusion rate of lithium ion. In the past few years, researchers have successfully prepared a variety of nanostructured LTO, which has shown excellent rate performance and cycle stability as anode of LIBs. However, the nanostructure design of LTO also has some shortcomings. Specifically, the design of nanomaterials significantly reduces the volume energy density of the battery, which is attributed to the high porosity of nanomaterials and the low loading density of the electrode. The secondary preparation of nano-LTO primary particles to obtain micron-sized secondary particles is an effective method to solve this problem. The design of the micro-nanostructure can not only effectively increase the tap density and loading density of electrode level, but also maintain the advantages of nanostructure of the primary particles. More importantly, the porous LTO with spherical morphology can significantly minimize the diffusion path of lithium ions, thereby greatly improving the volumetric energy density and electrochemical performance.

Researchers have found that TiO₂ has excellent lithium ion intercalation/deintercalation performance, and its crystal structure is diverse, including rutile, anatase, and brookite (Wang et al. 2019a). Similar to LTO, TiO₂ exhibits excellent safety performance, which attributed to its high electrochemical lithiation potential (1.5 V vs. Li⁺/Li). Unlike LTO, TiO₂ shows a much higher capacity (330 mA h g⁻¹). The analytical reason is that 1 mol of lithium can theoretically be inserted into TiO₂, corresponding to the formation of LiTiO₂. However, the research results showed that it is very difficult to realize the theoretical capacity (Wang et al. 2019a). Generally speaking, the crystal structure, particle size, and morphology of TiO₂ will greatly affect its electrochemical performance. Recently, researchers reported that the electrochemical properties of TiO₂ nanoparticles could be enhanced by adjusting the shape and size of TiO₂ nanoparticles. A simple solution preparation process is used to synthesize TiO₂-B nanowires or nanotubes with a length of up to several microns and a diameter in the range of 40–60 nm. As anode of LIBs, it exhibited the outstanding rate capability and high reversible capacity (Armstrong et al. 2005).

Conversion Anode: Transition Metal Oxide

The transition metal oxide based on the conversion mechanism is an important part of the current high-capacity anode material. The basic reaction is: M_xO_y + 2yLi⁺ + 2ye⁻ → yLi₂O + xM. Researchers have developed a variety of conversion materials, including fluorides, oxides, and sulfides (Lu et al. 2018). These materials that serve as anodes of LIBs provide the high volumetric capacities of 4100–5600 mA h cm⁻³ and specific capacities of 600–1200 mA h g⁻¹. However, this type of conversion anode has some disadvantages, including instability of the SEI layer, material pulverization, and huge volume changes during charging and discharging. In addition, the conversion mechanism also has the problem of a large voltage hysteresis ~1 V, which is mainly attributed to the slow path in the charging and discharging process. It is worth noting here that the large voltage hysteresis refers to the difference between the charge/discharge voltages. On the other hand, due to the interconversion of multiple solid phases (Li₂O, MO_x, and M) with different structures in this type of material. These solid phases involve the breaking of strong chemical bonds during the mutual conversion process, which causes a large voltage hysteresis.

In order to solve the above problems, researchers have realized the mutual transformation of various solid phases through the nano-design of materials (Lu et al. 2018). The small size of the nanostructure shortens the distance of atomic diffusion and the strain of solid transformation. It is worth noting here that Tarascon et al. found that after the first conversion reaction, MO_x would be divided into very small Li₂O and M particles (Taberna et al. 2006). The cycle performance of MO_x materials largely depends on the initial structure and morphology of the material. Researchers designed chemical bonds to attach small MO_x nanoparticles to reduced graphene oxide (RGO), demonstrating excellent cycle stability and rate performance (Wang et al. 2010). The reason for the analysis is that RGO provides MO_x with strong chemical contact sites and good electronic conductivity. On the other hand, researchers design void spaces in MO_x materials. The space design of the void could alleviate the volume change during the charging and discharging process, thereby showing excellent rate performance and cycle stability performance (Zhang et al. 2014).

Lithium Metal Anode

As the anode of LIBs, lithium metal has an ultra-high theoretical specific capacity of 3860 mAh g⁻¹, so it has aroused widespread interest. However, low coulomb efficiency and lithium dendrite growth are prone to occur during charging and discharging, which greatly hinders the practical application of lithium metal (Huang et al. 2010; Yao et al. 2011; Harry et al. 2014; Zheng et al. 2014b). In order to realize the wide application of lithium metal as anode, more basic research needs to be done to study these issues in depth. In this part, the focus is on the latest developments in electrolyte additives, interface material design, and solid electrolyte methods in limiting dendrite formation.

In order to solve the above-mentioned problems, a variety of new characterization techniques have been developed for the research of lithium metal anodes in recent years. In 2010, in situ transmission electron microscopy (TEM) was first reported, which is a new type of characterization technique. This characterization technique realizes the in situ study of the electrochemical process of SnO₂ nanowire batteries under TEM conditions (Huang et al. 2010). Specifically, in the in situ TEM battery configuration process, an ionic liquid electrolyte with low vapor pressure or Li₂O on lithium metal is used as a solid-state battery. For example, in order to help guide the design of a stable interface layer on lithium metal, researchers have adopted the dry solid-state battery configurations to analyze lithium metal deposition on a copper substrate with a hollow carbon sphere coating, revealing that lithium metal deposition occurs under carbon (Yao et al. 2011).

In recent years, researchers have developed synchrotron hard X-ray microtomography technology, which is another new type of characterization technology. Barsala et al. developed a symmetrical lithium-polymer-lithium battery that can be cycled under 90 °C (Harry et al. 2014). The research results showed that dendrites are located in the electrode and below the electrode/polymer interface in the early stage of dendrite formation. With the increase in the number of charge and discharge cycles, the dendrites are partially protruded into the polymer. As the charge-discharge cycle test continues to increase, the dendrites will exceed the thickness of the polymer electrolyte, which will eventually cause a short circuit. In situ nuclear magnetic resonance (NMR) technology is a new technology used to characterize lithium metal batteries.

In addition, the scientific design of nanoscale interface materials can also effectively solve the problem of lithium metal anodes (Zheng et al. 2014b). In order to obtain a stable interface layer, it is necessary to satisfy chemical and mechanical stability. This interface layer needs to be pre-deposited before the battery is manufactured, not just formed by electrolyte additives. Recently, researchers have made good research progress. One example is to promote the deposition of lithium metal between copper current collectors and carbon. The researchers designed interconnected hollow carbon spheres as the interface layer (Zheng et al. 2014b). The experimental results showed that the deposited lithium has a large columnar structure, and no dendrites are found. Through the testing of electrochemical performance, it exhibited the excellent cycle stability.

Separator

Polyolefin Separator

Polyolefin membranes have the disadvantage of thermal instability, which is also a problem that needs to be solved urgently. The most commonly used polyolefin membranes are PP and PE, but there is still a problem of poor thermal stability. The melting point (Tm) is just∼135 °C for PE and∼165 °C for PP, respectively (Luo et al. 2021). When the temperature reaches its melting point, the size of the diaphragm will shrink significantly. Batteries are prone to thermal runaway problems, which are attributed to the fact that the internal short circuit is further amplified by the “positive feedback loop.” In order to solve this safety issue, researchers have tried a variety of strategies to alleviate the thermal instability of polyolefin-based separators. When overheating occurs, it is an effective strategy to design a multilayer structure to close the conduction path of Li⁺ through the separator. In this multilayer structure, the separator with a three-layer structure of PP/PE/PP exhibits excellent performance. The porous middle PE layer partially melts when the internal temperature of the battery exceeds 130 °C, and functions to close the pores of the separator. This effectively prevents the diffusion of Li⁺ between cathode and anode, and the PP layer has good mechanical properties to maintain the stability of the entire separator, thereby avoiding the problem of battery short circuit. Experimental results exhibited that the safety of LIBs is effectively enhanced by the three-layer structure design of the separator (Luo et al. 2021). However, it still has some problems. For example, because the melting temperature difference between PP and PE is very small, the battery is prone to the phenomenon that the internal temperature rises too fast in actual use, which leads to the melting of the entire separator.

It is actually too late to shut down the battery after overheating, so another important research direction is to design the polyolefin membrane structure to avoid the risk of battery overheating. To enhance the safety of the battery, it is necessary to customize various important functions through the scientific design of the separator structure. A problem that seriously affects battery safety is the formation of dendritic lithium dendrites on the anode. Researchers have conducted a systematic study on the formation of lithium dendrites. During each charging and discharging process, uneven deposition/exfoliation occurs on the surface of lithium metal anode. As the number of cycles increases, lithium dendrites are gradually formed from the lithium metal anode (Lin et al. 2017). A series of exothermic reactions are prone to occur between lithium dendrites and the liquid electrolyte, so the formation of lithium dendrites is considered extremely dangerous. More importantly, the lithium dendrites have sharp shapes, which could penetrate the polymer separator and cause a short circuit inside the battery, leading to the problems of fire and even explosion. It is worth noting here that when charge-discharge cycling is under low temperature, high current density, or overcharge conditions, the commercial graphite anodes of LIBs cannot completely avoid the formation of lithium dendrites.

In situ detection of the formation of lithium dendrites during charging and discharging is another severe challenge. In particular, in the traditional battery structure, it is difficult to detect the growth of lithium dendrites by in situ detection methods. Recently, researchers have carefully designed the battery diaphragm structure to achieve in situ detection of lithium dendrites inside the battery (Wu et al. 2014). Specifically, the researchers sputtered a thin layer of copper on the surface of the PE separator to prepare a “dual functional separator” with a polymer-metal-polymer three-layer structure. The design of the middle metal layer can be used as the third electrode in addition to the traditional cathode and anode, thus having the voltage-sensing function. As the charge and discharge cycles increase, lithium dendrites are gradually formed and contact the intermediate metal layer. Because the lithium dendrites connect the anode and the metal layer in this process, researchers can detect the instant voltage drop between them and obtain the danger signal inside the battery. In addition, based on this principle, the red phosphorous layer can also be used as a voltage sensing layer for in situ detection of lithium dendrites (Wang et al. 2019b).

Polymers with High Melting Points for Separators

The development of new polymer porous membranes is an effective method to enhance the thermal stability of separators, such as cellulose, poly(ester), polyimide, and so on. Research results exhibited that polyimide has high tensile strength, outstanding thermal stability (stable above 400 °C), flame retardancy, and excellent electrolyte wettability (Lin et al. 2016). The production of interpenetrating micro/nanopores is an important step in the preparation of such high-performance separator. These pore structures can provide a rapid transmission channel for lithium ions. Recently, researchers have used lithium bromide to construct nanopore structures (Lin et al. 2016). The material is compatible with the preparation process of polyimide, which shows good practicability. Specifically, the lithium bromide (LiBr) salt and the polyimide intermediate are uniformly mixed, and a composite film is prepared through a chemical condensation reaction. By simply removing lithium bromide in the water bath, uniform and interconnected nanopores can be formed on the film. It is worth noting here that the LiBr obtained by dissolution can be recycled. This simple preparation process can provide the possibility for commercial large-scale production. Although researchers have developed a variety of new high-performance separator materials, there is still a big gap between them and their practical application. For commercial applications, it is necessary to comprehensively consider the performance of the separator, including melting point, mechanical strength, price, and processing ability.

Electrolytes

Aqueous Electrolytes

The results show that water can be used as an electrolyte solvent for LIBs, which exhibits the advantages of low cost and high safety (Suo et al. 2015; Yang et al. 2017). However, the use of aqueous alkaline electrolytes in LIBs has the problem of a narrow electrochemical stability window, which is usually only 1.23 V. In order to overcome this problem, the researchers designed a “water-in-salt” electrolyte by dissolving lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in water with a high molar concentration (Suo et al. 2015). Therefore, a dense SEI is formed on the anode surface based on the reduction of anions, which is attributed to some TFSI anions occupying the Li⁺ solvent. The electrolyte with this “water-in-salt” design shows a wider electrochemical stability window, which is significantly increased to ~3.0 V. This innovative strategy opens up a way for the possibility of using water as the electrolyte for LIBs. However, the use of these water-based electrolytes in LIB cannot match high-energy density anode materials such as silicon, graphite, and lithium metal. This is mainly because their electrochemical stability window limits are between 1.7 and 1.9 V. In order to solve the above problems, the researchers developed a new type of highly fluorinated electrolyte additives by adding 1,1,2,2-tetrafluoroethyl-2′,2′,2′-trifluoroethyl ether into the aqueous electrolyte to form a nonuniform electrolyte (Yang et al. 2017). Experimental results show that lithium metal and graphite as anode materials can achieve reversible cycles in aqueous electrolytes. The reason for the analysis is that after reductive decomposition during the first charging process, the additive forms a protective interphase, which is composed of inorganic fluoride LiF and fluorinated hydrocarbon species. More importantly, by matching these high-capacity and low-potential anode materials with various cathode materials, a 4.0 V water-based LIBs with high reversibility and efficiency can be assembled, and there is no safety issue during the charge-discharge cycle.

Flame-Retardant Additives for Carbonates

Flame-retardant properties are an important indicator for evaluating the quality of electrolyte (Granzow 1978; Pires et al. 2015; Xia et al. 2015). At present, adding flame retardant additives to commercial electrolytes is the most direct and effective strategy to reduce the flammability of conventional commercial electrolytes. Organic halogenated compounds or organic phosphorus compounds are used as common flame retardant additives in liquid electrolytes. Compared with halogens that are harmful to the human health and environment, due to the characteristics of environmental friendliness and high flame retardancy, organophosphorus compounds as flame retardant additives have more application potential in commercial LIBs. Commonly used organic phosphorus-containing compounds include phosphonate, phosphate, cyclotriphosphazene, phosphite, etc. The experimental results showed that the phosphorus-containing radical species are obtained through the thermal decomposition of phosphorus-containing molecules during the combustion process, avoiding the hydrogen and hydroxide radicals generated during the combustion process, which is essential for the propagation of chain reactions (Granzow 1978). It is found that the flammability of organic electrolytes can be effectively reduced by the addition of organophosphorus-based compounds. However, the addition of flame retardants will cause side reactions on the electrodes and lead to an increase in electrolyte viscosity, which is detrimental to the electrochemical properties of LIBs.

In order to overcome the above problem, the researchers used a variety of methods to modify the molecular architecture of flame retardants, which are as follows: (1) Preparation of flame retardants containing polymerizable or reducing groups, such as tris(2,2,2-trifluoro) Ethyl) phosphite) (Pires et al. 2015). On the premise of maintaining the flame retardant function, these can facilitate the formation of SEI on the electrode surface, thereby avoiding the occurrence of harmful side reactions. (2) Choose cyclic phosphazenes (such as fluorinated phosphazenes (Xia et al. 2015)) to replace linear organic phosphorus additives. The cyclic molecular structure can improve the electrochemical stability of the battery, which is attributed to the fact that it can decompose on the two electrodes and form a protective thin layer. It is worth noting here that the electrolyte effectively improves its flame-retardant efficiency, which is attributed to the extremely high atomic composition of flame-retardant P, N, and F elements and their synergistic effect (Xia et al. 2015). (3) It is found that the enrichment of fluoride components (such as LiF) helps to build a stable SEI in the anode, so fluorinated alkyl phosphate as an additive can significantly improve the electrochemical stability of the battery (Pires et al. 2015). In addition, research results show that the presence of fluoride elements can also improve the flame retardant effect of the compound. It should be noted that reactive H• is the key free radical to maintain combustion. The analytical reason is that during the combustion process, fluoride will undergo decomposition reaction under high temperature conditions, and the generated free radicals will combine with reactive H•, thus avoiding the spread of flame (Pires et al. 2015).

Intrinsically Nonflammable Organic Liquid Electrolytes

The development of nonflammable electrolytes is an effective strategy to solve the safety problems of liquid electrolytes in LIBs. Researchers have carried out a lot of research work on the nonflammable electrolytes, for example, electrolytes containing silicone and nonvolatile and nonflammable ionic liquids (Wang et al. 2018; Fan et al. 2018; Wong et al. 2014). However, there are still many shortcomings, such as low lithium transfer number, high viscosity, high cost, and high instability, which limit further applications. The use of flame retardants as the electrolyte for LIBs seems to be an ideal solution to overcome safety problems. However, the free solvent molecules in the flame retardant can interact with the active material, resulting in a significant drop in the electrochemical properties of the battery. Therefore, the traditional “diluted salt electrolyte” method has not been successful in solving the safety problem of battery.

It is worth noting that the researchers proposed a “salt-concentrated electrolyte” strategy to try to solve this problem by designing the solvation structure of Li⁺. The research results show that this strategy protects the electrodes from flame retardant corrosion by regulating the SEI microstructure. For example, the research group used the flame retardant trimethyl phosphate (TMP) as the sole solvent for organic electrolytes in LIBs (Wang et al. 2018). Specifically, the electrolyte is prepared by dissolving 1.0 M LiN(SO₂F)₂ (LiFSA) in TMP, and many free TMP molecules in the solution can be co-embedded and exfoliated with the graphite layered architecture. However, TMP molecules are likely to undergo coordination reactions with Li⁺ cations, and FSA⁻ anions are more likely to undergo decomposition reactions under ultra-high concentration conditions. This is attributed to the unique solution structure that improves the LUMO structure to the concentrated electrolyte system. The formation of a uniform and strong salt-derived inorganic SEI on the anode surface is essential to improve electrochemical properties. Research results show that TMP-based electrolytes have better flame retardancy than traditional carbonate electrolytes. This “concentrated salt electrolyte” strategy can promote the development of high-performance and high-safety LIBs. In addition to the design of high-concentration electrolyte, the traditional dilute salt electrolyte can also be modified to meet high-efficiency flame-retardant performance without affecting the electrochemical properties.

In recent years, researchers have developed a perfluorinated electrolyte (Fan et al. 2018). This fully fluorinated electrolyte forms a highly fluorinated interphase with a thickness of 5–10 nm, which promotes lithium metal to exhibit good electrochemical properties, and supports the stable cycle of high capacity and high voltage LiCoPO₄ or LiNi_0.8Mn_0.1Co_0.1O₂. It has also been found that electrolyte modification can effectively avoid transition metal dissolution and electrolyte oxidation. In addition, other research groups have successfully applied molecular modifications to nonflammable perfluoropolyether as electrolytes for LIBs (Wong et al. 2014). It was found that this was beneficial to the formation of SEI and the solvation of the lithium salt on the electrode, which was attributed to the modification of two methyl carbonate groups at the end of the chain. It is worth noting that due to its unique molecular structure, the number of electrolyte transfers is close to uniform.

Smart Electrolyte for Mechanical and Thermal Abuse

LIBs is often prone to some mechanical crushing and thermal abuse in the practical applications. For example, it may be crushed in an EV/HEV car accident, and the battery may be overheated when the battery is short-circuited internally or externally. This will cause a continuous exothermic reaction, which may eventually cause the battery to catch fire and explode. Therefore, the scientific design of the self-protection function of the liquid electrolyte of the LIBs is of great significance for solving the safety problem of the battery. In recent years, researchers have carried out a lot of research work on the mechanical crushing and thermal abusing of batteries, and have made good research progress (Ding et al. 2013; Yang et al. 2018). In order to solve the negative impact caused by mechanical crushing of the battery, the researchers prepared a shear thickening liquid electrolyte by adding fumed silica to the carbonate electrolyte (Ding et al. 2013). Surprisingly, this new type of liquid electrolyte has the characteristic that the shear viscosity increases with the applied shear stress. The electrolyte exhibits fluid shear thickening properties, which is attributed to the fact that the prepared fumed silica is composed of nanoparticles. When the battery is subjected to mechanical shock, the electrolyte-based shear thickening effect increases the viscosity and dissipates the destructive impact energy, which improves the safety of the battery.

In order to solve the negative impact caused by thermal abuse of batteries, researchers designed thermally responsive polymers with reversible changes in physical properties, which can effectively avoid the phenomenon of battery thermal runaway (Yang et al. 2018). Due to the free migration of ions between cathode and anode, the electrochemical storage device can operate well at room temperature. However, the molecular conformation of the polymer would change with the increase of temperature during the process of thermal runaway, resulting in significant changes in its hydrophobic-hydrophilic properties. Along with the change of polymer properties, the physical properties of the electrolyte solution will also change significantly, such as mechanical storage modulus, viscosity, and mechanical loss modulus. These changes can effectively inhibit ion transmission between cathode and anode, thereby avoiding the problem of fire/explosion caused by temperature rise. In short, researchers have carried out a lot of research work in electrolyte and made good progress in recent years.

Conclusions

In summary, the rapid development of electrode materials in recent years has greatly promoted the development of LIBs. Based on the different types of Li⁺ (ion) diffusion pathways, electrode materials are divided into different types such as spinel, olivine, and layered materials. After the commercialization of LCO, the research direction of batteries focused on the development of batteries that achieved a balance between discharge capacity, thermal stability, cycle life, and capacity retention. Mixed transition metal oxide cathodes have synergistic advantages over single transition metal cathodes. Therefore, mixed transition metal oxide cathodes (such as NMC) have developed rapidly in recent years. It is worth noting that the development of mixed transition metal oxide cathodes requires modification of the structure and composition of traditional materials. With the rapid expansion of the electric vehicle (HEV or EV) market, it is necessary to develop high-energy-density LIBs.

To develop high-energy-density LIBs, nickel-rich layered oxide batteries have attracted wide interest from researchers due to high reversible capacity (220 mA h g⁻¹). When Ni concentration in the NMC cathode increases, the capacity of the cathode material will increase, which is attributed to the two-stage oxidation change between Ni³⁺/Ni⁴⁺ and Ni²⁺/Ni³⁺ that can play an important role in charge compensation. In order to develop a LIBs with long cycle and high rate characteristics, it is necessary to optimize the ratio of Mn, Ni, and Co elements to obtain the best stoichiometric ratio. Therefore, a large number of research results show that lithium-rich and nickel-rich layered oxides can effectively enhance the power density, energy density, and cycle stability of LIBs. Although good research progress has been made, there are still some problems in these battery systems. Among them, the lithium-rich layered oxide as a cathode is assembled into a full battery, which exhibits irreversible capacity loss during the initial cycle. The reason for the analysis is that lithium oxide (Li₂O) is extracted from Li₂MnO₃ through phase change under the high cutoff voltage (>4.5 V), which leads to a rapid decrease in capacity during the charge-discharge cycle. Furthermore, the nickel-rich layered oxide used as cathode to form a battery will have the problem of decreased cycle performance, which is attributed to the adverse side reaction of NiO with the electrolyte. To further improve the electrochemical properties of the battery, the researchers tried to substitute other transition metals or use surface coating methods to scientifically design the material composition and structure. In addition, it is worth noting that the development and design of matching anode materials, separators, and electrolytes can also effectively improve the power density, energy density, cycle performance, and safety of the battery. In short, this chapter introduced the development trend of LIBs technology and the latest research progress in recent years. Although overall LIBs has made great progress, there are still some key issues that continue to be resolved. Therefore, future research will continue to push the limits of power density, energy density, cycle stability, rate performance, cost, and safety of LIBs.

Outlook

With the rapid growth in demand for LIBs, the issue of insufficient lithium supply in the future has also become the focus of attention. In order to alleviate the problem of insufficient lithium supply, the recycling technology of LIB has also attracted wide attention from researchers in recent years. Furthermore, the waste of LIBs has an adverse effect on the humans or environment, which has also attracted the attention of researchers. Fortunately, it was found that the heavy metals in the LIBs can be recovered through biological leaching, chemical, and physical methods, thereby avoiding the pollution of LIBs to the humans or environment. It is worth noting that various compounds can be prepared by these methods, which brings good economic benefits to manufacturers. To fundamentally solve the potential lithium supply crisis, researchers also have carried out a large number of studies on non-lithium-based secondary batteries, such as Na, Ca, Mg, K, and Al. In particular, sodium-ion batteries have made rapid progress and are expected to become a strong competitor of LIBs.

Although other battery technologies have also been developed, LIBs still play an irreplaceable role in the revolution of electric vehiclesand consumer electronics, and may continue to do so in the foreseeable future. Therefore, in the future, it is still necessary to conduct in-depth research on LIBs from the perspectives of material design, mechanism analysis, and device assembly, so as to further enhance the electrochemical properties of LIBs. In short, the purpose of this chapter is to comprehensively analyze the development history of LIBs. From the currently booming LIBs technology to the new type of lithium metal battery technology, its energy density, power density, cost, cycle life, safety, and other indicators largely determine the future development of LIBs technology.