Research Progress of All-Solid-State Lithium-Ion Batteries

. In order to reach the peak in carbon dioxide emissions by 2030 and achieve carbon neutrality by 2060, it is necessary to perform technical research to reduce carbon emissions. Key core technologies such as zero emissions/reductions, hydrogen industry, and energy storage are particularly important in energy conservation and emissions reduction. In terms of energy storage, lithium-ion batteries (LIBs) are more advanced. However, traditional LIBs have risks such as swelling, leakage, and flammability. The creation of solid-state lithium-ion batteries (SSLBs) will be thoroughly described in this article, along with the benefits and drawbacks of various electrolytes and electrode materials. Additionally, the future development prospects of SSLBs will be discussed. In the long run, with the continuous optimization of SSLBs performance, the positive electrode material system is a higher specific capacity of lithium-rich materials, and the lithium-ion batteries with negative metal lithium will become mainstream, and the SSLBs technology with a higher energy density, lower cost, more security, and better stability will play a vital supporting role in the clean energy transformation


Introduction
China currently holds the top position globally in terms of both its total annual carbon dioxide (CO2) emissions and its share of the global total (Figure 1).China is planning to implement more stringent rules and procedures as part of its efforts to reach a peak in CO2 emissions by 2030 and achieve carbon neutrality by 2060.The successful execution of the "dual carbon" action plan relies heavily on technological advancements.Key technologies such as zero emissions/reductions, the hydrogen industry, and energy storage play a particularly vital role in driving energy-saving and emissionreduction initiatives.Lithium-ion batteries (LIBs) have become a wellestablished form of energy storage technology.The most common kind of energy storage in modern society is provided by LIBs, which have a non-aqueous electrolyte solution and a cathode made of metallic lithium or a lithium alloy.However, solid-state lithium-ion batteries (SSLBs) represent a departure from the conventional structure of LIBs by replacing the electrolyte material with a solid, thereby introducing a new type of LIB that does not rely on any fluid.
In 1967, Kummer reported the first sodium/sulfur battery, utilizing β -alumina as the electrolyte, which marked the inception of high-conductivity solid-state electrolytes (SSEs) [1].In 2018, Zhang reported the potential for isotropic three-dimensional fast ion conduction in Na11Sn2PS12 material, demonstrating the highest conductivity among similar materials at that time [2].To address the interface challenges between the electrode and SSEs in all-solid-state batteries (ASSBs), the investigators propose various improvements.These include the addition of plasticizers, inorganic ceramic fillers, and other methods, all of which can significantly reduce the resistance to lithium-ion transport between phases.
The unique physical characteristics of SSLBs make them exceptionally promising in fields such as wearable devices, intelligent robots, and new energy vehicles, as they meet the increasingly stringent requirements for electrolyte sealing and battery safety.Traditional LIBs predominantly employ liquid electrolytes (LEs) composed of lithium salts and organic solvents.However, over extended periods of use, bubbles or dendrites may form within the battery, leading to battery swelling or leakage.Additionally, there is a risk of puncturing the battery separator, resulting in a short circuit between the positive and negative electrodes.SSLBs effectively mitigate these concerns by eliminating electrolyte volatility or leakage, thereby addressing the flammability and explosiveness issues associated with traditional LIBs.Moreover, they offer improved durability and environmental friendliness.
This article provides an overview of SSLBs, encompassing their materials, preparation methods, and current development status.Furthermore, it offers insights into the future prospects and recommendations for the advancement of this technology.The study of solid-state polymer electrolytes (SSPEs) began in the early 1970s.Fenton et al. reported PEO-based sodium and potassium crystalline complexes [3].Now, after decades of investigations, the nature of SSPEs has been better understood and their electrochemical performance has been improved.
The SSPEs usually have good flexibility, scalability, and low density, these properties allow solid-state polymer batteries to be made into different shapes.Typically, the pristine PEO/LiTFSI has a Young's modulus of 0.4 MPa [5].Although it has good mechanical performance, the electrochemical performance cannot meet the requirement of commercial applications.Additionally, the SSPEs carry out a poor performance of thwarting dendritic formation at the electrode/electrolyte interface.
The SSPEs are usually used in soft-pack batteries.Figure 2 shows a simple soft-pack battery.The electrolyte and electrodes are stacked and folded so that they can be made into various shapes.This type of battery packaging technology can be used in the fields of wearable smart devices, electric vehicles, etc.

Fig. 2. The soft pack diagram for SSPEs batteries 3 Solid-state inorganic electrolytes
Generally, SSIEs are based on oxides or sulfides.They typically perform better than SSPEs in terms of ionic conductivity at approximately 300 K.
Ionic-conductivity for OSSEs typically can vary from 10 -5 S•cm -1 to 10 -3 S•cm -1 at approximately 300 K.In order to create a low resistance channel for Li + ion transport with low resistance, the sintering temperature of electrolytes needs to be increased.
Ideal OSSEs should have the following two additional properties: (i) the thermal expansion coefficients and sintering temperature of electrolytes match with that of electrode materials; (ii) keep stable with electrode materials at sintering temperature [9].
The stiff nature of ceramics has the disadvantage of poor interface contact with electrodes and is challenging to manufacture, despite the advantages of strong ionic-conductivity and chemical stability.Therefore, achieving mass production and further application of the oxide-based SSLBs is still a challenge.To improve the electrolyte's contact with electrodes, some in-situ preparation processes of the cathode like screen-printing and physical vapor deposition were developed.Ohta et al. used a screenprinting process to coat LLZONb (SSE) with LBO (SSE in the cathode layer) [10].The battery's charge and discharge capacities ended up being about 74% of its theoretical capacity.
Figure 3 shows the basic structure of button cell batteries.Because of the rigid electrolytes, it is easier to shape it into a round button than other geometric shapes.The electrolyte and electrodes are stacked and sealed by the shell.To better improve the contact of electrolyte and shell with electrodes, a spring washer can be added to the button cell battery to apply pressure from the inside of the battery [11].
However, the low oxidation stability and sensitivity to H2O limit its application.In order to keep SSSEs in their best working condition, a sealed mold with the ability to apply pressure and control temperature is often needed.Figure 4 shows the basic structure of the mold.The electrolyte and electrodes are stacked and sealed in the sleeve which is also a heater.The pressure is applied in the vertical direction.The sleeve can also be connected to pipes in order to carry out the gas phase detection.Liu et al. reported an SSPE with well-aligned ceramic nanowires that have an ionic-conductivity of 6.05 × 10 -5 S • cm -1 at 30 ℃ [13].By manually redesigning the structure of the composite layer, the electrochemical and mechanical performance can be improved.
The research of SSCEs is still at an early stage.Unlike SSPEs or SSIEs, the models focused on pure SSEs are no longer suitable.For SSCEs, the ionic conduction mechanisms between different phases and the three-dimensional structure play a more important role.To achieve the commercial application, the physical models and technical maturity still needs to be improved.

Gel polymer electrolytes
Generally, the gel polymer electrolytes (GPEs) which contain LEs cannot be classified as typical all-solidstate electrolytes (ASSEs).However, some studies still include them in the ASSE category because of their similar ionic conductivity and safety performance to ASSEs.
Li et al. reported a novel perovskite electrolyte, Li0.38Sr0.44Ta0.7Hf0.3O2.95F0.05(LSTHF5), which was created by spark plasma sintering (SPS) and covered with a thin coating of a Li+-conducting polymer or with 2 wt%t of LiF added to the LLZO layer, followed by the addition of a gel polymer layer to create an LLZO-LiF/gel polymer solid electrolyte [14].
The gel polymer batteries may not be as suitable for harsh working conditions as those ASSBs does, but they are still a good category for some civil use and commercial application with gentle working conditions.

Advantages and disadvantages of various SSES
To improve mechanical stability and electrochemical performance, researchers try to combine SSIEs and SSPEs.Therefore, SSCE batteries are considered to be the closest category to practical and commercial applications.J Liimatainen et al. invented an electrochemical energy storage device using lithium [16].The device uses an anode composed of ISSE with lithium as the main material and connects the anode and cathode components together through a combination of different pressures and temperatures.This method allows pressure and/or temperature to be coupled to the machined component by a roll-to-roll method and various SSIEs are produced by different methods.The invention can be used in production.
As shown in Table 1, The SSSEs usually have a good room temperature ionic conductivity, which might be on par with or even exceed those of organic LEs.However, they have strict environmental requirements for application.Although SSPEs have strong electrode interface contact, their weak ionicconductivity at ambient temperature will restrict their usefulness.Kanno et al. reported that LIBs using SSSEs have exceptionally high conductivity of lithium superconductors and high stability [17].It was found that this ASSB has a very small internal resistance at 100 °C and is also superior to traditional LE batteries in many ways.
A novel polymer SSCE was formed by J Zhang et al.
by adding inorganic-organic hybrid polyphosphazene microspheres with an active hydroxyl group to polyphosphazene [18].The experimental findings demonstrate that the newly created polyPZS microspheres with the active hydroxyl group have dramatically increased the conductance of ions, The amount of lithium ions delivered is also more than with the ceramic fillers previously used.
Li Gang et al. prepared a high-safety polyimide gel polymer (PI-gel) electrolyte through the method of thermal phase conversion and integrated molding [19].The electrolyte not only has a rigid supporting skeleton existing in the original material but also has a flexible ion transport channel.These two characteristics make it exhibit good mechanical strength and ionic conductivity.
In general, the advantages of most SSEs are mainly: high ionic conductivity, while improving electrode interface compatibility.The main disadvantages are: the lithium dendrites cannot be inhibited, and the impedance at the interface between the electrode and the SSE is increased.

The future development prospects
The semi-solid electrolyte system will likely take the place of the conventional LE system in the immediate run, according to the material system.

Anode materials
The anode and cathode materials maintain the ternary anode and silicon carbon cathode system; In the medium term, it will be silicon-rich and Li-rich cathode materials, and the anode material system will remain unchanged; In the long run, the cathode materials system will be replaced by higher specific capacity lithium-rich materials.

Cathode materials
From the perspective of the cathode material system, in the short term, the system is the silicon carbon cathode.With the further improvement of the battery energy density requirements and technological progress, the metal lithium as cathode and SSEs as the electrolyte will be the future mainstream in the long term.

Conclusion
LIBs are a mature technology in the field of energy storage.However, traditional LIBs are prone to issues such as bubble formation or dendrite growth within the battery during long-term use.These problems can lead to leakage, swelling, and short circuits between the anode and cathode, posing a risk of flammability and explosion.SSLBs offer an effective solution to address these concerns.They primarily consist of electrodes and solid electrolytes and can be prepared in various forms such as press-molded, pouch, and button batteries, depending on different working environments and applications.Solid electrolytes can be classified into inorganic, polymer, and composite types.The advantages of ASSBs include higher ion conductivity and improved electrode-interface compatibility.
Major countries around the world are heavily investing in the research and development of ASSB technology.Japan has been an early entrant in the field of SSLBs, with a significantly higher number of patent applications compared to other countries.China ranks second in terms of patent applications in the field of ASSB technology, with the Chinese Academy of Sciences holding the second position globally.Among the top ten patent holders in this field worldwide, China occupies four seats, demonstrating its strong technological research and development capabilities.
However, there are still numerous challenges that need to be addressed in current SSLBs.For instance, they suffer from low ionic conductivity at room temperature, poor electrochemical stability, inadequate electrode/electrolyte interface contact, and mismatched thermal expansion coefficients.Researchers should focus on improving room temperature ionic conductivity, expanding the electrochemical stability window, and enhancing electrode/electrolyte compatibility.The development of battery materials that offer higher safety, wider applicability, and increased efficiency is crucial.There is no doubt that with the continuous progress of future science and technology, ASSBs will play an important role in cleaning the world's environment.

Fig. 1 .
Fig. 1.The CO2 emissions and its proportion of global total emissions from 1990 to 2019 in China

Fig. 5 .
Fig. 5. Top 20 institutions in the world based on the SSLB technology publications aggregated by their countries The top 20 organizations worldwide according to SSLB technology publications are shown in Figure 5 (aggregated by their countries).The number of priority patents in the world is divided into three camps, with Japan and China far ahead of the other countries.

Fig. 6 .
Fig. 6.Number of SSLB technology patent applications in four major countries at different periods Figure 6 shows the distribution of patent applications for SSLB technology in four major countries at different periods.In general, Japan's SSLB technology started earlier and has continued to this day.The number of Chinese patent applications began to enter the embryonic stage between 2005 and 2015 and began to develop rapidly after 2016r.All in all, China's SSLBs started late, but the development momentum is rapid.

Table 1 .
[15]s of solid electrolytes and summary of advantages and disadvantages[15]