Polyimide-Based Materials for Lithium-Ion Battery Separator Applications: A Bibliometric Study

Abstract

Polyimide (PI) has excellent thermal stability, high porosity, and better high-temperature resistance. It has the potential to become a more high-end separator material, which has attracted the attention of the majority of researchers. This review is aimed at identifying the research progress and development trends of the PI-based material for separator application. We searched the published papers (2012–2021) from the WOS core collection database for analysis and analyzed their research progress and development trend based on CiteSpace text mining and visualization software. The analysis shows that the PI-based composite separator material is a research hotspot in the future and the combination of nanofiber and cellulose materials with PI is also an important research direction in the future.

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

With the gradual shortage of nonrenewable resources such as oil and the increasing environmental pollution, people’s pursuit of green renewable energy has become a research hotspot. For electronic devices, electric vehicles, and power grids, the high energy density of rechargeable lithium-ion batteries (LIBs) has aroused great concern [1–3]. As a green and renewable energy, LIBs have achieved rapid development. It is widely used in laptops, smartphones, digital cameras, and new energy vehicles. It has become indispensable power energy in daily life [4–6].

The main components of LIBs include an anode, cathode, organic electrolyte, and separator, which play a direct and comprehensive role in determining and influencing key performance indicators such as energy density, cycling performance, rate performance, and internal resistance, as well as safety performance such as high-temperature resistance, flame retardant, and electrochemical stability of LIBs [7]. When the LIBs are discharged, Li⁺ is separated from the anode side of graphite and transmitted to the cathode side through the electrolyte. To maintain charge balance, electrons are embedded from anode to cathode in the external circuit. The charging process is the opposite. The function of the separator is to prevent short circuits caused by direct contact between the cathode and the anode. As a migration channel of Li⁺, it allows Li⁺ in the electrolyte to freely pass through micropores during charge and discharge [8–10]. Therefore, the safety performance, cycle life, and discharge rate of LIBs are closely related to the separator.

The commonly used materials for separators are polyolefin microporous films, usually uniaxial stretch polyethylene (PE) and polypropylene (PP), biaxial stretch PE, or multiaxial stretch PP/PE/PP [11]. As the key inner component with the most technical barrier in the LIB industry chain, the separator needs to meet the following conditions [12–14]: (1) having good electronic insulation to ensure the physical separation of anode and cathode materials and prevent internal short circuit of the battery, (2) having proper pore size and pore size distribution and good permeability to Li⁺ during charge and discharge, so as to ensure low resistance and high ion conductivity, (3) having chemical stability to ensure that the separator will not be corroded by electrolyte during use, (4) having electrochemical stability to maintain the normal use of the battery, (5) having good wettability of electrolyte, sufficient liquid absorption rate, liquid retention rate, and ionic conductivity; (6) having appropriate mechanical properties, including puncture strength and tensile strength, (7) having appropriate thickness to obtain low internal resistance, (8) and having good thermal stability and thermal shutdown performance to ensure the safety of the battery during use.

The polyolefin separator has become a kind of separator with the highest degree of marketization because of its good chemical stability, appropriate pore size and pore size distribution, and low price [15]. With the progress of science and technology, people put forward higher requirements for the comprehensive performance and safety of LIBs, such as high current and high rate discharge, long-time stable output, high heat resistance, low heat shrinkage, and good electrolyte wettability. High energy density power batteries need thinner polyolefin separators, which means that it is easier to cause thermal shrinkage deformation and internal short circuits at high temperatures [16–18]. Therefore, the traditional polyolefin separator can no longer meet the requirements of today’s 3C products and power batteries and the development of separators that can meet the requirements of high-performance LIBs has become an urgent demand of the lithium battery industry.

According to the development needs of LIB technology, researchers have developed a variety of new separator materials based on the traditional polyolefin separator. Natural materials and synthetic materials have been widely used to prepare new separators. Natural materials mainly include cellulose and its derivatives. Synthetic materials include polyethylene terephthalate (PET), polyvinylidene fluoride (PVDF), polyvinylidene fluoride hexafluoropropylene (PVDF-HFP), polyamide (PA), polyimide (PI), and aramid [8]. Polyimide (PI) is a new polymer used to manufacture LIB polymer separators. Because it contains a rigid aromatic ring and polar imide ring, it has heat resistance, chemical resistance, and good wettability, so it has a very important application prospect. The combination of electrospinning and PI is expected to make the PI nanofiber film become the next generation of battery separator material [19–21]. Cao et al. [22] prepared the PI separator by electrospinning. The porosity of the prepared separator is greater than 90% and the electrolyte absorption is relatively high, showing good thermal stability at 500°C, no obvious shrinkage, and sufficient tensile strength of 11 MPa, which can meet the requirements of battery assembly and use. Li et al. [23] successfully prepared the porous PI separator by non-solvent-induced phase separation. Compared with commercial PE membrane, the membrane shows excellent thermal stability, higher Li⁺ passing rate, and better wettability to the electrolyte.

Traditional PI materials are difficult to dissolve in most organic solvents and have very high melting temperature $\text{Tm}$ and glass transition temperature $\text{Tg}$ [24, 25]. This greatly limits the processing of PI materials into films. The polyimide film will contact with glass, copper foil, and other materials during device preparation. After the high-temperature process, obvious thermal stress will often form at the interface between the two, resulting in dimensional deformation problems such as peeling and warping of the device, which seriously affects the performance and reliability of the device [26, 27]. At present, the preparation methods of the PI separator mainly include the template method [28, 29], phase conversion method [30], and electrospinning method [31]. However, the degree of template removal is not complete. The immersion precipitation method can avoid the defects caused by stretching, but the interaction between the precursor and the solvent is often strong, and the separation process takes a long time. The nanofiber membrane prepared by electrospinning technology also has the problems of poor uniformity and low mechanical strength. In addition, the spinning process has strict environmental requirements. Therefore, in the future, it is also necessary to tackle key technical problems and optimize the cost of the preparation process of the PI-based separator.

The development of LIB separators is rapid, and the literature on the preparation and performance characterization of PI-based separators is emerging one after another. However, in the face of a large number of documents, there are some limitations, subjectivity, and one-sidedness in the analysis through reading and induction. The emergence of modern scientometrics and information metrology technology can objectively and comprehensively analyze massive literature data. Therefore, based on the WOS core collection database, this paper retrieved relevant literature on PI-based separator materials (2012-2021) and used CiteSpace visualization software to analyze the research progress and development trend, to provide theoretical reference for scholars to understand the research status of the PI-based separator and explore new research frontiers.

2. Materials and Methods

CiteSpace is an information visualization analysis software developed by Chen [32–35] using the Java language, which can analyze the potential dynamic mechanism of discipline evolution, predict the frontier of discipline development, and find out the key literature of discipline evolution through pathfinding operation of frontier terms, dynamic identification of cocitation clustering, and research hotspots. This paper takes the WOS core collection database as the data source. Set the retrieval formula “(lithium ion batter${}^{\ast }$) AND (separator) AND (polyimide)” for retrieval. The retrieval time span is 2012–2021 (slice $\text{length}=1$). 129 relevant literature were obtained through retrieval. CiteSpace 5.8 R3 (64 bit) software further collates and analyzes the obtained literature.

3. Results and Discussion

3.1. Annual Publication Trend

According to the statistical analysis of the literature’s year distribution (Figure 1), it can be seen that related studies on pi-based septum can be roughly divided into two stages. The first stage is from 2012 to 2017, during which the number of the literature shows a slow increase with a small increase, accounting for about 31% of the total output. The second stage is from 2018 to 2021, during which the number of literature increases rapidly and rapidly. In particular, 27 research articles were published in 2021. This shows that more and more scholars have devoted themselves to this direction of research. According to this trend, it can be predicted that related research will not decrease in the next few years.

[figure omitted; refer to PDF]

Figure 2 shows the dual map of published papers with the references cited in the content, with the cited map on the left and the cited map on the right. The curve is the citation line, which completely shows the context of the citation. As can be seen from the left side in the figure, the cited literature is mainly concentrated in the fields of physics, chemistry, and material science. The cited literature mainly focuses on the fields of materials, chemistry, physics, environment, and so on. At the same time, it can be seen that the research of PI-based separators does not involve interdisciplinary topics.

[figure omitted; refer to PDF]

Table 1 shows the number of articles published in different countries, time of first publication, and centrality. Centrality is an indicator to measure the importance of a node in the network (if the centrality is greater than 0.1, the node is marked purple). The greater the centrality, the greater the importance of the node to the network. The centrality of China is 0.98, indicating that China has made a great contribution to this research direction, followed by Australia, South Korea, and the United States. In terms of the number of articles published, there is an obvious scientific research cooperation between countries and important achievements are often discovered through active cooperation between multiple countries [36–38]. It is worth mentioning that although the United States began to study the application of PI-based separator materials in 2014 and published 15 relevant articles, the centrality was significantly lower than that of Australia, which only published 2 articles. This means that while the United States had a time advantage in the research process, it did not have a significant impact on subsequent research.

Table 1

The number, started time, and centrality of published papers in different countries.

County	Paper no.	Started time	Centrality
People’s Republic of China	95	2012	0.98
South Korea	21	2012	0.23
USA	15	2014	0.02
Japan	8	2015	0
Canada	2	2019	0
Australia	2	2016	0.4

Figure 4 shows the cooperation network of scientific research institutions in various countries, from which it can be seen that many small research networks have been formed by relevant research institutions. These small research networks are not connected with each other, forming a research team led by a certain research institution, such as the research team led by Tsinghua University and Xiangtan University, the research team led by Zhejiang University and Xiamen University, and the research team led by the Chinese Academy of Sciences, Shanghai Jiaotong University and Donghua University,and the research teams led by Tokyo Metropolitan University and National Hanba University. China’s scientific research institutions account for the majority and have a close cooperative relationship with scientific research institutions in other countries, but there is no obvious cooperative relationship between China’s scientific research teams. It can be predicted that Chinese scientific research institutions will make important contributions to this research direction in the future.

[figure omitted; refer to PDF]

From the author’s cooperation network (Figure 5), we can see that many academic groups have been formed among scholars and there has been close cooperation among academic groups. Four large and four small scientific research teams and some scattered research teams have been formed among scholars engaged in the research of PI-based separator materials. There is a discrete distribution among scientific research teams, indicating that there is no cooperative relationship between various teams. As can be seen in Table 2, Dezhen Wu, Shengli Qi, Guofeng Tian, and others have published a large number of documents and most of them began to study from the second stage. Dezhen Wu has the largest number of articles, reaching 14, indicating that he has been engaged in the research of PI-based separator materials. He has a close cooperative relationship with Shengli Qi, Guofeng Tian, Bingxue Lui, Guohua Sun, and others and has made great contributions [39–43]. Myunghyun Ryou and Kiyoshi Kanamura lead their scientific research teams. Myunghyun Ryou has participated in the work of the two scientific research teams at the same time and has become a bridge for communication between the two groups [44–46]. It can also be seen that the contribution of Chinese scholars is relatively large and the speed of output is relatively fast.

[figure omitted; refer to PDF]

Table 4 shows the frequency distribution and centrality of the first 22 keywords of published articles. It can be seen from the table that some properties of PI-based separator materials, such as thermal stability, safety, and conductivity, have attracted special attention. This is because these properties determine the application of PI-based separator materials, which has an important impact on the cycle performance and service life of lithium-ion batteries. At the same time, we found that new separator materials such as nonwoven separators and composite separators have also attracted the attention of researchers. If PI is compounded with some substances to make the PI composite nanofiber film, the porosity, wettability, insulation, and mechanical strength of the PI composite nanofiber film are improved compared with the pure PI film [51–58]. It can be seen from the diagram that PI-based separator materials can also be applied in gel polymer electrolytes because PI has excellent thermal stability and excellent mechanical strength. PI can be manufactured into a uniform polymer film by adjusting parameters, including concentration and viscosity of polymer solution, feed rate, nozzle size, and the distance between the tip and the collector [59–61]. In addition, we also found that the research on the separator focuses more on the effect on the anode. This is because the lithium metal anode will produce lithium dendrites during circulation, which is easy to pierce the separator, resulting in battery short circuits and serious fire accidents. Therefore, in the separator, we often focus on the influence on the anode.

Table 4

Top 22 keyword frequency distribution of published papers.

Keywords	Count	Centrality	Keywords	Count	Centrality
Lithium-ion batteries	56	0.22	Polyimide	11	0.08
Nonwoven separator	46	0.12	Cathode	11	0.08
Membrane	45	0.08	Gel polymer electrolyte	11	0.10
Performance	41	0.10	Li	11	0.07
Thermal stability	25	0.10	Composite	10	0.11
Electrolyte	24	0.18	Polyethylene separator	10	0.04
Composite separator	19	0.10	High performance	9	0.04
Polymer electrolyte	18	0.12	Cell	8	0.10
Anode	14	0.23	Separator	8	0.07
Stability	14	0.09	Composite membrane	8	0.04
High safety	13	0.08	Conductivity	7	0.11

Figure 8 is the timeline of keyword clustering. It can be seen from the figure that the current research directions of PI-based separator materials are divided into 9 categories, mainly focusing on lithium-ion batteries [62–66], lithium-sulfur batteries [67–69], and lithium metal batteries [70–72]. In addition to common lithium-ion batteries, other battery systems have also been widely studied, such as potassium ion batteries [73] and zinc ion batteries [74]. It can also be seen from the figure that nanofibers and cellulose are the hot research directions at present. Nanofibers have entered the research scope of PI-based separator materials since 2013. The research heat around the PI-based nanofiber separator has not decreased and continues until now. It can also be seen from the keywords of this cluster that the Pi-based nanofiber membrane is mainly to improve the wettability and high-temperature performance of the membrane to the electrolyte. Wang et al. [75] prepared the polymer electrolyte of the PI separator by electrospinning technology. Compared with the commercial Celgard separator, the separator showed excellent thermal stability and flame retardancy and showed high porosity and high absorption when activated with liquid electrolyte. The resulting PI-based electrolyte has an ionic conductivity of up to $2.0\times {10}^{-3}\text{S}·\text{c}{\text{m}}^{-1}$ at 25°C; Tian and his colleagues [76] prepared highly oriented polyimide/graphene oxide by electrospinning (PI/graphite oxide (Go)) nanofibers. As the separator of LIBs, PI/Go nanofibers have excellent thermal stability, good wettability to the organic liquid electrolyte, and excellent electrochemical properties compared with unprocessed PI and commercial battery separators.

[figure omitted; refer to PDF]

Cocitation analysis is a statistical method used to analyze the internal relationship between literature and reveal the citation frequency and authority of published papers in journals. Figure 9 shows the cocitation map of the literature. Through literature cocitation analysis, we can understand the influential articles in a certain research field. Figure 9 is the cocitation diagram of the literature. The size of each node indicates the frequency of literature cocitation. The lines between different kinds of literature indicate that these literature are cited in the same article. As can be seen in Figure 9, most of the articles with the highest cocitation frequency are from 2013 to 2018, indicating that this period is a key development stage in the field of PI-based separator materials, with many influential research results [77–90].

[figure omitted; refer to PDF]

4. Conclusions

In this work, we used bibliometrics software CiteSpace to mine and analyze the literature published on PI-based materials for separator applications. The peak of this research area is 2017–2021, and it is easy to predict that related research will continue to increase in the coming years. The work of this period focuses on the application of Pi-based separators in lithium batteries, with special attention to their thermal stability, infiltration to electrolytes, and electrochemical stability. Research is concentrated in the fields of physics, chemistry, and materials science. China, South Korea, and Japan are pioneers in this field. China is not only the pioneer in this field but also the country with the largest contribution, and its influence in this field can be predicted to grow. Dezhen Wu, Shengli Qi, Guofeng Tian, and Bingxue Lui are the most representative authors in this field, and their work has had an important impact on the whole field. PI-based composite separator materials will be the focus of future research, and the combination of nanofibers and cellulose materials with PI will also be an important research direction in the future. In addition to the need to focus on the design of the PI molecular structure and modification mechanism, the preparation technology and process production of the PI separator also need to strengthen research and development. Secondly, reducing the production cost of the PI separator by expanding raw material production capacity, optimizing process flow, and improving processing efficiency is also the key to realizing the rapid promotion and application of the PI separator.

Acknowledgments

The work reported here was supported by the Fundamental Research Funds for the Central Universities (no. FRF-MP-20-28).

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