Thermal Management Techniques for Lithium-Ion Batteries Based on Phase Change Materials: A Systematic Review and Prospective Recommendations

: Thermal management systems for lithium-ion batteries based on the cooling and heating of phase change materials have become a popular research topic. However, the low thermal conductivity, ﬂame resistance, high and low temperature adaptability of phase change materials, as well as the thermal runaway mechanisms and lightweight design of phase change material-based systems remain to be explored. The aim of this paper is to conduct a publication-wide macro bibliometric review on thermal management systems for lithium-ion batteries based on phase change material to date. Total of 583 associated publications were retrieved from the Web of Science Core Collection database for the period 2006–2022. A bibliometric study was conducted through the visualization software VOSviewer. The ﬁndings were derived from annual publication trends, geographical and institutional distribution, authors and their collaborative networks, keyword network analysis and analysis of highly cited publications as well as reference co-citation analysis. The ﬁndings provide a comprehensive overview of the evolution of research hotspots in the ﬁeld and can help researchers who would like to work in the ﬁeld to quickly grasp the research frontiers and the overall picture. Furthermore, some suggestions for future work are summarized.


Introduction
Energy storage technology with lithium-ion batteries as the core equipment belongs to one of the electrochemical energy storage technologies, using the conversion between electrical and chemical energy to achieve the storage and output of electrical energy [1].The lithium-ion battery has the technical characteristics of fast response and bi-directional regulation, showing advanced environmental adaptability, compact size, and a short construction cycle [2][3][4].However, in recent years, the safety accidents caused by the thermal runaway of lithium-ion have attracted widespread public attention [5,6].According to relevant data, in the past ten years (2011-2021), 32 explosions caused by the thermal runaway of energy storage batteries occurred at energy storage power plants worldwide, respectively, one in Japan, two in the United States, one in Belgium, three in China and twenty-four in South Korea.Among them, in 2018 alone, there were 16 explosions of energy storage power plants in South Korea, causing heavy losses of people's lives and properties.Domestically, in April 2021, an energy storage power plant in Fengtai, Beijing, Energies 2023, 16, 876 2 of 23 caught fire and exploded during the construction and commissioning, resulting in two deaths, one injury, and one loss.Therefore, it is essential to employ advanced technology to develop a stable and reliable lithium-ion battery thermal management system (BTMs) for comprehensive temperature control of the energy storage system to ensure its safe and stable operation [7][8][9].
The thermal management of lithium-ion batteries is mainly divided into active cooling and passive cooling or active and passive composite cooling.Active cooling is divided into air cooling with air as the cooling medium and liquid cooling with liquid cooling medium.Passive cooling includes phase change material (PCM) cooling and heat pipe cooling [10][11][12].The coupling of phase change material cooling with other active cooling methods has become the mainstream of current research, the most common of which is the combination with air cooling or liquid cooling.Table 1 shows a comparison and analysis of the advantages and disadvantages of different types of battery thermal management methods.In the air battery thermal management system, the temperature of the lithium-ion battery is controlled by using the air as a cooling and heating medium [13][14][15].Its main advantages are simple structure, low cost and high safety.However, it also has obvious disadvantages, which are demonstrated by the low heat transfer coefficient due to the low thermal conductivity and low specific heat capacity of air.Therefore, air cooling is less effective in heat transfer at high heat flow density [16,17].Liquid as a cooling and heating medium has better heat transfer performance than air [18].However, its comprehensive battery thermal management system requires heat exchangers and extra pumps, and has a complex, costly system with the risk of leakage [19].For heat pipe cooling and heating, the heat transfer efficiency is relatively high, but the arrangement of heat pipes needs to be closely fitted to the heat source, thus increasing the complexity and weight of the thermal management system [20].The research on air cooling in lithium-ion battery thermal management systems is gradually decreasing because of the poor effect of air cooling and the poor temperature uniformity of the battery.At the same time, air cooling or heating in high temperature or extremely cold environment is difficult to control the battery temperature in a suitable range.To solve these problems, the research of composite cooling technology with better thermal management performance is gradually increasing.Malik et al. studied EVs and HEVs and summarized their use of PCM or carbon nanotube (CNT) materials [23].Liu et al. reviewed various system techniques for increasing the thermal conductivity of native PCM.In recent years, BTMs have also used lower-cost hydrated salt inorganic phase change material thanks to technological advancements [24].
Numerous studies have been conducted on the applications of PCM in BTMs.Additionally, the research on PCM-based BTMs is growing annually, indicating updated keywords and creating an extensive knowledge network.However, there is no systematic overview of current works at a macro level and a summary of future trends to visualize their structure of knowledge.Bibliometrics is a well-established research technique that searches the document and can be used to analyze them for critical information, including authors, countries, institutions, etc., in a given field.Through the use of mathematical, statistical and metrological methods, bibliometrics has now become a popular method for recognizing and forecasting the trajectory of science and technology.By analyzing the links between titles, abstracts, keywords, and references, it is possible to demonstrate the relevance of the literature to research trends [25][26][27] and establish a systematic, methodical and reproducible review process, thereby improving the quality of the review [28,29].
In summary, PCM or phase change material-based battery thermal management systems have become a trend and research hotspot in the development of battery thermal management.Based on a literature information visualization approach, this paper uses VOSviewer visualization software to explore research hotspots related to phase change cooling of lithium-ion batteries through geographical and institutional distribution analysis, co-authorship analysis, keyword co-occurrence analysis, high citation analysis and cocitation analysis, in an effort to provide a structured overview and detailed analysis of existing publications and to identify potential research areas.

Data and Methods
The Web of Sciences database is one of the world's leading search databases for highquality journal and bibliographic information on a wide range of subjects [30].A total of 965 research papers on passive phase change thermal storage of lithium-ion batteries were obtained from the Web of Sciences core database, with the search formula "lithium-ion batter*" OR "Lithium ion batter*" OR "Li ion batter*" OR "Li-ion batter*" OR "Lithiumion power batter*" OR "lithium-ion cell*" OR "Lithium ion cell*" OR "Li ion cell*" OR "Li-ion cell*" OR "batter* pack" OR "batter* module" and "phase change material*" OR "PCM*" OR "*PCM" OR "passive cooling".The former of these search formulas is a search term for lithium-ion batteries, and the latter is an expression of phase change materials.The above search formula identified keywords relating to lithium-ion batteries and phase change materials in the titles, abstracts, and keywords of the publications searched.It should be emphasized that the retrieval ranges from 2006 to 2022, after eliminating porous carbon matrix, partially carbonized melamine and other acronyms that are the same as phase change materials and without mentioning interference items like lithium batteries.A total of 583 papers, comprising 91 reviews and 492 research articles, will be gathered by 20 December 2022.This paper uses the visualization software Vosviewer to analyze the country, institution, author, keyword and reference co-citation of the 583 retrieved publications on lithium-ion battery research on phase change materials, and to give correlations and network relationships between them.

Annual Publication Trends
The chronological classification of the findings, based on the eight hundred and nine publications over the years, provides a clear picture of their historical evolution and future trends [31,32].Figure 1 shows the year distribution of the retrieved research results (2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018)(2019)(2020)(2021)(2022).In 2000, Al-Hallaj, S. and Selman, J.R. published "A novel thermal management system for electric vehicle batteries using phase-change material" which used PCM thermal management system for the first time to numerically simulate the Energies 2023, 16, 876 4 of 23 thermal behavior of electric vehicle batteries [21].From 2006 to 2013, there were no more than ten annual research articles on this topic.However, some researchers began to focus on the application of phase change materials to lithium-ion batteries.Since 2014, the number of annual research literature has shown a rapid upward trend and reached more than one hundred articles for the first time in 2021, with more and more scholars investigating different perspectives on the application of phase change materials in the thermal management of lithium-ion batteries.In general, the main type of article in this field is research articles, accounting for 84.4%.The first review of this field appeared in 2011, Bandhauer T.M. et al., which mentioned the application of phase change materials in the thermal management system of lithium-ion batteries in "A critical review of thermal issues in lithium-ion batteries" [7].The trend in Figure 1 shows that research on PCM-based BTMS has received widespread attention from scholars worldwide and will continue to flourish in the future.

Annual Publication Trends
The chronological classification of the findings, based on the eight hundred and nine publications over the years, provides a clear picture of their historical evolution and future trends [31,32].Figure 1 shows the year distribution of the retrieved research results (2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018)(2019)(2020)(2021)(2022).In 2000, Al-Hallaj, S. and Selman, J.R. published "A novel thermal management system for electric vehicle batteries using phase-change material" which used PCM thermal management system for the first time to numerically simulate the thermal behavior of electric vehicle batteries [21].From 2006 to 2013, there were no more than ten annual research articles on this topic.However, some researchers began to focus on the application of phase change materials to lithium-ion batteries.Since 2014, the number of annual research literature has shown a rapid upward trend and reached more than one hundred articles for the first time in 2021, with more and more scholars investigating different perspectives on the application of phase change materials in the thermal management of lithium-ion batteries.In general, the main type of article in this field is research articles, accounting for 84.4%.The first review of this field appeared in 2011, Bandhauer T.M. et al., which mentioned the application of phase change materials in the thermal management system of lithium-ion batteries in "A critical review of thermal issues in lithium-ion batteries" [7].The trend in Figure 1 shows that research on PCM-based BTMS has received widespread attention from scholars worldwide and will continue to flourish in the future.

Geographical and Institutional Distribution
The geographical location analysis of these 583 papers according to the number of publications per country is provided.It is worth noting that, because an article may come from a collaboration of researchers from more than one country, the statistics doublecount the number of articles [33].The current statistics show that these articles come from a total of forty-nine countries.Table 2 gives the data for the five countries with the highest number of published reviews and research articles.

Geographical and Institutional Distribution
The geographical location analysis of these 583 papers according to the number of publications per country is provided.It is worth noting that, because an article may come from a collaboration of researchers from more than one country, the statistics double-count the number of articles [33].The current statistics show that these articles come from a total of forty-nine countries.Table 2 gives the data for the five countries with the highest number of published reviews and research articles.
Table 2 shows that China, the United States and India all have a position in both reviews and research articles on the application of phase change materials in the thermal management of lithium-ion batteries.Due to the enormous number of citizens, the rising need for energy and the international responsibility to reduce carbon emissions, China has vigorously developed new energy industry in recent years, and thus has taken a leading position in this field of research.Figure 2 shows the international cooperation network among 24 countries that have published more than 5 papers for phase change material-based lithium-ion batteries research.The size of the nodes in the diagram represents the number of publications from each country.The density, thickness, and shaded lines reflect the closeness of cooperation between countries, with more lines indicating a more complex cooperation network [34][35][36].Table 2 shows that China, the United States and India all have a position in both reviews and research articles on the application of phase change materials in the thermal management of lithium-ion batteries.Due to the enormous number of citizens, the rising need for energy and the international responsibility to reduce carbon emissions, China has vigorously developed new energy industry in recent years, and thus has taken a leading position in this field of research.

Country
Figure 2 shows the international cooperation network among 24 countries that have published more than 5 papers for phase change material-based lithium-ion batteries research.The size of the nodes in the diagram represents the number of publications from each country.The density, thickness, and shaded lines reflect the closeness of cooperation between countries, with more lines indicating a more complex cooperation network [34][35][36].As seen in Figure 2, there is close cooperation among the countries involved in the research on lithium-ion batteries and phase change materials, with stronger cooperation among the larger countries, such as China and the United States, which have made important contributions to solving the global energy crisis and carbon emission issues.China also cooperates with other countries around the world, which is in line with China's As seen in Figure 2, there is close cooperation among the countries involved in the research on lithium-ion batteries and phase change materials, with stronger cooperation among the larger countries, such as China and the United States, which have made important contributions to solving the global energy crisis and carbon emission issues.China also cooperates with other countries around the world, which is in line with China's current efforts to develop new energy electric vehicles and to achieve the goal of "carbon neutrality and emission peak" [37,38].
Figure 3 gives a network diagram of inter-country collaborations with a total of more than five publications in different years.The figure shows that the United States, Canada and Singapore have carried out earlier studies on the thermal management of lithium-ion batteries based on phase change materials, while China and some European countries such as the United Kingdom, France and Germany have been the main sources of publications in this field in the past two years.Despite the late start of the study in China, attention in this area is growing.It has the highest percentage of publications due to the increasing demand for new forms of transportation represented by electric vehicles in China.
than five publications in different years.The figure shows that the United States, Canada and Singapore have carried out earlier studies on the thermal management of lithium-ion batteries based on phase change materials, while China and some European countries such as the United Kingdom, France and Germany have been the main sources of publications in this field in the past two years.Despite the late start of the study in China, attention in this area is growing.It has the highest percentage of publications due to the increasing demand for new forms of transportation represented by electric vehicles in China.The papers' sources, which included 492 research articles from 363 institutions and 91 reviews from 175 institutions, were counted using the bibexcel 1.00.The top 10 institutions are listed in Table 3, the majority of which are located in China.In addition to having published a large number of articles, Guangdong University of Technology, South China University of Technology, and the Chinese Academy of Sciences also received a significant number of citations, showing that these three institutions have established themselves in the field of phase change materials and lithium battery combination.In 2019, the Chinese Academy of Sciences and Guangdong University of Technology began working together on research in this field.As early as 2011, Guangdong University of Technology and South China University of Technology cooperated on heat energy management of phase change materials for lithium-ion batteries [39].Both of them concentrated on the characteristics of composite phase change material (CPCM) and their applications in the thermal management of lithium-ion batteries during the course of the subsequent ten years.
"A Critical Review of Thermal Issues in Lithium-Ion Batteries" in 2011 by the Georgia Institute of Technology and the University System of Georgia were cited 1123 times in 91 reviews, making them the most-cited institutions on average [7].University of Quebec Trois Rivieres, with an average citation frequency of 524 times [40].City University of Hong Kong, Nanyang Technological University, Nanyang Technological University & National Institute of Education (NIE) Singapore, and University of Electronic Science & Technology of China ranked third with an average of 476 citations [24].While some institutions published only one article, the number of citations for that one article was substantially higher than for other articles, indicating that these were high-quality articles.The top 10 institutions, as determined by the average citations in 492 research articles, are listed in Table 4.These institutions are mainly from the United States, and nearly two-thirds of the highly cited articles were published before 2014.The high average citation rate suggests that their work assists successors in a significant way.

Authors and Their Cooperation Network
The thermal management of phase change materials for lithium-ion batteries is a thriving field, which is sufficiently attractive, as evidenced by the number of researchers involved.The 91 reviews collected were contributed by a total of 456 authors whose research has contributed to the development of the field.Chen Jingwei, Yang Shichun, Lin Jiayuan, Liu Xinhua are the authors with more than 3 reviews.The average citations of studies published as first author was 1123 for Bandhauer Todd M in the United States [7], 524 for Jaguemont J in Canada [40], 629 for Rao Zhonghao and 476 for Liu Huaqiang in China [11,24].They each provide a thorough overview of the application of phase change materials in the field of lithium batteries.
The highly productive authors have made significant contributions to the field, among the authors with more than 10 articles are mainly from three teams.There are Zhang Guoqing, Yang Xiaoqing, lixinxi and Huang Qiqiu from Guangdong University of Technology, among which Professor Zhang Guoqing has published 51 articles with an average citation of 44.4.In order to improve the thermal management performance of lithium batteries, he has conducted extensive research to improve the thermal stability, shape stability, and flame retardancy of composite phase change materials [41][42][43].The other team is Zhang Zhengguo, Ling Ziye, and Fang Xiaoming, who are from the South China University of Technology.Professor Zhang Zhengguo mainly studies the composite heat transfer characteristics of phase change materials combined with liquid cooling, air cooling and other cooling methods.In recent years, he also studies the thermal management performance of phase change materials at low temperatures [44][45][46].Rao Zhonghao has published 16 articles, each of which received an average of 81.7 citations, who graduated from South China University of Technology and worked at China University Mining and Technology.His research focuses on improving the thermal performance of phase change materials by using copper foam and porous medium, coupled heat pipes, microchannels with phase change material-based battery thermal management systems [47][48][49].Weng Jingwen and Wang Jian from the University of Science and Technology of China each published 14 articles [50][51][52].
As shown in Table 5, Mills Andrew in 2006 used graphite matrix and paraffin to enhance the thermal conductivity of composite phase change materials [53].Sabbah Rami proved that the PCM passive lithium-ion battery thermal management system is superior to the traditional air-cooled active cooling system [14].Goli Pradyumna used a hybrid phase change material with graphene fillers to significantly raise the efficiency and dependability of heat control in lithium-ion batteries [54].Chen Liangjie penetrated paraffin into porous and deformable carbon nanotube sponges to form phase change materials with high enthalpy and high thermal conductivity [55].Heyhat MohammadMahdi investigated the thermal properties of phase change materials with porous metal foams [56,57].Wilke Stephen and Kizilel, Riza separately demonstrated that phase change materials can effectively prevent the spread of thermal runaway in lithium-ion battery packs [58][59][60].Samimi Fereshteh and Babapoor, Aziz added carbon fiber to the PCM, which considerably improved thermal conductivity, made uniform distribution of the battery's temperature [61,62].They have continued to have an impact on the lithium-ion battery field as first authors by high-caliber publications that have considerable global appeal.A network analysis of author collaborations identifies the core authors of all current research findings in this area and their collaborative relationships.It is beneficial for researchers to quickly integrate into the field and seek their help.Figure 4 shows a diagram of the collaborative network relationships between authors.It should be noted that this diagram does not involve authors with a limited number of publications, only 24 authors who published more than one review and 55 authors who published more than four research articles.In this case, the color of the nodes indicates a closely collaborating team.The size of the nodes indicates the number of published articles, and the lines represent collaborative relationships between collaborators [34].

Research Hotsports Analysis
To further filter the hotspots and key points in the field of phase change material-based battery thermal management, keywords with a frequency of more than 2 times in the review and keywords with a frequency of more than 5 times in the research articles were extracted respectively, and high-frequency words with general significance were excluded for density and co-occurrence network analysis.The size of the nodes is positively correlated with the frequency of keyword occurrences, and the lines between nodes represent keyword dependencies.Figure 5 shows the network diagram between 45 keywords in 91 reviews.It is clear from the graph that "lithium-ion battery", "battery thermal management system", "phase change material" and "electric vehicle" are the four main keywords in this field, and the four are closely related to each other.More researchers have summarized and discussed the thermal management of lithium-ion battery based on phase change material in electric vehicle.In the cluster of electric vehicles and phase change materials, "temperature distribution" and "latent heat" follow closely.Numerous research conclusions show that, compared to other battery thermal management methods, phase change materials can absorb or release latent heat during phase change, which makes the battery surface temperature distribution more uniform.Future research will continue to concentrate on understanding the heat generation mechanism of lithium-ion batteries and prevent thermal runaway by cooling.
Figure 6 shows six clusters of 69 high-frequency keywords extracted from 492 research articles.The first cluster are "phase change material", "lithium-ion battery", "passive cooling", "heat pipe", "heat transfer", "thermal performance", and "thermal runaway" in green.This cluster illustrates that phase change materials are widely used in lithium-ion batteries as a high latent heat storage material and the combination of this passive cooling method and other cooling methods such as heat pipe, air cooling, and liquid cooling can enhance the heat transfer of phase change materials and the thermal performance of the battery.Preventing thermal runaway is another extremely important research area in lithium-ion batteries.The second cluster, in orange, uses "battery thermal management system" as the core keyword, a hot issue in recent years for optimizing battery cooling is adding fins, metal foams, and porous materials to phase change materials.The third cluster in blue consists of "hybrid thermal management system", "composite phase change materials", "liquid cooling" and other keywords.The fourth cluster in red links "numerical simulation", "battery module", and "heat dissipation" are grouped together, the construction of battery model for numerical simulation is still the primary method of current research.The fifth cluster, in pink, groups "electric vehicle", "low temperature", "thermal energy storage", "thermal model", and "battery pack".The key to promoting the use of electric vehicles in cold regions is determining how to guarantee the smooth operation of battery packs at low temperature.The sixth cluster in purple, "thermal conductivity", "paraffin", "expanded graphite", and "copper foam" are grouped together.Single phase change material cooling is no longer the optimum option for battery heat dissipation as a result of advances in research.Significant efforts have been made in two different directions: First, the development of composite phase change materials with high thermal conductivity, stability, and flame retardance; second, the combination of other active cooling techniques to develop composite battery thermal management systems.The current research on the thermal management performance of lithium-ion batteries will eventually serve the new energy industry, such as electric vehicles and energy storage power plants.
By counting the time of keyword appearance, the regularity of keyword evolution is obtained.Through numerical simulation, researchers preliminarily investigated the advantages of PCM battery cooling in comparison to other active cooling methods, such as liquid cooling and air cooling, between 2006 and 2013.From 2015 to 2017, scholars began to investigate the coupled battery thermal management system of phase change materials combined with other cooling methods to enhance heat transfer.At this stage, composite phase change materials also became the focus of research, but mostly focused on expanded graphite, paraffin and other materials.A variety of composite phase transition materials have been utilized extensively since 2018.Researchers now pay more attention to the thermal stability, flame retardant, leakage prevention, and other properties of phase change materials rather than merely improving heat transfer capacity.At the same time, the research on the thermal management of lithium-ion batteries based on phase change materials has become pluralism, such as thermal runaway, adding fins, reducing weight, etc. Figure 6 shows six clusters of 69 high-frequency keywords extracted from 492 research articles.The first cluster are "phase change material", "lithium-ion battery", "passive cooling", "heat pipe", "heat transfer", "thermal performance", and "thermal runaway" in green.This cluster illustrates that phase change materials are widely used in lithium-ion batteries as a high latent heat storage material and the combination of this passive cooling method and other cooling methods such as heat pipe, air cooling, and liquid cool- change material cooling is no longer the optimum option for battery heat dissipation as a result of advances in research.Significant efforts have been made in two different directions: First, the development of composite phase change materials with high thermal conductivity, stability, and flame retardance; second, the combination of other active cooling techniques to develop composite battery thermal management systems.The current research on the thermal management performance of lithium-ion batteries will eventually serve the new energy industry, such as electric vehicles and energy storage power plants.

Reference Co-Citation Analysis
Co-citation of a reference is the formation of a co-citation relationship between two papers when they are both cited by a third paper [63,64].The number of simultaneous citations indicates the strength of the co-citation, with a greater strength indicating a closer relationship between the two papers [65].Reference co-citation analysis provides information on the frontiers of the research field.The size of the nodes and the distance between the two papers show the frequency of occurrence of the references and the cooccurrence relationship, and the different colors represent the different clusters.A total of 7626 references were cited in the 91 reviews retrieved in this paper, and 60 references with a citation frequency greater than twenty were selected for co-citation analysis.As shown in Figure 7, three clusters are generated based on the relationship of references co-cited.The red cluster node in the upper right corner has the most nodes, which is the basic research on phase change material application in the field of lithium battery, and this part of references is mainly concentrated in the period before 2015.Followed by the green clusters on the left are mainly for thermal management structure optimization between 2014 and 2018.This cluster mainly focuses on improving the thermal conductivity of phase change materials and optimizing composite cooling methods.The blue clusters below the center, scholars' research on phase change material lithium-ion batteries between 2015 and 2019 shows diversification, mainly with studies related to the thermal management performance of lithium-ion batteries under extreme environments and prevention of thermal runaway, and the comparative analysis of various thermal management forms of lithium-ion batteries.
A total of 9391 references were cited in the 492 research articles retrieved in this paper, and 183 references with a citation frequency greater than twenty were selected for co-citation analysis.In Figure 8, the red and green in the three clusters here are consistent with those in the review, but the difference is that the references cover different times.Exploring the currence relationship, and the different colors represent the different clusters.A total of 7626 references were cited in the 91 reviews retrieved in this paper, and 60 references with a citation frequency greater than twenty were selected for co-citation analysis.As shown in Figure 7, three clusters are generated based on the relationship of references co-cited.The red cluster node in the upper right corner has the most nodes, which is the basic research on phase change material application in the field of lithium battery, and this part of references is mainly concentrated in the period before 2015.Followed by the green clusters on the left are mainly for thermal management structure optimization between 2014 and 2018.This cluster mainly focuses on improving the thermal conductivity of phase change materials and optimizing composite cooling methods.The blue clusters below the center, scholars' research on phase change material lithium-ion batteries between 2015 and 2019 shows diversification, mainly with studies related to the thermal management performance of lithium-ion batteries under extreme environments and prevention of thermal runaway, and the comparative analysis of various thermal management forms of lithium-ion batteries.A total of 9391 references were cited in the 492 research articles retrieved in this paper, and 183 references with a citation frequency greater than twenty were selected for cocitation analysis.In Figure 8, the red and green in the three clusters here are consistent with those in the review, but the difference is that the references cover different times.Exploring the application of phase change materials in the thermal management of lithium-ion batteries, optimizing the thermophysical properties and storage structures of phase change materials is the focus of scholars' early efforts.In addition, the blue cluster focuses on the period from 2016 to 2021, with intermittent work, delayed cooling and

Discussion
In this study, publications on thermal management of lithium-ion batteries based on heat transfer from phase change materials in the Web of Science database were surveyed using bibliometric methods.This study discussed the current research and shortcomings in multilayer PCMs, PCMs, structural optimization, lightweight design, low temperature

Discussion
In this study, publications on thermal management of lithium-ion batteries based on heat transfer from phase change materials in the Web of Science database were surveyed using bibliometric methods.This study discussed the current research and shortcomings in multilayer PCMs, PCMs, structural optimization, lightweight design, low temperature heating, thermal runaway, algorithm and some additional technical aspects, which will facilitate researchers to understand and overview current and future research on phase change materials for lithium-ion batteries.

Multilayer PCMs
In other fields, such as architecture, structures using multiple layers of phase change materials have been proposed to meet the different thermal loads of buildings in the winter and summer.Meng et al. [66] proposed a phase change material arrangement for a room in Shanghai, where the walls of the room are arranged with high and low-temperature phase change materials.The high-temperature phase change material is suitable for summer, with a phase change temperature of 29 • C, and the low-temperature phase change material is suitable for winter, whose phase change temperature is 18 • C. The results of the study show that this new phase change room can reduce indoor temperature fluctuations by 4.3 • C in summer and 14.2 • C in winter.Jin et al. proposed a two-layer phase change material arrangement in the floor, where two phase change material plates are arranged between the floor surface and the concrete layer, and the two-phase change materials plates for high and low temperature phase change materials, respectively [67].The results of the study show that the optimum phase change point is 38 • C for high-temperature phase change materials and 18 • C for low-temperature phase change materials.The optimal phase change temperature varies depending on the placement of the phase change material, and the application of the phase change material to the floor can also be a useful way to reduce energy consumption and achieve energy savings.Some articles investigate the energy saving effect and the improvement of indoor thermal comfort in summer and winter using a double-layer PCM.This was inspired by the design of the multilayer PCM in architecture [68][69][70].It can be expected that the multilayer PCM design will be more adaptable to the diversity of battery storage environments.

CPCM
Phase change materials can be divided into solid-solid, solid-liquid, solid-gas and liquid-gas according to the form of phase change.Compared to the other three, solid-liquid PCMs have the advantages of high latent heat and good thermal stability.Solid-liquid PCMs can be divided into organic PCMs, inorganic PCMs and eutectic PCMs according to the composition of materials.Inorganic phase change materials include hydration salts, metals, etc., organic phase change materials include paraffin, fatty acids, etc.At present, in the field of BTM, research on phase change materials is mainly focused on paraffin, expanded graphite (EG), and organic acids.When only paraffin was used as PCM, the thermal management performance of the battery was lower than that of the PCM with expanded graphite or organic acids [71].Through the unremitting efforts of research scholars, a large number of results have been achieved in the study of the application of PCMs in batteries, and some of the defects of PCMs themselves have been solved to a certain extent, such as thermal conductivity, thermal stability and leakage during phase change.Specifically, the problem of low thermal conductivity can be solved by creating composite phase change materials, one is to add carbon fiber, nanoparticles to phase change materials, the other is to add copper foam, aluminum foam, hexagonal boron nitride and so on to build metal skeleton, the latter effectively solves the problem of phase change materials leakage [61][62][63][64][65][66][67][68][69][70][71][72][73].Using the advantages of some materials make out thermochemical properties stable, stable shape of the composite phase change materials Huang et al. synthesized an anti-leakage and anti-vibration thermally induced flexible composite phase change material, the cross-linking of styrene-butadiene block copolymer was increased by ethylene-allyl-diene monomer, improve the adsorption property of paraffin wax [74].
However, the flame resistance of PCMs is now becoming a key issue for thermal runaway.Paraffin and other organic PCM compounds are currently the most widely used PCM materials.This is extremely undesirable for preventing the thermal runaway propagation of the battery.Further support materials are the many metallic structures, which have high electrical conductivity and may have the potential for short circuit circuits in the face of shock, vibration, and extrusion from the external environment.Therefore, exploring a new generation of inorganic PCMs or composite PCMs suitable for BTMs has become an urgent issue.

Structural Optimization
Enhanced heat exchange is the focus of current research in the thermal management of Li-ion batteries.Structural optimization has been the main way to enhance internal battery heat dissipation, and a lot of work has been carried out by many scholars in this area.The arrangement of batteries, spacing of batteries, thickness of PCM and the form of wrapped batteries are the main points of the structural optimization of the PCM cooling system [75][76][77].In addition to optimizing the performance of the battery and the phase change material itself, the researchers added fins inside the PCM to reduce the problem of heat accumulation inside the battery due to insufficient heat dissipation from the PCM.Sun et al. combined the straight fin and the arc-shaped fin into a new fin structure to increase the heat exchange area.He also proved that increasing the radial distance and length of the arc-fin helps to expand the heat conduction network and enhance the heat transfer effect of the phase change material [78].At present, the research on the insertion of fins in phase change materials to enhance heat transfer mainly focuses on the shape of fins, such as rectangle, triangle, trapezoid, as well as the number of fins, fin spacing and fin structure layout [79,80].Hybrid thermal management can provide better thermal performance than any active or passive cooling/heating alone, so researchers have conducted extensive, multidimensional, and in-depth studies on the structural forms of phase-changing materials coupled to air/liquid/heat pipes.First of all, there have been numerous studies on the inlet diameter, form and airflow direction of air-cooling channels, and the structural optimization of phase change materials combined with air cooling has formed a mature system of studies.Secondly, scholars optimized and improved the structure of the liquid-cooled plate/liquid-cooled tube, such as micro-channel liquidcooled plate, bionic micro-channel, spiral channel, wave channel, etc.On this basis, we further studied the quantity, size and combination form of liquid-cooled plate/liquidcooled tube, so as to optimize the thermal management performance of the battery based on phase-change material-liquid cooling [81].Chen et al. proposed a liquid cooled phase change material-hybrid battery thermal management system containing different expanded graphite contents, which improved the temperature uniformity and cooling performance of the battery [82].While the addition of liquid cooling can significantly reduce the surface temperature of the battery, there are also problems such as heavy weight, high energy consumption, and uneven heating.The combination of phase change material and heat pipe can effectively solve the problem of heat dissipation outside the battery and phase change material, But the heat pipe activation will affect the uneven temperature distribution of the battery module at the high discharge rate of the battery [20].However, there are relatively few studies on heat pipe structure optimization because of the complex structure of the heat pipe.Therefore, the structure of the heat pipe can be optimized in the future to better support the heat dissipation of the phase change material and improve the thermal management performance of the phase change material-based Li-ion battery.

Lightweight Design
The thermal system of the battery should meet the lightweight design as much as possible while ensuring operation within the appropriate temperature range and better temperature uniformity.Lightweight design can reduce the physical mass of lithium-ion batteries and thus effectively reduce the energy consumption of the main equipment.For example, reducing the weight of new energy vehicles can improve driving distance and reduce energy consumption.Current research on phase change material-based battery thermal management systems often ignores lightweight design.In battery thermal management systems, lightweight design is usually achieved in the following ways: (1) incorporating lightweight and high thermal conductivity such as fibers and nanoparticles to prepare composite phase change materials.(2) Switching to lighter metals such as aluminum for cooling pipes.(3) Adding fins to reduce the thickness of the phase change material.Luo et al. used a tubeless cooling system combining a shape-stabilized phase change material channel and phase change emulsion to make the battery thermal management system small and lightweight [83].Youssef et al. used jute fiber as a phase change material for a battery thermal management system for lithium-ion battery pack temperature and conducted an optimization study on weight, demonstrating that jute fiber is a cheap, environmentally friendly, and lightweight phase change material [84].Ling et al. developed an optimization method based on the response surface method and numerical heat transfer model, which minimizes the weight and volume of the battery thermal management system, saving up to 94.1% PCM mass and up to 55.6% volume [45].Numerous studies have shown that the individual phase change material cooling method cannot meet the requirements of improving the balance between heat dissipation efficiency and the weight of lithium-ion batteries.

Low Temperature Heating
Research on thermal management systems for lithium-ion batteries is mostly focused on high-temperature control.As the development of power sources represented by lithiumion batteries becomes more popular in cold or extremely cold regions, research on heating technology for lithium-ion batteries in low-temperature environments is also in need of a breakthrough.The efficient, safe, and stable operation of lithium-ion batteries at low temperatures is a great challenge due to poor conductivity, increased internal resistance, and the tendency to precipitate lithium metal during charging.For this reason, many scholars have devoted their efforts to the problem of low-temperature heating.At present, there are two main heating methods for lithium-ion batteries, internal heating, and external heating.Internal heating uses the heat of electrochemical reaction generated in large quantities during charging and discharging for heating, including self-heating, mutual pulse heating and alternating current heating, etc. Guo et al. determined the optimum current amplitude and frequency of alternating current at different temperatures for the internal heating of lithium-ion batteries [85].External heating is the use of an external heat source for cell heating by convection or heat conduction.The main heating methods are liquid, air, phase change materials and heat pipes.The high latent heat released by phase change materials during the curing process can effectively maintain the battery temperature for a certain period and plays an important role in balancing the temperature and voltage distribution between cells, providing better battery thermal management performance at low temperatures compared to other heating methods.Wang et al. analyzed and compared various low-temperature preheating technologies of lithium-ion batteries from five dimensions (temperature rise rate, temperature difference, cost, battery friendliness, safety, and reliability) [86].He et al. designed a battery heating system coupled with a hot plate and PCM to investigate the effect of thermal conductivity of phase change materials on the temperature and temperature uniformity of lithium-ion batteries [87].Ling et al. simulated the temperature and voltage distribution of the battery pack at different discharge multipliers at low-temperature, and the results showed that the thermal insulation performance of phase change materials with low thermal conductivity was better [44].Additionally, phase change materials with low thermal conductivity have better insulation performance.In contrast, phase change materials with high thermal conductivity can improve the temperature uniformity of the cell and shorten the charging and discharging time.However, there is relatively little literature on this topic overall and no mature theory has yet been developed.

Thermal Runaway
Lithium-ion batteries are extremely temperature sensitive and thermal runaway behavior is eventually activated when the internal temperature of the battery rises to a certain level, which can cause serious safety accidents if the propagation of thermal runaway is not stopped in time.The main factors that cause thermal runaway of lithium-ion batteries are: Physical factors that lead to deformation of the battery due to external forces such as collision and extrusion, electrical factors related to overcharge and over discharge, and thermal factors due to high or low ambient temperatures [88].Recent studies on thermal runaway in extreme environments have found that the precipitation of lithium metal in low temperature environments, which leads to battery aging, and the release of oxygenated materials from the cathode material in high temperature environments, which can react with the electrolyte and release large amounts of heat, are both important causes of thermal runaway [89].To mitigate the hazards of thermal runaway in lithium-ion batteries, safety measures such as closing the separator, adding fuses and heat-resistant coatings, and using thermal hysteresis electrolytes and cathode materials can be used in lithium-ion batteries [90].
The thermal runaway design of battery thermal management systems based on phase change materials has also become a research area.For example, in terms of phase change material selection, the application of the new flexible composite phase change materials can effectively mitigate the problem of battery deformation caused by external forces such as crushing or impact.Niu et al. found that composite phase change materials with low thermal conductivity were effective in suppressing thermal runaway propagation.However, this is in conflict with the high thermal conductivity required to improve the thermal management performance of lithium-ion batteries based on phase change materials [91].At the same time, most of the high thermal conductivity phase change materials currently used for the thermal management of lithium-ion batteries are combustible organic phase change materials, which are not only ineffective in preventing thermal runaway of lithium-ion batteries but also exacerbate the propagation of thermal runaway in lithium-ion batteries.Therefore, there is an urgent requirement to prepare new composite phase change materials with high thermal conductivity and flame retardancy.Weng et al. designed the tubular structure of CPCMs using physically flame-retardant modified CPCMs [92].Zhang et al. evaluated the cell temperature, temperature difference, and thermal runaway propagation of a hybrid battery thermal management system based on phase change materials and liquid cooling under extreme environments [93].The results show that phase change materials not only reduce the energy consumption of the battery thermal management system but also act as a buffer in the thermal runaway propagation of the battery.Weng et al. investigated thermal runaway propagation in a battery thermal management system with aluminum honeycomb coupled with air cooling and phase change materials by infrared imaging and concluded that aluminum honeycomb helps to mitigate thermal runaway propagation [51].Therefore, balancing the thermal conductivity of phase change materials and optimizing the fin structure to reduce the risk of thermal runaway is the focus of future research.

Algorithm
Current research into battery thermal management systems is mainly through experimental studies and simulation analysis and their combined methods.In order to obtain the performance of the system under multiple parameters, large number of resources are required, and algorithms can solve the above problems.It has the advantage that the performance parameters of the object can be analyzed and predicted quickly based on a certain sample size, reducing the research time [94].There is mainly neural network algorithm, multi-objective particle swarm optimization algorithm, genetic algorithm, non-dominated sorting genetic algorithm, orthogonal design and fuzzy grey relation analysis algorithm, weighted product method of multi-attribute decision algorithm, and multi-objective response surface algorithm.Mehrdad et al. developed a pattern-based neural networks (PBANNs) algorithm by combining physics-informed neural networks with visual tracking to predict different cooling rates for of the battery temperature at different cooling rates [95].This work significantly reduces the computational cost of transient case studies and the coding complexity of numerical simulations.Lin et al. used artificial neural network (ANN) to describe the relationship between BTMS parameters (intake speed, intake temperature, PCM thickness, battery spacing and discharge rate) and the thermal characteristics of battery pack.Aiming at minimizing the maximum temperature, the thermal performance of BTMS cooled by air-phase change material was optimized by genetic algorithm (GA) [96].Kolodziejczyk et al. tested the thermal conductivity of a phase change material (CPCM) consisting of paraffin and copper foam composites by building a microscopic model using the finite element method (FEM) with convolutional neural networks (CNN) for image recognition and evaluated the effectiveness of a system designed based on this method at different charge and discharge multipliers [97].The results show that the battery models developed on the basis of convolutional neural network algorithms are more reliable.Bao et al. used a fast non-dominated classification genetic algorithm (NSGA-II) for the multi-objective optimization of the thermal management system of a lithium battery pack and verified the accuracy of the optimization results experimentally [98].Zhu et al. used a continuous multi-attribute decision making (MADM) algorithm to optimize the temperature difference, the maximum temperature and the mass and curing rate of the composite phase change material, resulting in the maximum temperature and maximum temperature difference of the battery at 4 • C discharge multiplier being reduced to 317.51 K and 3.45 K respectively [99].The use of big data, artificial intelligence, and advanced algorithms to build the thermal management system of lithium-ion batteries will play a significant role in the role of phase change materials.

Additional Technical Aspects
Future research on the application of phase change materials in lithium batteries should also focus on delayed cooling, the effect of vibration on phase change materials, and how to balance the relationship between thermal management performance, economy, and safety.The delayed cooling system can effectively improve the performance of lithium-ion batteries under the long-term charge-discharge cycle.Cao et al. designed a delayed liquid cooling system for lithium-ion battery packs with 40 cylindrical batteries, which shortened the liquid cooling time, thus saving the power consumption of the system and significantly reducing the temperature difference between and within batteries [100].At present, there is little research on phase change material delay cooling system, so this is a future research direction.Joshy et al. conducted a vibration test of a plug-in hybrid electric vehicle on the road during normal operation and found that the temperature of the battery surface increased as the frequency and amplitude of the vibration increased [101].It is worth noting that the smaller vibration amplitude is beneficial to strengthen the heat transfer of phase change material and reduce the operating temperature of the battery.Vibration can accelerate the dispersion and collision of high thermal conductivity particles in PCM and improve the heat dissipation efficiency of battery pack [102].Considering the wide application of lithium-ion batteries in new energy vehicles, it is extremely important to explore the relationship between vibration and battery temperature to ensure the smooth operation of electric vehicles.At the same time, finding a reasonable vibration frequency conducive to enhancing heat transfer is also the research direction in the field of CPCMs in the future.

Conclusions
Over the past years, countries worldwide, represented by China, have focused their attention on the thermal management performance and thermal runaway mechanism of lithium batteries made of phase change materials.Firstly, the thermophysical properties of phase change materials are the main subject of investigation.The performance of lithiumion batteries' thermal management might be greatly enhanced by using inorganic PCMs with excellent thermal conductivity, flame retardancy, and electrical insulations.However, the PCM arrangement, filling quantity, fin structure and cyclic charge and discharge stability of the multilayer PCM, and how to ensure the stability of the performance and shape retention of PCMs during repeated heat storage and release need to be further investigated.Secondly, research has focused on optimizing the cooling method in phase change material-based battery thermal management systems, by adding metals such as aluminum foam to enhance cooling performance to prevent leakage of phase change materials and reduce system weight.At the same time, there are also bottlenecks in the hybrid cooling method that need to be broken through, such as the obvious weight increase of the liquid cooling method, the limited thermal efficiency of air cooling, and the large space occupied by heat pipes.
In addition, most of the current research on phase change materials for battery systems is focused on a single environment and mostly high-temperature environments.Given the prevalence of electric vehicles in cold areas, with the spread of electric vehicles in colder regions, the low-temperature performance of lithium-ion batteries, balancing the heating performance at low temperatures with the cooling performance at high temperatures will also be a key focus of future research.Finally, future research on battery thermal management can consider combining advanced algorithms with artificial intelligence and big data to establish an excellent battery thermal management system to ensure the battery's safe, efficient and reliable operation.

Figure 1 .
Figure 1.Trends in publications per year.

Figure 1 .
Figure 1.Trends in publications per year.

Figure 3 .
Figure 3. Relationship between countries and years.

Figure 3 .
Figure 3. Relationship between countries and years.
Energies 2023, 16, 876 9 of 23 of the collaborative network relationships between authors.It should be noted that this diagram does not involve authors with a limited number of publications, only 24 authors who published more than one review and 55 authors who published more than four research articles.In this case, the color of the nodes indicates a closely collaborating team.The size of the nodes indicates the number of published articles, and the lines represent collaborative relationships between collaborators[34].

Figure 4 .
Figure 4. Collaborative network of distinct authors.(a) Author cooperative network in reviews; (b) author cooperative network in research article.

Figure 4 .
Figure 4. Collaborative network of distinct authors.(a) Author cooperative network in reviews; (b) author cooperative network in research article.As shown in Figure 4a, Zhu Hao, Deng Yuanwang, E Jiaqiang, Chen Lingwei, Zhang Feng and Liao Gaoling have close cooperation among the authors of the review articles, and Yang Shichun, Zhang Cheng, Lin Jiayuan, Li shen, and Liu Xinhua is another group of related authors.Otherwise, the research was relatively independent among the authors.As shown in Figure 4b, the collaboration of authors shows several local aggregations.Most of the aggregation points represent authors from China, indicating that Chinese schol-

23 Figure 5 .
Figure 5. Keywords co-occurrence network of the review.

Figure 5 .
Figure 5. Keywords co-occurrence network of the review.

Figure 6 .
Figure 6.Keywords cluster of the research articles.By counting the time of keyword appearance, the regularity of keyword evolution is obtained.Through numerical simulation, researchers preliminarily investigated the advantages of PCM battery cooling in comparison to other active cooling methods, such as liquid cooling and air cooling, between 2006 and 2013.From 2015 to 2017, scholars began to investigate the coupled battery thermal management system of phase change materials combined with other cooling methods to enhance heat transfer.At this stage, composite phase change materials also became the focus of research, but mostly focused on expanded graphite, paraffin and other materials.A variety of composite phase transition materials have been utilized extensively since 2018.Researchers now pay more attention to the thermal stability, flame retardant, leakage prevention, and other properties of phase change materials rather than merely improving heat transfer capacity.At the same time, the research on the thermal management of lithium-ion batteries based on phase change materials has become pluralism, such as thermal runaway, adding fins, reducing weight, etc.

Figure 6 .
Figure 6.Keywords cluster of the research articles.
change materials in the thermal management of lithium-ion batteries, optimizing the thermophysical properties and storage structures of phase change materials is the focus of scholars' early efforts.In addition, the blue cluster focuses on the period from 2016 to 2021, with intermittent work, delayed cooling and other technologies added.A large number of scattered points in the clusters indicates the multidimensional and independent nature of the research, indicating a certain extent of the leading-edge research in the field, such as reducing the weight of phase change materials, neural networks, etc.

Figure 7 .
Figure 7. Reference co-citation network of reviews.

Figure 7 .
Figure 7. Reference co-citation network of reviews.

Figure 8 .
Figure 8. Reference co-citation network of research articles.

Figure 8 .
Figure 8. Reference co-citation network of research articles.

Table 1 .
Comparison of battery thermal management methods.

Table 2 .
Top five countries with more publications.

Table 3 .
Top 10 institutions with more publications.
* AC means average citations.

Table 4 .
Top 10 institutions with more average citations in research articles.

Table 5 .
Top 10 authors with more average citations in the research articles.