ANTI-MIGRANTION LAW

How the 2024 Migration Pact reconfigured European Union migration governance

Study finds that EU migration governance is increasingly shaped by political bargaining among national leaders rather than by technocratic coordination

Sophia University

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 This study shows how the 2024 New Pact on Migration and Asylum shifted European Union (EU) migration governance from expert-led coordination to political bargaining among member states, reshaping how migration and asylum decisions are taken.

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Credit: Professor Midori Okabe from Sophia University, Japan

Migration and asylum policy have long challenged the European Union (EU), exposing deep divisions among member states over how responsibility for asylum seekers should be shared and how the bloc's external borders should be managed. After years of political deadlock, the EU adopted the New Pact on Migration and Asylum in 2024, a landmark agreement designed to reform the region's migration system. However, a new study suggests that the Pact's most important legacy may not be the policies it introduced, but the way it transformed migration governance itself.

 

In a study published online in the JCMS: Journal of Common Market Studies on May 17, 2026, Professor Midori Okabe from Sophia University, Japan, examined how negotiations surrounding the Pact reshaped decision-making within the EU. Rather than relying primarily on expert-led coordination among national officials and EU institutions, migration governance increasingly came to be driven by political bargaining among national leaders.

 

Reflecting on the motivation behind the research, Prof. Okabe says, "I was interested in why and how the EU was able to reach an agreement on migration and asylum after years of political deadlock. The negotiation process suggested that migration governance was shaped not only by technocratic coordination, but increasingly by political bargaining among member states."

 

For decades, migration and asylum policies were largely coordinated through networks of Justice and Home Affairs officials. These officials worked through technical negotiations and administrative cooperation to develop common approaches across the EU. While political leaders occasionally intervened during crises, day-to-day policymaking generally remained in the hands of specialists and government officials.

 

The 2024 Pact, however, emerged under very different circumstances. The migration crisis of 2015 exposed weaknesses in the EU's asylum system and intensified disagreements between member states. Efforts to reform the existing rules repeatedly stalled, particularly over the question of whether asylum seekers should be redistributed across countries. According to the study, traditional mechanisms of coordination were no longer sufficient to overcome these political divisions. To understand how the deadlock was eventually broken, Prof. Okabe analyzed the negotiations that led to the adoption of the Pact. The study examined policy documents, institutional developments, existing scholarship, and evidence from the negotiation process to trace how decision-making evolved during the reform.

 

Rather than viewing the Pact simply as a reform of migration and asylum rules, the study argues that it marked a deeper institutional shift in how decisions are negotiated, justified, and implemented within the EU. The analysis revealed a significant change in the center of decision-making, which moved away from networks of officials and toward high-level political forums involving heads of state and government.

 

The study also found that negotiations increasingly relied on political trade-offs and package deals that linked migration policies to broader issues, including border management and cooperation with non-EU countries. This reflected what the study describes as a shift toward "transactional diplomacy," a form of negotiation in which political compromises are exchanged across multiple policy areas to secure agreement among member states.

 

As a result, migration governance evolved from a primarily technocratic process into a central arena of intra-EU diplomacy. Political leaders played a much greater role in shaping outcomes, while negotiations became increasingly focused on securing agreements that could accommodate diverse national interests. "This study examines the structural transformation through which EU migration governance has become a central agenda of intra-EU diplomacy, shifting from technocratic coordination led by Justice and Home Affairs officials to high-level political bargaining among member states," explains Prof. Okabe.

 

The findings also point to broader implications for the future of European integration. As migration continues to be a politically sensitive issue across the continent, decision-making may increasingly depend on negotiations among national leaders rather than on technical coordination among policy experts. This could reshape how the EU responds to future migration challenges and how solidarity among member states is defined in practice.

 

The study suggests that the 2024 Pact did more than resolve a legislative deadlock. It redefined who shapes migration policy within the EU and how political consensus is built. As future migration challenges emerge, the Pact may be remembered not only for the policies it introduced but also for the new model of governance it established.

 

 

About Professor Midori Okabe from Sophia University, Japan

Midori Okabe earned a Ph.D. in International Relations from the Graduate School of Arts and Sciences, the University of Tokyo, Japan. She currently serves as Professor in the Department of International Legal Studies, Faculty of Law, Sophia University, Tokyo, and as Professional Research Associate at SOAS University of London. With more than 20 years of experience in research and higher education, her work focuses on migration governance, international relations, European Union studies, regional integration, and global governance. Professor Okabe has published 16 scholarly articles and book chapters on these topics and has held visiting appointments at the University of Cambridge, the University of Oxford, and Johns Hopkins University. Her research examines the political and institutional dynamics that shape migration governance and international cooperation on migration in Europe and beyond.

 

About Sophia University

Established as a private Jesuit-affiliated university in 1913, Sophia University is one of the most prestigious universities located in the heart of Tokyo, Japan.  Imparting education through 29 departments in 9 faculties and 25 majors in 10 graduate schools, Sophia hosts more than 13,000 students from around the world.

Conceived with the spirit of “For Others, With Others,” Sophia University truly values internationality and neighborliness, and believes in education and research that go beyond national, linguistic, and academic boundaries. Sophia emphasizes on the need for multidisciplinary and fusion research to find solutions for the most pressing global issues like climate change, poverty, conflict, and violence. Over the course of the last century, Sophia has made dedicated efforts to hone future-ready graduates who can contribute their talents and learnings for the benefit of others, and pave the way for a sustainable future while “Bringing the World Together.”

Website: https://www.sophia.ac.jp/eng/

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Journal

JCMS Journal of Common Market Studies

DOI

10.1111/jcms.70129 

Method of Research

Literature review

Subject of Research

Not applicable

Article Title

Reconfiguring European Union Migration Governance: From Technocratic Policy Harmonisation to Transactional Diplomacy Under the 2024 Pact on Migration and Asylum

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Lancet papers call for Japan to redesign health and diplomacy by 2040

Tohoku University

On July 16, 2026, an international research team led by Tohoku University published two policy review papers that examine Japan's health system and global health strategy in The Lancet Regional Health - Western Pacific ― the Lancet group's first Japan-focused feature in about 15 years. Building on the world-leading healthy longevity Japan has achieved under universal health coverage, the papers argue that keeping the system sustainable to 2040 will take a full redesign, not just a small adjustment.

Japan's universal health coverage, which aims to provide everyone with necessary care without serious financial hardship, has helped build one of the longest-living societies on earth. However, new challenges are arising that may put a strain on Japan's healthcare system. By 2040, more than one in three people in Japan will be aged 65 or older, and medical and long-term care needs are shifting from short-term hospital treatment to years of daily support for older adults living with chronic illness, dementia and frailty. The papers argue that today's incentives, budgets and data systems were shaped for an earlier era, and that meeting these needs will require looking beyond cost containment to how well the system supports people's everyday lives. How can Japan keep universal health coverage fair and sustainable as its population rapidly ages?

A working group of 25 researchers and policy practitioners from Japan and the Western Pacific reviewed evidence and discussed what must change. The first paper describes a pattern in which widely recognised pressures have yet to translate into reform of the system's core architecture ― what it terms "institutional stagnation." It proposes that Japan moves from a hospital- and service-volume-centred system to one focused on people's lives, health outcomes and communities, with better links between medical, long-term care and welfare data; independent scientific advice; clearer evaluation of policy effects; and public spending decisions that consider broader social value, such as family burden and quality of life.

The second paper looks outward. Japan has delivered notable results at G7 and G20 summits. However, the paper's concern ― which it calls "strategic inertia" ― is connecting that momentum to everyday policy, multi-year budgets and skilled personnel. It argues that health should not be treated only as foreign aid; instead, Japan should make health a pillar of "human security diplomacy": diplomacy that protects people's lives, health and dignity while strengthening regional stability and economic security. The paper calls for stronger cross-ministry coordination, stable multi-year funding and common regional rules in areas such as infectious disease control, antimicrobial resistance, medicine quality and supply, health data, artificial intelligence and misinformation. It also calls for cooperation with Western Pacific partners in areas where common technical rules can be agreed.

The researchers suggest three main changes: from hospital-centred care to community-centred support; from aid to human security diplomacy; and from one-way knowledge transfer to mutual learning. The papers give particular weight to that third shift: Japan can share experience in areas such as universal health coverage, long-term care and disaster response, and can in turn learn from Asian partners in areas such as efficient payment systems, digital health tools and community-based care developed under tight resource constraints.

"The central question is not how Japan can protect the old system, but how it can redesign the social contract behind universal health coverage," says Shuhei Nomura, a professor at Tohoku University's International Research Institute of Disaster Science (IRIDeS).

The papers offer a 2040 roadmap linking domestic reform, regional partnership and preparedness for challenges such as population ageing, climate change, disasters and future health crises. Offered as a basis for dialogue rather than a prescription, the researchers hope this work can inform the policy choices ahead.

 

 

Paper 1: Japan's health system
Title: Japan's health system toward 2040: structural challenges and a renewed social contract

Authors: Lisa Yamasaki; Haruka Sakamoto; Hideki Hashimoto; Naoki Kondo; Osamu Kunii; Yusuke Tsugawa; Sarah Krull Abe; Satoshi Ezoe; Shinichi Egawa; Renzo Guinto; Ataru Igarashi; Angkana Lekagul; Kaung Suu Lwin; Mitsuru Mukaigawara; Ho Namkoong; Yoshitaka Nishino; Santosh Kumar Rauniyar; Eiko Saito; Kenji Shibuya; Keizo Takemi; Kun Tang; Takahiro Tabuchi; Hana Tomoi; Tomoko Suzuki; Shuhei Nomura

Journal: The Lancet Regional Health - Western Pacific

DOI: 10.1016/j.lanwpc.2026.101920

Paper 2: Japan's global health strategy
Title: Japan's global health engagement in the Western Pacific: diagnosing structural inertia and a framework for human security diplomacy

Authors: Haruka Sakamoto; Lisa Yamasaki; Sarah Krull Abe; Satoshi Ezoe; Shinichi Egawa; Renzo Guinto; Hideki Hashimoto; Ataru Igarashi; Angkana Lekagul; Kaung Suu Lwin; Mitsuru Mukaigawara; Ho Namkoong; Yoshitaka Nishino; Naoki Kondo; Osamu Kunii; Santosh Kumar Rauniyar; Eiko Saito; Kenji Shibuya; Keizo Takemi; Kun Tang; Takahiro Tabuchi; Hana Tomoi; Yusuke Tsugawa; Tomoko Suzuki; Shuhei Nomura

Journal: The Lancet Regional Health - Western Pacific

DOI: 10.1016/j.lanwpc.2026.101921

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Journal

The Lancet Regional Health - Western Pacific

DOI

10.1016/j.lanwpc.2026.101920 

Article Title

Japan's health system toward 2040: structural challenges and a renewed social contract

Article Publication Date

16-Jul-2026

 

World's largest study reveals the genetic diversity of Parkinson's disease

Researchers from the University of Lübeck and international partners show that the genetic causes of Parkinson's vary widely across ancestries; this is a key step towards globally equitable diagnostics and therapies

University of Lübeck

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Prof Dr Christine Klein (Institute of Neurogenetics, University of Lübeck) co-led the world's largest study on the genetic diversity of Parkinson's disease; here next to a modern sequencing instrument at her institute.

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Credit: Anja Staehle, University of Luebeck

Parkinson's disease is the second most common neurodegenerative disease worldwide after Alzheimer's and, according to the World Health Organization, one of the fastest-growing neurological disorders. Yet genetic research into the disease has so far relied almost exclusively on people of European ancestry. This is a problem, because the genetic causes behind the disease depend strongly on where a person comes from.

The work is part of the Global Parkinson's Genetics Program (GP2), an international consortium that brings together genetic data from people with Parkinson's around the world to advance diagnostics and precision medicine. "GP2 makes it possible to include numerous cohorts from different world regions and thus achieve a previously unattainable breadth and depth of genetic analyses," explains Prof Dr Christine Klein, adding: "Without close collaboration with the many GP2 partner centres and their cohorts, a study of this size and diversity would not have been possible."

Large regional differences in the frequency of gene variants

Two genes are at the centre of the study: GBA1 and LRRK2, which are already targets of new drugs. Both provide the blueprints for enzymes responsible for "waste disposal" in nerve cells. When they do not work properly, harmful protein deposits can accumulate and contribute to the development of Parkinson's. The study shows that while these genes are relevant worldwide, which variants occur where differs considerably from region to region.

Why this matters for patients

For the study, the team analysed genetic data from almost 100,000 people across eleven world regions. Nearly a third came from population groups that have hardly featured in previous research, such as those in Africa, Latin America and Asia. The findings have immediate consequences: if genetic diagnostics are geared only to the variants known in Europe, disease causes in people of other ancestries often go undetected. As a result, they also fall through the cracks when it comes to new, genetically targeted therapies, which so far have been tested almost exclusively in Europe and North America.

"Our study is an important first step towards a truly global precision medicine for Parkinson's. Before we can make genetically targeted therapies available worldwide, we need to understand which genetic causes play a role in different population groups and where patients potentially suitable for future genetically stratified clinical trials can be found," adds Dr Lara M. Lange.

Lübeck's contribution

Data from the University of Lübeck's own patient cohorts were included in the analyses. In addition, Prof Klein of the Institute of Neurogenetics leads the Monogenic Network within GP2, through which genetically particularly well-characterised Parkinson's cases from many international cohorts were contributed to the study. As a postdoctoral researcher at the Institute of Neurogenetics in Lübeck, Dr Lange played a key role in building this Monogenic Network and decisively shaped its structure and scientific direction. GP2 and the many participating cohorts thus form the basis that enabled the study to make the genetic diversity of Parkinson's visible across eleven population groups.

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Journal

The Lancet Neurology

DOI

10.1016/S1474-4422(26)00198-5 

Method of Research

Observational study

Subject of Research

People

Article Title

Parkinson's disease genetics across diverse ancestries: an observational genetic study of causal and risk variants with translational implications

Article Publication Date

16-Jul-2026

 

Study finds mixed associations between breastfeeding and childhood allergies

Six months of exclusive breastfeeding was associated with a higher prevalence of food allergy and atopic dermatitis

Toyama Regional Center for the Japan Environment and Children's Study, University of Toyama

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Using data from 88,037 mother–infant pairs, the study found that exclusive breastfeeding for the first 6 months of life was associated with a lower prevalence of bronchial asthma and allergic rhinoconjunctivitis but a higher prevalence of food allergy—particularly in male infants—and atopic dermatitis.

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Credit: Professor Emeritus Hidekuni Inadera from University of Toyama, Japan

Breastfeeding is widely recognized for its important role in supporting a child's early growth and immune system development. Previous studies have found that exclusive breastfeeding for 4 to 6 months is associated with a lower risk of bronchial asthma and allergic rhinitis during early childhood. Recognizing these health benefits, the World Health Organization recommends exclusive breastfeeding for the first 6 months of life. Despite its positive effects, its impact on food allergy remains inconclusive.

To address this question, a research team led by Professor Emeritus Hidekuni Inadera, together with doctoral candidate Hitomi Inano, Dr. Kanako Shimada, and Assistant Professor Akiko Tsuchida from the University of Toyama, Japan, examined the association between breastfeeding patterns and allergic diseases using data from a birth cohort of 88,037 mother–infant pairs followed from birth until the children reached 6 years of age. Their findings show that breastfeeding during the first 6 months of life was associated with a lower prevalence of some allergic diseases but a higher prevalence of others. This paper was published in Volume 26 of the journal BMC Pediatrics on April 21, 2026.

"Beneficial associations of breastfeeding were observed for the prevalence of bronchial asthma and allergic rhinoconjunctivitis, while adverse associations were detected for food allergy, particularly in male infants," says Prof. Inadera.

The researchers analyzed data from the Japan Environment and Children's Study (JECS), a large-scale birth cohort that enrolled pregnant women from 15 regions in Japan between January 2011 and March 2014. Participants were divided into four groups based on their feeding pattern during the first 6 months of life: infants who were exclusively formula-fed, breastfed for less than 6 months, breastfed for 6 months while also receiving formula, or exclusively breastfed for 6 months.

The researchers then examined the association between breastfeeding patterns and the development of allergic diseases until the children reached 6 years of age using annual questionnaires completed by caregivers, who reported whether a physician had diagnosed their child with bronchial asthma, allergic rhinoconjunctivitis, food allergy, or atopic dermatitis.

They found that children who were exclusively breastfed for the first 6 months of life were less likely to develop bronchial asthma and allergic rhinoconjunctivitis than children in the other feeding groups. The association with bronchial asthma was strongest during the first 2 years of life and gradually weakened as the children grew older.

In contrast, exclusive breastfeeding for 6 months was also associated with a higher prevalence of food allergy and atopic dermatitis. The association with food allergy was strongest in boys and remained evident until approximately 3 years of age.

The researchers note that one possible explanation for the higher prevalence of food allergy is the timing of complementary food introduction. Previous studies have shown that introducing common food allergens, such as eggs and peanuts, between 4 and 6 months of age may help infants develop immune tolerance, thereby reducing the likelihood of food allergies.

“Consistent with our results and some previous observations, current guidelines recommend that solid food be introduced when infants are between 4 and 6 months while breastfeeding is continued,” says Prof. Inadera.

The findings suggest that breastfeeding practices should be considered alongside the timing of complementary food introduction when developing strategies to reduce childhood allergies. Further research is needed to better understand how these factors interact.

 

Reference
DOI: https://doi.org/10.1186/s12887-026-06864-w

 

About University of Toyama
The University of Toyama located in Toyama Prefecture, Japan is comprised of 3 former national universities; Toyama University (founded in 1949), Toyama Medical and Pharmaceutical University (founded in 1975) and Takaoka National College (founded in 1983), which were integrated in October 2005. This integrated university houses 9 schools, 8 graduate schools, the University Laboratory, Hospital, and Library, among others like the Organization for International Education and Exchange. There are approximately 9,300 students (including 300 international students) studying at the university.

Website: https://www.u-toyama.ac.jp/en/

 

About Professor Emeritus Hidekuni Inadera from University of Toyama
Professor Emeritus Hidekuni Inadera is a public health researcher in the Department of Public Health, Faculty of Medicine, University of Toyama, Japan. He has authored more than 190 scientific publications, with research focusing on children's environmental health, environmental chemicals, the prevention of non-communicable diseases, and occupational health.

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Journal

BMC Pediatrics

DOI

10.1186/s12887-026-06864-w 

Method of Research

Observational study

Subject of Research

People

 

Cancer type–specific contributions to the reduction in cancer mortality gap between Black individuals and white individuals

JAMA Oncology

About The Study:

In this cross-sectional temporal analysis, the marked reductions in excess mortality indicate meaningful progress achieved for several major cancer types through population-wide cancer control efforts. Persistent or worsening disparities in breast cancer and uterine corpus cancer and substantial residual disparities in prostate cancer highlight priorities for equitable interventions. These findings may guide more targeted and efficient cancer prevention and control strategies to further accelerate progress. 

Corresponding Author: To contact the corresponding author, Chenxi Jiang, MPH, email chenxi.jiang@cancer.org.

To access the embargoed study: Visit our For The Media website at this link https://media.jamanetwork.com/

(doi:10.1001/jamaoncol.2026.2400)

Editor’s Note: Please see the article for additional information, including other authors, author contributions and affiliations, conflict of interest and financial disclosures, and funding and support.

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Journal

JAMA Oncology

 

A multidisciplinary study of the Guadalquivir River assesses water quality along its entire course

University of Córdoba

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The study of water quality, from its source to its mouth, combines physicochemical and biological parameters, indicating a gradual deterioration throughout the river, despite generally good quality

The Guadalquivir is the most important river in Andalusia. Stretching 657 kilometers, it forms the backbone of Andalusia, from its source in the Sierra de Cazorla mountains to its mouth in Sanlúcar de Barrameda. These facts, along with its name, of Arabic origin (Wadi al-Kabir), and its strategic role in the economy, agriculture, and tourism, are widely known. Few studies, however, have comprehensively assessed the quality of the "Great River" along its entire course.

To address this knowledge gap, the "Molecular Biology of Stress Response Mechanisms" and the "Waste Bioengineering: Green Engineering" groups at the University of Córdoba have joined forces to study the river's entire course. For the first time, they are carrying out a holistic, multidisciplinary analysis that considers not only the water's physicochemical variables, but also microbial factors to assess water quality and identify critical sections where it may be compromised. The Ecotoxicology, Ecophysiology, and Biodiversity of Aquatic Systems group at the Institute of Marine Sciences of Andalusia (ICMAN-CSIC) is also participating alongside them.

"To integrate the physicochemical parameters, we used the Water Quality Index, a standardized system used internationally that includes variables such as pH, dissolved oxygen concentration, water turbidity, and the presence of coliforms," explained researcher José Ángel Siles. Twenty-five different physicochemical variables were analyzed, and the nine most important were selected and applied to the water quality index, with weighted values (not all the variables are of the same importance). "This index showed us that the river's quality is generally good, although we did identify specific sections where, as in other river basins, the quality drops slightly."

For the first time, a biological analysis of the microbial community was added to this physicochemical one. "We characterized the microbiome along the entire river and calculated biological indices to determine water quality," explained Marina Barbudo, the lead researcher on this study, published in the Journal of Contaminant Hydrology. "We integrated the physicochemical parameters with biological ones and detected a gradual decline in quality along the river's course, though it always remained at adequate levels," added researcher José Alhama.

The groundbreaking aspect of this work is its combination of these two perspectives, as physicochemical analyses can fail to detect contaminants when they are present in very low concentrations; even in these cases, however, pollutants can interact with each other in complex ways, generating toxicity, an effect that will be reflected in the biological component. According to Carmen Michán, another participating researcher, "Biological characterization helps reveal what sometimes goes undetected by physicochemical analysis alone."

Results of the river's "X-ray"

"In the study, water samples were taken from 20 different points along the entire course of the river at the end of a dry season. This was important because weather conditions affect water quality, and this is usually when it’s at its worst," added researcher María Ángeles Martín.

The results showed "a gradual, though not concerning, deterioration along the Guadalquivir River, with poorer water quality in the middle-lower section of the river," which is common in all freshwater waterways, according to the team.

A striking finding for the team was that, in the middle-upper reaches of the river, while the water quality index (the physicochemical parameters) was good, there was a change detected in its microbiological parameters: in that section, there was a significant decrease in bacterial diversity, indicating ecological deterioration, along with a high prevalence of tolerant opportunistic microorganisms, particularly those in the Comamonadaceae family. While the team is still seeking an explanation for this microbiological change, it demonstrates how microbiological parameters reveal something that eludes physicochemical evaluation. "It's very difficult to detect chemically, but the biological changes show us that something is happening." Analyzing other tributaries that flow into the river, or the surrounding agricultural activity, where olive monoculture predominates, could be avenues of investigation to explain this discrepancy between parameters.

"This study demonstrates the suitability of this multidisciplinary and integrative method for analyzing water quality in freshwater waterways," added Arturo Chica, and the team recommends its use by the research community. It could serve as a model to create risk maps that pinpoint critical areas and uncover potential environmental threats not detected by more conventional methods.

Barbudo-Lunar M, Arjona GL, Trombini C, Pérez-González L, Chica AF, Martín MÁ, Blasco J, Michán C, Siles JÁ, Alhama J. A multidisciplinary approach to risk assessment enables the identification of deteriorated sections within a major waterway: The Guadalquivir River as a case study. J Contam Hydrol. 2026 Jun 6;282:105019. doi: 10.1016/j.jconhyd.2026.105019.  

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Journal

Journal of Contaminant Hydrology

DOI

10.1016/j.jconhyd.2026.105019 

Method of Research

Meta-analysis

Subject of Research

Not applicable

Article Title

A Multidisciplinary Study of the Guadalquivir River Assesses Water Quality Along its Entire Course

 

Finding the sweet spot for safer, longer-lasting lithium metal batteries

Researchers have discovered an optimal lithium concentration in lithium metal batteries, overcoming a long-standing barrier to widespread adoption

Tohoku University

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Cooperative ion transport balances ion transport kinetics and interfacial stability, enabling dense and flat lithium deposition with high reversibility. 

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Credit: Hongyi Li

Lithium metal has long been considered the ideal material for next-generation rechargeable batteries because it can store far more energy than the graphite anodes used in today's batteries. However, bringing lithium metal batteries into widespread use has proven difficult because lithium tends to grow into needle-like structures called dendrites during charging, reducing battery life and creating potential safety hazards.

Now, researchers from Tohoku University's Institute for Materials Research (IMR) have identified a key factor that could help overcome this challenge. Rather than simply increasing the amount of lithium salt in the electrolyte, the team discovered that there is an optimal concentration that allows lithium to deposit evenly while forming a stronger protective layer on the battery's surface.

Details of their discovery was published in ACS Electrochemistry on June 29, 2026.

Electrolytes carry lithium ions between a battery's electrodes during charging and discharging. Scientists have often focused on highly concentrated electrolytes because they can suppress dendrite formation. However, why some electrolytes perform better than others has remained unclear.

To investigate this, the researchers systematically examined electrolytes containing different concentrations of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) dissolved in a mixture of ethylene carbonate and propylene carbonate. By combining multiple analytical techniques--including pulsed-field gradient nuclear magnetic resonance (PFG-NMR), electrochemical measurements, electron microscopy, impedance spectroscopy, and nanoindentation--they linked ion transport in the electrolyte with the mechanical properties of the protective solid electrolyte interphase (SEI).

The team found that electrolytes containing 1-2 molar (M) LiTFSI produced the best results. At these concentrations, lithium ions and negatively charged TFSI ions moved together through the electrolyte at nearly the same rate. This cooperative transport created a more uniform flow of ions to the electrode surface, allowing lithium to deposit as smooth, dense layers instead of uneven, dendritic structures.

The researchers also found that this balanced ion transport produced a harder, more mechanically stable SEI layer. In comparison, dilute electrolytes formed weaker protective layers that allowed porous lithium deposits to develop, while highly concentrated electrolytes reduced ion mobility and hindered electron transport, ultimately leading to non-uniform lithium growth.

"Our results show that achieving stable lithium metal deposition is not simply a matter of increasing the salt concentration," said Hongyi Li, an assistant professor at Tohoku University's Institute for Materials Research. "Instead, the key is creating a balance where lithium ions and anions move cooperatively while maintaining a mechanically robust interfacial layer. This provides a new design principle for developing practical lithium metal batteries."

The findings offer a new way of designing electrolytes by optimizing both ion transport and the stability of the interface between the electrolyte and the electrode. Rather than relying solely on highly concentrated electrolytes, researchers can target this intermediate concentration regime to improve battery performance, safety, and lifespan.

Additionally, the study provides important insights for the development of next-generation lithium metal batteries for electric vehicles, portable electronics, and large-scale renewable energy storage. By identifying the fundamental relationship between ion transport and interfacial stability, the researchers have established practical guidelines for designing safer, longer-lasting, high-energy-density rechargeable batteries.

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Journal

ACS Electrochemistry

DOI

10.1021/acselectrochem.6c00140 

Article Title

Correlated Ion-Pair Diffusion Enables Balanced Transport Kinetics and Interfacial Stability for Lithium Metal Anodes