The "Holy Grail" of Solid-State Batteries: How a Tsinghua University Team Broke the 600 Wh/kg Barrier with a Single Electrolyte

Introduction

Solid-state batteries are widely regarded as the ultimate solution for next-generation energy storage, yet their path to practical application has long been blocked by two major hurdles: 1) poor solid-solid interfacial contact, leading to high ionic resistance; and 2) the lack of a single electrolyte that is simultaneously compatible with both high-voltage cathodes and lithium metal anodes. Conventional solutions, like applying high pressure, add system complexity and cost, hindering large-scale adoption.

Recently, a team led by Prof. Qiang Zhang and Dr. Chenzi Zhao at Tsinghua University published a breakthrough in Nature, proposing an innovative "anion-rich solvation structure" design strategy. They developed a novel fluorinated polyether electrolyte that, through simple in-situ polymerization, not only solves the interfacial contact problem but also achieves full compatibility with a 4.7V high-voltage cathode and a lithium metal anode. This enabled a record-breaking energy density of over 600 Wh/kg in a pouch cell with excellent intrinsic safety.

Schematic of the fluorinated polyether electrolyte design with an anion-rich solvation structure

The Key Innovation: From Molecular Design to "Anion-Rich Solvation"

The research team first designed and synthesized a fluorinated polyether copolymer (PTF-PE) with key advantages:

• Wide Electrochemical Window: The strong electron-withdrawing effect of fluorine atoms expands the polymer's oxidation stability window from ~3.6 V to an impressive 5.0 V, making it compatible with high-energy-density Li-rich manganese-based (LRMO) cathodes.

• Excellent Physicochemical Properties: The material is amorphous with a very low glass transition temperature (-75 °C), excellent thermal stability (>328 °C), and is flame-retardant.

Design and preparation of the fluorinated polyether polymer electrolyte

The core of the design is the creation of a unique "anion-rich solvation structure." In this new design, fluorine atoms participate in coordinating with the Li⁺ ion, forming a "–F∙∙∙Li⁺∙∙∙O–" Li-bond structure. This structure weakens the interaction between Li⁺ and the polymer, allowing the electrolyte salt's anions (TFSI⁻) to more easily enter the Li⁺ solvation sheath. This microscopic structural change is foundational to forming a high-quality interphase.

Structural and performance characterization of the fluorinated polyether (PTF-PE) electrolyte

The Anion-Derived Interphase: The Key to Stability

The most significant benefit of the "anion-rich solvation" is that during the formation of the interphase layers (CEI/SEI), the primary species to decompose is no longer the vulnerable polymer chain, but rather the anions that have been "pushed" to the forefront. This results in an anion-derived, dense, and uniform fluorine-rich interphase (mainly LiF).

This high-quality interphase plays three critical roles:

1. Physical Protection: A ~3 nm ultra-thin CEI on the cathode effectively suppresses the structural degradation and irreversible phase transition of the LRMO cathode at high voltages.

2. Chemical Passivation: F atoms from the interphase can even enter the LRMO sub-surface lattice to form stable Mn-F bonds, which suppresses oxygen release and enhances the reversibility of the oxygen redox reaction.

3. Anode Stabilization: A LiF-rich SEI on the lithium metal anode effectively suppresses dendrite growth and pulverization.

CEI composition analysis for LRMO cathodes based on PE-SPE and PTF-PE-SPE

Breakthrough Electrochemical and Safety Performance

Based on these unique interfacial chemistry advantages, the solid-state battery demonstrated exceptional performance:

• High Energy Density & Long Cycle Life: The team successfully fabricated an 8.96 Ah anode-free pouch cell that achieved a gravimetric energy density of 604 Wh kg⁻¹ and a volumetric energy density of 1027 Wh L⁻¹. In coin cells, it maintained 72.1% capacity retention after 500 cycles at 0.5C.

• Excellent Intrinsic Safety: Under a fully charged state, the pouch cell passed the nail penetration test without fire or explosion. Its thermal runaway onset temperature reached 216.0 °C, far superior to traditional liquid electrolyte batteries, demonstrating exceptionally high safety.

Electrochemical and safety performance of the PTF-PE-SPE electrolyte

Conclusion and Outlook

This work, starting from the fundamentals of Li-bond solvation chemistry, uses precise molecular design to develop a nearly "omnipotent" new polymer electrolyte. It solves multiple long-standing challenges in solid-state batteries—including interfacial contact, electrolyte incompatibility, and poor cycle stability—in a concise and highly effective manner. The achievement of over 600 Wh kg⁻¹ energy density combined with high safety marks a solid and critical step toward the practical application of high-energy solid-state batteries.

Literature Information & Author Biographies

• Article Link: https://www.nature.com/articles/s41586-025-09565-z

• Corresponding Author: Prof. Qiang Zhang, Tsinghua University. A long-time researcher in energy chemistry and materials, he proposed concepts like Li-bond chemistry, accelerating the development of solid-state and Li-S pouch cells.

• Corresponding Author: Dr. Chenzi Zhao, Tsinghua University. His research focuses on the chemical mechanisms, material synthesis, and device applications of solid-state lithium metal batteries and he has been named a Clarivate Global Highly Cited Researcher for four consecutive years.

• First Author: Dr. Xueyan Huang, Tsinghua University. Her research focuses on the molecular design of fluorine-modified polyether electrolytes and interface regulation for high-energy Li-rich solid-state batteries.