[Background introduction]

With the rapid growth of consumer electronics markets such as portable electronic equipment and electric vehicles (EVs), the market demand for long-term, efficient and safe batteries has been greatly promoted. Although batteries using metals such as sodium (Na), potassium (K), and magnesium (Mg) as anodes have also been developed, compared to lithium (Li) batteries, they are still in the development stage. However, Li batteries have not achieved inherently high capacity due to their high reactivity and potential safety hazards (battery fire, explosion, etc.). Therefore, designing Li batteries with high energy density and high safety is very important for the application of next-generation batteries.

As we all know, Sony developed the first commercial Li battery with organic solvent as liquid electrolyte in 1990. Although these organic liquid electrolytes can provide high room temperature conductivity and satisfactory electrochemical stability to most electrode systems (LFP, LCO, etc.), they are extremely flammable and volatile, and they have poor stability at high voltages. Based on these issues, researchers are trying to change these shortcomings from the perspective of material design. However, currently commercialized Li batteries still have disadvantages such as low capacity, potential safety, and undesired cycle life. Therefore, designing and developing materials for Li batteries with higher energy density and longer service life is the research focus of scientists in the field of energy materials. Among them, solid-state Li batteries have high expectations due to their advantages of portability, miniaturization, and higher safety! Therefore, it is very necessary to summarize and summarize the development status of solid electrolyte Li metal batteries based on organic electrolytes.

[Achievement Profile]

Recently, Professor Patrick C. Howlett and Researcher Wang Xiaoen (co-corresponding author) of Deakin University , Australia summarized and reported their latest progress in the development of new organic electrolytes in recent years. Li battery technology strategy. Based on new insights into the ionic conduction and design principles of organic solid electrolytes, the author has developed new organic electrolyte materials suitable for Li metal negative electrodes, high energy density positive electrode materials (such as high-voltage materials), and outlined optimization of positive electrode formulations. Finally, the outlook for the next generation of high-performance solid-state electrolytes is prospected. The research results were published in the internationally renowned journal Adv. Mater. Under the title "Toward High-Energy-Density Lithium Metal Batteries: Opportunities and Challenges for Solid Organic Electrolytes" .

[Graphic analysis]

FIG. 1. In shape the organic ionic crystal material (OIPC) [P _122i4 ] [the PF _{. 6} ], the ion transport mechanism and solid - solid transition dependent

(a) a [P _122i4 ] [the PF _{. 6} ] ion pair ion structure;

(B) Relationship between solid-solid phase transition, ionic conductivity, temperature, and solid-solid phase transition in OIPC;

(C) Models of the possible motions of different ions in different solid phases (IV-I phases).

Figure 2. The role of polymer nanofillers in OIPC composite electrolytes.

(A) Chemical structure of OIPC [C ₂ mpyr] [BF ₄ ] and polymer fibers used in composite electrolytes.

(B) Pure [C ₂ mpyr] [BF ₄ ], 10 mol% doped [C ₂ mpyr] [BF ₄ ] (10Li- [C ₂ mpyr] [BF ₄ ]) and Comparison of conductivity of composite electrolyte;

(C) Cycle performance of Li∣Li symmetrical battery assembled from OIPC composite materials doped with different nanofibers;

(D) The discharge performance of Li∣NMC111 battery using 50Li- [C ₂ mpyr] [FSI] / PVDF fiber composite electrolyte (C / 15, cut-off voltage 2.5-4.6 V, 50 ^o C).

Figure 3. [C ₂ mpyr] [FSI] -based composite electrolyte based on PVDF nanoparticles for Li metal full cells

.

(B) ionic conductivity and composition dependence of nanocomposites;

(C) ⁷ Li static NMR spectrum of 10 mol% LiFSI-doped [C ₂ mpyr] [FSI] and corresponding composite electrolyte with PVDF particles

(D) The discharge performance of a Li∣LFP battery containing 10 mol% LiFSI composite electrolyte at room temperature.

Figure 4. Polycarbonate polymer electrolyte

top: chemical structure of single ion PEO / PC and conventional PEO / PC polymer electrolyte;
bottom: a) single ion PEO / PC and b) conventional PEO / PC electrolyte containing LiTFSI Performance comparison of assembled Li symmetrical battery under 70 ^o C and 0.2 mA cm ^-2 polarization.

Figure 5. Chemical structure of polymer electrolyte

(ac) LiFSI salt, [P _111i4 ] [FSI], [C ₃ mpyr] [FSI], and PDDMA-TFSI host with polyionic liquid as polymer host ;

(D) Photographs and SEM images of a composite electrolyte containing high Li concentration [P _111i4 ] [FSI] and Al ₂ O ₃ nanoparticles;

(E) Digital pictures and SEM cross-sections of a composite electrolyte composed of high-concentration Li [C ₃ mpyr] [FSI] and PVDF nanofibers;

(Fg) (d) Li∣Li symmetrical cycling performance and conductivity dependence of the composite electrolyte;

(H) The relationship between the conductivity of the composite electrolyte and t _{Li ⁺} and the salt concentration.

Figure 6. Polymer electrolyte with polyIL as the polymer main body

(a) Chemical structure of polyionic liquid PDDMA-FSI and LiFSI salts;

(B) Dependence of electrolyte conductivity on LiFSI salt concentration;

(C) Li ⁺ mobility calculated based on the method proposed by Watanabe et al .;

(D) Cycle performance of Li∣NMC batteries assembled with PDDMA-LiFSI composite electrolyte;

(E) A typical charge-discharge curve corresponding to (d).

Figure 7. PolyIL block copolymer electrolyte

(a) Molecular structure of PolyIL block copolymer S-PIL64-16, ionic liquid and Li salt;

(Bc) The relationship between the phase behavior and structural relationship of S-PIL64-16 electrolyte and the IL content and salt concentration;

(D) Charge / discharge curves of Li∣LFP batteries using polyIL block copolymer electrolyte at 50 ° C and C / 20 C rate at an area capacity of 1.8 mAh cm ^-2 .

Figure 8. Designing a new lithium salt to improve Li ⁺ transport

(ab) Anion chemical structure of different Li salts, and calculated Li ⁺ conductivity (70 ^o C) and dissociation energy of LiX / PEO electrolyte containing different Li salts .

Figure IX. Understanding of OIPC at the molecular scale based on computer simulation

(ab) The single-pulse ¹ H-NMR (a) and single-pulse ¹⁹ F-NMR of OIPC [P _122i4 ] [PF ₆ ] as a function of test temperature.

Figure X. Computer-based understanding of OIPC at molecular scale

(ac) _{Snapshots of} Na-doped [P _122i4 ] [PF ₆ ] electrolyte system at three time points (t ₁ , t ₂ and t ₃), illustrating The jumping process of metal ions involves its first solvating shell;

_{Snapshots of} (df) Li-doped [P _122i4 ] [PF ₆ ] electrolyte system at three time points (t ₁ , t _2, and t ₃) illustrate that the jumping process of metal ions involves its first solvation shell.

Figure XI. Molecular dynamics (MD) simulation comparison of coordination mechanism and ion mobility between PDADMA-FSI-based electrolyte and PEO-based electrolyte

(a) Polycation, Li ion, and FSI anion in PDADMA FSI-LiFSI binary electrolyte Coordination

(B) Coordination structure between PEO oxygen and Li ⁺ ;

(C) Diffusion coefficients of Li + and TFSI anions in PEO electrolyte;

(D) Mean square displacement (MSD) of Li ⁺ and FSI (N atom) in PDDMA FSI-LiFSI electrolyte at 400 K;

(Ef) The snapshot captures the jump between two adjacent Li ⁺ (A) and Li ⁺ (B) in the PDDMA FSI-LiFSI electrolyte .

Figure 12: [C ₂ mpyr] [FSI]: LiFSI composite electrolyte Li∣NMC111 battery cycle based on PVDF nanoparticles

(a) first and 100th charge and discharge curves;

(B) Comparison of the discharge capacity of the traditional liquid electrolyte LP30 and the composite electrolyte when cycling at a rate of 1 C at 50 ° C;

(C) Charge and discharge curves of Li | NMC111 battery based on PDDMA-TFSI: LiFSI: [C ₃ mpyr] [FSI] composite electrolyte at 1, 2 and 10 cycles;

(D) Cycle performance of 50 cycles at 0.05C and 50 ° C.

Figure 13: High-energy SSLB prototype development

(a) Customized robotic stacking unit in Battery Technology Research and Innovation Center (BatTRI-Hub) at Deakin University

(B) Composite electrolyte based on polymer-ionic liquid PDDMA TFSI: LiFSI: [C ₃ mpyr] [FSI].

(C) The assembled lithium metal pouch battery;

(De) Charge / discharge curves and cycle performance of Li∣NMC111 pouch battery using PDDMA TFSI: LiFSI: [C ₃ mpyr] [FSI] composite electrolyte.

[Summary and Prospect]

In summary, due to the increasing demand for battery energy density and safety, all-solid-state Li metal batteries are considered to be one of the most promising next-generation energy storage technologies. Among them, polymer electrolytes are viable alternatives to ceramic or glassy electrolytes, with more ideal mechanical properties, better electrode / electrolyte interfaces, and simpler manufacturing methods. But to truly realize the potential of these materials, t _{Li +} and overall conductivity still need to be improved . In this article, the authors show several complementary design methods and have proposed new polymer systems, with a particular focus on polymer chains with an ionic charge, which can be used both as the body of the Li salt and as the monomer. Ionic conductor. Reinforcing polymers to improve mechanical properties and thermal stability, improving compatibility with electrodes, and moving towards solvent-free or water-based treatments will also be important to achieve high-performance solid-state lithium battery technology.

Currently, researchers are seeking a way to use a single lithium ion conductive fiber or particle to combine with OIPC to form an ion conductive composite with improved mechanical properties, doping OIPC at the interface and providing a conductive path for Li ⁺ . Although the modeling work of new solid electrolytes based on ionic polymer bodies has initially demonstrated the ability to obtain new transmission mechanisms, its research is still in its early stages. In addition, in terms of potential ionic polymers and proposed new anions and salts, the diversity of chemistry offers opportunities to develop new materials. Therefore, molecular simulation will become one of the most effective tools to explore new chemical methods and new strategies to improve the performance of solid electrolytes.

However, in order to achieve practical all-solid-state and high-energy-density devices, the next step is to design an electrochemically more stable electrolyte system and improve the design of the positive electrode assembly to incorporate solid-state ion conductors. Polymer electrolytes based on high-concentration salt systems show excellent stability under high pressure. In view of their softness and softness properties, polymer electrolytes and OIPC are both good candidates for replacing traditional PVDF binders in positive electrodes. In addition, structural design and positive electrode formulation will be another important issue for achieving high energy and high performance batteries in practical applications. However, although these solid electrolytes have high mechanical modulus and various interface problems that occur during the charge-discharge cycle, they have been shown to have the same dendrite problems as other systems. Similarly, the use of volatile or toxic organic solvents in ultra-concentrated electrolytes has proven to be a viable and safe alternative. Further progress in developing soft and non-volatile systems, as described herein, addressing chemical / electrochemical stability, dendrite control, efficiency, interfacial contact, and interfacial compatibility is urgently needed. Combining traditional composite systems such as polymers, ionic liquids, plastic crystals, ceramics, and glass, as well as methods that use simulation and chemical design to understand and control ion association and dynamics, provides a wide range of material solutions, Further research on polymers required for new high-energy batteries will be advanced.

Literature link: Toward High-Energy-Density Lithium Metal Batteries: Opportunities and Challenges for Solid Organic Electrolytes ( Adv. Mater. , 2020 , DOI: 10.1002 / adma.201905219)

About the Author

Xiaoen Wang, PhD in Materials Science, Wuhan University of Technology. He is currently a researcher at the Institute of Frontier Materials (IFM) at Deakin University, Australia, and is co-supervised by Professor Maria Forsyth . His research interests include polymer-based electrolytes, structural design of nano- and polymer composites, and applications in energy storage and energy conversion devices (such as metal batteries, fuel cells, etc.). At this stage, it is mainly committed to the development of new polymer electrolytes, high-performance ionic plastic crystal composite materials, and applications in all-solid-state lithium and sodium metal batteries.

Maria Forsyth, Professor Alfred Deakin, Deakin University, Australian Laureate Fellow, Fellow of the Australian Academy of Sciences, Fellow of the Royal Australian Chemical Society, Fellow of the International Electrochemical Society (ISE fellow), Electronic Materials Science Research, Australian Research Council Deputy Director of the Center . The Energy Storage and Corrosion Research Group led by Professor Forsyth has long been dedicated to the study of charge transport mechanisms at the metal / electrolyte interface and the development of high-performance electrolytes suitable for next-generation energy devices.

Patrick C. Howlett, Professor, Deakin University, Australia . Professor Howlett‘s research areas include the development of electrochemical energy storage devices and electrode engineering based on new material designs. He is currently the director of Battery Technology Research and Innovation Center (BatTRI-Hub) at Deakin University. His current research work includes the prototype and industrialization technology of high-performance batteries, mainly including the Performance lithium, sodium battery technology.

Source of information: material cattle