Thermodynamic analysis and kinetic optimization of high-energy batteries based on multi-electron reactions

Abstract

Multi-electron reaction can be regarded as an effective way of building high-energy systems (>500 W h kg^-1). However, some confusions hinder the development of multi-electron mechanisms, such as clear concept, complex reaction, material design and electrolyte optimization and full-cell fabrication. Therefore, this review discusses the basic theories and application bottlenecks of multi-electron mechanisms from the view of thermodynamic and dynamic principles. In future, high-energy batteries, metal anodes and multi-electron cathodes are promising electrode materials with high theoretical capacity and high output voltage. While the primary issue for the multi-electron transfer process is sluggish kinetics, which may be caused by multiple ionic migration, large ionic radius, high reaction energy barrier, low electron conductivity, poor structural stability, etc., it is urgent that feasible and versatile modification methods are summarized and new inspiration proposed in order to break through kinetic constraints. Finally, the remaining challenges and future research directions are revealed in detail, involving the search for high-energy systems, compatibility of full cells, cost control, etc.

Keywords: high-energy density; kinetics; metal anodes; multi-electron reactions; thermodynamic.

Figure 1.

Schematic illustration of different kinds of multi-electron reaction mechanisms and the corresponding electrode materials. (a) Single-ion intercalation reaction. (b) Dual-ion reaction. (c) Chemical bond reaction. (d) Anionic redox reaction. (e) Conversion reaction. (f) Alloying reaction. (g) Conversion-alloying reaction. Four key factors for optimizing multi-electron reaction: (h) carrier species; (i) host materials; (j) electrolyte compositions; (k) guest-host interaction.

Figure 2.

Theoretical specific gravimetric capacities, redox potentials, molar electron transfer numbers and chemical structures of promising multi-electron host materials for high-energy density batteries, which are discussed based on the lithium metal counter electrode. The representative metal anodes can be applied to high-energy density batteries. The structures of solvated cations are shown in the inset, which is an important factor for multi-ion and high-valence cations storage.

Figure 3.

Schematic illustration of complex kinetic processes during ionic transfer and storage in the general multi-electron system with high-energy density, involving metal plating/stripping, ionic solvation effect, ionic migration rate, electrolyte degradation, ionic de-solvation effect, pseudocapacitive effect, ionic diffusion in electrode bulk and phase conversion processes.

Figure 4.

DFT calculation for different migration energy barriers of representative multi-electron materials, involving Li₃V₂(PO₄)₃, FeFe(CN)₆, MoS₂ and sulfur electrodes. (a–d) Front views of possible single Li⁺ migration paths between adjacent storage sites. (e–h) Front views of possible double Li⁺ migration paths between adjacent storage sites. (i–l) Front views of possible single Mg²⁺ migration paths between adjacent storage sites. (m–p) Front views of possible double Mg²⁺ migration paths between adjacent storage sites. The corresponding migration energy barriers of the above paths are shown under them.

Figure 5.

Summary of advanced materials, mechanisms and methods for constructing high-energy density batteries via improved multi-electron reactions and enhanced kinetic process. (a) Composite materials. (b) Structural design. (c) Morphology control. (d) Electrolyte. (e) Storage mechanism. (f) External disturbance.

Figure 6.

Evaluation of representative high-energy density batteries based on the multi-electron cathodes and metal anodes. ICE: initial coulombic efficiency.