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REPORT OF THE BASIC RESEARCH NEEDS WORKSHOP 3.1.2 SCIENTIFIC CHALLENGES AND OPPORTUNITIES There are multiple research areas in which sustained effort could result in significant advances towards simultaneous high energy and power performance in next generation electrochemical systems. Three topics with significant scientific challenges and opportunities are discussed here. New Materials and Architectures for Simultaneous High Power and Energy: One of the most promising pathways to high power and energy is the discovery of new materials and architectures that feature high capacity while also enabling fast ion motion. Fundamental questions that should drive research in this area are as follows: ☐ How can materials and architectures be arranged so that ion and electron transport is facile, enabling rapid charge/discharge to enhance power and energy simultaneously? ☐ What synthesis and fabrication pathways, from the nanoscale level to mesoscale assembly, can be identified as viable? These questions pertain to all components of a battery, including active materials, binder, additives, electrolyte, and current collectors. Rethinking the function of each component, designing new materials with multiple functions, and controlling behavior across length scales each have the potential to lead to simultaneous high power and high energy. Various research areas of interest are detailed below. Controlling nanoscale architecture: A variety of electrode materials with high specific capacity have been developed in recent years, such as LiFePO4 and nickel-rich layered lithium transition-metal oxides. However, their applications for high-power and long-cycle-life lithium-ion batteries have been hindered by kinetic limitations or poor stability. Developing nanostructured electrode materials is a promising way to overcome these drawbacks owing to the unusual mechanical, electrical, and chemical properties of nanomaterials endowed by their confined dimensions.18 In batteries, nanostructured electrode materials offer the potential for high electrode/electrolyte interfacial area, the ability to accommodate mechanical strain upon lithium insertion, and reduced path lengths for lithium-ion/electron transport through the material, which can lead to high rate capability.19 For example, the rate behavior of bulk (~1-10 μm dimensions) LiFePO4 materials is significantly restricted by sluggish electron and lithium-ion transport kinetics. Reducing the particle size to the nanoscale (~100 nm) can significantly shorten the diffusion time of Li+ ions in LiFePO4. When further combined with carbon coating, the power performance of LiFePO4 can be greatly enhanced (Figure 3.1.2a).20 In another example, to overcome poor thermal stability of nickel-rich layered lithium-transition metal oxides, nanoscale composition gradient structures have been created in which the nickel concentration decreases linearly and the manganese concentration increases linearly from the center to the outer layer of each particle. This complex structure enables high rate capability and cycling stability (Figure 3.1.2b).21 Although nanomaterials can provide power and energy advantages, their higher specific surface area often results in increased side reactions and lower coulombic efficiency (especially for negative electrode materials). Such disadvantages are a significant challenge that needs to be overcome for maximum technological impact. For examples like this, a worthy goal is to identify design guidelines that prescribe energy and power metrics as a function of the materials distribution in electrode particles. a 2Cb 4.0 3.5 3.0 2.5 2.0 10C 20C 30C 40C 50C 50C 0 20 40 60 80 100 120 140 160 180 Capacity (mA h g-1) Inside Ni-rich composition: high capacity Full gradient composition Gradual Ni decrease and Mn increase from inner part to outer part of a particle Surface Mn-rich composition: high thermal statbility 2C Figure 3.1.2. (a) Galvanostatic discharge curves at different rates for nanoscale carbon-coated LiFePO4. From Ref. 20. (b) Controlled nanoscale composition gradients in a nickel-rich layered lithium-transition metal oxide particle. From Ref. 21. 82 PANEL 1 REPORT Voltage (V)PDF Image | Next Generation Electrical Energy Storage
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