Comparison of Lithium-ion Battery Cathode Materials and the Internal Stress Development

The need for development and deployment of reliable and efficient energy storage devices, such as lithium-ion rechargeable batteries, is becoming increasingly important due to the scarcity of petroleum. Lithium-ion batteries operate via an electrochemical process in which lithium ions are shuttled between cathode and anode while electrons flowing through an external wire to form an electrical circuit. The study showed that the development of lithium-iron-phosphate (LiFePO4) batteries promises an alternative to conventional lithium-ion batteries, with their potential for high energy capacity and power density, improved safety, and reduced cost. However, current prototype LiFePO4 batteries have been reported to lose capacity over ~3000 charge/discharge cycles or degrade rapidly under high discharging rate. In this study, we report that the mechanical and structural failures are attributed to dislocations formations. Analytical models and crystal visualizations provide details to further understand the stress development due to lithium movements during charging or discharging. This study contributes to the fundamental understanding of the mechanisms of capacity loss in lithium-ion battery materials and helps the design of better rechargeable batteries, and thus leads to economic and environmental benefits. INTRODUCTION The Need for Rechargeable Batteries For over a century, petroleum-derived fuels have been the first choice as an energy source for transportation, and accounted for more than 71.4% of U.S. petroleum use in 2009 [1]. Although the petroleum-based fuel energy resource is convenient and technically matures, researchers started looking for alternative energy sources such as batteries due to the shortage of petroleum and because burning fossil fuels have become an environmental issue. It is reported that 98% of carbon dioxide emissions come from petroleum fuels [2]. Since carbon dioxide accounts for the largest share of greenhouse gases, to meet the stated goal of reducing total U.S. greenhouse gas emissions to 83% below 2005 levels by 2050, an alternative energy storage system is required. One of the most promising energy storage solutions for future automotive technology is the rechargeable battery. Compared with other resources such as flywheels, capacitors, biofuel, solar cells, and fuel cells, rechargeable batteries are more portable and provide quick energy storage and release [35]. Moreover, it is more difficult to use these other resources globally than it is to use rechargeable batteries, due to the operating environment limitations for these other energy sources [3]. Compared with capacitors, rechargeable batteries have lower self-discharge rates [3, 5], thus holding their charge for longer periods of time. Therefore, to best serve as a future automotive technology, rechargeable batteries should have both high energy and power densities [4], the ability to output high current for a long period of time, and to be fully charged quickly. The durability and environmental friendliness of rechargeable batteries is also very important. They should work for several years safely under different climatic conditions, and even if involved in an unfortunate car collision. Fundamental Science in Li-ion Battery Materials Among the rechargeable batteries, Li-ion batteries have dominated the field of advanced power sources due to their high gravimetric and volumetric energy density [6]. The most common Li-ion battery applications in the market are for portable electronics, power tools, and transportation. Li-ion battery contains three main parts: cathode, anode, and the electrolyte (Fig. 1). It operates via an electrochemical process