Lithium Ion Batteries For Stationary Application

LITHIUM ION BATTERIES FOR STATIONARY APPLICATIONS: A SAFETY PERSPECTIVE

Ashish Arora, Joyelle Harris, Bala Pinnangudi Exponent Failure Analysis Associates Phoenix, AZ 85027

ABSTRACT After gaining dominance in the consumer electronics industry, lithium ion technology is slowly making inroads into other industries such as reserve power systems, automobiles, etc. While its many advantages makethe lithium ion technology highly desirable for use as a stationary storage solution, the chemistry brings with it a relatively higher safety risk when compared to the traditional workhorses - lead acid and nickel based chemistries. A complete understanding of the risks associated with the lithium ion technology aids in the design of a safe lithium ion based backup power storage system. Thispaper will introduce the safety aspects of the Li-ion cells and its typical failure modes. The paper will end with a discussion on energy-management system designs specifically for large format lithium ion based energy storage systems. INTRODUCTION Lithium-ion (Li-ion) technology is increasingly being adopted by a variety of industries including reserve power applications. The use of Li-ion batteriesas an energy storage medium brings with it many advantages. However, the technology also has safety concerns that need to be addressed. In the consumer electronics industry alone, over 2 million products containing Liion batteries have been recalled over the past few years due to safety concerns1,2. The amount of energy stored in a reserve power system is at least an order of magnitude higher thanthe amount of energy stored in a typical consumer electronic product. This increases the hazard posed by these batteries in the event of a failure. Understanding the failure modes of the chemistry allows a system designer to incorporate controls to protect against conditions that can cause the battery system to fail. This paper will discuss the failure modes of the Li-ion chemistry and typicalprotection system architectures employed in large format Li-ion energy storage systems. The Li-ion chemistry differs from other battery chemistries in that both the positive and negative electrodes serve as host structures for the charge carrying species (lithium ions, Li+) as shown in Figure 1. A layered transition metal oxide (commonly LiCoO2) is used as the cathode material, while graphite is thematerial of choice for the anode. The mechanism by which Li+ enters and leave the host electrode materials is known as intercalation, and the materials that are capable of hosting Li+ are referred to as intercalation compounds. During normal battery operation, Li+ shuttle back and forth between the positive and negative electrodes without breaking chemical bonds or forming new interphasesurfaces, thereby minimizing side-reactions and physical changes within the microstructure of the electrodes. A well designed Li-ion battery is capable of a much longer cycle life compared to other battery systems. The active positive electrode material, the active negative electrode material, the separator and the electrolyte form the four primary functional components of a Li-ion cell. The performance,safety and reliability of the cell depend to a large extent on the choice of the materials. The choice of electrode materials has the largest influence on the capacity, voltage, energy, and power capability of the cell. Nearly all commercially available Li-ion cells utilize a nano-porous polymer separator (typically a bi or tri-layered film of polyethylene and polypropylene6) and an electrolytecomprised of lithium hexafluorophosphate (LiPF6) dissolved in an organic solvent consisting of a mixture of alkal carbonates and small amounts of various additives.

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Figure 1: Schematic of a conventional Li-ion cell utilizing graphite as the negative electrode and a layered transition metal oxide as the positive electrode (courtesy of Exponent).

THERMAL RUNAWAY IN LI-ION CELLS A...