|Fig. 1: Low density polyethylene film embedded with spiky Ni microparticles. (Source: A. Rojas)|
As the global usage of intermittent renewable energy continues to increase, battery technology must simultaneously develop in order to store this energy. This will necessitate batteries with significantly higher energy densities, yet safety issues serve as a major roadblock to their adoption since existing technologies are slow to respond to overheating and have small operating voltage windows. 
One example of this is high-performance lithium-ion batteries that represent a frontier in effective energy storage and management. In addition to consumer devices, these or similar batteries used in grid storage may prove essential in enabling the economic viability of renewable energy given the intermittency of alternative sources.  Unfortunately, current safety technology requires external sensors, whose response times often are too slow to reliably prevent thermal runaway.  Therefore, an internal sensor that breaks the battery's circuit at high temperature before thermal runaway occurs may present a key advancement addressing current safety issues, particularly if it can return to normal functionality after the temperature returns to normal. Many ideas have been explored to solve this issue, but most currently suffer from irreversibility, low room-temperature conductivity, or small operating voltage windows.
One of the more promising approaches involves using a conductive, thermally responsive (thermoresponsive) polymer, pictured in Fig. 1. Previous work done by the Bao group at Stanford has shown the development of a highly conductive thermoresponsive polymer, utilizing spiky nickel particles embedded in polyethylene, that exhibits a switch-like (7-8 orders of magnitude) increase in resistivity at specific temperatures.  Such films are known as quantum-tunneling composites (QTCs) due to the mechanism of conduction, quantum tunneling of electrons between microparticle tips. This allows for quick current cutoff as small increases in distance between the tips, caused by polymer expansion, will prevent electron tunneling. The temperature at which this switch occurs was shown to be a function of the metal loading fraction and type of polymer used, allowing it to be tuned for the desired application such that it cuts off the current when the temperature goes above the normal range.
However, the use of Ni particles in QTCs has some disadvantages, including oxidation in the operating voltage windows of Li ion batteries, low conductivity relative to other base metals, and high cost relative to conductive alternatives.  In order to overcome these shortcomings, the Ni microparticles require significant processing, which involves coating the particles with a thin layer of graphene to increase its electrochemical stability. 
|Fig. 2: Dendritic Cu microparticles. (Source: A. Rojas)|
One alternative to the use of Ni microparticles is the implementation of dendritic copper (Cu) particles within the polymer, as seen in Fig. 2. Cu could be explored as an alternative to Ni due to its higher reduction potential, which translates to higher electrochemical stability and its lower cost.  Because of this, the use of Cu would theoretically eliminate the need of a graphene coating and the associated processes; thus, maintaining its conductive properties under the batterys operating potential.
In addition, a key requirement for these composites to serve in industrial applications is high baseline (room temperature) conductivity so as to not degrade battery performance under normal operation. Thus, Cu's four-fold higher innate conductivity compared to Ni makes it an attractive alternative for use in the QTC.
In summary, Ni microparticle QTCs have shown the potential to prevent thermal runaway in batteries given their thermoresponsive ability to rapidly increase its resistivity, effectively shutting off the circuit. However, Ni microparticles face several limitations due to its inherent physical properties, such as low conductivity and poor electrochemical stability. Therefore, further characterization of these metal-filled QTCs is needed to identify advantageous alternatives, such as the substitution of Ni for Cu. The exploration of alternative metals could serve as a pragmatic cornerstone in further understanding and developing a valuable solution to a problem in the battery space.
© Arturo Rojas. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 X. M. Feng, X. P. Ai, and H. X Yang, "A Positive-Temperature-Coefficient Electrode With Thermal Cut-Off Mechanism For Use in Rechargeable Lithium Batteries," Electrochem. Commun. 6, 1021 (2004).
 B. Dunn, H. Kamath, and J.-M. Tarascon, "Electrical Energy Storage For the Grid: A Battery of Choices," Science 334, 928 (2011).
 Z. Chen et al., "Fast and Reversible Thermoresponsive Polymer Switching Materials For Safer Batteries," Nat. Energy 1, 15009 (2016).
 "Metal Prices in the United States Through 2010," U.S. Geological Survey, Scientific Investigations Report 2012-5188, 2013.
 R. A. Levy, Principles of Solid State Physics (Academic Press, 1968).