a, A normal LIB consists of an anode, a separator, a cathode, and a flammable electrolyte. On abnormal heating, the separator will melt, resulting in internal shorting of the battery. The marked increase in temperature will cause thermal runaway and permanently damage the battery structure. b, The safe battery has one or two current collectors coated with a thin TRPS layer. It operates normally at room temperature. However, in the case of a high temperature or a large current, the TRPS will be activated, greatly increasing its resistance and shutting down the battery. The battery structure can thus be protected without damage. c, Thermal switching mechanism of the TRPS material. The polymer composite film has a high electrical conductivity at room temperature due to the quantum tunnelling effect enabled by the spiky nanostructure (GrNi). On heating, the polymer matrix expands, thus separating the conductive particles, which can decrease the value of σ by a factor of 107–108. On cooling, the polymer shrinks and regains the original conductive pathways. The symbol (×) illustrates blocking of electron or ion transport. Source: Chen et al. Click to enlarge.

To ensure good performance, LIBs [lithium-ion batteries] generally operate within a limited range of current density, voltage and temperature. However, at an abnormal temperature (for example, >150 ∘C), typically caused by shorting, overcharging or other abuse conditions, a series of exothermic reactions can be initiated and rapidly propagate to further increase the internal cell temperature and pressure, which results in catastrophic battery explosion and fire. Commercial LIBs are equipped with external pressure release vents and positive temperature coefficient (PTC) resistors on their cases to prevent overpressure and overheating. However, pressure and temperature increases inside cells can occur at much higher speeds than can be detected by these external devices. Thus, internal safety strategies are more effective in preventing thermal runaway.

There have been considerable efforts to design internal functional components to address battery safety issues, including novel separators, electrolyte additives and PTC-modified current collectors. … Clearly, in spite of efforts made thus far, battery safety remains an important concern, thus calling for new approaches.

—Chen et al.

Professor Yi Cui, a co-corresponding author of the paper, noted that existing techniques used to prevent battery fires, such as adding flame retardants to the electrolyte, are irreversible—i.e.., the battery is no longer functional after it overheats. (In 2014, Cui created a “smart” battery that provides ample warning before it gets too hot.)

People have tried different strategies to solve the problem of accidental fires in lithium-ion batteries. We’ve designed the first battery that can be shut down and revived over repeated heating and cooling cycles without compromising performance.

—Zhenan Bao, professor of chemical engineering and co-corresponding author

To address the problem Cui, Bao and postdoctoral scholar Zheng Chen turned to nanotechnology. Bao recently invented a wearable sensor to monitor human body temperature. The sensor is made of a plastic material embedded with tiny particles of nickel with nanoscale spikes protruding from their surface.

For the battery experiment, the researchers coated the spiky nickel particles with graphene and embedded the particles in a thin film of elastic polyethylene. To conduct electricity, the spiky particles have to physically touch one another. During thermal expansion, the polyethylene stretches, causing the particles to spread apart, thereby making the film nonconductive so that electricity can no longer flow through the battery.

Bunched together, as shown here, nanoparticles of graphene-coated nickel conduct electricity. When the battery overheats, the particles separate and electric current stops flowing. During cooling, the particles reunite and the battery starts producing electricity again. Credit: Zheng Chen. Click to enlarge.

When the researchers heated the battery above 70 ˚C, the polyethylene film quickly expanded, separating the particles and shutting down the battery. When the temperature dropped back down to 70 ˚C, the polyethylene shrunk, the particles came back into contact, and the battery started generating electricity again.

The temperature threshold can be tuned higher or lower depending on the number of particles and the type of polymer, Bao said.

Compared with previous approaches, our design provides a reliable, fast, reversible strategy that can achieve both high battery performance and improved safety. This strategy holds great promise for practical battery applications.

—Yi Cui

Other Stanford co-authors of the study are postdoctoral scholars Nan Liu, Chao Wang, Sean Andrews and Jia Liu; and graduate students Po-Chun Hsu, Jeffrey Lopez, Yuzhang Li and John To.

The research was supported by the SLAC National Accelerator Laboratory and the Precourt Institute for Energy at Stanford.

Resources

• Zheng Chen, Po-Chun Hsu, Jeffrey Lopez, Yuzhang Li, John W. F. To, Nan Liu, Chao Wang, Sean C. Andrews, Jia Liu, Yi Cui & Zhenan Bao (2016) “Fast and reversible thermoresponsive polymer switching materials for safer batteries” Nature Energy 1, Article number: 15009 doi: 10.1038/nenergy.2015.9