Nickel/Carboxymethylcellulose/Styrene-Butadiene-Rubber Matrix As In-Situ Volume-Expansion Sensors for Intermetallic Li-Ion Battery Anodes

As mobile phones, laptops, and other devices become more universally prevalent, high capacity lithium ion batteries with even better cycling efficiency will be in greater demand.  It is well established in the scientific community that intermetallic anodes show increased capacity while maintaining a high coulombic efficiency, however they suffer from large volumetric changes during charge cycling, causing degradation of the anode through cracking and delamination from the current collector [1].  Being able to detect this evolved stress in-situ would greatly aid researchers in understanding how changing material composition affects anode life and utility.  Piezoresistive materials are substances that change in electrical resistance in response to applied pressure.  They are commonly used as pressure sensors in microelectronic systems.  Our goal is to develop an in-situ piezoresistive sensor for detecting the stress from anodic volumetric expansion by a measureable resistance change.

Piezoresistive nickel micro-particle/polymer composites have been shown to be able to detect stresses as low as 50kPa [2], with a resistance change up to 9 orders of magnitude with greater applied stress [3].  Piezoresistive sensitivity relies heavily on the composite’s percolation threshold and sample thickness, which can be tuned to match operating requirements [3]. This lends credence to the use of nickel micro-particle/polymer composites as piezoresistive in-situ stress sensors.

We demonstrate that by distributing nickel microparticles within a mixture of carboxymethylcellulose (CMC) and styrene-butadiene-rubber (SBR), a piezoresistive material can be made that drops in resistance when strain is induced by an external force.  Furthermore we demonstrate the ferromagnetic nature of the nickel particles is advantageous in tuning the piezoresistive material’s sensitivity.

The resistance drop through a sample as a function of applied strain is demonstrated with an in-house testing apparatus.  This data is directly correlated to the stress vs. strain curve generated for the respective samples using a material testing system (MTS).  Therefore this allows us to relate the stress to a measurable resistance change. 

Additionally we induce anisotropic alignment of the nickel microparticles by a magnetic field during the material fabrication process.  We exhibit that this leads to a larger change in the resistance with applied stress, and a faster response time in resistance change; that is, a piezoresistive sensor with higher sensitivity and a lower detection limit can be obtained.

As a proof of concept, this methodology will be applied to an in-situ electrochemical half-cell setup. Using various compositions of nickel/tin planar anodes, the evolved stress with lithiation can be detected with the abovementioned piezoresistive material placed between the anode and lithium metal counter electrode.  We can directly relate charge/discharge curve to the resistance of the system. The effect of various depths of discharge and the associated stresses imparted on the piezoresistive material will be analyzed. The relationship between the different C rates and the stress exertion will also be discussed during the presentation.  Anode compositions and crystal structure will be analyzed with Energy-Dispersive X-ray Spectroscopy (EDS) with ZAF elemental composition correlation and XRD, respectively.

Our results demonstrate the feasibility of using nickel microparticle based piezoresistive sensors for accurately and quickly determining the in-situ stress generated in intermetallic anodes.

[1] J. Tarascon and M. Armand, Nature, 2001, 414, 359-367.

[2] B. Tee, C. Wang, R. Allen, and Z. Bao, Nature Nanotechnology, 2012, 7, 825-832.

[3] G. Canavese, S. Stassi, M. Stralla, C. Bignardi, and C.F. Pirri, Sensors and Actuators A, 2012, 186, 191-197.