Characterization of Sila Nanotechnologies’ new Silicon composite Anode Technology (from the WHOOP 4.0)

Sila Nanotechnologies

Over the last decade, Sila Nanotechnologies Inc. has been working on developing silicon nanoparticle technology to enhance Lithium-Ion Batteries (LIBs). In 2021, we saw the first commercial product using Sila’s Si-based anode material, the WHOOP 4 fitness tracker. Sila claims that their new anode composite material increases energy density by 20% today and up to 40% improvement in future releases.They also believe that Sila’s Si-anode can support full graphite replacement to achieve maximum performance or partial graphite replacement for faster speed to market. By partially replacing conventional graphite material with Sila’s Si-anode, WHOOP claims to provide radical product innovation with increased energy density while alleviating the squeeze of a big, underperforming LIB.

SiOx-based Technologies

Increasing the silicon-graphite ratio to achieve higher capacity is one of the main streams in lithium-ion battery technology development. We have discussed the advantages of SiOx-based composite anodic materials in previous TechStream blogs. We concluded that the cyclic life of silicon-graphite composite is a function of the properties of the solid electrolyte interphase (SEI) at the anode. The silicon ratio significantly influences the SEI expansion seen during cycling. With a composite graphite-silicon anode, a rigid framework made from graphite particles that structurally suppresses the expansion of the anode during lithiation. The silicon nanoparticles are accommodated by the carbon scaffold, preventing the delamination of the electrode upon repeated cycling, and improving cyclic life. However, if the silicon-graphite ratio is too high, this approach may not successfully prevent volume expansion which will disturb the SEI layer and reduce the cyclic life of the battery.

The first commercial smartphone using a silicon-oxygen anode was released in 2021. The Xiaomi Mi 11 Ultra claims “a new super-fast charging silicon-oxygen anode battery technology…enabling the battery to be thinner, while also being capable of faster-charging rates as well”. While a full analysis of the Amperex BM55 cell from this phone is available in a TechInsights Battery Essentials Report, we will also be using it in this blog as a comparison with the Sila-based battery. Although the Amperex BM55 claims to benefit from using 2nd generation nano-silicon anode material, the amount of silicon is not significant based on our analysis.

Sila’s Si-anode based battery and the WHOOP 4.0

At TechInsights, we opened a WHOOP 4.0 fitness tracker to characterize Sila nanotechnologies battery. Figure 1 shows the inside of the device along with its battery. The battery pack was removed and tested using charge-discharge, differential capacity analysis and electrochemical impedance spectroscopy measurements (EIS).

Energy Density vs. Apple Watch Series 7

This Sila battery exhibits a capacity of 192 mAh, which translates to 258 Wh/kg energy density. Surprisingly, the energy density of the WHOOP 4.0’s battery (Sila nanotechnologies) is ~2% lower than that of the Apple Watch Series 7’s battery (263Wh/kg). It is possible that the energy density gain in the Sila Cell from the silicon anode is offset by the graphite framework which aims to increase cyclic life.

Differential Capacity Analysis: Sila (WHOOP 4.0) vs. Amperex BM55 (Xiaomi Mi 11)

Differential capacity analysis (DCA) of WHOOP 4.0’s battery (Sila nanotechnologies) was performed at C/20. To compare the results with those of the other batteries, we normalized dq/dV values by the battery’s capacity (dq/dV divided by battery’s capacity). The results were plotted in Figure 2 and compared with the Amperex BM55 cell. As shown in this Figure, the chemistry of the Sila-based cell is mainly based on lithium cobalt oxide at the cathode and silicon-graphite at the anode, similar to the Amperex BM55. Figure 2 also highlights the peaks associated with Si alloy delithiation (indicated by diamond points). Under the cycling protocol of C/20, voltage segments with a discharge voltage above 3.65 V result in only the graphite component of the electrode undergoing lithiation and delithiation as the Si alloy remains lithiated. A comparison between normalized DCA results of the WHOOP 4.0’s battery (Sila nanotechnologies) and Amperex BM55 reveals that the WHOOP 4.0’s battery is more enriched with silicon-based particles as the peaks below 3.65 V during discharge are pronounced. To determine the silicon-graphite ratio, we are performing further material characterizations. In the full report, we present a comprehensive analysis of the silicon-graphite ratio and the oxidation state of silicon particles.

Electrochemical Impedance Spectroscopy: Sila (WHOOP 4.0) vs. Apple Watch Series 7

The impedance and internal resistance of the WHOOP 4.0’s battery (Sila nanotechnologies) were analyzed using electrochemical impedance spectroscopy (EIS) at different states of charge (SOC). EIS measurements were performed for a frequency range of 3 kHz to 50 mHz by applying a sinusoidal signal of 5 mV amplitude. The respective results are given in Figure 3 in the form of a Nyquist plot. A comparison of the different spectra reveals that they share a similar trend. Generally, each spectrum consists of two semi-circles at high-to-medium frequencies followed by a 45◦ line in the low-frequency region. The interception of the real and imaginary axis shows the overall ohmic resistance, equal to 434 mΩ. The first semi-circle represents the solid electrolyte interphase of the battery, while the second semi-circle represents the electrochemical reactions at anode and cathode. The 45◦ line corresponds to the diffusion of lithium ions. The sum of diameters of each semi-circle represents the resistances against electrochemical phenomena. For the fully discharged battery, this value is found to be ~ 1 Ω; as the battery charges, the resistance against the charge drops by 50%.

Usually, batteries with smaller capacities feature higher internal resistance than that large ones. For Sila, the minimum internal resistance was found to be 910 mΩ which is considered high for a cell of this capacity. This may lead to issues during fast charging. For charging at 1C (192 mA), this leads to a 174.7 mV drop and would generate 33.5 mW heat from internal resistance alone.

For comparison, we performed a similar EIS analysis on Apple Watch Series 7’s battery (Figure 4). As shown in the figure, in contrast to WHOOP 4’s battery, the ohmic resistance of the Apple Watch Series 7’s battery (the intersection of imaginary and real axis on the Nyquist diagram) changes with the state of charge. For a fully discharged battery, the ohmic resistance was found to be 123 mΩ for a fully discharged battery and increases to 141 mΩ to SOC=75% and slightly falls as the battery becomes fully charged. In comparison to Whoop’s 4 battery with a minimum internal resistance of 910 mΩ, the overall impedance of the battery is 433 mΩ and drops to 197 mΩ and 208 mΩ for SOC=75% and 100%, respectively.

The Apple Watch Series 7’s features minimum internal resistance of ~345 mΩ, significantly lower than that of the WHOOP 4.0’s battery (Sila nanotechnologies). The Apple Watch Series 7deliver capacity of 302 mAh. Therefore, at a 1C charge, the minimum voltage drop is 104.2 mV, which is 70.51 mV lower than the 1C charge of the WHOOP 4.0’s battery (Sila nanotechnologies). The higher resistance of the WHOOP 4.0’s battery (Sila nanotechnologies) can be attributed to differences in the electrolyte, separator, anode binder, electrode loading, and properties of the silicon-graphite composite, etc. The true nature of the impedance difference is described in the full report.

Summary and ongoing work

Our electrochemical measurements reveal that the WHOOP 4.0’s battery has a more Si-enriched anode than other existing SiOx-graphite batteries such as Amperex BM55 (used in Xiaomi Mi 11 Ultra). However, this battery features relatively high internal resistance and energy density close to that Apple Watch Series 7’s battery. It is possible that the energy density gain in the Sila Cell is offset by the graphite framework which aims to increase cyclic life. Cell structure and additional materials characterization on the Sila Nanotechnologies (WHOOP 4.0) can be found in the Sila Nanotechnologies (Whoop 4.0) Battery Essentials reports available from the TechInsights Battery Essentials subscription.