Xiaomi and Battery Innovation

Xiaomi and the Mi brand of mobile phones have a history of demonstrating battery innovation. The Xiaomi Mi 10 Ultra (released April 2021 and profiled in the TechInsights Battery Essentials subscription) supports 120 W wired charging using Mi Turbo Charge technology. Its battery is a double-cell, butterfly arrangement, with two 2250 mAh cells connected in series for faster wired and wireless charging. The approach of splitting charging flows avoids subjecting the battery cells to the full load of 120 W and instead each battery is subjected to 60 W.

The Xiaomi Mi 11 Ultra uses a traditional battery arrangement but includes novel cell chemistry with a cobalt oxide-based cathode and a graphite/Si-based anode from Amperex Technology Limited (ATL). Xiaomi claims it is the first smartphone to use 2nd generation nano-silicon anode material resulting in ten times higher capacity than traditional graphite anode cells. The battery is said to charge to 100% in 36 minutes @67 watts (wired and wireless).

Figure 1: Inside the Xiaomi Mi 11 Ultra.

The electrochemical performance and material characteristics of the battery were evaluated using Differential capacity (dQ/dV vs V) and electrochemical impedance spectroscopy. Figure 2 shows a plot of dQ/dV vs V for the Amperex BM55 cell. This graph shows that cathode chemistry is based on lithium cobalt oxide, while the anode is made up of a graphite-silicon composite. Highlighted areas (broad peaks shown by dashed circles) in Figure 2 are associated with Si alloy delithiation. Under the cycling protocol of C/20, voltage segments with a discharge voltage above 3.8 V result in only the graphite component of the electrode undergoing lithiation and delithiation, as the Si alloy remains in a lithiated state. This graph reveals that the anode of this battery is not highly enriched with silicon particles since Si- delithiation peaks are not remarkable.

Figure 2: Differential capacity curve of a Amperex BM55 cell with the corresponding Si alloy delithiation peaks marked by diamonds.

This observation is in a good agreement with electrochemical impedance spectroscopy (EIS) analysis at different states of charges (5%, 50% and 100%). EIS measurements were performed for a frequency range of 50 mHz to 3 kHz by applying a sinusoidal signal of 5 mV amplitude. The measured Nyquist diagrams are plotted in Figure 3. Each plot features two semicircles 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, which is found to be 16 mΩ. This value is reasonably low for a battery for fast charge capability. The first semicircle represents the solid electrolyte interphase of the battery (RSEI), while the second semicircle represents the electrochemical reactions at anode and cathode, indicating charge transfer resistance (RCT). The 45◦ line corresponds to the diffusion of lithium ions. In the case of using a significant amount of silicon in the graphite, the time constants of the anode and the cathode becomes different at high state of charges for a fresh cell. Therefore, three semicircles represent all the electrochemical phenomena. Thus, in Figure 3, the presence of two semicircles indicates the time constants of the anode and cathode remain close to each other, meaning that the anode is not highly enriched with silicon-based particles.

Figure 3: Nyquist plots of an Amperex BM55 at states of charge of 5%, 50% and 100%.

The cell structure, the inner texture of the electrode and distribution of Si-based particles were characterized by taking SEM (Scanning Electron Microscope) images of the electrode cross-section as presented in our report in TechInsights Li-Ion Battery Essential subscription. This report also provides details on the characterization of the electrolyte, cathode, separator and most importantly, the oxidation state of the silicon-based particles using the x-ray photoelectron spectroscopy (XPS) method.

Summary and future works

One promising method to increase the energy density of the LIB (Lithium-ion batteries) is the application of higher capacity active materials for anode instead of graphite. Among many alternative materials, Silicon is an attractive candidate due to its high energy-density of nearly 4200 mAh/g compared to only 372 mAh/g for the graphite-based anode material. However, the application of pure Si leads to short-term stability and low-capacity retention. One promising method to address these challenges is to utilize SiOx and Si -alloys. Xiaomi Mi 11 Ultra cell phone benefits from a battery (Amperex BM55) with a 2nd generation nano-silicon anode material. This battery has been fully characterized from the materials points of view, and the presence of SiOx was confirmed. More information on the Amperex BM55 can be found in our Battery Essentials subscription.

Looking Forward

As we see from BM55, consumer product manufacturers are taking advantage of composite Si-anode LIBs and are now releasing commercial products based on these chemistries. Another recent device of interest is the Whoop 4 fitness wearable. For this product, Whoop selected battery technology from Sila Nanotechnologies Inc (Alameda California) which has been developing novel silicon materials for LIB over the last decade. The Sila chemistry claims to improve energy density by 17%. This, combined with other improvements, is said to have shrunk the product size by 33% while achieving 5 days of battery life. The Sila-based LIB battery cell from the Whoop 4 will be profiled in a TechInsights Battery Essentials report as part of the TechInsights Battery Essentials subscription.