Elina Pohjalainen

Fully integrated DC-DC converter and a 0.4V 32-bit CPU with timing-error prevention supplied from a prototype 1.55V Li-ion battery

We introduce an ultra-low-energy system comprised of a prototype 1.55V Li-ion battery, fully integrated switched-capacitor (SC) DC-DC 3:1 converter, and a 32-bit RISC CPU with timing-error prevention (TEP). The DC-DC converter and CPU are manufactured in 28nm UTBB FD-SOI. The DC-DC converter uses the batterys flat discharge curve and low nominal voltage to achieve a peak efficiency of 85%. The CPU operates from 0.3V-0.5V and with energy as low as 4.9pJ/cyc. The battery, DC-DC converter, and CPU system is able to operate with an average energy of 8pJ/cyc over 95% of the batterys discharge curve in the temperature range of -20oC to 70oC.

Electrochemically anodized porous silicon: Towards simple and affordable anode material for Li-ion batteries

Silicon is being increasingly studied as the next-generation anode material for Li-ion batteries because of its ten times higher gravimetric capacity compared with the widely-used graphite. While nanoparticles and other nanostructured silicon materials often exhibit good cyclability, their volumetric capacity tends to be worse or similar than that of graphite. Furthermore, these materials are commonly complicated and expensive to produce. An effortless way to produce nanostructured silicon is electrochemical anodization. However, there is no systematic study how various material properties affect its performance in LIBs. In the present study, the effects of particle size, surface passivation and boron doping degree were evaluated for the mesoporous silicon with relatively low porosity of 50%. This porosity value was estimated to be the lowest value for the silicon material that still can accommodate the substantial volume change during the charge/discharge cycling. The optimal particle size was between 10–20 µm, the carbide layer enhanced the rate capability by improving the lithiation kinetics, and higher levels of boron doping were beneficial for obtaining higher specific capacity at lower rates. Comparison of pristine and cycled electrodes revealed the loss of electrical contact and electrolyte decay to be the major contributors to the capacity decay.

Battery development for ultra-low-voltage systems

Ultra-low voltage is the key to ultra-low power consumption. Hence, there is a drive to lower the operating voltage of wireless sensor System-on-Chips as much as possible. State-of-the-art components already operate below 1V and some below 0.5V. However, commercial rechargeable lithium-ion batteries show a high nominal voltage in the range of 3.4-4.1V. Were such batteries paired with ultra-low-voltage systems, cost effective power-conversion design (without off-chip components) becomes difficult and energy will in all likelihood be lost in the conversion. While sub-2V lithium-ion batteries exist, sub-1V water-based electrolyte batteries with high energy density should also be developed. Shown here is experimental low voltage lithium battery with an ultra-low-voltage DC-DC converter / processor test system. Simulations on the DC-DC show from 12% to 74% efficiency improvement depending on the configuration. Measurements on the processor show 83% energy-delay product and 39% energy / operation improvement.