|Fig. 1: Schematic of integrated PV and battery device.|
In 1997, my first remote control hobby car was powered by a relatively large nickel metal hydride (Ni-MH) battery that had a 20-minute useful life on a full-charge.  Fast-forward to 2001, when the Asahi Kasei Company developed the first lithium-ion (LIB) battery and the Sony Corporation produced it for commercialization. The development if lithium-ion batteries has made it possible to make cellular phones, laptops, and tablets thinner and lighter than it was previously possible with other battery types such as Ni-MH or nickel-cadmium (Ni-Cd).  Another advantage of LIBs have over Ni-Cd or Ni-MH is that it has a higher volumentric and gravimetric energy storage capability.
Solar cells have been used extensively for in space applications such as orbital satellites and the International Space Station. The steep learning curve and capital pumped in the solar industry has made it possible for citizens to purchase or lease solar panels to help supplement the electrical grid already powering their homes.
While both devices on their own are revolutionary, combined they constitute a device that can continuously generate power and store the power that is not used up.  Scientists at the Georgia Institute of Technology have been working on an integrated PV device and Li-ion battery module having a common electrode.
Like most batteries, a lithium-ion battery is divided into the anode, cathode and electrolyte. Generally, the anode of a conventional lithium-ion cell is made from carbon. The cathode is a metal oxide, and the electrolyte is a lithium salt in an organic solvent. The electrochemical roles of the electrodes reverse between anode and cathode, depending on the direction of current flow through the cell. For example, during discharge, the lithium ions move from the anode to the cathode and move from the cathode to the anode during charging. 
|Fig. 2: Schematic of a dye-sensitized solar cell. (Source: Wikimedia Commons)|
The motivation to study the combined PV and battery device was to be able to power small sensors. The sensor would rely on a lithium-ion battery for power, as needed. The battery would then be recharged via the integrated photovoltaic cell during the day.  Combined PV and lead-acid batteries have been studied, but these required the use of separate electronics and packaging. This results in an overall system that is larger, heavier, and potentially more costly than the integrated PV and Li-ion battery with a common electrode.
The construction of the integrated device consists of a transparent top layer that is used for surface protection. Sandwiched between the top layer and the common silicon substrate are the multi-junction silicon photovoltaic cells. Attached to the common substrate are silicon nanowire anodes that allow the flow of electrons.  Much effort has been put to using silicon because it has a very high specific capacity, roughly 3579 mAh g-1 for Li15Si4.  On the other hand, a carbon anode has a gravimetric capacity of 370 mAh g-1. The cathode is LiCoO2 with a solid electrolyte separating the anode and cathode.  Silicon is widely used in the photovoltaic industry because it is readily available and its material properties are well defined. A schematic is shown in Fig. 1.
Another type of integrated photovoltaic and battery is the dye-sensitized solar cell and lithium battery on double-sided TiO2 nanotube arrays. Fig. 2 shows how light interacts with dye molecules to produce current. Unlike the design discussed earlier, this does not use silicon but the stacking sequence is very similar with the solar cell in tandem with lithium battery.  The top layer is made of platinum and is the electrode where holes accumulate. The bottom layer is made of aluminum and acts as the anode. Sandwiched in between are the TiO2 nanotubes and titanium anode that separates the DSCC from the battery. TiO2 nanotubes are also grown on the battery side of the Ti-anode to transport positive ions. During irradiation, electrons are generated from the dye-sensitized solar cells and are transported to the Ti-anode via the TiO2 nanotubes. Holes are then generated and accumulate on the platinum electrode. In the battery side, the electrons generated flow via an external circuit to combine with the holes in the platinum electrode. 
The silicon nanowire and Li-ion integrated device has an open-circuit voltage of 423 mV with a short circuit current of 19.3 mA/cm2.  With the dye-sensitized solar cell and Li-ion device an open-circuit voltage of 3.39 V and a short-circuit current density of 1.01 mA/cm2 has been achieved. This results in an efficiency of 0.82%.
A drawback of using silicon nanowire photovoltaic cells is their lower efficiency compared to their planer counterparts.  Si nanowire PV cells are 9% efficient compared to 25% for planer solar cell designs. What is attractive about using nanowires is that they have a larger surface area for reaction with a smaller footprint compared to its planar counterpart. While the lower cost of silicon makes it an attractive material to use, it's anisotropic swelling during lithiation results in unwanted structural degradation.  During the charge and discharge cycle silicon nanowires can expand in a preferential direction up to 300% its original volume. The expansion is needed to reduce the radial strain induced during lithiation of the silicon structure. This results in pulverization of the material and severe reduction in the number of cycles it can undergo.  This side effect may only be of small concern with small wireless sensors because they only require intermittent power.
© Paul Ditiangkin. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 M. Yoshio, R. J. Broad and A. Kozawa, eds., Lithium-Ion Batteries (Springer, 2009), pp. 1-8.
 V. Chakrapani et al., "A Combined Photovoltaic and Li Ion Battery Device For Continuous Energy Harvesting and Storage," J. Power Sources, 216, 84 (2012).
 W. Guo et al., "An Integrated Power Pack of Dye-Sensitized Solar Cell and Li Battery Based on Double-Sided TiO2 Nanotube Arrays," Nano Lett. 5, 2520 (2012).