Does Heat Play a Role in the Observed Behavior of Aqueous Photobatteries?

Light-rechargeable photobatteries have emerged as an elegant solution to address the intermittency of solar irradiation by harvesting and storing solar energy directly through a battery electrode. Recently, a number of compact two-electrode photobatteries have been proposed, showing increases in capacity and open-circuit voltage upon illumination. Here, we analyze the thermal contributions to this increase in capacity under galvanostatic and photocharging conditions in two promising photoactive cathode materials, V2O5 and LiMn2O4. We propose an improved cell and experimental design and perform temperature-controlled photoelectrochemical measurements using these materials as photocathodes. We show that the photoenhanced capacities of these materials under 1 sun irradiation can be attributed mostly to thermal effects. Using operando reflection spectroscopy, we show that the spectral behavior of the photocathode changes as a function of the state of charge, resulting in changing optical absorption properties. Through this technique, we show that the band gap of V2O5 vanishes after continued zinc ion intercalation, making it unsuitable as a photocathode beyond a certain discharge voltage. These results and experimental techniques will enable the rational selection and testing of materials for next-generation photo-rechargeable systems.

Three-dimensional operando optical imaging of particle and electrolyte heterogeneities inside Li-ion batteries.

Understanding (de)lithiation heterogeneities in battery materials is key to ensure optimal electrochemical performance. However, this remains challenging due to the three-dimensional morphology of electrode particles, the involvement of both solid- and liquid-phase reactants and a range of relevant timescales (seconds to hours). Here we overcome this problem and demonstrate the use of confocal microscopy for the simultaneous three-dimensional operando measurement of lithium-ion dynamics in individual agglomerate particles, and the electrolyte in batteries. We examine two technologically important cathode materials: LixCoO2 and LixNi0.8Mn0.1Co0.1O2. The surface-to-core transport velocity of Li-phase fronts and volume changes are captured as a function of cycling rate. Additionally, we visualize heterogeneities in the bulk and at agglomerate surfaces during cycling, and image microscopic liquid electrolyte concentration gradients. We discover that surface-limited reactions and intra-agglomerate competing rates control (de)lithiation and structural heterogeneities in agglomerate-based electrodes. Importantly, the conditions under which optical imaging can be performed inside the complex environments of battery electrodes are outlined.

Interrogating the Light-Induced Charging Mechanism in Li-Ion Batteries Using Operando Optical Microscopy.

Photobatteries, batteries with a light-sensitive electrode, have recently been proposed as a way of simultaneously capturing and storing solar energy in a single device. Despite reports of photocharging with multiple different electrode materials, the overall mechanism of operation remains poorly understood. Here, we use operando optical reflection microscopy to investigate light-induced charging in LixV2O5 electrodes. We image the electrode, at the single-particle level, under three conditions: (a) with a closed circuit and light but no electronic power source (photocharging), (b) during galvanostatic cycling with light (photoenhanced), and (c) with heat but no light (thermal). We demonstrate that light can indeed drive lithiation changes in LixV2O5 while maintaining charge neutrality, possibly via a combination of faradaic and nonfaradaic effects taking place in individual particles. Our results provide an addition to the photobattery mechanistic model highlighting that both intercalation-based charging and lithium concentration polarization effects contribute to the increased photocharging capacity.

A mechanistic study of the dopant-induced breakdown in halide perovskites using solid state energy storage devices.

Doping halide perovskites (HPs) with extrinsic species, such as alkali metal ions, plays a critical, albeit often elusive role in optimising optoelectronic devices. Here, we use solid state lithium ion battery inspired devices with a polyethylene oxide-based polymer electrolyte to dope HPs controllably with lithium ions. We perform a suite of operando material analysis techniques while dynamically varying Li doping concentrations. We determine and quantify three doping regimes; a safe regime, with doping concentrations of <1020 cm-3 (2% Li : Pb mol%) in which the HP may be modified without detrimental effect to its structure; a minor decomposition regime, in which the HP is partially transformed but remains the dominant species; and a major decomposition regime in which the perovskite is superseded by new phases. We provide a mechanistic description of the processes mediating between each stage and find evidence for metallic Pb(0), LiBr and LiPbBr2 as final decomposition products. Combining results from synchrotron X-ray diffraction measurements with in situ photoluminescence and optical reflection microscopy studies, we distinguish the influences of free charge carriers and intercalated lithium independently. We find that the charge density is equally as important as the geometric considerations of the dopant species and thereby provide a quantitative framework upon which the future design of doped-perovskite energy devices should be based.

Solid-State Lithium-Ion Batteries as a Method for Doping Halide Perovskites with an In Situ Optical Readout of Dopant Concentration.

Controlled doping of halide perovskites is a longstanding challenge for efficient optoelectronic applications. Here, a solid-state lithium-ion battery (LIB) inspired device is used as a method of extrinsically doping a halide perovskite in a controlled and measurable fashion. The Burstein-Moss band gap shift induced by the electronic doping is measured using in situ optical spectroscopy to monitor the fraction of injected charges that successfully n-type dope the perovskite. By comparing the optical and electrochemical readouts of the charge density, we demonstrate a 96% doping efficiency during the insertion process. Subsequent charge removal steps demonstrate only a partial "undoping" of the perovskite, providing insights into the capacity degradation pathways in perovskite LIB electrodes.

Photo-Rechargeable Zinc-Ion Capacitor Using 2D Graphitic Carbon Nitride.

Off-grid energy storage devices are becoming increasingly important to power distributed applications, such as the Internet of things, and smart city ubiquitous sensor systems. To date, this has been achieved by combining an energy storage device, e.g., a battery or capacitor with an energy harvester, e.g., a solar cell. However, this approach inherently increases the device footprint and the output voltages of energy harvesters often do not match those required by energy storage device. Here we propose the first photo-rechargeable zinc-ion capacitors, where graphitic carbon nitride acts simultaneously as the capacitor electrode and light harvesting material. This approach allows light to be used to recharge the capacitor directly and they can be operated in a continuous light powered mode. These capacitors show a photo-rechargeable specific capacitance of ∼11377 mF g-1, a photo-charging voltage response of ∼850 mV, and a cyclability with ∼90% capacitance retention over 1000 cycles.