WO2018215470A1

WO2018215470A1 - Rechargeable electrochemical cells, methods for their manufacture and operation

Info

Publication number: WO2018215470A1
Application number: PCT/EP2018/063392
Authority: WO
Inventors: Shahab AHMAD; Jeremy John Baumberg; Michaël Franciscus DE VOLDER; Chandramohan GEORGE
Original assignee: Cambridge Enterprise Limited
Priority date: 2017-05-22
Filing date: 2018-05-22
Publication date: 2018-11-29
Also published as: GB201708175D0

Abstract

A rechargeable electrochemical cell is disclosed, comprising an anode, a cathode and an electrolyte in contact with the anode and cathode and permitting conduction of guest ions. The anode includes an anode active material. During discharging, guest ions are incorporated into the anode active material. During charging, guest ions are released from the anode active material. The anode active material comprises a perovskite structured material and is capable of being illuminated, e.g. via a light-transmissive anode current collector. During operation, illumination of the anode active material causes charging of the electrochemical cell by release of guest ions from the anode active material. The anode active material may comprise an organic-inorganic hybrid perovskite structured material, such as a two dimensional organic-inorganic hybrid perovskite represented by the general chemical formula (RNH3)2(A)n-1MnX3n+1 where: n is an integer of at least 1; RNH3 is a primary aliphatic or aromatic alkylammonium cation; A is selected from Cs+, CH3NH3 +;M is selected from Ge2+, Sn2+, Pb2+; X is selected from CI-, Br-, I-.

Description

RECHARGEABLE ELECTROCHEMICAL CELLS. METHODS FOR THEIR

MANUFACTURE AND OPERATION

The work leading to this invention has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007- 2013)/ERC grant agreement number 337739.

BACKGROUND TO THE INVENTION Field of the invention

The present invention relates to rechargeable electrochemical cells, methods for the manufacture of rechargeable electrochemical cells and methods for the operation of rechargeable electrochemical cells. It has particular, but not exclusive, applicability to Li ion rechargeable electrochemical cells.

Related art

The Internet of Things (loT), smart cities and other emerging connected devices require continuous remote power sources for sensing, data processing and communication

[Haight et al (2016)]. To date, electronic devices with built-in solar panels typically have a separate battery, which introduces charge transport losses, adds complexity, weight and cost. Most solar cells have an open circuit voltage of 0.6-1.0 V, which is typically insufficient to charge batteries [Ahmad et al (2016)], thus requiring additional DC-to-DC converters or stacked cells in series, and increasing current losses. Combinations of energy generation and storage devices have previously found applications ranging from wristwatches or calculators, to solar power plants. However, this marriage of two technologies suffers from transport losses between the components and more importantly is a bulky and expensive solution [Xu et al (2014); Vlad et al (2014)]. Materials that combine both energy storage and charging could be disruptive for numerous applications. A solar-battery device eliminates many issues and is well-suited to the emerging autonomous devices for the Internet of Things, where power requirements are typically low and device size and weight need to be reduced.

Satellites, solar planes and drones could also benefit from this integrated approach where weight is key and devices must be easily recharged [Malaver et al (2015)]. The core need for developing solar-batteries lies in the requirement of a material that can generate energy (photovoltaic functionality) and simultaneously store energy (battery functionality). Present research has not demonstrated any viable system which can accomplish both these functions.

Terakado and Tanaka (201 1 ) have proposed a thin-film solar-chemical battery, using amorphous AgAsS2 films. Their battery produced low photo-voltage, had relatively low energy density and poor charge retention capability.

Lithium ion (Li-ion) batteries have been intensively researched. They typically use a Li intercalation material as the active anode material, providing high energy density and good cycle life. Nagai and Sato (2016) report a transparent thin film Li-ion battery with titania (T1O2) as the anode and L1C0O2 as the cathode on a fluorine-doped tin oxide (FTO) glass substrate. The battery was capable of charging under solar irradiation. However, the battery had relatively low energy density, it only absorbs in the UV spectrum, and the device fabrication require high temperature processing. SUMMARY OF THE INVENTION

The present inventors have realised that there is scope for light-rechargeable electrochemical cells which provide substantial and practical charging and discharging performance compared with prior disclosures of the possibility of light-rechargeable electrochemical cells. Furthermore, and optionally separately from the performance characteristics, the present inventors have realised that there is scope for light- rechargeable electrochemical cells which can be manufactured by practical and scalable processing steps. The present invention has been devised in order to address at least one of the above problems. Preferably, the present invention reduces, ameliorates, avoids or overcomes at least one of the above problems.

Accordingly, in a first preferred aspect, the present invention provides a rechargeable electrochemical cell comprising:

an anode;

a cathode; and

an electrolyte in contact with the anode and cathode and permitting conduction of guest ions,

wherein the anode includes an anode active material, the rechargeable electrochemical cell being operable during discharging to incorporate guest ions into the anode active material and during charging to release guest ions from the anode active material, wherein the anode active material comprises a perovskite structured material and is capable of being illuminated, wherein, during operation, illumination of the anode active material causes charging of the electrochemical cell by release of guest ions from the anode active material.

In a second preferred aspect, the present invention provides a method for operating a rechargeable electrochemical cell, the cell comprising:

an anode;

a cathode;

wherein the anode includes an anode active material, the anode active material comprising a perovskite structured material, the method including the steps:

at least partially discharging the cell to incorporate guest ions into the anode active material;

at least partially charging the cell by exposing the cell to light illumination so that the anode active material receives illumination, to cause charging of the electrochemical cell by release of guest ions from the anode active material.

In a third preferred aspect, the present invention provides a method of manufacturing a rechargeable electrochemical cell according to the first aspect wherein the anode active material is formed as a layer using solution processing.

The first, second and/or third aspect of the invention may have any one or, to the extent that they are compatible, any combination of the following optional features. In some embodiments, the anode may further include a light-transmissive anode current collector. For example, the anode current collector may be formed of a light-transmissive electrically conductive material.

Suitable materials for the anode current collector include ITO, FTO, graphene. The anode current collector may be a porous electrode. The anode current collector may have the form of a light-transmissive substrate coated with electrically conductive nanoparticles. Suitable nanoparticles include CNTs, graphene, silver nanowires, gold nanoparticles. Additionally or alternatively, the substrate may be coated with a conductive film of suitable thickness to permit suitable light transmission. For example, thin Al, thin Au or other thin conductive films can be used. The thickness of the anode current collector typically varies depending on the material selected. For example FTO glass substrates used in exemplary embodiments of the invention have a layer of about 300nm thick FTO and are 85 % transparent in the visible spectrum region with a sheet resistance of about 8 Ohm/sq. However, similar properties in terms of light transmission and sheet resistance can be achieved with spin coated Ag-nanowire substrates with a much thinner layer (less than about 50 nm).

In some embodiments, the anode current collector may be formed of an arrangement of non-light-transmitting electrically conductive material portions with light-transmitting apertures formed between the non-light-transmitting electrically conductive material portions. Suitable approaches to form such an anode current collector include the use of metals to form the non-light transmitting electrically conductive material portions. For example, Al/Ag can be used, as known from solar cells. However, one point to note is that the use of such electrodes to extract charge carriers is straightforward where the diffusion length of charge carriers is large, as for Si or other inorganic solar cells. In the case of perovskites in general, it is considered that charge carrier diffusion lengths (for both electrons and holes) are small. This suggests that where possible the anode current collector should take the form of an electrically conducting light transmissive material (such as FTO), in order that the small diffusion length of charge carriers in the perovskite anode active material does not deleteriously affect the chances of charge carriers reaching part of the anode current collector.

Where the anode current collector is formed of an arrangement of non-light-transmitting electrically conductive material portions with light-transmitting apertures formed between the non-light-transmitting electrically conductive material portions, a grid-like shape of conductive material can be formed, for example from metal and/or conductive polymer. Suitable materials can be screen printed or wire bar coated. As one example, the material could comprise Ag nanowires, CNTs, or a Cu mesh.

Preferably, the anode active material comprises an organic-inorganic hybrid perovskite structured material. The anode active material may comprise an organic-inorganic metal halide based perovskite material. The anode active material may comprise an organic- inorganic metal halide layered perovskite material. The perovskite structured material of the anode active material is preferably a two dimensional organic-inorganic hybrid perovskite represented by the general chemical formula (RNH₃)2(A)n-iM_nX3n₊i

where:

n is an integer of at least 1

RNH3 is a primary aliphatic or aromatic alkylammonium cation

A is selected from Cs⁺, CH₃NH₃ ⁺

M is selected from Ge²⁺, Sn²⁺, Pb²⁺

X is selected from CI^", Br, l^~

Depending on the value of n, it is possible to obtain, for example, a perfect 2D or combination of 2D and 3D perovskite.

In the anode active material, inorganic monolayers of corner-shared [ΜΧβ]^4" octahedra are preferably confined between interdigitating bilayers of organic cations.

Preferably, the perovskite structured material is a 2D perovskite (n=1 ), represented by where R = C6H9C2H4NH3

M=Pb

X=lodine or Bromine.

Therefore the preferred materials are

Other suitable materials are other cyclic or aromatic organic amine based perovskites.

Suitable materials may be selected based on their optical and electrical properties. Suitable perovskite materials absorb a wide range of light spectra (typically UV to NIR), should be stable in electrolyte medium and should not be electrically insulating. The anode active material may further comprise an electron transport material. Suitable electron transport materials include those which are compatible with the solvents used in the solution processing for forming the anode active material layer. For example, low- band gap polymer, organic and inorganic materials can be used. Specific example materials include rGO, carbon nanotubes, carbon particles and PCBM. Preferably, the electron transport material is chemically inert to the electrolyte.

The anode active material may be operable during discharging to incorporate guest ions into the anode active material by one or more of intercalation, alloying or conversion.

The method of operating the cell may further include the step of charging the cell by applying an external electrical potential difference across the cell. Thus, the cell may be charged by illuminating the cell and/or by applying an external electrical potential difference across the cell. This provides added flexibility in the end use of the cell.

In the method of operating the cell, preferably the cell is only discharged partially before a subsequent charging, the cell being discharged to an open cell voltage of not less than 0.5V. Discharging to a lower open cell voltage is considered to reduce the operational lifetime of the cell, by reducing the cyclability and peak charging voltage.

More preferably, in order further to improve the operational lifetime of the cell, the cell may be discharged to an open cell voltage of not less than 0.6V, not less than 0.7V, not less than 0.8V, not less than 0.9V, not less than 1 .0V, not less than 1.1V, not less than 1.2V, not less than 1.3V, not less than 1.4V, not less than 1 .5V, not less than 1 .6V, not less than 1.7V, not less than 1.8V, not less than 1 .9V, or not less than 2.0V.

For example, for good cyclability and less decay in the charging potential, it may be preferred to operate the cell between about 3.0 to 1.4 V. In the method of manufacturing the cell, preferably the anode active material is formed as a layer. During the formation of the anode active material layer, the layer is preferably not subjected to a temperature greater than 100^°C. In particular, the layer preferably does not require an annealing step at high temperature, unlike anode active material layers disclosed in the prior art. By taking this approach, the anode active material layer can be formed by solution processing, in which a solution or dispersion of the anode active material layer, or its precursors, are deposited onto a suitable substrate and dried to remove the solvent. Such an approach is typically compatible with the formation of the other layers of the cell, and lends itself well to a scaled-up industrial manufacturing process.

The inventors are aware of the disclosure of Betz et al (1984), which discloses a device incorporating an electrode formed of what is referred to as Ti02(B). In the view of the present inventors, Betz et al (1984) discloses a different approach (compared with the present disclosure) to perform light induced exchange of protons. The Ti02(B) material in Betz et al (1984) is formed from K2T14O9 by hydrolysis, filtration and thermolysis. It is disclosed as having a "perovskite-related structure", where the built up units are compared with Re03. According to the well-known crystal structure of cubic ABX3 perovskite, ΒΧβ forms corner sharing octahedra and element 'A' occupies a position at the middle of the cube. However, in the case of the Ti02(B) crystal structure of Betz et al (1984), the space at the middle of cube lattice site is left vacant. In addition to being different to the crystal structure of cubic ABX3 perovskite, this structure is also different to the 2D metal halide based perovskite (A2BX4) of use in some embodiments of the present invention.

Further optional features of the invention are set out below.

BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

Fig. 1 shows a crystal structure representation of 2D layered perovskite 2-(1 - cyclohexenyl) ethylammonium lead iodide

(CHPI).

Fig. 2 shows optical absorption spectra of CHPI thin film and emission spectra of white light source. Transmission spectra of FTO glass and Graphene substrate are shown for comparison.

Fig. 3 shows a schematic representation of perovskite photo-battery according to an embodiment of the invention.

Fig. 4 shows an SEM image of drop-cast 2D perovskite electrode taken at 45° tilt angle, illustrating the substantially perpendicular alignment of the crystal plates relative to the substrate surface (not shown). The inset of Fig. 4 shows a PL image of corresponding perovskite film with A_ex of about 300 nm LED source.

Fig. 5 shows an energy level diagram of a perovskite photo-battery according to an embodiment of the invention.

Fig. 6 shows cyclic-voltammetry plots of CHPI and CHPB 2D perovskites at 0.05mV/s, measured in standard coin cell configuration in the dark.

Fig. 7 shows the first photocharge-discharge voltage profile of CHPI based photo-battery under light (100mW/cm²) and dark condition (21.5 kQ) respectively. The inset shows the extended cyclability of the photo-battery under similar conditions.

Fig. 8 shows the results of extended lifetime testing of photo-battery for a different range of voltage under similar condition as in Fig. 7.

Fig. 9 shows in-situ potential discharge curve for a photo-battery under different conditions.

Fig. 10 shows a schematic representation of photo-charge generation, transfer and storage mechanism in a perovskite photo-battery.

Fig. 1 1 shows a crystal structure representation of CHPI showing spacing between Pb^ octahedra providing access to Li⁺ -ion for intercalation (discharge) and de-intercalation (photo-charge). Fig. 12 shows in-situ confocal Photo-luminescence (PL) vs Open Circuit Voltage (OCV) measurement under discharge by resistive load. The step profile in dotted arrows is added to guide the eye. Inset photographs show PL response (A_ex of about 365nm LED source), of the photobattery when initially photo-charged to 2.90 V and after discharge to 2.26 V.

Fig. 13 shows an SEM image (at 0° angle) of perovskite photo-battery electrode (B4) prepared on precleaned FTO substrate.

Fig. 14 shows a higher magnification SEM image of the electrode of Fig. 13 showing porosity due to the vertical assembly of crystalline platelets of 2D perovskite.

Fig. 15 shows an SEM image of an ordinary hot-casted film of pristine CHPI solution (S3), on FTO substrates (at 0° angle), for comparison. The method of hot-casting is adopted from recently published paper from Nie et.al (2015).

Fig. 16 shows transmission spectra of perovskite photo-battery electrode (B4) prepared on preclean FTO substrate.

Fig. 17 shows the same electrode as in Fig. 14 when seen at 45° tilt angle.

Fig. 18 shows an SEM image of drop casted films of pristine CHPI solution (S3), on FTO substrates (at 0° angle), for comparison. The inset in Fig. 18 shows the hexagonal shape 2D perovskite crystal which generally forms when dropcasting CHPI thick films, representing hexagonal crystal structure of CHPI [Ahmad and Prakash (2014)].

Fig. 19 shows first and second charge-discharge voltage profile of CHPI and CHPB 2D perovskites at about 30 mA/g and about 40 mA/g respectively from corresponding standard coin cells, the measurements were done in dark conditions.

Fig. 20 shows cyclic-voltammetry of CHPI photo-battery at 0.1 mV/s, measured in dark condition.

Fig. 21 shows photocharge-discharge voltage profiles of perovskite photobattery electrode with PCBM (B2) as additive. When discharged to below 1 .4V the colour of the electrode was seen to turn dark brown.

Fig. 22 shows photocharge-discharge voltage curves profile of perovskite photobattery electrode (B4) fabricated on graphene substrate. The device has shown similar performance as in the case of FTO as transparent substrate. The electrode has turned into black when discharged below 1 .4V, therefore confirming that the perovskite is getting degraded due to Li-ion intercalation, not the FTO.

Fig. 23 shows photocharge-discharge voltage curves of perovskite photobattery electrode (B4) when discharged by 3V white light LED as load in dark conditions. The LED has turn on voltage around 2.2 V. This demonstrates that perovskite photobattery is capable of powering LED for almost an hour when fully charged.

Fig. 24 shows the retention in the capacity when the photo-battery was left in charged state after initial photo-charge and discharge cycles. The photo-battery was charged to about 2.75 V by light illumination, further light was turned OFF and photo-battery was left in charged state under dark and no load condition. The photo-battery has maintained almost similar capacity and underwent through less than 10 % loss in the output potential (about 2.60 V) in around 13 hours.

Fig. 25 shows another in-situ discharge potential curve produced from the device already used for 5 cycles (shown in Fig. 8). Here, unlike previous case as demonstrated in Fig. 9, after full photo-charge the device was first discharged in dark to 1 .4 V (grey region) and later simultaneous photo-charging was initiated. As soon as the light was turned ON a sudden rise in potential is observed which, after obtaining maxima at about 1.9 V, has started reducing and formed a slopping plateau ending at about 0.6 V. Since, being an already used device the performance was not as good as shown in case of new device (Fig. 9), still a stable potential value of about 0.5 V (η of about 0.01 1 %) is achieved for more than 13 hours until the light was turned OFF and voltage dropped to about 0.15 V (grey region).

Fig. 26 shows the confocal PL set-up designed to measure in-situ PL of perovskite film (B4) while discharging the photo-battery by a resistive load (21.5 kQ) under dark condition.

Fig. 27 shows exciton PL spectra collected against corresponding values of the open circuit voltage (OCV) of the photo-battery.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS. AND FURTHER OPTIONAL FEATURES OF THE INVENTION Solar panels are commonly used to power batteries for applications ranging from low- tech gadgets to high-end satellites and from very low power consumer electronic products to high power solar power plants. It is further anticipated that these

autonomous powered devices will become even more prominent with the rise of the Internet of Things and smart cities. The necessity of having a solar cell and battery as two separate devices has never been questioned to date because there are no materials available that fulfill simultaneously the requirements for energy generation and storage. However, integrating these functionalities could reduce losses and the size of these devices. In this disclosure, in summary, we disclose a rechargeable photo-battery system comprising a perovskite structured material, preferably an organic-inorganic hybrid perovskite, which at the same time acts as the energy storing device and can be fully charged by light. In a preferred embodiment, a highly photoactive two-dimensional lead halide perovskite is used as the anode active material, namely 2-(1-cyclohexenyl) ethylammonium lead iodide

(CHPI)), in order to simultaneously achieve photo-charging and Li-ion storage in a simplified device.

In the preferred embodiments of the present invention, there is used a material that can generate energy (solar functionality) and simultaneously store energy (battery

functionality).

Recently attempts have been made to co-integrate dye sensitized solar cells (DSSC) and Li-ion storage electrode to create hybrid batteries for standalone electronics [Thimmappa et al (2015); Xu et al (2014); Mendez et al (2014)]. In these devices the generation of photo-charges takes place in a separate photoactive layer which after separation gets collected at the battery storage electrode, which is another material system. Similar approaches are disclosed in Paolella et al (2016) and Nguyen et al (2017). Such co- integration provides added advantages such as reduced ohmic transport losses, shared common interface, electrodes and encapsulation units, higher gravimetric energy density, etc. However, mixing different materials for energy harvesting and storage gives rise to several fundamental limitations, including: (a) the energy levels of the light absorbing material should match with the storage material to allow efficient energy transfer, (b) the storage material should not block light into the optically active material, and (c) both materials should form good interfacial contacts and should not phase separate. Such requirements govern their working and set limitations on performances of these hybrid devices.

As described in more detail below, embodiments of the present invention address this challenge by using polycrystalline metal halide based 2D perovskite materials of type (RNH₃)2MX4 [R-organic, M-metal, X-halide]. Such materials are found, remarkably, to allow for simultaneous solar charging and energy storage in a practical solid-state device.

In recent years the three-dimensional organic-inorganic hybrid perovskites (AMX3 with A = Cs⁺, CH₃NH₃ ⁺, M = Ge²⁺, Sn²⁺, Pb²⁺; X = CI^", Br, I^") solar cells and in particular methylammonium lead iodide (CHsNHsPb ) has achieved power conversion efficiency (PCE) records of 22.1 % in 2014 [Saliba et al (2016); Akkerman et al (2016); Stranks and Snaith (2015)] with the stability of these cells being one of the main technologic challenges. Two-dimensional perovskite have shown improved stability, but poor efficiencies of only 4.73 % [Cao et al (2015); Ahmad et al (2015); Safdari et al (2016)]. Such two dimensional perovskites can be represented by (RNH3)2(A)n-iMnX3n+i (n is an integer, RNH3 is a primary aliphatic or aromatic alkylammonium cation), form

Ruddlesden-Popper phases [Tsai et al (2016)]. 2D perovskites (n =1 ; (RNH₃)₂MX4) self- assemble into layered structure where inorganic monolayers of corner-shared [MX₆]^4" octahedra are confined between interdigitating bilayers of organic cations, resulting in very robust crystal structure. 2D perovskites are regarded as 'natural' multiple-quantum- wells in which the semiconducting inorganic monolayers acting as potential wells and insulating organic layers act as potential barriers [Ishihara et al (1990); Ishihara (1994); Dou et al (2015)]. The crystal structure of the 2D layered perovskite CHPI is illustrated in Fig. 1 . The electronic and dielectric confinement effects generate excitons with high binding energies (about 300 meV) and remarkable room-temperature stability [Ahmad (2013)]. The relatively poor efficiency of pure 2D perovskite based PSCs is attributed to the inhibition of out-of-plane charge transport by the organic cations [Ahmad et al (2015); Safdari et al (2016)].

In general, organo-halide perovskites have not previously been considered to be appealing materials for Li ion application due to their inherently poor electrochemical stability. For example, a recent report exploiting 3D perovskites indicated that in particular lead iodide based organo-halide perovskites exhibit very low electrochemical performance in Li-ion half cells [Xia et al (2015)]. However, to the knowledge of the present inventors, they have not been disclosed as being tested as a solar material at the same time.

The present disclosure focuses on 2D perovskites because they are structurally more robust and form layered structure with tunable large interlayer spacing (see Fig. 1 ). The present inventors have found that 2D perovskites are not only good solar energy collectors but are also able to act as a battery electrode material, thus enabling photo- charging. The capacity value in the half coin cell Li-ion battery configuration disclosed here was estimated to be about 90-100 mAh/g, for lead iodide based 2D perovskite (CHPI) in the first cycle. This fades rapidly to as low as about one fifth of the initial capacity in the second cycle due to very low lithiation stability. Lead bromide based 2D perovskite (CHPB) achieved up to about 410 mAh/g, which is higher than graphite used in commercial Li-ion batteries, but its capacity fades to about one quarter in the second cycle.

It is of particular interest that these materials are solution processable, which is considered to make these materials compatible with industrial scale processes for battery electrode materials. Fig. 3 shows a schematic view of a photo-battery having a substrate with Al film and stainless steel layer formed over it. A Li metal layer is then formed and a perovskite layer formed over the Li metal layer with a separator (frit) interposed between. An FTO transparent current collector is formed over the perovskite layer, with a copper electrode sandwiched between part of the perovskite layer and part of the FTO layer. Unlike conventional coin cell batteries, the photo-battery is designed with a transparent FTO current collector to provide a window to incoming photons for optical excitation in the photoactive perovskite layer. For energetically favorable transport of electrons from perovskite to the electron selective electrode (Cu), it is found that using rGO (reduced graphene oxide) or PCBM (phenyl-C61-butyric acid methyl ester) as an additive in the perovskite material provided suitable electron transport.

The optical absorption of a CHPI thin film shows characteristic strong exciton absorption peak at about 509 nm with a broad band to band absorption peak at about 380nm (see Fig. 2). A broad band light source (420 nm-650 nm) is used to excite the charge carriers at and above the band-edge of the perovskite. Additives (rGO) and binder (PVDF) were incorporated in the anode active material to provide electrical connectivity and mechanical integrity to the perovskite crystalline platelets respectively. It is also found that the additive and binder cause the crystalline platelets to assemble non-parallel with the substrate (glass) surface. Specifically, the platelets were found to be packed in an orientation substantially perpendicular to the substrate surface. This is shown in Figs. 4, 13, 14 and 17. Note that this is unusual compared to pristine drop-cast thick films of the perovskite material - compare with Figs. 15 and 18.

It is considered, without wishing to be bound by theory, that the perpendicularly aligned crystal arrangement increases the available surface area and introduces porosity in the about 10 μηη thick film, which provides additional advantages by enhancing the interaction of light with perovskite and potentially also Li-ion diffusion. The transmittance of the film is about 14%, as shown in Fig. 16. The inset of Fig. 4 show the photoluminescence (PL) image of a corresponding perovskite film excited with a 300 nm LED source. A strong green colour emission (A_em about 518nm) is characteristic room temperature excitonic emission from CHPI.

The ability of solution processed 2D perovskites to act as active storage material in rechargeable Li-ion batteries has been investigated. The electrochemical properties of both iodide and bromide based 2D perovskites have been studied first on standard coin cells.

Fig. 6 shows cyclic voltammetry curves for lead iodide and lead bromide based materials. These curves show oxidation and reduction peaks, due to Li-ion insertion and extraction followed by Li reaction with organo-halide perovskites. Fig. 19 shows the corresponding galvanostatic charge-discharge curves which show that these materials are clearly able to act as battery electrodes but limited by capacity-fading and instability.

The inventors have investigated the combined solar-battery operation using the photo- battery configuration shown in Fig. 3. In all cases, the photo-battery is photo-charged until saturation by a broad band light source (intensity about 100mW/cm²). The photo- battery was discharged into an electrical resistor of 21.5 kQ or a 3V LED in dark conditions. Initially, the photo-battery was photocharged to 3.05 V and discharged to 0.4 V and subsequently photo-charged again. It was found that, after discharging the device to below 1.4 V, the dark yellow color of CHPI turns into black, due to which the perovskite loses some of its photoactivity. It is considered that this is most likely due to structural changes introduced into the perovskite due to Li-ion insertion. This affects the cyclability and resulted in a decrease of the maximum charging potential. To ensure that FTO does not contribute to this degradation during discharge, devices with graphene, instead of FTO, were fabricated to verify that the decay in performance is due to perovskites rather than FTO. Fig. 22 shows the photocharge-discharge voltage profile of the graphene based perovskite photo-battery, and verifies that the decay in performance is due to the perovskite.

When testing the device between about 3.0 to 1.4 V, better cyclability is achieved with a slight decay in the photo-charging potential (Fig. 7). This is due to rapid degradation of these perovskites at potentials below 1V vs. Li/Li⁺ where Li-ion insertion induces further reaction with the perovskite, therefore a cut-off voltage above 1.4V is preferred, to improve the reversibility of charge/discharge cycles. Evidently, the photo-battery CV reveals that reversible Li-ion intercalation and de-intercalation processes tend to dominate in 2D lead iodide perovskites at voltages above 1 .0 V - see Fig. 20. When cycled between safe voltage limits of 2.0-2.95 V, no decay in the photo-charging potential is observed after more than 10 cycles, however the capacity of the electrode shows some fading (Fig. 8, in accordance with the results obtained from coin cells - see Fig. 19. The photo-battery can efficiently power a commercial 3V white light LED after the 1 st cycle of photo-charging. Discharge-potential curves of photo-batteries with LED as a load have shown immediate drops in potential from 2.95 V to 2.5 V after connecting LED (see Fig. 23, which stabilize later for more than 1 .5 hours, demonstrating stable performance of perovskite photo-batteries. We further test the ability of the battery to retain its charge by keeping it in the charged state for approximately 13 hours in dark conditions without any load and find a decrease of < 10% of its output voltage during this time - see Fig. 24.

To further explore the photo-charging and sustained capacity in perovskite

photobatteries, the batteries were discharged while illuminated. Fig 9 shows the potential vs time curve where the battery is initially fully photo-charged (yellow region, left hand side), and then connected to a resistive load while illuminated at 100 mW/cm² intensity, (orange region, middle). Compared to the discharge potential curve obtained in dark condition (dotted line), a higher output potential (about 1.75V) is maintained along with an almost three times higher discharge time from about 3.0 to 1.4 V. A large sloping plateau at around 1.1 V is observed, which stabilizes at 0.8V. At this stage the battery is only relying on photo generated charge carriers (conversion efficiency, η of about 0.034 %). This is confirmed by a sudden drop in voltage to 0.2V when the light was turned off, after around 19 hours of operation (decay time of about 14 mins, grey region (right hand side) in Fig. 9). A more extensive photo-charge discharge voltage curve is shown in Fig. 21.

The mechanism of the photo-chargeable perovskite battery can be understood by the schematics shown in Fig. 10. Upon illumination, below exciton wavelength, the perovskite generates electron and holes pairs. Spin-coated compact perovskite films have absorption coefficients of about 2.5x10⁴ cm^"1 corresponding to a penetration depth of about 400nm at exciton wavelength (A_excabout 508nm for CHPI) [Tsai et al (2016); Milot et al (2016); Green et al (2014); Ishihara (1994)]. However, the enhanced morphology, surface area and porosity of the perovskite active layer (Fig. 3) improves both the penetration depth and the accessibility to the Li-ions, in comparison to drop-cast films.

The partially discharged photoanode undergoes photo-charging upon exposure to light with the photo-generated electrons spatially separated by the conducting rGO/PCBM, which acts as electron transport medium. This energetically favorable landscape of perovskite/rGO (or PCBM)/FTO/Cu smoothly transports photo-generated electrons from perovskite to FTO/Cu interface. The blend of perovskite with rGO (or PCBM) provides extensive conducting electronic pathways through grain boundaries and interfaces, although short carrier lifetimes (about 200 ps) and short diffusion lengths (less than 100 nm) of electrons and holes in 2D perovskites mean that most either recombine or are trapped (giving the low efficiency [Milot et al (2016)]). However, some of the photo- generated holes diffuse towards the perovskite-electrolyte interface and provide sufficient repulsion to drive Li- ions out of the perovskite matrix, thereby enabling photo-charging in perovskites. Intercalation dynamics in lead halide 2D perovskites were recently reported, and showed that organic moieties rapidly use the space (>37A²) between adjacent Pb^ octahedra to initially form the 2D perovskite [Ahmad et al (2014)]. Similarly in case of the demonstrated perovskite photo-battery, Li-ion (ionic radius of about 0.73 A) intercalation (discharging) and de-intercalation (photo-charging) occurs through the spacing between lead halide octahedrons along the c-axis, as represented schematically in Fig. 1 1 , and perpendicular to the platelets.

The inventors have verified that Li-ions are indeed intercalating in the perovskite material by in-situ measurements of PL during operation of the battery. For this, a fully photo- charged battery is connected with a resistive load while confocal PL is measured using a diode laser excitation source (A_ex of about 447nm) (Fig. 27). As expected from experiments discussed in relation to Fig. 7, the intensity of exciton PL (A_em of about 518 nm) decreases immediately after connecting the load because of the disruption from the Li-ions and after some time perovskite electrode has turned into non-emitting material (Fig. 26) [Jiang et al (2017)] . Substantially different PL responses of the device were seen when the device was fully photo-charged to 2.90 V and after discharging to 2.26 V, as indicated in Fig. 12.

The exciton PL intensity changes non-linearly with decreasing output potential in a steplike fashion. This non-linear trend matches well with the discharge potential curve of the electrode, which suggests that during discharge the process of Li-ion intercalation into the perovskite matrix takes places in a quantized manner, as also observed in the CV (Fig. 20).

It is therefore considered that photo-chargeable batteries are made available for manufacture by using solution processable perovskites. This has particular applicability to allow powering of autonomous devices and to meet the demand of sustainable energy alternatives.

Taking references Paolella et al (2016) and Nguyen et al (2017) as examples, these approaches use two separate materials for the individual purposes of light harvesting and energy storage. In the preferred embodiments of the present invention described above, a single material, layered perovskite, can do both jobs simultaneously.

It is of interest to consider how the work presented here differs from certain other prior art. Nagai et al (2016) demonstrated the use of single anode material (titania (ΤΊΟ2)) and fabricated a semi-transparent battery electrode. The photo-battery comprised of titania (anode) and UC0O2 (cathode) thin films fabricated on a conducting glass substrate (FTO) via a spin-coating method at ambient temperature using a two-step process, and they were pre-heated in a drying oven at 70 °C for 10 min. The precursor films were heat treated in air for 30 min at 500 and 550 °C, respectively. In terms of performance, the photo-battery was found to show very poor cyclability, capacity and charge retention. The averaged potential at 2.34 V was observed by applying a constant current of 1.0 mA, then that at 2.01 V was detected after 20 sec during the sequential self-discharge process. An illustration of the characteristics of such a photo-battery can be seen from Figure 6 of Chapter 6 of "Alkali-ion Batteries", by Nagai and Sato (2016).

Thus, the preferred embodiments of the present invention are considered to have high capacity and better performance than the cells disclosed in Nagai et al (2016). However, additionally, in the present work, the processing of the layered perovskite material and film does not require any high temperature annealing step. Another technical

disadvantage in Nagai et al (2016) is the active material, which is T1O2 which absorbs light only in the UV region, making it less efficient compare to layered perovskite (e.g. Iodide based) which are excellent in light absorbing over a large region of visible spectra (about 550 nm).

Materials and methods:

The following chemicals were purchased from Sigma-Aldrich: Lead iodide (Pb ), lead bromide (PbBr2), hydroiodic acid (HI), hydrobromic acid (HBr), cyclo-hexyl-ethylamine (C6H9C2H4NH2) and reduced graphene oxide (rGO). Electrochemical grade propylene carbonate, ethylene carbonate, diethyl carbonate, N-methyl-2-pyrrolidone, polyvinylidene fluoride (PVDF), and Al, Cu and Li metal foils were purchased from Sigma-Aldrich. Polypropylene layers were purchased from Cell Guard. The phenyl-C61 butyric acid methyl ester (PCBM) of >99.5 % purity grade was purchased from Solenne B.V. (The Netherlands).

Synthesis of perovskites: The synthesis of 2D perovskite is performed by using sol-gel method as reported earlier [Safdari et al (2016); Mitzi]. In short, first

(X=l or Br) was synthesized from the mixture of 1 :1 molar ratio of aqueous solutions of HX (HI or HBr) and cyclo-hexyl-ethylamine (C6H9C2H4NH2) (100°C and stirring at 2000 rpm). The precipitate from the reactant solution is separated and then washed with diethyl ether. To synthesize 2D perovskite

stoichiometric amounts of

and corresponding lead halide (Pb or PbBr2) were dissolved in Ν,Ν-dimethylformamide (DMF) (hereafter, the resultant compounds are termed as CHPI and CHPB, respectively). Perovskite powder is extracted from above prepared solution by drying overnight in vacuum oven at 60 °C.

Preparation of electrodes: Perovskite photo-battery electrodes were prepared by using different additives, rGO and PCBM. For both lead halide perovskites similar recipe is followed. 10 mg of rGO or PCBM is dissolved in 1 ml N-methy-2-pyrrolidone (NMP, in glove box) and sonicated for 1 hour. 85mg of perovskite is dissolved in the same solution and kept for overnight stirring. Later, 5 mg of polyvinylidene fluoride (PVDF) binder is added followed by 2 hours of stirring. CHPI electrode with additives

PCBM/PVDF is named as B1 and rGO/PVDF as B2 respectively. Both B2 and B4 were transferred in glove box for electrode preparation. 60 μΙ of B1 or B2 is drop-casted on pre-cleaned and UV-ozone plasma treated FTO substrate and left on hot-plate for drying overnight at 45 °C. All devices were made on FTO substrates of 1 .0 cm x 1 .5 cm.

Fabrication of photo-battery: Perovskite photo-batteries were assembled in an Ar filled glovebox. Al-metal foil and Li-metal foil (25μηη) were stacked on stainless steel (SS) disk. Whatman borosilicate paper, soaked with 1 M LiPF6 as electrolyte, was placed on Li-metal as a separator. On top of the separator, the perovskite electrodes with a Cu-foil extended electrode were gently flipped and covered with another glass slide (2.5 cm x 3.7 cm) to hold all layers. Finally, edges were sealed and clips were used to maintain the interfacial contacts between all components of device.

Characterizations: All photo-battery measurements were done manually and performed in air by using Biologic VMP-3 galvanostat. For photo-charging LED based broad band (λ of about 420-650 nm; intensity about 100mW/cm²) source is used for irradiation. Open circuit voltage (OCV) is measured to obtain photo-charge and discharge potential curves. For discharge, photobattery is connected with either a resistor of 21.5 kQ load or a commercial 3V white light LED in dark. 447nm diode laser is coupled to Olympus BX-51 microscope for confocal PL measurements. For PL images, Deep UV (300 nm,

Thorlabs-M300L4) and UV (365nm, Thorlabs-M365L2) mounted LEDs are used. A Leo variable pressure scanning electron microscope is used for scanning electron

microscopy (SEM).

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While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

All references referred to above and/or below are hereby incorporated by reference.

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Claims

1. A rechargeable electrochemical cell comprising:

an anode;

a cathode; and

2. A rechargeable electrochemical cell according to claim 1 wherein the anode further includes a light-transmissive anode current collector.

3. A rechargeable electrochemical cell according to claim 2 wherein the anode current collector is formed of a light-transmitting electrically conductive material.

4. A rechargeable electrochemical cell according to claim 2 wherein the anode current collector is formed of an arrangement of non-light-transmitting electrically conductive material portions with light-transmitting apertures formed between the non- light-transmitting electrically conductive material portions.

5. A rechargeable electrochemical cell according to any one of claims 1 to 4 wherein the anode active material comprises an organic-inorganic hybrid perovskite structured material.

6. A rechargeable electrochemical cell according to any one of claims 1 to 5 wherein the anode active material comprises an organic-inorganic metal halide based perovskite structured material.

7. A rechargeable electrochemical cell according to any one of claims 1 to 6 wherein the anode active material comprises an organic-inorganic metal halide layered perovskite material.

8. A rechargeable electrochemical cell according to any one of claims 1 to 7 wherein the perovskite structured material of the anode active material is a two dimensional organic-inorganic hybrid perovskite represented by the general chemical formula

where:

n is an integer of at least 1

RNH3 is a primary aliphatic or aromatic alkylammonium cation

A is selected from Cs⁺, CH₃NH₃ ⁺

M is selected from Ge²⁺, Sn²⁺, Pb²⁺

X is selected from CI^", Br, l^~

9. A rechargeable electrochemical cell according to claim 8 wherein, in the anode active material, inorganic monolayers of corner-shared [ΜΧβ]^4" octahedra are confined between interdigitating bilayers of organic cations.

10. A rechargeable electrochemical cell according to any one of claims 1 to 9 wherein the anode active material further comprises an electron transport material.

1 1. A rechargeable electrochemical cell according to any one of claims 1 to 10 wherein the anode active material is operable during discharging to incorporate guest ions into the anode active material by one or more of intercalation, alloying or conversion.

12. A method for operating a rechargeable electrochemical cell, the cell comprising: an anode;

a cathode;

wherein the anode includes an anode active material, the anode active material comprising a perovskite structured material,

the method including the steps:

13. A method according to claim 12, further including the step of charging the cell by applying an external electrical potential difference across the cell.

14. A method according to claim 12 or claim 13 wherein the cell is only discharged partially before a subsequent charging, the cell being discharged to an open cell voltage of not less than 0.5V.

15. A method of manufacturing a rechargeable electrochemical cell according to any one of claims 1 to 1 1 wherein the anode active material is formed as a layer using solution processing.

16. A method according to claim 15 wherein, the anode active material is formed as a layer and during the formation of the anode active material layer, the layer is not subjected to a temperature greater than 100^°C.