In the search for new positive-electrode materials for lithium-ion batteries, recent research has focused on nanostructured lithium transition-metal phosphates that exhibit desirable properties such as high energy storage capacity combined with electrochemical stability. Only one member of this class--the olivine LiFePO(4) (ref. 3)--has risen to prominence so far, owing to its other characteristics, which include low cost, low environmental impact and safety. These are critical for large-capacity systems such as plug-in hybrid electric vehicles. Nonetheless, olivine has some inherent shortcomings, including one-dimensional lithium-ion transport and a two-phase redox reaction that together limit the mobility of the phase boundary. Thus, nanocrystallites are key to enable fast rate behaviour. It has also been suggested that the long-term economic viability of large-scale Li-ion energy storage systems could be ultimately limited by global lithium reserves, although this remains speculative at present. (Current proven world reserves should be sufficient for the hybrid electric vehicle market, although plug-in hybrid electric vehicle and electric vehicle expansion would put considerable strain on resources and hence cost effectiveness.) Here, we report on a sodium/lithium iron phosphate, A(2)FePO(4)F (A=Na, Li), that could serve as a cathode in either Li-ion or Na-ion cells. Furthermore, it possesses facile two-dimensional pathways for Li+ transport, and the structural changes on reduction-oxidation are minimal. This results in a volume change of only 3.7% that--unlike the olivine--contributes to the absence of distinct two-phase behaviour during redox, and a reversible capacity that is 85% of theoretical.

Redox flow batteries (RFBs) are particularly well suited for storing the intermittent excess supply of renewable electricity, so-called "junk" electricity. Conventional RFBs are charged and discharged electrochemically, with electricity stored as chemical energy in the electrolytes. In the RFB system reported here, the electrolytes are conventionally charged but are then chemically discharged over catalytic beds in separate external circuits. The catalytic reaction of particular interest generates hydrogen gas as secondary energy storage. For demonstration, indirect water electrolysis was performed generating hydrogen and oxygen in separate catalytic reactions. The electrolyte containing V(II) was chemically discharged through proton reduction to hydrogen on a molybdenum carbide catalyst, whereas the electrolyte comprising Ce(IV) was similarly discharged in the oxidation of water to oxygen on a ruthenium dioxide catalyst. This approach is designed to complement electrochemical energy storage and may circumvent the low energy density of RFBs especially as hydrogen can be produced continuously whilst the RFB is charging. Broader contextRenewable energy technologies have evolved to deliver hundreds of terawatt hours of electricity, yet without its direct utilization in the grid part of that energy could be lost. In order to establish a thriving renewable energy economy it is of paramount importance that intermediate energy storage systems be developed. Mediating electricity production and usage will overcome the issues relating to intermittency, which presently limits widespread dependence on wind and photovoltaic power. Various approaches are under development, but no single approach is liable to address the issue as a whole. Combining technologies and hybridizing storage systems to adapt to a multifaceted energy future is the more viable option. This paper discusses one such hybrid system, in which electrochemical energy storage is combined with renewable hydrogen production, delivering a dual platform for energy storage as an electrochemical and chemical medium. † Electronic supplementary information (ESI) available: Charging and discharging curves, cyclic voltammetry, UV-vis spectra of V(II) and V(III), a picture of the cell, and details on the kinetics measurement and calculations. See

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tation currently accounts for 23% of global energy-related CO 2 emissions. [1] Electric vehicles (EVs) thus represent a rapidly expanding market, with at least 20% of road vehicles estimated to be electrically powered by 2030. [1] LIB technology takes great prominence within the automobile industry, due to its unbeatable electrochemical performance and lightweight, portable nature. Its impressive performance can be attributed, in part, to the low weight and small ionic radius of the Li + ions (0.76 Å), allowing fast ion transport. This fast transport, along with its low reduction potential (-3.04 V vs standard hydrogen electrode (SHE)), [2] allows for high power density as well as volumetric and gravimetric capacity. Such properties are of critical importance for EVs. [3] With the increased demand for high energy density LIBs for EVs, comes reductions in battery cost and subsequent volatility in material supply. In light of the immense scale of transport electrification that is being proposed in order to meet CO 2 emission targets, considerable attention is being directed toward the socioenvironmental and economic impact of such an increase in material demand. Of particular focus are lithium-ion cathode materials, many of which are composed of lithium (Li), nickel (Ni), manganese (Mn), and cobalt (Co), in varying concentrations (Figure 1a). The cathode constitutes more than 20% of LIB's overall cost and is a key factor in determining the energy and power density of the battery (Figure 1b). [3,4] It is, therefore, vital to maximise the cathode's performance while minimizing its cost, to make EVs more accessible for society.The high cost of cathode materials is largely attributed to the presence of cobalt-a rare and expensive element mined primarily in the Democratic Republic of the Congo (DRC)-which has been deemed necessary in the past to deliver high energy densities in LIBs. For example, the active material within the commercial NMC111 cathode (LiNi 0.33 Mn 0.33 Co 0.33 O 2 ) costs ca. £17 kg −1 , producing 3.88 kWh kg −1 . [5] This high cost is largely attributed to the relatively large amount of cobalt within the electrode (£ 25 kg −1 ). [6] This cost is over 350 times greater than that of iron (£0.068 kg −1 ), [7] which reflects its relative high natural abundance. A combination of political instability within the DRC, social impacts within the mining sector, and supply chain volatility and ambiguity have driven a decrease in cobalt content in NMC cathodes (e.g., going from NMC 111 to NMC811 (LiNi 0.8 Mn 0.1 Co 0.1 O 2 )) and zero-cobalt alternatives such as LiNi 0.5 Mn 1.5 O 4 spinels, LiMO 2 disordered rocksalts and Electric vehicles powered by lithium-ion batteries are viewed as a vital green technology required to meet CO 2 emission targets as part of a global effort to tackle climate change. Positive electrode (cathode) materials within such batteries are rich in critical metals-particularly lithium, cobalt, and nickel. The large-scale mining of such metals, to meet increasing battery demands, poses con...