A cheap, safe, and effective potassium-ion battery system with promising characteristics has been described in a Nature Energy article by Yaxiang Lu, Yong-Sheng Hu and co-workers in Beijing, bringing the renewable energy grids of the future closer to the realization.
Green energy storage
One of the obstacles to total green energy reliance is that many sources of carbon-neutral energy, such as solar and wind power, are unpredictable and intermittent. A possible solution is to build energy storage facilities that can charge up while excess energy is being generated, then discharge when demand overtakes supply.
Battery-based grid-storage facilities using a range of battery types have already been built for this reason, but there is still a need to develop an economical, safe, and long-term solution. Li-ion systems such as the 129 MWh Hornsdale power reserve are the current state of the art, benefiting from high energy density of Li-ion batteries. Na-S cells are a close competitor (e.g. the 300 MWh Buzen substation), having excellent energy density and low cost. Both of these battery types use highly flammable parts which increases their cost due to safety considerations. The main goal is optimizing the cost per MWh over the whole lifetime of the battery, for which non-flammable aqueous battery systems are a tempting prospect. They may also be manufactured more cheaply on larger scales than other batteries, which require rigorously dry conditions.
The cathodes in the cells Lu and Hu et al. demonstrated are more stable than in other systems, retaining 90% of their energy storage capacity after 10,000 cycles. One reason for their stability is that part of the distortion prone manganese in the Prussian Blue (KxFeyMn1 − y[Fe(CN)6]) material is substituted with iron. Another reason is the use of an electrolyte containing more potassium salt than water, to inhibit the dissolution of the cathode material over its long life. The anode used in the cells, an organic paint pigment (PTCDI), also has the potential to be manufactured cheaply.
Beyond the lithium-ion battery
The most remarkable feature of the cells was their tolerance of high rates of charging and discharging, comparable to Li-ion battery performance, without losing much of their capacity. Although the overall energy density of the cells is moderate, due to their comparatively low voltage of 1.3 V, there is potential for optimization in this system, both in increasing the voltage by adjusting the cathode metal and anode composition, and by lowering material costs to produce a very low cost per MWh system.