Abstract: The adoption of variable renewable energy generation based on solar and wind power is rapidly growing. Together, these sources are projected to provide up to 10% of global energy demand by 2023.1International Energy AgencyRenewables.https://www.iea.org/renewables2018/Date: 2018Google Scholar Wind and solar provide intermittent energy,2Sovacool B.K. The intermittency of wind, solar, and renewable electricity generators: Technical barrier or rhetorical excuse?.Util. Policy. 2009; 17: 288-296Crossref Scopus (158) Google Scholar subject to the Earth’s day and night cycles, weather patterns, and other environmental conditions. To sustain and accelerate this growth of intermittent energy production, electricity systems will require increased flexibility, including the potential to store power over long durations of time. Economical long-term energy storage for stationary applications is a pivotal missing element toward enabling a predominantly renewable energy powered future society. Existing long-duration energy storage has historically relied on pumped hydro. Here, large amounts of water are pumped uphill and released during times of increased demand on the electric grid, powering downstream turbines. It represents the largest installed capacity of stationary electricity storage today, comprising over 95% of the entire operational market.3International Renewable Energy AgencyElectricity storage and renewables: costs and markets to 2030.https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2017/Oct/IRENA_Electricity_Storage_Costs_2017.pdfDate: 2018Google Scholar Despite its high round-trip efficiency (of order 70%), the adoption and implementation of pumped hydro must cater to geographic constraints, of which there is limited room for additional deployment or further reductions in cost. New stationary energy storage solutions that can be deployed economically at scale are needed to aid the growth of renewables. The global energy storage market anticipates rapid growth in the coming years, with value estimates of $7 billion per year by 2025 to beyond $26 billion annually by 2022.4Deloitte Center for Energy SolutionsSupercharged: Challenges and opportunities in global battery storage markets.https://www2.deloitte.com/content/dam/Deloitte/bg/Documents/energy-resources/gx-er-challenges-opportunities-global-battery-storage-markets.pdfDate: 2018Google Scholar Li-ion batteries, which are already having a disruptive impact in the electrification of the transport sector, are further a nascent application in stationary energy storage. Figure 1 shows the increase in deployed electrochemical energy storage capacity overlaid with the recent cost reductions in Li-ion batteries. The prices of Li-ion batteries decreased over 80% from 2010 to 2018 while the broader technological deployment continues to increase exponentially. Li-ion batteries face unique challenges for stationary storage. The installed cost is higher due to less present demand and more complicated and variable output requirements in need of additional management systems. For stationary applications, developers paid near $300/kWh in 2017, a 51% increase over the average for electric vehicle producers.4Deloitte Center for Energy SolutionsSupercharged: Challenges and opportunities in global battery storage markets.https://www2.deloitte.com/content/dam/Deloitte/bg/Documents/energy-resources/gx-er-challenges-opportunities-global-battery-storage-markets.pdfDate: 2018Google Scholar Already traversing down a significant experience curve, assisted by its dominance and ubiquity in portable/mobile energy storage, it is predicted that the total installed cost of Li-ion batteries for stationary applications could decrease by an additional 54%–61% by 2030.3International Renewable Energy AgencyElectricity storage and renewables: costs and markets to 2030.https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2017/Oct/IRENA_Electricity_Storage_Costs_2017.pdfDate: 2018Google Scholar There is an invariable delay between rapid technological progress and the implementation of policy that can accelerate its deployment. Stakeholders must rely on predictive models—not today’s status quo, which is changing on an almost monthly basis—about the future cost of energy storage to make informed decisions. There is therefore a pressing need for improved forecasting models that accurately predict where a technology will be in the coming years to decades. A recent article from Schmidt et al. reports on the development of a levelized cost of storage model to evaluate 9 competing stationary storage technologies across 12 power system applications (https://doi.org/10.1016/j.joule.2018.12.008). Their model, which is free to explore on a website built by the researchers (www.EnergyStorage.ninja), predicts that Li-ion batteries will be the most economical form of stationary energy storage for nearly all end applications by 2030. The growing cost reductions and overall utility of Li-ion batteries led to numerous recent large-scale investments from industry leaders. French multinational automotive manufacturer Groupe Renault recently announced the launch of their “Advanced Battery Storage” program that plans to develop the largest stationary energy storage plant (70 MW) based on Li-ion electric vehicle batteries in Europe by 2020. The World Bank Group has now committed $1 billion USD for stationary battery storage in developing and middle-income countries, which are also some of the most important markets for new photovoltaic installations.5The World BankWorld Bank Group Commits $1 Billion for Battery Storage to Ramp Up Renewable Energy Globally.https://www.worldbank.org/en/news/press-release/2018/09/26/world-bank-group-commits-1-billion-for-battery-storage-to-ramp-up-renewable-energy-globallyDate: 2018Google Scholar Battery vendors such as Samsung, LG Chem, Panasonic, Toshiba, GE, Tesla, and BYD are ramping up production for stationary applications, each not wanting to be left behind by their competitors in this emerging market. While research into improving Li-ion batteries continues, such as through using high-capacity silicon anodes to improve cells’ volumetric energy density,6Chae S. Ko M. Kim K. Ahn K. Cho J. Confronting issues of the practical implementation of Si anode in high-energy lithium-ion batteries.Joule. 2017; 1: 47-60Abstract Full Text Full Text PDF Scopus (227) Google Scholar there are considerable efforts to develop “beyond Li-ion” storage solutions. These include batteries based on abundant and low-cost sodium,7Chen L. Fiore M. Wang J.E. Ruffo R. Kim D.-K. Longoni G. Readiness level of sodium-ion battery technology: a materials review.Adv. Sustain. Syst. 2018; 2: 1700153Crossref Scopus (109) Google Scholar or high-energy-density magnesium (https://doi.org/10.1016/j.joule.2018.10.028). Specifically for stationary applications, redox flow batteries are making strides toward long-term stability and lower capital costs. Recently, Liu and colleagues demonstrated an aqueous flow battery based on an ammonium ferrocyanide catholyte with nearly 100% capacity at 1,000 cycles (https://doi.org/10.1016/j.joule.2018.10.010), surpassing the projected lifetime of state-of-art quinone flow batteries.8Kwabi D.G. Lin K. Ji Y. Kerr E.F. Goulet M.-A. De Porcellinis D. Tabor D.P. Pollack D.A. Aspuru-Guzik A. Gordon R.G. Aziz M.J. Alkaline quinone flow battery with long lifetime at pH 12.Joule. 2018; 2: 1894-1906Abstract Full Text Full Text PDF Scopus (221) Google Scholar Much like new contenders in the photovoltaic market are fighting against the experience curve of silicon, new approaches in electrochemical energy storage face a scalable, stable, and increasingly economical competitor in Li-ion batteries. Policymakers and investors must remain mindful of the dynamic, non-linear growth of Li-ion batteries for all applications, including stationary systems. The same can be said for researchers working on storage solutions based on hydrogen, redox flow, sodium, magnesium, CO2, and other competing technologies. The applied commercial goals are not static and, consequently, research objectives are a moving target. Still, it is sometimes the uphill battles that are most worth fighting, provided one can see the top of battery mountain through the dispersing fog.