Sustainable battery materials and chemistries are required to complement the growing demand for renewable energy from solar farms, wind mills, and hydroelectric power stations which are characterized by either demand fluctuations or periodic supply interruptions. [1] To keep a balance between supply and demand at all times, the intermittent nature of renewables should be leveled using stationary batteries deploying abundant, inexpensive, and nontoxic materials. [2, 3] Current stationary

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... s rely heavily on expensive transition metals (e.g., nickelcadmium or nickel-metal hydride batteries, and vanadium redox flow batteries), toxic elements (e.g., nickelcadmium and lead-acid batteries), or require high temperature to operate (e.g., sodium-sulfur batteries operating at 300À350 C). [3, 4] In the interest of avoiding toxic and expensive minerals, there is a pressing need for sustainable battery materials that can provide comparable performance and cycle life. In this regard, the dual-ion battery (DIB) concept has emerged as a promising chemistry for future energy storage applications. In contrast to the "rocking chair" model in lithium-ion batteries, the energy storage mechanism in an archetype of a DIB is underpinned by the simultaneous intercalation of cations and anions from the electrolyte into, respectively, negative and positive electrodes containing graphite. [5, 6] In a lithium DIB, anion intercalation and extraction require an operating voltage window ranging from 3 to 5.2 V versus Li þ /Li with the extent of reversibility ultimately depending on the type of graphite, electrolyte-salt concentration, type of electrolyte solvent, type of anion in the electrolyte, working temperature, and amount of electrolyte in the cell. [7, 8] Reported gravimetric capacities for half-cells generally vary between 80 and 140 mAh g À1 with discharge voltages averaging 4.5 V. [6, 9] On the basis of these metrics, lithium metal DIBs can be optimized to deliver cell-level energy density and specific energy above 200 Wh L À1 and 100 Wh kg À1 , respectively, better than lead-acid batteries (50-80 Wh L À1 or 20-55 Wh kg À1 ) and comparable with Ni-metal hydride batteries (150-220 Wh L À1 or 50-70 Wh kg À1 ) or Na-S batteries (150-300 Wh L À1 or 80-150 Wh kg À1 ). [6, 10] Using graphite as the anion-hosting electrode, a wide selection of materials can be utilized in the negative electrode, which allows for increasingly diverse electrodeelectrolyte combinations in the design of DIBs. In addition, the fact that the negative and positive electrodes host different ions during cell operation relaxes the requirements on the type of electrolytes used. [13] These benefits are, in turn, anticipated to facilitate the transition to beyond Li-ion technologies harnessing more abundant Na þ -, K þ -, Ca 2þ -, or Al 3þ -based resources. [6, 14, 15] o maximize the energy density of KDIBs, the graphite cathode should ideally be paired with a K metal or graphite electrode. The use of K metal in KDIBs ensures unlimited supply of K þ to