A water softening device includes a container configured to contain water, first and second electrodes arranged in fluid communication with the water, and a power source. The first electrode includes a conversion material that has a first composition and a second composition coexisting with the first composition. The first composition includes calcium ions bonded thereto and the second composition includes sodium ions bonded thereto. The power source supplies current in a first operating state such that the second composition exchanges sodium ions for calcium ions in the water to generate a soft water solution. The first and second electrodes are connected in a second operating state such that the first composition exchanges calcium ions for sodium ions in the water to generate a wastewater solution. The conversion material undergoes a reversible conversion reaction to convert between the first and second compositions within the water stability window.