Local Structure and Dynamics in Solvent‐Free Molten Salt Ca2+$\left(\text{Ca}\right)^{2 +}$‐Electrolytes

Calcium batteries (CaBs) fundamentally offer a promise of sustainable high energy density storage. However, the development of functional CaB electrolytes remains a key challenge. Here, molecular simulations are used to investigate structural and dynamic properties of solvent-free molten salt electrolytes (MSEs) containing <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow> <mml:msup><mml:mrow><mml:mtext>Ca</mml:mtext></mml:mrow> <mml:mrow><mml:mn>2</mml:mn> <mml:mo>+</mml:mo></mml:mrow> </mml:msup> </mml:mrow> <mml:annotation>$\left(\text{Ca}\right)^{2 &#x00026;amp;amp;amp;amp;amp;amp;plus;}$</mml:annotation></mml:semantics> </mml:math> and alkali cations ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow> <mml:msup><mml:mrow><mml:mtext>Li</mml:mtext></mml:mrow> <mml:mo>+</mml:mo></mml:msup> </mml:mrow> <mml:annotation>$\left(\text{Li}\right)^{&#x00026;amp;amp;amp;amp;amp;amp;plus;}$</mml:annotation></mml:semantics> </mml:math> , <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow> <mml:msup><mml:mrow><mml:mtext>Na</mml:mtext></mml:mrow> <mml:mo>+</mml:mo></mml:msup> </mml:mrow> <mml:annotation>$\left(\text{Na}\right)^{&#x00026;amp;amp;amp;amp;amp;amp;plus;}$</mml:annotation></mml:semantics> </mml:math> , <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:msup><mml:mi>K</mml:mi> <mml:mo>+</mml:mo></mml:msup> </mml:mrow> <mml:annotation>$\left(\text{K}\right)^{&#x00026;amp;amp;amp;amp;amp;amp;plus;}$</mml:annotation></mml:semantics> </mml:math> ), paired with either FSI or TFSI anions. Two equimolar MSEs, [Li, Na, K, Ca]FSI and [Li, Na, K, Ca] TFSI, are examined across a range of temperatures to better understand cation-anion interactions, coordination and local structure, and ion mobility, in particular with respect to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow> <mml:msup><mml:mrow><mml:mtext>Ca</mml:mtext></mml:mrow> <mml:mrow><mml:mn>2</mml:mn> <mml:mo>+</mml:mo></mml:mrow> </mml:msup> </mml:mrow> <mml:annotation>$\left(\text{Ca}\right)^{2 &#x00026;amp;amp;amp;amp;amp;amp;plus;}$</mml:annotation></mml:semantics> </mml:math> . The interplay between cation charge density, anion structure, and thermal effects provides valuable insights into the MSEs' macroscopic behavior. These insights inform the design of advanced electrolytes that enhance <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow> <mml:msup><mml:mrow><mml:mtext>Ca</mml:mtext></mml:mrow> <mml:mrow><mml:mn>2</mml:mn> <mml:mo>+</mml:mo></mml:mrow> </mml:msup> </mml:mrow> <mml:annotation>$\left(\text{Ca}\right)^{2 &#x00026;amp;amp;amp;amp;amp;amp;plus;}$</mml:annotation></mml:semantics> </mml:math> mobility, supporting the development of next-generation CaBs.