Exploring Calcium Manganese Oxide as a Promising Cathode Material for Calcium-Ion Batteries

The dependence on lithium for the energy needs of the world, coupled with its scarcity, has prompted the exploration of postlithium alternatives. Calcium-ion batteries are one such possible alternative owing to their high energy density, similar reduction potential, and naturally higher abundance. A critical gap in calcium-ion batteries is the lack of suitable cathodes for intercalating calcium at high voltages and capacities while also maintaining structural stability. Transition metal oxide postspinels have been identified as having crystal structures that can provide low migration barriers, high voltages, and facile transport pathways for calcium ions and thus can serve as cathodes for calcium-ion batteries. However, experimental validation of transition metal oxide postspinel compounds for calcium ion conduction remains unexplored. In this work, calcium manganese oxide (CaMn2O4) in the postspinel phase is explored as an intercalation cathode for calcium-ion batteries. CaMn2O4 is first synthesized via solid-state synthesis, and the phase is verified with X-ray diffraction (XRD). The redox activity of the cathode is investigated with cyclic voltammetry (CV) and galvanostatic (GS) cycling, identifying oxidation potentials at 0.2 and 0.5 V and a broad insertion potential at −1.5 V. CaMn2O4 can cycle at a capacity of 52 mAh/g at a rate of C/33, and calcium cycling is verified with energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) and modeled with density functional theory (DFT) simulations. The results from the investigation concluded that CaMn2O4 is a promising cathode for calcium-ion batteries.

Figure S1a shows the cyclic voltammetry (CV) of a three-electrode system with 50 mM ferrocene dissolved in Ca(TFSI) 2 in DME.The working electrode was platinum while the counter and reference electrodes were activated carbon and Ag/Ag + , respectively.The ferrocene potential was calculated using equation ( 1 were the cathodic and anodic potentials, respectively.The results from the CV yielded a redox potential at 0.065 V.The value of the ferrocene reaction against the standard hydrogen electrode is 0.4 V. Using the results of the ferrocene CV and the established 0.4 V (vs.SHE) redox reaction, the potential of the Ag/Ag + reference electrode was adjusted from its standard reduction potential of 0.8 V to 1.135 V.The calibration also establishes that the Ca/Ca 2+ redox reaction would occur at -3.205 V vs Ag/Ag + .Following the calibration of the Ag/Ag + reference electrode, an open circuit voltage (OCV) measurement of the activated carbon was performed and found the potential to be -0.187V (vs Ag/Ag+).(Figure S1b) Using the OCV, the voltage of the activated carbon would be 3.018 V (vs Ca/Ca 2+ ).Values from the ferrocene CV and activated carbon OCV are in close agreement with previous studies on reference electrode calibrations. [1,2]After the DFT calculations were completed, structure files were generated using VASP.These files were opened with VESTA [3] and XRD patterns were generated using powder diffraction pattern functionality.The simulation incident light has wavelength of 1.54059 Å (X-ray) and the XRD plot was generated based on atomic positions.
The method for simulating the XRD diffraction pattern required the use of eq. 2. After performing Fourier transformation of the structural information, the phase of incident light and the structure were combined.The intensity of reflected light can be calculated using structure factor, , where is the atomic position, is the atomic form factor, and is the scattering vector.A plot     of intensity with respect to angle was generated.The results for the theoretical XRD patterns 2 from CaMn 2 O 4 at 0% and 25% decalciation are outlined in Figure S10b.The generated lattice parameters, coordinates and theoretical XRD patterns from DFT calculations are summarized in

Figure S1 -
Figure S1 -Calibration of reference electrodes a) CV test of Ag/Ag + reference electrode (0.01 M AgNO 3 in 0.5 M Ca(TFSI) 2 DME) with 50 mM ferrocene dissolved in 0.5 M Ca(TFSI) 2 in DME with Pt working electrode and AC counter electrode b) OCV of the activated carbon is -0.187V vs. Ag/Ag +

Figure S2 -
Figure S2 -Linear Stability Window of Ca(TFSI) 2 in DME using 316 stainless steel blocking and calcium nonblocking electrode at 0.5 mV/s

Figure S5 -
Figure S5 -Nyquist plots of impedance for CaMn 2 O 4 after charge and discharge cycling

Figure S6 -
Figure S6 -DFT Calculations of CaMn 2 O 4 a) Voltage profile of CaMn 2 O 4 as calcium is removed.Voltage values were calculated with stable concentrations of calcium in CaMn 2 O 4 with the convex hull.b) Convex hull of CaMn 2 O 4 .Three concentrations of calcium were on the hull and connected by the line.

Table S2 -
Atomic x, y, z coordinates and occupancy of Ca 1-x Mn 2 O 4 deinsertion with x = 0 (a)

Table S3 -
Atomic x, y, z coordinates and occupancy of Ca 1-x Mn 2 O 4 deinsertion with x = 0.25 (b)

Table S5 -
Atomic x, y, z coordinates and occupancy of Ca 1-x Mn 2 O 4 deinsertion with x = 0.5 (d)

Table S6 -
Atomic x, y, z coordinates and occupancy of Ca 1-x Mn 2 O 4 deinsertion with x = 0.5 (e)

Table S8 -
Atomic x, y, z coordinates and occupancy of Ca 1-x Mn 2 O 4 deinsertion with x = 1.0 (g)

Table S10 -
Refined parameters for Mn 3 O 4