New Compounds for Li-Ion Battery Cathode Materials Based on Phosphite Chemistry

Polyanion compounds of transition metals are currently acquiring interests as candidate materials for next generation Li-ion cells positive electrodes for both mobile and stationary applications. This is mainly due to the inherently higher safety of these materials along with the possibility to use a wider range of transition metals, including iron, which is cheap and abundant and is conducive for large scale applications. Moreover, the possibility to implement different polyanionic moieties in the formula brings about interesting opportunities for manipulation of cathode crystal engineering and its lithium insertion potential. In this regard different polyanions including PO₄^3-, SO₄^2-, SiO₄^4-, BO₃^3-, etc. have been examined either individually or in combination and the effect of the polyanion moiety on cell performance and voltage has been demonstrated (1). However, no systematic work has been done to date to study the electrochemical properties of cathode electrodes based on phosphite (HPO₃) moiety, despite the vast occurrence of phosphite compounds.

In this work, we focus on two new compounds that we have synthesized successfully and demonstrated their electrochemical performance toward reversible de(intercalation) of Li ions and the effect of secondary anion and crystal geometry on the cell voltage.

The first compound is lithium iron(II) phosphite chloride with the formula Li₃Fe₂(HPO₃)₃Cl. The crystal structure has been solved in Pnma space group and composed of dimers of FeO₅Cl octahedral units along the b-axis of the crystal, where these polyhedral units are bridged together via pseudo-tetrahedral HPO₃ groups. This arrangement creates channels in the b-axis of the crystal were Li⁺ ions can diffuse back and forth upon charge and discharge. In a variation, we have substituted 25% of the Fe sites in the crystals with Mn and its effect on the cell voltage is under investigation. The two Fe(II) atoms in the formula can be oxidized to Fe(III), giving rise to a theoretical capacity of 123 mAh.g^-1. Figure 1 depicts the first discharge curve of the compound in red as well as the one doped with manganese shown in green, emphasizing the initial high discharge voltage of the cell, though about 50% of the theoretical capacity can be achieved.

The second compound in this series is LiFe(HPO₃)₂. The crystal structure has been solved in I-42d space group and composed of FeO₆ and HPO₃ polyhedral chains connected in 3D. The aforementioned arrangement creates large channels along all 3 crystallographic directions with the Li⁺ ions located in channels along the a-axis. The Fe(III) in the formula unit can undergo reductive insertion reversibly and this leads to a theoretical capacity of 120 mAh.g^-1. The first discharge curve of this compound is shown in blue in Figure 1, demonstrating that the voltage is lower than the previous compound by almost 0.5 V, depicting the inductive effect of the chloride and the iron polyhedral connectivity on the cell voltage. For this compound the achievable capacity at this point is limited to almost 30% of the theoretical value, but higher capacities are anticipated by further optimization of cathode preparation procedure.

REFERENCES
1. C. Masquelier and L. Croguennec, Chem. Rev., 113, 6552 (2013).