Anion Reactivity in Cation‐Disordered Rocksalt Cathode Materials: The Influence of Fluorine Substitution

Abstract

The demand for high energy‐density, mass‐producible cathode materials has spurred the exploration of new material structures and compositions. Lithium‐excess, cation‐disordered rocksalt (DRX) materials are a new class of transition metal oxides that display high capacity and environmental friendly composition. These materials achieve their high capacities partially through oxygen redox, which leads to oxygen loss and detrimental reactivity with the electrolyte. It has previously been shown that oxygen loss can be suppressed by partial substitution of the lattice oxygen for fluorine, but the explicit mechanism behind this effect remains unknown. In this work, differential electrochemical mass spectrometry (DEMS) and titration mass spectrometry are used to quantify the primary electrochemical reactions occurring during the first cycle in DRX materials. Comparing a DRX oxide and a DRX oxyfluoride, it is shown that fluorination limits oxygen redox and suppresses oxygen loss. Additionally, DEMS is coupled with fluoride‐scavenging to demonstrate that small amounts of fluorine dissolve from DRX oxyfluorides during the first cycle. Finally, these techniques are extended over the first several cycles, demonstrating that CO2 evolution persists and fluoride dissolution continues to a diminishing extent during the first few cycles. These findings motivate surface modifications to control interfacial reactivity and improve long‐term cycling.

Fluorination‐Enhanced Surface Stability of Cation‐Disordered Rocksalt Cathodes for Li‐Ion Batteries

“…The most dramatic difference between these two materials was the strong near-surface degradation in LTMO, which is correlated to the significant O loss. [20,21,42] In fact, our EELS measurements indicate that the thickness of the O-deficient surface layer in LTMO (≈14 nm) (Figure 2d) after 50 cycles was much larger than that in LTMOF (≈8 nm) (Figure 2h), confirming that much more severe O loss occurred in LTMO. Strong O loss from Li-excess materials is usually triggered by excess usage of O redox.…”

Experimental considerations to study Li-excess disordered rock salt cathode materials

“…The low coulombic efficiencies can come from different factors, such as electrolyte decomposition, 45 surface species decomposition, 36,46 and gassing. 31,47 For disordered rock salt materials, O2 gas generation has been shown in the literature to occur mainly during the rst cycle, while CO2 gas generation can also happen during further cycles. 47 XPS was carried out to understand the changes in chemical composition of the CEI layer with the choice of cathode materials and electrolytes.…”

Exposure History and its Effect Towards Stabilizing Li Exchange Across Disordered Rock Salt Interfaces

“…No O 2 evolution is observed from LMNOF during the first charge (up to 4.8 V) and subsequent discharge. We know from previously published literature [45] that pristine LMNOF does not release any O 2 at potentials lower than 4.4 V but an onset of O 2 gas is observed starting at 4.45 V and continues as the cell is charged to 4.8 V. It should be noted, that the amount of total oxygen released in Mn-based DRX materials is already significantly lower when compared to Li-rich layered oxides. [49] The lack of O 2 gas release from the pre-treated sample illustrated in Figure 8 indicates that surface exposure to electrolyte tends to stabilize the oxygen based redox reaction.…”

“…For the pristine LMNOF a smooth sloped curve indicative of a single phase reaction is observed from 3 À 4:25 V vs: Li, corresponding to the removal of 0.625 moles of Li from the structure as shown previously by Chen et al [22] The second plateau beyond 4.25 V vs. Li is ascribed to an anionic oxygen based redox reaction. [45] The first charge results a total capacity of 330 mAh g À 1 . The theoretical capacity of the material based on full Li extraction from the DRX structure is approximately 347 mAh g À 1 .…”

“…LMNOF was synthesized based on a solid state high temperature route previously detailed. [20,25,45] Stoichiometric amounts of lithium carbonate (Li 2 CO 3 , 99 %, Sigma Aldrich), manganese (III) oxide (Sigma 98 % pure), niobium(V) oxide (99 %, Sigma Aldrich) and lithium fluoride (Sigma 99 % pure) were used as precursors. In order to adjust for lithium sublimation at high temperatures 10 % Li excess was used.…”