Improved Cycling Performance of Li‐Excess Cation‐Disordered Cathode Materials upon Fluorine Substitution

Abstract

research on lithium-ion battery cathodes has been largely dominated by layered rock salt materials in the Li x (Ni-Mn-Co-Al) 2−x O 2 (NMCA) compositional space, [3,4] in which redox activity is limited to Co and Ni. Cobalt in particular is expensive and relatively scarce compared to other 3d transition metals, such as Fe or Mn. [1,3,5] The fact that the cathode structure has to be layered and remain layered upon cycling greatly restricts the changes which can be made to NMCA-type rock salt chemistries.Recent progress in the development of Li percolation theory for rock salt compounds, in which Li transport still takes place even when the cations are disordered, has greatly enlarged the design space for cathode materials. [6,7] Lifting the requirement that cations form an ordered (layered) structure enables the use of various transition metal (TM) redox centers, including Mn 3+ /Mn 4+ , [8,9] Mn 2+ /Mn 4+ , [5,10] Cr 3+ /Cr 5+ , [6,11] Mo 3+ /Mo 6+ , [12] and V 3+ /V 5+ . [11,13] Because these compounds need Li excess to achieve Li percolation, [6,7] they typically also contain high valent charge compensators, such as Nb 5+ , [8,9] Sb 5+ , [14] Mo 6+ , [15,16] and Ti 4+ . [16][17][18] In addition, fluorine substitution is facile inThe recent discovery of Li-excess cation-disordered rock salt cathodes has greatly enlarged the design space of Li-ion cathode materials. Evidence of facile lattice fluorine substitution for oxygen has further provided an important strategy to enhance the cycling performance of this class of materials. Here, a group of Mn 3+ -Nb 5+ -based cation-disordered oxyfluorides, Li 1.2 Mn 3+ 0.6+0.5x Nb 5+ 0.2−0.5x O 2−x F x (x = 0, 0.05, 0.1, 0.15, 0.2) is investigated and it is found that fluorination improves capacity retention in a very significant way. Combining spectroscopic methods and ab initio calculations, it is demonstrated that the increased transition-metal redox (Mn 3+ /Mn 4+ ) capacity that can be accommodated upon fluorination reduces reliance on oxygen redox and leads to less oxygen loss, as evidenced by differential electrochemical mass spectroscopy measurements. Furthermore, it is found that fluorine substitution also decreases the Mn 3+ -induced Jahn-Teller distortion, leading to an orbital rearrangement that further increases the contribution of Mn-redox capacity to the overall capacity.

Effect of Fluorination on Lithium Transport and Short‐Range Order in Disordered‐Rocksalt‐Type Lithium‐Ion Battery Cathodes

“…Previous reports have established that most DRX systems can achieve at least 5% of fluorination by solid state synthesis, a value that is within the solubility limits predicted by our calculations in Table . Therefore, we first evaluate the impact of 5% F‐incorporation at 1273 K by comparing the 0‐TM percolation in Li 1.2 M x 1 M' x 2 O 1.9 F 0.1 and Li 1.2 M x 1 M' x 2 O 2 for the seven (M, M') pairs.…”

“…The phase diagram using the qualitative pair‐interaction model of Section 2.1 is shown in Figures S3 and S4 (Supporting Information) and defines the temperature and F‐contents achievable. We chose 7.5% F substitution which was previously estimated to be the thermal F solubility limit at 1273 K in several DRX materials . The percolation map in Figure shows the predicted 0‐TM capacity as function of temperature and Li‐excess.…”

“…Such a system with coupled disorder on multiple sublattices can be well studied with the coupled cluster‐expansion approach . The cluster‐expansion model has been demonstrated to be an effective method to capture long range order in intercalation cathodes as well as SRO in DRX systems . In a cluster expansion, the configurational energy dependence is captured by an expansion into different cluster functions, which can be formulated as …”

“…One aspect of these materials that has been enormously beneficial to their energy density and cycling stability is their ability to incorporate fluorine . Fluorine lowers the average cation valence enabling a higher fraction of the capacity to be derived from transition metal redox, rather than oxygen redox, which has been shown to be highly beneficial to cycle life of DRX compounds . In addition, it may have stabilizing effects on the surface chemistry of these cathode materials as some highly fluorinated compositions can be charged to 5 V without any significant oxygen loss …”

Gassing Behavior of High‐Entropy Oxide Anode and Oxyfluoride Cathode Probed Using Differential Electrochemical Mass Spectrometry

“…They combine stable capacity retention with good energy density and efficiency, making them first choice for a variety of applications, such as in electric vehicles . Nevertheless, especially energy‐demanding applications necessitate the development of new low‐voltage anode and high‐voltage cathode materials with improved specific capacities (where energy storage is based on some form of battery storage, such as insertion, conversion or alloying reactions) . However, despite the possibility of achieving high specific capacities, currently used materials often fall short of capacity retention over cycling.…”