Fluorine chemistry in lithium-ion and sodium-ion batteries
1Key Laboratory of Hydraulic Machinery Transients, Ministry of Education, School of Power and Mechanical Engineering, Wuhan University, Wuhan 430072, Hubei, China.
2Hubei Key Laboratory of Electrochemical Power Sources, College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, Hubei, China.
*Correspondence to: Prof./Dr. Zhongxue Chen, Key Laboratory of Hydraulic Machinery Transients, Ministry of Education, School of Power and Mechanical Engineering, Wuhan University, No. 299 Bayi Road, Wuchang District, Wuhan 430072, Hubei, China. E-mail:
As the peculiar element in the Periodic Table of Elements, fluorine gas owns the highest standard electrode potential of 2.87 V vs. F-, and a fluorine atom has the maximum electronegativity. Benefiting from the prominent property, fluorine plays an important role in the development of lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) in terms of cathode materials (transition metal fluorides, fluorinated polyanionic compounds), electrolytes, and interfaces. In cathode materials, the highly electronegative renders enhanced ionic character of transition metal fluorine bonds and correspondingly high working potential in electrolytes; fluorinated electrolytes possess good antioxidant ability and flame retardance, which can significantly improve the thermal safety of a battery. On an electrode-electrolyte interface, the fluorine-rich inorganic component (such as LiF and NaF) is essential for the formation of a robust and stable solid electrolyte interface on anodes. Despite the remarkable advances achieved in fluorinated cathodes, electrolytes, and interfaces, there is still a lack of comprehensive understanding of the function of fluorides in LIBs and SIBs. Accordingly, this review briefly summarized the recent progress of fluorine-based electrodes, electrolytes, and interfaces and highlighted the correlation between the composition, property, and function to reveal the fluorine chemistry in LIBs and SIBs. This review will provide guidance for the rational design and targeted regulation of fluorine-dominated high-performance electrode materials, functionalized electrolytes, and consolidated interfaces.
As major challenges facing humanity, climate change and exhaustion of fossil fuels today are facilitating the development of renewable resources such as wind and solar energy. However, the intermittent nature of these renewable energies could only generate electricity under favorable circumstances. In recent years, there has been a significant focus on the development of multiscale electrochemical energy storage systems (EESSs) based on rechargeable batteries. This approach aims to address the increasing energy demand while maximizing the utilization of sustainable energy sources.
Lithium/sodium ion batteries (LIBs/SIBs) consist of four parts: cathodes, anodes, separators, and electrolytes. Li+/Na+ ions are inserted/extracted between the cathode/anode by the electrolyte to store or release energy. Fluorine plays an important role in the electrode, electrolyte, and electrode-electrolyte interface (EEI) of LIBs and SIBs. As the most electronegative element, fluorine atoms have a strong tendency to accept electrons, which allows the introduction of fluorine into the electrode to bring the advantages of high voltage and high energy density to the battery. Fluorinated electrodes, such as CuF2 and CoF3 cathodes, were studied early in the development of lithium batteries. These materials adopting combined insertion-conversion type reactions were studied for space-oriented lithium batteries. Later, FeF3, a cheap and high-capacity cathode material, was further reported by Arai et al. The other fluorine-involved cathodes are fluorine-substituted polyanionic compounds. Vanadium fluorine phosphates, LiVPO4F and NaVPO4F, were first reported by Baker in 2003. This discovery paved the way for the exploration and development of various derivative materials for LIBs and SIBs, including LiFeSO4F, NaFeSO4F,
In addition to electrodes, it is well recognized that an electrolyte is an indispensable component in any electrochemical device. Numerous pieces of literature in recent years have also verified the pivotal role of fluorine in electrolytes. As to electrolytes, fluorinated electrolytes have two main merits in terms of high voltage stability and forming fluorinated solid electrolyte interphase (SEI). The former renders electrolytes able to be operated under high voltage, which backs up higher specific energy density. While the latter not only acts as excellent electronic insulators and good ion conductors but also presents high interfacial energy to the anode metal to suppress dendrite, for instance, LiF could promote Li+ migration along the interface and encourage growth of the deposited Li in a parallel direction.
The purpose of this review is to briefly summarize the unique roles of fluorine in battery cathode materials and electrolytes [Figure 1] to provide perspectives and strategies for future design of battery functional with long lifespan, high energy density, high power, and high safety under different application conditions.
The role of fluorine in cathode materials
The voltage trend of transition metal-based cathode materials roughly follows the formal charges of the central atoms in the structure, consisting of the idea of the inductive effect. F has the strongest electronegativity (3.98 on the Pauling scale), thus endowing the metal fluorine (M-F) bond with a high ionic character. The presence of strong M-F bonding stabilizes the antibonding Mn+/Mn+1 state through a M-F inductive effect to generate high operating voltage.
Transitional metal fluoride cathodes
Metal fluorides (denoted as MFn) are a class of cathode materials that have attracted extensive research interests. These materials have a large specific capacity due to the possibility of multiple electron transfer at a single redox center, such as FeF3, FeF2 to Fe0, In LIBs and SIBs, an overall chemical reaction equation is given as: nA+ +ne- + MFn → nAF + M (A denotes Na or Li, M denotes transition metals such as Fe, Cu, etc. if not specified, the same below). Specifically, both insertion and conversion reactions may happen during this progress [Figure 2A]. Simultaneously, metal fluorides typically provide a higher working potential compared to oxides and sulfides as a result of the high electronegativity of fluorine [Figure 2A]. To illustrate, in the case of LIBs, FeF3 boasts a theoretical potential of 2.74 V (vs. Li+/Li), while Fe2O3 displays a lower theoretical potential of 1.63 V (vs. Li+/Li). This highlights the superior performance of metal fluorides in terms of working potential.
Figure 2. (A) Discharge curves of FeF3 and Fe2O3, which show that metal fluorides have higher operation potential than oxides. This figure is quoted with permission from Lemoine et al.. (B) False-color composite image of the converted electrode showing LiF (red) and Cu (green) phases separate from each other. This figure is quoted with permission from Wang et al.. (C) Voltage profiles of
CuF2 was initially introduced as a cathode material for primary lithium batteries and possesses a high working potential of 3.55 V, which gives it a high energy density of 1,874 Wh kg-1. However, the lithium storage in CuF2 is irreversible: during the first lithiation process, copper will form large isolated particles when the conversion reaction occurs, which separates Cu from LiF and isolates the electron transport path
Wang et al. have proposed that iron-based metal fluoride forms a bicontinuous iron network during discharge, which provides a large LiF/Fe interface area and a highly conductive pathway for local electron transport, thus facilitating the conversion in subsequent cycles. In contrast to FeFx, the converted product from CuF2 consists of larger Cu nanoparticles isolated from the LiF phase, which is mainly responsible for the lack of reversibility in CuF2. To avoid the irreversibility issue of CuF2, various iron-based metal fluorides with much better reversibility have been explored. The ReO3-type FeF3 (Figure 3A, the first one) was tested for lithium storage in 1997 with a discharging capacity of 140 mAh g-1 for the first week in the voltage range of 4.5-2.5 V (vs. Li+/Li) and then exhibited a capacity of 80 mAh g-1 in subsequent cycles[3,19]. Until 2003,
Figure 3. (A) Crystal structures of FeF3, FeF2, FeF3·0.33H2O, and FeF3·0.5H2O. This figure is quoted with permission from Liu et al. (ReO3-type and HTB-type), Huang et al. (Rutile-type), and Rao et al. (Pyrochlore-type). (B) Reaction pathways for both discharging and charging of the FeF3 electrode and their crystal structures. These summaries are based on the empirical data and DFT calculation results, where Li, Fe, and F atoms are represented by green, blue, and red spheres. This figure is quoted with permission from Li et al.. (C) Performance during cycling of FeF2@CNT under different current densities, which shows cycling stability and rate performance of FeF2@CNT. This figure is quoted with permission from Zhou et al.. (D) Voltage vs. capacity profiles of FeF3·0.33H2O electrodes for the first ten cycles with a current density of 14 mA g-1. This figure is quoted with permission from Li et al.. (E and F) Voltage vs. capacity profiles of FeF3·0.5H2O cathodes wired by 5 wt % SWNTs at 0.1 C during the first few cycles (vs. Li/Na Anode, respectively). This figure is quoted with permission from Li et al..
Li+ +1 e- + FeIIIF3 → LiFeIIF3 (4.5-2.5 V vs. Li+/Li)
LiFeIIF3 + 2Li → 3LiF + Fe0 (2.5-1.5 V vs. Li+/Li)
Despite the fact that FeF3 exhibits attractive lithium storage properties, it shows severe polarization and sluggish kinetics during cycling. Considerable efforts have been undertaken to comprehend the mechanism and, hence, mitigate these issues. Through in-situ X-ray absorption spectroscopy, in-situ transmission electron microscopy, and density functional theory (DFT) calculations, Li et al. proposed a new reaction mechanism: FeF3 → Li0.25FeF3 ↔ LiF + FeF2 ↔ Fe + 3LiF [Figure 3B]. In this process, the uneven distribution of the electrochemically active phases brought about a serious ohmic voltage drop. To further, Hua et al. found that FeF3 undergoes a three-phase reaction upon Li+ insertion, generating a small amount of FeF2 through a replacement-like mechanism. During the deeper lithiation process, Fe3+ is gradually reduced to Fe2+ as Li+ is inserted, which is accompanied by the migration of Fe to form FeF2. The reaction mechanism is closely related to the size of FeF3 in this process: the FeF3 at the micrometer scale undergoes a process that involves a two-phase reaction, whereas FeF3 at the nanoscale displays a mechanism based on a solid solution, suggesting the irreversibility of FeF3 during charging and discharging may be mitigated by rationally designing FeF3 nanostructures and reducing the Fe migration distance. FeF3 nanoparticles (40-106 nm) were prepared in carbon nanocages (40-110 nm) by Yang et al. The results showed that the FeF3 nanoparticles provided an excellent reversible capacity of 410 mAh g-1 over 120 cycles at 100 mA g-1.
FeF2 has received attention as an intermediate product of the charge/discharge process of FeF3. In rutile-type FeF2, the three-dimensional (3D) network of FeF6 octahedra is described from trans-chains of edge-sharing octahedra along . The chains are further linked together by opposite vertices (Figure 3A, the second one)[24,25]. FeF2 also has an excellent theoretical specific capacity (571 mAh g-1), but it shows very poor cycling performance, with a severe decrease in capacity after only five cycles. Its poor cycling performance can be attributed to two main factors. Firstly, the severe volume change that FeF2 undergoes during the conversion reaction can cause instability. Secondly, FeF2 dissolves easily in the electrolyte, making it flake off from the current collector. This detachment can disrupt the operation and overall performance. It has been shown that the cycling performance of FeF2 in microporous/mesoporous carbon improves dramatically. Zhou et al. encapsulated FeF2 in carbon nanotubes (CNTs). The CNTs endure the stress brought on by the volume alteration during Li insertion to preserve the shape of the material and secure the contact between the material, the current collector, and the conductive carbon, which enhances the electrochemical performance of FeF2 [Figure 3C]. In order to reduce the dissolution of FeF2, Xiao et al. used lithium bis(fluorosulfonyl)imide (LiFSI)-based electrolytes to form a protective layer in situ to mitigate the dissolution of the cathode, which successfully enabled the Li-FeF2 battery to exhibit more than 1,000 stable cycles. Quantum chemical calculations show that the decomposition of LiFSI at the cathode side produces a LiF-rich protective layer.
Another strategy to extend the application of FeF3 is incorporating structural water. The crystal structure of FeF3 changes significantly when it contains crystalline water, and several special structures can even accommodate the insertion of Na+. Hexagonal bronze tungsten (HTB) type FeF3·0.33H2O, which has FeF6 octahedra connected by shared F atoms to form a hexagonal cavity, forms a tunnel structure along the  direction (Figure 3A, the third one)[19,30]. In the tunnel, water molecules occupy 1/3 of the cavity position, and the remaining space can accommodate ion insertion and extraction. This unique structure facilitates rapid ion transport in HTB-type FeF3[19,30]. Conte et al. monitored the reaction by Mossbauer spectroscopy to unveil the reaction mechanism. Their findings implied that during the first discharge, the reduction of Fe3+ to Fe2+ occurs through two different routes: one portion maintains the original HTB structure, while another portion has a disordered rutile structure. The rutile phase is then restored to the HTB phase during charging. When the voltage is below 2 V, the Fe phase appears, which increases the capacity but decreases the cycling stability. DFT calculations also show that water molecules in FeF3·0.33H2O are necessary for structural stability, and only 0.66Li+ can be inserted to form Li0.66FeF3·0.33H2O. In 2010, Li et al. reported a non-aqueous low-temperature synthesis of HTB-FeF3 based on ionic liquids, which exhibited a capacity of 154 mAh g-1 in the first ten cycles, with a typical charge/discharge plateau observed at around 2.7 V without significant polarization during the cycle [Figure 3D]. Zhang et al. packaged FeF3·0.33H2O in 3D ordered mesoporous carbon (3D-OMCs) as the cathode material for SIBs, and the first discharge capacity was
The one-dimensional channels of HTB-type FeF3·0.33H2O indeed facilitate ion transport. However, the single ion channels are easily partially blocked by water molecules, which limits the application of HTB-type FeF3 in SIBs. When the water of crystallization in FeF3 increases to 0.5, we get pyrochlore-type FeF3·0.5H2O, whose structure becomes similar to that of AlF3·0.5H2O. FeF6, as the basic unit in the crystal, is connected to form hexagonal cavities by shared F atoms, and these cavities are connected to form a 3D porous structure (Figure 3A, the last one)[29,34]. This structure has a larger cell volume (~1,130 Å3) than HTB-FeF3 (710 Å3) and ReO3-type FeF3 (310 Å3). At the same time, the interconnected 3D channels exhibit a higher pore density, which eliminates the need for stringent considerations in terms of pore size, especially for ions with a large radius, such as Na+. The water molecules in them are limited by the zigzag channels to have less effect on the ion storage. Li et al. obtained 10 nm FeF3 nanospheres with graphene coating in the presence of single-walled CNTs (SWNT), Fe(NO3)3·9H2O, and the ionic liquid, BmimBF4 or C10mimBF4. The material exhibited excellent lithium storage performance (143 mAh g-1 at the first discharge, and the capacity was maintained at 135 mA g-1 for the next 300 cycles. Figure 3E) and sodium storage performance
In addition to Fe-based fluorides, Ti, Mn, and Co fluorides were tested for LIBs and SIBs, but none of them can be recharged after the initial discharge. The electrochemical properties of metal fluoride cathodes are summarized in Table 1. Iron-based fluorides show very high specific capacity during charging and discharging, but the occurrence of the conversion reaction will seriously affect the reversibility of the material, and limiting the discharge cutoff voltage to above 2 V can improve the cycling performance but will reduce the energy density, which requires us to make reasonable trade-offs in practical applications. As it does not contain lithium/sodium ions, it needs to be pre-treated when combined with graphite and hard carbon to form a full battery.
Electrochemical properties of typical metal fluorides in lithium-ion batteries.
(V vs. Li+/Li)
density (Wh kg-1)
Fluorinated polyanionic cathodes
Metal fluorides have great capacity but are limited by their extremely low conductivity and slow kinetics, bringing difficulties in practical applications. In order to take advantage of the high redox potential brought by the high electronegativity of fluorine while waiving the slow kinetics of conversion reactions, researchers have turned their attention to fluorine-based polyanionic compounds. Polyanionic materials exhibit good thermal stability[36,37]. In polyanionic cathode materials, a number of anionic groups are reported ((XO4)mn-, X = P, S, Si, As, etc.)[38,39]. The elevation of the electronegativity of the X atom in the anionic group enhances the ionic character of the redox metal species, thereby increasing the redox potential, which allows us to modulate the working voltage of a material by changing the anionic group type or by combining multiple anionic groups. lithium-/sodium- fluorophosphates (AxMy(PO4)mFn) and fluorosulfates (AxMy(SO4)mFn) are the most typical fluorine-based polyanionic compounds in LIBs and SIBs, known for their high operating voltage and excellent thermal stability.
Lithium-/sodium- fluorophosphates (AxMy(PO4)mFn)
Exploring fluorophosphate chemistry, LiVPO4F (LVPF) was first introduced as a cathode material for LIBs by Barker et al. in 2003. As shown in Figure 4A, LiVPO4F has a triclinic structure consisting of VO4F2 octahedral chains that are interconnected by PO4 groups to form a 3D framework, of which 18% of the Li occupies the penta-coordination sites (LiO5) while the remaining 82% occupies the octahedral sites
Figure 4. (A) Crystal structures of LiVPO4F, NaFePO4F, and Na2FePO4F. (B) Electrochemical curves of LiVPO4F/C composite cycled vs. Li+/Li starting in discharge show Li-ion insertion/extraction potential (as shown by the red arrow). This figure is quoted with permission from Ellis et al.. (C) The characteristic discharge profiles of the 5 wt% ZrO2 coated Li2CoPO4F cathode materials between 2 and
As analogs to LiVPO4F, LiFePO4F and LiTiPO4F share isostructural structures to LiVPO4F. LiFePO4F can exhibit an insert (extract) Li+ potential of 2.71 V (vs. Li+/Li) with a first discharge capacity of 129 mAh g-1 at a current density of 150 mA g-1. LiTiPO4F demonstrates two reversible charge/discharge plateaus at 2.8 V and 1.7 V (vs. Li+/Li) and can provide a capacity of 157 mAh g-1. However, their practical application is hindered by the limited energy density caused by a low operating voltage. As fully discharged states (Li-rich states), another type of material known as Li2MPO4F (where M can be Mn, Fe, Co, or Ni) holds the potential to reach high energy density by accommodating more than one Li+ ion on a single transition metal[46,47]. In particular, Li2CoPO4F exhibits a high discharge plateau above 4.5 V and good performance (109 mAh g-1) at 2.0-5.5 V (vs. Li+/Li, Figure 4C).
Lithium-/sodium- fluorophosphates are also widely used in SIBs. Similar to LiVPO4F, NaVPO4F is used as a cathode material in SIBs. NaVPO4F has a monoclinic structure with two PO4 tetrahedra, sharing two O atoms connected to two different VO4F2 octahedra. When used as the cathode for SIBs, NaVPO4F synthesized by hydrothermal method showed the best performance, delivering a capacity of ~140 mAh g-1 (at 0.1 C) and a discharge plateau of 3.4 V [Figure 4D]. In its fully discharged state, Na2FePO4F belongs to the orthorhombic crystal system, which has a layered structure with two different Na sites consisting of FeO4F2 and Fe2O7F2 octahedra[43,50]. This material can exhibit efficient Na+ insertion and extraction at 3.0 V, with an initial discharge capacity of 120 mAh g-1. Besides, Na2MPO4F (M = Co, Mn) with the isostructural structure was also developed. Zou et al. synthesized pure Na2CoPO4F by spray drying methods, which exhibit a voltage plateau up to 4.3 V and a discharge capacity of 107 mAh g-1. Wu et al. synthesized carbon-coated hollow spheres Na2MnPO4F using the same method, which could observe a plateau of 3.6 V at room temperature and an initial discharge capacity of 102.4 mAh g-1 at a current density of 15 mA g-1. As another sodium fluorophosphate with different stoichiometry, Na3V2(PO4)2F3 is a 3D network consisting of [V2O8F3] octahedra and [PO4] tetrahedra. Upon the extraction of two Na+ ions from the framework of
The inherently low conductivity discussed above greatly limits the practical application of fluorophosphate materials. In order to solve this obstacle and improve the electrochemical performance, three major strategies with respect to nanostructure engineering, surface coating, and metal doping are developed.
Lithium-/sodium- fluorosulfates (AxMy(SO4)mFn)
Another promising catalog of the family of fluorosubstituted polyanionic compounds, fluorosulfates, is considered as higher voltage cathode materials for batteries due to the enhanced inductive effect of sulfate than phosphate. Tavorite-type LiFeSO4F, reported as a cathode material for LIBs, has a triclinic crystal structure consisting of shared-angle FeO4F2 octahedral chains oriented alternately along the c-axis. These chains connect isolated SO4 tetrahedra to form a 3D framework and form diffusion channels along the , , and  directions [Figure 5A][59-61]. In LIBs, it exhibits a reversible capacity of
Figure 5. (A) Crystal structures of tavotite and triplite-type LiFeSO4F and triplite-type type NaFeSO4F. This figure is quoted with permission from Ati et al.. and Tripathi et al.. (B) The tavorite LiFeSO4F has a stable cycling performance. Charge/discharge profile of LiFeSO4F cells cycled between 2.5 and 4.2 V at 15 mA g-1. This figure is quoted with permission from Recham et al.. (C) Charge and discharge profiles for triplite-type NaFeSO4F discharge at 0.01 C (1C = 138 mA g-1). This figure is quoted with permission from
Besides the above-mentioned crystalline fluorosulfides, amorphous fluorosulfates have also received attention from researchers. Heo et al. reported a-LiFeSO4F, a stable and reversible amorphous fluorinated iron sulfate electrode. During the discharge process, the amorphous LiFeSO4F consists of two processes that can be represented by the following equation:
a-FeSO4 + Li+ → LiFeSO4 (Intercalation reaction at ~3.59 V vs. Li+/Li)
b-LiFeSO4F + 2Li+ + 2e- → Fe + LiFeSO3F + Li2O (Conversion reaction at ~2.18 V vs. Li+/Li)
This mechanism is also evidenced by the dQ/dV profile [Figure 5D], where sharp redox peaks can be observed at 3.6 V and 3.9 V for tavorite- and triplite-type LiFeSO4F, but a-LiFeSO4F does not display similar peaks in this interval; instead, it exhibits a clear redox peak at 2.2 V and a capacity of 300 mAh g-1 at 25 °C (current density: 40 mA g-1) without significant capacity attenuation in 200 cycles [Figure 5E]. The superior cycling stability can be credited to the distinctive amorphous structure of a-LiFeSO4F that can remain intact after the conversion reaction and serve as the main host for subsequent Li+ insertion.
The electrochemical properties of fluorinated polyanionic compounds are summarized in Table 2. It can be seen that these electrodes generally have high operating voltages. Additionally, they also have good cycling performance and thermal stability due to their stable structural framework.
Electrochemical performance of fluorinated polyanionic cathodes
(V vs. Li+/Li or Na+/Na)
density (Wh kg-1)
THE ROLE OF FLUORINE IN ELECTROLYTES AND INTERFACE
The electrolyte undertakes the function of ion transport, so the stability of the electrolyte and its properties are crucial to the performance of the battery. The introduction of fluorine into the electrolyte and the construction of a robust EEI are two effective ways from the application perspective. The introduction of fluorine can improve the oxidative stability of the electrolyte. At the same time, the introduction of fluorine into the electrolyte can change the physicochemical properties of the electrolyte, such as melting point, flash point, etc., which makes the electrolyte have some unique functions, such as a wide range of operating temperatures, high safety, etc. The construction of a stable EEI can overcome the thermodynamic instability between the electrolyte and the electrode, promote the uniform deposition of ions, and reduce the loss of active metals.
Functionalized fluorinated electrolytes
Functionalized fluorinated electrolytes could extend the application scenarios of rechargeable batteries, such as high/low temperature operation, high safety, high power, etc., as discussed below:
Previous studies have shown that fluorinated solvents can effectively enhance the low-temperature performance of electrolytes. The substitution of fluorine typically lowers the freezing point of most solvents, enhancing the conductivity of the electrolyte during cold temperatures. This, in turn, improves the battery performance under low-temperature conditions. Fluorine, as a strong electron absorbing group, weakens the electron-giving ability of the solvent/co-solvent when there is a fluorine substitution on them. Fluorine-substituted solvent/co-solvent molecules have lower binding energies (in absolute terms) when coordinated with Li+, which means that introducing fluorine into solvent/co-solvent molecules can make it easier to detach Li+ from the solvation structure. Yang et al. proposed an ethyl trifluoroacetate (ETFA)-based electrolyte. Due to the high electronegativity of the fluorine, the binding energy of this solvent to Li+ is much lower than that of ethylene carbonate (EC), which allows the rapid de-solvation of Li+ from the solvation structure. Owing to the fast de-solvation process, Li||LTO cells with this electrolyte achieve capacities of 154 mAh g-1 and 78 mAh g-1 at -50 °C and -70 °C, respectively [Figure 6A].
Figure 6. (A) Charge/discharge profiles of LTO||Li cells using the ETFA electrolyte under different temperatures. This figure is quoted with permission from Yang et al.. (B) Schematic diagram of fluorine flame retardant principle; (C) Charge/discharge profiles of the LiNi0.8Co0.15Al0.05O2/Li half cells at various temperatures, which demonstrates the excellent high temperature stability of fluorinated solvents. This figure is quoted with permission from Shiga et al.. (D) Nail penetration tests of the 5 Ah graphite||NMC pouch use LiPF6-EC/EMC (left) and coordination number-rule electrolyte. This figure is quoted with permission from Chen et al.. (E) The linear sweep voltammetry curves of the electrolyte without and with F-EPE, which shows fluoride electrolytes could operate under high potential. This figure is quoted with permission from Luo et al..
Introduction of fluorinated molecules into the electrolyte also could extend the high temperature adaptability. Generally, organic molecules become less combustible as the degree of F substitution increases, which makes the electrolyte safer. In addition, F atoms also have a strong attraction to H atoms, which allows F to absorb H radicals during combustion and quench the chain reaction, increasing the flash point and flame retardant properties of the solvent [Figure 6B]. An ultra-concentrated electrolyte composed of LiFSA and fluorinated alkyl phosphate was prepared by Shiga et al. This electrolyte was self-extinguishing and allowed reversible insertion of Li+ into graphite, and the electrolyte also exhibited ultra-high thermal stability. Such features allow graphite/Li half-cells with this electrolyte to operate at temperatures exceeding 100 °C [Figure 6C]. Our group has proposed an electrolyte using trifluoroethyl phosphate and trifluoroethyl methyl carbonate as solvents. After nail penetration, pouch cells using this electrolyte still keep compact, while pouch cells using commercial 1m LiPF6-EC/EMC (EMC: Ethyl Methyl Carbonate) electrolytes immediately catch fire [Figure 6D]. Notably, due to the advantages of the F-substitute solvent described earlier, the pouch battery using the electrolyte can also operate stably within a temperature range of -40-60 °C.
The electron-withdrawing inductive effect results in a decrease in the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels when fluorine is introduced into the solvent molecule[70,71]. The decrease of the HOMO energy level enhances the anti-oxidative properties of the molecule, which enables this substance to be compatible with high potential cathodes, thus meeting the high-energy requirements. Luo et al. expanded the electrochemical window by adding 10% of fluorinated 3-(1,1,2,2-tetrafluoroethoxy)-1,1,2,2-tetrafluoropropane (F-EPE) solvent to 1.2 M LiPF6 in EC/EMC electrolytes. With the introduction of F-EPE, the potential window of the electrolyte extends above 5.2 V [Figure 6E]. This high potential window allows Li||LiNi0.5Mn1.5O4 cells to have higher stability and coulombic efficiency.
Fluorinated interface engineering
Briefly, the essence of the formation of the EEI is a redox reaction between the electrode and the electrolyte. Taking LIBs as an example, SEI has many components, such as Li2O, LiOH, Li2CO3, Li3N,
HOMO and LUMO directly reflect the redox stability of the electrolyte. Meanwhile, both the cathode and anode have their own electrochemical potentials. If the anode potential is higher than LUMO, then the electrons on the anode will tend to transfer to the electrolyte, thus causing the electrolyte to reduce, creating a solid-electrolyte interface and preventing the reaction from proceeding further. A similar situation occurs at the cathode, creating a cathode-electrolyte interface (CEI) [Figure 7A]. In addition to preventing the electrolyte from further decomposition, the current study also found that a fluorinated interface has a positive role in protecting the electrodes and promoting the uniform deposition of Li [Figure 7B].
Figure 7. (A) Schematic diagram of the mechanism of SEI and CEI formation. This figure is quoted with permission from
Many efforts have been made by researchers to obtain stable fluorinated interfaces. The straight ward method to get robust EEI is pretreatment by ex-situ coating a layer of LiF on Cu or Li to improve electrochemical performance. Zhang et al. plated LiF on the surface of Cu collectors. LiF-rich SEIs guided the deposited Li into a stabilized structure, which greatly contributed to the stability of such anodes.
The second way to generate a fluorinated interface is from organic components (either organic additives or solvents). As we mentioned before, fluorine substitution reduces the LUMO energy levels of the components in the electrolyte, which makes the fluorinated electrolyte components easier to reduce and thus generate stable SEIs. In addition, theoretical calculations also show that fluorinated solvent molecules that are in the primary solvated shell layer of the Li cation are more easily reduced. EC is the most widely used solvent commercially, and its fluorine-substituted product, fluoroethylene carbonate (FEC), is the most widely used film-forming additive for a variety of batteries, including LIBs, LMBs, and SIBs[85-87].
To improve the effectiveness of FEC, the researchers increased the amount of FEC in the electrolyte.
The third strategy for achieving a fluorinated interface comes from the inorganic component. The salt is another important component in the electrolyte. It has been shown that most of the F-containing lithium salts (e.g., LiPF6, LiBF4, LiFSI, etc.) can involve the F in them in the generation of LiF-rich SEI[91-93]. The decomposition of these salts produces an electrodeless fluorinated interface with excellent physical and chemical properties. Chen et al. designed an electrolyte [2.0 m LiPF6 in 1:1 v/v mixture of tetrahydrofuran and 2-methyltetrahydrofuran (mixTHF)]. The use of ether with a low thermodynamic reduction potential (0.0-0.3 V vs. Li+/Li) as a solvent allowed PF6- to become preferentially decomposed relative to being in a carbonate-based electrolyte [Figure 8A-C]. The SEI produced by the decomposition of inorganic salts greatly improves the cycling stability of the battery.
Figure 8. (A) Schematic of three general categories of solvent-salt interactions. (B) Distribution of the Li+ solvates for the LiPF6-THF (2.0 M) from MD simulations. This figure is quoted with permission from Chen et al.. (C) Reduction potentials of key electrolyte components with the first Li+ solvation shell, which shows that fluorinated salts will decompose preferentially. This figure is quoted with permission from Chen et al.. (D and E) Calculated electronic structures (projected density of states, pDOS) of dilute D and concentrated E LiTFSA/AN electrolytes, which suggests a gradual transfer of LUMO to anion with increasing concentration. This figure is quoted with permission from Yamada et al.. (F) Charge/discharge profiles of Na||HC(hard carbon) cells using the concentrated NaFSA/TMP electrolyte. This figure is quoted with permission from Wang et al.. (G) Cycling performance with different electrolytes at 2 C and 25 °C. This figure is quoted with permission from Piao et al..
When the salt concentration is increased, the anion decomposition tendency will be improved. DFT- molecular dynamics (MD) simulations illustrate that the LUMO energy level of the electrolyte gradually shifts from the solvent to the anion when the salt concentration is gradually increased [Figure 8D and E]. Wang et al. developed highly concentrated electrolytes for LIBS and SIBs, using trimethyl phosphate (TMP) as solvents and NaN(SO2F)2 (NaFSA) or LiN(SO2F)2 (LiFSA) as salts. Both electrolytes showed excellent performance (almost no degradation for 1,000 cycles, Figure 8F) when the NaFSA and LiFSA concentrations were increased to 3.3 m and 5.3 m, respectively.
The excellent SEI brought by high concentration electrolytes can significantly improve battery performance, but the higher viscosity and cost greatly limit the practical application of high concentration electrolytes. To address these two drawbacks, researchers have proposed an electrolyte known as localized high concentration electrolyte (LHCE). By adding one or more diluents that are immiscible with salt but miscible with solvents to the electrolyte, the electrolyte is saturated at a low concentration and exhibits low viscosity and ionic conductivity. Piao et al. added 1,1,2,2-Tetrafluoroethyl-2,2,3,3-tetrafluoropropylether (TTE) as a diluent to LiFSI/DMC electrolyte to form LHCE. MD simulations revealed that the introduction of diluent enhances the interaction between Li+ and FSI- so that the FSI- can be preferentially reduced to form LiF-rich SEI. This electrolyte enables stable cycling of Li||NMC622 cells at both 25 and 60 °C [Figure 8G].
CONCLUSION AND OUTLOOK
Fluorine chemistry plays a pivotal role in the pursuit of high energy density, high stability, and high safety batteries. Figure 9 displays a comprehensive comparison of the potential, specific capacity, and specific energy images of fluorinated electrodes. Transition metal fluorides exhibit excellent specific capacity and specific energy through conversion reactions, and fluorinated polyanion compounds have higher operating voltages as a consequence of the inductive effect of F-. Fluorinated polyanionic compounds also have a high working potential due to the inductive effect of fluorine, while the stable backbone can withstand the insertion/extraction of lithium-/sodium- ions in them. However, the introduction of fluorine also poses challenges for both types of materials. The high electronegativity of fluorine gives transition metal fluorides and fluorinated polyanion compounds very low conductivity, which results in very high polarization during charging and discharging. Transition metal fluorides also suffer from uneven distribution of the active substance, severe volume changes, and dissolution of the active substance in the conversion reaction.
Aiming to facilitate fluorinated electrodes towards practical applications, more efforts should be devoted to the following topics:
(1) Transition metal fluorides generally cannot supply lithium ions; hence, they are unable to couple with lithium-free anodes, such as graphite and silicon, in LIBs. Nevertheless, if matched with lithium metal anode in solid-state lithium batteries, the high-capacity advantage of the fluoride cathodes will be more prominent. One thing that needs to be emphasized is that the sluggish reaction kinetics and large voltage hysteresis of transition metal fluoride cathodes always lead to low energy conversion efficiency and insufficient output power, which is detrimental to their competitiveness.
(2) Fluorinated polyanionic cathode materials have received particular attention in SIBs due to their superiority of high operating voltage compared to the commonly studied layered transition metal oxides and Prussian blue analog cathodes. To pursue high electrochemical performance, strategies such as structural regulation and morphology tailoring are often incorporated into the synthesis procedure of fluorinated materials, which would undoubtedly bring increased manufacturing costs.
(3) Only in at least Ah-grade batteries can we accurately evaluate the practical electrochemical properties of fluorinated electrodes; the data obtained from the button battery test are not representative and may even be deceptive. In addition, fluoride ions involved in cathode materials may cause safety and environmental issues in material production, battery manufacturing, and recycling processes, which should be paid more attention to.
Fluorine also occupies a pivotal position in functional electrolytes. The physical and chemical properties of electrolyte solvents are significantly altered after fluorine-containing components are incorporated. The lower freezing point and the ability to bond with cations enable the fluorine electrolyte to exhibit excellent performance at low temperatures. The flame retardant property of fluorine also makes the electrolyte show excellent safety performance at high temperatures. The decreased HOMO energy level allows the electrolyte to operate at higher voltages, thus realizing high specific energy.
Regarding the EEI, a fluorinated interface is essential due to its high ionic conductivity, electron insulation, and mechanical robustness. A stable EEI could be built from the electrolyte by designing solvated structures.
Although fluorinated electrolytes have made remarkable contributions to the advances in battery technology, their potential has not yet been fully explored. Future research should focus on the following aspects.
(1) Fluorine-containing components in the electrolyte, such as salts, solvents, additives, etc., enable LIBs and SIBs to achieve improved performance, but their potential toxicity and corrosion will also raise concerns; hence, robust battery pack components should be rationally designed, and advanced battery recycling technologies should be developed.
(2) The interaction between solvents and salts greatly affects the performance of electrolytes. Considering the complexity and variability of the electrolyte composition, it is not a realistic way to optimize the composition of the electrolyte through experiments. Theoretical calculations, such as MD simulations and DFT, are powerful tools to investigate the intermolecular interactions in electrolytes. The in-depth inspection of the solvation structure can provide theoretical guidance for us to better design functionalized electrolytes.
(3) Fluorinated components endow the electrode/electrolyte interfaces with excellent mechanical/chemical/electrochemical stability. However, the interface is susceptible to electron radiation, X-rays, oxygen, and moisture and gradually ages during the cycle, causing a decline in battery performance. Therefore, monitoring the evolution of interface composition and structure is the basis for designing a robust and reliable interface.
In this review, we discuss the characteristics of fluorine and its unique role in the electrode, electrolyte, and EEI. The high electronegativity and strong inductive effect of fluorine can effectively increase the electrode operating voltage, and the strong electron-absorbing tendency of fluorine in the electrolyte can give the electrolyte the advantages of wide operating temperature and high potential window. The characteristics of LiF, such as wide forbidden band, high ion diffusion rate, low solubility, and high Young's modulus, make LiF a star component of EEIs. These unique roles of fluorine presented here may provide new ideas for future battery design.
Proposed the topic of this review: Chen Z
Prepared the manuscript: Pan Z, Pu X
Collectively discussed and revised the manuscript: Pan Z, Chen H, Zeng Y, Ding Y, Pu X, Chen Z
Availability of data and materials
Financial support and sponsorship
This work was financially supported by the National Natural Science Foundation of China (U22A20438, U22A20193), the Key R&D Plan of Hubei Province (2023BAB036), and the Fundamental Research Funds for the Central Universities.
Conflicts of interest
All authors declared that there are no conflicts of interest.
Ethical approval and consent to participate
Consent for publication
© The Author(s) 2023.
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Cite This Article
Pan Z, Chen H, Zeng Y, Ding Y, Pu X, Chen Z. Fluorine chemistry in lithium-ion and sodium-ion batteries. Energy Mater 2023;3:300054. http://dx.doi.org/10.20517/energymater.2023.61
Pan Z, Chen H, Zeng Y, Ding Y, Pu X, Chen Z. Fluorine chemistry in lithium-ion and sodium-ion batteries. Energy Materials. 2023; 3(6): 300054. http://dx.doi.org/10.20517/energymater.2023.61
Pan, Zibing, Huaqi Chen, Yubin Zeng, Yan Ding, Xiangjun Pu, Zhongxue Chen. 2023. "Fluorine chemistry in lithium-ion and sodium-ion batteries" Energy Materials. 3, no.6: 300054. http://dx.doi.org/10.20517/energymater.2023.61
Pan, Z.; Chen H.; Zeng Y.; Ding Y.; Pu X.; Chen Z. Fluorine chemistry in lithium-ion and sodium-ion batteries. Energy Mater. 2023, 3, 300054. http://dx.doi.org/10.20517/energymater.2023.61
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